WO2017054037A1 - Controlled growth of three-dimensional heterogeneous nanocrystals - Google Patents

Abstract

Disclosed is a method for controlled growth of three-dimensional nanocrystals, and the nanocrystals resulting therefrom. In one form, there is provided a method of controlled nanocrystal growth, comprising the steps of: partial dissolution of a core part of a nanocrystal; and epitaxial growth of new nanocrystal onto a selective surface of the nanocrystal. In examples, the nanocrystal is a hybrid nanocrystal, and/or is or includes an alkaline rare-earth fluoride (AREF₄) nanocrystal(s). A surfactant can be used to control the nanocrystal growth.

Description

CONTROLLED GROWTH OF THREE-DIMENSIONAL

HETEROGENEOUS NANOCRYSTALS

Technical Field

[001] The present invention generally relates to methods for controlled growth of three- dimensional nanocrystals and/or the produced three-dimensional nanocrystals with integrated functions. Background

[002] A general goal for nanomaterials engineering is to achieve reproducible and scalable techniques for synthesis of nanostructures, for example three-dimensional nanocrystals. Desirably, the nanocrystals would be synthesized according to a design with controlled size, uniformity, morphology, composition, distribution, and/or physical/chemical properties, so that functionalities can be tailored, integrated and/or optimized leading to a higher level of performance in a given application.

[003] More specifically, as an example non-limiting application, rare-earth-doped upconversion nanocrystals have recently emerged as a new generation of functional nanomaterials, because they exhibit exceptional optical, magnetic and chemical properties underpinning their diverse applications. In particular, alkaline rare-earth fluoride (AREF₄) nanocrystals, including hexagonal-phase P-NaYF₄, P-NaGdF₄, P-NaNdF₄ or P-NaLuF₄ are used in full-colour displays, photovoltaics, security inks, forensic science, autofluorescence-free biomolecular sensing, multimodal in vivo bio-imaging (fluorescence, MRI, X-ray, SPECT, etc.), and theranostics.

[004] Previously, a trial-and-error approach has been used to produce nanocrystals with a desired shape, for example spherical or rod-like shapes by varying the concentration of reagents, the reaction time or the temperature. However, this trial-and-error approach to sampling of a vast, multidimensional parameter space is inefficient and occurs without any proper understanding of the underpinning growth mechanisms.

[005] The reference in this specification to any prior publication (or information derived from the prior publication), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from the prior publication) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. Summary

[006] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Preferred Embodiments. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[007] In one aspect, there is provided a method for controlled growth of three- dimensional nanocrystals. In another aspect, there is provided three-dimensional nanocrystals, synthesized using a method for controlled growth of the three-dimensional nanocrystals.

[008] In another aspect, there is provided a method for controlled growth of monodisperse three-dimensional nanocrystals, and/or the produced monodisperse three- dimensional nanocrystals. In another aspect, there is provided a method for controlled growth of heterogeneous three-dimensional nanocrystals, and/or the produced heterogeneous three-dimensional nanocrystals.

[009] In yet another aspect, there is provided a method for controlled growth of three- dimensional nanocrystals less than about 50 nm in size, in at least one dimension, and/or the produced three-dimensional nanocrystals less than about 50 nm in size in at least one dimension.

[010] In another aspect, there is provided a method for controlled growth of monodisperse heterogeneous three-dimensional nanocrystals less than about 50 nm in size, and/or the produced monodisperse heterogeneous three-dimensional nanocrystals less than about 50 nm in size.

[011] It should be appreciated that reference to a nanocrystal(s) can also be considered as a reference to a nanoparticle(s), nanostructure(s) or nanomaterial(s). [012] In one form, there is provided a method of controlled nanocrystal growth, comprising the steps of: partial dissolution of a core part of a nanocrystal; and epitaxial growth of new nanocrystal onto a selective surface of the nanocrystal. [013] In another form, there is provided a nanocrystal, produced by partial dissolution of a core part of a nanocrystal; and epitaxial growth of new nanocrystals onto a selective surface of the nanocrystal.

[014] In another form, there is provided a method of controlled nanocrystal growth, comprising the steps of: partial dissolution of a core part of a nanocrystal; and epitaxial growth of new nanocrystal onto at least one preferential surface of the nanocrystal; wherein a surfactant in solution is used to control the nanocrystal growth.

[015] Preferably, selecting or controlling, or otherwise altering or moderating, a ratio of surfactant ions to surfactant molecules (by concentration) is used to control the nanocrystal growth. In an example, selecting the ratio of surfactant ions concentration to surfactant molecules concentration, of the surfactant in solution, is used to control anisotropic nanocrystal growth by the partial dissolution of the core part and the epitaxial growth of new nanocrystals onto the at least one preferential surface of the nanocrystal. Preferably, though not necessarily, the surfactant is oleic acid.

[016] Preferably, though not necessarily, the nanocrystal is a heterogeneous nanocrystal. In one example, the nanocrystal is or includes an alkaline rare-earth fluoride (AREF₄) nanocrystal.

[017] In another example aspect, the partial dissolution of the core part of the nanocrystal and the epitaxial growth of new nanocrystal(s) are controlled by selecting a ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration. [018] According to one example, the ratio of oleate ions (OA^") concentration to oleic acid molecules (OAH) concentration is greater than or equal to 1 : 3, or is greater than or equal to 1 :4, or is greater than or equal to 1 :5, or is greater than or equal to 1 :6, or is greater than or equal to 1 :7, or is greater than or equal to 1:7.6. [019] According to another example, the ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration is less than or equal to 1 :40, or is less than or equal to 1 :30, or is less than or equal to 1 :25, or is less than or equal to 1 :20. [020] According to another example, the ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration is about 1:3, preferably about 1:3.2.

[021] According to another example, the ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration is between about 1:40 and about 1:3, or is between about 1:30 and about 1:3, or is between about 1:30 and about 1:4, or is between about 1:30 and about 1:5, or is between about 1:30 and about 1:6, or is between about 1:30 and about 1:7.

[022] According to another example, the ratio of oleate ions (OA^") concentration to oleic acid molecules (OAH) concentration is between about 1:20 (for example 75 μιηοΙ/mL: 1508 μπιοΙ/mL) and about 1:7.6.

[023] In various aspects: the shape of the nanocrystal is controlled; the composition of the nanocrystal is controlled; and/or the aspect ratio of the nanocrystal is controlled.

[024] In one example, the epitaxial growth is longitudinal epitaxial growth. In another example, longitudinal epitaxial growth is induced by selecting a ratio of oleate ions (OA^") concentration to oleic acid molecules (OAH) concentration greater than or equal to about 1 :7, preferably about 1 :7.6.

[025] Optionally, KOH is added to increase longitudinal epitaxial growth. In another example, transversal epitaxial growth is induced by selecting a ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration less than or equal to about 1:20.

[026] In another example, the nanocrystal is or includes a NaYF₄ nanocrystal.

[027] In another example, the nanocrystal growth includes deposition of shells onto end surfaces of the core part.

[028] In another example, the shells are formed of NaGdF₄, and the core part is formed of NaYF₄. In another example, the nanocrystal is or includes a p-NaREF₄ nanocrystal. [029] In other examples, the RE in the alkaline rare-earth fluoride (AREF₄) nanocrystal is Y, Gd, Lu and/or Nd. [030] Optionally, the nanocrystal is a hybrid nanocrystal. In various examples, the hybrid nanocrystal is: NaYF₄-NaGdF₄-NaNdF₄; NaYF₄-NaLuF₄; or NaYF₄-NaLuF₄-NaGdF₄.

[031] In another example, the nanocrystal is less than about 50 nm in size in at least one direction. In another example, dissolution occurs at one or more side surfaces.

[032] In various examples, the shape of the nanocrystal is or approximates: a rod shape; a bamboo shape; an hour-glass shape; a flower shape; a co-axial cylinder shape; a pin with double ring shape; a disc shape or a dumbbell shape. [033] In another example, the ratio of OAVOAH concentration is selected to control the direction of epitaxial shell growth.

[034] In another example, the ratio of OAVOAH concentration is altered by changing the amount of NaOH, and/or the amount of OA, and/or the amount of rare earth salt.

[035] In another example, the temperature is selected to influence growth along the transverse direction. In further examples, the selected temperature is within the range of from about 290 °C to about 310 °C. In another example, a reagent concentration is selected to influence growth.

Brief Description of Figures

[036] Example embodiments should become apparent from the following description, which is given by way of example only, of at least one preferred but non-limiting embodiment, described in connection with the accompanying figures.

[037] Figure 1 illustrates example molecular bonding models of OAH and OA^" surfactants to the crystalline facets of P-NaYF₄.

[038] Figure 2 shows Transmission Electron Microscopy (TEM) characterization of transversal, longitudinal and successive heterogeneous growth of example P-NaREF₄ nanocrystals. [039] Figure 3 illustrates evolution of morphology and composition of the example heterogeneous NaREF₄ nanocrystals after different reaction time lengths. [040] Figure 4 illustrates example programmable routes for fabricating of example three- dimensional nanoscale architectures of particles by using longitudinal, transversal and/or migration growth techniques.

[041] Figure 5 illustrates an example method of providing controlled nanocrystal growth.

[042] Figure 6 illustrates (a) SEM images of example NaYF₄ disks and rods synthesized under hydrothermal conditions and with different amounts of NaOH (Scale bars: 1 μιη). (b) The observed aspect ratio of NaYF₄ crystals graphed against the amount of NaOH used in the reaction mix.

[043] Figure 7 illustrates real-time monitoring of the epitaxial growth process of example crystals from their NaYF₄ nanocrystal cores to their longitudinal form as NaYF₄ nanorods. TEM images of the NaYF₄ core (a) and their products after step-by-step sampling at after 1 minute (b), 10 minutes (c), 20 minutes (d), 30 minutes (e), 40 minutes (f), 50 minutes (g) and 60 minutes (h) of reaction time (Scale bars: 100 nm).

[044] Figure 8 illustrates a chart graph showing the size evolution of both a phase NaYF₄ nanocrystals in diameter and P-NaYF₄ nanorods in their width and length as function of reaction time from 0 minutes to 60 minutes.

[045] Figure 9 illustrates TEM images and size distributions of example NaYF₄ core nanocrystals (a) and the NaYF₄ nanocrystals after epitaxial growth in the longitudinal direction (b). The size of the NaYF₄ visibly increases in the longitudinal direction after the secondary growth of NaYF₄ shell from 24 nm to 47 nm. This was accompanied by a slight increase (1.8 nm) in their transversal dimension. (Scale bar is 100 nm)

[046] Figure 10 illustrates TEM images of example NaYF₄ core (a) and example NaYF₄ coated with NaYF₄ shell using NaOA as a sodium source (b) and example NaYF₄ nanorods synthesized with NaOH as a sodium source (c). [047] Figure 11 illustrates real-time monitoring morphology evolution in an example process of longitudinal growth of heterogeneous NaGdF₄ shell onto NaYF₄ cores. The TEM images of the NaYF₄ cores (a) and the evolution of its heterogeneous products are shown by step-by-step sampling during the reaction at 310°C after 1 min (b), 15 min (c), 30 min (d), 45 min (e), 60 min (f) of reaction time. (Scale bars: 100 nm)

[048] Figure 12 illustrates TEM images, size distributions and characterization of example elemental compositions of heterogeneous growth of NaGdF₄ onto the NaYF₄ core nanocrystals. The (a) NaYF₄ core nanocrystals; and (b) the NaGdF₄ - NaYF₄ nanocrystals after epitaxial growth of NaGdF₄ in the longitudinal direction. The (c) HAADF-STEM image of a single dumbbell shaped NaYF₄ - NaGdF₄ nanocrystal is shown with its elemental mapping images.

[049] Figure 13 illustrates TEM images of an example NaYF₄ core (a) and the NaYF₄ coated with an example NaYF₄ shell without KOH present (b) and with 0.4 mmol of KOH present (c). (Scale bars: 50 nm)

[050] Figure 14 illustrates TEM images of the NaYF₄ nanorods acting as the core (a) and the longitudinal growth of the NaGdF₄ shell onto the NaYF₄ core with 0.4 mmol KOH present in the mix. (Scale bars: 50 nm)

[051] Figure 15 illustrates TEM images, size distribution and characterization of the example elemental composition of periodical NaYF₄-NaGdF₄ nanocrystals which form a bamboo-like nanostructure. (a) five-section NaYF₄-NaGdF₄ nanocrystals in a bamboo shape obtained by an epitaxial growth of NaYF₄ and NaGdF₄ in a longitudinal direction; (b) seven-section NaYF₄-NaGdF₄ nanocrystals in an bamboo shape; the length of the five- section NaYF₄-NaGdF₄ nanocrystals can reach 122 nm and its width, 35 nm. With two more sections, the seven-section NaYF₄-NaGdF₄ nanocrystals grow to 173 nm and the width increases to 42 nm. (c) HAADF-STEM image of three single NaYF₄-NaGdF₄ nanocrystals and their elemental mapping images. The HAADF-STEM image demonstrates the contrast in density between the NaGdF₄ and the NaYF₄ as well as their elemental corresponding mapping images, which confirm the distribution of the elements Y and Gd. (Scale bars: 200 nm in (a) and (b), 100 nm in (c)) [052] Figure 16 illustrates (a-b): TEM images and size distribution histograms, (a): the example NaYF₄ core nanocrystals (scale bar is 100 nm). b): the example NaYF₄-NaGdF₄ nanocrystals with transversal growth of NaGdF₄ on their side surfaces (scale bar is 100 nm). (c-d) High magnification TEM images and EDS elemental mapping images of the example NaYF₄-NaGdF₄ nanocrystals (scale bar is 50 nm); from the top view (c) and the side view (d). The EDS elemental mapping images clearly show that that NaGdF₄ is located well outside the NaYF₄ core nanocrystals in the middle part of its structure.

[053] Figure 17 illustrates TEM and elemental composition characterization of the example NaYF4-NaLuF₄-NaGdF₄ heterogeneous nanocrystals which form a unique shape of two NaGdF₄ rings onto a NaLuF₄@NaYF₄ nanorod. (a) example NaLuF₄ nanocrystal shells transversally coated onto NaYF₄ core nanocrystals; (b) an overview image of the example NaYF₄-NaLuF₄-NaGdF₄ nanocrystals and a single example NaYF₄-NaLuF₄- NaGdF₄ nanocrystal (top-left inset); (c) HAADF-STEM image of a single NaYF₄-NaLuF₄- NaGdF₄ nanocrystal and its elemental mapping images. The elemental mapping images confirm the distribution of the Y ions in the middle of the rod and the Lu ions as a thin layer coating surrounding the nanorod. The Gd³⁺ ions are present as a double ring surrounding the NaYF₄-NaLuF₄ nanorods in the transversal direction, (d) A schematic illustration of the formation process "selective mask - etching - epitaxial growth" of the NaYF₄-NaLuF₄-NaGdF₄ heterogeneous nanocrystals is provided.

[054] Figure 18 illustrates real-time montitoring of the migration growth process in the formation of example NaYF₄-NaGdF₄-NaNdF₄ heterogeneous nanoscale hourglass particles. TEM images of NaYF₄ nanocrystal (a), NaYF₄ -NaGdF₄ core-shell nanocrystals (b), the migration growth after 5 minutes (c), 15 minutes (d), 30 minutes (e), 40 minutes (f), 50 minutes (g), 60 minutes (h) and the elemental mapping of Y(h'y), Gd (h'o_d) and Nd (h' d) elements.

[055] Figure 19 illustrates example detailed schematic processes of the simultaneous erosion of the NaYF₄-NaGdF₄, epitaxial growth of NaNdF₄ in the longitudinal direction and the migration growth of F^", Y³⁺ and Gd³⁺ ions. The etching of NaGdF₄-NaYF₄ nanocrystals is initially triggered by the OA^" ions strongly bonding to the side surfaces at a high reaction temperature (310 °C). As a result, F^", Na⁺ and Y³⁺ ions are released into the solution. [056] Figure 20 illustrates TEM images of example NaYF₄/NaGdF₄ cores and their migration growth with Nd for 10 min, 20 min, 30 min, 45 min and 60 min after the start of the reaction. (Scale bars: 100 nm) [057] Figure 21 illustrates TEM images of example NaYF₄/NaGdF₄ nanocrystals as cores, and migration growth with the Nd element at 300°C after 10 minutes, 25 minutes and 45 minutes of reaction time. (Scale bars: 50 nm)

[058] Figure 22 illustrates TEM images, elemental composition characterization and schematic illustration of example NaYF₄-NaGdF₄-NaNdF₄ dumbbell-shape nanocrystals with sharp tips via migration growth of NaNdF₄.

[059] Figure 23 illustrates TEM images and schematic illustration of example NaYF₄- NaGdF₄-NaNdF₄ dumbbell nanocrystals with round polished tips.

Preferred Embodiments

[060] The following modes, given by way of example only, are described in order to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments.

[061] A frontier in nanomaterials engineering is to realize controlled fabrication or synthesis of nanostructures, such as nanocrystals. It has been sought to achieve the ability to produce integrated multifunctional nanocrystals with desirable size, shape, surface, and/or composition placements with atomic scale precision. Such control, if achieved, would be even more useful when growing hybrid nanocrystals with integrated multiple functionalities.

[062] The Applicant has achieved the required degree of control in fabrication or synthesis of nanocrystals. This is exemplified using a family of alkaline rare-earth fluoride nanomaterials. In a particular example application, the Applicant has verified the coexistence and different functionalities of oleate anions (OA^") and oleic acid molecules (OAH) in crystal formation (e.g. surfactant ions and the surfactant molecules from which the ions are dissociated). The Applicant has identified that control over the ratio of oleate anions (OA ) (i.e. surfactant ions) to oleic acid molecules (OAH) (i.e. surfactant molecules) can be used to directionally inhibit, promote, and/or etch the crystallographic facets of the produced nanocrystals, which can also be generally referred to as nanoparticles, nanostructures or nanomaterials.

[063] In an example aspect, such control enables selective grafting of shells with complex morphologies grown over nanocrystal cores, thus allowing for access to a diverse library of nanocrystals. In particular forms the nanocrystals can be heterogeneous nanocrystals, and/or monodisperse nanoparticles with a size of less than about 100 nm, or preferably less than about 50 nm, at least in one dimension, or in two dimensions, or in three dimensions. With such programmable additive and subtractive engineering three-dimensional shapes can be designed and scaled from the "bottom-up". This approach can yield a large quantity of identical single nanocrystals with three-dimensional morphology and doping, such as rare-earth doping, composition synergistically controlled.

[064] The Applicant has identified a new nanocrystal growth mechanism, termed herein "migration growth", which involves the dissolution, or termed herein "de-growth", of a core nanocrystal, migration of ions and then formation of new crystals and epitaxial growth onto the selective surfaces of the core crystal. This facilitates controlled growth of new generation nanocrystals with enhanced properties, for example enhanced optical, magnetic and/or physical properties for a broad range of applications. Example applications include MRI biomedical imaging, nanophotonics sensing, nanomedicine drug delivery, photo-catalysis, and analytical technologies.

[065] The migration growth process is based on the thermodynamic stability difference of different materials within a certain growth environment and conditions. This migration growth process can be used as a de-growth approach to selectively trim the core nanocrystal while new epitaxial growth of nanocrystals simultaneously produces heterogeneous nanostructures. Therefore, combining the migration growth with directed epitaxial growth, programmable design and growth of heterogeneous nanocrystals have been realized.

[066] It was identified that oleate anions (OA^"), as the dissociated form of oleic acid molecules (OAH), have variable, dynamic roles in mediating the growth of alkaline rare- earth fluoride (AREF₄) nanocrystals. This allows a molecular approach to tailoring the shape and/or composition of AREF₄ nanocrystals. In this particular example, the method is based on a selective epitaxial core-shell growth process in the presence of oleic acid, which is used as a surfactant during the synthesis of P-AREF₄ nanocrystals. A selection of, or change in, the ratio of OA^" to OAH, preferably measured by concentration, can be used to influence the interaction of these ligands with the particle surface and hence the resulting morphology.

[067] Computational modelling and experimental results demonstrate that the preferential affinity of OAH and OA^" to different crystalline facets dictates the formation of nanocrystals of different shape. Importantly, precise control over shell thickness and particle shape can be achieved by deliberately switching the passivative, additive and subtractive roles of these surfactants.

[068] In a general example method, and referring to Figure 5, there is provided a method 500 of controlled nanocrystal growth. A surfactant, preferably in solution, is used to control the nanocrystal growth. This offers a method of nanocrystal growth in solution where at step 510 control, selection, modification or determination of a surfactant (e.g. a concentration ratio of surfactant ions to surfactant molecules) in turn controls growth of the nanocrystal by preferential or selective growth on at least one surface or crystal plane of the nanocrystal. Selective or preferential growth could be on two or more surfaces or crystal planes. The method 500 includes partial dissolution (i.e. removal) of a core part of a nanocrystal at step 520, and epitaxial growth of new nanocrystal onto at least one preferential surface of the nanocrystal at step 530. The growth is preferably anisotropic. Thus, the surfactant, such as the ratio of surfactant ions concentration to surfactant molecules concentration in solution, is used to control a growth direction.

[069] Selecting or controlling a ratio of surfactant ions to surfactant molecules can be used to control the nanocrystal growth. The ratio of surfactant ions concentration to surfactant molecules concentration can be used to control anisotropic nanocrystal growth by the partial dissolution of the core part of the nanocrystal and the epitaxial growth of new nanocrystal(s) onto the at least one preferential or selective surface of the nanocrystal. In a preferred example the surfactant is oleic acid. The partial dissolution of the core part of the nanocrystal and the epitaxial growth of new nanocrystal(s) can be controlled by selecting or controlling a ratio of oleate ions (OA ) to oleic acid molecules (OAH). Thus, the ratio of OA70AH concentration is selected, chosen, modified, controlled or determined to control the direction of epitaxial growth. [070] The nanocrystal may be a heterogeneous nanocrystal. The nanocrystal can be or can include an alkaline rare-earth fluoride (AREF₄) nanocrystal. The shape of the nanocrystal, and/or the aspect ratio of the nanocrystal, can thus be controlled by the use of the surfactant. In another example, the composition of the nanocrystal can be controlled. The epitaxial growth can be, in one example, longitudinal epitaxial growth. In another example, the nanocrystal growth includes deposition of shells onto end surfaces of the core part. [071] In various examples, the RE in the alkaline rare-earth fluoride (AREF₄) nanocrystal is Y, Gd, Lu and/or Nd. The nanocrystal may be a hybrid nanocrystal. For example, the hybrid nanocrystal can be: NaYF₄-NaGdF₄-NaNdF₄; NaYF₄-NaLuF₄; or NaYF₄-NaLuF₄- NaGdF₄. [072] In various particular examples, the nanocrystal is less than about 100 nm in size in at least one direction, or is less than about 50 nm in size in at least one direction, or is less than about 10 nm in size in at least one direction.

[073] The dissolution can occur at one or more side surfaces of the core part of the nanocrystal. In various examples, the shape of the nanocrystal is or approximates: a rod shape; a bamboo shape; an hour-glass shape; a flower shape; a co-axial cylinder shape; a pin with double ring shape; a disc shape or a dumbbell shape. The overall shape may be combinations of these shapes. [074] There is synthesized, consequently, a nanocrystal produced by partial dissolution of a core part of a nanocrystal; and epitaxial growth of new nanocrystal onto at least one preferential surface of the nanocrystal; wherein a surfactant in solution is used to control the nanocrystal growth.

[075] According to one example, the ratio of oleate ions (OA^") concentration to oleic acid molecules (OAH) concentration is greater than or equal to 1 : 3, or is greater than or equal to 1 :4, or is greater than or equal to 1 : 5, or is greater than or equal to 1:6, or is greater than or equal to 1 :7, or is greater than or equal to 1 :7.6. [076] According to another example, the ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration is less than or equal to 1 AO, or is less than or equal to 1 :30, or is less than or equal to 1 :25, or is less than or equal to 1 :20. [077] According to another example, the ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration is about 1 :3, preferably about 1 :3.2.

[078] According to another example, the ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration is between about 1 :40 and about 1 :3, or is between about 1 :30 and about 1 :3, or is between about 1 :30 and about 1 :4, or is between about 1 :30 and about 1 :5, or is between about 1 :30 and about 1 :6, or is between about 1 :30 and about 1 :7.

[079] According to another example, the ratio of oleate ions (OA^") concentration to oleic acid molecules (OAH) concentration is between about 1 :20 (for example 75 μηιοΙ/mL : 1508 μηιοΙ/mL) and about 1 :7.6.

[080] In one example, the epitaxial growth is longitudinal epitaxial growth. In another example, longitudinal epitaxial growth is induced by selecting a ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration greater than or equal to about 1 :7, preferably about 1 :7.6. Optionally, KOH can be added to increase longitudinal epitaxial growth. In another example, transversal epitaxial growth is induced by selecting a ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration less than or equal to about 1 :20.

[081] Figure 1 illustrates example molecular bonding models of OAH and OA^" surfactants to the crystalline facets of P-NaYF₄. Figure 1(a) illustrates the schematic shape of a P-NaYF₄ nanocrystal chosen as the core for directional epitaxial growth. The hexagonal cylinder includes the (001) facets at the ends and identical (100) and (1 10) facets around the cylinder sides. Figure 1(b) illustrates the Y arrangements and binding energies (see insert table) of OAH and OA^" on the most stable (001) and (100) facets. The Y³⁺ atoms form equilateral triangles with a length of 6 A in the relaxed (001) surface, while rectangles are observed in the (100) surface with a shorter length of 3.51 or 3.69 A. Figure 1(c) illustrates SEM characterization of submicron-sized nanocrystals synthesized using the hydrothermal route (scale bar: 500 nm).

[082] To quantify the surface coordination chemistry between P-NaYF₄ surface and OAH and OA^" ligands, first-principles calculations were performed based on density functional theory (DFT) using CASTEP. As shown in Figure lb, the (001) and (100) planes of the β- NaYF₄ nanocrystals terminated with specific atomic arrangement were treated as the most stable facets according to the calculated surface energies. Considering that the oxygen moiety in the ligands has a strong binding affinity to Y³⁺ ions at the particle surface, the interactions between the OAH and OA^" molecules and the Y³⁺ ions were modelled under a number of conditions, such as different adsorption configurations, ligand chain length and ligand coverage. A conclusion from these simulations is that OA^" preferentially binds to RE³⁺ (Rare Earth) ions exposed on the (100) facet of the hexagonal fluoride nanocrystal, with a much higher binding energy (-35.4 eV) than on its (001) facet (-21.8 eV). It should be noted that the OAH molecule would bind with a higher probability to the (001) facet than the (100) facet and has relatively small binding energies of -9.4 eV and -4.6 eV, respectively. Charge analysis further indicates that such selective binding is attributed to the difference in the atomic arrangements of these two facets (Figure 1 (b)), giving rise to different paths of charge transfer between the ligands and the surface ions.

[083] The binding preferences of OAH and OA^" molecules to different facets were used to induce longitudinal epitaxial growth. With reference to Figure 1(c), sub-micrometer- sized NaYF₄ crystals of different aspect ratios could be prepared by tuning the relative concentrations of OAH and OA^". Higher concentrations of OA^" encourage epitaxial growth along a longitudinal direction.

[084] A similar effect was obtained in the synthesis of sub-50 nm NaYF₄ nanoparticles (i.e. nanocrystals of size less than about 50 nm) prepared by a co-precipitation method. Figures 2(a) and 2(b) show that a high concentration of NaOH leads to longitudinally grown nanocrystals because of a large amount of passivation of OA^" ions onto the (100) facets. The zeta potential of + 20 mV in NaYF₄ nanocrystals after removal of ligands, indicates that the RE³⁺ (Rare Earth) cations are more abundant than the F^" ions on the crystal surfaces. To rule out the effect of OH^" on longitudinal growth, sodium oleate was added as the sodium source instead of hydroxide and identical results were obtained. It was also confirmed that a high ratio of OA70AH concentration directs longitudinal deposition of heterogeneous shells (NaGdF₄) onto the end surfaces of NaYF₄ core. Interestingly, when atoms are removed from the nanostructures, subtractive growth (dissolution) is observed from their side (100) surfaces. This results in concurrent decrease in the core width from 26 nm to 18 nm, thus producing dumbbell-shaped nanocrystals.

[085] Figure 2 illustrates electron microscopy characterization of the transversal, longitudinal and successive heterogeneous growth of example P-NaREF₄ nanocrystals. Figure 2(a) illustrates NaYF₄ core and homogenous NaYF₄ nanocrystals after epitaxial growth of NaYF₄ in a longitudinal direction with 0.5 mmol NaOH and 9.5 mmol OA at 310 °C for 1 hour. Figure 2(b) illustrates successive heterogeneous growth of periodical shells of NaGdF₄ onto NaYF₄ in the longitudinal direction with 0.5 mmol NaOH and 9.5 mmol OA at 310 °C; right: the addition of 0.4 mmol KOH was used to promote further growth in the longitudinal direction. Figure 2(c) illustrates NaYF₄ core and heterogeneous NaYF₄-NaGdF₄ nanocrystals after epitaxial growth of NaGdF₄ in the transversal direction with 0.15 mmol NaOH and 19 mmol OA at 290 °C for 3 hours; Top: elemental mapping of Y and Gd (scale bar: 50 nm).

[086] Moreover, the addition of KOH further accelerates longitudinal growth rate due to a higher dissociation constant of KOH than NaOH, which increases the dissociation of OAH producing more OA^". With the aid of KOH, heterogeneous "bamboo-shaped" nanorods with sharp edges were formed in a stepwise manner with a length of up to 173 nm (Figure 2(b)). [087] Inducement of transversal epitaxial growth was achieved by increasing the amount of OAH and reducing the amount of NaOH. At a reaction temperature of 290 °C, transversal growth was observed and NaGdF₄ rings of about 7 nm thickness around the NaYF₄ cores formed without a measurable change in the longitudinal direction (Figure 2(a), 2(b). Notably, dissolution from the (100) facets of the cores took place as well, and the width of the core was, again, reduced from about 49 nm to about 43 nm at both ends. The observed dissolution always occurred on the (100) facets for both cases of longitudinal and transversal growth. This is consistent with the strong chelating character of OA^" on the (100) facet, and with the fact that NaYF₄ is dissolved faster than NaGdF₄ because NaYF₄ is comparably less energetically stable. [088] By combining the approaches of longitudinal and transversal growth and selective dissolution with consideration of lattice mismatch, a variety of three-dimensional (3D) hybrid nanostructures/nanocrystals can be synthesized.

[089] Figure 3 illustrates evolution of morphology and composition of example heterogeneous NaREF₄ nanocrystals after different reaction time lengths. Figures 3(a) and 3(b) show NaGdF₄ growth along the longitudinal direction onto the ends of the NaYF₄ core. Figure 3(c) shows a TEM of the sample stopped 5 minutes after reacting with NaGdF₄ @ NaYF₄ nanocrystals in the presence of Na⁺, K⁺, Nd³⁺, OA^" and in the absence of F^" at a temperature of 310 °C, dissolution occurs first. Figures 3(d)-(g) show real-time monitoring of the epitaxial growth of NaNdF₄ along the longitudinal direction onto NaYF₄-NaGdF₄ nanocrystals, involving the dissolution of NaYF₄ and NaGdF₄ from the transversal surfaces of the crystal and their subsequent re-growth onto the NaNdF₄ nanocrystals in the presence of Na⁺, K⁺, Nd³⁺, OA^" and absence of F^" ions at 310 °C. Figure 3(h) shows HAADF-STEM images with elemental mapping results of the samples stopped after 60 minutes of reaction to confirm the distributions of Y³⁺, Gd³⁺, Nd³⁺ ions within a single NaYF₄-NaGdF₄-NaNdF₄ nanocrystal. Figure 3(e) shows schematic processes of dissolution of NaYF₄-NaGdF₄ and the sequential epitaxial growth of NaNdF₄ in the longitudinal direction and the migration growth of F^", Y and Gd ions (Scale bar: 100 run).

[090] Figure 3 shows a typical example of real-time evolution of morphology and composition of the example NaYF₄-NaGdF₄-NaNdF₄ nanocrystals, including the dissolution process of the NaYF₄-NaGdF₄ nanocrystals and subsequent longitudinal growth of NaNdF₄. The dissolution of NaYF₄-NaGdF₄ is initiated by the OA^" adsorbed on the surface of the nanocrystals. The concomitant depletion of dissolved F^" ions used for longitudinal growth of NaNdF₄ in the presence of high concentration of OA^" facilitates the dissolution of NaYF₄-NaGdF₄ nanocrystals and this, in turn, promotes longitudinal growth of NaNdF₄. Following the dissolution of the Y³⁺ and Gd³⁺ ions from the surface of NaYF₄- NaGdF₄ nanocrystals, these ions then participate in the epitaxial growth of NaNdF₄ nanocrystals, as evidenced by the elemental mapping (Figure 3 (h)). Moreover, real-time sampling TEM data further confirmed the underpinning mechanism (Figures 3 (a)-(c)). [091] The size of nanocrystal core decreased significantly in the first 5 minutes, indicating that the dissolution rate of the nanocrystals is faster than their growth rate. After about 15 minutes, new material started to form at the top and at the bottom ends of the core with simultaneous decrease of the nanocrystal core width. This observation rules out "surface mobility" ("atom diffusion") as the possible driving force behind the formation of the final shell, otherwise it is expected that the dissolution of NaYF₄ and growth of NaNdF₄ would occur at the same time. The only mechanism which explains the shape of this nanocrystal is that the absence of F^" source in the reaction solution at its beginning prevents growth of NaNdY₄ until the concentration of released F^" source exceeds a certain threshold.

[092] Control experiments further support the mechanism of OA^'-induced dissolution in that a firm bonding of the surfactant OA^" to the surface RE³⁺ cations is the main factor responsible for the removal of the surface crystalline layers. By applying a transversal growth approach to first grow a layer of NaGdF₄ on the side surfaces of NaYF₄ core, it was observed that there is smaller mismatch of NaGdF₄ vs. NaNdF₄ compared to the NaYF₄ vs. NaNdF₄. Instead, dissolution occurs in the first 10 minutes of the reaction and both dissolution from the side surfaces and epitaxial growth of NaNdF₄ on the end surfaces of NaGdF₄ @ NaYF₄ cores result in a thinner and longer nanocrystal.

[093] Thus, the Applicant has surprisingly found that the ratio of OAVOAH concentration controls the direction of epitaxial shell growth. Furthermore, a low ratio of OAVOAH concentration at a lower temperature directs the migration growth along the transverse direction. This enables formation of, for example, heterogeneous NaYF₄/NaGdF₄/NaNdF₄ nanocrystals in a flower shape, though in this case the dissolution process is much less efficient from the side surfaces of nanocrystal because there are too few OA^" ligands bound to RE cations on the (100) facet.

[094] Other parameters, such as reagent concentration, can be further applied to fine-tune the programmable protocols for other types of heterogeneous nanocrystals. During the formation of hourglass-shaped nanocrystals, by way of example, the decrease in the amount of Nd³⁺ source is found to hinder the migration growth process to yield sharper tips, while a supply of additional F^" ions in the reaction increases the diameter of dumbbell ends with round tips. [095] Figure 4 illustrates example programmable routes for fabricating the 3D nanoscale architectures of nanocrystals by using longitudinal, transversal and/or migration growth treatments. The four digital condition codes (R, T, F, RE) represent different reaction conditions where: R = 0, represents a low ratio of OAVOAH concentration; R = 1, represents a high ratio of OAVOAH concentration; T = 0, where the temperature is at 290 °C; T = 1, where the temperature is at 310 °C; F = 0, which designates the absence of an F^" ion source; F = 1, indicates the presence of an F^" ion source; RE = Y, with a rare earth Y³⁺ ion source; RE= Gd, with Gd³⁺ ion source; RE= Lu, with a Lu³⁺ source; RE=Nd, with Nd³⁺ source.

[096] By combining these different growth processes into a synthesis procedure, a variety of complex NaREF₄ nanostructures are fabricated as shown in the TEM images, including hour-glass shaped nanocrystals (NaYF₄-NaGdF₄-NaNdF₄), flower shaped nanocrystals (NaYF₄-NaGdF₄-NaNdF₄), co-axial cylinder shaped nanocrystals (NaYF₄-NaLuF₄), pins with double ring shaped nanocrystals (NaYF4-NaLuF4-NaGdF4), and dumbbell shaped nanocrystals (NaYF₄-NaGdF₄-NaNdF₄) with smooth or sharp ends (scale bar: 50 nm).

[097] Figure 4 shows an array of heterogeneous NaREF₄ nanostructures synthesized by carrying out specific sequences of longitudinal, transversal growth, selective dissolution, and directional migration growth of epitaxial shells in the presence of various OAVOAH ratios. As far as the Applicant is aware, these sub-50 nm nanoparticles are the smallest 3D objects prepared by a bottom-up additive and subtractive process. [098] To illustrate an example application of the method, multifunctional NaYF₄/NaLuF₄/NaGdF₄ heterogeneous nanocrystals with two NaGdF₄ rings on NaLuF₄ @ NaYF₄ nanorods were designed and synthesized. The hexagonal-phase NaYF₄ nanocrystal is an efficient luminescence upconversion material. The addition of NaLuF₄ enables X-ray computed tomography imaging while using NaGdF₄ enables magnetic resonance imaging. As far as the Applicant is aware, this presents the first controlled fabrication of sub-50 nm 3D shaped heterogeneous nanocrystals logically programmed by the combinational approaches of OA^"-assisted longitudinal growth, transversal growth and/or selective crystalline facet dissolution with consideration of crystallographic mismatch rates. [099] The nanoscale engineering capability presented herein enables the performance of quantitative studies which are inaccessible by convention approaches. For example, optical properties of the produced nanomaterials can be designed to precisely promote or inhibit particle energy transfers. Similarly, for example, magnetic properties of nanostructures can be optimised to enhance MRI by correlating the morphology and surface distributions of magnetic signals. Additionally, such hybrid nanomaterials could be used as a platform for transporting biologically important molecules across cell membranes. This process could be further facilitated by harnessing the anisotropic properties of different types of nanoparticles that permit diverse surface functionalizations and multi-modal bio- conjugations. This provides a significant advance over current capabilities of nanoscale programmable and reproducible engineering to new classes of heterogeneous materials in scalable quantities. New classes of multifunctional nanomaterials can be achieved and provide for new applications of nanoparticles with complex programmable shapes and surface properties.

[0100] Embodiments provide for programmable design and fabrication of nanocrystals including for morphology and/or composition control, which offers the capability for design of a variety of nanocrystals according to specific applications. Further Examples

[0101] The following examples provide a more detailed discussion of particular example embodiments. The examples are intended to be merely illustrative and not limiting to the scope of the present invention. 1. Hydrothermal synthesis of micro-sized NC1YF4 crystals

[0102] In one example, micro-sized NaYF₄ crystals were synthesized using the hydrothermal route and different ratios of OAVOAH were used to validate the selective roles of surfactant ions, for example oleate anions, in the formation of NaYF₄ crystals with different aspect ratios.

[0103] -NaYF₄ discs were synthesized via a modified hydrothermal reaction. In a typical process, NaOH (0.15 g; 3.75 mmol) was first dissolved into 1.5 mL of double distilled water, followed by the addition of 2.5 mL of oleic acid (7.5 mmol) and 2.5 mL of ethanol while undergoing vigorous stirring. Thereafter, an aqueous solution of NaF (0.5 M; 2 mL) was added to form a turbid mixture. Subsequently, a 1.2 mL aqueous solution of YC1₃

(Yb /Tm = 10/0.5 mol%; 0.2 M) was added and the solution was stirred for 20 min. The resulting mixture was then transferred into a 14 mL Teflon-lined autoclave and heated to 220 °C and the temperature maintained for 12 h. After cooling down to room temperature, the reaction product was isolated by centrifugation and washed with ethanol. Different amounts of NaOH were added to adjust the ratio of OAVOAH concentration by its reaction with OAH to form OA^". [0104] The aspect ratio of the disks was readily tuned using this method by varying the amount of NaOH (3.75, 5, 6.25, and 7.5 mmol) added in the reaction mixture. The aspect ratio was used as a general metric for the evaluation of crystal growth along either axis. For example, the aspect ratio of 0.2 suggested that the epitaxy rate along a crystallographic a axis was five times faster than that along a perpendicular c axis when 3.75 mmol NaOH was added to the reaction system; when 7.5 mmol of NaOH was added, the epitaxy rates along a and c axis were almost the same; the resulting aspect ratio was 0.95. Referring to Figure 6, there is illustrated (a) SEM images of the NaYF₄ disks and rods synthesized under hydrothermal conditions and with different amounts of NaOH (Scale bars: 1 μιη). (b) The observed aspect ratio of NaYF₄ crystals is graphed against the amount of NaOH used in the reaction mix.

[0105] The crystals formed from the hydrothermal method showed that the direction of growth corresponded closely to the ratio of OAVOAH concentration used in the initial reaction solution. It was found that a high concentration of OA^" encouraged epitaxial growth in the longitudinal direction of the crystal. It was found that changes to the shape of the NaYF₄ crystals closely matched the change in the OAVOAH concentration ratio of the reaction mix.

2. Longitudinal epitaxial-growth of NaYF4 nanocrystals via the co-precipitation route

[0106] The oleic acid in the co-precipitation method plays a dual role in directing the epitaxial growth of NaYF₄ nanocrystal and in forming a variety of nanostructures, such as nanorods, nanodiscs, hour-glass shaped nanocrystals, top-spin shaped nanocrystals, and flower-shaped nanocrystals. The details of this synthesis which involved changes to the ratio of oleate anion and oleic acid (OA70AH ratios) added, the reaction temperature of the synthesis, and the concentration of the reagents are summarized below for (1) anisotropic epitaxial-growth of crystals in their longitudinal and transversal directions and also for (2) their migration growth.

[0107] The conditions were first established for controlling the epitaxial growth of NaYF₄ purely along [001] direction (longitudinal), and in parallel we then controlled the epitaxial shell growth along [100] directions (transverse) by adjusting the ratio of OAVOAH concentration. The synthesis detail, result and discussion for morphologically different crystals are described in the following sections.

3. Longitudinal epitaxial-growth of homogeneous NaYF₄ nanocrystals

3.1 Method for growth of NaYF₄ nanocrystal cores

[0108] In a typical procedure, 4 mL of methanol solution of YC1₃ (2.0 mmol) was magnetically mixed with OA (38 mmol) and ODE (93 mmol) in a 100 mL three-neck round-bottom flask. The mixture was then degassed under an Ar flow and then heated to 150 °C for 30 min to form a clear solution, before cooling to room temperature. 15 mL of methanol solution containing NH₄F (8 mmol) and NaOH (5 mmol) was added to the solution of YC1₃ in ODE and stirred for 60 min. The mixture solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to completely remove methanol and any residual water. The mixture solution was then quickly heated to the reaction temperature of 300 °C and aged for 1 hour. After the solution was left to cool down to room temperature, ethanol was added to precipitate the nanocrystals. The product was washed with cyclohexane, ethanol and methanol for at least 4 times, before the final NaYF₄ nanocrystals were re- dispersed in 10 mL cyclohexane in preparation for their further use.

3.2 Method for longitudinal growth to form NaYF4 nanorods

[0109] 0.2 mmol of YC1₃ in 1 ml methanol solution was magnetically mixed with OA (9.5mmol) and ODE (25mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 150 °C for 30 min to form a clear solution, and then cooled to room temperature. 5 mL of methanol solution containing NH₄F (0.8 mmol) and NaOH (0.5 mmol) was added and stirred for 60 minutes. The solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to completely remove methanol and residual water. The solution was then injected with 0.2 mmol NaYF₄ of nanocrystals in cyclohexane and the mixture kept at 110 °C for another 10 min to evaporate the cyclohexane. Then, the reaction mixture was quickly heated to 310 °C and aged for 1 hour.

3.3 Method for characterization of rare earth ions on the nanocrystal surface

[0110] Rare earth doped fluoride nanocrystals were usually synthesized by reacting lanthanide precursors in organic media in the presence of capping ligands to increase their size and shape uniformity. These nanocrystals are often heavily aggregated in aqueous solutions owing to the hydrophobic nature of the capping ligands; surfactants are typically introduced during the reaction to keep the crystals colloidally stable as they grow in reaction solvent. To test the amount of rare earth ions on their surfaces, we first removed surface capping ligands including both Oleic Acid and Oleate ions from NaYF₄ nanocrystals with a procedure based on acid treatment. The as-prepared ligands-capped nanocrystals are first dispersed in a 2 mL HC1 solution (0.1 M) and sonicated for 30 min, followed by their centrifugation at 14,500 rpm for 10 min and their purification by adding an acidic ethanol solution (at a pH of 4; prepared by adding 0.1 M HC1 aqueous solution to the absolute ethanol). The resulting products of ligand-free nanocrystals were further washed with ethanol and deionized water three times, before they were re-dispersed in deionized water.

3.4 Results and Discussions

[0111] The TEM characterization (Figure 7) for the sample nanocrystals obtained confirmed that the epitaxial growth of shells onto the core was initiated by the formation of a-NaYF₄ nanocrystals at the beginning of the epitaxial growth, before these transformed into the stable P-NaYF₄ nanocrystals. Figure 8 is a graph showing the size evolution of both a phase NaYF₄ nanocrystals in diameter and P-NaYF₄ nanorods in their width and length as function of their reaction time from 0 minutes to 60 minutes. [0112] The size of a-NaYF₄ nanocrystals increased for the first 20 minutes, and decreased for the following 20 minutes until being completely consumed and epitaxial grown onto the P-NaYF₄ cores which then transformed into the P-NaYF₄ nanorods after 50 minutes of the start of the reaction. At the same time, the length of P-NaYF₄ cores gradually grows up for the whole reaction process longitudinally from 25 nm to 35 nm with the width of β- NaYF₄ nanocrystals remaining at the similar values about 25-27 nm.

[0113] Figure 9 further illustrates that mono-disperse nanorods with high aspect ratio (>2) can be step-by-step synthesized by the secondary epitaxial growth in a high concentration of oleate ions when reacted at high temperature by the co -precipitation method. Referring to Figure 9, there is shown TEM images and size distributions of the NaYF₄ core nanocrystals (a) and the NaYF₄ nanocrystals after their epitaxial growth in the longitudinal direction (b). The size of the NaYF₄ visibly increases in the longitudinal direction after the secondary growth of NaYF₄ shell from 24 nm to 47 nm. This was accompanied by a slight increase (1.8 nm) in their transversal dimension. (Scale bar is 100 nm)

[0114] Due to the fact that oleate and oleic acid are very similar in chemical structure, standard measurements, such as XPS, could not differentiate OA from OA- ions on the surface of nanocrystal. But there is indirect evidence supporting the existence of OA^"; the hydrophobic nanocrystals become hydrophilic in water after treatment with diluted HCl solution and after sonication. This indicates that the strongly binding oleate ions (OA ) react with H⁺ to form OAH at a lower binding energy, and is removeable by sonication treatment.

[0115] After both OAH molecules and OA^" oleate ions have been removed from the crystal facets, the zeta potential of ligand free NaYF₄ nanocrystals in MQ water was about +20 mV, which suggests that the naked nanocrystals are highly positively charged by their exposed rare earth ions on their surface. At high concentration of oleate ions (OA^"), the deposition and epitaxial growth of the shells have a preference for the bottom and top surfaces (001) of the crystal particles.

4. Further verification of O ions as a determining factor in longitudinal growth of NaYF4 nanorods

Method for longitudinal growth to form NaYF4 nanorods using Na-OA

[0116] To generate longitudinal epitaxial growth of nanocrystals, the approach was adopted of: increases in the ratio of OA^" to OAH by increasing the amount of NaOH lead to the conversion of more OAH into OA^" and to changes in the morphology of the crystals. In order to rule out interference from OH^" (from NaOH) that may play a role in directional epitaxial growth and as well to further verify if it is the oleate ion (OA^") that does indeed play a major role in passivating the side crystal surfaces (100), we used sodium oleate (NaOA) to replace NaOH as the sodium source. As a comparison, in one control experiment, the NaYF₄ nanocrystals were grown with a NaYF₄ shell using NaOH as the sodium source, and in the other experiment, the same molar amount of Na-OA (sodium oleate) was used as a sodium source and as a replacement for the NaOH.

[0117] 0.2 mmol YC1₃ in 1 ml methanol solution was magnetically mixed with OA (9.5mmol) and ODE (25mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under an Ar flow and heated to 150 °C for 30 min to form a clear solution, and then cooled to room temperature. 5 mL of methanol solution containing NH₄F (0.8 mmol) and sodium oleic acid Na-OA (0.5 mmol) was added and stirred for 60 min. The solution was slowly heated to 110 °C and kept at 110 °C for 30 min to completely remove any methanol and any remaining water. Then, 0.2 mmol of NaYF₄ in cyclohexane was injected into the nanocrystals in cyclohexane and the mixture was kept at 110 °C for another 10 minutes so that the cyclohexane could be completely removed. Then, the reaction mixture was quickly heated to 310 °C and aged before aging for 1 hour.

[0118] As shown in Figure 10 (b) and in (c), identical results were obtained with NaYF₄ shell grown longitudinally onto the NaYF₄ core no matter if OH^" was present in the reaction or not, ruling out OH^" as a key factor for longitudinal growth in the reaction system described here. In other reaction systems for synthesis of nanorods, the ratio of OA^" to OAH was a key factor in directing longitudinal growth of the crystals. 5. Longitudinal epitaxial growth of heterogeneous NaGdF₄ shells onto NaYF₄ cores Method for longitudinal growth ofNaGdF₄ -NaYF₄ nanorods

[0119] 0.2 mmol of GdCl₃ in 1 mL methanol solution was magnetically mixed with OA (9.5 mmol) and ODE (25 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under an Ar flow and heated to 150 °C for 30 min to form a clear solution, and then cooled to room temperature. 4 mL of methanol solution containing NH₄F (0.8 mmol) and NaOH (0.5 mmol) was added to the OA and the ODE solution and stirred for 60 min. The solution is slowly heated to 110 °C and kept at 110 °C for 30 minutes to remove methanol and the remaining water completely. Then, 0.2 mmol of NaYF₄ core nanocrystals in cyclohexane was infected into the reaction solution. After holding the reaction temperature at 110 °C for further 10 min to evaporate all cyclohexane, the reaction mixture was quickly heated to 310 °C and aged for 1 hour. [0120] Figure 11 shows that the process of longitudinal growth of NaGdF₄ shell onto the NaYF₄ core is similar to that seen in the NaYF₄ - NaYdF₄ nanorods structure. The contrast that can be achieved with a TEM between the NaGdF₄ shell and NaYF₄ crystal provides better resolution to record this process. [0121] Small a phase NaGdF₄ nanocrystals are formed at the beginning of the reaction step. After 15 minutes of reaction the shape of NaYF₄ cores changes from spherical to hexagonal. At 30 minutes of reaction, thin layers of NaGdF₄ have formed on the bottom and top surfaces (001) of the NaYF₄ hexagonal prism and the NaGdF₄ layer has also formed along the c axis. At the completion of the reaction, NaGdF₄@NaYF₄ dumbbell- shape nanocrystals have formed which can be attributed to the selective growth of the NaGdF₄ shells along the longitudinal direction. The concentration of shell source reagents in the reaction mix decreases rapidly as it was consumed by the growth of the P-NaGdF₄ shell and the cc-NaGdF₄ nanocrystals. With this decrease in the concentration, a-NaGdF₄ nanocrystals were rapidly dissolved and formed the P-NaGdF₄ shell.

[0122] The results of the characterization (Figure 12) of another validation experiment showed that the heterogeneous nanocrystals exhibit a dumbbell shape with NaGdF₄ nanocrystals at the two ends. The dissolution (etching) phenomenon from the side surfaces (100) of NaYF₄ core was observed, which was attributed to the strong binding of the oleate ions (OA^") to the exposed rare earth ions on the side surfaces of the particles. The length of the NaYF₄-NaGdF₄ nanocrystals was observe to visibly increase in the longitudinal direction from 42 nm to 62 nm, while the width of NaYF₄ in the middle of each nanorod decreased by 8 nm, due to the dissolution of the NaYF₄ crystal during growth of the NaGdF₄. The width of the newly formed NaGdF₄ crystals at each end was measured at 29 nm, which was similar to the original width of the NaYF₄ core nanocrystals.

[0123] The HAADF-STEM images (Figure 12(c)) demonstrate the density contrast of the NaGdF₄ at the ends and the NaYF₄ in the middle and the elemental mapping images confirm the distribution of the elements Y and Gd. The combined elemental mapping image confirms that the two compositions are well aligned. At a high concentration of oleate ions (OA^"), the deposition and epitaxial growth of heterogeneous shells (NaGdF₄) still prefer the bottom and top surfaces (001) of NaYF₄ crystal core. 6. Accelerated longitudinal growth of NaGdF4 -NaYF4 nanorods by adding KOH

Method for accelerated longitudinal growth ofNaGdF4 -NaYF4 nanorods by adding KOH

[0124] 0.2 mmol of GdCl₃ in 1 mL of methanol solution was magnetically mixed with OA (9.5 mmol) and ODE (25 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 150 °C for 30 min to form a clear solution, before cooling to room temperature. 5 mL of methanol solution containing NH₄F (0.8 mmol), KOH (0.4 mmol) and NaOH (0.5 mmol) was added into the OA and ODE solution and stirred for 60 min. The solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to remove the methanol and water completely. The reaction mix was then injected with 0.2 mmol of NaYF₄ core nanocrystals (or nanorods as shown in Figure 10), both suspended in cyclohexane, into the reaction solution. After holding the reaction mix at 110 °C for further 10 min to evaporate all cyclohexane, the mixture was heated rapidly to 310 °C before aging for 1 hour at this temperature. [0125] KOH has a higher dissociation constant than NaOH, adding additional KOH increases the amount of OA^" dissociated from OAH and increases the passivation effect on the side surfaces of the particles. This accelerates epitaxial growth in a longitudinal direction. As verification, 0.4 mmol of KOH was added to the reaction mix. [0126] With the aid of KOH, the longitudinal growth of the NaYF₄ and NaGdF₄ - NaYF₄ nanocrystals became faster and the end particles were longer in contrast to the particles formed when KOH was not present in the mix. The final nanorods or nanocrystal dumbbells have sharper edges compared to the ones produced without the presence of KOH. This suggests that the amount of the OA^" ions available on the side surfaces during the reaction to passivate the side surfaces (100) by acting as surfactants was sufficient.

[0127] Figure 13 shows TEM images of the NaYF₄ core (a) and the NaYF₄ coated with a NaYF₄ shell without KOH present (b) and with 0.4 mmol of KOH present (c) (Scale bars: 50 run). Figure 14 shows TEM images of the NaYF₄ nanorods acting as the core (a) and the longitudinal growth of the NaGdF₄ shell onto the NaYF₄ core with 0.4 mmol KOH present in the mix (Scale bars: 50 nm).

[0128] KOH can quickly supply sufficient OA^" ions on the side surfaces and therefore it promotes the longitudinal growth of the nanocrystals. KOH reacts with oleic acid (OA) and forms KOA, which significantly increasing the ratio of OA^" to OAH. The higher ratio of OAVOAH promotes this trend in longitudinal growth.

7. Longitudinal epitaxial growth of multiple-section NaYF4-NaGdF4 nanorods

7.1 Method for longitudinal synthesis of five-section NaYF4 - NaGdF4 - NaYF4 bamboolike nanorods

[0129] 0.2 mmol of YC1₃ in 1 mL of methanol solution was magnetically mixed with OA (9.5 mmol) and ODE (25 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 150 °C for 30 minutes to form a clear solution, and then cooled to room temperature. 5 mL of methanol solution containing NH₄F (0.8 mmol), KOH (0.4 mmol) and NaOH (0.5 mmol) was added into the OA and ODE solution and stirred for 60 minutes. The solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to remove the methanol and water completely. The reaction solution was then injected with 0.2 mmol of NaGdF₄ and of NaYF₄ nanorods in cyclohexane solution. After the reaction at 110 °C for a further 10 minutes to evaporate all the cyclohexane, the reaction mixture was quickly heated to 310 °C and held at this temperature for 1 hour.

7.2 Method for longitudinal synthesis of seven-section NaGdF4 - NaYF4 - NaGdF4 - NaYF4 bamboo-like nanorods

[0130] The same procedure was repeated, and then followed by the injection of 0.2 mmol of the five-section NaYF4 - NaGdF4 - NaYF4 nano-bamboo which acted as the core, all in cyclohexane solution, into the reaction solution. After holding at 110 °C for a further 10 minutes to evaporate all cyclohexane, the reaction mixture was quickly heated to 310 °C and held again for 1 hour.

[0131] The co-precipitation method is useful for the synthesis of sub- 100 nm nanocrystals, while the hydrothermal method is suitable for synthesizing larger sized nanocrystals in the micron size range. With the aid of high concentration of NaOH or KOH in the reaction OA^" ions can effectively passivate the side surfaces (100) (010) of nanorods and so grow very long homogeneous or heterogamous nanocrystals with some control. [0132] Figure 15 shows TEM images, size distribution and characterization of the elemental composition of periodical NaYF₄-NaGdF₄ nanocrystals which form a bamboolike nanostructure. (a) five-section NaYF₄-NaGdF₄ nanocrystals in a bamboo shape obtained by an epitaxial growth of NaYF₄ and NaGdF₄ in a longitudinal direction; (b) seven-section NaYF₄-NaGdF₄ nanocrystals in an bamboo shape; the length of the five- section NaYF₄-NaGdF₄ nanocrystals can reach 122 nm and its width, 35 nm. With two more sections, the seven-section NaYF₄-NaGdF₄ nanocrystals grow to 173 nm and the width increases to 42 nm. (c) HAADF-STEM image of three single NaYF₄-NaGdF₄ nanocrystals and their elemental mapping images. The HAADF-STEM image demonstrates the contrast in density between the NaGdF₄ and the NaYF₄ as well as their elemental corresponding mapping images, which confirm the distribution of the elements Y and Gd. (scale bars: 200 nm in (a) and (b), 100 nm in (c)).

8. Transversal epitaxial growth of heterogeneous NaGdF4 shell onto NaYF4 core [0133] To develop a set of approaches for programmable growth of arbitrary shapes of nanocrystals, and according to the selective preference of OAH molecules and OA^" ions to different facets of nanocrystal cores, we identified the condition of the lower ratio of OA^" /OAH concentration at slightly lower temperature (290 °C vs. 310 ⁰ C) which better passivate the top and bottom surfaces (001) of the nanocrystal cores so that the epitaxial shell growth is based on the side surfaces (100) along their transversal direction.

Method for transversal synthesis ofNaGdF₄ shell onto NaYF₄ core

[0134] O.lmmol of GdCl₃ in 1 mL methanol solution was magnetically mixed with OA (19.0 mmol) and ODE (18.7 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 150 °C for 30 min to form a clear solution, and then cooled to room temperature. 3 mL of methanol solution containing NH₄F (0.4 mmol) and NaOH (0.15 mmol) was added into the OA and ODE solution and stirred for 60 min. The solution was slowly heated to 110 °C and kept at 110 °C for 30 min to remove completely the methanol and water. Then 0.1 mmol of the NaYF₄ cores in cyclohexane solvent were injected into the reaction mix. After being kept at 110 °C for further 10 min to evaporate all cyclohexane, the reaction mixture was quickly heated up to 290 °C and held at that temperature for 3 hours.

[0135] TEM characterization results shown in Figure 16 confirm a core-shell structure when viewed from the top of the nanocrystals, while the insert image (Figure 16(b)) and high resolution image of (Figure 16(d)) reveal a core-ring structure when viewed from the side, with the shell grown only around the side surfaces of the nanocrystal cores.

[0136] The length of the nanocrystal slightly reduces from 41 nm to 39 nm after transversal growth, while the size in diameter increases from 50 nm to 65 nm, indicating that the thickness of NaGdF₄ ring layer was 7.5 nm. The height of NaGdF₄ ring was measured and found to be 18.2 nm.

[0137] It is worth noting that the diameter of the NaYF₄ core nanocrystals at its two ends was reduced from 50 nm to 29 nm, suggesting a dissolution (etching) phenomenon occurred on the end edges during the epitaxial growth of NaGdF₄ rings in the middle part of the core. The elemental mapping images in Figure 16(c) and (d) demonstrate that the ion Gd was deposited within the rings with the ion Y deposited in the core. Figure 16 shows (a-b): TEM images and size distribution histograms, (a): the NaYF₄ core nanocrystals (scale bar is 100 nm). b): the NaYF₄-NaGdF₄ nanocrystals with transversal growth of NaGdF₄ on their side surfaces (scale bar is 100 nm). (c-d) High magnification TEM images and EDS elemental mapping images of the NaYF₄-NaGdF₄ nanocrystals (scale bar is 50 nm); from the top view (c) and the side view (d). The EDS elemental mapping images clearly show that that NaGdF₄ is located well outside the NaYF₄ core nanocrystals in the middle part of its structure.

[0138] From high resolution TEM images, it was confirmed that the growth direction of NaGdF₄ shell was vertical to the [001] direction. The (110) plane distance at the top edge area was 2.97 A while the (110) distance at the middle area was 3.02 A, which is consistent with the standard parameters of P-NaYF₄ and P-NaGdF₄.

[0139] According to computational modelling results, the binding energy of OAH on the (001) facet was two times stronger than that calculated on the (100) facets, and the relatively higher concentration of OAH molecules generated a stronger passivation effect to block the access for epitaxial growth of shells onto the end surfaces. Under relatively low reaction temperature that facilitates the binding of OAH molecules, the difference in binding strength of the surfactants covering on each facet caused the anisotropic shell formation. Even though the difference was smaller than that observed in the longitudinal growth, the result was still very clear when a shorter reaction was performed.

[0140] With a lower ratio of OAVOAH (which equates to a relatively low OA^" concentration) and at lower reaction temperature, the OAH molecules were the main surface ligands preferably on the top and bottom surfaces of nanocrystal cores, inhibiting the end surfaces (001) and therefore promoting the epitaxial growth of NaGdF₄ shell in its transversal direction.

[0141] The etching phenomenon observed on the ends of the side surfaces (100) of the NaYF₄ was due to the strong binding of the oleate ions (OA^") to the exposed rare earth ions on the side surfaces of the particles.

9. Programmable growth of 3D shapes of heterogeneous nanocrystals

[0142] Rare earth doped nanocrystals have emerged as a means to provide exceptional optical, magnetic and physical-chemical properties, for photon upconversion, background- free biological assays, multimodal in vivo bio-imaging (fluorescence, MRI, X-ray, SPECT, etc.), targeted drug delivery as carriers, cancer therapy, full colour displays, infrared upconversion photovoltaic and photo catalysis for energy management, security inks, and photonics.

10. Crystal lattice mismatch for crystallographic parameters c and a & b for hexagonal phase NaREF4 crystals

[0143] Towards developing scalable synthesis protocols for multifunctional heterogeneous nanostructures with a high degree of control in size, shape, surface and composition, we have further studied the lattice mismatch for crystallographic parameters c and a for hexagonal phase NaREF₄ crystals. [0144] Table 1 shows the crystal lattice parameters of hexagonal phase NaREF₄ crystals. The difference between the two crystallographic parameters (a & b, and c) can affect the epitaxial growth of subsequent layers. For example, the NaNdF₄ crystals can be grown with no hindrance onto NaGdF₄ than onto NaYF₄ due to the close similarities in their crystallographic parameters; likewise NaGdF₄ crystals prefer to grow onto the NaYF₄ crystal lattice than onto the NaLuF₄ crystal lattice. By calculating the mismatch rate for the difference in the crystal lattice units, we can quantify the possibility of direct epitaxial growth of different crystal types onto the core crystal when forming a heterogeneous rare- earth doped single nanocrystals.

[0145] Table 2 summarizes the lattice mismatch for crystallographic parameters c and a for hexagonal phase NaREF₄ crystals. A first area represents the pair crystals with a lattice mismatch that is less than 3%, which suggests a close crystal lattice match, while the second and third areas mark a higher lattice mismatch rate of either 3.1% to 5% or 5.1% respectively, indicating greater difficulty in the direct epitaxial growth of a heterogeneous nanocrystal from these two constituents.

Table 1. The crystal lattice parameters of hexagonal phase NaREF₄ crystals.

Table 2. Summary of the lattice mismatch for crystallographic parameters c for a hexagonal phase NaREF₄ crystals. The crystallographic potential for high quality epitaxial growth increases from the lower diagonal border to the top left corner.

11. Design and fabrication of heterogeneous Na YF^aLuF^Na GdF₄ nanocrystals: a showcase synthesis for multifunctional single heterogeneous nanocrystal

[0146] Using the combinational approaches of longitudinal growth and transversal growth as well as taking the consideration of crystallographic mismatch rates, we demonstrate here a series of examples to illustrate rational design and programmable epitaxial growth techniques for bottom-up fabrication of three-dimensional heterogeneous nanostructures.

[0147] The hexagonal-phase NaYF₄ nanocrystal is acknowledged elsewhere as the most efficient photon upconversion host. Recent pioneer work has revealed that the incorporation of other functions in these nanocrystals can enrich their hybrid applications, for example, by providing X-ray computed tomography imaging through the use of NaLuF₄ as a host or Magnetic Resonance Imaging using NaGdF₄ as a host. [0148] A first example shows the design and synthesis of NaYF₄/NaLuF₄/NaGdF₄ heterogeneous nanocrystals with two NaGdF₄ rings on a NaLuF₄ @ NaYF₄ nanorod.

[0149] The design logic is described as:

1. First employ the approach of longitudinal growth to grow NaYF₄ nanoparticles to NaYF₄ nanorods;

2. Then employ the protocol for transversally coating the side surfaces of NaYF₄ nanorods with a thin layer of NaLuF₄. This thin layer of NaLuF₄ functions as a mask;

3. The surface of NaLuF₄ @ NaYF₄ nanorods at the top edge areas are less stable and are easier to dissolved after the transversal epitaxial shell growth, which leaves the NaLuF₄ mask at the end edges also becoming removed so that NaYF₄ was exposed;

4. According to Table 2, the mismatch rate between NaGdF₄ and NaLuF₄ was about 4.3% while the mismatch rate between NaGdF₄ and NaYF₄ was about 2%, which suggests that the NaGdF₄ will prefer to grow on NaYF₄ rather than on NaLuF₄;

5. To facilitate the etching process, in a modified protocol towards transversal growth of NaLuF₄ onto the side surfaces of NaYF₄ nanorods, absence of F source causes a faster erosion occur at the top edge area;

6. Finally, by injecting the a-NaGdF₄ nanocrystals with F element source by slowing dissolving it into the solution, P-NaGdF₄ nanocrystal shells start to grow onto the exposed NaYF₄ areas, forming double rings of NaGdF₄ shells transversally grown around the top and bottom edges of NaLuF₄ @ NaYF₄ nanorods.

Method for synthesis of pure a-NaGdF₄ nanocrystals

[0150] 2 mL of the methanol solution of GdCl₃ (1.0 mmol) was magnetically mixed with OA (19 mmol) and ODE (47 mmol) in a 100 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 150 °C for 30 minutes to form a clear solution, and then cooled to room temperature. 10 mL of the methanol solution containing NH₄F (4 mmol) and NaOH (2.5 mmol) was added and stirred for 60 min. Then, the solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to remove the methanol and water completely. After that, the reaction mixture was quickly heated to 240 °C and aged for 45 minutes.

Method for synthesis ofNaLuF4@NaYF4 nanorods [0151] 0.1 mmol of LuCl₃ in 1 ml method solution was magnetically mixed with OA (9.50 mmol) and ODE (25 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 150 °C for 30 min to form a clear solution, and then cooled to room temperature. 2 mL of methanol solution containing NaOH (0.15 mmol) was added and stirred for 60 min. The solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to completely remove the methanol and some of the water. It was then injected with 0.4 mmol of NaYF₄ seed particles in a cyclohexane solution. After holding the reaction mix at 110 °C for a further 10 minutes to evaporate cyclohexane, the reaction mixture was quickly heated to 290 °C and held at that temperature for a further 1 hour.

Method for synthesis of NaGdF₄ double-ring structure onto the NaLuF₄@NaYF₄ nanorods [0152] 0.1 mmol of GdCl₃ in 1 ml methanol solution was magnetically mixed with OA (19.0 mmol) and ODE (18.7 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 150 °C for 30 minutes to form a clear solution, and then cooled to room temperature. 2 mL of the methanol solution containing NaOH (at 0.15 mmol) was added and stirred for 60 minutes. The solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to completely remove the methanol and some of the water. It was then injected with 0.1 mmol of theNaYF₄ seed particles, in a cycloexane solution, into the reaction solution. After having been held the reaction mix at 110 °C for another 10 minutes to evaporate cyclohexane, the reaction mixture was quickly heated to 300 °C. It was then, injected with 0.02 mmol of a-NaGdF4 nanocrystals into the reaction system. This was done every 10 minutes for 5 times at 300 C. The reaction mix was held at this temperature for another 10 minutes after the last injection.

Results and Discussion

[0153] The elemental mapping images in Figure 17(c) further confirms that the three elements of Y³⁺ , Lu³⁺ and Gd³⁺ are separated in three partitions within one single nanocrystal, with Y³⁺ only in the inner core of the rod, Lu³⁺ only in the outer shell of the rod, and Gd³⁺ only in the two rings. The two rings can be seen to grown around the [001] crystallographic direction and the select area electron diffraction pattern has a close match to thep-NaYF₄, p-NaLuF₄ and -NaGdF₄. [0154] After a thin layer of NaLuF₄ shell is transversally formed onto the NaYF₄ core nanorods at 290°C, the F element source was removed so that NaLuF₄ layer can start to dissolve slowly, particularly at the sharp end areas of nanorods where the NaYF₄ is firstly exposed. With the drop-wise addition of a-NaGdF₄ (as the F source) into the reaction system, NaGdF₄ prefers to grow onto the (100) (010) facets of the exposed NaYF₄ cores with NaLuF₄ shell acting as a mask because of the inherent difference in crystalline mismatch rate between NaGdF₄ vs. NaYF₄ and NaGdF₄ vs. NaLuF₄.

[0155] Figure 17 shows TEM and elemental composition characterization of the NaYF₄- NaLuF₄-NaGdF₄ heterogeneous nanocrystals which form a unique shape of two NaGdF₄ rings onto a NaLuF₄@NaYF₄ nanorod. (a) NaLuF₄ nanocrystal shells transversally coated onto NaYF₄ core nanocrystals; (b) an overview image of the NaYF₄-NaLuF₄-NaGdF₄ nanocrystals and a single NaYF₄-NaLuF₄-NaGdF₄ nanocrystal (top-left inset); (c) HAADF-STEM image of a single NaYF₄-NaLuF₄-NaGdF₄ nanocrystal and its elemental mapping images. The elemental mapping images confirm the distribution of the Y³⁺ ions in the middle of the rod and the Lu ions as a thin layer coating surrounding the nanorod.

The Gd ions is present as a double ring surrounding the NaYF₄-NaLuF₄ nanorods in the transversal direction, (d) A schematic illustration of the formation process "selective mask - etching - epitaxial growth" of the NaYF₄-NaLuF₄-NaGdF₄ heterogeneous nanocrystals is provided.

[0156] Thus, for the first time there is provided a "bottom-up" programmable controlled fabrication of 3-D shaped heterogeneous nanocrystals using the combinational approaches of an oleate ion (OA^") assisted longitudinal growth, transversal growth and selective etching which includes controlling for and using the crystallographic mismatch rates.

12. Design and fabrication of heterogeneous the NaYF4/NaGdF4/NaNdF4 nanocrystals in an hourglasses shape: investigation on the migration growth mechanism [0157] In a second example, we show the design and synthesis of NaYF₄/NaGdF₄/NaNdF₄ heterogeneous nanocrystals to form heterogeneous nanoscale hourglasses. We have observed for the first time a new epitaxial shell growth process and a migration growth pattern, which includes the dissolution of NaYF₄ with subsequent growth of the NaNdF₄ on the other facet. [0158] The design logic is described as:

1. First employ the approach of longitudinal growth of NaGdF₄ nanocrystal shells onto the top and bottom ends of the NaYF₄ nanocrystal cores;

2. Then continue to longitudinally grow NaNdF₄ nanocrystals, but in a condition of absence of F^" source; in such a condition, direct epitaxial shell growth of NaNdF₄ was stopped due to the lack of an F^" ion source;

3. In contrast, OA^" ligands were strongly anchored onto the surface of the rare earth ions on their side surfaces (100) facets which become unstable with the erosion of NaYF₄ being the most obvious; this causes the middle part of nanorods to be trimmed off;

4. A minimum amount of F^" ions gradually released from NaYF₄ into the reaction solution under the high reaction temperature of 310 °C helps to form NaNdF₄ nanocrystals longitudinally; this end-surface preference was also caused by the crystalline mismatch rate of NaNdF₄ vs. NaYF₄ was greater than that for the

NaNdF₄ vs. NaGdF₄;

5. Finally, F^" ions and part of the Y³⁺ and Gd³⁺ ions are migrated from the middle part of nanorods to the NaNdF₄ section on each end of the heterogeneous nanocrystals. Method for synthesis of NC1YF4 /NaGdF₄/NaNdF₄ nanocrystals in hourglasses shape

[0159] 0.4 mmol of the NdCl₃ in 2 mL of methanol solution was magnetically mixed with OA (9.5 mmol) and ODE (25 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 150 °C for 30 min to form a clear solution, and then cooled to room temperature. 5 mL of the methanol solution containing KOH (0.8 mmol) and NaOH (0.8 mmol) was added and stirred for 60 min. The solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to completely remove the methanol and some of the water. It was then injected with 0.1 mmol 50 nm x 60 nm NaYF₄/NaGdF₄ nano-prisms particles, in a solution of cyclohexane, into the reaction mix. After having been kept at 110 °C for another 10 minutes to evaporate all cyclohexane, the reaction mixture was quickly heated to 310 °C. 500 samples of the reaction solution were collected each time with a syringe at 5 minutes, 15 minutes, 30 minutes, 40 minutes, 50 minutes, and 60 minutes after the start of the reaction. Results and discussion

[0160] By real-time monitoring of the migration growth process, Figure 18 provides direct evidence that migration growth consists of a two-step reaction, (1) the dissolution of NaYF₄ with subsequent (2) growth of NaNdF₄ on another facet. The size of nanocrystal core decreased significantly in the first 5 minutes, but without forming new component crystal on the crystal surface, indicating that the speed of dissolution of the nanocrystals was faster than crystal growth speed in this reaction step. After 15 minutes, new crystals started to form onto the top and bottom ends of the core as the width of the nanocrystal cores decreased.

[0161] This result directly rules out "surface mobility" ("atom diffusion") as the possible driving force, since if this was the case the decrease of NaYF₄ and the increase of NaNdF₄ would happen at the same time, which was not the case with the sample taken after 5 minutes. The only mechanism that could explain the 5 minute sample was that the absence of the F^" ion source in the reaction mix at beginning makes NaNdY₄ growth impossible until the concentration of released F^" ions reaches a certain threshold.

[0162] Figure 18 shows the evolution process of the NaYF₄/NaGdF₄ cores as they form the NaYF₄-NaGdF₄-NaNdF₄ heterogeneous nanoscale hourglasses. In the modified protocol for longitudinal growth of heterogeneous nanocrystal rods, the absence of an F^" ion source with unbalanced F^" ion concentration causes the accelerated erosion of the NaYF₄ rods and the selective trimming off the NaYF₄ and the NaGdF₄ from the (100) facets on the side of the nanorods particles. [0163] The elemental mapping images in Figure 18(h) further reveals the distributions of the three ions Y³⁺, Gd³⁺ and Nd³⁺ within one single nanocrystal, with the majority of the Y³⁺ ions in the middle, the Gd³⁺ ions as a bridge, and the minority of the Y³⁺ and Gd³⁺ ions migrating to the end section with the Nd ions found only on the end of each nanocrystal. [0164] The crystal lattice was confirmed as a NaNdF₄ (100) plane by analysis which matched the lattice distance. A small difference of lattice distance at different areas was also measured, a lattice distance in a first observed area was 5.25 A that is close to the standard value for the NaNdF₄ (100) plane distance, while a lattice distance in another area was 5.09 A which is close to the standard value of the NaYF₄ (100) plane distance. [0165] Oleate ions (OA^") acting as surfactant ligands firmly bond to the rare earth ions on the side surfaces (100) facet planes; in the absence of an F^" source ion and at a high temperature (310 °C), the nanocrystal side surfaces become unstable and there is an observed accelerated site-selective erosion phenomenon. With the release of the F^" ions, the migration growth occurs when NaYF₄/NaGdF₄ nanocrystals as cores are fabricated in the condition for their longitudinal growth with NaNdF_4j but without a F^" ion source.

[0166] Figure 19 illustrates schematic processes of the simultaneous erosion of the NaYF₄- NaGdF₄, epitaxial growth of NaNdF₄ in the longitudinal direction and the migration growth of F^", Y³⁺ and Gd³⁺ ions. The etching of NaGdF₄-NaYF₄ nanocrystals is initially triggered by the OA^" ions strongly bonding to the side surfaces at a high reaction temperature (310 °C). As a result, F^", Na⁺ and Y³⁺ ions are released into the solution. 13. Design and fabrication of "pupa-like" or capsule-like heterogeneous NaYF₄/NaGdF₄/NaNdF₄ nanocrystals: a further verification that oleate ions (OA^') on side surfaces cause dissolution

[0167] The observed dissolution (etching) phenomenon could possibly be caused by the relative crystal stability difference between NaYF₄ and NaGdF₄. To provide further insight into its formation, an experiment was designed which selectively protected the side surface of NaYF₄ nanocrystal using a more stable NaGdF₄ nanocrystal shell. We provide evidence that the surfactant oleate ions (OA^") firmly bonds to the side-surface rare earth ions as the main factor which causes the loss of NaYF₄ and NaGdF₄ from the side surface of the particles.

[0168] The approach to the design of the particles is described as:

1. First employ the approach of the transversal growth of NaGdF₄ nanocrystal shells onto the side surface of NaYF₄ nanocrystal cores. This provided a relatively stable side surface protected by the NaGdF₄ nanocrystal leaving the top and bottom surfaces of NaYF₄ exposed;

2. Then continue to longitudinally grow the NaNdF₄ nanocrystals, but without the F^" ion source; in such a condition, the direct epitaxial shell growth of NaNdF₄ was stopped because of the lack of an F^" ion source; 3. With OA^" ligands strongly anchored to the surface Gd ions on each side surface (100) facet and under the high reaction temperature condition of 310 °C, if the etching was caused by relative less stable NaYF₄, the exposed top and bottom surfaces of NaYF₄ will be first etched; while if the etching was caused by the OA^" ligands, the NaGdF₄ shell on the side surface will be first etched.

Method to verify the driving force for the selective-surface etching

[0169] 0.4 mmol of NdCl₃ in 2 mL methanol solution was magnetically mixed with OA (9.5 mmol) and ODE (25 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 150 °C for 30 minutes to form a clear solution, and then cooled to room temperature. 5 mL of methanol solution containing KOH (0.8 mmol) and NaOH (0.8 mmol) was added and stirred for 60 minutes. The solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to completely remove the methanol and some of the water. The reaction mix was then injected with 0.1 mmol of 50 nm x 60 nm NaYF₄/NaGdF₄ core/shell nano-prisms (NaGdF₄ growing on the lateral faces of NaYF₄ nanocrystal), suspended in cyclohexane solution, into the reaction solution. After holding at 110 °C for a further 10 minutes to evaporate all the cyclohexane, the reaction mixture was quickly heated to 310 °C. 500 samples of the reaction solution was collected each time with a syringe at 5 minutes, 15 minutes, 30 minutes, 40 minutes, 50 minutes, and 60 minutes after the reaction started.

[0170] By a transversal growth approach, NaGdF₄ shell was first grown on the side surfaces of NaYF₄, as shown in Figure 20(a). Figure 20(b) to (f) recorded the evolution process of the epitaxial growth of the NaNdF₄ nanocrystals onto the NaGdF₄ @ NaYF₄ nano-prisms in the absence of the F^" ions and at a high temperature (310 °C). The reduced diameter of NaGdF₄ @ NaYF₄ was observed by comparing Figures 20(a) and (b) which clearly show that the dissolution (etching) only happens on the side surfaces for the first 10 minutes of the reaction. Figure 20(c) shows simultaneous etching from the side surfaces and epitaxial growth of the NaNdF₄ onto the end surfaces of the NaGdF₄ @ NaYF₄ cores, resulting in a thinner and longer "pupa-like" or capsule-shaped crystal. This indicates that the etching of NaYF₄ was relatively faster than the more stable NaGdF₄ nanocrystals, but it should be noted that the etching still mostly occurs on the side surfaces of nanocrystals with the OA^" ligands acting as a surfactant. Once the NaGdF₄ was removed (as seen in Figure 20(d), the etching process appears more even on the side surface of NaYF₄ with a relatively smooth side surface created between the NaNdF₄ nanocrystal caps. Finally, the NaYF₄ crystals completely disappear after 60 minutes of reaction with only hexagonal- shape NaNdF₄ nanocrystals as the only yield.

[0171] Although the side surface of NaGdF₄ was much more stable than the NaYF₄ and has a smaller mismatch than the NaNdF₄, etching still occurs on the side surface and the migration growth direction was still observed to occur from the side and the end surfaces. This suggests that in a highly unstable growth environment, such as a reaction at a high temperature, in absence of an F^" ion source, the strongly binding surfactant oleate ions (OA^") remove the rare earth ions from the side surfaces of the particles which act as the main factor driving behind the observed etching.

14. Design and fabrication of heterogeneous NaYF₄/NaGdF₄/NaNdF₄ nanocrystals in a flower shape: the direction of migration growth is also determined by the ratio of OA^' /OAH concentration

[0172] The next example demonstrates the design and synthesis of NaYF₄/NaGdF₄/NaNdF₄ heterogeneous nanocrystals to form heterogeneous nanoscale flower-shaped particles. We demonstrate migration growth of the NaNdF₄ to the transversal structure rather than to the end surfaces. Using the principle that the direction of epitaxial shell growth can be controlled through adjustment of the ratio of OAVOAH, it is demonstrated that a low ratio of OAVOAH concentration at relatively lower temperature can direct the migration growth instead along the transverse direction.

[0173] The design logic is described as:

1. First transversally grow NaGdF₄ nanocrystal shells onto the side surface of the NaYF₄ nanocrystal core;

2. In absence of the F^" ion source, the direct epitaxial shell growth of NaNdF₄ was stopped;

3. Considering the weak erosion process due to the low ratio of OAVOAH, increase the reaction temperature from 290 °C to 300 °C to promote the erosion of NaYF₄/NaGdF₄ nanocrystals to release F^" ions. This has the effect of trimming off part of the NaGdF₄ shells around the NaYF₄ from its side surfaces;

4. A minimum amount of F^" ions slowly released from NaYF₄/NaGdF₄ nanocrystals into the reaction solution helps form the NaNdF₄ nanocrystals transversally from NaGdF₄ crystalline shell on side surfaces of the hexagonal prism;

5. Finally, F^" ions and part of Y³⁺ and Gd³⁺ ions migrate from the NaYF₄ to the NaNdF₄ section on the lateral faces of heterogeneous nanocrystals, forming a flower shape nanostructure.

Method: Synthesis of NaYF₄/NaGdF₄/NaNdF₄ flower-shaped nanocrystals

[0174] 0.1 mmol of NdCl₃ in 1 mL of methanol solution was magnetically mixed with OA (19 mmol) and ODE (18.7 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 150 °C for 30 minutes to form a clear solution, and then cooled to room temperature. 5 mL of methanol solution containing NaOH (0.6 mmol) was added and stirred for 60 minutes. The solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to completely remove the methanol and some of the water. Then, 0.1 mmol of 50 nm NaYF₄/NaGdF₄ nano-prisms (NaGdF₄ growing on the lateral faces of NaYF₄ nanocrystal), suspended in a cyclohexane solution, was injected into the reaction mix. After holding at 110 °C for another 10 minutes to evaporate all cyclohexane, the reaction mixture was quickly heated to 300 °C. 500 μL samples of the reaction solution were collected each time with a syringe after 10 minutes, 25 minutes and 45 minutes of the reaction time.

Results and Discussion

[0175] By this transversal growth approach, NaGdF₄ shell was first grown on the side surfaces of NaYF₄, as shown in Figure 21(a). Figure 21(b) to (d) record the evolution process of epitaxial growth of NaNdF₄ nanocrystals onto NaGdF₄/ NaYF₄ nano-prisms in absence of F^" ions and at lower ratio of OAVOAH concentration. The slightly reduced diameter of NaGdF₄/NaYF₄ is compared in Figures 21(a) and in (b) some parts of NaGdF₄ was etched from the side surfaces after the first 10 minutes of reaction. Figure 21(c) shows the simultaneous etching of the side surfaces and the epitaxial growth of NaNdF₄ onto the NaGdF₄ area forming side-lobes, protrusions or bulbous shapes on the side surfaces of NaGdF₄/NaYF₄ cores. Figure 21(d) shows that more NaNdF₄ was growth around the anchors which form a heterogeneous flower-shape nanocrystal.

[0176] Because there was few OA^" ligands on the side surface, OA^" mediated etching process for NaGdF₄/NaYF₄ nanocrystals dissolution of the side surfaces becomes very slow at this relatively lower reaction temperature. Despite a high concentration of OAH on the end surfaces, no etching phenomenon was observed at the top and bottom end surface of NaGdF₄/NaYF₄ of the nano-prisms. This indirectly provides further evidence for the mechanism of OA^" mediated etching process from side surfaces.

[0177] Programmable longitudinal growth of heterogeneous nanocrystals was much easier and faster than programmable transversal growth in yielding mono-disperse morphology. The migration growth in transversal direction was made possible through the use of a low ratio of OA /OAH concentration and a low reaction temperature, though the etching process was much less efficient at the side surfaces of nanocrystal due to fewer OA^" ligands bounding to rare earth ions to these side surfaces.

75. Design and fabrication of heterogeneous NaYF₄/NaGdF₄/NaNdF₄ nanocrystals in dumbbell shapes: fine tuning experiments for more choice of morphology engineering

[0178] Processes for programmable growth of heterogeneous nanocrystals can be further refined to meet more specific needs.

[0179] The design logic of the reaction method is described as:

1. For example, during the process of forming the NaYF₄/NaGdF₄/NaNdF₄ heterogeneous nanoscale hourglass particles, reduction of the amount of Nd ion source can slow down the migration growth process which includes dissolving NaYF₄ and reforming NaNdF₄ nanocrystals;

2. In the longitudinal growth environment (high ratio of OAVOAH; high temperature 310 °C), with a slowly released F^" ion source and a small amount of Nd³⁺ ion source, the longitudinal growth along c axis direction of the NaYFVNaGcU^ nanorods is quite limited in the formation of stable sharper tips; 3. While instead of relying on the released F^" ion source supplied by dissolved NaYF₄, supply of F^" the ion source in the reaction will increase the diameter of dumbbell ends, and will form round smoothed tips.

Method for synthesis of NaYF₄-NaGdF₄-NaNdF₄ nanocrystals in sharp-end dumbbell shape

[0180] 0.1 mmol of NdCl₃ in 1 mL of methanol solution was magnetically mixed with OA (9.5 mmol) and ODE (25 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 160 °C for 30 minutes to form a clear solution, and then cooled to room temperature. 5 mL of the methanol solution containing KOH (0.2 mmol) and NaOH (0.2 mmol) was added and stirred for 60 minutes. Note: in this reaction no NH₄F was added to the solution. The solution was slowly heated to 110 °C and kept at 110 °C for 30 min to remove the methanol and the water completely. It was then injected with 0.1 mmol of NaYF₄/NaGdF₄ nanorod particle in suspended in cyclohexane solvent into the reaction solution. After holding at at 110 °C for a further 10 minutes to evaporate all cyclohexane, the reaction mixture was quickly heated to 310 °C and held at this temperature for a further 30 minutes.

Method for synthesis of NaYF₄-Na.GdF₄-Na.NdF 4 nanocrystals in round-end dumbbell shape

[0181] 0.1 mmol of NdCl₃ in 1 mL of methanol solution was magnetically mixed with OA (9.5 mmol) and ODE (25 mmol) in a 50 mL three-neck round-bottom flask. The mixture was degassed under Ar flow and heated to 160 °C for 30 min to form a clear solution, and then cooled to room temperature. 5 mL of methanol solution containing NH₄F (0.3 mmol) KOH (0.2 mmol) and NaOH (0.2 mmol) was added and the mixture was stirred for 60 min. The solution was slowly heated to 110 °C and kept at 110 °C for 30 minutes to remove the methanol and the water completely. Then, it was injected with 0.1 mmol of NaYF₄- NaGdF₄ nanorods suspended in cyclohexane into the reaction solution. After being held at 110 °C for further 10 minutes to evaporate all cyclohexane, the reaction mixture was quickly heated to 310 °C and held for 30 minutes at room temperature.

Results and Discussion [0182] The longitudinal growth direction was determined by adding a higher ratio of oleate (OA^") to oleic acid (OA) ligands at a higher reaction temperature (310 °C), which selectively passivates the nanorods side surface (100) planes. A lower concentration of Nd³⁺ ion source will slow down the epitaxial growth on the ends and lead to a reduction in the diameter of the NdYF₄ nanocrystals. Moreover the absence of an F^" ion source will make the tips sharper while the presence of additional an F^" ion source leads to the formation of rounded tips.

[0183] A decrease of an F^" and Nd³⁺ ion source in the reaction mix accelerates the etching of the NaYF₄. Since the NaNdF₄ is relatively more stable than NaYF₄, a released F^" ion source will form NaNdF₄ nanocrystals with NaYF₄ located in the rod middle area continuously becoming dissolved. This migration growth process of dissolving NaYF₄ and re-growing NaNdF₄ gradually diminishes as the Nd³⁺ ion source is consumed, which leads to the formation of sharp tips.

[0184] Figure 22 shows TEM images, elemental composition characterization and schematic illustrations of the NaYF₄-NaGdF₄-NaNdF₄ dumbbell-shape nanocrystals with sharp tips via migration growth of NaNdF₄ (a) the cores of NaYF₄-NaGdF₄ nanocrystals with a thin layer of NaGdF₄ at the ends of NaYF₄ nanorods; (b) the high magnification image and (c) the overview image of the NaYF₄-NaGdF₄-NaNdF₄ dumbbell shape nanocrystals with sharp tips; (d) HAADF-STEM image and elemental mapping images of NaYF₄-NaGdF₄-NaNdF₄ nanocrystals. (e) A schematic illustration of the formation process of the NaYF₄-NaGdF₄-NaNdF₄ dumbbell nanocrystals with sharp tips (Scale bar: 50 nm for a and b; 200 nm for c and 25 nm for d).

[0185] Figure 23 shows TEM images and a schematic illustration of the NaYF₄-NaGdF₄- NaNdF₄ dumbbell nanocrystals with round polished tips, (a) NaYF₄-NaGdF₄ nanocrystals with a thin layer of NaGdF₄ at the ends of NaYF₄ nanorods; (b) high magnification image and (c) overview image of NaYF₄-NaGdF₄-NaNdF₄ nanocrystals. (d) Schematic illustration of the formation process of the NaYF₄-NaGdF₄-NaNdF₄ nanocrystals with round ends. (Scale bar: 50 nm for a and b; 200 nm for c).

[0186] Figures 22(c) and 23(c) show highly uniform heterogeneous nanocrystals. The STEM image (Figure 22(d)) illustrates two separate partitions of tips and core particles. The elemental mapping images show the distributions of the Y³⁺, Gd³⁺ and Nd³⁺ions The combined elemental mapping image shows three well aligned components, consistent with the formation mechanism already described.

[0187] The oleate ions (OA ) mediated longitudinal growth, transversal growth, etching process and migration growth in consideration of crystal lattice mismatching rates and crystal stability have jointly formed a toolbox for highly controlled nanoscale materials engineering to fabricate rare-earth doped heterogeneous nanocrystals. The switches between these growth mechanisms are externally controllable by fine-tuning the reaction temperature, concentration and/or ratio of surfactant ligands, and/or elemental concentrations and balance in the reaction environment.

[0188] Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

[0189] Although a preferred embodiment has been described in detail, it should be understood that many modifications, changes, substitutions or alterations will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims

The claims:

1. A method of controlled nanocrystal growth, comprising the steps of: partial dissolution of a core part of a nanocrystal; and epitaxial growth of new nanocrystal onto at least one preferential surface of the nanocrystal; wherein a surfactant in solution is used to control the nanocrystal growth.

2. The method of claim 1, wherein selecting or controlling a ratio of surfactant ions concentration to surfactant molecules concentration, of the surfactant in solution, is used to control the nanocrystal growth.

3. The method of claim 2, wherein selecting the ratio of surfactant ions concentration to surfactant molecules concentration is used to control anisotropic nanocrystal growth by the partial dissolution of the core part of the nanocrystal and the epitaxial growth of the new nanocrystal onto the at least one preferential surface of the nanocrystal.

4. The method of any one of claims 1 to 3, wherein the surfactant is oleic acid.

5. The method of claim 4, wherein the partial dissolution of the core part of the nanocrystal and the epitaxial growth of the new nanocrystal is controlled by selecting or controlling a ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration.

6. The method of any one of claims 1 to 5, wherein the nanocrystal is a heterogeneous nanocrystal.

7. The method of any one of claims 1 to 6, wherein the nanocrystal is or includes an alkaline rare-earth fluoride (AREF₄) nanocrystal.

8. The method of any one of claims 1 to 7, wherein the shape of the nanocrystal is controlled.

9. The method of any one of claims 1 to 8, wherein the composition of the nanocrystal is controlled.

10. The method of any one of claims 1 to 9, wherein the aspect ratio of the nanocrystal is controlled.

11. The method of any one of claims 1 to 10, wherein the epitaxial growth is longitudinal epitaxial growth.

12. The method of claim 11, wherein KOH is added to increase the longitudinal epitaxial growth.

13. The method of any one of claims 1 to 12, wherein the nanocrystal is or includes a NaYF₄ nanocrystal.

14. The method of any one of claims 1 to 13, wherein the nanocrystal growth includes deposition of shells onto end surfaces of the core part of the nanocrystal.

15. The method of claim 14, wherein the shells are formed of NaGdF₄, and the core part of the nanocrystal is formed of NaYF₄.

16. The method of any one of claims 1 to 12, wherein the nanocrystal is or includes a P-NaREF₄ nanocrystal.

17. The method of claim 7, wherein the RE in the alkaline rare-earth fluoride (AREF₄) nanocrystal is Y, Gd, Lu and/or Nd.

18. The method of any one of claims 1 to 17, wherein the nanocrystal is a hybrid nanocrystal.

19. The method of claim 18, wherein the hybrid nanocrystal is: NaYF₄-NaGdF₄- NaNdF₄; NaYF₄-NaLuF₄; or NaYF₄-NaLuF₄-NaGdF₄.

20. The method of any one of claims 1 to 19, wherein the nanocrystal is less than about 50 nm in size in at least one direction.

21. The method of any one of claims 1 to 20, wherein dissolution occurs at one or more side surfaces.

22. The method of any one of claims 1 to 21, wherein the shape of the nanocrystal is: a rod shape; a bamboo shape; an hour-glass shape; a flower shape; a co-axial cylinder shape; a pin with double ring shape; a disc shape; or a dumbbell shape.

23. The method of claim 5, wherein the ratio of oleate ions (OA^") concentration to oleic acid molecules (OAH) concentration is selected to control the direction of epitaxial growth.

24. The method of any one of claims 1 to 23, wherein the ratio of oleate ions (OA^") concentration to oleic acid molecules (OAH) concentration is greater than or equal to 1:3.

25. The method of any one of claims 1 to 23, wherein the ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration is greater than or equal to 1:7.

26. The method of any one of claims 1 to 23, wherein the ratio of oleate ions (OA ) concentration to oleic acid molecules (OAH) concentration is less than or equal to 1 :40.

27. The method of any one of claims 1 to 23, wherein the ratio of oleate ions (OA^") concentration to oleic acid molecules (OAH) concentration is less than or equal to 1 :20.

28. The method of any one of claims 1 to 23, wherein the ratio of oleate ions (OA^") concentration to oleic acid molecules (OAH) concentration is between about 1:40 and about 1:3.

29. The method of any one of claims 1 to 23, wherein the ratio of oleate ions (OA^") concentration to oleic acid molecules (OAH) concentration is between about 1:30 and about 1:7.

30. A nanocrystal, produced by partial dissolution of a core part of a nanocrystal and epitaxial growth of new nanocrystal onto at least one preferential surface of the nanocrystal, wherein a surfactant in solution is used to control the nanocrystal growth.

31. A nanocrystal, produced by the method of any one of claims 1 to 29.