WO2019008108A1

WO2019008108A1 - Atomic force microscopy probes with an assembly of metal nanowires and dielectrophoretic method for attaching and detaching said metal nanowires to and from said probes

Info

Publication number: WO2019008108A1
Application number: PCT/EP2018/068282
Authority: WO
Inventors: Peter Walke; Yasuhiko Fujita; Steven DE FEYTER; Hiroshi UJII
Original assignee: Katholieke Universiteit Leuven
Priority date: 2017-07-07
Filing date: 2018-07-05
Publication date: 2019-01-10
Also published as: GB201710982D0

Abstract

The present invention relates to (i) a commercially available atomic force microscopy cantilever with an assembly of chemically-synthesized noble metal nanowires attached to it, without the use of an adhesive, (ii) controllable methods to attach an assembly of chemically-synthesized noble metal nanowires to said cantilever and (iii) the use of the commercially available atomic force microscope cantilever with assembly of nanowires in techniques such as surface topographic scanning with atomic force microscopy, nanoscale chemical analysis using near-field optical microscopy and with particular reference to chemical analysis such as DNA mapping, mapping active sites of catalysts, evaluating anisotropy of surfaces and surface properties, such as graphene.

Description

ATOMIC FORCE MICROSCOPY PROBES WITH AN ASSEMBLY OF METAL NANOWIRES AND DIELECTROPHORETIC METHOD FOR ATTACHING AND DETACHING SAID METAL NANOWIRES

TO AND FROM SAID PROBES

FIELD OF THE INVENTION

In a first aspect, the invention relates to a commercially available atomic force microscopy cantilever with an assembly of chemically-synthesized noble metal nanowires attached to it, without the use of an adhesive, and controllable methods to attach an assembly of chemically-synthesized noble metal nanowires to said cantilever. In a second aspect, the invention relates to controllable methods for attaching an assembly of chemically- synthesized noble metal nanowires to a commercially-available atomic force microscope cantilever without using an adhesive to attach the nanowires to said cantilever. In a third aspect, the invention relates to the use of said cantilever with assembly of nanowires attached to it in techniques such as surface topographic scanning with atomic force microscopy, nanoscale chemical analysis using near-field optical microscopy and with particular reference to chemical analysis such as DNA mapping, mapping active sites of catalysts, evaluating anisotropy of surfaces and surface properties, such as graphene.

BACKGROUND OF THE INVENTION

The family of scanning probe microscopies (SPM), such as scanning tunneling microscopy (STM) or atomic force microscopy (AFM) has gained increasing precedence for nanoscale analysis due to the high-resolution images they can provide of surfaces. However, these methods typically do not have the chemical sensitivity afforded by optical techniques such as Raman or IR, precluding their use to answer many outstanding questions in the physical and chemical sciences.

Apertureless near-field optical microscopies (a-SNOM), such as tip-enhanced Raman spectroscopy (TERS), seek to alleviate the problems of traditional SPM by introducing highly specific chemical analysis at the nanoscale.

With the rapid development of three-dimensional (3D) nanophotonics and nanoelectronic devices, there has been a growing demand to use AFM for nondestructive and rapid topographical characterization of nanostructured 3D surfaces. Cantilever-based AFM (tapping and/or contact mode) is by far the most common and versatile form of SPM, as it allows scanning of 3D nanostructures with fast speeds. This high-aspect-ratio high- resolution (HAR-HR) topographical imaging of such 3D nanostructures requires AFM probes with not only a small apex radius for high spatial resolution but also a high aspect ratio (> 10 : 1) for deep trench access, such as compound or elemental nanowires. This necessitates the attachment of elemental or compound nanowires to conventional cantilevers. Currently, attachment of such nanowires to conventional AFM cantilevers is performed through a benchtop manipulation procedure as described in Ma X, Zhu Y, Kim S, Liu Q, Byrley P, Wei Y, Zhang J, Jiang K, Fan S, Yan R, Liu M, 2016, Nano Letters, 16(l l) : 6896-6902. To this end, metal nanowires are dispersed onto a fresh polydimethylsiloxane (PDMS) substrate and subsequently a tungsten probe mounted on a motorized four-axis micromanipulator is used to pick up one nanowire, to place the nanowire in the desired position on the pyramidal tip of the AFM cantilever and to finely adjust the protruding length by gently pushing along the nanowire. After the nanowire is mounted, an amount of epoxy glue is applied on the nanowire-cantilever junction to improve the adhesion and prevent the detachment of the nanowire during the tapping- mode measurement. This micromanipulation method is a time-intensive procedure, limiting the possibility for batch processing. In addition, removal of the epoxy glue requires high temperatures or the use of toxic solvents, both time consuming and costly procedures. Since the epoxy glue is not easy to remove, controlled removal of nanowires is not possible meaning that the expensive cantilevers can not be re-used and mistakes during attachments can not be resolved. Moreover, adhesives such as epoxy glue are polymeric and organic in nature, leading to rich optical (and particularly Raman) signatures. This is a significant limitation to the use of such tips for near-field optical measurements, where any measured signal can often be saturated by contaminants such as the adhesive used. In addition, TERS and related techniques are increasingly being conducted under low-temperature and ultra-high vacuum conditions, conditions in which a standard adhesive is not stable, or inside liquids. Both these situations require all sources of contamination to be minimized, making it impossible to use a cantilever with nanowires attached to it using an adhesive. Hence, there is a need for AFM cantilevers with metal nanowires attached to it without using an adhesive.

SUMMARY OF THE INVENTION In accordance with the purpose of the invention, as embodied and broadly described herein, the invention is broadly drawn to an atomic force microscopy cantilever with an assembly of metal nanowires attached to it characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever. The length of the assembly of nanowires varies preferably between 0.5 and 10 μιτι.

The present invention solves the problems of the related art to produce atomic force microscopy cantilever with an assembly of nanowires comprising nobel metals by a method comprising immersing said atomic force microscopy cantilever in a conductive solution containing said metal nanowires at a concentration higher than 106 NW/L; applying an electric field during 1 to 10 seconds and retracting the atomic force microscopy cantilever from the conductive solution. This way the nanowires can be attached to said cantilever without using an adhesive material. They do not have to be made of magnetic material. Hereby the number of metal nanowires attached to the atomic force microscopy cantilever is controlled by the concentration of nanowires in solution and the the atomic force microscopy cantilever in which the length and number of exposed nanowires from the cantilever apex is quantified by determining the contrast of the meniscus in an optical image.

In one aspect of the invention, the concerns an atomic force microscopy cantilever with an assembly of metal nanowires attached to it, characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever and further characterised in that the metal nanowires of the assembly are made in total or in part of a noble metal.

Another aspect of the invention is directed to an atomic force microscopy cantilever with an assembly of metal nanowires attached to it, characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever and further characterised in that the metal nanowires are noble metal nanowires. In still another aspect of the invention, concerns an atomic force microscopy cantilever with an assembly of metal nanowires attached to it, characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever and further characterised in that the metal nanowires are in total or in part of silver or gold.

In still another aspect of the invention, concerns an atomic force microscopy cantilever with an assembly of metal nanowires attached to it, characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever and further characterised in that the metal nanowires consist of silver

In still another aspect of the invention, concerns an atomic force microscopy cantilever with an assembly of metal nanowires attached to it, characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever and further characterised in that the metal nanowires consist of gold

In still another aspect of the invention, concerns an atomic force microscopy cantilever with an assembly of metal nanowires attached to it, characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever and further characterised in that the metal nanowires consist of gold silver alloy In still another aspect of the invention, concerns an atomic force microscopy cantilever with an assembly of metal nanowires attached to it, characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever and further characterised in that the metal nanowires consist essentially of gold

In still another aspect of the invention, concerns an atomic force microscopy cantilever with an assembly of metal nanowires attached to it, characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever and further characterised in that the metal nanowires consist essentially of gold silver alloy.

In still another aspect of the invention, concerns an atomic force microscopy cantilever with an assembly of metal nanowires attached to it, characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever and further characterised in that detachable from the cantilever by immersing the cantilever with attached assembly into a liquid, for instance an aqueous solution. Such detachment liquid can also be a polar solvent, an aqueous liquid and a polar solvent, an apolar solvent or an organic solvent. The atomic force microscopy cantilever with metal nanowires attached to it as described above can be used for any of the following techniques: surface topographic scanning with atomic force microscopy and nanoscale chemical analysis using near-field optical microscopy. In yet another embodiment the atomic force microscopy cantilever here above described is used for DNA mapping; mapping active sites of catalysts or evaluating anisotropy of surfaces and surface properties, such as graphene.

DETAILED DESCRIPTION OF THE INVENTION LEGENDS TO FIGURES

Figure 1 is a schematic representation of the nanowire attachment procedure before the attachment. It illustrates the situation immediately prior to nanowire attachment. An AFM cantilever [1] is submerged into an appropriate solution [2] containing the dispersed metal nanoparticles of interest on a counter electrode [3] . The solution can be water-based, a polar-solvent, or a mixture of the two containing dispersed nanowires. Particularly, in the case of the attachment of chemically-synthesized noble metal nanowires, such as silver nanowires, a mixture of water and polar-solvent, such as ethanol or iso-propylalcohol can be used. Both the cantilever and conductive surface are electrically contacted to an external voltage supply [4] . A camera [5] to monitor the attachment process can be used for fine control of the attachment. Manual/electrical translation stage at the cantilever and/or the substrate can be used for fine positioning.

Figure 2 is a schematic representation of the nanowire attachment procedure during the attachment. It illustrates the process during attachment of metal nanowires to the AFM cantilever. To attach a metal nanowire to the AFM cantilever, the AFM cantilever is submerged into the solution and a voltage of direct/alternative current (DC/AC) is applied between AFM cantilever and the counter electrode. Specifically, an AC pulsed voltage of more than IV with higher than 1kHz can give a high success rate of attachment. More specifically for higher production efficiency, a squared AC voltage of 4 V at 2 MHz for 2-5 s is often used. Due to the electric force formed between the cantilever apex and the counter electrode, this electric field induces attachment. Given the decreased surface area -and consequent increased electric field- at the AFM cantilever tip apex, the electric force will preferentially induce attachment at the tip apex that is broadly aligned along the direction of the tip. Also, the length of the exposed nanowire can be checked in situ. This is advantageous when the nanowire cannot be seen optically because of the diffraction limit of light (especially when the nanowire's diameter is smaller than 100 nm). After the nanowire attachment, the functionalised cantilever is dipped into the solution again, with the point at which the nanowire enters the solution being clearly visible by the breaking of the water meniscus beneath the AFM tip. The length of the nanowire can thereby be deduced by the distance of the tip above the surface.

Figure 3 is a schematic representation of the nanowire attachment procedure after the attachment. After switching off the voltage, the cantilever is retracted from the solution. Figure 4 shows optical transmission images of a commercially available AFM cantilever before (a), during (b), after (c, d) the attachment.

Figure 5 is a schematic representation of the in situ detachment procedure of the attached nanowires. Detachment is undertaken by simply submerging the AFM cantilever deeper into the solution. Upon withdrawal, the downward movement of the solution (resulting from the surface tension between it and the AFM cantilever) is sufficient to remove the nanowires from the apex of the AFM cantilever, facilitating the re-use of the AFM cantilever. This is useful for discriminating nanowires based on length and shape for selected applications, such as a-SNOM, as well as to more generally allow for the re-use of cantilevers. The short wire can be also used for other forms of a-SNOM, such as tip- enhanced fluorescence microscopy, or pump-probe spectroscopy. Note also that for some applications, tips with somewhat longer nanowire lengths could be preferred. This could help for topographic or optical mapping of trench-like or truly 3-D samples, where the increased aspect ratio of the tip affords a reduced convolution effect during imaging. Figure 6(a) and (b) shows a scanning electron microscope image of two nanowire- functionalised AFM cantilevers with short (a) and long (b) nanowire assemblies.

Figure 7(a) and (c) show AFM images of a calibration grating (TGZ3, NT-MDT) obtained with a standard AFM cantilever and a chemically-synthesized silver nanowire attached AFM cantilever under tapping-mode AFM feedback. It can be seen that this kind of nanowire tip shows a greatly reduced tip-convolution effect, resulting from the ID structure and consequent higher aspect ratio.

Figure 8 (a) and (b) show AFM images of graphite and DNA on mica obtained by a chemically-synthesized silver nanowire attached AFM cantilever under tapping-mode AFM feedback. This clearly demonstrates the tips are effective at imaging nanoscale structures and so can be used for standard AFM microscopy.

Figure 9(a) and (b) show a Tip-enhanced Raman image of an Azobenzenethiol monolayer on an Au(l l l) surface along with representative spectra recorded with and without the tip in contact with the surface. The TERS spectrum and image were recorded with the tip in contact-mode. The TERS image displays the intensity profile of the band at ~ 1141cm ^_1, the in-phase stretching of the two C-N bonds of the azobenzene moiety.

Figure 10(a), (b) and (c) show an AFM image, representative spectra and Tip-enhanced Raman image (c), respectively, of a single-walled carbon nanotube on a gold surface obtained by a chemically-synthesized silver nanowire attached AFM cantilever. The topographic and Raman image were obtained in tapping and contact-mode feedback, respectively. The TERS image displays the G-band at approximately 1590cm ¹.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to the devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly, it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination. In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein.

It is intended that the specification and examples be considered as exemplary only.

Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus, the claims are part of the description and are a further description and are in addition to the preferred embodiments of the present invention.

Each of the claims set out a particular embodiment of the invention.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention. DEFINITIONS

The term "metal nanowires" as used in this patent application refers to a quasi-lD solid structure with at least one dimension on the scale of nanometres (i.e. that is laterally confined in 1 dimension to the nanoscale)

The term "assembly of metal nanowires" as used in this patent application refers to several or numerous individual nanowires arranged or combined sequentially into a superstructure, which may be quasi-lD in nature.

The term "length of the assembly" as used in this patent application refers to length of the nanowire superstructure, as defined above, that extends beyond the AFM cantilever apex. The term "width of the assembly" as used in this patent application refers to the average diameter of the assembly measured midway of the assembly.

The term "Atomic Force Microscopy cantilever" as used in this patent application refers to the commercially available microscopic probe that performs the active measurements carried out by an atomic-force microscope in tapping or contact feedback mode, such as ACCESS-(N)C-A, AppNano, OMCL-AC55TS, Olympus or AC160TS-R3, HQ-300-Au Asylum Research. Such cantilevers are semiconducting in nature as they are composed of positively/negatively doped silicon, and therefore have significant lower conductivity than a typical metal. The term "conductive solution" as used in this patent application refers to any electrolytic solution capable of carrying an electric current.

A noble metal is one belonging to the group consisting of rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold.

A non-noble metal is a metal not belonging to the aforementioned group of noble metals.

A first aspect of the invention describes an atomic force microscopy (AFM) cantilever with an assembly of metal nanowires attached to it in such a way that at least one of the metal nanowires makes direct surface to surface contact with the AFM cantilever. The assembly of metal nanowires can exist in various forms. This assembly can be an aggregation of metal nanowires that is centered around one metal nanowire or it can be a sequence of metal nanowires where one metal nanowires is attached to another metal nanowire in a superstructure or it can be a combination of both forms. This assembly of metal nanowires is attached to the pyramidal tip of the AFM cantilever

In certain embodiments of the first aspect of the invention, said assembly of metal nanowires is attached to said AFM cantilever without using an adhesive material.

In certain embodiments of the first aspect of the invention, the attachment of said assembly of metal nanowires to said AFM cantilever is realized by applying an electrical field. The voltage of said electrical field is between about 0.1V to about 20V, preferably between about 0.5V to about 15V, more preferably between about IV to about 12V, most preferably between about IV to about 4V. The frequency of said electrical field is between about 1 kHz to about 10 MHz, preferably between about 100 kHz to about 5 MHz, more preferably between 1 MHz to about 5 MHz, most preferably between 1 MHz to 2 MHz. Said electrical field is applied during a time of between 0.1s and 60s, preferably between 0.5s and 30s, more preferably between Is and 20s, most preferably between Is and 10s. An example is the diectrophoresis (DEP) method. This is a general technique to control the movement of any charged object by an electric field generated between two electrodes. In certain embodiments of the first aspect of the invention, the assembly of metal nanowires comprises 1 to about 50 nanowires, preferably between 1 to about 25 nanowires, more preferably between 1 to about 10 nanowires, most preferably between 1 to about 5 nanowires.

In certain embodiments of the first aspect of the invention, the length of the assembly of metal nanowires that extends beyond the AFM cantilever apex varies between about 0.1 μιτι and 50 μιτι, preferably between 0.1 μιτι and 25 μιτι, more preferably between 0.1 μιτι and 15 μιτι, most preferably between 0.5 μιτι and 10 μιτι. In addition, the width of the assembly varies between about 50 nm to lOOOnm, preferably between 75 nm and 750 nm, more preferably between 100 nm and 600 nm.

In certain embodiments of the first aspect of the invention, said assembly of metal nanowires can be removed from said AFM cantilever. To this end, said AFM cantilever with said assembly of metal nanowires attached to it is immersed into a liquid. This liquid can comprise water, aqueous solutions and polar solvents. Said AFM cantilever with said assembly of metal nanowires is immersed into the liquid in such a way that the liquid contacts the AFM cantilever above the point where said assembly is attached to said AFM cantilever. Said AFM cantilever is immersed in the liquid during between about 0.1s to about 10s, preferably between about 0.1s to about 5s, more preferably between 0.1s to about 3 s, most preferably between about 0.1 s to Is.

In certain embodiments of the first aspect of the invention, said liquid in which the AFM cantilever is immersed is water and an aqueous solution.

In certain embodiments of the first aspect of the invention, said liquid in which the AFM cantilever is immersed is a polar solvent. Examples are ethanol or iso-propylalcohol.

In certain embodiments of the first aspect of the invention, said liquid in which the AFM cantilever is immersed is a mixture of an aqueous solution and a polar solvent. Examples of these mixtures are water and ethanol or water and iso-propylalcohol in any volume fraction.

In certain embodiments of the first aspect of the invention, said metal nanowire are made in total of silver or in part of silver or in total of gold or in part of gold.

A second aspect of the invention describes the method that is used to attach an assembly of metal nanowires to an AFM cantilever without using an adhesive such as glue. The method comprises immersing said AFM cantilever in a conductive solution containing said metal nanowires at a concentration higher than 10⁶ NW/L. The surfaces of metal nanowires dispersed in solution are always positively or negatively charged, depending on the particular surfactant used during synthesis. Said surfactant can be any used during the wet-chemical synthesis of nanowires (for example, if using the polyol synthesis for silver nanowires, it can be PolyvinylPyrrodone (PVP) or Citirc acid), or any charged ligand for which replacement of the original surfactant is viable, such as Cysteamine or an alkylthiol. Subsequently, an electrical field is applied between the AFM cantilever, which needs to be conductive, and a counter electrode in the conductive solution. For the counter electrode, a conductive wire, such as a carbon or metal wire, or any conductive substrate, such as a metal plate or conductive coated substrate, can be used. Examples are Indium Tin Oxide or Fluorine-doped Tin Oxide. For the cantilever, as an example, positively/negatively doped AFN cantilevers can be used. The voltage of said electrical field is between about 0.1V to about 20V, preferably between about 0.5V to about 15V, more preferably between about IV to about 10V, most preferably between about IV to about 4V. The frequency of said electrical field is between about 1 kHz to about 10 MHz, preferably between about 100 kHz to about 5 MHz, more preferably between 1 MHz to about 5 MHz, most preferably between 1 MHz to 2 MHz. The electrical field is applied during a time of between 0.1s and 60s, preferably between 0.5s and 30s, more preferably between Is and 20s, most preferably between Is and 10s. An example is the diectrophoresis (DEP) method . This is a general technique to control the movement of any charged object by an electric field generated between two electrodes. Subsequently, the AFM cantilever is retracted from the conductive solution.

In certain embodiments of the second aspect of the invention, the number of metal nanowires in the assembly is quantified by determining the contrast of the meniscus in an optical image. This is done by, upon moving down gradually, checking the point at which the nanowire breaks the liquid meniscus, as can be seen by a change in contrast in optical image used for visualisation of the cantilever, and then determining the distance from this to the apex of the cantilever. The cantilever and its reflection can be clearly visualised in the optical image, and the position of the meniscus or liquid layer is situated equidistant between the two. The system can be visualised using, for example, a high-magnification zoom lens (MVL12X3Z, THORLABS) and a CMOS camera (DCC1645C, THORLABS), with the image being produced on a standard computer monitor. The distance between the meniscus and the end of the AFM cantilever can be determined by determining the field of view (i.e. the distance subtended by the image) of the produced optical image by using a stage micrometre target (R1L3S2P, THORLABS) (I.e. A transparent sample marked with micrometre rulings to accurately calibrate the size of the image in both the vertical and horizontal directions). This then allows the distance between the meniscus and the end of the cantilever to be read out by measuring the distance in the displayed image relative to the total size of the image.

In certain embodiments of the second aspect of the invention, the assembly of metal nanowires can be detached from the AFM cantilever by immersing the AFM cantilever with attached assembly of metal nanowires into a liquid . Said liquid can comprise any of the following : water, aqueous solutions, polar solvents or organic solvents, or any combination therein. Said AFM cantilever with said assembly of metal nanowires is immersed into the liquid in such a way that the liquid contacts the AFM cantilever above the point where said assembly is attached to said AFM cantilever. Said AFM cantilever is immersed in the liquid during between about 0.1s to about 20s, preferably between about 0.1s to about 10s, more preferably between 0.1s to about 5 s, most preferably between about 0.1 s to 3s. In certain embodiments of the first aspect of the invention, said liquid in which the AFM cantilever is immersed is an aqueous solutions.

In certain embodiments of the second aspect of the invention, said liquid in which the AFM cantilever is immersed is a polar solvent. Examples are ethanol or iso-propylalcohol. In certain embodiments of the second aspect of the invention, said liquid in which the AFM cantilever is immersed is an organic solvents. Examples are n-tetradecane, hexane or toluene .

In certain embodiments of the second aspect of the invention, said liquid in which the AFM cantilever is immersed is an apolar solvent. Examples are n-tetreadecane, hexane or toluene.

In certain embodiments of the second aspect of the invention, said liquid in which the AFM cantilever is immersed is a mixture of an aqueous liquid and a polar solvent. Examples of these mixtures are water and iso-propylalcohol or water and ethanol mixed in any volume fraction.

In certain embodiments of the second aspect of the invention, the number of metal nanowires in the assembly attached to the AFM cantilever is controlled by the concentration of metal nanowires in the solution in which the AFM cantilever is immersed in. In brief, a concentration between 10¹⁰ NW/L and 10⁶ NW/L will promote nanowire attachment. More preferably, for single or attachment of an assembly consisting of a maximum of 5 nanowries, a concentration between 10⁷ NW/L and 10⁹ NW/L is used. Most preferably, a concentration between 10⁷ NW/L and 10⁸ NW/L is used.

In a third aspect of the invention, said AFM cantilever with said assembly of metal nanowires attached to it is used for any of the following techniques: surface topographic scanning with atomic force microscopy and nanoscale chemical analysis using near-field optical microscopy. Chemical analysis can comprise DNA mapping, mapping active sites of catalysts, evaluating anisotropy of surfaces and surface properties such as graphene.

EXAMPLES EXAMPLE 1

Figure 4 shows optical transmission images of a commercially available AFM cantilever before (a), during (b), after (c, d) the attachment. A commercially-available AFM cantilever from AppNano (ACCESS-NC-A) is used. Chemically synthesized silver nanowires prepared from a polyol synthesis were dispersed into a mixture of water/isopropanol with a volume ratio of 1 : 1. This solution was dropped onto a cleaned FTO substrate. The cantilever, held by a clamp equipped on a xyz-translation stage, is brought to the field of view of a camera, with a transmission configuration used for illumination. To attach the nanowire, a stage equipped onto a Z-axis translation stage, upon which Fluorine-doped Tin Oxide (FTO) is fixed, is moved towards the AFM cantilever until the cantilever is submerged. Then, an AC square voltage of 4 V at 2 MHz is applied between the AFM cantilever and FTO for 5 s. After this, the voltage is turned off and the stage is gradually moved down so as to check the nanowire attachment and/or the nanowire length by the meniscus. This is done by, upon moving down gradually, checking the point at which the nanowire breaks the liquid meniscus, as can be seen by a change in contrast in the optical image used for visualisation of the cantilever, and then determining the distance from this to the apex of the cantilever. The cantilever and it's reflection can be clearly visualised in the optical image, and the position of the meniscus or liquid layer is situated equidistant between the two. The cantilever and its reflection can be clearly visualised in the optical image, and the position of the meniscus or liquid layer is situated equidistant between the two. The system can be visualised using, for example, a high-magnification zoom lens (MVL12X3Z, THORLABS) and a CMOS camera (DCC1645C, THORLABS), with the image being produced on a standard computer monitor. The distance between the meniscus and the end of the AFM cantilever can be found by determining the field of view (i.e. the distance subtended by the image) of the produced optical image by using a stage micrometre target (R1L3S2P, THORLABS) (I.e. A transparent sample marked with micrometre rulings to accurately calibrate the size of the image in both the vertical and horizontal directions). This then allows the distance between the meniscus and the end of the cantilever to be read out by measuring the distance in the image relative to the total size of the image. Finally, the stage is lowered further and the functionalised AFM cantilever is removed and ready for use.

EXAMPLE 2

Figure 6 shows a scanning electron microscope image of several kinds of nanowire- functionalised AFM cantilevers. Depending on the specific conditions, such as voltage, time and concentration of the nanowires, the length of the assembly of nanowires attached to the AFM cantilever apex can be controlled . For example, while voltage and concentration are fixed to 4 V_p-_P and < 10⁹ NW/L at 2 MHz respectively, single nanowire attachment became dominant at 5 s, whereas multiple nanowire attachment is promoted at higher concentrations than 10¹⁰ NW/L. Possible applications of especially a relatively short nanowire tip is for high-aspect ratio AFM probe and for sensitive surface optical analysis, such as tip-enhanced fluorescence or tip-enhanced Raman scattering microscopy, when noble metal nanowires, such as gold/silver nanowires, are attached. A tip having a nanowire longer than 10 μιτι could be used for, e.g. imaging truly 3D structures with reduced tip-convolution or for endoscopic applications (inserting a thin needle into cells or any matrix which is difficult to reach by typical conical tips).

EXAMPLE 3

Figure 7(a) and (c) show AFM images of a calibration grating (TGZ3, NT-MDT) obtained with a standard AFM tip and a chemically-synthesized silver nanowire attached AFM cantilever under tapping-mode AFM feedback. The images are obtained using a commercial AFM system and by raster-scanning the tip across the sample. It can be seen that this kind of nanowire tip shows a greatly reduced tip-convolution effect, resulting from the ID structure and consequently higher aspect ratio.

Figure 8 (a) and (b) show AFM images of graphite and DNA on mica obtained by chemically-synthesized silver nanowire attached AFM cantilever under tapping-mode AFM feedback. This clearly demonstrates the tips are effective at imaging nanoscale structures and so can be used for standard AFM microscopy. The images are obtained using a commercial AFM system and by raster-scanning the tip across the sample.

EXAMPLE 4

Figure 9(a) and (b) showTip-enhanced Raman image of an Azobenzenethiol monolayer on an Au(l l l) surface along with representative spectra recorded with and without the tip in contact with the surface. The TERS spectrum and image were recorded with the tip in contact-mode. The TERS image was recorded by raster scanning. A laser operating in the visible range, such as a 633nm Helium-Neon gas laser, is focused through an objective onto the apex of the nanowire in direct contact with the sample surface. At each point in the image, a Raman spectrum is recorded by capturing the Raman scattered light through the same objective and directing it onto a Raman spectrometer and charged-coupled device. This hyperspectral 3D map can then be decomposed into a 2D image as seen here by fitting any relevant Raman peaks in the recorded spectra with an appropriate mathematical function -such as a Gauss or Lorentzian function - to obtain relevant parameters (such as the intensity of the Raman band of interest), which can then in turn be displayed as the false-colour visible in the image.

EXAMPLE 5

Figure 10 (a), (b) and (c) show an AFM image, representative spectra and Tip-enhanced Raman image (c), respectively, of a single-walled carbon nanotube on a gold surface obtained by a chemically-synthesized silver nanowire attached AFM cantilever. The topographic and Raman image were obtained in tapping and contact-mode feedback, respectively. The TERS image displays the G-band at approximately 1590cm ¹. The image in (c) was produced using the same procedure as described in figure 9.

Claims

1. An atomic force microscopy cantilever with an assembly of pre-fabricated noble metal nanowires attached to it, characterized in that at least one of the metal nanowires makes direct surface to surface contact with the atomic force microscopy cantilever and further characterised in that the metal nanowires of the assembly are made in total or in part of a noble metal.

2. The atomic force microscopy cantilever of claim 1, characterised in that the metal nanowires are noble metal nanowires.

3. The atomic force microscopy cantilever of claim 1, characterised in that the metal nanowires are in total or in part of silver or gold.

4. The atomic force microscopy cantilever of claim 1, characterised in that the metal nanowires consist of silver

5. The atomic force microscopy cantilever of claim 1, characterised in that the metal nanowires consist of gold

6. The atomic force microscopy cantilever of claim 1, characterised in that the metal nanowires consist of gold silver alloy

7. The atomic force microscopy cantilever of claim 1, characterised in that the metal nanowires consist essentially of silver

8. The atomic force microscopy cantilever of claim 1, characterised in that the metal nanowires consist essentially of gold

9. The atomic force microscopy cantilever of claim 1, characterised in that the metal nanowires consist essentially of gold silver alloy

10. The atomic force microscopy cantilever of any one of the previous claims in which an assembly of metal nanowires is attached to said cantilever without using an adhesive material.

11. The atomic force microscopy cantilever of any one of claims 1 to 10 in which the assembly of metal nanowires is attached to the atomic force microscopy cantilever by applying an electrical field.

12. The atomic force microscopy cantilever of any of claims 1 to 11 in which the assembly of metal nanowires comprises 1 to 50 metal nanowires.

13. The atomic force microscopy cantilever of any of claims 1 to 12 in which the length of the assembly of nanowires varies between 0.5 and 10 μιτι.

14. A method of producing the atomic force microscopy cantilever according of any one of the previous claims 1 to 13, characterised in that the method comprises attaching an assembly of metal nanowires to an atomic force microscopy cantilever by

a. immersing said atomic force microscopy cantilever in a conductive solution containing said metal nanowires at a concentration higher than 10⁶ NW/L; b. applying an electric field during 1 to 10 seconds; c. retracting the atomic force microscopy cantilever from the conductive solution.

15. The method of claim 14 in which the number of metal nanowires attached to the atomic force microscopy cantilever is controlled by the concentration of nanowires in solution, the immersing time in nanowire solution, or tuning the applying voltage and repetition rate.

16. Use of the atomic force microscopy cantilever with metal nanowires attached to it of any of claims 1 to 13 for any of the following techniques: surface topographic scanning with atomic force microscopy and nanoscale chemical analysis using near- field optical microscopy.

17. Use of the atomic force microscopy cantilever of claim 16 in which chemical analysis is DNA mapping; mapping active sites of catalysts; evaluating anisotropy of . surfaces and surface properties, such as graphene.

18. A method of detaching the metal nanowires of the cantilever of any of claims 1 to 13 by immersing the cantilever with attached assembly into a liquid.

19. The method of claim 18, in which the assembly of metal nanowires is detachable from the cantilever by immersing the cantilever with attached assembly into a aqueous solution

20. The method of claim 18, in which the assembly of metal nanowires is detachable from the cantilever by immersing the cantilever with attached assembly into a organic solution

21. The method of claim 18, in which the assembly of metal nanowires is detachable from the cantilever by immersing the cantilever with attached assembly into a liquid of the group a polar solvent, an aqueous liquid with a polar solvent, an apolar solvent, an organic solvent, and their mixture.