KR20050085437A - Sacrificial template method of fabricating a nanotube - Google Patents

Abstract

Methods of fabricating uniform nanotubes are described in which nanotubes were synthesized as sheaths over nanowire templates, such as using a chemical vapor deposition process. For example, single-crystalline zinc oxide (ZnO) nanowires are utilized as templates over which gallium nitride (GaN) is epitaxially grown. The ZnO templates are then removed, such as by thermal reduction and evaporation. The completed single-crystalline GaN nanotubes preferably have inner diameters ranging from 30 nm to 200 nm, and wall thicknesses between 5 and 50 nm. Transmission electron microscopy studies show that the resultant nanotubes are single-crystalline with a wurtzite structure, and are oriented along the direction. The present invention exemplifies single-crystalline nanotubes of materials with a non-layered crystal structure. Similar "epitaxial-casting" approaches could be used to produce arrays and single-crystalline nanotubes of other solid materials and semiconductors. Furthermore, the fabrication of multi- sheath nanotubes are described as well as nanotubes having multiple longitudinal segments.

Description

Sacrificial Molding Method for Fabricating Nanotubes {SACRIFICIAL TEMPLATE METHOD OF FABRICATING A NANOTUBE}

Related application cross-reference

This application claims priority from US provisional application serial number 60 / 461,346, filed April 8, 2003, which is hereby incorporated by reference in its entirety.

This application claims priority from US Provisional Application Serial No. 60 / 454,038, filed March 11, 2003, which is hereby incorporated by reference in its entirety.

This application claims priority from US Provisional Application Serial No. 60 / 432,104, filed December 9, 2002, which is hereby incorporated by reference in its entirety.

Statement of research or development sponsored by the federal government

The invention has been made with government sponsorship under contract number DE-AC03-76SF00098 awarded by the Ministry of Energy and award number DMR-0092086 awarded by the National Science Foundation. The government has certain rights in this invention.

Integration by Reference to Submissions on Compact Discs

Not applicable

The present invention generally relates to the fabrication of nanotubes, and more particularly to methods of fabricating nanotubes on sacrificial nanowire templates.

Since the discovery of carbon nanotubes (incorporated here by reference to Iijima, S., spiral microtubules of graphite carbon, Nature, 354, 56 (1991)), significant research has been poured into nanoscale tubulars of various solids ( Tenne, R. & Zettl, AK, Nanotubes from Inorganic Materials, Top.Appl. Phys. 80, 81-112 (2001); Tenne, R., Inorganic Nanoclusters and Nanotubes with Fluorine-like Structures, Prig. Inure. Chem. 50, 269-315 (2001); Partake, GR, Cromlech, F. & Nester, R., Anisotropic Modules for Oxidation Nanotubes and Nanorods-Future Nanotechnology, Angew.Chem.Int.Ed. 41, 2446-2461 (2002); Martin, CR, Nanomaterial-Membrane Based Synthesis Approach, Science, 266, 1961-65 (1994); Ajayan, PM et al., Removable for Metal Oxidized Nanocomposites and Nanostructures Carbon nanotubes as templates, Nature, 375, 564-566 (1996); Yang SM et al., Hollow Helix Formation in Mesoporous Silica: Shoe Supramolecular Origami, Adv. Mater. 11, 1427-30 (1999); Kondo, Y. & Takanayagi, K., Synthesis and Characterization of Spiral Multishell Gold Nanowires, Science, 289, 606- 608 (2000); Li Y. et al., Bismuth Nanotubes, J. Am. Chem. Soc. 123, 9904-05 (2001); and Wu, Y. & Yang, P., Semiconductor Nanowires in Nanotubes Dissolution and Welding, Adv. Mater. 13, 520-523 (2001), the above references are incorporated herein by reference).

In general, the formation of tubular nanostructures requires layered or anisotropic crystal structures (Tenne, R. & Zettl, AK, nanotubes from inorganic materials, Top. Appl. Phys. 80. 81-112 (2001) Tenne, R., Inorganic Nanoclusters and Nanotubes with Fluor-like Structures Prig.Inure.Chem. 50, 269-315 (2001); Partake, GR, Cromlech, F. & Nester, R., Nanotube Oxides And anisotropic modules for nanorod-future nanotechnology, Angew. Chem. Int. Ed. 41, 2446-2461 (2002), incorporated herein by reference).

Solid carbon nanotube templates and porous membranes or thin film rolling (Schmidt, OG & Eberl, K., thin films rolled into nanotubes) lacking layered crystal structures such as silica, alumina, silicon and metals, Nature, 410, 168 (2001), incorporated herein by reference).

However, the nanotubes made by the process are amorphous or polycrystalline, or exist only in the highest vacuum.

The importance of hollow inorganic nanotubes is recognized and they have wide applicability in biodegradation and catalysts (Lee, SB; Mitcell, DT; Trofin, L .; Nevanen, TK; Soderlund, H .; Martin, CR Science 2002 , 296, 2198, integrated here by reference). Among these hollow nanotubes, silica nanotubes are of particular interest because of their hydrophilic properties, colloidal suspension formation, and easy access to surface functionalization for the inner and outer walls. Such modified silica nanotubes and nanotube membrane examples have applicability to biodegradation and biocatalysts (Mitchell, DT; Lee, SB; Trofin, L .; Li, NC; Nevanen, TK; Soderlund, H .; Martin Incorporated herein by reference, CRJ Am. Chem. Soc. 2002, 124, 11864).

Recently, bright luminescence from sol-gel template synthesized silica nanotubes was observed by Zhang et al. (Zhang, M .; Ciocan, E .; Bando, Y .; Wada, K .; Cheng Pirouz, P. Appl. Phys. Lett. 2002, 80, 491; incorporated herein by reference). In addition, the study of the physical and chemical properties of molecules or ions trapped with inorganic nanotubes is of great interest.

 Silica nanotubes have typically been synthesized in the pores of porous alumina membrane templates using sol-gel coating techniques (incorporated herein by Martin, C. R. Chem. Mater. 1996, 8, 1739, reference). Alumina templates can be dissolved to liberate single silica nanotubes. Nanotubes produced at such low temperatures have relatively porous walls with porous walls. Once the template is removed, the silica nanotubes will typically be wound up and less directional. The same applies to silica nanotubes prepared at low temperatures using other templates (Obare, SO; Jana, NR; Murphy, CJ Nano Lett. 2001, 1, 601; Jung, JH; Shinkai, S .; Shimizu, T. Nano Lett. 2002, 2, 17; Yin, YD; Lu, Y .; Sun, YG; Xia, YN Nano Lett. 2002, 2, 427, incorporated herein by reference).

Thus, the growth of single crystalline semiconductor nanotubes offers many advantages for nanoscale electronics, optoelectronics and biochemical sensing applications. The present invention satisfies these needs as well as others and overcomes the deficiencies of previously developed nanoscale growth methods.

The invention will be better understood with reference to the following drawings, which are for purposes of explanation only:

1A-1C are cross-sectional views of an epitaxial casting process for fabricating nanotubes according to one aspect of the present invention showing GaN nanotubes formed on ZnO nanowires.

2A is an image of a nanowire template arrangement in accordance with an aspect of the present invention made from ZnO, showing the cross-section of the nanowire arrangement.

FIG. 2B is an image of a nanotube array formed with the sacrificial nanowire array of FIG. 2A according to one aspect of the present invention fabricated from GaN seen inserted at a broken interface between GaN nanotubes and a substrate. FIG.

FIG. 3 is a diffraction plot of the GaN nanotube array of FIG. 2B in accordance with an aspect of the present invention showing a nanotube composite. FIG.

4A-4C are images of the nanotubes of FIG. 2B according to one aspect of the present invention, showing relatively uniform in diameter and wall thickness.

4D is a high resolution image of the outer wall structure in the GaN nanotubes of FIG. 2B in accordance with an aspect of the present invention.

4E is a high resolution image of the inner wall structure in the GaN nanotubes of FIG. 2B according to aspects of the present invention, shown with the insertion of an electron diffraction pattern taken on the nanotubes along the [110] zone axis.

5 is a plot of a nanotube composite across a nanotube side in accordance with an aspect of the present invention, as tested by energy dispersive X-ray spectroscopy.

6 is an image and front view of a nanotube formed according to one aspect of the present invention.

7 is an image of a single crystalline GaN nanotube made in accordance with an aspect of the present invention and shows an approximate feature thereof.

FIG. 8 is a plot of the electron energy loss spectrum collected in the GaN nanotubes of FIG. 7.

9A is an image of a nanotube made in accordance with an aspect of the present invention, showing that the nanowire template was partially removed.

FIG. 9B is an image of a nanotube fabricated in accordance with an aspect of the present invention with the nanowire template partially removed and shows electron diffraction patterns recorded in the core sheath and pure tube regions along the [110] zone.

FIG. 10 is a plot of the line sides of the core sheath of the nanotubes at the upper arrow position in FIG. 9B, showing Ga and Zn signals.

FIG. 11 is a plot of the line sides of the core sheath of the nanotubes at the lower arrow position in FIG. 9B and shows Ga and Zn signals.

12 is a plot of photoluminescence spectra collected on GaN nanotubes according to one aspect of the present invention, showing spectra from thin wall and thick wall nanotubes.

13 is a plot of the temperature dependent curve of a single GaN nanotube in accordance with an aspect of the present invention.

14A-14G are steps of forming SiO ₂ nanotubes in accordance with one embodiment, showing a parylene deposition step during etching.

15A-15D are images of silicon nanotube array formation in accordance with aspects of the present invention, and are shown in detail in FIGS. 15B-15D.

16A-16B are images of silica nanotubes according to aspects of the present invention.

FIG. 17 is a cross-sectional view of a multi-layer nanotube in accordance with an aspect of the present invention, shown with a gallium nitride sheath superimposed on an insulating aluminum nitride layer.

18 is a cross-sectional view of a multi-layer nanotube in accordance with an aspect of the present invention and shows having a P-doped sheath over an N-doped sheath surrounding the sacrificial core.

19 is a cross-sectional view of a multi-layer nanotube in accordance with an aspect of the present invention and shows having an N-doped sheath over a P-doped sheath surrounding the sacrificial core.

20 is a perspective view of a sacrificial core covered with a solid sheath and has multiple longitudinal nanotube sections in accordance with aspects of the present invention.

21 is a perspective view of a sacrificial core covered with multiple sheaths and has multiple longitudinal nanotube segments in accordance with aspects of the present invention.

FIG. 22 is a cross-sectional view of fabricating a nanotube device in accordance with an aspect of the present invention, showing that it includes a hollow core NPN transistor.

The present invention includes single and multiple layer nanotube structures and fabrication methods. In the case of multiple layered nanotube structures, the interface between the cylindrical layers of nanotubes in the structure may form insulated or non-insulated device junctions, or provide other material properties. In addition, the longitudinal sections of the nanotubes can be made differently to make longitudinal bonds along a given nanotube.

An important aspect of the present invention is that the nanotubes and synthetic structures are formed on a sacrificial template, which is preferably a nanowire core. The general fabrication process of the present invention involves making a core (nanotube mold), on which a sheath is formed. Multiple methods will be used to make the core and form one or more sheaths. It is appreciated that the core and sheath portions can be formed from various materials.

For example, the core may be selected from materials including ZnO, Si, GaN, Ge, Ag, Group 2-6 materials, Group 3-5 materials, basic Group 4 materials (eg Si, Ge) and metals. Can be. The classification is considered to describe a group of substances as shown in the periodic table of elements.

For example, the sheath may be gallium nitride (GaN), silicon oxide (Si0 ₂ ), a group 2-6 material, a group 3-5 material, a basic group 4 material (eg, Si, Ge), a metal, the material Oxides and materials constituting the polymer. It is also highly appreciated that the sheath material can be preferably doped (eg, during formation) to change the properties of the base material.

Therefore, the invention generally relates to (a) forming nanowire templates; (b) depositing a sheath on the nanowire template; And (c) removing the nanowire template. Two aspects of the method will now be described, forming GaN nanotubes on zinc oxide nanowire templates, and Si0 ₂ nanotubes on Si nanowire templates. In general, nanotubes according to the invention are formed using a casting process, an etching process or a combination thereof. By way of example, an epitaxial casting process will first be described to make GaN nanotubes. Next, the oxidation and etching process for making silicon oxide (SiO ₂ ) nanotubes will be described.

In one aspect of the nanotube fabrication process according to the present invention, the "epitaxial casting" approach is based on the technically important gallium (III) nitrile (GaN) having an internal diameter of approximately 30 nm to 200 nm and a wall thickness of approximately 5 nm to 50 nm. Likewise, it is used for the synthesis of single crystalline nanotubes. Within such nanowire arrays, nanowires are used as templates for epitaxial surface determination of thin GaN layers in chemical vapor deposition systems. By way of example, the nanowire template may be fabricated from hexagonal zinc (II) oxide (ZnO) materials, in which GaN nanotubes are grown. The template material is preferably removed continuously by simple thermal reduction and evaporation steps, resulting in an ordered arrangement of GaN nanotubes on the substrate. Arrays of ZnO nanowires are grown using a vapor deposition process, on substrates such as sapphire wafers. The same approach to synthesizing nanotubes described herein can work for most of Group III nitrides.

In another aspect, the nanotubes are formed of silicon oxide (SiO ₂ ) during the oxidation process and the nanowire cores are removed during the etching process. Nanotube cores (templates) are made using thermal oxidation and etching from silicon (Si) nanowires, with caps (eg, Au). The process involves thermal oxidation of the Si nanowire array resulting in an array of thin Si nanowires covered by a hard silicon oxide (SiO ₂ ) layer. This oxidized nanowire array is then selectively etched away leaving an array of aligned silicon dioxide nanotubes with an adjustable inner diameter, such as removing the silicon nanowire core with xenon fluorine (XeF2). The inner diameter is controlled by the initial diameter and thermal oxidation process of the silicon nanowires. The inner tube diameter of the nanotubes is controlled in the range of approximately 10 nm to 200 nm. With purification of oxidation and etching processes, nanotubes with an internal diameter of less than 5 nm can be produced in this way.

Various aspects of the invention are raised herein, but are not limited to the following.

One aspect of the present invention is the formation of nanotube structures.

One aspect of the invention is the formation of single crystalline nanotube structures.

Another aspect of the invention is the formation of gallium nitride (GaN) nanotubes.

Another aspect of the invention is the formation of silica nanotubes (SiO ₂ ).

Another aspect of the present invention is to form nanowires that are used as templates for forming nanotubes.

Another aspect of the invention is the use of zinc oxide (ZnO) nanowires as a template for forming nanotubes.

Another aspect of the invention is the use of silicon (Si) nanowires as a template for forming nanotubes.

Another aspect of the invention is to use an epitaxial casting process for forming sheath on nanowires, such as GaN on ZnO nanowires.

Another aspect of the invention is the use of oxidation and etching processes to form a sheath of material over the nanowires, such as SiO ₂ for Si nanowires.

Another aspect of the invention is the formation of multiple sheath layers over a sacrificial mold (core).

Another aspect of the invention is the formation of a plurality of sheath layers over the sacrificial core.

Another aspect of the invention is the formation of a sheath layer in longitudinal sections along the length of the sacrificial core.

Another aspect of the invention is a method for forming single crystalline nanotubes that can be used in electronic devices, nanofluidic devices, or combinations thereof.

Further aspects of the invention are set forth in the following part of the specification, and the description is intended to disclose, in its entirety, more preferred aspects of the invention without being limited thereto.

According to the present invention, nanotubes are formed by making at least one sheath layer around a nanowire template. The nanowire template acts as a sacrificial core that is later removed for the opening of the center through the nanotubes. After the sacrificial core is removed, the nanotubes can be used in any conventional means.

By way of example, two aspects of a method for fabricating nanotubes using the sacrificial core according to the present invention will be described without limitation. However, the present invention is highly appreciated in that it has designed a method in which the sacrificial core is used as a template for nanotube fabrication. In a first aspect, a layer of material such as gallium nitride (GaN) grows epitaxially outside of a nanowire core such as zinc oxide (ZnO), and the nanowire core is removed. In a second aspect, nanowire cores, such as silicon (Si), are oxidized to form a SiO ₂ sheath layer, and the nanowire cores are removed to leave the oxide sheath.

Epitaxial casting method

1A-1C show a general procedure referred to as the "epitaxial casting" approach. 1A shows a substrate 10 in which nanowires 12, preferably a single crystalline nanowire, are formed. FIG. 1B shows the deposition of a single crystalline sheath 14, preferably onto nanowire 12. 1C shows forming nanotube 14 'by removing nanowire template (core) 12. FIG.

In one embodiment, nanowire 12, such as prefabricated hexagonal single crystalline nanowires (preferably ZnO), is used as a template for tubular deposition of materials such as GaN. Since both ZnO and GaN have fibrous zinc crystal structures and similar crystal constants (ZnO: a = 3.249Å, c = 5.207Å; GaN: a = 3.189Å, c = 5.185Å), GaN is the ZnO nanocylinder It can grow epitaxially on the {110} side of and naturally forms a single crystalline thin GaN. It is worth noting that many combinations of materials have crystalline structures and lattice constants similar enough to allow epitaxial growth of sheath material in nanowire materials.

After the ZnO nanocylinder was covered with a thin GaN sheath 14 (FIG. 1B), template 12 (FIG. 1A) was removed continuously, leaving GaN nanotubes 14 by a thermal process. By way of example and not limitation, two possible mechanisms for the removal of the ZnO template can be used.

In one approach, ZnO is chemically etched by hot edge ammonia (NH ₃ ) (Hamdani F. et al., Effect of buffer layer and substrate surface polarity on growth by molecular beam epitaxy of GaN at ZnO, Appl. Phys Lett. 71, 3111-13 (1997) incorporated herein by reference). Extended heating of the sample after GaN is coated with ammonia (NH ₃ ) readily produces pure GaN nanotubes (FIG. 1C).

Another approach is to use a heat reduction process at high temperatures (eg 600 ° C. in hydrogen gas H ₂ ). Single Crystalline Fibrous Zinc GaN Nanotubes

A fundamentally different from GaN nanotubes, a metastable graphite GaN structure has been proposed (Lee SM, Stability and Electronic Structure of GaN Nanotubes from Density-Function Measurements, Phys. Rev. B, 60,7788-7791 ( 1999), integrated here by reference).

Example 1

The nanowire cores used in the present invention can be formed by any conventional method. For example, an array of zinc oxide (ZnO) nanowires was grown on a substrate material such as a (110) sapphire wafer, preferably using a vapor deposition process (Huang M. et al., Room Temperature Ultraviolet Nanowire Nanolasers, Science , 292, 1897-99 (2001), incorporated herein by reference) This ZnO nanowire array is placed in a reaction tube (MOCVD reaction tube) for GaN chemical vapor deposition. Trimethylgallium and ammonia were used as precursors and injected into the system with argon or nitrogen transport gas. The deposition temperature was set at 600 ° C to 700 ° C.

After GaN deposition, the sample is treated in a hydrogen environment at elevated temperature, such as 600 ° C. with 10% H ₂ in argon, to remove the ZnO nanowire template. Other methods and materials form nanowires, cover nanowires with nanotube material, and sacrificially remove nanowire material (only one part of the nanowire material needs to be removed in accordance with the present invention in selective applications). It can be appreciated that it can be used (although less preferred in some instances) for the purpose of

2A shows an image of a scanning electron microscope (SEM) starting a ZnO nanowire array template, which has a uniform length, such as in the 2-5 μm range, and a diameter within the nanowire array in the range of 30-200 nm. It was found to have a uniform diameter. The nanowires have a well-cut face, as shown in FIG. 2A with a hexagonal cross section, which shows the {110} face in the side. After GaN deposition and template removal to form the nanotubes, the color of the sample changes from white to yellowish or darker.

FIG. 2B is an exemplary image showing that the shape of the initial Nawire arrangement is maintained within the nanotubes, except for an increase in the diameter of the resulting nanostructure. Nanostructures appear less shaved compared to the original ZnO nanowire template. Chemical analysis of the final product shows only a relatively minimal Zn signal.

FIG. 3 shows the results of X-ray diffraction (XRD) in samples showing only (001) diffraction peaks of fibrous zinc GaN structures showing good epitaxy / weaving for GaN coatings.

4A-4C show images of dispersing the FIG. 2B GaN nanotube sample in an electron microscope (TEM) grid for structural analysis. It has been found that most of the nanostructures show tubular structures with uniform wall thickness, which is generally seen from FIG. 4A. These nanotubes have been found to have internal diameters in the range of 30 nm to 200 nm and wall thicknesses between 5 nm and 50 nm, similar to ZnO nanowire arrays.

Most of the nanotubes have only one open end, but a tube with open ends on both sides was also observed. This observation is consistent with the SEM study, as shown in FIG. 2B, round shape and less shrunk ends are observed after GaN coating. It was concluded that the open nanotube ends were originally located at the interface of GaN and the substrate, which broke open during TEM sample preparation. Indeed we have often observed these open ends on the substrate surface with the corresponding nanotubes, as shown in the example shown in FIG. 2B. TEM studies also show that the inner cross section of the nanotubes is pseudo-hexagonal after template removal.

Considerably, electron diffraction (ED) for these GaN nanotubes indicates that these tubes are single crystalline. Returning to FIG. 4E, one ED pattern along the [110] zone axis is shown. This means that the nanotubes are oriented along the c-axis of the fibrous zinc GaN structure. This is consistent with XRD data where only (001) peak is observed. Along the tube axis, the lattice spacing of 0.51 nm for the (001) plane of the fibrous galvanic structure is readily represented by the high resolution TEM images of the tube space FIG. 4D and the interior of the tube.

FIG. 5 is a line profile of a compound tested by energy dispersive X-ray spectroscopy (EDX) showing correlated gallium and nitrogen against the tube wall, which is an indicator of stoichiometric GaN formation during deposition. This is also clearly reflected in the electron energy loss spectra (EELS) recorded in these nanotubes, as shown in FIG. 8, and strong nitrogen signals are observed. Interfacial dispersion between the GaN layer and the ZnO nanowire template results in a small amount of Zn or O bonds in the GaN tube wall.

6 is an electron microscope image of the front view of several GaN nanotubes. At least two important features can be seen in the image: (1) the inner cross section of the tube is pseudo hexagonal, and (2) the nanotubes are connected at the base of the porous GaN layer and their base, which is zinc and It is believed to be the fundamental route for oxygen leakage.

7 is an electron microscope image of a single crystalline GaN nanotube showing a very smooth inner and outer surface.

FIG. 8 is a plot of the nitrogen K-edge electron energy loss spectrum collected in the GaN nanotubes of FIG. 7.

Taken together, it is commendable that high density arrays of single crystalline nanotubes can be successfully fabricated for GaN nanotubes fabricated on sapphire substrates as described. It is important to point out that the GaN nanotube formation process described here is a significant departure from previous work on inorganic nanotubes. (lijima, S., Spiral Microtubules of Graphite Carbon, Nature, 354,56 (1991); Tenne, R. & Zettl, AK, Nanotubes from Inorganic Materials, Top.Appl Phys. 80,81-112 (2001) Tenne, R., Inorganic Nanoclusters and Nanotubes with Florin-like Structures, Prig. Inure.Chem. 50,269-315 (2001); Partake, GR, Cromlech, F. & Nester, R., Future Nanotechnology Nanotube and Nanorod-Anisotropic Modules for Oxidation, Angew. Chem. Int. Ed. 41,2446-2461 (2002); Martin, CR, Nanomaterial-Membrane-Based Synthesis Approach, Science, 266,1961-65 (1994) Ajayan, PM et al., Carbon Nanotubes as Removable Templates for Metal Oxide Nanocomposites and Nanostructures, Nature, 375,564-566 (1996), incorporated herein by reference).

Previous studies on inorganic nanotubes have been pointed out for materials with layered structures (eg, VO _x, MoS ₂ , NiCI ₂ , BN). These studies of materials that do not have structural anisotropy (in porous alumina), the template approach (Caruso, R. A & Antonietti, M. sol-gel nanocoatings: an approach to the manufacture of structural materials, Chem. Mater. , 3272-3282 (2001), incorporated herein by reference), are commonly used, which predominantly results in amorphous or polycrystalline tubes. Differences between amorphous or polycrystalline tubes and advantageous single crystalline tubes preferably made in accordance with the present invention will be readily appreciated by those skilled in the art.

9A, 9B and 10, 11 illustrate the removal of nanowire templates within a single crystalline nanotube. The "epitaxial casting" mechanism described by the present invention is established with the TEM study. In FIG. 9A, the arrangement of GaN nanotubes is shown with partially removed ZnO nanowire templates. Underneath these nanotubes, it is important that there is a thin layer of porous GaN film. In addition, the rest of the ZnO nanowire template resides on top of the sealed GaN nanotubes. These two observations suggest that zinc and oxygen species (which occur during the thermal chemical etching process) essentially escape the GaN nanotubes through the porous GaN layer (shown in FIG. 6).

In FIG. 9B a detailed illustration of the nanotubes with the template partially removed is shown at the boundary between the filled portion (up arrow) and the empty portion (down arrow) of the nanotubes. The electron diffraction shown in FIG. 9B for the filled and empty portions of the nanotubes indicates that they are the same set of diffraction patterns for the tube and core sheath regions, indicating that the fibrogal GaN growth is epitaxial.

Core sheath nanostructures can be considered as a single, uniform range of quality of the fibrous zinc GaN / ZnO structure type. In addition, a comparison of the EDX line profile (low arrow) through the GaN nanotubes shown in FIG. 11 and the ZnO-GaN core sheath structure shown in the upper arrow and shown in FIG. 10 shows the growth mechanism of the GaN nanotubes in the ZnO nanowire template. It clearly supports After the ZnO nanocylinder is removed, a single crystalline tube of GaN is created. As can be seen, the formation of such single crystalline GaN nanotubes has many advantages over the use of polycrystalline nanotubes (Li, JY et al. Synthesis of GaN nanotubes, J. Mater. Sci. Lett. 20,1987-1988 (2001), incorporated herein by reference), in particular in the fact that such polycrystalline nanotubes generally have an amorphous shape. It is interesting that the microscale tubes of ZnO have been prepared in solution through a first chemical dissolution process (Vayssieres, L., Keis, K., Hagfeldt, A. & Lindquist, S. Three-dimensional of highly oriented ZnO microtubes) Ever Placement, Chem. Mater. 13,4395-4398 (2001), incorporated herein by reference).

Importantly, the electrical and optical properties of these single crystalline GaN nanotubes are characterized by epitaxial GaN on ZnO (0001) grown by high quality GaN epilayers (Hamdani F. et al., Reaction Molecular Beam Epitaxy) growing on ZnO substrates. Microstructure and optical properties, J. Appl. Phys. 83, 983-990 (1998), incorporated herein by reference), as well as GaN nanowires (Huang, Y., Duan, X., Cui, Y. & Lieber, CM Gallium Nitride Nanowire Nanodevice, Nano. Leff. 2, 101-104 (2002); Kim, J. et al., The electrophoretic properties of each gallium nitrite nanowire synthesized by chemical vapor deposition, Appl. Phys. Lett. 80, 3548-3550 (2002), incorporated herein by reference).

12 shows a low temperature photoluminescence (PL) spectra plot of the produced nanotubes measured using the fourth harmonic output of a YAG laser (266 nm) as the excitation source. Note that no midgap yellow emission was observed. Band edge emission was observed with thinner tubes emitting at shorter wavelengths in these nanotube samples between these 375 nm and 360 nm. Slightly blue shift of this emission (Hamdani F. et al., ZnO (0001) grown by reactive molecular beam epitaxy, microstructure and optical properties of epitaxial GaN, see J. Appl. Phys. 83,983-990 (1998), Is part of the nanotube, which has a wall shorter than 5 nm, which is smaller than the excitation bore radius of GaN, which can be attributed to the quantum confinement effect.

Incorporated herein by reference, the photoluminescence spectra are collected on GaN nanotubes at 10K. The sample is excited by a 266 nm line of pulsed Nd: YAG laser (eg Spectra Physics ^™ ). The photoluminescence signal is transmitted to the 0.3 meter imaging monochromator by the optical fiber and detected by enhanced CCD walking under gate mode. The only band edge radiation is the spectra drawn on the right corresponding to the spectra collected from the relatively thick wall (≥10 nm) GaN nanotubes, with the spectra drawn on the left corresponding to the spectra collected from the thin wall (<10 nm) GaN nanotubes. Is observed. The spin spectra of thin tubes are relatively wide due to the wide distribution of tube wall thicknesses in the samples tested.

FIG. 13 shows that one embodiment of an electron transfer measurement showing the resistance of such nanotubes is in units of 10 MΩ and increases with decreasing temperature, similar to high quality GaN nanowires. Referring to the figure, the temperature dependent I-V curve of a single GaN nanotube is shown. Electrodes for electrical measurements (20 nm titanium, Ti and 80 nm gold, Au) are fabricated using e-beam lithography and thermal evaporation, although other techniques may be used. In order to form a secure contact, a rapid thermal annealing step is performed for 450 ° C. for 30 seconds, although any convenient means for contact formation can be used.

Successful production of a single crystalline GaN nanocapillary using the present epitaxial casting process has resulted in the formation of inorganic solid nanotubes / nanocapillaries, especially single crystalline nanotubes / nanocapillaries. It shows the ability to manufacture. It is anticipated that this new class of semiconductor nanotubes / nanocapillaries could be used for many advantageous technical applications in the field of chemistry used with nanoscale electronics, optoelectronics, and fluid systems. The present invention provides a sturdy semiconductor nanotube having a uniform inner diameter, an inner wall that can be easily functionalized, and both ends of the nanotube capable of application of fluid flow.

Oxidation and Etching Methods

Referring now to FIGS. 14A-14G, a second method of fabricating nanotubes using the sacrificial template according to the present invention is gathered. This method is called "oxidation and etching" because it forms a robust nanotube array by converting a vertical nanowire array into an oxide nanotube array. In one embodiment, the nanotube cores (moulds) are formed from silicon (Si) nanowires with metal caps (eg, Au), often fabricated using thermal oxidation and etching. Next, the Si nanowire array is thermally oxidized resulting in an array of thin Si nanowires covered by a rigid layer of silicon oxide (SiO ₂ ). This oxidized nanowire array is selectively etched into something like xenon fluoro (XeF ₂ ) to remove the silicon nanowire core, leaving an array of organized silicon dioxide nanotubes with an adjustable inner diameter. The internal diameter is controlled by the initial diameter and thermal oxidation process of the silicon nanowires. The inner tube diameter of the nanotubes will be in the range of approximately 10 nm to 200 nm.

It is commendable that a single nanotube or random sample can be formed as an alternative to forming nanotubes in an array. Other nanotube mixtures may be fabricated in a manner that includes, but is not limited to, GaO, InO and other oxides and insulating materials.

Example 2

FIG. 14A shows a silicon nanowire arrangement fabricated using chemical vapor deposition (CVD) epitaxial growth using silicon tetrachloride (SiC ₄ , Aldrich, 99.99%) as the silicon source. Hydrogen (10% stabilized by argon) was used to reduce SiC ₄ at high temperatures (900-950 ° C.). Au thin films were coated on Si (111) substrates 30 to initiate growth of silicon nanowires 32 through a vapor-liquid-solid growth mechanism. Gold is present as cap 34 in Si nanowires. This approach to obtaining Si nanowires was developed and used in our lab for the synthesis of vertical Si / SiGe transient lattice nanowire arrays (Wu, Y .; Fan, R .; Yang, PD Nano Lett. 2002, 2 , 83; Wu, Y .; Yan, H .; Huang, M .; Messer, B .; Song, J.; Yang, P. Chem.-Eur. J. 2002, 8, 1260; here by reference integrated). The silicon nanowire array samples were heated, as they were placed in a tube furnace and heated at 800-1000 ° C. for one hour under a continuous flow of pure oxygen (O ₂ ).

14B shows Nanowire 32 after being uniformly oxidized to provide a SiO ₂ sheath 36 into a continuous silicon core. During oxidation, nanowire tips 34 are preferably oxidized to provide an oxide cap 34 'in each vertical wire to prevent selective etching of the silicon core. Therefore, the first step after thermal oxidation selectively removes SiO ₂ cap 34 'from the Si / SiO ₂ core sheath nanowires.

14C shows a more preferred form of removing the SiO ₂ cap. Polymer 38 is deposited to fill the spaces between the nanowires so that SiO ₂ sidewall 36 is protected by a matrix polymer, such as an etch-resistant material. In this example, parylene dimer (di-para-xylene, (-CH ₂ -Ph-CH ₂ ) ₂ ) is thermally evaporated at 160 ° C., separated at 650 ° C., and parylene (poly-para- It is deposited on an Si / SiO ₂ core sheath nanowire array for approximately 5 hours to produce a continuous coating of xylene, (—CH ₂ —Ph—CH ₂ ) _n ) polymer. This parylene deposition is conformal, starting with a thin layer coating on the surface of the nanowires and then filling all the gap spaces between the nanowires. This process leads to highly conformal coverage of the nanowires without pinholes or cracks.

FIG. 14D shows a core-sheath arrangement subsequent to oxygen plasma etching of the surface of polymer fill 38, such as parylene, to expose the tip of the Si / SiO ₂ nanowires.

FIG. 14E shows the core-sheath arrangement after immersion in buffered hydrofluoric acid solution for about 2 minutes to selectively remove the SiO ₂ cap 34 'and expose the silicon core 32. FIG.

FIG. 14F shows the sheath arrangement after the silicon nanowire core 32 is removed by an etchant, such as an XeF ₂ etchant gas. Although any material has been removed, it will still be important for the layer of etch-resistant material 38 'to protect the solution of the nanotube wall. Etching is preferably performed by placing the core-sheath array into the XeF ₂ etch chamber with a chamber temperature controlled at 40 ° C. After flushing with nitrogen, XeF ₂ vapor was introduced with nitrogen gas N ₂ (XeF ₂ : N ₂ = 4: 5) to perform etching for 30 seconds at a total pressure of about 9 Torr. The chamber is then emptied and poured with nitrogen and etching is performed for the second cycle. In this aspect eight cycles are performed to achieve a complete etch of the silicon core.

According to the above procedure, it was obtained that the silica nanotube array was embedded in the parylene membrane 38 and a continuous hole exited through the entire polymer membrane.

FIG. 14G shows the resulting nanotube array 36 'after parylene matrices are etched using high power oxygen plasma handling for 30 minutes to produce a vertically oriented robust silica nanotube matrix attached to substrate 30. FIG.

Example 3

15A-15D are scanning electron microscopy (SEM), which are images of the formation of nanotubes according to the present invention. Silicon nanowire arrays are shown in FIG. 15A, with Si nanowires oriented vertically to form a substantially perfect array. Typical sizes of silicon nanowires are 50-200 nm and their length is about 8 μm. At the top of each nanowire is a bright gold tip, an indicator of vapor-liquid-solid growth (Wu, Y .; Yan, H .; Huang, M .; Messer, B .; Song, J .; Yang, P. Chem.-Euro. J. 2002, 8, 1260, incorporated herein by reference).

FIG. 15B depicts nanotubes after parylene deposition, SiO ₂ cap removal, and etching of the silicon core, resulting in an array of silica nanotubes sandwiched in the parylene film. The hole is easily visible on the polymer surface. The bright spots in the image corresponding to the gold nanoparticle tips are almost hemispherical in shape. The film has a relatively flat surface. 15B shows a high magnification of the two silica nanotubes sandwiched in the parylene membrane, clearly showing the silica walls and hollow holes.

15C and 15D are perspective and top views of the nanotube array after oxygen (O ₂ ) plasma etching of parylene, and a standing silica nanotube array is obtained without braces. As can be seen, the nanotubes remain straight in the vertical direction of the starting silicon nanotube array. 15C is an enlarged view of a high magnification SEM image clearly showing the shape of the vertical nanotube array. The image shows that the Si nanowires have a uniform diameter along the length range of approximately 50 nm to 200 nm, and are oriented vertically in an array of lengths up to about 8 μm and an average length of 5 μm. The average diameter of the resulting silica nanotubes exceeds that of the template silicon nanowires and is the result of structural expansion caused by thermal oxidation. 15D is a detailed top view showing the hexagonal shape of the tube. The scale bars in FIGS. 15A, 15B, and 15C are 10 μm, 1 μm, and 10 μm, respectively. The silica walls of the nanotubes show well-drawn hexagonal shapes and anisotropic in-plan etch ratios representing the <111> direction of the original Si nanowires.

16A and 16B are electron microscope images that further illustrate the high quality of silica nanotube formation. In Figure 16A a uniform inner diameter is shown and generally exists along the entire length of the nanotubes. The pore size of the nanotubes ranges from about 10 nm to 200 nm, with smooth inner and outer walls.

The nanotube thickness was found to be around 70 nm at 1000 ° C heat treatment, despite the range of pore sizes in the nanotubes. This result is due to the fact that the thermal oxidation of silicon is a self-limiting process and the oxide layer thickness is expected to be the same as that of nanowires under constant heat treatment conditions. Self-limiting of the process is advantageous for controlling tube size and wall thickness by adjusting the characteristics of the thermal handling process, such as the handling temperature.

As an example of how nanotube features can be controlled, a sample oxidized at 900 ° C. has a typical wall thickness on the order of 55-65 nm, while a temperature of about 800 ° C. produces a wall thickness on the order of 30-35 nm. The nanotubes shown in FIG. 16B have a pore size of approximately 20 nm, but as shown, have an evenly uniform smooth inner wall. In general, branched nanotubes have been produced and it is appreciated that these nanotubes will provide an advantage in selecting nanofluidic and electronic applications.

The versatile approach to making silica nanotube array templates from these silicon nanowires is a well controlled process that can control the pore size and array height, while the resulting nanotubes are easily presented to other surface modifications on the inner and outer walls. Each surface modification of the inner and outer walls may be important for applications such as bioseparation and molecular transport. In addition, the walls of such nanotubes are formed from pinhole-free abbreviated thermal oxides, which can be advantageous in terms of their mechanical robustness and fluid stability.

As a result, the new class of semiconductor nanotubes presented by the present invention are mechanically robust, electrically and optically active. Thus, these nanotubes could provide additional opportunities for fundamental research as well as nanocapillary electrophoresis, biochemical sensing of nanofluids, nanoscale electronics and optoelectronics (Schoening, M & Poghossian, A. Biological Recent advances in sensitive field effect transistors (BioFETs), Analyst, 127, 1137-1151 (2002), incorporated herein by reference). It is highly appreciated that the successful preparation of single crystalline GaN nanotubes using this “epitaxial casting” approach suggests that it is possible to produce single crystalline nanotubes of inorganic solids with generally unlayered crystalline structures. Lauhon, LJ, Gudiksen, MS, Wang, D. & Lieber, CM epitaxial coreshell and core-multishell nanowire heterostructures, Nature, 420,57-61 (2002); and He, R., Law , M., Fan, R., Kim, F. & Yang, P. Functional Release Compound Nanotape, Nano. Lett. 2, 1109-1112 (2002), incorporated herein by reference).

It is worth noting that the techniques described herein can be further extended by forming multiple sheath layers. Each of these sheath layers includes different materials, different doping constructs, or values. Furthermore, the vertical portion (compartment) of the nanotubes may proceed differently to produce different properties between the nanotube structures or the compartments of the multi-layer nanotube structure. The following nanotube structures are provided by way of example and not limitation.

FIG. 17 shows a multi-layered nanotube 50 wherein gallium nitride (GaN) sheath 54 comprises sacrificial ZnO nanowires 12 (before removal) between two sheaths 52 and 56 of aluminum nitride (AlN). It is appreciated that the sacrificial nanowires can be removed at least after the first sheath layer is deposited over the nanowires, and subsequently removed to deposit the next sheath layer.

18 and 19 show forming a sheath of generally doped material 60. FIG. 18 shows P-doped GaN 62 and N-doped GaN material 64 over P-doped material on sacrificial core 12 (prior to removal) such as ZnO. Similarly, FIG. 19 shows the inverse of FIG. 18 with P-doped material over N-doped material that is sheath core 12 (before removal). It is appreciated that various methods can be fabricated by the method, including diodes, light emitters, light detectors, electron transfer devices (eg, bipolar transistors, FETs, insulated gate FETs, etc.) and combinations thereof. The connection can also be embedded into the material layer, while the connection to the device layer can be provided from the core, or from a connection around the outside. The process method can be continued to make any desired multiple nested sheaths within a given nanotube.

20 and 21 illustrate forming nanotube sheaths partitioned by the present invention, and other compartments are formed from different materials, different dopants, different values of doping or combinations thereof. Such sheaths can be fabricated in compartment to compartment by any conventional means, such as using traditional masking techniques.

In FIG. 20, nanotube 80 is depicted having two compartments of different sheath materials 84, 86 positioned vertically on the sacrificial core 82. 21 shows nanotubes 90 formed from three or more vertical sections of material disposed to provide different materials, different doping materials or other properties. In addition, nanotubes have been shown to have at least two material sheaths.

The core 92 is shown with the upper inner sheath 94, the upper outer sheath 96, the middle inner sheath 98, the middle outer sheath 100, the lower inner sheath 102, and the lower outer sheath 104 before removal. It should be appreciated that any desired number of sheath layers may be deposited and nanotubes may be fabricated with any number of vertical sections. It is appreciated that electrical connections to the insulator and sheath layers can be formed as part of other sheath compartments. In addition, the removed core of the nanotubes can be used as a fluid or as a material such as a metal to form another layer.

22 illustrates, in an illustrative manner, a cross-section of a nested sheath layer 110 forming a bipolar transistor. The hollow tube 12 'shows that the sacrificial nanowire core was removed. The interior of the hollow tube 12 'is shown as a metallic contact 112. Three sheaths are shown in the picture. P-doped semiconductor inner sheath 114 is shown. Separate intermediate sheaths 116, 118 of the N-doped semiconductor indicate that the central insulating ring 120 surrounds the inner sheath 114. Finally the conductive outer sheath is shown with the upper conductor 122 and the lower conductor 124 separated by an insulating sheath structure 126. A simple example shows the shape of a bipolar NPN transistor along the nanotube length with an external emission contact 122 and a collector contact 124 and a base contact 112 lining the hollow core 12 '. The thickness of the layer can be varied to achieve the desired electrical properties or to enhance the firmness as provided by the outer sheath sections 122 and 124.

Transistors are provided by way of example and a wide variety of devices can be fabricated in accordance with the techniques of the present invention. It is appreciated that various materials and electrical properties can be performed using the method of the present invention. In addition, various electronic devices such as diodes, light emitting diodes, lasers, transistors, field effect transistors and the like can be made in accordance with the techniques of the present invention.

Therefore, as shown, the present invention includes a method for fabricating nanotubes by forming a sheath on a sacrificial core and removing the core. Two general methods are described: (i) epitaxial casting and (ii) oxidation and etching, as well as specific nanotube structures using GaN nanotubes (on ZnO sheath) and oxidation and etching methods using epitaxial casting methods. And appear as SiO ₂ nanotubes (on Si sheath). However, other materials are also included and used without limitation. GaN, Ge, Ag, Group 2-6, Group 3-5, Basic Group 4 (Si, Ge) and metals are used as core materials, Group 2-6, Group 2-5, Basic Group 4, metals, materials Oxides and polymers are used as sheath materials. Remember that all sheaths can be doped during formation.

Although the above detailed description includes many details, it does not limit the scope of the present invention but merely provides some of the more preferred aspects of the present invention. Therefore, the scope of the present invention includes other aspects that are apparent to those skilled in the art, and the scope of the present invention is not limited by any other than the appended claims, and the specific elements mean "one and only one." Rather, it means "one or more." It is claimed that all structural, chemical and functional equivalents for the elements of the preferred embodiments described above, generally known to those skilled in the art, are included herein and are covered by the claims. Moreover, there is no need for an apparatus or method for solving each and every problem that is covered by the present claims and that is believed to be solved by the present invention. In addition, any component, composition or method step herein is dedicated to the public regardless of which component, composition or method step is cited in the claims. No claim element herein is claimed unless a claim element is expressly recited using the phrase "means for". 112, it is not made under the provisions of paragraph 6.

Claims (35)

Forming nanowires; Depositing at least one sheath material over the nanowires; And Removing the nanowires; Wherein the remaining sheath material is a nanotube. The method of claim 1, And wherein said nanowires are sacrificed during said removal step. The method of claim 1, Wherein said nanowire is a sacrificial template to form said nanotubes. The method of claim 1, Wherein said nanowires are formed as a single crystalline nanowire structure. The method of claim 1, Wherein said nanotubes are formed as a single crystalline sheath structure. The method of claim 1, The nanowires are made of zinc oxide (ZnO), silicon (Si), gallium nitride (GaN), germanium (Ge), silver (Ag), gold (Au), Group 2-6 materials, Group 3-5 materials, basic And a material selected from the group consisting of a Group 4 material and a material comprising a metal. The method of claim 6, The sheath comprises gallium nitride (GaN), silicon oxide (SiO ₂ ), Group 2-6 materials, Group 3-5 materials, basic Group 4, metals, oxides of the materials, dopants introduced into the materials, and polymers. And a material selected from the group of constituent materials. The method of claim 7, wherein Any one material selected for the nanotube sheath has a crystalline structure and lattice constant sufficiently similar to any one material selected for the nanowire to allow epitaxial growth of the sheath on the nanowire. How to. The method of claim 1, And the sheath is a single vertical section covering the nanowires. The method of claim 1, And the sheath is a plurality of vertical sections covering the nanowires. The method of claim 10, And wherein said plurality of vertical segments are formed using a masking technique. The method of claim 1, The array of nanotubes is fabricated by depositing a sheath on the array of nanowires; Wherein said arrangement is formed on a substrate. Forming a sacrificial nanowire template of zinc oxide (ZnO); Depositing at least one gallium nitride (GaN) sheath on the nanowires; And Removing the nanowires; The sheath is a method for manufacturing a nanotube, characterized in that the gallium nitride (GaN) nanotube structure. The method of claim 13, Wherein said nanowires comprise a single crystalline zinc oxide (ZnO). The method of claim 13, Said gallium nitride (GaN) sheath is deposited on said nanowires by epitaxial casting. The method of claim 15, The epitaxial casting is gallium nitride (GaN) chemical vapor deposition. The method of claim 16, Trimethylgallium and ammonia are used as precursors in the chemical vapor deposition and are injected with argon or nitrogen transfer gas; Chemical vapor deposition of GaN is carried out at approximately 600 ℃ to 700 ℃. The method of claim 13, Gallium nitride (GaN) nanotubes have an internal diameter in the range of approximately 30 nm to 200 nm; And said gallium nitride (GaN) nanotubes have a wall thickness in the range of approximately 5 nm to 50 nm. The method of claim 13, And the zinc oxide (ZnO) nanowires are removed by an elevated temperature in an atmosphere containing hydrogen gas. The method of claim 19, The elevated temperature is approximately 600 ° C .; Wherein the atmosphere comprises approximately 10% hydrogen gas in an argon gas atmosphere. The method of claim 13, And the zinc oxide (ZnO) nanowires are removed by chemical etching the array. The method of claim 21, Wherein said chemical etching is an ammonia etching performed at a temperature sufficiently elevated for removal of said zinc oxide (ZnO) nanowires. Forming a sacrificial nanowire template of a first material; Forming a sheath made of the first material deformed on the nanowires; And Removing the nanowires; The sheath is a method for manufacturing a nanotube, characterized in that the nanotube structure. The method of claim 23, wherein Wherein said nanowires comprise a single crystalline material. The method of claim 23, wherein The sheath is formed on the nanowires by thermal oxidation. The method of claim 23, The nanowires are removed by an etching process. The method of claim 23, The first material comprises silicon (Si); The modified first material comprises silicon oxide (SiO ₂ ). The method of claim 27, The sheath is formed on the nanowires by a thermal oxidation process in which a temperature determines the thickness of the sheath. The method of claim 28, The temperature of said thermal oxidation is in the range of approximately 800 ° C to 1000 ° C. The method of claim 29, The nanowires are Covering the combination of nanowires with the sheath and etch resistant material; Removing the upper end of the sheathed nanowires while the sheathed wall of the nanotubes is protected by the etch resistant material; Removing silicon (Si) nanowire material from the silicon oxide (SiO ₂ ) nanotubes; And And removing the etch resistant material by an etching process. The method of claim 30, And the etch resistant material comprises a dimer or a polymer. The method of claim 31, wherein Wherein said etch resistant material comprises perylene. The method of claim 30, Removal of the upper end of the covered nanowires is Etching in an oxygen plasma to remove the etch resistant material deep enough to expose the covered nanowires; And By etching in a hydrofluoric acid solution to remove the metal cap of the nanowires. The method of claim 33, wherein Removing said silicon ((Si) nanowires) by etching in xenon fluorine (XeF ₂ ) gas. The method of claim 30, Removing the etch resistant material by oxygen plasma etching.