JP2006512218A - Sacrificial template method for producing nanotubes - Google Patents

Abstract

A method for producing uniform nanotubes is described in which the nanotubes are synthesized as a sheath on a nanowire template, such as using a chemical vapor deposition process. For example, single crystal zinc oxide (ZnO) nanowires are utilized as templates on which gallium nitride (GaN) is epitaxially grown. The ZnO template is then removed, such as by thermal reduction and evaporation. The completed single crystal GaN nanotubes preferably have an inner diameter in the range of 30 nm to 200 nm and a wall thickness of 5 nm to 50 nm. Transmission electron microscopy studies show that the resulting nanotubes are single crystals with a wurtzite structure and oriented along the <001> direction. The present invention illustrates single crystal nanotubes of a material having a crystal structure that is not layered. Similarly, the “epitaxial casting” approach can be used to make other solid material and semiconductor arrays and single crystal nanotubes. In addition, the manufacture of nanotubes having multiple sheaths as well as multiple longitudinal segments is described.

Description

  This application claims priority to US Provisional Application No. 60 / 461,346, filed Apr. 8, 2003, which is hereby incorporated by reference in its entirety.

  This application claims the priority of US Provisional Application No. 60/454038 filed Mar. 11, 2003, which is incorporated herein by reference in its entirety.

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

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT This invention is a governmental under contract number DE-AC03-76SF00098 awarded by the US Department of Energy under grant number DMR-0092086 and by the National Science Foundation. Was done with the support of The government has certain rights in the invention.

Incorporation by reference of materials submitted on compact discs Not applicable.

  The present invention relates generally to the manufacture of nanotubes, and more particularly to a method of manufacturing nanotubes on a sacrificial nanowire template.

  From the discovery of carbon nanotubes (see Iijma, S. “Helical microtubules of graphic carbon”, Nature, 352, 56 (1991), incorporated herein by reference), various solid nanoscale tubes Considerable research effort has been devoted to form. (Tenne, R. and Zettle, AK "Nanotubes from organic materials", Top. Appl. Phys. 80, 81-112 (2001); “Organic nanoclusters with fluorine-like structure and nanotubes”, Prig. Inure. Chem. 50, 269-315 (2001); Partake, G. R., n. Es and R. o. -Anisotropic modules for a future nanotechnology ", Ang eb.Chem.Int.Ed.41,2466-2461 (2002); "Nanomaterials-a membrane-based synthetic approach", Martin, CR, Science, 266,1961-65 (1994); PM, et al, “Carbon nanotubes as removable templates for metal-oxide nanocomposites and nanostructures”, Nature, 375, 564-566 (1996); Supramolecular Origa i ", Adv, Mater. 11.1427-30 (1999); Kondo, Y. and Takanagi, K." Synthesis and charac- terization of helical multi-shell gold nanores ", Science, 60000, 289, 860. Li, Y et al., “Bismuth nanotubes”, J. Am. Chem. Soc. 123, 9904-05 (2001); and Wu, Y. and Yang, P. “Melting and welding semiconductors innoires in annoires in annoires innes. Adv. Mater. 13, 520-523 (2001). )

  Formation of tubular nanostructures generally requires a layered or asymmetric crystal structure. (Tenne, R. and Zettle, AK "Nanotubes from organic materials", Top. Appl, Phys. 80, 81-112 (2001); “Organic nanoclusters with fluorine-like structure and nanotubes”, Prig. Inure. Chem. 50, 269-315 (2001); Partake, G. R., n. Es and R. o. -Anisotropic modules for a future nanotechnology ", Ang ew.Chem.Int, Ed. 41, 2246-2461 (2002).)

  There are reports of the formation of solid nanotubes, such as silica, alumina, silicon, and metals, that have lost the layered crystal structure via templating or thin film rolling of carbon nanotubes and porous membranes. (See Schmidt, OG and Eberl, K., “Thin solid films roll up into nanotubes,” Nature, 410, 168 (2001), incorporated herein by reference.)

  However, nanotubes made by the above method are either amorphous, polycrystalline, or exist only in an ultra-high vacuum environment.

  The importance of hollow inorganic nanotubes is recognized and they are widely applicable to biochemical analysis and catalysis. (Lee, SB; Mitcell, DT; Trofin, L .; Nevanen, TK; Soderlund, H .; Martin, CR Science 2002, incorporated herein by reference. 296, 2198.) Among these hollow nanotubes, silica nanotubes are particularly preferred due to the hydrophilic properties, the formation of colloidal suspensions, and the accessibility of the surface functionality with respect to the inner and outer walls. I am interested. These modified silica nanotubes and nanotube membranes have applicability in, for example, biochemical separations and biochemical catalysts. (Mitchell, DT; Lee, SB; Trofin, L; Li, NC; Nevanen, TK; Soderlund, H .; Martin, incorporated herein by reference) (See CRJ Am.Chem.Soc. 2002, 124, 11864.)

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

  Silica nanotubes were generally synthesized within the pores of a porous alumina membrane template using a sol-gel coating technique. (See Martin, CR Chem. Mater. 1996, 8, 1739, incorporated herein by reference.) Alumina templates can be dissolved to release single silica nanotubes. it can. These nanotubes prepared at low temperature have porous walls and are relatively fragile. When the template is removed, the silica nanotubes are generally bundled and less oriented. This is true for silica nanotubes prepared at low temperatures using other templates. (Obare, S.O .; Jana, N.R .; Murphy, C. J. Nano Lett. 2001, 1, 601; Jung, J. H .; Shinkai, S., incorporated herein by reference. Shimizu, T. Nano Lett., 2002, 2, 17; Yin, YD; Lu, Y .; Sun, Y.G .; Xia, Y. N. Nano Lett., 2002, 2, 427; I want.)

  Thus, the growth of single crystal semiconductor nanotubes offers a number of advantages for nanoscale electronic, optoelectronic, and biochemical detection applications. The present invention satisfies these and other needs and eliminates the disadvantages of previously developed nanoscale growth methods.

  The present invention includes single and multi-walled nanotube structures and manufacturing methods. In the case of a multi-walled nanotube structure, the interface between the cylindrical layers of nanotubes in the structure can form an insulating or non-insulating device junction or provide other material properties. Furthermore, the longitudinal portions (segments) of the nanotubes can be treated differently to produce a longitudinal bond along a given nanotube.

  An important aspect of the present invention is that the nanotubes and composite structures are formed on a sacrificial template, preferably a nanowire core. The general manufacturing process of the present invention involves making a core (nanotube template), where a sheath is formed on the core. A number of methods can be utilized for both making the core and forming one or more sheaths. It will be appreciated that the core and sheath sections can be formed from a variety of materials.

  For example, the core may be selected from materials consisting of ZnO, Si, GaN, Ge, Ag, Group II-VI materials, Group III-V materials, Group IV material elements (ie, Si, Ge), and metals. it can. Grouping is considered to describe a group of materials as shown in the periodic table of elements.

On the other hand, the sheath is, for example, gallium nitride (GaN), silicon oxide (SiO ₂ ), II-VI group material, III-V group material, IV group material element (ie, Si, Ge), metal, oxide of the above materials , And polymers. It should also be understood that the sheath material can be doped as desired (ie, during formation) to alter the properties of the base material.

Accordingly, it is understood that the present invention generally includes a method of manufacturing nanotubes, the method comprising: (a) forming a nanowire template; (b) depositing a sheath on the nanowire template; ) Removing the nanowire template. Two embodiments of the present invention are described, one embodiment is to form GaN nanotubes on a zinc oxide nanowire template, and one embodiment is to form SiO ₂ nanotubes on a Si nanowire template. Is. In general, the nanotubes according to the present invention can be formed utilizing a casting process, an etching process, or a combination thereof. As an example, an epitaxial casting process is first described to make GaN nanotubes. Next, the oxidation and etching processes are described, such as to make silicon oxide (SiO ₂ ) nanotubes.

  In one embodiment of the nanotube manufacturing process according to the present invention, an “epitaxial casting” approach has a technically important gallium nitride (III) (GaN) with an inner diameter of about 30 nm to 200 nm and a wall thickness of about 5 nm to 50 nm. Used to synthesize single crystal nanotubes such as nanotubes. Nanowires, such as in nanowire arrays, are used as templates for epitaxial overgrowth of thin GaN layers in chemical vapor deposition systems. As an example, a nanowire template can be fabricated from a hexagonal zinc (II) oxide (ZnO) material on which GaN nanotubes are grown. The template material is then removed, preferably by simple thermal reduction and evaporation steps, resulting in an array of GaN nanotubes aligned on the substrate. An array of ZnO nanowires is grown on a substrate such as a sapphire wafer using a vapor deposition process. The same approach to synthesize nanotubes described in detail herein can work with a large number of III-nitrides.

In other embodiments, the nanotubes are formed with silicon oxide (SiO ₂ ) in an oxidation process and the nanowire core is removed with an etching process. Nanotube cores (templates) are made from silicon (Si) nanowires with caps (ie Au), such as manufactured using thermal oxidation and etching. The process involves thermal oxidation of Si nanowire arrays, which result in thin Si nanowire arrays that are sheathed by a thin layer of silicon oxide (SiO ₂ ). This oxidized nanowire array is then selectively etched, such as with xenon fluoride (XeF ₂ ) to remove the silicon nanowire core, to form an array of aligned silicon oxide nanotubes having a controllable inner diameter. leave. The inner diameter is controlled by the initial diameter of the silicon nanowire and the thermal oxidation process. The inner tube diameter of the nanotube can be controlled in the range of about 10 nm to 200 nm. With further improvements in the oxidation and etching processes, it is envisaged that nanotubes with an inner diameter of less than 5 nm will be made in this way.

  Numerous aspects of the invention are addressed herein, including but not limited to the following.

  One aspect of the present invention is the formation of a nanotube structure.

  One aspect of the present invention is the formation of a single crystal nanotube structure.

  Another aspect of the present invention is to form nanotubes of gallium nitride (GaN).

Another aspect of the present invention is to form silica (SiO ₂ ) nanotubes.

  Another aspect of the present invention is to form nanowires for use as a template for forming nanotubes.

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

  Another aspect of the present invention is to utilize silicon (Si) nanowires as templates for forming nanotubes.

  Another aspect of the invention is to utilize an epitaxial casting process to form a sheath on the nanowire, such as GaN on a ZnO nanowire.

Another aspect of the invention is to utilize oxidation and etching processes to form a sheath of material on the nanowire, such as SiO ₂ on the Si nanowire.

  Another aspect of the present invention is the formation of multiple sheath layers on the sacrificial template (core).

  Another aspect of the invention is the formation of multiple sheath layers on the sacrificial core.

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

  Yet another aspect of the present invention is a method of forming single crystal nanotubes that can be utilized in electronic devices, nanofluidic devices, or combinations thereof.

  Further aspects of the invention will be apparent from the following portions of the specification and the detailed description is intended to fully disclose the preferred embodiments of the invention without limitation.

  The invention will be more fully understood with reference to the following drawings, which are for illustrative purposes only.

  According to the present invention, nanotubes are formed by creating at least one sheath layer around the nanowire template. The nanowire template functions as a sacrificial core that is later removed to establish a central opening through the nanotube. Once the sacrificial core is removed, the nanotubes can be used in any conventional manner.

By way of non-limiting example, two embodiments of a nanotube production method using a sacrificial core according to the present invention are described. However, it is understood that the present invention contemplates any method in which the sacrificial core is used as a template for nanotube manufacture. In a first embodiment, a layer of material such as gallium nitride (GaN) is epitaxially grown outside of a nanowire core such as zinc oxide (ZnO), and then the nanowire core is removed. In a second embodiment, a nanowire core such as silicon (Si) is oxidized to form a SiO ₂ sheath layer, and then the nanowire core is removed leaving the oxide sheath.

Epitaxial Casting Method FIGS. 1A-1C illustrate the general steps referred to as an “epitaxial casting” approach. FIG. 1A shows a substrate 10 on which nanowires 12, preferably single crystal nanowires, are formed. FIG. 1B illustrates depositing a preferred single crystal sheath 14 on the nanowire 12. FIG. 1C shows that the nanowire template (core) 12 is removed, thereby forming the nanotube 14 ′.

  According to one embodiment, a prefabricated nanowire 12 such as a hexagonal single crystal nanowire (preferably ZnO) is utilized as a template for a tubular deposit of material such as GaN. Both ZnO and GaN have a wurtzite crystal structure and similar lattice constants (ZnO: a = 3.249２４, c = 5.207Å; GaN: a = 3.189Å, c = 5. 185)), GaN can be epitaxially grown on the side {110} planes of these ZnO nanocylinders to form a thin GaN layer that is naturally a single crystal. It is understood that many combinations of materials have sufficiently similar crystal structures and lattice constants to allow epitaxial growth of the sheath material on the nanowire material.

  When the ZnO nanocylinder is coated with a thin GaN sheath 14 (FIG. 1B), the template 12 (FIG. 1A) is subsequently removed, such as by a thermal process, leaving the GaN nanotubes 14 '. As a non-limiting example, two possible mechanisms for removal of the ZnO template can be used.

In one approach, ZnO is chemically etched with ammonia (NH ₃ ) at high temperatures. (Effect of buffer layer and substrate surface polarity on the growth by molecular beam epil. 71, incorporated by reference, Hammond F et al. ) Extended heating of the sample after coating GaN in ammonia (NH ₃ ) readily yields pure GaN nanotubes (FIG. 1C).

Another approach is to utilize a high temperature thermal reduction process (eg, 600 ° C. in hydrogen gas H ₂ ). Here, single crystal wurtzite GaN nanotubes are fundamentally different from theoretically simulated GaN nanotubes that are provided with metastable graphite GaN structures. (Lee SM, “Stability and electronic structure of GaN nanotubes from density-functional calculations, et al.”, Phys. Rev. B, 77, 88, 77, 77, incorporated herein by reference.) Please refer to.)

Example 1
The nanowire core used in the present invention can be formed by any conventional method. For example, an array of zinc oxide (ZnO) nanowires is grown on a substrate material, such as a (110) sapphire wafer, preferably using a vapor deposition process. (See Huang M et al., “Room-temperature ultraviolet nanosphere nanoassers”, Science, 292, 1897-99 (2001), incorporated herein by reference.) These ZnO nanowire arrays are GaN chemical vapor deposited. It is placed inside the reaction tube for the method (ie, the MOCVD reaction tube). Trimethylgallium or ammonia is used as a precursor and is supplied into the system along with argon and nitrogen carrier gases. The deposition temperature is set to 600 ° C to 700 ° C.

After GaN deposition, the sample is treated in a hydrogen atmosphere at an elevated temperature, such as 600 ° C., in argon with 10% H ₂ to remove the ZnO nanowire template. Other methods and materials may be used to form nanowires, coat nanowires with nanotube materials, and sacrificial removal of nanowire materials (selective application of only a portion of nanowire materials that need to be removed by the present invention It is to be understood that it can be utilized (although in some instances it is hardly preferred).

  FIG. 2A shows a scanning electron microscope (SEM) image of the starting ZnO nanowire array template, where the starting ZnO nanowire array template was found to have a uniform length, such as in the range of 2 μm to 5 μm, each 30 μm to 200 μm. Having a uniform diameter with a diameter within a row of nanowires. The nanowires are sufficiently multifaceted as seen in the inset of FIG. 2A with a hexagonal cross-section, showing the {110} face on the side. After GaN deposition and removal of the template to form the nanotubes, the sample color changes from white to yellowish or darker.

  FIG. 2B is an exemplary image showing that the morphology of the initial nanowire array is maintained in the nanotubes, except for the resulting increase in nanostructure diameter. The nanostructure appears to be less multifaceted compared to the original ZnO nanowire array template. Composition analysis for the final product shows only a relatively small Zn signal.

  FIG. 3 shows the X-ray diffraction (XRD) results for the sample and shows only the diffraction peak (001) of the wurtzite GaN structure, showing excellent epitaxy / organization for the GaN coating.

  4A-4C show images of dispersed GaN nanotube samples in FIG. 2B on a transmission electron microscope (TEM) grid for further structural analysis. It can be seen that the majority of the nanostructures show tubular structures with uniform wall thickness that can be seen generally from FIG. 4A. These nanotubes were found to have an inner diameter in the range of 30 nm to 200 nm, similar to ZnO nanowire arrays, and wall thicknesses of 5 nm to 50 nm.

  It has been found that the majority of nanotubes have only one open end, but are observed in a tube open at both ends. These observations are consistent with SEM studies where rounded edges and few edged polyhedral edges are observed after GaN coating, as shown in FIG. 2B. It is concluded that the end of the opened nanotube was originally located at the open GaN and substrate interface that was folded during TEM sample preparation. Indeed, these open ends of the substrate surface were often observed, with an example of the corresponding nanotube shown in the inset of FIG. 2B. TEM studies also show that the inner cross section of the nanotube remains a pseudo hexagon after the template is removed.

  Significantly, electron diffraction (ED) taken on these GaN nanotubes indicates that these tubes are single crystals. Returning to FIG. 4E, the inset shows one ED pattern taken along the [−110] region axis. It can easily be seen that the nanotubes are oriented along the c-axis of the wurtzite GaN structure. This is consistent with XRD data where only the (001) peak is observed. Along the tube axis, a 0.51 nm lattice spacing for the (001) plane of the wurtzite structure can be easily analyzed with high resolution TEM images both on the tube surface in FIG. 4D and inside the tube in FIG. 4E. it can.

  FIG. 5 was examined by an energy dispersive x-ray spectrometer (EDX) showing well-correlated gallium and nitrogen signals across the tube wall, which is an indicator of stoichiometric GaN formation during deposition. A composition line profile is shown. This clearly reflects the electron energy loss spectrum (EELS) recorded with these nanotubes, as shown in FIG. 8 where a strong nitrogen signal is also observed. It should be noted that interfacial diffusion between the GaN layer and the ZnO nanowire template results in a small amount of Zn or O incorporated into the GaN tube wall.

  FIG. 6 is a transmission electron microscope image of a diagram in which the tips of several GaN nanotubes are directed. At least two important features can be seen in the image. That is, (1) the inner cross-section of the tube is pseudo-hexagonal, (2) the nanotubes are connected to the porous GaN layer at their base, and the porous GaN layer is free of zinc and This is considered to be the main route for leakage of oxygen species.

  FIG. 7 is a transmission electron microscope image of a single crystal GaN nanotube showing its very gentle inner and outer surfaces.

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

  It is also understood that high density arrays of single crystal nanotubes are successfully prepared such as described for GaN nanotubes fabricated on sapphire substrates. It is important to point out that the GaN nanotube formation process described herein differs significantly from previous work on inorganic nanotubes. (Ihelima, S., “Helical microtubules of graphic carbon”, Nature, 354, 56 (1991); Tenne, R. and Zettl, A. K., “Nanotubes inminator minor,” incorporated herein by reference. , Top. Appl. Phys. 80, 81-112 (2001); “Inorganic nanoclusters with fluorine-like structure and nanotubes” of Tenne, R., Pri. Inure. Partake, GR, Cromech, F. and Nester, R., “Oxidi nano-ands and nanorods-Anisotropic modules for a future nanotechnology, Angew.Chem.Int.Ed, 41,246-2461 (2002); Malein, C.R. ter. 266, 1961-65 (1994): Ajayan, PM et al., “Carbon nanobes as removable templates for metaloxides and nanostructures”, Nature, 375. )

Previous work on inorganic nanotubes has been directed to materials with a layered structure (eg, VO _x , MoS ₂ , NiCl ₂ , BN). For these studies on materials without structural asymmetry, a template-forming approach (in porous alumina) (Caruso, RA and Antonietti, M., “Sol-Gel nanocoating: incorporated herein by reference: anapproach to the preparation of structured materials ", Chem. Mater. 13, 3272-3282 (2001)) is commonly used, resulting in predominantly amorphous or polycrystalline tubes. The distinction between amorphous or polycrystalline tubes and advantageously single crystal tubes shown as preferably manufactured according to the present invention will be readily recognized by those skilled in the art.

  9A, 9B, and FIGS. 10, 11 show details of removal of the nanowire template within the single crystal nanotube. The “epitaxial casting” mechanism described by this document has been confirmed in TEM studies. In FIG. 9A, rows of GaN nanotubes are shown with their ZnO nanowire templates partially removed. Note that there is a thin layer of porous GaN film at the bottom of these nanotubes. Furthermore, the remainder of the ZnO nanowire template remains in the upper part of the sealed GaN nanotube. These two observations show that zinc and oxygen species (generated during the thermochemical etching process) leak from the GaN nanotubes that primarily pass through the underlying porous GaN layer (as shown in FIG. 6). .

  In FIG. 9B, a detailed view of a nanotube with a partially removed template is shown at the boundary between the filled (upper arrow) and empty (lower) portion of the nanotube. The electron diffraction shown in the inset of FIG. 9B for the filled and unfilled portions of the nanotubes shows an identical set of diffraction patterns for both the tube and core sheath regions, indicating that wurtzite GaN growth is epitaxial. Show.

  The core sheath nanostructure can be thought of as a seamless single domain of the wurtzite GaN / ZnO structure type. Furthermore, a comparison of the EDX line profile across the GaN nanotubes (aligned with the down arrow) shown in FIG. 11 with the ZnO-GaN core sheath structure aligned with the up arrow shown in FIG. It clearly supports the growth mechanism of the above GaN nanotubes. When the ZnO nanocylinder is removed, a GaN single crystal tube is produced. The formation of these single crystal GaN nanotubes as taught herein has numerous advantages over the use of polycrystalline nanotubes, particularly with respect to the fact that these polycrystalline nanotubes generally have a non-uniform shape. (See Li, JY et al., “Synthesis of GaN nanotubes”, J. Mater. Sci. Lett. 20, 1987-1988 (2001), incorporated herein by reference). It is also interesting to note that ZnO microscale tubes are prepared in solution through a preferred chemical dissolution process. (Three-dimensional array of highly oriented crystals ZnO microC. 95, Vaissieres, L., Keis, K., Hagfeldt, A. and Lindquist, S., incorporated herein by reference. (See -4398 (2001).)

  The electronic and optical properties of these polycrystalline GaN nanotubes that are important are the electronic and optical properties of these high quality GaN epilayers grown on a ZnO substrate (Handani, F. et al., “Incorporated herein by reference”. Microstructure and optical properties of epitaxy GaN on ZnO (0001) grown by reactive molecular beam epitaxy ", J. Appl. Phys. It is equivalent. (Hang, Y., Duan, X., Cui, Y. and Lieber, CM, “Gallium nanoride nanodevices,” Nano. Lett. 2, 101-104 (2002, incorporated herein by reference). Kim, J. et al., “Electrical transport properties of individual gallium nitride nanosynthesized by the synthesis by chemical vapor deposition. 50”, Appl. 35. Appl.

  FIG. 12 shows a low temperature photoluminescence (PL) spectral plot of the fabricated nanotubes measured using the fourth harmonic output of a YAG laser (266 nm) as the excitation source. It should be noted that no midgap yellow emission is observed. Band edge emission is observed in these nanotube samples between 375 nm and 360 nm, with thinner tubes emitting shorter wavelengths. Slightly blue shift of emission (Hamdani, F. et al., “Microstructures and optical properties of epitaxial GaN on ZnO (0001) grown by reactive mole., Incorporated herein by reference”). 83, 983-990 (1998)) can contribute to the quantum confinement effect because some nanotubes have walls as thin as 5 nm, which is smaller than the excitation Bohr diameter of GaN.

  Referring to the drawings, photoluminescence spectra are collected on GaN nanotubes at 10K. The sample is excited by a 266 nm line of a pulsed Nd: YAG laser (eg, Spectra Physics®). The photoluminescence signal is transmitted by optical fiber to a 0.3 meter imaging monochromator and detected by an amplified CCD operating under gated mode. Each of the spectra shown on the left side corresponding to the spectrum collected with thin wall (<10 nm) GaN nanotubes, while the spectrum shown on the right side corresponding to the spectrum collected with thick wall (≧ 10 nm) GaN nanotubes, Only band edge emissions are observed. It should be understood that the emission spectrum for a thin tube is relatively broad due to the wide distribution of tube wall thickness for the sample being tested.

  FIG. 13 shows an example of electron transport measurements, showing the resistance of these nanotubes on the order of 10 MΩ at room temperature and increasing with decreasing temperature, similar to the resistance of high quality GaN nanowires. Referring to the drawing, a temperature dependent IV curve of a single GaN nanotube is shown. Electrodes for electrical measurements (20 nm titanium Ti and 80 nm gold Au) are manufactured using electron beam lithography and thermal evaporation, although other techniques can be utilized. Any conventional contact forming means can be utilized to form a stable contact, but the rapid thermal annealing step is performed at 450 ° C. in about 30 seconds.

  The successful preparation of single crystal GaN nanocapillaries utilizing the epitaxial casting process of the present invention has the ability to prepare inorganic solid nanotubes / nanocapillaries with an unlayered crystal structure, especially single crystal nanotubes / nanocapillaries. Show. It is understood that this new class of semiconducting nanotubes / nanocapillaries can be utilized in a number of useful technical applications in the nanoscale electronic, optoelectronic, and chemical fields in addition to use with fluid systems. . The present invention provides a rigid semiconducting nanotube with both ends of the nanotube being accessible for fluid flow applications but having a uniform inner diameter and an inner wall that can be easily functionalized.

Oxidation and Etching Method Referring now to FIGS. 14A-14G, a second method of manufacturing nanotubes using a sacrificial template according to the present invention is illustrated. This method is called “oxidation and etching” because it forms a solid nanotube array by transferring vertical nanowire arrays to the oxide nanotube array. In one embodiment, the nanotube core (template) is formed from silicon (Si) nanowires using a metal cap (ie, Au), such as commonly manufactured using thermal oxidation and etching. The Si nanowire array is then thermally oxidized, resulting in an array of thin Si nanowire sheaths formed by a thick layer of silicon oxide (SiO ₂ ). This oxidized nanowire array is then selectively etched, such as with xenon fluoride (XeF ₂ ), to remove the silicon nanowire core and create a specified array of silicon dioxide nanotubes having a controllable inner diameter. leave. The inner diameter is controlled by the initial diameter of the silicon nanowire and the thermal oxidation process. The inner tube diameter of the nanotubes can range from about 10 nm to 200 nm.

  It should be understood that single nanotubes or random samples can be formed as an alternative to forming nanotubes in a row. Other nanotube compositions including, but not limited to, GaO, InO, and other oxides, as well as insulating materials can be produced in this manner as well. The following describes the implementation details of an embodiment of the manufacturing process of the present invention.

Example 2
FIG. 14A shows a silicon nanowire array prepared using chemical vapor deposition (CDV) epitaxial growth with silicon tetrachloride (SiCl ₄ , Aldrich, 99.99%) as the silicon source. Hydrogen (10% equilibrated with argon) is used to reduce SiCl ₄ at high temperature (900-950 ° C.). A gold (Au) thin film is coated on the Si (111) substrate 30 to begin the growth of the silicon nanowires 32 via a gas liquid solid growth mechanism. The gold remains as a cap 34 on the Si nanowire. This approach to grow Si nanowires was developed and used in the laboratory for the synthesis of vertical Si / SiGe superlattice nanowire arrays (Wu, Y .; Fan, R, incorporated herein by reference). Yang, P.D. Nano Lett., 2002, 2, 83; Wu, Y .; Yan, H .; Huang, M.; Messer, B.; Song, J.; Yang, P. Chem.-Eur J. 2002, 8, 1260). The silicon nanowire array sample is loaded into a tube furnace and heated, such as being heated at 800 ° C. to 1,000 ° C. for 1 hour under a continuous flow of pure oxygen (O ₂ ).

FIG. 14B shows the nanowire 32 after it has been uniformly oxidized to provide a SiO ₂ sheath 36 with a continuous inner silicon core. After oxidation, the nanowire tip 34 is preferably oxidized to provide an oxide cap 34 'for each vertical wire to prevent selective etching of the silicon core. Thus, the first step after thermal oxidation is to selectively remove the SiO ₂ cap 34 ′ from the Si / SiO ₂ core sheath nanowire.

FIG. 14C shows a preferred mode of removing the SiO ₂ cap. The polymer 38 is deposited to fill the spaces between the nanowires so that the SiO ₂ sidewalls 36 are protected by the matrix polymer as an etch resistant material. In this example, the perylene dimer (di-para-xylene, (—CH ₂ —Ph—CH ₂ —) ₂ ) is thermally deposited at 160 ° C., decomposed at about 650 ° C., and perylene (poly-para- To produce a continuous coating of xylene, (—CH ₂ —Ph—CH ₂ —) _n ) polymer, it is deposited on the Si / SiO ₂ core sheath nanowire array sample for about 5 hours. This perylene deposition is conformal and begins with a thin layer coating on the surface of the nanowire and then fills all intervening spaces between the nanowires. This process leads to high conformal wrapping of nanowires without pinholes or cracks.

FIG. 14D shows the core sheath row after oxygen plasma etching of the surface of a polymer fill 38 such as perylene to expose the tips of the Si / SiO ₂ nanowires.

FIG. 14E shows the core sheath row after selective removal of the SiO ₂ cap 34 ′ and immersion in a buffered hydrofluoric acid solution for about 2 minutes to expose the silicon core 32.

FIG. 14F shows the sheath row after the silicon nanowire core 32 has been removed by an etchant such as a XeF ₂ etchant gas. Note that after some material has been removed, the layer of etch resistant material 38 'protects the bulk of the nanotube walls. Etching is preferably performed by loading the core sheath row into a XeF ₂ etching chamber having a chamber temperature adjusted to 40 ° C., for example. After purging and flushing with nitrogen, XeF ₂ vapor is introduced with nitrogen gas N ₂ (XeF ₂ : N ₂ = 4: 5) for etching for 30 seconds at a total pressure of about 9 Torr. The chamber is then evacuated and flushed with nitrogen, and etching is performed for the second cycle. In this embodiment, 8 cycles are performed to achieve complete etching of the silicon core.

  The process described above results in a row of silica nanotubes embedded in the perylene film 38, with continuous pores extending through the entire polymer film.

  FIG. 14G shows the result after the perylene matrix has been etched away, such as using a high power oxygen plasma treatment for 30 minutes, to produce rigid silica nanotubes attached to the substrate 30 and oriented vertically. The resulting nanotube row 36 'is shown.

Example 3
15A to 15D are images of nanotube formation according to the present invention recorded on a scanning electron microscope (SEM) or the like. An array of silicon nanowires is shown in FIG. 15A, and the Si nanowires are oriented vertically to make a substantially complete array. The typical size of silicon nanowires is 50 nm to 200 nm and the length is about 8 μm. At the top of each nanowire, a bright gold tip showing gas liquid solid growth can be seen. (Wu, Y .; Yan, H .; Huang, M .; Messer, B .; Song, J .; Yang, P. Chem.-Euro. J. 2002, 8, incorporated herein by reference. (See 1260.)

FIG. 15B shows the nanotubes after perylene has been deposited, the SiO ₂ cap has been removed, and the silicon core has been etched, forming a column of silica nanotubes embedded in the perylene film. The pores can be easily seen on the polymer surface. The brightness can be seen in the image corresponding to the tip of the gold nanoparticle, which has a substantially hemispherical shape. The membrane has a relatively flat surface. The inset in FIG. 15B shows two silica nanotubes embedded in a high magnification perylene membrane, clearly showing a hollow hole in the silica wall.

15C and 15D are a perspective view and a top view, respectively, of a nanotube row after oxygen (O ₂ ) plasma etching of perylene from which silica nanotubes standing upright are obtained. As can be seen, the nanotubes are well aligned and retain the vertical orientation of the starting silicon nanowire template. The inset of FIG. 15C shows an enlarged view of the nanotubes in a high magnification SEM image, clearly showing the morphology of the vertical nanotube rows. The image shows that the Si nanowires are oriented vertically in a row and have a uniform diameter, a length of up to about 8 μm, and an average length of about 5 μm along a length in the range of about 50 nm to 200 nm. The resulting average diameter of the silica nanowires exceeds the average diameter of the template silicon nanowires as a result of structural expansion caused by thermal oxidation. The inset of FIG. 15D is a detailed top view from which the hexagonal shape of the tube can be seen. The scale bars in FIGS. 15A, 15B, and 15C are 10 μm, 1 μm, and 10 μm, respectively. The silica walls of the nanotubes were found to exhibit a well-defined hexagonal shape in the <111> orientation of the original Si nanowires and exhibit an asymmetric in-plane etch rate.

  16A and 16B are transmission electron microscope (TEM) images showing higher quality silica nanotube formation. In FIG. 16A, a uniform inner diameter is shown that is generally maintained along the entire length of the nanotube. The pore size for nanotubes is in the range of about 10 nm to 200 nm and has gentle inner and outer walls.

  Nanotube thickness is found to be about 70 nm for 1000 ° C. heat treatment, regardless of the range of nanotube pore sizes. The oxide layer thickness is expected to be the same as the nanowire thickness under certain heat treatment conditions because thermal oxidation of silicon is a self-limiting process, and this result is considered reasonable. Process self-limiting can have the advantage of controlling tube size and wall thickness by adjusting characteristics of the heat treatment process, such as process temperature.

  As an example of how nanotube characteristics can be controlled, a sample oxidized at 900 ° C. has a typical wall thickness of about 55 nm to 65 nm, while a temperature of about 800 ° C. is about 30 nm. A wall thickness of ˜35 nm is produced. The nanotubes shown in FIG. 16B have a pore size of about 20 nm, but as can be seen, the nanotubes are still uniform and have a gentle inner wall. It should be understood that sometimes branched nanotubes are produced and these nanotubes provide advantages for the choice of nanofluidic and electronic applications.

  This multi-use approach to making silica nanotube array templates from silicon nanowire arrays is a well-controlled process that can control the pore size and array height, while the resulting nanotubes have different surface modifications on the inner and outer walls. Can be easily received. Surface modification of the inner and outer walls, respectively, can be important in applications such as biochemical separation and smart molecular transport. Furthermore, the walls of these nanotubes are formed from agglomerated thermal oxides without pinholes, which can be advantageous with respect to mechanical robustness and fluid stability.

  Thus, the new class of semiconducting nanotubes presented by the present invention is mechanically robust and electronic and optically active. Thus, these nanotubes can provide additional basic research and additional opportunities for nanocapillary electrophoresis, nanofluidic biochemical sensing, nanoscale electronic devices and optical devices. (See Schoening, M. and Pogossian, A., “Recent advanced in biologically sensitive-effect transistors (BioFETs)”, Analyst, 127, 1151-1115, incorporated herein by reference. .) Understand that the successful preparation of single crystal GaN nanotubes using this “epitaxial casting” approach suggests that it is generally possible to prepare inorganic solid single crystal nanotubes with a non-layered crystal structure Should. ("Epitaxial core-shell and core-multiwell nanostructures of Lauhon, LJ, Gudiksen, MS, Wang, D. and Lieber, CM, incorporated herein by reference," , 420, 57-61 (2002); and He, R., Law, M., Fan, R., Kim, F. and Yang, P., "Functional bimorph composite nanotapes", Nano. Lett. 1109-1112 (2002).)

  It should also be understood that the techniques described herein can be further expanded by forming multiple sheath layers. Each of these sheath layers can include different materials, different doping compositions or levels. Furthermore, the longitudinal portions (segments) of the nanotubes can be treated differently to produce different properties between the segments of the nanotube structure or the multi-wall nanotube structure. The following nanotube structures are given as examples and are not limiting.

  FIG. 17 shows a multi-walled nanotube 50 in which a gallium nitride (GaN) sheath 54 includes sacrificial ZnO nanowires 12 (before removal) held between two sheaths 52, 56 of aluminum nitride (AlN). It should be understood that the sacrificial nanowire can be removed at any time after at least the first sheath layer has been deposited on the nanowire, which can be removed after the last sheath layer has been deposited. is there.

  18 and 19 illustrate the formation of a sheath of alternatingly doped material 60. FIG. FIG. 18 shows a P-doped GaN 62 on the sacrificial core 12 (before removal), such as ZnO, and an N-doped GaN material 64 that is P-doped. Similarly, FIG. 19 shows the reverse of FIG. 18 with P-doped material 74 on N-doped material 72 of seascore 12 (before it is removed). It should be understood that numerous circuits can be fabricated from this method, including diodes, light emitters, photodetectors, electron transport devices (ie, bipolar FETs, insulated gate FETs, etc.), and combinations thereof. It is. The connection to the device layer can be provided from a core or external peripheral connection, but the connection can also be embedded in the material layer. The process configuration described above can be continued to create any desired number of nested sheaths within a given nanotube.

  20 and 21 show segmented nanotube sheaths according to the present invention, wherein different segments are formed from different materials, different dopants, different doping levels, or combinations thereof. These sheaths can be manufactured segment by segment in any conventional manner, such as using conventional masking techniques.

  In FIG. 20, the nanotube 80 is shown having two segments of different sheath materials 84, 86 disposed longitudinally on the sacrificial core 82. FIG. 21 shows a nanotube 90 formed from three or more longitudinal segments of different materials, differently doped materials, or other materials configured to provide different properties. Further, the nanotubes are shown having a sheath of at least two materials.

  Core 92 is shown prior to removal, along with upper inner sheath 94, upper outer sheath 96, middle inner sheath 98, middle outer sheath 100, lower inner sheath 102, and lower outer sheath 104. It should be appreciated that any desired number of sheath layers can be deposited and the nanotubes can be fabricated with any number of longitudinal segments. It should also be understood that the insulator and electrical connections on the sheath layer can be formed as part of different sheath segments. Furthermore, the core from which the nanotubes have been removed can be utilized through a material such as a metal or as a fluid lined with a material to form another layer (ie, a conductive contact layer).

  FIG. 22 shows as an example a cross section of a nested sheath layer 110 forming a bipolar transistor. Hollow 12 'indicates where the sacrificial nanowire core has been removed. The inside of the hollow 12 ′ is shown lined as a metal contact 112. Three sheaths are shown in the drawing. A P-doped semiconductor inner sheath 114 is shown. A separate intermediate sheath of N-doped semiconductors 116, 118 is shown, and a central insulating ring 120 is shown surrounding the inner sheath 114 between the intermediate sheaths. Finally, the conductive outer sheath is shown having an upper conductor 122 and a lower conductor 124 separated by an insulating sheath segment 126. A simple example shows the configuration of a bipolar NPN transistor along the length of the nanotube, with an external emitter contact 122, a collector contact 124, and a base contact 112 backing the hollow core 12 '. The layer thickness can be varied to achieve the desired electrical properties or to enhance strength, such as provided by the outer sheath segments 122, 124, 126.

  Transistors are provided by way of example, and various devices can be manufactured by the techniques of the present invention. It should be understood that a variety of materials and electrical properties can be achieved utilizing 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 teachings of the present invention.

Thus, as can be seen, the present invention includes a method of manufacturing nanotubes by forming a sheath over a sacrificial core and then removing the core. Two general methods have been described. That is, (i) epitaxial casting, and (ii) oxidation and etching. In addition, examples of specific nanotube structures are described, such as GaN nanotubes using epitaxial casting methods (on ZnO sheaths) and SiO ₂ nanotubes using oxidation and etching methods (on Si sheaths). However, other materials are not limited and include and are limited to GaN, Ge, Ag, II-VI, III-V, IV (eg, Si, Ge) elements and metals as core materials. Furthermore, it can be used including a II-VI group, II-V group, IV group element, metal, and polymer as a sheath material. Note also that all sheaths can be doped during formation.

  While the above description includes many details, they should not be construed as limiting the scope of the invention, but merely as providing some illustrations of the presently preferred embodiments of the invention. Should not be done. The scope of the present invention fully encompasses other embodiments that may be apparent to those skilled in the art, and the scope of the present invention should therefore not be limited to anything other than the appended claims, Reference to an element is not intended to mean “one and only one” unless explicitly stated, but rather is intended to mean “one or more”. All structural, chemical, and functional equivalents for the elements of the above-described preferred embodiments known by those skilled in the art are incorporated herein by reference and are intended to be encompassed by the present claims. Intended. Moreover, there is no need for a device or method that addresses each and every problem sought to be solved by the present invention for inclusion in the current claims. Further, any element, component, or method step in the present disclosure shall be dedicated to the public regardless of which element, component, or method step is explicitly mentioned in the claims. Not intended. Any element claimed herein shall not be construed as invalid unless the element is explicitly referred to using the expression “means for”. S. C. It should not be construed under the provisions of the sixth paragraph of 122.

1A-1C are cross-sectional views of an epitaxial casting process for producing nanotubes according to embodiments of the present invention showing GaN nanotubes formed on ZnO nanowires. FIG. 2 is an image of a nanowire template array made from ZnO according to an embodiment of the present invention showing a cross section of the nanowire array in the inset. 2B is an image of a column of nanotubes formed on the sacrificial nanowire column of FIG. 2A made from GaN according to an embodiment of the present invention showing a folded interface between the GaN nanotube and the substrate in the inset. 3 is a diffraction plot of the GaN nanotube array of FIG. 2B according to an embodiment of the present invention showing nanotube composition. 2D is an image of the nanotube of FIG. 2B according to an embodiment of the present invention showing relative uniformity in diameter and wall thickness. 2D is an image of the nanotube of FIG. 2B according to an embodiment of the present invention showing relative uniformity in diameter and wall thickness. 2D is an image of the nanotube of FIG. 2B according to an embodiment of the present invention showing relative uniformity in diameter and wall thickness. 3 is a high resolution image of an outer wall structure in the GaN nanotube of FIG. 2B according to an embodiment of the invention. FIG. 2D is a high resolution image of the inner wall structure of the GaN nanotube of FIG. 2B according to an embodiment of the present invention showing an inset of electron diffraction patterns taken with the nanotube along the [−110] region axis. 2 is a plot of nanotube composition across a nanotube profile according to an embodiment of the invention, as examined by an energy dispersive x-ray spectrometer. 2 is an image of a nanotube formed according to an embodiment of the present invention shown with its tip directed. 2 is an image of a single crystal GaN nanotube produced according to an embodiment of the present invention showing gentle features. 8 is a plot of electron energy loss spectrum collected with the GaN nanotubes of FIG. 2 is an image of a column of nanotubes produced according to an embodiment of the invention, shown with a partially removed nanowire template. Image of nanotubes produced according to embodiments of the present invention with partially removed nanowire templates showing insets of electron diffraction patterns recorded in core sheath and pure tube regions along the [−110] region axis It is. FIG. 9B is a plot of a line profile for the nanotube core sheath at the top arrow position in FIG. 9B showing Ga and Zn signals. FIG. 9B is a plot of a line profile for the nanotube core sheath at the down arrow position in FIG. 9B showing Ga and Zn signals. 2 is a plot of photoluminescence spectra collected on GaN nanotubes according to an aspect of the present invention showing spectra from both thin and thick wall nanotubes. 2 is a plot of a temperature dependence curve of a single GaN nanotube according to an aspect of the present invention. 14A-14G are steps in the formation of SiO ₂ nanotubes according to embodiments of the present invention, shown with a perylene deposition stage during etching. 2 is an image of silicon nanotube array formation according to an aspect of the present invention. FIG. 15B is an image of silicon nanotube array formation according to an embodiment of the invention shown in detail in the inset of FIG. 15B. FIG. 15C is an image of silicon nanotube array formation according to an embodiment of the invention shown in detail in the inset of FIG. 15C. FIG. 15D is an image of silicon nanotube array formation according to an embodiment of the invention shown in detail in the inset of FIG. 15D. 2 is an image of a silica nanotube according to an embodiment of the present invention. 2 is an image of a silica nanotube according to an embodiment of the present invention. 1 is a cross-sectional view of a multi-walled nanotube according to an embodiment of the invention shown with a gallium nitride sheath sandwiched between insulating aluminum nitride layers. FIG. 1 is a cross-sectional view of a multi-walled nanotube according to an embodiment of the invention shown with a P-doped sheath on an N-doped sheath surrounding a sacrificial core. FIG. 1 is a cross-sectional view of a multi-walled nanotube according to an embodiment of the invention shown with an N-doped sheath on a P-doped sheath surrounding a sacrificial core. 1 is a perspective view of a sacrificial core having two longitudinal nanotube segments and covered with a solid sheath according to an embodiment of the present invention. FIG. 1 is a perspective view of a sacrificial core having a plurality of longitudinal nanotube segments and covered with a plurality of sheaths in accordance with aspects of the present invention. 1 is a cross-sectional view of a nanotube device manufacture in accordance with an aspect of the present invention shown as comprising a hollow core NPN transistor.

Claims (35)

Forming a nanowire; and
Depositing at least one sheath material on the nanowire;
Removing the nanowires, comprising the steps of:
The method wherein the remaining sheath material is the nanotube.   The method of claim 1, wherein the nanowire is sacrificed during a removal step.   The method of claim 1, wherein the nanowire consists of a sacrificial template for forming the nanotubes.   The method of claim 1, wherein the nanowire is formed as a single crystal nanowire structure.   The method of claim 1, wherein the nanotubes are formed from a single crystal sheath structure.   The nanowire is essentially zinc oxide (ZnO), silicon (Si), gallium nitride (GaN), germanium (Ge), silver (Ag), gold (Au), II-VI group material, III-V group. The method of claim 1, comprising a material selected from the group of materials consisting of a material, a Group IV material element, and a metal. The sheath is made of gallium nitride (GaN), silicon oxide (SiO ₂ ), II-VI group material, III-V group material, group IV material element, metal, oxide of the material, dopant introduced into the material, 7. A method according to claim 6, comprising a material selected from the group of materials consisting of and a polymer.   The material selected for the sheath of nanotubes has a crystal structure and lattice constant that is sufficiently similar to the material selected for the nanowire to allow epitaxial growth of the sheath on the nanowire. 8. The method according to 7.   The method of claim 1, wherein the sheath comprises a single longitudinal segment covering the nanowire.   The method of claim 1, wherein the sheath comprises a plurality of longitudinal segments covering the nanowires.   The method of claim 10, wherein the plurality of longitudinal segments are formed utilizing a masking technique. Forming a plurality of rows of said nanotubes by depositing a sheath over a plurality of rows of nanowires;
The method of claim 1, wherein the rows are formed on a substrate. Forming a sacrificial nanowire template of zinc oxide (ZnO);
Depositing at least one gallium nitride (GaN) sheath on the nanowire;
Removing the nanowires, comprising the steps of:
The method wherein the sheath comprises a gallium nitride (GaN) nanotube structure.   The method of claim 13, wherein the nanowire comprises single crystal zinc oxide (ZnO).   The method of claim 13, wherein the gallium nitride (GaN) sheath is deposited on the nanowire by epitaxial casting.   The method of claim 15, wherein the epitaxial casting comprises gallium nitride (GaN) chemical vapor deposition. Trimethylgallium and ammonia are used as precursors for the chemical vapor deposition and are supplied with argon or nitrogen carrier gas,
The method of claim 16, wherein the chemical vapor deposition of gallium nitride is performed at about 600 degrees Celsius (600 degrees Celsius) to 700 degrees Celsius (700 degrees Celsius). The gallium nitride (GaN) nanotube has an inner diameter in the range of about 30 nm to about 200 nm;
14. The method of claim 13, wherein the gallium nitride (GaN) nanotube has a wall thickness in the range of about 5 nm to about 50 nm.   14. The method of claim 13, wherein the zinc oxide (ZnO) nanowires are removed by exposing the zinc oxide (ZnO) nanowires to an elevated temperature in an atmosphere containing hydrogen gas. The elevated temperature is about 600 degrees Celsius (600 ° C.)
The method of claim 19, wherein the atmosphere comprises about 10% hydrogen gas in an argon gas atmosphere.   14. The method of claim 13, wherein the nanowires of zinc oxide (ZnO) are removed by exposing the row to chemical etching.   The method of claim 21, wherein the chemical etching comprises an ammonia etching performed at a temperature sufficiently elevated for removal of the nanowires of zinc oxide. Forming a sacrificial nanowire template of a first material;
Forming a modified sheath of the first material on the nanowire;
Removing the nanowires, comprising the steps of:
A method wherein the sheath is a nanotube structure.   24. The method of claim 23, wherein the nanowire comprises a single crystal material.   24. The method of claim 23, wherein the sheath is formed on the nanowire by thermal oxidation.   24. The method of claim 23, wherein the nanowire is removed with an etching process. The first material is made of silicon (Si),
Wherein the modified first material consists of silicon oxide (SiO _2), The method of claim 23.   28. The method of claim 27, wherein the sheath is formed on the nanowire by a thermal oxidation process whose temperature determines the thickness of the sheath.   29. The method of claim 28, wherein the thermal oxidation temperature is in the range of about 800 degrees Celsius (800 degrees Celsius) to about 1000 degrees Celsius (1000 degrees Celsius). Coating the combination of the sheath and nanowire with an etch resistant material;
Removing the upper end of the nanowire with the sheath formed thereon, while protecting the wall with the nanotube sheath formed with the etching resistance material;
Removing silicon (Si) nanowire material from within the silicon oxide (SiO ₂ ) nanotubes;
30. The method of claim 29, wherein the nanowires are removed in an etching process comprising removing the etch resistant material.   The method of claim 30, wherein the etch resistant material comprises a dimer or a polymer.   32. The method of claim 31, wherein the etch resistant material comprises perylene. Removing the upper end of the nanowire having a sheath formed thereon,
Etching with oxygen plasma to remove the etch resistant material to a sufficient depth to expose the nanowire with the sheath formed thereon;
31. Etching with hydrofluoric acid to remove the metal cap of the nanowire. The removal of the nanowires silicon (Si) comprises etching with xenon difluoride (XeF _2), The method of claim 33.   The method of claim 30, wherein the removal of the etch resistant material comprises an oxygen plasma etch.

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