| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | March 8, 2005 |
| PATENT TITLE |
Semiconducting oxide nanostructures |
| PATENT ABSTRACT | Briefly described, new types of nanostructures and methods of fabrication thereof are disclosed. A representative nanostructure includes a free-standing, helical semiconductor oxide nanostructure. The free-standing, helical semiconductor oxide nanostructure includes a nanobelt having a substantially rectangular cross-section. The the nanobelt is about 5 nanometers to about 200 nanometers in width and about 3 nanometers to about 50 nanometers in height, and the radius of the helical semiconductor oxide nanostructure is about 200 to 5000 nanometers |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | January 13, 2004 |
| PATENT REFERENCES CITED |
Dai, Pan and Wang; Gallium Oxide Nanoribbons and Nanosheets; pp. 1-14 (no date). Pan, Dai and Wang; Lead Oxide Nanobelts and Phase Transformation Induced by Electron Beam Irradiation, Aug., 2001; pp. 1-13. Pan, Dai and Wang; Nanobelts of Semiconducting Oxides; Mar. 9, 2001; pp. 1947-1949. Ginley and Bright; Transparent Conducting Oxides; Aug., 2000; pp. 15-18. Coutts, Young and Li; Characterization of Transparent Conducint Oxides; Aug., 2000; Pagse 58-65. Lewis and Paine; Applications and Processing of Transparent Conducting Oxides; Aug., 2000; pp. 22-26. Gordon; Criteria for Choosing Transparent Conductors; Aug., 2000; pp. 52-57. Kawazoe, Yanagi; Ueda, and Hosono; Transparent p-Type Conducting Oxides; Design and Fabrication of p-n Heterojunctions; Aug. 2000; pp. 28-35. Minami; New n-Type Transparent Conducting Oxides; Science Magazine Aug. 2000; p. 38-43. Wang; Semiconducting Oxides Prepared in the Form of Nanobelts; MRS Bulletin Aug., 2001; pp. 603-604. Kong, et al.; Spontaneous Polarization-Induced Nanohelixes, Nanosprings, and Nanorings of Piezoelectric Nanobelts; Nano Letters 2003, vol. 3, No. 12; pp. 1625-1631. Kong, et al.; Polar-Surface Dominated ZnO Nanobelts and the Electrostatic Energy Induced Nanohelixes, Nanosprings, and Nanospirals; Applied Physics Letters, vol. 84, No. 6, Feb. 9, 2004; pp 975-977. Wang, et al.; Induced Growth of Asymmetric Nanocantilever Arrays on Polar Surfaces; Physical Review Letters, vol. 91, No. 18; pp 185502-1-185502-4. |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
Therefore, having thus described the invention, at least the following is claimed: 1. A nanostructure, comprising: a free-standing, helical semiconductor oxide nanostructure including a nanobelt having a substantially rectangular cross-section, wherein the nanobelt is about 5 nanometers to about 200 nanometers in width and about 3 nanometers to about 50 nanometers in height, and wherein the radius of the helical semiconductor oxide nanostructure is about 200 to 5000 nanometers. 2. The nanostructure of claim 1, wherein the semiconductor oxide is chosen from oxides of zinc, cadmium, mercury, gallium, indium, tellurium, germanium, tin, and lead. 3. The nanostructure of claim 1, wherein the semiconductor oxide is zinc oxide. 4. The nanostructure of claim 1, wherein the nanobelt is a single crystalline structure. 5. The nanostructure of claim 1, wherein the nanobelt is a polar surface dominated zinc oxide nanobelt. 6. The nanostructure of claim 1, wherein the nanobelt includes polarized .+-.(0001) facets. 7. The nanostructure of claim 1, wherein the nanobelt has a substantially uniform width along the length of the free-standing helical semiconductor oxide nanostructure. 8. The nanostructure of claim 1, wherein the semiconductor oxide is zinc oxide, wherein the nanobelt has a top .+-.(0001) surface, bottom .+-.(0001) surface, a right side .+-.(1010) surface, and a left side .+-.(1010) surface. 9. The nanostructure of claim 1, wherein the semiconductor oxide is zinc oxide, wherein the nanobelt is described by characteristics selected from an interior (0001)-Zn surface and an exterior (0001)-O surface, and an interior surface (0001)-O and exterior surface (0001)-Zn. 10. A nanostructure comprising: a free-standing semiconductor oxide nanoring, wherein the nanoring has a radius of about 500 to 10,000 nanometers, a height of about 5 to 2000 nanometers, and a width of about 50 to 7500 nanometers. 11. The nanostructure of claim 10, wherein the semiconductor is chosen from ZnS, GaN, CdSe, and oxides of zinc, cadmium, gallium, indium, tin, lead, and, and combinations thereof. 12. The nanostructure of claim 10, wherein the semiconductor oxide is zinc oxide. 13. The nanostructure of claim 12, wherein the nanoring includes a nanobelt having a substantially rectangular cross-section, wherein the nanobelt is about 5 nanometers to about 200 nanometers in width and about 3 nanometers to about 50 nanometers in height. 14. The nanostructure of claim 13, wherein the nanoring includes about 1 to 250 loops of the nanobelt. 15. The nanostructure of claim 13, wherein the semiconductor oxide is zinc oxide, and wherein the nanobelt includes a top .+-.(0001) surface, a bottom .+-.(0001) surface, a right side .+-.(1210) surface, and a left side .+-.(1210) surface. 16. The nanostructure of claim 13, wherein the semiconductor oxide is zinc oxide, wherein the nanobelt has an interior (0001)-Zn surface and an exterior .+-.(0001)-O surface. 17. The nanostructure of claim 11, wherein the nanoring is a single crystalline structure. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
TECHNICAL FIELD This disclosure is generally related to nanostructures and, more particularly, is related to semiconductive and piezoelectric oxide nanostructures and fabrication thereof. BACKGROUND Binary semiconducting oxides often have distinctive properties and can be used as transparent conducting oxide (TCO) materials and gas sensors. Current studies of semiconducting oxides have been focused on two-dimensional films and zero-dimensional nanoparticles. For example, fluorine-doped tin oxide films are used in architectural glass applications because of their low emissivity for thermal infrared heat. Tin-doped indium oxide (ITO) films can be used for flat panel displays (FPDs) due to their high electrical conductivity and high optical transparency; and zinc oxide can be used as an alternative material for ITO because of its lower cost and easier etchability. Tin oxide nanoparticles can be used as sensor materials for detecting leakage of several inflammable gases owing to their high sensitivity to low gas concentrations. In contrast, investigations of wire-like semiconducting oxide nanostructures can be difficult due to the unavailability of nanowire structures. Wire-like nanostructures have attracted extensive interest over the past decade due to their great potential for addressing some basic issues about dimensionality and space confined transport phenomena as well as related applications. In geometrical structures, these nanostructures can be classified into two main groups: hollow nanotubes and solid nanowires, which have a common characteristic of cylindrical symmetric cross-sections. Besides nanotubes, many other wire-like nanomaterials, such as carbides, nitrides, compound semiconductors, element semiconductors, and oxide nanowires have been successfully fabricated. However, the nanostructures discussed above can have a variety of deficiencies. For example, often it is difficult to control the structure and morphology of many nanostructures. Further, many nanostructures are not defect and/or dislocation free. These deficiencies can cause problems such as, for example, uncontrolled properties due to uncontrolled structure and/or morphology, scattering from dislocations in electric transport applications, and degraded optical properties. Thus, a heretofore unaddressed need exists in the industry to address at least the aforementioned deficiencies and/or inadequacies. SUMMARY Briefly described, this disclosure provides for new types of nanostructures and methods of fabrication thereof. A representative nanostructure includes a free-standing, helical semiconductor oxide nanostructure. The free-standing, helical semiconductor oxide nanostructure includes a nanobelt having a substantially rectangular cross-section. The the nanobelt is about 5 nanometers to about 200 nanometers in width and about 3 nanometers to about 50 nanometers in height, and the radius of the helical semiconductor oxide nanostructure is about 200 to 5000 nanometers. Another representative nanostructure includes a free-standing semiconductor oxide nanoring. The nanoring has a radius of about 500 to 10,000 nanometers, a height of about 5 to 2000 nanometers, and a width of about 50 to 7500 nanometers. This disclosure also involves a method of preparing nanostructures. A representative method includes: providing a homogeneous metal oxide powder mixture; exposing the homogeneous metal oxide powder mixture to thermal conditions of about 900 to 1600.degree. C. at a pressure of about 10.sup.-3 to 10.sup.-2 torr for about 5 to 100 minutes; flowing an inert gas over the homogeneous metal oxide powder mixture; and forming a free-standing semiconductor oxide nanostructure via a condensation reaction at a pressure of about 50 to 800 torr at thermal conditions of about 100 to 700.degree. C., each of the free-standing semiconductor oxide nanostructures having a substantially rectangular cross-section. Other systems, methods, features, and advantages of this disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of this disclosure, and be protected by the accompanying claims. BRIEF DESCRIPTION OF THE DRAWINGS Many aspects of this disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of this disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. FIG. 1 includes schematics that illustrate a perspective view, a top view, a side view, and, an end view of a nanobelt. FIG. 2 includes schematics that illustrate a perspective view, a top view, a side view, and, an end view of a nanosheet. FIG. 3 is a schematic that illustrates an apparatus that can be used to fabricate the nanobelt and/or the nanosheet shown in FIGS. 1 and 2. FIG. 4 is a flow diagram illustrating a representative method for fabricating nanostructures as shown in FIGS. 1 and 2. FIG. 5 illustrates scanning electron microscope (SEM) images of the as-synthesized ZnO nanobelts. FIGS. 6A through 6G illustrate the controlled growth of (0001) polar surface dominated ZnO nanobelts. FIG. 6A illustrates a low-magnification transmission electron microscope (TEM) images and the corresponding electron diffraction patterns recorded from the areas as indicated by a sequence number from a TEM grid without tilting, showing their unanimous [0001] orientation on a flat carbon substrate. FIG. 6B illustrates a TEM image and the corresponding electron diffraction pattern, showing that the nanobelt grows along [2110] (a-axis), with .+-.(0001) top and bottom surfaces, and .+-.(0110) side surfaces. FIG. 6C illustrates a high-resolution TEM image recorded from the center of the nanobelt given in FIG. 6B, showing its dislocation-free volume. The inset is the projected model of the wurtzite ZnO along [0001]; the positions of the Zn atoms are in correspondence to the white dots observed in the image. FIG. 6D illustrates a [0001] profile high-resolution TEM image recorded from the edge of the nanobelt given in FIG. 6B, showing the flatness of the surface. FIG. 6E illustrates a low magnification TEM image and the corresponding electron diffraction patterns recorded from the circled regions, displaying the geometry of the nanobelt. The difference between the two electron diffraction patterns is due to the bending in the local regions. The contrast observed in the image is the bending contour in electron imaging produced by the deformation of atomic planes. FIG. 6F illustrates a TEM image showing the helical twist of a nanobelt. The dark contrast at the top is due to the local strain. FIG. 6G illustrates the structure model of the ZnO nanobelt. FIGS. 7A through 7D illustrate SEM images of the as-synthesized ZnO nanobelts, showing helical nanostructure. FIGS. 8A through 8E illustrate TEM images and the corresponding electron diffraction patterns recorded from the regions marked in the figure from an incomplete ring, a single-looped ring, double-looped ring, and multiple-looped ring, respectively, without tilting the specimen. Enlarged images from the nanobelts are inserted, from which the number of loops and the contact between the nanobelts can be directly imaged. Electron diffraction patterns recorded from the nanorings unanimously prove that the normal direction of the circular ring plane is [0110], and the [0001] c-axis is pointing to the ring center and it rotates following the arc of the ring. Electron diffraction patterns recorded from the straight nanobelts or substantially straight nanobelts are [0001] pattern. FIG. E illustrates convergent beam electron diffraction from a nanobelt, showing the polarity of the nanobelt as evidenced by the asymmetric intensity distribution in the (0002) and (0002) disks. The result shows that the interior of the nanoring is positive charged and the exterior is negatively charged. FIG. 9A illustrates a TEM image of a helical nanostructure formed by multiple-loop rolling up of a nanobelt. FIG. 9B illustrates a schematic model showing the relative orientation for recording high-resolution TEM images along the axial direction of the helical nanostructure ([1210]), and the [0110] perpendicular to the nanobelt and parallel to its polar surface. FIG. 9C illustrates a [1210] high-resolution TEM image of a nanobelt showing the dislocation-free volume but with the presence of stacking faults (SFs). FIG. 9D illustrates two possible atomic models in correspondence to the HRTEM image given in FIG. 9C, showing that stacking fault has no effect on the polarity of the nanobelt. FIG. 9E illustrates a [0110] high-resolution TEM image of a nanobelt and the profile of its side surface, showing dislocation-free volume. FIG. 9F illustrates an enlargement of the image given in FIG. 9E and the projected atomic structure model. FIG. 10A illustrates a low-magnification SEM image of the as-synthesized ZnO nanorings. FIGS. 10B through 10D illustrate high-magnification SEM images of freestanding, single-crystal ZnO nanorings, showing substantially uniform geometrical shapes. FIGS. 11A through 11I illustrate the structure of the type I ZnO single-crystal nanoring. FIG. 11A illustrates a TEM image of a ZnO nanoring viewed with the electron beam parallel to the plane of the nanoring, while (a.sub.1) illustrates an electron diffraction pattern recorded from area a.sub.1 marked in FIG. 11A, (a.sub.2) illustrates an enlargement of area a.sub.2 marked in (A), showing a loose end at the left-hand side, and (a.sub.3) illustrates electron diffraction pattern recorded from the loose end (area a.sub.3 marked in a.sub.2). FIGS. 11B and 11C illustrate bright-field and dark-field TEM images recorded from the nanoring after tilting about 15.degree., while (b) illustrates electron diffraction pattern recorded from the area (b) marked in FIG. 11B. FIGS. 11D through 11F illustrate larger images from the areas (d, e, f), respectively, marked in FIG. 11A, after slightly tilting the nanoring. FIG. 11G illustrates an enlarged TEM image of the nanoring tilted about 10.degree.. FIG. 11H illustrates an enlargement of the area h indicated in FIG. 11C, which shows a uniform distribution of stacking faults across the entire width of the nanoring. FIG. 11I illustrates a high-resolution TEM image recorded from the nanoring when the incident electron beam is parallel to the ring-plane, showing stacking faults inside the nanobelt and at the interface between the coiled loops. FIGS. 12A through 12H illustrate the structure of the type II ZnO single-crystal nanoring. FIGS. 12A and 12B illustrate bright-field and dark-field TEM images, respectively, recorded from the nanoring with the incident electron beam parallel to the ring plane. FIG. 12C illustrates an electron diffraction pattern recorded from the nanoring. The pattern shows vertical mirror symmetry, and the extra diffraction spots at the two sides are from the cylindrical bending of the single-crystal ribbon. FIG. 12D illustrates a high-resolution TEM image recorded from the central symmetric line in FIG. 12A. FIGS. 12E and 12F illustrate enlarged TEM images from the areas (e) and (f) marked in FIG. 12A, respectively, showing the coiling layers. FIGS. 12E and 12F illustrate bright-field and dark-field TEM images recorded form the nanoring after tilting about 15.degree.. FIG. 13A illustrates a structure model of ZnO and the corresponding crystal planes showing the .+-.(0001) polar surfaces. FIGS. 13B through 13E illustrate proposed growth processes and corresponding experimental results showing the initiation and formation of the single-crystal nanoring via self-coiling of a polar nanobelt. DETAILED DESCRIPTION This disclosure describes free-standing nanostructures and methods of fabrication thereof. In general, the free-standing nanostructures have substantially rectangular cross-sections. Embodiments of the free-standing nanostructure may be defect free, dislocation free, and/or structurally uniform, while the surfaces of the free-standing nanostructure are specific crystallographic planes. In addition, the structure and the morphology of the free-standing nanostructure can be controlled using embodiments of the method of fabrication. In this manner, the free-standing nanostructures and methods of fabrication thereof may overcome some of the deficiencies described above. In general, the free-standing nanostructures can be nanobelts, nanosheets, nanodiskettes, helical nanostructrues, or nanorings, that have a substantially rectangular cross-section. FIG. 1 illustrates a perspective view (A), a top view (B), a side view (C), and an end view (D) of a nanobelt 10. The perspective view (A) illustrates a top 12, a side 14, and an end 16 of the nanobelt 10. The top view (B), side view (C), and the end view (D) illustrate the top 12, the side 14, and the end 16 of the nanobelt 10. FIG. 2 illustrates a perspective view (A), a top view (B), a side view (C), and an end view (D) of a nanosheet 20. The perspective view (A) illustrates a top 22, a side 24, and an end 26 of the nanosheet 20. The top view (B), the side view (C), and the end view (D) illustrate the top 22, the side 24, and the end 26 of the nanosheet 20. Nanobelts 10 can be characterized as "ribbon-like" structures, while the nanosheets 20 can take the form of a variety of polygonal shapes such as, for example, a rectangle, a square, a triangle, etc. Nanodiskettes (not shown) are similar to nanosheets 20 except that nanodiskettes are "coin-shaped" structures. The structure of the helical nanostructure (as shown in FIG. 5 in Example 1B) can be described as a nanobelt configured as helix (e.g., coil or ring structure) having a substantially uniform radius along the length of the helix. The structure of the nanoring (as shown in FIG. 10 in Example 1C) can be described as a plurality of contiguous side-by-side loops of a nanobelt. This disclosure does not describe in any definite dimensions the difference between nanobelts 10, nanosheets 20, and nanodiskettes. For clarity, this disclosure refers to nanobelts, nanosheets, and nanodiskettes as "nanostructures," unless the structure being referred to is specifically denoted as a nanobelt, a nanosheet, a nanodiskette, a helical nanostructure, or a nanoring. The nanostructures are fabricated of at least one semiconductor oxide and/or at least one doped semiconductor oxide. The semiconductor oxide includes oxides of zinc, cadmium, mercury, gallium, indium, tellurium, germanium, tin, and lead. The nanostructure fabricated of at least one semiconductive oxide can be, for example, a binary or a ternary complex of the semiconductor oxide. The doped semiconductor oxide includes at least one semiconductive oxide that can be doped with at least one dopant that may be chosen from aluminum, gallium, boron, yttrium, indium, scandium, silicon, germanium, titanium, zirconium, hafnium, antimony, tin, nitrogen, and fluorine. The nanostructure can be fabricated of at least one doped semiconductor oxide, for example, a binary or a ternary complex of the doped semiconductor oxide. The size (e.g., length, width, and height (thickness)) of the nanostructure can vary within a type of semiconductor oxide and among each of the semiconductor oxides. The size of the nanostructure can be controlled to fit certain criteria for a particular application. However, in general, the nanostructures can be about 20 nanometers to about 6000 nanometers in width, about 5 nanometers to about 100 nanometers in height, and about 100 nanometers to about 3 millimeters in length. The nanostructures can have a width-to-height ratio of about 5 to about 15. In addition to the dimensions described above, the following examples describe illustrative sizes of the nanostructures for some of the semiconductor oxides. As mentioned above, the helical nanostructure includes a nanobelt having a substantially rectangular cross-section. In addition, the helical nanostructure is a single crystalline structure. The radius of the helical nanostructure is about 200 nanometers to 5000 nanometers, and specifically can be about 400 nanometers to 800 nanometers. The width of the nanobelt is about 5 nanometers to 200 nanometers, and specifically can be about 10 nanometers to 60 nanometers. The height of the nanobelt is about 3 nanometers to about 50 nanometers, and specifically can be about 5 nanometers to 20 nanometers. The nanobelt is about 100 nanometers to 3 millimeters in length. An exemplary embodiment of the helical nanostructure includes a helical zinc oxide nanostructure that includes a polar surface dominated zinc oxide nanobelt. The zinc oxide nanobelt includes polarized .+-.(0001) facets and, in particular, the zinc oxide nanobelt includes a top .+-.(0001) surface, a bottom .+-.(0001) surface, a right side (1010) surface, and a left side -(1010) surface. As mentioned above, the nanoring includes a nanobelt having a substantially rectangular cross-section. In addition, the nanoring is a single crystalline structure. The structure of the nanoring can be described as a plurality of contiguous side-by-side loops of the nanobelt. The dimensions of the nanobelt are similar to those described above for the helical nanobelt. The nanoring includes about 1 to 250 loops of the nanobelt, and suitably about 10 to 200 loops of the nanobelt. The nanoring has a radius of about 500 nanometers to 10,000 nanometers, and suitably about 600 nanometers to 2000 nanometers. The nanoring has a height of about 5 nanometers to 2000 nanometers, and suitably about 5 nanometers to 30 nanometers. The nanoring has a width of up to about 7500 nanometers, and suitably about 200 nanometers to 1000 nanometers. An exemplary embodiment of the nanoring includes a polar surface dominated zinc oxide nanobelt. The zinc oxide nanobelt includes polarized .+-.(0001) facets and, in particular, the zinc oxide nanobelt has an interior (0001)-Zn surface and an exterior .+-.(0001)-O surface. The zinc oxide nanobelt includes a top .+-.(0001) surface, a bottom .+-.(0001) surface, a right side (1210) surface, and a left side -(1210) surface. In general, the methods for fabricating nanostructures can be based on thermal evaporation of oxide powders under controlled conditions that can be performed on the apparatus 30 shown in FIG. 3. The apparatus 30 includes a horizontal tube furnace 32 that has an alumina tube 36 therein and is wrapped in a heating coil 34. Inside the alumina tube 36 are one or more alumina plates 38 and an alumina crucible 40, which contains the oxide powder 42 and/or other chemicals used to fabricate the nanostructures. To measure the temperature at various locations in the furnace 32, a thermocouple 44 or other temperature measuring device can be moved within the furnace 32. The apparatus 30 is also equipped with input 46 and output tubes 48 to introduce and pump-out a flow gas such as Argon (Ar). Additional features known by one skilled in the art are also included in the apparatus such as vacuum pumps, vacuum manifolds, reactant gas inputs, reactant gas manifolds, etc., and will not be discussed here. In practice, the desired oxide powder is placed in the aluminum crucible 40 in the center of an alumina tube 36. The temperature, pressure, and evaporation time are controlled. Typically, the evaporation is performed without a catalyst. Except for the evaporation temperature that can be determined based on the melting point of the oxides used, the following parameters are typically kept constant: evaporation time (e.g., 2 hours), alumina tube 36 pressure (e.g., 300 Torr), and flow gas flow rate (e.g., Argon flowed at approximately 50 standard cubic centimeter per minute (sccm)). During evaporation, the products of the evaporation are deposited onto the alumina plates 38 located at the downstream end of the alumina tube 36. Typically, the as-deposited products can be characterized and analyzed by x-ray diffraction (XRD) (Philips PW 1800 with Cu K.alpha. radiation), scanning electron microscopy (SEM) (Hitachi S800 FEG), transmission electron microscopy (TEM) (Hitachi HF-2000 FEG at 200 kV and JEOL 4000EX high resolution TEM (HRTEM) at 400 kV), and energy dispersive x-ray spectroscopy (EDS). Reference will now be made to the flow diagram of FIG. 4. FIG. 4 illustrates a representative method of preparing a plurality of semiconductor oxide nanostructures having a substantially rectangular cross-section from an oxide powder. Initially, the oxide powder is heated to an evaporation temperature of the oxide powder for about 1 hour to about 3 hours at about 200 torr to about 400 torr in an atmosphere comprising an inert gas such as Argon, as shown in block 42. Then, the oxide powder is evaporated, as shown in block 44. Thereafter, the plurality of semiconductor oxide nanostructures is formed, as shown in block 46. In regard to helical nanostructures and nanorings, the following describes a representative method for preparing helical nanostructures and nanorings. Additional details for the method are described in reference to Examples 1B and 1C. Initially, a metal oxide powder (e.g., zinc oxide with a small amount of lithium oxide, indium oxide, and/or lithium carbonate) is exposed to thermal conditions of about 900 to 1600.degree. C. (in particular, about 1350 to 1400.degree. C.) at a pressure of about 50 to 800 torr for about 5 to 100 minutes (in particular, about 30 minutes). After the metal oxide powder is heated at a pressure of about 10.sup.-3 to 10.sup.-2 torr, an inert gas, such as Argon, is flowed (e.g., about 25 to 50 standard cubic centimeters per second) over the decomposing and evapoating metal oxide powder. Subsequently, free-standing semiconductor oxide nanostructures (e.g., helical nanostructures and nanorings) are formed via a condensation reaction at a pressure of about 50 to 800 torr (particularly, about 250 to 500 torr) at thermal conditions of about 100 to 700.degree. C. (particularly, about 200 to 500.degree. C.). The processing conditions can be adjusted to control the amount of each type of nanostructure formed as described below in Examples 1B and 1C. Having summarized the nanostructures and methods of fabrication thereof above, reference will now be made in detail to six illustrative examples of the semiconductor oxide nanostructures. While the invention is described in connection with these examples, there is no intent to limit the invention to the following examples. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of the invention. Examples 1-4 are discussed in the following papers entitled "Nanobelts of Semiconducting Oxides," published in Science Vol. 291, 9, March 2001, "Spontaneous polarization induced nanohelicals, nanosprings, and nanorings of piezoelectric nanoblets," (Nano Letters, December 2003), and "Single-crystal cylindrical nanorings formed by epitaxial self-coiling of polar-nanoblets," (Science, in review), which are herein incorporated by reference. Example 5 is discussed in the paper entitled "Gallium Oxide Nanoribbons and Nanosheets," and is in-press at the Journal of Physical Chemistry B, which is herein incorporated by reference. Example 6 is discussed in the paper entitled "Lead Oxide Nanobelts and Phase Transformation Induced by Electron Beam Irradiation," and is in-press at Applied Physics Letters, which is herein incorporated by reference. |
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