DETAILED DESCRIPTION OF THE INVENTION

[0065] Definitions:

[0066] Definition of the symmetry symbols: In the description of symmetry symbols such as, for example, 4S ^*1 , are described herein. The numeral, 4, refers to a 4-fold crystal symmetry of the metallic oxide forming a central nanostructural spine. The letter, S (or M), refers to single (S) or multiple (M) rows of the secondary metallic oxide nanorods such as for example ZnO. The absence of any symbol or numerals after the letter S (or M), is indicative of all secondary nanorods being perpendicular to the linear axis of the central nanostructural spine. The symbol * after the letter S (or M), is indicative of secondary nanorods forming a finite non-perpendicular angle with the central nanostructural spine. The numeral 1 or 2 after the symbol * indicates that not all secondary nanorod branches form a finite non-perpendicular angle with the central nanostructural spine.

[0067] The term “hierarchical metal oxide” material refers to microscale ordered structure of metallic oxides having a three-dimensional periodic orientation. Metal oxide materials having a nanostructural morphology including but not limited to nanobrushes, nanobridges, nanonails, naowalls, nanobelts nanowires, nanocrystal chains and nanocircuits are alternativley refered to as “hierarchical metal oxide” materials

[0068] The terms “nanobrush” and “nanocomb” refer to metal oxide materials comprising a “nanostructural spine”, and “secondary nanostructural rods”, that are attached to the nanostructural spine, such that the secondary nanostructural rods are extended linearly from the core crystal and are substantially non-parallel to to the nanowire backbone axis. The term, nanostructural spine, is alternatively referred to as a “core crystal” or “nanowire”. The term, secondary nanostructural rods, is alternatively referred to as “secondary crystals”, “secondary nanorods” or “secondary nanonails”.

[0069] The term “nanonail” refers to a metal oxide microscopic structure on a nanoscale level, bearing a resemblance to a nail. In one embodiment, a nanonail comprises a metal oxide nanorod shaft and nanorod head, alternativle referred to as a nanorod cap. Typically, the nanonail shaft diameter decreases gradually from the cap at one end of the shaft, to the opposite end, where it is attached to the substrate.

[0070] The term “nanobelt” refers to a metal oxide microscopic structure on a nanoscale level, bearing a resemblance to a belt, strip or ribbon. Alternatively, a nanobelt is referred to as a “nanoribbon”. For example, an Sn-doped nanobelt of the invention comprises an array of multiple parallel nanorodsadhered to one another.

[0071] The term “nanobridge” refers to a metal oxide microscopic structure on a nanoscale level, bearing a resemblance to a bridge. Typically a nanobridge comprises a nanobelt having one or more rows of nanorods extending from the nanobelt. Typically the nanorods are perpendicular to the plane of the nanobelt. The rows of nanorods are optionally on one or both faces of the nanobelt. The rows of nanorods are optionally on one or both edges of the nanobelt. For example, a metal oxide nanobridge of the present invention has two rows of c-axis nanorods epitaxially grown perpendicularly on the edges of the {0001} plane of a ZnO nanobelt.

[0072] The term “roller coaster like nanobridge” refers to a nanobridge in that the nanobelt forms one or more loops or a ring around the center of the nanobridge.

[0073] The term “heteroepitaxial” as referred to herein, is afforded the meaning typically provided for in the art. Typically, the term heteroepitaxial, refers to an epitaxial relationship between two or more metallic oxides in which the composition of each metallic oxide is different from the metallic oxide adjacent to itself. An example of a heteroepitaxial metal oxide composite is that of a hierarchical metal oxide composite in which the core crystal is major In ₂ O ₃ or pure In ₂ O ₃ , and the secondary crystal is major ZnO or pure ZnO.

[0074] The term “homoepitaxial” as referred to herein, is afforded the meaning typically provided for in the art. Typically, the term homoepitaxial, refers to an epitaxial relationship between two or more metallic oxides in which the composition of each metallic oxide is the same or similar to the composition of the metallic oxide adjacent to itself. An example of a homoepitaxial metal oxide composite is that of a hierarchical metal oxide composite in which the core crystal is major ZnO or pure ZnO, and the secondary crystal is major ZnO or pure ZnO.

[0075] The term “nanocrystal chain” as referred to herein, is afforded the meaning typically provided for in the art. In particular, a nanocrystal chain refers to a metal oxide microscale structure comprising a plurality of metal oxide microcrystals linked together to form a linear or one-dimensional (1D) array. In one embodiment, a nanocrystal chain comprises interconnected individual nanocrystals epitaxial with each other. In a currently preferred embodiment, the nanocrystal chain form with a growth direction of [001].

[0076] The term “nanowire circuit” as referred to herein, is afforded the meaning typically provided for in the art. The term nanowire circuit, typically refers to a metal oxide microscale structure comprising a network of metal oxide network junctions and connected by metal oxide nanowires or metal oxide nanocrystal chains. Metal oxide network junctions are typically nanocrystals or bigger microcrystals, alternatively referred to as “big crystals”. Alternative embodiments of either nanowire and nanocrystal chains are formed by varying the growth temperature gradient of the thermal evaporation and condensation. The term “nanowire circuit” is alternatively referred to as a “nanowire network circuit” or a “nanowire network”.

[0077] A “metal oxide source material” as used herein, is a mixture of one or more metallic oxides that one used as metal oxide vapor source in the thermal evaporation and condensation methods of invention. The metal oxide source material alternatively includes non-metal oxide source material, including but not limited to, graphite. The metal oxide source material can have any suitable morphology. In one embodiment, the metal oxide source material comprises a metal oxide source powder.

[0078] The present invention including metal oxide materials, specific embodiments thereof, specific attributes thereof and advantages of thereof, as well as methods for their preparation, are described below with reference to the relevant figures.

[0079] The present invention provides a metal oxide material comprising at least one metallic oxide wherein said metallic oxide is arranged in a three-dimensionally periodic orientation so as to confer nanostructural morphology to said metal oxide material. Preferably, the metal oxide material has a pre-determined symmetry.

[0080] In one embodiment, the present invention provides a metal oxide material comprising a first metallic oxide and a second metallic oxide, wherein said first metallic oxide forms a central nanostructural spine comprising a linear axis in a three-dimensional orientation, whereupon said second metallic oxide forms terminally attached three-dimensional periodically oriented nanostructural rods, the linear axes of said nanostructural rods being orientedsubstantially non-parallel to the linear axis of said nanostructural spine formed by said first metallic oxide.

[0081] In another embodiment, the central nanostructural spine is a nanowire. The term “nanowire” as referred to herein, is afforded the meaning typically provided for in the art. In one embodiment, the nanowire is a single crystal metal oxide. In another embodiment, the nanowire single crystal metal oxide in an indium oxide. In a currently preferred embodiment, the indium oxide is In ₂ O ₃ . In yet another embodiment, the nanowire single crystal metal oxide comprises zinc oxide. In another currently preferred embodiment, the zinc oxide is ZnO. A nanowire alternatively comprises a “nanocrystal chain.” Nanowires of the present invention can be grown to different lengths. In one embodiment, the nanowires have an average diameter of ranging from about 10 nanometers (nm) to about 1000 nm (1 μm). In another embodiment, the nanowires have a diameter ranging from about 50 to about 500 nm. In one embodiment, the nanowires have an average length of about 0.01 micrometers (μm) to about 100 μm. In another embodiment, the nanowires have a length ranging from about 1 μm to about 20 μm.

[0082] The central nanostructural spine in the metal oxide materials of the invention can have a varity of morphologies including but not limited to, cylindrical, rod, barrel-shaped, conical, rectangular cross-sectional, square cross-sectional and hexagonal cross-sectional morphologies. The central nanostructural spine of the invention can have a plurality of facets. The term “facet” as referred to herein, is afforded the meaning typically provided for in the art. Typically a facet refers to a planar external surface of the crystal structure of the nanowires. In one embodiment, the central nanostructural spine of the invention can have 2, 4 or 6 facets. The central nanostructural spine of has a structural symmetry. Typically the central nanostructural spine of the invention can have 2-fold, 4-fold or 6-fold symmetry, wherein the geometries are rectangular cross- sectional, square cross-sectional or hexagonal cross-sectional respectively.

[0083] The periodically oriented nanostructural rods in the metal oxide materials of the invention can also have a plurality of facets. In one embodiment, the number of facets comprised by the secondary crystals corresponds to the number of facets of the core crystal. In onather embodiment the periodically oriented nanostructural rods are nanonails.

[0084] The term “nanorod” refers to a nanoscale crystalline metal oxide. In a currently preferred embodiment, the nanorod is a single crystal metal oxide. Typically the nanorod comprises a zinc oxide. In a currently preferred embodiment, the zinc oxide is ZnO.

[0085] In one embodiment of the invention, the nanorods have an average diameter ranging from about 10 to about 1000 nm (1 μm). In another embodiment of the invention, the nanorods have a diameter of about 20 to about 200 nanometers (nm). The nanorods of the invention typically have an average length of about 0.01 to about 100 μm. Preferably, the nanorods have a length ranging from about 0.2 μm to about 5 μm.

[0086] The periodically oriented nanostructural rods of the invention have proximal and distal ends such that the nanostructural rods of the invention are attached to the core crystal at the distal ends as to extend laterally form the central nanostructural spine.

[0087] The periodically oriented nanostructural rods of the invention are aligned to the nanostructural spine either in single rows or in multiple rows on the nanostructural spine. The nanostructural rods also are aligned in a direction either perpendicular to the linear axis of the nanostructural spine or at a finite non-perpendicular angle to (slanted). In one embodiment, the nanostructural rods grow at an angle ranging from about 45° to about 160° to the nanostructural spine. In a currently preferred embodiment, the secondary crystals grow perpendicular to the core crystal. In another currently preferred embodiment, the secondary crystals grow at an angle rangeing from about 60° or about 120° to the core crystal.

[0088] FIG. 1 shows the typical scanning electron microscopy (SEM) images of the ZnO nanostructures of the invention at low and medium magnifications, respectively. The low magnification image in FIG. 1 ( a ) shows the large quantity of such nanostructures. The medium magnification in FIG. 1 ( b ) shows that there are 3 major structural symmetries, 6-fold, 4-fold, and 2-fold and additionally shows a plurality of sub-symmetries associated with each major symmetry. For 6-fold symmetry, 3 sub-symmetries are identified as 6S-fold, 6M-fold, and 6M*-fold. For 4-fold symmetry, 5 sub-symmetries as 4S ^*1 -fold, 4S ^*2 -fold, 4S ^*2 fold, 4M-fold, and 4M*-fold are identified. For 2-fold symmetry, 3 sub-symmetries as 2S-fold, 2S*-fold, and 2M-fold are identified. The length of the In ₂ O ₃ core nanowires along the axis can be as long as tens of micrometers as shown in FIG. 1 ( a ). The diameter of the In ₂ O ₃ core nanowire ranges from about 50 to about 500 nm. The length of the secondary ZnO nanorods grown on the major In ₂ O ₃ core nanowire ranges from about 0.2 to a few micrometers, with diameters ranging from about 20 to about 200 nm. Powder X-ray diffraction (XRD) measurements show that the samples are mixtures of hexagonal ZnO (wurtzite) and cubic In ₂ O ₃ . From the XRD spectra, lattice constants for ZnO are derived as a=3.249 Å and c=5.206 Å, consistent with the standard values for bulk ZnO. The lattice constant for the cubic In ₂ O ₃ is a=10.118 Å which is in good agreement with the reported bulk value. For standard bulk values, see, for example, Powder Diffraction File Release 2000, PDF Maintenance 6.0 (International Center for Diffraction Data, Pennsylvania).

[0089] During SEM examination, observed are areas with particular symmetry as the majority. In FIG. 2 , SEM and transmission electron microscopy (TEM) images are presented in medium and high magnifications to show the 6-fold nanostructures in details. FIG. 2 ( a ) is a medium magnification of the area with the basic 6S-fold nanostructure symmetry as the majority and FIG. 2 ( b ) is a medium magnification of the area with the basic 6M-fold nanostructure symmetry as the majority. When the major core nanowire is small, the secondary nanorods grow in a single row as shown in FIG. 2 ( a ) and FIG. 2 ( c ). When the major core nanowire is large enough, multiple rows of secondary nanorods form on the major nanowire as shown in FIG. 2 ( b ) and FIG. 2 ( d ). The secondary nanorods grow into 6-fold symmetry because of the hexagonal symmetry of the major core as shown in FIG. 2 ( e ) and FIG. 2 ( f ). It is shown that not only the major core is hexagonal, but also the secondary nanorods are hexagonal and aligned in the same direction with each other as shown in FIG. 2 ( f ). FIG. 2 ( g ) is the SEM image of 6M* -fold nanostructure symmetry, in which the nanorods are not perpendicular to the major core. The TEM contrast image shown in FIG. 2 ( i ) indicates the hexagonal nature of the In ₂ O ₃ nanocore (lighter contrast in the center). Energy dispersive X-ray spectroscopy (EDS) composition analysis shows that all the secondary nanorods are pure ZnO and the major core nanowire is In ₂ O ₃ . Structural studies by electron diffraction indicate that the major core nanowire is cubic In ₂ O ₃ with lattice parameter of a=10.1 Å. Selected-area electron diffraction patterns shown in FIG. 20 ), point out that the major cores are along [110] or [001] zone axes of In ₂ O ₃ . Secondary nanorods grow along the [0001] direction of ZnO.

[0090] As shown in FIG. 3, 4-fold nanostructures are observed as the majority. FIG. 3 ( a ) is the medium magnification SEM image showing the abundance of 4-fold nanostructures. Under close examination of high magnification, at least 5 variations are found for the 4-fold symmetry. FIG. 3 ( b ) is a high magnification SEM image of the basic 4S-fold to show the single row of the secondary ZnO nanorods perpendicular to the major In ₂ O ₃ core nanowire. When the major In ₂ O ₃ core nanowire is large enough, multiple rows of ZnO nanorods are observed perpendicular to the major In ₂ O ₃ core nanowire. This sub-symmetry, having 4-fold symmetry with multiple rows of ZnO nanorods, is identified as 4M-fold as shown in FIG. 3 ( c .) In addition, the secondary ZnO nanorods are observed to be not always perpendicular to the major In ₂ O ₃ core nanowire, but grow at finite non-perpendicular angles with respect to the linear axis of the In ₂ O ₃ core nanowire. Again, when the major In ₂ O ₃ core nanowire is small, the secondary ZnO nanorods grow in a single row as shown in FIG. 3 ( d ) and FIG. 3 ( e ). In FIG. 3 ( e ), the same angle exists on all 4 directions, whereas in FIG. 3 ( d ), the same angle only exists in the 2 opposite directions (parallel to the page), and the other 2 opposite directions are perpendicular to the linear axis of major In ₂ O ₃ core nanowire (in-to and out-of the page). These two nanostructures are defined as 4S ^*1 -fold and 4S ^*2 -fold, respectively. When the major In ₂ O ₃ core nanowire is large enough, multiple rows of ZnO nanorods grow with an angle in all 4 directions as shown in FIG. 3 ( f ). This nanostructure is defined as 4M*-fold symmetry. FIG. 3 ( g ) is the growth model. FIG. 3 ( h ) and FIG. 3 ( i ) are the TEM bright field image and selected area diffraction patterns, respectively.

[0091] In addition to the 6-fold and 4-fold symmetrical nanostructures, the basic 2-fold nanostructures are observed as shown in FIG. 4 . FIG. 4 ( a ) is a medium magnification SEM image to show the abundance of the basic 2S-fold symmetry. FIG. 4 ( b ) is a high magnification SEM that shows that the secondary ZnO nanorods are perpendicular to the linear axis of the major core nanowire. 2S-fold symmetry variations have been observed and identified so far as 2S*-fold and 2M-fold. FIG. 4 ( c ) and FIG. 4 ( d ) show the SEM images in low and high magnifications of the 2M-fold symmetry, respectively. FIG. 4 ( e ) and FIG. 4 ( f ) are the low and high magnification SEM images showing the abundance and close up look of the 2S*-fold, respectively.

[0092] Additional variations of the 6-, 4-, and 2-fold symmetries are contemplated. Orientation relationships between the major In ₂ O ₃ core nanowire and the secondary ZnO nanorods are obtained from selected-area diffraction patterns shown in FIG. 2 ( j ) for the 6-fold symmetry and FIG. 3 ( i ) for the 4-fold symmetry. In each diffraction pattern, two sets of diffraction spots are observed; one from the major In ₂ O ₃ core nanowire and the other from the secondary ZnO nanorods. The diffraction pattern in FIG. 2 ( j ) is indexed using the [110] zone axis of In ₂ O ₃ and the [11{overscore (2)}0] zone axis of ZnO. Therefore, the crystallographic relationship is [110] _In2O3 //[11{overscore (2)}0] _ZnO for the 6-fold symmetry. In FIG. 3 ( i ), the diffraction pattern is indexed using the [001] zone axis of In ₂ O ₃ and the [63{overscore (9)}2] zone axis of ZnO. Therefore, the [001] zone axis of In ₂ O ₃ nanowire is parallel to the [63{overscore (9)}2] zone axis of the ZnO nanorods.

[0093] When In ₂ O ₃ nanowire is along the [001] direction, the core nanowire is enclosed by ±(100) and ±(010) facets ( FIG. 3 ( i )). ZnO nanorods grow on each facet of the In ₂ O ₃ nanowire according to the orientation relationships as listed in Table 1. There are two major orientation relationships, corresponding to the observed 4-fold and 4*-fold symmetries, respectively. The orientation relationship in the 4-fold symmetry is: [001] _In2O3 //[10{overscore (1)}0] _ZnO , [100] _In2O3 //[{overscore (1)}2{overscore (1)}0] _ZnO . In this case, [001] _In2O3 ⊥[0001] _ZnO , therefore, ZnO nanorods in the four arms grow perpendicular to the core nanowire as shown in FIG. 3 ( b ) since ZnO nanorods grow along [0001] direction. In the case of 4*-fold symmetry, [001] _In2O3 //[63{overscore (9)}2] _ZnO , so that the angle between ZnO nanorod and core nanowire is equal to the angle between [63{overscore (9)}2] _ZnO , and [0001] _ZnO , which is about 60°, explaining why a tilted 4*-fold symmetry is often observed in SEM images. Since there is no difference between ±[63{overscore (9)}2] _ZnO , for the growth of a nanorod on a [001] core In ₂ O ₃ nanowire, nanorods can grow with an angle of either 60 or 120 degree, which result in all the variations of the tilted growth as shown in FIG. 3 ( d ) and FIG. 3 ( e ).

[0094] When In ₂ O ₃ is along [110] directions, the core nanowire is enclosed by ±(1{overscore (1)}2), ±(1{overscore (1)}{overscore (2)}), and ±(1{overscore (1)}0) facets. The angle between each of these adjacent facets is about 60°. Therefore, a quasi 6-fold symmetry is observed when an In ₂ O ₃ nanowire grows along the [110] direction. In addition to the [001] and [110] directions, In ₂ O ₃ nanowires also grow along the [111] direction, as shown in FIG. 2 ( e ) and FIG. 2 ( f ). Hexagon end planes at the ends of the core nanowire ( FIG. 2 ( e )) are identified in the SEM images, showing that the core nanowire grows along the [111] zone axis of In ₂ O ₃ . When this type of nanostructure lies in the observation plane, hexagon end planes are observed at the end of nanorods ( FIG. 2 ( f )). The observation indicates that the nanorods grow along the [0001] zone axis of ZnO. The orientation relationship determined by the facets appeared in FIG. 2 ( e ) and FIG. 2 ( f ) is also listed in Table 1. 1

[0095] Several orientation relationships between ZnO nanorods and In ₂ O ₃ nanowire are found. The orientation relationships, as schematically shown in FIG. 2 ( h ) and FIG. 3 ( g ) as listed in Table 1 are understood using the theory of the near coincidence-site lattice. For example, the In ₂ O ₃ a plane is 4-fold symmetry with a=10.18 Å and the ZnO c plane is 6-fold symmetry with a=3.24 Å, which results in a lattice mismatch to about 3.7% (a factor of 3 for In ₂ O ₃ a axis to ZnO a axis), a reasonable value for epitaxial growth.

[0096] The heteroepitaxial nature of ZnO nanorods from In ₂ O ₃ cores gives several possible crystal orientation relations between the cores and nanorods, thus resulting in several different ZnO nanorods orientations with respect to the core. Therefore, the symmetry of these hierarchical nanostructures is dependent on the crystallographic orientation of the In ₂ O ₃ core nanowires. The orientation of the In ₂ O ₃ nanowire along the [110] or [111] direction creates 6-fold symmetries, whereas the orientation of the In ₂ O ₃ nanowire along the [001] direction produced 4-fold symmetries. No catalyst is used in this system. Therefore, the In ₂ O ₃ nanowire growth is based on the vapor-solid mechanism. Compared to the aligned ZnO grown by vapor-liquid-solid mechanism with source temperature of about 900° C., the metal and/or metal oxide vapor pressure here is much higher. This high vapor pressure is optimal for the growth of the hierarchical structures. The growth conditions such as temperature, pressure and source component ratios are correlated to affect the supersaturation rate and the structure formed.

[0097] In yet another embodiment, the invention provides Zn—In—O nanostructures having secondary ZnO nanorods grown on core nanowires/nanobelts. Zn—In—O nanostructures having secondary ZnO nanorods grown on core nanowires/nanobelts, are synthesises by thermal vaporization and condensation. In particular, 2-fold, and 6-fold ZnO nanonail hierarchical nanostructures can also been synthesized by reducing the synthesis pressure of the thermal vaporization and condensation. FIG. 36 ( a ) shows the large amount of 2-fold, and 6-fold ZnO nanonail hierarchical nanostructures on graphite foil. The structures are in length of tens of microns and width in about 1-2 microns. FIG. 36 ( b ) is the SEM image of a 2-fold structure in which the nanonails grow on both sides of the core nanobelt. The nanonails have a tip diameter of about 200 nm and length of about 0.5 micron to one micron. FIG. 36 ( c ) shows another embodiment of the 2-fold structure in which the nanonails grow on one side of the core nanobelt. Factors such as surface nucleation and vapor access to the surface effect formation of such structures. From the geometry of the structures, it is shown that the 2-fold structure grows along the [11{overscore (2)}0] direction and the nanonails grow epitaxially from the (0001) surface of the major core nanobelt along the [0001] direction. FIG. 36 ( d )shows the 6-fold nanostructure with nanonails as the secondary branches. FIG. 36 ( e ) shows the low magnification TEM image of the 2-fold structure in which the branched nanonails are dense. FIG. 36 ( f ) shows the high magnification TEM image of the core nanobelt with a contrast in the center caused by the indium rich composition.

[0098] In yet another embodiment, the invention provides substantially pure 2-fold hierarchical ZnO nanostructures wherein multiple rows of ZnO nanorods grow on the nanobelt surface. Substantially pure 2-fold hierarchical ZnO nanostructures where multiple rows of ZnO nanorods grow on the nanobelt surface when the amount of the In ₂ O ₃ in the metal oxide source mixture is reduced. FIG. 37 ( a ) shows a SEM image of large amount of such structures. The structure is tens of micron long and microns in width. FIG. 37 ( b ) shows the high magnification SEM image of such structures, wherein the ZnO nanorods have a diameter from about 50 nm to about 1000 nm and have a length from about 100 nanometers to several micrometers. FIG. 37 ( c ) shows the TEM bright field image of such structures The dark contrast dots represent the secondary nanorods. The associated SAD pattern [0001] direction illustrates nanobelts grown along the [0{overscore (1)}10] direction. The associated SAD pattern that the secondary nanorods are epitaxial with the core nanobelts.

[0099] In yet another embodiment, the invention provides hierarchical nanostructures synthesized using a ZnO, SnO ₂ and graphite powder mixture as the metallic oxide source. Such structures are Sn-doped ZnO structures. FIG. 38 ( a ) shows the SEM image of the structures typically obtained. The majority of the materials are nanobelts. Such nanobelts comprise nanorods welded together, and are alternatively straight or twisted. The individual nanorods are alternatively either perpendicular to the linear axis of the nanobelt growth direction as shown in FIG. 38 ( b ) or form non-perpendicular angles to the linear axis of the nanobelt. FIG. 38 ( c ) shows a nanobelt with three rows of nanorods growing on one side of the nanobelt surface. The middle row of the secondary nanorods is perpendicular to the nanobelt surface and the other two rows form an angle of about 45 degrees to the linear axis of the nanobelt. FIG. 38 ( d ) shows a nanobelt with only one row of ZnO nanorods perpendicular to the to the linear axis of the nanobelt surface, Such a structure is a four fold structure. The nanorods in FIG. 38 ( c ) and FIG. 38 ( d ) are grown from the core line of the nanobelt which is also the junction of the individual nanorods. FIG. 38 ( e ) shows the 2-fold structure wherein only one side of the core nanorod has secondary nanorods. FIG. 38 ( e ) shows tree leaf-like structure.

[0100] The present invention also provides hierarchical nanostructures having a ternary composition wherein the symmetric metal oxide material is formed form three metallic oxides. In one embodiment, ZnO, GeO ₂ , In ₂ O ₃ and graphite powder mixture are used as the metallic oxide source. FIG. 39 ( a ) shows a large amount of random hierarchical nanostructures obtained from the ZnO, GeO ₂ , In ₂ O ₃ and graphite powder mixture. The secondary nanorods grow on the major core nanorod without any order. FIG. 39 ( b ) is the higher magnification image of the structure. Typically the structures have a length of tens of microns. Typically, the core nanorods have a diameter of about 100 nm. The secondary nanorods are in smaller diameter and the length varies from about 100 nm to about 100 μm.

[0101] FIG. 40 ( a ) shows the large amount of such structures and FIG. 40 ( b ) shows a high magnification image of such structures. The comb-like nanostructure grows along the [11{overscore (2)}0] direction, which is different from the [01{overscore (1)}0] direction which is common in the hierarchical structure formed from binary sources.

[0102] In yet another embodiment, the present invention provides symmetric metal oxide materials formed from a MgO metallic oxide source. FIG. 41 ( a ) shows an image of MgO 2-fold hierarchical nanostructures, wherein a row of nanorods is substantially perpendicular to the core nanobelt.

[0103] The Zn—Sn—O and Zn—Sn—Ge—O nanostructures are also formed in two steps. The Zn—Sn—O core nanobelt forms first as shown in FIG. 38 ( a - d ) (core nanowire for FIG. 38 ( e - f )), then the secondary nanorods grow on the nanobelt epitaxailly. The same two-step growth also applies to the MgO 2-fold structure.

[0104] In yet another embodiment, the invention provides metal oxide materials having a nanobridge or nanonail nanostructural morphology. FIG. 10 ( a ) shows the SEM image of the ZnO nanobridges rich area. Random 3D ZnO nanostructures are observed in the same area. The typical nanobridges can be tens of microns long, and up to microns in width and height. Two rows of nanorods grow at the edge of the belt along the belt growth direction and the nanorods are typically substantially perpendicular to the belt surface. The nanorods can grow on either one side of the belt, or both sides of the belt. Nanorods have diameters ranging from about 50 to about 200 nm and lengths ranging from several hundred nanometers to about 2 μm. The density of nanorods on the belt varies. FIG. 10 ( b ) is the side view of part of a nanobridge. The nanobelt is several hundred nanometers thick and the nanorods are uniform in length and diameter. FIG. 10 (