Patent Grant
8723407
The present application is directed to the field of nanostructures, for example nanowires and methods of nanowire growth.
Self-assembled nanostructures have a wide range of potential applications in the areas of optics and electronics. For example, arrays of self-assembled nanostructures can be used in applications such as nanoscale transistors, sensors, light emitting devices, and field emitting devices. These and other applications can benefit from nanostructure synthesis techniques that permit nanostructure alignment, allow for control of nanostructure size, and enable selective growth of nanostructures while reducing cost and facilitating large-scale fabrication.
Current state of the art is deficient in several respects. For example, methods using evaporated metal films as a catalyst for nanostructure growth typically lack control over the size and density of the grown nanostructures and can generate significant metal waste.
Thus, there is a need to develop new methods for nanostructure growth and nanostructure assembly.
The present disclosure describes nanostructures, functionalized nanoparticles, and embodiments of a method of nanostructure synthesis using functionalized catalyst particles.
Disclosed herein are embodiments of a method for selectively depositing nanowires on a support layer. In one embodiment, a monolayer of catalyst particles is deposited on the support layer and nanostructure growth is catalyzed by the catalyst particles. In one aspect of the embodiment, the catalyst particles are functionalized to selectively bond to the support layer. In some examples, the support layer is a patterned support layer. In some embodiments, the catalyst particles are nanoparticles modified with a functionalized thiol ligand shell. In some examples, the catalyst particles are functionalized with a phosphonic acid.
Patterned arrays of self-assembled nanostructures are also disclosed herein. In some examples, nanostructures are attached to catalyst particles via linker molecules. In some examples, the linker molecule includes a thiol group and a phosphonic acid group. In some examples, the nanostructures are zinc oxide nanowires and the catalyst particles are gold nanoparticles.
Nanoscale devices including nanostructure arrays are also described. Disclosed nanoscale devices can include patterned arrays of self-assembled nanowires. In some examples, the nanowires are chemically bonded to nanoparticles via ligands.
The foregoing and other objects, features, and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.
As used in this application and in the claims, the singular forms “a,” “an,” and “the” include the plural forms unless the context clearly dictates otherwise. Additionally, the term “includes” means “comprises.” Further, the term “coupled” means electrically, electromagnetically, or optically coupled or linked and does not exclude the presence of intermediate elements between the coupled items.
The described systems, apparatus, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another. The disclosed systems, methods, and apparatus are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed systems, methods, and apparatus require that any one or more specific advantages be present or problems be solved.
Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus. Additionally, the description sometimes uses terms like “produce” and “provide” to describe the disclosed methods. These terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art.
Theories of operation, scientific principles, or other theoretical descriptions presented herein in reference to the apparatus or methods of this disclosure have been provided for the purposes of better understanding and are not intended to be limiting in scope. The apparatus and methods in the appended claims are not limited to those apparatus and methods which function according to scientific principles or theoretical descriptions presented herein.
In general, a nanostructure is an object or structure having one or more dimensions on the nanoscale such having a length scale in a nanometer or micrometer range. For example, a nanostructure can have one or more dimensions which are between about 0.1 nm and about 10 μm. As used herein, a “nanowire” is a nanostructure having at least two dimensions that are on the nanoscale. For example, a two-dimensional cross-section of a nanowire can be on the nanoscale and much smaller than a third dimension or length of the nanowire. Nanowires can have a range of lengths and effective diameters and nanowires can have cross-sections of various shapes. For example, nanowires can have circular, square, trapezoidal, or other shaped cross-sections which can be characterized by an effective diameter. In some embodiments, a nanowire can have an average length between about 40 nm and about 20 μm, such as between about 100 nm and about 2 μm. In some examples, nanowires can have lengths greater than 20 μm. A nanowire can have an average effective diameter between about 10 nm and about 400 nm, such as between about 20 nm and about 200 nm. A nanowire has an aspect ratio (i.e., a ratio of the structure length to the structure width) that is greater than 1 and typically greater than about 5, such as between about 5 and about 100, or greater than about 50, such as from about 50 to 200. In some embodiments, nanowires have an aspect ratio larger than 200, such as between about 200 and 2000.
As used herein, a “nanoparticle” is a particle having a diameter of less than about one micron. In some embodiments, nanoparticles can have a diameter of from about 0.5 nm to about 500 nm, such as from about 0.7 nm to about 5 nm or from about 5 nm to about 200 nm.
Disclosed nanostructures and embodiments of the disclosed method for nanostructure growth are not limited to nanostructures of a particular material, though particular materials are described herein. Exemplary nanostructure materials include nitrides, carbon, and oxides such as zinc oxide or other metallic oxides. In general, disclosed nanostructures can be generated based on a catalyst growth mechanism and therefore include materials that can be catalyzed for nanostructure growth.
Any catalytic methods for nanostructures growth can be used with or combined with disclosed embodiments. Exemplary catalytic methods of nanostructure growth include, but are not limited to, vapor-liquid-solid (VLS) methods, vapor-solid (VS) methods, and chemical vapor deposition (CVD) methods. Therefore, in some embodiments of the disclosed method, nanostructures can be grown rapidly and without the need for vacuum systems.
In general, once a nanostructure material is chosen, an appropriate catalyst material and an appropriate support layer composition can be determined, and the determination can be based on a catalytic growth method, on the nanostructure material chosen, or both. Disclosed embodiments can include any possible nanostructure material/catalyst combinations. For example, catalyst particles can include Group VI metals, Group VII metals, gold, copper, silver, and combinations thereof. In some embodiments, catalyst particles are nanoparticles.
In general, a support layer is a layer of material that provides a surface for nanostructure growth. A support layer can include a substrate, the support layer can be a substrate, or the support layer can be a layer of material deposited onto a substrate. For example, the support layer can be in contact with the substrate or with other materials that have been deposited or otherwise formed or situated on a prepared substrate. A substrate can include silicon, silicon dioxide, titanium, sapphire, and combinations thereof. For example, the substrate can be a silicon substrate having a layer of silicon dioxide on a surface of the substrate. The support layer can include a nanostructure precursor. In some examples, the support layer includes an oxophilic metal or an oxide such as hafnium-modified silicon dioxide, indium tin oxide, or zinc oxide. For example, the support layer can be a zinc oxide film deposited onto a substrate. The support layer, the substrate, or both can be patterned such as by using lithography techniques.
Exemplary nanostructure material/catalyst/support layer combinations include carbon nanotubes grown from nickel catalyst particles on a titanium or silicon substrate using plasma enhanced CVD (PE-CVD), and zinc oxide nanowires grown from gold catalyst nanoparticles on a zinc oxide support layer using VLS growth mechanism.
Typically, once a suitable catalyst material is selected, catalyst particles can be synthesized and subsequently functionalized to enable the catalyst particles to selectively bond with a surface. For example, catalyst particles can include functional groups that enable the particles to selectively couple to a support layer. Disclosed nanostructures and embodiments of the disclosed method for nanostructure growth are not limited to catalyst particles with particular functionalities. In general, any functional group/support layer, or ligand/support layer, interactions can be used with disclosed nanostructures and in embodiments of the disclosed method.
In general, a functionalized catalyst particle includes a particle and a linker molecule. Linker molecules can include one or more ligands and such ligands can be for bonding to catalyst particles, for bonding to a support layer, or both. In general, particles can be functionalized by directly forming such particles having the appropriate ligands attached thereto. Nanoparticles can be functionalized by first forming ligand-stabilized nanoparticles, which act as precursors for ligand exchange reactions. Ligand-stabilized nanoparticles generally include a nanoparticle having one or more exchangeable ligands.
Ligand exchange reactions form functionalized nanoparticles by replacing stabilizing ligands with ligands that are more useful for coupling nanoparticles to a support layer. To perform ligand-exchange reactions, typically, a reaction mixture is formed comprising the nanoparticle having exchangeable ligands attached thereto and the ligands to be attached to the nanoparticle, such as thiols. A precipitate generally forms upon solvent removal, and this precipitate is then isolated by conventional techniques.
Ligands suitable for forming nanoparticles may be selected, without limitation, from the group consisting of sulfur-bearing compounds, such as thiols, thioethers, thioesters, disulfides, and sulfur-containing heterocycles; selenium bearing molecules, such as selenides; nitrogen-bearing compounds, such as 1°, 2° and perhaps 3° amines, aminooxides, pyridines, nitriles, and hydroxamic acids; phosphorus-bearing compounds, such as phosphines; and oxygen-bearing compounds, such as carboxylates, hydroxyl-bearing compounds, such as alcohols, and polyols; and mixtures thereof. Particularly effective ligands for metal nanoparticles may be selected from compounds bearing elements selected from the chalcogens. Of the chalcogens, sulfur is a particularly suitable ligand, and molecules comprising sulfhydryl moieties are particularly useful ligands for stabilizing metal nanoparticles. Additional guidance concerning the selection of ligands can be obtained from Michael Natan et al's Preparation and Characterization of Au Colloid Monolayers, Anal. Chem. 1995, 67, 735-743, which is incorporated herein by reference.
Sulfur-containing molecules (e.g., thiols, sulfides, thioesters, disulfides, sulfur-containing heterocycles, and mixtures thereof) comprise a particularly useful class of ligands. Thiols, for example, are a suitable type of sulfur-containing ligand for several reasons. Thiols have an affinity for gold, and gold, including gold particles and gold nanoparticles, may be used as a catalyst in nanostructure growth. Moreover, thiols are good ligands for stabilizing gold nanoparticles, and many sulfhydryl-based ligands are commercially available. The thiols form ligand-stabilized metal nanoparticles having a formula Mx(SR)n wherein M is a metal, R is an aliphatic group, typically an optionally substituted chain (such as an alkyl chain) or aromatic group, x is a number of metal atoms that provide metal nanoparticles, and n is the number of thiol ligands attached to the ligand-stabilized metal nanoparticles.
A linker molecule is adapted to bind to a substrate, a support layer, and/or an oxophilic metal deposited thereon, thereby linking the nanoparticle to the support layer. Functionalized nanoparticles include a wide variety of nanoparticles of the general formulas CORE-L-(S—X)n, wherein L is a linker and X is a functional group or chemical moiety that serves to couple the nanoparticle to a the support layer, and n is at least one.
For example, X may include without limitation phosphonic acid groups, carboxylic acid groups, sulfonic acid groups, peptide groups, amine groups, and ammonium groups. Other functional groups that may be part of X include aldehyde groups and amide groups. Functionalized nanoparticles can be prepared from phosphine-stabilized nanoparticles of the formula CORE-(PR3)n, where the R groups are independently selected from the group consisting of aromatic, such as phenyl and aliphatic groups, such as alkyl, typically such alkyl groups have 20 or fewer carbons, for example, cyclohexyl, t-butyl or octyl, and n is at least one.
Linker molecules can be bifunctional. Such linker molecules have one functional group adapted to coordinate such as covalently or non-covalently bond with a nanoparticle and a second functional group adapted to coordinate to a support layer. The first and second functional groups may be the same or different. One example of such bifunctional linker molecules has the formula
wherein R comprises an aliphatic group. R can include a lower alkyl group, and/or an aryl group, such as a phenyl or biphenyl moiety or R can represent an alkylene group, optionally interrupted with one or more heteroatoms, such as oxygen or nitrogen. Examples of such alkylene groups interrupted with oxygen include polyethylene glycol (PEG) and/or polypropylene glycol (PPG) chains. As used herein, PEG and PPG refer to oligomeric groups having as few as two glycol subunits. Exemplary R groups include, without limitation, —CH2CH2—, —CH2CH2OCH2CH2— and —CH2CH2OCH2CH2OCH2CH2—.
In some embodiments, functionalized catalyst particles are catalyst particles modified with a terminally functionalized ligand shell such as a thiol ligand shell. Because terminal functionality of a ligand shell can dictate the interactions that occur between the catalyst particles and the substrate or support layer, a functional group can be chosen based on properties of a substrate and/or a support layer. In one embodiment, catalyst particles are functionalized with a phosphonic acid such as alkylphosphonic acid or 2-mercaptoethylphosphonic acid (2-MEPA). For example, a phosphonic acid functionalized gold nanoparticle can have a thiol group attached to the gold nanoparticle and a phosphonic acid group attached to the thiol group, wherein the phosphonic acid group selectively binds to a support layer. In this example, the functionalized catalyst particle includes a linker molecule and a gold nanoparticle, wherein the linker molecule includes a thiol group and a phosphonic acid group.
Functionalized gold nanoparticles can be produced using methods described by Weare, W. W.; Reed, S. M.; Warner, M. G.; Hutchison, J. E., J. Am. Chem. Soc. 2000, 122, 12890-12891, which is incorporated herein by reference, and Hutchison, J. E.; Foster, E. W.; Warner, M. G.; Reed, S. M.; Weare, W. W., in Inorg. Syn.; Buhro, W., Yu, H., Eds., 2004; Vol. 34, pp 228, which is incorporated herein by reference.
Methods disclosed herein can be optionally combined with methods disclosed in PCT Patent Application No. PCT/US2006/018716, entitled METHOD FOR FUNCTIONALIZING SURFACES, filed May 12, 2006, in the names of James E. Hutchison, Christina E. Imnan, Gregory J. Kearns and Evan Foster, which is incorporated herein by reference in its entirety. Also incorporated herein by reference is PCT Patent Application No. PCT/US2006/019861, entitled NANOPARTICLES AND METHOD TO CONTROL NANOPARTICLE SPACING, and filed May 22, 2006.
With respect to
At 120, the support layer is contacted with functionalized catalyst particles. For example, the support layer can be contacted with a solution containing functionalized catalyst particles. The functionalized catalyst particles include catalyst particles that have been functionalized to selectively chemically anchor to the support layer. For example, the functionalized particles can be functionalized to bond with the support layer and to generally not bond with the substrate.
The functionalized catalyst particles that bond with the support layer can be referred to as selectively deposited particles. The selectively deposited particles can be considered to be self-assembled because the particles assemble or couple to the support layer based on functional groups coupled to the particles. The selectively deposited particles can form a monolayer of catalyst particles on the support layer. The catalyst particles include materials that can catalyze nanostructure growth on the support layer.
At 130, nanostructures are grown. The growth of the nanostructures is assisted by the catalyst particles, and the nanostructures can be grown using any catalytic method of nanostructure growth. Nanostructure size, such as nanostructure height and diameter, can depend on parameters of the chosen catalytic method of growth and on the size of the functionalized catalyst particles. In some examples, the size and density of nanostructures can be tuned such as by controlling catalyst particle diameter, linker molecule size (e.g., ligand shell length), growth temperature, and growth time.
At 210, a substrate is patterned. The substrate can be patterned using lithographic techniques. For example, a layer of resist can be deposited onto the substrate and patterned using photolithography techniques.
At 220, a support layer film is deposited onto the patterned substrate. The support layer can be deposited using standard deposition techniques. For example, the patterned substrate can be spin-coated with the support layer material.
At 230, at least a portion of the support layer film is lifted-off from the patterned substrate. For example, the patterned substrate, including the deposited support layer film, can be contacted with a solvent capable of dissolving resist on the substrate. Portions of the support layer film deposited onto the resist are released from the substrate as the resist dissolves, while portions of the support layer film in contact with the substrate are generally not removed by the solvent. The resulting substrate can include portions of exposed substrate and remainder portions of the support layer that were not lifted-off.
At 240, the substrate including remainder portions of the support layer film is contacted with functionalized catalyst particles. The catalyst particles are functionalized to selectively anchor to the remainder portions of the support layer and not to the substrate.
At 250, nanostructures are grown on the remainder portions of the support layer, wherein the growth is catalyzed by the selectively anchored catalyst particles. In this manner, the remainder portions of the support layer provide a user defined template for nanostructure growth.
Using method 200, nanostructures can be selectively placed or grown on a surface without patterning a catalyst film.
With regard to
At 330, the substrate is contacted with a solvent to dissolve the patterned resist layer. Portions of the zinc oxide film that were deposited onto the resist layer are lifted off during contact with the solvent, thereby forming a patterned zinc oxide seed layer on the substrate surface. The patterned zinc oxide seed layer can be annealed to form a patterned c-oriented zinc oxide seed layer. Seed layers having other orientations can also be used. At 340, the patterned zinc oxide seed layer is contacted with functionalized gold nanoparticles. The gold nanoparticles are functionalized such that they selectively anchor to the zinc oxide seed layer and not to the substrate surface. For example, the gold nanoparticles can be functionalized with a phosphonic acid. At 350, zinc oxide nanowires are grown using vapor-liquid-solid techniques, wherein the gold nanoparticles catalyze the nanowire growth.
i)-4(iii) include elevational and cross-sectional views that illustrate substrate processing during three stages of zinc oxide nanowire growth according to embodiments of the method described herein.
ii) illustrates chemically anchored gold nanoparticles 420-423 forming a selectively deposited monolayer on a surface 412 of the patterned zinc oxide seed layer 410. The gold nanoparticles 420-423 are functionalized with 2-MEPA and selectively anchor to the patterned zinc oxide seed layer 410 and not to a substrate surface 414.
In some examples described herein, the amount of catalyst material used during nanostructure synthesis is greatly reduced from conventional methods. The examples demonstrate that less catalyst waste can be generated and, therefore, nanostructure device production costs can be potentially reduced. For example, when only sufficient catalyst material to cover a seed layer such as a patterned seed layer is used, the amount of generated catalyst waste can be much smaller than methods such as vapor deposition in which a catalyst film is deposited over an area larger than the seed layer.
In this example, zinc oxide nanowires were grown using self-assembled arrays of gold nanoparticles. First, the gold nanoparticles were synthesized and modified with a terminally functionalized thiol ligand shell. Then, a patterned zinc oxide seed layer was prepared on a substrate and the substrate was immersed in an aqueous solution of gold nanoparticles. The gold nanoparticles selectively deposited onto the patterned zinc oxide seed layer and zinc oxide nanowires were grown by a vapor-liquid-solid method.
To synthesize the gold nanoparticles, hydrogen tetrachloroaurate trihydrate (HAuCl4.3H2O, 1.00 g, 2.54 mmol) in water was reacted with triphenylphosphine (PPh3, 2.33 g, 8.88 mmol) in toluene in the presence of the phase-transfer catalyst tetraoctylammonium bromide (TOAB, 1.40 g, 2.56 mmol). Reduction with NaBH4 (1.99 g, 52.6 mmol) yielded PPh3-stabilized gold nanoparticles with an average diameter of about 1.5 nm (1.5 nm±0.4 nm).
Ligand exchange was performed by dissolving the PPh3-stabilized gold nanoparticles in dichloromethane and mixing the dissolved nanoparticles with one mass equivalent of 2-mercaptoethylphosphonic acid (2-MEPA) dissolved in water. The ligand exchange solution was stirred for 48 hours. When an organic layer in the ligand exchange solution was nearly colorless, an aqueous layer was separated, washed with dichloromethane, and purified by diafiltration using a 10 kD diafiltration capsule (from Pall Life Sciences) and approximately 50 volume equivalents of deionized water. The resulting nanoparticle solution included gold nanoparticles functionalized with 2-MEPA. The functionalized gold nanoparticles were considered pure when no free ligand was evident after analysis using proton nuclear magnetic resonance (1H NMR) spectroscopy. The functionalized gold nanoparticles included thiol groups attached to gold nanoparticles and to phosphonic acid groups. The functionalized gold nanoparticle solution was then diluted with deionized water to achieve a desired concentration.
A zinc oxide sol-gel solution was prepared by first dissolving zinc acetate dihydrate ((CH3COO)2Zn.2H2O) (5 g, 22 mmol) in 2.5 mL ethylene glycol. This mixture was then heated at 150° C. for 15 minutes in a condensation system. After the solution cooled to room temperature and became transparent, 8 mL 1-propanol and 0.2 mL glycerol were added, followed by 5 mL triethylamine and 0.1 mL water. The resulting solution was stirred at 35° C. for 30 hours. This sol-gel precursor solution was then diluted with isopropanol to a concentration of 50 mM.
A silicon substrate (a 1 cm2 wafer) possessing a 3-μm layer of SiO2 was patterned using photolithographic patterning techniques. A zinc oxide seed film was then deposited onto the patterned substrate by spin-coating the wafer with the sol-gel precursor solution at 3000 rpm for 60 seconds. Lift-off of photoresist was performed using standard techniques to create a patterned seed layer on the wafer. The patterned wafers were pre-baked at 150° C. for 10 minutes to drive off any remaining solvent and then annealed at 350° C. for 30 minutes. The resulting films were patterned c-oriented zinc oxide seed layers. Patterned zinc oxide surfaces were treated with UV-ozone for 5 minutes and rinsed with deionized water to remove adventitious carbon contamination.
Wafers with patterned c-oriented zinc oxide seed layers were then immersed into a 0.25 mg/mL solution of functionalized gold nanoparticles for 10 seconds. The samples were then rinsed with copious amounts of deionized water to remove physisorbed or unbound particles and then dried under a stream of nitrogen prior to further modification or analysis. Because zinc oxide exhibits spontaneous polarization in the wurtzite structure and silicon and SiO2 have no surface charge, the functionalized gold nanoparticles assembled onto the zinc oxide surface selectively. During nanoparticle self-assembly, it is currently understood that a thiol group attaches to a phosphonic acid group and to a gold nanoparticle, and the phosphonic acid group allows the nanoparticle to anchor to the zinc oxide film and not to exposed silicon or SiO2. The resulting wafers, having selectively deposited nanoparticles, are sometimes referred to as nanoparticle modified wafers or as having a nanoparticle modified surface.
Less than 50 μL of the 0.25 mg/mL gold nanoparticle solution, or less than 12.5 μg of gold, was used in this example. Typically, 0.2 g of gold is used by methods employing an evaporation system. Therefore, the mass of gold consumed in this example was at least 16,000 times less than the mass of gold typically consumed in an evaporation system for deposition of patterned gold films.
a)-7(b) are positive ion mapping images using time-of-flight secondary ion mass spectrometry (TOF-SIMS) of Zn2+ and Au3+ ions, respectively. The TOF-SIMS images were acquired with an ION-TOF Model IV Spectrometer using a bismuth liquid metal ion gun as the primary ion beam, operated at an accelerating voltage of 5 kV. These images demonstrate that gold nanoparticles were indeed selectively bonded or anchored onto the patterned zinc oxide seed layer. The zinc oxide surface was covered by a nanoparticle layer within 10 seconds, and no gold particles were observed on the bare SiO2 substrate. The 2-MEPA ligand was, therefore, the anchoring agent between the gold nanoparticles and the zinc oxide surface.
Table 1 shows a TOF-SIMS quantitative analysis of ion yields for a series of immersion times of the substrate in a gold nanoparticle solution. The TOF-SIMS quantitative analysis provides the film composition or ion yield as a function of the immersion time. In this analysis, the peak intensities for ionic fragments of interest were divided by total ion intensity in each measurement to compare the relative compositions among samples. Peaks for Zn2+ and Au3+ positive ions and PO3− and S2− negative ions were observed even if the immersion time was only 1 second. Table 1 also demonstrates that the ratio of gold to zinc was constant and independent of immersion time, indicating that the reaction time could have been less than one second.
Zinc oxide nanowires were grown by the vapor-liquid-solid (VLS) method on the patterned and gold nanoparticle modified wafers. A mixture of zinc oxide and carbon powder was placed in a small quartz tube as a zinc oxide source, and wafers with patterned zinc oxide seed layers and selectively deposited gold nanoparticles were placed downstream from the source. The wafer temperature was controlled at 600° C. The source temperature was raised to 900° C. and held for 20 minutes in a N2 gas flow (2.5 SCFH). Then, the furnace was shut down and cooled to room temperature while maintaining the nitrogen flow.
a)-8(b) are SEM images of VLS-deposited zinc oxide nanowires, grown as described in the example.
The images in
In the example, nanowires with an average diameter of about 40 nm or with an average diameter of about 30 nm were grown, the size distribution of the diameter being less than about 10 nm. In this example, the average height of the zinc oxide nanowires was varied from about 100 nm to a few micrometers by changing the growth time. For example, nanowires with an average height about 1 μm and with an average height of about 600 nm were grown.
In this example, zinc oxide nanowires grown according to Example III are compared to zinc oxide nanostructures grown by the VLS method on a bare SiO2 substrate, on an unmodified zinc oxide seed layer, and on a modified zinc oxide seed layer on a c-sapphire substrate. The bare SiO2 substrate is immersed in a gold nanoparticle solution during preparation. The resulting nanostructures are compared in the images of
In
a) shows that no structures were produced when zinc oxide was deposited by VLS method onto a bare SiO2 substrate without a zinc oxide seed layer.
d) shows zinc oxide nanowires grown by VLS on a c-sapphire substrate with a nanoparticle-modified zinc oxide seed layer. Relative to the nanowires in
In this example, photoluminescence (PL) properties of zinc oxide nanowires grown according to Example III are compared to PL properties of zinc oxide nanostructures grown directly on a SiO2 substrate and of zinc oxide nanostructures grown on an unmodified zinc oxide seed layer. PL measurements were performed at room temperature using a xenon lamp as the light source, with an excitation wavelength of 300 nm.
Only spectra (c) in
In view of the many possible embodiments to which the disclosed principles may be applied, it should be recognized that the illustrated embodiments are only examples and should not be taken as limiting the scope of the disclosure. We claim all that comes within the scope and spirit of the appended claims.
This application claims the benefit of the earlier filing date of U.S. provisional patent application No. 61/028,144, filed Feb. 12, 2008, which is incorporated herein by reference.
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