The present application relates generally to nanostructures such as nanowires and carbon nanotubes and more particularly to branched nanostructures.
The design and controlled synthesis of complex nanostructures, such as nanowires and carbon nanotubes (“CNTs”) will impact developments in nanotechnology applications. The prior art synthesis approaches, however, limit the degree of complexity that can be controllably configured into these nanostructures. Fabrication inside rationally designed porous templates, such as anodic aluminum oxide (“AAO”) templates, may be used to produce nanostructure morphologies. However, it is believed that only linear nanostructures and Y-branched CNTs (i.e., carbon nanotubes having one stem and two branches) have been grown inside the rationally designed porous templates (see Li, J., Papadopoulos, C. & Xu, J. (1999) Nature 402, 253-254 and Papadopoulos, C., Rakitin, A., Li, J., Vedeneev, A. S. & Xu, J. M. (2000) Phys. Rev. Lett. 85, 3476-3479, both incorporated herein by reference in their entirety).
One embodiment of the invention includes a branched nanostructure, includes at least one of (a) a stem and at least two levels of branches; or (b) a stem connected to three of more branches; or (c) a nanowire nanostructure comprising a stem and two or more branches; or (d) a stem connected to two or more branches, where the stem and the branches comprise a different material composition or structure.
The embodiments of the invention provide a generic synthetic approach to rationally design multiply connected and hierarchically branched nanopores inside nanopore arrays in a template material, such as inside anodic aluminum oxide templates. By using these nanopores or nanochannels, a large variety of branched nanostructures are fabricated, and which are believed to be more complex than prior art nanostructures. These nanostructures include carbon nanotubes and nanowires. The term nanowire includes metal, semiconductor, conductive polymer (such as polyaniline, polypyrrole, etc.), insulating polymer or other insulating material nanowires, but excludes hollow carbon nanotubes. The term nanostructures also includes quasi-nanotube or quasi-nanowire structures, such as nanohorns and nanowhiskers.
The nanostructures of the embodiments of the invention may include several hierarchical levels of multiple branching or more than two branches for each stem. The number and frequency of branching, dimensions, and the overall architecture are controlled precisely through pore design and templated assembly. The technique provides a powerful approach to produce nanostructures of greater morphological complexity, which could have far-reaching implications in the design of future nanoscale systems.
The branched nanostructures of the embodiments of the invention may comprising at least one of: (a) a stem and at least two levels of branches, or (b) a stem connected to three of more branches, or (c) a nanowire nanostructure comprising a stem and two or more branches, or (d) a stem connected to two or more branches, wherein the stem and the branches comprise a different material composition or structure. The nanostructure may contain any combination of one, two, three or all four of above features. The nanostructure is preferably formed by a method which includes forming the nanostructure in a branched nanopore located in a template material, and selectively removing the template material.
The term “stem” as used herein refers to the part of the nanostructure which is formed in the nanopore before the branches. The term “branches” refer to the parts of the nanostructure which are directly or indirectly connected to the stem. For example, in a nanostructure that has two levels of branches, each of the branches in the first level are directly connected to the stem and each of the branches in the second level are connected to one of the branches in the first level. Thus, the branches in the second level are indirectly connected to the stem through the branches of the first level. Preferably the stem is connected to more than one branch of the first level, and each branch of the first level is connected to more than one branch of the second level. It should be noted that the nanostructure may contain only one level of branches.
In one embodiment, the nanostructure contains three or more branches connected to the stem. In another embodiment, the nanostructure contains a stem and two or more levels of branches, where the stem may be connected to two or more branches in the first level, and each branch in the first level is connected to two or more branches in the second level. If desired, the embodiments may be combined such that the nanostructure contains a stem and two or more levels of branches, where the stem may be connected to three or more branches in the first level, and/or each branch in the first level is connected to three or more branches in the second level.
It should be noted that the term “stem” is not limited to the bottom most part of the nanostructure. For example, each branch located in the middle of the nanostructure can be considered to be a stem with respect to the higher level branches to which it is connected. Thus, a nanostructure can have several stems. Furthermore, the original stem of the nanostructure may be removed, leaving a plurality of branches, the lowest of which becomes the new stem.
Thus, a method of making the multilevel nanopore array includes anodically oxidizing the template material 1 at a first voltage to form a first level of stem nanopores 3 in the template material. The method further includes anodically oxidizing the template material 1 at a second voltage lower than the first voltage to form a second level of branch nanopores 5 connected to the first level of stem nanopores 3. The method further includes anodically oxidizing the template material 1 at third voltage lower than the second voltage to form a third level of branch nanopores 7 connected to the second level of branch nanopores 5, and so on until a desired number of levels is formed.
The nanopore array formed by the method of the first four steps of
The structures in
Thus, as shown in
Up to four generations of Y-branching have been fabricated onto individual nanotube structures (12222), as shown in the scanning electron microscope (SEM) images in
The high-magnification image from each of these interfaces (shown in
The diameters of the primary stems and the branches depend on the corresponding anodizing voltages. For any two consecutive branches (at each interface), the ratio of the diameters is approximately √{square root over (2)}, as seen from the Figures. The details of diameter evolution during anodization at different voltages is provided in Table I below. The diameter ratios of the smallest branch and the primary stem in several of the architectures that have been fabricated are presented in
Next, in another embodiment, the present inventors generated templates where the individual pores divide into predetermined multiple numbers of branches, such as three or more branches. The above described process allows growth of nanotubes and nanowires with a predetermined numbers of branches, such as more than two branches. The anodizing voltage controls the pore size and pore density during the anodization, since the pore diameter is proportional to the anodizing voltage. A simple calculation, based on the fact that the original total area of the template will not change during the anodization, shows that the anodizing voltage to form a number of (n) smaller branch pores from a single stem pore can be expressed as (1/√{square root over (n)})×Vs, where Vs is the anodizing voltage for stem pores, and n is the number of branch pores that branch away from that stem. Based on this rationale, the present inventors have successfully prepared AAO templates with different numbers of branch pores emanating from individual stem pores and grown nanotubes in them. Once again, the precise location (depth) inside the template where the branching occurs is controlled by the sequence and timing of voltage reduction, and the branching can be made to occur abruptly or gradually based on the voltage-reduction procedure.
Thus, in reference to
The term “about the same diameter” include exactly the same diameters and diameters which differ by a small amount due to inherent small spatial non-uniformity during anodization.
In another embodiment, a combination of Y-shapes and multiple branches can lead to a wealth of new nanoscale architectures. This configuration achieved by reducing the anodizing voltages in steps, by factors of 1/√{square root over (2)} and 1/√{square root over (n)} (where n>2) sequentially, generating Y-shapes and n-branched pores in the template consecutively. The sequences can be interchangeable (for example, the stem can be split into multiple branches first, and each of the branches can subdivide as Y-shapes or vice versa) and recurring (several levels) so that many complicated nanostructures become possible, as illustrated for example in
Additional branched nanostructures are shown in
Typically, multi-walled nanotubes are formed in the AAO templates because the smallest pore size that can be developed using AAO templates is about 10 nm, which is much greater than a single-walled nanotube diameter. However, the nanotubes made in the pores have very few walls, and, theoretically, the number of walls may be controlled (to a single layer) by controlling the deposition time. Alternatively, single-walled nanotubes (“SWNT”) or SWNT bundles may be deposited by seeding small catalyst particles within or at the bottom of the pores.
Thus, by using the templates with tailored pores, various nanostructures, such as nanotubes and nanowires can be grown. This use of the tailored pores in a template material serves as a generic method for creating complex nanowires of most materials that can either be deposited by means of vapor phase deposition or electrodeposition. The following references, which are incorporated herein by reference in their entirety, describe nanostructure deposition by vapor phase deposition: Li, J., Papadopoulos, C. & Xu, J. (1999) Nature 402, 253-254; Davydov, D. N., Sattari, P. A., AlMawlawi, D., Osika, A., Haslett, T. L. & Moskovits, M. (1999) J. Appl. Phys. 86, 3983-3987; and Sui, Y. C., Cui, B. Z., Martinez, L., Perez, R. & Sellmyer, D. J. (2002) Thin Solid Films 406, 64-69. The following references, which are incorporated herein by reference in their entirety, describe nanostructure deposition by electrodeposition: Martin, C. R. (1994) Science 266, 1961-1966; Routkevitch, D., Tager, A. A., Haruyama, J., Almawlawi, D., Moskovits, M. & Xu, J. M. (1996) IEEE Trans. Electron Devices 43, 1646-1658; Huczko, A. (2000) Appl. Phys. A 70, 365-376; Schmid, G. (2002) J. Mater. Chem. 12, 1231-1238; and Choi, J., Sauer, G., Nielsch, K., Wehrspohn, R. B. & Gosele, U. (2003) Chem. Mater. 15, 776-779.
Insulating, semiconducting, and polymeric materials also may be controllably synthesized into the complex nanowires by using the above templates with infiltration processes described in the above references and in Kovtyukhova, N., Mallouk, T. E. & Mayer, T. (2003) Adv. Mater. 15, 780-785; and Park, S., Lim, J. H., Chung, S. W. & Mirkin, C. A. (2004) Science 303, 348-351.
In another embodiment, in addition to single-component nanowire and nanotube architectures, it also should be possible to make hetero-nanowire junctions, for example by electrodepositing metal nanowires in the stems and then growing nanotubes or other material nanowires as the branches. Thus, the stem comprises one of nanowire or nanotube material and the branches comprise the other one of nanowire or nanotube material. Alternatively, the stem and the branches may comprise nanowires of a different material composition and/or structure. This configuration includes different level branches having a different material composition and/or structure, where the lower branches are viewed as the stems for the upper branches. In multilevel nanostructures, the stem and each level of branches may be made of a different material composition and/or structure.
For example, the stem may comprise a nanowire made of one metal, polymer, semiconductor or other insulating material which the branches may comprise a nanowire made of a different metal, polymer, semiconductor or other insulating material. This difference in material composition may be embodied in a different type of material (for example, metal stem and polymer branches) or in different materials of the same type (for example, GaAs semiconductor stem and InGaAs semiconductor branches).
Furthermore, the difference in composition may be embodied in a different doping composition or concentration of the stem and branches. Thus, the stem and branches may comprise the same nanowire or nanotube material, but doped with a different dopant and/or containing a different concentration of the same dopant and/or where one of the stem or branches is undoped and the other one is doped.
For example, in a Y-branched semiconductor nanowire or semiconductor nanotube, the stem can be low doped with a dopant of one conductivity type (i.e., p or n) and the two branches can be highly doped with a dopant of the opposite conductivity type (i.e., n or p). In this case, the nanostructure acts as a p-n-p or n-p-n diode or as a bipolar transistor. Alternatively, the stem acts as the channel and the branches act as source and drain regions of a field effect transistor with an additional gate electrode being provided near or around the channel stem. In this case, the stem may be low doped and the branches may be highly doped. The branches are then connected to separate electrodes. A 13 nanowire structure may act as a complete field effect transistor with the middle branch acting as a gate electrode, the end branches acting as source and drain regions and the stem acting as a channel, if the middle branch can be formed to avoid physical contact with the end branches. It may be desirable to implement a separate doping step to dope the “gate” branch with an opposite conductivity dopant type from the “source and drain” branches.
For multilevel nanowires and semiconductor nanotubes, the middle level branches may have one doping type (p or n) to act as a middle of a diode, or as a base of a bipolar transistor, or as a channel of a field effect transistor, and the stem and the upper level branches may have an opposite doping type (i.e., n or p) to act as ends of a two junction diode, or as emitter and collector regions of a bipolar transistor, or as source and drain regions of a field effect transistor. A separate gate electrode may be provided near or around the middle level branch of the structure to complete the field effect transistor. Of course the stem and branches may also be made of different semiconductor materials to make a heterojunction diode or transistor if desired.
Alternatively, the stem and the branches may be made of the same material but may have a different structure. Different structure includes different crystal structure, different grain size for polycrystalline nanowires, different number of walls for multi-walled nanotubes, different chiralities for nanotubes, etc.
In summary, a powerful, rational, synthetic approach for the design and fabrication of hierarchical nanopore/nanostructure architectures is provided. The nanopore architectures should complement materials, such as zeolites, that contain interconnected ordered pore frameworks of different dimensionality, chemistry, and structure. The rational approach for creating hierarchically branched ordered nanoporous AAO templates allows fabrication of a whole generation of branched nanowires and nanotubes inside these templates.
The nanostructures described herein should open up new opportunities for both fundamental research and building of various nanoscale architectures for applications. The hierarchically branched nanotube/nanowire constructs with tree-like morphology could impart similar functions as polymer dendrimers, which are used to build large supramolecular constructs for applications such as drug delivery. In other words, the individual branches can be differentially chemically functionalized and terminated to create complex multiple chemical sensors in one unit. Such constructs also can be the core structure to build complex nanoscale biomaterials. The multiply branched nanotube/nanowire architectures could be key to building components of complex nanoelectronics circuits and nanoelectromechanical systems.
It should be noted that the branched nanopore arrays may be used without growing the nanostructures, such as nanotubes or nanowires in the nanopores. For example, the ordered straight pore arrays of traditional AAO templates have been used effectively to build flow-through-type DNA arrays. The template structures with hierarchically branched nanopores may have applications in biotechnology, such as nanoscale separation technologies, and in fundamental diffusion studies where the multiply divided pores can act as selective barriers in a multicomponent diffusion process.
The following exemplary materials and methods are provided for illustration of the embodiments of the invention should not be considered to be limiting on the scope of the invention.
Preparation of Templates. Anodically oxidized alumina (“AAO”) is a preferred template material. However, other metals, such as scandium or niobium, which can be anodically oxidized to form a controlled nanopore array can be used. In the specific examples of the present invention, AAO templates are prepared by using a two-step anodization process. The first-step anodization is the same for all templates. High-purity Al foils are anodized in 0.3 M oxalic acid solution at 8-10° C. under a constant voltage (in the range of 40-72 Vdc) for 8 h. Then, the formed anodic aluminum layer is removed. In the second-step anodization, templates with different pore architectures undergo different processes of anodization as follows.
AAO Templates with Multiple Levels or Generations of Y-Branched Pores. The anodizing voltage is reduced multiple times (i.e., more than twice) in the second-step anodization. Initially, the anodization is performed under the same conditions as those in the first step to create the primary stem pores. Then, the anodizing voltage is reduced by a factor of 1/√{square root over (2)} to form Y-branched pores (i.e., a stem pore connected to two branch pores). Two-, three-, and four-generation or level Y-branched pores can be obtained by further sequential reduction of anodizing voltages by a factor of 1/√{square root over (2)}, over prior voltages. It is noted that if a subsequent anodizing voltage is ≦25 V, after any prior anodization, the samples should be washed in deionized water for about 30 min. to clean the remaining oxalic acid solution in the pores, and then the anodization should be conducted in 0.3 M sulfuric acid at the same temperature used previously.
AAO Templates with Multiply Branched Pore Structure. To form templates with three or more branch pores for each stem pore, after the initial anodization to form the stem pores, the anodizing voltage is reduced by a factor of 1/√{square root over (n)} to create multiply branched pores containing n branches. For n>2, there are more than two branch pores for each stem pore. If the voltage is reduced slowly, the stem pores divide branched pores gradually (at several depths), but if the voltage is reduced suddenly, the stem pores will be divided abruptly (sharp interface). Typically, after the anodization for the stem pores, the remaining oxalic acid solution in the pores is cleaned in deionized water and the barrier layer at the pore bottom is thinned by immersing the samples in a 5% (wt) phosphoric acid solution at 31° C. for 30-70 min. It should also be noted that if the anodizing voltage for branched pores is ≦25 V, a 0.3-M sulfuric acid electrolyte should be used instead of oxalic acid.
AAO Templates with Several Levels of Multiply Branched Pores. To form templates with three or more branch pores for each stem pore and with two or more levels or branch pores, after the initial anodization for primary stem pores, the anodizing voltage is reduced by a factor of 1/√{square root over (n)} to create first-generation multibranched (n) pores, and the anodizing voltage is subsequently reduced again by a factor of 1/√{square root over (m)}, to generate the second-generation multibranched pores growing from each of the first-generation multibranched pores. The numbers n and m are integers which are equal to or are greater than 2. n may be equal to or not equal to m. Thus, n may be greater than, less than or equal to m. At least one of n or m may be greater than 2, such as 3 to 16, for example. Preferably, but not necessarily, both n and m are greater than 2.
Growth of Carbon Nanotubes in AAO Template. Multiwalled carbon nanotubes are grown inside the pores of the AAO templates by the pyrolysis of acetylene at 650° C. for 1-2 hours with a flow of gas mixture of Ar (85%) and C2H2 (15%) at a rate of 35 ml/min. The nanotubes are multiwalled (having about 4 to 10 walls), have a diameter in a range of about 20 to about 120 nm, and are graphitic in nature. From the observations of several branched multiwalled nanotube structures presented herein, the wall thickness (and hence the number of walls) falls within a very narrow range of about 1-4 nm. The present inventors normally observe a small reduction in the number of walls (approximately two or three walls) as a larger tube changes into smaller ones, and this reduction seems to happen quite abruptly.
It should be noted that other suitable process conditions may be used. Furthermore, other carbon containing source gases, such as ethylene for example, may be used instead of acetylene to deposit the nanotube using the chemical vapor deposition process. Finally processes other than chemical vapor deposition, such as laser ablation for example, may be used.
Electrochemical Deposition of Ni Nanowires in AAO Template. Nickel nanowires are grown inside the pores by the following method. It should be noted that while nickel is used as an example, nanowires made from other metal or non-metal materials may be used instead. After the final anodization, the remaining Al layer at the bottom of AAO templates is removed in a saturated SnCl4 solution. Before removing the barrier layer, the top surface of the templates is covered with nail polish to protect the pores if the barrier layer is thinned before the anodization for the branched pores. An adhesion layer of Ti (10 nm) and Cu film (1 μm) is coated onto the stem pore side of the AAO templates (i.e., the back side of the template) by electron-beam evaporation to cover the pores completely and to serve as the working electrode in electrochemical deposition. Ni nanowires are electrodeposited into the pores of AAO templates by using standard electrodeposition procedures described in Whitney, T. M., Jiang, J. S., Searson, P. C. & Chien, C. L. (1993) Science 261, 1316-1319, incorporated by reference in its entirety. It should be noted that other nanowire growth methods may be used instead.
Template Removal. Nanotubes are released from AAO templates by dissolving the templates in a 20% (wt) HF solution for 12 h, and then washing with deionized water several times. Ni nanowires are released from AAO templates by immersing the templates in a 10% (wt) NaOH solution for 1 h, and then washing with deionized water several times. For other nanowire materials, selective etching solutions other than NaOH which selectively etch the anodized aluminum (i.e., aluminum oxide) over the nanowire material may be used instead.
Without wishing to be bound by a particular theory, the present inventors believe that the pore diameter developed inside the template depends on the following three processes:
1. For the anodization process, the pore diameter is proportional to the anodizing voltage, and the diameter attributed by the anodization can be expressed as Dκ×I′ (nm), where V refers to the anodizing voltage, and κ is a constant (nmV−1) as reported, for example, in O'Sullivan, J. P. & Wood, G. C. (1970) Proc. R. Soc. London A 317, 511-543; Furneaux, R. C., Rigby, W. R. & Davidson, A. P. (1989) Nature 337, 147-149; Broughton, J. & Davies, G. A. (1995) J. Membr. Sci. 106, 89-101; and Choi, J., Sauer, G., Nielsch, K., Wehrspohn, R. B. & Gösele, U. (2003) Chem. Mater. 15, 776-779.
2. Thinning the barrier layer process before further anodization for multibranched pores also widens the existing pores. The diameter increase depends on the pore widening rate and thinning barrier layer time.
3. Removing the barrier layer in the final process of template preparation also will increase the pore diameter if the top surface is not covered with nail polish. The diameter increase depends on the pore widening rate and removing barrier layer time.
Templates with different pore structures undergo different processes, so the final diameters of the pores (and correspondingly, the outer diameter of the grown nanotubes or other nanostructures) in different structured templates can be calculated separately.
The diameter ratios of the nanotube structures (ratio of the smallest diameter of the highest or final branch level to the stem diameter) that are produced experimentally are provided in Table I, below. Both theoretically calculated ratios and experimentally observed values are shown, and there is an good correspondence between them.
In Table I, the anodizing voltage for the primary stem is about 70 V for all architectures. In the second column of Table I, the first two numbers are average values of diameters (in nanometers). For the structures marked with the “∀” symbol in Table I, an intermediate step (thinning barrier layer process) produced widening of the primary stem.
Although the foregoing refers to particular preferred embodiments, it will be understood that the present invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the present invention. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety.
This application claims priority to U.S. Provisional Patent Application No. 60/681,743, filed. May 17, 2005, which is incorporated herein by reference in its entirety.
This invention was made with U.S. government support under the National Science Foundation grant No. ______. The United States government may have rights in this invention.
Number | Date | Country | |
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60681743 | May 2005 | US |