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US20120034410A1 - Multiple walled nested coaxial nanostructures - Google Patents

Multiple walled nested coaxial nanostructures Download PDF

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US20120034410A1
US20120034410A1 US13/265,427 US201013265427A US2012034410A1 US 20120034410 A1 US20120034410 A1 US 20120034410A1 US 201013265427 A US201013265427 A US 201013265427A US 2012034410 A1 US2012034410 A1 US 2012034410A1
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nanostructure
coaxial
layer
substrate
nanotube
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Helmut Baumgart
Gon Namkoong
Diefeng Gu
Tarek Abdel-Fattah
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Old Dominion University Research Foundation
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Old Dominion University Research Foundation
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/02—Inorganic material
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/42—Electrodialysis; Electro-osmosis ; Electro-ultrafiltration; Membrane capacitive deionization
    • B01D61/427—Electro-osmosis
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0053—Inorganic membrane manufacture by inducing porosity into non porous precursor membranes
    • B01D67/006—Inorganic membrane manufacture by inducing porosity into non porous precursor membranes by elimination of segments of the precursor, e.g. nucleation-track membranes, lithography or laser methods
    • B01D67/0062—Inorganic membrane manufacture by inducing porosity into non porous precursor membranes by elimination of segments of the precursor, e.g. nucleation-track membranes, lithography or laser methods by micromachining techniques, e.g. using masking and etching steps, photolithography
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0072—Inorganic membrane manufacture by deposition from the gaseous phase, e.g. sputtering, CVD, PVD
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/02—Inorganic material
    • B01D71/0213—Silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/02—Inorganic material
    • B01D71/022—Metals
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
    • C23C16/01—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes on temporary substrates, e.g. substrates subsequently removed by etching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
    • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
    • C23C16/30—Deposition of compounds, mixtures or solid solutions, e.g. borides, carbides, nitrides
    • C23C16/40—Oxides
    • C23C16/403—Oxides of aluminium, magnesium or beryllium
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
    • F04B—POSITIVE-DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS
    • F04B19/00—Machines or pumps having pertinent characteristics not provided for in, or of interest apart from, groups F04B1/00 - F04B17/00
    • F04B19/006—Micropumps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2313/00—Details relating to membrane modules or apparatus
    • B01D2313/34—Energy carriers
    • B01D2313/345—Electrodes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/08—Patterned membranes

Definitions

  • Nanostructures including nanotubes, exhibit novel physical properties and play an important role in fundamental research.
  • nanostructures and nanotubes find many practical applications because of their restricted size and high surface area. See R. Kelsall et al., Nanoscale Science and Technology , Wiley, Chichester, (2006); C. R. Martin, Acc. Chem. Mater. 28, 61 (1995); J. Goldberger et al., Nature, 422 599 (2003); and S. B. Lee et al., Science, 296, 2198 (2002).
  • Nanotubes may be formed from a variety of materials, including different classes of materials such as insulators, semiconductors, and metals, including transition metal oxides.
  • hafnium oxide hafnia, HfO 2
  • aluminum oxide alumina, Al 2 O 3
  • titanium oxide TiO 2
  • zirconium oxide zirconia, ZrO 2
  • the semiconductor zinc oxide (ZnO)
  • ZnO zinc oxide
  • multiple walled nested coaxial nanostructures Provided herein are multiple walled nested coaxial nanostructures, methods for making the multiple walled nested coaxial nanostructures, and devices incorporating the multiple walled nested coaxial nanostructures.
  • the disclosed multiple walled nested coaxial nanostructures have extremely high aspect ratios and surface areas. Consequently, devices incorporating these multiple walled nested coaxial nanostructures exhibit superior and novel properties as compared with conventional devices. These advantages are further discussed below with respect to specific devices incorporating the coaxial nanostructures.
  • the disclosed multiple walled nested coaxial nanostructures may be formed using atomic layer deposition (ALD) or other suitable chemical vapor deposition (CVD) techniques to deposit different materials by coating the inner walls of the pores of various nanoporous substrates (also referred to herein as nanoporous templates or nanoporous membranes), one atomic layer at a time.
  • Nanoporous substrates or templates may be formed from nanoporous alumina, polycarbonate membranes, porous silicon, or any other suitable porous substrate.
  • the ability to achieve multiple walled nested coaxial nanostructures with such high aspect ratios is derived, in part, from the use of long ALD deposition dwell times and the use of sacrificial spacer layer technology to open up all surfaces of such multiple walled nested coaxial nanostructures.
  • the use of long ALD deposition dwell times is contrary to conventional wisdom, since longer ALD deposition times can clog the pores of the underlying porous substrates.
  • multiple walled nested coaxial nanostructures are provided.
  • the multiple walled nested coaxial nanostructure may include an inner nanostructure, a first outer nanotube disposed around the inner nanostructure, and a first annular channel between the inner nanostructure and the first outer nanotube.
  • the coaxial nanostructure may further include a second outer nanotube disposed around the first outer nanotube and a second annular channel between the first outer nanotube and the second outer nanotube.
  • a third outer nanotube may be disposed around the second outer nanotube, a fourth outer nanotube may be disposed around the third outer nanotube, and so forth, up to an n th outer nanotube.
  • the aspect ratio of the coaxial nanostructures may range from about 5 to about 1,200, or about 300 to about 1200, although other aspect ratios are possible.
  • the materials used to form the inner nanostructure and the outer nanotubes may vary and may include a conductor, an insulator, or a semiconductor. Specific examples of conductors, insulators, and semiconductors are provided herein.
  • a sacrificial spacer material including Al 2 O 3 , may be disposed within the annular channel of any of the coaxial nanostructures in order to create annular channels to open up all surfaces (inner and outer wall) of these multiple walled nested coaxial nanostructures.
  • the multiple walled nested coaxial nanostructures may be coupled to other components, including various substrates.
  • the substrate may be a porous anodic aluminum oxide (AAO) substrate.
  • AAO anodic aluminum oxide
  • a porous silicon substrate or any other suitable porous template may be used.
  • the substrate can be macroporous. Also provided herein are arrays comprising two or more of any of the disclosed coaxial nanostructures and devices incorporating any of the disclosed coaxial nanostructures.
  • the method may include forming a layer of a first material on an inner surface of a nanopore of a nanoporous substrate using atomic layer deposition, forming a first layer of a sacrificial material on the layer of the first material using atomic layer deposition, and forming a layer of a second material on the first layer of the sacrificial material using atomic layer deposition.
  • the method may further include forming a second layer of a sacrificial material on the layer of the second material and forming a layer of a third material on the second layer of the sacrificial material (until the n th layer in the most general case).
  • the aspect ratio of the coaxial nanostructures provided by such a method may range from about 5 to about 1,200, or about 300 to about 1200, although other aspect ratios are possible.
  • the methods may further include removing the nanoporous template and/or the layers of the sacrificial spacer material by a variety of methods, including by chemical etching.
  • the nanoporous substrates and the compositions of the first material, the second material, the third material, the n th material and the sacrificial spacer material may vary as described above with respect to the multiple walled nested coaxial nanostructures.
  • an electroosmotic pump may include a nanoporous membrane having one or more nanopores, a layer of a first material deposited on an inner surface of the nanopore, a first electrode (anode) coupled to a first side of the nanoporous substrate, and a second electrode (cathode) coupled to a second side of the nanoporous template.
  • the electroosmotic pump may further include a layer of a second material deposited on the layer of the first material.
  • a variety of nanoporous substrates and compositions for the first material, the second material, and the electrodes may be used. Specific examples are provided herein. The performance of the disclosed electroosmotic pumps is superior to conventional pumps.
  • FIGS. 1A and 1B show SEM images of a porous anodic aluminum oxide (AAO) substrate.
  • FIG. 1A shows the surface of the substrate after ion milling.
  • FIG. 1B shows a cross-section of a cleaved AAO substrate.
  • FIG. 2 shows a cross-sectional SEM image of ALD (atomic layer deposited) zirconia coated AAO substrate (A) and a corresponding EDS Zr mapping showing uniform distribution of zirconia throughout the entire thickness of the 60 ⁇ m AAO substrate (B).
  • ALD atomic layer deposited
  • FIG. 3 shows a top-down SEM image of an uncoated AAO substrate (A); the same AAO substrate with a thin film ALD coating of ZrO 2 (B); and the same coated AAO substrate after the alumina substrate walls have been removed to provide single ZrO 2 nanotubes (C).
  • FIG. 4 shows a SEM micrograph of HfO 2 tube-in-tube coaxial nanostructures. A top-down view of the sample surface and a partial side-view from a cleavage site is shown by tilting the sample.
  • FIG. 5 is a TEM micrograph of a separated HfO 2 tube-in-tube coaxial nanostructure shown in FIG. 4 .
  • FIG. 6 is a top-down SEM image showing three concentric metal oxide (ZrO2) nanotubes inside large AAO pores following the dissolution of the 2 separating spacer Al 2 O 3 layers in order to expose the sidewalls of the coaxial (ZrO2) nanotubes.
  • ZrO2 concentric metal oxide
  • These five coaxial nanostructures were formed by layering ZrO 2 /Al 2 O 3 /ZrO 2 /Al 2 O 3 /ZrO 2 and removing the Al 2 O 3 layers by chemical etching.
  • FIG. 7 is a top-down SEM image of hafnia/zirconia coaxial nanostructures disposed in the nanopores of a AAO substrate showing the simultaneous coating of the AAO surface and the inner walls of the nanopores.
  • FIG. 8 is an illustration of an exemplary coaxial nanostructure having an inner nanotube of ZnO, a first outer nanotube of ZrO 2 disposed around the inner nanotube, and a first annular channel between the inner nanotube and the outer nanotube.
  • FIG. 9 is an illustration of an exemplary structure that may be used to provide a chemical sensor.
  • FIG. 10 is an SEM image of free-standing single walled nanotubes.
  • FIG. 11 is a schematic depiction of a process sequence for synthesizing free-standing HfO2 nested coaxial tube-in-tube nanostructures.
  • FIG. 12 shows coaxial HfO 2 nanotubes formed by, for example, the method depicted in FIG. 11 .
  • FIG. 13 provides thermodynamic modeling diagrams showing the distributions of ionic species to represent, for example, the removal of AAO substrate from ZnO nanotubes using NaOH of various pH values.
  • FIG. 14 is an SEM micrograph showing large numbers of high aspect ratio coaxial ALD HfO 2 nanotubes.
  • FIG. 15 is a top-down SEM image showing five nested coaxial ALD layers such as nested nanotube structures.
  • multiple walled nested coaxial nanostructures Provided herein are multiple walled nested coaxial nanostructures, methods for making the multiple walled nested coaxial nanostructures, and devices incorporating the multiple walled nested coaxial nanostructures.
  • the multiple walled nested coaxial nanostructures include an inner nanostructure and at least one outer nanotube disposed around the inner nanostructure.
  • the multiple walled nested coaxial nanostructures may include multiple outer nanotubes (up to n outer nanotubes) arranged concentrically around the inner nanostructure. This includes embodiments in which the coaxial nanostructure includes a first outer nanotube disposed around an inner nanostructure, a second outer nanotube disposed around the first outer nanotube, a third outer nanotube disposed around the second outer nanotube, and so forth.
  • the inner nanostructure may also be a nanotube.
  • the innermost nanostructure may also be a nanorod.
  • the multiple walled nested coaxial nanostructures may also include an annular channel between the inner nanostructure and the at least one outer nanotube.
  • the coaxial nanostructure may include additional annular channels between the additional outer nanotubes.
  • a multiple walled nested coaxial nanostructure may include a first outer nanotube disposed around an inner nanostructure, a first annular channel between the inner nanostructure and the first outer nanotube, a second outer nanotube disposed around the first outer nanotube, a second annular channel between the first outer nanotube and the second outer nanotube, and so forth.
  • the annular channel comprises air, after the sacrificial spacer material has been removed from the annular channel.
  • the annular channel may comprise a sacrificial material. Sacrificial materials are further described below.
  • the materials used to form the coaxial nanostructures may vary.
  • the inner nanostructure and any of the outer nanotubes may comprise a conductor, an insulator, or a semiconductor.
  • a variety of conductors may be used, including metals or nitrides of metals.
  • metals include Ti, Au, Pt, Al, Cu, Ag, and W.
  • Non-limiting examples of conducting metal nitrides include TiN and TaN and conducting metal oxides include ITO (indium tin oxide) and RuO 2 .
  • a variety of insulators may be used, including metal oxides.
  • Non-limiting examples of insulating oxides and metal oxides include SiO 2 , HfO 2 , ZrO 2 , Al 2 O 3 , Ta 2 O 5 , La 2 O 3 , Y 2 O 3 , MoO 2 , In 2 O 3 , V 2 O 5 ,
  • a variety of semiconductors may also be used, including, but not limited to ZnO, TiO 2 , WO 3 , NiO, GaAs, GaP, GaN, InP, InAs, AlAs, and Ge.
  • the inner nanostructure and any of the outer nanotubes are substantially free of carbon. By “substantially free of carbon,” it is meant that the nanostructures do not include, and are not formed of, carbon.
  • nanostructure may include trace amounts of carbon that may be unavoidable due to the methods used to form the nanostructures.
  • the structures can be different from and not comprise carbon nanotubes including multi-walled carbon nanotubes, single walled carbon nanotubes, and other types of carbon nanotubes.
  • the inner nanostructure and any of the outer nanotubes are completely free of carbon.
  • the inner nanostructure and each of the outer nanotubes may be formed of the same material.
  • the inner nanostructure and each of the outer nanotubes may be each formed of different materials.
  • some of outer nanotubes and the inner nanostructure may be formed of the same material while others are formed of different materials.
  • the dimensions of the coaxial nanostructures may also vary.
  • the diameter of the coaxial nanostructures may range from about 50 nm to about 300 nm for alumina templates and at the upper range pore diameters may range as large as several micrometers for porous silicon templates.
  • the pore diameter range that is achievable depends on the material parameters of the porous template material and the electro-chemical parameters of the fabrication method used. This includes embodiments in which the diameter is about 60 nm, 75 nm, 90 nm, 125 nm, 150 nm, 175 nm, 200 nm, 250 nm, or 300 nm (including 300 nm for AAO case).
  • the length of the coaxial nanostructures may range from about 15 ⁇ m to about 75 ⁇ m.
  • the length is about 20 ⁇ m, 30 ⁇ m, 40 ⁇ m, 50 ⁇ m, 60 ⁇ m, or 70 ⁇ m.
  • the aspect ratio (the ratio of the length of the coaxial nanostructure to the diameter of the coaxial nanostructure) may also vary. In some embodiments, the aspect ratio ranges from about 5 to about 1,200, or about 300 to about 1200. This includes embodiments in which the aspect ratio is about 400, 500, 600, 700, 800, 900, or 1000.
  • the cross-sectional shape of the coaxial nanostructures may vary. In some embodiments, the cross-sectional shape is a polyhedron, such as an octahedron. In other embodiments, the cross-sectional shape is substantially circular. By “substantially circular,” it is meant that shape is circular-elliptical, but not necessarily perfectly circular.
  • the dimensions of the outer nanotubes and the inner nanorod or nanotube forming the multiple walled nested coaxial nanostructures may vary, depending upon number of such structures present in the coaxial nanostructure and the overall dimensions of the coaxial nanostructure itself.
  • the width of the walls of the nanotubes and the width of the annular spacer channel (if present) may also vary. In some embodiments, the width ranges from about 5 nm to about 30 nm. This includes embodiments in which the width is about 10 nm, 15 nm, 20 nm, 25 nm and 35 nm.
  • Multiple walled nested coaxial nanostructures may be coupled to other elements.
  • the multiple walled nested coaxial nanostructure is coupled to a substrate.
  • a variety of substrates may be used, including any of the metals described above.
  • the substrate is an Al substrate.
  • the coaxial nanostructure may be attached to the substrate at one of the ends of the coaxial nanostructure.
  • the substrate may be a nanoporous substrate and the multiple walled nested coaxial nanostructure may be disposed within a pore of the nanoporous substrate.
  • nanoporous substrates may be used, including, but not limited to, porous anodic aluminum oxide (AAO) substrates, polycarbonate nanoporous templates (membranes), and porous silicon.
  • AAO anodic aluminum oxide
  • Such nanoporous (substrates) templates are known and AAO is commercially available.
  • the multiple walled nested coaxial nanostructure may be coupled to both a metal substrate, such as an Al substrate, and a nanoporous substrate, such as an AAO substrate.
  • the coaxial nanostructure may be disposed within a pore of the nanoporous substrate and attached to the metal substrate at one of the ends of the coaxial nanostructure.
  • anodic aluminum oxide can be formed by electrochemical oxidation of aluminum in acidic solutions to form regular porous channels, which are parallel to each other. See H. Masuda and K. Fukuda, Science, 268, 1466 (1995); V. P. Menon and C. R. Martin, Anal. Chem., 67, 1920 (1995); and M. A. Cameron, I. P. Gartland, J. A. Smith, S. F. Diaz and S. M. George, Langmuir, 16, 7435 (2000).
  • the individual pore diameters inside the porous alumina membrane are mainly defined by the anodization voltage.
  • the diameter of the pore depends on the electrolyte nature, its temperature and concentration, the current density and other parameters of the anodization process. Aside from the modulation of the pore diameters by variation of the electrolyte composition and anodization conditions, it is possible to further enlarge the pore diameters by another subsequent selective etching of the porous template walls.
  • the Examples below provide an exemplary method for making a suitable AAO substrate.
  • arrays of two or more of any of the coaxial nanostructures described above may be coupled to any of the substrates described above.
  • FIG. 8 A non-limiting exemplary multiple walled nested coaxial nanostructure is illustrated in FIG. 8 .
  • a first layer of a metal oxide e.g., ZrO 2
  • a second layer of a metal oxide e.g., Al 2 O 3
  • a third layer of a metal oxide e.g., ZnO
  • both the AAO substrate and the second layer of the metal oxide may be removed by etching to provide a coaxial nanostructure comprising an inner nanotube of ZnO, a first outer nanotube of ZrO 2 disposed around the inner nanotube, and a first annular channel between the inner nanotube and the outer nanotube.
  • the multiple walled nested coaxial nanostructures described above may be prepared according to the following methods.
  • the methods can use atomic layer deposition or other suitable chemical vapor deposition (CVD) techniques to deposit layers (also referred to as films herein) of the types of materials described above on the inner surface of the nanopores of a nanoporous substrate.
  • ALD is a known technique. Briefly, ALD technology deposits thin films using pulses of chemical precursor gases to adsorb at the target surface one atomic layer at a time. ALD is based on the sequential deposition of individual monolayers or fractions of a monolayer in a controlled fashion.
  • ALD the growth substrate surface is alternately exposed to the vapors of one of two chemical reactants (complementary chemical precursors), which are supplied to the reaction chamber one at a time.
  • the exposure steps are separated by inert gas purge or pump-down steps in order to remove any residual chemical precursor or its by-product before the next chemical precursor can be introduced into the reaction chamber.
  • ALD involves a repetition of individual growth cycles. See also Ritala, M., “Atomic Layer Deposition”, p.
  • ALD atomic layer deposition
  • a variety of chemical precursors may be used with ALD, depending upon the desired film.
  • the general requirements and properties of useful chemical precursors are known. See Sneh, O., Clark-Phelps, R. B., Londergan, A. R., Winkler J., and Seidel, T., “Thin film atomic layer deposition equipment for semiconductor processing,” Thin Solid Films , Vol. 402, Issues 1-2, p. 248-261, 2002 and Leskela, M., and Ritala, M., “Atomic Layer Deposition (ALD): from precursor to thin film structures,” Thin Solid Films, 409, p. 138-146, 2002. Specific chemical precursors are provided in the Examples below.
  • the method comprises forming a layer of a first material on an inner surface of a nanopore of a nanoporous substrate using atomic layer deposition and forming a layer of a second material on the layer of the first material using atomic layer deposition.
  • a layer of a third material may be formed on the layer of the second material
  • a layer of a fourth material may be formed on the layer of the third material, and so forth.
  • the layer of the first material corresponds to an outer nanotube of the coaxial nanostructures described above.
  • the layer of the second material provides either an additional outer nanotube, or an inner nanostructure, depending upon the number of layers of materials deposited.
  • the first material, second material, and third material may include any of the conductors, insulators, and semiconductors described above. Similarly, any of the nanoporous substrates described above may be used with the disclosed method.
  • the method may further comprise removing the nanoporous (substrate) template after the multiple walled nested coaxial nanostructure is formed.
  • a variety of methods may be used to remove the nanoporous (substrate) template, including, but not limited to chemical etching.
  • a variety of chemical etchants may be used, depending upon the composition of the nanoporous substrate.
  • NaOH may be used to remove the porous template (substrate).
  • the method comprises forming a layer of a first material on an inner surface of a nanopore of a nanoporous substrate using atomic layer deposition, forming a first layer of a sacrificial material on the layer of the first material using atomic layer deposition, and forming a layer of a second material on the first layer of the sacrificial material using atomic layer deposition.
  • Other sacrificial spacer layers and layers of additional materials may be deposited.
  • a second layer of a sacrificial spacer material may be formed on the layer of the second material
  • a layer of a third material may be formed on the second layer of the sacrificial material, and so forth.
  • sacrificial spacer material it is meant a material that is capable of being substantially removed (i.e., removed, but not necessarily completely removed) by a chemical etchant.
  • a non-limiting example of a sacrificial material is Al 2 O 3 , which is capable of being substantially removed by a variety of chemical etchants, including NaOH.
  • the sequence of synthesizing the multiple walled nested coaxial nanostructures comprises alternating sacrifical spacer material annular rings with the next nested coaxial nanotube material of choice.
  • the first material, second material, and third material may include any of the conductors, insulators, and semiconductors described above.
  • any of the nanoporous templates (substrates) described above may be used with the disclosed method.
  • the method may further comprise removing any or all of the sacrificial layers by chemical etching.
  • Such a method provides the multiple walled nested coaxial nanostructures having one or more annular channels comprising air, as described above.
  • the method may further comprise removing the nanoporous substrate after the coaxial nanostructure is formed, as described above.
  • the description of the coaxial nanostructures, AAO substrates, and ALD process make clear that the dimensions of the coaxial nanostructures are both a function of the pore sizes of the AAO substrates as well as the number of cycles and length of each cycle of the ALD process.
  • the length of the cycle may be maximized to ensure deposition along the entire length of the nanopore. Long cycle times, however, are contrary to the conventional wisdom that cycle times should be minimized to prevent clogging the pores of the AAO substrates.
  • the multiple walled nested coaxial nanostructures described above may be incorporated into a variety of devices for use in a variety of applications.
  • the multiple walled nested coaxial nanostructures may be used in electroosmotic pumps, chemical sensors, photovoltaic devices, and photonic crystals.
  • the multiple walled nested coaxial nanostructures may also find use as extremely hard and highly durable nanometer-sized pipette tips for various medical applications.
  • devices incorporating the disclosed coaxial nanostructures are expected to exhibit superior properties over conventional devices due to the high aspect ratio and high surface area of the coaxial nanostructures. These devices are further described below.
  • Electroosmosis is the motion of ionized liquid relative to a stationary charged surface by an externally applied electric field.
  • Electroosmotic (EO) flows are useful in microfluidic systems, since they enable fluid pumping and flow control without the need for mechanical pumps or valves, and they also minimize the sample dispersion effects. See Karniadakis, G. E., Beskok, A., and Alum, N., Microflows and Nanoflows: Fundamentals and Simulation , Springer, N.Y., 2005.
  • conventional EO pumps suffer from a number of drawbacks, including the need for large operating voltages (on the order of 1 kV to 10 kV), electrolysis of water, oxidation of electrode surfaces, and Joule heating.
  • the need for a high voltage supply limits the use of conventional EO pumps in lab-on-a-chip (LoC) type portable devices, designed for bio-medical, pharmaceutical, environmental monitoring and homeland-security applications.
  • a two-terminal electroosmotic pump comprises a nanoporous substrate having one or more nanopores and a layer of a first material deposited on an inner surface of the nanopore.
  • the layer of the first material provides a nanotube disposed within the nanopore of the nanoporous substrate. Electrodes may be coupled to both sides of the nanoporous substrate. Any of the nanoporous substrates described above may be used.
  • the aspect ratio of the nanopores of the nanoporous substrate ranges from about 5 to about 1,200, or about 300 to about 1200. This includes embodiments in which the aspect ratio is about 400, 500, 600, 700, 800, 900, or 1000.
  • the composition of the first material may vary.
  • the first material comprises a metal oxide or a metal nitride.
  • Any of the metal oxides disclosed above may be used, including, but not limited to HfO 2 , ZrO 2 , Al 2 O 3 , ZnO, TiO 2 , TiN, or SiO 2 .
  • the composition of the electrodes may vary.
  • the electrodes comprise a metal. Examples of useful metals, include, but are not limited to, Au, Pt, and W. As noted above, the performance of the disclosed two-terminal electroosmotic pump exceeds that of conventional electroosmotic pumps.
  • a three-terminal electroosmotic pump comprises a nanoporous substrate having one or more nanopores, a layer of a first material deposited on an inner surface of the nanopore, and a layer of a second material deposited on the layer of the first material.
  • the layer of the first material provides an outer nanotube and the second material provides an inner nanotube, resulting in a coaxial nanostructure disposed within the nanopore of the nanoporous substrate.
  • Electrodes may be coupled to both sides of the nanoporous substrate. Any of the nanoporous substrates described above may be used.
  • the aspect ratio of the nanopores of the nanoporous substrate ranges from about 5 to about 1,200, or about 300 to about 1200. This includes embodiments in which the aspect ratio is about 400, 500, 600, 700, 800, 900, or 1000.
  • the composition of the first material, the second material, and the electrodes may vary.
  • the first material comprises a metal, a metal nitride, or a semiconductor.
  • metals and metal nitrides include Ti, Au, Pt, Al, Cu, Ag, and nitrides thereof.
  • a non-limiting example of a semiconductor includes ZnO.
  • the second material comprises a metal oxide.
  • oxides and metal oxides include HfO 2 , ZrO 2 , Al 2 O 3 , TiO 2 , and SiO 2 .
  • Other possible metals, metal nitrides, semiconductors, or metal oxides include, but are not limited to, those described above.
  • the electrodes comprise a metal.
  • useful metals include, but are not limited to, Au, Pt, and W. Similar to the two-terminal electroosmotic pumps described above, the performance of the three-terminal electroosmotic pumps exceeds that of conventional electroosmotic pumps.
  • electroosmotic pumps The methods for forming these and other electroosmotic pumps is similar to the methods described above, involving the use of atomic layer deposition to deposit the desired number of layers of materials in nanoporous substrates. Methods for depositing electrodes and patterning contacts on the electroosmotic pumps using photolithography or wire bonding techniques are known.
  • a sensor may include a coaxial nanostructure having an inner nanotube formed of ZnO and an outer nanotube formed of ZrO 2 , wherein the inner and outer nanotubes are separated by an annular channel.
  • ZnO is an ideal material for detecting carbon monoxide
  • ZrO 2 is an ideal material for detecting oxygen. Accordingly, such a sensor is capable of detecting multiple chemicals simultaneously.
  • the “tube-in-tube” or “nested” design increases the reactive surface area by at least four times, thereby providing a sensor with a greater capacity and lifetime than conventional sensors.
  • the nested coaxial nanotube design can be extended to include up to n-times nested detector nanotubes each separated by empty annular spacer channel, where each coaxial nanotube is custom tailored to sense a different chemical. In this fashion, multi-functional broadband sensors and detectors can be prepared.
  • Photovoltaic cells and the components used to form the cells are known. See Luque, A., et al., Handbook of Photovoltaic Science and Engineering , Wiley (2003). Any of the coaxial nanostructures described above, including the multiple walled nested coaxial nanostructures comprising an annular channel, may be incorporated into a photovoltatic cell and coupled to components such as an anode, cathode, and supporting substrate.
  • any of the coaxial nanostructures described above, including the multiple walled nested coaxial nanostructures comprising an annular channel, may be incorporated into a photonic crystals and coupled to various components such as a supporting metal substrate.
  • Two-dimensional photonic crystals may be formed from coaxial nanostructures having an outer nanotube disposed around an inner nanotube, wherein the nanotubes are separated by an annular channel.
  • Three-dimensional photonic crystals may be similarly formed, using a nanoporous substrate having branched channels connecting the main nanopores.
  • Non-limiting exemplary devices are illustrated in FIG. 9 .
  • a layer of a metal oxide e.g., ZrO 2
  • a layer of a metal e.g., Pt
  • the AAO substrate may be dissolved by chemical etching.
  • the structure shown may be used as an oxygen sensor.
  • FIG. 10A shows an SEM micrograph of partially released, single-walled, and ALD synthesized nanotubes of insulating high-k ZrO2.
  • FIG. 10B shows SEM views of cleaved samples of partially released, single-walled, and ALD synthesized nanotubes of semiconductor ZnO.
  • FIG. 10C shows SEM views of cleaved samples of partially released, single walled, and ALD synthesized metallic Pt nanotubes obtained by dissolving an alumina template in an NaOH solution.
  • a sensor capable of simultaneously detecting a plurality of chemicals comprises any of the coaxial nanostructures described herein.
  • a sensor based on multiple walled nested nanotubes such as a multiple walled nanotube comprising an inner nanostructure, at least one of an outer nanotube disposed around the inner nanostructure, and a first annular channel between the inner nanostructure and the at least one first outer nanotube, is capable of detecting several different chemicals, for example several different hazardous or dangerous gases.
  • each of the at least one of an outer nanotubes may comprise a material capable of targeting specific chemical compounds.
  • the at least one outer nanotube may be a plurality of nanotubes, wherein each subsequent nanotube is disposed around the previous nanotube, and an annular channel is formed between each of the plurality of nanotubes.
  • each of the nanotubes is capable of targeting one or more chemical compounds, and may be capable of targeting the same or different chemical compound as subsequent nanotubes. Accordingly, a sensor having broadband sensing capabilities can be engineered by substituting a specific sensor material for one of the multiple tubes.
  • multi-layered tube-in-tube nanostructures described herein may be used in applications including sensors and detectors, MEMS, nano-capacitors, photonic crystals, Microfluidic electroosmotic pumps for drug delivery and general medical applications and photovoltaic devices. Additional embodiments include the use of the methods described herein in applications such as commercial fabrication and assembly of extremely hard and durable ZnO 2 nanometer pipette tips for medical research needed for injecting chemicals from aqueous solutions into cancer cells, or for fertilization of egg cells in reproductive medicine.
  • FIG. 11 Non-limiting exemplary methods of forming nested coaxial tube-in-tube nanostructures are illustrated in FIG. 11 .
  • FIG. 11A a nanoporous AAO substrate is formed.
  • FIG. 11B shows a subsequent step of using ALD to coat the inner pore walls of the AAO substrate of FIG. 11A with HfO 2 .
  • An ALD deposition of a sacrifical spacer layer consisting of, for example, Al 2 O 3 over the HfO 2 layer is shown FIG. 11C .
  • FIG. 11D using an ALD process, a second layer of HfO 2 is coated on the sacrificial spacer layer and AAO template walls.
  • FIG. 11E shows an ion milling sputter removal step of the ALD composite layers from surfaces of the AAO template in order to expose the sacrifical spacer layer and the AAO template walls.
  • FIG. 11F shows a step of releasing and separating formed coaxial HfO 2 nanotubes by chemical dissolution of the alumina AAO template walls and the sacrifical ALD Al 2 O 3 spacer layers using an Aqueous NaOH solution.
  • FIG. 12A is a high-magnification tilted SEM top view of resultant coaxial HfO 2 nanotubes following release from an AAO template and after removing an sacrifical spacer Al 2 O 3 layer as in the method of FIG. 11 .
  • FIG. 12B is a schematic model depicting an array of free standing coaxial nested nanotubes.
  • FIG. 13 a represents a thermodynamic model showing the distributions of the fraction of all Al 3+ species at different pH values calculated at 298K for 0.001 mM Al 3+ solution.
  • the formation of solid alumina is in the pH range of 4.2 to 9.8 and the maximum solubility of Al 2 O 3 is at pH below 4.2 and above pH 9.8.
  • thermodynamic model showing the distributions of the fraction of all Zn 2+ species at different pH values calculated at 298 K for 0.001 mM Zn 2+ solution.
  • the thermodynamic modeling of Zn 2+ species indicates that zinc always has soluble species at any pH value, the existence of crystalline ZnO is in the pH range of 9.2 to 11.5 and the maximum solubility of crystalline ZnO is at pH below 9.2 and above pH 11.5.
  • an NaOH solution at pH 13 partially eteches and degrades a ZnO surface.
  • an NaOH solution in the range of pH 10.3-11.0 for example a pH of 11.0 may be used to successfully remove AAO.
  • the free-standing coaxial nanotubes may be released from the template as shown in FIG. 14 .
  • the embedded nanotubes have to be released in order to collect and incorporate them into device structures.
  • Certain applications call for attached upright standing coaxial nanotubes, while other application require completely chemically released and detached coaxial nanotubes.
  • Large numbers of completely detached and individual nanotubes of the present embodiments can be harvested by, for example, sonication in an aqueous solution.
  • Such nanotubes may be completely detached and may have high aspect ratios of about 5 or above, or about 300 and above.
  • FIGS. 5 , and 15 A-B show that the annular channel between the nested coaxial nanotubes provides sufficient space to continue a process of growing additional nanotubes by ALD.
  • a synthesis and assembly of nested multiple tube-in-tube nanostructures can be extended to n-layers, where n is more than one.
  • n is more than one.
  • a total number of five nested coaxial nanotube structures may be provided.
  • the number of nested nanotubes may be increased.
  • the structures shown in FIG. 15 consist of triple coaxial HfO 2 nanotubes separated by a gap and two sacrifical ALD AL 2 O 3 spacer layers.
  • the nanoporous AAO substrate was prepared by a two-step anodization procedure. High purity aluminum sheets (0.5 mm thick) were degreased in acetone. The Al sheets were then electropolished in a solution of HClO 4 and ethanol (1:4, v/v) at 20 V for 5-10 min or until a mirror like surface was achieved. The first anodization step was carried out in a 0.3 M oxalic acid solution electrolyte under a constant direct current (DC) voltage of 80 V at 17° C. for 24 h. The porous alumina layer was then stripped away from the Al substrate by etching the sample in a solution containing 6 wt % phosphoric acid and 1.8 wt % chromic acid at 60° C.
  • DC direct current
  • the second anodization step was carried out in a 0.3 M oxalic acid solution under a constant direct current (DC) voltage of 80 V at 17° C. for 24 h.
  • the AAO substrates with highly ordered arrays of nanopores were then obtained by selectively etching away the unreacted Al in a saturated HgCl 2 solution.
  • FIG. 1A shows the SEM image of the pore structure of the AAO after the surface was planarized by ion milling.
  • the pore size is in the range of 200-300 nm and the wall width between pores is around 50 nm. Some of the pores were connected through thinning of the wall.
  • the cross-sectional SEM image shown in FIG. 1B reveals that the pores are all parallel to each other and across the whole substrate of 60 ⁇ m thickness.
  • the inset to FIG. 1B shows the formation of branches in some of the pores. These branches may be eliminated with shorter anodization times, which results in a shorter pore length.
  • a closer view of tube opening showed that the side connected to the cathode has smaller pore size, to a depth of a few micrometers.
  • This thin layer can be removed by etching to achieve uniform pore diameter across the entire substrate depth.
  • High magnification FE-SEM of a cleavage sample highlights the microstructure of partially split open nanopores of AAO. The smooth morphology of the inside walls of the AAO nanopores can be clearly seen. Excellent surface finish of the inner pore walls of the template is useful for obtaining highly ordered tube-in-tube nanostructures, since the ALD thin film coating technique replicates the surface finish on an Angstrom scale.
  • the AAO substrates were subsequently transferred to the ALD chamber for ZrO 2 , HfO 2 and ZnO coating of the inside surfaces of the nanopores.
  • the ZrO 2 and HfO 2 deposition was performed at 250° C. using water vapor as the oxidant and tetrakis (dimethylamido) hafnium (IV) and tetrakis (dimethylamido) zirconium (IV) as the precursor, respectively.
  • the deposition rate is about 1 ⁇ /cycle at this temperature.
  • ZnO was grown with diethyl zinc (DEZ) as precursor and water vapor as oxidation source.
  • the optimum ALD process window for ZnO was determined to be in the temperature range between 110° C. and 160° C.
  • the entire nanopores may not be coated uniformly unless an extended ALD cycle time is used.
  • AAO pores coated with 20 nm HfO 2 cross sectional energy dispersive spectroscopy (EDS) mapping demonstrated that Hf signal was detected up to a depth of about 15 ⁇ m from the sample surface without any added ALD exposure time.
  • EDS energy dispersive spectroscopy
  • the surface pore diameter was reduced after ZrO 2 deposition, indicating that ZrO 2 was also deposited on AAO template.
  • Increased ALD exposure times were used for the Zr precursor to reach saturation of precursor species on the inside walls of the pores and ensure uniform coating along the length of the pores.
  • FIGS. 2A and 2B show the cross-sectional SEM image and EDS mapping of the AAO substrate coated with 20 nm ZrO 2 using 30 s additional ALD exposure time. It can be observed that there is still a gradient in the Zr signal following the length of the metal oxide the nanotubes. This is because the AAO substrate was placed in the ALD chamber flat on one side so that access of the Zr precursor to the backside opening was blocked. The uniformity of coating can be improved by lifting the AAO substrate so that the precursor can access both sizes of the pore opening during ALD deposition.
  • FIGS. 3A and 3B show an AAO substrate before being coated with ZrO 2 (A) and after being coated with 20 nm ZrO 2 (B).
  • a comparison of the figures shows that the pore size has been reduced because the wall thickness has been increased by growing a ZrO 2 film.
  • the alumina walls between the pores were dissolved by a 6M NaOH solution.
  • the porous AAO surface was first cleared of its ZrO 2 films by ion milling to expose the AAO wall to the etchant.
  • 3C shows the free-standing ZrO 2 nanotubes after ion milling and chemical dissolution of alumina walls.
  • the SEM image clearly shows the empty trenches in place of the former alumina side walls.
  • the dimensions of the nanotubes are dependent upon the thickness and pore diameter of the AAO substrate and the ALD deposition time. Smaller tubes or even rods can be fabricated using this method by using AAO substrates with smaller pores. Different materials can still be deposited inside of the nanotubes depending on the application.
  • a second nanotube having a smaller dimension was deposited inside of the aforementioned HfO 2 nanotubes.
  • two layers of 10 nm HfO 2 films were deposited inside of the AAO pores and separated by 25 nm of a layer of Al 2 O 3 , which was deposited by ALD at 300° C. using [Al(CH 3 ) 3 ] (TMA) and water vapor as the aluminum and oxygen source, respectively.
  • Al 2 O 3 is the same material as the AAO substrate and can be easily etched away.
  • the sample surface was again polished by ion milling and then dipped into NaOH solution to etch both the AAO substrate and Al 2 O 3 layer between HfO 2 layers.
  • FIG. 4 shows a double-walled HfO 2 tube-in-tube structure after wet etching in NaOH solution.
  • the HfO 2 tube thicknesses are very uniform from both the top and cross section.
  • the expected wall thicknesses for both tubes are 10 nm, as determined from the number of ALD cycles.
  • the HfO 2 tubes look much thicker from the SEM image due to the gold coating for charging release.
  • FIG. 5 shows TEM high magnification micrographs of double-walled HfO 2 tube-in-tube structure.
  • the tube-in-tube structure shown in FIG. 5 was achieved even from AAO pores with branches or dead ends.
  • Example 3 The method of Example 3 was modified to provide two nanotubes having a smaller dimension deposited inside of the aforementioned ZrO 2 nanotubes. Three layers of ZrO 2 films were deposited inside of AAO pores, separated by a layer of Al 2 O 3 . The Al 2 O 3 layers were removed as described above. The resulting tube-in-tube-in-tube coaxial nanostructure is shown in FIG. 6 .
  • FIG. 7 shows the surface morphology and tube size after the two layer coating.
  • the AAO substrate is transferred to an ALD reaction chamber in order to grow nested multiple-walled nanotubes within the AAO pores.
  • Pt may be used for metal nanotubes and ZnO and TiO 2 may be used for semiconducting metal oxide nanotubes.
  • the transition metal oxides of ZrO 2 , HfO 2 , and Al 2 O 3 may be used.
  • ALD is a thin film growth technique that requires the sequential exposure of the sample to two chemical precursors to saturate the sample surface and to react with each other. The technical details of the ALD process conditions and the different chemical precursors and deposition parameters utilized for all of the nested nanotubes investigated in this study are listed in Table 1 below.
  • the AAO sample surfaces may be polished by ion milling to expose the template surface and the ALD grown alumina spacer to the NaOH solution.
  • a 1M NaOH solution is used to etch alumina for all ALD nanotube materials except for ZnO nanotubes.
  • 0.1 M NaOH is used to achieve etching of the alumina template while minimizing the etch attack of the ZnO nanotubes. It is also essential to perform a post-ALD deposition annealing procedure for ZnO nanotubes at 600° C. for 10 min in air, in order to obtain high quality smooth surface morphologies of the ZnO nanotubes.

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