Patent Application
20140332392
The present invention relates to an anodized aluminum oxide template that is used to grow periodic nanostructure and method of fabrication of the said template. The invention further relates to the fabrication of the respective periodic nanostructures from diverse materials using hydrothermal and/or CVD method for growing the said nanostructure.
The periodic nanostructures find use in the diverse applications is need of the hour in electronics, telecommunication, energy, sensing elements, field emission devices, wave guides, solar cells, LEDs, display, diodes etc. The periodic structure is defined as a structure that is repeated without substantial variation in dimensions and/or distance between the adjacent structures. The nano structures are in diverse forms such as nanowires, nanorods, nanotubes, nano-colloid. Furthermore morphological transformations of the nanorods are in the form of pencil-like, rods, tubular, dot, needle-like, tree-like, ball, Urchin-like, spherical shapes
Conventional methods of creating the periodic nanostructure primarily comprises of use of template. The desired pattern is created on this template using lithography techniques such as electron beam, laser, nano imprint, mask lithography of porous alumina, soft lithography, interference lithography for creating/forming individual openings at desired pitch. It is well established fact that this process is time consuming and therefore very expensive.
Yung-Huang Chang et. al. report Fabrication and Characteristics of Self-Aligned ZnO Nanotube and Nanorod Arrays on Si Substrates by Atomic Layer Deposition.
There are several methods for fabricating nano-rods/nanostructures using AAO template. Following are the details:
One of the conventional methods uses AAO membrane that is used for gold sputtering to create impression on Silicon substrate. However the membrane is substantially low (of the order of 200 nm) to enable passing through gold particles through the pores of the membrane to reach the Si substrate. The characteristics of the Si substrate (smoothness and crystalline structure) is used in this method. However, it suffers from the drawback that the superthin membrane poses problem in handling leading to limitations of scaling up the process or use of the membrane on a concentrated area (of the order of few micrometer).
In nano imprint technology seed layer is dispersed on the substrate (Si) and further photoresist (adhesive) is placed on the seed layer. An external nano-stamp is fabricated having desired periodic structure in the form of raised/protruding portions. This stamp is pressed on the said photoresist layer to enable nano-indents on the said layer. Etching process is carried out to open the indents for the seed layer that creates periodic arrangement of the seed layer used for growing nano rods.
In yet another method reported in the literature, the nano-stamp is inserted inside the seeds that are adhered on the surface of the nano-stamp. This seed loaded nano-stamp is then pressed on the substrate (such as Si). This results in the transfer of the seeds from the said raised portions of the nano-stamp on the substrate surface creating a periodic arrangement of the seeds. Further, the seeds are processed using methods such as CVD, hydrothermal method for growing nanorods or structures.
In yet another method disclosed in US Patent application number 20060270229 aluminum layer is deposited on the top of the Si wafer. Further the Aluminum is anodized to form AAO. Further the barrier layer is removed to expose Si in the nanochannels of AAO. The desired material is electrodeposited in the nano-channels of AAO from top of the AAO. The Si is the substrate. The AAO is chemically etched to obtain nano-rods that substantially perpendicular to the surface of Si substrate. However this method suffers from limitations that substantially expensive Si substrate is necessary due to its inherent characteristics such as smoothness. This method is not capable of providing crystalline hexagonal geometry of ZnO nano-rods because the said nano-rods are oxidised after formation in the AAO.
The Korean patent application number KR 20060103924 discloses method for fabricating the nano-rod by AAO template and the nano-rod using the method. The AAO is grown on Al. Further, gold is sputtered to form conducting seed layer on the top surface of AAO. Further, the gold layer thickness is increased by electrochemical deposition for enhancing conductivity. The Al is etched chemically followed by etching of barrier layer. The desired material is electrochemically deposited in the opened nano-pores of the AAO. Further, the AAO is etched chemically to form the metallic nano-rods. However, there is limitation of using the thick membrane in this method. The rods are formed using electrochemical deposition that limits use for only metals. The nano-rod length is limited by the membrane thickness. That is the rod length is the function/dependent on the membrane thickness. This method can-not be used for hydrothermal growth or CVD growth of rods.
Muhammad Arif et. al. report method that uses electro-deposition to form rods on the flexible substrate of PDMS. The graphene is used as transparent electrode for connecting nano-rods. However this method suffers from drawback that it is not possible to achieve crystalline structure of the nano-rods because of the use of electrochemical deposition.
US Patent application number 20050276743 discloses method for fabrication of porous metal templates and growth of carbon nanotubes and utilization thereof. The seed for Carbon Nanotube (CNT) is placed inside the nano-channels of AAO and further CNT is grown using CVD. Further, the top of the AAO including grown CNT is attached with electrode. The Al and barrier layer of AAO is etched chemically and CNT is further grown for increasing the length of the tube. However, in this process the tube is grown in two directions. In the first phase the direction for the growth is towards the top surface of the AAO. Further the tube is grown from the bottom surface of the AAO upon removal of the barrier layer. However there is limitation of this method in terms of precisely placing the seed in the nano-channels without attachment with AAO surface
The methods disclosed in the prior art suffer from following drawbacks:
The main object of the invention is to provide AAO template assisted substantially uniform nanostructures, method of formation/manufacture of the nanostructures and AAO template thereof.
The main object of the invention is to provide a method for the growth of periodic nanostructures using AAO as a template. Further object of the invention is to provide the AAO template to this effect.
Another object of the invention is to provide a method to form an AAO template.
Yet another object of the invention is to enable substrate disposition for the seeds/nanostructures on the top of the AAO membrane.
Yet another object of the invention is to enable substrate disposition for the seeds and thereby nanostructures on the top of the AAO membrane and yet enable the growth of the nanostructures using AAO as a template.
Yet another object of the invention is to utilize AAO membrane barrier layer side to enable growth of the nanostructure and thereby as a template.
Yet another object of the invention is to integrate the process of seeding for nanostructure growth, substrate disposition and AAO template formation.
Yet another object of the invention is to enable growth of the nanostructures from the barrier layer side of the AAO membrane.
Yet another object of the invention is to enable maintain substantially lower thickness of the AAO template and yet achieve substantially large surface area for formation of the substantially uniform nanostructures.
Yet another object of the invention is to enable growth of the nanostructures from respective nano-channel/nano pore of the AAO membrane to achieve substantially uniform nanostructure near periodic distribution.
Yet another object of the invention is to obviate uncontrolled and undesirable growth of nanostructures from seeds from substrate.
Another object of the invention is to provide a method to selectively orient and direct nanostructure to achieve periodicity using AAO template.
Yet another object of the invention is to utilize AAO nano-channels as a guide to selectively direct the nanostructure to achieve periodicity.
Yet another object of the invention is to obviate use of substantially thick AAO membrane as a template.
Yet another object of the invention is to achieve length of the nanostructure independent of the membrane thickness.
Yet another object of the invention is to use AAO template over substantially large area yet maintaining substantially lower thickness of the template.
Yet another object of the invention is to obviate limitation of the membrane thickness obviating the limitation of the AAO channel thickness consideration.
Yet another object of the invention is to provide a method of fabrication of the AAO template.
Another object of the invention is to provide a method to grow periodic nanostrucuture/s from diverse metal oxides.
Yet another object of the invention is to obviate use of specific material properties such as smoothness and crystalline structure for substrate to achieve periodic structure formation.
Yet another object of the invention to enable use of flexible substrate for the formation of periodic nanostructure.
Yet another object of the invention is to enable use of substrate material to sustain substantially higher temperatures.
Thus in accordance with the present invention the AAO template enabled nanostructure comprises of
wherein the method of AAO template assisted substantially uniform nanostructure fabrication comprises steps of:
Optionally hard anodization is used in place of the said mild anodization steps wherein the hard anodization comprises steps of:
chemical etching of the anodized aluminum oxide comprising steps of:
etching for the time duration in the range of 5 to 10 minutes in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein phosphoric acid is in the range of 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt %;
second step hard anodization comprising steps of applying voltage in the range of 120V to 150V for 1 to 2 minutes wherein hexagonally arranged nanoporous structures are formed with one end blocked with barrier layer
to form the AAO template enabled substantially uniform near periodic nanostructures
wherein optional use of the CVD for the growth of the nanostructure comprises steps of selection of carrier and active gas
wherein the carrier gas is selected from a non-reacting gases such as Argon, Nitrogen and the active gas that enables reaction is selected from Oxygen, Carbon dioxide and therelike,
the source that is disposed in the CVD chamber is in the form of powder or gas; it is selected depending on the material of the seed,
optionally the source is a gas itself depending on the nature and material of the nanostructure such as carbon nanotube, zinc oxide, CdS to form the AAO template enabled substantially uniform near periodic nanostructures.
In one of the aspects of the invention the seeding for the growth of nanostructure/s is carried out using spin coating process wherein
optionally depositing nano-particles on the top surface of the said Alumina (AAO) structure using sputtering process
optionally a combination of the said deposition of the solution and deposition of the nanoparticles using respective spin coating and/or sputtering
Features and advantages of this invention will become apparent in the following detailed description and the preferred embodiments with reference to the accompanying drawings. The embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings.
a) illustrates the schematic of the seeding
b) illustrates schematic of the substrate disposition
In the following description, various embodiments will be disclosed. However, it will be apparent to those skilled in the art that the embodiments may be practiced with only some or shall disclosed subject matter. For purposes of explanation, specific numbers, materials, and/or configuration are set forth in order to provide a thorough understanding of the embodiments. However, it will also be apparent to one skilled in the art that the embodiments may be practiced without one or more of the specific details, or with other approaches, materials, components etc. In other instances, well-known structures, materials, and/or operations are not shown and/or described in detail to avoid obscuring the embodiments. Accordingly, in some instances, features are omitted and/or simplified in order to not obscure the disclosed embodiments. Further more, it is understood that the embodiments shown in the Figures are illustrative representation and are not necessarily drawn to scale.
As illustrated in the schematic in
The method of AAO template assisted substantially uniform nanostructure fabrication comprises steps of:
electro-polishing of the substrate,
The process of electro polishing of Aluminum substrate (Al) comprising steps of:
First step mild anodization comprising steps of:
The chemical etching of the anodized aluminum oxide comprising steps of etching for the time duration in the range of 5 to 10 minutes in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein phosphoric acid is in the range of 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt %;
The second step mild anodization comprising steps of repeating the process in the first step anodization wherein hexagonally arranged nanoporous Alumina (AAO) structures are formed with one end blocked with barrier layer wherein process time depends on the membrane thickness, it can range from 5 minutes to 25 minutes.
In one of the embodiments, optionally hard anodization is used in place of the said mild anodization steps wherein the two step hard anodization process comprises steps of:
Chemical etching of the anodized aluminum oxide comprising steps of etching for the time duration in the range of 5 to 10 minutes in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein phosphoric acid is in the range of 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt %.
The second step hard anodization comprising steps of applying voltage in the range of 120V to 150V for 1 to 2 minutes wherein hexagonally arranged nanoporous structures are formed with one end blocked with barrier layer.
The process of seeding for the growth of nanostructure/s comprises steps of:
The disposition of the substrate comprises steps of selection of substrate material depending on the process of growth of the nanostructures. The schematic of this aspect is illustrated in
Optionally if the process of hydrothermal growth of the nanostructure is selected a flexible or hard substrate is selected.
In one of the embodiments the hard material is selected from glass, thick metal layer and therelike that do not tend to bend under force, a person skilled in art can easily contemplate meaning of the hard material in the context of the substrate in this art. In another embodiment the flexible material is selected from Polydimethyl Silexene (PDMS), rubber, polymer based materials, plastics and therelike that tend to bend under force and regain original shape, a person skilled in art can easily contemplate meaning of the flexible material in the context of the substrate in this art.
The process of removal of Al and barrier layer comprising steps of chemical etching of the Al in cupric chloride (CuCl2)and hydrochloric acid (HCL) solution wherein the temperature is in the range of 30° C. to 40° C., concentration of CuCl2 is in the range of 0.2M to 0.25M and that of HCL is in the range of 5.5M to 6.2M wherein the time duration depends on the thickness of Al to be etched optionally saturated mercuric chloride is used for the said chemical etching process and the temperature is in the range of 50 to 60° C. The result of the barrier layer is depicted in
The process of barrier layer (BL) removal by chemical etching comprises steps of placing of AAO in 5 wt % to 6 wt % Phosphoric acid for time duration in the range of 200 to 250 min at a temperature in the range of 32° C. to 33° C.
As depicted in
The process of growth of the nanostructures from the said opened nanopores is effected by use of hydrothermal growth or Chemical Vapour Deposition process.
The process of hydrothermal growth comprises steps of:
to form the AAO template enabled substantially uniform near periodic nanostructures as depicted in
The CVD process for the growth of the nanostructure comprises steps of selection of carrier and active gas
wherein the carrier gas is selected from a non-reacting gases such as Argon, Nitrogen and the active gas that enables reaction is selected from Oxygen, Carbon dioxide and therelike,
The source that is disposed in the CVD chamber is in the form of powder or gas; it is selected depending on the material of the seed,
optionally the source is a gas itself depending on the nature and material of the nanostructure such as carbon nanotube, zinc oxide, CdS cadmium sulphide to form the AAO template enabled substantially uniform near periodic nanostructures.
In one of the embodiments the nanostructure is in the form of nanowires, nanorods, nanotubes, nano-colloid. wherein morphological transformations of the nanorods are in the form of pencil-like, rods, tubular, dot, needle-like, tree-like, ball, Urchin-like, spherical shapes.
In another embodiment the zinc oxide nanostructure is formed.
