Patent Application
20150292080
This application relates to an apparatus for the generation of a nanocluster film or a compound film.
The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.
Nanoclusters are aggregates of atoms that contain from a few atoms to a few thousand atoms, in which crucial size-dependent properties such as the quantum confinement effect can be engineered for nanotechnology applications3. However, due to the large surface-to-volume ratio of nanoclusters, drawbacks such as material deterioration, cluster aggregation and chemical instability become an inevitable challenge to control4. Through the infusion of Nanoclusters into a matrix material, the nanoclusters are encapsulated from ambient conditions and hence are applicable for device fabrications5.
Conventionally, there are various methods to synthesize nanoclusters, such as, chemical reactions6, ion sputtering, implantation7 and cluster beam deposition8. In general, the process used in the synthesis of the nanoclusters is strongly dependent on the chemical structure of the material, generally provides insufficient control over the density and the uniformity of the nanocluster and also little or no intermixing capability to enable compound nanocluster generation.
Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure that possess extraordinary intrinsic properties for thermal, electrical and mechanical applications15. Commercial CNTs are commonly produced by the Chemical Vapor Deposition (CVD) method. For CVD, typically, the process involves the preparation of the metal catalyst which requires the deposition of a thin layer of metal catalyst (˜1-10 nm) and subsequently a thermal treatment (˜600° C.) to produce metal clusters before the growth of CNTs. The CVD method involves multiple process steps and the CNTs usually suffer from large boundary resistances (thermal and electrical)16.
Owing to their high chemical and thermal stabilities, carbon encapsulated Ni nanoparticles have recently received considerable attention17, 18, 19. This attention has been especially focussed on magnetic applications because of the potential to have small magnetic clusters with preserved surface spin, large coercive force, enhanced signal to noise ratio when reading/writing and reduced magnetic coupling between individual clusters, all of which leads towards ultra-high density magnetic storage applications 20, 21, 22, 23, 24. In addition, carbon thin films possess versatile mechanical, thermal and electrical properties dependent on the configuration of its hybridization25. For example, graphite possesses one of the highest thermal conductivities in the horizontal direction (parallel to its basal planes) but four orders of magnitude lower thermal conductivity in the perpendicular direction, due to the anisotropic nature of the crystalline configuration. Vertically orientated nanocrystalline graphite (NCG) films are known to have excellent thermal conductivity in the vertical direction, due to the formation of graphitic basal planes perpendicular to the substrate. The thermal conductivity of such a film was found to be 16.72 W/m K, as compared to amorphous carbon film of 0.95 W/m K26. However, no methods are currently known that can make a textured carbon as the matrix for nickel nanoclusters, such a method would enable a higher quality film. There currently exists a need to find better ways to make compound nanoclusters, carbon nanotubes and carbon encapsulated Ni nanoparticles.
Filtered Cathodic Vacuum Arc (FCVA) techniques, an arc-evaporation plasma deposition method2, have been previously disclosed by the current inventors1 and others. The inventors have surprisingly found that the FCVA technique and apparatus can be adapted for use in the formation of compound nanoclusters, carbon encapsulated Ni nanoparticles and compound nanoclusters for use as a catalyst to make superior carbon nanotubes.
The present invention aims to provide a new method for the production of nanoclusters, and their infusion into a film to form a nanocluster film. It further aims to provide a hardware design, prototype development and a new methodology for the generation of compound nanoclusters.
This technology is based on the FCVA technique, whereby parameters that govern the deposition, such as the ion density and energy of the plasma, can be adjusted according to the required application. In general terms the invention proposes a new design of the magnetic filter (a Y-bend filter) that couples with two target sources for the simultaneous production of two separate plasmas in a single deposition process. An electromagnetic component that is based on the engineering of the magnetic field has also been developed to provide an additional medium to control the intermixing of the two plasmas and therefore, the properties of the generated nanoclusters.
In embodiments of the invention, the ion density and energy of the two plasma sources can be adjusted independently to provide a mode of control for the parameters of the nanoclusters. Furthermore, the FCVA technique is capable of depositing a wide range of materials2. As discussed herein, the interaction of the two plasmas is established with the Y- bend filter by channelling the two plasma beams into one. This interaction is further enhanced with a controllable condensation of the plasma density before the nucleation and the growth of the nanocluster film.
Furthermore, with the capability of intermixing two different sources, nanoclusters in a matrix film can be formed and hence, provide a new platform for the development of complex atomic nanocluster physics.
In an aspect of the invention, there is disclosed a filtered cathodic vacuum arc (FCVA) apparatus for the generation of a nanocluster film or a compound film on a substrate, comprising: a deposition chamber; a substrate holder for holding a substrate; means for simultaneously generating a first beam of plasma and a second beam of plasma from a first and a second plasma source, respectively; a Y-bend magnetic filter that connects the first and second plasma sources with the deposition chamber, the Y-bend filter comprising a single stem, a first arm and a second arm, with the first arm connecting the first plasma source to the stem, and the second arm connecting the second plasma source to the stem, and the stem connecting the first and second arms to the deposition chamber, at least the first and second arms containing respective magnetic field generation means for guiding the first beam of plasma and the second beam of plasma respectively along the first and second arms; and an anti-Helmholtz coil set-up within the deposition chamber that is adapted to provide a magnetic confinement for the plasma generated from the first and second plasma sources, and which comprises a first and a second electromagnetic coil, wherein, the Y-bend magnetic filter is adapted to channel the plasma from the first and second plasma sources together in a first stage interaction and the anti-Helmholtz coil set-up is adapted to establish a second stage interaction between the respective the first and second plasmas by magnetic confinement. In embodiments of the invention, the apparatus is for the generation of a nanocluster film.
In an embodiment, the Y-bend magnetic filter further comprises a focusing coil attached to the stem. In yet further embodiments, the first and second arms of the Y-bend magnetic filter each comprise a first bend and a second bend.
In yet further embodiments, the means for simultaneously generating a beam of positive ions in the form of plasma from a first and a second plasma source is a first anode-cathode assembly attached to the first plasma source and a second anode-cathode assembly attached to the second plasma source.
In embodiments of the invention, the first plasma source is carbon or a metal and the second plasma source is a metal. For example, the second plasma source is selected from zinc, nickel, aluminium and iron. For example, when the first source is a metal, it can be selected from zinc, nickel, aluminium and iron, provided that the first source is not the same as the second source. In yet still further embodiments, the apparatus further comprises: means for independently controlling the energy of the first and second plasma beams, optionally wherein the means is supplied by a first and second current controlled attached to the first plasma source and the second plasma source, respectively; and/or means for independently controlling the density of the first and second plasma beams, optionally wherein the means is supplied by a first and second tuning controller that tune the magnetic field produced in the first arm and the second arm of the Y-bend magnetic filter, respectively.
In yet still further embodiments, the anti-Helmholtz coil further comprises a current controller for independent control of the first and second electromagnetic coils.
In a further aspect of the invention, there is disclosed a process for the deposition of a compound nanocluster film onto a substrate, using the herein before disclosed FCVA apparatus, comprising the steps of:
In embodiments of the invention, the magnetic containment field is generated by said anti-Helmhotz coil set-up, said first and second electromagnetic coils independently producing substantially identical but opposed electromagnetic fields. In a further embodiment, the first and second electromagnetic coils operate using a current from 0.5 A to 20 A, optionally from 2.5 A to 15 A, such as from 5 A to 10 A.
In a further embodiment of the invention, the process can further comprise a step (g) where carbon nanotubes are formed from the nanocluster film deposited onto the substrate in step (f) of the process disclosed hereinbefore.
In yet a further embodiment of the invention, wherein the first plasma source is carbon and the second plasma source is a metal. For example the second plasma source is selected from zinc, nickel, aluminium and iron. For example, when the first source is a metal, it can be selected from zinc, nickel, aluminium and iron, provided that the first source is not the same as the second source. In a yet further aspect of the invention, there is disclosed a compound nanocluster thin film and/or compound nanocluster thin film catalyst for the synthesis of carbon nanotubes obtained or obtainable by the processes disclosed hereinbefore. For example, the nanocluster film used in said process is made of cobalt nanoclusters embedded in a carbon matrix and the carbon nanotubes are formed by a chemical vapour deposition process using C2H2 and NH3 at 650° C. In a yet further aspect, there is disclosed a carbon nanotube obtained or obtainable from the process of generating a carbon nanotube from a compound nanocluster catalyst as disclosed hereinbefore.
In a yet further aspect, there is disclosed a carbon encapsulated metal nanocluster obtained or obtainable by the process disclosed hereinbefore.
Preferred embodiments will now be described with reference to the accompanying figures in which:
a is a schematic of the Y-bend FCVA deposition apparatus;
b is an image of the Y-bend FCVA deposition apparatus;
a is a schematic of the Y-bend magnetic filter, showing the double bends in each of the arms of the filter;
b is an image of the Y-bend magnetic filter;
a is a simulation of the formation of the magnetic trap;
b depicts the density of plasma observed in a functioning magnetic trap;
a (prior art) is a schematic depicting the preparation of a catalyst and growth of carbon nanotubes using conventional technology;
b is a schematic depicting the preparation of catalyst and growth of carbon nanotubes using the apparatus and method of the current invention; and
Embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings. As those skilled in the art would realize, the described embodiments may be modified in various different ways without departing from the scope of the present invention.
In the drawings, dimensions of layers, regions, etc., are exaggerated for clarity. Like reference numerals designate like elements throughout the specification and the drawings.
When referred to herein the terms “plasma source”, “plasma target” are interchangeable unless otherwise indicated.
When referred to herein the terms “arc source” and “arc supply” are interchangeable unless otherwise indicated.
In all embodiments of the invention disclosed herein, terms such as “comprises” or “comprising of” can be replaced by terms such as “consists of” and “consisting of”.
In an embodiment of the invention, the FCVA apparatus is described with reference to
The schematic of the deposition apparatus (100) is shown in
As depicted, the substrate holder contains a substrate (108) for deposition of the plasmas generated from the plasma sources (101, 102). The anode and cathode assembly (103) comprises a striker (109) and arc supply (110). In the embodiment shown, the first plasma source can be a carbon source and the second plasma source can be a metal source for the generation of carbon and metal ions, respectively. Heating and a source of electricity are supplied to the deposition chamber via the heater (112) and electrical bias supply (113), respectively. Figure lb shows the image of the actual setup of the apparatus.
a. The generation of plasma and the Y-bend magnetic filter
The plasma sources (or targets) are connected to an arc supply (110) within the anode-cathode assembly (103) while the striker (109) is connected to ground. When a contact is made, an arc is generated and plasma is formed. The ion energy of the two independent plasmas can be controlled by adjusting the electrical current of the respective arc source. The anode-cathode assembly is of standard construction and is described in US 2003/085123 and U.S. Pat. No. 6,262,539.
Due to the charged-nature of the plasma, the magnetic field generated by the Y-bend magnetic filter (104) produces a steering force (Lorentz force), acting perpendicularly to the momentum of the charged particle and hence provides a spirally guided motion for the plasma to travel towards the deposition chamber. The density of these plasma beams can be controlled by tuning the strength of the magnetic field at the Y-bend filter. These two plasma beams are combined at the junction of the Y-bend filter and therefore, engage in the first stage of their interaction. Unwanted macro-particles, being neutrally charged, are trapped and filtered by the Y-bend magnetic filter.
The Y-bend magnetic filter (104), consists of two off-plane double bend magnetic filters (104a, 104b), manufactured as mirror images to each other and partially attached together at the upper end, as shown in
Due to the bending angle of filters used in FCVA systems, the guided plasma beam is usually off-center as it leaves the filter. This is true for conventional FCVA systems with a single plasma beam. This is not a significant problem for single-beam systems because the substrate can be aligned with respect to the plasma beam. However, when two plasma beams are used, each beam deflects to a different off-center position. To ensure that the beams are focused properly onto the magnetic trap discussed below, a focusing coil is used to realign the beams.
The focusing coil is an electromagnetic coil and is connected to a DC power supply. The operational range of the focusing coil is about 10 A and it is placed at the end of the Y-bend magnetic filter, as shown in
b. Anti-Helmholtz coil and the intermixing of the two plasmas
The second stage of interaction between the two plasmas is established at the magnetic confinement created by the anti-Helmholtz coil within the deposition chamber. The purpose of the magnetic confinement is to condense the density of the intermixed plasmas and therefore increases the probability of the interaction, as illustrated in
An anti-Helmholtz coil (106) consists of two electromagnet coils (i.e. a first and a second electromagnetic coil; 200, 201), arranged in an anti-parallel configuration. Analogously with a permanent magnet (300), when the magnetic fields from the two coils are aligned in an opposition direction a net field of zero is created at the center of the coils (magnetic trap; 301), as shown in
The simulated result from the formation of the magnetic trap can be seen in
The operational current of the anti-Helmholtz coils is directly proportional to the strength of the magnetic confinement and therefore, the density of the nanoclusters. Increasing the magnetic confinement will cause a decrease in the confinement volume and results in a smaller deposition area. As described below the system disclosed has been tested up to 10 A but higher currents are achievable using a cooling system. As a general guideline, the current of the anti-Helmholtz coils can be anything greater than 0 A up to the maximum current carrying capacity of the coil before it melts down.
In embodiments of the invention, maximum deposition efficiency is achieved when the center of the focusing coil, the anti-Helmholtz coils and the substrate holder are aligned together in a straight line.
In operation, the anode cathode array generates first (101) and second (102) plasma sources are ignited using the strikers (109) and arc sources (110) of the anode-cathode assembly (103) to simultaneously and independently generating a first beam and a second beam of plasma from a first plasma source and a second plasma source, respectively. The resultant first beam of plasma is directed through the first arm (104a) of the Y-bend magnetic filter (104), while the resultant second beam of plasma is directed through the second arm (104b) of the Y-bend magnetic filter (104). The energy of the first beam and the second beam is independently controlled by adjusting the electrical current of the first or second plasma source, respectively.
The first and second beams of plasma intersect and mix together in a first stage interaction in the stem (104c) of the Y-bend magnetic filter (104). The direction of the first beam of plasma and the second beam of plasma is accomplished by the use of magnetic filters attached to the first (104a) and second (104b) arms of the Y-bend magnetic filter (104). Additionally the density of the first beam of plasma and the second beam of plasma is controlled by tuning the strength of the magnetic field of the Y-bend filter.
The resultant mixed plasma is then directed towards an electromagnetic magnetic containment field situated in a deposition chamber, optionally the orientation and direction of the beam may be controlled by a focusing coil (111) which helps to align and converge the plasma beams. The magnetic containment field is generated by the electromagnetic coils (200, 201) of the anti-Helmholtz set-up (105), which provide substantially identical electromagnetic fields, and is described herein with reference
The substrate holder (107), which holds the substrate is positioned and oriented so that it is placed within the anti-Helmholtz coils, near to the magnetic trap, as shown in
When discussed herein, the magnetic trap or magnetic confinement is not an absolute confinement. The strength of the magnetic field varies at a gradient, from zero at the center and increases as it moves away from the center of the trap, towards the coil, as shown in the simulation result at
In a typical system according to the invention, the system operates at a base pressure of 2-6×10−6 Torr, achieved with a rotary pump and a turbo-molecular pump. Three separate DC power sources independently provide electrical current to the Y-bend filter, focusing coil and the anti-Helmholtz coils, respectively, to create the magnetic guiding field and trap. The amount of the current for the Y-bend magnetic filter and the focusing coil are dependent on the magnetic field strength required and different systems may have different requirements.
In an embodiment of the invention, 60 A is supplied to the Y-bend filter, 10 A is supplied to the focusing coil and a current from 0 A to 10 A is supplied to the anti-Helmholtz coils.
To generate and sustain plasma from the material target placed within the anode-cathode assembly, it is necessary to maintain an electrical arcing across the target material and the amount of voltage is material dependent, associated to their ionization energy. Typically for a carbon target and operating voltage of 50V is applied, for a Zinc target the operating voltage will be about 70V and for aluminum the operating voltage will be around 130V.
As discussed in the examples below, the apparatus described above can be used to make:
Compound nanocluster thin films have widespread of potential applications in microelectronic9, optoelectronics10, magnetic applications11, gas sensors12, catalysis13 and even biotechnological applications14.
It is speculated that the use of a compound nanocluster catalyst prepared using the apparatus and process of the currently disclosed application will result in improved thermal boundary resistance between CNT and substrate for use in the production of next-generation thermal interface materials.
Theoretical studies and experimental results have both demonstrated that the thermal boundary resistance between carbon nanotubes and substrate is inversely proportional to their contact surface27. Also, current carbon nanotube growth techniques on many substrates require a barrier layer (typically SiO2, AlN) to prevent the catalyst layer from diffusing into the substrate during growth. This means that the thermal boundary resistance between the carbon nanotube and the substrate is unnecessarily increased. In order to circumvent this, a hybrid interface layer (e.g. the Ni:C nanocluster thin film embodied in this application) is introduced. By embedding a segment of the base of the carbon nanotubes into a carbon-based matrix, the contact surface area is increased while the barrier layer is minimised. Therefore, it is speculated that the overall thermal performance will significantly improve.
a shows the conventional methodology used to make carbon nanotubes. Carbon nanotubes are conventionally prepared by chemical vapor deposition (CVD) of a metal layer (˜1-10 nm) on a substrate step (1) in
As discussed above, carbon encapsulated metal nanoclusters (particularly carbon encapsulated nickel nanoclusters) are useful in magnetic applications, due to their properties (particularly high-temperature magnetic storage properties in a high temperature environment).
In
Scanning Electron Microscope (SEM) is used to examine the cluster distribution.
X-Ray Diffraction—XRD is the method to determine the crystal structure of the clusters and the carbon matrix.
High Resolution Transmission Electron Microscopy (HRTEM) is the technique to tell the detail microstructure of the nanocluster film, the detail arrangement of nanocluster and the orientation of the NCG carbon can be seen using this technique.
The operation of FCVA apparatus is initiated by igniting an arc by making momentary contact the target surface (cathode) with a graphite striker connected to anode, the spot is heated rapidly due to the high local current density. A flux of ions with energies exceeding the cathode-anode potential difference and amounting to roughly 10% of the arc current will be emitted from the target arc spots. The ion plasma will then travel through the magnetic filter, and all neutral particles will be filtered away by the filter, only the desired ions will pass through and landed on the deposition substrate in the vacuum chamber. The ions will then form the film with different microstructures (sp2, sp3 content, and degree of graphitization) depends on the ion energy (controlled by the substrate bias, heating).
A Metal (Zinc)—Carbon nanocluster film is deposited with the Y-bend FCVA system described herein, with both targets having a purity of 99.99%. Three different conditions of the magnetic trap are tested with the electrical current at both of the electromagnetic coils being adjusted to 0 A, 5 A and 10 A. The thickness of each deposited film is 100 nm, approximately. The scanning electron microscopy (SEM) images of these films are shown in
To verify the chemical characteristic of these deposited nanocluster films, energy-dispersive X-ray (EDX) spectroscopy is performed on the material deposited using the anti-Helmholtz coils at a current of 10 A (also depicted in
In a conventional carbon nanotube (CNT) synthesis approach, a thin layer of catalytically active metal layer (˜10 nm) is firstly deposited onto the substrate and subsequently, a thermal annealing process at the range of 550-600° C. This annealing process is to transform the thin layer of metal into island-like nanostructures. The diameters of these island-like structures are typically in the range of 10 nm. For the growth of CNTs, the nanostructure film then undergoes a chemical vapor deposition (CVD) process by having chemical reaction with C2H2 and NH3 in a controlled environment at 650-700° C. The diameter of the grown CNTs is closely related to the diameter of the island-like nanostructure.
In the current example, Cobalt (Co) is used as the catalytically active metal and it is directly embedded into the carbon matrix film as a form of nanoclusters, identical to the previously mentioned island-like nanostructures, using the FCVA apparatus of this invention. This deposition process is performed by having one source as Co (at 90V) and the other source as carbon (at 50V). To synthesise CNTs, the nanocluster film undergoes the same CVD process with C2H2 and NH3 at 650° C. as per the conventional method. By varying the strength of the magnetic trap (0-10A), the density of the nanoclusters varies, which corresponds directly to the density of the grown CNTs. An advantage of this approach is that the island-like nanostructures are now embedded within the carbon matrix, therefore, the contact surface area is increased and as a result, the electrical/thermal boundary resistance between the CNTs and the substrate is reduced.
The SEM image of the CNTs synthesized with the nanocluster film described above is shown in
We have successfully infused Nickel (Ni) nanoclusters into vertically orientated nanocrystalline graphite (NCG) film to study its potential for high temperature magnetic storage applications, as shown in the schematic in
For the application of high-density magnetic storage, we embed Nickel (Ni) nanoclusters into a NCG matrix. Since the objective is to increase the magnetic storage capacity, the magnetic trap operates at its maximum strength (10A) to deliver a high density Ni nanocluster thin film. Ni (at 90V) is used one of the sources and carbon (at 50V) is used for the other. A substrate temperature of 400° C. is applied during the deposition process in order to produce the NCG film.28
1. Patents of the FCVA Technology (Nanyang Technological University, Singapore)
i Filtered cathodic arc source (1995) Description : Invention of double bend solenoid filter Shi X, Flynn D, Tay B K, Tan H S Patent No: U.S. Pat. No. 6,031,239
ii Filtered cathodic arc source deposition apparatus (1995) Description: Invention of FCVA coating system to deposit amorphous diamond films Flynn D, Fulton M, Shi X, Tan H S, Tay B K Patent No: US 2003085123
iii Cathodic arc source and graphite target (1996) Description: Invention of electromagnetic field configuration used for FCVA technology Shi X, Tan H S, Tay B K Patent No: UK 97932940.6
iv Cathodic arc source with target feeding apparatus (1997) Description: Invention of continuous target feeding for production of amorphous diamond coating Shi X, Tay B K, Tan H S Patent No: U.S. Pat. No. 6,262,539
v Enhanced Macroparticle filter And cathodic Arc Source (1997) Description: Invention of filter to improve filtering efficiency for FCVA technology Shi X, Tay B K, Tan H S Patent No: U.S. Pat. No. 6,511,585
vi Cathodic Arc Source For Metallic and Dielectric coatings (1997) Description: Invention of metal and dielectric coatings using FCVA technology Shi X, Tay B K, Tan H S Patent No: U.S. Pat. No. 6,413,387
vii Composite Coatings (1999) Description: Invention of carbon based composite coatings using FCVA technology Shi X, Tan H S, Tay B K Patent No: U.S. Pat. No. 6,143,142
viii Ignition Means for a cathodic arc source (1995) Flynn D, Shi X, Tan HS, Tay B K Patent No: U.S. Pat. No. 6,319,369
ix Heat Transfer Surface (1999) Description: Heating element having a heat transfer surface comprising a layer of tetrahedral amorphous carbon and/or diamond-like carbon, and for a composite such a heating element has a prolonged service life and requires less frequent cleaning to remove deposits which may otherwise cause a reduction in the heat coefficient of the element and an increase in the consumption of electricity Shi X, Tan HS Patent No: U.S. Pat. No. 6,571,865
x Low Fouling & Drop-wise Coatings for Heat Exchangers (1999) Description: Invention of coating process for dropwise condensation Shi X, Tan H S, Tay B K Patent No:
xi Arc Monitoring (1997) Description: Invention on the control and monitoring of FCVA process Shi X, Jin X Z, Tan H S Patent No: U.S. Pat. No. 6,338,779
xii Field Emission Device and Method of Fabricating Same (2001) Description: A method of fabrication a field emission component for use in flat panel displays and vacuum microelectronics applications Sun Z, Tay B K, Li Y J, Lau S P Patent No:
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| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2013/000050 | 2/6/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61595346 | Feb 2012 | US |
