Abstract:
Known techniques for forming nanoparticles require a multiple-step process to coat a surface with nanoparticles. The present invention provides a single-step process that requires the deposition of a substrate in a mixture of a solvent, ligands and organometallic precursors. The mixture containing the substrate is heated under pressure in a dihydrogen environment for a predetermined period of time, during which supercrystals of nanoparticles form on the substrate.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a method of coating a surface with nanoparticles of the type, for example, that overlies a substrate. 
       BACKGROUND OF THE INVENTION 
       [0002]    Thin film technology, wherein inorganic particles with sizes on the order of 1-100 nm are arranged in layers to form a film, is currently being used for an increasingly large number of different technological applications, including: information storage systems, chemical and biological sensors, fibre-optic systems, and magneto-optic and optical device. 
         [0003]    A number of techniques are currently known for the preparation of such films. U.S. Pat. No. 6,805,904 discloses a method and structure that forms a multilayer nanoparticle thin film assembly by functionalizing a substrate with functional molecules and then replacing a stabilizer on a bottom surface of first nanoparticles with the functional molecules via surface ligand exchange to make a first nanoparticle. U.S. Pat. No. 6,676,729 relates to a method of making nanoparticles via metal salt reduction by firstly mixing metal salts in a solvent and then adding a reducing agent to the solvent. The nanoparticle dispersion is then stabilized and the nanoparticles precipitated from the nanoparticle dispersion. Finally, the nanoparticles are re-dispersed into the solvent. “Alternation of cationic and anionic polymeric materials and metal nanoparticles” (Decher et al., Science 1997, vol. 277: page 1232) describes alternation of cationic and anionic polymeric materials and metal nanoparticles. This is also described in “Layer-by-Layer Growth of Polymeric Nanoparticle Films Containing Monolayer Protected Gold Clusters” (Jocelyn F. Hicks, Young Seok-Shon, and Royce W. Murray, Langmuir, 2002, 18, 2288-2294) and “Rapid deposition of gold nanoparticle films with controlled thickness and structure by convective assembly” (B. G. Prevo, J. C. Fuller, III and O. D. Velev, Chem. Mater, in press (2004)). U.S. Pat. No. 6,162,532 describes a layer-by-layer formation of a film of compact arrays of magnetic nanoparticles for a magnetic storage medium. 
         [0004]    However, all of the above described techniques comprise multiple steps to achieve formation of the nanoparticles on the substrate employed, including formation of the nanoparticles, selection of the size of nanoparticles and then deposition of the nanoparticles. At each step, loss or degradation of the end-product is risked and, in particular, when the nanoparticles are air-sensitive, resulting in restricting possible applications of the nanoparticles formed due to the resulting degradation of the nanoparticles formed caused by an ambient atmosphere. Some solutions have been proposed to obviate this disadvantage, for example, coating the particles with an amorphous and inert substance, such as silica or a polymer, or forming an oxide coating on the nanoparticles as described in U.S. Pat. No. 6,045,925. However, such coating results in a loss of some of magnetic properties of the nanoparticle film. 
       STATEMENT OF INVENTION 
       [0005]    According to the present invention, there is provided a method of forming nanoparticles as set forth in the claims herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0007]      FIG. 1  is a schematic diagram of an apparatus for forming nanoparticles; 
           [0008]      FIG. 2  is a flow diagram of a method of forming nanoparticles constituting an embodiment of the invention; 
           [0009]      FIG. 3  is a scanning electron micrograph image of nanoparticles formed in accordance with the embodiment of  FIG. 2 ; and 
           [0010]      FIG. 4  is a scanning electron micrograph image of nanoparticles formed in accordance with another embodiment of the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0011]    Referring to  FIG. 1 , an apparatus  100  for the manufacture of nanoparticles comprises a bath  102  of oil  103  in which a flask  104  is partially immersed. The oil  103  can be any suitable oil that has a boiling point that is higher than the temperature of the reaction to take place. The flask  104  contains a predetermined quantity of a mixture  106 , the mixture being immersed below a level  107  of the oil  103  of the oil bath  102 . In addition, an opening  108  of the flask  104  is connected to a source of gas  110 , for example dihydrogen (H 2 ), via a tube  112  coupled to a pump (not shown) for pressuring the flask  104 . 
         [0012]    A seed material  114  is also disposed within the mixture  106  adjacent a base of the flask  104 . The seed material  114  has at least one surface exposed to the mixture  106  and, in this example, is a silicon (Si) substrate. However, other materials can be used to form the seed substrate, for example glass, alumina, carbon, ceramics, or sapphire. 
         [0013]    Turning to  FIG. 2 , the nanoparticles are formed as follows. 
         [0014]    The bath  102  is firstly heated {Step  200 ) to an appropriate temperature for formation of nanoparticles  116  ( FIG. 1 ) to take place, for example, between room temperature and 260° C. In this example, the bath is heated to a temperature of 150° C. The mixture  106  is then formed in the flask  104  by filling the flask  104  with ligands (Step  202 ), serving as a stabilising agents, for example an organic amine or acid, such as Oleylamine and/or hexadecylamine and/or carboxylic acid. In this example, the flask  104  is first filled with 1 mmol hexadecylamine and then 1 mmol of a carboxylic acid, such as oleic acid. Thereafter, a first metal-organic (organometallic) precursor, for example, a cobalt precursor, such as 1 mmol of cyclooctadiene-cyclooctenyl cobalt (Co(η 3 —C 8 H 13 ) (η 4 —C 8 H 12 )), is then added (Step  204 ) to the flask  104 . 
         [0015]    The silicon substrate  114  is then immersed (Step  206 ) in the mixture created so far. 50 ml of a degassed and distilled solvent, for example, an ether or an aromatic solvent, such as mesytilene, toluene, or anisole is then added (Step  208 ) to the flask  104 . In this example, the solvent is toluene, but the skilled person will appreciate that other solvents can be used. To complete the mixture  106 , a second metal-organic (organometallic) precursor, for example, an iron precursor, such as 2 mmol of iron pentacarbonyl (Fe(CO 5 )), is then added (Step  210 ) to the flask  104 . 
         [0016]    Although the above described example refers to specific precursor compounds, the skilled person will appreciate that, depending upon the type of nanoparticles that are being produced, different precursor compounds can be employed. Examples of iron precursors are: carbonyls; olefins, such as iron indenyl, iron cyclopentadienyl (FeCp 2 ), or iron fluorenyl; and amides, such as bis(bistrimethylsilyl)amide iron. Examples of cobalt precursors are: carbonyls, such as cobalt carbonyle (CO 2 (CO) 8 ); olefins, such as cobalt indenyl, cobalt cyclopentadienyl (CoCp 2 ), or cobalt fluorenyl; and amides, such as bis(bistrimethylsilyl)amide cobalt. 
         [0017]    The mixture  106  is subsequently pressurised (Step  212 ), for example, with 3 Bars of H 2 . It should be appreciated that other pressures can be applied, for example between about 1 and about 5 Bars. Further, other reducing gasses can be employed, for example carbon monoxide. Thereafter, the mixture  106  is heated (Step  214 ), for example at 150° C. for 48 hours (Step  216 ). 
         [0018]    During heating, the organometallic precursor therefore decomposes in the reductive atmosphere of dihydrogen to form nanoparticles  116 . The substrate  114 , located inside the flask  104 , is in situ whilst the decomposition takes place, resulting in the nanoparticles  116  adhering to the silicon substrate  114  in an organised manner. Indeed, the nanoparticles  116 , stabilised by the ligand, are self-organising in nature and crystallise on the substrate  114  as millimetre-scale super-crystals (sometimes known as superlattices) of bimetallic Fe/Co nanoparticles. The super-crystals are partially air-stable, having a slow oxidation rate. As can be seen from  FIG. 3 , the nanoparticles  116  display a compact arrangement and are adjacent the exposed surfaces of the silicon substrate  114 . In this respect, layers of nanoparticles at least about 100 nm thick can be achieved. Moreover, the crystallisation of the nanoparticles results in a self-selection process that causes the size of the nanoparticles forming part of a given super-crystal to be homogeneous. 
         [0019]    Once the above process has been completed, the dihydrogen is evacuated from the flask  104  and supernatant formed removed from the flask  104 . The silicon substrate  114  is then removed (Step  218 ) from the flask  104  in an inert atmosphere to prevent contamination of the nanoparticles  116  that have formed on the surface of the substrate  114 . 
         [0020]    In another embodiment, a chemical binder, for example aminopropyltrimethoxysilane (APTMS), is deposited onto the substrate  114  to pattern the surface of the substrate  114  selectively, thereby controlling adhesion of the nanoparticles  116  to the substrate, as can be seen, for example, in  FIG. 4 . 
         [0021]    In the embodiments described above, the layers of nanoparticles produced possess magnetic permeability values at high frequency, for example less than 1 GHz, that make the layers of nanoparticles produced particularly suitable for RF applications, such as to form RF inductors, transformers or other magnetic structures, such as magnetic shields. In particular, ferromagnetic nanoparticles films formed in accordance with the above described technique can be used to integrate RF inductors into circuits. Indeed, such nanoparticle materials can be used in high-frequency Integrated Circuit (IC) applications, such as for wireless portable electronic devices, to enhance magnetic field confinement in a variety of passive and active devices. 
         [0022]    In a further embodiment of the invention, the above described layers and multiple layers of nanoparticles can be annealed (Step  220 ) during processing, resulting in the ligands becoming burnt, thereby coating substantially each nanoparticle with a continuous carbon film that is, for example, 1-2 nm thick so as to isolate the nanoparticles from the ambient atmosphere. The nanoparticles therefore become completely air-stable. 
         [0023]    It is thus possible to provide a method of forming nanoparticles in a single process step in a way that is low-cost and does not required additional re-dispersion and deposition of the nanoparticles. Organised nanoparticles can therefore be formed directly on a substrate surface in a simpler manner than hitherto possible and without the need for separate filtering and dispensing of the nanoparticles. Additionally, by obviating the need for slow evaporation of a solvent, the speed of the process is increased. Consequently, the above method is particularly suitable for the formation of high permeability films, where lateral dimensions are about 100 times greater than vertical dimensions, for improving thin-film inductor/transformer/transmission-line performance. RF components formed from the above-described nanoparticle films are low-loss. The high permeability of the nanoparticles films results in improved component and circuit performance through reduced parasitics and a high quality (Q) factor, thus impacting upon many circuit performance specifications, for example amplifier gain and Voltage Controlled Oscillator (VCO) phase noise. Also, improved signal isolation is possible for circuits employing high permeability nanoparticle films, resulting in reduced power consumption, as well as a reduction in the scale of circuit designs that consequently can have a positive impact upon cost and/or form factor as well as yield of electronic circuits, for example analogue, mixed-signal and RF-(Bi)CMOS integrated circuit. 
         [0024]    Although the above examples have been described in the context of forming bimetallic Co/Fe nanoparticles  116 , it should be appreciated that the above technique can be adapted for use in relation to other applications, for example to the field of optoelectronics by fabricating Gallium Arsenide (GaAs), Gallium Phosphide (GaP), Indium Phosphide (InP), Cadmium Telluride (CdTe) or Germanium (Ge) nanoparticles, and/or in relation to passive components, for example capacitors, inductors or a voltage dependent resister. Additionally, the ability to arrange quantum dots on a substrate can be used to create high-efficiency Light Emitting Diodes (LEDs), lasers and/or very high resolution detectors having, for example, a pixel size in the nanometre range. 
         [0025]    It should be appreciated that although the use a of substrate has been described herein, other seed materials of different physical forms and dimensions can be used, for example wires or metal lines. It should also be appreciated that whilst the formation of a particular type of ferromagnetic nanoparticles is being described herein, the above described technique is applicable to the formation of other types of ferromagnetic nanoparticles, for example Iron-Platinum (FePt), Iron-Nickel (FeNi), Iron-Cobalt-Nickel, or Cobalt-Platinum. 
         [0026]    Whilst specific, and preferred, implementations of the present invention are described above, it is clear that one skilled in the art could readily apply variations and modifications of such inventive concepts.