Sputtering

Sputtering is a convenient method for preparing films, by which different elementary materials are ionized or heated to form the plasma before depositing on substrates. Sputtering can be employed for preparing the nanoparticles embedded in the thin films, or so-called granular films. The mechanism of the sputtering is as follows: the direct current of hundreds of voltages applied between the cathode and the anode in atmosphere of inert and active gases leads to glow discharge. The ions during the process of discharge bump the target that serves as the cathode, so that the atoms of the target materials could evaporate from the surface. The cooling and the condensation in inert gases and/or the reaction in active gases of the evaporation atoms result in the formation of nanoparticles and nanocapsules, or thin films. The advantages of sputtering are as follows: no crucible is needed; particles of metals with a high melting point can be prepared; the evaporation surface can be large; particles of alloys can be synthesized using active gases; granular thin films can be prepared; the narrow size distribution of the particles can be controlled well; nanocomposite materials can be produced if several different materials are used as targets; etc. The voltage, current, gas pressure, and target are the most important factors which affect the formation of nanoparticles/nanocapsules.

A method was reported for fabricating thin films of carbon nanocapsules and onionlike graphitic particles [155], which consisted of cosputtering Ni and graphite and subsequent thermal annealing in vacuum. The films were composed of Ni-filled carbon nanocapsules with graphitic coatings several nanometers in thickness. Ni particles were removed by acid etching, resulting in the formation of thin films composed of onionlike graphitic particles. Electron field emission properties of the onionlike graphitic-particle films were studied.

C-Ag thin films were synthesized by cosputtering of a silver-graphite target [156]. The deposition temperature ranged from 77 to 773 K, while the silver concentration varied from 10 to 71 at%. The homogeneously distributed silver nanoparticles, having an elongated shape along the direction of the thin film growth, formed within a more or less graphitized carbon matrix. A graphitization led to the encapsulation of the silver nanoparticles in graphite-like carbon when the depositions were performed at 773 K for lower silver concentrations without ion-beam assistance and below 573 K for upper silver concentrations with ion-beam assistance.

(Ni66Fe22Co12)x C1-x nanocomposite films with x = 10-75 at% were prepared by dc magnetron cosputtering [157]. Subsequent thermal annealing was performed in a vacuum (<2 x 10-3 Pa) furnace for 1 h at various temperatures. The phase transition with increase of annealing temperature was closely dependent on the composition. Films with NiFeCo less than 20 at% showed an amorphous structure in the as-deposited samples and ones annealed up to 400 °C. After being annealed at 500 °C, a small amount of fcc crystalline NiFeCo precipitated while carbon remained amorphous. For the films with NiFeCo concentration of 30-55 at%, the as-deposited films consist of very small NiFeCo nanocrystals encapsulated in amorphous carbon. After being annealed, the crystal grain size of the alloys increased with increasing annealing temperature while carbon was graphitized. For the films with more than 62 at% NiFeCo, the as-deposited films went through a metastable stage at which a rhombohedral Ni3C phase and fcc NiFeCo coexisted upon annealing to a temperature between approximately 300 and 400 °C (dependent on composition). Upon further annealing to a sufficiently high temperature between approximately 350 and 500 °C, the carbide phase decomposed and only the fcc NiFeCo nanocrystals encapsulated in graphite existed in the films.

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