Structural and phase composition of fullerene films deposited by high-power ion beams

Deposition of carbon films containing C₆₀ and C₇₀ fullerenes is an urgent problem, related to development of nanotechnologies and new nanomaterials. Such films have been obtained by ultrafast deposition of dense ablation plasma on a substrate; the plasma was generated as a result of irradiation of a graphite target by pulsed high-power ion beams. The structural and phase composition of the deposited films has been investigated.     

Hybrid ICP/sputter deposition of TiC/CaO nanocomposite films for biomedical application

TiC/a-C nanocomposite films doped with CaO have been deposited by means of a hybrid PVD/PACVD technique, which combines dc magnetron sputtering of a TiC_0.5+10% CaO target, with a subsequent high density inductively coupled plasma (ICP) in order to excite and ionize the sputtered species to a high degree. The films were characterized according to their morphology, structure and thickness by scanning electron microscopy, their composition and bonding structure by X-ray photoelectron spectroscopy, and their crystalline properties by X-ray diffraction. The films consist of fcc titanium carbide nanocrystallites with grain sizes of 5–15 nm and an amorphous carbon phase. The average composition is Ti_0.43C_0.35Ca_0.02O_0.15N_0.05. The influence of the ICP plasma power and the bias voltage applied to the substrate on the major film properties has been investigated. A number of such TiC/a-C/CaO nanocomposite films on silicon substrates have been subjected to a 60 day cell test with human osteoblastic cells in order to investigate their suitability for the coating of prostetic implants. The results of these cell tests, some of which turned out to be rather promising, are discussed in terms of film properties such as surface roughness and biaxial stress. PACS 68.55.-a; 81.07.-b; 87.68.+z     

微波等离子体化学气相沉积法制备C3N4薄膜的结构研究

采用微波等离子体化学气相沉积法,用高纯氮气(99.999%)和甲烷(99.9%)作反应气体,在单晶Si(100)基片上沉积C₃N₄薄膜.利用扫描电子显微镜观察薄膜形貌,表明薄膜由密排的六棱晶棒组成.X射线衍射和透射电子显微镜结构分析说明该薄膜主要由β-C₃N₄和α-C₃N₄组成,并且这些结果与α-C₃N₄相符合较好.由虎克定律近似关 关键词： 3N₄')" href="#">C₃N₄ 微波等离子体化学气相沉积法 薄膜沉积     

C60 growth on Si(100), GaSe(0001) and GeS(001)

C₆₀ films have been grown in ultra high vacuum on various crystalline substrates and the structure of the films has been investigated by low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS). The C₆₀ films form randomly oriented nanocrystals on Si(100), mesoscopic polycrystals on GaSe(0001) and microscopic single crystals on GeS(001). The vibrational structure of the C₆₀/substrate interfaces is analyzed in detail by HREELS carried out in the dipole and impact scattering regimes. It is shown that the epitaxy of C₆₀ on GeS(001) is induced by the weak van der Waals bonding and the peculiar corrugation of the substrate surface.     

Phase transitions in the CN<Subscript>x</Subscript>-TiN system attendant on the variation of the bias voltage during ion-stimulated growth

The structure of amorphous CN_x-TiN films grown by ion-stimulated deposition at a bias voltage U = 200–500 V is studied by X-ray diffraction. As the bias voltage increases in the range U = 300–360 V, the CN_x-TiN films are shown to undergo a phase transition in the amorphous phase having different order scales (20–50 Å): this transition is related to an increase in the content of the fraction of medium-cell (4 Å) carbon clusters as compared to the fractions of clusters with large (8 Å) and small (2 Å) cells. Under these conditions, 80–150 Å crystalline clusters undergo the phase transition from the Ti₂C(N) carbide into graphite (C _g) and diamond (C_d); the last two phases are represented by 100-Å clusters.     

Theoretical and experimental investigations of the properties of Ge2Sb2Te5 and indium-doped Ge2Sb2Te5 phase change material

We have carried out comprehensive computational and experimental study on the face-centered cubic Ge₂Sb₂Te₅ (GST) and indium (In)-doped GST phase change materials. Structural calculations, total density of states and crystal orbital Hamilton population have been calculated using first-principle calculation. 5 at.% doping of In weakens the Ge–Te, Sb–Te and Te–Te bond lengths. In element substitutes Sb to form In–Te-like structure in the GST system. In–Te has a weaker bond strength compared with the Sb–Te bond. However, both GST and doped alloy remain in rock salt structure. It is more favorable to replace Sb with In than with any other atomic position. X-ray diffraction (XRD) analysis has been carried out on thin film of In-doped GST phase change materials. XRD graph reveals that In-doped phase change materials have rock salt structure with the formation of In₂Te₃ crystallites in the material. Temperature dependence of impedance spectra has been calculated for thin films of GST and doped material. Thickness of the as-deposited films is calculated from Swanepoel method. Absorption coefficient (α) has been calculated for amorphous and crystalline thin films of the alloys. The optical gap (indirect band gap) energy of the amorphous and crystalline thin films has also been calculated by the equation \( \alpha h\nu = \beta (h\nu - E_{\text{g }} )^{2} \) . Optical contrast (C) of pure and doped phase change materials have also been calculated. Sufficient optical contrast has been found for pure and doped phase change materials.     

Evidence for a solid phase of dodecahedral C 20

Evidence is presented for the formation of a solid phase based on the smallest fullerene, C₂₀, in thin diamond-like carbon films deposited by ultraviolet laser ablation from diamond onto nickel substrates at room temperature in the presence of 10^-4 torr of cyclohexane or benzene. Laser desorption mass spectrometry from the films shows the presence of C₂₀, C₂₁ and C₂₂ species, while micro-Raman spectroscopy and electron diffraction from selected particles together with first principle density-functional calculations, indicate a cubic solid with dodecahedral C₂₀ cages as building blocks. Unlike solid C₆₀ and fully protonated C₂₀, which are bound by van der Waals forces, the proposed structure is stabilized by linking of the C₂₀ dodecahedra with bridging carbon atoms at interstitial tetrahedral sites to form a face-centered-cubic lattice with 22 carbon atoms per unit cell. Received 10 October 2002 / Received in final form 24 December 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: zafar.iqbal@njit.edu     

A valence-band and core-level photoemission study of a-SixC1-x thin films grown by low-temperature low-pressure chemical vapour deposition

Amorphous Si_xC_1-x thin films have been grown by low-pressure chemical vapour deposition at 800 K from Si₂H₆ and C₂H₂ in the x concentration range 0.5≤x<0.7. Measurements of the valence-band and core-level photoemission spectra using X-ray photoemission spectroscopy and synchrotron radiation have shown a clear change in the electronic structure for 0.55xC_1-x films obtained by excimer laser annealing during growth have shown the formation of a dominant 3C-SiC phase up to x=0.6, while for higher x the growth of a poly-Si phase has been observed . PACS 81.15.Gh; 79.60.Dp     

Giant fullerenes formed on C60 films irradiated with electrons field-emitted from scanning tunneling microscope tips

It has been found that spherical large clusters of carbon atoms are formed by irradiation of crystalline C₆₀ films grown on Si(1 1 1)-(7 × 7) surfaces with electrons field-emitted from a scanning tunneling microscope probe tip. The size distribution of the clusters deduced from surface profile measurements suggests that the dominant clusters were not necessarily C_60n (n = 2-4) expected from the simple fusion of C₆₀ molecules. It was proposed that electronic excitations of C₆₀ molecules caused the fragment and coalescence of the molecules to form the giant fullerenes as in the photo-induced similar effects.     

Influence of electromagnetic radiation on the phase composition of clustered CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

The effect of white and UV radiation on the phase composition of amorphous CN _x films are studied by X-ray diffraction analysis and visible-range spectroscopy. The films have variable-range atomic order and consist of amorphous graphite clusters (30 Å) crystalline clusters (50–100 Å) of graphite, diamond, and carbon nitride phases; and intercluster medium with long-range (1–2 Å) atomic order. It is shown that irradiation of the films by white light facilitates the growth of fine graphite clusters. Irradiation by UV light suppresses the growth of the graphite and carbon nitride phases, favoring the growth of the diamond phase (1.5%). It is demonstrated that a change in the mesoscopic phase composition of the CN _x films causes a change in the energy gap width in the visible range from E _g = 0.75 eV for the films irradiated by white light to E _g = 1.75 eV for those exposed to UV radiation.     

Neutron diffraction study of the interaction of iron with amorphous fullerite

The amorphous fullerite C₆₀ has been prepared by mechanical activation (grinding in a ball mill), and its interaction with iron during sintering of powders with 0–95 at % Fe has been studied. After sintering in the range 800–1200°C under a pressure of 70 MPa, the samples have nonequilibrium structures different from the structures of both annealed and quenched steels. In this case, the carbon phase, i.e., amorphous fullerite, undergoes a polyamorphous transition to amorphous graphite. It has also been shown that the interaction of amorphous fullerite with iron is weaker compared to crystalline fullerite or crystalline graphite.     

Growth of Bi2Sr2Cu1Ox films by laser ablation

The formation and the properties of thin BiSrCuO films grown on MgO single crystal substrates by the pulsed laser ablation of a Bi₂Sr₂Cu₁O₆ target have been studied. The precise influence of the oxygen pressure and substrate temperature on the atomic composition, nature and structure of the grown phases, crystalline quality and superconductivity of the films has been analyzed. In the 600–750°C temperature range and 0.05–0.5 mbar oxygen pressure range, highly textured films of the 2201 phase (Bi₂Sr₂Cu₁O₆ phase) were formed despite large composition deviations (Bi enrichment and Sr depletion) with respect to the ideal composition. A high crystalline quality was evidenced for films grown at low and intermediate pressure and high temperature. χ_min values in the films deduced from channeling experiments were similar to the case of single crystal material. Despite this high crystalline quality, incomplete or very low T_c superconducting transitions were observed in the resistivity measurements.     

Determination of C60/C70 ratios in fullerene mixtures and film characterization by scanning tunneling microscopy

Fullerene powder mixtures with different C₆₀/C₇₀ ratios have been analyzed by a variety of techniques, and results have been compared. The fullerence mixtures have been characterized as solutions in n-hexane by high-pressure liquid chromatography (HPLC) and UV-VIS spectroscopy. Thin films of fullerenes on Au(111) have been prepared from the mixtures by sublimation. The sublimation process has been studied by simultaneous thermogravimetric and differential thermal analyses. Thin fullerene films on Au(111) have been investigated by scanning tunneling microscopy (STM). The STM images show primarily two types of ballshaped molecules arranged in a lattice with hexagonal symmetry (fcc(111) face, nearest neighbour distance: 1 nm). The two species differ in diameter. STM images of films made of mixtures of different C₆₀/C₇₀ ratios show that C₇₀ molecules display a larger apparent diameter (0.8 nm) and corrugation than C₆₀ molecules (0.7 nm). The C₆₀/C₇₀ ratios obtained by counting the corresponding molecular species in the STM images of the thin films are compared to the C₆₀/C₇₀ ratios determined by HPLC on hexane solutions of the mixtures. The observed differences might be explained by different rates of sublimation for the two species. The STM images reveal film defects (vacancies and boundaries) and dynamic processes (displacement of C₇₀ molecules and vacancies). In films prepared to have a C₆₀ coverage of less than one monolayer, stable structural units of the C₆₀(111) surface consisting of three or seven C₆₀ molecules are revealed by STM. Occasionally, substructure within individual fullerene molecules is observed.     

Synthesis and characterization of nanostructured (Pb1− x Sr x )TiO3 thin films by a modified chemical route

The coating solutions of nanostructured (Pb_{1– x} Sr _x )TiO₃ (PST) thin films have been prepared by the sol–gel combined metallo-organic decomposition method. The coating solutions were deposited on Pt/Ti/SiO₂/Si substrates using a spin-coating technique with spinning speed of 4300 rpm and annealed at 650°C. The effect of Sr content in reducing the grain size and tetragonal distortion of PST films has been studied. The optimum conditions for crystalline phase formation in the films have been analyzed by thermogravimetric, differential thermal analysis and Fourier transform infrared spectroscopy. The phase and microstructure of the films were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The XRD pattern shows that the PST thin films are crystallized into tetragonal structures without any impurity phase and the distortion ratio reduces with increasing Sr concentration. The AFM results indicate an increase in grain size with increasing annealing temperature of the film and reduction in grain size with increasing Sr concentration.     

Chemical and phase compositions of silicon oxide films with nanocrystals prepared by carbon ion implantation

The chemical and phase compositions of silicon oxide films with self-assembled nanoclusters prepared by ion implantation of carbon into SiO _x (x < 2) suboxide films with subsequent annealing in a nitrogen atmosphere have been investigated using X-ray photoelectron spectroscopy in combination with depth profiling by ion sputtering. It has been found that the relative concentration of oxygen in the maximum of the distribution of implanted carbon atoms is decreased, whereas the relative concentration of silicon remains almost identical over the depth in the layer containing the implanted carbon. The in-depth distributions of carbon and silicon in different chemical states have been determined. In the regions adjacent to the layer with a maximum carbon content, the annealing results in the formation of silicon oxide layers, which are close in composition to SiO₂ and contain silicon nanocrystals, whereas the implanted layer, in addition to the SiO₂ phase, contains silicon oxide species Si²⁺ and Si³⁺ with stoichiometric formulas SiO and Si₂O₃, respectively. The film contains carbon in the form of SiC and elemental carbon phases. The lower limit of the average size of silicon nanoclusters has been estimated as ∼2 nm. The photoluminescence spectra of the films have been interpreted using the obtained results.     

Polymerization of solid C<Subscript>60</Subscript> under C<Stack><Subscript>60</Subscript><Superscript>+</Superscript></Stack> cluster ion bombardment

The structure transformation occurring in fullerene film under bombardment by 50 keV C₆₀⁺ cluster ions is reported. The Raman spectra of the irradiated C₆₀ films reveal a new peak rising at 1458 cm⁻¹ with an increase in the ion fluence. This feature of the Raman spectra suggests linear polymerization of solid C₆₀ induced by the cluster ion impacts. The aligned C₆₀ polymeric chains composing about 5–10 fullerene molecules have been distinguished on the film surface after the high-fluence irradiation using atomic force microscopy (AFM). The surface profiling analysis of the irradiated films has revealed pronounced sputtering during the treatment. The obtained results indicate that the C₆₀ polymerization occurs in a deep layer situated more than 40 nm below the film surface. The deep location of the C₆₀ polymeric phase indirectly confirms the dominant role of shock waves in the detected C₆₀ phase transformation.     

多晶Si0.9654Mn0.0346:B薄膜的磁性研究

利用磁控溅射和快速热处理的方法制备了Mn,B共掺的多晶硅薄膜(Si_0.9654Mn_0.0346:B).磁性和结构研究发现薄膜有两个铁磁相.低温铁磁相来源于杂相Mn₄Si₇,高温铁磁相(居里温度T_C～250K)是由Mn原子掺杂进入Si晶格导致.晶化后的薄膜利用射频等离子体增强化学气相沉积系统(PECVD)进行短暂(4min)的氢化处理后发现,薄膜的微结构没有发生变化而饱和磁化强度却随着 关键词： 磁性半导体 硅 氢化     

Laser crystallization of amorphous silicon films investigated by Raman spectroscopy and atomic force microscopy

The intrinsic and phosphorous (P)-doped hydrogenated amorphous silicon thin films were crystallized by laser annealing. The structural properties during crystallization process can be investigated. Observed redshifts of the Si Raman transverse optical phonon peak indicate tensile stress present in the films and become intense with the effect of doping, which can be relieved in P-doped films by introducing buffer layer structures. Based on experimental results, the established correlation between the stress and crystalline fraction (X_C) suggests that the relatively high stress can limit the increase in X_C and the highest crystalline fraction is obtained by a considerable stress release. At high laser energy density of 1250 mJ/cm², the poorer crystalline quality and disordered structure of the film originating from the irradiation damage and defects lead to the low electron mobility.     

Chromium carbide thin films deposited by ultra-short pulse laser deposition

Pulsed laser deposition performed by a laser with a pulse duration of 250 fs has been used to deposit films from a Cr₃C₂ target. Due to the different processes involved in the laser ablation when it is performed by an ultra-short pulse source instead of a conventional short pulse one, it has been possible to obtain in vacuum films containing only one type of carbide, Cr₃C₂, as shown by X-ray photoelectron spectroscopy. On the other hand, Cr₃C₂ is not the only component of the films, since a large amount of amorphous carbon is also present. The films, deposited at room temperature, are amorphous and seem to be formed by the coalescence of a large number of particles with nanometric size. The film composition can be explained in terms of thermal evaporation from particles ejected from the target.     

Double-pulse laser ablation applied to reactive PLD method for synthesis of carbon nitride film: A second laser shot delay

Carbon nitride films were deposited using ablation of graphite target by second harmonic radiation of Nd:YAG laser in nitrogen atmosphere. To produce high hardness films, the deposited particles should have sufficient kinetic energy to provide their efficient diffusion on a substrate surface for formation of crystal structure. However, a shock wave is arisen in ambient gas as a consequence of laser plasma explosive formation. This shock wave reflected from the substrate interacts with plume particles produced by the first laser pulse and decreases their kinetic energy. This results in decrease of film crystallinity. To improve film quality, two successive laser pulses was proposed to be used. At adjusting time delay, the particles induced by the second pulse wilt serve as a piston, which will push forward both stopped particles ablated by the first pulse and arisen from chemical reactions in ambient gas. An X-ray photoelectron spectroscopy (XPS) analysis of deposited films has shown an increase of content of sp ³ carbon atoms corresponding to crystalline phase, if double-pulse configuration is employed. The luminescence of excited C₂ and CN molecules in laser plume at different distances from the target was studied to optimize the delay between laser pulses.