Structure and electronic properties of Hn@C20 molecule

Density functional theory has been employed to optimize the structure of endohedral doped C₂₀ fullerene. We have also investigated electronic properties. We have found that C₂₀ cage can accommodate up to 8 hydrogen atoms. Some hydrogen atoms get chemisorbed on the inner surface of C₂₀ cage and form C-H bond. Structural deformation is found to increase with increase in H-atoms. From the analysis of electronic properties, we observe that due to endohedral doping of hydrogen atoms inside C₂₀, H-atoms acquire net negative charge by accepting electrons and fullerene molecules acquire positive charge by donating electrons to H-atoms. For endohedral complexes where H₃ triangular molecule formation takes place, the nature of net charge transfer changes, i.e. fractional electronic charge is transferred from H-atoms to fullerene. C₂₀ doped with odd number of H-atoms should be more reactive compared to the even number case. Most of the present results are similar to those of endohedral C₆₀.     

STM study of the initial stages of C60 adsorption on the Pt(1 1 0)-(1 × 2) surface

We have studied the initial stages of adsorption of C₆₀ on the Pt (1 1 0)-(1 × 2) surface by means of STM. At room temperature, fullerene molecules adsorb in the troughs between two adjacent Pt rows of the missing row reconstruction. Mobility over the terraces is negligible, denoting strong bonding with the surface, also testified by a well-defined orientation of fullerene monomers with respect to the substrate. Upon annealing at 750 K, molecular migration towards kinks and step edges occurs, where small islands nucleation begins. A commensurate registry with the substrate is maintained by small (5-10 molecules) C₆₀ aggregates, leading to expanded nearest-neighbour distances with respect to those found in hexagonal close packed fullerene ad-islands grown on other metallic substrates.     

Adsorption of N@C60 on Si(1 0 0)

The interactions between endohedrally doped N@C₆₀ molecules and the Si(1 0 0) surface have been explored via ab initio total energy calculations. Configurations which have the cage located upon the dimer row bonded to two dimers (r2) and within the dimer trench bonded to four dimers (t4) have been investigated, as these have previously been found to be the most stable for the C₆₀ molecule. We have investigated the differences between the adsorption of the C₆₀ and N@C₆₀ molecules upon the Si(1 0 0) surface and found that there are only minimal differences. Two interesting cases are the r2g and t4d configurations, as they both exhibit differences that are not present in the other configurations. These subtle differences have been explored in-depth. It is shown that the effects on the endohedral nitrogen atom, due to its placement within the fullerene cage, are small. Bader analysis has been used to explore differences between the C₆₀ and N@C₆₀ molecules.     

Characterization of Al/Si junctions on Si(100) wafers with chemical vapor deposition-based sulfur passivation

Chemical vapor deposition-based sulfur passivation using hydrogen sulfide is carried out on both n-type and p-type Si(100) wafers. Al contacts are fabricated on sulfur-passivated Si(100) wafers and the resultant Schottky barriers are characterized with current–voltage (I–V), capacitance–voltage (C–V) and activation-energy methods. Al/S-passivated n-type Si(100) junctions exhibit ohmic behavior with a barrier height of <0.078 eV by the I–V method and significantly lower than 0.08 eV by the activation-energy method. For Al/S-passivated p-type Si(100) junctions, the barrier height is ~0.77 eV by I–V and activation-energy methods and 1.14 eV by the C–V method. The discrepancy between C–V and other methods is explained by image force-induced barrier lowering and edge-leakage current. The I–V behavior of an Al/S-passivated p-type Si(100) junction remains largely unchanged after 300 °C annealing in air. It is also discovered that heating the S-passivated Si(100) wafer before Al deposition significantly improves the thermal stability of an Al/S-passivated n-type Si(100) junction to 500 °C.     

Molecular depth profiling of trehalose using a C60 cluster ion beam

Molecular depth profiling of organic overlayers was performed using a mass selected fullerene ion beam in conjunction with time-of-flight (TOF-SIMS) mass spectrometry. The characteristics of depth profiles acquired on a 300-nm trehalose film on Si were studied as a function of the impact kinetic energy and charge state of the C₆₀ projectile ions. We find that the achieved depth resolution depends only weakly upon energy.     

Low-dimensional nanostructures and fullerene films on semiconductor surface

The morphology and atomic structures of C₆₀ fullerene films on a Bi(0001)/Si(111)-7 × 7 surface and adsorption of fluorofullerene C₆₀F _x molecules on a Si(111)-7 × 7 surface have been studied by scanning tunneling microscopy/spectroscopy and low-energy electron microscopy under ultra high-vacuum conditions. It has been shown that initial nucleation of C₆₀ islands on the surface of an epitaxial Bi film occurs on double steps and domain boundaries, while tunnel spectra do not exhibit any significant charge transfer to the lowest unoccupied molecular orbital states. Fluorofullerene molecules allow local (at the nanoscale level) modification of Si surface through local etching.     

Comparative study of optical parameters of fullerene C60 film at different temperatures

Fullerene C₆₀ thin films on glass substrate (around 2000 ? thickness) were prepared by thermal evaporation technique. The structural, surface morphology and optical properties of the films were studied. The optical properties of fullerene C₆₀ were investigated in the spectral range 200 nm to 900 nm using a UV-Vis spectrophotometer at room temperature as well as at liquid nitrogen temperature (77 K). The optical band gap at room temperature is found to be 2.30 eV, which gradually decreases with lowering the temperature and reaches to 2.27 at 77 K. The thickness and refractive index of fullerene C₆₀ film were calculated by ellipsometry. From the X-ray analysis, we have calculated the grain size, dislocation density, number of crystallite per unit area, and strain of the film at room temperature. The surface morphology of film was analyzed by scanning electron microscope (SEM). The present result show that the fullerene C₆₀ film becomes more conducting at low temperature.     

Epitaxial growth of C<Subscript>60</Subscript> thin films on the Bi(0001)/Si(111) surface

The morphology and atomic structure of C₆₀ fullerene films on the Bi(0001)/Si(111)?7 × 7 surface with different coverages have been studied by scanning tunneling microscopy and spectroscopy and low-energy electron microscopy in high vacuum. It is shown that the most favorable sites for nucleation of C₆₀ islands are double steps and domain boundaries on the surface of epitaxial Bi film.     

ELECTRON SPIN RESONANCE CHARACTERISTICS OF NITROGEN-DOPED FULLERENE

The nitrogen-doped fullerene has been obtained by arc discharge between two high-purity graphite rods in the atmosphere of N₂ and He, The electron spin resonance(ESR) characteristics of N-doped fullerene have been investigated. The results show that the ESR spectra of N-doped fallerene axe composed of two parts, a paramagnetic signal of N-center and a paramagnetic signal of C-center, For comparing with N-doped fullerene, we have also studied ESR spectra of C₆₀ powder, C₆₀ sublimed film and H-doped C₆₀ film. For C₆₀ powder and H-doped C₆₀ film, their in situ ESR. measurements are carried out at various temperatures, and reasonable explanations are proposed.     

硅衬底GaN基LED N极性n型欧姆接触研究

在Si衬底GaN基垂直结构LED的N极性n型面上,利用电子束蒸发的方法制作了Ti/Al电极,通过了I-V曲线研究了有无AlN缓冲层对这种芯片欧姆接触的影响.结果显示,去除AlN缓冲层后的N极性n型面与Ti/Al电极在500到600 ℃范围内退火才能形成欧姆接触.而保留AlN缓冲层的N极性n型面与Ti/Al电极未退火时就表现为较好的欧姆接触,比接触电阻率为2×10^-5 Ω·cm²,即使退火温度升高至600 ℃,也始终保持着欧姆接触特性.因此,AlN缓冲层的存在是Si衬底GaN基垂直结构LED获得高热稳定性n型欧姆接触的关键. 关键词： 硅衬底 N极性 AlN缓冲层 欧姆接触     

Doping and photoelectric properties of C60 films prepared by ionized cluster beam deposition

60 films by means of ionized cluster beam (ICB) deposition. X-ray diffraction (XRD) measurement showed the C₆₀ films to be polycrystalline. The films show negative resistance–temperature coefficients, and their room-temperature resistivity is greater than 10² Ω cm. The films were implanted with 80-keV phosphorus, BBr₃, Ar, and He ions, under doses ranging up to 10¹⁶ cm^-2. The resistivity of the implanted films decreases with increasing doses. n-type electrical conduction was observed for phosphorus-implanted C₆₀ films. The interaction of impinging ions with C₆₀ clusters was found to force the C₆₀ molecules to disintegrate and the films to amorphize. p-type conduction was observed for the C₆₀ films doped with aluminum by simultaneously sputtering aluminum during deposition. C₆₀/Si structures show heterojunction characteristics that can be influenced by light illumination. The photoelectric properties of the films were found to be improved by doping with aluminum. Received: 12 January 1998/Accepted: 24 March 1998     

NMR study of acceptor doped fullerenes (MF6)2C60 (M=As,Sb)

We report on ¹³C and ¹⁹F nuclear magnetic resonance study of acceptor doped fullerenes showing p-type semiconductor behavior. Room temperature ¹⁹F spectra are characteristic for rotating (MF₆⁻) complexes, while the ¹³C spectra show that fullerene molecules are not mobile at temperatures up to 365 K. Reduction of the ¹³C chemical shielding anisotropy of (MF₆)₂C₆₀ in comparison to solid C₆₀ is assigned to the charge transfer between C₆₀ and intercalated species.     

Orientation-dependent ionization potential of CuPc and energy level alignment at C60/CuPc interface

The electronic structure evolution of interfaces of fullerene (C₆₀) with copper phthalocyanine (CuPc) on highly oriented pyrolytic graphite (HOPG) and on native silicon oxide has been investigated with ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. The LUMO edge of C₆₀ was found to be pinned at the interface with CuPc on SiO₂. A substantial difference in the electron affinity of CuPc on the two substrates was observed as the orientation of CuPc is lying flat on HOPG and standing up on SiO₂. The ionization potential and electron affinity of C₆₀ were not affected by the orientation of CuPc due to the spherical symmetry of C₆₀ molecules. We observed band bending in C₆₀ on the standing-up orientation of CuPc molecules, while the energy levels of C₆₀ on the flat-lying orientation of CuPc molecules were observed to be flat. The observation points to a dependence of photoexcited charge transfer on the relative molecular orientation at the interface.     

Permeability of C60 films deposited on polycarbonatesyloxane to N2, O2, CH4, and He gases

In this study, we report on the gas permeability of non-polymerized and polymerized fullerene films (thickness about 0.5 μm) grown on an organic polymer substrate, polycarbonatesyloxane (PCS), using a high vacuum deposition method. The photopolymerized C₆₀ films were prepared by a simultaneous thin film deposition and UV-vis irradiation method which was reported previously [V.A. Karachevtsev, P.V. Mateichenko, N.Y. Nedbailo, A.V. Peschanskii, A.M. Plokhotnichenko, O.M. Vovk, E.N. Zubarev, A.M. Rao, Carbon 42 (2004) 2091]. Raman spectroscopy revealed that ∼90% of the C₆₀ molecules are covalently linked to neighboring C₆₀ molecules in the photopolymerized film after 20 h of film deposition/irradiation. Permeability of the resulting membranes consisting of polymer PCS base and fullerene films to the N₂, O₂, CH₄, and He gases has been investigated. Our experiments revealed that the gas permeability properties are dependent on the age of the membrane. In particular, the aged membrane exhibited an enhanced permeability for O₂ and He gases in comparison to N₂ and CH₄, respectively.     

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.     

The interface reaction of high-k La2Hf2O7/Si thin film grown by pulsed laser deposition

The La₂Hf₂O₇ films have been deposited on Si (1 0 0) substrate by using pulsed laser deposition (PLD) method. X-ray diffraction (XRD) demonstrates that the as-grown film is amorphous and crystallizes after 1000 °C annealing. The interface structure is systematically studied by Synchrotron X-ray reflectivity (XRR), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS). Silicide, silicate and SiO_x formations from interfacial reaction are observed on the surface of the Si substrate in the as-grown film. The impact of silicide formation on the electrical properties is revealed by capacitance-voltage (C-V) measurements. By post-deposition annealing (PDA), silicide can be effectively eliminated and C-V property is obviously improved.     

Self-assembly of C60 fullerenes on quasi-one-dimensional Si(111)4 × 1-In surface

Using scanning tunneling microscopy observations, self-assembly of C₆₀ fullerenes in the course of room-temperature adsorption onto Si(111)4 × 1-In reconstruction and after subsequent annealing at temperatures ranging from 150 to 450 °C has been studied. Adsorbed C₆₀ fullerenes have been found to occupy off-centered positions on In-atom rows forming linear chains with a maximal length of eight C₆₀ molecules. Intermolecular spacing within the regular chains equals three lattice constants of Si(111) surface. Two energetically different adsorption states of C₆₀ have been detected, one of which is occupied preferentially at room temperature, while occupation of the second (more tight) state dominates at temperature above ~ 150 °C. In the first state, C₆₀ fullerene resides plausibly in a continuous rotation, while in the second state a C₆₀ molecule is fixed tightly in a single orientation with a C₆₀ hexagon pointing upward. Transition of C₆₀ fullerenes to the more stable state is accompanied by expelling In atoms from the Si(111)4 × 1-In reconstruction.     

Adsorption of helium on isolated C60 and C70 anions

Adsorption of helium on free, negatively charged fullerenes is studied in this work. Helium nanodroplets have been doped with fullerenes and ionised by electron attachment. For suitable experimental conditions, C^?₆₀ and C^?₇₀ anions are found to be complexed with a large number of helium atoms. Prominent anomalies in the ion abundances indicate the high stability of the commensurate 1×1 phase in which all hollow adsorption sites are occupied by one atom each. The adsorption energy for an additional helium atom is about 40% less than for atoms in the commensurate layer, similar to our previous findings for fullerene cations and in agreement with theoretical dissociation energies. Similarly, an anomaly in the adsorption energy occurs when 60 helium atoms are attached to C^?₆₀ or 65 to C^?₇₀. For C₆₀, the anomaly coincides with the one observed for cationic complexes but for C₇₀ it does not. Implications of these features are discussed in light of several theoretical studies of neutral and positively charged helium–fullerene complexes.     

Effect of a buffer layer, composition and annealing on the performance of a bulk heterojunction polythiophene/fullerene composite photovoltaic device

Post-annealing of P3HT/C₆₀-based plastic solar cells, composite ratio and, incorporation of a buffer layers at the ITO–polymer/fullerene composite interface and the polymer/fullerene composite–Al interface have shown significant improvement in the performance of the photovoltaic device, both in terms of open-circuit voltage and short-circuit current. The annealing temperature, time duration and P3HT/C₆₀ composite ratios have been optimized for the best performance of typical ITO/PEDOT:PSS/P3HT:C₆₀/LiF/Al solar cell. The C₆₀ content is very crucial because it improves the photocurrent in the beginning but after a certain value it leads to decrease in absorption, thereby decreasing the photocurrent.     

Electronic non-equilibrium conditions at C60–pentacene heterostructures

The electronic structure of vacuum-sublimed layered organic heterostructures of pentacene (PEN) and fullerene (C₆₀) on conducting polymer substrates was investigated using ultraviolet photoelectron spectroscopy (UPS). The conditions at the PEN/C₆₀interface changed from thermodynamic non-equilibrium (i.e. the onset of the PEN highest occupied molecular orbital above the substrate Fermi-energy) for thin PEN coverages on C₆₀ to thermodynamic equilibrium for thicker PEN coverages (i.e. Fermi-level pinning of PEN). This finding is attributed to a coverage-dependent pinhole connection of PEN through the C₆₀ layer with the substrate. The experiments demonstrate the importance of organic thin film morphology for UPS measurements to assess the energy level alignment at organic/organic heterointerfaces.