Electron-energy-loss spectroscopy of C60 monolayer films on active and inactive surfaces

The characteristics of interaction between C₆₀ molecules and Si(1 1 1)-7×7, Ag/Si(1 1 1)-√3×√3 R30° and layered material MoS₂ surfaces have been investigated using electron-energy-loss spectroscopy (EELS). The EEL spectrum of C₆₀/Si(1 1 1)-7×7 shows a new peak at loss energy of 2.7 eV. This indicates the existence of charge transfer from the substrate to C₆₀ molecules. The EEL spectrum of a C₆₀ monolayer film grown on a cleaved surface of MoS₂ is almost the same as that of bulk C₆₀. The EEL spectrum of a C₆₀ monolayer film on an Ag/Si(1 1 1) surface is quite different from that on a clean Si(1 1 1)-7×7 surface, although the films on those substrates have the same epitaxial arrangement. Furthermore, intensities of energy-loss peaks of C₆₀/Ag/Si(1 1 1) are slightly smaller than those of C₆₀/MoS₂ in spite of having the same loss-energy. This suggests that the interaction between C₆₀ molecules and the Ag/Si(1 1 1) surface is stronger than that between C₆₀ molecules and the MoS₂ surface.     

Structural modification of C60 films induced by 300-keY Xe-ion irradiation

The structural modification of C60 films induced by 300-keV Xe-ion irradiation was investigated. The irradiated C60 films were analysed using Fourier transform infrared spectroscopy, the Raman scattering technique, ultraviolet/visible spectrophotometry and atomic force microscopy. The analysis results indicate that the Xe-ion irradiation induces polymerization and damage of the C60 molecule and significantly modifies the surface morphology and the optical property of the C60 films. The damage cross-section for the C60 molecule was also evaluated.     

Electronic structure of C60 films

A complete set of fundamental optical functions of fullerite (C₆₀) films in energy ranges of 2.5–5.0 and 4.0–9.0 eV is calculated using the known spectra of the imaginary and real parts of the dielectric constant. An integrated spectrum of the dielectric constant is decomposed into elementary components. Three basic parameters of each component (the maximum and halfwidth energies and oscillator strength) are determined. Based on the known theoretical calculations of fullerite zones, a scheme of the nature of these components of the dielectric constant is suggested. Udmurt State University, 71, Krasnogeroiskaya Str., Izhevsk, 426034, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 227–232, March–April, 1999.     

Oxygen diffusion in C60 films

We have performed detailed resistivity measurements as a function of temperature in the range from 12 to 300 K on oxygen loaded C₆₀ films. We observe that two ordering phase transitions (i.e.,T ₀=260 K andT _g =90 K) are present in (T), which, in addition, strongly depends on the oxygen content. We find a decrease of both ordering temperatures with increasing oxygen concentrations. The mechanisms of oxygen diffusion are greatly enhanced in the ordered phase on heating. Finally, the transition to a glassy state atT _g is detected as a point of reversibility of the resistivity curve as a function of temperature.     

Thermal expansion of thin C60 films

The thermal expansion coefficient a and structure of C₆₀ films with thickness t∼3–10 nm were investigated in the temperature interval from room to liquid-nitrogen temperature by electron-optical methods. The thermal expansion coefficient was determined from the temperature shift of the diffraction maxima in the electron diffraction patterns. The objects of investigation were epitaxial C₆₀ films condensed in vacuum on a (100) NaCl cleavage surface and oriented in the (111) plane. A surface-induced size effect in the thermal expansion coefficient was observed. It was established that as t decreases α _f increases and is described well by the relation α _f=17·10⁻⁶ K⁻¹+8.3·10⁻⁵ nm K⁻¹ t ⁻¹. This relation was used to estimate the linear expansion coefficient α _s of the C₆₀ surface in the (111) plane as α _s=60·10⁻⁶K⁻¹, which is several times larger than the bulk value. The experimental results agree satisfactorily with the theoretical calculations of the mean-square displacements of molecules located in a region near the surface. Zh. éksp. Teor. Fiz. 114, 1868–1875 (November 1998)     

Carbon nitride films synthesized by nitrogen ion beam bombarding on C60 films

\valunit*{400}{\eV} or ) nitrogen ion beam was used to bombard films to synthesize carbon nitride films. The bombarded films were examined by Raman and X-ray photoelectron spectroscopy (XPS) measurements. The experimental results showed that the destroyed carbon species chemically combined with nitrogen ions to form stable carbon nitride. An appropriate beam energy (possibly ) was proposed in this method. Received: 1 July 1996/Accepted: 27 November 1996     

The characterization of plasma-polymerized C60 thin films

Plasma-polymerized C₆₀ thin films were investigated in the form of a field effect transistor (FET) structure. In the FET device, the C₆₀ polymer acts as a p-type semiconductor, whereas C₆₀ is an n-type semiconductor. Its conduction mechanism can be described as due to variable range hopping. As a sensing device, the C₆₀ polymer can behave as a gas sensor for electrophoric gases and can also be operated as a photo-sensing device in air. Received: 21 April 2001 / Accepted: 23 July 2001 / Published online: 2 October 2001     

Specific features of the evaporation of C60 films

The evaporation of pure C₆₀ films is investigated. The rates of evaporation of the fullerene are experimentally measured, and the saturated vapor pressures are determined in the temperature range 250–290°C. The influence of atmospheric oxygen and ultraviolet irradiation on the rate of evaporation of the fullerene is studied. It is shown that the evaporation parameters of pure C₆₀ films differ substantially from the reference values.     

Structure and electrical characteristics of ICBD C60 films

Polycrystalline C₆₀ films are deposited onto a variety of substrates by ionized cluster beam deposition (ICBD) technique. The structure of the ICBD C₆₀ films are studied by transmission electron microscopy (TEM). The electrical characteristics of the ICBD C₆₀ films on silicon substrates are investigated by current-voltage (I–V) measurements. TheICBD C₆₀/p-Si and C₆₀/n-Si heterostructures show strong current rectification, which is analyzed using band theory.     

Optical and electrical properties of C60Tex films

The structure, composition, and electrical and optical properties of thin tellurium-intercalated fullerene films C₆₀Te_x are investigated. The samples of compositions from C₆₀Te_0.1 to C₆₀Te₆ are prepared by thermal evaporation. The sample composition and the impurity distribution are controlled by the Rutherford backscattering technique. The Raman vibrational spectra indicate changes in the symmetry of a C₆₀ molecule: the strain of the molecule increases with a decrease in the tellurium concentration and decreases as the tellurium impurity concentration increases. The evolution of the optical absorption spectra and the electrical conductivity suggests that intercalation of a tellurium impurity leads to modification of the electronic structure of the material. This process is accompanied by a shift and change in shape of the optical absorption edge and a change in the electrical conductivity of films by several orders of magnitude depending on the composition. The electrical conductivity is minimum at a low tellurium impurity content.