Properties Of C60 Thin Films Research Articles

The effect of substrate deposition temperature on the electrical and optical properties of C60 thin films

We report the effects of the substrate deposition temperature on the photovoltaic performance of fullerene (C60) thin films. These thin films were deposited via thermal evaporation at substrate temperatures from 20 to 250 °C. The optical and electrical properties of the films were dependent on the substrate deposition temperature, and improvements in film optical properties were noted for certain temperatures. Both the optical constants n and k and the dielectric constants show theoretical and experimental decreases with the substrate temperature due to the creation of crystalline structures in the thin films. Despite differences between the theoretical and experimental optical constants, the nature of the decrease is similar. The film porosity calculated from the optical constants increased more than 50% for greater substrate temperatures. The conductivity of the films also increased with the substrate temperature and the activation energy ranged from 0.25 to 0.31 eV. We demonstrate an increased energy gap with temperature using a Tauc-plot, and the photopic and solar transmittances improved with the increased substrate temperature during deposition.

Electrostatic properties of two-dimensional C60 polymer thin films under an external electric field

Electrostatic properties of different C60 thin films under external electric fields have been investigated from first-principle total-energy calculations. Density functional theory calculations combined with the effective screening medium method reveal that the electrostatic properties of C60 thin films in an electric field strongly depend on the arrangement and conformation of the C60 molecules. The relative permittivity across the thin films exhibits clear a positional dependence resulting from the π electron distribution within the films. An electrostatic polarization is uniformly induced by weak electric fields, typically 0.1 V nm−1, because of the semiconducting electronic structure of the thin films, whereas the polarization is highly concentrated in the outermost C60 layer under strong electric fields of 0.5 V nm−1.

Substrate Temperature Influence on Optical Properties of C60 Thin Films Within the Visible Range

Fullerene thin films of about 200 nm thicknesses have been deposited by thermal evaporation method on soda lime glass at substrate temperature 303 and 403K under pressure about 10-5 mbar. This study concentrated on the influence of substrate temperature on the optical properties of C60 thin films within the visible range. Optical characterization has been carried out at room temperature using the absorption spectra, at normal incidence, in range (200-900) nm.
 The absorption and extinction coefficients of the samples have been evaluated according to the variation in the UV- Visible spectrum. Increasing substrate temperature causes decreasing in optical band gap energy, for direct allowed transitions, and slightly changing in refractive index. This incident was due to the reducing of interatomic intervals, which may be correlating a decrease in the amplitude of atomic vibrations around their equilibrium sites.

Mechanical properties of C60 thin films at the air–water interface

Understanding the mechanical properties of ultra-thin films formed by the self-assembly of molecules/nanoparticles/colloids at fluid–fluid interfaces is central to many technological applications. Here, we have carried out interfacial rheology measurements to systematically investigate the concentration dependent viscoelastic response of 2D films of Fullerene C60 at the air–water interface. With increasing C60 concentration, amplitude sweep measurements show that the films undergo a transition from viscoelastic liquid-like to viscoelastic solid-like behaviour. Interestingly, for high C60 concentrations's, the loss modulus G′′ reaches a maximum before the onset of power-law shear-thinning in G′, the storage modulus, and G′′. The power-law exponents have a ratio 2. This response is typical of systems that show soft glassy behaviour. We also observe a power-law increase in G′ and G′′ at low frequencies in the frequency response measurements and a transition from Newtonian to shear-thinning behaviour, with increasing shear rate, in steady shear measurements. Our results are in qualitative agreement with the phenomenological soft glassy rheology model.

The preparation of oxygenated fullerene film and its photoelectrical properties

In studies of photoelectrochemical properties of C60 thin films, researchers have pointed out that the presence of oxygen in C60 thin films causes problems in electron and hole drift mobilities as well as in the recombination lifetime at a moderate steady-state illumination level; therefore, it is necessary to understand the characterization of photoelectronic and transport processes in oxygenated C60 thin films. From previous studies, researchers have pointed out that C60 fullerene thin film has the n-type property. Therefore, by using oxygenated C60 fullerene thin film, we prepared p–n junctions (C60/p-Si) in order to obtain its photoelectronic conversion properties. We analyzed the conductivity, the I–V curves, the open-circuit voltage and close-circuit current versus film thickness, and the fill factor and conversion efficiency.As a result, we found the photoelectronic conversion properties of the p–n junctions (C60/p-Si). However, the photoelectronic properties of n-type C60 fullerene thin film are lower than the characteristics of other photoelectronic devices. Those that have low photoelectronic conversion properties have the possibility of being good photoelectronic devices. Therefore, C60 fullerene thin film can be applied to photoelectronic device fields.

Correlation of crystalline and structural properties of C60 thin films grown at various temperature with charge carrier mobility

Transistors fabricated from C60 films grown by hot wall epitaxy at higher substrate temperature, showed an order of magnitude increased charge carrier mobility up to 6cm2∕Vs. In this letter, the authors present an extensive study of morphology and crystallinity of the fullerene films using atomic force microscopy and grazing-incidence x-ray diffraction. A clear correlation of crystalline quality of the C60 film and charge carrier mobility was found. A higher substrate temperature leads to a single crystal-like faceted fullerene crystals. The high crystalline quality solely brings a drastic improvement in the charge carrier mobility. A gate voltage independent mobility is also observed in these devices which can be attributed to the highly conjugated nature of the C60 thin film.

Disorder/order phase transition in C60 thin films studied by surface photovoltage spectroscopy

The electronic properties of C60 thin films have been studied using surface photovoltage (SPV) spectroscopy at 120–300 K. Temperature variations of the SPV spectra are correlated with temperature-dependent x-ray diffraction patterns of the same samples, which indicate the first-order phase transition at Tc=250 K. Absolute values of both the band-to-band and band (or band tail) -to-band tail SPV signals are shown to exhibit a clear minimum at Tc=250 K in contrast with the well-known increasing background of the SPV intensity with decreasing temperature. The energy positions of the thresholds of the band-to-band and band (or band tail) -to-band tail regions in the SPV spectra also exhibit nonmonotonic behavior with a mirror symmetry and distinct extrema near Tc. On the basis of the presented results, we discuss possible reasons behind the effect of rotational and orientational states of C60 molecules on the electronic structure of the C60 fullerite and charge carrier transport in C60 thin films.

Effect of the disorder/order phase transition on the electrical and photoelectrical properties of C60 thin films

This article reports the temperature dependence of the structural and photoelectrical properties of C60 thin films with varying crystal structure and oxygen content near the orientational disorder/order phase transition at about 260 K. X-ray diffraction data demonstrate that highly crystalline oxygen-free C60 films undergo the first-order phase transition at 252 K with a lattice parameter discontinuity Δa/a of 0.22%; no discontinuity was observed in C60 films with coexisting amorphous and nanocrystalline phases. A strong effect of the phase transition on the dark- and photoconductivity in those films is presented: In highly crystalline C60 films both dark- and photoconductivity increase as the temperature is lowered through the transition region. However, opposite temperature changes of the conductivity are observed in nanocrystalline/amorphous C60 films. In situ exposure of samples to oxygen gas suppresses any critical behavior. The experimental results are interpreted in terms of trap-controlled mobility of the dominant charge carriers such that the temperature changes of the conductivity are attributed to variations in the concentration, energy distribution, and population of the carrier traps at the phase transition. Differences in the parameters of the traps, which are associated with the freeze-out of the molecular orientations, may also explain the strong variations in the temperature dependence of the conductivity observed in our C60 films and reported by other authors.

In Search of the Molecular Triplet of C60 using Low Temperature Raman Spectroscopy.

ABSTRACTThe excited state properties of C60 thin films have been probed in the temperature range 77–273K using Raman spectroscopy. The change in the Raman, 2Ag mode of C60 (whose position is largely independent of temperature) was monitored as a function of the excitation intensity at 514.5nm. This mode normally positioned at 1469cm-1, was seen to shift reversibly to a lower Raman frequency with increasing laser intensity. Two excited state species have been identified. The first, at 1466cm-1 has been associated with the molecular triplet of C60. The second species at 1463cm-1, has been speculated to be an excited state co-operative involving two or more excited states in the solid and is seen to be intrinsic to solid state C60 below the phase transition.