Temperature-dependent infrared ellipsometry of Mo-doped VO2 thin films across the insulator to metal transition.

We present a spectroscopic ellipsometry study of Mo-doped VO2 thin films deposited on silicon substrates for the mid-infrared range. The dielectric functions and conductivity were extracted from analytical fittings of Ψ and Δ ellipsometric angles showing a strong dependence on the dopant concentration and the temperature. Insulator-to-metal transition (IMT) temperature is found to decrease linearly with increasing doping level. A correction to the classical Drude model (termed Drude-Smith) has been shown to provide excellent fits to the experimental measurements of dielectric constants of doped/undoped films and the extracted parameters offer an adequate explanation for the IMT based on the carriers backscattering across the percolation transition. The smoother IMT observed in the hysteresis loops as the doping concentration is increased, is explained by charge density accumulation, which we quantify through the integral of optical conductivity. In addition, we describe the physics behind a localized Fano resonance that has not yet been demonstrated and explained in the literature for doped/undoped VO2 films.

Note: A simple multi-channel optical system for modulation spectroscopies.

Photoreflectance-difference (PR/PRD) and reflectance-difference (RD) spectroscopies employ synchronic detection usually with lock-in amplifiers operating at moderate (200-1000 Hz) and high (50-100 KHz) modulation frequencies, respectively. Here, we report a measurement system for these spectroscopies based on a multichannel CCD spectrometer without a lock-in amplifier. In the proposed scheme, a typical PRD or RD spectrum consists of numerical subtractions between a thousand CCD captures recorded, while a photoelastic modulator is either operating or inhibited. This is advantageous and fits the slow response of CCD detectors to high modulation frequencies. The resulting spectra are processed with Savitzky-Golay filtering and compared well with those measured with conventional scanning systems based on lock-in amplifiers.

Adsorption energetics of CO on supported Pd nanoparticles as a function of particle size by single crystal microcalorimetry.

The heat of adsorption and sticking probability of CO on well-defined Pd nanoparticles were measured as a function of particle size using single crystal adsorption microcalorimetry. Pd particles of different average sizes ranging from 120 to 4900 atoms per particle (or from 1.8 to 8 nm) and Pd(111) were used that were supported on a model in situ grown Fe(3)O(4)/Pt(111) oxide film. To precisely quantify the adsorption energies, the reflectivities of the investigated model surfaces were measured as a function of the thickness of the Fe(3)O(4) oxide layer and the amount of deposited Pd. A substantial decrease of the binding energy of CO was found with decreasing particle size. Initial heat of adsorption obtained on the virtually adsorbate-free surface was observed to be reduced by about 20-40 kJ mol(-1) on the smallest 1.8 nm sized Pd particles as compared to the larger Pd clusters and the extended Pd(111) single crystal surface. This effect is discussed in terms of the size-dependent properties of the Pd nanoparticles. The CO adsorption kinetics indicates a strong enhancement of the adsorbate flux onto the metal particles due to a capture zone effect, which involves trapping of adsorbates on the support and diffusion to metal clusters. The CO adsorption rate was found to be enhanced by a factor of ∼8 for the smallest 1.8 nm sized particles and by ∼1.4 for the particles of 7-8 nm size.

Growth and optical properties of Ag clusters deposited on poly(ethylene terephthalate).

The growth and concomitant evolution of the optical properties of Ag nano-clusters deposited on biaxially extruded poly(ethylene terephthalate) films is studied by reflectance difference spectroscopy. It is demonstrated by low energy ion scattering and simulated optical spectra that the clusters form a two-dimensional layer buried beneath the surface of the substrate. The experimental spectra are described by simulations in which different configurations of the host such as anisotropy, amorphization, and dilution are considered in an effective medium approach. The contribution of the anisotropic substrate is used to explain the resulting line shapes. We also discuss the role of the rate of change of the filling fraction with Ag coverage in the evolution of the spectra and the detection of the onset of coalescence by optical means.

A rotating-compensator based reflectance difference spectrometer for fast spectroscopic measurements.

We present a new type of reflectance difference (RD) spectrometer for fast spectroscopic measurements based on a rotating-compensator (RC) design. The instrument uses a 1024 element Si photodiode linear array for simultaneous multiwavelength detection. High quality RD spectra covering a spectral range from 1.5 to 4.5 eV can be acquired within a few seconds. A detailed description of the working principle, the instrumentation, and the algorithms used for data collection and reduction is presented, followed by a discussion of errors introduced by lamp instability and optical imperfections of the compensator. Finally, to demonstrate the performance of the new RCRD spectrometer, we illustrate its application for the in situ, real-time monitoring of the initial stages of organic thin film growth of para-sexiphenyl (p-6P) on the Cu(110)-(2 x 1)O surface.

Retardation correction for photoelastic modulator-based multichannel reflectance difference spectroscopy.

The wavelength dependence of the retardation induced by a photoelastic modulator (PEM) is a central issue in multichannel modulator-based spectroscopic ellipsometry and reflectance difference spectroscopy (RDS), where the optical signal is detected simultaneously at different wavelengths. Here we present a refined analysis of the modulator crystal's retardation and its effect on the signal quality. Two retardation correction schemes that take into account the actual wavelength dependence of the stress-optic coefficient are introduced. It is demonstrated experimentally that both methods provide a better correction than the procedure currently used in multichannel RDS. We define quality factors to evaluate the actual performance of the multichannel detection system as compared with a wavelength adaptive single-channel experiment. These quality factors thus provide a useful guideline for choosing the appropriate PEM retardation or reference wavelength in a multichannel experiment.

Measurement of the surface strain induced by reconstructed surfaces of GaAs (001) using photoreflectance and reflectance-difference spectroscopies.

We report photoreflectance-difference and reflectance-difference measurements on reconstructed GaAs (001) surfaces. From these data the linear and quadratic electro-optic coefficient spectra are determined in the important 2.8-3.4 eV spectral region. The surface strain and fields induced by the surface reconstruction are also determined. We show experimentally that between c(4x4) and (2x4) surfaces, there is an inversion of the surface electric field which we attribute to a direct piezo-electric effect related to the surface strain induced by reconstruction.