X-ray Photoelectron Spectroscopy Sampling Research Articles

Gamma-induced stress, strain and p-type doping in MBE-grown thin film MoTe2.

A thorough examination of the stability of a 2D MoTe2 thin film exposed to high-dose gamma radiation (γ) is addressed in this study. This study compares the film before and after irradiation (10-600 kGy dosage) to report the impact of γ radiation on the surface morphology, work function, tensile strain and charge redistribution in the MoTe2 thin film. The radiation damage to the film is monitored by optical micrographs (OM) and with atomic force microscopy (AFM) and conceptualized by thermal strain in the film. Raman spectroscopy has been used to analyze the shifting and to address the strain that arises due to irradiation in its vibrational mode. Kelvin probe force microscopy (KPFM) has been performed to evaluate the work function of the film, which increases by 0.14 eV for the 600 kGy γ-irradiated sample, implying shifting of the Fermi-level to the valence band of the spectrum and thus it results in p-type doping in the film. Owing to the reduced atomic mass and high energy of tellurium atoms, γ-irradiation causes tellurium vacancies, which lead to the formation of dangling bonds at unoccupied sites. When oxygen is adsorbed at these reactive spots, a charge-transfer mechanism takes place. This mechanism involves the transfer of electrons from the thin MoTe2 film to the adsorbed oxygen, forming oxides and causing p-type doping. Furthermore, p-doping is verified by the valence band shifting by 1.27 eV in 600 kGy in γ-irradiated samples in X-ray photoelectron spectroscopy. This comprehensive study shows how γ irradiation affects the chemical and physical characteristics of the MoTe2 thin film. Consequently, it shows that if devices integrating MoTe2 thin films are meant to be used in high-dose radiation conditions, the adsorbate concentrations, radiation shielding and required lifetimes must be carefully evaluated.

Surface charging of insulating samples in x-ray photoelectron spectroscopy

Electrical charging of insulating samples during XPS is of direct concern for referencing the binding energy scale. Results are presented on charging of composite insulating samples that consist of an organic overlayer of polystyrene (PS) or polydimethylsiloxane (PDMS) deposited on NaCl particles. The samples were analysed using small-spot monochromatized radiation, the surface charge being stabilized with an electron flood gun. The charging shift of the NaCl substrate is influenced by three different factors: the total photoelectron flux, the energy of the electrons produced by the flood gun and the flux of electrons originating from the overlayer. The overlayer is acting as an additional electron source, which makes the substrate more negative and reduces inhomogeneities of the local potential affecting the peak full width at half-maximum. Comparison of substrate peak intensities with computed values showed that PS overlayers were less uniform compared to PDMS overlayers. The overlayer thickness and uniformity affect both the total photoelectron flux and the flux of electrons from the overlayer to the substrate. Overlayer charging, like substrate charging, is probably determined by the balance of electron fluxes in the phase considered. The kinetic energy of the photoelectron is determined by the electrical potential in the phase where photoemission occurs and not with respect to the sample surface. This was demonstrated by differential charging between the NaCl substrate and PS overlayer. (C) 1998 John Wiley & Sons, Ltd.

Charge correction by gold deposition onto non-conducting samples in X-ray photoelectron spectroscopy

Gold deposited onto sample materials has been used as a calibrant for determining their absolute binding energies, and the optimum thickness of the gold decoration investigated. Polytetrafluoroethylene film and NaF and graphite pellets were used as the substrate materials. A series of depositions of gold layers up to ∼60 Å thick was performed on each sample surface. The thickness of gold deposited influences the apparent binding energies, the precision of the charge-corrected binding energies, and the FWHM's of the Au 4 f, C 1 s and F 1 s peaks. It seems that, for the instrument used in this study (which records photoelectrons at 45° to the sample surface), the optimum thickness of gold deposited onto a non-conducting sample surface is ∼ 6 Å (ie., ∼9 Å path-length through the layer).

A method to reduce the hydrocarbon contamination of samples in X-ray photoelectron spectroscopy

A method to reduce the hydrocarbon contamination of samples in X-ray photoelectron spectroscopy

The unsuitability of gold as a standard for non-conducting samples in X-ray photoelectron spectroscopy

The bombardment of non-conducting substances with X-rays causes charging which alters the kinetic energies of the ejected photoelectrons. Some means of