D. R. T. Zahn

Photoluminescence emission and Raman response of monolayer MoS 2 , MoSe 2 , and WSe 2

We mechanically exfoliate mono- and few-layers of the transition metal dichalcogenides molybdenum disulfide, molybdenum diselenide, and tungsten diselenide. The exact number of layers is unambiguously determined by atomic force microscopy and high-resolution Raman spectroscopy. Strong photoluminescence emission is caused by the transition from an indirect band gap semiconductor of bulk material to a direct band gap semiconductor in atomically thin form.

Vibrational spectroscopy of bulk and supported manganese oxides

Manganese oxide catalysts, both bulk and supported on γ-Al2O3 and SiO2, have been studied by Raman and FTIR spectroscopies, their phase composition being determined by X-ray diffraction. The supported catalysts were prepared via pore volume impregnation from nitrate precursors, the atomic ratio of manganese to aluminium and silicon, respectively, being in the range from 0.5/100 to 18/100. The use of Raman analysis in the microscopic configuration allowed the spectra to be taken at different points of the surface and revealed the inhomogeneity of the catalyst. Besides the Raman features of the β-MnO2 and α-Mn2O3 phases, other signals were assigned to isolated Mn2+ ions accommodated in tetrahedral vacancies on the support surface and to some epitaxial layers of γ-Mn2O3 and manganese silicate, respectively. The FTIR spectra, though not as useful because of the strong bands of the support that overlap those of manganese oxides, support these findings.

Enhancement of the thermoelectric properties of PEDOT:PSS thin films by post-treatment

In this work, the thermoelectric (TE) properties of poly(3,4-ethylenedioxylthiophene):poly(styrene sulfonate) (PEDOT:PSS) thin films at room temperature are studied. Different methods have been applied for tuning the TE properties: 1st addition of polar solvent, dimethyl sulfoxide (DMSO), into the PEDOT:PSS solution; 2nd post-treatment of thin films with a mixture of DMSO and ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4). It is verified that DMSO post-treatment is more efficient than DMSO addition in improving the electrical conductivity with a trivial change in the Seebeck coefficient. The power factor is increased up to 30.1 μW mK−2 for the film with DMSO post-treatment, while the optimized power factor by DMSO addition is 18.2 μW mK−2. It is shown that both DMSO addition and post-treatment induce morphological changes: an interconnected network of elongated PEDOT grains is generated, leading to higher electrical conductivity. In contrast, for those films post-treated in the presence of EMIMBF4, an interconnected network of short and circular PEDOT grains with increased polaron density is created, resulting in the improvement in the Seebeck coefficient and a concomitant compromise in the electrical conductivity. An optimized power factor of 38.46 μW mK−2 is achieved at 50 vol% of EMIMBF4, which is the highest reported so far for PEDOT:PSS thin films to our knowledge. Assuming a thermal conductivity of 0.17 W mK−1, the corresponding ZT is 0.068 at 300 K. These results demonstrate that post-treatment is a promising approach to enhance the TE properties of PEDOT:PSS thin films. Furthermore, ionic liquid, EMIMBF4, shows the potential for tuning the TE properties of PEDOT:PSS thin films via a more environmentally benign process.

Raman spectroscopy investigation of size effects in cubic boron nitride

Raman spectra were taken of cubic boron nitride (c-BN) crystals with diameters between 100 nm and 1 mm. The Raman line shape of the optical phonons was found to become increasingly asymmetric towards lower frequency shifts, broader and weaker with decreasing crystal diameters. The results can be explained in terms of a spatial correlation model. The corresponding correlation lengths lie in the nanometer range, i.e. several orders of magnitude below the actual crystallite sizes as determined by electron microscopy, thus revealing a high defect density. An additional examined typical c-BN film on a Si(100) substrate exhibits even weaker and broader structures consistent with an even higher defect density.

Cubic boron nitride films by d.c. and r.f. magnetron sputtering: layer characterization and process diagnostics

Abstract Cubic boron nitride films have been deposited by three different magnetron sputtering methods: r.f. sputtering of an h-BN target, r.f. sputtering as well as d.c. sputtering of a pure boron target. By heating the boron target up to 800 °C its intrinsic electrical conductivity becomes sufficiently high to sustain a stable d.c. magnetron sputter discharge. The process parameters required for maximum c-BN formation were different for the three methods investigated. This could be correlated to the process parameters measured by means of Langmuir probes and optical emission spectroscopy. The films were investigated utilizing Fourier-transformed infrared spectroscopy with respect to their phase composition and compressive stress. Investigating the delamination of c-BN films, humidity was found to play a crucial role.

Analysis of molecular‐beam epitaxial growth of InAs on GaAs(100) by reflection anisotropy spectroscopy

The molecular‐beam epitaxial growth of InAs on GaAs(100) was investigated in situ using reflection anisotropy spectroscopy (RAS) and simultaneously reflection high‐energy electron diffraction. The RAS spectra of the GaAs c(4×4) and (2×4) and the InAs (4×2) and (2×4) reconstructions are reported. During InAs deposition, the RAS signal shows significant changes for InAs coverages as low as 1/6 of a monolayer. At this coverage surface reconstructions are responsible for the signal variation. For InAs coverages larger than four monolayers, the RAS signal is essentially determined by the anisotropic roughness of the three‐dimensional growing surface. This is verified using a three‐layer model which gives an excellent description of the experimental spectra at large coverages.

Resonant Raman scattering study of CdSe nanocrystals passivated with CdS and ZnS

CdSe nanocrystals (NCs) were obtained from cadmium sulfate and sodium selenosulfate in aqueous gelatin solutions. A near-bandgap emission of CdSe NCs was noticeably enhanced after passivation with CdS or ZnS. Resonant Raman scattering spectra of the passivated NCs revealed new peaks attributed to the formation of the sulfide shells around CdSe cores. The peaks observed for the CdSe/CdS core–shell NCs near 280 cm−1 were attributed to LO vibrations within a thin CdS passivating layer. Observation of the peak in the same frequency range for CdSe/ZnS is discussed within an assumption of alloying at the core–shell interface. Notable changes in the Raman spectra at different excitation wavelengths and shell parameters were attributed to the resonant and size-selective nature of the Raman process.

Experimental investigation and simulation of hybrid organic/inorganic Schottky diodes

We have investigated electronic transport in hybrid organic/inorganic Schottky diodes. In order to derive from basic principles the transport properties of the organic semiconductors, we have use a two-dimensional drift-diffusion simulator which properly accounts for transport in both organic and inorganic layers. We have calculated the I–V characteristics of Ag/PTCDA/GaAs Schottky diodes as a function of PTCDA thickness and compared the results with experimental in situ measurements. The interplay between barrier height, PTCDA thickness, space-charge-limited current, and image charge is outlined.

Formation of interfacial layers in InSb‐CdTe heterostructures studied by Raman scattering

The formation of a thin interfacial layer of indium telluride in InSb‐CdTe heterostructures has been previously suggested by soft x‐ray photoemission (SXPS) results. However, the detailed nature of this layer was difficult to interpret by SXPS. Therefore, samples being previously investigated by SXPS were studied by Raman spectroscopy. Spectra were taken of heterostructures grown by deposition of CdTe on (100) InSb substrates at room and elevated temperatures. For the room‐temperature sample the vibrational modes of CdTe and crystalline Te were detected. In the case of elevated substrate temperatures it was possible to identify the interfacial layer rich in indium and tellurium as consisting largely of In2Te3 using the Raman spectrum of In2Te3 as a fingerprint. Furthermore, the segregation of antimony was confirmed by the detection of the vibrational modes of antimony at the interface.

Comparison of techniques to characterise the density, porosity and elastic modulus of porous low-k SiO 2 xerogel films

A range of mesoporous xerogel low-k dielectric films were prepared and characterised using complementary techniques: Laser-generated surface acoustic waves, ellipsometric porosimetry, Rutherford backscattering and nanoindentation. The density, porosity, pore size distribution, cumulative surface area, elastic modulus and hardness of the films were measured as well as their dielectric constants. Dielectric constant values of k = 1.7-2.3 were measured for samples with porosities of 36-55%. Mean pore radii values of 2.2-4.2 nm and surface areas of 280-240 m3 cm-3 were also obtained. Using porosity and mean film density values determined using different techniques, the film skeletal density of these samples were calculated to be ≈ 1.4 g cm-3, almost 40% lower than that of dense SiO2. The elastic moduli of the films were found to be E < 4 GPa.