Yaroslav E. Romanyuk

Nanoscale Infrared Absorption Spectroscopy of Individual Nanoparticles Enabled by Scattering-Type Near-Field Microscopy

Infrared absorption spectroscopy is a powerful and widely used tool for analyzing the chemical composition and structure of materials. Because of the diffraction limit, however, it cannot be applied for studying individual nanostructures. Here we demonstrate that the phase contrast in substrate-enhanced scattering-type scanning near-field optical microscopy (s-SNOM) provides a map of the infrared absorption spectrum of individual nanoparticles with nanometer-scale spatial resolution. We succeeded in the chemical identification of silicon nitride nanoislands with heights well below 10 nm, by infrared near-field fingerprint spectroscopy of the Si-N stretching bond. Employing a novel theoretical model, we show that the near-field phase spectra of small particles correlate well with their far-field absorption spectra. On the other hand, the spectral near-field contrast does not scale with the volume of the particles. We find a nearly linear scaling law, which we can attribute to the near-field coupling between the near-field probe and the substrate. Our results provide fundamental insights into the spectral near-field contrast of nanoparticles and clearly demonstrate the capability of s-SNOM for nanoscale chemical mapping based on local infrared absorption.

Enhanced Carrier Collection from CdS Passivated Grains in Solution-Processed Cu2ZnSn(S,Se)4 Solar Cells

Solution processing of Cu2ZnSn(S,Se)4 (CZTSSe)-kesterite solar cells is attractive because of easy manufacturing using readily available metal salts. The solution-processed CZTSSe absorbers, however, often suffer from poor morphology with a bilayer structure, exhibiting a dense top crust and a porous bottom layer, albeit yielding efficiencies of over 10%. To understand whether the cell performance is limited by this porous layer, a systematic compositional study using (scanning) transmission electron microscopy ((S)TEM) and energy-dispersive X-ray spectroscopy of the dimethyl sulfoxide processed CZTSSe absorbers is presented. TEM investigation revealed a thin layer of CdS that is formed around the small CZTSSe grains in the porous bottom layer during the chemical bath deposition step. This CdS passivation is found to be beneficial for the cell performance as it increases the carrier collection and facilitates the electron transport. Electron-beam-induced current measurements reveal an enhanced carrier collection for this buried region as compared to reference cells with evaporated CdS.

Effect of band gap on the red luminescence capability of Pr-doped InGaN layers grown by molecular beam epitaxy

The effect of In concentration on the red photoluminescence (PL) from trivalent Pr rare-earth (RE) ions in InGaN layers is investigated. A series of Pr-doped InxGa1−xN thin layers with 0⩽x⩽0.15 is grown by radio-frequency plasma-assisted molecular beam epitaxy on GaN∕sapphire templates. Photoexcitation with a He–Cd laser results in characteristic red emission at 652nm, corresponding to the P03−F23 transition of Pr3+. The Pr PL is the highest for samples grown under slightly N-rich conditions, but the “yellow luminescence” caused by Ga vacancies is also present. The intensity of the characteristic Pr PL in InxGa1−xN:Pr decreases by one order of magnitude when increasing the In content from x=0 to 0.15, even though the matching of the band gap to the Pr3+ energy states is better. Low-temperature PL measurements show that the de-excitation energy backtransfer process from excited Pr3+ ions is more severe for In-richer samples, although the amount of backtransfer cannot be the sole reason for the significant ...

New sulphide precursors for Zn(O,S) buffer layers in Cu(In,Ga)Se2 solar cells for faster reaction kinetics

The development of a novel chemistry for the chemical bath deposition of Zn(O,S) buffer layers for Cu(In,Ga)Se2 (CIGS) solar cells is desired for a higher growth rate, hence reduced deposition time, while reducing simultaneously the required concentration of reactants. State-of-the-art recipes are based on thiourea as sulphide precursor requiring a high molarity of reactants and relatively long deposition times due to the slow decomposition rate of thiourea. In this contribution thioamide based sulphide precursors were investigated for their decomposition and growth behaviour. A co-solvent approach in an ethanolic/aqueous ammonia medium was evaluated omitting the need for additional complexants. By replacing thiourea with the investigated thioamides, homogeneous dense layers of around 30 nm were grown with a greatly decreased deposition time of 8 min compared to 25 min for thiourea. Likewise, the concentration of the sulphide precursor was 40-fold reduced. The photovoltaic performance as characterized by external quantum efficiency and current–voltage measurements, showed conversion efficiencies of 15% comparable to the thiourea based process.

Imaging of InGaN inhomogeneities using visible apertureless near-field scanning optical microscope

The optical properties of epitaxially grown islands of InGaN are investigated with nanometer-scale spatial resolution using visible apertureless near-field scanning optical microscopy. Scattered light from the tip-sample system is modulated by cantilever oscillations and detected at the third harmonic of the oscillation frequency to distinguish the near-field signal from unwanted scattered background light. Scattered near-field measurements indicate that the as-grown InGaN islanded film may exhibit both inhomogeneous In composition and strain-induced changes that affect the optical signal at 633 nm and 532 nm. Changes are observed in the optical contrast for large 3D InGaN islands (100s of nm) of the same height. Near-field optical mapping of small grains on a finer scale reveals InGaN composition or strain-induced irregularities in features with heights of only 2 nm, which exhibit different near-field signals at 633 nm and 532 nm incident wavelengths. Optical signal contrast from topographic features as small as 30 nm is detected.