Effect of the crystallographic c-axis orientation on the tribological properties of the few-layer PtSe2

Two-dimensional (2D) transition metal dichalcogenides are potential candidates for ultrathin solid-state lubricants in low-dimensional systems owing to their flatness, high in-plane mechanical strength, and low shear interlayer strength. Yet, the effects of surface topography and surface chemistry on the tribological properties of 2D layers are still unclear. In this work, we performed a comparative investigation of nanoscale tribological properties of ultra-thin highly-ordered PtSe2 layers deposited on the sapphire substrates with the in-plane and out-of-plane crystallographic orientation of the PtSe2 c-axis flakes, and epitaxial PtSe2 layers. PtSe2 c-axis orientation was found to has an impact on the nanotribological, morphological and electrical properties of PtSe2, in particular the change in the alignment of the PtSe2 flakes from vertical (VA) to horizontal (HA) led to the lowering of the coefficient of friction from 0.21 to 0.16. This observation was accompanied by an increase in the root-mean-square surface roughness from 1.0 to 1.7 nm for the HA and VA films, respectively. The epitaxial films showed lower friction caused by lowering adhesion when compared to other investigated films, whereas the friction coefficient was similar to films with HA flakes. The observed trends in nanoscale friction is attributed to a different distribution of PtSe2 structure

Polyethylene Glycol-Modified Poly(Styrene-co-Ethylene/Butylene-co-Styrene)/Carbon Nanotubes Composite for Humidity Sensing

Polymeric composites of the linear triblock copolymer poly(styrene-co-ethylene/butylene-co-styrene) grafted with maleic anhydride units (SEBS-MA) or MA modified by hydrophilic polyethylene glycol (PEG) and containing various amounts of multiwall carbon nanotubes (MWCNTs) as conducting filler—were prepared by solvent casting. The MWCNT surface was modified by a non-covalent approach with a pyrene-based surfactant to achieve a homogeneous dispersion of the conducting filler within the polymeric matrix. The dispersion of the unmodified and surfactant-modified MWCNTs within the elastomeric SEBS-MA and SEBS-MA-PEG matrices was characterized by studying the morphology by TEM and SAXS. Dynamical mechanical analysis was used to evaluate the interaction between the MWCNTs and copolymer matrix. The electrical conductivity of the prepared composites was measured by dielectric relaxation spectroscopy, and the percolation threshold was calculated. The prepared elastomeric composites were characterized and studied as humidity sensor. Our results demonstrated that at MWCNTs concentration slightly above the percolation threshold could result in large signal changes. In our system, good results were obtained for MWCNT loading of 2 wt% and an ~0.1 mm thin composite film. The thickness of the tested elastomeric composites and the source current appear to be very important factors that influence the sensing performance

Cavity ring-down spectroscopy using telecom diode lasers

Continuous wave (cw) distributed feedback (DFB) telecom diode lasers are increasingly being used as light sources in the near-IR regions of the spectrum for cavity ring-down spectroscopy (CRDS). They are compact, operate economically at room temperature, are able to generate suitable narrowband tunable coherent radiation on a single longitudinal mode and are compatible with a variety of optical components, including fibre-optic devices. The very high sensitivities enabling to measure weak absorption with cw CRDS plus the quantitative nature of the absorption measurements are enabling an expanding range of applications of cw CRDS in molecular spectroscopy, atmospheric chemistry, plasma and medical diagnosis.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.info:eu-repo/semantics/publishe

Smart diode laser switching for cw-CRDS

info:eu-repo/semantics/nonPublishe

2D Halide Perovskite Phase Formation Dynamics and Their Regulation by Co‐Additives for Efficient Solar Cells

Abstract The incorporation of large organic ammonium ions renders the crystallization dynamics and layer formation process of halide perovskites complex, difficult to control, and leads to problems of suppressed charge transport with the formation of tiny‐sized grains. In this paper, the use of methylammonium chloride (MACl) and an excess of PbI2 is introduced as a co‐additives in the precursor solution for the control of phenylmethylammonium or benzylammonium (PMA+ spacer) and formamidinium (FA+)‐based quasi‐2D PMA2FAn−1PbnI3n+1 (n = 5) perovskite layers formation. By this method, the morphology of the layer, the inner phase distribution, and the charge transport properties are improved. By employing glow discharge‐optical emission spectroscopy (GD‐OES) and other techniques, it is revealed that the quasi‐2D perovskites prepared in the presence of co‐additives exhibit uniform removal dynamics of the solvent across the film. Furthermore, the grain growth mode, upon thermal annealing, is lateral. It results in large, monolithic grains with low‐trap state density and excellent substrate coverage. Particularly, co‐additives improve the cations dispersion upon the crystallization process, thus suppressing the low‐n phase formed through the aggregation of spacer cations and accelerating the formation of the high‐n phase

Thickness effect on structural defect-related density of states and crystallinity in P3HT thin films on ITO substrates

We report on a study of thickness effect on the formation of structural defect-related density of states (DOS) in the band gap of poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films spincoated on ITO substrates. The energy-resolved electrochemical impedance spectroscopy and grazing-incidence wide-angle X-ray scattering were used to correlate the DOS with the degree of crystallinity in P3HT thin films. We found an exponential increase of the defect DOS in the band gap with increasing fraction of the amorphous phase when decreasing the film thickness. The exponent increases abruptly when reducing the thickness down to 30 nm, which indicates two thickness regions with different dynamics of the defect DOS formation driven by increasing the fraction of the amorphous phase. Moreover, we observed the co-existence of two P3HT polymorphic crystalline phases with different backbone spacings, which results in the appearance of a peculiar DOS satellite peak above the highest occupied molecular orbital. The volume of the minor, more dense, crystalline phase exhibits a thickness dependence with a maximum plateau around 40 nm. These results suggest an important effect of the substrate roughness on the crystallinity and polymorphism of P3HT thin films depending on the film thickness with general implications for polymer thin films. © 2018 American Chemical Society.P3HT, SAS Institute; COFORD, Programme of Competitive Forestry Research for Development; APVV-0096-11, APVV, Agentúra na Podporu Výskumu a Vývoja; 2/0092/18; 1/0501/15; 2/0163/17; 26240220047; FEDER, European Regional Development FundSlovak Research and Development Agency [APVV-0096-11]; Scientific Grant Agency VEGA [1/0501/15, 2/0163/17, 2/0092/18]; Research and Development Operational Programme - ERDF [26240220047

Tunable Subnanometer Gaps in Self-Assembled Monolayer Gold Nanoparticle Superlattices Enabling Strong Plasmonic Field Confinement

Nanoparticle superlattices produced with controllableinterparticlegap distances down to the subnanometer range are of superior significancefor applications in electronic and plasmonic devices as well as inoptical metasurfaces. In this work, a method to fabricate large-area(& SIM;1 cm(2)) gold nanoparticle (GNP) superlattices witha typical size of single domains at several micrometers and high-densitynanogaps of tunable distances (from 2.3 to 0.1 nm) as well as variableconstituents (from organothiols to inorganic S2-) is demonstrated. Our approach is based on the combination of interfacialnanoparticle self-assembly, subphase exchange, and free-floating ligandexchange. Electrical transport measurements on our GNP superlatticesreveal variations in the nanogap conductance of more than 6 ordersof magnitude. Meanwhile, nanoscopic modifications in the surface potentiallandscape of active GNP devices have been observed following engineerednanogaps. In situ optical reflectance measurementsduring free-floating ligand exchange show a gradual enhancement ofplasmonic capacitive coupling with a diminishing average interparticlegap distance down to 0.1 nm, as continuously red-shifted localizedsurface plasmon resonances with increasing intensity have been observed.Optical metasurfaces consisting of such GNP superlattices exhibittunable effective refractive index over a broad wavelength range.Maximal real part of the effective refractive index, n (max), reaching 5.4 is obtained as a result of the extremefield confinement in the high-density subnanometer plasmonic gaps.ISSN:1936-0851ISSN:1936-086

Kinetics of Polymer–Fullerene Phase Separation during Solvent Annealing Studied by Table-Top X‑ray Scattering

Solvent annealing is an efficient way of phase separation in polymer–fullerene blends to optimize bulk heterojunction morphology of active layer in polymer solar cells. To track the process in real time across all relevant stages of solvent evaporation, laboratory-based in situ small- and wide-angle X-ray scattering measurements were applied simultaneously to a model P3HT:PCBM blend dissolved in dichlorobenzene. The PCBM molecule agglomeration starts at ∼7 wt % concentration of solid content of the blend in solvent. Although PCBM agglomeration is slowed-down at ∼10 wt % of solid content, the rate constant of phase separation is not changed, suggesting agglomeration and reordering of P3HT molecular chains. Having the longest duration, this stage most affects BHJ morphology. Phase separation is accelerated rapidly at concentration of ∼25 wt %, having the same rate constant as the growth of P3HT crystals. P3HT crystallization is driving force for phase separation at final stages before a complete solvent evaporation, having no visible temporal overlap with PCBM agglomeration. For the first time, such a study was done in laboratory demonstrating potential of the latest generation table-top high-brilliance X-ray source as a viable alternative before more sophisticated X-ray scattering experiments at synchrotron facilities are performed

A Non-Equilibrium Transient Phase Revealed by in Situ GISAXS Tracking of the Solvent-Assisted Nanoparticle Self-Assembly

We report on a time-resolved study of the colloidal nanoparticle self-assembly into a high-quality nanoparticle crystal with the face-centered cubic crystallographic symmetry. In particular, the grazing-incidence small-angle X-ray scattering technique was employed to track kinetics of the solvent evaporation driven self-assembly on casting a drop of plasmonic Ag nanoparticles on a silicon substrate. The short-range (cumulative) disorder typical for paracrystal structures before the complete solvent evaporation at 300–350 s from the drop casting was found with the exception of the time window of 125–150 s where a highly regular transient phase with the long-range order was observed. This temporary improvement of the nanoparticle crystal perfection occurring shortly before the complete solvent evaporation is the main message of the paper. It is attributed to interaction between the surfactant shells of the neighboring nanoparticles getting into contact in the presence of solvent residua to the end of the solvent evaporation which results in a larger nanoparticle hydrodynamic diameter with a smaller dispersion and improvement of the crystallization. This process has direct impact on the quality of the resulting nanoparticle crystal and tailoring its properties