We show that small π-conjugated molecules adopt a lying-down orientation when deposited on few-layer MoS2 with horizontally oriented layers. In contrast, for vertically aligned MoS2 layers, DIP molecules are arranged in a standing-up manner.
Ongoing interest in two-dimensional (2D) layered materials has motivated extensive studies of transitional metal dichalcogenides, especially its most pronounced representative, MoS 2 . The few-layer MoS 2 exhibits distinct properties from those in bulk, which predetermine its potential usage in optoelectronics and flexible devices. Recently, it was found that the layer orientation in MoS 2 thin films is a key parameter for their utilization in specific devices. Thus far, the alignment of MoS 2 layers has been detected mostly by transmission electron microscopy (TEM). The drawback of this method is that it requires elaborate sample preparation and probes only a nanometer-scale area of the sample surface. Here we present polarized Raman spectroscopy which provides information about the MoS 2 layer orientation on the area a few orders larger than in TEM. We show that the depolarization ratio of the significant Raman peaks A 1g and E 2g show specific values for the vertical and horizontal alignment of the MoS 2 layers. We also analytically calculated the depolarization ratio for a thin MoS 2 layer, which is in good agreement with the measured values. Polarized Raman spectroscopy thus provides a simple, reliable, and general way for specifying the layer alignment in various 2D layered materials.
Chalcogenide perovskites (CPs), with the general composition
ABX3, where A and B are metals and X = S and Se, have recently
emerged as promising materials for application in photovoltaics. However,
the development of CPs and their applications has been hindered by
the limitations of available preparation methods. Here we present
a new approach for the synthesis of CPs, based on the sulfurization
of ternary and binary oxides or carbonates with in situ formed boron
sulfides. In contrast to the previously described approaches, the
method presented here uses chemically stable starting materials and
yields pure-phase crystalline CPs within several hours, under low
hazard conditions. CP yields over 95% are obtained at temperatures
as low as 600 °C. The generality of the approach is demonstrated
by the preparation of CPs with compositions BaZrS3, β-SrZrS3, BaHfS3, SrHfS3, and EuHfS3. Mechanistic insights about the formation of CPs are discussed.
Thin films of transition-metal
dichalcogenides are potential materials
for optoelectronic applications. However, the application of these
materials in practice requires knowledge of their fundamental optical
properties. Many existing methods determine optical constants using
predefined models. Here, a different approach was used. We determine
the sheet conductance and absorption coefficient of few-layer PtSe
2
in the infrared and UV–vis ranges without recourse
to any particular model for the optical constants. PtSe
2
samples with a thickness of about 3–4 layers were prepared
by selenization of 0.5 nm thick platinum films on sapphire substrates
at different temperatures. Differential reflectance was extracted
from transmittance and reflectance measurements from the front and
back of the sample. The film thickness, limited to a few atomic layers,
allowed a thin-film approximation to calculate the optical conductance
and absorption coefficient. The former has a very different energy
dependence in the infrared, near-infrared, and visible ranges. The
absorption coefficient exhibits a strong power-law dependence on energy
with an exponent larger than three in the mid-infrared and near-infrared
regions. We have not observed any evidence for a band gap in PtSe
2
thin layers down to an energy of 0.4 eV from our optical
measurements.
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