2 research outputs found
Anisotropic Thermoelectric Response in Two-Dimensional Puckered Structures
Two-dimensional
semiconductor materials with puckered structure
offer a novel playground to implement nanoscale thermoelectric, electronic,
and optoelectronic devices with improved functionality. Using a combination
of approaches to compute the electronic and phonon band structures
with Green’s function based transport techniques, we address
the thermoelectric performance of phosphorene, arsenene, and SnS monolayers.
In particular, we study the influence of anisotropy in the electronic
and phononic transport properties and its impact on the thermoelectric
figure of merit <i>ZT</i>. Our results show no strong electronic
anisotropy, but a strong thermal one, the effect being most pronounced
in the case of SnS monolayers. This material also displays the largest
figure of merit at room temperature for both transport directions,
zigzag (<i>ZT</i> ∼ 0.95) and armchair (<i>ZT</i> ∼ 1.6), thus hinting at the high potential of these new materials
in thermoelectric applications
Effect of Surface Properties on the Microstructure, Thermal, and Colloidal Stability of VB<sub>2</sub> Nanoparticles
Recent
years have seen an increasing research effort focused on nanoscaling
of metal borides, a class of compounds characterized by a variety
of crystal structures and bonding interactions. Despite being subject
to an increasing number of studies in the application field, comprehensive
studies of the size-dependent structural changes of metal borides
are limited. In this work, size-dependent microstructural analysis
of the VB<sub>2</sub> nanocrystals prepared by means of a size-controlled
colloidal solution synthesis is carried out using X-ray powder diffraction.
The contributions of crystallite size and strain to X-ray line broadening
is separated by introducing a modified Williamson–Hall method
taking into account different reflection profile shapes. For average
crystallite sizes smaller than ca. 20 nm, a remarkable increase of
lattice strain is observed together with a significant contraction
of the hexagonal lattice decreasing primarily the cell parameter <i>c</i>. Exemplary density-functional theory calculations support
this trend. The size-dependent lattice contraction of VB<sub>2</sub> nanoparticles is associated with the decrease of the interatomic
boron distances along the <i>c</i>-axis. The larger fraction
of constituent atoms at the surface is formed by boron atoms. Accordingly,
lattice contraction is considered to be a surface effect. The anisotropy
of the size-dependent lattice contraction in VB<sub>2</sub> nanocrystals
is in line with the higher compressibility of its macroscopic bulk
structure along the <i>c</i>-axis revealed by theoretical
calculations of the respective elastic properties. Transmission electron
microscopy indicates that the VB<sub>2</sub> nanocrystals are embedded
in an amorphous matrix. X-ray photoelectron spectroscopy analysis
reveals that this matrix is mainly composed of boric acid, boron oxides,
and vanadium oxides. VB<sub>2</sub> nanocrystals coated with these
oxygen containing amorphous species are stable up to 789 °C as
evidenced by thermal analysis and temperature dependent X-ray diffraction
measurements carried out under Ar atmosphere. Electrokinetic measurement
indicates that the aqueous suspension of VB<sub>2</sub> nanoparticles
with hydroxyl groups on the surface region has a good stability at
neutral and basic pH arising from electrostatic stabilizatio