4 research outputs found
Efficient Photothermoelectric Conversion in Lateral Topological Insulator Heterojunctions
Tuning
the electron and phonon transport properties of thermoelectric materials
by nanostructuring has enabled improving their thermopower figure
of merit. Three-dimensional topological insulators, including many
bismuth chalcogenides, attract increasing attention for this purpose,
as their topologically protected surface states are promising to further
enhance the thermoelectric performance. While individual bismuth chalcogenide
nanostructures have been studied with respect to their photothermoelectric
properties, nanostructured p–n junctions of these compounds
have not yet been explored. Here, we experimentally investigate the
room temperature thermoelectric conversion capability of lateral heterostructures
consisting of two different three-dimensional topological insulators,
namely, the n-type doped Bi<sub>2</sub>Te<sub>2</sub>Se and the p-type
doped Sb<sub>2</sub>Te<sub>3</sub>. Scanning photocurrent microscopy
of the nanoplatelets reveals efficient thermoelectric conversion at
the p–n heterojunction, exploiting hot carriers of opposite
sign in the two materials. From the photocurrent data, a Seebeck coefficient
difference of Δ<i>S</i> = 200 μV/K was extracted,
in accordance with the best values reported for the corresponding
bulk materials. Furthermore, it is in very good agreement with the
value of Δ<i>S</i> = 185 μV/K obtained by DFT
calculation taking into account the specific doping levels of the
two nanostructured components
Raman Characterization of the Charge Density Wave Phase of 1T-TiSe<sub>2</sub>: From Bulk to Atomically Thin Layers
Raman
scattering is a powerful tool for investigating the vibrational
properties of two-dimensional materials. Unlike the 2H phase of many
transition metal dichalcogenides, the 1T phase of TiSe<sub>2</sub> features a Raman-active shearing and breathing mode, both of which
shift toward lower energy with increasing number of layers. By systematically
studying the Raman signal of 1T-TiSe<sub>2</sub> in dependence of
the sheet thickness, we demonstrate that the charge density wave transition
of this compound can be reliably determined from the temperature dependence
of the peak position of the E<sub>g</sub> mode near 136 cm<sup>–1</sup>. The phase transition temperature is found to first increase with
decreasing thickness of the sheets, followed by a decrease due to
the effect of surface oxidation. The Raman spectroscopy-based method
is expected to be applicable also to other 1T-phase transition metal
dichalcogenides featuring a charge density wave transition and represents
a valuable complement to electrical transport-based approaches
Spectroscopic Determination of the Electrochemical Potentials of n-Type Doped Carbon Nanotubes
Understanding the doping mechanism that involves substantial
charge
transfer between carbon nanotubes and chemical adsorbent is of critical
importance for both basic scientific knowledge and nanodevice applications.
Nevertheless, it is difficult to estimate the modification of electronic
structures of the doped carbon nanotubes. Here we report measurements
of electrochemical potentials of n-doped single-walled carbon nanotubes
(SWCNTs) by using photoluminescence (PL) measurement. The change of
the measured PL intensity before and after n-type doping was used
to extract the electrochemical potential using the Nernst equation.
The measured electrochemical potentials of SWCNTs approached the theoretical
reduction potential of SWCNTs as the mole concentration of the dopant
increased. The doping effect was also confirmed by the change of absorption
spectroscopy. The quenching of the PL and absorption intensity was
strongly correlated to the standard reduction potential of the dopant
and its concentration. This investigation could be a cornerstone for
SWCNTs-based electronic device applications such as solar cells, light-emitting
diodes, and nanogenerators
Stranski–Krastanov and Volmer–Weber CVD Growth Regimes To Control the Stacking Order in Bilayer Graphene
Aside
from unusual properties of monolayer graphene, bilayer has been shown
to have even more interesting physics, in particular allowing bandgap
opening with dual gating for proper interlayer symmetry. Such properties,
promising for device applications, ignited significant interest in
understanding and controlling the growth of bilayer graphene. Here
we systematically investigate a broad set of flow rates and relative
gas ratio of CH<sub>4</sub> to H<sub>2</sub> in atmospheric pressure
chemical vapor deposition of multilayered graphene. Two very different
growth windows are identified. For relatively high CH<sub>4</sub> to
H<sub>2</sub> ratios, graphene growth is relatively rapid with an
initial first full layer forming in seconds upon which new graphene
flakes nucleate then grow on top of the first layer. The stacking
of these flakes versus the initial graphene layer is mostly turbostratic.
This growth mode can be likened to Stranski–Krastanov growth.
With relatively low CH<sub>4</sub> to H<sub>2</sub> ratios, growth
rates are reduced due to a lower carbon supply rate. In addition bi-,
tri-, and few-layer flakes form directly over the Cu substrate as
individual islands. Etching studies show that in this growth mode
subsequent layers form beneath the first layer presumably through
carbon radical intercalation. This growth mode is similar to that
found with Volmer–Weber growth and was shown to produce highly
oriented AB-stacked materials. These systematic studies provide new
insight into bilayer graphene formation and define the synthetic range
where gapped bilayer graphene can be reliably produced