4 research outputs found
Second Harmonic Generation from Artificially Stacked Transition Metal Dichalcogenide Twisted Bilayers
Optical second harmonic generation (SHG) is known as a sensitive probe to the crystalline symmetry of few-layer transition metal dichalcogenides (TMDs). Layer-number dependent and polarization resolved SHG have been observed for the special case of Bernal stacked few-layer TMDs, but it remains largely unexplored for structures deviated from this ideal stacking order. Here we report on the SHG from homo- and heterostructural TMD bilayers formed by artificial stacking with an arbitrary stacking angle. The SHG from the twisted bilayers is a coherent superposition of the SH fields from the individual layers, with a phase difference depending on the stacking angle. Such an interference effect is insensitive to the constituent layered materials and thus applicable to hetero-stacked bilayers. A proof-of-concept demonstration of using the SHG to probe the domain boundary and crystal polarity of mirror twins formed in chemically grown TMDs is also presented. We show here that the SHG is an efficient, sensitive, and nondestructive characterization for the stacking orientation, crystal polarity, and domain boundary of van der Waals heterostructures made of noncentrosymmetric layered materials
Enhanced Thermoelectric Performance of PEDOT:PSS Flexible Bulky Papers by Treatment with Secondary Dopants
For inorganic thermoelectric materials,
Seebeck coefficient and
electrical conductivity are interdependent, and hence optimization
of thermoelectric performance is challenging. In this work we show
that thermoelectric performance of PEDOT:PSS can be enhanced by greatly
improving its electrical conductivity in contrast to inorganic thermoelectric
materials. Free-standing flexible and smooth PEDOT:PSS bulky papers
were prepared using vacuum-assisted filtration. The electrical conductivity
was enhanced to 640, 800, 1300, and 1900 S cm<sup>–1</sup> by
treating PEDOT:PSS with ethylene glycol, polyethylene glycol, methanol,
and formic acid, respectively. The Seebeck coefficient did not show
significant variation with the tremendous conductivity enhancement
being 21.4 and 20.6 μV K<sup>–1</sup> for ethylene glycol-
and formic acid-treated papers, respectively. This is because secondary
dopants, which increase electrical conductivity, do not change oxidation
level of PEDOT. A maximum power factor of 80.6 μW m<sup>–1</sup> K<sup>–2</sup> was shown for formic acid-treated samples,
while it was only 29.3 μW m<sup>–1</sup> K<sup>–2</sup> for ethylene glycol treatment. Coupled with intrinsically low thermal
conductivity of PEDOT:PSS, ZT ≈ 0.32 was measured at room temperature
using Harman method. We investigated the reasons behind the greatly
enhanced thermoelectric performance
Monolayer MoS<sub>2</sub> Heterojunction Solar Cells
We realized photovoltaic operation in large-scale MoS<sub>2</sub> monolayers by the formation of a type-II heterojunction with p-Si. The MoS<sub>2</sub> monolayer introduces a built-in electric field near the interface between MoS<sub>2</sub> and p-Si to help photogenerated carrier separation. Such a heterojunction photovoltaic device achieves a power conversion efficiency of 5.23%, which is the highest efficiency among all monolayer transition-metal dichalcogenide-based solar cells. The demonstrated results of monolayer MoS<sub>2</sub>/Si-based solar cells hold the promise for integration of 2D materials with commercially available Si-based electronics in highly efficient devices
Ultrafast Transient Terahertz Conductivity of Monolayer MoS<sub>2</sub> and WSe<sub>2</sub> Grown by Chemical Vapor Deposition
We have measured ultrafast charge carrier dynamics in monolayers and trilayers of the transition metal dichalcogenides MoS<sub>2</sub> and WSe<sub>2</sub> using a combination of time-resolved photoluminescence and terahertz spectroscopy. We recorded a photoconductivity and photoluminescence response time of just 350 fs from CVD-grown monolayer MoS<sub>2</sub>, and 1 ps from trilayer MoS<sub>2</sub> and monolayer WSe<sub>2</sub>. Our results indicate the potential of these materials as high-speed optoelectronic materials