Jeffrey D. Cain

High thermoelectric figure of merit in the Cu3SbSe4-Cu3SbS4 solid solution

We report thermoelectric properties of selected compounds from the Cu3SbSe4-Cu3SbS4 system. Additional phonon scattering due to the disordered arrangement of Se and S atoms reduces the lattice thermal conductivity to near its minimum possible value at high temperature. The hole concentration is optimized at approximately 2.0 × 1020 cm−3 by doping with 3% Ge on the Sb site. Compounds of the form Cu3Sb1−yGeySe4−xSx (x = 0.8 and 1.2, y = 0.02 and 0.03) all have dimensionless thermoelectric figure of merit in excess of 0.8 at 650 K, with a maximum value of 0.89 for x = 1.2, y = 0.03.

High thermoelectric figure of merit in the Cu 3 SbSe 4 -Cu 3 SbS 4 solid solution

We report thermoelectric properties of selected compounds from the Cu3SbSe4-Cu3SbS4 system. Additional phonon scattering due to the disordered arrangement of Se and S atoms reduces the lattice thermal conductivity to near its minimum possible value at high temperature. The hole concentration is optimized at approximately 2.0 × 1020 cm−3 by doping with 3% Ge on the Sb site. Compounds of the form Cu3Sb1−yGeySe4−xSx (x = 0.8 and 1.2, y = 0.02 and 0.03) all have dimensionless thermoelectric figure of merit in excess of 0.8 at 650 K, with a maximum value of 0.89 for x = 1.2, y = 0.03.

Lattice thermal conductivity of the Cu3SbSe4-Cu3SbS4 solid solution

The compositional dependence of the crystal structure and lattice thermal conductivity in the Cu3SbSe4-Cu3SbS4 system has been studied. The lattice parameters of the Cu3SbSe4-xSx compounds decrease linearly with x, and the tetragonal structure (space group 14−2m no. 121) of the end compounds is maintained at all compositions. The lattice thermal conductivity is much lower than that predicted by a simple rule of mixtures, which is typical for a solid solution. The Debye model produces a very reasonable fit to the experimental lattice thermal conductivity data when phonon scattering due to atomic mass and size differences between Se and S is taken into account. Compounds in this series are likely to improve upon the thermoelectric performance of Cu3SbSe4, which has shown ZT = 0.72 when optimized.

Structural effects on the lattice thermal conductivity of ternary antimony- and bismuth-containing chalcogenide semiconductors

The lattice thermal conductivities Cu3SbSe4 and Cu3SbSe3 have been measured. While the former compound exhibits classical behavior, the lattice thermal conductivity of Cu3SbSe3 is anomalously low. We speculate that, similar to the case of AgSbTe2, the low thermal conductivity in Cu3SbSe3 has its origin in strong anharmonicity induced by the presence of the two additional nonbonding electrons in the valence shell of the Sb3+ ions. Anomalously low thermal conductivity is also demonstrated in compounds in which Bi occurs in the trivalent state. The results have implications in the design of thermoelectric semiconductors with intrinsically low thermal conductivity.

Improved Thermoelectric Performance in Cu-Based Ternary Chalcogenides Using S for Se Substitution

Cu-based ternary chalcogenide semiconductors are a promising class of p-type thermoelectric materials. Here we show that S for Se substitution is a promising route for improving the thermoelectric performance of these compounds, in particular for Cu3SbSe4 and Cu2SnSe3. Phonon scattering due to the combined effects of the atomic mass and size difference between S and Se produces a significant reduction in lattice thermal conductivity over a wide temperature range, and limits the phonon mean free path to its minimum possible value at high temperature. The small electronegativity difference between S and Se is ideal for conserving carrier mobility, as demonstrated by the reasonably large hole mobility in the disordered Cu3Sb(Se1−xSx)4 compounds. These effects along with the low-cost, environmentally benign nature of S, make S for Se substitution a simple yet effective method of improving the thermoelectric performance of Cu-based ternary selenides.

Valley-polarized exciton–polaritons in a monolayer semiconductor

Exploitation of the valley electronic structure of transition metal dichalcogenides with exciton–polaritons is an elusive challenge. Now, valley-polarized exciton–polaritons in monolayers of MoS2 have been demonstrated.

Evaporative thinning: A facile synthesis method for high quality ultrathin layers of 2D crystals

The palette of two-dimensional materials has expanded beyond graphene in recent years to include the chalcogenides among other systems. However, there is a considerable paucity of methods for controlled synthesis of mono- and/or few-layer two-dimensional materials with desirable quality, reproducibility, and generality. Here we show a facile top-down synthesis approach for ultrathin layers of 2D materials down to monolayer. Our method is based on controlled evaporative thinning of initially large sheets, as deposited by vapor mass-transport. Rather than optimizing conditions for monolayer deposition, our approach makes use of selective evaporation of thick sheets to control the eventual thickness, down to a monolayer, a process which appears to be self-stopping. As a result, 2D sheets with high yield, high reproducibility, and excellent quality can be generated with large (>10 μm) and thin (∼ 1-2 nm) dimensions. Evaporative thinning promises to greatly reduce the difficulty involved in isolating large, mono- and few-layers of 2D materials for subsequent studies.

Quantifying Plasmon-Enhanced Light Absorption in Monolayer WS2 Films

Transition metal dichalcogenide semiconductors hold great promise in photonic and optoelectronic applications, such as flexible solar cells and ultrafast photodetectors, because of their direct band gap and few-atom thicknesses. However, it is crucial to understand and improve the absorption characteristics of these monolayer semiconducting materials. In this study, we conducted a systematic numerical and experimental investigation to demonstrate and quantify absorption enhancement in WS2 monolayer films, in the presence of silver plasmonic nanodisk arrays. Our analysis combining full-field electromagnetic simulations and optical absorption spectroscopy measurements indicates a fourfold enhancement in the absorption of an WS2 film near its band edge, close to the plasmonic resonance wavelength of Ag nanodisk arrays. The proposed Ag/WS2 heterostructure exhibited a 2.5-fold enhancement in calculated short-circuit current. Such hybrid plasmonic or two-dimensional (2D) materials with enhanced absorption pave the way for the practical realization of 2D optoelectronic devices, including ultrafast photodetectors and solar cells.

Superior Plasmonic Photodetectors Based on Au@MoS2 Core–Shell Heterostructures

Integrating plasmonic materials into semiconductor media provides a promising approach for applications such as photosensing and solar energy conversion. The resulting structures introduce enhanced light-matter interactions, additional charge trap states, and efficient charge-transfer pathways for light-harvesting devices, especially when an intimate interface is built between the plasmonic nanostructure and semiconductor. Herein, we report the development of plasmonic photodetectors using Au@MoS2 heterostructures-an Au nanoparticle core that is encapsulated by a CVD-grown multilayer MoS2 shell, which perfectly realizes the intimate and direct interfacing of Au and MoS2. We explored their favorable applications in different types of photosensing devices. The first involves the development of a large-area interdigitated field-effect phototransistor, which shows a photoresponsivity ∼10 times higher than that of planar MoS2 transistors. The other type of device geometry is a Si-supported Au@MoS2 heterojunction gateless photodiode. We demonstrated its superior photoresponse and recovery ability, with a photoresponsivity as high as 22.3 A/W, which is beyond the most distinguished values of previously reported similar gateless photodetectors. The improvement of photosensing performance can be a combined result of multiple factors, including enhanced light absorption, creation of more trap states, and, possibly, the formation of interfacial charge-transfer transition, benefiting from the intimate connection of Au and MoS2.

Substrate-induced strain and charge doping in CVD-grown monolayer MoS2

Due to its electronic-grade quality and potential for scalability, two-dimensional (2D) MoS2 synthesized by chemical vapor deposition (CVD) has been widely explored for electronic/optoelectronic applications. As 2D MoS2 can be considered a 100% surface, its unique intrinsic properties are inevitably altered by the substrate upon which it is grown. However, systematic studies of substrate-layer interactions in CVD-grown MoS2 are lacking. In this study, we have analyzed built-in strain and charge doping using Raman and photoluminescence spectroscopy in 2D MoS2 grown by CVD on four unique substrates: SiO2/Si, sapphire, Muscovite mica, and hexagonal boron nitride. We observed decreasing strain and charge doping in grown MoS2 as the substrates become less rough and more chemically inert. The possible origin of strain was investigated through atomic force microscopy roughness measurements of the as-grown layer and substrate. Our results provide direction for device optimization through careful selection of the ...