Two-dimensional 2D Transition Metal Chalcogenides Research Articles

Room-Temperature Excitonic Nanolaser Array with Directly Grown Monolayer WS2

Two-dimensional (2D) transition-metal chalcogenides (TMDCs) with strong excitonic emission are ideal candidates for small-volume gain materials in nanolasers, which can be realized by integrating 2D TMDCs with optical microcavities. However, at present, the integration is usually realized by the transfer method, which is not suitable for large-scale production, and the quality factor of the microcavity usually drops sharply in the process of transfer. Therefore, scalable fabrication of nanolasers remains an urgent issue to be solved. In this paper, by embedding continuous monolayer WS2 between silicon nitride (Si3N4) microdisks and Al2O3, we demonstrated the room-temperature transfer-free excitonic nanolaser array. Lasing at room temperature is achieved with the help of the high-quality-factor Si3N4 microdisks and the physical vapor deposition method for growing 2D gain material directly. This work provides a practical solution for large-scale and low-cost TMDC-based nanolaser arrays in the integrated optical communication systems.

Intensive and broad bound exciton emission at cryogenic temperature in suspended monolayer transition metal dichalcogenides

Two-dimensional (2D) transition metal chalcogenides (TMDCs) are expected to play an important role in next-generation optoelectronic devices. However, the properties of 2D TMDCs can be largely affected by defects, and thus, many efforts have been made to characterize and eliminate the defects. Here, we investigate defect-related trap states, particularly in large-area suspended ${\mathrm{WS}}_{2}$ monolayers at cryogenic temperatures, using steady-state and time-resolved photoluminescence (TRPL) spectroscopy. We observed an intensive and broad (full width half maximum $\ensuremath{\sim}170\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$) photoluminescence (PL) emission centered at 1.83 eV, 0.3 eV lower than the free exciton transition energy. From temperature-dependent PL spectra, the thermal activation energy of excitons escaping from trap states is determined to be 30 meV. The TRPL spectrum shows an ultralong lifetime at 13 K. The long-lived emission with the broad PL linewidth is attributed to excitons bound to defects of possibly chalcogenide-site substitution and sulfur vacancies. Compared with the supported monolayer, the defect-related trap states are found to be deeper in the suspended layer and can trap excitons more efficiently due to less dielectric screening. In this letter, we provide further understanding of trap states and decay channels in monolayer TMDCs and show that suspended 2D TMDCs can provide a pure platform for studying defect-related properties.

Ultrasonically derived WSe2 nanostructure embedded MXene hybrid composites for supercapacitors and hydrogen evolution reactions

Two-dimensional (2D) transition metal chalcogenides (TMDCs) and carbide have validated boundless prospective as multi-functional constituents for high performance energy storing and/or conversion devices. However, pure TMDCs exhibit poor performance for electrochemical applications due to low electrical conductivity, scarce electrochemically active edges and inadequate cycling stability. Therefore, this paper fabricated MXene/WSe2 hybrids with strong interfacial interactions and conductivities using a one-step chemical reaction as electrodes for supercapacitors and hydrogen evolution. Fabricated supercapacitors achieved high specific capacitance = 840 F g−1 at 2 A g−1, with symmetric capacitance = 246 F g−1 at 2 A g−1 for MXene/WSe2 hybrids. Hydrogen evolution achieved low overpotentials = 76 and 62 mV to drive 10 mA cm−2 current with small Tafel slopes = 78 and 84 mV.dec−1 in acid and base media, respectively. 2D and 2D hybrid WSe2 nanoparticle composite embedded MXene scaffolds achieved excellent electron/ion intercalation owing to its distinctive 2D-layered structure, increasing interlayer spacing and retaining large electrode/electrolyte contact to enhance efficiency. Two-dimensional TMDCs/MXene composites were verified as potentially efficient electrode materials for energy storing and exchange uses.

Chemical vapor deposition of WS2/Mo1-xWxS2/MoS2 lateral heterostructures

The growth of lateral heterostructures of two-dimensional (2D) transition metal chalcogenides (TMDCs) opens up new paths for promising electronic and optoelectronic devices. Here, we demonstrate the one-step growth of 2D WS2/Mo1-xWxS2/MoS2 lateral heterostructures prepared on Si substrates by chemical vapor deposition (CVD), and the growth mechanism of WS2/Mo1-xWxS2/MoS2 lateral heterostructures are also systematically analyzed. The variation of radial Raman and photoluminescence (PL) spectrum of crystals on Si substrates are attributed to the three different structures of WS2, Mo1-xWxS2 and MoS2. Clear structure and optical modulation can be observed by Raman and PL mapping to further confirm the formation WS2/Mo1-xWxS2/MoS2 lateral heterostructures. According to the analysis of the optical properties and growth mechanism of the WS2/Mo1-xWxS2/MoS2 lateral heterostructures, it can be concluded that the formation of the lateral heterostructures depends on the growth temperature and chronology.