Jiangtan Yuan

Bio: Jiangtan Yuan is an academic researcher from Rice University. The author has contributed to research in topics: Monolayer & Graphene. The author has an hindex of 20, co-authored 45 publications receiving 1989 citations. Previous affiliations of Jiangtan Yuan include Chinese Academy of Sciences & Northwestern University.

Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2 heterostructures.

Abstract: Van der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2 heterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices.

Facile Synthesis of Single Crystal Vanadium Disulfide Nanosheets by Chemical Vapor Deposition for Efficient Hydrogen Evolution Reaction

Abstract: A facile chemical vapor deposition method to prepare single-crystalline VS2 nanosheets for the hydrogen evolution reaction is reported. The electrocatalytic hydrogen evolution reaction (HER) activities of VS2 show an extremely low overpotential of -68 mV at 10 mA cm(-2), small Tafel slopes of ≈34 mV decade(-1), as well as high stability, demonstrating its potential as a candidate non-noble-metal catalyst for the HER.

Long-lived nanosecond spin relaxation and spin coherence of electrons in monolayer MoS2 and WS2

Abstract: A range of semiconductors can host both spin and valley polarizations. Optical experiments on single layers of transition metal dichalcogenides now show that inter-valley scattering can accelerate spin relaxation. The recently discovered monolayer transition metal dichalcogenides (TMDCs) provide a fertile playground to explore new coupled spin–valley physics1,2,3. Although robust spin and valley degrees of freedom are inferred from polarized photoluminescence (PL) experiments4,5,6,7,8, PL timescales are necessarily constrained by short-lived (3–100 ps) electron–hole recombination9,10. Direct probes of spin/valley polarization dynamics of resident carriers in electron (or hole)-doped TMDCs, which may persist long after recombination ceases, are at an early stage11,12,13. Here we directly measure the coupled spin–valley dynamics in electron-doped MoS2 and WS2 monolayers using optical Kerr spectroscopy, and reveal very long electron spin lifetimes, exceeding 3 ns at 5 K (two to three orders of magnitude longer than typical exciton recombination times). In contrast with conventional III–V or II–VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. Supported by a model of coupled spin–valley dynamics, these results indicate a novel mechanism of itinerant electron spin dephasing in the rapidly fluctuating internal spin–orbit field in TMDCs, driven by fast inter-valley scattering. Additionally, a long-lived spin coherence is observed at lower energies, commensurate with localized states. These studies provide insight into the physics underpinning spin and valley dynamics of resident electrons in atomically thin TMDCs.

Unveiling Active Sites for the Hydrogen Evolution Reaction on Monolayer MoS2

Abstract: Here, the hydrogen evolution reaction (HER) activities at the edge and basal-plane sites of monolayer molybdenum disulfide (MoS2 ) synthesized by chemical vapor deposition (CVD) are studied using a local probe method enabled by selected-area lithography. Reaction windows are opened by e-beam lithography at sites of interest on poly(methyl methacrylate) (PMMA)-covered monolayer MoS2 triangles. The HER properties of MoS2 edge sites are obtained by subtraction of the activity of the basal-plane sites from results containing both basal-plane and edge sites. The catalytic performances in terms of turnover frequencies (TOFs) are calculated based on the estimated number of active sites on the selected areas. The TOFs follow a descending order of 3.8 ± 1.6, 1.6 ± 1.2, 0.008 ± 0.002, and 1.9 ± 0.8 × 10-4 s-1 , found for 1T'-, 2H-MoS2 edges, and 1T'-, 2H-MoS2 basal planes, respectively. Edge sites of both 2H- and 1T'-MoS2 are proved to have comparable activities to platinum (≈1-10 s-1 ). When fitted into the HER volcano plot, the MoS2 active sites follow a trend distinct from conventional metals, implying a possible difference in the reaction mechanism between transition-metal dichalcogenides (TMDs) and metal catalysts.

Photoluminescence quenching and charge transfer in artificial heterostacks of monolayer transition metal dichalcogenides and few-layer black phosphorus.

Abstract: Transition metal dichalcogenides monolayers and black phosphorus thin crystals are emerging two-dimensional materials that demonstrated extraordinary optoelectronic properties. Exotic properties and physics may arise when atomic layers of different materials are stacked together to form van der Waals solids. Understanding the important interlayer couplings in such heterostructures could provide avenues for control and creation of characteristics in these artificial stacks. Here we systematically investigate the optical and optoelectronic properties of artificial stacks of molybdenum disulfide, tungsten disulfide, and black phosphorus atomic layers. An anomalous photoluminescence quenching was observed in tungsten disulfide–molybdenum disulfide stacks. This was attributed to a direct to indirect band gap transition of tungsten disulfide in such stacks while molybdenum disulfide maintains its monolayer properties by first-principles calculations. On the other hand, due to the strong build-in electric fields...

Recent Advances in Two-Dimensional Materials beyond Graphene

Abstract: The isolation of graphene in 2004 from graphite was a defining moment for the “birth” of a field: two-dimensional (2D) materials In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement Here, we review significant recent advances and important new developments in 2D materials “beyond graphene” We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (ie, silicene, phosphorene, etc) and transition metal carbide- and carbon nitride-based MXenes We then discuss the doping and functionalization of 2

Valleytronics in 2D materials

Abstract: Semiconductor technology is currently based on the manipulation of electronic charge; however, electrons have additional degrees of freedom, such as spin and valley, that can be used to encode and process information. Over the past several decades, there has been significant progress in manipulating electron spin for semiconductor spintronic devices, motivated by potential spin-based information processing and storage applications. However, experimental progress towards manipulating the valley degree of freedom for potential valleytronic devices has been limited until very recently. We review the latest advances in valleytronics, which have largely been enabled by the isolation of 2D materials (such as graphene and semiconducting transition metal dichalcogenides) that host an easily accessible electronic valley degree of freedom, allowing for dynamic control. The energy extrema of an electronic band are referred to as valleys. In 2D materials, two distinguishable valleys can be used to encode information and explore other valleytronic applications.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Abstract: Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Colloquium : Excitons in atomically thin transition metal dichalcogenides

Abstract: Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.