Yuewen Sheng

Biexciton Formation in Bilayer Tungsten Disulfide

Monolayer transition metal dichalcogenides (TMDs) are direct band gap semiconductors, and their 2D structure results in large binding energies for excitons, trions, and biexcitons. The ability to explore many-body effects in these monolayered structures has made them appealing for future optoelectronic and photonic applications. The band structure changes for bilayer TMDs with increased contributions from indirect transitions, and this has limited similar in-depth studies of biexcitons. Here, we study biexciton emission in bilayer WS2 grown by chemical vapor deposition as a function of temperature. A biexciton binding energy of 36 ±4 meV is measured in the as-grown bilayer WS2 containing 0.4% biaxial strain as determined by Raman spectroscopy. The biexciton emission was difficult to detect when the WS2 was transferred to another substrate to release the stain. Density functional theory calculations show that 0.4% of tensile strain lowers the direct band gap by about 55 meV without significant change to the indirect band gap, which can cause an increase in the quantum yield of direct exciton transitions and the emission from biexcitons formed by two direct gap excitons. We find that the biexciton emission decreases dramatically with increased temperature due to the thermal dissociation, with an activation energy of 26 ± 5 meV. These results show how strain can be used to tune the many-body effects in bilayered TMD materials and extend the photonic applications beyond pure monolayer systems.

Revealing Defect-State Photoluminescence in Monolayer WS2 by Cryogenic Laser Processing

Understanding the stability of monolayer transition metal dichalcogenides in atmospheric conditions has important consequences for their handling, life-span, and utilization in applications. We show that cryogenic photoluminescence spectroscopy (PL) is a highly sensitive technique to the detection of oxidation induced degradation of monolayer tungsten disulfide (WS2) caused by exposure to ambient conditions. Although long-term exposure to atmospheric conditions causes massive degradation from oxidation that is optically visible, short-term exposure produces no obvious changes to the PL or Raman spectra measured at either room temperature or even cryogenic environment. Laser processing was employed to remove the surface adsorbents, which enables the defect states to be detected via cryogenic PL spectroscopy. Thermal cycling to room temperature and back down to 77 K shows the process is reversible. We also monitor the degradation process of WS2 using this method, which shows that the defect related peak can be observed after one month aging in ambient conditions.

Layer-dependent modulation of tungsten disulfide photoluminescence by lateral electric fields.

Large single-crystal domains of WS2 are grown by chemical vapor deposition, and their photoluminescent properties under a lateral electric field are studied. We demonstrate that monolayer and bilayer WS2 have opposite responses to lateral electric fields, with WS2 photoluminescence (PL) substantially reduced in monolayer and increased in bilayers with increasing lateral electric field strength. Temperature-dependent PL measurements are also undertaken and show behavior distinctly different than that of the lateral electric field effects, ruling out heating as the cause of the PL changes. The PL variation in both monolayer and bilayer WS2 is attributed to the transfer of photoexcited electrons from one conduction band extremum to another, modifying the resultant recombination pathways. This effect is observed in 2D transition metal dichalcogenides due to their large exciton binding energy and small energy difference between the two conduction band extrema.

Photoluminescence Segmentation within Individual Hexagonal Monolayer Tungsten Disulfide Domains Grown by Chemical Vapor Deposition

We show that hexagonal domains of monolayer tungsten disulfide (WS2) grown by chemical vapor deposition (CVD) with powder precursors can have discrete segmentation in their photoluminescence (PL) emission intensity, forming symmetric patterns with alternating bright and dark regions. Two-dimensional maps of the PL reveal significant reduction within the segments associated with the longest sides of the hexagonal domains. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics. Monolayers of WS2 hexagonal islands transferred to new substrates still exhibit this PL segmentation, ruling out local strain in the regions as the dominant cause. High-power laser irradiation causes preferential degradation of the bright segments by sulfur removal, indicating the presence of a more defective region that is higher in oxidative reactivity. Atomic force microscopy (AFM) images of topography and amplitude modes show uniform thickness of the WS2 domains and no signs of segmentation. However, AFM phase maps do show the same segmentation of the domain as the PL maps and indicate that it is caused by some kind of structural difference that we could not clearly identify. These results provide important insights into the spatially varying properties of these CVD-grown transition metal dichalcogenide materials, which may be important for their effective implementation in fast photo sensors and optical switches.

Electroluminescence Dynamics across Grain Boundary Regions of Monolayer Tungsten Disulfide

We study how grain boundaries (GB) in chemical vapor deposition (CVD) grown monolayer WS2 influence the electroluminescence (EL) behavior in lateral source-drain devices under bias. Real time imaging of the WS2 EL shows arcing between the electrodes when probing across a GB, which then localizes at the GB region as it erodes under high bias conditions. In contrast, single crystal WS2 domains showed no signs of arcing or localized EL. Analysis of the eroded GB region shows the formation of micro- and nanoribbons across the monolayer WS2 domains. Comparison of the EL spectrum with the photoluminescence spectrum from the monolayer WS2 shows close agreement, indicating the EL emission comes from direct band gap excitonic recombination. These results provide important insights into EL devices that utilize CVD grown monolayer transition metal dichalcogenides when GBs are present in the active device region.

Distinguishing Lead and Molecule States in Graphene-Based Single-Electron Transistors

Graphene provides a two-dimensional platform for contacting individual molecules, which enables transport spectroscopy of molecular orbital, spin, and vibrational states. Here we report single-electron tunneling through a molecule that has been anchored to two graphene leads. Quantum interference within the graphene leads gives rise to an energy-dependent transmission and fluctuations in the sequential tunnel-rates. The lead states are electrostatically tuned by a global back-gate, resulting in a distinct pattern of varying intensity in the measured conductance maps. This pattern could potentially obscure transport features that are intrinsic to the molecule under investigation. Using ensemble averaged magneto-conductance measurements, lead and molecule states are disentangled, enabling spectroscopic investigation of the single molecule.

Oligomeric aminoborane precursors for the chemical vapour deposition growth of few-layer hexagonal boron nitride

We explore the use of stable, pre-formed, oligomeric aminoboranes as precursors for the chemical vapour deposition growth of few-layered hexagonal boron nitride (h-BN) films on Cu foils under atmospheric pressure conditions. Dimeric diborazane H3B·NH2BH2·NH3 (DAB), and trimeric triborazane H3B·(NH2BH2)2·NH3 (TAB), derivatives of ammonia borane, H3B·NH3 (AB), are compared with AB, a commonly used precursor for the CVD growth of h-BN. Both DAB and TAB show similar effectiveness to AB in growing h-BN few layered films. Using DAB as the precursor instead of AB leads to fully continuous h-BN films in a shorter period of time. Analysis of the surface of the h-BN films reveals that DAB and TAB precursors deposit more nanoparticles on the surface of the h-BN films during their CVD growth within the same time period as when using AB. The viability of these two new h-BN precursors (DAB and TAB), opens up a wider range of solid-state sources for growing wide band gap h-BN films using CVD techniques.

Lateral Graphene‐Contacted Vertically Stacked WS2/MoS2 Hybrid Photodetectors with Large Gain

A demonstration is presented of how significant improvements in all-2D photodetectors can be achieved by exploiting the type-II band alignment of vertically stacked WS2 /MoS2 semiconducting heterobilayers and finite density of states of graphene electrodes. The photoresponsivity of WS2 /MoS2 heterobilayer devices is increased by more than an order of magnitude compared to homobilayer devices and two orders of magnitude compared to monolayer devices of WS2 and MoS2 , reaching 103 A W-1 under an illumination power density of 1.7 × 102 mW cm-2 . The massive improvement in performance is due to the strong Coulomb interaction between WS2 and MoS2 layers. The efficient charge transfer at the WS2 /MoS2 heterointerface and long trapping time of photogenerated charges contribute to the observed large photoconductive gain of ≈3 × 104 . Laterally spaced graphene electrodes with vertically stacked 2D van der Waals heterostructures are employed for making high-performing ultrathin photodetectors.

Uniformity of large-area bilayer graphene grown by chemical vapor deposition

Graphene grown by chemical vapor deposition (CVD) on copper foils is a viable method for large area films for transparent conducting electrode (TCE) applications. We examine the spatial uniformity of large area films on the centimeter scale when transferred onto both Si substrates with 300 nm oxide and flexible transparent polyethylene terephthalate substrates. A difference in the quality of graphene, as measured by the sheet resistance and transparency, is found for the areas at the edges of large sheets that depends on the supporting boat used for the CVD growth. Bilayer graphene is grown with uniform properties on the centimeter scale when a flat support is used for CVD growth. The flat support provides consistent delivery of precursor to the copper catalyst for graphene growth. These results provide important insights into the upscaling of CVD methods for growing high quality graphene and its transfer onto flexible substrates for potential applications as a TCE.

Field-Effect Control of Graphene–Fullerene Thermoelectric Nanodevices

Although it was demonstrated that discrete molecular levels determine the sign and magnitude of the thermoelectric effect in single-molecule junctions, full electrostatic control of these levels has not been achieved to date. Here, we show that graphene nanogaps combined with gold microheaters serve as a testbed for studying single-molecule thermoelectricity. Reduced screening of the gate electric field compared to conventional metal electrodes allows control of the position of the dominant transport orbital by hundreds of meV. We find that the power factor of graphene-fullerene junctions can be tuned over several orders of magnitude to a value close to the theoretical limit of an isolated Breit-Wigner resonance. Furthermore, our data suggest that the power factor of an isolated level is only given by the tunnel coupling to the leads and temperature. These results open up new avenues for exploring thermoelectricity and charge transport in individual molecules and highlight the importance of level alignment and coupling to the electrodes for optimum energy conversion in organic thermoelectric materials.