Yingqiu Zhou

Ultrathin 2D Photodetectors Utilizing Chemical Vapor Deposition Grown WS2 With Graphene Electrodes

In this report, graphene (Gr) is used as a 2D electrode and monolayer WS2 as the active semiconductor in ultrathin photodetector devices. All of the 2D materials are grown by chemical vapor deposition (CVD) and thus pose as a viable route to scalability. The monolayer thickness of both electrode and semiconductor gives these photodetectors ∼2 nm thickness. We show that graphene is different to conventional metal (Au) electrodes due to the finite density of states from the Dirac cones of the valence and conduction bands, which enables the photoresponsivity to be modulated by electrostatic gating and light input control. We demonstrate lateral Gr-WS2-Gr photodetectors with photoresponsivities reaching 3.5 A/W under illumination power densities of 2.5 × 10(7) mW/cm(2). The performance of monolayer WS2 is compared to bilayer WS2 in photodetectors and we show that increased photoresponsivity is achieved in the thicker bilayer WS2 crystals due to increased optical absorption. This approach of incorporating graphene electrodes in lateral TMD based devices provides insights on the contact engineering in 2D optoelectronics, which is crucial for the development of high performing ultrathin photodetector arrays for versatile applications.

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.

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.

Doping Graphene Transistors Using Vertical Stacked Monolayer WS2 Heterostructures Grown by Chemical Vapor Deposition

We study the interactions in graphene/WS2 two-dimensional (2D) layered vertical heterostructures with variations in the areal coverage of graphene by the WS2. All 2D materials were grown by chemical vapor deposition and transferred layer by layer. Photoluminescence (PL) spectroscopy of WS2 on graphene showed PL quenching along with an increase in the ratio of exciton/trion emission, relative to WS2 on SiO2 surface, indicating a reduction in the n-type doping levels of WS2 as well as reduced radiative recombination quantum yield. Electrical measurements of a total of 220 graphene field effect transistors with different WS2 coverage showed double-Dirac points in the field effect measurements, where one is shifted closer toward the 0 V gate neutrality position due to the WS2 coverage. Photoirradiation of the WS2 on graphene region caused further Dirac point shifts, indicative of a reduction in the p-type doping levels of graphene, revealing that the photogenerated excitons in WS2 are split across the heterostructure by electron transfer from WS2 to graphene. Kelvin probe microscopy showed that regions of graphene covered with WS2 had a smaller work function and supports the model of electron transfer from WS2 to graphene. Our results demonstrate the formation of junctions within a graphene transistor through the spatial tuning of the work function of graphene using these 2D vertical heterostructures.

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.

Hydrogen-Assisted Growth of Large-Area Continuous Films of MoS2 on Monolayer Graphene

We show how control over the chemical vapor deposition (CVD) reaction chemistry of molybdenum disulfide (MoS2) by hydrogen addition can enable the direct growth of centimeter-scale continuous films of vertically stacked MoS2 monolayer on graphene under atmospheric pressure conditions. Hydrogen addition enables longer CVD growth times at high temperature by reducing oxidation effects that would otherwise degrade the monolayer graphene. By careful control of nucleation density and growth time, high-quality monolayer MoS2 films could be formed on graphene, realizing all CVD-grown vertically stacked monolayer semimetal/semiconducting interfaces. Photoluminescence spectroscopy shows quenching of MoS2 by the underlying graphene, indicating a good interfacial charge transfer. We utilize the MoS2/graphene vertical stacks as photodetectors, with photoresponsivities reaching 2.4 A/W under 135 μW 532 nm illumination. This approach provides insights into the scalable manufacturing of high-quality two-dimensional electronic and optoelectronic devices.

Revealing Strain-Induced Effects in Ultrathin Heterostructures at the Nanoscale

Two-dimensional materials are being increasingly studied, particularly for flexible and wearable technologies because of their inherent thickness and flexibility. Crucially, one aspect where our understanding is still limited is on the effect of mechanical strain, not on individual sheets of materials, but when stacked together as heterostructures in devices. In this paper, we demonstrate the use of Kelvin probe microscopy in capturing the influence of uniaxial tensile strain on the band-structures of graphene and WS2 (mono- and multilayered) based heterostructures at high resolution. We report a major advance in strain characterization tools through enabling a single-shot capture of strain defined changes in a heterogeneous system at the nanoscale, overcoming the limitations (materials, resolution, and substrate effects) of existing techniques such as optical spectroscopy. Using this technique, we observe that the work-functions of graphene and WS2 increase as a function of strain, which we attribute to the Fermi level lowering from increased p-doping. We also extract the nature of the interfacial heterojunctions and find that they get strongly modulated from strain. We observe that the strain-enhanced charge transfer with the substrate plays a dominant role, causing the heterostructures to behave differently from two-dimensional materials in their isolated forms.

Electrical Breakdown of Suspended Mono- and Few-Layer Tungsten Disulfide via Sulfur Depletion Identified by In-Situ Atomic Imaging

The high-bias and breakdown behavior of suspended mono- and few-layer WS2 was explored by in situ aberration-corrected transmission electron microscopy. The suspended WS2 devices were found to undergo irreversible breakdown at sufficiently high biases due to vaporization of the WS2. Simultaneous to the removal of WS2 was the accompanying formation of few-layer graphene decorated with W and WS2 nanoparticles, with the carbon source attributed to organic residues present on the WS2 surface. The breakdown of few-layer WS2 resulted in the formation of faceted S-depleted WS2 tendrils along the vaporization boundary, which were found to exhibit lattice contraction indicative of S depletion, alongside pure W phases incorporated into the structure, with the interfaces imaged at atomic resolution. The combination of observing the graphitization of the amorphous carbon surface residue, W nanoparticles, and S-depleted WS2 phases following the high-bias WS2 disintegration all indicate a thermal Joule heating breakdown mechanism over an avalanche process, with WS2 destruction promoted by preferential S emission. The observation of graphene formation and the role the thin amorphous carbon layer has in the prebreakdown behavior of the device demonstrate the importance of employing encapsulated heterostructure device architectures that exclude residues.

Chemical Vapor Deposition Growth of Two-Dimensional Monolayer Gallium Sulfide Crystals Using Hydrogen Reduction of Ga2S3

Two-dimensional gallium sulfide (GaS) crystals are synthesized by a simple and efficient ambient pressure chemical vapor deposition (CVD) method using a single-source precursor of Ga2S3. The synthesized GaS structures involve triangular monolayer domains and multilayer flakes with thickness of 1 and 15 nm, respectively. Regions of continuous films of GaS are also achieved with about 0.7 cm2 uniform coverage. This is achieved by using hydrogen carrier gas and the horizontally placed SiO2/Si substrates. Electron microscopy and spectroscopic measurements are used to characteristic the CVD-grown materials. This provides important insights into novel approaches for enlarging the domain size of GaS crystals and understanding of the growth mechanism using this precursor system.