Atmospheric-pressure chemical vapor deposition (CVD) is used to grow monolayer MoS 2 two-dimensional crystals at elevated temperatures on silicon substrates with a 300 nm oxide layer. Our CVD reaction is hydrogen free, with the sulfur precursor placed in a furnace separate from the MoO 3 precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. We intentionally establish a sharp gradient of MoO 3 precursor concentration on the growth substrate to explore its sensitivity to the resultant MoS 2 domain growth within a relatively uniform temperature range. We find that the shape of MoS 2 domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. These results improve our understanding of the factors that influence the growth of MoS 2 domains and their shape evolution.

A simple method is presented for synthesizing large single crystal graphene domains on melted copper using atmospheric pressure chemical vapor deposition (CVD). This is achieved by performing the reaction above the melting point of copper (1090 °C) and using a molybdenum or tungsten support to prevent balling of the copper from dewetting. By controlling the amount of hydrogen during growth, individual single crystal domains of monolayer graphene greater than 200 μm are produced within a continuous film. Stopping growth before a complete film is formed reveals individual hexagonal domains of graphene that are epitaxially aligned in their orientation. Angular resolved photoemission spectroscopy is used to show that the graphene grown on copper exhibits a linear dispersion relationship and no sign of doping. HRTEM and electron diffraction reveal a uniform high quality crystalline atomic structure of monolayer graphene.

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.

We show that controlling the introduction time and the amount of sulphur (S) vapour relative to the WO3 precursor during the chemical vapour deposition (CVD) growth of WS2 is critical to achieving large crystal domains on the surface of silicon wafers with a 300 nm oxide layer. We use a two furnace system that enables the S precursor to be separately heated from the WO3 precursor and growth substrate. Accurate control of the S introduction time enabled the formation of triangular WS2 domains with edges up to 370 μm which are visible to the naked eye. The WS2 domains exhibit room-temperature photoluminescence with a peak value around ∼635 nm and a full-width at half-maximum (FWHM) of ∼12 nm. Selected area electron diffraction from different regions of the triangular WS2 domains showed that they are single crystal structures.

Many potential applications of monolayer transition metal dichalcogenides (TMDs) require both high photoluminescence (PL) yield and high electrical mobilities. However, the PL yield of as prepared TMD monolayers is low and believed to be limited by defect sites and uncontrolled doping. This has led to a large effort to develop chemical passivation methods to improve PL and mobilities. The most successful of these treatments is based on the nonoxidizing organic “superacid” bis(trifluoromethane)sulfonimide (TFSI) which has been shown to yield bright monolayers of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) but with trap-limited PL dynamics and no significant improvements in field effect mobilities. Here, using steady-state and time-resolved PL microscopy we demonstrate that treatment of WS2 monolayers with oleic acid (OA) can greatly enhance the PL yield, resulting in bright neutral exciton emission comparable to TFSI treated monolayers. At high excitation densities, the OA treatment allows for bright trion emission, which has not been demonstrated with previous chemical treatments. We show that unlike the TFSI treatment, the OA yields PL dynamics that are largely trap free. In addition, field effect transistors show an increase in mobilities with the OA treatment. These results suggest that OA serves to passivate defect sites in the WS2 monolayers in a manner akin to the passivation of colloidal quantum dots with OA ligands. Our results open up a new pathway to passivate and tune defects in monolayer TMDs using simple “wet” chemistry techniques, allowing for trap-free electronic properties and bright neutral exciton and trion emission.

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 WS /MoS semiconducting heterobilayers and finite density of states of graphene electrodes. The photoresponsivity of WS /MoS 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 WS and MoS , reaching 10 A W under an illumination power density of 1.7 × 10 mW cm . The massive improvement in performance is due to the strong Coulomb interaction between WS and MoS layers. The efficient charge transfer at the WS /MoS heterointerface and long trapping time of photogenerated charges contribute to the observed large photoconductive gain of ≈3 × 10 . Laterally spaced graphene electrodes with vertically stacked 2D van der Waals heterostructures are employed for making high-performing ultrathin photodetectors.

We examine the temperature dependence of graphene edge terminations at the atomic scale using an in situ heating holder within an aberration-corrected transmission electron microscope. The relative ratios of armchair, zigzag, and reconstructed zigzag edges from over 350 frames at each temperature are measured. Below 400 °C, the edges are dominated by zigzag terminations, but above 600 °C, this changes dramatically, with edges dominated by armchair and reconstructed zigzag edges. We show that at low temperature chemical etching effects dominate and cause deviation to the thermodynamics of the system. At high temperatures (600 and 800 °C), adsorbates are evaporated from the surface of graphene and chemical etching effects are significantly reduced, enabling the thermodynamic distribution of edge types to be observed. The growth rate of holes at high temperature is also shown to be slower than at room temperature, indicative of the reduced chemical etching process. These results provide important insights into the role of chemical etching effects in the hole formation, edge sputtering, and edge reconstruction in graphene.

Hexagonal boron nitride (h-BN) is a 2D, wide band-gap semiconductor that has recently been shown to display bright room-temperature emission in the visible region, sparking immense interest in the material for use in quantum applications. In this work, we study highly crystalline, single atomic layers of chemical vapour deposition (CVD)-grown hexagonal boron nitride and find predominantly one type of emissive state. Using a multidimensional super-resolution fluorescence microscopy technique we simultaneously measure spatial position, intensity and spectral properties of the emitters, as they are exposed to continuous wave illumination over minutes. As well as low emitter heterogeneity, we observe inhomogeneous broadening of emitter linewidths and power law dependency in fluorescence intermittency, this is in striking similarity to previous work on quantum dots. These results show that high control over h-BN growth and treatment can produce a narrow distribution of emitter type, and that surface interactions heavily influence the photodynamics. Furthermore, we highlight the utility of spectrally-resolved wide-field microscopy in the study of optically-active excitons in atomically thin two-dimensional materials.an EPSRC Doctoral Training Award (EP/M506485/1). J.C.