Direct formation of high-quality and wafer scale graphene thin layers on insulating gate dielectrics such as SiO(2) is emergent for graphene electronics using Si-wafer compatible fabrication. Here, we report that in a chemical vapor deposition process the carbon species dissociated on Cu surfaces not only result in graphene layers on top of the catalytic Cu thin films but also diffuse through Cu grain boundaries to the interface between Cu and underlying dielectrics. Optimization of the process parameters leads to a continuous and large-area graphene thin layers directly formed on top of the dielectrics. The bottom-gated transistor characteristics for the graphene films have shown quite comparable carrier mobility compared to the top-layer graphene. The proposed method allows us to achieve wafer-sized graphene on versatile insulating substrates without the need of graphene transfer.

A nine-layer WS/MoS heterostructure is established on a sapphire substrate after sequential growth of large-area and uniform five- and four-layer MoS and WS films by using sulfurization of predeposited 1.0 nm molybdenum (Mo) and tungsten (W), respectively. By using the results obtained from the ultraviolet photoelectron spectroscopy and the absorption spectrum measurements of the standalone MoS and WS samples, a type-II band alignment is predicated for the WS/MoS heterostructure. Increasing drain currents and enhanced field-effect mobility value of the transistor fabricated on the heterostructure suggested that a channel with higher electron concentration compared with the standalone MoS transistor channel is obtained with electron injection from WS to MoS under thermal equilibrium. Selective 2D crystal growth with (I) blank sapphire substrate, (II) standalone MoS, (III) WS/MoS heterostructure, and (IV) standalone WS was demonstrated on a single sapphire substrate. The results have revealed the potential of this growth technique for practical applications.

A ten-stacked self-assembled InAs/GaAs quantum-dot infrared photodetector operated in the 2.5-7 m range by photovoltaic and photoconductive mixed-mode near-room-temperature operation ͑у250 K͒ was demonstrated. The specific peak detectivity D* is 2.4ϫ10 8 cm Hz 1/2 /W at 250 K. The use of high-band-gap Al 0.3 Ga 0.7 As barriers at both sides of the InAs quantum-dot structure and the long carrier recombination time are the key factors responsible for its near-room-temperature operation.

Single-crystal antimonene flakes are observed on sapphire substrates after the postgrowth annealing procedure of amorphous antimony (Sb) droplets prepared by using molecular beam epitaxy at room temperature. The large wetting angles of the antimonene flakes to the sapphire substrate suggest that an alternate substrate should be adopted to obtain a continuous antimonene film. By using a bilayer MoS/sapphire sample as the new substrate, a continuous and single-crystal antimonene film is obtained at a low growth temperature of 200 °C. The results are consistent with the theoretical prediction of the lower interface energy between antimonene and MoS. The different interface energies of antimonene between sapphire and MoS surfaces lead to the selective growth of antimonene only atop MoS surfaces on a prepatterned MoS/sapphire substrate. With similar sheet resistance to graphene, it is possible to use antimonene as the contact metal of 2D material devices. Compared with Au/Ti electrodes, a specific contact resistance reduction up to 3 orders of magnitude is observed by using the multilayer antimonene as the contact metal to MoS. The lower contact resistance, the lower growth temperature, and the preferential growth to other 2D materials have made antimonene a promising candidate as the contact metal for 2D material devices.

A growth model is proposed for the large-area and uniform MoS2 film grown by using sulfurization of pre-deposited Mo films on sapphire substrates. During the sulfurization procedure, the competition between the two mechanisms of the Mo oxide segregation to form small clusters and the sulfurization reaction to form planar MoS2 film is determined by the amount of background sulfur. Small Mo oxide clusters are observed under the sulfur deficient condition, while large-area and complete MoS2 films are obtained under the sulfur sufficient condition. Precise layer number controllability is also achieved by controlling the pre-deposited Mo film thicknesses. The drain currents in positive dependence on the layer numbers of the MoS2 transistors with 1-, 3- and 5- layer MoS2 have demonstrated small variation in material characteristics between each MoS2 layer prepared by using this growth technique. By sequential transition metal deposition and sulfurization procedures, a WS2/MoS2/WS2 double hetero-structure is demonstrated. Large-area growth, layer number controllability and the possibility of hetero-structure establishment by using sequential metal deposition and following sulfurization procedures have revealed the potential of this growth technique for practical applications.

Ten-stacked InAs/GaAs quantum-dot infrared photodetectors with single Al0.3Ga0.7As blocking layers at either side of the stacked dots are investigated. With peak responsivity 214 mA/W and specific detectivity 1.17×1010 cm Hz1/2/W at 6 μm, quantum-dot infrared photodetectors with single-sided blocking layers are superior in responsivity with compatible detectivity as compared to those without blocking layers. Enhancement of the photoelectron avalanche process and the absence of negative differential conductance are observed. The devices exhibit two different infrared absorption regions at 2–6 and 6–10 μm, which indicates a wide detection window of the device.

No abstract

Excitons in monolayer transition metal dichalcogenides (TMDs) have exceptionally large binding energies and dominate the optical properties of materials. Exploring the relaxation behavior of excitons is crucial for understanding the fundamental physics as well as the performance of TMD-based optoelectronic devices. However, ultrafast carrier dynamics is sensitive to the structural defects and surface conditions of TMDs, depending on the growth or transfer process. Here, we utilized pump-probe transient absorption (TA) spectroscopy with a white-light probe to investigate the dynamics of excitons in monolayer MoS2 synthesized by the metal sulfurization method. The sulfurization method was used for the fabrication of large-scale, continuous, and uniform thin films with a controllable number of layers. The excitation dynamics of the wafer-size monolayer MoS2 is found to be comparable to that of monolayer MoS2 flakes grown by chemical vapor deposition (CVD). The dominant processes of carrier relaxation in the monolayer MoS2 are exciton–exciton annihilation (hundreds of femtoseconds), the trapping of the excitons by surface states (a few picoseconds), and interband carrier-phonon scattering (tens of picoseconds). Moreover, the induced absorption due to mid-gap defects, which is often observed for samples fabricated by growth methods, such as CVD, is not observed for our continuous and uniform monolayer films. Understanding the charge carrier dynamics of the exciton in the scalable and uniform monolayer MoS2 can provide physical insights that are valuable in the design and development of complex 2D devices.