Carrier dynamics in monolayer MoS2 have been investigated using broadband femtosecond transient absorption spectroscopy (FTAS). A tunable pump pulse was used while a broadband probe pulse revealed ground and excited state carrier dynamics. Interestingly, for pump wavelengths both resonant and non-resonant with the A and B excitons, we observe a broad ground state bleach around 2.9 eV, with decay components similar to A and B. Associating this bleach with the band nesting region between K and  in the band structure indicates significant k-space delocalization and overlap among excitonic wave functions identified as A, B, C, and D. Comparison of time dynamics for all features in resonance and non-resonance excitation is consistent with this finding.

Scalable fabrication of two-dimensional materials-based devices with consistent characteristics remains a significant impediment in the field. Here, we report on as-grown monolayer MoS2 metal-semiconductor-metal photodetectors produced using a CVD process which results in self-contacted two-dimensional material-based devices. The photodetectors show high responsivity (∼1 A/W) even at a low drain-source voltage (VDS) of 1.5 V and a maximum responsivity of up to 15 A/W when VDS = 4 V with an applied gate voltage of 8 V. The response time of the devices is found to be on the order of 1 μs, an order of magnitude faster than previous reports. These devices demonstrate the potential of this simple, scalable, and reproducible method for creating as-grown two-dimensional materials-based devices with broad implications for basic research and industrial applications.

While new species and properties of two-dimensional (2D) materials are being reported with extraordinary regularity, a significant bottleneck in the field is the ability to controllably process material into working devices. We report a chemical vapor deposition based procedure to selectively grow 2D material in a deterministic manner around lithographically defined metallic patterns which concurrently provide as-grown contacts to the material.Monolayer films, with lateral extent of up to hundreds of microns are controllably grown on and around patterned regions of bulk transition metals. By using different combinations of metallic pattern and oxide precursor, heterostructured MoS2/WS2 growth has been observed as well. The materials display strong luminescence, monolayer Raman signatures, and relatively large crystal domains. In addition to producing high optical quality monolayer material deterministically and selectively over large regions, the metallic patterns have the advantage of providing as-grown contacts to the material, offering a path for device fabrication and large scale production. 2The contemporary prominence of layered materials is driven by their technological and scientific potential in the 2D, monolayer, limit. [1,2] In addition to properties such as high mobilities, semiconducting and superconducting behavior, and excellent thermal properties, many of these materials have the potential for transformative opto-electronic applications, with large absorption, strong room-temperature emission, non-linear response, and optical control of spin and valley degrees of freedom. [3] Many seminal results of 2D material based device fabrication have involved the isolation of monolayers followed by making metal contact with the film, commonly using e-beam lithography. The customary method for isolating monolayers is micromechanical exfoliation which produces high quality crystalline flakes, on the order of up to tens of microns. However, this method provides no deterministic control over sample thickness, size, or location and provides no path to scalability. Liquid exfoliation uses ionic species as intercalating agents, which facilitates a breakdown of van der Waal forces and results in sub-micrometer sized monolayers of transition metal dichalcogenides (TMDs). [4,5] In addition to small sizes, liquid exfoliated monolayers are often found to have different structural and electronic properties, requiring further processing. [4] Chemical Vapor Deposition (CVD) has emerged as one of the most promising and preferred synthesis processes for TMD growth. [6] Two general methods of CVD growth include heating of a metal oxide powder in the presence of sulfur, [7] or direct sulfurization of thin layers of either metal or metal oxide. [8,9] Regardless of the technique used to produce monolayer TMDs, once they are grown or isolated, devices are then typically fabricated using lithography and metal deposition, which may require further processing in order to obtain useful metal-semiconductor contacts. Here we ...

The difficulty of processing two-dimensional (2D) transition metal dichalcogenide (TMD) materials into working devices with any scalability is one of the largest impediments to capitalizing on their industrial promise. Here, we describe a versatile, simple, and scalable technique to directly grow self-contacted thin-film materials over a range of TMDs (MoS 2 , MoSe 2 , WS 2 , and WSe 2 ), where predeposited bulk metallic contacts serve as the nucleation site for the TMD material to grow, forming naturally contacted device structures in a single step. The conditions for growth as well as optical and physical properties are reported. Because the material grows controllably around the lithographically defined patterns, wafer scale circuits and complex device geometries can be envisioned, including lateral heterostructures of different TMD materials.

The need for a fast and robust method to characterize nanostructure thickness is growing due to the tremendous number of experiments and their associated applications. By automatically analyzing the microscopic image texture of MoS2 and WS2, it was possible to distinguish monolayer from few-layer nanostructures with high accuracy for both materials. Three methods of texture analysis (TA) were used: grey level histogram (GLH), grey levels co-occurrence matrix (GLCOM), and run-length matrix (RLM), which correspond to first, second, and higher-order statistical methods, respectively. The best discriminating features were automatically selected using the Fisher coefficient, for each method, and used as a base for classification. Two classifiers were used: artificial neural networks (ANN), and linear discriminant analysis (LDA). RLM with ANN was found to give high classification accuracy, which was 89% and 95% for MoS2 and WS2, respectively. The result of this work suggests that RLM, as a higher-order TA method, associated with an ANN classifier has a better ability to quantify and characterize the microscopic structure of nanolayers, and, therefore, categorize thickness to the proper class.