AVS 61st International Symposium & Exhibition
    2D Materials Focus Topic Wednesday Sessions
       Session 2D+EM+NS+SS+TF-WeM

Paper 2D+EM+NS+SS+TF-WeM6
Synthesis and Properties of Large Scale, Atomically Thin Tungsten Diselenide (WSe2)

Wednesday, November 12, 2014, 9:40 am, Room 310

Session: Novel 2D Materials 
Presenter: Sarah Eichfeld, The Pennsylvania State University
Authors: S.M. Eichfeld, The Pennsylvania State University
Y.C. Lin, The Pennsylvania State University
L. Hossain, The Pennsylvania State University
A. Piasecki, The Pennsyvania State University
A. Azcati, University of Texas, Dallas
S. McDonnell, University of Texas at Dallas
R.M. Wallace, University of Texas at Dallas
J.A. Robinson, The Pennsyvania State University
Correspondent: Click to Email
Transition metal dichalcogenides (TMDs), such as tungsten diselenide (WSe2) are of interest due to their intriguing properties including the transition from indirect gap to direct gap as the material is thinned to a single atomic layer. Stacking of these layered TMDs also allows for the possibility of bandgap tuning. These properties can suit a large range of flexible and low temperature electronic and optoelectronic devices. Current methods of WSe2 research involve exfoliation or vaporization of WO3 and Se powder, which limits industrial scalability. This work is focused on development of a metal-organic chemical vapor deposition process that can controllably produce highly-crystalline, atomically thin WSe2 on large area substrates.

Growth of controlled monolayer tungsten diselenide (WSe2) was carried out using chemical vapor deposition in a cold wall vertical reactor. Tungsten hexacarbonyl (W(CO)6) and dimethylselenium (DMSe) served as the tungsten and selenium precursors, respectively. Use of MOCVD precursors provides a means to independently control the W and Se precursors allowing for more precise control of the individual species during growth. Process variables including temperature (500-950°C), pressure (100-700 Torr), and carrier gas, which were correlated with grain size, growth rate, and nucleation density of the WSe2 to identify optimal parameters for atomically controlled synthesis. Increasing the growth pressure from 100-700 Torr results in a decrease in growth rate and nucleation density, leading to a >50x increase in grain size. Increased growth temperatures yield an increase in grain size, however, it was found that above temperatures of 800 °C the sapphire substrate begins to decompose in the growth environment, resulting in a degradation of WSe2 above 800°C. Synthesis using 100% hydrogen, and also hydrogen/nitrogen mixtures was carried out. It was found that 100% hydrogen was necessary in order to achieve low carbon incorporation in the WSe2 films. Characterization of these samples via Raman and photoluminescence spectroscopy verified that high quality, monolayer WSe2 is readily achievable. Additional characterization (i.e. scanning electron microscopy, atomic force microscopy, etc.) verify the quality, grain size, and nucleation density of the atomic layers. Finally, we will discuss the impact of substrate choice on the quality of the WSe2 atomic layers, as well as providing direct evidence that synthesis on graphene results in highly textured films, with nearly 100% commensurability to the underlying graphene.