Synthesis and Characterization of Few Layer Semiconducting Transition Metal Dichalcogenides
The intense interest in graphene as the prototypical 2D electronic material has recently been accompanied by the investigation of layered transition metal dichalcogenides (TMDC), most notably MoS2 and MoSe2. Like graphene, they can be prepared in a stable form down to monolayer thickness. These materials provide favorable mechanical properties similar to graphene, but exhibit an intrinsic indirect band gap that crossovers to a direct band gap in the monolayer limit without the need for nanostructuring,[1, 2] chemical functionalization, or application of a high electric field to bilayers. In addition to this interesting electronic structure, certain transition metal dichalcogenides, such as MoS2, have established applications in catalysis, as in the case of hydrodesulfurization [5, 6]. In addition, MoS2 recently received attention as an electrode material for water splitting [7, 8].
There are several published techniques for obtaining monolayer MoS2. These methods include the preparation of single layer films by laser-based thinning, plasma thinning, liquid exfoliation,[28-31] graphene assisted growth, and sulfurization of molybdenum films from e-beam evaporation , dip coating , mechanical exfoliation, [9, 10] and chemical vapor deposition (CVD) [12, 13]. A variety of substrates have been used successfully with CVD, including Cu , Au[11, 15-17], SiO2 [11, 18], and various other insulators [11, 19, 20]. In addition, other Molybdenum-sulfur compounds with stoichiometry different from MoS2 have been reported in CVD deposition, including Mo6S6 nanowires [21, 22] and Mo2S3 films [14, 23]. In this work, I present various CVD techniques and a pre-patterned Mo film sulfurization technique to attempt to create MoS2 structures without the need for lithography.
One of the most promising applications of thin TMDs is the creation of viable filed effect transistors. Single-layer MoS2 field effect transistors have been fabricated with mobilities on the order of 1 cm2 V-1 s-1 and higher [19, 33-35] as well as on-off ratios up to 108 at room temperature. Bulk MoS2, and most mono- or few-layer MoS2 materials examined to date, exhibit n-doping [19, 33-37] but p-doping has also been observed . Ambipolar operation has been achieved by gating with an ionic liquid . Another distinctive electronic property is the possibility of selective valley population of the monolayer, which has been achieved using excitation by circularly polarized light [39-42].
The electronic structure of TMDs of the form MX2 (M = Mo, W; X = S, Se) differs significantly from that of graphene. While the latter is a semi-metal with a linear energy dispersion near the K point, monolayer TMDs have a direct band gap between 1 and 2 eV, with valence band maxima and conduction band minima at the K point. Excitons and charged excitons (trions) can be created in TMDs by optical excitation and the use of circular polarized light resulting in valley polarization[40, 41, 44] which may be used to develop valleytronics.
For all of the unique properties of TMDS to be explored and utilized in future technologies, the synthesis of these materials must be developed and perfected. A technique that allows for economical industrial level scaling while simultaneously having high crystallinity and large area growth would be ideal. This work is an attempt to develop synthesis techniques that will allow for the full utilization of the promise these materials.