Ultrafast growth of large single crystals of monolayer WS2 and WSe2

Abstract

Monolayer transition metal dichalcogenides (TMDs) have attracted considerable attention as atomically thin semiconductors for the ultimate transistor scaling. For practical applications in integrated electronics, large monolayer single crystals are essential for ensuring consistent electronic properties and high device yield. The TMDs available today are generally obtained by mechanical exfoliation or chemical vapor deposition (CVD) growth, but are often of mixed layer thickness, limited single crystal domain size or have very slow growth rate. Scalable and rapid growth of large single crystals of monolayer TMDs requires maximization of lateral growth rate while completely suppressing the vertical growth, which represents a fundamental synthetic challenge and has motivated considerable efforts. Herein we report a modified CVD approach with controllable reverse flow for rapid growth of large domain single crystals of monolayer TMDs. With the use of reverse flow to precisely control the chemical vapor supply in the thermal CVD process, we can effectively prevent undesired nucleation before reaching optimum growth temperature and enable rapid nucleation and growth of monolayer TMD single crystals at a high temperature that is difficult to attain with use of a typical thermal CVD process. We show that monolayer single crystals of 450 μm lateral size can be prepared in 10 s, with the highest lateral growth rate up to 45 μm/s. Electronic characterization shows that the resulting monolayer WSe2 material exhibits excellent electronic properties with carrier mobility up to 90 cm2 V−1 s−1, comparable to that of the best exfoliated monolayers. Our study provides a robust pathway for rapid growth of high-quality TMD single crystals.

In Situ Ultrafast and Patterned Growth of Transition Metal Dichalcogenides from Inkjet‐Printed Aqueous Precursors

“…The growth rate ( R ) can be estimated by dividing the domain size by the growth time at elevated temperature. [ 33,52 ] Over 1 mm size single domain TMDC could be obtained within 30 s at the elevated temperature of 1050 °C, corresponding to a growth rate up to 36.4 µm s −1 , which is comparable with the ultrafast growth rate of WS 2 with controllable reverse flow (45 µm s −1 ) [ 33 ] and graphene using an oxygen supply (60 µm s −1 ). [ 52 ] For comparison, Figure S6 in the Supporting Information shows the optical images of MoSe 2 sheets fabricated on the glass substrates at 1050 °C with a growth time of 5 min, using the same precursor placed upstream in the furnace.…”

“…[43,45,46] On the other hand, to accelerate the growth rate, some unique processes as well as ultrahigh temperatures were adopted to prepare large-grain TMDCs. [25,33] Despite these rapid achievement in the CVD growth of TMDCs, the reported precursor usage can be varied over a wide range, [2,24,31,32,43] , e.g., from 1 mg to 1200 mg, strongly depending on the CVD parameters, such as the outside diameter (OD) of the quartz tube, the distance between the precursor and the substrate, as well as the growth temperature and the gas flow rates. Moreover, the patterned growth of TMDC films usually involves a complicated lithographic process with organic contaminations and a limited dimension.…”

“…[ 14,15 ] So far, a number of approaches have been developed to obtain TMDCs, including the micromechanical cleavage method [ 16 ] and liquid‐phase exfoliation [ 17–19 ] due to the weak van der Waals interactions between the neighboring layers. Among them, chemical vapor deposition (CVD) has been widely demonstrated to be an effective technique to synthesize high‐quality TMDCs as well as their in‐plane and vertical heterostructures by using different precursors, [ 20,21 ] such as the solid powders of MoO 3 , [ 22–25 ] MoO 3 solution, [ 26,27 ] MoCl 5 , [ 28 ] WO 3 , [ 29,30 ] WS 2 , WSe 2 , MoS 2 , and MoSe 2 [ 31–33 ] with the additional alkali metal halides as the growth promoters, [ 2,34 ] the gaseous chemical precursors including molybdenum hexacarbonyl (MHC), [ 4 ] as well as the liquid precursors like the solution of ammonium thiomolybdate (NH 4 ) 2 MoS 4 , [ 35,36 ] ammonium molybdate tetrahydrate (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and ammonium tungstate hydrate (NH 4 ) 10 W 12 O 41 · x H 2 O. [ 37–39 ] For the CVD process, the growth substrate plays an important role in the resulting TMDCs, for example, centimeter‐scale MoS 2 monolayer on has been achieved on Au(111), [ 40 ] the c ‐plane sapphire has been used for epitaxial growth of monolayer MoS 2 .…”

Ultrafast Growth of Large‐Area Uniform, Millimeter‐Size MoSe2 Single Crystals on Low‐Cost Soda‐Lime Glass

“…[ 8–12 ] Chemical vapor deposition (CVD) has proven to be promising for synthesizing high‐quality TMDCs monolayers, due to its excellent controllability, scalability, and cost effectiveness. [ 11,13–17 ] To obtain large‐sized single crystals via CVD, remarkably reducing the nucleation density in the initial growth process is critical, as commonly realized by keeping sufficiently low precursor supply rate (e.g., inserting precursor diffusion barriers, [ 18 ] introducing etching agent (e.g., O 2 ), [ 19 ] or using an extremely small amount of precursors [ 20 ] ), to minimize undesired nucleation and multi‐layer growth. However, this growth strategy usually leads to a slow lateral growth rate (typically ≈1 µm s −1 or less) and ultra‐low production efficiency.…”

“…Ultrafast growth is also effective in reducing the nucleation density and enhancing the lateral growth of monolayer crystals, [ 15,16,22–25 ] since a precursor can rapidly diffuse to and attach onto existing nuclei (or growing crystals) before new nuclei form, [ 16 ] allowing the synthesis of large‐sized single crystals in a quite short growth period. In addition, CVD is an energy‐consuming process in terms of the high growth temperature, hereby, ultrafast growth is more compatible with low‐cost industrial production.…”