Growth of two-dimensional high-quality large edge length WS2

Unlike graphene, tungsten disulfide is a semiconductor that belongs to two-dimensional transition metal dichalcogenides (2D-TMDs). Tungsten disulfide has an intrinsic indirect bandgap of ∼1.3eV (bulk), which is tuned to a direct bandgap of ∼2.1 eV when the number of layers decreased to mono. An atmospheric pressure chemical vapor deposition (APCVD) approach was employed to realize WS2 on Si/SiO
2
substrate with a 300 nm thick oxide layer (University Wafers). A custom-build 12-zone horizontal split furnace (Quazar Technologies Pvt. Ltd.) with solid-phase precursors (tungsten trioxide and elemental sulfur) were used. A set of growths were conducted with the WO
3
zone heated to 900-1100°C with the Ar as carrier gas (0–300 sccm). Besides monolayer, spiral multilayer, pyramidal, nanoflower, and dendritic WS2 were also observed. The as-grown WS2 was characterized by optical microscopy, scanning electron microscope, Raman, and PL spectroscopy. The number of layers was confirmed by Raman spectroscopy (Horiba Labram) with a laser source of 514 nm and with much lesser power (1mW) to prevent any damage to the sample. The in-plane (
$E_{2g}^{1}$
) and out-of-plane (
$A_{1g}$
) peaks were observed at ∼352 cm
⁻¹
and ∼418 cm
⁻¹
confirming monolayer WS2. The
$E_{2g}^{1}$
and
$A_{1g}$
peaks shift with the increase in the number of layers (∼352 cm
⁻¹
and ∼419 cm
⁻¹
for bilayer WS2). The field emission scanning electron microscopy (FEI Quanta 200 F SEM, 10–20
$k$
V) and optical imaging (Olympus BX53M) show the distribution of as-grown WS2 on the substrate. The maximum monolayer domain size obtained in this was approach
$\sim 30\ \mu \mathrm{m}$
, along with a spiral and pyramidal structured multilayer.