The important role of water in growth of monolayer transition metal dichalcogenides

Abstract

The important role of water in growth of monolayer transition metal dichalcogenides Interest in transition metal dichalcogenides (TMDs) has been renewed by the discovery of emergent properties when reduced to single, two-dimensional (2D) layers. The transition to direct band gap [1,2], emerging charge density waves [3,4], high mobility [5][6][7], and valley polarization [8][9][10] are some of the many exciting properties that have been reported in the TMD literature recently. A major bottleneck to this research is the lack of reproducible and large scale synthetic methods for high quality, consistent monolayer TMD samples. The dominant growth method is the vaporization and subsequent chalcogenization of solid metal oxides in the presence of gaseous chalcogen precursors. This process is commonly referred to as chemical vapor deposition (CVD) or powder vaporization [11][12][13][14]. Due to its simplicity, CVD is extensively used by the TMD community to produce high quality, micron-sized single crystals [11][12][13][15][16][17][18][19]. Understanding the vaporization chemistry of solid transition metal precursors and vapor transport of volatilized precursors, particularly with respect to the influence of water vapor, is critical. Humidity, i.e. water content of the reaction environment, is an important parameter in the gas phase synthesis of inorganic materials, and while it is typically thought of as a contaminant, water is also an effective transport agent [20][21][22][23]. In this communication, we describe the synthesis of luminescent monolayer TMD islands by introducing water vapor as a simple means of controlling the volatilization and transport of the metal oxide precursor. Our experiments demonstrate a direct correlation between gas phase water content and the morphology of the resulting films. In particular, explicit control of the in situ water vapor concentration allows us to switch between two modes of growth: one in an effectively dry environment, in which the transition metal oxide source is converted directly to TMD material through a solid state reaction with the chalcogen source, and another in which the transition metal oxide undergoes vapor transport followed by reaction with the chalcogen source. We show that a small amount of water enhances the volatilization, and hence vapor transport, of the oxides of tungsten and molybdenum at the elevated temperatures (500-800 °C) used in the conversion or growth of their TMD counterparts. We attribute this effect to the enhanced vaporization of WO 3 and MoO 3 in the presence of water, first demonstrated in the 1930s and Our results show a direct correlation between gas phase water content and the morphology of TMD films. In particular, we show that the presence of water enhances volatilization, and therefore the vapor transport of tungsten and molybdenum oxide. Surprisingly, we find that water not only plays an important role in volatilization but is also compatible with TMD growth. In fact, carefully controlled humidity can consistently produce high qual...

Effects of Strategically Placed Water Droplets on Monolayer Growth of Molybdenum Disulfide

“…Source. In contrast to previous works, where water was introduced as vapor during the entire growth process [20], controlled amounts of water were introduced into the system before growth. Prior to its loading in the growth tube, the MoO 3 source was wet with water, volumes ranging from 0 to 200 μL.…”

“…Using MoO 3 and S as source materials, Si substrates with water droplets on their surface were used to study the impact of water on the growth of MoS 2 monolayers. It has been reported that a continuous supply of water vapor into the growth region promotes the growth of high-quality MoS 2 layers [20]. The technique proposed in this work explores a novel way of introducing controlled amounts of water vapor into the growth region to facilitate large-area growth of MoS 2 monolayers.…”

Resonant and bound states of charged defects in two-dimensional semiconductors

“…STM/STS measurements were performed under ultrahigh vacuum (< 2 × 10 −10 mbar) at low temperature (∼6 K). WS 2 samples were grown ex situ by chemical vapor deposition (CVD) on epitaxial graphene on silicon carbide substrates [17] and subsequent annealing in vacuum at ∼200 • C. A representative overview image of the surface is shown in Fig. 1(a).…”

Giant defect emission enhancement from ZnO nanowires through desulfurization process

“…Temperature dependent PL spectroscopy shows a dominant excitonic feature at ~3.24 eV at low temperature, which is related to phonon replicas of both a defect related transition and the free exciton. The green emission at ~2.5 eV presents a negative thermal quenching (NTQ) with a thermal activation energy The CVD furnace used in the experiment has been reported previously 29 . For the sulfurization process, the ZnO nanowire sample was exposed to an H 2 S flow of 2 sccm and an Ar flow of 100 sccm at 550 °C for 10 min.…”