Abstract
Compared with their bulk counterparts, atomically thin two-dimensional (2D) crystals exhibit new physical properties, and have the potential to enable next-generation electronic and optoelectronic devices. However, controlled synthesis of large uniform monolayer and multi-layer 2D crystals is still challenging. Here, we report the controlled synthesis of 2D GaSe crystals on SiO2/Si substrates using a vapor phase deposition method. For the first time, uniform, large (up to ~60 μm in lateral size), single-crystalline, triangular monolayer GaSe crystals were obtained and their structure and orientation were characterized from atomic scale to micrometer scale. The size, density, shape, thickness, and uniformity of the 2D GaSe crystals were shown to be controllable by growth duration, growth region, growth temperature, and argon carrier gas flow rate. The theoretical modeling of the electronic structure and Raman spectroscopy demonstrate a direct-to-indirect bandgap transition and progressive confinement-induced bandgap shifts for 2D GaSe crystals. The 2D GaSe crystals show p-type semiconductor characteristics and high photoresponsivity (~1.7 A/W under white light illumination) comparable to exfoliated GaSe nanosheets. These 2D GaSe crystals are potentially useful for next-generation electronic and optoelectronic devices such as photodetectors and field-effect transistors.
Highlights
The 2D Gallium selenide (GaSe) crystals were synthesized in a tube furnace system equipped with a 10 diameter quartz tube (Figure S1)
The triangular flakes are composed of Ga and Se with an atomic ratio of 151 (GaSe) as determined by energy-dispersive x-ray spectroscopy (EDS) (Figure S3; the bright, small particles on the flakes are Se nanoparticles deposited during the synthesis, and can be removed by heat treatment at 300uC in vacuum)
The result demonstrates that large, uniform monolayer GaSe crystals were grown on the SiO2/Si substrate for the first time (the darker domains shown in Figure 1c correspond to multi-layer flakes, which will be discussed in detail later (Figure 3))
Summary
In order to overcome these limitations, a variety of 2D materials beyond graphene with different bandgaps have been synthesized in recent years[4,8], including insulating h-BN9,10 and semiconducting layered transition metal dichalcogenides, e.g., MoS29,11–14, WS215,16, and WSe217,18 These materials show many unique optical and electrical properties, e.g., indirect-to-direct bandgap transitions and valley polarization[19,20], and enrich the number of building blocks that may be used for next-generation electronic and optoelectronic devices. The Raman and theoretical modeling of the electronic structure of 2D GaSe crystals demonstrate a direct-toindirect bandgap transition and a progressive confinement-induced bandgap shift, which are significantly different from the widely studied transitional metal dichalcogenides such as MoS2 These 2D GaSe crystals show a high photoresponse with white light illumination, and great potential for next-generation electronic and optoelectronic devices
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