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Controlled Vapor Phase Growth of Single Crystalline, Two-Dimensional GaSe Crystals With High Photoresponse

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Controlled Vapor Phase Growth of Single Crystalline, Two-Dimensional GaSe Crystals With High Photoresponse

Xufan Li et al. Sci Rep.

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.

Figures

Figure 1
Figure 1. Growth of 2D GaSe crystals.
(a–c) SEM images of monolayer triangular GaSe crystals grown for 2 min (a), 5 min (b), and 10 min (c). (d–f) AFM images of monolayer GaSe crystals. Insets are line profiles in the direction of the dashed arrows. Images (d) and (f) show an individual and two merged triangular monolayer flakes, respectively, while image (e) shows an enlarged view of the tip of the triangle. (g) High-resolution ADF-STEM image of monolayer GaSe. The lattice is composed of hexagonal rings of gallium and selenium atoms. Top and side views of monolayer GaSe structure are overlaid. Inset is the corresponding FFT image. (h) High-resolution ADF-STEM image of multi-layer GaSe. Top view of multi-layer GaSe structure with ε-type stacking is overlaid. Inset is the corresponding FFT image.
Figure 2
Figure 2. Grain structures in monolayer GaSe crystals.
(a) Bright-field TEM image of a single monolayer triangular flake. Inset is the SAED pattern of the flake, showing a single set of spots in a hexagonal pattern. (b) DF-TEM image of the flake in (a). (c) Bright field TEM image showing two monolayer triangular flakes merging together. Inset is the SAED pattern obtained from the common area of the two flakes as indicated by a dashed circle. The pattern shows two sets of spots in a hexagonal pattern (indicated by red and green dashed-lines, respectively) with orientated ~30° apart. (d) Color-coded overlay of DF-TEM images corresponding to the red- and green-circled diffraction spots in the inset of (c). (e) Bright-field TEM image of an area containing both monolayer triangular flakes and large islands of merged flakes. Inset is the electron diffraction pattern obtained from the whole area in (e). (f) Color-coded overlay of DF-TEM image of the area in (e). The overlapped crystal grains are indicated by the white arrows and the clear grain boundaries are indicated by the red arrows.
Figure 3
Figure 3. Influence of growth conditions on monolayer GaSe crystals.
Optical micrographs of 2D monolayer GaSe crystals synthesized at a growth temperature of (a–c) ~710–720°C, (d–f) ~700–710°C, and (g–i) ~660–670°C, with an argon gas flow rate of 50 sccm and a growth time of 5 min. The images were obtained near the downstream side (a, d, g), middle (b, e, h), and upstream side (c, f, i) of the substrates. Insets of (d) and (g) are AFM images of individual crystals.
Figure 4
Figure 4. Multi-layer 2D GaSe crystals.
(a–c) SEM images of GaSe crystals grown with an 80 sccm argon flow for 5 min. Images (a) and (b) were obtained from the region close to the downstream side of the substrate. The flakes with lighter contrast are monolayer, while darker flakes indicate additional layers grown on monolayer flakes. (b) is the enlarged image of the area contained in the dashed square in (a). Image (c) shows thicker multi-layer GaSe crystals on a continuous monolayer GaSe film grown in the region close to the upstream side of the substrate. (d–f) AFM images of multi-layer GaSe crystals. Insets are line profiles along the dashed arrows.
Figure 5
Figure 5. Optical properties of 2D GaSe.
(a) AFM image of 2D GaSe crystals with different layer numbers (from 1 to ~30 L). The scale bar is 5 μm. (b) Raman spectra (532 nm laser excitation) of 2D GaSe crystals with 1, 3, 7, 12, and ~30 L as indicated in (a) and a bare substrate. Note that the spectra were offset for clarity. (c) Raman mapping of the crystals included in the dashed square in (a) by monitoring A11g peak in the Raman spectra. The scale bar is 5 μm.
Figure 6
Figure 6. Electronic band structures of 2D GaSe from theoretical calculations.
(a) Energy band plots of monolayer and bulk GaSe along the high symmetry k-points. (b) Energy bands near the valence band maximum. (c) Bandgap energy as a function of layer numbers.
Figure 7
Figure 7. Photoconducting properties of 2D GaSe crystals.
(a) Illustration of a 2D GaSe-based device illuminated with white light. (b) Optical image of a 2D GaSe device with patterned electrodes. The red arrows indicate the two electrodes used for measurements. (c) Ids-Vds characteristics of the 2D GaSe device in the dark (black curve) and with white light illumination (power density: 1.2 mW/cm2) (red curve). (d) The transfer characteristics of the 2D GaSe FET with Vds = −10 V in the dark (black curve) and with white light illumination (red curve). Calculated responsivity as function of the Vg also shown (blue curve).

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