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Selective Growth of Two-Dimensional Heterostructures of Gallium Selenide on Monolayer Graphene and the Thickness Dependent p- and n- type Nature Su Kong Chong, Fei Long, Gaoxue Wang, Yung-Chang Lin, Shiva Bhandari, Reza Shahbazian-Yassar, Kazu Suenaga, Ravindra Pandey, and Yoke Khin Yap ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00504 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018
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ACS Applied Nano Materials
Selective Growth of Two-Dimensional Heterostructures of Gallium Selenide on Monolayer Graphene and the Thickness Dependent p- and ntype Nature Su Kong Chong1, Fei Long2, Gaoxue Wang1, Yung-Chang Lin3, Shiva Bhandari1, Reza Shahbazian-Yassar2†, Kazu Suenaga3, Ravindra Pandey1, Yoke Khin Yap1* 1
Department of Physics, Michigan Technological University Houghton, MI 49931, USA
2
Department of Mechanical Engineering–Engineering Mechanics, Michigan Technological
University, Houghton, MI 49931, USA 3
Nano-Materials Research Institute, National Institute of Advanced Industrial Science and
Technology (AIST), Tsukuba 305-8565, Japan
KEYWORDS. van der Waals heterostructures; GaSe; graphene; Chemical Vapor Deposition; Schottky barrier junction.
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ABSTRACT. GaSe crystals were grown on graphene domains with few-layer graphene (FLG) grains at the centers of larger monolayer graphene (MLG) grains. We found that GaSe are selectively grown on the MLG and not on the adjacent FLG and the oxidized Si substrates. Nucleation of GaSe was preferentially occurred at the steps of FLG/MLG and MLG/SiO2, due to the presence of dangling bonds/graphene edges as supported by density function theory (DFT) calculation. We also evidenced that wrinkles on graphene were not the preferred nucleation site for GaSe if there is no dangling bond. Subsequent growth of the GaSe nuclei on MLG was favorable due to the higher migration tendency of adatoms on the MLG, as supported by DFT calculation, which promoted lateral growth of larger GaSe. The surface roughness and defects on SiO2 may also promote nucleation of GaSe on MLG. We further investigated the work functions of the GaSe/graphene heterostructures using Kelvin probe force microscopy. We have detected a unique thickness-dependent work function of GaSe on MLG, which suggests for a shift of Fermi level due to n-type to p-type conversion. This is a promising route to prepare GaSe p-n junction on MLG, and an approach to match the work function of GaSe and MLG by controlling the Schottky barrier height for application in electrical devices.
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Two-dimensional (2D) van der Waals (vdW) heterostructures of transition metal chalcogenics (TMCs) on graphene have generated tremendous research interest in fundamental science and applications [1,2]. For instance, MoS2/graphene has gained significant attention for the formation of Schottky barrier solar cells [3]. The vdW heterostructures can be constructed by top down and bottom up approaches. The top down approach is convenient and involves the exfoliation/transfer/stacking procedure. However, this approach is limited by the quality deterioration after the transfer and stacking processes. In contrast, chemical vapor deposition (CVD) is a promising bottom up method for clean interlayer interfaces and scalable production of high-quality vdW heterostructures [2]. The CVD approach offers a way to control the number of layers, the dimensional, and the crystallinity of 2D materials. In addition, direct CVD deposition of heterostructures could prevent transfer-induced defects. However, the science and engineering of such an ideal bottom up approach is still at the infancy. Layered structures of gallium-based TMCs such as GaS, GaSe and GaTe have started to attract attention for their application in optoelectronic and photovoltaic devices [4]. Theoretical calculation and experimental data suggested that 2D GaSe is a promising materials for photodetectors [5-13]. The band gap of GaSe is about 2.1 eV in bulk [14], and is predicted to increase with the decrease of crystal thickness [7]. Furthermore, GaSe undergoes a direct-toindirect band gap transition as the number of layers reduced below 7 layers [7]. This transition is attributed to the inversion of the valence band maximum, causing a minimum dip to occur at the Γ point. Experimentally, the inverted valence band at Γ has been detected in bilayer GaSe deposited on GaAs substrates [15]. Systematic downshift of the valence band edge with the decrease of the number of GaSe layers is also observed in a GaSe/bilayer graphene system [16].
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This suggests that GaSe/graphene heterostructures would offer tunable electronic properties for novel device application. Apparently, CVD synthesis of GaSe crystals on graphene would be an ideal approach for scalable production of GaSe/graphene heterostructures. These heterojunctions are energetically stable due to the large binding energy of GaSe-graphene (47 meV) [17]. Here we report the selective growth of GaSe on monolayer graphene (MLG), instead of the adjacent few layer graphene (FLG) grains. Due to the use of our unique FLG on MLG domains, the mechanism of such a selective growth can be thoroughly investigated. A selective growth model is proposed based on the difference in migration and desorption of adatoms on the MLG and FLG. Furthermore, the electrical properties of these GaSe/graphene heterostructures are investigated and discussed.
RESULTS AND DISCUSSION Analysis on the graphene domains. Two types of single crystalline graphene domains are investigated for the formation of our GaSe/graphene heterostructures: i) monolayer graphene (MLG) domains [Figure 1(a) and (b)], and ii) few layer graphene (FLG) domains [Figure 1(c) and (d)]. As shown, each FLG domain has a smaller and thicker graphene grain grown on the center of the bottom MLG grain. All these graphene samples are synthesized on Cu foils, and then transferred onto SiO2/Si substrates prior to the synthesis of GaSe on them (see experimental section). Atomic force microscope (AFM) is used to characterize the thickness of these MLG and FLG domains. As shown in Figure 1(b) the MLG domain has an average thickness of about 1.0 nm, thicker than the theoretical value of ~3.4 Å due to the existence of the interlayer air gap between graphene and SiO2/Si substrate [18]. For the FLG domain [Figure 1(d)], the larger grain
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at the base is identified as MLG, whereas the smaller top grain at the center has a stack height of 1.3±0.1 nm, corresponding to two or three-layered graphene. As shown, both the MLG and FLG domains preserve the equilateral hexagonal shape, which signifies their single crystallinity.
Figure 1. Two types of graphene domains. FESEM (a, and c) and AFM (b, and d) images of monolayer graphene (MLG, a and b) domains and few layer graphene (FLG, c and d) domains. (e) Typical Raman spectra recorded from the (i) MLG domain on the Cu substrate, (ii) MLG domain on the SiO2/Si substrate, and (iii) FLG domain on the SiO2/Si substrate. (f) Plots of I2D/IG ratio and FWHM of the 2D peak for spectrum (i), (ii), and (iii). (g) The number of graphene layer at the center region of the FLG domain are determined from the FWHMs of the associated 2D band and the AFM topography images.
Micro-Raman spectroscopy is used to verify the AFM data. Figure 1(e) shows the Raman spectra of (i) MLG on a Cu foil, transferred (ii) MLG, and (iii) FLG on SiO2/Si substrates. The insignificant D band in (i) indicates that nearly defect-free MLG domains are grown on the Cu foil. Trace D band appears after transferring the graphene onto the SiO2/Si substrate (ii), with an
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intensity ratio of G to D band, ID/IG, 2) and the FWHM2D as low as 29 cm-1 (on Cu) and 44 cm-1 (on SiO2) strongly suggested for the formation of MLG. The FWHM2D is used to determine the number of layer of the FLG [21]. To better estimate the number of graphene layer on the FLG grain, we have used 18 FLG domains to estimate the FWHM2D and 8 FLG domains for AFM measurement. As shown in Figure 1(g), both methods suggested that the center FLG domains consist of 2-3 graphene layers.
Analysis of GaSe on graphene. GaSe crystals are grown directly on the transferred graphene domains by using our CVD reactor (see Figure S1). We used two types of substrates, pristine and graphene-coated SiO2/Si substrates, in every CVD deposition. The GaSe crytals grown on the pristine SiO2/Si are studied by various technques (see Figure S2-S6), and are used as the reference. Figure 2(a) shows GaSe crystals deposited on a MLG domain. Due to the size of the GaSe crystals (
