Letter pubs.acs.org/NanoLett
Low-Temperature Growth of Two-Dimensional Layered Chalcogenide Crystals on Liquid Yubing Zhou,† Bing Deng,† Yu Zhou,† Xibiao Ren,‡ Jianbo Yin,† Chuanhong Jin,‡ Zhongfan Liu,† and Hailin Peng*,† †
Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences (BNLMS), College of Chemistry and Molecular Engineering, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China ‡ State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *
ABSTRACT: The growth of high-quality two-dimensional (2D) layered chalcogenide crystals is highly important for practical applications in future electronics, optoelectronics, and photonics. Current route for the synthesis of 2D chalcogenide crystals by vapor deposition method mainly involves an energy intensive hightemperature growth process on solid substrates, often suffering from inhomogeneous nucleation density and grain size distribution. Here, we first demonstrate a facile vapor-phase synthesis of large-area high-quality 2D layered chalcogenide crystals on liquid metal surface with relatively low surface energy at a growth temperature as low as ∼100 °C. Uniform and large-domain-sized 2D crystals of GaSe and GaxIn1−xSe were grown on liquid metal surface even supported on a polyimide film. As-grown 2D GaSe crystals have been fabricated to flexible photodetectors, showing high photoresponse and excellent flexibility. Our strategy of energy-sustainable low-temperature growth on liquid metal surface may open a route to the synthesis of high-quality 2D crystals of Ga-, In-, Bi-, Hg-, Pb-, or Sn-based chalcogenides and halides. KEYWORDS: low-temperature growth, layered chalcogenides, liquid metal
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substrate. Considering the dissociation of Se from Ga−Se chalcogenides due to its intrinsically high vapor pressure, the use of high growth temperature (>500 °C) may lead to the synthesis of compositionally inhomogeneous and nonstoichiometric nanostructures from incongruent melting compounds via traditional vapor-phase method.23 To overcome the abovementioned issues, energy-sustainable low-temperature growth methods of high-quality 2D crystals is still desirable. On the other hand, when depositing onto liquid substrates with relatively low surface energy, the clusters diffuse laterally across the surface until they reach binding sites, where they nucleate and grow into 2D crystals. Ideally, it would be beneficial to reduce the grain boundary density of as-grown 2D crystals on homogeneous substrates with low surface energy, leading to the synthesis of large-area high-quality 2D crystals at relatively low temperatures. Here, we report for the first time on a low-temperature growth of 2D layered chalcogenide crystals such as GaSe and GaxIn1−xSe on liquid metals with relatively low surface energy. As an example, gallium or eutectic indium−gallium alloy with
he isolation of atomically thin two-dimensional (2D) layered crystals has opened up new realms in chemistry, physics, material science, and engineering.1−5 Among versatile 2D crystals, layered III−VI chalcogenides such as gallium selenide and indium selenide (GaSe, In2Se3, and InSe) have attracted increasing attention because of their unique optical and electronic properties, which may lead to numerous applications in optoelectronics and phase-change memory.6−13 Several preparation methods of 2D layered III−VI chalcogenides have been proposed, including top-down mechanical7,10,11 or chemical exfoliation14 from their bulk counterparts, and bottom-up liquid-phase or vapor-phase growth.8,9,15−18 Recently, van der Waals epitaxial growth of GaSe and In2Se3 nanoplates on a layered substrate without surface dangling bonds has also been reported by our group and others.9,13,18 The vapor-phase growth approach is a powerful method for the synthesis of high-quality and largescale 2D crystals from clusters.19,20 In usual case studied, the clusters preferred to nucleate at high-surface energy sites of substrates, such as defects, facet steps, or grain boundaries associated with a solid substrate, resulting in the growth of 2D crystals with inhomogeneous nucleation density and grain size distribution.21,22 Thus, high growth temperature should be usually used because the clusters need sufficient energy to diffuse to energetically preferred locations on the growth © XXXX American Chemical Society
Received: January 26, 2016 Revised: February 23, 2016
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DOI: 10.1021/acs.nanolett.6b00324 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
representative scanning electron microscope (SEM) image in Figure 1c shows that a uniform GaSe film covered over the entire liquid Ga surface. The GaSe film became rippled when the sample was under vibration because of certain fluidity of the underlying liquid Ga. High-magnification SEM images of different locations of the film show highly crystalline film with the regular facet angle of approximately 120 degree, matching well with the hexagonal phase of layered GaSe (Figure 1d). Remarkably, the whole film exhibits nearly identical crystalline orientation, strongly suggesting that liquid Ga is an excellent substrate for the low-temperature growth of large-area high-quality 2D GaSe crystals. We also notice that triangular GaSe nanoplates with identical orientation were formed and merged on the surface at early stages of growth (Figure 1e). Besides separated GaSe nanoplates, large-area continuous GaSe films were obtained and gradually approached to bulk crystals after a long growth time (Figure 1f). The crystal structure of the 2D GaSe crystal was further examined by X-ray diffraction spectroscopy (XRD). Figure 1g shows a typical XRD pattern of the GaSe film grown on liquid Ga without transfer. The two peaks are indexed as (0002) and (0004) of hexagonal GaSe, indicating the 2D layered crystals are preferentially oriented in the c-axis direction. The 2D GaSe crystals can be easily transferred onto arbitrary substrates using a PMMA-mediated transfer technique26 by gently etching the underlying Ga in a dilute hydrochloric acid. To characterize the microstructures and chemical compositions of the 2D GaSe crystals, we have carried out transmission electron microscopy (TEM) and selective area electron diffraction (SAED) studies. Figure 2a shows a typical lowmagnification TEM image of 2D GaSe crystals supported by the lacey carbon support film. Extensive high-resolution TEM (HRTEM) and SAED studies recorded from the same GaSe film indicate that the 2D GaSe crystals are structurally uniform and highly crystalline. Figure 2b shows a typical HRTEM image of the 2D GaSe crystals with clear hexagonal lattice fringes. The
very low melting points was used as both substrate and reactant for the growth of 2D GaSe or GaxIn1−xSe. Gallium or eutectic gallium−indium alloy keeps liquid state at room temperature, which provides a low-energy liquid surface during growth. 2D crystals of GaSe or GaxIn1−xSe can be readily grown when Ga− Se clusters vaporized from the Ga2Se3 source meet the Ga or In−Ga on the surface of liquid metals at very low temperature (about 100 to 150 °C). Such low growth temperatures can be used for polymer substrates. Figure 1a illustrates the schematic of 2D GaSe crystals growth on the liquid gallium surface. Gallium is a soft, silvery
Figure 1. (a) Schematic of low-temperature growth of 2D crystals on liquid metal. (b) Photographs of liquid gallium on tungsten foil held in hand before growth (left); on tungsten foil (middle) and a transparent polyimide substrate after growth (right) at about 150 °C. (c) SEM image of 2D GaSe crystal on liquid Ga after growth. (d) Corresponding high-magnification SEM images recorded at four different positions indicated as four boxes in (c). Scale bars: 2 μm. (e, f) Typical SEM images of GaSe nanoplates grown at early stages and for a long time, respectively. (g) XRD spectra of pure Ga before and after growth of the 2D GaSe crystals with a reference diffractogram of layered GaSe.
metal and melts at 29.76 °C (slightly above room temperature). Ga supported by a tungsten foil will melt if held in hand (Figure 1b, left panel). To grow large-area 2D GaSe crystals via a modified vapor phase deposition method, the source material Ga2Se3 powder (99.99%, Alfa Aesar) is placed into the center of a typical horizontal single zone tube furnace, where the temperature is 840−860 °C. Ar carrier gas transports the vapor of Ga−Se precursor downstream, and 2D GaSe crystals are grown on the liquid Ga substrate placed 19−21 cm away from heating center, where temperature is about 100−150 °C. Note that Ga 2 Se 3 is the suitable source material to grow stoichiometric GaSe on liquid Ga surface, where GaSe is the sole stable compound in a Ga-rich melt according to the Ga−Se phase diagram.24,25 Large-area 2D GaSe crystals can form on the liquid Ga supported by the tungsten foil (middle of Figure 1b). The Ga still spreads over the tungsten foil with small shrinkage because Ga wets tungsten. Due to the use of low growth temperature (100−150 °C), 2D GaSe crystal can grow on liquid Ga supported by a polyimide film, a flexible transparent polymer with good chemical resistance (Figure 1b, right panel). A
Figure 2. Typical TEM image(a), HRTEM image (b) and SAED pattern (c) of 2D GaSe crystal grown on liquid Ga. (d) AFM image and height profile of 2D GaSe crystal transferred onto 300 nm thick SiO2 of Si substrate. (e) Micro-Raman spectrum of the GaSe crystal after transferred onto 300 nm SiO2/Si. Inset is the optical image. (f) Corresponding Raman map of the area in the optical image in (e). B
DOI: 10.1021/acs.nanolett.6b00324 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
3e further shows a low magnification high-angle annular dark field (HAADF) image of 2D GaxIn1−xSe crystal. The spatially resolved EDX elemental mapping of the Ga, In, and Se elements show relatively uniform distribution, indicating composition uniformity across the entire nanosheets (Figure 3f−h). Typical X-ray photoelectron spectrum (XPS) of asgrown 2D GaxIn1−xSe crystals further revealed In 3d peaks (Figure 3i). Figure 3j show the quantification analysis of XPS depth profile of 2D GaxIn1−xSe crystals. The concentration of In, Ga, and Se along vertical direction in 2D GaxIn1−xSe crystal has scarcely changed with the In remaining at low content, which indicates the uniformity of composition along the direction of crystal thickness. Large-area, high crystalline quality 2D chalcogenide crystals can provide a platform for the fabrication of flexible optoelectronic devices because of their sizable bandgaps, the absence of dangling bonds and considerable flexibility. Layered GaSe has a bandgap of 2.1 eV, suitable for visible-light harvest in flexible photodetectors.8,30 The 2D GaSe crystals were transferred onto flexible and transparent EVA/PET substrates via hot lamination method for practical flexible photodetector applications (Figure S3).31 The GaSe photodetector arrays were directly fabricated by photolithography on the transparent EVA/PET substrates. The typical photograph and optical images of the device arrays are shown in Figure 4a,b, respectively. The spatial photocurrent map was recorded by a focused laser beam scanning over the corresponding photodetector under a fixed bias (Figure 4c). It is distinct that the photocurrent was strongly generated in the whole 2D GaSe crystal channel, weakly in the two electrodes, which can be attributed to the ohmic contacts between the GaSe channel and metal electrodes. To gain further insight into the characteristics of the 2D GaSe photodetector, we extracted the representative I−V output curves (Figure 4d) and calculated the photoresponsivity (Figure 4e) of 2D GaSe crystals illuminated at 532 nm under different incident light intensities. More excited photocarriers are produced by interband transition and transported to the electrodes with the light intensity increasing, resulting in larger photocurrent. Under low incident power (Pin = 0.2 nW), the measured photoresponsivity of 2D GaSe crystal devices is about 5 A/W, which is comparable to that of exfoliated GaSe nanosheets and superior to the previously reported grown 2D GaSe.7,15 As shown in Figure 4f, the flexible GaSe photodetector exhibits a remarkable photoresponse after switching incident visible light on and off many times. The on−off ratio of ∼50 is invariant of bending. However, the slight decrease of on−off switching ratio was observed with bending radii of 25 mm, in agreement with the decrease in effective irradiance. In addition, we investigated the mechanical durability of the 2D GaSe photodetectors after repeated bending. Figure 4g plots the current as a function of bending cycles with the light on and off at a bias voltage of 10 V. The 2D GaSe photodetectors kept their structural integrity with little variation in both photocurrent and dark current after repeated bending, which shows its excellent flexibility and durability. In summary, we have reported a low-temperature vaporphase growth of large-area high crystalline quality 2D layered chalcogenide crystals such as GaSe and GaxIn1−xSe on liquid metal surface. The use of Ga or eutectic Ga−In liquid metals as both the efficient reactant and the substrate with relatively low surface energy is found to be a facile approach to the lowtemperature production of uniform, large-domain-sized 2D
lattice spacing is measured as 0.32 nm, agreeing well with that of (10−10) plane of the hexagonal GaSe looking along the [0001] zone axis (Figure 2c). Energy dispersive X-ray (EDX) spectra revealed that the as-grown 2D crystals consist of Ga and Se with an atomic ratio of 1:1 (Figure S1a). Atomic force microscopy (AFM) was performed to determine the thickness of 2D GaSe crystals transferred onto 300 nm SiO2/Si substrate. As shown in Figure 2d, 2D GaSe crystal sample has a flat top surface with a uniform height of about 4.1 nm, corresponding to five layers of GaSe. Note that the thickness of 2D GaSe crystals can be tuned from few layers to multiple layers by adjusting the growth time (Figure S2). The structure and quality of few-layer GaSe is further characterized by Raman spectroscopy. The characteristic peaks for A11g mode (133.0 cm−1), E12g mode (211.9 cm−1), E21g mode (250.0 cm−1), and A21g mode (306.8 cm−1) were clearly observed (Figure 2e).27−29 The corresponding Raman map of the A11g band intensity has a uniformly distributed color (Figure 2f), indicating the structural integrity and uniformity of the 2D GaSe crystal. In addition to the use of pure gallium liquid metal, we employed eutectic gallium−indium (Sigma-Aldrich, 75% Ga 25% In by weight, ∼15.5 °C melting point) as liquid growth substrates aiming to develop a facile synthesis for 2D GaxIn1−xSe alloy crystals (Figure 3a). The microstructure of
Figure 3. (a) Schematic of the growth of GaxIn1−xSe on liquid eutectic gallium−indium alloy. Typical TEM image (b), HRTEM image (c), and SAED pattern (d) of 2D GaxIn1−xSe crystal. (e) HAADF image of one typical 2D GaxIn1−xSe. (f−h) EDX mapping for the three detected elements: Ga, In, and Se, respectively. (i) XPS In 3d spectrum of 2D GaxIn1−xSe crystal. (j) XPS depth profiles of 2D GaxIn1−xSe crystal.
as-grown 2D GaxIn1−xSe alloy was examined by TEM analysis (Figure 3b−d). Determined from the distance between adjacent lattice fringes, the lattice spacing of (10−10) of GaxIn1−xSe is 0.33 nm, slightly larger than that of GaSe (0.32 nm), suggesting a high crystallinity of 2D GaxIn1−xSe with the doping of larger In atom. Figure S1b shows a typical EDX spectrum taken from GaxIn1−xSe crystals, with x = 0.95. Figure C
DOI: 10.1021/acs.nanolett.6b00324 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21525310, 51222202, and 51472215), the National Basic Research Program of China (Nos. 2014CB932500 and 2015CB921000), and National Program for Support of Top-Notch Young Professionals. Part of this work was performed at the Center of Electron Microscopy of Zhejiang University.
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Figure 4. (a, b) Photograph and optical microscopy images of the photodetector arrays on the transparent and flexible PET substrate. (c) Spatial photocurrent map recorded by a focused laser beam scanning over the corresponding device in (b). GaSe channel indicated as dashed lines. VSD = 15 V, Vg = 0 V, Pinc = 480 nW. (d) Current− voltage (ISD−VSD) curves of the individual device in the dark and under different illumination intensities with a 532 nm laser. (e) Dependence of photoresponse on different illumination intensities. (f) Time trace of source−drain current when visible light was toggled on and off before, during, and after bending to a 25 mm bending radius at bias voltage of 10 V. (g) Optoelectronic characterization of a flexible photodetector. Current as a function of bending cycles (before/after bending to a 25 mm radius) of GaSe devices on PET with the light on or off.
crystals, even on the polyimide film substrate. In addition, photodetectors based on the 2D GaSe crystals show high photoresponsivity and excellent flexibility. Our strategy may pave the way to the low-temperature production of 2D layered crystals of Ga-, In-, Bi-, Hg-, Pb-, or Sn-based chalcogenides and halides on various liquid metals (Ga, In, Bi, Hg, Pb, Sn, and their alloy) for applications in future electronics and optoelectronics.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b00324. Experimental details and supplementary figures. (PDF)
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DOI: 10.1021/acs.nanolett.6b00324 Nano Lett. XXXX, XXX, XXX−XXX
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