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Growth of 2D GaN Single Crystals on Liquid Metals Yunxu Chen, Keli Liu, Jinxin Liu, Tianrui Lv, Bin Wei, Tao Zhang, Mengqi Zeng, Zhongchang Wang, and Lei Fu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08351 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018
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Growth of 2D GaN Single Crystals on Liquid Metals Yunxu Chen,§,† Keli Liu,§,‡ Jinxin Liu,† Tianrui Lv,† Bin Wei,# Tao Zhang,† Mengqi Zeng†, Zhongchang Wang# and Lei Fu*,† †College
of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China The Institute for Advanced Studies (IAS), Wuhan University, Wuhan 430072, China # Department of Quantum and Energy Materials, International Iberian Nanotechnology Laboratory (INL), 4715-330 Braga, Portugal _____________________________________________________ ‡
ABSTRACT: Two dimensional (2D) gallium nitride (GaN) has been highly anticipated because its quantum confinement effect enables desirable deep-ultraviolet emission, excitonic effect and electronic transport properties. However, the currently obtained 2D GaN can only exist as intercalated layers of atomically thin quantum wells or nanometer-scale islands, limiting further exploration of its intrinsic characteristics. Here, we report, for the first time, the growth of micron-sized 2D GaN single crystals on liquid metals via a surface-confined nitridation reaction and demonstrate that the 2D GaN shows uniformly incremental lattice, unique phonon modes, blue shifted photoluminescence emission and improved internal quantum efficiency, providing direct evidence to the previous theoretical predictions. The as-grown 2D GaN exhibits an electronic mobility of 160 cm2 V–1 s–1. These findings pave the way to potential optoelectronic applications of 2D GaN single crystals. ____________________________________________________ Gallium nitride (GaN) with a wide direct band gap, high critical breakdown electric field and fast saturated migration rate has emerged as one of the primary materials for constructing photoelectric devices.1-4 Inspired by the birth of graphene,5 the researches of the transport of charge carriers,6, 7 and photon modes that are restricted in two dimensional (2D) plane have attracted enormous attention. Excitedly, the characteristics of GaN in 2D limit will present remarkable changes caused by quantum confinement. Compared with its bulk counterpart, 2D GaN shows enlarged bandgap, thus allowing electronic components to hold their durability even at a higher voltage.8 The atomically thin GaN quantum wells can emit photons in the deep-ultraviolet (UV) range. Theoretically, the enhanced excitonic effects will be appeared in 2D GaN, further improving the internal quantum efficiency.9 Given these distinctive superiorities of 2D GaN, uncovering the geometric structure and correlated functionalities of the 2D GaN systems is timely and of ultimate relevance. Yet there are no 2D GaN single crystals with a suitable size to reveal their physical characters and even their electronic applications. Traditional epitaxy approaches,10-12 or metal organic chemical vapor deposition (MOCVD),13 can easily generate bulk GaN films, or thin GaN quantum wells inserted in the aluminum nitride (AlN) layers. However, it remains difficult to directly thin GaN bulk via cleaving the tetrahedrally bonded surface along the (0001) plane.9 Notably, Redwing and Robinson et al. synthesized 2D GaN with nanometer-scale domains through graphene encapsulating but suffered the hardship in separating its two constituting components.8 Hence, 2D GaN single crystals are still highly required to explore their intrinsic physico-chemical properties.
Herein, we reported, for the first time, the successful growth of 2D GaN single crystals and investigated the properties of GaN single crystals in 2D limit. The growth of 2D GaN single crystals was achieved by a surface-confined nitridation reaction (SCNR) via chemical vapor deposition (CVD). We employed urea as the nitrogen source and conducted the synthesis on the liquid Ga (Figure S1), which is an accessible substrate for synthesizing 2D materials.14-16 The as-obtained 2D GaN exhibits uniformly incremental lattice as compared to the bulk GaN. Unique phonon modes, blue-shifted photon energy and improved internal quantum efficiency can be observed.
Figure 1. Morphology and chemical composition of 2D GaN single crystals. (a) A typical AFM image of the 2D GaN crystal, the inset shows the corresponding thickness profile of the AFM image. (b) A low-magnification TEM image of 2D GaN crystals. (c) EDS elemental mapping of Ga–K, Ga–L, and N–K for a hexagonal GaN single crystal.
To identify the morphology and thickness of the as-grown 2D GaN, atomic force microscope (AFM) characterization was utilized (Figure 1a). The hexagonal GaN single crystal with 50-μm lateral size exhibits a thickness of 4.1 nm. The statistical information of the lateral size and thickness of GaN single crystals was shown in Figure S2. Meanwhile, the high-resolution transmission electron microscopy (HRTEM) image (Figure 1b) confirms the ultrathin hexagon morphology of the 2D GaN crystals. Energy dispersive Xray spectroscopic (EDS) elemental mapping indicates a uniform distribution of Ga and N element in the GaN crystal (Figure 1c). In addition, X-ray photoelectron spectroscopy (XPS) spectra were collected to survey the element composition of the GaN crystals (Figure S3). GaN single crystals exhibit the typical wurtzite structure with the c-elongated hexagon (0001) plane and the C6v-symmetric (1010)
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Figure 2. Atomic structure of 2D GaN single crystals. (a) Scheme of the crystalline structure of GaN. (b, c) HAADF (b) and BF STEM (c) images of the 2D GaN crystal acquired along [0001] zone axis. (d) Atomic resolution EDS elemental mapping of Ga, N, and their combination along [0001] zone axis. (e, f) HAADF (e) and BF STEM (f) images of the 2D GaN crystal acquired along [1010] zone axis.
plane (Figure 2a).17, 18 To disclose the atomic structure of the asgrown 2D GaN, the high angle annular dark-field scanning transmission electron microscopy (HAADF STEM) is shown in Figure 2b and bright-field STEM (BF EE2STEM) image of GaN crystal viewed from the [0001] direction is shown in Figure 2c. The Ga and N atoms are arranged in a hexagonal close packing, which is also indicated by the structure stimulation (Figure S4). A HAADF STEM image as well as the corresponding elemental mapping at the atomic level were utilized to investigate the structure of the (0001) plane (Figure 3d), further confirming the close packing of the atoms in the 2D hexagonal structure. We also obtained the HAADF STEM and BF STEM images of the crystal along the (10 1 0) plane, as displayed in Figure 2e, f, further clarifying the atomic structure of 2D GaN, where the Ga atoms locate at the tetrahedral sites (point group 3m). To gain more insights into the features of the 2D GaN single crystals, the selected area electron diffraction (SAED), Raman and photoluminescence (PL) spectra were performed. Here, the bulk GaN single crystals obtained in a prolonged growth process were utilized for comparison. The lattice distance on the
Figure 3. Characterization of the 2D GaN single crystals. (a) Overlay of the SAED spots of bulk GaN and 2D GaN crystals. (b) Intensity profiles extracted from the SAED spots in (a). (c) Raman spectra for bulk GaN and the 2D GaN with a thickness of 5.2 nm. The inset shows enlarged region of the E2 model peaks for both samples. (d) PL spectra of commercial bulk GaN and the 2D GaN with a thickness of 5.2 nm.
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(0001) plane recorded from the 2D GaN is 2.900 Å (Figure S5), which is obviously larger than that of the bulk GaN (2.790 Å). SAED patterns (Figure 3a) collected from the ultrathin hexagonal crystal (yellow pots) reveal a lattice constant of 3.323 Å, which is much larger than that extracted from the bulk one (blue pots, 3.190 Å). The intensity profiles extracted from the corresponding SAED spots are presented in Figure 3b for a clearer comparison. An enlargement ratio of 4.17 % of the lattice constant along the (0001) plane is achieved for the 2D GaN crystal structure. In addition, the SAED patterns recorded from three different regions of a 2D GaN crystal verified the uniformly distributed enlargement of the lattice owing to the same spots spacing and coincident orientation of the patterns (Figure S6). The results are consistent with the previous prediction that the lattice constant is enlarged once the GaN is in 2D limit.19 The Ga–N bond along the c axis will be broken when the thickness of GaN goes downs to few layers, which can destroy the sp3 orbital hybridization in the original GaN4 tetrahedron,20, 21 further leading to uniformly elongated lattice of 2D GaN crystals. Moreover, the fast Fourier transform (FFT) patterns (Figure S7) collected from the (1010) plane indicate a lattice constant c value of 5.364 Å, which is larger than that of bulk (5.186 Å). This manifests that the lattice is enlarged in the entire structure. The phonon modes for the 2D GaN single crystals also alter obviously compared with those for the bulk.22 As the Raman spectra shown in Figure 3c, an exclusive peak at 566.2 cm–1 (E2) appears in 2D GaN, whereas three peaks of bulk GaN are observed at 567.2 (E2), 560.7 (E1), and 529.3 (A1) cm–1. On one hand, the symmetry and strength of E2 confirm the high crystallinity of the 2D GaN. On the other hand, this distinction indicates the variation of phonons modes of GaN in 2D limit. Its position at 566.2 cm–1 is obviously blue shifted compared with that of the bulk, suggesting a tensilestrain state in 2D limit.23 The presence of A1 and E1 modes reveal that the Raman polarization selection rules are broken, which is caused by the light disorder scattering in bulk phase. Conversely, it can be deduced that the Raman polarization selection rules maintain in the 2D GaN crystals due to the disappearance of two peaks. Room-temperature PL spectra were obtained to explore the optical properties of the resulting 2D GaN crystals (Figure 3d). Pronounced PL emission of 2D GaN is located at the position of 330 nm, which indicates a photon energy of 3.76 eV, further demonstrating the light emission in a shorter UV wavelength. The evident blue shift compared with the commercial bulk GaN (3.40 eV) exhibits a signature of quantum confinement effect in the 2D limit. And the PL intensity of 2D GaN is about 48 times stronger than that of the commercial bulk GaN (Figure 3d). The obviously improved intensity suggests a greatly improved internal quantum efficiency for 2D GaN,24 which can be attributed to the enhanced excitonic effects. According to previous theoretical prediction,9 this root cause lies in the enhanced strength of electron-hole interaction induced by the quantum confinement effect. In addition, the PL spectra of 2D GaN with different thickness were collected (Figure S8), revealing that the phone energy increases with the decrease of the thickness. Moreover, temperature dependent PL measurements (Figure S9) indicate the absence of compositional inhomogeneity or clusters in the 2D GaN crystals.25, 26 To acquire the 2D GaN, we design a surface tension-induced stratified Ga/Ga–W structure, which enables the SCNR, as illustrated in Figure S10. Thereinto, the surficial Ga layers serve as templates to grow micron-sized 2D GaN, whereas the sub-surficial Ga–W solid solution can impede the thickening of 2D GaN owing to the stronger nitridation ability of W atoms than Ga atoms. Thus, the confined growth behavior for 2D GaN on the surface of molten system could be realized. XPS depth profile analysis was performed
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to shed light on the SCNR process. Here, the molten Ga/Ga–W structure can be formed when a droplet of Ga spreads out on tungsten (W) metals at 1080 oC. The XPS depth profile in Figure S11 demonstrates the stratified structure consisting of an outmost ultrathin Ga layer and an underlying Ga–W solid solution bulk. Importantly, the surface tension of molten W (2361.5 mNm–1) is far larger than that of Ga (628.7 mNm–1),27 which demonstrates a stronger interaction among W atoms. Hence, the surface layer with atomic thickness is occupied by the Ga atoms with weak interactions. In addition, due to the lower Gibbs free energy of WN (–121 kJmol–1) than GaN (–18 kJmol–1), the W atoms in the subsurface can easily capture the N atoms to preferentially form the W–N bonds, thus confining the nitridation reaction of Ga atoms in the outmost surface layer. The XPS depth profile analysis was conducted on the molten Ga/Ga–W system after nitridation, as shown in Figure S12. The GaN can be detected only on the outer surface, while the WNx species can only be detected inside. Thus, the 2D GaN single crystals can be synthesized via SCNR on molten Ga/Ga–W system.
Experimental details and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *
[email protected] ORCID Lei Fu: 0000-0003-1356-4422
Author Contributions §
These authors contributed equally to this work
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT The research was supported by the National Natural Science Foundation of China (Grants Nos. 21673161 and 21473124), the Science and Technology Department of Hubei Province (Grant No. 2017AAA114), and the Sino-German Center for Research Promotion (Grant No. 1400).
REFERENCES (1) Chung, K.; Lee, C.-H.; Yi, G.-C. Transferable GaN layers grown on ZnO-coated graphene layers for optoelectronic devices. Science 2010, 330, 655. (2) Kobayashi, Y.; Kumakura, K.; Akasaka, T.; Makimoto, T. Layered boron nitride as a release layer for mechanical transfer of GaN-based devices. Nature 2012, 484, 223. (3) Kako, S.; Santori, C.; Hoshino, K.; Götzinger, S.; Yamamoto, Y.;
Figure 4. FET performance of 2D GaN single crystal. (a) Schematic of 2D GaN FET. (b) The false-colored SEM image of a 2D GaN crystal FET. (c) Ids–Vg characteristic curve recorded at Vds = 1V. (d) Gatedependent Ids–Vds characteristic curves. The gate voltages of individual Ids–Vds curve are given.
Field effect transistor (FET) device was constructed on a 2D GaN crystal to probe the intrinsic electronic property (Figure 4). Mechanically exfoliated graphene films are utilized as the contact layers of 2D GaN crystal to get a better Ohmic contact.28 The presented curve in Figure 4c exhibits the typical n-type conduction of 2D GaN crystal and Figure 4d displays the Ids–Vds characteristic curves with various Vg values. The mobility of the 2D crystal is calculated to be 160 cm2 V–1 s–1 with an on/off ratio of ~106. In summary, we report the successful growth of the highly crystalline 2D GaN single crystals with lateral size up to 50 μm via a surface-confined reaction and investigate the unique characteristics of GaN singe crystals in 2D limit, such as enlarged lattice parameter, special phonon vibration modes, blue-shifted PL emission and improved internal quantum efficiency, which pave the way to further exploration of the optoelectronic properties for 2D GaN. The fabricated 2D GaN single crystals show a high electronic mobility. Our strategy paves a new route to produce highly crystalline 2D GaN single crystals with a large lateral size, which in association with their intriguing characters differing from their bulk counterparts suggests that they hold substantial promise for the future nanoelectronics.
ASSOCIATED CONTENT Supporting Information
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