Template Approach to Crystalline GaN Nanosheets - ACS Publications

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Template approach to crystalline GaN nanosheets Baodan Liu, Wenjin Yang, Jing Li, Xinglai Zhang, Pingjuan Niu, and Xin Jiang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00754 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 17, 2017

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Template approach to crystalline GaN nanosheets Baodan Liu,a,*,# Wenjin Yang,a,b,# Jing Li,a,# Xinglai Zhang,a Pingjuan Niu,c Xin Jianga,* a

Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal

Research (IMR), Chinese Academy of Sciences (CAS), No. 72 Wenhua Road, Shenyang 110016 China b

School of Materials Science and Engineering, University of Science and Technology

of China, No. 72 Wenhua Road, Shenyang 110016 China c

School of Electrical Engineering and Automation, Tianjin polytechnical university

No. 399, Binshuixi Road, Tianjin,300387, China

Abstract

Crystalline GaN nanosheets hold great challenge in growth and promising application in optoelectronic nanodevices. In this work, we reported an accessible template approach toward the rational synthesis of GaN nanosheets through the nitridation of metastable γ-Ga2O3 nanosheets synthesized from a hydrothermal reaction. The cubic γ-Ga2O3 nanosheets with smooth surface and decent crystallinity can be directly converted into hexagonal GaN nanosheets with similar morphology framework and comparable crystal quality in NH3 at 850oC. UV-Vis spectrum measurement reveals that the GaN nanosheets show a band gap of 3.30 eV with 1

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strong visible absorption in the range of 370-500 nm. The template synthetic strategy proposed in this work will open up more opportunities for the achievement of a variety of sheet-like nanostructures which can not be obtained through conventional routines and will undoubtedly further promote the fundamental research of newly emerging sheet-like nanostructures and nanotechnology. Key words: GaN; nanosheets; template synthesis; HRTEM; optical property; γ-Ga2O3;

Introduction

The discovery of graphene in 2004 has attracted global research interest in two-dimensional (2D) nanosheets for their layered crystallographic structure, peculiar physical and chemical properties and promising applications in diverse fields ranging from electronics, optoelectronics, photocatalysis and so on.1-8 The layered structure of graphene and related materials such as h-BN, WS2, MoS2 etc, which is bonded with weak Van der Waals' force in interlayers, facilitates an easy synthetic approach toward single layer or multilayers through mechanical peeling, epitaxial growth and so on.9-15 However, the obtaining of sheet-like semiconductors without intrinsic graphene-like layer structures still remains a key challenge even though extensive research interests focus on this field.16 As a key semiconductor with a wide band-gap of 3.4 eV and technologically important applications in LED, laser, solar-cell, UV-detector,17-23 GaN nanosheets with few layers attract specific attention for their peculiar optoelectronic properties predicted in theory, including the thickness-dependent 2


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energy band gap and the p-type conductivity, and the diverse functionality in the construction of optoelectronic nanodevices.24-27 Unfortunately, traditional GaN crystal growth methods such as hydride vapor phase epitaxy (HVPE),28 metal-organic chemical vapor deposition (MOCVD)29 and molecular beam epitaxy (MBE)30 etc are not applicable in the direct synthesis of GaN nanosheets, and the cleaving of tetrahedral-coordinated GaN bulk crystal along the (0001) plane is also impossible. Therefore, it is essential and urgent to explore an effective approach to synthesize GaN nanosheets and investigate their optoelectronic properties. Recently, the challenge in preparing

GaN layers has been overcome by the

research groups led by Redwing and Robinson et al using a migration-enhanced encapsulated growth (MEEG) method, in which the GaN monolayer is created beneath the graphene layer generated by the Si sublimation from SiC (0001) surface.26 However, the complicated synthetic process and the inhomogeneity of GaN layers in size and layer thickness further hinder the property investigation and practical applications in optoelectronic nanodevices. Additionally, the removal of graphene layer on the top of GaN layer and the peeling off of GaN layers from SiC substrate still remain a key challenge. As a result, a feasible and easily accessible strategy for the scalable and controllable growth of GaN nanosheets is still required. In this work, we demonstrated the template approach to the synthesis of

GaN

nanosheets through the nitrification reaction of metastable γ-Ga2O3 layers. Metastable γ-Ga2O3 phase with an ultrathin layer thickness of 5-10 nm was first synthesized through a feasible hydrothermal reaction. The γ-Ga2O3 layers with body-centered 3


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cubic (bcc) structure are crystalline and show triangular or hexagonal geometrical configuration. High-resolution transmission electron microscope is utilized to confirm the planar growth of γ-Ga2O3 in (111) plane. The GaN nanosheets are in-situ obtained through the solid replacement reaction of γ-Ga2O3 with ammonia gas due to the less lattice mismatching between (220) plane of cubic γ-Ga2O3 and (101ത 0) plane of hexagonal GaN. The confined template reaction enables the maintaining of layered framework of γ-Ga2O3 phase and leads to the direct formation of

GaN nanosheets.

Additionally, the less lattice mismatching also promotes the structure transition from cubic γ-Ga2O3 phase to hexagonal GaN phase. The template synthetic strategy proposed in this work opens up more opportunities for the achievement of a variety of sheet-like nanostructures which can not be obtained through conventional routines and will undoubtedly further promote the fundamental research of newly emerging sheet-like nanostructures and nanotechnology.

Results and discussion

Inspired by the template synthesis method to a variety of nanostructures,31, 32 a layered structure of Ga2O3 is considered as the ideal template for the formation of GaN nanosheets via atomic rearrangement. To realize the indirect synthesis of GaN nanosheets, metastable γ-Ga2O333 is first selected for its cubic structure and the close lattice constant of (220) plane to the value of (101ത 0) plane of wurtzite-type hexagonal GaN (a=0.318 nm, c=0.518 nm), as illustrated in Figure 1a and 1c. The unstable structure and cubic crystallographic symmetry of γ-Ga2O3 phase, as well as the close 4


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lattice constants, enable an easy structure transition from cubic to hexagonal under proper nitridation process. In this way, the structure and phase transition from γ-Ga2O3 to wurtzite GaN will take place from the surface to the inside through solid diffusion/replacement reaction. Most importantly, the framework of γ-Ga2O3 phase in the form of nanosheets can be well maintained and layered GaN nanostructures can be obtained, as schematically illustrated in Figure 1d. To synthesize γ-Ga2O3 nanosheets, a modified hydrothermal method used to prepare ZnGa2O4 nanosheets is employed and a detailed experimental process is schematically described in Figure 1b.34 Series of growth experiments under different hydrothermal conditions find that the ratio of H2O and ethylenediamine in the initial solvent is of great importance in the morphology control of γ-Ga2O3 nanostructures. A higher H2O content in the solvent will lead to the formation of GaOOH nanocrystals with wire-like morphology, as shown in Figure S1 a. Increasing the ethylenediamine ratio in the initial solvent will result in the appearance of γ-Ga2O3 phase with sheet-like morphology and further increasing the ethylenediamine content will make the γ-Ga2O3 nanosheet more compact to form continuous colloidal nanocrystal film (Figure S1b-e). X-ray diffraction measurements on these samples also confirm the formation of GaOOH nanowires and γ-Ga2O3 nanosheets as a dependence of H2O/ethylenediamine ratio (Figure S2). It should be noted here that either GaOOH nanowires or γ-Ga2O3 nanosheets show decent phase purity. Figure 2a-c shows the morphology of γ-Ga2O3 nanosheets under different magnifications. It can be seen that the γ-Ga2O3 nanosheets are more stable to 5


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self-assemble into microspheres (Figure 2b). From the low-magnification SEM image observations, we can see that these γ-Ga2O3 microspheres have an average diameter of 3-5 µm and good dispersibility (Figure 2a and Figure S3), while the thickness of single γ-Ga2O3 nanosheet is roughly estimated to be less than 10 nm, as shown in Figure 2c. In addition, it can be noticed that γ-Ga2O3 nanosheets also exhibit regular geometrical shapes with hexagonal or triangular contours and the included angle between neighbored edges is around 120o. TEM characterizations on the dispersed γ-Ga2O3 nanosheets further verify our SEM observation on the morphology of γ-Ga2O3 nanosheets (Figure S4 a,c) and it can be seen again the hexagonal or triangular shapes (Figure 2d-f). These featured morphologies of γ-Ga2O3 nanosheets suggest that they are possibly oriented the [111] direction of cubic γ-Ga2O3, which will allow for an easy structural transition to hexagonal GaN. High-resolution transmission electron microscopy (HRTEM) analysis performed on the individual γ-Ga2O3 nanosheet verifies the decent crystal quality and the single crystal nature (Figure S4b, d). Figure 2g shows the HRTEM image of γ-Ga2O3 nanosheet taken along the [111] zone axis (perpendicular to plane) and the distance between adjacent lattice planes is measured to be 0.29 nm, matching well with the d-spacing value of (220) planes in cubic γ-Ga2O3. The corresponding Fast Fourier Transition (FFT) pattern shown in Figure 2h also demonstrates the superior crystallinity of the thin γ-Ga2O3 nanosheets. Accurate composition analysis of the γ-Ga2O3 nanosheet using an energy-dispersive X-ray (EDX) spectrometer and spatially-resolved elemental mapping in scanning transmission electron microscopy (STEM) mode reveal that the 6


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γ-Ga2O3 nanosheets have a very high chemical purity and a uniform distribution of each element inside the nanosheets (Figure2 i). The crystalline feature and the thin layer of γ-Ga2O3 nanosheets provide predominant advantages in the template synthesis of crystalline GaN nanosheets through diffusion/replacement reaction in ammonia gas. Considering the smaller thickness of γ-Ga2O3 nanosheets and possible quantum size effect, the nitridation process was carried out a temperature range of 800-850oC, which is much lower than conventional chemical vapor deposition reaction for GaN nanostructures using Ga2O3 powder as reactant.35-37 The nitridation experiment results find that γ-Ga2O3 phase has completely converted into wurtzite GaN phase at 850oC and only the diffraction peaks corresponding to wurtzite GaN phase were observed,38 suggesting the phase purity of GaN (Figure 3b). SEM observations reveal that the morphology and framework of γ-Ga2O3 nanosheets have been well maintained after template synthesis (Figure 3a,c-h), and the GaN nanosheets still exhibit a good dispersivity (Figure 3c, 3f). Spatially resolved elemental mapping analysis indicates that Ga and N elements have a uniform distribution inside the GaN nanosheets (Figure S5). Similar template synthesis works have also been reported in cubic GaN nanotubes and GaN-ZnO solid solutions, in which the final products have the same geometrical shape as the reactants.39, 40 However, the surface of GaN nanosheets is rather rough compared with the smooth morphology of initial γ-Ga2O3 nanosheets and the flat γ-Ga2O3 layers have been heavily destroyed into some broken holes, as shown in Figure 3e, 3h. We can still notice that some tiny crystalline islands with a pyramid-like shape were formed 7


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on the GaN nanosheet surface during the nitridation process. The formation of GaN nano-pyramids can be regarded as a self-organized process of surface adsorbed Ga and N atoms during the nitridation reaction of γ-Ga2O3 nanosheets and similar crystalline evolution from GaN nucleus have been observed in various GaN and related nanocrystals.35,

41, 42

The dimensional size and density of these GaN

nano-pyramids show an obvious increasing tendency with the increase of reaction time. The transition from cubic γ-Ga2O3 nanosheets to hexagonal GaN nanosheets involves the rearrangement and substitution of atoms between different structure symmetries. As a result, possible crystallinity deterioration and structural defect can be created in GaN nanosheets during the process. To further study the crystallinity of GaN nanosheets and the possible structure/phase transition process from cubic γ-Ga2O3 nanosheets to hexagonal GaN layers, detailed structural analysis using atomically-scaled high-resolution TEM was carried out under an accelerated voltage of 200 kV. Figure 4 shows the typical low-magnification TEM images and atomic resolution lattice fringes of GaN nanosheets nitrified at 850oC for 30 and 60 min, respectively. Apparently, the completely smooth surface and regular edge of γ-Ga2O3 nanosheets has been converted into rough and broken morphology during the nitridation process at the first 30 min (Figure S6), which is related to the surface nitridation of Ga2O3 with reactive ammonia gas and element volatilization. The local surface chemical reaction or atomic diffusion will result in the reorganization of Ga and N atoms within the framework of γ-Ga2O3 nanosheets. It should be noted that the 8


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two processes are expected to occur in γ-Ga2O3 nanosheets simultaneously. In some local areas, the reaction of surface atoms in γ-Ga2O3 nanosheets with NH3 gas will result in the collapse of γ-Ga2O3 nanosheets to generate tiny holes, as observed in Figure 4a and the SEM images in Figure 2. Correspondingly, the atomic stacking sequence “ABCABC” in cubic γ-Ga2O3 nanosheets will reorganize into “ABAB” in hexagonal GaN. Similar cubic-to-hexagonal symmetry transition is also observed in diverse semiconductors existed in the form of zinc-blende and wurtzite structures.43, 44 In addition, the structural conversion from cubic γ-Ga2O3 to wurtzite hexagonal GaN also leads to the formation of numerous tiny GaN pyramids on the nanosheet surface, as shown in Figure S7a and SEM image (Figure 3h). The appearance of such GaN nano-pyramids further demonstrates the volatilization and reorganization of Ga and N atoms during the nitridation reaction. When the reaction starts, the O atoms in γ-Ga2O3 nanosheet framework will be replaced with N atom to bond with Ga to form GaN. At this stage, the aggregation of surface adsorbed Ga atoms via a self-assembled diffusion process is also expected to form Ga-rich nucleus and further lead to the generation of GaN nano-pyramids. With the prolonging of reaction time, the morphology collapse of GaN nanosheets becomes more obvious due to the continuous reaction and the ongoing atom migration/aggregation and volatilization, as shown in Figure 4e,f. In spite of the morphology collapse, the GaN nanosheets obtained from the conversion of γ-Ga2O3 nanosheets still maintains a decent crystallinity. As shown in Figure 4c, the atoms in GaN nanosheets synthesized at 850oC for 30 min are regularly 9


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arranged in a stacking sequence of “ABAB” when it is observed along the [0001] orientation. The magnified HRTEM image in inset of Figure 4c further verifies the crystalline nature and the hexagonal structure of wurtzite GaN nanosheets. The distance of 0.28 nm measured from adjacent lattice fringes (Figure 4c), which is close to the 0.29 nm interplanar distance of (220) plane in γ-Ga2O3 nanosheets, matches well with the d-spacing of (101ത0) plane in wurtzite-type GaN. In addition, the FFT pattern with succinct spots also demonstrates the good crystal quality of GaN nanosheets. Similarly, the GaN nanosheets synthesized at 850oC for 60 min and with a severe morphology collapse also exhibit the same crystallinity as the one in 30 min (Figure 4g, h). However, the impurity O signal is still detected in GaN nanosheets in EDS spectrum (Figure S8) even though the Ga2O3 phase is not monitored during XRD and TEM analyses (Figure 3b and Figure 4c,g). This means that the residual oxygen in GaN host lattice does not affect the crystallographic symmetry of wurtzite GaN and the O signal may originate from the oxygen adsorbed on the nanosheet surface. It is reasonable to understand that the escaped oxygen atoms from γ-Ga2O3 nanosheets can be adsorbed again on the surface of GaN nanosheets or partially doping into GaN host lattice, as reported in GaN nanostructures synthesized from Ga2O3 precursor.42 In addition, it is also found that the dislocation can be occasionally formed during the structural transition and atom rearrangement, as illustrated in Figure S7b. As a key semiconductor, GaN has a direct band gap of 3.4 eV at room temperature and the intrinsic band-to-band emission luminescence with a peak 10


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wavelength of 365-369 nm has been observed in various GaN bulk and nanostructures.37 However, recent theoretical study on

GaN layers predicts a

thickness-dependent band-gap in the range of 4.79-4.89 eV and a band-gap as large as 4.90 eV has been experimentally observed in GaN monolayer synthesized from a MEEG technique using ultraviolet visible reflectance measurement.26 This means that the electronic band structure of GaN layers may show extinguished difference compared to that of conventional GaN bulk crystal and the quantum-size effect may be observed in as-synthesized GaN nanosheets. To further examine the optical emission and the band-gap, similar method using UV-Vis reflectance spectrum is utilized to calculate the energy band-gap of GaN nanosheets. Figure 5 shows the absorption spectra of γ-Ga2O3 nanosheet precursor and GaN nanosheets nitrified at 850oC for 30 and 60 min, respectively. It can be noted that the γ-Ga2O3 nanosheets show a strong absorption in the UV range with a wavelength shorter than 246 nm, which corresponds to an energy band-gap of 5.04 eV, as calculated from the F-E curve in the inset. The band-gap value of γ-Ga2O3 nanosheets is also close to that of pure β-Ga2O3 crystal, suggesting their chemical purity and crystallinity perfection. After converted into GaN nanosheets, the absorption edge shifts to about 370 nm besides the absorption tail extending to 500 nm. Correspondingly, the band-gap of GaN nanosheets can be derived from the F-E curve in inset to be 3.30 eV. Apparently, the band-gap of GaN nanosheets is smaller than the theoretically predicted value 4.79-4.89 eV of GaN layers26 and even smaller than the 3.4 eV of conventional GaN crystal.45 Considering the oxygen impurity and dislocation in GaN host lattice, it is 11


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easy to understand that these compositional and structural defects may dominate the band structure of GaN nanosheets and the quantum size effect may be neglected since the GaN nanosheets still exhibit a bulky feature. As a result, the strong visible absorption from the defect-related energy levels is observed, as shown in Figure 5. The above results suggest that the template method provides an easily-accessible and feasible approach toward the rational synthesis of thermodynamically unstable GaN nanosheets which can only be obtained through precise control of the complex growth technology. Even though the GaN nanosheets show a rough surface and broken morphology, further improvement on the morphology smoothness and crystal quality can still be expected through long time nitridation under lower temperature. The strategy proposed in this work can also be applied to the layered synthesis of a variety of other semiconductors which have no graphene-like layered structures. In addition, the peculiar optoelectronic properties and functionality distinguished from their bulky peers are also anticipated, and the potential applications of

GaN

nanosheets or related thin layers in advanced electronic, optical and optoelectronic nanodevices can be expected.

Conclusion

To summarize, thin

GaN nanosheets featured a mesoporous morphology have

been synthesized through a feasible template approach. γ-Ga2O3 nanosheets with smooth surface, crystalline nature and very thin layer thickness are first prepared using the hydrothermal method and are further utilized as the template for GaN 12


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growth. The nitridation of γ-Ga2O3 nanosheets in NH3 gas leads to the direct structure transition from cubic γ-Ga2O3 phase to hexagonal GaN phase with the sheet-like morphology partially collapsed, but the layer framework is well maintained. HRTEM analysis indicates that the GaN nanosheets with collapsed morphology still show the crystalline feature with ordering atomic arrangement after structure transition. UV-Vis measurements find that γ-Ga2O3 nanosheets have a strong absorption in UV range while the GaN nanosheets exhibit an obvious visible absorption in the range of 370-500 nm, and the quantum-size effect leading to the band-gap enlargement of GaN nanosheets is not observed due to the oxygen doping in GaN host lattice. The method proposed in this work provides an efficient and accessible avenue toward the synthesis of

GaN nanosheets or other semiconductor thin layers with peculiar

optoelectronic properties, diverse functionality and promising applications of in advanced electronic, optical and optoelectronic nanodevices.

Experimental section

Synthesis of γ-Ga2O3 nanosheets γ-Ga2O3 nanosheets are synthesized from the hydrothermal reaction of Ga(NO3)3 in solvents containing H2O and ethylenediamine. In a typical reaction, 0.52 g of Ga(NO3)3 powder was added into the mixed solvent of deionized H2O (15 mL) and ethylenediamine (30 mL). All the chemicals were bought from Aladdin without further purification. The mixed solution was first stirred in beaker for 10 min at room temperature and then transferred to a 100 mL stainless Teflon-lined autoclave. The 13


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solvothermal reaction was carried out at 180°C for 24 h, and then cooled naturally to room temperature. The γ-Ga2O3 nanosheets was finally collected by centrifuging the suspension solution and washing the white powder with alcohol several times followed by drying at 80°C for 2 h. Conversion of γ-Ga2O3 nanosheets to GaN nanosheets The GaN nanosheets were obtained through the direct nitridation of γ-Ga2O3 nanosheets inside a resistance furnace. Typically, the γ-Ga2O3 nanosheets were dispersed uniformly on a quartz substrate and reacted with NH3 gas (100 sccm) at 850oC for 30-60 min. Morphology, structure and optical characterizations The morphology of γ-Ga2O3 and GaN nanosheets is characterized by using a field-emission scanning electron microscope (FE-SEM, Zeiss, Supra 55) operating at 20 kV. The phase, microstructures and crystallinity of γ-Ga2O3 and GaN nanosheets are measured using an X-ray powder diffractometer (XRD, Rigaku RINT 2000) operating at 40 kV and 40 mA by using Cu Kα radiation (λ=1.54056 Å) and a transmission electron microscopy (TEM, FEI, Tecnai G2 F20). The structure transition from cubic γ-Ga2O3 to hexagonal GaN phase is convinced by high-resolution TEM and local FFT pattern. Composition analysis on γ-Ga2O3 and GaN nanosheets is performed using the X-ray energy dispersive spectrometer installed in the TEM. The optical properties of as-synthesizedγ-Ga2O3 and GaN nanosheets are measured using a UV-Vis spectrometer (Hitachi U-3900) at room temperature. 14


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ASSOCIATED CONTENT

Supporting Information. SEM images and XRD patterns of Ga2O3 nanostructures under different H2O/ethylenediamine ratios; Low-magnification SEM images and TEM images of γ-Ga2O3 and GaN nanosheets; HRTEM results, elemental mapping and EDS spectrum of GaN nanosheets. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Corresponding Author: [email protected];

[email protected]

Notes #

These authors contribute equally to this work.

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by the Knowledge Innovation Program of Institute of Metal Research, Chinese Academy of Sciences with grants No. Y2NCA111A1 and Y3NCA111A1, the Youth Innovation Promotion Association, Chinese Academy of Sciences (Grant No.Y4NC711171) and National Torch Plan with a grant number 2015GHA100042B00. References 1.

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Figures

Figure 1 (a,c) crystal structural models of cubic γ-Ga2O3 and hexagonal GaN; (b) experimental process for the formation of γ-Ga2O3 and GaN nanosheets; (d) 17


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Schematic diagram describing the structure/phase transformation of GaN nanosheet from γ-Ga2O3 nanosheet;

Figure 2 (a-c) SEM images of cubic γ-Ga2O3 nanosheets under different magnifications; (d-f) representative TEM images of thin γ-Ga2O3 nanosheets featured with hexagonal and triangular configurations; (g) atomically-resolved lattice image of γ-Ga2O3 nanosheet and (h) its corresponding Fast Fourier Transformation (FFT) pattern; (i) scanning transmission electron microscope (STEM) image of γ-Ga2O3 nanosheets and their corresponding elemental mappings of Ga and O elements;

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Figure 3 (a) Typical SEM image of flower-like γ-Ga2O3 nanosheets; (b) XRD patterns of γ-Ga2O3 nanosheets and GaN nanosheets nitrified at 850oC for 30 and 60 minutes; (c-e) and (f-h) different-magnification TEM images of thin GaN nanosheets obtained at 850oC for 30 and 60 minutes, respectively; The yellow arrows indicate the formation of GaN nano-pyramids.

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Figure 4 (a,b) Typical low-magnification TEM images and (c) high-resolution TEM image and (d) its corresponding FFT pattern of GaN nanosheets obtained 850oC for 30 minutes; (e,f) Typical low-magnification TEM images and (g) high-resolution TEM image and (h) its corresponding FFT pattern of GaN nanosheets obtained 850oC for 60 minutes; Insets of (c,g) show enlarged TEM images of GaN nanosheets taken along [0001] zone axis;

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Figure 5 UV-Vis spectra of γ-Ga2O3 and GaN nanosheets synthesized at different temperatures; Inset is the corresponding (F(R)hν)2-Energy (F-E) curve for band gap calculations.

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Template Approach to Crystalline GaN Nanosheets - ACS Publications

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