Wafer-sized ultrathin gallium and indium nitride nanosheets through

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Wafer-sized ultrathin gallium and indium nitride nanosheets through the ammonolysis of liquid metal derived oxides Nitu Syed, Ali Zavabeti, Kibret A. Messalea, Enrico Della Gaspera, Aaron Elbourne, Azmira Jannat, Md Mohiuddin, Bao Yue Zhang, Guolin Zheng, Lan Wang, Salvy P. Russo, Dorna Esrafilzadeh, Chris F. McConville, Kourosh Kalantar-Zadeh, and Torben Daeneke J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11483 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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Journal of the American Chemical Society

Wafer-sized ultrathin gallium and indium nitride nanosheets through the ammonolysis of liquid metal derived oxides Nitu Syed, †,§ Ali Zavabeti, †,§ Kibret A. Messalea,† Enrico Della Gaspera,‡ Aaron Elbourne, ‡ Azmira Jannat,† Md Mohiuddin,† Bao Yue Zhang, † Guolin Zheng, ‡ Lan Wang, ‡ Salvy P. Russo, ‡ Dorna Esrafilzadeh, *† Chris F. McConville,‡ Kourosh Kalantar-Zadeh, *ǂ Torben Daeneke *,† School of Engineering, RMIT University, Melbourne, VIC 3001, Australia School of Science, RMIT University, Melbourne, VIC 3001, Australia ǂ School of Chemical Engineering, University of New South Wales (UNSW), Kensington, NSW 2052, Australia Supporting Information Placeholder † ‡

ABSTRACT: We report the synthesis of centimeter sized

ultrathin GaN and InN. The synthesis relies on the ammonolysis of liquid metal derived two-dimensional (2D) oxide sheets that were squeeze-transferred onto desired substrates. Wurtzite GaN nanosheets featured typical thicknesses of 1.3 nm, an optical bandgap of 3.5 eV and a carrier mobility of 21.5 cm2V-1s-1, while the InN featured a thickness of 2.0 nm. The deposited nanosheets were highly crystalline, grew along the (001) direction and featured a thickness of only three unit cells. The method provides a scalable approach for the integration of 2D morphologies of industrially important semiconductors into emerging electronics and optical devices.

Since the emergence of graphene, a large variety of two-dimensional (2D) materials has been reported, leading to new paradigms in the design of electronic and optical systems.1-3 Until recently, the discovery of new 2D crystals has been restricted to intrinsically layered crystals that can be exfoliated into 2D nanosheets, significantly limiting the choice of materials.2, 4 However, a considerable need exists for the creation of ultrathin nanosheets of non-layered materials, to allow accessing new atomically-thin systems with capabilities that have been predicted through computational methods.4-6 Herein, we report the synthesis of extraordinarily large area ultrathin gallium nitride, using an ammonolysis strategy on liquid metal-derived ultrathin Ga2O3 nanosheets. We also show that the method can be

expanded to include the fabrication of 2D InN nanosheets. Gallium nitride is a semiconductor of great technological importance with diverse applications due to its excellent electronic and optical properties.7-8 GaN is the material of choice for the fabrication of phosphorescent light emitting diodes, due to its wide bulk bandgap (3.4 eV), which facilitates the efficient emission of blue and near UV light.7 The high carrier mobility, chemical robustness together with the piezoelectric and pyroelectric properties of GaN have also led to other important applications, including the design of optoelectronic quantum wells, a variety of field effect transistors, energy harvesting systems, sensors and photocatalysts.9-13 Crystalline GaN adopts the wurtzite structure, featuring covalent bonds in all three dimensions, and hence it cannot be mechanically exfoliated.14 Conventional methods for depositing thin films of GaN are various, and amongst them, processes based on elemental epitaxial or chemical vapor deposition techniques are the most common.15-19 While epitaxial methods allow the controlled formation of ultrathin films, they involve considerable costs. Conversely, lower costs CVD methods do not allow thin films of several unit-cell thickness to be achieved due to inherent nucleation. As such, the development of a substrate independent, scalable method for the synthesis of 2D GaN which allows for the deposition of large area, highly crystalline GaN nanosheets, is necessary in order to grant access to 2D GaN at low cost.

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Figure 1. Synthesis process 2D GaN. (a) Schematic illustration of the squeeze-printing technique for depositing oxide skins onto substrates. The liquid gallium droplet is squeezed between two substrates, transferring the Ga2O3 layer. An optical image of exfoliated Ga2O3 on 300nm SiO2/Si is shown on the right (top). (b) Synthesis process of the 2D GaN nanosheet from 2D Ga2O3 using ammonolysis. The crystal structure of GaN exhibits a unit cell (dotted box) with cell parameters of 5.19 Å along the long axis and 3.19 Å along both short axes.20 Optical image of the synthesized GaN on 300nm SiO2/Si is shown on the right.

Herein we report the synthesis of centimeter scale 2D GaN nanosheets which can be deposited directly onto flat substrates. The synthesis relies on a two-step process utilizing the squeeze-printing of 2D Ga2O3, which is then converted into GaN using ammonolysis in a tubular furnace. The Ga2O3 deposition process was adopted from our previous work,4, 21-22 utilizing liquid gallium metal that is allowed to oxidize in air. Gallium oxidation in air follows Cabrera-Mott kinetics, leading to selftermination after a nanometer-thin oxide layer has formed.4, 22 Surface oxides on liquid metals feature weak adherence to the parent metal and can be easily transferred to desired substrates through van der Waals adhesion.4, 23 In this work, a small droplet (~1mm in radius) of liquid gallium (m.p. 29.8 °C) was placed on a SiO2 terminated, atomically-flat silicon wafer (Figure 1a). A second SiO2/Si wafer was firmly pressed onto the droplet, effectively squeezing the droplet into the shape of a thin metal film.24-25 Upon removal of the top substrate, a large and continuous ultrathin gallium oxide film which reached lateral dimensions exceeding several centimeters (Figure S1) was revealed. The small remaining liquid metal microdroplets were removed using a solvent-assisted mechanical cleaning protocol (Figure S3). The Ga2O3 sheet remained attached to the SiO2 surface due to strong van der Waals forces.22

Substrates featuring high surface roughness were found to be not conducive to the process (Figure S2). Atomic force microscopy (AFM) was conducted on the deposited Ga2O3 sheets which revealed that the oxide is continuous atomically-flat nanosheet with a thickness of ~1.4 nm (Figure S1), which is slightly thinner than sheets isolated through previously published liquid metal printing techniques.4, 21 Isolated oxide sheets were then converted into GaN using a high temperature ammonolysis reaction (Figure 1b). Urea was used as an ammonia precursor, with the reaction occurring at 800 °C (Figure S3). Synthesis at lower temperatures was found to not successfully convert Ga2O3 into wurtzite GaN (Figure S4 and S5). Optical micrographs of the final product (synthesized at 800 °C) revealed that the nanosheet retained its original morphology, while the color contrast had changed, indicating a change in reflective index of the material (Figure 1).26-27 AFM analysis revealed a sheet thickness of ~1.3 nm (Figure 2a), which corresponds to three wurtzite GaN unit cells. Atomic resolution AFM imaging (insets) identified a repeated crystal pattern, across the substrate, with a lattice constant of 5.18 Å which corresponds to a wurtzite GaN nanosheet that grew along the (001) direction.28 The 2D nanosheet was found to be highly crystalline.


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To gain further insight into the atomic structure of the synthesized nanosheets, high resolution transmission electron microscopy (HRTEM) was conducted. Gallium oxide was directly deposited onto a Si3N4 TEM grid using a previously published van der Waals exfoliation method4, 22, with the sample being subjected to the ammonolysis process. Laterally large, ultrathin nanosheets could be found during TEM imaging, with the sheets being highly translucent, indicating the thin nature of the final GaN sample (Figure 2b). Atomic resolution images of the crystal lattice and the associated fast Fourier transform (FFT) analysis also indicate high crystallinity of the synthesized nanosheets with the lattice parameters matching wurtzite GaN (Figure 2b insets). Observation of the (002) plane in the HRTEM images is consistent with a sheet growth along the (001) direction.28-29 In order to ensure that the TEM samples of GaN produced with the reported van der Waals exfoliation are comparable to the squeeze transferred samples, a detailed analysis of the GaN nanosheet synthesized using the previous method4 was conducted. Both methods lead to indistinguishable products (Figure S6).

Figure 2. Morphology and structural characterization of 2D GaN. (a) AFM topography of the nanosheet with height profile along the blue line. The top inset shows the highresolution AFM image of the sheet, the FFT pattern is displayed in the lower inset. (b) TEM micrograph of the GaN film. The insets represent the lattice fringes (top inset) and the FFT pattern (lower inset).

To confirm that the process results in a phase pure product, a sample of Ga2O3 nanoplatelets30 was further utilized as a GaN precursor. The powder XRD of these GaN nanoplatelets closely resembled the XRD pattern of wurtzite GaN with no impurity phases (detailed results are provided in Figure S8). To ascertain that a quantitative conversion of Ga2O3 to GaN occurred, X-ray photoelectron spectroscopy (XPS) was utilized (Figure 3a, b). The Ga 2p and N 1s peak locations are indicative for GaN,31 and no peaks associated with elemental Ga and Ga2O3 were observed.22 Interestingly, the XPS valence band spectrum featured a high density of states in the region between 8 and 12 eV (Figure 3c). This feature indicates that a substantial fraction of the nitrogen sites has been substituted by oxygen atoms.32-33 In ambient air, oxygen can occupy nitrogen sites in the GaN lattice due to the similar bond length of the Ga-N and Ga-O bonds, without causing detectable distortions of the wurtzite lattice, rendering the XRD patterns and HRTEM images indistinguishable.32, 34 Higher synthesis temperatures (up to 950°C) were explored in order to investigate if the degree of oxygen substitution may be decreased. Higher synthesis temperatures were found to lead to minor decreases in oxygen substitution; however, the complete removal of oxygen could not be achieved. The optical properties (band gap) of the 2D GaN sheets synthesized at higher temperatures were similar to the properties of samples synthesized at 800°C (see Supplementary note 1 and Figure S9-S10). Due to the ultimately thin nature of the synthesized GaN nanosheets, this surface substitution is expected to have significant effects on the optical and electronic properties. Tauc plots obtained during UV-vis analysis (Figure 3d), revealed that the bandgap of the 2D nanosheet is ~ 3.5 eV, which is considerably lower than that of previously reported graphene encapsulated 2D GaN nanosheets.35 This lower bandgap may be expected since oxygen substitution in wurtzite GaN samples is well known to lead to an upward shift of the valence band due to O2p-N2p orbital hybridization.36 The 2D GaN samples exhibit considerable signs of quantum confinement,37-38 since the determined bandgap of the synthesized nanosheets was wider than that of published bulk morphologies of oxygen doped GaN (bandgap between 2.5 and 3 eV), while also being wider than the aforementioned GaN nanoplatelets, which featured an increased sheet thickness of around 8 nm and an optical bandgap of ~ 3.2 eV (Figure S8). The density of states for the synthesized 2D GaN nanosheet was obtained using density functional theory calculations to assess the effect of quantum confinement and oxygen substitution on the electronic properties of the prepared 2D GaN (see Supplementary note 2 and Figure S11). The DFT revealed that oxygen substitution leads to a reduction of the gap, while



quantum confinement results in a widening of the bandgap.

Figure 3. Material characterization and electronic band properties of 2D GaN. (a-b) XPS results of the 2D GaN for the regions of interest, (a) Ga 2p and (b) N 1s. The characteristic doublets for Ga 2p region, 2p1/2 and 2p3/2, are positioned at ~1144.8 and ~1118.1 eV. The broad N 1s peak centered at ~397.6 eV corresponds to the expected binding energy for nitrogen in GaN. (c) XPS valence band spectrum affirming an energy difference of ~2.7 eV between the valence band maximum (VBmax) and Fermi level (EF). The spectrum was deconvoluted into three peaks associated with different bonds. (d) Tauc plot used for ascertaining the electronic band gap for GaN with simplified electronic band diagram.

The room temperature carrier mobility for GaN nanosheets grown at 800 °C was determined using Halleffect measurements (Supplementary note 3 and Figure S12). The observed carrier mobility was 21.5 cm2V-1s-1 which is comparable to large area CVD grown MoS2.3940 As such, the developed synthesis technique and the isolated 2D GaN should be considered suitable for the development of optoelectronic and electronic 2D devices. In order to assess the versatility of the synthesis method, an adapted synthesis process utilizing liquid indium instead of liquid gallium was explored. InN is a semiconductor with interesting electronic and optical properties that may find applications in laser diodes, light emitting diodes and several further optoelectronic devices.41 InN is less stable than GaN and undergoes decomposition to In and N at elevated temperatures (above 650°C),42-43 while also undergoing morphological transformations following the vapor-solid mechanism. Due to these limitations, direct conversion of 2D In2O3 to

2D InN was found to be not practical, and an intermediate bromination step was utilized that effectively converted In2O3 into the more reactive InBr3 through the exposure of the 2D In2O3 nanosheet to HBr vapor (Figure S13).21 Ammonolysis was then successfully carried out utilizing urea as the NH3 source at a temperature of ~630 °C. XPS analysis of the product confirmed the successful synthesis of InN as shown in Figure S14, with an AFM determined thickness of ~2 nm. High resolution AFM analysis was utilized to confirm the crystallographic properties of the final product over HRTEM imaging, since the necessary bromination step rendered the 2D sheet to be too fragile to sustain sheet integrity during synthesis. A crystal lattice with a constant of 5.5 Å was determined, which is in excellent agreement with the structure of wurtzite InN (Figure S14).43-44 In conclusion, a liquid metal-based van der Waals squeeze-printing technique exploiting Cabrera-Mott oxidation processes was reported that allowed depositing centimeter sized 2D gallium and indium oxide sheets. These sheets could then be converted into highly crystalline 2D Wurtzite GaN and InN, using a urea based ammonolysis reaction. The 2D GaN sheet was found to be oxygen doped leading to a narrower than expected bandgap, while featuring a high carrier mobility of 21.5 cm2V-1s-1. This work provides a pathway towards integrating 2D morphologies of well-known high performance semiconductors into emerging 2D material based electronic and optoelectronic devices and heterostructures using a liquid metal based techniques. ASSOCIATED CONTENT Supporting Information Full experimental details, data associated with the InN and GaN nanoplatelets synthesis as well as supplementary notes and supplementary figures 1-14 are located in the supporting information. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected] [email protected] Author Contributions §N.S. and A. Z. contributed equally.

Funding Sources The authors would also like to thank the Australian Research Council (ARC) that supported this project under The ARC center of excellence FLEET - CE170100039, the Discovery Project DP180102752 and Laureate Fellowship - FL180100053. N. S. and A.Z acknowledge funding from through the Australian Government Research Training Program Scholarship scheme. D.E thanks ViceChancellor's Research Fellowship funding at RMIT University. S.P.R


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Journal of the American Chemical Society acknowledges the support by computational resources provided by the Australian Government through the National Computational Infrastructure National Facility and the Pawsey Supercomputer Centre.

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