Solution-Based, Template-Assisted Realization of Large-Scale

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Solution-Based, Template-Assisted Realization of Large-Scale Graphitic ZnO Kyle B. Tom,†,‡ Shuren Lin,†,‡ Liwen F. Wan,§ Jie Wang,⊥ Nolan Ahlm,†,‡ Alpha T. N’Diaye,∥ Karen Bustillo,# Junwei Huang,∇ Yin Liu,†,‡ Shuai Lou,†,‡ Rui Chen,†,‡ Shancheng Yan,⊗ Hui Wu,= Dafei Jin,⊥ Hongtao Yuan,∇ David Prendergast,§ and Jie Yao*,†,‡ †

Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Center for Nanoscale Materials, Nanoscience and Technology, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States ∥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States # National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ∇ National Laboratory of Solid-State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China ⊗ Nanjing University of Posts and Telecommunications, Nanjing 210023, P. R. China = State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China

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‡

S Supporting Information *

ABSTRACT: With a honeycomb single-atomic-layer structure similar to those of graphene and hexagonal boron nitride (hBN), the graphitic phase of ZnO (gZnO) have been predicted to offer many advantages for engineering, including high-temperature stability in ambient conditions and great potential in heterostructure applications. However, there is little experimental data about this hexagonal phase due to the difficulty of synthesizing large-area gZnO for characterization and applications. In this work, we demonstrate a solution-based approach to realize gZnO nanoflakes with thicknesses down to a monolayer and sizes up to 20 μm. X-ray photoelectron spectroscopy, X-ray absorption near-edge spectroscopy, photoluminescence, atomic force microscopy, and electron microscopy characterizations are conducted on synthesized gZnO samples. Measurements show significant changes to the electronic band structure compared to its bulk phase, including an increase of the band gap to 4.8 eV. The gZnO nanosheets also exhibit excellent stability at temperatures as high as 800 °C in ambient environment. This wide band gap layered material provides us with a platform for harsh environment electronic devices, deep ultraviolet optical applications, and a practical alternative for hBN. Our synthesis method may also be applied to achieve other types of 2D oxides. KEYWORDS: materials science, nanotechnology, semiconductors, synthesis, solids, crystals field-effect tunneling transistors and the observation of the quantum Hall effect.2 Of the single atomic layer family of materials, most of the systems are mostly or entirely covalent, including graphene, which prevents them from showing the advantageous characteristics typical of highly ionic solids.

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ingle atomic layer materials, such as the widely investigated graphene and hexagonal boron nitride (hBN), have been the subjects of an incredible number of studies, revealing revolutionary physics and intriguing device designs. This is due to their many favorable and interesting properties, such as band gap tuning via thickness variations, the observation of exotic quantum effects, and strong exciton behavior.1 Additionally, multilayer structures of these materials have been used for interesting physics and applications, such as © XXXX American Chemical Society

Received: May 21, 2018 Accepted: July 16, 2018 Published: July 16, 2018 A

DOI: 10.1021/acsnano.8b03835 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Structure and AFM characterization of gZnO nanosheets. Crystal structure of (A) wZnO and (B) gZnO. (C) SEM image of a monolayer gZnO nanosheet on Si (scale bar 1 μm). (D) AFM image of the monolayer nanosheet (scale bar 1 μm). (E) Corresponding linescan of the dashed white line in (D). Optical image of the (F) ZnO/rGO composite and (G) ZnO nanosheets (scale bar 3 μm). Yellow lines indicate the outline of the composite shown in (F). (H) AFM image of the dashed box area in (F) and (G) (scale bar 3 μm). (I) Corresponding linescan of the dashed white line in (H).

lack of data is primarily due to the difficulty in synthesis. Even with the utilization of high vacuum and specialized setups, large, free-standing samples have not been achieved, prohibiting the characterization and application of this material.12,13 As an alternative, template-based synthesis has been shown to create nanostructures of ZnO and other materials with well-controlled morphologies; this provides inspiration for forming 2D sheets in a comparable way.14−16 In this paper, we demonstrate the solution-based synthesis of uniform, free-standing gZnO with lateral sizes up to 20 μm using a graphene oxide (GO) template method, and we present the measurements of intrinsic properties. This synthesis approach allows gZnO sheets to be placed on a variety of nonmetal substrates, which allows a wide range of characterizations and enables potential device applications. Through X-ray absorption spectroscopy (XAS) measurements and X-ray photoemission spectroscopy (XPS), we show significant changes in the bonding and electronic structure of the gZnO relative to its wurtzite phase. Additionally, we show the stability of the ultrathin samples at elevated temperatures, which indicates that gZnO may be a great platform for hightemperature applications in ambient conditions.

Additionally, many layered materials are unstable in ambient or are susceptible to oxidation, particularly at elevated temperatures.2 Crystals with stronger ionicity, particularly at the nanoscale, often hold many interesting properties that are particular to ionic systems. This includes wide band gaps for high-power applications, high-temperature stability, piezoelectricity, low thermal conductivity, enhanced electron− phonon scattering,3 and strong polaron interactions.4 ZnO is a widely studied oxide due to its favorable properties for electronics, optics, and electromechanics, and it normally crystallizes in a three-dimensional (3D) wurtzite crystal (wZnO, Figure 1A). However, below 1 nm in thickness, ZnO will adopt a graphitic layered phase (graphitic ZnO or gZnO, Figure 1B)5 that is stable in ambient conditions. gZnO is more ionic than graphene, h-BN, silicene, or germanene, which may lead to drastically different physical and chemical properties compared to other atomic layer materials. This phase has been predicted to have optoelectronic applications in the deep UV range, at wavelengths shorter than its bulk phase, as well as favorable properties for thermovoltaics,6 hydrogen storage,7 magnetism,8 and catalysis.9 This material is also inherently oxidation resistant at high temperatures, something that is not currently possible for conventional 2D materials.2 Additionally, the wide band gap nature of gZnO makes it an excellent alternative of hBN. For example, first-principles calculations predict that the electron mobility of graphene, when forming heterostructures with gZnO, can reach a level comparable to that achieved with freestanding graphene.10 This material may also offer additional engineering opportunities in the recently reported quantum-state control via 2D superlattice formation.11 From a materials perspective, it can act as a model system for understanding how the 3D-to-2D transformation can change the properties of a material. While this hexagonal gZnO phase may have many advantages, it is currently unknown how this phase transformation from wZnO will change the properties of ZnO, making it difficult to utilize this material in applications. This

RESULTS Synthesis and AFM Verification of Thickness. The general growth procedure of the gZnO nanosheets is summarized in the Methods. In brief, GO sheets serving as a template are placed on a substrate or suspended on a holey electron microscopy grid and intercalated with a Zn acetate dihydrate aqueous solution. After intercalation, the samples are subjected to a high-temperature treatment (500 °C, argon atmosphere) in order to form the crystals under confinement, with the required oxygen coming from the GO. The template is then removed by high-temperature annealing in air (625 °C, air calcination). Figure 1C,D shows SEM and AFM images of a monolayer ZnO sample on a Si wafer at the end of synthesis, B


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by fabricating lamella of a thick composite with a focused ion beam (FIB) process (Figure 2B). Due to the waviness of both 2D ZnO and graphene layers in our sample, we were only able to see locally, but not perfectly, aligned zinc and carbon atoms. The STEM image, and corresponding intensity profile, shows clearly two-layer and three-layer ZnO crystals between layers of rGO, which confirms the crystallinity of sub-nm sheets. The interlayer spacing of the ZnO is 0.25 nm, which is slightly less that the spacing for wZnO (0.26 nm) and within the values for gZnO (0.23−0.25 nm).20 The image also shows that the number of layers between ZnO and rGO is irregular, ranging from 1 to 4 layers. To further confirm the graphitic phase, we pursue a spectroscopic approach to verify the structural change. To obtain high-precision information regarding the bonding configuration of these ultrathin samples, we utilized X-ray absorption near-edge spectroscopy (XANES) measurements at the Zn L3 edge (Figure 3A). Measurements were performed on a dispersion of mostly sub-nm samples on a Si/SiO2 substrate, as well as bulk ZnO and a composite of the alternating ZnO/rGO structure, where the rGO keeps the ZnO constrained as gZnO. The bulk ZnO spectrum matches other reports of XANES on wurtzite ZnO.21 In contrast, the composite and ultrathin ZnO show a markedly different spectrum, with a pronounced pre-edge peak and a shift to higher energies. There is no previous XANES measurement of gZnO or similar structures for Zn, so we cannot directly compare our spectra to a standard. As such, we turn to performing XANES simulations (details in Methods). A bilayer of planar gZnO sheet (3×3 unit cell) was initially sandwiched between two graphene sheets, and after structure optimization, the gZnO sheet is no longer perfectly planar, instead it becomes rippled such that almost half of the Zn atoms deviate from trigonal planar coordination as in the perfect planar gZnO to form tetrahedral coordination with 3 in-plane oxygen atoms and 1 out-of-plane oxygen, as shown in the Figure 3B inset. The computed spectrum of bulk wZnO (black) and gZnO/graphene (red) are presented in Figure 3B, along with the experimental spectrum collected for reference wZnO (green) and nanosheet gZnO/rGO (blue). For bulk wZnO, the simulated spectrum match reasonably well with the experiment, given that the no thermal fluctuations or structure variations are currently included in the simulated spectrum that is based on a 0 K ground-state structure. The simulated spectrum of the gZnO/Graphene structure, based on the density functional excited core-hole approach,22 shows a strong pre-edge feature with s-state characteristics, similar to that of the experimental data, (Figure 3A), which agrees well with previous spectroscopic analysis of wZnO and amorphous ZnO.21 The enhanced pre-edge feature in gZnO is due to the broken Zn−O bond on the surface that leads to a change of local Zn coordination from tetrahedral into a pseudo-trigonal planar environment. This phenomenon has also been observed in amorphous ZnO, as discussed in ref 21. which explains that the growth of the Zn 4s pre-edge feature is attributed to stronger structural disorder in amorphous ZnO than in wZnO. Large discrepancies of the Zn L-edge spectra are observed for the gZnO/graphene composite, where the pre-edge feature is red-shifted in the simulated spectrum compared to the experimental data. This is likely due to the semi-local generalized gradient approximation used in the simulation that fails to correctly predict the relative positions of Zn 4s and 4d states at the bottom of conduction bands. A number of simple Hubbard

showing excellent uniformity. The linescan, Figure 1E, shows a measured thickness of about 2.8 Å, which is within the range of measured monolayer heights seen in literature.17,18 This value may be slightly larger than the true value due to the tapping mode AFM offsets and effects from a different substrate as is the case for graphene.19 Figure 1F,G shows optical images of the rGO/ZnO composite and of the ZnO after the air bake. After burning in air, thicker areas of the sample are still visible; however, thinner parts (below 2 nm in thickness) become invisible in an optical microscope. This is quite different from other 2D materials and can be explained by differences in the refractive indices (see Figure S1). Figure 1H shows an AFM image of the boxed area in Figure 1G, showing that the ZnO nanosheets possess a layered structure well defined by the ZnO/rGO template seen in Figure 1G. This indicates that the sheets form due to the intercalation of the zinc precursor in between GO sheets, and that thicker templates will create thicker ZnO sheets. Figure 1I shows the step-like profile of the line scan along the dashed line in Figure 1H. From the linescan, the thickness of the thin piece (1st step) is about 5.3 Å, which corresponds closely to the thickness of bilayer gZnO (the interlayer distance is about 2.4 Å).20 The AFM image also clearly shows a second bilayer, which shows a very clear indication of a flat, layered structure of gZnO sheets. A trilayer sample with a thickness of 8 Å and a lateral size of nearly 20 μm is also observed and shown in Figure S2. Structural Characterization of gZnO Nanosheets and Composites. To confirm the crystallinity of these sheets, we grew composite samples directly onto holey carbon transmission electron microscopy (TEM) grids to perform TEM measurements. Figure 2A shows a nanobeam electron

Figure 2. TEM imaging of ZnO nanosheets. (A) Nanobeam electron diffraction pattern of the composite structure showing clear features from both rGO and ZnO (scale bar 5 nm-1). (B) HRSTEM image of a composite of ZnO and rGO with the corresponding intensity profile. The layers of both are clearly resolved. The interlayer spacing of rGO is close to the interlayer spacing of graphite and about half that of GO.

diffraction (NBED) pattern of a plan view composite structure suspended over a hole in the grid. The pattern shows a primarily single crystal GO with randomly oriented polycrystalline ZnO features, showing only diffraction from the [0002] zone axis (additional images found in Figure S3). From the diffraction pattern, the d spacing of the ZnO was determined to be around 2.85 ± 0.08 Å. This value is much closer to the spacing of the {101̅0} gZnO (2.86 Å) than that of wZnO (2.81 Å). It is also evident that the gZnO crystals are growing in between the sheets of GO. We also obtained side view images C


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Figure 3. Optoelectronic properties of ultrathin gZnO. (A) XANES measurements of bulk and nanosheet ZnO and a composite structure. (B) Simulated Zn L-edge XAS spectra. The experimental spectra are reproduced from (A). Insets: Top and side view of a representative excitation state at ∼1020 eV (indicated by red arrow) of bilayer gZnO that is sandwiched between two graphene sheets. A clear Zn 4s characteristic that is weakly coupled with the nearby O 2p state is observed in the excited-state plot. (C) Photoluminescence spectra of ZnO sheets of varying thickness. (D) XPS scan of the Zn 2p peak of a dispersion of mostly sub-nm gZnO nanosheets, bulk ZnO, and an rGO/ gZnO composite. (E) Valence band XPS measurements on bulk ZnO, nanosheet ZnO, composite structures, rGO and SiO2. (F) Band alignment of wZnO and gZnO relative to vacuum. Conduction band = Red, Valence Band = Blue.

X-ray-based measurements. We measured the photoluminescence from the ZnO nanosheets as a function of thickness to observe behavioral changes as a result of a different phase. Figure 3C shows the PL spectra of the ZnO nanosheets as a function of thickness. Above 1 nm, the PL signal is strong and is located close to the bulk band gap of ZnO at 3.27 eV. As the thickness decreases, the PL signal blue shifts due to quantum confinement. The intensity drops with thickness, primarily as a result of a decreased absorption and emission. At 1 nm the signal is very weak and has blue-shifted further. However, below 1 nm, the photoluminescence signal has completely disappeared. This may imply a very large band gap increase below 1 nm, presumably larger than the 266 nm excitation energy of our laser. In order to determine the band gap of the nanosheets, we turn to X-ray-based techniques. From the XANES spectra in Figure 3A, the peak at 1222 eV indicates that the 4s states of this phase are more available than those found in the wurtzite ZnO. The shift of the onset of absorption also indicates an upward shift of the conduction band, as it is formed by the Zn 4s states for both gZnO and wZnO.25,26 As the spectrum indicates transitions from the Zn 2p to Zn 4s states, the core

U models have been applied to correct the band alignment in ZnO, and if we pick a set of U parameters (UZn,4d = 12.8 and UO,2p = 5.29) as recommended in ref 23, the pre-edge feature indicated by the red arrow in Figure 3B will be blue-shifted by ∼0.5 eV. A similar U model has even been proposed for the Zn 4s state.24 Ideally, more sophisticated hybrid functional or advanced excited-state theory should be used to better predict the position of Zn 4s and 4d under photon excitation, which is currently under investigation and is beyond the scope of this paper. However, the pre-edge feature does indicate the formation of gZnO, confirming that our method synthesizes gZnO. The ZnO/rGO composite and ZnO/SiO2/Si nanosheets show almost identical spectroscopic signatures, which further verifies the formation of gZnO in the composite, agreeing with the nanobeam electron diffraction images above. We expect that the GO has a negligible effect on the properties of the gZnO. We then turn to investigating the intrinsic properties. Band Structure Characterization of gZnO Nanosheets and Composites. The large-scale gZnO sheets provide an excellent platform for their optical property investigations and allows us to confirm the behavior of our D


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Figure 4. Annealing of ZnO sheets of different thicknesses. SEM images of (A) as synthesized nanosheets and (B) nanosheets annealed at 800 °C with corresponding roughness values. (C) and (D) correspond to linescans shown in (A) and (B), respectively. Lettered regions indicate areas of different thicknesses: A = 6 nm, B = 1.5 nm, C = 0.54 nm, sub = Si substrate. Scale bar is 2 μm.

measurements of composite samples, shown in Figure S5, shows a value similar to those determined by the X-ray-based techniques. High-Temperature Annealing Behavior. Due to its chemical stability and oxidation resistance, gZnO shows great potential for applications at high temperatures in ambient or oxidative environments. We proceeded to test the hightemperature behavior of ZnO sheets of different thicknesses. The samples are imaged using AFM and SEM before and after a 1 hanneal in air at 800 °C. SEM images and RMS roughness values, shown superimposed on the corresponding sample, before and after the 800 °C annealing are shown in Figure 4A,B. From the SEM images, it is clear that after the 800 °C anneal, area A and, more noticeably, area B show an increase in the roughness and a coarsening to larger particles. The streaks are caused by a charging effect that was enhanced due to the coarsening effect, i.e., it breaks the continuous film and increases the thickness of low-conductivity ZnO. RMS values from AFM measurements confirm a significant change in the roughness. On the other hand, the thin sheet, area C, remains flat and thin, and is comparable to the substrate roughness. The sub-nm thickness and low roughness are confirmed in the linescans shown in Figure 4C,D (topographical images of the sub-nm sheet are found in Figure S6). It should be noted that the slight decrease in the substrate and area C RMS values with annealing temperature may be a result of high-temperature smoothing of the surface, such as the case seen in silicon wafers.33 We believe that the different experimentally observed roughening (or lack of) behaviors are due to the surface energy differences between gZnO and its bulk counterpart. Due to its layered structure, the {0001} surface has a very low surface energy, and it is unlikely to undergo coarsening or ripening to reduce the surface energy.34 The wurtzite phase, however, has a high surface energy from unfulfilled bonds and a large dipole energy, so the nanosheets will ripen and coarsen to thicker nanoparticles at high temperature, which has been observed

level shifts must be taken into account. From X-ray photoelectron spectroscopy (XPS) measurements (Figure 3D) the Zn 2p core binding energy increases approximately 0.5 eV from bulk to nanosheet, either as a result of a quantum size effect27 or from an increase in the ionicity of the bonding. Both composite and nanosheet structures show the same shift, once again indicating the rGO has little effect on the gZnO. From the XANES data, the Zn 4s states shift up approximately 1.2 eV, determined from the crossing of the linear region of the spectra with the x axis and, alternatively, using the shift in the first inflection point of the low-energy part of the curve.28 Both methods agree within 0.2 eV. After correcting for the core level shifts, the actual CB shift is about 0.7 eV to higher energies. Valence band XPS measurements were performed on a dispersion of primarily sub-nm samples to determine the valence band (VB) positions. As can be seen from Figure 3E, the ultrathin sheets and composite show a 0.8 eV shift to higher energies, determined from the crossing of the linear region of the spectra with the x-axis, when compared to the wZnO. They also differ quite significantly from the SiO2 substrate and rGO VB shapes, indicating that the signal does not come from the substrate or rGO. The conduction band and valence band locations of bulk wZnO and ultrathin gZnO, using known locations of the wZnO bands,29 are compared in Figure 3F. From the band locations, the band gap of ultrathin gZnO is found to be approximately 4.8 eV. This is significantly higher than bulk wZnO (∼3.3 eV) and greatly differs from reported rGO/ZnO composites that have shown emission at 3.3 eV30 and 2.95 eV.31 This substantial change is consistent with theoretical predictions of the ultrathin gZnO band gap energy,32 as well as our transport measurement results (Figure S4). The increased band gap, together with a lattice structure similar to graphene and hBN, makes gZnO an intriguing alternative to hBN in various applications, such as 2D heterostructures. Additional low-loss electron energy loss spectroscopy (low-loss EELS) E


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ACS Nano and reported previously for wurtzite ZnO films.35 A schematic for these two differences are shown in Figure S7. This hightemperature robustness of gZnO is very different from most 2D materials, such as graphene or transition metal chalcogenides, that tend to oxidize and/or decompose at elevated temperatures in ambient conditions.2 This allows gZnO to be a promising 2D candidate for high-temperature ambient environment applications. As mentioned previously, initial work has demonstrated the successful fabrication of a simple gZnO device showing typical wide band gap behavior (Figure S4). This technique also can be used to dope gZnO sheets, as can be seen in the incorporation of Al in a composite structure shown in Figure S8, for further functionalization and improvement of electrical properties.

Supermarket). After, the solution is diluted further, typically to a concentration of 0.03 mg/mL. Then, the solution is dropcast onto a silicon or thermal oxide wafer chip and dried on a hot plate for at least 1 h at 80 °C. The chip is placed in a precursor solution of Zn acetate dihydrate (99.999% pure, Sigma-Aldrich) mixed with water at 10 wt% for at least 8 h. After that, the templates are rinsed in DI water and dried using nitrogen and then in a vacuum oven at 80 °C for at least 1 h. The samples are then baked in argon in order to form the crystals without loss of the GO template. The samples are baked in argon in a tube furnace at 500 °C with a ramp rate of 5 °C/min, and then are cooled down naturally. This is a sufficiently high temperature for crystal formation detectable by NBED. This forms the rGO/ZnO composite. After, the GO is calcined in a tube furnace at 625 °C with a ramp rate of 5 °C/min in an air environment. High-temperature annealing was performed with a ramp rate of 5 °C/min in an air environment up to 800 °C for 1 h. Material Characterization. AFM measurements were performed in tapping mode using a Veeco D3100 atomic force microscope. SEM images were taken with a FEI Quanta 3D FEG with a secondary electron detector. Plan view TEM Imaging and nanobeam diffraction was performed at the Molecular Foundry at the National Center for Electron Microscopy using a FEI TitanX 60-300 microscope at 60 kV. Side view lamella were fabricated at Argonne National Laboratory using FEI Nova 600 NanoLab focused ion beam system at 30 and 5 kV. The HRSTEM images were performed on a FEI Talos F200X TEM/STEM at 200 kV. XAS measurements have been performed at beamline 6.3.1 at the Advanced Light Source. Spectra are recorded at room temperature in total electron yield mode with a 30° grazing incidence angle between X-ray beam and sample surface. Micro-PL measurements were performed using a custom-built setup using 266 nm excitation from a Coherent Chameleon femtosecond laser and tripler with a spot size of about 10 μm. XPS measurements were obtained from a PHI 5600 XPS using a 2 mm Al monochromatic source at 15 kV and 350 W with a neutralizer with a spot size of 400 μm. EELS measurements were performed at the National Center for Electron Microscopy using a 200 kV FEI monochromated F20 UT Tecnai instrument. XAS Simulations. The structure relaxations were performed using the VASP software package39,40 and the X-ray absorption spectra were computed using the PWscf code, distributed as part of the Quantum espresso source code package.41 Both of the structure relaxation and X-ray absorption simulations were performed using plane waves to represent the electronic wave function, and generalized gradient approximation to approximate the exchange-correlation functional.42 PAW type potentials were used in the structure relaxation in VASP43,44 and ultrasoft pseudopotentials are used in the PWscf X-ray absorption calculations.45 To simulate the Zn L-edge spectra, a modified Zn pseudopotential was used with a removed core−electron from the Zn 2p state.

CONCLUSIONS In conclusion, as a more ionic counterpart to graphene, gZnO possesses advantageous material properties that introduces an additional platform into the family of graphene-like 2D materials and their applications. In this article we demonstrate the successful synthesis of large-area graphitic ZnO nanosheets. The sub-nm step structure, XANES measurements, and hightemperature stability all indicate the transition of ZnO from the wurtzite phase to the graphene-like phase as the thickness decreases. From X-ray-based measurements, we see significant changes in the bonding characteristics and electronic structure that are attributed to the graphitic phase. This facile synthesis method of large-area gZnO independent of substrate facilitates characterization by various optical, electrical, and other techniques. Sub-nm sheets also show a higher morphological stability than thicker wurtzite sheets at high temperature. This material expands the parameter space of 2D materials and adds some additional benefits, including enhanced harsh condition stability, chemical/biological compatibility, and a wider range of band alignments for electronic devices. Our synthesis method also inherently creates high-quality heterostructures between gZnO and graphene, which offers a platform for studying gZnO heterostructures, and can be expanded further by mechanically stacking other 2D materials. This may provide additional engineering opportunities for multilayer structure applications such as twistronics. Also, using the gZnO as a model system, we can better predict how 3D-to-2D transitions affect properties of a material. Additionally, this method of synthesis may be extended to other oxide materials and help realize the 2D phases of a large variety of conventional materials, some of which have already been predicted. Currently, this technique is limited to oxides due to the relatively high oxygen concentration atmosphere caused by the reducing GO. However, the use of the gaps in GO could be used for more general nanosheet synthesis. This could further improve the parameter space of non-layered nanosheets. The development of various additional synthesis methods, such as the template-based approach, liquid exfoliation approach,36,37 and other non-van der Waals exfoliation approaches,38 will enhance our capability of achieving low-dimensional morphologies of various materials both within and outside the layered material family.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b03835. Figures S1−S8, showing the low contrast of a sub-nm ZnO sheet on a 300 nm SiO2Si wafer, AFM images of an 18 μm three-layer gZnO nanosheet, STEM image of the plan-view composite structure and the corresponding NBED pattern, electronic transport of an ionic liquid gating transistor based on gZnO, low-loss EELS spectrum of a gZnO/rGO composite, topographical images of sub-nm ZnO before and after 800 °C anneal and a schematic depicting the coarsening effect in regards to surface energy, and HAADF and EDX chemical mapping of an Al-doped gZnO composite (PDF)

METHODS Materials Synthesis. To synthesize the gZnO nanosheets, first, GO powder (ACS materials) is placed in water at a ratio of 3 mg/mL. The solution is then sonicated for at least 15 min. Alternatively, commercially available graphene oxide solutions were used (Graphene F


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(8) Ren, J.; Zhang, H.; Cheng, X. Electronic and Magnetic Properties of All 3d Transition-metal-doped ZnO Monolayers. Int. J. Quantum Chem. 2013, 113, 2243−2250. (9) Ma, D.; Wang, Q.; Li, T.; Tang, Z.; Yang, G.; He, C.; Lu, Z. CO Catalytic Oxidation on Al-Doped Graphene-like ZnO Monolayer Sheets: A First-Principles Study. J. Mater. Chem. C 2015, 3, 9964− 9972. (10) Hu, W.; Li, Z.; Yang, J. Electronic and Optical Properties of Graphene and Graphitic ZnO Nanocomposite Structures. J. Chem. Phys. 2013, 138, 124706. (11) Cao, Y.; Fatemi, V.; Fang, S.; Watanabe, K.; Taniguchi, T.; Kaxiras, E.; Jarillo-Herrero, P. Unconventional Superconductivity in Magic-Angle Graphene Superlattices. Nature 2018, 556, 43. (12) Ta, H. Q.; Zhao, L.; Pohl, D.; Pang, J.; Trzebicka, B.; Rellinghaus, B.; Pribat, D.; Gemming, T.; Liu, Z.; Bachmatiuk, A.; Rummeli, M. Graphene-Like ZnO: A Mini Review. Crystals 2016, 6, 100. (13) Quang, H. T.; Bachmatiuk, A.; Dianat, A.; Ortmann, F.; Zhao, J.; Warner, J. H.; Eckert, J.; Cunniberti, G.; Rümmeli, M. H. In Situ Observations of Free-Standing Graphene-like Mono-and Bilayer ZnO Membranes. ACS Nano 2015, 9, 11408−11413. (14) Thurn-Albrecht, T.; Schotter, J.; Kästle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Ultrahigh-Density Nanowire Arrays Grown in Self-Assembled Diblock Copolymer Templates. Science 2000, 290, 2126−2129. (15) Liu, C.; Zapien, J. A.; Yao, Y.; Meng, X. M.; Lee, C. S.; Fan, S. S.; Lifshitz, Y.; Lee, S. T. High-Density, Ordered Ultraviolet Lightemitting ZnO Nanowire Arrays. Adv. Mater. 2003, 15, 838−841. (16) Saito, Y.; Luo, X.; Zhao, C.; Pan, W.; Chen, C.; Gong, J.; Matsumoto, H.; Yao, J.; Wu, H. Filling the Gaps between Graphene Oxide: A General Strategy toward Nanolayered Oxides. Adv. Funct. Mater. 2015, 25, 5683−5690. (17) Martynova, Y.; Liu, B.-H.; McBriarty, M. E.; Groot, I. M. N.; Bedzyk, M. J.; Shaikhutdinov, S.; Freund, H.-J. CO Oxidation over ZnO Films on Pt (1 1 1) at near-Atmospheric Pressures. J. Catal. 2013, 301, 227−232. (18) Deng, X.; Yao, K.; Sun, K.; Li, W.-X.; Lee, J.; Matranga, C. Growth of Single-and Bilayer ZnO on Au (111) and Interaction with Copper. J. Phys. Chem. C 2013, 117, 11211−11218. (19) Nemes-Incze, P.; Osváth, Z.; Kamarás, K.; Biró, L. P. Anomalies in Thickness Measurements of Graphene and Few Layer Graphite Crystals by Tapping Mode Atomic Force Microscopy. Carbon 2008, 46, 1435−1442. (20) Tusche, C.; Meyerheim, H. L.; Kirschner, J. Observation of Depolarized ZnO (0001) Monolayers: Formation of Unreconstructed Planar Sheets. Phys. Rev. Lett. 2007, 99, 026102. (21) Cho, D.-Y.; Kim, J. H.; Na, K. D.; Song, J.; Hwang, C. S.; Park, B.-G.; Kim, J.-Y.; Min, C.-H.; Oh, S.-J. Spectroscopic Evidence for Limited Carrier Hopping Interaction in Amorphous ZnO Thin Film. Appl. Phys. Lett. 2009, 95, 261903. (22) Prendergast, D.; Galli, G. X-Ray Absorption Spectra of Water from First Principles Calculations. Phys. Rev. Lett. 2006, 96, 215502. (23) Agapito, L. A.; Curtarolo, S.; Nardelli, M. B. Reformulation of DFT+ U as a Pseudohybrid Hubbard Density Functional for Accelerated Materials Discovery. Phys. Rev. X 2015, 5, 011006. (24) Paudel, T. R.; Lambrecht, W. R. L. First-Principles Calculation of the O Vacancy in ZnO: A Self-Consistent Gap-Corrected Approach. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 205202. (25) Klingshirn, C. ZnO: Material, Physics and Applications. ChemPhysChem 2007, 8, 782−803. (26) Kang, J.; Zhang, Y.; Wen, Y.-H.; Zheng, J.-C.; Zhu, Z.-Z. FirstPrinciples Study on the Structural and Electronic Properties of Ultrathin ZnO Nanofilms. Phys. Lett. A 2010, 374, 1054−1058. (27) Sykes, E. C. H.; Tikhov, M. S.; Lambert, R. M. Quantum Size Effects in Catalysis by TiO 2/Platinum: The Switch from Partial Oxidation to Partial Hydrogenation of Styrene. Catal. Lett. 2002, 82, 169−173.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Liwen F. Wan: 0000-0002-5391-0804 Shuai Lou: 0000-0001-8797-7516 Hui Wu: 0000-0002-4284-5541 Jie Yao: 0000-0003-0557-759X Author Contributions

Original concept was created by K.T. and J.Y.; K.T., R.C., and N.A. performed synthesis of all samples; K.T., N.A., S.L., J.W., Y.L., and A.N. performed structural measurements and characterization; K.T. and A.N. performed the optical and Xray characterization. L.F.W. and D.P. performed the XAS simulations. J.H. and H.Y. performed the electric tuning measurements; Y.L. obtained the low-loss EELS spectrum. Data analysis was performed by K.T., J.Y., J.W., A.N., L.F.W., D.P., J.H., H.Y., S.Y. and H.W. J.Y. supervised the project. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory under U.S. Department of Energy Contract No. DE-AC02-05CH11231. This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. Devices were fabricated in the UC Berkeley Marvell Nanofabrication Laboratory. We thank the Biomolecular Nanotechnology center for access and assistance with measurement systems. We thank Joonki Suh and Maribel Jaquez for useful discussion. We also thank the Dubon Group for assistance with their AFM system. REFERENCES (1) Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; Heinz, T. F.; Hong, S. S.; Huang, J.; Ismach, A. F.; et al. Progress, Challenges, and Opportunities in Two-Dimensional Materials beyond Graphene. ACS Nano 2013, 7, 2898−2926. (2) Geim, A. K.; Grigorieva, I. V. Van Der Waals Heterostructures. Nature 2013, 499, 419. (3) Ma, N.; Tanen, N.; Verma, A.; Guo, Z.; Luo, T.; Xing, H.; Jena, D. Intrinsic Electron Mobility Limits in β-Ga2O3. Appl. Phys. Lett. 2016, 109, 212101. (4) Ipatova, I. P.; Maslov, A. Y.; Proshina, O. V. Polaron in Quantum Nanostructures. Surf. Sci. 2002, 507, 598−602. (5) Freeman, C. L.; Claeyssens, F.; Allan, N. L.; Harding, J. H. Graphitic Nanofilms as Precursors to Wurtzite Films: Theory. Phys. Rev. Lett. 2006, 96, 066102. (6) Li, Y.-L.; Fan, Z.; Zheng, J.-C. Enhanced Thermoelectric Performance in Graphitic ZnO (0001) Nanofilms. J. Appl. Phys. 2013, 113, 083705. (7) Si, H.; Peng, L. J.; Morris, J. R.; Pan, B. C. Theoretical Prediction of Hydrogen Storage on ZnO Sheet. J. Phys. Chem. C 2011, 115, 9053−9058. G


Article

ACS Nano (28) Bostedt, C.; Van Buuren, T.; Willey, T. M.; Franco, N.; Terminello, L. J.; Heske, C.; Möller, T. Strong Quantum-Confinement Effects in the Conduction Band of Germanium Nanocrystals. Appl. Phys. Lett. 2004, 84, 4056−4058. (29) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338. (30) Chen, X.; He, Y.; Zhang, Q.; Li, L.; Hu, D.; Yin, T. Fabrication of Sandwich-Structured ZnO/Reduced Graphite Oxide Composite and Its Photocatalytic Properties. J. Mater. Sci. 2010, 45, 953. (31) Kole, A. K.; Biswas, S.; Tiwary, C. S.; Kumbhakar, P. A Facile Synthesis of Graphene Oxide−ZnS/ZnO Nanocomposites and Observations of Thermal Quenching of Visible Photoluminescence Emission and Nonlinear Optical Properties. J. Lumin. 2016, 179, 211−221. (32) Topsakal, M.; Cahangirov, S.; Bekaroglu, E.; Ciraci, S. FirstPrinciples Study of Zinc Oxide Honeycomb Structures. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 235119. (33) Shive, L. W.; Gilmore, B. L. Impact of Thermal Processing on Silicon Wafer Surface Roughness. ECS Trans. 2008, 16, 401−405. (34) Claeyssens, F.; Freeman, C. L.; Allan, N. L.; Sun, Y.; Ashfold, M. N. R.; Harding, J. H. Growth of ZnO Thin Filmsexperiment and Theory. J. Mater. Chem. 2005, 15, 139−148. (35) Sengupta, J.; Sahoo, R. K.; Bardhan, K. K.; Mukherjee, C. D. Influence of Annealing Temperature on the Structural, Topographical and Optical Properties of Sol−gel Derived ZnO Thin Films. Mater. Lett. 2011, 65, 2572−2574. (36) Yadav, T. P.; Woellner, C. F.; Sinha, S. K.; Sharifi, T.; Apte, A.; Mukhopadhyay, N. K.; Srivastava, O. N.; Vajtai, R.; Galvao, D. S.; Tiwary, C. S.; Ajayan, P. M. Liquid Exfoliation of Icosahedral Quasicrystals. Adv. Funct. Mater. 2018, 28, 1801181. (37) Balan, A. P.; Radhakrishnan, S.; Woellner, C. F.; Sinha, S. K.; Deng, L.; de los Reyes, C.; Rao, B. M.; Paulose, M.; Neupane, R.; Apte, A.; et al. Exfoliation of a Non-van Der Waals Material from Iron Ore Hematite. Nat. Nanotechnol. 2018, 13, 602−609. (38) Kochat, V.; Samanta, A.; Zhang, Y.; Bhowmick, S.; Manimunda, P.; Asif, S. A. S.; Stender, A. S.; Vajtai, R.; Singh, A. K.; Tiwary, C. S.; Ajayan, P. M. Atomically Thin Gallium Layers from Solid-Melt Exfoliation. Sci. Adv. 2018, 4, e1701373. (39) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (40) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558. (41) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; et al. QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. (42) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (43) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953. (44) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758. (45) Vanderbilt, D. Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 41, 7892.

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