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Kyle Tom1 2 Shuren Lin1 2 Nolan Ahlm1 Alpha N'Diaye2 Liwen Wan2 Maged Abdelsamie3 Shuai Lou1 Shancheng Yan4 Hui Wu5 Michael Toney3 David Prendergast2 Jie Yao1 2

1, University of California, Berkeley, Berkeley, California, United States
2, Lawrence Berkeley National Laboratory, Berkeley, California, United States
3, Stanford University, Menlo Park, California, United States
4, Nanjing University, Nanjing, , China
5, Tsinghua University, Beijing, , China

Zinc oxide has shown great promise due to its favorable properties for piezoelectricity, UV optics, and catalysis as well as its wide array of crystal morphologies and phases. It has been shown [1,2] that below approximately 1 nm in thickness, ZnO will preferentially form a planar honeycomb structure similar to BN or graphene, called graphitic ZnO (gZnO), as a way to counteract the instability of its polar surfaces. These ultrathin sheets of ZnO have been predicted to hold many advantages over conventional atomically thin layers, including a wider bandgap and high temperature stability in ambient conditions. Additionally, its high ionicity relative to most other conventional 2D materials offers intriguing opportunities for study, including stronger electromechanical coupling and enhanced phonon scattering. However, little is known about this phase due to the difficulty of synthesizing large area gZnO for characterization and applications. In this work, we demonstrate solution-based synthesis of polycrystalline ZnO nanoflakes down to monolayer thicknesses and sizes up to 20 µm using a graphite oxide template process. The process can be performed on a variety of non-metal, flat substrates. TEM imaging on suspended structures show polycrystalline samples with grain sizes on the order of 15 nm. X-Ray Absorption Near Edge spectroscopy (XANES) also shows a very distinct change that indicates a large change in the local structure from buckled wurtzite to a graphitic phase. XPS measurements are performed on synthesized gZnO samples, and, in addition to the XANES spectra, show significant changes to the electronic band structure compared to its bulk phase, including an enlarged band gap. The gZnO sheets also exhibit excellent stability at temperatures as high as 800oC in ambient environment. This new wide band gap, atomically thin material provides us a platform for harsh environment electronic devices and deep ultra-violet optical applications. Also, the growth process is able to easily incorporate dopants and inherently forms a heterostructure of reduced graphene oxide and gZnO, simplifying the fabrication process for heterostructures and offering a unique platform for study.

Acknowledgements:
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.
References:
Freeman, C. L., et al. Graphitic nanofilms as precursors to wurtzite films: theory. Physical review letters 96(6), 066102/1-066102/4 (2006).
Ta, H. Q., et al. Graphene-Like ZnO: A Mini Review. Crystals 6(8), 100 (2016).

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