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Hexagonal metal oxide monolayers derived from the metal–gas interface

Nature Materials volume 20, pages 1073–1078 (2021)Cite this article

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Abstract

Two-dimensional (2D) crystals are promising materials for developing future nano-enabled technologies1,2,3,4,5,6. The cleavage of weak, interlayer van der Waals bonds in layered bulk crystals enables the production of high-quality 2D, atomically thin monolayers7,8,9,10. Nonetheless, as earth-abundant compounds, metal oxides are rarely accessible as pure and fully stoichiometric monolayers owing to their ion-stabilized ‘lamellar’ bulk structure11,12,13,14. Here, we report the discovery of a layered planar hexagonal phase of oxides from elements across the transition metals, post-transition metals, lanthanides and metalloids, derived from strictly controlled oxidation at the metal–gas interface. The highly crystalline monolayers, without the support of ionic dopants or vacancies, can easily be mechanically exfoliated by stamping them onto substrates. Monolayer and few-layered hexagonal TiO2 are characterized as examples, showing p-type semiconducting properties with hole mobilities of up to 950 cm2 V−1 s−1 at room temperature. The strategy can be readily extended to a variety of elements, possibly expanding the exploration of metal oxides in the 2D quantum regime.

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Fig. 1: Synthesis and characterization of the exfoliated 2D transition metal oxides.
Fig. 2: Characterization of exfoliated 2D post-transition metal, metalloid and lanthanide oxides.
Fig. 3: Atomic structures of mono- and few-layered h-TiO2.
Fig. 4: Electronic band structure and charge carrier transport properties of 2D h-TiO2.

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Data availability

The authors declare that the main data supporting the findings of this study are available within the article and the Supplementary Information files. Extra data are available from the corresponding author upon request.

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Acknowledgements

B.Z. and J.O. acknowledge the support received from the Australian Research Council (DE160100715). A.Z. acknowledges the financial support received through the McKenzie Fellowship programme. B.Z., A.Z. and J.O. acknowledge the support from ARC Future Low Energy Electronics Technologies (FLEET) centre of excellence (CE170100039). Q.Y. acknowledges the financial support of National Natural Science Foundation of China (11904026) and Beijing Natural Science Foundation (1194021). This work was performed in part at RMIT Micro Nano Research Facility (MNRF) in the Victorian Node of the Australian National Fabrication Facility (ANFF), and the facilities of RMIT Microscopy & Microanalysis Facility (RMMF). We thank P. Rummel, Z. Li, Y. Hu, Y. Cao, E. Mayes and N. Pillai for their technical support.

Author information

Authors and Affiliations

  1. School of Engineering, RMIT University, Melbourne, Victoria, Australia

    Bao Yue Zhang, Kai Xu, Azmira Jannat, Guanghui Ren & Jian Zhen Ou

  2. Beijing Academy of Quantum Information Sciences, Beijing, China

    Qifeng Yao

  3. Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, China

    Qifeng Yao & Jian Zhen Ou

  4. Integrated Photonics and Applications Centre (InPAC), RMIT University, Melbourne, Victoria, Australia

    Guanghui Ren

  5. RMIT Microscopy & Microanalysis Facility, RMIT University, Melbourne, Victoria, Australia

    Matthew R. Field

  6. Centre for Translational Atomaterials, Swinburne University of Technology, Hawthorn, Victoria, Australia

    Xiaoming Wen & Chunhua Zhou

  7. Department of Chemical Engineering, The University of Melbourne, Parkville, Victoria, Australia

    Ali Zavabeti

  8. School of Science, RMIT University, Melbourne, Victoria, Australia

    Ali Zavabeti

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Contributions

The project was planned by J.Z.O. and A.Z. AFM, Raman, TEM, XPS and STM/STS measurements were conducted by B.Y.Z., A.Z. and K.X. PL experiments were conducted by B.Y.Z., X.W. and C.Z. DFT simulations and the resulting interpretation were carried out by Q.Y. SEM measurements were conducted by M.R.F., G.R. and A.Z. Optical profiling experiments were carried out by G.R. HAADF–STEM imaging and analysis were performed by B.Y.Z. and K.X. FET device fabrication and measurements were carried out by B.Y.Z., A.J., K.X., G.R. and A.Z. All authors participated in experiment data analysis, result interpretation and manuscript preparation.

Corresponding authors

Correspondence to Ali Zavabeti or Jian Zhen Ou.

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The authors declare no competing interests.

Additional information

Peer review information Nature Materials thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Surface roughness analysis of machine-polished metal samples for the growth of 2D planar layered h-MO.

a-i, profiler images (up) and roughness profiles (down) along x and y directions indicated by the white dashed lines in the profiler images of Ti (a), Mn (b), Fe (c), Co (d), Ni (e), Cu (f), Al (g), Ge (h) and Gd (i).

Extended Data Fig. 2 Experimental oxygen concentration threshold with respect to the oxygen Ec on transition metal surfaces.

The Ec data was extracted from Toyoshima et al.25. For metal elements with higher Ec such as Ti, Mn, and Fe, we performed the thermal oxidation at ppt levelled oxygen; for elements with lower Ec such as Co, Ni, and Cu, the ppm levelled oxygen environment was utilised for thermal oxidation.

Extended Data Fig. 3 Representative appearance and crystal structure of an exfoliated h-MO.

a, TEM imaging with a scale bar of 200 nm. b, Corresponding SAED pattern with a scale bar of 4 nm-1 of an exfoliated 2D hexagonal Cu2O sheet from the polished Cu chunk annealed at 200 °C for 20 min in the ppm levelled oxygen environment.

Extended Data Fig. 4 Morphological characterisations of a representative multi-layered h-MO.

a, A TEM image of a few-layered h-TiO2 sheet exfoliated on a holey-carbon TEM grid. b, A HAADF-STEM image of the curved edge of a free-standing exfoliated h-TiO2 sheet showing the layered feature. The interlayer distance is not measurable here as the edge is at a tilted angle to the camera. The h-TiO2 sheets were firstly obtained from the polished Ti metal annealed at 200 °C for 20 mins in the ppt levelled oxygen environment, subsequently treated at 100 °C for 5 min in the ambient air environment, then transferred on a holey carbon grid.

Extended Data Fig. 5 Raman analysis of layered h-TiO2.

a-c, Spatially resolved scanning Raman spectra of individual monolayered (1 L) (a), bi-layered (2 L) (b), and tri-layered (3 L) (c) h-TiO2 sheet. ‘▲’ signs specify Raman peaks obtained from DFT simulations for comparison (Supplementary Table 1).

Extended Data Fig. 6

Drain-source current Ids versus drain-source voltage Vds as a function of the gate voltage Vgs from 0 to -1 V for the device presented in Fig. 4h.

Extended Data Fig. 7 Conductivity analysis of FET devices with selective thicknesses.

Conductivity versus gate voltage Vgs measured from representative devices based on 2 (yellow), 3.5 (purple) and 5 nm (green) thick 2D h-TiO2 sheets. The corresponding field-effect mobilities are measured to be 32, 210, and 737 cm2 V-1 s-1, respectively.

Extended Data Fig. 8 The conductance of a 2D h-TiO2 based FET as a function of the gate voltage Vgs at different operation temperatures.

The length and width of the device conduction channel are 3.1 and 8.0 μm, respectively.

Supplementary information

Supplementary Information

Supplementary Notes 1–7, Figs. 1–25, Tables 1–5 and references.

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Zhang, B.Y., Xu, K., Yao, Q. et al. Hexagonal metal oxide monolayers derived from the metal–gas interface. Nat. Mater. 20, 1073–1078 (2021). https://doi.org/10.1038/s41563-020-00899-9

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