Ultrafast formation of a transient two-dimensional diamondlike structure in twisted bilayer graphene

Duan Luo, Dandan Hui, Bin Wen, Renkai Li, Jie Yang, Xiaozhe Shen, Alexander Hume Reid, Stephen Weathersby, Michael E. Kozina, Suji Park, Yang Ren, Troy D. Loeffler, S. K. R. S. Sankaranarayanan, Maria K. Y. Chan, Xing Wang, Jinshou Tian, Ilke Arslan, Xijie Wang, Tijana Rajh, and Jianguo Wen

Phys. Rev. B 102, 155431 – Published 29 October 2020

Abstract

Due to the absence of matching carbon atoms at honeycomb centers with carbon atoms in adjacent graphene sheets, theorists predicted that a sliding process is needed to form AA, $A B^{'}$ , or ABC stacking when directly converting graphite into $s p^{3}$ bonded diamond. Here, using twisted bilayer graphene, which naturally provides AA and $A B^{'}$ stacking configurations, we report the ultrafast formation of a transient two-dimensional diamondlike structure (which is not observed in aligned graphene) under femtosecond laser irradiation. This photoinduced phase transition is evidenced by the appearance of bond lengths of 1.94 and 3.14 Å in the time-dependent differential pair distribution function using MeV ultrafast electron diffraction. Molecular dynamics and first-principles calculation indicate that $s p^{3}$ bonds nucleate at AA and $A B^{'}$ stacked areas in a moiré pattern. This work sheds light on the direct graphite-to-diamond transformation mechanism, which has not been fully understood for more than 60 years.

Received 18 March 2020
Accepted 5 October 2020

DOI:https://doi.org/10.1103/PhysRevB.102.155431

Physics Subject Headings (PhySH)

Chemical bonding Dynamical phase transitions Lattice dynamics Structural phase transition Structural properties

Bilayer films Graphene

Electron diffraction Femtosecond laser irradiation Molecular dynamics

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Duan Luo ^1,2,3, Dandan Hui ^2,3, Bin Wen ⁴, Renkai Li^5,*, Jie Yang⁵, Xiaozhe Shen⁵, Alexander Hume Reid⁵, Stephen Weathersby ⁵, Michael E. Kozina⁵, Suji Park⁵, Yang Ren⁶, Troy D. Loeffler¹, S. K. R. S. Sankaranarayanan¹, Maria K. Y. Chan¹, Xing Wang ², Jinshou Tian^2,7, Ilke Arslan¹, Xijie Wang⁵, Tijana Rajh^1,†, and Jianguo Wen ^1,‡

¹Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, USA
²Key Laboratory of Ultra-fast Photoelectric Diagnostics Technology, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China
³University of Chinese Academy of Sciences, Beijing 100049, China
⁴State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China
⁵SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
⁶X-ray Science Division, Argonne National Laboratory, Lemont, Illinois 60439, USA
⁷Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China

^*Corresponding author: lrk@slac.stanford.edu
^†Corresponding author: rajh@anl.gov
^‡Corresponding author: jwen@anl.gov

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Vol. 102, Iss. 15 — 15 October 2020

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Images

Figure 1
Photoinduced structural transition from twisted bilayer graphene (TBG) to 2D diamond structure. (a) Schematic of a TBG moiré pattern at 6°, which consists of regions with AB, AA, and $A B^{'}$ stackings as structural details shown in (b). (c) Schematic illustration of the photoinduced structural transition in TBG, taking AA stacking as an example. Subsequent to the fs laser irradiation, interlayer contraction and buckling are induced by the generated electron-hole plasma which consists of electrons and holes in distinct graphene layers and then the transformation of $s p^{2}$ bonds to $s p^{3}$ bonds occurs. Such a transformation occurs preferentially at regions with AA and $A B^{'}$ stacking rather than at AB stacked areas due to the lack of matching carbon atoms at honeycomb centers between adjacent AB stacked graphene layers [top view in (b)].
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Figure 2
Ultrafast structural dynamics in twisted bilayer graphene (TBG) (upper row) and aligned graphene layers (lower row). (a) Typical diffraction pattern of TBG observed by MeV UED, labeled the first two most visible rings ({100} and {110}). (b) Diffraction intensity changes as a function of time delays. A rapid intensity rise in the {100} and drop in the {110} indicate that this process is not a thermal effect. (c) Time-dependent Q shift of TBG showing the ultrafast structural transformation within 330 fs (from {100}). (d) Typical diffraction pattern of aligned graphene sheets observed by MeV UED. (e) Oscillatory dynamics in {100} family showing out-of-phase shear mode vibration between (−110) and (100) peaks. (f) Extracted modulation frequency. (g) Q-dependent peak intensity decay.
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Figure 3
Temporal evolution of difference pair distribution functions (ΔPDF). (a) Experimental 2D ΔPDF of twisted bilayer graphene (TBG) showing the intensity increase (warm color) and decrease (cool color) at different interatomic distances and different time delays. (b) One-dimensional experimental and simulated ΔPDF at different time delays. The observed reduction of 1.42 Å (I) and 2.46 Å (III) bonds in (b) matches with the bonds in graphene (c) before the transformation. The appearance of 1.94 Å (II) and 3.14 Å (IV) bonds observed in TBG matches the new diaphitene structure (c) extracted from simulated transient structures (Fig. S9 [21]).
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Figure 4
Simulated structural evolution and energy pathway. (a) Snapshot of the top view at 255 fs (during compression) showing the preferential formation $s p^{3}$ bonds (warm colors) at regions with AA and $A B^{'}$ stacking. (b) Snapshot of the side view at 800 fs showing the strong interlayer C-C bonds slow down the relaxation. (c)–(e) Generalized stacking fault energy surfaces for interlayer spacing of 3.4, 2.2, and 2.0 Å. When the layer spacing is compressed from 3.4 to 2.0 Å, the lowest free-energy configuration gradually changes from AB stacking (c) to $A B^{'}$ (d) and AA (e) stacking. This 2D map of bilayer graphene with different stacking sequences is obtained by shifting top graphene layer along $a$ -axis [100] and $b$ -axis [010] directions with respect to the bottom layer (fixed). Four corners correspond to configurations with AB stackings (Fig. S10 [21]).
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