Research

Introduction

Computational materials science team are doing research on the structural and electronic properties of various two-dimensional materials such as graphene, MoS₂, and h-BN using ab initio calculation and Tight-binding molecular dynamics (TBMD) simulation. In addition, many of the problems arising from the miniaturization of semiconductor devices have been caused by atomic-scale phenomenon. Then, we are also conducting research to understand these problems at the atomic-scale and suggest improvements. Our research team has derived excellent research results through joint research with world-class transmission electron microscope experimental groups and industry.

Research

Study on the structural and electronic properties of various 2D materials
Prediction of efficient nanocatalysis (HER, CO2RR, etc.)
Developing hydrogen storage materials
Understanding atomic-scale mechanism of semiconductor process (ALD, CVD, etc.)
Development of tight-binding molecular dynamics potentials
New physics in van der Waals space
Mechanism of unconventional high Tc superconductivity

1. Direct observation and catalytic role of mediator atom in 2D materials [Gun-Do Lee, Alex W. Robertson, Sungwoo Lee, Yung-Chang Lin, Jeong-Wook Oh, Hwanyeol Park, Young-Chang Joo, Euijoon Yoon, Kazu Suenaga, Jamie H. Warnerand Christopher P. Ewels*. Science Advances, Vol 6, No 24, pp. eaba4942, 2020]

The structural transformations of graphene defects have been extensively researched through aberration-corrected transmission electron microscopy (AC-TEM) and theoretical calculations. For a long time, a core concept in understanding the structural evolution of graphene defects has been the Stone-Thrower-Wales (STW)–type bond rotation. In this study, we show that undercoordinated atoms induce bond formation and breaking, with much lower energy barriers than the STW-type bond rotation. We refer to them as mediator atoms due to their mediating role in the breaking and forming of bonds. Here, we report the direct observation of mediator atoms in graphene defect structures using AC-TEM and annular dark-field scanning TEM (ADF-STEM) and explain their catalytic role by tight-binding molecular dynamics (TBMD) simulations and image simulations based on density functional theory (DFT) calculations. The study of mediator atoms will pave a new way for understanding not only defect transformation but also the growth mechanisms in two-dimensional materials. [Science Advances, Vol 6, No 24, pp. eaba4942, 2020]

Fig. 1 Mediator atom formation from carbon adatom inclusion and subsequent mediator atom–induced structural change.

(A and B) Formation of mediator atom from an incoming adatom and subsequent mediator atom–induced change of the defect structure. (C) Mediator atom–induced structural change and its sputtering. Dotted red arrows indicate the position at which the adatom will sit at the following image. Red arrows indicate the position of the adatom. Blue solid arrows indicate atoms forming an sp² bond, which were adatoms before. Green arrows indicate newly formed mediator atoms. Black dotted lines and black solid bars indicate the forming and breaking of bonds, respectively. A red thick arrow in (B) indicates the propagation direction of the mediator atom. The process of structural change by mediator atom in (B) is suggested in detail in fig. S1.

2. Diffusion, Coalescence, and Reconstruction of Vacancy Defects in Graphene Layers [Gun-Do Lee, C. Z. Wang, Euijoon Yoon, Nong-Moon Hwang, Doh-Yeon Kim, and K. M. Ho. Physical Review Letters, 95(20), 205501, 2005]

Diffusion, coalescence, and reconstruction of vacancy defects in graphene layers are investigated by tight-binding molecular dynamics (TBMD) simulations and by ﬁrst principles total energy calculations. It is observed in the TBMD simulations that two single vacancies coalesce into a 5-8-5 double vacancy at the temperature of 3000 K, and it is further reconstructed into a new defect structure, the 555-777 defect, by the Stone-Wales type transformation at higher temperatures. First principles calculations conﬁrm that the 555-777 defect is energetically much more stable than two separated single vacancies, and the energy of the 555-777 defect is also slightly lower than that of the 5-8-5 double vacancy. In TBMD simulation, it is also found that the four single vacancies reconstruct into two collective 555-777 defects which is the unit for the hexagonal haeckelite structure proposed by Terrones et al. [Phys. Rev. Lett. 84, 1716 (2000)].

Fig. 1 Diffusion, coalescence, and reconstruction process of vacancy defects on graphene layer studied using tight-binding molecular dynamics simulation. (f)-(g) shows a process of coalescence of two vacancy defects and (g)-(h) shows a process of 555-777 defect formation by reconstruction.

3. Atomic Scale Imaging of Reversible Ring Cyclization in Graphene Nanoconstrictions [Ja Kyung Lee, Gun-Do Lee, Sungwoo Lee, Euijoon Yoon, Harry L. Anderson, G. Andrew D. Briggs, and Jamie H. Warner. ACS Nano, 13(2), 2379-2388, 2019]

We present an atomic level study of reversible cyclization processes in suspended nanoconstricted regions of graphene that form linear carbon chains (LCCs). Before the nanoconstricted region reaches a single linear carbon chain (SLCC), we observe that a double linear carbon chain (DLCC) structure often reverts back to a ribbon of sp2 hybridized oligoacene rings, in a process akin to the Bergman rearrangement. When the length of the DLCC system only consists of ∼5 atoms in each LCC, full recyclization occurs for all atoms present, but for longer DLCCs we find that only single sections of the chain are modified in their bonding hybridization and no full ring closure occurs along the entire DLCCs. This process is observed in real time using aberration-corrected transmission electron microscopy and simulated using density functional theory and tight binding molecular dynamics calculations. These results show that DLCCs are highly sensitive to the adsorption of local gas molecules or surface diffusion impurities and undergo structural modifications.

Fig. 2 Density functional theory (DFT) calculation of short DLCCs with hydrogen molecules and the corresponding energy states. The DFT calculation was performed on initially 4-atom length double carbon chains and the corresponding dynamic reconstruction of the atomic structures are captured and shown in the consecutive panels (a–j) and the energy barrier plot versus each transient state is shown in (k). Solid and dotted red lines represent bond breakage and formation, respectively. When hydrogen molecules (shaded in violet) are considered in the system, they dissociate and attach to the carbon atoms in atomic chains with a small energy barrier. The carbon atoms in the chains next to the ones attached with hydrogen make a bonding, eventually lowering the energy of the system (see a dip in the energy curve in (k), reaction coordinates from c to d, g to h, and i to j). Finally, as shown in the final image in (j), double carbon chains revert back to the ring structures. The two different colors (yellow and green) used to display each atom represent the difference in the hybridization states of carbon atoms. (l) Schematic flow-chart describing the reversible ring cyclization process: (1) a 2-D network of aromatic rings, (2) the formation of DLCCs, (3) reverting back to the aromatic rings with the attachment of hydrogen atoms, and (4) the removal of carbon–hydrogen bondings by incident electron beam energy brings the structure back to the initial stage of the entire cycle (1).

4. Atomic Structure and Dynamics of Epitaxial Platinum Bilayers on Graphene [Alex W. Robertson, Gun-Do Lee, Sungwoo Lee, Parker Buntin, Matthew Drexler, Ali A. Abdelhafiz, Euijoon Yoon, Jamie H. Warner, and Faisal M. Alamgir. ACS Nano, 13(10), 12162-12170, 2019]

Platinum atomic layers grown on graphene were investigated by atomic resolution transmission electron microscopy (TEM). These TEM images reveal the epitaxial relationship between the atomically thin platinum layers and graphene, with two optimal epitaxies observed. The energetics of these epitaxies influences the grain structure of the platinum film, facilitating grain growth via in-plane rotation and assimilation of neighbor grains, rather than grain coarsening from the movement of grain boundaries. This growth process was enabled due to the availability of several possible low-energy intermediate states for the rotating grains, the Pt-Gr epitaxies, which are minima in surface energy, and coincident site lattice grain boundaries, which are minima in grain boundary energy. Density functional theory calculations reveal a complex interplay of considerations for minimizing the platinum grain energy, with free platinum edges also having an effect on the relative energetics. We thus find that the platinum atomic layer grains undergo significant reorientation to minimize interface energy (via epitaxy), grain boundary energy (via low-energy orientations), and free edge energy. These results will be important for the design of two-dimensional graphene-supported platinum catalysts and obtaining large-area uniform platinum atomic layer films and also provide fundamental experimental insight into the growth of heteroepitaxial thin films.

Fig. 3 Energetics of the Pt bilayer epitaxial relationship with graphene. (a) DFT modeled energy diagram showing the optimal interlayer distance between the platinum and graphene. Red is for Pt₁₀₀Gr_zz and blue for Pt₁₁₀Gr_zz. (b) Calculated relative energies for Pt bilayer on graphene for different rotational alignments

5. Atomic Structure and Dynamics of Metal Dopant Pairs in Graphene [Zhengyu He, Kuang He, Alex W. Robertson, Angus I. Kirkland, Dongwook Kim, Jisoon Ihm, Euijoon Yoon, Gun-Do Lee, and Jamie H. Warner. Nano Letters, Vol. 14, pp. 3766, 2014]

We present an atomic resolution structural study of covalently bonded dopant pairs in the lattice of monolayer graphene. Two iron (Fe) metal atoms that are covalently bonded within the graphene lattice are observed and their interaction with each other is investigated. The two metal atom dopants can form small paired clusters of varied geometry within graphene vacancy defects. The two Fe atoms are created within a 10 nm diameter predefined location in graphene by manipulating a focused electron beam (80 kV) on the surface of graphene containing an intentionally deposited Fe precursor reservoir. Aberration-corrected transmission electron microscopy at 80 kV has been used to investigate the atomic structure and real time dynamics of Fe dimers embedded in graphene vacancies. Four different stable structures have been observed; two variants of an Fe dimer in a graphene trivacancy, an Fe dimer embedded in two adjacent monovacancies and an Fe dimer trapped by a quadvacancy. According to spin-sensitive DFT calculations, these dimer structures all possess magnetic moments of either 2.00 or 4.00 μ_B. The dimer structures were found to evolve from an initial single Fe atom dopant trapped in a graphene vacancy.

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