Gun-do Lee - Academia.edu (original) (raw)

Papers by Gun-do Lee

ACS Nano, Oct 13, 2015

Thermally induced dislocation movements are important in understanding the effects of high temper... more Thermally induced dislocation movements are important in understanding the effects of high temperature annealing on modifying the crystal structure. We use an in-situ heating holder in an aberration corrected transmission electron microscopy to study the movement of dislocations in suspended monolayer graphene up to 800 o C. Control of temperature enables the differentiation of electron beam induced effects and thermally driven processes. At room temperature the dynamics of dislocation behavior is driven by the electron beam irradiation at 80kV, however at higher temperatures increased movement of the dislocation is observed and provides evidence for the influence of thermal energy to the system. An analysis of the dislocation movement shows both climb and glide processes, including new complex pathways for migration and large nanoscale rapid jumps between fixed positions in the lattice. The improved understanding of the high temperature dislocation movement provides insights into annealing processes in graphene and the behavior of defects with increased heat.

Bulletin of the American Physical Society, Mar 16, 2016

boundaries (GBs) in graphene can migrate when irradiated by electron beams from a transmission el... more boundaries (GBs) in graphene can migrate when irradiated by electron beams from a transmission electron microscope (TEM). Here, we present an ab initio study on the atomic scale mechanism for the GB motion with misorientation angle of 30 in graphene. From total energy calculations and energy barrier calculations, we find that a Stone-Wales(SW)-type transformation can occur more easily near GBs than in pristine graphene due to a reduced energy barrier. There are other cases of migration which can be understood by other type of transformation, named evaporation of a carbon dimer. We also find that a mismatch in the crystalline orientation at GBs can drive the evaporation of a carbon dimer easily by greatly reducing the corresponding overall energy barrier. After evaporation of the carbon dimer, the GBs can be stabilized through a series of SW-type transformations that result in GB motion. The GB motion induced by evaporation of the dimer is in excellent agreement with recent TEM experiments.

Nanotechnology, 2021

For graphene-based 2D materials, charge transfer at the interface between graphene and ferromagne... more For graphene-based 2D materials, charge transfer at the interface between graphene and ferromagnetic metal leads to many intriguing phenomena. However, because of the unidirectional spin orientation in ferromagnetic transition metals, interface interaction plays a detrimental role in diminishing the magnetic parameters on 2D surfaces. To overcome this issue, we have synthesized ultrathin 2D weak antiferromagnetic β-NiOOH layers on a graphene surface. By exploiting the charge transfer effect and tuning the thickness of the thin β-NiOOH layers, conversion of ferromagnetism along with giant coercivity and the thermo-remnant magnetic memory effect were observed. As antiferromagnets have two spin orientations, transfer of charge at the interface breaks the nullifying effect of zero magnetization in antiferromagnets and the combined system behaves like a 2D ferrimagnet. Whenever, the sandwich structure of β-NiOOH/graphene/β-NiOOH is formed, it also shows interlayer exchange coupling those results in huge exchange bias and anomalous temperature dependence of coercivity. Due to the strong exchange interaction between the layers, the combined system also shows a robust temperature-based memory effect. Spin-polarized density functional theory was also calculated to confirm the interface interaction and its quantitative evaluation by means of Bader charge analysis and charge-density mapping.

2006 International SiGe Technology and Device Meeting

Journal of Materials Chemistry A, 2020

Cu acetate/PAN nanofibers were transformed into porous C nanofibers with doped N and Cu particles... more Cu acetate/PAN nanofibers were transformed into porous C nanofibers with doped N and Cu particles,viaO2partial pressure-controlled calcination. N atoms next to Cu trigger the CO2RR by increasing the amount of CO* on the Cu, lowering the energy needed for CO dimerization.

ACS Applied Nano Materials, 2021

Small, 2021

Precise controlled filling of point vacancies in hBN with carbon atoms is demonstrated using a fo... more Precise controlled filling of point vacancies in hBN with carbon atoms is demonstrated using a focused electron beam method, which guides mobile C atoms into the desired defect site. Optimization of the technique enables the insertion of a single C atom into a selected monovacancy, and preferential defect filling with sub‐2 nm accuracy. Increasing the C insertion process leads to thicker 3D C nanodots seeded at the hBN point vacancy site. Other light elements are also observed to bind to hBN vacancies, including O, opening up a wide range of complex defect structures that include B, C, N, and O atoms. The ability to selectively fill point vacancies in hBN with C atoms provides a pathway for creating non‐hydrogenated covalently bonded C molecules embedded in the insulating hBN.

Inorganic Chemistry Frontiers, 2020

The nature and the mechanism of the film interaction with the substrate at the film/substrate int... more The nature and the mechanism of the film interaction with the substrate at the film/substrate interface are still far from being fully understood.

Applied Surface Science, 2021

Abstract The nature of the electronic interaction of precursor molecules with functionalized surf... more Abstract The nature of the electronic interaction of precursor molecules with functionalized surfaces are still uncertain. Here, DFT calculations were performed to study the dissociative reactions of TiCl4 on non-hydroxylated and hydroxylated α-Al2O3 (0 0 0 1) surface based on DFT calculation. For non-hydroxylated α-Al2O3 (0 0 0 1) surface, residual Cl atoms from the dissociative reactions hinder another TiCl4 molecule to be adsorbed on non-hydroxylated α-Al2O3 surface, which are known to greatly degrade the performance of memory devices. We found that the removal of those Cl adatoms is energetically difficult on the non-hydroxylated α-Al2O3 (0 0 0 1) surface due to the strong bonding nature of Cl Al bond, which was additionally confirmed by charge density difference and Bader charge analysis. In contrast, hydroxyl functional group worked as a catalysis for the dissociative reactions of TiCl4 molecules by lowering activation energy for the successive removal process of Cl adatoms as HCl gas on the hydroxylated surface. These results imply that the electronic interaction between highly reactive molecule and surface functional group should be deeply understood in order to design the ALD process of future memory devices.

Nanotechnology, 2020

Point defects in freestanding graphene monolayers such as monovacancies (MVs) and divacancies hav... more Point defects in freestanding graphene monolayers such as monovacancies (MVs) and divacancies have been investigated at atomic scale with aberration-corrected transmission electron microscopy and theoretical calculations. In general, these defects can be formed simply by the absence of individual carbon atoms and carbon bond reconstructions in the graphene lattice under electron and ion irradiation. However, in this study, we found that oxygen and hydrogen atoms can be involved in the formation of these point defects caused by the simultaneous detachment of oxygen–carbon atoms. Here we report the effect of the oxygen and hydrogen atoms on the graphene surface forming the point defects under electron beam irradiation, and their role of stabilizing other MVs when composed of 13–5 ring pairs. In addition, theoretical analysis using density functional theory calculations demonstrates that the participating atoms can form the point defects in the intermediate states and stabilize 13–5 ri...

Carbon, 2019

An amorphous carbon hardmask was fabricated by DC sputtering to evaluate the etching characterist... more An amorphous carbon hardmask was fabricated by DC sputtering to evaluate the etching characteristics for semiconductor microstructure patterning. The bonding structure of the carbon films deposited by sputtering was modified by the DC sputtering conditions and the deposition pressure. In the case of a low-pressure deposition process, an sp 3-bonding-rich amorphous carbon film was fabricated and had excellent etching resistance. On the other hand, during the high-pressure deposition process, an sp 2bonding-rich amorphous carbon film was fabricated and had poor etching resistance. To understand the degradation process of the carbon hardmask induced by the penetration of fluorine ions into the film during dry etching, the phenomenon of fluorine penetration into the film and the interaction between fluorine and the carbon bonds were studied by density functional theory (DFT). Through the DFT calculations, it is unveiled that the energy barrier for the migration of a fluorine atom through the sp 3 bonding path is much larger than that of a fluorine atom through the sp 2 bonding path in amorphous carbon.

Journal of Materials Chemistry A, 2019

With the combination of first principle calculations and thermodynamics, a predictive fabrication... more With the combination of first principle calculations and thermodynamics, a predictive fabrication to modify structure and phase was developed.

Nanoscale, Jan 23, 2016

There has been significant research interest in controlling and imaging molecular dynamics, such ... more There has been significant research interest in controlling and imaging molecular dynamics, such as translational and rotational motions, especially at a single molecular level. Here we applied aberration-corrected transmission electron microscopy (ACTEM) to actuate and directly image the rotational motions of molecules anchored on a single-layer-graphene sheet. Nanometer-sized carbonaceous molecules anchored on graphene provide ideal systems for monitoring rotational motions via ACTEM. We observed the preferential registry of longer molecular axis along graphene zigzag or armchair lattice directions due to the stacking-dependent molecule-graphene energy landscape. The calculated cross section from elastic scattering theory was used to experimentally estimate the rotational energy barriers of molecules on graphene. The observed energy barrier was within the range of 1.5-12 meV per atom, which is in good agreement with previous calculation results. We also performed molecular dynamic...

ACS nano, Jan 2, 2015

The atomic structure of subnanometer pores in graphene, of interest due to graphene's potenti... more The atomic structure of subnanometer pores in graphene, of interest due to graphene's potential as a desalination and gas filtration membrane, is demonstrated by atomic resolution aberration corrected transmission electron microscopy. High temperatures of 500 °C and over are used to prevent self-healing of the pores, permitting the successful imaging of open pore geometries consisting of between -4 to -13 atoms, all exhibiting subnanometer diameters. Picometer resolution bond length measurements are used to confirm reconstruction of five-membered ring projections that often decorate the pore perimeter, knowledge which is used to explore the viability of completely self-passivated subnanometer pore structures; bonding configurations where the pore would not require external passivation by, for example, hydrogen to be chemically inert.

Carbon, 2015

Grain boundaries (GBs) in graphene can migrate when irradiated by electron beams from a transmiss... more Grain boundaries (GBs) in graphene can migrate when irradiated by electron beams from a transmission electron microscope (TEM). Here, we present an ab initio study on the atomic scale-mechanism for motion of GB with misorientation angle of $30°in graphene. From total energy calculations and energy barrier calculations, we find that a Stone-Wales (SW)-type transformation can occur more easily near GBs than in pristine graphene due to a reduced energy barrier of 7.23 eV; thus, this transformation is responsible for the motion of GBs. More interestingly, we find that a mismatch in the crystalline orientation at GBs can drive the evaporation of a carbon dimer by greatly reducing the corresponding overall energy barrier to 11.38 eV. After evaporation of the carbon dimer, the GBs can be stabilized through a series of SW-type transformations that result in GB motion. The GB motion induced by evaporation of the dimer is in excellent agreement with recent TEM experiments. Our findings elucidate the mechanism for the dynamics of GBs during TEM experiments and enhance the controllability of GBs in graphene.

ACS nano, Jan 27, 2015

Thermally induced dislocation movements are important in understanding the effects of high temper... more Thermally induced dislocation movements are important in understanding the effects of high temperature annealing on modifying the crystal structure. We use an in situ heating holder in an aberration corrected transmission electron microscopy to study the movement of dislocations in suspended monolayer graphene up to 800 °C. Control of temperature enables the differentiation of electron beam induced effects and thermally driven processes. At room temperature, the dynamics of dislocation behavior is driven by the electron beam irradiation at 80 kV; however at higher temperatures, increased movement of the dislocation is observed and provides evidence for the influence of thermal energy to the system. An analysis of the dislocation movement shows both climb and glide processes, including new complex pathways for migration and large nanoscale rapid jumps between fixed positions in the lattice. The improved understanding of the high temperature dislocation movement provides insights into...

ACS Nano, 2015

Vacancy defects play an important role in influencing the properties of graphene and understandin... more Vacancy defects play an important role in influencing the properties of graphene and understanding their detailed atomic structure is crucial for developing accurate models to predict their impact. Divacancies (DVs) are one of the most common defects in graphene and can take three structural different forms through various sequences of bond rotations to minimize the energy. Using aberration-corrected transmission electron microscopy with monochromation of the electron source, we resolve the position of C atoms in graphene and measure the CC bond lengths within the three DVs, enabling a map of bond strain to be generated. We show that bond rotations reduce the maximum single bond strain reached within a DV and help distribute the strain over a larger number of bonds to minimize the peak magnitude.

Defects in graphene have become a subject of intensive investigation because those affect the mec... more Defects in graphene have become a subject of intensive investigation because those affect the mechanical and electronic properties of graphene. In order to observe and control the defects in graphene, many state-of-the-art techniques such as high resolution transmission electron microscopy (HR-TEM) have been devoted to the study of the structure and formation process. However, it is very difficult to observe the detail of the formation process even within the state-of-the-art microscopy methods because the dynamics of defective structures such as vacancy, adatom, and edge atoms is completed in very short time. Various simulation methods have been employed to elucidate the hidden process of defect formation and dynamics [1]. In the study of defect formation and dynamics in graphene, we performed the cooperative research of HR-TEM and simulation methods. In the simulation methods, we used the tight-binding molecular dynamics simulation and density functional theory (DFT) calculation. In this study, it is found the hydrogen-free graphene edges and our TBMD simulation results are in excellent agreement with images from HR-TEM. These results are expected to make an effective way toward the functionalized graphene [2]. We also found from tight-binding calculation and HR-TEM study that the dislocation core with pentagon-heptagon pair originates ripples which are an out of plane distortion that help stabilize suspended monolayer graphene [3]. We also show that the introduction of atomic vacancies in graphene disrupts the uniformity of CC bond lengths immediately surrounding linear armchair defects in graphene. The measured changes in CC bond lengths are related to DFT calculations of charge density variation and corresponding DFT calculated structural models [4]. If time allows, we will introduce briefly our recent results on metal dopants in graphene which show interesting results on magnetic property.

The formation of haeckelite structures induced by vacancy defects in graphene layers of carbon na... more The formation of haeckelite structures induced by vacancy defects in graphene layers of carbon nanotube 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 3,000 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.

Nano Letters, 2014

Vacancy defects in graphene with an odd number of missing atoms, such as the trivacancy, have bee... more Vacancy defects in graphene with an odd number of missing atoms, such as the trivacancy, have been imaged at atomic resolution using aberration corrected transmission electron microscopy. These defects are not just stabilized by simple bond reconstructions between undercoordinated carbon atoms, as exhibited by even vacancies such as the divacancy. Instead we have observed reconstructions consisting of under-coordinated bridging carbon atoms spanning the vacancy to saturate edge atoms. We report detailed studies of the effect of this bridging atom on the configuration of the trivacancy and higher order odd number vacancies, as well as its role in defect stabilization in amorphous systems. Theoretical analysis using density functional theory and tight-binding molecular dynamics calculations demonstrate that the bridging atom enables the low energy reconfiguration of these defect structures.

ACS Nano, Oct 13, 2015

Thermally induced dislocation movements are important in understanding the effects of high temper... more Thermally induced dislocation movements are important in understanding the effects of high temperature annealing on modifying the crystal structure. We use an in-situ heating holder in an aberration corrected transmission electron microscopy to study the movement of dislocations in suspended monolayer graphene up to 800 o C. Control of temperature enables the differentiation of electron beam induced effects and thermally driven processes. At room temperature the dynamics of dislocation behavior is driven by the electron beam irradiation at 80kV, however at higher temperatures increased movement of the dislocation is observed and provides evidence for the influence of thermal energy to the system. An analysis of the dislocation movement shows both climb and glide processes, including new complex pathways for migration and large nanoscale rapid jumps between fixed positions in the lattice. The improved understanding of the high temperature dislocation movement provides insights into annealing processes in graphene and the behavior of defects with increased heat.

Bulletin of the American Physical Society, Mar 16, 2016

boundaries (GBs) in graphene can migrate when irradiated by electron beams from a transmission el... more boundaries (GBs) in graphene can migrate when irradiated by electron beams from a transmission electron microscope (TEM). Here, we present an ab initio study on the atomic scale mechanism for the GB motion with misorientation angle of 30 in graphene. From total energy calculations and energy barrier calculations, we find that a Stone-Wales(SW)-type transformation can occur more easily near GBs than in pristine graphene due to a reduced energy barrier. There are other cases of migration which can be understood by other type of transformation, named evaporation of a carbon dimer. We also find that a mismatch in the crystalline orientation at GBs can drive the evaporation of a carbon dimer easily by greatly reducing the corresponding overall energy barrier. After evaporation of the carbon dimer, the GBs can be stabilized through a series of SW-type transformations that result in GB motion. The GB motion induced by evaporation of the dimer is in excellent agreement with recent TEM experiments.

Nanotechnology, 2021

For graphene-based 2D materials, charge transfer at the interface between graphene and ferromagne... more For graphene-based 2D materials, charge transfer at the interface between graphene and ferromagnetic metal leads to many intriguing phenomena. However, because of the unidirectional spin orientation in ferromagnetic transition metals, interface interaction plays a detrimental role in diminishing the magnetic parameters on 2D surfaces. To overcome this issue, we have synthesized ultrathin 2D weak antiferromagnetic β-NiOOH layers on a graphene surface. By exploiting the charge transfer effect and tuning the thickness of the thin β-NiOOH layers, conversion of ferromagnetism along with giant coercivity and the thermo-remnant magnetic memory effect were observed. As antiferromagnets have two spin orientations, transfer of charge at the interface breaks the nullifying effect of zero magnetization in antiferromagnets and the combined system behaves like a 2D ferrimagnet. Whenever, the sandwich structure of β-NiOOH/graphene/β-NiOOH is formed, it also shows interlayer exchange coupling those results in huge exchange bias and anomalous temperature dependence of coercivity. Due to the strong exchange interaction between the layers, the combined system also shows a robust temperature-based memory effect. Spin-polarized density functional theory was also calculated to confirm the interface interaction and its quantitative evaluation by means of Bader charge analysis and charge-density mapping.

2006 International SiGe Technology and Device Meeting

Journal of Materials Chemistry A, 2020

Cu acetate/PAN nanofibers were transformed into porous C nanofibers with doped N and Cu particles... more Cu acetate/PAN nanofibers were transformed into porous C nanofibers with doped N and Cu particles,viaO2partial pressure-controlled calcination. N atoms next to Cu trigger the CO2RR by increasing the amount of CO* on the Cu, lowering the energy needed for CO dimerization.

ACS Applied Nano Materials, 2021

Small, 2021

Precise controlled filling of point vacancies in hBN with carbon atoms is demonstrated using a fo... more Precise controlled filling of point vacancies in hBN with carbon atoms is demonstrated using a focused electron beam method, which guides mobile C atoms into the desired defect site. Optimization of the technique enables the insertion of a single C atom into a selected monovacancy, and preferential defect filling with sub‐2 nm accuracy. Increasing the C insertion process leads to thicker 3D C nanodots seeded at the hBN point vacancy site. Other light elements are also observed to bind to hBN vacancies, including O, opening up a wide range of complex defect structures that include B, C, N, and O atoms. The ability to selectively fill point vacancies in hBN with C atoms provides a pathway for creating non‐hydrogenated covalently bonded C molecules embedded in the insulating hBN.

Inorganic Chemistry Frontiers, 2020

The nature and the mechanism of the film interaction with the substrate at the film/substrate int... more The nature and the mechanism of the film interaction with the substrate at the film/substrate interface are still far from being fully understood.

Applied Surface Science, 2021

Abstract The nature of the electronic interaction of precursor molecules with functionalized surf... more Abstract The nature of the electronic interaction of precursor molecules with functionalized surfaces are still uncertain. Here, DFT calculations were performed to study the dissociative reactions of TiCl4 on non-hydroxylated and hydroxylated α-Al2O3 (0 0 0 1) surface based on DFT calculation. For non-hydroxylated α-Al2O3 (0 0 0 1) surface, residual Cl atoms from the dissociative reactions hinder another TiCl4 molecule to be adsorbed on non-hydroxylated α-Al2O3 surface, which are known to greatly degrade the performance of memory devices. We found that the removal of those Cl adatoms is energetically difficult on the non-hydroxylated α-Al2O3 (0 0 0 1) surface due to the strong bonding nature of Cl Al bond, which was additionally confirmed by charge density difference and Bader charge analysis. In contrast, hydroxyl functional group worked as a catalysis for the dissociative reactions of TiCl4 molecules by lowering activation energy for the successive removal process of Cl adatoms as HCl gas on the hydroxylated surface. These results imply that the electronic interaction between highly reactive molecule and surface functional group should be deeply understood in order to design the ALD process of future memory devices.

Nanotechnology, 2020

Point defects in freestanding graphene monolayers such as monovacancies (MVs) and divacancies hav... more Point defects in freestanding graphene monolayers such as monovacancies (MVs) and divacancies have been investigated at atomic scale with aberration-corrected transmission electron microscopy and theoretical calculations. In general, these defects can be formed simply by the absence of individual carbon atoms and carbon bond reconstructions in the graphene lattice under electron and ion irradiation. However, in this study, we found that oxygen and hydrogen atoms can be involved in the formation of these point defects caused by the simultaneous detachment of oxygen–carbon atoms. Here we report the effect of the oxygen and hydrogen atoms on the graphene surface forming the point defects under electron beam irradiation, and their role of stabilizing other MVs when composed of 13–5 ring pairs. In addition, theoretical analysis using density functional theory calculations demonstrates that the participating atoms can form the point defects in the intermediate states and stabilize 13–5 ri...

Carbon, 2019

An amorphous carbon hardmask was fabricated by DC sputtering to evaluate the etching characterist... more An amorphous carbon hardmask was fabricated by DC sputtering to evaluate the etching characteristics for semiconductor microstructure patterning. The bonding structure of the carbon films deposited by sputtering was modified by the DC sputtering conditions and the deposition pressure. In the case of a low-pressure deposition process, an sp 3-bonding-rich amorphous carbon film was fabricated and had excellent etching resistance. On the other hand, during the high-pressure deposition process, an sp 2bonding-rich amorphous carbon film was fabricated and had poor etching resistance. To understand the degradation process of the carbon hardmask induced by the penetration of fluorine ions into the film during dry etching, the phenomenon of fluorine penetration into the film and the interaction between fluorine and the carbon bonds were studied by density functional theory (DFT). Through the DFT calculations, it is unveiled that the energy barrier for the migration of a fluorine atom through the sp 3 bonding path is much larger than that of a fluorine atom through the sp 2 bonding path in amorphous carbon.

Journal of Materials Chemistry A, 2019

With the combination of first principle calculations and thermodynamics, a predictive fabrication... more With the combination of first principle calculations and thermodynamics, a predictive fabrication to modify structure and phase was developed.

Nanoscale, Jan 23, 2016

There has been significant research interest in controlling and imaging molecular dynamics, such ... more There has been significant research interest in controlling and imaging molecular dynamics, such as translational and rotational motions, especially at a single molecular level. Here we applied aberration-corrected transmission electron microscopy (ACTEM) to actuate and directly image the rotational motions of molecules anchored on a single-layer-graphene sheet. Nanometer-sized carbonaceous molecules anchored on graphene provide ideal systems for monitoring rotational motions via ACTEM. We observed the preferential registry of longer molecular axis along graphene zigzag or armchair lattice directions due to the stacking-dependent molecule-graphene energy landscape. The calculated cross section from elastic scattering theory was used to experimentally estimate the rotational energy barriers of molecules on graphene. The observed energy barrier was within the range of 1.5-12 meV per atom, which is in good agreement with previous calculation results. We also performed molecular dynamic...

ACS nano, Jan 2, 2015

The atomic structure of subnanometer pores in graphene, of interest due to graphene's potenti... more The atomic structure of subnanometer pores in graphene, of interest due to graphene's potential as a desalination and gas filtration membrane, is demonstrated by atomic resolution aberration corrected transmission electron microscopy. High temperatures of 500 °C and over are used to prevent self-healing of the pores, permitting the successful imaging of open pore geometries consisting of between -4 to -13 atoms, all exhibiting subnanometer diameters. Picometer resolution bond length measurements are used to confirm reconstruction of five-membered ring projections that often decorate the pore perimeter, knowledge which is used to explore the viability of completely self-passivated subnanometer pore structures; bonding configurations where the pore would not require external passivation by, for example, hydrogen to be chemically inert.

Carbon, 2015

Grain boundaries (GBs) in graphene can migrate when irradiated by electron beams from a transmiss... more Grain boundaries (GBs) in graphene can migrate when irradiated by electron beams from a transmission electron microscope (TEM). Here, we present an ab initio study on the atomic scale-mechanism for motion of GB with misorientation angle of $30°in graphene. From total energy calculations and energy barrier calculations, we find that a Stone-Wales (SW)-type transformation can occur more easily near GBs than in pristine graphene due to a reduced energy barrier of 7.23 eV; thus, this transformation is responsible for the motion of GBs. More interestingly, we find that a mismatch in the crystalline orientation at GBs can drive the evaporation of a carbon dimer by greatly reducing the corresponding overall energy barrier to 11.38 eV. After evaporation of the carbon dimer, the GBs can be stabilized through a series of SW-type transformations that result in GB motion. The GB motion induced by evaporation of the dimer is in excellent agreement with recent TEM experiments. Our findings elucidate the mechanism for the dynamics of GBs during TEM experiments and enhance the controllability of GBs in graphene.

ACS nano, Jan 27, 2015

Thermally induced dislocation movements are important in understanding the effects of high temper... more Thermally induced dislocation movements are important in understanding the effects of high temperature annealing on modifying the crystal structure. We use an in situ heating holder in an aberration corrected transmission electron microscopy to study the movement of dislocations in suspended monolayer graphene up to 800 °C. Control of temperature enables the differentiation of electron beam induced effects and thermally driven processes. At room temperature, the dynamics of dislocation behavior is driven by the electron beam irradiation at 80 kV; however at higher temperatures, increased movement of the dislocation is observed and provides evidence for the influence of thermal energy to the system. An analysis of the dislocation movement shows both climb and glide processes, including new complex pathways for migration and large nanoscale rapid jumps between fixed positions in the lattice. The improved understanding of the high temperature dislocation movement provides insights into...

ACS Nano, 2015

Vacancy defects play an important role in influencing the properties of graphene and understandin... more Vacancy defects play an important role in influencing the properties of graphene and understanding their detailed atomic structure is crucial for developing accurate models to predict their impact. Divacancies (DVs) are one of the most common defects in graphene and can take three structural different forms through various sequences of bond rotations to minimize the energy. Using aberration-corrected transmission electron microscopy with monochromation of the electron source, we resolve the position of C atoms in graphene and measure the CC bond lengths within the three DVs, enabling a map of bond strain to be generated. We show that bond rotations reduce the maximum single bond strain reached within a DV and help distribute the strain over a larger number of bonds to minimize the peak magnitude.

Defects in graphene have become a subject of intensive investigation because those affect the mec... more Defects in graphene have become a subject of intensive investigation because those affect the mechanical and electronic properties of graphene. In order to observe and control the defects in graphene, many state-of-the-art techniques such as high resolution transmission electron microscopy (HR-TEM) have been devoted to the study of the structure and formation process. However, it is very difficult to observe the detail of the formation process even within the state-of-the-art microscopy methods because the dynamics of defective structures such as vacancy, adatom, and edge atoms is completed in very short time. Various simulation methods have been employed to elucidate the hidden process of defect formation and dynamics [1]. In the study of defect formation and dynamics in graphene, we performed the cooperative research of HR-TEM and simulation methods. In the simulation methods, we used the tight-binding molecular dynamics simulation and density functional theory (DFT) calculation. In this study, it is found the hydrogen-free graphene edges and our TBMD simulation results are in excellent agreement with images from HR-TEM. These results are expected to make an effective way toward the functionalized graphene [2]. We also found from tight-binding calculation and HR-TEM study that the dislocation core with pentagon-heptagon pair originates ripples which are an out of plane distortion that help stabilize suspended monolayer graphene [3]. We also show that the introduction of atomic vacancies in graphene disrupts the uniformity of CC bond lengths immediately surrounding linear armchair defects in graphene. The measured changes in CC bond lengths are related to DFT calculations of charge density variation and corresponding DFT calculated structural models [4]. If time allows, we will introduce briefly our recent results on metal dopants in graphene which show interesting results on magnetic property.

The formation of haeckelite structures induced by vacancy defects in graphene layers of carbon na... more The formation of haeckelite structures induced by vacancy defects in graphene layers of carbon nanotube 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 3,000 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.

Nano Letters, 2014

Vacancy defects in graphene with an odd number of missing atoms, such as the trivacancy, have bee... more Vacancy defects in graphene with an odd number of missing atoms, such as the trivacancy, have been imaged at atomic resolution using aberration corrected transmission electron microscopy. These defects are not just stabilized by simple bond reconstructions between undercoordinated carbon atoms, as exhibited by even vacancies such as the divacancy. Instead we have observed reconstructions consisting of under-coordinated bridging carbon atoms spanning the vacancy to saturate edge atoms. We report detailed studies of the effect of this bridging atom on the configuration of the trivacancy and higher order odd number vacancies, as well as its role in defect stabilization in amorphous systems. Theoretical analysis using density functional theory and tight-binding molecular dynamics calculations demonstrate that the bridging atom enables the low energy reconfiguration of these defect structures.