Bulletin of the American Physical Society, Mar 3, 2015
microscopically understand the mechanisms of electron-induced NO dissociations, we performed firs... more microscopically understand the mechanisms of electron-induced NO dissociations, we performed first-principles density-functional theory (DFT) calculations for NO-CoTPP on Au(111). We explain the scanning tunneling microscopy (STM) results that the dissociations of NO were induced by both positive and negative voltage pulses with threshold voltages, +0.68 V and 0.74 V, respectively, at 0.1 nA tunneling current, showing power law relations between tunneling current and dissociation yield. To evaluate first-principles thermodynamics of the NO dissociation, we considered not only adsorption-desorption energetics, zero-point energy, and vibrational free energy at experiment temperature from first-principles, but also the chemical potential of NO gas at the cryogenic ultra-high vacuum condition. Using first-principles thermodynamics for the NO dissociation, we argue that the dissociations are induced with inelastic electron tunneling through molecular orbital resonances.
Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have rec... more Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial-phonons in an iron-based superconductor Sr2VO3FeAs (Tc 33 K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in VO2 layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.
Epitaxial graphene on metal substrates provides excellent platforms to study its atomic and elect... more Epitaxial graphene on metal substrates provides excellent platforms to study its atomic and electronic structures, and can be grown either by surface segregation of carbon or by chemical vapor deposition. The growth behaviors of the two methods, however, have not been directly compared each other. Here, we studied domain structures of graphene grown by three different methods, surface segregation, post-annealing with adsorbed ethylene, and high-temperature dose of ethylene, using scanning tunneling microscopy. The first two methods resulted in graphene regions with areas of 100 nm 2 , whereas the third method showed large area graphene (＞10 4 nm 2) with regular hexagonal Moiré patterns, implying that high-temperature dose of ethylene is preferable for further studies on graphene such as additional growth of organic molecules.
Graphene has inspired remarkable advances in nanotechnology due to its unusual electronic band st... more Graphene has inspired remarkable advances in nanotechnology due to its unusual electronic band structures represented by massless Dirac cones. Graphene can be epitaxially grown on metal surfaces by chemical vapor deposition method. Due to the lattice mismatch, epitaxial graphene grown on Ru(0001) shows hexagonal Moir'e patterns with subatomic height variations. We studied local electronic structures of the epitaxial graphene using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Different spectra were observed at top, bridge, and hollow regions of the Moir'e patterns. Observed STS data will be explained with structural models.
The symmetry requirement and the origin of magnetic orders coexisting with superconductivity have... more The symmetry requirement and the origin of magnetic orders coexisting with superconductivity have been strongly debated issues of iron-based superconductors (FeSCs). Observation of C$_4$-symmetric antiferromagnetism in violation of the inter-band nesting condition of spin-density waves in superconducting ground state will require significant change in our understanding of the mechanism of FeSC. The superconducting material Sr$_2$VO$_3$FeAs, a bulk version of monolayer FeSC in contact with a perovskite layer with its magnetism (T$_N$ ~ 50 K) and superconductivity (T$_c$ ~ 37 K) coexisting at parent state, has no reported structural orthorhombic distortion and thus makes a perfect system to look for theoretically expected C$_4$ magnetisms. Based on variable temperature spin-polarized scanning tunneling microscopy (SPSTM) with newly discovered imaging mechanism that removes the static surface reconstruction (SR) pattern by fluctuating it rapidly with spin-polarized tunneling current, w...
Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have rec... more Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial-phonons in an iron-based superconductor Sr2VO3FeAs (Tc 33 K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in VO2 layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.
Intermolecular interactions of supramolecular structures were studied in anthraquinone molecules ... more Intermolecular interactions of supramolecular structures were studied in anthraquinone molecules on Au(1 1 1) using scanning tunneling microscopy. Molecular chains of linear and zigzag structures were observed and explained with simple models of hydrogen bonds. In two-dimensional islands, square and chevron structures were observed, and their molecular models were reproduced by density functional theory calculations. Square structures were made of four hydrogen bonds per molecule, whereas chevron structures were explained with four hydrogen bonds per molecule and additional van der Waals interactions. Our study shows that van der Waals interactions play cooperative roles in determining the hydrogen bonded networks of the system.
the synthesis of CVD-grown h-BN on Cu can be found in our previous report. [ 13 ] A CVD growth of... more the synthesis of CVD-grown h-BN on Cu can be found in our previous report. [ 13 ] A CVD growth of graphene on this h-BN fi lm on Cu is then done (CVD growth conditions are described in the Supporting Information). Before it was transferred onto a SiO 2 substrate, sequentially grown CVD graphene/h-BN on Cu foil was studied with STM and scanning tunneling spectroscopy (STS), which show the almost ideal (i.e., unperturbed) electronic properties of graphene by the high crystallinity, epitaxy, and the clean interface achieved in this sequentially CVD-grown graphene/h-BN fi lm. After transfer, transmission electron microscopy (TEM) and Raman spectroscopy confi rmed that this structure is robust enough for a reliable transfer process. In addition, electrical transport measurements revealed that transistors based on this sequential CVD-grown graphene/h-BN fi lm exhibit better electrical performance than those based on graphene mechanically transferred onto h-BN fi lms or SiO 2 substrates. Transferring graphene onto dielectric substrates can thus be avoided by this direct growth method. The interface between graphene and h-BN, therefore, is also protected from external impurities during transfer of this hybrid structure onto any substrate. A direct growth of h-BN fi lm on graphene was reported, [ 15 ] which can be combined with our method to make a new recipe to synthesize large area super lattice of graphene and h-BN. [ 16 ] As this direct growth
We explore a new mechanism for switching magnetism and superconductivity in a magnetically frustr... more We explore a new mechanism for switching magnetism and superconductivity in a magnetically frustrated iron-based superconductor using spin-polarized scanning tunneling microscopy (SPSTM). Our SPSTM study on single-crystal Sr_{2}VO_{3}FeAs shows that a spin-polarized tunneling current can switch the Fe-layer magnetism into a nontrivial C_{4} (2×2) order, which cannot be achieved by thermal excitation with an unpolarized current. Our tunneling spectroscopy study shows that the induced C_{4} (2×2) order has characteristics of plaquette antiferromagnetic order in the Fe layer and strongly suppresses superconductivity. Also, thermal agitation beyond the bulk Fe spin ordering temperature erases the C_{4} state. These results suggest a new possibility of switching local superconductivity by changing the symmetry of magnetic order with spin-polarized and unpolarized tunneling currents in iron-based superconductors.
Electronic properties of graphene can be controlled by using elastic strain. Theories predicted t... more Electronic properties of graphene can be controlled by using elastic strain. Theories predicted two major effects of strain, pseudo-magnetic field and spatially-varying Fermi velocity effects in graphene, but only the former was confirmed by experiments. Here, we report the experimental evidences of the latter, observed in strained graphene weakly coupled to a hexagonal boron nitride (BN) using scanning tunneling microscopy (STM) and spectroscopy (STS). V-shaped STS spectra showed spatially-varying slopes at lines across the strained ridges, whereas they showed constant slopes at lines along the long directions of ridges. The observed slope variations in the former were explained with the reduced Fermi velocity which is linear with the uniaxial strain less than 5% in agreement with theoretical predictions. Spatially-varying Fermi velocity that is controllable by local strain can provide an accessible component for the electronic engineering of graphene.
Molecular networks connected by halogen bonds have been actively studied due to their ubiquity in... more Molecular networks connected by halogen bonds have been actively studied due to their ubiquity in biological systems and complementary role to hydrogen bonds. Although networks of planar molecules with halogen ligands have been studied using scanning tunneling microscopy (STM), those of non-planar molecules have not. Here, we report on the network structures of non-planar molecules containing Brligands, tetrabromo-spirobifluorene on Au (111), studied using STM. One and two-dimensional networks were observed and their intermolecular interactions were investigated. In two-dimensional networks, a molecule forms 3.5 Br• • •Br halogen bonds and 2 Br• • •H hydrogen bonds, as supported by our density functional theory calculation results. Our study demonstrates that intermolecular structures of nonplanar molecules with halogen bonds can be probed on surfaces using STM.
Supramolecular structures of two rod-like molecules whose both ends are terminated with Br-ligand... more Supramolecular structures of two rod-like molecules whose both ends are terminated with Br-ligands were studied using scanning tunneling microscopy (STM). Rectangular and square porous networks were observed in 4,4-dibromo-p-terphenyl (DBTP) and 4,4-dibromo-p-quaterphenyl (DBQP) molecules on Au(1 1 1) with the void areas of 3.5 and 5.6 nm 2 , respectively. Both networks had chiral quadrupole nodes that could be explained with four Br• • •Br halogen bonds and four Br• • •H hydrogen bonds. The reversible transformations of network structures were observed in consecutive STM images, possibly mediated by a liquid phase surrounding the networks. The molecules also formed close-packing ladder-like networks with triple nodes that could be explained with four Br• • •H bonds. Our study shows that Br-ligands play key roles in forming molecular networks for both rod-like molecules.
Metal-supported high crystalline Bi 2 Se 3 quintuple layers
Nanotechnology, 2011
Atomically flat thin films of Bi(2)Se(3) were grown on Au(111) metal substrate using molecular be... more Atomically flat thin films of Bi(2)Se(3) were grown on Au(111) metal substrate using molecular beam epitaxy. Hexagonal atomic structures and quintuple layer steps were observed at the surfaces of grown films using scanning tunneling microscopy. Multiple sharp peaks from (003) family layers were characterized by x-ray diffraction measurements. The atomic stoichiometry of Bi and Se was considered using x-ray photoemission spectroscopy. Moiré patterns were obtained at the surfaces of one quintuple layer films due to lattice mismatch between Bi(2)Se(3) and Au. Our experiments suggest that Au is a reasonable material for electrodes in Bi(2)Se(3) devices.
Bulletin of the American Physical Society, Mar 3, 2015
microscopically understand the mechanisms of electron-induced NO dissociations, we performed firs... more microscopically understand the mechanisms of electron-induced NO dissociations, we performed first-principles density-functional theory (DFT) calculations for NO-CoTPP on Au(111). We explain the scanning tunneling microscopy (STM) results that the dissociations of NO were induced by both positive and negative voltage pulses with threshold voltages, +0.68 V and 0.74 V, respectively, at 0.1 nA tunneling current, showing power law relations between tunneling current and dissociation yield. To evaluate first-principles thermodynamics of the NO dissociation, we considered not only adsorption-desorption energetics, zero-point energy, and vibrational free energy at experiment temperature from first-principles, but also the chemical potential of NO gas at the cryogenic ultra-high vacuum condition. Using first-principles thermodynamics for the NO dissociation, we argue that the dissociations are induced with inelastic electron tunneling through molecular orbital resonances.
Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have rec... more Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial-phonons in an iron-based superconductor Sr2VO3FeAs (Tc 33 K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in VO2 layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.
Epitaxial graphene on metal substrates provides excellent platforms to study its atomic and elect... more Epitaxial graphene on metal substrates provides excellent platforms to study its atomic and electronic structures, and can be grown either by surface segregation of carbon or by chemical vapor deposition. The growth behaviors of the two methods, however, have not been directly compared each other. Here, we studied domain structures of graphene grown by three different methods, surface segregation, post-annealing with adsorbed ethylene, and high-temperature dose of ethylene, using scanning tunneling microscopy. The first two methods resulted in graphene regions with areas of 100 nm 2 , whereas the third method showed large area graphene (＞10 4 nm 2) with regular hexagonal Moiré patterns, implying that high-temperature dose of ethylene is preferable for further studies on graphene such as additional growth of organic molecules.
Graphene has inspired remarkable advances in nanotechnology due to its unusual electronic band st... more Graphene has inspired remarkable advances in nanotechnology due to its unusual electronic band structures represented by massless Dirac cones. Graphene can be epitaxially grown on metal surfaces by chemical vapor deposition method. Due to the lattice mismatch, epitaxial graphene grown on Ru(0001) shows hexagonal Moir'e patterns with subatomic height variations. We studied local electronic structures of the epitaxial graphene using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Different spectra were observed at top, bridge, and hollow regions of the Moir'e patterns. Observed STS data will be explained with structural models.
The symmetry requirement and the origin of magnetic orders coexisting with superconductivity have... more The symmetry requirement and the origin of magnetic orders coexisting with superconductivity have been strongly debated issues of iron-based superconductors (FeSCs). Observation of C$_4$-symmetric antiferromagnetism in violation of the inter-band nesting condition of spin-density waves in superconducting ground state will require significant change in our understanding of the mechanism of FeSC. The superconducting material Sr$_2$VO$_3$FeAs, a bulk version of monolayer FeSC in contact with a perovskite layer with its magnetism (T$_N$ ~ 50 K) and superconductivity (T$_c$ ~ 37 K) coexisting at parent state, has no reported structural orthorhombic distortion and thus makes a perfect system to look for theoretically expected C$_4$ magnetisms. Based on variable temperature spin-polarized scanning tunneling microscopy (SPSTM) with newly discovered imaging mechanism that removes the static surface reconstruction (SR) pattern by fluctuating it rapidly with spin-polarized tunneling current, w...
Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have rec... more Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial-phonons in an iron-based superconductor Sr2VO3FeAs (Tc 33 K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in VO2 layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.
Intermolecular interactions of supramolecular structures were studied in anthraquinone molecules ... more Intermolecular interactions of supramolecular structures were studied in anthraquinone molecules on Au(1 1 1) using scanning tunneling microscopy. Molecular chains of linear and zigzag structures were observed and explained with simple models of hydrogen bonds. In two-dimensional islands, square and chevron structures were observed, and their molecular models were reproduced by density functional theory calculations. Square structures were made of four hydrogen bonds per molecule, whereas chevron structures were explained with four hydrogen bonds per molecule and additional van der Waals interactions. Our study shows that van der Waals interactions play cooperative roles in determining the hydrogen bonded networks of the system.
the synthesis of CVD-grown h-BN on Cu can be found in our previous report. [ 13 ] A CVD growth of... more the synthesis of CVD-grown h-BN on Cu can be found in our previous report. [ 13 ] A CVD growth of graphene on this h-BN fi lm on Cu is then done (CVD growth conditions are described in the Supporting Information). Before it was transferred onto a SiO 2 substrate, sequentially grown CVD graphene/h-BN on Cu foil was studied with STM and scanning tunneling spectroscopy (STS), which show the almost ideal (i.e., unperturbed) electronic properties of graphene by the high crystallinity, epitaxy, and the clean interface achieved in this sequentially CVD-grown graphene/h-BN fi lm. After transfer, transmission electron microscopy (TEM) and Raman spectroscopy confi rmed that this structure is robust enough for a reliable transfer process. In addition, electrical transport measurements revealed that transistors based on this sequential CVD-grown graphene/h-BN fi lm exhibit better electrical performance than those based on graphene mechanically transferred onto h-BN fi lms or SiO 2 substrates. Transferring graphene onto dielectric substrates can thus be avoided by this direct growth method. The interface between graphene and h-BN, therefore, is also protected from external impurities during transfer of this hybrid structure onto any substrate. A direct growth of h-BN fi lm on graphene was reported, [ 15 ] which can be combined with our method to make a new recipe to synthesize large area super lattice of graphene and h-BN. [ 16 ] As this direct growth
We explore a new mechanism for switching magnetism and superconductivity in a magnetically frustr... more We explore a new mechanism for switching magnetism and superconductivity in a magnetically frustrated iron-based superconductor using spin-polarized scanning tunneling microscopy (SPSTM). Our SPSTM study on single-crystal Sr_{2}VO_{3}FeAs shows that a spin-polarized tunneling current can switch the Fe-layer magnetism into a nontrivial C_{4} (2×2) order, which cannot be achieved by thermal excitation with an unpolarized current. Our tunneling spectroscopy study shows that the induced C_{4} (2×2) order has characteristics of plaquette antiferromagnetic order in the Fe layer and strongly suppresses superconductivity. Also, thermal agitation beyond the bulk Fe spin ordering temperature erases the C_{4} state. These results suggest a new possibility of switching local superconductivity by changing the symmetry of magnetic order with spin-polarized and unpolarized tunneling currents in iron-based superconductors.
Electronic properties of graphene can be controlled by using elastic strain. Theories predicted t... more Electronic properties of graphene can be controlled by using elastic strain. Theories predicted two major effects of strain, pseudo-magnetic field and spatially-varying Fermi velocity effects in graphene, but only the former was confirmed by experiments. Here, we report the experimental evidences of the latter, observed in strained graphene weakly coupled to a hexagonal boron nitride (BN) using scanning tunneling microscopy (STM) and spectroscopy (STS). V-shaped STS spectra showed spatially-varying slopes at lines across the strained ridges, whereas they showed constant slopes at lines along the long directions of ridges. The observed slope variations in the former were explained with the reduced Fermi velocity which is linear with the uniaxial strain less than 5% in agreement with theoretical predictions. Spatially-varying Fermi velocity that is controllable by local strain can provide an accessible component for the electronic engineering of graphene.
Molecular networks connected by halogen bonds have been actively studied due to their ubiquity in... more Molecular networks connected by halogen bonds have been actively studied due to their ubiquity in biological systems and complementary role to hydrogen bonds. Although networks of planar molecules with halogen ligands have been studied using scanning tunneling microscopy (STM), those of non-planar molecules have not. Here, we report on the network structures of non-planar molecules containing Brligands, tetrabromo-spirobifluorene on Au (111), studied using STM. One and two-dimensional networks were observed and their intermolecular interactions were investigated. In two-dimensional networks, a molecule forms 3.5 Br• • •Br halogen bonds and 2 Br• • •H hydrogen bonds, as supported by our density functional theory calculation results. Our study demonstrates that intermolecular structures of nonplanar molecules with halogen bonds can be probed on surfaces using STM.
Supramolecular structures of two rod-like molecules whose both ends are terminated with Br-ligand... more Supramolecular structures of two rod-like molecules whose both ends are terminated with Br-ligands were studied using scanning tunneling microscopy (STM). Rectangular and square porous networks were observed in 4,4-dibromo-p-terphenyl (DBTP) and 4,4-dibromo-p-quaterphenyl (DBQP) molecules on Au(1 1 1) with the void areas of 3.5 and 5.6 nm 2 , respectively. Both networks had chiral quadrupole nodes that could be explained with four Br• • •Br halogen bonds and four Br• • •H hydrogen bonds. The reversible transformations of network structures were observed in consecutive STM images, possibly mediated by a liquid phase surrounding the networks. The molecules also formed close-packing ladder-like networks with triple nodes that could be explained with four Br• • •H bonds. Our study shows that Br-ligands play key roles in forming molecular networks for both rod-like molecules.
Metal-supported high crystalline Bi 2 Se 3 quintuple layers
Nanotechnology, 2011
Atomically flat thin films of Bi(2)Se(3) were grown on Au(111) metal substrate using molecular be... more Atomically flat thin films of Bi(2)Se(3) were grown on Au(111) metal substrate using molecular beam epitaxy. Hexagonal atomic structures and quintuple layer steps were observed at the surfaces of grown films using scanning tunneling microscopy. Multiple sharp peaks from (003) family layers were characterized by x-ray diffraction measurements. The atomic stoichiometry of Bi and Se was considered using x-ray photoemission spectroscopy. Moiré patterns were obtained at the surfaces of one quintuple layer films due to lattice mismatch between Bi(2)Se(3) and Au. Our experiments suggest that Au is a reasonable material for electrodes in Bi(2)Se(3) devices.
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