Sangku Kwon

Enhanced Nanoscale Friction on Fluorinated Graphene

Atomically thin graphene is an ideal model system for studying nanoscale friction due to its intrinsic two-dimensional (2D) anisotropy. Furthermore, modulating its tribological properties could be an important milestone for graphene-based micro- and nanomechanical devices. Here, we report unexpectedly enhanced nanoscale friction on chemically modified graphene and a relevant theoretical analysis associated with flexural phonons. Ultrahigh vacuum friction force microscopy measurements show that nanoscale friction on the graphene surface increases by a factor of 6 after fluorination of the surface, while the adhesion force is slightly reduced. Density functional theory calculations show that the out-of-plane bending stiffness of graphene increases up to 4-fold after fluorination. Thus, the less compliant F-graphene exhibits more friction. This indicates that the mechanics of tip-to-graphene nanoscale friction would be characteristically different from that of conventional solid-on-solid contact and would be dominated by the out-of-plane bending stiffness of the chemically modified graphene. We propose that damping via flexural phonons could be a main source for frictional energy dissipation in 2D systems such as graphene.

Superlubric Sliding of Graphene Nanoflakes on Graphene

The lubricating properties of graphite and graphene have been intensely studied by sliding a frictional force microscope tip against them to understand the origin of the observed low friction. In contrast, the relative motion of free graphene layers remains poorly understood. Here we report a study of the sliding behavior of graphene nanoflakes (GNFs) on a graphene surface. Using scanning tunneling microscopy, we found that the GNFs show facile translational and rotational motions between commensurate initial and final states at temperatures as low as 5 K. The motion is initiated by a tip-induced transition of the flakes from a commensurate to an incommensurate registry with the underlying graphene layer (the superlubric state), followed by rapid sliding until another commensurate position is reached. Counterintuitively, the average sliding distance of the flakes is larger at 5 K than at 77 K, indicating that thermal fluctuations are likely to trigger their transitions from superlubric back to commensurate ground states.

Hot Carrier-Driven Catalytic Reactions on Pt–CdSe–Pt Nanodumbbells and Pt/GaN under Light Irradiation

Hybrid nanocatalysts consisting of metal nanoparticle-semiconductor junctions offer an interesting platform to study the role of metal-oxide interfaces and hot electron flows in heterogeneous catalysis. Here, we report that hot carriers generated upon photon absorption significantly impact the catalytic activity of CO oxidation. We found that Pt-CdSe-Pt nanodumbbells exhibit a higher turnover frequency by a factor of 2 during irradiation by light with energy higher than the bandgap of CdSe, while the turnover rate on bare Pt nanoparticles did not depend on light irradiation. We found that Pt nanoparticles deposited on a GaN substrate under light irradiation exhibit changes in catalytic activity of CO oxidation that depends on the type of doping of the GaN. We suppose that hot electrons are generated upon the absorption of photons by the semiconducting nanorods or substrates, whereafter the hot electrons are injected into the Pt nanoparticles, resulting in the change in catalytic activity. The results imply that hot carrier flows generated during light irradiation significantly influence the catalytic activity of CO oxidation, leading to potential applications as a hot electron-based catalytic actuator.

Work function engineering of single layer graphene by irradiation-induced defects

We report the tuning of electrical properties of single layer graphene by α-beam irradiation. As the defect density increases upon irradiation, the surface potential of the graphene changes, as determined by Kelvin probe force microscopy and Raman spectroscopy studies. X-ray photoelectron spectroscopy studies indicate that the formation of C/O bonding is promoted as the dose of irradiation increases when at atmospheric conditions. Our results show that the surface potential of the graphene can be engineered by introducing atomic-scale defects via irradiation with high-energy particles.

Probing nanoscale conductance of monolayer graphene under pressure

The correlation between charge transport and mechanical deformation of the single layer graphene layer was studied with conductive probe atomic force microscopy/friction force microscopy in ultra-high vacuum. By measuring the current and friction on a graphene layer that is under mechanical stress, we identify crossover of two regimes in the current density that depend on the applied pressure. We suggest that the difference in work function under mechanical deformation as well as a change in the density of state and formation of a dipole field are responsible for this crossover behavior.

Reversible bistability of conductance on graphene/CuOx/Cu nanojunction

We report that a nanojunction composed of graphene, copper oxide, and Cu substrate exhibits resistive switching behavior, revealed with conductive probe atomic force microscopy at ultrahigh vacuum. The current-voltage curve measured between the titanium nitride-coated tip and the nanojunction exhibited reversible bistable resistance states. We propose that the switching behavior is controlled by the migration of oxygen ions in the copper oxide layer, leading to the reversible formation/disruption of a CuOx-associated charge tunneling barrier, which is consistent with glancing-angle x-ray photoelectron spectroscopy analysis.

Internal and External Atomic Steps in Graphite Exhibit Dramatically Different Physical and Chemical Properties

We report on the physical and chemical properties of atomic steps on the surface of highly oriented pyrolytic graphite (HOPG) investigated using atomic force microscopy. Two types of step edges are identified: internal (formed during crystal growth) and external (formed by mechanical cleavage of bulk HOPG). The external steps exhibit higher friction than the internal steps due to the broken bonds of the exposed edge C atoms, while carbon atoms in the internal steps are not exposed. The reactivity of the atomic steps is manifested in a variety of ways, including the preferential attachment of Pt nanoparticles deposited on HOPG when using atomic layer deposition and KOH clusters formed during drop casting from aqueous solutions. These phenomena imply that only external atomic steps can be used for selective electrodeposition for nanoscale electronic devices.

Strain effects on in-plane conductance of the topological insulator Bi2Te3

We investigated the correlation between electrical transport and mechanical stress in a topological insulator, Bi2Te3, using conductive probe atomic force microscopy in an ultrahigh vacuum environment. After directly measuring charge transport on the cleaved Bi2Te3 surface, we found that the current density varied with applied load. Current mapping revealed a variation of the current on different terraces. The current density increased in the low-pressure regime and then decreased in the high-pressure regime. This variation of current density was explained in light of the combined effect of changes in the in-plane conductance due to spin–orbit coupling and hexagonal warping.

Large changes of graphene conductance as a function of lattice orientation between stacked layers

Using the conductive tip of an atomic force microscope as an electrode, we found that the electrical conductance of graphite terraces separated by steps can vary by large factors of up to 100, depending on the relative lattice orientation of the surface and subsurface layers. This effect can be attributed to interlayer interactions that, when stacked commensurately in a Bernal sequence (ABAB...), cause the band gap to open. Misaligned layers, on the other hand, behave like graphene. Angular misorientations of a few degrees were found to cause large increases in the conductance of the top layer, with the maximum occurring around 30°. These results suggest new applications for graphene multilayers by stacking layers at various angles to control the resistance of the connected graphene ribbons in devices.

Local conductance mapping of water-intercalated graphene on mica

We report that the conductance of graphene is influenced by intercalated water layers using current sensing atomic force microscopy (AFM). We obtained a confined water layer between chemical vapor deposition graphene and mica by transferring graphene onto mica in a liquid water bath. Atomic force microscopy topographic images confirm high coverage by a single water layer, and scanning tunneling microscopy (STM) verifies a clean surface without contamination by measuring the honeycomb lattice structure of the graphene. We show that the surface conductance is perturbed by the presence of a water layer between the graphene and mica, which is not found in the STM topographic image. We found that the graphene on the edge and at pinholes of the water layer exhibits lower conductance, compared with that of graphene on the water terrace. We attribute the perturbation of conductance to structural defects from the water film and a variation of interaction between the edge of the water and graphene.