The Effect of Dye Molecules and Surface Plasmons in Photon-Induced Hot Electron Flows Detected on Au/TiO₂ Nanodiodes

We report the combined influence of dye molecules and surface plasmons on hot electron flows generated by the absorption of photons. The study was enabled by the deposition of merbromin and rhodamin B on Au/TiO₂ Schottky diodes that exhibit connected island-shaped gold thin films. The short-circuit photocurrent and incident photon-to-electron conversion efficiency (IPCE) measured on Au/TiO₂ diodes show significant amplification of the photocurrent when the dye molecules are present on the modified Au islands/TiO₂, indicating that the combination of dye molecules and surface plasmons increases the energy conversion efficiency. The positions of the IPCE peaks are in good agreement with the absorbance spectrum of the dye molecules and surface plasmons, suggesting that the detection of hot electron flows can be used as a novel sensing mechanism to characterize charge transport through organic–inorganic and metal–oxide interfaces

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

Tuning Hydrophobicity of TiO₂ Layers with Silanization and Self-Assembled Nanopatterning

The wettability of TiO₂ layers is controlled by forming highly ordered arrays of nanocones using nanopatterning, based on self-assembly and dry etching. Nanopatterning of TiO₂ layers is achieved via formation of self-assembled monolayers of SiO₂ spheres fabricated using the Langmuir–Blodgett technique, followed by dry etching. Three types of TiO₂ layers were fabricated using the sol–gel technique, sputtering, and thermal process in order to address the relationship between the wettability and the structure of TiO₂ nanostructures. Compared to a thin film TiO₂ layer, the nanopatterned TiO₂ samples show a smaller static water contact angle (i.e., where the water contact angle decreases as the etching time increases), which is attributed to the Wenzel equation. When TiO₂ layers are coated by 1H,1H,2H,2H-perfluorooctyltrichlorosilane, we observed the opposite behavior, exhibiting superhydrophobicity (up to contact angle of 155°) on the nanopatterned TiO₂ layers. Self-assembled nanopatterning of the TiO₂ layer may provide an advanced method for producing multifunctional transparent layers with self-cleaning properties

In Situ Observations of UV-Induced Restructuring of Self-Assembled Porphyrin Monolayer on Liquid/Au(111) Interface at Molecular Level

Porphyrin-derived molecules have received much attention for use in solar energy conversion devices, such as artificial leaves and dye-sensitized solar cells. Because of their technological importance, a molecular-level understanding of the mechanism for supramolecular structure formation in a liquid, as well as their stability under ultraviolet (UV) irradiation, is important. Here, we observed the self-assembled structure of free-base, copper­(II), and nickel­(II) octaethylporphyrin formed on Au(111) in a dodecane solution using scanning tunneling microscopy (STM). As evident in the STM images, the self-assembled monolayers (SAMs) of these three porphyrins on the Au(111) surface showed hexagonal close-packed structures when in dodecane solution. Under UV irradiation (λ = 365 nm), the porphyrin molecules in the SAM or the dodecane solution move extensively and form new porphyrin clusters on the Au sites that have a high degree of freedom. Consequently, the Au(111) surface was covered with disordered porphyrin clusters. However, we found that the porphyrin molecules decomposed under UV irradiation at 254 nm. Molecular-scale observation of the morphological evolution of the porphyrin SAM under UV irradiation can provide a fundamental understanding of the degradation processes of porphyrin-based energy conversion devices

Enhancement of Hot Electron Flow in Plasmonic Nanodiodes by Incorporating PbS Quantum Dots

The enhancement of hot electron generation using plasmonic nanostructures is a promising strategy for developing photovoltaic devices. Here, we show that hot electron flow generated in plasmonic Au/TiO₂ nanodiodes by incident light can be amplified when PbS quantum dots are deposited onto the surface of the nanodiodes. The effect is attributed to efficient extraction of hot electrons via a three-dimensional Schottky barrier, thus giving new pathways for hot electron transfer. We also demonstrate a correlation between the photocurrent and Schottky barrier height when using PbS quantum dots with varying size and ligand treatments that allow us to control the electric properties (e.g., band gap and Fermi level, respectively) of the PbS quantum dots. This simple method introduces a new technique for further improving the power conversion efficiency of thin-film photovoltaic devices

Thermal Evolution and Instability of CO-Induced Platinum Clusters on the Pt(557) Surface at Ambient Pressure

Carbon monoxide (CO) is one of the most-studied molecules among the many modern industrial chemical reactions available. Following the Langmuir–Hinshelwood mechanism, CO conversion starts with adsorption on a catalyst surface, which is a crucially important stage in the kinetics of the catalytic reaction. Stepped surfaces show enhanced catalytic activity because they, by nature, have dense active sites. Recently, it was found that surface-sensitive adsorption of CO is strongly related to surface restructuring via roughening of a stepped surface. In this scanning tunneling microscopy study, we observed the thermal evolution of surface restructuring on a representative stepped platinum catalyst, Pt(557). CO adsorption at 1.4 mbar CO causes the formation of a broken-step morphology, as well as CO-induced triangular Pt clusters that exhibit a reversible disordered–ordered transition. Thermal instability of the CO-induced platinum clusters on the stepped surface was observed, which is associated with the reorganization of the repulsive CO–CO interactions at elevated temperature

In Situ Observation of Competitive CO and O₂ Adsorption on the Pt(111) Surface Using Near-Ambient Pressure Scanning Tunneling Microscopy

We investigated the competitive coadsorption of CO and O₂ molecules on a Pt model surface using a catalytic reactor integrated with a scanning tunneling microscope at elevated pressure. CO-poisoned incommensurate atom-resolved structures are observed on the terrace sites of the Pt(111) surface under gaseous mixtures of CO and O₂. However, in situ surface measurements revealed that segmented local structures were influenced by the CO/O₂ partial pressures in the catalytic reactor at a total pressure of a few Torr. This could be related to the expected formation of the theoretical oxygen precursor intermediates during dissociation of O₂ on the surface before the chemical reaction. These findings provide microscopic insights into the early steps of the catalytic reaction pathways on the Pt surface during CO oxidation in an industrial chemical reactor

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

Support Effect of Arc Plasma Deposited Pt Nanoparticles/TiO₂ Substrate on Catalytic Activity of CO Oxidation

The smart design of nanocatalysts can improve the catalytic activity of transition metals on reducible oxide supports, such as titania, via strong metal–support interactions. In this work, we investigated two-dimensional Pt nanoparticle/titania catalytic systems under the CO oxidation reaction. Arc plasma deposition (APD) and metal impregnation techniques were employed to achieve Pt nanoparticle deposition on titania supports, which were prepared by multitarget sputtering and sol–gel techniques. APD Pt nanoparticles with an average size of 2.7 nm were deposited on sputtered and sol–gel-prepared titania films to assess the role of the titania support on the catalytic activity of Pt under CO oxidation. In order to study the nature of the dispersed metallic phase and its effect on the activity of the catalytic CO oxidation reaction, Pt nanoparticles were deposited in varying surface coverages on sputtered titania films using arc plasma deposition. Our results show an enhanced activity of Pt nanoparticles when the nanoparticle/titania interfaces are exposed. APD Pt shows superior catalytic activity under CO oxidation, as compared to impregnated Pt nanoparticles, due to the catalytically active nature of the mild surface oxidation and the active Pt metal, suggesting that APD can be used for large-scale synthesis of active metal nanocatalysts

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