Charge Trapping States at the SiO₂–Oligothiophene Monolayer Interface in Field Effect Transistors Studied by Kelvin Probe Force Microscopy

Using Kelvin probe force microscopy (KPFM) we studied the local charge trapping states at the SiO₂–oligothiophene interface in a field effect transistor (FET), where SiO₂ is the gate dielectric. KPFM reveals surface potential inhomogeneities within the oligothiophene monolayer, which correlate with its structure. A large peak of trap states with energies in the oligothiophene’s band gap due to hydroxyl groups is present at the oxide surface. We show that these states are successfully eliminated by preadsorption of a layer of (3-aminopropyl)triethoxysilane (APTES). Time-resolved surface potential transient measurements further show that the charge carrier injection in the nonpassivated FET contains two exponential transients, due to the charge trapping on the oxide surface and in the bulk oxide, while the APTES-passivated FET has only a single-exponential transient due to the bulk oxide. The results demonstrate that APTES is a good SiO₂ surface passivation layer to reduce trap states while maintaining a hydrophilic surface, pointing out the importance of dielectric surface passivation to bridge the gap between soft materials and electronic devices

Dynamic Charge Carrier Trapping in Quantum Dot Field Effect Transistors

Noncrystalline semiconductor materials often exhibit hysteresis in charge transport measurements whose mechanism is largely unknown. Here we study the dynamics of charge injection and transport in PbS quantum dot (QD) monolayers in a field effect transistor (FET). Using Kelvin probe force microscopy, we measured the temporal response of the QDs as the channel material in a FET following step function changes of gate bias. The measurements reveal an exponential decay of mobile carrier density with time constants of 3–5 s for holes and ∼10 s for electrons. An Ohmic behavior, with uniform carrier density, was observed along the channel during the injection and transport processes. These slow, uniform carrier trapping processes are reversible, with time constants that depend critically on the gas environment. We propose that the underlying mechanism is some reversible electrochemical process involving dissociation and diffusion of water and/or oxygen related species. These trapping processes are dynamically activated by the injected charges, in contrast with static electronic traps whose presence is independent of the charge state. Understanding and controlling these processes is important for improving the performance of electronic, optoelectronic, and memory devices based on disordered semiconductors

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

Tolerance of Intrinsic Defects in PbS Quantum Dots

Colloidal quantum dots exhibit various defects and deviations from ideal structures due to kinetic processes, although their band gap frequently remains open and clean. In this Letter, we computationally investigate intrinsic defects in a real-size PbS quantum dot passivated with realistic Cl-ligands. We show that the colloidal intrinsic defects are ionic in nature. Unlike previous computational results, we find that even nonideal, atomically nonstoichiometric quantum dots have a clean band gap without in-gap-states provided that quantum dots satisfy electronic stoichiometry

Catalyst Chemical State during CO Oxidation Reaction on Cu(111) Studied with Ambient-Pressure X‑ray Photoelectron Spectroscopy and Near Edge X‑ray Adsorption Fine Structure Spectroscopy

The chemical structure of a Cu(111) model catalyst during the CO oxidation reaction in the CO+O₂ pressure range of 10–300 mTorr at 298–413 K was studied <i>in situ</i> using surface sensitive X-ray photoelectron and adsorption spectroscopy techniques [X-ray photoelectron spectroscopy (XPS) and near edge X-ray adsorption fine structure spectroscopy (NEXAFS)]. For O₂:CO partial pressure ratios below 1:3, the surface is covered by chemisorbed O and by a thin (∼1 nm) Cu₂O layer, which covers completely the surface for ratios above 1:3 between 333 and 413 K. The Cu₂O film increases in thickness and exceeds the escape depth (∼3–4 nm) of the XPS and NEXAFS photoelectrons used for analysis at 413 K. No CuO formation was detected under the reaction conditions used in this work. The main reaction intermediate was found to be CO₂^δ−, with a coverage that correlates with the amount of Cu₂O, suggesting that this phase is the most active for CO oxidation

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

Scanning Tunneling Microscopy Study of the Structure and Interaction between Carbon Monoxide and Hydrogen on the Ru(0001) Surface

We use scanning tunneling microscopy (STM) to investigate the spatial arrangement of carbon monoxide (CO) and hydrogen (H) coadsorbed on a model catalyst surface, Ru(0001). We find that at cryogenic temperatures, CO forms small triangular islands of up to 21 molecules with hydrogen segregated outside of the islands. Furthermore, whereas for small island sizes (3–6 CO molecules) the molecules adsorb at <i>hcp</i> sites, a registry shift toward <i>top</i> sites occurs for larger islands (10–21 CO molecules). To characterize the CO structures better and to help interpret the data, we carried out density functional theory (DFT) calculations of the structure and simulations of the STM images, which reveal a delicate interplay between the repulsions of the different species

Reaction of CO with Preadsorbed Oxygen on Low-Index Copper Surfaces: An Ambient Pressure X‑ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study

The reaction of CO with chemisorbed oxygen on three low-index faces of copper was studied using ambient pressure X-ray photoelectron spectroscopy (XPS) and high-pressure scanning tunneling microscopy. At room temperature, the chemisorbed oxide can be removed by reaction with gas-phase CO in the 0.01–0.20 Torr pressure range. The reaction rates were determined by measuring the XPS peak intensities of O and CO as a function of time, pressure, and temperature. On Cu(111) the rate was found to be one order of magnitude faster than that on Cu(100) and two orders of magnitude faster than that on Cu(110). The apparent activation energies for CO oxidation were measured as 0.24 eV for O/Cu(111), 0.29 eV for O/Cu(100), and 0.51 eV for O/Cu(110) in the temperature range between 298 and 473 K. These energies are correlated to the oxygen binding energies on each surface

Revealing Correlation of Valence State with Nanoporous Structure in Cobalt Catalyst Nanoparticles by <i>In Situ</i> Environmental TEM

Simultaneously probing the electronic structure and morphology of materials at the nanometer or atomic scale while a chemical reaction proceeds is significant for understanding the underlying reaction mechanisms and optimizing a materials design. This is especially important in the study of nanoparticle catalysts, yet such experiments have rarely been achieved. Utilizing an environmental transmission electron microscope equipped with a differentially pumped gas cell, we are able to conduct nanoscopic imaging and electron energy loss spectroscopy <i>in situ</i> for cobalt catalysts under reaction conditions. Studies reveal quantitative correlation of the cobalt valence states with the particles’ nanoporous structures. The <i>in situ</i> experiments were performed on nanoporous cobalt particles coated with silica, while a 15 mTorr hydrogen environment was maintained at various temperatures (300–600 °C). When the nanoporous particles were reduced, the valence state changed from cobalt oxide to metallic cobalt and concurrent structural coarsening was observed. <i>In situ</i> mapping of the valence state and the corresponding nanoporous structures allows quantitative analysis necessary for understanding and improving the mass activity and lifetime of cobalt-based catalysts, for example, for Fischer–Tropsch synthesis that converts carbon monoxide and hydrogen into fuels, and uncovering the catalyst optimization mechanisms

In Situ Scanning Tunneling Microscopy and X‑ray Photoelectron Spectroscopy Studies of Ethylene-Induced Structural Changes on the Pt(100)-hex Surface

We have studied the structures of the Pt(100) surface in the presence of gas-phase ethylene at room temperature. High-pressure scanning tunneling microscopy shows that the hexagonal reconstruction on the clean Pt(100) surface is preserved under 1 Torr of C₂H₄, which produces an ethylidyne and di-σ-bonded ethylene saturated surface. At 5 × 10^–6 Torr of C₂H₄, coadsorbed CO from the background gases lifts the reconstruction, with the excess Pt atoms from the hexagonal surface forming islands on the surface. The chemisorption of CO from background gases in the vacuum system, in the nominally pure C₂H₄, is revealed by ambient-pressure X-ray photoelectron spectroscopy