Thermal-Processing-Induced Structural Dynamics of Thiol Self-Assembly in Solution

Using ultrahigh vacuum scanning tunneling microscopy (UHV-STM), we studied the thermal-processing-induced structural changes that occur during formation of a 1-octanethiol (OT) self-assembled monolayer (SAM) on Au(111) near melting temperature in an OT solution. A favorable ordered phase of a c(4√3 × 2√3) structure was achieved at high temperature (e.g., 343 K) below the melting temperature of (e.g., 353 K) alkanethiol SAMs, while a favorably ordered phase of a (√3 × √3) structure was achieved at the melting temperature. The high resolution STM observation indicated the following: (1) the growth process of OT SAMs in an OT solution induced a change in the structural phase via diffusion of OT molecules on gold by thermal energy at a high temperature below the melting temperature; (2) application of the melting temperature resulted in partial desorption of OT molecules from the surface due to melting of OT SAMs, showing a striped phase and a disordered phase; and (3) after melting of the SAM, time-dependent rearrangement of OT molecules adsorbed on Au(111) occurred through relaxation of surface Au atoms, which caused thiolate–gold complexes to diffuse and reform a (√3 × √3) phase. Furthermore, the ultimate structural transition to a c(4√3 × 2√3) structure at 343 K revealed the translational transition of molecular adsorption sites induced by lateral movements of OT–Au complexes at a high temperature in a solution. Also, the absence of domain boundaries among the three mobile phases (i.e., disordered, striped, and ordered (√3 × √3)) at 353 K revealed that there is no transition of molecular adsorption sites after melting of the SAM

Electrochemical Nanoscale Templating: Laterally Self-Aligned Growth of Organic–Metal Nanostructures

The electrodeposition of Ag into organized surfactant templates adsorbed onto (22 × √3) reconstructed Au(111) is investigated by in situ electrochemical scanning tunneling microscopy. Ag⁺ concentrations of as low as 2.5 × 10^–6 M allow the visualization of the electrochemical molecular templating effect of a sodium dodecyl sulfate (SDS) adlayer. The SDS hemicylindrical stripes determine the adsorption sites of the Ag⁺ ions and the directionality of Ag nanodeposition. The SDS-Ag nanostructures grow along the long axis of SDS hemicylindrical stripes, and an interaction of Ag with the Au(111) substrate leads to a structural change in the SDS stripe pattern. The SDS-Ag nanostructures undergo dynamic rearrangement in response to changes in the applied electrode potential. At negative potentials, the orientations of SDS-Ag nanostructures are pinned by the (22 × √3) reconstructed pattern. Furthermore, observed differences in Ag nanostructuring on Au(111) without molecular templates (i.e., on a bare Au(111) surface) confirm the role of self-assembled organic templates in producing metal–organic nanostructures under control of the surface potential, which can determine the feature size, shape, and period of the metal nanostructure arrays

Nitrogen-Doped Partially Reduced Graphene Oxide Rewritable Nonvolatile Memory

As memory materials, two-dimensional (2D) carbon materials such as graphene oxide (GO)-based materials have attracted attention due to a variety of advantageous attributes, including their solution-processability and their potential for highly scalable device fabrication for transistor-based memory and cross-bar memory arrays. In spite of this, the use of GO-based materials has been limited, primarily due to uncontrollable oxygen functional groups. To induce the stable memory effect by ionic charges of a negatively charged carboxylic acid group of partially reduced graphene oxide (PrGO), a positively charged pyridinium N that served as a counterion to the negatively charged carboxylic acid was carefully introduced on the PrGO framework. Partially reduced N-doped graphene oxide (PrGO_DMF) in dimethylformamide (DMF) behaved as a semiconducting nonvolatile memory material. Its optical energy band gap was 1.7–2.1 eV and contained a sp² CC framework with 45–50% oxygen-functionalized carbon density and 3% doped nitrogen atoms. In particular, rewritable nonvolatile memory characteristics were dependent on the proportion of pyridinum N, and as the proportion of pyridinium N atom decreased, the PrGO_DMF film lost memory behavior. Polarization of charged PrGO_DMF containing pyridinium N and carboxylic acid under an electric field produced N-doped PrGO_DMF memory effects that followed voltage-driven rewrite-read-erase-read processes

Nonvolatile Memory Device Using Gold Nanoparticles Covalently Bound to Reduced Graphene Oxide

Nonvolatile memory devices using gold nanoparticles (AuNPs) and reduced graphene oxide (rGO) sheets were fabricated in both horizontal and vertical structures. The horizontal memory device, in which a singly and doubly overlayered semiconducting rGO channel was formed by simply using a spin-casting technique to connect two gold electrodes, was designed for understanding the origin of charging effects. AuNPs were chemically bound to the rGO channel through a π-conjugated molecular linker. The π-conjugated bifunctional molecular linker, 4-mercapto-benzenediazonium tetrafluoroborate (MBDT) salt, was newly synthesized and used as a molecular bridge to connect the AuNPs and rGOs. By using a self-assembly technique, the diazonium functional group of the MBDT molecular linker was spontaneously immobilized on the rGOs. Then, the monolayered AuNPs working as capacitors were covalently connected to the thiol groups of the MBDT molecules, which were attached to rGOs (AuNP-frGO). These covalent bonds were confirmed by XPS analyses. The current–voltage characteristics of both the horizontal and vertical AuNP-frGO memory devices showed noticeable nonlinear hysteresis, stable write–multiple read–erase–multiple read cycles over 1000 s, and a long retention time over 700 s. In addition, the vertical AuNP-frGO memory device showed a large current ON/OFF ratio and high stability

Ultrasensitive Carbon Monoxide Gas Sensor at Room Temperature Using Fluorine-Graphdiyne

Currently, most carbon monoxide (CO) gas sensors work at high temperatures of over 150 °C. Developing CO gas sensors that operate at room temperature is challenging because of the sensitivity trade-offs. Here, we report an ultrasensitive CO gas sensor at room temperature using fluorine-graphdiyne (F-GDY) in which electrons are increased by light. The GDY films used as channels of field-effect transistors were prepared by using chemical vapor deposition and were characterized by using various spectroscopic techniques. With exposure to UV light, F-GDY showed a more efficient photodoping effect than hydrogen-graphdiyne (H-GDY), resulting in a larger negative shift in the charge neutral point (CNP) to form an n-type semiconductor and an increase in the Fermi level from −5.27 to −5.01 eV. Upon CO exposure, the negatively shifted CNP moved toward a positive shift, and the electrical current decreased, indicating electron transfer from photodoped GDYs to CO. Dynamic sensing experiments demonstrated that negatively charged F-GDY is remarkably sensitive to an electron-deficient CO gas, even with a low concentration of 200 parts per billion. This work provides a promising solution for enhancing the CO sensitivity at room temperature and expanding the application of GDYs in electronic devices

Low-Temperature Layer-by-Layer Growth of Semiconducting Few-Layer γ‑Graphyne to Exploit Robust Biocompatibility

The sp-hybridized carbon network in single- or few-layer γ-graphyne (γ-GY) has a polarized electron distribution, which can be crucial in overcoming biosafety issues. Here, we report the low-temperature synthesis, electronic properties, and amyloid fibril nanostructures of electrostatic few-layer γ-GY. ABC stacked γ-GY is synthesized by layer-by-layer growth on a catalytic copper surface, exhibiting intrinsic p-type semiconducting properties in few-layer γ-GY. Thickness-dependent electronic properties of γ-GY elucidate interlayer interactions by electron doping between electrostatic layers and layer stacking-involved modulation of the band gap. Electrostatic few-layer γ-GY induces high electronic sensitivity and intense interaction with amyloid beta (i.e., Aβ40) peptides assembling into elongated mature Aβ40 fibrils. Two-dimensional biocompatible nanostructures of Aβ40 fibrils/few-layer γ-GY enable excellent cell viability and high neuronal differentiation of living cells without external stimulation

Efficient and Stable Solar Hydrogen Generation of Hydrophilic Rhenium-Disulfide-Based Photocatalysts <i>via</i> Chemically Controlled Charge Transfer Paths

Effective charge separation and rapid transport of photogenerated charge carriers without self-oxidation in transition metal dichalcogenide photocatalysts are required for highly efficient and stable hydrogen generation. Here, we report that a molecular junction as an electron transfer path toward two-dimensional rhenium disulfide (2D ReS2) nanosheets from zero-dimensional titanium dioxide (0D TiO2) nanoparticles induces high efficiency and stability of solar hydrogen generation by balanced charge transport of photogenerated charge carriers. The molecular junctions are created through the chemical bonds between the functionalized ReS2 nanosheets (e.g., −COOH groups) and −OH groups of two-phase TiO2 (i.e., ReS2–C6H5C­(O)–O–TiO2 denoted by ReS2–BzO–TiO2). This enhances the chemical energy at the conduction band minimum of ReS2 in ReS2–BzO–TiO2, leading to efficiently improved hydrogen reduction. Through the molecular junction (a Z-scheme charge transfer path) in ReS2–BzO–TiO2, recombination of photogenerated charges and self-oxidation of the photocatalyst are restrained, resulting in a high photocatalytic activity (9.5 mmol h–1 per gram of ReS2 nanosheets, a 4750-fold enhancement compared to bulk ReS2) toward solar hydrogen generation with high cycling stability of more than 20 h. Our results provide an effective charge transfer path of photocatalytic TMDs by preventing self-oxidation, leading to increases in photocatalytic durability and a transport rate of the photogenerated charge carriers

Molecular-Linked Z‑Scheme Heterojunction of Ti³⁺-Doped TiO₂ and WO₃ Nanoparticles for Photocatalytic Removal of Acetaldehyde

Photocatalytic removal of indoor organic air pollutants is effective, but there are practical limits to catalytic activation by indoor conditions. Here, we report a molecular-linked heterojunction of semiconducting metal oxide nanoparticles (e.g., Blue TiO2 and WO3) that can be activated by wide-range light including an indoor light-emitting diode (LED) under ambient conditions. Chemically reduced Blue TiO2 improves visible light absorption of white TiO2 by regulating the electronic structure with self-doping of Ti3+. The heterojunction between Blue TiO2 and WO3 is formed via a molecular linker, and a hybridized electronic structure of a molecular-linked Z-scheme alignment is generated without changes in chemical characteristics, increasing utilization of indoor light and effectively improving electron–hole separation. WO3 sufficiently adapts to the photooxidative degradation of air pollutants by •OH, while Blue TiO2 leads to the effective generation of •O2–, leading to the complete decomposition of gaseous acetaldehyde (CH3CHO) to CO2 and CO without remaining organic byproducts (e.g., formaldehyde). As a robust interfacial contact, molecular-linked heterojunctions provide efficient charge separation and highly stable performance and enhance solution-processable homogeneous coatings of metal oxide photocatalysts on real surfaces

Vertical Alignments of Graphene Sheets Spatially and Densely Piled for Fast Ion Diffusion in Compact Supercapacitors

Supercapacitors with porous carbon structures have high energy storage capacity. However, the porous nature of the carbon electrode, composed mainly of carbon nanotubes (CNTs) and graphene oxide (GO) derivatives, negatively impacts the volumetric electrochemical characteristics of the supercapacitors because of poor packing density (<0.5 g cm^–3). Herein, we report a simple method to fabricate highly dense and vertically aligned reduced graphene oxide (VArGO) electrodes involving simple hand-rolling and cutting processes. Because of their vertically aligned and opened-edge graphene structure, VArGO electrodes displayed high packing density and highly efficient volumetric and areal electrochemical characteristics, very fast electrolyte ion diffusion with rectangular CV curves even at a high scan rate (20 V/s), and the highest volumetric capacitance among known rGO electrodes. Surprisingly, even when the film thickness of the VArGO electrode was increased, its volumetric and areal capacitances were maintained

Inductive Effect of Lewis Acidic Dopants on the Band Levels of Perovskite for a Photocatalytic Reaction

Band-edge modulation of halide perovskites as photoabsorbers plays significant roles in the application of photovoltaic and photochemical systems. Here, Lewis acidity of dopants (M) as the new descriptor of engineering the band-edge position of the perovskite is investigated in the gradiently doped perovskite along the core-to-surface (CsPbBr3–CsPb1–xMxBr3). Reducing M–bromide bond strength with an increase in hardness of acidic M increases the electron ability of basic Br, thus strengthening the Pb–Br orbital coupling in M–Pb–Br, noted as the inductive effect of dopants. Especially, the highly hard Lewis acidic Mg localized in the outer position of the perovskite induces the increase of work function and then shifts band edge upward along the core-to-surface of the perovskite. Thus, charge separation driven by the dopant-induced internal electric field induces the slow annihilation of the excited holes, improving the slow aromatic Csp3–H dissociation in the photocatalytic oxidation process by ∼211% (491.39 μmol g–1 h–1) enhancements, compared with undoped nanocrystals