Detection of Low-Density Surface Sites on Silica: Experimental Evidence of Intrinsic Oxygen-Vacancy Defects

Low-density sites on planar fused silica surfaces are studied by titration with fluorescent probe molecules in an ultrahigh vacuum environment. Intrinsic sites chemically distinct from either hydroxyl or strained siloxane sites are identified by titration with a perylene derivative containing a vinyl functional group. Evidence is presented that these sites are oxygen vacancy defect (OVD) sites, which have previously been difficult to detect experimentally. The density of intrinsic OVD sites is shown to depend on pretreatment temperature, with an estimated density of approximately 10¹¹ sites/cm² for fused silica heated to 700 °C in vacuum. The influence of molecular and atomic deuterium exposure on various silica surface sites is also explored

Coverage-Dependent Luminescence from Two-Dimensional Systems of Covalently Attached Perylene Fluorophores on Silica

Photophysical processes occurring in two-dimensional systems of perylene derivatives covalently attached to planar silica surfaces are studied using a combination of steady-state and time-resolved measurements. Monomeric emission is observed for low fluorophore densities (below 0.15 nm^–2). A broad emission feature centered around 530 nm is observed for high fluorophore densities (above 0.44 nm^–2) and is attributed to excimer-like emission from partially overlapping perylene moieties. No distinct emission feature from the fully relaxed excimer state is observed, although the presence of a small fraction of such sites is inferred from both steady-state and time-resolved measurements. A one-carbon chain is used to anchor the fluorophore to the surface by the reaction of perylene-3-methanol with free hydroxyl and strained siloxane sites, likely hindering the ability of neighboring molecules to form a fully relaxed excimer state. Analysis of fluorescence decay profiles indicates energy migration between sites occurs even for low fluorophore densities

Selective Growth of Titanium Nitride on HfO₂ across Nanolines and Nanopillars

This work targets the area selective atomic layer deposition (AS-ALD) of TiN onto HfO₂ for use as the word line in a memory device. Unlike other patterning processes, AS-ALD eliminates etching steps and also allows for growth of patterned films with precise thickness control. This study investigates how AS-ALD differs on planar and nonplanar surfaces. Using a combination of X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy, we demonstrate a way to confer selectivity to a substrate using surface features. Self-assembled monolayers form defects at regions of high curvature, allowing nucleation of TiN films in ALD. This is in contrast to a treated planar surface with no features, which exhibits complete blocking of TiN up to a certain limit of ALD cycles

Functionalized Polycyclic Aromatic Polymers for High Temperature Wireless Chemical Memory Threshold Sensors

A pair of new polymers, poly­(di-<i>t</i>-butylacenaphthylene) (pTBAcN) and poly­(dipropylacenaphthylene) (pPAcN), were made via Friedel–Crafts alkylation of polyacenaphthylene. These polymers exhibit thermal stability beyond 250 °C and solubility in excess of 25 wt % in aliphatic hydrocarbons. Films of pPAcN over 10 μm thick were successively applied to planar surfaces via brush coating. Chemical memory for a passive wire resonant sensor was utilized to detect aliphatic hydrocarbons in high temperature environments. The coated threshold sensor showed a lower, stable resonant frequency before and after exposure to water at 130 °C for 3 h and showed an increased triggered resonant frequency after aliphatic exposure, consistent with an uncoated sensor device

Highly Controllable and Stable Quantized Conductance and Resistive Switching Mechanism in Single-Crystal TiO₂ Resistive Memory on Silicon

TiO₂ is being widely explored as an active resistive switching (RS) material for resistive random access memory. We report a detailed analysis of the RS characteristics of single-crystal anatase-TiO₂ thin films epitaxially grown on silicon by atomic layer deposition. We demonstrate that although the valence change mechanism is responsible for the observed RS, single-crystal anatase-TiO₂ thin films show electrical characteristics that are very different from the usual switching behaviors observed for polycrystalline or amorphous TiO₂ and instead very similar to those found in electrochemical metallization memory. In addition, we demonstrate highly stable and reproducible quantized conductance that is well controlled by application of a compliance current and that suggests the localized formation of conducting Magnéli-like nanophases. The quantized conductance observed results in multiple well-defined resistance states suitable for implementation of multilevel memory cells

Enhanced Photoluminescence of Monolayer WS₂ on Ag Films and Nanowire–WS₂–Film Composites

Monolayer transition metal dichalcogenides (TMDCs), due to their structural similarity to graphene, emerge as a promising alternative material of integrated optoelectronic devices. Recently, intense research efforts have been devoted to the combination of atomically thin TMDCs with metallic nanostructures to enhance the light–matter interaction in TMDCs. One crucial parameter for semiconductor–metallic nanostructure hybrids is the spacer thickness between the gain media and the plasmonic resonator, which needs to be optimized to balance radiation enhancement and radiation quenching. In current investigations of TMDCs–plamonic coupling, one often adopts a spacer thickness of ∼5 nm or larger, a typical value for transitional gain media–plasmonic composites. However, it is unclear whether this typical spacer thickness represents the optimal value for TMDCs–plasmonic hybrids. Here we address this critical issue by studying the spacer thickness dependence of the luminescent efficiency in the monolayer tungsten-disulfide (WS₂)–Ag film hybrids. Surprisingly, we discovered that the optimal thickness occurs at ∼1 nm spacer, much smaller than the typical value used previously. In a WS₂–Ag film system, at this optimal spacer thickness, the photoluminescence (PL) is increased by more than an order of magnitude due to exciton-coupled surface plasmon polaritons (SPPs), as compared to the as-grown WS₂ on sapphire. We further explore a new composite system comprising Ag nanowires on top of a WS₂–Ag film and observe additional enhancement of the PL (by a factor of 3) contributed by SPPs that are reflected from the end of the wires. Interestingly, in such a composite system, the additional improvement of the PL signal is observed only when the underlying Ag film is an epitaxial film instead of a commonly available thermal film. This is attributed to the reduction of propagation loss of the SPPs on atomically smooth, epitaxial films

A Low-Leakage Epitaxial High‑κ Gate Oxide for Germanium Metal–Oxide–Semiconductor Devices

Germanium (Ge)-based metal–oxide–semiconductor field-effect transistors are a promising candidate for high performance, low power electronics at the 7 nm technology node and beyond. However, the availability of high quality gate oxide/Ge interfaces that provide low leakage current density and equivalent oxide thickness (EOT), robust scalability, and acceptable interface state density (<i>D</i>_it) has emerged as one of the most challenging hurdles in the development of such devices. Here we demonstrate and present detailed electrical characterization of a high-κ epitaxial oxide gate stack based on crystalline SrHfO₃ grown on Ge (001) by atomic layer deposition. Metal–oxide–Ge capacitor structures show extremely low gate leakage, small and scalable EOT, and good and reducible <i>D</i>_it. Detailed growth strategies and postgrowth annealing schemes are demonstrated to reduce <i>D</i>_it. The physical mechanisms behind these phenomena are studied and suggest approaches for further reduction of <i>D</i>_it

Pulsed Laser Deposition of Epitaxial and Polycrystalline Bismuth Vanadate Thin Films

We report pulsed laser deposition (PLD) synthesis of epitaxial and polycrystalline monoclinic bismuth vanadate (BiVO₄, BVO) thin films. X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy were used to characterize the samples. Epitaxial, <i>c</i>-axis oriented growth was achieved using single crystal yttria-stabilized zirconia (100), a substrate temperature of 575–600 °C, and an oxygen pressure of 7.8 mTorr. The volatility of Bi necessitated a large excess (Bi:V = ∼6:1) of this element in the ceramic targets to obtain stoichiometric films. XRD confirmed a BVO (001)||YSZ (001) and BVO [100]||YSZ [100] epitaxial relationship. Film growth was 3-D, and the morphology was discontinuous, consisting of irregular, smooth grains. Additionally, dense, continuous polycrystalline films were deposited on fluorine-doped tin oxide (FTO) on glass substrates at room temperature with stoichiometric targets and postdeposition annealing in air. Evaluation of these samples as photoanodes yielded photocurrents of ∼0.15 and ∼0.05 mA cm^–2 at 1.23 V vs RHE under backside AM1.5G illumination with and without a hole scavenger (Na₂SO₃), respectively. We argue that the photocurrents are due to the high oxygen content inherent in the PLD process and suggest that these continuous films may be well-suited to investigating oxygen-related defects in BVO