Thin Cap Layer Research Articles

Long-term effects of thin layer capping in the Grenland fjords, Norway: Reduced uptake of dioxins in passive samplers and sediment-dwelling organisms

The Grenlandfjords in South East Norway are severely contaminated with dioxins from a magnesium smelter operated between 1950 and 2001. In 2009, the proposal of thin-layer capping as a potential mitigation method to reduce spreading of dioxins from the fjord sediments, resulted in the set-up of a large-scale field experiment in two fjord areas at 30 and 100 m depth. After capping, several investigations have been carried out to determine effects on benthic communities and bioavailability of dioxins. In this paper we present the results on uptake of dioxins and furans (PCDD/F) in passive samplers and two sediment-dwelling species exposed in boxcores collected from the test plots during four surveys between 2009 (after cap placement) and 2018. Sediment profile images (SPI) and analyses of dioxins revealed that the thin (1–5 cm) cap layers became buried beneath several centimeters of sediments resuspended from adjacent bottoms and deposited on the test plots after capping. Uptake reduction ratios (R) were calculated as dioxins accumulated in cores collected from capped sediments divided by dioxins accumulated in cores collected from uncapped reference sediments. Cap layers with dredged clay or crushed limestone had only short-term positive effect with R-values increasing to about 1.0 (no effect) 1–4 years after capping. In spite of the recontamination, cap layers with clay and activated carbon had significant long-term effects with R-values slowly increasing from 0.12–0.33 during the first three years to 0.39–0.46 in 2018, showing 54–61% reduced uptake of dioxins (PCDD/F-TE) nine years after capping with AC.

SrSnO3 Field-Effect Transistors With Recessed Gate Electrodes

Fabrication of gate-recessed SrSnO3 (SSO) metal-semiconductor field-effect transistors (MESFETs) with Ni Schottky gates is reported on bi-layer epitaxial SSO films with a thin heavily-doped cap layer. Devices with 0.5- $\mu \text{m}$ gate length showed enhancement-mode behavior with a saturation drain current, ${I}_{\text {DSAT}}$ , of 33 mA/mm and peak transconductance, ${g}_{\text {m,max}}$ , of 65 mS/mm. The ${g}_{\text {m,max}}$ value is a roughly $2\times $ improvement over control devices fabricated on single-layer films. Gate-recessed SSO MESFETs with Pt Schottky gates were also explored on the bi-layer films. Devices with 1- $\mu \text{m}$ gate length displayed ${I}_{\text {DSAT}} =133$ mA/mm and $g_{\text {m,max}} =73$ mS/mm, after thermal annealing.

Anomalous minority carrier behavior induced by chemical surface passivation solution in p-type GaSb metal–oxide–semiconductor capacitors on Si substrates

p-type GaSb metal–oxide–semiconductor capacitors with thin InAs surface capping layers were prepared on Si(001) substrates. Epitaxial structures with superlattice metamorphic buffer layers were grown by molecular beam epitaxy. Chemical surface treatment and atomic layer deposition methods were employed for a semiconductor surface passivation and Al2O3 high-k oxide fabrication, respectively. Capacitance-voltage measurements and scanning and transmission electron microscopies were used to correlate electrical properties with the oxide-semiconductor interface structure of the capacitors. Unexpectedly, fast minority carrier response present down to liquid nitrogen temperature was observed in the capacitors passivated by an ammonium sulfide solution. This fast response was found to be related to etch pitlike surface morphology developed upon chemical passivation at the surface steps formed by microtwins and antiphase domain boundaries. Preferential InAs etching by ammonium sulfide at the surface defects was confirmed by analytical TEM studies. Very low activation energy of minority carrier response suggests the presence of electron sources under the gate; they result from growth-related surface defects that give rise to potential fluctuations of as high as half the GaSb bandgap.

Energetics and Energy Loss in 2D Ruddlesden–Popper Perovskite Solar Cells

Abstract2D Ruddlesden–Popper perovskites (RPPs) are emerging as potential challengers to their 3D counterpart due to superior stability and competitive efficiency. However, the fundamental questions on energetics of the 2D RPPs are not well understood. Here, the energetics at (PEA)2(MA)n−1PbnI3n+1/[6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) interfaces with varying n values of 1, 3, 5, 40, and ∞ are systematically investigated. It is found that n–n junctions form at the 2D RPP interfaces (n = 3, 5, and 40), instead of p–n junctions in the pure 2D and 3D scenarios (n = 1 and ∞). The potential gradient across phenethylammonium iodide ligands that significantly decreases surface work function, promotes separation of the photogenerated charge carriers with electron transferring from perovskite crystal to ligand at the interface, reducing charge recombination, which contributes to the smallest energy loss and the highest open‐circuit voltage (Voc) in the perovskite solar cells (PSCs) based on the 2D RPP (n = 5)/PCBM. The mechanism is further verified by inserting a thin 2D RPP capping layer between pure 3D perovskite and PCBM in PSCs, causing the Voc to evidently increase by 94 mV. Capacitance–voltage measurements with Mott–Schottky analysis demonstrate that such Voc improvement is attributed to the enhanced potential at the interface.

The emergence of local wrinkling or global buckling in thin freestanding bilayer films

Periodic wrinkling of a rigid capping layer on a deformable substrate provides a useful method for templating surface topography for a variety of novel applications. Many experiments have studied wrinkle formation during the compression of a rigid film on a relatively soft pre-strained elastic substrate, and most have focused on the regime where the substrate thickness can be considered semi-infinite relative to that of the film. As the relative thickness of the substrate is decreased, the bending stiffness of the film dominates, causing the bilayer to transition to either local wrinkling or a global buckling instability. In this work optical microscopy was used to study the critical parameters that determine the emergence of local wrinkling or global buckling of freestanding bilayer films consisting of a thin rigid polymer capping layer on a pre-strained elastomeric substrate. The thickness ratio of the film and substrate as well as the pre-strain were controlled and used to create a buckling phase diagram which describes the behaviour of the system as the ratio of the thickness of the substrate is decreased. A simple force balance model was developed to understand the thickness and strain dependences of the wrinkling and buckling modes, with excellent quantitative agreement being obtained with experiments using only independently measured material parameters.

Instability of UV Photoresponse in MgZnO Thin Films and Its Improvement by MgO Capping

Herein, it is shown that the UV photoresponse characteristics of MgZnO thin films deposited by pulsed laser deposition change rapidly with time if the films are exposed to ambient air and that the response can be stabilized significantly by depositing a thin capping layer of MgO on the surface of MgZnO thin film. The decay time of photocurrent in as‐deposited MgZnO thin films increases from 1.8 to 229 s and the value of peak responsivity increases by about three orders of magnitude as the film is exposed to air for about 106 s. On application of MgO capping layer, the decay time of the photocurrent marginally increases from ≈0.2 to ≈0.4 s and the value of peak responsivity increases only by one order of magnitude over the span of 106 s, thereafter showing tendency of saturation, implying much better stability in performance. The MgO capping layer is transparent to UV, but acts as a diffusion barrier for oxygen, stabilizing the UV photoresponse.

Hierarchically Ordered Silicon Metastructures from Improved Self-Assembly-Based Nanosphere Lithography.

We developed an improved self-assembly method to obtain a large-area, high-quality templated monolayer mask using the polystyrene spheres. On the basis of the templated mask, hierarchically ordered Si metastructures with different nanosteps are fabricated using cyclic inductively coupled plasma etching technique. By evaporating a thin gold capping layer on these Si metastructures, their optical properties are comparatively studied using the surface-enhanced Raman scattering spectroscopy. Our proposed technique is highly promising for fabricating a variety of periodic three-dimensional hierarchically ordered metastructures, which could be further utilized for applications in SERS-based biosensors, optical absorbers, metamaterial/metasurface devices, etc.

Mixed polyvinyl pyrrolidone hydrogel-mediated synthesis of high-quality Ag nanowires for high-performance transparent conductors

A novel mixed PVP hydrogel-mediated polyol reduction method was developed for the synthesis of high-quality Ag nanowires with negligible Ag nanoparticles produced, high aspect ratio (exceeding 1200), and a thin (∼1 nm) capping layer.

Interfacial-Redox-Induced Tuning of Superconductivity in YBa2Cu3O7-δ.

Solid-state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here, we report a simple, scalable approach allowing for manipulation of the superconducting transition in optimally doped YBa2Cu3O7-δ (YBCO) films via a chemically driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. This work demonstrates an effective step toward total ionic tuning of superconductivity in oxides, an interface-induced effect that goes well into the quasi-bulk regime, opening-up possibilities for electric field manipulation.

Controllable Perovskite Crystallization via Antisolvent Technique Using Chloride Additives for Highly Efficient Planar Perovskite Solar Cells

AbstractThe presence of surface and grain boundary defects in organic–inorganic halide perovskite films can be detrimental to both the performance and operational stability of perovskite solar cells (PSCs). Here, the effect of chloride additives is studied on the bulk and surface defects of the mixed cation and halide PSCs. It is found that using an antisolvent technique, the perovskite film is divided into two layers, i.e., a bottom layer with large grains and a thin capping layer with small grains. The addition of formamidinium chloride (FACl) into the precursor solution removes the small‐grained perovskite capping layer and suppresses the formation of bulk and surface defects, providing a perovskite film with enhanced crystallinity and large grain size of over 1 µm. Time‐resolved photoluminescence measurements show longer lifetimes for perovskite films modified by FACl and subsequently passivated by 1‐adamantylamine hydrochloride as compared to the reference sample. Impedance spectroscopy measurements show that these treatments reduce the recombination in the PSCs, leading to a champion device with power conversion efficiency (PCE) of 21.2%, an open circuit voltage of 1152 mV and negligible hysteresis. The Cl treated PSC also shows improved operational stability with only 12% PCE loss after 700 h under continuous illumination.

Set compliance current induced resistive memory characteristics of W/Hf/HfOx/TiN devices

In this paper, the authors have investigated the effect of current compliance during the set process on the resistive memory characteristics and switching mechanism of W/Hf/HfOx/TiN devices. The presence of an Hf thin cap layer enables the stable and uniform bipolar resistive switching behavior. Compliance current can modify the barrier height at the oxide-electrode interface by increasing or reducing the oxygen vacancies and induce different switching mechanisms. Low compliance current (50 μA) based switching confirms the Schottky conduction mechanism due to the interfacial effects, while high compliance current (500 μA) involves the ohmic conduction mechanism, signifying the formation of a conductive filament. No significant dispersion of reset current and reset voltage has been found for each set compliance current varying from 50 to 500 μA, indicating uniform performance of the devices. The devices also exhibited a read endurance up to 2000 cycles.

Exciton and Electron-Hole Liquid Emission under Conditions of Resonant Near-Field Coupling with Localized Plasmon Polaritons

The effect of gold nanoparticles on the emission of many-particle states in SiGe/Si quantum wells with thin capping layers is studied. It is found that near the plasmon resonance of the nanoparticles the enhancement factor for the visible-range luminescence of many-particle states in SiGe/Si quantum wells depends on the thickness of the Si capping layer and the type of the many-particle state.

Environmentally Tight TiO2–SiO2 Porous 1D‐Photonic Structures

AbstractAlthough thin film porosity is the basis of many optical sensors, it can be deleterious for a stable optical behavior of passive optical elements due to the condensation of water and other vapors in their pores. This paper proposes a new strategy for the magnetron sputtering (MS) fabrication of environmentally tight SiO2–TiO2 porous multilayers. Thin films of these two oxides deposited in an oblique angle configuration (MS‐OAD) present a nanocolumnar and highly porous nanostructure and, as a consequence, experience significant changes in their optical properties when exposed to water vapor. Similarly, the optical properties of Bragg reflectors and Bragg microcavities made of the stacking of porous and compact SiO2 and TiO2 thin films experience reversible changes when these 1D‐photonic structures are exposed to water pressure. A key finding of this work is that a very thin capping layer of SiO2 deposited on the surface of porous SiO2 films in the stack, at the interlayer between the two oxides, efficiently seals the pores making the photonic structures environmentally tight. This capping layer approach is a useful strategy to incorporate porosity as an additional parameter to design the optical behavior of planar photonic structures while preserving optical and environmental stability.

Different natures of sub-gap states in 135-CIGS/112-CIGS and 112-CIGS/135-CIGS heterostructures investigated by photoluminescence technique

CuIn1-xGaxSe2 (112-CIGS) has been proposed as a material for high efficiency thin film solar cells with reliably applicable photovoltaic properties. Higher efficiency devices can be achieved by various surface treatments and are believed to have Cu deficiency at the surface. One of the possible techniques is to have a very thin Cu-depleted surface, e.g. Cu(In1-xGax)3Se5 (135-CIGS) layer on top of 112-CIGS layer, thus the heterostructure of 135-CIGS/112-CIGS is formed. Our previous work show that the photoluminescence (PL) spectra of 135-CIGS with various thicknesses on 112-CIGS (135-CIGS/112-CIGS) heterostructure are identified as donor-to-acceptor pairs (DAPs) and free (conduction band) -to-bound (acceptor) transitions and show the temperature and excitation power dependence on the PL spectra. On the contrary, when the thin 112-CIGS layer is deposited on top of 135-CIGS (112-CIGS/135-CIGS), the PL spectra show more pronounced and resolved peaks which are surprisingly independent of temperature and excitation power and significantly distinguishable in its natures observed in those broad emissions of 135-CIGS/112-CIGS heterostructures. When the thickness of 112-CIGS on 135-CIGS is increased, the red shift in the order of ten meV is noticed in the emission lines from both 112-CIGS and 135-CIGS layers. These results suggest that the strains play more important roles in thinner capping layer and freeze out the temperature and excitation power dependence in the 112-CIGS/135-CIGS rather than the 135-CIGS/112-CIGS heterostructures.

Structural Evolution and Phase Change Properties of C-Doped Ge2Sb2Te5 Films During Heating in Air☆☆Supported by the National Natural Science Foundation of China under Grant No 11704161, the Natural Science Foundation of Jiangsu Province under Grant Nos BK20170309 and BK20151172, and the Changzhou Science and Technology Bureau under Grant Nos CJ20159049 and CJ20160028.

We elucidate the importance of a capping layer on the structural evolution and phase change properties of carbon-doped Ge2Sb2Te5 (C-GST) films during heating in air. Both the C-GST films without and with a thin SiO2 capping layer (C-GST and C-GST/SiO2) are deposited for comparison. Large differences are observed between C-GST and C-GST/SiO2 films in resistance-temperature, x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectra, data retention capability and optical band gap measurements. In the C-GST film, resistance-temperature measurement reveals an unusual smooth decrease in resistance above 110 °C during heating. X-ray diffraction result has excluded the possibility of phase change in the C-GST film below 170 °C. The x-ray photoelectron spectroscopy experimental result reveals the evolution of Te chemical valence because of the carbon oxidation during heating. Raman spectra further demonstrate that phase changes from an amorphous state to the hexagonal state occur directly during heating in the C-GST film. The quite smooth decrease in resistance is believed to be related with the formation of Te-rich GeTe4−nGen (n = 0, 1) units above 110 °C in the C-GST film. The oxidation of carbon is harmful to the C-GST phase change properties.

Designing pH-Responsive Biodegradable Polymer Coatings for Controlled Drug Release via Vapor-Based Route.

We present the design of a novel pH-responsive drug release system that is achieved by solventless encapsulation of drugs within a microporous membrane using a thin capping layer of biodegradable poly(methacrylic anhydride) (PMAH) coating. The coating was synthesized via a mild vapor polymerization process, namely, initiated chemical vapor deposition, which allowed perfect retention of the anhydride groups during deposition. The synthesized polyanhydride underwent degradation upon exposure to aqueous buffers, resulting in soluble poly(methacrylic acid). The degradation behavior of PMAH is highly pH-dependent, and the degradation rate under pH 10 is 15 times faster than that under pH 1. The release profile of a model drug rifampicin clearly exhibited two stages: the initial stage when the coatings were being degraded but the drugs were well stored and the second stage when drugs were gradually exposed to the medium and released. The drug release also showed strong pH responsiveness where the duration of the initial stage under pH 1 was more than 7 and 3 times longer than that under pH 10 and 7.4, respectively, and the release rates at pH 7.4 and 10 were significantly faster than that at pH 1. The pH-dependent degradation of the encapsulant thus enabled good preservation of drugs under low-pH environment but high drug release efficiency under neutral and alkaline environment, suggesting potential applications in site-specific drug delivery systems.

Bromine Doping as an Efficient Strategy to Reduce the Interfacial Defects in Hybrid Two-Dimensional/Three-Dimensional Stacking Perovskite Solar Cells.

Solar-to-electricity conversion efficiency, power conversion efficiency (PCE), and stability are two important aspects of perovskite solar cells (PSCs). However, both aspects are difficult to simultaneously enhance. In the recent two years, two-dimensional (2D)/three-dimensional (3D) stacking structure, designed by covering the 3D perovskite with a thin 2D perovskite capping layer, was reported to be a promising method to achieve both a higher PCE and improved stability simultaneously. However, when reducing the surface defects of 3D perovskite, the thin 2D capping layer itself may probably introduce additional interfacial defects in a 2D/3D stacking structure, which is thought to be able to trigger trap-assisted nonradiative recombination or ion migration. Thus, efforts should be paid to reduce the interfacial defects of 2D hybrid perovskite when serving as a modification layer in a 2D/3D stacking structure PSCs. Here, we demonstrate that bromine (Br) doping of the 2D perovskite capping layer is an efficient strategy to passivate interfacial defects robustly, by which the photoluminescence lifetime is enhanced notably, whereas the interfacial charge recombination is suppressed a lot. As a result, the PCE is enhanced from 18.01% (3D perovskite) to 20.07% (Br-doped 2D/3D perovskite) along with improved moisture stability.

Water-Repellent Low-Dimensional Fluorous Perovskite as Interfacial Coating for 20% Efficient Solar Cells.

Hybrid perovskite solar cells have been capturing an enormous research interest in the energy sector due to their extraordinary performances and ease of fabrication. However, low device lifetime, mainly due to material and device degradation upon water exposure, challenges their near-future commercialization. Here, we synthesized a new fluorous organic cation used as organic spacer to form a low-dimensional perovskite (LDP) with an enhanced water-resistant character. The LDP is integrated with three-dimensional (3D) perovskite absorbers in the form of MA0.9FA0.1PbI3 (FA = NH2CH = NH2+, MA = CH3NH3+) and Cs0.1FA0.74MA0.13PbI2.48Br0.39. In both cases, a LDP layer self-assembles as a thin capping layer on the top of the 3D bulk, making the perovskite surface hydrophobic. Our easy and robust approach, validated for different perovskite compositions, limits the interface deterioration in perovskite solar cells yielding to >20% power conversion efficient solar cells with improved stability, especially pronounced in the first hours of functioning under environmental conditions. As a consequence, single and multijunction perovskite devices, such as tandem solar cells, can benefit from the use of the waterproof stabilization here demonstrated, a concept which can be further expanded in the perovskite optoelectronic industry beyond photovoltaics.

Photoluminescence from GeSn nano-heterostructures

We investigate the distribution of Sn in GeSn nano-heteroepitaxial clusters deposited at temperatures well exceeding the eutectic temperature of the GeSn system. The 600 °C molecular beam epitaxy on Si-patterned substrates results in the selective growth of GeSn nano-clusters having a 1.4 ± 0.5 at% Sn content. These nano-clusters feature Sn droplets on their faceted surfaces. The subsequent deposition of a thin Ge cap layer induced the incorporation of the Sn atoms segregated on the surface in a thin layer wetting the nano-dots surface with 8 ± 0.5 at% Sn. The presence of this wetting layer is associated with a relatively strong photoluminescence emission that we attribute to the direct recombination occurring in the GeSn nano-dots outer region.

(Invited) Inverted Metamorphic Multijunction III-Vs for Photo-Electrochemical Hydrogen Production Systems: Challenges in Absorber Stabilization and Device Scale-up

While III-V semiconductors have achieved the highest photo-electrochemical solar-to-hydrogen conversion efficiencies, they are remarkably unstable during operation in a harsh electrolyte[1]. The first half of this talk will focus on the degradation mechanism of inverted metamorphic multijunction (IMM) III-V cells and surface modification strategies aimed at protecting them from photocorrosion. We applied noble metal catalysts, oxide coatings by atomic layer deposition, and MoS2 in an effort to protect the GaInP2 surface that was in contact with acidic electrolyte. We also grew thin epitaxial capping layers from III-V alloys that should be more intrinsically stable than GaInP2. The ability of the various modifications to protect the IMM’s surface was evaluated by operating at each electrode at short circuit for extended periods of time. The second part of this talk will identify the challenges encountered while scaling the IMM III-V absorber areas of from ~0.15 cm2 up to 16 cm2 and incorporating them in a photoreactor capable of generating 3 standard liters of hydrogen in 8 hours under natural sunlight. To successfully scale photo-electrochemical water-splitting technologies from bench to demonstration size requires addressing predictable and unpredictable complications. Despite using Comsol multiphysics to model our photoreactor and identify suitable specifications for a prototype, several practical issues were uncovered during testing that led to multiple iterations of photoreactor design between the initial and final generation. Several bottlenecks that ranged from counter electrode composition and orientation to bubble removal needed redress in order to meet our performance targets. Ultimately, the demonstration-scale system was able to generate nearly twice the target volume of hydrogen in an 8-hour outdoor trial. [1] NATURE ENERGY 2, 17028 (2017)