Ultrathin ZnO Layer Research Articles

Manipulating Trion and Biexciton Emissions in Monolayer WS2 by Sandwiching with Ultrathin ZnO Layers for Excitonic Light Emission Applications

Manipulating Trion and Biexciton Emissions in Monolayer WS₂ by Sandwiching with Ultrathin ZnO Layers for Excitonic Light Emission Applications

Surface states modulation of ZnTe via ultrathin ZnO layer as efficient photocathodes for CO2 reduction reaction

Surface states strongly affect charge transfer at the semiconductor/electrolyte interface and thus the total power conversion efficiency of photoelectrochemical CO2 reduction. In this study, we report surface states modulation of ZnTe via ultrathin ZnO layer formed with a one-step hydrothermal method. The resulting photocathode achieved an impressive performance (−7.70 mA cm−2, 93.88 % CO selectivity at −2.58 V vs. Fc+/Fc) and stability (10 h) in CO2-saturated acetonitrile with 0.1 M TBAPF6 electrolyte. Detailed investigation including electrochemical impedance spectroscopy and DFT calculation reveals the surface modification of ZnTe with ultrathin ZnO layer reduces the energy disorder, thereby increasing the electron utilization involved in the CO2 reduction reaction. This work provides an in-depth analysis of the charge transfer process at the semiconductor/electrolyte interface from the perspective of surface states and has the potential to stimulate further excellent research in this area.

High-Resolution Multicolor Patterning of InP Quantum Dot Films by Atomic Layer Deposition of ZnO

This paper presents the high-resolution (>2000 PPI) multicolor patterning of InP quantum dot films using a conventional photolithography process with a positive photoresist (PR). The solvent resistance of the quantum dot (QD) film is achieved by depositing an ultrathin ZnO layer through atomic layer deposition. This is different from previous studies, which lack high-resolution patterning or compatibility with indium phosphide (InP) QDs owing to chemical weaknesses. By employing a positive PR with a photoacid generator, the side-by-side patterning process yields multicolor patterns of red- and green-colored InP-based QDs. Additionally, the stacking of each color QD film is achieved. The patterning process can be used to fabricate QD light-emitting diode devices without degrading their performance. This process can be used not only for thin (<100 nm) QD films, which are used in QD-LED devices, but also for thick (>1 μm) QD films, which can be used in the color-conversion layer with a backlight.

Effect of surface modification via sol-gel spin coating of ZnO nanoparticles on the performance of WO3 photoanode based dye sensitized solar cells

In this paper, we have reported the improvement of electro-optical performance of dye sensitized solar cells based on highly porous WO3 nanoparticles as photoanode by an ultrathin coating of ZnO nanoparticles with varying precursor solution concentration over WO3 surface and the results were compared with the performance of a Dye Sensitized Solar Cell (DSSC) fabricated with bare WO3 photoanode. The semiconducting material WO3 was chosen in search of a photoanode material for DSSC alternative to TiO2. But the performance of pure WO3 based DSSC was found to be extremely poor in spite of having several advantageous properties. To improve the photovoltaic performance, we have coated the WO3 surface with varying concentrations of the ZnO precursor solution. It was observed that the concentration of the precursor solution of ZnO highly controls the performance of the DSSC. From the electrochemical measurements, it was found that the bare WO3 cell suffers high electron recombination. But the coating of an ultrathin layer of ZnO over the WO3 surface introduces an energy barrier and reduces the electron recombination and thereby enhances the cell performance. The solar cell energy conversion efficiency was found to be highest for 5 mM ZnO precursor solution concentration and it decreases with the increase in concentration further and becomes very low at a concentration of 25 mM. This may be attributed to the poor dye adsorption on the WO3 surface due to complete screening by the thicker ZnO layer.

Advanced Hybrid GaN/ZnO Nanoarchitectured Microtubes for Fluorescent Micromotors Driven by UV Light.

The development of functional microstructures with designed hierarchical and complex morphologies and large free active surfaces offers new potential for improvement of the pristine microstructures properties by the synergistic combination of microscopic as well as nanoscopic effects. In this contribution, dedicated methods of transmission electron microscopy (TEM) including tomography are used to characterize the complex hierarchically structured hybrid GaN/ZnO:Au microtubes containing a dense nanowire network on their interior. The presence of an epitaxially stabilized and chemically extremely stable ultrathin layer of ZnO on the inner wall of the produced GaN microtubes is evidenced. Gold nanoparticles initially trigger the catalytic growth of solid solution phase (Ga1- x Znx )(N1- x Ox ) nanowires into the interior space of the microtube, which are found to be terminated by AuGa-alloy nanodots coated in a shell of amorphous GaOx species after the hydride vapor phase epitaxy process. The structural characterization suggests that this hierarchical design of GaN/ZnO microtubes could offer the potential to exhibit improved photocatalytic properties, which are initially demonstrated under UV light irradiation. As a proof of concept, the produced microtubes are used as photocatalytic micromotors in the presence of hydrogen peroxide solution with luminescent properties, which are appealing for future environmental applications and active matter fundamental studies.

Near zero-bias MIIM diode based on TiO2/ZnO for energy harvesting applications

Energy harvesting rectennas require ultrafast rectifying diodes that are efficiently matched to the optical nanoantenna. These diodes should possess low on-resistance and high responsivity. Here, we introduce a metal-insulator-metal diode composed of a new material, Ti-TiO2-Al. This diode has a 1.0 nm ultrathin insulator layer fabricated using atomic layer deposition (ALD). It has a zero-bias resistance of 275 Ω and a maximum responsivity of 3.1 A/W. To further improve its performance, another ultrathin layer of ZnO was added. The proposed Ti-TiO2/ZnO-Al metal-insulator-insulator-metal diode has a zero-bias resistance of 312 Ω and a maximum responsivity of 5.1 A/W. The two types of diodes are fabricated on a SiO2 substrate using conventional photolithography and ALD. Between 20 °C and 55 °C, the I-V characteristics did not show much temperature dependence. The effective barrier height, dielectric constant, and electron effective mass in each insulator are extracted using a constrained and derivative-based optimization algorithm.

Synergistic Effect of 3D Current Collectors and ALD Surface Modification for High Coulombic Efficiency Lithium Metal Anodes

Improving the performance of Li metal anodes is a critical bottleneck to enable next‐generation battery systems beyond Li‐ion. However, stability issues originating from undesirable electrode/electrolyte interactions and Li dendrite formation have impaired long‐term cycling of Li metal anodes. In this work, we have developed a bottom‐up fabrication process of vertically aligned and highly uniform Cu pillar arrays on a Cu foil via templated electrodeposition for use as a 3D current collector.[1] By rationally controlling geometric parameters of the 3D current collector architecture, including pillar diameter, spacing, and length, we have demonstrated the morphology of Li plating/stripping upon cycling can be controlled and optimal cycling performance can be achieved. In addition, we showed that deposition of an ultrathin layer of ZnO by atomic layer deposition on the current collector surface can facilitate the initial Li nucleation, which dictates the morphology and reversibility of subsequent cycling. This core–shell pillar architecture allows for the effects of geometry and surface chemistry to be decoupled and individually controlled to optimize the electrode performance in a synergistic manner. Using this model platform, we have demonstrated Li metal anodes with Coulombic efficiency up to 99.5% at 0.5 mA/cm2 and 99.4% at 1 mA/cm2, which are among the highest reported values to date. The results gained from this work can thus provide a pathway toward high‐efficiency and long‐cycle life Li metal batteries with reduced excess Li loading. [1] K.-H. Chen, A.J. Sanchez, E. Kazyak, A.L. Davis, N.P. Dasgupta, Adv. Energy Mater. 9, 1802534 (2019).

Ultralow Schottky Barrier Height Achieved by Using Molybdenum Disulfide/Dielectric Stack for Source/Drain Contact.

Energy barrier formed at a metal/semiconductor interface is a critical factor determining the performance of nanoelectronic devices. Although diverse methods for reducing the Schottky barrier height (SBH) via interface engineering have been developed, it is still difficult to achieve both an ultralow SBH and a low dependence on the contact metals. In this study, a novel structure, namely, a metal/transition-metal dichalcogenide (TMD) interlayer (IL)/dielectric IL/semiconductor (MTDS) structure, was developed to overcome these issues. Molybdenum disulfide (MoS2) is a promising TMD IL material owing to its interface characteristics, which yields a low SBH and reduces the reliance on contact metals. Moreover, an ultralow SBH is achieved via the insertion of an ultrathin ZnO layer between MoS2 and a semiconductor, thereby inducing an n-type doping effect on the MoS2 IL and forming an interface dipole in the favorable direction at the ZnO IL/semiconductor interfaces. Consequently, the lowest SBH (0.07 eV) and a remarkable improvement in the reverse current density (by a factor of approximately 5400) are achieved, with a wide room for contact-metal dependence. This study experimentally and theoretically validates the effect of the proposed MTDS structure, which can be a key technique for next-generation nanoelectronics.

Constructing Multifunctional Interphase between Li1.4Al0.4Ti1.6(PO4)3 and Li Metal by Magnetron Sputtering for Highly Stable Solid‐State Lithium Metal Batteries

AbstractDue to high ionic conductivity and low cost, Li1.4Al0.4Ti1.6(PO4)3 (LATP) has emerged as a promising solid‐state electrolyte for next‐generation lithium (Li) metal solid‐state batterie with high safety performance and energy density. However, the extremely high impedance and surface instability of LATP with Li metal retard its practical application. Herein, a novel method is proposed to construct an ultrathin ZnO layer that is tightly coated on the LATP pellets, surface (ZnO@LATP) via magnetron sputtering, which in situ reacts with Li to form a low electronic conductivity and multifunctional solid electrolyte interphase (SEI). The formed SEI can not only effectively lower the interfacial resistance, but also overcome the side reactions of LATP with the Li metal anode and suppress the Li dendrite growth. Specifically, the interface resistance decreases from 80 554 to 353 Ω and the overpotential reduces from 1 V to 20 mV. As a result, the Li/ZnO@LATP@ZnO/Li symmetric batteries can stably cycle for more than 2000 h without short circuit at 0.05 mA cm−2 and Li/ZnO@LATP/LiFePO4 batteries show excellent cycle stability for 200 cycles at 0.1 C. This work highlights the significance of multifunctional interphase between LATP and Li metal for improvement of interfacial impedance and instability.

Resolving the Correlation between Tip-Enhanced Resonance Raman Scattering and Local Electronic States with 1 nm Resolution.

Low-temperature tip-enhanced Raman spectroscopy (TERS) enables chemical identification with single-molecule sensitivity and extremely high spatial resolution even down to the atomic scale. The large enhancement of Raman scattering obtained in TERS can originate from physical and/or chemical enhancement mechanisms. Whereas physical enhancement requires a strong near-field through excitation of localized surface plasmons, chemical enhancement is governed by resonance in the electronic structure of the sample, which is also known as resonance Raman spectroscopy. Here we report on tip-enhanced resonance Raman spectroscopy (TERRS) of ultrathin ZnO layers epitaxially grown on a Ag(111) surface, where both enhancement mechanisms are operative. In combination with scanning tunneling spectroscopy (STS), it is demonstrated that the TERRS intensity strongly depends on the local electronic resonance of the ZnO/Ag(111) interface. We also reveal that the spatial resolution of TERRS is dependent on the tip–surface distance and reaches nearly 1 nm in the tunneling regime, which can be rationalized by strong-field confinement resulting from an atomic-scale protrusion on the tip apex. Comparison of STS and TERRS mapping clearly shows a correlation between resonantly enhanced Raman scattering and the local electronic states at near-atomic resolution. Our results suggest that TERRS is a new approach for the atomic-scale optical characterization of local electronic states.

Synergistic Effect of 3D Current Collectors and ALD Surface Modification for High Coulombic Efficiency Lithium Metal Anodes

AbstractImproving the performance of Li metal anodes is a critical bottleneck to enable next‐generation battery systems beyond Li‐ion. However, stability issues originating from undesirable electrode/electrolyte interactions and Li dendrite formation have impaired long‐term cycling of Li metal anodes. Herein, a bottom‐up fabrication process is demonstrated for a current collector for Li metal electrodeposition and dissolution composed of highly uniform vertically aligned Cu pillars. By rationally controlling geometric parameters of the 3D current collector architecture, including pillar diameter, spacing, and length, the morphology of Li plating/stripping upon cycling can be controlled and optimal cycling performance can be achieved. In addition, it is demonstrated that deposition of an ultrathin layer of ZnO by atomic layer deposition on the current collector surface can facilitate the initial Li nucleation, which dictates the morphology and reversibility of subsequent cycling. This core–shell pillar architecture allows for the effects of geometry and surface chemistry to be decoupled and individually controlled to optimize the electrode performance in a synergistic manner. Using this platform, Li metal anodes are demonstrated with Coulombic efficiency up to 99.5%, providing a pathway toward high‐efficiency and long‐cycle life Li metal batteries with reduced excess Li loading.

Enhanced light absorption in simulations of ultra-thin ZnO layers structured by a SiO2 photonic glass

Hierarchically organized nanostructures are often employed to improve the energy conversion efficiency of photovoltaic and photoelectrochemical cells. Ultra-thin semiconductors can improve the internal carrier collection yield in materials with poor carrier lifetimes by reducing the characteristic length scales of collection. However, reducing the dimension of the light absorber requires strategies to increase absorption and the overall photogeneration when the material is to be used in broadband solar energy conversion applications. Here, we explore a strategy for improving light absorption in nanometer-scale, ultra-thin film ZnO layers by integrating them into a SiO2 colloidal crystal-based photonic glass. We use three-dimensional finite-difference electromagnetic simulations to study the local and total absorption improvements on composite films of close-packed, randomized colloidal structures coated with a thin layer of ZnO. These simulations show that the near band-gap absorption in the ZnO coating is dependent on the degree of vacancies in the colloidal crystal that templates the photonic glass. With these results, we show that disordered, defective colloidal composites can potentially be used to fabricate nanostructured photoelectrodes based on ultra-thin semiconductor layers with improved light absorption characteristics.

Enhanced resolution imaging of ultrathin ZnO layers on Ag(111) by multiple hydrogen molecules in a scanning tunneling microscope junction

Molecular hydrogen in a scanning tunneling microscope (STM) junction has been found to enhance the lateral spatial resolution of the STM imaging, referred to as scanning tunneling hydrogen microscopy (STHM). Here we report atomic resolution imaging of 2- and 3-monolayer (ML) thick ZnO layers epitaxially grown on Ag(111) using STHM. The enhanced resolution can be obtained at a relatively large tip to surface distance and resolves a more defective structure exhibiting dislocation defects for 3-ML-thick ZnO than for 2 ML. In order to elucidate the enhanced imaging mechanism, the electric and mechanical properties of the hydrogen molecular junction (HMJ) are investigated by a combination of STM and atomic force microscopy. It is found that the HMJ shows multiple kinklike features in the tip to surface distance dependence of the conductance and frequency shift curves, which are absent in a hydrogen-free junction. Based on a simple modeling, we propose that the junction contains several hydrogen molecules and sequential squeezing of the molecules out of the junction results in the kinklike features in the conductance and frequency shift curves. The model also qualitatively reproduces the enhanced resolution image of the ZnO films.

Methanol Oxidation to Formaldehyde Promoted at the Step Sites of Ultrathin ZnO

Adsorption and oxidation of methanol on ultrathin ZnO layers supported on Au(111) have been investigated using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. In the TPRS experiments, following adsorption of methanol-18O at T = 100 K, only molecular methanol-18O desorbed from the planar ZnO bilayer surface at T = 220 and 260 K, whereas a partial oxidation product, formaldehyde-18O (~ 95% selectivity), and a small amount of carbon dioxide (C16O18O) were produced at T = 580 K at the bilayer–trilayer step sites. The DFT calculations were used to identify the adsorption configurations of methanol on the planar ZnO surface and at the step sites, as well as the reaction pathways to gaseous formaldehyde. The most stable adsorption configuration corresponds to methanol molecule adsorbed at the bilayer–trilayer step sites with its C–O axis parallel to the upper terrace edge, forming a bond between its O atom and a Zn site on the lower terrace, and also a hydrogen bond between its H atom in the OH group and a lattice O anion at the upper terrace edge. Starting from the most stable adsorption configuration at the step sites, formation of gaseous formaldehyde was found to take place preferentially via a methoxy (CH3O(ad)) intermediate. This process follows the pathways CH3OH(ad) → CH3O(ad) + H(ad) → CH2O(g) + 2H(ad) and has an overall barrier of 19.0 kcal/mol. The reaction pathway to produce a lattice O-bonded formaldehyde (H2COOlattice(ad)), the proposed precursor leading to CO2, was found to be energetically less favorable with a barrier of ~ 38 kcal/mol. The preference to produce gaseous formaldehyde from the DFT calculations agrees well with the high selectivity toward formaldehyde observed in the TPRS experiments.

Free-Standing Atomically Thin ZnO Layers via Oxidation of Zinc Chalcogenide Nanosheets.

Monolayer ZnO represents a class of new two-dimensional (2D) materials that are expected to exhibit unique optoelectronic properties and applications. Here we report a novel strategy to synthesize free-standing atomically thin ZnO layers via the oxidation of hydrothermally grown ultrathin zinc chalcogenide nanosheets. With micrometer-scaled lateral size, the obtained ultrathin ZnO layer has a thickness of ∼2 nm, and the layered structure still maintained well after high temperature oxidation. The thermal treatment strongly improves the crystal quality as well without inducing cracks or pinholes in the ultrathin layers. The atomically thin ZnO layers are highly luminescent with dominant green emission. High quality white light is obtained from the mixed phosphors containing the ZnO layers, exhibiting their potential as compelling ultraviolet-excited phosphors.

Photoactivity enhancement of zinc sulfide ceramics thin films through ultrathin buffering engineering

Zinc-sulfide (ZnS) thin films 200nm-thick with various crystal features were fabricated using RF sputtering onto patterned sapphire substrates with and without ultrathin homo-ZnS and hetero-zinc oxide (ZnO) ultrathin buffer layers (approximately 45nm in thickness). Microstructural analyses revealed that the crystalline ZnS thin films with a columnar grain feature were deposited on the various ultrathin buffer layers-coated substrates through RF sputtering. The surface morphology of the ZnS thin films became rough and the crystal defect density of the ZnS thin films increased when the ZnS thin films were grown on the buffer layers. Comparatively, the rugged and island-like ZnO buffer layer engendered the crystal growth of the ZnS thin film with a higher degree of structural disorder than that of the crystal growth on the ZnS buffer layer. An increased crystal defect number together with the highly rugged film surface of the ZnS thin film buffered with ultrathin ZnO layers efficiently enhanced the photoactivity of the 200nm-thick ZnS thin film in this study.

Enhanced Performance of Nanowire-Based All-TiO2 Solar Cells using Subnanometer-Thick Atomic Layer Deposited ZnO Embedded Layer

In this paper, the effect of angstrom-thick atomic layer deposited (ALD) ZnO embedded layer on photovoltaic (PV) performance of Nanowire-Based All-TiO2 solar cells has been systematically investigated. Our results indicate that by varying the thickness of ZnO layer the efficiency of the solar cell can be significantly changed. It is shown that the efficiency has its maximum for optimal thickness of 1 ALD cycle in which this ultrathin ZnO layer improves device performance through passivation of surface traps without hampering injection efficiency of photogenerated electrons. The mechanisms contributing to this unprecedented change in PV performance of the cell have been scrutinized and discussed.

Local Characterization of Ultrathin ZnO Layers on Ag(111) by Scanning Tunneling Microscopy and Atomic Force Microscopy

We have studied the local structure of ultrathin ZnO layers grown on Ag(111) by the reactive deposition method using low-temperature scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM) at 5 K. The characteristic Moiré patterns arising from the lattice mismatch between the ZnO(0001) layers and Ag(111) appear in STM, but it is not pronounced in nc-AFM images. This indicates an atomically flat geometrical structure of the ZnO layer and a dominant contribution of the electronic state to the Moiré patterns imaged by STM. We found that the apparent height of STM for the ZnO layers strongly depends on the bias voltage and becomes comparable with that of nc-AFM when the bias voltage is below the conduction band edge of the ZnO layers. The ZnO layers with the STM (AFM) apparent height of 3.8 (4.0) ± 0.3 and 5.8 (6.1) ± 0.3 Å were observed. On the other hand, mapping the onset of the resonance state of the ZnO layer by scanning tunneling spectroscopy provides a basis for determining its thickness. Our results suggest that the ZnO layers on Ag(111) grow predominantly as bi- and trilayers under the conditions used.

Aqueous Solution Processed, Ultrathin ZnO Film with Low Conversion Temperature as the Electron Transport Layer in the Inverted Polymer Solar Cells

Ultrathin ZnO layer of ca. four nm thick deposited from an aqueous zinc oxide hydrate (ZnO·xH2O) solution is developed as the electron transport layer (ETL) in the inverted polymer solar cells (PSCs). Because of the low energy metal–ammine dissociation and hydroxide condensation/dehydration chemistry, a conversion temperature (TA) as low as 80 °C is achieved. With the active layer of poly(N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-3′,2′,1′-benzothiadiazole)) doped with [6,6]-phenyl C71 butyric acid methyl ester (PCDTBT:PC71BM), the average power conversion efficiency of the devices is up to 6.48%, and the short circuit current density reaches 11.4 mA cm–2. In comparison, the devices with traditional sol–gel processed ZnO electron transport layer has a power conversion efficiency of 5.53% at an annealing temperature of 200 °C. The device performance improvement is attributed to the better charge transportation and smoother surface of the ZnO film deposited from ZnO·xH2O aqueous solution. ...

Tailoring the interface quality between HfO2 and GaAs via in situ ZnO passivation using atomic layer deposition.

We investigated ZnO surface passivation of a GaAs (100) substrate using an atomic layer deposition (ALD) process to prepare an ultrathin ZnO layer prior to ALD-HfO2 gate dielectric deposition. Significant suppression of both Ga-O bond formation near the interface and As segregation at the interface was achieved. In addition, this method effectively suppressed the trapping of carriers in oxide defects with energies near the valence band edge of GaAs. According to electrical analyses of the interface state response on p- and n-type GaAs substrates, the interface states in the bottom half of the GaAs band gap were largely removed. However, the interface trap response in the top half of the band gap increased somewhat for the ZnO-passivated surface.