Ultrathin ZnO Research Articles

Spectroscopic analysis of ultrathin amorphous ZnO films grown by atomic layer deposition

Ultrathin amorphous Zinc Oxide films were prepared on glass substrates by homemade atomic layer deposition system using Diethyl Zinc and Deionized water as precursors and high pure Argon gas as purging gas. The substrate temperature was set as 125℃ and cycle number was varied. Different spectroscopic techniques were employed to unveil the structural and optical properties of films. The effect of quantum confinement on band gap energy values reduces with increase in thickness. Resonant Raman scattering was observed for all the samples when excited with laser wavelength of 325 nm.

Strain and support effects on phase transition and surface reactivity of ultrathin ZnO films: DFT insights

Strain and support effects play a crucial role in heterogeneous catalysis, which has been intensively studied over metal-based catalysts. In contrast, there is little discussion about the two effects in oxide systems. In this work, using an ultrathin ZnO film as an example, we investigate strain and support effects on the structure and surface reactivity of oxide catalysts through density functional theory calculations. Our results suggest that tensile strain increases the surface reactivity of ZnO films as indicated by enhanced CO and NH3 adsorptions and compressive strain renders an early phase transition from an inert graphene-like phase to a more reactive wurtzite-like phase. The support (Au, Pt, and Ru) can promote the phase transition and surface reactivity concurrently, which exhibits a larger effect on the reactivity than the strain. The support effect can be ascribed to the increasing rumple and polarization of ZnO films through the strong ZnO–substrate interaction, which enhances the surface reactivity. The insight helps us to develop advanced oxide-based catalysts through the strain and/or substrate engineering.

Hydrogen sensing characteristics of Pd-decorated ultrathin ZnO nanosheets

Pristine and Pd-decorated two-dimensional (2D) ZnO nanosheets were used for hydrogen detection. The 2D ZnO nanosheets were prepared through a facile hydrothermal method, and Pd-decoration was carried out using UV irradiation. Formation of ZnO nanosheets with the desired morphology, chemical composition, and crystallinity was demonstrated through different characterization techniques. In particular, the thickness of each ZnO nanosheet was approximately 1 nm, corresponding to a few atomic layers. Sensing studies on hydrogen gas revealed the positive effects of Pd-decoration on response, sensing temperature, and selectivity. The Pd-decorated nanosheet sensor successfully sensed the low concentrations of hydrogen gas. Moreover, the gas sensor was able to detect hydrogen in a gaseous mixture with benzene. Moreover, a mixture of H2 and interfering gas exhibited a noticeable detection, exhibiting higher and lower responses than pure interfering gas and pure H2 gas, respectively. Gas sensing experiments in self-heating mode were conducted and showed that the optimal voltage for self-heating operation decreased from 20 to 5 V after Pd-decoration. At a low temperature, Pd-decorated ultrathin ZnO nanosheets showed the p-type sensing, presumably from the combined effects of enhanced oxygen adsorption and ZnO-to-Pd electron transfer. In addition, we examined the flexibility by means of bending, tilting, and stretching tests. Sensing enhancement of the Pd-decorated sensor originates from the presence of Pd–ZnO heterojunctions, formation of PdHx, and the high catalytic activity of Pd.

Growth and Raman spectroscopy of ultrathin ZnO(0001) films on Ag(001)

The growth and vibrational properties of ultrathin ZnO(0001) films on Ag(001) were investigated by X-ray and ultraviolet photoemission spectroscopy (XPS/UPS), low-energy electron diffraction (LEED), and Raman spectroscopy. Our results indicate the formation of well-defined stoichiometry and crystalline phase ~5 ML thick ZnO(0001) films characterized by two domains rotated by 30°. Further film analysis using Raman spectroscopy showed that the ultrathin films display substantial intensity enhancement of their Raman combination modes. Our findings suggest a significant role of the finite-size film electronic and structural properties on its vibrational properties. These observations may serve as a pathway toward further investigations dependent on film thickness and orientation. More important, the modification of the vibrational properties is expected to be relevant to understanding general phenomena in ZnO nanostructures (i.e. thin films, nanoparticles, nanorods).

Modulating the band alignment and current conduction mechanism of ZrO2/In0.2 Ga0.8As gate stack by atomic-layer-deposited ZnO passivation layer

Effect of an ultrathin ZnO interface layer (IL) on the band alignment and carrier transportation in atomic-layer-deposited ZrO2 high-k gate dielectrics on p-In0.2Ga0.8As substrates has been investigated. Based on x-ray photoelectron spectroscopy (XPS), the valence-band offset and conduction-band offset of ZrO2/ZnO bilayer with In0.2Ga0.8As are found to be 2.36 and 1.86 eV, respectively. Meanwhile, introducing ZnO IL has made the conduction-band offset increased by 0.26 eV as compared with monolayer ZrO2, thus remarkably suppressing the gate leakage current. It has been revealed by the current density-voltage characteristics measured from 27 °C to 102 °C that Schottky emission is dominant at low positive bias, while Ohm's law dominates the conduction mechanism at high positive bias for ZnO passivated p-In0.2Ga0.8As. However, for ZrO2/In0.2Ga0.8As, the leakage current follows Schottky emission at low positive bias and space-charge-limited conduction at high bias, which is in good agreement with the energy-band parameters extracted by XPS. The ZnO passivation dependent energy-band diagrams of the ZrO2/p-In0.2Ga0.8As heterostructures presented in this paper could be very valuable and beneficial for making ZrO2/ZnO/InGaAs great potential gate stacks in future III-V-based high-performance and low-power devices.

Enhanced electrical properties of ZrO2-TiN based capacitors by introducing ultrathin metal oxides

Owing to the scaling down of dynamic random-access-memory, the development of new high-k dielectrics as well as the reduction in the equivalent-oxide-thickness value using ZrO2 and TiN electrode-based capacitors have become crucial. Because the unwanted interfacial layer between ZrO2 and TiN electrodes can induce the degradation of electrical properties, we propose a new approach for improving capacitor properties by introducing ultrathin metal oxides as the buffers. Each ultrathin TiO2, Ta2O5, and ZnO is inserted between ZrO2 thin films and the top or bottom electrodes, and the variations in their electrical properties are investigated. We discovered that the electrical properties of ZrO2-based capacitors, such as the dielectric constant and leakage current density, can be improved by introducing certain types of buffers without requiring a noble metal electrode and higher-k dielectrics. Furthermore, we discovered that the EOT scaling of approximately 0.15 nm is achievable only through the introduction of the appropriate buffer.

Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation.

Tip-enhanced Raman scattering (TERS) in ångström-scale plasmonic cavities has drawn increasing attention. However, Raman scattering at vanishing cavity distances remains unexplored. Here, we show the evolution of TERS in transition from the tunneling regime to atomic point contact (APC). A stable APC is reversibly formed in the junction between an Ag tip and ultrathin ZnO or NaCl films on the Ag(111) surface at 10 K. An abrupt increase of the TERS intensity occurs upon APC formation for ZnO, but not for NaCl. This remarkable observation is rationalized by a difference in hybridization between the Ag tip and these films, which determines the contribution of charge transfer enhancement in the fused plasmonic junction. The strong hybridization between the Ag tip and ZnO is corroborated by the appearance of a new vibrational mode upon APC formation, whereas it is not observed for the chemically inert NaCl.

Urea mediated synthesis and acetone-sensing properties of ultrathin porous ZnO nanoplates

Facile synthesis of advanced porous nanomaterials for enhanced gas sensing performance has gained increasing interest. Here, we report our study on the preparation of ultrathin porous ZnO nanoplates by a urea mediated hydrothermal method for acetone gas-sensor application. The synthesized ZnO nanomaterials were nanoplates with a porous structure, high crystallinity, and ultrathin thickness of approximately 15 nm. The gas-sensing characteristics of the ultrathin porous ZnO nanoplates were studied at 350–450 °C. The sensor showed the highest response to acetone among the tested gases (methanol, ethanol, acetone, toluene and ammonia); the response to 2.5 ppm acetone was 1.8. The sensor also showed a low 45 ppb detection limit with excellent stability and selectivity at 450 °C. The acetone-sensing mechanism of the ultrathin porous ZnO nanoplates was also discussed. Overall, the findings indicated that the ultrathin porous ZnO nanoplates were excellent materials for sensing applications (e.g., breath analysis) of volatile organic compounds.

Design and construction of hierarchical α-Co(OH)2-coated ultra-thin ZnO flower nanostructures on nickel foam for high performance supercapacitors

Abstract Most pseudocapacitor materials have only near-surface parts participated into the Faradic-reaction, and their low contribution limits the development of pseudocapacitors. In this situation, hierarchical α-Co(OH)2-coated ultra-thin ZnO flower nanostructures on nickel foam (NF@ZnO@Co(OH)2) are designed and constructed to enhance the efficiency of charge and ion-transfer process as well as to improve the low contribution of pseudoactive materials during the Faradic-reaction. The ultra-thin petal ZnO flower-like nanostructures can take the role of the effective backbones for succeeding synthesis of electrochemical active materials. The optimized electrodes exhibited an outstanding pseudocapacitive performance, including an excellent durability, wide working potential window (0.55 V in galvanostatic charge-discharge measurement) and a high specific capacitance of 2396.4 F g−1. Additionally, we have constructed an asymmetric supercapacitor device and it delivers a prominent energy density of 62.57 Wh kg−1 at a power density of 711.28 W kg−1 with a cell voltage of 1.5 V and an excellent durability. Thereby, the NF@ZnO@Co(OH)2 is one promising supercapacitors electrode material for practical application.

Ultrathin Al-Doped ZnO Films Coated Li4Ti5O12 As an Anode Material with Excellent Cycling Stability and Rate Capability for Lithium-Ion Batteries

Lithium titanate oxide (Li4Ti5O12, LTO) is a lithium ion intercalated compound, which was identified as a promising anode material for LIBs with a theoretical capacity of 175 mAh g-1 within a typical potential range of 1.0 V–3.0 V. Extending the end voltage from 1.0 V to 0.01 V is an effective way to improve the electrochemical performance of LTO; the tetrahedral (8a) sites of LTO would be available for lithium ion storage to have an “extra” reversible capacity, when the end potential is lower than 0.6 V. However, this approach also leads to some other problems: (1) a lower end potential will cause a more intensive decomposition of electrolyte to form the solid electrolyte interface (SEI), which means that the initial irreversible capacity loss will increase, and (2) the change in lattice dimension during the lithium ion insertion/extraction will be increased and thus the structure stability of LTO will become worse. In this study, an Al-doped ZnO (AZO) film with various thicknesses and compositions were deposited on LTO particles by atomic layer deposition (ALD) method with different deposition temperatures and different ratios of the number of Al2O3 ALD to that of ZnO ALD. The electrochemical characterization results indicated that the coating of AZO significantly improved the electrochemical performance of LTO between 0.1 V and 3.0 V with a proper coating thickness. After 250 cycles of charge/discharge at a 1 C rate, the capacity of the optimum AZO-coated LTO sample decreased from 203 to 190 mAh g-1 with a capacity retention of ~94% at room temperature and from 224 to 216 mAh g-1 with a capacity retention of ~96% at 55 ℃. The AZO coating layer with an appropriate thickness not only increased the capacity of LTO by enhancing conductivity, but also assisted the LTO electrodes to form a beneficial interface layer to protect the LTO from the continuous attack by harmful components in the electrolyte, especially under extreme conditions, such as high temperature and high current rates.

Flexible Supercapacitor-Type Rectifier-free Self-Charging Power Unit Based on a Multifunctional Polyvinylidene Fluoride-ZnO-rGO Piezoelectric Matrix.

The development of an effective mechanical to electrical energy conversion device and its functional integration with an energy storage device for self-powered portable gadgets are cutting-edge research fields. However, the generated power and the mechanical stability of these integrated devices are still not efficient to power up portable electronics. We fabricated a rectifier-free piezoelectric nanogenerator (NG) integrated with a supercapacitor (SC). A multifunctional composite matrix was prepared by the incorporation of ultrathin (<10 nm) ZnO nanoflakes and reduced graphene oxide in polyvinylidene fluoride to enhance the piezoelectric output characteristics and mechanical stability of the device while minimizing the additional energy losses during the integration. The as-fabricated SC-based power unit through the energy conversion and storage processes showed a remarkable self-charging performance. We obtained the maximum output voltage, current density, and power density of about 44 V, 1000 nA cm-2, and 193.6 μW cm-2 under the applied mechanical force of 10 N, respectively. The self-charging behavior of the device showed that it can store 1.5 × 10-3 mC within 100 s without resorting to a rectifier. We obtained the total energy density of about 10.34 mW h kg-1 under palm impact. Our results present a step forward in the development of the NG and SC-based flexible and self-charging devices.

Development of ALD ZnO:Al as transparent conductive films

We report the deposition of ultrathin Al-doped ZnO (AZO) films by using atomic layer deposition (ALD) and results from X-ray photoelectron spectroscopy (XPS) and photoelectrical measurements. The AZO films were deposited on glass, silicon and polyethylene terephthalate (PET) substrates and their structural and photoelectrical properties were measured and discussed. Based on the results, selected AZO films deposited on PAT were implemented as transparent electrodes in a polymer dispersed liquid crystal display device (PDLC). The electro-optical modulation characteristics of these devices are comparable to those of devices using commercial ITO electrodes.

Rational design of hierarchical C–MgO@ZnO nanofiber as sulfur host for high-performance lithium–sulfur batteries

The development of sulfur cathode materials for lithium–sulfur (Li–S) batteries has significant technical challenges owing to poor conductivity and “shuttle effect” of the polysulfide dissolution. In recently, many efforts have been made to improve the electrochemical performance of cathode materials for Li–S batteries. In the present investigation, MgO-doped and nitrogen-rich carbon nanofibers have been prepared via electrospinning technology under carbonization treatment. Ultrathin ZnO nanolayer is further coated on the surface of the carbon nanofiber by atomic layer deposition and then sulfur is loaded inside the nanofibers to achieve a C–MgO/S@ZnO composite. This composite was used as cathode material in Li–S batteries. They are exhibited maximum rate capabilities of 1066.08 and 846.75 mA h g−1 at 0.5 and 2 C, respectively. Moreover, the discharge capacity is 812.67 mA h g−1 after 100 cycles at 2 C and the capacity decay after 100 cycles is as small as 0.04% per cycle. The excellent electrochemical performance of C–MgO/S@ZnO nanofibers could be ascribed by the rational design due to the structure and composition of sulfur host.

Atomic layer deposition for improved lithiophilicity and solid electrolyte interface stability during lithium plating

Uneven deposition and stripping of Lithium (Li) can lead to dendrite growth and instability of solid electrolyte interphase (SEI), which severely prevents the Li metal battery from practical applications. In this paper, atomic layer deposition (ALD) method is used to alter the lithiophilicity of carbon fiber network by depositing ultra-thin conformal ZnO layer at a low mass loading (5.9 ​wt%). The thin layer ZnO by ALD could provide uniform Li nucleation sites and guide Li deposition along the carbon fibers without formation of dendrites, and release Li from top of the electrode without formation of dead Li during dissolution. As a result, the ALD ZnO-modified carbon fiber/Li (ALD/C–Li) composite anode can cycle stably at a large current density as high as 15 ​mA/cm2 giving low overpotential (105 ​mV) and long-running lifespan with stable stripping/plating profiles nearly 500 cycles at 5 ​mA/cm2. Tests of X-ray photoelectron spectroscopy (XPS) combined with argon ion etching show that the ALD/C–Li anode can effectively stabilize the SEI with a negligible increase in thickness after 100th cycle, while that of bare Li electrode doubles under the same cycling conditions. The full cell with LiFePO4 cathode also shows a much low hysteresis at 2 ​C over 300 cycles. The strategy developed in this work provides a novel alternative to Li anodes with long lifespan and opens up a new avenue for Li metal batteries with high energy density and high power density.

Fermi-Level Engineering of Atomic Layer-Deposited Zinc Oxide Thin Films for a Vertically Stacked Inverter

We report here atomic layer-deposited ZnO FETs and an effective electron doping method based on Al2O3 layer for high-performance transistors and its circuit application. Through the atomic layer deposition, an ultrathin and flat ZnO film (5 nm) has been deposited successfully where our ZnO FETs initially show n-type enhancement-mode transfer characteristics. The threshold voltage and the electron field-effect mobility of the device are 42 V and 2.8 cm2 V–1 s–1, respectively. After the deposition of the top Al2O3 layer, our ZnO FETs exhibit depletion-mode transfer characteristics with enhanced mobility. The threshold voltage (Vth) shifts to −38 V and the electron field-effect mobility increases by a factor of 2. Analyzed by a four-point probe and Hall and temperature-variable current–voltage measurements, Al2O3-induced doping is considered the main reason for the noticeable changes by minimizing the trap states in our ZnO films. Furthermore, a vertically stacked low-voltage n-type ZnO inverter was successfully fabricated utilizing the enhancement- and depletion-mode ZnO FETs with a voltage gain of ∼1.5 at low supplied voltage. We thus conclude that the atomic layer-deposited ZnO FETs with Al2O3-induced doping offers practical benefits for the high-performance transistors and its circuit applications.

Ultrathin agaric-like ZnO with Pd dopant for aniline sensor and DFT investigation

Aniline detection is of great importance in many industries, but most of the aniline sensors suffers from tedious and time consuming process. Herein, we present an efficient aniline sensor based on Pd decorated ZnO nanomaterials. Ultrathin ZnO nanosheets were synthesized by a facile one-step hydrothermal method. The nanosheets were corrugated into a unique agaric morphology, endorsing the nanomaterials with high surface area that is ideal for gas sensing applications. The obtained ZnO nanosheets were then uniformly decorated with uniform Pd nanoparticles (Pd NPs) around 5 nm in diameter. Gas sensing experiment on the ZnO decorate with different amount of Pd nanoparticles were systematically evaluated. The sample decorated with 0.3 % Pd NPs (Pd-ZnO-0.3) exhibited the highest sensitivity to aniline, which is about two orders higher than that of the pure ZnO nanosheet. The gas sensor based on Pd-ZnO-0.3 has a detection limit to aniline down to 0.5 ppm, with very short response and recovery times of 29 s and 23 s, respectively to 100 ppm aniline. First-principles DFT study was employed to provide the sensing mechanism. The improved sensing performance could be attributed to the increasing adsorbed oxygen and tunable band alignment for Pd-ZnO materials. This work provides new insights to the design strategy of Pd-decorated ZnO nanomaterials for high performance gas sensors.

A pulse electrodeposited amorphous tunnel layer stabilises Cu2O for efficient photoelectrochemical water splitting under visible-light irradiation

An ultrathin amorphous ZnO layer is introduced on Cu2O by pulsed electrodeposition, which helps stabilise the Cu2O photocathode for water splitting.

Clean Interface Contact Using a ZnO Interlayer for Low-Contact-Resistance MoS2 Transistors.

Two-dimensional transition metal dichalcogenides (TMDCs) have emerged as promising materials for next-generation electronics due to their excellent semiconducting properties. However, high contact resistance at the metal-TMDC interface plagues the realization of high-performance devices. Here, an effective metal-interlayer-semiconductor (MIS) contact is demonstrated, wherein an ultrathin ZnO interlayer is inserted between the metal electrode and MoS2, providing damage-free and clean interfaces at electrical contacts. Using TEM imaging, we show that the contact interfaces were atomically clean without any apparent damages. Compared to conventional Ti/MoS2 contacts, the MoS2 devices with a Ti/ZnO/MoS2 contact exhibit a very low contact resistance of 0.9 kΩ μm. These improvements are attributed to the following mechanisms: (a) Fermi-level depinning at the metal/MoS2 interface by reducing interface disorder and (b) presence of interface dipole at the metal/ZnO interface, consequently reducing the Schottky barrier and contact resistance. Further, the contact resistivity of a Ti/ZnO/MoS2 contact is insensitive to the variation of ZnO thickness, which facilitates large-scale production. Our work not only elucidates the underlying mechanisms for the operation of the MIS contact but also provides a simple and damage-free strategy for conventional aggressive metal deposition that is potentially useful for the realization of large-scale 2D electronics with low-resistance contacts.

Photocatalytic producing dihydroxybenzenes from phenol enabled by gathering oxygen vacancies in ultrathin porous ZnO nanosheets

As an energy-efficient and environmental friendliness method, solar sunlight-driven photo-oxidation catalysis process for organic chemicals synthesis has gained enormous attention, but still faces huge challenge in developing highly-efficient photocatalysts material. Two-dimensional materials engineering and surface defect engineering of photocatalysts both provide an effective strategy to improve the catalytic activity. Inspired by this pathway, we design and synthesize ultrathin porous ZnO nanosheets featuring abundant oxygen vacancies specific to producing dihydroxybenzenes based on a photocatalytic oxidation process. Several valid characterizations had been employed to discern the structural character of the obtained model catalyst, revealing that the resultant ZnO sheets afford an average thickness of 3 nm, and abundant surface porosity, thereby contributing to the rich oxygen vacancies. Such a structure could generate a synergistic effect to enhance the optical absorption and improve the transportation rate of photogenerated charge carriers from the materials design. As expected, the specific ultrathin ZnO nanosheets exhibited a greatly-improved photocatalytic activity for oxidation of phenol to dihydroxybenzenes (31.5% conversion & almost 76.7% selectivity of DHB), near 3 and 4 times higher, respectively than its counterparts that one with few oxygen vacancies and Bulk-ZnO. Impressively, the obtained catalyst showed durable catalytic activity without any activity loss during the five recycling. Finally, the feasible oxidation mechanism was proposed and testified by the controlled scavenger experiments. This study provides a novel reference on how to design high-performance photocatalytic material.

Fabrication and characterization of Schottky diode on ultra thin ZnO film and its application for UV detection

In this paper we report a Schottky contact based UV detector over ultrathin (∼25 nm) radio frequency (RF) sputtered ZnO thin film probably for the first time. The substrate used was IC compatible p-type silicon instead of widely used sapphire substrate. The surface morphology, crystal phases, optical properties of the thin film were studied by atomic force microscopy, Field emission scanning electron microscopy, x-ray diffraction and photo luminance. The planner Schottky diode (Pd/ZnO/Ti/Au) was fabricated by photolithography e-beam evaporator and DC sputtering. The electrical properties of the diode were investigated by semiconductor parameter analyzer with micro probing arrangement. Different parameters of the diode like barrier height, ideality factor and reverse saturation current were estimated from I-V characteristics using the Schottky Barrier Height theory. UV light of fixed frequencies and intensity were exposed on the diode and it was found that its different parameters were changed.