Metal Oxide Nanowires Research Articles

Pseudocapacitive Charge Storage in Thin Nanobelts

This article reports that extremely thin nanobelts (thickness ~ 10 nm) exhibit pseudocapacitive (PC) charge storage in the asymmetric supercapacitor (ASC) configuration, while show battery-type charge storage in their single electrodes. Two types of nanobelts, viz. NiO–Co3O4 hybrid and spinal-type NiCo2O4, developed by electrospinning technique are used in this work. The charge storage behaviour of the nanobelts is benchmarked against their binary metal oxide nanowires, i.e., NiO and Co3O4, as well as a hybrid of similar chemistry, CuO–Co3O4. The nanobelts have thickness of ~ 10 nm and width ~ 200 nm, whereas the nanowires have diameter of ~ 100 nm. Clear differences in charge storage behaviours are observed in NiO–Co3O4 hybrid nanobelts based ASCs compared to those fabricated using the other materials—the former showed capacitive behaviour whereas the others revealed battery-type discharge behaviour. Origin of pseudocapacitance in nanobelts based ASCs is shown to arise from their nanobelts morphology with thickness less than typical electron diffusion lengths (~ 20 nm). Among all the five type of devices fabricated, the NiO–Co3O4 hybrid ASCs exhibited the highest specific energy, specific power and cycling stability.

Bifunctionalized novel Co-V MMO nanowires: Intrinsic oxidase and peroxidase like catalytic activities for antibacterial application

The development of effective and low-cost enzyme-like nanomaterials for killing bacteria is essential for human health and environmental protection. We report the synthesis of novel Co-V mixed metal oxide (MMO) nanowires composited of Co3V2O8 uniform dispersing among Co3O4 by calcining Co6(CO3)2(OH)8·H2O@Co3V2O8 nanowires in air. Co-V MMO nanowires can possess intrinsic oxidase-like and peroxidase-like catalytic activity. When the Co/V molar ratio is 3 and the calcination temperature is 400 °C, Co-V MMOs have superior activity. Utilizing this unique bifunctional activity, an antibacterial system combining Co-V MMOs with a low level of H2O2 has been put forward. The activity originates from their ability to catalyze the generation of O2―, which hydrolyze to HO2. Thus, the intracellular reactive oxygen species level can be enhanced and toxicity of high level H2O2 can be avoided. This work would be indicative to construct Co-V MMOs in the application of new energy and environmental catalysis.

Optimization of the surface coverage of metal nanoparticles on nanowires gas sensors to achieve the optimal sensing performance

The functionalization of noble metal nanoparticles (NPs) is a highly efficient method for increasing the sensing performance of metal oxide nanowire (NWs) gas sensors. Despite the well-established strategy, the level of the optimal functionalization for obtaining the maximum sensing response has rarely been reported. Herein, the surfaces of SnO2 NWs were functionalized with Pt and Pd NPs, and the gas sensing characteristics were then investigated using NO2 as an example gas. The sensing responses obtained at the optimal temperature showed a bell-shaped dependency on the proportion of surface coverage by Pd and Pt NPs. The sensing mechanism is explained and the results were also fitted to a simple theoretical model based on modulation in the conduction channel via the chemical and electronic sensitization on NWs. This study provides guidelines on the amount of metal NPs for achieving the optimal sensing responses in metal oxide NWs gas sensors.

Field Stress Influenced Conduction Behavior of Narrow Diameter Gate-All-Around (GAA) Silicon Nanowire n-MOSFET

Narrow diameter, n-channel gate-all-around (GAA) silicon nanowire metal–oxide–semiconductor-field-effect-transistor (GAA-SNWFET) shows partial surface conduction under constant field stress, which was validated from the low frequency noise measurements. The said conduction mode is different than the complete bulk mode of conduction as found in pre-stressed devices, and had been assessed from the normalized drain current power spectral density (PSD) versus frequency (f) and gate voltage overdrive [(VG − VT)] characteristics. In all instances followed by constant field stressing for different times, Lorentzian PSD was found, the low frequency component of which furthermore shows a direct deviation from its dependence on (VG − VT)−1, in contrary to the pre-stressed device. We explained the rationale of Lorentzian PSD from fresh trap creation inside the gate oxide of silicon nanowire under the influence of stress. Further, observed correlated variation between the normalized PSD versus time averaged drain current ( ) and, normalized transconductance versus supports trap influenced transport in the stressed device. Transient drain current with its statistical distribution of current levels for different VG − VT furthermore shows trap assisted partial surface conduction in stressed nanowire.

Highly Sensitive and Selective Gas Sensors Using p-Type Oxide Nanoparticles Decorated ZnO Nanowire Network Sensors

Metal oxide nanowire network structures are promising chemoresistive materials with a highly sensitive response, good stability and fast response due to their large surface area and gas accessible networked structures. In general, highly crystalline oxides nanowire (NW) networks were prepared by a thermal evaporation process. Those have lots of advantages for the detection of volatile organic compounds (VOCs) and environmental pollution gases. Especially, n-type oxide NWs such as SnO2, ZnO, and In2O3, and Ga2O3 are widely investigated due to their easy synthetization and high gas response. In other to enhance gas response of n-type oxides NWs, doping or loading of noble metals of Pt , Pd, Ag or p-type oxide semiconductor of CuO , Co3O4, NiO, and MnO2. In this study, we investigated for the detection using p-type metal oxide nanoparticles decorated ZnO NWs. Consequently, the surface modification of oxide NWs via various methods provides new approach to accomplish the selective detection of a specific gas, and after further researches, this approach could provide a new strategy in the field of gas sensors.

Manipulation of a Single Crystal Nanowire on an Atomic Level

In recent years, metal oxide nanostructures (MO) have been intensively studied due to a wide variety of applications on multiple fields, for example in optoelectronics, gas sensing, photovoltaics, field-effect transistors, UV lasers, field emission sources etc. [1]. However, for applications, other types of structures are also interesting such as sulfides and nitrides. Here the question that raises is can we manipulate nanomaterials like nanowires on an atomic level. Knowing the exact mechanism of sulfur-oxygen exchange in the metal oxide crystal lattice can be utilized for the synthesis of metal sulfides with specific morphologies. When compared to metal oxides, single-crystalline metal sulfide nanostructures of specific morphologies are usually harder to produce than MOs. Generally, metal sulfide nanostructures represent unconventional materials in areas such as batteries or photovoltaics with yet unexploited potential, where their efficiency is mainly limited by the purity of crystalline phases and structural defects [2]. A promising path for novel synthesis route is incorporation of the sulfur in the crystal structure of a metal oxide nanoparticle, while during this transition the morphology of the nanostructure does not change. In this way, it is possible to obtain single crystal metal sulfide nanostructures from metal oxide nanostructures in the single crystal to single crystal (SCSC) transformation. A promising method to achieve such transformation in a short period of time is a treatment of metal oxide nanostructures with the plasma of sulfur-containing gas. In this research, we studied the mechanism of sulfur incorporation in the metal oxide crystal structure. The procedure was carried out by synthesis of ultra-thin metal oxide nanowires, which were then treated with sulfur-containing gases under increased temperature (in thermodynamic equilibrium conditions) and in the plasma of the same gases (under thermodynamically non-equilibrium conditions). Experiments were conducted at different temperatures and pressures to evaluate the influence of plasma on transformation process. Mechanisms involved in the interaction of sulfur-containing species with metal oxides were studied on isolated single ultra-thin metal oxide nanowire. Furthermore, changes in crystal structures and their effect on the material properties before and after sulfurization were studied. [1] Umar A. & Hahn Y. B. (Eds.). (2010), Metal Oxide Nanostructures and their Applications. American Scientific Publishers, CA, USA [2] Zavašnik, J., & Rečnik, A. (2013). Electron microscopy study of CVT grown Fe-sulphides. Journal of Crystal Growth, 367, 18-23.

Multi-metal functionalized tungsten oxide nanowires enabling ultra-sensitive detection of triethylamine

Sensitization of metal oxide semiconductors is crucial for developing high performance electronic gas sensors. Herein, a unique dual sensitization protocol is proposed to boost the sensing performances of metal oxide nanowires. As a proof-of-concept, W18O49 nanowires are selected as the host materials, and Ag, Pt nanoparticles are loaded on W18O49 by an in-situ redox reaction to fabricate the novel Ag/Pt/W18O49 hybrid nanowires. Gas sensing tests demonstrate the superiority of the ternary nanowires in detecting triethylamine vapor against the binary Ag/W18O49 and Pt/W18O49 nanowires. The sensor based on Ag/Pt/W18O49 nanowires manifests fast response and recovery features and very high response over a wide concentration of 0.1–50 ppm, as well as good selectivity and stability. An ultrahigh sensitivity of 1.13 ppm−1 is registered by the Ag/Pt/W18O49 nanowires and a low detection limit of 71 ppb is obtained. The excellent performances are attributed to the dual sensitization mechanism, i.e., a synergy of both electronic and chemical interactions derived from the Ag and Pt functionalization. This work establishes a new strategy for constructing multi-metal functionalized nanowires and their utilization in functional nanodevices with enhanced performances.

A general strategy for the preparation of semiconductor-oxide-nanowire photoanodes

Semiconductor photoelectrodes for photoelectrochemical water splitting are at the heart of the area of renewable energy. Among various semiconductors as photoelectrode materials, metal oxide nanowires have attracted exceptional attention due to their favorable band-edge positions, wide distribution of bandgaps, low cost, and outstanding stability. In this study, we develop a general strategy for the preparation of semiconductor-oxide-nanowire photoanodes. Various metal oxide nanowires are grown directly from and on the metal or alloy substrates by thermal oxidation at ultra-low pressure using the corresponding metal or alloy foil as substrates. These metal oxide nanowires and their underlying conductive substrates are used directly as integrated photoanodes, which effectively avoids the detachment of the active light-harvesting materials from the substrates during the photoelectrochemical test. For example, a water photo-oxidation current density of the iron oxide photoanode reaches 0.71 mA/cm2 over a period of 20 h under 100 mW cm−2 illumination at the potential of 1.23 V versus the reversible hydrogen electrode. The successful preparation of these semiconductor-oxide-nanowire photoanodes have important implications for further development of photoelectrodes and highly efficient solar water-splitting devices.

Selective H2S sensing without external heat by a synergy effect in self-heated CuO-functionalized SnO2-ZnO core-shell nanowires

Future gas sensors require minimal power consumption to enable their integration into portable electronics such as smart mobile phones. We developed H2S gas sensors based on a self-heating effect using metal oxide nanowires (NWs). We fabricated bare SnO2 NWs, CuO functionalized SnO2 NWs, and a CuO functionalized SnO2-ZnO core-shell (C–S) NW sensor, and tested their sensor response towards H2S gas by applying different external voltages at room temperature. It was found that the CuO functionalized SnO2-ZnO C-S NW gas sensor had higher response to H2S gas relative to other tested sensors due to higher self-heating effect, formation of heterojunctions, phase transformation, and spillover effects of CuO nanoparticles. Without external heat, the selective H2S detection obtained in this work demonstrates the possibility of embedding low power consumption gas sensors in portable devices for detection of H2S as a biomarker for early diagnosis of diseases.

Nano-manipulation and nano-assembling using shape memory alloy nanogripper of metal oxide and semiconductor single nanowires and nanoparticles for biological nanosensors∗∗Report at 9th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2018), 7-11 November 2018, Ninh Binh City, Vietnam.

The nanomaterials based on metal oxides or semiconductor nanoparticles and nanowires appear to demonstrate the unique physical and also functional properties. Those properties, in general, allow constructing a large number of individual nanodevices for promising applications in nanoelectronics, photonics and biomedical technology. Such individual nanodevices represent interesting building blocks for new type of hierarchical mechanical nano-assembling. Mechanical hierarchical nano-assembly of functional nanoscale/meso-macroscale devices into macroscopic devices opens up the new perspectives for the creating of micro-meso and macro-devices and their arrays for many applications. The report describes the physical principle and design of the shape memory based nanogrippers as well as the preliminary results of the manipulation of CNTs, InP, ZnO nanowires and nanoparticles as building blocks for biomedical sensors.

Efficient Synthesis of Graphene Nanoscrolls for Fabricating Sulfur-Loaded Cathode and Flexible Hybrid Interlayer Toward High Performance Li−S Batteries

Graphene oxide; Sulfur/carbon composite; Graphene nanoscroll; Metal oxides nanowires; Interlayer; Li−S batteries Nowadays, the growing grievous environmental problems together with the exhaustion of energy resources put urgent demands for developing high energy density devices, whereas traditional lithium-ion batteries (LIBs) are getting close to their energy density. Consideration of the factors including capacity, resource and environment, lithium sulfur (Li-S) batteries are the most potential lithium secondary batteries.[1] However, many drawbacks need to be overcome before its industrial application, such as low electronic conductivity of sulfur, serious shuttling effects of dissolution polysulfides, volume change of electrode during charging-discharging process and problems such as lithium dendrites, passivation and corrosion of lithium anode. This work aims to improve the properties of Li-S batteries by preparation of sulfur/carbon composite cathode materials and building high-efficiency interlayer .[2-4] Firstly, a modified lyophilization approach is developed and used for highly efficient transformation of 2D graphene oxide sheet into 1D graphene nanoscroll (GNS) with high topological transforming efficiency (94%). Because of the unique open tubular structure and large specific surface area (545 m2 g−1), GNS is utilized for the first time as a porous cathode scaffold for encapsulating sulfur with a high loading (81 wt%). The interlayer space and pores of obtained GNS provide large sites for sulfur loading, and this unique structure can accommodate volume expansion and prevent the dissolution of polysulfides. As a result, the composite deliver good rate performance and cycling stability (the capacity decay is only 0.04% per cycle after 400 cylces at 2C).[5] Furthermore, a flexible self-standing hybrid interlayer is engineered by intertwining one-dimensional (1D) metal oxides nanowires (NWs) with GNS into a robust interconnected 3D network to be used as both an upper current collector and physical/chemical polysulfide-trapper. Combining GNS with MnO2, V2O5 and MoO3 respectively, free-standing membranes are obtained to serve as interlayers for Li-S battery (namely, GNSMn, GNSV and GNSMo). The highly conductive and open GNS framework provides bicontinuous transfer channels for rapid electron and ion transport across the cathode/separator interface. Meanwhile, the metal oxides nanowires act as ideal redox mediators to dynamically block polysulfide dissolution and facilitate their conversion into sulfides via formation of active intermediate polythionate complexes. Even though GNSV is used as an interlayer for lithium sulfur battery with bare sulfur as cathodethese cells with 70 wt% sulfur content in the whole cathode have significantly improved performance with a long cycling life (1000 cycles), low capacity decay (0.041% per cycle), and considerable areal capacity (>4 mA h cm- 2), even at a high sulfur loading of 5.5 mg cm- 2. This novel interlayer design strategy has great potential for promoting the practical use of Li–S batteries.[6] In summary, the work described here shows a potential of GNS as a promising functional carbon material to realize high sulfur loading and electrochemically stable electrode configuration aimed toward developing high-performance Li−S batteries. Figure 1 (a) SEM images and (b) TEM images of GNS, (c) schematic illustration on the advantages in entrapping polysulfides of the S@GNS (insert: typical colors of the cycled S@GNS and Gs-S cathodes after soaking in a mixture of DOL/DME for 24 h), (d) schematic model of the cell (insert: photos of the free-standing and flexible GNSM interlayer).

Rational Design and in-Situ Electrochemical Characterization of Low Dimensional Nanomaterials and Devices for Energy Storage

Developing high performance energy storage devices such as metal ion batteries and supercapacitors are important for portable electronics, vehicle electrification and smart grid, while developing techniques for electrochemical monitoring that offer higher spatial and temporal resolution would open up new ways to study electrochemical interfaces and reaction kinetics for further developing novel energy storage devices. In this talk, I will introduce our recently developed materials optimization strategies and techniques to address their critical performance parameters related to energy density, power density, cycle, calendar life, and realize electrochemical monitoring, which includes (1) development of wafer-scale large-scale, shape & strain-controlled deterministic one-dimensional material assembly technique[1], which can be used for fabrication of single crystal nano-/microscale electrochemical devices array for in-situ electrochemical probing[2]; (2) synthesis and in-situ TEM characterization of self-adaptive strain-relaxed structure with high-stretchy protective shells through homogeneously crumpling of 3D graphene electrode showing high specific capacity, fast charge-discharge rate and long cycling stability for high-energy lithium battery materials[3]; and (3) operando observation of coupled discontinuous–continuous transitions in a series of ion-stabilized intercalation cathode[4], showing that the continuous transition at the intralayer could inhabit the irreversible ion intercalation and the consequent irreversible phase transition, while the discontinuous transitions at the interlayer of layered cathode provides the rapid adjustments in diffusion channels for the fast charge and discharge[5]. Reference: Y. Zhao, J. Yao, C.M. Lieber, et al. Shape-controlled deterministic assembly of nanowires. Nano Letters 2016, 16.4, 2644.L. Mai, M. Yan, Y. Zhao. Track batteries degrading in real time. Nature 2017, 546.7659, 469Y. Zhao, J. Feng, X. Liu, et al. Self-adaptive strain-relaxation optimization for high-energy lithium storage material through crumpling of graphene. Nature Communications 2014, 5.Y. Zhao, C. Han, et al. Stable alkali metal ion intercalation compounds as optimized metal oxide nanowire cathodes for lithium batteries. Nano Letters 2015, 15.3, 2180.G. Zhang, T. Xiong, Y. Zhao. Operando observation of coupled discontinuous–continuous transitions in ion-stabilized intercalation cathode. Submitted.

Low-Power Detection of Food Preservatives by a Novel Nanowire-Based Sensor Array.

Food preservatives are compounds that are used for the treatment of food to improve the shelf life. In the food industry, it is necessary to monitor all processes for both safety and quality of the product. An electronic nose (or e-nose) is a biomimetic olfactory system that could find numerous industrial applications, including food quality control. Commercial electronic noses are based on sensor arrays composed by a combination of different sensors, which include conductometric metal oxide devices. Metal oxide nanowires are considered among the most promising materials for the fabrication of novel sensing devices, which can enhance the overall performances of e-noses in food applications. The present work reports the fabrication of a novel sensor array based on SnO2, CuO, and WO3 nanowires deposited on top of μHPs provided by ams Sensor Solutions Germany GmbH. The array was tested for the discrimination of four typical compounds added to food products or used for their treatment to increase the shelf life: ethanol, acetone, nitrogen dioxide, and ozone. Results are very promising; the sensors array was able to operate for a long time, consuming less than 50 mW for each single sensor, and principal component analysis (PCA) confirmed that the device was able to discriminate between different compounds.

Detection mechanism in highly sensitive ZnO nanowires network gas sensors

Metal-oxide nanowires are showing a great interest in the domain of gas sensing due to their large response even at a low temperature, enabling low-power gas sensors. However their response is still not fully understood, and mainly restricted to the linear response regime, which limits the design of appropriate sensors for specific applications. Here we analyse the non-linear response of a sensor based on ZnO nanowires network, both as a function of the device geometry and as a response to oxygen exposure. Using an appropriate model, we disentangle the contribution of the nanowire resistance and of the junctions between nanowires in the network. The applied model shows a very good consistency with the experimental data, allowing us to demonstrate that the response to oxygen at room temperature is dominated by the barrier potential at low bias voltage, and that the nanowire resistance starts to play a role at higher bias voltage. This analysis allows us to find the appropriate device geometry and working point in order to optimize the sensitivity. Such analysis is important for providing design rules, not only for sensing devices, but also for applications in electronics and opto-electronics using nanostructures networks with different materials and geometries.

Multi-Phase Field Model of Localized Corrosion Kinetics with Corrosion Products Formation

Localized corrosion is one of the complex forms of corrosion which makes it difficult to detect and design-against. During metal corrosion in the corrosive environment, corrosion products are also formed as a result of electrochemical reactions inside the electrolyte. These products can precipitate on the corroding surface and stop/ slowdown the overall corrosion process. In order to understand this complex phenomenon, a multi-phase field model is proposed to simulate metal corrosion with corrosion products formation. Both anodic (metal oxidation) and cathodic (oxygen reduction) reactions along with other electrochemical reactions in the electrolyte are considered. The free energy of the system is described in terms of its metal ion concentration and the order parameters. Rather than considering linear kinetics (Allen-Cahn equation), inspired from classical rate theory, non-linear (Butler-Volmer) kinetics is considered to describe the temporal evolution of order parameters. The time dependent evolution of ionic species is governed by Nernst-Plank equations while electrostatic potential is governed by Poisson equation. The model results are compared with experimental findings and several examples are presented to show the practical applications of this model. One interesting example, which can be studied in detail with this methodology, is the formation of metal oxide nanowires (zinc/ aluminum oxide).

Enhanced photosensitivity of highly spectrum selective cylindrical gate In1−xGaxAs nanowire MOSFET photodetector

In this study, an [Formula: see text] nanowire metal oxide semiconductor field effect transistor (MOSFET) photosensor with zinc oxide (metallic ZnO), which act as a transparent optical window over channel has been investigated. The electrical characterization of [Formula: see text] nanowire MOSFET was compared to their silicon counterpart. [Formula: see text] nanowire MOSFET responds to visible-near infrared (600 nm to 1.7 nm) under light illumination as opposite to silicon nanowire MOSFET, which respond to ultraviolet to visible spectra (Peak @ 400 nm). ZnO–[Formula: see text]–InGaAs device is characterized by responsivity [Formula: see text] of [Formula: see text], photoconductive gain of [Formula: see text]%, with reasonable available photocurrent of [Formula: see text] and sensitivity [Formula: see text] of [Formula: see text]. The present work demonstrates the potential for high performance visible to near-infrared [Formula: see text] detectors with tunable band gaps for applications like interactive display, indoor communication and optical sensors.

Rational Method of Monitoring Molecular Transformations on Metal-Oxide Nanowire Surfaces.

Metal-oxide nanowires have demonstrated excellent capability in the electrical detection of various molecules based on their material robustness in liquid and air environments. Although the surface structure of the nanowires essentially determines their interaction with adsorbed molecules, understanding the correlation between an oxide nanowire surface and an adsorbed molecule is still a major challenge. Herein, we propose a rational methodology to obtain this information for low-density molecules adsorbed on metal oxide nanowire surfaces by employing infrared p-polarized multiple-angle incidence resolution spectroscopy and temperature-programmed desorption/gas chromatography-mass spectrometry. As a model system, we studied the surface chemical transformation of an aldehyde (nonanal, a cancer biomarker in breath) on single-crystalline ZnO nanowires. We found that a slight surface reconstruction, induced by the thermal pretreatment, determines the surface chemical reactivity of nonanal. The present results show that the observed surface reaction trend can be interpreted in terms of the density of Zn ions exposed on the nanowire surface and of their corresponding spatial arrangement on the surface, which promotes the reaction between neighboring adsorbed molecules. The proposed methodology will support a better understanding of complex molecular transformations on various nanostructured metal-oxide surfaces.

Recent Developments and Perspectives for Memristive Devices Based on Metal Oxide Nanowires

AbstractMemristive devices are considered one of the most promising candidates to overcome technological limitations for realizing next‐generation memories, logic applications, and neuromorphic systems in the modern nanoelectronics and information technology. Despite the continuous efforts, understanding the resistive switching mechanism underlying memristive/neuromorphic behavior still represents a challenge. Metal oxide nanowire‐based memristors appear suitable model systems for a deeper understanding of the involved physical/chemical phenomena due the possibility for localizing and investigating the switching mechanism. In practical aspects, nanowire‐based devices can be grown using a bottom‐up approach, thus being considered reliable candidates for going beyond the current scaling limitations of the top‐down approach by the standard lithography. In addition, taking into advantage of the high surface‐to‐volume ratio of these nanostructures, new device functionalities can be achieved by exploiting surface effects. In the literature, a variety of nanowire‐based devices such as single nanowires, nanowire arrays, and networks are reported to exhibit memristive behavior, explained by different switching mechanisms. This work provides a comparative review and a comprehensive analysis of device performances and physical phenomena responsible for memristive and neuromorphic behavior in such nanostructures. The analysis of the state‐of‐art of memristor devices based on nanowires and nanorods represent a milestone toward the development of nanowire‐based artificial neural networks.

Unusual Oxygen Partial Pressure Dependence of Electrical Transport of Single-Crystalline Metal Oxide Nanowires Grown by the Vapor-Liquid-Solid Process.

In general, the electrical conductivities of n-type semiconducting metal oxide nanostructures increase with the decrease in the oxygen partial pressure during crystal growth owing to the increased number of crystal imperfections including oxygen vacancies. In this paper, we report an unusual oxygen partial pressure dependence of the electrical conductivity of single-crystalline SnO2 nanowires grown by a vapor-liquid-solid (VLS) process. The electrical conductivity of a single SnO2 nanowire, measured using the four-probe method, substantially decreases by 2 orders of magnitude when the oxygen partial pressure for the crystal growth is reduced from 10-3 to 10-4 Pa. This contradicts the conventional trend of n-type SnO2 semiconductors. Spatially resolved single-nanowire electrical transport measurements, microstructure analysis, plane-view electron energy-loss spectroscopy, and molecular dynamics simulations reveal that the observed unusual oxygen partial pressure dependence of the electrical transport is attributed to the intrinsic differences between the two crystal growth interfaces (LS and VS interfaces) in the critical nucleation of the crystal growth and impurity incorporation probability as a function of the oxygen partial pressure. The impurity incorporation probability at the LS interface is always lower than that at the VS interface, even under reduced oxygen partial pressures.

Nanophotonic enhancement and improved electron extraction in perovskite solar cells using near-horizontally aligned TiO2 nanorods

While vertically oriented metal oxide nanowires have been intensely researched for use as electron transport layers (ETLs) in halide perovskite solar cells (HPSCs), horizontal nanowires (oriented roughly parallel to the substrate) have received much less attention despite their higher photonic strength due to overlapping electric and magnetic dipolar Mie resonance modes. Herein, we demonstrate the fabrication of an assembly of horizontally aligned TiO2 nanorods (HATNRs) on FTO substrates via a facile hydrothermal route. The HATNRs are employed as the ETL to achieve 15.03% power conversion efficiency (PCE) in HPSCs which is higher than the PCE of compact TiO2 based devices (10.12%) by a factor of nearly 1.5. A mixed halide, mixed cation organometal perovskite FA0.83MA0.17Pb(Br0.17I0.83)3 with optimized composition is used as the active layer. The excellent refractive index matching between the perovskite and TiO2, coupled with strong Mie scattering in the nanorod geometry results in broadband near-zero backscattering and high forward scattering, upon coating of HATNRs with perovskite. The maximum suppression of backscattering is found at ∼600 nm. The HATNRs ETL also improves the extraction of electrons from the perovskite layer and results in superior blocking of carrier recombination at the perovskite layer/FTO interface.