Modification Of Material Properties Research Articles

One‐Step Photo‐Patterning Process for Multi‐Modal and Transformative Structural Coloration: Toward Anti‐Counterfeiting Applications

AbstractThe selective interactions of light with matter enable the existence of colors in nature. Through the precise modification of material properties, photonic crystals, plasmonic nanoparticles, and dielectric metasurfaces can produce bright and vivid structural colors. However, this approach is rendered inadequate by its complex fabrication processes, limited working area, and lack of inherent material‐property tuning. Here, a novel one‐step fabrication process based on ultraviolet‐curable chitosan (UVCC) for generating structural multicolor patterns in polydimethylsiloxane in a single bending event is presented. UV exposure through a grayscale photomask controls the polymerization of the UVCC, enabling manipulation of its thickness, thereby allowing the production of size‐, wavelength‐, and amplitude‐controllable wrinkles under compressive stress. This enables the exhibition of selective structural coloration in response to incoming incident light. Upon the release of applied stresses, the wrinkles disappear, and the bilayer device becomes transparent again. Notably, repeating the process allows for dynamic covert–overt switching of the structural colors. This firm control over the active‐layer thickness, simple device structure, cost‐effectiveness, facile fabrication process, and vibrant‐color tuning ability make this one‐step photopatterning approach more competitive for structural‐color applications. Finally, the authenticity of a cosmetic cream is evaluated to demonstrate the anti‐counterfeiting effect of the UVCC circumference wrinkles.

Separated Electronic and Strain Interfaces in Core/Dual‐Shell Nanowires: Unlocking the Potential of Strained GaAs for Applications Across Near‐Infrared

AbstractSemiconductor nanowires have inspired plenty of novel nanotechnology device concepts in photonics, electronics, and sensing, owing to their unique functionalities and integrability in heterogeneous platforms. Lattice‐mismatched core/shell heterostructures, in particular, open new avenues for strain engineering and material properties modification. A notable case is the widely tunable tensile strain in the core of GaAs/InxAl1‐xAs core/shell nanowires, which can be used to tailor the GaAs bandgap for applications across near‐infrared, like optical fiber telecommunication, imaging, photovoltaics, etc. As it is shown here, though, the bandgap narrowing under high tensile strain in the GaAs core is accompanied by fast non‐radiative recombination, which is undesirable for any device application. The limiting role of the lattice‐mismatched core/shell interface is revealed, and a novel core/dual‐shell heterostructure that employs an intermediate AlyGa1‐yAs shell (spacer) is proposed. This spacer decouples the GaAs/AlyGa1‐yAs interface, which confines electrons and holes into GaAs, from the lattice‐mismatched AlyGa1‐yAs/InxAl1‐xAs one, whereas the strain in GaAs is unaffected. Choosing the optimal spacer thickness, the photoluminescence yield increases significantly, with longer emission decay lifetimes and slower carrier cooling rates. Besides unlocking the potential of GaAs for photonic applications across near‐infrared, the proposed heterostructure concept can also be adopted for other material systems.

Machinability and Surface Properties of Cryogenic Poly(methyl methacrylate) Machined via Single-Point Diamond Turning.

Poly(methyl methacrylate) (PMMA), with a glass transition temperature (Tg) over 100 °C, shows good mechanical and optical properties and has broad applications after being machined with single-point diamond turning (SPDT) at room temperature. Because of the high Tg, current efforts mostly focus on optimizing machining parameters to improve workpiece precision without considering the modification of material properties. Cryogenic cooling has been proven to be an effective method in assisting ultra-precision machining for certain types of metals, alloys, and polymers, but has never been used for PMMA before. In this work, cryogenic cooling was attempted during the SPDT of PMMA workpieces to improve surface quality. The machinability and surface properties of cryogenically cooled PMMA were investigated based on the mechanical properties at corresponding temperatures. Nanoindentation tests show that, when temperature is changed from 25 °C to 0 °C, the hardness and Young's modulus are increased by 37% and 22%, respectively. At these two temperature points, optimal parameters including spindle speed, feed rate and cut depth were obtained using Taguchi methods to obtain workpieces with high surface quality. The surface quality was evaluated based on the total height of the profile (Pt) and the arithmetic mean deviation (Ra). The measurement results show that the values of Pt and Ra of the workpiece machined at 0 °C are 124 nm and 6 nm, respectively, while the corresponding values of that machined at 25 °C are 291 nm and 11 nm. The test data show that cryogenic machining is useful for improving the form accuracy and reducing the surface roughness of PMMA. Moreover, the relationship between temperature, material properties and machinability weas established with dynamic mechanical analysis (DMA) data and a theoretical model. This can explain the origin of the better surface quality of the cryogenic material. The basis of this is that temperature affects the viscoelasticity of the polymer and the corresponding mechanical properties due to relaxation. Then, the material property changes will affect surface profile formation during machining. The experimental results and theoretical analysis show that cryogenically cooled PMMA has good machinability and improved surface quality when using SPDT compared to that at ambient temperature.

Consideration of tungsten recrystallization in plasma facing components design

Tungsten recrystallization leads to a modification of material properties including a reduction in hardness, strength and thermal shock resistance. The recrystallization assisted material degradation compared to the as-received state has made it very undesirable as demonstrated in high heat flux testing. Consequently, tungsten recrystallization temperature has been widely used as the upper limit of material surface temperature under steady state loading conditions. However, this study shows that recrystallization is inevitable where steady state or transient loads are high enough. The efforts to retard recrystallization will only reduce the recrystallized depth but cannot completely avoid it. While it is demonstrated the material will be weakened by recrystallization, the recrystallized depth plays a relatively small role in deciding the fatigue lifetime once it is recrystallized. Therefore, it is recommended that the geometry and fracture toughness of the design is optimised to reduce crack growth into the component. The design criterion against tungsten recrystallization under steady state loading conditions has also been proposed for engineering design stages.

Characteristics of sintered calcium deficient hydroxyapatite scaffolds produced by 3D printing

This study investigates the parametric quality and reliability of 3D-printed scaffolds using a composite filament comprising a thermoplastic polymer and hydroxyapatite in 1:1 wt ratio. Employing a fused filament fabrication printer, we verified the 3D printing strategy, sintering temperature range, and preparation of hydroxyapatite scaffolds. To complete the study, in vitro cytotoxicity tests using animal and human cell models were conducted. The calcium-deficient hydroxyapatite (Ca/P ∼ 1.54) used in preparation of filament exhibited after sintering differences in crystalochemical phases in different rations, comparing to stoichiometric hydroxyapatite (Ca/P ≅ 1.67). The FFF process successfully produced scaffold macropores suitable for tissue vascularization (∼380 µm or smaller). Optimal sintering temperature for cell proliferation was identified at 1300 °C, especially effective for used investigated calcium-deficient hydroxyapatite. Cytotoxicity assessment with murine and human fibroblastic cells demonstrated differing behaviour, emphasizing the need for careful material property modifications for practical scaffold utilization.

Modification of fibre materials properties with the use of nanotechnologies

Nanotechnologies play an important role in the modern world economy. As they deal, as a rule, together with other convergent (nano-, bio-, info-, cognitive) technologies. This connection causes a synergy effect, i.e. non linear development of innovations. The growth dynamics of the world's nanotechnology products is impressive. The world market of nanotechnologies in 2021 was 85 billion dollars, in 2024 (plan) will be 140 billion dollars. The forecast for 2030 is $288 billion. The production of nanoparticles of different nature and their use in different industries, fields of science, and technology takes a special place in nanotechnology. Both nanotechnology itself and the production and application of nanoparticles are interdisciplinary and cross-sectoral ones. Their users and customers are developed industries, including the textile industry. Metal nanoparticles are used both in the form of colloidal solutions and as part of microcapsules containing functional substances of different nature in the core. The article considers some methods of obtaining metal nanoparticles and microcapsule synthesis. The authors list the technologies of microcapsules application for textile functionalization.

Investigating Immunomodulatory Biomaterials for Preventing the Foreign Body Response.

Implantable biomaterials represent the forefront of regenerative medicine, providing platforms and vessels for delivering a creative range of therapeutic benefits in diverse disease contexts. However, the chronic damage resulting from implant rejection tends to outweigh the intended healing benefits, presenting a considerable challenge when implementing treatment-based biomaterials. In response to implant rejection, proinflammatory macrophages and activated fibroblasts contribute to a synergistically destructive process of uncontrolled inflammation and excessive fibrosis. Understanding the complex biomaterial-host cell interactions that occur within the tissue microenvironment is crucial for the development of therapeutic biomaterials that promote tissue integration and minimize the foreign body response. Recent modifications of specific material properties enhance the immunomodulatory capabilities of the biomaterial and actively aid in taming the immune response by tuning interactions with the surrounding microenvironment either directly or indirectly. By incorporating modifications that amplify anti-inflammatory and pro-regenerative mechanisms, biomaterials can be optimized to maximize their healing benefits in harmony with the host immune system.

A robust optimised multi-material 3D inkjet printed elastic metamaterial

This paper presents and validates a novel elastic metamaterial design, that is optimised for broadband robust vibration control of a structure in the presence of uncertainties, and realised using multi-material additive manufacturing. A novel concept resonator design that allows the resonance frequency to be flexibly tuned via both geometrical and material property modifications is presented and characterised. A unit cell consisting of 12 of these resonators is then proposed. The resonance frequencies and damping ratios of this elastic metamaterial unit cell when attached to a parametrically uncertain example structure are then optimised using a Particle Swarm Optimisation to maximise the mean attenuation in kinetic energy of a structure with parametric uncertainties, based on an analytical model of the system. The performance of the optimised metamaterial is then validated experimentally, and it is shown that the realised metamaterial design is able to achieve a mean of 3.5 dB of broadband attenuation in the presence of uncertainties in the structure. In addition, in the presence of structural uncertainties the robustly optimised design achieves 0.5 dB greater mean attenuation than a design optimised on the nominal structural response alone, and reduced variation in attenuation for different levels of uncertainty.

Investigation of Friction Hydro-Pillar Processing as a Repair Technique for Offshore Mooring Chain Links

Repairing links of offshore mooring chains has presented a significant industry challenge, primarily arising from modifications in material properties, encompassing alterations in microstructure, hardness, and residual stress. In this context, the present work investigates the method of friction hydro-pillar processing (FHPP) applied to R4 grade mooring chain steel. Joints in as-repaired and post-weld heat treatment (PWHT) conditions were subjected to residual stress (RS) tests using the neutron diffraction technique, microhardness mapping, and microstructural evaluations. The process generated peaks of tensile and compressive stresses in different directions and hardness below that of the parent material in the softening zone. The friction zone promoted high hardness levels in the thermo-mechanically affected zone (TMAZ) with a maximum of 19% of the ultimate tensile strength of the parent material. As expected, the PWHT restored the RS and reduced the hardness; however, 4 h PWHT allowed the elimination of a hardness higher than that of the base material.

Lithium-Ion Capacitors: A Review of Strategies toward Enhancing the Performance of the Activated Carbon Cathode

Lithium-ion capacitors (LiC) are promising hybrid devices bridging the gap between batteries and supercapacitors by offering simultaneous high specific power and specific energy. However, an indispensable critical component in LiC is the capacitive cathode for high power. Activated carbon (AC) is typically the cathode material due to its low cost, abundant raw material for production, sustainability, easily tunable properties, and scalability. However, compared to conventional battery-type cathodes, the low capacity of AC remains a limiting factor for improving the specific energy of LiC to match the battery counterparts. This review discusses recent approaches for achieving high-performance LiC, focusing on the AC cathode. The strategies are discussed with respect to active material property modifications, electrodes, electrolytes, and cell design techniques which have improved the AC’s capacity/capacitance, operating potential window, and electrochemical stability. Potential strategies and pathways for improved performance of the AC are pinpointed.

Characterization of the Mechanical Properties of Selectively Embossed Sheet Metal Materials Under Multi-Axial Loads

The application of embossed structures close to the sheet surface allows for work hardening to be introduced into sheet metal materials, leading to an improvement in their mechanical properties. So far, this was demonstrated in previous research work via tensile tests with differently embossed dual-phase steel specimens. Since tensile tests only reproduce uniaxial loading of the sheet metal material, no conclusions can yet be drawn about the property modification that can be achieved by near-surface embossing concerning real-life, multi-axial loading scenarios. To get deeper knowledge about the deformation behaviour of embossed components, it is important to consider multi-axial load cases. In the investigations reported in this paper, a multi-axial load case was realized by testing embossed bending specimens of DP600 and DP800 using a three-point bending device. The tests showed an overall improvement in the maximum bending force needed for the three-point bending test due to embossing of the materials investigated. Further, the tests indicated that lower-strength materials get more influenced by embossing than higher-strength materials. By simulating the process of embossing the bending specimens and the subsequent three-point bending test, an even deeper understanding of the material property modification could be generated.

Effects of post-fracture repeated impacts and short-term temperature gradients on monolithic glass elements bonded by safety films

Weathering and operational conditions, as known, have significant impact on typical constituent materials which are used for many construction applications. Among others, structural glass solutions can suffer for these effects in terms of major modification of material properties of interlayers, bonds, connections, gaskets and polymeric components in general. In this paper, the attention is given to the effects of repeated low-amplitude impacts and short-term temperature gradients for the characterization of load-bearing capacity in monolithic glass elements retrofitted by safety films. Especially for existing glass systems which are made of monolithic glass with limited strength and resistance capacity against ordinary and accidental mechanical loads, safety films are commercially available for retrofit interventions. They are primarily expected to keep together glass fragments in case of breakage, and thus minimize possible injuries. Besides, after first fracture, the so obtained glass-film composite elements have uncertain residual mechanical capacity against ordinary loads, given that it mostly depends on thin films composed of Polyethylene terephthalate (PET)-layers and pressure sensitive adhesives (PSAs). To this aim, a set of experiments (for a total of 950 configurations) is carried out in laboratory conditions (30 °C) on small-scale samples of fractured annealed monolithic glass elements bonded by commercial safety films, under repeated low-amplitude impacts / vibrations (S1-TR series), or additionally subjected to preliminary short-term thermal gradients (S2-TC1 series cooled at +5 °C and S3-TC2 series at −20 °C). Localized impacts are quantified in acceleration peaks in the range of 2 ÷ 14 m/s2 and rotations at supports in the order of 15 ÷ 20°. The interpretation of dynamic experimental results is carried out in terms of post-fracture vibration frequency (based on classical operational modal analysis techniques) and used, with the support of simplified analytical models or Finite Element (FE) numerical simulations, to characterize the response of cracked glass-film samples. Most importantly, the vibration frequency decrease is used to quantify their residual load-bearing capacity under unfavourable conditions, and to quantify the post-critical benefit of thin bonding safety films under unfavourable conditions.

Effect of latent heat storage on thermal comfort and energy consumption in lightweight earth-based housings

Embedding PCMs in building materials has attracted great interest due to their ability to store thermal energy. Storage of heat and cold is an efficient way to save energy. Considering a lightweight earth-based material, mixture of soil with a high straw content, the present work aims to improve its thermal properties by using phase change materials (PCM). In this investigation, a laboratory experimental campaign and subsequent numerical analysis were carried out to examine the impact of incorporating PCM within the lightweight earth-based material. Materials' hygro-thermal properties (thermal conductivity, specific heat capacity and water vapor permeability), determined experimentally in laboratory, have been considered to calculate energy consumption and occupant's comfort in a typical single-family housing. Experimental results showed an improvement of the lightweight earth-based material thermal conductivity and specific heat capacity, as well as a slight reduction in its water vapor permeability depending on the PCM content. These modifications in the lightweight earth-based material properties lead to predict an improvement of occupants' thermal comfort and a reduction of energy consumption in buildings that will be made with such a material. Finally, this study underlined the existence of an optimal phase change temperature regarding climate in which the building evolves.

Harnessing the Metal-Insulator Transition of VO2 in Neuromorphic Computing.

Future-generation neuromorphic computing seeks to overcome the limitations of von Neumann architectures by colocating logic and memory functions, thereby emulating the function of neurons and synapses in the human brain. Despite remarkable demonstrations of high-fidelity neuronal emulation, the predictive design of neuromorphic circuits starting from knowledge of material transformations remains challenging. VO2 is an attractive candidate since it manifests a near-room-temperature, discontinuous, and hysteretic metal-insulator transition. The transition provides a nonlinear dynamical response to input signals, as needed to construct neuronal circuit elements. Strategies for tuning the transformation characteristics of VO2 based on modification of material properties, interfacial structure, and field couplings, are discussed. Dynamical modulation of transformation characteristics through in situ processing is discussed as a means of imbuing synaptic function. Mechanistic understanding of site-selective modification; external, epitaxial, and chemical strain; defect dynamics; and interfacial field coupling in modifying local atomistic structure, the implications therein for electronic structure, and ultimately, the tuning of transformation characteristics, is emphasized. Opportunities are highlighted for inverse design and for using design principles related to thermodynamics and kinetics of electronic transitions learned from VO2 to inform the design of new Mott materials, as well as to go beyond energy-efficient computation to manifest intelligence.

Flat-Band-Induced Many-Body Interactions and Exciton Complexes in a Layered Semiconductor.

Interactions among a collection of particles generate many-body effects in solids that result in striking modifications of material properties. The heavy carrier mass that yields strong interactions and gate control of carrier density over a wide range makes two-dimensional semiconductors an exciting playground to explore many-body physics. The family of III-VI metal monochalcogenides emerges as a new platform for this purpose because of its excellent optical properties and the flat valence band dispersion. In this work, we present a complete study of charge-tunable excitons in few-layer InSe by photoluminescence spectroscopy. From the optical spectra, we establish that free excitons in InSe are more likely to be captured by ionized donors leading to the formation of bound exciton complexes. Surprisingly, a pronounced red shift of the exciton energy accompanied by a decrease of the exciton binding energy upon hole-doping reveals a significant band gap renormalization induced by the presence of the Fermi reservoir.

Spark plasma sintered Mg-4Y-3Nd with exceptional tensile performance

This work utilizes the spark plasma sintering (SPS) technique and exploits the physical mechanisms leading to the preparation of a fully compacted Mg-4wt.%Y-3wt.%Nd (WN43) material to exhibit superb mechanical properties, including outstanding plastic elongation in tension of more than 10 % among the SPS-ed Mg-based materials. To this end, the detrimental effect of oxide shells present on the Mg-Y-Nd powder particles is purposefully suppressed. The essential principle underlying the strong interconnection of the former powder particles is based on the combination of sufficiently high pressure and temperature approaching the eutectic points of Mg-Y/Nd systems. Thus-prepared WN43 material also retains the initial fine powder microstructure and its microstructural features are translated into good ductility and strength of the final product comparable with the conventionally-prepared (e.g. wrought) Mg-RE alloys. At the same time, the advantages of SPS – the production of a relatively large volume of near net-shape material with weak crystallographic texture – are retained. The deformation mechanisms are investigated by means of complementary in-situ techniques (acoustic emission, digital image correlations) and semi in-situ electron backscatter diffraction. A relatively low tension–compression asymmetry is revealed owing to the weak texture and the activity of deformation mechanisms (dislocation slip and twinning) follows the conventions of bulk Mg. In support of potential practical applications of the material, no significant cracking of the residual oxide phases is observed although they seem to play some role in the initiation of critical crack leading to the fracture. The SPS technique offers undemanding fine-tuning of processing parameters leading, in turn, to expeditious modification of material properties and, given the understanding of microstructure evolution unveiled in this work, it can be effectively utilized for the preparation of different Mg-based (or other) materials.

The Degradation of Synthetic Polymeric Scaffolds With Strut-like Architecture Influences the Mechanics-dependent Repair Process of an Osteochondral Defect in Silico

Current clinical treatments of osteochondral defects in articulating joints are frequently not successful in restoring articular surfaces. Novel scaffold-based tissue engineering strategies may help to improve current treatment options and foster a true regeneration of articulating structures. A frequently desired property of scaffolds is their ability to degrade over time and allow a full restoration of tissue and function. However, it remains largely unknown how scaffold degradation influences the mechanical stability of the tissue in a defect region and, in turn, the regenerative process. Such differing goals–supporting regeneration by degrading its own structure–can hardly be analyzed for tissue engineered constructs in clinical trials and in vivo preclinical experiments. Using an in silico analysis, we investigated the degradation-induced modifications in material and architectural properties of a scaffold with strut-like architecture over the healing course and their influence on the mechanics-dependent tissue formation in osteochondral defects. The repair outcome greatly varied depending on the degradation modality, i.e. surface erosion or bulk degradation with and without autocatalysis, and of the degradation speed, i.e. faster, equal or slower than the expected repair time. Bulk degradation with autocatalysis, independently of degradation speed, caused the mechanical failure of the scaffold prior to osteochondral defect repair and was thereby deemed inappropriate for further application. On the other hand, scaffolds with strut-like architecture degrading by both surface erosion and bulk degradation with slow degradation speed resulted in comparably good repair outcomes, thereby indicating such degradation modalities as favorable for the application in osteochondral defects.

Free electron gas in cavity quantum electrodynamics

Cavity modification of material properties and phenomena is a novel research field largely motivated by the advances in strong light-matter interactions. Despite this progress, exact solutions for extended systems strongly coupled to the photon field are not available, and both theory and experiments rely mainly on finite-system models. Therefore, a paradigmatic example of an exactly solvable extended system in a cavity becomes highly desirable. To fill this gap we revisit Sommerfeld's theory of the free electron gas in cavity quantum electrodynamics. We solve this system analytically in the long-wavelength limit for an arbitrary number of noninteracting electrons, and we demonstrate that the electron-photon ground state is a Fermi liquid which contains virtual photons. In contrast to models of finite systems, no ground state exists if the diamagentic ${\mathbf{A}}^{2}$ term is omitted. Further, by performing linear response we show that the cavity field induces plasmon-polariton excitations and modifies the optical and the DC conductivity of the electron gas. Our exact solution allows us to consider the thermodynamic limit for both electrons and photons by constructing an effective quantum field theory. The continuum of modes leads to a many-body renormalization of the electron mass, which modifies the fermionic quasiparticle excitations of the Fermi liquid and the Wigner-Seitz radius of the interacting electron gas. Last, we show how the matter-modified photon field leads to a repulsive Casimir force and how the continuum of modes introduces dissipation into the light-matter system. Several of the presented findings should be experimentally accessible.

Functionally Graded Non-Periodic Cellular Structure Design and Optimization

Abstract Topological tailoring of materials at a micro-scale can achieve a diverse range of exotic physical and mechanical properties that are not usually found in nature. Modification of material properties through customizing the structural pattern paves an avenue for novel functional products design. This paper explores a non-periodic microstructure design framework for functional parts design with high-strength and lightweight. To address the geometric frustration problem commonly found in non-periodic microstructure designing, we employ a smooth transition layer to connect distinct structural patterns and thus achieve functional gradation among adjacent microstructures. The concept of spatial control points is introduced for the interpolation of this transition layer. To achieve a high-strength macro-structural performance for designing functional parts, we formulate the control points as the design variables and encapsulate them into a macro-structural design optimization problem. Given that our objective function involves expensive finite element (FE) simulations, a Bayesian optimization scheme is exploited to address the computational challenge brought by the FE simulation. Experimental results demonstrate that the proposed design framework can yield both functionally graded lightweight structures and high-strength macro-mechanical performance for the designing parts. The compatibility issue of non-periodic microstructure design is well-addressed. Comparative studies reveal that the proposed framework is robust and can achieve superior mechanical performance to design functional parts with spatially varying properties.

Doping effect of Zn on structural and optical properties of CuO nanostructures prepared by wet chemical precipitation process

CuO exhibits superior properties in semiconducting technology. The preparation with thermal activation can be found in several previous reports. Consequently, the synthesizing process at room temperature is interesting because of no energy consumption, reducing production costs. For material property modification, incorporated metal atoms affect the morphology and properties of the host lattice. Accordingly, the purpose of this work is to study the consequence of Zn doping on the structural and optical properties of CuO, prepared via room temperature synthesizing process of wet chemical precipitation without further calcination. Various analytical techniques studied crystal structure, morphology, chemical composition, and optical properties. X-ray Diffraction (XRD) analysis revealed the formation of a pure monoclinic CuO phase with crystallite size in the range of 25–38 nm. As increasing Zn content, structural and optical properties of doped CuO were changed comparing with pure CuO. Scanning electron microscope (SEM) displayed morphology transformation from irregular plates to nanoparticles. TEM images elucidated the formation of crystals in the different directions of block aggregation. Specific surface area (SSA) revealed a reducing value from 17.2567 to 16.6128 m2/g. PL emission assigned to near band-edge emission at 445 nm and oxygen vacancy existence at 519 nm. Comparison with pristine CuO, PL intensity quenching was found in doped CuO. The reduction of electron-hole recombination corresponds to the increasing energy bandgap with linear tendency, appearing at 2.80–2.94 eV. Finally, this synthesizing route is effective and can extend synthesizing technology to other doped metal oxide nanostructures.