Increase In Defect Density Research Articles

Nickel doping induced amorphization of brookite TiO2: Photoluminescence enhancement

In this work, we have elaborated nickel doped TiO2 thin films by low temperature spray pyrolysis technique from ethanol based precursor. For the sample free of nickel, The FTIR spectrum reveals a large amount of organic species bonded to TiO2, resulting in a carbon-doped brookite. Ni doping causes extinction of the organic related peak that fade away for highly (> 10 %) Ni doped films. XRD spectra reveal the presence of brookite phase for low Ni amounts. While increasing Ni rate, the crystalline phase undergoes a transition to amorphous state and brookite XRD peaks vanishes. Optical and electronic (SEM) micrographs show a smooth surface with small brookite nanoparticles, for low Ni rate. Increasing Ni rate results in a fractured morphology composed of micrometric smooth ships. Optical absorption obtained by transmittance-reflectance spectra is enhanced in the transparency region of TiO2, by Ni doping due to the increase of defect density and the brookite to amorphous phase transformation. Visible and UV emission spectra are obtained from PL measurements at room temperature. A spectacular PL magnification is obtained for high Ni doping levels due to increase of oxygen vacancies in the amorphous film. However, thermal annealing at 500 °C causes recrystallization to anatase phase, leading to a dramatic PL quenching.

Effect of in situ degradation on the atomic structure and optical properties of GaN-based green light-emitting diodes

The structure at the atomic scale and optical properties of GaN-based green light-emitting diodes (LEDs) before and after in situ degradation were investigated by spherical aberration corrected scanning transmission electron microscopy and temperature-dependent micro-photoluminescence. Indium (In) interstitial atoms existed in the degraded sample, due to the small-bond-energy In atoms deviating from their normal lattice position, caused by the relaxation of the InGaN well. Both the peak wavelengths of the original and degraded green LEDs had similar temperature-dependent behaviors, due to the localization states in the InGaN well. These wavelengths indicate that the degradation had little influence on the localization states. However, the emission peak of the degraded green LED redshifted by 1.6 nm at 300 K, and the integrated intensity decreased by 36.8%, compared to the peak and intensity of the original sample, respectively. Based on first-principles calculations, the calculated bandgap for the relaxation of the InGaN well was small. Therefore, the wavelength redshifted, and the luminous efficiency of the green LED decreased after degradation. These features are attributed to a decreased bandgap due to the relaxation of the InGaN well; increased defect density, resulting from In interstitial atoms; and an increase in the InGaN well thickness.

Strain Tuning via Larger Cation and Anion Codoping for Efficient and Stable Antimony-Based Solar Cells.

Strain induced by lattice distortion is one of the key factors that affect the photovoltaic performance via increasing defect densities. The unsatisfied power conversion efficiencies (PCEs) of solar cells based on antimony chalcogenides (Sb‐Chs) are owing to their photoexcited carriers being self‐trapped by the distortion of Sb2S3 lattice. However, strain behavior in Sb‐Chs‐based solar cells has not been investigated. Here, strain tuning in Sb‐Chs is demonstrated by simultaneously replacing Sb and S with larger Bi and I ions, respectively. Bi/I codoped Sb2S3 cells are fabricated using poly[2,6‐(4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b']dithiophene)‐alt‐4,7‐(2,1,3‐enzothiadiazole)] as the hole‐transporting layer. Codoping reduced the bandgap and rendered a bigger tension strain (1.76 × 10−4) to a relatively smaller compression strain (−1.29 × 10−4). The 2.5 mol% BiI3 doped Sb2S3 cell presented lower trap state energy level than the Sb2S3 cell; moreover, this doping amount effectively passivated the trap states. This codoping shows a similar trend even in the low bandgap Sb2(SxSe1‐x)3 cell, resulting in 7.05% PCE under the standard illumination conditions (100 mW cm−2), which is one of the top efficiencies in solution processing Sb2(SxSe1‐x)3 solar cells. Furthermore, the doped cells present higher humidity, thermal and photo stability. This study provides a new strategy for stable Pb‐free solar cells.

Exfoliation and Defect Control of Two-Dimensional Few-Layer MXene Ti3C2Tx for Electromagnetic Interference Shielding Coatings.

Defect-controlled exfoliation of few-layer transition-metal carbide (f-Ti3C2Tx) MXene was demonstrated by optimizing chemical etching conditions, and electromagnetic interference (EMI) shielding coatings were explored. The structural features such as layer morphology, lateral size, layer thickness, defect density, and mechanical stability of the exfoliated f-Ti3C2Tx were strongly dependent on exfoliation conditions. By selecting appropriate exfoliation conditions, moderate etching time leads to the formation of quality f-Ti3C2Tx with lesser defects, whereas longer etching time can break the layer structure and increase defect density, structural misalignment, and oxidative products of f-Ti3C2Tx. The resultant fabricated free-standing flexible f-Ti3C2Tx films exhibited electrical conductivity and electromagnetic interference (EMI) shielding effectiveness (SE) in the X-band of about 3669 ± 33 S/m and 31.97 dB, respectively, at a thickness of 6 μm. The large discrepancy in EMI SE performance between quality (31.97 dB) and defected (3.164 dB) f-Ti3C2Tx sheets is attributed to interconnections between f-Ti3C2Tx nanolaminates interrupted by defects and oxidative products, influencing EMI attenuation ability. Furthermore, the demonstrated solution-processable high-quality f-Ti3C2Tx inks are compatible and, when applied for EM barrier coating on various substrates, including paper, cellulose fabric, and PTFE membranes, exhibited significant EMI shielding performance. Moreover, controlling defects in f-Ti3C2Tx and assembly of heterogeneous disordered carbon-loaded TiO2-Ti3C2Tx ternary hybrid nanostructures from f-Ti3C2Tx by tuning etching conditions could play an enormous role in energy and environmental applications.

Enhancing interface doping in graphene-metal hybrid devices using H2 plasma clean

Synthetic graphene suffers from surface contamination after the removal of transfer polymer. This impacts the chemical and electronic properties at the interface of graphene-metal structures. Hydrogen plasma treatment is promising to effectively clean, dope and pattern graphene. However, it also causes irreversible damage to the graphene lattice or reversible graphene hydrogenation which primarily depends on the flux and energy of plasma radicals/ions. Here, we optimize a downstream Ar/H2 plasma with moderate flux and low energy thermal H radicals to eliminate polymer residues and enhance the charge transfer at the graphene-metal interface. A systematic study is carried out to evaluate the plasma-induced doping and defectivity in ruthenium capped graphene devices. Single layer graphene is found to be n-doped after plasma treatment accompanied by an increase in defect density. Similarly, bilayer graphene (BLG) also undergoes n-doping but does not incur significant plasma-induced damage. Sheet resistance of Ru capped plasma cleaned BLG is found to decrease by 18%. After device fabrication, a final plasma clean reduces the surface roughness of the capping metal, and further improves the electrical conductivity. This study provides a viable basis to effectively clean and dope graphene-metal interface with a fab-compliant plasma technique while preserving the high quality of graphene.

Microstructural Refinement and Mechanical Properties of Ferritic Stainless Steel Processed by Equal-Channel Angular Pressing

Ferritic stainless steel (FSS 430) has been deformed by equal-channel angular pressing (ECAP) at room temperature at an equivalent strain (evm) of 1.2 by adopting the route Bc. Microstructural developments were studied by optical microscopy, transmission electron microscopy, and microtexture by electron backscattered diffraction. Typical ultrafine-grains of less 0.2 µm could be identified. The XRD patterns confirm the presence of a BCC phase alone. An increase in peak broadening is attributed to the reduction of crystallite size and increase of lattice strain arising from deformation during ECAP. The grain refinement takes place through simple shear deformation by elongation of grains, splitting of grains into bands, the subdivision of bands into subgrains by dynamic recovery, and conversion of subgrains into grains by progressive lattice rotation (PLR). Weakly textured rolled ferritic stainless steel is strongly textured at an imposed equivalent strain of 0.6 with the development of ideal shear components of D2θ and $$\bar{E}_{\theta }$$ for bcc materials. On the second pass of ECAP texture is partially randomized and the intensity of components was decreased and new components of D1θ and Fθ are developed due to subdivision of grains into bands, bands into subgrains, and conversion of subgrains into grains of high angle of misorientation by PLR. The yield strength, tensile strength, and hardness are increased almost 1.5 times that of as-received coarse-grained material due to a high degree of grain refinement and an increase in defect density or strain. The yield strength of 687 MPa of as-received material was enhanced to 1093 MPa of ECAPed ferritic stainless steel. However, the material has lost its ductility significantly due to high defect density and a significant amount of non-equilibrium nature of grain boundaries.

Impact of surface defects in electron beam evaporated ZnO thin films on FET biosensing characteristics towards reliable PSA detection

In this work, the impact of surface defects in zinc oxide (ZnO) thin films on the performance of field effect transistor (FET) biosensors has been explored. Here, ZnO thin films have been fabricated using electron beam evaporation with various deposition parameters to tune the surface roughness and defect state density. The films have been deployed for sensing prostate specific antigen (PSA) after anti-PSA antibody immobilization. Interestingly, it has been observed that though the transconductance of the sensors degrade monotonically with increasing defect density, the antibody binding density has a non-monotonic behaviour due to enhanced steric hindrance effect beyond a certain limit of surface roughness. There is a variation of sensitivity by 15 times with different physical properties of ZnO and the optimized sensor has a PSA detection limit of 0.06 fM. Further, the sensor with the best sensitivity retains its performance upto 2 months while a non-optimized sensor with initial PSA detection limit of 0.1 fM has longevity of 4 months. Thus, it can be envisaged that tuning the surface defect state in ZnO FET biosensor is an essential requirement not only to ensure a low detection limit and high sensitivity but also to assure a reliable long term performance.

Defect-mediated ionic hopping and green electricity generation in Al2−xMgxO3-based hydroelectric cell

A direct evidence of enhanced water molecule splitting by increasing defect concentration in Al2−xMgxO3 hydroelectric cells (HECs) has been elaborated for green electricity generation. Existence of F, F+ and [Mg]0 defect centers in nanoporous Al2−xMgxO3 (x = 0–0.5) cell pellets has been confirmed by optical spectroscopy. Increased defect density from ~ 1.45 × 1015 cm−3 to 5.4 × 1016 cm−3 induced by increasing Mg doping concentration is found to be a key factor to control water molecule dissociation/splitting at alumina surface. Small polaron hopping-assisted ionic conduction for enhanced current density is analyzed by impedance spectroscopy. The maximum, 15 mA, current is obtained in Al2−xMgxO3 HEC for x = 0.5 concentration resulting in 13.5 mW off-load peak output power as compared to 4.95 mW peak output power in pure alumina HEC. A theoretical modeling of Nyquist spectra analyzes ion–solid interaction for real charge transfer process in HEC. Deliberate defect creation in alumina devised for water molecule dissociation at room temperature paves the way to fabricate a facile green electricity generation source in the form of HEC. Al2−xMgxO3-based hydroelectric cell is a very low-cost device to generate green electricity besides providing eco-friendly by-products without use of photocatalytic activity, acid/alkali or electrolyte. Moreover, presented innovative alumina-based HEC would be a big step for mitigating industrial waste consisting of alumina. Polaron assisted hopping of OH− ions in the lattice of Al2-xMgxO3 HEC and green electricity generation in the form of V-I plot.

Investigation of the interface defect density at pm-Si:H/c-Si and the surface recombination speeds on silicon heterojunction solar cells

We carefully analyze in this article, the influence of the interface defect density of the defective surface between crystalline silicon and hydrogenated polymorphous silicon (i-pm-Si:H/n-c-Si) on the current density-voltage (J–V) characteristic of n-type HIT solar cells (ITO/p-a-SiC:H/i-pm-Si:H/n-c-Si/Al). We have also studied the variation of the electrons and holes surface recombination speeds on the front and the back face of the studied cell. The results show that the recombination rate is higher on the front surface of the c-Si wafer when the defect density increases. Indeed, we have found that for the two values of the front contact barrier height at the ITO/p-a-SiC:H, when the interface defect density increases, the Voc and the FF drop. Thus, the efficiency goes from almost 20 to 2 %. These results allow us to conclude that to have a good Voc, the surface of the active layer should be passivated to have a interface defect density less than 1011 cm−2. For the variation of the surface recombination speed SnL with the interface defect density for electrons at the back face of the solar cell, no sensitivity is observed. For small values of these defects, the efficiency increases when the holes surface recombination speed SpL is decreased, but for large values of interface defect density, no improvement is observed, even for low surface recombination speeds.

The Influence of Thermomechanical Treatment on the Strain Behavior of the Fe–Ni–Co–Ti Ferromagnetic Alloy Nanocomposite with Shape Memory Effect

The paper describes mechanical tests of a nanocomposite produced from the ferromagnetic shape memory Fe−Ni−Co−Ti alloy under uniaxial tension over a wide temperature range. The production of the nanocomposite was preceded by preliminary thermomechanical treatment (TMT), involving drawing, quenching, and ageing, for precipitation hardening. The TMT imparted high superelastic strain and shape memory effect to the nanocomposite. The preliminary TMT with strain ψ = = 7.4−22.5% aged at T = 650°C for 5−10 min was experimentally found to correspond to the optimal combination of the maximum superelastic strain and shape memory effect. This contributed to the phase and twinning plastic deformation of the nanocomposite over the test temperature range Ms < Ttest < Af (where Ms is the start temperature of forward martensitic transformation on cooling and Af is the finish temperature of reverse martensitic transformation on heating). A plateau with constant stress was found in the two-phase Mf < Ttest < Ms region on the tensile curve at drawing strain ψ = 22.5%. A significant increase in the preliminary strain (more than 40%) substantially stabilized the austenitic matrix, thus inhibiting the martensitic transformation and reducing reversible effects because the austenite grain size refines when the lattice defect density increases. The austenite grain size distribution was assessed versus the chosen TMT conditions. When the austenite grain size increased, the superelastic strain recovery became higher. The factors leading to greater superelasticity were analyzed within different phenomenological models. The TMT has a crucial role in the variation of structure and mechanical properties, in turn promoting inelastic effects at different temperatures.

Influence of carbon on microstructure and mechanical properties of magnetron sputtered TaW coatings

(Ta,W) and (Ta,W):C films with ~5 at.% C were deposited by non-reactive magnetron sputtering. They crystallised in a bcc structure with a columnar microstructure. The solid solubility of C in (Ta,W) alloys is very low, which suggests that the (Ta,W):C films are supersaturated with respect to carbon. This was confirmed by diffraction and atom probe tomography (APT) showing that carbon is in the as-deposited (Ta,W):C films homogeneously distributed in the structure without carbide formation or carbon segregation. Annealing at 900 °C for 2 h showed no significant column coarsening but an increased defect density at the column boundaries in the (Ta,W):C films. The films were still supersaturated with respect to carbon but APT showed a partial segregation of carbon presumably to defect-rich column boundaries after annealing. The (Ta,W) films exhibited a hardness of ~12–13 GPa. Alloying with carbon increased the hardness to ~17 GPa. The hardness increased to ~19 GPa for the annealed (Ta,W):C films. This annealing-induced hardness increase was explained by C segregation to the more defect-rich column boundaries, which restricts dislocation movements. (Ta,W):C coatings may be a potential alternative to ceramic coatings, worth exploring further by small scale mechanical testing to investigate if these materials are ductile.

Characterization of oxide film in P92 ferritic-martensitic steel exposed to high temperature and pressure water

This paper investigated oxide film of P92 steel in 300 °C/10 MPa water at different corrosion time. The surface morphology, phase structure and micro-defects were investigated by scanning electron microscope, X-ray diffraction and positron annihilation spectroscopy. The results reveal the formation of double oxide film, the outer oxide film was mainly composed of magnetite and the inner oxide film was composed of magnetite and FeCr spinel. The positron annihilation lifetimes spectra shows that the average size of defects increases with the increase of corrosion time. Doppler broadening spectroscopy of slow positron beam indicates that the outer oxide film structure was relatively dense in the initial stage of corrosion. The inner oxide film was characterized with relative loose structure due to the increase of defects size and density.

Influence of temperature on growth of graphene on germanium

Growth of high-quality graphene on germanium is to date only reported at growth temperatures near the substrate melting point. Direct integration of graphene growth into technological processes would, however, require a significantly lower growth temperature. Accordingly, we investigated the influence of growth temperature on the quality of graphene on Ge(001), Ge(110), and Ge(111). We found that increased defect density as indicated by Raman spectroscopy correlates with topographically protruding carbon defect clusters as indicated by scanning tunneling microscopy. The Raman quality of graphene on Ge(001) and Ge(110) grown at 850 °C is clearly limited by defects within the relatively large grains and not by grain boundaries, whereas the quality of graphene on Ge(111) additionally suffers from small grain size. We explain the decreased graphene quality by too weak substrate-mediated etching of defective carbon structures. Finally, we discuss potential ways to increase the rate of carbon etching.

Confocal laser scanning microscopy as a real-time quality-assessment tool for industrial graphene synthesis

For the industrial quality control (QC) of the chemical vapor deposition (CVD) graphene, it is essential to develop a method to screen out unsatisfactory graphene films as efficiently as possible. However, previously proposed methods based on Raman spectroscopy or optical imaging after chemical etching are unable to provide non-invasive and fast analysis of large-area graphene films as grown on Cu foil substrates. Here we report that the reflection mode of confocal laser scanning microscopy (CLSM) provides a high-contrast image of graphene on Cu, enabling the real-time evaluation of the coverage and quality of graphene. The reflectance contrast, Rc, was found to be dependent on the incident laser wavelength, of which the maximum was obtained at 405 nm. In addition, Rc decreases with increasing defect density of graphene. The dependence of Rc on the graphene’s quality and laser wavelengths were explained by the tight-binding model calculation based on the Fresnel’s interference formula. Thus, we believe that the reflection mode CLSM would be a very powerful quality-assessment tool for the mass production of CVD graphene films grown on Cu.

Phonon localization in single wall carbon nanotube: Combined effect of 13C isotope and vacancies

The combined effect of 13C isotope doping and vacancies on the phonon properties of a single-wall carbon nanotube is theoretically investigated using the forced oscillation method. The phonon density of states (PDOS) is calculated for all (0%–100%) 13C isotope contents and wide (0%–30%) vacancy concentrations. We found a redshift of the Raman active E2g peak in the PDOS with increasing isotope contents, while the disappearance of the E2g peak and the appearance of a new sharp peak in the low-energy region with increasing combined defects. Both 13C isotope and combined defects cause the localization of the high-energy optical phonons. We calculated the typical mode patterns for the in-plane longitudinal optical phonon to visualize the localization phenomena elaborately at the presence of isotope and vacancies. The calculated average localization length shows an asymmetric behavior with increasing 13C isotope concentrations which is in good agreement with the 13C isotope dependence localization length of single-layer graphene. We noticed that a typical localization length is on the order of ∼1 nm at 70% isotope concentrations. The combined effect of 13C isotope and vacancies shows an abruptly decreasing localization length with increasing defect densities. These results are important to understand the heat conduction as well as nanoscopic vibrational studies such as tip-enhanced Raman spectra in carbon nanotubes where the local phonon energies may be mapped.

The formation of methanol from glycerol bio-waste over doped ceria-based catalysts.

A series of ceria-based solid solution metal oxides were prepared by co-precipitation and evaluated as catalysts for glycerol cleavage, principally to methanol. The catalyst activity and selectivity to methanol were investigated with respect to the reducibility of the catalysts. Oxides comprising Ce-Pr and Ce-Zr were prepared, calcined and compared to CeO2, Pr6O11 and ZrO2. The oxygen storage capacity of the catalysts was examined with analysis of Raman spectroscopic measurements and a temperature programmed reduction, oxidation and reduction cycle. The incorporation of Pr resulted in significant defects, as evidenced by Raman spectroscopy. The materials were evaluated as catalysts for the glycerol to methanol reaction, and it was found that an increased defect density or reducibility was beneficial. The space-time yield of methanol normalized to surface area over CeO2 was found to be 0.052 mmolMeOH m-2 h-1, and over CeZrO2 and CePrO2, this was to 0.029 and 0.076 mmolMeOH m-2 h-1, respectively. The inclusion of Pr reduced the surface area; however, the carbon mole selectivity to methanol and ethylene glycol remained relatively high, suggesting a shift in the reaction pathway compared to that over ceria. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Simulation studies to quantify the impacts of point defects: An investigation of Cs2AgBiBr6 perovskite solar devices utilizing ZnO and Cu2O as the charge transport layers

In this investigation, we have applied SCAPS and wxAMPS to simulate defects and probe a photovoltaic device utilizing Cs2AgBiBr6 as the active photovoltaic layer and ZnO and Cu2O as the electron transport layer (ETL) and hole transport layer (HTL) respectively. At the Cs2AgBiBr6 bulk we find that with increasing defect density, each defect level has increasing impact on all device performance parameters. At a given defect density however, we find that that deeper defects have more profound impacts on Jsc and FF, and minimal effects on Voc. Specific to the Cs2AgBiBr6 structure, we have investigated VAg (shallow defect), VBi (deep defect) and Bri (quasi-deep defect). Our results provide insight into the growth conditions of Cs2AgBiBr6, with a need to have both Br-poor and Bi-rich conditions, and a preference for the latter over the former to suppress the deeper defect. Exploring the performance kinetics at the ZnO/Cs2AgBiBr6 and Cs2AgBiBr6/Cu2O interfaces due to defect type, location and density, we showcase a remarkably stable behavior in both Voc and Jsc across both interfaces. We attribute this to much higher charge mobilities in the ZnO and Cu2O compared to the Cs2AgBiBr6 layer combined with similar defect densities across the layers, leading to effective charge extraction and minimal charge recombination.

Structural Modifications in Epitaxial Graphene on SiC Following 10 keV Nitrogen Ion Implantation

Modification of epitaxial graphene on silicon carbide (EG/SiC) was explored by ion implantation using 10 keV nitrogen ions. Fragments of monolayer graphene along with nanostructures were observed following nitrogen ion implantation. At the initial fluence, sp3 defects appeared in EG; higher fluences resulted in vacancy defects as well as in an increased defect density. The increased fluence created a decrease in the intensity of the prominent peak of SiC as well as of the overall relative Raman intensity. The X-ray photoelectron spectroscopy (XPS) showed a reduction of the peak intensity of graphitic carbon and silicon carbide as a result of ion implantation. The dopant concentration and level of defects could be controlled both in EG and SiC by the fluence. This provided an opportunity to explore EG/SiC as a platform using ion implantation to control defects, and to be applied for fabricating sensitive sensors and nanoelectronics devices with high performance.

Variations in structural, optical, and dielectric properties of CuO nanostructures with thermal decomposition

Herein, we report the characterization of CuO nanostructures annealed at different temperatures, synthesized by chemical co-precipitation method. It is found in the XRD patterns that the peaks were broader at low temperatures due to increased defect density and poor crystallization suggesting that increasing annealing temperature enhances the degree of crystallization of the samples. Scanning electron microscopy (SEM) suggests that the CuO nanostructures change with increasing annealing temperature from rods to disc-shaped particles. It is observed in PL that with the increase in annealing temperature, the emission intensities increase for the same morphology due to the increase in oxygen vacancies (O.Vs) and improvement of crystallization. The dielectric constant has been increased with increasing temperature for the same morphology and is maximum for the sample annealed at 600 °C which is mainly due to the RDP and SCP. The relaxation peaks are shifted to the higher frequencies due to defect dipoles. Enhancement in conductivity is due to the growth of conducting grains. Our findings may provide comprehensive insight in designing high-performance electronic devices.

Experimental and first-principles study of defect-induced electronic and magnetic properties of ZnO nanocrystals

We investigate defect-induced electronic and magnetic properties of zinc oxide (ZnO) nanocrystalline powders prepared by mechanical milling. The average crystallite size decreases as the milling time is increased but the wurtzite structure is stable even after 60 hours of milling. The structural properties are characterized by X-ray diffraction and transmission electron microscopy. The chemical composition of pure unmilled and as-milled powders is determined by X-ray photoelectron spectroscopy. Raman scattering spectra were analyzed to find evidence of crystallinity and defect structures in the milled powders. The results obtained from electronic paramagnetic resonance studies demonstrate that the defects are due to both oxygen and zinc vacancies. Since defect-induced magnetism has potential application in spintronic devices, we measured the M-H loops of ZnO powders at different temperatures. We observed that there is a transition from paramagnetic to ferromagnetic behavior due to the vacancies caused by the crystallite size reduction. We also calculated the band structures and density of states within the framework of density functional theory with Hubbard U correction to understand the electronic properties of ZnO for both pure crystals and crystals with defects. A simple mechanical milling process can produce a crystallite size of approximately 10 nm after 40 hours of milling, and the resulting significant changes in electronic properties favor superior application in devices as compared with the bulk. This change of intrinsic properties in ZnO is largely due to the size effect and the increase of defect density.