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Abstract

We report here investigations of crystal and electronic structure of as-synthesized and annealed ZnO nanobelts by an in-situ high-resolution transmission electron microscope equipped with a scanning tunneling microscopy probe. The in-situ band gap measurements of individual ZnO nanobelts were carried out in scanning tunneling spectroscopy mode using the differential conductance dI/dV–V data. The band gap value of the as-synthesized ZnO nanobelts was calculated to be ∼2.98 eV, while this property for the annealed nanobelts (∼3.21 eV) was close to the band gap value for bulk ZnO materials (∼3.37 eV). The difference in the band gap value of the as-synthesized ZnO nanobelts and annealed ones was attributed to the planar defects (e.g. stacking faults and twins). These defects can alter the electronic structure by producing localized resonant states that result in band gap reduction.

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... One can see that the values of optical band gaps are a little smaller than that of the bulk ZnO (~3.37 eV at room temperature [61]). Such lower values of band gap have their origin in the intrinsic defects (confirmed by PL spectroscopy study given below) present in ZnO and Zn 1-x Ag x O NPs [62]. Further, the redshift of the optical band gap of ZnO NPs with Ag doping has been explained in different ways [26,58,63] viz (i) enhancement of p-type conductivity due to Ag doping with ZnO NPs [62]. ...
... Such lower values of band gap have their origin in the intrinsic defects (confirmed by PL spectroscopy study given below) present in ZnO and Zn 1-x Ag x O NPs [62]. Further, the redshift of the optical band gap of ZnO NPs with Ag doping has been explained in different ways [26,58,63] viz (i) enhancement of p-type conductivity due to Ag doping with ZnO NPs [62]. p-type conductivity has its origin in the formation of oxygen vacancies on Ag doping that arise due to differences in charges of Ag + and Zn 2+ [58]. ...
Article
Understanding the effect of doping concentration on the various properties of metal-doped ZnO nanoparticles (NPs) near the shape transition point (sphere to the rod) is essentially required for their possible applications. Given these, we have successfully synthesized spherical to rod-shaped Zn1-xAgxO employing a simple chemical route considering Ag doping with concentrations of 1, 2, and 3 mol percentage (%). Although, the shape transition from spherical to rod has been triggered at doping concentration of 2 mol%, the shape transition has been found to be completed for 3 mol% generating nanorods with an aspect ratio of ∼1.33. Structural, optical and antimicrobial properties of Zn1-xAgxO NPs show significant changes at the transition concentration of Ag. The shape transition occurs mainly due to the process of synthesis and change in the surface states of the NPs in presence of excess Ag⁺ ions in the local environment of the nucleation sites of ZnO.
... TiO2 loaded on ZnO nanorod film had a staggered gap (type II), as proposed by previous researchers [33,59]. TiO2-ZnO film exhibited band gap reduction due to the existence of planar defects [60]. Twin boundaries and stacking faults (planar defects) are correlated in band gap reduction. ...
... TiO 2 loaded on ZnO nanorod film had a staggered gap (type II), as proposed by previous researchers [33,59]. TiO 2 -ZnO film exhibited band gap reduction due to the existence of planar defects [60]. Twin boundaries and stacking faults (planar defects) are correlated in band gap reduction. ...
Article
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Efficient solar driven photoelectrochemical (PEC) response by enhancing charge separation has attracted great interest in the hydrogen generation application. The formation of one-dimensional ZnO nanorod structure without bundling is essential for high efficiency in PEC response. In this present research work, ZnO nanorod with an average 500 nm in length and average diameter of about 75 nm was successfully formed via electrodeposition method in 0.05 mM ZnCl2 and 0.1 M KCl electrolyte at 1 V for 60 min under 70 °C condition. Continuous efforts have been exerted to further improve the solar driven PEC response by incorporating an optimum content of TiO2 into ZnO nanorod using dip-coating technique. It was found that 0.25 at % of TiO2 loaded on ZnO nanorod film demonstrated a maximum photocurrent density of 19.78 mA/cm² (with V vs. Ag/AgCl) under UV illumination and 14.75 mA/cm² (with V vs. Ag/AgCl) under solar illumination with photoconversion efficiency ~2.9% (UV illumination) and ~4.3% (solar illumination). This performance was approximately 3-4 times higher than ZnO film itself. An enhancement of photocurrent density and photoconversion efficiency occurred due to the sufficient Ti element within TiO2-ZnO nanorod film, which acted as an effective mediator to trap the photo-induced electrons and minimize the recombination of charge carriers. Besides, phenomenon of charge-separation effect at type-II band alignment of Zn and Ti could further enhance the charge carrier transportation during illumination.
... Various semiconducting nanomaterials such as TiO 2 , ZnO, WO 3 , V 2 O 5 , MO 3 , CdS, BiVO 4 , etc. have been reported as photocatalysts so far [16][17][18][19][20][21][22]. Among those, an excessive care has been rewarded to ZnO due to its availability, low cost to prepare, chemical stability, biocompatibility, strong oxidizing capability and harmless makeup [23,24]. However, its wide optical band gap (3.22 eV) is not suitable for the electron-hole generation by visible light illumination, which hinders to use ZnO as a visible light driven photocatalyst [25]. ...
... ZnO films showed excellent efficiency against the inactivation of S. aureus (Park et al. 2017). nZnO has a bandgap of 3.37 eV and bond energy of 60 meV; however, the bandgap depends entirely on the synthesis method as it significantly impacts the electronic structure and properties of ZnO (Asthana et al. 2011). nZnO has been considered an effective antimicrobial agent, and it has been applied in the biomedical field (Zhou et al. 2008). ...
Article
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Nanotechnology has been rapidly developing in the past decade, and metal nanomaterials have shown promising improvement in microbial control. Metal nanoparticles have been applied in medical settings for adequate disease spread control and to overcome the challenges of multidrug-resistant microorganisms. Recently, the demand for safe water supply has increased, requiring higher sanitation of the water treatment technology as well as being environmentally sustainable. However, the employed water disinfection technologies cannot meet the elevated demand due to limitations including chemical byproducts, immobility, energy consumption, etc. Metal nanomaterials are considered to be an alternative disinfection technology considering their high efficiency, mobility, and stability. A significant amount of research has been carried out on enhancing the antimicrobial efficiency of metal nanomaterials and determining the underlying antimicrobial mechanisms. This paper provides an overview of emerging metal nanomaterials development, including the synthesis method, material characteristics, disinfection performance, environmental factors, potential mechanism, limitations, and future opportunities in the water disinfection process.
... Efficient computational tools have been developed to perform simulations within timespans not accessible to experimental studies [22,23]. We may refer to large-scale continuum simulations , phase-field simulations for capturing the microstructure [48][49][50][51][52][53][54][55], MD simulations to capture the atomistic mechanisms [2,47,[56][57][58][59][60][61][62][63][64][65][66][67], and multiscale simulations to capture the broad spectrum of materials and processes response [24,25,46,[68][69][70][71][72][73]. Out of several computational methods, molecular dynamics simulation allows the capturing of materials evolution with atomistic accuracy, including the radiation damage mechanism. ...
Article
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Ferritic-martensitic steels, such as T91, are candidate materials for high-temperature applications, including superheaters, heat exchangers, and advanced nuclear reactors. Considering these alloys’ wide applications, an atomistic understanding of the underlying mechanisms responsible for their excellent mechano-chemical properties is crucial. Here, we developed a modified embedded-atom method (MEAM) potential for the Fe-Cr-Si-Mo quaternary alloy system—i.e., four major elements of T91—using a multi-objective optimization approach to fit thermomechanical properties reported using density functional theory (DFT) calculations and experimental measurements. Elastic constants calculated using the proposed potential for binary interactions agreed well with ab initio calculations. Furthermore, the computed thermal expansion and self-diffusion coefficients employing this potential are in good agreement with other studies. This potential will offer insightful atomistic knowledge to design alloys for use in harsh environments.
... A lower band gap value of ZnO NPs synthesized using a chemical method followed by thermal treatment (solvothermal route) was also reported by Khlifi et al [78]. This can be due to planar defects (such as stacking faults or twin boundaries) present in the crystal or intrinsic defects (such as zinc vacancies, Zn and O interstitials, etc.) possessed by the synthesized NPs [79]. The band gap of ZnO NPs has an increasing nature (blue-shift) with an increase in Li doping level up to a concentration of 8 mol%. ...
Article
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The mode of incorporation of lithium (Li) (as substitution or interstitial position) in zinc oxide (ZnO) has its own importance as far as the potential applications of Li-doped ZnO nanoparticles (NPs) are concerned. Fabrication of p-type ZnO based semiconductors as well as defect engineering based applications demand substitution of Zn2+ by Li+. However, doping of ZnO by Li with interstitial positions can play an important role in controlling the different properties of it. In the present study, we report the successful doping of Li in ZnO NPs up to a Li concentration of 10 mol% employing a simple wet chemical precipitation method in water. Up to a Li concentration of 8 mol%, doping by substitution of Li to the Zn sites have been observed. However, for 10 mol% of Li concentration, doping by incorporation of interstitial sites in addition to the substitution have been confirmed through the complementary characterization techniques. The effects of interstitial Li in ZnO on structural, optical, and antimicrobial properties have been studied in details systematically. For all the cases (structural, optical, and antimicrobial), the properties of Li-doped ZnO NPs have been changed reversibly in the ZnO NPs after incorporation of interstitial sites by Li as compared to the substitution of Li. For example, the microstrain, band gap, and antimicrobial activity have been found to increase with increase in Li concentration up to 8 mol%. However, the microstrain, band gap, and antimicrobial activity are found the decrease for 10 mol% of Li as compared to 8 mol% of Li. This study indicated that the different properties of Li-doped ZnO NPs can be controlled suitably as per the requirements for the practical applications of ZnO based materials.
... The variety of nanostructures of ZnO makes it as an attractive and new material with potential applications in many fields of nanotechnology. One-dimensional structures are considered the largest group of ZnO nanostructures including nanorods [5], nanowires [6], nanobelts [7] and nanotubes [8]. Various methods have been developed during several years for the synthesis of one-dimensional ZnO that have included hydrothermal method [9], chemical vapor deposition [10], green synthesis [11], sol-gel method [12] and electrochemical technique [13]. ...
Article
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One-dimensional ZnO nanostructures have been attracting growing research effort worldwide over the last few years due to their various morphologies, easy synthesis and excellent physical properties for fabricating optoelectronic devices. In this study, we have reported the synthesis of Holmium coated ZnO nanostructures. The structural, morphological and optical properties of our materials were studied using different techniques. The obtained materials show promising properties and the remarkable effects of the coated material. The experimental results obtained for our material exhibit signficant enhancement in properties, which is suitable for the optoelectronic applications.
... The band gap energy of the commercial and the synthesized sample was measured to be 3.10 and 3.14 eV, respectively. The results are in good agreement with literature [16]. ...
Article
Herein, a comparative study focusing on the effects of measuring atmosphere on ultraviolet (UV) photodetection performance of ZnO nanostructures is presented. Various morphologies of ZnO nanostructures were synthesized by changing the hydrothermal temperature within 343–423 K. The ultraviolet photodetection performance of the samples was studied against dark ultraviolet light (360 nm, 800 μW/cm²) under 200 sccm gas flows of argon and oxygen. The nanoflowers grown at 373 and 393 K showed the best photosensitivity and photoresponsivity under both argon and oxygen flows. Concerning the importance of active surface area, it was shown that the sample grown at 393 K serves higher accessible surface-to-volume ratio (qualitatively observable by SEM and confirmed by BET), higher concentration of freed electron due to oxygen vacancies (confirmed by PL) and consequently, the best UV photosensitivity. The comparative study between photodetection parameters in argon and oxygen atmospheres showed that although higher partial pressure of oxygen is favorable for all photodetection parameters like photosensitivity, photoresponsivity and decay time, it has adverse effects on rising time because rising time is directly coupled with the density of desorbed oxygen atoms.
... where E bg = band gap energy of the nanoparticles = 5.68  10 À19 J (from UV-Visible absorbance spectrum), E bg(bulk) = band gap energy of the bulk ZnO = 3.37 eV [19], which has the value of 5.39  10 À19 J, h = Planck's Constant = 6.625  10 À34 J s, r = particle radius (nm), m à e = effective mass of electron of ZnO = 0.29m e , m à h = effective mass of hole in ZnO = 1.21m e , e = elementary charge, e o = permittivity of free space, e = relative permittivity of ZnO = 8.12 [20,21]. Using above effective mass model and UV-Visible spectrum, calculated radius of ZnO nanoparticles is 1.35 nm, with particle size of 2.7 nm. ...
Article
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Cr-doped ZnO (0.6molarity) nanosized particles are synthesized by sol–gel dip coating method on borosilicate glass tube. The structural and optical characterization of the particles has been investigated by TEM, SEM, UV–Vis and FTIR spectroscopy. TEM reveals that the Cr-doped ZnO occurs in uniform surface growth along with spherical in shape having size ∼3nm. The SEM image shows the growth of nanostructured cluster (average size 75nm) with large surface area. The UV–Vis spectrum of nanoparticles suggests the use of such nanostructured coating for better solar energy harvesting. FT-IR results show the structural and optical correlation of nanoparticles. This observation excludes the presence of any other Cr or ZnO concerned impurity. The present investigation, deals with potential applications toward the motivation for using nanostructured materials in solar energy conversion.
... Zinc oxide nanoparticle is the multifunctional nanomaterial and is used for many technological applications. There are several methods have been developed for the synthesis of ZnO nanopowders/nanoparticles [4]. Sonochemical method is based on the acoustic cavitation from the continuous formation and collapse of the bubbles in a liquid. ...
Conference Paper
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With the miniaturization of crystal size, the fraction of under-coordinated surface atoms becomes dominant, and hence, materials in the nano-regime behave very differently from the similar material in a bulk. Zinc oxide (ZnO), particularly, exhibits extraordinary properties such as a wide direct band gap (3.37 eV), large excitation binding energy (60 meV), low refractive index (1.9), stability to intense ultraviolet (UV) illumination, resistance to high-energy irradiation, and lower toxicity as compared to other semiconductors. This very property makes Zinc Oxide a potential candidate in many application fields, particularly as a prominent semiconductor. Zinc Oxide plays a significant role in many technological advances with its application in semiconductor mediated photocatalytic processes and sensor, solar cells and others. In present study, Zinc Oxide (ZnO) has been synthesized using three different precursors by sonochemical method. Zinc Acetate Dihydrate, Zinc Nitrate Hexahydrate and Zinc Sulphate Heptahydrate used as a precursor for the synthesis process. The synthesized ZnO nanoparticle has been found under the range of ∼50 nm. Zinc oxide nanoparticles were characterized using different characterizing tools. The as-synthesized ZnO was characterized by Fourier Transform-Infrared Spectroscopy (FT-IR) for the determination of functional group; Scanning Electron Microscopy equipped with Energy Dispersive Spectroscopy (SEM-EDS) for Morphology and elemental detection respectively, Transmission Electron Microscopy for Particle size distribution and morphology and X-Ray Diffraction (XRD) for the confirmation of crystal structure of the nanomaterial. The optical properties of the ZnO were examined by UV-VIS spectroscopy equipped with Diffuse Reflectance spectroscopy (DRS) confirmed the optical band gap of ZnO-3 around 3.23 eV resembles with the band gap of bulk ZnO (3.37eV). The TEM micrograph of the as-synthesized material showed perfectly spherical shaped nanoparticle under the size range of 50nm. The XRD data showed that the ZnO-3 which was synthesized using Zinc Nitrate Hexahydrate as precursor showed the hexagonal wurtzite crystal structure. The XRD data obtained were compared with the JCPDS standard data. The precursor Zinc Nitrate Hexahydrate (ZnO-3) showed the good yield, monodispersity and size of nanoparticle under the range of 50 nm. The ZnO nanoparticles synthesize using different precursor was found effective in order of ZnO-3, followed by ZnO-1 & ZnO-2. The Synthesized ZnO has wider application in environmental remediation and clean-up as a potential nano-catalyst.
... The hetero-interface may be assumed to consist of localized low and high potential barrier patches, which may originate from the defects due to dislocations, stacking faults due to lattice mismatch, or difference in the thermal expansion coefficients between n-ZnO and p-Si at the hetero-interfaces [29]. It should be noted that the defects such as stacking faults, twin boundaries, and vacancies in ZnO nanostructures also induce piezoelectric nano-domains within the ZnO crystal, which also alter the electromechanical properties of the nanostructures [30][31][32]. Such piezoelectric nano-domains also contribute to the inhomogeneity at the interface. ...
Article
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Heterojunction diodes are fabricated using a low-temperature chemical bath deposition of oriented and crystalline ZnO nanowires on a <1 1 1> p-silicon substrate. The electrical transport properties of the heterojunction are investigated at various temperatures by measuring current–voltage (I–V) characteristics in the range of 90–390 K. A thermionic emission (TE) model is used to analyze the transport behavior. The deviation in the experimental value of Richardson's constant for ZnO nanowires is obtained from I–V–T measurement. The temperature dependence of the effective barrier height and ideality factor is attributed to the inhomogeneous barrier height distribution at the n-ZnO NW/p-Si hetero-interface. The TE and barrier inhomogeneity model are simultaneously used to extract the appropriate value of the Richardson's constants in three different temperature regions. Linear fittings for three different temperature regions suggest multiple Gaussian distributions of barrier heights at the junction.
... Electronic tunneling behavior of these Fe QDs-BNNTs is tested by using a STM-TEM system 16,30,31 . In this case, Fe QDs-BNNTs are transferred to the tip of a gold wire (250 μ m in diameter) by mechanically scratching on the QDs-BNNT sample. ...
Article
Full-text available
Tunneling field effect transistors (TFETs) have been proposed to overcome the fundamental issues of Si based transistors, such as short channel effect, finite leakage current, and high contact resistance. Unfortunately, most if not all TFETs are operational only at cryogenic temperatures. Here we report that iron (Fe) quantum dots functionalized boron nitride nanotubes (QDs-BNNTs) can be used as the flexible tunneling channels of TFETs at room temperatures. The electrical insulating BNNTs are used as the one-dimensional (1D) substrates to confine the uniform formation of Fe QDs on their surface as the flexible tunneling channel. Consistent semiconductor-like transport behaviors under various bending conditions are detected by scanning tunneling spectroscopy in a transmission electron microscopy system (in-situ STM-TEM). As suggested by computer simulation, the uniform distribution of Fe QDs enable an averaging effect on the possible electron tunneling pathways, which is responsible for the consistent transport properties that are not sensitive to bending.
... They can also explain why the band gap of undoped ZnO (3.26 eV) is smaller than the expected value of 3.3 eV [1]. Note that recent studies showed that planar defects such as twins and stacking faults can lead as well to a reduction of the band gap due to resonant states produced by an altered electronic structure [58]. The existence of all these structural defects in pure and Mo-doped ZnO was already suggested by the XRD and TEM data. ...
... They can also explain why the band gap of undoped ZnO (3.26 eV) is smaller than the expected value of 3.3 eV [1]. Note that recent studies showed that planar defects such as twins and stacking faults can lead as well to a reduction of the band gap due to resonant states produced by an altered electronic structure [58]. The existence of all these structural defects in pure and Mo-doped ZnO was already suggested by the XRD and TEM data. ...
Article
Cr2O3 is considered to be a promising cathodic material for supercapacitor applications on the account of its fast redox kinetics, mesoporous structure, and high electrochemical stability. Using a one-step synthesis of Cr2O3, a unique cactus-like morphology can be easily obtained. Such particles improve the specific surface area and electrolyte ion diffusion, leading to enhanced capacitance values. The prepared cactus-like (Cr2O3-c) particles are able to return a specific capacitance of 68 F/g at 5 mV s⁻¹, in three-electrode measurement. This can be attributed to the redox couple of Cr⁴⁺/ Cr³⁺ displayed by Cr2O3 at the electrode–electrolyte interface. Further, the electrochemical performance of cactus-like Cr2O3 material (Cr2O3-c) and solid Cr2O3 spheres (Cr2O3-s) is also compared. This study proves that much superior performance is delivered by protruded nanoparticles. The spikes on the top of the particle surface ensures that two particles are well separated, which stimulates stable channels for ion intercalation–deintercalation. The physics can be explained using a modified free electron model for three-dimensional lattice. Further, using density functional theory (DFT) calculations the electronic band structure, density of states (DOS) and their influence on the electrochemical performance of Cr2O3 are evaluated.
Article
Herein, innovative visible-light-driven nano-photocatalysts for accelerated-activation of H2O2 and persulfate (PS) are explored towards degradation of ionic-dyes from wastewater. Highly-defective yttria-layers (D-Y2O3) are customized and used for stabilization of ZnO and Sm-doped ZnO (Sm-ZnO) nanoparticles. Comparing to [email protected]2O3, [email protected]2O3 nanocomposite possesses advanced surface-area and porosity with significant surface-heterogeneity, elevated negatively-charged nanoparticles (ζav=-36.3mV), large populations of oxygen vacancies, and enhanced visible-light absorptivity. PS-activation over visible-light-driven [email protected]2O3 photocatalyst ([email protected]2O3/PS/Vis) displays superior degradation efficiency (~100%) of isosulfan-blue, Bismarck-brown, and methylene-blue dyes within 10 min, near-complete mineralization performance, and great cyclability. Such excellent photodegradability results from (i) synergistic combination of nanocomposite-PS-visible light, yielding highly-developed synergy-index values (SI=3-20), (ii) abundant generation of reactive-oxidizing-species (SO4•-/h⁺/•OH synergistic ratio 1:1.8:1.9), and (iii) facilitated transference of photoexcited-electrons across oxygen vacancies in yttria-moiety, prohibiting electron-hole pairs recombination. Hence, [email protected]2O3/PS/Vis system is an extraordinarily candidate for complete destruction of organic-pollutants via newly photocatalytic-mechanism, whatever it treats it degrades.
Article
In the present work, individual zinc oxide (ZnO), tin oxide (SnO2) and vanadium oxide (V2O5), their binary and ternary combinations were prepared via hydrothermal route. The structure and the morphology of all the samples were characterised using X-ray Diffractometer (XRD) and Field Emission Scanning Electron Microscopy (FE-SEM) and Raman spectroscopic tools. UV–Visible spectroscopic analysis was recorded for all the samples. The samples were also tested for ethanol sensing (0 ppm–300 ppm) at room temperature. It has been focussed on to investigate the Raman and UV spectroscopic studies on the ethanol sensing properties of these oxides. It is observed that the Raman peaks of the binary and ternary systems shifted to lower wavenumber compared to their bulk and it is attributed to the tensile stress experienced by the nanocomposite. The peculiar hierarchical nanostructures of zinc – tin – vanadium oxide (ZTV) nanocomposite with larger surface area (167.3 m²/g) provided the required active surface sites for the adsorption of ethanol molecules. The band gap of ZTV is calculated as 1.97 eV. This band gap narrowing observed in ZTV might be due to the competing effects of high free carrier concentration and Burstein-Moss shift. Hence ZTV shows pronounced sensitivity of 98% at a response time of 32 s and recovery time of 6 s. Moreover the synergistic effect of ZTV nanocomposite enhanced the sensitivity at a faster rate by overcoming the problems associated with the energy consumption, reversibility, adsorptive capacity and fabrication cost.
Article
CdS nanoparticles (NPs), ZnO layers, and CdS/ZnO composite were successfully prepared by a two-step chemical method. From the analysis of XRD, cubic CdS and the hexagonal ZnO phase were described for the synthesized samples. The CdS/ZnO composite exhibited larger surface area (99.9 m² g⁻¹) than pure CdS NPs (50.5 m² g⁻¹) and ZnO layers (67.3 m² g⁻¹), respectively. The band gap energies evaluated from the DRS results were 3.40, 2.13 and 1.78 eV for pure ZnO, CdS and CdS/ZnO composite respectively. The photocatalytic activity of the prepared materials was studied by the degradation of dye in aqueous solution under UV–Visible light exposure. CdS/ZnO composite exhibited the superior photocatalytic activity than ZnO and CdS. The photocatalytic performance was achieved 91.5% degradation in composite sample.
Article
Photocatalytic properties of ZnO nanoparticles (NPs) strongly depend on their electronic characteristics which can be tuned by varying size, crystal defects and dopings. In this work, ZnO NPs were prepared using a simple chemical precipitation method. Effect of some independent process/synthesis parameters (i.e. reaction time and precursor concentration) on size and ROS generation in ZnO NPs was investigated. Based on TEM analysis and UV-Vis absorption spectra, the synthesized nanoparticles ranged from 2.3 to 4.0 nm in size with the spherical morphologies. Moreover, XRD and SAED showed mono-crystal structures with noticeable diffraction peak at 2θ = 36.8° attributed to (101) plane. Using response surface methodology, OH-/Zn+ molar ratio, Zn+ concentration and reaction time were shown to have significant impacts on size of ZnO NPs. At fixed Zn+ concentrations, reaction time was found to be the most effective parameter on ROS generation and photocatalytic activity of NPs. Obtained results demonstrated that ROS generation is strongly dependent on synthesis reaction time as an indicator of oxygen deficiency and crystal formation in ZnO NPs. No clear relation was obtained between size of NPs and their ROS generation/photocatalytic activity in this size range.
Article
Undoped CdO films were prepared on glass substrate and p-type silicon wafer using sol-gel spin coating method. The structural and optical properties of the films were investigated as a function of the annealing temperature. X-ray diffraction (XRD) patterns reveal that the films are formed from CdO with cubic crystal structure and (1 1 1) preferred orientation. It is seen that good crystallinity is due to the high annealing temperature. The surface morphology of the CdO films was found to be depending on the annealing temperature, showing cauliflower like structure. Optical band gaps for annealing temperature of 250 °C and 450 °C were found to be 2.49 eV and 2.27 eV, respectively, showing a decrease with raising temperature. Optics parameters such as extinction coefficient, refractive index, and surface-volume energy loss were determined with spectrophotometric analysis as a function of annealing temperature. CdO/p-Si heterojunction structure showed weak rectifying behavior. The diode parameters were found to be depending on annealing temperature. The results are encouraging to get better conjunction with CdO thin film component at optimize annealing temperature.
Article
The structural, optical and catalytic properties of undoped (CuO–ZnO) and CdS doped CuO–ZnO (CdS/CuO–ZnO) nanoparticles were studied. The blue shifting of optical band gap in CuO–ZnO nanoparticles as compared to their respective bulk oxides (CuO: 1.21–1.5 eV, ZnO: 3.37 eV) was observed as 3.9 eV, while red shifting after doping of CdS was found from 3.9 to 3.7 eV. The angle of diffraction and FWHM values were used to observe crystallite phase and to calculate crystallite size (using Scherer and Williamson–Hall equations) and other parameters like strain, dislocation density and bond length of nanoparticles. The particle size of CuO–ZnO and CdS/CuO–ZnO nanoparticles using transmission electron microscopy (TEM) was found 12.54 and 6.93 nm, respectively. It was concluded that decrease in particle size cause red shifting which increase the catalytic efficiency of nanoparticles.
Article
In the current study, XRD peak profile analysis, optical and catalytic properties of pure ZnO–NiO and CdS doped ZnO-NiO nanocomposites were investigated. Average crystallite size, strain, dislocation density and bond length were determined with X-ray peak profile analysis. Optical properties such as band gap, extinction coefficient, refractive index, optical conductivity, and dielectric constants were studied by solid phase spectroscopy (SPS). The blue shift was observed in ZnO–NiO as compare to bulk ZnO due to the quantum confinement while red shift was found in CdS/ZnO–NiO nanocomposites as compare to ZnO–NiO nanocomposites is due to bulk defects inducing delocalization and pressure induced effect. The optical conductivity of ZnO–NiO nanocomposites was observed increased with doping of CdS on ZnO–NiO from 4.57 × 10¹⁸ to 6.71 × 10¹⁸ S–1, respectively. It was observed that catalytic efficiency depends on the particle size and band gap of the nanocomposites.
Article
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Moving to nanoscale is a path to get perfect materials with superior properties. Yet defects, such as stacking faults (SFs), are still forming during the synthesis of nanomaterials and, according to common notion, degrade the properties. Here, we demonstrate the possibility of engineering defects to, surprisingly, achieve mechanical properties beyond those of the corresponding perfect structures. We show that introducing SFs with high density increases the Young’s Modulus and the critical stress under compressive loading of the nanowires above those of a perfect structure. The physics can be explained by the increase in intrinsic strain due to the presence of SFs and overlapping of the corresponding strain fields. We have used the molecular dynamics technique and considered ZnO as our model material due to its technological importance for a wide range of electromechanical applications. The results are consistent with recent experiments and propose a novel approach for the fabrication of stronger materials.
Article
Gas sensing properties of ZnO nanobelts synthesized using carbothermal reduction method has been investigated. At room temperature (28 °C), the sensor films exhibit an appreciable response towards H2S and NO and response of these two gases were studied as a function of concentration. For NO the sensor films exhibit a complete reversible curve for the concentration range between 1 and 60 ppm. However, for H2S a complete recovery was obtained for concentration < 5 ppm and for higher concentration a partial recovery of the baseline resistance was observed. The reason for the incomplete recovery was investigated using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) studies of the sample before and after the H2S exposure. After exposure, appearance of an additional peak at 26.6 degree corresponding to the formation of ZnS was observed in XRD. Formation of additional phase was further corroborated using the results of XPS. H2S exposure causes decrease in the intensity of O 1 s peak and appearance of sulphide peaks at binding energies of 162.8 and 161.8 eV corresponding to S-2p peaks − 2p3/2 and 2p1/2, confirms the formation of ZnS upon exposure.
Article
Metal oxide nanoparticles (MONPs) are considered to have the potency to generate reactive oxygen species (ROS), one of the key mechanisms underlying nanotoxicity. However, the nanotoxicology literature demonstrates a lack of consensus on the dominant toxicity mechanism(s) for a particular MONP. Moreover, recent literature has studied the correlation between band structure of pristine MONPs to their ability to introduce ROS and thus has downplayed the ROS-mediated toxicological relevance of a number of such materials. On the other hand, material science can control the band structure of these materials to engineer their electronic and optical properties and thereby is constantly modulating the pristine electronic structure. Since band structure is the fundamental material property that controls ROS-producing ability, band tuning via introduction of dopants and defects needs careful consideration in toxicity assessments. This commentary critically evaluates the existing material science and nanotoxicity literature and identifies the gap in our understanding of the role of important crystal structure features (i.e., dopants and defects) on MONPs' electronic structure alteration as well as their ROS-generation capability. Furthermore, this commentary provides suggestions on characterization techniques to evaluate dopants and defects on the crystal structure and identifies research needs for advanced theoretical predictions of their electronic band structures and ROS-generation abilities. Correlation of electronic band structure and ROS will not only aid in better mechanistic assessment of nanotoxicity but will be impactful in designing and developing ROS-based applications ranging from water disinfection to next-generation antibiotics and even cancer therapeutics. Full Text Available at: http://www.sciencedirect.com/science/article/pii/S0048969716313195
Article
Semiconducting materials are of great technological importance in electronics industry and environmental remediation due to their ability to generate charge carriers when activated with certain energy. The favorable combination of electronic structure, light absorption properties, charge transport characteristics, and excited lifetimes of some semiconductors has made possible its application as photocatalyst. Although many publications define the fundamental aspects of the semiconductors from different points of view, in this chapter the definition of photocatalytic semiconductor, its structural characteristics, and the requirements to be used as photocatalyst are briefly recalled. In addition, the classification of photocatalytic materials and the mechanism of the photocatalytic process are also reviewed. The advantages and disadvantages of the most used photocatalyst TiO2 and the second-generation TiO2-based materials are also addressed.
Article
ZnO microcrystals with hexagonal morphologies are synthesized via a fast and facile hydrothermal route using hexamethylene tetramine (HMTA) as reducing agent and hydroxylpropyl methyl cellulose (HPMC) as morphology directing agent. In the absence of HPMC, hexagonal rod shaped ZnO microcrystals are formed whereas hexagonal bar and both end open hexagonal bar shaped structures are obtained in the presence of different amounts of HPMC. Synthesized ZnO microstructures are characterized using XRD, SEM and fluorescence spectroscopic study. The strong asymmetric blue emission band from ZnO microrod has been explained due to the presence of extended Zni states within the microcrystals. Photocatalytic activities of the microcrystals are investigated by monitoring the photochemical degradation of Methylene Blue. It has been observed that the catalytic efficiency of hexagonal both end open bar shaped ZnO microcrystals is higher than the other ZnO structures.
Article
We report here, the in-situ field emission (FE) property measurement on the individual ZnO nanobelts inside a high resolution transmission electron microscope (TEM) using a special scanning tunneling microscopy (STM)-TEM system. The field emission properties were found to be size scale dependent. It was found that the threshold voltage decreases and the field enhancement factor increases with the decrease in the diameter of the tip of the nanobelt and increase in the sharpness of the tip. The field emission parameter was estimated following the Fowler–Nordheim (F –N) theory. The ZnO nanobelt with the sharp agave like tip structure (d = 10 nm) showed the highest value of the field enhancement factor, β ≈ 4562, and a high field emission current of ∼502 μA.
Article
We present a home-built scanning tunneling microscope (STM) which allows us to precisely position the tip on any specified small sample or sample feature of micron scale. The core structure is a stand-alone soft junction mechanical loop (SJML), in which a small piezoelectric tube scanner is mounted on a sliding piece and a "U"-like soft spring strip has its one end fixed to the sliding piece and its opposite end holding the tip pointing to the sample on the scanner. Here, the tip can be precisely aligned to a specified small sample of micron scale by adjusting the position of the spring-clamped sample on the scanner in the field of view of an optical microscope. The aligned SJML can be transferred to a piezoelectric inertial motor for coarse approach, during which the U-spring is pushed towards the sample, causing the tip to approach the pre-aligned small sample. We have successfully approached a hand cut tip that was made from 0.1 mm thin Pt∕Ir wire to an isolated individual 32.5 × 32.5 μm(2) graphite flake. Good atomic resolution images and high quality tunneling current spectra for that specified tiny flake are obtained in ambient conditions with high repeatability within one month showing high and long term stability of the new STM structure. In addition, frequency spectra of the tunneling current signals do not show outstanding tip mount related resonant frequency (low frequency), which further confirms the stability of the STM structure.
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The band-gap shift of GaN has been examined as a function of uniaxial compression along the c axis using time-resolved, optical absorption measurements in shock wave experiments. The hydrostatic deformation potential acz−D1 (parallel to the c axis), has been determined independently from act−D2 (perpendicular to the c axis). Based on the experimental results, a set of deformation potentials has been obtained: acz−D1=−9.6 eV, act−D2=−8.2 eV, D3=1.9 eV, and D4=−1.0 eV. These values indicate that the deformation potentials in wurtzite GaN are anisotropic.
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Three different values (3.1, 3.2, and 3.3 eV) have been reported for the optical band gap of zinc oxide single crystals at room temperature. By comparing the optical properties of ZnO crystals using a variety of optical techniques it is concluded that the room temperature band gap is 3.3 eV and that the other values are attributable to a valence band-donor transition at ∼ 3.15 eV that can dominate the optical absorption when the bulk of a single crystal is probed. © 1998 American Institute of Physics.
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The investigation of the field emission (FE) properties of carbon nanotube (CNT) films by a scanning anode FE apparatus, reveals a strong dependence on the density and morphology of the CNT deposit. Large differences between the microscopic and macroscopic current and emission site densities are observed, and explained in terms of a variation of the field enhancement factor β. As a consequence, the emitted current density can be optimized by tuning the density of CNTs. Films with medium densities (on the order of 107 emitters/cm2, according to electrostatic calculations) show the highest emitted current densities. © 2000 American Institute of Physics.
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Bulk ZnO single crystals grown by the hydrothermal and flux methods have been characterized by scanning tunnelling spectroscopy performed in the different crystalline faces. Normalized differential conductance has been found to depend on the face considered. Polar O-terminated surfaces show an intrinsic conduction behaviour and surface bandgap in the range 0.4-0.8 eV, which depends on the position probed. The Zn polar surfaces show mainly n-type conduction. The non-polar m regions present either intrinsic or p-type behaviour. The differences observed are attributed to the nature of impurities and defects appearing in the polar and non-polar surfaces during crystal growth.
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We report direct observation of the strong exciton-photon coupling in a ZnO tapered whispering gallery (WG) microcavity at room temperature. By scanning excitations along the tapered arm of the ZnO tetrapod using a micro-photoluminescence spectrometer with different polarizations, we observed a transition from the pure WG optical modes in the weak interaction regime to the excitonic polariton in the strong coupling regime. The experimental observations are well described by using the plane wave model including the excitonic polariton dispersion relation. This provides a direct mapping of the polariton dispersion, and thus a comprehensive picture for coupling of different excitons with differently polarized WG modes.
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ZnO thin films were grown on silicon (100) by pulsed laser deposition. Highly textured crystalline ZnO thin films can be grown at 600 °C. The films were then annealed at 600 °C in oxygen. The effects of annealing on chemical composition of the ZnO films were investigated by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS spectra indicate that water has been adsorbed and then dissociated into H and OH groups. The surface properties of ZnO were studied both by scanning tunneling microscopy and scanning tunneling spectroscopy (STS). A narrow potential well has been formed on the surface of the ZnO thin films due to high density of surface states and negatively biasing the ZnO thin films during STS measurement. The discrete energy levels can be measured by STS.
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The green and yellow luminescence centres in ZnO and Mn-doped ZnO are investigated. Water vapour enhances green luminescence of ZnO in the surface, and causes a two band luminescence phenomenon (green and yellow35in the bulk. Mn behaves as quencher for both the green and the yellow luminescence of ZnO. Positron lifetime measurements show the existence of at least two distinct vacancy-type defects in all materials. At the present time it seems that a VZn · V0 divacancy exists, and that luminescence is due to interstitial zinc and oxygen.
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The excellent field emission characteristics of carbon nanotube films, i.e., the property that the tips of the nanotubes emit electrons when placed in an electric field, are discussed in some detail. The experimental configurations used (see Figure) and the results obtained are reviewed. It is suggested that carbon nanotube films should be considered as good candidates for display applications.
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The tunneling current is measured as a function of voltage, lateral position, and vertical separation between a tungsten probe tip and a Si(111)2 × 1 surface. A rich spectrum is obtained in the ratio of differential to total conductivity, revealing the structure of the surface-state bands. The magnitude of the parallel wave vector for certain surface states is determined from the decay length of the tunneling current. Real-space images of the surface states reveal a phase reversal between those states on either side of the surface-state band gap.
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The nature of defect states associated with group III impurities (Ga, In, and Tl) in PbTe, a narrow band-gap semiconductor, has been studied within density functional theory and supercell model. For all three impurities (both substitutional—at the Pb site and interstitial—at the tetrahedral site), there is a hyper-deep defect state which lies about 0.5–1.0eV below the valence band. It is a highly localized bonding state between the impurity s-orbital and the surrounding p-orbitals of the Te atoms. The corresponding anti-bonding state, denoted as the deep defect state, lies in the band-gap region. Its precise position vis-à-vis the conduction- and valence-band extrema controls the unusual properties exhibited by these defects.
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Semiconducting zinc oxide nanowires (NWs) and nanobelts (NBs) are a unique group of quasi-one-dimensional nanomaterial. This review mainly focuses on the rational synthesis, structure analysis, novel properties and unique applications of zinc oxide NWs and NBs in nanotechnology. First, we will discuss rational design of synthetic strategies and the synthesis of NWs via vapor phase and chemical growth approaches. Secondly, the vapor–solid process for synthesis of oxide based nanostructures will be described in details. We will illustrate the polar surface dominated growth phenomena, such as the formation of nanosprings, nanorings and nanohelices of single-crystal zinc oxide. Third, we will describe the unique and novel electrical, optoelectronic, field emission, and mechanical properties of individual NWs and NBs. Finally, we will illustrate some novel devices and applications made using NWs as ultra-sensitive chemical and biological nanosensors, solar cell, light emitting diodes, nanogenerators, and nano-piezotronic devices. ZnO is ideal for nanogenerators for converting nano-scale mechanical energy into electricity owing to its coupled piezoelectric and semiconductive properties. The devices designed based on this coupled characteristic are the family of piezotronics, which is a new and unique group of electronic components that are controlled by external forces/pressure.
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Carbon nanotubes can be considered as single graphene sheets wrapped up into cylinders. Theoretical studies have shown that nanotubes can be either metallic or semiconducting, depending on minor differences in wrapping angle and diameter. We have obtained scanning tunneling microscopy and spectroscopy results on individual nanotubes which verify this prediction( J.W.G.Wildöer, L.C. Venema, A.G. Rinzler, R.E. Smalley, and C. Dekker, Nature, aimed for publication in Januari 1998). The combination of spectroscopy measurements and atomically resolved images allow to relate the electronic spectra to the wrapping angle and diameter. Tubes with various wrapping angles appear to be either metallic or semiconducting. Carbon nanotubes are expected to be one-dimensional conductors. Sharp peaks in the tunneling density of states that can be associated with the onsets of one-dimensional subbands are indeed observed. Furthermore, we are able to control the length of carbon nanotubes by STM nanostructuring( L.C. Venema, J.W.G. Wildöer, H.J. Temminck Tuinstra, A.G. Rinzler, R.E. Smalley and C. Dekker, Appl. Phys. Lett. (1997)). By applying voltage pulses to the STM tip above a nanotube, the tube can cut into a shorter section. In this way the electronic properties of nanotubes of various lengths can be investigated. footnotetext[0] email: venema@qt.tn.tudelft.nl Work done in collaboration with J.W.G. Wildöer, A.G. Rinzler, R.E. Smalley and C. Dekker.
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Scanning tunneling spectroscopy has been used to observe electronic band features of the Si(111)2×1 surface, focusing on the size of the surface-state band gap. It is shown that peak positions in the normalized conductance, (dI/dV)/(I/V), which are used to characterize the tunneling spectrum, are shifted slightly towards zero volts compared to the corresponding band structure features. A corrected band gap of 0.59±0.04 eV is obtained for this surface.
Article
Band-gap narrowing has been measured optically for semiconducting zinc-oxide films. All films were n type with carrier densities of 5 × 1017 - 2 × 1020 cm-3. The narrowing appeared suddenly at n∼2×1019 cm-3, a carrier density consistent with that expected for the onset of a semiconductor-metal transition. However the gap-shrinkage dependence on carrier concentration was not n1/3 as expected from predictions based on an electron-gas model, but could be described by the same empirical relation proposed for Si:As and Si:B.
Article
By combining electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy, a strong correlation is observed between the green 510 nm emission, the free‐carrier concentration, and the density of singly ionized oxygen vacancies in commercial ZnO phosphor powders. From these results, we demonstrate that free‐carrier depletion at the particle surface, and its effect on the ionization state of the oxygen vacancy, can strongly impact the green emission intensity. The relevance of these observations with respect to low‐voltage field emission displays is discussed. © 1996 American Institute of Physics.
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The photoluminescence (PL) spectra of the undoped ZnO films deposited on Si substrates by dc reactive sputtering have been studied. There are two emission peaks, centered at 3.18 eV (UV) and 2.38 eV (green). The variation of these peak intensities and that of the I–V properties of the ZnO/Si heterojunctions were investigated at different annealing temperatures and atmospheres. The defect levels in ZnO films were also calculated using the method of full-potential linear muffin-tin orbital. It is concluded that the green emission corresponds to the local level composed by oxide antisite defect OZn rather than oxygen vacancy VO, zinc vacancy VZn, interstitial zinc Zni, and interstitial oxygen Oi. © 2001 American Institute of Physics.
Article
Using infrared irradiation to heat an industrial brass (Cu–Zn alloy) disk in moderate vacuum, ZnO nanobelts were directly prepared on a Si substrate. The nanobelts had a single-crystal hexagonal structure and grew along the [0001] direction. The nanobelts had two distinct widths along their entire length. Photoluminescence measurement showed that the nanobelts had an intensive near-band ultraviolet emission at 379 nm. Large-area growth and high quality indicate that the prepared ZnO nanobelts have potential application in optoelectronic devices. © 2002 American Institute of Physics.
Article
ZnO can appear as nanowires, nanobelts, and nanocombs, which are attractive for various applications. However, this has prevented the growth of desired nanostructures without other trace morphologies. Here we demonstrated a mechanism for selective growth of pure and long ZnO nanowires. This was obtained by placing a gold film at a high-temperature zone so that ZnO nanowires with controllable densities can be grown on adjacent bare substrates at lower temperature zones. The concentration gradients of gold and ZnO vapors are responsible for this selective growth, which could be applicable for selective growth of ZnO nanobelts and nanocombs in the future.
Article
The light emission of ZnO under high one- and two-quantumexcitation is investigated in the temperature region from 4 to 300 K. Emission bands are observed which can be attributed to the interactions of free excitons with phonons, free excitons and free electrons, and of bound excitons with phonons and free and bound electrons.Die Lichtemission von ZnO-Kristallen bei starker Bin- und Zweiquantenanregung wird im Temperaturbereich von 4 bis 300 K untersucht. Dabei werden Emissionsbanden beob-achtet, die zurückgeführt werden können auf die Wechselwirkungen von freien Exzitonen mit Phononen, freien Exzitonen und freien Elektronen und von gebundenen Exzitonenmit Phononen und freien und gebundenen Elektronen.
Article
A rapid initial brightness slump and occasional subsequent recovery observed for the zinc‐doped zinc oxide phosphor undergoing cathodoluminescence is shown to depend on the surface work function. A quantitative model leads to an explanation of the dependence on surface potential in terms of field‐aided or retarded surface recombination. Evidence is given that temporarily recovered surfaces result from dynamic surface adsorption of barium.
Article
Quasi-one-dimensional (1D) nanostructures, such as nanowires, nanobelts and nanorods, are the forefront materials for nanotechnology. To date, such nanostructures have been synthesized for a wide range of semiconductors and oxides, and they are potential building blocks for fabricating numerous nano-scale devices. 1D ZnO nanostructures, due to its unique semiconducting, piezoelectric, and bio-safe properties, have received wide attention. From structure point of view, a common characteristic of ZnO nanostructures is that they are mostly dislocation-free. However, planar and point defects do frequently exist in such nanostructures. The objective of this paper is to present detailed electron microscopy study about the structures of planar defects, such as stacking faults, twins, inversion domain walls that existed in 1D ZnO nanostructures. These planar defects are important for understanding the growth mechanism and relevant physical and possibly chemical properties of 1D ZnO nanostructures.
Article
Using a scanning tunneling microscope, the tunneling current versus voltage is measured at fixed values of separation between a tungsten probe-tip and a Si(111)2 × 1 surface. Rectification is observed in the I - V curves and is quantitatively accounted for by an electric-field enhancement due to the finite radius-of-curvature of the probe-tip. The parallel wave-vector of certain states is obtained from the decay length of the tunneling current. A rich spectrum is obtained in the ratio of differential to total conductivity, yielding a direct measure of the Si surface density-of-states. Small shifts are observed in the spectrum as a function of doping, and are attributed to shifts in the position of the surface Fermi level.
Article
The development of the field of spectroscopic measurement with the scanning tunneling microscope (STM) is discussed. A historical review of early experimental results in this field is presented, with emphasis on the techniques for data acquisition and interpretation. The applicability of STM spectroscopic measurement to surface structural determination is addressed. The role of geometric versus electronic contributions to STM images is discussed, with reference to studies of Si(111)7 × 7, Si(111)2 × 1, and Ge(111)c(2 × 8) surfaces. It is concluded that, for semiconductor surfaces, the observed corrugations are dominated by electronic effects. Issues of dynamic range in spectroscopic measurement, and interpretation of spectroscopic images, are examined.
Article
We report on a universal band gap modulation by radial deformation found for semiconductor single-walled carbon nanotubes (SWNTs). The plausible radial deformation of an individual SWNT under hydrostatic pressure is predicted using the method developed in the previous work [M. Hasegawa and K. Nishidate, Phys. Rev. B 74, 115401 (2006)]. It is found by ab initio electronic-structure calculations that the band gap of zigzag SWNTs is dictated by the shape of the high-curvature edge region of a deformed cross section perpendicular to the tube axis: If we let Rmin be an averaged curvature radius in that region, the band gap at the Γ point remains almost unchanged when Rmin≥3.2Å , and its closure occurs at Rmin≈2.4Å irrespective of tube size and cross-sectional shape as a whole. It is also confirmed that the band gap closure is accompanied by the concentration, in the high-curvature region, of the lowest conduction state at the Γ point. Possible implications of these results are discussed.
Article
The efficacy of pre- and postexposure treatment with the antiviral compound (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) was tested against simian immunodeficiency virus (SIV) in macaques as a model for human immunodeficiency virus (HIV). PMPA was administered subcutaneously once daily beginning either 48 hours before, 4 hours after, or 24 hours after virus inoculation. Treatment continued for 4 weeks and the virologic, immunologic, and clinical status of the macaques was monitored for up to 56 weeks. PMPA prevented SIV infection in all macaques without toxicity, whereas all control macaques became infected. These results suggest a potential role for PMPA prophylaxis against early HIV infection in cases of known exposure.
Article
The synthesis of massive arrays of monodispersed carbon nanotubes that are self-oriented on patterned porous silicon and plain silicon substrates is reported. The approach involves chemical vapor deposition, catalytic particle size control by substrate design, nanotube positioning by patterning, and nanotube self-assembly for orientation. The mechanisms of nanotube growth and self-orientation are elucidated. The well-ordered nanotubes can be used as electron field emission arrays. Scaling up of the synthesis process should be entirely compatible with the existing semiconductor processes, and should allow the development of nanotube devices integrated into silicon technology.
Article
A study is presented of the bias dependence of the tunneling current between two planar metal electrodes, each of which has an adsorbed atom. (One atom can be thought of as the tip, the other as the sample.) The positions of the peaks in a plot of the calculated dlnI/dlnV versus V show a close correspondence to the positions of the resonances in the densities of states of both sample and tip.
Article
We have obtained the first energy-resolved real-space images of the filled and empty surface states of the Si(111)-(7×7) surface, with 3-′A lateral resolution. This ability to resolve spatially these surface states with a scanning tunneling microscope depends upon a new method to acquire and separate geometric and electronic information. Our results not only are in good agreement with previous spectroscopic studies but also directly reveal the atomic location and geometric origin of the Si(111)-(7×7) surface states.
Article
The emission spectrum of individual high-quality ZnO nanowires consists of a series of Fabry-Pérot-like eigenmodes that extend far below the band gap of ZnO. Spatially resolved luminescence spectroscopy shows that light is emitted predominantly at both wire ends, with identical spectra reflecting standing wave polariton eigenmodes. The intensity of the modes increases supralinearly with the excitation intensity, indicating that the mode population is governed by scattering among polaritons. Due to strong light-matter interaction, light emission from a ZnO nanowire is not dictated by the electronic band diagram of ZnO but depends also on the wire geometry and the excitation intensity. Delocalized polaritons provide a natural explanation for the pronounced subwavelength guiding in ZnO wires that has been reported previously.
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