Physical mechanism of blue-shift of UV luminescence of a single pencil-like ZnO nanowire

Coupling Reliable Interfacial Carrier Migration Channels with Visible-Light Response Antennas in ZnO-Based Heterostructure for Ameliorated Photocatalytic Hydrogen Generation

Engineering targeted and reliable charge transfer pathways in multiphase photocatalysts remains a challenge. Herein, we conceptualize the Cd@CdS-ZnO/reduced graphene oxide (rGO)/ZnS heterostructures coupled with reliable carrier migration channels and visible-light response antennas by building rGO-integrated electrochemical nanoreactors and an ion-exchange process. In this ternary catalyst, the Cd clusters and rGO perform as charge relays to boost carrier transport via the Z-scheme route and accelerate photogenerated carriers to react with surface-adsorbed substances. Meanwhile, thanks to CdS, the heterostructures have photocatalytic properties under visible light illumination and can also inhibit self-corrosion by shielding Cd clusters to avoid disrupting charge transfer channels. Therefore, the special heterostructure demonstrates fascinating photocatalytic hydrogen production activity without the intervention of cocatalysts. This work provides a feasible protocol for improving the interfaces between metals and semiconductors to achieve efficient photocatalytic hydrogen generation.

Boosting Gas-Phase TiO2 Photocatalysis with Weak Electric Field Strengths of Volt/Centimeter

Among semiconductor nanomaterials, titanium dioxide is at the forefront of heterogeneous photocatalysis, but its catalytic activity greatly suffers from the loss of photoexcited charge carriers through deleterious recombination processes. Here, we investigate the impact of an external electric field (EEF) applied to conventional P25 TiO2 nanopowder with or without Au nanoparticles (NPs) to circumvent this issue. The study of two redox reactions in the gas phase, water splitting and toluene degradation, reveals an enhancement of the photocatalytic activity with rather modest electric fields of a few volt/centimeters only. Such an improvement arises from the electric-field-induced quenching of the green emission in anatase, allowing the photoexcited charge carriers to be transferred to the adsorbed reactants instead of pointless radiative recombinations. Applying an EEF across a trap-rich metal oxide material, such as TiO2, which, when impregnated with Au NPs, leads, respectively, to 12- and 6-fold enhancements in the production of hydrogen and the oxidation of toluene for an electric field of 8 V/cm, without any electrolysis, is a simple and elegant strategy to meet higher photocatalytic efficiencies.

Vertically Well-Aligned ZnO Nanoscintillator Arrays with Improved Photoluminescence and Scintillation Properties

ZnO nanoarrays were grown via a low-temperature hydrothermal method. Solutions, each with different additive combinations, were prepared and evaluated. The effects of the additives involved in the growth procedure, i.e., ammonium hydroxide and sodium citrate, were studied in terms of the morphological, optical and scintillation properties of the ZnO nanostructures. Measurement of the nanorod (NR) length, corresponding photoluminescence (PL) and scintillation spectra and their dependence on the additives present in the solution are discussed. ZnO NRs grown on a silica substrate, whose UV transmission was found to be better than glass, showed high-quality structural and optical properties. It was found that the addition of sodium citrate significantly reduced defects and correspondingly increased the intrinsic near-band-edge (NBE) UV emission intensity at ~380 nm. To obtain high-quality nanostructures, samples were annealed in a 10% H2 + 90% N2 atmosphere. The anneal in the forming gas atmosphere enhanced the emission of the UV peak by reducing defects in the nanostructure. NRs are highly tapered towards the end of the structure. The tapering process was monitored using time growth studies, and its effect on PL and reflectance spectra are discussed. A good alpha particle response was obtained for the grown ZnO NRs, confirming its potential to be used as an alpha particle scintillator. After optimizing the reaction parameters, it was concluded that when ammonium hydroxide and sodium citrate were used, vertically well-aligned and long ZnO nanoarrays with highly improved optical and scintillation properties were obtained.

Modification of Tomato Photosystem II Photochemistry with Engineered Zinc Oxide Nanorods

We recently proposed the use of engineered irregularly shaped zinc oxide nanoparticles (ZnO NPs) coated with oleylamine (OAm), as photosynthetic biostimulants, to enhance crop yield. In the current research, we tested newly engineered rod-shaped ZnO nanorods (NRs) coated with oleylamine (ZnO@OAm NRs) regarding their in vivo behavior related to photosynthetic function and reactive oxygen species (ROS) generation in tomato (Lycopersicon esculentum Mill.) plants. ZnO@OAm NRs were produced via solvothermal synthesis. Their physicochemical assessment revealed a crystallite size of 15 nm, an organic coating of 8.7% w/w, a hydrodynamic diameter of 122 nm, and a ζ-potential of -4.8 mV. The chlorophyll content of tomato leaflets after a foliar spray with 15 mg L-1 ZnO@OAm NRs presented a hormetic response, with an increased content 30 min after the spray, which dropped to control levels 90 min after the spray. Simultaneously, 90 min after the spray, the efficiency of the oxygen-evolving complex (OEC) decreased significantly (p < 0.05) compared to control values, with a concomitant increase in ROS generation, a decrease in the maximum efficiency of PSII photochemistry (Fv/Fm), a decrease in the electron transport rate (ETR), and a decrease in the effective quantum yield of PSII photochemistry (ΦPSII), indicating reduced PSII efficiency. The decreased ETR and ΦPSII were due to the reduced efficiency of PSII reaction centers (Fv'/Fm'). There were no alterations in the excess excitation energy at PSII or the fraction of open PSII reaction centers (qp). We discovered that rod-shaped ZnO@OAm NRs reduced PSII photochemistry, in contrast to irregularly shaped ZnO@OAm NPs, which enhanced PSII efficiency. Thus, the shape and organic coating of the nanoparticles play a critical role in the mechanism of their action and their impact on crop yield when they are used in agriculture.

Husk-like Zinc Oxide Nanoparticles Induce Apoptosis through ROS Generation in Epidermoid Carcinoma Cells: Effect of Incubation Period on Sol-Gel Synthesis and Anti-Cancerous Properties

This study effectively reports the influence of experimental incubation period on the sol-gel production of husk-like zinc oxide nanoparticles (ZNPs) and their anti-cancerous abilities. The surface morphology of ZNPs was studied with the help of SEM. With the use of TEM, the diameter range of the ZNPs was estimated to be ~86 and ~231 nm for ZNP^A and ZNP^B, prepared by incubating zinc oxide for 2 and 10 weeks, respectively. The X-ray diffraction (XRD) investigation showed that ZNPs had a pure wurtzite crystal structure. On prolonging the experimental incubation, a relative drop in aspect ratio was observed, displaying a distinct blue-shift in the UV-visible spectrum. Furthermore, RBC lysis assay results concluded that ZNP^A and ZNP^B both demonstrated innoxious nature. As indicated by MTT assay, reactive oxygen species (ROS) release, and chromatin condensation investigations against the human epidermoid carcinoma (HEC) A431 cells, ZNP^B demonstrated viable relevance to chemotherapy. Compared to ZNP^B, ZNP^A had a slightly lower IC₅₀ against A431 cells due to its small size. This study conclusively describes a simple, affordable method to produce ZNP nano-formulations that display significant cytotoxicity against the skin cancer cell line A431, suggesting that ZNPs may be useful in the treatment of cancer.

Resonant defect recombination-localized surface plasmon energy transfer and exciton dominated fluorescence in ZnO-Au-ZnO multi-interfaced heteronanocrystals

The semiconductor-metal heteronanocrystals (HNCs) that possess a perfect epitaxial interface can accommodate novel and interesting physical phenomena owing to the strong interaction and coupling between the semiconductor excitons and metal plasmons at the interface. Here, we fabricate the pyramidal ZnO-Au HNCs and study their unique photophysical properties. Several Au nanospheres are perfectly epitaxially bound with a single ZnO nanocrystal owing to the small lattice mismatch between them and there are also ZnO-Au-ZnO sandwiched HNCs. There is strong coupling between the green defect-associated recombination in the ZnO nanocrystal and the localized surface plasmon resonance (LSPR) of the Au nanoshpere at the interface of the HNC. This leads to resonant defect recombination-LSPR energy transfer and resultant nearly complete quenching of the green defect luminescence of the ZnO NCs in the HNCs, leaving only the UV exciton luminescence. The lifetimes of both the green and UV emission bands decrease significantly in the ZnO-Au HNCs relative to that of the pure ZnO nanocrystals owing to the combined effect of resonance energy transfer and surface plasmon enhanced radiative transition. The exponent of the luminescence intensity-excitation intensity power function for the green emission band is remarkably smaller than unity and this suggests that the involved defects have an intermediate concentration.

Charge transfer-induced photoluminescence in ZnO nanoparticles

The quality of solution-processed zinc oxide (ZnO) nanoparticles (NPs) is often correlated with their photoluminescence (PL) spectral characteristics. However, the reported PL spectral characteristics lack consistency and remain controversial. Here we report that "defect-emission free" PL spectra can even be obtained in thin films composed of as-synthesized ZnO NPs. It is found that both the PL spectral line-shape and intensity are extremely sensitive to nitrogen and oxygen. By conducting time-dependent PL (t-PL) and photothermal deflection spectroscopy (PDS) measurements under vacuum and different gases, it is proposed that both inert (N2) and reactive (O2) molecules can be absorbed on the ZnO NP surface and induce charge transfer (CT). The CT states induced by N2 are non-radiative which significantly reduces the band emission. Whereas the CT states induced by O2 are radiative at the visible region, and the exciton transfer is efficient which increases the overall PL quantum yield. Owing to such effects, the previously reported correlation between defects and PL emission becomes questionable and needs to be revisited. Particularly, the visible emission from the ZnO NPs is proved to be facilitated by external effects, instead of direct recombination from defect states.

Control of electrical conductivity of highly stacked zinc oxide nanocrystals by ultraviolet treatment

Zinc oxide (ZnO) nanocrystals (NCs) were synthesized using a modified sol-gel method. Ultraviolet (UV) treatment was performed under various atmospheres on the highly stacked ZnO NCs. The prepared NCs were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, photoluminescence spectroscopy, and atomic force microscopy to investigate their structural, electrical, and electrochemical properties. Through these analyses, the effect of the UV treatment on the chemical and electrical characteristics of ZnO NCs was established. According to the analyses, the organic ligands in the NCs were decomposed, and the particles were densified. The mobility of UV-treated ZnO NCs thin films increased to 1.4 cm²/Vs, almost 2 orders higher than the UV untreated ZnO thin films. It was confirmed that the recombination from oxygen vacancies of ZnO could be controlled by UV irradiation. As decreased oxygen vacancies, the band gap of ZnO NCs was increased from 3.2 eV to 3.27 eV.

Tunable synthesis of ultrathin AuAg nanowires and their catalytic applications

Metallic nanowires (NWs) are very interesting and important nanomaterials with unique properties and a number of potential applications. Herein we report a tunable synthesis of water soluble ultrathin AuAg NWs. By using TEM and UV-vis spectroscopy, we demonstrate that these NWs can be produced by a new two-step process, which involves the formation of NW templates during the aging period and the subsequent formation of thicker NWs by a solvent driven fusion and wetting process. Our control studies further show that silver concentration plays a key role in the formation of these nanowires. We also demonstrate that these nanowires can effectively catalyse the reduction of 4-nitrophenol to the corresponding 4-aminophenol. Interestingly, the larger diameter ultrathin nanowires (av. 8 nm) exhibit a greater catalytic performance than the thinner nanowires (av. 3 nm). We believe that these studies are important for further development of one dimensional metal based nanomaterials, which may find a range of potential research and technological applications.

The Effects of Doping Density and Temperature on the Optoelectronic Properties of Formamidinium Tin Triiodide Thin Films

Optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI₃ ) thin films, whose background hole doping density is varied through SnF₂ addition during film fabrication. Monomolecular charge-carrier recombination exhibits both a dopant-mediated part that grows linearly with hole doping density and remnant contributions that remain under tin-enriched processing conditions. At hole densities near 10²⁰ cm^-3 , a strong Burstein-Moss effect increases absorption onset energies by ≈300 meV beyond the bandgap energy of undoped FASnI₃ (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (10²⁰ cm^-3 ), temperature-dependent measurements indicate that the effective charge-carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 10¹⁹ cm^-3 and below, the charge-carrier mobility increases with decreasing temperature according to ≈T^-1.2 , suggesting that it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2 × 10¹⁸ cm^-3 , charge-carrier mobilities reach a value of 67 cm² V^-1 s^-1 at room temperature and 470 cm² V^-1 s^-1 at 50 K. Intraexcitonic transitions observed in the THz-frequency photoconductivity spectra at 5 K reveal an exciton binding energy of only 3.1 meV for FASnI₃ , in agreement with the low bandgap energy exhibited by this perovskite.

NH4F-assisted one-pot solution synthesis of hexagonal ZnO microdiscs for efficient ultraviolet photodetection

One-pot solution method to grow large hexagonal ZnO microdiscs with the aid of ammonium fluoride (NH₄F) mineralizer has been realized. The size, morphology, crystallinity and optical properties of the synthesized ZnO microdiscs can be efficiently modulated by the concentration of NH₄F. X-ray diffraction and scanning electron microscopy analyses illustrate that hexagonal ZnO microdiscs achieved at 0.03 M NH₄F concentration have larger disc size and narrower full-width value at half maximum of (002) peak. It implies better crystal quality compared with those from other additive concentrations. Photoluminescence results also demonstrate the same trend. These results indicate that with proper addition of NH₄F, the crystal quality of ZnO microdiscs has been improved and defects have been suppressed. Furthermore, a UV photodetector has been fabricated by simply transferring the ZnO microdiscs grown with 0.03 M NH₄F onto a p-type silicon substrate. The device exhibits photosensitive behaviour at 365 nm UV light illuminating when -0.6 V is applied. The response time as well as recovery time is less than 0.1 s. The relatively large photoresponsivity of 1.19 A W^-1 with power consumption less than 10 nW makes it possible in application field of highly efficient low power consumption UV detection.

Surface-Guided CsPbBr3 Perovskite Nanowires on Flat and Faceted Sapphire with Size-Dependent Photoluminescence and Fast Photoconductive Response

All-inorganic lead halide perovskite nanowires have been the focus of increasing interest since they exhibit improved stability compared to their hybrid organic-inorganic counterparts, while retaining their interesting optical and optoelectronic properties. Arrays of surface-guided nanowires with controlled orientations and morphology are promising as building blocks for various applications and for systematic research. We report the horizontal and aligned growth of CsPbBr₃ nanowires with a uniform crystallographic orientation on flat and faceted sapphire surfaces to form arrays with 6-fold and 2-fold symmetries, respectively, along specific directions of the sapphire substrate. We observed waveguiding behavior and diameter-dependent photoluminescence emission well beyond the quantum confinement regime. The arrays were easily integrated into multiple devices, displaying p-type behavior and photoconductivity. Photodetectors based on those nanowires exhibit the fastest rise and decay times for any CsPbBr₃-based photodetectors reported so far. One-dimensional arrays of halide perovskite nanowires are a promising platform for investigating the intriguing properties and potential applications of these unique materials.

Growth and properties of ZnO nanorods by RF-sputtering for detection of toxic gases

ZnO polycrystalline nanorods were easily prepared via RF sputtering and proved excellent sensors for H₂S and other toxic/explosive gases.

The preparation of γ-crystalline non-electrically poled photoluminescant ZnO-PVDF nanocomposite film for wearable nanogenerators

Polyvinylidene fluoride (PVDF) films are filled with various mass fractions (wt%) of zinc oxide nanoparticles (ZnO-NPs) to fabricate the high performance of a wearable polymer composite nanogenerator (PCNG). The ZnO-NPs can induced a fully γ-crystalline phase in PVDF, where traditional electrical poling is not necessary for the generation of piezoelectric properties. The PCNG delivers up to 28 V of open circuit voltage and 450 nA of short circuit current by simple repeated human finger imparting (under a pressure amplitude of 8.43 kPa) that generates sufficient power to turn on at least 48 commercial blue light emitting diodes (LEDs) instantly. Furthermore, it also successfully charged the capacitors, signifying practical applicability as a piezoelectric based nanogenerator for self-powering devices. The applicability of PCNG by wearable means is clarified when it gives rise to a sensible response, say up to 400 mV of output voltage synchronized with the PCNG embedded human finger in a bending and releasing gesture. UV-visible absorption spectral analysis revealed the possibility of estimating a change in the optical band gap value (E _g), refractive index (n) and optical activation energy (E _a) in different concentrations of ZnO-NP incorporated PVDF nanocomposite films, and it possesses a useful methodology where ZnO-NPs can be used as an optical probe. Near blue light emission is observed from photoluminescence spectra, which are clearly shown from a Commission Internationale de L'Eclairage (CIE) diagram. The piezoelectric charge coefficient of the nanocomposite film is estimated to be -6.4 pC/N, where even electrical poling treatment is not employed. In addition, dielectric properties have been studied to understand the role of molecular kinetic and interfacial polarization occurring in nanocomposite films at different applied frequencies.

Burstein-Moss Effect Behind Au Surface Plasmon Enhanced Intrinsic Emission of ZnO Microdisks

In this paper, ZnO microdisks with sputtering of Au nanoparticles were prepared to explore their plasmon/exciton coupling effect. An obvious blue shift and enhanced excitonic emission intensity were observed in the PL spectra of as-grown and Au-sputtered ZnO samples at room temperature. The investigation on the absorption spectra and temperature-dependent PL spectra has been demonstrated the Burstein-Moss effect behind the optical phenomena. These results revealed the coupling dynamics between the metal localized surface plasmon and semiconductor exciton.

Eco-friendly synthesis of ZnO nanopencils in aqueous medium: a study of photocatalytic degradation of methylene blue under direct sunlight

Pomegranate peel extract stabilized ZnO nanopencils were shown to exhibit a better photocatalytic degradation of methylene blue under sun light irradiation than commercial ZnO and TiO₂ nanoparticles.

Role of Zn-interstitial defect states on d0 ferromagnetism of mechanically milled ZnO nanoparticles

An impurity defect level formed by interstitial zinc at the surfaces of undoped ZnO nanoparticles plays a crucial role for d0 ferromagnetism.

Understanding quantum confinement in nanowires: basics, applications and possible laws

A comprehensive investigation of quantum confinement in nanowires has been carried out. Though applied to silicon nanowires (SiNWs), it is general and applicable to all nanowires. Fundamentals and applications of quantum confinement in nanowires and possible laws obeyed by these nanowires, have been investigated. These laws may serve as backbones of nanowire science and technology. The relationship between energy band gap and nanowire diameter has been studied. This relationship appears to be universal. A thorough review indicates that the first principles results for quantum confinement vary widely. The possible cause of this variation has been examined. Surface passivation and surface reconstruction of nanowires have been elucidated. It has been found that quantum confinement owes its origin to surface strain resulting from surface passivation and surface reconstruction and hence thin nanowires may actually be crystalline-core/amorphous-shell (c-Si/a-Si) nanowires. Experimental data available in the literature corroborate with the suggestion. The study also reveals an intrinsic relationship between quantum confinement and the surface amorphicity of nanowires. It demonstrates that surface amorphicity may be an important tool to investigate the electronic, optoelectronic and sensorial properties of quantum-confined nanowires.

Maximizing integrated optical and electrical properties of a single ZnO nanowire through native interfacial doping

A native interfacial doping layer introduced in core-shell type ZnO nano-wires by a simple vapor phase re-growth procedure endows the produced nano-wires with both excellent electrical and optical performances compared to conventional homogeneous ZnO nanowires. The unique Zn-rich interfacial structure in the core-shell nanowires plays a crucial role in the outstanding performances.

Amino acid-mediated synthesis of zinc oxide nanostructures and evaluation of their facet-dependent antimicrobial activity

ZnO nanostructures (ZnO-NSs) of different morphologies are synthesized with the amino acids L-alanine, L-threonine, and L-glutamine as capping agents. X-ray diffraction (XRD) shows the formation of a crystalline wurtzite phase of ZnO-NSs. The surface modification of ZnO-NSs due to the capping agents is confirmed using Fourier transform infrared (FTIR) spectroscopy. Photoluminescence spectroscopy reveals that the concentration of surface defects correlates positively with the number of polar facets in ZnO-NSs. The antimicrobial activity of the ZnO-NSs has been tested against Escherichia coli and the common pathogens Staphylococcus aureus, Klebsiella pneumoniae, and Bacillus subtilis. Culture-based methods in rich medium show up to 90% growth inhibition, depending on the ZnO-NSs. Flow cytometry analyses indicate that the reactive oxygen species (ROS) generated by ZnO-NSs contribute mostly to the antibacterial activity. Control experiments in minimal medium show that amino acids and other reducing agents in Luria-Bertani (LB) medium quench ROS, thereby decreasing the antimicrobial activity of the ZnO-NSs.

Wavelength tunable single nanowire lasers based on surface plasmon polariton enhanced Burstein-Moss effect

Wavelength tunable semiconductor nanowire (NW) lasers are promising for multifunctional applications ranging from optical communication to spectroscopy analysis. Here, we present a demonstration of utilizing the surface plasmon polariton (SPP) enhanced Burstein-Moss (BM) effect to tune the lasing wavelength of a single semiconductor NW. The photonic lasing mode of the CdS NW (with length ~10 μm and diameter ~220 nm) significantly blue shifts from 504 to 483 nm at room temperature when the NW is in close proximity to the Au film. Systematic steady state power dependent photoluminescence (PL) and time-resolved PL studies validate that the BM effect in the hybrid CdS NW devices is greatly enhanced as a consequence of the strong coupling between the SPP and CdS excitons. With decreasing dielectric layer thickness h from 100 to 5 nm, the enhancement of the BM effect becomes stronger, leading to a larger blue shift of the lasing wavelength. Measurements of enhanced exciton emission intensities and recombination rates in the presence of Au film further support the strong interaction between SPP and excitons, which is consistent with the simulation results.

Diffuse reflection inside a hexagonal nanocavity

Geometrical diffuse reflection is a common optical phenomenon that occurs when a reflecting surface has roughness of order of hundreds of micrometres. Light rays thus reflect uniformly in all directions with each ray obeying Snell's law. Of interest is knowing what happens when light reflects off surfaces with roughness of nanometres. Here, by introducing nanoscaled roughness on the hexagonal faces of ZnO nanocavities, we observe luminescent profiles with flowery patterns, replacing the usual whispering gallery modes. The unique profile for these nanocavities is attributed to wave diffuse reflection, which occurs when the features on the reflecting surfaces are typically nanometre-sized. Light with wavelengths of similar scale "sees" these nano-perturbations, and undergoes scattering rather than geometrical diffuse reflection. These findings could benefit the fields of nanoscale topography and nanoscopic uniform lighting by using wave diffuse reflection.

Tuning physical and optical properties of ZnO nanowire arrays grown on cotton fibers

This article reports the first systematic study on the quantitative relationship between the process parameters of solution concentration ratio, structure, and physical and optical properties of ZnO nanowires grown on cotton surfaces. To develop a fundamental understanding concerning the process-structure-activity relations, we grew a series of well-defined, radially oriented, highly dense, and uniform single-crystalline ZnO nanorods and nanoneedles on cotton surfaces by a simple and inexpensive two-step optimized hydrothermal process at a relatively low temperature. This process involves seed treatment of a cotton substrate with ZnO nanocrystals that will serve as the nucleation sites for subsequent anisotropic growth of single crystalline ZnO nanowires. All of the ZnO nanowires exhibit wurtzite crystal structure oriented along the c-axis. For investigating structure-controlled properties, seed-to-growth solutions concentrations ratio ([S]/[G]) of the synthesis process was varied over six different values. Superhydrophobicity was achieved for all morphologies after 1-dodecanethiol modification, which was highly durable after prolonged UV irradiation. Durability of the ZnO materials under laundry condition was also verified. Variation of the [S]/[G] ratio resulted in a morphological transform from nanorods to needle-like structures in conjunction with a drastic change in the physical and optical properties of the ZnO modified cotton surfaces. Higher [S]/[G] ratios yielded formation of ZnO nanoneedles with high degree of crystallinity and higher aspect ratio compared to nanorods. Increasing [S]/[G] ratio resulted in the amount of ZnO grown on the cotton surface to drop significantly, which also caused a decrease in the surface hydrophobicity and UV absorption. In addition, room temperature photoluminescence measurements revealed that the band gap of ZnO widened and the structural defects were reduced as the morphology changed from nanorods to nanoneedles. A similar trend was observed in the UV-vis absorption of nanorods and nanoneedles, the onset of the latter exhibiting a blue-shift that correlates with the widening of band gap with nanoneedle formation.

Novel hollow Pt-ZnO nanocomposite microspheres with hierarchical structure and enhanced photocatalytic activity and stability

Noble metal/semiconductor nanocomposites play an important role in high efficient photocatalysis. Herein, we demonstrate a facile strategy for fabrication of hollow Pt-ZnO nanocomposite microspheres with hierarchical structure under mild solvothermal conditions using Zn (CH(3)COO)(2)·2H(2)O and HPtCl(4) as the precursors, and polyethylene glycol-6000 (PEG-6000) and ethylene glycol as the reducing agent and solvent, respectively. The as-synthesized ZnO and Pt-ZnO composite nanocrystals were well characterized by powder X-ray diffraction (XRD), nitrogen-physical adsorption, scanning electron microscopy (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectra (DRS), and photoluminescence (PL) emission spectroscopy. It was found that Pt content greatly influences the morphology of Pt-ZnO composite nanocrystals. Suitable concentration of HPtCl(4) in the reaction solution system can produce well hierarchically hollow Pt-ZnO nanocomposite microspheres, which are composed of an assembly of fine Pt-ZnO nanocrystals. Photocatalytic tests of the Pt-ZnO microspheres for the degradation of the dye acid orange II revealed extremely high photocatalytic activity and stability compared with those of pure ZnO and corresponding Pt deposited ZnO. The remarkable photocatalytic performance of hollow Pt-ZnO microspheres mainly originated from their unique nanostructures and the low recombination rate of the e(-)/h(+) pairs by the platinum nanoparticles embedded in ZnO nanocrystals.

Surface depletion induced quantum confinement in CdS nanobelts

We investigate the surface depletion induced quantum confinement in CdS nanobelts beyond the quantum confinement regime, where the thickness is much larger than the bulk exciton Bohr radius. From room temperature to 77 K, the emission energy of free exciton A scales linearly versus 1/L(2) when the thickness L is less than 100 nm, while a deviation occurs for those belts thicker than 100 nm due to the reabsorption effect. The 1/L(2) dependence can be explained by the surface depletion induced quantum confinement, which modifies the confinement potential leading to a quasi-square potential well smaller than the geometric thickness of nanobelts, giving rise to the confinement effect to exciton emission beyond the quantum confinement regime. The surface depletion is sensitive to carrier concentration and surface states. As the temperature decreases, the decrease of the electrostatic potential drop in the surface depletion region leads to a weaker confinement due to the decrease of carrier concentration. With a layer of polymethyl methacrylate (PMMA) passivation, PL spectra exhibit pronounced red shifts due to the decrease of the surface states at room temperature. No shift is found at 10 K both with or without PMMA passivation, suggesting a much weaker depletion field due to the freezing-out of donors.

Size dependence of dielectric constant in a single pencil-like ZnO nanowire

Scanning conductance microscopy (SCM) is used to measure the dielectric constant of a single pencil-like zinc oxide (ZnO) nanowire with the diameters ranging from 85 to 285 nm. As the diameter decreases, the dielectric constant of ZnO nanowire is found to decrease from 6.4 to 2.7, which is much smaller than that of the bulk ZnO of 8.66. A core-shell composite nanowire model in terms of the surface dielectric weakening effect is proposed to explore the origin of the size dependence of dielectric constant, and the experimental results are well explained.

Magnetism in dopant-free ZnO nanoplates

It is known that bulk ZnO is a nonmagnetic material. However, the electronic band structure of ZnO is severely distorted when the ZnO is in the shape of a very thin plate with its dimension along the c-axis reduced to a few nanometers while keeping the bulk scale sizes in the other two dimensions. We found that the chemically synthesized ZnO nanoplates exhibit magnetism even at room temperature. First-principles calculations show a growing asymmetry in the spin distribution within the distorted bands formed from Zn (3d) and O (2p) orbitals with the reduction of thickness of the ZnO nanoplates, which is suggested to be responsible for the observed magnetism. In contrast, reducing the dimension along the a- or b-axes of a ZnO crystal does not yield any magnetism for ZnO nanowires that grow along c-axis, suggesting that the internal electric field produced by the large {0001} polar surfaces of the nanoplates may be responsible for the distorted electronic band structures of thin ZnO nanoplates.

ZnO nanocone: application in fabrication of the smallest whispering gallery optical resonator

ZnO semiconductors at the micro- and nanometre scales are attractive in optical, magnetic, and electronic applications because of their particular features and excellent properties. The whispering gallery mode (WGM) is a general and effective type to amplify the intensity of the luminescence emission, and has gained extensive application in lasing and microcavities. In this contribution, we reported that the smallest whispering gallery optical resonator has been achieved in an individual ZnO nanocone whose diameter gradually reduces from bottom to top in the range of 700 to 50 nm. Using the monochromatic cathodoluminescence (CL) equipment attached at a scanning electron microscopy, we observed the alternating patterns of bright and dark rings from the monochromatic CL image of an individual ZnO nanocone, which is attributed to the WGM-like enhanced luminescence emission when the ZnO nanocone is considered as an optical resonator. The smallest mode number of WGM, N=0, was observed in the ZnO nanocone with a radius of 55 nm for the considered light wavelength of 380 nm, and with a radius of 81 nm for the considered light wavelength of 500 nm, respectively. These results showed that the smallest whispering gallery optical resonator from an individual ZnO nanocone has been fabricated. Experiments are in good agreement with both theoretical predictions and computer simulations based on the finite-difference time domain method with perfectly matched layer boundary conditions. These findings provided valuable information for applications of ZnO micro- and nanostructures in optoelectronic devices.

A ZnO nanorod inorganic/organic heterostructure light-emitting diode emitting at 342 nm

An inorganic/organic heterostructure light-emitting diode consisting of the hole-transporting layer N, N'-di(naphth-2-yl)- N, N'-diphenylbenzidine (NPB) and n-type ZnO nanorods fabricated by hydrothermal decomposition is reported. Poly(methyl methacrylate) was used to form a smooth surface on top of ZnO nanorod array with ZnO nanorod tops exposed for subsequent NPB deposition. An unusual ultraviolet emission at 342 nm was observed in the electroluminescence spectrum. Compared to band gap energy of ZnO (3.37 eV), the excitonic emission is blue-shifted and broadened. The mechanism of the blue shift is discussed in terms of the energy band diagram of the heterostructure.