Aspect Ratio Of Nanorods Research Articles

Influence of the Aspect Ratio of Iron Oxide Nanorods on Hysteresis-Loss-Mediated Magnetic Hyperthermia.

Owing to the problems associated with conventional cancer treatment methods, magnetic hyperthermia-based cancer therapy has gained importance recently. Achieving the desired heating effect at the site of the tumor with a minimal concentration of iron oxide nanoparticles (IONPs) and a safer field is necessary to explore the advantages of hyperthermia. For one to address this challenge, biocompatible IONPs with a desirable magnetic response at a tolerable field are necessary. In this work, magnetic shape anisotropy of iron oxide nanorods (NR) of different lengths (70, 115, 170, and 210 nm) with different aspect ratios ranging from 1.55 to 3.2 was explored to achieve higher hysteresis loss, in turn leading to better hyperthermia efficiency. The magnetic properties of the NRs with respect to the applied field were studied using micromagnetic simulation. Even though the nanorods with high aspect ratio showed a higher hysteresis loss of 69485 J/m3 at 2000 Oe, the field required to attain it was high and well beyond the safety limit. From nanorods of various aspect ratios, the nanorod with a lower aspect ratio of 1.55 and a length of 70 nm exhibited a better hysteresis loss and specific absorption rate (SAR) value of 4214 W g-1 was achieved at a frequency and alternating magnetic field of 400 kHz and 800 Oe, respectively. The PEGylated GO-Nanorod of 70 nm exhibited excellent antitumor efficacy in 4T1 tumor model mice by obstructing the tumor progression within a safer dosage and field.

Environment-Friendly Zinc Oxide Nanorods-Grown Cellulose Nanofiber Nanocomposite and Its Electromechanical and UV Sensing Behaviors.

This paper reports a genuine environment-friendly hybrid nanocomposite made by growing zinc oxide (ZnO) nanorods on cellulose nanofiber (CNF) film. The nanocomposite preparation, characterizations, electromechanical property, and ultraviolet (UV) sensing performance are explained. CNF was extracted from the pulp by combining the 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) oxidation and the aqueous counter collision (ACC) methods. The CNF film was fabricated using doctor blade casting, and ZnO nanorods were grown on the CNF film by seeding and by a hydrothermal method. Morphologies, optical transparency, mechanical and electromechanical properties, and UV sensing properties were examined. The nanocomposite’s optical transparency was more than 80%, and the piezoelectric charge constant d31 was 200 times larger than the CNF film. The UV sensing performance of the prepared ZnO-CNF nanocomposites was tested in terms of ZnO concentration, UV irradiance intensity, exposure side, and electrode materials. A large aspect ratio of ZnO nanorods and a work function gap between ZnO nanorods and the electrode material are essential for improving the UV sensing performance. However, these conditions should be compromised with transparency. The use of CNF for ZnO-cellulose hybrid nanocomposite is beneficial not only for electromechanical and UV sensing properties but also for high mechanical properties, renewability, biocompatibility, flexibility, non-toxicity, and transparency.

Magneto-optical properties of a magneto-plasmonic nanofluid based on superparamagnetic iron oxide and gold nanoparticles

In this investigation the magnetic and plasmonic properties of nanomagnetic Fe3O4 and gold nanorods are combined by synthesizing a magneto plasmonic nano fluid. They are then examined by probing the enhanced magneto-optical properties such as linear dichroism and Faraday rotation. The nanofluid is excited to transverse excitation of gold nanorods by a laser and the magneto-optical response of this ferrofluid is investigated. The results are compared with gold nanospheres incorporated magnetic nano fluids. The structural cascading of rods into chain-like structure in the presence of an applied magnetic field enables the electric field enhancement leading to improved dichroism and faraday rotation. These fluids can be used for biomedical applications such as targeted drug delivery and magnetic hyperthermia. The aspect ratio of the gold nanorods can be tuned to suit the absorption and hence such fluids can be a potential via media where in magnetic hyperthermia and photodynamic therapy can be combined.

Statistical morphological identification of low-dimensional nanomaterials by using TEM

Nanomaterials with low-dimensional morphology display unique properties in catalysis and related fields, which are highly dependent on the structure and aspect ratio. Thus, accurate identification of the structure and morphology is the basis to correlate to the performance. However, the widely adopted techniques such as XRD is incapable to precise identify the aspect ratio of low-dimensional nanomaterials, not even to quantify the morphological uniformity with statistical deviation value. Herein, ZnO nanorod and nanosheet featured with one- and two-dimensional morphology were selected as model materials, which were prepared by the hydrothermal method and statistically characterized by transmission electron microscopy (TEM). The results indicate that ZnO nanorods and nanosheets display rod-like and orthohexagnal morphology, which mainly encapsulated with {100} and {001} planes, respectively. The 7.36±0.20 and 0.39±0.02 aspect ratio (c/a) of ZnO nanorods and nanosheets could be obtained through the integration of the (100) and (002) diffraction rings in selected area electron diffraction (SAED). TEM combining with the SAED is favorable compare with XRD, which not only provides more accurate aspect ratio results with standard deviation values but also requires very small amounts of sample. This work is supposed to provide a convenient and accurate method for the characterization of nanomaterials with low-dimensional morphology through TEM.

A robust low data solution: Dimension prediction of semiconductor nanorods

Precise control over dimension of nanocrystals is critical to tune the properties for various applications. However, the traditional control through experimental optimization is slow, tedious and time consuming. Herein a robust deep neural network-based regression algorithm has been developed for precise prediction of length, width, and aspect ratios of semiconductor nanorods (NRs). Given there is limited experimental data available (28 samples), a Synthetic Minority Oversampling Technique for regression (SMOTE-REG) is employed first for data generation. Deep neural network is further applied to develop regression model which demonstrated the well performed prediction on both the original and generated data with a similar distribution. The prediction model is further validated with additional experimental data, showing accurate prediction results. Additionally, Local Interpretable Model-Agnostic Explanations (LIME) is used to interpret the weight for each sample, corresponding to its importance towards the target dimension, which is well validated by experimental observations.

Oxidase-like Nanozyme-Mediated Altering of the Aspect Ratio of Gold Nanorods for Breaking through H2O2-Supported Multicolor Colorimetric Assay: Application in the Detection of Acetylcholinesterase Activity and Its Inhibitors

A convenient, fast, and colorful colorimetric platform with high resolution for acetylcholinesterase (AChE) activity and its inhibitors detection based on the regulation of oxidase-like nanozyme-mediated etching of gold nanorods (AuNRs) has been proposed in this work. MnO2 nanosheets are selected as the nanozyme. Their excellent oxidase-like activity enables the etching process to proceed smoothly without the usage of unstable H2O2. When AChE is present, it catalytically hydrolyzes acetylthiocholine (ATCh) to thiocholine (TCh). With high reducing ability, TCh induces the decomposition of MnO2 nanosheets, causing them to lose their oxidase-like activity. Thus, the etching of AuNRs is hampered. Consequently, with the increasing concentration of AChE, an apparent change in the AuNRs solution color is observed. The proposed platform achieves high-sensitivity detection of AChE (limit of detection = 0.18 mU/mL). Furthermore, the proposed platform also has been demonstrated its applicability for its inhibitors detection. Benefiting from the advantages of convenient and high resolution of visual readout, the proposed platform holds great potential for the detection of AChE and its inhibitors in clinical diagnosis.

Phase behavior of polymer-nanorod composites: A comparative study using PRISM theory and molecular dynamics simulations.

Well-dispersed composites of polymer and nanorods have many emerging applications and, therefore, are an important area of research. Polymer reference interaction site model (PRISM) theory and molecular dynamics simulations have become powerful tools in the study of the structure and phase behavior of polymer nanocomposites. In this work, we employ both PRISM theory and molecular dynamics simulations to determine the structure and spinodal phase diagram of 1% volume fraction of nanorods in a polymer melt. We make quantitative comparisons between the phase diagrams, which are reported as a function of nanorod aspect ratio and polymer-nanorod interactions. We find that both PRISM theory and molecular dynamics simulations predict the formation of contact aggregates at low polymer-nanorod attraction strength (γ) and bridged aggregates at high polymer-nanorod attraction strength. They predict an entropic depletion-driven phase separation at low γ and a bridging-driven spinodal phase separation at high γ. The polymer and nanorods are found to form stable composites at intermediate values of the polymer-nanorod attraction strength. The fall of the bridging boundary and the gradual rise of the depletion boundary with the nanorod aspect ratio are predicted by both PRISM theory and molecular dynamics simulations. Hence, the miscible region narrows with increasing aspect ratio. The depletion boundaries predicted by theory and simulation are quite close. However, the respective bridging boundaries present a significant quantitative difference. Therefore, we find that theory and simulations qualitatively complement each other and display quantitative differences.

Facile approach for developing gold nanorods with various aspect ratios for an efficient photothermal treatment of cancer

Here, we report a facile preparation of high purity gold nanorods using a cetyl trimethylammonium bromide (CTAB) assisted colloidal approach and functionalization with mercaptoproponic acid. The latter replaces CTAB on the gold nanorod surface, allowing an exceptionally stable aqueous dispersion of the nanorods due to the formation of Au-S bonds. The aspect ratio of the gold nanorods is easily controlled by the reaction time with the maximum of 4.3, allowing absorption in the near-infrared (NIR) region and efficient heat release. This leads to a temperature gradient of 3 °C/min and 42 °C reached for only 6 min in the case of the gold nanorod concentration of 20 μg/mL. The studies of cell viability demonstrated a low cytotoxicity of the gold nanorods with the 4.3 aspect ratio, allowing 92% survival of living cells for 48 h. Furthermore, in mice injected with these nanorods and subjected to NIR-laser irradiation for 10 min, the cancer tumor progressively decreased in volume and fully disappeared within four weeks. A robust fabrication of these gold nanorods with a desirable aspect ratio leading to an efficient heat generation by a NIR laser, makes them exceptionally promising for photothermal ablation of cancer.

High Performance Gold Dimeric Nanorods for Plasmonic Molecular Sensing

A metal dimeric nanostructure is an alternative design for a Localized Surface Plasmon Resonance sensor. At the dimer nanojunction, a high E-field enhancement (hot spot) is observed. In this work, nanostructure size parameters such as aspect ratio, junction, and length of gold dimeric nanorods (AuDNR) linked end-to-end by a thin dielectric junction were evaluated numerically, optimizing the sensing parameters of the plasmonic nanoplatform. The computational approach focused on the assessment of the LSPR extinction spectral peak shift to their local surrounding medium in order to estimate the behavior of sensing parameters such as the figure of merit (FoM), the bulk sensitivity ( η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> ) and the η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> × FoM. The nanostructure aspect ratio, junction, and rod length were evaluated, optimizing the plasmonic nanoplatforms sensing parameters. The optimized AuDNR achieved a high sensing performance with η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> = 544 nm/RIU and FoM = 12.4 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Moreover, the obtained η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> × FoM = 6745.6 nm/RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is the highest reported value in the literature. For molecular sensing, Campbell's model was explored to evaluate the wavelength shift as a result of molecular adsorption on the nanoparticle surface. We demonstrate that the dimeric nanostructure exhibit twice LSPR peak shift due to a thin molecular adsorption layer rather than a monomer of equivalent size. Our results show that optimized dimeric nanostructure is highly attractive for molecular identification, opening the possibility of establishing new nanoplatforms for medical diagnostic.

Trapping of rare earth-doped nanorods using quasi Bessel beam optical fiber tweezers

We demonstrate optical trapping of rare earth-doped NaYF4:Er/Yb nanorods of high aspect ratio (length 1.47 μm and diameter 140 nm) using a quasi Bessel beam (QBB) generated by positive axicon optical fiber tips. Propulsion or trapping of the nanorods is demonstrated using either single or dual fiber nano-tip geometries. The optical force exerted on the trapped nanorods, their velocities, and their positions have been analyzed. We determine the trap stiffness for a single nanorod to be 0.12 pN/μm (0.003 pN/μm) by power spectrum analysis and 0.13 pN/μm (0.015 pN/μm) by Boltzmann statistics in the direction perpendicular to (along) the fiber axes for an average optical power of 34 mW. The experiments illustrate the advantage of using a QBB for multiple nanorod trapping over a large distance of up to 30 μm.

Modulated 3D Cross-Correlation Dynamic Light Scattering Applications for Optical Biosensing and Time-Dependent Monitoring of Nanoparticle-Biofluid Interactions

This paper reviews dynamic light scattering (DLS) of gold nanoparticle-protein interactions for the model protein bovine serum albumin (BSA), as well as in complex biofluids, at the example of mouse serum. DLS data of nanorods of various aspect ratio, of proteins and of mouse serum are discussed in terms of the analysis of their hydrodynamic radii, leading to the distinction of rotational and translational motion as well as to the detection of agglomerates. We address in particular advances obtained by modulated 3D cross correlation dynamic light scattering and recent progress using the CORENN algorithm for analysis of DLS data.

Investigation of electron transfer between single plasmon and graphene by dark field spectroscopy

We investigated the electron transfer time between single plasmonic gold nanoparticles and graphene with our home-build spectral imaging dark-field microscope. The process of electron transfer is supposed to be shuttling of hot electrons on the nanoparticle-graphene interface, resulting in a slight broadening of the scattering spectrum. For detecting the minor spectrum broadening, we firstly characterized our setup systematically and then calibrated its intrinsic error. We found the mechanism of a common but normally neglected setup error, scattering spectrum broadening, which is caused by the bandwidth of the incident light and could exist in most fast dark-field microscopy setups. We corrected the linewidth of plasmon scattering spectra theoretically by both numerical and analytical solution, and then realized it experimentally by tuning the bandwidth of the incident light. After calibration, we revisited scattering spectra of 700 small aspect ratio nanorods on glass and monolayer graphene revealing a typical 14.3 meV linewidth broadening. Furthermore, we measured four other kinds of gold nanoparticles on glass, mono- and bilayer graphene for deeper understanding of the electron transfer. A common linewidth broadening is found for each kind of particle agreeing well with previous theory. However, an unconventional linewidth narrowing is also discovered for big particles whose resonance wavelength is close to the near infrared region. It implies a competitive mechanism in the electron transfer process which could not only increase the damping of small particles, causing a linewidth broadening, but also simplify the electric field pattern for big particles, leading to a linewidth narrowing, according to our Mie theory simulation.

Osteogenic and antibacterial surfaces on additively manufactured porous Ti-6Al-4V implants: Combining silver nanoparticles with hydrothermally synthesized HA nanocrystals

The recently developed additively manufacturing techniques have enabled the fabrication of porous biomaterials that mimic the characteristics of the native bone, thereby avoiding stress shielding and facilitating bony ingrowth. However, aseptic loosening and bacterial infection, as the leading causes of implant failure, need to be further addressed through surface biofunctionalization. Here, we used a combination of (1) plasma electrolytic oxidation (PEO) using Ca-, P-, and silver nanoparticle-rich electrolytes and (2) post-PEO hydrothermal treatments (HT) to furnish additively manufactured Ti-6Al-4V porous implants with a multi-functional surface. The applied HT led to the formation of hydroxyapatite (HA) nanocrystals throughout the oxide layer. This process was controlled by the supersaturation of Ca2+ and PO43− during the hydrothermal process. Initially, the high local supersaturation resulted in homogenous nucleation of spindle-like nanocrystals throughout the surface. As the process continued, the depletion of reactant ions in the outermost surface layer led to a remarkable decrease in the supersaturation degrees. High aspect-ratio nanorods and hexagonal nanopillars were, therefore, created. The unique hierarchical structure of the microporous PEO layer (pore size < 3 μm) and spindle-like HA nanocrystals (<150 nm) on the surface of macro-porous additively manufactured Ti-6Al-4V implants provided a favorable substrate for the anchorage of cytoplasmic extensions assisting cell attachment and migration on the surface. The results of our in vitro assays clearly showed the important benefits of the HT and the spindle-like HA nanocrystals including a significantly stronger and much more sustained antibacterial activity, significantly higher levels of pre-osteoblasts metabolic activity, and significantly higher levels of alkaline phosphatase activity as compared to similar PEO-treated implants lacking the HT.

Two-Stage Synthesis of Structured Microsystems Based on Zinc-Oxide Nanorods by Ultrasonic Spray Pyrolysis and the Low-Temperature Hydrothermal Method

The specific features are presented of the two-stage synthesis of structured microsystems based on zinc-oxide nanorods by ultrasonic spray pyrolysis and the low-temperature hydrothermal method, with nucleation suppressed in the bulk of the solution. It is shown that the use of two-stage synthesis provides control over the size and aspect ratio of separate nanorods and over the structure of the microsystem of nanorods as a whole. It is proposed to monitor the concentration of intrinsic surface defects by measuring the photocurrent under irradiation through narrow-band interference light filters.

Non-aqueous liquid crystals of hydroxyapatite nanorods

Hydroxyapatite (HA) nanorods in the collagen matrix of bone have a macroscopically ordered structure that has many similarities to the ordered structure of anisotropic nano-units in inorganic liquid crystals (LCs). Inspired by these similarities, we conducted the first (to our best knowledge) synthesis of HA LCs in non-polar solvents (such as cyclohexane and toluene), thus expanding the range of applicable monomers and polymers. We synthesized HA nanorods by a simple, effective, and oleic-acid-assisted hydrothermal route. The hydrothermal temperature directly modulates the aspect ratio of the HA nanorods, and indirectly modulates their LC behavior. The LC phase transition has no size limitation. Thus, our approach may be used to develop high solid content, macroscopically assembled, large-scale polymer-based bio(mimetic)-materials.

GaN-Based LEDs Grown on Graphene-Covered SiO2/Si (100) Substrate

The growth of nitride on large-size and low-cost amorphous substrates has attracted considerable attention for applications in large-scale optoelectronic devices. In this paper, we reported the growth of GaN-based light-emitting diodes (LEDs) on amorphous SiO2 substrate with the use of nanorods and graphene buffer layers by metal organic chemical vapor deposition (MOCVD). The effect of different growth parameters on the morphology and vertical-to-lateral aspect ratio of nanorods was discussed by analyzing growth kinetics. Furthermore, we tuned nanorod coalescence to obtain continuous GaN films with a blue-LED structure by adjusting growth conditions. The GaN films exhibited a hexagonal wurtzite structure and aligned c-axis orientation demonstrated by X-ray diffractometer (XRD), Raman, and transmission electron microscopy (TEM) results. Finally, five-pair InGaN/GaN multi-quantum-wells (MQWs) were grown. The photoluminescence (PL) showed an intense emission peak at 475 nm, and the current–voltage (I-V) curve shows a rectifying behavior with a turn-on voltage of 5.7 V. This work provides a promising fabrication method for the large-area and low-cost GaN-based devices on amorphous substrates and opens up the further possibility of nitride integration with Si (100) complementary metal oxide semiconductor (CMOS) electronics.

Heterometallic Seed-Mediated Growth of Monodisperse Colloidal Copper Nanorods with Widely Tunable Plasmonic Resonances.

We report a heterometallic seed-mediated synthesis method for monodisperse penta-twinned Cu nanorods using Au nanocrystals as seeds. Elemental analyses indicate that resultant nanorods consist predominantly of copper with a gold content typically below 3 atom %. The nanorod aspect ratio can be readily adjusted from 2.8 to 13.1 by varying the molar ratio between Au seeds and Cu precursor, resulting in narrow longitudinal plasmon resonances tunable from 762 to 2201 nm. Studies of reaction intermediates reveal that symmetry-breaking is promoted by rapid nanoscale diffusion in Au-Cu alloys and the formation of a gold-rich surface. The growth pathway features coevolving shape and composition whereby nanocrystals become progressively enriched with Cu concomitant with nanorod growth. The availability of uniform colloidal Cu nanorods with widely tunable aspect ratios opens new avenues toward the synthesis of derivative one-dimensional metal nanostructures, and applications in surface-enhanced spectroscopy, bioimaging, and electrocatalysis, among others.

The aspect ratio of gold nanorods as a cytotoxicity factor on Raphidocelis subcaptata

Gold nanorods (AuNRs) are promising nanoscale materials for several technological and biomedical applications. The physicochemical properties of AuNRs, including size, shape and surface features, are crucial factors affecting their cytotoxicity. In this study, we investigated the effects of different aspect ratios of AuNRs (1.90, 2.35, 3.25 and 3.50) at concentrations of 2 and 10 μg mL−1 on their cytotoxicity and cellular uptake in green algae Raphidocelis subcaptata. The experiment was performed in oligotrophic freshwater medium in a growth chamber with constant agitation of 80 rpm under controlled conditions (120 μEm−2s−1 illumination; 12:12h light dark cycle and constant temperature of 22 ± 2 °C). The algal growth was monitored daily for 96 h via electronic absorbance scanning at 600–750 nm. Oxidative stress, cell viability and autofluorescence were evaluated using a flow cytometer. Oxidative stress quantified by loading cultures with the fluorescent dye 2′, 7′-dichlorofluorescein diacetate. To assess algal cell viability, propidium iodide was selected as the fluorescent probe. Our results indicated that the aspect ratio of AuNRs mediates their biological effects in green algae R. subcaptata. A positive correlation between oxidative stress and increase of aspect ratio was found at concentration of 10 μg mL−1. Higher cytotoxicity and mortality were observed for algae incubated with higher aspect ratios AuNRs (3.50). These findings may be useful to understand the impact of the AuNRs in aquatic environments, contributing to ecosystem management and nanomaterials regulation.

Thiol-suppressed I2-etching of AuNRs: acetylcholinesterase-mediated colorimetric detection of organophosphorus pesticides.

For the first time it isdemonstrated that sulfhydryl compounds can suppress longitudinal etching of gold nanorods via consuming oxidizers, which provides a new signaling mechanism for colorimetric sensing. As a proof of concept, a colorimetric assay is developed for detecting organophosphorus pesticides, which are most widely used in modern agriculture to improve food production but with high toxicity to animals and the ecological environment. Triazophos was selected as a model organophosphorus pesticide. In the absence of triazophos, the active acetylcholinesterase can catalyze the conversion of acetylthiocholine iodide to thiocholine whose thiol group can suppress the I2-induced etching of gold nanorods. When triazophos is present, the activity of AchE is inhibited, and I2-induced etching of gold nanorods results in triazophos concentration-dependent color change from brown to blue, pink, and red. The aspect ratio of gold nanorods reduced with gradually blue-shifted longitudinal absorption. There was a linear detection range from 0 to 117nM (R2 = 0.9908), the detection limit was 4.69nM, and a good application potential was demonstrated by theassay of real water samples. This method will not only contribute to public monitoring of organophosphorus pesticides but also has verified a new signaling mechanism which will open up a new path to develop colorimetric detection methods. It has been first found that sulfhydryl compounds can suppress longitudinal etching of gold nanorods (AuNRs) via consuming oxidizers, which provides a new signaling mechanism for colorimetric sensing. As a proof of concept, a colorimetric assay is developed for sensitively detecting organophosphorus pesticides (OPs). It will not only contribute to public monitoring of OPs but also has verified a new signaling mechanism which will open up a new path to develop multicolor colorimetric methods.

Role of oxygen vacancies on the green photoluminescence of microwave-assisted grown ZnO nanorods

Mechanism of commonly seen defect mediated photoluminescence (PL) from ZnO nanostructures is a topic of much discussion mainly due to its broad nature originating from several types of defects. Identification of such surface related defects requires suitable specimen having high surface to volume ratio characterized by complimentary techniques. Here, ZnO nanorods of different aspect ratio within the quantum confinement regime have been prepared using microwave-assisted synthesis method by varying time in the range of 1–4 min. The band gap as well as intensity of visible PL emission is found to decrease with the increase in size of nanorods obtained by allowing longer preparation time. Excitation wavelength dependent PL shows a strong green emission at ∼540 nm for above the band gap excitation (ABE) as well as below the band gap excitation (BBE) near to the band edge. Whereas a blue emission at ∼485 nm is seen for excitations far below the band gap. A systematic shift in emission from green to blue is observed for decreasing values of BBE, whereas almost no shift was seen for green emission with the variation of ABE. It is proposed that the green PL can be seen when ABE generated electrons in the conduction band transit rediatively to singly ionized oxygen vacancy (Vo+) and finally to reach the valence band whereas the blue emission may appear by direct excitation of electrons to a sub-band within the band gap by BBE. The presence of Vo+ in ZnO nanorods is confirmed by x-ray photoelectron spectroscopic studies and correlated with the green PL that shows increased intensity for higher concentration of Vo+ in smaller nanorods.