Array of ZnO nanoparticle-sensitized ZnO nanorods for UV photodetection

Vertically aligned ZnO nanorod (NR) arrays have been successfully synthesized on ITO-glass substrate by hydrothermal growth. Chemical bath deposition method was used to deposit ZnO nanoparticles (NPs) onto the ZnO NRs. These structures were applied in fabricating ZnO NPs sensitized ultraviolet (UV) photodetectors (PDs). Incorporation of ZnO NPs onto ZnO NRs results in distinct improvement of optical properties of ZnO NRs, i.e., significant enhancement of emission as well as effective suppression of defects emission in ZnO. Furthermore, there is a noticeable blue-shift in absorption spectra compared with that of ZnO NRs structure. I–V characteristics show that the sensitized structure improved photocurrent almost twice that of unsensitized ZnO NRs. Consequently, these findings may open new opportunities for the integration of different ZnO nanostructures for application in UV region particularly fabrication of UV PDs.     

Optoelectronics behaviour of ZnO nanorods for UV detection

In this study ZnO nanorods have been synthesized by a chemical precipitation method. The room temperature UV–Vis absorption spectra of the ZnO nanorods indicated two absorption peaks in the UV region, one in the near UV region and the other attributed to the band gap of ZnO. The Photoluminescence spectra of ZnO nanorods show two emission bands, one ultraviolet emission band at 378 nm and the other in the defect related yellow emission band near 550 nm. The stimulated yellow luminescence of ZnO nanorods were affected by the synthesis time and annealing temperature. The same ZnO nanorods were deposited onto the ITO substrate to form a UV photoconductive detector. The ratio of the UV photogenerated current to dark current was as high as nine times under 3 V bias. Hence, these nanorods can be promising materials in the use of UV radiation detection.     

Large surface dipole moments in ZnO nanorods

A self-consistent linear scaling three-dimensional fragment (LS3DF) method is used to study the dipole moments and internal electric fields of large ZnO nanorods. Our ab initio calculations reveal that the ZnO nanorod with unpassivated (1010) side surface has a side surface contribution per Zn-O dimer 10 times larger than the bulk contribution per Zn-O bond. Detailed analysis is used to decompose the total dipole moment into different contributions. It is found that the total dipole moment of a nanorod can be calculated from the different parts with their dielectric screening described by a continuous model. We also show the effect of the dipole moment on the interior electronic structure of the nanorod.     

Direct synthesis of well dispersed ZnO nanorods without using additional surfactant

Without using additional surfactant as structure-directing agent, well dispersed ZnO nanorods were directly synthesized in the dimethyl sulfoxide (DMSO) and alcohol mixture solution by a one-step wet chemical method. The experiment results demonstrated that alcohol molecular with longer chain is helpful for the growth of ZnO nanorods. The absorption and emission spectra of as-synthesized ZnO nanorods were also presented in this paper.     

Ultrafast carrier dynamics of near-band-edge emission in single-crystal ZnO nanorods

We report on rational synthesis and optical characteristics of highly crystallined ZnO nanorods which were grown by a facile chemical vapor transport method. Temperature-dependent photoluminescence spectra of as-fabricated ZnO nanorods are dominated by near-band-edge emission with a characteristic fine structure due to high crystallinity. Furthermore, the recombination emission involving carrier dynamics of near-band-edge emission in ZnO nanorods was systematically investigated by temperature-dependent time-resolved photoluminescence spectroscopy. Recombination peaks pertaining to the exciton emissions are monitored and resolved in both temporal and spatial regimes.     

Effects of residual gas exposure on the field emission properties of ZnO nanorods

We compare the effects of O2, CO2, N2, H2, and Ar residual gas exposure on the field emission (FE) properties of ZnO nanorods. In contrast to carbon nanotubes and Mo metal microtips, we find that O2 and CO2 exposures do not significantly degrade the FE properties of ZnO nanorods. However, N2 exposure significantly degrades the FE properties. We propose that this could be due to the dissociation of N2 into atomic nitrogen species and the reaction of such species with ZnO. H2 and Ar exposures are not observed to significantly degrade the FE properties.     

Epitaxial growth of ZnO layers using nanorods with high crystalline quality

Epitaxial ZnO films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition using a ZnO multi-dimensional structure having the sequence of ZnO film/ZnO nanorods/sapphire. The vertically well-aligned one-dimensional ZnO nanorods were grown epitaxially on the sapphire substrate with in-plane alignment under suitable growth conditions and then used as seeds for the subsequent epitaxial ZnO layer. For the transition of the ZnO structures from the nanorods to the film, the growth temperature and working pressure were controlled, while keeping the other conditions fixed. The growth of the ZnO films on the well-aligned ZnO nanorods results in homoepitaxial growth with the identical orientation relationship along the in-plane direction as well as the same c-axis orientation. The microstructural analysis of the multi-dimensional structure and analysis of the microstructural evolution from the one-dimensional nanorods to the two-dimensional film were conducted using transmission electron microscopy.     

Enhancement of green emission from Sn-doped ZnO nanowires

Oxygen vacancies play a crucial role in the emission characteristics of oxide nanomaterials. In this study, we found that the green emission intensity of ZnO nanowires can be enhanced through a Sn-doping concentration which increases the number of oxygen vacancies. Undoped ZnO nanowires showed blue emission at 380 nm, but as the concentration of Sn was increased, the green emission peak at around 500 nm, which is attributed to oxygen vacancies, showed drastic enhancement. On the basis of XPS compositional analysis, it was confirmed that the green luminescence intensity was closely related to the number of oxygen vacancies in Sn-doped ZnO nanowires. These results pave the way to a greater understanding of tunable light emission from nanowires, which could be a key technology for next-generation display devices, including flexible and transparent displays.     

Tuning photoluminescence properties of ZnO nanorods via surface modification

Zinc oxide (ZnO) is a versatile material that has been used in photocatalysis, solar cells, chemical sensors, and piezoelectric transducers. All these are directly related to its surface properties. Here ZnO nanorod arrays were successfully synthesized by electrochemical deposition method, the surface of which was modified by dopamine, a robust anchor. Compared with pristine ZnO sample, the surface modification can greatly enhance the ultraviolet and visible-light photoluminescence. This is due to the formation of polydopamine on the nanorod surface, which may act as a dye that can be photoexcited. The resultant photogenerated electrons can inject into the conduction band of ZnO and take part in the luminescent process. These results may provide a foundation for real applications of ZnO nanomaterials in optoelectronic devices and, especially, for the applications in biological field as both the dopamine and ZnO are biocompatible materials.     

Growth of aligned arrays of ZnO nanorods by low temperature solution method on Si surface

We report growth of ZnO nanorods by low temperature (<100°C) solution growth method. The substrates (Si, glass and fused Quartz) were seeded by pre-coating with ZnO nanoparticles (4–7 nm diameter) prepared by chemical precipitation route. Nanorods were grown on the seeded substrate in aqueous solution of Zinc Nitrate and Hexamethylenetetramine (HMT). The growth process lasts for up to 8 h and at the maximum time of growth, the nanorods have a width of ∼230–250 nm and length of ∼1.5–1.6 μm. The growth process after some initial growth (<2 h) preserves the aspect ratio and leads to about 90% texturing along the (002) direction. The growth of the nanorods was studied with time and observed growth data suggests a two-stage growth process. The nanorods have a well-defined hexagonal morphology and have a Wurtzite structure. The nanorods were characterized by different techniques and have a band gap of 3.25 eV.     

A high photocurrent gain in UV photodetector based on Cu doped ZnO nanorods on PEN substrate

Vertical well-aligned Cu-doped ZnO nanorods were successfully synthesized by chemical bath deposition (CBD) method on low cost and flexible polyethylene naphthalate (PEN) substrate. The structural and optical investigations exhibited the high quality of the Cu-doped ZnO nanorods on a flexible PEN substrate. The metal-semiconductor-metal (MSM) configuration was used to fabricate UV photodetector based on the Cu-doped ZnO nanorods grown on PEN substrate. Under a 5 V applied bias, the values of dark current and photocurrent of the Cu-doped ZnO nanorods photodetector were 14.9 µA and 3.27 mA, respectively. Meanwhile, calculated photocurrent gain of the UV photodetector was 219 at 5 V bias voltage. Upon exposure to 365 nm UV light, the UV device exhibited fast response time and recovery time of 0.317 and 0.212 s, respectively, at a bias voltage of 5 V.     

Effects of seed layers on structural, morphological, and optical properties of ZnO nanorods

We studied the effects of seed layers on the structural and optical properties of ZnO nanorods. ZnO and Ag-doped ZnO (ZnO:Ag) seed layers were deposited on glass substrates by magnetron co-sputtering. ZnO nanorods were grown on these seed layers by the chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethyltetramine. SEM micrographs clearly reveal that ZnO nanorods were successfully grown on both kinds of seed layers. The XRD patterns indicate that crystallization of ZnO nanorods is along the c-axis. Meanwhile, the packing density and the vertical alignment of the ZnO nanorods on the ZnO seed layer are better than those of the ZnO nanorods on ZnO:Ag. The enhanced growth of nanorods is thought to be due to the fact that the ZnO layer exhibits a higher crystalline quality than the ZnO:Ag layer. According to the low-temperature photoluminescence spectra, the ZnO nanorods on the ZnO seed layer show a narrow strong ultraviolet emission band centered at 369 nm, while those on ZnO:Ag exhibit multiple bands. These results are thought to be related with the crystallinity of ZnO nanorods, the morphologies of ZnO nanorods, and the reflectivities of seed layers. More detailed studies for clarification of the seed layer effect on the growth of ZnO nanorods are desirable.     

Epitaxial growth of ZnO nanorods on electrospun ZnO nanofibers by hydrothermal method

In this paper, we report a new ZnO nanofibers-nanorods structure which was successfully prepared by the electrospun ZnO nanofibers as seed to guide hydrothermal epitaxial growth of the ZnO nanorods. The structure was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL). The XRD results indicate that ZnO nanofibers obtained at 600° have high crystallinity with a typical hexagonal wurtzite structure. Furthermore compared with the strongest diffraction of ZnO nanofibers in (101) plane, the diffraction from (002) plane of ZnO nanofibers-nanorods becomes the strongest. The SEM shows that the diameters of epitaxial-grown ZnO nanorods on ZnO nanofibers were approximately 100–200?nm. The PL spectrum shows that the ZnO nanofibers-nanorods have a broad green-yellow emission around 537?nm, in contrast to that of ZnO nanofibers, the peak had obvious redshift about 24?nm and the luminous intensity weakened.     

Spontaneous and stimulated emission of vertically aligned ZnO nanorods of different lengths

The effect of the length of zinc oxide nanorods on their photoluminescence (PL) has been studied in the visible through UV spectral region. Hexagonally faceted columnar nanorods grown on (100) Si substrates have been shown to be aligned almost vertically. The chemistry of point defects in the nanorods depends on their position in the reactor during growth. The room-temperature PL spectrum of the nanorods shows narrow peaks due to stimulated free-exciton emission. The threshold optical pump power density for lasing in the longer ZnO nanorods is 8000 kW/cm², and the laser radiation is directed predominantly along their axis.     

Enhancement in ethanol sensing response by surface activation of ZnO with SnO2

A surface functionalized gas sensing material convincingly giving enhanced response to ethanol is demonstrated by SnO₂ activated ZnO. Zinc oxide was synthesized by a chemical route, deposited on an alumina substrate and activated by tin dioxide obtained by on-site oxidation of tin chloride. The XRD study of samples confirmed wurtzite hexagonal structure of zinc oxide and FESEM investigation revealed that surface of activated ZnO microrods was covered by nanoparticles of tin dioxide. Sensing response of sensing elements activated with different concentrations of tin chloride solution has been investigated. It was found that response to ethanol vapor significantly enhanced (eight times) by surface activation with tin dioxide, which optimized at a concentration of 3 wt.%.     

Aptamer-based immunosensor on the ZnO nanorods networks

This paper presents the fabrication and characteristics of a new aptamer-based electrochemical immunosensor on the patterned zinc oxide nanorod networks (ZNNs) for detecting thrombin. Aptamers are single-stranded RNA or DNA sequence that binds to target materials with high specificity and affinity. An antibody-antigen-aptamer sandwich structure was employed to this immunosensor for detecting thrombin. First, hydrothermally grown ZNNs were patterned on the patterned 0.02 cm2 Au/Ti electrodes on a glass substrate by lift-off process. The high isoelectric point (IEP, approximately 9.5) of nanostructured ZnO makes it suitable for immobilizing proteins with low IEP. Then 5 microL of the 500 nM antibody was immobilized on the ZNNs electrode. 5 micro/L of the mixture of 1 microM aptamer labeled by ferrocene (Fc) and thrombin was dropped on the electrode for antibody-antigen binding. The peak oxidation currents of the immunosensors at various thrombin concentrations were measured by using cyclic voltammetry. The peak oxidation current was observed at 340 mV versus Ag/AgCl electrode, and the peak oxidation current increased linearly from 62.26 nA to 354.13 nA with the logarithmic concentration of thrombin in the range from 100 pM to 250 nM. Fabrication of an aptamer-based immunosensor for thrombin detection is a new attempt and the characteristics of the fabricated immunosensors showed that the fabricated aptamer-baded immunosensor worked electrochemically well and had a low detection limit (approximately 91.04 pM) and good selectivity.     

Sulphur doping induced band gap narrowing and enhancement of green emission in ZnO nanorods

Journal of Materials Science: Materials in Electronics - Sulphur doped ZnO nanostructures were synthesized by a cost-effective wet chemical method. Structural characterization indicated that the...     

Surface plasmon enhanced bandgap emission of electrochemically grown ZnO nanorods using Au nanoparticles

Electrochemical deposition of ZnO nanorods having a diameter of 80-150 nm and length ~ 2 μm has been carried out. Au particles were sputtered on the ZnO nanorods for different sputtering times (from 0 to 100 s). The Photoluminescence spectra of bare ZnO nanorods showed a weak bandgap emission at around 375 nm and a broad defect-related emission band centered at ~ 596 nm. After the Au sputtering, the defect-related emission disappeared for all the samples. Moreover, the band edge emission intensity was enhanced with Au sputtering time 50 s. The enhancement factor reached a maximum value for the Au sputtering time of 50 s The enhancement in band edge emission is due to the transfer of electrons from defect states to the Au nanoparticles that cause not only an increase of resonant electron density, but also creates energetic electrons in the higher energy states. These resonant electrons can escape from the surface of the Au nanoparticles to conduction band of ZnO nanorods leading to the suppression of defect related emission intensity.     

Structural analysis of vertically-aligned single crystalline ZnO nanorods grown on different seed layers with chemical solution deposition

We report the structural properties of the vertically-oriented ZnO nanorods fabricated on various ZnO seed layers with chemical solution deposition (CSD) technique. The ZnO nanorods were prepared using an aqueous solution with Zinc nitrate (Zn(NO3)2 x 6H2O, Aldrich) and hexamethylenetetramine (HMT, Aldrich) in a convection oven. A-plane sapphire substrates with a deposited ZnO thin film were placed upside down in a quartz holder to avoid any micro-crystalline contamination. Especially, our hydro-thermal syntheses are automatically processed on precision pump drive systems (Masterflex) to accurately control the pH of the aqueous solution. The [002] crystal orientation of the ZnO seed layer was observed by the X-ray diffraction pattern. Structural features of ZnO nanorods were systematically analyzed by scanning electron microscopy and tunneling electron microscopy, together with selective area electron diffraction patterns. Experimental observations clearly demonstrated the dependence of the growth direction of the ZnO nanorods on the crystal structures of the ZnO seed layers.