Nucleation control for ZnO nanorods grown by catalyst-driven molecular beam epitaxy

We report a study of the annealing temperature and time on Ag catalyst size and density for subsequent growth of ZnO nanorods by catalyst-driven molecular beam epitaxy (MBE). Two different substrates (SiO₂ and SiN_X) for the Ag deposition were used and the thickness of the Ag held constant at 25 Å. Annealing between 600 and 800 °C produced Ag cluster sizes in the range 8-30 nm diameter on SiO₂ and 10-65 nm on SiN_X with a cluster density from 100 to 2500 mm⁻² for SiO₂ and 30 to 1900 mm⁻² for SiN_X. ZnO nanorods grown on these clusters show single-crystal, wurtzite-phase nature and strong band-edge photoluminescence at 380 nm. The nanorods can also be grown selectively on lithographically-patterned dielectric stripes with Ag clusters formed on top by e-beam evaporation and annealing.     

Novel waveguide design for high brightness optoelectronic applications

We describe a novel compound ridge waveguide in which the highest order propagating mode is guided in the low index region of the structure and exhibits a narrow single lobe. For a prototype AlGaAs/GaAs was waveguide designed for operation at = 980 nm we show that due to the narrow lobed emission this mode produces significant improvements in source brightness and hence fibre coupling efficiency. In the case of a 61 m aperture laser diode operating in this mode 20% coupling efficiency is predicted between a structure and standard single mode fibre designed for = 980 nm operation.     

Growth and optical study of ZnO nanorods

ZnO nanorods were grown by a simple near room temperature, chemical solution method on ZnO-seeded silicon substrates. Study of the ZnO nanorods over different growth times by electron microscopy methods revealed that the resulting ZnO nanorods were single crystalline with a highly preferential growth perpendicular to the substrate and a very good c-axis alignment. The size of the nanorods increased with increasing growth time. The growth mechanism is briefly discussed. Post-annealing in oxygen slightly improved the surface roughness of the ZnO nanorods. Photoluminescence experiments at 1.6 K revealed a major emission peak of the nanorods at around 3.36 eV which is attributed to the band edge transition of ZnO, while defect-related emission is relatively weak.     

Selective growth of ZnO nanorods in aqueous solution

ZnO nanorod arrays find applications in solar energy conversion, light emission and other promising areas. One approach to generate ZnO nanorods is the cost efficient aqueous chemical growth (ACG). Usually the ACG process is based on a nucleation step followed by growth of ZnO nanorods in aqueous solution at temperatures below 95 C.We report on the fabrication of homogeneous, large scale arrays of nanorods on various substrate materials (Si, glass, polymer) by ACG. PL-measurements show surprisingly good optical quality although the rods were grown at low temperature.Even though we have developed patterning of these arrays with photolithographic techniques, a bottom up approach for lateral patterning is important concerning further applications especially for mass-production. The substrates with patterned metal layers were employed to realize selective growth of nanorods. The experiments were carried out on Ti-, Ag- and Pt-patterned substrates. Selective growth on metal structured glass substrates was developed and is described.     

Catalyst-free synthesis and luminescence of aligned ZnO nanorods

Quasi-aligned undoped ZnO nanorods with diameter in the range 100–300 nm and length of several micrometers have been grown catalyst-free on Si(1 0 0) wafer in a one-step process by direct heating of Zn powders. All nanowires are single crystals and are aligned vertically to the substrate surface with c-axis preferred orientation. XRD, HRTEM and Raman studies revealed that the ZnO nanorods have wurtzite phase, are highly crystalline and well aligned with the lattice parameters a=0.32 nm and c=0.52 nm. The PL spectra measured at different temperatures are dominated by excitonic emission at 380 nm and less intense below band gap emission band centered at 520 nm.     

Synthesis and photoluminescence properties of ZnO nanorods and nanotubes

Different morphologies of zinc oxide (ZnO) nanorods and nanotubes, which were grown under the same conditions but different dissolving processes, are prepared in our experiment through hydrothermal method. After the growth process, cooling down the reactor naturally or dissolving at a constant temperature of 40 °C, preferential dissolution will occur at different places on the tip of ZnO nanorods. During the dissolution process, different dissolution rates on the entire surface of nanorod will lead to different nanostructures. ZnO nanorods and nanotubes on Cu substrates display the same PL property with strong green emission but weak UV emission, while ZnO nanorods on Si substrates exhibits a relatively strong UV emission.     

Electrodeposition of ZnO nanorods for device application

We report the electrochemical growth of zinc oxide nanorods in a zinc nitrate/hexamethylenetetramine solution at 70 °C. High-density vertical nanorods were grown on Au films on silicon substrates with a texture coefficient better than 99.9%. By varying the reactant concentration the diameter can be varied between 100 and 250 nm, with corresponding lengths of 1 to 4 μm. Furthermore, this approach was used for the selective growth on Ti/Au strip conductors ordered in an interdigitated structure on an insulating substrate. We achieved the growth of ZnO nanorods between neighbouring strip conductors bridging the gap between them. In this configuration the nanorods are already contacted and electrical measurements can be directly performed. First I–V measurements show a good conductivity of the as-grown nanorods and the resistance could be estimated to be 0.1 Ω cm. Under UV illumination the ZnO nanorods demonstrate a photoconductivity, but only after annealing the sample at 300 °C in N₂. PACS 61.05.cp; 73.63.-b; 78.55.Et; 81.15.Pq; 82.45.Yz     

Optically stimulated optical effects in the ZnO:Tm nanorods

We have performed the studies of photoinduced second harmonic generation in the ZnO nanorods doped by Thulium with different content (0.1; 0.2;0.5% of Tm). The illumination was performed with the Er:glass laser emitting at 1540 nm wavelength of the 10 ns laser and the output second harmonic generation (SHG) was studied. The AFM data analysis was performed. The role of the rare earth Tm ions on the morphology and the optically stimulated SHG is discussed. The SHG yield was found to be strongly dependent on Thulium content and Root Mean Square roughness of the studied layers.     

Detection of hydrogen with SnO2-coated ZnO nanorods

SnO₂-coated ZnO nanorods on c-plane sapphire substrates were synthesized by pulsed laser deposition. The thickness of the polycrystalline SnO₂ was ∼10 nm, as determined by high-resolution transmission electron microscopy, while the diameter of the ZnO nanorods was ∼30 nm. The sensitivity of the SnO₂/ZnO structures to hydrogen was tested by depositing Ti/Au Ohmic contacts on a random array of the nanorods and measuring the current at fixed voltage. There was no response to 500 ppm H₂ in N₂ at room temperature, but we obtained a sensitivity of ∼70% at 400 °C. The SnO₂/ZnO structures exhibit drift in their recovery characteristics and for sequential detection of hydrogen, as generally reported for SnO₂ thin film sensors.     

Growth, properties and dye-sensitized solar cells–applications of ZnO nanorods grown by low-temperature solution process

Hexagonal-shaped small ZnO nanorods were grown in a large-quantity via simple aqueous solution process by using zinc nitrate as a source of zinc ions at low temperature under stirring. The as-grown hexagonal-shaped ZnO nanorods were characterized in detail in terms of their structural, optical and photovoltaic properties. The detailed structural investigations by HRTEM, SAED and FFT revealed that the as-synthesized ZnO nanorods are well-crystalline, possessing a perfect hexagonal ideal growth habits of wurtzite zinc oxide and grown along the [0001] direction in preference. The optical properties, composition and quality of the as-synthesized nanorods were examined by using UV-visible and FTIR spectroscopy. Moreover, films of as-grown nanorods were used as photoanode materials to fabricate the dye sensitized solar cells (DSSCs). An overall light to electricity conversion efficiency of 0.70% with a fill factor of 47.2%, short-circuit current of 1.8 mA/cm² and open-circuit voltage of 0.76 V were achieved for the solar cell based on hexagonal-shaped small ZnO nanorods.     

Fabrication and characterization of graphene oxide/zinc oxide nanorods hybrid

This work presented a hybrid architecture of graphene oxide (GO)/ZnO nanorods (ZNs) with ZNs attached parallel onto GO sheets. ZNs were synthesized by refluxing zinc acetate dehydrate in methanol solution under basic conditions followed by surface modification of 3-aminopropyl triethoxysilane (ATS), and then the preformed ZNs were attached onto GO sheets by reaction of the amino groups on the outer wall of ZNs with the carboxyl groups on the GO surface. Transmission electron microscopy (TEM) image of the as-prepared hybrid reveals the morphology of the architecture of GO/ZNs hybrid. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) ultraviolet-visible (UV-vis) and fluorescence spectroscopy were also performed to characterize the structure and properties of the GO/ZNs hybrid. It was shown that ZNs maintained their initial morphology and crystallinity in the hybrid and the luminescence quenching of yellow-green emission of ZNs confirmed the electron transfer from excited ZnO to GO sheets.     

Thermo-electrochemical selective growth of ZnO nanorods on any noble metal electrodes

Selective growth of ZnO nanorods has been successfully performed on the patterned Au/Ti metal electrode regions on a glass substrate by using a seeded thermo-electrochemical method in an acidic growth solution. The selective growth mechanism of the thermo-electrochemical method was proposed by using a series of chemical reactions for the first time. The thermo-electrochemical selective ZnO growth was performed on the cathode electrode at a temperature below 90 °C. A ZnO seed layer was precoated and selectively etched away from the non-metal regions in order to create the patterned selective nucleation sites on which the precursors are transferred and crystallized into ZnO nanorods. Both the dimensions and the placements of the ZnO nanorods have been simultaneously controlled. Energy dispersive X-ray spectrometry showed that the selectively grown ZnO nanorods consist of only Zn and O, indicating that the selectively grown ZnO nanorods are pure and contamination free. XRD and electron diffraction patterns revealed that the obtained ZnO nanorods have a wurtzite single-crystal structure.     

Photoluminescence study of aligned ZnO nanorods grown using chemical bath deposition

The photoluminescence study of self-assembled ZnO nanorods grown on a pre-treated Si substrate by a simple chemical bath deposition method at a temperature of 80 °C is hereby reported. By annealing in O₂ environment the UV emission is enhanced with diminishing deep level emission suggesting that most of the deep level emission is due to oxygen vacancies. The photoluminescence was investigated from 10 K to room temperature. The low temperature photoluminescence spectrum is dominated by donor-bound exciton. The activation energy and binding energy of shallow donors giving rise to bound exciton emission were calculated to be around 13.2 meV, 46 meV, respectively. Depending on these energy values and nature of growth environment, hydrogen is suggested to be the possible contaminating element acting as a donor.     

Fabrication of diamond nanorods for gas sensing applications

Diamond nanorods were fabricated for a sensing device by utilizing reactive ion etching in CF₄/O₂ radio frequency plasma. The length of the nanorods has been controlled by the ion etching time. The obtained morphologies were investigated by scanning electron microscopy. The gas sensing properties of the H-terminated diamond-based sensor structures are indicating that we have achieved high sensitivity to detect phosgene gas. Also, our sensor exhibited good selectivity between humid air and phosgene gas if the measurement is conducted at elevated temperatures, such as 140 °C. Furthermore, such sensor response rating could reach as high value as 4344 for the phosgene gas, which was evaluated for the sample consisting of the longest nanorods (up to 200 nm).     

Color tunable ZnO nanorods by Eu^{3+} and Tb^{3+} co-doping for optoelectronic applications

Eu \(^{3+}\) /Tb \(^{3+}\) co-doped ZnO nanorods were prepared by co-precipitation method and the effect of Eu–Tb co-doping was studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy, Fourier transform infrared spectroscopy (FTIR), UV-Vis-NIR diffuse reflectance (DR) and photoluminescence (PL) spectroscopy. The XRD pattern shows typical peak pattern for pure hexagonal wurtzite structure to match with the JCPDS data. The samples are found to be consisting of nanorods of diameter 20–30 nm as revealed by the TEM image. The FTIR pattern confirms the formation of the compounds. The DR study was carried to show the variation of absorption edge and the variation in band gap values, which showed the crystal size effect in the co-doped sample of different rare-earth ratios. The room temperature PL study shows bright emission spectra for the samples with different rare-earth ratios. It shows a very good energy transfer from Tb \(^{3+}\) to Eu \(^{3+}\) ions. The energy transfer mechanism and color tunability were discussed thoroughly.     

Effects of curing temperature on physical properties of hydrothermally-grown yttrium-doped ZnO nanorods

We investigated the effects of the curing temperature on physical properties of hydrothermal synthesized yttrium-doped ZnO (YZO) nanorods. Compared to the as-grown sample, the c-axis preference (i.e., the peak intensity of x-ray diffraction at the (002) Bragg angle) was much enhanced after curing in vacuum at 200 °C. In addition, 200-°C-cured YZO nanorods showed a significant decrease in the number of metastable oxygen bonds. These resulted in both the increase in ultra-violet emission and the decrease in oxygen-related deep-level emission.     

A novel deposition method to grow ZnO nanorods: Spray pyrolysis

ZnO layers were deposited by chemical spray pyrolysis (CSP) using zinc chloride aqueous solutions onto indium tin oxide (ITO) glass substrates at growth temperatures in the region of 400–580 C. The layers were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and low-temperature () photoluminescence (PL) measurements. The flat film of ZnO obtained at 400 C evolves to a structured layer by raising the temperature up to 500 C. Deposition around 550 C and above results in a layer comprising well-shaped hexagonal ZnO nanorods with diameter of 100–150 nm and length of up to 1 micron. XRD shows strong c-axis orientation of ZnO being in accordance with the SEM study. Deposition of nanorods was successful using ITO with grain size around 100 nm, whereas on fine-grained ITO (grain size < 50 nm) with smooth surface fat crystals with diameter up to 400 nm and length of about 300 nm were formed. Sharp near band edge (NBE) emission peaks centered at 3.360 and 3.356 eV dominated the PL spectra of ZnO at , originating from the exciton transition bound to neutral donors. PL and XRD results suggest that ZnO rods prepared by spray pyrolysis are of high optical and crystalline quality.     

ZnO nanorods/plates on Si substrate grown by low-temperature hydrothermal reaction

The zinc oxide (ZnO) nanorods/plates are obtained via hydrothermal method assisted by etched porous Al film on Si substrate. The products consist of nanorods with average diameter of 100 nm and nanoplates with thickness of 200-300 nm, which are uniformly distributed widely and grown perpendicularly to the substrate. The ZnO nanoplates with thickness of 150-300 nm were grown on Si substrate coated with a thin continuous Al film (without etching) in the same aqueous solution. The growth mechanism and room temperature photoluminescence (PL) properties of ZnO nanorods/plates and nanoplates were investigated. It is found that the introduction of the etched Al film plays a key role in the formation of ZnO nanorods/plates. The annealing process is favorable to enhance the UV PL emissions of the ZnO nanorods/plates.     

Fabrication of ZnO nanorod array-based photodetector with high sensitivity to ultraviolet

Large scale densely packed and vertically oriented ZnO nanorod arrays were grown on F-doped SnO₂ (FTO) substrates through a simple hydrothermal synthesis route. Based on the arrays of hexagonal ZnO nanorod with size of 60100 nm in diameter, and 1.5 μm in length, a prototypical photoelectrical device was fabricated for ultraviolet detection, showing good reproducibility and a large photocurrent of around 6.71 mA at the applied voltage of 0.4 V. The large photocurrent and the ohmic I–V characteristics of the ZnO nanorods under the illumination could be ascribed to the decrease of the barrier height among the ZnO nanorods and the Schottky barrier between the nanorods and the Au electrodes and, in particular, to the accumulation of conduction electrons, resulted from the neutralization between photogenerated holes and negatively charged oxygen ions. The photoresponse curve is well fitted to an exponential curve with the relaxation time constant of 9 s in rising edge and 90 s in decaying one, representing the accumulation of conduction electrons. These well-aligned ZnO nanostructures of high quality could be easily fabricated by a cost-effective chemical route and used for constructing nanoscale devices with excellent performances.     

Structural investigations of InGaAs/InGaAs SLSs for optoelectronic device applications

InGaAs/InGaAs strained layer supelattices (SLSs) are interesting candidates for optoelectronic device applications working in the 1.55 μm range. However, it was recognized very early that both the miniband widths and interband transition energies in these very shallow systems are extremely sensitive to the final value of all structural parameters. As a consequence, any unwanted modification of the composition and thickness of the wells and barriers, with respect to the nominal ones, should be carefully avoided and, at the same time, systematically checked. Toward this end, since the most usefull and non-destructive techniques are X-ray and optical spectroscopy, we present in this work a comparative evaluation of their respective sensitivity.