Doped ZnO Nanowires Research Articles

Electrical conduction mechanisms in natively doped ZnO nanowires (II)

We have measured the intrinsic electrical resistivities,ρ(T), of three individual single-crystalline ZnO nanowires (NWs) from 320 downto 1.3 K. The NWs were synthesized via carbon thermal chemical vapordeposition and the four-probe Pt contacting electrodes were made by thefocused-ion-beam technique. Analysis of the overall temperature behavior ofρ(T) confirms that the charge transport processes in natively doped ZnO NWs are due to acombination of the thermal activation conduction and the nearest-neighbor hoppingconduction processes, as proposed and explained in a recent work (Chiu et al 2009Nanotechnology 20 015203) where the ZnO NWs were grown by a different thermalevaporation method and the four-probe electrodes were made by the electron-beamlithography technique. Taken together, the observations of these two complementarystudies firmly establish that the electrical conduction mechanisms in natively doped ZnONWs are unique and now satisfactorily understood.

Optical Investigation of ZnO Nanowires

In this study we report the application of synchrotron X-ray fluorescence, photoluminescence and Raman scattering techniques to the analysis of the incorporation of impurities in unintentionally doped ZnO nanowires. Highly ordered one-dimensional ZnO arrays were fabricated by an oxidation process of Zn metal electrodeposited in nanoporous anodic alumina template. X-ray fluorescence data show the contribution of residual elements into the ZnO nanowires growth. A rough analytical quantification of the main light and heavy chemical contents derives impurity concentrations below 1%. The optical eciency of ZnO nanowires is strongly aected by non-radiative centers up to temperatures as low as 100 K. The photoluminescence was found to be totally dominated by optical transitions associated with the anodic alumina template. Finally, the Raman scattering provides no evidence of local vibrational modes or secondary phases, but it shows the unambiguous signature of the ZnO hexagonal phase.

Thermal growth and cathodoluminescence of Bi doped ZnO nanowires and rods

Bi doped ZnO nanowires and rods have been grown by a catalyst free evaporation–deposition method with precursors containing either ZnO and Bi2O3 or ZnS and Bi2O3 powders. The use of ZnS as a precursor was found to lead to a higher density of nano- and microstructures at lower temperatures than by using ZnO. Energy dispersive x-ray spectroscopy (EDS) shows that the Bi content in the wires and rods is in the range 0.15–0.35 at%. Bi incorporation was found to induce a red shift of the near band gap luminescence but no quantitative correlation between the shift and the amount of Bi, as measured by EDS, was observed. The I–V curves of single Bi doped wires had linear behaviour at low current and non-linear behaviour for high currents, qualitatively similar to that of undoped wires.

Trapping of Ce electrons in band gap and room temperature ferromagnetism of Ce4+ doped ZnO nanowires

Rare-earth (RE) metal doped ZnO nanowires have been fabricated through a simple, quick, and versatile low temperature soft chemical method. The average length and diameter of nanowires lie in range of 5μm and 60nm, respectively. Raman and x-ray photoelectron spectroscopy studies demonstrate that Ce has 4+ oxidation state and successfully substitutes Zn up to 2.5% into ZnO single phase wurtzite structure. Doping of Ce shows a remarkably prominent large redshift of 22nm in the UV region of the band gap, with an increase in the intensity of green emission band due to charge transfer of Ce4+ dopant. In addition, it has been interestingly found that RE (Ce) doped ZnO nanowires exhibit room temperature ferromagnetism, which makes them potential for spintronic devices with excellent optical characteristics.

Controlled synthesis and luminescence of Eu doped ZnO nanowires and nanorods via hydrothermal method

Undoped and Eu3+ doped ZnO nanostructures were prepared by single-step mild hydrothermal conditions. The nanostructures were deposited on a Si or glass substrate without template. The reaction was carried out for 10–20h which resulted in morphology and aspect ration of the final products depend on the concentration of the precursors and reaction time. The concentration of dopant Eu3+ in the synthesized nanostructures was below 4 at.%. The crystal structure and the microstructure of the doped nanostructures were studied and compared with undoped ZnO. Photoluminescence measurements of the nanowires exhibit a strong band gap emission at room temperature. In partcular, relative intensity of band gap and defect emission changes drastically in ZnO:Eu and ZnO nanostructures with different morphology. The highly crystalline ZnO nanorods, nanowires can be a good one-dimensional candidate to study optical and electronic properties

Visible Light Response of Unintentionally Doped ZnO Nanowire Field Effect Transistors

A significant visible light response of unintentionally doped ZnO nanowire (NW) field effect transistors (FETs) has been observed in a reversible manner (for illumination source on and off). In particular, under white light illumination (wavelength longer than 400 nm), the threshold voltage (VT) of the ZnO NW FET shifts greatly to the negative direction, suggesting a remarkable increase in carrier concentration. A photon-assisted oxygen molecule desorption mechanism is proposed to explain the observed sub-bandgap photoresponse on the basis of the behavior of the experimental devices in different gas atmospheres (air, vacuum, pure N2, and pure O2) and with/without nanowire surface modifications (coated with PMMA).

Optical and structural properties of Eu-diffused and doped ZnO nanowires

Single crystal ZnO nanowires diffused with europium (Eu) from a solid source at 900 °C for 1 h or doped with Eu during growth have been characterized. The ZnO nanowires were grown by chemical vapor deposition on Si substrates employing Au as a catalyst. The diameter of the resulting nanowires was ∼200 nm with a length of 1 μm. Photoluminescence spectra excited by a He–Cd laser at room temperature showed the green luminescence at 515 nm in Eu-diffused nanowires. A small red shift of near-band-edge emission of ZnO nanowires was observed in the diffused wires, but sharp emission from Eu 3 ions was not present. Transmission electron microscopy shows crystalline Eu 2O 3 formation on the diffused nanowire surface, which forms a coaxial heterostructure system. When Eu was incorporated during the nanowire growth, the sharp 5D O– 7F 2 transition of the Eu 3+ ion at around 615 nm was observed.

Fabrication, characterization and photocatalytic activity of La-doped ZnO nanowires

La-doped ZnO nanowires with different doping concentrations were synthesized via a solvothermal synthesis route, using ethanol as the solvent. The products were characterized by X-ray diffraction (XRD), field scanning electron microscopy (FESEM) together with an energy dispersion X-ray spectrum (EDS) analysis, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Raman spectrum, X-ray photoelectron spectrum (XPS) and UV–vis spectroscopy. The results of XRD, EDS and XPS revealed that La 3+ was successfully doped into ZnO lattice. The observation of TEM and FESEM showed that the obtained La 3+-doped ZnO nanowires were homogenous with an average diameter of 15–25 nm. The photocatalytic activity of pure and La 3+-doped ZnO samples was studied by the degradation of Rhodamine B (RhB). The result showed that La 3+ doping concentration had a remarkable effect on the efficiency of photocatalytic activity, with the optimal doping content being determined to be 2 at% in terms of the photocatalytic activity efficiency.

Pre-avalanche Ultraviolet Photoconduction Properties of Transitional-Metal-Doped ZnO Nanowires

The ultraviolet (UV) photoelectric characteristics of transitional metal (Cu) doped ZnO nanowires produced by the self-catalytic vapor–liquid–solid (VLS) method were investigated by performing a series of photoconduction and time-resolved measurements. The photocurrent voltage characteristics obtained on the nanowires configured as two-terminal metal–semiconductor–metal photodetectors exhibited a nonmonotonic behavior attributed to the interplay of several limiting mechanisms: Schottky contacts and trapping/detrapping effects that take place at low and intermediate (pre-avalanche) bias regimes, respectively. In the intermediate biases, the photocurrent was power-law dependent, i.e., changed with voltage as \( I \propto V^{3.3}\hbox{,} \)\( I \propto V^{3.0}\hbox{,} \) and \( I \propto V^{2.8} \) for excitation wavelengths of 365 nm, 302 nm, and 254 nm, respectively. The dependence of the exponent on the wavelength of the light is analyzed and explained based on the detailed consideration of the contribution of different deep-defect Cu levels formed within the band gap of ZnO. The study will be important to those working in the area of ZnO-based nanophotodetectors, optical switches, and sensors.

Electrical and optical properties of single As-doped ZnO nanowire field effect transistors

ZnO nanowires with the average diameter of 20 nm and the length of longer than 10 μm were synthesized on GaAs substrate by chemical vapor deposition（CVD）. As-doped ZnO nanowires were obtained by annealing the samples at 600 ℃ for 30 min in oxygen. Single ZnO nanowire field effect transistors（FET） were fabricated by electron beam exposure and magnetron sputtering deposition and Ti/Au deposited as ohmic-contact electrodes. Based on the electrical properties of the single ZnO nanowire FET before and after annealing, we verified that p-type ZnO nanowire can be obtained by As doping effectively. The parameters of the single As doped ZnO nanowire FET were as follows: the transconductance was 35 nA/V, the hole density was 1.4×1018 cm-3, and the mobility was 6.0 cm2/V·s. We also obtained the photoluminescence spectrum of single As-doped ZnO nanowire: strong ultraviolet light at 383 nm, weaker yellow-green light, and red light due to the existence of AsZn-2VZn shallow acceptors.

Non-catalytic growth of high-aspect-ratio Sb-doped ZnO nanowires by simple thermal evaporation process: Structural and optical properties

Well-crystallized high-aspect-ratio antimony (Sb)-doped ZnO nanowires have been successfully synthesized on Si(1 0 0) substrates in a large-quantity via simple thermal evaporation process by using metallic zinc and Sb powders in the presence of oxygen. It is observed from the detailed structural characterizations that the grown nanowires are well-crystallized with the wurtzite hexagonal phase and preferentially grown along the [0 0 0 1] direction. It was clearly seen from the high-resolution TEM images that the Sb-atoms are successfully doped into the lattices of ZnO nanowires. The room-temperature photoluminescence (PL) spectrum exhibited a broad band in the visible region with a suppressed UV emission, indicating the presence of structural defects due to insertion of Sb-atoms in the lattices of as-grown nanowires. Due to the enhancement of green emission in the formed nanowires, these structures show great interest for typical applications of ZnO-based phosphors, such as field emissive display technology, etc.

Preparation and Properties of Cobalt Doped ZnO Nanowires

The template method was used for preparing Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> O nanowires with x ranging from 0.01 to 0.05. Thus, electrochemical deposition was employed for filling up the pores of polycarbonate ion track nanoporous membranes with the desired material. The method allows a good control over the morphology and composition of the deposited nanowires, as evidenced by scanning electron microscopy (SEM) and energy dispersive X ray analysis (EDX). Measurements of the magnetic properties showed a paramagnetic behavior of the nanowire arrays for the whole set of temperatures and Co concentrations.

Preparation and Properties of Transition Metal Doped ZnO Nanowires

We employed template replication by electrochemical deposition as the technique to prepare transition metals doped zinc oxide nanowires. Polycarbonate nanoporous membranes obtained by swift heavy ions irradiation and subsequent etching were used as templates. The pores were filled with the desired material by electrochemical deposition from an aqueous bath. The properties of the nanowires were investigated by scanning electron microscopy, energy dispersive X ray analysis and optical spectroscopy. We investigated the effect of preparation condition (i.e. deposition potential, deposition bath composition) on nanostructures properties

Raman and photoluminescence properties of highly Cu doped ZnO nanowires fabricated by vapor-liquid-solid process

Highly Cu doped ZnO nanowires have been fabricated by vapor-liquid-solid (VLS) process. The average concentration of Cu in the ZnO nanowires is estimated to be 6 at. %. The ultrafine synthesized nanowires have diameters nearly 80 nm, while their average length lies in the range of 40 to 90 mum. Raman spectroscopy shows that the Cu doped ZnO nanowires have a typical wurtzite structure. High resolution transmission electron microscopy (HRTEM) investigations of individual nanowires demonstrate that the nanowires have single crystalline structure in which the growth direction is oriented along the c axis. Room temperature photoluminescence spectrum of as prepared nanowires shows two emissions in UV and visible regions that can be ascribed to the near band edge (NBE) transition and defects respectively, while the spectrum of the annealed nanowires exhibits a red shift in UV and a suppression in visible bands. Furthermore, the low temperature (10 K) PL spectrum illustrates a novel dominant blue emission relating to the different valence states of Cu atoms in ZnO, which is explained on the basis of Dingle model.

Preparation and Properties of Transition Metal Doped ZnO Nanowires

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Synthesis and characterization of Zn1−xMnxO nanowires

Mn doped ZnO nanowires (NWs) were fabricated by a one-step vapor-solid process at 500°C. The doped Mn exists in the wurtzite lattice as substitutional atom without forming secondary phases. X-ray absorption near-edge structure reveals that the doped Mn atoms occupy the Zn sites, and they lead to an expansion in lattice constants. The I-V characteristic of a single Zn1−xMnxO NW shows a typical Ohmic contact with gold electrodes. The as-received NWs could be suitable for studying spintronics in one-dimensional diluted magnetic semiconductors.

Temperature dependent photoluminescence study on phosphorus doped ZnO nanowires

We report temperature dependent photoluminescence studies on phosphorus doped ZnO nanowires. The shape of the spectra is very similar to those of phosphorus doped ZnO films. The photoluminescence spectrum at 10K is dominated by neutral acceptor bound exciton (AX0) emissions. The acceptor binding energy determined also agrees with the corresponding value in phosphorus doped films. Studies on the AX0 intensity show two quenching channels, associated with the thermal dissociations of AX0 to a free exciton and of shallow residual donors. The residual donors revealed provide a clue for the difficulty in p doping of ZnO.

Acceptor related photoluminescence from ZnO:Sb nanowires fabricated by chemical vapor deposition method

Single-crystal Sb doped ZnO nanowires were fabricated on Si (1 0 0) substrate by a chemical vapor deposition method. The photoluminescence properties of ZnO nanowires were studied with the temperature ranging from 81 to 306 K. At 81 K, the recombination of the acceptor-bound exciton was predominant in PL spectrum, which was attributed to the transition of the (Sb Zn–2V Zn) complex bound exciton. The activation of A 0X and the acceptor binding energy had been calculated to be 16.8 and 168 meV, respectively.

Nanomaterial electronic structure investigation by valence electron energy loss spectroscopy—An example of doped ZnO nanowires

The effects of doping (by ion implantation) on the electronic structure of ZnO nanowires, particularly on the defect states generation in the band gap of ZnO, are investigated using valence electron energy loss spectroscopy (VEELS) performed in a transmission electron microscope (TEM). The improved spectrum energy resolution via the introduction of a gun monochromator, together with the reduced intensity in the zero loss peak tail as realized by spectrum acquisition at non-zero momentum transfer, enable us to extract such electronic structure information from the very low loss region of the EEL spectra. We have compared the doping effects of several dopant elements, i.e., Er, Yb, and Co, and found that generation of the band tail states (∼2–3.3 eV) is a common consequence of the ion implantation process. On the other hand, specific mid-gap state(s) in the lower energy range are created only in the rare earth element doped ZnO nanowires, suggesting the dopant-sensitive nature of such state.

Phosphorus acceptor doped ZnO nanowires prepared by pulsed-laser deposition

Phosphorus-doped ZnO (ZnO:P) nanowires were successfully prepared by a novelhigh-pressure pulsed-laser deposition process using phosphorus pentoxide as the dopantsource. Detailed cathodoluminescence studies of single ZnO:P nanowires revealedcharacteristic phosphorus acceptor-related peaks: neutral acceptor-bound exciton emission(A0, X, 3.356 eV), free-to-neutral-acceptor emission (e,A0, 3.314 eV), and donor-to-acceptor pair emission (DAP,∼3.24 and ∼3.04 eV). This means that stable acceptor levels with a binding energy of about 122 meV havebeen induced in the nanowires by phosphorus doping. Moreover, the induced acceptors aredistributed homogeneously along the doped nanowires.