ZnO Nanocolumns Research Articles

Ultra‐Inclined Nanocolumnar ZnO Films Sputtered Using a Novel Masking Configuration Providing Controlled and Restricted Oblique Angle Deposition for Enhanced Sensing Platforms

AbstractOblique angle deposition (OAD) of inclined thin films is mainly performed using electron beam evaporation due to its accurate point source control over the incoming evaporated flux angle α, leading to thin films with a nanocolumnar inclination angle β. However, the utilization of magnetron sputtering (MS) with an extended source for OAD is not extensively studied and reported. This work presents a thorough analysis of ZnO inclined thin films deposited by a novel restricted DC‐reactive MS‐OAD technique. OAD‐inclined films are deposited at α ranged 60°‐88°, where incoming flux is restricted using a patented masking configuration enabling tunable control of deposited nanocolumn angular range. The described technique provides accurate control over the resulting β (99.5% reproducibility), allowing demonstrated βmax of 47.3°, close to theoretical limits predicted for ZnO. The approach discussed here probes enhanced control of β comparable to that observed in evaporation, however using an extended source, resulting in high‐quality reproducible nanocolumnar‐inclined films. The mentioned improvements result from the exploration of operational parameters such as magnetron power, working pressure, and chamber temperature, as well as the design of the restricting configuration and substrate holders and their influence on the resulting inclined thin film crystallinity, and morphology.

Enhanced UV emission and photocatalytic activity due to morphology evolution of ZnO thin films

Tuning the optical properties of nanomaterials through morphology evolution has attracted extensive attention. In this study, ZnO films were deposited on substrates using sol-gel approach, and ZnO nanocolumn arrays were formed on the film surface induced by doped ions. XRD results show that the nanocolumns all have a growth orientation along the c-axis and good crystallization quality. Compared with pure ZnO film, the ones with nanocolumn arrays on the film surface exhibit higher UV emission efficiency and photocatalytic activity. The enhanced UV emission efficiency is attributed to the higher crystalline quality and fewer surface defects of the nanocolumn arrays. The improved photocatalytic activity is mostly ascribed to two factors: (1) nanocolumns are more conducive to the migration of photogenerated electrons and holes to the photocatalyst surface to participate in redox reactions; (2) the formation of nanocolumn arrays improves light absorption of the ZnO films.

Structural, morphological, and luminescent properties of ZnO nanocolumns growth over ZnO NPs seed layer deposited at different temperature

Structural, morphological, and luminescent properties of ZnO nanocolumns growth over ZnO NPs seed layer deposited at different temperature

Correlation of conductivity enhancement and Al-site defects in nanocolumnar ZnO films under vacuum annealing by experimental and calculations

The variation of Al-site defects was experimentally and theoretically investigated to explain the significant conductivity enhancement of vacuum-annealed Al-doped nanocolumnar ZnO (AZO) films, as compared to as-deposited AZO films. From the analyses of XPS and XAS, several defects including Al substitution (AlZn), Al interstitial site (Ali), and Zn vacancy (VZn) were detected in the as-deposited AZO films. After annealing, the transition from 2AlZn-VZn-Ali to AlZn-Ali complex defects was found. Moreover, FT-EXAFS also indicated the significant decrease of VZn in the annealed films. The formation energy computed by DFT calculation suggested that the AlZn-VZn-Ali complex defects could be formed in the as-deposited films. By heat treatment, an increase of AlZn is expected because more Ali could substitute to the VZn site forming the AlZn-Ali complex defects in the annealed films. The DOS calculation, the presence of Ali defects induced donor states in the AZO structure while and additional VZn acted as an acceptor state which diminished the Ali states. Hence, the increase of carrier concentration in the annealed films can be explained by the reduction of VZn defects.

The effects of copper doping on morphology and room-temperature photoluminescence of ZnO nanocolumns.

In this study, a versatile vapor phase transport method for the synthesis and copper-doping of ZnO nanocolumns is demonstrated. Doping percentage (up to 5%) showed no effect on the wurtzite structural phase of ZnO nanocolumns. However, a decrease in nanocolumn diameter (cross-sectional length of longest side or diagonal) due to doping was observed by scanning electron microscopy. Reduced rate of electron-hole recombination was inferred from a decrease in the intensity of the near-band edge emission peak shown in room-temperature photoluminescence spectra. Expression of structural defects in both doped and undoped nanocolumns suggest p-type conductivity. Observed copper-doping effects show promise for utilizing such structures as electrode components in dye-sensitized solar cells.

Micro-Raman and micro-Photoluminescence study of ZnO thin films

We present the experimental results of optical analysis of nanostructured ZnO thin films grown onto commercial glass by reactive sputtering. Films with 20, 50, and 100 nm in thickness were analyzed by micro-Raman and micro-photoluminescence spectroscopies. Raman and photoluminescence bands were deconvoluted with Lorentzian profiles, in order to obtain information about response of films to excitation with laser light, occurring changes in position, full width half maximum (FWHM), and area of each phonon and emission bands of ZnO, correlating them with its nanostructure nature, and packing morphology of ZnO nanocolumns.

Sm3+ driven enhancement in photocatalytic degradation of hazardous dyes and photoluminescence properties of hexagonal-ZnO nanocolumns

Samarium doped ZnO (Sm-ZnO) nanocolumns were grown by wet chemical method and the doping effect of Sm3+ on visible light photocatalytic (PC) and photoluminescence (PL) properties of ZnO was investigated. Methylene blue (MB) and methyl orange (MO) were considered for the degradation study as a step initiated towards the remediation of industrial wastewater. Subsequent characterization studies by x-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) enumerate that the hexagonal-wurtzite structured Sm-ZnO is highly crystalline in nature and possesses hexagonal column like nano-architectures. Although, a charge imbalance exists between the ions, the substitutional effects of Sm3+ at Zn2+ sites have been observed from the XRD spectra and discussed by suitable mechanisms. FTIR measurement gives the information on the evolution of peaks related to metal-oxygen bond in Sm-ZnO which may be linked with Sm ions. PC measurement shows that the degradation efficiency of 95% can be achieved by Sm-ZnO photocatalysts in degrading MB and 91% for MO. Sm doping induces high charge separation efficiency and generation of OH ions in ZnO leading to such improvisation in degradation efficiency. The prepared Sm-ZnO nanocatalysts possess high degree of photostability and reusability even after fourth cycle of photodegradation. PL spectra show the suppression of the sharp and intense excitonic emission band of ZnO in Sm-ZnO due to low rated direct recombination of carriers. Incorporation of Sm3+ ends up with intrinsic defect mediated enhancement in the visible emissions especially in the blue, yellow and red region of light spectrum.

Soft-lithographically line-patterned In-doped ZnO quantum dots with hydrothermally grown ZnO nanocolumns for acetone detection

We introduced a new approach to enhance the sensing performance of sensors based on metal oxide semiconductors. We synthesized In-doped ZnO quantum dots (IZO QDs) by a hydrothermal method and fabricated line-patterned IZO QD layers with polydimethylsiloxane (PDMS) molds. Additional hydrothermal growth was conducted to create ZnO nanocolumns (NCs) on the patterned surfaces. Thus, drop-cast and line-patterned sensors and line-patterned samples with NCs grown for 0.5 h (NC(0.5 h)/Line) and 4 h (NC(4 h)/Line) were prepared. Among these four different sensors, the NC(0.5 h)/Line sensor exhibited an excellent response of 26,000 with fast response times of less than 1 s to 10 ppm of acetone. In addition, the detection limit was approximately 0.1 ppm of acetone, and the resistance was almost constant even after repeatability tests. According to UV–vis and X-ray photoelectron spectroscopic analyses, the extraordinary sensing characteristics of NC(0.5 h)/Line were mainly because this sensor had the largest optical band energy and the highest ratio of oxygen vacancies among the tested sensors. On the other hand, the NC(4 h)/Line sensor showed the lowest response of the four sensors. During the long-term growth of NCs, OH groups are produced on the surface of the material, and Zn(OH)2 NCs are formed instead of ZnO NCs, resulting in a large decrease in the carrier concentration and active sites. In conclusion, we developed highly sensitive acetone sensors by constructing special morphologies.

ZnO nanocolumns synthesized by chemical bath process and spray pyrolysis: Ultrasonic and mechanical dispersion of ZnO seeds and their effect on optical and morphological properties

Hexagonal ZnO nanocolumns were synthesized by chemical bath deposition on previously deposited ZnO nanoseeds on Corning glass substrates, at deposition temperatures from 300 to 550 °C in steps of 50 °C, by ultrasonic spray pyrolysis technique. The ZnO nanoseeds were previously suspended in the spraying solution, with and without ultrasonic dispersion. Scanning Electron Microscopy study of surface morphology was carried out revealing the formation of hexagonal ZnO nanocolumns; the smaller diameters were obtained for ZnO seed layers deposited at substrate temperature between 400 and 450 °C. The best vertical alignment was observed on nanocolumns grown from ultrasonic-dispersed ZnO nanoseeds deposited at 400 °C. Raman spectroscopy was used to evaluate the crystal quality of the ZnO nanocolumns, a relationship was determined between nanocolumns stress with a shift to lower wavenumbers of E2H mode, the ZnO seeds deposition temperature and the nanocolumn diameters. Intrinsic defects of ZnO nanocolumns were studied with photo and cathodoluminescence spectroscopy. Also, photoluminescence life time measurements were carried out. Samples grown from ultrasonically dispersed ZnO seeds generated nanocolumns with greater stoichiometry and purity.

Fabrication of high responsivity deep UV photo-detector based on Na doped ZnO nanocolumns

We report a variety of the hydrothermally synthesized ZnO nanostructures with a significant suppression in defect-related emission and huge enhancement in the photo-current to the dark current ratio (approximately six orders of magnitude) upon UV light illumination. Interestingly, the photo-detector shows lower dark current of 1.6 nA with high responsivity of 507 A W−1 at 254 nm. Here, a systematic analysis of the growth process as well as the physical, chemical and electrical properties of as-grown ZnO nanostructures has been performed. We have utilized the duo effect of both the inorganic (KMnO4) and organic (Na3C6H5O7) additives, which has facilitated the precise tuning of the morphology and intrinsic defects in nanostructures that have made an impact on the photo-responsivity, photoluminescence (PL) and adhesivity of the film on to the underlying substrate. PL analysis of as-grown ZnO nanostructures has suggested 11 times improvement in the near band emission (NBE) to defect level emission (DLE) ratio. Interestingly, thermal annealing of the samples has shown a dramatic change in the morphology with significant improvement in the crystallinity. Notably, the band gap was observed to be modulated from 3.3 eV to 3.1 eV after annealing. In addition to UV photo-detector based applications, the work presented here has provided a subtle solution towards the rectification of various problems pertaining to hydrothermal processes like poor adhesivity, feeble UV emission and problem in precise tuning of the morphology along with the bandgap in one go. Therefore, these investigations assume critical significance towards the development of next-generation optoelectronic devices.

Electrically pumped random laser from ZnO nanocolumn based on back-to-back Schottky structure

Electrically pumped random laser has been achieved in a Au/ZnO nanocolumn/Au structure, which is back-to-back Schottky barrier. The origin of the lasing is that the electron from the reverse-biased Au/ZnO Schottky contact recombines radiatively with the hole from the forward-biased Au/ZnO Schottky contact, then the generated light is amplified in the ZnO nanocolumn, ultimately random lasing is realized. The threshold of the lasing is about 2 A/cm2, which is among the lowest threshold current of ZnO random lasing. The lasing peaks are randomly distributed between 375 and 420 nm, whose full width at half maximum (FWHM) can reach as narrow as 2 nm. The results reported in this paper indicate that ZnO nanocolumn based on back to back Schottky sturcture is suitable for achieving random laser.

Role of the radio frequency magnetron sputtered seed layer properties on ultrasonic spray pyrolyzed ZnO thin films

Herein, we investigated structural and electrical properties of ultrasonic spray pyrolyzed ZnO thin films (ZnO(USP)) deposited on radio frequency sputtered ZnO nanocolumns (ZnO(Seed)) under various oxygen atmosphere on p-Si substrates. X-ray diffraction data of the samples showed that the samples had a hexagonal structure with a strong (002) preferred orientation. The grain size of ZnO(USP) samples prepared on the seed layer sputtered in 25% and 50% oxygen atmosphere decreased and surface morphology was nanopebbles. ZnO samples prepared on the seed layers exhibited sharp and predominant ultraviolet luminescence at approximately 380nm. The current–voltage characteristics of the n-ZnO(USP)/n-ZnO(seed)/p-Si heterojunctions were significantly affected by the seed layer preparation conditions. Backward diode behavior was observed for the n-ZnO(USP)/n-ZnO(seed)/p-Si heterojunctions with high donor concentration, in which the seed layers prepared in 100% and 75% argon atmosphere. The carrier concentration value was decreased for the films with the seed layer, obtained at higher oxygen content in the sputtering atmosphere.

Electronic structure, optical and magnetic studies of PLD-grown (Mn, P)-doped ZnO nanocolumns at room temperature

We prepared well-aligned Zn1−xMnxO:yP nanocolumns (x = 0–0.02, and y = 0 and 1 mol%) on SiO2/Si(0 0 1) substrates by using pulsed laser deposition (PLD) and then investigated their electronic structure and optical and magnetic properties at room temperature. The analyses of x-ray photoelectron and x-ray absorption fine structure spectra revealed Mn2+ and/or P ions existing in nanocolumns, where Mn2+ ions are situated in the Zn2+ site of the ZnO-wurtzite structure. Although the incorporation of Mn2+ and/or P ions did not form secondary phases, as confirmed by x-ray and electron diffraction patterns, more lattice defects were created, and consequently changed the band-gap energy as well as the electron–phonon interactions in the nanocolumns. Magnetization versus magnetic-field measurements revealed that all the samples exhibited FM order. In particular, the (Mn, P) co-doping with x = 0.02 and y = 1 remarkably enhanced the magnetic moment up to 2.92 µB/Mn. Based on the results obtained from analyzing the electronic structures, UV–Vis absorption and resonant Raman scattering spectra, and theoretical calculations, we believe that the enhancement of the FM order in (Mn, P)-doped ZnO nanocolumns is due to exchange interactions taking place between vacancy-mediated Mn2+ ions.

Photoluminescence study of Eu+3 doped ZnO nanocolumns prepared by electrodeposition method

Europium doped ZnO (ZnO:Eu) nanocolumns were grown on indium tin oxide substrates (ITO) by electrodeposition method in zinc nitrate aqueous solution with different concentration of Eu. According to XRD results electrodeposited ZnO:Eu films exhibit thigh crystalline quality with a preferential orientation parallel to c-axis. Moreover, the (002) peak position shifts toward to lower angles confirming the incorporation of Eu+3 ions into the ZnO lattice. The crystallite size of a pure ZnO decreases when increasing the Eu doping. SEM images reveal a well dense nanocolumns arrays perpendicular to the surface. Furthermore, doping with Eu+3 ions leads to a change in the top of nanocolumns from hexagonal to conic-hexagonal shape. The average diameters of nanocolumns are found to be around 200nm and 180nm for undoped ZnO and Eu doped ZnO, respectively. Photoluminescence (PL) emission from ZnO:Eu thin films reveals the typical peaks related to excitons and intrinsic defects in ZnO and four narrow peaks in the red region attributed to 4f-shell transition in Eu+3 ions. As Eu doping increased, PL peaks related to Eu+3 emission showed an increase in intensity and a shift in its wavelength position. A schematic drawing of the excitation mechanism and related emission process is proposed.

Study of the surface properties of ZnO nanocolumns used for thin-film solar cells.

Densely packed ZnO nanocolumns (NCs), perpendicularly oriented to the fused-silica substrates were directly grown under hydrothermal conditions at 90 °C, with a growth rate of around 0.2 μm/h. The morphology of the nanostructures was visualized and analyzed by scanning electron microscopy (SEM). The surface properties of ZnO NCs and the binding state of present elements were investigated before and after different plasma treatments, typically used in plasma-enhanced CVD solar cell deposition processes, by X-ray photoelectron spectroscopy (XPS). Photothermal deflection spectroscopy (PDS) was used to investigate the optical and photoelectrical characteristics of the ZnO NCs, and the changes induced to the absorptance by the plasma treatments. A strong impact of hydrogen plasma treatment on the free-carrier and defect absorption of ZnO NCs has been directly detected in the PDS spectra. Although oxygen plasma treatment was proven to be more efficient in the surface activation of the ZnO NC, the PDS analysis showed that the plasma treatment left the optical and photoelectrical features of the ZnO NCs intact. Thus, it was proven that the selected oxygen plasma treatment can be of great benefit for the development of thin film solar cells based on ZnO NCs.

Development and surface characterization of a glucose biosensor based on a nanocolumnar ZnO film

Highly oriented nanostructured ZnO films were grown on the surface of stainless steel plates (ZnO/SS) by chemical bath deposition (CBD). The films consisted of vertically aligned ZnO nanocolumns, ∼1μm long and ∼80nm wide, as observed by SEM (scanning electron microscopy) and FIB (focused ion beam). XRD (X-ray diffraction) confirmed the c-axis preferred orientation of the ZnO columns, which were functionalized with the glucose oxidase (GOx) enzyme into a biosensor of glucose. The electrochemical response studied by CV (cyclic voltammetry) proved that the biosensor was capable of detecting glucose from 1.5 up to 16mM concentration range. XPS (X-ray photoelectron spectroscopy) analysis, excited with synchrotron radiation, probed the atom specific chemical environment at the electrode’s surface and shed some light on the nature of the ZnO-GOx interaction.

Photoluminescent properties of electrochemically synthetized ZnO nanotubes

ZnO nanotubes were prepared by a sequential combination of electrochemical deposition, chemical attack and regeneration. ZnO nanocolumns were initially electrodeposited on conductive substrates and then converted into nanotubes by a process involving chemical etching and subsequent regrowth. The morphology of these ZnO nanocolumns and derived nanotubes was monitored by Scanning Electron Microscopy and their optical properties was studied by photoluminescence spectroscopy. Photoluminescence were measured as a function of temperature, from 6 to 300K, for both nanocolumns and nanotubes. In order to study the behaviour of induced intrinsic defect all ZnO films were annealed in air at 400°C and their photoluminescent properties were also registered before and after annealing. The behaviour of photoluminescence is explained taking into account the contribution of different point defects. A band energy diagram related to intrinsic defects was proposed to describe the behaviour of photoluminescence spectra.

Effect of annealing temperature on the structural, optical and electrical properties of ZnO thin films grown chemically on PS substrate

ZnO thin films were successfully produced on porous silicon (PS) substrates by a chemical bath deposition method. ZnO thin films were then annealed at 300, 500, and 700 °C for 20 min in nitrogen (N2) atmosphere. X-ray diffraction, field emission scanning electron microscopy (FESEM), and photoluminescence (PL) were utilized to investigate the effect of post-annealing temperature on the structural, optical, and electrical properties of ZnO thin films. The lattice constant, full width at half maximum, and strain (the grain size) of (0 0 2) peak of ZnO thin films decreased (increased) with increasing the annealing temperature, respectively. The FESEM images revealed that the ZnO thin films are compact array ZnO nanocolumns and were perpendicularly grown to the PS substrate. The biggest ratio of the PL intensity of UV emission to that of visible emission is observed from ZnO thin films annealed at 500 °C. The PL results of the ZnO thin films exhibit that the UV peak positions shift slightly toward lower wavelengths with increase of the annealing temperature. The current–voltage measurements demonstrate that the current level increases as the annealing temperature increases to 500 °C, and then decreases when the temperature further increases up to 700 °C.

Structural and Photoluminescence Properties of Mn-doped ZnO Nanocolumns Grown by Cathodic Electrodeposition

Mn-doped ZnO nanocrystal was prepared on Si substrates by cathodic electrodeposition. X-ray diffraction (XRD) study suggests that the Mn ions are substituted for Zn position into ZnO crystal. Field-emission scanning electron microscopy (SEM) images show the column-like morphology of ZnO nanocrystal. In photoluminescence spectrum, the visible emission band is broadened because of the doping effect. The fitted result of the temperature-dependent PL spectra in the range from 122.8 K to 302.2 K indicates that Mn dopant could lead to the decrease of the exciton binding energy.

Effect of oxygen flow rate on the ultraviolet sensing properties of zinc oxide nanocolumn arrays grown by radio frequency magnetron sputtering

Highly transparent metal–semiconductor–metal ultraviolet (UV) photoconductive sensors were fabricated using thin (less than 100nm in thickness), dense, small-diameter ZnO nanocolumn arrays prepared via low-power, catalyst-free radio frequency (RF) magnetron sputtering at different oxygen flow rates ranging from 0 to 25sccm. The FESEM images revealed the average nanocolumn diameter decreased with increasing oxygen flow rate. The transmittance spectra show that with the introduction of oxygen, the transmittance of the nanocolumn arrays in the visible region improves relative to that of a film prepared in the absence of oxygen with values greater than 95%. The UV responsivity and sensitivity were significantly improved for sputtered ZnO nanocolumn arrays prepared at oxygen flow rates up to 10sccm, with the highest values of 9.70mA/W and 2.20×104. Furthermore, the responsivity and sensitivity decreased at oxygen flow rates greater than 10sccm, which can be attributed to the increased electrical resistance of the nanocolumn arrays. Our findings indicate that a high-performance UV photoconductive sensor can be realised using very thin sputtered ZnO nanocolumn arrays and that such a sensor would exhibit high sensitivity.