Nanocolumnar Films Research Articles

Optical properties of Sm-doped ceria nanostructured films grown by electrodeposition at low temperature

Nanostructured undoped and samarium doped ceria thin nanocolumnar films are electrodeposited onto (FTO) glass substrates at low-temperature (30°C) with a subsequent thermal annealing at 600°C for 1h. Films are obtained from mixed Sm3+/Ce3+ aqueous nitrate solutions, applying a −0.8V/(SCE) potential for 1h. Cubic fluorite type ceria nanostructured films of high crystal quality are synthesized as confirmed by X-ray diffraction and Raman spectroscopy. SEM analysis demonstrates that doping with Sm improves the quality of the film with respect to crack formation. The incorporation and activation of the Sm3+ ions in the ceria host as well as the Stark splitting of the manifolds responsible for emission in the red-orange spectral range are investigated by means of photoluminescence spectroscopy.

Synthesis and magnetic properties of nanocolumnar nickel films deposited in argon-nitrogen atmosphere

Ferromagnetic nickel films with a structure comprising nanocolumns grown perpendicular to a substrate have been obtained by magnetron sputtering of a nickel target in argon atmosphere containing 2 vol % nitrogen. Measurements of X-ray diffraction and investigation of magnetic properties showed that the obtained films consist of a solid solution of nitrogen in nickel. The saturation magnetization of the films is 1.5–2 times smaller and their Curie temperature is 30 K lower than the corresponding values for pure nickel. The film material exhibits a dimensional effect: as the grain size decreases below 10 nm, the magnetization sharply drops; at a grain size of 0.5 nm, the film possesses no magnetic order.

Charge collection properties of dye-sensitized solar cells based on 1-dimensional TiO2 porous nanostructures and ionic-liquid electrolytes

Abstract Dye solar cells consisting of mesoporous TiO 2 nanocolumnar films sensitized with a highly absorptive indoline dye were studied to estimate the charge collection efficiency provided by porous 1-dimensional (1-D) nanostructures in combination with viscous, fast-recombining electrolytes. The TiO 2 mesoporous nanostructured films were prepared by physical vapor deposition at glancing incidence (GLAD-PVD). Electrochemical Impedance Spectroscopy (EIS) in the dark was utilized to extract the transport and recombination properties of the fabricated devices. Due to their high porosity, the TiO 2 nanocolumnar electrodes incorporated a dye amount similar to that admitted by nanoparticulated electrodes with higher thickness. This fact, together with the longer lifetimes of electrons obtained for the GLAD-PVD electrodes, lead to an overall improvement of the charge collection and photovoltaic properties with respect to randomly packed electrodes. Measured diffusion lengths were improved by a factor between 2 and 3 with respect to the disordered nanostructure. The present results demonstrate the capability of partially ordered nanostructures to improve charge collection in devices constructed with fast-recombining electrolytes.

Morphology and structural properties of nanocrystalline and nanocolumnar aluminum thin films grown on glass and Ti/glass substrates

Nanocrystalline and nanocolumnar aluminum thin films were deposited on glass and Ti/glass substrates at normal and oblique angles of incidence in vacuum (1.0 × 10−6 Torr) by using an electron-beam evaporator. The average grain size of the Al thin films produced at a normal angle of incidence increased from 25 nm to 50 nm when the substrate was changed from glass to Ti/glass. The average aspect ratio of the aluminum nanocolumns (Al nanorods) grown on the glass substrate increased from 1.65 to 3.65 as the deposition angle increased from 65° to 85° while the aspect ratio of Al nanorods grown on Ti/glass substrates increased from 1.80 to 4.00 as the deposition angle increased from 65° to 85°. The X-ray diffraction pattern showed a broad baseline in nanocrystalline and nanocolumnar thin films. In the case of Al thin films on glass, the full width at half maximum (FWHM) was observed to be 14.34 for all depositions while in the case of Al thin films on Ti/glass, the FWHM was decreased from 17.66 to 14.02 as the deposition angle increased from 0° to 85°. The X-ray photoelectron spectroscopy analysis confirmed that the peak for Al nanorods was observed at a binding energy of 74.44 eV.

Tilting angle of nanocolumnar films fabricated by oblique angle deposition

The tilting angle, β, of nanocolumns during oblique angle deposition (OAD) as a function of vapor incident angle, θ, has been investigated through both literature survey and systematic experiments. Most experimental data cannot be fit by either the tangent or cosine rule and show strong material dependence. The material-dependent fan angle, φ, can be extracted using the fan-out model proposed by Tanto et al. [J. Appl. Phys. 108, 026107 (2010)]. This fan angle, φ, depends closely on the melting temperature for single element materials, the heat of formation for compound materials, lattice structure, and the complexity of the materials, and is a critical parameter in understanding the fundamental growth mechanism of OAD.

Effect of in situ ultraviolet irradiation on the formation of nanostructural carbon nitride films

AbstractWe report the effect of in situ ultraviolet (UV) irradiation on the formation of carbon nitride CNx films. The nanocolumnar films were produced under UV irradiation without a metal catalyst by dc magnetron sputtering of a graphite target in an N2 atmosphere with 3% of O2 on a quartz glass substrate. It was shown that films have dense nanostructure with columns of 60–80 nm in diameter and with nitrogen concentration of ∼4%. Scanning electron microscopy (SEM) and X‐ray photoemission spectroscopy (XPS) data show the coexistence of amorphous and columnar phases. In situ UV irradiation was found to be effective for growing the pure nanocolumnar structure of carbon nitride by etching the amorphous phase.

Effect of Si Doping and Porosity on Hematite’s (α-Fe2O3) Photoelectrochemical Water Oxidation Performance

The photoelectrochemical water oxidation performance under simulated solar irradiation of hematite (α-Fe2O3) films synthesized by coevaporation of pure Si and Fe in an oxygen ambient, a process known as reactive ballistic deposition, is studied as a function of Si doping level and film porosity, ranging from dense films to nanocolumnar films. It is found that Si segregates to the hematite surface, does not improve the bulk conductivity, and lowers the optical absorption coefficient. Nevertheless, the photoelectrochemical performance of Si-doped, porous films is significantly improved relative to undoped, porous films. However, the improvement relative to dense, undoped films is marginal. It is concluded that Si acts to passivate the hematite surface and aids charge transfer to the solution. Additionally, from incident photon conversion efficiency measurements it is found that Si doping and porosity have little effect on the normalized spectral response of 100 nm thick hematite films.

Thin Film Nanomorphology Tailored by Physical Vapor Deposition

Shadowing growth by glancing angle deposition (GLAD) has been providing self-assembled nanostructures over much larger area for much lower costs since much earlier than the recent advanced top down processes do. In these two decades, significant progress has been made in the development of the well-controlled 3D nanomorphologies such as zigzag and helix. Much effort for theoretical and numerical understanding of the growth mechanism has been also paid in order to improve the morphology. Many researches in academia have been investigating useful properties of nanocolumnar thin films in their laboratory. On the other hand, most companies seem hesitate to introduce GLAD technique into the factory due to the prejudice that the obliquely deposited thin films are not durable and reproducible. In this article, we discuss the progress in glancing angle deposition technology for the practical applications.

Flexible and mechanical strain resistant large area SERS active substrates

We report a cost effective and facile way to synthesize flexible, uniform, and large area surface enhanced Raman scattering (SERS) substrates using an oblique angle deposition (OAD) technique. The flexible SERS substrates consist of 1 μm long, tilted silver nanocolumnar films deposited on flexible polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET) sheets using OAD. The SERS enhancement activity of these flexible substrates was determined using 10(-5) M trans-1,2-bis(4-pyridyl) ethylene (BPE) Raman probe molecules. The in situ SERS measurements on these flexible substrates under mechanical (tensile/bending) strain conditions were performed. Our results show that flexible SERS substrates can withstand a tensile strain (ε) value as high as 30% without losing SERS performance, whereas the similar bending strain decreases the SERS performance by about 13%. A cyclic tensile loading test on flexible PDMS SERS substrates at a pre-specified tensile strain (ε) value of 10% shows that the SERS intensity remains almost constant for more than 100 cycles. These disposable and flexible SERS substrates can be integrated with biological substances and offer a novel and practical method to facilitate biosensing applications.

Tilted Aligned Epitaxial La0.7Sr0.3MnO3 Nanocolumnar Films with Enhanced Low-Field Magnetoresistance by Pulsed Laser Oblique-Angle Deposition

Single-phase epitaxial La0.7Sr0.3MnO3 (LSMO) thin films with significantly enhanced low-field magnetoresistance (LFMR) properties (23% at 20 K) are demonstrated in this work. The LSMO films on SrTiO3 (001) substrates exhibit tilted and well-aligned nanocolumn structure achieved by pulsed laser oblique-angle deposition (PLOAD) followed by subsequent postannealing. The tilted aligned nanocolumnar (TAN) arrays have been achieved at relative high deposition angles (≥30o) and low deposition temperatures (≤450 °C). More attractively, the tilted grain boundaries (GBs) can be systematically manipulated by the postannealing process and so can the LFMR values of the LSMO TAN films. These results demonstrate that the tilted nanocolumnar films achieved by PLOAD and the GB tailoring by postannealing may provide a new approach to control and manipulate the magnetotransport properties of single-phase manganite perovskite films for device applications that require large LFMR effects, high epitaxial quality, and low resistivity.

MgZnO/MgO strained multiple-quantum-well nanocolumnar films: Stress-induced structural transition and deep ultraviolet emission

MgZnO/MgO strained multiple-quantum-wells (S-MQWs) with different structures are grown by pulsed laser deposition. The S-MQW films are composed of closely arranged, vertically oriented nanocolumns. Z-contrast scanning transmission electron microscopy observations and line-scan compositional analysis reveal that the nanocolumns are compositionally modulated along their length and have a MQW structure. A coherent epitaxial relationship with a sharp interface is established between MgZnO and MgO layers with large lattice mismatch. It is worth noting that the MgZnO layer undergoes a structural transition from hexagonal to cubic phase with its thickness decreasing. The calculations reveal that a large in-plane compressive stress dominates such an interesting phase transition process, stabilizes the low Mg-content MgZnO alloy in the anomalous cubic phase, and also leads to a broadening of the band gap. As a result, the wavelength-tunable deep-ultraviolet emission in the range of 261–314 nm is obtained from these S-MQW nanocolumnar films. The 261 nm is known to be the shortest emission wavelength ever reported for MgZnO material.

Slanted S-shaped nano-columnar thin films for broadband and wide-angle polarization conversion

A slanted S-shaped nano-columnar (SSNC) thin film sculptured by oblique angle deposition is applied as an enhanced polarization converter. A growth model of a SSNC film is developed using available empirical equations for columnar thin films. The optimum sculpture parameters for a SSNC film to have high efficient polarization conversion are theoretically estimated and applied in fabrication. The designated SSNC films are sculptured and demonstrated to have high polarization conversion reflectance over a broadband and wide-angle range.

Highly luminescent columnar ZnO films grown directly on n-Si and p-Si substrates by low-temperature electrochemical deposition

In this study, nanocolumnar zinc oxide thin films were catalyst-free electrodeposited directly on n-Si and p-Si substrates, what makes an important junction for optoelectronic devices. We demonstrate that ZnO thin films can be grown on Si at low cathodic potential by electrochemical synthesis. The scanning electron microscopy SEM showed that the ZnO thin films consist of nanocolumns with radius of about 150 nm on n-Si and 200 nm on p-Si substrates, possess uniform size distribution and fully covers surfaces. X-ray diffraction (XRD) measurements show that the films are crystalline material and are preferably grown along (0 0 2) direction. The impact of thermal annealing in the temperature range of 150–800 °C on ZnO film properties has been carried out. Low-temperature photoluminescence (PL) spectra of the as-prepared ZnO/Si samples show the extremely high intensity of the near bandgap luminescence along with the absence of visible emission. The optical quality of ZnO thin films was improved after post-deposition thermal treatment at 150 °C and 400 °C in our experiments, however, the luminescence intensity was found to decrease at higher annealing temperatures (800 °C). The obtained results indicate that electrodeposition is an efficient low-temperature technique for the growth of high-quality and crystallographically oriented ZnO thin films on n-Si and p-Si substrates for device applications.

Surface Morphology Induced Localized Electric Field and Piezoresponse Enhancement in Nanostructured Thin Films

Nanostructured piezoelectric and ferroelectric thin films are being increasingly used in sensing and actuating microdevices. In this work, we report the experimental discovery of localized electric field enhancement in nanocolumnar piezoelectric thin films and its significant impact on piezoresponse. The magnitude of electric field enhancement is associated with nonflat surface morphologies and is in agreement with theoretical and finite element models. The influence of this surface morphology induced enhancement on piezoresponse is demonstrated using phase field simulations, which also illustrates surface morphology induced strain enhancement. The observed enhancement can be effectively harnessed to improve the sensitivity of related piezoelectric thin film applications.

Highly efficient ultraviolet photodetection in nanocolumnar RF sputtered ZnOfilms: a comparison between sputtered, sol–gel and aqueous chemically grownnanostructures

Highly efficient ultraviolet (UV) photodetection has been realized in nanocolumnarradio frequency (RF) sputtered ZnO thin films as compared to sol–gelnanocrystalline films and aqueous chemically grown nanowire (NW) arrays. Thephoto-to-dark current ratio in the columnar films reaches a value as high as106 which is at least 2–4 orders of magnitude higher than that of the sol–gel films and NWs.Such a high value is attributed to a dense compact columnar morphology of the sputteredfilms with higher surface-related trap states. Within 2 s of UV illumination, five orders ofchange in the photocurrent have occurred in the sputtered film in contrast to three ordersof change in the NWs. The photocurrent decay under steady UV illumination in thesputtered films is only 23%, compared to 50% in the NWs. However, the fall in current underthe UV ‘off’ condition in the sputtered film takes a longer time than in the NWs.These changes are correlated to the deep defect levels of the nanocolumnar films.

Tunable In-Plane Optical Anisotropy of Ag Nanoparticles Deposited by DC Sputtering onto SiO2 Nanocolumnar Films

This work reports an easy-to-handle method for growing two-dimensional assemblies of Ag nanostructures presenting a tunable in-plane optical anisotropy. Ag is deposited by DC sputtering in an Ar plasma at room temperature onto bundled nanocolumnar SiO2 thin films grown by glancing angle physical vapor deposition. In contrast with previously reported processes involving the grazing angle deposition of the metal, DC sputtering is performed at normal incidence. By varying the deposition angle of SiO2 and the Ar pressure, it was possible to tune the deposited amount of Ag and thus the topology of the Ag deposit from isolated spherical Ag nanoparticles with isotropic optical properties to strongly dichroic Ag nanostripes oriented along the bundling direction of the SiO2 nanocolumns. Based on simple calculations taking into account the shadowing effects during metal deposition, it is proposed that the width and shape of the tip of the bundled SiO2 nanocolumns influence significantly the metal local atom flux arriving to them and thus the final structure of the deposit.

Photoelectrochemical behaviour of anatase nanoporous films: effect of the nanoparticle organization

The photoelectrochemical behaviour of anatase thin films with different nanoarchitectures and the same active surface area (or thickness) has been studied in acidic media in the absence and in the presence of formic acid. The electrodes were composed of either wire-like nanocrystal aggregates or commercial TiO(2) nanoparticles. Cyclic voltammetry in the dark reveals a larger trap concentration in the band gap for the nanoparticulate (NP) electrodes, which can be ascribed to a larger number of intergrain boundaries. Also under illumination, the behaviour for both types of anatase structures significantly differs: water photooxidation arises at more negative potentials for the nanocolumnar (NC) electrodes. In the presence of an efficient hole acceptor such as HCOOH, significantly larger photocurrents were noted for the NC films as compared with those for the NP electrodes, with the photocurrent onset also shifted towards more positive potentials for the latter. These results point to a diminished electron recombination, which can be related with a smaller concentration of intergrain boundaries, together with a more efficient HCOOH hole transfer for the wire-like nanocrystal aggregate architecture. In addition, the oxygen reduction reaction is also favoured in the case of NC electrodes.

Enhancement of surface morphology and optical properties of nanocolumnar ZnO films

Nanocolumnar ZnO films were prepared by electrodeposition (ED) on a glass substrate covered with a conductive layer of thin oxide doped with fluorine (FTO). After deposition the samples were annealed in oxidizing or reducing atmosphere, at temperatures between 100 to 500 ∘C, in order to follow the evolution of optical properties and morphology. The optical properties of these films were studied by means of photoluminescence spectroscopy (PL) and the morphology by scanning electron microscopy (SEM). Films annealed at 300 ∘C exhibit a higher ultraviolet emission peak, originating from exciton transitions. A green band related to deep-level emission centered at 500 nm, shows a drastic increase at 500 ∘C. These results are independent of the annealing atmosphere. An increase of coalescence is also observed after annealing at 500 ∘C. These results are explained taking into account the contribution of different point defects.

Nanocolumnar titania thin films uniquely incorporated with carbon for visible light photocatalysis

Carbon-doped or carbon-covered titania can enhance photocatalytic performance under visible light. Here we report the first instance of synergistic effect of the carbon incorporation in anatase titania (TiO2−xCx:C) films with both substitutional doping and surface covered characteristics, by reactively co-sputtering Ti metal and graphite targets. The nature of incorporated carbon is characterized by Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. The absorption edge of the carbon incorporated titania thin films shifted from ultraviolet to visible region and directly depended on the carbon content. The photocatalytic performance, e.g., photodegradation of methylene blue (MB), photo-reduction of silver-ions and super hydrophilicity, was greatly enhanced with increasing carbon content. The best photocatalytic activity is obtained in the TiO2−xCx:C film of the most carbon concentration about 9.3at.% with a degradation rate-constant of 0.108h−1 for MB under visible-light illumination.

Studies on Crystal Orientation of ZnO Film on Sapphire Using High-throughout X-ray Diffraction

The orientation of the nano-columnar ZnO films grown on sapphire using the technique of metal-organic chemical vapor deposition (MOCVD) exhibits deviation because of the mismatch between the crystal lattices of the films and the sapphire substrate. A high-throughout X-ray diffraction method was employed to determine the crystal orientation of the ZnO films at a time scale of the order of minutes based on the general area detection diffraction system (GADDS). This rapid, effective, and ready method, adapted for characterizing the orientation of the nano-columnar crystals is used to directly explain the results of observation of the X-ray diffraction images, by the measurements of the orientations of the crystal columns of the ZnO films along c -axis and in parallel to ab plane.