Oblique Angle Deposition Technique Research Articles

Spatially selective narrow band and broadband absorption in Ag/SiO2/Ag based trilayer thin films by oblique angle deposition of SiO2 layer

We have experimentally demonstrated spatially selective absorption in Ag-SiO2-Ag based trilayer thin films by tuning the deposition angle of SiO2 layer. These structures generate cavity resonance which can be tuned across the substrate locations due to spatially selective thickness and refractive index of silicon oxide (SiO2) film sandwiched between metallic silver (Ag) mirrors. Spatially selective property of SiO2 film is obtained by oblique angle deposition technique using an electron beam evaporation system. The resonance wavelength of absorption in this trilayer structure shifts across the substrate locations along the direction of oblique deposition. The extent of shift in resonance increases with increase in angle of deposition of SiO2 layer. 4.14 nm mm−1 average shift of resonance wavelength is observed when SiO2 is deposited at 40° whereas 4.76 nm mm−1 average shift is observed when SiO2 is deposited at 60°. We observed that the width of resonance increases with angle of deposition of the cavity layer and ultimately the resonant absorption disappears and becomes broadband when SiO2 is deposited at glancing angle deposition (GLAD) configuration. Our study reveals that there is a suitable range of oblique angle of deposition from 40° to 60° for higher spatial tunability and resonant absorption whereas the absorption becomes broadband for glancing angle deposition.

Fractal and structural analysis of the different sculptured Mn-based nanostructures

Surface morphology and fractal properties of 3D nanostructures of the circular helical, square helical, and chevron were investigated. The nanostructures were deposited on the glass substrate using the oblique angle deposition technique. The surface properties of the nanostructures were investigated by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). The autocorrelation function (ACF), height-height correlation function (H(r)), power density function (PSDF), and Minkowski function are used for fractal characterization. The results showed that the circular helical nanostructure has more surface roughness, roughness exponent, and grain size compared to the square helical and chevron nanostructures. The fractal dimension of the surface of all three nanostructures showed that the chevron nanostructure with Df=2.88 has higher spatial complexity. The Minkowski functionals showed similar behavior for all three nanostructures. However, more negative and positive Minkowski connectivity was obtained for the circular helical nanostructure compared to the other two nanostructures.

Structural, morphological, and optical properties of AgxO thin films deposited via obliquely angle deposition

Abstract This work reports on the optical, structural, and morphological properties of silver oxide thin films obtained by postoxidation of silver deposited previously by the thermal evaporation technique. The samples were deposited on glass substrates using the oblique angle deposition technique for different angles of incidence γ (γ=0°, 20°, 40°, 60°, 75°, and 85°). γ is defined as the angle between the particle flux and the normal to the substrate. The resulting thin films were annealed in the free air at two temperatures (300°C and 400°C). X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-visible-NIR spectrophotometer were performed to study the crystal structure, as well as the morphological and optical properties (transmittance and reflectance), of the AgxO samples. X-ray diffraction analysis revealed the presence of the AgxO phase for the silver films deposited at a high angle of incidence and for the annealing temperature 300°C. In contrast, the diffractograms of the silver films annealed at 400°C show an amorphous behavior. Optical results indicated that the direct band gap energy increases pursuant to increasing the angle of incidence γ. The absorption coefficients of AgxO thin films were found to be in the range of 103–105 cm−1. Additionally, we determined the birefringence for the layers annealed at 400°C and found that the highest value of birefringence is obtained corresponding to the angle of incidence 60°. Morphological analysis indicated that the porosity increases with the angle of incidence and highlights the amorphous nature of the films, which is attributed to the columnar structure.

Ultimate figures of merit broadband self-powered obliquely deposited antimony thin film laser detectors

Fabrication of a fast and high detectivity infrared detector operating at room temperature represents a big challenge. Due to the small energy gap of the semiconducting materials used for infrared detectors, the noise becomes considerable factor and the possibility of operating the detector at room temperature is very limited. A study of the figures of merit antimony thin films detector grown by oblique angle deposition technique is presented. Polycrystalline antimony thin films were thermally evaporated on the glass substrates at a angles of 0, 10, 30, and70°. The aim was to develop a wideband (0.649–10.6) µm self-powered laser detectors; operating at room temperature. The deposition angle had a decisive role in the detector specifications, namely, its detectivity, responsivity, linearity, and response time. At θ = 70° deposition angle; maximum detectivity and fastest response were achieved. The variation of rise time with deposition angle was linear, and the rise time was around 50 ns at 70°. The antimony detectors showed about the same specific detectivity ~ 109 Jones at 300 k for the wavelength range of 1.064–10.6 µm.

3-5 µm mid-infrared broadband absorbers composed of layered ITO nanorod arrays with high visible light transmittance.

A mid-infrared broadband absorber with high visible light transmittance is proposed in this paper. The absorber is composed of layered ITO nanorod arrays with increasing angles fabricated by oblique angle deposition technique. The experimental results show that the average transmittance of the absorber reaches 80% in the 400-800 nm band and the integrated absorption reaches 82.9% in the 3-5 µm band, when the QCM thickness of the first layer of film is 100 nm and the deposition angle θ is 10°, the QCM heights of the second to fifth layers of nanorods are all 330 nm, and their deposition angles are 55°, 68°, 80°, and 87°, respectively. The high transmittance in the visible band is attributed to the gradient of the refractive index. The broadband absorption in the mid-infrared band results from different resonances in the empty cavities with different sizes. Such a simple and large-area absorber has potential applications in window materials and infrared cloaking.

Subwavelength Densely Packed Disordered Semiconductor Metasurface Units for Photoelectrochemical Hydrogen Generation

For most semiconductors, especially the visible-light-absorbing ones, the carrier diffusion length is significantly shorter than the light penetration depth, limiting their photoactivities. This limitation could be mitigated through the use of subwavelength semiconductor-based metasurfaces and metamaterials. In this paper, a large-scale compatible metasurface photocathode, made of densely packed disordered p-type chromium oxide (CrOX), is developed to be utilized in photoelectrochemical (PEC) hydrogen generation. For this purpose, first, tightly packed random Cr nanorods are fabricated using an oblique angle deposition technique. Afterward, an annealing step is applied to the sample to transform these metallic units into a semiconducting p-type CrOX-based metasurface. Based on the experimental characterization results and numerical simulations, the proposed design can provide strong light–matter interactions in an ultra-broadband-wavelength range, mainly due to its multidimensional random geometry and ultrasmall gap sizes. Finally, to substantiate the activity of the CrOX nanorods, a core–crown geometry is developed where the NiOX capping layer catalyzes the hydrogen evolution reaction (HER). The proposed heterostructure metasurface absorber can impose photocurrent values as large as 50 μA cm–2 with a photocurrent spectral response extended up to 500 nm. Moreover, the electrode shows outstanding operation under light irradiation for 9 hours. This work demonstrates a simple, scalable design strategy to fabricate low-cost and stable photocathodes for PEC hydrogen evolution.

Structural development and phase transformation behavior of thermally-oxidization Ti by sputtering power and OAD technique

Anatase to rutile phase transition (ARPT) of thermally-oxidized Ti films have been investigated via effect of sputtering power and oblique angle deposition (OAD) technique. ARPT is controlled via the sputtering power of Ti films in a range of 50–200 W. Mechanism of ARPT is proposed in terms of oxidation rate on the different morphological structures of Ti films. Increase in sample mass with different oxygen content of thermally-oxidized Ti films is controlled via sputtering power. The evidence of FE-SEM, GIXRD, and EDS corresponded well and in agreement with the optical evolution of transmittance spectra and the correlation of energy band gap (Eg) with the feature of the TiO2 phases. The TiO2 films showed the lowest water contact angle of 7° under UV irradiation for mixed anatase-rutile phases condition.

Fabrication of an acetone gas sensor based on Si-doped WO3 nanorods prepared by reactive magnetron co-sputtering with OAD technique

Gas sensing technology is currently applied in a variety of applications. In medical applications, gas sensors can be used for the detection of the biomarker in various diseases, metabolic disorders, diabetes mellitus, asthma, renal, liver diseases, and lung cancer. In this study, we present acetone sensing characteristics of Si-doped WO3 nanorods prepared by a DC reactive magnetron co-sputtering with an oblique-angle deposition (OAD) technique. The composition of Si-doped in WO3 has been studied by varying the electrical input power applied to the Si sputtered target. The nanorods film was constructed at the glancing angle of 85°. After deposition, the films were annealed at 400 °C for 4 h in the air. The microstructures and phases of the materials were characterized by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and field-emission scanning electron microscopy (FESEM). The results showed that 1.43 wt% Si-doped WO3 thin film exhibited the maximum response of 5.92 towards 100 ppm of acetone at performing temperature (350 °C), purifying dry air carrier. The process exposed in this work demonstrated the potential of high sensitivity acetone gas sensor at low concentration and may be used as an effective tool for diabetes non-invasive monitoring.

Nanostructure optimization of Zr-W-Ti metallic glass thin films via multitarget co-sputtering with oblique angle deposition approach

In this work, we systematically investigated the Zr-W-Ti nanostructured metallic glasses (NMGs) thin films on silicon (Si) wafer substrate fabricated via multitarget co-sputtering with oblique angle deposition (OAD) technique. By varying the substrate angles (θs) from 0° to 55°, the evolution of Zr-W-Ti NMGs from dense to nanocolumnar and nanorod film were observed by field-emission scanning electron microscope (FE-FEM), which created by the self-shadowing effect. The critical point of substrate angles was around 15° to produce the nanocolumnar and 40° to form the nanorod structure. The relationship among the substrate angles and the Zr-W-Ti NMGs tilted angle (β) was determined and discussed in detail with two theoretical correlations as the "Tangent rule" and "Cosine rule." The X-ray photoelectron spectroscopy (XPS) studied revealed the different chemical compositions. The results obtained from grazing incidence X-ray diffraction (GIXRD) and transmission electron microscope (TEM) indicated that the prepared Zr-W-Ti NMGs thin films exhibit an amorphous structure. Ultimately, it suggests that the multitarget co-sputtering with the OAD technique provides NMGs thin films, enabling applications in gas storage and sensor devices due to their well-defined nanostructured morphological and chemical composition characteristics.

Electrochromic property of tungsten trioxide films prepared by DC magnetron sputtering with oblique angle deposition and thermal oxidation

In this work, WO3 thin films were prepared in two steps: First, tungsten (W) films with thickness of 100-300 nm were deposited by DC magnetron sputtering with oblique angle deposition (OAD) technique at 0 and 85. Second, sputtered W films were annealed under air atmosphere at a temperature of 500℃ and different oxidation times for 1-3 h. The structure of WO3 thin films were examined by X-ray diffraction and field emission scanning electron microscope. In addition, the optical and electrochromic properties of the WO3 thin films were measured by a spectrophotometer before and after testing in potassium hydroxide electrolytes. The results showed that the OAD technique can enhance porosity and exert high oxidation in W films. The increase in film thickness and oxidation time indicated that the crystallinity of WO3 films increased. The condition of WO3 films for OAD at 85, thickness of 300 nm, and oxidation time of 1 h showed the best electrochromic property with the highest optical modulation and current density.

Multifunctional hard coatings based on CrNx for temperature sensing applications

This paper presents reports on the preparation of a multifunctional coating system, CrNx, with temperature sensing capability. A systematic study of the thermo-resistive effect of chromium nitride (CrNx) thin films with a negative temperature coefficient of resistance (TCR) has been carried out. The CrNx nanostructures were grown by reactive magnetron sputtering under distinct Ar + N2 conditions. To confer a zigzag columnar morphology to the CrNx we explored the oblique angle deposition technique. The structural properties have been studied through X-ray diffraction and Scanning Electron Microscopy. The thermo-resistive response was evaluated by measuring the electrical resistivity as a function of temperature by the two-point method. The results observed for the CrNx films produced with N2 flux between 4 and 8 sccm present an stable and negative TCR. Values of −9.17 × 10−4, −5.31 × 10−3, and −1.476 × 10−2 were observed for the films grown with 4, 6, and 8 sccm, respectively. The grain-boundary was used to theoretically describe our results. The results open new technological possibilities for the application of CrNx coatings for esistive temperature detector (RTD) systems.

Polarized photoluminescence of dicyanomethylene pyran and its derivatives thin films obtained by the oblique angle deposition technique

Polarized photoluminescence of dicyanomethylene pyran and its derivatives thin films obtained by the oblique angle deposition technique

Highly stable and reproducible Au nanorod arrays for near-infrared optofluidic SERS sensor

Surface-enhanced Raman spectroscopy (SERS) is a sensitive vibrational spectroscopy technique that can enable fast and non-destructive detection of trace molecules. SERS substrates are critical for the advancement of the SERS application. By incorporating SERS substrates into microfluidic devices, the function of microfluidic devices can be extended, and an efficient on-site trace analysis platform with powerful sensing capabilities can be realized. In this paper, we report the fabrication of a rapid and sensitive optofluidic SERS device using a unique Au nanorod array (AuNRA) with a plasmon resonance frequency in the near IR region. The highly stable and reproducible AuNRA were fabricated by a facile dynamic oblique angle deposition technique. A typical spectrum of 4,4-bipyridine (BPY) with enhanced peaks was observed within a few seconds after the injection of an aqueous solution BPY. Time-course measurements revealed an outstandingly quick response of SERS in this system. Using the AuNRA microfluidic device, approximately 2 × 10−12mol molecules were enough to produce detectable SERS signals, and BPY was detected down to a concentration of 15.6 × 10−9 g/mL. This work demonstrates rapid and sensitive chemical sensing using an optofluidic device equipped with a unique noble metal nanorod array.

Photoresponsivity enhancement of SnS porous film

Tin (II) sulfide (SnS) films have been deposited on glass substrates by oblique angle deposition (OAD) technique. The structural properties of the prepared samples were investigated by using XRD, Raman, and FESEM characterization techniques. The optical properties of the specimens were studied by UV–Vis spectroscopy technique. The photosensitivity properties of the samples were measured by red and amber lights with different intensities. The OAD technique was found able to reduce the response time of the samples from 2.15s to 0.69s for the amber light, and from 3.59s to 1.49s for the red light. Besides, a significant reduction in the decay time of the specimens was found from 12.21s to 8.48s and from 11.18s to 8.77s for amber and red lights, respectively.

Multilayer thin-film flake dispersion gel for surface-enhanced Raman spectroscopy

Ag nanorod arrays/dielectrics/mirror-structured multilayer thin-film are well known, highly sensitive surface-enhanced Raman scattering (SERS) substrates that enhance the Raman scattering cross-section by the interference of light. However, extracting biomarkers directly from human skin using these solid substrates is difficult. To overcome this problem, we propose a multilayer thin-film flake dispersion gel by centrifugal mixing of the multilayer thin-film and hydroxyethyl cellulose (HEC) gel. The multilayer thin-film was prepared by serial bideposition using the dynamic oblique angle deposition technique. The mixing process was optimized to obtain flakes of ~10 {\mu}m so that the optical properties of the multilayer film can be preserved, and there is no risk of adverse effects on humans. The SERS features of the flakes dispersion gel were tested using 4, 4'-bipyridine (BPY). The BPY molecules diffused through the highly porous gel within a few seconds, generating significant SERS signals. The multilayer film flakes dispersion gel showed a SERS signal about 20 times better than the gel-dispersed Ag nanorod arrays without a multilayer film structure. These SERS active flakes dispersion gel can be used directly on the skin surface to collect body fluids from sweat, for biomarker sensing.

Increasing the circular dichroism of the planar chiral nanostructure by inducing coupling between the coverage layer and the planar nanostructure.

Circular dichroism (CD) has been widely studied in recent decades because of its wide application in biomedical detection. Nanostructures with different heights (NDH) usually increase the transmission CD effect. To achieve such nanostructures, one needs to repeatedly perform the electron-beam lithography (EBL) method twice or more, layer-by-layer, which is a very complicated process. Here, we propose a method to prepare NDH by combining the EBL and oblique angle deposition (OAD) techniques. L-shaped planar silver nanostructures are prepared using EBL and normal electron beam deposition, and the OAD method is then used to partially cover one arm of the L-shaped nanostructure. Numerical simulations reveal that the height difference in the two arms of the L-shaped NDH (LSNDH) causes a difference in the polarization directions of the left- (LCP) and right-circularly polarized (RCP) incident light, thereby, generating CD effects. A 2D material is used to cover the LSNDH to further increase the charge polarization direction differences, which considerably increases the CD effect. These results are useful in simplifying and increasing the convenience of the preparation method of 3D chiral nanostructures. Furthermore, the proposed nanostructure may have potential application in biosensor, such as chiral enantiomer sensors.

Plasmonic Nanoslit Arrays Fabricated by Serial Bideposition: Optical and Surface-Enhanced Raman Scattering Study.

Recently, studies have been carried out to combine surface-enhanced Raman spectroscopy substrates that are based on either localized surface plasmon or surface plasmon polariton structures. By combining these two systems, the individual drawbacks of each can be overcome. However, the manufacturing methods involved so far are sophisticated, labor-intensive, expensive, and technically demanding. We propose a facile method for the fabrication of a flexible plasmonic nanoslit surface-enhanced Raman scattering (SERS) sensor. We utilized the pattern on periodic optical disks as an inexpensive substitute for printing the periodic pattern on polydimethylsiloxane with soft imprint lithography. The Ag nanoslits were fabricated by serial bideposition using the dynamic oblique angle deposition technique. The nanoslit structures were physically and optically characterized, and the experimental results were compared to the results of the numerical simulation: Monte Carlo and finite-difference time-domain simulation. The AgNS samples showed excellent SERS performance with an enhancement factor of ∼105 and a limit of detection of 5 × 10-7 g/mL for a Rhodamine 6G solution. Their biosensing capability was demonstrated by the sensing of bilirubin.

Enhancing electrochemical performance of thin film lithium ion battery via introducing tilted metal nanopillars as effective current collectors

Novel Li2MnO3 (LMO)-Au nanocomposite thin film with tilted Au pillars has been synthesized by using an oblique angle deposition technique (OAD) in pulsed laser deposition. The tilt angle and dimension of the Au nanopillars can be tuned by varying the inclination angle of the incoming flux and the growth rate. The obtained LMO-Au nanocomposite structure exhibits an initial volumetric discharge capacity of 35.78 μAh cm−2 μm−1 for the 1st cycle but increases to 62.32 μAh cm−2 μm−1 at the 100th cycle. Such an increase in discharge capacity upon cycling is attributed to significantly increased reaction depth upon cycling, indicating that Au pillars function as effective current collectors and the LMO-Au interfaces improve the cycling stability. In addition, the LMO-Au nanocomposites display highly anisotropic optical complex dielectric function in-plane and out-of-plane, reduced bandgap, and high hardness of 10 GPa which almost doubled that of pure Li2MnO3. This study presents a novel approach for processing nanocomposite thin films with tunable tilted current collectors towards advanced thin film battery cathode, nanoscale plasmonic systems, and other oxide-metal hybrid electrochemical systems.

Texture and Stress Evolution in HfN Films Sputter-Deposited at Oblique Angles

In this study, polycrystalline hafnium nitride (HfN) thin films were grown by oblique angle deposition (OAD) technique to investigate the relationship between column tilt angle, texture development and residual stress evolution with varying inclination angle α of the substrate. The films (~1 μm thickness) were grown at various angles (α = 5°, 25°, 35°, 65°, 75°, and 85°) with respect to the substrate normal by reactive magnetron sputtering at 0.3 Pa and 300 °C. The film morphology, crystal structure and residual stress state were characterized by scanning electron microscopy and X-ray diffraction (XRD), including pole figure and sin2ψ measurements. All HfN films had a cubic, NaCl-type crystal structure with an [111] out-of-plane orientation and exhibited a biaxial texture for α ≥ 35°. XRD pole figures reveal that the crystal habit of the grains consists of {100} facets constituting triangular-base pyramids, with a side and a corner facing the projection of the incoming particle flux (indicative of a double in-plane alignment). A columnar microstructure was formed for α ≥ 35°, with typical column widths of 100 nm. It is observed that the column tilt angle β increases monotonously for α ≥ 35°, reaching β = 34° at α = 85°. This variation at microscopic scale is correlated with the tilt angle of the (111) crystallographic planes, changing from −24.8 to 11.3° with respect to the substrate surface. The residual stress changes from strongly compressive (~−5 GPa at α = 5°) to negligible or slightly tensile for α ≥ 35°. The observed trends are compared to previous works of the literature and discussed based on existing crystal growth and stress models, as well as in light of energy and angular distribution of the incident particle flux calculated by Monte Carlo. Importantly, a decrease of the average kinetic energy of Hf particles from 22.4 to 17.7 eV is found with increasing α due to an increase number of collisions.

L-shaped ITO structures fabricated by oblique angle deposition technique for mid-infrared circular dichroism.

This paper proposes a mid-infrared chiral structure, which consists of L-shaped indium tin oxide (ITO) films formed on self-assembled monolayer polystyrene microspheres in two orthogonal directions by oblique angle deposition technique. Experimental results demonstrate that the structure exhibit circular dichroism (CD) responses in the range of 2.5 - 4 µm. As the thickness difference of the ITO films in the two orthogonal directions increases, the CD response enhances. The reason is that the ITO films produce cross dipoles and their bigger differences in thickness bring to bigger phase differences in optical chirality. The experimental results also demonstrate that the CD signals are evidently stronger than those of the structure consisting of silver in the mid-infrared band. This work provides a new idea for the fabrication of mid-infrared chiral structures, which have potential applications in the polarization state control of mid-infrared lasers.