Oblique Angle Deposition Research Articles

Optofluidic liquid sensing on electromicrofluidic devices

Electromicrofluidic (EMF) devices are used to handle and move tiny amounts of liquids by electrical actuation, including electrowetting-on-dielectric (EWOD) and dielectrophoresis (DEP). Monitoring the liquid characteristics in one of these devices requires suitable sensing transducers incorporated within the microfluidic structure. In the present work, we describe the incorporation of an optofluidic photonic transducer in an EMF device to monitor the refractive index of a liquid during its manipulation. The incorporated transducer consists of a responsive porous Bragg Microcavity (BM) deposited via physical vapor oblique angle deposition. Besides reporting the manufacturing procedure of the sensing-EMF device combining liquid handling and monitoring, the performance of the BM is verified by infiltrating several liquids dripped on its surface and comparing the responses with those of liquid droplets electrically moved from the delivery part of the chip to the BM location. This study proved that modified EMF devices can incorporate photonic structures to analyze very low liquid volumes (∼0.2 μL) during its handling.

On the importance of light scattering for high performances nanostructured antireflective surfaces

An antireflective coating presenting a continuous refractive index gradient is theoretically better than its discrete counterpart because it can give rise to a perfect broadband transparency. This kind of surface treatment can be obtained with nanostructures like moth-eye. Despite the light scattering behavior must be accounted as it can lead to a significant transmittance drop, no methods are actually available to anticipate scattering losses in such nanostructured antireflective coatings. To overcome this current limitation, we present here an original way to simulate the scattering losses in these systems and routes to optimize the transparency. This method, which was validated by a comparative study of coatings presenting refractive indices with either discrete or continuous gradient, shows that a discrete antireflective coating bilayer made by oblique angle deposition allows reaching ultra-high mean transmittance of 98.97% over the broadband [400–1800] nm. Such simple surface treatment outperforms moth-eye architectures thanks to both interference effect and small dimensions nanostructures that minimize the scattering losses as confirmed by finite-difference time domain simulations performed on reconstructed volumes obtained from electron tomography experiments.

A 4-view imaging to reveal microstructural differences in obliquely sputter-deposited tungsten films

We report on the morphological disparity of the columnar growth in W thin films sputter-deposited by oblique angle deposition. Oriented tungsten thin films (400 ± 50 nm thick) are prepared using a tilt angle α of 80° and a sputtering pressure of 0.25 Pa. Inclined columns (β = 38 ± 2°) are produced and the microstructure is observed by scanning electron microscopy. A 4-view imaging is performed in order to show inhomogeneous growing evolutions in the columns. Morphological features vs. viewing direction are also investigated from a growth simulation of these tilted W columns. Experimental and theoretical approaches are successfully compared and allow understanding how the direction of the W particle flux leads to dense or fibrous morphologies, as the column apexes are in front of the flux or in the shadowing zone.

Физические основы наклонного напыления фосфатов кальция из плазмы ВЧ-магнетронного разряда

The possibility of titanium surface functionalization by an oblique angle deposition of nanostructured bioactive calcium phosphate coatings is shown in the present work. The coatings were obtained by an RF magnetron sputtering of hydroxyapatite and antibacterial zinc-substituted hydroxyapatite-based targets. The morphology and growth features of coatings were studied in relation to the process parameters, such as the angle of substrates inclination relative to the particle flux, an RF power supplied to the target, and the distance from the target surface to the surface of the substrates. With an increase in the angle of inclination, the surface morphology of the coating’s changes significantly. It resulted inappearance of nanoscale structural elements tilted towards the source of the material flux. The arrangement of surface features is ordered in accordance with the initial surface of the substrate before sputtering. The internal structure of the coatings is represented by columnar elements inclined relative to the plane of the substrate.

Surface tuning of the photoelectrochemical properties of oblique angle co-sputtered ZnxFeyO films by Fe concentration

Complex three-dimensional nanosheet structure of ZnxFeyO was prepared by highly stable co-sputtering oblique angle deposition. Scanning electron microscopy was employed to observe surface morphology evolution of ZnxFeyO with different Fe concentrations. X-ray diffraction was employed to analyze compositions of ZnO, ZnFe2O4, and ZnFe2O4/Fe2O3 with Fe doping. Furthermore, specific nanostructures of ZnxFeyO decreased band gap and increased visible-light absorption ability. The ZnFe2O4/Fe2O3 sample exhibited higher photocatalytic efficiency than those of other films for the degradation of methylene blue. Addition of Fe led to the enhancement of photoelectrochemical properties of ZnFe2O4/Fe2O3 compared to pure ZnO and Fe2O3, and photocurrent response of ZnFe2O4/Fe2O3 was ~10 times than that of pure ZnO at constant potential of −0.2 V (vs. Ag/AgCl).

Biaxially Textured Titanium Thin Films by Oblique Angle Deposition: Conditions and Growth Mechanisms

Growing highly crystalline nanowires over large substrate areas remains an ambiguous task nowadays. Herein, a time‐efficient and easy‐to‐handle bottom‐up approach is demonstrated that enables the self‐assembled growth of biaxially textured Ti thin films composed of single‐crystalline nanowires in a single‐deposition step. Ti thin films are deposited under highly oblique incidence angles by electron beam evaporation on amorphous substrates. Substrate temperature, angle of the incoming particle flux, and working pressure are varied to optimize the crystallinity in those films. Height‐resolved structure information of individual nanowires is provided by a transmission electron microscopy (TEM) nanobeam, high‐resolution TEM, and electron diffraction. Ti nanowires are polycrystalline at 77 K, whereas for ≥300 K, single‐crystalline nanowires are tendentially found. The Ti crystals grow along the thermodynamically favored c‐direction, but the nanowires’ tilt angle is determined by shadowing. Biaxially textured Ti thin films require a certain temperature range combined with highly oblique deposition angles, which is proved by X‐ray in‐plane pole figures. A general correlation between average activation energy for surface self‐diffusion and melting point of metals is given to estimate the significant influence of surface self‐diffusion on the evolution of obliquely deposited metal thin films.

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.

Tuning optical properties of ITO films grown by rf sputtering: Effects of oblique angle deposition and thermal annealing

Abstract Indium tin oxide (ITO) thin films were prepared using the technique of rf-sputtering with oblique angle deposition (OAD). The films were as-deposited and thermally treated at 250 °C. The combination of substrate inclination and annealing was used for modifying morphological and structural properties that lead to changes of the optical properties. The resulting films show morphology of tilted nanocolumn, fissures among columns, and structural changes. The as-deposited films are structurally disordered with an amorphous component and the annealed films are crystallized and more ordered and the film diffractograms correspond to the cubic structure of In2O3. The refractive index could be modified up to 0.3 in as-deposited films and up to 0.15 in annealed films as functions of the inclination angle of the nanocolumns. Similarly, the band gap energy increases up to about 0.4 eV due to the reduction of the microstrain distribution. It is found that the microstrain distribution, which is related to lattice distortions, defects and the presence of fissures in the films, is the main feature that can be engineered through morphological modifications for achieving the adjustment of the optical properties.

Generalized ellipsometry characterization of Ag nanorod arrays prepared by oblique angle deposition

Silver nanorods arrays (AgNRs) prepared by oblique angle deposition were characterized by the generalized ellipsometry method in the spectral range from 370 to 950 nm. Three structure models were used to fit the ellipsometry data, the uniaxial model, the biaxial orthorhombic model, and the biaxial monoclinic model. Unlike the uniaxial model reported in most literature, the biaxial models are found to give better fitting results. The optical properties along the three principle axes are different: along long axis it displays predominantly metallic behavior; along one short axis it approaches to a lossless dielectric while along the other it behaves as an absorbance dielectric. The AgNRs also demonstrate epsilon-near-zero property with the real part of dielectric constant along the rod being very close to zero at wavelength of 416 nm, which is expected to be tuned with changing of the vapor incident angles.

Robust label-free CuxCoyOz electrochemical sensors for hexose detection during fermentation process monitoring

Label free electrochemical sensors of glucose are used whenever long-term operation and stable response are required. For this purpose, various metals and oxides of the first transition series have been proposed as alternative to more expensive noble metal electrochemical sensors. In this work we propose a new formulation consisting of copper-cobalt mixed oxides which, in the form of porous and nanostructured thin films with well controlled Co/Cu ratio, are prepared at room temperature in one step by a modification of the magnetron sputtering oblique angle deposition procedure. Films with various compositions were electrochemically characterized by cyclic voltammetry to determine their amperometric response to glucose as a function of voltage and NaOH electrolyte concentration. This analysis showed that films with a Co/Cu atomic ratio equal 3.4 presented a maximum sensitivity (0.710 A M−1 cm−2), a small limit of detection (0.105 μM) and a resilient behaviour upon cycling operation and long storage periods that clearly overpassed the performance of copper and cobalt single oxides. The CuxCoyO electrocatalysts also presented a good selectivity towards glucose and fructose in the presence of common interference compounds found in biological fluids (e.g., ascorbic acid, acetaminophen and uric acid), sucrose and ethanol, this latter present in many agrofood liquids. The possibilities of this sensor electrocatalyst have been tested for the analysis of a wine synthetic fermentation process. The comparison of the electrochemical results with conventional analytical methods showed a lineal amperometric response with respect hexose contents in a must at different stages of its transformation into wine.

Tailoring the structural, electrical, optical and wettability properties of ZnSe films by oblique angle thermal evaporation

ZnSe films were prepared using thermal evaporation by keeping the substrates at angles of 0°, 60° and 75° with the horizontal direction. The film prepared at 0° had zinc-blende cubic crystal structure with an average crystallite size of 27 nm while the other two films showed very weak crystalline peaks due to the presence of a small amount of nano-crystallites. The Raman spectra of all the thin film samples show peaks at 256 cm−1 due to the longitudinal optical (LO) mode that is blue shifted as compared to the LO peak at 245 cm−1 in the commercial ZnSe powder sample. Globular structures protruding from the surface were observed in the FESEM and AFM images and the size of the globules decreased with an increase in the deposition angle. The root mean square surface roughness is 21 nm, 48 nm and 33 nm for films deposited at 0°, 60° and 75° respectively. The optical band gap is in the range: 2.71 to 2.75 eV and all the films were highly transparent in the mid-infrared region. The electrical conductivity increases with an increase in the deposition angle and the values are 4.84 × 10−9 ohm−1 cm−1, 9.51 × 10−9 ohm−1 cm−1 and 2.12 × 10−7 ohm−1 cm−1 (at 308 K) for the films deposited at 0°, 60° and 75° respectively. The water contact angle measurements found that the films grown at oblique angles have higher hydrophobicity as compared to the normally deposited film. The study elaborates the influence of oblique angle deposition in the modification of the structural, surface roughness, electrical, optical and wettability properties of ZnSe thin films.

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.

Efficient excitation of exchange dominated spin waves in oblique deposited CoFeB thin films

Spin waves play a vital role as potential data carriers in low power wave-like computing and high-frequency spintronic devices, which stimulate intensive studies on the efficient excitation of spin waves in magnetic thin films. Here, we suggest a controllable approach for the efficient excitation of perpendicular standing spin waves (PSSWs) in a series of CoFeB/Si thin films by the oblique angle deposition technique. It is experimentally demonstrated that the intensity of the PSSW mode can be enhanced significantly with increased oblique angle, and a comparable intensity of spin wave to that of uniform FMR mode is achieved at an oblique angle of 39°. Moreover, the dependence of in-plane anisotropy field Hk, exchange constant A, as well as intrinsic Gilbert damping α on oblique deposition angle are also investigated.

Sensitive and label-free detection of protein secondary structure by amide III spectral signals using surface-enhanced Raman spectroscopy

Proteins and peptides perform a vital role in living systems, however it remains a challenge for accurate description of proteins at the molecular level. Despite that surface-enhanced Raman spectroscopy (SERS) can provide the intrinsic fingerprint information of samples with ultrahigh sensitivity, it suffers from the poor reproducibility and reliability. Herein, we demonstrate that the silver nanorod array fabricated by an oblique angle deposition method is a powerful substrate for SERS to probe the protein secondary structures without exogenous labels. With this method, the SERS signals of two typical proteins (lysozyme and cytochrome c) are successfully obtained. Additionally, by analyzing the spectral signals of the amide III of protein backbone, the influence of concentration on the folding status of proteins has been elucidated. With the concentration increasing, the components of α-helix and β-sheet structures of lysozyme increase while the secondary structures of cytochrome c almost keep constant. The SERS method in this work offers an effective optical marker to characterize the structures of proteins.

A bilayer Al nanowire-grid polarizer integrated with an IR-cut filter

A bilayer Al nanowire-grid polarizer integrated with an IR-cut filter has been developed to exhibit high polarization sensitivity in a visible region. The IR-cut filter consisting of Ti3O5/SiO2 multilayer oxide film completely blocks the incoming light over 680 nm wavelength and shows high transmittance over 95% in the visible range. By incorporating the reversible nanostructures molding lithography and the two-step oblique angle deposition of the e-beam evaporator, we are able to secure the nanowire-grid patterning with 50-nm resolution in a form of bilayer Al grating. Then, it is found that the plasmonic coupling between the nearby Al layers leads to the high transmittance of TM-mode light passing through the IR-cut filter, while there is no plasmonic coupling between them for TE-mode case. Both the Finite-Difference Time-Domain simulation and experimental results show that the transmittance of the TM polarization for the grid height of 110 nm reaches up to 90% and the extinction ratio is around 300∶1 in the visible range. Therefore, the observed results can provide an insight in designing the bilayer Al based plasmonic polarizer combined with the IR-cut filter in order to achieve better performance utilizing the plasmonic coupling effect.

Oblique angle deposition of nickel thin films by high-power impulse magnetron sputtering.

Background: Oblique angle deposition is known for yielding the growth of columnar grains that are tilted in the direction of the deposition flux. Using this technique combined with high-power impulse magnetron sputtering (HiPIMS) can induce unique properties in ferromagnetic thin films. Earlier we have explored the properties of polycrystalline and epitaxially deposited permalloy thin films deposited under 35° tilt using HiPIMS and compared it with films deposited by dc magnetron sputtering (dcMS). The films prepared by HiPIMS present lower anisotropy and coercivity fields than films deposited with dcMS. For the epitaxial films dcMS deposition gives biaxial anisotropy while HiPIMS deposition gives a well-defined uniaxial anisotropy.Results: We report on the deposition of 50 nm polycrystalline nickel thin films by dcMS and HiPIMS while the tilt angle with respect to the substrate normal is varied from 0° to 70°. The HiPIMS-deposited films are always denser, with a smoother surface and are magnetically softer than the dcMS-deposited films under the same deposition conditions. The obliquely deposited HiPIMS films are significantly more uniform in terms of thickness. Cross-sectional SEM images reveal that the dcMS-deposited film under 70° tilt angle consists of well-defined inclined nanocolumnar grains while grains of HiPIMS-deposited films are smaller and less tilted. Both deposition methods result in in-plane isotropic magnetic behavior at small tilt angles while larger tilt angles result in uniaxial magnetic anisotropy. The transition tilt angle varies with deposition method and is measured around 35° for dcMS and 60° for HiPIMS.Conclusion: Due to the high discharge current and high ionized flux fraction, the HiPIMS process can suppress the inclined columnar growth induced by oblique angle deposition. Thus, the ferromagnetic thin films obliquely deposited by HiPIMS deposition exhibit different magnetic properties than dcMS-deposited films. The results demonstrate the potential of the HiPIMS process to tailor the material properties for some important technological applications in addition to the ability to fill high aspect ratio trenches and coating on cutting tools with complex geometries.

Improvement in DC and pulse characteristics of AlGaN/GaN HEMT by employing dual metal gate structure

This paper presents the fabrication and electrical characterization of dual metal gated high electron mobility transistors (DMG-HEMTs) on AlGaN/GaN/SiC hetero-structure using oblique and normal angle deposition of Nickel and Titanium respectively as gate metals. A noteworthy advance in the device characteristics has been achieved including DC and pulse drain current, transconductance, ON-resistance. The upsurge in DC drain current and transconductance is ∼8% and 11% respectively. Pulse I–V measurements have been executed to evaluate the dynamic performance of DMG-HEMTs. Pulse I–V unveils significant improvement in drain lag, gate lag and current collapse in these devices which is due to the redistribution of electric field under the gate.

Detection of overtone and combined peaks using Mn/Cu helical star-shaped (pine-tree-like) sculptured thin films in surface-enhanced Raman spectroscopy

Mn helical star-shaped (pine-tree-like) nano-sculptured thin films with three-, four-and fivefold symmetry on copper substrates were produced using oblique angle deposition method in conjunction with rotation of sample holder at certain angles. Raman spectroscopy of the samples which were subjected to impregnation by 4,4′-bipyridine (C10H8N2) solution was carried out by 632.8 nm wavelength laser. The analysis of the Raman spectra showed enhancement for the 4,4′-bipyridine main bands (1592 and 1297 cm−1) belonging to the C=C stretching mode, aromatic ring stretching ring and in-plane ring mode of 4,4′-bipyridine, same as results obtained for Ag nano-structures. In addition, overtone and combined peaks are detected that may be related to the particular sculptured structure of the thin films fabricated in this work and the laser wavelength used for Raman spectroscopy. UV–Vis (absorption mode) spectra showed consistent observations with the SERS results leading to important role of surface electromagnetic selection rule in intensification of SERS, in addition pointing out the importance of the effect of the size of hot spots on the frequency of the localized surface plasmonic oscillations.

Fabrication of Au Nanorods by the Oblique Angle Deposition Process for Trace Detection of Methamphetamine with Surface-Enhanced Raman Scattering.

Oblique angle deposition (OAD) is a simple, low cost, effective, and maskless nanofabrication process. It can offer a reliable method for the mass fabrication of uniform metal nanorods which can be used as the surface-enhanced Raman scattering (SERS) substrate with an excellent enhancing performance. Up to now, Ag nanorods SERS substrates have been extensively studied. However, Ag is chemically active and easy to oxidize under atmospheric conditions. Comparatively, Au is chemically stable and has better biocompatibility than Ag. In this paper, we in detail, studied the electromechanical (EM) field distribution simulation, fabrication, and application of Au nanorods (AuNRs) on trace detection of methamphetamine. According to the finite-difference time-domain (FDTD) calculation results, the maximum EM intensity can be obtained with the length of AuNRs to be 800 nm and the tilting angle of AuNRs to be 71° respectively. The aligned Au nanorod array substrate was fabricated by the OAD process. The two key process parameters, deposition angle, and deposition rate were optimized by experiments, which were 86° and 2 Å/s, respectively. Using 1,2-bis (4-pyridyl) ethylene (BPE) as the probe molecule, the limit of detection (LOD) was characterized to be 10−11 M. The AuNRs were also used to detect methamphetamine. The LOD can be down to M (i.e., 14.92 pg/ml), which meet the requirements of the on-site rapid detection of the methamphetamine in human urine (500 ng/ml).