Nano-sculptured Thin Films Research Articles

On the characteristics and optical properties of Mn-based (MnO2) helical conical nanostructure thin films

Mn-based helical conical nano-sculptured thin films were fabricated by means of oblique angle deposition method in conjunction with the rotation of sample holder with different speeds at different pitches of each revolution. Atomic force microscopy, field emission scanning electron microscopy and X-ray diffraction (XRD) analyses were used to obtain morphology, nanostructure and crystallography of the samples. Formation of MnO2 film by the proposed method of this work is confirmed by the XRD results. Optical spectra of the films were obtained using s- and p-polarized incident lights at three different incident light angles of 0°, 20° and 30°. Bruggeman homogenization method was employed to obtain spectra for different optical parameters. The observed peaks/oscillations in these spectra are related to the different radii of the deposited sculptured structure with helical conical shape. Results show that by engineering of Mn-based (MnO2) helical conical sculptured thin film the absorption is increased. Hence, by engineering of this type of thin films the absorption increases due to entrapment of light between the elements of the structure (rings) and results in a high broadband absorption.

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.

Impedance spectroscopy characterization of anisotropic nano-sculptured copper oxide Cu2O thin films for optoelectronic applications

In this work, we report a simple way of copper oxide Cu2O elaboration, by first depositing copper using thermal vacuum evaporation at different substrate angles. This technique is known as GLAD (glancing angle deposition). In order to obtain the oxide phase, the copper nanostructures obtained were annealed in air at 523 K for 2 h. After air annealing, the films were characterized by x-ray diffraction, scanning electron microscopy, four-point probe method and UV–vis-NIR spectrophotometry. XRD study showed that Cu2O phase was obtained. The optical direct band gap energies for Cu2O films calculated from optical absorption measurements are between 2.45 and 2.54 eV. Optical and electrical anisotropy versus the incident angle of the Cu2O thin films were reported. An enhancement of the birefringence was observed for the incident angle around 60°. Electrical properties of the Cu2O films were investigated by AC impedance spectroscopy over a wide range of temperature up to 468 K starting from room temperature in the frequency range 5Hz–13 MHz. Nyquist diagrams of all samples display two semicircles that highlight the influences of single nanocrystals and grain boundaries and interface as well as porosity of samples. Impedance analysis showed that the resistance decreased by increasing temperature. Both analysis of AC and DC conductivities showed that the conduction mechanism was assured by hopping between localized states.

Gold coated metal nanostructures grown by glancing angle deposition and pulsed electroplating

Nickel based nanostructures are grown by glancing angle deposition (GLAD) on flat and pre-patterned substrates. These fabricated porous thin films were subsequently coated by pulsed electroplating with gold. The morphology and conformity of the gold coating were investigated by scanning electron microscopy and X-ray diffraction. Controlled growth of closed gold layers on the nanostructures could be achieved, while the open-pore structure of the nanosculptured thin films was preserved. Such gold coated nanostructures are a candidate for optical sensing and catalysis applications. The demonstrated method can be applied for numerous material combinations, allowing to provide GLAD thin films with new surface properties.

Design and fabrication of highly hydrophobic Mn nano-sculptured thin films and evaluation of surface properties on hydrophobicity

The wettability of solid surfaces is important from the aspects of both science and technology. The Mn nano-sculptured thin films were designed and fabricated by oblique angle deposition of Mn on glass substrates at room temperature. The obtained structure was characterized by field emission scanning electron microscopy and atomic force microscopy. The wettability of thin films samples was investigated by water contact angle (WCA). The 4-pointed helical star-shaped structure exhibits hydrophobicity with static WCAs of more than 133° for a 10-mg distilled water droplet. This sample also shows the rose petal effect with the additional property of high adhesion. The Mn nano-sculptured thin films also act as a sticky surface which is confirmed by hysteresis of the contact angle obtained from advancing and receding contact angles measurements. Physicochemical property of liquid phase could effectively change the contact angle, and polar solvents in contact with hydrophobic solid surfaces do not necessarily show high contact angle value.

Enhanced intrinsic fluorescence from carboxidized nano-sculptured thin films of silver and their application for label free dual detection of glycated hemoglobin.

Enhanced intrinsic fluorescence (~x103) from novel carboxidized nanosculptured thin films (CO-nSTFs) of silver is reported. The sources of intrinsic fluorescence, confirmed by X-ray photoelectron spectroscopy, are Ag2O grains and residual carbon formed on the outer layer of silver nSTFs when exposed to air, while the localized surface plasmons on silver nSTFs enhance this intrinsic fluorescence. The CO-nSTFs are optimized with respect to porosity for the maximum enhancement. A sensor developed by using the self-assembled monolayer technique on optimized CO-nSTF is used for the label free detection of glycated hemoglobin, performed by simultaneously using fluorescence imaging and spectroscopy. The specificity of the sensor is established from control experiments on hemoglobin. These novel nanorod like intrinsically fluorescent CO-nSTFs pose huge potential in label free biosensing, light sources, imaging and many more applications.

Liquid crystals alignment with PbS nanosculptured thin films

ABSTRACTLead sulphide (PbS) nano-sculptured thin films (nSTFs) are prepared for the first time using glancing angle deposition technique by physical vapour deposition process. An anisotropic morphology is obtained, which provides an orientational effect. Due to this anisotropic nature of PbS nSTF an attempt has been made to check its effect on the alignment of nematic liquid crystals (LCs). Two different LC devices were fabricated, that is, anti-parallel aligned or electrically-controlled birefringence and hybrid twisted nematic. It is noticed that in both types of devices, good alignment is obtained with high extinction and contrast. A significant effect of these alignments on the electro-optic properties of the LC is reported such as the threshold voltage, response time and contrast ratio.

Design and engineering of sculptured nano-structures for application in hydrophobicity

Abstract The design and engineering of suitable structures for enhancement of the hydrophobic property of a surface is one of the most challenging problems. In order to achieve a superhydrophobic structure we have designed and fabricated Mn nano-sculptured thin films with different shapes and dimensions, namely helical squares and helical pentagons on glass substrates. The contact angle (CA) of three liquids; α–bromonaphtalene (apolar), water and formamide (polar) to these surfaces was measured and the surface free energy was calculated. Changes to the geometry of the structure produced results ranging from hydrophilic (CA = 51°) to superhydrophobic (CA = 152°). The superhydrophobic structure is a helical square shaped structure with high porosity (deposited at 83°) which also shows the rose petal effect with the additional property of high adhesion. The resemblance of this structure to that of gecko feet, which shows both high adhesion forces and superhydrophobic property is discussed. All structures investigated in this work showed negative spreading coefficients with highest values for the largest contact angle for each type/shape of structure. The superhydrophobic sample also acts as a sticky surface which is confirmed by hysteresis of the contact angle obtained from advancing and receding contact angles measurements. The influence of the volume of liquid drop and different surface morphologies on the wetting transition from Cassie–Baxter to Wenzel states is also reported.

Highly sensitive and specific detection of E. coli by a SERS nanobiosensor chip utilizing metallic nanosculptured thin films.

A nanobiosensor chip, utilizing surface enhanced Raman spectroscopy (SERS) on nanosculptured thin films (nSTFs) of silver, was shown to detect Escherichia coli (E. coli) bacteria down to the concentration level of a single bacterium. The sensor utilizes highly enhanced plasmonic nSTFs of silver on a silicon platform for the enhancement of Raman bands as checked with adsorbed 4-aminothiophenol molecules. T-4 bacteriophages were immobilized on the aforementioned surface of the chip for the specific capture of target E. coli bacteria. To demonstrate that no significant non-specific immobilization of other bacteria occurs, three different, additional bacterial strains, Chromobacterium violaceum, Paracoccus denitrificans and Pseudomonas aeruginosa were used. Furthermore, experiments performed on an additional strain of E. coli to address the specificity and reusability of the sensor showed that the sensor operates for different strains of E. coli and is reusable. Time resolved phase contrast microscopy of the E. coli-T4 bacteriophage chip was performed to study its interaction with bacteria over time. Results showed that the present sensor performs a fast, accurate and stable detection of E. coli with ultra-small concentrations of bacteria down to the level of a single bacterium in 10 μl volume of the sample.

Plasmonic Sensing Using Metallic Nano‐Sculptured Thin Films

Nano-sculptured thin films (nSTFs) is a group of meterials prepared by the oblique or the glancing angle deposition technique. They take the form of rods having different shapes such as nanocolumns, nanoscrews, nanozigzags and many other nanoshapes. Their potential for biosensing is highlighted in this review particularly the metallic ones due to their remarkable plasmonic properties. The techniques that have been shown so far to be of high potential are: extended surface plasmon resonance (SPR), localised SPR, surface enhanced flourescence (SEF) and Raman scattering (SERS). The use of metal nSTFs in SPR biosensors with Kretschmann-Raether configuration enhances both the angular and the spectral sensitivities due to the porosity and adds more degrees of freedom in designing evanescent waves based techniques. The metallic nSTFs, exhibit remarkable localised plasmonic properties that make them a promising substrate for enhanced spectroscopies. Their long term stability in water environment makes them suitable candidates for biosensing in water as it is already demonstrated for several water pollutants. The influences of the nanostructures' size, topology, the substrate features, and the preparation conditions on the enhancement of SEF and SERS are highlighted with emphases on the unresolved issues and future trends.

SERS biosensor using metallic nano-sculptured thin films for the detection of endocrine disrupting compound biomarker vitellogenin.

A biosensor chip is developed for the detection of a protein biomarker of endocrine disrupting compounds, vitellogenin (Vg) in aquatic environment. The sensor chip is fabricated by immobilizing anti-Vg antibody on 4-Aminothiophenol (4-ATP) coated nanosculptured thin films (nSTFs) of silver on Si substrates. The biosensor is based on the SERS of 4-ATP, enhanced by the Ag nSTFs. Before the fabrication of the sensor, the performance of the enhancement is optimized with respect to the porosity of nSTFs. Further, the biosensor is developed on the nSTF with optimized enhancement. The SERS signals are recorded from the sensor chip for varying concentrations of Vg. A control experiment is performed on another similar protein Fetuin to confirm the specificity of the sensor. The repeatability and reusability of the sensor, along with its shelf life are also checked. The limit of detection of the sensor is found to be 5 pg mL −1 of Vg in PBS within our experimental window. Apart from high sensitivity, specificity and reusability, the present sensor provides additional advantages of miniaturization, requirement of very small volumes of the analyte solution (15 μL) and fast response as compared to conventional techniques e.g., ELISA, as its response time is less than 3 minutes.

Nanostructure and Optical Properties of Silver Helical Pentagon Nanosculptured Thin Films

Silver helical pentagon shaped nanosculptured thin films (HPNSTFs) were produced using oblique angle deposition method in conjunction with the rotation of sample holder under controlled conditions. Thes-polarization extinction spectra obtained at different azimuthal angles (φ) and low incidence angle (i.e.,10∘) from the Ag (HPNSTF) did not show significant change in the plasmon peak position, while at higher incidence angle (i.e.,70∘) a blue shift appeared for the broad peak which was observed for lower incidence angle (i.e.,10∘) and occurred at lower wavelength. In case ofp-polarized light a very broad peak was obtained for the70∘incidence angle and for differentφangles and when compared with the lower incidence angle results it can be concluded that it is gone under a red shift. Polar diagrams of the samples showed slight anisotropy that should be due to high symmetry of the pentagon helical structure.

On the fabrication and characterization of graded slanted chiral nano-sculptured silver thin films

Graded slanted chiral sculptured silver thin films are produced using oblique angle deposition together with rotation of substrate holder about its surface normal, plus a shadowing block, fixed at the center of the substrate holder. Scanning electron microscope (SEM) and atomic force microscope (AFM) were used for characterization of these films. The results showed a structural gradient with distance from the edge of the shadowing block, which in turn is responsible for the decrease in the volume of void fraction and increase of grain size. Plasmon absorption peaks observed in the optical analysis of these nano-structures showed that their wavelength region and intensity depend on the polarization and the incident angle of light, as well as the distance from the edge of the shadowing block. Photoluminescence (PL) study of the samples showed that there is an inverse size dependence of the PL spectra intensity.

Detailed study of surface-enhanced Raman scattering from metallic nanosculptured thin films and their potential for biosensing

Surface-enhanced Raman scattering (SERS) from silver nanosculptured thin films (STF) was studied in detail for biosensing. The influences of the nanostructures' sizes, topology, the substrate features, and the preparation conditions on the enhancement were examined. Enhancement factors on the order of 10 7 were obtained from silver nanorods deposited on bare silicon substrates with respect to their dense counterparts, using 4-aminotheophenol (4-ATP) for the Raman emission. The low detection limit that can be achieved with STFs is below 1 μg/lit of the probe molecule 4-ATP in Ethanol solution. Theoretical modeling based on a single small spheroidal nanoparticle helped in explaining the main properties of SERS from STFs. Stability of the films was noticed over a period of one year without significant degradation.

Fabrication, characterization and some applications of graded chiral zigzag shaped nano-sculptured silver thin films

Graded chiral zig–zag shaped nano-sculptured silver thin films (GCZSSTF) were produced in two stages using oblique deposition technique together with rotation of substrate about its surface normal while a shadowing block was also fixed at the center of the substrate holder. Chrystallographic and morphological structure of these films were obtained using X-ray diffraction (XRD) and atomic force microscopy (AFM). Spectrophotometry was used to obtain their optical behavior while their application in both hydrophobicity and gas sensing was also investigated. XRD results showed a dominant (1 1 1) orientation growth on the zig arm of the structure while by addition of the second arm (zag) the crystallographical growth orientation changed to (2 2 0). The anisotropic nano-structure of these films was also distinguished through (1 − R) spectra. A common peak at about 350 nm related to the TM mode of plasmon resonances and a broad shoulder at about 420 nm for the s-polarized light and at 620 nm for the p-polarized light corresponding to the LM mode of plasmon resonances are observed. These peaks are directly related to the nano-columns topography. The film system used here proved to act as a physical method for producing layer-by-layer structure for obtaining enhanced hydrophobic surfaces rather than the usual chemical methods reported in the literature. In addition, the GCZSSTF also acted as good as reported results for nano-tubes when applied as cathode in the field ionization gas sensing setup.

Optical anisotropy of tilted columns thin films of chromium deposited at oblique incidence

Several chromium thin films with thicknesses comprised between 840 nm and 440 nm have been deposited by magnetron sputtering at glancing angle on silicon substrates. The deposition angle was varied between 0° and 80° resulting in nanosculptured thin films with column angles comprised between 0° and 25°. The surface topography of the films has been characterized by atomic force microscopy. It revealed different structurations of the surface originating from: the cross-section of the tilted columns with the surface, lateral ordering of the columns and faceting. The optical properties of the films have been determined from generalized ellipsometric measurements performed along different azimuths. The inspection of the off-diagonal elements of the Jones matrix as a function of columns tilt angle showed that both in-plane and out-of-plane anisotropy were present, associated with the columnar growth.

Thickness and density evaluation for nanostructured thin films by glancing angle deposition

Thickness evaluation is a particular challenge encountered in the fabrication of nanosculptured thin films fabricated by glancing angle deposition (GLAD). In this article, we deduce equations which allow for accurate in situ thickness monitoring of GLAD thin films deposited onto substrates tilted with respect to the direction of incoming vapor. Universal equations are derived for the general case of Gaussian vapor flux distribution, off-axis sensors, variable substrate tilt, and nonunity sticking coefficient. The mathematical description leads to an incidence angle dependence of thickness and density, allowing for quantitative prediction of porosity in samples with different morphologies and thickness calibrations. In addition, variation of sticking probability with the incidence angle creates a nonmonotonic variation of the film thickness and porosity with the substrate tilt. We discuss the implications of the substrate type, sensor type, and source geometry in a precise quantitative determination of the thickness of thin films fabricated on tilted substrates. Our equations can be particularized for the case of films fabricated at normal incidence.