Oblique Angle Vapor Deposition Research Articles

Silver thin film deposited 3-dimensional gold nanorod arrays for plasmonic catalysis

Plasmonic metal nanoparticles are of great importance in heterogeneous catalysis applications including partial oxidations, dehydrogenations, and hydrocarbon reforming. Although utilization of such nanoparticulate metals seems to be a simple and cheap approach for catalysis, their low surface area, un-controlled aggregation, and limited recovery and recycling abilities prevent their further applications. Herein, to overcome these limitations, vertically aligned, highly ordered, and unidirectional 3-dimensional (3D) gold nanorod (AuNR) arrays were fabricated through an oblique angle vapor deposition method. The plasmonic features and catalytic efficiencies of 3D AuNR arrays were further improved by combining them with a thin layer of silver having varying thicknesses. The catalytic activity of fabricated samples was evaluated by the reduction of 4-nitrophenol to 4-aminophenol with NaBH4 in the absence and presence of laser illumination with different wavelengths. All the results indicate that the current Ag@AuNR substrates are expected to have a broad impact in a range of fields including plasmonic catalysis, sensing, and many others.

Ag Nanorod Arrays for SERS: Aspects of Spectral Reproducibility, Surface Contamination, and Spectral Sensitivity

Ag nanorod arrays prepared by oblique angle vapor deposition (OAD) represent regular, large area substrates for surface‐enhanced Raman scattering (SERS) spectroscopy. We studied uniformity and spectral reproducibility of silver OAD‐fabricated substrates (AgOADs) by spectral mapping of methylene blue. The results demonstrate good reproducibility apart from occasional “hot‐spot” sites where the intensity is higher. The number of “hot‐spots” represents 2%–6% of SERS‐active sites of mapping substrate area. We were able to obtain good SERS spectra of testing amino acid tryptophan at 1 × 10−5 M concentration and three different free‐base porphyrins down to ∼10−7 M concentration. We found out that keeping the AgOADs in a vacuum chamber overnight prevents the surface from binding any contaminants from the ambient atmosphere, without significant reduction in the SERS enhancement. Such substrates provide stable SERS enhancement even when stored for 1 year after preparation.

A study of Parylene coated Pd/Mg nanoblabes for reversible hydrogen storage

Catalyst Pd decorated Mg nanoblades grown by oblique angle vapor deposition has been shown to possess low desorption temperature and fast kinetics in the dehydrogenation process. Using chemical vapor deposition, we coated these Pd/Mg nanoblades with a shell of ultrathin Parylene N layer that allows high permeation of hydrogen but low permeation of non-hydrogen gases in an attempt to protect the Pd/Mg nanoblades. The temperature programmed desorption experiments demonstrated that hydrogen can permeate through the Parylene shelled Pd/Mg nanoblades and is capable for reversible hydrogen cycling under a low dehydrogenation temperature of 403 K. Comparing the Pd/Mg nanoblades without Parylene coating to the Pd/Mg nanoblades with Parylene coating exhibit a comparable hydrogen capacity but a slower kinetics of hydrogen desorption. To understand the kinetics of hydrogen desorption, a modified Johnson–Mehi–Avrami (JMA) model was employed. From data fitting, we found that the experimental observation of a slowing-down kinetics can be interpreted in terms of simultaneously increased activation energy and impingement factor in Parylene coated samples. Oxidation characteristics under thermal annealing in ambient air between Parylene coated and no Parylene coated Mg nanoblades were studied by X-ray photoelectron spectroscopy. It shows that the increased amount of contaminations and oxygen near sample surface in Parylene coated samples are responsible for the slower kinetics of desorption. Our experimental work shows the feasibility that conformal Parylene coating of nanoscale Pd/Mg nanoblades can be used for reversible hydrogen storage.

Ag nanorod based surface‐enhanced Raman spectroscopy applied to bioanalytical sensing

Recent progress in substrate nanofabrication has led to the development of Ag nanorod arrays as uniform, reproducible, large area SERS-active substrates with high signal enhancement. These novel nanostructures fabricated by oblique angle vapor deposition (OAD) offer a robust platform for the rapid detection of biological agents and open new perspectives for the development and integration of biomedical diagnostic for clinical and therapeutic applications. Ag nanorod arrays have been investigated as SERS-active substrates for the detection and identification of pathogens, including bacteria and viruses, as well as to evaluate the potential of this biosensing platform for bio-recognition of high affinity events using oligonucleotide-modified substrates. This review summarizes the various nanostructured substrates designed for SERS-based applications, highlights the nanofabrication methodology used to produce Ag nanorod arrays, outlines their morphological and physical properties, and provides a summary of the most recent uses of these substrates for clinical diagnostic and biomedical applications.

Temperature dependent biaxial texture evolution in Ge films under oblique angle vapor deposition

The morphology and texture of Ge films grown under oblique angle vapor deposition on native oxide covered Si(001) substrates at temperatures ranging from 230°C to 400°C were studied using scanning electron microscopy, X-ray diffraction and X-ray pole figure techniques. A transition from polycrystalline to {001}<110> biaxial texture was observed within this temperature range. The Ge films grown at substrate temperatures <375°C were polycrystalline. At substrate temperatures of 375°C and 400°C, a mixture of polycrystalline and biaxial texture was observed. The 230°C sample consisted of isolated nanorods, while all other films were continuous. The observed biaxial texture is proposed to be a result of the loss of the interface oxide layer, resulting in epitaxial deposition of Ge on the Si and a texture following that of the Si(001) substrates used. The rate of oxide loss was found to increase under oblique angle vapor deposition.

Removal of Surface Contamination and Self-Assembled Monolayers (SAMs) from Silver (Ag) Nanorod Substrates by Plasma Cleaning with Argon

Surface contamination of surface-enhanced Raman (SERS)-active metallic substrates has been a limitation to the utility of SERS as an analytical technique, potentially affecting surface coverage, spectral reproducibility, and analytical limits of detection. We have developed a simple and versatile cleaning method for SERS-active Ag nanorod arrays that consists of a short (4 min) exposure of the substrate to an Ar(+) plasma in a low-pressure environment. The findings presented here demonstrate that this cleaning procedure essentially eliminates organic background contamination. This procedure works equally well for self-assembled monolayers of thiolates that strongly adsorb onto Au and Ag surfaces. For SERS-active surfaces composed of arrays of Ag nanorods prepared by oblique-angle vapor deposition, we investigated the (1) Raman band intensities, (2) nanorod morphology via scanning electron microscopy, and (3) surface hydrophobicity via static contact angle measurements, as a function of exposure time of the Ag nanorods to the Ar(+) plasma. Short (4 min) exposure to Ar(+) plasma eliminated background contamination but decreased the observed SERS intensity for re-adsorbed analytes by approximately a factor of 2 while leaving the nanorod morphology essentially unchanged. Prolonged exposure to Ar(+) plasma (>10 min) resulted in substantial morphological changes of the Ag nanorod lattice and led to a decrease in the observed SERS intensities by a factor of 10. The results presented here suggest that Ar(+) plasma cleaning is an efficient process for removing carbonaceous and organic contamination as well as thiolate monolayers from SERS-active Ag surfaces, as long as the plasma conditions and exposure times are carefully monitored.

Identification of individual genotypes of measles virus using surface enhanced Raman spectroscopy

A spectroscopic assay based on surface-enhanced Raman spectroscopy (SERS) has been developed for rapid genotyping of the measles virus (MeV). Silver nanorods fabricated using an oblique angle vapor deposition method acted as the SERS-active substrate. The SERS spectra of four separate MeV genotypes, i.e. A, H1, D4 and D9, and two separate negative media control samples were analyzed using multivariate statistical methods. Principal components analysis (PCA) and hierarchical cluster analysis (HCA) successfully separated three of the four MeV genotypes studied. The MeV genotypes used in this study had >96% sequence similarity as monitored using the MeV hemagglutinin (H) gene, and the clustering seen in PCA and HCA mirrored this sequence diversity. For example, the MeV genotypes with the highest sequence diversity (~3%, A and H1) were the most widely separated in the PCA scores plot and HCA dendogram. Conversely, the MeV genotypes with the lowest sequence diversity (~0.5%, D4 and D9) could not be statistically differentiated. However, a supervised chemometric method, partial least squares-discriminant analysis (PLS-DA) was able to separate each of the four MeV strains, the two negative controls, and the background, with >90% sensitivity and >96% selectivity based solely on their inherent SERS spectra. These results demonstrate that SERS, in combination with multivariate statistical methods, is a highly sensitive and rapid viral identification and classification method that can be applied to MeV genotyping.

Quantitative Surface-Enhanced Raman Spectroscopy Based Analysis of MicroRNA Mixtures

We have developed a rapid, sensitive, and quantitative method for identification of microRNA (miRNA) sequences in multicomponent mixtures using surface-enhanced Raman spectroscopy (SERS). The method uses Ag nanorod array substrates prepared by oblique angle vapor deposition as the SERS platform. We show that Ag nanorod-based SERS spectra are uniquely characteristic for each miRNA sequence studied, and that the spectral reproducibility is sufficient for quantitative analysis of miRNA profiles in multicomponent mixtures using partial least squares (PLS) regression analysis. This method was applied to individual sample mixtures consisting of two, three, and five miRNAs. Separate PLS models were generated for the two-, three-, and five-component mixtures from >150 calibration spectra covering a concentration range of 6 to 150 microM for each miRNA. The PLS models were externally validated with independent test samples resulting in root mean square errors of prediction (RMSEP) of 7.4, <7.4, and <10 microM for the two-, three-, and five-component models, respectively. These results demonstrate the applicability of SERS for quantitative detection and profiling of miRNAs and suggest that SERS may prove to be a novel, label-free method for identification of disease biomarkers.

Growth of CdTe Films on Amorphous Substrates Using CaF2 Nanorods as a Buffer Layer

Continuous biaxially textured CdTe films were grown on biaxial CaF 2 buffer layers. The CaF 2 nanorods were grown by oblique angle vapor deposition and possessed a {111} biaxial texture. The CdTe film was deposited by metal organic chemical vapor deposition (MOCVD). Film morphology and the CdTe/ CaF 2 interface were studied by scanning electron microscopy and transmission electron microscopy. Characterization showed that small CdTe grains formed initially from the CaF 2 surfaces. These small grains then merged into large columnar grains during growth. Analysis revealed that the crystalline orientation of the CdTe film followed the biaxial texture of the CaF 2 nanorods.

Novel Ultrathin Mg Nanoblades for Hydrogen Storage

AbstractWe describe the growth of novel ultrathin Mg crystalline nanoblades by oblique angle vapor deposition. These nanoblades were then coated with catalyst Pd and hydrogenated into magnesium hydride MgH2. In situ thermal desorption spectroscopy study showed a low H desorption temperature at ∼365 K. In situ reflection high energy electron diffraction patterns were used to study the temperature dependent structure and composition changes during the de-hydrogenation of Pd coated MgH2 nanoblades. The diffraction rings reveal the formation of alloys of Pd and Mg when the temperature is over ∼480 K. Transmission electron microscopy diffraction also supports the formation of Pd and Mg alloys. This alloying reduces the cycling capability of Mg hydride. The de-hydrogenation of MgH2 introduces a strain at the bilayer interface between MgH2 and Mg resultant from 30% volume reduction from MgH2 to Mg and formed curved nanoblades as evident by scanning electron microscopy images. Designing factors of recyclable simple hydrides will be discussed.

Trace detection and differentiation of uranyl(VI) ion cast films utilizing aligned Ag nanorod SERS substrates

Aligned Ag nanorod arrays prepared by oblique angle vapor deposition (OAD) were utilized to observe the SERS response for cast films of adsorbed uranyl ion complexes. These nanorod arrays consisted of preferentially aligned and tilted cylindrically shaped nanorods (average nanorod length = 858 nm). Routine SERS spectra of cast films of uranyl nitrate resulted in 0.87 pg of material analyzed with a detection limit estimated at 0.87 fg of adsorbed uranyl nitrate. The main spectral feature observed was a strong, broad peak centered at 700 cm −1 assigned to the ν 1 symmetric stretch of the uranyl ion, ν s(UO 2). The complex shape of this band is attributed to multiple uranyl complexes adsorbed onto the Ag surface. Uranyl nitrate cast films formed from ethanol solutions were more uniform and had stronger adsorption to the Ag surface compared to films formed from aqueous solutions. Differentiation of uranyl ion cast films formed from four different uranyl systems (i.e., uranyl nitrate hexahydrate, uranyl acetate dihydrate, uranyl formate monohydrate, and uranyl chloride trihydrate) was accomplished using univariate spectral analysis as well as Partial Least Squares Discriminant Analysis (PLSDA). A simple PLSDA calibration was able to correctly classify 27 SERS spectra of unknown uranyl ion cast films based on the type of anionic ligand present.

Emergent properties of spatially organized poly( p-xylylene) films fabricated by vapor deposition

A novel bottom-up process to deposit structured poly( p-xylylene) (PPX) polymer films, based on oblique angle vapor deposition polymerization, is introduced. In this process, monomer vapors produced by pyrolysis of chemically functionalized p-xylylene precursors are directed at an oblique angle towards a surface to initiate structured polymer growth. Inclined deposition induces growth of PPX nanofibers leading to a nanostructured surface comprising clusters of ∼100–150 nm diameter nanocolumns. Morphology of these films can be controlled by manipulating the substrate rotation during oblique angle deposition (i.e., helical, chevron and columnar morphology). The morphology of the nanostructured PPX films are characterized by atomic force microscopy and scanning electron microscopy.

Identification and classification of respiratory syncytial virus (RSV) strains by surface-enhanced Raman spectroscopy and multivariate statistical techniques

There is a critical need for a rapid and sensitive means of detecting viruses. Recent reports from our laboratory have shown that surface-enhanced Raman spectroscopy (SERS) can meet these needs. In this study, SERS was used to obtain the Raman spectra of respiratory syncytial virus (RSV) strains A/Long, B1, and A2. SERS-active substrates composed of silver nanorods were fabricated using an oblique angle vapor deposition method. The SERS spectra obtained for each virus were shown to possess a high degree of reproducibility. Based on their intrinsic SERS spectra, the four virus strains were readily detected and classified using the multivariate statistical methods principal component analysis (PCA) and hierarchical cluster analysis (HCA). The chemometric results show that PCA is able to separate the three virus strains unambiguously, whereas the HCA method was able to readily distinguish an A2 strain-related G gene mutant virus (DeltaG) from the A2 strain. The results described here demonstrate that SERS, in combination with multivariate statistical methods, can be utilized as a highly sensitive and rapid viral identification and classification method.

The Use of Aligned Silver Nanorod Arrays Prepared by Oblique Angle Deposition as Surface Enhanced Raman Scattering Substrates

Aligned Ag nanorod arrays have been fabricated on glass substrates using an oblique angle vapor deposition (OAD) method. The surface morphology and structure of the Ag rods were studied by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Individual nanorods were found to be primarily cylindrical, albeit with irregularly shaped surface protrusions of the rods. The surface enhanced Raman scattering (SERS) response as a function of nanorod length was investigated using trans-1,2-bis(4-pyridyl)ethene as a probe molecule at an excitation wavelength of 785 nm. The enhancement factors reached a maximum of ∼5 × 108 as the nanorod length approached 868 nm and then decreased to ∼3 × 106 as the nanorod length increased further to 1900 nm. For the 868 nm nanorod array, signals collected over multiple spots within the same substrate resulted in a relative standard deviation (RSD) of 10%, while an RSD of 15% was measured in signals collected from different substrates. When stored in a food vacuum bag, the substrates are stable with respect to SERS intensity for greater than 2 months. This study demonstrates that the OAD Ag nanorod arrays are highly sensitive, uniform, and stable SERS substrates that are suitable for a variety of surface chemical analysis applications.

Growth of nanostructured thin films of poly( p-xylylene) derivatives by vapor deposition

Nanostructured thin films of poly( p-xylylene) derivatives are deposited by oblique angle vapor deposition method under low-vacuum conditions. We showed deposition of columnar nanostructured poly(chloro- p-xylylene) and poly(bromo- p-xylylene) thin films, and co-deposition of nanostructured poly( o-trifluoroacetyl- p-xylylene- co- p-xylylene) thin film. Characterization of both the nanostructured and planar thin films of poly( p-xylylene) are performed with different experimental methods. We developed a generalized strategy towards depositing nanostructured poly( p-xylylene) derivatives and thus promise a new generation of thin films suitable for biomedical and antifouling applications.

Surface pole figures by reflection high-energy electron diffraction

The authors demonstrated that it is possible to construct a reflection high-energy electron diffraction (RHEED) pole figure of a polycrystalline film by recording multiple RHEED patterns as they rotate the substrate around the surface normal. Since electrons have limited penetration depth, the pole figure constructed is a surface pole figure. It is in contrast with the conventional x-ray pole figure which gives an average texture information of the entire film. Surface texture change, particularly the evolution of multiple preferred orientations, in polycrystalline Ru films grown by oblique angle vapor deposition has been observed using this RHEED surface pole figure technique.

Aligned Silver Nanorod Arrays as Substrates for Surface-Enhanced Infrared Absorption Spectroscopy

Preferentially aligned silver nanorod arrays prepared by oblique angle vapor deposition were evaluated as substrates for surface-enhanced infrared absorption (SEIRA) spectroscopy. These nanorod arrays have an irregular surface lattice and are composed of tilted, cylindrically shaped nanorods that have an average length of 868 nm +/- 95 nm and an average diameter of 99 nm +/- 29 nm. The overall enhancement factor for chemisorbed organic films of para-nitrobenzoic acid (PNBA) deposited onto the Ag nanorod arrays analyzed by external reflection SEIRA was calculated to be 31 +/- 9 compared to infrared reflection-absorption spectroscopy (IRRAS) obtained from a 500 nm Ag film substrate. This enhancement is attributed to the unique optical properties of the nanorod arrays as well as the increased surface area provided by the nanorod substrate. SEIRA reflection-absorbance intensity was observed with both p- and s-polarized incident radiation with angles of incidence ranging from 25 degrees to 80 degrees . The largest intensity was achieved with p-polarization and incident angles larger than 75 degrees . Polarization-dependent ultraviolet/visible/near-infrared (UV/Vis/NIR) spectra of the nanorod arrays demonstrate that the red-shifted surface plasmon peaks of the elongated nanorods may be partially responsible for the observed SEIRA response. The SEIRA detection limit for the Ag nanorod arrays was estimated to be 0.08 ng/cm(2). Surface-enhanced Raman scattering (SERS) and SEIRA analysis of chemisorbed PNBA utilizing the same nanorod substrate is demonstrated.

Polarized Surface Enhanced Raman and Absorbance Spectra of Aligned Silver Nanorod Arrays

Polarized surface-enhanced Raman scattering (SERS) and UV-vis absorbance spectra were measured for a nonplanar Ag nanorod array substrate prepared by oblique angle vapor deposition. The anisotropy of the SERS polarization was shown to differ from that of the polarized UV-vis absorbance. The maximum SERS intensity was observed in the polarization direction perpendicular to the long axis of the Ag nanorods, while the UV-vis absorbance was strongly polarized along the direction of the long axis of the nanorod array. Analysis of the polarization data showed that molecular orientation was not the cause of the anisotropic SERS scattering. Rather, the SERS anisotropy was primarily attributed to the lateral arrangement of the three-dimensional tilted nanorod lattice in which highly localized plasmon modes are created by strong electromagnetic coupling between adjacent metallic nanorods.

Aligned silver nanorod arrays produce high sensitivity surface-enhanced Raman spectroscopy substrates

Substrates consisting of silver nanorod arrays with an irregular surface lattice (i.e., random nucleation sites) and with varying rod lengths were fabricated by an oblique angle vapor deposition method. These arrays were evaluated as potential surface-enhanced Raman spectroscopy (SERS) substrates using trans-1,2-bis(4-pyridyl)ethene as a reported molecule. SERS activity was shown to depend upon the length of the nanorods. The Ag nanorods with average lengths of 508.29±44.86nm, and having aspect ratios of 5.69±1.49 exhibited the maximum SERS enhancement factors of greater than 108. Theoretical calculations indicate that this large SERS enhancement may be partially explained by the shape, density, and lateral arrangement of the Ag nanorod arrays.

AFM, SEM andin situRHEED study of Cu texture evolution on amorphous carbon by oblique angle vapor deposition

The evolution of the crystal orientation of a Cu film grown on an amorphous carbon substrate without intentional heating under 75±6° oblique angle vapor deposition was investigated ex-situ by atomic force microscopy AFM and scanning electron microscopy SEM, and in-situ by reflection high-energy electron diffraction RHEED. At the initial stage of growth 12 nm thick the diffraction patterns showed uniform rings indicating random nucleations of crystals. With more Cu depositions 100 nm thick the diffraction pattern started to break symmetrically from the middle of the 111 and 200 rings representing the absence of 111 and 200 planes parallel to the substrate. However, after this transition stage, at the thickness of 410 nm, the intensity distribution of diffraction patterns appeared asymmetric about the middle of the rings, which is interpreted as the appearance of a tilted 111 texture. Finally the diffraction patterns developed into separated short arcs and showed only a 1111 ¯10 II-O two-orientation texture. By comparing RHEED patterns with the SEM and AFM images of the final film, we argue that the tilted columns having tilted 111 top faces dominate in the later stage of growth. Furthermore, considering the geometry of crystals and shadowing effects, we argue that the vertices of columns having the highest growth velocity normal to the substrate and therefore receiving the maximum flux will dominate the film growth and determine the tilt angle of the texture and the preference of the azimuthal angle orientation.