Al Nanostructures Research Articles

Field Enhancement Around Al Nanostructures in the DUV–Visible Region

Aluminium supports strong and tunable surface plasmon resonances in the deep ultraviolet (DUV)–visible–near infrared (NIR) region. We have used theoretical modelling based on finite difference time domain (FDTD) to study localized surface plasmon resonance (LSPR) and local field enhancement in isolated, dimer and multimer Al nanostructures. The spherical shape nanostructure shows a larger field enhancement in the DUV–UV region, whereas the anisotropic shape nanostructure shows a larger field enhancement with tunability in the peak wavelength towards the visible region. The extinction spectra and the electric field distribution profiles of interacting nanoparticles are compared with those of corresponding monomers to reveal the extent of plasmon coupling. The dimer configuration shows polarization-dependent higher field enhancement ~108 at 196 nm, whereas a multimer forming a rhombus shape nanostructure shows field enhancement ~107 at 411 nm. By optimizing size, interparticle separation and shape of the aluminium nanostructure, a larger field enhancement factor in the DUV–visible region can be achieved for sensitive surface-enhanced Raman scattering (SERS) detection of biomolecules.

Light interaction in sapphire/MgF2/Al triple-layer omnidirectional reflectors in AlGaN-based near ultraviolet light-emitting diodes.

This study examined systematically the mechanism of light interaction in the sapphire/MgF2/Al triple-layer omnidirectional reflectors (ODR) and its effects on the light output power in near ultraviolet light emitting diodes (NUV-LEDs) with the ODR. The light output power of NUV-LEDs with the triple-layer ODR structure increased with decreasing surface roughness of the sapphire backside in the ODR. Theoretical modeling of the roughened surface suggests that the dependence of the reflectance of the triple-layer ODR structure on the surface roughness can be attributed mainly to light absorption by the Al nano-structures and the trapping of scattered light in the MgF2 layer. Furthermore, the ray tracing simulation based upon the theoretical modeling showed good agreement with the measured reflectance of the ODR structure in diffuse mode.

Cathodoluminescence-activated nanoimaging: noninvasive near-field optical microscopy in an electron microscope.

We demonstrate a new nanoimaging platform in which optical excitations generated by a low-energy electron beam in an ultrathin scintillator are used as a noninvasive, near-field optical scanning probe of an underlying sample. We obtain optical images of Al nanostructures with 46 nm resolution and validate the noninvasiveness of this approach by imaging a conjugated polymer film otherwise incompatible with electron microscopy due to electron-induced damage. The high resolution, speed, and noninvasiveness of this "cathodoluminescence-activated" platform also show promise for super-resolution bioimaging.

Size- and morphology-dependent optical properties of ZnS:Al one-dimensional structures.

Typical morphology substrates can improve the efficiency of surface-enhanced Raman scattering; the need for SERS substrates of controlled morphology requires an extensive study. In this paper, one-dimensional ZnS:Al nanostructures with the width of approximately 300 nm and the length of tens um, and micro-scale structures with the width of several um and the length of tens um were synthesized via thermal evaporation on Au-coated silicon substrates and were used to study their size effects on Raman scattering and photoluminescent spectra. The photoluminescence spectra reveal the strongest green emission at a 5 at% Al source, which originates from the Al-dopant emission. The Raman spectra reveal that the size and morphology of the ZnS:Al nanowires greatly influences the Raman scattering, whereas the Al-dopant concentration has a lesser effect on the Raman scattering. The observed Raman scattering intensity of the saw-like ZnS:Al nanowires with the width of tens nm was eight times larger than that of the bulk sample. The enhanced Raman scattering can be regarded as multiple scattering and weak exciton—phonon coupling. The branched one-dimensional nanostructure can be used as an ideal substrate to enhance Raman scattering.

Fabrication and Characterization of Electronic and Optical Properties of Polyaniline Nanopatterns.

The authors conducted polyaniline (HA) polymerization on a micro-scale patterned Si water and nano-scale patterned Al surface. Polymerization was performed using a microliter solution droplet made of aniline, HCI and oxidation agent ammonium peroxodisulfate (APS). The droplet was dropped on a flat Si wafer, a micro-patterned Si wafer and a nanostructured Al surface. The SEM image showed that PA was densely polymerized on the circle edge of the dropped 1 mm sized droplet on the flat Si wafer because of large surface tension due to the flat surface. On the other hand, a droplet was broken on a circular trench pattern of 100 µm in diameter fabricated on a Si wafer. The width and depth of the trench were 1 µm and 1 µm, respectively. Tree-like polymer was intensively polymerized along the circular trench. Droplet was also dropped on a lattice trench pattern whose pitch was 10 µm. The width and the depth of the trench were 1 µm and 1 µm, respectively. The SEM image showed that dots of PA were fabricated along the trenches. Far smaller dots of PA were also observed on the flat area of the lattice. Thus, micro-scale structure affects the shape and size of PA in polymerization. Nanoscopic polymerization of PA was conducted locally in a nanoscale highly-oriented line pattern with nanoscale trenches formed on an Al surface. One of the characteristic fabricated patterns was a highly conductive PA line pattern whose pitch was 100 nm. In this case, point-contact IV characteristic measurement, step-like curve was observed. PL spectra of the PA line-pattern exhibited significantly enhanced emission peaks at 380, 450 anc 550 nm due to PA which were overlapped by the rippled PL pattern due to the Al nanostructure.

Growth process of nanosized aluminum thin films by pulsed laser deposition for fluorescence enhancement

Pulsed laser deposition was used to deposit aluminum thin films of various thicknesses (tAl) ranging from 5 to 40 nm and to investigate their growth process when they are deposited onto SiO2 and Y2O3. Atomic force microscopy and x-ray reflectivity measurements show that the structure of the Al films are related to the wettability properties of the underlaying layer. Onto SiO2, ultra-smooth layers of aluminum are obtained, due to a perfect wetting of SiO2 by Al. In contrast when deposited onto Y2O3, percolated Al layers are observed with apparent pore size decreasing from 200 to 82 nm as is increased from 5 to 40 nm, respectively. This particular morphology is related to partial dewetting of Al on Y2O3. These two different growth mechanisms of aluminum depend therefore on the surface properties of SiO2 and Y2O3. The plasmon resonance of such Al nanostructures in the UV region was then analyzed by studying the coupling between Eu3+ rare earth emitters and Al.

Radiative effects in plasmonic aluminum and silver nanospheres and nanorods

We explore localized surface plasmon resonances in small (5–30 nm radius) aluminum and silver nanoparticles using classical electrodynamics simulations, focusing on radiative (far-field scattering) effects and the unique characteristics of aluminum as a plasmonic material. In Al spheres, higher-order plasmon resonances (e.g. quadrupoles) are significant at smaller sizes (>15 nm) than in Ag spheres. Additionally, although the plasmon width is minimized at a radius of about 15 nm for both materials, the Al plasmon linewidth (~1.4 eV) for the dipole mode is much larger than that observed in Ag (~0.3 eV). The radiative contribution to damping dominates over non-radiative effects for small (5–20 nm) Al spheres (>95%) whereas for similar size Ag spheres damping is almost entirely attributed to the bulk dielectric function (non-radiative). For Al nanorods the linewidths can be narrowed by increasing aspect ratio such that for an aspect ratio of 4.5, the overall Al (0.75 eV) linewidth is reasonably close to that of the same size Ag rod (0.35 eV). This narrowing arises from frequency dispersion in the real part of the Al dielectric function, and is associated with a 65% (1.5 to 0.5 eV) decrease in the radiative contribution to the linewidth for Al. Concurrently, an increase in the non-radiative width occurs as the aspect ratio increases and the plasmon tunes to the red. This demonstrates that anisotropy can be used as a parameter for controlling Al plasmon dephasing where the composition of the plasmon linewidth (radiative or non-radiative) can be tailored with aspect ratio. Overall, these data suggest that localized surface plasmon resonance dephasing mechanisms in Al nanostructures are inherently different from those in the noble metals, which could allow for new applications of plasmonic materials, tunable plasmon lifetimes, and new physics to be observed.

Stress-induced growth of aluminum nanowires with a range of cross-sections

The use of aluminum (Al) as a plasmonic building block has drawn increasing attention of late, due to its natural abundance, and extended tunability into the ultraviolet range. However, a controllable way to grow Al nanowires (NWs) in a bottom-up manner has not been reported. Here a facile, stress-induced growth process for Al nanowires, influenced by the concentration of applied hydrofluoric acid, is reported. Physical characterizations show the nanowires to be polycrystalline fcc Al with ultrasmooth surfaces, with many possible cross-section geometries, making them potential candidates for Al plasmonic investigations and applications. Dark-field optical analysis of a crescent-shaped Al NW demonstrates its plasmonic nature. These results may stimulate new interest in the fabrication of unconventional, nearly one dimensional Al nanostructures.

Efficient confinement of ultraviolet light into a self-assembled, dielectric colloidal monolayer on a flat aluminum film

Herein, we propose the efficient confinement of ultraviolet (UV) light into a self-assembled dielectric colloidal monolayer on a flat aluminum (Al) film. Using a numerical approach, we analyzed each different resonant mode at the UV wavelengths, including the surface plasmon polariton (SPP) and waveguided (WG) mode. The calculated quality-factors (Q-factors) of each resonant mode were at least one order of magnitude higher than those of the existing Al nanostructures. Finally, we examined the influence of the oxide contamination, induced during the Al deposition, on both the SPP and WG modes in order to develop strategies for a realistic experimental fabrication.

UV plasmonic-based sensing properties of aluminum nanoconcave arrays

For several decades the plasmonic behavior of materials has been almost exclusively studied in visible region. Emerging applications require, however, the development of efficient materials operating in UV range. In UV nanoplasmonics aluminum (Al) can play a leading role due to its advantageous electronic properties. Yet, there is still lack of reproducible method to obtain Al nanostructures with desired parameters. Al nanoconcaves can provide a way to overcome these limitations. Here, two different periodicities of the Al nanoconcaves arrays were analyzed. It was observed that the Al concaves can dramatically reduce the optical reflectivity as compared to flat, unstructured Al. At the same time pronounced reflectivity dips were discernible, which were ascribed to (0,1) plasmonic mode. The positions of the dips were at around 250 nm and 350 nm for Al concaves with interpores distance (Dc) of 246.3 nm and 456.7 nm, respectively. The refractive index sensitivity (RIS) was: ∼191 nm/RIU for the Al concaves with Dc = 246.3 nm, and ∼291 nm/RIU for the Al nanoconcaves arrays with Dc = 456.7 nm.

Enhancement of Molecular Fluorescence in the UV Spectral Range Using Aluminum Nanoantennas

We investigate the fluorescence rate of a dipolar emitter coupled to Al nanoparticles of varying shapes and sizes and to dimer nanoantennas in the deep-ultraviolet (UV) spectral range, using the surface integral equation method. In particular, we show that the shape of the Al nanostructures plays a fundamental role in controlling the complex interplay between the excitation rate and the quantum yield in radiative plasmonic systems. In addition, we also investigate the role of the near-field interaction of two coupled Al nanoparticles in enhancing the fluorescence rate of the dipole. This study is important for the engineering of more efficient light-emitting nanostructures in the UV spectral range, such as Al-based material systems or light-emitting fluorophores for biodetection.

Mechanochemical synthesis and shock wave consolidation of TiN(Al) nanostructure solid solution

This paper aims to report the study of the synthesis of a titanium nitride nanostructure solid solution through the reduction of aluminum nitride with titanium based on the stoichiometric reaction of 2Ti + AlN by mechanical alloying (MA) process at a ball-to-powder weight ratio of 15:1. A nanostructure solid solution of in-situ titanium nitride was formed through exchange reaction between Ti and AlN at the initial time of MA. XRD, SEM, EPMA, TEM, and particle size analysis (PSA) were used to characterize the products. It was found that the amount of Al resulting from decomposition of aluminum nitride dissolved in the TiN lattice increased in accordance with milling time, leading to the formation of TiN(Al) solid solution and a reduction of the TiN lattice interplanar distance. The milled powder displayed equiaxed morphology and a narrow size distribution of about 1 μm at the end of the milling process. In-situ TiN(Al) crystallites were of an average size of 6 ± 2 nm. Subsequent to MA, an underwater shock compaction method was applied to the prolonged milled powders to obtain bulk sample. The effect of this shock compaction on the selected sample resulted in the preservation of its nanostructure characteristics with no additional phase transformation which are considered advantageous in the use of high dynamic compaction method for ceramic materials.

Influence of precursor type, deposition time and doping concentration on the morphological, electrical and optical properties of ZnO and ZnO:Al thin films grown by ultrasonic spray pyrolysis

The influence of precursor type, deposition time and doping concentration on the morphological, optical and electrical characteristics of ZnO and ZnO:Al thin films and nanostructures deposited by ultrasonic spray pyrolysis at 400°C on glass substrates was investigated. A deposition period of 30min resulted in compact, uniform samples with thickness of 30 and 45nm, and growth rates in the order of 1 and 1.5nm/min for the Zn acetate and Zn nitrate precursors, respectively. The as-grown films showed good adhesion and excellent optical transmission. Moreover, films grown using Zn acetate precursor exhibited considerably low resistivity, being 14Ω·cm for a deposition time of 30min. Surface roughness was observed to increase with deposition time, while Al doping up to 5% reduced the resistivity down to 4Ω·cm without affecting the optical transmission of the films. Finally, films grown using Zn nitrate were observed to exhibit better photoconductivity than those grown with Zn acetate.

Loss mitigation in plasmonic solar cells: aluminium nanoparticles for broadband photocurrent enhancements in GaAs photodiodes

We illustrate the important trade-off between far-field scattering effects, which have the potential to provide increased optical path length over broad bands, and parasitic absorption due to the excitation of localized surface plasmon resonances in metal nanoparticle arrays. Via detailed comparison of photocurrent enhancements given by Au, Ag and Al nanostructures on thin-film GaAs devices we reveal that parasitic losses can be mitigated through a careful choice of scattering medium. Absorption at the plasmon resonance in Au and Ag structures occurs in the visible spectrum, impairing device performance. In contrast, exploiting Al nanoparticle arrays results in a blue shift of the resonance, enabling the first demonstration of truly broadband plasmon enhanced photocurrent and a 22% integrated efficiency enhancement.

Formation of Al Nanostructures on Alq3: An in Situ Grazing Incidence Small Angle X-ray Scattering Study during Radio Frequency Sputter Deposition

The formation of metal/organic interfaces is a complicated process involving chemical interaction, physical nucleation and diffusion, and thin film growth. It is closely related to the performance of organic electronic devices. To understand this process, we investigate the system of aluminum (Al) and tris(8-hydroxyquinolinato)aluminum (Alq3) as a model, owing to the well-known strong chemical interaction between both and their close technological relevance to organic light emitting devices. By using grazing small angle incidence X-ray scattering (GISAXS), we follow the Al thin film development on top of Alq3 during radio frequency (rf) sputter deposition in real-time and without interrupting the growth process. Three growth stages have been clearly distinguished: Al diffusion into Alq3, Al/Alq3 complex agglomeration and self-assembled Al pillar nanostructure thin film development. Thus in situ GISAXS yields the fundamental insights into the formation of the metal/organic interface for small organic semiconductor devices, prepared via vacuum based deposition techniques.

Effect of aluminum-nano structures on the optoelectronic and structural properties of the AZO layer with various substrate heating temperatures

In this study, Al doped ZnO layers are deposited on glass substrates with various substrate heating temperatures, using RF magnetron sputtering. The conductivity, microstructure and optoelectronic properties are investigated. X-ray diffraction and field emission scanning electron microscopy are used to analyze the crystal orientation and structural characteristics. The optoelectronic properties are measured by UV–Vis–NIR spectroscopy and Hall effect measurement, respectively. The results show that the preferred orientation of the ZnO crystal is changed for different substrate heating temperatures. For lower temperatures, between 200 and 400 °C, the main peak is located at (103) and at substrate temperatures of more than 400 °C, the main peak returns to (002). A comparison of the different substrate heating conditions shows that the conductivity obviously increases with the substrate heating temperatures, especially at 300 °C, where the lowest resistance of 2.356 × 10−3 Ω is achieved, and the average transmittance in visible light, from 400 to 800 nm, is about 92 %. Well-distributed Al nano-structures with lower density are observed by high-resolution transmission electron microscopy, which play an important role.

Polymeric photovoltaics with various metallic plasmonic nanostructures

Broadband light absorption enhancement is numerically investigated for the active light harvesting layer of an organic photovoltaic (OPV), which consists of a blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Periodic plasmonic nanostructures placed above and below the active layer incorporate Ag, Al, Au, or a combination of two different metals. Three dimensional (3D) full-field electromagnetic simulations are applied to determine the effect of varying the metal employed in the plasmonic nanostructures on the absorption enhancement of the OPV. In addition, the geometric parameters (e.g., film thickness, period, and diameter) of the symmetrically distributed top and bottom metal (Ag, Al, or Au) nanostructures were varied to optimize the device structure and delineate the mechanism(s) leading to the absorption enhancement. A spectrally broadband, polarization-insensitive, and wide-angle absorption enhancement is obtained using a double plasmonic nanostructure and is attributed to the combined excitation of localized and single-interface surface plasmon polariton modes. The total photon absorption of the OPV with the optimized double plasmonic Ag nanostructures was found to be enhanced by as much as 82.8% and 80.4% under normal (0°) and 60° light incidence, respectively.

Tailoring plasmon resonances in the deep-ultraviolet by size-tunable fabrication of aluminum nanostructures

Localized surface plasmon resonances were controlled at deep-ultraviolet (DUV) wavelengths by fabricating aluminum (Al) nanostructures in a size-controllable manner. Plasmon resonances were obtained at wavelengths from near-UV down to 270 nm (4.6 eV) depending on the fabricated structure size. Such precise size control was realized by the nanosphere lithography technique combined with additional microwave heating to shrink the spaces in a close-packed monolayer of colloidal nanosphere masks. By adjusting the microwave heating time, the sizes of the Al nanostructures could be controlled from 80 nm to 50 nm without the need to use nanosphere beads of different sizes. With the outstanding controllability and versatility of the presented fabrication technique, the fabricated Al nanostructure is promising for use as a DUV plasmonic substrate, a light-harvesting platform for mediating strong light–matter interactions between UV photons and molecules placed near the metal nanostructure.

Deep-Ultraviolet–Blue-Light Surface Plasmon Resonance of Al and Alcore/Al2O3shell in Spherical and Cylindrical Nanostructures

The surface plasmon resonance (SPR) properties in the deep-ultraviolet (UV) to blue-light region of Al and Alcore/Al2O3shell in spherical and cylindrical nanostructures have been investigated using the discrete dipole approximation method. Simulation results show that the extremely short resonance wavelength of the Al nanostructures means the SPR is highly sensitive to the particle size and results in significant phase retardation and multipole resonance. Cylindrical Al nanoparticles show multipole resonance peaks with extremely strong intensity and narrow widths that are blue-shifted and then red-shifted with an increase in the cylinder length. An expression of the SPR peak wavelength, λmax, is derived and related to the peak shift. Cylindrical Alcore/Al2O3shell nanostructures are shown to modulate the SPR through changes to the aspect ratio and Al2O3 shell thickness. The results will assist greatly in modulating or optimizing SPR in the deep-UV to blue-light region for multimode optical filter, enhanced...

Material Characteristics of Zinc Oxide Doped Aluminum for Microharvesting

Material characteristics of Zinc Oxide doped Al are investigated for microharvester. The microharvester include two parts, the first part is zigzag structures on silicon wafer using MEMS fabrication, and the second part is epitaxial ZnO with doping Al nanostructures on ITO glass using aqueous solution. The effects of the growth temperature, growth concentration, Al-doped for ZnO epitaxial growth, and AZO microharvesting are determined. Results show the percent transmittance decreases with increasing growth concentration ratio and the peak intensity of the (002) orientation increased with increasing growth temperature. The ZnO doped Al with zigzag structures have good efficiency of microharvesting due to its larger geometrical strain to area ratio.