Metal Nanopillars Research Articles

Tunable Magnetic and Optical Anisotropy in ZrO2‐Co Vertically Aligned Nanocomposites

AbstractMetamaterials have gained great research interest in recent years owing to their potential for property tunability, multifunctionality, and property coupling. As a new group of self‐assembled thin films, vertically aligned nanocomposite (VAN)‐based hybrid metamaterials have been demonstrated with significant anisotropic physical properties and a broad range of property tailorability, such as optical anisotropy, magnetic anisotropy, hyperbolic dispersion, and enhanced second harmonic generation properties. Herein, self‐assembled ZrO2‐Co nanocomposite films, with high epitaxial quality and ultra‐fine vertically aligned Co nanopillars (with an average diameter of ≈2 nm) embedded in a ZrO2 matrix, are fabricated using a pulsed laser deposition (PLD) method. The Co pillar density can be effectively tuned by varying the Co concentration in the target, which results in tunable optical properties and magnetic properties. Specifically, a high saturation magnetization of 100 emu cm−3, strong out‐of‐plane magnetic anisotropy and tailorable magnetization properties are achieved via tuning the Co nanopillar density. Coupled with hyperbolic dispersion of dielectric constant from 950 to 1500 nm in wavelength, plasmonic Co metal nanopillars, and the unique dielectric ZrO2 matrix, this new nanoscale hybrid metamaterial shows great potential for future integrated optical and magnetic device designs.

Organometallic hotspot engineering for ultrasensitive EC-SERS detection of pathogenic bacteria-derived DNAs

The sensitivity and limit-of-detection (LOD) of the traditional surface-enhanced Raman spectroscopy (SERS) platform suffer from the requirement of precise positioning of small analytes, including DNAs and bacteria, into narrow hotspots. In this study, a novel SERS sensor was developed using electrochemical deposition onto metal nanopillars (ECOMPs) combined with complementary DNAs (cDNAs) for the detection of pathogenic bacteria. Applying a redox potential to AuCl4− ions actively engineered the organometallic hotspots based on the cDNAs in a short time (<10 min) and simultaneously produced SERS signals. Because of the influence of potential-driven morphological properties on the SERS efficiency in the cDNA domains and the resonant coupling of internal fields with the fields confined between adjacent ECOMPs-cDNAs, the optimum growth time was determined to be 5 min. The EC-SERS detection and discrimination of Enterococcus faecium and Staphylococcus aureus were successfully carried out because of the DNA complementarity. Compared with plasmonic metal nanopillars (MPs)-cDNAs, the enhancement factor of the ECOMPs-cDNAs was estimated to be ∼2.0 × 103. A quantitative investigation revealed that a highly linear progression in the target DNA concentration range (0.05–100 nM) and a LOD of ∼0.035 nM were achieved. The specificity of the ECOMPs-cDNAs was validated by cross-hybridization. The platform was also used to assay human whole blood containing 0.1 nM bacterial DNAs. The proposed strategy provides the potential for highly sensitive SERS-based multiplex DNA detection in clinical diagnostics.

Figures of merit of plasmon lattice resonance sensors: shape and material matters

The figure of merit (FOM) of plasmon lattice resonance (PLR) sensors based on the array of metal/Si/SiO2 nanoparticles has been investigated. We demonstrate the shape and material of metal nanoparticles have remarkable effects on the PLR and FOM. FOM is governed by full-widths at half maximum (FWHM) and sensitivity of the PLR. Three different types of PLR can be generated by changing Ag nanoparticles’ shapes (pillars, cubes, spheres). One (named PLR1) is mainly originated from the coupling between Mie resonance of individual Si nanopillars and diffraction waves. PLR1 of Ag/Si/SiO2 nanoparticle arrays is limited in sensing applications due to lower intensity (for Ag pillars and Ag cubes), or smaller FOM (for Ag spheres). The other two are named PLR2. PLR2 of Ag/Si/SiO2 nanoparticle array with Ag pillars (or Ag cubes) is mainly originated from the coupling between the quadrupole resonance of individual Ag nanopillars (or Ag cubes) and diffraction waves. While PLR2 of Ag/Si/SiO2 nanoparticle array with Ag spheres is mainly originated from the coupling between dipole resonance of individual Ag nanospheres and diffraction waves. The optimal Ag nanoparticles’ shape in FOM is pillar due to the smallest FWHM of PLR2 of Ag/Si/SiO2 nanoparticle array with Ag pillars. Meanwhile, a comparison of FOM between Au, Ag and Al nanopillars of fixed size is made. The optimal material of metal nanopillars to obtain a high FOM is Ag due to higher sensitivity and narrower FWHM.

Nanopillar composite electrodes for solar-driven water splitting

Spontaneous noble metal dopant segregation in an oxide lattice can lead to the formation of metallic clusters and extended acicular inclusions. In a thin-film process, the shape and orientation of such noble metal inclusions are governed by the crystal growth direction, giving rise to a composite material with lattice-matched metal nanopillars embedded vertically in an insulating or semiconducting oxide matrix. An interesting application of such composites is in photoelectrochemical cell electrodes, where the metallic nanopillars take on three distinct roles: forming a Schottky junction with the host matrix, providing a low-loss current path from bulk to surface, and creating an efficient electrocatalytic active site on the electrode surface. In particular, we discuss the application of vertically aligned metal–oxide nanopillar composites in photoelectrochemical water-splitting cells used for direct solar-powered hydrogen generation.

Kinetin Detection Enhancement Based on Photonic Nanojets and Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) technique can increase the Raman signals of the analytes owing to the electric field enhancements on the nanostructures. The strong light intensity, called photonic nanojet, can be formed at the backlight of the dielectric microstructures. Recently, some researchers have applied the photonic nanojets in SERS. In this paper, a layer of synthesized low-cost polystyrene microspheres with the average of small particle size of 4.26 μm and high refractive index 1.5875 was prepared by dispersion polymerization method and coating on metal nanopillars SERS substrates using air-water interfacial floating method, and it was applied to improve the Raman signal sensitivity of kinetin (6-furfurylaminopurine) which is one kind of plant growth regulators and can function an effective compound for improving plant stress tolerance. The Raman intensity increased from 841 to 4449 for the Raman shift at 1327 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , about 5.29 times stronger. The SERS experiments using the kinetin of ultralow concentrations are measured and it shows that femto Molar can be measured. The numerical simulations indicated that the enhancement factor was about 4 times for plane wave illumination and 3.1 times for focused Gaussian beam. It has considerable potential for different analytes in sensing applications.

3D Hybrid Plasmonic Framework with Au Nanopillars Embedded in Nitride Multilayers Integrated on Si

AbstractIntegration of nanoscale photonic and plasmonic components on Si substrates is a critical step toward Si‐based integrated nanophotonic devices. In this work, a set of unique complex 3D metamaterials with intercalated nanolayered and nanopillar structures with tunable plasmonic and optical properties on Si substrates is designed. More specifically, the 3D metamaterials combine metal (Au) nanopillars and alternating metal‐nitride (Au‐TiN and Au‐TaN) nanolayers, epitaxially grown on Si substrates. The ultrafine Au nanopillars (d ≈ 3 nm) continuously grow throughout all the nanolayers with high epitaxial quality. Novel optical properties, such as highly anisotropic optical property, high absorbance covering the entire visible spectrum regime, and hyperbolic property in the visible regime, are demonstrated. Furthermore, a waveguide based on a silicon nitride (Si3N4) ridge with a multilayer structure is successfully fabricated. The demonstration of 3D nanoscale metamaterial design integrated on Si opens up a new route toward tunable metamaterials nanostructure designs with versatile material selection for various optical components in Si integrated photonics.

Phase-Regulated Sensing Mechanism of MoS2 Based Nanohybrids toward Point-of-Care Prostate Cancer Diagnosis.

Alpha-methylacyl-CoA racemase (AMACR) has been proven to be consistently overexpressed in prostate cancer epitheliums, and is expected to act as a positive biomarker for the diagnosis of prostate carcinoma in clinical practice. Here, a strategy for specific determination of AMACR in real human serum by using an electrochemical microsensor system is presented. In order to implement the protocol, a self-organized nanohybrid consisting of metal nanopillars in a 2D MoS2 matrix is developed as material for the sensing interface. The testing signal outputs are strongly enhanced with the presence of the nanohybrids owing to that the metal pillars provide an efficient mass difussion and electron transfer path to the MoS2 film surface. Furthermore, the phase-regulated sensing mechanism over MoS2 is noticed and demonstrated by density functional theory calculation and experiments. The explored MoS2 based nanohybrids are employed for the fabrication of an electrochemical microsensor, presenting good linear relationship in both ng µL-1 and pg µL-1 ranges for AMACR quantification. The sampling analysis of human serum indicates that this microsensor has good diagnostic specificity and sensitivity toward AMACR. The proposed electrochemical microsensor system also demonstrates the advantages of convenience, cost-effectiveness, and disposability, resulting in a potential integrated microsystem for point-of-care prostate cancer diagnosis.

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.

Enhanced photoelectrochemical water splitting by self-organized metal nanopillars in an oxide semiconductor

Enhanced photoelectrochemical water splitting by self-organized metal nanopillars in an oxide semiconductor

Photoelectrochemical water splitting enhanced by self-assembled metal nanopillars embedded in an oxide semiconductor photoelectrode

Production of chemical fuels by direct solar energy conversion in a photoelectrochemical cell is of great practical interest for developing a sustainable energy system. Various nanoscale designs such as nanowires, nanotubes, heterostructures and nanocomposites have been explored to increase the energy conversion efficiency of photoelectrochemical water splitting. Here we demonstrate a self-organized nanocomposite material concept for enhancing the efficiency of photocarrier separation and electrochemical energy conversion. Mechanically robust photoelectrodes are formed by embedding self-assembled metal nanopillars in a semiconductor thin film, forming tubular Schottky junctions around each pillar. The photocarrier transport efficiency is strongly enhanced in the Schottky space charge regions while the pillars provide an efficient charge extraction path. Ir-doped SrTiO3 with embedded iridium metal nanopillars shows good operational stability in a water oxidation reaction and achieves over 80% utilization of photogenerated carriers under visible light in the 400- to 600-nm wavelength range.

Optical properties of plasmonic nanopillars in extended quasi-static limits

An extended method based on the long-wavelength approximations is introduced to investigate the optical properties of finite-size metal nanopillars for different incident light polarizations. In this proposed method, the optical depolarization factors are the key parameters and strongly depend on the position of the considered point dipole inside the irregular-shaped nanoparticles. Interestingly, depending on the polarization, it is shown that the conventional point dipole positioned at the center of a nonspherical nanoparticle seldom is the proper choice as its simple electromagnetic model. The adequate off-center point, though, is determined and physically associated with the electric-field distribution inside the nanoparticle, which is supported by numerical computations. Remarkably, the proposed extended MLWA could extend the quasi-static limits and, consequently, more accurately predict the optical properties of the finite-size plasmonic nanopillar.

Periodically Varying Height in Metal Nano-pillars for Enhanced Generation of Localized Surface Plasmon Field

Localized surface plasmon resonance (LSPR) field condition study in metal nanostructure is important for different fields of applications which include chemical and bio-sensing investigations and in surface-enhanced Raman scattering (SERS)-based sensing investigations. Generally, sharp metal nano-tips, metal nano-colloids, nano-pillars and other sharp structures were used to generate different magnitudes of enhanced LSPR conditions, and all such structures usually provide tightly bound LSPR field conditions. In this paper, we demonstrate that with periodically varying height in metal nano-pillars, enhanced LSPR field condition can also be obtained and the enhancement scale of the generated field are found to be more than that of the uniformly structured metal nano-pillars. Different parameters which govern the enhancement factor for LSPR field condition have been thoroughly studied here. Since, periodic pattern of nano-pillars can be fabricated using lithographic tool such as e-beam lithography or focus-ion beam lithography, we envision that our simulation results would be useful in getting desired structure of the proposed nano-pillars for which it provides enhanced LSPR field condition as compared to uniformly structured metal nano-pillars.

Study of metal pillar nanostructure formation with thin porous alumina template

In the present paper, the nickel pillared nanostructure fabrication by electrochemical deposition of Ni into the pores of thin porous anodic alumina is considered. The main characteristics of these structures, obtained by scanning electron microscopy and atomic-force microscopy, are presented. Information on geometrical parameters of porous host and pillar nanostructure elements has been obtained. The influence of the barrier layer thinning at the pore bottom on nucleation and growth of the ordered metal nanopillars is discussed. The process of functional layer formation based on thin aluminum and Al2O3 films with incorporated nickel pillars is analyzed. This process may be used for fabrication of advanced high density magnetic memory devices.

The influence of laser-induced nanosecond rise-time stress waves on the microstructure and surface chemical activity of single crystal Cu nanopillars

An apparatus and test procedure for fabrication and loading of single crystal metal nanopillars under extremely high pressures (>1 GPa) and strain rates (>107 s-1), using laser-generated stress waves, are presented. Single-crystalline Cu pillars (∼1.20 μm in tall and ∼0.45 μm in diameter) prepared via focused ion beam milling of Cu(001) substrates are shock-loaded using this approach with the dilatational stress waves propagating along the [001] axis of the pillars. Transmission electron microscopy observations of shock-loaded pillars show that dislocation density decreases and that their orientation changes with increasing stress wave amplitude, indicative of dislocation motion. The shock-loaded pillars exhibit enhanced chemical reactivity when submerged in oil and isopropyl alcohol solutions, due likely to the exposure of clean surfaces via surface spallation and formation of surface steps and nanoscale facets through dislocation motion to the surface of the pillars, resulting in growth of thin oxide films on the surfaces of the pillars.

Critical-temperature/Peierls-stress dependent size effects in body centered cubic nanopillars

The size-dependent plasticity of body centered cubic (bcc) metals is different from face centered cubic (fcc) metals: the size-effect exponent n varies for different bcc metal nanopillars (n = 0.8–1.0 for V, Nb; n = 0.3–0.5 for Ta, Mo, W). This inconsistency is first explained through a simple model based on the temperature-dependent Peierls stress. The bcc V nanopillars with a low critical temperature and Peierls stress showed a fcc-like size effect with n = 0.79, and our in-situ TEM compression study revealed that fcc-like dislocation starvation occurred in bcc V nanopillars, indicating that a small Peierls stress in V contributes to the fcc-like behavior.

High aspect ratio metal micro and nano pillars for minimal footprint MEMS suspension

Vertical nano and micro pillars perpendicularly rising from a substrate offer two lateral translatory–rotatory degrees of freedom. Electroforming allows their production as small footprint integrated suspension elements of micro to nano scale. This paper demonstrates the design of a novel inertial sensor concept with acceleration sensor and gyroscope function using only one inertial mass. Experimental results using UV Direct LIGA with AZ 125 nXT show the feasibility of a technology demonstrator with a copper micro pillar of 400 μm length and 40 μm diameter. Further work using x-ray Direct LIGA is scheduled for the production of the pillar with a length of 100 μm and a diameter of 3–6 μm. Fabrication concepts and pilot tests show promising possibilities for miniaturization towards nano scale pillars for minimal footprint suspension in MEMS.

Deformation mechanisms in nanotwinned metal nanopillars

Nanotwinned metals are attractive in many applications because they simultaneously demonstrate high strength and high ductility, characteristics that are usually thought to be mutually exclusive. However, most nanotwinned metals are produced in polycrystalline forms and therefore contain randomly oriented twin and grain boundaries making it difficult to determine the origins of their useful mechanical properties. Here, we report the fabrication of arrays of vertically aligned copper nanopillars that contain a very high density of periodic twin boundaries and no grain boundaries or other microstructural features. We use tension experiments, transmission electron microscopy and atomistic simulations to investigate the influence of diameter, twin-boundary spacing and twin-boundary orientation on the mechanical responses of individual nanopillars. We observe a brittle-to-ductile transition in samples with orthogonally oriented twin boundaries as the twin-boundary spacing decreases below a critical value (∼3-4 nm for copper). We also find that nanopillars with slanted twin boundaries deform via shear offsets and significant detwinning. The ability to decouple nanotwins from other microstructural features should lead to an improved understanding of the mechanical properties of nanotwinned metals.

Application of Cylinder Forming Block Copolymers as Templates for Formation of Bit Patterned and Graded Media

AbstractBit patterned media, including media fabricated with a gradient in composition, is being developed as a potential path to higher information storage density. The formation of metal nanopillars with 20-30 nm repeat spacing and precisely controlled magnetic properties presents a significant challenge to current fabrication methods. We have been developing cylinder forming block copolymer phases as a method to generate the desired patterns coupled with the processing steps necessary to transfer the pattern into magnetic material. This involves spin coating of the polymer on an appropriate orienting layer, annealing to allow the pattern to form by self-organization of the block copolymer, solvent processing to remove the minority domain, electrodeposition to form a hard mask, followed by ion-milling to transfer the pattern to the magnetic material. We have demonstrated each step in this process and report on the quality of the pattern achieved.

Controlled Fabrication of Hollow Metal Pillar Arrays Using Colloidal Masks

A 2D hollow metal nanopillar array was fabricated using reactive ion etching through the silica nanosphere arrays self-organized over a polymeric film for higher etching contrast. The present approach is applicable to diverse hollow metal nanopillars.

Controlled Normal and Inverse Current-Induced Magnetization Switching and Magnetoresistance in Magnetic Nanopillars

By combining pairs of ferromagnetic metals with the same or different signs of scattering anisotropies in ferromagnetic-nonmagnetic-ferromagnetic metal nanopillars, we independently invert just the magnetoresistance, just the direction of current-induced magnetization switching, or both together, at room temperature (295 K) and at 4.2 K. In all cases studied, the switching direction is correctly predicted from the net scattering anisotropy of the fixed ferromagnet, including both bulk and interfacial contributions.