Au Nanopillars Research Articles

Dynamically Tunable Localized Surface Plasmon Resonance in Self‐Assembled SrCoOx‐Au Vertically Aligned Nanocomposite Thin Films

AbstractWhile the physical principles for regulating localized surface plasmon resonance (LSPR) are well established, dynamically tuning the LSPR in a given material post synthesis remains challenging. Herein, this study demonstrates a strategy for dynamically tuning the LSPR of Au nanostructures by selectively altering the dielectric environment. Au is integrated with an electrochemically gatable oxide, i.e., SrCoOx (SCO), into vertically aligned nanocomposite thin films via a self‐assembly growth mechanism, where Au develops into nanopillars embedded in the SCO matrix. By selectively controlling the tri‐state phase transitions of SCO matrix via varying the bias voltage polarity in ionic liquid gating (ILG), the LSPR behavior of Au nanopillars can be dynamically tuned. Specifically, gating with a negative bias fully suppresses the LSPR, while a positive bias leads to a continuous blueshift of the LSPR wavelength upon increasing the ILG duration. This work not only opens new directions for the dynamic control of LSPR of noble metal nanostructures, but also offers insight to the voltage control of multifunctionalities via structural and physical intercoupling between different phases in self‐assembled nanocomposites.

TiN-Au/HfO2 -Au Multilayer Thin Films with Tunable Hyperbolic Optical Response.

Hyperbolic metamaterials (HMM) possess significant anisotropic physical properties and tunability and thus find many applications in integrated photonic devices. HMMs consisting of metal and dielectric phases in either multilayer or vertically aligned nanocomposites (VAN) form are demonstrated with different hyperbolic properties. Herein, self-assembled HfO2 -Au/TiN-Au multilayer thin films, combining both the multilayer and VAN designs, are demonstrated. Specifically, Au nanopillars embedded in HfO2 and TiN layers forming the alternative layers of HfO2 -Au VAN and TiN-Au VAN. The HfO2 and TiN layer thickness is carefully controlled by varying laser pulses during pulsed laser deposition (PLD). Interestingly, tunable anisotropic physical properties can be achieved by adjusting the bi-layer thickness and the number of the bi-layers. Type II optical hyperbolic dispersion can be obtained from high layer thickness structure (e.g., 20 nm), while it can be transformed into Type I optical hyperbolic dispersion by reducing the thickness to a proper value (e.g., 4 nm). This new nanoscale hybrid metamaterial structure with the three-phase VAN design shows great potential for tailorable optical components in future integrated devices.

ZnO-Au Hybrid Metamaterial Thin Films with Tunable Optical Properties

ZnO-Au nanocomposite thin films have been previously reported as hybrid metamaterials with unique optical properties such as plasmonic resonance properties and hyperbolic behaviors. In this study, Au composition in the ZnO-Au nanocomposites has been effectively tuned by target composition variation and thus resulted in microstructure and optical property tuning. Specifically, all the ZnO-Au nanocomposite thin films grown through the pulsed laser deposition (PLD) method show obvious vertically aligned nanocomposite (VAN) structure with the Au nanopillars embedded in the ZnO matrix. Moreover, the average diameter of Au nanopillars increases as Au concentration increases, which also leads to the redshifts in the surface plasmon resonance (SPR) wavelength and changes in the hyperbolic behaviors of the films. As a whole, this work discusses how strain-driven tuning of optical properties and microstructure resulted through a novel Au concentration variation approach which has not been previously attempted in the ZnO-Au thin film system. These highly ordered films present great promise in the areas of sensing, waveguides, and nanophotonics to name a few.

On the splitting of the surface plasmon resonance band of gold nanopillars developed on rippled Si surface

In this work, we reported split surface plasmon resonance (SPR) bands originated from the Au nanopillars developed on rippled Si substrates. This is the first report of gold SPR splitting on a pre-structured Si surface. The Si surface was subjected to a bombardment of Ar ions to fabricate ripples on the surface. A 5 nm thin layer of Au was then deposited on the rippled surface, and the SPR was studied using UV–visible spectroscopy. Further, X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS) were used to characterize the prepared samples. The AFM confirmed that the deposited gold is in nanopillar form, and the rippled surface governs the growth, shape, and size of gold nanopillars. A large aspect ratio has been observed for the nanopillars, resulting in a red-shifted and split SPR band. The experimental XANES of Au L3-edge were successfully mimicked by simulated XANES, confirming the existence and signature of the nanostructure. For the first time, the local structure information of Au nanopillars constructed on rippled Si substrate has been obtained using the EXAFS approach in different directions and correlated with the plasmonic bands.

A Surface Enhanced Raman Scattering (SERS) Sensing Method Enhanced by All-metal Metasurface

In this study, we developed a simple surface-enhanced Raman Scattering (SERS) composite substrate, composed of an all-metal metasurface and on-surface random distributing silver nanoparticles (AgNPs), to enhance the performance of SERS sensing. The metasurface is made up of gold (Au) nanopillars array on aluminum (Al), it can localize the electromagnetic field energy with a resonant absorption peak near the 630 nm wavelength, which contributes to the SERS performance of AgNPs deposited into the structure. The composite substrate can significantly improve the sensing performance, and the SERS Enhancement Factor (EF) reaches 2.81 × 106. The substrate also has good stability and reproducibility. The research is based on the combination of Localized Surface Plasmon Resonance (LSPR) and SERS effects, providing a method and idea for improving the sensitivity of SERS detection, and achieving the trace detection of Rhodamine 6G (R6G) and Thiram at 10−10 M, respectively.

Development of Nano-Micro Fused LSPR Chip for In Situ Single-Cell Secretion Analysis.

Single-cell analysis has become increasingly important in uncovering cell heterogeneity, which has great implications in medicine and biology for a deep understanding of cell characteristics. Owing to its significance, it is vital to create novel devices that can reveal special or unique cells. In this work, we developed a single-cell secretion detection chip consisting of microwells that can trap single cells. Each well is surrounded by Au nanopillars capable of localized surface plasmon resonance (LSPR) measurement. Using microfabrication and nanofabrication techniques, Au nanopillar and microwell structures were fabricated on a COP film. The Au nanopillar was modified with IL-6 antibodies for the direct detection of single-cell secreted IL-6 via LSPR absorbance peak shift. Specific IL-6 detection was successfully demonstrated using a null and IL-6 oversecreting Jurkat cell. A high single-cell trapping efficiency of over 80% was also achieved. Overall, the development of this single-cell secretion detection chip with a simple LSPR measurement setup represents a significant development in the field of cell biology and immunology, providing researchers with a powerful tool for studying individual cells and their secreted cytokines, and is useful for point-of-care testing (POCT) diagnostics.

Energetics of nanoscale films with self–assembled oxide/metal pillars in nitride matrix

Vertically aligned nanocomposite (VAN) thin films consisting of NiO and Au nanopillars in a TiN matrix exhibits excellent optical anisotropy and tunability along with strong magneto–optical coupling properties. A recent experimental study showed that NiO deposited on a template of TiN matrix containing Au nanopillars nucleates NiO pillars on the already existing Au pillars. Due to the oxygen–deficient NiO, Au and Ni interdiffusion develops a unique Au shell/NiO core structure in the top region and segregated agglomerates of Ni in the lower Au pillars. As a step toward understanding the growth of such a multi–phase film configuration, we report a comparative energetic investigation confirming that configurations with interdiffusion of Au and Ni is energetically favorable compared to other possible configurations without the interdiffusion of Au and Ni. In this work, a thermodynamic model accounting for elastic energy due to lattice mismatch strain, capillarity effects, and interfacial and chemical energies is utilized to compare various film configurations energetically. Among all the configurations examined, the ones that closely resemble the experimental observations possessed the least energy. It was also found that Au–Ni interdiffusion is immediate upon the deposition of oxygen–deficient NiO on TiN–Au template, and a critical ratio of Au and NiO pillar heights favors core–shell structure. These findings are vital for understanding the formation mechanisms of complex VANs and future designs of new three–phase VAN structures with complex morphologies.

Optical properties of hollow plasmonic nanopillars for efficient solar photothermal conversion

Plasmonic nanofluids, which have superior optical properties, provide a novel way to achieve efficient solar thermal utilization. Here, the optical properties and photothermal conversion performance of hollow nanopillar structures based on four plasmonic materials (Ag, Au, Cu, TiN) were investigated and compared with the corresponding solid nanopillar structures. Numerical results show that the most significant increase in photothermal conversion efficiency is the plasmonic nanofluid based on hollow Au nanopillars, with an increase of 34.14%. The underlying physical mechanism is that the local and propagating surface plasmon resonances are further promoted in the hollow structure. In contrast, the increase of the photothermal conversion efficiency is only 1.41% for the hollow TiN nanopillars. This is due to the fact that the solid TiN nanopillars already have a high photothermal conversion efficiency of 97.19%. Thus, we can consider the enhancement of the photothermal properties of the plasmonic nanofluid by the hollow structure, especially for precious metal materials with narrow-band absorption, such as Au and Ag. For plasmonic materials with intrinsic broad-spectrum absorption (TiN, Cu), the enhancement effect is not significant. It is believed that this work will provide theoretical guidance for high-performance direct absorption solar collectors.

High sensitivity biosensing scheme based on a GHz phononic crystal waveguide

We propose a high sensitivity biosensor based on a GHz phononic crystal (PnC) waveguide, and demonstrate its operation by numerical simulations. The geometry consists of a micron-scale freestanding PnC silica waveguide plate with embedded Au nanopillars for bioparticle attachment, the PnC plate lying between two groups of periodic metal strips for GHz Lamb-wave acoustic generation and detection with ultrashort light pulses. By precise choice of the waveguide defect width, this biosensor is designed to work using a single, isolated waveguide mode. We study the influence of the waveguide defect width on the acoustic dispersion and transmission of this mode. Bioparticle attachment is simulated by investigation of the Au nanopillar mass loading, and is shown to shift the waveguide transmission peak to lower frequencies. We thereby demonstrate femtogram detection, showing that our approach provides a new methodology for label-free ultra-sensitive biosensing.

Abnormal in-plane epitaxy and formation mechanism of vertically aligned Au nanopillars in self-assembled CeO2-Au metamaterial systems.

Metamaterials present great potential in the applications of solar cells and nanophotonics, such as super lenses and other meta devices, owing to their superior optical properties. In particular, hyperbolic metamaterials (HMMs) with exceptional optical anisotropy offer improved manipulation of light-matter interactions as well as a divergence in the density of states and thus show enhanced performances in related fields. Recently, the emerging field of oxide-metal vertically aligned nanocomposites (VANs) suggests a new approach to realize HMMs with flexible microstructural modulations. In this work, a new oxide-metal metamaterial system, CeO2-Au, has been demonstrated with variable Au phase morphologies from nanoparticle-in-matrix (PIM), nanoantenna-in-matrix, to VAN. The effective morphology tuning through deposition background pressure, and the corresponding highly tunable optical performance of three distinctive morphologies, were systematically explored and analyzed. A hyperbolic dispersion at high wavelength has been confirmed in the nano-antenna CeO2-Au thin film, proving this system as a promising candidate for HMM applications. More interestingly, a new and abnormal in-plane epitaxy of Au nanopillars following the large mismatched CeO2 matrix instead of the well-matched SrTiO3 substrate, was discovered. Additionally, the tilting angle of Au nanopillars, α, has been found to be a quantitative measure of the balance between kinetics and thermodynamics during the depositions of VANs. All these findings provide valuable information in the understanding of the VAN formation mechanisms and related morphology tuning.

Enhanced Optical Response in Au Nanopillar/La0.67Sr0.33MnO3 Film Composites: Implications for Magnetooptical Devices

Metal oxide vertically aligned nanocomposite thin films have been of considerable interest for hot-carrier-based light harvesting and active plasmonic applications. Herein, we present femtosecond pump–probe spectroscopy studies on a nanocomposite thin film with plasmonic Au nanopillars vertically aligned in a ferromagnetic La0.67Sr0.33MnO3 (LSMO) matrix. Our measurements reveal that the carrier decay of Au/LSMO films qualitatively follows that of pure LSMO with a two-component relaxation. In the presence of Au nanopillars, the charge generation within the LSMO film is enhanced by a factor of ∼40, and the spin–lattice relaxation time associated with demagnetization is substantially decreased from 259.6 to 7.7 ps. The enhanced nonequilibrium dynamics and energy transfer between the lattice and spin system in the Au/LSMO films can be well modeled using an extended three-temperature model. These Au/LSMO thin films can be potentially useful in ultrafast photoelectric and magnetooptical applications.

Enhanced Light Absorption and Efficient Carrier Collection in MoS2 Monolayers on Au Nanopillars.

We fabricated hybrid nanostructures consisting of MoS2 monolayers and Au nanopillar (Au-NP) arrays. The surface morphology and Raman spectra showed that the MoS2 flakes transferred onto the Au-NPs were very flat and nonstrained. The Raman and photoluminescence intensities of MoS2/Au-NP were 3- and 20-fold larger than those of MoS2 flakes on a flat Au thin film, respectively. The finite-difference time-domain calculations showed that the Au-NPs significantly concentrated the incident light near their surfaces, leading to broadband absorption enhancement in the MoS2 flakes. Compared with a flat Au thin film, the Au-NPs enabled a 6-fold increase in the absorption in the MoS2 monolayer at a wavelength of 615 nm. The contact potential difference mapping showed that the electric potential at the MoS2/Au contact region was higher than that of the suspended MoS2 region by 85 mV. Such potential modulation enabled the Au-NPs to efficiently collect photogenerated electrons from the MoS2 flakes, as revealed by the uniform positive surface photovoltage signals throughout the MoS2 surface.

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.

A generalized 3D elastic model for nanoscale, self-assembled oxide-metal thin films with pillar-in-matrix configurations

In recent years, functional oxide-metal based vertically aligned nanocomposite (VAN) thin films have gained interest due to their intriguing physical properties and multifunctionalities stemming from the complex interactions between the two phases in the film and the substrate. In this work, we develop a model for studying the energetics of these thin film systems, including the effects of both lattice mismatch and capillary forces due to interface curvature. Each phase is incorporated into the model using a phase indicator function, and we introduce the capillary forces as body forces using a vector density representation of the interface. The model is implemented using the finite element method to study the deformation of the thin film which is composed of Au nanopillars embedded in a La0.7Sr0.3MnO3 (LSMO) matrix on an SrTiO3 (STO) substrate. The results suggest that the total energy is lowest for random configurations of pillars compared to ordered square and hexagonal lattice configurations, consistent with the random distribution of pillars found in experiments. Furthermore, we find that the interfacial energy dominates the total energy of each configuration, suggesting that interfacial energy in the system is an important design parameter for nanocomposite growth, along with the lattice mismatch.

Ultrahigh in-plane thermoelectric performance in self-assembled PbSe:Au films with vertically aligned nanopillars

Self-assembled nanocomposites with tunable physical properties have potential applications in advanced devices. In this work, we fabricated self-assembled PbSe:Au films with vertically aligned nanopillars (VANs) by pulsed laser deposition. We used scanning tunneling microscopy to evaluate detailed heterostructures at atomic resolution. The results indicate that we prepared self-assembled Au nanopillars in a PbSe matrix. Compared with the PbSe matrix, the PbSe:Au vertical interface can facilitate the energy-filter effect and lower the in-plane thermal conductivity. The maximum ZT values for an n-type self-assembled VAN film is ∼2.6 at 523 K, which is ∼520% higher than that of a pure PbSe film (ZT = 0.5). This study suggests that the vertical interface in the unique VAN described here provides a viable route to manipulate the thermoelectric properties of thermoelectric materials.

Self-assembled HfO2-Au nanocomposites with ultra-fine vertically aligned Au nanopillars.

Oxide-metal-based hybrid materials have gained great research interest in recent years owing to their potential for multifunctionality, property coupling, and tunability. Specifically, oxide-metal hybrid materials in a vertically aligned nanocomposite (VAN) form could produce pronounced anisotropic physical properties, e.g., hyperbolic optical properties. Herein, self-assembled HfO2-Au nanocomposites with ultra-fine vertically aligned Au nanopillars (as fine as 3 nm in diameter) embedded in a HfO2 matrix were fabricated using a one-step self-assembly process. The film crystallinity and pillar uniformity can be obviously improved by adding an ultra-thin TiN-Au buffer layer during the growth. The HfO2-Au hybrid VAN films show an obvious plasmonic resonance at 480 nm, which is much lower than the typical plasmonic resonance wavelength of Au nanostructures, and is attributed to the well-aligned ultra-fine Au nanopillars. Coupled with the broad hyperbolic dispersion ranging from 1050 nm to 1800 nm in wavelength, and unique dielectric HfO2, this nanoscale hybrid plasmonic metamaterial presents strong potential for the design of future integrated optical and electronic switching devices.

A generalizable machine learning potential of Ag–Au nanoalloys and its application to surface reconstruction, segregation and diffusion

Owing to the excellent catalytic properties of Ag–Au binary nanoalloys, nanostructured Ag–Au, such as Ag–Au nanoparticles and nanopillars, has been under intense investigation. To achieve high accuracy in molecular simulations of Ag–Au nanoalloys, the surface properties must be modeled with first-principles precision. In this work, we constructed a generalizable machine learning interatomic potential for Ag–Au nanoalloys based on deep neural networks trained from a database constructed with first-principles calculations. This potential is highlighted by the accurate prediction of Au (111) surface reconstruction and the segregation of Au toward the Ag–Au nanoalloy surface, where the empirical force field (EFF) failed in both cases. Moreover, regarding the adsorption and diffusion of adatoms on surfaces, the overall performance of our potential is better than the EFFs. We stress that the reported surface properties are blind to the potential modeling in the sense that none of the surface configurations is explicitly included in the training database; therefore, the reported potential is expected to have a strong generalization ability to a wide range of properties and to play a key role in investigating nanostructured Ag–Au evolution, where accurate descriptions of free surfaces are necessary.

Ultrahigh In-Plane Thermoelectric Performance in Self-Assembled PbSe:Au Films With Vertically Aligned Nanopillar

Self-assembled nanocomposites with tunable physical properties have potential applications in advanced devices. In this work, self-assembled PbSe:Au films with vertically aligned nanopillars (VAN) were fabricated by pulsed laser deposition. Detailed heterostructures with atomic-resolution have been investigated by cross-sectional scanning tunnelling microscopy. The results show that self-assembled Au nanopillars in a PbSe matrix is realized. Compared with PbSe matrix, PbSe:Au vertical interface can facilitate energy filter effect and lower the in-plane thermal conductivity. The maximum ZT values for n-type self-assembled VAN film is ~2.6 at 523 K, which is ~400% higher than that of pure PbSe film (ZT=0.5). This study suggests that the vertical interface in the unique VAN provide a viable route to manipulate the thermoelectric properties for the thermoelectric materials.

Rapid and sensitive multiplex molecular diagnosis of respiratory pathogens using plasmonic isothermal RPA array chip

The current clinically available multiplex molecular diagnostic technologies are difficult to apply to onsite diagnostic platforms due to their large and sophisticated instrumentation, long amplification times and limited number of simultaneous detections. We developed a plasmonic isothermal recombinase polymerase amplification (RPA) array chip for rapid and sensitive multiplex molecular detection. The 3D plasmonic substrate composed of Au nanoparticles (NPs) on dense Au nanopillars (NPOP) showed highly enhanced plasmon-enhanced fluorescence (PEF) of RPA products with long DNA amplicons (~200 bp). The plasmonic 4-plex RPA array chip successfully detected bacterial DNA within 30 min and viral RNA within 40 min; the sensitivity of the plasmonic RPA array chip was comparable to or 10-fold higher than that of the 4-pelx liquid-phase RPA and 4-plex liquid-phase PCR techniques. Additionally, no cross-reactivity was observed. The 4-plex plasmonic RPA array chip was preliminary evaluated using clinical respiratory viral-positive nasopharyngeal swab samples. This approach enables rapid, sensitive and high-multiplex molecular detection and can be used in the realization of a simplified and miniaturized platform for onsite multiplex molecular diagnostics.

On the yielding and densification of nanoporous Au nanopillars in molecular dynamics simulations

We report on the plastic deformation characteristics of nanoporous gold nanopillars under compression in molecular dynamics simulations. The stress-strain curves demonstrate an initial non-linear regime up to compressive true strains of about 5%, followed by a stress plateau and a strong hardening stage. The plateau stress is increasing with the ligament size. In addition, a power-law is fitted to the stress variation during the plateau and hardening stages. In order to relate the mechanical response to the dislocation activity, an in-house post-processing technique was developed to skeletonize the structure and to identify the yielded ligaments. We found that the elastic-plastic transition starts at a strain substantially lower than that of the plateau. The load-bearing capacity is further retained by the unyielded ligaments within the three-dimensionally connected network, allowing further increase in the stress and at the onset of the plateau stress 40% of the ligaments have already yielded plastically. Calculation of the genus of the subnetwork of unyielded ligaments shows that it drops to zero at the onset of the plateau stage. Additionally, the rate of dislocation nucleation is maximum at the onset of the plateau, allowing to continue the plastic flow without increasing the stress. The stress starts increasing again as the ligaments coalesce and the nanoporous structure densifies. Based on the analysis of the genus of the whole structure during compression, we propose a correlation between the hardening and the topology of the structure, suggesting that hardening is dominated by the coalescence of ligaments. During coalescence, grain boundaries were formed between coalesced ligaments, some are either removed or retained. As a result, at very large compressive strains the single crystal nanoporous nanopillar turns into a polycrystalline specimen with some disconnected pores.