Strong Plasmonic Field Research Articles

Tunable Subnanometer Gaps in Self-Assembled Monolayer Gold Nanoparticle Superlattices Enabling Strong Plasmonic Field Confinement.

Nanoparticle superlattices produced with controllable interparticle gap distances down to the subnanometer range are of superior significance for applications in electronic and plasmonic devices as well as in optical metasurfaces. In this work, a method to fabricate large-area (∼1 cm2) gold nanoparticle (GNP) superlattices with a typical size of single domains at several micrometers and high-density nanogaps of tunable distances (from 2.3 to 0.1 nm) as well as variable constituents (from organothiols to inorganic S2-) is demonstrated. Our approach is based on the combination of interfacial nanoparticle self-assembly, subphase exchange, and free-floating ligand exchange. Electrical transport measurements on our GNP superlattices reveal variations in the nanogap conductance of more than 6 orders of magnitude. Meanwhile, nanoscopic modifications in the surface potential landscape of active GNP devices have been observed following engineered nanogaps. In situ optical reflectance measurements during free-floating ligand exchange show a gradual enhancement of plasmonic capacitive coupling with a diminishing average interparticle gap distance down to 0.1 nm, as continuously red-shifted localized surface plasmon resonances with increasing intensity have been observed. Optical metasurfaces consisting of such GNP superlattices exhibit tunable effective refractive index over a broad wavelength range. Maximal real part of the effective refractive index, nmax, reaching 5.4 is obtained as a result of the extreme field confinement in the high-density subnanometer plasmonic gaps.

Chemical Enhancement and Quenching in Single‐Molecule Tip‐Enhanced Raman Spectroscopy

AbstractDespite intensive research in surface enhanced Raman spectroscopy (SERS), the influence mechanism of chemical effects on Raman signals remains elusive. Here, we investigate such chemical effects through tip‐enhanced Raman spectroscopy (TERS) of a single planar ZnPc molecule with varying but controlled contact environments. TERS signals are found dramatically enhanced upon making a tip–molecule point contact. A combined physico‐chemical mechanism is proposed to explain such an enhancement via the generation of a ground‐state charge‐transfer induced vertical Raman polarizability that is further enhanced by the strong vertical plasmonic field in the nanocavity. In contrast, TERS signals from ZnPc chemisorbed flatly on substrates are found strongly quenched, which is rationalized by the Raman polarizability screening effect induced by interfacial dynamic charge transfer. Our results provide deep insights into the understanding of the chemical effects in TERS/SERS enhancement and quenching.

Chemical Enhancement and Quenching in Single-Molecule Tip-Enhanced Raman Spectroscopy.

Despite intensive research in surface enhanced Raman spectroscopy (SERS), the influence mechanism of chemical effects on Raman signals remains elusive. Here, we investigate such chemical effects through tip-enhanced Raman spectroscopy (TERS) of a single planar ZnPc molecule with varying but controlled contact environments. TERS signals are found dramatically enhanced upon making a tip-molecule point contact. A combined physico-chemical mechanism is proposed to explain such an enhancement via the generation of a ground-state charge-transfer induced vertical Raman polarizability that is further enhanced by the strong vertical plasmonic field in the nanocavity. In contrast, TERS signals from ZnPc chemisorbed flatly on substrates are found strongly quenched, which is rationalized by the Raman polarizability screening effect induced by interfacial dynamic charge transfer. Our results provide deep insights into the understanding of the chemical effects in TERS/SERS enhancement and quenching.

Impacts of plasmonic nanoparticles incorporation and interface energy alignment for highly efficient carbon-based perovskite solar cells

This work utilizes a realistic electro-optical coupled simulation to study the (i) impact of mesoporous TiO2 removal; (ii) the embedding of Ag@SiO2 and SiO2@Ag@SiO2 plasmonic nanoparticles; (iii) utilization of solution-processed inorganic p-type copper(I) thiocyanate (CuSCN) layer at the perovskite/carbon interface; and (iv) the increase of the work function of carbon electrodes (via incorporation of suitable additives/binders to the carbon ink) on the performance of carbon-based PSCs. Removal of mesoporous TiO2 increased the power conversion efficiency (PCE) of the device from 14.83 to 16.50% due to the increase in exciton generation rate and charge carriers’ mobility in the vicinity of the perovskite-compact TiO2 interface. Subsequently, variable mass ratios of Ag@SiO2 and SiO2@Ag@SiO2 plasmonic nanoparticles are embedded in the vicinity of the perovskite-compact TiO2 interface. In the optimum cases, the PCE of the devices increased to 19.72% and 18.92%, respectively, due to light trapping, scattering, and strong plasmonic fields produced by the plasmonic nanoparticles. Furthermore, adding the CuSCN layer remarkably increased the PCE of the device with a 0.93% mass ratio of Ag@SiO2 nanoparticles from 19.72 to 26.58% by a significant improvement of Voc and FF, due to the proper interfacial energy band alignment and the reduction of the recombination current density. Similar results were obtained by increasing the carbon work function, and the cell PCE was enhanced up to 26% in the optimal scenario. Our results pave the way to achieve high efficiencies in remarkably stable printable carbon-based PSCs.

Chiral biosensing at both interband transition and plasmonic extinction regions using twisted-stacked nanowire arrays.

Chiral metal nanostructures that exhibit strong chiroptical properties and enhanced light-matter interactions have recently attracted great interest due to their potential applications including chiral sensing and asymmetric synthesis. Most studies in this field focused on chiral sensing using circular dichroism (CD) responses at the plasmonic extinction region. In comparison, little is known about their CD responses at interband transition regions and their utility in chiral biosensing. Herein, we constructed a series of twisted-stacked silver nanowire arrays (TNAs) featuring CD signals at both the interband transition and plasmonic extinction regions and that are independently controllable. These TNAs are highly sensitive towards protein secondary structures. Proteins containing more β-sheets are more sensitive toward strong chiral plasmonic fields, whereas proteins rich in α-helices tend to generate larger CD shifts at the interband transition region of TNAs. The mutually independent optical activities at the interband transition and plasmonic extinction regions complement each other, providing more sensitivity and reliability in chiral biosensing.

Ultrasensitive detection of vitamin E by signal conversion combined with core-satellite structure-based plasmon coupling effect.

The rapid and sensitive surface-enhanced Raman scattering (SERS) detection of molecular biomarkers from real samples is still a challenge because the intrinsically trace analytes may have a low molecular affinity for metal surfaces. Herein, we develop a smart signal conversion and amplification strategy based on silver-gold-silica core-satellite structure nanoparticles (Ag@Au@SiO2 NPs) to sensitively detect low adsorptive vitamin E using SERS, which has been considered a biomarker of neuromuscular disorders when its abnormal content is measured in the serum of patients. Through the reducibility of vitamin E, Ag+ ions are rapidly reduced to Ag atoms, resulting in the epitaxial growth of Ag nanocrystals on gold nanoparticles forming satellite particle-particle gap-narrowed Ag@Au@SiO2 NPs. The generated strong plasmonic field dramatically enhances the Raman signal of the Raman reporter molecule 4-aminothiophenol (4-ATP) and the detected vitamin E molecules at an estimated level of 58.19 nmol L-1. The sensitivity of this operational SERS strategy provides tremendous prospects for the screening of neuromuscular disorders.

Hydrogen-Doping-Induced Metal-Like Ultrahigh Free-Carrier Concentration in Metal-Oxide Material for Giant and Tunable Plasmon Resonance.

The practical utilization of plasmon-based technology relies on the ability to find high-performance plasmonic materials other than noble metals. A key scientific challenge is to significantly increase the intrinsically low concentration of free carriers in metal-oxide materials. Here, a novel electron-proton co-doping strategy is developed to achieve uniform hydrogen doping in metal-oxide MoO3 at mild conditions, which creates a metal-like ultrahigh free-carrier concentration approaching that of noble metals (1021 cm-3 in H1.68 MoO3 versus 1022 cm-3 in Au/Ag). This bestows giant and tunable plasmonic resonances in the visible region to this originally semiconductive material. Using ultrafast spectroscopy characterizations and first-principle simulations, the formation of a quasi-metallic energy band structure that leads to long-lived and strong plasmonic field is revealed. As verified by the surface-enhanced Raman spectra (SERS) of rhodamine 6G molecules on Hx MoO3 , the SERS enhancement factor reaches as high as 1.1 × 107 with a detection limit at concentration as low as 1 × 10-9 mol L-1 , representing the best among the hitherto reported non-metal systems. The findings not only provide a set of metal-like semiconductor materials with merits of low cost, tunable electronic structure, and plasmonic resonance, but also a general strategy to induce tunable ultrahigh free-carrier concentration in non-metal systems.

Synergetic enhancement in optical nonlinearity of Au nanoparticle decorated MoS2 via interaction between excitonic and surface plasmon resonances

Local intensive field by surface plasmon resonance on gold nanoparticles boosted the nonlinear property of MoS2. Au decorated MoS2 nanohybrid material was prepared using facile photo-chemical method with MoS2 dispersion and HAuCl4 to create Au nanoparticles specifically at the edge of MoS2 sheets. The prominent nonlinear features of samples was investigated by Z-scan measurement and supercontinuum generation. The Au and MoS2 nanohybrid provided outstanding nonlinear refractive index, n2, of 1.45 × 10−5 cm2/W and the third-order nonlinear susceptibility, χ(3), of 8.18 × 10−3 esu, which were about four times larger than those of intrinsic MoS2. The excitonic resonance in MoS2 shows the synergetic interaction with surface plasmon resonance of Au nanoparticles at the edge, which results in concentrated nonlinear process with strong plasmonic field by localization and polarity. The hollow core fiber with optical bandgap was employed to make the supercontinuum generation from sample materials. From the 3-cm short fiber filled with Au decorated MoS2, the broad spectrum covering whole visible and near-infrared range was obtained, while that with MoS2 only exhibited emission about 200 nm of bandwidth. Our nanohybrid material and filament will be an important aspect in exploring nonlinear optical properties for the combination of transition-metal dichalcogenides and metal nanoparticles.

Large plasmonic absorption enhancement effect of triangular silver nanowires in silicon.

Two-dimensional finite difference time domain (FDTD) simulations were performed for evaluating optical absorption enhancement and loss effects of triangular silver (Ag) nanowires embedded in silicon (Si) thin-film photovoltaic device structures. Near-bandgap absorption enhancement in Si was much larger than the reported values of other nanostructures from similar simulations. A nanowire with equal sides of 20 nm length showed 368-fold absorption enhancement whereas only 5× and 15× values of solid spherical and two-dimensional core-shell type nanostructures, respectively. Undesirable absorption loss in the metal of the nanowire was 3.55× larger than the absorption in Si which was comparable to the value reported for the spherical nanoparticle. Interestingly, as the height of the nanowire was increased to form a sharper tip, absorption loss showed a significant drop. For a nanowire with 20 nm base and 20 nm height, absorption loss was merely 1.91× larger than the absorption in Si at the 840 nm plasmon resonance. This drop could be attributed to weaker plasmon resonance manifested by lower metallic absorption in the spatial absorption map of the nanowire. However, absorption enhancement in Si was still large due to strong plasmonic fields at the sharper and longer tip, which was effective in enhancing absorption over a larger area in Si. Our work shows that the shape of a nanostructure and its optimization can significantly affect plasmonic absorption enhancement and loss performance in photovoltaic applications.

Strong and tunable plasmonic field coupling and enhancement generating from the protruded metal nanorods and dielectric cores

Abstract Numerical investigation of resonant peak by modifying the number of protruded metal nanorods (MNRDs) and the core material in metal nanoshells for near field intensity are explored by means of the finite element method. The surface plasmon effect arising from the core cases with/without the protruded MNRDs was compared along with that arose from their solid counterparts. A strong and tunable field coupling and enhancement effects corresponding to the transverse surface plasmon resonance (SPR) and cavity plasmon resonance (CPR) modes are observed. It is found that the near field intensity spectra obtained from the silver and silver-shell nanorods with the protruded silver nanorods is approximately 2.5–3.0 times larger than that of the silver nanorods without the protruded silver nanorods. The peak resonance wavelength shows a red shift as the increasing of the permittivity in the core regions and the number of the protruded MNRDs. This indicates that the core regions and the protruded MNRDs can significantly mediate the EM waves coupling and enhancing the plasmonic effects and play a key role to tune the peak resonance wavelengths position. The investigated nanostructures offer multiple design freedoms to modulate the peak resonant wavelength and electromagnetic wave properties.

Controlling DNA translocation speed through graphene nanopore via plasmonic fields

We propose a novel plasmonic-based method for controlling translocation speed of DNA molecule through graphene nanopore. Dynamic properties of a double-stranded DNA molecule passage through a graphene nanopore are investigated by employing molecular dynamics simulation. Also, the effect of plasmonic fields parallel to the graphene plane on the translocation speed of the DNA molecule is studied. The DNA translocation speed is calculated for different values of confinement, spectral width, and power of the plasmonic field. Results show the potential of the method for controlling translocation speed of DNA via surface plasmons in graphene nanopore. The plasmon field power, confinement depth, and spectral width can increase translocation time of DNA up to 107, 62 and 15 %. Also, strong plasmon field can trap the DNA molecule in the nanopore. The suggested method can be utilized to solve the fast-translocation challenge of the nanopore DNA sequencers.

Controlled release of targeted chemotherapeutic drug dabrafenib for melanoma cancers monitored using surface-enhanced Raman scattering (SERS) spectroscopy

The advanced skin cancer melanoma, which is primarily caused by the mutation of BRAF gene, has a high mortality rate and requires high doses of chemotherapeutic drugs. To mitigate the drug toxicity to healthy cells and other side effects, the development of alternative modes of treatment has been extensively sought after. Herein, we describe a new targeted drug delivery system with controlled release, based on nanoparticle nanocarriers functionalized with folate and transferrin ligands for recognition of the respective receptors overexpressed in cancer cell membrane. We have investigated the immobilization of a new drug dabrafenib onto the nanocarriers and its controlled release, aided with surface-enhanced Raman scattering (SERS) spectroscopy which affords ultra-sensitive in situ measurement ability owing to the high signal amplification, associated with strong plasmonic fields of the nanocarrier gold nanoparticle (AuNP) cores. The nanocarriers were equipped with Raman reporters: mercaptobenzoic acid (MBA) and para-aminothiophenol (PATP) forming a mixed thiolate monolayer shell on AuNPs. The dabrafenib was covalently attached to MBA via an amide bond which is pH sensitive and enables the drug release at lower pH encountered in cancer cells. This arrangement in the drug binding to the nanocarrier protects the dabrafenib amine group against deactivation until the drug release in the target tumor cells.

Imaging the Nonlinear Plasmoemission Dynamics of Electrons from Strong Plasmonic Fields.

We use subcycle time-resolved photoemission microscopy to unambiguously distinguish optically triggered electron emission (photoemission) from effects caused purely by the plasmonic field (termed "plasmoemission"). We find from time-resolved imaging that nonlinear plasmoemission is dominated by the transverse plasmon field component by utilizing a transient standing wave from two counter-propagating plasmon pulses of opposite transverse spin. From plasmonic foci on flat metal surfaces, we observe highly nonlinear plasmoemission up to the fifth power of intensity and quantized energy transfer, which reflects the quantum-mechanical nature of surface plasmons. Our work constitutes the basis for novel plasmonic devices such as nanometer-confined ultrafast electron sources as well as applications in time-resolved electron microscopy.

(Invited) Novel Nanophotonic Devices By Engineering Light-Matter Interactions in Low-Dimensional Systems

Strongly confined electrical, optical and thermal excitations drastically modify material’s properties and break local symmetries that can enable precisely tunable novel responses and new functionalities. With an emphasis on low-dimensional materials such as nanowires and monolayer MoS2, we will discuss how extreme confinement of fields interacting with materials produces new and unexpected materials response. For example, we will discuss how the strong plasmonic fields can lead to a new paradigm of nanoscale Si photonics such as optical emission in the visible region and nonlinear optical devices. Furthermore, by utilizing the fundamental symmetry breaking properties of fields, new quantum phenomena such as chirality-dependent optical and electronic properties will be discussed in non-chiral materials and utilized to enable new functionalities that are only possible in strong spin-orbit coupled materials. The role of geometry in low-dimensional systems to produce new properties in the presence of symmetry breaking fields will be discussed. Finally, effect of plasmons on light matter interactions in 2D excitonic crystals will be discussed, which can be engineered to produce novel responses such as enhanced and tunable emission, Fano resonances and strong exciton-plasmon polaritons, which can be precisely controlled by geometry and applied fields to produce novel device concepts.

Responsivity Enhancement of Metal-Insulator-Semiconductor Photodetectors on Silicon-on-Insulator Substrates by Plasmonic Nanoantennas

We report metal-insulator-semiconductor photodetectors fabricated on a silicon-on-insulator substrate, exhibiting an enhanced responsivity in the near-infrared due to the incorporation of Au-nanoantennas at the optical input port. The optimal antenna design and the photoresponse enhancements have been calculated using detailed finite-difference time-domain simulations. These nanoantennas modify the propagation of the input optical signals. In this regime, strong plasmonic local field enhancements are observed. The maximum responsivity enhancement, 40%, was obtained at 780 nm; while at wavelengths shorter than 720 nm, the responsivity is reduced due to absorption losses in the Au-nanoantennas.

A Facile Strategy for All-Optical Controlling Platform by Using Plasmonic Perfect Absorbers

Dual-band light absorption with the maximal absorptivity up to 99.7% and the minimal spectral bandwidth down to 3 nm is obtained in the plasmonic absorbers consisting of triple-layer plasmonic crystal-nonlinear medium cavity-metal substrate structure, where the intercalated dielectric material is chosen to be a Kerr medium cavity. Efficient all-optical controlling with high spectral intensity change ratios and detecting signal-to-noise is achieved for the system after a slight increase of pumping intensity. These impressive results mainly result from the strong plasmonic resonant field confinement in the middle nonlinear Kerr medium cavity and the near-perfect relative intensity change response by the ultra-sharp anti-reflection spectrum. This work can lay a foundation for advanced all-optical devices by exploiting light perfect absorption behavior and resonant optical field enhancement.

Plasmonic nanostructures for organic photovoltaic devices

Due to the increasing demand on high-efficiency organic photovoltaic (OPV) devices, light management technique has become an active research subject. Especially, plasmonic approach was proven to be suitable for application in OPV and has shown lots of successful results. In this review, we summarize recent studies on plasmonic nanostructures for OPV with their underlying enhancement mechanisms. Optical absorption enhancement by the resonant scattering and the strong plasmonic near field will be discussed for various implementation geometries including metal nanoparticles, patterned electrodes, and plasmonic metamaterials. In addition, we will also look into the electrical effects originating from plasmonic nanostructures, which inevitably affect the device’s efficiency. Future research directions will be also discussed.

Dipping into a drink: Basil-seed supported silver nanoparticles as surface-enhanced Raman scattering substrates for toxic molecule detection

High-density plasmonic Ag nanoparticles (Ag-NPs) have been synthesized on a three-dimensional framework of natural basil seeds as the inexpensive substrates for surface-enhanced Raman scattering (SERS). The Ag-NPs decorated basil seeds (denoted as Ag-NPs@basil-seeds) are dipped into the analyte solution for rapid detection of methyl parathion in orange juice. Alternatively, they are incorporated into a microfluidic chip for online measurement of melamine in milk. The porous basil seeds can absorb trace melamine into the micro-cavity in the seeds, which load the melamine molecules into the gap where high-density “hot spots” appear under laser excitation. Hence the Ag-NPs@basil-seeds not only separate and pre-concentrate the trace analyte but also expose the analyte into the strong plasmonic field. This unique feature can eliminate the pre-treatment of analyte prior to SERS detection, and improve the sensitivity of the sensor.

Breakdown of the Dipole-Dipole Approximation at Short Distances and Hot Spot Formation between a Pair of Silver Nanocubes

ABSTRACTAg or Au nanocubes are known to be plasmonic nanoparticles with strong plasmonic fields concentrated around their corners1. When these nanoparticles aggregate the individual plasmonic oscillations of each particle begin to couple. The coupling between the two plasmonic nanoparticles is assumed to be dipolar in nature which results in an exponential red shift dependence of their localized surface plasmon resonance (LSPR) on the dimer separation2. Unfortunately, this exponential behavior is shown to fail as the separation distance between the two 42 nm nanocube dimer becomes 6nm or smaller3. Hooshmand et al4 have noted that these separation distances are marked by the formation of hot spots between the facets of the dimer.This dipolar exponential behavior results from a treatment of the coupling between the two excited nanocubes as a coupling between two oscillating dipole moments2. As a result, the vectorial addition of all the oscillating electronic dipoles is assumed to interact with the nearest nanoparticle as a single oscillating electronic dipole. Herein we suggest that as the separation distance becomes increasingly small, the coupling between the individual oscillating dipoles on the different nanocubes becomes significant. Thus, the dipolar exponential behavior fails to accurately predict the near field coupling between two nanoparticles with small separation distances.This leads to the realization that the interaction between the individual oscillating dipoles on the two nanocubes changes in a complicated manner as a function of separation distance. At 2nm, a good fraction of the oscillating dipoles are between the adjacent facets of the nanocubes as well as between the the corners. While at 3 nm less are in between the two facets of the nanocubes and a larger portion are localized at the corners. Thus, the coupling is not only dependent on the separation distance but also on what the separation does to the net interaction between the oscillating dipoles on each facet of the two coupled nanocubes. This results in the failure of the exponential behavior as the dipole moment on each nanocube is changing with distance in a complicated manner.

Are Hot Spots between Two Plasmonic Nanocubes of Silver or Gold Formed between Adjacent Corners or Adjacent Facets? A DDA Examination.

Of all the plasmonic solid nanoparticles, single Ag or Au nanocubes are known to be plasmonic nanoparticles with strong plasmonic fields, which are concentrated around their corners. However, when nanoparticles aggregate, they do so in a face-to face arrangement. The formation of hot spots between plasmonic nanoparticles in close proximity to each other is known to greatly enhance their plasmonic fields which are important in the field of imaging. Thus, what is the structural development of hot spots between two nanocubes in a dimer? Do they form between the corners or are they between the adjacent facets, and what does this depend on? A detailed discrete dipole approximation (DDA) simulation of Au-Au and Ag-Ag dimers suggests that there is a competition between their formation in these two locations, which depends on the polarization direction of the exciting light (with respect to the interparticle axis in the dimer), its intensity and the wavelength as well as the interparticle separation of the dimer.