Nanoparticle-on-mirror Research Articles

Observation of domain‐selective defect effects from a CVD‐grown graphene monolayer sandwiched at individual nanoparticle‐on‐mirror plasmonic junctions

AbstractThe author examines the different defect effects of chemical vapor deposition graphene monolayer (CVD‐GM) relying on domain position at an individual nanoparticle‐on‐mirror (NPoM) plasmonic junction. The NPoM, composed of Au nanoparticle (NP)/CVD‐GM/Au thin film (TF), could previously show the mode‐selective Raman enhancement, in which the center domain exhibited the highest Raman enhancement of both radial breathing like mode (RBLM) and D mode considering the local z‐directional electromagnetic (EM) field enhancement effect in the NPoM. Meanwhile, the outer domain of the NPoM also displays the near‐two symmetrical deep pits via atomic force microscope (AFM) with the accompanying lower I[D]/I[D*], higher FWHM[RBLM], I[RBLM], and relatively red‐shifted (tensile strained) Max [RBLM] from surface‐enhanced Raman scattering (SERS) information than the central case. With these observations, the physical loading of Au NP on CVD‐GM/Au TF may induce the threading effect on the most outer position of CVD‐GM at the NPoM, leading to sp2 defect‐rich domains. In contrast, the central domains, where the sp3 defects are rich, exhibit the opposite SERS spectral phenomena, indicating that the near‐flat surface shape of Au NP bottom has a higher probability to exert a strong charge transfer along the z‐direction, resulting in higher I[RBLM] and I[D]/I[D*] than the most outer domain. As such, the author anticipates various chemical sensing using subtle changes in CVD‐GM morphology at both the most outer and center positions in the NPoM with the comparison of each SERS spectral signature.

Tuning magnetic Mie-exciton interaction from the intermediate to strong coupling regime in a WSe2 monolayer coupled with dielectric-metal nanoresonators

Manipulation of coherent light-matter interactions at the nanoscale with the suppression of the incoherent damping process is of great importance for both fundamental research and future applications of quantum information devices. Here, we propose a low-loss dielectric nanoparticle-on-mirror (NPoM) system for flexible control of the coherent interaction between magnetic Mie resonances and the excitonic mode in a monolayer of $\mathrm{W}{\mathrm{Se}}_{2}$. We demonstrate that the dielectric NPoM system simultaneously enables a large field enhancement and low absorption in the dielectric nanoresonator, thus greatly facilitating the coherent magnetic Mie-exciton coupling. Importantly, the efficiency of radiating the magnetic mode to the far field can be readily manipulated by changing the thickness of the dielectric spacer and the particle size, therefore offering flexible tuning of the interaction from the intermediate to strong coupling regime. Moreover, the highly directional emission at the Mie resonance dramatically suppresses the incoherent damping process between the two individual systems via the continuum reservoir. Such a NPoM-based hybrid system is expected to offer not only the possibility to manipulate light-matter interactions but also new avenues for low-loss, high-efficiency coherent light control at the deep nanoscale.

Quantitative Investigation of the Morphologically Corrugated CVD-Grown Graphene Monolayer Surface with a Nanoparticle-on-Mirror System

Graphene can be used as a starting material for the synthesis of useful nano-complexes for flexible, transparent electrodes, therapeutic, bio-diagnostics, and bio-sensing. In order to apply graphene in the medical field, the chemical vapor deposition (CVD) method has been mainly utilized considering its large and near-homogeneous carbon constituents. Especially, the less degree of perturbation of graphene monolayer (GM), which is followed by the underneath catalytic Cu surface morphology, is very crucial in terms of providing the suspended GM and relatively fluent lateral carrier mobility with lower sheet resistance value. In this work, we can suggest a surface-enhanced Raman spectroscopic (SERS) indicator in a quantitative way on the status of z-directional morphological corrugation of a CVD-grown GM (CVD-GM) by applying a nanoparticle-on-mirror (NPoM) system composed of Au nanoparticle (NP)/CVD-GM/Au thin film (TF) plasmonic junction structure. A new (or enhanced) radial breathing-like mode (RBLM) SERS signal around ~ 150 cm−1 from CVD-GM spaced in NPoM is clearly observed by employing a local z-polarized incident field formed at the Au NP–Au TF plasmonic gap junctions. With this observation, the value of I[out-of-plane, RBLM]/I[in-plane, [2D] at certain domains, can be suggested as a new optical nano-metrology value to relatively determine between lower z-directional morphological corrugation (or protrusion) status of a CVD-GM spaced in our NPoM system (lower I[RBLM]/I[2D] value) and a higher degree of lateral carrier mobility of the CVD-GM associated with lower sheet resistance values as a result of higher blue-shifted Raman in-plane (G, 2D) peak maximum position. Furthermore, we will also expect the bio-sensing performances by utilizing the high specific surface area and ultrahigh flexibility of the CVD-GM in one of the future prospective works such as pressure-strain, strain-to-electricity, and chemical-coupled sensor via I[RBLM]/I[2D] values.Graphical abstract

On-demand nanoparticle-on-mirror (NPoM) structure for cost-effective surface-enhanced Raman scattering substrates

We report a robust technique to fabricate a cost-efficient Raman substrate which is composed of polyvinylpyrrolidone (PVP) coated gold nanoparticles layer on commercial aluminum foil. The layer of metal nanoparticles on the aluminum foil, i.e., the nanoparticle-on-mirror (NPoM) structure was fabricated by spraying nanoparticle colloidal solution directly on the foil. The detection limit (LOD) of NPoM substrate is investigated by performing the SERS for Rhodamine 6G (R6G) with the concentration ranging from mM to nM without any post treatment of the substrate. The findings show that the LOD of 1 nM and maximum intensity enhancement factor of ~ 24 is accomplished. Field enhancement owing to reflection from the metallic mirror is the reason behind the signal enhancement and it would be beneficial for routine clinical applications, trace chemical detection, and disease diagnostics.

Multimetallic Nanoparticles on Mirrors for SERS Detection

Nanoparticle on mirror (NPoM) architectures are attractive for optical applications–particularly for sensing–due to their simplicity and reliability of their multicomponent plasmonic interactions. However, to date, work has predominantly focused on monometallic nanoparticles on different metallic films. While these studies have investigated the effect of geometry on the optical responses of NPoM structures, understanding the effect of nanoparticle composition is needed to gain a full understanding and complete control over these systems. To elucidate the role of composition on localized plasmonic resonances, a surface-based nanoparticle synthetic method, scanning block copolymer lithography (SPBCL), was used to synthesize and characterize single Au0.5Ag0.5 particles on a five-layer, thermally stable plasmonic mirror substrate. The use of SPBCL is important because it can easily be extended to almost a limitless number of polyelemental systems. We demonstrate the potential of the approach by using surface-enhanced Raman spectroscopy to detect benzene-1,4-dithiol adsorbed onto individual alloyed AuAg NPs in an NPoM geometry.

Portable Nanoplasmonic Sensor for Real-Time SARS-CoV-2 RNA Detection

Introduction Fast, accurate, and in-situ environmental detection of SARS-CoV-2, the virus which causes COVID-19, is currently an unmet challenge. The gold standard detection method, polymerase chain reaction (PCR) is highly sensitive, but requires centralized lab infrastructure, specialized training, and is time consuming (> 3 hrs. time to result) [1].Other approaches more conducive to real-time and portable sensing are in development, including electrochemical [2] and plasmonic sensors [3]. However, improvements are still needed in device portability and robustness. Here, we introduce a new nanoplasmonic sensor configuration for SARS-CoV-2 RNA detection, which leverages commercially available surface plasmon readout instrumentation [4] for portable, real-time operation. Methods The sensor is configured as a plasmonic nanoparticle on mirror (NPoM) system, formed by a gold nanoparticle attached to a gold mirror by a peptide nucleic acid (PNA) probe complementary to the target RNA sequence (Figure 1A).The PNA capture probes are conjugated to the gold nanoparticles and then attached to the gold mirror using a two-step process [5]. Simple amine-coupling is used to attach the probes to the particle first, and then onto the activated gold mirror. Particle quantification is used to determine conjugation efficacy. The PNA probes act as a spacer layer, separating the nanoparticles from the mirror by approximately 10 nm.Upon hybridization between the PNA and RNA, the nanoparticle is pulled to within approximately 7 nm of the mirror below. The electromagnetic coupling between the nanoparticle and mirror is strongly dependent on the distance between the two and is transduced with nanometer precision by a pronounced shift in the resonance dip (Figure 1B). A change in distance of 3 nm is predicted to yield a ~40 nm shift in resonance wavelength.The NPoM system can either be interrogated in reflection mode (Figure 1B inset), or by total internal reflection with oblique illumination from the back side of the gold mirror. The latter enables integration with commercially available hardware for a portable, robust detection platform. Results and Conclusion Baseline experimental SARS-CoV-2 RNA detection has been obtained using gold nanoparticles on a glass substrate (Figure 1C). Standardized positive control material including the entire RNA genome of SARS-CoV-2 was linearly diluted to clinically relevant viral loads, and spiked into buffer for testing. The nanoparticles were functionalized with PNA capture probes complementary to a CDC-approved PCR panel sequence, and, upon RNA hybridization, a change in local refractive index was transduced by a shift in the extinction spectra peak (Figure 1C). The limit of detection of this system approached clinically relevant RNA loads (100,000 copies/mL).The NPoM geometry is expected to improve sensitivity and on-chip/portable system readout, owing to the narrower resonance width (cf. Figure 1B and 1C top). It is envisioned that this integrated platform could also be applied more broadly to the detection of DNA/RNA of other pathogens of interest including other viruses and bacterial species.

Structural Order of the Molecular Adlayer Impacts the Stability of Nanoparticle-on-Mirror Plasmonic Cavities.

Immense field enhancement and nanoscale confinement of light are possible within nanoparticle-on-mirror (NPoM) plasmonic resonators, which enable novel optically activated physical and chemical phenomena and render these nanocavities greatly sensitive to minute structural changes, down to the atomic scale. Although a few of these structural parameters, primarily linked to the nanoparticle and the mirror morphology, have been identified, the impact of molecular assembly and organization of the spacer layer between them has often been left uncharacterized. Here, we experimentally investigate how the complex and reconfigurable nature of a thiol-based self-assembled monolayer (SAM) adsorbed on the mirror surface impacts the optical properties of the NPoMs. We fabricate NPoMs with distinct molecular organizations by controlling the incubation time of the mirror in the thiol solution. Afterward, we investigate the structural changes that occur under laser irradiation by tracking the bonding dipole plasmon mode, while also monitoring Stokes and anti-Stokes Raman scattering from the molecules as a probe of their integrity. First, we find an effective decrease in the SAM height as the laser power increases, compatible with an irreversible change of molecule orientation caused by heating. Second, we observe that the nanocavities prepared with a densely packed and more ordered monolayer of molecules are more prone to changes in their resonance compared to samples with sparser and more disordered SAMs. Our measurements indicate that molecular orientation and packing on the mirror surface play a key role in determining the stability of NPoM structures and hence highlight the under-recognized significance of SAM characterization in the development of NPoM-based applications.

Hydrophobic Plasmonic Nanoacorn Array for a Label-Free and Uniform SERS-Based Biomolecular Assay.

A surface-enhanced Raman scattering (SERS) aptasensor based on a hydrophobic assembled nanoacorn (HANA) was developed with improved reproducibility and reduced nonspecific binding effect. In the fabrication process, a hexagonal-packed gold film over nanosphere (AuFON) arrays was first obtained and used as a hydrophobic plasmonic substrate. Then, a uniform sub-3 nm molecular spacer array (containing Raman reporters) was prepared by patterning nanometric hydrophilic ultrathin patches onto the hydrophobic AuFON, in which the hydrophilic thin layer is composed of polymers and aptamers. During the sensing process, the HANA aptasensor smartly impedes the adsorption of SERS probes as Au@Ag nanocubes (Au@Ag NCs) in the absence of targets. In the presence of targets, the displacement of aptamers occurs due to the specific interaction between the targets and the aptamers, and the Au@Ag NCs can be assembled onto the hydrophilic patches on AuFON through electrostatic interactions with polymers. Thus, SERS signals of reporter molecules inside the spacer can be dramatically enhanced due to the formation of a nanoparticle-on-mirror (NPoM) array. In such a SERS aptasensor, the well-ordered distribution of SERS probes ensures excellent repeatability, while the precise subnanometer junctions guarantee high sensitivity. More importantly, since the hydrophobic surface can greatly reduce nonspecific adsorption, the tedious process of nonspecific blocking that is employed in traditional biosensors is no longer needed. Using such a SERS HANA platform, human epidermal growth factor receptor 2 (HER2) and three exosomal proteins were analyzed with high sensitivity and good reproducibility (RSD < 7%) in whole-blood samples.

Plasmon-driven protodeboronation reactions in nanogaps.

Boronic acids are the key compounds in Suzuki coupling reactions and in the detection of monosaccharides. The C-B bond cleavage deboronation is an important side reaction that lowers the Suzuki coupling reaction yield and even disables saccharide detection. Here, we report that protodeboronation occurs for 4-mercaptophenylboronic acid (MPBA) within narrow nanogaps between gold nanoparticles (AuNPs) and planar gold substrates. The irradiation of such nanoparticle-on-mirror (NPoM) systems at 785 nm drives the protodeboronation reaction to form benzenethiol (BT). Wavelength-dependence experiments, combined with dark-field single-particle scattering spectroscopy, reveal that excitation of the bonding dipole plasmon mode of the NPoM leads to the best efficiency. Among the excited plasmon decay pathways, the generation of hot charge carriers induces the protodeboronation of MPBA. The possibility of plasmonic thermal reactions is ruled out because external heating of the substrates does not cause the reaction to take place. A comparison of the reaction yield under ambient, Ar, and oxygen gas conditions reveals that hot charge carriers directly transfer to MPBA, which subsequently produces BT, but the presence of oxygen promotes the reaction by opening another hot-electron transfer channel. The protodeboronation reaction of MPBA is an important addition to the catalog of plasmon-driven chemical reactions, not only because the reaction is relevant to organic and analytical chemistry but also because it deepens our understanding of the hot carrier dynamics at the interface between plasmonic nanoparticles and molecules.

Anomalously Large Spectral Shifts near the Quantum Tunnelling Limit in Plasmonic Rulers with Subatomic Resolution.

The resonance wavelength of a coupled plasmonic system is extremely sensitive to the distance between its metallic surfaces, resulting in "plasmon rulers". We explore this behavior in the subnanometer regime using self-assembled monolayers of bis-phthalocyanine molecules in a nanoparticle-on-mirror (NPoM) construct. These allow unprecedented subangstrom control over spacer thickness via choice of metal center, in a gap-size regime at the quantum-mechanical limit of plasmonic enhancement. A dramatic shift in the coupled plasmon resonance is observed as the gap size is varied from 0.39 to 0.41 nm. Existing theoretical models are unable to account for the observed spectral tuning, which requires inclusion of the quantum-classical interface, emphasizing the need for new treatments of light at the subnanoscale.

Rabi Splitting in a Plasmonic Nanocavity Coupled to a WS2 Monolayer at Room Temperature

A large Rabi splitting (∼145 meV) is demonstrated in a plasmonic nanocavity coupled to a WS2 monolayer at room temperature. The nanocavity is composed of a silver nanocube and a silver film with an Al2O3 spacer of a few nanometers, which belongs to a nanoparticle on mirror (NPoM) type. The surface plasmon resonance (SPR) of the nanocavity can be tuned by controlling the thickness of nanogap and the size of silver nanocubes, which allows to successively adjust the SPR to accurately match the exciton energy of WS2 monolayers (2.02 eV). A mode splitting can be clearly observed from the dark-field scattering spectrum of the single hybrid nanocavity, which is ascribed to a strong coupling between the nanocavity mode and the excitonic mode. Furthermore, the anticrossing curves of the hybrid system are obtained by recording the scattering spectra with varied sizes of silver nanocubes, which further validate the interaction regime. It presents a strong coupling platform for two-dimensional monolayers, which is of...

Fluorescence enhancement and strong-coupling in faceted plasmonic nanocavities

Emission properties of a quantum emitter can be significantly modified inside nanometre-sized gaps between two plasmonic nanostructures. This forms a nanoscopic optical cavity which allows single-molecule detection and single-molecule strong-coupling at room temperature. However, plasmonic resonances of a plasmonic nanocavity are highly sensitive to the exact gap morphology. In this article, we shed light on the effect of gap morphology on the plasmonic resonances of a faceted nanoparticle-on-mirror (NPoM) nanocavity and their interaction with quantum emitters. We find that with increasing facet width the NPoM nanocavity provides weaker field enhancement and thus less coupling strength to a single quantum emitter since the effective mode volume increases with the facet width. However, if multiple emitters are present, a faceted NPoM nanocavity is capable of accommodating a larger number of emitters, and hence the overall coupling strength is larger due to the collective and coherent energy exchange from all the emitters. Our findings pave the way to more efficient designs of nanocavities for room-temperature light-matter strong-coupling, thus providing a big step forward to a non-cryogenic platform for quantum technologies.

Comparative Study of Plasmonic Resonances between the Roundest and Randomly Faceted Au Nanoparticles-on-Mirror Cavities

Over the past decade, the synthesis of a relatively large-sized (>50 nm), spherical gold nanoparticles (Au NPs) has undergone significant progress, from the initial demonstration of the hydroquinone-mediated synthesis of randomly faceted Au nanospheres (NSs) to iterative growth and dissolution of highly spherical and ultrasmooth Au NSs. The iterative growth and dissolution method can synthesize the roundest Au NSs. However, the roundest Au NSs have not been used in nanoparticle-on-mirror (NPoM) cavities; thus, the effects of Au NS roundness and facets on the plasmonic resonance of an NPoM cavity needs to be understood. In this work, we synthesized the Au NSs of the same size but with different facets and used them in NPoM cavities. Using these plasmonic models, we systematically compared round and randomly faceted Au NSs in terms of plasmonic resonance and spectral reliability. On the basis of these experimental results, we theoretically defined the accessible plasmonic mode volume with an NPoM cavity.

Nanoparticle-on-mirror cavity modes for huge and/or tunable plasmonic field enhancement

We present a careful numerical study of nanoparticle (NP) faceting, highlighting the great influence of small morphological changes of NP-mirror cavities on near-field enhancement in the nanoparticle-on-mirror (NPOM) system. Using a 3D finite element method (FEM) plasmon mapping method, the active transverse cavity modes can be confirmed. For the dominant mode, we have found that, by increasing the facet width, the resonance can be tuned linearly to the red with little decrease of the peak near-field intensity. It is further demonstrated that by increasing the NP size, the near-field intensity can be strongly enhanced. Understanding of such extreme optics benefits significantly both the optimized design of potential plasmonic devices and the fundamental understanding of nano-optics. Collaborative experimental considerations are expected with the rapid development of nanotechnology.

Tracking Optical Welding through Groove Modes in Plasmonic Nanocavities.

We report the light-induced formation of conductive links across nanometer-wide insulating gaps. These are realized by incorporating spacers of molecules or 2D monolayers inside a gold plasmonic nanoparticle-on-mirror (NPoM) geometry. Laser irradiation of individual NPoMs controllably reshapes and tunes the plasmonic system, in some cases forming conductive bridges between particle and substrate, which shorts the nanometer-wide plasmonic gaps geometrically and electronically. Dark-field spectroscopy monitors the bridge formation in situ, revealing strong plasmonic mode mixing dominated by clear anticrossings. Finite difference time domain simulations confirm this spectral evolution, which gives insights into the metal filament formation. A simple analytic cavity model describes the observed plasmonic mode hybridization between tightly confined plasmonic cavity modes and a radiative antenna mode sustained in the NPoM. Our results show how optics can reveal the properties of electrical transport across well-defined metallic nanogaps to study and develop technologies such as resistive memory devices (memristors).

Ultrasmall Mode Volumes in Plasmonic Cavities of Nanoparticle-On-Mirror Structures.

The mode volume and Purcell factor are two important parameters to assess the performance of optical nanocavities. Achieving small mode volumes and high Purcell factors for nanocavity structures while simplifying their fabrication has been a major task to realize high-performance and large-scale photonic devices and systems. Different optical resonators based on nanoparticle-on-mirror (NPoM) structures are systematically analyzed, which are easy to fabricate and flexible to use. Direct comparison of these optical resonators is made through finite-difference time-domain (FDTD) simulations. The achievement of ultrasmall mode volumes below 10-7 (λ/n)3 based on the NPoM structure through FDTD simulations is demonstrated by rationally selecting the structural parameters. Such NPoM structures provide a decent Purcell factor on the order of 107 , which can effectively enhance spontaneous emission and facilitate a number of photonic applications. The simulation results are confirmed by dark field scattering and second-harmonic generation measurements. This work is scientifically important and offers practical guidelines for the design of optical resonators for state-of-the-art optical and photonic devices.

Controllable Tuning Plasmonic Coupling with Nanoscale Oxidation.

The nanoparticle on mirror (NPoM) construct is ideal for the strong coupling of localized plasmons because of its simple fabrication and the nanometer-scale gaps it offers. Both of these are much harder to control in nanoparticle dimers. Even so, realizing controllable gap sizes in a NPoM remains difficult and continuous tunability is limited. Here, we use reactive metals as the mirror so that the spacing layer of resulting metal oxide can be easily and controllably created with specific thicknesses resulting in continuous tuning of the plasmonic coupling. Using Al as a case study, we contrast different approaches for oxidation including electrochemical oxidation, thermal annealing, oxygen plasma treatments, and photo-oxidation by laser irradiation. The thickness of the oxidation layer is calibrated with depth-mode X-ray photoemission spectroscopy (XPS). These all consistently show that increasing the thickness of the oxidation layer blue-shifts the plasmonic resonance peak while the transverse mode remains constant, which is well matched by simulations. Our approach provides a facile and reproducible method for scalable, local and controllable fabrication of NPoMs with tailored plasmonic coupling, suited for many applications of sensing, photochemistry, photoemission, and photovoltaics.