Tip-enhanced Spectroscopy Research Articles

Quantum mechanical effects in plasmonic sub-nanometer cavities formed by a tip and substrate structure

Enhancing local field intensity through light field compression is one of the core issues in surface plasmon-enhanced spectroscopy. The theoretical framework for the nanostructure composed of a tip and a substrate has predominantly relied on classical electromagnetic models, ignoring the electron tunneling effect. In this paper, we investigate the plasmonic near-field characteristics in the sub-nanometer cavity formed by the tip and the substrate using a quantum-corrected model. Additionally, we analyze the local electric field and Raman enhancement when hexagonal boron nitride (h-BN) monolayer is used as a decoupling layer for the nanocavity. The results indicate that classical electromagnetic theory fails to accurately describe the plasmonic electric field in smaller sub-nanometer gaps. When the gap is reduced to 0.32 nm, the quantum-corrected model shows that the local electric field in the sub-nanometer cavity is significantly reduced due to the tunneling current, aligning more closely with experimental results. Moreover, adding a high-barrier h-BN layer effectively prevents the occurrence of tunneling current, allowing for a strong local electric field even when the gap is less than 0.32 nm. The calculated maximum Raman enhancement reaches up to 15 orders of magnitude. Our research results provide a deep understanding of quantum mechanical effects in tip-enhanced spectroscopy systems, enabling the potential applications based on quantum plasmons in nanocavity.

Effect of thin MoS2 film with different layer numbers on tip-enhanced photoluminescence spectroscopy

As a novel semiconductor material, Molybdenum disulfide (MoS2) has great application potential in sensors, catalysts and optoelectronic devices. In this study, we aim to investigate the unique optical properties of MoS2 by theoretically exploring the impact of thin MoS2 films with varying layer numbers on the tip-enhanced fluorescence of a single molecule confined within the nanocavity formed by an Au tip over a substrate. The influence of incidence angle, layer numbers, tip radius and tip-MoS2 film distance on the excitation field, quantum yield and fluorescence enhancement are quantitatively analyzed in detail. The results show that the presence of multilayer MoS2 film can significantly improve tip-enhanced fluorescence spectroscopy. In the strong gap-mode plasmon coupling effect, the maximum fluorescence enhancement reaches approximately five orders of magnitude when it consists of six layers under excitation wavelength of 660 nm. Furthermore, the spatial resolution of Raman and fluorescence increases with decreasing tip radius and gap distance, reaching a maximum of 2.6 nm and 3.8 nm, respectively. Our results provide a theoretical guidance for further understanding and application of the two-dimensional material in optical experiments.

Submolecular-scale control of phototautomerization.

Optically activated reactions initiate biological processes such as photosynthesis or vision, but can also control polymerization, catalysis or energy conversion. Methods relying on the manipulation of light at macroscopic and mesoscopic scales are used to control on-surface photochemistry, but do not offer atomic-scale control. Here we take advantage of the confinement of the electromagnetic field at the apex of a scanning tunnelling microscope tip to drive the phototautomerization of a free-base phthalocyanine with submolecular precision. We can control the reaction rate and the relative tautomer population through a change in the laser excitation wavelength or through the tip position. Atomically resolved tip-enhanced photoluminescence spectroscopy and hyperspectral mapping unravel an excited-state mediated process, which is quantitatively supported by a comprehensive theoretical model combining ab initio calculations with a parametric open-quantum-system approach. Our experimental strategy may allow insights in other photochemical reactions and proof useful to control complex on-surface reactions.

Sub-Tip-Radius Near-Field Interactions in Nano-FTIR Vibrational Spectroscopy on Single Proteins.

Tip-enhanced vibrational spectroscopy has advanced to routinely attain nanoscale spatial resolution, with tip-enhanced Raman spectroscopy even achieving atomic-scale and submolecular sensitivity. Tip-enhanced infrared spectroscopy techniques, such as nano-FTIR and AFM-IR spectroscopy, have also enabled the nanoscale chemical analysis of molecular monolayers, inorganic nanoparticles, and protein complexes. However, fundamental limits of infrared nanospectroscopy in terms of spatial resolution and sensitivity have remained elusive, calling for a quantitative understanding of the near-field interactions in infrared nanocavities. Here, we demonstrate the application of nano-FTIR spectroscopy to probe the amide-I vibration of a single protein consisting of ∼500 amino acid residues. Detection with higher tip tapping demodulation harmonics up to the seventh order leads to pronounced enhancement in the peak amplitude of the vibrational resonance, originating from sub-tip-radius geometrical effects beyond dipole approximations. This quantitative characterization of single-nanometer near-field interactions opens the path toward employing infrared vibrational spectroscopy at the subnanoscale and single-molecule levels.

Kapitza-resistance-like exciton dynamics in atomically flat MoSe2-WSe2 lateral heterojunction

Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challenging task. Here we demonstrate that an atomically sharp TMD-ML lateral heterostructure (MoSe2-WSe2) transforms the isotropic exciton diffusion into a unidirectional excitonic flow through the junction. Using tip-enhanced photoluminescence spectroscopy (TEPL) and a modified exciton transfer model, we show a discontinuity of the exciton density distribution on each side of the interface. We introduce the concept of exciton Kapitza resistance, by analogy with the interfacial thermal resistance referred to as Kapitza resistance. By comparing different heterostructures with or without top hexagonal boron nitride (hBN) layer, we deduce that the transport properties can be controlled, over distances far greater than the junction width, by the exciton density through near-field engineering and/or laser power density. This work provides a new approach for controlling the neutral exciton flow, which is key toward the conception of excitonic devices.

Tip-enhanced two-photon excited fluorescence spectroscopy of monolayer black phosphorus

The optical properties of monolayer black phosphorus (BP) in two-photon excited fluorescence (TPEF) are pivotal for photoluminescence applications at the single molecular level. In this study, we theoretically examine the TPEF properties of monolayer BP using a tip-enhanced spectroscopy system. We detail the influence of parameters such as the tip radius, the distance between the tip and the BP film, the incidence angle and the tip half-cone angle on the fluorescence excitation and emission enhancement of monolayer BP. This examination employs the finite element method. Our findings reveal that the signal enhancement in TPEF is substantially improved, approximately two orders of magnitude greater than that in single-photon excited fluorescence (SPEF), attributed to the high-order nonlinear coupling effect. By optimising the system configuration, we determined that the TPEF enhancement factor with monolayer BP can reach as high as eight orders of magnitude under an excitation wavelength of 800 nm. The highest spatial resolutions achievable for TPEF and SPEF spectroscopy are also 4.4 nm and 5.3 nm, respectively. These results provide an in-depth understanding of the TPEF enhancement mechanism and are invaluable for designing and applying BP-based optoelectronic devices.

Competing mechanisms of local photoluminescence quenching and enhancement in the quantum tunneling regime at 2D TMDC/hBN/plasmonic interfaces

Owing to the extraordinary physical and chemical properties, and the potential to couple with nanoplasmonic structures, two-dimensional (2D) transition metal dichalcogenides are promising materials for next-generation (opto-)electronic devices. Targeting the application stage, it is essential to understand the mechanisms of photoluminescence (PL) quenching and enhancement at the nanoscale. In this work, using monolayer MoSe2/hBN heterostructure on Au nanotriangles (NTs) as an example, we report on the local PL quenching and enhancement in the quantum tunneling regime at MoSe2/hBN/plasmonic nanostructure interfaces. By exploiting tip-enhanced photoluminescence spectroscopy, we were able to resolve and image the nanostructures locally. Moreover, by studying the different near-field emission behavior of MoSe2/SiO2, MoSe2/hBN, MoSe2/NT, and MoSe2/hBN/NT, we investigate the localized surface plasmon resonance, electron tunneling, and highly localized strain as the three competing mechanisms of local PL quenching and enhancement in the quantum tunneling regime at the nanoscale.

Plexcitonics: plasmon-exciton coupling for enhancing spectroscopy, optical chirality, and nonlinearity.

Plexcitonics is a rapidly developing interdisciplinary field that holds immense potential for the creation of innovative optical technologies and devices. This field focuses on investigating the interactions between plasmons and excitons in hybrid systems. In this review, we provide an overview of the fundamental principles of plasmonics and plexcitonics and discuss the latest advancements in plexcitonics. Specifically, we highlight the ability to manipulate plasmon-exciton interactions, the emerging field of tip-enhanced spectroscopy, and advancements in optical chirality and nonlinearity. These recent developments have spurred further research in the field of plexcitonics and offer inspiration for the design of advanced materials and devices with enhanced optical properties and functionalities.

Tip-Enhanced Dark Exciton Nanoimaging and Local Strain Control in Monolayer WSe2.

Dark excitons in transition-metal dichalcogenides, with their long lifetimes and strong binding energies, provide potential platforms from photonic and optoelectronic applications to quantum information science even at room temperature. However, their spatial heterogeneity and sensitivity to strain is not yet understood. Here, we combine tip-enhanced photoluminescence spectroscopy with atomic force induced strain control to nanoimage dark excitons in WSe2 and their response to local strain. Dark exciton emission is facilitated by out-of-plane picocavity Purcell enhancement giving rise to spatially highly localized emission, providing for higher spatial resolution compared to bright exciton nanoimaging. Further, tip-antenna-induced dark exciton emission is enhanced in areas of higher strain associated with bubbles. In addition, active force control shows dark exciton emission to be more sensitive to strain with both compressive and tensile lattice deformation facilitating emission. This interplay between localized strain and Purcell effects provides novel pathways for nanomechanical exciton emission control.

Evidence of exciton-libron coupling in chirally adsorbed single molecules

Interplay between motion of nuclei and excitations has an important role in molecular photophysics of natural and artificial structures. Here we provide a detailed analysis of coupling between quantized librational modes (librons) and charged excited states (trions) on single phthalocyanine dyes adsorbed on a surface. By means of tip-induced electroluminescence performed with a scanning probe microscope, we identify libronic signatures in spectra of chirally adsorbed phthalocyanines and find that these signatures are absent from spectra of symmetrically adsorbed species. We create a model of the libronic coupling based on the Franck-Condon principle to simulate the spectral features. Experimentally measured librational spectra match very well the theoretically calculated librational eigenenergies and peak intensities (Franck-Condon factors). Moreover, the comparison reveals an unexpected depopulation channel for the zero libron of the excited state that can be effectively controlled by tuning the size of the nanocavity. Our results showcase the possibility of characterizing the dynamics of molecules by their low-energy molecular modes using µeV-resolved tip-enhanced spectroscopy.

Controlling excitons in the quantum tunneling regime in a hybrid plasmonic/2D semiconductor interface

The electromagnetic field confinement and amplification typical of nano-sized metallic objects supporting localized surface plasmon resonances, i.e., light-induced collective electronic oscillations, can significantly strengthen the interaction of light with atomically thin transition metal dichalcogenides. In view of the realization of plasmon-enhanced devices, it is crucial to investigate the effects induced by light confinement within metallic nanostructures on the excitonic properties of these materials at the nanoscale. Here, we exploit tip-enhanced photoluminescence spectroscopy to locally control the excitons of monolayer molybdenum disulfide (MoS2) coupled with gold nanotriangles in the quantum tunneling regime. The spatial resolution of 10 nm in the tip-enhanced photoluminescence measurements made it possible to image the light-emission related properties of monolayer MoS2 across one single metallic nanostructure and to investigate the effect of the plasmonic enhancement on its photoluminescence peak. Moreover, by taking advantage of the degree of freedom given by the tuning of the tip-sample distance; it was possible to probe the effect of the plasmonic pico-cavity size on the photoluminescence quenching rate of monolayer MoS2.

Enhancing electromagnetic field gradient in tip-enhanced Raman spectroscopy with a perfect radially polarized beam.

Tip-enhanced Raman spectroscopy (TERS) is a promising label-free super-resolving imaging technique, and the electric field gradient of nanofocusing plays a role in TERS performance. In this paper, we theoretically investigated the enhancement and manipulation of the electric field gradient in a bottom-illumination TERS configuration through a tightly focused perfect radially polarized beam (PRPB). Improvement and manipulation in electric field enhancement and field gradient of the gap-plasmon mode between a plasmonic tip and a virtual surface plasmons (SPs) probe are achieved by adjusting the ring radius of the incident PRPB. Our results demonstrate that the method of optimizing the ring radius of PRPB is to make the illumination angle of incident light as close to the surface plasmon resonance (SPR) excitation angle as possible. Under the excitation of optimal parameters, more than 10 folds improvement of field enhancement and 3 times of field gradient of the gap-plasmon mode is realized compared with that of the conventional focused RPB. By this feat, our results indicate that such a method can further enhance the gradient Raman mode in TERS. We envision that the proposed method, to achieve the dynamic manipulation and enhancement of the nanofocusing field and field gradient, can be more broadly used to control light-matter interactions and extend the reach of tip-enhanced spectroscopy.

Tip-enhanced two-photon-excited fluorescence of monolayer MoS2

High-order nonlinearity enables larger signal enhancements in two-photon spectroscopy compared to one-photon spectroscopy, which is important for conducting photoluminescence measurements at the single-molecule level. Herein, the two-photon-excited fluorescence (2pF) of monolayer MoS2 in various tip-enhanced spectroscopy configurations was theoretically investigated using the finite-difference time-domain method. Large enhancement factor for 2pF on the order of 105 was achieved by optimizing the radius and cone angle of the tip together with the distance between the tip and MoS2 film. The enhancement factor for 2pF is 40 times as much as that for one-photon-excited fluorescence because of the high-order nonlinear effect. Radiative and nonradiative decay rates were further calculated for determining quantum yields and elucidating the 2pF emission mechanism. These results provide a comprehensive understanding of 2pF behavior, which is highly useful for the design of MoS2-based optoelectronic devices.

Circular nanocavity substrate-assisted plasmonic tip for its enhancement in nanofocusing and optical trapping.

Plasmonic tip nanofocusing has widely been applied in tip-enhanced Raman spectroscopy, optical trapping, nonlinear optics, and super-resolution imaging due to its capability of high local field enhancement. In this work, a substrate with a circular nanocavity is proposed to enhance the nanofocusing and optical trapping characteristics of the plasmonic tip. Under axial illumination of a tightly focused radial polarized beam, the circular nanohole etched on a metallic substrate can form a nanocavity to induce an interference effect and further enhance the electric field intensity. When a plasmonic tip is placed closely above such a substrate, the electric field intensity of the gap-plasmon mode can further be improved, which is 10 folds stronger than that of the conventional gap-plasmon mode. Further analysis reveals that the enhanced gap-plasmon mode can significantly strengthen the optical force exerted on a nanoparticle and stably trap a 4-nm-diameter dielectric nanoparticle. Our proposed method can improve the performance of tip-enhanced spectroscopy, plasmonic tweezers and extend their applications. We anticipate that our methods allow simultaneously manipulating and characterizing single nanoparticles in-situ.

Etching-Free Transfer and Nanoimaging of CVD-Grown MoS2 Monolayers

The transfer of two-dimensional materials from grown substrates onto target substrates is critical for device applications and postgrown analysis. The traditional transfer method for as-grown two-dimensional materials requires a wet chemical etching process that damages the crystals and the substrates. These issues deteriorate the electrical and optical performances of two-dimensional-material-based devices fabricated afterward. Herein, we developed an etching-free method and nanoimaging for transferring and analyzing monolayers of MoS2 onto arbitrary substrates using polyurethane as a sacrifice polymer. The polymer layer and the MoS2 crystals can be peeled off from the substrate by tweezers and transferred to any substrate. We analyzed the transferred samples to a glass coverslip substrate with optical microscopy, atomic force microscopy, tip-enhanced Raman spectroscopy, and tip-enhanced photoluminesce spectroscopy. Also, we transferred monolayers of MoS2 to a transmission electron microscopy grid and acquired scanning electron microscopy together with high-resolution scanning transmission electron microscopy images. The results of nanoimaging indicate that the method preserved the sample’s optical and structural properties and avoid undesirable cracks, wrinkles, and polymer residues. The etching-free transfer method of as-grown two-dimensional materials will improve the quality of transferred samples enhance the devices’ performance.

Lighting up lattice vibrations

An article in Nature reports measurements of lattice vibrations in bilayer graphene twisted at very small angles by tip-enhanced nano-Raman spectroscopy.

Nanocavity Clock Spectroscopy: Resolving Competing Exciton Dynamics in WSe2/MoSe2 Heterobilayers.

Transition-metal dichalcogenide heterostructures are an emergent platform for novel many-body states from exciton condensates to nanolasers. However, their exciton dynamics are difficult to disentangle due to multiple competing processes with time scales varying over many orders of magnitude. Using a configurable nano-optical cavity based on a plasmonic scanning probe tip, the radiative (rad) and nonradiative (nrad) relaxation of intra- and interlayer excitons is controlled. Tuning their relative rates in a WSe2/MoSe2 heterobilayer over 6 orders of magnitude in tip-enhanced photoluminescence spectroscopy reveals a cavity-induced crossover from nonradiative quenching to Purcell-enhanced radiation. Rate equation modeling with the interlayer charge transfer time as a reference clock allows for a comprehensive determination from the long interlayer exciton (IX) radiative lifetime τIXrad = (94 ± 27) ns to the 5 orders of magnitude faster competing nonradiative lifetime τIXnrad = (0.6 ± 0.2) ps. This approach of nanocavity clock spectroscopy is generally applicable to a wide range of excitonic systems with competing decay pathways.

Sample induced intensity variations of localized surface plasmon resonance in tip-enhanced Raman spectroscopy.

Tip-enhanced spectroscopy techniques, in particular tip-enhanced Raman spectroscopy (TERS), rely on a localized surface plasmon resonance (LSPR). This LSPR depends on the near field antenna, its material and shape, and the surrounding medium with respect to its relative permittivity and the volume fraction of the optical near field occupied by the sample. Here, we investigate the effects of the surface composition and topography on the change of the LSPR intensity in tip-enhanced spectroscopy on SrTiO3 nanoislands by monitoring the LSPR enhanced luminescence of gold tips. Our experimental results and analytical estimates indicate that by affecting the effective permittivity of the dielectric environment at the tip apex, the material composition as well as topography of the studied sample induce a change in LSPR intensity. This result significantly helps the understanding of the evolution or origin of the LSPR intensity during a typical TERS measurement, which in turn leads to a more accurate assessment of the relative intensity of different Raman modes in TERS.

Nanofocusing of Surface Plasmon Polaritons on Metal-Coated Fiber Tip Under Internal Excitation of Radial Vector Beam

We theoretically present the nanofocusing of the metal-coated fiber tip under internal excitation of the radial vector beam within visible band based on the finite difference time domain (FDTD) analysis. The electric field intensity enhancement factor of the localized surface plasmons (LSP) mode at the tip apex is quantitatively shown in relation with incident wavelength, coating material, conical angle of tip, and coating film thickness/length. Specially, the evolution of fiber radial vector mode to surface mode with respect to the radius of metal-coated fiber tip is calculated under typical excitation wavelengths of 633 nm and 785 nm. Furthermore, the reason of the tip eliminating far-field background signal is explained, and the transverse electric field distributions of LSP mode and the tip-substrate coupling are also given at the optimal excitation wavelength. These calculation results will be a good reference for the fabrication of metal-coated fiber tips and for the experimental design of the tip-enhanced spectroscopy (TES) system.

Plasmonic enhancement and directional emission for side-illumination tip-enhanced spectroscopy

Realization of single-molecule sensitivity is crucial for tip-enhanced spectroscopy (TES) and its applications. Plasmonic enhancement and directional emission of TES, which are related to the improvement of detection sensitivity, have been theoretically investigated. Calculated enhancement factors for Raman and fluorescence spectroscopy are of the order 1010 and 104 respectively. Directional TES emission reveals an optimal collection angle of ±25º, although tradeoffs must be made between the emission and laser incident angles to ensure high-sensitivity TES. The proposed method enhances detection sensitivity by considering the contributions of plasmonic enhancement and directional emission of TES, useful for the design of high-sensitivity TES platforms.