Probing Bias-Induced Electron Density Shifts in Metal-Molecule Interfaces via Tip-Enhanced Raman Scattering

Impact of Surface Enhanced Raman Spectroscopy in Catalysis

Catalysis stands as an indispensable cornerstone of modern society, underpinning the production of over 80% of manufactured goods and driving over 90% of industrial chemical processes. As the demand for more efficient and sustainable processes grows, better catalysts are needed. Understanding the working principles of catalysts is key, and over the last 50 years, surface-enhanced Raman Spectroscopy (SERS) has become essential. Discovered in 1974, SERS has evolved into a mature and powerful analytical tool, transforming the way in which we detect molecules across disciplines. In catalysis, SERS has enabled insights into dynamic surface phenomena, facilitating the monitoring of the catalyst structure, adsorbate interactions, and reaction kinetics at very high spatial and temporal resolutions. This review explores the achievements as well as the future potential of SERS in the field of catalysis and energy conversion, thereby highlighting its role in advancing these critical areas of research.

Extensive photochemical restructuring of molecule-metal surfaces under room light

The molecule-metal interface is of paramount importance for many devices and processes, and directly involved in photocatalysis, molecular electronics, nanophotonics, and molecular (bio-)sensing. Here the photostability of this interface is shown to be sensitive even to room light levels for specific molecules and metals. Optical spectroscopy is used to track photoinduced migration of gold atoms when functionalised with different thiolated molecules that form uniform monolayers on Au. Nucleation and growth of characteristic surface metal nanostructures is observed from the light-driven adatoms. By watching the spectral shifts of optical modes from nanoparticles used to precoat these surfaces, we identify processes involved in the photo-migration mechanism and the chemical groups that facilitate it. This photosensitivity of the molecule-metal interface highlights the significance of optically induced surface reconstruction. In some catalytic contexts this can enhance activity, especially utilising atomically dispersed gold. Conversely, in electronic device applications such reconstructions introduce problematic aging effects.

Photoluminescence emission and Raman enhancement in TERS: an experimental and analytic revisiting

An analytic model is used to calculate the Raman and fluorescence enhancement of a molecule in between two closely spaced gold nanospheres. Instead of using the conventional approach that only the dipolar plasmonic mode is considered, we calculate the electric field enhancement in the nanometre sized gap, by taking account of the higher order modes in one gold sphere, which couples to the dipolar mode of the other sphere. The experimental confirmation is performed by gap-dependent tip-enhanced Raman spectroscopy (TERS) measurements. The photoluminescence and Raman enhancement are both observed with different growing trends as the gap width decreases. Red-shift of the background spectra is observed and implies the increasing coupling between the nanospheres. This analytic model is shown to be able to interpret the enhancement mechanisms underlying gap-dependent TERS experimental results.

Remote-Controllable Interfacial Electron Tunneling at Heterogeneous Molecular Junctions via Tip-Induced Optoelectrical Engineering

Molecular electronics enables functional electronic behavior via single molecules or molecular self-assembled monolayers, providing versatile opportunities for hybrid molecular-scale electronic devices. Although various molecular junction structures are constructed to investigate charge transfer dynamics, significant challenges remain in terms of interfacial charging effects and far-field background signals, which dominantly block the optoelectrical observation of interfacial charge transfer dynamics. Here, tip-induced optoelectrical engineering is presented that synergistically correlates photo-induced force microscopy and Kelvin probe force microscopy to remotely control and probe the interfacial charge transfer dynamics with sub-10 nm spatial resolution. Based on this approach, the optoelectrical origin of metal-molecule interfaces is clearly revealed by the nanoscale heterogeneity of the tip-sample interaction and optoelectrical reactivity, which theoretically aligned with density functional theory calculations. For a practical device-scale demonstration of tip-induced optoelectrical engineering, interfacial tunneling is remotely controlled at a 4-inch wafer-scale metal-insulator-metal capacitor, facilitating a 5.211-fold current amplification with the tip-induced electrical field. In conclusion, tip-induced optoelectrical engineering provides a novel strategy to comprehensively understand interfacial charge transfer dynamics and a non-destructive tunneling control platform that enables real-time and real-space investigation of ultrathin hybrid molecular systems.

A Full Quantum Mechanical Approach Assessing the Chemical and Electromagnetic Effect in TERS

Tip-enhanced Raman spectroscopy (TERS) is a valuable method for surface analysis with nanometer to angstrom-scale resolution; however, the accurate simulation of particular TERS signals remains a computational challenge. We approach this challenge by combining the two main contributors to plasmon-enhanced Raman spectroscopy and to the high resolution in TERS, in particular, the electromagnetic and the chemical effect, into one quantum mechanical simulation. The electromagnetic effect describes the sample's interaction with the strong, highly localized, and inhomogeneous electric fields associated with the plasmonic tip and is typically the thematic focus for most mechanistic studies. On the other hand, the chemical effect covers the different responses to the extremely close-range and highly position-sensitive chemical interaction between the apex tip atom(s) and the sample, and, as we could show in previous works, plays an often underestimated role. Starting from a (time-dependent) density functional theory description of the chemical model system, comprised of a tin(II) phthalocyanine sample molecule and a single silver atom as the tip, we introduce the electromagnetic effect through a series of static point charges that recreate the electric field in the vicinity of the plasmonic Ag nanoparticle. By scanning the tip over the molecule along a 3D grid, we can investigate the system's Raman response on each position for nonresonant and resonant illumination. Simulating both effects on their own already hints at the achievable signal enhancement and resolution, but the combination of both creates even stronger evidence that TERS is capable of resolving submolecular features.

Ultrastable tip-enhanced hyperspectral optical nanoimaging for defect analysis of large-sized WS2 layers

Optical nanoimaging techniques, such as tip-enhanced Raman spectroscopy (TERS), are nowadays indispensable for chemical and optical characterization in the entire field of nanotechnology and have been extensively used for various applications, such as visualization of nanoscale defects in two-dimensional (2D) materials. However, it is still challenging to investigate micrometer-sized sample with nanoscale spatial resolution because of severe limitation of measurement time due to drift of the experimental system. Here, we achieved long-duration TERS imaging of a micrometer-sized WS₂ sample for 6 hours in a reproducible manner. Our ultrastable TERS system enabled to reveal the defect density on the surface of tungsten disulfide layers in large area equivalent to the device scale. It also helped us to detect rare defect-related optical signals from the sample. The present study paves ways to evaluate nanoscale defects of 2D materials in large area and to unveil remarkable optical and chemical properties of large-sized nanostructured materials.

Charge Transfer-Mediated Dramatic Enhancement of Raman Scattering upon Molecular Point Contact Formation

Charge-transfer enhancement of Raman scattering plays a crucial role in current-carrying molecular junctions. However, the microscopic mechanism of light scattering in such nonequilibrium systems is still imperfectly understood. Here, using low-temperature tip-enhanced Raman spectroscopy (TERS), we investigate how Raman scattering evolves as a function of the gap distance in the single C₆₀-molecule junction consisting of an Ag tip and various metal surfaces. Precise gap-distance control allows the examination of two distinct transport regimes, namely tunneling regime and molecular point contact (MPC). Simultaneous measurement of TERS and the electric current in scanning tunneling microscopy shows that the MPC formation results in dramatic Raman enhancement that enables one to observe the vibrations undetectable in the tunneling regime. This enhancement is found to commonly occur not only for coinage but also transition metal substrates. We suggest that the characteristic enhancement upon the MPC formation is rationalized by charge-transfer excitation.

Investigation of SERS and Electron Transport Properties of Oligomer Phenylacetyne-3 Trapped in Gold Junctions

Molecular junctions hold great potential for future microelectronics and attract people’s attention. Here, we used density functional theory calculations (DFT) to investigate the surface-enhanced Raman spectroscopy (SERS) and electron transport properties of fully π-conjugated oligomers (phenylacetylene)-3 (OPE-3) trapped in gold junctions. The effects of charge injection, an applied electric field, and molecular deformation are considered. We found that a new Raman peak located at around 1400 cm⁻¹ appears after the injection of a charge, which agrees well with the experiment. The external electric field and configurational deformation hardly affect the Raman spectra, indicating that the electronic rather than the geometrical structure determines the Raman response. Nonequilibrium Green’s function (NEGF) calculations show that both the rotation of the benzene groups and an increased electrode distance largely reduced the conductivity of the studied molecular junctions. The present investigations provide valuable information on the effect of charging, electric field, and deformation on the SERS and conductivity of molecular junctions, helping the development of molecular devices.

A bio-sensing surface with high biocompatibility for enhancing Raman scattering signals as enabled by a Mo–Ag film

The sensitive SERS sensing of molecules and bacteria was acquired through a Mo–Ag film with high cytocompatibility and hydrophilicity.

Interfacial Engineering of Metal/Metal Oxide Heterojunctions toward Oxygen Reduction and Evolution Reactions

Oxygen reduction reactions (ORR) and oxygen evolution reactions (OER) are two very important electrochemical processes for renewable energy conversion and storage devices. Electrocatalysts are needed to accelerate their sluggish kinetics to improve energy conversion efficiencies. Hence, extensive efforts have been devoted to the development of OER and ORR electrocatalysts with high activity and stability as well as low cost. Among these developed electrocatalysts, metal/metal oxide heterostructures attract a great deal of research interest because their catalytic performances can be tuned by interface engineering. In this Review, the latest achievements in interface engineering of metal/metal oxides heterostructures toward ORR and OER are described. The effects of the metal/metal oxide interface on catalysis are first discussed. Then, the approaches for interface engineering are illustrated. The developments of interface engineering in OER and ORR catalysis as well as bifunctional electrocatalysis are further introduced. Lastly, a perspective for future development of interface engineering in metal/metal oxide for OER and ORR is discussed.