Fast Nano-IR Hyperspectral Imaging Empowered by Large-Dataset-Free Miniaturized Spatial-Spectral Network.

The emerging field of nanoscale infrared (nano-IR) offers label-free molecular contrast, yet its imaging speed is limited by point-by-point traverse acquisition of a three-dimensional (3D) data cube. Here, we develop a spatial-spectral network (SS-Net), a miniaturized deep-learning model, together with compressive sampling to accelerate the nano-IR imaging. The compressive sampling is performed in both the spatial and spectral domains to accelerate the imaging process. The SS-Net is trained to learn the mapping from small nano-IR image patches to the corresponding spectra. With this elaborated mapping strategy, the training can be finished quickly within several minutes using the subsampled data, eliminating the need for a large-labeled dataset of common deep learning methods. We also designed an efficient loss function, which incorporates the image and spectral similarity to enhance the training. We first validate the SS-Net on an open stimulated Raman-scattering dataset; the results exhibit the potential of 10-fold imaging speed improvement with state-of-the-art performance. We then demonstrate the versatility of this approach on atomic force microscopy infrared (AFM-IR) microscopy with 7-fold imaging speed improvement, even on nanoscale Fourier transform infrared (nano-FTIR) microscopy with up to 261.6 folds faster imaging speed. We further showcase the generalization of this method on AFM-force volume-based multiparametric nanoimaging. This method establishes a paradigm for rapid nano-IR imaging, opening new possibilities for cutting-edge research in materials, photonics, and beyond.

Charge Delocalization and Vibronic Couplings in Quadrupolar Squaraine Dyes.

Squaraines are prototypical quadrupolar charge-transfer chromophores that have recently attracted much attention as building blocks for solution-processed photovoltaics, fluorescent probes with large two-photon absorption cross sections, and aggregates with large circular dichroism. Their optical properties are often rationalized in terms of phenomenological essential state models, considering the coupling of two zwitterionic excited states to a neutral ground state. As a result, optical transitions to the lowest S1 excited state are one-photon allowed, whereas the next higher S2 state can only be accessed by two-photon transitions. A further implication of these models is a substantial reduction of vibronic coupling to the ubiquitous high-frequency vinyl-stretching modes of organic materials. Here, we combine time-resolved vibrational spectroscopy, two-dimensional electronic spectroscopy, and quantum-chemical simulations to test and rationalize these predictions for nonaggregated molecules. We find small Huang-Rhys factors below 0.01 for the high-frequency, 1500 cm-1 modes in particular, as well as a noticeable reduction for those of lower frequency modes in general for the electronic S0 → S1 transition. The two-photon allowed state S2 is well separated energetically from S1 and has weak vibronic signatures as well. Thus, the resulting pronounced concentration of the oscillator strength in a narrow region relevant to the lowest electronic transition makes squaraines and their aggregates exceptionally interesting for strong and ultrastrong coupling of excitons to localized light modes in external resonators with chiral properties that can largely be controlled by the molecular architecture.

Correlating the orbital overlap area and vibrational frequency shift of an isocyanide moiety adsorbed on Pt and Pd covered Au(111) surfaces.

Orbital interactions between adsorbed molecules and the underlying metal surfaces play critical roles in a wide range of surface and interfacial processes. Establishing a correlation between an experimental observable (e.g., vibrational frequency shift of the adsorbed molecule) and the orbital interactions is of vital importance. Herein, theoretical calculations are used to investigate the vibrational frequency shift of phenyl isocyanide molecules as a probe molecule adsorbed on mono- and bi-layer Pt and Pd covered Au(111) surfaces and Pd2Au4 and Pt2Au4 clusters. By analyzing the density of states (DOS) of the adsorption system, we show that the orbital overlap area of d electronic DOS with a molecular σ or π* orbital, particularly their ratio (Rd-σ/d-π*), can be a meaningful descriptor to explain the frequency shift of the CN moiety. This hypothesis has been verified by simulations for phenyl isocyanide with electron donating NH2- and withdrawing CF3- substituent groups, formonitrile and carbon monoxide. Quasi-linear dependence of the frequency shift on Rd-σ/d-π* is observed for both the red and blue shift regions. Our findings build up on previous notions of electronic interactions, which will provide a more quantitative and solid footing to understand and analyze the frequency shift of adsorbed molecules on metal surfaces and the related electronic interactions and catalytic properties.

Probing the Local Generation and Diffusion of Active Oxygen Species on a Pd/Au Bimetallic Surface by Tip-Enhanced Raman Spectroscopy.

Active oxygen species (AOS) play key roles in many important catalytic reactions relevant to clean energy and environment. However, it remains challenging to characterize the active sites for producing AOS and to image the surface properties of AOS, especially on multicomponent metallic catalyst surfaces. Herein, we utilize tip-enhanced Raman spectroscopy (TERS) to probe the local generation and diffusion of OH radicals on a Pd/Au(111) bimetallic catalyst surface. The reactive OH radicals can be catalytically generated from hydrogen peroxide (H2O2) at the metal surface, which then oxidizes the surface adsorbed thiolate, a reactant that is used as the TERS probe. By TERS imaging of the spatial distribution of unreacted thiolate molecules, we demonstrate that the Pd surface is active for generation of OH radicals and the Pd step edge shows much higher activity than the Pd terrace, whereas the Au surface is inactive. Furthermore, we find that the locally generated OH radicals at the Pd step edge could diffuse to both the Au and the Pd surface sites to induce oxidative reactions, with a diffusion length estimated to be about 5.4 nm. Our TERS imaging with few-nanometer spatial resolution not only unravels the active sites but also characterizes in real space the diffusion behavior of OH radicals. The results are highly valuable to understand AOS-triggered catalytic reactions. The strategy of using reactants with large Raman cross sections as TERS probes may broaden the application of TERS for studying catalysis with reactive small molecules.

Tip-enhanced Raman spectroscopy for surfaces and interfaces.

Surface and interfaces play key roles in heterogeneous catalysis, electrochemistry and photo(electro)chemistry. Tip-enhanced Raman spectroscopy (TERS) combines plasmon-enhanced Raman spectroscopy with scanning probe microscopy to simultaneously provide a chemical fingerprint and morphological information for the sample at the nanometer spatial resolution. It is an ideal tool for achieving an in-depth understanding of the surface and interfacial processes, so that the relationship between structure and chemical performance can be established. We begin with the background of surfaces and interfaces and TERS, followed by a detailed discussion on some issues in experimental TERS, including tip preparation and TERS instrument configuration. We then focus on the progress of TERS for studying the surfaces and interfaces under different conditions, from ambient, to UHV, solid-liquid and electrochemical environments, followed by a brief introduction to the current understanding of the unprecedented high spatial resolution and surface selection rules. We conclude by discussing the future challenges for TERS practical applications in surfaces and interfaces.

Real-Space Observation of Atomic Site-Specific Electronic Properties of a Pt Nanoisland/Au(111) Bimetallic Surface by Tip-Enhanced Raman Spectroscopy.

Resolving atomic site-specific electronic properties and correlated substrate-molecule interactions is challenging in real space. Now, mapping of sub-10 nm sized Pt nanoislands on a Au(111) surface was achieved by tip-enhanced Raman spectroscopy, using the distinct Raman fingerprints of adsorbed 4-chlorophenyl isocyanide molecules. A spatial resolution better than 2.5 nm allows the electronic properties of the terrace, step edge, kink, and corner sites with varying coordination environments to be resolved in real space in one Pt nanoisland. Calculations suggest that low-coordinate atomic sites have a higher d-band electronic profile and thus stronger metal-molecule interactions, leading to the observed blue-shift of Raman frequency of the N≡C bond of adsorbed molecules. An experimental and theoretical study on Pt(111) and mono- and bi-atomic layer Pt nanoislands on a Au(111) surface reveals the bimetallic effect that weakens with the increasing number of deposited Pt adlayer.

Revealing Intermolecular Interaction and Surface Restructuring of an Aromatic Thiol Assembling on Au(111) by Tip-Enhanced Raman Spectroscopy.

Controlling the packing structure and revealing the intermolecular interaction of self-assembled monolayers (SAMs) on solid surfaces are crucial for manipulating its properties. We utilized tip-enhanced Raman spectroscopy (TERS) to address the challenge in probing the subtle change of the intermolecular interaction during the assembly of a pyridine-terminated aromatic thiol on the single crystal Au(111) surface that cannot produce enhanced Raman signal, together with electrochemical methods to study the charge transfer properties of SAM. We observed that the aromatic C═C bond stretching vibration can be a marker to monitor the strength of the intermolecular interaction of SAMs, because this Raman peak is very sensitive to the intermolecular π-π stacking. Our results indicate that the SAM experiences a surface restructuring after the formation of a densely packed monolayer. We propose that the intermolecular electrostatic repulsion governs the restructuring when the packing density is high. The correlated TERS and electrochemical studies also suggest that the intermolecular interaction may have some impact on the charge transfer properties of SAM. This study provides a molecular-level insight into understanding and exploiting the intermolecular interactions toward better control over the assembling process and tuning the electrical properties of aromatic thiols.

Electrochemical Tip-Enhanced Raman Spectroscopy.

Interfacial properties are highly important to the performance of some energy-related systems. The in-depth understanding of the interface requires highly sensitive in situ techniques that can provide fingerprint molecular information at nanometer resolution. We developed an electrochemical tip-enhanced Raman spectroscopy (EC-TERS) by introduction of the light horizontally to the EC-STM cell to minimize the optical distortion and to keep the TERS measurement under a well-controlled condition. We obtained potential-dependent EC-TERS from the adsorbed aromatic molecule on a Au(111) surface and observed a substantial change in the molecule configuration with potential as a result of the protonation and deprotonation of the molecule. Such a change was not observable in EC-SERS (surface-enhanced), indicating EC-TERS can more faithfully reflect the fine interfacial structure than EC-SERS. This work will open a new era for using EC-TERS as an important nanospectroscopy tool for the molecular level and nanoscale analysis of some important electrochemical systems including solar cells, lithium ion batteries, fuel cells, and corrosion.

Quantitative correlation between defect density and heterogeneous electron transfer rate of single layer graphene.

Improving electrochemical activity of graphene is crucial for its various applications, which requires delicate control over its geometric and electronic structures. We demonstrate that precise control of the density of vacancy defects, introduced by Ar(+) irradiation, can improve and finely tune the heterogeneous electron transfer (HET) rate of graphene. For reliable comparisons, we made patterns with different defect densities on a same single layer graphene sheet, which allows us to correlate defect density (via Raman spectroscopy) with HET rate (via scanning electrochemical microscopy) of graphene quantitatively, under exactly the same experimental conditions. By balancing the defect induced increase of density of states (DOS) and decrease of conductivity, the optimal HET rate is attained at a moderate defect density, which is in a critical state; that is, the whole graphene sheet becomes electronically activated and, meanwhile, maintains structural integrity. The improved electrochemical activity can be understood by a high DOS near the Fermi level of defective graphene, as revealed by ab initio simulation, which enlarges the overlap between the electronic states of graphene and the redox couple. The results are valuable to promote the performance of graphene-based electrochemical devices. Furthermore, our findings may serve as a guide to tailor the structure and properties of graphene and other ultrathin two-dimensional materials through defect density engineering.

Growth of adlayer graphene on Cu studied by carbon isotope labeling.

The growth of bilayer and multilayer graphene on copper foils was studied by isotopic labeling of the methane precursor. Isotope-labeled graphene films were characterized by micro-Raman mapping and time-of-flight secondary ion mass spectrometry. Our investigation shows that during growth at high temperature, the adlayers formed simultaneously and beneath the top, continuous layer of graphene and the Cu substrate. Additionally, the adlayers share the same nucleation center and all adlayers nucleating in one place have the same edge termination. These results suggest that adlayer growth proceeds by catalytic decomposition of methane (or CH(x), x < 4) trapped in a "nano-chemical vapor deposition" chamber between the first layer and the substrate. On the basis of these results, submillimeter bilayer graphene was synthesized by applying a much lower growth rate.