Visualizing nanoscale heterogeneity in perylene thin films via tip-enhanced photoluminescence with unsupervised machine learning.

We investigate the properties of ultrathin 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) films using a combination of tip-enhanced photoluminescence and unsupervised machine learning. We expose nanoscale spectral heterogeneities that can be understood on the basis of the interplay between vibronic effects, intermolecular excitons, and intramolecular excitons in PTDCI films.

Tunable Localized Charge Transfer Excitons in Nanoplatelet-2D Chalcogenide van der Waals Heterostructures.

Observation of interlayer, charge transfer (CT) excitons in van der Waals heterostructures (vdWHs) based on 2D-2D systems has been well investigated. While conceptually interesting, these charge transfer excitons are highly delocalized and spatially localizing them requires twisting layers at very specific angles. This issue of localizing the CT excitons can be overcome via making nanoplate-2D material heterostructures (N2DHs) where one of the components is a spatially quantum confined medium. Here, we demonstrate the formation of CT excitons in a mixed dimensional system comprising MoSe2 and WSe2 monolayers and CdSe/CdS-based core/shell nanoplates (NPLs). Spectral signatures of CT excitons in our N2DHs were resolved locally at the 2D/single-NPL heterointerface using tip-enhanced photoluminescence (TEPL) at room temperature. By varying both the 2D material and the shell thickness of the NPLs and applying an out-of-plane electric field, the exciton resonance energy was tuned by up to 100 meV. Our finding is a significant step toward the realization of highly tunable N2DH-based next-generation photonic devices.

Core/Shell-Like Localized Emission at Atomically Thin Semiconductor-Au Interface.

Localized emission in atomically thin semiconductors has sparked significant interest as single-photon sources. Despite comprehensive studies into the correlation between localized strain and exciton emission, the impacts of charge transfer on nanobubble emission remains elusive. Here, we report the observation of core/shell-like localized emission from monolayer WSe2 nanobubbles at room temperature through near-field studies. By altering the electronic junction between monolayer WSe2 and the Au substrate, one can effectively adjust the semiconductor to metal junction from a Schottky to an Ohmic junction. Through concurrent analysis of topography, potential, tip-enhanced photoluminescence, and a piezo response force microscope, we attribute the core/shell-like emissions to strong piezoelectric potential aided by induced polarity at the WSe2-Au Schottky interface which results in spatial confinement of the excitons. Our findings present a new approach for manipulating charge confinement and engineering localized emission within atomically thin semiconductor nanobubbles. These insights hold implications for advancing the nano and quantum photonics with low-dimensional semiconductors.

Classical vs. quantum plasmon-induced molecular transformations at metallic nanojunctions.

Chemical transformations near plasmonic metals have attracted increasing attention in the past few years. Specifically, reactions occurring within plasmonic nanojunctions that can be detected via surface and tip-enhanced Raman (SER and TER) scattering were the focus of numerous reports. In this context, even though the transition between localized and nonlocal (quantum) plasmons at nanojunctions is documented, its implications on plasmonic chemistry remain poorly understood. We explore the latter through AFM-TER-current measurements. We use two molecules: i) 4-mercaptobenzonitrile (MBN) that reports on the (non)local fields and ii) 4-nitrothiophenol (NTP) that features defined signatures of its neutral/anionic forms and dimer product, 4,4'-dimercaptoazobenzene (DMAB). The transition from classical to quantum plasmons is established through our optical measurements: It is marked by molecular charging and optical rectification. Simultaneously recorded force and current measurements support our assignments. In the case of NTP, we observe the parent and DMAB product beneath the probe in the classical regime. Further reducing the gap leads to the collapse of DMAB to form NTP anions. The process is reversible: Anions subsequently recombine into DMAB. Our results have significant implications for AFM-based TER measurements and their analysis, beyond the scope of this work. In effect, when precise control over the junction is not possible (e.g., in SER and ambient TER), both classical and quantum plasmons need to be considered in the analysis of plasmonic reactions.

Controlling the Fluctuating Tip-Enhanced Raman Spectra of Chloramben on Silver Nanocubes.

Tip-enhanced Raman (TER) scattering from molecules residing at plasmonic junctions can be used to detect, identify, and image single molecules. This is most evident for flat molecules interrogated under conditions of extreme temperatures and pressure. It is also the case for (bio)molecular systems that feature preferred orientations/conformations under ambient laboratory conditions. More complex molecules that can adopt multiple conformations and/or feature different protonation or charge states give rise to complex TER spectra. We illustrate how the latter can be controlled in the case of chloramben molecules coated onto plasmonic silver nanocubes. We show that characteristic molecular Raman spectra cannot be obtained when tunneling plasmons are operative, i.e., when the tip is in direct contact with the chemically functionalized plasmonic nanoparticles. We rationalize these observations and propose an approach to less invasive and hence more analytical TER spectral imaging.

Chemical characterization of microplastic particles formed in airborne waste discharged from sewer pipe repairs.

Microplastic particles are of increasing environmental concern due to the widespread uncontrolled degradation of various commercial products made of plastic and their associated waste disposal. Recently, common technology used to repair sewer pipes was reported as one of the emission sources of airborne microplastics in urban areas. This research presents results of the multi-modal comprehensive chemical characterization of the microplastic particles related to waste discharged in the pipe repair process and compares particle composition with the components of uncured resin and cured plastic composite used in the process. Analysis of these materials employs complementary use of surface-enhanced Raman spectroscopy, scanning transmission X-ray spectro-microscopy, single particle mass spectrometry, and direct analysis in real-time high-resolution mass spectrometry. It is shown that the composition of the relatively large (100 µm) microplastic particles resembles components of plastic material used in the process. In contrast, the composition of the smaller (micrometer and sub-micrometer) particles is significantly different, suggesting their formation from unintended polymerization of water-soluble components occurring in drying droplets of the air-discharged waste. In addition, resin material type influences the composition of released microplastic particles. Results are further discussed to guide the detection and advanced characterization of airborne microplastics in future field and laboratory studies pertaining to sewer pipe repair technology.

Probing Local Optical Fields via Ultralow Frequency Raman Scattering from a Corrugated Probe.

We revisit nanoscale local optical field imaging via tip-enhanced Raman scattering (TERS). Rather than taking advantage of molecular reporters to probe different aspects of the local fields, we show how ultralow frequency Raman (ULF) scattering from the (nanocorrugated) metallic probe itself can be used for the same purpose. The bright ULF-TERS response we record allows non-invasive (tapping mode feedback) local field imaging, enables visualization of the local fields of small (≥20 nm) isolated plasmonic particles, and can also be exploited to distinguish between Si and SiO2 domains with 5 nm spatial resolution. We describe our approach and its limitations, particularly when it comes to using all-metallic versus molecular reporters.

Subnanometer Visualization of Spatially Varying Local Field Resonances that Drive Tip-Enhanced Optical Spectroscopy.

Our knowledge of the electromagnetic fields that power modern nanoscale optical measurements, including (non)linear tip-enhanced Raman and photoluminescence, chiefly stems from numerical simulations. Aside from idealized in silico vs heterogeneous (nano)structures in the laboratory, challenges in quantitative descriptions of nanoscale light-matter interactions more generally stem from the very nature of the problem, which lies at the interface of classical and quantum theories. This is particularly the case in ultrahigh spatial resolution measurements that are sensitive to local optical field variations that take place on subnanometer length scales. This work approaches this challenge through extinction-based spectral nanoimaging experiments. We demonstrate <1 nm spatial resolution in hyperspectral extinction measurements that track spatially varying plasmon resonances. We describe the principles behind our experiments and highlight more general implications of our observations.

Influence of surface and intermolecular interactions on the properties of supported polyoxometalates.

Polyoxometalates (POMs) with localized radical or open-shell metal sites have the potential to be used as transformative electronic spin based molecular qubits (MQs) for quantum computing (QC). For practical applications, MQs have to be immobilized in electronically or optically addressable arrays which introduces interactions with supports as well as neighboring POMs. Herein, we synthesized Keggin POMs with both tungsten (W) and vanadium (V) addenda atoms. Ion soft landing, a highly-controlled surface modification technique, was used to deliver mass-selected V-doped POMs to different self-assembled monolayer surfaces on gold (SAMs) without the solvent, counterions, and contaminants that normally accompany deposition from solution. Alkylthiol, perfluorinated, and carboxylic-acid terminated monolayers were employed as representative model supports on which different POM-surface and POM-POM interactions were characterized. We obtained insights into the vibrational properties of supported V-doped POMs and how they are perturbed by interactions with specific surface functional groups using infrared reflection absorption and scattering-type scanning near-field optical microscopy, as well as tip enhanced Raman spectroscopy. Different functional groups on SAMs and nanoscale heterogeneity are both shown to modulate the observed spectroscopic signatures. Spectral shifts are also found to be dependent on POM-POM interactions. The electronic structure of the V-doped POMs was determined in the gas phase using negative ion photoelectron spectroscopy and on surfaces with scanning Kelvin probe microscopy. The chemical functionality and charge transfer properties of the SAMs are demonstrated to exert an influence on the charge state and electronic configuration of supported V-doped POMs. The geometric and electronic structure of the POMs were also calculated using density functional theory. Our joint experimental and theoretical findings provide insight into how V substitution as well as POM-surface and POM-POM interactions influence the vibrational properties of POMs.

Tip-Enhanced Raman Chemical and Chemical Reaction Imaging in H2O with Sub-3-nm Spatial Resolution.

Reproducible chemical and chemical reaction nanoimaging at solid-liquid interfaces remains challenging, particularly when resolutions on the order of a few nanometers are sought. In this work, we demonstrate the latter through liquid-tip-enhanced Raman (TER) measurements that target gold nanoplates functionalized with 4-mercaptobenzonitrile (MBN). In addition to chemical imaging and local optical field nanovisualization with high spatial resolution, we observe the signatures of 4-mercaptobenzoic acid, which forms as a result of plasmon-induced hydrolysis of MBN. Evidently, the solvent leads to distinct plasmon-induced/enhanced chemical reaction pathways that have not been documented. This work shows that such reactions that take place at solid-liquid interfaces can be tracked with a record sub-3-nm spatial resolution via TER spectral nanoimaging in liquids.

High spatial resolution ambient tip-enhanced (multipolar) Raman scattering.

This article summarizes lessons learnt from ambient tip-enhanced Raman (TER) mapping of molecules interacting with plasmonic nanostructures. It is shown that numerous physical and chemical phenomena contribute to high-resolution TER spectral images. As a result, selectively tracking interfacial chemical transformations via TERS is more challenging than currently appreciated.

Ambient Tip-Enhanced Two Photon Photoluminescence from CdSe/ZnS Quantum Dots.

Nonlinear nano-optical measurements that combine ultrafast spectroscopy with tools of scanning probe microscopy are scarce. This is particularly the case when high spatial resolution on the order of a few nanometers is sought after in experiments performed under ambient laboratory conditions. In this work, we demonstrate the latter through measurements that track two-photon photoluminescence from aggregates of CdSe/ZnS quantum dots with sub-5 nm spatial resolution. Our proof-of-principle measurements that only take advantage of a plasmonic probe (as opposed to a gap mode) pave the way for nonlinear photoluminescence-based spectral nanoimaging of realistic/heterogeneous (bio) molecular and (bio) material systems.

Multipolar Raman Scattering vs Interfacial Nanochemistry: Case of 4-Mercaptopyridine on Gold.

Caution needs to be exercised in associating changes in plasmon-enhanced Raman spectra with chemical transformations. This is demonstrated through a detailed analysis of tip-enhanced Raman (TER) scattering from 4-mercaptopyridine (MPY) on gold. The substrate used consists of gold nanoplates atop a gold surface featuring heterogeneous grooves, all coated with a monolayer of MPY. The brightest spectra across the substrate exhibit features that can only be recovered by considering the generalized polarizability of oriented MPY molecules. The complex TER spectra we observe do not mark interfacial chemistry but rather multipolar TER scattering driven by local field gradients.

Excited-State-Selective Ultrafast Relaxation Dynamics and Photoisomerization of trans-4,4'-Azopyridine.

Excited-state dynamics of trans-4,4'-azopyridine in ethanol is studied using femtosecond transient absorption with 30 fs temporal resolution. Exciting the system at three different wavelengths, 460 and 290 (275) nm, to access the S1 nπ* and S2 ππ* electronic states, respectively, reveals a 195 cm-1 vibrational coherence, which suggests that the same mode is active in both nπ* and ππ* relaxation channels. Following S1-excitation, relaxation proceeds via a nonrotational pathway, where a fraction of the nπ* population is trapped in a planar minimum (lifetime, 2.1 ps), while the remaining population travels further to a second shallow minimum (lifetime, 300 fs) prior to decay into the ground state. Population of the S2 state leads to 30 fs nonrotational relaxation with a concurrent buildup of nπ* population and nearly simultaneous formation of hot ground-state species. An increase in the cis-isomer quantum yield upon ππ* versus nπ* excitation is observed, which is opposite to trans-azobenzene.

Atmospheric emission of nanoplastics from sewer pipe repairs.

Nanoplastic particles are inadequately characterized environmental pollutants that have adverse effects on aquatic and atmospheric systems, causing detrimental effects to human health through inhalation, ingestion and skin penetration1-3. At present, it is explicitly assumed that environmental nanoplastics (EnvNPs) are weathering fragments of microplastic or larger plastic debris that have been discharged into terrestrial and aquatic environments, while atmospheric EnvNPs are attributed solely to aerosolization by wind and other mechanical forces. However, the sources and emissions of unintended EnvNPs are poorly understood and are therefore largely unaccounted for in various risk assessments4. Here we show that large quantities of EnvNPs may be directly emitted into the atmosphere as steam-laden waste components discharged from a technology commonly used to repair sewer pipes in urban areas. A comprehensive chemical analysis of the discharged waste condensate has revealed the abundant presence of insoluble colloids, which after drying form solid organic particles with a composition and viscosity consistent with EnvNPs. We suggest that airborne emissions of EnvNPs from these globally used sewer repair practices may be prevalent in highly populated urban areas5, and may have important implications for air quality and toxicological levels that need to be mitigated.

Multimodal (Non)Linear Optical Nanoimaging and Nanospectroscopy.

This Perspective highlights recent advances in linear and nonlinear spectral nanoimaging. The described developments are motivated by the need to characterize molecular and material systems noninvasively with nanometer spatial and femtosecond temporal resolution. Indeed, the ability to image and chemically characterize heterogeneous interfaces with joint nano-femto resolution is a prerequisite to advancing our fundamental understanding of processes as diverse as heterogeneous catalysis, microbial communication, and energy flow in pristine/defect-containing low-dimensional quantum materials, to name a few. We describe pioneering work and recent demonstrations of (non)linear optical nanoimaging and nanospectroscopy, with an emphasis on high spatial resolution measurements conducted under ambient laboratory conditions.

Resonant Coherent Raman Scattering from WSe2.

Low-dimensional transition-metal dichalcogenides (TMDs) continue to comprise a subject of intense research because of their unique optical and electronic properties that may be harnessed in modern devices. Intense photoluminescence (PL) from few-/monolayer TMDs rendered PL-based micro- and nanospectroscopic characterization ideal in the quest to understand the correlation between structure and function in these materials. Nonlinear optical methods are by comparison far less utilized for this purpose. In this work, we describe an approach based on electronically resonant four-wave-mixing that allows spatio-spectral characterization of excitons in monolayer WSe2. Due to the coherent nature of the response that we exploit to trace exciton resonances, and recent demonstrations of electronic four-wave-mixing-based nanoimaging and nanospectroscopy, our present work is an important step toward characterizing TMDs on the nano-femto scale using light.

Integrated photoelectrochemical energy storage cells prepared by benchtop ion soft landing.

The exceptional photochromic and redox properties of polyoxometalate anions, PW12O403-, have been exploited to develop an integrated photoelectrochemical energy storage cell for conversion and storage of solar energy. Elimination of strongly coordinating cations using benchtop ion soft landing leads to a ∼370% increase in the maximum power output of the device. Additionally, the photocathode displayed a pronounced color change from clear to blue upon irradiation, which warrants the potential application of the IPES cell in advanced smart windows and photochromic lenses.

Nano-optical Visualization of Interlayer Interactions in WSe2/WS2 Heterostructures.

The interplay between excitons and phonons governs the optical and electronic properties of transition metal dichalcogenides (TMDs). Even though a number of linear and nonlinear optical-, electron-, and photoelectron-based approaches have been developed and/or adopted to characterize excitons and phonons in single/few-layer TMDs and their heterostructures, no existing method is capable of directly probing ultralow-frequency and interlayer phonons on the nanoscale. To this end, we developed ultralow-frequency tip-enhanced Raman spectroscopy, which allows spectrally and spatially resolved chemical and structural nanoimaging of WSe2/WS2 heterostructures. In this work, we apply this method to analyze phonons in nanobubbles that are sustained in these heterobilayers. Our method is capable of directly probing interlayer (de)coupling using our novel structurally sensitive nano-optical probe and the interplay between excitons and interlayer/intralayer phonons through correlation analysis of the recorded spectral images.

A Closer Look at Tip-Enhanced Raman Chemical Reaction Nanoimages.

Tip-enhanced Raman spectroscopy (TERS) is a powerful technique that enables ultrahigh spatial resolution and ultrasensitive chemical imaging. This technique's ability to track plasmon-induced/enhanced chemical reactions in real space has gained increasing popularity in recent years. In this study, we expose inherent difficulties associated with assigning TERS signatures that accompany chemical transformations. Namely, distinct selection rules as well as the possibility of multiple physical processes/chemical reaction pathways complicate spectral assignments and necessitate caution in assigning the experimental observables. We illustrate the latter using 4,4'-dimercaptostilbene-functionalized plasmonic silver nanocubes, wherein we identify the TERS signatures of product formation, molecular charging, multipolar Raman scattering, and preferred molecular orientations that all lead to distinct and assignable spectral patterns.