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Defects can induce drastic changes of the electronic properties of two-dimensional transition metal dichalcogenides and influence their applications. It is still a great challenge to characterize small defects and correlate their structures with properties. Here, we show that tip-enhanced Raman spectroscopy (TERS) can obtain distinctly different Raman features of edge defects in atomically thin MoS2, which allows us to probe their unique electronic properties and identify defect types (e.g., armchair and zigzag edges) in ambient. We observed an edge-induced Raman peak (396 cm−1) activated by the double resonance Raman scattering (DRRS) process and revealed electron–phonon interaction in edges. We further visualize the edge-induced band bending region by using this DRRS peak and electronic transition region using the electron density-sensitive Raman peak at 406 cm−1. The power of TERS demonstrated in MoS2 can also be extended to other 2D materials, which may guide the defect engineering for desired properties. Probing inevitable defects in two- dimensional materials is challenging. Here, the authors tackle this issue by using tip-enhanced Raman spectroscopy (TERS) to obtain distinctly different Raman features of edge defects in atomically thin MoS2, and further probe their unique electronic properties as well as identify the armchair and zigzag edges.

The dynamics of exciton formation in transition metal dichalcogenides is difficult to measure experimentally, since many momentum-indirect exciton states are not accessible to optical interband spectroscopy. Here, we combine a tuneable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS$_2$. This provides momentum-, energy- and time-resolved access to excited states on an ultrafast timescale. The high temporal resolution of the setup allows us to trace the early-stage exciton dynamics on its intrinsic timescale and observe the formation of a momentum-forbidden dark K$\Sigma$ exciton a few tens of femtoseconds after optical excitation. By tuning the excitation energy we manipulate the temporal evolution of the coherent excitonic polarization and observe its influence on the dark exciton formation. The experimental results are in excellent agreement with a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS$_2$., Comment: 18 pages, 5 figures

Twist-engineering of the electronic structure in van-der-Waals layered materials relies predominantly on band hybridization between layers. Band-edge states in transition-metal-dichalcogenide semiconductors are localized around the metal atoms at the center of the three-atom layer and are therefore not particularly susceptible to twisting. Here, we report that high-lying excitons in bilayer WSe2 can be tuned over 235 meV by twisting, with a twist-angle susceptibility of 8.1 meV/°, an order of magnitude larger than that of the band-edge A-exciton. This tunability arises because the electronic states associated with upper conduction bands delocalize into the chalcogenide atoms. The effect gives control over excitonic quantum interference, revealed in selective activation and deactivation of electromagnetically induced transparency (EIT) in second-harmonic generation. Such a degree of freedom does not exist in conventional dilute atomic-gas systems, where EIT was originally established, and allows us to shape the frequency dependence, i.e., the dispersion, of the optical nonlinearity., Here, the authors report on the large twist-angle susceptibility of excitons involving upper conduction bands in transition metal dichalcogenide bilayers. These high-lying excitons couple with band-edge excitons, and give rise to nonlinear quantum-optical processes that become tuneable by twisting.

By probing internal 1s-2p transitions with phase-locked mid-infrared pulses, we reveal the internal structure and binding energy of photo-generated excitons in transition metal dichalcogenide bilayers. Furthermore, we trace how the twist angle precisely controls the ultrafast formation of interlayer excitons and tunes excitonic hybridization in hetero- and homobilayers, respectively.

Underpotential deposition offers a predominant way to tailor the electronic structure of the catalytic surface at the atomic level, which is key to engineering materials with a high activity for (electro)catalysis. However, it remains challenging to precisely control and directly probe the underpotential deposition of a (sub)monolayer of atoms on nanoparticle surfaces. In this work, we in situ observe silver electrodeposited on gold nanocrystals surface from sub-monolayer to one monolayer by designing a highly sensitive electrochemical dark field scattering setup. The spectral variation is used to reconstruct the optical “cyclic voltammogram” of every single nanocrystal for understanding the underpotential deposition process on nanocrystals, which cannot be achieved by any other methods but are essential for creating novel nanomaterials., Underpotential deposition (UPD) is important to modify the surface properties of nanocrystals. Here, the authors show the application of in situ electrochemical dark field spectroscopy in identifying the UPD processes of silver on different facets of gold nanocrystals at the single nanoparticle level.

The large surface-to-volume ratio in atomically thin 2D materials allows to efficiently tune their properties through modifications of their environment. Artificial stacking of two monolayers into a bilayer leads to an overlap of layer-localized wave functions giving rise to a twist angle-dependent hybridization of excitonic states. In this joint theory-experiment study, we demonstrate the impact of interlayer hybridization on bright and momentum-dark excitons in twisted WSe$_2$ bilayers. In particular, we show that the strong hybridization of electrons at the $\Lambda$ point leads to a drastic redshift of the momentum-dark K-$\Lambda$ exciton, accompanied by the emergence of flat moir\'e exciton bands at small twist angles. We directly compare theoretically predicted and experimentally measured optical spectra allowing us to identify photoluminescence signals stemming from phonon-assisted recombination of layer-hybridized dark excitons. Moreover, we predict the emergence of additional spectral features resulting from the moir\'e potential of the twisted bilayer lattice.

The recent discovery of artificial phase transitions induced by stacking monolayer materials at magic twist angles represents a paradigm shift for solid state physics. Twist-induced changes of the single-particle band structure have been studied extensively, yet a precise understanding of the underlying Coulomb correlations has remained challenging. Here we reveal in experiment and theory, how the twist angle alone affects the Coulomb-induced internal structure and mutual interactions of excitons. In homobilayers of WSe2, we trace the internal 1s–2p resonance of excitons with phase-locked mid-infrared pulses as a function of the twist angle. Remarkably, the exciton binding energy is renormalized by up to a factor of two, their lifetime exhibits an enhancement by more than an order of magnitude, and the exciton-exciton interaction is widely tunable. Our work opens the possibility of tailoring quasiparticles in search of unexplored phases of matter in a broad range of van der Waals heterostructures., The crystallographic orientation of monolayers in van der Waals multi-layers controls their electronic and optical properties. Here the authors show how the twist angle affects Coulomb correlations governing the internal structure and the mutual interaction of excitons in homobilayers of WSe2.

Precise measurement of the temperature right at the surface of thermoplasmonic nanostructures is a grand challenge but extremely important for the photochemical reaction and photothermal therapy. We present here a method capable of measuring the surface temperature of plasmonic nanostructures with surface-enhanced Raman spectroscopy, which is not achievable by existing methods. We observe a sensitive shift of stretching vibration of a phenyl isocyanide molecule with temperature (0.232 cm–1/°C) as a result of the temperature-dependent molecular orientation change. We develop this phenomenon into a method capable of measuring the surface temperature of Au nanoparticles (NPs) during plasmonic excitation, which is validated by monitoring the laser-induced desorption process of the adsorbed CO on Au NP surface. We further extend the method into a more demanding single living cell thermometry that requires a high spatial resolution, which allows us to successfully monitor the extracellular temperature distribut...

Tip-enhanced Raman spectroscopy (TERS), known as nanospectroscopy, has received increasing interest as it can provide nanometer spatial resolution and chemical fingerprint information of samples simultaneously. Since Ag tips are well accepted to show a higher TERS enhancement than that of gold tips, there is an urgent quest for Ag TERS tips with a high enhancement, long lifetime, and high reproducibility, especially for atomic force microscopy (AFM)-based TERS. Herein, we developed an electrodeposition method to fabricate Ag-coated AFM TERS tips in a highly controllable and reproducible way. We investigated the influence of the electrodeposition potential and time on the morphology and radius of the tip. The radii of Ag-coated AFM tips can be rationally controlled at a few to hundreds nanometers, which allows us to systematically study the dependence of the TERS enhancement on the tip radius. The Ag-coated AFM tips show the highest TERS enhancement under 632.8 nm laser excitation and a broad localized surface plasmon resonance (LSPR) response when coupled to a Au substrate. The tips exhibit a lifetime of 13 days, which is particularly important for applications that need a long measuring time.

Electromagnetically induced transparency (EIT) arises because of quantum interference between electronic transitions. While the phenomenon is a “gold standard” in atomic quantum optics, it is hard to probe in bulk condensed matter and difficult to control in quantum-confined systems—prerequisites for exploitation in devices. EIT arises in excitonic transitions of single-layer transition-metal dichalcogenide crystals, which, in effect, constitute giant two-dimensional exfoliated molecules. We exploit the characteristic sensitivity of molecules to their immediate dielectric environment to demonstrate how chemical tuning of the exciton resonance over 5% of the exciton energy allows unprecedented control over quantum interference. EIT is probed in second-harmonic generation (SHG) of monolayer WSe2, where it gives rise to resonant suppression of SHG in response to the immediate surrounding. This solid-state solvatochromic effect arises primarily from changes in electronic band gap and exciton binding energy of monolayer WSe2. Surprisingly, the EIT resonance shifts linearly with exciton energy in response to the dielectric nonlocal manipulation. The approach demonstrates that concepts from atomic quantum optics can be ported directly to condensed-phase materials, stimulating synthetic challenges to develop materials to tune quantum-coherent phenomena.

Heterostructures of atomically thin van der Waals bonded monolayers have opened a unique platform to engineer Coulomb correlations, shaping excitonic, Mott insulating, or superconducting phases. In transition metal dichalcogenide heterostructures, electrons and holes residing in different monolayers can bind into spatially indirect excitons with a strong potential for optoelectronics, valleytronics, Bose condensation, superfluidity, and moir\'e-induced nanodot lattices. Yet these ideas require a microscopic understanding of the formation, dissociation, and thermalization dynamics of correlations including ultrafast phase transitions. Here we introduce a direct ultrafast access to Coulomb correlations between monolayers; phase-locked mid-infrared pulses allow us to measure the binding energy of interlayer excitons in WSe2/WS2 hetero-bilayers by revealing a novel 1s-2p resonance, explained by a fully quantum mechanical model. Furthermore, we trace, with subcycle time resolution, the transformation of an exciton gas photogenerated in the WSe2 layer directly into interlayer excitons. Depending on the stacking angle, intra- and interlayer species coexist on picosecond scales and the 1s-2p resonance becomes renormalized. Our work provides a direct measurement of the binding energy of interlayer excitons and opens the possibility to trace and control correlations in novel artificial materials., Comment: This is a post-peer-review, pre-copyedit version of an article published in Nature Materials. The final authenticated version is available online at https://doi.org/10.1038/s41563-019-0337-0

We report the observation of the undocumented visible one-photon photoluminescence (PL) of single silver nanorods excited by 532 and 633 nm continuous wave lasers with single-nanoparticle spectroscopy. We attribute the PL of silver nanorods to the intraband transition excited hot-electron radiative decay. The PL of silver nanorods closely resembles the corresponding LSPR scattering spectrum, and both are dependent on their aspect ratio. The good correlation between the quantitative PL intensity and the absorption cross section at the excitation wavelength of each nanorod leads to an aspect ratio independent PL quantum yield. The PL quantum yield of silver nanorods is similar to that of gold nanorods (10–6), indicating an efficient intraband excitation of hot electrons. The understanding of the PL mechanism of Ag nanorods points to the high-energy nature of the hot electrons excited via intraband transition, which has important indications in utilizing hot electrons for energy harvesting and photocatalysis.

Surface-enhanced Raman spectroscopy (SERS) has attracted tremendous interest as a label-free highly sensitive analytical method. For optimization of SERS activity, it is highly important to systematically investigate the size effect of nanoparticles on the SERS enhancement, which appears to be challenging in experiment, as the localized surface plasmon resonance (LSPR) of nanoparticles also changes with the change of the particle size. This challenge can be overcome by utilizing the unique property of gold nanorods, whose LSPR wavelength can be controlled to be the same by properly choosing the size and aspect ratio of the nanorods. We obtained the correlated SEM images, scattering spectra, and SERS spectra on a home-built single nanoparticle spectroscopy system and systematically investigate the size effect on SERS of individual gold nanorods using the adsorbed malachite green isothiocyanate (MGITC) molecule as the probe molecule. The dark field scattering intensity was found to increase with the increas...

Surface-enhanced Raman scattering (SERS) spectroscopy has attracted tremendous interests as a highly sensitive label-free tool. The local field produced by the excitation of localized surface plasmon resonances (LSPRs) dominates the overall enhancement of SERS. Such an electromagnetic enhancement is unfortunately accompanied by a strong modification in the relative intensity of the original Raman spectra, which highly distorts spectral features providing chemical information. Here we propose a robust method to retrieve the fingerprint of intrinsic chemical information from the SERS spectra. The method is established based on the finding that the SERS background originates from the LSPR-modulated photoluminescence, which contains the local field information shared also by SERS. We validate this concept of retrieval of intrinsic fingerprint information in well controlled single metallic nanoantennas of varying aspect ratios. We further demonstrate its unambiguity and generality in more complicated systems of tip-enhanced Raman spectroscopy (TERS) and SERS of silver nanoaggregates., We acknowledge support from NSFC (21633005, 21621091 and J1310024), MOST (2013CB933703 and 2016YFA0200601), MOE (IRT13036), and MINECO (FIS2016-80174-P) and Basque Government grant of UPV/EHU Grupos Consolidados IT-756-13. R.E. also acknowledges Fellow Gipuzkoa of the Gipuzkoako Foru Aldundia through the European Union’s FEDER program ‘Una manera de hacer Europa’.