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Nanostructure-based plasmon-enhanced Raman spectroscopy for surface analysis of materials
Since 2000, there has been an explosion of activity in the field of plasmon-enhanced Raman spectroscopy (PERS), including surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). In this Review, we explore the mechanism of PERS and discuss PERS hotspots — nanoscale regions with a strongly enhanced local electromagnetic field — that allow trace-molecule detection, biomolecule analysis and surface characterization of various materials. In particular, we discuss a new generation of hotspots that are generated from hybrid structures combining PERS-active nanostructures and probe materials, which feature a strong local electromagnetic field on the surface of the probe material. Enhancement of surface Raman signals up to five orders of magnitude can be obtained from materials that are weakly SERS active or SERS inactive. We provide a detailed overview of future research directions in the field of PERS, focusing on new PERS-active nanomaterials and nanostructures and the broad application prospect for materials science and technology.
Assisted by rationally designed novel plasmonic nanostructures, surface-enhanced Raman spectroscopy has presented a new generation of analytical tools (that is, tip-enhanced Raman spectroscopy and shell-isolated nanoparticle-enhanced Raman spectroscopy) with an extremely high surface sensitivity, spatial resolution and broad application for materials science and technology.
Journal Article
Lossless enrichment of trace analytes in levitating droplets for multiphase and multiplex detection
Concentrating a trace amount of molecules from liquids, solid objects, or the gas phase and delivering them to a localized area are crucial for almost any trace analyte detection device. Analytes within a liquid droplet resting on micro/nanostructured surfaces with liquid-repellent coatings can be concentrated during solvent evaporation. However, these coatings suffer from complex manufacturing procedures, poor versatility, and limited analyte enrichment efficiency. Here, we report on the use of an acoustic levitation platform to losslessly concentrate the analyte molecules dissolved in any volatile liquid, attached to solid objects, or spread in air. Gold nanoparticles can be simultaneously concentrated with the analytes in different phases, realizing sensitive, surface-enhanced Raman scattering detection even at attomolar (10
−18
mol/L) concentration levels. The acoustic levitation platform-enabled, lossless analyte enrichment can significantly increase the analytical performance of many conventional microsensing techniques.
Efficient enrichment of molecules from liquids, solid objects, or the gas phase is critical for their detection at trace concentrations. Here, the authors report on the lossless enrichment of analytes in droplets using acoustic levitation for multiphase and multiplex SERS detection.
Journal Article
Chemically sensitive bioimaging with coherent Raman scattering
Coherent Raman imaging techniques have evolved to become powerful tools for biomedical imaging without the need for labelling.
Raman scattering provides an intrinsic fingerprint of chemical composition. Spontaneous Raman spectroscopy has been used for many decades to interrogate biological materials and systems. In spite of its valuable information content, Raman imaging is rarely used compared with modalities such as fluorescence microscopy because of its relatively slow signal acquisition. Coherent Raman imaging technologies have evolved over the past fifteen years to now capture rich chemical information with improved acquisition speeds. As a result, coherent Raman imaging methods are now poised to begin emerging as widely used tools for obtaining functional information in a label-free manner from biological systems. We briefly review the development and application of these methods.
Journal Article
Advances in Brillouin–Mandelstam light-scattering spectroscopy
Recent years have witnessed a much broader use of Brillouin inelastic light-scattering spectroscopy for the investigation of phonons and magnons in novel materials, nanostructures and devices. Driven by the developments in instrumentation and the strong need for accurate knowledge on the energies of elemental excitations, Brillouin–Mandelstam spectroscopy is rapidly becoming an essential technique that is complementary to Raman inelastic light-scattering spectroscopy. We provide an overview of recent progress in the Brillouin light-scattering technique, focusing on the use of this photonic method for the investigation of confined acoustic phonons, phononic metamaterials and magnon propagation and scattering. This Review emphasizes the emerging applications of Brillouin–Mandelstam spectroscopy for phonon-engineered structures and spintronic devices, and concludes with a perspective on future directions.Nearly 100 years after the prediction of Brillouin light-scattering spectroscopy, or Brillouin–Mandelstam light-scattering spectroscopy, the effect has proved itself a powerful tool for decades. Now its application to probing confined acoustic phonons, phononic metamaterials and magnons is reviewed.
Journal Article
Rapid antibiotic susceptibility testing of bacteria from patients’ blood via assaying bacterial metabolic response with surface-enhanced Raman spectroscopy
Blood stream infection is one of the major public health issues characterized with high cost and high mortality. Timely effective antibiotics usage to control infection is crucial for patients’ survival. The standard microbiological diagnosis of infection however can last days. The delay in accurate antibiotic therapy would lead to not only poor clinical outcomes, but also to a rise in antibiotic resistance due to widespread use of empirical broad-spectrum antibiotics. An important measure to tackle this problem is fast determination of bacterial antibiotic susceptibility to optimize antibiotic treatment. We show that a protocol based on surface-enhanced Raman spectroscopy can obtain consistent antibiotic susceptibility test results from clinical blood-culture samples within four hours. The characteristic spectral signatures of the obtained spectra of
Staphylococcus aureus
and
Escherichia coli
—prototypic Gram-positive and Gram-negative bacteria—became prominent after an effective pretreatment procedure removed strong interferences from blood constituents. Using them as the biomarkers of bacterial metabolic responses to antibiotics, the protocol reported the susceptibility profiles of tested drugs against these two bacteria acquired from patients’ blood with high specificity, sensitivity and speed.
Journal Article
Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr2IrO4
The dynamics of the magnetic correlations after photo-doping Sr
2
IrO
4
are studied through ultrafast time-resolved resonant inelastic X-ray spectroscopy. The timescales are found to depend on the dimensionality of the correlations.
Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity
1
,
2
,
3
,
4
. Recently, photo-excitation has been used to induce similarly exotic states transiently
5
,
6
,
7
. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr
2
IrO
4
. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. The marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.
Journal Article
Mapping of Low-Frequency Raman Modes in CVD-Grown Transition Metal Dichalcogenides: Layer Number, Stacking Orientation and Resonant Effects
Layered inorganic materials, such as the transition metal dichalcogenides (TMDs), have attracted much attention due to their exceptional electronic and optical properties. Reliable synthesis and characterization of these materials must be developed if these properties are to be exploited. Herein, we present low-frequency Raman analysis of MoS
2
, MoSe
2,
WSe
2
and WS
2
grown by chemical vapour deposition (CVD). Raman spectra are acquired over large areas allowing changes in the position and intensity of the shear and layer-breathing modes to be visualized in maps. This allows detailed characterization of mono- and few-layered TMDs which is complementary to well-established (high-frequency) Raman and photoluminescence spectroscopy. This study presents a major stepping stone in fundamental understanding of layered materials as mapping the low-frequency modes allows the quality, symmetry, stacking configuration and layer number of 2D materials to be probed over large areas. In addition, we report on anomalous resonance effects in the low-frequency region of the WS
2
Raman spectrum.
Journal Article
Uncovering low-frequency vibrations in surface-enhanced Raman of organic molecules
Accessing the terahertz (THz) spectral domain through surface-enhanced Raman spectroscopy (SERS) is challenging and opens up the study of low-frequency molecular and electronic excitations. Compared to direct THz probing of heterogenous ensembles, the extreme plasmonic confinement of visible light to deep sub-wavelength scales allows the study of hundreds or even single molecules. We show that self-assembled molecular monolayers of a set of simple aromatic thiols confined inside single-particle plasmonic nanocavities can be distinguished by their low-wavenumber spectral peaks below 200 cm
−1
, after removal of a bosonic inelastic contribution and an exponential background from the spectrum. Developing environment-dependent density-functional-theory simulations of the metal-molecule configuration enables the assignment and classification of their THz vibrations as well as the identification of intermolecular coupling effects and of the influence of the gold surface configuration. Furthermore, we show dramatically narrower THz SERS spectra from individual molecules at picocavities, which indicates the possibility to study intrinsic vibrational properties beyond inhomogeneous broadening, further supporting the key role of local environment.
Low frequency vibrations of molecules are collective motions of many atoms, and thus very sensitive to their surroundings. Here, authors use light ultra-confined to the nanoscale, to measure single molecule vibrations below 1 THz. These show tapping and shearing motions, like waves on a string.
Journal Article
High-resolution tip-enhanced Raman scattering probes sub-molecular density changes
Tip-enhanced Raman spectroscopy (TERS) exhibits new selection rule and sub-nanometer spatial resolution, which is attributed to the plasmonic near-field confinement. Despite recent advances in simulations of TERS spectra under highly confined fields, a simply physical mechanism has remained elusive. In this work we show that single-molecule TERS images can be explained by local sub-molecular density changes induced by the confined near-field during the Raman process. The local sub-molecular density changes determine the spatial resolution in TERS and the gradient-based selection rule. Using this approach we find that the four-fold symmetry of
meso
-tetrakis(3,5-di-
tert
-butylphenyl)porphyrin (H
2
TBPP) TERS images observed in experiments arises from the combination of degenerate normal modes localized in the functional side groups rather than the porphyrin ring as previously considered. As an illustration of the potential of the method, we demonstrate how this new theory can be applied to microscopic structure characterization.
Despite recent advances in simulations of tip-enhanced Raman spectroscopy (TERS) under highly confined fields, a simply physical mechanism has remained elusive. Here, the authors show that single molecule TERS images can be explained by local sub-molecular density changes induced by the confined near-field during the Raman process.
Journal Article