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Surface plasmon induced hot‐carrier injection to semiconducting transition metal dichalcogenide (TMD) monolayers has been extensively studied. However, comprehensive understanding of the injection kinetics by fully considering the weak metal−TMD interaction and the TMDs' exciton formation kinetics is missing. Here, a hot‐carrier injection pathway is elucidated by systematically investigating the interfacial interaction kinetics among different plasmonic metals and TMDs. The pathway highlights the exciton formation timescale as a threshold for interfacial carrier injection, before which plasmonic hot carriers and free electron−hole pairs are relaxed incoherently across the interface. The injected hot carriers will interact with excitons to form charged quasiparticles as trions, which have extended lifetime. The pathway reveals the fundamental mechanism of the plasmonic hot‐carrier−TMD interactions, opens the possibility of controllable manipulation of hot‐carrier injection process, and allows future research toward opto‐electrical guidance of trions in metal−TMD systems. [ABSTRACT FROM AUTHOR]

[Display omitted] Two-dimensional (2D) van der Waals (vdW) heterostructures offer new platforms for exploring novel physics and diverse applications ranging from electronics and photonics to optoelectronics at the nanoscale. The studies to date have largely focused on transition-metal dichalcogenides (TMDCs) based samples prepared by mechanical exfoliation method, therefore it is of significant interests to study high-quality vdW heterostructures using novel materials prepared by a versatile method. Here, we report a two-step vapor phase growth process for the creation of high-quality vdW heterostructures based on perovskites and TMDCs, such as 2D Cs 3 Bi 2 I 9 /MoSe 2 , with a large lattice mismatch. Supported by experimental and theoretical investigations, we discover that the Cs 3 Bi 2 I 9 /MoSe 2 vdW heterostructure possesses hybrid band alignments consisting of type-I and type-II heterojunctions because of the existence of defect energy levels in Cs 3 Bi 2 I 9. More importantly, we demonstrate that the type-II heterojunction in the Cs 3 Bi 2 I 9 /MoSe 2 vdW heterostructure not only shows a higher interlayer exciton density, but also exhibits a longer interlayer exciton lifetime than traditional 2D TMDCs based type-II heterostructures. We attribute this phenomenon to the reduced overlap of electron and hole wavefunctions caused by the large lattice mismatch. Our work demonstrates that it is possible to directly grow high-quality vdW heterostructures based on entirely different materials which provide promising platforms for exploring novel physics and cutting-edge applications, such as optoelectronics, valleytronics, and high-temperature superfluidity. [ABSTRACT FROM AUTHOR]

Second harmonic generation (SHG) microscopy is useful for visualizing interfaces and sub-structures within a wide range of materials due to the propensity for SHG to occur in non-centrosymmetric environments. However, since SHG is a nonlinear process generally necessitating small focal sizes for higher peak powers, a raster scanning approach is usually needed to build an SHG image over a significant sample size. While raster scanning is effective, there is a cost in terms of the time needed to acquire the image and, also, some materials cannot withstand the higher optical intensities within the small focal volume. In order to address this, we describe a SHG microscopy approach based on ghost imaging (GI), which enables imaging data to be collected in parallel rather than sequentially as in raster scanning techniques. We experimentally demonstrate the approach and combine GI-SHG with compressive sensing to make further substantial gains in reducing the amount of sampling required for image reconstruction. Furthermore, GI-SHG is shown to have significant advantages for imaging in highly scattering environments, partly because GI is a background-free approach requiring spatial correlations between photons that travel two paths, with one path entirely devoid of sample interaction. This basic property of GI means that only the photons that travel unimpeded through the sample preserve the spatial correlations needed to reconstruct the image, while more scattered photons do not contribute to the overall GI signal. Finally, we compare the image quality and sampling properties of three different reconstruction algorithms used for compressive sensing. [ABSTRACT FROM AUTHOR]

In general, intermolecular distancesin condensed phases at theangstrom scale are difficult to measure. We were able to do so byusing the vibrational energy transfer method, an ultrafast vibrationalanalogue of Förster resonance energy transfer. The distancesamong SCN–anions in KSCN crystals and ion clustersof KSCN aqueous solutions were determined with the method. In thecrystalline samples, the closest anion distance was determined tobe 3.9 ± 0.3 Å, consistent with the XRD result. In the 1.8and 1 M KSCN aqueous solutions, the anion distances in the ion clusterswere determined to be 4.4 ± 0.4 Å. The clustered anion distancesin aqueous solutions are very similar to the closest anion distancein the KSCN crystal but significantly shorter than the average aniondistance (0.94–1.17 nm) in the aqueous solutions if ion clusteringdid not occur. The result suggests that ions in the strong electrolyteaqueous solutions can form clusters inside of which they have directcontact with each other. [ABSTRACT FROM AUTHOR]

The low‐energy layer edge states (LESs) from quasi 2D hybrid perovskite single crystals have shown great potential because of their nontrivial photoelectrical properties. However, the underlying formation mechanism of the LESs still remains controversial. Also, the presence or creation of the LESs is of high randomness due to the lack of proper techniques to manually generate these LESs. Herein, using a single crystals platform of quasi‐2D (BA)2(MA)n−1PbnI3n+1 (n > 1) perovskites, the femtosecond laser ablation approach to design and write the LESs with a high spatial resolution is reported. Fundamentally, these LESs are of smaller bandgap 3D MAPbI3 nanocrystals which are formed by the laser‐induced BA escaping from the lattice and thus the lattice shrinkage from quasi‐2D to 3D structures. Furthermore, by covering the crystal with tape, an additional high‐energy emission state corresponding to the reformation of (BA)2PbI4 (n = 1) within the irradiation region is generated. This work presents a simple and efficient protocol to manually write LESs on single crystals and thus lays the foundation for utilizing these LESs to further enhance the performance of future photoelectronic devices. [ABSTRACT FROM AUTHOR]

Interactions between model molecules representing buildingblocksof proteins and the thiocyanate anion, a strong protein denaturantagent, were investigated in aqueous solutions with intermolecularvibrational energy exchange methods. It was found that thiocyanateanions are able to bind to the charged ammonium groups of amino acidsin aqueous solutions. The interactions between thiocyanate anionsand the amide groups were also observed. The binding affinity betweenthe thiocyanate anion and the charged amino acid residues is about20 times larger than that between water molecules and the amino acidsand about 5–10 times larger than that between the thiocyanateanion and the neutral backbone amide groups. The series of experimentsalso demonstrates that the chemical nature, rather than the macroscopicdielectric constant, of the ions and molecules plays a critical rolein ion/molecule interactions in aqueous solutions. [ABSTRACT FROM AUTHOR]

Atomically thin quantum dots from layered materials promise new science and applications, but their scalable synthesis and separation have been challenging. We demonstrate a universal approach for the preparation of quantum dots from a series of materials, such as graphite, MoS2, WS2, h-BN, TiS2, NbS2, Bi2Se3, MoTe2, Sb2Te3, etc., using a cryo-mediated liquid-phase exfoliation and fracturing process. The method relies on liquid nitrogen pretreatment of bulk layered materials before exfoliation and breakdown into atomically thin two-dimensional quantum dots of few-nanometer lateral dimensions, exhibiting size-confined optical properties. This process is efficient for a variety of common solvents with a wide range of surface tension parameters and eliminates the use of surfactants, resulting in pristine quantum dots without surfactant covering or chemical modification. [ABSTRACT FROM AUTHOR]

Energy-gap-dependentvibrational-energy transfers among the nitrilestretches of KSCN/KS13CN/KS13C15Nin D2O, DMF, and formamide liquid solutions at room temperaturewere measured by the vibrational-energy-exchange method. The energytransfers are slower with a larger energy donor/acceptor gap, independentof the calculated instantaneous normal mode (“phonons”in liquids) densities or the terahertz absorption spectra. The energy-gapdependences of the nonresonant energy transfers cannot be describedby phonon compensation mechanisms with the assumption that phononsare the instantaneous normal modes of the liquids. Instead, the experimentalenergy-gap dependences can be quantitatively reproduced by the dephasingmechanism. A simple theoretical derivation shows that the fast molecularmotions in liquids randomize the modulations on the energy donor andacceptor by phonons and diminish the phonon compensation efficiencyon energy transfer. Estimations based on the theoretical derivationssuggest that, for most nonresonant intermolecular vibrational-energytransfers in liquids with energy gaps smaller than the thermal energy,the dephasing mechanism dominates the energy-transfer process. [ABSTRACT FROM AUTHOR]

Themolecular conformations of crystalline l-cysteineprepared in its orthorhombic form were determined by the vibrationalcross angle measurements. Its major dihedral angles of chemical bondsdetermined by this method are consistent with the results from diffractionexperiments. In addition, the relative orientations of the chemicalbonds associated with the hydrogen atoms of the NH3+group and the thiol group are also determined. The resultsdemonstrate that the vibrational cross angle method based on the multiple-modeapproach can potentially become a structural tool for determiningmolecular conformations. The major challenges for the method to becomea general molecular structural tool are discussed, and some approachesto address them are proposed. [ABSTRACT FROM AUTHOR]

Inthis work, through investigating a series of liquid, glassy,and crystalline samples with ultrafast multiple-mode 2D IR and IRtransient absorption methods, we demonstrated that the signal anisotropyof vibrational relaxation-induced heat effects is determined by bothrelative molecular orientations and molecular rotations. If the relativemolecular orientations are randomized or molecular rotations are fastcompared to heat transfer, the signal anisotropy of heat effects iszero. If the relative molecular orientations are anisotropic and themolecular rotations are slow, the signal anisotropy of heat effectscan be nonzero, which is determined by the relative orientations ofthe energy source mode and the heat sensor mode within the same moleculeand in different molecules. We also demonstrated that the correlationbetween the anisotropy value of heat signal and the relative molecularorientations can be quantitatively calculated. [ABSTRACT FROM AUTHOR]

Waiting time dependent rotationalanisotropies of SCN–anions and water moleculesin alkali thiocyanate (XSCN, X = Li,Na, K, Cs) aqueous solutions at various concentrations were measuredwith ultrafast infrared spectroscopy. It was found that cations cansignificantly affect the reorientational motions of both water moleculesand SCN–anions. The dynamics are slower in a solutionwith a smaller cation. The reorientational time constants follow theorder of Li+> Na+> K+≃Cs+. The changes of rotational time constants of SCN–at various concentrations scale almost linearly withthe changes of solution viscosity, but those of water molecules donot. In addition, the concentration-dependent amplitudes of dynamicalchanges are much more significant in the Li+and Na+solutions than those in the K+and Cs+solutions. Further investigations on the systems with the ultrafastvibrational energy exchange method and molecular dynamics simulationsprovide an explanation for the observations: the observed rotationaldynamics are the balanced results of ion clustering and cation/anion/waterdirect interactions. In all the solutions at high concentrations (>5M), substantial amounts of ions form clusters. The structural inhomogeneityin the solutions leads to distinct rotational dynamics of water andanions. The strong interactions of Li+and Na+because of their relatively large charge densities with water moleculesand SCN–anions, in addition to the likely geometricconfinements because of ion clustering, substantially slow down therotations of SCN–anions and water molecules insidethe ion clusters. The interactions of K+and Cs+with water or SCN–are much weaker. The rotationsof water molecules inside ion clusters of K+and Cs+solutions are not significantly different from those of otherwater species so that the experimentally observed rotational relaxationdynamics are only slightly affected by the ion concentrations. [ABSTRACT FROM AUTHOR]

Ultrafast multiple-dimensional vibrational spectroscopy has been extensively applied to studies of molecular structures and dynamics in condensed phases. Along with the developments of new laser sources and new concepts, increasing improvements and applications of this technique have brought the understanding of molecular systems to a new level. In this review, we first briefly introduce the basic concepts, experimental setups and applications of the technique. The most recent progresses in applying vibrational energy transfers to determine intermolecular distances and vibrational couplings to determine three dimensional molecular conformations with our high power multiple-mode multiple-dimensional vibrational spectroscopy are then introduced in more details. [ABSTRACT FROM AUTHOR]