Atomically terminated and nanoscopically smooth silver tips effectively focus light on the angstrom scale, allowing tip-enhanced Raman spectromicroscopy (TER-sm) with single molecule sensitivity and submolecular spatial resolution. Through measurements carried out on cobalt-tetraphenylporphyrin (CoTPP) adsorbed on Au(111), we highlight peculiarities of vibrational spectromicroscopy with light confined on the angstrom scale. Field-gradient-driven spectra, orientational fingerprinting, and sculpting of local fields by atomic morphology of the junction are elucidated through measurements that range from 2D arrays at room temperature to single molecule manipulations at 5 K. Notably, vibrational Stark tuning within molecules, reflecting intramolecular charge distributions, becomes accessible when light is more localized than the interrogated normal modes. The Stark images of CoTPP reveal that it is saddled, and the distortion is accompanied by charge transfer to gold through the two anchoring pyrroles.Keywords: confined light; field-gradient-driven Raman; quadrupolar scattering; scanning tunneling microscopy; spectromicroscopy; Stark shift; tip-enhanced Raman spectroscopy;

Comprehensive interpretation of vibrational sum-frequency-generation (SFG) spectroscopic features of SFG-active nanodomains interspersed in amorphous bulk requires the knowledge of nonlinear susceptibility, χijk(2), of the SFG-active phase as a function of its spatial arrangement in the bulk as well as the polarizations of the probe lights. This study reports the full analysis of the azimuth angle and polarization dependence of SFG signals from a control sample consisting of uniaxially aligned cellulose Iβ crystals. The χijk(2) terms of cellulose were estimated from quantum mechanics calculations using time-dependent density functional theory (TD-DFT), and a simple structural model was built with truncated glucose dimers. The theoretical azimuth angle and polarization dependences of characteristic CH/CH2 and OH stretch modes of cellulose were compared with the experimentally observed trends. These comparisons revealed that the relative polarity of crystallites within the SFG coherence length, the random quasi phase-matching of polycrystalline domains, and the preferential packing of crystallites in the bulk play important roles governing the spectral features. Compared to that of small molecules, the difference between chiral and achiral responses in SFG spectra is more difficult to observe because of the inhomogeneous distribution of crystallites in the bulk sample.

Herein, we utilize surface-enhanced hyper-Raman scattering (SEHRS) under resonance conditions to probe the adsorbate geometry of rhodamine 6G (R6G) on silver colloids. Our results show resonance SEHRS is highly sensitive to molecular orientation due to non-Condon effects, which do not appear in its linear counterpart surface-enhanced Raman scattering. Comparisons between simulated and measured SEHRS spectra reveal R6G adsorbs mostly perpendicular to the nanoparticle surface along the ethylamine groups with the xanthene ring oriented edgewise. Our results expand upon previous studies that rely on indirect, qualitative probes of R6G’s orientation on plasmonic substrates. More importantly, this work represents the first determination of adsorbate geometry by SEHRS and opens up the possibility to study the orientation of single molecules in complex, plasmonic environments.

The two-photon absorption (TPA) cross-sections of small thiolate-protected gold clusters have been shown to be much larger than typical small organic molecules. In comparison with larger nanoparticles, their TPA cross-sections per gold atom are also found to be larger. Theoretical simulations have suggested that the large enhancement of these TPA cross-sections comes from a one-photon double-resonance mechanism. However, it remains difficult to simulate TPA cross-sections of thiolate-protected gold clusters due to their large system size and a high density of states. In this work, we report a time-dependent density functional theory (TDDFT) study of the TPA spectra of the Au25(SR)18− cluster based on a damped response theory formalism. Damped response theory enables a consistent treatment of on- and off-resonance molecular properties even for molecules with a high density of states, and thus is well-suited for studying the TPA properties of gold clusters. Our results indicate that the one- and two-photon double-resonance effect is much smaller than previously found, and thus is unlikely to be the main cause of the large TPA cross-sections found experimentally. The effect of symmetry breaking of the Au25(SR)18− cluster due to the ligands on the TPA cross-sections has been studied and was found to only slightly increase the cross-section. Furthermore, by comparing with larger nanoparticles we find that the TPA cross-section per gold atom scales linearly with the diameter of the particles, and that the Kerr non-linear response of the Au25(SR)18− cluster is on the same order as that of bulk gold films.

Vibrational sum-frequency-generation (SFG) spectroscopy is capable of selectively detecting crystalline biopolymers interspersed in amorphous polymer matrices. However, the spectral interpretation is difficult due to the lack of knowledge on how spatial arrangements of crystalline segments influence SFG spectra features. Here we report time-dependent density functional theory (TD-DFT) calculations of cellulose crystallites in intimate contact with two different polarities: parallel versus antiparallel. TD-DFT calculations reveal that the CH/OH intensity ratio is very sensitive to the polarity of the crystallite packing. Theoretical calculations of hyperpolarizability tensors (βabc) clearly show the dependence of SFG intensities on the polarity of crystallite packing within the SFG coherence length, which provides the basis for interpretation of the empirically observed SFG features of native cellulose in biological systems.

Surface-enhanced hyper-Raman scattering (SEHRS) is the two-photon analogue of surface-enhanced Raman scattering (SERS), which has proven to be a powerful tool to study molecular structures and surface enhancements. However, few theoretical approaches to SEHRS exist and most neglect the atomistic descriptions of the metal surface and molecular resonance effects. In this work, we present two atomistic electrodynamics-quantum mechanical models to simulate SEHRS. The first is the discrete interaction model/quantum mechanical (DIM/QM) model, which combines an atomistic electrodynamics model of the nanoparticle with a time-dependent density functional theory description of the molecule. The second model is a dressed-tensors method that describes the molecule as a point-dipole and point-quadrupole object interacting with the enhanced local field and field-gradients (FG) from the nanoparticle. In both of these models, the resonance effects are treated efficiently by means of damped quadratic response theory. Using these methods, we simulate SEHRS spectra for benzene and pyridine. Our results show that the FG effects in SEHRS play an important role in determining both the surface selection rules and the enhancements. We find that FG effects are more important in SEHRS than in SERS. We also show that the spectral features of small molecules can be accurately described by accounting for the interactions between the molecule and the local field and FG of the nanoparticle. However, at short distances between the metal and molecule, we find significant differences in the SEHRS enhancements predicted using the DIM/QM and the dressed-tensors methods.

A general implementation for damped cubic response properties is presented using time-dependent density functional theory (TDDFT) and Slater-type orbital basis sets. To directly calculate two-photon absorption (TPA) cross sections, we also present an implementation of a reduced damped cubic response approach. Validation of the implementations includes a detailed comparison between response theory and the sum-over-states approach for calculating the nonlinear optical properties of LiH, as well as a comparison between the simulated and experimental TPA and third-harmonic generation (THG) spectra for the dimethylamino-nitrostilbene (DANS) molecule. The study of LiH demonstrates the incorrect pole structure obtained in response theory due to the adiabatic approximation typically employed for the exchange-correlation kernel. For DANS, we find reasonable agreement between simulated and experimental TPA and THG spectra. Overall, this work shows that care must be taken when calculating higher-order response functions in the vicinity of one-photon poles due to the approximate kernels typically used in the simulations.

Subsystem density functional theory (subsystem DFT) is a DFT partitioning method that is exact in principle, but depends on approximations to the kinetic energy density functional (KEDF). One may avoid the use of approximate KEDFs by ensuring that the inter-subsystem molecular orbitals are orthogonal, termed external orthogonality (EO). We present a method that extends a subsystem DFT method, that includes EO, into the time-dependent DFT (TDDFT) regime. This method therefore removes the need for approximations to the kinetic energy potential and kernel, and we show that it can accurately reproduce the supermolecular TDDFT results for weakly and strongly coupled subsystems, and for systems with strongly overlapping densities (where KEDF approximations traditionally fail).

Accurately describing the electronic structure of molecules on metal nanostructures is key to modeling their surface-enhanced properties. Particularly difficult is the modeling of the coupling between molecular excited states and plasmons. Here we present a computational efficient approach to study the renormalization effects on the molecular electronic structure and its optical properties due to the interactions with the metal surface. Accurate simulations of the renormalization effects are achieved by employing a hybrid atomistic electrodynamics and time-dependent density functional model. The coupling between the molecular absorption and the plasmon excitation depends strongly on the spectral overlap. Here we show that the renormalization effect for the benzene–tetracyanoethylene donor–acceptor complex interacting with a metal nanoparticle causes a 0.6 eV shift in the absorption band. Furthermore, we show that the coupling between the molecular absorption and the plasmon excitation is caused by interference between the molecular absorption, the image field of the metal nanoparticle, and the near field due to the plasmon excitation. The results presented here illustrate the importance of using first-principles simulations to understand in detail the coupling between molecular absorption and plasmon excitation.

The ability to simulate surface-enhanced Raman scattering (SERS) is a vital tool in elucidating the chemistry of molecules near the vicinity of plasmonic metal nanoparticles. However, typical methods do not include the dynamics of the molecule(s) of interest and are often limited to a single or few molecules. In this work, we combine molecular dynamics simulations with the dressed-tensor formalism to simulate the SERS spectra of Ag nanoparticles coated with a full monolayer of pyridine molecules. This method allows us to simulate the ensemble-averaged SERS spectra of more realistic large scale systems, while accounting for the organization of molecules in the hotspots. Through these simulations, we find that the preferential binding location and orientation of the molecules, the choice of electrodynamics method, and the inclusion of field gradient effects influence both the enhancement distribution and the spectral signatures. We also show that both the translational and rotational motions of a pyridine molecule near a nanoparticle junction may be effectively tracked through its SERS spectrum.

The surface-enhanced hyper-Raman scattering spectra of crystal violet are experimentally measured and theoretically calculated for excitation energies spanning the two lowest-lying electronic states (12,700–27,400 cm–1). The theory and experiment are in qualitative agreement over the measured energy range, indicating that first-principles calculations capture many of the complex resonance contributions in this prototypical octupolar system. The discrepancies between theory and experiment are investigated by comparing spectra obtained in different local environments as well as from higher-order surface-enhanced spectroscopies. A comparison between relative surface-enhanced hyper-Raman scattering band ratios plotted as a function of excitation wavelength and crystal violet’s absorption spectra elucidates correlations between groups of vibrations and the excited-electronic states. Our results suggest that the spectral features across the range of resonance excitation energies (∼15,500–27,400 cm–1) are dominated by strong A-term scattering.

Soluble, monomeric Ir(III/IV) complexes strongly affect the photoelectrochemical performance of IrOx·nH2O-catalyzed photoanodes for the oxygen evolution reaction (OER). The synthesis of IrOx·nH2O colloids by alkaline hydrolysis of Ir(III) or Ir(IV) salts proceeds through monomeric intermediates that were characterized using electrochemical and spectroscopic methods and modeled in TDDFT calculations. In air-saturated solutions, the monomers exist in a mixture of Ir(III) and Ir(IV) oxidation states, where the most likely formulations at pH 13 are [Ir(OH)5(H2O)]2– and [Ir(OH)6]2–, respectively. These monomeric anions strongly adsorb onto IrOx·nH2O colloids but can be removed by precipitation of the colloids with isopropanol. The monomeric anions strongly adsorb onto TiO2, and they promote the adsorption of ligand-free IrOx·nH2O colloids onto mesoporous titania photoanodes. However, the reversible adsorption/desorption of electroactive monomers effectively short-circuits the photoanode redox cycle and thus dramatically degrades the photoelectrochemical performance of the cell. The growth of a dense TiO2 barrier layer prevents access of soluble monomeric anions to the interface between the oxide semiconductor and the electrode back contact (a fluorinated tin oxide transparent conductor) and leads to improved photoanode performance. Purified IrOx·nH2O colloids, which contain no adsorbed monomer, give improved performance at the same electrodes. These results explain earlier observations that IrOx·nH2O catalysts can dramatically degrade the performance of metal oxide photoanodes for the OER reaction.

Frozen density embedding (FDE) has become a popular subsystem density functional theory (DFT) method for systems with weakly overlapping charge densities. The failure of this method for strongly interacting and covalent systems is due to the approximate kinetic energy density functional (KEDF), although the need for approximate KEDFs may be eliminated if each subsystem’s Kohn–Sham (KS) orbitals are orthogonal to the other, termed external orthogonality (EO). We present an implementation of EO into the FDE framework within the Amsterdam density functional program package, using the level-shift projection operator method. We generalize this method to remove the need for orbital localization schemes and to include multiple subsystems, and we show that the exact KS-DFT energies and densities may be reproduced through iterative freeze-and-thaw cycles for a number of systems, including a charge delocalized benzene molecule starting from atomic subsystems. Finally, we examine the possibility of a truncated basis for systems with and without charge delocalization, and found that subsystems require a basis that allows them to correctly describe the supermolecular delocalized orbitals.

Motivated to explore the ultimate limits of surface-enhanced nonlinear spectroscopies, we report on the first observation of molecular second hyper-Raman scattering with the aid of surface enhancement and provide a new theoretical framework for first-principles calculations of the second hyper-Raman effect. Second hyper-Raman enhancement factors, determined to be a minimum of 105 times stronger than those in Raman scattering, demonstrate a clear trend toward larger enhancements for nonlinear phenomena, and the nearly quantitative agreement between simulation and experiment provides a unique spectroscopic window into higher-order molecular responses.

Plasmonic circular dichroism (CD) of chiral molecules in the near field of plasmonic nanoparticles (NPs) may be used to enhance molecular CD signatures or to induce a CD signal at the plasmon resonance. A recent few-states theory explored these effects for model systems and showed an orientation dependence of the sign of the induced CD signal for spherical NPs. Here, we use the discrete interaction model/quantum mechanical (DIM/QM) method to simulate the CD and plasmonic CD of the 310- and α-helix conformations of a short alanine peptide. We find that the interactions between the molecule and the plasmon lead to significant changes in the CD spectra. In the plasmon region, we find that the sign of the CD depends strongly on the orientation of the molecule as well as specific interactions with the NP through image dipole effects. A small enhancement of the CD is found in the molecular region of the spectrum, however, the molecular signatures may be significantly altered through interactions with the NP. We also show that the image dipole effect can result in induced plasmonic CD even for achiral molecules. Overall, we find that the specific interactions with the NP can lead to large changes to the CD spectrum that complicates the interpretation of the results.

Surface-enhanced Raman scattering (SERS) is a technique that has broad implications for biological and chemical sensing applications by providing the ability to simultaneously detect and identify a single molecule. The Raman scattering of molecules adsorbed on metal nanoparticles can be enhanced by many orders of magnitude. These enhancements stem from a twofold mechanism: an electromagnetic mechanism (EM), which is due to the enhanced local field near the metal surface, and a chemical mechanism (CM), which is due to the adsorbate specific interactions between the metal surface and the molecules. The local field near the metal surface can be significantly enhanced due to the plasmon excitation, and therefore chemists generally accept that the EM provides the majority of the enhancements.While classical electrodynamics simulations can accurately simulate the local electric field around metal nanoparticles, they offer few insights into the spectral changes that occur in SERS. First-principles simulations can directly predict the Raman spectrum but are limited to small metal clusters and therefore are often used for understanding the CM. Thus, there is a need for developing new methods that bridge the electrodynamics simulations of the metal nanoparticle and the first-principles simulations of the molecule to facilitate direct simulations of SERS spectra.In this Account, we discuss our recent work on developing a hybrid atomistic electrodynamics–quantum mechanical approach to simulate SERS. This hybrid method is called the discrete interaction model/quantum mechanics (DIM/QM) method and consists of an atomistic electrodynamics model of the metal nanoparticle and a time-dependent density functional theory (TDDFT) description of the molecule. In contrast to most previous work, the DIM/QM method enables us to retain a detailed atomistic structure of the nanoparticle and provides a natural bridge between the electronic structure methods and the macroscopic electrodynamics description.Using the DIM/QM method, we have examined in detail the importance of the local environment on molecular excitation energies, enhanced molecular absorption, and SERS. Our results show that the molecular properties are strongly dependent not only on the distance of the molecule from the metal nanoparticle but also on its orientation relative to the nanoparticle and the specific local environment. Using DIM/QM to simulate SERS, we show that there is a significant dependence on the adsorption site. Furthermore, we present a detailed comparison between enhancements obtained from DIM/QM simulations and those from classical electrodynamics simulations of the local field. While we find qualitative agreement, there are significant differences due to the neglect of specific molecule–metal interactions in the classical electrodynamics simulations. Our results highlight the importance of explicitly considering the specific local environment in simulations of molecule–plasmon coupling.

We report first-principles simulations of the doubly resonance sum-frequency generation (DR-SFG) spectrum for rhodamine 6G (R6G). The simulations are done using a time-dependent formalism that includes both Franck–Condon (FC) and Herzberg–Teller (HT) terms in combination with time-dependent density functional theory (TDDFT) calculations. The simulated spectrum matches experiments, allowing a detailed assignment of the DR-SFG spectrum. Our work also shows that non-Condon effects are important and the DR-SFG spectrum of R6G is highly dependent on both FC and HT modes. This is surprising as R6G is known to be a strong FC resonant Raman scatterer. The simulations predict an orientation where the xanthene plane of R6G is perpendicular to the surface with binding through one of the ethyl amine groups. Our results show the importance of first-principles simulations for providing a detailed assignment of DR-SFG experiments, especially for large molecules where such an assignment is complicated due close-lying vibrational modes.Keywords: doubly resonant sum-frequency generation; non-Condon effects; rhodamine 6G; surface orientation; time-dependent density functional theory (TDDFT);

Raman optical activity has proven to be a powerful tool for probing the geometry of small organic and biomolecules. It has therefore been expected that the same mechanisms responsible for surface-enhanced Raman scattering may allow for similar enhancements in surface-enhanced Raman optical activity (SEROA). However, SEROA has proved to be an experimental challenge and mirror-image SEROA spectra of enantiomers have so far not been measured. There exists a handful of theories to simulate SEROA, all of which treat the perturbed molecule as a point-dipole object. To go beyond these approximations, we present two new methods to simulate SEROA: the first is a dressed-tensors model that treats the molecule as a point-dipole and point-quadrupole object; the second method is the discrete interaction model/quantum mechanical (DIM/QM) model, which considers the entire charge density of the molecule. We show that although the first method is acceptable for small molecules, it fails for a medium-sized one such as 2-bromohexahelicene. We also show that the SEROA mode intensities and signs are highly sensitive to the nature of the local electric field and gradient, the orientation of the molecule, and the surface plasmon frequency width. Our findings give some insight into why experimental SEROA, and in particular observing mirror-image SEROA for enantiomers, has been difficult.

A parallel implementation of analytical time-dependent density functional theory gradients is presented for the quantum chemistry program NWChem. The implementation is based on the Lagrangian approach developed by Furche and Ahlrichs. To validate our implementation, we first calculate the Stokes shifts for a range of organic dye molecules using a diverse set of exchange-correlation functionals (traditional density functionals, global hybrids, and range-separated hybrids) followed by simulations of the one-photon absorption and resonance Raman scattering spectrum of the phenoxyl radical, the well-studied dye molecule rhodamine 6G, and a molecular host–guest complex (TTF⊂CBPQT4+). The study of organic dye molecules illustrates that B3LYP and CAM-B3LYP generally give the best agreement with experimentally determined Stokes shifts unless the excited state is a charge transfer state. Absorption, resonance Raman, and fluorescence simulations for the phenoxyl radical indicate that explicit solvation may be required for accurate characterization. For the host–guest complex and rhodamine 6G, it is demonstrated that absorption spectra can be simulated in good agreement with experimental data for most exchange-correlation functionals. However, because one-photon absorption spectra generally lack well-resolved vibrational features, resonance Raman simulations are necessary to evaluate the accuracy of the exchange-correlation functional for describing a potential energy surface.

Here we describe an approach to the synthesis of small ligand stabilized Al nanoclusters by catalytic decomposition of alane using Ti(OiPr)4 as catalyst. The selected area electron diffraction (SAED) and elemental analysis are consistent with the presence of Al in the clusters. The cluster sizes are measured by the small-angle X-ray scattering method in air-free conditions. The absorption maximum exhibits red shifts when cluster sizes decrease from 4 to 1.5 nm. A two-layer Mie theory model indicates that the electron conductivity in the Al core is reduced due to a combination of quantum size effects and chemical interaction with the ligand shell resulting in the observed red shift with decreasing size. The red shift is shown to scale with the inverse radius in good agreement with a spill-out model. Furthermore, the results are consistent with time-dependent density functional simulations for a small ligand stabilized Al cluster. Remarkably, we find that the absorption maximum is significantly red-shifted compared with that expected from simulations based on the bulk dielectric constant. This is true even for the larger nanoclusters with diameters of 4 nm. This indicates that small ligand protected Al clusters behave significantly different from similar Ag and Au clusters.

The inhomogenous electric field near the metal surface of plasmonic nanoparticles allows molecular orientation to be determined from surface-enhanced Raman scattering (SERS). We illustrate this by simulating the effects of the field-gradient on the SERS spectrum of benzene and pyridine. To do this, we present an origin-independent formalism describing the effects of the local electric-field gradient in SERS. Using this formalism, we found that the field-gradient led to observation of Raman-inactive modes in benzene and allowed for extraction of orientation information from the SERS spectra of both benzene and pyridine. It was also observed that the SERS electromagnetic enhancement factor, when considering field-gradient effects, depends on the field-gradient magnitudes and is only approximately described by |E|4 for certain modes. The field-gradient mechanism may also lead to a weakening of intensities as compared to a homogeneous local field. Thus, inclusion of field-gradient effects are crucial in understanding relative intensity changes in SERS.

Tris(2,2′-bipyridine)ruthenium(II), Rubpy, an important transition metal complex for its robust photochemistry, is studied using simulated resonance Raman scattering (RRS) and resonance hyper-Raman scattering (RHRS) in comparison to measured surface-enhanced Raman scattering (SERS) and surface-enhanced hyper-Raman scattering (SEHRS). Detailed examination of the simulated data shows that many of the observed features in the experiments are captured by the theory. For the metal-to-ligand charge transfer (MLCT) absorption band at 452 nm, it is demonstrated that the shoulder on the absorption band at 425 nm is not a vibronic feature and that the line shape should be considered as coming from two separate MLCT states. We find that good agreement can be obtained by comparing simulated spectra to the SERS and SEHRS spectra on resonance with the absorption band. However, the simulations do not capture the high sensitivity of relative peak intensities observed during wavelength scanned SERS and SEHRS experiments. This result is interpreted on the basis of discussion of the literature and the approximations made in the vibronic model, where it is concluded that the simulations underestimate interference effects. These results demonstrate the complexity of using theoretical methods for accurately describing the electronic structure of large molecules, and that commonly used exchange-correlation functionals like B3LYP and LC-PBE cannot completely describe all of the vibronic features in the Raman scattering of Rubpy.

The surface-enhanced hyper-Raman (SEHRS) spectra of Rhodamine 6G (R6G) are measured for two different excitation energies: ∼25 000 cm–1 (∼810 nm) and ∼19 000 cm–1 (1030 nm). The collected spectra are compared to time-dependent density functional theory simulations of the resonance hyper-Raman spectra for the same excitation energies. The analysis of molecular orbital changes in these nonlinear transitions elucidates the mechanism of vibronic enhancement. This analysis is used to elucidate features in the two-photon absorption spectra of R6G.

We identify and control the photoreaction paths of self-assembled monolayers (SAMs) of thiolate-linked anthracene phenylethynyl molecules on Au substrate surfaces, and study the effects of nanoscale morphology of substrates on regioselective photoreactions. Two types of morphologies, atomically flat and curved, are produced on Au surfaces by controlling substrate structure and metal deposition. We employ surface-enhanced Raman spectroscopy (SERS), combined with Raman mode analyses using density functional theory, to identify the different photoreaction paths and to track the photoreaction kinetics and efficiencies of molecules in monolayers. The SAMs on curved surfaces exhibit dramatically lower regioselective photoreaction kinetics and efficiencies than those on atomically flat surfaces. This result is attributed to the increased intermolecular distances and variable orientations on the curved surfaces. Better understanding of the morphological effects of substrates will enable control of nanoparticle functionalization in ligand exchange in targeted delivery of therapeutics and theranostics and in catalysis.

The enhancement mechanism due to the molecule–surface chemical coupling in surface-enhanced Raman scattering (SERS) is governed to a large extent by the energy difference between the highest occupied molecular orbital (HOMO) of the metal and the lowest unoccupied molecular orbital (LUMO) of the molecule. Here, we investigate the importance of correctly describing charge-transfer excitations, using time-dependent density functional theory (TDDFT), when calculating the chemical coupling in SERS. It is well-known that TDDFT, using traditional functionals, underestimates the position of charge-transfer excitations. Here, we show that this leads to a significant overestimation of the chemical coupling mechanism in SERS. Significantly smaller enhancements are found using long-range corrected (LC) functionals as compared with a traditional generalized gradient approximation (GGA) and hybrid functionals. Enhancement factors are found to be smaller than 530 and typically less than 50. Our results show that it is essential to correctly describe charge-transfer excitations for predicting the chemical enhancement in SERS.Keywords: enhancement; long-range corrected DFT; pyridines; silver cluster; surface-enhanced Raman scattering (SERS); time-dependent density functional theory (TDDFT);

We establish the role of tether conductivity on the photoisomerization of azobenzene-functionalized molecules assembled as isolated single molecules in well-defined decanethiolate self-assembled monolayer matrices on Au{111}. We designed the molecules so as to tune the conductivity of the tethers that separate the functional moiety from the underlying Au substrate. By employing surface-enhanced Raman spectroscopy, time-course measurements of surfaces assembled with azobenzene functionalized with different tether conductivities were independently studied under constant UV light illumination. The decay constants from the analyses reveal that photoisomerization on the Au{111} surface is reduced when the conductivity of the tether is increased. Experimental results are compared with density functional theory calculations performed on single molecules attached to Au clusters.Keywords: azobenzene; nanohole array; quenching; surface plasmons; surface-enhanced Raman spectroscopy; tether conductivity;

We apply in situ surface-enhanced Raman spectroscopy (SERS) to probe the reversible photoswitching of azobenzene-functionalized molecules inserted in self-assembled monolayers that serve as controlled nanoscale environments. Nanohole arrays are fabricated in Au thin films to enable SERS measurements associated with excitation of surface plasmons. A series of SERS spectra are recorded for azobenzene upon cycling exposure to UV (365 nm) and blue (450 nm) light. Experimental spectra match theoretical calculations. On the basis of both the simulations and the experimental data analysis, SERS provides quantitative information on the reversible photoswitching of azobenzene in controlled nanoscale environments.

Experimentally measured resonance hyper-Raman (RHR) spectra spanning the S1 ← S0, S2 ← S0, and S3 ← S0 transitions in rhodamine 6G (R6G) have been recorded. These spectra are compared to the results of first-principles calculations of the RHR intensity that include both Franck–Condon (A-term) and non-Condon (B-term) scattering effects. Good agreement between the experimental and theoretical results is observed, demonstrating that first-principles calculations of hyper-Raman intensities are now possible for large molecules such as R6G. Such agreement indicates that RHR spectroscopy will now be a routine aid for probing multiphoton processes. This work further shows that optimization of molecular properties to enhance either A- or B-term scattering might yield molecules with significantly enhanced two-photon properties.

Localization of large electric fields in plasmonic nanostructures enables various processes such as single-molecule detection, higher harmonic light generation, and control of molecular fluorescence and absorption. High-throughput, simple nanofabrication techniques are essential for implementing plasmonic nanostructures with large electric fields for practical applications. In this article we demonstrate a scalable, rapid, and inexpensive fabrication method based on the salting-out quenching technique and colloidal lithography for the fabrication of two types of nanostructures with large electric field: nanodisk dimers and cusp nanostructures. Our technique relies on fabricating polystyrene doublets from single beads by controlled aggregation and later using them as soft masks to fabricate metal nanodisk dimers and nanocusp structures. Both of these structures have a well-defined geometry for the localization of large electric fields comparable to structures fabricated by conventional nanofabrication techniques. We also show that various parameters in the fabrication process can be adjusted to tune the geometry of the final structures and control their plasmonic properties. With advantages in throughput, cost, and geometric tunability, our fabrication method can be valuable in many applications that require plasmonic nanostructures with large electric fields.Keywords: colloidal doublets; colloidal lithography; large electric field; nanocusp structures; nanoparticle dimers; salting-out quenching; surface enhanced Raman scattering

The optical properties involving charge-transfer states of the donor−acceptor electron-transfer complexes carbazole/tetracyanoethylene (carbazole/TCNE) and hexamethylbenzene/tetracyanoethylene (HMB/TCNE) were investigated by utilizing the time-dependent theory of Heller to simulate absorbance and resonance Raman spectra. Excited-state properties were obtained using time-dependent density functional theory (TDDFT) using the global hybrid B3LYP and the long-range corrected LC- ωPBE functionals and compared with experimental results. It is shown that, while reasonable simulations of the absorbance spectra can be made using B3LYP, the resonance Raman spectra for both complexes are poorly described. The LC-ωPBE functional gives a more accurate representation of the excited-state potential energy surfaces in the Franck−Condon region for charge-transfer states, as indicated by the good agreement with the experimental resonance Raman spectrum. For the carbazole/TCNE complex, which includes contributions from two overlapping excited states on its absorbance spectrum, interference effects are discussed, and it is found that detuning from resonance with an excited state results in interference along with other factors. Total vibrational reorganization energy for both complexes is discussed, and it is found that both B3LYP and LC-ωPBE yield reasonable estimates of this quantity compared with experiment.

We present the absolute resonance Raman scattering (RRS) intensities of uracil, rhodamine 6G (R6G), and iron(II) porphyrin with imidazole and CO ligands (FePImCO) calculated using density functional theory (DFT). The spectra are calculated using both the vibronic theory and the short-time approximation. We find that the absolute RRS intensities calculated using the short-time approximation are severely overestimated, as compared with results obtained using the vibronic theory. This issue is attributed to the sensitivity of the absolute RRS intensities to the adjustable damping factor within the short-time approximation. This is illustrated for uracil, for which the relative intensities were predicted accurately using the short-time approximation, but the absolute intensities were still overestimated. Although intensities comparable to that obtained with the vibronic theory could be obtained using the short-time approximation, it requires a large damping factor, roughly twice that estimated from the absorption spectrum, to be used in the simulations. Furthermore, we find that DFT underestimates the absolute RRS intensities for R6G as compared to experiments, which is most likely due to the neglect of solvent effects in the calculations. For R6G and FePImCO, vibronic effects are shown to enhance the low-frequency modes relatively more, improving the agreement with experiments.

In this work we present a detailed investigation of the Raman properties of a dithienylethene photoswitch interacting with a small gold cluster (Au19+) using time-dependent density functional theory (TD-DFT). The enhancement mechanism (CHEM) due to the molecule–surface chemical coupling in surface-enhanced Raman scattering (SERS) has been characterized for this system. We demonstrate that it is possible to control the CHEM enhancement by switching the photoswitch from its closed form to its open form. The open form of the photoswitch is found to be the strongest Raman scatterer when adsorbed on the surface whereas the opposite is found for the free molecule. This trend is explained using a simple two-state approximation. In this model the CHEM enhancement scales roughly as (ωX/e)4, where ωX is the HOMO–LUMO gap of the free molecule and e is an average excitation between the HOMO of the photoswitch and the LUMO of the metal. We propose that the ability of this photoswitch to switch reversibly from open to closed will make it an excellent probe to control the CHEM enhancement of SERS.

In this work we present the first simulations of the surface-enhanced Raman optical activity (SEROA) using time-dependent density functional theory (TDDFT). A consistent treatment of both the chemical and electromagnetic enhancements is achieved by employing a recently developed approach based on a short-time approximation for the Raman and ROA cross-sections. As an initial application we study a model system consisting of adenine interacting with a Ag20 cluster. Because both the silver cluster and adenine in the absorption geometry are achiral, the chiroptical properties are due to the interactions between the two systems. Our results show that the total enhancement is on the order of 104 both for SEROA and SERS. However, the chemical enhancement is found to be larger for SEROA than for SERS. The results presented here show that SEROA can be a useful approach for studying induced chirality in small metal clusters due to the absorption of molecules.

In this work, we present a study of the excitation energies of adenine, cytosine, guanine, thymine, and the adenine-thymine (AT) and guanine-cytosine (GC) base pairs using long-range corrected (LC) density functional theory. We compare three recent LC functionals, BNL, CAM-B3LYP, and LC-PBE0, with B3LYP and coupled cluster results from the literature. We find that the best overall performance is for the BNL functional based on LDA. However, in order to achieve this good agreement, a smaller attenuation parameter is needed, which leads to nonoptimum performance for ground-state properties. B3LYP, on the other hand, severely underestimates the charge-transfer (CT) transitions in the base pairs. Surprisingly, we also find that the CAM-B3LYP functional also underestimates the CT excitation energy for the GC base pair but correctly describes the AT base pair. This illustrates the importance of retaining the full long-range exact exchange even at distances as short as that of the DNA base pairs. The worst overall performance is obtained with the LC-PBE0 functional, which overestimates the excitations for the individual bases as well as the base pairs. It is therefore crucial to strike a good balance between the amount of local and long-range exact exchange. Thus, this work highlights the difficulties in obtained LC functionals, which provides a good description of both ground- and excited-state properties.

In this work, we show using both experiments and classical electrodynamic simulations that plasmon splitting in resonant molecule-coated nanoparticles increases linearly as the square root of absorbance of the molecular layer. This linear relationship shows the same universal behavior established in analogous systems such as cavity-polariton and surface plasmon polariton systems. To explain this behavior, a simple physical mechanism based on linear dispersion and absorption is proposed. The insights obtained in this study can be used as a general principle for designing resonant molecule-coated nanoparticles for realizing tunable nanophotonic devices and molecular sensing.

In this work we have presented an atomistic electrodynamics model for describing the optical properies of silver clusters in the size range of 1−5 nm. The model consists of interacting atom-type capacitances and polarizabilities that combined describe the total response of the nanoclusters. A double Lorentzian oscillator is used to describe the frequency-dependent atomic polarizabilities, while a single Lorentzian oscillator is used to describe the frequency-dependent atomic capacitances. All atomic parameters have been optimized using reference data obtained from time-dependent density functional theory (TDDFT) calculations. As a comprehensive test of our model, we have studied the frequency-dependent polarizabilities of quasi-spherical silver nanoclusters having different structural motifs, i.e., icosahedra, truncated Ino and Marks decahedra, and regular truncated octahedra and cubocahedra. We have shown that clusters in all five structural motifs exhibit a strong absorption peak in the spectral region of 2.4−4.8 eV, although the size evolution of absorption peak location and peak width depends strongly on the number of atoms and the atomic arrangements of the clusters.

This critical review highlights recent advances in using electronic structure methods to study surface-enhanced Raman scattering. Examples showing how electronic structure methods, in particular time-dependent density functional theory, can be used to gain microscopic insights into the enhancement mechanism are presented (150 references).

In this work we have presented a capacitance-polarizability interaction model for describing the polarizability of large metal clusters. The model consists of interacting atomic capacitances and polarizabilities that are optimized to reproduce the full polarizability tensors of medium sized (N ≤ 68) gold and silver clusters. The reference polarizability tensors have been calculated using time-dependent density functional theory and shown good agreement with experimental results. We have shown that very good agreement between the model and the DFT results can be achieved both for the isotropic and anisotropic polarizability as a function of size, thus providing an accurate description of the polarizability of noble metal clusters. The model is computationally efficient and can easily handle cluster several nm in radius, thus, provides a natural bridge between the quantum mechanical methods and the macroscopic electrodynamic description. This allowed us to study the polarizability of silver and gold clusters having different shapes, i.e., spheres, rods, and disks, and sizes having diameters as large as 4.5 nm, thus, reaching the saturation of the polarizability. By partitioning the total polarizability into effective atomic polarizabilities that depend on the atomic position in the cluster, we provide a physical picture of the saturation of the polarizability of metal clusters as a function of size. This illustrates that the onset of the saturation occurs as the cluster starts to have a core of atoms showing bulk-like polarizabilities, surrounded by layers of atoms with surface-like polarizabilities. For larger clusters we see that the bulk core grows whereas the surface layers keep roughly the same thickness thus leading to a saturation of the polarizability as the size of the cluster increases.

Dihydroazulenes are photochromic molecules that reversibly switch between two distinct geometric and conductivity states. Molecular design, surface attachment, and precise control over the assembly of such molecular machines are critical in order to understand molecular function and motion at the nanoscale. Here, we use surface-enhanced Raman spectroscopy on special atomically flat, plasmonically enhanced substrates to measure the photoreaction kinetics of isolated dihydroazulene-functionalized molecules assembled on Au{111}, which undergo a ring-opening reaction upon illumination with UV light and switch back to the initial isomer via thermal relaxation. Photokinetic analyses reveal the high efficiency of the dihydroazulene photoreaction on solid substrates compared to other photoswitches. An order of magnitude decrease in the photoreaction cross section of surface-bound dihydroazulenes was observed when compared with the cross sections of these molecules in solution.

Subsystem density functional theory (subsystem DFT) is a DFT partitioning method that is exact in principle, but depends on approximations to the kinetic energy density functional (KEDF). One may avoid the use of approximate KEDFs by ensuring that the inter-subsystem molecular orbitals are orthogonal, termed external orthogonality (EO). We present a method that extends a subsystem DFT method, that includes EO, into the time-dependent DFT (TDDFT) regime. This method therefore removes the need for approximations to the kinetic energy potential and kernel, and we show that it can accurately reproduce the supermolecular TDDFT results for weakly and strongly coupled subsystems, and for systems with strongly overlapping densities (where KEDF approximations traditionally fail).

The two-photon absorption (TPA) cross-sections of small thiolate-protected gold clusters have been shown to be much larger than typical small organic molecules. In comparison with larger nanoparticles, their TPA cross-sections per gold atom are also found to be larger. Theoretical simulations have suggested that the large enhancement of these TPA cross-sections comes from a one-photon double-resonance mechanism. However, it remains difficult to simulate TPA cross-sections of thiolate-protected gold clusters due to their large system size and a high density of states. In this work, we report a time-dependent density functional theory (TDDFT) study of the TPA spectra of the Au25(SR)18− cluster based on a damped response theory formalism. Damped response theory enables a consistent treatment of on- and off-resonance molecular properties even for molecules with a high density of states, and thus is well-suited for studying the TPA properties of gold clusters. Our results indicate that the one- and two-photon double-resonance effect is much smaller than previously found, and thus is unlikely to be the main cause of the large TPA cross-sections found experimentally. The effect of symmetry breaking of the Au25(SR)18− cluster due to the ligands on the TPA cross-sections has been studied and was found to only slightly increase the cross-section. Furthermore, by comparing with larger nanoparticles we find that the TPA cross-section per gold atom scales linearly with the diameter of the particles, and that the Kerr non-linear response of the Au25(SR)18− cluster is on the same order as that of bulk gold films.

In this work we present a detailed investigation of the Raman properties of a dithienylethene photoswitch interacting with a small gold cluster (Au19+) using time-dependent density functional theory (TD-DFT). The enhancement mechanism (CHEM) due to the molecule–surface chemical coupling in surface-enhanced Raman scattering (SERS) has been characterized for this system. We demonstrate that it is possible to control the CHEM enhancement by switching the photoswitch from its closed form to its open form. The open form of the photoswitch is found to be the strongest Raman scatterer when adsorbed on the surface whereas the opposite is found for the free molecule. This trend is explained using a simple two-state approximation. In this model the CHEM enhancement scales roughly as (ωX/e)4, where ωX is the HOMO–LUMO gap of the free molecule and e is an average excitation between the HOMO of the photoswitch and the LUMO of the metal. We propose that the ability of this photoswitch to switch reversibly from open to closed will make it an excellent probe to control the CHEM enhancement of SERS.

This critical review highlights recent advances in using electronic structure methods to study surface-enhanced Raman scattering. Examples showing how electronic structure methods, in particular time-dependent density functional theory, can be used to gain microscopic insights into the enhancement mechanism are presented (150 references).