Quantum dynamical simulations of local field enhancement in metal nanoparticles

Interfacial Charge Transfer in Photoexcited QD-Molecule Composite of Tetrahedral CdSe Quantum Dot Coupled with Carbazole

Photoexcited charge transfer dynamics in CdSe quantum dots (QDs) coupled with carbazole were explored to model QD-molecule systems for light-harvesting applications. The absorption spectra of QDs with different sizes, i.e., Cd35Se20X30L30 (T1), Cd56Se35X42L42 (T2), and Cd84Se56X56L56 (T3) were simulated with quantum dynamical methods, which qualitatively match the reported experimental spectra. The carbazole is attached with a 3-amino group at the apex position of T1 (namely T1-3A-Cz), establishing proper electronic communication between T1 and carbazole. The spectra of T1-3A-Cz is 0.22 eV red-shifted compared to T1. A time-dependent perturbation was applied in tune with the lowest energy peak (3.63 eV) of T1-3A-Cz to investigate the charge transfer dynamics, which revealed an ultrafast charge separation within the femtosecond time scale. The electronic structure showed a favorable energy alignment between T1 and carbazole in T1-3A-Cz. The LUMO of carbazole was situated below the conduction band of the QD, while the HOMO of carbazole mixed perfectly with the top of the valence band of the QD, developing the interfacial charge transfer states. These states promoted the photoexcited electron transfer directly from the CdSe core to carbazole. A rapid and enhanced charge separation occurred with the laser field strength increasing from 0.001 to 0.005 V/Å. However, T1 connected to the other positions of carbazole did not show charge separation effectively. The photoinduced charge transfer is negligible in the case of T2-carbazole systems due to poor electronic coupling, and it is not observed in T3-carbazole systems. So, the T1-3A-Cz model acts as a perfect donor-acceptor QD-molecule nanocomposite that can harvest photon energy efficiently. Further enhancement of charge transfer can be achieved by coupling more carbazoles to the T1 QD (e.g., T1-3A-Cz2) due to the extension of hole delocalization between T1 and the carbazoles.

First-Principles Simulations of Tip Enhanced Raman Scattering Reveal Active Role of Substrate on High-Resolution Images

Tip-enhanced Raman scattering (TERS) has emerged as a powerful tool to obtain subnanometer spatial resolution fingerprints of atomic motion. Theoretical calculations that can simulate the Raman scattering process and provide an unambiguous interpretation of TERS images often rely on crude approximations of the local electric field. In this work, we present a novel and first-principles-based method to compute TERS images by combining Time Dependent Density Functional Theory (TD-DFT) and Density Functional Perturbation Theory (DFPT) to calculate Raman cross sections with realistic local fields. We present TERS results on free-standing benzene and C60 molecules, and on the TCNE molecule adsorbed on Ag(100). We demonstrate that chemical effects on chemisorbed molecules, often ignored in TERS simulations of larger systems, dramatically change the TERS images. This observation calls for the inclusion of chemical effects for predictive theory-experiment comparisons and an understanding of molecular motion at the nanoscale.

Tunable Size Dependence of Quantum Plasmon of Charged Gold Nanoparticles

The quantum behavior of surface plasmons has received extensive attention, benefiting from the development of exquisite nanotechnology and the diverse applications. Blueshift, redshift, and nonshift of localized surface plasmon resonances (LSPRs) have all been reported as the particle size decreases and enters the quantum size regime, but the underlying physical mechanism to induce these controversial size dependences is not clear. Herein, we propose an improved semiclassical model for modifying the dielectric function of metal nanospheres by combining the intrinsic quantized electron transitions and surface electron injection or extraction to investigate the plasmon shift and LSPR size dependence of the charged Au nanoparticles. We experimentally observe that the nonmonotonic blueshift of LSPRs with size for Au nanoparticles is turned into an approximately monotonic blueshift by increasing the electron donor concentration in the reduction solution, and it can also be transformed to an approximately monotonic redshift after surface passivation by ligand molecules. Moreover, we demonstrate controlled blueshift and redshift for the electron and hole plasmons in Cu_{2-x}S@Au core-shell nanoparticles by injecting electrons. The experimental observations and the theoretical calculations clarify the controversial size dependences of LSPR reported in the literature, reveal the critical role of surface electron injection or extraction in the transformation between the different size dependences of LSPRs, and are helpful for understanding the nature of surface plasmons in the quantum size regime.

Density-functional tight-binding: basic concepts and applications to molecules and clusters

The scope of this article is to present an overview of the Density Functional based Tight Binding (DFTB) method and its applications. The paper introduces the basics of DFTB and its standard formulation up to second order. It also addresses methodological developments such as third order expansion, inclusion of non-covalent interactions, schemes to solve the self-interaction error, implementation of long-range short-range separation, treatment of excited states via the time-dependent DFTB scheme, inclusion of DFTB in hybrid high-level/low level schemes (DFT/DFTB or DFTB/MM), fragment decomposition of large systems, large scale potential energy landscape exploration with molecular dynamics in ground or excited states, non-adiabatic dynamics. A number of applications are reviewed, focusing on -(i)- the variety of systems that have been studied such as small molecules, large molecules and biomolecules, bare orfunctionalized clusters, supported or embedded systems, and -(ii)- properties and processes, such as vibrational spectroscopy, collisions, fragmentation, thermodynamics or non-adiabatic dynamics. Finally outlines and perspectives are given.

Trap-Door-Like Irreversible Photoinduced Charge Transfer in a Donor-Acceptor Complex

For efficient conversion of light into useful energy sources, it is very important to study and describe the first steps of primary charge-transfer process in natural structures and artificial devices. The time scale of these processes in artificial photosynthetic and photovoltaic devices is on the order of femto- to picoseconds and involves vibronic coupling of electrons and nuclei and also nuclear alleviation to enhance charge separation. Here we present an atomistic description of the photoexcited electron dynamics in a noncovalently bonded system formed by an hydrogenated nanodiamond as donor and a perylene diimide as an acceptor. The complex shows extremely fast charge transfer, separation, and stabilization within 90 fs. This stabilization is purely electronic in nature. To the best of our knowledge, these results show for the first time that it is possible to stabilize charge without polaron formation or nuclear relaxation, reaching a steady state enhanced by a pure electronic reorganization.

Atomic-Scale Lightning Rod Effect in Plasmonic Picocavities: A Classical View to a Quantum Effect

Plasmonic gaps are known to produce nanoscale localization and enhancement of optical fields, providing small effective mode volumes of about a few hundred nm³. Atomistic quantum calculations based on time-dependent density functional theory reveal the effect of subnanometric localization of electromagnetic fields due to the presence of atomic-scale features at the interfaces of plasmonic gaps. Using a classical model, we explain this as a nonresonant lightning rod effect at the atomic scale that produces an extra enhancement over that of the plasmonic background. The near-field distribution of atomic-scale hot spots around atomic features is robust against dynamical screening and spill-out effects and follows the potential landscape determined by the electron density around the atomic sites. A detailed comparison of the field distribution around atomic hot spots from full quantum atomistic calculations and from the local classical approach considering the geometrical profile of the atoms' electronic density validates the use of a classical framework to determine the effective mode volume in these extreme subnanometric optical cavities. This finding is of practical importance for the community of surface-enhanced molecular spectroscopy and quantum nanophotonics, as it provides an adequate description of the local electromagnetic fields around atomic-scale features with use of simplified classical methods.

Plasmon-Induced Ultrafast Hydrogen Production in Liquid Water

Hydrogen gas production from solar water splitting provides a renewable energy cycle to address the grand global energy challenge; however, its dynamics and fundamental mechanism remain elusive. We directly explore by first-principles the ultrafast electron-nuclear quantum dynamics on the time scale of ∼100 fs during water photosplitting on a plasmonic cluster embedded in liquid water. Water molecule splitting is assisted by rapid proton transport in liquid water in a Grotthuss-like mechanism. We identify that a plasmon-induced field enhancement effect dominates water splitting, while charge transfer from gold to the antibonding orbital of a water molecule also plays an important role. "Chain-reaction" like rapid H₂ production is observed via the combination of two hydrogen atoms from different water molecules. These results provide a route toward a complete understanding of water photosplitting in the ultimate time and spatial limit.

Multiscale Study of Plasmonic Scattering and Light Trapping Effect in Silicon Nanowire Array Solar Cells

Nanometallic structures that support surface plasmons provide new ways to confine light at deep-subwavelength scales. The effect of light scattering in nanowire array solar cells is studied by a multiscale approach combining classical electromagnetic (EM) and quantum mechanical simulations. A photovoltaic device is constructed by integrating a silicon nanowire array with a plasmonic silver nanosphere. The light scatterings by plasmonic element and nanowire array are obtained via classical EM simulations, while current-voltage characteristics and optical properties of the nanowire cells are evaluated quantum mechanically. We found that the power conversion efficiency (PCE) of photovoltaic device is substantially improved due to the local field enhancement of the plasmonic effect and light trapping by the nanowire array. In addition, we showed that there exists an optimal nanowire number density in terms of optical confinement and solar cell PCE.

Plasmonic Nanogap-Enhanced Raman Scattering with Nanoparticles

Plasmonic coupling-based electromagnetic field localization and enhancement are becoming increasingly important in chemistry, nanoscience, materials science, physics, and engineering over the past decade, generating a number of new concepts and applications. Among the plasmonically coupled nanostructures, metal nanostructures with nanogaps have been of special interest due to their ultrastrong electromagnetic fields and controllable optical properties that can be useful for a variety of signal enhancements such as surface-enhanced Raman scattering (SERS). The Raman scattering process is highly inefficient, with a very small cross-section, and Raman signals are often poorly reproducible, meaning that very strong, controllable SERS is needed to obtain reliable Raman signals with metallic nanostructures and thus open up new avenues for a variety of Raman-based applications. More specifically, plasmonically coupled metallic nanostructures with ultrasmall (∼1 nm or smaller) nanogaps can generate very strong and tunable electromagnetic fields that can generate strong SERS signals from Raman dyes in the gap, and plasmonic nanogap-enhanced Raman scattering can be defined as Raman signal enhancement from plasmonic nanogap particles with ∼1 nm gaps. However, these promising nanostructures with extraordinarily strong optical signals have shown limited use for practical applications, largely due to the lack of design principles, high-yield synthetic strategies with nanometer-level structural control and reproducibility, and systematic, reliable single-molecule/single-particle-level studies on their optical properties. All these are extremely important challenges because even small changes (<1 nm) in the structure of the coupled plasmonic nanogaps can significantly affect the plasmon mode and signal intensity. In this Account, we examine and summarize recent breakthroughs and advances in plasmonic nanogap-enhanced Raman scattering with metal nanogap particles with respect to the design and synthesis of plasmonic nanogap structures, as well as ultrasensitive and quantitative Raman signal detection using these structures. The applications and prospects of plasmonic nanogap particle-based SERS are also discussed. In particular, reliable synthetic and measurement strategies for plasmonically coupled nanostructures with ∼1 nm gap, in which both the nanogap size and the position of a Raman-active molecule in the gap can be controlled with nanometer/sub-nanometer-level precision, can address important issues regarding the synthesis and optical properties of plasmonic nanostructures, including structural and signal reproducibility. Further, single-molecule/single-particle-level studies on the plasmonic properties of these nanogap structures revealed that these particles can generate ultrastrong, quantifiable Raman signals in a highly reproducible manner.