Hot Adsorbate-Induced Retardation of the Internal Thermalization of Nonequilibrium Electrons in Adsorbate-Covered Metal Nanoparticles

Molecular dynamics study of plasmon-mediated chemical transformations

Heterogeneous catalysis of adsorbates on metallic surfaces mediated by plasmons has potential high photoelectric conversion efficiency and controllable reaction selectivity. Theoretical modeling of dynamical reaction processes enables in-depth analyses complementing experimental investigations. Especially for plasmon-mediated chemical transformations, light absorption, photoelectric conversion, electron-electron scattering, and electron-phonon coupling occur simultaneously on different timescales, making it very challenging to delineate the complex interplay of different factors. In this work, a trajectory surface hopping non-adiabatic molecular dynamics method is used to investigate the dynamics of plasmon excitation in an Au₂₀-CO system, including hot carrier generation, plasmon energy relaxation, and CO activation induced by electron-vibration coupling. The electronic properties indicate that when Au₂₀-CO is excited, a partial charge transfer takes place from Au₂₀ to CO. On the other hand, dynamical simulations show that hot carriers generated after plasmon excitation transfer back and forth between Au₂₀ and CO. Meanwhile, the C-O stretching mode is activated due to non-adiabatic couplings. The efficiency of plasmon-mediated transformations (∼40%) is obtained based on the ensemble average of these quantities. Our simulations provide important dynamical and atomistic insights into plasmon-mediated chemical transformations from the perspective of non-adiabatic simulations.

Chemical Interface Damping in Nonstoichiometric Semiconductor Plasmonic Nanocrystals: An Effect of the Surrounding Environment

Semiconductor plasmonic nanocrystals (NCs) have been utilized for an enormous number of plasmon-enhanced spectroscopic and energy conversion applications. Plasmonic NCs are extremely high light absorbers, and optical properties can be easily manipulated across the UV-vis-NIR spectrum region by changing mere chemical compositions and the surrounding environment of the NCs. This feature article focuses on reassessing plasmon dynamics by changing the interface composition between NCs and the surrounding medium to ascertain the damping contribution from chemical interface damping (CID). Also, this feature article deciphers a fundamental understanding of hot-carrier relaxation and extraction from plasmonic materials. On the route to determining the different relaxation dynamics of nonstoichiometric Cu_2-xS/Se NCs, we have employed a transient ultrafast pump-probe broadband spectrometer. First, we have described the ultrafast plasmon relaxation dynamics of nonstoichiometric Cu_2-xS NCs by varying the copper to sulfur ratio, and then we carefully compare how two surface ligands (oleylamine and 3-mercaptopropionic acid) lead to significantly different transient kinetics of the same plasmonic (Cu_2-xSe) NCs because of different capping agents. Along with this, we have described the impact of a molecular adsorbate (methylene blue) on ultrafast plasmon relaxation dynamics of the nonstoichiometric Cu_2-xSe NCs system. Finally, the chemical interface damping effect has been compared in the Cu_2-xS NCs system after capping with two distinct capping ligands: oleylamine and oleic acid. For the proof of concept, plasmonic thin-film devices were fabricated and exhibited higher conductivity/photoconductivity performance in oleic acid-capped NCs because of a deprotonated carboxyl functional group. We have also introduced a model and mechanism of chemical interface damping in a nonstoichiometric plasmonic semiconductor (Cu_2-xS/Se) NC system. This feature article highlights the importance of the surface functionalization of nonstoichiometric plasmonic semiconductors to develop new advanced semiconductor-based devices such as infrared photodetectors, plasmonic solar cells, and efficient NIR phototransistors.

Experimental characterization techniques for plasmon-assisted chemistry

Plasmon-assisted chemistry is the result of a complex interplay between electromagnetic near fields, heat and charge transfer on the nanoscale. The disentanglement of their roles is non-trivial. Therefore, a thorough knowledge of the chemical, structural and spectral properties of the plasmonic/molecular system being used is required. Specific techniques are needed to fully characterize optical near fields, temperature and hot carriers with spatial, energetic and/or temporal resolution. The timescales for all relevant physical and chemical processes can range from a few femtoseconds to milliseconds, which necessitates the use of time-resolved techniques for monitoring the underlying dynamics. In this Review, we focus on experimental techniques to tackle these challenges. We further outline the difficulties when going from the ensemble level to single-particle measurements. Finally, a thorough understanding of plasmon-assisted chemistry also requires a substantial joint experimental and theoretical effort.

Prospects of Coupled Organic-Inorganic Nanostructures for Charge and Energy Transfer Applications

We review the field of organic-inorganic nanocomposites with a focus on materials that exhibit a significant degree of electronic coupling across the hybrid interface. These nanocomposites undergo a variety of charge and energy transfer processes, enabling optoelectronic applications in devices which exploit singlet fission, triplet energy harvesting, photon upconversion or hot charge carrier transfer. We discuss the physical chemistry of the most common organic and inorganic components. Based on those we derive synthesis and assembly strategies and design criteria on material and device level with a focus on photovoltaics, spin memories or optical upconverters. We conclude that future research in the field should be directed towards an improved understanding of the binding motif and molecular orientation at the hybrid interface.

Plasmonic hot electrons for sensing, photodetection, and solar energy applications: A perspective

In plasmonic metals, surface plasmon resonance decays and generates hot electrons and hot holes through non-radiative Landau damping. These hot carriers are highly energetic, which can be modulated by the plasmonic material, size, shape, and surrounding dielectric medium. A plasmonic metal nanostructure, which can absorb incident light in an extended spectral range and transfer the absorbed light energy to adjacent molecules or semiconductors, functions as a "plasmonic photosensitizer." This article deals with the generation, emission, transfer, and energetics of plasmonic hot carriers. It also describes the mechanisms of hot electron transfer from the plasmonic metal to the surface adsorbates or to the adjacent semiconductors. In addition, this article highlights the applications of plasmonic hot electrons in photodetectors, photocatalysts, photoelectrochemical cells, photovoltaics, biosensors, and chemical sensors. It discusses the applications and the design principles of plasmonic materials and devices.

Femtosecond Time-Resolved Dynamics of trans-Azobenzene on Gold Nanoparticles

We report a first femtosecond time-resolved transient absorption study of the photoinduced ultrafast dynamics of trans-azobenzene (AB) on gold nanoparticles (AuNPs). The observed changes in optical density following excitation at λ = 357 nm were analyzed by using temperature-dependent Mie theory and by Lorentzian band fitting to disentangle the ultrafast relaxation of the local surface plasmon resonance (LSPR) excitation of the Au core and the electronic deactivation of the attached AB ligands. The analysis of the dynamics associated with the AB photochrome yielded lifetime constants of τ1 = 1.2 ± 0.2 ps and τ2 = 4.7 ± 1.1 ps. Both values together indicate surprisingly little difference in the dynamics of the AB ligand on the AuNPs vs in solution. Our results thus highlight the extraordinarily efficient electronic decoupling of the azo chromophore and the Au core by the alkyl linker chain.

Modulation of the electron transfer processes in Au-ZnO nanostructures

Plasmonic nanostructures comprising Au and ZnO nanoparticles synthesized by the spontaneous reduction of HAuCl4 in ethylene glycol were used to assess the possibility of modulating the direction of the electron transfer processes at the interface. One electron UV reduction and visible oxidation of the reversible couple TEMPOL/TEMPOL-H was confirmed by EPR spectroscopy. The apparent quantum yield for TEMPOL-H conversion under continuous wave visible excitation depends on the irradiation wavelength, being 0.57% and 0.27% at 450 ± 12 and 530 ± 12 nm, respectively. These results indicate that both the surface plasmon resonance and the interband transition from the 5d to the 6s level of Au nanoparticles contribute to the visible activity of the nanostructure. In addition, by detecting free electron conduction band electrons in ZnO, after the visible excitation of Au/ZnO nanostructures, we provide direct evidence of the photoexcited electron transfer from gold nanoparticles to ZnO.

Identification of parameters through which surface chemistry determines the lifetimes of hot electrons in small Au nanoparticles

This paper describes measurements of the dynamics of hot electron cooling in photoexcited gold nanoparticles (Au NPs) with diameters of ∼3.5 nm, and passivated with either a hexadecylamine or hexadecanethiolate adlayer, using ultrafast transient absorption spectroscopy. Fits of these dynamics with temperature-dependent Mie theory reveal that both the electronic heat capacity and the electron-phonon coupling constant are larger for the thiolated NPs than for the aminated NPs, by 40% and 30%, respectively. Density functional theory calculations on ligand-functionalized Au slabs show that the increase in these quantities is due to an increased electronic density of states near the Fermi level upon ligand exchange from amines to thiolates. The lifetime of hot electrons, which have thermalized from the initial plasmon excitation, increases with increasing electronic heat capacity, but decreases with increasing electron-phonon coupling, so the effects of changing surface chemistry on these two quantities partially cancel to yield a hot electron lifetime of thiolated NPs that is only 20% longer than that of aminated NPs. This analysis also reveals that incorporation of a temperature-dependent electron-phonon coupling constant is necessary to adequately fit the dynamics of electron cooling.

Photoinduced thermal copper reduction onto gold nanocrystals under potentiostatic control

We study the photoreduction of adsorbed copper ions onto Au nanoparticles, on an indium tin oxide (ITO) electrode in an aqueous electrochemical cell, as a function of applied voltage and laser intensity. The photocurrent is a nonlinear function of laser intensity and increases sharply with cathodic voltage in the underpotential deposition region. The photoreduction is attributed to laser heating of the Au nanoparticles rather than "hot electron" processes. Numerical simulation of the Butler-Volmer kinetic equation using experimental parameters predicts a several orders of magnitude increase in current for a temperature rise of a few Kelvin.