Charge Induced Chi(3) Susceptibility in Interfacial Nonlinear Optical Spectroscopy Beyond the Bulk Aqueous Contributions: The Case for Silica/Water Interface

The electric field induced (EFI) bulk Chi(3) contribution to the second harmonic generation (SHG) signal from charged interfaces was discovered and applied to study the interfacial chemistry of various charged interfaces three decades ago. For both the buried fused silica/water interface and the exposed charged monolayer covered air/water interface, such bulk Chi(3) contribution was all attributed to the Chi(3) term of the polarized water molecules near the charged interfaces. The puzzling experimental observation of the more than one-order of magnitude difference of the EFISHG intensity between the fully charged silica/water interface and the charged molecular covered air/water interface was generally overlooked in the EFISHG literature. Nevertheless, this significant signal difference suggests additional source for the Chi(3) contribution at the fully charged silica/water interface other than the polarized water molecules as in the case of charged monolayer covered air/water interface. In this report, we re-examine the treatment of the Chi(3) mechanism at the charged silica/water interface by including the contributions from the bulk silica using proper boundary condition and image charge distributions for the change screening effects inside bulk silica phase. We show that the Chi(3) contribution from the bulk silica is in similar form as that of the aqueous bulk phase, and it is with more than one-order of magnitude and with opposite sign. The treatment reported here can be extended to other charged interfaces.

Magnetoelectric boundary simulated by a Chern–Simons-like model

In this work we study some physical phenomena that emerge in the vicinity of a magnetoelectric boundary. For simplicity, we restrict to the case of a planar boundary described by a coupling between the gauge field with a planar external Chern–Simons-like potential. The results are obtained exactly. We compute the correction undergone by the photon propagator due to the presence of the Chern–Simons coupling and we investigate the interaction between a stationary point-like charge and the magnetoelectric boundary. In the limit of a perfect mirror, where the coupling constant between the field and the potential diverges, we recover the image method. For a non perfect mirror, we show that we have an attenuated image charge and, in addition, an image magnetic monopole whose field strength does not exhibit the presence of the undesirable and artificial divergences introduced by Dirac strings. We also study the interaction between the plate and a quantum particle with spin. In this case we have a kind of charge-magnetic dipole interaction due to the magnetoelectric properties of the plate.

Trapping of Electrons around Nanoscale Metallic Wires Embedded in a Semiconductor Medium

We predict that conduction electrons in a semiconductor film containing a centered square array of metal nanowires normal to its plane are bound in quantum states around the central wires, if a positive bias voltage is applied between the wires at the square vertices and the latter. We obtain and discuss the eigenenergies and eigenfunctions of two models with different dimensions. The results show that the eigenstates can be grouped into different shells. The energy differences between the shells is typically a few tens of meV, which corresponds to frequencies of emitted or absorbed photons in a range of 3THz to 20THz approximately. These energy differences strongly depend on the bias voltage. We calculate the linear response of individual electrons on the ground level of our models to large-wavelength electromagnetic waves whose electric field is in the plane of the semiconductor film. The computed oscillator strengths are dominated by the transitions to the states in each shell whose wave function has a single radial node line normal to the wave electric field. We include the effect of the image charge induced on the central metal wires and show that it modifies the oscillator strengths so that their sum deviates from the value given by the Thomas-Reiche-Kuhn rule. We report the linear response, or polarizability, versus photon energy, of the studied models and their absorption spectra. The latter show well-defined peaks as expected from the study of the oscillator strengths. We show that the position of these absorption peaks is strongly dependent on the bias voltage so that the frequency of photon absorption or emission in the systems described here is easily tunable. This makes them good candidates for the development of novel infrared devices.

Effect of Surface Roughness on Electrochemical Adsorption/Desorption of Dopamine by Carbonaceous Electrodes

Electrochemical adsorption/desorption of dopamine by carbonaceous electrodes upon voltage variation is the key process of neurotransmitter detection through fast scan cyclic voltammetry. In the present study, <i>ab initio</i> molecular dynamics simulation empowered by image-charge method was applied to calculate the adsorption/desorption free energy profile of dopamine and dopamine o-quinone at fixed electrode potentials using our newly developed open-source CP2K simulation package. It was found that the activation barriers for both adsorption and desorption were substantially reduced with increasing surface roughness of the carbonaceous electrodes. For example, on the flat graphene electrode, the activation barrier for dopamine adsorption at V₀=−0.4V is 1.34 kcal/mol, while its counterpart on the curved nanotube electrode drops to 0.82 kcal/mol. Moreover, the diffusion coefficient of dopamine decreases by approximately 60% when it is moving close to the graphene electrode, while its diffusion is accelerated by up to 100% when the nanotube electrode is adopted. The faster diffusion alongside the reduced activation barrier greatly facilitates the electrochemically driven adsorption/desorption of dopamine by nanotube electrodes, in consistent with experimental findings that a rougher carbonaceous surface is critical for fast scan cyclic voltammetry.

Effect of Surface Roughness on Electrochemical Adsorption/Desorption of Dopamine by Carbonaceous Electrodes

Electrochemical adsorption/desorption of dopamine by carbonaceous electrodes upon voltage variation is the key process of neurotransmitter detection through fast scan cyclic voltammetry. In the present study, <i>ab initio</i> molecular dynamics simulation empowered by image-charge method was applied to calculate the adsorption/desorption free energy profile of dopamine and dopamine o-quinone at fixed electrode potentials using our newly developed open-source CP2K simulation package. It was found that the activation barriers for both adsorption and desorption were substantially reduced with increasing surface roughness of the carbonaceous electrodes. For example, on the flat graphene electrode, the activation barrier for dopamine adsorption at V₀=−0.4V is 1.34 kcal/mol, while its counterpart on the curved nanotube electrode drops to 0.82 kcal/mol. Moreover, the diffusion coefficient of dopamine decreases by approximately 60% when it is moving close to the graphene electrode, while its diffusion is accelerated by up to 100% when the nanotube electrode is adopted. The faster diffusion alongside the reduced activation barrier greatly facilitates the electrochemically driven adsorption/desorption of dopamine by nanotube electrodes, in consistent with experimental findings that a rougher carbonaceous surface is critical for fast scan cyclic voltammetry.