The electrical conductance, thermal conductance, thermal power and figure of merit (ZT) of semiconductor quantum dots (QDs) embedded into an insulator matrix connected with metallic electrodes are theoretically investigated in the Coulomb blockade regime. The multilevel Anderson model is used to simulate the multiple QDs junction system. The charge and heat currents in the sequential tunneling process are calculated by the Keldysh Green function technique. In the linear response regime the ZT values are still very impressive in the small tunneling rates case, although the effect of electron Coulomb interaction on ZT is significant. In the nonlinear response regime, we have demonstrated that the thermal rectification behavior can be observed for the coupled QDs system, where the very strong asymmetrical coupling between the dots and electrodes , large energy level separation between dots and strong interdot Coulomb interactions are required.

Two-dimensional metallic broadband absorbers on a SiO(2)/Ag/Si substrate were experimentally studied. The absorptivity of such structure can be increased by tailoring the ratio of disk size to the unit cell area. The metallic disk exhibits a localized surface plasmon polariton (LSPP) mode for both TE and TM polarizations. A broadband thermal emitter can be realized because the LSPP mode is independent of the periodicities. By manipulating the ratios and disk sizes, a high-performance, wide-angle, polarization-independent dual band absorber was experimentally achieved. The results demonstrated a substantial flexibility in absorber designs for applications in thermal photovoltaics, sensors, and camouflage.

Excitons and trions (or exciton-polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violate the conservation of momentum. Here we reveal the intervalley recombination of dark excitons and trions through more than one path in monolayer WSe2. We observe the intervalley dark excitons, which can recombine by the assistance of defect scattering or chiral-phonon emission. We also reveal that a trion can decay in two distinct pathsthrough intravalley or intervalley electron-hole recombination -into two different final valley states. Although these two paths are energy degenerate, we can distinguish them by lifting the valley degeneracy under a magnetic field. In addition, the intra-and intervalley trion transitions are coupled to zone-center and zone-corner chiral phonons, respectively, to produce distinct phonon replicas. The observed multipath optical decays of dark excitons and trions provide much insight into the internal quantum structure of trions and the complex excitonic interactions with defects and chiral phonons in monolayer valley semiconductors.

Global-warming issues coupled with high oil prices have become a major driving force for the use of advanced solar power technology, where a key component lies in the development of high-efficiency and low-cost photovoltaic cells. Next generation photovoltaics, hence, demand an efficiency-boosting mechanism in order to render solar energy cost competitive with conventional sources of electricity.[1] Fundamentally, the conversion efficiency of a solar cell depends on the photon absorption, carrier separation, and carrier collection. [2,3] Therefore, an effective antireflection (AR) coating, minimized recombination loss, and good Ohmic contacts are particularly important. Metal grids that inevitably block the transmission of solar energy also require optimization in order to reduce the series resistance. The trade-off between the electrode and the AR coating areas is one of the efficiency-limiting factors in a conventional solar cell.The conventional AR coating is usually composed of a quarter wavelength stack of dielectrics with different refractive indices. Broad angular and spectral AR is achievable at the price of multiple layers.[ [4][5][6][7] Over the past few years, versatile subwavelength structures (SWS) have emerged as promising candidates for AR coatings, due to the characteristics of zero-order gratings, or the so-called moth-eye effects. [8][9][10][11][12][13][14] However, the fabrication costs, which involve either electron-beam (e-beam) lithography or various etching processes, can be significant. In addition, the resulting surface-recombination loss due to dry or wet etching could further hinder the applications of SWS in commercial solar cells. Recently, multiple studies have been carried out on indium tin oxide (ITO), titanium dioxide (TiO 2 ), and silicon dioxide (SiO 2 ) nanostructures employing oblique-angle deposition methods, [15][16][17] where the refractive indices of the nanoporous materials can be engineered by adjusting the air volume ratio. Still, the materials require multiple layers to effectively suppress the Fresnel reflection.In this paper, we demonstrate a practical photovoltaic application of ITO nanocolumns serving as a conductive AR layer for GaAs solar cells. As in standard GaAs cells, the use of a nanostructured AR layer could be otherwise limited due to severe front-surface recombination. The characteristic ITO nanocolumns, prepared by glancing-angle deposition with an incident nitrogen flux, offer omnidirectional and broad-band AR properties for both s-and p-polarizations, up to an incidence angle of 708 for the 350-900 nm wavelength range. Calculations based on a rigorous coupled-wave analysis (RCWA) method indicate that the superior AR characteristics arise from the tapered column profiles, which collectively function as a graded-refractive-index layer. The conversion efficiency of the GaAs solar cell with the nanocolumn AR layer increases by 28% compared to a cell without any AR treatment. Moreover, nearly 42% enhancement is achieved for photocurrents generated at wavele...

The analytical expressions for the van der Waals potential energy and force between two crossed carbon nanotubes are presented. The Lennard-Jones potential for two carbon atoms and the method of the smeared out approximation suggested by L.A. Girifalco were used. The exact formula is expressed in terms of rational and elliptical functions. The potential and force for carbon nanotubes were calculated. The uniform potential curves for single-and multi-wall nanotubes were plotted. The equilibrium distance, maximal attractive force, and potential energy have been estimated.

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We develop a scheme for building the scalar exchange-correlation ͑XC͒ kernel of time-dependent density functional theory ͑TDDFT͒ from the tensorial kernel of time-dependent current density functional theory ͑TDCDFT͒ and the Kohn-Sham current density response function. Resorting to the local approximation to the kernel of TDCDFT results in a nonlocal approximation to the kernel of TDDFT, which is free of the contradictions that plague the standard local density approximation ͑LDA͒ to TDDFT. As an application of this general scheme, we calculate the dynamical XC contribution to the stopping power of electron liquids for slow ions to find that our results are in considerably better agreement with experiment than those obtained using TDDFT in the conventional LDA.

Monolayer WSe 2 hosts a series of exciton Rydberg states denoted by the principal quantum number n = 1, 2, 3, etc. While most research focuses on their absorption properties, their optical emission is also important but much less studied.Here we measure the photoluminescence from the 1s -5s exciton Rydberg states in ultraclean monolayer WSe 2 encapsulated by boron nitride under magnetic fields from -31 T to 31 T. The exciton Rydberg states exhibit similar Zeeman shifts but distinct diamagnetic shifts from each other. From their luminescence spectra, Zeeman and diamagnetic shifts, we deduce the binding energies, g-factors and radii of the 1s -4s exciton states. Our results are consistent with theoretical predictions and results from prior magneto-absorption experiments.