optoelectronics. [ 1,2 ] Advantages including reduced materials consumption, relaxed requirements of materials purity, and ability to form large-area devices on unlimited classes of module substrates make them particularly useful as building blocks for realizing high-effi ciency, low-cost photovoltaic systems. [3][4][5] The photovoltaic performance of ultrathin silicon solar cell is, however, inherently limited by incomplete absorption of longer wavelength photons near its bandgap. [ 6,7 ] While light trapping methods based on various diffractive and/ or refl ective optical elements can greatly help to improve the absorption of optically thin silicon, [8][9][10][11][12][13] complementary means to additionally capitalize on such low energy photons are desirable to further improve the performance of ultrathin silicon solar cells. In particular, spectral upconversion, a concept proposed for addressing the sub-bandgap transparency of solar cells, [14][15][16][17][18][19][20][21] is an attractive approach that provides an additional pathway to enhance the quantum effi ciency of above-bandgap longer wavelength photons by converting them into high energy photons that can be more strongly absorbed by the ultrathin silicon. One of key challenges for the practical application of spectral upconversion in photovoltaics (PVs), however, is that the intensity of natural sunlight in relevant wavelengths (i.e., near-infrared) is often too weak to yield meaningful effects of upconversion. [22][23][24] In this regard, recent advances in light manipulation using metallic nanostructures, A type of composite photovoltaic system that can improve the absorption of longer wavelength photons for ultrathin silicon solar cells is presented by synergistically exploiting spectral upconversion and plasmonic light manipulation under a reconfi gurable platform where individual module components can be independently optimized and strategically combined by printing-based deterministic materials assemblies. The ultrathin (≈8 µm) nanostructured silicon solar cells are embedded in a thin polymeric medium containingNaYF 4 :Yb 3+ ,Er 3+ nanocrystals, coated on a plasmonically engineered substrate that incorporates hybrid nanostructures of cylindrical nanoholes and truncated-cone-shaped nanoposts. Both excitation and emission processes of upconversion luminophores are signifi cantly enhanced by combined effects of surface plasmon resonance to amplify the light intensity at the excitation wavelength as well as to facilitate the far-fi eld outcoupling at the emission wavelengths, respectively. The performance of the integrated solar module is improved by ≈13% compared to devices on a nanostructured plasmonic substrate without luminophores due to collective contributions from plasmonically enhanced spectral upconversion, together with effects of waveguiding and fl uorescence of NaYF 4 :Yb 3+ ,Er 3+ . Detailed studies on optical properties of engineered plasmonic nanostructures and device performance in both experiments and numerical modeling provide quanti...

Understanding the relaxation and injection dynamics of hot electrons is crucial to utilizing them in photocatalytic applications. While most studies have focused on hot carrier dynamics at metal/semiconductor interfaces, we study the in situ dynamics of direct hot electron injection from metal to adsorbates. Here, we report a hot electron-driven hydrogen evolution reaction (HER) by exciting the localized surface plasmon resonance (LSPR) in Au grating photoelectrodes. In situ ultrafast transient absorption (TA) measurements show a depletion peak resulting from hot electrons. When the sample is immersed in solution under −1 V applied potential, the extracted electron−phonon interaction time decreases from 0.94 to 0.67 ps because of additional energy dissipation channels. The LSPR TA signal is redshifted with delay time because of charge transfer and subsequent change in the dielectric constant of nearby solution. Plateau-like photocurrent peaks appear when exciting a 266 nm linewidth grating with p-polarized (on resonance) light, accompanied by a similar profile in the measured absorptance. Double peaks in the photocurrent measurement are observed when irradiating a 300 nm linewidth grating. The enhancement factor (i.e., reaction rate) is 15.6× between p-polarized and s-polarized light for the 300 nm linewidth grating and 4.4× for the 266 nm linewidth grating. Finite-difference time domain (FDTD) simulations show two resonant modes for both grating structures, corresponding to dipolar LSPR modes at the metal/fused silica and metal/water interfaces. To our knowledge, this is the first work in which LSPR-induced hot electron-driven photochemistry and in situ photoexcited carrier dynamics are studied on the same plasmon resonance structure with and without adsorbates.

By discharging nanosecond high-voltage (5 kV) pulses across an insulating substrate containing Au, Pt, or Cu nanoparticles, a 3 order of magnitude (1000×) enhancement in the generation of plasma can be achieved through local field enhancement on the surface of the nanoparticles. The lowtemperature nature of this transient plasma is crucial to maintaining the structural integrity of these delicate nanoparticles. These nanoparticles provide up to a 1000-fold enhancement in the generation of the plasma, which is localized to the surface of the nanoparticles where it is potentially useful (e.g., for catalysis). We performed both time-domain and frequency-domain calculations of the electromagnetic response of the nanoparticles based on high-resolution transmission electron microscope (HRTEM) images, which show local field enhancement of the nanosecond high-voltage pulse on the order of 3×. Since the plasma initiation depends exponentially on the peak electric field strength, this 3-fold increase in the local electric field can result in a several orders of magnitude increases in the generation of plasma at a given applied external field strength. In order to rule out plasmon-resonance enhancement, which is often associated with small metal nanoparticles, we performed finite difference time domain (FDTD) simulations in the optical frequency domain, which show that the effect of plasmon resonance is negligible for Pt nanoparticles. We therefore attribute the nanoparticle-based enhancement to the generation of plasma (an electrostatic effect) rather than enhanced coupling of light from the near field to the far field via the plasmon resonance phenomenon (an optical effect).

We report spectroscopic measurements of the local electric fields and local charge densities at electrode surfaces using graphene-enhanced Raman spectroscopy (GERS) based on the Stark-shifts of surface-bound molecules and the G band frequency shift in graphene. Here, monolayer graphene is used as the working electrode in a three-terminal potentiostat while Raman spectra are collected in situ under applied electrochemical potentials using a water immersion lens. First, a thin layer (1 Å) of copper­(II) phthalocyanine (CuPc) molecules are deposited on monolayer graphene by thermal evaporation. GERS spectra are then taken in an aqueous solution as a function of the applied electrochemical potential. The shifts in vibrational frequencies of the graphene G band and CuPc are obtained simultaneously and correlated. The upshifts in the G band Raman mode are used to determine the free carrier density in the graphene sheet under these applied potentials. Of the three dominant peaks in the Raman spectra of CuPc (i.e., 1531, 1450, and 1340 cm–1), only the 1531 cm–1 peak exhibits Stark-shifts and can, thus, be used to report the local electric field strength at the electrode surface under electrochemical working conditions. Between applied electrochemical potentials from −0.8 V to 0.8 V vs NHE, the free carrier density in the graphene electrode spans a range from −4 × 1012 cm–2 to 2 × 1012 cm–2. Corresponding Stark-shifts in the CuPc peak around 1531 cm–1 are observed up to 1.0 cm–1 over a range of electric field strengths between −3.78 × 106 and 1.85 × 106 V/cm. Slightly larger Stark-shifts are observed in a 1 M KCl solution, compared to those observed in DI water, as expected based on the higher ion concentration of the electrolyte. Based on our data, we determine the Stark shift tuning rate to be 0.178 cm–1/ (106 V/cm), which is relatively small due to the planar nature of the CuPc molecule, which largely lies perpendicular to the electric field at this electrode surface. Computational simulations using density functional theory (DFT) predict similar Stark shifts and provide a detailed atomistic picture of the electric field-induced perturbations to the surface-bound CuPc molecules.

In situ surface-enhanced Raman scattering (SERS) spectroscopy is used to identify the key reaction intermediates during the plasma-based removal of NO and SO 2 under dry and wet conditions on Ag nanoparticles. Density functional theory (DFT) calculations are used to confirm the experimental observations by calculating the vibrational modes of the surface-bound intermediate species. Here, we provide spectroscopic evidence that the wet plasma increases the SO 2 and the NO x removal through the formation of highly reactive OH radicals, driving the reactions to H 2 SO 4 and HNO 3 , respectively. We observed the formation of SO 3 and SO 4 species in the SO 2 wet-plasma-driven remediation, while in the dry plasma, we only identified SO 3 adsorbed on the Ag surface. During the removal of NO in the dry and wet plasma, both NO 2 and NO 3 species were observed on the Ag surface; however, the concentration of NO 3 species was enhanced under wet-plasma conditions. By closing the loop between the experimental and DFT-calculated spectra, we identified not only the adsorbed species associated with each peak in the SERS spectra but also their orientation and adsorption site, providing a detailed atomistic picture of the chemical reaction pathway and surface interaction chemistry.