Light Absorption Mechanisms Research Articles

Photocurrent enhancement of HgTe quantum dot photodiodes by plasmonic gold nanorod structures.

The near-field effects of noble metal nanoparticles can be utilized to enhance the performance of inorganic/organic photosensing devices, such as solar cells and photodetectors. In this work, we developed a well-controlled fabrication strategy to incorporate Au nanostructures into HgTe quantum dot (QD)/ZnO heterojunction photodiode photodetectors. Through an electrostatic immobilization and dry transfer protocol, a layer of Au nanorods with uniform distribution and controllable density is embedded at different depths in the ZnO layer for systematic comparison. More than 80 and 240% increments of average short-circuit current density (Jsc) are observed in the devices with Au nanorods covered by ∼7.5 and ∼4.5 nm ZnO layers, respectively. A periodic finite-difference time-domain (FDTD) simulation model is developed to analyze the depth-dependent property and confirm the mechanism of plasmon-enhanced light absorption in the QD layer. The wavelength-dependent external quantum efficiency spectra suggest that the exciton dissociation and charge extraction efficiencies are also enhanced by the Au nanorods, likely due to local electric field effects. The photodetection performance of the photodiodes is characterized, and the results show that the plasmonic structure improves the overall infrared detectivity of the HgTe QD photodetectors without affecting their temporal response. Our fabrication strategy and theoretical and experimental findings provide useful insight into the applications of metal nanostructures to enhance the performance of organic/inorganic hybrid optoelectronic devices.

Research on mechanism for high light absorption of micro-structured silicon

The mechanism of the high light absorption of surface-microstructure silicon is discussed using optical simulation based on Monte Carlo method in this article. Calculation results indicate that the micro-structured surface and high refractive index of material are the two key factors that effectively reduce the reflection, especially transmittance, of the material instead of surface wave and sulfur. In addition, this shows clearly a development in direction for manufacturing new type detectors and increasing the efficiency of traditional detector effectively in the future.

Fluorescent carbon nanomaterials: "quantum dots" or nanoclusters?

Despite many efforts, the mechanisms of light absorption and emission of small fluorescent carbon nanoparticles (C-dots) are still unresolved and are a subject of active discussion. In this work we address the question as to whether the fluorescence is a collective property of these nanoparticles or they are composed of assembled individual emitters. Selecting three types of C-dots with "violet", "blue" and "green" emissions and performing a detailed study of fluorescence intensity, lifetime and time-resolved anisotropy as a function of excitation and emission wavelengths together with the effect of viscogen and dynamic fluorescence quencher, we demonstrate that the C-dots represent assemblies of surface-exposed fluorophores. They behave as individual emitters, display electronic anisotropy, do not exchange their excited-state energies via homo-FRET and possibly display sub-nanosecond intra-particle mobility.

Light Absorption Enhancement in Organic Solar Cell by Embedding Ag Nanoparticles and Nanochains within the Active Layer

We numerically investigate the light absorption enhancement of organic photovoltaic cells by embedding Ag nanoparticles and nanochains within the active layer using a finite element method. We analyze the enhancement mechanism of light absorption and systematically study the influence of factors such as the size and the period of silver nanoparticles. The result shows the localized surface plasmon resonance of the particles has a significant influence on the light absorption. Under AM1.5 illumination condition, a relative enhancement with a factor of 107.1% is observed for nanoparticles with a diameter of 30 nm and a period of 200 nm. In addition, different types of nanochain structures have been studied, and we find that, comparing to monodisperse nanoparticles, nanochain structures can further enhance the light absorption because of the stronger light harvesting in the long wavelength range of 600–800 nm.

Synthesis and Light Absorption Mechanism in Si or Ge Nanoclusters for Photovoltaics Applications

Photon absorption in the solar energy range has been investigated in semiconductor nanostructures. Different synthesis techniques (magnetron sputtering, plasma enhanced chemical vapor deposition, ion implantation) followed by thermal annealing, have been employed to fabricate Si or Ge nanoclusters (1-25 nm in size) embedded in SiO2or Si3N4matrices. The thermal evolution in the formation of Si nanoclusters (NCs) in SiO2was shown to depend on the synthesis technique and to significantly affect the light absorption. Experimentally measured values of optical bandgap (EgOPT) in Si NCs evidence the quantum confinement effect which significantly increases the value ofEgOPTin comparison to bulk Si.EgOPTspans over a large range (1.6-2.6 eV) depending on the Si content, on the deposition technique and, in a most significant way, on the structural phase of NC. Amorphous Si NCs have a lowerEgOPTin comparison to crystalline ones. The matrix effect on the synthesis and light absorption in semiconductor NCs was investigated for Ge NCs. Large difference in the Ge NCs synthesis occurred when using SiO2or Si3N4matrices, essentially due to a much lower Ge diffusivity in the latter, which slows down the formation and growth of Ge NCs in comparison to silica matrix. Light absorption in NCs is also shown to be largely affected by the host matrix. Actually, Ge NCs embedded in Si3N4material absorb photons in the solar energy range with a higher efficiency than in silica, due to the different confinement effect. In fact, Si3N4host offers a lower potential barrier to photogenerated carriers in comparison to silica, thus a lower confinement effect is expected, leading to slightly smaller optical bandgap. These effects have been presented and discussed for potential application in light harvesting purposes.

Algal photoreceptors: in vivo functions and potential applications

Many algae, particularly microalgae, possess a sophisticated light-sensing system including photoreceptors and light-modulated signaling pathways to sense environmental information and secure the survival in a rapidly changing environment. Over the last couple of years, the multifaceted world of algal photobiology has enriched our understanding of the light absorption mechanisms and in vivo function of photoreceptors. Moreover, specific light-sensitive modules have already paved the way for the development of optogenetic tools to generate light switches for precise and spatial control of signaling pathways in individual cells and even in complex biological systems.

Enhanced visible-light activity of F-N co-doped TiO2 nanocrystals via nonmetal impurity, Ti3+ ions and oxygen vacancies

A visible light active F-N co-doped TiO2 photocatalysts with varying molar ratio of TiO2/NH4F were successfully prepared using an effective yet simple ethanol–water solvothermal method. The photocatalytic activities of the prepared catalysts were tested for the degradation of methylene blue (MB) under visible light irradiation. Superior photocatalytic activity relative to that of undoped TiO2 was observed on the F-N co-doped catalyst with TiO2:NH4F molar ratio of 1.0. Based on the characterization results including FT-IR, XRD, Raman analysis, UV–vis DRS, XPS and ESR, the F and N dopants were effectively incorporated into the anatase TiO2 lattice, and resulted in a decrease in the energy gap, especially for the TONF-2 sample, which shows two optical absorption edge at 2.32eV and 2.99eV, respectively. The ESR data showed that the presence of fluorine and nitrogen in the lattice induces the formation of reduced Ti3+ centers and oxygen vacancies. In addition, the surface O2− radicals were also detected by ESR analysis, which together with OH radicals are responsible for the MB solution decoloration under visible light. According to the experimental results, it is believed that the enhanced visible light activity arises from the synergistic effect of F-N co-doping, the presence of Ti3+ centers and oxygen vacancies, and thus possible mechanism for both visible light absorption and charge separation were proposed.

A Cell‐Based Light Interaction Model for Human Blood

AbstractThe development of predictive appearance models for organic tissues is a challenging task due to the inherent complexity of these materials. In this paper, we closely examine the biophysical processes responsible for the appearance attributes of whole blood, one the most fundamental of these materials. We describe a new appearance model that simulates the mechanisms of light propagation and absorption within the cellular and fluid portions of this specialized tissue. The proposed model employs a comprehensive, and yet flexible first principles approach based on the morphological, optical and biochemical properties of blood cells. This approach allows for environment driven changes in the cells' anatomy and orientation to be appropriately included into the light transport simulations. The correctness and predictive capabilities of the proposed model are quantitatively and qualitatively evaluated through comparisons of modeled results with actual measured data and experimental observations reported in the scientific literature. Its incorporation into rendering systems is illustrated through images of blood samples depicting appearance variations controlled by physiologically meaningful parameters. Besides the contributions to the modeling of material appearance, the research presented in this paper is also expected to have applications in a wide range of biomedical areas, from optical diagnostics to the visualization and noninvasive imaging of blood‐perfused tissues.

Light absorption mechanism in single c-Si (core)/a-Si (shell) coaxial nanowires

We have carried out detailed investigations on the light absorption mechanism in singlecrystalline silicon (c-Si) (core)/amorphous Si (a-Si) (shell) coaxial nanowires (NWs). Basedon the Lorenz–Mie light scattering theory, we have found that the light absorption in thecoaxial NWs relies on the leaky mode resonances and that the light absorption can beoptimized towards photovoltaic applications when the a-Si shell thickness is about twicethe c-Si core radius. The photocurrent has been found to be enhanced up to ∼ 560% compared to c-Si NWs, and to be further enhanced up to ∼ 60% by coating the nonabsorbing dielectric shells.

The Regulation of TiO2 Nanoparticles on the Expression of Light-Harvesting Complex II and Photosynthesis of Chloroplasts of Arabidopsis thaliana

Recent studies demonstrated that titanium dioxide nanoparticles (TiO2 NPs) could significantly promote photosynthesis and plant growth, but its mechanism is still unclear. In this article, we studied the mechanism of light absorption and transfer of chloroplasts of Arabidopsis thaliana caused by TiO2 NPs treated. The results showed that TiO2 NPs could induce significant increases of light-harvesting complex II (LHCII) b gene expression and LHCII II content on the thylakoid membrane in A. thaliana, and the increases in LHCII were higher than the non-nano TiO2 (bulk-TiO2) treatment. Meanwhile, spectroscopy assays indicated that TiO2 NPs obviously increased the absorption peak intensity of the chloroplast in red and blue region, the fluorescence quantum yield near 680 nm, the excitation peak intensity near 440 and 480 nm and/or near 650 and 680 nm of the chloroplast. TiO2 NPs treatment could reduce F480/F440 ratio and increase F650/F680 ratio and accelerate the rate of whole chain electron transport and oxygen evolution of the chloroplast. However, the photosynthesis improvement of the non-nanoTiO2 treatment was far less effective than TiO2 NPs treatment. Taken together, TiO2 NPs could promote the light absorption of chloroplast, regulate the distribution of light energy from PS I to PS II by increasing LHCII and accelerate the transformation from light energy to electronic energy, water photolysis, and oxygen evolution.

High photosensitivity and low dark current of photoconductive semiconductor switch based on ZnO single nanobelt

Photoconductive semiconductor switch (PCSS) based on ZnO single nanobelt (NB) was fabricated by controlling the concentration of mixed solution and using the probe technique, and applied into a test circuit to control the circuit state. The current–voltage characteristics and voltage spectra were investigated by system source meter and oscillograph, and the results show that the PCSS is of high photosensitivity of ∼10 4, low dark current of ∼10 −3 μA, low power consumption of ∼2.45 μW, typical rise time 0.12 s, and decay time 0.15 s. Within the wavelength range of 280–340 nm, the shorter the wavelength is, the higher the voltage response is. The test circuit state conversion between “1” and “0” is obviously corresponding to UV illumination “on” and “off”. The high photosensitivity and low dark current of PCSS can be reasonably explained by using the view point of light absorption and oxygen chemisorption mechanism.

SrTiO3 Recombination-Inhibiting Barrier Layer for Type II Dye-Sensitized Solar Cells

Type II dye-sensitized solar cells (DSSCs) differ from conventional DSSCs by their mechanism of light absorption and electron injection. Instead of photoexcitation of the dye, followed by electron injection to the semiconductor conduction band, in type II DSSCs, there is a direct electron injection from the HOMO level of the sensitizer into the conduction band of the semiconductor. The main drawback of such cells is their extremely rapid back-electron-transfer rate. Herein, we present a new approach for inhibiting back electron transfer in a catechol-sensitized type II DSSC using a thin layer barrier coating of SrTiO3 between the semiconductor and the sensitizer. A 70% improvement in charge collection efficiency is reported. A proposed mechanism for the operation of the SrTiO3 barrier layer is presented.

Modeling the structure and spectral properties of sensitizing black dye for nanocrystalline TiO2 solar cells

Spectral and electronic properties of a sensitizing black dye; a 4,4′,4′′-tricarboxy-2,2′:6′,2′-terpyridine)tris(isocyanato) ruthenium(II) complex; for nanocrystalline TiO 2 solar cells have been investigated by modern methods of quantum chemistry. The light absorption mechanism in the lowest excited triplet states of the dye was studied. The efficiency of electron injection into nanocrystalline TiO 2 is shown to depend on both the nature of charge-transfer states and asymmetric deformation of an excited triplet term of the black dye.

Sharpening Conical Tungsten Nanotips in KOH Solution under Laser Irradiation

Laser-enhanced chemical etching with a 532 nm neodymium-doped yttrium aluminum garnet pulsed laser in KOH solution was used for sharpening conical tungsten nanotips which were prefabricated with a conventional electrochemical etch. The laser irradiation was incident axially on the tip apex. Due to different light absorption mechanisms by objectives in different sizes, the laser irradiation generates a controllable temperature gradient profile from the tip apex to the tip base. In the KOH solution, the etching rate varies with the temperature profile along the tip axis. The tip apex radius was greatly reduced, with the shape and aspect ratio in good control.

Modelling of organic nanograting heterojunctions for photoelectric conversion

Organic nanograting heterojunctions for photoelectric conversion were studied based on a two-dimensional analytic exciton transport model. The photocurrent and external quantum efficiency were investigated as the function of materials and geometric parameters. It showed that the introduction of the complicated heterointerfaces changed the exciton distribution within the nanostructures and enhanced significantly the exciton dissociation probability. The photoelectric conversion was determined by the cooperation of the two mechanisms of light absorption and exciton migration. The possibility of the nanograting heterojunctions applied to photovoltaic devices was evaluated quantitatively with the recombination-dominated assumption at donor–acceptor interface. The results suggested that the photovoltaic performance in these nanograting heterojunctions might be improved significantly after optimization when compared to that in a planar heterojunction.

Homogeneity assessment of large‐diameter III‐V compound semiconductor substrates by near‐infrared transmittance measurement

The light absorption mechanisms below the fundamental energy gap in undoped and Si-doped GaAs crystals have been investigated to assess EL2 defects and the carrier concentration distribution in III-V compound semiconductor substrates. It is demonstrated, that the near-infrared transmittance mapping equipment consisting of a high-brigthness LED, a high-sensitivity CCD camera, and a digital image processing system is capable to reveal surface scratches and local crystal imperfections such as inclusions, voids, and twining inside the substrate, besides EL2 defects and the carrier concentration distribution. Hence, the homogeneity of III-V compound semiconductor substrates with diameter up to 6 inches can be assessed in less than several seconds with high spatial resolution of 640×480 pixels. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of plastic strain on the absorption spectrum of germanium crystals

This paper shows that the plastic strain of germanium crystals changes their absorption spectrum in the entire transmission region. These changes are ascribed to the influence of strain on three light-absorption mechanisms: direct interband, direct intraband, and free-carrier absorption.

Photocurrent Spectral Distribution and Relaxation in CdS/CdTe Heterojunctions

AbstractPhotocurrent spectral distribution and relaxation in CdS/CdTe heterojunctions Sergiu Vatavu, Petru Gasin, Iuliana Caraman. Thin film CdS/CdTe solar cells have proved their importance in use as solar energy converters achieving 16.5 % efficiency. Thin film CdS/CdTe heterojunctions have been deposited by close spaced sublimation method (CSS), onto SnO2 covered glass plates (2õ2 cm2). The thickness of CdS and CdTe thin films was 0.3-1.6 m and 2.3-6.6 m respectively. For to enhance CdS/CdTe heterojunctions photoelectrical parameters and photosensitivity, the deposition procedure was followed by annealing in presence of CdCl2 at 420C for 15-60 min. Ni has been used as back contact to CdTe. After annealing, the photoelectrical parameters at 100 mW/ñm2 are: Uoc=0.73 V, Isc=21.7 mA/cm2, ff=0.43.The analysis of the photosensitivity for samples with different thicknesses of the component layers in the 78-293K temperature range showed, that the increase of the photoconductivity for photon energies ħw>1.9 eV is determined by the generation-recombination processes in CdSxTe1-x interface layer, and in the 1.42 eV-1.85 eV region, by light absorption mechanism in CdTe.At direct biases of the CdS/CdTe heterojunctions (2Uoc), the photocurrent spectral distribution is determined by the generation-recombination processes in CdS layer. The annealing of the heterojunction in presence of CdCl2 results in photocurrent increase more than one order of magnitude. CdS/CdTe at reverse biases, having a constant sensitivity in visible and near infrared spectral region, can be used in photometry as radiation detector.The analysis of the absorption spectra and the photocurrent spectral distribution at direct biases resulted in determination of the holes mean free path in CdTe layer, being equal to 1.05 mm.The ratio of the ambipolar diffusion coefficient and surface recombination velocity for CdS/CdTe interface is 0.3 m and the diffusion length is 0.6 m at 78K. The same parameters for Ni/CdTe interface are: 0.025 m and 0.05 m respectively.The phococurrent relaxation curves have been studied for illumination throught both sides of the heterojunctions at different biases, temperatures and wavelength, determining different penetration depths and materials excitation. Nonequilibrium charge carriers lifetime has been determined. The thermal annealing of the CdS/CdTe heterojunctions in presence of CdCl2 determine the nonequilibrium charghe carrier lifetime increase by more than one order of magnitude from 22 s up to 280 s.

Temperature changes in the photoluminescence and the intrinsic absorption edge of the (Ga0.1In0.9)2Se3 crystal

The photoluminescence and the intrinsic absorption edge of the (Ga0.1In0.9)2Se3 uniaxial crystal are investigated in the temperature range 77–300 K. Exciton and impurity luminescence bands are revealed at low temperatures and the Urbach tail of the absorption edge is found in the temperature range under study. The temperature dependences of the spectral position and the half-width of the luminescence bands, as well as the optical pseudogap and the energy width of the absorption edge, are studied. The mechanisms of radiative recombination and light absorption and the processes of lattice disordering in the (Ga0.1In0.9)2Se3 crystal are discussed.

Mechanisms of light absorption in gamma-irradiated blanks for optical fibers based on high-purity quartz glass

The IR optical absorption spectra and the Raman spectra of rods made of high-purity quartz glass of the KU-1 and KS-4V grades have been studied in comparison to those measured for the blanks of optical fibers fabricated from this glass using the plasma outside deposition (POD) technology. Based on these data, a discussion is presented of the most probable mechanisms responsible for an increase in the γ-radiation-induced absorption in high-purity quartz glasses as a result of their processing in the course of optical fiber fabrication using the POD technology.