Photon Absorption Efficiency Research Articles

Advances in Semiconductor Nanostructures for Photonic Applications

In this paper we present the concept and demonstration of novel photovoltaic and electro-optic devices, and photoelectrochemical cells based on various semiconductor nanostructures, specifically compound semiconductor quantum wells and nanowires, and the use of plasmonic and related scattering effects from metal or dielectric nanoparticles to increase efficiency of optical absorption. Quantum-well solar cells were fabricated with scattering from metallic or dielectric nanostructures incorporated to direct incident photons into lateral, optically confined paths with high electromagnetic field intensity within relatively thin multiple-quantum-well regions to maximize quantum efficiency of photon absorption. The internal structure of quantum wells in quantum-well solar cells was also analyzed and characterized; the incorporation of a suitable potential step within each quantum well was explored for improvement in power conversion efficiency. Vertical nanowire arrays were engineered to optimize optical confinement within the nanowires, and core-shell heterostructures were employed to achieve broad-spectrum absorption while maintaining high open-circuit voltages. Large linear electro-optic effect is observed in the nanowire arrays. Branched nanowire photoelectrochemical cells were also made and characterized for their spectral incident photon-to-current conversion efficiency. These works have been sponsored by U.S. Department of Energy and National Science Foundation.

Advanced Materials for Sustainable Energy and a Greener Environment

Advanced Materials for Sustainable Energy and a Greener Environment

Dye-Sensitized Solar Cells Fabricated from Atomic Layer Deposited Photoanodes on Aerogel Scaffolds

Dye-sensitized solar cells (DSSCs) based on mesoporous, high surface area nanocrystalline titania (TiO2) exhibiting up to 11% solar energy-conversion efficiencies are promising materials for cost-effective devices. Our objective here is to realize a cost-effective fabrication technique coupled with suitable device architecture of nanostructured DSSCs to achieve high photoconversion efficiencies. Atomic layer deposition (ALD) offers an attractive synthetic route for fabricating conformal photoanode coating materials on a nanoporous scaffold. Here, we used silica aerogels processed by doctor blading as structural scaffolds on transparent conducting oxide substrates, to promote subsequent conformal growth of thin films of TiO2 and aluminum-doped zinc oxide (AZO) by ALD. The resulting interdigitated architecture of the photoanodes can facilitate fast electron transfer. Dye sensitization of the photoanodes was accomplished using di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) ("N719", Dyesol, B2 dye) and assembled into photocells using liquid electrolytes. Co-sensitization with a second dye was found to increase the photon absorption and cell efficiency.

Titanium nitride films for ultrasensitive microresonator detectors

Titanium nitride (TiNx) films are ideal for use in superconducting microresonator detectors for the following reasons: (a) the critical temperature varies with composition (0<Tc<5 K); (b) the normal-state resistivity is large, ρn∼100 μΩ cm, facilitating efficient photon absorption and providing a large kinetic inductance and detector responsivity; and (c) TiN films are very hard and mechanically robust. Resonators using reactively sputtered TiN films show remarkably low loss (Qi>107) and have noise properties similar to resonators made using other materials, while the quasiparticle lifetimes are reasonably long, 10–200 μs. TiN microresonators should therefore reach sensitivities well below 10−19 W Hz−1/2.

Radiation Absorption and Optimization of Solar Photocatalytic Reactors for Environmental Applications

This study provides a systematic and quantitative approach to the analysis and optimization of solar photocatalytic reactors utilized in environmental applications such as pollutant remediation and conversion of biomass (waste) to hydrogen. Ray tracing technique was coupled with the six-flux absorption scattering model (SFM) to analyze the complex radiation field in solar compound parabolic collectors (CPC) and tubular photoreactors. The absorption of solar radiation represented by the spatial distribution of the local volumetric rate of photon absorption (LVRPA) depends strongly on catalyst loading and geometry. The total radiation absorbed in the reactors, the volumetric rate of absorption (VRPA), was analyzed as a function of the optical properties (scattering albedo) of the photocatalyst. The VRPA reached maxima at specific catalyst concentrations in close agreement with literature experimental studies. The CPC has on average 70% higher photon absorption efficiency than a tubular reactor and requires 39% less catalyst to operate under optimum conditions. The "apparent optical thickness" is proposed as a new dimensionless parameter for optimization of CPC and tubular reactors. It removes the dependence of the optimum catalyst concentration on tube diameter and photocatalyst scattering albedo. For titanium dioxide (TiO(2)) Degussa P25, maximum photon absorption occurs at apparent optical thicknesses of 7.78 for CPC and 12.97 for tubular reactors.

A ternary hybrid CdS/Pt–TiO 2 nanotube structure for photoelectrocatalytic bactericidal effects on Escherichia Coli

A ternary hybrid CdS/Pt–TiO 2 nanotube (NT) photoelectrode was developed by dipping and deposition technique as well as successive ionic layer adsorption and reaction (SILAR). Small Pt nanoparticles (NPs) and CdS NPs were effectively deposited on both the inside and outside of the TiO 2 NTs. The as-prepared hybrid shows enhanced photon absorption and photocurrent generation efficiency. Higher bactericidal effect to Escherichia coli was observed on the ternary hybrid CdS/Pt–TiO 2 NTs as compared with Pt–TiO 2 NTs or pure TiO 2 NTs.

Efficient photon detectors using surface acoustic waves

An efficient photon detector based on the ambipolar transport of photogenerated electrons and holes by surface acoustic waves (SAWs) has been investigated. In the experiments, the photogenerated electrons and holes are separated and transported in a GaAs channel and electrically detected by a lateral p–i–n junction. We show that the acoustic transport efficiency improves by using biased metallic guides along the SAW beam to create independent transport channels for electrons and holes. By optimizing the photon absorption efficiency and the amplitude of the SAW field, we demonstrated overall transport efficiencies above 85% for transport lengths exceeding 200 μ m .

Electrical detection of ambipolar acoustic carrier transport by surface acoustic waves

We have investigated the efficiency of the ambipolar transport of photogenerated electrons and holes in (Al,Ga)As structures by surface acoustic waves (SAWs). In the experiments, the photogenerated electrons and holes transported by the SAW are collected by a lateral p-i-n junction and detected by electrometers. Carrier recombination during transport was also studied by detecting the photoluminescence emitted along the SAW path by the transported carriers. We show that the acoustic transport efficiency improves by using biased metallic guides along the SAW beam to create independent transport channels for electrons and holes. By optimizing the photon absorption efficiency and the amplitude of the acoustic fields, we demonstrated overall transport efficiencies above 85% for transport lengths on the order of 300 μm.

Ultraviolet Photodissociation at 355 nm of Fluorescently Labeled Oligosaccharides

Ultraviolet photodissociation (UVPD) produces complementary fragmentation to collision-induced dissociation (CID) when implemented for activation of fluorescently labeled oligosaccharide and glycan ions. Reductive amination of oligosaccharides with fluorophore reagents results in efficient photon absorption at 355 nm, producing fragment ions from the nonreducing end that do not contain the appended fluorophore. In contrast to the fragment ions observed upon UVPD (A- and C-type ions), CID produces mainly reducing end fragments retaining the fluorophore (Y-type ions). UVPD affords better isomeric differentiation of both the lacto-N-fucopentaoses series and the lacto-N-difucohexaoses series, but in general, the combination of UVPD and CID offers the most diagnostic elucidation of complex branched oligosaccharides. Four fluorophores yielded similar MS/MS results; however, 6-aminoquinoline (6-AQ), 2-amino-9(10 H)-acridone (AMAC) and 7-aminomethylcoumarin (AMC) afforded more efficient photon absorption and subsequent dissociation than 2-aminobenzamide (2-AB). UVPD also was useful for characterization of glycans released from ribonuclease B and derivatized with 6-AQ. Lastly, electron photodetachment dissociation of oligosaccharides derivatized with 7-amino-1,3-naphthalenedisulfonic acid (AGA) yielded unique cross-ring cleavages similar to those obtained by electron detachment dissociation.

Experimental studies of Ge1−xCx and Ge1−x−yCxAly thin films

Experimental results on thin films of the materials GexC1−x, and Ge1−x−yCxAly deposited by a unique multiplasma hollow cathode sputtering technique, are presented. These films have been characterized by several techniques, the most important being in situ and ex situ spectroscopic ellipsometry, Auger electron spectroscopy (AES), and x-ray diffraction (XRD). The spectroscopic ellipsometry measurements of the films indicate that this material is a very strong absorber of photons. The XRD measurements reveal that the carbon is entering the lattice in substitutional sites, to a certain extent, and the AES depth profiling measurements determine the concentration and location of the different constituents. The materials appear to grow such that the concentration of the different elements is uniform throughout the films. The photon absorption inferred from the ellipsometry measurements corroborated the fact that GeC has a photon absorption efficiency much greater than crystalline Si or Ge and even more than amorphous Si in the wavelength range most useful for photovoltaic applications. In fact, the addition of aluminum increased this absorption efficiency substantially.Experimental results on thin films of the materials GexC1−x, and Ge1−x−yCxAly deposited by a unique multiplasma hollow cathode sputtering technique, are presented. These films have been characterized by several techniques, the most important being in situ and ex situ spectroscopic ellipsometry, Auger electron spectroscopy (AES), and x-ray diffraction (XRD). The spectroscopic ellipsometry measurements of the films indicate that this material is a very strong absorber of photons. The XRD measurements reveal that the carbon is entering the lattice in substitutional sites, to a certain extent, and the AES depth profiling measurements determine the concentration and location of the different constituents. The materials appear to grow such that the concentration of the different elements is uniform throughout the films. The photon absorption inferred from the ellipsometry measurements corroborated the fact that GeC has a photon absorption efficiency much greater than crystalline Si or Ge and even more than amor...

Photopolymerization of N, N-dimethylaminobenzyl alcohol as amine co-initiator for light-cured dental resins

Objective The present study was carried out in order to assess the suitability of N, N-dimethylaminobenzyl alcohol (DMOH) as co-initiator of camphorquinone (CQ) and 1-phenyl-1,2-propanedione (PPD) in light-cured dental resins. Methods DMOH was synthesized and used as co-initiator for the photopolymerization of a model resin based on {2,2-bis[4-(2-hydroxy-3-methacryloxyprop-1-oxy)phenyl]propane} (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA). Experimental formulations containing CQ or PPD in combination with DMOH at different concentrations were studied. The photopolymerization was carried out by means of a commercial light-emitting diode (LED) curing unit. The evolution of double bonds consumption versus irradiation time was followed by near-infrared spectroscopy (NIR). The photon absorption efficiency (PAE) of the photopolymerization process was calculated from the spectral distribution of the LED unit and the molar absorption coefficient distributions of PPD and CQ. Results DMOH is an efficient photoreducer of CQ and PPD resulting in higher polymerization rate and higher double bond conversion compared with dimethylaminoethylmethacrylate. The PAE for PPD was higher than that for CQ. However, the polymerization initiated by PPD progressed at a lower rate and exhibited lower values of final conversion compared with the resins containing CQ. This observation indicates that the lower polymerization rate of the PPD/amine system should be explained in terms of the mechanism of generating primary radicals by PPD, which is less efficient compared with CQ. Significance The DMOH/benzoyl peroxide redox system, has recently been proposed as a more biocompatible accelerator for the polymerization of bone cements based on poly(methyl methacrylate), because cytotoxity tests have demonstrated that DMOH possesses better biocompatibility properties compared with traditional tertiary amines. The results obtained in the present study reveal the suitability of the CQ/DMOH initiator system for the polymerization of light-cured dental composites.

Resonant-Cavity-Enhanced Far-Infrared Upconversion Imaging Devices

We have carried out a detailed investigation on the application of resonant cavities to the photon-frequency-upconversion-based far-infrared (FIR) semiconductor imaging devices. The employment of a bottom mirror (BM) enhances the FIR photon absorption efficiency and, therefore, increases the quantum efficiency of GaAs homojunction interfacial work- function internal photoemission (HIWIP) FIR detectors. Significant improvement of the extraction efficiency could be achieved in resonant cavity enhanced (RCE) GaAs-AlGaAs near-infrared (NIR) light-emitting diodes (LEDs) through redirecting as many NIR photons as possible into the escape cone. Under the optimal structural parameters, we have predicted that the upconversion quantum efficiency of the integrated HIWIP-BM-RCE-LED imaging device could be boosted to 5-6 times of the normal HIWIP-LED upconverter without any resonant cavities. As a consequence of few reincarnation cycles needed by NIR photons to escape in the photon recycling process, we can further expect sharp and high-resolution imaging in HIWIP-BM-RCE-LED

Investigation of CdTe x and Cd1−x Zn x Te Schottky Barrier Diode Structure Based γ-Ray Detectors

Cadmium Telluride (CdTe) and Cadmium Zinc Telluride (CdZnTe) based detectors have been developed for hard X-ray and γ -ray detection. These semiconducting materials have high resistivity because of the wide bandgap and also have high photon absorption efficiency because of the large atomic number (ZCd = 48, ZTe = 52). CdTe and CdZnTe substrates (7 mm × 9 mm × 0.5 mm) with different stoichiometry were taken for the fabrication of γ-ray detectors. The substrate was prepared by polishing the bulk crystals grown by the rotational Bridgman method. Crystals with maximum electrical resistivity were grown in this way. For fabrication of Schottky barrier diode structures, the Schottky contacts were made by electroless deposition for gold (Au) and thermal evaporation for Indium (In). The Au/CdTe/In and Au/CdZnTe/In Schottky barrier diodes were linked to the charge sensitive preamplifier by gold wires. Then, I–V measurement and detector efficiency like charge collection performance with energy resolutions were analyzed at room temperature by using 57Co and 137Cs gamma sources. The good energy resolutions of 57Co (122 KeV) and 137Cs (662 KeV) sources are obtained for both CdTe and CdZnTe diode detectors.

Radiation field optimization in photocatalytic monolith reactors for air treatment

AbstractThe tools for optimal design of photocatalytic air purifiers are still being established. In this article, the optimal design of monolith photocatalytic reactors irradiated by cylindrical UV lamps was investigated. The radiative transfer equation in dimensionless monoliths was solved by the Monte‐Carlo method to yield the radiation field in the reactor. The effect of the dimensionless design parameters, monolith aspect ratio, geometric ratios, catalyst reflectivity and number of lamps were investigated and correlated in terms of photon absorption efficiency (ϕ), uniformity of radiation distribution (η), and overall monolith photonic efficiency (Φ = η ϕ). Optimal design is achieved for aspect ratios of 2.5 and monolith‐to‐lamp‐length ratios in the range from 1 to 1.1. The largest number of lamps compatible with uniform air distribution over the monoliths should always be used. The optimal monolith‐to‐lamp‐distance ratio, βoptimum, was correlated as: [βoptimum = 1.698 (n + 1) − 0.279],where n is the number of lamps. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Dynamics of generating transients of delayed fluorescence induction signal and photosynthetic antennas: A possible relationship: Mathematical modeling approach

A mathematical model was developed for resolved temporal transients of experimentally recorded delayed fluorescence (DF) induction signal. During an intermittent light regime, antennas of the photosynthetic apparatus were treated as targets, repeatedly hit by potentially absorbable photons within a series of consecutive light flashes. Formulas were derived for the number of antennas, cumulatively hit by a specific number of photons, as function of the flash serial number (time). Model parameters included: number of absorbable photons in one flash, antenna sizes and numbers. A series of induction curves were analyzed, obtained from a ZeamaysL. leaf segment and differing in the previous dark period (td). Each curve, consisting of the two most prominent DF transients (C and D), was fitted with several model types, differing in the number of absorbed photons. For both transients, the best fitting result was achieved when DF induction was linked to the second absorbed photon. As expected, model parameters related to antenna sizes showed weaker dependence on td than those referring to antenna numbers. With restrictions applied in this model, the two DF induction transients may be related to two classes of photosynthetic antennas. Their different sizes may have a predominant influence on the efficiency of photon absorption, and possibly time-dependent appearance of DF transients.

Preparation and characterization of Fe-incorporated titanate nanotubes

A simple hydrothermal procedure is developed for incorporating Fe intoH2Ti3O7 nanotubes. Detailed characterizations have been carried out using scanningelectron microscopy, transmission electron microscopy, energy dispersive x-rayspectroscopy, electron energy loss spectroscopy and x-ray diffractometry, andresults showed that the specimens have the same structural framework asH2Ti3O7, into whichFe atoms are successfully incorporated. Ultraviolet–visible absorption spectra show that the Fe-incorporatedH2Ti3O7 nanotube has a much improved photon absorption efficiency in the visible region compared with thepure H2Ti3O7 nanotube. Other metals have also been incorporated intoH2Ti3O7 nanotubes, and similar improvements are observed in some of these metal-incorporatednanotubes.

A photoelectrochemical study of nanostructured Cd-doped titanium oxide

The photoelectrochemical properties of Cd-doped titanium oxide thin film, prepared by sol–gel method, were investigated as a function of doping concentration and film thickness (number of coatings). Doping with 1.0 at% Cd resulted in best photoresponse. Further increase or decrease in doping concentration led to fall in the photoresponse. The observed rise in photocurrent density with increase in film thickness suggests efficient absorption of photons with thicker films. However, the photocurrent decreased beyond four coatings, (thickness ∼ 1.3 μ m ), which can be attributed to the higher recombination rate of photogenerated carriers due to reduction in electric field gradient in much thicker films. The observed band gap energy decreased from 3.5 to 2.9 eV on increasing the dopant concentration from 0.5 to 2.0 at%. Flatband potential and donor density, determined from Mott–Schottky plots, were found to vary from - 0.98 to - 1.46 V/SCE and 5.7 × 10 18 to 2.9 × 10 19 cm - 3 , respectively.

Synthesis of new donor-fluorene-donor type organic dyes and their nonlinear absorption properties

We have synthesized two new donor-fluorene-donor type organic dyes (JFJ and TFT) which contain an alkyl fluorene unit bridging two donor groups. In order to enhance the two photon absorption efficiency the molecules were designed for facile donation of electrons from the substituent groups as well as for high coplanarity of the molecule. The wavelength of maximum absorption of TFT and JFJ appeared at 412 and 406nm, respectively. The photoluminescence (PL) spectra show maximum emission wavelengths of TFT and JFJ in chloroform solution at 465 and 495nm, respectively. From optical power limiting experiments, the nonlinear coefficients (σ2) of JFJ and TFT were found to be 8×10−21 and 4×10−21 cm4/GW, respectively.

Transients of delayed fluorescence induction signal and photosynthetic antennas: A possible relationship. Mathematical modeling approach

A mathematical model was developed for resolved temporal transients of experimentally recorded delayed fluorescence (DF) induction signal. During an intermittent light regime, antennas of the photosynthetic apparatus were treated as targets, repeatedly hit by potentially absorbable photons within a series of consecutive light flashes. Formulas were derived for the number of antennas, cumulatively hit by a specific number of photons, as a function of the flash serial number (time). Model parameters included number of absorbable photons in one flash, antenna sizes, and their number. A series of induction curves were analyzed, obtained from a Zea mays leaf segment and differing in the previous dark period (t d). Each curve, consisting of the two most prominent DF transients (C and D), was fitted with several model types, differing in the number of absorbed photons. For both transients, the best fitting result was achieved when DF induction was linked to the second absorbed photon. As expected, model parameters related to antenna sizes showed weaker dependence on t d than those referring to antenna number. With restrictions applied to this model, the two DF induction transients may be related to two classes of photosynthetic antennas. Their different sizes may have a predominant influence on the efficiency of photon absorption and possibly time-dependent appearance of DF transients.

The initiating radical yields and the efficiency of polymerization for various dental photoinitiators excited by different light curing units

To evaluate the efficiency of the photopolymerization of dental resins it is necessary to know to what extent the light emitted by the light curing units is absorbed by the photoinitiators. On the other hand, the efficiency of the absorbed photons to produce species that launch the polymerization process is also of paramount importance. Therefore, the previously determined PAE (photon absorption efficiency) is used in conjunction with the polymerization quantum yields for the photoinitiators, in order to be able to compare the total process on an equivalent basis. This parameter can be used to identify the best performance for the photochemical process with specific photoinitiators. The efficiency of LED (Ultrablue IS) and QTH (Optilux 401) lamps were tested comparing their performances with the photoinitiators camphorquinone (CQ); phenylpropanedione (PPD); monoacylphosphine oxide (Lucirin TPO); and bisacylphosphine oxide (Irgacure 819). The extent of photopolymerization per absorbed photon was determined from the polymerization quantum yields obtained by using the photoinitiators to polymerize methyl methacrylate, and afterwards combined with the previously determined PAEs. Although CQ presents a rather low polymerization quantum yield, its photopolymerization efficiency is practically the highest when irradiated with the Ultrablue LED. On the other hand, Lucirin is much more efficient than the other photoinitiators when irradiated with a QTH lamp, due to its high quantum yield and the overlap between its absorption spectrum and the output of the visible lamp light. Difference in photopolymerization efficiencies arise when combinations of photoinitiators are used, and when LED sources are used in preference to QTH. Mechanistic understanding is essential to optimal initiator formulation.