Photon Capture Cross Section Research Articles

Au-Pt Nanoparticle Formulation as a Radiosensitizer for Radiotherapy with Dual Effects.

BackgroundRadiotherapy occupies an essential position as one of the most significant approaches for the clinical treatment of cancer. However, we cannot overcome the shortcoming of X-rays which is the high value of the oxygen enhancement ratio (OER). Radiosensitizers with the ability to enhance the radiosensitivity of tumor cells provide an alternative to changing X-rays to protons and heavy ion radiotherapy.Materials and MethodsWe prepared the Au-Pt nanoparticles (Au-Pt NPs) using a one-step method. The characteristics of the Au-Pt NPs were determined using TEM, HAADF-STEM, elemental mapping images, and DLS. The enhanced radiotherapy was demonstrated in vitro using MTT assays, colony formation assays, fluorescence imaging, and flow cytometric analyses of the apoptosis. The biodistribution of the Au-Pt NPs was analyzed using ICP-OES, and thermal images. The enhanced radiotherapy was demonstrated in vitro using immunofluorescence images, tumor volume and weigh, and hematoxylin & eosin (H&E) staining.ResultsPolyethylene glycol (PEG) functionalized nanoparticles composed of the metallic elements Au and Pt were designed to increase synergistic radiosensitivity. The mechanism demonstrated that heavy metal NPs possess a high X-ray photon capture cross-section and Compton scattering effect which increased DNA damage. Furthermore, the Au-Pt NPs exhibited enzyme-mimicking activities by catalyzing the decomposition of endogenous H2O2 to O2 in the solid tumor microenvironment (TME).ConclusionOur work provides a systematically administered radiosensitizer that can selectively reside in a tumor via the EPR effect and enhances the efficiency of treating cancer with radiotherapy.

To see the invisible: Image of the event horizon within the black hole shadow

How the supermassive black hole SgrA* in the Milky Way Center looks like for a distant observer? It depends on the black hole highlighting by the surrounding hot matter. The black hole shadow (the photon capture cross-section) would be viewed if there is a stationary luminous background. The black hole event horizon is invisible directly (per se). Nevertheless, a more compact (with respect to black hole shadow) projection of the black hole event horizon on the celestial sphere may be reconstructed by detecting the highly redshifted photons emitted by the nonstationary luminous matter plunging into the black hole and approaching the event horizon. It is appropriate to call this reconstructed projection of the event horizon on the celestial sphere for a distant observer as the “lensed event horizon image”, or simply the “event horizon image”. This event horizon image is placed on the celestial sphere within the position of black hole shadow. Amazingly, the event horizon image is a gravitationally lensed projection on the celestial sphere of the whole surface of the event horizon globe. As a result, the black holes may be viewed at once from both the front and back sides. The lensed event horizon image may be considered as a genuine silhouette of the black hole. For example, a dark northern hemisphere of the event horizon image is the simplest model for a black hole silhouette in the presence of a thin accretion disk.

A switchable self-assembling and disassembling chiral system based on a porphyrin-substituted phenylalanine-phenylalanine motif.

Artificial light-harvesting systems have until now not been able to self-assemble into structures with a large photon capture cross-section that upon a stimulus reversibly can switch into an inactive state. Here we describe a simple and robust FLFL-dipeptide construct to which a meso-tetraphenylporphyrin has been appended and which self-assembles to fibrils, platelets or nanospheres depending on the solvent composition. The fibrils, functioning as quenched antennas, give intense excitonic couplets in the electronic circular dichroism spectra which are mirror imaged if the unnatural FDFD-analogue is used. By slightly increasing the solvent polarity, these light-harvesting fibres disassemble to spherical structures with silent electronic circular dichroism spectra but which fluoresce. Upon further dilution with the nonpolar solvent, the intense Cotton effects are recovered, thus proving a reversible switching. A single crystal X-ray structure shows a head-to-head arrangement of porphyrins that explains both their excitonic coupling and quenched fluorescence.

Photoinduced electron transfer through peptide-based self-assembled monolayers chemisorbed on gold electrodes: directing the flow-in and flow-out of electrons through peptide helices.

Photoinduced electron transfer (PET) experiments have been carried out on peptide self-assembled monolayers (SAM) chemisorbed on a gold substrate. The oligopeptide building block was exclusively formed by C(α)-tetrasubstituted α-aminoisobutyric residues to attain a helical conformation despite the shortness of the peptide chain. Furthermore, it was functionalized at the C-terminus by a pyrene choromophore to enhance the UV photon capture cross-section of the compound and by a lipoic group at the N-terminus for linking to gold substrates. Electron transfer across the peptide SAM has been studied by photocurrent generation experiments in an electrochemical cell employing a gold substrate modified by chemisorption of a peptide SAM as a working electrode and by steady-state and time-resolved fluorescence experiments in solution and on a gold-coated glass. The results show that the electronic flow through the peptide bridge is strongly asymmetric; i.e., PET from the C-terminus to gold is highly favored with respect to PET in the opposite direction. This effect arises from the polarity of the Au-S linkage (Au(δ+)-S(δ-), junction effect) and from the electrostatic field generated by the peptide helix.

Deep Level Saturation Spectroscopy

We review the “Deep Level Saturation Spectroscopy” (DLSS) as the nonlinear method to study the deep local defects in semiconductors. The essence of a method is determined by the processes of sufficiently strong laser modulation (up to saturation) of quasistationar two-step absorption of the probe light via deep levels (DLs). DLSS is based on nonequilibrium processes of the optically induced population changes for deep levels which lead to the changes in an impurity absorption. This method allows us the separation of the spectral contributions from different deep centers (even in the case of their full spectral overlap), on the basis of the difference of their optical activity (photon capture cross-sections) and of their electroactivity difference (carriers capture coefficients). As shown, DLSS is allowed to determine directly the main set of phenomenological parameters (cross-sections, concentration, bound energy, etc.) for deep local defects, their content and energy position in the band gap. Some important aspects of DLSS were shown also: the possibility to connect directly the measured data to the local centers which are participating in radiative recombination, and also the possibility to study directly the phonon relaxation processes in the localized states of deep defects.

Deep-trap discharge in semiconductors in response to concurrent heat and light flows

We have numerically solved differential equations describing the detrapping kinetics of impurity traps during heating (thermally stimulated conductivity) under illumination (induced impurity photoconductivity) with arbitrary trap parameters. An algorithm has been formulated for determining parameters of impurity traps in semiconductors (ionization energy, photon capture cross section, and concentration) from isothermal detrapping curves measured under illumination.

Photoinduced electron transfer at liquid|liquid interfaces. Part VII. Correlation between self-organisation and structure of water-soluble photoactive species

The self-organisation of a variety of dyes at the water|1,2-dichloroethane interface was studied by admittance measurements, photocurrent–potential curves and light polarisation anisotropy of the photocurrent. The heterogeneous photo-oxidation of ferrocene was studied at interfaces sensitised by Sn(IV) meso-tetra-(4-carboxyphenyl) porphyrin dichloride (SnTPPC), chlorin e-6, protoporphyrin IX (protoIX) and Fe(III) protoporphyrin IX chloride (Fe-protoIX). Cyclic voltammograms and capacitance voltage curves exhibit different features associated with the self-assembly of the dye species at the liquid|liquid boundary. In the case of SnTPPC, the capacitance curves displayed the characteristic responses commonly associated with the specific adsorption of ionic species. On the other hand, chlorin e-6, protoIX and Fe-protoIX show rather complex behaviour suggesting not only changes in the excess charge but also in the dielectric permittivity of the interface. Differences in the photocurrent efficiency were also observed under the same experimental conditions. The relative magnitude of the photocurrent responses were rationalised in terms of the phenomenological electron transfer rate constant, the photon capture cross-sections and the lifetime of the triplet state as obtained from nanosecond flash photolysis. Finally, the average molecular orientation of the adsorbed photoactive species was estimated from the photocurrent dependence on the angle of light polarisation in total internal reflection. The results show a clear correlation between the orientation of the transition dipole and the distribution of the peripheral carboxyl groups responsible for the hydrophilic nature of the dyes.

Barrier photo-emf studies of electronic transitions in α-RbAg 4I 5

A study has been made of the photo-emf of α-RbAg 4I 5 superionic crystals exposed to light from the impurity excitation region. The effects of local light irradiation and silver saturation on the photo-emf excitation spectrum were investigated. It was found that irradiation with light having energies 2.78 eV and 2.92 eV causes color centers formation in the crystals. Illumination intensity and time dependences of the photo-emf for the main excitation bands were measured at room temperature. The estimation of the photon capture cross sections for the electronic transitions gives values of the order of magnitude of 10 −17 cm 2. A mechanism for the origin of the photo-emf is proposed.

Deep Centers Influence On Photoresponse Characteristics In High-Resistivity ZnSe

AbstractThe photocurrent peak after preliminary optical excitation with specific wavelength in high resistivity (ρ≍1012 Ω∗cm), unintentionally doped ZnSe monocrystals was observed. To study photocurrent “flash” phenomenon, the photocurrent relaxation spectra, photon-capture crosssection (PCCS) spectra and photo-electron paramagnetic resonance (photo-EPR) were investigated. Two-levels model involving the recombination of an S-center and an acceptor A has been developed to obtain the relation between recombination probability and instantaneous photocurrent value. The S-centers ionization energies (1.06eV and 1.14eV from the bottom of the conduction band) and charge carriers life times were determined. The acceptor's positions in ZnSe band gap are equal to 2.05, 2.11, 2.16 and 2.21eV from the bottom of the conduction band. The photocurrent and PCCS “flash” nature as well as complicated photo-EPR are associated with the relaxation of S-centers attributed to Fe ion in the band gap of semi-insulated ZnSe.

Photovoltaic effect and electronic transitions in RbAg4I5

Abstract A study has been made of the effect of local light irradiation and silver saturation on the contact photo-emf excitation spectrum of α-RbAg4I5 superionic crystals at room temperature. It was found that irradiation with light having energies 2.75 eV and 2.92 eV causes color centers formation in the crystals. The possible nature of the bands in the photo-emf excitation spectrum is discussed. Time dependences of the photo-emf for the main excitation bands were measured at various illumination levels. A characteristic asymmetry of the relaxation curves governed by the excitation kinetics was observed at high illumination intensities. The estimation of the photon capture cross sections for the electronic transitions gives the values of the order of magnitude of 10−17cm2. The transient experiments have shown that the photocarrier lifetimes in the crystals are of the order of 1000 sec.

Particle capture cross sections for a Reissner--Nordström black hole

Expressions are derived for the capture cross sections of photons (or uncharged ultrarelativistic particles) and of uncharged particles that are slowly moving at infinity by a spherically symmetric, charged black hole. The well known expressions for the photon capture cross sections and for capture cross sections of slowly moving particles in the Schwarzschild metric and extreme Reissner--Nordström metric are obtained by taking the appropriate limits in the equations derived.

Effect of boron implantation in CdTe on the spectrum of deep levels in intercrystalline barriers

The effect of boron ion implantation on the spectrum of deep levels and the properties of potential barriers of obliquely sputtered cadmium telluride films are studied. By comparing experimental data on the spectral dependence of short circuit current and photoconductivity to the theoretical curves for photon capture cross sections the optical ionization energy is determined for four levels. It is established that ion implantation and thermal processing in vacuum at 100°C of obliquely sputtered cadmium telluride films substantially impacts the parameters of 1.13–1.16 eV deep levels located in the region of potential barriers and leads to the transformation of impurity complexes in intercrystalline barriers and in microcrystals. The nature of these levels is discussed.

Uv-stimulated photocurrent spectroscopy and trapping kinetics of a 2.1-eV trap in anodic Ta2O5 films

A new experimental technique, uv-stimulated photocurrent spectroscopy, has been used to study a trapping effect observed in the anodic oxide of sputtered β-Ta films. Experimental results indicate the presence of a discrete trap level peaked at 2.1 eV below the oxide conduction band, in addition to the 1.5-eV trap band as previously reported by Hickmott and later by Thomas. A kinetic model involving a single trap level in the oxide was used to analyze transient data. The results of this analysis indicate that the 2.1-eV trap is a positive Coulombic center having an electron capture cross section of 3.2×10−14 cm2, a photon capture cross section (at 2.1 eV) of 10−17 cm2, a density of states of 3×1018/cm3, and a ratio of filled to unfilled states of 0.06 during uv stimulation.

Free Carrier Absorption in n-GaAs

The infrared absorption between 400 and 4,000 cm-1 is measured as a function of carrier concentration for n-type GaAs at three different temperatures of 90°, 300° and 450K. The absorption in the 800 to 2,000 cm-1 region is only due to the optical transition of free carriers. The free carrier absorptions arising from three scattering sources, i.e. acoustic and optical phonons, and ionized impurities, are numerically evaluated in the infrared region. At a constant wave number, 1,000 cm-1, a comparison of the theoretical cross section with the experimental value is performed and the degree of compensation of impurities (Nimp/Ne) for each sample is graphically determined using the total photon capture cross section vs. carrier concentration chart. The numerical results of the wave number and temperature dependences of free carrier absorption are in good agreement with the experimental data in the wide range of carrier concentrations from 1×1016 to 1×1018 cm-3. A relatively sharp peak is detected at ν̄=3,200 cm-1 at 90K. The absorption is suggested to occur by a transition from a deep impurity level (0.4 eV) to the conduction band.

Photoconductivity in Rigid Organic Solutions. III. The Triplet Intermediate and a Kinetic Model Involving Secondary Matrix Trap Photoionization

The original studies of photoconductivity in rigid organic solutions [G. E. Johnson and A. C. Albrecht, J. Chem. Phys. 44, 3162, 3179 (1966)] have been extended to a detailed experimental and theoretical analysis of the rise kinetics of the photocurrents. The principal system studied is an ∼ 10−3M solution of N,N,N′,N′-tetramethylparaphenylenediamine (TMPD) in 3-methylpentane (3-MP) at 77°K. In the earlier work it was found that the charge-carrier production is a biphotonic step involving a one-photon-produced intermediate state which did not appear to be the triplet state. In the present work it is found that under monochromatic excitation the triplet-state intermediate is clearly indicated. However, when broad-band ionizing light is used, entirely different rise behavior is seen which, as before, can exhibit, at low light levels, rise times which considerably exceed the triplet lifetime. However, under conditions in which the matrix-trapped electrons are either stabilized (by O2 or CO2) or destabilized (lower viscosities) the triplet-state intermediate is evident under all conditions of ionizing excitation. A kinetic model is developed which embraces these various observations while still maintaining the triplet intermediate under all conditions. Central to the model is the role of matrix-trapped electrons which find themselves variously distributed across the Coulomb potentials of their partner cationic molecular centers. It is argued how the position of the trapped electron in the Coulomb well is a major factor in determining its dark lifetime, its photoionization cross section, but not its photon-capture cross section for detrapping. An important distinction is made between detrapping which simply mobilizes the electron leading to charge recombination and detrapping which leads to full charge separation and conductivity. The model accounts for the new kinetics seen under broad-band excitation through the relatively increased contribution to the photocurrent of the photoionization of matrix-trapped electrons in the Coulomb well. The rate constant for the exponential photocurrent rise is now strongly light-intensity dependent and reflects a composite of individual processes which include ionization of the triplet state as the first-produced intermediate state. Some parameters are determined including estimates of the trapping efficiencies of matrix cavities and the ionization efficiency of matrix-trapped electrons near the edge of the Coulomb well. The kinetic model is fairly complete and provides the analytic context calling for numerous additional kinetic studies. While the present model has been able to incorporate the rise kinetics into a triplet-state intermediate model, it is still not able to explain the unusual excitation spectrum seen in the previous work for the first photon step. For this reason an alternate possibility that parallel biphotonic paths towards ionization are occurring is examined—one involving a nontriplet intermediate (a charge-transfer state). This calls for a shift from one path to another depending on the characteristics of the ionizing radiation. Until certain definitive experiments are carried out this second mechanism must remain in contention, whenever matrix trapping is important.

Conductivity storage in CdS

A model for conductivity storage in CdS is proposed. It involves a storage level from which electrons are excited while the collection of these electrons by a receiving level is accompanied by local crystal deformation allowing the quasi-equilibrium state, which stored conductivity represents, to be set up. Published literature dealing with the sensitivity of CdS electrical properties to thermal or mechanical stress lends credibility to the model.Various techniques of photoelectronic analysis are used to obtain values of 5 × 10−16 cm2 for the photon-capture cross section and 10−30 cm2 for the recapture cross section of a stored conductivity carrier. Storage level densities ranged from 1016 to 1019 cm−3 with various crystals.

Photoconductive Time Constants and Related Characteristics of p-Type Gold-Doped Germanium

To measure the very short photoconductive time constants of p-type gold-doped germanium, two alternative methods were applied and compared. The first is indirect, using the relationship between the magnitude of generation-recombination noise and carrier lifetimes. The second method is direct, employing a high-speed light-pulsing technique. If no other noise sources are important, the results of the indirect method approach those of the direct method as a lower limit. A combination of such time-constant measurements was performed on a series of crystals in which impurity densities and carrier concentrations had been evaluated by Hall coefficient and conductivity measurements. From these data quantum yields of carrier generation, and cross sections for photon capture and carrier recombination were evaluated. The photon capture cross section of the 0.15 ev gold acceptor level at 5 μ is 1.3×10−16 cm2, averaging 0.9×10−16 cm2 for 2–9 μ. The hole capture cross section by the Au− ion in germanium was found to be 2.3×10−14 cm2.