Effect of Fast Electron Irradiation and Annealing on the Luminescence in GaAs(Zn) crystals. Irradiation-induced 1.26 and 1.39 eV Emission Bands

The effect of 2.2 MeV electron irradiation and subsequent annealings on the photoluminescence in zinc-doped p-type GaAs crystals is studied and analyzed. Rather strong emission bands peaked at hv_m (77 K) near 1.26 eV (induced by electron irradiation) and 1.39 eV (induced by annealing of irradiated crystals) are observed. Evidence is presented that the 1.26 and 1.39 eV emission bands occur due to radiative electronic transitions in As_iZn_Ga and V_AsZn_Ga pairs induced by irradiation and annealing of irradiated crystals, accordingly. The observed variations in the intensities of the 1.26 and 1.39 eV emission bands upon irradiation and subsequent annealings of GaAs(Zn) crystals are explained in terms of irradiation and annealing-induced variations in the amount of 1.26 and 1.39 eV radiative centres resulting from: a) the effective interaction of mobile radiation-induced defects in the arsenic sublattice with zinc atoms leading to the formation of As_iZn_Ga and V_AsZn_Ga pairs; b) the thermal dissociation of As_iZn_Ga and V_AsZn_Ga pairs on individual components.     

Characterization of the linear optical properties of a multiple quantum well structure in the sheet-model approximation

The possibility of determining the linear optical response tensor of a multiple quantum well (MQW) structure from optical reflection spectra is discussed. Taking into account local-field effects, the optical parameters of the MQW system are derived directly from the sheet conductivity tensor of the individual quantum well. For an amorphous Si/SiO₂ MQW structure deposited on a crystalline Si substrate, both s- and p-polarized reflection spectra were measured at different angles of incidence in the near-infrared and visible frequency regions. It was shown that the s-polarized spectra can be characterized by means of only one (complex) parameter, whereas in general it is impossible to account for the p-polarized spectra parametrically in a full range of angles of incidence even with four angular independent parameters. Finally, it was demonstrated that Feibelman's d-parameters can only be used to describe the experimental spectra at near-normal and grazing angles of incidence.     

An integrated relay/nitrate reductase field-effect transistor for the sensing of nitrate (NO3-)

An integrated enzyme-functionalized field-effect transistor (ENFET) device for the sensing of nitrate ions is described. An aminosiloxane-functionalized gate interface is modified with N-methyl-N'-(carboxyalkyl)-4,4'-bipyridinium relay units. The complex formed between nitrate reductase and the bipyridinium units on the gate surface is crosslinked with glutaric dialdehyde to yield a stable relay-enzyme layer on the gate interface. In the presence of sodium dithionite as electron donor, the biocatalyzed reduction of nitrate to nitrite ion is stimulated. The ratio between the oxidized and reduced states of the bipyridinium sites regulates the gate potential, and is controlled by the concentration of NO3- ions in the system. The effect of the chain length tethering the N-methyl-N'-(carboxyalkyl)-4,4'-bipyridinium units to the gate surface on the biocatalyzed reduction of NO3- ions, and on the NO3- FET sensor performance is discussed. The devices that include the bipyridinium units tethered to the gate interface with methylene chain length, -(CH2)n, where n > or = 7, reveal a detection limit of 7 x 10(-5) M for nitrate and a sensitivity of 52 +/- 2 mV dec-1. The response time of the device is as low as 50 s, and the operational time of the system is ca. 85 s. We estimate the surface coverage of nitrate reductase on the gate surface to be ca. 1.2 x 10(-12) mol cm-2.     

Probing photoelectrochemical processes in Au-CdS nanoparticle arrays by surface plasmon resonance: application for the detection of acetylcholine esterase inhibitors

The photoelectrochemical charging of Au-nanoparticles (NP) in a Au-nanoparticle/CdS-nanoparticle array assembled on a Au-coated glass surface is followed by means of surface plasmon resonance (SPR) spectroscopy upon continuous irradiation of the sample. The charging of the Au-NPs results in the enhanced coupling between the localized surface plasmon of the Au-NP and the surface plasmon of the bulk surface, leading to a shift in the plasmon angle. The charging effect of the Au-NPs is supported by concomitant electrochemical experiments in the dark. Analysis of the results indicates that ca. 4.2 electrons are associated with each Au-nanoparticle under steady-state irradiation. The photoelectrochemical charging effect of the Au-NPs in the Au-CdS NP array is employed to develop a SPR sensor for acetylcholine esterase inhibitors.     

Electroconductivity of Submicron Solid Electrolytes Ce1−x GdxO2−δ as a Function of Their Density and the Gadolinium Content

The effect of the composition and density of ceramics on the electroconductivity of solid electrolytes Ce_{1 − x} Gd_xO_{2 − δ}(x = 0.09–0.31) with a submicron (0.1–0.3 µm) grain size, which are prepared from nanosized powders, is investigated. The powders are synthesized by two methods, specifically, by evaporation of a target with the aid of the radiation of a pulsed carbon dioxide laser and by a chemical method of burning. The samples of ceramics are prepared by magnetopulsed or quasi-isostatic (200–500 MPa) pressing and subsequent caking in air at a temperature of 1100–1300°C. It is established that the electroconductivity of the obtained solid electrolytes is higher and the effective activation energy for conduction is considerably lower as compared with analogous electrolytes with a micron grain size.__________Translated from Elektrokhimiya, Vol. 41, No. 6, 2005, pp. 694–701.Original Russian Text Copyright © 2005 by Ivanov, Kotov, Gorelov, Borisov, Murzakaev, Samatov, Medvedev, Khrustov, Ivin, Zayats, Balakireva, Sharova, Kuz’min, Vaganov, Moskalenko.Published on the basis of a contribution delivered at the VII Meeting on Fundamental Problems in Solid-State Ionics (Chernogolovka-2004).     

Motion of protons in the central opening of porphyrins

The distribution of the electrostatic potential of the molecule of porphine (P) and the anions formed by the successive elimination of one and two central protons from it has been studied by the SCF-MO-LCAO method in the all-valence-electron CNDO/2 approximation. The electrostatic potential of the potential of the (P-2HO^2– dianion is characterized by the presence of four minima located at a distance of about 1 Å from the nitrogen atoms. The potential in them is equal to –991 kJ/mole. The value of the potential at the center of the opening is –978 kJ/mole. The distribution of the electrostatic potential of the (P-H)^– anion with a fixed position of the proton near one of the nitrogen atoms is characterized by the presence of a deep valley situated at the oppositely lying nitrogen atom perpendicularly to the N-H bond. The potential of the molecule of P in the plane of the ring does not have negative values, attesting to the energetic unfavorability of the planar conformations of the (P+H)⁺ cation.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 216–220, March–April, 1986.     

Nano‐opto‐mechanical effects in plasmonic waveguides

In order to achieve interaction between light beams, a mediating material object is required. Nonlinear materials are commonly used for this purpose. Here a new approach to control light with light, based on a nano‐opto‐mechanical system integrated in a plasmonic waveguide is proposed. Optomechanics of a free‐floating resonant nanoparticle in a subwavelength plasmonic V‐groove waveguide is studied. It is shown that nanoparticle auto‐oscillations in the waveguide induced by a control light result in the periodic modulation of a transmitted plasmonic signal. The modulation depth of 10% per single nanoparticle of 25 nm diameter with the clock frequencies of tens of MHz and the record low energy‐per‐bit energies of 10⁻¹⁸ J is observed. The frequency of auto‐oscillations depends on the intensity of the continuous control light. The efficient modulation and deep‐subwavelength dimensions make this nano‐optomechanical system of significant interest for opto‐electronic and opto‐fluidic technologies.     

Controlling the direction of photocurrents by means of CdS nanoparticles and cytochrome c-mediated biocatalytic cascades

Cathodic or anodic photocurrents are generated by a monolayer of CdS nanoparticles in the presence of the oxidized or reduced states of cytochrome c, respectively, and the photocurrents are amplified by enzyme-generated biocatalytic cascades mediated by cytochrome c.     

Realization of a sonic black hole analog in a Bose-Einstein condensate

We have created an analog of a black hole in a Bose-Einstein condensate. In this sonic black hole, sound waves, rather than light waves, cannot escape the event horizon. A steplike potential accelerates the flow of the condensate to velocities which cross and exceed the speed of sound by an order of magnitude. The Landau critical velocity is therefore surpassed. The point where the flow velocity equals the speed of sound is the sonic event horizon. The effective gravity is determined from the profiles of the velocity and speed of sound. A simulation finds negative energy excitations, by means of Bragg spectroscopy.