Effect of surface states on the amphoteric behavior of Sn in vapor epitaxial GaAs

The amphoteric behavior of Sn, a commonly-used dopant in AsCl₃GaH₂ vapor epitaxy, is examined for Sn concentration from 5 × 10¹⁴ to 5 × 10¹⁷cm^?3. The compensation ratio $\text{(}\text{N}{}_{\mathrm{A}}\text{N}{}_{\mathrm{D}}\text{)}$ remains constant at 0.23 for low concentrations and begins to increase in the 10¹⁶cm^?3 range. This behavior can be explained quantitatively with non-equilibrium impurity incorporation model which takes into account 3 × 10¹¹ cm^?2 surface states.     

The diagnetic circular dichroism of KBr: Sn2+

The magnetic circular dichroism (MCD) of thoroughly-quenched KBr: Sn²⁺ crystals has been measured over the temperature range 10–230 K. Theoretical calculations of the MCD line shape have verified that such crystals contain Sn²⁺ ions at centres that are principally of either cubic or tetragonal symmetry. The possibility that a small fraction of rhombic Sn²⁺ centres is also present is examined. The calculated spectra are in reasonable agreement with experiment over a wide temperature range, apart from the spectral region above 4.85 eV. The factors that might contribute to this discrepancy are discussed.     

Photothermal ionization fourier transform spectroscopy of shallow donor states in III–V semiconductors

The technique of photothermal ionization Fourir transform spectroscopy of shallow donors is very useful for the detection and identification of residual impurity species in very high purity compound semiconductors, especially GaAs. However, the use of this technique in less pure or intentionally doped samples has resulted many incorrect impurity identifications. In this paper we show how the photothermal ionization spectra change with impurity concentration, thickness, and magnetic field, and present a model of the dielectric response of shallow impurity states which explains many of the previously anomalous results.     

Luminescence characteristics of InAlP---InGaP heterostructures having native-oxide windows

Data are presented on the luminescence characteristics of InGaP/InAlP heterostructures with oxidized InAlP cladding layers grown by metalorganic chemical vapor deposition. The structures are grown on GaAs substrates and consist of either a 20 nm thick In_0.5Ga_0.5P quantum well or a 0.75 μm InGaP layer sandwiched between two InAlP bulk barriers or between two 10-period In_0.5Al_0.5P/In_xGa_1−xP strain-modulated superlattice heterobarriers, where x varies from 0.5 to 0.45 and the period of the superlattice is 3 nm. The top InAlP cladding layer of the InAlP/InGaP heterostructures is oxidized for 2–5.5 h at 500°C in an ambient of H₂O vapor saturated in a N₂ carrier gas. Photoluminescence and time-resolved photoluminescence studies at room temperature show that, as a result of the oxidation of a portion of the top InAlP cladding layer, the photoluminescence emission intensity and lifetime from the InGaP QWs increase significantly.     

Metallobiological necklaces: mass spectrometric and molecular modeling study of metallation in concatenated domains of metallothionein

The ubiquitous protein metallothionein (MT) has proven to be a major player not only in the homeostasis of Cu(I) and Zn(II), but also binds all the Group 11 and 12 metals. Metallothioneins are characterised by the presence of numerous cys-x-cys and cys-cys motifs in the sequence and are found naturally with either one domain or two, linked, metal-binding domains. The use of chains of these metal-thiolate domains offers the possibility of creating chemically tuneable and, therefore, chemically dependent electrochemical or photochemical surface modifiers or as nanomachinery with nanomechanical properties. In this work, the metal-binding properties of the Cd(4)-containing domain of alpha-rhMT1a assembled into chains of two and three concatenated domains, that is, "necklaces", have been studied by spectrometric techniques, and the interactions within the structures modelled and interpreted by using molecular dynamics. These chains are metallated with 4, 8 or 12 Cd(II) ions to the 11, 22, and 33 cysteinyl sulfur atoms in the alpha-rhMT1a, alphaalpha-rhMT1a, and alphaalphaalpha-rhMT1a proteins, respectively. The effect of pH on the folding of each protein was studied by ESI-MS and optical spectroscopy. MM3/MD simulations were carried out over a period of up to 500 ps by using force-field parameters based on the reported structural data. These calculations provide novel information about the motion of the clustered metallated, partially demetallated, and metal-free peptide chains, with special interest in the region of the metal-binding site. The MD energy/time trajectory conformations show for the first time the flexibility of the metal-sulfur clusters and the bound amino acid chains. We report unexpected and very different sizes for the metallated and demetallated proteins from the combination of experimental data, with molecular dynamics simulations.     

Application of MCD spectroscopy and TD-DFT to nonplanar core-modified tetrabenzoporphyrins: effect of reduced symmetry on nonplanar porphyrinoids

The optical spectra of a series of core-modified tetrabenzoporphyrins were analyzed to determine the effects of core modification, ligand folding, and partial benzo substitution at the ligand periphery on the electronic structure by using magnetic circular dichroism (MCD) and NMR spectroscopy, X-ray crystallography, cyclic and differential pulse voltammetry, and TD-DFT calculations. Planar 21-carba-, 21-thia-, 21,23-dithia-, and 21-oxa-23-thiatetrabenzo[b,g,l,q]porphyrins reported previously were studied together with the previously unreported 21-oxa- and 21-carba-23-thiatetrabenzo[b,g,l,q]porphyrins. The optical properties of these compounds are compared to those of tetrabenzo[b,g,l,q]-, 5,10,15,20-tetraphenyl-, 5,10,15,20-tetraphenyltetrabenzo[b,g,l,q]-21-thia-, 5,10,15,20-tetraphenyltetrabenzodithia-, 5,10,15,20-tetraphenyldibenzo[g,q]-21,23-dithia-, 5,10,15,20-tetraphenyldibenzo[b,l]-21,23-dithia-, 5,10,15,20-tetraphenyltribenzo[g,q,l]-21-thia-, and 5,10,15,20-tetraphenylbenzo[b]-21-thiaporphyrins. Michl's perimeter model and Gouterman's four-orbital model are used to conceptualize the results and to account for red shifts commonly observed in the spectral bands of nonplanar porphyrinoids.     

Spectroscopy and electronic structure of electron deficient zinc phthalocyanines

The effect of introduction of perfluoro alkyl groups into phthalocyanines, as evidenced by the spectroscopic properties of 1,4,8,11,15,18,22,25-octa-fluoro-2,3,9,10,16,17,23,24-octa-perfluoro isopropyl zinc phthalocyanine, ZnF(64)Pc(-2) and its ring-reduced radical anion species, [ZnF(64)Pc(-3)](-), are reported. A combination of UV-visible absorption and magnetic circular dichroism (MCD) spectroscopy, ESI and MALDI-TOF mass spectrometry, cyclic and differential pulse voltammetry, and complete theoretical calculations using INDO/S and DFT techniques reveals that the substitution of all sixteen hydrogen atoms in protio ZnPc(-2) by eight F and eight i-C(3)F(7) groups red shifts the Q and pi --> pi transitions and narrows the HOMO-LUMO gap while simultaneously preventing ring photooxidation and stabilizing the radical anion. The [ZnF(64)Pc(-3)](-) species, which is in equilibrium in solution with the neutral complex when a reducing agent is present, is unusually stable. The above effects are attributed to the strong electron withdrawing properties of the peripheral substituents, which render ZnF(64)Pc extremely electron deficient.