Ioannis Koutselas

Electronic properties of three- and low-dimensional semiconducting materials with Pb halide and Sn halide units

Band-structure calculations, semiempirical as well as ab initio, have been applied to study the electronic band gap of the new exotic natural low-dimensional MX systems (where M = Pb or Sn and X = I, Br or Cl). Moreover, variational calculations are employed to calculate the excitonic binding energies, whose amplification is due not only to the quantum confinement of the excitons but also to a dielectric enhancement effect. A single set of semiempirical parameters is sought to describe the materials; comparison of the calculations with experimental data shows this to be successful in the case of the PbI- and PbBr-containing compounds. .

Some new organic–inorganic hybrid semiconductors based on metal halide units: structural, optical and related properties†

The structural, optical and related properties of organic–inorganic hybrid semiconductors based on the organic molecules (cations) CH3NH3, H3N(CH2)6NH3, C6H5CH2NH3, CH3C6H4CH2NH3, CH3OC6H4CH2CH2NH3, (H2N)2CS(CH2)4SC(NH2)2, H3NCH2CH2OCH2CH2NH3, C6H5CH2CH2SC(NH2)2, C10H21SC(NH2)2, O2NC6H4CH2SC(NH2)2, 1-naphthylmethylammonium C10H7CH2NH3, 9-anthrylmethylammonium C14H9CH2NH3 and 1-pyrenylmethylammonium C16H9CH2NH3 and the inorganic units MXn (anions) (M≡Bi, Sb, Sn, Pb, Cu, Ag; X≡I, Br, Cl) are described. There is evidence that the position, intensity and shape of the excitonic bands depend on the order/disorder of the inorganic component and on the electronic interaction between organic and inorganic components. Copyright

Structural and Electronic-Properties of the Natural Quantum-Well System (C6h5ch2ch2nh3)2sni4

Abstract The structural and electronic properties of the natural multi-quantum-well system (C 6 H 5 CH 2 CH 2 NH 3 ) 2 SnI 4 were investigated by determining its single crystal x-ray structure, measuring its dc-electrical-conductivity and electronic spectra, as well as by calculating its electronic band structure and its excitonic binding energy.

Nanocrystalline/microcrystalline materials based on lead-halide units

The preparation, photoluminescence (PL) spectra and related properties of particulate (nanocrystalline/microcrystalline) materials based on Pb(BrxCl1−x)3, Pb(BrxI1−x)3, and Pb(ClxI1−x)3 (x = 0–1) units, in several macroscopic forms, are reported. The materials in the form of suspensions in toluene or in toluene containing polymethyl methacrylate (PMMA) were obtained from the corresponding n-layer materials (or their precursors), mainly, by a titration-like method. This is the injection of solutions of the corresponding compounds with the Pbn(BrxCl1−x)3n+1, Pbn(BrxI1−x)3n+1, and Pbn(ClxI1−x)3n+1 (n > 2) units, respectively, into neat toluene or toluene–PMMA, with parallel observation of the PL-spectra. With this method, the mass of the lead-halide materials is controlled and the position of the low frequency PL band could be tuned in a wide spectral range, i.e., from ca. 400 to ca. 700 nm. From the suspensions in toluene the corresponding materials in the form of precipitates (powders) were obtained by centrifugation and decantation. From suspensions in toluene–PMMA, the corresponding materials in the form of composite thin films (of PMMA-matrix) were obtained by drop-casting, deep-coating and/or spin-coating techniques. So, blue, green, yellow and red narrow PL-band (of 20–35 nm) emitters (in forms of suspensions, powders and films) were obtained and their PL and optical absorption spectra were investigated further. The PL-intensity varies as in the case of CsPbBr3 particles in a CsBr matrix. Also, some similarities with the spectra of metal chalcogenide quantum dots were found. They were suggested as candidate materials for several optical and optoelectronic applications.

Excitonic Bands in the Spectra of Some Organic-Inorganic Hybrid Compounds Based on Metal Halide Units

The optical absorption, photoluminescence and photoconductivity spectra of some compounds of the formulas [R(CH 2 ) n NH 3 ] x M y X z , [R(CH 2 ) n NH(CH 3 ) 2 ] x M y X z , [R(CH 2 ) n S(CH 3 ) 2 ] x M y X z , [R(CH 2 ) n SC(NH 2 ) 2 ] x M y X z , and [R(CH 2 ) n SeC(NH 2 ) 2 ] x M y X z (where R=organic-group M=Bi(III), Pb(II), Sn(II), Cu(I), Ag(I) etc; X=I, Br, Cl; n,x,y,z=0,1,2,3,...) are reported. The position, intensity and shape of the excitonic bands depend on the dimensionality and size of the inorganic network, as well as on the nature of M, X, R and onium-group.

Preparation and Characterization of (C6H5CH2CH2NH3)4BiI7 · H2O, (C6H5CH2CH2NH3)3BiBr6 and (C6H5CH2CH2NH3)3BiCl6

The title compounds were prepared and characterized analytically, structurally, and spectroscopically. They contain isolated BiX6 (X = I. Br, Cl) anions. Their optical absorption spectra show excitonic bands in the UV-visible spectral region. These bands occur at higher energies than the corresponding bands of R3M2X9 (R = MemNH4-m; m = 1,2, 3, 4).

Structural and Theoretical Study of Strontium Borophosphate Glasses Using Raman Spectroscopy and ab Initio Molecular Orbital Method

Strontium borophosphate glasses of composition xSrO·(1 - x)·[0.68B2O3·0.32P2O5], 0.40 ≤ x ≤ 0.68, have been prepared by fast quenching of high-temperature melts and studied using Raman spectroscopy. In order to comprehend and confirm the obtained spectroscopic Raman data, crystalline compounds and glass-ceramics of analogous compositions were also prepared and studied. Also, ab initio molecular electronic structure theory was used to predict and confirm the experimental vibrational spectra The comparison between theoretical and experimental results showed a good overall agreement. The analysis has focused on a new detailed interpretation of the P-O-B Raman bands. Also, the analysis has revealed a divergent modification of the reported glasses near the meta-stoichiometry where the dominant species in the glass network were found to be borophosphate chains [BP2O9]5-, pyrophosphate P2O74-, and orthophosphate PO43- units.

Graphene/Carbon Dot Hybrid Thin Films Prepared by a Modified Langmuir–Schaefer Method

The special electronic, optical, thermal, and mechanical properties of graphene resulting from its 2D nature, as well as the ease of functionalizing it through a simple acid treatment, make graphene an ideal building block for the development of new hybrid nanostructures with well-defined dimensions and behavior. Such hybrids have great potential as active materials in applications such as gas storage, gas/liquid separation, photocatalysis, bioimaging, optoelectronics, and nanosensing. In this study, luminescent carbon dots (C-dots) were sandwiched between oxidized graphene sheets to form novel hybrid multilayer films. Our thin-film preparation approach combines self-assembly with the Langmuir–Schaefer deposition and uses graphene oxide nanosheets as template for grafting C-dots in a bidimensional array. Repeating the cycle results in a facile and low-cost layer-by-layer procedure for the formation of highly ordered hybrid multilayers, which were characterized by photoluminescence, UV–visible, X-ray photoelectron, and Raman spectroscopies, as well as X-ray diffraction and atomic force microscopy.

Encapsulation and protection of carbon dots within MCM-41 material

The effective encapsulation of Carbon Dots (CDs) within spherical-like MCM-41 nanoparticles is reported in this work, using a simple, low cost synthetic procedure, here focusing on blue luminescent CDs. CDs of 2 nm average size, with strong photoluminescence, are embedded into MCM-41 nanoparticles during the sol-gel procedure, using tetrathylorthosilicate as the precursor for the glass structure, whereas a cationic surfactant is used as the structure directing agent under basic conditions. The white powder of the final hybrid material showed long term, stable photoluminescence due to the presence of the CDs, indicating the protective character of the silica matrix. Most importantly, it is reported that the photoluminescence of the final hybrid material is not significantly affected by a thermal treatment at 550 °C. The transparent nature of the MCM-41, allows loading this type of mesoporous materials with CDs, thus, providing some new key composite photoluminescent materials for energy and other related applications.Graphical Abstract

Micro-fabrication by laser radiation forces: a direct route to reversible free-standing three-dimensional structures.

The origins and first demonstration of structurally stable solids formed by use of radiation forces are presented. By experimentally proving that radiation forces can indeed produce stable solid material forms, a novel method enabling two- and three-dimensional (2d and 3d) microfabrication is introduced: An optical, non-contact single-step physical operation, reversible with respect to materials nature, based on the sole use of radiation forces. The present innovation is elucidated by the formation of polyisoprene and polybutadiene micro-solids, as well as plasmonic and fluorescent hybrids, respectively comprising Au nanoparticles and CdS quantum dots, together with novel concepts of polymeric fiber-drawing by radiation forces.