Year: 2019

Unraveling the Radiative Pathways of Hot Carriers upon Intense Photoexcitation of Lead Halide Perovskite Nanocrystals
Papagiorgis P, Manoli A, Michael S, Bernasconi C, Bodnarchuk MI, Kovalenko MV, Othonos A, Itskos G

ACS Nano, DOI: 10.1021/acsnano.9b01398
The slowdown of carrier cooling in lead halide perovskites (LHP) may allow the realization of efficient hot carrier solar cells. Much of the current effort focuses on the understanding of the mechanisms that retard the carrier relaxation, while proof-of-principle demonstrations of hot carrier harvesting have started to emerge. Less attention has been placed on the impact that the energy and momentum relaxation slowdown imparts on the spontaneous and stimulated light-emission process. LHP nanocrystals (NCs) provide an ideal testing ground for such studies as they exhibit bright emission and high optical gain, while the carrier cooling bottleneck is further pronounced compared to their bulk analogues due to confinement. Herein, the luminescent properties of CsPbBr3, FAPbBr3, and FAPbI3 NCs in the strong photoexcitation regime are investigated. In the former two NC systems, amplified spontaneous emission is found to dominate over the radiative recombination at average carrier occupancy per nanocrystal larger than 5–10. On the other hand, under the same photoexcitation conditions in the FAPbI3 NCs, a longer lived population of hot carriers results in a competition between hot luminescence, stimulated emission, and defect recombination. The dynamic interplay between the aforementioned three emissive channels appears to be influenced by various experimental and material parameters that include temperature, material purity, film morphology, and excitation pulse width and wavelength.

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Epitaxial highly ordered Sb:SnO2 nanowires grown by the vapor liquid solid mechanism on m-, r- and a-Al2O3
Zervos M, Lathiotakis N, Kelaidis N, Othonos A, Tanasa E, Vasile E

Nanoscale Adv., DOI: 10.1039/C9NA00074G
Epitaxial, highly ordered Sb:SnO2 nanowires were grown by the vapor–liquid–solid mechanism on m-, r- and a-Al2O3 between 700 °C and 1000 °C using metallic Sn and Sb with a mass ratio of Sn/Sb = 0.15 ± 0.05 under a flow of Ar and O2 at 1 ± 0.5 mbar. We find that effective doping and ordering can only be achieved inside this narrow window of growth conditions. The Sb:SnO2 nanowires have the tetragonal rutile crystal structure and are inclined along two mutually perpendicular directions forming a rectangular mesh on m-Al2O3 while those on r-Al2O3 are oriented in one direction. The growth directions do not change by varying the growth temperature between 700 °C and 1000 °C but the carrier density decreased from 8 × 1019 cm−3 to 4 × 1017 cm−3 due to the re-evaporation and limited incorporation of Sb donor impurities in SnO2. The Sb:SnO2 nanowires on r-Al2O3 had an optical transmission of 80% above 800 nm and displayed very long photoluminescence lifetimes of 0.2 ms at 300 K. We show that selective area location growth of highly ordered Sb:SnO2 nanowires is possible by patterning the catalyst which is important for the realization of novel nanoscale devices such as nanowire solar cells.

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High-Temperature Pb Doping of SnO₂ and Growth Limitations of PbₓSn₁–ₓO₂ Nanowires Versus Low-Temperature Growth of PbₓSn₁–ₓO for Energy Storage and Conversion
Zervos M, Othonos A, Tanasă E

Journal of physical chemistry, DOI: 10.1021/acs.jpcc.9b02865
Pb doping of SnO₂ nanowires grown by the vapor–liquid–solid mechanism on 1 nm Au/Si has been investigated between 500 and 1000 °C via the reaction of Sn-containing Pb with O₂ at 10–¹ mbar. The SnO₂ nanowires have diameters of 50 nm, lengths up to 100 μm, and a tetragonal rutile crystal structure, but they do not contain Pb because of its significant depletion during the temperature ramp and re-evaporation from the surface of the SnO₂ nanowires. Consequently, we do not observe a semiconductor to semimetal transition and band gap narrowing. Instead, the Pb reacts with O₂, leading to the deposition of PbO directly on Si but not on the SnO₂ nanowires, which have carrier densities of ≈10¹⁶ cm–³. Furthermore, one-dimensional growth was completely suppressed by increasing the amount of Pb in Sn. As such, Pb doping of SnO₂ and the growth of PbₓSn₁–ₓO₂ nanowires is difficult, if not impossible, because PbO₂ nanowires cannot be grown by the vapor–liquid–solid mechanism irrespective of the growth temperature. In contrast, we find that the composition of PbₓSn₁–ₓO nanostructures may be tuned over a broad range by low-temperature growth at 400 °C. We discuss the properties and prospects of this ternary oxide for energy conversion and storage.

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Year: 2018

Efficient Optical Amplification in the Nanosecond Regime from Formamidinium Lead Iodide Nanocrystals
Papagiorgis P, Manoli A, Protesescu L, Achilleos C, Violaris M, Nicolaides K, Trypiniotis T, Bodnarchuk MI, Kovalenko MV, Othonos A and others

ACS Photonics, DOI: 10.1021/acsphotonics.7b01159
Lead halide perovskites nanocrystals (NCs) were recently found to exhibit extraordinary optical amplification properties. The great majority of such studies, implemented ultrashort photon pulses in the femtosecond regime to initiate the stimulated emission process. Yet the realization of practical lasing applications based on such materials is crucially dependent on their ability to sustain optical amplification at significantly longer timescales, at which major losses associated with spontaneous emission and non-radiative recombination occur. Herein we demonstrate highly efficient amplified spontaneous emission (ASE) from closed-packed films of formamidinium lead iodide perovskite (FAPbI3) NCs under excitation in the nanosecond regime. Systematic optimization of the NCs processing and thermal treatment, yields solids that exhibit high ASE net modal gain up to 604 cm-1 and nearly-temperature insensitive ASE thresholds with room temperature values as low as 140 μJ cm-2. The efficient optical amplification using excitation pulses comparable to the exciton lifetime combined with the excellent chemical durability and air stability of FAPbI3 NCs renders them as outstanding gain media for continuous wave lasers in the red and near-infrared.

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SnO2/PbOx (x = 1, 2) Core–Shell Nanowires and Their Growth on C-Fiber Networks for Energy Storage
Zervos M, Othonos A, TanasÄ? E, Vasile E, Leontidis E

The Journal of Physical Chemistry C, DOI: 10.1021/acs.jpcc.8b07526
SnO2 nanowires were grown on Si, fused SiO2, and C fibers by the vapor–liquid–solid mechanism at 800 °C and 10–1 mbar, and SnO2/PbO core–shell nanowires were obtained by the deposition of 50 nm Pb over the SnO2 nanowires followed by annealing between 100 and 200 °C. The SnO2/PbO nanowires have diameters of 100–300 nm and lengths up to 100 μm and consist mainly of tetragonal rutile SnO2 and PbO. Higher temperatures between 300 and 500 °C resulted in the formation of Pb2O3 and Pb3O4 with monoclinic and orthorhombic crystal structures, but the SnO2/PbO and SnO2/Pb2O3 nanowires had low conductivities of 10–1 S/cm. In contrast, highly conductive SnO2/PbO2 nanowires were obtained by electrodeposition of PbO2 in 0.3 M HNO3 and 1 M Pb(NO3)2 (aq). PbO2 forms a straddling-type heterojunction with SnO2, and the one-dimensional (1D) electron gas distribution is confined in the PbO2 shell for sufficiently thick shells, as shown by the self-consistent solution of the Poisson–Schrödinger equations in the effective mass approximation. The SnO2/PbO2 nanowires exhibit an open-circuit potential of 1.8 V versus C-fiber networks in 5 M H2SO4 (aq) and show symmetric cyclic voltammetry curves, suggesting a suppression of the redox reactions related to SnO2 and a high specific capacity of 206 mAh/g. We discuss the potential of both SnO2 and SnO2/PbO2 nanowires on C fibers for the attainment of even higher specific capacity in a Li-ion battery.

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Year: 2017

Photovoltaic limitations of BODIPY:fullerene based bulk heterojunction solar cells
Baran D, Tuladhar S, Economopoulos SP, Neophytou M, Savva A, Itskos G, Othonos A, Bradley DD, Brabec CJ, Nelson J and others

Synthetic Metals , DOI:
Abstract The photovoltaic performance of blends of a series of 4,4′-Difluoro-4-bora-3a,4a-diaza-s-indacenes)-based (BODIPY) conjugated polymers donors with fullerene electron acceptors is investigated. Despite the high Voc values observed, solar cell device yields relatively low power conversion efficiencies. Our study takes into account the materials’ structure-property relationship, light harvesting capabilities, charge transport, collection properties and morphological characteristics to elucidate factors affecting the photovoltaic performance in this class of polymers. We show that elimination of low molecular weight species and suitable electrodes for hole collection can be used to overcome some of the observed limitations on photovoltaic performance.

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The influence of additives in the stoichiometry of hybrid lead halide perovskites
Burgués-Ceballos I, Savva A, Georgiou E, Kapnisis K, Papagiorgis P, Mousikou A, Itskos G, Othonos A, Choulis SA

AIP Advances, DOI: 10.1063/1.5010261
We investigate the employment of carefully selected solvent additives in the processing of a commercial perovskite precursor ink and analyze their impact on the performance of organometal trihalide perovskite (CH3NH3PbI3−xClx) photovoltaic devices. We provide evidence that the use of benzaldehyde can be used as an effective method to preserve the stoichiometry of the perovskite precursors in solution. Benzaldehyde based additive engineering shows to improve perovskite solid state film morphology and device performance of CH3NH3PbI3−xClx based solar cells.

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Long-Lived Hot Carriers in Formamidinium Lead Iodide Nanocrystals
Papagiorgis P, Protesescu L, Kovalenko MV, Othonos A, Itskos G

The Journal of Physical Chemistry C, DOI: 10.1021/acs.jpcc.7b02308
The efficient harvesting of hot carrier energy in semiconductors is typically inhibited by their ultrafast thermalization process. Recently, highly promising experiments reported on the slowdown of the intraband relaxation in hybrid metal halide perovskites. In this work, we report on the presence of long-lived hot carriers in weakly confined colloidal nanocrystals (NCs) of formamidinium lead iodide perovskite (FAPbI3). The effect is apparent from the excitation-dependent lengthening of the rise time and broadening of the high-energy tail of the transient absorption bleaching signal, yielding a retardation of the carrier relaxation by 2 orders of magnitude compared to typical time scales in colloidal semiconductor NCs. Three distinct cooling stages are observed, occurring at sub-picosecond, ∼5 ps, and ∼40 ps time scales, which we attribute to scattering from LO-phonons, contribution from a hot phonon bottleneck effect and Auger heating, respectively. Thermalization appears also influenced by the FAPbI3 NCs purity, with trapping at unreacted precursor impurities further reducing the carrier energy loss rate.

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Sn:In2O3 and Sn:In2O3/NiS2 Core–Shell Nanowires on Ni, Mo Foils and C Fibers for H2 and O2 Generation
Zervos M, Leontidis E, Tanasă E, Vasile E, Othonos A

The Journal of Physical Chemistry C, DOI: 10.1021/acs.jpcc.7b09587
Sn:In2O3 nanowires have been grown by the vapor liquid solid mechanism on Si, Ni, Mo, and C fibers. These were used to obtain Sn:In2O3/NiS2 core–shell nanowires by the deposition of 10 nm Ni over the Sn:In2O3 nanowires followed by post growth processing under H2S between 100 and 200 °C. The Sn:In2O3/NiS2 nanowires have diameters of ≈100 nm and lengths up to ≈100 μm and consist of cubic bixbyite Sn:In2O3 surrounded by 3 nm NiS2 crystalline quantum dots with a cubic crystal structure. Higher temperatures of 300–500 °C result in the formation of NiS2 quantum dots and cubic In3S4 branches around the Sn:In2O3. We find that the p-type NiS2 in contact with n-type Sn:In2O3 NWs gives rectifying current–voltage (IV) characteristics due to the formation of a p–n heterojunction with a straddling type band alignment where electrons are confined to the n-type Sn:In2O3 core and holes in the p-type NiS2, as shown by self-consistent Poisson–Schrödinger calculations in the effective mass approximation. The gas evolution of O2 and H2 was measured using the Sn:In2O3/NiS2 nanowires as the anode and Pt as the cathode in a two-compartment photoelectrochemical cell containing 1 M KOH (aq) and 0.5 M H2SO4 (aq), respectively, under light of 1 sun. We obtain 7.8 μL/min of O2 and 15.0 μL/min of H2 at an overpotential of 0.2 V and 25 °C from the Sn:In2O3/NiS2 nanowires on C. These are ≈35% larger than those obtained from plain Sn:In2O3 nanowires attributed to the existence of the p–n junction.

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Year: 2016

The Influence of Doping on the Optoelectronic Properties of PbS Colloidal Quantum Dot Solids
Papagiorgis P, Stavrinadis A, Othonos A, Konstantatos G, Itskos G

Scientific Reports, DOI:
We report on an extensive spectroscopic investigation of the impact of substitutional doping on the optoelectronic properties of PbS colloidal quantum dot (CQD) solids. N-doping is provided by Bi incorporation during CQD synthesis as well as post-synthetically via cation exchange reactions. The spectroscopic data indicate a systematic quenching of the excitonic absorption and luminescence and the appearance of two dopant-induced contributions at lower energies to the CQD free exciton. Temperature-dependent photoluminescence indicates the presence of temperature-activated detrapping and trapping processes of photoexcitations for the films doped during and after synthesis, respectively. The data are consistent with a preferential incorporation of the dopants at the QDs surface in the case of the cation-exchange treated films versus a more uniform doping profile in the case of in-situ Bi incorporation during synthesis. Time-resolved experiments indicate the presence of fast dopant- and excitation-dependent recombination channels attributed to Auger recombination of negatively charged excitons, formed due to excess of dopant electrons. The data indicate that apart from dopant compensation and filling of dopant induced trap states, a fraction of the Bi ionized electrons feeds the QD core states resulting in n-doping of the semiconductor, confirming reported work on devices based on such doped CQD material.

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Pb doping of In2O3 and their conversion to highly conductive PbS/In2S3−3xO3x nanowires with infra red emission
Zervos M, Othonos A, Gianetta V, Nassiopoulou AG

Materials Letters , DOI:
Abstract We have grown Pb doped In2O3 nanowires at 800 °C which have the cubic bixbyite crystal structure of In2O3 and contain orthorhombic α-PbO. These had resistances up to ≈10 Ω and exhibited photoluminescence at 2.5 eV but we observed the gradual emergence of infra red emission from 0.8 μm to 1.2 μm after processing under \{H2S\} between 200 °C and 400 °C. The resultant PbS/In2S3−3xO3x nanowires had similar resistances and consist of stacked crystals with sizes that depend on the Pb content. We discuss the potential of Pb for doping metal oxide semiconductors and the importance of PbS/In2O3 p–n tunnel junctions for solar cells.

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Core–shell PbS/Sn:In2 O3 and branched PbIn2S4 /Sn:In2O3nanowires in quantum dot sensitized solar cells
Zervos M, Vasile E, Vasile E, Othonos A

Nanotechnology, DOI:
Core–shell PbS/Sn:In 2 O 3 and branched PbIn 2 S 4 /Sn:In 2 O 3 nanowires have been obtained via the deposition of Pb over Sn:In 2 O 3 nanowires and post growth processing under H 2 S between 100 °C–200 °C and 300 °C–500 °C respectively. The PbS/Sn:In 2 O 3 nanowires have diameters of 50–250 nm and consist of cubic PbS and In 2 O 3 while the PbIn 2 S 4 /Sn:In 2 O 3 nanowires consist of PbIn 2 S 4 branches with diameters of 10–30 nm and an orthorhombic crystal structure. We discuss the growth mechanisms and also show that the density of electrons in the n-type Sn:In 2 O 3 core is strongly dependent on the thickness of the p-type PbS shell, which must be smaller than 30 nm to prevent core depletion, via the self-consistent solution of the Poisson–Schrödinger equations in the effective mass approximation. The PbS/Sn:In 2 O 3 and PbIn 2 S 4 /Sn:In 2 O 3 nanowire networks had resistances of 100–200 Ω due to the large carrier densities and exhibited defect related photoluminescence at 2.2 eV and 1.5 eV respectively. We show that PbS in contact with polysulfide electrolyte has ohmic like behavior but the PbS/Sn:In 2 O 3 nanowires gave, rectifying current voltage characteristics as a counter electrode in a quantum dot sensitized solar cell using a conventional ITO/TiO 2 /CdS/CdSe photo anode, an open circuit voltage of ≈0.5 V, and short circuit current density of ≈1 mA cm −2 . In contrast the branched PbIn 2 S 4 /Sn:In 2 O 3 nanowires exhibited a higher current carrying capability of ≈7 mA cm −2 and higher power conversion efficiency of ≈2%.

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Year: 2015

Förster resonant energy transfer from an inorganic quantum well to a molecular material: Unexplored aspects, losses, and implications to applications
Itskos G, Othonos A, Choulis SA, Iliopoulos E

The Journal of Chemical Physics, DOI: 10.1063/1.4935963

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Ultrafast Spectroscopy and Red Emission from β-Ga2O3 β-Ga2S3 Nanowires
Othonos KM, Zervos M, Christofides C, Othonos A

Nanoscale Research Letters, DOI: 10.1186/s11671-015-1016-y
Ultrafast pump-probe and transient photoluminescence spectroscopy were used to investigate carrier dynamics in Î{\texttwosuperior}-Ga2O3 nanowires converted to Î{\texttwosuperior}-Ga2O3/Ga2S3 under H2S between 400 to 600Â?Â{\textdegree}C. The Î{\texttwosuperior}-Ga2O3 nanowires exhibited broad blue emission with a lifetime of 2.4Â?ns which was strongly suppressed after processing at 500â??600Â?Â{\textdegree}C giving rise to red emission centered at 680Â?nm with a lifetime of 19Â?Î{Œ}s. Differential absorption spectroscopy reveals that state filling occurs in states located below the conduction band edge before sulfurization, but free carrier absorption is dominant in the Î{\texttwosuperior}-Ga2O3/Ga2S3 nanowires processed at 500 to 600Â?Â{\textdegree}C for probing wavelengths >500Â?nm related to secondary excitation of the photo-generated carriers from the mid-gap states into the conduction band of Ga2S3.

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Compositional tuning, properties and conversion of In2xga2-2xo3 Nanowires into I–III–VI2 Chalcopyrite Cu (Inxga1-X) S2
Zervos M, Othonos A

Frontiers in Nanoscience and Nanotechnology, DOI: 10.15761/FNN.1000106
In2xGa2-2xO3 nanowires were grown at 800°C via the vapor-liquid-solid mechanism on Si(001) using 1 nm Au as a catalyst and by varying systematically the In to Ga ratio. The In2xGa2-2xO3 nanowires have average diameters of » 50 nm, lengths up to 100 mm and consist of a mixture of phases belonging to the cubic bixbyite In2O3 and monoclinic b-Ga2O3. The nanowires exhibited room temperature photoluminescence at 3.1 eV which shifted to the blue upon increasing the content of Ga. In contrast we observe a strong red-shift from 3.1 eV to 1.8 eV after processing under H2S at 700°C due to the diffusion of S into oxygen vacancies and the formation of Ga rich In2xGa2-2xS3. We find that the deposition of Cu over In2xGa2-2xO3 and conversion under H2S between 100°C to 500°C resulted into the formation of Cu(InxGa1-x)S3 nanowires with smaller resistances and a stronger red shift in the photoluminescence from 3.1 eV to 1.5 eV close to the energy gap of Cu(InxGa1-x)S2.

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Sulfur doping of M/In2O3 (M=Al,W) nanowires with room temperature near infra red emission
Zervos M, Mihailescu C, Giapintzakis J, Othonos A, Travlos A

AIP Advances, DOI: 10.1063/1.4930188

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Sn doped β-Ga2O3and β-Ga22S3 nanowires with red emission for solar energy spectral shifting
Zervos M, Othonos A, Gianneta V, Travlos A, Nassiopoulou AG

Journal of Applied Physics, DOI: 10.1063/1.4935633

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Surface passivation and conversion of SnO2 to SnS2 nanowires
Zervos M, Mihailescu CN, Giapintzakis J, Othonos A, Luculescu CR

Materials Science and Engineering: B , DOI:
Abstract SnO2 nanowires have been grown on Si(0 0 1) via the vapour–liquid–solid mechanism at 800 °C and then exposed to \{H2S\} between 300 and 600 °C. The SnS2/SnO2 nanowires obtained at 300 °C consist of tetragonal rutile SnO2 and hexagonal SnS2, exhibited defect related photoluminescence at 2.4 eV and have smaller resistances than the SnO2 nanowires. We show how the Fermi level pinning at the surface of a SnS2/SnO2 nanowire would lead to an increase of the one dimensional electron gas density, smaller barrier height and resistance. The SnO2 nanowires are fully converted into hexagonal SnS2 at 400 °C resulting into photoluminescence at 2.4 and 2.8 eV but have considerably larger resistances than the SnO2 nanowires which are eliminated and converted into SnS2 crystals above 400 °C.

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Electrical, structural, and optical properties of sulfurized Sn-doped In2O3 nanowires
Zervos M, Mihailescu CN, Giapintzakis J, Othonos A, Travlos A, Luculescu CR

Nanoscale Research Letters, DOI: 10.1186/s11671-015-0995-z
Sn-doped In2O3 nanowires have been grown on Si via the vapor-liquid-solid mechanism at 800Â?Â{\textdegree}C and then exposed to H2S between 300 to 600Â?Â{\textdegree}C. We observe the existence of cubic bixbyite In2O3 and hexagonal SnS2 after processing the Sn:In2O3 nanowires to H2S at 300Â?Â{\textdegree}C but also cubic bixbyite In2O3, which remains dominant, and the emergence of rhombohedral In2(SO4)3 at 400Â?Â{\textdegree}C. The resultant nanowires maintain their metallic-like conductivity, and exhibit photoluminescence at 3.4Â?eV corresponding to band edge emission from In2O3. In contrast, Sn:In2O3 nanowires grown on glass at 500Â?Â{\textdegree}C can be treated under H2S only below 200Â?Â{\textdegree}C which is important for the fabrication of Cu2S/Sn:In2O3 core-shell p-n junctions on low-cost transparent substrates such as glass suitable for quantum dot-sensitized solar cells.

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Current Transport Properties of CuS/Sn:In2O3 versus CuS/SnO2 Nanowires and Negative Differential Resistance in Quantum Dot Sensitized Solar Cells
Zervos M, Vasile E, Vasile E, Karageorgou E, Othonos A

The Journal of Physical Chemistry C, DOI: 10.1021/acs.jpcc.5b08306
The structural, optical, and electrical transport properties of nanowires obtained by the deposition of Cu over Sn doped In2O3 and SnO2 nanowires followed by processing under H2S between 100 and 500 °C have been investigated for their use in quantum dot sensitized solar cells. We find that the CuS/Sn:In2O3 nanowires obtained between 100 and 200 °C consist of hexagonal CuS and cubic In2O3 but higher temperatures lead to the formation of Cu0.23In2.59S4 nanowires. Moreover, we observed the existence of SnO2 quantum dots in tetragonal Cu2SnS3 nanowires obtained at 400–500 °C which are responsible for ultraviolet emission at 3.65 eV and a breakdown of the dipole forbidden rule in SnO2.The CuS/Sn:In2O3 nanowires obtained at lower temperatures exhibit better rectifying current–voltage characteristics and higher currents, but we did not observe negative differential resistance, as expected from a p–n tunnel junction, although this occurred by bringing Sn:In2O3 nanowires in weak contact with p-type CuS, similar to a cat’s whisker device. We discuss the origin of the negative differential resistance which was also observed in connection with the TiO2 barriers deposited on the transparent conducting oxide anode and its importance for quantum dot sensitized solar cells.

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Year: 2014

Ultraviolet emission from low resistance Cu2SnS3/SnO2 and CuInS2/Sn:In2O3 nanowires
Karageorgou E, Zervos M, Othonos A

APL Materials, DOI: 10.1063/1.4901295
SnO2 and Sn:In2O3 nanowires were grown on Si(001), and p-n junctions were fabricated in contact with p-type Cu2S which exhibited rectifying current–voltage characteristics. Core-shell Cu2SnS3/SnO2 and CuInS2/Sn:In2O3 nanowires were obtained by depositing copper and post-growth processing under H2S between 100 and 500 °C. These consist mainly of tetragonal rutile SnO2 and cubic bixbyite In2O3. We observe photoluminescence at 3.65 eV corresponding to band edge emission from SnO2 quantum dots in the Cu2SnS3/SnO2 nanowires due to electrostatic confinement. The Cu2SnS3/SnO2 nanowires assemblies had resistances of 100 Ω similar to CuInS2/In2O3 nanowires which exhibited photoluminescence at 3.0 eV

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Photophysics of PbS Quantum Dot Films Capped with Arsenic Sulfide Ligands
Tsokkou D, Papagiorgis P, Protesescu L, Kovalenko MV, Choulis SA, Christofides C, Itskos G, Othonos A

Advanced Energy Materials, DOI: 10.1002/aenm.201301547
PbS quantum dots (QDs) of different sizes capped with short (NH4)3AsS3 inorganic ligands are produced via ligand exchange processes from oleate-capped PbS QDs. The solid-state photophysical properties of the control organic-capped and the inorganic-ligand-capped QDs are investigated to determine their potential for optoelectronic applications. Ultrafast transient transmission shows that in the oleate-capped QDs, carrier recombination at sub-nanosecond scales occurs via Auger recombination, traps, and surface states. At longer times, intense signals associated with radiative recombination are obtained. After ligand exchange, the QDs become decorated with (NH4)3AsS3 complexes and relaxation is dominated by efficient carrier transfer to the ligand states on timescales as fast as ≈2 ps, which competes with carrier thermalization to the QD band edge states. Recombination channels present in the oleate-capped QDs, such as radiative and Auger recombination, appear quenched in the inorganic-capped QDs. Evidence of efficient carrier trapping at shallow ligand states, which appears more intense under excitation above the (NH4)3AsS3 gap, is provided. A detailed band diagram of the various relaxation and recombination processes is proposed that comprehensively describes the photophysics of the QD systems studied.

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Broad compositional tunability of indium tin oxide nanowires grown by the vapor-liquid-solid mechanism
Zervos M, Mihailescu CN, Giapintzakis J, Luculescu CR, Florini N, Komninou P, Kioseoglou J, Othonos A

APL Materials, DOI: 10.1063/1.4875457
Indium tin oxide nanowires were grown by the reaction of In and Sn with O2 at 800 °C via the vapor-liquid-solid mechanism on 1 nm Au/Si(001). We obtain Sn doped In2O3 nanowires having a cubic bixbyite crystal structure by using In:Sn source weight ratios > 1:9 while below this we observe the emergence of tetragonal rutile SnO2 and suppression of In2O3 permitting compositional and structural tuning from SnO2 to In2O3 which is accompanied by a blue shift of the photoluminescence spectrum and increase in carrier lifetime attributed to a higher crystal quality and Fermi level position.

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Year: 2013

Zn3N2 nanowires: growth, properties and oxidation
Zervos M, Karipi C, Othonos A

, DOI: 10.1186/1556-276X-8-221
Zinc nitride (Zn3N2) nanowires (NWs) with diameters of 50 to 100 nm and a cubic crystal structure have been grown on 1 nm Au/Al2O3 via the reaction of Zn with NH3 including H2 between 500°C and 600°C. These exhibited an optical band gap of ≈ 3.2 eV, estimated from steady state absorption-transmission spectroscopy. We compared this with the case of ZnO NWs and discussed the surface oxidation of Zn3N2 NWs which is important and is expected to lead to the formation of a Zn3N2/ZnO core-shell NW, the energy band diagram of which was calculated via the self-consistent solution of the Poisson-Schrödinger equations within the effective mass approximation by taking into account a fundamental energy band gap of 1.2 eV. In contrast, only highly oriented Zn3N2 layers with a cubic crystal structure and an optical band gap of ≈ 2.9 eV were obtained on Au/Si(001) using the same growth conditions.

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Structural properties of SnO2 nanowires and the effect of donor like defects on its charge distribution
Zervos M, Othonos A, Tsokkou D, Kioseoglou J, Pavlidou E, Komninou P

physica status solidi (a), DOI: 10.1002/pssa.201200403
Tin oxide (SnO2) nanowires (NWs) with diameters of 50 nm, lengths up to 100 µm and a tetragonal rutile crystal structure have been grown by low pressure reactive vapour transport on 1 nm Au/Si(001). The free carrier density of the SnO2 NWs measured by THz absorption spectroscopy was found to be n = (3.3 ± 0.4) × 1016 cm−3. Based on this we have determined the one-dimensional (1D) sub-band energies, overall charge distribution and band bending via the self-consistent solution of the Poisson–Schrödinger equations in cylindrical coordinates and in the effective mass approximation. We find that a high density of 1018–1019 cm−3 donor-like defect related states is required to obtain a line density of 0.7 × 109 close to the measured value by taking the Fermi level to be situated ≈0.7 eV below the conduction band edge at the surface which gives a surface depletion shell thickness of 15 nm. We discuss the origin of the donor-like states that are energetically located in the upper half of the energy band gap as determined by ultrafast, time-resolved absorption–transmission spectroscopy.

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