Publications

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: http://dx.doi.org/10.1016/j.synthmet.2017.01.006
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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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: http://dx.doi.org/10.1016/j.matlet.2015.12.041
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: http://dx.doi.org/10.1016/j.mseb.2015.03.006
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

Size-Dependent Charge Transfer in Blends of PbS Quantum Dots with a Low-Gap Silicon-Bridged Copolymer
Itskos G, Papagiorgis P, Tsokkou D, Othonos A, Hermerschmidt F, Economopoulos SP, Yarema M, Heiss W, Choulis S

Advanced Energy Materials, DOI: 10.1002/aenm.201300317
The photophysics of bulk heterojunctions of a high-performance, low-gap silicon-bridged dithiophene polymer with oleic acid capped PbS quantum dots (QDs) are studied to assess the material potential for light harvesting in the visible- and IR-light ranges. By employing a wide range of nanocrystal sizes, systematic dependences of electron and hole transfer on quantum-dot size are established for the first time on a low-gap polymer–dot system. The studied system exhibits type II band offsets for dot sizes up to ca. 4 nm, whch allow fast hole transfer from the quantum dots to the polymer that competes favorably with the intrinsic QD recombination. Electron transfer from the polymer is also observed although it is less competitive with the fast polymer exciton recombination for most QD sizes studied. The incorporation of a fullerene derivative provides efficient electron-quenching sites that improve interfacial polymer-exciton dissociation in ternary polymer–fullerene–QD blends. The study indicates that programmable band offsets that allow both electron and hole extraction can be produced for efficient light harvesting based on this low-gap polymer-PbS QD composite.

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Structure, morphology, and photoluminescence of porous Si nanowires: effect of different chemical treatments
Leontis I, Othonos A, Nassiopoulou AG

, DOI: 10.1186/1556-276X-8-383
The structure and light-emitting properties of Si nanowires (SiNWs) fabricated by a single-step metal-assisted chemical etching (MACE) process on highly boron-doped Si were investigated after different chemical treatments. The Si nanowires that result from the etching of a highly doped p-type Si wafer by MACE are fully porous, and as a result, they show intense photoluminescence (PL) at room temperature, the characteristics of which depend on the surface passivation of the Si nanocrystals composing the nanowires. SiNWs with a hydrogen-terminated nanostructured surface resulting from a chemical treatment with a hydrofluoric acid (HF) solution show red PL, the maximum of which is blueshifted when the samples are further chemically oxidized in a piranha solution. This blueshift of PL is attributed to localized states at the Si/SiO2 interface at the shell of Si nanocrystals composing the porous SiNWs, which induce an important pinning of the electronic bandgap of the Si material and are involved in the recombination mechanism. After a sequence of HF/piranha/HF treatment, the SiNWs are almost fully dissolved in the chemical solution, which is indicative of their fully porous structure, verified also by transmission electron microscopy investigations. It was also found that a continuous porous Si layer is formed underneath the SiNWs during the MACE process, the thickness of which increases with the increase of etching time. This supports the idea that porous Si formation precedes nanowire formation. The origin of this effect is the increased etching rate at sites with high dopant concentration in the highly doped Si material.

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Ultrafast transient spectroscopy and photoluminescence properties of V2O5 nanowires
Othonos A, Christofides C, Zervos M

Applied Physics Letters, DOI: http://dx.doi.org/10.1063/1.4823506
The properties of V2O5 semiconductor nanowires have been investigated using ultrashort transient absorption spectroscopy in conjunction with time resolved photoluminescence. Femtosecond pulse excitation has been utilized to generate non equilibrium carrier densities above the band edge of the Nanowires (NWs), and non-degenerate pump probe techniques have been employed to follow carrier relaxation through the conduction band and defects states located within the band gap of the semiconductor NWs. Photoluminescence revealed three relaxation mechanisms with time constants ranging from a single to tens of ns providing evidence of the importance of radiative and non-radiative decay channels associated with states within the nanowires.

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Ultrafast pulsed laser deposition of carbon nanostructures: Structural and optical characterization
Pervolaraki M, Komninou P, Kioseoglou J, Othonos A, Giapintzakis J

Applied Surface Science , DOI: http://dx.doi.org/10.1016/j.apsusc.2013.03.015
Carbon nanostructured materials were obtained by high-repetition rate pulsed laser ablation of a graphite target using a train of 10-ps duration pulses at 1064 nm in different pressures of high-purity Ar gas. It is demonstrated that their microstructure and optical properties vary as a function of the argon pressure. High-resolution transmission electron microscopy revealed the existence of onion-like carbon nanostructures embedded in a matrix of amorphous carbon nanofoam for samples prepared at 300 Pa. In comparison samples prepared at 30 Pa show evidence of both onion-like and turbostratic carbon coexisting in a matrix of amorphous carbon nanofoam whereas samples prepared in vacuum are continuous films of amorphous carbon. Transient transmission spectroscopy measurements suggested that free carrier absorption is the dominant effect following photo-excitation for probing wavelengths in the range of 550–1000 nm and its magnitude varies among the materials investigated due to their different microstructures.

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Concentration and excitation effects on the exciton dynamics of poly(3-hexylthiophene)/PbS quantum dot blend films
Tsokkou D, Itskos G, Choulis S, Yarema M, Heiss W, Othonos A

Nanotechnology, DOI:
The dynamics of photoexcitations in hybrid blends of poly(3-hexylthiophene) (P3HT) conjugated polymer donor and oleic-acid capped lead sulfide (PbS) quantum dot (QD) acceptors of different concentrations—for light harvesting applications—were investigated using time-resolved transmission and photoluminescence spectroscopies. Following excitation at 400 nm and probing in the 500–1000 nm region, we find that geminate excitation recombination in the blend of P3HT/PbS QDs dominates the transient decays at sub-ns times while intermaterial interactions such as charge transfer processes appear at longer times in the 1–50 ns regime. For the hybrid blend films with lower QD concentrations (

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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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Year: 2012

A systematic study of the nitridation of SnO2 nanowires grown by the vapor liquid solid mechanism
Zervos M, Othonos A

Journal of Crystal Growth, DOI: 10.1016/j.jcrysgro.2011.11.063Download
SnO2 nanowires (NWs) with diameters of 50 nm and lengths ≥10 μm have been grown at 800 °C on 1.0 nm Au/Si(001) via the vapor liquid solid mechanism and the low pressure chemical vapor deposition. These exhibited clear peaks in the X-ray diffraction corresponding to the tetragonal rutile crystal structure of SnO2 and a broad-symmetric photoluminescence (PL) spectrum, centered around 560 nm due to structural-related defect states, energetically located in the upper half-band-gap of SnO2. We find that post-growth thermal annealing of the SnO2 NWs over a broad range of temperatures, i.e. 400–1000 °C and high flow of O2 does not change their crystal structure or optical properties. In contrast the nitridation of SnO2 NWs using NH3 leads to their elimination above 500 °C. Lower temperatures did not favor the nitridation even using extended nitridation times, hydrogen, lower ramp rates or a two-step-temperature process which are effective in the case of In2O3 and Ga2O3. However the nitridation of SnO2 NWs was promoted by HCl, supplied in-situ via the sublimation of NH4Cl, which reacts with Sn and SnO2 leading to the formation of the intermediate SnCl4, which reacts in turn with NH3 giving tin nitride at temperatures between 400 and 500 °C . We discuss the effect of the nitridation and thermal annealing on the PL spectra.

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Synthesis of hybrid polymethacrylate-noble metal (M = Au, Pd) nanoparticles for the growth of metal-oxide semiconductor nanowires
Zervos M, Demetriou M, Krasia-Christoforou T, Othonos A, Turcu RP

RSC Adv., DOI: 10.1039/C2RA01072K
Metal-oxide semiconductor nanowires (NWs) such as ZnO{,} [small beta]-Ga2O3 and SnO2 with diameters of tens of nanometres and lengths of many micrometres have been grown using micellar nanohybrids consisting of methacrylate-based diblock copolymers and noble metal nanoparticles (MNPs). Micellar Au and Pd MNPs with diameters as small as 2.3 +/- 0.3 nm were deposited on Si(001) by spin coating or drop casting and metal-oxide NWs were grown by reactive vapor transport. A high yield of [small beta]-Ga2O3 NWs with diameters of approximately 40 nm{,} lengths > 10 [small mu ]m and a monoclinic crystal structure were obtained at 900 [degree]C with the largest MNPs. These exhibited a broad{,} symmetric photoluminescence (PL) spectrum centred at 2.3 eV attributed to defect states situated energetically in the energy band gap of [small beta]-Ga2O3. We find that a reduction in the size of the MNPs below 10 nm leads to the formation of necklace like [small beta]-Ga2O3 NWs via the encapsulation of the MNPs which act as catalytic centres for the formation of branched nanostructures along the length of the [small beta]-Ga2O3 NWs that are also responsible for a blue shift in the PL at 2.8 eV as a result of quantum confinement. This was not observed upon reducing the density of MNPs or in the case of ZnO or SnO2 NWs grown with the smallest of MNPs probably due to differences in surface energy. We show that polymethacrylate-noble MNPs may be patterned directly by electron beam lithography and may be exploited for the selective location growth of semiconductor NWs while we also discuss the difference between the sizes of the hybrid polymer-MNPs and MO NWs which is attributed to agglomeration.

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The nitridation of ZnO nanowires
Zervos M, Karipi C, Othonos A

, DOI: 10.1186/1556-276X-7-175Download
ZnO nanowires (NWs) with diameters of 50 to 250 nm and lengths of several micrometres have been grown by reactive vapour transport via the reaction of Zn with oxygen on 1 nm Au/Si(001) at 550°C under an inert flow of Ar. These exhibited clear peaks in the X-ray diffraction corresponding to the hexagonal wurtzite crystal structure of ZnO and a photoluminescence spectrum with a peak at 3.3 eV corresponding to band edge emission close to 3.2 eV determined from the abrupt onset in the absorption-transmission through ZnO NWs grown on 0.5 nm Au/quartz. We find that the post growth nitridation of ZnO NWs under a steady flow of NH3 at temperatures ≤600°C promotes the formation of a ZnO/Zn3N2 core-shell structure as suggested by the suppression of the peaks related to ZnO and the emergence of new ones corresponding to the cubic crystal structure of Zn3N2 while maintaining their integrity. Higher temperatures lead to the complete elimination of the ZnO NWs. We discuss the effect of nitridation time, flow of NH3, ramp rate and hydrogen on the conversion and propose a mechanism for the nitridation.

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