Publications

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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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

Excitation dynamics of a low bandgap silicon-bridged dithiophene copolymer and its composites with fullerenes
Othonos A, Itskos G, Neophytou M, Choulis SA

Applied Physics Letters, DOI: 10.1063/1.3703601Download
We report on excitation dynamics in pristine and bulk heterojunction films of the low bandgap silicon-bridged dithiophene copolymer poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′, 3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] with methanofullerene derivatives. The combination of ultrafast transient transmission and photoluminescence allows us to probe the relaxation of both exciton and polaron states in a relatively wide spectral and temporal range. Measurements reveal that the majority of excitations undergo ultrashort non-radiative relaxation while a small fraction of the photoexcited species decays slowly within hundreds of ps. In the blend films, significantly longer decays are observed suggesting the presence of long lived holes and/or charged-transfer type of excitons.

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Carrier dynamics and conductivity of SnO2 nanowires investigated by time-resolved terahertz spectroscopy
Tsokkou D, Othonos A, Zervos M

Applied Physics Letters, DOI: 10.1063/1.3698097Download
THz spectroscopy has been applied to investigate the photo-induced and intrinsic conductivity in SnO2 nanowires using the Drude-Smith model. The refractive index of the nanowires was found to decrease from 2.4 to 2.1 with increasing THz frequency and the dc mobility of the non-excited nanowires was determined to be 72 ± 10 cm2/Vs. Measurements reveal that scattering times are carrier density dependent, while a strong suppression of long transport is evident. Intensity-dependent measurements provided an estimate of the Auger coefficient found to be γ = (7.2 ± 2.0) × 10−31 cm6/s.

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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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Year: 2011

Optical Properties of Organic Semiconductor Blends with Near-Infrared Quantum-Dot Sensitizers for Light Harvesting Applications
Itskos G, Othonos A, Rauch T, Tedde SF, Hayden O, Kovalenko MV, Heiss W, Choulis SA

Advanced Energy Materials, DOI: 10.1002/aenm.201100182Download
We report an optical investigation of conjugated polymer (P3HT)/fullerene (PCBM) semiconductor blends sensitized by near-infrared absorbing quantum dots (PbS QDs). A systematic series of samples that include pristine, binary and ternary blends of the materials are studied using steady-state absorption, photoluminescence (PL) and ultrafast transient absorption. Measurements show an enhancement of the absorption strength in the near-infrared upon QD incorporation. PL quenching of the polymer and the QD exciton emission is observed and predominantly attributed to intermaterial photoinduced charge transfer processes. Pump-probe experiments show photo-excitations to relax via an initial ultrafast decay while longer-lived photoinduced absorption is attributed to charge transfer exciton formation and found to depend on the relative ratio of QDs to P3HT:PCBM content. PL experiments and transient absorption measurements indicate that interfacial charge transfer processes occur more efficiently at the fullerene/polymer and fullerene/nanocrystal interfaces compared to polymer/nanocrystal interfaces. Thus the inclusion of the fullerene seems to facilitate exciton dissociation in such blends. The study discusses important and rather unexplored aspects of exciton recombination and charge transfer processes in ternary blend composites of organic semiconductors and near-infrared quantum dots for applications in solution-processed photodetectors and solar cells.

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Ultrafast hole carrier relaxation dynamics in p-type CuO nanowires
Othonos A, Zervos M

Nanoscale Research Letters, DOI: doi:10.1186/1556-276X-6-62Download
Ultrafast hole carrier relaxation dynamics in CuO nanowires have been investigated using transient absorption spectroscopy. Following femtosecond pulse excitation in a non-collinear pump-probe configuration, a combination of non-degenerate transmission and reflection measurements reveal initial ultrafast state filling dynamics independent of the probing photon energy. This behavior is attributed to the occupation of states by photo-generated carriers in the intrinsic hole region of the p-type CuO nanowires located near the top of the valence band. Intensity measurements indicate an upper fluence threshold of 40 μJ/cm2 where carrier relaxation is mainly governed by the hole dynamics. The fast relaxation of the photo-generated carriers was determined to follow a double exponential decay with time constants of 0.4 ps and 2.1 ps. Furthermore, time-correlated single photon counting measurements provide evidence of three exponential relaxation channels on the nanosecond timescale.

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An investigation into the conversion of In2O3 into InN nanowires

Nano Research Letters, DOI: 10.1186/1556-276X-6-311Download
Straight In2O3 nanowires (NWs) with diameters of 50 nm and lengths ≥2 μm have been grown on Si(001) via the wet oxidation of In at 850°C using Au as a catalyst. These exhibited clear peaks in the X-ray diffraction corresponding to the body centred cubic crystal structure of In2O3 while the photoluminescence (PL) spectrum at 300 K consisted of two broad peaks, centred around 400 and 550 nm. The post-growth nitridation of In2O3 NWs was systematically investigated by varying the nitridation temperature between 500 and 900°C, flow of NH3 and nitridation times between 1 and 6 h. The NWs are eliminated above 600°C while long nitridation times at 500 and 600°C did not result into the efficient conversion of In2O3 to InN. We find that the nitridation of In2O3 is effective by using NH3 and H2 or a two-step temperature nitridation process using just NH3 and slower ramp rates. We discuss the nitridation mechanism and its effect on the PL.

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Gallium hydride vapor phase epitaxy of GaN nanowires
Zervos M, Othonos A

Nano Express, DOI: 10.1186/1556-276X-6-262Download
Straight GaN nanowires (NWs) with diameters of 50 nm, lengths up to 10 μm and a hexagonal wurtzite crystal structure have been grown at 900°C on 0.5 nm Au/Si(001) via the reaction of Ga with NH3 and N2:H2, where the H2 content was varied between 10 and 100%. The growth of high-quality GaN NWs depends critically on the thickness of Au and Ga vapor pressure while no deposition occurs on plain Si(001). Increasing the H2 content leads to an increase in the growth rate, a reduction in the areal density of the GaN NWs and a suppression of the underlying amorphous (α)-like GaN layer which occurs without H2. The increase in growth rate with H2 content is a direct consequence of the reaction of Ga with H2which leads to the formation of Ga hydride that reacts efficiently with NH3 at the top of the GaN NWs. Moreover, the reduction in the areal density of the GaN NWs and suppression of the α-like GaN layer is attributed to the reaction of H2 with Ga in the immediate vicinity of the Au NPs. Finally, the incorporation of H2 leads to a significant improvement in the near band edge photoluminescence through a suppression of the non-radiative recombination via surface states which become passivated not only via H2, but also via a reduction of O2-related defects.

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Enhanced growth and photoluminescence properties of SnxNy (x>y) nanowires grown by halide chemical vapor deposition
Zervos M, Othonos A

Journal of Crystal Growth, DOI: DOI: 10.1016/j.jcrysgro.2010.12.029
Tin nitride nanowires have been grown by halide chemical vapor deposition via the reaction of Sn with NH4Cl at 425 °C under a steady flow of NH3 using small ramp rates c=5.193 Å, a=3.725 Å. The excitation of the SnxNy NWs with UV light of λ=300 nm at T=300 and 77 K gave a broad photoluminescence (PL) spectrum covering 450–750 nm attributed to optical transitions between shallow and deep traps located within the band gap. These traps are most likely related to surface and nitrogen vacancy states. Time correlated, single photon counting PL measurements taken between 450 and 750 nm, showed that the PL decay has a multi-exponential structure, suggesting the existence of complex, non-radiative relaxation paths with relaxation times that are found to become shorter at smaller wavelengths. Finally no significant differences were observed between the PL spectra of the SnxNy and In doped SnxNy NWs most likely due to the low level of incorporation of In attributed to differences in the ionic radii of In and Sn but also the larger energy and growth temperatures required for the formation of In–N bonds.

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Year: 2010

Carrier dynamics in InS nanowires grown via chemical vapor deposition
Othonos A, Zervos M

Physica Status Solidi (a), DOI: 10.1002/pssa.201026048Download
Transient femtosecond absorption spectroscopy and time correlating single photon counting (TCSPC) photoluminescence (PL) were employed to study InS nanowires (NWs) grown by chemical vapor deposition (CVD) and determine the relaxation mechanisms in these nanostructures. Intensity dependent measurements revealed that Auger recombination plays an important role in the relaxation of photogenerated carriers at fluences larger than 0.4x1015 photons/cm2. Calculations provided an estimated of the Auger recombination coefficient to be 1.1x10-31 cm6/s. At the low fluence regime TCSPC PL revealed three relaxation mechanisms with time constants ranging from ps to nanosecond providing evidence of the importance of non-radiative decay channels associated with defect/trap states within the NWs.

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A systematic investigation into the conversion of beta -Ga2O3 to GaN nanowires using NH3 and H2: Effects on the photoluminescence properties
Othonos A, Zervos M, Christofides C

Journal of Applied Physics, DOI: 10.1063/1.3525562Download
GaN nanowires (NWs) with a hexagonal wurtzite crystal structure, diameters of 50 nm and lengths of 10 μm have been obtained from postgrowth nitridation of monoclinic β-Ga2O3 NWs using NH3 between 700–1090 °C. The conversion of β-Ga2O3 to GaN NWs has been investigated in a systematic way by varying the temperature, gas flows and nitridation times using Ar or N2:10% H2. We find that nitridation is most effective at temperatures ≥ 900 °C using NH3 with N2:10% H2 which promotes the efficient conversion of β-Ga2O3 to GaN, resulting into the enhancement of the band edge emission, suppression of the broad-band photoluminescence (PL) related to oxygen defects and the appearance of red emission due to deep-acceptorlike states. The gradual evolution of the PL spectra from that of β-Ga2O3 to GaN exhibited a clear, systematic dependence on the nitridation temperature and gas flows and the band to band emission lifetime which was found to be τ ≈ 0.35 ns in all cases. In contrast the nitridation of β-Ga2O3 NWs using NH3 and Ar is less effective. Therefore, H2 is essential in removing O2 and also effective since it lead to the complete elimination of the β-Ga2O3 NWs at 1000 °C in the absence of NH3.

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Carrier dynamics in beta -Ga2O3 nanowires
Othonos A, Zervos M, Christofides C

Journal of Applied Physics, DOI: 10.1063/1.3520589Download
Carrier dynamics have been investigated in β-Ga2O3 nanowires (NWs) grown by the vapor-liquid-solid mechanism, using ultrashort transient absorption spectroscopy in conjunction with time-correlating single photon counting photoluminescence. UV femtosecond pulse excitation has been utilized to generate nonequilibrium carrier distributions near the band edge of the NWs and nondegenerate pump-probe techniques have been employed to follow carrier relaxation through the defect related states located within the band gap of the NW semiconductor. Relaxation of the photogenerated carriers through these states appears to be biexponential with a fast component on the order of 3–5 ps and the slower component around 40–90 ps depending on the states being probed. Transient absorption intensity measurements reveal that recombination mechanisms such as Auger and bimolecular become contributing factors to the relaxation dynamics for absorbed fluences larger than 90 μJ/cm2. In the low fluence regime, time-correlated single photon counting photoluminescence measurements revealed a nonradiative relaxation mechanism with time constants ranging from 0.6–1 ns and a radiative relaxation with a time constant of approximately 4 ns.

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Hydride-assisted growth of GaN nanowires on Au/Si(001)via the reaction of Ga with NH3 and H2
Zervos M, Othonos A

Journal of Crystal Growth, DOI: 10.1016/j.jcrysgro.2010.05.040Download
High quality, straight GaN nanowires (NWs) with diameters of 50 nm and lengths up to 3 μm have been grown on Si(0 0 1) using Au as a catalyst and the direct reaction of Ga with NH3 and N2:H2 at 900 °C. These exhibited intense, near band edge photoluminescence at 3.42 eV in comparison to GaN NWs with non-uniform diameters obtained under a flow of Ar:NH3, which showed much weaker band edge emission due to strong non-radiative recombination. A significantly higher yield of β-Ga2O3 NWs with diameters of ≤50 nm and lengths up to 10 μm were obtained, however, via the reaction of Ga with residual O2 under a flow of Ar alone. The growth of GaN NWs depends critically on the temperature, pressure and flows in decreasing order of importance but also the availability of reactive species of Ga and N. A growth mechanism is proposed whereby H2 dissociates on the Au nanoparticles and reacts with Ga giving GaxHy thereby promoting one-dimensional (1D) growth via its reaction with dissociated NH3 near or at the top of the GaN NWs while suppressing at the same time the formation of an underlying amorphous layer. The higher yield and longer β-Ga2O3 NWs grow by the vapor liquid solid mechanism that occurs much more efficiently than nitridation.

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High yield-low temperature growth of indium sulphide nanowires via chemical vapor deposition

Journal of Crystal Growth, DOI: DOI: 10.1016/j.jcrysgro.2009.12.023Download
Indium sulphide nanowires (NWs) have been grown on Si via the reaction of In and InCl3 with H2S using chemical vapor deposition at temperatures as low as 250 °C. We find that the growth of InxSy NWs via the direct reaction of In with H2S is hindered by the formation of InxSy around the source of In which limits its vapor pressure. Thus a low yield of InxSy NWs with diameters of ≈100 nm, lengths up to ≈5 μm and hexagonal crystals measuring ≈500 nm across, were obtained between 500 and 600 °C, but their growth was not uniform or reproducible. These exhibited weak, but nevertheless clear peaks, in the X-ray diffraction (XRD) spectrum corresponding to tetragonal β-In2S3 and orthorhombic InS. No NWs were obtained for TG≤500 °C while for TG>600 °C we obtained a polycrystalline layer with oriented grains of triangular shape. In contrast, a high yield of InS NWs with diameters ≤200 nm and lengths up to ≈2 μm were obtained at temperatures as low as 250 °C via the reaction of In and InCl3 with H2S. The sublimation of InCl3 enhances the vapor pressure of In and the growth of InS NWs, which organize themselves in urchin like structures at 300 °C, exhibiting very intense peaks in the XRD spectrum, corresponding mainly to orthorhombic InS. Optical transmission measurements through the InS NWs gave a band-gap of 2.4 eV.

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Year: 2009

Ultrafast Dynamics of Localized and Delocalized Polaron Transitions in P3HT/PCBM Blend Materials: The Effects of PCBM Concentration
Lioudakis E, Alexandrou I, Othonos A

Nanoscale Research Letters, DOI: 10.1007/s11671-009-9423-6Download
Nowadays, organic solar cells have the interest of engineers for manufacturing flexible and low cost devices. The considerable progress of this nanotechnology area presents the possibility of investigating new effects from a fundamental science point of view. In this letter we highlight the influence of the concentration of fullerene molecules on the ultrafast transport properties of charged electrons and polarons in P3HT/PCBM blended materials which are crucial for the development of organic solar cells. Especially, we report on the femtosecond dynamics of localized (P2 at 1.45 eV) and delocalized (DP2 at 1.76 eV) polaron states of P3HT matrix with the addition of fullerene molecules as well as the free-electron relaxation dynamics of PCBM-related states. Our study shows that as PCBM concentration increases, the amplified exciton dissociation at bulk heterojunctions leads to increased polaron lifetimes. However, the increase in PCBM concentration can be directly related to the localization of polarons, creating thus two competing trends within the material. Our methodology shows that the effect of changes in structure and/or composition can be monitored at the fundamental level toward optimization of device efficiency.

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