Publications

Year: 2013

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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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

Monitoring Charge Exchange in P3HT-Nanotube Composites Using Optical and Electrical Characterisation
Alexandrou I, Lioudakis E, Delaportas D, Zhao C, Othonos A

Nanoscale Research Letters, DOI: 10.1007/s11671-009-9287-9Download
Charge exchange at the bulk heterojunctions of composites made by mixing single wall nanotubes (SWNTs) and polymers show potential for use in optoelectronic devices such as solar cells and optical sensors. The density/total area of these heterojunctions is expected to increase with increasing SWNT concentration but the efficiency of solar cell peaks at low SWNT concentrations. Most researchers use current–voltage measurements to determine the evolution of the SWNT percolation network and optical absorption measurements to monitor the spectral response of the composites. However, these methods do not provide a detailed account of carrier transport at the concentrations of interest; i.e., near or below the percolation threshold. In this article, we show that capacitance–voltage (C–V) response of (metal)-(oxide)-(semiconducting composite) devices can be used to fill this gap in studying bulk heterojunctions. In an approach where we combine optical absorption methods with C–V measurements we can acquire a unified optoelectronic response from P3HT-SWNT composites. This methodology can become an important tool for optoelectronic device optimization.

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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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Ultrafast Carrier Relaxation in InN Nanowires Grown by Reactive Vapor Transport
Othonos A, Zervos M, Pervolaraki M

Nanoscale Research Letters, DOI: 10.1007/s11671-008-9211-8Download
We have studied femtosecond carrier dynamics in InN nanowires grown by reactive vapor transport. Transient differential absorption measurements have been employed to investigate the relaxation dynamics of photogenerated carriers near and above the optical absorption edge of InN NWs where an interplay of state filling, photoinduced absorption, and band-gap renormalization have been observed. The interface between states filled by free carriers intrinsic to the InN NWs and empty states has been determined to be at 1.35 eV using CW optical transmission measurements. Transient absorption measurements determined the absorption edge at higher energy due to the additional injected photogenerated carriers following femtosecond pulse excitation. The non-degenerate white light pump-probe measurements revealed that relaxation of the photogenerated carriers occurs on a single picosecond timescale which appears to be carrier density dependent. This fast relaxation is attributed to the capture of the photogenerated carriers by defect/surface related states. Furthermore, intensity dependent measurements revealed fast energy transfer from the hot photogenerated carriers to the lattice with the onset of increased temperature occurring at approximately 2 ps after pulse excitation.

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Ultrafast time-resolved spectroscopy of ZnSe nanowires: Carrier dynamics of defect-related states
Othonos A, Lioudakis E, Tsokkou D, Philipose U, Ruda HE

Journal of Alloys and Compounds, DOI: DOI: 10.1016/j.jallcom.2008.07.197Download
In recent years, ZnSe nanowires have been widely investigated for their potential applications in optoelectronics. A typical room temperature photoluminescence spectrum of ZnSe nanowires grown by vapor–liquid–solid growth under different growth conditions shows that the spectrum is dominated by two characteristic emission peaks. The first peak is attributed to the band edge emission peak at 2.68 eV whereas the second to the broad deep defect-related emission peak in the region of 1.8–2.4 eV. In thiswork,we present a study of ultrafast time-resolved spectroscopy of defect states of ZnSe nanowires grown under Se-rich growth conditions. We investigate in detail the carrier dynamics of these nanostructure materials using selective optical excitation femtosecond pulses from a wavelength tunable optical parametric amplifier system. The effects of intrinsic point defects inherent in the manufacturing of these materials and in particular the relaxations of the photogenerated carriers occupying these defect states are examined.Temporal dynamics on a few picoseconds time-scale provided information on effects such as state filling and secondary excitation and their contribution to the overall induced absorption. Long time-scale probing of induced absorption provided information on the defect states associated with the observed photoluminescence in this material.

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Femtosecond Carrier Dynamics in In2O3 Nanocrystals
Othonos A, Zervos M, Tsokkou D

Nanoscale Research Letters, DOI: Download
We have studied carrier dynamics in In2O3 nanocrystals grown on a quartz substrate using chemical vapor deposition. Transient differential absorption measurements have been employed to investigate the relaxation dynamics of photo-generated carriers in In2O3 nanocrystals. Intensity measurements reveal that Auger recombination plays a crucial role in the carrier dynamics for the carrier densities investigated in this study. A simple differential equation model has been utilized to simulate the photo-generated carrier dynamics in the nanocrystals and to fit the fluence-dependent differential absorption measurements. The average value of the Auger coefficient obtained from fitting to the measurements was γ = 5.9 ± 0.4 × 10−31 cm6 s−1. Similarly the average relaxation rate of the carriers was determined to be approximately τ = 110 ± 10 ps. Time-resolved measurements also revealed ~25 ps delay for the carriers to reach deep traps states which have a subsequent relaxation time of approximately 300 ps.

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Influence of surface-related states on the carrier dynamics in (Ga,In)N/GaN single quantum wells
Othonos A, Itskos G, Bradley DD, Dawson MD, Watson IM

Applied Physics Letters, DOI: 10.1063/1.3139079Download
We report on the influence of surface-related states on the relaxation of carriers within single (Ga,In)N/GaN quantum wells. Two identical samples that differ only in the thickness of the top GaN cap layer were studied. Photoluminescence and pump-probe measurements reveal significant variations in the quantum well integrated emission and the carrier relaxation decay times in the two samples, when probing both the ground and excited states of the wells. The variations are attributed to the presence of an efficient nonradiative relaxation channel associated with the proximity of the quantum well excitations to the surface-related states in the thin-cap sample.

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Tin Oxide Nanowires: The Influence of Trap States on Ultrafast Carrier Relaxation
Othonos A, Zervos M, Tsokkou D

Nanoscale Research Letters, DOI: 10.1007/s11671-009-9323-9Download
We have studied the optical properties and carrier dynamics in SnO2 nanowires (NWs) with an average radius of 50 nm that were grown via the vapor–liquid solid method. Transient differential absorption measurements have been employed to investigate the ultrafast relaxation dynamics of photogenerated carriers in the SnO2 NWs. Steady state transmission measurements revealed that the band gap of these NWs is 3.77 eV and contains two broad absorption bands. The first is located below the band edge (shallow traps) and the second near the center of the band gap (deep traps). Both of these absorption bands seem to play a crucial role in the relaxation of the photogenerated carriers. Time resolved measurements suggest that the photogenerated carriers take a few picoseconds to move into the shallow trap states whereas they take ~70 ps to move from the shallow to the deep trap states. Furthermore the recombination process of electrons in these trap states with holes in the valence band takes ~2 ns. Auger recombination appears to be important at the highest fluence used in this study (500 μJ/cm2); however, it has negligible effect for fluences below 50 μJ/cm2. The Auger coefficient for the SnO2 NWs was estimated to be 7.5 ± 2.5 × 10−31 cm6/s.

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Defect states of chemical vapor deposition grown GaN nanowires: Effects and mechanisms in the relaxation of carriers
Tsokkou D, Othonos A, Zervos M

Journal of Applied Physics, DOI: 10.1063/1.3212989Download
Carrier relaxation in GaN nanowires, grown by atmospheric pressure chemical vapor deposition, via direct nitridation of Ga with NH3 at 950 °C has been investigated in detail. Differential absorption measurements reveal a large number of defect states located within the band gap. The relaxation dynamics of the photogenerated carriers suggest three distinct regions of energy states below the band edge identified as shallow donor states, midgap states, and deep acceptor states. Measurements suggest that Auger recombination is not a contributing factor in carrier relaxation even at the highest fluence ( ∼ 1 mJ/cm2) used in this work for carriers located within the conduction band. On the contrary, Auger recombination has been observed when probing the shallow donor states for fluences above 40 μJ/cm2. Measurements at the lowest fluence reveal a biexponential relaxation for the donor states with the fast component ( ∼ 50 ps) corresponding to the relaxation of carriers into the midgap states and the slow component of 0.65 ns associated with the relaxation into the deep acceptor states. Measurements reveal free-carrier absorption contribution from the deep acceptor states to the U-valley with an observed threshold limit of 3.5 eV suggesting the U-valley is located approximately 4.7 eV from the valence band.

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Low Temperature Growth of In2O3 and InN Nanocrystals on Si(111) via Chemical Vapour Deposition Based on the Sublimation of NH4Cl in In
Zervos M, Tsokkou D, Pervolaraki M, Othonos A

Nanoscale Research Letters, DOI: 10.1007/s11671-009-9266-1Download
Indium oxide (In2O3) nanocrystals (NCs) have been obtained via atmospheric pressure, chemical vapour deposition (APCVD) on Si(111) via the direct oxidation of In with Ar:10% O2 at 1000 °C but also at temperatures as low as 500 °C by the sublimation of ammonium chloride (NH4Cl) which is incorporated into the In under a gas flow of nitrogen (N2). Similarly InN NCs have also been obtained using sublimation of NH4Cl in a gas flow of NH3. During oxidation of In under a flow of O2 the transfer of In into the gas stream is inhibited by the formation of In2O3 around the In powder which breaks up only at high temperatures, i.e. T > 900 °C, thereby releasing In into the gas stream which can then react with O2 leading to a high yield formation of isolated 500 nm In2O3 octahedrons but also chains of these nanostructures. No such NCs were obtained by direct oxidation for TG ≤ 900 °C. The incorporation of NH4Cl in the In leads to the sublimation of NH4Cl into NH3 and HCl at around 338 °C which in turn produces an efficient dispersion and transfer of the whole In into the gas stream of N2 where it reacts with HCl forming primarily InCl. The latter adsorbs onto the Si(111) where it reacts with H2O and O2 leading to the formation of In2O3 nanopyramids on Si(111). The rest of the InCl is carried downstream, where it solidifies at lower temperatures, and rapidly breaks down into metallic In upon exposure to H2O in the air. Upon carrying out the reaction of In with NH4Cl at 600 °C under NH3 as opposed to N2, we obtain InN nanoparticles on Si(111) with an average diameter of 300 nm.

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