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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Carrier relaxation dynamics in SnxNy nanowires grown by chemical vapor deposition
Othonos A, Zervos M

Journal of Applied Physics, DOI: 10.1063/1.3264721Download
Carrier relaxation dynamics in tin nitride (SnxNy) nanowires have been investigated using femtosecond transient absorption spectroscopy. The nanowires were grown directly on quartz using chemical vapor deposition and had diameters ≤ 200 nm and lengths up to 2 μm. Steady state optical transmission measurements suggest that the band gap is ∼ 2.9 eV while time resolved measurements reveal that free carrier absorption dominates the carrier dynamics and overcomes state filling within 0.5 ps of the incoming excitation pulse even when probing above the band edge. This is a unique and markedly different behavior compared to what we have observed in other semiconductor nanowires and it is attributed to fast scattering of the photogenerated carriers out of the excitation energy region and possible rise in the lattice temperature due to energy relaxation. Carrier relaxation occurs through two channels with a fast time constants of ≈ 200 ps and a slow time constant ranging between 5 and 8 ns while intensity measurements reveal negligible contribution from nonlinear effects such as Auger recombination.

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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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Ultrafast time-resolved spectroscopy of In2O3 nanowires
Tsokkou D, Othonos A, Zervos M

Journal of Applied Physics, DOI: 10.1063/1.3245339Download
Ultrafast carrier dynamics in In2O3 nanowires with an average diameter of ≈ 100±20 nm grown by the vapor-liquid-solid method have been investigated in detail using differential absorption spectroscopy with femtosecond resolution. Measurements revealed that state filling is important for states above the band gap and states just below the band edge, thus demonstrating the critical role that shallow traps play in the relaxation of the photogenerated carriers. Furthermore, time-resolved intensity measurements revealed the importance of Auger recombination in the relaxation of carriers in the In2O3 nanowires and provided the maximum fluence ( ∼ 3 μJ/cm2) where this recombination mechanism may be considered negligible. Transient measurements in this low-fluence regime for carriers above the band gap revealed single exponential recovery ( ∼ 1.5 ns) associated with recombination of the photogenerated carriers. Similar behavior has been observed for the photogenerated carriers distributed within the shallow traps just below the band edge. Furthermore, measurements at longer probing wavelengths provided an estimate of the nonradiative relaxation of carriers ( ∼ 300 ps), which are distributed among the midgap states. Finally, long-lived oscillations in the transient reflection were detected, which corresponds to the presence of longitudinal acoustic phonons in the In2O3 nanowires.

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Synthesis of Tin Nitride SnxNy Nanowires by Chemical Vapour Deposition
Zervos M, Othonos A

Nanoscale Research Letters, DOI: 10.1007/s11671-009-9364-0Download
Tin nitride (SnxNy) nanowires have been grown for the first time by chemical vapour deposition on n-type Si(111) and in particular by nitridation of Sn containing NH4Cl at 450 °C under a steady flow of NH3. The SnxNy nanowires have an average diameter of 200 nm and lengths ≥5 μm and were grown on Si(111) coated with a few nm’s of Au. Nitridation of Sn alone, under a flow of NH3 is not effective and leads to the deposition of Sn droplets on the Au/Si(111) surface which impedes one-dimensional growth over a wide temperature range i.e. 300–800 °C. This was overcome by the addition of ammonium chloride (NH4Cl) which undergoes sublimation at 338 °C thereby releasing NH3 and HCl which act as dispersants thereby enhancing the vapour pressure of Sn and the one-dimensional growth of SnxNy nanowires. In addition to the action of dispersion, Sn reacts with HCl giving SnCl2 which in turn reacts with NH3 leading to the formation of SnxNy NWs. A first estimate of the band-gap of the SnxNy nanowires grown on Si(111) was obtained from optical reflection measurements and found to be ≈2.6 eV. Finally, intricate assemblies of nanowires were also obtained at lower growth temperatures.

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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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Year: 2008

Time-resolved ultrafast carrier dynamics in as-grown nanocrystalline silicon films: the effect of film thickness and grain boundaries

, DOI: 10.1002/pssr.200701219Download
In this letter, we have studied transient photoinduced absorption in as-grown nanocrystalline silicon films with thickness varied from 5 to 30 nm. Effects of quantum confinement (QC) in z -direction and grain boundary distortions alter the carrier dynamics of these films considerably. Based on the determination of critical points in the first Brillouin zone of the band structure of materials, we have time-resolved the relaxation times of surface-related states and indirect valleys. When decreasing the film thickness down to the QC limit (∼10 nm) new ultrafast relaxation mechanisms start to play a dominant role in carrier dynamics due to the topological disordering of these ultrathin films. These relaxation mechanisms seem to be related with the traping/de-traping of the excited carriers prior to recombination

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Femtosecond Dynamics in Single Wall Carbon Nanotube/Poly(3-Hexylthiophene) Composites
Lioudakis E, Christofide C, Othonos A

Nanoscale Research Letters, DOI: Download
We have studied the influence of implantation energy and subsequent isochronal annealing temperature on the optical and structural properties of implanted Si wafers employing a multiwavelength spectroscopic ellipsometer. A temperature-dependent multilayer optical model is used to explain the ellipsometric data for all implantation energies (20 to 180 keV) and annealing temperatures (300 to 1100 °C) of this work. This work completely characterizes the structural and optical properties of these implanted samples via the pseudodielectric functions and the integrated damage depth profile. For the highest implantation sample self-annealing phenomena have appeared, reducing the integrated damage depth profile. Finally, the dynamics of isochronal annealing temperature on the integrated damage depth profile of these wafers exhibit an abrupt drop in the transition temperature where a long-range ordering is obtained and pseudodielectric functions approach the crystallinity shapes

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Direct observation of excitons in polymer/carbon nanotube composites at room temperature: The influence of nanotube concentration
Lioudakis E, Kanari C, Othonos A, Alexandrou I

Diamond and Related Materials, DOI: DOI: 10.1016/j.diamond.2008.01.028Download
In this work, we have employed spectroscopic ellipsometry technique to study the optical properties of polymer/carbon nanotube (CN) composites as a function of nanotube concentration. Using a two-layer structural model based on Airy rigorous equations, the optical constants in various CN concentrations have been extracted. In the optical absorption spectra, we have observed a tuning of the excitonic transitions with the addition of single wall CN concentration. Based on theoretical calculations for the interaction of CNs with the surrounding effective media, the spectral evolution of the binding energy of optically active excitonic transitions with the addition of CNs suggests that the effective dielectric function of surrounding media decreases. Furthermore, using an oscillator Lorentz model, the absorption peaks, oscillator strengths and energy broadenings of the excitonic transitions have been extracted

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Optical properties of polyelectrolyte quantum dot multilayer films prepared using the layer by layer self-assembly method
Lioudakis E, Koupanou E, Kanari C, Leontidis E, Othonos A

Journal of Applied Physics, DOI: 10.1063/1.2906121Download
In this work, we have used spectroscopic ellipsometry to study the optical properties of polyelectrolyte-PbS quantum dot (QD) multilayer films prepared using the layer by layer self-assembly method. The optical results provide information about the absorption coefficients of the materials as a function of the number of layers deposited on a quartz substrate. We have found that the fundamental energy gap of the films decreases linearly upon addition of each layer due to the formation of nanoclusters at the surface. Furthermore, the influence of PbS QD concentration in colloidal dispersion on the energy gap of the materials is examined in detail, and it is found that the optical band gap in the films is in agreement with the linear absorption measurements in the PbS colloidal dispersion from which the film deposition takes place. Finally, the observed electronic transitions of the films corroborate that nanoparticles in the regime of strong quantum confinement are present in the films. This comprehensive fundamental study provides important information, necessary for photovoltaic applications, about the absorption tunability of these nanofilms.

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Determination of critical points on silicon nanofilms: surface and quantum confinement effects
Lioudakis E, Othonos A, Nassiopoulou AG

, DOI: 10.1002/pssc.200780109Download
In this work, we present a comprehensive study of the optical properties of nanocrystalline silicon films with thickness varied from 5 to 30 nm. Spectroscopic ellipsometry is employed to determine the dielectric functions of these films using a structural two-layer model based on the rigorous Airy formula. Our investigation gives an important insight of the origin of critical points for direct and indirect gaps of nanocrystalline silicon films as well as the evolution of them with decreasing the film thickness. The influence of the quantum confinement effect due to the nanoscale grain size and the surface vibrations at the interface on the optical properties are examined in detail

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Femtosecond Dynamics in Single Wall Carbon Nanotube/Poly(3-Hexylthiophene) Composites
Lioudakis E, Othonos A, Alexandrou I

Nanoscale Research Letters, DOI: 10.1007/s11671-008-9149-xDownload
Femtosecond transient absorption measurements on single wall carbon nanotube/poly(3-hexylthiophene) composites are used to investigate the relaxation dynamics of this blended material. The influence of the addition of nanotubes in polymer matrix on the ultrashort relaxation dynamics is examined in detail. The introduction of nanotube/polymer heterojunctions enhances the exciton dissociation and quenches the radiative recombination of composites. The relaxation dynamics of these composites are compared with the fullerene derivative-polymer composites with the same matrix. These results provide explanation to the observed photovoltaic performance of two types of composites.

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Transient Photoinduced Absorption in Ultrathin As-grown Nanocrystalline Silicon Films
Lioudakis E, Othonos A, Lioutas C, Vouroutzis N

Nanoscale Research Letters, DOI: 10.1007/s11671-007-9105-1Download
We have studied ultrafast carrier dynamics in nanocrystalline silicon films with thickness of a few nanometers where boundary-related states and quantum confinement play an important role. Transient non-degenerated photoinduced absorption measurements have been employed to investigate the effects of grain boundaries and quantum confinement on the relaxation dynamics of photogenerated carriers. An observed long initial rise of the photoinduced absorption for the thicker films agrees well with the existence of boundary-related states acting as fast traps. With decreasing the thickness of material, the relaxation dynamics become faster since the density of boundary-related states increases. Furthermore, probing with longer wavelengths we are able to time-resolve optical paths with faster relaxations. This fact is strongly correlated with probing in different points of the first Brillouin zone of the band structure of these materials.

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Surface-Related States in Oxidized Silicon Nanocrystals Enhance Carrier Relaxation and Inhibit Auger Recombination
Othonos A, Lioudakis E, Nassiopoulou A

Nanoscale Research Letters, DOI: 10.1007/s11671-008-9159-8Download
We have studied ultrafast carrier dynamics in oxidized silicon nanocrystals (NCs) and the role that surface-related states play in the various relaxation mechanisms over a broad range of photon excitation energy corresponding to energy levels below and above the direct bandgap of the formed NCs. Transient photoinduced absorption techniques have been employed to investigate the effects of surface-related states on the relaxation dynamics of photogenerated carriers in 2.8 nm oxidized silicon NCs. Independent of the excitation photon energy, non-degenerate measurements reveal several distinct relaxation regions corresponding to relaxation of photoexcited carriers from the initial excited states, the lowest indirect states and the surface-related states. Furthermore, degenerate and non-degenerate measurements at difference excitation fluences reveal a linear dependence of the maximum of the photoinduced absorption (PA) signal and an identical decay, suggesting that Auger recombination does not play a significant role in these nanostructures even for fluence generating up to 20 carriers/NC.

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Year: 2007

Instrumentation for the monitoring of toxic pollutants in water resources by means of neural network analysis of absorption and fluorescence spectra
Kuzniz T, Halot D, Mignani AG, Ciaccheri L, Kalli K, Tur M, Othonos A, Christofides C, Jackson DA

Sensors and Actuators B: Chemical, DOI: DOI: 10.1016/j.snb.2006.09.012Download
The concentration of several pollutants, usually present in industrial waste waters, is predicted by the neural network data processing of absorption and fluorescence measurements in the visible spectral range. Proper network training provides quantitative analysis of many pollutants with sub-ppm resolution. Compact optical fibre instrumentation for absorption spectroscopy and an innovative flowcell for fluorescence measurements enable cost-effective, in situ, nonstop monitoring of waste waters.

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Time-resolved ultrafast carrier dynamics in as-grown nanocrystalline silicon films: the effect of film thickness and grain boundaries
Lioudakis E, Nassiopoulou AG, Othonos A

physica status solidi (RRL) – Rapid Research Letters, DOI: Download
We have studied the ultrafast optical response of highly implanted and highly annealed polycrystalline silicon films using 400 nm ultrashort amplified pulses with fluence ranging between 8 mJ cm−2 to 56 mJ cm−2. Transient reflection measurements reveal differences both in the short and long temporal behaviour between the implanted non-annealed and annealed samples. Important contributing factors to the dynamics of the non-annealed sample are the carrier recombination centres and traps induced by ion implantation. In contrast to the non-annealed sample, the Auger recombination process is a key factor in the dynamics in the first few picoseconds for the sample annealed at 1100 °C. A model based on two coupled differential equations has been employed to investigate in detail the carrier dynamics in these systems. Parameters including carrier trapping times, diffusion coefficients and the Auger coefficient have been extracted

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Femtosecond carrier dynamics of InxGa1- xN thin films grown on GaN (0001): Effect of carrier-defect scattering
Lioudakis E, Iliopoulos E, Georgakilas A, Othonos A

Journal of Applied Physics, DOI: Download
Ultrafast carrier dynamics in the In0.33Ga0.67N epilayer were investigated in detail, using femtosecond transient differential non-degenerate optical absorption measurements. Following an excitation at 400 nm with fluence ranging from 25 µJ cm−2 to 3000 µJ cm−2, probing was carried out above and near the bandgap using different wavelengths generated from a super continuum source. We have found that bandgap renormalization plays a key role when probing at photon energies well above the bandgap and it is clearly distinct from other effects at the lowest fluence. The critical carrier density for the onset of noticeable bandgap renormalization effects in this material when probing well above the bandgap is approximately 5 × 1018 carriers cm−3. We have observed a decrease in the energy loss rate of this material as a function of photogenerated carrier density which is attributed to phonon bottleneck effect. For the lowest carrier density, we have extracted an optical phonon lifetime to be approximately 45 ± 9 fs.

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Quantum confinement and interface structure of Si nanocrystals of sizes 3-5 nm embedded in a-SiO2
Lioudakis E, Othonos A, Hadjisavvas GC, Kelires PC, Nassiopoulou AG

Physica E: Low-dimensional Systems and Nanostructures, DOI: DOI: 10.1016/j.physe.2006.12.020Download
Spectroscopic ellipsometry and Monte Carlo simulations are employed to answer the fundamental question whether the energy gaps of Si nanocrystals with sizes in the range of 3–5 nm, which are embedded in amorphous silica, follow or deviate from the quantum confinement model, and to examine their interfacial structure. It is shown that the optical properties of these nanocrystals are well described by the Forouhi–Bloomer interband model. Analysis of the optical measurements over a photon-energy range of 1.5–5 eV shows that the gap of embedded nanocrystals with a mean size of not, vert, similar3.9 nm follows closely quantum confinement theory. A large band gap expansion (not, vert, similar0.65 eV) compared to bulk Si is observed. The Monte Carlo simulations reveal a non-abrupt interface and a large fraction of interface oxygen bonds. This, in conjunction with the experimental observations, indicates that oxygen states and the chemical disorder at the interface have a negligible influence on the optical properties of the material in this size regime.

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Optical properties of conjugated poly(3-hexylthiophene)/[6,6]-phenylC[sub 61]-butyric acid methyl ester composites
Lioudakis E, Othonos A, Alexandrou I, Hayashi Y

Journal of Applied Physics, DOI: 10.1063/1.2799049Download
In this work, we present the evolution of optical constants as a function of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) concentration for conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites. The PCBM concentration of the utilized samples varies from 1 to 50 wt %. The dielectric functions for all these composites reveal electronic structural changes as a result of the addition of PCBM. We have deconvoluted the contribution of the substrate using a two-layer Fabry-Pérot structural model. The extracted optical properties contain crucial absorption peaks of singlet exciton states and vibronic sidebands for poly(3-hexylthiophene) (P3HT) conjugated polymer as well as two PCBM-related states at higher energies. With the addition of PCBM, we have observed a limit of 20 wt % PCBM beyond which two discrete energy levels (3.64 and 4.67 eV) appear in the spectrum. For the highest concentration composite, the results suggest that the interchain interactions provide a small excitonic contribution in the absorption spectrum at energies where the conjugated polymer absorbs (1.85–2.7 eV) and a strong rise of PCBM states (3.64 and 4.67 eV) which are responsible for the subsequent exciton dissociation. In addition, the energy gap between the higher occupied molecular orbitals and the lower unoccupied molecular orbitals of the highest concentration composite (50 wt %) is 1.85 eV. The tuning of the optical properties of P3HT with the addition of PCBM shows that ellipsometry can be used to monitor layer concentration toward optimization of plastic solar cells.

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The role of surface vibrations and quantum confinement effect to the optical properties of very thin nanocrystalline silicon films
Lioudakis E, Antoniou A, Othonos A, Christofides C, Nassiopoulou AG, Lioutas CB, Frangis N

Journal of Applied Physics, DOI: 10.1063/1.2800269Download
We report on a spectroscopic study of very thin nanocrystalline silicon films varying between 5 and 30 nm. The role of quantum confinement effect and surface passivation of nanograins in optical properties are examined in detail. The coupling between surface vibrations and fundamental gap Eg as well as the increase of interaction between them at the strong confinement regime ( ⩽ 2 nm) are proposed for the observable pinning of Eg in luminescence measurements.

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