Emmanouil Lioudakis

Ultrafast transient photoinduced absorption in silicon nanocrystals: Coupling of oxygen-related states to quantized sublevels

The authors have studied transient photoinduced absorption in single monolayers of oxidized silicon nanocrystals. Transient photoinduced absorption measurements along with optical absorption and photoluminescence (PL) emission reveal that the light-absorption process takes place in defects related to strong PL emission, suggesting that the photoexcited carriers are in oxygen-related interface states. They have time-resolved ultrafast relaxation paths in oxygen-related states and quantized sublevels, which have important implications in the understanding of fundamental optical properties for this system.

The role of surface vibrations and quantum confinement effect to the optical properties of very thin nanocrystalline silicon films

We report on a spectroscopic study of very thin nanocrystalline silicon films varying between 5 and 30nm. 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 (⩽2nm) are proposed for the observable pinning of Eg in luminescence measurements.

Optical properties of conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites

In this work, we present the evolution of optical constants as a function of [6,6]-phenylC61-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 1to50wt%. 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-Perot 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 20wt% PCBM beyond which two discrete energy levels (3.64 and 4.67eV) appear in the spectrum. For the highest concentration composite, the results suggest that the interchain interactions provide a small excitonic c...

Ultrafast carrier dynamics on conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites

The authors have studied ultrafast carrier dynamics on poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester (PCBM) composites up to 50% PCBM concentration. They have resolved the exciton intraband relaxation of composites as well as the subsequent electron/polaron relaxation of dissociated excitons. The observed fast time constant of intraband relaxation is found to be 0.5–1.5ps, whereas the second long-live relaxation (0.5–1ns) is strongly fullerene related. A wavelength dependent ultrafast study is performed giving fundamental information on the nonradiative exciton relaxation, exciton dissociation, and electron relaxation of PCBM-related states.

Ultrafast carrier dynamics in band edge and broad deep defect emission ZnSe nanowires

Ultrafast carrier dynamics of ZnSe nanowires grown under different growth conditions have been studied. Transient absorption measurements reveal the dependence of the competing effects of state filling and photoinduced absorption on the probed energy states. The relaxation of the photogenerated carriers occupying defect states in the stoichiometric and Se-rich samples are single exponentials with time constants of 3–4ps. State filling is the main contribution for probe energies below 1.85eV in the Zn-rich grown sample. This ultrafast carrier dynamics study provides an important insight into the role that intrinsic point defects play in the observed photoluminescence from ZnSe nanowires.

Surface-Related States in Oxidized Silicon Nanocrystals Enhance Carrier Relaxation and Inhibit Auger Recombination

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.

Influence of grain size on ultrafast carrier dynamics in thin nanocrystalline silicon films

The ultrafast carrier dynamics in thin nanocrystalline silicon films (10 and 20nm thick) on quartz over a broad spectral range using optical pumping at a moderate fluence of 2.5mJ∕cm2 were studied. The films were composed of randomly oriented silicon nanocrystals of various sizes and shapes. The authors probed a fast and a slow relaxation mechanism. The slow decay (∼300ps) was attributed to bulk nanocrystal states similar to those of bulk silicon, while the faster one (∼3ps) was attributed to surface states at grain boundaries, more dominant in the smaller nanocrystals due to their larger surface/volume ratio.

Study of the annealing kinetic effect and implantation energy on phosphorus-implanted silicon wafers using spectroscopic ellipsometry

In this work, we have studied the changes in the optical properties on crystalline silicon implanted wafers (1×1013–1×1016P+∕cm2) using an extensive ellipsometric analysis. The effects of implantation energy (20–180KeV) and subsequent isochronical annealing temperature (300–1100°C) on the electronic band structure of material are investigated. The evolution of pseudodielectric functions is studied using a temperature dependent multilayer model for each implantation dose and energy. The temperature evolution of integrated damage depth profile for each wafer is presented depicting the amorphous/crystalline transition temperatures. Finally, the critical implantation dose and energy of crystalline to amorphous silicon phase are given.

Femtosecond Dynamics in Single Wall Carbon Nanotube/Poly(3-Hexylthiophene) Composites

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.

Ultrafast carrier dynamics in InxGa1−xN (0001) epilayers: Effects of high fluence excitation

Ultrafast carrier dynamics in InxGa1−xN (0001) epilayers were investigated, using femtosecond transient differential transmission and reflection measurements for x=0.07, 0.15, and 0.33, over a fluence range of 1–12mJ∕cm2. Stimulated emission as well as band gap renormalization play a crucial role in the dynamics of the photogenerated carriers. Threshold fluence leading to saturation of the differential reflectivity and transmission signals related to the In mole fraction has been observed, which is attributed to band gap renormalization, Auger process, and carrier recombination through In-rich nanoclusters. Furthermore, coherent acoustic phonon oscillations have also been observed in the In0.15Ga0.85N at high fluence excitation.