Photoluminescence Dynamics Research Articles

Photosensitized generation of singlet oxygen in powders and aqueous suspensions of silicon nanocrystals

The photoluminescence spectra and kinetics in powders and aqueous suspensions produced from porous silicon layers are studied. The systematic features of photosensitized generation of singlet oxygen by silicon nanocrystals in the samples are established. The dependence of the efficiency of generation of singlet oxygen on the pressure of molecular oxygen is analyzed. It is concluded that the generation can be described on the basis of concepts of energy transfer from photoexcited silicon nanocrystals to oxygen molecules adsorbed at the nanocrystal surface to the concentration described by Langmuir's adsorption model. The processes limiting the efficiency of photosensitized generation of singlet oxygen in the systems are discussed.

Photoluminescence dynamics in GaAs/AlGaAs quantum wells under pulsed intersubband excitation

We investigate the time-resolved photoluminescence (PL) dynamics of an undoped GaAs/AlGaAs multiple quantum well under mid-infrared (MIR) irradiation. A time-delayed MIR laser pulse from a free-electron laser, tuned to the intersubband transition energy of the quantum well, induces temporal quenching of the PL intensity with subsequent recovery. The experimental data can be accurately described by a simple rate-equation model, which accounts for the cooling of the non-radiative states to radiative states. By performing polarization sensitive measurements, we are able to discriminate the contributions of free-carrier absorption from that of intersubband absorption, where the latter is about 50 times more efficient.

Ultrafast photoluminescence spectroscopy of H- and O-terminated nanocrystalline diamond films

We use femtosecond photoluminescence spectroscopy to study the light-induced changes in the sub-gap energy states of nanocrystalline diamond samples (thickness ~ 500 and 1000 nm) prepared on a spectral-grade fused silica substrate by microwave plasma enhanced chemical vapour deposition technique. The decay of photoluminescence in the visible spectral interval excited by blue femtosecond light pulses (405 nm,70 fs) shows that photoexcited charge carrier dynamics depend strongly on the ambient air pressure and on the light irradiation by the laser pulses. Specifically, at lower ambient air pressure (0.5–300 Pa) the irradiation leads to the peak photoluminescence intensity increase and to its faster decay. At higher air pressures (> 600 Pa) the photoluminescence intensity slightly decreases with no change in decay rates. O- and H-termination of nanocrystalline diamond films had negligible effect on their photoluminescence dynamics. The photoluminescence decay curves are described very well by the power-law decay reflecting the importance of the carrier trapping in the dynamics. Based on our results we propose a model of surface and sub-surface structure of nanodiamond films.

Kinetics of long-time photoluminescence in quantum dots

Some quantum dot samples show a long-time behavior of their luminescence intensity decay. This effect has been recently explained as being due to a cooperation of many tunneling channels transferring electrons from quantum dots with triplet exciton to quantum dots at which the electrons can recombine with the holes in the valence band states. In this work, the authors show that the long-time character of the sample luminescence decay can also be caused by an intrinsic property of a single dot, namely, by a nonadiabatic effect of the up-conversion caused by the electron-phonon multiple scattering mechanism.

Optical gain observation on silicon nanocrystals embedded in silicon nitride under femtosecond pumping

We report the observation of positive optical gain in silicon nanocrystals (Si-nc) embedded in silicon nitride measured by the variable stripe length technique. We evidence the onset of stimulated emission and report gain coefficients up to 52 cm−1 at the highest excitation power (6.5 W/cm2). Photoluminescence dynamics presents two distinct recombination lifetimes in the nanosecond and the microsecond ranges. This was interpreted in terms of fast carrier trapping in nitrogen-induced localized states in the Si-nc surface and subsequent slow radiative recombination, suggesting that carrier trapping in radiative surface states plays a crucial role in the optical gain mechanism of Si-nc.

Optical properties of ferromagnetic ytterbium‐doped III‐nitride epilayers

AbstractWe have studied the optical and magnetic properties of ytterbium (Yb) implanted GaN epilayers grown on (0001) sapphire by different epitaxial growth methods. Samples were implanted at room temperature with Yb ions and subsequently thermally annealed at 1000 °C in N2 at atmospheric pressure. The characteristic Yb3+ ion photoluminescence (PL) spectra were observed in the spectral range between 970 nm and 1050 nm. Analysis of the PL temperature quenching and PL kinetics suggest that Yb3+ ions are involved in at least two major luminescence centers. The magnetizations versus magnetic field curves show an enhancement of magnetic order for Yb‐implanted GaN epilayer containing intrinsic impurities and structural defects. The hysteretic behavior with a weak coercivity is enhanced when the magnetic field is out‐of‐plane indicating magnetic anisotropy of the Yb‐implanted GaN epilayers (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Edge photoluminescence of single-crystal silicon with a p-n junction: Structures produced by high-efficiency solar cell technology

The systematic features and kinetics of edge photoluminescence of silicon structures produced by the high-efficiency solar cell technology is studied at different voltages applied to the p-n junction. It is shown that the effect of modulation of the edge photoluminescence intensity by a dc voltage applied to the p-n junction is qualitatively similar to the effect induced by excitation of photoluminescence by laser radiation at the wavelengths 658 and 980 nm. The possibility of modulating the edge photoluminescence power by varying the resistance parallel to the p-n junction is demonstrated. It is found that, at zero voltage, the rise time constant of the photoluminescence intensity far exceeds the decay time constant. However, as the dc forward current is increased, the decay time constant approaches the rise time constant. To interpret the results, the concepts of the second, more efficient saturable recombination channel coexisting with the common Shockley-Read-Hall recombination channel in the structure are developed. The study extends the functional capabilities of the luminescence technique in determining the effective lifetimes of charge carriers.

Microwave-assisted synthesis of surface-passivated doped ZnSe quantum dots with enhanced fluorescence

A microwave-assisted synthesis route is established to prepare ZnSe:Cu and ZnSe:Mn quantum dots (QDs) with efficient surface passivation. The products exhibit over an order of magnitude stronger photoluminescence (PL) than the samples without surface passivation. The PL dynamics for fluorescence enhancement was studied by using time-resolved PL measurement. Considering the ultrafast growth rates (reaction time of ∼25min) and the high quantum yields (over 12%) of the products, our proposed synthesis route suggests a facile method for obtaining various doped QDs, and the synthesized Cu (or Mn) doped ZnSe QDs with greatly improved biocompatibility may find use in biological applications.

Optical properties and luminescence dynamics ofEu3 + -doped terbium orthophosphate nanophosphors

The development of luminescent inorganic nanocrystals (NCs) doped withrare-earth (RE) ions has attracted increasing interest owing to their distinctoptical properties and versatile applications in time-resolved bioassays,multiplex biodetection, DNA hybridization and bioimaging. HexagonalTbPO4:Eu3 + NCs (10–30 nm) were synthesized via a facile hydrothermal method assisted with oleic acid (OA)surfactants, which exhibit tunable emissions from green to red by varying the concentration ofEu3 + . TheTb3 + -to-Eu3 + energy transfer efficiency observed reaches up to 94%. Different from theirbulk counterparts, a new interface-state band (316 nm) in addition to thecommonly observed spin-forbidden 4f–5d transition band (265 nm) ofTb3 + wasfound to be dominant in the excitation spectrum of NCs due presumably to the formation of surfaceTbPO4/OA complexes,which provides an additional excitation antenna in practical utilization. Two kinds of luminescence sitesof Eu3 + inTbPO4 NCs, with the sitesymmetry of C2 or C1, were identified based on the emission spectra at 10 K and roomtemperature. Furthermore, the photoluminescence (PL) dynamics ofTb3 + ions inpure TbPO4 NCs have been revealed. Compared to the exponential PL decay in bulk counterpartsinduced by very fast energy migration, the non-exponential decay from 5D4 ofTb3 + inTbPO4 NCsis mainly attributed to the diffusion-limited energy migration due to more rapid energy transfer fromTb3 + to defects than theenergy migration among Tb3 + .

White upconversion luminescence in nanocrystalline (Ho,Tm,Yb):KLu(WO4)2 phosphor

AbstractHighly crystalline codoped Ho3+,Tm3+,Yb3+ KLu(WO4)2 monoclinic nanocrystals with the C 2/c space group symmetry were synthesized by the modified Pechini method. Powder X‐ray diffraction, transmission electron microscopy, upconversion photoluminescence spectra, and kinetic decay were used to characterize the samples. The nanocrystals belonged to the monoclinic phase, with a crystallite size of 30‐70 nm. Under 930 nm laser excitation, red, green and blue upconversion emissions were simultaneously observed, due to the energy transfer from the Yb3+ 2F5/2 excited level. The decay times of the emitting states were studied at room temperature to describe the photoluminescence dynamics. The influence of excitation power and calcination conditions on CIE chromaticity coordinates were evaluated. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Blue light emission from strongly photoexcited and electron-doped SrTiO3

We studied photoluminescence (PL) spectrum and dynamics of undoped and electron-doped SrTiO3. At low temperatures below 100 K, we observed two band-edge PL peaks and a broad PL band in strongly photoexcited SrTiO3 and electron-doped SrTiO3 samples. In electron-doped samples, the peak energy of the broad PL band depends on the excitation photon energy at low temperatures: it changes from 2.5 to 2.9 eV with the excitation photon energy. On the other hand, the PL dynamics is independent of the excitation photon energy and is explained by a simple model involving single-carrier trapping and Auger recombination. From the PL spectrum and dynamics under different excitation photon energies, the PL processes in SrTiO3 are discussed.

Photoluminescence dynamics of ensemble and individual CdSe/ZnS quantum dots with an alloyed core/shell interface

A comprehensive study of the photoluminescence dynamics in newly developed CdSe/ZnS quantum dots with alloyed core/shell interfaces is presented. Time-correlated single photon counting is used to measure the decay of exciton luminescence from both the ensemble and individual quantum dots. For decreasing emission wavelength (i.e., for smaller dots), the ensemble data reveal increasing total decay rates with greater variation. This systematic change is expected for emitters with stronger quantum confinement and more influenced by the surface/interface trap states. In experiments performed on single quantum dots, the photoluminescence trajectories exhibit two-state blinking behavior. The distributions of the “off”-state probability density are described by an average power-law exponent of 1.5 ± 0.2, while the average decay rate of emission from the threshold-discriminated “on”-states is estimated to be 0.035 ± 0.004 ns−1. We suggest that in core/shell quantum dots with a large bandgap offset, the compositionally graded energy profile at the interface may not be smooth enough to suppress nonradiative Auger recombination and prevent blinking.

Charge‐Separation Dynamics in Inorganic–Organic Ternary Blends for Efficient Infrared Photodiodes

AbstractKnowledge about the working mechanism of the PbS:P3HT:PCBM [P3HT=poly(3‐hexylthiophene), PCBM=[6,6]‐phenyl‐C61 ‐butyric acid methyl ester] hybrid blend used for efficient near‐infrared photodiodes is obtained from time‐resolved photoluminescence (PL) studies. To understand the role of each component in the heterojunction, the PL dynamics of the ternary (PbS:P3HT:PCBM) blend and the binary (PbS:P3HT, PbS:PCBM and P3HT:PCBM) blends are compared with the PL of the pristine PbS nanocrystals (NCs) and P3HT. In the ternary blend the efficiency of the charge transfer is significantly enhanced compared to the one of PbS:P3HT and PbS:PCBM blends, indicating that both hole and electron transfer from excited NCs to the polymer and fullerene occur. The hole transfer towards the P3HT determines the equilibration of their population in the NCs after the electron transfer towards PCBM, allowing their re‐excitation and new charge transfer process.

Size-dependent visible absorption and fast photoluminescence decay dynamics from freestanding strained silicon nanocrystals

In this article, we report on the visible absorption, photoluminescence (PL), and fast PL decay dynamics from freestanding Si nanocrystals (NCs) that are anisotropically strained. Direct evidence of strain-induced dislocations is shown from high-resolution transmission electron microscopy images. Si NCs with sizes in the range of approximately 5-40 nm show size-dependent visible absorption in the range of 575-722 nm, while NCs of average size <10 nm exhibit strong PL emission at 580-585 nm. The PL decay shows an exponential decay in the nanosecond time scale. The Raman scattering studies show non-monotonic shift of the TO phonon modes as a function of size because of competing effect of strain and phonon confinement. Our studies rule out the influence of defects in the PL emission, and we propose that owing to the combined effect of strain and quantum confinement, the strained Si NCs exhibit direct band gap-like behavior.

Self-Assemblies of Novel Magnesium Porphyrins Mimicking Natural Chlorosomal Bacteriochlorophylls

Self-assembling porphyrins are promising materials to mimic natural bacteriochlorophylls c, d, or e encountered in the chlorosomes of photosynthetic bacteria. We have studied four novel magnesium porphyrins mimicking this chlorosomal antenna system. In contrast to previous articles reporting synthetic Zn-porphyrins, our studies focus on porphyrins with Mg as the central atom, which mimic more closely the natural bacteriochlorophylls. The analysis of time-resolved photoluminescence dynamics based on the decay-associated emission spectra shows a short-lived component associated with the supramolecular architecture. It confirms the ordered nature of the investigated porphyrins and suggests that these self-assemblies can be used as artificial antennae in light-harvesting devices like biomimetic solar cells.

Specific features of amorphous silicon layers grown by plasma-enhanced chemical vapor deposition with tetrafluorosilane

Hydrogenated amorphous silicon layers with crystalline nanoparticles have been produced by plasma-enhanced chemical vapor deposition, with tetrafluorosilane added to the gas mixture. The photoluminescence kinetics and photoelectric properties of structures based on these layers have been studied. The structures have a substantial photoresponse efficiency in the visible spectral range, with the position of the photoresponse maximum dependent on the applied bias.

Thermally induced defect photoluminescence in hydrogenated amorphous silicon

The intrinsic defect photoluminescence of hydrogenated amorphous silicon (a-Si:H) films has been investigated at high intensities of optical pumping that lead to heating of the film. It has been revealed that, for short heating times, the intensity of the defect photoluminescence increases exponentially with an increase in the temperature with an activation energy of 0.85 eV, which is considerably higher than the activation energy (∼0.2 eV) determined from experiments on classical annealing. This and other experimental results on the temperature dependence of the intensity and kinetics of the defect photoluminescence have been explained in terms of the “hydrogen glass” model by thermally induced generation of intrinsic defects in amorphous silicon. The results of the calculations are in good agreement with the experimental data on the defect photoluminescence that reflects the formation and annihilation of defects for short heating times under optical excitation.

Ultrafast photoluminescence dynamics of blue‐emitting silicon nanostructures

AbstractWe report on ultrafast photoluminescence (PL) spectroscopy of blue and red‐emitting silicon nanostructures. We prepared samples of porous Si with different dominating PL bands in time‐integrated PL spectrum. These samples show also striking differences in ultrafast PL dynamics. For the red‐emitting samples we observe rapid PL onset followed by a two‐exponential decay, which is spectrally dependent. For the blue‐emitting porous Si the PL builds up several picoseconds and decays mono‐exponentially with spectrally independent PL rise and decay times.Our results indicate that a dominant part of the blue and red emission comes from two distinct species with different energy states structures. This is furthermore supported by two‐photon absorption induced PL mea‐surements. The two‐photon and one‐photon absorption induced PL spectra, which are identical for the red‐emitting samples, differ substantially for the blue‐emitting ones. However, polarization properties of PL suggest that the blue PL band originates from several overlapping PL bands with sample‐dependent importance. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Defect dynamics and spectral observation of twinning in single crystalline LaAlO3 under subbandgap excitation

We have investigated the photoluminescence and ultrafast dynamics of LaAlO3 crystal. The photoluminescence consists of a broad spectrum and two sharp peaks, which arise from various defect levels within the bandgap. A doublet splitting of roughly 6 nm is seen in these two sharp peaks. An Al displacement of 0.09 Å in a sublattice, which is possible because of twinning, is adequate to explain the spectral splitting. Femtosecond pump probe experiments reveal further that many of these defect levels have a few picosecond decay times while the lowest defect states have decay times longer than nanosecond to the valence band.

Photoluminescence spectrum and dynamics in highly photoexcited rutile TiO2

AbstractWe studied the temperature dependence of the photoluminescence (PL) spectrum and dynamics of rutile‐type TiO2 single crystals. Rutile TiO2 shows sharp exciton‐PL lines at low temperatures. A broad blue PL band appears at around 2.8 eV under high‐density photoexcitation using a pulse laser at room temperature. We discussed the temperature‐dependent photocarrier dynamics of rutile TiO2 in terms of Auger recombination and single‐carrier trapping processes (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)