Photoluminescence transients

Very fast recombination process can be studied by photoluminescence transients [182,199]. In this experiment, very short laser pulses are used and ultra-fast recombination studied. For a mechanically polished but unetched sample of n-CdSe, luminescence decay times of 50 ps were found due to recombination in the damaged surface layer. On etching, the quantum yield for luminescence and the decay lifetime were both substantially increased [182]. 

Experimentally, local vibrational modes associated witli a defect or impurity may appear in infra-red absorjrtion or Raman spectra. The defect centre may also give rise to new photoluminescence bands and otlier experimentally observable signature. Some defect-related energy levels may be visible by deep-level transient spectroscopy (DLTS) [23]. 

Transient UV-vis absorption spectra showed that theTi02/Ru(II) films yield prompt electron injection upon photolysis ( >108s 1) These same films displayed photoluminescence decays with parallel first- and second-order components, the first-order component having a rate constant of about lxl06s-1. These two sets of results provide further support for the existence of at least two populations of adsorbed Ru(II), one of which injects electrons rapidly and another which does not inject electrons and is thus capable of luminescing on a longer time scale. The second-order component of the luminescence decay is attributed to bimolecular triplet-triplet annihilation of surface-bound Ru(II). (Note that the second-order rate constants reported for luminescence decay have units of s-1 because they are actually values for k2(Asi))... 

As expected, the coordination of Pt markedly influences the photophysical characteristics of the PPE. The photoluminescence is efficiently quenched, and the absorption maximum in the visible regime experiences a hypsochromic shift. The charge-carrier mobility of different EHO-OPPE-Pt samples was determined by TOE measurements as described above for the neat EHO-OPPE. The shape of the photocurrent transients of all EHO-OPPE-Pt samples was similar to those shown in Figs. 6 and 7 for the neat EHO-OPPE. This indicates that these organometallic conjugated polymers networks are also characterized... 

FRET interactions are typically characterized by either steady-state or transient fluorescence emission signals from the donor or acceptor species. Efficient nonradiative energy transfer results in donor PL loss associated with acceptor gain in photoluminescence intensity (if the acceptor is an emitter). The rate of this energy transfer is related to the intrinsic lifetime of the isolated donor and depends strongly on the donor-acceptor separation distance ... 

Fig. 4. Energy below the conduction band of levels reported in the literature for GaP. States are arranged from top to bottom chronologically, then by author. At the left is an indication of the method of sample growth or preparation liquid phase epitaxy (LPE), liquid encapsulated Czochralski (LEC), irradiated with 1-MeV electrons (1-MeV e), and vapor phase epitaxy (VPE). Next to this the experimental method is listed photoluminescence (PL), photoluminescence decay time (PLD), junction photocurrent (PCUR), photocapacitance (PCAP), transient capacitance (TCAP), thermally stimulated current (TSC), transient junction dark current (TC), deep level transient spectroscopy (DLTS), photoconductivity (PC), and optical absorption (OA).

The technique of up-conversion photoluminescence allows one to record the transient PL of a system at the temporal resolution of the laser pulse. It is used to study very fast processes below the picosecond time domain. A typical set-up for this experiment is shown in Fig. 3. The sample is excited at frequency uq by a femtosecond laser pulse and its PL at ujj- is mixed with that of an optically... 

Figure 10. Transient photoluminescence of opalCT (a) and opal A (b) and kinetics at maximum...

Figure 11. Transient photoluminescence of a para-hexaphenyle layer with molecules lying on the substrate, for increasing time delays. The dotted curve corresponds to the ultrafast response of a film with molecules standing almost perpendicular to the substrate.

A3.5 Time-resolved photoluminescence studies of GaN A3.6 Persistent photoconductivity in GaN A3.7 Electrical transport in wurtzite and zincblende GaN A3.8 Characterisation of III-V nitrides by capacitance transient spectroscopy... 

Related to our work on the bipyridyl acetylides, we have also demonstrated that proper selection of the acetylide ligand makes possible the design of Ptn terpyridyl complexes that exhibit acetylide 3IL excited states [20]. The perylene complexes 3.7 and 3.8 do not display photoluminescence, however, indirect evidence that the triplet excited state is indeed populated was indicated through the sensitization of singlet oxygen. Transient absorption measurements (Fig. 7) confirmed that regardless of the polyimine ligand used, the lowest excited state in these molecules is 3IL localized in the perylenylacetylide moiety. It is clear in Fig. 7a and b that the identical features are observed in the absorption difference spectra of 3.7 and 3.8, whereas the difference spectrum of the phenylacetylide complex is clearly distinct, illustrative of the marked differences between 3IL and 3CT excited absorptions. 

Photoluminescence is the radiation emitted by the recombination process and as such is a direct measure of the radiative transition. Information about non-radiative recombination can often be inferred from the luminescence intensity, which is reduced by the competing processes (Street 1981a). The most useful feature of the luminescence experiment is the ability to measure the emission spectrum to obtain information about the energy levels of the recombination centers. The transition rates are found by measuring the transient response of the luminescence intensity using a pulsed excitation source. Time resolution to about 10 s is relatively easy to obtain and is about the maximum radiative recombination rate. The actual recombination times of a-Si H extend over a wide range, from 10 s up to at least 10- s. 

Wang R.S., Gu Q.L., Ling C.C., and Ong H.C. (2008) Studies of oxide / ZnO near-interfacial defects by photoluminescence and deep level transient spectroscopy Appl. Phys. Lett. 92 042105. 

Figure 19.5 Schematic diagram showing decomposition of total phosphorescence enhancement of PtOEP on silver films into absorption enhancement E X. ) and emissive rate enhancement E (%.2) based on the photophysical model described in the text and data from steady state and transient spectroscopy of PtOEP films with various thicknesses and excitation wavelengths as labeled. The lines represent the possible combinations that could explain the experimentally observed changes in photoluminescence where each position on the line represents a different choice of fQ, the fraction of the excited states that are quenched nonradiatively by interactions between the molecule and the metallic surface. The blue shaded region on the vertical axis is the range of possibilities allowed by constraints from extinction and excitation spectra as explained in the text. The dotted oval is what we believe to be the most likely decomposition for the 6 nm films characterized in Figure 19.4 as discussed in the text. Reprinted from reference 45 with permission of the American Chemical Society.

Dynamics of photoluiiiinesceiice refers to the behavior of photoluminescence in the presence of reactant molecules as the pressures of reactant molecules and/or diffusion rates are changed. It is possible to obtain detailed information about the excited state of the emitting sites as a function of time during their transient lifetimes. 

TA-transient absorption TRANIP = time-resolved anisotropy TRPL = time-resolved photoluminescence TRIR = time-resolved IR TR = EPR-time-resolved electron paramagnetic resonance spectroscopy. 

The carrier generation mechanism in PPV has been addressed by studying the transient photoconductivity and the photoluminescence as a function of the external electric field, E, in samples oriented by tensile drawing [166]. The transient photocurrent is proportional to E at low fields, but increases nonlinearly for E> 10 V/cm. The field at which the photoconductivity becomes nonlinear (the onset field, EP ) depends on the degree of alignment the higher the draw... 

The photoconductivity (AIp /Ip, ) and the photoluminescence quenching ( - AIl(E)/Il) obtained at 77 K are plotted versus the bias field in Fig. VD-1. The data indicate clearly that the onset field for the nonlinear photocurrent, EP " = 0.77 X 10 V/cm, is lower by about 50% than the onset field of the luminescence quenching, EP =1.7x 10 V/cm. Below E5 the photocurrent is linearly dependent on E, and below E, the luminescence is field independent. At the highest electric fields employed in the transient photoconductivity experiment (E = 2.8x10 V/cm), - AIl(E)/Il 0.30, whereas the photocurrent increases beyond the linear extrapolation by a factor of = 6.3. 

The spectral peak locations at 580 nm and 630 nm are identical to what can usually be seen with photoluminescence and regular EL measurements of MEH-PPV. Therefore we conclude that microscopic light emission mechanism in transient... 

Additional experiments observing MEG have been reported for CdSe (Schaller et al., 2005b), PbTe (Murphy et al., 2006) and InAs (Pijpers et al, 2007). In addition to transient absorption studies, these optical experiments use time-resolved photoluminescence, terahertz spectroscopy, and quasi-CW spectroscopy. 

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