Academic literature on the topic 'TIME-RESOLVED PHOTOLUMINESCENCE (TRPL)'

The time-resolved photoluminescence (TRPL) of red HgI 2 single crystal has been measured to determine the carrier lifetimes and to reveal the energy relaxation of excitons. Sharp near-bandgap luminescence lines due to free and bound excitons are observed at 530 nm, and a broad luminescence band appears at 630 nm at low temperatures. TRPL experiments of the near-bandgap luminescence have revealed that the luminescence comprise fast (30 to 200 ps) and slow (100 to 400 ps) decay components, showing several relaxation processes in free and bound exciton annihilation. TRPL of the broad band at 630 nm has shown that the luminescence is ascribed to the radiative recombination of donor-acceptor (DA) pairs.

Temperature dependence of photoluminescence (PL) and time resolved photoluminescence (TRPL) were obtained by two experimental systems. The relative intensity and peak position of PL show S-shift variation with increasing temperature, which may result from temperature induce carriers redistribution. Fast decay time and slow decay time were fitted by double exponential function from decay curves of TRPL at different emission energy, and the decreasing trend of both fast decay and slow decay time with increasing photon energy is attributed to various channels of recombination in shallow and deep localized states.

Time-resolved photoluminescence studies of nitride semiconductors and ultraviolet light emitters comprised of these materials are performed as a function of pump intensity as a means of understanding and evaluating device performance. Comparison of time-resolved photoluminescence (TRPL) on UV LED wafers prior to fabrication with subsequent device testing indicate that the best performance is attained from active regions that exhibit both reduced nonradiative recombination due to saturation of traps associated with point and extended defects and concomitant lowering of radiative lifetime with increasing carrier density. Similar behavior is observed in optically pumped UV lasers. Temperature and intensity dependent TRPL measurements on a new material, AlGaN containing nanoscale compositional inhomogeneities (NCI), show that it inherently combines inhibition of nonradiative recombination with reduction of radiative lifetime, providing a potentially higher efficiency UV emitter active region.

The luminescence investigations on the calcinated zinc tungstate nanopowder (ZnWO4 NP) synthesized by microwave-assisted synthesis are presented using photoluminescence (PL) and time-resolved photoluminescence (TRPL) analyses. The X-ray diffraction (XRD) patterns exhibit that the significant wolframite structure of ZnWO4 NP can be detected at calcination temperatures above 300 °C. The 12 K PL and TRPL results demonstrated that the deformation of WO6 octahedra is responsible for the low-energy side of PL spectra and dominate the red-shifted PL spectra with increasing calcination temperatures.

We report a correlation between the structural phase transition of CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) and their temperature dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL).

In this study, we report the optical properties and carrier dynamics of different surface dimensionality n-type wurtzite InN with various carrier concentrations using photoluminescence (PL) and an energy-dependent, time-resolved photoluminescence (ED-TRPL) analysis. Experimental results indicated that the InN morphology can be controlled by the growth temperature, from one-dimensional (1D) nanorods to two-dimensional (2D) films. Moreover, donor-like nitrogen vacancy (VN) is responsible for the increase in carrier concentration due to the lowest formation energies in the n-type InN samples. The PL results also reveal that the energies of emission peaks are higher in the InN samples with 2D features than that with 1D features. These anomalous transitions are explained as the recombination of Mahan excitons and localized holes, and further proved by a theoretical model, activation energy and photon energy-dependent lifetime analysis.

Studying the carrier recombination process in MAPb(Br1−yIy)3 single crystals (SCs) is important for its application in the optoelectronic field. In this work, a series of MAPb(Br1−yIy)3 SCs with varied Br/I compositions have been studied. Steady-state photoluminescence (PL) spectra, time-resolved photoluminescence (TRPL) spectra and time-resolved microwave photoconductivity (TRMC) were used to understand the radiative and non-radiative recombination processes of MAPb(Br1−yIy)3 SCs. By comparing the dynamics of TRPL and TRMC, we conclude that the dynamics of TRPL is dominated by the electron trapping process, which is in accordance with the fast decay component of TRMC kinetics, whereas the slower decay component in TRMC is dominated by the hole trapping process. Moreover, we find both the electron and hole trapping rates in mixed-halide perovskite MAPb(Br1−yIy)3 (0 < y < 1) SCs are higher than that of mono-halide perovskite MAPbBr3 SCs and MAPbI3 SCs. This suggests mixed-halide crystals could introduce additional electron and hole trapping densities, which could be related to the fluctuation of Br/I compositions in the crystals. This work is helpful for understanding carrier recombination process in mixed-halide perovskite SCs.

Luminescence properties of blue emission InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs), grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD), were studied by time-resolved photoluminescence (TRPL) spectroscopic technique. Samples involved have similar basic structures of three QWs but different well-composition and barrier/well dimensions. TRPL results show that PL intensity and decay time increase with the number of QWs and the indium composition. Correlation of physical properties with crystalline perfection open the way for optimized designs of InGaN MQW LED, with controlled the indium composition and QW numbers.

7.1 Summary The research topic of the thesis entitled “Investigations of Metal Assisted Titanium Dioxide (TiO2) Nanocrystals” disclosed the structural, morphological, compositional and optical properties of TiO2 nanoparticles (NPs) and discussed their utility for photocatalytic applications. In the current thesis, TiO2 NPs were prepared using the sol-gel method and comprehensively explored their properties. The crystalline structural and optical characteristics of TiO2 NPs (like X-ray diffraction, absorption, photoluminescence, and time-resolved photoluminescence) along with photocatalytic applications have also been discussed in detail in the previous chapter. 7.2 Important Findings of Research Work Nanocrystals of anatase, mixed and rutile phases of TiO2 and metal-doped TiO2 have been synthesized via the sol-gel method. The prepared samples were characterized by various analytical tools. For the structural and surface morphology analysis, essential tools such as XRD, SEM and TEM were used. EDX analysis has been carried out for elemental identification present in prepared nanomaterials. TGA, FTIR spectroscopy, Raman spectroscopy, UV-visible & PL spectroscopy, time-resolved photoluminescence (TRPL) spectroscopy at varying temperatures were used to understand the structural and transitions following photoexcitation. TiO2 NPs were prepared by the sol-gel method with titanium isopropoxide as a precursor at different annealing temperatures. The analyzed XRD patterns, Raman and Fourier transform infrared spectra demonstrated the structural transformation from 160 amorphous to anatase and further to rutile phase while increasing annealing temperature. In addition, a mixed-phase of TiO2 NPs is formed, which consists of both phases. The absorption and photoluminescence (PL) spectra of mixed and rutile phases are shifted towards a longer wavelength region. The indirect band gap structure changed into the direct band gap during the structural transformation. Both absorption and PL spectra shifted towards lower energy regions, which might be due to the increase in size or the induced oxygen vacancies produced at a higher temperature. Furthermore, the photocatalytic activity of all the three different structural TiO2 NPs was examined. Furthermore, the photocatalytic performance of the different types of TiO2 NPs was examined through the degradation of a dye, rhodamine B (RhB), under UV radiation and measuring changes in absorption and PL spectra. The anatase phase structure shows higher photocatalytic activity than the rutile phase. However, the mix phase has the highest photocatalytic activity among all the structures, which degraded RhB entirely at a faster rate. On the other hand, the rutile phase is unable to take part in this process. Thus, the mix phase of TiO2 NPs is beneficial for industrial and environmental applications. The transition metal ions (Ag+, Cu2+, and Ni2+) doped and undoped TiO2 NPs have been synthesized using a cost-effective sol-gel method with a 1.0 wt% dopant concentration. The microstructure and chemical compositions of these NPs were examined using various techniques such as x-ray diffractometric, field emission scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared and absorption and photoluminescence (PL) spectroscopy. The absorption and photoluminescence (PL)-excitation spectra of metal-doped TiO2 NPs are shifted to the longer wavelength region, which indicates a reduced bandgap than the bare TiO2 NPs. The absorption and PL spectra of methylene blue (MB) in the presence of undoped and metal ions doped TiO2 NPs show dramatic changes upon UV-irradiation. The absolute absorption 161 intensity reduced entirely and the solution of MB became colorless in the presence of UV irradiation. The PL of the degraded dye exhibits a new band in the shorter wavelength region, which has a multi-exponential decay function and an increased average PL lifetime. The dye degradation rate is higher for metal ions doped catalyst and highest for Cu2+ doped TiO2 NPs. Thus, Cu ions-doped TiO2 shows the highest photocatalytic activity. The order of catalytic degradation rate under UV irradiation was found to be Cu– TiO2 > Ni– TiO2 >Ag– TiO2 >anatase TiO2. The analysis of the PL spectra and PL-decays reveals the formation of smaller species that emits at a shorter wavelength region, thus helps in understanding the degradation of dye molecules. TiO2 NPs synthesized by employing the sol-gel routes and annealed at a different temperature from 400 to 900 °C. Three different nanostructures were formed, namely anatase, mixed (anatase/rutile) and rutile phases. The structure and morphology of as-synthesized NPs were confirmed using XRD and FESEM analysis. The XRD analysis of TiO2 NPs was carried out in the 290 K to 77 K temperature range and found no significant change in XRD patterns that means thermally stable TiO2 NPs. The PL spectra and contour maps of TiO2 NPs show that the anatase phase falls in the visible region. However, mixed and rutile phases fall in both visible and NIR regions as the temperature decreases from 290 K to 77 K. The visible PL band is ascribed to donor-acceptor recombination. In contrast, oxygen vacancies serve as donors and hydroxyl groups function as accepter sites. NIR PL band attributed to the trapped electrons in rutile TiO2, which recombine with free holes and intrinsic defects. The fast component of the decay processes was aided by the immediate formation of trapped electrons in luminescence sites. The indirect trap processes were responsible for the power-law component in the rutile phase, which was the recombination of trapped electrons formed via a deep trap state. It was observed that the electron-hole pairs thermally separated, preventing the formation of STEs directly. The PL 162 and PL decay studies under weak excitation conditions prove to be a more valuable and appropriate method to evaluate the trap states distribution and their carrier dynamic effects, which were vibrant to understand the photocatalytic processes better. Two-dimensional (2D) layered MoS2 nanosheets (NSs), which possess a vast range of unique properties and hold great potential for various applications. MoS2 NSs were synthesized by a hydrothermal method and the obtained NSs bear crystalline and layered structure. Absorption and electroabsorption (E-A) spectra of MoS2 doped in a PMMA thin film were measured at different temperatures (290-40 K). The E-A spectra observed at the second harmonic of the modulation frequency of the applied electric field (1.0 kHz) were analyzed with an integral method by assuming the Stark effect as a dominant feature. The absorption spectra consist of multiple transitions, among which five transitions are contributed to the E-A spectra. The changes in electric dipole moment (Δμ) and polarizability (Δα) of each transition were determined at different temperatures. Two electronic resonance states were identified for two excitonic bands of MoS2 NSs, which showed a strong E-A signal.

An all-solid-state, microscope-based time-resolved photoluminescence (TRPL) system has been developed for studying excess carrier dynamics, from the picosecond to the microsecond time domains, in III-V and n-VI semiconductor structures. The system employs laser diode excitation sources and a silicon single photon avalanche diode detector, and it allows TRPL measurements in the spectral region from 430 nm to ~1 μm. By using a 780 nm excitation source, the high spatial resolution of the system (<3 μm) has facilitated the examination of GaAs/AlGaAs MQW microresonator structures fabricated by alloy mixing techniques. These structures have important applications as surface-emitting lasers and as all-optical non-linear devices based on electronic band-filling nonlinearities. For each of these, a knowledge of carrier lifetime is of fundamental importance. Effective lateral carrier confinement is demonstrated in individual square pixels of various sizes, between 2 and 50 μm, with no significant reduction in carrier lifetime. The increasing importance of "wide-gap" II-VI semiconductors for the fabrication of blue light emitting diodes and laser diodes has led to the requirement for routine TRPL measurements on these materials. The integration of a frequency-doubled laser diode source, emitting at 420 nm, has permitted TRPL measurements to be performed on both n-and p-type ZnSe of various doping densities.

Time-resolved photoluminescence (TRPL) spectroscopy has been utilized by various groups to determine resonant and nonresonant tunneling times of electrons as well as of holes in asymmetric double quantum well (ADWQ) structures /1-4/. The main disadvantage of the TRPL method arises from the fact that it is impossible to measure electron and hole tunneling times simultaneously or to determine hole tunneling times directly in the absence of an electric field. These drawbacks are inherent to the linear TRPL technique since the photoluminescence (PL) decay time of the narrow quantum well (QWn) depends only on the tunneling time of the carrier type which tunnels faster.

In this paper we report direct measurements of carrier recombination at 1.58 µm via time-correlated single photon counting using a Ge single-photon avalanche photodiode with time resolution of 300 ps. Under high optical excitation, this provides a means for assessing the contribution of Auger processes. These are believed to play a role in the high temperature sensitivity of 1.5 µm lasers although to what extent remains unclear. The strained (1% compressive) and unstrained samples mimicked our laser structures (four and 5 30 Å InGaAs quantum wells with 150 Å InGaAsP barriers lattice matched to InP) but were nominally undoped throughout (background~5 × 1015 cm−3,n-type) Fits to the data were calculated using the initial photogenerated earner density (Δn(t=0)) and Auger coefficient (CA) as fitting parameters and included radiative, extrinsic non-radiative and Auger recombination. For example, fitting the room temperature photoluminescence decay for a strained sample gave CA= 1.0 × 10−28 cm6 s−1 and Δn(t=0) = 3.5 ×1017 cm−3, typical of the sample set. At this initial carrier density, Auger accounts for 13% of the total recombination. Fit quality is degraded if Auger recombination is omitted. This value for CA is greater than previously measured (1.3 × 10-29 cm6 s-1 [1]), and has important implications for laser operation. In order to explore the relation between the time-resolved photoluminescence results (TRPL) and the laser devices in more detail, we have studied lasers with similar layer structures to the time-resolved photoluminescence samples and cavity lengths the range 300- 1160 µm. The temperature dependence of the threshold current shows two distinct regions; for T=230-340K threshold current increases exponentially with temperature, whilst above 340K it increases super-exponentially. By comparing these results with the TRPL studies as a function of temperature we are able to assess the relative contribution of Auger recombination in these samples.

A novel instrument has recently been developed for picosecond time-resolved photoluminescence (TRPL) investigation of GaAs with 3 μm spatial resolution : the Photoluminescence Lifetime Microscope Spectrometer (PLμS). The PLμS is based on time-correlated single photon counting (TCSPC) with a single photon avalanche diode (SPAD) detector. Sensitivity of the PLμS, especially in the near infrared wavelength region (800-1000nm), is several orders of magnitude better than for synchroscan streak cameras. A signal-to-noise ratio of better than 1000:1 is typically obtained from a GaAs sample region of 3 μm diameter at room temperature and at excess carrier densities (at peak excitation) as low as 1015cm-3. As a result of the very low optical power requirements, a pulsed diode laser can be used as the excitation source. All signals are conveniently handled via optical fiber, which makes the PLμS a unique instrument for routine assessment of semiconductor materials and devices in an industrial environment.

We report time-resolved photoluminescence (TRPL) measurements of the radiative lifetime (τr) at 1.8K of resonantly excited heavy-hole excitons in a 27nm GaAs single quantum well as a function of exciton density. By the use of AlAs interface smoothing layers [1], the sample exhibits very little inhomogeneous broadening (≈0.15meV). The exciton centre of mass motion is therefore not significantly restricted by localisation in well width fluctuation islands. However, measurements of the exciton homogeneous linewidth (Γ h ) using time-resolved degenerate four wave mixing in the self-diffraction configuration (Fig.1) [2], additionally show that the sample imperfection scattering contribution to the Γ h of these delocalised excitons is also small (0.2meV). Thus, as indicated by the dependence of Γ h on exciton density (N x ), even at moderate exciton densities, exciton-exciton scattering is the dominant dephasing interaction within the photocreated exciton ensemble.

While the speed at which a SEED device may be switched is limited fundamentally by carrier vertical transport times and RC effects, in a practical parallel processing system it is likely to be limited by the ratio of incident optical power and device switch energy. Since high speed SEED operation is desirable for a high processing rate, it is important that the device performance is not degraded at high incident power. One of the important mechanisms which result in this degradation is the build-up of a carrier density in the quantum wells (QWs). This carrier density weakens the excitonic absorption feature, resulting in a reduction of the reflectivity contrast ratio. In references [1] and [2] several QW structures were compared and the saturation effects were found to decrease as the sweep-out time [3] was reduced by either lowering or thinning the barrier between wells. An MQW consisting of 100Å GaAs wells and 35Å Ga0.7Al0.3As barriers was therefore developed in [1] and for this structure, at the intensities used in that work, no saturation effects were observed. We have extended the results by examining a similar structure and measuring the saturation of the responsivity and reflectivity at incident intensities up to 40kW/cm2 (1400μW in a spot with 2.1μm 1/e2-1/e2 intensity). Thermal effects were also observed. Using time resolved photoluminescence (TRPL) we have directly measured carrier sweep-out times and so have been able to calculate the carrier densities generated in the saturation experiments.