Free Exciton Photoluminescence Research Articles

Hybrid Photoexcitation in Undoped Diamond by Mid-Infrared Femtosecond Laser Pulses

Direct interband and intragap photoexcitation by intense mid-infrared (4.0 and 4.7 μm) femtosecond (fs) laser pulses was explored in ultrapure chemical-vapor deposited (CVD) diamond via acquisition of characteristic ultraviolet photoluminescence of free excitons and A-band photoluminescence of electrons anchored at deep donor–acceptor or dislocation-related traps, respectively. At lower laser intensities (<10 TW/cm2) the excitonic photoluminescence yields exhibit highly nonlinear dependences with Iλ2-scaling and power slopes N ≈ 17 (4.0 μm) and 14 (4.7 μm), still insufficient to cross over the direct bandgap (≥6.5 eV) by ≈1.2 and 2.8 eV, respectively. Similarly high slope of ≈9 (4.7 μm) for intragap (≥3.5 eV) photo-population of donor–acceptor traps is still insufficient for their direct excitation by ≈1 eV. At the intermediate Iλ2-dependent values of the Keldysh parameter γ ∼ 1 such incomplete multiphoton excitation anticipates the hybrid total “multiphoton + tunneling” photoexcitation generally predicted by the Keldysh theory, but never unambiguously experimentally demonstrated. At higher laser intensities (>10 TW/cm2) both the excitonic and A-band photoluminescence yields exhibit (sub)linear slopes, apparently, indicating formation of more strongly absorbing electron–hole plasma. These findings shed light on the hybrid multiphoton + tunneling character of Keldysh photoexcitation at intermediate values γ and pave the way to defect/impurity band engineering of intragap nonlinear optical properties in bulk dielectrics for their precise fs-laser nanomodification.

Pressure-Driven Phase Transition in Two-Dimensional Perovskite MHy2PbBr4

The application of high pressure allows tuning physicochemical properties of materials by changing interatomic distances. Pressure may also induce structural phase transitions into new phases with enhanced or novel functional properties. Here, we report complementary high-pressure single-crystal X-ray diffraction, Raman spectroscopy, and optical studies of a two-dimensional (2D) perovskite, MHy2PbBr4, comprising a very small spacer cation (methylhydrazinium, MHy+). This crystal exhibits highly desired ferroelectric and extraordinary multiple linear and nonlinear optical (NLO) properties. Single-crystal X-ray diffraction shows that MHy2PbBr4 undergoes an unusual Pmn21 → P21 phase transition near 4 GPa, associated with the extrusion of some MHy+ cations from the interlayer space into voids located within the inorganic sheets, not reported for any 2D hybrid perovskite. The transport of counter cations leads to a significant increase of Pb–NH2 interactions, an unprecedented threefold increase of positive linear compressibility perpendicular to the polyanionic layers and a large negative linear compressibility of −22.39 TPa–1 within the layers. The Raman data confirm the association of the phase transition with strong distortion of the crystal structure and reorganization of the hydrogen bond network, while the absorption spectra of the compressed ambient-pressure Pmn21 phase show the band gap narrowing, followed by its widening in the high-pressure P21 phase. A similar change in the pressure dependence from a red shift to a blue shift is also observed for the free-exciton (FE) photoluminescence (PL). Furthermore, the pressure-induced phase transition leads to a giant enhancement of PL intensity, especially pronounced for the broad-band emission attributed to the self-trapped excitons (STEx). We attribute the effects, observed in absorption and PL spectra, to the shortening of Pb–Br bonds in the ambient pressure phase and increased distortion of the inorganic layers and tilts of PbBr6 octahedra in the high-pressure phase. Overall, our results for a 2D hybrid compound comprising very small spacer cations extend the understanding of the pressure effect on the properties of 2D hybrid perovskites in general and demonstrate a very different behavior under compression compared to the analogues with large organic cations. They revealed that the structure–strain mechanism can be used for engineering new high-pressure phases with unusual structural, mechanical, and optoelectronic properties.

Signatures of ultrafast electronic and atomistic dynamics in bulk photoluminescence of CVD and natural diamonds excited by ultrashort laser pulses of variable pulsewidth

Inter-band photoexcitation in bulk natural and synthetic diamonds by tightly focused near-IR (1030 nm) ultrashort laser pulses of variable duration (0.3–6.3 ps) promotes characteristic UV photoluminescence A-band and phonon-mediated free-exciton recombination bands, respectively, with distinct phonon progressions. The photoluminescence yield in these diamonds is very similar and highly-nonlinear versus laser intensity with the power slopes, gradually decreasing at higher intensities for all the laser pulsewidths to indicate varying contributions of multi-photon photoexcitation, free-carrier impact ionization and Auger recombination processes in electron-hole plasma (EHP). The peak position of the main free-excitonic photoluminescence band in the synthetic diamond demonstrates universal, pulsewidth-independent “red” spectral shift as a function of laser intensity, representing the electronic bandgap renormalization at the increasing plasma density. The related phonon progression indicates the predominating interaction of free carriers with near-center-zone optical phonons, exerting distinct dynamic increase in phonon energy at the increasing plasma density. Dynamic photoluminescence micro-spectroscopy appears as a simple, but informative experimental tool, paving the way to clarification and modeling of ultrafast electronic and lattice dynamics, as well as laser energy deposition, in different types of diamonds for their micromarking and tracing.

Combination of optical transitions of polarons with Rashba effect in methylammonium lead trihalide perovskites under high magnetic fields

We investigate photoluminescence (PL) transitions of ${\text{MA}\text{Pb}X}_{3}$ ($X=\text{I}$, Br, and Cl) organic-inorganic hybrid perovskite single crystals under magnetic fields of up to 60 T. In these materials, sharp free-exciton transition peaks emerge at a low temperature (4.2 K). Under strong magnetic fields, the free-exciton PL transitions of three different halogens show dramatic differences. The free-exciton transitions of the ${\mathrm{MAPbCl}}_{3}$ crystal undergo negative energy shifts, while those of the ${\mathrm{MAPbBr}}_{3}$ crystal show normal diamagnetic shifts. To obtain the variation from Cl to Br, we attempt to measure PL transitions of ${\mathrm{MAPbCl}}_{x}{\mathrm{Br}}_{3\ensuremath{-}x}$. For ${\mathrm{MAPbI}}_{3}$, the transition-energy shifts for both ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}}$ transitions at 4.2 K exhibit a power-law dependence on the magnetic field. Such inconsistent magnetic-field effects on different halogens make it difficult to understand the transition-energy behavior through a unified model. We propose a possible mechanism for the field effects that is based on a combination of the Rashba effect induced by strong spin-orbit coupling and the polaron effect caused by the polar nature of the inorganic elements.

How free exciton–exciton annihilation lets bound exciton emission dominate the photoluminescence of 2D-perovskites under high-fluence pulsed excitation at cryogenic temperatures

Photoluminescence (PL) spectra of atomically thin 2D lead iodide perovskite films are shown to depend on excited-state density, especially at cryogenic temperatures. At high excited-state densities and low temperatures, free exciton (FE) emission is so suppressed by exciton–exciton annihilation (EEA) that other—normally much weaker—emissions dominate the PL spectrum, such as emission from bound excitons (BEs) or PbI2 inclusions. In the Ruddlesden–Popper perovskite with phenethylammonium (PEA) ligands (PEA2PbI4, PEPI), FE emission dominates at all temperatures at the excited-state densities reached with continuous wave excitation. At higher excited state densities reached with femtosecond pulsed excitation, the PL at temperatures under 100 K is dominated by BE emission redshifted from that of FE by 40.3 meV. Weak emission from PbI2 inclusions 170 meV higher in energy than FE PL is also observable under these conditions. Equilibrium between BE and FE states explains why FE emission first increases with decreasing temperature from 290 until 140 K and then decreases with decreasing temperature as the BEs become stable. A Dion–Jacobson (DJ) material based on 1,4-phenyl-enedimethanammonium (PDMA) supports the reduction of FE emission by EEA at cryogenic temperatures. However, in the PDMA-based DJ material, BE emission is never as pronounced. At low temperatures and high-excited state densities caused by pulsed excitation, a broad emission redshifted by 390 meV from the FE dominates. Based on comparison with temperature-dependent measurements of PbI2 films, this emission is suggested to arise from PbI2 inclusions in the material. Possible avenues for improving PL at room temperature are discussed concerning these findings.

First-principles study of the phonon replicas in the photoluminescence spectrum of 4H -SiC

The free-exciton photoluminescence (PL) spectrum of $4H$-SiC exhibits a detailed fine structure due to the different phonons involved in the indirect gap transition. Here a first-principles calculation of these phonons, their symmetry labeling and their contribution to the photoluminescence spectrum is presented. The calculation uses phonons and electron-phonon coupling matrix elements computed via density functional perturbation theory and energy bands and optical phonon matrix elements calculated in density functional theory. The results are in excellent agreement with the experiment for the phonon energies and the polarization dependence of the spectrum. The relative intensities are also in fair agreement if we allow for some phonons within a few meV to be interchanged. There is however a remarkable discrepancy that the experimental spectrum shows a distinct behavior for phonons with energy below $\ensuremath{\sim}55$ meV and above that energy, which is absent in the theory. The experimental PL lines corresponding to phonon energies below 55 meV are about a factor 5--10 smaller in intensity. This is not found in our calculations. The calculations show a similar peak distribution as the experiment in this range but with intensities comparable to those above 55 meV. This indicates that another mechanism outside the scope of the electron-phonon mediated transitions is operative for photon energies of the PL lines closer to the indirect exciton gap than this 55 meV cutoff, which reduces the overall intensity of these lines. We propose that this may result from competition between the phonon-assisted PL and trapping of the electron in the available unoccupied hexagonal site N-donor shallow level at 53-meV binding energy. As part of this study, we also present the phonon dispersions and density of states in $4H$-SiC and the electronic band structure including quasiparticle corrections.

Revealing the competing contributions of charge carriers, excitons, and defects to the non-equilibrium optical properties of ZnO.

Due to its wide band gap and high carrier mobility, ZnO is, among other transparent conductive oxides, an attractive material for light-harvesting and optoelectronic applications. Its functional efficiency, however, is strongly affected by defect-related in-gap states that open up extrinsic decay channels and modify relaxation timescales. As a consequence, almost every sample behaves differently, leading to irreproducible or even contradicting observations. Here, a complementary set of time-resolved spectroscopies is applied to two ZnO samples of different defect density to disentangle the competing contributions of charge carriers, excitons, and defects to the nonequilibrium dynamics after photoexcitation: time-resolved photoluminescence, excited state transmission, and electronic sum-frequency generation. Remarkably, defects affect the transient optical properties of ZnO across more than eight orders of magnitude in time, starting with photodepletion of normally occupied defect states on femtosecond timescales, followed by the competition of free exciton emission and exciton trapping at defect sites within picoseconds, photoluminescence of defect-bound and free excitons on nanosecond timescales, and deeply trapped holes with microsecond lifetimes. These findings not only provide the first comprehensive picture of charge and exciton relaxation pathways in ZnO but also uncover the microscopic origin of previous conflicting observations in this challenging material and thereby offer means of overcoming its difficulties. Noteworthy, a similar competition of intrinsic and defect-related dynamics could likely also be utilized in other oxides with marked defect density as, for instance, TiO2 or SrTiO3.

Swift heavy ion irradiation to ZnO nanoparticles: Steep degradation at low fluences and stable tolerance at high fluences

A mono-layer of ZnO nanoparticles (NPs), each of which does not mostly overlap with one another, was formed on the surface of a silica glass by implantation with 60 keV Zn+ ions and subsequent thermal oxidation. Then, the sample was irradiated with swift heavy ions (SHIs) of 200 MeV Xe14+ ions in the fluence range of 1 × 1011–5 × 1013 ions/cm2. The X-ray diffraction intensity of the {002} peak from ZnO NPs shows a steep drop to 67% of the unirradiated value at the fluence of 1 × 1012 ions/cm2 but maintains almost the same value up to 50 times higher fluence of 5 × 1013 ions/cm2. The behavior could be ascribed to high susceptibility of this material ZnO for recrystallization in the cooling stage of the thermal spike: While damage remains at the central region of SHI impact, recrystallization is induced in large surrounding regions. The interplay between the damage generation in the core regions and the recovery in the surrounding regions reaches a dynamical equilibrium at the fluence exceeding 1 × 1012 ions/cm2. While it is known that free excitons are sensitive to defect registration, the free exciton photoluminescence (PL) with 20% of the unirradiated intensity still survives up to the highest fluence 5 × 1013 ions/cm2. The stable tolerance of this material in optical absorption and PL against SHI irradiation could be attractive for applications.

Photoluminescence properties of zinc white: an insight into its emission mechanisms through the study of historical artist materials

While the photophysical properties of ZnO nanostructures have been widely explored, less research has focused on the bulk material present in artist pigments. This study is based on the analysis of historical pastels, representative of artist materials available at the turn of the twentieth century, and of the pure powder pigment as the control sample. The study of the intensity of the photoluminescence emission as a function of the fluence and of the nanosecond and microsecond emission decay kinetic properties allows the elucidation of the emission mechanisms in control ZnO and historical samples containing ZnO. Data suggest that in historical samples the near-band-edge free-exciton photoluminescence emission, typically occurring in the pure semiconductor, is influenced by the interaction of the pigment with surrounding organic binding material. Conversely, crystal defects, typically expected in historical samples following the imperfect synthesis process available at the beginning of the twentieth century, introduce minor modifications to the photoluminescence emission. The study further suggests that zinc carboxylates, detected in all historical samples and known to introduce characteristic groups on the surface of ZnO, could be responsible for changes in emission mechanisms. Research demonstrates how photoluminescence decay kinetics and the study of the dependence of the emission intensity on the fluence are powerful methods for elucidating the nature of the mechanism processes in luminescent semiconductor pigments.

Bound exciton and free exciton states in GaSe thin slab.

The photoluminescence (PL) and absorption experiments have been performed in GaSe slab with incident light polarized perpendicular to c-axis of sample at 10 K. An obvious energy difference of about 34 meV between exciton absorption peak and PL peak (the highest energy peak) is observed. By studying the temperature dependence of PL and absorption spectra, we attribute it to energy difference between free exciton and bound exciton states, where main exciton absorption peak comes from free exciton absorption, and PL peak is attributed to recombination of bound exciton at 10 K. This strong bound exciton effect is stable up to 50 K. Moreover, the temperature dependence of integrated PL intensity and PL lifetime reveals that a non-radiative process, with activation energy extracted as 0.5 meV, dominates PL emission.

Thermodynamic origin of the slow free exciton photoluminescence rise in GaAs

We use time-resolved photoluminescence (TRPL) spectroscopy to unequivocally clarify the microscopic origin of the nanosecond free exciton photoluminescence rise in GaAs at low temperatures. In crucial distinction from previous work, we examine the TRPL of the GaAs free exciton second LO-phonon replica. This enables us to simultaneously monitor the unambiguous time evolution of the total exciton population and the cooling dynamics of the initially hot free exciton ensemble. We demonstrate by a model based on the Saha equation and the experimentally determined cooling behavior that the long-debated slow photoluminescence rise is caused by time-dependent shifts in the thermodynamic quasiequilibrium between free excitons and the uncorrelated electron-hole plasma.

Spatially Resolved Thermodynamics of the Partially Ionized Exciton Gas in GaAs.

We report on the observation of macroscopic free exciton photoluminescence (PL) rings that appear in spatially resolved PL images obtained on a high purity GaAs sample. We demonstrate that a spatial temperature gradient in the photocarrier system, which is due to nonresonant optical excitation, locally modifies the population balance between free excitons and the uncorrelated electron-hole plasma described by the Saha equation and accounts for the experimentally observed nontrivial PL profiles. The exciton ring formation is a particularly instructive manifestation of the spatially dependent thermodynamics of a partially ionized exciton gas in a bulk semiconductor.

Systematic investigation of effects of exciton–acoustic-phonon scattering on photoluminescence rise times of free excitons in GaAs/Al0.3Ga0.7As single quantum wells

We have systematically investigated the photoluminescence (PL) dynamics of free excitons in GaAs/Al0.3Ga0.7As single quantum wells, focusing on the energy relaxation process due to exciton–acoustic-phonon scattering under non-resonant and weak excitation conditions as a function of GaAs-layer thickness from 3.6 to 12.0 nm and temperature from 30 to 50 K. The free exciton characteristics were confirmed by observation that the PL decay time has a linear dependence with temperature. We found that the free exciton PL rise rate, which is the reciprocal of the rise time, is inversely linear with the GaAs-layer thickness and linear with temperature. This is consistent with a reported theoretical study of the exciton–acoustic-phonon scattering rate in the energy relaxation process in quantum wells. Consequently, it is conclusively verified that the PL rise rate is dominated by the exciton–acoustic-phonon scattering rate. In addition, from quantitative analysis of the GaAs-layer thickness and temperature dependences, we suggest that the PL rise rate reflects the number of exciton–acoustic-phonon scattering events.

Features of free carrier and exciton recombination, diffusion, and photoluminescence in undoped and phosphorus-doped diamond layers

We investigated carrier dynamics and photoluminescence in undoped and n-type phosphorus-doped diamond epilayers under interband picosecond-pulse photoexcitation at 213nm. Carrier/exciton lifetime and diffusion coefficient D were determined in 4×1017 to 4×1019cm−3 carrier density range. The low D=7÷11cm2/s value in the undoped epilayer was ascribed to exciton formation and contribution of many-body effects. At low excitations, photoluminescence spectra in the undoped layer exhibited exciton emission, while at the higher ones (above 2×1018cm−3) free carrier emission became dominant. The calculated radiative lifetime of excitons (860ns) was found much higher than the experimentally measured nonradiative lifetime of 11ns, governed by excess carrier diffusive flow to the surface, with S=(3±2)×105cm/s surface recombination velocity. In the n-type layer, donor-related emission dominated and the phosphorus-related defect complexes diminished the carrier lifetime to 30–50ps. Optically recharged phosphorus states resulted in millisecond long decay tails of trapped carriers.

Nonlinear photoluminescence spectroscopy of carbon nanotubes with localized exciton states.

We report distinctive nonlinear behavior of photoluminescence (PL) intensities from localized exciton states embedded in single-walled carbon nanotubes (SWNTs) at room temperature. We found that PL from the local states exhibits strong nonlinear behavior with increasing continuous-wave excitation power density, whereas free exciton PL shows only weak sublinear behavior. The strong nonlinear behavior was observed regardless of the origin of the local states and found to be nearly independent of the local state density. These results indicate that the strong PL nonlinearity arises from a universal mechanism to SWNTs with sparse local states. The significant nonlinear PL is attributed to rapid ground-state depletion of the local states caused by an efficient accumulation of photogenerated free excitons into the sparse local states through one-dimensional diffusional migration of excitons along the nanotube axis; this mechanism is verified by Monte Carlo simulations of exciton diffusion dynamics.

Temperature and density dependence of exciton dynamics in IIa diamond: Experimental and theoretical study

We studied dependence of time‐resolved and time‐integrated photoluminescence of free excitons in IIa type monocrystalline diamond on sample temperature and carrier density. Numerical model based on rate equations for time evolution of excited carrier and occupied traps populations was used for theoretical modelling. The density dependent diffusion and free carrier/free exciton equilibrium were shown to be important factors in exciton recombination dynamics. Room temperature quantum yield of excitonic photoluminescence was found to be strongly dependent on the carrier density with the highest measured value of 13% at n = 1017 cm−3.

Temperature dependent photoluminescence of nanocrystalline γ-CuCl hybrid films

Organic–inorganic hybrid films combine the basic properties of organic and inorganic materials and offer special advantages that enhance optical, thermal and mechanical properties. We have studied the temperature dependent photoluminescence (PL) of nanocrystalline γ-CuCl hybrid films from 15K to room temperature in order to investigate the electronic transitions of the hybrid films. The I1 impurity bound exciton peak is the most intense emission peak at 15K but the peak intensity decreases rapidly with increasing temperature due to the low binding energy of this exciton bound to an impurity center and above 80K all three excitonic and biexcitonic peaks, except the Z3 free exciton emission peak disappeared. The biexciton emission peak intensity follows a quadratic dependency on power in the excitation power range <10 kWcm−2. The integrated Z3 excitonic PL intensity is almost independent of the temperature below 80K, while above 100K the PL emission intensity decreases rapidly. Thermal quenching of the Z3 free exciton PL emission in hybrid films has been observed. The full width at half maximum (FWHM) of the free exciton peak was investigated as a function of temperature and was explained by a theoretical model which considers the scattering of excitons with acoustic phonons and longitudinal optical phonons. The FWHM of the Z3 free exciton emission peak increases with increasing temperature, and a value of ~76meV was deduced for the FWHM at room temperature, which is comparatively better than ZnO (106meV) and GaN (100meV) nanostructures. The Z3 free exciton energy of the hybrid films exhibits a blue shift of 3meV at 15K compared to the bulk CuCl samples which may be due to a dead layer effect near the CuCl nanocrystal surface. The exciton energy also presents a blue shift of ~41meV with increasing temperature from 15K to room temperature. The results obtained for the γ-CuCl hybrid films are comparable to those of vacuum evaporated and sputtered CuCl films reported in the literature.

Effect of spin polarization of excitons on the energy spectra of GaAs/AlGaAs heterostructures

The evolution of the energy position of the maximum of the free exciton photoluminescence line in high-quality GaAs/AlGaAs heterostructures under optical excitation by short laser pulses of high density 5 · 1014–2 · 1018 cm−3 is considered. The influence of screening of the Coulomb interaction and interexciton exchange interaction are discussed. These effects give corrections to the exciton peak energy position of opposite signs. The second effect is sensitive to the exciton spin orientation and is manifested as splitting of the energy peaks of excitons with angular momentum projections +1 and −1. The value of splitting is proportional to the density of excitons and to the degree of their spin polarization and reaches 1.5 meV.

Spectroscopic features of nonlinear AgGaSe2 crystals

When producing large AgGaSe2 single crystals of high optical quality, the shadow and light scattering techniques were found effective. Annealing in proper atmosphere allows one to affect the crystal composition and the state of submicron-sized inclusions and point defects. Stoichiometric crystals demonstrate mainly photoluminescence (PL) of free excitons. New PL bands, a 727/745nm doublet and a 950nm broad band, are associated with selenium vacancy, VSe, and cation antisite defect, GaAg, respectively. Using the thermoactivation spectroscopy, the traps of charge carriers were revealed in AgGaSe2: those with thermal activation energy ET>0.4eV were found responsible for photoinduced absorption on the 0.1cm−1 level.

Piezotronic Effects on the Optical Properties of ZnO Nanowires

We report the piezotronic effects on the photoluminescence (PL) properties of bent ZnO nanowires (NWs). We find that the piezoelectric field largely modifies the spatial distribution of the photoexcited carriers in a bent ZnO NW. This effect, together with strain-induced changes in the energy band structure due to the piezoresistive effects, results in a net redshift of free exciton PL emission from a bent ZnO NW. At the large-size limit, this net redshift depends only on the strain parameter, but it is size-dependent if the diameter of the NW is comparable to that of the depletion layer. The experimental data obtained using the near-field scanning optical microscopy technique at low temperatures support our theoretical model.