Secondary Excitons Research Articles

The formation of direct Z-scheme Ag/BiOCl/AgIO3 heterojunction and its degradation stability

Ag and AgIO3 were simultaneously deposited on the (001) facet of BiOCl by photoreduction method. The difference of work function between Ag and BiOCl led the electrons to flow from Ag to BiOCl, which raised the Fermi energy level of BiOCl and constructed direct Z-scheme Ag/BiOCl/AgIO3 heterojunction. The electrons that flowed from Ag to BiOCl scrambled for the holes in the primary excitons of BiOCl and formed the secondary excitons, while the electrons in the primary excitons were released and reached the conduction band of BiOCl to participate in the reduction reaction. Finally, the dissociation of excitons and separation of carriers were accelerated, and the degradation rate under solar light of Ag/BiOCl/AgIO3 to RhB was 2.8 and 4.6 times that of BiOCl and AgIO3, respectively. It could also degrade RhB under near-infrared light. Furthermore, the released electrons from the primary excitons could be accepted by AgIO3 to produce AgI, and the type-II Ag/BiOCl/AgI heterojunction was formed. The Ag/BiOCl/AgI exhibited excellent degradation stability and could still degrade RhB under near-infrared light.

The enhanced photocatalytic activity of Ag-OVs-(0 0 1) BiOCl by separating secondary excitons under double SPR effects

The Ag-OVs-(0 0 1) BiOCl photocatalysts were synthesized by photodeposition method. The strong singlet oxygen signal meant that BiOCl only produce very few free electrons and not enough to reduce Ag+ to Ag. Inevitably, the “hot electrons” of oxygen vacancies reduced Ag+ and Ag occupied oxygen vacancies of BiOCl. Under light, high-energy electrons of Ag were propagated to BiOCl by propagating surface plasmon resonance (TSPR), and the high-energy electrons scrambled for the holes of primary excitons in BiOCl to form secondary excitons. The electrons in the primary excitons were released, while the secondary excitons were captured by oxygen vacancies and the holes of secondary excitons were released. The electrons released from primary excitons and the holes released from secondary excitons were accelerated to separate under the local electric field (LSPR) effect of Ag, and lots of superoxide radicals and holes participated in the catalytic reaction. The photocatalytic activity of Ag-OVs-(0 0 1) BiOCl was improved under the polarization charges, TSPR and LSPR effects of Ag, and the degradation rate of RhB by Ag-OVs-(0 0 1) BiOCl was 2.64 times that of BiOCl under the simulated sunlight.

Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.

Perovskite Excitonics: Primary Exciton Creation and Crossover from Free Carriers to a Secondary Exciton Phase

AbstractUnderstanding exciton formation is of fundamental importance for emerging optoelectronic materials, like hybrid organic–inorganic perovskites, as excitons are the lowest‐energy photoexcitations in semiconductors, are electrically neutral, and do not directly contribute to charge transport, but can emit light more efficiently than free carriers. However, despite the increasing attention toward these materials, experimental results on the processes of formation of an exciton population in perovskites are still elusive. Here, an ultrafast differential photoluminescence technique is presented that is able to track the kinetics of exciton formation and dissociation in CH3NH3PbBr3. Data show the presence of geminate excitons, i.e., primary excitons directly created upon photon absorption, and their dissociation into free electron–hole pairs. The formation is demonstrated of a secondary exciton phase through pairing of the initial population of free carriers. The analysis of the generation of secondary excitons provides an estimate of the Langevin factor, the parameter governing the charge‐pairing rate. Understanding and controlling the formation of a bright exciton population instead of a highly conductive free carrier population may help to design new hybrid perovskite materials with tailored optoelectronic functionalities.

Transport of dipolar excitons in (Al,Ga)N/GaN quantum wells

We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially- and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we measure a continuous red shift of the emission, as excitons propagate away from the excitation spot. This shift corresponds to a steady-state gradient of exciton density, measured over several tens of micrometers. Time-resolved micro-photoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons. We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 microns is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the exciton emission pattern on the scale up to 100 microns. The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.

Prompt and delayed secondary excitons in rare gas solids

Direct and indirect creation of excitons in rare gas solids has been investigated with reflectivity and luminescence spectroscopy. For the heavy rare gas solids Kr and Xe, new and more reliable exciton parameters have been deduced. With time-resolved luminescence spectroscopy, fast and delayed secondary-exciton creation has been established and separated. Thermalization of photocarriers and their delayed recombination have been analyzed, including a first attempt to investigate the influence of excitation density on the carrier dynamics. The existence of excitonic sidebands of the ionization limits Ei (either band-gap or inner-shell ionization limits) in prompt secondary exciton creation has been established. The threshold energies of these sidebands are given by Eth≈Ei+nEex (n is an integer, Eex is the exciton energy). The sidebands are ascribed to the formation of electronic polaron complexes, superimposed on the inelastic scattering of photoelectrons.

Creation of free excitons in solid krypton investigated by time-resolvedluminescence spectroscopy

The creation and relaxation of secondary excitons in solid Kr was investigatedusing energy-and time-resolved luminescence spectroscopy in the vacuumultraviolet region. The spectrally selected emission of the free exciton (FE) wasused as a probe for an investigation of the different exciton creation processes.Delayed FE creation via electron–hole recombination and ‘prompt’ (in terms ofthe time-resolution of the experiment) creation of excitons were separated. The‘prompt’ creation of a FE appears in the region above threshold energyEth,which is equal to the sum of the band gap energy and thefree exciton energy. ‘Prompt’ creation of excitons aboveEth isascribed to a superposition of two processes: (i) creation of the electronic polaroncomplex (one-step process) and (ii) inelastic scattering of photoelectrons describedin the framework of the multiple-parabolic-branch band model (two-step process).In addition, the ratio spectrum of the time-integrated FE and self-trapped exciton(STE) emission was analysed. The behaviour of the ratio spectrum is a proof thatelectron–hole recombination leads to STE states through FE states as precursors.

Multiplication Processes of Electronic Excitations in PbCl2Crystals Excited by Vacuum Ultraviolet Radiation

The excitation spectra for two intrinsic luminescence bands at 3.76 eV (UV) and 2.62 eV (BG) in PbCl 2 have been measured using synchrotron radiation in the energy range up to 25 eV at 10 K. Since the UV and BG bands are connected to excitonic luminescence and electron–hole ( e – h ) recombination luminescence, respectively, an analysis of the difference in their excitation spectra allows us to separate excitonic and e – h mechanisms in the multiplication of electronic excitations. It is found that the intensity of the UV emission is sharply increased at 9.8 eV, which is higher than twice the band gap energy. Secondary excitons are formed by inelastic scattering of hot photoelectrons in the region above 9.8 eV. On the other hand, the BG emission is gradually enhanced in the range of 9–12.5 eV, and the intensity doubles at around 12.5 eV. This result indicates that the creation of two e – h pairs per photon predominantly causes in the region above 12.5 eV. The e – h multiplication originates not only in in...

CATION AND ANION ELECTRONIC EXCITATIONS IN MgO AND BaF2 CRYSTALS UNDER EXCITATION BY PHOTONS UP TO 75 eV

The creation of primary and secondary cation excitations was investigated for MgO and BaF 2 crystals using the energy-and time-resolved luminescence method. It was shown that the primary cation excitations in MgO created by photons of 50–65 eV decay with the formation of anion free excitons and electron–hole pairs. The formation of secondary cation holes by hot photoelectrons in the Ba 2+ 5p shell causes the increase of cross-luminescence intensity whereas the creation of neither primary nor secondary cation excitons causes the appearance of cross-luminescence in BaF 2.

AN ANALYSIS OF ELECTRON–HOLE RECOMBINATION IN SOLID KRYPTON USING TIME-RESOLVED VUV-LUMINESCENCE SECTROSCOPY

For the first time, the free exciton (FE) VUV-luminescence decay curves were systematically investigated in solid Kr using photoexcitation in the energy region above band gap energy. Delayed electron–hole recombination and "prompt" (in terms of the time resolution of the experimental setup) creation of secondary excitons were separated using the time-resolved experimental technique. A detailed model for the dynamics of electron–hole recombination into the FE state was developed. The delayed component of the free exciton decay curve was reproduced with model calculations, including thermalization of the carriers via scattering on acoustic phonons, and the recombination cross-section, which depends on the actual carrier temperatures. A satisfactory agreement between experiment and theory was found.

Creation of electronic excitations and defects by vuv radiation (6-40 eV) in wide-gap solids

The processes of multiplication of electronic excitations (MEE), connected with the creation of secondary excitons or electron-hole (e-h) pairs by hot conduction electrons, are realized in wide-gap metal halides and oxides. In oxides, secondary e-h pairs can be also formed by 27-40 v eV photons due to L 1 VV Auger transitions (with the participation of 2s oxygen holes). The excitation spectra of luminescence and the creation spectra of electron F centres or hole V centres have been measured for Na 6 Al 6 Si 6 O 24 (NaI) 2x sodalites and MgO:Be, respectively, at 8-80 v K. A high local density of excitations has been revealed under MEE conditions in KBr and Br sodalites with self-trapping excitons and holes.

INNER-SHELL TRANSITIONS AND SECONDARY EXCITONS IN SOLID XE AND KR

Time-resolved photoluminescence of the exciton emission in rare gas solids has been measured for excitation in the range of inner-shell transitions. The excitons are created both 'promptly' (within the experimental time resolution) and 'delayed' through electron-hole recombination. Excitation spectra at the 4d edge in Xe and the 3d edge in Kr are discussed. An outstanding feature of the time resolved spectra is the strong resonance that appears in a short time window above a threshold energy, which is the sum of the ionisation energy of the core level and the energy of the (valence) free exciton.

Creation of secondary excitons in solid Ar observed in the luminescence of molecular self-trapped excitons (M-STE)

Secondary exciton formation in Ar polycrystals was investigated. Using time-resolved luminescence methods the `prompt’ creation of secondary excitons was experimentally separated from delayed electron–hole recombination. The experimentally observed threshold of secondary exciton production is compared to the values provided by various theoretical approaches. Furthermore luminescence excitation spectra of Ar crystals and small Ar clusters in the threshold region of secondary excitations are compared.

Excitonic and electron-hole processes in NaCl and NaCl:Ag crystals under conditions of multiplication of electronic excitations

The excitation spectra of and emissions of self-trapped excitons (3.4 eV and 5.4 eV, respectively) as well as the excitation spectra of 5.17 eV luminescence of Ag+ impurity centres were measured in NaCl and NaCl:Ag crystals using synchrotron radiation of 5 - 38 eV. Fast and slow components of these emissions were detected. An analysis of the differences in the excitation spectra measured at 8 and 295 K allowed us to separate the excitonic and electron-hole (e-h) mechanisms of the multiplication of electronic excitations. A photon of 17 - 19 eV forms an e-h pair and a secondary exciton, while the absorption of a 21 - 27 eV photon causes the creation of two e-h pairs. Using luminescent and photoelectric methods, it was shown that a 2p3s Na+ cation exciton, formed at the absorption of a 33.4 eV photon, decays with the creation of an anion exciton with a 3p hole component and two e-h pairs. Three e-h pairs are formed after the absorption of a 31 eV photon by a chlorine ion.

Creation of electronic polaron complexes in solid xenon observed in free-exciton luminescence under selective photon excitation

Abstract Time-resolved excitation spectra of the free exciton luminescence of solid xenon are measured in the energy range up to 30 eV at 7.1 K and 9.3 K. The pronounced structures observed near band gap and two band gap energies are assigned to primary and secondary excitons formed via various mechanisms. For the first time, the creation of free secondary excitons is directly observed in the excitation spectra. The results are assigned to creation of electronic polaron complexes and to inelastic scattering of photocarriers.

Peculiarities of stable Frenkel defect formation in RbCl crystals

Low creation efficiency of stable F–H and α–I pairs in RbCl is determined by the microstructure of H centres (Cl 2 − molecule is not interacting with neighbouring anions). The spectrum of F-centre creation has been measured for RbCl : Tl using synchrotron radiation 6–20 eV. At 295 K the creation efficiency is especially high in the regions 14.5–15.7 and 15.7–17.0 eV where the energy of hot photoelectrons is sufficient for the formation of near-impurity excitations or secondary excitons, respectively. A new mechanism of F–H pair creation has been revealed in RbCl : Ag. The heating of an irradiated crystal up to 210–240 K provides the tunnel recharging of V K and Ag 0 centres with the creation of Frenkel defects.

Creation of groups of spatially correlated defects in a KBr crystal at 8 K

The creation spectrum of triplets of spatially correlated defects (F centre + self-trapped hole, centre + interstitial halogen ion, I centre) was measured for the first time in a KBr crystal at 8 K, using synchrotron radiation of 6-25 eV and highly sensitive luminescent methods. The spectrum of spatially correlated F and H centre creation by synchrotron radiation at 8 K was measured as well. The efficiency of triplet creation is especially high at crystal irradiation by photons under the conditions of multiplication of electronic excitations when the absorption of one photon leads, respectively, to the formation of an electron-hole (e-h) pair and a secondary exciton (15.0-16.5 eV), or two e-h pairs (16.7-19.0 eV), or to the formation of a cation exciton (19.5-20.5 eV). It is shown that the formation of an triplet is caused by the recharging of a primary F-H pair by an e-h pair or by a free exciton.

Creation of Electronic Polaron Complexes in Solid Xenon Observed in Free–Exciton Luminescence under Selective Photon Excitation

Time–resolved excitation spectra of the free exciton luminescence of solid Xenon are measured in the energy range up to 25 eV at 9.5 K. The pronounced structures observed near band gap and two band gap energies are assigned to primary and secondary excitons formed via various mechanisms. For the first time, the creation of free secondary excitons is directly observed in the excitation spectra. The results are assigned to creation of electronic polaron complexes and to inelastic scattering of photocarriers.

Secondary excitons in alkali halide crystals.

The peculiarities of multiplication of electronic excitations have been investigated in wide-gap alkali halide crystals where, in contrast to semiconductors, free and self-trapped excitons as well as free and self-trapped holes coexist. In particular, the luminescent methods for separation and quantitative investigation of the formation processes of secondary excitons have been elaborated. These methods are based on the investigation of the dependence of the intensity ratio for two different emissions from the same crystal as a function of the energy of exciting photons. The excitation spectra of triplet ($\ensuremath{\pi}$) and singlet ($\ensuremath{\sigma}$) emissions of self-trapped excitons as well as of impurity luminescence have been measured, using synchrotron radiation of 12 to 32 eV, for high-purity KI, KBr, and doped KI:Na, KI:Tl, KBr:I, KBr:Tl crystals at 8 K. The analysis of the intensity ratio spectra for two emissions allowed us to separate the regions of secondary triplet exciton formation in KI (13-15 and 26.5-29 eV) and KBr (15-17 and 28-30 eV). The threshold photon energy for the formation of a secondary exciton has been experimentally determined to be equal to 13.1\ifmmode\pm\else\textpm\fi{}0.2 eV in KI and 15.2\ifmmode\pm\else\textpm\fi{}0.2 eV in KBr. The probability of secondary exciton formation by hot photoelectrons reaches the main maximum at 0.5-0.7 eV above the value of threshold energy. The energy transport to impurity centers by free triplet secondary excitons has been revealed in KI:Na and KBr:I crystals. The decay of $3{p}^{5}4s$ cation excitons in KI and KBr, generated by 20-eV photons, leads to the formation of a double amount of electron-hole pairs.

Energy transfer by exciton-polaritons and secondary excitons in a KBr:I crystal

The excitation spectra of I −-centre luminescence and self-trapped exciton π and σ emissions have been measured in the region 6.7–33 eV for a KBr:I crystal at 8 K. The energy transfer to the I −-centre by secondary triplet excitons has been revealed. This process is detected in the region of Br − and K + ionisation by 15–17 or 28–30 eV photons, respectively. Formation of orthoexcitons by 6.79 eV photons leads to the effective excitation of σ emission. The impurity luminescence can be effectively excited in the region of lower polaritonic branch and paraexciton formation.