Electronic Relaxation Research Articles

Time-resolved photoelectron spectroscopy at surfaces

Light is still an important spectroscopic tool for investigating the electronic structure of surfaces. Time-resolved photoelectron spectroscopy has mainly been developed in the last 30 years. It is therefore not surprising that the topic was hardly mentioned in the last issue on “The first thirty years” of surface science. In the second thirty years, however, we have seen tremendous progress in the development of time-resolved photoelectron spectroscopy on surfaces. Femtosecond light pulses and advanced photoelectron detection schemes are increasingly being used to study the electronic structure and dynamics of occupied and unoccupied electronic states and dynamic processes such as the energy and momentum relaxation of electrons, charge transfer at interfaces and collective processes such as plasmonic excitation and optical field screening. Using spin- and time-resolved photoelectron spectroscopy, we were able to study ultrafast spin dynamics, electron-magnon scattering and spin structures in magnetic and topological materials. Light also provides photon energy as well as electric and magnetic fields that can influence molecular surface processes to steer surface photochemistry and hot-electron-driven catalysis. In addition, we can consider light as a chemical reagent that can alter the properties of matter by creating non-equilibrium states and ultrafast phase transitions in correlated materials through the coupling of electrons, phonons and spins. Electric fields have also been used to temporarily change the electronic structure. This opened up new methods and areas such as high harmonic generation, light wave electronics and attosecond physics. This overview certainly cannot cover all these interesting topics. But also as a testimony to the cohesion and constructive exchange in our ultrafast community, a number of colleagues have come together to share their expertise and views on the very vital field of dynamics at surfaces. Following the introduction, the interested reader will find a list of contributions and a brief summary in Section 1.3.

Radiation-Induced Paramagnetic Centers in Meso- and Macroporous Synthetic Opals from EPR and ENDOR Data

The paramagnetic defects and radiation-induced paramagnetic centers (PCs) in silica opals can play a crucial role in determining the magnetic and electronic behavior of materials and serve as local probes of their electronic structure. Systematic investigations of paramagnetic defects are essential for advancing both theoretical and practical aspects of material science. A series of silica opal samples with different geometrical parameters were synthesized and radiation-induced PCs were investigated by means of the conventional and pulsed X- and W-band electron paramagnetic resonance, and 1H/2H Mims electron-nuclear double resonance. Two groups of PCs were distinguished based on their spectroscopic parameters, electron relaxation characteristics, temperature and time stability, localization relative to the surface of silica spheres, and their origin. The obtained data demonstrate that stable radiation-induced E’ PCs can be used as sensitive probes for the hydrogen-containing fillers of the opal pores, for the development of compact radiation monitoring equipment, and for quantum technologies.

Enhanced near-field radiative heat transfer between borophene sheets on different substrates

Abstract Near-field radiative heat transfer (NFRHT) has the potential to exceed the blackbody limit by several orders of magnitude, offering significant opportunities for energy harvesting. In this study, we have examined the NFRHT between two borophene sheets through the calculation of heat transfer coefficient (HTC). Due to the tunneling of evanescent waves, borophene sheet allows for enhanced heat flux and adjustable NFRHT by varying its electron density and electron relaxation time. Additionally, the near field coupling is further examined when the borophene is deposited on dielectric or lossy substrates. The maximum HTC is closely related to the real part of the dielectric substrate. As a case study, the HTC on the lossy substrate of MoO3, ZnSe and SiC are calculated for comparison. Our results indicate that MoO3 is the optimal substrate to get the enhanced energy transfer coefficient. It results in a remarkable value of 1737 times higher than the blackbody limit owing to the enhanced photon tunneling probability. Thus, our study reveals the effect of substrate on the HTC between borophene sheets and provides a theoretical guidance for the design of near-field thermal radiation devices.

Dielectric behavior and defects of nitrogen-containing single crystal diamond films

Single crystal diamond (SCD) film, as a new substrate material, has attracted growing attention because of its low dielectric loss. The nitrogen was usually introduced into microwave plasma chemical vapor deposition (MPCVD) process in order to enhance the growth rate of SCD films. So SCD films with nitrogen were prepared by MPCVD compared with the undoped film. The impurities and defects of diamond films were characterized by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy (PL). The dielectric behavior of SCD films was measured using the LCR measurement in the frequency range of 1 kHz to 110 MHz. The results indicate that the primary dielectric loss in SCD films is conductivity loss and space charge polarization below 1 MHz, while the main dielectric loss is dipole orientation polarization and electronic relaxation polarization in the frequency range of 1 MHz to 110 MHz. Point defects like substitutional N+, NV− and SiV− exert a significant influence on polarization loss. This is essential to improve the growth of SCD with excellent dielectric properties.

Slowing down the hot electron cooling to activate near-infrared photocatalytic activity in potassium/carbon co-doped polymeric carbon nitride

The research on hot electron cooling in photocatalysis has been neglected. Here, taking polymeric carbon nitride as an example, a potassium/carbon (K/C) co-doping strategy is proposed to slow down hot electron cooling. The K/C co-doping reduces the specific arrangement and overlap of electronic band structures, and decreases the degeneracy of conduction band energy levels, thus exciting phonon bottleneck effect to effectively suppress the relaxation of hot electrons by reducing the interaction between hot electrons and phonons. Our work benefits to improve the efficiency of photocatalytic energy conversion.

Metasurface Absorber for Blood Hemoglobin Concentration.

In this study, a biosensing scenario is developed for monitoring blood quality based on the detection of blood hemoglobin concentration. The procedure involves considering the blood sample as the dielectric with different refractive indexes for different concentrations of hemoglobin. Usually, the sensitivity to design parameters is the major issue with the metasurface-based detection. To address this issue, a three-layer graphene-based wave absorber is designed and modeled using passive circuit elements. The major idea behind this work is to maximize the device sensitivity against the blood sample. The research methodology involves impedance matching between the device and the surrounding environment, while full-wave simulation is also performed and compared to ensure circuit view accuracy. The findings suggest that the proposed graphene-based absorber can efficiently monitor blood quality via dual absorption peaks. The simulation results extracted from impedance matching and the full-wave method indicate frequency shifts of the second absorption peak. These shift values are interpreted based on hemoglobin concentration. Additionally, ample analyses are provided to show the reliability of the proposed absorber against geometrical aspects, incident angle, external stimulation, and the graphene electron relaxation time.

Modulating intramolecular charge transfer via π-d conjugative effect in coordination polymers toward photo-thermo-electric conversion

The prospect of harnessing sunlight to generate cost-effective heat and electricity presents a captivating opportunity to address water scarcity and electricity shortages. Photothermal materials with a broad absorption spectrum are indispensable for effectively harnessing solar energy and converting it into heat/electricity. Herein, we developed an intramolecular charge transfer strategy via conjugated effect to regulate solar absorption and photothermal conversion ability of M-DABDT (M = Fe/Co/Ni/Cu/Zn, DABDT = 2,5-diaminobenzene-1,4-dithiol), overcoming a common limitation of narrow absorption and low photothermal efficiency in traditional metal-organic assemblies. The density functional theory calculations reveal that altering transition metal ions induces the structural torsion of organic linkers, resulting in a significant change in dihedral angle from 89.96° to 2.57°. The structural change significantly facilitates the charge transfer and electron relaxation, ultimately promoting the conversion of light to heat. Under one sun irradiation, the maximum temperature of Fe-DABDT can reach approximately 92.5 ℃. More significantly, the integration of M-DABDT CPs and thermoelectric devices under normal solar irradiation results in an extraordinary open circuit voltage (238 mV) and maximum power density (364.5 μW m−2), processing a peak output voltage density of 76.9 V m−2 at 11:00 a.m. in the presence of outdoor sunlight. This strategy presents an avenue for developing metal-organic solar absorbers for photo-thermo-electric conversion systems.

Internal Conversion Cascade in a Carbon Nanobelt: A Multiconfigurational Quantum Dynamical Study.

Carbon nanobelts feature intriguing photophysical properties, due to their high symmetry and structural rigidity. Here, we consider a (6,6) armchair carbon nanobelt, i.e., the very first carbon nanobelt to be synthesized [Povie et al., Science 2017, 356, 172] and characterize the internal conversion dynamics using multiconfigurational quantum dynamics via the multi-layer multiconfiguration time-dependent Hartree (ML-MCTDH) method. A symmetry-adapted linear vibronic coupling Hamiltonian for 26 electronic states and 210 vibrational modes is employed. Electronic excitations are found to decay through a dense manifold of excited states, which interact via multiple conical intersections, while inducing minimal geometry change. It is shown that a rapid coherent decay, exhibiting a nonvanishing quantum flux on a time scale of less than 50 fs, transitions toward a slower, decoherent decay at longer times. As previously suggested in the literature, electronic relaxation is hindered by phonon bottlenecks such that a stepwise internal conversion cascade is observed. The computed vibronic absorption spectrum is shown to be in good agreement with the experimental spectrum.

Light funneling into localized acoustic graphene plasmons for extreme infrared and terahertz field enhancement and confinement

Enhancing light-matter interaction through deep subwavelength-scale confinement is crucial for numerous applications like molecular sensing, optoelectronic devices, and non-linear optics. Here, we report the excitation of localized acoustic graphene plasmons (LAGPs) confined in a sub-micro- wide, nanometer-thick layer using a metal slit antenna. This approach enables light funneling in the infrared and terahertz regimes, leading to strong field enhancement and confinement. LAGPs exhibit broad-band excitation characteristics, with the number of excited modes adjustable via the symmetry of the relative positioning between graphene and the metal slit. Detailed analysis indicates that the local field intensities of LAGPs are critically influenced by both the periodicity of the device structure and the electron relaxation time of graphene. These findings are effectively elucidated using temporal coupled mode theory. In comparison to conventional non-localized acoustic graphene plasmons, LAGPs demonstrate significantly improved field confinement and enhancement attributed to the funneling effect. Our study presents a promising avenue for achieving robust light-matter interaction and holds potential for various applications in the infrared and terahertz domains.

Unraveling the electronic properties in SiO2 under ultrafast laser irradiation

First-principles simulations were conducted to explore various electronic properties of crystalline SiO2 (α-quartz) under ultrafast laser irradiation. Employing Density Functional Perturbation Theory and the many-body (GW) approximation, we calculated the impact of thermally excited electrons on the electronic specific heat, electron pressure, effective mass, deformation potential, electron-phonon coupling and electron relaxation time of quartz, covering a wide range of electron temperatures, up to 100,000 K. We show that the electron-phonon relaxation time of highly-excited quartz becomes twice faster compared to low-excited states. The deformation potential, which dictates atomic displacement, has a non-monotonic behavior with a well-pronounced minimum at around 16,000 K (2.7 × 1021 cm−3 of excited electrons) where the bond ionicity of the Si-O starts decreasing followed by a cohesion loss at 35,000 K due to the pressure exerted by the excited electrons on the lattice. Consequently, our calculated data, illustrating the evolution of physical parameters, can facilitate simulations of laser-matter interactions and provide predictive insights into the behavior of quartz under experimental conditions.

Influence of Aliphatic versus Aromatic Ligand Passivation on Intersystem Crossing in Au25(SR)18.

The electronic relaxation dynamics of gold monolayer protected clusters (MPCs) are influenced by the hydrocarbon structure of thiolate protecting ligands. Here, we present ligand-dependent electronic relaxation for a series of Au25(SR)18- (SR = SC8H9, SC6H13, SC12H25) MPCs using femtosecond time-resolved transient absorption spectroscopy. Relaxation pathways included a ligand-independent femtosecond internal conversion and a competing ligand-dependent picosecond intersystem crossing process. Intersystem crossing was accelerated for the aliphatic (SC6H13, SC12H25) thiolate MPCs compared to the aromatic (SC8H9) thiolate MPCs. Additionally, a 1.2 THz quadrupolar acoustic mode and a 2.4 THz breathing acoustic mode was identified in each cluster, which indicated that differences in ligand structure did not result in significant structural changes to the metal core of the MPCs. Considering that the difference in relaxation rates did not result from ligand-induced core deformation, the accelerated intersystem crossing was attributed to greater electron-vibrational coupling to Au-S vibrational modes. The results suggested that the organometallic semiring was less rigid for the aliphatic thiolate MPCs due to reduced steric effects, and in turn, increases in nonradiative decay rates were observed. Overall, these results imply that the protecting ligand structure can be used to modify carrier relaxation in MPCs.

Effect of high-temperature annealing on terahertz optoelectronic properties of nitrogen-doped polycrystalline diamond

Nitrogen-doped polycrystalline diamond (N-PCD) is one of the most important carbon-based electronic materials. Here we present a systmatic investigation of the effect of high-temperature annealing (HTA) on basic physical properties of N-PCD wafer grown by microwave plasma-enhanced chemical vapor deposition (MPECVD). The metallographic and optical microscopes, X-ray diffraction,Raman spectroscopy and X-ray photoelectron spectroscopy are applied for the characterization of N-PCD before and after HTA. Particularly, we study the optoelectrnic properties of N-PCD before and after HTA by using terahertz (THz) time-domain spectroscopy. THz transmittance, dynamical and static dielectric constants and optical conductivity for N-PCD are measured. For N-PCD before HTA, we can observe a THz absorption peak induced by weak H-bonds in N-PCD, which was predicted theoretically in 1999. For N-PCD after HTA, we find that the corresponding optical conductivity can be rightly descriped by the Drude-Lorentz formula. Thus, via fitting the experimental results with the theoretical formula, we can determine optically the key electronic parameters of N-PCD after HTA, such as the electron density, the electronic relaxation time and the Lorentz frequency. The temperature dependence of these parameters is examined in the range of 80–300 K. This work demondtrates that HTA is a practical and efficient way in improving the electronic and optoelectronic properties of N-PCD. The results obtained from this research can benefit an in-depth understanding of the effect of HTA on physical properties of N-PCD from a viewpoint of semiconductor physics.

Full-spectrum plasmonic semiconductor with stabilized oxygen vacancies enables VOCs purification for durable clean water capture

To mitigate health risks associated with volatile organic compounds (VOCs) in solar-driven water evaporation, we developed Au/WO2.72 material with full-spectrum absorption capabilities and dual photothermal and photocatalytic properties. Injecting hot electrons from Au nanoparticles (NPs) into WO2.72 preserves oxygen vacancies, thus maintaining the plasmonic effect and significantly reducing hot electron relaxation. This leads to enhanced purification of VOCs. The MF-Au/WO2.72 water evaporator, constructed on a melamine–formaldehyde (MF) substrate, demonstrated a water evaporation rate of 1.36 kg m-2 h−1 and stability over 6 h daily for 12 days under one sun irradiation. Importantly, it achieved a 92.9 % removal efficiency for phenol VOCs. Outdoor experiments validated its effectiveness in organic wastewater purification and seawater desalination, highlighting its potential for efficient and clean water production by simultaneously removing VOCs during the evaporation process. This work provides valuable insights into utilizing plasmonic-coupling materials for solar water purification.

Ultrafast dynamics and internal processing mechanism of silica glass under double-pulse femtosecond laser irradiation

Femtosecond laser-induced plasma filaments have potential for various applications including attosecond physics, spectroscopy, and microprocessing. However, the use of plasma filaments to generate high-aspect-ratio internal modifications remains low-efficiency. Here, we experimentally demonstrated high-efficiency internal processing using plasma filaments induced by a double-pulse femtosecond laser. The processing mechanism was revealed through an investigation of the ultrafast dynamics of plasma filaments in experiments and simulations. We found that the excitation region of the first pulse (P1) exerted a temporal effect on the propagation and absorption of the second pulse (P2) due to the evolution of excited electrons, thus resulting in different processing characterizations. At a smaller inter-pulse delay (IPD), electrons and self-trapped excitons induced by P1 improved the absorption of P2 in the shallow region. Consequently, the main excitation regions of P1 and P2 were separated, resulting in a lower density of energy deposition and weak modifications. Whereas, at a larger IPD, P2 penetrated a deeper region with the relaxation of electrons and excitons induced by P1, leading to a better overlap of excitation regions between P2 and P1, thus improving the density of energy deposition and achieving efficient microprocessing. Besides, at an infinite IPD, P2 behaved like P1, but no modification was obtained owing to the complete energy diffusion of P1. Therefore, controlling the electron dynamic and energy diffusion contributes to the improvement of modification efficiency. Furthermore, the distribution of electron densities on the cross section was estimated to precisely analyze the microprocessing. These results are expected to aid in a better understanding of the interaction mechanism between dielectrics and intense ultrafast lasers and be useful for microprocessing applications.

(Invited) Ultrafast Excited Electron Relaxation in Triply Fused Ni(II) Porphyrin-Nanographene Conjugates for Energy Conversion

Porphyrins are appealing building blocks for solar energy applications, although their photosensitization capability is limited by their large optical energy gap, resulting in a mismatch in absorption for efficient harvesting of the solar spectrum. Porphyrin π-extension by edge-fusing with nanographenes can be employed to narrow their optical energy gap, enabling the development of porphyrin-based panchromatic dyes with an optimized energy onset for solar energy conversion in dye-sensitized solar fuel and solar cell configurations. Metalloporphyrin-nanographene conjugates based on open-shell transition metals, such as Ni(II), exhibit panchromatic absporption covering the entire visible and part of the near-IR spectral range. Dispite this interesting property, they exhibit fast excited-state electronic relaxation across the Ni d levels, which hampers harnessing photoexcited electrons to trigger photophysical or photochemical reactions. In this work, we shed light into the underlying mechanism behind fast non-radiative relaxation in these systems. Variable temperature transient absorption and global fit analysis are combined with time-dependent density functional theory to produce a picture of the relevant excited states and underlying relaxation pathways in the conjugates. At room temperature, photoexcitation to the lowest S1* energy level is followed by vibrational cooling in 1-2 ps, and intersystem crossing in > 10 ps, setting a short temporal window wherein a small fraction of relaxed singlets decay to the ground state. Following intersystem crossing and triplet internal conversion, triplets relax rapidly to the ground state (S0) in few tens of picoseconds. By performing measurements at low temperature, we provide evidences for the interplay of a lowest terminal triplet level with > 1 ns lifetime which acts as a bottleneck, and a sloped conical intersection to S0* which is thermally accessible at room temperature from the former energy level. The overall triplet decay is dictated by the contributions that fall along these two photophysical branches. This observation bears significance in understanding how nanographene functionalization leads to a profound effect, opening avenues for potential applications in Optoelectronics and related fields.[1] Garcia-Orrit, S., Vega-Mayoral, V., Chen, Q., Serra, G., Paternò, G. M., Cánovas, E., Narita, A., Müllen, K., Tommasini, M., Cabanillas-González, J., Nanographene-Based Decoration as a Panchromatic Antenna for Metalloporphyrin Conjugates. Small 2023, 19, 2301596.[2] Garcia-Orrit, S. et al., in preparation Figure 1

(Invited) Photophysical Properties and Photodynamics in CdSe Quantum Dots as Photocatalysts and Photosensitizers

Interactions between dye molecules and the semiconductor quantum dots (QDs) has a strong effect on both solar-to-electrical and solar-to-chemical energy conversion processes. In both types of applications, the charge transfer from the photoexcited QD to the molecule play a crucial role. Applying DFT-based non-adiabatic molecular dynamics, we have revealed ultrafast hole transfer (~10 fs) from the CdSe QD to the Ru(II) dye, following by a slow component (~100 fs) associated with the redistribution of population throughout QD states. The ultrafast interfacial QD-to-dye hole transfer is rationalized by strong non-adiabatic couplings between the QD surface states and the high frequency vibrational modes of isocyanide ligands in the dye. We also have studied the effect of the complex heterostructures of Janus QDs, where a half of the QD is made from CdSe and another half is from PbSe forming an interface along a specific crystalline direction. Our calculations show that the Pb-enriched (111) crystalline direction of the interface enhances optical response of the QDs. The hole relaxation is much slower than the electron relaxation, followed by the hole transfer to the dye. The hole transfer is sensitive to the polarity of the environment, rather than the ways of dye’s attachment to the surface. We also found that in nonstoichiometric QDs, a loss of a passivating ligand affects the electronic structure in the same way as injection of an electron in Cd-rich QDs or hole injection in Se-rich QDs. Moreover, electron injection to the Se-rich QDs results in highly optically active redshifted transitions associated with the introduced charge. Overall, our calculations provide atomistic insights into QD-to-molecule charge transfer, giving researchers an additional handle for enhancing QD photophysical properties by means of the QD stoichiometry, interface structure, and QD-molecule engineering.

Exciton-Assisted Relaxation of Hot Carriers in Semiconductor Quantum Dots

The work provides computational arguments in support of excitonic approach for the treatment of the photo-induced processes in semiconductor quantum dots. The non-radiative relaxation, non-radiative recombination, and photo-luminescence quantum yield are computed for a range of atomistic models of semiconductor quantum dots (QDs) in the quantum confinement regime. The excitonic (EX) approach is compared to independent orbital approximation (IOA) approach. Both approaches address dissipation of the electronic energy from electronic degrees of freedom to thermal vibrations of the lattice. The difference of two approaches appears in treatment of energies of electronic states and in a way how the electron-phonon interaction is taken into account. IOA approach uses energies of Kohn-Sham orbitals and on the fly non-adiabatic couplings. [1-3] EX approach uses Bethe-Salpeter equation (BSE) for energies.[4-6] The excitonic wavefunctions from BSE is used to construct a linear transformation matrix that transforms IOA-based non-adiabatic couplings into an excitonic basis. Both approaches are compared in application to untrasmall 1 nm diameter Si QD.Results include an evidence that hot excitons relax sooner in the excitonic picture than in the IOA picture. The observed effect is rationalized via smaller subgaps and different available relaxation pathways in the excitonic picture. The most surprising result is found for the simulated emission spectrum. The spectum in the excitonic picture demonstrates intensity in several 5 orders of magnitude higher than in the IOA picture. This observation is related to formation of a bright exciton in the lowest excitation of the ultra-small Si QDs. Obtained evidence favors excitonic approach and promises a reliable interpretation and prediction of time-dependent observables in a range of semiconductor quantum dots of different composition, sizes, and surface environment.[7] Most intriguing results are expected for QDs representing interface between PbSe and CdSe. [8]Support of National Science foundation via NSF CHE-2004197 is gratefully acknowledged.[1] D. S. Kilin and D. A. Micha, “Relaxation of photoexcited electrons at a nanostructured Si(111) surface”, J. Phys. Chem. Lett. 1, 1073-1077 (2010).[2] D. J. Vogel and D. S. Kilin, "First-Principles Treatment of Photoluminescence in Semiconductors" J. Phys. Chem. C 119, 50, 27954–27964 (2015).[3] D. J. Vogel, A. B. Kryjevski, T. M. Inerbaev, and D. S. Kilin, "Photoinduced Single- and Multiple-Electron Dynamics Processes Enhanced by Quantum Confinement in Lead Halide Perovskite Quantum Dots", J. Phys. Chem. Lett. 8, 13, 3032–3039(2017).[4] A. B. Kryjevski and Dmitri Kilin, "Enhanced multiple exciton generation in amorphous silicon nanowires and films", Molec. Phys. 114, 365-379 (2016).[5] M. Rohlfing and S. G. Louie, "Electron-Hole Excitations in Semiconductors and Insulators", Phys. Rev. Lett. 81, 2312-2315 (1998).[6] T. Sander, G. Kresse, "Macroscopic dielectric function within time-dependent density functional theory—Real time evolution versus the Casida approach", J. Chem. Phys. 146, 064110 (2017).[7] S. V. Kilina, P. K. Tamukong, and D. S. Kilin, "Surface Chemistry of Semiconducting Quantum Dots: Theoretical Perspectives", Acc. Chem. Res. 49, 10, 2127–2135 (2016).[8] H. B. Griffin, A. B. Kryjevski, and Dmitri S. Kilin, "Ab initio calculations of through-space and through-bond charge-transfer properties of interacting Janus-like PbSe and CdSe quantum dot heterostructures", Molec. Phys., e2273415 (2023).

(Invited) Luminescent InP Quantum Dots: They're Not Just Like CdSe

Time-resolved photoluminescence (PL) and transient absorption (TA) spectroscopy have been used to elucidate the electron and hole dynamics in high quality InP/ZnSe/ZnS quantum dots (QDs) at room temperature. These dynamics depend critically on the overall In:P ratio. Stoichiometric QDs (those having an In:P ratio close to unity), have PL quantum yields of > 90% and exhibit dynamics that are very simple: both electron and hole relaxation to the bandedge is fast compared to the ~ 40 ps time-correlated photon-counting instrument response and the PL follows a single exponential decay having a time constant of about 26 ns. However, QDs having an In:P ratio greater than about 1.1 maintain PL quantum yields > 90%, but show much more complicated PL kinetics. In the non-stoichiometric QDs, a significant fraction of the PL exhibits a slower risetime. The PL rises over a range of timescales varying from close to the instrument response to approximately 2.0 ns. The time constant of the slowest component increases with increasing InP core size and the fraction of the PL that has the slow risetime increases with excess indium in the particle. The slow risetime is absent in the TA results, indicating that the slow dynamics may be assigned to relaxation in the valence band. The risetime increases with the thickness of the ZnSe shell, varying from < 40 ps for the thinnest (1.1 nm) to about 2.0 ns for the thickest (2.7 nm) ZnSe shells. In addition to the slow risetime, the QDs having excess indium show a significant long-lived (>100 ns) decay component. The magnitude and time constant of this slow decay also increases with ZnSe shell thickness.These results suggest that holes are transiently trapped at sites associated with indium dopants in the ZnSe shell. The trapped hole has negligible overlap with the conduction band electron, making this an optically dark state. The holes are in an equilibrium between the shell traps and the valence band, giving rise to the delayed emission. Several possible assignments of the trapping species can be considered, and a substitutional indium adjacent to a zinc vacancy, In3+/VZn 2-, is the most likely. This assignment is consistent with the observation that trapping occurs only when the QD has excess indium and is supported by experiments showing that the addition of zinc oleate or acetate decreases the extent of trapping, presumably by filling some of the vacancy traps. Further experiments show that addition of alkyl carboxylic acids causes increased trapping, presumably by the formation of zinc oleate, thereby creating additional zinc vacancies. Chloride ion is a common impurity in the synthesis of InP/ZnSe/ZnS QDs and the possibility of In3+/Cl- impurities acting as the hole trap can be also considered. However, additional experimental data on InP/ZnSe/ZnS QDs synthesized in the absence of chloride show the same PL risetimes and delayed emission, eliminating this possibility. Density functional theory calculations further corroborate assignment of the trapping species to In3+/VZn 2-. These calculations show that either a single In2+ ion or an In2+-In3+ dimer is much too easily oxidized to form the reversible traps observed experimentally, while In3+ is far too difficult to oxidize. However, a zinc vacancy adjacent to a substitutional indium is calculated to have its highest occupied orbitals about 1 eV above the top of the valence band of bulk ZnSe, in the appropriate energy range to act as reversible traps for quantum confined holes in the InP valence band.

Prediction of Shockley–Read–Hall lifetimes in strained layer superlattices for mid-wave and long-wave infrared photodetectors

We have calculated carrier nonradiative recombination lifetimes limited by Shockley–Read–Hall (SRH) centers in strained layer superlattices (SLSs) for mid-wave and long-wave infrared applications. The capture rate of an electron (hole) in the SLS's conduction (valence) band by the defect level is dominated by a multi-phonon process, which is orders-of-magnitude more efficient than the radiative process. Long-range polar coupling between electrons and optical phonons can account for the observed SRH lifetimes in a variety of SLSs reported in the literature. The capture rate depends on temperature rather weakly, consistent with experimental observations. The efficient capture is caused by the comparable electronic difference and lattice relaxation energy, Ect∼Sℏω, with S and ℏω being the Huang–Rhys factor and optical photon energy in the SLSs. A weaker polar coupling would give rise to a smaller capture cross section, which, for InAs/InAsSb SLSs, can be achieved by increasing Sb in the alloy region.

High-Order Harmonic Generation in Photoexcited Three-Dimensional Dirac Semimetals.

High-order harmonic generation (HHG) in condensed matter is highly important for potential applications in various fields, such as materials characterization, all-optical switches, and coherent light source generation. Linking HHG to the properties or dynamic processes of materials is essential for realizing these applications. Here, a bridge has been built between HHG and the transient properties of materials through the engineering of interband polarization in a photoexcited three-dimensional Dirac semimetal (3D-DSM). It has been found that HHG can be efficiently manipulated by the electronic relaxation dynamics of 3D-DSM on an ultrafast time scale of several hundred femtoseconds. Furthermore, time-resolved HHG (tr-HHG) has been demonstrated to be a powerful spectroscopy method for tracking electron relaxation dynamics, enabling the identification of electron thermalization and electron-phonon coupling processes and the quantitative extraction of electron-phonon coupling strength. This demonstration provides insights into the active control of HHG and measurements of the electron dynamics.