Electron-hole overlap dictates the hole spin relaxation rate in nanocrystal heterostructures

Theoretical calculations of nonlinear optical responses for interpreting nonconjugated molecular systems to affect non-optimal properties

Understanding the intricacies of polymorphic origins in nonconjugated crystal systems is crucial for optimizing their properties. This study focuses on the crystal growth, characterization, and nonlinear optical (NLO) responses of a system analyzed using single crystal X-ray analysis, revealing a monoclinic geometry. Hirschfeld surface analysis emphasized the significance of intermolecular interactions in driving polymorph development, shedding light on the structural nuances influencing the material's properties. Through the evaluation of density functional theory parameters, the research found that the NLO responses of the system were as efficient as those of widely recognized materials like urea and KDP. Moreover, the stability of the system was confirmed through (NBO) analysis, showcasing its potential for practical applications. By manipulating the polymorphic crystal forms, researchers can potentially unveil new crystalline materials with tailored properties suitable for applications in optical and optoelectronic devices. This work underscores the importance of exploring novel crystal engineering strategies to harness the full potential of materials in the realm of advanced technologies.

Design and Exploration of Benzene Like Azobis Triazoles for Long-range Push-Pull Photo-Switching Attributes

This research paper presents a comprehensive study on the design and photovoltaic parameters of azobenzene type 24 photo switches (PSs) of triazole by density functional theory (DFT). The focus was on investigating how to create a long-range push-pull effect of different substituents on the PS properties for their application in photovoltaics by further substituent decoration. Their range of values for the maximum wavelength (λmax) ranged 315-556 nm while their HOMO-LUMO energies (Egaps) were 0.57-6.35eV. The stability of the PS was evaluated by measuring hardness (η) and softness (σ) values. Additionally, photovoltaic parameters such as open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and maximum power (Pmax) were calculated to assess the performance of the PS as photovoltaic materials. The results revealed that PSs 6 exhibited promising photovoltaic parameters to include Voc values ranging from 0.4-1.63eV, FF values ranging from 0.5438-0.929, Jsc values ranging from 19.27-50.75 mA/cm2, and Pmax values ranging from 14.72-75.91W. This indicates its potential as an efficient light-harvesting material for photovoltaic applications. Moreover, this study presents a pioneering investigation on the correlation between rotational velocity (R) and Mayer bond index (MBI) for the first time. The findings revealed a significant correlation between R and MBI, providing valuable insights into the structural dynamics of the PS. This novel finding opens up new avenues for understanding the structural dynamics of PS and their potential applications in various fields, including photovoltaics. The study provides valuable insights into the structure-property relationships of azobenzene-based PS and their suitability for photovoltaic devices. Further investigations are warranted to optimize the design of the PS, enhance their photovoltaic performance, and explore the underlying mechanisms of the correlation between R and MBIs.

Spin-polarized lasing in manganese doped perovskite microcrystals

Spin-polarized lasers have demonstrated many superiorities over conventional lasers in both performance and functionalities. Hybrid organic-inorganic perovskites are emerging spintronic materials with great potential for advancing spin-polarized laser technology. However, the rapid carrier spin relaxation process in hybrid perovskites presents a major bottleneck for spin-polarized lasing. Here we report the identification and successful suppression of the spin relaxation mechanism in perovskites for the experimental realization of spin-polarized perovskite lasers. The electron-hole exchange interaction is identified as the decisive spin relaxation mechanism hindering the realization of spin-polarized lasing in perovskite microcrystals. An ion doping strategy is employed accordingly to introduce a new energy level in perovskites, which enables a long carrier spin lifetime by suppressing the electron-hole exchange interaction. As a result, spin-polarized lasing is achieved in the doped perovskite microcrystals. Moreover, the doped cation is a magnetic species allowing for the magnetic field control of the spin-polarized perovskite lasing. This work unlocks the potential of perovskites for spin-polarized lasers, providing guidance for the design of perovskites towards spintronic devices.

Molecular Engineering for UV-Vis to NIR Absorption/Emission Bands of Pyrazine-based A-π-D- π-A Switches to Design TiO2 Tuned Dyes: DFT Insights

This study investigates the tuning of the UV-Vis/NIR absorption bands of pyrazine-based A-D-A switches for designing efficient UV retardancy over TiO2 surfaces. The electronic properties and optical characteristics of seven dyes (DP1-DP7) were analyzed using computational methods. The results indicate that the dyes possessed distinct UV-Vis/NIR absorption properties. Their absorption wavelengths ranged from 389 to 477 nm, with corresponding energies ranging from 2.59 to 3.19 eV. The major contributions to the absorption were found to be the HOMO-LUMO transitions, varying from 86 to 96%. The dyes exhibited different donor (D) and acceptor (A) groups, influencing their electronic properties and absorption characteristics. The tunable electronic and optical properties of these dyes make them promising candidates for applications requiring UV protection for TiO2-based materials. The results contribute to understand the structure-property relationships in the design of UV-Vis/NIR absorbers and provide a foundation for further experimental investigations in the field of UV retardancy.

Dielectric and Wavefunction Engineering of Electron Spin Lifetime in Colloidal Nanoplatelet Heterostructures

Colloidal semiconductor nanoplatelets (NPLs) have emerged as low-cost and free-standing alternates of traditional quantum wells. The giant heavy- and light-hole splitting in NPLs allows for efficient optical spin injection. However, the electron spin lifetimes for prototypical CdSe NPLs are within a few picoseconds, likely limited by strong electron-hole exchange in these quantum- and dielectric-confined materials. Here how this hurdle can be overcome with engineered NPL-heterostructures is demonstrated. By constructing type-I CdSe/ZnS core/shell NPLs, dielectric screening inside the core is strongly enhanced, prolonging the electron spin polarization time (τesp) to over 30 ps (or 60 ps electron spin-flip time). Alternatively, by growing type-II CdSe/CdTe core/crown NPLs to spatially separate electron and hole wavefunctions, the electron-hole exchange is strongly suppressed, resulting in τesp as long as 300 ps at room temperature. This study not only exemplifies how the well-established synthetic chemistry of colloidal heterostructures can aid in spin dynamics control but also establishes the feasibility of room-temperature coherent spin manipulation in colloidal NPLs.

Spin Quantum Dot Light-Emitting Diodes Enabled by 2D Chiral Perovskite with Spin-Dependent Carrier Transport

Chiral-induced spin selectivity (CISS) effect provides innovative approach to spintronics and quantum-based devices for chiral materials. Different from the conventional ferromagnetic devices, the application of CISS effect is potential to operate under room temperature and zero applied magnetic field. Low dimensional chiral perovskites by introducing chiral amines are beginning to show significant CISS effect for spin injection, but research on chiral perovskites is still in its infancy, especially on spin-light emitting diode (spin-LED) construction. Here, the spin-QLEDs enabled by 2D chiral perovskites as CISS layer for spin-dependent carrier injection and CdSe/ZnS quantum dots (QDs) as light emitting layer are reported. The regulation pattern of the chirality and thickness of chiral perovskites, which affects the circularly polarized electroluminescence (CP-EL) emission of spin-QLED, is discovered. Notably, the spin injection polarization of 2D chiral perovskites is higher than 80% and the CP-EL asymmetric factor (gCP-EL ) achieves up to 1.6 × 10-2 . Consequently, this work opens up a new and effective approach for high-performance spin-LEDs.

Optical control of exciton spin dynamics in layered metal halide perovskites via polaronic state formation

One of the open challenges of spintronics is to control the spin relaxation mechanisms. Layered metal-halide perovskites are an emerging class of semiconductors which possess a soft crystal lattice that strongly couples electronic and vibrational states and show promise for spintronic applications. Here, we investigate the impact of such strong coupling on the spin relaxation of excitons in the layered perovskite BA₂FAPbI₇ using a combination of cryogenic Faraday rotation and transient absorption spectroscopy. We report an unexpected increase of the spin lifetime by two orders of magnitude at 77 K under photoexcitation with photon energy in excess of the exciton absorption peak, and thus demonstrate optical control over the dominant spin relaxation mechanism. We attribute this control to strong coupling between excitons and optically excited phonons, which form polaronic states with reduced electron-hole wave function overlap that protect the exciton spin memory. Our insights highlight the special role of exciton-lattice interactions on the spin physics in the layered perovskites and provide a novel opportunity for optical spin control.

Spintronic devices will require long spin lifetimes, but the effect of exciton-lattice coupling on spin lifetime in metal-halide perovskites is not well understood. Here, the authors find a 100-fold increase in the lifetime of exciton spins in a 2D perovskite by exciting with excess energy, resulting from strong coupling between excitons and optically excited phonons.

Ultrafast separation of multiexcitons within core/shell quantum dot hybrid systems

We investigated the electron transfer processes in methylene blue-CdTe and methylene blue-CdTe/CdSe complexes by steady state and femtosecond transient absorption spectroscopy by selective excitation of the quantum dot (QD) moiety. The ultrafast electron transfer is accelerated by the shell growth due to the separation of the charge carriers and the resulting increase of electron density in the shell. Transmission electron microscope images show that the successive addition of shell material deforms the spherical QDs until they adopt a tetrapodal shape. The increased donor-acceptor distance in the tetrapodal CdTe/CdSe QDs leads to a slower electron transfer. Photon flux dependent transient absorption measurements indicate the separation of two electrons for the QDs with a thin shell and thus demonstrate that charge carrier multiplication can be directly utilized for increased charge transfer in this type of QD hybrid system.

Linearly Polarized Luminescence of Atomically Thin MoS2 Semiconductor Nanocrystals

Atomically thin layers of transition-metal dichalcogenides semiconductors, such as MoS₂, exhibit strong and circularly polarized light emission due to inherent crystal symmetries, pronounced spin-orbit coupling, and out-of-plane dielectric and spatial confinement. While the layer-by-layer confinement is well-understood, the understanding of the impact of in-plane quantization in their optical spectrum is far behind. Here, we report the optical properties of atomically thin MoS₂ colloidal semiconductor nanocrystals. In addition to the spatial-confinement effect leading to their blue wavelength emission, the high quality of our MoS₂ nanocrystals is revealed by narrow photoluminescence, which allows us to resolve multiple optically active transitions, originating from quantum-confined excitons (coupled electron-hole pairs). Surprisingly, in stark contrast to monolayer MoS₂, the luminescence of the lowest-energy levels is linearly polarized and persists up to room temperature, meaning that it could be exploited in a variety of light-emitting applications.

Lattice Disorder-Engineered Energy Splitting between Bright and Dark Excitons in CsPbBr3 Quantum Wires

Excitons in nanostructured semiconductors often undergo strong electron-hole exchange interaction, resulting in bright-dark exciton splitting with the dark exciton usually being the lower energy state. This unfavorable state arrangement has become the major bottleneck for achieving high photoluminescence quantum yield (PLQY). However, the arrangement of dark and bright exciton states in lead halide perovskites is under intense debate due to the involvement of many complicated factors. We present here the first experimental evidence to demonstrate that the strain is a crucial factor in tuning the energy splitting of the bright and dark excitons, resulting in different PL properties.

A thin CdSe shell boosts the electron transfer from CdTe quantum dots to methylene blue

CdTe core and CdTe/CdSe core/shell quantum dots (QD) are investigated with steady state and time-resolved spectroscopic methods. The coating of the CdTe core with a 0.7 nm thick CdSe shell shifts the lowest exciton absorption band to the red by more than 70 nm making the CdTe/CdSe QD an interesting candidate for application in solar energy conversion. Femtosecond transient absorption measurements are applied to study the photoinduced electron transfer (ET) to the molecular acceptor methylene blue (MB). ET times after single excitation of the QD are determined for different MB : QD ratios. The ET reaction is significantly faster in the case of the MB-CdTe/CdSe QD complexes, indicative of an altered charge distribution in the photoexcited heterostructure with a higher electron density in the CdSe shell. As a result of the efficient absorption of incoming light and the faster ET reaction, the amount of reduced MB in the time resolved experiments is higher for CdTe/CdSe QD compared to CdTe QD.

Bright type-II photoluminescence from Mn-doped CdS/ZnSe/ZnS quantum dots with Mn2+ ions as exciton couplers

Mn²⁺ ions were introduced as exciton couplers to enhance the quantum yield (QY) of type-II photoluminescence (PL) from CdS/ZnSe/ZnS quantum dots (QDs) with slow hot-exciton cooling and low radiative rate. Transient absorption spectroscopy verifies the faster bleach recovery and faster peak red-shifting at the charge-transfer state. And the transient PL peak of the QDs changes from blue-shifting to red-shifting due to Mn²⁺ doping. The QY of type-II PL can be enhanced from ∼35% to ∼60% by Mn²⁺ doping. As the energy-transfer-stations of hot excitons during rapid ET (∼100 ps), Mn²⁺ ions transform more excitons from hot to cold for emission. As the couplers of cold excitons during long thermal equilibrium (∼100 ns), Mn²⁺ ions further decrease exciton trapping by strong bidirectional coupling. This work provides a unique way of acquiring high QY of type-II PL, and highlights the general law of PL enhancement in Mn-doped QDs.

Relationships between Exciton Dissociation and Slow Recombination within ZnSe/CdS and CdSe/CdS Dot-in-Rod Heterostructures

Type-II and quasi type-II heterostructure nanocrystals are known to exhibit extended excited-state lifetimes compared to their single material counterparts because of reduced wave function overlap between the electron and hole. However, due to fast and efficient hole trapping and nonuniform morphologies, the photophysics of dot-in-rod heterostructures are more rich and complex than this simple picture. Using transient absorption spectroscopy, we observe that the behavior of electrons in the CdS "rod" or "bulb" regions of nonuniform ZnSe/CdS and CdSe/CdS dot-in-rods is similar regardless of the "dot" material, which supports previous work demonstrating that hole trapping and particle morphology drive electron dynamics. Furthermore, we show that the longest lived state in these dot-in-rods is not generated by the type-II or quasi type-II band alignment between the dot and the rod, but rather by electron-hole dissociation that occurs due to fast hole trapping in the CdS rod and electron localization to the bulb. We propose that specific variations in particle morphology and surface chemistry determine the mechanism and efficiency of charge separation and recombination in these nanostructures, and therefore impact their excited-state dynamics to a greater extent than the heterostructure energy level alignment alone.

Dynamic Evolution from Negative to Positive Photocharging in Colloidal CdS Quantum Dots

The optical properties of colloidal semiconductor nanocrystals are largely influenced by the trapping of charge carriers on the nanocrystal surface. Different concentrations of electron and hole traps and different rates of their capture to the traps provide dynamical charging of otherwise neutral nanocrystals. We study the photocharging formation and evolution dynamics in CdS colloidal quantum dots with native oleic acid surface ligands. A time-resolved technique with three laser pulses (pump, orientation, and probe) is developed to monitor the photocharging dynamics with picosecond resolution on wide time scales ranging from picoseconds to milliseconds. The detection is based on measuring the coherent spin dynamics of electrons, allowing us to distinguish the type of carrier in the QD core (electron or hole). We find that although initially negative photocharging happens because of fast hole trapping, it eventually evolves to positive photocharging due to electron trapping and hole detrapping. The positive photocharging lasts up to hundreds of microseconds at room temperature. These findings give insight into the photocharging process and provide valuable information for understanding the mechanisms responsible for the emission blinking in colloidal nanostructures.

Effect of Electron-Hole Overlap and Exchange Interaction on Exciton Radiative Lifetimes of CdTe/CdSe Heteronanocrystals

Wave function engineering has become a powerful tool to tailor the optical properties of semiconductor colloidal nanocrystals. Core-shell systems allow to design the spatial extent of the electron (e) and hole (h) wave functions in the conduction- and valence bands, respectively. However, tuning the overlap between the e- and h-wave functions not only affects the oscillator strength of the coupled e-h pairs (excitons) that are responsible for the light emission, but also modifies the e-h exchange interaction, leading to an altered excitonic energy spectrum. Here, we present exciton lifetime measurements in a strong magnetic field to determine the strength of the e-h exchange interaction, independently of the e-h overlap that is deduced from lifetime measurements at room temperature. We use a set of CdTe/CdSe core/shell heteronanocrystals in which the electron-hole separation is systematically varied. We are able to unravel the separate effects of e-h overlap and e-h exchange on the exciton lifetimes, and we present a simple model that fully describes the recombination lifetimes of heteronanostructures (HNCs) as a function of core volume, shell volume, temperature, and magnetic fields.

Enhanced electron spin rotation in CdS quantum dots

We studied the spin rotation of electrons in CdS quantum dots (QDs) and CdS QDs with charge acceptors by means of time-resolved Faraday rotation (TRFR) at room temperature.

Observation of the full exciton and phonon fine structure in CdSe/CdS dot-in-rod heteronanocrystals

Light emission of semiconductor nanocrystals is a complex process, depending on many factors, among which are the quantum mechanical size confinement of excitons (coupled electron-hole pairs) and the influence of confined phonon modes and the nanocrystal surface. Despite years of research, the nature of nanocrystal emission at low temperatures is still under debate. Here we unravel the different optical recombination pathways of CdSe/CdS dot-in-rod systems that show an unprecedented number of narrow emission lines upon resonant laser excitation. By using self-assembled, vertically aligned rods and application of crystallographically oriented high magnetic fields, the origin of all these peaks is established. We observe a clear signature of an acoustic-phonon assisted transition, separated from the zero-phonon emission and optical-phonon replica, proving that nanocrystal light emission results from an intricate interplay between bright (optically allowed) and dark (optically forbidden) exciton states, coupled to both acoustic and optical phonon modes.

Controllable growth of Ag2S–CdS heteronanostructures

High quality Ag₂S–CdS heteronanostructures were prepared via a facile one-pot method.

Ultrafast exciton dynamics in Type II ZnTe-ZnSe colloidal quantum dots

Ultrafast transient absorption spectroscopy is used to investigate the exciton dynamics of Type II ZnTe-ZnSe core-shell colloidal quantum dots. Surface-trapping is shown to occur within a few picosecond for hot electrons and with a few 10s of picoseconds for electrons cooled to the band-edge, and is the dominant process in the decay of the band-edge bleach for well-stirred samples pumped at moderate powers. The surface-trapped electrons produce a broad photo-induced absorption that spectrally overlaps with the band-edge, distorting and partially cancelling out the bleach feature. At high pump powers and for unstirred samples, these surface-trapped electrons can survive sufficiently long within the pumped volume to accumulate under repeated excitation of the sample, resulting in the formation of an additional exciton decay channel.

The birth of a type-II nanostructure: carrier localization and optical properties of isoelectronically doped CdSe:Te nanocrystals

CdTe/CdSe core/shell nanocrystals are the prototypical example of type-II nanoheterostructures, in which the electron and the hole wave functions are localized in different parts of the nanostructure. As the thickness of the CdSe shell increases above a few monolayers, the spectroscopic properties of such nanocrystals change dramatically, reflecting the underlying type-I → type-II transition. For example, the exciton Stokes shift and radiative lifetime increase, while the decreasing biexciton binding energy changes sign from positive to negative. Recent experimental results for CdSe nanocrystals isoelectronically doped with a few Te substitutional impurities, however, have revealed a very different dependence of the optical and electronic properties on the nanocrystal size. Here we use atomistic calculations based on the pseudopotential method for single-particle excitations and the configuration-interaction approach for many-particle excitations to investigate carrier localization and electronic properties of CdTe/CdSe nanocrystals as the size of the CdTe core decreases from a few nm (characteristic of core/shell CdTe/CdSe nanocrystals) to the single impurity limit. We find that the unusual spectroscopic properties of isoelectronically doped CdSe:Te nanocrystals can be rationalized in terms of the change in the localization volume of the electron and hole wave functions as the size of the nanocrystal increases. The size dependence of the exciton Stokes shift, exciton radiative lifetime, and biexciton binding energy reflects the extent of carrier localization around the Te impurities.

Calculation of electron-hole recombination probability using explicitly correlated Hartree-Fock method

The electron-hole explicitly correlated Hartree-Fock method (eh-XCHF) is presented as a general strategy for investigation of electron-hole correlation and computation of electron-hole recombination probability. The eh-XCHF method is a variational method which uses explicitly correlated wavefunction that depends on the electron-hole inter-particle distances. It is shown that the explicitly correlated ansatz provides a systematic route to variationally minimize the total energy. The parabolic quantum dot is used as the benchmark system and the eh-XCHF method is used for computation of the ground state energy and electron-hole recombination probability. The results are compared to Hartree-Fock and explicitly correlated full configuration interaction (R12-FCI) calculations. The results indicate that an accurate description of the electron-hole wavefunction at short electron-hole inter-particle distances is crucial for qualitative description of the electron-hole recombination probability. The eh-XCHF method successfully addresses this issue and comparison of eh-XCHF calculations with R12-FCI shows good agreement. The quality of the mean field approximation for electron-hole system is also investigated by comparing HF and R12-FCI energies for electron-electron and electron-hole systems. It was found that performance of the mean field approximation is worse for the electron-hole system as compared to the corresponding electron-electron system.

Contribution of Femtosecond Laser Spectroscopy to the Development of Advanced Optoelectronic Nanomaterials

Femtosecond laser spectroscopy has now been a powerful technique for over a decade to investigate charge carrier dynamics in nanoscale optoelectronic systems with a temporal resolution of 100 fs (10(-13) s) or better. Both transient absorption and time-resolved photoluminescence spectroscopy are now popular spectroscopic techniques, which are well-established and provide direct insight into the charge carrier dynamics of nanomaterials. In this Perspective, we focus mainly on the developments with regard to studies of semiconductor nanostructures. Controlling the charge carrier dynamics, including hot carrier relaxation, trapping, interfacial carrier transfer, carrier multiplication, and recombination, is essential for successful energy conversion or photocatalysis, to name two major optoelectronic applications. We will show how femtosecond laser spectroscopy evolved into techniques that unveil the dynamic charge carrier properties of semiconductor nanomaterials toward heterostructures and complex nanoarchitectures and that femtosecond time-resolved laser spectroscopy can shine light on the path to novel optoelectronic structures and emergent optoelectronic technologies.

Engineering the spin-flip limited exciton dephasing in colloidal CdSe/CdS quantum dots

We have measured the intrinsic exciton dephasing in high-quality zinc blende CdSe/CdS colloidal quantum dots in the temperature range from 5 to 170 K using a sensitive three-beam photon echo technique in heterodyne detection, which is not affected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing. From the temperature dependence of the zero-phonon line dephasing, the exciton lifetime, and the exciton thermalization within its fine structure, we show that the zero-phonon line dephasing of the lowest bright state originates from the phonon-assisted spin-flip to dark exciton states. Importantly, we can control the dephasing by tailoring the exciton fine structure through its dependence on the dot core size and shell thickness, as expected from the spin-flip mechanism. By reducing the electron-hole exchange interaction with increasing core size and delocalization of the electron wave function in the quasi-type-II core/shell band alignment, we find the longest zero-phonon line dephasing time of ∼110 ps at 5 K in dots with the largest core diameter (5.7 nm) and the thickest CdSe shell (9 monolayers) in the series studied.

Exciton superposition states in CdSe nanocrystals measured using broadband two-dimensional electronic spectroscopy

Coherent superpositions among eigenstates are of interest in fields as diverse as photosynthesis and quantum computation. In this report, we used two-dimensional electronic spectroscopy (2D ES) to measure the decoherence time of a superposition of the two lowest-energy excitons in colloidal CdSe nanocrystals (cubic phase) in solution at room temperature. In the electron-hole representation, the quantum coherence is, remarkably, a twelve-particle correlation. By comparing the measured 2D ES to simulations, we also explored the effects of inhomogeneous broadening and examined the spectroscopic signatures of biexcitons.

Nano-engineered electron-hole exchange interaction controls exciton dynamics in core-shell semiconductor nanocrystals

A strong electron–hole exchange interaction (EI) in semiconductor nanocrystals (NCs) gives rise to a large (up to tens of meV) splitting between optically active ('bright') and optically passive ('dark') excitons. This dark–bright splitting has a significant effect on the optical properties of band-edge excitons and leads to a pronounced temperature and magnetic field dependence of radiative decay. Here we demonstrate a nanoengineering-based approach that provides control over EI while maintaining nearly constant emission energy. We show that the dark–bright splitting can be widely tuned by controlling the electron–hole spatial overlap in core–shell CdSe/CdS NCs with a variable shell width. In thick-shell samples, the EI energy reduces to <250 μeV, which yields a material that emits with a nearly constant rate over temperatures from 1.5 to 300 K and magnetic fields up to 7 T. The EI-manipulation strategies demonstrated here are general and can be applied to other nanostructures with variable electron–hole overlap.

Electron–hole exchange interaction is an intrinsic property of semiconductors, which affects their fine structure. Brovelli et al. demonstrate a nanoengineering-based approach that provides control over the exchange interaction energy at nearly constant emission energy, which cannot be carried out using core-only nanocrystals.

Measuring electron and hole transfer in core/shell nanoheterostructures

Using femtosecond transient absorption and time-resolved photoluminescence spectroscopy, we studied the electron versus hole dynamics in photoexcited quasi-type-II heterostructured nanocrystals with fixed CdTe core radii and varying CdSe shell coverage. By choosing the pump wavelength in resonance with the core or the shell states, respectively, we were able to measure the excited electron and hole dynamics selectively. Both, the core- and the shell-excited CdTe/CdSe nanocrystals showed the same spectral emission and photoluminescence lifetimes, indicating that ultrafast electron and hole transfer across the core/shell interface resulted in the identical long-lived charge transfer state. Both charge carriers have subpicosecond transfer rates through the interface, but the subsequent relaxation rates of the hole (τ(dec) ∼ 800 ps) and electron (τ(avg) ∼ 8 ps) are extremely different. On the basis of the presented transient absorption measurements and fitting of the steady-state spectra, we find that the electron transfer occurs in the Marcus inverted region and mixing between the CdTe exciton and charge transfer states takes place and therefore needs to be considered in the analysis.

Emergent properties resulting from type-II band alignment in semiconductor nanoheterostructures

The development of elegant synthetic methodologies for the preparation of monocomponent nanocrystalline particles has opened many possibilities for the preparation of heterostructured semiconductor nanostructures. Each of the integrated nanodomains is characterized by its individual physical properties, surface chemistry, and morphology, yet, these multicomponent hybrid particles present ideal systems for the investigation of the synergetic properties that arise from the material combination in a non-additive fashion. Of particular interest are type-II heterostructures, where the relative band alignment of their constituent semiconductor materials promotes a spatial separation of the electron and hole following photoexcitation, a highly desirable property for photovoltaic applications. This article highlights recent progress in both synthetic strategies, which allow for material and architectural modulation of novel nanoheterostructures, as well as the experimental work that provides insight into the photophysical properties of type-II heterostructures. The effects of external factors, such as electric fields, temperature, and solvent are explored in conjunction with exciton and multiexciton dynamics and charge transfer processes typical for type-II semiconductor heterostructures.