Enhanced carrier multiplication in engineered quasi-type-II quantum dots

Multiphoton-pumped UV-Vis transient absorption spectroscopy of 2D materials: basic concepts and recent applications

2D materials are considered a key element in the development of next-generation electronics (nanoelectronics) due to their extreme thickness in the nanometer range and unique physical properties. The ultrafast dynamics of photoexcited carriers in such materials are strongly influenced by their interfaces, since the thickness of 2D materials is much smaller than the typical depth of light penetration into their bulk counterparts and the mean free path of photoexcited carriers. The resulting collisions of photoexcited carriers with interfacial potential barriers of 2D materials in the presence of a strong laser field significantly alter the overall dynamics of photoexcitation, allowing laser light to be directly absorbed by carriers in the conduction/valence band through the inverse bremsstrahlung mechanism. The corresponding ultrafast carrier dynamics can be monitored using multiphoton-pumped UV-Vis transient absorption spectroscopy. In this review, we discuss the basic concepts and recent applications of this spectroscopy for a variety of 2D materials, including transition-metal dichalcogenide monolayers, topological insulators, and other 2D semiconductor structures.

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Recent Advances and Challenges of Colloidal Quantum Dot Light-Emitting Diodes for Display Applications

Colloidal quantum dots (QDs) exhibit tremendous potential in display technologies owing to their unique optical properties, such as size-tunable emission wavelength, narrow spectral linewidth, and near-unity photoluminescence quantum yield. Significant efforts in academia and industry have achieved dramatic improvements in the performance of quantum dot light-emitting diodes (QLEDs) over the past decade, primarily owing to the development of high-quality QDs and optimized device architectures. Moreover, sophisticated patterning processes have also been developed for QDs, which is an essential technique for their commercialization. As a result of these achievements, some QD-based display technologies, such as QD enhancement films and QD-organic light-emitting diodes, have been successfully commercialized, confirming the superiority of QDs in display technologies. However, despite these developments, the commercialization of QLEDs is yet to reach a threshold, requiring a leap forward in addressing challenges and related problems. Thus, representative research trends, progress, and challenges of QLEDs in the categories of material synthesis, device engineering, and fabrication method to specify the current status and development direction are reviewed. Furthermore, brief insights into the factors to be considered when conducting research on single-device QLEDs are provided to realize active matrix displays. This review guides the way toward the commercialization of QLEDs.

New Theoretical Model to Describe Carrier Multiplication in Semiconductors: Explanation of Disparate Efficiency in MoTe2 versus PbS and PbSe

We present a theoretical model to compute the efficiency of the generation of two or more electron-hole pairs in a semiconductor by the absorption of one photon via the process of carrier multiplication (CM). The photogeneration quantum yield of electron-hole pairs is calculated from the number of possible CM decay pathways of the electron and the hole. We apply our model to investigate the underlying cause of the high efficiency of CM in bulk 2H-MoTe2, as compared to bulk PbS and PbSe. Electronic band structures were calculated with density functional theory, from which the number of possible CM decay pathways was calculated for all initial electron and hole states that can be produced at a given photon energy. The variation of the number of CM pathways with photon energy reflects the dependence of experimental CM quantum yields on the photon energy and material composition. We quantitatively reproduce experimental CM quantum yields for MoTe2, PbS, and PbSe from the calculated number of CM pathways and one adjustable fit parameter. This parameter is related to the ratio of Coulomb coupling matrix elements and the cooling rate of the electrons and holes. Large variations of this fit parameter result in small changes in the modeled quantum yield for MoTe2, which confirms that its high CM efficiency can be mainly attributed to its extraordinary large number of CM pathways. The methodology of this work can be applied to analyze or predict the CM efficiency of other materials.

Axially lattice-matched wurtzite/rock-salt GaAs/Pb1-xSnxTe nanowires

We investigate the full and half-shells of Pb1-xSnxTe topological crystalline insulator deposited by molecular beam epitaxy on the sidewalls of wurtzite GaAs nanowires (NWs). Due to the distinct orientation of the IV-VI shell with respect to the III-V core the lattice mismatch between both materials along the nanowire axis is less than 4%. The Pb1-xSnxTe solid solution is chosen due to the topological crystalline insulator properties above some critical concentrations of Sn (x ≥ 0.36). The IV-VI shells are grown with different compositions spanning from binary SnTe, through Pb1-xSnxTe with decreasing x value down to binary PbTe (x = 0). The samples are analysed by scanning transmission electron microscopy, which reveals the presence of (110) or (100) oriented binary PbTe and (100) Pb1-xSnxTe on the sidewalls of wurtzite GaAs NWs.

Spin-exchange carrier multiplication in manganese-doped colloidal quantum dots

Carrier multiplication is a process whereby a kinetic energy of a carrier relaxes via generation of additional electron-hole pairs (excitons). This effect has been extensively studied in the context of advanced photoconversion as it could boost the yield of generated excitons. Carrier multiplication is driven by carrier-carrier interactions that lead to excitation of a valence-band electron to the conduction band. Normally, the rate of phonon-assisted relaxation exceeds that of Coulombic collisions, which limits the carrier multiplication yield. Here we show that this limitation can be overcome by exploiting not 'direct' but 'spin-exchange' Coulomb interactions in manganese-doped core/shell PbSe/CdSe quantum dots. In these structures, carrier multiplication occurs via two spin-exchange steps. First, an exciton generated in the CdSe shell is rapidly transferred to a Mn dopant. Then, the excited Mn ion undergoes spin-flip relaxation via a spin-conserving pathway, which creates two excitons in the PbSe core. Due to the extremely fast, subpicosecond timescales of spin-exchange interactions, the Mn-doped quantum dots exhibit an up-to-threefold enhancement of the multiexciton yield versus the undoped samples, which points towards the considerable potential of spin-exchange carrier multiplication in advanced photoconversion.

Epitaxial CdSe/PbSe Heterojunction Growth and MWIR Photovoltaic Detector

A novel Epitaxial Cadmium Selenide (CdSe) on Lead Selenide (PbSe) type-II heterojunction photovoltaic detector has been demonstrated by Molecular Beam Epitaxy (MBE) growth of n-type CdSe on p-type PbSe single crystalline film. The use of Reflection High-Energy Electron Diffraction (RHEED) during the nucleation and growth of CdSe indicates high-quality single-phase cubic CdSe. This is a first-time demonstration of single crystalline and single phase CdSe growth on single crystalline PbSe, to the best of our knowledge. The current-voltage characteristic indicates a p-n junction diode with a rectifying factor over 50 at room temperature. The detector structure is characterized by radiometric measurement. A 30 μm × 30 μm pixel achieved a peak responsivity of 0.06 A/W and a specific detectivity (D*) of 6.5 × 10⁸ Jones under a zero bias photovoltaic operation. With decreasing temperature, the optical signal increased by almost an order of magnitude as it approached 230 K (with thermoelectric cooling) while maintaining a similar level of noise, achieving a responsivity of 0.441 A/W and a D* of 4.4 × 10⁹ Jones at 230 K.

Impact of Morphological Inhomogeneity on Excitonic States in Highly Mismatched Alloy ZnSe1-XTeX Nanocrystals

ZnSe_1-XTe_X nanocrystals (NCs) are promising photon emitters with tunable emission across the violet to orange range and near-unity quantum yields. However, these NCs suffer from broad emission line widths and multiple exciton decay dynamics, which discourage their practicable use. Here, we explore the excitonic states in ZnSe_1-XTe_X NCs and their photophysical characteristics in relation to the morphological inhomogeneity of highly mismatched alloys. Ensemble and single-dot spectroscopic analysis of a series of ZnSe_1-XTe_X NC samples with varying Te ratios coupled with computational calculations shows that, due to the distinct electronegativity between Se and Te, nearest-neighbor Te pairs in ZnSe_1-XTe_X alloys create localized hole states spectrally distributed approximately 130 meV above the 1S_h level of homogeneous ZnSe_1-XTe_X NCs. This forms spatially separated excitons (delocalized electron and localized hole in trap), accounting for both inhomogeneous and homogeneous line width broadening with delayed recombination dynamics. Our results identify photophysical characteristics of excitonic states in NCs made of highly mismatched alloys and provide future research directions with potential implications for photonic applications.

Carrier Multiplication in Transition Metal Dichalcogenides Beyond Threshold Limit

Carrier multiplication (CM), multiexciton generation by absorbing a single photon, enables disruptive improvements in photovoltaic conversion efficiency. However, energy conservation constrains the threshold energy to at least twice bandgap (2E g $ E_\text{g}$ ). Here, a below threshold limit CM in monolayer transition metal dichalcogenides (TMDCs) is reported. Surprisingly, CM is observed with excitation energy of only 1.75E g $E_\text{g}$ due to lattice vibrations. Electron-phonon coupling (EPC) results in significant changes in electronic structures, which favors CM. Indeed, the strongest EPC in monolayer MoS2 leads to the most efficient CM among the studied TMDCs. For practical applications, chalcogen vacancies can further lower the threshold by introducing defect states within bandgap. In particular, for monolayer WS2 , CM occurs with excitation energy as low as 1.51E g $E_\text{g}$ . The results identify TMDCs as attractive candidate materials for efficient optoelectronic devices with the advantages of high photoconductivity and efficient CM.

Dye-Sensitized Multiple Exciton Generation in Lead Sulfide Quantum Dots

Multiple exciton generation (MEG), the generation of multiple excitons from the absorption of a single high-energy photon, is a strategy to go beyond the limiting efficiencies that define current-day solar cells by harvesting some of the thermalization energy losses that occur when photons with an energy greater than the semiconductor bandgap are absorbed. In this work, we show that organic dyes can sensitize MEG in semiconductor quantum dots (QDs). In particular, we found that surface-anchored pyrene ligands enhanced the photon-to-charge carrier quantum yield of PbS QDs from 113 ± 3% to 183 ± 7% when the photon energy was 3.9 times the band gap. A wavelength dependence study shows that the enhancement is positively correlated with the pyrene absorptivity. Transient absorption and steady-state photoluminescence measurements suggest that the MEG sensitization is based on an initial fast electron transfer from the pyrene ligands to the PbS QDs producing hot-electrons in the QDs that subsequently undergo MEG. This work demonstrates that hybrid and synergistic organic/inorganic interactions can be a successful strategy to enhance MEG.

Insight into morphology dependent charge carrier dynamics in ZnSe-CdS nanoheterostructures

Semiconductor nanoheterostructures (NHSs) are being increasingly used for the photocatalytic conversion of solar energy in which photo-induced charge separation is an essential step and hence it is necessary to understand the effect of various factors such as size, shape, and composition on the charge transfer dynamics. Ultrafast transient absorption spectroscopy is used to investigate the nature and dynamics of photo-induced charge transfer processes in ZnSe-CdS NHSs of different morphologies such as nanospheres (NSs), nanorods (NRs), and nanoplates (NPs). It demonstrates the fast separation of charge carriers and localization of both charges in adjacent semiconductors, resulting in the formation of a charge-separated (CS) state. The lifetime of the charge-separated state follows the order of NSs < NPs < NRs, emphasizing the effect of morphology on the enhancement of photo-induced charge separation and suppression of backward recombination. The separated charge carriers have been utilized in visible light driven hydrogen production and the hydrogen generation activity follows the same order as that for the lifetime of the CS state, underlining the role of charge separation efficiency. Therefore, the variation of the morphology of NHSs plays a significant role in their charge carrier dynamics and hence the photocatalytic hydrogen production activity.

Atomic Identification of Interfaces in Individual Core@shell Quantum Dots

CdSe@CdS Core@shell quantum dots (QDs) have been widely studied in recent years, due to their architecture which allows to tailor properties by controlling structure and composition. However, since CdSe and CdS have the same crystal structure, same cations, and similar lattice parameters, it is very challenging to image the interface. Herein, high-resolution transmission electron microscopy, high-angle annular dark-field imaging, and energy-dispersive X-ray spectroscopy elemental mapping are combined to characterize the core@shell structure and identify the interface in the CdSe@CdS QDs with different CdS shell thicknesses. By examining changes in lattice spacing in an individual CdSe@CdS quantum dot, the atomic core@shell interface is identified. For thin-shelled QDs, an ideal coherent interface forms between core and shell due to the small lattice mismatch, and the lattice spacing remains unchanged at the core and shell regions. For thick-shelled QDs, the lattice spacing is different at the core and shell regions, while the heterostructured interface is still coherent and cannot be clearly imaged. As the shell thickness further increases, a sharp core@shell interface appears. The results define an approach to characterize the heterostructure of two materials with the same crystalline structure and cations.

Multiple exciton generation in isolated and interacting silicon nanocrystals

An important challenge in the field of renewable energy is the development of novel nanostructured solar cell devices which implement low-dimensional materials to overcome the limits of traditional photovoltaic systems. For optimal energy conversion in photovoltaic devices, one important requirement is that the full energy of the solar spectrum is effectively used. In this context, the possibility of exploiting features and functionalities induced by the reduced dimensionality of the nanocrystalline phase, in particular by the quantum confinement of the electronic density, can lead to a better use of the carrier excess energy and thus to an increment of the thermodynamic conversion efficiency of the system. Carrier multiplication, i.e. the generation of multiple electron-hole pairs after absorption of one single high-energy photon (with energy at least twice the energy gap of the system), can be exploited to maximize cell performance, promoting a net reduction of loss mechanisms. Over the past fifteen years, carrier multiplication has been recorded in a large variety of semiconductor nanocrystals and other nanostructures. Owing to the role of silicon in solar cell applications, the mission of this review is to summarize the progress in this fascinating research field considering carrier multiplication in Si-based low-dimensional systems, in particular Si nanocrystals, both from the experimental and theoretical point of view, with special attention given to the results obtained by ab initio calculations.

Highly versatile near-infrared emitters based on an atomically defined HgS interlayer embedded into a CdSe/CdS quantum dot

The availability of colloidal quantum dots with highly efficient, fast and 'non-blinking' near-infrared emission would benefit numerous applications, from advanced optical communication and quantum networks to biomedical diagnostics. Here, we report high-quality near-infrared emitters that are based on well known CdSe/CdS heterostructures. By incorporating an HgS interlayer at the quantum dot core/shell interface, we convert normally visible emitters into highly efficient near-infrared fluorophores. Employing thermodynamically controlled sequential deposition of metal and chalcogen ions, we achieve atomic-level precision in defining the thickness of the HgS interlayer (H). This manifests in 'quantized' jumps of the photoluminescence spectrum when H changes in discrete, atomic steps. The synthesized structures show highly efficient photoluminescence, tunable from 700 to 1,370 nm, and fast radiative rates of ~1/60 ns^-1. The emission from individual CdSe/HgS/CdS colloidal quantum dots is virtually blinking free and exhibits nearly perfect single-photon purity. In addition, when incorporated into a light-emitting-diode architecture, these quantum dots demonstrate strong electroluminescence with a sub-bandgap turn-on voltage.

Giant Photoluminescence Enhancement and Resonant Charge Transfer in Atomically Thin Two-Dimensional Cr2Ge2Te6/WS2 Heterostructures

Hybridization of two-dimensional (2D) magnetic semiconductors with transition-metal dichalcogenides (TMDC) monolayers can significantly engineer the light-matter interactions and provide a promising platform for enhanced excitonic systems with artificially tailored band alignments. Here, we report the fabrication of heterostructures with monolayer WS₂ on 2D Cr₂Ge₂Te₆ (CGT), which displayed giant photoluminescence enhancement at specific CGT layer numbers. The highly enhanced quantum yield obtained can be explained by novel photoexcited carrier dynamics, facilitated by alternate relaxation channels, resulting in resonance charge transfer at the heterointerface. 2D CGT revealed a strongly layer-dependent work function (up to ∼750 meV), which greatly modulates the band positioning in the heterostructure. These heterostructures conceived both type I and type II band alignments, which are verified by Kelvin probe force microscopy and PL measurements. In addition to layer modulation, we uncover temperature and power dependence of the resonance charge transfer in the multilayer heterostructure. Our findings provide further insights into the ultrafast charge dynamics occurring at the atomic interfaces. The results may pave the way for novel optoelectronics based on van der Waals heterostructures.

Manipulating the Optoelectronic Properties of Quasi-type II CuInS2/CdS Core/Shell Quantum Dots for Photoelectrochemical Cell Applications

Colloidal core/shell heterostructured quantum dots (QDs) possessing quasi-type II band structure have demonstrated effective surface passivation and prolonged exciton lifetime, leading to enhanced charge separation/transfer efficiencies that are promising for photovoltaic device applications. Herein, we synthesized CuInS₂ (CIS)/CdS core/shell heterostructured QDs and manipulated the optoelectronic properties via controlling the CdS shell thickness. The shell-thickness-dependent optical properties indicate the existence of a quasi-type II band structure in such core/shell QDs, which was verified by ultrafast spectroscopy and theoretical simulations. These quasi-type II core/shell QDs having various shell thicknesses are used as light absorbers for the fabrication of solar-driven QDs-based photoelectrochemical (PEC) devices, exhibiting an optimized photocurrent density of ∼6.0 mA/cm² and excellent stability under simulated AM 1.5G solar illumination. The results demonstrate that quasi-type II CIS/CdS core/shell heterostructured QDs with tailored optoelectronic properties are promising to realize high-performance QDs-based solar energy conversion devices for the production of solar fuels.

Efficient Carrier Multiplication in Low Band Gap Mixed Sn/Pb Halide Perovskites

Carrier multiplication (CM) generates multiple electron-hole pairs in a semiconductor from a single absorbed photon with energy exceeding twice the band gap. Thus, CM provides a promising way to circumvent the Shockley-Queisser limit of solar cells. The ideal material for CM should have significant overlap with the solar spectrum and should be able to fully utilize the excess energy above the band gap for additional charge carrier generation. We report efficient CM in mixed Sn/Pb halide perovskites (band gap of 1.28 eV) with onset just above twice the band gap. The CM rate outcompetes the carrier cooling process leading to efficient CM with a quantum yield of 2 for photoexcitation at 2.8 times the band gap. Such efficient CM characteristics add to the many advantageous properties of mixed Sn/Pb metal halide perovskites for photovoltaic applications.

Tailoring the Heterostructure of Colloidal Quantum Dots for Ratiometric Optical Nanothermometry

Colloidal quantum dots (QDs) are a fascinating class of semiconducting nanocrystals, thanks to their optical properties tunable through size and composition, and simple synthesis methods. Recently, colloidal double-emission QDs have been successfully applied as competitive optical temperature sensors, since they exhibit structure-tunable double emission, temperature-dependent photoluminescence, high quantum yield, and excellent photostability. Until now, QDs have been used as nanothermometers for in vivo biological thermal imaging, and thermal mapping in complex environments at the sub-microscale to nanoscale range. In this Review, recent progress for QD-based nanothermometers is highlighted and perspectives for future work are described.

Phonon-Driven Energy Relaxation in PbS/CdS and PbSe/CdSe Core/Shell Quantum Dots

We study the impact of the chemical composition on phonon-mediated exciton relaxation in the core/shell quantum dots (QDs), with 1 nm core made of PbX and the monolayer shell made of CdX, where X = S and Se. For this, time-domain nonadiabatic molecular dynamics (NAMD) based on density functional theory (DFT) and surface hopping techniques are applied. Simulations reveal twice faster energy relaxation in PbS/CdS than PbSe/CdSe because of dominant couplings to higher-energy optical phonons in structures with sulfur anions. For both QDs, the long-living intermediate states associated with the core-shell interface govern the dynamics. Therefore, a simple exponential model is not appropriate, and the four-state irreversible kinetic model is suggested instead, predicting 0.9 and 0.5 ps relaxation rates in PbSe/CdSe and PbS/CdS QDs, respectively. Thus, 2 nm PdSe/CdSe QDs with a single monolayer shell exhibit the phonon-mediated relaxation time sufficient for carrier multiplications to outpace energy dissipation and benefit the solar conversion efficiency.

Encapsulation of Dual Emitting Giant Quantum Dots in Silica Nanoparticles for Optical Ratiometric Temperature Nanosensors

Accurate temperature measurements with a high spatial resolution for application in the biomedical fields demand novel nanosized thermometers with new advanced properties. Here, a water dispersible ratiometric temperature sensor is fabricated by encapsulating in silica nanoparticles, organic capped PbS@CdS@CdS “giant” quantum dots (GQDs), characterized by dual emission in the visible and near infrared spectral range, already assessed as efficient fluorescent nanothermometers. The chemical stability, easy surface functionalization, limited toxicity and transparency of the silica coating represent advantageous features for the realization of a nanoscale heterostructure suitable for temperature sensing. However, the strong dependence of the optical properties on the morphology of the final core–shell nanoparticle requires an accurate control of the encapsulation process. We carried out a systematic investigation of the synthetic conditions to achieve, by the microemulsion method, uniform and single core silica coated GQD (GQD@SiO2) nanoparticles and subsequently recorded temperature-dependent fluorescent spectra in the 281-313 K temperature range, suited for biological systems. The ratiometric response—the ratio between the two integrated PbS and CdS emission bands—is found to monotonically decrease with the temperature, showing a sensitivity comparable to bare GQDs, and thus confirming the effectiveness of the functionalization strategy and the potential of GQD@SiO2 in future biomedical applications.

Extremely Efficient Photocurrent Generation in Carbon Nanotube Photodiodes Enabled by a Strong Axial Electric Field

Carbon nanotube (CNT) photodiodes have the potential to convert light into electrical current with high efficiency. However, previous experiments have revealed the photocurrent quantum yield (PCQY) to be well below 100%. In this work, we show that the axial electric field increases the PCQY of CNT photodiodes. Under optimal conditions, our data suggest PCQY > 100%. We studied, both experimentally and theoretically, CNT photodiodes at room temperature using optical excitation corresponding to the S₂₂, S₃₃, and S₄₄ exciton resonances. The axial electric field inside the pn junction was controlled using split gates that are capacitively coupled to the suspended CNT. Our results give new insight into the photocurrent generation pathways in CNTs and the field dependence and diameter dependence of PCQY.

Photoexcited carrier dynamics in colloidal quantum dot solar cells: insights into individual quantum dots, quantum dot solid films and devices

The certified power conversion efficiency (PCE) record of colloidal quantum dot solar cells (QDSCs) has considerably improved from below 4% to 16.6% in the last few years. However, the record PCE value of QDSCs is still substantially lower than the theoretical efficiency. So far, there have been several reviews on recent and significant achievements in QDSCs, but reviews on photoexcited carrier dynamics in QDSCs are scarce. The photovoltaic performances of QDSCs are still limited by the photovoltage, photocurrent and fill factor that are mainly determined by the photoexcited carrier dynamics, including carrier (or exciton) generation, carrier extraction or transfer, and the carrier recombination process, in the devices. In this review, the photoexcited carrier dynamics in the whole QDSCs, originating from individual quantum dots (QDs) to the entire device as well as the characterization methods used for analyzing the photoexcited carrier dynamics are summarized and discussed. The recent research including photoexcited multiple exciton generation (MEG), hot electron extraction, and carrier transfer between adjacent QDs, as well as carrier injection and recombination at each interface of QDSCs are discussed in detail herein. The influence of photoexcited carrier dynamics on the physiochemical properties of QDs and photovoltaic performances of QDSC devices is also discussed.

Engineering the Band Alignment in QD Heterojunction Films via Ligand Exchange

Colloidal quantum dots (QDs) allow great flexibility in the design of optoelectronic devices, thanks to their size-dependent optical and electronic properties and the possibility to fabricate thin films with solution-based processing. In particular, in QD-based heterojunctions, the band gap of both components can be controlled by varying the size of the QDs. However, control over the band alignment between the two materials is required to tune the dynamics of carrier transfer across a heterostructure. We demonstrate that ligand exchange strategies can be used to control the band alignment of PbSe and CdSe QDs in a mixed QD solid, shifting it from a type-I to a type-II alignment. The change in alignment is observed in both spectroelectrochemical and transient absorption measurements, leading to a change in the energy of the conduction band edges in the two materials and in the direction of electron transfer upon photoexcitation. Our work demonstrates the possibility to tune the band offset of QD heterostructures via control of the chemical species passivating the QD surface, allowing full control over the energetics of the heterostructure without requiring changes in the QD composition.

Carrier multiplication in van der Waals layered transition metal dichalcogenides

Carrier multiplication (CM) is a process in which high-energy free carriers relax by generation of additional electron-hole pairs rather than by heat dissipation. CM is promising disruptive improvements in photovoltaic energy conversion and light detection technologies. Current state-of-the-art nanomaterials including quantum dots and carbon nanotubes have demonstrated CM, but are not satisfactory owing to high-energy-loss and inherent difficulties with carrier extraction. Here, we report CM in van der Waals (vdW) MoTe₂ and WSe₂ films, and find characteristics, commencing close to the energy conservation limit and reaching up to 99% CM conversion efficiency with the standard model. This is demonstrated by ultrafast optical spectroscopy with independent approaches, photo-induced absorption, photo-induced bleach, and carrier population dynamics. Combined with a high lateral conductivity and an optimal bandgap below 1 eV, these superior CM characteristics identify vdW materials as an attractive candidate material for highly efficient and mechanically flexible solar cells in the future.

During carrier multiplication, high-energy free carriers in a given material relax by generation of additional electron-hole pairs. Here, the authors report evidence of carrier multiplication in multilayer MoTe₂ and WSe₂ films with up to 99% conversation efficiency.

Hot-electron dynamics in quantum dots manipulated by spin-exchange Auger interactions

The ability to effectively manipulate non-equilibrium 'hot' carriers could enable novel schemes for highly efficient energy harvesting and interconversion. In the case of semiconductor materials, realization of such hot-carrier schemes is complicated by extremely fast intraband cooling (picosecond to subpicosecond time scales) due to processes such as phonon emission. Here we show that using magnetically doped colloidal semiconductor quantum dots we can achieve extremely fast rates of spin-exchange processes that allow for 'uphill' energy transfer with an energy-gain rate that greatly exceeds the intraband cooling rate. This represents a dramatic departure from the usual situation where energy-dissipation via phonon emission outpaces energy gains due to standard Auger-type energy transfer at least by a factor of three. A highly favourable energy gain/loss rate ratio realized in magnetically doped quantum dots can enable effective schemes for capturing kinetic energy of hot, unrelaxed carriers via processes such as spin-exchange-mediated carrier multiplication and upconversion, hot-carrier extraction and electron photoemission.

Optimal Blending of PbSe and CdSe in Polycrystalline PbCdSe Nanocomposite Film: Improved Carrier Multiplication and Enhanced Photoconversion Efficiency

The present work reports galvanostatic electro-co-deposition of n-PbCdSe semiconductor (SC) films on FTO substrate from the respective precursors. Self-designed matrices were formulated at variable concentrations of Pb²⁺ in the deposition medium. The semiconductor films constitute an intermixed structure of close-packed PbSe and CdSe nanoparticles (NPs), and the band gap (E_g) was effectively tuned in the range 0.99-1.47 eV for the variable compositions. Energy dispersive spectroscopy studies revealed that Cd exists in low level in the film matrix compared to Pb, presumably due to competitive deposition kinetics of the two chalcogenide compounds and the crystallite sizes determined from XRD studies, ranges between 15 and 12 nm, which corresponds to the size quenching of SC-NPs with increased Pb²⁺ concentration. The durability studies identify the most stable film developed at 0.025 M Pb²⁺ concentration. PbSe materials are typically characterized with impact ionization which effectively induces carrier multiplication (CM) in the quasi Type-II PbCdSe composite, exhibiting reasonably high photoconversion efficiency (PCE) of 6.14% with current output of 19.2 mA cm^-2 for the optimal PbCdSe film.

Persuasive Evidence for Electron-Nuclear Coupling in Diluted Magnetic Colloidal Nanoplatelets Using Optically Detected Magnetic Resonance Spectroscopy

The incorporation of magnetic impurities into semiconductor nanocrystals with size confinement promotes enhanced spin exchange interaction between photogenerated carriers and the guest spins. This interaction stimulates new magneto-optical properties with significant advantages for emerging spin-based technologies. Here we observe and elaborate on carrier-guest interactions in magnetically doped colloidal nanoplatelets with the chemical formula CdSe/Cd_1-xMn_xS, explored by optically detected magnetic resonance and magneto-photoluminescence spectroscopy. The host matrix, with a quasi-type II electronic configuration, introduces a dominant interaction between a photogenerated electron and a magnetic dopant. Furthermore, the data convincingly presents the interaction between an electron and nuclear spins of the doped ions located at neighboring surroundings, with consequent influence on the carrier's spin relaxation time. The nuclear spin contribution by the magnetic dopants in colloidal nanoplatelets is considered here for the first time.

Oxygen Plasma-Induced p-Type Doping Improves Performance and Stability of PbS Quantum Dot Solar Cells

PbS quantum dots (QDs) have been extensively studied for photovoltaic applications, thanks to their facile and low-cost fabrication processing and interesting physical properties such as size dependent and tunable band gap. However, the performance of PbS QD-based solar cells is highly sensitive to the humidity level in the ambient air, which is a serious obstacle toward its practical applications. Although it has been previously revealed that oxygen doping of the hole transporting layer can mitigate the cause of this issue, the suggested methods to recover the device performance are time-consuming and relatively costly. Here, we report a low-power oxygen plasma treatment as a rapid and low-cost method to effectively recover the device performance and stability. Our optimization results show that a 10 min treatment is the best condition, resulting in an enhanced power conversion efficiency from 6.9% for the as-prepared device to 9% for the plasma treated one. Moreover, our modified device shows long-term shelf-life stability.

Multiphoton Excitation of CsPbBr3 Perovskite Quantum Dots (PQDs): How Many Electrons Can One PQD Donate to Multiple Molecular Acceptors?

Metastable multiexcitonic states (MESs) of semiconductor quantum dots can be involved in multielectron transfer reactions, which opens new perspectives in nanomaterials-based optoelectronic applications. Herein, we demonstrate the generation of a MES in CsPbBr₃ perovskite quantum dots (PQDs) and its dissociation dynamics through multiple electron transfers to molecular electron acceptors, anthraquinones (AQs), bound to the PQD surface by a carboxylic anchor. As many as 14 excitons are produced at an excitation density of roughly 220 μJ cm^-2 without detectable PQD degradation. Addition of AQ results in the formation of PQD-AQ hybrids with excess of AQs (PQD:AQ ≈ 1:20), which opens the possibility of multielectron transfer acts from MES to AQs. We found that the electron transfer saturates after roughly five transfer acts and that the first electron transfer (ET) time constant is as short as 1 ps. However, each ET increases the Coulomb potential barrier for the next ET, which decreases the rate of ET, resulting in a saturation after five ETs.

Infrared Quantum Dots: Progress, Challenges, and Opportunities

Infrared technologies provide tremendous value to our modern-day society. The need for easy-to-fabricate, solution-processable, tunable infrared active optoelectronic materials has driven the development of infrared colloidal quantum dots, whose band gaps can readily be tuned by dimensional constraints due to the quantum confinement effect. In this Perspective, we summarize recent progress in the development of infrared quantum dots both as infrared light emitters ( e.g., in light-emitting diodes, biological imaging, etc.) as well as infrared absorbers ( e.g., in photovoltaics, solar fuels, photon up-conversion, etc.), focusing on how fundamental breakthroughs in synthesis, surface chemistry, and characterization techniques are facilitating the implementation of these nanostructures into exploratory device architectures as well as in emerging applications. We discuss the ongoing challenges and opportunities associated with infrared colloidal quantum dots.

Enhancement of Biexciton Emission Due to Long-Range Interaction of Single Quantum Dots and Gold Nanorods in a Thin-Film Hybrid Nanostructure

Semiconductor quantum dots (QDs) are known for their ability to exhibit multiphoton emission caused by recombination of biexcitons (BX). However, the quantum yield (QY) of BX emission is low due to the fast Auger process. Plasmonic nanoparticles (PNPs) provide an attractive opportunity to accelerate BX radiative recombination. Here, we demonstrate the PNPs induced distance-controlled enhancement of BX emission of single QDs. Studying the same single QD before and after its integration with the PNPs, we observed a plasmon-mediated increase in the QY of BX emission. Remarkably, the enhancement of BX emission remains pronounced even at distances of 170 nm. We attribute this effect to efficient coupling, which results in the trade-off between resonance energy transfer from QD to gold nanorods and the Purcell effect at small QD-PNP separations and the predominant influence of the Purcell effect at longer distances. Our findings constitute a reliable approach to managing the efficiency of multiexciton emission over a wide span of distances, thus paving the way for new applications.

Combined Computational and Experimental Study of CdSeS/ZnS Nanoplatelets: Structural, Vibrational, and Electronic Aspects of Core-Shell Interface Formation

In the past few years, core-shell nanoparticles have opened new perspectives for the optoelectronic applications of semiconductor quantum dots. In particular, it has become possible to localize electrons in either part of these heterostructures. Understanding and controlling this phenomenon require a thorough characterization of the interfaces. In this study, we prepared quasi-2D CdSeS/ZnS core-shell nanoplatelets (NPLs) by colloidal atomic layer deposition. This technique allows fine control over the quantum confinement, the surfaces, and the interfaces. The layer-by-layer formation of a the ZnS shell around the CdSeS core was monitored using UV-vis absorption, XRD, and Raman spectroscopy. The measured band gaps and structural distortions were compared with results obtained from density functional theory (DFT) calculations. Modeling has also shown that 34% of the photoexcited electrons are delocalized into the ZnS shell. The herein presented combined modeling and experimental characterization strategy is of general interest since it can be applied to a large choice of layered semiconductor heterostructures in optoelectronics. The present approach paves the way for the synthesis of nanocrystals with precisely engineered properties for light-emitting diodes and solar cells.

Enhanced Multiple Exciton Generation in PbS|CdS Janus-like Heterostructured Nanocrystals

Generating multiple excitons by a single high-energy photon is a promising third-generation solar energy conversion strategy. We demonstrate that multiple exciton generation (MEG) in PbS|CdS Janus-like heteronanostructures is enhanced over that of single-component and core/shell nanocrystal architectures, with an onset close to two times the PbS band gap. We attribute the enhanced MEG to the asymmetric nature of the heteronanostructure that results in an increase in the effective Coulomb interaction that drives MEG and a reduction of the competing hot exciton cooling rate. Slowed cooling occurs through effective trapping of hot-holes by a manifold of valence band interfacial states having both PbS and CdS character, as evidenced by photoluminescence studies and ab initio calculations. Using transient photocurrent spectroscopy, we find that the MEG characteristics of the individual nanostructures are maintained in conductive arrays and demonstrate that these quasi-spherical PbS|CdS nanocrystals can be incorporated as the main absorber layer in functional solid-state solar cell architectures. Finally, based upon our analysis, we provide design rules for the next generation of engineered nanocrystals to further improve the MEG characteristics.

Quantum Dot Solar Cells: Small Beginnings Have Large Impacts

From a niche field over 30 years ago, quantum dots (QDs) have developed into viable materials for many commercial optoelectronic devices. We discuss the advancements in Pb-based QD solar cells (QDSCs) from a viewpoint of the pathways an excited state can take when relaxing back to the ground state. Systematically understanding the fundamental processes occurring in QDs has led to improvements in solar cell efficiency from ~3% to over 13% in 8 years. We compile data from ~200 articles reporting functioning QDSCs to give an overview of the current limitations in the technology. We find that the open circuit voltage limits the device efficiency and propose some strategies for overcoming this limitation.

Tailoring the interfacial structure of colloidal "giant" quantum dots for optoelectronic applications

Colloidal semiconductor quantum dots (QDs) are promising building blocks for the realization of future optoelectronic technologies, thanks to their size-tunable electronic and optical properties. Among various types of QDs, colloidal "giant" QDs (g-QDs, core/thick-shell) have been widely used in different applications, such as solar cells, light emitting devices, luminescent solar concentrators and photoelectrochemical (PEC) hydrogen production. However, g-QDs have a thick-shell which serves as a physical barrier for electron and hole transfer, leading to a slow charge transfer rate. In this work, we synthesized CdSe/CdSexS1-x/CdS core/shell/shell g-QDs with an intermediate CdSexS1-x alloyed layer. The presence of this interfacial layer largely improves the absorption of CdSe/CdS QDs, particularly in the 300-650 nm range. By engineering the interfacial layer, the holes can leak more into the CdS shell region compared to that of CdSe/CdS QDs. PEC devices based on alloyed g-QDs exhibit a 20% higher saturated photocurrent density (11 ± 0.5 mA cm-2) compared to CdSe/CdS QDs. In addition, after one-hour illumination (100 mW cm-2), the PEC cell based on alloyed g-QDs still exhibits a photocurrent density of 7.5 mA cm-2, maintaining 70% of its initial value. Such alloyed g-QDs are very promising for several emerging optoelectronic applications, where charge separation, transfer and transport play a critical role for the realization of high performance devices.

New Pathway for Hot Electron Relaxation in Two-Dimensional Heterostructures

Two-dimensional (2D) heterostructures composed of transition-metal dichalcogenide atomic layers are the new frontier for novel optoelectronic and photovoltaic device applications. Some key properties that make these materials appealing, yet are not well understood, are ultrafast hole/electron dynamics, interlayer energy transfer and the formation of interlayer hot excitons. Here, we study photoexcited electron/hole dynamics in a representative heterostructure, the MoS₂/WSe₂ interface, which exhibits type II band alignment. Employing time-dependent density functional theory in the time domain, we observe ultrafast charge dynamics with lifetimes of tens to hundreds of femtoseconds. Most importantly, we report the discovery of an interfacial pathway in 2D heterostructures for the relaxation of photoexcited hot electrons through interlayer hopping, which is significantly faster than intralayer relaxation. This finding is of particular importance for understanding many experimentally observed photoinduced processes, including charge and energy transfer at an ultrafast time scale (<1 ps).

GaAs Nanowires Grown by Catalyst Epitaxy for High Performance Photovoltaics

Photovoltaics (PVs) based on nanostructured III/V semiconductors can potentially reduce the material usage and increase the light-to-electricity conversion efficiency, which are anticipated to make a significant impact on the next-generation solar cells. In particular, GaAs nanowire (NW) is one of the most promising III/V nanomaterials for PVs due to its ideal bandgap and excellent light absorption efficiency. In order to achieve large-scale practical PV applications, further controllability in the NW growth and device fabrication is still needed for the efficiency improvement. This article reviews the recent development in GaAs NW-based PVs with an emphasis on cost-effectively synthesis of GaAs NWs, device design and corresponding performance measurement. We first discuss the available manipulated growth methods of GaAs NWs, such as the catalytic vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) epitaxial growth, followed by the catalyst-controlled engineering process, and typical crystal structure and orientation of resulted NWs. The structure-property relationships are also discussed for achieving the optimal PV performance. At the same time, important device issues are as well summarized, including the light absorption, tunnel junctions and contact configuration. Towards the end, we survey the reported performance data and make some remarks on the challenges for current nanostructured PVs. These results not only lay the ground to considerably achieve the higher efficiencies in GaAs NW-based PVs but also open up great opportunities for the future low-cost smart solar energy harvesting devices.

Wave Function Engineering in CdSe/PbS Core/Shell Quantum Dots

The synthesis of epitaxial CdSe/PbS core/shell quantum dots (QDs) is reported. The PbS shell grows in a rock salt structure on the zinc blende CdSe core, thereby creating a crystal structure mismatch through additive growth. Absorption and photoluminescence (PL) band edge features shift to lower energies with increasing shell thickness, but remain above the CdSe bulk band gap. Nevertheless, the profiles of the absorption spectra vary with shell growth, indicating that the overlap of the electron and hole wave functions is changing significantly. This leads to over an order of magnitude reduction of absorption near the band gap and a large, tunable energy shift, of up to 550 meV, between the onset of strong absorption and the band edge PL. While the bulk valence and conduction bands adopt an inverse type-I alignment, the observed spectroscopic behavior is consistent with a transition between quasi-type-I and quasi-type-II behavior depending on shell thickness. Three effective mass approximation models support this hypothesis and suggest that the large difference in effective masses between the core and shell results in hole localization in the CdSe core and a delocalization of the electron across the entire QD. These results show the tuning of wave functions and transition energies in CdSe/PbS nanoheterostructures with prospects for use in optoelectronic devices for luminescent solar concentration or multiexciton generation.

Asymmetric Optical Transitions Determine the Onset of Carrier Multiplication in Lead Chalcogenide Quantum Confined and Bulk Crystals

Carrier multiplication is a process in which one absorbed photon excites two or more electrons. This is of great promise to increase the efficiency of photovoltaic devices. Until now, the factors that determine the onset energy of carrier multiplication have not been convincingly explained. We show experimentally that the onset of carrier multiplication in lead chalcogenide quantum confined and bulk crystals is due to asymmetric optical transitions. In such transitions most of the photon energy in excess of the band gap is given to either the hole or the electron. The results are confirmed and explained by theoretical tight-binding calculations of the competition between impact ionization and carrier cooling. These results are a large step forward in understanding carrier multiplication and allow for a screening of materials with an onset of carrier multiplication close to twice the band gap energy. Such materials are of great interest for development of highly efficient photovoltaic devices.

Multiple Exciton Generation in Nanostructures for Advanced Photovoltaic Cells

This paper reviews both experimental and theoretical work on nanostructures showing high quantum yields due to the phenomenon of multiple exciton generation. It outlines the aims and barriers to progress in identifying further such nanostructures and also includes important developments concerning solar devices where nanostructures act as the light-absorbing component. It reports on both semiconductor and carbon structures, both monocomposite (of various dimensionalities) and heterogeneous. Finally, it looks at future directions that can be taken to push solar cell efficiency above the classic limit set by Shockley and Queisser in 1961.

Colloidal Dual-Diameter and Core-Position-Controlled Core/Shell Cadmium Chalcogenide Nanorods

To capitalize on shape- and structure-dependent properties of semiconductor nanorods (NRs), high-precision control and exquisite design of their growth are desired. Cadmium chalcogenide (CdE; E = S or Se) NRs are the most studied class of such, whose growth exhibits axial anisotropy, i.e., different growth rates along the opposite directions of {0001} planes. However, the mechanism behind asymmetric axial growth of NRs remains unclear because of the difficulty in instant analysis of growth surfaces. Here, we design colloidal dual-diameter semiconductor NRs (DDNRs) under the quantum confinement regime, which have two sections along the long axis with different diameters. The segmentation of the DDNRs allows rigorous assessment of the kinetics of NR growth at a molecular level. The reactivity of a terminal facet passivated by an organic ligand is governed by monomer diffusivity through the surface ligand monolayer. Therefore, the growth rate in two polar directions can be finely tuned by controlling the strength of ligand-ligand attraction at end surfaces. Building on these findings, we report the synthesis of single-diameter CdSe/CdS core/shell NRs with CdSe cores of controllable position, which reveals a strong structure-optical polarization relationship. The understanding of the NR growth mechanism with controllable anisotropy will serve as a cornerstone for the exquisite design of more complex anisotropic nanostructures.

Biexciton and trion energy transfer from CdSe/CdS giant nanocrystals to Si substrates

Observation of energy transfer (ET) from multiexcitonic (MX) complexes in nanocrystal quantum dots (NQDs) has been severely restricted due to efficient nonradiative Auger recombination leading to very low MX emission quantum yields. Here we employed "giant" CdSe/CdS NQDs with suppressed Auger recombination to study ET of biexcitons (BX) and charged excitons (trions) into Si substrate. Photoluminescence (PL) measurements of (sub)monolayers of gNQDs controllably assembled on various interacting surfaces and augmented by single gNQD's imaging reveal appearance of BX spectral signatures and progressive acceleration of PL lifetimes of all excitonic species on Si substrates. From statistical analysis of a large number of PL lifetime traces, representative exciton, trion and BX ET efficiencies are measured as ∼75%, 55% and 45% respectively. Detailed analysis of the MX's radiative rates demonstrate the crucial role of the radiative (waveguide) ET in maintaining high overall transfer efficiency despite the prevalent Auger recombination. Our observations point towards practical utilization of MX-bearing nanocrystals in future optoelectronics architectures.

Hot carrier-enhanced interlayer electron-hole pair multiplication in 2D semiconductor heterostructure photocells

Strong electronic interactions can result in novel particle-antiparticle (electron-hole, e-h) pair generation effects, which may be exploited to enhance the photoresponse of nanoscale optoelectronic devices. Highly efficient e-h pair multiplication has been demonstrated in several important nanoscale systems, including nanocrystal quantum dots, carbon nanotubes and graphene. The small Fermi velocity and nonlocal nature of the effective dielectric screening in ultrathin layers of transition-metal dichalcogenides (TMDs) indicates that e-h interactions are very strong, so high-efficiency generation of e-h pairs from hot electrons is expected. However, such e-h pair multiplication has not been observed in 2D TMD devices. Here, we report the highly efficient multiplication of interlayer e-h pairs in 2D semiconductor heterostructure photocells. Electronic transport measurements of the interlayer I-V_SD characteristics indicate that layer-indirect e-h pairs are generated by hot-electron impact excitation at temperatures near T = 300 K. By exploiting this highly efficient interlayer e-h pair multiplication process, we demonstrate near-infrared optoelectronic devices that exhibit 350% enhancement of the optoelectronic responsivity at microwatt power levels. Our findings, which demonstrate efficient carrier multiplication in TMD-based optoelectronic devices, make 2D semiconductor heterostructures viable for a new class of ultra-efficient photodetectors based on layer-indirect e-h excitations.

Assessment of toxicity of selenium and cadmium selenium quantum dots: A review

This paper reviews the current understanding of the toxicity of selenium (Se) to terrestrial mammalian and aquatic organisms. Adverse biological effects occur in the case of Se deficiencies, associated with this element having essential biological functions and a narrow window between essentiality and toxicity. Several inorganic species of Se (-2, 0, +4, and +6) and organic species (monomethylated and dimethylated) have been reported in aquatic systems. The toxicity of Se in any given sample depends not only on its speciation and concentration, but also on the concomitant presence of other compounds that may have synergistic or antagonistic effects, affecting the target organism as well, usually spanning 2 or 3 orders of magnitude for inorganic Se species. In aquatic ecosystems, indirect toxic effects, linked to the trophic transfer of excess Se, are usually of much more concern than direct Se toxicity. Studies on the toxicity of selenium nanoparticles indicate the greater toxicity of chemically generated selenium nanoparticles relative to selenium oxyanions for fish and fish embryos while oxyanions of selenium have been found to be more highly toxic to rats as compared to nano-Se. Studies on polymer coated Cd/Se quantum dots suggest significant differences in toxicity of weathered vs. non-weathered QD's as well as a significant role for cadmium with respect to toxicity.

Bright alloy type-II quantum dots and their application to light-emitting diodes

Type-II quantum dots (QDs) are emerging as a promising candidate for full color light sources owing to their advantages in achieving full color light by tuning the heterostructures. Despite the recent developments in type-II QDs, the choices of proper materials are limited for the composition of a high-quality QD and it still remains a big challenge to enhance the photoluminescence (PL) quantum yields (QYs) of type-II QDs for light-emitting diode (LED) applications. Here, we develop Cd_xZn_1-xS/ZnSe/ZnS type-II QDs with a maximum quantum yield as high as 88.5%. Time-resolved PL results show that the ZnS shell suppresses non-radiative pathways by passivating the surface of Cd_xZn_1-xS/ZnSe, thus leading to a high QY. Moreover, our results demonstrate that the outer ZnS also benefits the charge injection and radiative recombinations of the Cd_xZn_1-xS/ZnSe. The LED based on green Cd_0.2Zn_0.8S/ZnSe/ZnS QDs achieves a current efficiency (CE) of 9.17cdA^-1, an external quantum efficiency (EQE) of 8.78% and a low turn-on voltage of ∼2.3V.

Carrier relaxation dynamics in type-II ZnO/CdSe quantum dot heterostructures

Semiconductor heterostructures with type-II band alignment are well known for their engineering property of efficient charge separation in photogenerated carriers. Herein, type-II CdSe/ZnO core/shell quantum dot heterostructures with CdSe shells of different thicknesses have been synthesized and a study of carrier dynamics is carried out using femtosecond transient absorption and picosecond emission spectroscopy. Carrier lifetime measurements by transient emission spectroscopy have revealed reduced electron-hole overlap in the type-II localization regime of ZnO/CdSe heterostructures. Femtosecond transient absorption studies have revealed hot electron transfer from CdSe shell to ZnO core prior to electron cooling in the CdSe shell. In addition, a surface channel for the hole cooling process has been identified in the transient absorption measurements. Effects of carrier trapping at interfacial defect states and type-II localization on carrier recombination have been recognized in our transient absorption and emission studies of ZnO/CdSe QDs heterostructure.