arXiv subjects

Victor I. Klimov

Publications and source records attributed to Victor I. Klimov.

14 recordsLinked to original sources

arXiv2025

Interplay of Conventional and Spin-Exchange Auger Recombination in Magnetically Doped Quantum Dots

Auger processes play a critical role in the behavior of colloidal quantum dots (QDs). While nonradiative Auger recombination is a key performance-limiting factor in light-emitting applications, Auger effects also present exciting opportunities for hot-carrier manipulation in emerging optoelectronic and photochemical technologies. Recent studies have shown that incorporating magnetic manganese (Mn) dopants into QDs enables an ultrafast spin-exchange (SE) mechanism that significantly accelerates Auger recombination and related processes. However, the fundamental dynamics of SE-Auger recombination, including its dependence on QD size, exciton multiplicity, and Mn-ion content, remain largely unexplored. Here, we investigate SE-Auger recombination in Mn-doped QDs across three regimes defined by the energetic alignment between the QD band-edge exciton and the internal Mn spin-flip transition: resonant, downhill, and uphill, corresponding to the exciton energy being approximately equal to, greater than, or smaller than the Mn spin-flip transition energy, respectively. We find that SE-Auger recombination dominates multiexciton dynamics in both resonant and downhill regimes, proceeding on ultrafast, sub-picosecond timescales, that is, significantly faster than conventional Auger decay. Notably, SE-Auger lifetimes are primarily governed by the occupancy of intrinsic excitonic states, with minimal dependence on QD size or the number of excited Mn ions. In the uphill regime, SE-Auger recombination coexists with conventional Auger processes, enabling direct comparison of their timescales alongside co-occurring hot-carrier cooling. These findings establish that SE-Auger recombination outpaces phonon-assisted cooling, making it uniquely suited for the generation and manipulation of hot carriers, and opening new avenues in advanced photoconversion and photochemistry.

cond-mat.mes-hall↗

arXiv2024

Wetting-Layer-Assisted Synthesis of Inverted CdSe/PbSe Quantum Dots and their Photophysical and Photo-Electrical Properties

Heterostructured quantum dots (QDs) based on narrow-gap PbSe and wide-gap CdSe have been studied with an eye on their prospective applications in near-infrared (NIR) light sources, photodetectors, and solar cells. The most common structural motif is a spherical QD comprising a PbSe core enclosed into a CdSe shell. However, the potential barrier created by the CdSe shell complicates extraction of band-edge charge carriers from the QD. Therefore, conventional PbSe/CdSe QDs are not suitable for applications in practical photoconversion devices. Here we report inverted CdSe/PbSe core/shell QDs that overcome this drawback. In these structures, both photocarriers (electron and hole) exhibit a significant degree of shell localization and are therefore free to move within the QD solid and be extracted into an external circuit. To create such QDs, we employ a novel synthetic method in which a thin, atomically controlled wetting layer is used to homogenize the surface of the CdSe core and thus promote directionally uniform growth of the PbSe shell. Unlike noninverted QDs, inverted core/shell structures exhibit highly efficient photocarrier transport, making them excellent candidates for applications in practical photoconversion including photovoltaics, photodetection, and photochemistry.

cond-mat.mes-hall↗

arXiv2024

Photocurrent Enhancement due to Spin-Exchange Carrier Multiplication in Films of Manganese-Doped 'Inverted' CdSe/HgSe Quantum Dots

Incorporation of manganese impurities into II-VI semiconductors results in a dramatic change in their properties due to strong exchange interactions between the Mn ion and the semiconductor host. In colloidal quantum dots (QDs), these interactions result in a rapid bidirectional energy transfer between the magnetic impurity and the QD intrinsic states, which is characterized by an extremely high energy transfer rate of more than ~5 eV/ps. This rate is higher than the rate of energy loss due to phonon emission (typically, ~1 eV/ps or less), so Mn-QD interactions could in principle be used to capture and utilize the kinetic energy of the hot carrier before it is lost to phonons. Here, we demonstrate that by using Mn-doped CdSe/HgSe core/shell QDs, we can efficiently convert the kinetic energy of a hot exciton into an additional electron-hole pair (exciton). This carrier multiplication process occurs through rapid capture of a hot exciton by a Mn ion, which then undergoes spin flip relaxation, producing two excitons near the QD band edge. Due to the inverted geometry of the CdSe/HgSe QDs, both electrons and holes resulting from carrier multiplication occupy the QD shell, allowing them to be easily extracted from the QD for potential use in electro-optical devices or chemical reactions.

cond-mat.mtrl-sci↗

arXiv2024

Liquid-State Semiconductor Lasers Based on Type-(I+II) Colloidal Quantum Dots

Present-day liquid-state lasers are based on organic dyes. Here we demonstrate an alternative class of liquid lasers that employ solutions of colloidal quantum dots (QDs). Previous efforts to realize such devices have been hampered by fast nonradiative Auger recombination of multi-carrier states needed for optical gain. We overcome this challenge using type-(I+II) QDs that feature a trion-like optical-gain state with strongly suppressed Auger recombination. When combined with a Littrow optical cavity, static (non-circulated) solutions of these QDs exhibit stable lasing tunable from 634 nm to 594 nm. These results point towards the feasibility of technologically viable dye-like QD lasers that feature wide spectral tunability and, importantly, allow for stable operation without the need for a bulky circulation system, a standard attribute of traditional dye lasers.

physics.optics↗

arXiv2022

Optically Excited Two-Band Amplified Spontaneous Emission from a High-Current-Density Quantum-Dot LED

Laser diodes based on solution-processable materials could benefit numerous technologies including integrated electronics and photonics, telecommunication, and medical diagnostics. An attractive system for implementing these devices is colloidal semiconductor quantum dots (QDs). The primary challenge that hampered progress towards a QD laser diode (QLD) has been fast nonradiative Auger decay of optical-gain-active multicarrier states. Recently, this problem has been resolved by employing continuously graded QDs (cg-QDs) wherein Auger recombination is strongly suppressed. The use of these structures allowed for demonstrations of optical gain with electrical pumping and optically-excited lasing in multilayered LED-like devices. Here we report on achieving the next critical milestone towards a QLD, which is the demonstration of optically excited amplified spontaneous emission from a fully functional high-current density electroluminescent device. This advance has become possible due to excellent optical gain properties of novel 'compact' cg-QDs and a new LED architecture, which allows for concerted optimization of its optical and electrical properties. The results of this work strongly suggest the feasibility of the final step towards a functional QLD, which is the demonstration of lasing with electrical pumping.

physics.optics↗

arXiv2018

Transient Spectroscopy of Glass-Embedded Perovskite Quantum Dots: Novel Structures in an Old Wrapping

Semiconductor doped glasses had been used by the research and engineering communities as color filters or saturable absorbers well before it was realized that their optical properties were defined by tiny specs of semiconductor matter known presently as quantum dots (QDs). Nowadays, the preferred type of QD samples are colloidal particles; however, there is still a number of applications that would benefit from the availability of high-quality glass-based QD samples. These applications include fiber optics, optically pumped lasers and amplifiers, and luminescent solar concentrators. Here we conduct optical studies of a new type of all-inorganic CsPbBr3 perovskite QDs fabricated directly in glasses by high-temperature precipitation. These samples are scattering free and exhibit excellent waveguiding properties. However, the presently existing problem is their low room-temperature emission quantum yields (QY) of only 1-2%. Here we investigate the reasons underlying the limited QY by conducting transient photoluminescence (PL) and absorption measurements across a range of temperatures from 20-300 K. We observe that the low-temperature PL QY of these samples can be as high as ~25%. However, it quickly drops with increasing temperature. Interestingly, experimental observations cannot be explained in terms of a thermally activated nonradiative rate but rather suggest the existence of two QD sub-ensembles of emissive and completely nonemissive particles. The temperature-induced variation in the PL efficiency is likely due to a structural transformation of the QD surfaces or interior leading to formation of trapping sites or nonemissive phases resulting in conversion of emissive QDs into nonemissive. Thus, future efforts on improving emissivity of glass-based perovskite QD samples might focus on approaches for extending the range of stability of the low-temperature up to room temperature.

cond-mat.mes-hall↗

arXiv2013

Influence of the Core/Shell Interface on Biexciton Auger Recombination in Individual Nanocrystal Quantum Dots

Previous single-particle spectroscopic studies of colloidal quantum dots have indicated a significant spread in biexciton lifetimes across an ensemble of nominally identical nanocrystals. It has been speculated that in addition to dot-to-dot variation in physical dimensions, this spread is contributed to by variations in the structure of the quantum dot interface, which controls the shape of the confinement potential. Here we directly evaluate the effect of the composition of the core-shell interface on single- and multi-exciton dynamics via side-by-side measurements of individual core-shell CdSe-CdS nanocrystals with a sharp vs. smooth (graded) interface. To realize the latter type of structures, we incorporate a CdSexS1-x alloy layer of controlled composition and thickness between the CdSe core and the CdS shell. We observe that while having essentially no effect on single-exciton decay, the interfacial alloy layer leads to a systematic increase in biexciton lifetimes. This observation provides direct experimental evidence that in addition to the size of the quantum dot, its interfacial properties also significantly affect the rate of Auger recombination, which governs biexciton decay. These findings help rationalize previous observations of a significant heterogeneity in the biexciton lifetimes across similarly sized quantum dots and should facilitate the development of Auger-recombination-free colloidal nanostructures for a range of applications from lasers and light-emitting diodes to photodetectors and solar cells.

cond-mat.mtrl-sci↗

arXiv2013

The influence of Auger recombination on the performance of quantum-dot light-emitting diodes

A growing interest in colloidal quantum dot (QD) based light-emitting diodes (QD-LEDs) has been motivated by the exceptional color purity and spectral tunability of QD emission as well as the amenability of QD materials to highly scalable and inexpensive solution processing. One current challenge in the QD-LED field has been a still incomplete understanding of the role of extrinsic factors (e.g., recombination via QD surface defects) versus intrinsic processes such as multicarrier Auger recombination or electron-hole separation due to applied electric field in defining device efficiency. Here, we address this problem with a study of excited-state dynamics in a series of structurally engineered QDs, which is performed in parallel with characterization of their performance upon incorporation into LEDs. The results of this study indicate that under both zero and forward bias, a significant fraction of the QDs within the active emitting layer is negatively charged and therefore, Auger recombination represents an important factor limiting the efficiency of these devices. We further observe that the onset of the LED efficiency roll-off is also controlled by Auger recombination and can be shifted to higher currents by using newly developed QDs with an intermediate alloy layer at the core-shell interface introduced for suppression of Auger decay. Our findings suggest that further improvement in the performance of QD-LEDs can be achieved by developing effective approaches for controlling Auger recombination and/or minimizing the effects of QD charging via improved balancing of electron and hole injection currents.

cond-mat.mtrl-sci↗

arXiv2008

Tunable magnetic exchange interactions in manganese-doped inverted core/shell ZnSe/CdSe nanocrystals

Magnetic doping of semiconductor nanostructures is actively pursued for applications in magnetic memory and spin-based electronics. Central to these efforts is a drive to control the interaction strength between carriers (electrons and holes) and the embedded magnetic atoms. In this respect, colloidal nanocrystal heterostructures provide great flexibility via growth-controlled `engineering' of electron and hole wavefunctions within individual nanocrystals. Here we demonstrate a widely tunable magnetic sp-d exchange interaction between electron-hole excitations (excitons) and paramagnetic manganese ions using `inverted' core-shell nanocrystals composed of Mn-doped ZnSe cores overcoated with undoped shells of narrower-gap CdSe. Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the band-edge exciton that, surprisingly, are tunable in both magnitude and sign. Effective exciton g-factors are controllably tuned from -200 to +30 solely by increasing the CdSe shell thickness, demonstrating that strong quantum confinement and wavefunction engineering in heterostructured nanocrystal materials can be utilized to manipulate carrier-Mn wavefunction overlap and the sp-d exchange parameters themselves.

cond-mat.mtrl-sci↗

arXiv2008

Nanoplasmonic Renormalization and Enhancement of Coulomb Interactions

Nanostructured plasmonic metal systems are known to enhance greatly variety of radiative and nonradiative optical processes, both linear and nonlinear, which are due to the interaction of an electron in a molecule or semiconductor with the enhanced local optical field of the surface plasmons. Principally different are numerous many-body phenomena that are due to the Coulomb interaction between charged particles: carriers (electrons and holes) and ions. These include carrier-carrier or carrier-ion scattering, energy and momentum transfer (including the drag effect), thermal equilibration, exciton formation, impact ionization, Auger effects, etc. It is not widely recognized that these and other many-body effects can also be modified and enhanced by the surface-plasmon local fields. A special but extremely important class of such many-body phenomena is constituted by chemical reactions at metal surfaces, including catalytic reactions. Here, we propose a general and powerful theory of the plasmonic enhancement of the many-body phenomena resulting in a closed expression for the surface plasmon-dressed Coulomb interaction. We illustrate this theory by computing this dressed interaction explicitly for an important example of metal-dielectric nanoshells, which exhibits a reach resonant behavior in both the magnitude and phase. This interaction is used to describe the nanoplasmonic-enhanced Foerster energy transfer between nanocrystal quantum dots in the proximity of a plasmonic nanoshell. Catalysis at nanostructured metal surfaces, nonlocal carrier scattering and surface-enhanced Raman scattering are discussed among other effects and applications where the nanoplasmonic renormalization of the Coulomb interaction may be of principal importance.

physics.optics↗

arXiv2007

Carrier Multiplication in Nanocrystals via Photostimulated Generation of Biexcitons from Vacuum

We propose a novel mechanism for photogeneration of multiexcitons by single photons (carrier multiplication) in semiconductor nanocrystals. In this mechanism, the Coulomb interaction between two valence-band electrons involving their transfer to the conduction band creates a virtual biexciton from vacuum that is then converted into a real biexciton by photon absorption on an intraband optical transition. This mechanism is inactive in bulk semiconductors as momentum conservation suppresses intraband absorption. However, it becomes highly efficient in zero-dimensional nanocrystals and can provide a significant contribution to carrier multiplication in these materials.

cond-mat.mes-hall↗

arXiv2006

The effect of Auger heating on intraband carrier relaxation in semiconductor quantumrods

The rate at which excited charge carriers relax to their equilibrium state affects many aspects of the performance of nanoscale devices, including switching speed, carrier mobility and luminescent efficiency. Better understanding of the processes that govern carrier relaxation therefore has important technological implications. A significant increase in carrier-carrier interactions caused by strong spatial confinement of electronic excitations in semiconductor nanostructures leads to a considerable enhancement of Auger effects, which can further result in unusual, Auger-process-controlled recombination and energy-relaxation regimes. Here, we report the first experimental observation of efficient Auger heating in CdSe quantum rods at high pump intensities, leading to a strong reduction of carrier cooling rates. In this regime, the carrier temperature is determined by the balance between energy outflow through phonon emission and energy inflow because of Auger heating. This equilibrium results in peculiar carrier cooling dynamics that closely correlate with recombination dynamics, an effect never before seen in bulk or nanoscale semiconductors.

cond-mat.mes-hall↗

arXiv2005

Time- and Polarization-Resolved Optical Spectroscopy of Colloidal CdSe Nanocrystal Quantum Dots in High Magnetic Fields

In an effort to elucidate the spin (rather than charge) degrees of freedom in colloidal semiconductor nanocrystal quantum dots, we report on a series of static and time-resolved photoluminescence measurements of colloidal CdSe quantum dots in ultra-high magnetic fields up to 45 Tesla. At low temperatures (1.5 K - 40 K), the steady-state photoluminescence (PL) develops a high degree of circular polarization with applied magnetic field, indicating the presence of spin-polarized excitons. Time-resolved PL studies reveal a marked decrease in radiative exciton lifetime with increasing magnetic field and temperature. Except for an initial burst of unpolarized PL immediately following photoexcitation, high-field time-resolved PL measurements reveal a constant degree of circular polarization throughout the entire exciton lifetime, even in the presence of pronounced exciton transfer via Forster energy transfer processes.

cond-mat.mtrl-sci↗

arXiv2003

Picosecond energy transfer in quantum dot Langmuir-Blodgett nanoassemblies

We study spectrally resolved dynamics of Forster energy transfer in single monolayers and bilayers of semiconductor nanocrystal quantum dots assembled using Langmuir-Blodgett (LB) techniques. For a single monolayer, we observe a distribution of transfer times from ~50 ps to ~10 ns, which can be quantitatively modeled assuming that the energy transfer is dominated by interactions of a donor nanocrystal with acceptor nanocrystals from the first three shells surrounding the donor. We also detect an effective enhancement of the absorption cross section (up to a factor of 4) for larger nanocrystals on the red side of the size distribution, which results from strong, inter-dot electrostatic coupling in the LB film (the light-harvesting antenna effect). By assembling bilayers of nanocrystals of two different sizes, we are able to improve the donor-acceptor spectral overlap for engineered transfer in a specific (vertical) direction. These bilayers show a fast, unidirectional energy flow with a time constant of ~120 ps.

cond-mat.mes-hall↗