Despite three decades of research, colloidal quantum dot (QD) lasers are still at the stage of exploratory devices rather than a practical technology. A primary complication is nonradiative Auger recombination of gain-active multicarrier states. To overcome this challenge, we introduce type-(I+II) QDs that feature a trion-like optical-gain state with strongly suppressed Auger recombination. Using a single QD sample, we achieve stable lasing tunable 634 nm (red) to 590 nm (orange-yellow). These results point towards the feasibility of technologically viable QD lasers that combine wide dye-laser-like spectral tunability with high operational stability and unparallel diversity of optical characteristics of zero-dimensional semiconductors.
Colloidal quantum dots (QDs) are attractive materials for implementing solution-processable laser diodes. In addition to being compatible with inexpensive chemical techniques, they offer multiple advantages derived from a 0D character of their electronic states including size-tunable emission wavelengths and low optical-gain thresholds. Here we report on the realization of amplified spontaneous emission (ASE) with electrically driven QDs – an important milestone towards a QD laser diode. This effect is realized using compact, continuously graded QDs with strongly suppressed Auger recombination incorporated into a low-loss photonic waveguide integrated into a pulsed, high-current density light-emitting diode.
Single photon emitters (SPE) are essential building blocks for proposed quantum technologies. An ideal SPE emits only one photon at a time and the photons are indistinguishable. CdSe core/shell quantum dots (QDs) provide a physical system that approximates a SPE. We discuss progress to improve the SPE properties of CdSe QDs. We present synthesis and spectroscopic analysis of wurtzite-CdSe/CdS, mixed shell CdSe/Cd_0.5Zn_0.5S, and continuously graded CdSe/Cd_xZn_1-xSe/ZnSe_yS_1-y QDs. We compare their behaviors at room and cryogenic temperatures using photoluminescence spectroscopy, Michelson interferometry, and Hanbury Brown and Twiss interferometry. We also address efforts to mitigate undesirable behaviors observed in QDs such as charging.
The realization of solution-processable laser diodes will open new opportunities in a range of photonic and optoelectronic applications. Colloidal quantum dots (QDs) have gained considerable attention for attaining this goal because of excellent optical properties such as readily tunable emission wavelengths and a near-unity photoluminescence quantum yield. However, demonstration of electrically pumped lasers using colloidal QDs has not been achieved mainly due to their fast nonradiative Auger recombination that plagues generation of optical gain. Important milestones toward QD laser diodes have been achieved as a result of great progress in suppressing Auger recombination; achievement of low lasing threshold under optical excitation and population inversion and optical gain with electrical injection. These achievements guide us to the next step, which is the incorporation of an optical cavity into the LED structure. Here, we demonstrate a dual functional QD-LED with an integrated optical cavity as a promising device platform for realizing solution-processable laser diodes. These devices work well as both LEDs and optically pumped lasers as a result of careful optimization of refractive-index profile across the device. The remaining challenges to realize QD laser diodes will be also discussed.
Understanding and controlling carrier transport and recombination dynamics in colloidal quantum dot films is key to their application in electronic and optoelectronic devices. Towards this end, we have conducted transient photocurrent measurements to monitor transport through quantum confined band edge states in lead selenide quantum dots films as a function of pump fluence, temperature, electrical bias, and surface treatment. Room temperature dynamics reveal two distinct timescales of intra-dot geminate processes followed by non-geminate inter-dot processes. The non-geminate kinetics is well described by the recombination of holes with photoinjected and pre-existing electrons residing in mid-gap states. We find the mobility of the quantum-confined states shows no temperature dependence down to 6 K, indicating a tunneling mechanism of early time photoconductance. We present evidence of the importance of the exciton fine structure in controlling the low temperature photoconductance, whereby the nanoscale enhanced exchange interaction between electrons and holes in quantum dots introduces a barrier to charge separation. Finally, side-by-side comparison of photocurrent transients using excitation with low- and high-photon energies (1.5 vs. 3.0 eV) reveals clear signatures of carrier multiplication (CM), that is, generation of multiple excitons by single photons. Based on photocurrent measurements of quantum dot solids and optical measurements of solution based samples, we conclude that the CM efficiency is unaffected by strong inter-dot coupling. Therefore, the results of previous numerous spectroscopic CM studies conducted on dilute quantum dot suspensions should, in principle, be reproducible in electronically coupled QD films used in devices.
Multicarrier dynamics in colloidal quantum dots (QDs) are normally controlled by nonradiative Auger recombination wherein the energy of an electron-hole pair is converted not into a photon but instead transferred to a third carrier (an electron or a hole). Auger decay is extremely fast in QDs (time scales of tens-to-hundreds of picoseconds) due to both close proximity between interacting charges and elimination of restrictions imposed by translational momentum conservation. Photoluminescence (PL) quenching by nonradiative Auger processes complicates realization of applications that require high emissivity of multicarrier states such as light-emitting diodes (LEDs) and lasers. Therefore, the development of “Auger-recombination-free” QDs is an important current challenge in the field of colloidal nanostructures.
Previous single-dot spectroscopic studies have indicated a significant spread in Auger lifetimes across an ensemble of nominally identical QDs. 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 QD 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. We observe that while having essentially no effect on single-exciton decay, the interfacial alloy layer leads to a systematic increase in the biexciton lifetime indicating suppression of Auger recombination. We demonstrate that using QDs with “engineered interfaces” we can considerably improve the performance of QD LEDs and lasers.
Energy-resolving gamma-ray detectors are of particular interest for the detection of illicit radioactive materials at border crossings and other portals because they offer fast, contactless screening that can discriminate between dangerous and benign materials. Among detector classes, scintillators offer an intriguing balance between cost and performance, but current technologies rely on single-crystal materials that are not scalable to portal-relevant detector sizes. Thus, there is a recognized need for novel, processible, high-performance scintillating materials or composites. Composites based on semiconductor nanocrystal quantum dots (QDs) are of interest because of their potentially high gamma-stopping power, high emission quantum yields, and low-cost solution synthesis and processing. Yet the performance of these and other granular nanomaterials has not met expectations. We suggest that this is due to the general lack of insight into the gamma-to-photons transduction process within these inherently more complex materials, which reduces the development and refinement of candidates to simple trial-and-error. Here, we describe the development of ultrafast transient cathodoluminescence as a unique spectroscopic tool for probing the population of excited states formed within a material during scintillation, and thus determining the major sources of energy loss. Our analysis shows that in the case of CdSe/ZnS core/shell QDs, any efficiency loss due to previously blamed factors of low-stopping power and high reabsorptive losses are likely dwarfed by the losses attributable to efficient, non-radiative Auger recombination. We examine how we reached this conclusion, and how this insight defines the characteristics needed in the next generation of scintillating QD composites.
Using semiconductor nanocrystals (NCs) one can produce extremely strong spatial confinement of electronic wave functions not accessible with other types of nanostructures. As a result, NCs exhibit important physical properties which, in combination with the chemical stability and solution processability, make this class of functional materials particularly appealing for several technological fields, such as solid-state lighting, lasers, photovoltaics, and electronics. Generally, the tunability of their physical properties is achieved through particle-size control of the quantum confinement effect. Wavefunction engineering adds a degree of freedom for manipulating the physical properties of NCs by selectively confining the carriers in specific domains of the material, thereby controlling the spatial overlap between the electron and hole wavefunctions. This design has been applied to several material systems in different geometries and has been shown to successfully control the emission energy and recombination dynamics as well as to reduce nonradiative Auger recombination, a process in which, as a consequence of strong spatial confinement, the energy of one electron-hole pair is nonradiatively transferred to a third charge carrier. The focus of this presentation is on nanocrystal heterostructures that comprise a small CdSe core overcoated with a thick shell of wider-gap CdS. These quasi-type II structures show greatly suppressed Auger recombination, which allows us to realize broadband optical gain (extends over 500 meV)1, and are a remarkable class of model compounds for investigating the influence of nanoengineered electron-hole overlap on the exciton fine structure.2 We indeed recently showed that this quasi-type II motif can be used to tune the energy splitting between optically active (“bright”) and optically passive (“dark”) excitons due to strong electron-hole exchange interaction, which is typical of quantum-confined semiconductor nanocrystals. This design provides a new tool for controlling excitonic dynamics including absolute recombination time scales and temperature and magnetic field dependences separately from the confinement energy.
As a result of reduced Auger recombination, in combination with essentially complete suppression of energy-transfer in thick-shell NCs films, we recently fabricated bright, monochrome LEDs based on these nanostructures. Our results indicate that the luminance and efficiency can be improved dramatically by increasing the shell thickness without detrimental effects of increased turn-on voltage.3 Detailed structural and spectroscopic studies reveal a crucial role of interfaces on the Auger recombination process ion these heterostructures. Specifically, we observe a sharp transition to Auger-recombination-free behavior for shell thickness ~1.8-2.5 nm, accompanied by the development of an intense phonon mode characteristic of a CdSeS alloy.4 These results suggest that the likely reason for suppressed Auger recombination in these nanostructures is the “smoothing out” of the otherwise sharp confinement potential due to formation of a graded interfacial CdSeS layer between the CdSe core and the CdS shell, as was recently proposed by theoretical calculations by Cragg and Efros.5
We reported for the first time that key nanocrystal quantum dot (NQD) optical properties - quantum yield, photobleaching and blinking - can be rendered independent of NQD surface chemistry and environment by growth of a very thick, defect-free inorganic shell (Chen, et al. J. Am. Chem. Soc. 2008). Here, we show the precise shell-thickness
dependence of these effects. We demonstrate that 'giant-shell' NQDs can be largely non-blinking for observation times as long as 54 minutes and that on-time fractions are independent of experimental time-resolution from 1-200 ms. These effects are primarily demonstrated on (CdSe)CdS (core)shell NQDs, but we also show that alloyed shells comprising CdxZn1-xS and terminated with a non-cytotoxic ZnS layer exhibit similar properties. The mechanism for suppressed
blinking and dramatically enhanced stability is attributed to both effective isolation of the NQD core excitonic
wavefunction from the NQD surface, as well as a quasi-Type II electronic structure. The unusual electronic structure
provides for effective spatial separation of the electron and hole into the shell and core, respectively, and, thereby, for
reduced efficiencies in non-radiative Auger recombination.
In this paper 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 dressed interaction explicitly for an important example of metal-dielectric nanoshells which
exhibits a rich resonant behavior in magnitude and phase. This interaction is used to describe the nanoplasmonic-enhanced
F¨orster resonant energy transfer (FRET) between nanocrystal quantum dots near a nanoshell.
We report a novel type of nanocomposite of conjugated polymer (regio-regular polythiophene) with infrared-sensitive, PbSe quantum dots (QD), which have size-tunable lowest-energy absorption bands between 0.3 and 1 eV. Thin film devices show very good diode characteristics and sizable photovoltaic response with an open circuit voltage, Voc, of ~ 0.3-0.4 V and short circuit current density, Jsc, of ~ 0.2mA/cm2, which is significantly higher than recently reported in PbS QD-based devices. This is the evidence of a quite efficient photoinduced charge transfer between the polymer and QD, with infrared sensitivity. Photocurrent under reverse bias is significantly enhanced to Jph ~ 1 mA/cm2 indicating that the polythiophene/PbSe QD system can be used as effective infrared photodetectors. Detailed spectroscopic studies of photoresponse over a wide spectral range are presented. Quenching of photoluminescence by PbSe QDs has also been studied to gain more understanding of energy and charge transfer in this system.
Using subpicosecond transient absorption spectroscopy, we have investigated the primary photoexcitations in thin films and solution of several phenylene-based conjugated polymers and an oligomer. We identify two features in the transient absorption spectra and dynamics that are common to all of the materials which we have studied from this family. The first spectral feature is a photoinduced absorption (PA) band peaking near 1 eV which has intensity-dependent dynamics which match the stimulated emission dynamics exactly over two orders of magnitude in excitation density. This band is associated with singlet intrachain excitons. The second spectral feature (observed only in thin films and aggregated solutions) is a PA band peaking near 1.8 eV, which is longer-lived than the 1 eV exciton PA band, and which has dynamics that are independent (or weakly-dependent) on excitation density. This feature is attributed to charge separated (interchain) excitations. These excitations are generated through a bimolecular process. By comparing to samples in which charged excitations are created deliberately by doping with C6O, we assign these secondary species as bound polarons.
We present our recent advances toward the development of high-performance solid-state optical limiting devices using reverse saturable absorption (RSA) dyes doped into optical host materials. Femtosecond transient absorption spectroscopy was employed to determine both the spectral regions of strong RSA, and the singlet-triplet excited-state dynamics. The optical limiting in the visible spectrum in both metallo-phthalocyanines and metallo-porphyrins is due to a combination of singlet and triplet RSA. Optical limiting performance was studied for RSA dyes in dual tandem limiters (both in solution and solid-state). Our best results in the solid-state yielded an attenuation of 400X, and a damage threshold of up to several mJ at f/5 focusing. The optical limiting at f/5 is further enhanced, particularly in the solid-state, by self-defocusing thermal nonlinearities.
We report femtosecond transient-absorption studies of a five-ring oligomer of polyphenylenevinylene prepared in two different forms: as solid-state films and dilute solutions. Both types of samples exhibit a photoinduced absorption (PA) band with dynamics which closely match those of the stimulated emission (SE), demonstrating unambiguously that these features originate from the same species, namely from intrachain singlet excitons. Photochemical degradation of the solid-state samples is demonstrated to dramatically shorten the SE dynamics above a moderate incident pump fluence, whereupon the dynamics of the SE and the long- wavelength PA no longer coincide. In contrast to solutions, solid-state films exhibit an additional short-wavelength PA band with pump-independent dynamics, indicating the efficient formation of non-emissive inter-chain excitons. Correlations in the subpicosecond dynamics of the two PA features, as well as the pump intensity-dependence provide strong evidence that the formation of inter-chain excitons is mediated by intrachain two-exciton states. At high pump levels, we see a clear indication of interaction between excited states also in dilute solutions. This is manifested as a superlinear pump-dependence and shortening of the decay dynamics of the SE. We attribute this behavior to the formation of biexcitons resulting from coherent interaction between two excitons on a single chain.
We present recent results of broadband femtosecond (fs) transient absorption (TA) and broadband nanosecond (ns) optical limiting (OL) studies of C60 and derivatized C60. Improvements in measurement techniques for fs TA spectra provide sensitivity to 10-5 in differential transmission, allowing detailed comparison of excited-state spectra with established energy level diagrams, as well as comparison of the ratio of triplet to singlet excited-state absorption cross sections from TA spectra with those obtained by modeling time transients at different wavelengths. For derivatized fullerenes, which provide enhanced solubility and a ground-state absorption extended into the IR compared with C60 there is no spectral region where the triplet absorption cross section dominates the singlet as strongly as in C60. Wavelength-dependent studies show that the OL response improves monotonically at longer wavelengths, demonstrating broadband limiting in all 6,6 mono-adducts and neat C60. We report new approaches to processing sol-gel encapsulated fullerenes to improve the OL performance of solid-state materials to approach the response of solution limiters.
We compare detailed dynamics of the excited-state absorption for C60 in solution, thin films, and entrapped in an inorganic sol-gel glass matrix. Our results demonstrate that the microscopic morphology of the C60 molecule plays a crucial role in determining the relaxation dynamics. This is a key factor for applications in optical limiting for nanosecond pulses using reverse saturable absorption. We find that the dynamics of the C60-glass composite occur on long timescales, comparable to that in solution; thin film samples, by contrast, show rapid decay. These results demonstrate that the C60-sol-gel glass composites contain C60 in a molecular dispersion, and are suitable candidates for solid-state optical limiting. Multispectral analysis of the decay dynamics in solution allows accurate determination of both the intersystem crossing time and the relative strengths of the singlet and triplet excited-state cross sections as a function of wavelength from 450-950 nm. The triplet excited-state cross section is greater than that for the singlet excited-state over the range form 620-810 nm.
We present measurements of intensity-dependent transmission (optical limiting), Z-scan, and time resolved excited-state absorption, for several fullerenes and fullerene derivatives in solution, and for Co thin films and porous glass composites. Specific derivatives investigated include the isomers 5,6 pheny1-C-butyñc acid cholesteryl ester (5,6 PCBCR) and 6,6 PCBCR. We report optical limiting results for several wavelengths in the visible and near-infrared. Excited-state absorption is measured from 500-1000 nm with <1 picosecond resolution. The spectral and temporal information about the excited state can be used in combination with ground state measurements of absorption cross section to predict optical limiting behavior due to reverse saturable absorption. These predictions are confirmed by intensity dependent transmission measurements made at different wavelengths. We find that fullerenes functionalized for optimized solubility display enhanced optical limiting properties.
Duncan McBranch, Laura Smilowitz, Victor Klimov, Aaron Koskelo, Jeanne Robinson, Benjamin Mattes, Jan Hummelen, Fred Wudl, James Withers, Nicholas Borrelli
We report the ground-state and excited-state optical absorption spectra in the visible and near infrared for several substituted fullerenes and higher fullerenes in toluene solutions. Based on these measurements, broadband predictions of the optical limiting performance of these molecules can be deduced. These predictions are then tested in the wavelength range from 532 nm to 700 nm in intensity-dependent transmission measurements. We observe optical limiting in all fullerenes measured; higher fullerenes show the greatest potential for limiting in the near infrared (650 - 1000 nm), while substituted C60 shows optimal limiting in the visible (450 - 700 nm). We observe dramatically reduced limiting for solid forms of C60 (thin films and C60-doped porous glasses), indicating that efficient optical limiting in fullerenes requires true molecular solutions.
Changes in the transmission of commercially available semiconductor doped glasses and porous silicon layers are studied by using picosecond pump and probe measurements. Bleaching bands attributed to the saturation of optical transitions in semiconductor nanostructures (crystallites or wires) are registered in time-resolved differential transmission spectra for both of the materials under investigation. It is found that porous silicon exhibits strong and fast optical nonlinearity (third-order nonlinear susceptibility is about 10-s esu; transmission recovery time is 30 - 40 ps) which can be used for optical switching.
Transmission spectra recovery of CdSe, CdSxSe1-x nanocrystals and porous silicon wires optically excited by ultrashort laser pulses have been studied with picosecond time resolution using pump and probe technique. The transitions between levels of electrons and holes spatially confined within nanocrystals and thin wires were observed as bleaching bands in nonlinear transmission spectra (saturation effect). Spectra and values of third order resonant susceptibility were determined for nanocrystals of different size and porous silicon using the experimentally measured differential transmission spectra.
Transmission spectra recovery and time-resolved luminescence of CdSe microcrystals optically excited by ultrashort laser pulses have been studied with picosecond time resolution. The transitions between levels of electrons and holes spatially confined within microcrystals were observed as bleaching bands in nonlinear transmission spectra. The significant shortening of the carrier lifetime in microcrystals of smaller size was detected. The regime of laser emission at the transition between the lowest levels of size quantization was achieved.
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