Nanoparticles, Nanocrystals, and Quantum Dots: What are the Implications of Size in Colloidal Nanoscale Materials?

Overcoming the EQE × Li-Fi Frequency Constraint by Modulating the PbS CQDs Distribution in Perovskite Film

Advanced photodetectors are crucial for high-fidelity optical communication. However, the tradeoff between high external quantum efficiency (EQE) and high light fidelity (Li-Fi) frequency often limits data transmission accuracy and timeliness. Here, we report a photodetector consisting of lead sulfide (PbS) colloidal quantum dots (CQDs) with near-infrared responsiveness and perovskite frameworks responsible for the charge transport to overcome the EQE × Li-Fi constraint. Optimizing the PbS CQDs distribution and trap depth in the perovskite layer enhances charge injection, achieving a device gain of 11892% for 1200 nm photons and a response frequency of 24 kHz at -2 V. The device exhibits a record EQE × Li-Fi frequency product of 106 Hz. We have applied the detector to near-infrared optical communications at a data transfer rate of 2000 bits per second (2 kbps) to demonstrate the advances in high fidelity, the device retains over 98% of the original waveform information in its output.

Size Dependent Photocatalytic Activity of Mesoporous ZnIn2S4 Nanocrystal Networks

Understanding of the band-edge electronic structure and charge-transfer dynamics in size-confined nanostructures is vital in designing new materials for energy conversion applications, including green hydrogen production, decomposition of organic pollutants and solar cells. In this study, a series of mesoporous materials comprising continuous networks of linked zinc indium sulfide (ZnIn2S4) nanocrystals with a tunable diameter (ranging from 4 to 12 nm) is reported. These nanomaterials demonstrate intriguing size-dependent electronic properties, charge-transfer kinetics and photocatalytic behaviors. Our extensive characterizations uncover strong size effects on the catalytic activity of constituent ZnIn2S4 nanocrystals in the photochemical hydrogen evolution reaction. As an outcome, the optimized single-component ZnIn2S4 mesostructure produces hydrogen at a 7.8 mmol gcat -1 h-1 release rate under ultraviolet (UV)-visible light irradiation associated with an apparent quantum yield (AQY) of 17.2% at 420 ± 10 nm, far surpassing its microstructured counterpart by a factor of 10.7×. These findings provide a valuable perspective for the rational design of semiconductor nanostructures through synthetic engineering, aiming at the development of high-performance catalysts for zero-carbon energy-related applications.

Advances in Nanoparticles as Non-Viral Vectors for Efficient Delivery of CRISPR/Cas9

The clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 system is a gene-editing technology. Nanoparticle delivery systems have attracted attention because of the limitations of conventional viral vectors. In this review, we assess the efficiency of various nanoparticles, including lipid-based, polymer-based, inorganic, and extracellular vesicle-based systems, as non-viral vectors for CRISPR/Cas9 delivery. We discuss their advantages, limitations, and current challenges. By summarizing recent advancements and highlighting key strategies, this review aims to provide a comprehensive overview of the role of non-viral delivery systems in advancing CRISPR/Cas9 technology for clinical applications and gene therapy.

Unraveling the excitonics of light emission from metal-halide perovskite quantum dots

Metal halide semicondictor perovskites have been under intense investigation for their promise in light absorptive applications like photovoltaics. They have more recently experienced interest for their promise in light emissive applications. A key aspect of perovskites is their glassy, ionic lattice that exhibits dynamical disorder. One possible result of this dynamical disorder is their strong coupling between electronic and lattice degrees of freedom which may confer remarkable properties for light emission such as defect tolerance. How does the system, comprised of excitons, couple to the bath, comprised of lattice modes? How does this system-bath interaction give rise to novel light emissive properties and how do these properties give insight into the nature of these materials? We review recent work from this group in which time-resolved photoluminescence spectroscopy is used to reveal such insights. Based upon a fast time resolution of 3 ps, energy resolution, and temperature dependence, a wide variety of insights are gleaned. These insights include: lattice contributions to the emission linewidths, multiexciton formation, hot carrier cooling, excitonic fine structure, single dot superradiance, and a breakdown of the Condon approximation, all due to complex structural dynamics in these materials.

Strongly-confined colloidal lead-halide perovskite quantum dots: from synthesis to applications

Colloidal semiconductor nanocrystals enable the realization and exploitation of quantum phenomena in a controlled manner, and can be scaled up for commercial uses. These materials have become important for a wide range of applications, from ultrahigh definition displays, to solar cells, quantum computing, bioimaging, optical communications, and many more. Over the last decade, lead-halide perovskite nanocrystals have rapidly gained prominence as efficient semiconductors. Although the majority of studies have focused on large nanocrystals in the weak- to intermediate-confinement regime, quantum dots (QDs) in the strongly-confined regime (with sizes smaller than the Bohr diameter, which ranges from 4-12 nm for lead-halide perovskites) offer unique opportunities, including polarized light emission and color-pure, stable luminescence in the region that is unattainable by perovskites with single-halide compositions. In this tutorial review, we bring together the latest insights into this emerging and rapidly growing area, focusing on the synthesis, steady-state optical properties (including exciton fine-structure splitting), and transient kinetics (including hot carrier cooling) of strongly-confined perovskite QDs. We also discuss recent advances in their applications, including single photon emission for quantum technologies, as well as light-emitting diodes. We finish with our perspectives on future challenges and opportunities for strongly-confined QDs, particularly around improving the control over monodispersity and stability, important fundamental questions on the photophysics, and paths forward to improve the performance of perovskite QDs in light-emitting diodes.

Synthesis, characterization, and practical applications of perovskite quantum dots: recent update

This review paper provides an in-depth analysis of Perovskite quantum dots (PQDs), a class of nanomaterials with unique optical and electronic properties that hold immense potential for various technological applications. The paper delves into the structural characteristics, synthesis methods, and characterization techniques of PQDs, highlighting their distinct advantages over other Quantum Dots (QDs). Various applications of PQDs in fields such as solar cells, LEDs, bioimaging, photocatalysis, and sensors are discussed, showcasing their versatility and promising capabilities. The ongoing advancements in PQD research and development point towards a bright future for these nanostructures in revolutionizing diverse industries and technologies.

Strong high-energy exciton electroluminescence from the light holes of polytypic quantum dots

High-energy exciton emission could allow single-component multi-colour display or white light-emitting diodes. However, the thermal relaxation of high-energy excitons is much faster than the photon emission of them, making them non-emissive. Here, we report quantum dots with light hole-heavy hole splitting exhibiting strong high-energy exciton electroluminescence from high-lying light holes, opening a gate for high-performance multi-colour light sources. The high-energy electroluminescence can reach 44.5% of the band-edge heavy-hole exciton emission at an electron flux density Φe of 0.71 × 1019 s-1 cm-2 - 600 times lower than the photon flux density Φp (4.3 × 1021 s-1 cm-2) required for the similar ratio. Our simulation and experimental results suggest that the oscillator strength of heavy holes reduces more than that of light holes under electric fields. We attribute this as the main reason for strong light-hole electroluminescence. We observe this phenomenon in both CdxZn1-xSe-ZnS and CdSe-CdS core-shell quantum dots exhibiting large light hole-heavy hole splittings.

Ultrafast Dynamics Across Pressure-Induced Electronic State Transitions, Fluorescence Quenching, and Bandgap Evolution in CsPbBr3 Quantum Dots

This work investigates the impact of pressure on the structural, optical properties, and electronic structure of CsPbBr3 quantum dots (QDs) using steady-state photoluminescence, steady-state absorption, and femtosecond transient absorption spectroscopy, reaching a maximum pressure of 3.38 GPa. The experimental results indicate that CsPbBr3 QDs undergo electronic state (ES) transitions from ES-I to ES-II and ES-II to ES-III at 0.38 and 1.08 GPa, respectively. Intriguingly, a mixed state of ES-II and ES-III is observed within the pressure range of 1.08-1.68 GPa. The pressure-induced fluorescence quenching in ES-II is attributed to enhanced defect trapping and reduced radiative recombination. Above 1.68 GPa, fluorescence vanishes entirely, attributed to the complete phase transformation from ES-II to ES-III in which radiative recombination becomes non-existent. Notably, owing to stronger quantum confinement effects, CsPbBr3 QDs exhibit an impressive bandgap tuning range of 0.497 eV from 0 to 2.08 GPa, outperforming nanocrystals by 1.4 times and bulk counterparts by 11.3 times. Furthermore, this work analyzes various carrier dynamics processes in the pressure-induced bandgap evolution and electron state transitions, and systematically studies the microphysical mechanisms of optical properties in CsPbBr3 QDs under pressure, offering insights for optimizing optical properties and designing novel materials.

DNA and Nanomaterials: A Functional Combination for DNA Sensing

Recent decades have experienced tough situations due to the lack of reliable diagnostic facilities. The most recent cases occurred during the pandemic, where researchers observed the lack of diagnostic facilities with precision. Microorganisms and viral disease's ability to escape diagnosis has been a global challenge. DNA always has been a unique moiety with a strong and precise base-paired structure. DNA in human and foreign particles makes identification possible through base pairing. Since then, researchers have focused heavily on designing diagnostic assays targeting DNA in particular. Moreover, DNA nanotechnology has contributed vastly to designing composite nanomaterials by combining DNA/nucleic acids with functional nanomaterials and inorganic nanoparticles exploiting their physicochemical properties. These nanomaterials often exhibit unique or enhanced properties due to the synergistic activity of the many components. The capabilities of DNA and additional nanomaterials have shown the combination of robust and advanced tailoring of biosensors. Preceding findings state that the conventional strategies have exhibited certain limitations such as a low range of target detection, less biodegradability, subordinate half-life, and high susceptibility to microenvironments; however, a DNA-nanomaterial-based biosensor has overcome these limitations meaningfully. Additionally, the unique properties of nucleic acids have been studied extensively due to their high signal conduction abilities. Here, we review recent studies on DNA-nanomaterial-based biosensors, their mechanism of action, and improved/updated strategies in vivo and in situ. Furthermore, this review highlights the recent methodologies on DNA utilization to exploit the interfacial properties of nanomaterials in DNA sensing. Lastly, the review concludes with the limitations/challenges and future directions.

Hot Excitons Cool in Metal Halide Perovskite Nanocrystals as Fast as CdSe Nanocrystals

The idea of phonon bottlenecks has long been pursued in nanoscale materials for their application in hot exciton devices, such as photovoltaics. Decades ago, it was shown that there is no quantum phonon bottleneck in strongly confined quantum dots due to their physics of quantum confinement. More recently, it was proposed that there are hot phonon bottlenecks in metal halide perovskites due to their physics. Recent work has called into question these bottlenecks in metal halide perovskites. Here, we compare hot exciton cooling in a range of sizes of CsPbBr3 nanocrystals from weakly to strongly confined. These results are compared to strongly confined CdSe quantum dots of two sizes and degrees of quantum confinement. CdSe is a model system as a ruler for measuring hot exciton cooling being fast, by virtue of its efficient Auger-assisted processes. By virtue of 3 ps time resolution, the hot exciton photoluminescence can now be directly observed, which is the most direct measure of the presence of hot excitons and their lifetimes. The hot exciton photoluminescence decays on nearly the same 2 ps time scale on both the weakly confined perovskite and the larger CdSe quantum dots, much faster than the 10 ps cooling predicted by transient absorption experiments. The smaller CdSe quantum dot has still faster cooling, as expected from quantum size effects. The quantum dots of perovskites show extremely fast hot exciton cooling, decaying faster than detection limits of <1 ps, even faster than the CdSe system, suggesting the efficiency of Auger processes in these metal halide perovskite nanocrystals and especially in their quantum dot form. These results across a range of sizes of nanocrystals reveal extremely fast hot exciton cooling at high exciton density, independent of composition, but dependent upon size. Hence these metal halide perovskite nanocrystals seem to cool heavily following quantum dot physics.

Enhancing Multiexcitonic Emission in Metal-Halide Perovskites by Quantum Confinement

Semiconductor metal halide perovskite nanocrystals have been under intense investigation for their promise in a variety of optoelectronic applications, which arises from their remarkable properties of defect tolerance and efficient light emission. Recently, quantum dot versions of perovskite nanocrystals have been available, enabling investigation of how quantum size effects control optical function and performance in these quantum dots (QD), past their well-known covalent II-VI analogues. We perform time-resolved photoluminescence (t-PL) experiments on CsPbBr3 perovskite nanocrystals spanning in diameter from 5.8 nm strongly confined quantum dots to 18 nm weakly confined quantum dots. Experiments are performed with sufficient time resolution of 3 ps to observe the interaction energies and recombination kinetics from excitons to multiexcitons. Comparing the same sized QD reveals that perovskite QD have a larger radiative rate constant for emission from X than CdSe QD due to a larger oscillator strength. The multiexciton (MX) regime reveals that perovskite QD emit brightly and with more focused bandwidth than equivalent sized CdSe QD enabling more spectrally pure brightness. The MX kinetics reveals that the perovskite QD maintain efficient radiative decay, effectively competing with Auger recombination. These experiments reveal that the strongly confined QD of perovskites can be efficient multiexcitonic emitters, such as in high brightness light emitting diodes, especially in the blue.

Perturbed free induction decay obscures early time dynamics in two-dimensional electronic spectroscopy: The case of semiconductor nanocrystals

Two-dimensional electronic spectroscopy (2DES) has recently been gaining popularity as an alternative to the more common transient absorption spectroscopy due to the combination of high frequency and time resolution of 2DES. In order to advance the reliable analysis of population dynamics and to optimize the time resolution of the method, one has to understand the numerous field matter interactions that take place at an early and negative time. These interactions have historically been discussed in one-dimensional spectroscopy as coherent artifacts and have been assigned to both resonant and non-resonant system responses during or before the pulse overlap. These coherent artifacts have also been described in 2DES but remain less well-understood due to the complexity of 2DES and the relative novelty of the method. Here, we present 2DES results in two model nanocrystal samples, CdSe and CsPbI₃. We demonstrate non-resonant signals due to solvent response during the pulse overlap and resonant signals, which we assign to perturbed free induction decay (PFID), both before and during the pulse overlap. The simulations of the 2DES response functions at early and negative time delays reinforce the assignment of the negative time delay signals to PFID. Modeling reveals that the PFID signals will severely distort the initial picture of the resonant population dynamics. By including these effects in models of 2DES spectra, one is able to push forward the extraction of early time dynamics in 2DES.

Direct Observation of Ultrafast Lattice Distortions during Exciton-Polaron Formation in Lead Halide Perovskite Nanocrystals

The microscopic origin of slow hot-carrier cooling in lead halide perovskites remains debated and has direct implications for applications. Slow hot-carrier cooling of several picoseconds has been attributed to either polaron formation or a hot-phonon bottleneck effect at high excited carrier densities (>1018 cm-3). These effects cannot be unambiguously disentangled with optical experiments alone. However, they can be distinguished by direct observations of ultrafast lattice dynamics, as these effects are expected to create qualitatively distinct fingerprints. To this end, we employ femtosecond electron diffraction and directly measure the sub-picosecond lattice dynamics of weakly confined CsPbBr3 nanocrystals following above-gap photoexcitation. While we do not observe signatures of a hot-phonon bottleneck lasting several picoseconds, the data reveal a light-induced structural distortion appearing on a time scale varying between 380 and 1200 fs depending on the excitation fluence. We attribute these dynamics to the effect of exciton-polarons on the lattice and the slower dynamics at high fluences to slower sub-picosecond hot-carrier cooling, which slows down the establishment of the exciton-polaron population. Further analysis and simulations show that the distortion is consistent with motions of the [PbBr3]- octahedral ionic cage, and closest agreement with the data is obtained for Pb-Br bond lengthening. Our work demonstrates how direct studies of lattice dynamics on the sub-picosecond time scale can discriminate between competing scenarios proposed in the literature to explain the origin of slow hot-carrier cooling in lead halide perovskites.

A spectroscopic overview of the differences between the absorbing states and the emitting states in semiconductor perovskite nanocrystals

Semiconductor perovskites have been under intense investigation for their promise in optoelectronic applications and their novel and unique physical properties. There have been a variety of material implementations of perovskites from thin films to single crystals to nanocrystals. The nanocrystal form, in particular, is attractive as it enables solution processing and also spectroscopically probes both absorptive and emissive transitions. Broadly, the literature is comprised of experiments of either form, but the experiments are rarely performed in concert and are not discussed in a unified picture. For example, absorptive experiments are typically transient absorption measurements, which aim to measure carrier kinetics and dynamics. In contrast, the emissive experiments largely focus on excitonic fine structures and coupling to phonons. The time resolved emission experiments report on excited state lifetimes and their dependence on temperature. There are broad differences in the spectroscopy techniques and the questions asked in both classes of experiments. Yet there is one measure in common that suggests there are mysteries in our understanding of how the absorbing and emitting states are connected. The linewidth of emission spectra is always larger than the linewidth of absorption spectra. The question of the physics underlying linewidths is complex and is one of the central issues in perovskite nanocrystals. So why are the absorptive and emissive linewidths different? At present even this simple question has no clear answer. The more complex questions of the structure and dynamics of absorptive and emissive states are even more ambiguous. Hence there is a need to connect these experiments and the relevant states. Here, we provide an overview of the salient absorptive and emissive spectroscopy techniques in an effort to begin connecting these two disparate areas of inquiry.

Modification of Liquid Separation Membranes Using Multidimensional Nanomaterials: Revealing the Roles of Dimension Based on Classical Titanium Dioxide

Membrane technology has become increasingly popular and important for separation processes in industries, as well as for desalination and wastewater treatment. Over the last decade, the merger of nanotechnology and membrane technology in the development of nanocomposite membranes has emerged as a rapidly expanding research area. The key motivation driving the development of nanocomposite membranes is the pursuit of high-performance liquid separation membranes that can address the bottlenecks of conventionally used polymeric membranes. Nanostructured materials in the form of zero to three-dimensions exhibit unique dimension-dependent morphology and topology that have triggered considerable attention in various fields. While the surface hydrophilicity, antibacterial, and photocatalytic properties of TiO2 are particularly attractive for liquid separation membranes, the geometry-dependent properties of the nanocomposite membrane can be further fine-tuned by selecting the nanostructures with the right dimension. This review aims to provide an overview and comments on the state-of-the-art modifications of liquid separation membrane using TiO2 as a classical example of multidimensional nanomaterials. The performances of TiO2-incorporated nanocomposite membranes are discussed with attention placed on the special features rendered by their structures and dimensions. The innovations and breakthroughs made in the synthesis and modifications of structure-controlled TiO2 and its composites have enabled fascinating and advantageous properties for the development of high-performance nanocomposite membranes for liquid separation.

Synthesis of Weakly Confined, Cube-Shaped, and Monodisperse Cadmium Chalcogenide Nanocrystals with Unexpected Photophysical Properties

Zinc-blende CdSe, CdS, and CdSe/CdS core/shell nanocrystals with a structure-matched shape (cube-shaped, edge length ≤30 nm) are synthesized via a universal scheme. With the edge length up to five times larger than exciton diameter of the bulk semiconductors, the nanocrystals exhibit novel properties in the weakly confined size regime, such as near-unity single exciton and biexciton photoluminescence (PL) quantum yields, single-nanocrystal PL nonblinking, mixed PL decay dynamics of exciton and free carriers with sub-microsecond monoexponential decay lifetime, and stable yet extremely narrow PL full width at half maximum (FWHM < 0.1 meV) at 1.8 K. Their monodisperse edge length, shape, and facet structure enable demonstration of unexpected yet size-dependent PL properties at room temperature, including unusually broad and abnormally size-dependent PL FWHM (∼100 meV), nonmonotonic size dependence of PL peak energy, and dual-peak single-exciton PL. Calculations suggest that these unusual properties should be originated from the band-edge electron/hole states of the dynamic-exciton, whose exciton binding energy is too small to hold the photogenerated electron-hole pair as a bonded Wannier exciton in a weakly confined nanocrystal.

Three Millennia of Nanocrystals

The broad deployment of nanotechnology and nanomaterials in modern society is increasing day by day to the point that some have seen in this process the transition from the Silicon Age to a new Nano Age. Nanocrystals─a distinct class of nanomaterials─are forecast to play a pivotal role in the next generation of devices such as liquid crystal displays, light-emitting diodes, lasers, and luminescent solar concentrators. However, it is not to be forgotten that this cutting-edge technology is rooted in empirical knowledge and craftsmanship developed over the millennia. This review aims to span the major applications in which nanocrystals were consistently employed by our forebears. Through an analysis of these examples, we show that the modern-age discoveries stem from multimillennial experience passed on from our proto-chemist ancestors to us.

Chemical Transformations of Colloidal Semiconductor Nanocrystals Advance Their Applications

Recently, colloidal semiconductor nanocrystals (NCs) are finding more and more applications in optoelectronic devices. Their usage, however, is still very far from the great potential already demonstrated in many fields owing to their unique features. While researchers are still struggling to achieve a wider gamut of different semiconductor nanomaterials with more controllable properties, the library of already existing candidates is large enough to harness their potential. Modification of well-studied semiconductor NCs by means of their chemical transformations can greatly advance their practical exploitation. In this Perspective, the main types of chemical transformations represented by ligand and cation exchange reactions and their recent examples are summarized. While ligand exchange is used to adjust the surface of a semiconductor NC, cation exchange allows us to engineer its core composition. Both approaches greatly extend the range of properties of the resulting nanomaterials, advancing their further incorporation into optoelectronic devices.

Power-Dependent Photoluminescence Efficiency in Manganese-Doped 2D Hybrid Perovskite Nanoplatelets

Substitutional metal doping is a powerful strategy for manipulating the emission spectra and excited state dynamics of semiconductor nanomaterials. Here, we demonstrate the synthesis of colloidal manganese (Mn²⁺)-doped organic-inorganic hybrid perovskite nanoplatelets (chemical formula: L₂[APb_1-xMn_xBr₃]_n-1Pb_1-xMn_xBr₄; L, butylammonium; A, methylammonium or formamidinium; n (= 1 or 2), number of Pb_1-xMn_xBr₆^4- octahedral layers in thickness) via a ligand-assisted reprecipitation method. Substitutional doping of manganese for lead introduces bright (approaching 100% efficiency) and long-lived (>500 μs) midgap Mn²⁺ atomic states, and the doped nanoplatelets exhibit dual emission from both the band edge and the dopant state. Photoluminescence quantum yields and band-edge-to-Mn intensity ratios exhibit strong excitation power dependence, even at a very low incident intensity (<100 μW/cm²). Surprisingly, we find that the saturation of long-lived Mn²⁺ dopant sites cannot explain our observation. Instead, we propose an alternative mechanism involving the cross-relaxation of long-lived Mn-site excitations by freely diffusing band-edge excitons. We formulate a kinetic model based on this cross-relaxation mechanism that quantitatively reproduces all of the experimental observations and validate the model using time-resolved absorption and emission spectroscopy. Finally, we extract a concentration-normalized microscopic rate constant for band edge-to-dopant excitation transfer that is ∼10× faster in methylammonium-containing nanoplatelets than in formamidinium-containing nanoplatelets. This work provides fundamental insight into the interaction of mobile band edge excitons with localized dopant sites in 2D semiconductors and expands the toolbox for manipulating light emission in perovskite nanomaterials.

Gradual Carrier Filling Effect in "Green" InGaN/GaN Quantum Dots: Femtosecond Carrier Kinetics with Sequential Two-Photon Absorption

Quantum dots (QDs) allow for a significant amount of strain relaxation, which is helpful in GaN systems where a large lattice mismatch needs to be accommodated. InGaN QDs with a large indium composition are intensively investigated for light emitters requiring longer wavelengths. These are especially important for developing high-efficiency white light sources. Understanding the carrier dynamics in this large lattice-mismatched system is essential to improving the radiative efficiency while circumventing high defect density. This work investigates femtosecond carrier and photon dynamics in self-organized In_0.27Ga_0.73N/GaN QDs grown by molecular beam epitaxy using transient differential absorption spectroscopy, which measures the differential absorption coefficient (Δα) with and without an optical pump. Due to 3D quantum confinement and the small effective mass of InGaN, the low density of states in the conduction band is easily filled with electrons. In contrast, the GaN barrier region is replete with a high density of electrons due to a large effective mass. This contrast in carrier density creates a unique phenomenon in the dynamics, showing a change in the differential absorption coefficient (Δα) sign from negative to positive with time. The ultrafast microscopic processes indicate that right after the optical pump and first photon absorption, the valence (conduction) band states are depleted (replete) of electrons. This ground-state bleaching process makes Δα negative, and the probe beam is not absorbed. The electrons are then gradually transferred from the GaN barrier into InGaN QDs, which absorb the second photon from the probe beam (excited-state absorption), making Δα positive. The presence of excited-state carriers with a long lifetime is indicative of the enhanced availability of carriers for radiative recombination. This effect also promotes stimulated emission and amplified spontaneous emission, which can be used to develop lasers and superluminescent LEDs, respectively. Measurements with multiple pump powers and temperatures further confirm that the efficacy of InGaN QDs is enhanced by this effective mass contrast and 3D reservoir of carriers from the GaN barrier. This effect can be used to improve the internal quantum efficiency of GaN-based light emitters.

Resonance Raman Vibrational Mode Enhancement of Adsorbed Benzenethiols on CdSe Is Predominantly Franck-Condon in Nature and Governed by Symmetry

Here, we report mode-specific resonance Raman enhancements of ligands covalently bound to the surface of colloidal CdSe nanocrystals (NCs). By the systematic comparison of a set of structural derivatives, the extent of resonance Raman enhancement is shown to be directly related to the molecular symmetry of the bound ligands. The enhancement dependence on molecular symmetry is further discussed in terms of Franck-Condon and Herzberg-Teller contributions and their associated selection rules. We further show that resonance Raman may be used to distinguish between possible surface binding motifs of bidentate ligands under continuous wave excitation. More generally, this work demonstrates the usefulness of resonance Raman as a characterization tool when characterizing adsorbed molecular species on semiconductor NC surfaces.