Semiconductor Nanocrystal Quantum Dots Research Articles

(Invited) Transformation Mechanism of Quantum-Sized Semiconductor Nanocrystals Revealed by in Situ Transmission Electron Microscopy

Semiconductor nanocrystal quantum dots are renowned for their remarkable properties, including tunable bandgap, pure color emission with exceptionally narrow bandwidths, and high luminescence efficiency. A thorough understanding of the transformation mechanism of quantum-sized semiconductor nanocrystals is essential for their practical application, as their properties are determined by their structure. This presentation will focus on exploring these mechanisms using in-situ transmission electron microscopy (TEM). The first part of the talk will examine the moisture-induced degradation of quantum-sized semiconductor nanocrystals, highlighting a notable finding: the formation of amorphous intermediates on the surfaces of these nanocrystals during degradation. This insight is crucial for developing strategies to enhance the stability and durability of nanocrystals, thereby extending their practical application. Next, I will discuss the crystal phase transition mechanism in II-VI semiconductor nanocrystals, triggered by off-stoichiometry. This discussion will shed light on the determinants of the crystal phase in these nanocrystals, a key to understanding their functionality. Lastly, the presentation introduces the application of in-situ X-ray scattering. This technique offers structural insights that complement those obtained from in-situ TEM, significantly enriching our understanding of the structural dynamics in semiconductor nanocrystals.

Ligand-Induced Size-Dependent Circular Dichroism in Quantum Dots.

Recent experiments have probed the chiral properties of semiconductor nanocrystal (NC) quantum dots (QDs), but understanding the circular dichroism line shape, excitonic features, and chirality induction mechanism remains a challenge. We propose an atomistic pseudopotential method to model chiral ligand passivated QDs, computing circular dichroism (CD) spectra for CdSe QDs (2.6-3.8 nm). We find strong agreement between calculated and measured line shapes, predicting consistent bisignate line shapes with decreasing CD magnitude as size increases. Our analysis reveals the origin of bisignate line shapes, arising from nondegenerate excitons with opposing angular momenta. We also explore the impact of chiral ligand orientation on QD surfaces, observing changes in the optical activity magnitude and sign. This orientation sensitivity offers the means to distinguish ordered from disordered ligand configurations, facilitating the study of order-disorder transitions at ligand-QD interfaces.

Recent advances in photoelectrochemical hydrogen production using I-III-VI quantum dots.

Photoelectrochemical (PEC) water splitting, recognized for its potential in producing solar hydrogen through clean and sustainable methods, has gained considerable interest, particularly with the utilization of semiconductor nanocrystal quantum dots (QDs). This minireview focuses on recent advances in PEC hydrogen production using I-III-VI semiconductor QDs. The outstanding optical and electrical properties of I-III-VI QDs, which can be readily tuned by modifying their size, composition, and shape, along with an inherent non-toxic nature, make them highly promising for PEC applications. The performance of PEC devices using these QDs can be enhanced by various strategies, including ligand modification, defect engineering, doping, alloying, and core/shell heterostructure engineering. These approaches have notably improved the photocurrent densities for hydrogen production, achieving levels comparable to those of conventional heavy-metal-based counterparts. Finally, this review concludes by addressing the present challenges and future prospects of these QDs, underlining crucial steps for their practical applications in solar hydrogen production.

Correlation between Single-Photon Emission and Size of Cesium Lead Bromide Perovskite Nanocrystals.

Emission photon statistics of semiconductor nanocrystal quantum dots (QDs), including lead halide perovskite nanocrystals (PNCs), are important fundamental and practical optical properties. Single QDs exhibit high-probability single-photon emission owing to the efficient Auger recombination between generated excitons. Because the recombination rate depends on QD size, single-photon emission probability should be size-dependent. Previous studies have researched QDs smaller than their exciton Bohr diameters (twice the Bohr radius of excitons). Here, we investigated the relationship between the single-photon emission behavior and size of CsPbBr3 PNCs to elucidate their size threshold. Simultaneous single-nanocrystal spectroscopy and atomic force microscopy observations on single PNCs with approximately 5-25 nm edge length showed that those smaller than approximately 10 nm, which had size-dependent photoluminescence (PL) spectral shifts, exhibited high-probability single-photon emissions, which decreased linearly with PNC volume. Novel single-photon emission, size, and PL peak correlations of PNCs are important for understanding the relationship between single-photon emission and quantum confinement.

EPR spin trapping of nucleophilic and radical reactions at colloidal metal chalcogenide quantum dot surfaces.

The participation of the surfaces of colloidal semiconductor nanocrystal quantum dots (QDs) in QD-mediated photocatalytic reactions is an important factor that distinguishes QDs from other photosensitizers (e.g. transition metal complexes or organic dyes). Here, we probe nucleophilic and radical reactivity of surface sulfides and selenides of metal chalcogenide (CdSe, CdS, ZnSe, and PbS) QDs using chemical reactions and NMR spectroscopy. Additionally, the high sensitivity of EPR spectroscopy is adapted to study these surface-centered reactions through the use of spin traps like 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) under photoexcitation and thermal conditions. We demonstrate that DMPO likely adds to CdSe QD surfaces under thermal conditions by a nucleophilic mechanism in which the surface chalcogenides add to the double bond, followed by further oxidation of the surface-bound product. In contrast, CdS QDs more readily form surface sulfur-centered radicals that can perform reactions including alkene isomerization. These results indicate that QD surfaces should be an important consideration for the design of photocatalysis beyond simply tuning QD semiconductor band gaps.

Zn-Doped P-Type InAs Nanocrystal Quantum Dots.

Doped heavy metal-free III-V semiconductor nanocrystal quantum dots (QDs) are of great interest both from the fundamental aspects of doping in highly confined structures, and from the applicative side of utilizing such building blocks in the fabrication of p-n homojunction devices. InAs nanocrystals (NCs), that are of particular relevance for short-wave IR detection and emission applications, manifest heavy n-type character poising a challenge for their transition to p-type behavior. The p-type doping of InAs NCs is presented with Zn - enabling control over the charge carrier type in InAs QDs field effect transistors. The post-synthesis doping reaction mechanism is studied for Zn precursors with varying reactivity. Successful p-type doping is achieved by the more reactive precursor, diethylzinc. Substitutional doping by Zn2+ replacing In3+ is established by X-ray absorption spectroscopy analysis. Furthermore, enhanced near infraredphotoluminescence is observed due to surface passivation by Zn as indicated from elemental mapping utilizing high-resolution electron microscopy corroborated by X-ray photoelectron spectroscopy study. The demonstrated ability to control the carrier type, along with the improved emission characteristics, paves the way towards fabrication of optoelectronic devices active in the short-wave infrared region utilizing heavy-metal free nanocrystal building blocks.

Effects of the surface ligands of quantum dots on the intaglio transfer printing process

High-resolution semiconductor nanocrystal quantum dot (QD) patterns are required for applications in display devices. For this, the dry transfer printing of QDs is promising because it does not degrade the inherent properties of QDs. However, the effect of the surface ligands on this process remains poorly understood, despite its importance. Herein, we investigate the effect of the surface ligands on the intaglio transfer printing process. Colloidal QDs with organic (C8–C18) or inorganic (I−) ligands are prepared. In various pattern-printing tests, including patterns with a sub-10-μm width, the patterning yield is ∼100% for QDs with long-chain ligands (C18). However, the patterning yield decreases with decrease in the chain length for the organic-ligand-passivated QDs and is the lowest for the QDs passivated with I−. Using surface energy characterization and finite element method simulations, we suggest two printing failure mechanisms: (i) Inorganic-ligand-passivated QDs are not effectively picked-up by the stamp because of poor adhesion, and, (ii) for the organic-ligand-passivated QDs, internal crack formation is easier for QDs having short-chain ligands because of the weak interparticle attraction between QDs. Our findings reveal previously unknown defects of the intaglio printing process and provide guidance for mitigating these problems for preparing high-resolution QD patterns.

Effects of Mono- and Bifunctional Surface Ligands of Cu-In-Se Quantum Dots on Photoelectrochemical Hydrogen Production.

Semiconductor nanocrystal quantum dots (QDs) are promising materials for solar energy conversion because of their bandgap tunability, high absorption coefficient, and improved hot-carrier generation. CuInSe2 (CISe)-based QDs have attracted attention because of their low toxicity and wide light-absorption range, spanning visible to near-infrared light. In this work, we study the effects of the surface ligands of colloidal CISe QDs on the photoelectrochemical characteristics of QD-photoanodes. Colloidal CISe QDs with mono- and bifunctional surface ligands are prepared and used in the fabrication of type-II heterojunction photoanodes by adsorbing QDs on mesoporous TiO2. QDs with monofunctional ligands are directly attached on TiO2 through partial ligand detachment, which is beneficial for electron transfer between QDs and TiO2. In contrast, bifunctional ligands bridge QDs and TiO2, increasing the amount of QD adsorption. Finally, photoanodes fabricated with oleylamine-passivated QDs show a current density of ~8.2 mA/cm2, while those fabricated with mercaptopropionic-acid-passivated QDs demonstrate a current density of ~6.7 mA/cm2 (at 0.6 VRHE under one sun illumination). Our study provides important information for the preparation of QD photoelectrodes for efficient photoelectrochemical hydrogen generation.

Suppressing Non-Radiative Relaxation through Single-Atom Metal Modification for Enhanced Fluorescence Efficiency in Molybdenum Disulfide Quantum Dots.

To enhance the fluorescence efficiency of semiconductor nanocrystal quantum dots (QDs), strategies via enhancing photo-absorption and eliminating non-radiative relaxation have been proposed. In this study, we demonstrate that fluorescence efficiency of molybdenum disulfide quantum dots (MoS2 QDs) can be enhanced by single-atom metal (Au, Ag, Pt, Cu) modification. Four-fold enhancement of the fluorescence emission of MoS2 QDs is observed with single-atom Au modification. The underlying mechanism is ascribed to the passivation of non-radiative surface states owing to the new defect energy level of Au in the forbidden band that can trap excess electrons in n-type MoS2 , increasing the recombination probability of conduction band electrons with valence band holes of MoS2 . Our results open an avenue for enhancing the fluorescence efficiency of QDs via the modification of atomically dispersed metals, and extend their scopes and potentials in a fundamental way for economic efficiency and stability of single-atom metals.

Suppressing Non‐Radiative Relaxation through Single‐Atom Metal Modification for Enhanced Fluorescence Efficiency in Molybdenum Disulfide Quantum Dots

AbstractTo enhance the fluorescence efficiency of semiconductor nanocrystal quantum dots (QDs), strategies via enhancing photo‐absorption and eliminating non‐radiative relaxation have been proposed. In this study, we demonstrate that fluorescence efficiency of molybdenum disulfide quantum dots (MoS2 QDs) can be enhanced by single‐atom metal (Au, Ag, Pt, Cu) modification. Four‐fold enhancement of the fluorescence emission of MoS2 QDs is observed with single‐atom Au modification. The underlying mechanism is ascribed to the passivation of non‐radiative surface states owing to the new defect energy level of Au in the forbidden band that can trap excess electrons in n‐type MoS2, increasing the recombination probability of conduction band electrons with valence band holes of MoS2. Our results open an avenue for enhancing the fluorescence efficiency of QDs via the modification of atomically dispersed metals, and extend their scopes and potentials in a fundamental way for economic efficiency and stability of single‐atom metals.

Electronic transport in quantum-dot-in-perovskite solids.

We investigate theoretically the band transport of electrons and holes in a "quantum-dot-in-perovskite" solid, a periodic array of semiconductor nanocrystal quantum dots embedded in a matrix of lead halide perovskite. For concreteness we focus on PbS quantum dots passivated by inorganic halogen ligands and embedded in a matrix of CsPbI3. We find that the halogen ligands play a decisive role in determining the band offset between the dot and matrix and may therefore provide a straightforward way to control transport experimentally. The model and analysis developed here may readily be generalized to analyze band transport in a broader class of dot-in-solid materials.

Engineering Brightness Matched Indium Phosphide Quantum Dots.

The size-dependent optoelectronic properties of semiconductor nanocrystals quantum dots (QDs) are hugely beneficial for color tunability but induce an inherent relative PL brightness mismatch in QDs emitting different colors, as larger emitters absorb more incident photons than smaller particles. Here, we examine the effect of core composition, shell composition, and shell thickness on optical properties including high energy absorption, quantum yield (QY), and the relative brightness of InP/ZnS and InP/ZnSe core/shell and InP/ZnSe/ZnS core/shell/shell QDs at different excitation wavelengths. Our analysis reveals that the presence of an intermediate ZnSe shell changes the wavelength of enhanced absorption onset and leads to highly excitation wavelength dependent QYs. Switching from commercial CdSe/ZnS to InP/ZnS reduces the brightness-mismatch between green and red emitters from 33- to 5-fold. Incorporating a 4-monolayer thick optically absorbing ZnSe shell into the QD heterostructure and heating the QDs in a solution of zinc oleate and trioctylphosphine produces InP/ZnSe/ZnS QDs that are ~10-fold brighter than their InP/ZnS counterparts. In contrast to CdSe/CdS/ZnS core/shell/shell QDs, which only photoluminesce at red wavelengths with thicker CdS shells due to their Quasi-Type II bandstructure, Type I InP/ZnSe/ZnS QDs are uniquely suited to creating a rainbow of visible-emitting, brightness matched emitters. By tailoring the thickness of the intermediate ZnSe shell, heavy metal-free, brightness-matched green and red emitters are produced. This study highlights the ability to overcome the inherent brightness mismatch seen in QDs through concerted materials design of heterostructured core/shell InP-based QDs.

Comparison of the effects of different ionic liquid-gels on the efficiencies of the PbSe, PbS and PbTe quantum dot sensitized solar cells

Abstract The semiconductor nanocrystal quantum dots (QD) are used in the third generation solar cells due to their high photon conversion efficiency. In this study, we investigated the effects of ionic liquid on the efficiency of lead chalcogenide (PbSe, PbTe and PbS) QD solar cells. To achieve this, we looked at the interaction between the alkyl groups (R = CH3) and the thiol bonds (SH) of the selected organic electrolytes. Two different classes of liquids were prepared based on organic (960 and SB) and inorganic (AY1-3) ionic liquid (IL) where AY1-3 liquid include LiI, NaI, I2 materials; 960 and SB type liquids include BMII, tBP and GuNCS materials. These liquids provide redox reactions. 5 different ILs were applied to each of the 3 types of QDs and 15 solar cells were produced. Particle sizes and solar cell parameters like fill factor and efficiencies were determined through optical absorption, photoluminescence, TEM, SEM, EDS and J-V measurements. It was observed that the interaction of these groups affected the fill factor (FF), open circuit voltage (Voc) and efficiency of PbX (X = Se, S, Te) QD solar cells. The AY1 was found to be compatible with the PbX QDSSCs when used alone while the AY2 was not. LiI was an effective IL when used alone which resulted in with an efficiency of 7.853 for PbS QDSSC. NaI and GuSCN introduced different pairs of redox to the system, increasing the efficiency of PbSe solar cells to 8.257.

Nanohybrid Structures Based on Plasmonic or Fluorescent Nanoparticles and Retinal-Containing Proteins.

Rhodopsins are light-sensitive membrane proteins enabling transmembrane charge separation (proton pump) on absorption of a light quantum. Bacteriorhodopsin (BR) is a transmembrane protein from halophilic bacteria that belongs to the rhodopsin family. Potential applications of BR are considered so promising that the number of studies devoted to the use of BR itself, its mutant variants, as well as hybrid materials containing BR in various areas grows steadily. Formation of hybrid structures combining BR with nanoparticles is an essential step in promotion of BR-based devices. However, rapid progress, continuous emergence of new data, as well as challenges of analyzing the entire data require regular reviews of the achievements in this area. This review is devoted to the issues of formation of materials based on hybrids of BR with fluorescent semiconductor nanocrystals (quantum dots) and with noble metal (silver, gold) plasmonic nanoparticles. Recent data on formation of thin (mono-) and thick (multi-) layers from materials containing BR and BR/nanoparticle hybrids are presented.

(Invited) Colloidal Semiconductor Nanocrystal Photocatalysts: Teaching an Old Dot New Tricks

Colloidal semiconductor nanocrystal quantum dots (QDs) have shown great promise as photocatalysts for a variety of chemical transformations including oxidations, reductions, and most recently carbon-carbon bond forming reactions. QDs inherently possess several desirable properties for photocatalysis including oxidation and reduction potentials that are size tunable, and also that are largely independent of their surface chemistry. Due to their size, QDs can also interact with several substrates at once, enhancing charge-transfer rates. We recently found that CdSe QDs and simple aqueous Ni2+salts in the presence of a sacrificial electron donor form a highly efficient, active, and robust system for photochemical reduction of protons to molecular hydrogen. Under appropriate conditions, the nanocrystal-based system has undiminished activity for at least 360 hours with about a million turnovers. Ultrafast optical spectroscopy studies of electron transfer from the nanoparticles to a Ni-catalyst reveals the optimal nanoparticle size and shape for photochemical proton reduction, which was confirmed with photocatalytic experiments. We will also discuss recent measurements of carbon-carbon bond formation driven photochemically using CdSe QDs. A single-sized CdSe QD can replace several different dye catalysts needed for five different photoredox reactions (beta-alkylation, beta-aminoalkylation, dehalogenation, amine arylation, and decarboxylative radical formation).Even without optimization of the QDs or the reaction conditions, efficiencies rivaling the best available metal dyes were obtained. The facile customization afforded by QDs for photoredox catalysis suggests they could have important applications in future syntheses of novel pharmaceuticals.

Coherent Spectroscopy of Multiple Excitons in Colloidal Nanocrystal Quantum Dots

AbstractOver three decades of intense research on colloidal semiconductor nanocrystal quantum dots (QDs) have passed, and the multitude of physical effects in QD systems still makes fascinating discoveries even today. In particular, the use of multiple excitons and their coherence is a hot topic to be investigated. We explore the potential of colloidal QDs for next‐generation optoelectronic applications. Here, we summarize our recent studies on the multiple‐exciton harmonic quantum coherence in PbS QDs. The multiple excitonic coherence can be observed with our newly developed system for phase‐locked interference detection of transient absorption signals. We found that multiple excitons exhibit dipole oscillations with integer multiples of the exciton resonance frequency. An important practical feature of this harmonic quantum coherence of multiple excitons is that it assists photoabsorption processes. Therefore, harmonic quantum coherence is a key to realize advanced optoelectronic applications.

Optical Properties of Quantum Dots with a Core–Multishell Structure

In the last decade, colloidal semiconductor nanocrystals (quantum dots) have been not only studied fundamentally but also applied in photovoltaics, optoelectronics, and biomedicine. Beginning with simple approaches to the deposition of protective shells, e.g., ZnS on CdSe cores, searches for ways to increase the quantum yield of photoluminescence of quantum dots have resulted now in the development of new types of quantum dots characterized not only by record high extinction coefficients but also by high photoluminescence quantum yields. In this work, the optical properties of core–multishell quantum dots have been analyzed. These quantum dots have been specially designed to reach the maximum possible localization of excited charge carriers inside luminescent cores, which makes it possible to reach a photoluminescence quantum yield close to 100%. Core–multishell quantum dot samples with a shell thickness of 3–7 monolayers have been fabricated. Changes in the characteristics of optical transitions in such quantum dots with an increase in the number of layers of the shell have been studied. The effect of the thickness of the shell on the optical properties of prepared quantum dots has been analyzed. In particular, analysis of photoluminescence lifetimes of such quantum dots has revealed a possible alternative mechanism of radiation of core–multishell quantum dots based on the slow charge carrier transfer from the excited outer layer of the CdS shell to the CdSe core.

Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation.

Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II–VI and III–V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts. Optimized treatments with InCl3 or CdCl2 afford a near-unity (>90%) photoluminescence quantum yield (PL QY), whereas other metal halogen or carboxylate salts provide a smaller increase in PL QY as a result of weaker binding or steric repulsion. The addition of non-Lewis acidic ligands (amines, alkylammonium chlorides) systematically gives a much smaller but non-negligible increase in the PL QY. We discuss possible reasons for this result, which points toward a more complex and dynamic QD surface. Finally we show that Z-type metal halide ligand treatments also lead to a strong increase in the PL QY of CdSe, CdS, and InP QDs and can increase the efficiency of sintered CdTe solar cells. These results show that surface anions are the dominant source of trap states in II–VI and III–V QDs and that passivation with Lewis acidic Z-type ligands is a general strategy to fix those traps. Our work also provides a method to tune the PL QY of QD samples from nearly zero up to near-unity values, without the need to grow epitaxial shells.

Efficient Encoding of Matrix Microparticles with Nanocrystals for Fluorescent Polyelectrolyte Microcapsules Development

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Dependence of Photocurrent Enhancements in Quantum Dot (QD)‐Sensitized MoS2 Devices on MoS2 Film Properties

AbstractThis report demonstrates highly efficient nonradiative energy transfer (NRET) from alloyed CdSeS/ZnS semiconductor nanocrystal quantum dots (QDs) to MoS2 films of varying layer thicknesses, including pristine monolayers, mixed monolayer/bilayer, polycrystalline bilayers, and bulk‐like thicknesses, with NRET efficiencies of over 90%. Large‐area MoS2 films are grown on Si/SiO2 substrates by chemical vapor deposition. Despite the ultrahigh NRET efficiencies there is no distinct increase in the MoS2 photoluminescence intensity. However, by studying the optoelectronic properties of the MoS2 devices before and after adding the QD sensitizing layer photocurrent enhancements as large as ≈14‐fold for pristine monolayer devices are observed, with enhancements on the order of ≈2‐fold for MoS2 devices of mixed monolayer and bilayer thicknesses. For the polycrystalline bilayer and bulk‐like MoS2 devices there is almost no increase in the photocurrent after adding the QDs. Industrially scalable techniques are specifically utilized to fabricate the samples studied in this report, demonstrating the viability of this hybrid structure for commercial photodetector or light harvesting applications.