Colloidal Semiconductor Nanocrystal Quantum Dots Research Articles

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.

(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.

Uncovering active precursors in colloidal quantum dot synthesis

Studies of the fundamental physics and chemistry of colloidal semiconductor nanocrystal quantum dots (QDs) have been central to the field for over 30 years. Although the photophysics of QDs has been intensely studied, much less is understood about the underlying chemical reaction mechanism leading to monomer formation and subsequent QD growth. Here we investigate the reaction mechanism behind CdSe QD synthesis, the most widely studied QD system. Remarkably, we find that it is not necessary for chemical precursors used in the most common synthetic methods to directly react to form QD monomers, but rather they can generate in situ the same highly reactive Cd and Se precursors that were used in some of the original II-VI QD syntheses decades ago, i.e., hydrogen chalcogenide gas and alkyl cadmium. Appreciating this surprising finding may allow for directed manipulation of these reactive intermediates, leading to more controlled syntheses with improved reproducibility.

Surface Charge Control of Quantum Dot Blinking

A characteristic property of colloidal semiconductor nanocrystal quantum dots (QDs) is their emission intermittency. Although a unifying theory of QD photoprocesses remains elusive, the importance of charged states is clear. We now report a new approach to directly study the role of surface charge on QD emission by adding metal ions to individual, core-only QDs immobilized in aqueous solution in an agarose gel. The CdTe QDs show very stable emission in the absence of metal ions but a dramatic and reversible increase in blinking due to the presence of trivalent metal ions. Our results support a charge-separation model, in which the major blinking pathway is the surface trapping of electrons; transiently bound metal ions close to the QD surface enhance this process.

Deconstructing the photon stream from single nanocrystals: from binning to correlation

Prior to the advent of single-molecule fluorescence spectroscopy, many of the fundamental optical properties of colloidal semiconductor nanocrystal quantum dots were obscured by ensemble averaging over their inherent inhomogeneities. Single quantum dot spectroscopy has become a leading technique for the unambiguous determination of the governing excitonic physics of these quantum-confined systems. The analysis and interpretation of the timing and energies of photons emitted from individual nanocrystals have uncovered unexpected and fundamental electronic processes at the nanoscale. We review several different paradigms for deconstructing the photon stream from single nanocrystals, ranging from intensity "binning" techniques to more sophisticated methods based on single-photon counting. In particular, we highlight photon correlation - a powerful developing paradigm in single-nanocrystal studies. The application of photon-correlation techniques to single nanocrystals is changing the study of multiexcitonic recombination dynamics, uncovering the basic processes governing spectral linewidths and spectral diffusion, and enabling the extraction of single-nanocrystal properties directly from an ensemble with high statistical significance. These single-molecule techniques have proven invaluable for understanding the physics of nanocrystals and can provide unique insight into other heterogeneous and dynamical systems.

ChemInform Abstract: Colloidal Nanocrystal Quantum Dot Assemblies as Artificial Solids

AbstractReview: 298 refs.

Surface Doping Quantum Dots with Chemically Active Native Ligands: Controlling Valence without Ligand Exchange

One remaining challenge in the field of colloidal semiconductor nanocrystal quantum dots is learning to control the degree of functionalization or “valence” per nanocrystal. Current quantum dot surface modification strategies rely heavily on ligand exchange, which consists of replacing the nanocrystal’s native ligands with carboxylate- or amine-terminated thiols, usually added in excess. Removing the nanocrystal’s native ligands can cause etching and introduce surface defects, thus affecting the nanocrystal’s optical properties. More importantly, ligand exchange methods fail to control the extent of surface modification or number of functional groups introduced per nanocrystal. Here, we report a fundamentally new surface ligand modification or “doping” approach aimed at controlling the degree of functionalization or valence per nanocrystal while retaining the nanocrystal’s original colloidal and photostability. We show that surface-doped quantum dots capped with chemically active native ligands can be pre...

Colloidal nanocrystal quantum dot assemblies as artificial solids

The prospect of designing novel materials with electrical, optical, and magnetic properties by design has intrigued scientists and engineers for years. Building blocks for such “artificial solids” have emerged from recent advances in nanomaterial synthesis, characterization, and emerging understanding of their size-dependent properties. Colloidal semiconductor nanocrystal quantum dots (NQDs) stand out as an intellectually intriguing and experimentally advantageous system for the fundamental study of artificial solids and their technological development. The authors review the rapid evolution of artificial solids from an early theoretical concept towards the refined control of metamaterials with programmable electronic structure and their potential commercial applications, in particular, in next-generation energy technologies. The review is organized around the three independently adjustable parameters of artificial solids: (i) the electronic structure of NQD as artificial atom by tailoring the quantum confinement of the wave function, (ii) the interdot coupling as an artificial bond, and (iii) the self-assembly of NQDs into ordered superstructures as artificial crystals. The authors review elementary aspects of colloidal NQD synthesis as well as pertinent advances which have led to refined control over the NQD size, shape, and composition. Coupling between NQDs is reviewed in the context of an artificial bond; we summarize chemical and physical approaches to address the seemingly contradictory requirements of coupling nanostructures while preserving the effects of quantum-confinement. The authors review the self-assembly of NQDs into ordered superstructures in analogy to atomic crystal growth and discuss fundamental interactions between NQD and how they can be modulated to direct the growth of superlattices with predefined structures. Collectively, the experimental control over the properties of the artificial atom, bond, and crystal enable the systematic exploration of the electronic phase diagram of NQD solids. From an applied perspective, these advances have created an immensely fertile opportunity space technological applications of artificial solids in optoelectronic devices. The authors conclude with a perspective on three specific unresolved challenges ahead: (i) knowledge gaps concerning the detailed physiochemical nature of the NQD surface, (ii) limitations posed by the inherent inhomogeneity within the ensemble of NQDs, (iii) the true electronic structure of NQD solids, and (iv) the connection between NQD model systems in the laboratory and commercially deployable NQD technologies.

Quantum efficiency of stimulated emission in colloidal semiconductor nanocrystal quantum dots

External differential quantum efficiencies were studied for the stimulated emission of thin films of colloidal CdSe/CdS/ZnS nanocrystal quantum dots (NQDs) using the integrating sphere technique. A maximum efficiency of 34% was measured for the stimulated emission in NQDs which far exceeds that for spontaneous emission (9%). The competition between the radiative exciton/biexciton recombination and the carrier density-dependent nonradiative relaxation processes in NQDs was modeled to account for the observed difference in efficiency.

Cascade upconversion of photoluminescence in quantum dot ensembles

We present a theoretical model and Monte Carlo simulation results that naturally explain all the features of the thermally activated photoluminescence upconversion effect [also known as anti-Stokes photoluminescence (ASPL)] observed in ensembles of colloidal semiconductor nanocrystal quantum dots (QDs). The proposed ASPL mechanism includes the following principal ingredients: (i) optical-phonon-assisted absorption of an incident photon in a relatively large dot in the ensemble, (ii) emission of a higher-energy photon from the zero-phonon exciton-polaron state, with an upconversion equal to one optical-phonon energy, and (iii) cascade reabsorption and re-emission processes involving QDs of successively smaller sizes within the sample, rendering the experimentally observed large anti-Stokes shift of the energy of the photon that finally leaves the sample. The results obtained by the Monte Carlo modeling based on the proposed mechanism reproduce all the experimentally observed ASPL trends in colloidal QD solutions.

Circularly polarized emission from colloidal nanocrystal quantum dots confined in microcavities formed by chiral mirrors

Colloidal semiconductor nanocrystal quantum dots (NQDs) confined in a microcavity device emit resonance-enhanced, circularly polarized (CP) light, when the device consists of a pair of chiral sculptured-thin-film mirrors separated by layers of emissive nanocrystals and spacers that form a λ∕2-resonant cavity. The resonant emission from the NQDs within the microcavity is strongly directed normally to the mirror surfaces. It is speculated that the enhanced coupling between the NQD excitons and the confined electromagnetic field resulted in the observed CP emission from the embedded NQDs with significant improvements in the light output's directionality and color purity.

Microcavity light-emitting devices based on colloidal semiconductor nanocrystal quantum dots

This letter describes the design, fabrication, and characterization of a microcavity-electroluminescence (EL) device based on colloidal semiconductor nanocrystal quantum dots (NQDs). The device was fabricated by sandwiching a solution-cast film of light-emitting CdSe-CdS core-shell NQDs between two metal mirrors to form a resonant microcavity structure. We have observed a significant reduction in EL emission bandwidth from the fabricated device. Further improvement of the emission efficiency of the NQD-microcavity-EL devices can be achieved upon the minimization of the losses that are pertinent to metal mirrors.

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 ultrahigh magnetic fields up to 45 T. At low temperatures (1.5-40 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 Förster energy transfer processes.

Semiconductor Nanocrystal Quantum Dots on Single Crystal Semiconductor Substrates: High Resolution Transmission Electron Microscopy

We report on high-resolution transmission electron microscope structural studies of InAs colloidal semiconductor nanocrystal quantum dots (NCQDs) on ultrathin GaAs (001) semiconductor single-crystal substrates. We employ a benign method for preparing electron transparent specimens that is suitable for the study of such fragile samples. The image contrast comprises contributions from electron scattering from both the NCs and the GaAs substrate. Long-term electron exposure studies reveal different damage mechanisms operative in the nanocrystals and the substrate.

Ultrafast Coherent Terahertz Spectroscopy in High Magnetic Fields and Directed Energy Flows in Quantum Dot Assemblies

We describe the design, construction, and use of fiber-coupled terahertz antennas for performing ultrafast coherent THz spectroscopy directly in the cryogenic bore of high-field magnets. With an aim towards measuring the high-frequency (100 GHz to 2000 GHz) complex conductivity of correlated electron materials in the regime of low temperatures and high magnetic fields, these miniature THz emitters and receivers are demonstrated to work down to 1.5 K and up to 18 T, for eventual use in higher-field magnets. Results from a variety of semiconducting and superconducting samples are presented. This paper also describes a separate effort towards achieving coupling between colloidal semiconductor nanocrystal quantum dots, wherein we realize and study inter-dot communication via resonant (Forster) energy transfer. We present studies of the dynamics of resonant energy transfer in monodisperse and energy gradient (layered) assemblies of CdSe nanocrystal quantum dots. Time- and spectrally-resolved photoluminescence data directly reveal the energy-dependent transfer rate of excitons from smaller to larger dots. Results from layered nanocrystal quantum dot assemblies demonstrate unidirectional energy flows, a first step towards artificial light-harvesting structures. Lastly, time-resolved studies at millikelvin temperatures elucidate the nature of ground-state dark excitons in these quantum dots.

Electrochromic Nanocrystal Quantum Dots.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electrochromic nanocrystal quantum dots.

Incorporating nanocrystals into future electronic or optoelectronic devices will require a means of controlling charge-injection processes and an understanding of how the injected charges affect the properties of nanocrystals. We show that the optical properties of colloidal semiconductor nanocrystal quantum dots can be tuned by an electrochemical potential. The injection of electrons into the quantum-confined states of the nanocrystal leads to an electrochromic response, including a strong, size-tunable, midinfrared absorption corresponding to an intraband transition, a bleach of the visible interband exciton transitions, and a quench of the narrow band-edge photoluminescence.