Delayed luminescence involving charge-carrier trapping and detrapping has recently been identified as a widespread and possibly universal phenomenon in colloidal quantum dots. Its near-power-law decay suggests a relationship with blinking. Here, using colloidal CuInS2 and CdSe quantum dots as model systems, we show that short (nanosecond) excitation pulses yield less delayed luminescence intensity and faster delayed luminescence decay than observed with long (millisecond) square-wave excitation pulses. Increasing the excitation power also affects the delayed luminescence intensity, but the delayed luminescence decay kinetics appear much less sensitive to excitation power than to excitation pulse width. An idealized four-state kinetic model reproduces the major experimental trends and highlights the very slow approach to steady state during photoexcitation, stemming from extremely slow detrapping of the metastable charge-separated state responsible for delayed luminescence. The impacts of these findings on proposed relationships between delayed luminescence and blinking are discussed.

We examine the effects of CdS shell growth on photochemical reduction of colloidal CdSe quantum dots (QDs) and describe the spectroscopic properties of the resulting n-type CdSe/CdS QDs. CdS shell growth greatly slows electron trapping. Because of this improvement, complete two-electron occupancy of the 1Se conduction-band orbital is achieved in CdSe/CdS QDs and found to be much more stable than in past experiments. Simultaneous photoluminescence at two different energies is now observed from QDs possessing two excess conduction-band electrons, reflecting competing recombination of discretized 1Se and 1Pe conduction-band electrons within photogenerated four-carrier negative tetrons (three electrons and one hole). Stable occupancy of the 1Pe level is not achievable under these conditions, and possible reasons are discussed. The stability and accessibility of these multielectron configurations, and the facile spectroscopic detection of negative tetrons, both make photodoped core/shell QDs attractive for exploring the physical properties of free-standing heavily n-doped colloidal CdSe-based QDs.Keywords: photodoping; photoluminescence; quantum dots; tetron luminescence;

Impurity doping can be used to dramatically alter the physical properties of semiconductor nanostructures and endow them with promising new technological potential. This review summarizes recent progress toward the development of colloidal semiconductor quantum dots (QDs) doped with individual magnetic impurity ions. Such singly doped quantum dots (SDQDs), as well as related magic-sized nanoclusters, represent an exciting class of materials for cultivating unique physical effects arising from magnetic exchange coupling between delocalized charge carriers and the single impurity ions. Key exchange effects can be enhanced by carrier confinement in such small structures. The physical properties displayed by these materials may prove valuable for new technologies based on the manipulation of individual spins in semiconductor nanostructures, such as spintronics, spin-photonics, or quantum computing. Interesting chemical and analytical challenges also emerge when exploring this research frontier. Here, we describe cluster and QD results from recent literature related to these challenges. We summarize (magneto-)optical investigations of SDQDs on both the ensemble and single-particle levels. Comparisons to analogous bulk materials and self-assembled QDs are drawn and used to highlight some of the rich and unique characteristics found within this class of materials.

We present a general model for describing the properties of excess electrons in multiply charged quantum dots (QDs). Key factors governing Fermi-level energies and electron density distributions are investigated by treating carrier densities, charge compensation, and various material and dielectric medium properties as independently tunable parameters. Electronic interactions are described using a mean-field electrostatic potential calculable through Gauss’s Law by treating the quantum dot as a sphere of uniform charge density. This classical approximation modifies the “Particle in a Sphere” Schrödinger equation for a square well potential and reproduces the broken degeneracy and Fermi-level energies expected from experiment and first-principles methods. Several important implications emerge from this model: (i) excess electron density drifts substantially toward the QD surfaces with high electron densities and large radii and when solvated by a high dielectric medium. (ii) The maximum density of the conduction-band electrons depends strongly on the dielectric strength of the solvent and the electron affinity and dielectric strength of the QD material. (iii) Fermi-level energies stabilize with charge-balancing cations in close proximity to the QD surface.

Mid-gap luminescence in copper (Cu+)-doped semiconductor nanocrystals (NCs) involves recombination of delocalized conduction-band electrons with copper-localized holes. Silver (Ag+)-doped semiconductor NCs show similar mid-gap luminescence at slightly (∼0.3 eV) higher energy, suggesting a similar luminescence mechanism, but this suggestion appears inconsistent with the large difference between Ag+ and Cu+ ionization energies (∼1.5 eV), which should make hole trapping by Ag+ highly unfavorable. Here, Ag+-doped CdSe NCs (Ag+:CdSe) are studied using time-resolved variable-temperature photoluminescence (PL) spectroscopy, magnetic circularly polarized luminescence (MCPL) spectroscopy, and time-dependent density functional theory (TD-DFT) to address this apparent paradox. In addition to confirming that Ag+:CdSe and Cu+:CdSe NCs display similar broad PL with large Stokes shifts, we demonstrate that both also show very similar temperature-dependent PL lifetimes and magneto-luminescence. Electronic-structure calculations further predict that both dopants generate similar localized mid-gap states. Despite these strong similarities, we conclude that these materials possess significantly different electronic structures. Specifically, whereas photogenerated holes in Cu+:CdSe NCs localize primarily in Cu(3d) orbitals, formally oxidizing Cu+ to Cu2+, in Ag+:CdSe NCs they localize primarily in 4p orbitals of the four neighboring Se2– ligands, and Ag+ is not oxidized. This difference reflects a shift from “normal” to “inverted” bonding going from Cu+ to Ag+. The spectroscopic similarities are explained by the fact that, in both materials, photogenerated holes are localized primarily within covalent [MSe4] dopant clusters (M = Ag+, Cu+). These findings reconcile the similar spectroscopies of Ag+- and Cu+-doped semiconductor NCs with the vastly different ionization potentials of their Ag+ and Cu+ dopants.

Doping lanthanide ions into colloidal semiconductor nanocrystals is a promising strategy for combining their sharp and efficient 4f–4f emission with the strong broadband absorption and low-phonon-energy crystalline environment of semiconductors to make new solution-processable spectral-conversion nanophosphors, but synthesis of this class of materials has proven extraordinarily challenging because of fundamental chemical incompatibilities between lanthanides and most intermediate-gap semiconductors. Here, we present a new strategy for accessing lanthanide-doped visible-light-absorbing semiconductor nanocrystals by demonstrating selective cation exchange to convert precursor Yb3+-doped NaInS2 nanocrystals into Yb3+-doped PbIn2S4 nanocrystals. Excitation spectra and time-resolved photoluminescence measurements confirm that Yb3+ is both incorporated within the PbIn2S4 nanocrystals and sensitized by visible-light photoexcitation of these nanocrystals. This combination of strong broadband visible absorption, sharp near-infrared emission, and long (>400 μs) emission lifetimes in a colloidal nanocrystal system opens promising new opportunities for both fundamental-science and next-generation spectral-conversion applications such as luminescent solar concentrators.

The trapping dynamics of conduction-band electrons in colloidal degenerately doped n-CdSe nanocrystals prepared by photochemical reduction (photodoping) were measured by direct optical methods. The nanocrystals show spontaneous electron trapping with distributed kinetics that extend to remarkably long time scales. Shifts in nanocrystal band-edge potentials caused by quantum confinement and surface ion stoichiometry were also measured by spectroelectrochemical techniques, and their relationship to the slow electron trapping is discussed. The very long electron-trapping time scales observed in these measurements are more consistent with atomic rearrangement than with fundamental electron-transfer processes. Such slow and broadly distributed electron-trapping dynamics are reminiscent of the well-known distributed dynamics of nanocrystal photoluminescence blinking, and potential relationships between the two phenomena are discussed.

A fundamental understanding of the rich electronic structures of electronically doped semiconductor nanocrystals is vital for assessing the utility of these materials for future applications from solar cells to redox catalysis. Here, we examine the use of magnetic circular dichroism (MCD) spectroscopy to probe the infrared localized surface plasmon resonances of p-Cu2–xSe, n-ZnO, and tin-doped In2O3 (n-ITO) nanocrystals. We demonstrate that the MCD spectra of these nanocrystals can be analyzed by invoking classical cyclotron motions of their excess charge carriers, with experimental MCD signs conveying the carrier types (n or p) and experimental MCD intensities conveying the cyclotron splitting magnitudes. The experimental cyclotron splittings can then be used to quantify carrier effective masses (m*), with results that agree with bulk in most cases. MCD spectroscopy thus offers a unique measure of m* in free-standing colloidal semiconductor nanocrystals, raising new opportunities to investigate the influence of various other synthetic or environmental parameters on this fundamentally important electronic property.

Doping a semiconductor quantum dot with just a single impurity atom can completely transform its physical properties. Here, we report and analyze the magnetic circular dichroism (MCD) spectra of colloidal CdSe quantum dot samples containing on average fewer than one Mn2+ per quantum dot. Even at this sub-single-dopant level, the low-temperature low-field data are dominated by impurity-induced Zeeman splittings caused by dopant–carrier sp–d exchange. Unlike in more heavily doped quantum dots, however, the MCD intensity at the first CdSe exciton shows a field-induced sign flip as the field strength is increased, reflecting competition between sp–d exchange and the intrinsic Zeeman splittings of comparable magnitude. Most unusually, the competition between these two effects leads to a large apparent shift in the first MCD peak maximum, which we show is attributable to a difference in sign of the intrinsic excitonic g factor between the first and second excitons. Finally, the sp–d and intrinsic contributions to the excitonic Zeeman splittings each exhibit unique magnetic-field and temperature dependencies, allowing the MCD spectra of undoped, singly doped, and bi-doped quantum dot sub-ensembles to be analyzed.

Copper-doped semiconductors are classic phosphor materials that have been used in a variety of applications for many decades. Colloidal copper-doped semiconductor nanocrystals have recently attracted a great deal of interest because they combine the solution processability and spectral tunability of colloidal nanocrystals with the unique photoluminescence properties of copper-doped semiconductor phosphors. Although ternary and quaternary semiconductors containing copper, such as CuInS2 and Cu2ZnSnS4, have been studied primarily in the context of their photovoltaic applications, when synthesized as colloidal nanocrystals, these materials have photoluminescence properties that are remarkably similar to those of copper-doped semiconductor nanocrystals. This review focuses on the luminescent properties of colloidal copper-doped, copper-based, and related copper-containing semiconductor nanocrystals. Fundamental investigations into the luminescence of copper-containing colloidal nanocrystals are reviewed in the context of the well-established luminescence mechanisms of bulk copper-doped semiconductors and copper(I) molecular coordination complexes. The use of colloidal copper-containing nanocrystals in applications that take advantage of their luminescent properties, such as bioimaging, solid-state lighting, and luminescent solar concentrators, is also discussed.

Ion exchange, in which an in-diffusing ion replaces a lattice ion, has been widely exploited as a synthetic tool for semiconductor doping and solid-to-solid chemical transformations, both in bulk and at the nanoscale. Here, we present a systematic investigation of cation-exchange reactions that involve the displacement of Mn2+ from CdSe nanocrystals by Cd2+ or In3+. For both incoming cations, Mn2+ displacement is spontaneous but thermally activated, following Arrhenius behavior over a broad experimental temperature range. At any given temperature, cation exchange by In3+ is approximately 2 orders of magnitude faster than that by Cd2+, illustrating a critical dependence on the incoming cation. Quantitative analysis of the kinetics data within a Fick’s-law diffusion model yields diffusion barriers (ED) and limiting diffusivities (D0) for both incoming ions. Despite their very different kinetics, indistinguishable diffusion barriers of ED ≈ 1.1 eV are found for both reactions (In3+ and Cd2+). A dramatically enhanced diffusivity is found for Mn2+ cation exchange by In3+. Overall, these findings provide unique experimental insights into cation diffusion within colloidal semiconductor nanocrystals, contributing to our fundamental understanding of this rich and important area of nanoscience.

Colloidal semiconductor nanocrystals offer a unique opportunity to bridge molecular and bulk semiconductor redox phenomena. Here, potentiometric titration is demonstrated as a method for quantifying the Fermi levels and charging potentials of free-standing colloidal n-type ZnO nanocrystals possessing between 0 and 20 conduction-band electrons per nanocrystal, corresponding to carrier densities between 0 and 1.2 × 1020 cm–3. Potentiometric titration of colloidal semiconductor nanocrystals has not been described previously, and little precedent exists for analogous potentiometric titration of any soluble reductants involving so many electrons. Linear changes in Fermi level vs charge-carrier density are observed for each ensemble of nanocrystals, with slopes that depend on the nanocrystal size. Analysis indicates that the ensemble nanocrystal capacitance is governed by classical surface electrical double layers, showing no evidence of quantum contributions. Systematic shifts in the Fermi level are also observed with specific changes in the identity of the charge-compensating countercation. As a simple and contactless alternative to more common thin-film-based voltammetric techniques, potentiometric titration offers a powerful new approach for quantifying the redox properties of colloidal semiconductor nanocrystals.

Understanding the structural and compositional origins of midgap states in semiconductor nanocrystals is a longstanding challenge in nanoscience. Here, we report a broad variety of reagents useful for photochemical reduction of colloidal CdSe quantum dots, and we establish that these reactions proceed via a dark surface prereduction step prior to photoexcitation. Mechanistic studies relying on the specific properties of various reductants lead to the proposal that this surface prereduction occurs at oxidized surface selenium sites. These results demonstrate the use of small-molecule inorganic chemistries to control the physical properties of colloidal QDs and provide microscopic insights into the identities and reactivities of their localized surface species.

A potentiometric method for measuring redox potentials of colloidal semiconductor nanocrystals (NCs) is described. Fermi levels of colloidal ZnO NCs are measured in situ during photodoping, allowing correlation of NC redox potentials and reduction levels. Excellent agreement is found between electrochemical and optical redox-indicator methods. Potentiometry is also reported for colloidal CdSe NCs, which show more negative conduction-band-edge potentials than in ZnO. This difference is highlighted by spontaneous electron transfer from reduced CdSe NCs to ZnO NCs in solution, with potentiometry providing a measure of the inter-NC electron-transfer driving force. Future applications of NC potentiometry are briefly discussed.

We report a one-pot synthesis of high-quality colloidal copper-doped cadmium selenide nanocrystals (Cu+:CdSe NCs) by injection of a mixture of copper iodide (CuI) and trioctylphosphine (TOP) into solutions containing preformed CdSe NCs. This method allows NC doping to be separated from nucleation and growth, thereby simultaneously achieving large size tunability, narrow size dispersion, and exclusively copper-based photoluminescence (PL). The copper doping level is affected by both the reaction time and the relative concentrations of the cadmium precursor, CuI, and TOP. A correlation is demonstrated between the copper dopant concentration and the intensities of the characteristic near-IR PL and midgap absorption bands, both associated with metal-to-ligand (conduction band) charge-transfer (MLCBCT) excitation of Cu+ dopants. Mechanistic studies reveal that Cu2–xSe NCs are easily formed as kinetic intermediates under reaction conditions involving substantial copper and that these NCs then act as a copper source for the subsequent formation of Cu+:CdSe NCs in the same reaction mixture. We also observe postsynthetic loss of copper from the doped NCs during shell growth or exposure to phosphines and amines, reflecting the high mobility of Cu+ ions in colloidal NCs.

Understanding and controlling the redox properties of colloidal semiconductor nanocrystals is critical for application of this class of materials in many proposed technologies. Here, we use spectroelectrochemical potentiometry to analyze the redox potentials of free-standing colloidal n-type CdSe nanocrystals. We show that both the redox potentials and the maximum number of conduction-band electrons that can be accumulated through photodoping are strongly affected by the nanocrystal’s surface stoichiometry, varying reproducibly by over 400 meV with changes in relative Cd2+:Se2– surface concentration. The data suggest that Se2– enrichment generates electric dipoles at the nanocrystal surfaces that shift the CdSe nanocrystal band-edge potentials to more negative values, and these generated dipoles are largely eliminated upon Cd2+ binding. These results demonstrate the importance of nanocrystal surface stoichiometry in applications involving tuned nanocrystal redox potentials, band-edge alignment, or electron-transfer driving forces.

Solar water splitting using catalyst-modified semiconductor photoelectrodes is a promising approach to harvesting and storing solar energy. Prior studies have demonstrated that modification of α-Fe2O3 photoanodes with the water-oxidation electrocatalyst Co-Pi enhances photon-to-current conversion efficiencies, particularly at less positive potentials, but the mechanism underlying this enhancement remains poorly understood. Different experimental techniques have suggested very different interpretations of the microscopic origins of this improvement. Here, we report results from photoelectrochemical and impedance measurements aimed at understanding the Co-Pi/α-Fe2O3 interface of mesostructured composite photoanodes. Contrary to expectations, these measurements reveal that α-Fe2O3 water-oxidation kinetics actually slow upon deposition of Co-Pi, but electron–hole recombination slows even more, resulting in a net enhancement of water-oxidation quantum efficiency. The negative shift in the J–V curve caused by Co-Pi deposition is found to result from the introduction of an alternative pathway for water oxidation catalyzed by Co-Pi, which allows the composite photoanode to avoid positive charge accumulation at the α-Fe2O3 surface. We detail the role of Co-Pi thickness optimization in balancing the slower recombination against the slower water oxidation kinetics to achieve the lowest water-oxidation onset potential. These results provide new insights into the microscopic properties of the catalyst/semiconductor interface in Co-Pi/α-Fe2O3 composite solar water-splitting photoanodes.

The physical properties of semiconductor nanocrystals can be tuned dramatically via composition control. Here, we report a detailed investigation of the synthesis of high-quality colloidal Cd1–xMnxSe nanocrystals by diffusion doping of preformed CdSe nanocrystals. Until recently, Cd1–xMnxSe nanocrystals proved elusive because of kinetic incompatibilities between Mn2+ and Cd2+ chemistries. Diffusion doping allows Cd1–xMnxSe nanocrystals to be prepared under thermodynamic rather than kinetic control, allowing access to broader composition ranges. We now investigate this chemistry as a model system for understanding the characteristics of nanocrystal diffusion doping more deeply. From the present work, a Se2–-limited reaction regime is identified, in which Mn2+ diffusion into CdSe nanocrystals is gated by added Se2–, and equilibrium compositions are proportional to the amount of added Se2–. At large added Se2– concentrations, a solubility-limited regime is also identified, in which x = xmax = ∼0.31, independent of the amount of added Se2–. We further demonstrate that Mn2+ in-diffusion can be reversed by cation exchange with Cd2+ under exactly the same reaction conditions, purifying Cd1–xMnxSe nanocrystals back to CdSe nanocrystals with fine tunability. These chemistries offer exceptional composition control in Cd1–xMnxSe NCs, providing opportunities for fundamental studies of impurity diffusion in nanocrystals and for development of compositionally tuned nanocrystals with diverse applications ranging from solar energy conversion to spin-based photonics.Keywords: cation exchange; diffusion doping; giant Zeeman effect; magnetic circular dichroism; nanocrystals; quantum dots;

Single-nanocrystal and ensemble photoluminescence measurements on CuInS2 semiconductor nanocrystals reveal luminescence bandshapes that are broad compared to those typical of individual II–VI or related semiconductor nanocrystals. This finding is consistent with the hypothesis of strong electron–phonon coupling in the emissive excited state of these CuInS2 semiconductor nanocrystals. Blinking is observed that resembles that of other semiconductor nanocrystals. Ensemble luminescence measurements also reveal the existence of a remarkably long-lived excited state in these nanocrystals that continues to emit photons over several orders of magnitude in time following the excitation pulse. The delayed luminescence overlaps in time and shows similar distributed kinetics to the blinking “off” times of the same nanocrystal sample, supporting the proposal that these two phenomena arise from the same microscopic carrier-trapping and -detrapping processes. Excitation power dependence measurements illustrate that the delayed luminescence saturates at very low emission intensities under the excitation power densities used in the single-nanocrystal measurements, consistent with this metastable charge-trapped state being the “off” state of the luminescence blinking cycle.

The electronic structures of copper-doped CdSe nanocrystals (NCs) are investigated using time-dependent density functional theory. Comparison of the electronic structures of Cu+- and Cu2+-doped NCs indicates that only the Cu+ ground state is consistent with the experimental absorption and photoluminescence (PL) spectra of copper-doped NCs, Cu2+-doped NCs being characterized by low-energy charge-transfer and d–d excited states that quench visible PL. In the luminescent metal-to-conduction-band charge-transfer (MLCBCT) excited state of the Cu+-doped CdSe NCs, the photogenerated hole is calculated to be localized at the copper dopant. Strong electron–phonon coupling in this MLCBCT excited state causes substantial geometric distortion along totally symmetric and Jahn–Teller nuclear coordinates, with a correspondingly large excited-state nuclear reorganization energy. This excited-state nuclear reorganization causes the broad PL band shape and large PL Stokes shift observed experimentally. Singlet and triplet MLCBCT excited-state configurations are also examined computationally. The sign and strength of the computed magnetic exchange coupling between the conduction-band electron’s spin and the copper-localized spin are both consistent with experimental results. These calculations yield fundamental insights into the electronic structures and photophysical properties of copper-doped semiconductor NCs relevant to their potential application as spectral conversion phosphors in lighting and solar technologies.

The photoluminescence decay dynamics of colloidal CdSe, Cu+:CdSe, and CuInS2 nanocrystals have been examined as a function of temperature and magnetic field. All three materials show photoluminescence decay on time scales significantly longer than the intrinsic lifetimes of their luminescent excited states, i.e., delayed luminescence, involving formation of a metastable trapped excited state followed by detrapping to re-form the emissive excited state. Surprisingly, the delayed luminescence decay kinetics are nearly identical for these three very different materials, suggesting they reflect universal properties of the delayed luminescence phenomenon in semiconductor nanocrystals. By measuring luminescence decay over 8 decades in time and 6 decades in intensity, we observe for the first time a clear deviation from power-law dynamics in delayed luminescence. Furthermore, for all three materials, the delayed luminescence decay dynamics are observed to be nearly independent of temperature between 20 K and room temperature, reflecting tunneling as the dominant mechanism for detrapping from the metastable state. A kinetic model is introduced that invokes a log-normal distribution of tunneling rates and reproduces the full range of delayed luminescence decay dynamics well. These findings are discussed in relation to photoluminescence blinking, with which delayed luminescence appears closely associated.

Interfacing α-Fe2O3 photoanodes with the water-oxidation electrocatalyst Co-Pi is known to enhance their photon-to-current conversion efficiencies by reducing electron–hole recombination near their surfaces, particularly at more negative potentials, but the mechanism by which Co-Pi modification achieves this enhancement remains poorly understood. Conflicting experimental observations have been recorded with respect to the role of Co-Pi thickness and even the participation of Co-Pi in catalysis, raising important general questions concerning the fundamental properties of catalyst-modified PEC water-oxidation photoanodes for solar energy conversion. Here, we report results from electrochemical, spectroscopic, and microscopic measurements on mesostructured Co-Pi/α-Fe2O3 composite photoanodes that reveal evolving pathways of water oxidation with increasing Co-Pi thickness. These results highlight major fundamental differences between structured and planar Co-Pi/α-Fe2O3 composite photoanodes and help to reconcile previously conflicting mechanistic interpretations.

Electronic doping is one of the most important experimental capabilities in all of semiconductor research and technology. Through electronic doping, insulating materials can be made conductive, opening doors to the formation of p–n junctions and other workhorses of modern semiconductor electronics. Recent interest in exploiting the unique physical and photophysical properties of colloidal semiconductor nanocrystals for revolutionary new device technologies has stimulated efforts to prepare electronically doped colloidal semiconductor nanocrystals with the same control as available in the corresponding bulk materials. Despite the impact that success in this endeavor would have, the development of general and reliable methods for electronic doping of colloidal semiconductor nanocrystals remains a long-standing challenge.In this Account, we review recent progress in the development and characterization of electronically doped colloidal semiconductor nanocrystals. Several successful methods for introducing excess band-like charge carriers are illustrated and discussed, including photodoping, outer-sphere electron transfer, defect doping, and electrochemical oxidation or reduction. A distinction is made between methods that yield excess band-like carriers at thermal equilibrium and those that inject excess charge carriers under thermal nonequilibrium conditions (steady state). Spectroscopic signatures of such excess carriers, accessible by both equilibrium and nonequilibrium methods, are reviewed and illustrated. A distinction is also proposed between the phenomena of electronic doping and redox-potential shifting. Electronically doped semiconductor nanocrystals possess excess band-like charge carriers at thermal equilibrium, whereas redox-potential shifting affects the potentials at which charge carriers are injected under nonequilibrium conditions, without necessarily introducing band-like charge carriers at equilibrium. Detection of the key spectroscopic signatures of band-like carriers allows distinction between these two regimes. Both electronic doping and redox-potential shifting can be powerful tools for tuning the performance of nanocrystals in electronic devices. Finally, key chemical challenges associated with nanocrystal electronic doping are briefly discussed. These challenges are centered largely on the availability of charge-carrier reservoirs with suitable redox potentials and on the relatively poor control over nanocrystal surface traps. In most cases, the Fermi levels of colloidal nanocrystals are defined by the redox properties of their surface traps. Control over nanocrystal surface chemistries is therefore essential to the development of general and reliable strategies for electronically doping colloidal semiconductor nanocrystals. Overall, recent progress in this area portends exciting future advances in controlling nanocrystal compositions, surface chemistries, redox potentials, and charge states to yield new classes of electronic nanomaterials with attractive physical properties and the potential to stimulate unprecedented new semiconductor technologies.

Single-particle photoluminescence blinking is observed in the copper-centered deep-trap luminescence of copper-doped CdSe (Cu+:CdSe) nanocrystals. Blinking dynamics for Cu+:CdSe and undoped CdSe nanocrystals are analyzed to identify the effect of Cu+, which selectively traps photogenerated holes. Analysis of the blinking data reveals that the Cu+:CdSe and CdSe nanocrystal “off”-state dynamics are statistically identical, but the Cu+:CdSe nanocrystal “on” state is shorter lived. Additionally, a new and pronounced temperature-dependent delayed luminescence is observed in the Cu+:CdSe nanocrystals that persists long beyond the radiative lifetime of the luminescent excited state. This delayed luminescence is analogous to the well-known donor–acceptor pair luminescence of bulk copper-doped phosphors and is interpreted as revealing metastable charge-separated excited states formed by reversible electron trapping at the nanocrystal surfaces. A mechanistic link between this delayed luminescence and the luminescence blinking is proposed. Collectively, these data suggest that electron (rather than hole) trapping/detrapping is responsible for photoluminescence intermittency in these nanocrystals.

Luminescent solar concentrators (LSCs) harvest sunlight over large areas and concentrate this energy onto photovoltaics or for other uses by transporting photons through macroscopic waveguides. Although attractive for lowering solar energy costs, LSCs remain severely limited by luminophore reabsorption losses. Here, we report a quantitative comparison of four types of nanocrystal (NC) phosphors recently proposed to minimize reabsorption in large-scale LSCs: two nanocrystal heterostructures and two doped nanocrystals. Experimental and numerical analyses both show that even the small core absorption of the leading NC heterostructures causes major reabsorption losses at relatively short transport lengths. Doped NCs outperform the heterostructures substantially in this critical property. A new LSC phosphor is introduced, nanocrystalline Cd1–xCuxSe, that outperforms all other leading NCs by a significant margin in both small- and large-scale LSCs under full-spectrum conditions.

Electronically doped colloidal semiconductor nanocrystals offer valuable opportunities to probe the new physical and chemical properties imparted by their excess charge carriers. Photodoping is a powerful approach to introducing and controlling free carrier densities within free-standing colloidal semiconductor nanocrystals. Photoreduced (n-type) colloidal ZnO nanocrystals possessing delocalized conduction-band (CB) electrons can be formed by photochemical oxidation of EtOH. Previous studies of this chemistry have demonstrated photochemical electron accumulation, in some cases reaching as many as >100 electrons per ZnO nanocrystal, but in every case examined to date this chemistry maximizes at a well-defined average electron density of ⟨Nmax⟩ ≈ (1.4 ± 0.4) × 1020 cm–3. The origins of this maximum have never been identified. Here, we use a solvated redox indicator for in situ determination of reduced ZnO nanocrystal redox potentials. The Fermi levels of various photodoped ZnO nanocrystals possessing on average just one excess CB electron show quantum-confinement effects, as expected, but are >600 meV lower than those of the same ZnO nanocrystals reduced chemically using Cp*2Co, reflecting important differences between their charge-compensating cations. Upon photochemical electron accumulation, the Fermi levels become independent of nanocrystal volume at ⟨N⟩ above ∼2 × 1019 cm–3, and maximize at ⟨Nmax⟩ ≈ (1.6 ± 0.3) × 1020 cm–3. This maximum is proposed to arise from Fermi-level pinning by the two-electron/two-proton hydrogenation of acetaldehyde, which reverses the EtOH photooxidation reaction.

The performance of colloidal CuInS2/CdS nanocrystals as phosphors for full-spectrum luminescent solar concentrators has been examined. Their combination of large solar absorption, high photoluminescence quantum yields, and only moderate reabsorption produces the highest projected flux gains of any nanocrystal luminophore to date.

Impurity ions can transform the electronic, magnetic, or optical properties of colloidal quantum dots. Magnetic impurities introduce strong dopant-carrier exchange coupling that generates giant Zeeman splittings (ΔEZ) of excitonic excited states. To date, ΔEZ in colloidal doped quantum dots has primarily been quantified by analysis of magnetic circular dichroism (MCD) intensities and absorption line widths (σ). Here, we report ΔEZ values detected directly by absorption spectroscopy for the first time in such materials, using colloidal Cd1–xMnxSe quantum dots prepared by diffusion doping. A convenient method for decomposing MCD and absorption data into circularly polarized absorption spectra is presented. These data confirm the widely applied MCD analysis in the low-field, high-temperature regime, but also reveal a breakdown at low temperatures and high fields when ΔEZ/σ approaches unity, a situation not previously encountered in doped quantum dots. This breakdown is apparent for the first time here because of the extraordinarily large ΔEZ and small σ achieved by nanocrystal diffusion doping.

Spontaneous magnetization is observed at zero magnetic field in photoexcited colloidal Cd1–xMnxSe (x = 0.13) quantum dots (QDs) prepared by diffusion doping, reflecting strong Mn2+–exciton exchange coupling. The picosecond dynamics of this phenomenon, known as an excitonic magnetic polaron (EMP), are examined using a combination of time-resolved photoluminescence, magneto-photoluminescence, and Faraday rotation (TRFR) spectroscopies, in conjunction with continuous-wave absorption, magnetic circular dichroism (MCD), and magnetic circularly polarized photoluminescence (MCPL) spectroscopies. The data indicate that EMPs form with random magnetization orientations at zero external field, but their formation can be directed by an external magnetic field. After formation, however, external magnetic fields are unable to reorient the EMPs within the luminescence lifetime, implicating anisotropy in the EMP potential-energy surfaces. TRFR measurements in a transverse magnetic field reveal rapid (<5 ps) spin transfer from excitons to Mn2+ followed by coherent EMP precession at the Mn2+ Larmor frequency for over a nanosecond. A dynamical TRFR phase inversion is observed during EMP formation attributed to the large shifts in excitonic absorption energies during spontaneous magnetization. Partial optical orientation of the EMPs by resonant circularly polarized photoexcitation is also demonstrated. Collectively, these results highlight the extraordinary physical properties of colloidal diffusion-doped Cd1–xMnxSe QDs that result from their unique combination of strong quantum confinement, large Mn2+ concentrations, and relatively narrow size distributions. The insights gained from these measurements advance our understanding of spin dynamics and magnetic exchange in colloidal doped semiconductor nanostructures, with potential ramifications for future spin-based information technologies.Keywords: diluted magnetic semiconductors; doped quantum dots; magnetic polarons; magneto-luminescence; spin dynamics; sp−d exchange;

Colloidal impurity-doped quantum dots (QDs) are attractive model systems for testing the fundamental spin properties of semiconductor nanostructures relevant to future spin-based information processing technologies. Although static spin properties of this class of materials have been studied extensively in recent years, their spin dynamics remain largely unexplored. Here we use pulsed electron paramagnetic resonance (pEPR) spectroscopy to probe the spin relaxation dynamics of colloidal Mn2+-doped ZnO, ZnSe, and CdSe quantum dots in the limit of one Mn2+ per QD. pEPR spectroscopy is particularly powerful for identifying the specific nuclei that accelerate electron spin relaxation in these QDs. We show that the spin-relaxation dynamics of these colloidal QDs are strongly influenced by dipolar coupling with proton nuclear spins outside the QDs and especially those directly at the QD surfaces. Using this information, we demonstrate that spin-relaxation times can be elongated significantly via ligand (or surface) deuteration or shell growth, providing two tools for chemical adjustment of spin dynamics in these nanomaterials. These findings advance our understanding of the spin properties of solution-grown semiconductor nanostructures relevant to spin-based information technologies.

Colloidal diluted magnetic semiconductor (DMS) nanocrystals are model systems for studying spin effects in semiconductor nanostructures with relevance to future spin-based information processing technologies. The introduction of excess delocalized charge carriers into such nanocrystals turns on strong dopant–carrier magnetic exchange interactions, with important consequences for the physical properties of these materials. Here, we use pulsed electron paramagnetic resonance (pEPR) spectroscopy to probe the effects of excess conduction band electrons on the spin dynamics of colloidal Mn2+-doped ZnO nanocrystals. Mn2+ spin–lattice relaxation is strongly accelerated by the addition of even one conduction band electron per Zn1–xMnxO nanocrystal, attributable to the introduction of a new exchange-based Mn2+ spin relaxation pathway. A kinetic model is used to describe the enhanced relaxation rates, yielding new insights into the spin dynamics and electronic structures of these materials with potential ramifications for future applications of DMS nanostructures in spin-based technologies.

We report a systematic investigation of the size dependence of negative trion (T–) Auger recombination rates in free-standing colloidal CdSe nanocrystals. Colloidal n-type CdSe nanocrystals of various radii have been prepared photochemically, and their trion decay dynamics have been measured using time-resolved photoluminescence spectroscopy. Trion Auger time constants spanning 3 orders of magnitude are observed, ranging from 57 ps (radius R = 1.4 nm) to 2.2 ns (R = 3.2 nm). The data reveal a substantially stronger size dependence than found for bi- or multiexciton Auger recombination in CdSe or other semiconductor nanocrystals, scaling in proportion to R4.3.

Optical concentration can lower the cost of solar energy conversion by reducing photovoltaic cell area and increasing photovoltaic efficiency. Luminescent solar concentrators offer an attractive approach to combined spectral and spatial concentration of both specular and diffuse light without tracking, but they have been plagued by luminophore self-absorption losses when employed on practical size scales. Here, we introduce doped semiconductor nanocrystals as a new class of phosphors for use in luminescent solar concentrators. In proof-of-concept experiments, visibly transparent, ultraviolet-selective luminescent solar concentrators have been prepared using colloidal Mn2+-doped ZnSe nanocrystals that show no luminescence reabsorption. Optical quantum efficiencies of 37% are measured, yielding a maximum projected energy concentration of ∼6× and flux gain for a-Si photovoltaics of 15.6 in the large-area limit, for the first time bounded not by luminophore self-absorption but by the transparency of the waveguide itself. Future directions in the use of colloidal doped nanocrystals as robust, processable spectrum-shifting phosphors for luminescent solar concentration on the large scales required for practical application of this technology are discussed.Keywords: doped nanocrystals; solar concentrators; solar energy; transparent solar collector

Nanomaterials exhibiting plasmonic optical responses are impacting sensing, information processing, catalysis, solar, and photonics technologies. Recent advances have expanded the portfolio of plasmonic nanostructures into doped semiconductor nanocrystals, which allow dynamic manipulation of carrier densities. Once interpreted as intraband single-electron transitions, the infrared absorption of doped semiconductor nanocrystals is now commonly attributed to localized surface plasmon resonances and analyzed using the classical Drude model to determine carrier densities. Here, we show that the experimental plasmon resonance energies of photodoped ZnO nanocrystals with controlled sizes and carrier densities diverge from classical Drude model predictions at small sizes, revealing quantum plasmons in these nanocrystals. A Lorentz oscillator model more adequately describes the data and illustrates a closer link between plasmon resonances and single-electron transitions in semiconductors than in metals, highlighting a fundamental contrast between these two classes of plasmonic materials.Keywords: doped nanocrystals; photodoping; plasmons; quantum dots; zinc oxide

The electronic structures of n-type ZnO nanocrystals formed via photochemical reduction and by aliovalent doping with aluminum are investigated using time-dependent density functional theory. Connections between the density functional theory results and a simple quantum-mechanical particle-in-a-spherical-potential model are highlighted. Molecular orbitals obtained from density functional theory reveal the often-invoked S-, P-, D-, ... type “super” orbitals used to characterize the absorption spectra of these materials.

The roles of nanocrystal shape and crystalline anisotropy on carrier-mediated magnetism in diluted magnetic semiconductor nanocrystals have been examined in this work. A combination of density functional theory and analytical perturbation theory are used to investigate the electronic structures of Mn2+-doped nanocrystals of varied shapes and sizes. Density functional calculations are used to compute the magnetic exchange energies and to analyze the band edge splitting in these materials. To understand the observed anisotropic magnetism, we have derived a perturbative relationship between the magnetic exchange and nanocrystal anisotropy to illustrate the effect in an analytical and quantum mechanical expression. The first-principles calculations and the analytical predictions are in excellent agreement, and the anisotropic driving terms in the magnetic exchange are very well explained by the final perturbation equation.

The acceleration of Auger-type multicarrier recombination in semiconductor nanocrystals impedes the development of many quantum-dot photonics, solar-cell, lighting, and lasing technologies. To date, only multiexciton and charged-exciton Auger recombination channels are known to show strong size dependence in nanocrystals. Here, we report the first observation of strongly accelerated “trap-assisted” Auger recombination rates in semiconductor nanocrystals. Trap-assisted Auger recombination in ZnO nanocrystals, involving the recombination of conduction-band electrons with deeply trapped holes via nonradiative energy transfer to extra conduction-band electrons, has been probed using time-resolved photoluminescence and transient absorption spectroscopies. We demonstrate that this trap-assisted Auger recombination accelerates dramatically with decreasing nanocrystal size, having recombination times of >1 ns in the largest nanocrystals but only ∼80 ps in the smallest. These trap-assisted Auger recombination rates are shown to scale with inverse nanocrystal radius squared (1/τAuger ∼ R–2). Because surface carrier traps are ubiquitous in colloidal semiconductor nanocrystals, such fast trap-assisted Auger recombination is likely more prevalent in semiconductor nanocrystal photophysics than previously recognized.

Photodoped colloidal ZnO nanocrystals are model systems for understanding the generation and physical or chemical properties of excess delocalized charge carriers in semiconductor nanocrystals. Typically, ZnO photodoping is achieved photochemically using ethanol (EtOH) as a sacrificial reductant. Curiously, different studies have reported over an order of magnitude spread in the maximum number of conduction-band electrons that can be accumulated by photochemical oxidation of EtOH. Here, we demonstrate that this apparent discrepancy results from a strong size dependence of the average maximum number of excess electrons per nanocrystal, ⟨nmax⟩. We demonstrate that ⟨nmax⟩ increases in proportion to nanocrystal volume, such that the maximum carrier density remains constant for all nanocrystal sizes. ⟨nmax⟩ is found to be largely insensitive to precise experimental conditions such as solvent, ligands, protons or other cations, photolysis conditions, and nanocrystal or EtOH concentrations. These results reconcile the broad range of literature results obtained with EtOH as the hole quencher. Furthermore, we demonstrate that ⟨nmax⟩ depends on the identity of the hole quencher, and is thus not an intrinsic property of the multiply reduced ZnO nanocrystals themselves. Using a series of substituted borohydride hole quenchers, we show that it is possible to increase the nanocrystal carrier densities over 4-fold relative to previous photodoping reports. When excess lithium and potassium triethylborohydrides are used in the photodoping, formation of Zn0 is observed. The relationship between metallic Zn0 formation and ZnO surface electron traps is discussed.

A method for electronic doping of colloidal CdSe nanocrystals (NCs) is reported. Anaerobic photoexcitation of CdSe NCs in the presence of a borohydride hole quencher, Li[Et3BH], yields colloidal n-type CdSe NCs possessing extra conduction-band electrons compensated by cations deposited by the hydride hole quencher. The photodoped NCs possess excellent optical quality and display the key spectroscopic signatures associated with NC n-doping, including a bleach at the absorption edge, appearance of a new IR absorption band, and Auger quenching of the excitonic photoluminescence. Although stable under anaerobic conditions, these spectroscopic changes are all reversed completely upon exposure of the n-doped NCs to air. Chemical titration of the added electrons confirms previous correlations between absorption bleach and electron accumulation and provides a means of quantifying the extent of electron trapping in some NCs. The generality of this photodoping method is demonstrated by initial results on colloidal CdE (E = S, Te) NCs as well as on CdSe quantum dot films.

A diffusion-based synthesis of doped colloidal semiconductor nanocrystals is demonstrated. This approach involves thermodynamically controlled addition of both impurity cations and host anions to preformed seed nanocrystals under equilibrium conditions, rather than kinetically controlled doping during growth. This chemistry allows thermodynamic crystal compositions to be prepared without sacrificing other kinetically trapped properties such as shape, size, or crystallographic phase. This doping chemistry thus shares some similarities with cation-exchange reactions, but proceeds without the loss of host cations and excels at the introduction of relatively unreactive impurity ions that have not been previously accessible using cation exchange. Specifically, we demonstrate the preparation of Cd1–xMnxSe (0 ≤ x ≤ ∼0.2) nanocrystals with narrow size distribution, unprecedentedly high Mn2+ content, and very large magneto-optical effects by diffusion of Mn2+ into seed CdSe nanocrystals grown by hot injection. Controlling the solution and lattice chemical potentials of Cd2+ and Mn2+ allows Mn2+ diffusion into the internal volumes of the CdSe nanocrystals with negligible Ostwald ripening, while retaining the crystallographic phase (wurtzite or zinc blende), shape anisotropy, and ensemble size uniformity of the seed nanocrystals. Experimental results for diffusion doping of other nanocrystals with other cations are also presented that indicate this method may be generalized, providing access to a variety of new doped semiconductor nanostructures not previously attainable by kinetic routes or cation exchange.

Soluble luminescent temperature probes are promising candidates for optical thermometry and thermography applications requiring precise, passive, and spatially resolved temperature data. Dual-emitting temperature sensors overcome many of the obstacles encountered with absolute intensity-based luminescence sensors, including optical occlusion, concentration variation, or nonspecificity, by providing internally referenced (ratiometric) signals. Here, we provide an overview of the key mechanisms underpinning the dual emission of various nanostructures from recent literature and discuss their relationship to optical thermometry.Keywords: dual emission; energy transfer; Mn2+ luminescence; optical thermometry; ratiometric sensing;

Alloyed Zn1−x−yCdxMnySe nanocrystals exhibiting bright intrinsic dual emission attractive for ratiometric optical nanothermometry are reported. Relative to earlier materials with related dual emission, these alloy nanocrystals require shorter syntheses, contain less Cd2+, and show dual emission that is less sensitive to nanocrystal shape, size, or surfaces.

Colloidal reduced ZnO nanocrystals are potent reductants for one-electron or multielectron redox chemistry, with reduction potentials tunable via the quantum confinement effect. Other methods for tuning the redox potentials of these unusual reagents are desired. Here, we describe synthesis and characterization of a series of colloidal Zn1–xMgxO and Zn0.98–xMgxMn0.02O nanocrystals in which Mg2+ substitution is used to tune the nanocrystal reduction potential. The effect of Mg2+ doping on the band-edge potentials of ZnO was investigated using electronic absorption, photoluminescence, and magnetic circular dichroism spectroscopies. Mg2+ incorporation widens the ZnO gap by raising the conduction-band potential and lowering the valence-band potential at a ratio of 0.68:0.32. Mg2+ substitution is far more effective than Zn2+ removal in raising the conduction-band potential and allows better reductants to be prepared from Zn1–xMgxO nanocrystals than can be achieved via quantum confinement of ZnO nanocrystals. The increased conduction-band potentials of Zn1–xMgxO nanocrystals compared to ZnO nanocrystals are confirmed by demonstration of spontaneous electron transfer from n-type Zn1–xMgxO nanocrystals to smaller (more strongly quantum confined) ZnO nanocrystals.

Chemical reductants of sub-conduction-band potentials are demonstrated to induce large photoluminescence enhancement in colloidal ZnSe-based nanocrystals. The photoluminescence quantum yield of colloidal Mn2+-doped ZnSe nanocrystals has been improved from 14% to 80% simply by addition of an outer-sphere reductant. Up to 48-fold redox brightening is observed for nanocrystals with lower starting quantum yields. These increases are quickly reversed upon exposure to air and are temporary even under anaerobic conditions. This redox brightening process offers a new and systematic approach to understanding redox-active surface “trap states” and their contributions to the physical properties of colloidal semiconductor nanocrystals.

Spectroelectrochemical experiments on wide-gap semiconductor nanocrystals (ZnSe and Mn2+-doped ZnSe) have allowed the influence of trap electrochemistry on nanocrystal photoluminescence to be examined in the absence of semiconductor band filling. Large photoluminescence electrobrightening is observed in both materials upon application of a reducing potential and is reversed upon return to the equilibrium potential. Electrobrightening is correlated with the transfer of electrons into nanocrystal films, implicating reductive passivation of midgap surface electron traps. Analysis indicates that the electrobrightening magnitude is determined by competition between electron trapping and photoluminescence (ZnSe) or energy transfer (Mn2+-doped ZnSe) dynamics within the excitonic excited state, and that electron trapping is extremely fast (ktrap ≈ 1011 s–1). These results shed new light on the complex surface chemistries of semiconductor nanocrystals.

The “extra” electrons in colloidal n-type ZnO nanocrystals formed by aliovalent doping and photochemical reduction are compared. Whereas the two are similar spectroscopically, they show very different electron-transfer reactivities, attributable to their different charge-compensating cations (Al3+vs. H+).

Variable-temperature magnetic circular dichroism (MCD) spectroscopy is used to measure excitonic Zeeman splittings in colloidal Co2+- and Mn2+-doped CdSe quantum dots with low dopant concentrations. The data demonstrate that the competition between intrinsic and exchange contributions to the excitonic Zeeman splittings in doped quantum dots can be tuned using temperature, from being dominated by exchange at low temperatures to being dominated by intrinsic Zeeman interactions at room temperature, with inversion at easily accessible temperatures and fields. These results may have relevance to spin-based information processing technologies that rely on manipulating carrier spins in quantum dots.Keywords: doped quantum dot; excitonic Zeeman splitting; magnetic circular dichroism;

Photochemical charging of colloidal ZnO nanocrystals has been studied using continuous-wave and time-resolved photoluminescence spectroscopies in conjunction with electron paramagnetic resonance spectroscopy. Experiments have been performed with and without addition of alcohols as hole quenchers, focusing on ethanol. Both aerobic and anaerobic conditions have been examined. We find that ethanol quenches valence-band holes within ∼15 ps of photoexcitation, but does not quench the trapped holes responsible for the characteristic visible photoluminescence of ZnO nanocrystals. Hole quenching yields “charged” nanocrystals containing excess conduction-band electrons. The extra conduction-band electrons quench visible trap-centered luminescence via a highly effective electron/trap-state Auger-type cross-relaxation process. This Auger process is prominent even under aerobic photoexcitation conditions, particularly when samples are not stirred. Charging also reduces exciton nonradiative decay rates, resulting in increased UV luminescence. The dependence of charging on ethanol concentration and the reduced exciton nonradiative decay rates of charged ZnO nanocrystals are discussed. Finally, the results here provide a kinetic basis for understanding photochemical electron accumulation in colloidal ZnO nanocrystals.

Colloidal Mn2+-doped semiconductor nanocrystals such as Mn2+:ZnSe have attracted broad attention for potential applications in phosphor and imaging technologies. Here, we report saturation of the sensitized Mn2+ photoluminescence intensity at very low continuous-wave (CW) and quasi-CW photoexcitation powers under conditions that are relevant to many of the proposed applications. Time-resolved photoluminescence measurements and kinetic modeling indicate that this saturation arises from an Auger-type nonradiative cross relaxation between an excited Mn2+ ion and an exciton within the same nanocrystal. A lower limit of k = 2 × 1010 s–1 is established for the fundamental rate constant of the Mn2+(4T1)-exciton cross relaxation.

A quantitative method for analysis of fast Mn2+-centered Auger processes in Mn2+-doped semiconductors is introduced. Analytical expressions have been derived that describe the Coulomb (″direct″) and exchange (″sp–d″) contributions to Mn2+-electron Auger de-excitation rate constants. All of the quantities in these expressions can be calculated using existing electronic structure methods without the computational expense of multireference approaches. Using time-dependent density functional theory to generate input parameters, these expressions have been applied to calculate Auger de-excitation rate constants in Mn2+-doped CdS nanocrystals. The results of these calculations agree well with experiment. Analysis reveals that the large rate constants for Auger de-excitation in Mn2+-doped semiconductors are primarily attributable to effective s–d exchange coupling.

A photo-assisted electrodeposition approach was used to deposit a cobalt–phosphate water oxidation catalyst (“Co–Pi”) onto recently improved dendritic mesostructures of α-Fe2O3. A comparison between this approach, electrodeposition of Co–Pi, and Co2+ wet impregnation showed that photo-assisted electrodeposition of Co–Pi yields superior α-Fe2O3 photoanodes for photoelectrochemical water oxidation. Stable photocurrent densities of 1.0 mA cm−2 at 1.0 V and 2.8 mA cm−2 at 1.23 V vs. RHE measured under standard illumination and basic conditions were achieved. By allowing deposition only where visible light generates oxidizing equivalents, photo-assisted electrodeposition provides a more uniform distribution of Co–Pi onto α-Fe2O3 than obtained by electrodeposition. This approach of fabricating catalyst-modified metal-oxide photoelectrodes may be attractive for optimization in conjunction with tandem or hybrid photoelectrochemical cells.

We use time-resolved Faraday rotation spectroscopy to probe the electron spin dynamics in ZnO and magnetically doped Zn1–xCoxO sol–gel thin films. In undoped ZnO, we observe an anomalous temperature dependence of the ensemble spin dephasing time T2*, i.e., longer coherence times at higher temperatures, reaching T2* ∼ 1.2 ns at room temperature. Time-resolved transmission measurements suggest that this effect arises from hole trapping at grain surfaces. Deliberate addition of Co2+ to ZnO increases the effective electron Landé g factor, providing the first direct determination of the mean-field electron-Co2+ exchange energy in Zn1–xCoxO (N0α = +0.25 ± 0.02 eV). In Zn1–xCoxO, T2* also increases with increasing temperature, allowing spin precession to be observed even at room temperature.

Colloidal ZnO nanocrystals capped with dodecylamine and dissolved in toluene can be charged photochemically to give stable solutions in which electrons are present in the conduction bands of the nanocrystals. These conduction-band electrons are readily monitored by EPR spectroscopy, with g* values that correlate with the nanocrystal sizes. Mixing a solution of charged small nanocrystals (e−CB:ZnO−S) with a solution of uncharged large nanocrystals (ZnO−L) caused changes in the EPR spectrum indicative of quantitative electron transfer from small to large nanocrystals. EPR spectra of the reverse reaction, e−CB:ZnO−L + ZnO−S, showed that electrons do not transfer from large to small nanocrystals. Stopped-flow kinetics studies monitoring the change in the UV band-edge absorption showed that reactions of 50 μM nanocrystals were complete within the 5 ms mixing time of the instrument. Similar results were obtained for the reaction of charged nanocrystals with methyl viologen (MV2+). These and related results indicate that the electron-transfer reactions of these colloidal nanocrystals are quantitative and very rapid, despite the presence of ∼1.5 nm long dodecylamine capping ligands. These soluble ZnO nanocrystals are thus well-defined redox reagents suitable for studies of electron transfer involving semiconductor nanostructures.

Multishell semiconductor nanocrystals have been synthesized that display intrinsic dual emission with robust photo and thermal stability and attractive thermal sensitivity. Dual emission is demonstrated following phase transfer into aqueous media. These nanocrystals are suitable for diverse optical thermometric or thermographic applications in biotechnology or other areas.

The influence of an earth-abundant water oxidation electrocatalyst (Co-Pi) on solar water oxidation by W:BiVO4 has been studied using photoelectrochemical (PEC) techniques. Modification of W:BiVO4 photoanode surfaces with Co-Pi has yielded a very large (∼440 mV) cathodic shift in the onset potential for sustained PEC water oxidation at pH 8. PEC experiments with H2O2 as a surrogate substrate have revealed that interfacing Co-Pi with these W:BiVO4 photoanodes almost completely eliminates losses due to surface electron–hole recombination. The results obtained for W:BiVO4 are compared with those reported recently for Co-Pi/α-Fe2O3 photoanodes. The low absolute onset potential of ∼310 mV vs RHE achieved with the Co-Pi/W:BiVO4 combination is promising for overall solar water splitting in low-cost tandem PEC cells, and is encouraging for application of this surface modification strategy to other candidate photoanodes.

Absorption spectra of Mn2+-doped ZnO quantum dots have been studied with the linear-response time-dependent density functional theory. Spectral changes caused by excited state dopant-carrier and dopant–dopant magnetic exchange couplings are investigated. The excitonic transition maximum shifts to higher energy and decreases in intensity with increasing Mn2+ concentration. The lowest excitonic transitions split in the spin-up and spin-down manifolds due to sp–d magnetic exchange between the Mn2+ and ZnO conduction and valence band carriers. Increased Mn2+ concentration leads to a broadening and increase in the intensity of the midgap charge-transfer electronic absorption band. The charge-transfer band broadening results from excited-state splitting arising from double exchange magnetic interactions involving Mn2+ ions and the photogenerated hole. The excited-state double exchange leads to stabilization of the ferromagnetic configuration in the charge-transfer state. The strength of this ferromagnetic double exchange interaction depends on the inter-Mn2+ distance within the quantum dot.

Auger processes in colloidal semiconductor nanocrystals have been scrutinized extensively in recent years. Whether involving electron−exciton, hole−exciton, or exciton−exciton interactions, such Auger processes are generally fast and hence have been considered prominent candidates for interpreting fast processes relevant to photoluminescence blinking and multiexciton decay. With recent advances in the chemistries of nanocrystal doping, increasing attention is now being paid to analogous photophysical properties of colloidal-doped semiconductor nanocrystals. Here, we report the first investigation of the effects of electron-dopant exchange interactions on dopant luminescence in doped semiconductor nanocrystals. Using electrochemical techniques, electrical control of charge-carrier densities in films of colloidal Mn2+-doped CdS quantum dots has been achieved and used to demonstrate remarkably effective Auger de-excitation of photoexcited Mn2+. The doped nanocrystals are found to be substantially more sensitive to Auger de-excitation than their undoped analogues, a result shown to arise primarily from the long Mn2+ excited-state lifetime. This observation of exceptionally effective Auger quenching has broader implications in areas of high-power, single-particle, or electrically driven luminescence of doped semiconductor nanocrystals, and also suggests interesting opportunities for modulating Mn2+ photoluminescence intensities on sublifetime time scales, or for imaging charge carriers in nanocrystal-based devices.Keywords: auger de-excitation; charged quantum dots; cyclic voltammetry; electrochemistry; manganese-doped nanocrystals; photoluminescence

Photoelectrochemical (PEC) water splitting is an attractive approach to capturing and storing the earth's abundant solar energy influx. The challenging four-electron water-oxidation half-cell reaction has hindered this technology, giving rise to slow water oxidation kinetics at the photoanode surfaces relative to competitive loss processes. In this perspective, we review recent efforts to improve PEC efficiencies by modification of semiconductor photoanode surfaces with water-oxidation catalysts that can operate at low overpotentials. This approach allows separation of the tasks of photon absorption, charge separation, and surface catalysis, allowing each to be optimized independently. In particular, composite photoanodes marrying nanocrystalline and molecular/non-crystalline components provide flexibility in adjusting the properties of each component, but raise new challenges in interfacial chemistries.

Colloidal manganese-doped semiconductor nanocrystals have been developed that show pronounced intrinsic high-temperature dual emission. Photoexcitation of these nanocrystals gives rise to strongly temperature dependent luminescence involving two distinct but interconnected emissive excited states of the same doped nanocrystals. The ratio of the two intensities is independent of nonradiative effects. The temperature window over which pronounced dual emission is observed can be tuned by changing the nanocrystal energy gap during growth. This unique combination of properties makes this new class of intrinsic dual emitters attractive for ratiometric optical thermometry applications.

A cobalt−phosphate water oxidation catalyst (“Co−Pi”) has been electrodeposited onto mesostructured α-Fe2O3 photoanodes. The photoelectrochemical properties of the resulting composite photoanodes were optimized for solar water oxidation under frontside illumination in pH 8 electrolytes. A kinetic bottleneck limiting the performance of such photoanodes was identified and shown to be largely overcome by more sparse deposition of Co−Pi onto the α-Fe2O3. Relative to α-Fe2O3 photoanodes, a sustained 5-fold enhancement in the photocurrent density and O2 evolution rate was observed at +1.0 V vs RHE with the Co−Pi/α-Fe2O3 composite photoanodes. These results demonstrate that integration of this promising water oxidation catalyst with a photon-absorbing substrate can provide a substantial reduction in the external power needed to drive the catalyst’s electrolysis chemistry.

Electron spin relaxation dynamics in colloidal ZnO quantum dots containing additional delocalized conduction band electrons (n-type) have been studied using electron paramagnetic resonance (EPR) spectroscopy. Variation of the 67Zn (I = 5/2) nuclear isotope content within the quantum dots allows the effects of the electron−nuclear hyperfine interaction on spin−spin and spin−lattice relaxation dynamics to be explored. Long room-temperature spin−spin relaxation times of T2 = 87 ns are observed in ZnO quantum dots almost completely depleted of 67Zn.

The lowest-energy electronic excited states of ZnO quantum dots containing 1−3 Mn2+ ions have been studied by time-dependent density functional theory. The calculations show that these excited states involve holes with predominantly manganese 3d character and electrons delocalized in conduction-band-like orbitals, consistent with description of these lowest-energy excited states as charge-transfer (photoionization) states. When the quantum dot contains two or more distant Mn2+ ions, spin-dependent hole delocalization among the dopants is observed, with parallel Mn2+ spin alignment maximizing hole delocalization and excited state energy stabilization. This effect is proposed to arise from double exchange in the charge-transfer excited state.Keywords (keywords): Anderson−Hasegawa model; diluted magnetic semiconductor; double-exchange; excited state; linear response TDDFT; quantum dot;

Dopant−carrier magnetic exchange interactions in semiconductor nanostructures give rise to unusually large Zeeman splittings of the semiconductor band levels, raising possibilities for spin-based electronics or photonics applications. Here we evaluate the recently highlighted possibility of confinement-induced kinetic s−d exchange coupling in doped ZnSe/CdSe inverted core/shell nanocrystals. Magneto-optical studies of a broad series of Co2+- and Mn2+-doped core, inverted core/shell, and isocrystalline core/shell nanocrystals reveal that the dominant spectroscopic effects caused by CdSe shell growth around doped ZnSe core nanocrystals arise from hole spatial relaxation, being essentially independent of the electron−dopant interaction or the heterointerface itself. The general criteria for observation of kinetic s−d exchange coupling in doped nanocrystals are discussed in light of these results.

The photoluminescence of colloidal Mn2+-doped CdSe nanocrystals has been studied as a function of nanocrystal diameter. These nanocrystals are shown to be unique among colloidal doped semiconductor nanocrystals reported to date in that quantum confinement allows tuning of the CdSe bandgap energy across the Mn2+ excited-state energies. At small diameters, the nanocrystal photoluminescence is dominated by Mn2+ emission. At large diameters, CdSe excitonic photoluminescence dominates. The latter scenario has allowed spin-polarized excitonic photoluminescence to be observed in colloidal doped semiconductor nanocrystals for the first time.

Colloidal Mn2+-doped CdSe quantum dots showing long excitonic photoluminescence decay times of up to τexc = 15 μs at temperatures over 100 K are described. These decay times exceed those of undoped CdSe quantum dots by ∼103 and are shown to arise from the creation of excitons by back energy transfer from excited Mn2+ dopant ions. A kinetic model describing thermal equilibrium between Mn2+ 4T1 and CdSe excitonic excited states reproduces the experimental observations and reveals that, for some quantum dots, excitons can emit with near unity probability despite being ∼100 meV above the Mn2+ 4T1 state. The effect of Mn2+ doping on CdSe quantum dot luminescence at high temperatures is thus completely opposite from that at low temperatures described previously.

Magnetic-ion-rich nanoscale inclusions formed by spinodal decomposition have been observed in many diluted magnetic semiconductors and have recently been implicated in the ferromagnetic ordering observed in some of these materials. In this study, colloidal nanocrystals of the ternary alloy wurtzite Zn1−xCoxO, with x ranging from 0.0 (w-ZnO) to 1.0 (w-CoO), have been synthesized as model systems for the proposed spinodal decomposition nanostrucures of ferromagnetic Zn1−xCoxO thin films and powders. As free-standing nanocrystals, these phases do not show any signs of ferromagnetism or superparamagnetism at any value of x. Changes in the electronic absorption and magnetic circular dichroism (MCD) spectra with x are described that should allow optical identification of spinodal decomposition in other Zn1−xCoxO samples. Optical and magneto-optical spectroscopic results are presented for the end member of this series (w-CoO), apparently for the first time, and show this binary oxide to be an indirect gap charge-transfer insulator with Eg ≈ 2.3 eV.

Recent advances in nanocrystal doping chemistries have substantially broadened the variety of photophysical properties that can be observed in colloidal Mn2+-doped semiconductor nanocrystals. A brief overview is provided, focusing on Mn2+-doped II–VI semiconductor nanocrystals prepared by direct chemical synthesis and capped with coordinating surface ligands. These Mn2+-doped semiconductor nanocrystals are organized into three major groups according to the location of various Mn2+-related excited states relative to the energy gap of the host semiconductor nanocrystals. The positioning of these excited states gives rise to three distinct relaxation scenarios following photoexcitation. A brief outlook on future research directions is provided.Mn2+-doped semiconductor nanocrystals are organized into three major groups according to the location of various Mn2+-related excited states relative to the energy gap of the host semiconductor nanocrystals. The positioning of these excited states gives rise to three distinct relaxation scenarios following photoexcitation.

Additional unpaired electrons have been introduced into colloidal ZnO quantum dots (QDs) photochemically and investigated by experimental and theoretical methods to test various possible descriptions of their wave functions. For n-type ZnO QDs with diameters between 3.0 and 7.0 nm, electron paramagnetic resonance (EPR) spectroscopy reveals size-dependent g* values in the range 1.960 < g* < 1.968 that are temperature independent and that rule out highly localized wave function descriptions. The size dependence of g* is described well using a k·p perturbation expression, indicating similarity between these quantum confined electrons and free carriers in bulk ZnO. Model calculations confirm that significant electron density can reside on the QD surfaces only for surface well depths that can be experimentally excluded. Together, these results allow a firm experimental description of the photogenerated electrons as delocalized within the conduction bands of the colloidal ZnO QDs, making them excellent candidates for investigation of spin effects in semiconductor nanostructures.

Plasmonic doped semiconductor nanocrystals promise exciting opportunities for new technologies, but basic features of the relationships between their structures, compositions, electronic structures, and optical properties remain poorly understood. Here, we report a quantitative assessment of the impact of composition on the energies of localized surface plasmon resonances (LSPRs) in colloidal tin-doped indium oxide (Sn:In2O3, or ITO) nanocrystals. Using a combination of aliovalent doping and photodoping, the effects of added electrons and impurity ions on the energies of LSPRs in colloidal In2O3 and ITO nanocrystals have been evaluated. Photodoping allows electron densities to be tuned post-synthetically in ITO nanocrystals, independent of their Sn content. Because electrons added photochemically are easily titrated, photodoping also allows independent quantitative determination of the dependence of the LSPR energy on nanocrystal composition and changes in electron density. The data show that ITO LSPR energies are affected by both electron and Sn concentrations, with composition yielding a broader plasmon tuning range than achievable by tuning carrier densities alone. Aspects of the photodoping energetics, as well as magneto-optical properties of these ITO LSPRs, are also discussed.

The electronic and magnetic properties of the luminescent excited states of colloidal Cu+:CdSe, Cu+:InP, and CuInS2 nanocrystals were investigated using variable-temperature photoluminescence (PL) and magnetic circularly polarized luminescence (MCPL) spectroscopies. The nanocrystal electronic structures were also investigated by absorption and magnetic circular dichroism (MCD) spectroscopies. By every spectroscopic measure, the luminescent excited states of all three materials are essentially indistinguishable. All three materials show very similar broad PL line widths and large Stokes shifts. All three materials also show similar temperature dependence of their PL lifetimes and MCPL polarization ratios. Analysis shows that this temperature dependence reflects Boltzmann population distributions between luminescent singlet and triplet excited states with average singlet–triplet splittings of ∼1 meV in each material. These similarities lead to the conclusion that the PL mechanism in CuInS2 NCs is fundamentally different from that of bulk CuInS2 and instead is the same as that in Cu+-doped NCs, which are known to luminesce via charge-transfer recombination of conduction-band electrons with copper-localized holes. The luminescence of CuInS2 nanocrystals is explained well by invoking exciton self-trapping, in which delocalized photogenerated holes contract in response to strong vibronic coupling at lattice copper sites to form a luminescent excited state that is essentially identical to that of the Cu+-doped semiconductor nanocrystals.

Alloyed Zn1−x−yCdxMnySe nanocrystals exhibiting bright intrinsic dual emission attractive for ratiometric optical nanothermometry are reported. Relative to earlier materials with related dual emission, these alloy nanocrystals require shorter syntheses, contain less Cd2+, and show dual emission that is less sensitive to nanocrystal shape, size, or surfaces.

The “extra” electrons in colloidal n-type ZnO nanocrystals formed by aliovalent doping and photochemical reduction are compared. Whereas the two are similar spectroscopically, they show very different electron-transfer reactivities, attributable to their different charge-compensating cations (Al3+vs. H+).

Solar water splitting using catalyst-modified semiconductor photoelectrodes is a promising approach to harvesting and storing solar energy. Prior studies have demonstrated that modification of α-Fe2O3 photoanodes with the water-oxidation electrocatalyst Co-Pi enhances photon-to-current conversion efficiencies, particularly at less positive potentials, but the mechanism underlying this enhancement remains poorly understood. Different experimental techniques have suggested very different interpretations of the microscopic origins of this improvement. Here, we report results from photoelectrochemical and impedance measurements aimed at understanding the Co-Pi/α-Fe2O3 interface of mesostructured composite photoanodes. Contrary to expectations, these measurements reveal that α-Fe2O3 water-oxidation kinetics actually slow upon deposition of Co-Pi, but electron–hole recombination slows even more, resulting in a net enhancement of water-oxidation quantum efficiency. The negative shift in the J–V curve caused by Co-Pi deposition is found to result from the introduction of an alternative pathway for water oxidation catalyzed by Co-Pi, which allows the composite photoanode to avoid positive charge accumulation at the α-Fe2O3 surface. We detail the role of Co-Pi thickness optimization in balancing the slower recombination against the slower water oxidation kinetics to achieve the lowest water-oxidation onset potential. These results provide new insights into the microscopic properties of the catalyst/semiconductor interface in Co-Pi/α-Fe2O3 composite solar water-splitting photoanodes.

The performance of colloidal CuInS2/CdS nanocrystals as phosphors for full-spectrum luminescent solar concentrators has been examined. Their combination of large solar absorption, high photoluminescence quantum yields, and only moderate reabsorption produces the highest projected flux gains of any nanocrystal luminophore to date.

Doping a semiconductor quantum dot with just a single impurity atom can completely transform its physical properties. Here, we report and analyze the magnetic circular dichroism (MCD) spectra of colloidal CdSe quantum dot samples containing on average fewer than one Mn2+ per quantum dot. Even at this sub-single-dopant level, the low-temperature low-field data are dominated by impurity-induced Zeeman splittings caused by dopant–carrier sp–d exchange. Unlike in more heavily doped quantum dots, however, the MCD intensity at the first CdSe exciton shows a field-induced sign flip as the field strength is increased, reflecting competition between sp–d exchange and the intrinsic Zeeman splittings of comparable magnitude. Most unusually, the competition between these two effects leads to a large apparent shift in the first MCD peak maximum, which we show is attributable to a difference in sign of the intrinsic excitonic g factor between the first and second excitons. Finally, the sp–d and intrinsic contributions to the excitonic Zeeman splittings each exhibit unique magnetic-field and temperature dependencies, allowing the MCD spectra of undoped, singly doped, and bi-doped quantum dot sub-ensembles to be analyzed.