Potentiometric Measurements of Semiconductor Nanocrystal Redox Potentials

How Depletion Layers Govern the Dynamic Plasmonic Response of In-Doped CdO Nanocrystals

Doped metal oxide nanocrystals exhibit a localized surface plasmon resonance that is widely tunable across the mid- to near-infrared region, making them useful for applications in optoelectronics, sensing, and photocatalysis. Surface states pin the Fermi level and induce a surface depletion layer that hinders conductivity and refractive index sensing but can be advantageous for optical modulation. Several strategies have been developed to both synthetically and postsynthetically tailor the depletion layer toward particular applications; however, this understanding has primarily been advanced in Sn-doped In2O3 (ITO) nanocrystals, leaving open questions about generalizing to other doped metal oxides. Here, we quantitatively analyze the depletion layer in In-doped CdO (ICO) nanocrystals, which is shown to have an intrinsically wide depletion layer that leads to broad plasmonic modulation via postsynthetic chemical reduction and ligand exchange. Leveraging these insights, we applied depletion layer tuning to enhance the inherently weak plasmonic coupling in ICO nanocrystal superlattices. Our results demonstrate how an electronic band structure dictates the radial distribution of electrons and governs the response to postsynthetic modulation, enabling the design of tunable and responsive plasmonic materials.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Photoinduced Ligand-to-Metal Charge Transfer of Cobaltocene: Radical Release and Catalytic Cyclotrimerization

Irradiation of cobalt metallocenes at the ligand-to-metal charge transfer energies results in the labilization of the cyclopentadienyl-cobalt bond and radical release. The cyclopentadienyl radical is detected by electron paramagnetic resonance (EPR) spectroscopy using a spin trap and can also be chemically trapped using hydrogen-atom-donating reagents. This reaction presents a new photochemical method of generating new cobalt complexes or of forming cyclopentadienyl cobalt(I) species that are active for catalytic [2 + 2 + 2] cyclotrimerization reactions. More importantly, these results also show that cobaltocene should not be considered as a photostable redox reagent under many conditions, including those relevant to photovoltaics or photocatalysis.

The Electronic Origin of the Zeta Potential is Supported by the Redox Mechanism on an Aqueous Dispersion of Exfoliated Graphite

Herein we have proposed that a redox mechanism can produce surface charges and negative zeta potential on an aqueous graphite dispersion. Graphite was kept in contact with a concentrated ammonia aqueous solution, washed, and exfoliated in water, resulting in a dispersion with lyophobic nature. Ammonia treatment did not provide functional groups or nitrogen doping to graphite. Moreover, this material was washed twice before sonication to remove most hydroxide. Therefore, neither functional groups, nitrogen atoms, nor hydroxide excess is responsible for the zeta potential. Kelvin probe force microscopy has shown that the ammonia-treated and exfoliated graphite has higher Fermi level than the water-treated material, indicating that the contact between ammonia and graphite promotes redox reactions that provide electrons to graphite. These electrons raise the Fermi level of graphite and generate the negative zeta potential, consequently, they account for the colloidal stability.

Solvent-controlled ion-coupled charge transport in microporous metal chalcogenides

Interactions between ions and itinerant charges govern electronic processes ranging from the redox chemistry of molecules to the conductivity of organic semiconductors, but remain an open frontier in the study of microporous materials. These interactions may strongly influence the electronic behavior of microporous materials that confine ions and charges to length scales comparable to proton-coupled electron transfer. Yet despite mounting evidence that both solvent and electrolyte influence charge transport through ion-charge interactions in metal-organic frameworks, fundamental microscopic insights are only just beginning to emerge. Here, through electrochemical analysis of two open-framework chalcogenides TMA2FeGe4S10 and TMA2ZnGe4S10, we outline the key signatures of ion-coupled charge transport in band-type and hopping-type microporous conductors. Pressed-pellet direct-current and impedance techniques reveal that solvent enhances the conductivity of both materials, but for distinct mechanistic reasons. This analysis required the development of a fitting method that provides a novel quantitative metric of concerted ion-charge motion. Taken together, these results provide chemical parameters for a general understanding of electrochemistry in nanoconfined spaces and for designing microporous conductors and electrochemical methods used to evaluate them.

Tunable Band Gaps in MUV-10(M): A Family of Photoredox-Active MOFs with Earth-Abundant Open Metal Sites

Titanium-based metal-organic frameworks (Ti-MOFs) have attracted intense research attention because they can store charges in the form of Ti³⁺ and they serve as photosensitizers to cocatalysts through heterogeneous photoredox reactions at the MOF-liquid interface. Both the charge storage and charge transfer depend on the redox potentials of the MOF and the molecular substrate, but the factors controlling these energetic aspects are not well understood. Additionally, photocatalysis involving Ti-MOFs relies on cocatalysts rather than the intrinsic Ti reactivity, in part because Ti-MOFs with open metal sites are rare. Here, we report that the class of Ti-MOFs known as MUV-10 can be synthetically modified to include a range of redox-inactive ions with flexible coordination environments that control the energies of the photoactive orbitals. Lewis acidic cations installed in the MOF cluster (Cd²⁺, Sr²⁺, and Ba²⁺) or introduced to the pores (H⁺, Li⁺, Na⁺, K⁺) tune the electronic structure and band gaps of the MOFs. Through the use of optical redox indicators, we report the first direct measurement of the Fermi levels (redox potentials) of photoexcited MOFs in situ. Taken together, these results explain the ability of Ti-MOFs to store charges and provide design principles for achieving heterogeneous photoredox chemistry with electrostatic control.

Photodoping of metal oxide nanocrystals for multi-charge accumulation and light-driven energy storage

The growing demand for self-powered devices has led to the study of novel energy storage solutions that exploit green energies whilst ensuring self-sufficiency. In this context, doped metal oxide nanocrystals (MO NCs) are interesting nanosized candidates with the potential to unify solar energy conversion and storage into one set of materials. In this review, we aim to present recent and important developments of doped MO NCs for light-driven multi-charge accumulation (i.e., photodoping) and solar energy storage. We will discuss the general concept of photodoping, the spectroscopic and theoretical tools to determine the charging process, together with unresolved open questions. We conclude the review by highlighting possible device architectures based on doped MO NCs that are expected to considerably impact the field of energy storage by combining in a unique way the conversion and storage of solar power and opening the path towards competitive and novel light-driven energy storage solutions.

Single Trap States in Single CdSe Nanoplatelets

Trap states can strongly affect semiconductor nanocrystals, by quenching, delaying, and spectrally shifting the photoluminescence (PL). Trap states have proven elusive and difficult to study in detail at the ensemble level, let alone in the single-trap regime. CdSe nanoplatelets (NPLs) exhibit significant fractions of long-lived "delayed emission" and near-infrared "trap emission". We use these two spectroscopic handles to study trap states at the ensemble and the single-particle level. We find that reversible hole trapping leads to both delayed and trap PL, involving the same trap states. At the single-particle level, reversible trapping induces exponential delayed PL and trap PL, with lifetimes ranging from 40 to 1300 ns. In contrast with exciton PL, single-trap PL is broad and shows spectral diffusion and strictly single-photon emission. Our results highlight the large inhomogeneity of trap states, even at the single-particle level.

Molecular-Level Insight into Semiconductor Nanocrystal Surfaces

Semiconductor nanocrystals exhibit attractive photophysical properties for use in a variety of applications. Advancing the efficiency of nanocrystal-based devices requires a deep understanding of the physical defects and electronic states that trap charge carriers. Many of these states reside at the nanocrystal surface, which acts as an interface between the semiconductor lattice and the molecular capping ligands. While a detailed structural and electronic understanding of the surface is required to optimize nanocrystal properties, these materials are at a technical disadvantage: unlike molecular structures, semiconductor nanocrystals lack a specific chemical formula and generally must be characterized as heterogeneous ensembles. Therefore, in order for the field to improve current nanocrystal-based technologies, a creative approach to gaining a "molecular-level" picture of nanocrystal surfaces is required. To this end, an expansive toolbox of experimental and computational techniques has emerged in recent years. In this Perspective, we critically evaluate the insight into surface structure and reactivity that can be gained from each of these techniques and demonstrate how their strategic combination is already advancing our molecular-level understanding of nanocrystal surface chemistry.

On the limit of proton-coupled electronic doping in a Ti(iv)-containing MOF

Ti^IV-containing metal–organic frameworks are known to accumulate electrons in their conduction bands, accompanied by protons, when irradiated in the presence of alcohols. The archetypal system, MIL-125, was recently shown to reach a limit of 2e⁻ per Ti₈ octomeric node. However, the origin of this limit and the broader applicability of this unique chemistry relies not only on the presence of Ti^IV, but also access to inorganic inner-sphere Lewis basic anions in the MOF nodes. Here, we study the loading of protons and electrons in MIL-125, and assess the thermodynamic limit of doping these materials. We find that the limit is determined by the reduction potential of protons: in high charging regimes the MOF exceeds the H⁺/H₂ potential. Generally, we offer the design principle that inorganic anions in MOF nodes can host adatomic protons, which may stabilize meta-stable low valent transition metals. This approach highlights the unique chemistry afforded by MOFs built from inorganic clusters, and provides one avenue to developing novel catalytic scaffolds for hydrogen evolution and transfer hydrogenation.

Photo-promoted doping of MIL-125 is limited by the potential of MOF-bound protons exceeding the hydrogen evolution reaction.

Hierarchical porous TiO2 single crystals templated from partly glassified polystyrene

Hierarchical macro-mesoporous anatase TiO₂ single crystal is one-pot synthesized in an EtOH-H₂O system using polystyrene (PS) as the single porogen both for macropore and mesopore and TiF₄ as the titanium precursor. The key to the simultaneous growth of single crystal and the introduction of hierarchical pores is the assembly of PS and titania at the glassification temperature of PS (100 °C). During the hydrolytic polymerization of TiF₄, PS is encapsulated inside titania and gradually glassified. The interference from elastic PS on the oriental growth of TiO₂ crystallite is thus minimized and the final removal of PS through calcination leaves interconnected macropore and mesopore inside the single crystal. According to XPS, EPR and fluorescence analyses, abundant oxygen vacancies are formed on the hierarchical porous single crystal, which presents extraordinary photocatalytic activity and stability in degrading organic pollutants under simulated sunlight irradiation using Rhodamine B as the model. The improved photocatalytic activity is a synergistic effect of improved separation of charge carrier and facilitated interfacial charge transfer benefitting from highly accessible porous single crystal structure.

Cation Effects on the Reduction of Colloidal ZnO Nanocrystals

The effects of a variety of monatomic cations (H⁺, Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺) and larger cations (decamethylcobaltocenium and tetrabutylammonium) on the reduction of colloidal ZnO nanocrystals (NCs) are described. Suspensions of "TOPO"-capped ZnO NCs in toluene/THF were treated with controlled amounts of one-electron reductants (CoCp*₂ or sodium benzophenone anion radical) and cations. Equilibria were quickly established and the extent of NC reduction was quantified via observation of the characteristic near-IR absorbance of conduction band electrons. Addition of excess reductant with or without added cations led to a maximum average number of electrons per ZnO NC, which was dependent on the NC volume and on the nature of the cation. Electrons are transferred to the ZnO NCs in a stoichiometric way, roughly one electron per monovalent cation and roughly two electrons per divalent cation. This shows that cations are charge-balancing the added electrons in ZnO NCs. Overall, our experiments provide insight into the thermodynamics of charge storage and relate the colloidal chemistry of ZnO with bulk oxide semiconductors. They indicate that the apparent band energies of colloidal ZnO are highly dependent on cation/electrolyte composition and concentration, as is known for bulk interfaces, and that electrons and cations are added stoichiometrically to balance charge, similar to the behavior of Li⁺-batteries.

Degenerately n-Doped Colloidal PbSe Quantum Dots: Band Assignments and Electrostatic Effects

We present a spectroscopic study of colloidal PbSe quantum dots (QDs) that have been photodoped to introduce excess delocalized conduction-band (CB) electrons. High-quality absorption spectra are obtained for these degenerately doped QDs with excess electron concentrations up to ∼10²⁰ cm^-3. At the highest doping levels, electrons have completely filled the 1S_e orbitals of the CB and partially populated the higher-energy 1P_e orbitals. Spectroscopic changes observed as a function of carrier concentration permit an unambiguous assignment of the second excitonic absorption maximum to 1P_h-1P_e transitions. At intermediate doping levels, a clear absorption feature appears between the first two excitonic maxima that is attributable to parity-forbidden 1S_h,e-1P_e,h excitations, which become observable because of electrostatic symmetry breaking. Redshifts of the main excitonic absorption features with increased carrier concentration are also analyzed. The Coulomb stabilization energies of both the 1S_h-1S_e and 1P_h-1P_e excitons in n-doped PbSe QDs are remarkably similar to those observed for multiexcitons with the same electron count. The origins of these redshifts are discussed.

Soluble Supercapacitors: Large and Reversible Charge Storage in Colloidal Iron-Doped ZnO Nanocrystals

Colloidal ZnO semiconductor nanocrystals have previously been shown to accumulate multiple delocalized conduction-band electrons under chemical, electrochemical, or photochemical reducing conditions, leading to emergent semimetallic characteristics such as quantum plasmon resonances and raising prospects for application in multielectron redox transformations. Here, we demonstrate a dramatic enhancement in the capacitance of colloidal ZnO nanocrystals through aliovalent Fe³⁺-doping. Very high areal and volumetric capacitances (33 μF cm^-2, 233 F cm^-3) are achieved in Zn_0.99Fe_0.01O nanocrystals that rival those of the best supercapacitors used in commercial energy-storage devices. The redox properties of these nanocrystals are probed by potentiometric titration and optical spectroscopy. These data indicate an equilibrium between electron localization by Fe³⁺ dopants and electron delocalization within the ZnO conduction band, allowing facile reversible charge storage and removal. As "soluble supercapacitors", colloidal iron-doped ZnO nanocrystals constitute a promising class of solution-processable electronic materials with large charge-storage capacity attractive for future energy-storage applications.

Purification technologies for colloidal nanocrystals

Almost all applications of colloidal nanocrystals require some type of purification or surface modification process following nanocrystal growth. Nanocrystal purification - the separation of nanocrystals from undesired solution components - can perturb the surface chemistry and thereby the physical properties of colloidal nanocrystals due to changes in solvent, solute concentrations, and exposure of the nanocrystal surface to oxidation or hydrolysis. For example, nanocrystal quantum dots frequently exhibit decreased photoluminescence brightness after precipitation from the growth solvent and subsequent redissolution. Consequently, purification is an integral part of the synthetic chemistry of colloidal nanocrystals, and the effect of purification methods must be considered in order to accurately compare and predict the behavior of otherwise similar nanocrystal samples. In this Feature Article we examine established and emerging approaches to the purification of colloidal nanoparticles from a nanocrystal surface chemistry viewpoint. Purification is generally achieved by exploiting differences in properties between the impurities and the nanoparticles. Three distinct properties are typically manipulated: polarity (relative solubility), electrophoretic mobility, and size. We discuss precipitation, extraction, electrophoretic methods, and size-based methods including ultracentrifugation, ultrafiltration, diafiltration, and size-exclusion chromatography. The susceptibility of quantum dots to changes in surface chemistry, with changes in photoluminescence decay associated with surface chemical changes, extends even into the case of core/shell structures. Accordingly, the goal of a more complete description of quantum dot surface chemistry has been a driver of innovation in colloidal nanocrystal purification methods. We specifically examine the effect of purification on surface chemistry and photoluminescence in quantum dots as an example of the challenges associated with nanocrystal purification and how improved understanding can result from increasingly precise techniques, and associated surface-sensitive analytical methods.

Electron Stability and Negative-Tetron Luminescence in Free-Standing Colloidal n-Type CdSe/CdS Quantum Dots

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 1S_e 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 1S_e and 1P_e conduction-band electrons within photogenerated four-carrier negative tetrons (three electrons and one hole). Stable occupancy of the 1P_e 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.

Extremely Slow Spontaneous Electron Trapping in Photodoped n-Type CdSe Nanocrystals

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 timescales. 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 timescales 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.