Characterizing the Ligand Shell Morphology of PEG-Coated ZnO Nanocrystals Using FRET Spectroscopy.

Poly(ethylene glycol) (PEG) ligands can inhibit proteins and other biomolecules from adhering to underlying surfaces, making them excellent surface ligands for nanocrystal (NC)-based drug carriers. Quantifying the PEG ligand shell morphology is important because its structure determines the permeability of biomolecules through the shell to the NC surface. However, few in situ analytical tools can reveal whether the PEG ligands form either an impenetrable barrier or a porous coating surrounding the NC.

Colloidal Approaches to Zinc Oxide Nanocrystals.

Zinc oxide is an extensively studied semiconductor with a wide band gap in the near-UV. Its many interesting properties have found use in optics, electronics, catalysis, sensing, as well as biomedicine and microbiology. In the nanoscale regime the functional properties of ZnO can be precisely tuned by manipulating its size, shape, chemical composition (doping), and surface states.

Sub-bandgap trap sites for high-density photochemical electron storage in colloidal SrTiO nanocrystals.

We report facile and reversible electron storage in colloidal SrTiO nanocrystals using photochemical and redox titration methods. A very high electron storage capacity (∼180 e per 7 nm nanocrystal) is achieved which we attribute to the localized nature of added electrons at sub-bandgap trap sites in these colloidal SrTiO nanocrystals. The rate of electron accumulation is also found to be much faster with ethylene glycol as the sacrificial reductant compared to ethanol.

Core-Doped [(CdCo)S(SPh)] Clusters from a Self-Assembly Route.

The incorporation of substitutional Co impurities in [CdS(SPh)] (Cd) molecular clusters prepared by the self-assembly method where NaS is the sulfur precursor and a redox method where elemental S is the sulfur precursor is studied. The Co ions provide unique spectroscopic and chemical handles to monitor dopant speciation during cluster formation and determine what role, if any, other cluster species play during Cd cluster formation. In contrast to the redox method that produces exclusively surface-exchanged Co-doped Cd (Co:Cd), the preparation of Cd by the self-assembly method in the presence of Co ions results in Co incorporation at both the surface and core sites of the Cd cluster.

Raising Dielectric Permittivity Mitigates Dopant-Induced Disorder in Conjugated Polymers.

Conjugated polymers need to be doped to increase charge carrier density and reach the electrical conductivity necessary for electronic and energy applications. While doping increases carrier density, Coulomb interactions between the dopant molecules and the localized carriers are poorly screened, causing broadening and a heavy tail in the electronic density-of-states (DOS). The authors examine the effects of dopant-induced disorder on two complimentary charge transport properties of semiconducting polymers, the Seebeck coefficient and electrical conductivity, and demonstrate a way to mitigate them.

Memristive Behavior of Mixed Oxide Nanocrystal Assemblies.

Recent advances in memristive nanocrystal assemblies leverage controllable colloidal chemistry to induce a broad range of defect-mediated electrochemical reactions, switching phenomena, and modulate active parameters. The sample geometry of virtually all resistive switching studies involves thin film layers comprising monomodal diameter nanocrystals. Here we explore the evolution of bipolar and threshold resistive switching across highly ordered, solution-processed nanoribbon assemblies and mixtures comprising BaZrO (BZO) and SrZrO (SZO) nanocrystals.

On the formation of superoxide radicals on colloidal ATiO (A = Sr and Ba) nanocrystal surfaces.

Controlling the surface chemistry of colloidal semiconductor nanocrystals is critical to exploiting their rich electronic structures for various technologies. We recently demonstrated that the hydrothermal synthesis of colloidal nanocrystals of SrTiO, a technologically-relevant electronic material, provided a strong negative correlation between the presence of an O-related surface defect and hydrazine hydrate [W. L.

Air-stable n-type Fe-doped ZnO colloidal nanocrystals.

The synthesis of Al and Fe codoped ZnO colloidal nanocrystals (NCs) using a modified etching-regrowth-doping method is presented. We show that the spectroscopic signatures associated with Fe in ZnO disappear upon introduction of Al donor defects into the ZnO lattice. The presence of Al is confirmed by the appearance of a localized surface plasmon resonance feature indicating excess free carriers in the codoped NCs.

Reversible Control of the Mn Oxidation State in SrTiO Bulk Powders.

We demonstrate a low-temperature reduction method for exhibiting fine control over the oxidation state of substitutional Mn ions in strontium titanate (SrTiO) bulk powder. We employ NaBH as the chemical reductant that causes significant changes in the oxidation state and oxygen vacancy complexation with Mn dopants at temperatures <350°C where lattice reduction is negligible. At higher reduction temperatures, we also observe the formation of Ti in the lattice by diffuse-reflectance and low-temperature electron paramagnetic resonance (EPR) spectroscopy.

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 Fe-doping. Very high areal and volumetric capacitances (33 μF cm, 233 F cm) are achieved in ZnFeO nanocrystals that rival those of the best supercapacitors used in commercial energy-storage devices.

Persistent radical anion polymers based on naphthalenediimide and a vinylene spacer.

Persistent n-doped conjugated polymers were achieved by doping the electron accepting PDNDIV and PFNDIV polymers with ionic (TBACN) or neutral (TDAE) dopants. The great electron affinities, as indicated by the low LUMO levels of PDNDIV (-4.09 eV) and PFNDIV (-4.

Chemical Stabilization of Perovskite Solar Cells with Functional Fulleropyrrolidines.

While perovskite solar cells have invigorated the photovoltaic research community due to their excellent power conversion efficiencies (PCEs), these devices notably suffer from poor stability. To address this crucial issue, a solution-processable organic chemical inhibition layer (OCIL) was integrated into perovskite solar cells, resulting in improved device stability and a maximum PCE of 16.3%.

Spectroscopic Study of the Reversible Chemical Reduction and Reoxidation of Substitutional Cr Ions in SrTiO.

The solid-state synthesis and controllable speciation of Cr dopants in the layered perovskite SrTiO is reported. We employed a chemical reduction procedure with NaBH at relatively mild temperatures (<450 °C) to impart sensitive control over the relative concentration of Cr dopants, the charge-state of oxygen-vacancy defects, and presence of Ti defects in highly reduced Cr-doped SrTiO. The electron paramagnetic resonance (EPR) spectra of the reduced powder samples reveal a 12-fold increase in the Cr concentration within the axially compressed Ti-site of the SrTiO host.

Transferable Memristive Nanoribbons Comprising Solution-Processed Strontium Titanate Nanocubes.

Memristors, often comprising an insulating metal oxide film between two metal electrodes (MIM), constitute a class of two-terminal devices that possesses tunable variations in resistance based on the applied bias history. Intense research remains focused on the metal-insulator interface, which serves as the crux of coupled electronic-ionic interactions and dictates the underpinning transport mechanisms at either electrode. Top-down, ultrahigh-vacuum (UVH) deposition approaches for MIM nanostructures yield highly crystalline, heteroepitaxial interfaces but limit the number of electrode configurations due to a fixed bottom electrode.

Reversible control of the chromium valence in chemically reduced Cr-doped SrTiO3 bulk powders.

The effect of chemical reduction by NaBH4 on the electronic structure of Cr-doped SrTiO3-δ bulk powders prepared by a solid-state reaction was systematically studied as a function of reduction temperature. Electron paramagnetic resonance (EPR) and diffuse reflectance spectroscopies (DRS) were utilized to monitor changes in the electronic structures of both intrinsic defects (oxygen vacancies and/or Ti(3+)) and extrinsic dopants (Cr(3+)) at different reduction temperatures. We identify the existence of two temperature regimes where changes occur within 30 min.

Electron trapping on Fe(3+) sites in photodoped ZnO colloidal nanocrystals.

The effects of photodoping on the electronic structure of Fe(3+)-doped ZnO colloidal nanocrystals are presented. We observe disappearance of the spectroscopic signatures attributed to both substitutional Fe(3+) and interstitial Fe(3+) in the ZnO host as a function of photodoping time, which precede the appearance of the well-known localized surface plasmon resonance from conduction band electrons in ZnO nanocrystals. These results suggest that the oxidation state of Fe(3+) defects can be reversibly switched in ZnO nanocrystals.

Substitution of Co(2+) ions into CdS-based molecular clusters.

We report on the metal ion exchange between Co(2+) and CdS-based molecular clusters. These studies demonstrate that exchange into the smaller [Cd4(SPh)10](2-) clusters is facile compared to the larger [Cd10S4(SPh)16](4-) and [Cd17S4(SPh)28](2-) clusters. This trend correlates with the rate of benzenethiolate interconversion and μ-S(2-) and μ-SPh(-) content among the clusters.

Utilizing Reversible Interactions in Polymeric Nanoparticles To Generate Hollow Metal-Organic Nanoparticles.

The use of reversible linkers in polymers has been of interest mainly for biomedical applications. Herein, we present a novel strategy to utilize reversible interactions in polymeric nanoparticles to generate hollow metal-organic nanoparticles (MOPs). These hollow MOPs are synthesized from self-assembled polymeric nanoparticles using a simple metal-comonomer exchange process in a single step.

Cation Exchange in Small ZnS and CdS Molecular Analogues.

The simplest means of altering the chemistry and electronic structure of any material, from molecular clusters to single crystals, is by the introduction of chemical impurities. We present a systematic study of the cation exchange reaction involving Co(2+) ions with metal benzenethiolate clusters, [M4(SPh)10](2-) (M = Zn, Cd), yielding diluted magnetic clusters having the general formula [(M1-xCox)4(SPh)10](2-). This method allows high concentrations of doping at the molecular level without forming concentrated magnetic clusters such as [Co4(SPh)10](2-).

Thin-film and bulk investigations of LiCoBO₃ as a Li-ion battery cathode.

The compound LiCoBO3 is an appealing candidate for next-generation Li-ion batteries based on its high theoretical specific capacity of 215 mAh/g and high expected discharge voltage (more than 4 V vs Li(+)/Li). However, this level of performance has not yet been realized in experimental cells, even with nanosized particles. Reactive magnetron sputtering was therefore used to prepare thin films of LiCoBO3, allowing the influence of the particle thickness on the electrochemical performance to be explicitly tested.

Optimizing millisecond time scale near-infrared emission in polynuclear chrome(III)-lanthanide(III) complexes.

This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF(3)SO(3))(2) (M = Cr, Zn) and Ln(CF(3)SO(3))(3) (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D(3)-symmetrical trinuclear [MLnM(L2)(3)](CF(3)SO(3))(n) complexes (M = Zn, n = 7; M = Cr, n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(III) chromophores in [CrLnCr(L2)(3)](9+) induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range.

Tuning the potentials of "extra" electrons in colloidal n-type ZnO nanocrystals via Mg2+ substitution.

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 Zn(1-x)Mg(x)O and Zn(0.

Ground-state electronic structure of vanadium(III) trisoxalate in hydrated compounds.

The ground-state electronic structures of K3V(ox)3.3H2O, Na3V(ox)3.5H2O, and NaMgAl1-xVx(ox)3.

Room-temperature electron spin dynamics in free-standing ZnO quantum dots.

Conduction band electrons in colloidal ZnO quantum dots have been prepared photochemically and examined by electron paramagnetic resonance spectroscopy. Nanocrystals of 4.6 nm diameter containing single S-shell conduction band electrons have g(*)=1.