Interplay of Homogeneous Reactions, Mass Transport, and Kinetics in Determining Selectivity of the Reduction of CO2 on Gold Electrodes.

Gold electrocatalysts have been a research focus due to their ability to reduce CO2 into CO, a feedstock for further conversion. Many methods have been employed to modulate CO2 reduction (CDR) vs hydrogen evolution reaction (HER) selectivity on gold electrodes such as nano-/mesostructuring and crystal faceting control. Herein we show that gold surfaces with very different morphologies (planar, leaves, and wires) lead to similar bell-shaped CO faradaic efficiency as a function of applied potential. At low overpotential (E > -0.85 V vs standard hydrogen electrode (SHE)), HER is dominant via a potential quasi-independent rate that we attribute to a rate limiting process of surface dissociation of competent proton donors. As overpotential is increased, CO faradaic efficiency reaches a maximal value (near 90%) because CO production is controlled by an electron transfer rate that increases with potential, whereas HER remains almost potential independent. At high overpotential (E < -1.2 V vs SHE), CO faradaic efficiency decreases due to the concurrent rise of HER via bicarbonate direct reduction and leveling off of CDR as CO2 replenishment at the catalyst surface is limited by mass transport and homogeneous coupled reactions. Importantly, the analysis shows that recent attempts to overcome mass transport limitations with gas diffusion electrodes confront low carbon mass balance owing to the prominence of homogeneous reactions coupled to CDR. The comprehensive kinetics analysis of the factors defining CDR vs HER on gold electrodes developed here provides an activation-driving force relationship over a large potential window and informs on the design of conditions to achieve desirable high current densities for CO2 to CO conversion while maintaining high selectivity.

Design of template-stabilized active and earth-abundant oxygen evolution catalysts in acid.

Oxygen evolution reaction (OER) catalysts that are earth-abundant and are active and stable in acid are unknown. Active catalysts derived from Co and Ni oxides dissolve at low pH, whereas acid stable systems such as Mn oxides (MnO x ) display poor OER activity. We now demonstrate a rational approach for the design of earth-abundant catalysts that are stable and active in acid by treating activity and stability as decoupled elements of mixed metal oxides. Manganese serves as a stabilizing structural element for catalytically active Co centers in CoMnO x films. In acidic solutions (pH 2.5), CoMnO x exhibits the OER activity of electrodeposited Co oxide (CoO x ) with a Tafel slope of 70-80 mV per decade while also retaining the long-term acid stability of MnO x films for OER at 0.1 mA cm-2. Driving OER at greater current densities in this system is not viable because at high anodic potentials, Mn oxides convert to and dissolve as permanganate. However, by exploiting the decoupled design of the catalyst, the stabilizing structural element may be optimized independently of the Co active sites. By screening potential-pH diagrams, we replaced Mn with Pb to prepare CoFePbO x films that maintained the high OER activity of CoO x at pH 2.5 while exhibiting long-term acid stability at higher current densities (at 1 mA cm-2 for over 50 h at pH 2.0). Under these acidic conditions, CoFePbO x exhibits OER activity that approaches noble metal oxides, thus establishing the viability of decoupling functionality in mixed metal catalysts for designing active, acid-stable, and earth-abundant OER catalysts.

Electrochemical Deposition of Conformal and Functional Layers on High Aspect Ratio Silicon Micro/Nanowires.

Development of new synthetic methods for the modification of nanostructures has accelerated materials design advances to furnish complex architectures. Structures based on one-dimensional (1D) silicon (Si) structures synthesized using top-down and bottom-up methods are especially prominent for diverse applications in chemistry, physics, and medicine. Yet further elaboration of these structures with distinct metal-based and polymeric materials, which could open up new opportunities, has been difficult. We present a general electrochemical method for the deposition of conformal layers of various materials onto high aspect ratio Si micro- and nanowire arrays. The electrochemical deposition of a library of coaxial layers comprising metals, metal oxides, and organic/inorganic semiconductors demonstrate the materials generality of the synthesis technique. Depositions may be performed on wire arrays with varying diameter (70 nm to 4 µm), pitch (5 µ to 15 µ), aspect ratio (4:1 to 75:1), shape (cylindrical, conical, hourglass), resistivity (0.001-0.01 to 1-10 ohm/cm2), and substrate orientation. Anisotropic physical etching of wires with one or more coaxial shells yields 1D structures with exposed tips that can be further site-specifically modified by an electrochemical deposition approach. The electrochemical deposition methodology described herein features a wafer-scale synthesis platform for the preparation of multifunctional nanoscale devices based on a 1D Si substrate.

Orthogonal Chemical Modification of Template-Synthesized Nanostructures with DNA.

Very few chemical strategies for the selective functionalization of nanostructures have been developed despite their potential for controlling high-order assembly processes. We report a novel approach for the selective chemical functionalization and localized assembly of one-dimensional nanostructures (rods), based upon the systematic activation (DNA functionalization) and passivation (self-assembled monolayers) of specific surface sites through the use of orthogonal chemical reactions on electrochemically grown metal nanorod arrays in porous anodic aluminum oxide templates. The ability to orthogonally functionalize the ends or the side of a nanorod, as well as the gaps between two rods, with different DNA strands allows one to synthesize nanostructure assemblies that would be difficult to realize any other way and that could ultimately be utilized for making a wide variety of device architectures.

Solution-Dispersible Metal Nanorings with Deliberately Controllable Compositions and Architectural Parameters for Tunable Plasmonic Response.

We report a template-based technique for the preparation of solution-dispersible nanorings composed of Au, Ag, Pt, Ni, and Pd with control over outer diameter (60-400 nm), inner diameter (25-230 nm), and height (40 nm to a few microns). Systematic and independent control of these parameters enables fine-tuning of the three characteristic localized surface plasmon resonance modes of Au nanorings and the resulting solution-based extinction spectra from the visible to the near-infrared. This synthetic approach provides a new pathway for solution-based investigations of surfaces with negative curvature.

Coaxial lithography.

The optical and electrical properties of heterogeneous nanowires are profoundly related to their composition and nanoscale architecture. However, the intrinsic constraints of conventional synthetic and lithographic techniques have limited the types of multi-compositional nanowire that can be created and studied in the laboratory. Here, we report a high-throughput technique that can be used to prepare coaxial nanowires with sub-10 nm control over the architectural parameters in both axial and radial dimensions. The method, termed coaxial lithography (COAL), relies on templated electrochemical synthesis and can create coaxial nanowires composed of combinations of metals, metal oxides, metal chalcogenides and conjugated polymers. To illustrate the possibilities of the technique, a core/shell semiconductor nanowire with an embedded plasmonic nanoring was synthesized--a structure that cannot be prepared by any previously known method--and its plasmon-excitation-dependent optoelectronic properties were characterized.

Systematic study of antibonding modes in gold nanorod dimers and trimers.

Using on-wire lithography to synthesize well-defined nanorod dimers and trimers, we report a systematic study of the plasmon coupling properties of such materials. By comparing the dimer/trimer structures to discrete nanorods of the same overall length, we demonstrate many similarities between antibonding coupled modes in the dimers/trimers and higher-order resonances in the discrete nanorods. These conclusions are validated with a combination of discrete dipole approximation and finite-difference time-domain calculations and lead to the observation of antibonding modes in symmetric structures by measuring their solution-dispersed extinction spectra. Finally, we probe the effects of asymmetry and gap size on the occurrence of these modes and demonstrate that the delocalized nature of the antibonding modes lead to longer-range coupling compared to the stronger bonding modes.

Hybrid semiconductor core-shell nanowires with tunable plasmonic nanoantennas.

Multi-segmented nanowires with optically active hybrid core-shell regions are fabricated between two metal nanoantennas. These nanowires generate significant photocurrent under illumination and are solution-dispersible.

Observation of selective plasmon-exciton coupling in nonradiative energy transfer: donor-selective versus acceptor-selective plexcitons.

We report selectively plasmon-mediated nonradiative energy transfer between quantum dot (QD) emitters interacting with each other via Förster-type resonance energy transfer (FRET) under controlled plasmon coupling either to only the donor QDs (i.e., donor-selective) or to only the acceptor QDs (i.e., acceptor-selective). Using layer-by-layer assembled colloidal QD nanocrystal solids with metal nanoparticles integrated at carefully designed spacing, we demonstrate the ability to enable/disable the coupled plasmon-exciton (plexciton) formation distinctly at the donor (exciton departing) site or at the acceptor (exciton feeding) site of our choice, while not hindering the donor exciton-acceptor exciton interaction but refraining from simultaneous coupling to both sites of the donor and the acceptor in the FRET process. In the case of donor-selective plexciton, we observed a substantial shortening in the donor QD lifetime from 1.33 to 0.29 ns as a result of plasmon-coupling to the donors and the FRET-assisted exciton transfer from the donors to the acceptors, both of which shorten the donor lifetime. This consequently enhanced the acceptor emission by a factor of 1.93. On the other hand, in the complementary case of acceptor-selective plexciton we observed a 2.70-fold emission enhancement in the acceptor QDs, larger than the acceptor emission enhancement of the donor-selective plexciton, as a result of the combined effects of the acceptor plasmon coupling and the FRET-assisted exciton feeding. Here we present the comparative results of theoretical modeling of the donor- and acceptor-selective plexcitons of nonradiative energy transfer developed here for the first time, which are in excellent agreement with the systematic experimental characterization. Such an ability to modify and control energy transfer through mastering plexcitons is of fundamental importance, opening up new applications for quantum dot embedded plexciton devices along with the development of new techniques in FRET-based fluorescence microscopy.

Long-range plasmophore rulers.

Using on-wire lithography, we studied the emission properties of nanostructures made of a polythiophene disk separated by fixed nanoscopic distances from a plasmonic gold nanorod. The intense plasmonic field generated by the nanorod modifies the shape of the polythiophene emission spectrum, and the strong distance dependence of this modulation forms the basis for a new type of "plasmophore ruler". Simulations using the discrete dipole approximation (DDA) quantitatively support our experimental results. Importantly, this plasmophore ruler is independent of signal intensity and is effective up to 100 nm, which is more than two times larger than any reported value for rulers based on photoluminescence processes.

Chemically isolating hot spots on concave nanocubes.

We report a simple and general strategy for selectively exposing and functionalizing the sharp corners of concave nanocubes, which are the SERS hot spots for such structures. This strategy takes advantage of the unique shape of the concave cubes by coating the particles with silica and then etching it away to expose only the corner regions, while maintaining the silica coating in the concave faces. These corner regions can then be selectively modified for improved enhancement and signal response with SERS.

Peptide-mediated constructs of quantum dot nanocomposites for enzymatic control of nonradiative energy transfer.

A bottom-up approach for constructing colloidal semiconductor quantum dot (QDot) nanocomposites that facilitate nonradiative Förster-type resonance energy transfer (FRET) using polyelectrolyte peptides was proposed and realized. The electrostatic interaction of these polypeptides with altering chain lengths was probed for thermodynamic, structural, and morphological aspects. The resulting nanocomposite film was successfully cut with the protease by digesting the biomimetic peptide layer upon which the QDot assembly was constructed. The ability to control photoluminescence decay lifetime was demonstrated by proteolytic enzyme activity, opening up new possibilities for biosensor applications.

Anisotropic emission from multilayered plasmon resonator nanocomposites of isotropic semiconductor quantum dots.

We propose and demonstrate a nanocomposite localized surface plasmon resonator embedded into an artificial three-dimensional construction. Colloidal semiconductor quantum dots are assembled between layers of metal nanoparticles to create a highly strong plasmon-exciton interaction in the plasmonic cavity. In such a multilayered plasmonic resonator architecture of isotropic CdTe quantum dots, we observed polarized light emission of 80% in the vertical polarization with an enhancement factor of 4.4, resulting in a steady-state anisotropy value of 0.26 and reaching the highest quantum efficiency level of 30% ever reported for such CdTe quantum dot solids. Our electromagnetic simulation results are in good agreement with the experimental characterization data showing a significant emission enhancement in the vertical polarization, for which their fluorescence decay lifetimes are substantially shortened by consecutive replication of our unit cell architecture design. Such strongly plasmon-exciton coupling nanocomposites hold great promise for future exploitation and development of quantum dot plasmonic biophotonics and quantum dot plasmonic optoelectronics.

Highly efficient nonradiative energy transfer mediated light harvesting in water using aqueous CdTe quantum dot antennas.

We present light harvesting of aqueous colloidal quantum dots to nonradiatively transfer their excitonic excitation energy efficiently to dye molecules in water, without requiring ligand exchange. These as-synthesized CdTe quantum dots that are used as donors to serve as light-harvesting antennas are carefully optimized to match the electronic structure of Rhodamine B molecules used as acceptors for light harvesting in aqueous medium. By varying the acceptor to donor concentration ratio, we measure the light harvesting factor, along with substantial lifetime modifications of these water-soluble quantum dots, from 25.3 ns to 7.2 ns as a result of their energy transfer with efficiency levels up to 86%. Such nonradiative energy transfer mediated light harvesting in aqueous medium holds great promise for future quantum dot multiplexed dye biodetection systems.

Non-radiative resonance energy transfer in bi-polymer nanoparticles of fluorescent conjugated polymers.

This work demonstrates the comparative studies of non-radiative resonance energy transfer in bi-polymer nanoparticles based on fluorescent conjugated polymers. For this purpose, poly[(9,9-dihexylfluorene) (PF) as a donor (D) and poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as an acceptor (A) have been utilized, from which four different bi-polymer nanoparticle systems are designed and synthesized. Both, steady-state fluorescence spectra and time-resolved fluorescence measurements indicate varying energy transfer efficiencies from the host polymer PF to the acceptor polymer MEH-PPV depending on the D-A distances and structural properties of the nanoparticles. The first approach involves the preparation of PF and MEH-PPV nanoparticles separately and mixing them at a certain ratio. In the second approach, first PF and MEH-PPV solutions are mixed prior to nanoparticle formation and then nanoparticles are prepared from the mixture. Third and fourth approaches involve the sequential nanoparticle preparation. In the former, nanoparticles are prepared to have PF as a core and MEH-PPV as a shell. The latter is the reverse of the third in which the core is MEH-PPV and the shell is PF. The highest energy transfer efficiency recorded to be 35% is obtained from the last system, in which a PF layer is sequentially formed on MEH-PPV NPs.

Quantum efficiency enhancement in film by making nanoparticles of polyfluorene.

We report on conjugated polymer nanoparticles of polyfluorene that were formed to exhibit higher fluorescence quantum efficiency in film (68%) and reduce undesired emission peak wavelength shifts in film (by 20 nm), compared to the solid-state polymer thin film made directly out of the same polymer solution without forming nanoparticles. Using the facile reprecipitation method, solutions of poly[9,9-dihexyl-9H-fluorene] in THF were added at different volume ratios to obtain different size distributions of nanoparticle dispersions in water. This allowed us to control the sizedependent optical emission of our polyfluorene nanoparticles. Such organic nanoparticles hold great promise for use as efficient emitters in optoelectronic device applications.

White emitting polyfluorene functionalized with azide hybridized on near-UV light emitting diode for high color rendering index.

We develop and demonstrate high-quality white light generation that relies on the use of a single-type simple conjugated polymer of polyfluorene functionalized with azide groups (PFA) integrated on a near-UV LED platform. The high-quality white emission from the polyfluorene is achieved by using the azide functionalization to facilitate cross-linking intentionally when cast into solid-state form. Hybridized on n-UV InGaN/GaN LED at 378 nm, the PFA emitters collectively generate a very broad down-converting photoluminescence at longer wavelengths across the entirety of the visible spectrum, yielding high color rendering indices up to 91.

Tuning shades of white light with multi-color quantum-dot-quantum-well emitters based on onion-like CdSe-ZnS heteronanocrystals.

We present white light generation controlled and tuned by multi-color quantum-dot-quantum-well emitters made of onion-like CdSe/ZnS/CdSe core/shell/shell heteronanocrystals integrated on InGaN/GaN light-emitting diodes (LEDs). We demonstrate hybrid white LEDs with (x, y) tristimulus coordinates tuned from (0.26, 0.33) to (0.37, 0.36) and correlated color temperatures from 27 413 to 4192 K by controlling the number of their integrated red-green-emitting heteronanocrystals. We investigate the modification of in-film emission from these multi-layered heteronanocrystals with respect to their in-solution emission, which plays a significant role in hybrid LED applications. Our proof-of-principle experiments indicate that these complex heteronanocrystals hold promise for use as nanoluminophors in future hybrid white LEDs.