Brett A. Helms

Tunable Infrared Absorption and Visible Transparency of Colloidal Aluminum-Doped Zinc Oxide Nanocrystals

Plasmonic nanocrystals have been attracting a lot of attention both for fundamental studies and different applications, from sensing to imaging and optoelectronic devices. Transparent conductive oxides represent an interesting class of plasmonic materials in addition to metals and vacancy-doped semiconductor quantum dots. Herein, we report a rational synthetic strategy of high-quality colloidal aluminum-doped zinc oxide nanocrystals. The presence of substitutional aluminum in the zinc oxide lattice accompanied by the generation of free electrons is proved for the first time by tunable surface plasmon absorption in the infrared region both in solution and in thin films.

Exceptionally Mild Reactive Stripping of Native Ligands from Nanocrystal Surfaces by Using Meerwein’s Salt†

Native coordinating ligands acquired during the chemical synthesis of colloidal nanocrystals are optimized primarily for their ability to exert control over nanocrystal size, composition, morphology, and dispersibility, and not necessarily for their final application. [1] In general, they are hydrophobic and highly insulating, and constitute a significant barrier for charge or ion transport in devices configured therefrom. Bare nanocrystal surfaces, while desirable for many applications, can be difficult to obtain reliably and without undesirable consequences. For example, removal of native ligands from nanocrystal dispersions usually results in aggregation or etching, [2] while in thin films their chemical displacement (e.g., by hydrazine or formic acid) often gives inefficient removal of surface ligands. [3] Thermal treatments inevitably leave behind an undesirable residue, require lengthy annealing times, or result in particle sintering. [4] Nevertheless, these approaches have demonstrated that near-bare nanocrystal surfaces are useful in a broad spectrum of advanced energy applications, from light-emitting diodes to field-effect transistors and photovoltaics. [5, 6] Dispersions of bare nanocrystals would also be useful as nanoinks and for facilitating their transfer into polar media for biomedical applications and catalysis. [7] In pursuit of a universal reagent for producing

Dual-Emitting Quantum Dot/Quantum Rod-Based Nanothermometers with Enhanced Response and Sensitivity in Live Cells

Temperature is a key parameter in physiological processes, and probes able to detect small changes in local temperature are necessary for accurate and quantitative physical descriptions of cellular events. Several have recently emerged that offer excellent temperature sensitivity, spatial resolution, or cellular compatibility, but it has been challenging to realize all of these properties in a single construct. Here, we introduce a luminescent nanocrystal-based sensor that achieves this with a 2.4% change/°C ratiometric response over physiological temperatures in aqueous buffers, with a precision of at least 0.2 °C. Thermoresponsive dual emission is conferred by a Förster resonant energy transfer (FRET) process between CdSe-CdS quantum dot-quantum rods (QD-QRs) as donors and cyanine dyes as acceptors, which are conjugated to QD-QRs using an amphiphilic polymer coating. The nanothermometers were delivered to live cells using a pH-responsive cationic polymer colloid, which served to both improve uptake and release nanocrystals from endosomal confinement. Within cells, they showed an unexpected enhancement in their temperature response and sensitivity, highlighting the need to calibrate these and similar probes within the cell.

Engineering of a synthetic electron conduit in living cells

Engineering efficient, directional electronic communication between living and nonliving systems has the potential to combine the unique characteristics of both materials for advanced biotechnological applications. However, the cell membrane is designed by nature to be an insulator, restricting the flow of charged species; therefore, introducing a biocompatible pathway for transferring electrons across the membrane without disrupting the cell is a significant challenge. Here we describe a genetic strategy to move intracellular electrons to an inorganic extracellular acceptor along a molecularly defined route. To do so, we reconstitute a portion of the extracellular electron transfer chain of Shewanella oneidensis MR-1 into the model microbe Escherichia coli. This engineered E. coli can reduce metal ions and solid metal oxides ∼8× and ∼4× faster than its parental strain. We also find that metal oxide reduction is more efficient when the extracellular electron acceptor has nanoscale dimensions. This work demonstrates that a genetic cassette can create a conduit for electronic communication from living cells to inorganic materials, and it highlights the importance of matching the size scale of the protein donors to inorganic acceptors.

Sub-10 nm Nanofabrication via Nanoimprint Directed Self-Assembly of Block Copolymers

Directed self-assembly (DSA) of block copolymers (BCPs), either by selective wetting of surface chemical prepatterns or by graphoepitaxial alignment with surface topography, has ushered in a new era for high-resolution nanopatterning. These pioneering approaches, while effective, require expensive and time-consuming lithographic patterning of each substrate to direct the assembly. To overcome this shortcoming, nanoimprint molds--attainable via low-cost optical lithography--were investigated for their potential to be reusable and efficiently template the assembly of block copolymers (BCPs) while under complete confinement. Nanoimprint directed self-assembly conveniently avoids repetitive and expensive chemical or topographical prepatterning of substrates. To demonstrate this technique for high-resolution nanofabrication, we aligned sub-10 nm resolution nanopatterns using a cylinder-forming, organic-inorganic hybrid block copolymer, polystyrene-block-polydimethylsiloxane (PS-b-PDMS). Nanopatterns derived from oxidized PDMS microdomains were successfully transferred into the underlying substrate using plasma etching. In the development phase of this procedure, we investigated the role of mold treatments and pattern geometries as DSA of BCPs are driven by interfacial chemistry and physics. In the optimized route, silicon molds treated with PDMS surface brushes promoted rapid BCP alignment and reliable mold release while appropriate mold geometries provided a single layer of cylinders and negligible residual layers as required for pattern transfer. Molds thus produced were reusable to the same efficacy between nanoimprints. We also demonstrated that shear flow during the nanoimprint process enhanced the alignment of the BCP near open edges, which may be engineered in future schemes to control the BCP microdomain alignment kinetics during DSA.

Processable Cyclic Peptide Nanotubes with Tunable Interiors

A facile route to generate cyclic peptide nanotubes with tunable interiors is presented. By incorporating 3-amino-2-methylbenzoic acid in the D,L-alternating primary sequence of a cyclic peptide, a functional group can be presented in the interior of the nanotubes without compromising the formation of high aspect ratio nanotubes. The new design of such a cyclic peptide also enables one to modulate the nanotube growth process to be compatible with the polymer processing window without compromising the formation of high aspect ratio nanotubes, thus opening a viable approach toward molecularly defined porous membranes.

Subnanometer Porous Thin Films by the Co-assembly of Nanotube Subunits and Block Copolymers

Porous thin films containing subnanometer channels oriented normal to the surface exhibit unique transport and separation properties and can serve as selective membranes for separation and protective coatings. While molecularly defined nanoporous inorganic and organic materials abound, generating flexible nanoporous thin films with highly aligned channels over large areas has been elusive. Here, we developed a new approach where the growth of cyclic peptide nanotubes can be directed in a structural framework set by the self-assembly of block copolymers. By conjugating polymers to cyclic peptides, the subunit of an organic nanotube can be selectively solubilized in one copolymer microdomain. The conjugated polymers also mediate the interactions between nanotube and local medium and guide the growth of nanotubes in a confined geometry. This led to subnanometer porous membranes containing high-density arrays of through channels. This new strategy takes full advantage of nanoscopic assembly of BCPs and the reversibility of organic nanotube growth and circumvents impediments associated with aligning and organizing high aspect ratio nano-objects normal to the surface. Furthermore, the hierarchical coassembly strategy described demonstrates the feasibility of synchronizing multiple self-assembly processes to achieve hierarchically structured soft materials with molecular level control.

Nanocomposite Architecture for Rapid, Spectrally-Selective Electrochromic Modulation of Solar Transmittance

Two active electrochromic materials, vacancy-doped tungsten oxide (WO(3-x)) nanocrystals and amorphous niobium oxide (NbOx) glass are arranged into a mesostructured architecture. In a strategy applicable across electrochemical applications, the critical dimensions and interfacial connections in the nanocomposite are designed to optimize pathways for electrochemical charging and discharging. The result is an unprecedented optical range for modulation of visible and near-infrared solar radiation with rapid switching kinetics that indicate the WO(3-x) nanocrystal framework effectively pumps charge out of the normally sluggish NbOx glass. The material is durable for at least 2000 electrochemical cycles.

Polysulfide-Blocking Microporous Polymer Membrane Tailored for Hybrid Li-Sulfur Flow Batteries

Redox flow batteries (RFBs) present unique opportunities for multi-hour electrochemical energy storage (EES) at low cost. Too often, the barrier for implementing them in large-scale EES is the unfettered migration of redox active species across the membrane, which shortens battery life and reduces Coulombic efficiency. To advance RFBs for reliable EES, a new paradigm for controlling membrane transport selectivity is needed. We show here that size- and ion-selective transport can be achieved using membranes fabricated from polymers of intrinsic microporosity (PIMs). As a proof-of-concept demonstration, a first-generation PIM membrane dramatically reduced polysulfide crossover (and shuttling at the anode) in lithium-sulfur batteries, even when sulfur cathodes were prepared as flowable energy-dense fluids. The design of our membrane platform was informed by molecular dynamics simulations of the solvated structures of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) vs lithiated polysulfides (Li2Sx, where x = 8, 6, and 4) in glyme-based electrolytes of different oligomer length. These simulations suggested polymer films with pore dimensions less than 1.2-1.7 nm might incur the desired ion-selectivity. Indeed, the polysulfide blocking ability of the PIM-1 membrane (∼0.8 nm pores) was improved 500-fold over mesoporous Celgard separators (∼17 nm pores). As a result, significantly improved battery performance was demonstrated, even in the absence of LiNO3 anode-protecting additives.

Assembly of ligand-stripped nanocrystals into precisely controlled mesoporous architectures.

The properties of mesoporous materials hinge on control of their composition, pore dimensions, wall thickness, and the size and shape of the crystallite building units. We create ordered mesoporous materials in which all of these parameters are independently controlled. Different sizes (from 4.5 to 8 nm) and shapes (spheres and rods) of ligand-stripped nanocrystals are assembled using the same structure-directing block copolymers, which contain a tethering domain designed to adsorb to their naked surfaces. Material compositions range from metal oxides (Sn-doped In(2)O(3) or ITO, CeO(2), TiO(2)) to metal fluorides (Yb,Er-doped NaYF(4)) and metals (FePt). The incorporation of new types of nanocrystals into mesoporous architectures can lead to enhanced performance. For example, TiO(2) nanorod-based materials withstand >1000 electrochemical cycles without significant degradation.