Pingwei Liu

Layered and scrolled nanocomposites with aligned semi-infinite graphene inclusions at the platelet limit

Stacking up the filler material In composite materials, a strong or stiff filler is added to a softer matrix to create a combined material with better mechanical or electrical properties. To minimize the filler content, it needs to be uniformly distributed in the composite, which is particularly challenging for nanoscale materials. Liu et al. alternately stacked sheets of graphene and polycarbonate to make a base composite. By further cutting and stacking, up to 320 aligned layers were made with a very uniform filler distribution. Alternatively, the initial stack could be rolled into a rod. In both cases, the properties exceeded what might be expected from a simple combination of the two materials. Science, this issue p. 364 Stacking and folding of layers of graphene and polycarbonate create a highly uniform, aligned composite. Two-dimensional (2D) materials can uniquely span the physical dimensions of a surrounding composite matrix in the limit of maximum reinforcement. However, the alignment and assembly of continuous 2D components at high volume fraction remain challenging. We use a stacking and folding method to generate aligned graphene/polycarbonate composites with as many as 320 parallel layers spanning 0.032 to 0.11 millimeters in thickness that significantly increases the effective elastic modulus and strength at exceptionally low volume fractions of only 0.082%. An analogous transverse shear scrolling method generates Archimedean spiral fibers that demonstrate exotic, telescoping elongation at break of 110%, or 30 times greater than Kevlar. Both composites retain anisotropic electrical conduction along the graphene planar axis and transparency. These composites promise substantial mechanical reinforcement, electrical, and optical properties at highly reduced volume fraction.

Nitroaromatic detection and infrared communication from wild-type plants using plant nanobionics

Plant nanobionics aims to embed non-native functions to plants by interfacing them with specifically designed nanoparticles. Here, we demonstrate that living spinach plants (Spinacia oleracea) can be engineered to serve as self-powered pre-concentrators and autosamplers of analytes in ambient groundwater and as infrared communication platforms that can send information to a smartphone. The plants employ a pair of near-infrared fluorescent nanosensors-single-walled carbon nanotubes (SWCNTs) conjugated to the peptide Bombolitin II to recognize nitroaromatics via infrared fluorescent emission, and polyvinyl-alcohol functionalized SWCNTs that act as an invariant reference signal-embedded within the plant leaf mesophyll. As contaminant nitroaromatics are transported up the roots and stem into leaf tissues, they accumulate in the mesophyll, resulting in relative changes in emission intensity. The real-time monitoring of embedded SWCNT sensors also allows residence times in the roots, stems and leaves to be estimated, calculated to be 8.3 min (combined residence times of root and stem) and 1.9 min mm-1 leaf, respectively. These results demonstrate the ability of living, wild-type plants to function as chemical monitors of groundwater and communication devices to external electronics at standoff distances.

Observation of Switchable Photoresponse of a Monolayer WSe2–MoS2 Lateral Heterostructure via Photocurrent Spectral Atomic Force Microscopic Imaging

In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chemical vapor deposition methods has provided an effective solution to producing an in-plane p-n junction, which is a critical component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy to image the current and photocurrent generated between a biased PtIr tip and a monolayer WSe2-MoS2 lateral heterostructure. Current measurements in the dark in both forward and reverse bias reveal an opposite characteristic diode behavior for WSe2 and MoS2, owing to the formation of a Schottky barrier of dissimilar properties. Notably, by changing the polarity and magnitude of the tip voltage applied, pixels that show the photoresponse of the heterostructure are observed to be selectively switched on and off, allowing for the realization of a hyper-resolution array of the switchable photodiode pixels. This experimental approach has significant implications toward the development of novel optoelectronic technologies for regioselective photodetection and imaging at nanoscale resolutions. Comparative 2D Fourier analysis of physical height and current images shows high spatial frequency variations in substrate/MoS2 (or WSe2) contact that exceed the frequencies imposed by the underlying substrates. These results should provide important insights in the design and understanding of electronic and optoelectronic devices based on quantum confined atomically thin 2D lateral heterostructures.

A conveniently synthesized polyethylene gel encapsulating palladium nanoparticles as a reusable high-performance catalyst for Heck and Suzuki coupling reactions

The synthesis of a polyethylene (PE) gel containing self-incarcerated palladium(0) nanoparticles is described. The PE gel matrix without any special functionality was synthesized by one-pot chain walking copolymerization of ethylene and 1,6-hexanediol diacrylate facilitated by a Pd–diimine complex (1). The Pd(II) species from 1 were immobilized in situ onto the gel matrix and further reduced to Pd(0) nanoparticles as a result of catalyst 1 deactivation during polymerization and methanol (a reducer) washing in the polymer purification process. The resulting Pd-containing PE gel (2) is shown to be a high-performance and facilely reusable heterogeneous catalyst for the Heck and Suzuki coupling reactions of iodobenzene or aryl bromides. An average TOF of 460 h−1 was achieved with an average 0.57 ppm Pd leaching in each cycle of the Heck reaction of iodobenzene when the PE gel 2 was recycled 10 times. A maximum TOF of 3.33 × 104 h−1 was reached and less than 0.64 ppm of Pd was leached in the Suzuki reactions of aryl bromides with water as solvent.

Well-controlled and stable emulsion ATRP of MMA with low surfactant concentration using surfactant–ligand design as the copper capture agent

In emulsion atom transfer radical polymerization (ATRP), escape of catalytic copper ion species from the particle to the aqueous phase represents one of the major challenges which causes uncontrollable polymerization and/or unstable latex. We addressed this issue by employing a functional surfactant–ligand (SL) design as the capture agent (CA), which prevented the copper species from escape in the emulsion ATRP. In this work, emulsion ATRP of methyl methacrylate (MMA) mediated CuCl2/4,4-di(5-nonyl)-2,2′-bipyridine (dNbpy) was carried out with Brij-98 as a surfactant in the presence of a small amount of CA. The CA molecules effectively captured dissociated copper ions inside the particle, thus efficiently decreasing the copper ion concentration in the aqueous phase. Only 3 wt% of surfactant loading (0.6 wt% CA and 2.4 wt% Brij-98 based on MMA) was needed to achieve well-controlled ATRP and to stabilize the emulsion system, which is a significant reduction from a typical 13–18 wt% often used in the studies reported in the literature.

Synthesis of polyethylene and polystyrene miktoarm star copolymers using an “in–out” strategy

Miktoarm star copolymers (MSPs) having multiple polyethylene (PE) and polystyrene (PS) arms joined at the crosslinked polydivinylbenzene (PDVB) core were synthesized using an “in–out” synthesis approach. The “in” step involved the atom transfer radical polymerization (ATRP) of divinylbenzene initiated by a narrowly distributed PE macroinitiator (MI), generated via Pd-catalyzed living polymerization, to produce star homoarm PEs containing the core-Br functionality. The star PEs were then employed as MIs to initiate ATRP of styrene in the “out” step to produce (PE)m–PDVB–(PS)n MSPs. A variety of (PE)m–PDVB–(PS)n MSPs were synthesized possessing PE arm lengths and numbers in the range of 7.3–13.7 kg mol−1 and 11.6–33.4, respectively, and PS arm lengths and numbers in the range of 6.7–32.5 kg mol−1 and 3.7–9.0, respectively. Star–star couplings were observed when the shielding protection of the PE arms on the PS propagating radicals vanished, leading to a rapid increase of molecular weights for the MSPs. Characterization of dilute MSP solutions indicates that the generated copolymers possess highly compact spherical chain conformations.

Electrical Energy Generation via Reversible Chemical Doping on Carbon Nanotube Fibers

Chemically modified carbon nanotube fibers enable unique power sources driven entirely by a chemical potential gradient. Electrical current (11.9 μA mg-1 ) and potential (525 mV) are reversibly produced by localized acetonitrile doping under ambient conditions. An inverse length-scaling of the maximum power as L-1.03 that creates specific powers as large as 30.0 kW kg-1 highlights the potential for microscale energy generation.

Hyperbranched polyethylene-supported L-proline: a highly selective and recyclable organocatalyst for asymmetric aldol reactions

A novel hyperbranched polyethylene (HBPE)-supported L-proline has been developed via a bottom-up copolymerization strategy. Copolymerization of ethylene and a protected proline acrylate comonomer with cationic Pd-diimine catalysts was conducted, followed by de-protection of the proline groups. Well-defined HBPE copolymers having molecular weights (MWs) of 10.3–50.3 kDa and 3.2–15.6 L-proline molecules per HBPE polymer chain were synthesized. The effects of the L-proline amount and HBPE MW on the catalyst performance were studied. The HBPE catalysts were efficient in asymmetric aldol reactions of p-nitrobenzaldehyde (p-NBA) or benzaldehyde derivatives with cyclohexanone. High p-NBA conversions of up to 98% and excellent product selectivities with anti/syn = 98/2 and ee > 99% were achieved. Moreover, the HBPE catalysts could be easily recovered by adding water into the product. The recovered catalysts could be reused multiple times with only a slight decline in reactivity and selectivity.

Ultra-high thermal effusivity materials for resonant ambient thermal energy harvesting

Materials science has made progress in maximizing or minimizing the thermal conductivity of materials; however, the thermal effusivity—related to the product of conductivity and capacity—has received limited attention, despite its importance in the coupling of thermal energy to the environment. Herein, we design materials that maximize the thermal effusivity by impregnating copper and nickel foams with conformal, chemical-vapor-deposited graphene and octadecane as a phase change material. These materials are ideal for ambient energy harvesting in the form of what we call thermal resonators to generate persistent electrical power from thermal fluctuations over large ranges of frequencies. Theory and experiment demonstrate that the harvestable power for these devices is proportional to the thermal effusivity of the dominant thermal mass. To illustrate, we measure persistent energy harvesting from diurnal frequencies, extracting as high as 350 mV and 1.3 mW from approximately 10 °C diurnal temperature differences.Ambient environmental thermal fluctuations offer an abundant yet difficult to harvest renewable energy source, when compared to static thermal gradients. Here, by tuning the thermal effusivity of composite phase change materials, the authors are able to harvest energy from diurnal ambient temperature changes.

Colloidal nanoelectronic state machines based on 2D materials for aerosolizable electronics

A previously unexplored property of two-dimensional electronic materials is their ability to graft electronic functionality onto colloidal particles to access local hydrodynamics in fluids to impart mobility and enter spaces inaccessible to larger electronic systems. Here, we demonstrate the design and fabrication of fully autonomous state machines built onto SU-8 particles powered by a two-dimensional material-based photodiode. The on-board circuit connects a chemiresistor circuit element and a memristor element, enabling the detection and storage of information after aerosolization, hydrodynamic propulsion to targets over 0.6 m away, and large-area surface sensing of triethylamine, ammonia and aerosolized soot in inaccessible locations. An incorporated retroreflector design allows for facile position location using laser-scanning optical detection. Such state machines may find widespread application as probes in confined environments, such as the human digestive tract, oil and gas conduits, chemical and biosynthetic reactors, and autonomous environmental sensors.Colloidal state machines, composed of 2D nanoelectronics grafted onto submillimetre-sized particles, act as autonomous electronic circuits capable of logical operation and information storage.