Zhiwei Peng

Chemical Gating of a Synthetic Tube-in-a-Tube Semiconductor

A critical challenge to translating field effect transistors into biochemical sensor platforms is the requirement of a gate electrode, which imposes restrictions on sensor device architectures and results in added expense, poorer scalability, and electrical noise. Here we show that it is possible to eliminate the need of the physical gate electrode and dielectrics altogether using a synthetic tube-in-a-tube (Tube∧2) semiconductor. Composed of a semiconducting single-walled carbon nanotube nested in a charged, impermeable covalent functional shell, Tube∧2 allows the semiconducting conduction pathway to be modulated solely by surface functional groups in a chemically gated-all-around configuration. The removal of physical gates significantly simplifies the device architecture and enables photolithography-free, highly scalable fabrication of transistor sensors in nonconventional configurations that are otherwise impossible. We show that concomitant FET sensitivity and single-mismatch selectivity can be achieved with Tube∧2 even in a two-terminal, thin film transistor device configuration that is as simple as a chemiresistor. Miniaturized two-terminal field effect point sensors can also be fabricated, using a straightforward dice-and-dip procedure, for the detection of tuberculosis biomarkers.

Superacid-Surfactant Exchange: Enabling Nondestructive Dispersion of Full-Length Carbon Nanotubes in Water

Attaining aqueous solutions of individual, long single-walled carbon nanotubes is a critical first step for harnessing the extraordinary properties of these materials. However, the widely used ultrasonication-ultracentrifugation approach and its variants inadvertently cut the nanotubes into short pieces. The process is also time-consuming and difficult to scale. Here we present an unexpectedly simple solution to this decade-old challenge by directly neutralizing a nanotube-chlorosulfonic acid solution in the presence of sodium deoxycholate. This straightforward superacid-surfactant exchange eliminates the need for both ultrasonication and ultracentrifugation altogether, allowing aqueous solutions of individual nanotubes to be prepared within minutes and preserving the full length of the nanotubes. We found that the average length of the processed nanotubes is more than 350% longer than sonicated controls, with a significant fraction approaching ∼9 μm, a length that is limited by only the raw material. The nondestructive nature is manifested by an extremely low density of defects, bright and homogeneous photoluminescence in the near-infrared, and ultrahigh electrical conductivity in transparent thin films (130 Ω/sq at 83% transmittance), which well exceeds that of indium tin oxide. Furthermore, we demonstrate that our method is fully compatible with established techniques for sorting nanotubes by their electronic structures and can also be readily applied to graphene. This surprisingly simple method thus enables nondestructive aqueous solution processing of high-quality carbon nanomaterials at large-scale and low-cost with the potential for a wide range of fundamental studies and applications, including, for example, transparent conductors, near-infrared imaging, and high-performance electronics.

Controlled synthesis of brightly fluorescent CH3NH3PbBr3 perovskite nanocrystals employing Pb(C17H33COO)2 as the sole lead source

Organometal halide perovskite nanocrystals hold vast potential for application in photovoltaics, light emitting diodes, low-threshold lasers, and photodetectors due to their size-tunable bandgap energies and photoluminescence as well as excellent electron and hole mobilities. However, the synthesis of such nanocrystals typically suffers from poor structural stability in solution and the coexistence of lamellate nanocrystals (nanoplatelets) and spherical nanocrystals (nanoparticles). Here we show that the pure nanoparticle morphology of CH3NH3PbBr3 nanocrystals can be realized by employing lead oleate (Pb(C17H33COO)2) as the sole lead source and controlled using short- and long-chain mixed alkyl ammonium. These nanocrystals are monodispersed (2.2 ± 0.4 nm in diameter), highly fluorescent (with a quantum yield approaching 85%), and highly stable in the solution (for more than 30 days). Comparative studies reveal that the shape of CH3NH3PbBr3 nanocrystals is strongly dependent on the lead source, PbBr2 and Pb(C17H33COO)2, and evolves as a function of the ratio of short- and long-chain alkyl ammoniums in the precursors. At an optimal short to long-chain alkyl ammonium ratio of 4u2006:u20066, the growth of CH3NH3PbBr3 nanoplatelets can be selectively suppressed with Pb(C17H33COO)2 as the sole lead source, enhancing the overall photoluminescence quantum yield of the produced CH3NH3PbBr3 nanocrystals. This work reveals important new insights for controlled synthesis of perovskite nanocrystals with pure crystal shape and significantly improved photoluminescence properties and stability.

Self-assembly and photoactivation of blue luminescent CsPbBr3 mesocrystals synthesized at ambient temperature

Mesocrystals, which are ordered superstructures composed of crystalline nanoparticles aligned along well-defined crystallographic directions, have recently been investigated for their novel or improved properties for different applications. All-inorganic halide perovskite CsPbX3 materials are believed to be promising materials for next generation optoelectronic devices. In this study, a facile and rapid synthesis of all-inorganic halide perovskite CsPbBr3 mesocrystals at ambient temperature in air is presented, which does not employ dimethylformamide (DMF). The results indicate that ligands have great influences on the growth kinetics and morphologies of CsPbBr3 mesocrystals with standard cubic structure and high luminescense. The destabilization of ligand matrix is responsible for the self-assembly of mesocrystals through oriented attachment into larger monocrystals because of mutual crystallographic orientations of individual building blocks with a defined long-range order on the atomic scale. Furthermore, photoactivation phenomenon was observed in CsPbBr3 mesocrystals and these intrinsic properties may help to understand the nucleation and growth mechanisms of metal-halide perovskites.

Phosphine-free synthesis and optical stabilities of composition-tuneable monodisperse ternary PbSe1−xSx alloyed nanocrystals via cation exchange

Compared to binary PbSe or PbS nanocrystals (NCs), ternary PbSe1−xSx alloyed NCs have unique electronic and optical properties such as high quantum efficiency, narrow band edge, and fast response time, and are widely used in infrared optoelectronic devices. In this work, high-quality composition-tunable monodisperse ternary PbSe1−xSx alloyed NCs were synthesized by employing cation exchange from host CdSe1−xSx alloyed NCs, which were pre-fabricated by using cadmium oxide, myristic acid and 1-octadecene (ODE) solutions of selenium/sulphur powders (Se/S-ODE), excluding toxic phosphine-containing solvents. The alloyed nature of PbSe1−xSx NCs was confirmed using powder X-ray diffraction (XRD). XRD patterns and field emission transmission electron microscopy (FETEM) images demonstrated that the average diameters of the resulting PbSe1−xSx alloyed NCs were not more than 5 nm, and ultraviolet-visible-near infrared (UV-vis-NIR) absorption spectra indicated that the NC size, hence the band gap, bared a linear relationship with the first absorption peak position. The temporal evolution of the absorption spectra revealed that the PbSe1−xSx alloyed NCs were air-stable due to the hybrid surface passivation of Cl− and Cd2+. X-ray fluorescence results indicated that slight stoichiometric deviations of PbSe1−xSx NCs occurred but never exceeded 15% of the expected composition, based on the amount of introduced hosts.

Photoactuated Pens for Molecular Printing

The photoactuation of pen arrays made of polydimethylsiloxane carbon nanotube composites is explored, and the first demonstration of photoactuated pens for molecular printing is reported. Photoactuation of these composites is characterized using atomic force microscopy and found to produce microscale motion in response to modest illumination, with an actuation efficiency as high as 200 nm mW-1 on the sub-1 s time scale. Arrays of composite pens are synthesized and it is found that local illumination is capable of moving selected pens by more than 3 µm out of the plane, bringing them into contact to perform controllable and high quality printing while completely shutting off the nonilluminated counterparts. In light of the scalability limitations of nanolithography, this work presents an important step and paves the way for arbitrary control of individual pens in massive arrays. As an example of a scalable soft actuator, this approach can also aid progress in other fields such as soft robotics and microfluidics.

Stimulus-Responsive Interfacial Chemistry in CNT/Polymer Nanocomposites

The enhancement of interfacial interactions in carbon nanotube (CNT)/polydimethylsiloxane (PDMS) polymer matrix composites was investigated. The approach taken was to functionalize the CNTs with the photoreactive molecule benzophenone in order to anchor the CNTs to the polymer chains on demand. The anchoring reaction was activated by the use of externally applied UV irradiation. A comparison was done on randomly dispersed and aligned CNTs in order to observe the effect of orientation on interface mechanics and overall enhancement. The effect of interfacial interaction on the mechanical response was determined through analysis of static mechanical experiments, as an increase in interfacial interaction resulted in an observable change in elastic modulus and yield stress. An increase of 22% in elastic modulus was observed in randomly oriented CNTs while an increase of 93% was observed in aligned CNT composites after exposure to UV light. In addition, alignment of CNTs lead to a more discreet yield stress which allowed for a clearer identification of the onset of interfacial failure. This work provides insight into the intelligent design of composites, starting at the nanoscale, to provide desired on-demand macroscale response.

Stretchable Transparent Conductive Films from Long Carbon Nanotube Metals

Flexible transparent conductors are an enabling component for large-area flexible displays, wearable electronics, and implantable medical sensors that can wrap around and move with the body. However, conventional conductive materials decay quickly under tensile strain, posing a significant hurdle for functional flexible devices. Here, we show that high electrical conductivity, mechanical stretchability, and optical transparency can be simultaneously attained by compositing long metallic double-walled carbon nanotubes with a polydimethylsiloxane substrate. When stretched to 100% tensile strain, thin films incorporating these long nanotubes (≈3.2 µm on average) achieve a record high conductivity of 3316 S cm-1 at 100% tensile strain and 85% optical transmittance, which is 194 times higher than that of short nanotube controls (≈0.8 µm on average). Moreover, the high conductivity can withstand more than 1000 repeated stretch-release cycles (switching between 100% and 0% strain) with a retention approaching 96%, whereas the short nanotube controls exhibit only 10%. Mechanistic studies reveal that long tubes can bridge the microscale gaps generated during stretching, thereby maintaining high electrical conductivity. When mounted on human joints, this elastic transparent conductor can accommodate large motions to provide stable, high current output. These results point to transparent conductors capable of attaining high electrical conductivity and optical transmittance under mechanical strain to allow large shape changes that may take place in the operation and use of flexible electronics.

Phosphine-free synthesis and shape evolution of MoSe2 nanoflowers for electrocatalytic hydrogen evolution reactions

MoSe2 represents important layered transition metal dichalcogenides (TMDs) that have high electrocatalytic activity in the hydrogen evolution reaction (HER). A key issue to achieve excellent electrochemical properties of MoSe2 is to synthesize nanostructures that are composed of few-layered MoSe2 with abundant exposure of active edge sites. Nanoflowers of layered materials are promising building blocks for photocatalysis due to their large specific surface area and a wealth of exposed edge sites. In this work, free-standing colloidal MoSe2 nanoflowers, with a size of 250 nm, were synthesized by employing a quick and nontoxic phosphine-free solution-processing approach. Oleic acid (OA), 1-octadecene (ODE) and 1-octylamine (OLA) were primarily used as solvents to control the morphology of MoSe2 nanostructures. Experimental results revealed that the shape evolution of MoSe2 nanostructures started with a first fast precipitation of amorphous materials followed by crystallization of a few layers of nanosheets. The prepared MoSe2 nanoflowers demonstrated excellent HER activities with low overpotentials and small Tafel slopes, and the chronopotentiometry responses revealed good stability, making them promising materials for the electrocatalytic HER.

Graphene as a functional layer for semiconducting carbon nanotube transistor sensors

Single-walled carbon nanotubes (SWCNTs) hold vast potential for future electronic devices due to their outstanding properties, however covalent functionalization often destroys the intrinsic properties of SWCNTs, thus limiting their full potential. Here, we demonstrate the fabrication of a functionalized graphene/semiconducting SWCNT (T@fG) heterostructured thin film transistor as a chemical sensor. In this structural configuration, graphene acts as an atom-thick, impermeable layer that can be covalently functionalized via facile diazonium chemistry to afford a high density of surface functional groups while protecting the underlying SWCNT network from chemical modification, even during a covalent chemical reaction. As a result, the highly functionalized carbon-based hybrid structure exhibits excellent transistor properties with a carrier mobility and ON/OFF ratio as high as 64 cm2/Vs and 5400, respectively. To demonstrate its use in potential applications, T@fG thin films were fabricated as aqueous ammonium sensors exhibiting a detection limit of 0.25 μM in a millimolar ionic strength solution, which is comparable with state-of-the-art aqueous ammonium nanosensors.