Project description:The behavior of materials in sliding contact is challenging to determine since the interface is normally hidden from view. Using a custom microfabricated device, we conduct in situ, ultrahigh vacuum transmission electron microscope measurements of crystalline silver nanocontacts under combined tension and shear, permitting simultaneous observation of contact forces and contact width. While silver classically exhibits substantial sliding-induced plastic junction growth, the nanocontacts exhibit only limited plastic deformation despite high applied stresses. This difference arises from the nanocontacts' high strength, as we find the von Mises stresses at yield points approach the ideal strength of silver. We attribute this to the nanocontacts' nearly defect-free nature and small size. The contacts also separate unstably, with pull-off forces well below classical predictions for rupture under pure tension. This strongly indicates that shearing reduces nanoscale pull-off forces, predicted theoretically at the continuum level, but not directly observed before.
Project description:A general method of separating isotopes by centrifuging dissolved chemical compounds in a liquid is introduced. This technique can be applied to almost all elements and leads to large separation factors. The method has been demonstrated in several isotopic systems including Ca, Mo, O, and Li with single-stage selectivities of 1.046 to 1.067 per neutron mass difference (e.g., 1.43 in 40Ca/48Ca), which are beyond the capabilities of various conventional methods. Equations are derived to model the process, and the results agree with those of the experiments. The scalability of the technique has been demonstrated by a three-stage enrichment of 48Ca with a total 40Ca/48Ca selectivity of 2.43, and the scalability is more broadly supported through analogies to gas centrifuge, whereby countercurrent centrifugation can further multiply the separation factor by 5 to 10 times per stage in a continuous process. Optimal centrifuge conditions and solutions can achieve both high-throughput and highly efficient isotope separation.
Project description:This article describes the stabilization and postsynthetic separation of gold nanostars (AuNS) synthesized with a morpholine-based Good's buffer, 3-(N-morpholino)propanesulfonic acid. Resuspension of AuNS in ultrapure water improved the shape stability of the particles over 30 days. We demonstrated the sorting of nanostars via rate-zonal centrifugation through a linear sucrose gradient based on branch length and number. We determined that one round of centrifugation was sufficient for separation. Also, we improved the structural homogeneity and stability of the nanoparticles through the optimization of the storage conditions and established a robust method to sort AuNS based on size and shape.
Project description:A comprehensive reaction-path search for the oligomerization of 5-(hydroxymethyl)furfural (HMF) based on quantum chemical calculations was conducted to clarify the mechanism of humin formation in the oxidation of HMF to furan-2,5-dicarboxylic acid (FDCA), in which humin is a typical macromolecular byproduct. The present procedure repeatedly utilizes the multi-component artificial-force-induced reaction (MC-AFIR) method to investigate multistep oligomerization reactions. Although humin formation has been reported even in reagent-grade HMFs with 97-99% purity during their storage at low temperatures, no direct addition path of two HMFs with <185 kJ mol-1 barrier has been found, suggesting humin formation is caused by a reaction with impurities. Based on the reaction conditions, we considered the reactions of HMF + H2O, HMF + OH-, and HMF + O2 and identified three reaction paths with <65 kJ mol-1 barrier for the reaction of HMF + OH-. Further, the suppression of humin formation by the acetal protection of HMF is computationally confirmed.
Project description:Sustainable production and use of carbon nanotube (CNT)-enabled materials require efficient assessment of CNT environmental hazards, including the potential for CNT bioaccumulation and biomagnification in environmental receptors. Microbes, as abundant organisms responsible for nutrient cycling in soil and water, are important ecological receptors for studying the effects of CNTs. Quantification of CNT association with microbial cells requires efficient separation of CNT-associated cells from individually dispersed CNTs and CNT agglomerates. Here, we designed, optimized, and demonstrated procedures for separating bacteria (Pseudomonas aeruginosa) from unbound multiwall carbon nanotubes (MWCNTs) and MWCNT agglomerates using sucrose density gradient centrifugation. We demonstrate separation of protozoa (Tetrahymena thermophila) from MWCNTs, bacterial agglomerates, and protozoan fecal pellets by centrifugation in an iodixanol solution. The presence of MWCNTs in the density gradients after centrifugation was determined by quantification of 14C-labeled MWCNTs; the recovery of microbes from the density gradient media was confirmed by optical microscopy. Protozoan intracellular contents of MWCNTs and of bacteria were also unaffected by the designed separation process. The optimized methods contribute to improved efficiency and accuracy in quantifying MWCNT association with bacteria and MWCNT accumulation in protozoan cells, thus supporting improved assessment of CNT bioaccumulation.
Project description:Unprecedented in situ formation of artificial moth-eye structure is demonstrated by spontaneous nano-phase separation of a silica-based system on substrate. The moth-eye thin film with a homogenously distributed nipples array shows broadband antireflection functionalities. The mechanism of nano-phase separation is unveiled as spinodal decomposition by chemical freezing method and thermodynamic analysis. The current method may provide a new avenue to ready fabrication of patterned nanostructures toward a variety of applications.
Project description:Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh2)2, DTS(F2BTTh2)2, DTS(PTTh2)2, DTG(FBTTh2)2 and DTG(F2BTTh2)2) with the fullerene derivative PC61BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh2)2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh2)2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. The higher BET triplet exciton population in the DTS(PTTh2)2 blend is in accordance with the slower charge separation dynamics observed in this blend.
Project description:The identification of scalable processes that transfer random mixtures of single-walled carbon nanotubes (SWCNTs) into fractions featuring a high content of semiconducting species is crucial for future application of SWCNTs in high-performance electronics. Herein we demonstrate a highly efficient and simple separation method that relies on selective interactions between tailor-made amphiphilic polymers and semiconducting SWCNTs in the presence of low viscosity separation media. High purity individualized semiconducting SWCNTs or even self-organized semiconducting sheets are separated from an as-produced SWCNT dispersion via a single weak field centrifugation run. Absorption and Raman spectroscopy are applied to verify the high purity of the obtained SWCNTs. Furthermore SWCNT - network field-effect transistors were fabricated, which exhibit high ON/OFF ratios (10(5)) and field-effect mobilities (17 cm(2)/Vs). In addition to demonstrating the feasibility of high purity separation by a novel low complexity process, our method can be readily transferred to large scale production.
Project description:Liquid-liquid phase separation is mainly dependent on temperature and composition. Electric fields have also been shown to influence demixing of binary liquid mixtures. However, a puzzling behavior that remains elusive is the electric field-induced phase separation in ion-containing solvents at low voltages, as predicted by Tsori and Leibler. Here, we report the first experimental study of such a phenomenon in ionic liquid-silane mixtures, which not only results in phase separation at the electrode-electrolyte interface (EEI) but also is accompanied by deposition of porous structures of micrometer size on the electrode. This multiscale phenomenon at the EEI was found to be triggered by an electrochemically induced process. Using several analytical methods, we reveal the involved mechanism in which the formation of new Si-N bonds becomes unstable and eventually decomposes into the formation of silane-rich and silane-poor phases. The deposition of porous structures on the electrode surface is therefore a realization of the silane-rich phase. The finding of an electrochemically induced phase separation not only brings a paradigm shift in understanding the EEI in ionic liquids but also provides alternative strategies toward designing porous surfaces.