Isolation of Single-Walled Carbon Nanotube Enantiomers by Density Differentiation

Current methods of synthesizing single-walled carbon nanotubes(SWNTs)result in racemic mixtures that have impeded the study of left-and right-handed SWNTs.Here we present a method of isolating different SWNT enantiomers using density gradient ultracentrifugation.Enantiomer separation is enabled by the chiral surfactant sodium cholate,which discriminates between left-and right-handed SWNTs and thus induces subtle differences in their buoyant densities.This sorting strategy can be employed for simultaneous enrichment by handedness and roll-up vector of SWNTs having diameters ranging from 0.7 to 1.5 nm.In addition,circular dichroism of enantiomer refined samples enables identification of high-energy optical transitions in SWNTs.

Intrinsic carrier multiplication in layered Bi_(2)O_(2)Se avalanche photodiodes with gain bandwidth product exceeding 1 GHz

Emerging layered semiconductors present multiple advantages for optoelectronic technologies including high carrier mobilities,strong light-matter interactions,and tunable optical absorption and emission.Here,metal-semiconductor-metal avalanche photodiodes(APDs)are fabricated from Bi2O2Se crystals,which consist of electrostatically bound[Bi2O2]2+and[Se]2−layers.The resulting APDs possess an intrinsic carrier multiplication factor up to 400 at 7 K with a responsivity gain exceeding 3,000 A/W and bandwidth of~400 kHz at a visible wavelength of 515.6 nm,ultimately resulting in a gain bandwidth product exceeding 1 GHz.Due to exceptionally low dark currents,Bi2O2Se APDs also yield high detectivities up to 4.6×1014 Jones.A systematic analysis of the photocurrent temperature and bias dependence reveals that the carrier multiplication process in Bi2O2Se APDs is consistent with a reverse biased Schottky diode model with a barrier height of~44 meV,in contrast to the charge trapping extrinsic gain mechanism that dominates most layered semiconductor phototransistors.In this manner,layered Bi2O2Se APDs provide a unique platform that can be exploited in a diverse range of high-performance photodetector applications.

Characterizing and Mitigating Chemomechanical Degradation in High-Energy Lithium-Ion Battery Cathode Materials

Lithium-ion batteries(LIBs)are nearly ubiquitous energy storage solutions,powering devices ranging from consumer electronics to electric vehicles.To advance these applications,current LIB research efforts are directed toward improving energy and power densities,cyclic lifetimes,charging speeds,and safety.These parameters are intrinsically tied to properties of the active electrode materials,such as the redox mechanism,chemical composition,and crystal structure.One particularly challenging issue is that the active electrode materials that possess higher theoretical energy densities are generally more susceptible to degradation during cycling.A notable example is the family of layered multicomponent transition metal oxides,which is the incumbent class of active LIB cathode materials for electric vehicles.To increase their theoretical capacities,the transition metal fraction in these materials is trending toward higher Ni content.However,Ni-rich chemistries suffer from electrochemical,crystallographic,and mechanical degradation that increase in severity with increasing Ni content.Furthermore,alternative high-energy cathode materials,including overlithiated layered oxides and disordered rock salt materials,present additional stability challenges that must be overcome before they can be realistically incorporated into LIB technology.The chemomechanical degradation in high-energy LIB cathode materials occurs at multiple length scales.Point defects,such as antisite defects or vacancies,are commonly generated during electrochemical cycling and can contribute to the loss of cyclable active material.At both the primary and secondary particle level,electrochemical cycling also induces significant volumetric changes and state-of-charge heterogeneity,generating regions of high stress and strain that are precursors to mechanical fracture.Finally,at the electrode level,nonuniform charge transfer reactions throughout the electrode can lead to locally overcharged regions that become sites of enhanced degradation.To address these issues,active cathode material design and electrode engineering are being heavily pursued to accelerate improvements in LIB energy density.To consolidate the current understanding of chemomechanical degradation and provide guidance on mitigation strategies,a comprehensive overview of degradation mechanisms across multiple length scales is critically needed.In this Account,we first outline the origins of chemomechanical degradation for high-energy LIB cathodes,including layered oxides,overlithiated layered oxides,and disordered rock salt structures.Specifically,we delineate the thermodynamic and kinetic origins of defect generation at the atomic level and then progress to the kinetic origins of broader degradation mechanisms at the particle level and electrode level.Next,we discuss strategies for minimizing chemomechanical degradation in high-energy LIB cathodes at multiple length scales.Finally,we provide a forward-looking perspective on how to accelerate progress toward practical high-energy LIB cathodes,including emerging methods to map state-of-charge heterogeneity,efficient data processing techniques,and improved strategies for spatially identifying chemomechanical degradation.We also propose engineering solutions for mitigating chemomechanical degradation,such as grain boundary engineering,modifying the active material particle morphology,and electrode architecture design.Since many of these suggestions can be applied irrespective of cathode chemistry,this Account is likely to be broadly applicable to the diverse set of ongoing efforts to realize high-energy LIBs.