Dynamics of growing carbon nanotube interfaces probed by machine learning-enabled molecular simulations.

Carbon nanotubes (CNTs), hollow cylinders of carbon, hold great promise for advanced technologies, provided their structure remains uniform throughout their length. Their growth takes place at high temperatures across a tube-catalyst interface. Structural defects formed during growth alter CNT properties. These defects are believed to form and heal at the tube-catalyst interface but an understanding of these mechanisms at the atomic-level is lacking. Here we present DeepCNT-22, a machine learning force field (MLFF) to drive molecular dynamics simulations through which we unveil the mechanisms of CNT formation, from nucleation to growth including defect formation and healing. We find the tube-catalyst interface to be highly dynamic, with large fluctuations in the chiral structure of the CNT-edge. This does not support continuous spiral growth as a general mechanism, instead, at these growth conditions, the growing tube edge exhibits significant configurational entropy. We demonstrate that defects form stochastically at the tube-catalyst interface, but under low growth rates and high temperatures, these heal before becoming incorporated in the tube wall, allowing CNTs to grow defect-free to seemingly unlimited lengths. These insights, not readily available through experiments, demonstrate the remarkable power of MLFF-driven simulations and fill long-standing gaps in our understanding of CNT growth mechanisms.

3,4-Ethylenedioxythiophene Hydrogels: Relating Structure and Charge Transport in Supramolecular Gels.

Ionic charge transport is a ubiquitous language of communication in biological systems. As such, bioengineering is in constant need of innovative, soft, and biocompatible materials that facilitate ionic conduction. Low molecular weight gelators (LMWGs) are complex self-assembled materials that have received increasing attention in recent years. Beyond their biocompatible, self-healing, and stimuli responsive facets, LMWGs can be viewed as a "solid" electrolyte solution. In this work, we investigate 3,4-ethylenedioxythiophene (EDOT) as a capping group for a small peptide library, which we use as a system to understand the relationship between modes of assembly and charge transport in supramolecular gels. Through a combination of techniques including small-angle neutron scattering (SANS), NMR-based Van't Hoff analysis, atomic force microscopy (AFM), rheology, four-point probe, and electrochemical impedance spectroscopy (EIS), we found that modifications to the peptide sequence result in distinct assembly pathways, thermodynamic parameters, mechanical properties, and ionic conductivities. Four-point probe conductivity measurements and electrochemical impedance spectroscopy suggest that ionic conductivity is approximately doubled by programmable gel assemblies with hollow cylinder morphologies relative to gels containing solid fibers or a control electrolyte. More broadly, it is hoped this work will serve as a platform for those working on charge transport of aqueous soft materials in general.

Why Carbon Nanotubes Grow.

Despite three decades of intense research efforts, the most fundamental question "why do carbon nanotubes grow?" remains unanswered. In fact, carbon nanotubes (CNTs) should not grow since the encapsulation of a catalyst with graphitic carbon is energetically more favorable than CNT growth in every aspect. Here, we answer this question using a theoretical model based on extensive first-principles and molecular dynamics calculations. We reveal a historically overlooked yet fundamental aspect of the CNT-catalyst interface, viz., that the interfacial energy of the CNT-catalyst edge is contact angle-dependent. The contact angle increases via graphitic cap lift-off, drastically decreasing the interfacial formation energy by up to 6-9 eV/nm, overcoming van der Waals cap-catalyst adhesion, and driving CNT growth. Mapping this remarkable and simple interplay allows us to understand, for the first time, why CNTs grow.

Scalable growth of single-walled carbon nanotubes with a highly uniform structure.

Here, a scalable floating catalyst chemical vapor deposition (FCCVD) method is developed for the production of single-walled carbon nanotubes (SWCNTs) with a controlled structure. For the first time, water is used as the growth promoter in the FCCVD process to modulate the growth of SWCNTs. At an optimum water concentration of ca. 115 ppm, the water-assisted FCCVD process synthesizes SWCNTs with a significantly narrow chirality distribution. In particular, the proportion of (9,8) and (8,7) semiconducting tubes was dramatically enhanced to 45% with 27% of the (9,8) tube in the end product. This is attributed to the changes in both the SWCNT diameter and the chiral angle. The experiment results and accurate quantum chemical molecular dynamics simulations show that the addition of water affects the nucleation and the size distribution of nanoparticle catalysts, thus resulting in the growth of SWCNTs with a highly uniform structure. This direct and continuous water-assisted FCCVD provides the possibility for the mass production of high-quality SWCNTs with a controlled structure.

The effects of moral/social identity threats and affirmations on psychological defensiveness following wrongdoing.

Defensive responses to transgressions can have a negative impact on decision-making within government and organizations, on relationships, and even an individual's well-being. Transgressors who are defensive are less likely to acknowledge or appreciate the extent of harm caused, and their responsibility in having contributed to it or in helping to repair it. It is therefore important to understand what situational factors increase or reduce defensiveness and, thus, offer solutions for those trying to foster responsibility-taking by individuals in relationships, organizations, and society. This paper presents two studies exploring what underpins defensive responses in the context of transgressions. In Study 1 (N = 202), participants recalled an interpersonal transgression, and in Study 2 (N = 143), omnivorous participants watched a guilt-eliciting documentary about meat production practices. Both studies demonstrate that defensiveness increases in response to social/moral threat. Further, Study 2 demonstrates that this defensiveness can be reduced by addressing the underlying threat to social/moral identity via value affirmation, encouraging moral engagement, and repair.

Boron Nitride Nanotube Nucleation via Network Fusion during Catalytic Chemical Vapor Deposition.

Despite boron nitride nanotubes (BNNTs) first being synthesized in the 1990s, their nucleation mechanism remains unknown. Here we report nonequilibrium molecular dynamics simulations showing how BNNT cap structures form during Ni-catalyzed chemical vapor deposition (CVD) of ammonia borane. BN hexagonal ring networks are produced following the catalytic evolution of H2 from the CVD feedstock, the formation and polymerization of B-N chain structures, and the repeated cleavage of homoelemental B-B/N-N bonds by the catalyst surface. Defect-free BNNT cap structures then form perpendicular to the catalyst surface via direct fusion of adjacent BN networks. This BNNT network fusion mechanism is a marked deviation from the established mechanism for carbon nanotube nucleation during CVD and potentially explains why CVD-synthesized BNNTs are frequently observed having sharper tips and wider diameters compared to CVD-synthesized carbon nanotubes.

Catalytic CVD synthesis of boron nitride and carbon nanomaterials - synergies between experiment and theory.

Low-dimensional carbon and boron nitride nanomaterials - hexagonal boron nitride, graphene, boron nitride nanotubes and carbon nanotubes - remain at the forefront of advanced materials research. Catalytic chemical vapour deposition has become an invaluable technique for reliably and cost-effectively synthesising these materials. In this review, we will emphasise how a synergy between experimental and theoretical methods has enhanced the understanding and optimisation of this synthetic technique. This review examines recent advances in the application of CVD to synthesising boron nitride and carbon nanomaterials and highlights where, in many cases, molecular simulations and quantum chemistry have provided key insights complementary to experimental investigation. This synergy is particularly prominent in the field of carbon nanotube and graphene CVD synthesis, and we propose here it will be the key to future advances in optimisation of CVD synthesis of boron nitride nanomaterials, boron nitride - carbon composite materials, and other nanomaterials generally.

Molecular Resolution in situ Imaging of Spontaneous Graphene Exfoliation.

All reported methods of graphene exfoliation require external energy input, most commonly from sonication,1 shaking,2 or stirring.3 The reverse process-aggregation of single or few layer graphene sheets-occurs spontaneously in most solvents. This makes producing, and especially storing, graphene in economic quantities challenging,4,5 which is a significant barrier to widespread commercialization. This study reveals ionic liquids (ILs) can spontaneously exfoliate graphene from graphite at room temperature. The process is thermally activated and follows an Arrhenius-type behavior, resulting in thermodynamically stable IL/graphene suspensions. Using atomic force microscopy, the kinetics of the exfoliation could be followed in situ and with subnanometer resolution, showing that both the size and the charge of the constituent IL ions play a key role. Our results provide a general molecular mechanism underpinning spontaneous graphene exfoliation at room temperature in electrically conducting ILs, paving the way for their adoption in graphene-based technology.

Nanostructure of [Li(G4)] TFSI and [Li(G4)] NO3 solvate ionic liquids at HOPG and Au(111) electrode interfaces as a function of potential.

Atomic force microscopy (AFM) force measurements have been used to study the solvate ionic liquid (IL) double layer nanostructure at highly ordered pyrolytic graphite (HOPG) and Au(111) electrode surfaces as a function of potential. Two solvate ILs are investigated, [Li(G4)] TFSI and [Li(G4)] NO3. Normal force versus apparent separation data indicates that both solvate ILs adopt a multilayered morphology at the electrode interface, similar to conventional ILs. Calculations of adsorption free energies indicate that at 0 V the ion layer in contact with the electrode surface is enriched in the more surface active cations. When a positive or negative surface bias is applied, the concentration of counterions in the innermost layer increases, and higher push-through forces are required to displace near surface layers, indicating a stronger interfacial structure. Generally, [Li(G4)] TFSI has a better defined structure than [Li(G4)] NO3 on both electrode surfaces due to stronger cohesive interactions within layers. Interfacial structure is also better defined for both solvate ILs on HOPG than Au(111) due to the greater surface roughness of Au(111). Further, at all negative potentials on both surfaces, a small final step is observed, consistent with either compression of the complex cation adsorbed structure or desolvation of the glyme from the Li(+).