27 research outputs found
Strain-Driven Mn-Reorganization in Overlithiated LixMn2O4 Epitaxial Thin-Film Electrodes
Lithium manganate LixMn2O4 (LMO) is a lithium ion cathode that suffers from the widely observed but poorly understood phenomenon of capacity loss due to Mn dissolution during electrochemical cycling. Here, operando X-ray reflectivity (low- and high-angle) is used to study the structure and morphology of epitaxial LMO (111) thin film cathodes undergoing lithium insertion and extraction to understand the inter-relationships between biaxial strain and Mn-dissolution. The initially strain-relieved LiMn2O4 films generate in-plane tensile and compressive strains for delithiated (x 1) charge states, respectively. The results reveal reversible Li insertion into LMO with no measurable Mn-loss for 0 1) reveals Mn loss from LMO along with dramatic changes in the intensity of the (111) Bragg peak that cannot be explained by Li stoichiometry. These results reveal a partially reversible site reorganization of Mn ions within the LMO film that is not seen in bulk reactions and indicates a transition in Mn-layer stoichiometry from 3:1 to 2:2 in alternating cation planes. Density functional theory calculations confirm that compressive strains (at x = 2) stabilize LMO structures with 2:2 Mn site distributions, therefore providing new insights into the role of lattice strain in the stability of LMO
Lawson criterion for ignition exceeded in an inertial fusion experiment
For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37Â MJ of fusion for 1.92Â MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion
Polynucleotide Adsorption to Negatively Charged Surfaces in Divalent Salt Solutions
Polynucleotide adsorption to negatively charged surfaces via divalent ions is extensively used in the study of biological systems. We analyze here the adsorption mechanism via a self-consistent mean-field model that includes the pH effect on the surface-charge density and the interactions between divalent ions and surface groups. The adsorption is driven by the cooperative effect of divalent metal ion condensation along polynucleotides and their reaction with the surface groups. Although the apparent reaction constants are enhanced by the presence of polynucleotides, the difference between reaction constants of different divalent ions at the ideal condition explains why not all divalent cations mediate DNA adsorption onto anionic surfaces. Calculated divalent salt concentration and pH value variations on polynucleotide adsorption are consistent with atomic force microscope results. Here we use long-period x-ray standing waves to study the adsorption of mercurated-polyuridylic acid in a ZnCl(2) aqueous solution onto a negatively charged hydroxyl-terminated silica surface. These in situ x-ray measurements, which simultaneously reveal the Hg and Zn distribution profiles along the surface normal direction, are in good agreement with our model. The model also provides the effects of polyelectrolyte line-charge density and monovalent salt on adsorption
Crystallization induced by electrostatic correlations in vesicles of mixed-valence ionic amphiphiles
Crystallization induced by electrostatic correlations in vesicles of mixed-valence ionic amphiphiles
High Aspect Ratio Nanotubes Assembled from Macrocyclic Iminium Salts
One-dimensional nanostructures such as carbon nanotubes rely on strong and directional interactions that stabilize their high aspect ratio shapes from fracture. This requirement has precluded making isolated, long, thin organic nanotubes by stacking molecular macrocycles, as their noncovalent stacking interactions are generally too weak. Here we report high aspect ratio (>103), lyotropic nanotubes of stacked, macrocyclic, iminium salts, which are formed by protonation of the corresponding imine-linked macrocycles. Iminium ion formation establishes cohesive interactions that are two orders-of-magnitude stronger than the neutral macrocycles, as estimated by molecular dynamics simulations. Nanotube formation stabilizes the iminium ions, which otherwise rapidly hydrolyze, and is reversed and restored upon addition of bases and acids. Acids generated by irradiating a photoacid generator or sonicating chlorinated solvents also induced nanotube assembly, allowing these nanostructures to be coupled to diverse stimuli, and, once assembled, they can be fixed permanently by crosslinking their pendant alkenes. As the largest, and the first macrocyclic chromonic liquid crystals, macrocyclic iminium salts are easily accessible through a modular design and provide a means to rationally synthesize structures that mimic the morphology and rheology of carbon nanotubes and biological tubules.</p
Counterion Distribution Surrounding Spherical Nucleic AcidâAu Nanoparticle Conjugates Probed by Small-Angle Xâray Scattering
The radial distribution of monovalent cations surrounding spherical nucleic acidâAu nanoparticle conjugates (SNA-AuNPs) is determined by <i>in situ</i> small-angle x-ray scattering (SAXS) and classical density functional theory (DFT) calculations. Small differences in SAXS intensity profiles from SNA-AuNPs dispersed in a series of solutions containing different monovalent ions (Na<sup>+</sup>, K<sup>+</sup>, Rb<sup>+</sup>, or Cs<sup>+</sup>) are measured. Using the âheavy ion replacementâ SAXS (HIRSAXS) approach, we extract the cation-distribution-dependent contribution to the SAXS intensity and show that it agrees with DFT predictions. The experimentâtheory comparisons reveal the radial distribution of cations as well as the conformation of the DNA in the SNA shell. The analysis shows an enhancement to the average cation concentration in the SNA shell that can be up to 15-fold, depending on the bulk solution ionic concentration. The study demonstrates the feasibility of HIRSAXS in probing the distribution of monovalent cations surrounding nanoparticles with an electron dense core (<i>e.g.</i>, metals)
Electrolyte-Mediated Assembly of Charged Nanoparticles
Solutions at high salt concentrations
are used to crystallize or
segregate charged colloids, including proteins and polyelectrolytes
via a complex mechanism referred to as âsalting-outâ.
Here, we combine small-angle X-ray scattering (SAXS), molecular dynamics
(MD) simulations, and liquid-state theory to show that salting-out
is a long-range interaction, which is controlled by electrolyte concentration
and colloid charge density. As a model system, we analyze Au nanoparticles
coated with noncomplementary DNA designed to prevent interparticle
assembly via WatsonâCrick hybridization. SAXS shows that these
highly charged nanoparticles undergo âgasâ to face-centered
cubic (FCC) to âglass-likeâ transitions with increasing
NaCl or CaCl<sub>2</sub> concentration. MD simulations reveal that
the crystallization is concomitant with interparticle interactions
changing from purely repulsive to a âlong-range potential wellâ
condition. Liquid-state theory explains this attraction as a sum of
cohesive and depletion forces that originate from the interelectrolyte
ion and electrolyteâionânanoparticle positional correlations.
Our work provides fundamental insights <i>into the effect of
ionic correlations</i> in the salting-out mechanism and suggests
new routes for the crystallization of colloids and proteins using
concentrated salts
Electrostatic Control of Polymorphism in Charged Amphiphile Assemblies
Stimuli-induced
structural transformations of molecular assemblies
in aqueous solutions are integral to nanotechnological applications
and biological processes. In particular, pH responsive amphiphiles
as well as proteins with various degrees of ionization can reconfigure
in response to pH variations. Here, we use in situ small and wide-angle
X-ray scattering (SAXS/WAXS), transmission electron microscopy (TEM),
and Monte Carlo simulations to show how charge regulation via pH induces
morphological changes in the assembly of a positively charged peptide
amphiphile (PA). Monte Carlo simulations and pH titration measurements
reveal that ionic correlations in the PA assemblies shift the ionizable
amine p<i>K</i> ⌠8 from p<i>K</i> âŒ
10 in the lysine headgroup. SAXS and TEM show that with increasing
pH, the assembly undergoes spherical micelle to cylindrical nanofiber
to planar bilayer transitions. SAXS/WAXS reveal that the bilayer leaflets
are interdigitated with the tilted PA lipid tails crystallized on
a rectangular lattice. The details of the molecular packing in the
membrane result from interplay between steric and van der Waals interactions.
We speculate that this packing motif is a general feature of bilayers
comprised of amphiphilic lipids with large ionic headgroups. Overall,
our studies correlate the molecular charge and the morphology for
a pH-responsive PA system and provide insights into the Ă
-scale
molecular packing in such assemblies