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Implications of room temperature oxidation on crystal structure and exchange bias effect in Co/CoO nanoparticles
In this study, we describe how the exchange bias effect in Co/CoO nanoparticles depends on the size focusing and temperature treatment of precursor Co nanoparticles before oxidation at ambient conditions. By appealing to magnetization, microscopy, neutron and synchrotron x-ray measurements we found that as-synthesized Co nanoparticles readily oxidize in air only after 20 days. The highest exchange bias field of 814 Oe is observed at T = 2K. When the same nanoparticles are centrifuged and annealed at 70 °C in vacuum prior to oxidation, the exchange bias field is increased to 2570 Oe. Annealing of Co nanoparticles in vacuum improves their crystallinity and prevents complete oxidation, so that Co-core/CoO-shell structure is preserved even after 120 days. The crystal structure of CoO shell in both samples is different from its bulk counterpart. Implications of such distorted CoO shells on exchange bias are discussed. Coating of Co nanoparticles with amorphous silica shell makes them resistant to oxidation, but ultimately modifies the crystal structure of both Co core and SiO 2 shell.
Methane Storage: Molecular Mechanisms Underlying Room-Temperature Adsorption in Zn 4 O(BDC) 3 (MOF-5)
Here, we study how the methane adsorption properties of the ionic MOF-5 are derived from the local structure of its coordinated metal-cluster. Density functional theory is used to study the adsorption process and identify the key interactions which drive it at ambient temperatures. A detailed adsorption model which represents the adsorption process is derived and used to extract thermodynamic properties from previously reported adsorption isotherms. We find that after adsorption of a single molecule to the face of the metal cluster, a nanostructured surface is formed which enables adsorption of additional CH 4 molecules at reduced entropic penalty thanks to on-surface hopping motions and retention of significant translational freedom. Binding of the CH 4 molecules to the MOF is dominated by electrostatic interactions with negatively charged carboxylate groups, while CH 4 -CH 4 dispersion interactions are important only at high pressures. Last, the MOF-specific adsorption model is compared against the single-site Langmuir model.
Solvents Polarity Governs Ion Interactions and Transport in a Solvated Room Temperature Ionic Liquid
We explore the influence of the solvent dipole moment on cation anion interactions and transport in 1-butyl-3-methyl-imidazolium bis-(trifluoromethylsulfonyl), [BMIM+][Tf2N ]. Free energy profiles derived from atomistic molecular dynamics (MD) simulations show a correlation of the cation anion separation and the equilibrium depth of the potential of mean force with the dipole moment of the solvent. Correlations of the ion diffusivity with the dipole moment and the concentration of the solvent were further demonstrated by classical MD simulations. Quasi-elastic neutron scattering experiments with deuterated solvents reveal a complex picture of nanophase separation into the ionic liquid-rich and solvent-rich phases. The experiment corroborates the trend of concentration- and dipole moment-dependent enhancement of ion mobility by the solvent, as suggested by the simulations. Despite the considerable structural complexity of ionic liquid solvent mixtures, we can rationalize and generalize the trends governing ionic transport in these complex electrolytes.
Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a Room-Temperature Flow Reactor
Chirped-pulse Fourier transform millimeter-wave spectroscopy is a potentially powerful tool for studying chemical reaction dynamics and kinetics. Branching ratios of multiple reaction products and intermediates can be measured with unprecedented chemical specificity; molecular isomers, conformers, and vibrational states have distinct rotational spectra. Here we demonstrate chirped-pulse spectroscopy of vinyl cyanide photoproducts in a flow tube reactor at ambient temperature of 295 K and pressures of 1-10 mu bar. This in situ and time-resolved experiment illustrates the utility of this novel approach to investigating chemical reaction dynamics and kinetics. Following 193 nm photodissociation of CH2CHCN, we observe rotational relaxation of energized HCN, HNC, and HCCCN photoproducts with 10 mu s time resolution and sample the vibrational population distribution of HCCCN. The experimental branching ratio HCN/HCCCN is compared with a model based on RRKM theory using high-level ab initio calculations, which were in turn validated by comparisons to Active Thermochemical Tables enthalpies.
Design of a One-Dimensional Stacked Spin Peierls System with Room-Temperature Switching from Quantum Mechanical Predictions
Planar bis-1,2-dithiolene complex anions of a transition metal (denoted as [M(dithiolato) 2 ]- and M = Ni, Pd, or Pt ion) favor forming columnar stacks of anions in the crystal that feature S = 1/2 spin-chains, and such a spin-chain compound often undergoes a spin-Peierls-type transition, making this a promising material for conducting and magnetic switching. However, current examples show the transition temperatures are far too low for most applications. Furthermore, we use quantum mechanics to predict that changing the cation arrangement from the boat-type to the chair-type packing configuration in a spin-Peierls-type [Ni(dithiolato) 2 ]- complex will substantially stabilize the antiferromagnetic coupling, dramatically increasing the transition temperature. We estimate that the [Ni(mnt)2]-based complexes (mnt = maleonitriledithiolate) with chair-type packing of cations will lead to critical temperatures of ~170, ~252, and ~310 K for S-, Se-, and Te-based mnt, respectively. We also suggest how to stabilize the chair-type configurations of these systems.
Large-Grained Cylindrical Block Copolymer Morphologies by One-Step Room-Temperature Casting
We report a facile method of ordering block copolymer (BCP) morphologies in which the conventional two-step casting and annealing steps are replaced by a single-step process where microphase separation and grain coarsening are seamlessly integrated within the casting protocol. This is achieved by slowing down solvent evaporation during casting by introducing a non-volatile solvent into the BCP casting solution what effectively prolongs the duration of the grain-growth phase. We demonstrate the utility of this solvent evaporation annealing (SEA) method by producing well-ordered large molecular-weight BCP thin films in a total processing time shorter than 3 minutes without resorting to any extra laboratory equipment other than a basic casting-device i.e., spin- or blade-coater. By analyzing the morphologies of the quenched samples, we identify a relatively narrow range of polymer concentration in the wet film, just above the order-disorder concentration, to be critical for obtaining large-grained morphologies. This finding is corroborated by the analysis of the grain-growth kinetics of horizontally-oriented cylindrical domains where relatively large growth exponents (1/2) are observed, indicative of a more rapid defect-annihilation mechanism in the concentrated BCP solution than in thermally-annealed BCP melts. Furthermore, the analysis of temperature-resolved kinetics data allows us to calculate the Arrhenius activation energy of the grain-coarsening in this one-step BCP ordering process.
Room-Temperature Fiber-Coupled Single-Photon Sources based on Colloidal Quantum Dots and SiV Centers in Back-Excited Nanoantennas
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Ultrafast Room-Temperature Single Photon Emission from Quantum Dots Coupled to Plasmonic Nanocavities
Not Available
Erbium-Implanted WS 2 Flakes with Room-Temperature Photon Emission at Telecom Wavelengths
Optically addressable spin impurities in crystals along with device engineering provide an attractive route to realizing quantum technologies in the solid state, but reconciling disparate emitter and host material constraints for a given target application is often challenging. Rare-earth ions in two-dimensional (2D) materials could mitigate this problem given the atomic-like transitions of the emitters and the versatile nature of van der Waals systems. Here we combine ion implantation, confocal microscopy, and ab initio calculations to examine the photon emission of Er-doped WS 2 flakes. Optical spectroscopy reveals narrow, long-lived photoluminescence lines in the telecom band, which we activate after low-temperature thermal annealing. Spectroscopic and polarization-selective measurements show a uniform response across the ensemble, while the fluorescence brightness remains mostly unchanged with temperature, suggesting nonradiative relaxation channels are inefficient. Our results create opportunities for novel solid state devices coupling 2D-hosted, telecom-band emitters to photonic heterostructures separately optimized for photon manipulation.
Room-Temperature Micron-Scale Exciton Migration in a Stabilized Emissive Molecular Aggregate
We report 1.6 ± 1 μm exciton transport in self-assembled supramolecular light-harvesting nanotubes (LHNs) assembled from amphiphillic cyanine dyes. We stabilize LHNs in a sucrose glass matrix, greatly reducing light and oxidative damage and allowing the observation of exciton–exciton annihilation signatures under weak excitation flux. Fitting to a one-dimensional diffusion model, we find an average exciton diffusion constant of 55 ± 20 cm 2 /s, among the highest measured for an organic system. We develop a simple model that uses cryogenic measurements of static and dynamic energetic disorder to estimate a diffusion constant of 32 cm 2 /s, in agreement with experiment. We ascribe large exciton diffusion lengths to low static and dynamic energetic disorder in LHNs. We argue that matrix-stabilized LHNS represent an excellent model system to study coherent excitonic transport.
Direct Intertube Cross-Linking of Carbon Nanotubes at Room Temperature
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