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Axion sensor
摘要： Optical tweezers are a popular tool for manipulating and sorting individual nanoparticles. Now, Avinash Kumar and John Bechhoefer from Simon Fraser University, Canada have shown that when equipped with a suitable feedback scheme tweezers can be used to create a more complicated force field, such as single- or double-well harmonic potentials, for controlling particle dynamics. In the experiments, a polarized 532-nm laser was used for trapping and detection. The polarization of the detection beam was rotated by 90° with a half-wave plate so that a polarized beam splitter could separate the detection beam from backscattered light arising from the trapping laser. Quadrant photodiodes were used to detect the particle’s fluctuation and dynamics. Two feedback loops continuously regulated a pair of acousto-optic deflectors to compensate for any fluctuation in the total intensity. The shape of the double-well potential was reconstructed from the Boltzmann distribution of the position measurements. The well separation was 10.6 nm, which is far below the diffraction limit (≈ 220 nm). The ability to create and control energy landscapes at scales comparable to the size of proteins offers intriguing possibilities for biophysical applications.
关键词： biophysical applications,nanoparticles,optical tweezers,force field
更新于2025-09-23 15:21:21
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Understanding the Surfaces and Crystal Growth of Pyrite FeS2
摘要： Pyrite is a common sulfide mineral, which has arisen early interest by its euhedral shape and by its metallic glow similar to gold. However, it is only in our century that we began to understand pyrite crystal growth, considering the thermodynamic and kinetic aspects of crystal formation as a function of temperature and concentration of the elements present in the medium. This article reports an analysis by molecular mechanics of 11 surfaces associated to observed morphologies in order to explain the growth of natural and synthesized minerals. The lowest surface and attachment energies (respectively 1.04 J/m2 and -20.3 kJ/mol) were obtained for the (001) plane, indicating that it is the most stable surface and that kinetic growth also preferentially occurs on this plane. Less known properties, such as crystal striations along the <100> directions, are also discussed.
关键词： pyritohedron,force field,striation,surface,morphology
更新于2025-09-23 15:21:01
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Atomistic theory of electronic and optical properties of InAsP/InP nanowire quantum dots
摘要： We present here an atomistic theory of the electronic and optical properties of hexagonal InAsP quantum dots in InP nanowires in the wurtzite phase. These self-assembled quantum dots are unique in that their heights, shapes, and diameters are well known. Using a combined valence-force-field, tight-binding, and configuration-interaction approach we perform atomistic calculations of single-particle states and excitonic, biexcitonic, and trion complexes as well as emission spectra as a function of the quantum dot height, diameter, and As versus P concentration. The atomistic tight-binding parameters for InAs and InP in the wurtzite crystal phase were obtained by ab initio methods corrected by empirical band gaps. The low energy electron and hole states form electronic shells similar to parabolic or cylindrical quantum confinement, only weakly affected by hexagonal symmetry and As fluctuations. The relative alignment of the emission lines from excitons, trions, and biexcitons agrees with that for InAs/InP dots in the zincblende phase in that biexcitons and positive trions are only weakly bound. The random distribution of As atoms leads to dot-to-dot fluctuations of a few meV for the single-particle states and the spectral lines. Due to the high symmetry of hexagonal InAsP nanowire quantum dots the exciton fine structure splitting is found to be small, of the order a few μeV with significant random fluctuations in accordance with experiments.
关键词： fine structure splitting,electronic properties,optical properties,emission spectra,tight-binding,single-particle states,excitonic complexes,InAsP quantum dots,InP nanowires,wurtzite phase,configuration-interaction,valence-force-field,atomistic theory
更新于2025-09-23 15:19:57
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Holographic interferometric microscopy for measuring Cu2+ concentration profile during Cu electrodeposition in a magnetic field
摘要： We measured the concentration boundary layer during copper electrodeposition in a copper sulfate solution under an applied magnetic field. Using a holographic laser interference microscope, we demonstrated the formation of the diffusion layer near the cathode. In the initial stage of electrodeposition, the diffusion layer was distorted in a magnetic field. An analysis of the electrode surface concentration supported the following assertions: without a magnetic field, dendrites formed when the surface concentration of Cu2+ was depleted; with a magnetic field, the unique convection phenomena of the field gradient force and localized Lorentz force promoted Cu2+ mass transport in the diffusion layer, producing large crystal grains.
关键词： Diffusion layer,Magnetohydrodynamic,Magnetic field,Localized Lorentz force,Field gradient force
更新于2025-09-04 15:30:14
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Interfacing CRYSTAL/AMBER to Optimize QM/MM Lennard–Jones Parameters for Water and to Study Solvation of TiO2 Nanoparticles
摘要： Metal oxide nanoparticles (NPs) are regarded as good candidates for many technological applications, where their functional environment is often an aqueous solution. The correct description of metal oxide electronic structure is still a challenge for local and semilocal density functionals, whereas hybrid functional methods provide an improved description, and local atomic function-based codes such as CRYSTAL17 outperform plane wave codes when it comes to hybrid functional calculations. However, the computational cost of hybrids are still prohibitive for systems of real sizes, in a real environment. Therefore, we here present and critically assess the accuracy of our electrostatic embedding quantum mechanical/molecular mechanical (QM/MM) coupling between CRYSTAL17 and AMBER16, and demonstrate some of its capabilities via the case study of TiO2 NPs in water. First, we produced new Lennard–Jones (LJ) parameters that improve the accuracy of water–water interactions in the B3LYP/TIP3P coupling. We found that optimizing LJ parameters based on water tri- to deca-mer clusters provides a less overstructured QM/MM liquid water description than when fitting LJ parameters only based on the water dimer. Then, we applied our QM/MM coupling methodology to describe the interaction of a 1 nm wide multilayer of water surrounding a spherical TiO2 nanoparticle (NP). Optimizing the QM/MM water–water parameters was found to have little to no effect on the local NP properties, which provide insights into the range of influence that can be attributed to the LJ term in the QM/MM coupling. The effect of adding additional water in an MM fashion on the geometry optimized nanoparticle structure is small, but more evident effects are seen in its electronic properties. We also show that there is good transferability of existing QM/MM LJ parameters for organic molecules–water interactions to our QM/MM implementation, even though these parameters were obtained with a different QM code and QM/MM implementation, but with the same functional.
关键词： water,geometry optimization,nanoparticles,QM/MM,molecular dynamics,force field parameters,titanium dioxide,multiscale
更新于2025-09-04 15:30:14
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A coherent derivation of the Ewald summation for arbitrary orders of multipoles: The self-terms
摘要： In this work, we provide the mathematical elements we think essential for a proper understanding of the calculus of the electrostatic energy of point-multipoles of arbitrary order under periodic boundary conditions. The emphasis is put on the expressions of the so-called self-parts of the Ewald summation where different expressions can be found in the literature. Indeed, such expressions are of prime importance in the context of new generation polarizable force field where the self-field appears in the polarization equations. We provide a general framework, where the idea of the Ewald splitting is applied to the electric potential and, subsequently, all other quantities such as the electric field, the energy, and the forces are derived consistently thereof. Mathematical well-posedness is shown for all these contributions for any order of multipolar distribution.
关键词： Ewald summation,periodic boundary conditions,multipoles,polarizable force field,electrostatic energy
更新于2025-09-04 15:30:14
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Swift heavy ion track formation in nanoporous Si: Wave packet molecular dynamics study
摘要： In this article we study the process of silicon relaxation after swift heavy ion (SHI) irradiation using ?rst-principles-based electron force ?eld eFF. We show that early stages of so-called nonthermal melting take place in the nanometric viscinity of SHI trajectory in both bulk Si and Si nanolayer, but result in structural modi?cations only for the last one. Our results shed light on recent experimental evidence of signi?cant reduction of threashold for SHI-induced damage creation in nanoporous Si.
关键词： nanoporous Si,nonthermal melting,wave packet molecular dynamics,electron force field,swift heavy ion
更新于2025-09-04 15:30:14