Nanocrystal Assemblies: Current Advances and Open Problems.

We explore the potential of nanocrystals (a term used equivalently to nanoparticles) as building blocks for nanomaterials, and the current advances and open challenges for fundamental science developments and applications. Nanocrystal assemblies are inherently multiscale, and the generation of revolutionary material properties requires a precise understanding of the relationship between structure and function, the former being determined by classical effects and the latter often by quantum effects. With an emphasis on theory and computation, we discuss challenges that hamper current assembly strategies and to what extent nanocrystal assemblies represent thermodynamic equilibrium or kinetically trapped metastable states.

Tempering stochastic density functional theory.

We introduce a tempering approach with stochastic density functional theory (sDFT), labeled t-sDFT, which reduces the statistical errors in the estimates of observable expectation values. This is achieved by rewriting the electronic density as a sum of a "warm" component complemented by "colder" correction(s). Since the warm component is larger in magnitude but faster to evaluate, we use many more stochastic orbitals for its evaluation than for the smaller-sized colder correction(s).

Stochastic embedding DFT: Theory and application to p-nitroaniline in water.

Over this past decade, we combined the idea of stochastic resolution of identity with a variety of electronic structure methods. In our stochastic Kohn-Sham density functional theory (DFT) method, the density is an average over multiple stochastic samples, with stochastic errors that decrease as the inverse square root of the number of sampling orbitals. Here, we develop a stochastic embedding density functional theory method (se-DFT) that selectively reduces the stochastic error (specifically on the forces) for a selected subsystem(s).

Energy window stochastic density functional theory.

Linear scaling density functional theory is important for understanding electronic structure properties of nanometer scale systems. Recently developed stochastic density functional theory can achieve linear or even sublinear scaling for various electronic properties without relying on the sparsity of the density matrix. The basic idea relies on projecting stochastic orbitals onto the occupied space by expanding the Fermi-Dirac operator and repeating this for N stochastic orbitals.

Continuous imaging of plasmon rulers in live cells reveals early-stage caspase-3 activation at the single-molecule level.

The use of plasmon coupling in metal nanoparticles has shown great potential for the optical characterization of many biological processes. Recently, we have demonstrated the use of "plasmon rulers" to observe conformational changes of single biomolecules in vitro. Plasmon rulers provide robust signals without photobleaching or blinking.

Broadband phase modulation by adiabatic pulses.

The use of inhomogeneous but spatially correlated static and radiofrequency (RF) magnetic fields offers a potential methodology for performing magnetic resonance spectroscopy of samples placed outside the bore of the magnet. However, its practical implementation still presents challenging problems, among them the control of nuclear spins over broad frequency offset intervals. The present study introduces an efficient method of encoding the phase of the magnetization when the variation of the static field along the sample is much larger than the RF amplitude.

Molecular packing of lysozyme, fibrinogen, and bovine serum albumin on hydrophilic and hydrophobic surfaces studied by infrared-visible sum frequency generation and fluorescence microscopy.

Infrared-visible sum frequency generation (SFG) vibrational spectroscopy, in combination with fluorescence microscopy, was employed to investigate the surface structure of lysozyme, fibrinogen, and bovine serum albumin (BSA) adsorbed on hydrophilic silica and hydrophobic polystyrene as a function of protein concentration. Fluorescence microscopy shows that the relative amounts of protein adsorbed on hydrophilic and hydrophobic surfaces increase in proportion with the concentration of protein solutions. For a given bulk protein concentration, a larger amount of protein is adsorbed on hydrophobic polystyrene surfaces compared to hydrophilic silica surfaces.

Polariton-biexciton transitions in a semiconductor microcavity.

The influence of four-particle correlations on the nonlinear optics of a semiconductor microcavity is determined by a pump-and-probe investigation. Experiments are performed on a nonmonolithic microcavity which contains a ZnSe quantum well. In this system the biexciton binding energy exceeds both the normal-mode splitting between exciton and cavity mode and all damping constants.

Demonstration of sixth-order coulomb correlations in a semiconductor single quantum well.

Six-wave mixing in a ZnSe quantum well is investigated and compared with microscopic theory. We demonstrate that sixth-order Coulomb correlations have a significant qualitative impact on the nonlinear optical response. Six-wave mixing is shown to be a uniquely sensitive tool for investigation of correlations beyond the four-point level.