18 research outputs found
Thermodynamics and equilibrium structure of Ne_38 cluster: Quantum Mechanics versus Classical
The equilibrium properties of classical LJ_38 versus quantum Ne_38
Lennard-Jones clusters are investigated. The quantum simulations use both the
Path-Integral Monte-Carlo (PIMC) and the recently developed
Variational-Gaussian-Wavepacket Monte-Carlo (VGW-MC) methods. The PIMC and the
classical MC simulations are implemented in the parallel tempering framework.
The VGW method is used to locate and characterize the low energy states of
Ne_38, which are then further refined by PIMC calculations. Unlike the
classical case, the ground state of Ne_38 is a liquid-like structure. Among the
several liquid-like states with energies below the two symmetric states (O_h
and C_5v), the lowest two exhibit strong delocalization over basins associated
with at least two classical local minima. Because the symmetric structures do
not play an essential role in the thermodynamics of Ne_38, the quantum heat
capacity is a featureless curve indicative of the absence of any structural
transformations. Good agreement between the two methods, VGW and PIMC, is
obtained.Comment: 13 pages, 9 figure
Explanation of quantum dot blinking without long-lived trap hypothesis
A simple model explaining the experimental data on QDs luminescence blinking
is suggested. The model does not assume the presence of the long-lived electron
traps. The bleaching of the QD luminescence is a result of the Auger assisted
radiationless relaxation of the excitation through the deep surface states.
Possible ways of the experimental verification of the model are discussed
Gaussian resolutions for equilibrium density matrices
A Gaussian resolution method for the computation of equilibrium density
matrices rho(T) for a general multidimensional quantum problem is presented.
The variational principle applied to the ``imaginary time'' Schroedinger
equation provides the equations of motion for Gaussians in a resolution of
rho(T) described by their width matrix, center and scale factor, all treated as
dynamical variables.
The method is computationally very inexpensive, has favorable scaling with
the system size and is surprisingly accurate in a wide temperature range, even
for cases involving quantum tunneling. Incorporation of symmetry constraints,
such as reflection or particle statistics, is also discussed.Comment: 4 page
Universal emission intermittency in quantum dots, nanorods, and nanowires
Virtually all known fluorophores, including semiconductor nanoparticles,
nanorods and nanowires exhibit unexplainable episodes of intermittent emission
blinking. A most remarkable feature of the fluorescence intermittency is a
universal power law distribution of on- and off-times. For nanoparticles the
resulting power law extends over an extraordinarily wide dynamic range: nine
orders of magnitude in probability density and five to six orders of magnitude
in time. The exponents hover about the ubiquitous value of -3/2. Dark states
routinely last for tens of seconds, which are practically forever on quantum
mechanical time scales. Despite such infinite states of darkness, the dots
miraculously recover and start emitting again. Although the underlying
mechanism responsible for this phenomenon remains an enduring mystery and many
questions remain, we argue that substantial theoretical progress has been made.Comment: 9 pages, 2 figures, Accepted versio
Self-Healing Ability of Perovskites Observed via Photoluminescence Response on Nanoscale Local Forces and Mechanical Damage
The photoluminescence (PL) of metal halide perovskites can recover after light or current-induced degradation. This self-healing ability is tested by acting mechanically on MAPbI3 polycrystalline microcrystals by an atomic force microscope tip (applying force, scratching, and cutting) while monitoring the PL. Although strain and crystal damage induce strong PL quenching, the initial balance between radiative and nonradiative processes in the microcrystals is restored within a few minutes. The stepwise quenching–recovery cycles induced by the mechanical action is interpreted as a modulation of the PL blinking behavior. This study proposes that the dynamic equilibrium between active and inactive states of the metastable nonradiative recombination centers causing blinking is perturbed by strain. Reversible stochastic transformation of several nonradiative centers per microcrystal under application/release of the local stress can lead to the observed PL quenching and recovery. Fitting the experimental PL trajectories by a phenomenological model based on viscoelasticity provides a characteristic time of strain relaxation in MAPbI3 on the order of 10–100 s. The key role of metastable defect states in nonradiative losses and in the self-healing properties of perovskites is suggested
Universality of the Fluorescence Intermittency in Nanoscale Systems: Experiment and Theory
A variety of optically active nanoscale objects show
extremely
long correlations in the fluctuations of fluorescence intensity (blinking).
Here we performed a systematic study to quantitatively estimate the
power spectral density (PSD) of the fluorescence trajectories of colloidal
and self-assembled quantum dots (QDs), nanorods (NRs), nanowires (NWs),
and organic molecules. We report for the first time a statistically
correct method of PSD estimation suitable for these systems. Our method
includes a detailed analysis of the confidence intervals. The striking
similarity in the spectra of these nanoscale systems, including even
a “nonblinking” quantum dot investigated by Wang and
collaborators (<i>Nature</i> <b>2009</b>, <i>459</i>, 685–689), is powerful evidence for the existence
of a universal physical mechanism underlying the blinking phenomenon
in all of these fluorophores (Frantsuzov et al. <i>Nat. Phys.</i> <b>2008</b>, <i>4</i>, 519–522). In this
paper we show that the features of this universal mechanism can be
captured phenomenologically by the multiple recombination center model
(MRC) we suggested recently for explaining single colloidal QD intermittency.
Within the framework of the MRCs we qualitatively explain all of the
important features of fluorescence intensity fluctuations for a broad
spectrum of nanoscale emitters