128 research outputs found
Screen printed PbâOâ films and their application to photoresponsive and photoelectrochemical devices
A new and simple procedure for the deposition of lead (II, IV) oxide films by screen printing was developed. In contrast to conventional electrochemical methods, films can be also deposited on non-conductive substrates without any specific dimensional restriction, being the only requirement the thermal stability of the substrate in air up to 500 °C to allow for the calcination of the screen printing paste and sintering of the film. In this study, films were exploited for the preparation of both photoresponsive devices and photoelectrochemical cell photoanodes. In both cases, screen printing was performed on FTO (Fluorine-Tin Oxide glass) substrates. The photoresponsive devices were tested with I-V curves in dark and under simulated solar light with different irradiation levels. Responses were evaluated at different voltage biases and under light pulses of different durations. Photoelectrochemical cells were tested by current densityâ»voltage (J-V) curves under air mass (AM) 1.5 G illumination, incident photon-to-current efficiency (IPCE) measurements, and electrochemical impedance spectroscopy
Excitons in van der Waals heterostructures: The important role of dielectric screening
The existence of strongly bound excitons is one of the hallmarks of the newly
discovered atomically thin semi-conductors. While it is understood that the
large binding energy is mainly due to the weak dielectric screening in two
dimensions (2D), a systematic investigation of the role of screening on 2D
excitons is still lacking. Here we provide a critical assessment of a widely
used 2D hydrogenic exciton model which assumes a dielectric function of the
form {\epsilon}(q) = 1 + 2{\pi}{\alpha}q, and we develop a quasi-2D model with
a much broader applicability. Within the quasi-2D picture, electrons and holes
are described as in-plane point charges with a finite extension in the
perpendicular direction and their interaction is screened by a dielectric
function with a non-linear q-dependence which is computed ab-initio. The
screened interaction is used in a generalized Mott-Wannier model to calculate
exciton binding energies in both isolated and supported 2D materials. For
isolated 2D materials, the quasi-2D treatment yields results almost identical
to those of the strict 2D model and both are in good agreement with ab-initio
many-body calculations. On the other hand, for more complex structures such as
supported layers or layers embedded in a van der Waals heterostructure, the
size of the exciton in reciprocal space extends well beyond the linear regime
of the dielectric function and a quasi-2D description has to replace the 2D
one. Our methodology has the merit of providing a seamless connection between
the strict 2D limit of isolated monolayer materials and the more bulk-like
screening characteristics of supported 2D materials or van der Waals
heterostructures.Comment: 14 pages, 13 figure
Cavity control of Excitons in two dimensional Materials
We propose a robust and efficient way of controlling the optical spectra of
two-dimensional materials and van der Waals heterostructures by quantum cavity
embedding. The cavity light-matter coupling leads to the formation of
exciton-polaritons, a superposition of photons and excitons. Our first
principles study demonstrates a reordering and mixing of bright and dark
excitons spectral features and in the case of a type II van-der-Waals
heterostructure an inversion of intra and interlayer excitonic resonances. We
further show that the cavity light-matter coupling strongly depends on the
dielectric environment and can be controlled by encapsulating the active 2D
crystal in another dielectric material. Our theoretical calculations are based
on a newly developed non-perturbative many-body framework to solve the coupled
electron-photon Schr\"odinger equation in a quantum-electrodynamical extension
of the Bethe-Salpeter approach. This approach enables the ab-initio simulations
of exciton-polariton states and their dispersion from weak to strong cavity
light-matter coupling regimes. Our method is then extended to treat van der
Waals heterostructures and encapsulated 2D materials using a simplified
Mott-Wannier description of the excitons that can be applied to very large
systems beyond reach for fully ab-initio approaches.Comment: 32 pages. 10 figures, 2 tabl
Solid solutions of rare earth cations in mesoporous anatase beads and their performances in dye-sensitized solar cells
Solid solutions of the rare earth (RE) cations Pr3+, Nd3+, Sm3+, Gd3+, Er3+ and Yb3+ in anatase TiO2 have been synthesized as mesoporous beads in the concentration range 0.1-0.3% of metal atoms. The solid solutions were have been characterized by XRD, SEM, diffuse reflectance UV-Vis spectroscopy, BET and BJH surface analysis. All the solid solutions possess high specific surface areas, up to more than 100 m2/g. The amount of adsorbed dye in each photoanode has been determined spectrophotometrically. All the samples were tested as photoanodes in dye-sensitized solar cells (DSSCs) using N719 as dye and a nonvolatile, benzonitrile based electrolyte. All the cells were have been tested by conversion efficiency (J-V), quantum efficiency (IPCE), electrochemical impedance spectroscopy (EIS) and dark current measurements. While lighter RE cations (Pr3+, Nd3+) limit the performance of DSSCs compared to pure anatase mesoporous beads, cations from Sm3+ onwards enhance the performance of the devices. A maximum conversion efficiency of 8.7% for Er3+ at a concentration of 0.2% has been achieved. This is a remarkable efficiency value for a DSSC employing N719 dye without co-adsorbents and a nonvolatile electrolyte. For each RE cation the maximum performances are obtained for a concentration of 0.2% metal atoms. © 2015, Nature Publishing Group. All rights reserved
Stark shift and electric-field-induced dissociation of excitons in monolayer MoS2 and hBN/MoS2 heterostructures
Efficient conversion of photons into electrical current in two-dimensional
semiconductors requires, as a first step, the dissociation of the strongly
bound excitons into free electrons and holes. Here we calculate the
dissociation rates and energy shift of excitons in monolayer MoS2 as a
function of an applied in-plane electric field. The dissociation rates are
obtained as the inverse lifetime of the resonant states of a two-dimensional
hydrogenic Hamiltonian which describes the exciton within the Mott-Wannier
model. The resonances are computed using complex scaling, and the effective
masses and screened electron-hole interaction defining the hydrogenic
Hamiltonian are computed from first principles. For field strengths above 0.1
V/nm the dissociation lifetime is shorter than 1 ps, which is below the
lifetime associated with competing decay mechanisms. Interestingly,
encapsulation of the MoS2 layer in just two layers of hexagonal boron nitride
(hBN), enhances the dissociation rate by around one order of magnitude due to
the increased screening. This shows that dielectric engineering is an
effective way to control exciton lifetimes in two-dimensional materials
Simple Screened Hydrogen Model of Excitons in Two-Dimensional Materials
We present a generalized hydrogen model for the binding energies () of
excitons in two-dimensional (2D) materials that sheds light on the fundamental
differences between excitons in two and three dimensions. In contrast to the
well-known hydrogen model of three-dimensional (3D) excitons, the description
of 2D excitons is complicated by the fact that the screening cannot be assumed
to be local. We show that one can consistently define an effective 2D
dielectric constant by averaging the screening over the extend of the exciton.
For an ideal 2D semiconductor this leads to a simple expression for that
only depends on the excitonic mass and the 2D polarizability . The
model is shown to produce accurate results for 51 transition metal
dichalcogenides. Remarkably, over a wide range of polarizabilities the
expression becomes independent of the mass and we obtain
, which explains the recently observed linear
scaling of exciton binding energies with band gap. It is also shown that the
model accurately reproduces the non-hydrogenic Rydberg series in WS and can
account for screening from the environment.Comment: 5 page
Electron-Photon Exchange-Correlation Approximation for QEDFT
Quantum-electrodynamical density-functional theory (QEDFT) provides a
promising avenue for exploring complex light-matter interactions in optical
cavities for real materials. Similar to conventional density-functional theory,
the Kohn-Sham formulation of QEDFT needs approximations for the generally
unknown exchange-correlation functional. In addition to the usual
electron-electron exchange-correlation potential, an approximation for the
electron-photon exchange-correlation potential is needed. A recent
electron-photon exchange functional [C. Sch\"afer et al., Proc. Natl. Acad.
Sci. USA, 118, e2110464118 (2021),
https://www.pnas.org/doi/abs/10.1073/pnas.2110464118], derived from the
equation of motion of the non-relativistic Pauli-Fierz Hamiltonian, shows
robust performance in one-dimensional systems across weak- and strong-coupling
regimes. Yet, its performance in reproducing electron densities in higher
dimensions remains unexplored. Here we consider this QEDFT functional
approximation from one to three-dimensional finite systems and across weak to
strong light-matter couplings. The electron-photon exchange approximation
provides excellent results in the ultra-strong-coupling regime. However, to
ensure accuracy also in the weak-coupling regime across higher dimensions, we
introduce a computationally efficient renormalization factor for the
electron-photon exchange functional, which accounts for part of the
electron-photon correlation contribution. These findings extend the
applicability of photon-exchange-based functionals to realistic cavity-matter
systems, fostering the field of cavity QED (quantum electrodynamics) materials
engineering.Comment: 15 pages, 4 figure
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