4 research outputs found
Exploring the phonon-assisted excitation mechanism of luminescent centres in hexagonal boron nitride by photoluminescence excitation spectroscopy
The two-dimensional material hexagonal boron nitride (hBN) hosts
single-photon emitters active at room temperature. However, the microscopic
origin of these emitters, as well as the mechanism through which they are
excited, remain elusive. We address these issues by combining \emph{ab initio}
calculations with low-temperature photoluminescence excitation spectroscopy. By
studying 26 defect transitions, we find excellent qualitative agreement of
experiments with the emission and absorption line shapes of the carbon trimers
and , while enabling us to exclude 24 defect
transitions for one luminescent centre. Furthermore, we show an enhanced
zero-phonon line intensity at two-phonon detuning. This unambiguously
demonstrates that luminescent centers in hBN, and by inference single-photon
emitters, are excited through a phonon-assisted mechanism. To the best of our
knowledge, this study provides the most comprehensive insight into the
excitation mechanism and the microscopic origin of luminescent centers in hBN
A facile strategy for the growth of high-quality tungsten disulfide crystals mediated by oxygen-deficient oxide precursors
Chemical vapor deposition (CVD) has been established as a versatile route for
the large-scale synthesis of transition metal dichalcogenides, such as tungsten
disulfide (WS2). Yet, the role of the precursor composition on the efficiency
of the CVD process remains largely unknown and yet to be explored. Here, we
employ Pulsed Laser Deposition (PLD) in a two-stage process to tune the oxygen
content in tungsten oxide (WO3-x) precursors and demonstrate that the presence
of oxygen vacancies in the precursor films leads to a more facile conversion
from WO3-x to WS2. Using a joint study based on ab initio density functional
theory (DFT) calculations and experiments, we unravel that the oxygen vacancies
in WO3-x can serve as niches through which sulfur atoms enters the lattice and
may facilitate an efficient growth of WS2 crystals. By solely modulating the
precursor stoichiometry, the photoluminescence emission of WS2 can be greatly
enhanced, while the size of WS2 domains increases significantly. Atomic
resolution scanning transmission electron microscopy (STEM) reveals that
tungsten vacancies are the dominant intrinsic defects in mono- and bilayers
WS2. Moreover, our data reveal that grain boundaries in bilayer WS2 emerge upon
the coalescence of AA' and AB-oriented crystals, while turbostatic moir\'e
patterns originate upon formation of distinct grain boundaries between the
bottom and bottom layers. The atomic resolution images show local strain
buildup in bilayer WS2 due to competing effects of complex grain boundaries.
Our study provides a means to tune the precursor composition in order to
control the lateral growth of TMDs, while revealing insights into the different
pathways for the formation of grain boundaries in bilayer WS2
Combining experiments on luminescent centres in hexagonal boron nitride with the polaron model and ab initio methods towards the identification of their microscopic origin
The two-dimensional material hexagonal boron nitride (hBN) hosts luminescent centres with emission energies of ∼2 eV which exhibit pronounced phonon sidebands. We investigate the microscopic origin of these luminescent centres by combining ab initio calculations with non-perturbative open quantum system theory to study the emission and absorption properties of 26 defect transitions. Comparing the calculated line shapes with experiments we narrow down the microscopic origin to three carbon-based defects: C2CB, C2CN, and VNCB. The theoretical method developed enables us to calculate so-called photoluminescence excitation (PLE) maps, which show excellent agreement with our experiments. The latter resolves higher-order phonon transitions, thereby confirming both the vibronic structure of the optical transition and the phonon-assisted excitation mechanism with a phonon energy ∼170 meV. We believe that the presented experiments and polaron-based method accurately describe luminescent centres in hBN and will help to identify their microscopic origin
Tuning Surface Defect States in Sputtered Titanium Oxide Electron Transport Layers for Enhanced Stability of Organic Photovoltaics
Nonfullerene acceptors (NFAs) have dramatically improved the power conversion efficiency (PCE) of organic photovoltaics (OPV) in recent years; however, their device stability currently remains a bottleneck for further technological progress. Photocatalytic decomposition of nonfullerene acceptor molecules at metal oxide electron transport layer (ETL) interfaces has in several recent reports been demonstrated as one of the main degradation mechanisms for these high-performing OPV devices. While some routes for mitigating such degradation effects have been proposed, e.g., through a second layer integrated on the ETL surface, no clear strategy that complies with device scale-up and application requirements has been presented to date. In this work, it is demonstrated that the development of sputtered titanium oxide layers as ETLs in nonfullerene acceptor based OPV can lead to significantly enhanced device lifetimes. This is achieved by tuning the concentration of defect states at the oxide surface, via the reactive sputtering process, to mitigate the photocatalytic decomposition of NFA molecules at the metal oxide interlayers. Reduced defect state formation at the oxide surface is confirmed through X-ray photoelectron spectroscopy (XPS) studies, while the reduced photocatalytic decomposition of nonfullerene acceptor molecules is confirmed via optical spectroscopy investigations. The PBDB-T:ITIC organic solar cells show power conversion efficiencies of around 10% and significantly enhanced photostability. This is achieved through a reactive sputtering process that is fully scalable and industry compatible