Halide
perovskites are promising optoelectronic materials. Despite
impressive device performance, especially in photovoltaics, the femtosecond
dynamics of elementary optical excitations and their interactions
are still debated. Here we combine ultrafast two-dimensional electronic
spectroscopy (2DES) and semiconductor Bloch equations (SBEs) to probe
the room-temperature dynamics of nonequilibrium excitations in CsPbBr3 crystals. Experimentally, we distinguish between excitonic
and free-carrier transitions, extracting a ∼30 meV exciton
binding energy, in agreement with our SBE calculations and with recent
experimental studies. The 2DES dynamics indicate remarkably short,
<30 fs carrier relaxation at a ∼3 meV/fs rate, much faster
than previously anticipated for this material, but similar to that
in direct band gap semiconductors such as GaAs. Dynamic screening
of excitons by free carriers also develops on a similarly fast <30
fs time scale, emphasizing the role of carrier–carrier interactions
for this material’s optical properties. Our results suggest
that strong electron–phonon couplings lead to ultrafast relaxation
of charge carriers, which, in turn may limit halide perovskites’
carrier mobilities.
We
studied and compared the hydrodeoxygenation (HDO) and depolymerization
of aldehyde-stabilized lignin and 4-propylguaiacol (PG), a model lignin
monomer. We demonstrated by liquid phase adsorption that PG HDO catalyzed
by Ru/C can be achieved in isooctane but not in 1,4-dioxane due to
competitive solvent binding to the active sites. Unfortunately, alkanes
cannot be used as solvents for real lignin due to limited solubility.
However, we show that competitive solvent binding is suppressed when
switching from activated carbon to oxophilic metal oxide supports
such as TiO2, yielding 32%mol of an equimolar mixture of
cyclohexanes and cyclohexanols from real lignin.
A systematic review and analysis of the most stable spatial arrangements of n carbon, n oxygen, and 2n hydrogen atoms including vibrational zero-point energy up to n = 5 shows that small-molecule aggregates win, typically followed by thermally unstable molecules, before kinetically stable molecules and finally carbohydrates are found. Near n ≈ 60 a crossover to carbon allotropes and ice as the global minimum structure is expected and the asymptotic limit is most likely graphite and ice. Implications for astrochemical and fermentation processes are discussed. Density functionals like B3LYPD3 are found to describe these energy sequences quite poorly, mostly due to an overestimated stability of carbon in high oxidation states.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.