115 research outputs found
Coherent optoelectronic control of single excitons
Steffen J. Michaelis de VasconcellosPaderborn, Univ., Diss., 200
Phonon Sidebands in Transition Metal Dichalcogenides
Excitons dominate the optical properties of monolayer transition metal
dichalcogenides (TMDs). Besides optically accessible bright exciton states,
TMDs exhibit also a multitude of optically forbidden dark excitons. Here, we
show that efficient exciton-phonon scattering couples bright and dark states
and gives rise to an asymmetric excitonic line shape. The observed asymmetry
can be traced back to phonon-induced sidebands that are accompanied by a
polaron redshift. We present a joint theory-experiment study investigating the
microscopic origin of these sidebands in different TMD materials taking into
account intra- and intervalley scattering channels opened by optical and
acoustic phonons. The gained insights contribute to a better understanding of
the optical fingerprint of these technologically promising nanomaterials
Lifetime vs. rate capability: Understanding the role of FEC and VC in high-energy Li-ion batteries with nano-silicon anodes
Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are the most frequently used electrolyte components to enhance the lifetime of anode materials in Li-ion batteries, but for silicon it is still ambiguous when FEC or VC is more beneficial. Herein, a nanostructured silicon/carbon anode derived from low-cost HSiCl3 is tailored by the rational choice of the electrolyte component, to obtain an anode material outperforming current complex silicon structures. We demonstrate highly reversible areal capacities of up to 5 mA h/cm2 at 4.4 mg/cm2 mass loading, a specific capacity of 1280 mA h/gElectrode, a capacity retention of 81% after 500 deep-discharge cycles versus lithium metal and successful full-cell tests with high-voltage cathodes meeting the requirements for real application. Electrochemical impedance spectroscopy and post-mortem investigation provide new insights in tailoring the interfacial properties of silicon-based anodes for high performance anode materials based on an alloying mechanism with large volume changes. The role of fluorine in the FEC-derived interfacial layer is discussed in comparison with the VC-derived layer and possible degradation mechanisms are proposed. We believe that this study gives a valuable understanding and provides new strategies on the facile use of additives for highly reversible silicon anodes in Li-ion batteries.Fil: Jaumann, Tony. Ifw Dresden; AlemaniaFil: Balach, Juan Manuel. Ifw Dresden; AlemaniaFil: Langklotz, Ulrike. Technische Universität Dresden; AlemaniaFil: Sauchuk, Viktar. Fraunhofer Institute for Ceramic Materials and Systems; AlemaniaFil: Fritsch, Marco. Fraunhofer Institute for Ceramic Materials and Systems; AlemaniaFil: Michaelis, Alexander. Technische Universität Dresden; AlemaniaFil: Teltevskij, Valerij. Leibniz Institute for Solid State and Materials Research; AlemaniaFil: Mikhailova, Daria. Leibniz Institute for Solid State and Materials Research; AlemaniaFil: Oswald, Steffen. Leibniz Institute for Solid State and Materials Research; AlemaniaFil: Klose, Markus. Leibniz Institute for Solid State and Materials Research; Alemania. Technische Universität Dresden; AlemaniaFil: Stephani, Guenter. Branch Lab Dresden. Fraunhofer Institute for Manufacturing Technology and Advanced Materials; ArgentinaFil: Hauser, Ralf. Branch Lab Dresden. Fraunhofer Institute for Manufacturing Technology and Advanced Materials; ArgentinaFil: Eckert, Jürgen. Technische Universität Dresden; Alemania. Leibniz Institute for Solid State and Materials Research; AlemaniaFil: Giebeler, Lars. Leibniz Institute for Solid State and Materials Research; Alemania. Technische Universität Dresden; Alemani
Ultrafast dynamics in monolayer TMDCs: the interplay of dark excitons, phonons and intervalley Coulomb exchange
Understanding the ultrafast coupling and relaxation mechanisms between
valleys in transition metal dichalcogenide semiconductors is of crucial
interest for future valleytronic devices. Recent ultrafast pump-probe
experiments showed an unintuitive significant bleaching at the excitonic
transition after optical excitation of the energetically lower excitonic
transition. Here, we present a possible microscopic explanation for this
surprising effect. It is based on the joint action of exchange coupling and
phonon-mediated thermalization into dark exciton states and does not involve a
population of the B exciton. Our work demonstrates how intra- and intervalley
coupling on a femtosecond timescale governs the optical valley response of 2D
semiconductors
SEI-component formation on sub 5 nm sized silicon nanoparticles in Li-ion batteries: The role of electrode preparation, FEC addition and binders
Silicon is a promising negative electrode for secondary lithium-based batteries, but the electrochemical reversibility of particularly nanostructured silicon electrodes drastically depends on their interfacial characteristics, commonly known as the solid electrolyte interface (SEI). The beneficial origin of certain electrolyte additives or different binders is still discussed controversially owing to the challenging peculiarities of interfacial post-mortem investigations of electrodes. In this work, we address the common difficulties of SEI investigations of porous silicon/carbon nanostructures and study the addition of a fluoroethylene carbonate (FEC) as a stabilizing additive as well as the use of two different binders, carboxymethyl cellulose/styrene-butadiene rubber (CMC/SBR) and polyacrylic acid (PAA), for the SEI formation. The electrode is composed of silicon nanocrystallites below 5 nm diameter allowing a detailed investigation of interfacial characteristics of silicon owing to the high surface area. We first performed galvanostatic long-term cycling (400 times) and carried out comprehensive ex situ characterization of the cycled nanocrystalline silicon electrodes with XRD, EDXS, TEM and XPS. We modified the preparation of the electrode for post-mortem characterization to distinguish between electrolyte components and the actual SEI. The impact of the FEC additive and two different binders on the interfacial layer is studied and the occurrence of diverse compounds, in particular LiF, Li2O and phosphates, is discussed. These results help to understand general issues in SEI formation and to pave the way for the development of advanced electrolytes allowing for a long-term performance of nanostructured Si-based electrodes
Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2
The research field of two dimensional (2D) materials strongly relies on
optical microscopy characterization tools to identify atomically thin materials
and to determine their number of layers. Moreover, optical microscopy-based
techniques opened the door to study the optical properties of these
nanomaterials. We presented a comprehensive study of the differential
reflectance spectra of 2D semiconducting transition metal dichalcogenides
(TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up
to six layers. We analyzed the thickness-dependent energy of the different
excitonic features, indicating the change in the band structure of the
different TMDC materials with the number of layers. Our work provided a route
to employ differential reflectance spectroscopy for determining the number of
layers of MoS2, MoSe2, WS2, and WSe2.Comment: Main text (3 Figures) and Supp. Info. (23 Figures
Biaxial strain tuning of the optical properties of single-layer transition metal dichalcogenides
Since their discovery single-layer semiconducting transition metal
dichalcogenides have attracted much attention thanks to their outstanding
optical and mechanical properties. Strain engineering in these two-dimensional
materials aims to tune their bandgap energy and to modify their optoelectronic
properties by the application of external strain. In this paper we demonstrate
that biaxial strain, both tensile and compressive, can be applied and released
in a timescale of a few seconds in a reproducible way on transition metal
dichalcogenides monolayers deposited on polymeric substrates. We can control
the amount of biaxial strain applied by letting the substrate expand or
compress. To do this we change the substrate temperature and choose materials
with a large thermal expansion coefficient. After the investigation of the
substrate-dependent strain transfer, we performed micro-differential
spectroscopy of four transition metal dichalcogenides monolayers (MoS2, MoSe2,
WS2, WSe2) under the application of biaxial strain and measured their optical
properties. For tensile strain we observe a redshift of the bandgap that
reaches a value as large as 95 meV/% in the case of single-layer WS2 deposited
on polypropylene. The observed bandgap shifts as a function of substrate
extension/compression follow the order MoSe2 < MoS2 < WSe2 < WS2. Theoretical
calculations of these four materials under biaxial strain predict the same
trend for the material-dependent rates of the shift and reproduce well the
features observed in the measured reflectance spectra.Comment: 10 pages, 5 figures, 2 tables, supporting informatio
Zeeman spectroscopy of excitons and hybridization of electronic states in few-layer WSe, MoSe and MoTe
Monolayers and multilayers of semiconducting transition metal dichalcogenides
(TMDCs) offer an ideal platform to explore valley-selective physics with
promising applications in valleytronics and information processing. Here we
manipulate the energetic degeneracy of the and
valleys in few-layer TMDCs. We perform high-field magneto-reflectance
spectroscopy on WSe, MoSe, and MoTe crystals of thickness from
monolayer to the bulk limit under magnetic fields up to 30 T applied
perpendicular to the sample plane. Because of a strong spin-layer locking, the
ground state A excitons exhibit a monolayer-like valley Zeeman splitting with a
negative -factor, whose magnitude increases monotonically when thinning the
crystal down from bulk to a monolayer. Using the
calculation, we demonstrate that the observed evolution of -factors for
different materials is well accounted for by hybridization of electronic states
in the and valleys. The mixing of the valence and
conduction band states induced by the interlayer interaction decreases the
-factor magnitude with an increasing layer number. The effect is the largest
for MoTe, followed by MoSe, and smallest for WSe.
Keywords: MoSe, WSe, MoTe, valley Zeeman splitting, transition
metal dichalcogenides, excitons, magneto optics.Comment: 14 pages, 5 figure
- …