Monolayer ${\mathrm{WSe}}_{2}$ is an intriguing material to explore dark exciton physics. We have measured the photoluminescence from excitons and trions in ultraclean monolayer devices encapsulated by boron nitride. The can be tuned continuously between negative positive with electrostatic gating. reveal their spin-triplet configuration distinct valley optical emission characteristic Zeeman splitting under a magnetic field. trion binding energies are 14--16 meV, slightly lower than bright...

We observe a set of three replica luminescent peaks at ~21.4 meV below the dark exciton, negative and positive trions (or exciton-polarons) in monolayer WSe2. The redshift energy matches zone-center E"-mode optical phonons. phonon replicas exhibit parallel gate dependence same g-factors as excitonic states, but follow valley selection rules bright states. While states out-of-plane transition dipole valley-independent linearly polarized emission in-plane directions, their valley-dependent...

Excitons and trions (or exciton-polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role the excitonic dynamics, but intervalley transitions rarely observed monolayer TMDs, because they violate conservation of momentum. Here we reveal dark excitons more than one path WSe$_2$. We observe excitons, which recombine by assistance defect scattering or...

Exciton polaron is a hypothetical many-body quasiparticle that involves an exciton dressed with polarized electron-hole cloud in the Fermi sea. It has been evoked to explain excitonic spectra of charged monolayer transition metal dichalcogenides, but studies were limited ground state. Here we measure reflection and photoluminescence MoSe2 WSe2 gating devices encapsulated by boron nitride. We observe gate-tunable polarons associated 1 s-3 s Rydberg states. The excited exhibit comparable...

Monolayer WSe$_2$ hosts a series of exciton Rydberg states denoted by the principal quantum number n = 1, 2, 3, etc. While most research focuses on their absorption properties, optical emission is also important but much less studied. Here we measure photoluminescence from 1s - 5s in ultraclean monolayer encapsulated boron nitride under magnetic fields -31 T to 31 T. The exhibit similar Zeeman shifts distinct diamagnetic each other. From luminescence spectra, and shifts, deduce binding...

We report the synthesis and transfer of epitaxial germanane (GeH) onto arbitrary substrates by electrochemical delamination investigate its optoelectronic properties. GeH films with thickness ranging from 1 to 600 nm (2–1000 layers) areas up ∼1 cm2 have been reliably transferred characterized photoluminescence, x-ray diffraction, energy-dispersive spectroscopy. Wavelength dependent photoconductivity measurements on few-layer exhibit an absorption edge provide a sensitive characterization...

We have investigated the interlayer shear and breathing phonon modes in MoS2 with pure 3R 2H stacking order by using polarization-dependent ultralow-frequency Raman spectroscopy.We observe up to three branches four thickness from 2 13 layers.The show same activity behavior for both polytypes, but frequencies are consistently several wavenumbers higher than frequencies, signifying that has slightly stronger lattice coupling MoS2.In contrast, shear-mode spectra strikingly different MoS2.While...

Atomically thin NbSe2 is a metallic layered transition metal dichalcogenide with novel charge-density-wave (CDW) and superconductive phases. Properties of atomic layers are sensitive to interlayer coupling. Here we investigate the phonons few-layer by ultralow-frequency Raman spectroscopy. We observe both breathing modes shear at frequencies below 40 cm−1 for samples 2–15 layers. Their frequency, activity, environmental instability depend systematically on layer number. account these results...

Exciton exchange between semiconductor quantum dots (QDs) and organic chromophores may find application in next-generation photovoltaics, probes, or photocatalysts. We study the effect of shell composition thickness on triplet energy transfer (TET) from PbS QDs to rubrene a hybrid organic–inorganic photon upconverison system show that defect states introduced by surface adsorbed Zn Cd result up 700 325 fold enhancements yield (QY). Time-correlated single-photon counting, photoluminescence...

We investigate Landau-quantized excitonic absorption and luminescence of monolayer WSe_{2} under magnetic field. observe gate-dependent quantum oscillations in the bright exciton trions (or polarons) as well dark their phonon replicas. Our results reveal spin- valley-polarized Landau levels (LLs) with filling factors n=+0, +1 bottom conduction band n=-0 to -6 top valence band, including Berry-curvature-induced n=±0 LLs massive Dirac fermions. The LL produces periodic plateaus energy shift...

We report the observation of QΓ intervalley exciton in bilayer WSe2 devices encapsulated by boron nitride. The resides at ∼18 meV below QK exciton. and excitons exhibit different Stark shifts under an out-of-plane electric field due to their interlayer dipole moments. By controlling field, we can switch energy ordering control which dominates luminescence WSe2. Remarkably, both unusually strong two-phonon replicas, are comparable or even stronger than one-phonon replicas. detailed...

The technique of focused ion beam (FIB) milling is used to cut two-dimensional shapes into single crystals composed the organic semiconductor molecule perylene. use an ultrathin Au coating and its removal using a KI/I2 etchant solution allow crystal be imaged in FIB apparatus while protecting it from electron damage. Using this approach, features with spatial resolution on order 130 nm can created retaining 90% original photoluminescence intensity surrounding regions. These...

We investigate spin relaxation in graphene by systematically comparing the roles of absorption, other contact-induced effects (e.g., fringe fields), and bulk for valves with MgO barriers, Al2O3 transparent contacts. obtain effective lifetimes fitting Hanle precession data two models that include or exclude effect absorption. Results indicate additional than absorption dominates contact effect. For tunneling contacts, we find reasonable agreement between median discrepancy ∼20% ∼10% Al2O3.

Fully fluorinated perfluorocarbon ligands are shown to modify the energetics and dielectric environment of quantum dots (QDs), resulting in a large hypsochromic shift optical gap. The original oleic acid (OA) on PbS QDs can be completely replaced with thiolate carboxylate-based perfluorocarbons, e.g., -SCF3 CF3(CF2)14COOH (pFA), respectively. Ultraviolet photoelectron spectroscopy indicates that work function varies by >1.3 eV depending electronegativity surface ligand, while cyclic...

The electronic exciton polaron is a hypothetical many-body quasiparticle formed by an dressed with polarized electron-hole cloud in the Fermi sea (FS). It predicted to display rich physics and unusual roton-like dispersion. Exciton polarons were recently evoked explain excitonic spectra of doped monolayer transition metal dichalcogenides (TMDs), but these studies are limited ground state. Excited-state can exhibit richer due their larger spatial extent, detection challenging inherently weak...

Atomically thin NbSe2 is a metallic layered transition metal dichalcogenide (TMD) with considerably different crystallographic structure and electronic properties from other TMDs, such as MoS2, MoSe2, WS2 WSe2. Properties of TMD atomic layers are sensitive to interlayer coupling. Here we investigate the phonons few-layer by ultralow-frequency Raman spectroscopy. We observe both breathing modes shear at frequencies below 40 cm-1 for samples 2 15 layers. Their frequency, activity,...

We report the direct observation of intervalley exciton between Q conduction valley and $Γ$ valence in bilayer WSe$_2$ by photoluminescence. The Q$Γ$ lies at ~18 meV below QK dominates luminescence WSe$_2$. By measuring spectra gate-tunable electric field, we reveal different interlayer dipole moments Stark shifts excitons. Notably, can use field to switch energy order dominant Both excitons exhibit pronounced phonon replicas, which two-phonon replicas outshine one-phonon due existence...

High resolution scanning tunneling microscopy investigation of the pentacene derivative, 5,6,7-trithiapentacene-13-one (TTPO), on close-packed gold surfaces reveals nanoscale structures ranging from molecular chains at low coverage to highly ordered self-assembled monolayers. Robust, thermally stable, and inert photo-oxidation, TTPO is a promising organic semiconductor with potential applications in high-temperature photovoltaic devices. When adsorbed electrode, experiment first-principle...

Ultrathin layers of hexagonal boron nitride (h-BN) are used to fully encapsulate single perylene microcrystals. The morphology and chemical stability for samples prepared using different encapsulation methods characterized electron, optical, atomic force microscopies. Only multilayer MBE-grown h-BN could protect the organic crystals from dissolution sublimation. To determine interaction two-dimensional material with underlying chromophores, a polymer film Lumogen Red dye molecules that act...