Ultrafast Charge Transfer and Recombination Dynamics in Monolayer-Multilayer WSe2 Junctions Revealed by Time-Resolved Photoemission Electron Microscopy.

The ultrafast carrier dynamics of junctions between two chemically identical, but electronically distinct, transition metal dichalcogenides (TMDs) remains largely unknown. Here, we employ time-resolved photoemission electron microscopy (TR-PEEM) to probe the ultrafast carrier dynamics of a monolayer-to-multilayer (1L-ML) WSe2 junction. The TR-PEEM signals recorded for the individual components of the junction reveal the sub-ps carrier cooling dynamics of 1L- and 7L-WSe2, as well as few-ps exciton-exciton annihilation occurring on 1L-WSe2. We observe ultrafast interfacial hole (h) transfer from 1L- to 7L-WSe2 on an ∼0.2 ps time scale. The resultant excess h density in 7L-WSe2 decays by carrier recombination across the junction interface on an ∼100 ps time scale. Reminiscent of the behavior at a depletion region, the TR-PEEM image reveals the h density accumulation on the 7L-WSe2 interface, with a decay length ∼0.60 ± 0.17 µm. These charge transfer and recombination dynamics are in agreement with ab initio quantum dynamics. The computed orbital densities reveal that charge transfer occurs from the basal plane, which extends over both 1L and ML regions, to the upper plane localized on the ML region. This mode of charge transfer is distinctive to chemically homogeneous junctions of layered materials and constitutes an additional carrier deactivation pathway that should be considered in studies of 1L-TMDs found alongside their ML, a common occurrence in exfoliated samples.

Nitroglycerin trapping in melamine matrices.

Melamine (Mel) was used as host matrix for liquid nitroglycerin (NG), to prepare Mel/NG solid powdered compounds containing up to 45 wt% of this explosive. The two preparation processes used for this purpose consisted in evaporating a solution of both components, either in ambient conditions or under reduced pressure by the Spray Flash-Evaporation (SFE) process. In Mel/NG materials, amorphous nitroglycerin is distributed in the crystallized melamine matrix as inclusions, which were found to be smaller in size in the material prepared by the SFE process. Mel/NG materials are not stable over time: they gradually lose the nitroglycerin they contain by evaporation.

Ultrafast Electronic Relaxation Dynamics of Atomically Thin MoS2 Is Accelerated by Wrinkling.

Strain engineering is an attractive approach for tuning the local optoelectronic properties of transition metal dichalcogenides (TMDs). While strain has been shown to affect the nanosecond carrier recombination dynamics of TMDs, its influence on the sub-picosecond electronic relaxation dynamics is still unexplored. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and nonadiabatic ab initio molecular dynamics (NAMD) to investigate the ultrafast dynamics of wrinkled multilayer (ML) MoS2 comprising 17 layers. Following 2.41 eV photoexcitation, electronic relaxation at the Γ valley occurs with a time constant of 97 ± 2 fs for wrinkled ML-MoS2 and 120 ± 2 fs for flat ML-MoS2. NAMD shows that wrinkling permits larger amplitude motions of MoS2 layers, relaxes electron-phonon coupling selection rules, perturbs chemical bonding, and increases the electronic density of states. As a result, the nonadiabatic coupling grows and electronic relaxation becomes faster compared to flat ML-MoS2. Our study suggests that the sub-picosecond electronic relaxation dynamics of TMDs is amenable to strain engineering and that applications which require long-lived hot carriers, such as hot-electron-driven light harvesting and photocatalysis, should employ wrinkle-free TMDs.

Raman spectroscopy of GaSe and InSe post-transition metal chalcogenides layers.

III-VI post-transition metal chalcogenides (InSe and GaSe) are a new class of layered semiconductors, which feature a strong variation of size and type of their band gaps as a function of number of layers (N). Here, we investigate exfoliated layers of InSe and GaSe ranging from bulk crystals down to monolayer, encapsulated in hexagonal boron nitride, using Raman spectroscopy. We present the N-dependence of both intralayer vibrations within each atomic layer, as well as of the interlayer shear and layer breathing modes. A linear chain model can be used to describe the evolution of the peak positions as a function of N, consistent with first principles calculations.

Weak Distance Dependence of Hot-Electron-Transfer Rates at the Interface between Monolayer MoS2 and Gold.

Electron transport across the transition-metal dichalcogenide (TMD)/metal interface plays an important role in determining the performance of TMD-based optoelectronic devices. However, the robustness of this process against structural heterogeneities remains unexplored, to the best of our knowledge. Here, we employ a combination of time-resolved photoemission electron microscopy (TR-PEEM) and atomic force microscopy to investigate the spatially resolved hot-electron-transfer dynamics at the monolayer (1L) MoS2/Au interface. A spatially heterogeneous distribution of 1L-MoS2/Au gap distances, along with the sub-80 nm spatial- and sub-60 fs temporal resolution of TR-PEEM, permits the simultaneous measurement of electron-transfer rates across a range of 1L-MoS2/Au distances. These decay exponentially as a function of distance, with an attenuation coefficient ß âˆ¼ 0.06 ± 0.01 Å-1, comparable to molecular wires. Ab initio simulations suggest that surface plasmon-like states mediate hot-electron-transfer, hence accounting for its weak distance dependence. The weak distance dependence of the interfacial hot-electron-transfer rate indicates that this process is insensitive to distance fluctuations at the TMD/metal interface, thus motivating further exploration of optoelectronic devices based on hot carriers.

Low Operating Voltage Carbon-Graphene Hybrid E-textile for Temperature Sensing.

Graphene-coated polypropylene (PP) textile fibers are presented for their use as temperature sensors. These temperature sensors show a negative thermal coefficient of resistance (TCR) in a range between 30 and 45 °C with good sensitivity and reliability and can operate at voltages as low as 1 V. The analysis of the transient response of the temperature on resistance of different types of graphene produced by chemical vapor deposition (CVD) and shear exfoliation of graphite (SEG) shows that trilayer graphene (TLG) grown on copper by CVD displays better sensitivity due to the better thickness uniformity of the film and that carbon paste provides good contact for the measurements. Along with high sensitivity, TLG on PP shows not only the best response but also better transparency, mechanical stability, and washability compared to SEG. Temperature-dependent Raman analysis reveals that the temperature has no significant effect on the peak frequency of PP and expected effect on graphene in the demonstrated temperature range. The presented results demonstrate that these flexible, lightweight temperature sensors based on TLG with a negative TCR can be easily integrated in fabrics.

Photocatalytic activity of exfoliated graphite-TiO2 nanoparticle composites.

We investigate the photocatalytic performance of composites prepared in a one-step process by liquid-phase exfoliation of graphite in the presence of TiO2 nanoparticles (NPs) at atmospheric pressure and in water, without heating or adding any surfactant, and starting from low-cost commercial reagents. These show enhanced photocatalytic activity, degrading up to 40% more pollutants with respect to the starting TiO2-NPs, in the case of a model dye target, and up to 70% more pollutants in the case of nitrogen oxides. In order to understand the photo-physical mechanisms underlying this enhancement, we investigate the photo-generation of reactive species (trapped holes and electrons) by ultrafast transient absorption spectroscopy. We observe an electron transfer process from TiO2 to the graphite flakes within the first picoseconds of the relaxation dynamics, which causes the decrease of the charge recombination rate, and increases the efficiency of the reactive species photo-production.

High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition.

We report high room-temperature mobility in single-layer graphene grown by chemical vapor deposition (CVD) after wet transfer on SiO2 and hexagonal boron nitride (hBN) encapsulation. By removing contaminations, trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to ∼70000 cm2 V-1 s-1 at room temperature and ∼120 000 cm2 V-1 s-1 at 9K. These are more than twice those of previous wet-transferred graphene and comparable to samples prepared by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycrystalline graphene, achieving room-temperature mobilities of ∼30 000 cm2 V-1 s-1. These results show that, with appropriate encapsulation and cleaning, room-temperature mobilities well above 10 000 cm2 V-1 s-1 can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.

Broadband, electrically tunable third-harmonic generation in graphene.

Optical harmonic generation occurs when high intensity light (>1010 W m-2) interacts with a nonlinear material. Electrical control of the nonlinear optical response enables applications such as gate-tunable switches and frequency converters. Graphene displays exceptionally strong light-matter interaction and electrically and broadband tunable third-order nonlinear susceptibility. Here, we show that the third-harmonic generation efficiency in graphene can be increased by almost two orders of magnitude by controlling the Fermi energy and the incident photon energy. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from resonant multiphoton transitions. Thanks to the linear dispersion of the massless Dirac fermions, gate controllable third-harmonic enhancement can be achieved over an ultrabroad bandwidth, paving the way for electrically tunable broadband frequency converters for applications in optical communications and signal processing.

Large-scale quantum-emitter arrays in atomically thin semiconductors.

Quantum light emitters have been observed in atomically thin layers of transition metal dichalcogenides. However, they are found at random locations within the host material and usually in low densities, hindering experiments aiming to investigate this new class of emitters. Here, we create deterministic arrays of hundreds of quantum emitters in tungsten diselenide and tungsten disulphide monolayers, emitting across a range of wavelengths in the visible spectrum (610-680 nm and 740-820 nm), with a greater spectral stability than their randomly occurring counterparts. This is achieved by depositing monolayers onto silica substrates nanopatterned with arrays of 150-nm-diameter pillars ranging from 60 to 190 nm in height. The nanopillars create localized deformations in the material resulting in the quantum confinement of excitons. Our method may enable the placement of emitters in photonic structures such as optical waveguides in a scalable way, where precise and accurate positioning is paramount.

Atomically thin quantum light-emitting diodes.

Transition metal dichalcogenides are optically active, layered materials promising for fast optoelectronics and on-chip photonics. We demonstrate electrically driven single-photon emission from localized sites in tungsten diselenide and tungsten disulphide. To achieve this, we fabricate a light-emitting diode structure comprising single-layer graphene, thin hexagonal boron nitride and transition metal dichalcogenide mono- and bi-layers. Photon correlation measurements are used to confirm the single-photon nature of the spectrally sharp emission. These results present the transition metal dichalcogenide family as a platform for hybrid, broadband, atomically precise quantum photonics devices.

Doping dependence of the Raman spectrum of defected graphene.

We investigate the evolution of the Raman spectrum of defected graphene as a function of doping. Polymer electrolyte gating allows us to move the Fermi level up to 0.7 eV, as directly monitored by in situ Hall-effect measurements. For a given number of defects, we find that the intensities of the D and D' peaks decrease with increasing doping. We assign this to an increased total scattering rate of the photoexcited electrons and holes, due to the doping-dependent strength of electron-electron scattering. We present a general relation between D peak intensity and defects valid for any doping level.

Mechanistic insights into heterogeneous zinc dicarboxylates and theoretical considerations for CO2-epoxide copolymerization.

Copolymerization of epoxides and CO(2) with heterogeneous zinc dicarboxylates is prominent since the early days of this area of chemistry. However, in over 30 years of research, the efficiency of this catalyst system could not be improved significantly. Furthermore, a huge activity difference between zinc glutarate and its lower homologue zinc succinate exists, which could not be explained so far. A detailed investigation of the underlying copolymerization mechanisms on heterogeneous catalysts is therefore necessary. Such investigations are so far lacking, which renders logical improvements of the catalysts difficult. We therefore decided to conduct a detailed investigation on the different zinc-dicarboxylic catalysts, their copolymerization efficiency, solid state structure and supplemented the results with theoretical calculations. The results imply that the widely discussed bimetallic mechanism (for homogeneous catalysts) is in place for heterogeneous zinc dicarboxylates as well. Theoretical calculations conducted to identify an "ideal" Zn-Zn distance suggest an optimal separation of Zn atoms in the range of 4.3-5.0 Å. The combined copolymerization experiments and calculated models give a consistent explanation for the difference in activity of the different zinc-dicarboxylate catalysts and give a hint why the activity of the heterogeneous zinc-dicarboxylate system is limited.

Synthesis and properties of (triptycenedicarboxylatio)zinc coordination networks.

(Triptycenedicarboxylato)zinc metal-organic frameworks (MOFs) based on paddle wheel secondary building units (SBUs) with different axial ligands have been prepared. The reproducible formation of the layered paddle-wheel structures from triptycenedicarboxylic acid (H(2)TDC) and zinc nitrate under various conditions seems to be characteristic of this acid and is utilized for the construction of 3D frameworks by a pillaring approach. We attempted to bring additional functionalities into MOFs by employing the appropriate pillaring ligands, for example, bis(4-pyridyl)-s-tetrazine and bis(4-pyridyl)-dimethoxy-p-phenylenedivinylene, and investigated certain properties of some MOF materials, such as guest-exchange behavior, luminescence, microporosity, and stability.