Fine-Tuning of the Excitonic Response in Monolayer WS2 Domes via Coupled Pressure and Strain Variation

We present a spectroscopic investigation into the vibrational and optoelectronic properties of WS2 domes in the 0-0.65 GPa range. The pressure evolution of the system morphology, deduced by the combined analysis of Raman and photoluminescence spectra, revealed a significant variation in the dome's aspect ratio. The modification of the dome shape caused major changes in the mechanical properties of the system resulting in a sizable increase of the out-of-plane compressive strain while keeping the in-plane tensile strain unchanged. The variation of the strain gradients drives a non-linear behavior in both the exciton energy and radiative recombination intensity, interpreted as the consequence of a hybridization mechanism between the electronic states of two distinct minima in the conduction band. Our results indicate that pressure and strain can be efficiently combined in low dimensional systems with unconventional morphology to obtain modulations of the electronic band structure not achievable in planar crystals.Comment: 11 pages, 4 figure

Experimental investigation of electronic interactions in collapsed and uncollapsed LaFe2As2 phases

The iron-based pnictide LaFe2As2 is not superconducting as-synthesized, but it becomes such below Tc around 12 K upon annealing, as a consequence of a structural transition from a phase with collapsed tetragonal crystal structure to an uncollapsed phase. In this work, we carry out specific heat, Raman spectroscopy and normal state electric and thermoelectric transport measurements in the collapsed and uncollapsed LaFe2As2 phases to gain insight into the electron interactions and their possible role in the superconducting pairing mechanism. Despite clear features of strong electron-phonon coupling observed in both phases, neither the low energy phonon spectra nor the electron-phonon coupling show significant differences between the two phases. Conversely, the Sommerfield constants are significantly different in the two phases, pointing to much higher electron correlation in the superconducting uncollapsed phase and confirming theoretical studies.Comment: In press on Physical Review

Enhanced coupling between massive fermions and zone-boundary phonons probed by infrared resonance Raman in bilayer graphene

Few-layer graphene possesses low-energy carriers which behave as massive fermions, exhibiting intriguing properties in both transport and light scattering experiments. By lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV we target these massive quasiparticles in the low-energy split bands close to the K point. The low excitation energy suppresses some of the Raman processes which are resonant in the visible, and induces a clearer frequency-separation of the sub-structures of the resonant 2D peak. Studying the different intensities of the sub-structures and comparing experimental measurements with fully ab initio theoretical calculations, in the case of bilayer graphene we unveil an enhanced coupling between the massive fermions and the lattice vibrations at the K point, in analogy to what found for the massless fermions of monolayer graphene, and also suggesting that what governs the enhancement is the vicinity of the electron-hole pair momentum to K rather than how small the electron-hole pair energy is.Comment: 14 pages, 10 figure

Infrared resonance Raman of Bilayer graphene. Signatures of massive Fermions and band structure on the 2D peak

Few-layer graphene possesses low-energy carriers that behave as massive Fermions, exhibiting intriguing properties in both transport and light scattering experiments. Lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV, we target these massive quasiparticles in the split bands close to the K point. The low excitation energy weakens some of the Raman processes that are resonant in the visible, and induces a clearer frequency-separation of the substructures of the resonance 2D peak in bi- and trilayer samples. We follow the excitation-energy dependence of the intensity of each substructure, and comparing experimental measurements on bilayer graphene with ab initio theoretical calculations, we trace back such modifications on the joint effects of probing the electronic dispersion close to the band splitting and enhancement of electron-phonon matrix elements

First- and second-order Raman scattering from MoTe2 single crystal

We report on Raman experiments performed on a MoTe 2 single crystal. The system belongs to the wide family of transition metal dichalcogenides which includes several of the most interesting two- dimensional materials for both basic and applied physics. Measurements were performed in the standard basal plane configuration, by placing the ab plane of the crystal perpendicular to the wave vector k i of the incident beam to explore the in-plane vibrational modes, and in the edge plane configuration with k i perpendicular to the crystal c axis, thus mainly exciting out-of-plane modes. For both configurations we performed a polarization-dependent study of the first-order Raman components and detailed computation of the corresponding selection rules. We were thus able to provide a complete assignment of the observed first-order Raman peaks, in agreement with previous literature results. A thorough analysis of the second- order Raman bands, as observed in both basal and edge plane configurations, provides new information and allows a precise assignment of these spectral structures. In particular, we have observed and assigned Raman active modes of the M point of the Brillouin zone previously predicted by ab initio calculations but never previously measured

Laser Ablation Nanoarchitectonics of Au–Cu Alloys Deposited on TiO2 Photocatalyst Films for Switchable Hydrogen Evolution from Formic Acid Dehydrogenation

The regulation of H2 evolution from formic acid dehydrogenation using recyclable photocatalyst films is an essential approach for on-demand H2 production. We have successfully generated Au–Cu nanoalloys using a laser ablation method and deposited them on TiO2 photocatalyst films (AuxCu100–x/TiO2). The Au–Cu/TiO2 films were employed as photocatalysts for H2 production from formic acid dehydrogenation under light-emitting diode (LED) irradiation (365 nm). The highest H2 evolution rate for Au20Cu80/TiO2 is archived to 62,500 μmol h–1 g–1 per photocatalyst weight. The remarkable performance of Au20Cu80/TiO2 may account for the formation of Au-rich surfaces and the effect of Au alloying that enables Cu to sustain the metallic form on its surface. The metallic Au–Cu surface on TiO2 is vital to supply the photoexcited electrons of TiO2 to its surface for H2 evolution. The rate-determining step (RDS) is identified as the reaction of a surface-active species with protons. The results establish a practical preparation of metal alloy deposited on photocatalyst films using laser ablation to develop efficient photocatalysts

Far-infrared signatures for a two-step pressure-driven metallization in transition metal dichalcogenides

We present a high-pressure investigation of the semiconductor-to-metal transition in MoS2 and WS2 carried out by synchrotron-based far-infrared spectroscopy, to reconcile the controversial estimates of the metallization pressure found in the literature and gain new insight into the mechanisms ruling this electronic transition. Two spectral descriptors are found indicative of the onset of metallicity and of the origin of the free carriers in the metallic state: the absorbance spectral weight, whose abrupt increase defines the metallization pressure threshold, and the asymmetric line shape of the E1u peak, whose pressure evolution, interpreted within the Fano model, suggests the electrons in the metallic state originate from n-type doping levels. Combining our results with those reported in the literature, we hypothesize a two-step mechanism is at work in the metallization process, in which the pressure-induced hybridization between doping and conduction band states drives an early metallic behavior, while the band gap closes at higher pressures

Experimental data for "Superradiant Thomson scattering from graphite in the extreme ultraviolet"

<p>Pretreated experimental data of an extreme ultraviolet (EUV) Thomson scattering experiment on highly oriented pyrolytic graphite (HOPG), described in the PNAS paper "Superradiant Thomson scattering from graphite in the extreme ultraviolet" by the same authors. Details on the experimental geometry and setup are available in the paper. </p> <p>The data contain information on the distribution of the Thomson scattering intensity, at varying the incident intensity. Given the observation that the ratio of the scattered to incident intensity is not constant throughout the experiment, but depends on the incident intensity, the pulse-by-pulse measured intensities are treated as follows. </p> <p>Each independent dataset (at given incident wavelength and polarization) of our experiment was treated by averaging the ratio of I_scat to the incoming I_0, R_scat (I_0) = I_scat (I_0)/I_0, over several different sample points. The average was taken in small enough intervals of I_0. Statistically meaningful data were obtained rejecting I_0 intervals with too few (<5) data within them and (rare) fluctuations larger than twice the standard deviations. These data are reported in the available files. </p> <p>The files are named after the value of the wavelength and polarization of the EUV free electron laser beam, exciting the HOPG sample.</p> <p>"lambda0-4nm" -> incident wavelength 4.08 nm<br>"lambda0-5nm" -> incident wavelength 4.74 nm<br>"pol-s" -> polarization orthogonal to the scattering plane<br>"pol-p" -> polarization parallel to the scattering plane</p> <p>The file column are as follows: <br>col. 1: incident EUV pulse intensity I_0 (central values for of the sampled intervals, see above)<br>col. 2: ratio between scattered and incident intensity R_scat = I_scat/I_0<br>col. 3: experimental error associated to col. 2, estimated as the standard deviation in the data relative to a given I_0 interval<br>col. 4: scattering angle.</p> <p>Col. 1,2,3 are in arbitrary units, as discussed in detail in the paper. </p> <p>Please note that in the case of lambda0-4nm, pol-s, two datasets are available, relative to two different graphite samples: these differ for a scaling value of I_0 (about the 20%), possibly associated to a slightly different surface quality and/or focusing conditions. </p> <p> </p&gt

Probing Enhanced Electron-Phonon Coupling in Graphene by Infrared Resonance Raman Spectroscopy

We report on resonance Raman spectroscopy measurements with excitation photon energy down to 1.16 eV on graphene, to study how low-energy carriers interact with lattice vibrations. Thanks to the excitation energy close to the Dirac point at K, we unveil a giant increase of the intensity ratio between the double-resonant 2D and 2D′ peaks with respect to that measured in graphite. Comparing with fully ab initio theoretical calculations, we conclude that the observation is explained by an enhanced, momentum-dependent coupling between electrons and Brillouin zone-boundary optical phonons. This finding applies to two-dimensional Dirac systems and has important consequences for the modeling of transport in graphene devices operating at room temperature