Low-Frequency Raman Modes and Electronic Excitations In Atomically Thin MoS2 Crystals

Atomically thin MoS$_{2}$ crystals have been recognized as a quasi-2D semiconductor with remarkable physics properties. This letter reports our Raman scattering measurements on multilayer and monolayer MoS$_{2}$, especially in the low-frequency range ($<$50 cm$^{-1}$). We find two low-frequency Raman modes with contrasting thickness dependence. With increasing the number of MoS$_{2}$ layers, one shows a significant increase in frequency while the other decreases following a 1/N (N denotes layer-number) trend. With the aid of first-principle calculations we assign the former as the shear mode $E_{2g}^{2}$ and the latter as the compression vibrational mode. The opposite evolution of the two modes with thickness demonstrates novel vibrational modes in atomically thin crystal as well as a new and more precise way to characterize thickness of atomically thin MoS$_{2}$ films. In addition, we observe a broad feature around 38 cm$^{-1}$ (~5 meV) which is visible only under near-resonance excitation and pinned at the fixed energy independent of thickness. We interpret the feature as an electronic Raman scattering associated with the spin-orbit coupling induced splitting in conduction band at K points in their Brillouin zone.Comment: 5 pages, 4 figure

4-[(5-Chloro-3-methyl-1-phenyl-1H-pyrazol-4-yl)methyl­idene­amino]-1,5-dimethyl-2-phenyl-1H-pyrazol-3(2H)-one

In the mol­ecule of the title compound, C22H20ClN5O, the atoms of the two pyrazole rings and the –C=N– group which joins them are essentially coplanar, with an r.m.s. deviation of 0.054 (2) Å. The phenyl rings form dihedral angles of 41.24 (5) and 55.53 (5)° with this plane. The crystal structure is stabilized by weak inter­molecular π–π inter­actions, with centroid-to-centroid distances of 3.6179 (13) Å between the imidazole rings

Bis[4-(1-imino­eth­yl)-3-methyl-1-phenyl-1H-pyrazol-5-olato-κ2 O,N 4]copper(II)

In the title complex, [Cu(C12H12N3O)2], the CuII ion is tetra­coordinated by two N atoms and two O atoms from two bis-chelating 4-(1-imino­eth­yl)-3-methyl-1-phenyl-1H-pyrazol-5-olate ligands in a square-planar geometry. The two N atoms and two O atoms around the CuII atom are trans to each other, as the CuII atom lies on an inversion centre. The six-membered ring composed of the Cu, an O, an N and three C atoms of the ligand and the pyrazole ring is nearly planar, the largest deviation being 0.037 (4) Å for an N atom. In the crystal, weak inter­molecular C—H⋯N hydrogen-bonding inter­actions link the mol­ecules into chains along the c axis

Bis(4-acetyl-3-methyl-1-phenyl-1H-pyrazol-5-olato-κ2 O,O′)bis­(N,N-dimethyl­formamide-κO)nickel(II)

The title complex, [Ni(C12H11N2O2)2(C3H7NO)2], lies on on an inversion center. The NiII ion is coordinated in a slightly distorted octa­hedral coordination enviroment by four O atoms from two bis-chelating 4-acety-3-methyl-1-phenyl-1H-pyrazol-5-olate ligands in the equatorial plane and two O atoms from two N,N-dimethyl­formamide ligands in the axial sites. In the crystal structure, weak inter­molecular π–π stacking inter­actions with centroid–centroid distances of 3.7467 (13) Å link mol­ecules into chains extending alongthe b axis

Anomalously Robust Valley Polarization and Valley Coherence in Bilayer WS2

Coherence is a crucial requirement to realize quantum manipulation through light-matter interactions. Here we report the observation of anomalously robust valley polarization and valley coherence in bilayer WS2. The polarization of the photoluminescence from bilayer WS2 inherits that of the excitation source with both circularly and linearly polarized and retains even at room temperature. The near unity circular polarization of the luminescence reveals the coupling of spin, layer and valley degree of freedom in bilayer system, while the linear polarized photoluminescence manifests quantum coherence between the two inequivalent band extrema in momentum space, namely, the valley quantum coherence in atomically thin bilayer WS2. This observation opens new perspectives for quantum manipulation in atomically thin semiconductors

Methyl 2-{[(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl­idene)(thio­phen-2-yl)meth­yl]amino}-3-phenyl­propionate

In the title compound, C25H23N3O3S, an intra­molecular N—H⋯O inter­action generates an S(6) ring, which stabilizes the enamine–keto form of the compound. This S(6) ring and the pyrazole ring are essentially coplanar, making a dihedral angle of 1.49 (6)°. The bond lengths within the S(6) ring of the mol­ecule lie between classical single- and double-bond lengths, indicating extensive conjugation. The structure exhibits a thienyl-ring flip disorder, with occupancy factors in the ratio 64.7 (3):35.3 (3)

1,5-Dimethyl-4-{[(3-methyl-5-oxo-1-phenyl-4,5-dihydro-1H-pyrazol-4-yl­idene)(thio­phen-2-yl)meth­yl]amino}-2-phenyl-1H-pyrazol-3(2H)-one

In the title compound, C26H23N5O2S, an intra­molecular N—H⋯O inter­action generates an S(6) ring. The essentially planar S(6) and pyrazole rings [maximum deviations = −0.0270 (14) and 0.0195 (15) Å, respectively] are nearly coplanar, making a dihedral angle of 3.94 (6)°. The S(6) ring makes dihedral angles of 23.79 (6), 78.53 (6) and 67.91 (6)° with the pyrazolone ring, the pyrazole ring and the benzene ring of anti­pyrine, respectively. The structure exhibits a thienyl-ring flip disorder with occupancy factors in the ratio 0.82:0.18