Signature of lattice dynamics in twisted 2D homo/hetero-bilayers

Twisted 2D bilayer materials are created by artificial stacking of two monolayer crystal networks of 2D materials with a desired twisting angle $\theta$. The material forms a moir\'e superlattice due to the periodicity of both top and bottom layer crystal structure. The optical properties are modified by lattice reconstruction and phonon renormalization, which makes optical spectroscopy an ideal characterization tool to study novel physics phenomena. Here, we report a Raman investigation on a full period of the twisted bilayer (tB) WSe$_2$ moir\'e superlattice (\textit i.e. 0{\deg} $\leq \theta \leq$ 60{\deg}). We observe that the intensity ratio of two Raman peaks, $B_{2g}$ and $E_{2g}/A_{1g}$ correlates with the evolution of moir\'e period. The Raman intensity ratio as a function of twisting angle follows an exponential profile matching the moir\'e period with two local maxima at 0{\deg} and 60{\deg} and a minimum at 30{\deg}. Using a series of temperature-dependent Raman and photoluminescence (PL) measurements as well as \textit{ab initio} calculations, the intensity ratio $I_{B_{2g}}/I_{{E_{2g}}/{A_{1g}}}$ is explained as a signature of lattice dynamics in tB WSe$_2$ moir\'e superlattices. By further exploring different material combinations of twisted hetero-bilayers, the results are extended for all kinds of Mo- and W-based TMDCs.Comment: 22 pages, 12 fugure

Hydrogen-induced sp2-sp3 rehybridization in epitaxial silicene

We report on the hydrogenation of (3×3)/(4×4) silicene epitaxially grown on Ag(111) studied by in situ Raman spectroscopy and state-of-the-art ab initio calculations. Our results demonstrate that hydrogenation of (3×3)/(4×4) silicene leads to the formation of two different atomic structures which exhibit distinct spectral vibrational modes. Raman selection rules clearly show that the Si atoms undergo a rehybridization in both cases from a mixed sp2-sp3 to a dominating sp3 state increasing the distance between the two silicene sublattices. This results in a softening of the in-plane and a stiffening of the out-of-plane phonon modes. Nevertheless, hydrogenated epitaxial silicene retains a two-dimensional nature and hence can be considered as epitaxial silicane. The level of hydrogenation can be determined by the intensity ratio of the Raman modes with different symmetries.Deutsche Forschungsgemeinschaft (VO1261/4-1); ESF Nachwuchsforschergruppe "E-PISA"; TUBITAK (116C073); The Science Academy, Turkey, under the BAGEP progra

Growth of Nanocrystalline MoSe2 Monolayers on Epitaxial Graphene from Amorphous Precursors

A new approach to the growth of MoSe2 thin films on epitaxial graphene on SiC(0001) by the use of modulated elemental reactants (MER) precursors has been reported. The synthesis applies a two-step process, where first an amorphous precursor is deposited on the substrate which self-assembles upon annealing. Films with a nominal thickness of about 1ML are successfully grown on epitaxial graphene monolayer as well as buffer layer samples. Characterization of the films is performed using XPS, LEED, AFM, and Raman spectroscopy. The films are nanocrystalline and show randomly rotated domains. This approach opens up an avenue to synthesize a number of new van-der-Waals systems on epitaxial graphene and other substrates

Charge transfer from and to manganese phthalocyanine: bulk materials and interfaces

Manganese phthalocyanine (MnPc) is a member of the family of transition-metal phthalocyanines, which combines interesting electronic behavior in the fields of organic and molecular electronics with local magnetic moments. MnPc is characterized by hybrid states between the Mn 3d orbitals and the π orbitals of the ligand very close to the Fermi level. This causes particular physical properties, different from those of the other phthalocyanines, such as a rather small ionization potential, a small band gap and a large electron affinity. These can be exploited to prepare particular compounds and interfaces with appropriate partners, which are characterized by a charge transfer from or to MnPc. We summarize recent spectroscopic and theoretical results that have been achieved in this regard

Charge transfer from and to manganese phthalocyanine: bulk materials and interfaces

Manganese phthalocyanine (MnPc) is a member of the family of transition-metal phthalocyanines, which combines interesting electronic behavior in the fields of organic and molecular electronics with local magnetic moments. MnPc is characterized by hybrid states between the Mn 3d orbitals and the π orbitals of the ligand very close to the Fermi level. This causes particular physical properties, different from those of the other phthalocyanines, such as a rather small ionization potential, a small band gap and a large electron affinity. These can be exploited to prepare particular compounds and interfaces with appropriate partners, which are characterized by a charge transfer from or to MnPc. We summarize recent spectroscopic and theoretical results that have been achieved in this regard

Surface- and tip-enhanced Raman spectroscopy reveals spin-waves in iron oxide nanoparticles

Nanomaterials have the remarkable characteristic of displaying physical properties different from their bulk counterparts. An additional degree of complexity and functionality arises when oxide nanoparticles interact with metallic nanostructures. In this context the Raman spectra due to plasmonic enhancement of iron oxide nanocrystals are here reported showing the activation of spin-waves. Iron oxide nanoparticles on gold and silver tips are found to display a band around 1584 cm−1 attributed to a spin-wave magnon mode. This magnon mode is not observed for nanoparticles deposited on silicon (111) or on glass substrates. Metal–nanoparticle interaction and the strongly localized electromagnetic field contribute to the appearance of this mode. The localized excitation that generates this mode is confirmed by tip-enhanced Raman spectroscopy (TERS). The appearance of the spin-waves only when the TERS tip is in close proximity to a nanocrystal edge suggests that the coupling of a localized plasmon with spin-waves arises due to broken symmetry at the nanoparticle border and the additional electric field confinement. Beyond phonon confinement effects previously reported in similar systems, this work offers significant insights on the plasmon-assisted generation and detection of spin-waves optically induced

Exciton tuning in monolayer WSe2_2 via substrate induced electron doping

We report on large exciton tuning in WSe$_2$ monolayers via substrate induced non-degenerate doping. We observe a redshift of $\sim$62 meV for the $A$ exciton together with a 1-2 orders of magnitude photoluminescence (PL) quenching when the monolayer WSe$_2$ is brought in contact with highly oriented pyrolytic graphite (HOPG) compared to the dielectric substrates such as hBN and SiO$_2$. As the evidence of doping from HOPG to WSe$_2$, a drastic increase of the trion emission intensity was observed. Using a systematic PL and Kelvin probe force microscopy (KPFM) investigation on WSe$_2$/HOPG, WSe$_2$/hBN, and WSe$_2$/graphene, we conclude that this unique excitonic behavior is induced by electron doping from the substrate. Our results propose a simple yet efficient way for exciton tuning in monolayer WSe$_2$, which plays a central role in the fundamental understanding and further device development.Comment: 14 pages, 10 figure