Two‐dimensional heterostructures are generally extremely sensitive to perturbations due to the large surface–volume ratio. A widely used non‐destructive experimental technique to characterize these perturbations is the Raman spectroscopy. However, vibrational frequencies of monolayer MoS2 is proven to be non‐sensitive to perturbations such as strain and doping. In order to characterize the effect of these perturbations to the spectra we compute the frequencies and Raman intensities of monolayer MoS2 on the ab initio level. In agreement with previous experimental works, we show that the frequencies of the Raman active peaks (A1′ and E′) are rather non‐sensitive to strain or doping. On the other hand, we demonstrate that the intensity ratio depends strongly on the strain − thus it can be used as a supplementary method to characterize strain in the samples.

MoS 2 single layers are valued for their sizeable direct bandgap at the heart of the envisaged electronic and optoelectronic applications. Here we experimentally demonstrate that moderate strain values (~2%) can already trigger an indirect bandgap transition and induce a finite charge carrier density in 2D MoS 2 layers. A conclusive proof of the direct-to-indirect bandgap transition is provided by directly comparing the electronic and optical bandgaps of strained MoS 2 single layers obtained from tunneling spectroscopy and photoluminescence measurements of MoS 2 nanobubbles. Upon 2% biaxial tensile strain, the electronic gap becomes significantly smaller (1.45 ± 0.15 eV) than the optical direct gap (1.73 ± 0.1 eV), clearly evidencing a strain-induced direct to indirect bandgap transition. Moreover, the Fermi level can shift inside the conduction band already in moderately strained (~2%) MoS 2 single layers conferring them a metallic character.

Graphene nanoribbons with sub-nanometer widths are extremely interesting for nanoscale electronics and devices as they combine the unusual transport properties of graphene with the opening of a band gap due to quantum confinement in the lateral dimension. Strong research efforts are presently paid to grow such nanoribbons. Here we show the synthesis of 6-and 7-armchair graphene nanoribbons, with widths of 0.61 and 0.74 nm, and excitonic gaps of 1.83 and 2.18 eV, by high-temperature vacuum annealing of ferrocene molecules inside single-walled carbon nanotubes. The growth of the so-obtained graphene nanoribbons is evidenced from atomic resolution electron microscopy, while their welldefined structure is identified by a combination of an extensive wavelength-dependent Raman scattering characterization and quantum-chemical calculations. These findings enable a facile and scalable approach leading to the controlled growth and detailed analysis of well-defined sub-nanometer graphene nanoribbons.

Patterning graphene into various mesoscopic devices such as nanoribbons, quantum dots, etc. by lithographic techniques has enabled the guiding and manipulation of graphene’s Dirac-type charge carriers. Graphene, with well-defined strain patterns, holds promise of similarly rich physics while avoiding the problems created by the hard to control edge configuration of lithographically prepared devices. To engineer the properties of graphene via mechanical deformation, versatile new techniques are needed to pattern strain profiles in a controlled manner. Here we present a process by which strain can be created in substrate supported graphene layers. Our atomic force microscope-based technique opens up new possibilities in tailoring the properties of graphene using mechanical strain.

Quantum spin Hall (QSH) insulators host edge states, where the helical locking of spin and momentum suppresses backscattering of charge carriers, promising applications from low-power electronics to quantum computing. A major challenge for applications is the identification of large gap QSH materials, which would enable room temperature dissipationless transport in their edge states. Here we show that the layered mineral jacutingaite (Pt2HgSe3) is a candidate QSH material, realizing the long sought-after Kane–Mele insulator. Using scanning tunneling microscopy, we measure a band gap in excess of 100 meV and identify the hallmark edge states. By calculating the invariant, we confirm the topological nature of the gap. Jacutingaite is stable in air, and we demonstrate exfoliation down to at least two layers and show that it can be integrated into heterostructures with other two-dimensional materials. This adds a topological insulator to the 2D quantum material library.

The pseudo-magnetic field generated by mechanical strain in graphene can have dramatic consequences on the behavior of electrons and holes. Here we show that pseudo-magnetic field fluctuations present in crumpled graphene can induce significant intravalley scattering of charge carriers. We detect this by measuring the confocal Raman spectra of crumpled areas, where we observe an increase of the D'/D peak intensity ratio by up to a factor of 300. We reproduce our observations by numerical calculation of the double resonant Raman spectra and interpret the results as experimental evidence of the phase shift suffered by Dirac charge carriers in the presence of a pseudo-magnetic field. This lifts the restriction on complete intravalley backscattering of Dirac fermions. arXiv:1801.08861v3 [cond-mat.mes-hall]

We model Raman processes in silicene and germanene involving scattering of quasiparticles by, either, two phonons, or, one phonon and one point defect. We compute the resonance Raman intensities and lifetimes for laser excitations between 1 and 3 eV using a newly developed third-nearest neighbour tight-binding model parametrized from first principles density functional theory. We identify features in the Raman spectra that are unique to the studied materials or the defects therein. We find that in silicene, a new Raman resonance arises from the 2.77 eV π − σ plasmon at the M point, measurably higher than the Raman resonance originating from the 2.12 eV π plasmon energy. We show that in germanene, the lifetimes of charge carriers, and thereby the linewidths of the Raman peaks, are influenced by spin-orbit splittings within the electronic structure. We use our model to predict scattering cross sections for defect induced Raman scattering involving adatoms, substitutional impurities, Stone-Wales pairs, and vacancies, and argue that the presence of each of these defects in silicene and germanene can be qualitatively matched to specific features in the Raman response.

The most relevant two‐phonon band in the Raman spectrum of graphene is the 2D band. In addition, there are numerous other combination bands with much smaller intensities in the observed spectra (e.g. 2D′ or normalD+D′′). Their origin can be understood within the double resonance theory. However, calculations on single‐layer graphene found in the literature so far took into account only the Stokes processes for both electron–phonon interaction steps. This leads to the creation of two phonons, thus it gives the positive combination of the phonon frequencies in the Raman shift. Here we calculate the Raman spectra considering the Stokes–anti‐Stokes combinations as well: one phonon is created and another is annihilated. As a consequence features at the differences of the phonon frequencies show up. Tight binding calculations are carried out including the electron–photon and electron–phonon matrix elements explicitly. Phonon normal‐modes and frequencies are calculated at the level of density functional theory.