The structural, electronic, and magnetic properties of pristine, defective, and oxidized monolayer TiS 3 are investigated using first-principles calculations in the framework of density functional theory. We found that a single layer of TiS 3 is a direct band gap semiconductor, and the bonding nature of the crystal is fundamentally different from other transition metal chalcogenides. The negatively charged surfaces of single layer TiS 3 makes this crystal a promising material for lubrication applications. The formation energies of possible vacancies, i.e. S, Ti, TiS, and double S, are investigated via total energy optimization calculations. We found that the formation of a single S vacancy was the most likely one among the considered vacancy types. While a single S vacancy results in a nonmagnetic, semiconducting character with an enhanced band gap, other vacancy types induce metallic behavior with spin polarization of 0.3−0.8 μ B . The reactivity of pristine and defective TiS 3 crystals against oxidation was investigated using conjugate gradient calculations where we considered the interaction with atomic O, O 2 , and O 3 . While O 2 has the lowest binding energy with 0.05−0.07 eV, O 3 forms strong bonds stable even at moderate temperatures. The strong interaction (3.9−4.0 eV) between atomic O and TiS 3 results in dissociative adsorption of some O-containing molecules. In addition, the presence of S-vacancies enhances the reactivity of the surface with atomic O, whereas it had a negative effect on the reactivity with O 2 and O 3 molecules.

The layer dependent structural, electronic and vibrational properties of the 1T phase of two dimensional (2D) platinum diselenide are investigated by means of state-of-the-art firstprinciples calculations. The main findings of the study are: (i) monolayer platinum diselenide has a dynamically stable 2D octahedral structure with 1.66 eV indirect band gap, (ii) the semiconducting nature of 1T-PtSe 2 monolayers remains unaffected even at high biaxial strains, (iii) top-to-top (AA) arrangement is found to be energetically the most favorable stacking of 1T-PtSe 2 layers, (iv) the lattice constant (layer-layer distance) increases (decreases) with increasing number of layers, (v) while monolayer and bilayer 1T-PtSe 2 are indirect semiconductors, bulk and few-layered 1T-PtSe 2 are metals, (vi) Raman intensity and peak positions of the A 1g and E g modes are found to be highly dependent on the layer thickness of the material, hence; the number of layers of the material can be determined via Raman measurements.

By employing density functional theory-based methods, the structural, vibrational, electronic, and magnetic properties of monolayer α-RuCl3 were investigated.

Ca(OH) 2 crystals, well known as portlandite, are grown in layered form, and we found that they can be exfoliated on different substrates. We performed first principles calculations to investigate the structural, electronic, vibrational, and mechanical properties of bulk, bilayer, and monolayer structures of this material. Different from other lamellar structures such as graphite and transition-metal dichalcogenides, intralayer bonding in Ca(OH) 2 is mainly ionic, while the interlayer interaction remains a weak dispersion-type force. Unlike well-known transition-metal dichalcogenides that exhibit an indirect-to-direct band gap crossover when going from bulk to a single layer, Ca(OH) 2 is a direct band gap semiconductor independent of the number layers. The in-plane Young's modulus and the in-plane shear modulus of monolayer Ca(OH) 2 are predicted to be quite low while the in-plane Poisson ratio is larger in comparison to those in the monolayer of ionic crystal BN. We measured the Raman spectrum of bulk Ca(OH) 2 and identified the high-frequency OH stretching mode A 1g at 3620 cm −1 . In this study, bilayer and monolayer portlandite [Ca(OH) 2 ] are predicted to be stable and their characteristics are analyzed in detail. Our results can guide further research on ultrathin hydroxites.

Using first-principles density functional theory calculations, we systematically investigate the structural, electronic and vibrational properties of bulk and potential single-layer structures of perovskite-like CsPb 2 Br 5 crystal. It is found that while Cs atoms have no effect on the electronic structure, their presence is essential for the formation of stable CsPb 2 Br 5 crystals. Calculated vibrational spectra of the crystal reveal that not only the bulk form but also the single-layer forms of CsPb 2 Br 5 are dynamically stable. Predicted single-layer forms can exhibit either semiconducting or metallic character. Moreover, modification of the structural, electronic and magnetic properties of single-layer CsPb 2 Br 5 upon formation of vacancy defects is investigated. It is found that the formation of Br vacancy (i) has the lowest formation energy, (ii) significantly changes the electronic structure, and (iii) leads to ferromagnetic ground state in the single-layer CsPb 2 Br 5 . However, the formation of Pb and Cs vacancies leads to p-type doping of the single-layer structure. Results reported herein reveal that single-layer CsPb 2 Br 5 crystal is a novel stable perovskite with enhanced functionality and a promising candidate for nanodevice applications.

Electronic states and their dynamics are of critical importance for electronic and optoelectronic applications. Here, we probe various relevant electronic states in monolayer MoS2, such as multiple excitonic Rydberg states and free-particle energy bands, with a high relative contrast of up to ≳200 via broadband (from ~1.79 to 3.10 eV) static third-harmonic spectroscopy, which is further supported by theoretical calculations. Moreover, we introduce transient third-harmonic spectroscopy to demonstrate that third-harmonic generation can be alloptically modulated with a modulation depth exceeding ~94% at ~2.18 eV, providing direct evidence of dominant carrier relaxation processes, associated with carrier-exciton and carrierphonon interactions. Our results indicate that static and transient third-harmonic spectroscopies are not only promising techniques for the characterization of monolayer semiconductors and their heterostructures, but also a potential platform for disruptive photonic and optoelectronic applications, including all-optical modulation and imaging.

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