Origin of the counterintuitive dynamic charge in the transition metal dichalcogenides

Abstract

We investigate the chemical bonding characteristics of the transition metal dichalcogenides based on their static and dynamical atomic charges within Density Functional Theory. The dynamical charges of the trigonal transition metal dichalcogenides are anomalously large, while in their hexagonal counterparts, their sign is even counterintuitive i.e. the transition metal takes the negative charge. This phenomenon cannot be understood simply in terms of a change in the static atomic charge as it results from a local change of polarization. We present our theoretical understanding of these phenomena based on the perturbative response of the system to a static electric field and by investigating the hybridization of the molecular orbitals near the Fermi level. Furthermore, we establish a link between the sign of the Born effective charge and the π-backbonding in organic chemistry and propose an experimental procedure to verify the calculated sign of the dynamical charge in the transition metal dichalcogenides. The expanding interest in the Transition Metal Dichalcogenides (TMDs) stems from their wide variety of applications, ranging from batteries to electronic devices [1-3]. Indeed, by adjusting either their chemical composition or the number of layers, one can tune their electronic, vibrational, and magnetic properties in a remarkable manner that cannot be imitated in other twodimensional and layered materials. In particular, the TMDs offer high carrier mobilities [4-6] and a straindependent indirect to direct band gap transition [7], that are crucial for future electronic and electro-optic applications [8,9]. Still, while the electronic properties of these materials are now relatively well-known [10], the character of their chemical bonds is, interestingly, quite diverse, and, to the best of our knowledge, not yet fully understood. For example, while ZrS 2 is reported as extremely ionic [11,12], MoS 2 and WS 2 are reported to possess both ionic and covalent characteristics [13,14]. Additionally, TiS 2 was recently reported as metallic and semiconducting, both experimentally and theoretically [15][16][17].In this Letter, we aim to provide a deeper understanding of the bonding characteristics and charge transfer in the TMDs thanks to Density Functional Theory (DFT) [18]. One common way to estimate the charge distribution within this theory is to partition the electronic density of the system into constituent atoms [19,20]. While conceptually simple, this notion of "static" charge is, unfortunately, ambiguous [21] and the corresponding charges cannot be measured experimentally. Contrarily, the Born effective charge (BEC) [22], arising from the change of dipole moment due to an atomic perturbation, is a physical observable as it corresponds to the dynamical charge response to a perturbation. It governs, for example, the splitting between the transverse optical (TO) and longitudinal optical (LO) vibrational modes [22]. It has been argued [23] that all the various operational charge definitions (including static and dyn...

Negative Piezoelectric Coefficient in Ferromagnetic 1H-LaBr2 Monolayer

“…Such counterintuitive BECs—Mo (S) shows a negative (positive) dynamical charge, opposite to its static positive charge—are also reported for bulk 2H-MoS 2 . 26 Interestingly, we notice that other good 2D piezoelectric transition metal (Mo, W, and Cr) dichalcogenide (S, Se, and Te) monolayers also exhibit counterintuitive BECs (see Table S1 ), making their e 11 ion values positive; thus, both e 11 ion and e 11 elc positively contribute to the total e 11 . We also observe the previously reported trend that the larger the chalcogen is, the larger the e 11 .…”

Vibrational and dielectric properties of monolayer transition metal dichalcogenides

“…The sign of the Born effective charge and its origin are discussed extensively in Ref. [126] and are counterintuitive for the h-and tc-TMDs. The Born effective charges calculated here agree with previous DFT and DFPT calculations [81,127,128], and with their bulk counterparts as shown in Fig.…”

Spontaneous interlayer compression in commensurately stacked van der Waals heterostructures

“…We even find that the BEC of WS 2 in mixed group II heterostructures become larger outof-plane than in-plane (yellow cells). In hexagonal systems, the counterintuitive sign and abnormal magnitude of the BEC observed in bulk is still apparent even under compression of the unit cell and heterostructure stacking [69]. To characterize the polarization effect in each layer of the group II materials, we calculate the electronic contribution to the dielectric tensor ( ∞ ) for our heterostructures and compare them to the out-ofplane component of ∞ of the corresponding bulk compounds in Table I.…”