Band Bending and Valence Band Quantization at Line Defects in MoS₂

Abstract

The variation of the electronic structure normal to 1D defects in quasi-freestanding MoS 2 , grown by molecular beam epitaxy, is investigated through high resolution scanning tunneling spectroscopy at 5 K. Strong upwards bending of valence and conduction bands towards the line defects is found for the 4|4E mirror twin boundary and island edges, but not for the 4|4P mirror twin boundary. Quantized energy levels in the valence band are observed wherever upwards band bending takes place. Focusing on the 1 arXiv:2007.06313v1 [cond-mat.mes-hall] 13 Jul 2020 common 4|4E mirror twin boundary, density functional theory calculations give an estimate of its charging, which agrees well with electrostatic modeling. We show that the line charge can also be assessed from the filling of the boundary-localized electronic band, whereby we provide a measurement of the theoretically predicted quantized polarization charge at MoS 2 mirror twin boundaries. These calculations elucidate the origin of band bending and charging at these 1D defects in MoS 2. The 4|4E mirror twin boundary not only impairs charge transport of electrons and holes due to band bending, but holes are additionally subject to a potential barrier, which is inferred from the independence of the quantized energy landscape on either side of the boundary. Keywords band bending, scanning tunnelling spectroscopy, MoS 2 , polarization charge, mirror twin boundary Coupled to the rise of MoS 2 and other transition metal dichalcogenide (TMDC) semiconductors as prospective two-dimensional (2D) device materials came the need to investigate their one-dimensional (1D) defect structures, e.g. grain boundaries (GBs). Depending on their structure, GBs impair device performance to differing degrees when positioned in the channel of a single layer MoS 2 field effect transistor. 1-4 It is thus evident that control of the type and concentration of GBs is of importance for device fabrication. Besides satisfying scientific curiosity, it therefore pays to understand their effect on band structure and charge carrier transport. The lowest energy GBs are those hardest to avoid during growth, as the energy penalty associated with their introduction is marginal. In the three-dimensional (3D) world, these low energy GBs are 2D stacking faults or twin planes. For the case of SiC devices such defects cause increased leakage current, reduced blocking voltage, and the degradation of bipolar devices. 5,6 In the world of 2D materials, the analog to twin planes is 1D mirror twin boundaries (MTBs). These structural defects have some surprising effects on the band structure of monolayer MoS 2 , to be investigated in this manuscript.

Engineering and probing atomic quantum defects in 2D semiconductors: A perspective

“…Apart from zero dimensional point defects, some TMDs also exhibit a low formation energy of line defects in the form of mirror-twin boundaries. Such mirror-twin boundaries, detected for instance in MoSe2 and MoS2, host 1D conducting channels within the semiconductor band gap that can be described as a charge density wave 3 or Tomonaga-Luttinger liquid 4,5 , respectively. More generally, TMDs can develop various types of structural 6,7 or compositional 8 domain boundaries that may host extended (topologically protected) conductive channels.…”

Anomalous Light-Induced Charging in MoS2 Monolayers with Cracks

“…Multiple reflections of incident light occur in the air gap and SiO 2 layer on Si wafers, and hence, the suspended and nonsuspended flakes exhibit a notable optical contrast difference. 22,23 The Raman mapping image (Figure 6a, right) reveals that the in-plane E 2g 1 phonon mode peak in the suspended MoS 2 flakes is redshifted and the E 1 2g peak positions in the nonsuspended and flat regions are similar. This suggests that the suspended flakes undergo NP-induced strain.…”

“…AFM measurements showed that the top dark area had suspended MoS 2 flakes and the bottom bright area had nonsuspended flakes. Multiple reflections of incident light occur in the air gap and SiO 2 layer on Si wafers, and hence, the suspended and nonsuspended flakes exhibit a notable optical contrast difference. , The Raman mapping image (Figure a, right) reveals that the in-plane E 2g 1 phonon mode peak in the suspended MoS 2 flakes is redshifted and the E 1 2g peak positions in the nonsuspended and flat regions are similar. This suggests that the suspended flakes undergo NP-induced strain. , The biaxial strain (ε) can be calculated from the measured Raman peak shift (Δω = 0.6 cm –1 ): ε = Δω/(2γω 0 ), where γ is the Grüneisen parameter .…”

Metal-insulator transition in monolayer MoS$_2$ via contactless chemical doping

“…The bending is smooth in the CB but occurs stepwise at characteristic energies in the VB due to quantization effects 59 .…”