PCSI2027 Session PCSI-WeM1: 2D Materials: Graphene & Beyond

Twisted trilayer van der Waals crystals enable the creation of hierarchical moiré-of-moiré lattices by overlapping two moiré patterns through controlled twist angles. At small twist angles, the interlayer and intralayer interactions at two distinct interfaces in the twisted trilayer crystals give rise to an intricate network of domain structures that exhibit exotic electronic properties. In this study, we present a comprehensive structural phase diagram of twisted trilayer graphene (TTG) with atomic-scale lattice reconstruction. Using transmission electron microscopy in conjunction with advanced interatomic potential simulations, we reveal a variety of large-scale moiré lattices, including triangular, kagome, and hexagram-shaped domain patterns.[1] Each domain is enclosed within a two-dimensional network of domain wall lattices. At small twist angles, competing rhombohedral and Bernal stacking orders, separated by a small energy difference, induce unconventional lattice reconstructions characterized by spontaneous symmetry breaking and nematic instability, emphasizing the role of long-range interactions across van der Waals layers.[1] The tunable tessellation of diverse domain networks by adjusting the twist angles establishes TTG as a versatile platform for investigating the relationship between engineered nontrivial domain lattice structures and their emergent physical properties.

[1] D. Park et al., Unconventional domain tessellations in moiré-of-moiré lattices, Nature 641, 896-903 (2025).

In 2013, we controversially reported strong electron–phonon coupling (EPC) in graphene’s σ-bands [PRL 111:216806]. The result was controversial. Combing theoretical support with measurements in neighbouring Brillouin zones, we showed that the mass enhancement, λ ≈ 0.7, was intrinsic [PRB 95:075430].

We subsequently predicted similarly strong EPC in the σ-bands of hBN [PRB 101:214513], whose electronic and phonon structures resemble those of graphene. Experimental confirmation further support-ed our understanding of EPC in graphene. Unlike graphene, hBN was predicted to exhibit strong EPC in its π-band, whose parabolic maximum leads to fundamentally different scattering phase space and much stronger coupling.

Most recently, we have developed improved our mathematical tools for analysing EPC. Established methods are implicitly based on the assumption of linear electronic bands, and are unsuitable near a parabolic maximum.

We reveal that π-band EPC depends strongly on doping, substrate choice and substrate interaction strength. In this talk, I will show our latest studies and analysis, as well as the open question which motivate ongoing ARPES, DFT and helium-scattering studies.

Two-dimensional transition metal dichalcogenides (2D-TMDs) are promising candidates for next-generation, nanoscale electronic devices due to their profound optoelectronic properties (e.g., direct and tunable bandgaps) [1]. However, these materials host atomic-scale defects (e.g., vacancies, substitutions, etc.), introducing electronic heterogeneities that can either enhance or hinder device performance [2], [3]. Thus, characterizing atomic defects within 2D-TMDs is essential for nanoscale device implementation. Current-voltage (I-V) spectroscopy via conductive atomic force microscopy (C-AFM) characterizes electronic properties under ambient conditions [4]. Thermal drift makes this method unsuitable, reducing confidence in assigning electronic features to specific defects. We present an alternative approach on molybdenum disulfide (MoS2), a representative 2D-TMD, that overcomes thermal drift [5]. Consecutive current maps are collected at various bias voltages, enabling direct assessment of electronic features for defects spanning single atomic sites to several nanometers. Spectroscopy data allows sorting defects into distinct groups, leading to their chemical identification.

[1] S. Manzeli, D. Ovchinnikov, D. Pasquier, O. V. Yazyev, and A. Kis, “2D transition metal dichalcogenides,” Nat. Rev. Mater., vol. 2, no. 8, p. 17033, Jun. 2017, doi: 10.1038/natrevmats.2017.33.

[2] Y. Zhu et al., “Point defects in semiconductors: Friends and foes for quantum technologies,” MRS Bull., vol. 51, no. 5, pp. 615–629, May 2026, doi: 10.1557/s43577-026-01095-x.

[3] B. Kumral et al., “Mechanically reliable and electronically uniform monolayer MoS2 by passivation and defect healing,” Nat. Commun., vol. 16, no. 1, p. 7105, Aug. 2025, doi: 10.1038/s41467-025-62370-0.

[4] G. Adabasi et al., “Strain-Modulated Conductivity and Work Function on Thin Crystals of Mo2 C,” ACS Appl. Nano Mater., vol. 8, no. 41, pp. 19810–19817, Oct. 2025, doi: 10.1021/acsanm.5c03237.

[5] J. R. Evans, D. A. Garibay, A. N. Kuhls, and M. Z. Baykara, “Electronic Spectroscopy of Atomic Defects in Molybdenum Disulfide under Ambient Conditions,” ACS Omega, in press, 2026.

Dielectric property in two-dimensional (2D) semiconductors is set by the external environment, charge carriers, and disorders, making defects a tuning parameter to modulate their electronic response. Here, we reveal quasiparticle electronic band gap (E_g,QP) renormalization in single-layer (1L) WS₂ by independently tuning sulfur vacancy density and substrate screening. Photoelectron spectroscopy and electron energy loss spectroscopy show that a defect density () of reduces E_g,QP from 2.4 eV to 2.2 eV in weakly-screened, suspended WS₂ with the diminished exciton binding energy (E_b) by 0.2 eV. In Au-supported WS₂ with strong external screening, E_g,QP remains ~2.0 eV showing no E_g,QP renormalization upon increasing . Enhancing disorder additionally drives the reduced photoelectron core-hole relaxation in suspended WS₂ when the exciton screening length (estimated from E_b) exceeds the mean interdefect spacing of ~4 nm at . This threshold signifies the crossover from weak to strong polarizability in 2D dielectric screening.