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Register a new userMultifaceted moire superlattice physics in twisted WSe$_2$ bilayers
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Added by
David Ruiz-Tijerina
Publication date
2021
fields
Physics
and research's language is
English
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Lattice reconstruction in twisted transition-metal dichalcogenide (TMD) bilayers gives rise to piezo- and ferroelectric moire potentials for electrons and holes, as well as a modulation of the hybridisation across the bilayer. Here, we develop hybrid $mathbf{k}cdot mathbf{p}$ tight-binding models to describe electrons and holes in the relevant valleys of twisted TMD homobilayers with parallel (P) and anti-parallel (AP) orientations of the monolayer unit cells. We apply these models to describe moire superlattice effects in twisted WSe${}_2$ bilayers, in conjunction with microscopic emph{ab initio} calculations, and considering the influence of encapsulation, pressure and an electric displacement field. Our analysis takes into account mesoscale lattice relaxation, interlayer hybridisation, piezopotentials, and a weak ferroelectric charge transfer between the layers, and describes a multitude of possibilities offered by this system, depending on the choices of P or AP orientation, twist angle magnitude, and electron/hole valley.
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The large surface-to-volume ratio in atomically thin 2D materials allows to efficiently tune their properties through modifications of their environment. Artificial stacking of two monolayers into a bilayer leads to an overlap of layer-localized wave functions giving rise to a twist angle-dependent hybridization of excitonic states. In this joint theory-experiment study, we demonstrate the impact of interlayer hybridization on bright and momentum-dark excitons in twisted WSe$_2$ bilayers. In particular, we show that the strong hybridization of electrons at the $Lambda$ point leads to a drastic redshift of the momentum-dark K-$Lambda$ exciton, accompanied by the emergence of flat moire exciton bands at small twist angles. We directly compare theoretically predicted and experimentally measured optical spectra allowing us to identify photoluminescence signals stemming from phonon-assisted recombination of layer-hybridized dark excitons. Moreover, we predict the emergence of additional spectral features resulting from the moire potential of the twisted bilayer lattice.
Twistronic van der Waals heterostrutures offer exciting opportunities for engineering optoelectronic properties of nanomaterials. Here, we use multiscale modeling to study trapping of charge carriers and excitons by ferroelectric polarisation and piezoelectric charges by domain structures in twistronic WX$_2$/MoX$_2$ bilayers (X=S,Se). For almost aligned 2H-type bilayers, we find that holes and electrons are trapped in the opposite -- WMo and XX (tungsten over molybdenum {it versus} overlaying chalcogens) -- corners of the honeycomb domain wall network, swapping their position at a twist angle $0.2^{circ}$, with XX corners providing $30$,meV deep traps for the interlayer excitons for all angles. In 3R-type bilayers, both electrons and holes are trapped in triangular 3R stacking domains, where WX$_2$ chalcogens set over MoX$_2$ molybdenums, which act as $130$,meV deep quantum boxes for interlayer excitons for twist angles $lesssim 1^{circ}$, for larger angles shifting towards domain wall network XX stacking sites.
Moire structures in van der Waals heterostructures lead to emergent phenomena including superconductivity in twisted bilayer graphene and optically accessible strongly-correlated electron states in transition metal dichalcogenide heterobilayers. Dual periodicity moire structures (DPMS) formed in layered structures with more than two layers have been shown to lead to ferromagnetism and multiple secondary Dirac points in TBG. Whilst in principle it is possible to obtain DPMS in bilayers there has not been clear experimental evidence of this yet. In this paper we present signatures of DPMS in a twisted MoSe$_2$/WSe$_2$ bilayer revealed by resonance Raman spectroscopy. We observed zone-folded acoustic and optical phonon modes with a wavevector twice of the moire wavevector, evidence of a dual periodicity moire heterostructure. These results simultaneously open up opportunities for new emergent phenomena and an optical method for characterising DPMS in a wide range of van der Waals heterostructures.
Fabricating van der Waals (vdW) bilayer heterostructures (BL-HS) by stacking the same or different two-dimensional (2D) layers, offers a unique physical system with rich electronic and optical properties. Twist-angle between component layers has emerged as a remarkable parameter that can control the period of lateral confinement, and nature of the exciton (Coulomb bound electron-hole pair) in reciprocal space thus creating exotic physical states including moire excitons. In this review article, we focus on opto-electronic properties of excitons in transition metal dichalcogenide (TMD) semiconductor twisted BL-HS. We look at existing evidence of moire excitons in localized and strongly correlated states, and at nanoscale mapping of moire superlattice and lattice-reconstruction. This review will be helpful in guiding the community as well as motivating work in areas such as near-field optical measurements and controlling the creation of novel physical states.
In twisted bilayers of semiconducting transition metal dichalcogenides (TMDs), a combination of structural rippling and electronic coupling gives rise to periodic moire potentials that can confine charged and neutral excitations. Here, we report experimental measurements of the structure and spectroscopic properties of twisted bilayers of WSe2 and MoSe2 in the H-stacking configuration using scanning tunneling microscopy (STM). Our experiments reveal that the moire potential in these bilayers at small angles is unexpectedly large, reaching values of above 300 meV for the valence band and 150 meV for the conduction band - an order of magnitude larger than theoretical estimates based on interlayer coupling alone. We further demonstrate that the moire potential is a non-monotonic function of moire wavelength, reaching a maximum at around a 13nm moire period. This non-monotonicity coincides with a drastic change in the structure of the moire pattern from a continuous variation of stacking order at small moire wavelengths to a one-dimensional soliton dominated structure at large moire wavelengths. We show that the in-plane structure of the moire pattern is captured well by a continuous mechanical relaxation model, and find that the moire structure and internal strain rather than the interlayer coupling is the dominant factor in determining the moire potential. Our results demonstrate the potential of using precision moire structures to create deeply trapped carriers or excitations for quantum electronics and optoelectronics.
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