Session T39: 2D Moiré Materials: Optical Properties

Towards strain-tunning correlations in van der Waals heterosrtuctures

Two-dimensional (2D) materials and heterostructures offer a rich environment to study and probe quantum phenomena with great tunability. Controllable in-situ strain in 2D materials offers a novel pathway to control those correlated effects. Here, we present a strain study on twisted homobilayers of WSe2 on hexagonal Boron nitride (BN). The twisted WSe2 structure exhibits a moire lattice that alters the electronic band structure and phonon modes. Placing the twisted homobilayers of WSe2/BN structure on flexible polyethylene terephthalate (PET) substrates enables the application of tunable tensile strain along the axis of substrate bending and orthogonal compressive strain. We then perform ultralow frequency Raman spectroscopy, photoluminescence spectroscopy, and lifetime measurements to characterize any changes with strain. We observe changes in the low-frequency phonon modes which we attribute to changes in the moire wavelength. Furthermore, we extract the strain applied to the homobilayer WSe2 by a Gruneisen parameter analysis on phonon mode shift in the E2g and A1g modes. We measure an applied strain of roughly 1% in our samples. We observe a small decrease in the exciton lifetime, but no significant shift in exciton energy at room temperature. Knowing the exact amount of strain applied to a moire lattice is pivotal to characterizing the strain-tunability in these structures. Our results could pave the way toward strain-tunable electron interactions in strongly correlated systems and the study of strain-driven phase transitions in van der Waals heterostructures.   

GW-BSE Study on the Interlayer Moiré Excitonic States in Twisted Bilayer WSe2

Recent experiments on twisted bilayer WSe₂ have shown signatures of interlayer moiré excitons in a wide range of twist angles. The emission spectra of the excitonic states manifest strong valley polarization which is absent in naturally occurring 2H stacked homobilayer counterparts. However, the underlying mechanism of the excitonic states and their exotic polarization properties is still elusive. In this work, we investigate the interlayer moiré excitonic states in the twisted bilayer WSe₂ using first-principles GW-BSE calculations. We first perform density functional theory (DFT) calculations and study how moiré potential and interlayer hybridization affect the spin-valley properties of the system. To investigate the quasiparticle and optical properties, we next perform GW and GW-BSE calculations with the recently developed pristine unit-cell matrix projection (PUMP) [1] method and explore the role of spin and valley degrees of freedom in the polarization properties of interlayer moiré excitons.   

Interacting interlayer-moiré-excitons in twisted WS2/WSe2 heterobilayer

Stacking transition metal dichalcogenide (TMDC) monolayers with an angle of twist [1,2] create artificial superlattices. In such twisted heterobilayers, the emergence of the moiré pattern forms spatially periodic deep potential traps, giving rise to moiré sub-bands. Due to type II band alignment in these heterobilayers, electrons and holes reside in different layers, leading to interlayer moiré-trapped bound excitonic states.

In this work, we create closely spaced moiré traps using a twisted heterobilayer of WS₂/WSe₂ with a twist angle of θ≈59°. We observe three equally spaced emission peaks arising from the moiré sub-bands. The lowest (highest) energy moiré exciton exhibits localized (delocalized) character, as identified by their sublinear (linear) power law and slow (fast) decay time. We probe the interaction among these moiré sub-band states using steady-state and time-resolved photoluminescence measurements with gate voltage and optical power as varying parameters. Our main observations are: (1) With increasing positive V_g (n-doping), the lifetime of the moiré-trapped exciton comes down from 100 ns to 10 ns, with a suppressed steady-state PL intensity – indicating an enhanced nonradiative process due to spatial flattening of the conduction band. (2) With an increase in V_g, the moiré trapped excitons exhibit an unconventional Stark shift. (3) With an increase in the incident optical power, the moiré localized exciton energy peak shows a spectral blue shift up to 20 meV, owing to coulomb interaction from neighboring moiré-trapped excitons.

References

1. Kha Tran et al., Nature, 567, 71-75, 2019.

2. Fengcheng Wu et al., Phys. Rev. B 97, 035306, 2018.   

Identification of moiré phonons in WSe2/WS2 van der Waals heterostructures

In Raman measurements on WSe₂/WS₂ van der Waals bilayers, two new satellite peaks are observed on either side of the well-known WSe₂ A_1g/E_2g peak.  We make three observations about these satellite peaks: 1) They are observed in samples with a small twist angle deviation from aligned 0^o (3R) or 60^o (2H) stacking and are not observed in samples with a large twist angle. 2) The frequencies of the satellite peaks are identical for 60^o+θ and 0^o+θ samples, where θ is the same in each sample. 3) The frequency of the satellite peaks shifts with small variations in twist angle. Using density functional theory calculations and geometric arguments, we show that all three of these observations can be explained if we identify these new satellite peaks as zone-folded optical phonon modes from the WSe₂ layer.    

Phason-assisted interlayer exciton diffusion in a WSe2/WS2 moiré superlattice

The moiré potential arising from the relative twist and lattice mismatch in heterobilayers of two-dimensional materials has led to the discovery of novel excitonic species. Twist-angle dependent transport measurements of these interlayer excitons have been reported for WSe2/WS2 heterobilayers [1]. In our experiment, we use time- and spatially resolved spectroscopies to investigate the temperature dependent lifetime and diffusion length of various interlayer excitons in WSe2/WS2 heterobilayers, where the individual monolayers have been aligned to produce long-range moiré superlattices. We observe that the temperature dependent formation of the interlayer exciton is reflected in the overall lifetime and diffusion lengths of exciton species in the WSe2/WS2 heterobilayer. The excitons diffusion is affected by the presence of the moiré trap and increasing the temperature, they acquire enough energy to escape the potential barrier. However, at low temperature the moiré phonons (phasons) allow for non-zero diffusion of the exciton providing enough energy for the exciton to hop from one site to another.

    

Nanoscale patterns at molecule/MoS2 heterostructure and their effects on the interlayer exciton dynamics.

The dynamics of interlayer excitons (IX) in transition metal dichalcogenides (TMD) heterostructures has shown to be influenced by the moiré potential formed at the interface. Similar periodic potentials can also be formed at van der Waals organic/TMD interfaces by controlling the nanoscale patterns formed by the molecule on the TMD layer. We investigated the IX dynamics in heterostructures formed by PTCDI and PTCDA with monolayer (ML) MoS₂. The two molecules have very similar electronic structures but form different lattice structures on MoS₂. Using photoemission spectroscopy, a large energy splitting in the molecular orbital was observed in PTCDI/MoS₂ but not in PTCDA/MoS₂. The energy level splitting can be attributed to the two different molecular patterns formed on the MoS₂ surface. Due to the energy level splitting, IX properties of PTCDI/MoS₂ is very different from that of PTCDA/MoS₂. Time-resolved photoemission spectroscopy measurements show that the electron within an IX is spatially trapped near the interface for PTCDI/MoS₂, whereas it is spatially delocalized across the molecular films for PTCDA/MoS₂. As a result of the trapping, photoluminescence intensity of an order of magnitude larger was observed from the IX in PTCDI/MoS₂, compared to PTCDA/MoS₂.   

Diverse nature of excitonic states in transition metal dichalcogenide moiré superlattices

Recent experimental measurements have demonstrated signatures of novel exciton states in the moiré superlattices of bilayer transition metal dichalcogenides. However, the microscopic nature and origin of these moiré excitons was not well understood, and previous studies relied often on empirically fit models that did not fully account for the electron and hole degrees of freedom of the exciton. We performed state-of-the-art first-principles GW-Bethe Salpeter equation calculations and discovered a rich diversity of excitonic states in large-area transition metal dichalcogenide moiré superlattices. These studies, which involve thousands of atoms in the reconstructed moiré unit-cell, are made possible by a novel computational approach we developed, the pristine unit-cell matrix projection (PUMP) method [1]. In rotationally aligned WSe₂/WS₂ moiré superlattice, we find some excitons of a modulated Wannier character and others of a previously unidentified intralayer charge-transfer character [1]. In 57.7° twisted bilayer WS₂, we discover layer-hybridized excitons with in-plane charge transfer character. These characteristics originate from the strong modulation of electron wavefunctions due to atomic reconstructions of the moiré superlattice. Due to the weaker binding and larger spatial extent of the in-plane charge-transfer excitons, they can be strongly modulated by external electrical field, doping charges, and substrate screening. Experimental reflection contrast [1], electron energy loss spectroscopy [2] and scanning tunneling spectroscopy confirm these predictions. 

[1] M. H. Naik, E. C. Regan, Z. Zhang, …, F. H. da Jornada, F. Wang and S. G. Louie, "Intralayer charge-transfer moiré excitons in van der Waals superlattices", Nature 609, 52–57 (2022)

[2] S. Susarla, M. H. Naik, …, F. H. da Jornada, S. G. Louie, P. Ercius and A. Raja, "Hyperspectral imaging of excitons within a moiré unit-cell with a sub-nanometer electron probe", arXiv:2207.13823 (2022)   

Electric-field-tunable type-I to type-II band transition in MoSe2/WS2 heterobilayer

We have measured the electric-field-dependent photoluminescence map of a dual-gate MoSe₂/ WS₂ heterobilayer device encapsulated by boron nitride. At zero electric field, the photoluminescence is dominated by intralayer excitons in the MoSe₂ layer, indicating type-I band alignment between MoSe₂ and WS₂ in the heterobilayer. However, when a strong electric field is applied in the out-of-plane direction, a bright photoluminescence peak appears below the MoSe₂ intralayer exciton energy. This new luminescence peak redshifts linearly with increasing electric field, indicating that it arises from interlayer excitons with type-II band alignment. Our results therefore demonstrate that the MoSe₂/ WS₂ heterobilayer transits from type-I to type-II band alignment under increasing out-of-plane electric field. Such unique electrically controllable band transition may find novel applications in (opto)electronics.

 

Bosonic Mott insulator of interlayer excitons in transition metal dichalcogenide heterostructure

A panoply of unconventional electronic states is recently observed in moiré superlattices. On the other hand, similar opportunities to engineer bosonic phases remain largely unexplored. Here we report the observation of a bosonic Mott insulator in WSe₂/WS₂ moiré superlattices composed of excitons, i.e., tightly bound electron-hole pairs. Using a novel pump probe spectroscopy, we find an exciton incompressible state at exciton filling v_ex = 1 and charge neutrality, which we assign to a bosonic Mott insulator. When further varying charge density, the bosonic Mott insulator continuously transitions into an electron Mott insulator at charge filling v_e = 1, suggesting a mixed Mott insulating state in between. Our observations are well captured by a mixed Hubbard model involving both fermionic and bosonic components, from which we extract the on-site Coulomb repulsion to be 15meV and 35meV for exciton-exciton and electron-exciton interactions, respectively. Our studies establish semiconducting moiré superlattices as intriguing platforms for engineering novel bosonic phases.   

Observation of quadrupolar and dipolar excitons in a semiconductor heterotrilayer

We report the observation of quadrupolar and dipolar excitons in a semiconductor heterotrilayer consisting of stacked, angle-aligned WSe₂/WS₂/WSe₂ monolayers. Characteristics of the quadrupolar excitons are manifested in tunable dipole moments under an external electric field, a decrease in exciton energy of 12 meV from  coherent hole tunneling between the two outer layers, and suppressed exciton-exciton interactions. At high exciton density, we also see signatures of a phase of staggered dipolar excitons, which is driven by the attractive interaction between oppositely aligned static dipole moments of dipolar interlayer excitons.