Session X57: 2D Semiconductors: Defects, Twists, and Optical Properties

First-principles studies of defect-induced electron-phonon interactions in 2D semiconductors

Point defects have played an important role in semiconductor physics to tune physical and chemical properties of the host material. A variety of point defects have been uncovered in monolayer transition-metal dichalcogenides (TMDs), showing unique fingerprints in structural, electronic, and vibronic properties. In this talk, I will summarize calculations of electron-phonon interactions in monolayer TMDs with point defects using density functional theory and density-functional perturbation theory. We find that local atomic-scale structure in the vicinity of point defects is distorted, leading to spatially-localized electronic states, which in turn possess strong electron-phonon coupling. We discuss our calculations in the context of ongoing scanning tunneling spectroscopy measurements.   

Defect hydrogenation in monolayer transition metal dichalcogenides

Structural defects in two-dimensional semiconducting transition metal dichalcogenides (TMDs) play an important role in modifying their physical properties. In this talk, we will present our optical spectroscopic study on the defect engineering of monolayer TMDs through hydrogenation. Photoluminescence spectra show the evolution of defect exciton upon hydrogenation, which behaves differently from that of free excitons. Time-resolved photoluminescence measurement further reveals the change of defect-induced exciton dynamics in monolayer TMDs after hydrogenation process. Based on the experimental observations, we propose a microscopic picture for the hydrogenation process in monolayer TMDs. Our work thus demonstrated the ability to tune the defect emission and gained the knowledge of structural defects in monolayer TMDs.   

Optical quantum emitters in monolayer MoS2 fabricated with nm-precision by He ion microscopy

Atomistic defects in semiconductors can serve as single photon emitters for applications in quantum technologies. However, in conventional semiconductors, defects are often buried in the bulk hampering integration of defect centers into photonic circuits. Recently, it was demonstrated that color centers with narrow emsission are generated in single layer MoS₂ by focused He-ion beam irradiation [1], opening a natural pathway for device integration.
Here, we investigate atomistic defects in MoS₂ deliberately induced by focused He ion bombardment. We use scanning tunneling microscopy (STM) to resolve the atomistic defect states. By imaging line patterns with atomic resolution, we can quantify the lateral resolution of the defect patterning to be better than 7 nm.

[1] Klein et al. Site-selectively generated photon emitters in monolayer MoS2 via local helium ion irradiation, Nature Communications 10, 2755 (2019).   

Crucial Role of many-body van der Waals interaction in understanding the stability of point defects in MoS2 monolayer

The present study investigates the stability of native point defects in MoS₂ monolayer by first-principles based approach under the framework of density functional theory (DFT). The property of band gap transition from indirect to direct on going from bulk to monolayer in MoS₂, establishes a huge potential in the optoelectronic industry. Defects being inevitable, their role in tuning the band gap for photovoltaics is an important approach. Initially, we have obtained the phase diagram that shows the most stable defect states with minimum Gibbs free energy of formation. We have considered charged and neutral states, of three vacancies and five antisites with the inclusion of many-body vdW interaction, which have posed a significant change in the stability pattern. The stable defect states as observed by HSE06 functional are, V_S (-2,+2), V_Mo (-2,+2), S_Mo (-2) antisite and S_2Mo (+2) antisite. They also have significant concentration at the finite temperature range of 50K-1000K, which is achieved by ab-initio atomistic thermodynamics along with the DFT input. Further, the S_Mo (-2) and S_2Mo (+2) antisites have tuned the band gap in the range 1.1 – 1.8 eV. These states have shown optical response in the visible region, hence increasing the absorption capability of MoS₂ monolayer.
  

Electrical control of neutral and charged excitons in few-layer InSe

Indium selenide(InSe), a layered metal chalcogenide semiconductor with a layer dependent band structure, high electron mobility and strong light-matter interaction, has gained considerable interests as a promising material system for optoelectronics applications. Monolayer InSe is predicted to be an indirect band gap material. However, few-layer InSe maintains direct band gap optical properties and enables electrostatic gating. Here we report the observation of the electrical field controlled neutral and charged excitons in high-quality hBN encapsulated few-layer InSe devices. Near the charge neutrality point, the photoluminescence (PL) spectra display a strong and narrow peak with 4meV linewidth (at 20K) associated with the recombination of neutral excitons. By adjusting the Fermi energy, we can tune the PL emission so that the PL spectra are dominated either by positively charged or negatively charged trions. We found relatively large trion binding energies of about 8meV, which further indicates the potential of few-layer InSe for optoelectronics.   

Trions in doped MoS2 monolayer.

Transition metal dichalcogenide monolayers are semiconductors with a direct transition at the K-point of the Brillouin zone. The band structure of these materials has unique features that makes them ideal candidates for valleytronics. Tightly bound negative trions, a quasiparticle composed of two electrons and a hole, can be optically optically created with valley and spin polarized holes. They possess a large binding energy and large oscillator strength, such that they dominate optical spectra even at room temperature. Here, we solve Bethe-Salpeter equation for the three particle wavefunction at finite momentum. Our results enable us to explain existing data on temperature and doping dependence and predict new spectroscopic features in doped MoS2.   

Visualizing Electrically Driven Photon Emission from Individual Defects in WS2 with Atomic Resolution

Point defects in two-dimensional semiconductors are exciting atomic quantum systems. Recently, we established the correlation of atomic structure, electronic and optical properties of native point defects in monolayer WS2 [1] and MoSe2 [2] using atomically resolved scanning probe microscopy techniques. We identified isoelectronic chalcogen and transition metal substitutions as the dominant defects based on their unique electronic fingerprint [1,2]. Sulfur vacancies that are absent in as-grown samples could be selectively generated by high temperature annealing in vacuum and exhibit strong spin-orbit splitting [3].
Here we demonstrate electrically driven photon emission from individual, atomically defined defects in a 2D semiconductor [4]. We show atomically resolved luminescence maps from sulfur vacancy defects and native chromium substituents. The widely tunable optical emission generated by charge carrier injection into localized defect states in a 2D material is a powerful platform for electrically driven single-photon emission.

[1] B. Schuler et al., ACS Nano 13, 10520 (2019)
[2] S. Barja et al., Nat. Commun. 10, 3382 (2019)
[3] B. Schuler et al., Phys. Rev. Lett. 123, 076801 (2019)
[4] B. Schuler et al., arxiv: 1910.04612 (2019)   

Unexpected symmetries in twisted bilayer MoSeθ

Spin-orbit interactions lead to a large spin-splitting of the valence band maximum at K in MoSe_² monolayers. However, on stacking a second layer of MoSe₂ in the same manner (2H) as found in the bulk, one finds that there is no spin splitting. This has been attributed to the presence of inversion symmetry. As exploiting the spin splitting at the K valleys allows us to increase the functionality, an obvious route to making the bilayers useful for exploring the coupled spin and valley physics is through breaking inversion symmetry. We examined this by rotating the top layer of the bilayer by an angle θ with respect to the lower layer. The choice of angles was resticted to those for which one had reasonable sized supercells and were otherwise arbitrary. Surprisingly, we found several instances where the spin splitting vanished, though there was no breaking of inversion symmetry. An unusual mechanism behind this is identified. Additionally, we found that while the spin splitting existed for theta, it vanished for 60 - θ. This unusual behavior, we find, is a consequence of the symmetry of the hexagonal Brillouin zone.   

Twist-Angle Dependence of Moiré Excitons in MoSe2/MoS2 Twisted Heterobilayers

Twisted heterobilayers (hBLs) of transition metal dichalcogenides can form a moiré superlattice of periodically varying atomic registry between the two layers. The excitonic properties in twisted hBLs can be altered drastically by the formation of moiré minibands that depends sensitively on the moiré periodicity controlled by the twist angle. To study the twist-angle dependence, conventional methods based on mechanical exfoliation and transfer of individual crystals are tedious to fabricate many hBLs with various twist angles within 5°. Here we report on the study of moiré excitons in MoSe₂/MoS₂ hBLs fabricated by stacking a large MoSe₂ flake on an ensemble of highly-oriented MoS₂ flakes grown by chemical vapor deposition. The small angle variations in the MoS₂ flakes form a large number of hBLs with various small twist angles, enabling the study of twist-angle dependence of moiré excitons. We observed systematic energy shifts and fine structures in intralayer excitons with the twist angle, signifying the formation of moiré minibands. The polarization selection rules of moiré excitons were also investigated by photoluminescence excitation spectroscopy. Understanding the twist-angle dependent properties of moiré excitons provides insights for future developments of “twistronics”.   

Twist-angle dependent interlayer exciton diffusion in MoS2-MoSe2 heterobilayers

Heterobilayers (HBLs) of transition metal dichalcogenides can form a moiré superlattice with a periodic potential landscape, which can modulate the electronic structure and confine excitons. HBLs can host interlayer excitons (IXs) with electrons and holes separated in different layers, forming a dipolar exciton gas with repulsive Coulomb interactions among the aligned vertical dipoles. The presence of moiré superlattice further imposes an additional length scale that can change the dynamics and transport properties of IXs in HBLs. Here we study the influence of moiré periodicity on the IX diffusion in MoS₂-MoSe₂ HBLs. We prepared a series of HBLs with small twist angles by stacking monolayer MoSe₂ onto MoS₂ with well-aligned orientations grown by chemical vapor deposition. Twist-angle dependent photoluminescence (PL) energy of interlayer exciton (IX) were observed, signifying the formation of moiré potential. We further investigated the IX diffusion by time-resolved PL and spatially resolved PL imaging. The interplay between moiré potentials and dipole-dipole interactions of IX leads to exciton-density and twist-angle dependent diffusion length. Our results provide insights in understanding of the localization and delocalization of interlayer excitons in the moiré superlattice.   

Exciton physics in organic-inorganic 2D perovskites

Organic-inorganic (hybrid) 2D perovskites (2DPKs) is an emerging class of layered materials that feature unique structural characteristics related to the soft and dynamic nature of their lattice structure and organic-inorganic interfaces. There is still little knowledge of the interplay between the physical and structural characteristics. Here, using optical spectroscopy and magneto-absorption, coupled with structural probes, we report the dependence of the formation, dynamics, and recombination of exciton states on the structural and compositional details of 2DPKs . Our work reveals the changes in the exciton properties due to the tuning of the 2DPKs thickness and the size of the organic molecules in the lattice [1]. Moreover, we demonstrate the existence of unique electronic states located at the edge surfaces of 2DPKs that are beneficial for optoelectronic applications [2,3]. Finally, we provide insight into the hetero-coupling between 2D perovskites and monolayer WS₂, which yield photoluminescence enhancement by more than one order of magnitude as compared to their constituent [4].

[1] J.-C. Blancon et al., Nat. Commun. 9, 2254 (2018).
[2] J.-C. Blancon et al., Science 355, 1288 (2017).
[3] H. Tsai et al., Nature 536, 312 (2016).
[4] A. Yang et al., Nano Lett. 19, 4852 (2019).   

Plasmon-Resonant Enhancement of Photocatalysis on Monolayer WSe2

We present plasmonic enhancement of photocatalysis by depositing 5 nm Au nanoislands onto tungsten diselenide (WSe₂) monolayer films.¹ Under 532 nm wavelength illumination, the bare WSe₂ film produces a relatively small photocurrent (20 nA). With the addition of Au nanoparticles, we observe enhancements of up to 7X (0.14 μA) in the measured photocurrent. Despite these relatively small photocurrents, it is remarkable that adequate charge separating fields are generated over just 7.3 Å of material. Here, the improvement in the photocatalytic performance is caused by the local electric field enhancement produced in the monolayer WSe₂ monolayer by the plasmonic Au nanoislands, as verified by electromagnetic simulations using the finite different time domain (FDTD) method. The near-field optical enhancement increases the electron−hole pair generation rate at the surface of WSe₂, thus, increasing the amount of photogenerated charge contributing to photoelectrochemical reactions.

1. Chen, J., C.S. Bailey, Y. Hong, L. Wang, Z. Cai, L. Shen, B. Hou, Y. Wang, H. Shi, J. Sambur, W. Ren, E. Pop, and S.B. Cronin., Plasmon-Resonant Enhancement of Photocatalysis on Monolayer WSe₂. ACS Photonics, 6, 787 (2019).   

First principles studies of magnetic ion-intercalated transition metal dichalcogenide bilayers

Monolayer transition metal dichalcogenides (TMDs) offer a promising framework for valleytronic applications given their degenerate, yet non-equivalent K,K’ points from broken inversion symmetry which exhibit significant spin-orbit interaction induced spin-valley coupling. Recent experimental work1,2 in transferring monolayer TMDs onto ferromagnetic substrates has shown splitting of the K-K’ band degeneracy associated with the breaking of time-reversal symmetry. Here we perform ab initio density functional theory calculations of the structural, electronic, and magnetic properties of magnetic ion intercalated MoS2 bilayers. Through investigation of intercalant element, intercalant density, and bilayer spacing and registry, we assess the promise of intercalated bilayer architectures for valley splitting and valleytronic applications.

1. Nature Nanotechnology volume 12, pages 757–762 (2017)
2. Science Advances volume 3, pages e1603113 (2017)