Session G55: Optical Properties of 2D Materials

Stacking-Dependent Optical Properties in bilayer WSe2

It has recently been demonstrated that the angle between layers of two-dimensional materials can strongly impact the resulting properties, inspiring the rapidly developing research area of twistronics. Here, we investigate stacking-dependent optical properties in bilayer WSe₂. Both 2H and 3R stacking orientations are synthesized by chemical vapor deposition. Samples are investigated using photoluminescence, Raman spectroscopy, and reflectivity measurements under ambient and cryogenic conditions. In both 2H and 3R systems, the A_1g Raman mode is sensitive to excitation conditions, with orders of magnitude enhancement observed for certain excitation wavelengths. However, the laser wavelength leading to maximum enhancement is distinctly different for the two stacking orientations, with 2H-WSe₂ exhibiting maximum enhancement under 514 nm excitation and 3R-WSe₂ at 520 nm excitation at cryogenic temperatures. DFT calculations and reflectivity measurements indicate differences in band structure between the two systems, evident by shifts in emission energy of excitonic features, and elucidate the source of variation in Raman spectra. Maximum Raman enhancement is achieved when the excitation wavelength is resonant with the stacking-dependent C-excitonic feature. This work provides a comprehensive investigation of optical properties in 2H- and 3R-WSe₂ bilayers.     

Magneto-Raman Spectroscopy of Antiferromagntic FePS3 and Ferromagnetic V-Doped WS2

The recent discovery that van der Waals-bonded magnetic materials retain long range magnetic ordering down to a single layer stimulates a thorough Raman spectroscopic study of such materials, including FePS₃ and V-doped WS₂. Bulk FePS₃ was shown to be a quasi-2D Ising antiferromagnet, with additional features in the Raman spectra emerging below the Néel temperature (T_N ~ 120 K). Using temperature and magnetic field-dependent Raman spectroscopy as an optical probe of magnetic structure, we demonstrate that one of these Raman-active modes (ψ₄) below T_N is a magnon with a frequency of ~ 3.7 THz (~122 cm^-1). Contrary to previous work, which interpreted this feature as a phonon, our Raman data shows the expected frequency shifting and splitting of the magnon as a function of temperature and magnetic field, respectively, where we find the g-factor ~2. Other Raman-active modes emerging below T_N are attributed to zone-folded phonons in the antiferromagnetic state. The anomalous temperature dependence of these modes along with an analysis of temperature dependent magnetization data, suggests a persistence of short-range magnetic order above T_N. In contrast to antiferromagnetic FePS₃, the transition metal dichalcogenide WS₂ doped is found to be ferromagnetic when doped with vanadium down to the monolayer. We compare our magneto-Raman measurements on V-doped WS₂ to circularly-polarized magneto-PL measurements.   

3D Geometry-driven optical properties of atomically thin semiconductors

3D architecture design and process with thin and lightweight components is promising for high-speed and low-power operation towards next generation electronics and optoelectronics. However, constructing functional 3D nanoscale structures from atomically thin semiconductors is challenging and requires different approach. Here, we present such an approach realized by adding three-dimensional (3D) nano-topography to 2D materials of monolayers of transition metal dichalcogenides (TMDs). Using this approach, we successfully reprogram their optical properties to produce atomically thin TMD films that are optically isotropic. For this, we use the conformal growth of monolayer TMDs (MoS₂, WS₂, and WSe₂) on surfaces with nanoscale half-spherical textures, producing wafer-scale optical films with distinct geometry at different length scales. Our films show optical flatness and uniformity at the macroscale, conformal and continuous films at the mesoscale, and atomic lattice configuration of monolayer TMDs at the microscale. The resulting films show an order-of-magnitude increase in the out-of-plane susceptibility for enhanced angular performance, compared to their flat-film counterparts. We further show that such 3D geometric programming of optical properties is applicable to different TMD materials, offering spectral generalization over the entire visible range.   

Observation of photoluminescence from Franckeite, a natural van der Waals heterostructure

Van der Waals heterostructures (vdWH) comprised of two-dimensional (2D) materials offer a platform to obtain materials by design with unique electronic properties. Franckeite (Fr) is a naturally occurring vdWH comprised of two distinct alternately stacked semiconducting layers; (i) SnS₀ layer, also known as H layer, and (ii) Pb₃SbS₄ also known as Q layers. Though both H and Q layers in the heterostructure are semiconductors, the photoluminescence from Fracnkeite remains elusive. In this work, we report the observation of photoluminescence (PL) from Franckeite at cryogenic temperature. The PL peak appears at 2.1 eV measured at 1.5 K. We measured PL at varying temperature from 1.5 K to 80 K. We found that both the PL peak and the integrated intensity does not depend on the temperature in the range of 1.5 K to 80 K. We have observed linear dependence of PL integrated intensity on laser power. Our study provides a fundamental understanding of the optical behavior in a complex naturally occurring vdWH and may pave an avenue toward developing nanoscale optical and optoelectronic devices with tailored properties.   

Optical-field induced transient optics and exciton dynamics in transition metal dichalcogenide hetero-bilayers

Two-dimensional (2D) heterostructures offer a new platform for exploring low-dimensional physics. Due to the strong light–matter interactions and direct-bandgap properties, mono- and few-layer transition-metal dichalcogenides (TMDs) further attract a lot of interest for possible optoelectronic applications. In particular, the type II band-alignment TMDs heterostructures provide interesting opportunities to investigate real-time dynamics of different excitons.  Although recent advanced pump–probe spectroscopies have provided rich experimental data, their accurate theoretical description and understanding remain under explored. In this work, we investigate from first-principles optical-field induced transient optics and exciton dynamics in type II TMD bilayers, employing the time-dependent adiabatic GW method (TD-aGW) with real-time propagation of the density matrix [1]. Strong excitonic effect (electron-hole interaction) is discovered to play a vital role on the dynamic processes. Our results reveal a new picture of fast dynamic coupling between inter- and intra-layer excitons, contrary to previously believed single-particle charge transfer scenario. We compute the transient optical properties from TD-aGW corresponding to the pump-probe setup (which are directly observable quantities in most experiments).

    

Uniaxial and Biaxial Strain Engineering in 2D Materials with Thin Film Stressors

2D materials present a wide variety of strain sensitive electrical, optical, mechanical, magnetic, superconducting, and topological properties. We introduce an approach to selectively strain MoS₂ (uniaxially or biaxially) by depositing lithographically patterned thin film stressors. This technique is analogous to the ones used in industrial strained silicon but applied to 2D materials with weak out-of-plane bonding. To study the strain distribution in MoS₂ with Raman spectroscopic mapping, we chose optically transparent stressor films. Tensile stressors with a stripe geometry produce large tensile strains in MoS₂ at the edges and compressive strain at the center of the stressor strip. Uniaxial strains are observed in MoS₂ at stripe edges through polarized Raman while biaxial strains occur at stripe centers. We show that strain in 2D materials can be engineered to selectively exhibit tension/compression, uniaxiality/biaxiality, and directionality relative to crystal axes through simple lithographic patterning of stressed thin films.¹ Using this strain engineering technique, we demonstrate moving strain solitons and reconfiguring the stacking order in trilayer graphene.   

Slicing and guiding of light waves by atomically thin materials

Light waves interacting with an optical medium are generally investigated in terms of transmission, reflection, and absorption by the material. However, when the propagation direction of light changes to be parallel and bisected by the material, the fundamental behavior of light-matter interaction becomes significantly distinct from that of the normal incidence. Here, we explored an extreme limit of this parallel configuration realized by using a monolayer MoS₂ film that is grown on fused silica substrate and then immersed inside index-matching liquid to isolate optical responses from atomically thin materials. With this system, we observed that a beam propagating along the film is sliced into two sub-beams leaving a nodal line at the film location, regardless of the photon energy, whether larger or smaller than the MoS₂ bandgap. In addition, by collecting field intensity elastically scattered from the film surface, we visualized intense light waves starting from the edge of film and then eventually dissipating while it propagates, which we identified as a guided wave by further evidence. Our results demonstrate that atomically thin materials slice and guide light waves, presenting an ideal platform to study optics and photonics in two-dimensional limit.   

Spontaneous polarization induced photovoltaic effect in 3R MoS2

Different stacking order in van der Waals materials has a large impact on the interlayer coupling and can lead to many novel physics. Recently, stacking-induced two-dimensional ferroelectricity has been observed in the zero-degree aligned hexagonal boron nitride (hBN) and graphene-hBN heterostructures. Here we report a spontaneous polarization induced photovoltaic effect in semiconducting MoS₂ in the 3R phase. We experimentally demonstrated that the spontaneous polarization is homogeneous throughout the exfoliated bilayer, which is much larger in size than the individual domain in artificially stacked homobilayers. A giant depolarization field induced by the spontaneous polarization allows us to build a scalable graphene-MoS₂ photovoltaic device. Utilizing the weak screening in graphene and the exciton-enhanced light-matter interaction and ultrafast interlayer relaxation in MoS₂, we demonstrate a high external quantum efficiency in the few-layer device. Our results indicate that transition metal dichalcogenides in the rhombohedral phase are a class of promising candidates for optoelectronic applications such as energy-efficient photo-detection with high speed and programmable polarity.   

Tuning the valley polarization in WS2 monolayers via control of active defect sites induced by photochlorination

Transition metal dichalcogenides (TMDs) arise as an exciting class of atomically thin, two-dimensional (2D) materials for electronics, optics, and optoelectronics. Apart from light-based applications, these materials are good candidates for quantum information processing based on valley degrees of freedom i.e., valleytronics. In this work [1], we demonstrate spin-valley polarization tunability by more than 40% in 1L-WS₂ on hBN via photochlorination. The PL intensity of the neutral exciton is significantly enhanced after several laser pulses in a chlorine environment and the circular polarization (P_circ) decreased systematically. Polarization PL spectroscopy was performed in the temperature range from 4K to 300K. The decrease of P_circ after the photochlorination treatment is attributed to the significant reduction of the active defect sites in the 1L-WS₂ and consequently to the increase of the non-radiative exciton lifetime. Ultrafast time-resolved transient absorption spectroscopy measurements support our findings. These results indicate a useful approach to controll the density of the active defect sites as well as the degree of the emitted circular polarization in all 2D-TMDs. [1] I. Demeridou et al, Applied Physics Letters 118, 123103 (2021).    

Nonlinear optical response in strain-engineered 3R-MoS2

Straintronics, which aims at the control of the electronic properties and functionalities of two dimensional materials by using the curved structures or mechanically induced stresses, is an emerging new trend of nanoscience. Among a variety of two dimensional materials, transition metal dichalcogenides have a lot of distinct poly types and quantum phases and are also know to withstand large strain up to 10% [1], thus offering an ideal material platform for straintronics research.

Here, we report straintronic control of the optical properties in 3R-MoS₂, which has a unique stacking sequence and resultant noncentrosymmetric structure in multilayer forms [2]. By applying the strain in 3R-MoS₂, symmetry can be further controlled, leading to the emergence of the in-plane polarization. In the presentation, we focus on the nonlinear optical response originating from this emergent in-plane polarization in strain-engineered 3R MoS₂.

[1] S. Bertolazzi, et al. ACS Nano 5, 9703-9709 (2011)

[2] R. Suzuki, et al. Nature. Nanotech. 9, 611-617 (2014)   

Dielectric and substrate engineered tuning of photoluminescence in monolayer WS2

The dielectric environment strongly influences the photoluminescence (PL) of transition metal dichalcogenide (TMD) monolayers. The PL spectra are usually modified by the defect states present in the monolayer and substrate interface. To study the effect of substrate defects on PL, we have gradually increased the separation between substrate and WS₂ by placing hBN of various thicknesses in between. Further dielectric engineering was done by patterning the substrate with holes and pillars. In this way, we were able to tune the contribution of trion and exciton in the PL of WS₂. An excitation power-dependent study was also done to understand the tuning mechanism of PL in various kinds of engineered substrates. The charge of the substrate defects is also important in determining the nature of the PL spectra in monolayer WS₂. This effect was also studied by treating the substrate with APTES which changes the nature of the charge carried by a SiO₂/Si substrate. This study will be important for the fabrication of valleytronic and excitonic interconnects.   

Tunable PL in W-based TMDs probed by optical vortex beam

Selection rules for optical transitions in monolayer transition metal dichalcogenides are governed by the spin and valley degree of freedom of excitons. The intravalley scattering dynamics is responsible for photoluminescence (PL) quenching in WS₂ and WSe₂ at low temperature. The PL intensity of bright exciton at steady state is determined by intravalley scattering rate between bright and dark exciton states. In this work we propose a technique to probe the PL spectrum of monolayer W-based TMDs via photo excitation utilizing optical vortex beam and investigate the effect of intravalley scattering.   

Second-Harmonic Generation in 2D Heterostructures Strongly Modulated by Interlayer Coupling

Second-harmonic generation (SHG) is a nonlinear optical process where two photons with the same energy convert into a new photon with twice the energy. Transition metal dichalcogenides (TMDs) have been spotlighted for their high SHG efficiency and atomic thickness circumventing the phase-matching requirement. Combinatorial stacking of TMDs also enables the fabrication of heterostructures with desired nonlinear optical properties. In this work, we report that interlayer coupling dramatically affects the SHG response of individual monolayers in MoSe₂/WS₂. We were able to selectively probe each monolayer of the heterostructure using polarization-resolved detection, which revealed SHG intensity and phase were remarkably different in hetero-stack regions compared to unstacked monolayers. The modulation of the former reached a maximum when the photon energy matched with the exciton resonance of the adjacent layer. The nature of the interlayer coupling will be discussed.    

Optically probing the band offset in rhombohedrally stacked MoS2 bilayer

Stacking van der Waals materials in different orders enables a new opportunity to realize two-dimensional (2D) ferroelectricity, where the stacking-induced out-of-plane spontaneous polarization can be electrically switched by interlayer sliding. In rhombohedrally stacked MoS₂ bilayer, such spontaneous polarization leads to an interlayer potential with a type-II band alignment at the K/K' point of Brillouin zone. Since the conduction and valence band offsets are not equal, the A exciton is split by about 10 meV, which has been observed through optical spectroscopy of an artificially stacked bilayer MoS₂. Here we study the band alignment of a natural MoS₂ bilayer with rhombohedral stacking by performing optical reflection and photoluminescence spectroscopy of A/B excitons and their charged species while precisely controlling the doping and displacement field in the bilayer. As a result, we can quantitatively measure the conduction and valence band offset individually as well as the 2D spontaneous polarization density. Our study provides a detailed understanding about the bandstructure of rhombohedral MoS₂ bilayer, which may contribute to the potential application of 2D ferroelectricity.