Session T64: Characterizations of 2D Materials by Optical Methods

Imaging ExcitonTransport with Ultrafast Microscopy in the Quantum Regime

At the most fundamental level, transport of energy carriers (such as electrons and excitons) in the solid state is determined by their wavefunctions and the interactions with the lattices and the environment.  The key difficulties in probing transport in the quantum regime in real materials lie in the fast (picosecond or shorter) dephasing processes and the nanoscale localization lengths. Thus, to image the motion of charges and excitons in their natural (quantum) time and length scales, experimental approaches combining spatial and temporal resolutions are necessary, which requires a paradigm shift from conventional spectroscopy and microscopy methods.

 

To address this challenge, my research group has developed the combined use of optical microscopy and ultrafast spectroscopy tools to image transport of charge carriers and excitons from the nanoscale to the mesoscale and over a wide range of temperatures. In my talk, I will discuss our recent progress on imaging the hot carrier transport in hybrid perovskites, environment-assisted quantum exciton transport in perovskite quantum dot superlattices, and exciton phase transitions in moiré superlattices of two-dimensional transition metal dichalcogenides. These results provide fundamental understandings of how excitons and charge carriers migrate in materials and how these processes can be manipulated quantum mechanically. The unique ability to measure and control coherent pathways are critical for both solar energy and quantum information applications.   

Improved Tip-Enhanced Photoluminescence (TEPL) Resolution Through Demodulation

Photoluminescence (PL) spectroscopy has proven a potent tool for investigating nanoscale structures and materials. Nevertheless, conventional PL techniques find themselves confined by the diffraction limit, which hinders their ability to unveil intricate details. To surmount this limitation, researchers have turned to tip-enhanced photoluminescence (TEPL) spectroscopy. By employing sharp tips that enhance both excitation and emission processes, TEPL empowers imaging and spectroscopy at resolutions exceeding the diffraction limit. In our study, we leverage a demodulation technique using an AFM tip in intermittent contact mode to meticulously segregate the PL signal from background noise. This strategic refinement substantially boosts the signal-to-noise ratio. Through the integration of a photomultiplier tube (PMT), our detection mechanism achieves heightened sensitivity and efficiency, facilitating the precise capture of even the faintest PL signals. We have validated the technique by PL imaging experiments involving fluorescent beads and monolayer MoS2. These preliminary experiments serve as prime examples of our enhanced spatial resolution and sensitivity.   

Combining ab initio accuracy and large-scale Molecular Dynamics with Machine Learning: an application on Transition Metal Dichalcogenides.

Amidst the exciting landscape of materials science, Transition Metal Dichalcogenides (TMDs) have emerged as exciting subjects of exploration. These versatile low-dimensional materials, which include monolayer TMDs like MoS2 and MoSe2, offer a rich tapestry of optical and dynamical properties with applications across various scientific domains. In our pursuit of understanding TMDs, we've employed a molecular dynamics method powered by neural network potentials. This innovative approach enables us to delve into these materials with remarkable precision, and we have determined phonon dispersion curves and frequency-dependent dielectric constants at finite temperatures for monolayer systems. Importantly, our approach is adaptable and can readily incorporate any new findings.   

Nearfield scanning optical microscopy of dynamically twistable TMDs

The periodic potential generated in transition metal dichalcogenide (TMD) moiré superlattices can trap electrons and holes to form an array of optically active single emitters and other exotic quantum states. The twist angle in TMD moiré superlattices critically determines the properties of the superlattice potential and thereby the optical properties of the emitters. However, twisted devices are generally fabricated with a fixed angle that cannot be modified once the device has been made. In addition, the Moiré unit cell is below optical wavelengths, making it difficult to address single emitters with free-space optics. In this work, we develop a new platform for reliable creation and read-out of Moiré emitters. This platform combines two capabilities: 1) a "Twistronics" apparatus that can bring two van der Waals layers into direct contact and rotate them relative to each other which allows for precise in-situ control of interlayer twist angle. 2) a near-field scanning optical probe that can collect light from a subwavelength scale spot, making it possible to address single photon emitters in moiré superlattices. We discuss the merits and challenges of the design and report our progress in using it to study the optical properties of TMD heterolayers.   

Magneto-Raman studies of chiral phonon in Fe-doped MoS2

The magnetic moment of the chiral phonon in transition metal dichalcogenides (TMDCs) holds promise for valleytronic and spintronic applications. As the TMDC lattice is achiral, an important question is how to lift the degeneracy of the chiral phonon. In this work, we study the monolayer iron-doped molybdenum disulfide (Fe:MoS₂) grown via chemical vapor deposition. Our magnetometry measurements demonstrate that Fe:MoS₂ is ferromagnetic at room temperature. Helicity-resolved Raman spectroscopy shows that at zero external magnetic field the chiral phonon in Fe:MoS₂ splits into two modes due to the lifted time-reversal symmetry.  The split chiral phonons further shift separately in the presence of an external magnetic field. Our results highlight magnetic dopants as an important approach of tuning chiral phonons in TMDCs.   

“Extraordinary” Phase Transition Revealed in a van der Waals Antiferromagnet

An uncommon and even counterintuitive situation in surface phase transitions is that the surface order emerges at a higher temperature than the bulk one, despite the two-dimensional (2D) Mermin-Wagner fluctuations. Such a phase transition, where the bulk order sets in after the surface order, has already been theoretically established and dubbed “extraordinary”. They can only happen if the surface interactions are much stronger than the bulk ones. While theoretically possible, they have been hardly realized in any materials so far. Here, we demonstrate the presence of an extraordinary phase transition in bulk CrSBr, a van der Waals (vdW) antiferromagnet (AFM). Using a combination of various second harmonic generation (SHG) techniques, we capture the surface and bulk magnetic phase transitions, spin correlations, as well as distinguish the two degenerate AFM domain states. Density functional theory calculations further identify key factors contributing to the enhanced surface magnetism.   

Nonlinear Optical Imaging of Few-Layer Hexagonal Boron Nitride

Few-layer hexagonal boron nitride (hBN) has attracted considerable attention for its nanophotonic and 2D dielectric applications. Because TEM and STM are limited to very small length scales and by demanding sample requirements, a wide-area nondestructive probe with high throughput is required to characterize crystalline domains and defects in wafer-scale hBN [1]. In this study, we report on polarimetric second-harmonic generation (SHG) imaging that uses a ratiometric detection of two orthogonally polarized SHG fields. The method revealed grain boundaries of polycrystalline CVD-grown hBN films on the length scale of hundreds of micron with crystallographic orientation and quality. A significant variation in the width of orientational distribution was observed for single-crystal samples from various sources, which implied varying crystallinity. Confirming a positive correlation between the widths of orientational distribution and Raman peak, we propose the nonlinear optical analysis can be a new structural probe for synthesized hBN films. Since the properties of hBN films are closely linked to their structure, the reported method will contribute to the controlled synthesis of hBN with specific material properties [2].   

Magneto-Raman signature of exciton-activated chiral phonon in MoS2

The concept of chirality has been extended to the description of circularly polarized phonons in which the atomic circular motions produced nonzero angular momentum. In monolayer transition metal dichalcogenides, the noncentrosymmetric hexagonal lattice allows chiral phonons at the high symmetry points of the Brillouin zone. In this work, we investigate the helicity-resolved Raman response in monolayer MoS₂ grown via chemical vapor deposition. We identify a doubly degenerate Brillouin-zone-center chiral phonon mode at ~270 cm^-1. We establish the selection rule of this chiral phonon and show that this mode is activated by the resonant transition of A exciton.  By applying an external magnetic field, we lift both the energy and intensity degeneracy of the chiral phonon. Remarkably, the phonon Zeeman splitting reveals a giant chiral phonon magnetic moment. Our study paves the route to excite and control chiral phonons in an achiral material.    

Raman Spectroscopic Fingerprint of Exfoliated S=1 Triangular Lattice Antiferromagnet NiGa2S4

Two-dimensional (2D) van der Waals (vdW) materials have been an exciting area of research ever since scientists first isolated a single layer of graphene. Now, since the emergence of 2D magnets, there has been even more potential for interesting properties to arise and result in applications ranging from spintronics to topological magnonics. Here we report the first successful exfoliation of bilayer and few-layer flakes of S=1 triangular lattice antiferromagnet NiGa₂S₄. We establish the reported number of layers of the material by performing atomic force microscopy (AFM) and detail a careful characterization using Raman spectroscopy to see how the optical, electronic, and structural properties of the crystal change as a function of sample thickness. Raman active phonons in the material shown to have a strong dependence on excitation wavelength due to resonance effects. Infrared (IR) active modes that had become Raman active due to disorder in the material are shown to have some dependence on sample thickness, however, their behavior with exfoliation has yet to be explained. These results provide a reliable method of determining the number of layers of the material by means of Raman spectroscopy.   

Infrared and Terahertz Nanospectroscopy of 2D Materials at the National Synchrotron Light Source II

2D materials provide unprecedented opportunities for tailoring material properties, e.g., via exfoliation, stacking of heterostructures, and twisting.  Many fundamental material excitations of 2D materials are found at energies of a few 10 meV to a few 100 meV, i.e., in the infrared and terahertz (THz) frequency range.  Infrared nanospectroscopy provides direct access to these excitations at a spatial resolution of ~10 nm.  At the National Synchrotron Light Source II, we have established a new infrared-to-THz nanospectroscopy setup, which uniquely enables spectroscopy in the ultrabroad frequency range from 5-242 THz (175-8000 cm^‑1, 22-1000 meV, ~1-57 µm).  To demonstrate our system’s capabilities for THz nanospectroscopy, we showcase ultrabroadband synchrotron nanospectroscopy of phonons in ZnSe (~7.8 THz) and BaF₂ (~6.7 THz), as well as polariton interferometry of hyperbolic phonon polaritons in the van der Waals material GeS (6-8 THz).

The NSLS-II near-field nanospectroscopy setup at the 22IR2/MET infrared beamline is open for general user beamtime proposals. See: https://www.bnl.gov/nsls2/userguide/   

Optically Enhanced Magnetization by Exciton Recombination in an Atomically Thin Antiferromagnet

Two-dimensional (2D) semiconductors offer a unique platform for investigating the interplay between light and magnetization. Excitons are usually responsive probes of magnetic order, yet whether magnetic order responds to the presence of excitons is less frequently examined. In this work, we synchronize optical pumping of a 2D magnet to an ac sensing protocol on nitrogen-vacancy (NV) centers in diamond. Using this spatiotemporal-resolved magnetometry, we discover an enhancement of the magnetic moments in a few-layer van der waals antiferromagnet. Together with time-resolved photoluminescence measurements and density functional theory calculations, we infer that this enhancement arises from a defect-assisted Auger recombination, which involves localized excitons and drives electron hopping into the spin-polarized band. This study offers insights into defect engineering in 2D magnets and encourages the development of spintronic devices modulated by light.