Session Y60: Advanced Optical Measurements of 2D Materials

Nanoscale Optical Imaging of 2D Semiconductor Heterostructures by Exciton Enhanced Second Harmonic Generation

Second harmonic generation (SHG) is a nonlinear optical response arising exclusively from broken inversion symmetry in the electric-dipole limit. Recently, SHG has attracted widespread interest as a versatile and noninvasive tool for characterization of crystal symmetry and emerging ferroic or topological orders in quantum materials. However, conventional far-field optics is unable to probe local symmetry at the deep subwavelength scale. Here, we demonstrate near-field SHG imaging of 2D semiconductors and heterostructures with the spatial resolution down to 20 nm using a scattering-type nano-optical apparatus. We show that near-field SHG efficiency is greatly enhanced by excitons in atomically thin transition metal dichalcogenides. Furthermore, by correlating nonlinear and linear scattering-type nano-imaging, we resolve nanoscale variations of interlayer stacking order in bilayer WSe₂, and reveal the stacking- tuned excitonic light-matter-interactions. Our work demonstrates nonlinear optical interrogation of crystal symmetry and structure-property relationships at the nanometer length scales relevant to emerging properties in quantum materials.   

Sub-diffraction nanoscale Raman imaging of the interface in a 2D semiconductor heterostructure

Transition metal dichalcogenide (TMD) semiconductors are solids composed of single sheets of atomic layers that are weakly bonded together via the van der Waals interaction. A single layer yields a 2D semiconductor with a direct band gap and strong light-matter interactions. This work studies the interface in single-layer 2D MoS₂/WS₂ lateral heterostructures with resonant and non-resonant tip-enhanced Raman scattering (TERS) imaging and spectroscopy at spatial resolutions better than 50 nm. In conventional confocal Raman spectroscopy, a spatial resolution on this scale is not possible because of the diffraction limit. With the sub-diffraction spatial resolution and the vibrational fingerprinting ability of Raman spectroscopy, TERS allows us to probe the composition, size, and heterogeneity of the 2D system on length scales that are most relevant for nanoscale optoelectronic technologies. We use TERS to reveal that the alloyed transition region varies in size from 50-600 nm within a single crystallite. TERS nanoscale imaging of the transition region allows for tracking of vibrational modes as they evolve across the MoS₂/Mo_xW_1-xS₂/WS₂ system. This work demonstrates the capabilities of TERS in characterizing monolayer lateral heterostructures on the nanoscale.   

Quantifying charge-carrier dynamics in hexagonal boron nitride by optimized cathodoluminescence of buried interfaces

We demonstrate high-resolution cathodoluminescence (CL) hyperspectral maps of the heterogeneous light emission from monolayers transition metal dichalcogenides (TMDs) encapsulated in hexagonal boron nitride (hBN). TMD CL crucially relies on the electron-hole (e-h) pair dynamics in the hBN encapsulation. However, there is no systematic study of the implication of these dynamics to advance high-resolution CL mapping.

Here, we address this issue by showing that the hBN thickness controls the CL spatial resolution and we estimate a minimum threshold of the e-h diffusion length in bulk hBN. We provide an optimal thickness range to achieve bright, sub-diffraction resolved CL and corroborate our approach with low-temperature hyperspectral maps, in which we isolate different forms of heterogeneity at the nanoscale.

Our approach enables the access to the properties of buried, optically active interfaces at the nanoscale and paves the way towards the full comprehension of the rich TMD optical properties.   

Low-frequency tip-enhanced Raman scattering of 2D materials

Two-dimensional materials such as transition metal dichalcogenides have unique optoelectronic properties with broad tunability via doping and stacking, which makes them promising platforms for functional nanodevices. Nanoscale structural characterization of 2D materials provides information about local exciton generation and defects via tip-enhanced photoluminescence (TEPL) and tip-enhanced Raman scattering (TERS). TERS of 2D materials has been previously performed in the high frequency range. However, low-frequency range is advantagous for investigating the interlayer interactions and stacking. Here, we performed low-frequency TERS measurements of twisted 2D spirals of lateral MoSe2-WSe2 heterostructures to understand the correlations between the local structural heterogeneities and nano-optical response. We compared the results with high-frequency TERS and discuss optimizationf of relevant plasmonic processes and enhancement mechanisms.   

Acoustic Phonons from Brillouin Spectroscopy of Chemically Tunable 2D Materials

Brillouin scattering is a non-invasive, laser-based scattering technique capable of determining the acoustic phonon properties of 2D materials at GHz frequencies. Here, we use Brillouin spectroscopy to map the entire angular dispersion curves of multiple acoustic phonon branches of 2D layered MoO3 and V2O5 - directly probing the effects of phonon quantum confinement within a 2D layered material. Measurements provide longitudinal and transverse sound velocities, refractive indices, and acoustic attenuations. Since acoustic phonons dictate elasticity, we can take advantage of Lamb theory to optically derive the complete elastic stiffness tensor of each 2D layered material as well as the thickness to within less than a monolayer. We demonstrate how the intercalation of metallic Sn, Co, and Cu can chemically tune quantized acoustic phonons and elasticity of 2D layered metal materials by substantially stiffening the van der Waals gap, with longitudinal moduli increasing by 20% or more with a complex effect on the stiffness of the 2D material.  This work establishes methodology to extract precise elastic constants from complex Brillouin scattering of 2D materials. It takes advantage of phonon confinement to capture the complete phonon response with minimal scattering geometries.   

Quantitative Thermal Characterization of Supported and Encapsulated 2D Semiconductors

Heat conduction in two-dimensional (2D) materials differs from their bulk counterparts due to the role of interfaces and microstructural defects, geometry-dependent phonon band structure, and phonon scattering at phase boundaries. For example, 2D semiconductors are placed in contact with dielectric gates and metal source and drain electrodes in practical applications, leading to significantly reduced thermal conductivity due to phonon scattering with the substrate and superstrate. Studies of the thermal conductivity of substrate-supported and encapsulated 2D materials are critical for managing heat dissipation in 2D field-effect transistors. This work reports the in-plane and cross-plane thermal conductivity measurements in MoS₂ and CrI₃ nanosheets, based on the combination of the frequency domain thermoreflectance (FDTR) and optothermal Raman spectroscopy (OTRS) techniques. Combining the two techniques overcomes the limitations of the individual methods by decoupling the in-plane and through-thickness thermal property measurements. We will show that this new measurement approach is suitable for quantitative characterization of the anisotropic thermal conductivity and thermal boundary conductance of supported and encapsulated quasi-2D and few-layer semiconducting films.   

Confined exciton photoluminescence in 2D transition metal dichalcogenide architectures from nano-strained regions revealed by scanning transmission electron microscopy

Assemblies comprised of materials with different dimensionalities can yield exceptional and highly tunable properties provided that the size, structure, and interfacial interaction between the constituents is precisely controlled. Here we fabricate a mixed-dimensional assembly by ‘draping’ CVD-grown 2D MoSe₂ monolayers over single-crystal silicon nanowires (NW), and by this process generate an extended region of localized strain in the monolayer along the edges of the NW. Near-field optical characterization reveals a photoluminescence (PL) peak characteristic of emission (at 1.57 eV) from the MoSe₂ A-exciton in regions away from this strained area. Notably, we uncover an anomalous PL feature at 1.38 eV coincident with the strained area of the 2D MoSe₂-NW assembly. To better understand the origin of this anomalous peak, we use aberration-corrected HAADF-STEM to determine the atomic structure of the MoSe₂ lattice near the Si NW.   

Observation and Exploitation of Coherent Phonon Frequency Combs in Eu(Fe.8Co.2)2As2

Iron pnictides have gained interest in recent years based on their display of unconventional superconductivity. However, ultrafast studies on many of the alloys of the parent compound EuFe₂As₂ are missing in the literature. In this talk, I will present some preliminary results on a Cobalt doped europium iron pnictide sample that presents a strong pulsed behavior on top of the typical electronic decay. The pulses are found to be caused by the propagation of a coherent phonon wavepacket, which forms a frequency comb upon Fourier transformation of the time domain signal. We find that the pulsed behavior is extremely robust and is primarily dependent on sample thickness, as in picosecond ultrasonics experiments. Additionally, we see evidence that this frequency comb may provide a sensitive measure of the order parameter during phase transitions in the material, as a shift is seen upon crossing the Neel temperature in our experiments.   

Spontaneous polarization induced photocurrent mechanisms in 3R-MoS2-graphene heterostructures

Rohmbohedrally stacked transition metal dichalcogenides (TMD) exhibit broken inversion symmetry and have been shown to be ferroelectric up to room temperature. When sandwiched between two graphene electrodes, the 3R-MoS2 develops a large depolarization field, which leads to a significant photovoltaic (PV) effect. On the other hand, the spontaneous polarization also induces image charges in top and bottom graphene electrodes, resulting in a chemical potential imbalance and a corresponding photo-thermoelectric (PTE) effect. These two photocurrents both occur at zero bias and therefore are difficult to distinguish in the steady state measurement. Here we perform the ultrafast photocurrent autocorrelation experiment in 3R-MoS2-graphene devices. We find the autocorrelation response of the PV and PTE effects are opposite in direction and have different time scales, which are limited by cooling of the lattice and electron, respectively. These results reveal the intrinsic mechanisms behind the spontaneous photocurrent in 3R-MoS2 devices and can help further optimize ferroelectric TMD heterostructures for practical optoelectronic applications.   

Direct Formation of Interlayer Exciton in MoSe2/WSe2 Heterostructure Measured via Broadband Transient Absorption Spectroscopy

In Transition Metal Dichalcogenide Heterostructures (TMD HS) with Type II band alignment, such as MoSe₂/WSe₂, photoexcited carriers can form interlayer excitons (ILX) with electrons/holes residing in different layers. Spatially-indirect ILXs exhibit longer recombination times than intralayer excitons (10 – 100x longer) making these HS a promising platform for observing possible Bose-Einstein condensation of excitons and for applications in spin/valley optoelectronics.

Our understanding of the ILX kinetics is limited to recombination resolved via photoluminescence spectroscopy. We directly study the early formation process of the ILX in a MoSe₂/WSe₂ HS on a sub-ps timescale using transient absorption spectroscopy. Upon selective photoexcitation of the MoSe₂ layer, we resolve interlayer hole scattering to WSe₂ and the signature of the ILX around ~1.4 eV, which grows in on a 400 fs timescale (i.e. slower than the interlayer charge transfer process). This suggests photoexcited carriers are not instantaneously converted into the ILX following interlayer scattering, but rather intermediate scattering processes take place. Moreover, valley-contrasting physics of intra/inter-layer excitons are explored.   

Ultrafast optical characterization of superconductivity in atomically thin Bi2Sr2CaCu2O8+x

Recently, atomically thin van der Waals superconducting material Bi₂Sr₂CaCu₂O_8+x (Bi-2212) has been isolated and proven to inherit the properties of its bulk counterpart. However,  in  the atomically thin limit, Bi-2212 is very fragile and easily deteriorates during usual nanofabrication processes. A non-invasive characterization technique is thus highly desired. Here we present an optical pump-probe method for identifying superconductivity of atomically thin Bi-2212 flakes. We confirm its applicability by comparison with resistivity measurements in thick flakes, in which both methods yield comparable superconducting transition temperature (Tc). To further validate this technique, our pump-probe results show the expected gap closing signature with increasing applied magnetic field. Direct pump-probe measurements from micron-sized trilayer and bilayer Bi-2212 samples yield Tc values that are very close to that of bulk materials. For these thin samples, low temperature exfoliation and hBN capping are used to preserve the sample quality. Our work provides an all-optical approach for characterizing superconductivity with diffraction limited spatial resolution in atomically thin Bi-2212, which is helpful for furthering the study of 2D superconductivity.   

Exciton Dynamics in Functionalized Germanane

Monoelemental 2D materials are fast emerging alongside the much better-characterized transition metal dichalcogenide semiconductors due to high electron mobility, a wide range of achievable band gaps, and the possibility to tune their morphology and physical properties via functionalization. Methyl-substituted Germanane GeCH₃ is a two-dimensional semiconductor with strong above-gap photoluminescence connected to water intercalation.  Here, we use time-resolved fluorescence spectroscopy at different temperatures to unravel the origin of this photoluminescence. We find two different populations of bright excitons localized at recombination centers within puddles of intercalated water. The two exciton populations involve different electronic levels and couple to different phonons. They both recombine exponentially, together with a thermally activated conversion from the shorter- to the longer-lived species. The latter diffuses within the energy distribution of recombination centers. We expect these results to spawn further research on the exciton dynamics, charge separation, and many-body physics in monoelemental 2d semiconductors that will underpin future applications in optoelectronics, light-harvesting, and sensing.   

Signature of Faraday effect in Raman spectra of MoS2

We perform Raman microscopy of a MoS₂ flake at 2K and in varying magnetic field (B), and for both parallel (VV) and perpendicular (VH) polarization configuration of polarizer and analyzer. We observe suppression of the A₁' Raman signal of MoS₂ in B=9T in the VH configuration. Raman maps were recorded in different B for both VV and VH configurations, and the intensity ratio of Raman signals A₁' and E' was inferred from them. The observed changes in suppression of A₁' are in good agreement with [1]. However, in [2] this change could only be detected at significantly higher B. Beside the influence of B on the oscillation modes of a crystal, one must also consider the impact of the Faraday effect. For this purpose, we recorded in addition a series of Raman spectra of the Si substrate as the function of B and polarization. The MoS₂ and Si peak ratios show approximately the same dependence on B, which indicates the dominant cause for the changes in the detected Raman signals being the Faraday effect. As the Faraday rotation angle is linear with B, it is straightforward to compensate it, which can even be automated in the cryogenic Raman microscope that we developed.   

Stimulated Raman spectroscopy for two-dimensional dielectric crystals

Spontaneous Raman spectroscopy of two-dimensional (2D) dielectric crystals is often challenged by weak signals because the crystals have a small Raman cross-section. In this work, we report a micro-spectroscopy setup for stimulated Raman scattering (SRS) of dielectric 2D crystals. Coherent nonlinear scattering gives SRS a substantial increase in overall sensitivity in addition to avoiding non-resonant background signals, unlike coherent anti-stokes Raman scattering. The spatial and temporal overlap of ps pump and Stokes beams were optimized for graphene using transient reflectance signals epi-collected with a microscope objective. The Raman loss or change in the pump beam was lock-in-detected with a fast photodiode while the Stokes beam was modulated at 20 MHz. SRS was verified with respect to the spontaneous Raman spectrum of hexagonal BN and further optimized to improve sensitivity. As quasi-2D model systems, thin crystals of hexagonal BN and calcium hydroxide were prepared and probed in ~1370 cm^-1 and 3650 cm^-1, respectively. We will discuss the results in comparison with their spontaneous Raman spectra and instrumental refinements to reach a 2D limit.   

The new standard for Raman spectroscopy of graphene over extreamly high biaxial strain.

Most conventional methods to measure the mechanical properties of 2D materials and thin films is nanoindentation using atomic force microscopy (AFM). Herein, we report the experimental process as well as measurement of Raman spectrum (G, 2D peak, respectively) of suspended graphene under the extremely high biaxial (e.g., isotropic) strain using AFM-Raman combined tool. In our nano-indentation, the Raman peaks shifting in G and 2D were founded to be 256 cm^-1, and 675 cm^-1, respectively, for suspended monolayer graphene. The bigger advantage is that our experiment is a method that can be repeatable and simultaneously extremely high strain. Moreover, relation between shifts (G, 2D peak) and strain (%) obtained through FEM simulation show a new standard to optical property of strained graphene.