Session L60: Experimental Optical Spectroscopic Measurements of 2D Materials I

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Photoemission electron micrscopy (PEEM) is a nanoscale, full-field imaging technique based on the photoelectric effect. PEEM provides real space imaging of surfaces with enhanced contrast mechanism based on topography and electronic properties. Compared to other electron microscopies, PEEM is not as ubiquitous and has been widely associated with synchrotron user facilities. Recent instrument advancements in energy filtering and integration with laser-based photon sources have meant that they are rising in laboratories beyond synchrotron facilities. Here, I will present on the current capabilities and trends of lab-based PEEMs both at NIST and elsewhere. Using epitaxial graphene, I will demonstrate our current instrument capabilities of high-resolution imaging, topographic-electronic contrast due to graphene layer number, and acquiring 3-dimensional E_kinetic, k_x, k_y band structure. I will present on our plan to advance PEEM-based metrology in 2021 by integrating it with an ultraviolet light source generated by a visible laser and non-linear optics. With this new development, it will enable additional, dynamic PEEM imaging modalities to probe new physical phenomena of surfaces and interfaces.   

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Anisotropic dielectric tensors of uniaxial van der Waals (vdW) materials are notoriously difficult to investigate at infrared frequencies. The small dimensions of high-quality exfoliated crystals prevent the practical use of diffraction-limited spectroscopies. Near-field microscopes coupled to broadband lasers can function as Fourier transform infrared spectrometers with nanometric spatial resolution (nano-FTIR). While dielectric functions of isotropic materials can be readily extracted from nano-FTIR spectra, the in- and out-of-plane permittivities of anisotropic vdW crystals cannot be easily distinguished. We show how to separate the in- and out-of-plane contributions by exploiting the information in the screening of substrate resonances by vdW crystals. As an example, we determine the dielectric tensor of a bulk 2H-WSe2 microcrystal in the mid-IR and demonstrate how to quantify the uncertainty on the extracted permittivities using likelihood-based confidence intervals.   

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Two-dimensional transition metal dichalcogenides are emerging opto-electronic materials that feature strong many-body coulomb interactions due to reduced dielectric screening and quantum confinement. These many-body interactions can be strongly modified by optically injecting large number of carriers, which provides a method to engineer the excited state optical response. Here, we explore new insights into excited state properties in monolayer CVD grown MoS₂ by implementing spectrally and temporarily resolved ultrafast pump-probe transient absorption spectroscopy. Our pump fluence dependent study reveals a drastic change in the optical response over a wide spectral region, which is manifestation of giant bandgap renormalization of around 1100 meV, one order higher than the conventional semiconductors. Further, we observe a transient redshift followed by an anomalous blueshift of exciton energy with an increase in carrier density and modeled using a phenomenological framework similar to Lennard-Jones potential with modified exponents. Our experimental findings suggest that MoS₂ can be a promising material for solid-state technologies by precise and efficient manipulation of electrons in the excited state.   

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Exciton-trion-polaritons are robust coherent hybrid excitations involving excitons, trions, and photons ^[1]. In these polaritons, the 2-body exciton states are coupled to the material ground state via exciton-photon interaction and the 4-body trion states are coupled to the exciton states via Coulomb interaction. The 4-body trion states are not directly optically coupled to the material ground state ^[2]. The energy-momentum dispersion of these polaritons exhibit three bands. We realized these polaritons by coupling a doped monolayer of 2D-MoSe₂ with a 1D photonic crystal waveguide. Strong coupling between the excitons, trions, and photons was observed in the energy-band dispersion obtained via reflectivity and photoluminescence measurements. Three Rabi-split positive-mass-dispersion polariton bands were observed. Our theoretical model [1] describing exciton-trion-polaritons matches our experimental results with remarkable accuracy. [1] arXiv:2009.13069. [2] Phys. Rev. B 102, 085304 (2020).   

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Frequency- and momentum-resolved detection of thermal acoustic phonons can be carried out with spontaneous Brillouin Light Spectroscopy (BLS) but many interesting applications of nanophononics occur far from thermal equilibrium [1]. In this talk, I will show how BLS, combined with femtosecond laser excitation, can provide frequency-, momentum- and space-resolved view of non-thermal acoustic phonons in semiconducting nanomembranes [2]. Using this technique, we find that the population of photoexcited GHz phonons displays a hundred-fold enhancement compared to thermal equilibrium, while their spectra reveal Stokes-anti-Stokes asymmetry due to transport, and strongly asymmetric Fano resonances due to coupling between the electron-hole plasma and the phonons.
[1] M. Sledzinska, et al. Adv. Funct. Mater. vol. 30, no. 8, 2020.
[2] Th. Vasileiadis et al. Science Advances 2020 (accepted).   

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Atomically thin two-dimension materials have emerged as a new class of quantum structures owing to its unique properties such as controllable bandgap, spin valley control, efficient light-electron coupling. Among these, ReS₂ a member of transition metal dichalcogenides (TMDCs) has become the center of research attraction due to its anomalous characteristics, for example, Charge decoupling from extra valence electron of Re atom creates its crystallization in a distorted 1T diamond-like chain structure. Further, the resilient bandgap created by the crystal distortion offers remarkable possibility to produce a large-area ReS₂ possessing properties that are virtually independent of number of layers, in stark contrast to other TMDCs which are prone to out of plane quantum confinement. Despite these benchmark characteristics a comprehensive and profound study of the carrier dynamics in ReS₂ mostly remain unexplored. In the present study using ultrafast pump-probe spectroscopy we have measured the excited state absorption in solution processed few layer ReS₂. The detailed analysis reveals excitation dependent crossover from reverse-saturable absorption to two-photon absorption. Our striking experimental findings show the potential of ReS₂ in optical limiting and switching applications.   

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The field of quantum information science promises to usher in the next age of technological breakthroughs. In order to do so, more work needs to be done on fundamental components of quantum systems. One such crucial element is the single-photon source (SPS). Many diverse systems—from quantum dots to nitrogen-vacancy color centers in diamond—have been shown to host SPSs; however, these typically require cryogenic temperature to operate, lack tunability, or are hard to integrate with other components. Herein we examine an alternative solid-state system based on highly strained monolayer sheets of WSe2, which has the potential to host tunable SPSs at room temperature. The single crystal nature and extremely low defect density of this van der Waals material allows it to tightly conform to sharp substrate features without breaking. Instead, nanoscale wrinkles are formed in the sheet, emanating from, and in some cases bridging the patterned structures. Following our recent work on strained nanobubbles, we show that these highly strained regions can effectively localize excitons in the material and produce low-energy emission observable at room temperature, which is consistent with the location of SPSs reported in cryogenic studies.   

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Quantum coherence associated with excitons and valleys in transition metal dichalcogenide (TMD) monolayers is lost on an ultrafast time scale (~ 200 fs), imposing limits on how such pseudospins can be manipulated. Here, we investigate how quantum coherence of excitons are modified in hBN encapsulated TMD bilayers. By applying two-dimensional coherent spectroscopic techniques, we separate different dephasing pathways and explore methods for controlling such decoherence.   

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Monolayer transition metal dichalcogenides (TMDs) exhibit exceptional optical properties such as high oscillator strength and narrow excitonic resonances. However, above effects have been so far explored only for structures produced by techniques involving mechanical exfoliation and encapsulation in hBN inevitably inducing considerable large-scale inhomogeneity. On the other hand, techniques which are essentially free from this disadvantage, such as molecular beam epitaxy (MBE), have to date yielded only structures characterized by considerable spectral broadening, which hinders most of interesting optical effects.
We report [1] for the first time on the MBE-grown TMD exhibiting narrow and fully resolved spectral lines of neutral and charged exciton. Moreover, our MBE-grown TMD exhibits unprecedented high spatial homogeneity of optical properties, with variation of the exciton energy as small as 0.16 meV over a distance of tens of micrometers. Our recipe for MBE growth is presented for MoSe2 and includes extremely slow growth rate and the use of atomically flat hexagonal boron nitride (hBN) substrate. Importantly, good optical properties are achieved for as-grown sample, without any post growth mechanical treatment.
[1] W. Pacuski et al., Nano Letters 20, 3058 (2020).   

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Monolayer transition metal dichalcogenides support various types of excitonic complexes due to the strong vertical confinement and reduced dielectric screening¹. Electrostatic gating plays a crucial role to control the excitonic complexes and enables a wide-range of optoelectronic and quantum applications². However, gating in a nanoscale region and its effect on the excitonic complexes remains an open question. Here, we show that a conductive nanoscale tip induces a local emission peak around 1.67 eV, which is clearly distinguished from the surrounding area in a monolayer WSe₂. Electrical and photophysical experiments reveal that the local peak is energy-tunable and different from trionic or biexcitonic emission. Our results open the possibility of confining and controlling excitonic complexes at cryogenic temperature.

1. M. Barbone et al., Nature Communications 9, 3721 (2018)
2. T. Mueller and E. Malic, npj 2D Materials and Applications 2, 29 (2018)   

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As an emerging 2D semiconductor, titanium trisulfide (TiS₃) has a strong anisotropic photoresponse and a bandgap in the near-infrared regime, which render it a promising candidate for polarized and infrared optoelectronics. However, due to its quasi-1D atomic structure, TiS₃ often exists in the form of nanoribbon, and the absorption measurement of the individual ribbon is challenging. In this work, by performing low-temperature polarization-dependent photocurrent spectroscopy, we characterize the polarization-resolved absorption spectra of both thick and thin TiS₃ flakes. We find that the absorption edge is 0.90 eV for a 200-nm-thick flake and 0.96 eV for a 15-nm-thick ribbon, indicating a modification of the bandgap by the thickness. While the thick TiS₃ flake possess two highly anisotropic resonance peaks at 1.23 eV and 1.41 eV, the thin ribbon only exhibit one resonance at 1.34 eV. The bandstructure and the absorption spectrum obtained from first principle calculations are in excellent agreement with our experimental results. Our findings have advanced the understanding of the optical properties of TiS₃ nanoflakes, which will inspire future optoelectronics applications.   

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Coulomb-bound electron-hole pairs, known as excitons, facilitate both energy and information transport. Controlling their motion in real space, however, remains one of the major hurdles on the path towards exciton-based integrated quantum circuitry. Among prime candidates to address this challenge are single layers of transition metal dichalcogenides that offer both highly robust and mobile excitons as well as excellent opportunities to tailor excitonic energy landscapes via proximity and substrate-induced effects. Here, we introduce a highly promising platform to realize directional propagation of excitons by employing a hybrid 2D/1D approach. We integrate a hBN-encapsulated monolayer of WSe2 together with a cylindrical GaAs nanowire to form strain-induced quasi-1D transport channels for mobile excitonic quasiparticles. Time-resolved emission microscopy reveals a striking anisotropy in the propagation of excitons both at cryogenic and room temperature. Whereas their motion is strongly suppressed across the channel, excitons are able to move very efficiently along the nanowire direction. The high contrast obtained for the exciton mobilities highlights the viability of artificially created channels for excitonic transport in 2D/1D hybrid structures.   

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We investigate the twist angle dependent interlayer exciton lifetimes in MoSe₂/WSe₂ twisted bilayers (TBLs) capsulated by hexagonal boron nitride (hBN). Three R-stacking style samples with different twist angles are prepared with careful angle alignment and measured by time-resolved photoluminescence at low-temperature. The interlayer exciton lifetimes are found to change significantly with a small change in the twist angle by one order of magnitude. Using a low-energy continuum model, we theoretically explain two possible mechanisms influencing twist angle dependent interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moiré potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments.