Symposium NM02—Reconfiguring the Properties of 2D Materials by Post-Synthesis Design

The physical properties of atomic monolayers are often very different from those of their parent bulk materials. Prime examples are graphene and monolayers of MoS2, as their ultimate thinness makes them extremely promising for applications in electronics and optics. At the same time, they give access to new degrees of freedom of the electronic system (such as the valley index) or interactions between quasiparticles such as excitons (Coulomb- bound electron–hole pairs). Here we discuss how our understanding and engineering of exciton states in multi-layers allows tuning the linear and non-linear optical properties in atomically thin materials in the context of collective quantum states. In particular, we study the interactions between intra-layer and inter-layer excitons with different dipoles, as they are tuned into resonance [1]. In homobilayers we show gate-tunable second harmonic generation mediated by interlayer excitons [2]. In heterobilayers we measure the twist angle between the layer in-situ by addressing intralayer exciton resonances in each layer. For heterostructures with close to zero twist angle, we observe changes of exciton resonance energies and the appearance of new resonances in the linear and non-linear susceptibilities [3].
[1] Sponfeldner et al, arXiv:2108.04248
[2] Shree et al, arXiv:2104.01225
[3] Paradisanos et al, arXiv:2110.08095

Two dimensional van der Waals materials have emerged as a playground for observing and manipulating collective quantum phases. A large part of this is due to the incredible diversity in materials and the number of tuning knobs available to manipulate electronic structure in these materials. In this talk, I will discuss recent developments in tuning electronic structure of this materials class by using strain and twist control in heterostructures of van der Waals materials, and the emergent quantum phases that can result from applying these tuning knobs.

Interlayer excitons (ILXs) in transition metal dichalcogenides heterostructures, correlated electron-hole pairs separated across adjacent vertically stacked monolayers, have been the focus of much recent interest. Not only do they present potential for information storage due to their exceptionally long lifetimes [1], but they are also highly tunable through out-of-plane electric fields [1,2] and through the twist-angle alignment between the layers [3]. Using an electromodulation strategy, we have directly measured absorption spectra of ILXs in WSe₂/MoSe₂. The absorption features show an expected linear Stark shift in energy with applied perpendicular electric field. Further, we observe that the ILX oscillator strength changes significantly with electric field strength due to modification of electron-hole wavefunction overlap. From our investigation of ILX absorption at different twist angles, we see strong evidence of a deep moiré potential that undergoes significant reconstruction at small twist angles. The measurement technique used here can be broadly applied to other heterobilayer systems.
References: [1] A. Jauregui et al., Science 366 (2019) [2] O. Karni et al., PRL. 123 (2019) [3] Choi et al. PRL 126 (2021)

The moiré potential arising from the relative twist and lattice mismatch in heterobilayers of two-dimensional materials has led to the discovery of novel excitonic species. Twist-angle dependent transport measurements of these interlayer excitons have been reported for WSe2/WS2 heterobilayers [1]. In our experiment, we use time- and spatially resolved spectroscopies to investigate the temperature dependent lifetime and diffusion length of various interlayer excitons in WSe2/WS2 heterobilayers, where the individual monolayers have been aligned to produce long-range moiré superlattices. We observe that the interlayer exciton diffusion length increases with temperature till it undergoes a phonon-driven transition where the intralayer exciton starts to dominate the photoluminescence spectrum. The temperature dependent formation of the interlayer exciton is reflected in the overall lifetime and diffusion lengths of exciton species in the WSe2/WS2 heterobilayer.
[1] Yuan, Long, et al. Nature materials 19.6 (2020): 617-623

The synthesis of quasi-two-dimensional materials, such as monolayer transition metal dichalcogenides (TMDCs), opened the door to the study of new classes of systems with nanoscale dimensionality confinement and weak electronic screening, leading to strongly enhanced electron interactions. In particular, heterobilayers of such materials host unique physics due to the emergence of a moiré potential that modulates such excitations. However, the interplay between the structural details in such twisted bilayer structures (including atomistic relaxation effects), extrinsic fields, and doping on the excited-state properties of such materials is poorly understood, and often relies on empirically fitted continuum models. In this talk, we present results obtained from recent formalisms and methods we developed to bridge these effects and phenomena. We show how moiré effects can lead to a surprising localization of excitons even for relatively large twist angles (~2°) – associated with a moiré lattice parameter of ~ 6 nm. We also develop a formalism to understand the spectroscopic signatures of moiré-confined excitons, in very good agreement with recent time- and angle-resolved photoemission spectroscopy measurements. We also show a new method to extract accurate moiré potentials from first principles, and show why previous ab initio approaches often yield potential depths that are significantly smaller than those deduced from experiment. Finally, we also show how such realistically deduced potentials lead to moiré-modulated interlayer excitons with qualitatively different characters, offering unique tuning capabilities for excitations in van-der-Waals-stacked materials.

Few-layer transition metal dichalcogenides (TMDCs) have emerged as the ideal experimental platform to study many-body interactions in low dimensions due to their weak dielectric screening, enhanced many-electron interactions, and straightforward optical characterization. When two layers of TMDCs are stacked with a finite twist angle, a superlattice-periodic moiré potential emerges that modulates the localized quasiparticle and optical excitations. Up to now, however, the experimental characterization of moiré-localized excitons has been indirect. Here, we derive the spectroscopic signatures of moiré-confined excitons in twisted 2D materials that can be measured in current time-resolved angle-resolved photoemission spectroscopy (TR-ARPES) setups. We show that, by simultaneously measuring the distribution of the holes and electrons bound to the photoexcited excitons, it is possible to directly extract both the center-of-mass and internal structure of an exciton. Our calculations are in good agreement with recent experiments, showing that moiré excitons are surprisingly localized even for relatively small moiré unit cells with a moiré lattice parameter of ~ 6 nm. Finally, we comment on a new method to extract accurate moiré potentials from first principles.

*This work was supported by C2SEPEM at LBNL, funded by the U.S. DOE under Contract No. DE-AC02-05CH11231. This research used resources of the National Energy Research Scientific Computing Center, operated under Contract No. DE-AC02-05CH11231.

Photoinduced charge transfer in van der Waals heterostructures occurs on ultrafast timescales of order 100 fs, despite the weak interlayer coupling and momentum mismatch. Little is understood about the microscopic mechanism behind this fast process and the role of the lattice in mediating it. Here, using femtosecond electron diffraction, we directly visualize lattice dynamics in photoexcited heterostructures of WSe₂/WS₂ monolayers. Following selective excitation of WSe₂, we measure unexpectedly concurrent heating of both WSe₂ and WS₂ on ps timescales, corresponding to an interlayer thermal conductance that is >100x larger than that due to phonons alone. Using first-principles calculations, we identify a fast channel, involving an electronic state hybridized across the heterostructure, enabling phonon-assisted interlayer transfer of photoexcited electrons. Phonons are emitted in both layers on femtosecond timescales via this channel, consistent with the simultaneous lattice heating observed experimentally. Taken together, our work indicates strong electron-phonon coupling via layer-hybridized electronic states – a novel route to control energy transport across atomic junctions.

The relative angular alignment between 2D layers of a van der Waals (vdW) heterostructure can dramatically alter its fundamental properties[1]. A striking example is the recent observation of strongly correlated states and intrinsic superconductivity in twisted bilayer graphene[2]. Another remarkable effect of angular layer alignment, predicted for certain vdW heterostructures, is the emergence of phases of matter with non-trivial topological properties, where charge carriers flow without dissipation, being protected against perturbations. In graphene aligned with boron nitride (BN), such a phase has been predicted, with topological protection linked not to the spin, as commonly observed, but rather to the valley degree of freedom.

The experimental observations of these topological valley currents [3] has been largely put in question by theorist, results of numerical simulation [4] and recent scanning SQUID results[5]. In these, the observed non-local signal have been attributed mostly to localized states on the edge of graphene. In this talk, we will show how these two pictures are not incompatible and can be re-conciliated if we take the angular layer alignment into account.
References
[1] Ribeiro-Palau et al., Science 361 (2018), 690
[2] Cao et al., Nature 556 (2018) 43.
[3] Gorbachev et al., Science 436 (2014), 448; Komatsu et al., Science Advances 4 (2018) eaaq0194
[4] J M Marmolejo-Tejada et al., J. Phys. Mater. 1 (2018) 015006
[5] A. Aharon-Steinberg et al., Nature 593 (2021), 528

Weak interlayer coupling in 2-dimensional layered materials such as graphene gives rise to rich mechanical and electronic properties, in particular in the case where the two atomic lattices at the interface are rotated with respect to one another. The reduced crystal symmetry leads to anti-correlations and cancellations of the atomic interactions across the interface, leading to low friction¹ and low interlayer electrical transport². Using our recent nanomanipulation technology, based on atomic force microscopy, we show that combined electro-mechanical characterization can uniquely address open fundamental questions related to electronic charge transport^2-3 through stacking faulted structures. To this end, we studied experimentally and theoretically the interlayer charge transport in twisted bilayer graphene systems separately for edges and bulk parts. We find that interlayer edge currents are several orders of magnitude larger than in the bulk and therefore govern the transport up until very large critical diameters depending on the potential across the adjacent layers and the angular mismatch angle. In addition, we show that the strong edge transport across the interface is governed by strong quantum mechanical interference effects as opposed to simple interlayer atomic interactions.

[1] E. Koren et al., Science, 6235 (2015) 679.
[2] E. Koren et al., Nature Nanotech., 9 (2016) 752.
[3] D. Dutta et al., Nature Comm, 11 (2020) 4746.

We explore how atomically sharp hetero structures such as boundaries, vacancies and substitutes in 2-D materials create new protected quantum states using photo low temperature Scanning Tunneling Microscopy and time resolved optical spectroscopy.
The concept of quantum coherence is well established in fields such as atomic physics and quantum optics. In solid state systems or the emerging field of quantum biology, quantum coherence plays a critical role defining new phenomena such as coherent transport through topological states, coherent exciton transport between organic antenna complexes or coherent emission from color centers to name just a few. However, in the solid state, the meaning of quantum coherence of a quasiparticle excitation is often not straightforward due to undefined decoherence channels, coherence time noise and the difficulty to access some of the quantum coherent properties experimentally directly.
Understanding how heterogeneous materials can create localized quantum states and how to control their interaction with the environment to foster coherent transport or coherent emission, is at the core of my research interest. In my presentation I will focus on our work on 0-D and 1-D heterostructures in 2-D Transition Metal Dichalcogenides (TMDs). 2-D TMDs provide atomically well-defined heterostructures and environments, making correlations between atomic structure and quantum phenomena easier compared to more complex material systems.
We have used photo STM/AFM and correlation microscopy spectroscopy to study vacancies, atomic substitutes and 1-D twin boundaries to correlate the local morphology with the resulting electronic and optical properties with atomic precision. I will discuss mirror twin boundaries in 2-D MoSe2 that from charge density waves in the midst of a 2-D semiconductor, how 2-D MoSe2 and 2-D WS2 carry a zoo of intrinsic point defects that modify substantially electronic properties, such as individual S vacancies that host very sharp defect states in the band gap with extremely high spin orbit coupling. These S defects can be artificially induced, and mediate single photon emission via optical stimulation as well as electric stimulation. C-H for S or Se substitutes in 2-D WS2 and 2-D WSe2 form locally charged hydrogen-like states. Upon deprotonation the localized carbon radical hosts a localized deep in gap state with a net spin and electron-phonon coupling that bears strong similarities to NV color centers in diamond, but in this case with atomistic control. Especially the control of point defects and substitutes opens exciting possibilities to create and study next generation color centers.

Due to strong quantum confinement and reduced screening, defects in 2D semiconductors are predicted to serve as a well-defined quantum system that is optically addressable, similar to NV centers in diamond. Interpretation of optical signatures of such defects, however, is often complicated by the presence of multiple defect species and large inhomogeneous broadening arising from disorder. Introduction of desired defects at appropriate concentrations is key to enable in-depth studies of their optical response. We utilize various in-situ and post-growth techniques to introduce a variety of atomic defects in 2D transition metal dichalcogenides in a controllable manner, and investigate their physical properties in absence of strong extrinsic disorder by hBN encapsulation. Both substitutional transition metal impurities (e.g. Re, Nb, V) and chalcogen vacancy defects give rise to subgap emission peaks near free exciton resonance. They exhibit different magnitude of circular dichroism, indicating partial hybridization with band edge states. Their dependence on chemical potential and temperature further offers insight into whether these states are exciton-like or donor-acceptor-pair-like. Strategies to achieving quantum emitters and their integration into photonic devices will be discussed.

Two-dimensional transition metal dichalcogenides (2D TMDs) are a remarkable class of materials with many applications, including electronic and optoelectronic devices, quantum information systems, and new catalytic materials. In particular, the introduction of zero-dimensional defects exhibit strong confinement and when combining 3d transition metal nanostructures with monolayer TMDs, magnetic interactions may allow spin-valley modulation. 2D TMDs have the benefit of large synthetic variability and the possibility of bottom-up or top-down impurity incorporation, which facilitates precise defect engineering. However, for both understanding how defects alter the material properties, and how engineered defects introduce specific functionalities, it is critical to study the correlation between atomic-scale morphology, altered electronic structure, and macroscopic functionality that is illusive for many of these systems. High resolution scanning probe microscopy (SPM) techniques have the unique ability to examine the structural, electronic, and optoelectronic properties of these materials on the atomic length scale. Previously, we have extensively used low temperature, high resolution SPM to study both intrinsic and induced point defects in TMDs, predominantly in monolayer WS₂, and here, we utilize scanning tunneling microscopy and spectroscopy (STM/S) to understand the electronic and structural properties and combine our experimental results with ab initio calculations to confirm defect identity. We are able to image and identify the adsorbed system at 4 K and show magnetic contrast between defective states due to the nature of our STM tip after preparation. Previously, the introduction of cobalt clusters on TMD by thermal deposition, and subsequent atomic manipulation of adsorbed atoms has been limited to 77 K. However, we are able to show both theoretically and experimentally that single-atom adsorbed states at liquid helium temperatures produce different magnetic anisotropy energies that contribute to the visualized contrast with STM/S. We also employ autonomous exploration measurement techniques, where dense spectroscopic volume is collected via Gaussian process regression and convolutional neural networks are used in tandem for spectroscopic classification and subsequent image segmentation.

Quantum emitters in solids are promising building blocks for quantum information processing, quantum communications, and quantum sensing. Atomically thin materials that host quantum emitters, when compared to three-dimensional materials, have the advantage of reduced total internal reflection and easy coupling with interconnects. In this talk, I will share the story of the discovery and control of quantum emitters in two-dimensional materials. The possibility of leveraging strain-, electric- and magnetic- field tunable van der Waals heterostructure for dynamic modulation of their photophysical properties will be discussed. Further, I will also discuss the progress in their ab-initio prediction, deterministic generation, and integration with photonic devices, which offers a compelling solution to scalable solid-state quantum photonics. Our work opens the frontier of quantum optics in two-dimensional materials with the potential to revolutionize solid-state quantum devices.

Spin defects in two-dimensional (2D) materials such as ultrathin hexagonal boron nitride have been found to be promising single-photon emitters and potential candidates for spin qubits. However, first-principles prediction of accurate defect properties in 2D materials remains challenging, mainly because of the highly anisotropic dielectric screening and strong many body interactions[1]. In this talk, we will show our recent progress on developing first-principles methods for spin defects in two-dimensional materials.

We will show how to correctly handle reduced dimensionality on charged defect formation energies and ionization energies from hybrid functionals and GW approximations, using quantum defects in hexagonal boron nitride as a test-bed system [2,3]. We will also discuss our recent development on radiative[4] and nonradiative recombination methods in low-dimensional quantum defects, including demonstrating the effect of strain on phonon-assisted non-radiative lifetimes of point defects in hexagonal boron nitride[5]. We will show our recent work on discovery of new spin defects with strong exciton-defect coupling and fast intersystem crossing rates[6]. With these predicted rates, we can predict quantum efficiency and spin-dependent PL contrast critical for readout of quantum information from spin defect. In addition, we will show how substrate and thickness affects quantum defects’ optoelectronic properties from first-principles many-body theory.

Lastly, we will briefly show our recently developed methodology for spin dynamics based on real-time density-matrix Lindblad dynamics with electron-electron, electron-phonon, electron-defect couplings and self-consistent spin-orbit coupling fully from first-principles [7,8], which is necessary to predict spin lifetime (relaxation and decoherence) for general systems, critical for quantum-information science, spintronics and valleytronics applications. We show its application on predicting spin and valley relaxation in transition metal dichalcogenides and Dirac 2D materials.

References:
[1] Y. Ping and T. Smart, Nat. Comput. Sci., 1, 646, (2021).
[2] F. Wu, A. Galatas, R. Sundararaman, D. Rocca, and Y. Ping, Phys. Rev. Mater., 1, 071001(R), (2017).
[3] T. Smart, F. Wu, M. Govoni and Y. Ping, Phys. Rev. Mater., 2, 124002, (2018).
[4] F. Wu, D. Rocca and Y. Ping, J. Mater. Chem. C, 7, 12891, (2019), Emerging investigator issue.
[5] F. Wu, T. Smart, J. Xu, Y. Ping, Phys. Rev. B, 100, 081407(R) (2019)
[6] T. Smart, K. Li, J. Xu, Y. Ping, NPJ Comput. Mater., 7, 59, (2021).
[7] J. Xu, A. Habib, S. Kumar, F. Wu, R. Sundararaman, and Y. Ping, Nat. Commun., 11, 2780, (2020)
[8] J. Xu, A. Habib, R. Sundararaman, and Y. Ping, Phys. Rev. B, in press, (2021), Editor’s suggestions. arXiv: 2012.08711 [cond-mat-mtrl-sci].

Layered Transition Metal Dichalcogenides (LTMDs) are an important materials class that exhibit a wide variety of opto-electronic properties. The ability to spatially tailor the expansive property-space (e.g., conduction behavior, optical emission, surface interactions etc…) is of special interest for applications in sensing, bioelectronics, spintronics/valleytronics and more. Current methods of property modulation focus on the modification of the basal surfaces and edge sites by the introduction of defects, functionalization with organic or inorganic moieties, alloying, heterostructure formation, and phase engineering. A majority of these methods lack the resolution for the development of next-generation nanoscale devices or are limited in the types of functionalities useful for efficient TMD property modification. In this study, we utilize electron-beam patterning on monolayer LTMDs in the presence of a controlled atmosphere enabled by an environmental scanning electron microscope (eSEM). A series of parametric studies show the effect of acceleration voltage, beam current, pressure, and electron dose on the optical and electronic properties of the TMDs. Raman and photoluminescence spectroscopies coupled with Kelvin Probe Force Microscopy reveal shifts in the emission linked to doping in the patterned regions. We further speculate on the mechanism of functionalization based on the changes in spectral signatures. Finally, we show the ability to achieve a pattern resolution of ~ 70 nm on the basal plane of the TMDs, demonstrating a robust method to achieve nanoscale functionalization with high fidelity.

Manipulation and Localization of excitons in 2-D semiconducting systems have far fetching impact on the ability to design novel optoelectronic devices, study fundamental phenomena such as excitons condensation, as well as to create next generation single photon emitters, key for quantum information transduction and quantum sensing [1, 2]. 2-D Transition metal dichalcogenides such as WSe₂ are a fascinating material class due to their direct band gap, high emissivity and strong exciton binding energies[3]. Various approaches to engineer the localization of excitons in these materials have been pursued, via strain[4], intrinsic and extrinsic defect creation[2] and heterostructures[5]. While fascinating in their own respect, these approaches are at least in the moment difficult to scale. Using nano optical elements, such as plasmonic nano antennas, to locally enhance the radiative recombination rate by over an order of magnitude is another approach that leads itself to be more scalable that we would like to pursue here. In this presentation, we will introduce a new type nano-cavity configuration which is based on the gold coated pyramid scanning probe and gold nanogrooves on the substrate. By using this near-field probe, we have managed to mapping the highly polarized and localized excitonic emission from the WSe₂, which is deposited on the top on the Gold nanogroove substrate, with an optical resolution below 50 nm. Our work helps to reveal intrinsic physic of the strong exciton-plasmon coupling and may contribute to designing many smart optoelectronic devices.
[1] B. Schuler, K.A. Cochrane, C. Kastl, E.S. Barnard, E. Wong, N.J. Borys, A.M. Schwartzberg, D.F. Ogletree, F.J.G.d. Abajo, A. Weber-Bargioni, Electrically driven photon emission from individual atomic defects in monolayer WS2, Science Advances 6 (2020) eabb5988.
[2] L. Peng, H. Chan, P. Choo, T.W. Odom, S.K.R.S. Sankaranarayanan, X. Ma, Creation of Single-Photon Emitters in WSe2 Monolayers Using Nanometer-Sized Gold Tips, Nano Letters 20 (2020) 5866-5872.
[3] J. Huang, T.B. Hoang, M.H. Mikkelsen, Probing the origin of excitonic states in monolayer WSe2, Sci Rep 6 (2016) 22414.
[4] Y. Koo, Y. Kim, S.H. Choi, H. Lee, J. Choi, D.Y. Lee, M. Kang, H.S. Lee, K.K. Kim, G. Lee, K.D. Park, Tip-Induced Nano-Engineering of Strain, Bandgap, and Exciton Funneling in 2D Semiconductors, Adv Mater 33 (2021) e2008234.
[5] C. Jin, E.C. Regan, A. Yan, M. Iqbal Bakti Utama, D. Wang, S. Zhao, Y. Qin, S. Yang, Z. Zheng, S. Shi, K. Watanabe, T. Taniguchi, S. Tongay, A. Zettl, F. Wang, Observation of moiré excitons in WSe2/WS2 heterostructure superlattices, Nature 567 (2019) 76-80.

Scanning transmission electron microscopy (STEM) provides uniquely powerful tools to study the interfacial structure and interactions of 2D materials up to atomic resolution. In my talk, I will discuss how we create and utilize 2D multilayer stacks to create nanoscale laboratories for studying the atomic structure, structural transformations, and properties of 2D interfaces inside the STEM. For example, we utilize graphene encapsulation in combination with a MEMS-based heating holder to conduct in-situ studies of solid-solid phase transformations and restructuring in 2D transition metal dichalcogenides (TMDCs). Here, graphene encapsulation protects the 2D materials of interest from the vacuum environment of the STEM, enabling in-situ high temperature studies up to 1000°C. We use these structures to directly visualize phenomena such as the layer-by-layer phase transformation of MoTe₂ and the lattice reconstruction of 2D moirés.
We also use custom-built 2D stacks to study the bending and bending-induced properties of 2D multilayers. We find that interlayer interactions play a major role in governing the bending of 2D stacks, where they lead to an unusually low bending stiffness in few-layer graphene and a curvature-dependence of the bending stiffness.[1] For example, we find that the bending stiffness of 4-layer heterostructures containing two layers each of graphene and MoS₂ can be tuned from 20 eV to 80 eV by simply changing the order and interface type of the layers present [2]. The key role of interfaces in the bending of 2D multilayers also indicates potential methods to tune the deformability of 2D systems though interfacial engineering. Our techniques should enable the rational design of interfacial properties of devices based on 2D materials, including illustrating how to incorporate slippable interfaces and create ultra-low bending stiffness structures for deformable, stress-resilient electronics.
[1] E. Han et al., Nature Materials 19, 305-310 (2020).
[2] J. Yu et al., Advanced Materials 33, 2007269 (2021).

Two-dimensional (2D) ferroelectric materials have recently garnered interest for their applications in nanoscale electronics and memory storage devices [1]. The unparalleled flexibility of 2D materials provides a unique opportunity to study coupling between bending and polarization in 2D ferroelectrics to control their electronic properties [2]. Understanding the bend-polarization coupling and electronic properties in 2D ferroelectric materials is crucial for the development of these next-generation devices. We focus on α-In₂Se₃, a van der Waals materials which was recently discovered to exhibit strong out-of-plane ferroelectricity [3,4]. In our work, we study the atomic structure and electronic properties of domain walls in 2D ferroelectric α-In₂Se₃ using a combination of aberration-corrected scanning transmission electron microscopy (STEM), piezoelectric force microscopy (PFM), and density functional theory (DFT).
We discover strong coupling between out-of-plane ferroelectricity and out-of-plane deformation (i.e. bending) in α-In₂Se₃. Using atomic resolution imaging and 4D-STEM, we map the polarization of α-In₂Se₃ in multilayer flakes containing nanoscale bends. We observe two types of domain boundaries: transverse domain walls (TDWs) and lateral domain walls (LDWs), which respectively are perpendicular and parallel to the basal plane. We show that out-of-plane bending can be used to switch the polarization of α-In₂Se₃, producing both TDWs and LDWs. We use atomic-resolution STEM imaging to show that TDWs are atomically sharp, localized at sharp bends, and are created through the formation of planar defects. We also observe strongly charged LDWs in both head-to-head and tail-to-tail configurations, which can readily travel between the In₂Se₃ layers to tune the net polarization of a multilayer flake. Using 4D-STEM supported by DFT simulations, we directly image the charged layers at domain walls in α-In₂Se₃. Our methods should be useful for understanding the atomic and nanoscale interplay between mechanical deformations and ferroelectric polarization in 2D ferroelectrics.
pReferences:
[1] Z. Guan, et al. Advanced Electronic Materials 6, 1900818 (2020).
[2] P. Zubko, G. Catalan, & A. K. Tagantsev. Annu. Rev. Mater. Res. 43, 387-421 (2013).
[3] Y. Zhou, et al. Nano Letters 17, 5508-5513 (2017).
[4] M. Soleimani & M. Pourfath. Nanoscale 12, 11688 (2020).

van der Waals 2D magnetic materials have emerged as a novel platform that offer unique optoelectronic, magnetic, and quantum properties.¹ This low-dimensional spin system has vast potential in applications such as spintronics and nanoscale magnetic devices. Therefore, being able to engineer the structure and defects with respect to magnetic, optical, and electronic properties is critical. For example, an optically active single defect may have potential as a quantum emitter interacting with and embedded in the novel magnetic environment. Air-stable 2D van der Waals magnets are relatively unexplored making the study of such materials intriguing and helpful for both fundamental research and device design.
Here, we exfoliate novel 2D magnets, specifically transition metal phosphorus trichalcogenides (MPX₃) in atmosphere down to mono- and few-layers and encapsulate them in hexagonal boron nitride (hBN). We image the structure of the 2D magnetic material in a scanning transmission electron microscope (STEM) with atomic resolution. We then modify the material using STEM electron probe patterning and/or controlled helium ion irradiation to create and explore different types of point defects, their local structure and concentration, and potential structural transitions arising from the defect introduction. Theoretical and computational modelling is performed to predict the structure and type of defects. Lastly, we relate the calculations to the results we obtain from STEM imaging to establish the fundamental properties of defects in these novel air-stable 2D magnetic materials as a basis for future control and application of magnetic and optical properties.

References
1. Gibertini, M., Koperski, M., Morpurgo, A. F. et al. Magnetic 2D materials an heterostructures. Nat. Nanotechnol. 14, 408-129 (2019).

Ion beam technology has emerged as an attractive option to tailor the properties of materials owing to the unique tunable parameters, including ion species, charge, temperature, and angle of incidence. Further advancements of these techniques and a more thorough understanding of the ion-matter interaction in two-dimensional (2D) MoS₂ is paramount to incorporating ion irradiation as a means to tune interface geometry. Here, we demonstrate that the size, morphology, and concentration of isolated point defects and nanopores can be selectively tuned by varying the metrics of the ion beam. Additionally, we discuss the advantages of ion irradiation to generate a controlled concentration of defects over electron beam irradiation, fabrication processes, and electromagnetic radiation. Our work aims to shed light on non-conventional, effective methods to enhance the optical, mechanical, and electrical properties of MoS₂.

Crystalline imperfections and defects are vital to understanding the structure-property relationship of low-dimensional chalcogenides and their heterostructures. We utilized scanning atomic electron tomography (sAET) to determine the 3D atomic coordinates in low-dimensional chalcogenides and their heterostructures with picometer precision. We measured the 3D atomic displacement and the full strain tensor of monolayers of Re-doped MoS₂ and MoS₂-WSe₂ lateral heterostructures. We observed strong correlations between local atomic strains and Re dopants, S vacancies, and heterostructure interfaces. Furthermore, the experimental 3D atomic coordinates were used as direct input to DFT calculations to correlate crystal defects with the electronic band structure and vibrational properties at the single-atom level. We observed stark differences between the band structures and phonon dispersions obtained from the experimental and relaxed atomic models. The local atomic strains from Re-dopants were found to drastically alter the overall electronic properties and were corroborated by photoluminescence spectra measurements. We expect sAET can be a useful tool to aid in theoretical ab initio calculations by supplying experimental atomic coordinates as direct input to reveal the physics behind physical, material, chemical, and electronic properties in 2D materials.

Monolayer group-VI transition metal dichalcogenides (ML TMDs) exhibit large quantum confinement effects and reduced dielectric screening [1], leading to many-body features such as neutral and charged excitons [2] with binding energies of several hundred of meV. The manipulation of these features via charge injection enables numerous optoelectronic and photonic functionalities such as single-photon generation from light-emitting diodes (LEDs) [3], or valleytronic-based devices for encryption and processing of quantum information [4], among others. However, it has so far been challenging to control excitons and trions in the two-dimensional limit, and a deeper physical comprehension is highly required for developing a feasible tool that enables to tune and enhance the optoelectronic behaviour of these promising novel materials. In this work we study the ultrafast generation and decay of both exciton and trion states in ML TMDs upon the variation of carrier density, introduced by means of the application of gate bias voltages in a FET configuration. We analyse the pump-probe spectra, obtained with a narrow pump above the bandgap and a broadband white probe, originated on a Ti:Sapphire laser with a 2 kHz repetition rate. The setup is coupled with a microscope to focus the pump and probe beams onto the few-micron transistor channel, and with a closed-cycle cryostat for cooling down to temperatures where the exciton and trion features are spectrally well separated. With this ultrafast spectroscopic technique we are able of spectrally resolving the many-body features and disentangle their different dynamics. We demonstrate the gate tunability of the optical response of monolayer TMDs, showing energy renormalization due to many-body effects, shifts on the excitonic peaks, and the sudden formation of trions by the interaction of excitons and injected free charge carriers, when the doping level of the monolayers is not in the neutral point due to the applied backgate voltages. The exciton and trion features form instantaneously within the instrument response function around 500 fs. Our results therefore demonstrate the possibility of electrically tuning the optical response of ML TMDs, and provides a deeper understanding of the ultrafast physical processes that take part in these materials. We also show the generation of stable positively and negatively charged excitons with binding energies of some tens of meV and tunable emission energies by means of applied voltage, and give some insight about their formation lifetimes and recombination mechanisms. This may impact the development of new optoelectronic and photonic devices, based on the control of the light-matter interaction in nanosized volumes of matter.
References
[1] G. Wang, A. Chernikov, M. M. Glazov, T. F. Heinz, X. Marie, T. Amand, and B. Urbaszek, Rev. Mod. Phys. 2018, 90, 021001.
[2] Mak, K., He, K., Lee, C. et al. Tightly bound trions in monolayer MoS2. Nature Mater 2013, 12, 207–211.
[3] X. Lin, X. Dai, C. Pu, Y. Deng, Y. Niu, L. Tong, W. Fang, Y. Jin, X. Peng, Nat. Commun. 2017, 8, 1132.
[4] Morozov, S., Wolff, C., Mortensen, N. A., Adv. Optical Mater. 2021, 2101305.

Transition metal dichalcogenides (TMDs) possess unique spin-valley locked and spin-split K/K’ valleys which have led to many fascinating valley-related phenomena. Furthermore, TMDs also exhibit other conduction band minima with similar properties, the Q/Q’ valleys. Previous reports demonstrated exciting physical phenomena involving K-Q scattering, but those effects are usually hindered in monolayer TMDs because of the energy difference between K and Q valleys. To unlock new multivalley phenomena in monolayer TMDs, it is desirable to reduce that energy difference, while being able to sensitively probe the valley shifts and the multivalley scattering processes. Here, we compress monolayer MoS₂ and WSe₂ at high-pressures and probe K-Q crossing and multivalley scattering via double resonance Raman scattering for the first time. The ability to probe K-K’ and K-Q scattering processes as a function of strain shall shed light on different multivalley phenomena in TMDs such as superconductivity, intervalley quantum interference and valley transport.

Transport of optical excitations in semiconducting solids plays a central role from both fundamental and technological perspectives. In systems with strong Coulomb interaction the propagation of optically injected carriers is dominated by excitons instead of free electrons and holes. In this context, two-dimensional van der Waals materials offer a broad range of strategies to tune excitonic movement employing a variety of approaches, including application of external fields, electrical charge injection, as well as dielectric and strain engineering. Here, I will discuss manipulation of mobile excitons using electrical gating and external strain fields. The focus will be placed on creating quasi-one-dimensional transport channels using combinations of monolayer materials and nanowires, as well as the impact of free charge carriers on the exciton propagation. I will present evidence for externally engineered anisotropic diffusion as well as non-trivial dependence of exciton mobility on free carrier density.

The formation of tightly bound excitons, dominating the electro-optical response of two-dimensional semiconductor materials, such as monolayers of transition metal dichalcogenides (TMDCs), have attracted attention in the solid-state community for the past decade due to a plethora of novel physical properties connected to their strong light-matter coupling, high binding energies and intriguing spin-valley physics. Yet, the local control of exciton propagation and the manipulation of their transport has manifested as a particular challenge so far. Although excitons in monolayer TMDCs are known to be quite mobile with observed diffusion coefficients up to 1-3 cm²/s [1,2], their lack of charge precludes common ways of creating directed transport by applying an electrical potential gradient. However, an alternative route for creating such potentials may be found within their inherent two-dimensional nature since the electrostatically bound excitons are particularly susceptible to changes in their local dielectric environment [3,4]. This opens a novel and convenient way to tailor the excitonic energy landscape in a controlled and non-invasive way, envisioning directed exciton propagation along a well-defined energy pathway created from dielectric patterns obtained through artificial, nanostructured substrates.
In our work we couple monolayers of WSe₂ to nano-patterned substrates with abrupt, repetitive changes in their dielectric constants created through ultra-sharp interfaces of alternating materials. The structures are conveniently created via block copolymer lithography and subsequent atomic layer deposition on large areas, easily scalable to 100's of micrometers in either direction, while achieving nano-pattern dimensions as small as 20nm. From a wide palette of possible patterns that can be fabricated, our initial, prototypical substrates of choice are 50nm interdigitated oxide/air line-grids. This pattern transfers into steep and narrow potential trenches in the excitonic energy landscape through distinct, local dielectric screening, creating a single preferred direction for the excitons to propagate along. We employ time and spatially resolved micro-spectroscopies to systematically monitor exciton propagation across this engineered energy landscape via the phonon-assisted PL emission from long-lived states at cryogenic temperatures.
[1] N. S. Ginsberg and W. A. Tisdale, Annual Review of Physical Chemistry 71, 1 (2020); 10.1146/annurev-physchem-052516-050703.
[2] K. Wagner et al., Physical Review Letters 127, 76801 (2021); 10.1103/PhysRevLett.127.076801.
[3] D. Rhodes, S. H. Chae, R. Ribeiro-Palau, and J. Hone, Nature Materials 18, 541 (2019); 10.1038/s41563-019-0366-8.
[4] M. I. B. Utama et al., Nature Electronics 2, 60 (2019); 10.1038/s41928-019-0207-4.

As technology has advanced, the rush to miniaturization has led to the ultimate limit of
atomically thin, two dimensional (2D) materials. Within this family of materials, the emergence
of atomically thin semiconducting transition metal dichalcogenides (TMDs) has provided
exciting possibilities for advanced 2D electronics, including devices comprised of a single
atomic layer. However, future technology based on TMDs hinges on the capability of creating
nanoscale lateral junctions with large built-in potentials. Following the paradigm of three-
dimensional counterparts, lateral heterojunctions have been directly synthesized by stitching two
different TMD monolayers together, but this approach considerably constrains the attainable
band offsets. However, the extreme 2D nature of TMDs provides other opportunities to
manipulate their electronic properties, as their quasiparticle band gap is not static and depends
strongly on the proximal environment. Recent studies have shown this effect can be harnessed to
engineer a lateral heterojunction in monolayer TMDs by controlling the proximal environment
through substrates, capping layers, and adsorbates. 1, 2, 3, 4 Here we demonstrate the creation of a
nanoscale lateral heterojunction in monolayer MoSe 2 by intercalating Se at the interface of a
hBN/Ru(0001) substrate. The Se intercalation modulates the local hBN/Ru work function, and
this sudden change in the local electrostatic environment is imprinted directly onto monolayer
MoSe 2 to create a large built-in potential of ~0.8 eV. We spatially resolve the MoSe 2 band profile
and work function using scanning tunneling spectroscopy to map out the nanoscale depletion
region. The Se intercalation also modifies the dielectric environment, influencing the local band
gap renormalization and increasing the MoSe 2 band gap by ~0.23 eV. This work illustrates that
environmental proximity engineering provides a robust method to indirectly manipulate the band
profile of 2D materials outsidethe limits of their intrinsic properties, opening alternative avenues for device design.

1. M. Utama et. al., Nat. Electron., 2 (2019) 60-65.
2. J.-W. Chen et. al., Nat. Comm., 9 (2018) 3143.
3. A. Raja et. al., Nat. Comm., 8 (2017) 15251.
4. Z. Song et.al, ACS Nano, 11 (2017) 9128-9135.

Charge-transfer (CT) excitons at hetero-interfaces play a critical role in light to electricity conversion using nanostructured materials. However, how CT excitons migrate at these interfaces is poorly understood. Atomically thin and two-dimensional (2D) nanostructures provide a new platform to create architectures with sharp interfaces for directing interfacial exciton transport. Here we investigate the formation and transport of interlayer CT excitons in van der Waals (vdW) heterostructures based on semiconducting transition metal dichalcogenides (TMDCs) employing transient absorption microscopy (TAM) with a temporal resolution of 200 fs and spatial precision better than 50 nm.
We have investigated interlayer exciton dynamics and transport modulated by the moiré potentials in WS₂-WSe₂ heterobilayers in time, space, and momentum domains using transient absorption microscopy combined with first-principles calculations. Experimental results verified the theoretical prediction of energetically favorable K-Q interlayer excitons and unraveled exciton-population dynamics that was controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. Spatially- and temporally-resolved exciton-population imaging directly visualizes exciton localization by twist-angle-dependent moiré potentials of ~100 meV. Exciton transport deviates significantly from normal diffusion due to the interplay between the moiré potentials and strong many-body interactions, leading to exciton-density- and twist-angle-dependent diffusion length. These results have important implications for designing vdW heterostructures for exciton and spin transport as well as for quantum communication applications.

The influence of external dielectric environments is well understood for 2D semiconductor materials but overlooked for colloidally grown II–VI nanoplatelets (NPLs). In this work, we synthesize MX (M = Cd, Hg; X = Se, Te) NPLs of varying thicknesses and apply the Elliott model to extract exciton binding energies—reporting values in good agreement with prior methods and extending to less studied cadmium telluride and mercury chalcogenide NPLs. We find that the exciton binding energy is modulated both by the relative effect of internal vs external dielectric and by the thickness of the semiconductor material. An analytical model shows dielectric screening increases the exciton binding energy relative to the bulk by distorting the Coulombic potential across the NPL surface. We further confirm this effect by decreasing and recovering the exciton binding energy of HgTe NPLs through washing in polarizable solvents. Our results illustrate NPLs are colloidal analogues of van der Waals 2D semiconductors and point to surface modification as an approach to control photophysics and device properties.

The optoelectronic and transport properties of two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs) are highly sensitive to minute atomic displacements and to their local environments. This sensitivity offers unique opportunities for post-synthesis reconfiguration of material function. In this talk, I will show that the motion of excitons (electron-hole pairs) in TMDs can be manipulated with nanoscale precision by creating nanoscale strain and dielectric inhomogeneities known as nanobubbles. Using ultrafast optical scattering microscopy, we directly monitor exciton funneling into these nanoscale regions with femtosecond resolution and nanometer precision. Our imaging approach reveals that interactions between bright and dark excitons in TMDs depend strongly on the dielectric properties of their local environment, providing new opportunities for sensing as well as a highly effective and non-invasive strategy to manipulate exciton transport at the nanoscale in 2D materials.

Tailoring exciton transport in two-dimensional (2D) semiconductors has been of significant interest in optical and optoelectronic research fields for ultrafast, energy-efficient information processing circuits and interconnects functioning at room temperature. Due to charge-neutral characteristics of exciton, electrical control of exciton transport is limited to exciton subtypes, such as charged trions or interlayer excitons, and has shown low transport efficiency. Elastic strain engineering is another promising mechanism to tune bandgap energy and induce exciton drift via built-in energy gradient. Although intensive efforts have been devoted to experimentally configuring strain-induced 2D exciton drift, its demonstration has mostly limited to indirect evidence of nanoscale confinement, such as spectroscopic interpretation within diffusion-limited length scale. Here, we report strain-induced exciton drift in 2D tungsten diselenide (WSe₂) at room temperature. We employed 3D wrinkle architecture to create reconfigurable local strain and optically resolvable strain gradient. Our 3D wrinkle structure enabled a substantial modulation of exciton energy (> 100 meV) by tensile and compressive strain (Δε = 2.4%) applied to apex and valley of monolayer WSe₂, respectively. Pump-probe photoluminescence (PL) measurement of wrinkled WSe₂ revealed micrometer-scale exciton drift (2.9 μm) and more than an order of magnitude higher fluorescence (>40%) of drifted exciton compared to electrically driven interlayer exciton drift (3%) at a same drift distance. We also demonstrated manipulation of exciton funneling as functions of funneling length and local strain, resulting in asymmetric funneling or tunable fluorescent efficiency. A theoretical model is proposed to elucidate the role of drift, diffusion and dark exciton on strain-coupled exciton dynamics. Our results of elastic strain-induced exciton transport offer future strategies of designing straintronics for optical logic circuits and quantum phase transitions, as well as new opportunities for strain engineering of diverse 2D materials.

Characterizing how heat and electronic energy interconvert and transport at the nanoscale is vital for understanding the fundamental properties of thermoelectric materials. In low-dimensional transition metal dichalcogenides (TMDCs), heat can be generated upon photoexcitation via exciton-exciton annihilation, thermalization and nonradiative decay. These processes can create a localized temperature gradient that drives exciton dynamics with a coupling strength given by the Seebeck coefficient. While this coupling has been observed indirectly by mapping exciton profiles in monolayer TMDCs, we simultaneously measure both heat and exciton transport in few-layer MoS₂ on relevant nanometer and picosecond length- and timescales using a stroboscopic optical scattering microscopy technique called stroboSCAT. To observe this microscopic heat transport for the first time, we tune the stroboSCAT probe wavelength far from excitonic resonances. We then capture a complementary measurement at a probe wavelength closer to the exciton resonances where stroboSCAT is sensitive to both excitonic and thermal energy. We decouple the excitonic and heat contributions to the stroboSCAT signal using separate temperature-dependent reflectance contrast measurements and find that the heat generated nearly depletes the center of the initial exciton distribution. We subsequently observe a recovery of a Gaussian exciton distribution over ~500 ps and a few-ns decay of the excitons. Based on known kinetics in TMDCs we assign the electronic species observed to the indirect exciton in MoS₂. Meanwhile, we observe that heat generated initially transfers to the encapsulating hexagonal boron nitride (hBN) within 300 ps and then transfers more slowly, limited by the rate of heat diffusion in the hBN. We capture these effects with a spatiotemporal model that includes the excitonic excited state dynamics in addition to the transport of heat and exciton species and their cross-couplings. With the ability to directly visualize and decouple heat and excitonic transport on relevant spatiotemporal scales, we are able to estimate the intrinsic Seebeck coefficient, an important material property for predicting thermoelectric performance in energy conversion technologies including power generation and cooling applications.

2D materials and their van der Waals (vdW) heterostructures weakly combined by vdW force have attracted widespread attention because of their unique properties and large degree of freedom in designing novel materials. In these material systems, we can dramatically change their properties by proximity effect. The proximity potential variation of 2D material templates has a significant impact on the growth and crystallization of the deposited material. In this talk, I will introduce how to modify the surfaces and interfaces of 2D materials and their vdW heterostructures to control their properties. We modify the surfaces of 2D materials by using plasma or highly active gas, leading to control of surface properties, such as hydrophilicity, conductivity, and band structure, which enable fabrication of low resistance contact and passivation/dielectric layer. By using encapsulation annealing with hexagonal boron nitride (hBN), we observed ordered recrystallization of metals and the reconstruction of the Moiré interface between two different transition metal dichalcogenides (TMDs) through strong interaction of matters. We also report an abnormal thickness-dependent phase transition in MoTe₂ by using encapsulation annealing. Our observations of the proximity effect on interaction at hetero-interfaces in 2D materials provide deeper insights into understanding behavior and properties of 2D materials and offers abundant choices in manipulation of 2D materials for advanced electronics.

The novel properties of 2D materials, such as graphene, are proposed to enable exciting applications in diverse areas such as sensing, electronics, and mechanics. However, the thinness of 2D materials and their sensitivity to environmental conditions present challenges when examining the fundamental liquid/surface interactions. For instance, it has been shown that the wettability, surface friction, and electrical conductivity of graphene on silicon dioxide (SiO₂) can be tuned by adding bond terminations like hydrogen and fluorine to the graphene lattice. However, there have been few studies examining the surface energy of these chemically functionalized forms of graphene. These studies are important to conduct as they would indicate the types of interactions (dispersive and/or polar) the samples are capable of participating in and the strength of the interaction potential. Further, graphene's wettability, typically characterized using water contact angle measurements, has also been shown to vary with its supporting substrate—a phenomenon known as wetting transparency. This wetting transparency can further be complicated by the adsorption of airborne volatile organic compounds (VOCs) that lower the surface energy of the sample, raising the contact angle, leading to irreproducible results. Thus, to fully leverage the exciting properties of 2D materials for novel applications, we must develop a more rigorous understanding of the interactions between the 2D material, its supporting substrate, and its surroundings.
In this work, we used microgoniometry to examine the wettability of supported bare, fluorinated, and hydrogenated graphene. Using contact angle measurements with deionized water and diiodomethane, we used Fowkes’s method to determine the dispersive and polar surface energies of all samples tested. The graphene samples were functionalized with hydrogen or fluorine bond terminations by indirect exposure to hydrogen plasma and xenon difluoride (XeF₂). The graphene samples were supported by two substrates: Parylene C, a low surface energy polymer commonly used in biomedical and printed circuit board applications, and annealed SiO₂, which has a higher surface energy. We used a dry transfer process combined with thermal annealing to ensure that the samples were cleaned properly to minimize the effects of contaminants on the measurements. The effect of the cleaning and quality of the graphene was verified using XPS and Raman spectroscopy. To limit the effect of VOC adsorption on the contact angle measurements, we stored and transported our samples in an airtight vessel filled with adsorbents and pressurized with argon. We systematically examined the effect of VOCs by exposing the samples to ambient conditions for specific amounts of time. The results indicate that the surface energy (polar and dispersive components) and wetting transparency of the samples depends on the specific bond termination, underlying substrate, and exposure to VOCs. Our results offer guidance to those seeking to use graphene in novel applications where the behavior of the liquid/surface interaction is of paramount importance, such as liquid phase sensors, actuators, and surface modifiers.

XeF2 gas is an isotropic and controlling etchant of Si, which has been widely used as an etching gas for micro-electro-mechanical systems (MEMS) processes, and it has recently been used for the fluorination of graphene and etching of h-BN.
When F radical is produced via dissociation from XeF2 molecules, graphene is fluorinated or h-BN is etched. Studies on both fluorination and etching via XeF2 gas treatment have been extensively reported, however, there is a lack of study reporting that Si acts as a chemical mediator in the dissociation process of XeF2.
Here, we report the role of Si as a chemical mediator for the dissociation of XeF2 gas. In Si-free environments, XeF2 was not dissociated, so graphene was not fluorinated and h-BN was not etched. In contrast, with Si, graphene was fluorinated and h-BN was etched. This experimentstudy showsfound that Si is the chemical mediator dissociating XeF2 to generate F radicals that is a key element for both fluorination of graphene and etching of h-BN.
This study is expected to provides highly useful information on the control of the XeF2-bases chemical reaction using Si and the mechanism in chemical reaction for on the fluoridation of graphene and the etching process control of h-BN using via XeF2 gas.

Oxygen evolution (OER) and oxygen reduction (ORR) reactions are the key electrocatalytic redox couple for advanced energy storage/conversion, including rechargeable metal-air batteries and regenerative fuel cells. Heteroatom doped carbon catalysts propose a promising candidate for such purposes along with the superior durability and cost-effectiveness. Unfortunately, exact identification of the catalytic site as well as the critical role of dopants is still controversial in the catalytic mechanism. Here we present bifunctional catalytic site of nitrogen pair-doped graphene nanoribbons for precisely switchable OER and ORR. Pyrazolated N2-edges of graphene nanoribbon serve as switchable dual-functional active sites for OER/ORR with efficient activities and extraordinary durability. Theoretical calculation reveals genuine catalytic mechanism originating from the electrochemical potential-dependent molecular absorption and conversion at the atomic level dopant site. This judiciously controllable bifunctional electrocatalytic activity of dopant catalyst fundamentally addresses the interference between ORR and OER and attains highly stable rechargeable metal-air battery with long-term stability.

Two-dimensional (2D) Janus materials are atomically thin membranes with asymmetric structures on surfaces, and those have attracted attention due to unique properties resulting from the different surfaces. However, the fabrication technique for selectively different functionalization is still challenging, and there is a lack of experimental study on the dipole moment in Janus graphene.
Here, we present the way to fabricate H-C-F type Janus monolayer graphene and its dipole properties. Hydrogen plasma and XeF2 gas treatment were used for hydrogenation and fluorination, respectively. We observed the strong dipole moment of H-C-F type Janus graphene via PiFM (Photo-induced Force Microscopy) and EFM (Electrostatic Force Microscopy). Moreover, we observed that the surface-enhanced Raman scattering (SERS) signal of the diluted rhodamine was strong on the Janus graphene surface. We expect this material will be useful for future electronic devices and sensing technologies.

Ferroelectric field-effect transistors (FeFETs) employing graphene on inorganic perovskite substrates received considerable attention due to their interesting electronic and memory properties. They are known to exhibit an unusual hysteresis of electronic transport that is not consistent with the ferroelectric polarization hysteresis and was previously shown to be associated with charge trapping at graphene-ferroelectric interface. In this study, we demonstrate an electrical measurement scheme that minimizes the effect of charge traps and reveals the polarization-dependent hysteresis of electronic transport in graphene-Pb(Zr,Ti)O₃ FeFETs. Observation of the polarization-dependent conductivity hysteresis is important for the fundamental understanding of the interplay between the ferroelectric polarization and charge carriers in graphene. It is also important for practical memory applications, because this hysteresis emulates the operation of nonvolatile memories and reveals the range of ON and OFF currents that can be achieved in long term data storage. We demonstrate that this measurement scheme can be used to optimize the memory performance of graphene-PZT FeFETs that can exhibit nonvolatile time-independent ON/OFF ratios. The described measurement technique can be potentially used in the studies of kinetics of charge trap dissipation, polarization-dependent properties and memory performance of FeFET devices comprising other two-dimensional materials and various ferroelectric substrates.

MoS₂ has attracted the attention of researchers owing to its high light absorption efficiency. However, the covered spectra of the reported MoS₂ are restricted to the visible range owing to their electronic bandgap. Strain engineering, which modulates the bandgap of a semiconductor, can extend the application coverage of MoS₂ to the infrared spectral range. The shrinkage of the bandgap because of the tensile strain on MoS₂ enhances the photoresponsivity in the visible range and extends its sensing capability beyond its fundamental absorption limit. In this talk, I report a MoS₂/graphene metal-semiconductor-metal photodetector (PD) array with a strain-modulated photo-response up to the spectral range of the near-infrared (NIR). MoS₂ PD array on a flexible substrate was stretched in the biaxial direction to a tensile strain level of 1.19% using a pneumatic bulging process. The MoS₂-based line-scanning system was implemented by digitizing the output photocurrent of the strained MoS₂ linear array with a low-noise complementary metal–oxide–semiconductor readout integrated circuit and successfully captured Vis-NIR images in foggy conditions. Therefore, we extended the application of the MoS₂ PD array to the NIR range and demonstrated its use in real-life imaging systems.

Two-dimensional (2D) materials have been explored to create the next generation of integrated electronic and photonic circuits. This class of materials possess a wide variety of strain-sensitive electrical, optical, mechanical, magnetic, superconducting, and topological properties available to control. We introduce a controllable approach to selectively strain (uniaxially or biaxially) MoS₂ by depositing e-beam evaporated thin film materials with intrinsic stress that are lithographically patterned in a stripe geometry. This type of strain engineering has been highly successful in commercial silicon-based CMOS processes to enhance carrier mobility by applying uniaxial strain in MOSFET channels. In these processes, it has been critical that within a single chip, the strain can be selectively controlled for magnitude, compression/tension, uniaxiality/biaxiality, and directionality relative to the silicon crystal axes for each transistor in a densely integrated system. We attempt to outline the basis for using a similar technique in 2D van der Waals materials with weak out-of-plane bonding. The stressor in this work is chosen to be optically transparent in order to examine the strain distribution within MoS₂ using Raman spectroscopic mapping. Tensile stressors partially covering MoS₂ flakes produce large tensile strains close to free edges and compressive strain at the center of the stressor strip. By varying strip width and MoS₂ flake thickness, the geometric distribution of both tensile and compressive strained regions can be controlled. The directionality of strain induced by the stressor strip is also explored through polarized Raman spectroscopy where MoS₂ shows uniaxial strains occurring at strip edges and biaxial strains occurring at strip centers using the same stressor. The strain magnitude at the edges is estimated to be 0.85% for stressor film force of 25 N/m and can be controlled with the stressor film thickness. Using these combined techniques, we show that strain in 2D materials can be uniquely engineered by design to selectively exhibit tension/compression, uniaxiality/biaxiality, and directionality relative to crystal axes of 2D materials through simple lithographic patterning of stressed thin films. ¹ Using this strain engineering technique, we demonstrate the possibility of moving strain solitons and reconfiguring the stacking order in trilayer graphene.
¹ A. Azizimanesh, T. Peña, A. Sewaket, W. Hou, and S. M. Wu, Applied Physics Letters, 118, 213104 (2021).
This work was supported by the National Science Foundation under Grant No. OMA-1936250, ECCS-1942815, and DGE-1939268.

Transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS₂) are promising candidates for scaled nanoelectronics but require improvements in their on-state transistor currents in order to compete with existing technologies. Theoretical studies have predicted that tensile strain can improve the mobility of TMDs, by modifying the band structure, reducing intervalley scattering and effective mass [1]. However, experimental studies of tensile strain induced in MoS₂ have largely been done using flexible substrates, which cannot be applied to conventional semiconductor industry manufacturing [2]. While process-induced strain techniques are extensively used for modern strained-silicon technology [3-4], such approaches have not yet been investigated for TMDs.

Here, for the first time, we implement uniaxial tensile strain locally at the metal source/drain contacts to improve the electrical performance of monolayer MoS₂ transistors on rigid, SiO₂/Si substrates compatible with conventional semiconductor manufacturing. Monolayer MoS₂ is first grown by chemical vapor deposition on 90 nm SiO₂ on p⁺ Si substrates [5] which also serve as back-gates. We fabricate transistors using e-beam lithography to define the channel by XeF₂ etching and to pattern the source/drain contacts using 50 nm e-beam evaporated Au. We then evaporate 50 nm Ni with high tensile stress onto the top contact metal stack, in order to induce tensile strain locally near the contacts of the device.

We characterize the magnitude of tensile strain in the MoS₂ transistors before and after Ni deposition using Raman and photoluminescence (PL) mapping of the channel region. The Raman E’ peak and PL A exciton peak reveal a spatial distribution of tensile strain induced by Ni deposition, with > 0.5 % tensile strain estimated to be present near the contact regions using empirical methods [6].

We also measure the electrical characteristics of transistors with channel lengths of 500 nm and 2 μm before and after Ni deposition, to investigate the effect of stressed metal contacts on electrical performance and its possible geometrical dependence. We obtain a drain current improvement up to ~33% after 50 nm Ni deposition, accompanied by ~10% improvement in field-effect mobility and shifting of the threshold voltage to more negative values. Finite-element simulations indicate a large increase in tensile stress in MoS₂ near the edge of the metal contacts and project a further increase in the tensile strain in the channel region, and thus the mobility, for channel dimensions below 50 nm.

These results demonstrate the first implementation of contact-metal induced strain in MoS₂ transistors to date and show that strain engineering is a promising way to tune the electrical performance of TMD-based devices.

[1] M. Hosseini et al., J. Phys. D: Appl. Phys. 48, 375104 (2015)
[2] S. Yang et al., InfoMat 3.4, 397-420 (2021)
[3] S. Thompson et al., IEEE Electron Device Lett. 25.4, 191-103 (2004)
[4] C.Y. Kang et al., IEEE Electron Device Lett. 29.5, 487-490 (2008)
[5] K. Smithe, E. Pop et al., 2D Materials 4, 011009 (2017)
[6] A. Michail et al., Appl. Phys. Lett. 108.17, 173102 (2016)

Two-dimensional (2D) bilayers have an extensive list of (opto)electronic properties that may be intentionally modified with strain and relative twist angle between the two layers. Industrial CMOS technology has long implemented process-induced straining techniques, to combat deteriorating transistor performance with geometric scaling. One technique often utilized is thin film stressor encapsulation, where a highly stressed SiN_x thin film is deposited, leading to strain transfer into the substrate underneath due to stress relaxation in the SiN_x layer. Thin film stressor encapsulations were adopted quickly, since they allow strain to be tailored to individual devices for strain magnitude and direction. We have proven that this concept can easily be applied to 2D systems, where we accomplish both tensile and compressive strain transfer into 2D MoS₂ with evaporated thin film dielectric encapsulations¹. We observed these 2D-bonded systems naturally have incomplete out-of-plane strain transfer, where the strength of the system’s interlayer coupling will determine how many succeeding layers strain will penetrate from the top strained layer². Graphene and MoS₂ have comparatively weak interlayer coupling, consequentially this allows for heterostrain to be present because only the top layer will be strained. Here, we probe heterostrain in MoS₂ and graphene homobilayers with highly stressed dielectric encapsulations and Raman spectroscopic mapping. We then inspect twisted bilayer graphene (tBLG) structures beyond the magic angle, this way we can probe heterostrain by quantifying peaks shifts of the twist induced Raman modes (R-bands) with thin film force application. We conclude by discussing the (opto)electronic device implications of this work, where we show that this method is both time and temperature stable, therefore any number of 2D strain-based devices can be actualized from these findings. More importantly, time and temperature-stability open the door to potentially using this method to further explore a wide-range of materials’ properties, such as low-temperature correlated electron physics in heterostrained low-angle tBLG, or even heterostrained moirè excitonic behavior in 2D heterobilayers.

[1] T. Peña, et. al., 2D Mater. 8, 045001 (2021).
[2] S. A. Chowdhury, et. al. J. Eng. Mater. Technol. 144, 011006 (2021).

We wish to acknowledge support from the National Science Foundation (OMA-1936250 and ECCS-1942815) and the National Science Foundation Graduate Research Fellowship Program (DGE-1939268). This work also made use of the Cornell Center for Materials Research Shared Facilities, which are supported through the NSF MRSEC program (DMR-1719875).

In our previous work, we introduced a new type of ‘straintronic’ device to control physical properties of 2D materials using electric-field controllable dynamic strain [1]. We demonstrated that multilayered transition metal dichalcogenide MoTe₂ can be reversibly switched between the 1T’ semimetallic phase and a semiconducting phase in a three-terminal transistor-like device geometry, achieving large non-volatile control of the channel conductivity. This was achieved by combining thin film stressed capping layers as contacts and relaxor ferroelectric Pb(Mg_1/3Nb_2/3)_0.71Ti_0.29O₃ (PMN-PT) as the gate dielectric, with the former setting a higher static strain in the MoTe₂ and the latter applying a gate controllable ferroelastic strain. Here, we show that the thin film stressor can also result in large strain gradients near the surface of the ferroelectric, causing a flexoelectric field comparable with the coercive field of PMN-PT. By controlling the film force of the thin film stressor, we can continuously tune the internal bias to control ferroelastic strain applied by the ferroelectric vs. applied electric field, thereby achieving control of ferroelastic non-volatility. Moreover, by combining thin film strain engineering with PMN-PT containing different intrinsic internal biases, we show MoTe₂ phase change devices with the same engineered non-volatility, demonstrating the full control of the non-volatile functionality in our ‘straintronic’ devices.
[1] W. Hou, A. Azizimanesh, A. Sewaket, T. Peña, C. Watson, M. Liu, H. Askari, and S. M. Wu, Strain-based room-temperature non-volatile MoTe₂ ferroelectric phase change transistor, Nature Nanotechnology 14, 668-673 (2019)
ACKNOWLEDGEMENTS
This work was supported by the National Science Foundation under Grant No. ECCS-1942815.

Flexible semiconductor materials, where structural fluctuations and transformation are tolerable and have a low impact on electronic properties, focus interest for future applications. Two-dimensional thin-layer lead halide perovskites are hailed for efficiency and a high degree of compliance in devices. I will show how strain engineering in two-dimensional CsPbBr₃ perovskite nanobelts results in a structural deformation when adsorbed on carbon substrates. The microstructure is indicative of buckled nanobelts is determined using transmission electron microscopy. We develop and use using our original selected orientation dark-field imaging (SODFI) technique.¹ This method enabled the collection of scattered electrons from solid angles and traced them back to the specific orientation of the crystal. Apparent emission was measured from the buckled nanobelt using cathodoluminescence, signifying tolerance to mechanical deformations of the electronic properties. By employing plate buckling theory, we approximate adhesion forces between the buckled nanobelt and the substrate, to be F_adhesion ~0.12uN, marking a limit to sustain such deformations. Post synthetic engineering of surface ligands may be used to control these forces and minimize the buckling. I will also demonstrate how near-field SNOM correlates between the strained structures and their optical and electronic properties by combining CAFM and SNOM measurements. This work opens possibilities for controlling the properties of halide perovskite nanostructures through rational strain engineering.
(1) Massasa, E. H. et al. Thin Layer Buckling in Perovskite CsPbBr₃ Nanobelts. Nano Lett. 21, 5564–5571 (2021).

The interfaces between two-dimensional (2D) materials and 2D-3D materials host rich and diverse phenomena and applications. However, most studies only show a rather local picture of these interfaces. The understanding of the interface interaction including interface flatness on a larger scale remains limited. Here we present the flatness of 2D-2D and 2D-3D interfaces in arrays of field-effect transistors with a total length of over 16 µm. This approach shows a more complete picture of the 2D interfaces from multiple metal contacts and transistor channels, with and without hexagonal boron nitride (hBN) encapsulation. By using cross-sectional transmission electron microscopy (TEM), we observe that transferring hBN onto contacted 2D materials can dramatically alter the interface structure. Intriguingly, the interface between 3D metal and 2D materials is also not flat. We observe highly non-uniform adhesion between evaporated metal on 2D materials. Finally, we correlate the interface characterization with electrical device measurements, highlighting that interface dynamics can profoundly impact device operation in transistors and beyond. Our results reveal the intricacy and complexity of 2D interfaces and challenge the conventional assumption that 2D interfaces are always flat and uniform.

Oxide materials have enormous potential as the fundamental building block of new generations of electronic, magnetic, optical and electromechanical devices. We create these materials by artificially layering epitaxial materials with atomic layer control, and also stacking epitaxial membranes of materials with incompatible structure and lattice constant [1]. Our goal is to create new materials and heterostructures with novel properties likely to advance basic science and future applications. I will present advances in assembled single crystal functional oxide membrane heterostructures, including giant piezo-driven low-voltage magnetoelectric coupling in ferromagnetic/piezoelectric membrane heterostructures [2] and electronically reconfigurable LaAlO₃/SrTiO₃ complex-oxide heterostructure free-standing membranes [3], and highlight several examples of how our research has played a role in understanding fundamental solid state phenomena and in the discovery of new materials. I will discuss the challenges and opportunities in this exciting field.
[1] H. S. Kum et al. Nature, 578, 78 (2020)
[2] S. Lindemann et al., Science Advances, in press (2021)
[3] K. Eom et al., Science Advances, 7, eabh1284 (2021)
This work has been done in collaboration with S. Lindemann, J. Irwin, G.-Y. Kim, B. Wang, J. J. Wang, J. M. Hu, L. Q. Chen, S. Y. Choi, M. S. Rzchowski, K. Eom, M. Yu, J. Seo, D. Yang, H. Lee, J.-W. Lee, P. Irvin, S. H. Oh, J. Levy.
This research is funded by the Gordon and Betty Moore Foundation’s EPiQS Initiative, Grant GBMF9065 to C.B.E., a Vannevar Bush Faculty Fellowship (N00014-15-1-2847), the Army Research Office through Grant W911NF-17-1-0462, and the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, (DE-FG02-06ER46327).

The ability to create and manipulate materials in two-dimensional (2D) form has repeatedly had transformative impact on science and technology. In parallel with the exfoliation and stacking of intrinsically layered crystals, the atomic-scale thin film growth of complex materials has enabled the creation of artificial 2D heterostructures with novel functionality and emergent phenomena, as seen in perovskite oxides. We present a general method to create freestanding complex oxide membranes and heterostructures using epitaxial water-soluble buffer layers, with millimeter-scale lateral dimensions and nanometer-scale thickness. This facilitates many new opportunities we are beginning to explore; here we will focus on the mechanical response of these membranes and the ability to tune their properties by bending and stretching.

Ferroelectric materials are useful for their application in transistors, nonvolatile memory, solar cells, energy harvesters, and sensors. Under mechanical deformation, the symmetry of a ferroelectric material is broken which offer a facile way to modulate their electronic properties. However, this phenomenon is not fully realized in conventional thin film ferroelectrics made of bulk materials due to their high bending stiffness. In this regard, two-dimensional ferroelectric materials, because of their ultrathin and flexible nature, offer a great opportunity to realize modulation of electronic properties by mechanical means. We focus on α-In₂Se₃, a van der Waals 2D material which has recently been confirmed to show strong out of plane ferroelectricity. The key questions are how does the ferroelectric dipole couple to changes in piezoelectric and flexoelectric moments under large curvatures, and how do energetics of mechanics and bending compare to the switching energy between ferroelectric domains.
In this study, we investigate the modulation of out-of-plane ferroelectric properties in highly localized bends in few layer α-In₂Se₃. We examine two structures: highly localized buckle delamination in multilayer flakes that are spontaneously created because of residual stress in the material during the mechanical exfoliation process, and smooth bends with controlled curvatures made by laminating multilayer α-In₂Se₃ onto graphite steps creating a heterostructure. We use piezoelectric force microscopy (PFM) and Kelvin Probe Force Microscopy (KPFM) to examine tuning of the piezoelectric moment in and near the bends, and scanning transmission electron microscopy (STEM), to examine the atomic structure within the bends.
PFM and KPFM distinguish the local polarization direction and converse piezoelectric coefficient of the α-In₂Se₃. We measure the out of plane converse piezoelectric coefficient of flat α-In₂Se₃ to be 4-6 pm/V consistent with the value mentioned in the literature [1]. For the curve-controlled structures, we observe curvature and thickness dependent tuning of the converse piezoelectric coefficient and surface potential at the bends. We observe that the piezoelectric coefficient decreased by 80% and the surface potential increased by 10% for a 25 nm flake on a 30 nm graphite step. For the highly localized buckled structures, STEM reveals a spontaneous flip of polarization across the buckle. The polarization switching resides only in the buckled few layers and does not propagate perpendicular to the basal plane and thus creates two types of domain boundaries, charged lateral domain walls and charge neutral transverse domain walls, which are parallel and perpendicular to the basal plane, respectively. PFM and KPFM reveal that the presence of lateral domain walls within the heterostructure leads to an increase in out of plane converse piezoelectric coefficient of the material by 100% to 800% and surface potential by 40% to 300%. The strong enhancement cannot be explained purely from the net change in polarization. We hypothesize that the difference is explained by the presence of conductive states at the charged domain boundaries.
Our study indicates the potential for dramatically reconfiguring the properties and structure of 2D ferroelectrics using curvature, both through continuous tuning and through domain flipping. The results have applications for 2D ferroelectrics in mechanically reconfigurable devices leading to next generation sensors, memory devices, and energy harvesters by actively utilizing out of plane deformation.


References:
[1] F. Xue, et al. ACS Nano 12_, 4976-4983 (2018)

Abstract: Miura-ori pattern is seen in nature be it the unfolding of a leaf or opening and closing of insect wings. Similar patterns in the form of wrinkles and ridges are also observed in biaxially compressed rigid thin films supported on soft substrates. We have investigated the formation of wrinkles in graphene placed on a polyethylene substrate using both quantum (QXMD) and classical molecular dynamics (MD) simulations. QXMD is a massively parallel quantum molecular dynamics software with various “eXtensions” such as non-adiabatic dynamics for the study of light-induced electronic excitations. Classical MD simulations are performed with quantum mechanically informed force fields. We have observed wrinkles and ridges in biaxially compressed graphene sheets and calculated the electronic, mechanical, and thermal properties of the system. We will present QXMD results for electronic properties, and large-scale MD simulation results for mechanical and thermal properties of the system.

Acknowledgments
This work was supported by the National Science Foundation, Future Manufacturing Program, Award 2036359. The simulations were performed at the Centre for Advanced Research and Computing of the University of Southern California.

2D Material based electro/optomechanical systems are emerging platforms for realizing next generation technologies such as advanced sensors, actuators, and reconfigurable quantum states. Graphene has been demonstrated as an ideal resonant element for such system due to its large surface area, low mass, and high strength. However, understanding and further tailoring the vibrational properties of a suspended graphene membrane are still rudimentary and urgently required for developing applications with new functionalities. In this work, we investigate the correlation between the graphene membrane geometry and its resonant modes. Various designs of graphene mechanical resonators are modeled by COMSOL and experimentally characterized using a Fabry–Pérot interferometry. This technique provides the guidance to precisely tune the resonant modes and further mechanical properties of a suspended graphene membrane, which paves the path for optimizing such optomechanical transducers for various applications.

Two-dimensional (2D) transition metal dichalcogenides (TMDs), especially monolayer (1L) MoS₂, have shown promise for future nanoelectronics, maintaining good mobility at sub-1 nm thickness [1]. However, due to lack of dangling bonds at the inert surface of 2D TMDs, it is difficult to use atomic layer deposition (ALD) for uniform ultrathin gate dielectrics on these materials [2]. Few efforts have demonstrated low equivalent oxide thickness (EOT) gate dielectric for top-gated (TG) field-effect transistors (FETs) using 1L TMDs as the channel material, but most approaches lack compatibility with modern complementary metal oxide semiconductor (CMOS) processing [3,4]. For example, we have shown that ultrathin Al films on MoS₂ oxidize into sub-stoichiometric AlO_x [4], which was used as a TG dielectric. However, this process lacks accurate control of the oxide thickness and is difficult to adapt for ultrathin EOT with low leakage.
Here, we investigate ultrathin electron (e)-beam evaporated Si, Ge, Al, and Hf seed layers to provide ALD nucleation sites for top-gated monolayer MoS₂ transistors. These seed layers must enable uniform deposition of a high-κ insulator, but must be thin enough to completely oxidize and not limit the EOT. We also characterize the damage or reactions between these seed layers and 1L MoS₂, as the channel material must not be harmed during this process. Samples are prepared by growing 1L MoS₂ using chemical vapor deposition (CVD) on SiO₂/Si substrates [5]. For first test purposes, ~2 nm of seed layer material (Si, Ge, Al or Hf) was evaporated at ~10^-7 Torr, and the interfaces were analyzed using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). We find that Si and Ge are completely benign to 1L MoS₂, with negligible changes to the Raman and XPS spectra after their evaporation. We also see evidence that Al (which oxidizes to AlO_x) dopes 1L MoS₂ n-type based on A₁’ peak and a Fano lineshape in the E’ peak, but previous works have shown that the mobility of the underlying MoS₂ is not affected [6]. On the other hand, we find by both Raman and XPS analysis that Hf reacts with 1L MoS₂, making it a poor seed layer candidate.
We next fabricate top-gated MoS₂ FETs with channel lengths from 200 nm to 3 μm. Devices are based on CVD-grown monolayer MoS₂ on SiO₂/Si substrates [5], with pure Au contacts [1] to source and drain. The top oxide is obtained by e-beam evaporating ~1 nm of Si, Ge, Al or Hf followed by ~5 nm of thermal ALD HfO₂. Top gates are patterned by e-beam lithography, e-beam evaporation of Pd, and lift-off. The best devices were obtained with the Si seed layer, which enabled < 2 nm EOT, subthreshold swing of ~120 mV/dec, and on/off current ratios >10⁷. In 200 nm MoS₂ FETs with Si seed layer, we achieve high on-state current of ~420 μA/μm at 2 V drain bias and high top- and back-gate bias. When both the top gate and drain are set to 1 V, we observe record drain current of ~220 μA/μm for this bias condition. Leakage current densities with 1 V top gate bias are < 5×10^-5 A/cm² with the Si seed layer, over 2x lower than recommended for low power logic application [3].
These results offer a semiconductor-manufacturing-compatible approach to realize ultrathin gate stacks on 2D TMDs, achieving low EOT of < 2 nm, transistors with high on-state current, and maintaining low gate leakage. With further improvements in contact resistance and ALD, these results could be extended to achieve 2D transistors with sub-1 nm EOT and even higher performance at low voltage.
[1] C. English, K. Saraswat, E. Pop et al., Nano Lett., 16, 3824 (2016)
[2] H. Kim, H. Lee, Chem. Mater., 29, 3809 (2017)
[3] W. Li et al., Nature Electronics, 2, 563 (2019)
[4] C. English, E. Pop et al., IEDM (2016)
[5] K. Smithe, E. Pop et al., 2D Materials 4, 011009 (2017)
[6] K. Schauble, E. Pop et al., ACS Nano, 14, 14798 (2020)

Real time monitoring of chemical exposure events is crucial for occupational monitoring. However, current hand-held monitors are bulky, cannot identify the specific hazard, and are not designed for continuous use. Therefore, conformal wearable sensors and devices that can identify and quantitate hazardous compounds need to be further developed. Novel sensing materials are required to provide the requisite selectivity and sensitivity for next generation wearable atmospheric monitors. Two-dimensional (2D) materials are an emerging class of nanomaterials that have utility in diverse applications, including biochemical sensors. 2D materials, including graphene (Gr) and molybdenum disulfide (MoS2), have been demonstrated in a multitude of applications, including in biochemical sensing. Most commonly, 2D materials are deposited as nanoflakes, creating a crystalline network that can have its properties modified by the composition/blend of 2D materials selected. However, substantial work remains to understand how electrode material and electrode geometry impact sensor performance. Impedance spectroscopy (IS) provides a robust method towards material characterization. More specifically for nanomaterial inks, IS can interrogate and resolve intraflake resistance (Rintraflake), interflake resistance (Rinterflake), and intraflake capacitance (Cintraflake). This is especially important for atmospheric sensing, as the interfaces of these 2D materials are sensitive to the electrostatic environment, including absorbed analytes. Combining these 2D material electrodes with semi-selective barriers has the promise to enable creation of sensitive, selective atmospheric sensors. Here we present progress towards development of flexible, real-time atmospheric sensing through optimization of 1) the sensor, comprised of 2D materials, 2) the electrode geometry, 3) the interrogation parameters, and 4) the polymeric barrier. In this proceeding, we present specific examples of the importance of each of these parameters, as well as in-depth discussions of key achievements.

Seamless integration of two-dimensional (2D) semiconductors with bulk materials is essential for preserving and exploiting their outstanding optoelectronic properties within functional devices. A major challenge is to contact mono- and few-layers with metals without introducing defects or otherwise impeding interfacial charge transport. Here, we demonstrate encapsulation and doping of monolayer MoS₂ with van der Waals (vdW) bonded aluminum oxide (AlO_x) by atomic layer deposition (ALD) at 40 °C. The 18 Å thin AlO_x coating increases the carrier concentration (from 1×10¹² cm^-2 to 2×10¹³ cm^-2), while it also protects monolayer MoS₂ from defect creation during metallization and processing. We utilize in situ spectroscopic ellipsometry for the first time to assess the effects of adsorbed reactants and film formation on the dielectric function and excitonic properties of a vdW material during ALD, thus allowing optimization of AlO_x film growth and adlayer modulation doping in real-time. Encapsulated monolayer MoS₂ field-effect transistors exhibit a reduced contact resistance and over an order of magnitude larger maximum drive current, I_ON. By alleviating the effects from the contact interfaces, we were able to reliably determine a field-effect room-temperature mobility of ~10 cm²/Vs for the applied monolayer MoS₂, synthesized by chemical vapor deposition on a large scale (SixCarbon Technology, Shenzen). Overall, this work demonstrates the scalable and damage-free encapsulation and doping of 2D materials with weakly bonded and ultrathin AlO_x by ALD near room temperature, as well as the fabrication of tunnel contacts, readily compatible with polymer and lift-off processing. Beyond the demonstrated application as a contact interfacial layer, the nanometer-thin conformal coating is potentially relevant for surface functionalization in chemical sensors and for modulation doping of 2D and organic materials and their heterojunctions implemented in optoelectronic devices.

Because of unfavorable scaling laws of friction, there are entire classes of mechanical devices such as rotating motors, gears, and actuators which cannot be easily realized at the micro and nanoscale. 2D materials offer a new opportunity because incommensurate van der Waals interfaces display structural superlubricity, with orders of magnitude lower friction than conventional materials [1] . While superlubricity has been extensively explored via nanotribology and external manipulation of nanostructures, there is very little work exploring how superlubricity may be actively incorporated into a nanoelectromechanical system. In addition, the extensive library of 2D materials and their heterostructures offer unprecedent opportunities for micro and nanoelectromechanical systems (MEMS/NEMS), such as sensors and actuators that transduce across multiple physical domains and are simultaneously smaller, more responsive, and consume less power than current technologies, as well as mechanically-reconfigurable optical and electronic properties. Two key questions are how interfacial slip and friction will affect the dynamics and dissipation of heterostructure-based 2D NEMS and what kinds of devices are now realizable which actively utilize slip in operation.
First, we directly probe the dissipation in different types of 2D interfaces by comparing the dynamic response of mechanical drumhead resonators made from Bernal-stacked and twisted bilayer, and monolayer graphene [2]. On average, Bernal-stacked and twisted bilayer graphene have dissipation rates which are 70 and 30% larger than the monolayer case respectively. Then, we build a lumped element model which calculates the effective dissipation in terms of hysteresis in stress during the resonance cycle resulting from interlayer slip and friction at the van der Waals interface. The model shows that the higher dissipation rate results from a combination of a stiffness change and energy dissipation arising from friction values that agree well with nanotribological measurements [1]. We find that even the smallest friction can significantly contribute to the dissipation in multilayer 2D NEMS, although it is not sufficient to completely damp motion.
Second, the model we developed is generalizable to predicting and designing the dissipation in 2D heterostructure NEMS which actively utilize slip. We will show preliminary modelling and experimental data of a new class of NEMS consisting of an electrostatically actuated gold/graphite heterostructure sliding in a boron-nitride substrate. Using this geometry, we will explore low-power switches and oscillators, as well as mechanically reconfigurable electronic devices incorporating static and dynamic slip at the microscale.
[1] Koren, E. et al. Adhesion and friction in mesoscopic graphite contacts. Science 348, 6235 (2015).
[2] Ferrari, P. F. et al. Dissipation from Interlayer Friction in Graphene Nanoelectromechanical Systems. Nano Letters 21, 19, 8058-8065 (2021).

Over the last three decades, carbon (one of the most abundant materials found on earth) and carbon nanomaterials (carbon allotrope forms such as 0D fullerenes, 1D carbon nanotubes, and 2D graphene) have attracted significant attention due to their unique electronic, thermal, mechanical, and chemical properties. Recent advances in the synthesis and assembly techniques have renewed interest in employing carbon nanomaterials as the basis of electronic applications, and the flexibility and the low cost of these materials provide the opportunity for many applications such as wearable, disposable, and next-generation electronics. In addition, in these material systems, we can dramatically change their properties by engineering the surface, defect, phase, and interface.
In this talk, I will deliver the recent progress regarding the synthesis of carbon nanomaterials and provide related applications for functional nanocomposite materials and electronic devices. Especially, this talk mainly affords studies on the chemical functionalization of graphene and introduces how to modify the surface and interface of 2D vdW heterostructures and control the defects for a broad range of practical applications such as electronics, spintronics, bio-sensors, and energy harvesting devices.

Transition metal dichalcogenides (TMDCs) such as MoS₂ and WS₂ have potential applications in new generations of optoelectronic devices. Treatment with the superacid bis(trifluoromethanesulfonyl)amide (known as TFSA, TFSI, or HNTf₂) enhances TMDC photoluminescence (PL) quantum yields greatly [1-6]. Despite extensive empirical reports on the topic, the underlying reasons for PL improvement are not clear and establishing these is key to maximizing enhancements and prolonging device stability.

This work aims to gain insight into the mechanisms responsible for PL enhancement following treatments of the superacid type by conducting a multi-material study using TFSA and a range of structurally related chemical analogues. Previous work identified several similarities between TFSA-based passivation of TMDCs and crystalline silicon [3, 6-8], and we first use silicon as a model system for passivation optimization using carrier lifetime and Kelvin probe measurements. The insight gained is then applied to monolayer MoS₂, and MoS₂ and WS₂ flakes.

Firstly, we find that PL enhancement is not dependent on the superacidic nature of TFSA, as MoS₂ and WS₂ luminescence both improved considerably following treatment with non-acidic analogues. This contradicts earlier studies, which claimed that Brønsted superacidity of TFSA was crucial for good enhancement of TMDCs, particularly MoS₂ [1, 3, 9]. Secondly, we test the hypotheses of recent computational reports that the strong enhancement following superacid treatment is achieved through fragmentation of TFSA in situ, releasing SO₂ which either ‘repairs’ sulfur defects or fills vacancies with free oxygen or sulfur [6, 10]. Our experimental results show that sulfur vacancy occupation by sulfur or oxygen does not explain the increase in photoluminescence following TFSA treatment, as enhancements arise from treatments with analogues which lack sulfonyl functionality. Passivating solutions are investigated by NMR which shows no evidence for superacid (or analogue) dissociation in solution prior to interaction with the surface.

An additional aim of this work is to address practical considerations for using this class of treatments in TMDC-based optoelectronics. Previously reported treatments involve immersion in a TFSA solution, generally followed by processing at elevated temperatures [1-3, 6]. Treatment solutions primarily incorporate 1,2-dichloroethane (DCE) [1, 3-5], the use of which is problematic due to its carcinogenic, flammable, toxic and mutagenic nature, and the necessity to avoid the use of DCE at industrial scale [11]. In our study we show that it is possible to enhance TMDCs effectively with a room temperature approach, whilst also avoiding DCE. The enhancement we achieve at room temperature using alternative solvents (~43x for monolayer MoS₂ and up to ~53x for few-layer MoS₂ flakes depending on processing conditions) exceeds that of many elevated temperature studies employing TFSA-DCE [2-4], providing a more realistic route for implementation of these processes in future device manufacturing.

References
1. M. Amani et al., Science 350, 6264, 1065-1068 (2015).
2. Y. Kim et al., Nanoscale 10, 18, 8851-8858 (2018).
3. D. Kiriya et al., Langmuir 34, 35, 10243-10249 (2018).
4. M. R. Molas et al., Scientific Reports 9, 1989 (2019).
5. M. Amani et al., ACS Nano 10, 7, 6535-6541 (2016).
6. S. Roy et al., Nano Letters 18, 7, 4523-4530 (2018).
7. J. Bullock et al., ACS Applied Materials & Interfaces 8, 36, 24205-24211 (2016).
8. A. I. Pointon et al., Solar Energy Materials & Solar Cells 183, 164-172 (2018).
9. H. Lu et al., APL Materials 6, 6, 066104 (2018).
10. C. Schwermann et al., Physical Chemistry Chemical Physics 20, 25, 16918-16923 (2018).
11. D. Prat et al., Green Chemistry 18, 288-296 (2016).

In this work, we utilize the internal valley degree of freedom in two-dimensional (2D) transition metal dichalcogenides (TMDC) to study the magnetic properties of van der Waals (vdW) ferromagnet. Semiconductor spintronics devices that rely on manipulating electronic spin for spin-based information processing and storage applications have seen significant progress over the past several decades. However, the advent of optically active 2D materials has enabled optically addressable electronic valley pseudospin degrees of freedom which have led to the concept of valleytronic devices with a wide range of applications in transport phenomenon, electrical, magnetic, and spin control. Specifically, valley manipulation has been demonstrated via a strong external magnetic field which is an essential component for creating valley-based logic gates. The recent rise of 2D vdW ferromagnets not only unlocks new methods to control such spin-valley degrees of freedom at room temperature but enables magnetic sensing via van der Waals heterostructure engineering. For example, the proximity effect can be harnessed for all-optical readout of the magnetic properties such as magnetic hysteresis, local magnetic field, domain dynamics, and the interlayer magnetic dipole, as the electronic valley transition follows the optical selection rule. Such heterostructure assembly also makes on-chip manipulation of spins and g-factor possible without the need for huge superconducting coils to generate an external magnetic field. In our work, we have assembled heterostructure of Fe₃GeTe₂(FGT) and mono/bi-layer MoS₂ (TMDC) and study the valley Zeeman splitting and valley polarization via photoluminescence spectroscopy of MoS₂/FGT heterostructure as a function of temperature. Such assembly will lead to all-optical room temperature sensing of van der Waals magnet via 2D valley degree of freedom and spin-valley polarization of 2D excitons via magnetic proximity effect of a van der Waals magnet.

at the atomic scale. In particular, twisted 2D materials has recently attracted a lot of interest due to the capability
to induce moiré superlattices and discovery of electronic correlated phases [1,2]. In this talk, we present
nanoscale optical techniques such as near-field optical microscopy and photocurrent nanoscopy, and reveal with
nanometer spatial resolution unique observations of the optical properties of twisted 2D materials. We report on
the topological domain wall boundaries [4] of small-angle twisted graphene and interband collective modes in
charge neutral twisted-bilayer graphene near the magic angle [3]. The freedom to engineer these so-called optical
and electronic quantum metamaterials [1] is expected to expose a myriad of unexpected phenomena.
We will also show record-small nanoscale polaritonic cavities [4,5], where the resonances are not associated to
the eigenmodes of the cavity. Rather, they are multi-modal excitations whose reflection is greatly enhanced due
to the interference of constituent modes. We demonstrate mid-IR cavities with volumes more than a billion
below the free-space mode volume, while maintaining quality factors above 100.
References
[1] Song, Gabor et. al., Nature Nanotechnology (2019)
[2] Cao et al., Nature (2018)
[3] Hesp et al., Nature Communications (2020)
[4] Hesp et al., Nature Physics (2021)
[5] Epstein et al., Science (2020)
[6] Herzig Sheinfux et al., under review

The large surface areas and interlayer gaps of 2D materials enable surface functionalization and intercalation as effective post-synthesis design knobs to tune the properties of 2D materials using ions, atoms, and organic molecules. For complete engineering control, detailed understanding of the interactions between the 2D materials and the molecules adsorbed on 2D materials surface or between the 2D materials and the intercalants is necessary.
I will first discuss surface functionalization to tune the electrical properties of 2D materials. We developed an experimental approach to quantitatively measure the doping powers of organic electron donors (OEDs) to monolayer MoS₂. Using novel and previously studied OEDs, we demonstrate experimentally that the measured doping power is a sensitive function of molecule’s reduction potential, size, surface coverage, and orientation to 2D materials [1, 2]. Because we can experimentally measure the doping powers of the molecules, our results can be compared with DFT calculations to validate and predict new OEDs.
I will then discuss electrochemical intercalation into 2D materials to induce novel phases that were previously undetected and to study heterointerface effects on the intercalation induced phase transition [3, 4]. We discover a new structural phase in T_d-WTe₂ with lithium intercalation and this new phase is semiconducting even though the initial WTe₂ is semimetallic and lithium ions donate electrons to WTe₂. In the lithium intercalation-induced phase transition from the 2H to 1T’ phase of MoS₂, we show that the nucleation of the 1T’ phase proceeds via heterogeneous nucleation where the nature of heterointerface dictates the thermodynamics of the phase transition.
[1] Advanced Electronic Materials 7, 2000873 (2021).
[2] arXiv: 2107.12470 (2021).
[3] ACS Applied Materials & Interfaces 13, p.10603-10611 (2021).
[4] arXiv:2107.02255 (2021).

Silver organochalcogenides (SOCs) are a novel class of hybrid organic-inorganic 2D semiconductors. Like transition metal dichalcogenides and 2D lead halide perovskites, SOCs can be synthesized in the form of three-dimensional crystals consisting of 2D layers held together by van der Waals forces. However, SOCs are distinguished from perovskites and other vdW crystals by the presence of organic ligands covalently bound to each 2D inorganic layer, which imparts chemical robustness and a unique handle for tuning the electronic properties. Silver phenylselenolate (AgSePh), a prototypical 2D SOC, exhibits narrow blue emission, large exciton binding energy, and in-plane anisotropy.
In this talk, I will discuss efforts from my lab to synthesize new AgSePh derivatives, optimize their morphology, and understand the effects of organic functionalization on photophysics in this unique class of materials.

Optical techniques have provided us with rich information about the exciton – a two-particle photoexcited state in semiconductors and insulators. Yet, they have left a fundamental degree of freedom of the exciton inaccessible – it’s momentum!
Over the past decade, we have developed novel time-resolved photoemission spectroscopy techniques that have yielded valuable insights into perovskite photovoltaic materials and other semiconductor structures [1, 2, 3, 4]. In this talk, I will discuss the application of these techniques to access the momentum coordinate of excitons in 2D semiconductors, thereby providing us the formation pathways of momentum-forbidden dark excitons [5], an image of the distribution of the electron around the hole in the exciton [6], and the degree of localization of the center of mass coordinate of the exciton in a moiré cell [7].
References
[1] M. K. L. Man, et al. Nature Nanotechnol. 12, 36 (2017)
[2] E. L. Wong, et al. Science Advances 4, eaat9722 (2018)
[3] T. Doherty^*, A. Winchester^*, et al. Nature 580, 360 (2020)
[4] S. Kosar, et al, Energy and Environment Sciences, DOI: 10.1039/d1ee02055b (2021)
[5] J. Madeo^*, M. K. L. Man^*, et al. Science 370, 1199 (2020).
[6] M. K. L. Man^*, J. Madeo^*, et al. Science Advances 7, eabg0192 (2021).
[7] O Karni^*, E. Barr^*, V. Pareek^*, J. D. Georgaras^*, M. K. L. Man^*, C. Sahoo^*, et al. arXiv:2108.01933 (2021)
^* equal authors

Strain engineering is of great significance in semiconductor science and engineering applications. Silicon usually breaks at about 1.5% strain, and two-dimensional (2D) atomic crystal materials can withstand strains of> 10%, making two-dimensional materials have important application potential in the flexible electronics industry. Integrating two-dimensional semiconductor materials into silicon devices to expand the functions of silicon-based devices has great application potential. However, silicon-based microelectromechanical systems (MEMS) devices are generally on the scale of tens of microns to millimeters, while two-dimensional semiconductors are generally on the scale of several microns and a thickness of several nanometers. Therefore, for two-dimensional semiconductors and silicon, the integration of basic MEMS actuators is very challenging, which greatly restricts the application of strain engineering in two-dimensional materials and devices. In this talk, we will present single-layer to multi-layer two-dimensional material optoelectronic devices, and the controllability of large strain on two-dimensional semiconductor optoelectronic devices through precise control of the voltage of MEMS actuators based on silicon-on-insulator (SOI) strain tuning. The two-dimensional semiconductor strain tuning device of the SOI-based MEMS actuator will play a vital role in promoting the integration of CMOS-MEMS of two-dimensional materials and the chip-level integration of single-photon emitters.

The synthesis of high-quality MoS₂ in a large area plays a crucial role in realizing industrial applications of nanoelectronics, flexible devices, and optoelectronics. However, the current wafer-scale MoS₂ using chemical vapor deposition (CVD) process requires a high growth temperature, which limits its application in device fabrications.¹ As indicated by the Mo-S binary phase diagram, semiconducting MoS₂ is considered as a width compound, in which the defects are drastically increased by reducing the growth temperature.² The lowest reported temperature at which MoS₂ still exhibits semiconductor properties is 320 °C.³ Subsequently, a transfer step is required to bring MoS₂ from as-grown substrate to a targeted substrate, especially for the realization of flexible electronics that requires a flexible substrate such as polymer or ultrathin glass. It raises the issue of quality degradation due to residues and wrinkles caused by the transfer step. By lowering the growth temperature, MoS₂ can be deposited on ultrathin glass, which can be used straightly for flexible electronics. Also, the quality of MoS₂ can be preserved by the direct fabrication without the transfer step. Herein, the development of a new method to synthesize high-quality MoS₂ using a metal-organic CVD (MOCVD) system at low temperature (i.e ~150 °C) is proposed, and low temperature grown MoS₂ is applied to electronic applications such as photodetector and logic circuits. This method offers not only cost-effective but can be used for low melting temperature substrates such as polymers.
References
[1] Li, Taotao, et al.: Epitaxial growth of wafer-scale molybdenum disulfide semiconductor single crystals on sapphire. Nature Nanotechnology, 2021.
[2] Hoang, Anh Tuan, et al.: Epitaxial growth of wafer-scale molybdenum disulfide/graphene heterostructures by metal–organic vapor-phase epitaxy and their application in photodetectors. ACS Applied Materials & Interfaces 12, pp. 44335-44344, 2020.
[3] Park, Ji-Hoon, et al.: Synthesis of high-performance monolayer molybdenum disulfide at low temperature. Small 5, 2000720, 2021.

Atomically thin quasi two-dimensional (2D) insulating materials exhibit novel exciton physics due to ineffective screening, quantum confinement, and topological effects. Such exciton physics has recently been studied in details experimentally and theoretically. Going beyond near-equilibrium set-up, one expects that excitonic effects also dominate the responses of out-of-equilibrium systems and can lead to interesting phenomena in optically-driven 2D materials. Using a newly developed real-time, non-equilibrium Green function method within the adiabatic GW approximation, we show that, for non-centrosymmetric 2D semiconductors, excitonic effects give rise to a strong DC current, the so-called shift current, upon even sub-bandgap frequency CW light illumination through a second-order nonlinear optical process. The frequency-dependent shift current coefficients can be enhanced by orders of magnitude by the strong e-h interactions, producing a bulk photovoltaic effect (i.e., without having to have a p-n junction) of promise for applications with appropriate materials. Furthermore, we show that, in optical-field-driven angle-resolved photoemission spectroscopy (ARPES) experiments, the energy and wavefunction of excitons may be measured directly under achievable laboratory conditions. With optical pump frequencies close to the resonance frequency for exciton excitations, distinct excitonic features manifest themselves dramatically as modulated replicas of the involved valence band states. We also find that, at higher pump intensity, the quasiparticle band energies are renormalized due to the driving optical fields.

This work was supported by the Center for Computational Study of Excited State Phenomena in Energy Materials (C2SEPEM), which is funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-AC02-05CH11231. Y.H.C. acknowledges support by the Ministry of Science and Technology, National Center for Theoretical Sciences (Grant No. 110-2124-M-002-012 and 110-2112-M-001-018 -MY3).

Twisted heterostructures of two-dimensional crystals offer a broad scope for the design of novel metamaterials. In this talk I will review our latest work [1] on twisted transition metal fichalcogenide bilayers and discuss atomic reconstruction that occurs when the twist angles are small. For small twist near the 2H stacking, stable 2H domains dominate, with nuclei of a second MM metastable phase. This appears as a kagome-like pattern at ~1 degree twist, transitioning below 0.3 degree to a hexagonal array of large 2H domains. Our tunnelling measurements show that such reconstruction creates piezoelectric textures, opening a new avenue for engineering of 2D material properties.
For twisted bilayer near 3R stacking (parrallel unit cells), a pattern of mirror reflected triangular 3R domains merges, featuring layer-polarized conduction band states caused by lack of both inversion and mirror symmetries. Surprisingly, the lack of inversion symmetry in 3R polytype leads to emergence of out-of-plane ferroelectricity due to layer-asymmetric interband hybridisation [2]. The resulting alternating domains can be moved by applying out-of-plane electrical fields, as visualized in situ using channelling contrast electron microscopy. The interfacial charge transfer for the observed ferroelectric domains is quantified using Kelvin probe force microscopy and agrees well with theoretical calculations. The movement of domain walls and their bending rigidity also agrees well with our modelling results. Furthermore, we demonstrate proof-of-principle field-effect transistors, where the channel resistance exhibits a pronounced hysteresis governed by pinning of ferroelectric domain walls. Our results show a potential venue towards room temperature electronic and optoelectronic semiconductor devices with built-in ferroelectric memory functions.
[1] Nat. Nanotechnol. 2020, 15 (7), 592–597.
[2] arXiv:2108.06489

The interplay of electronic correlations, lattice degrees of freedom and topology holds the promise for the realization of exotic states of quantum matter. Here, we discuss routes to understand and control this interplay in van der Waals superlattice systems. The nature of superconducting order presents a recurring overarching open question in this context. We will first address doping fingerprints of superconductivity arising from spin and lattice fluctuations in moiré superlattice systems [1]. We will show how doping-dependent measurements of the superconducting transition temperature provide direct access to probing the superconducting pairing mechanism in twisted Van der Waals materials. We will then analyze possibilities of Coulomb and superlattice engineering in these systems. We will discuss confinement and deconfinement [2] pathways to create correlated Dirac fermions in stacks of transition metal dichalcogenides and identify knobs to control topology, electron correlations and emergent order in these systems.
[1] N. Witt, J. M. Pizarro, T. Nomoto, R. Arita, T. O. Wehling, arXiv:2108.01121 (2021).
[2] J. M. Pizarro, S. Adler, K. Zantout, T. Mertz, P. Barone, R. Valentí, G. Sangiovanni, T. O. Wehling, npj Quantum Materials 5, 79 (2020).

Excitonic insulators arise from the formation of bound electron-hole pairs (excitons) in semiconductors and provide a solid-state platform for quantum many-boson physics. Although the concept of excitonic insulator has been understood for sixty years, it has been challenging to establish distinct experimental signatures of its realization. One problem is that exciton coherence in condensed phases inevitably couples to the crystal Hamiltonian so that condensation does not imply superfluidity. A second problem is that the exciton population of a particular material depends very sensitively on band structure details and cannot be controlled. In this talk, I will discuss a recent experiment in which we solve both problems by establishing electrical control of the chemical potentials of interlayer excitons in semiconducting transition metal dichalcogenide double layers and probing the thermodynamic properties of exciton fluids by capacitance measurements. We construct a phase diagram of strongly correlated excitons that reveals both the Mott transition and interaction-stabilized quasi-condensation. Our experiment paves the path for realizing the exotic quantum phases of excitons, as well as multi-terminal exciton circuitry for applications.

The emergence of two-dimensional (2D) magnetic crystals and moiré engineering has opened the door for devising new magnetic ground states via competing interactions in moiré superlattices. In this talk, I will discuss our recent experiment on twisted bilayer CrI₃, which is an A-type antiferromagnet in its natural form. We characterize the formation of large wavelength moiré pattern by TEM. We observe twist-angle-dependent magnetic ground states by magneto optical measurements. At large twist angle, CrI₃ bilayers are ferromagnetic (FM). Below the critical twist angle of about 3°, our result shows the coexistence of antiferromagnetic (AF) and FM domains on the moiré length scale. This arises from the stacking-dependent interlayer exchange interactions in CrI₃; and it is a result of the competing interlayer AF coupling in the monoclinic stacking regions of the moiré superlattice and the energy cost for forming AF-FM domain walls. The state can further be controlled by electrostatic gating through the doping-dependent interlayer AF interaction. Our results demonstrate the possibility of engineering moiré magnetism in twisted bilayer magnetic crystals, as well as gate-voltage-controllable high-density magnetic memory storage.

Atomically thin materials, such as graphene and transitional metal dichalcogenides (TMDs), have recently come to the forefront of research in materials physics. This is largely due to the ease with which they can be combined into artificially engineered heterostructures that exhibit emergent electronic and optical properties. Enhanced Coulomb interactions in moiré heterostructures makes TMDs, such as MoSe₂/WSe₂, promising to explore correlated quantum phases of matter.

Moiré-trapped dipolar excitons are quantum emitters whose emission energy responds to electric-field, making them local charge sensors. By analyzing emission of multiple moiré-trapped dipolar excitons, we uncover numerous Wigner-like crystalline phases of electrons at fractional fillings of the moiré lattice. Our approach offers higher resolution at higher sensitivity compared to the existing optical reflectance based techniques which average over several moiré sites.