Symposium EQ07—Defects and Strain Potential Enabled Emergent Behavior in Two-Dimensional Materials

Atomically thin materials such as graphene and other 2D materials are promising for a wide range of applications. Among many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both theoretically and experimentally. This tutorial aims to summarize the current understanding on the mechanics of 2D materials including linear and nonlinear elasticity, strength and toughness, as well as mechanical interactions such as adhesion and friction at the interfaces of 2D materials. Theoretical and computational models will be presented along with experimental methods for predicting and measuring the mechanical and interfacial properties of 2D materials.

Outline:
Linear and nonlinear elasticity
Strength and toughness
Adhesion and friction

When two or more van der Waals material layers are overlaid with small differences in lattice constant are overlaid with small relative twist angles they form two-dimensional moiré superlattices with unit cell areas that can be thousands of times larger than the unit cell areas of
the underlying two-dimensional crystals. When the moiré superlattice materials are semiconductors or semimetals, low-energy electronic properties are accurately described by low-energy models that have the periodicity of the superlattice. These moiré materials act like crystals with artificial giant atoms. One of the most attractive properties of moiré materials is that the number of electrons per unit cell can be changed by more than one using electrical gates. I will survey the electronic properties of moiré materials, focusing on systems that have been achieved experimentally, including graphene bilayers and multilayers and systems based on tradition metal dichalcogenide layer building blocks.

Two-dimensional (2D) van der Waals materials exhibit a rich variety of properties that can be further tailored to achieve novel functionality by strain and defect engineering. Here we describe a new paradigm of quantum straintronics based on strain engineering of monolayer (ML) graphene and 2H-phase transition metal dichalcogenides (TMDs), as well as novel photonic phenomena based on defect engineering of quasi-one dimensional (1D) graphene nanostripes (GNSPs).

Theory and experimental studies have found that non-trivial nanoscale strain in ML graphene can give rise to giant pseudo-magnetic fields (PMF) that dramatically modify the local electronic properties of graphene. Nonetheless, no realistic devices with spatially scaled-up PMF had been demonstrated until our recent work that achieved periodic graphene “wrinkles” by placing ML graphene on substrates with arrays of nanostructures and then by developing the strained graphene wrinkles into valley-Hall transistors via e-beam lithography. We found that the periodic graphene wrinkles formed a valley splitter that separated the K/K'-valley Dirac fermions and preserved the valley polarization. We further observed a wealth of novel quantum and topological phenomena, including flat bands of quantized Landau levels, spontaneous symmetry-breaking and spin polarization in the strong correlation limit, as well as quantum valley Hall and quantum anomalous Hall effects with different Chern numbers in the ballistic limit. Controllable electronic correlation could be further achieved by designing the magnitude and spatial distribution of strain and by varying the back-gate voltage. These findings suggest that nanoscale strain-engineering of ML graphene provides a new platform to control the topological electronic states and electronic correlation in 2D materials.

For ML semiconducting 2H-TMDs (e.g., MoS₂ and WS₂) with strong spin-valley coupling, strain has been shown to induce variations in the optical bandgaps and exciton lifetimes. Our molecular dynamics simulations revealed additional symmetry breaking effects, suggesting that nanoscale strain engineering of ML TMDs may be employed to design the landscape of optical bandgaps and to lift the valley degeneracy to produce valley-polarized excitons for topological photonics and opto-valleytronic/spintronic device applications.

For quasi-1D GNSPs directly grown on silicon by PECVD, we found a number of novel photonic phenomena: [1] The GNSPs exhibited strong and wavelength independent optical absorption (absorption coefficient ~ 10⁶ cm^-1) from IR to visible light (Vis). [2] The photoluminescence (PL) spectra revealed an emission gap around the excitation frequency as well as both red- and blue-shifted broadband signals. [3] Unusually long carrier lifetimes (up to ~ 12 ns) were found from the time-resolved PL studies with Vis lasers, and the lifetimes were nearly independent of temperature (from 15 K to 300 K). [4] The PL emissions were coherent, as manifested by the second-order temporal correlation function and the spatial interference patterns of the PL maps. [5] Under strong CW excitations, lasing phenomena with sharp peaks (FWHM linewidths < 0.1 THz) at discrete frequencies from FIR to Vis were observed. We attribute these phenomena to the presence of topological defects that either naturally formed during the PECVD synthesis or engineered in the material by selective plasma radiation. These defects behaved like two-level color centers that coupled strongly to the high-mobility GNSPs, and the GNSPs provided significant confinement of photons due to strong subwavelength scattering and internal reflection. The GNSP matrix thus acted as an effective optical cavity for the color centers, and the observed photonic phenomena were consistent with cavity-QED. These findings suggest that defect engineering of quasi-1D GNSPs on silicon is a promising new approach for on-chip nano-photonics that are compatible with CMOS technologies.

Layered Materials (LMs) have potential for quantum technologies, as scalable sources of single photon emitters (SPEs)[1,2]. LM heterostructures can be built with tuneable properties depending on the constituent materials and their relative crystallographic orientation[3,4]. Quantum emitters in LMs hold potential in terms of scalability, miniaturization, integration. Generation of quantum emission from the recombination of indirect excitons in heterostructures made of different LMs is also promising. I will discuss how LM combinations can be used to generate SPEs and confinement of interlayer excitons[5].

1.C. Palacios-Berraquero et al., Nat. Commun. 8, 15093 (2017)
2.C. Palacios-Berraquero, et al., Nat. Commun. 7, 12978 (2016)
3.P. Rivera et al., Nat. Nanotech. 13,1004 (2018)
4.M. Barbone et al. Nat. Commun 9, 3721 (2018)
5. A. R. P. Montblanch et al. Commun Phys. 4, 119 (2021)

The gap between light-based communication technology and state-of-the-art electronic devices on the nanoscale constitutes a major obstacle towards integrated optoelectronic circuits. Two-dimensional transition metal dichalcogenides (2D TMDCs) harbour an enormous potential to bridge that gap by unifying strong light-matter interaction with exceptionally robust and simultaneously highly mobile exciton quasiparticles. However, steering these neutral quasiparticles along predesigned pathways has proven to be exceptionally challenging and only recently strain was identified as a key element in 2D TMDCs to gain real-space control over the exciton flow. Here, we demonstrate guiding of excitons along strain-engineered quasi-one-dimensional (1D) potential channels by combining mechanically flexible van der Waals heterostructures with single-crystal nanowires in hybrid 1D/2D systems. Using ultrafast, all-optical injection and time-resolved readout, we show near 100% anisotropy in the exciton propagation both at cryogenic and room temperatures. The vanishing diffusivity of excitons perpendicular to the channels highlights their efficient localization by the simultaneous action of confinement potentials and reduced scattering in strained regions. Such artificially induced anisotropy in a pristine, otherwise fully 2D semiconductor offers a highly promising, non-invasive platform to steer the flow of optical excitations and opens a path towards rich quasiparticle transport phenomena in 1D.

Atomically thin transition metal dichalcogenides (TMDs), like MoS₂, with high career mobilities and tunable band gaps, are great candidates for next-generation optoelectronic devices. This study demonstrates the use of relatively low power (100 keV) ion beams to control the point defect levels in single and bilayer films of MoS₂. The use of ion irradiation towards controlling the localized strains and defect densities in TMDs is systematically probed to provide opportunities towards tuning the electronic and optical properties, like resistivity, and absorption and emission spectra. The use of relatively low power beams provides the scope to explore the novel phenomenon of ion implantation in 2D materials. Slight changes in the vibrational states and more significant changes in the photoluminescence (PL) of MoS₂ have been observed which might be explained by substrate straining as the ions penetrate the 2D materials before being trapped by the sapphire substrate. The effect of these strained microstructures on the vibrational states of the MoS₂ lattice is studied using PL and Raman spectroscopy. Recent unpublished alpha particle bombardment studies performed on WS₂ thin films also showed significant shifts in PL peaks (~5 nm). This novel capability provides insights into discerning the links between ion bombardment-induced mechanical straining of lattice and the emergent quantum properties. The study also combines experimental characterization with quantum mechanical simulations performed using density functional theory (DFT) framework to quantify the defect density and distributions; as well as further our understanding of the modifications observed in the optical and electronic band alignments with varying levels of atomistic defects.

In recent years, dynamical systems have proven to be a highly versatile platform for hosting exotic quantum phenomena. Specifically, high-dimensional topological physics, initially developed out of pure theoretical interest, has now been experimentally observed in physical systems, ranging from cold-atoms, to photonics, metamaterials and electrical circuits. At the heart of these experiments, the spatio-temporal evolution of the system's parameters induces a response proportional to a topological index defined in higher dimensions [1,2]. Drawing on this correspondence, we present a dynamical realization of an Axion field in a 2D material where strain is utilized towards engineering a controlled evolution. Such an approach extends the traditional description of 3D Axion insulators to lower-dimensional dynamical systems, in addition to predicting emergent responses in novel platforms.

Even though Axions are yet to be observed in particle physics and cosmology, they naturally arise in condensed matter systems. Specifically, recent research into the topological aspects of three-dimensional semiconductors and insulators has shown that electrons in certain materials are nontrivially coupled to an Axion field. Apart from the interesting responses to electromagnetic fields, e.g., fractional quantum Hall effect on the surface and magneto-electric polarizability, Axion field effects can be employed in numerous other fields, such as testing fundamental physics with dark matter detectors [3] or controlling electronic properties for technological applications [4]. Here, we present these effects in a completely different setting: in dynamical systems where physical observables are shown to depend stroboscopically on an Axion field defined over a higher-dimensional manifold – the phase-space of the system, i.e., the combined position, momentum and time coordinates.

Designing materials that can be easily controlled is of paramount importance when proposing realistic experiments. To this end, coupling electronic properties to strain offers a promising route towards tailoring dynamics. In particular, piezo-electromagnetic and deformation-induced symmetry breaking effects were shown to significantly change the electronic structure of materials. Here, we exploit such properties towards engineering systems that can be externally controlled via weak deformation fields.


References:
[1] Petrides, I., Price, H.M. and Zilberberg, O., Six-dimensional quantum Hall effect and three-dimensional topological pumps. Physical Review B, 98(12), p.125431, 2018.
[2] Petrides, I. and Zilberberg, O., Higher-order topological insulators, topological pumps and the quantum Hall effect in high dimensions. Physical Review Research, 2(2), p.022049, 2020.
[3] D. J. Marsh, K. C. Fong, E. W. Lentz, L. Šmejkal, and M. N. Ali, “Proposal to detect dark matter using axionic topological antiferromagnets,” Physical review letters, vol. 123, no. 12, p. 121601, 2019.
[4] B. Zhao, C. Guo, C. A. Garcia, P. Narang, and S. Fan, “Axion-field-enabled nonreciprocal thermal radiation in weyl semimetals,” Nano Letters, vol. 20, no. 3, pp. 1923–1927, 2020.

Controlling phase transitions in 2D materials and switching between structures opens uncharted avenues in the use of 2D materials in mechanical, optical, electronics, and thermal applications. The first demonstration of a metastable phase transformation in bilayer graphene into an ultra-stiff single layer diamond phase has been recently achieved in ambient conditions and room temperature, with the solely application of a localized pressure [REF}, this new phase was called diamane. This new single layer diamond phase is obtained in two-layer (one layer plus buffer layer) epitaxial graphene grown on SiC without chemical functionalization, and it is reversible to graphene upon release of pressure. Besides graphene, other 2D materials possess the structural characteristic necessary for sustaining a similar pressure-induced sp2-to-sp3 phase transition. In particular, two- and three-layer hexagonal boron nitride (h-BN) is shown to be convertible in a stable cubic phase (c-BN) under pressure. Furthermore, we conduct an electrical and mechanical investigation of 2D diamond phase formation as observed through pressure activation in hBN, single-layer epitaxial graphene, and quasi-free-standing H-intercalated two-layer graphene on SiC. In particular, we fabricate metal contacts on epitaxial graphene and hBN to study the change in electrical and optical properties before, during, and after the phase activation. This also allows to observe the time stability of the pressure-activated phase formation after the release of pressure.<script id="th-iframe-script" src="chrome-extension://ofdopmlmgifpfkijadehmhjccbefaeec/assets/comms/commsiframe.js"></script>

The bending stiffness of 2D materials and heterostructures governs their deformability, reconfigurability, and 3D structure. In particular, interlayer interactions play a major role in governing the bending of 2D materials, where they lead to an unusually low bending stiffness in few-layer graphene and a curvature-dependence of the bending stiffness [1]. The key role of interfaces in the bending of 2D multilayers also indicates potential methods to tune the deformability and 3D structures of 2D systems though interfacial engineering.

In this talk, we use aberration-corrected scanning transmission electron microscopy (STEM), density functional theory (DFT) simulations, and continuum mechanical models to examine the bending of 2D multilayers and heterostructures. We investigate the bending stiffness of graphene and MoS₂ multilayers, comparing the properties of stacks containing aligned, twisted, and heterointerfaces. We find that the bending properties of 2D materials and heterostructures is strongly tuned by the atomic arrangement of the interfaces present. 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 interfacial atomic registry of the layers present. In addition, our measurements show that 2D heterostructures containing only misaligned interfaces exhibit ultra-low bending stiffnesses that are the linear sum of the bending stiffness of each constituent monolayer, reaching the theoretical lower limit for stiffness in 2D materials. These properties arise from the low interlayer friction and sliding energy barrier of crystallographically misaligned interfaces, which DFT simulations indicate are two orders of magnitude lower in misaligned interfaces than in 2H-stacked MoS₂. Using these observations, we develop a model to predict and design the bending stiffness for arbitrary heterostructures as a function of thickness, system size, and the number and location of incommensurate interfaces [2]. Our techniques should enable the rational design of the out-of-plane structure 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).

Recently, nano-tents and nano-bubbles formed by two-dimensional (2D) materials have seen a surge of interest because they are able to induce in-plane strain as well as strain gradient via out-of-plane deformation.[1] Our previous work has unveiled what sets the in-plane strains in terms of the shape characteristics of nano-tents and nano-bubbles.[2-4] Moreover, out-of-plane poking or bulging, also known as indentation or blister tests, are popular methods for the measurement of in-plane elasticity of thin sheets. For linear elastic sheets, a load-cubic deflection relation has been frequently assumed so that the stiffness of the sheet could be readily extracted. However, we find that recent results of indentation and bulge tests on 2D materials do not follow this relation, which can be attributed to the slippage of atomically smooth 2D materials against their supporting substrates.[5, 6] Besides, the interfacial slippage could cause instabilities in the sheet such as radial wrinkles in suspended region, with finite lengths.[4] To gain a quantitative understanding, we assume constant interfacial shear traction and study the wrinkling extent and the effective stiffness of thin sheets upon poking and bulging. We identify a single dimensionless parameter governing these mechanical responses—the sliding number—defined by comparing the sheet tension (that drives the slippage) with the interfacial traction (that resists the slippage).[7] We discuss several useful asymptotic behaviors emerging at small and large sliding numbers. These understandings are helpful for determining when the effect of the interfacial slippage (as well as other substrate-associated subtleties) can be neglected in these tests. At the end, we propose a simple poking/bulging methodology immune to the complexities caused by the slippage, pretension, Poisson’s ratio, substrate roughness, etc., enabling a robust and accurate measure of 2D material stiffness.

1 Sanchez, D.A., Dai, Z., and Lu, N.: ‘2D Material Bubbles: Fabrication, Characterization, and Applications’, Trends in Chemistry, 2021, 3, (3), pp. 204-217
2 Dai, Z., Hou, Y., Sanchez, D.A., Wang, G., Brennan, C.J., Zhang, Z., Liu, L., and Lu, N.: ‘Interface-Governed Deformation of Nanobubbles and Nanotents Formed by Two-Dimensional Materials’, Physics Review Letters, 2018, 121, (26), pp. 266101
3 Sanchez, D.A., Dai, Z., Wang, P., Cantu-Chavez, A., Brennan, C.J., Huang, R., and Lu, N.: ‘Mechanics of spontaneously formed nanoblisters trapped by transferred 2D crystals’, Proceedings of the National Academy of Sciences, 2018, 115, (31), pp. 7884-7889
4 Dai, Z., Sanchez, D.A., Brennan, C.J., and Lu, N.: ‘Radial Buckle Delamination around 2D Material Tents’, J Mech Phys Solids, 2020, 137, pp. 103843
5 Akinwande, D., Brennan, C.J., Bunch, J.S., Egberts, P., Felts, J.R., Gao, H., Huang, R., Kim, J.-S., Li, T., Li, Y., Liechti, K.M., Lu, N., Park, H.S., Reed, E.J., Wang, P., Yakobson, B.I., Zhang, T., Zhang, Y.-W., Zhou, Y., and Zhu, Y.: ‘A review on mechanics and mechanical properties of 2D materials—Graphene and beyond’, Extreme Mechanics Letters, 2017, 13, pp. 42-77
6 Dai, Z., Lu, N., Liechti, K.M., and Huang, R.: ‘Mechanics at the interfaces of 2D materials: Challenges and opportunities’, Current Opinion in Solid State and Materials Science, 2020, 24, (4), pp. 100837
7 Dai, Z., and Lu, N.: ‘Poking and bulging of suspended thin sheets: Slippage, instabilities, and metrology’, J Mech Phys Solids, 2021, 149, pp. 104320

Although strain engineering in 2D transition-metal dichalcogenides (TMDs) has been emerged as a critical way to control the electronic/optoelectronic properties, the presence of an in-situ compressive or tensile strain-applied in the flat state of 2D materials has never been reported in terms of photo-detecting capability. Herein, we propose a technical way of inducing intentional stress during the wet-transfer method of MoS₂ monolayer by using the pre-bent PET (polyethylene terephthalate) substrate for the photodetector of Au/MoS₂/Au structure, ultimately to investigate the strain-dependent photosensitivity modulations. As a highlight, the photocurrent reached a maximum value of 1.48 AW^-1 with an in-situ tensile strain of +1.08%, which corresponds to nearly 8 times increment compared to the reference sample. The enhancement originates from realigned energy bands at the MoS₂-Au interface through the strain-induced bandgap modulation. It is assumed that the in-situ tensile strain promotes the separation of photo-generated charge carriers and facilitates their transport. With extra strain imposed by post-bending of the in-situ strained sample, the photocurrent performance was further found to be improved to 1.77 AW^-1 due to the lowered Schottky barrier by the known piezophototronic effect.

Low-dimensional Molybdenum Disulfide has shown remarkable potential for applications in flexible electronics and nanoscale photonics. However, a fundamental property that needs to be explored is the thermal expansion coefficient on free-standing MoS₂ because it is instrumental to the thermal management of nanomaterials. Currently, Raman spectroscopy methods and first-principles calculations provide order-of-magnitude differences, likely from the difficulty in obtaining accurate experimental measurements on 2D materials. In this contribution, we couple in-situ transmission electron microscopy (TEM) to supply uniform heating with selected area electron diffraction (SAED) patterns to analyze the defined regions upon heating. Next, we integrate advanced image analysis techniques to precisely align the diffraction pattern, outline Bragg peaks, and calculate the spacing of the d₁₀₀ and d₁₁₀ peaks. Through analysis of the uniaxial and biaxial changes of in-plane spacing, we show that properly indexing SAED peaks is a reliable tool to investigate some thermal properties of MoS₂.

Controlling the interlayer twist angle in an artificial two-dimensional (2D) van der Waals (vdW) interface offers an experimental route to create a moire superlattice. One can create exotic electronic states by minimizing electronic bandwidth with a tunable moire length scale. However, in the small twist angle regime, vdW interlayer interaction can cause significant structural reconfiguration at the interface, creating the arrays of domain structures. Particularly, in the crystal symmetry engineered twisted bilayer polar crystals, unconventional ferroelectricity can arise to exhibit the ferroelectric switching mechanism. Here we performed an in-situ transmission electron microscopy (TEM) investigation on dual gated twisted bilayer transition metal dichalcogenides (TMD) devices that enable real-time observation of polar domain dynamics in a 2D system. In combination with the theoretical investigation, we find the polarizability of the twisted bilayer TMD sensitively depends on the moire length and the domain shapes. We also report on the angle-dependent development of ferroelectricity and unconventional domain antiferroelectricity in the twisted bilayer TMD that exhibits intriguing distinction from conventional ferroelectricity and antiferroelectricity.

Two-dimensional materials (2DM) are atomically thin materials with extraordinary mechanical, electrical, and chemical properties that make them promising for next generation technologies in quantum communications, sensing, flexible and transparent electronics and optoelectronics, and energy conversion. The realization of new technologies based on 2DM requires both fundamental research on the materials science of 2DM and research that aims to bridge the gap between materials science and the engineering of real devices and systems. In this talk, I will describe our recent work on using atomic force microscopy (AFM) in conjunction with optical spectroscopy and transmission electron microscopy to understand the physics of strain and heterostructure interfaces in 2DM and leveraging that understanding to control material behavior. In the first part of this talk, I will discuss our work on using the AFM tip to write strain gradients into 2DM semiconductors with nanometer-scale precision. Briefly, we use the AFM tip to indent 2DM placed on top of a thin polymer layer, which results in strain texturing. We use this technique to deterministically place single photon emitters (SPEs) in WSe₂.[1] We find that electrical gating can lead to improved purity of SPEs created with this method. In the second part of the talk, I will discuss our investigations of the interfaces between 2DM layers, with a particular focus on the relative twist angle between layers. I will present experimental evidence of atomic reconstruction in WSe₂/MoSe₂ and WS₂/MoS₂ heterostructures.[2] We confirm the presence of atomic reconstruction through a combination of AFM and transmission electron microscopy. We find that the reconstructed domains have a pronounced dependence on the relative crystal orientation of the layers in the heterostructure. We also find intriguing electronic behavior of the domain boundaries between reconstructed domains. We use density functional theory modeling to understand the origin of the electronic behavior of the domain boundaries. These calculations suggest that flat bands in the conduction band may explain the observed electronic behavior.

[1] M. Rosenberger et al. ACS Nano, 2019, 13 (1), 904-912 (DOI: 10.1021/acsnano.8b08730)
[2] M. Rosenberger et al. ACS Nano, 2020, 14, (4), 4550–4558 (DOI: 10.1021/acsnano.0c00088)

Strain in the context of transition metal dichalcogenides (TMDs) has been shown to have a significant impact on the physics of both intralayer and interlayer excitons (ILE), e.g. in the former inhomogeneous strain can trap excitons in deep potential wells forming quantum-dot like localized states. For the monolayer TMDs these strain-induced electronic states have been investigated heavily with optical probes, revealing the emergence of single-quantum emitters at the inhomogeneous strain sites when cooled to cryogenic temperatures. However, to date strain induced localization of ILE's has predominately been investigated by broad far-field probes which obscure significant features of the strain field at the nanoscale. Because of the dramatic effect of strain on the opto-electronic properties of the ILE states, a detailed understand of the nanoscale variations present in heterolayers is crucial.
In this presentation I will detail a combined nano-PL and STM study of nanobubbles in MoSe₂/WSe₂ heterostructures. Utilizing scanning tunneling and the nano-PL spectroscopies we find a consistent shifting of the indirect gap and ILE photoluminescence from the edge of the nanobubble to the center. Our hyperspectral STM imaging further shows the emergence of a strain-induced in-gap state at the nanobubble edge, that to our knowledge has not been observed in the literature. Our results demonstrate the dramatic effect of, and importance of, nanoscopic strain on the interlayer excitons in theses state-of-the-art heterostructures.

A new nanoscale probing technique is employed using Scanning Tunneling Microscopy (STM) to study the local inter-layer potentials of different domains in moire landscapes on graphite surface. An anomalous tip-sample interaction based on local elastic deformation theory is exploited to deform the moire domains out of the plane on a scale of angstroms! This study of moire patterns under tip-induced strain not only reveals the VDW strength gradually varying on these patterns but also guides us in understanding the inter-layer bonding strength of other graphite features like step height and grain boundary.

The upper limit of efficiency that any semiconductors can achieve in a light-emitting device is directly dictated by photoluminescence quantum yield (PL QY), which is a strong function of photocarrier density. Optoelectronic devices that are ubiquitously used for display or lighting applications such as light-emitting diodes (LEDs) often operate in a high photocarrier density regime, where all semiconductors experience elevated nonradiative carrier recombination, and thus diminished PL QY. Previously we have shown that, although 2D transition metal dichalcogenide (TMDC) monolayers exhibit near-unity PL QY at low carrier densities, their PL QY lowers significantly at high carrier densities like all other semiconductors. Despite extensive research to advance their material quality and device architecture, the inherent PL QY degradation in 2D TMDC monolayers at high exciton densities remains a major challenge for their utility towards high-performance optoelectronic devices. At low densities, neutral excitons in monolayer semiconductors recombine entirely radiatively even in the presence of a high native defect concentration. However, at high densities, recombination of neutral excitons is dominated by exciton−exciton annihilation (EEA), where an exciton nonradiatively recombines while colliding with another exciton. We show that EEA is resonantly amplified in TMDC monolayers by van Hove singularities (VHSs) present in their joint density of states. Logarithmically divergent VHSs are a topological hallmark of two-dimensional semiconductors, arising from saddle points in the energy dispersion. By applying small mechanical strain, we shift the EEA process away from the VHS resonance and circumvent the enhanced nonradiative EEA that plagues the PL QY at high exciton densities, leading to near-unity PL QY at all exciton densities in 2D TMDC monolayers (e.g. MoS2, WS2, WSe2). This simple, scalable method suppresses all nonradiative recombination at all generation rates for both exfoliated and CVD-grown centimeter-scale TMDC monolayers, as long as they are direct, and their Fermi level is in the middle of the bandgap by electrostatic or chemical doping. Our results constitute the first critical step towards optoelectronic devices that retain their high efficiency at high carrier concentrations and put forth TMDC monolayers as a key material of choice for next-generation optoelectronic applications.

Quantum defects of vdW materials have advanced to the forefront of quantum communication and transduction research due to their unique potential to access the valley pseudo-spin degree of freedom and facilely integrate into quantum-photonic, electronic and sensing platforms via the layer-by-layer-assembly approach. Strain localized to a nm scale region of vdW has emerged as an effective means of creating quantum defects in vdW materials. This approach allows only a limited degree of control on placement, emission wavelength and quantum coherence properties of defects. Quantum defects realized to dates emit at wavelengths shorter than 1 micron while the defects operating in telecommunication wavelength range are highly desirable. More importantly, how and why localized strain creates the quantum defects is poorly understood. Studies have suggested that strain simply activates the optical properties of naturally existing point defects.¹ Aiming to address these issue, Center for Integrated Nanotechnologies is developing novel strain engineering approaches in conjunction with ion beam implantation capabilities for deterministic creation of defects in vdW materials. Here in this talk I will provide an overview of these capabilities development efforts and recently discovered novel quantum defects: telecomm quantum emitters of MoTe₂² and quantum defects of TMD/2D magnet heterostructures realized via local strain engineering.
¹ Linhart, L., Paur, M., Smejkal, V., Burgdörfer, J., Mueller, T. & Libisch, F. Phys. Rev. Lett. 123, 146401 (2019).
² Zhao, H., Pettes, M. T., Zheng, Y. & Htoon, H. arXiv preprint arXiv:2105.00576 (2021)

The controlled formation of structural defects, ranging from single-atom vacancies to nanopores, has been broadly implemented as a method of modulating the properties of two-dimensional materials (2DMs). Moreover, theoretical studies predict that point defects in 2DMs provide nucleation sites for metal adatoms and clusters^1,2 and that nanopores in graphene facilitate the growth of metal islands.³ These suggest that defect engineering may be extended to the site-selective growth of metals on 2DMs, enabling applications that require deterministic patterning of nanoscale features. Focused ion beams (FIBs) are well-suited for patterning 2DMs with nanometer precision and can be used to mill point defects⁴ and sub-10nm features⁵ in suspended 2DMs by tuning irradiation parameters that influence the ion dose and beam profile. To optimize the irradiation parameters for deterministic metal nucleation sites, an experimental investigation of defect creation mechanisms and metal-2DM interactions is required.

Here, we study the structural modifications that arise from FIB patterning of suspended 2DMs and the effects of patterning on subsequent metal nucleation and growth. Obtained by mechanical exfoliation of bulk crystals, layered 2DMs (graphene, MoS₂, WSe₂) are transferred to holey Si₃N₄ transmission electron microscope grids and annealed in ultra-high vacuum (UHV) to remove surface adsorbates, resulting in nearly pristine suspended membranes. Ions are delivered to the 2DM surface as arrays of spots and lines using He⁺ in the Zeiss ORION helium ion microscope and Si²⁺ and Au²⁺ in VELION FIB-SEM. Ion doses are calibrated and the resulting defects and nanopores are characterized with aberration-corrected scanning transmission electron microscopy. We evaporate metals (Au, Ti) onto the patterned 2DM surface in UHV and study, post-deposition, the extent to which the defects, ion species, dose rate, and 2DM thickness affect the metal nucleation and growth. We also heat the samples post-deposition to examine with in situ UHV transmission electron microscopy the effect of post-deposition annealing on island shape and size. The island geometry is well suited for automated image analysis to obtain statistics of the nucleation density and location. The materials preparation and analysis technique can be generalized to the site-selective growth of other materials, such as Si and Ge via chemical vapor deposition or metals via thermal and e-beam evaporation. This templating and nucleation control strategy opens routes towards the directed self-assembly of 3D nanoislands on 2DMs and other mixed-dimensional heterostructures.


¹ S. Malola, H. Häkkinen, and P. Koskinen, Appl. Phys. Lett. 94, (2009).
² W. Ju, T. Li, X. Su, H. Li, X. Li, and D. Ma, Phys. Chem. Chem. Phys. (2017).
³ S. Antikainen and P. Koskinen, Comput. Mater. Sci. 131, 120 (2017).
⁴ J.P. Thiruraman, P. Masih Das, and M. Drndić, Adv. Funct. Mater. 29, 1904668 (2019).
⁵ Y. Deng, G. Wang, Y. Qiu, D. He, J. Lin, J. He, and J. He, Nanotechnology 31, (2020).

There is a continuous pursuit for new materials to enable the development of small-scale, power-efficient electronics for next generation devices and applications. During the past decade and a half, two-dimensional (2D) materials have received particular attention in this regard with their extraordinary electronic characteristics. Defects in the form of vacancies, interstitials etc. have a profound influence on carrier transport and local conductance in these materials and as such, would constitute a key factor affecting device performance. Therefore, it is of critical importance to experimentally study defects in 2D materials, in terms of their atomic structure and influence on surface electronics. Motivated in this fashion, we present here an atomic-resolution study of native defects in thin crystals of α–Mo₂C, an emerging 2D transition metal carbide with attractive physical properties and potential to be employed in electronic applications. In particular, we perform conductive atomic force microscopy (C-AFM) experiments on α–Mo₂C crystals grown on copper foils via chemical vapor deposition, revealing the presence of two types of defects that either enhance or inhibit the local conductivity in their vicinity (referred to as “bright” and “dark” defects, respectively), over a lateral span of 2 to 10 nanometers. The simultaneous recording of topography maps together with atomic-resolution conduction maps allow us to make conclusions about the structure and location of defects in the crystals. Specifically, no features are detected in topography maps near both types of defects, suggesting that they are located below the top surface of the crystals. Additionally, while the influence of dark defects on the electronic landscape are diffuse such that the regular atomic structure of Mo₂C appears mainly undisturbed in conductivity maps, bright defects significantly distort the local conductivity, hinting at a stronger influence on local density of states. The findings presented here have the potential to provide critical fundamental information not only for electronic applications but also for other fields ranging from energy storage to catalysis where 2D transition metal carbides are envisioned to be used.

Rechargeable batteries are considered the key energy storage for portable electronics due to their ability to restore the charge and provide consistency in the voltage. Carbon-based materials remain the safest, cost-efficient, and environmentally stable anode material providing high-power density and energy compared to any other active materials.
I will discuss the advantageous layered graphenic structures versus commonly used graphitic negative electrodes in Lithium-ion batteries (LIB). The focus will be the study of interlayer structural features of multilayered graphene and its direct impact on Lithium intercalation. As-synthesized layered graphene with rotated layers is studied revealing that broken Van der Waals forces preceded intense Lithium diffusion and intercalation reaching up Li₂C₂ component formation. A large Li/electrolyte interface brings an enhanced charge-transfer kinetics resulting in effective charge/discharge cycling throughout hundreds of cycles with over 1800 mAh/g specific capacity.
This graphene anode's feasibility allows developing over 500 Wh/kg energy density in LIBs.

Emerging 2-dimensional (2D) magnets provide opportunities for reducing magnetism to atomic length scales. For 2D magnets, investigations of their local magnetic structure will be important for future generations of magnetic memory storage, where current technologies utilizing ferromagnetic materials will soon reach fundamental scaling limits. In particular, 4-dimensional scanning transmission electron microscopy (4D-STEM) with the electron microscope pixel array detector (EMPAD) have provided ways to image magnetic domains, topological structures such as skyrmions and local chirality. In my talk, I will highlight new developments for imaging magnetic structures with 4D-STEM and how we can apply these new techniques to the investigation of magnetic properties in 2D materials.

Van der Waals 2D magnets are recently discovered and have triggered significant research interests. These Magnetic 2D materials provide unique properties to study the magnetism response in the 2D limit, and thus an ideal platform for the integration with other atomic thin semiconductor heterostructures to explore the manipulate spin and photon degrees of freedom (DOF), facilitating control of spin degeneracy and light-matter interactions of well-defined quantum states. Magnetic proximity effect between monolayer transition metal dichalcogenides (TMDs) and 2D magnets have been proved in a few heterostructures (WSe₂/CrI₃ and MoSe₂/CrBr₃) based on manipulation of the PL intensity/linewidth and valley Zeeman splitting [1-3]. 0D strain-induced quantum emitters in WSe₂ are also employed to determine the proximity exchange interaction in WSe₂/CGT, WSe₂/FGT, and WSe₂/CrI₃ devices [4-6]. Band alignment and spin-dependent interlayer charge transfer are crucial for this effect which also strongly depends on the distance between layers due to the short range of the electronic wave function at the interface. However, most 2D magnets are sensitive to air which hinders the application of spintronic devices. In contract with these extremely air unstable magnetic 2D materials, multiple 2D magnetic materials including the MPS₃ family (M= Mn, Ni, Cr, Co, Fe), CrPS₄, and CrSBr have emerged as environmentally stable materials providing a new opportunity for assembling air–stable devices. Moreover, the observation of magnetic exciton in CrPS₄, NiPS₃, and CrSBr with strong anisotropy also expands research possibilities of band alignment and many-body exciton interactions between them and TMDs. Here, we use monolayer WSe₂ as a probe to investigate the interplay between the magnetic proximity effect and valley pseudospin dynamics in air-stable 2D antiferromagnetic magnets (NiPS₃, CrPS₄, and CrSBr) heterostructures via polarization-resolved low-temperature PL, time-resolved PL, and magneto-optical spectroscopies. We will also report the interesting properties of strain-induced quantum defects created in TMD/2D magnet heterostructures.

References:
[1] D. Zhong et. al Nature nanotechnology 2020, 15 (3), 187-191
[2] T. P. Lyons et. al Nature communications 2020, 11 (1), 1-9
[3] L. Ciorciar et. al Physical Review Letters 2020, 124 (19), 197401
[4] K. Shanyan et. al Nano letters 2019, 10, 7301-7308
[5] N. Liu et. al Nanoscale 2021, 13 (2), 832-841
[6] A. Mukherjee et. al Nature communications 2020, 11 (1), 1-8

2D hybrid organic-inorganic perovskites (HOIPs) are low-cost, high-performance semiconductor materials with great potential demonstrated in various fields, including photovoltaics, light-emitting diodes, lasers and field-effect transistors. During device fabrication and device operation, 2D HOIPs will go through various temperature statuses. Because of the hybrid organic and inorganic nature, the temperature fluctuation will cause significant influence on the mechanical behavior of 2D HOIPs, which is vital for material processing and device durability, but is largely unknown. We measured the in-plane elastic modulus E of a prototypical 2D HOIP, (C₄H₉-NH₃)₂(CH₃NH₃)₄Pb₅I₁₆ (abbreviated as C4n5) as a function of temperature across a range of technological interest (i.e., -15 ^oC to 65 ^oC) by AFM stretching suspended 2D HOIP nanosheets. Unlike their 3D counterpart, CH₃NH₃PbI₃, which shows negligible temperature-dependent mechanical property, C4n5 exhibits a strong thermo-mechanical response in the tested temperature range. E is a non-monotonic function of temperature, which peaks slightly above room temperature, but are lower both at high and low temperature range. This trend clearly deviates from the general anharmonic expectation of the thermo-mechanical behavior for materials. Further structural analysis suggests that the observed thermo-mechanical behavior is due to the formation of gauche defects in the organic spacer molecules at the van der Waals interfaces, which mediates the mechanical coupling between the 2D layers. We further demonstrated that exposure to moisture results in defects in the 2D HOIP nanosheets and the defect density can strongly modulate the thermo-mechanical behavior of C4n5, probably due to a mix-phase of 2D and 3D HOIPs.

Two-dimensional (2D) materials and van der Waals (vdW) heterostructures have emerged as promising candidates for post-Moore electronics due to their unique electronic properties and atomically thin geometry. In this talk, I will start with an important type of in-memory computing device, robust memristors with excellent memory performance and thermal stability, which can be created from a vdW heterostructure composed of graphene/MoS_2–xO_x/graphene.^[1] I will then discuss a prototype reconfigurable neural network vision sensor that operates via the gate-tunable positive and negative photoresponses of a WSe₂/BN heterostructure.^[2] A neuromorphic vision system with brain-inspired visual perception can be further realized by networking such retinomorphic sensors with a memristive crossbar array.^[3] Our latest results on an electrically tunable homojunction for reconfigurable circuits,^[4] as well as a scalable massively parallel computing scheme in crossbar arrays^[5] will be presented in the last part of my talk.

References:
1) M. Wang, S. Cai, C. Pan, C. Wang, X. Lian, Y. Zhuo, K. Xu, T. Cao, X. Pan, B. Wang, S. Liang, J. Yang, P. Wang, F. Miao, Nature Electronics 1, 130 (2018).
2) C. Wang, S. -J Liang, S. Wang, P. Wang, Z. Li, Z. Wang, A. Gao, C. Pan, C. Liu, J. Liu, H. Yang, X. Liu, W. Song, C. Wang, B. Cheng, X. Wang, K. Chen, Z. Wang, K. Watanabe, T. Taniguchi, J. Yang, F. Miao, Science Advances 6, eaba6173 (2020).
3) S. Wang, C. Wang, P. Wang, C. Wang, Z. Li, C. Pan, Y. Dai, A. Gao, C. Liu, J. Liu, H. Yang, X. Liu, B. Cheng, K. Chen, Z. Wang, K. Watanabe, T. Taniguchi, S. -J Liang, F. Miao, National Science Review 8, nwaa172 (2021).
4) C. Pan, C. Wang, S. -J Liang, Y. Wang, T. Cao, P. Wang, C. Wang, S. Wang, B. Cheng, A. Gao, E. Liu, K. Watanabe, T. Taniguchi, F. Miao, Nature Electronics 3, 383 (2020).
5) C. Wang, S.-J Liang, C. Wang, Z. Yang, Y. Ge, C. Pan, X. Shen, W. Wei, Z. Zhang, B. Cheng, C. Zhang, F. Miao, Nature Nanotechnology (2021) (in press).

The presentation will outline several chemical manipulation techniques developed in our laboratory to control the properties of graphene, including non-destructive functionalization, nanoparticle incorporation and biomolecular interfacing. The following topics will be discussed: (i) modification of phononic energy of antibody-coupled graphene when it interfaces with SARS-CoV-2 spike protein. (ii) phononic energy’s modification with ionic flow over graphene, (iii) unique eta-6 organometallic approach to functionalize graphene in a vapor-phase process, while retaining its structural and electrical properties and offering chemical sites for nano-interfacing of plasmonic centers for enhanced photovoltaics; (iv) biointerfacing of graphene with electrogenic bacteria for microbial energy devices; and (vi) Photovoltaics via electronic coupling of graphene atop a bulk semiconductor to induce interfacial energy-band reorganization for light-sensitive junctions only one atom below the front surface.

Interlayer excitons in 2D semiconductors have emerged as a promising candidate for engineering excitonic devices, due to the strong coupling with light, large exciton binding energy, and gate tunability. However, the exciton is a charge-neutral particle that can not be controlled by the in-plane electric field and thus inhibits the study of the exciton transport. Here, we demonstrate the directional transport of interlayer excitons in bilayer WSe₂ driven by the dynamic piezoelectric field induced by surface acoustic waves (SAW) on LiNbO3. We show that at 100 K, the SAW-driven excitonic transport is activated above a threshold acoustic power and reaches a distance up to 20 um which is far beyond the diffusion length, and only limited by the device size. Our work shows that acoustic waves are an effective contact-free approach to control exciton dynamics, which can provide a novel physical platform for the study of 2D excitons such as the exciton valley hall effect and also be promising for realizing 2D materials-based excitonic devices such as exciton transistors, switches, and transducers.

Measuring and manipulating circular polarization is central to numerous existing and emerging technologies, including spintronics, bio-inspired navigation, valleytronics, quantum computing and communications, and spectroscopy. Here, we demonstrate a method to tune the photoluminescence (PL) intensity (factor of x161), peak position (38.4meV range), and degree of circular polarization (DoCP) (39.4% range) with extraordinary precision using a low-power laser in Bi₂Se₃-WS₂ 2D heterostructures. Few-layer, crystalline Bi₂Se₃ is grown on monolayer WS₂ using chemical vapor deposition (CVD). The Bi₂Se₃ grows at a ~0° twist angle, and density functional theory (DFT) calculations predict a coupling between the layers that possibly facilitates non-radiative recombination. The as-grown 2D heterostructure PL intensity is low; however, when exposed to a low-power laser (0.8mW) in ambient, the PL smoothly increases intensity, the peak position shifts, and the DoCP decreases. Atmospheric measurements suggest oxygen is absorbing into the 2D heterostructure. We use first-principles calculations to elucidate the role of oxygen incorporation on the electronic structure. We show that the intensity and peak position changes can be reversed by desorbing the oxygen using a laser in a nitrogen (N₂) environment, and that it can be reused. Our technology can be used to locally modify the valley polarization with submicron precision, a promising advancement for spintronics and valleytronics applications.

Quantum systems, and the field of quantum information science, are quickly becoming a hotbed for technological breakthroughs. At the heart of many of these technologies is the single-photon source (SPS). While a variety of systems are currently used for this purpose—from quantum dots to nitrogen-vacancy centers in diamond—they are not without their limitations. Often, cryogenic temperatures are required, tunability is lacking, or integration with other components is difficult. In an attempt to address some of these challenges, we examine a solid-state system with the potential of realizing tunable SPSs at room temperature by locally inducing large amounts of strain in a monolayer sheet of WSe₂. Due to the very low defect density and single crystal nature of the material, it is able to strongly conform to patterned substrate features. In so doing, nanoscale wrinkles are formed on, around, and in-between the structures, with orientations influenced by the patterned array. These highly strained regions can effectively localize excitons and produce room temperature low-energy emission, similar to that reported in our previous work on strained nanobubbles. When cooled to cryogenic temperatures, a rich milieu of sharp spectral features is revealed in these areas, which suggests that the emission seen at room temperature is quantum in nature and tunable with strain.

Monolayer (ML) transition-metal dichalcogenides (TMDs) exhibit a direct bandgap and a relatively high carrier mobility. This makes them as excellent building blocks for applications in optoelectronics, photovoltaics and catalysis.

Electronic and optoelectronic applications with TMDs rely on the good functioning of the junction with metals. While the van der Waals nature of TMDs should in principle imply absence of states in the middle of the TMD bandgap, vacancies and point defects in the material may result in atom exchange and thus doping of the TMD by the metal. Chemical interaction at the interface, such as creation of metal-chalcogenide bonds, could lead to a configurational change in the TMD. In addition, ML TMDs can be easily perturbed by strain or charge transfer from the underlying layers due to their ultra-thin nature. Similar to classical low-dimensional structures, the physics of the metal- semiconductor junction shall dictate the behavior of the device. A fundamental understanding of the metal-TMD junction is thus extremely relevant for the reliability of TMD-based electronics and/or optoelectronics.

Raman spectroscopy is a versatile tool for characterization of 2D materials, especially in terms of quantifying the number of layers, and evaluating the effects of strain, doping and defects. In the case of monolayer TMDs, the most prominent features in the Raman spectra are in-plane (E_2g) and out-of- plane (A_1g) peaks. Among the recent Raman studies on TMD-metal interfaces, many have reported peak splitting of the A_1g and E_2g modes. A variety of attempts have been made to explain the peak splitting, usually in terms of strain and charge transfer effects. While these effects could cause splitting of individual peaks in some cases, they are insufficient to explain splitting in both A_1g and E_2g peaks simultaneously. To better understand the interaction at ML TMD-metal interfaces, which is extremely important for device performance, it is necessary to provide a full explanation of the mechanism causing the Raman peak splitting that occurs in this case.

In this work, we look into the origin of A_1g and E_2g peak splitting through a systematic measurement and simulation of Raman spectra, along with relevant literature review. We focus on MoS₂/Au as a representative system. We illustrate why strain and charge transfer cannot be the main contribution to the simultaneous splitting of both A_1g and E_2g Raman peaks. We propose a mechanism based on plasmon-phonon interaction and presence of extrinsic defects at the TMD-metal interface as the main driver leading to peak splitting. We attribute the new peaks to non-Γ phonon modes originating from in-plane (E_2g) and out-of-plane (A_1g) phonon branches at certain non-zero wave vector. We test the validity of our hypothesis by investigating samples with different interface configurations, including the exchange of Au with Al.

There has been a rising notion of the wave nature of phonons over the past decade. Notably, recent experiments and atomistic modeling have revealed ballistic transport of coherent phonons in superlattices containing dense interfaces. These coherent phonons allegedly arise from the interference of backscattered phonon waves at the densely packed interfaces, which should otherwise greatly hinder the transport of conventional, particle-like phonons. In this invited talk, we will discuss our recent findings of some exotic (and non-exotic) behaviors of coherent phonons in superlattices and phononic crystals, particularly in those made of two-dimensional materials. First, we will demonstrate that randomizing the layer thicknesses of a superlattice, which leads to localization of phonons, can reduce its thermal conductivity to even below the amorphous material limit, which can hardly be achieved by merely scattering phonons. Second, we will discuss the effect of reduced dimensionality on coherent phonon behaviors: can two-dimensional materials demonstrate more prominent coherent phonon transport than three-dimensional ones? what is the effect of anharmonic scattering on coherent phonons? can we find stronger phonon localization in two-dimensional systems than three-dimensional ones? We will explain these behaviors by decomposing thermal transport into coherent phonon and incoherent phonon contributions via the Landauer-Datta-Lundstrom approach. Detailed spectral phonon analysis will further reveal the interplay between coherent and incoherent phonons, as well as their respective effect on thermal transport.

Helical conductors, systems that have no bulk conduction but support dissipationless conducting states at their edges, may be engineered to realize Majorana statistics for quantum computation. Underlying these remarkable systems are the non-trivial topology of electronic structure in the bulk, arising band inversion in the bulk and crossing of conductance and valence bands at the system boundary. In helical conductors, these counterpropagating states carry different spin quantum numbers, protecting the crossing and preventing a gap from opening in the spectrum of edge states. Traditional helical conductors, such as those achieved in topological insulator systems, are typically not tunable in situ, and helical conduction is only achieved over a narrow range of parameters.
Here we show that helical edge states are achievable and tunable in few-layer van der Waals materials. In Bernal-stacked trilayer and tetralayer graphene, we observe helical edge states at moderate and strong magnetic fields, respectively, arising from the competing effects of inter-layer coherence, electrostatic polarization and exchange interaction. As the interlayer potential and magnetic field varies, we observe a series of quantum transitions among the phases that host 2, 1 and 0 helical edge states on each edge. Our work highlights the complex competing symmetries in few-layer graphene and the rich quantum phases in this seemingly simple system[1, 2]. Lastly, in thin exfoliated Bi4I4 samples, which is a quasi-1D topological insulator and candidate for higher order topological states, we observe gate tunable magneto-transport and Josephson current. Our combined transport, photoemission, and theoretical results indicate that the gate-tunable channels consist of novel gapped side surface states, a 2D TI in the bottommost layer, and helical hinge states of the upper layers[3].
References
[1] P. Stepanov, Y. Barlas, S. Che, K. Myhro, G. Voigt, Z. Pi, K. Watanabe, T. Taniguchi, D. Smirnov, F. Zhang, R. Lake, A. MacDonald, and C. N. Lau, Quantum parity Hall effect in Bernal-stacked trilayer graphene, Proc. Natl. Acad. Sci. 116, 10286 (2019).
[2] S. Che, Y. Shi, J. Yang, H. Tian, R. Chen, T. Taniguchi, K. Watanabe, D. Smirnov, C. N. Lau, E. Shimshoni, G. Murthy, and H. A. Fertig, Helical Edge States and Quantum Phase Transitions in Tetralayer Graphene, Phys. Rev. Lett. 125, 036803 (2020).
[3] Y. Liu, R. Chen, Z. Zhang, M. Bockrath, C. N. Lau, Y.-F. Zhou, C. Yoon, S. Li, X. Liu, N. Dhale, B. Lv, F. Zhang, K. Watanabe, T. Taniguchi, J. Huang, M. Yi, J. S. Oh, and R. J. Birgeneau, Gate-Tunable Transport and Unconventional Band Topology in Quasi-One-Dimensional a-Bi4I4 Field Effect Transistors, preprint, submitted (2021).

Since the experimental realization of graphene [1], there is an increasing number of works for new two-dimensional materials (2D) [2]. In this work, we investigated using ab initio methods, the structural, electronic, and mechanical properties of a new 2D semiconductor material, named (TTA-2D) [3], which is topologically based on the molecular structure of Thiophene- Tetrathia-Annulene (TTA), which has been already synthesized. Our results showed TTA-2D has a small indirect bandgap semiconductor of about 0.6 eV with a semiconductor-metal transition when a uniaxial strain is applied. We also carried out molecular dynamics at high temperatures and observed that TTA-2D is thermally stable up to T = 1000 K. From optical analysis, our results also show that TTA-2D absorbs in a large energy range, from infrared to ultraviolet regions. The analysis of refractive and reflectivity index shows that TTA-2D reflects only 10% of the incident light within the visible region. Therefore, these results suggest that TTA-2D is a promising material for solar cell applications.
[1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V.Grigorieva, and A. A. Firsov, Science, 306, 666 (2004).
[2] R. M Tromer, M. G. E. da Luz, M. S. Ferreira, L. F. C. Pereira, The Journal of Physical Chemistry C, 121,3055 (2017).
[3] R. M. Tromer, L. D. Machado, C. F. Woellner, and D. S. Galvao, Physica E 129, 114586 (2021).

Defect engineering in transition metal dichalcogenides (TMDs) is a key approach to controlling the properties of TMDs. Defects in TMDs like chalcogen vacancy can induce the phase conversion of TMDs. Because TMDs have different properties depending on the phase, controlling specific phase of TMDs through defect engineering is crucial to obtain specific properties.
For instance, group VI TMDs such as MoS₂ and WS₂ have thermodynamically stable H phase and unstable T phase. In application of these TMDs in hydrogen evolution reaction (HER), unstable T phase exhibits better performance. However, it is difficult to obtain and maintain the T phase, which causes stability problems. Many research groups have found the ways to tackle this issue. Formation of S vacancies (V_S) on TMD layer is one of the ways for the phase conversion of TMDs.
Recently, our colleagues proposed novel method of forming V_S for phase conversion of MoS₂ from stable H phase to unstable T phase. [1] In this method, S vacancy is formed on MoS₂ by the reaction between CO and S, and the phase conversion of MoS₂ occurs. This method was also applied to WS₂. It was found that the phase conversion of WS₂ required higher annealing temperature and CO mole fraction than that of MoS₂. To reveal the mechanism of CO-induced phase conversion and elucidate the difference in the phase conversion conditions, we conducted DFT calculations. We calculated the reaction barriers of CO-mediated V_S formation and S diffusion to octahedral site of MoS₂ [2] and WS₂. V_S on the edge sites of MoS₂ and WS₂ were formed. Then, S atoms at the trigonal prismatic site diffused to the octahedral site. The reaction barrier for V_S formation on edge site and S diffusion to octahedral site of WS₂ was higher than that of MoS₂. By DFT calculation, we revealed the mechanism for CO-induced phase conversion of MoS₂ and WS₂. And we identified the cause of the difference in experimental conditions. Consequently, this work provides insights into the defect engineering for phase conversion of several TMD materials using gas-solid reactions.

References
Nam, D. H.; Kim, J. Y.; Kang, S.; Joo, W.; Lee, S. Y.; Seo, H.; Kim, H. G.; Ahn, I. K.; Lee, G. B.; Choi, M.; Cho, E.; Kim, M.; Nam, K. T.; Han, S.; Joo, Y. C. Nano Lett. 19, (12), 8644-8652 (2019).
Lee, S.; Hong, D.; Kim, J. Y.; Nam, D. H.; Kang, S.; Han, S.; Joo, Y. C.; Lee, G. D. ACS Appl. Nano Mater. 4, (5), 5496-5502 (2021).

Large arrays of quantum dots at controlled positions could be pivotal for enabling practical applications of quantum dot systems. While single quantum dots have been widely studied in many material sysetms, creating a large array of quantum dots face different challenges. The engineering flexibility of atomically thin van der Waals crystals have brought about new possibilities to realize localized excitons. Here we explore two ways to create large arrays of quantum dots using transition metal dichalcogenides. First, we demonstrate rapid creation of large, rewritable arrays of localized emitters with high yield through strain engineering. Second, we show that uniform quantum dot arrays are naturally formed via moire potential confinement in twisted bilayers. Both types of quantum dot arrays are compatible with integration with other on-chip photonic structures and large scale fabrication.

Vacancy defects, despite being commonly present in nanomaterials, are inadequately understood in terms of their role in modulating interfacial fluid flow. In this work, we study the frictional properties of hexagonal boron nitride (hBN), an up-and-coming 2D material, by considering five different types of “realistic” vacancy defects present in it. In recent years, hBN has gained increasing attention for use in desalination membranes, oil-water separation devices, and osmotic energy harvesting, where hBN-water interfaces are ubiquitous. Thus, understanding solid–liquid friction on real hBN surfaces is important to design and develop such applications. Here, we present a combined quantum-classical approach to achieve this objective. To this end, we use quantum-mechanical density functional theory to obtain point charges to accurately represent the unique electric field produced by each of the five defects considered. Subsequently, using classical molecular dynamics simulations, we study the role of the vacancy composition and concentration in modulating the hBN–water friction coefficient, and the resultant slip length for water flow on hBN. Our findings demonstrate the use of vacancy defects to tune the water flow and slip on hBN, and indicate the promise of defective hBN as an alternative high-slip surface to graphene.

Defect engineering of atomically thin semiconductor crystals is an important approach to tailoring their optical properties for developing single-photon sources and valleytronic devices. Recently, He ion and electron beam irradiation of transition metal dichalcogenide (TMD) monolayers has been shown to induce single-photon emitting defects. Here, we report the generation of robust luminescent defects in chemical vapor deposition (CVD)-grown monolayer MoS₂ and WS₂ via proton beam irradiation with a wide range of ion beam fluences from 10¹¹ to 10¹⁴ cm^-2. These defects exhibit subgap photoluminescence peaks that are 100 to 200 meV below the neutral exciton peak. Remarkably, these emissions are robust even at room temperature, unlike the previously reported defect-induced emission. They exhibit sublinear power-dependent emission intensity and long emission lifetime (> 1ns), which are characteristics of localized emitters. Atomic resolution scanning transmission electron microscopy (STEM) images reveal that the areal density of sulfur vacancies is increased by ~10¹³ cm^-2 after ion beam irradiation, indicating that unpassivated sulfur vacancies are responsible for the observed localized emissions. We conduct DFT and GW-BSE calculations of TMDs with different defect configurations to further discuss the potential role of defect complexes.

Deterministic creation of quantum emitters in transition metal dichalcogenides (TMD) has become a key challenge for their implementation into quantum technology. One of the promising routes to generating quantum emitters is controlled defect engineering by ion beam irradiation. In this study, we investigate the effect of high energy proton beam irradiation on deterministic creation of optically active defects in monolayer MoSe₂. We show that proton beam irradiation results in emergence of three narrow sub-gap emission peaks, which are stable up to 160 K. We probe the nature of the defects by studying their valley selectivity and the effect of electrostatic doping. Gate-dependent photoluminescence suggests that the sub-gap emission is suppressed when states near the conduction band edges are depopulated. Our observations are in good agreement with the recent theoretical predictions for selenium-vacancy-induced optical transitions in MoSe₂. Our scanning transmission electron microscopy (STEM) imaging reveals that proton beam irradiation results in increase in selenium vacancies but no changes in metal vacancy concentration, which further corroborates that these emissions are related to unpassivated selenium vacancies created by proton beam irradiation. Our work demonstrates that proton beam irradiation can serve as a simple approach to creating optically active defects in TMDs.

We report the production of defects in suspended graphene that have an extremely high defect density and low activation healing energy using electron irradiation at 20 keV from a scanning electron microscope (SEM). The graphene was suspended over microchambers etched in SiO₂, measuring 5 microns in diameter and 400nm in depth. The graphene completely seals the chamber with air at atmospheric pressure. The samples were then placed in an SEM system and irradiated to a dosage of about 1 x 10¹⁷ electrons/cm². The graphene effectively seals the microchamber so that the air pressure inside does not significantly decrease from atmospheric pressure during the irradiation. This produced an extremely high defect density with a sharp Raman D peak and Raman D peak height to G peak height ratio, I_D/I_G, of over 4.5. Thermal studies showed that the defects almost completely heal after annealing the sample at only 200 ^oC for 5 minutes. In contrast, microchambers in which the graphene does not completely seal the chamber show I_D/I_G ratios of 0.14. The high defect density achieved in the sealed microchambers is attributed to the atmospheric pressure inside and strain induced by the curvature. The low healing temperature rules out damage in the form of lattice defects such as vacancies.The method by which the defects were produced involving SEM, which is compatible with nanolithography, makes it useful for device applications.

Hexagonal boron nitride (hBN), a van der Waals crystal with a honeycomb structure similar to graphene, has been widely investigated as a dielectric substrate for 2D-based devices. More recently, hBN has emerged as a prominent candidate for quantum nanophotonics due to the observation of a tunable single photon emission. These emitters have been associated with mid-gap states in ultra-wide bandgap hBN (~6eV) that originate from atomistic defects in the hBN lattice. Here, we report the observation of near-infrared (~ 800 nm) photoluminescence (PL) emission in hBN at room temperature caused by defects formed via focused He-ion beam irradiation. We tune PL intensity of the emitters by tailoring the defect generation in hBN as a function of the irradiation parameters. Raman spectroscopy and high-resolution scanning transmission electron microscopy imaging further confirm that the emerging PL response is correlated with point-defect states in hBN. We also demonstrate that similar results can be obtained by Ga-ion irradiation, confirming the PL emission to be related to intrinsic structural defects in hBN. Our work presents a platform to precisely create and control defect-related color centers in hBN that can serve as building blocks for next-generation nanophotonic and possible quantum devices.

Modulating physical (electrical, optical, and electrochemical) properties of transition metal dichalcogenides (TMDs) by oxygen plasma treatment has attracted a great deal of attention. However, the effect of exposure time on oxidation and etching process in oxygen plasma treatment and creating lattice defects still lacks systematic structural property investigations at the nanoscale. In this work, we have employed optical microscopy, scanning electron microscopy, micro-and nanoscale Raman (tip-enhanced, TERS) and photoluminescence (TEPL) spectroscopy, and scanning probe microscopy (atomic force and Kelvin Probe) to characterize changes in surface morphology, spectral signatures, electronic structure, oxidation, etching and/or clustering behaviors of atomically thin molybdenum disulfide (MoS₂) nanosheet layers, mechanically exfoliated on gold-coated substrates, treated by oxygen plasma with different times. For oxidation plasma (electron beam), the process is predominantly oxidation from 0 to 20s, while etching becomes the dominant behavior beyond 20s for monolayer (1L) to trilayer (3L) MoS₂ nanosheets under medium power oxygen plasma. By means of TERS, we uncover spectral heterogeneities, strain changes spatially localized to regions as small as 30 nm clustering domains due to sulfur vacancy followed by oxygenation and forming MoO₂/MoO₃, enhancement of certain Raman peaks due to strong electromagnetic confinement between the tip and MoS₂ and appearance of new peaks, often unobserved in confocal Raman spectroscopy. As for excitonic properties measured through photoluminescence spectroscopy, we observed evolution of trion peak and suppression or quenching of neutral exciton A peak with increased oxygen plasma exposure. We also estimated the concentration of sulfur vacancy via correlating the variation in characteristic E’ and A’ band position (~ 4 cm^-1) of MoS₂. We find that regions of maximum local strain correspond to the regions of topographic curvature extracted from AFM measurements. Likewise, we observed nanoscale variation in work function correlated with an alloy of MoS₂/MoO_3-x. The TERS study allowed us to determine various chemical compositional phases and built-in strain that arises when 2D materials interact with Au under oxygen plasma. This work is financially supported in parts by NSF and NSF-MRI Grants.

Modulating physical-chemical (electrical, optical, and electrochemical) properties of transition metal dichalcogenides (TMDs), particularly MoS₂, has attracted a great deal of attention for diverse technologies. In this work, we exposed exfoliated monolayer MoS₂ under electron beam irradiation with an energy of 15 keV for different times and investigated the possible effects of the structural defects intentionally created by electron-beam on electronic properties at the nanoscale. Among various ways in which an electron beam adversely affects the sample are: local heating, electrostatic charging, displacement damage, sputtering, and hydrocarbon contamination. However, the effects of exposure time on the oxidation and etching process in creating lattice defects by e-beam irradiation still lack systematic investigations. In this work, we have employed scanning electron microscopy, micro-and nanoscale Raman (tip-enhanced, TERS) and photoluminescence (TEPL) spectroscopy, atomic force, and Kelvin Probe microscopy to characterize changes in surface morphology, spectral signatures, oxidation, etching, and/or defect clustering, to obtain high-resolution images and quantitative measurements of the work function heterogeneities. We unravel the evolution of defects formed under electron beam irradiation and observe the formation of metastable defect domains with different work function values. The experiments show that material diffused outward forming ring-like morphologies which become prominent with increasing exposure time while sulfur vacancies formed after exposure to diffuse, coalesce, and migrate molybdenum atoms at the edge of the ring while leaving patches of MoS₂ in the center, suggested by work function variation bringing the system from a metastable to equilibrium ground state. In light of recent experimental evidence, the results provide the importance of defect control and determined that the amount of radiation damage is proportional to the electron dose; and the extent of the damage is proportional to the amount of energy deposited in the sample. The results of this study enhance understanding of the electrical properties of MoS₂ FET devices in terms of defect sites on the MoS₂ surface that work as trap sites, which can deteriorate the carrier mobility and carrier concentration similar to what occurs during oxygen plasma. This work is financially supported in parts by NSF and NSF-MRI Grants.

Thermoelectric performance in 2D transition metal dichalcogenides (TMDs) are usually optimized through controlling carrier concentration, band engineering, and/or scattering mechanism. Applying uniform strain to TMDs has been demonstrated to increase electrical performance and carrier mobility owing to the alteration of electronic band structure and effective carrier mass. Here, we study the effects of non-uniform strain on the thermoelectric performance of MoS₂ using SiN_x substrates with a crested morphology. The non-uniform strain is demonstrated to suppress phonon-scattering which drastically enhances carrier mobility and thermoelectric power factor in crested MoS₂ by up to 2 orders of magnitude depending on carrier concentration as compared to conventional MoS₂ on flat SiO₂ surfaces. We investigate gate- and temperature-dependent carrier mobility and density to determine the mechanisms responsible for the drastic enhancement in thermoelectric performance. Distinctively, the ultra-high carrier mobility achieved at low carrier concentrations weakens the dependence of thermoelectric parameters on carrier concentration, therefore resulting in a substantial improvement in electrical conductivity without greatly compromising Seebeck coefficient. Our work provides a universal pathway towards engineering carrier mobility and superior thermoelectric performance for a wide range of room-temperature applications.

The development of novel ultra-compact two-dimensional (2D) photonic technologies for application in quantum information processing, relies on our ability to fabricate single photon sources (SPS) in 2D van der Waals materials where we are able to on-demand position them as well as tune their optical emission properties¹. However, the challenge still remains to fully understand their physical origin in order to be able to efficiently utilize them and how to actively tune their optical properties. One solution is to implement elastic strain engineering through the introduction of a novel class of piezoelectric actuators^2,3.

In this work, we demonstrate site-controlled SPSs with reversible tuning of their emission energy by placing WSe2 monolayers on top of piezoelectric pillars capable of introducing in-plane isotropic strain fields⁴. Our findings prove that it is feasible to have a full control of the emission energy of these SPSs without affecting the single photon emission purity with relatively large energy shifts up to 18mev. The experimental results are further corroborated by finite element simulations along with analytical calculations showing that all SPSs can be energy blue-shifted. Interpretations on the origin of the SPSs will be discussed based on an exciton diffusion model accounting for the variations on the micro-photoluminescence spectra corresponding to individual quantum emitters. Our findings shed light on the understanding of the physical origin of SPSs in 2D monolayers and are of strong relevance for the practical implementation of single photon devices based on 2D materials for future applications in quantum information processing.

References:
[1] A. Srivastava et al. Nat. Nanotechnol. 10 (2015) 491
[2] J. Martín-Sánchez et al. Semicond. Sci. Technol. 33 (2018) 013001
[3] R. Trotta, J. Martín-Sánchez et al. Nature Commun. 7 (2016) 10375
[4] O. Iff, D. Tedeschi, J. Martín-Sánchez et al. Nano Lett. 19 (2019) 6931

In recent years, graphene has captured wide attention among engineers and researchers for its unique electronic, thermal, chemical, and mechanical properties, which could revolutionize a number of application areas, including electronics, nanotechnology, drug delivery, sensors, and catalysis. Since discovery, extensive research has been conducted theoretically, experimentally, and computationally to investigate the behavior of pristine graphene under no strain conditions. With the goal of applications in mind, it is critical to develop a fundamental understanding of the properties of defective graphene under deformed or strained conditions. While grain boundary defects within the graphene lattice are known to degrade mechanical properties and decrease both electrical and thermal conductivity, they can serve as an important source for making the graphene layer function as a chemically reactive 2D material, which may be unattainable otherwise. Furthermore, assessing and analyzing defects in the lattice is essential for application because the prevalence of defects (especially grain boundaries) causes graphene’s extrinsic effects to have greater influence on the material’s behavior than its intrinsic properties. In this work, using density functional theory simulations, we investigated the energetics of an isolated H2O molecule at a linear grain-boundary of pentagonal and heptagonal configurations formed between armchair and zigzag graphene sheets. Using SIESTA, we simulated a water molecule at an initial approach configuration parallel to the graphene lattice at different heights, and we analyzed the results of the relaxed configurations. Because we wanted to observe strain influence on the water molecule, we picked sites where the grain boundary induced tensions and compressions in the graphene lattice. Using these results, we investigated the total groundstate energies of the systems, the net force experienced by the carbon atoms, the orientations of the relaxed water molecules, and the electronic structure of the lattice. Our findings suggest that if the initial placement of the molecule is parallel to the lattice within the van der Waals interaction distance, the molecule undergoes a reorientation and the O atom faces the graphene lattice. On the other hand, if the initial position of the molecule is parallel but closer to the graphene lattice, the H atoms become more reactive and face toward the graphene lattice. We considered five different sites on the grain boundary defect and found similar behaviors throughout. An electronic structure analysis suggests that the electron redistribution at the grain boundary affects the behavior of the molecule; for electronic structure, we analyzed electronic charge and electron density to give us an indication of any polarization at the site and give insight into coulombic interactions at the defect. And through energy and force values that were calculated and plotted, we also found that chemical interactions between the water molecule and graphene lattice is negligible when the molecule is released more than 14.5Å above the sheet. Also, due to separation-distance dependent distinct chemical affinities between the C-O atom pairs vs. C-H atom pairs, the groundstate interaction is strongly affected by the initial position of the molecule. And finally, we looked at the possibilities of reorientation at relaxed states; we calculated the activation energies required to change the configurations of the relaxed water molecules and found that changes in such configurations is unattainable at room temperature. The results are expected to have important implications to our understanding of strain-chemistry of defective graphene. This talk will present the analysis of these findings as well as their potential applications in catalysis and water purification.

Bound electron-hole pairs, known as excitons, are critical to the optical and electronic responses of semiconductors. Just as for traditional materials, this principle applies to the newly-emerged Van der Waals semiconductors such as 2H-MoTe₂. Distinctinct from conventional bulk semiconductors, the quasi 2D semiconductors are soft and can get buckled spontaneously at micro or nano scale. The mechanism of spontaneous breaking of periodicity in 2D crystals has been proven by David Mermin [1], which means that real 2D crystals either break or buckle [2]. The curvature effect on excitons has been theoretically investigated for various systems [3-5] but few experimental efforts for nanoscaled bucking effect on excitons have been dedicated to Van der Waals semiconductors.
Here, we explore the relationshiop between Gaussian curvature and the excitonic behaviors in multilayer 2H-MoTe₂ by acquiring electron energy loss spectroscopy (EELS) and 4D scanning transmission electron microscopy (4D STEM) over a curved area of a 2H-MoTe₂ flake. The same types of data have also been acquired from a flat area of the same sample with a similar thickness. We carried out these experiements on a NION monochromated, aberration-corrected STEM, which provides both good energy and spatial resolutions. An energy resolution of 30 meV (with 3meV/pix) and a spatial resolution of 1.5 Å is used here. Each diffraction pattern in the 4D STEM data shows the normal direction of the tagent plane at each spot on the curved surface, and therefore the Gaussian curvature can be computed. Exciton peaks are observed from monochromated EELS with a high exposure for each spot in the scan. Unlike tensile strain effects which show redshift of the exciton peaks [6], the A exciton peaks in the curved area show a blueshift of 20 meV on average. Further results are being analyzed and will be discussed.

References
[1] N. D. Mermin, Phys Rev 176, 250 (1968).
[2] J. M. Carlsson, Nat Mater 6, 801 (2007).
[3] Y. Kayanuma and N. Saito, Solid State Commun 84, 771 (1992).
[4] T. G. Pedersen, Phys Rev B 67 (2003).
[5] T. G. Pedersen, Phys Rev B 81 (2010).
[6] O. B. Aslan et al., Nano Lett 18, 2485 (2018).

Two-dimensional transition-metal dichalcogenides (TMDCs) have attracted tremendous attention toward optoelectronic applications. Photoluminescence quantum yield (PL QY) in TMDC monolayer can approach unity at the low exciton generation rate, whereas the PL QY drastically droops at the high generation rate due to a nonradiative exciton-exciton annihilation (EEA), limiting their light-emitting device application. Here, we demonstrate a strain tunable transient-mode electroluminescence (EL) device composed of TMDC and gate oxide. WSe₂ and WS₂ monolayers exhibit near-unity PL QY even at the high exciton density by applying external tensile strain together with gate modulation due to the inhibition of van-Hove singularity resonance, which causes the EEA. With a small strain of 0.5%, notably, the WSe₂ monolayer shows EL enhancement by two order of magnitude and its internal quantum efficiency (QE) reaches 8% at maximum. Furthermore, this concept of EEA suppression by strain modulation is valid even in WSe₂ bilayer. Unstrained WSe₂ bilayer exhibits a low PL QY and EL QE at around 1% and 0.03%, respectively, because of its indirect bandgap nature. By strain modulation, however, we achieve ~50% PL QY and ~1.5% EL QE for a broad range of carrier densities. This improvement is attributed to the indirect-direct bandgap transition and EEA suppression, corroborated by density functional theory calculations. This approach will pave the way for practical optoelectronic device applications based on TMDC monolayer and even multilayers.

MXene, as a new type of 2-dimensional (2D) material, can be synthesized by removing A layers from MAX phases by HF etching. MAX phases are ternary carbides or nitrides with the general formula M_n+1AX_n, where M is an early transition metal, A is an A-group element (mostly group IIIA or IVA), X is either carbon or nitrogen with n value of 1, 2, or 3. During the selective etching process, the outer surfaces of the exfoliated MXene layers are always terminated with F, OH, and/or O groups. Thus, MXenes are referred to as M_n+1X_nT_x, where T represents the surface groups (F, OH, and/or O) and x is the number of terminations. Since now, about 20 different kinds of MXene have been prepared. More MXene materials are also expected to be exfoliated from more than 70 reported MAX phases in the future. To date, MXenes have shown attractive electronic, optical and magnetic properties, which have great potential in sensing, energy storage, and electromagnetic shielding applications. Their surface frictional properties are critically important to these applications involving layered structures and 2D interfaces. However, there has only been a few experiments conducted on MXenes. The interfacial frictional behaviors between MXenes and other 2D materials are still unknown.
Being a few atoms-thin, 2D materials have unique friction and wear properties that are different from those of their 3D counterparts. Lee et al. conducted the atomic force microscopy (AFM) friction experiments on interfaces between SiO₂ and a group of 2D materials (including graphene, MOS₂, NbSe₂ and h-BN). The results have shown that measured coefficient of frictions (COFs defined as the ratio between normal and tangential force) are dependent on both the surface conditions and the contact induced elastic deformations (which is a function of bending flexibility) of the 2D materials. Due to the rich surface chemistry of MXene based on various of possible terminating groups (F, OH, and/or O) and tunable monolayer thickness in terms of atomical structures (i.e., changing n in M_n+1X_nT_x), it was suspected that the friction between MXene and other 2D materials can be quite different from other 2D materials based on computational investigations. However, there have not been any experiments conducted on the frictional behaviors of MXenes with 2D materials. Most of the experiments conducted so far were limited to MXenes’ frictional behaviors with 3D materials.
In this paper, we experimentally investigated the frictional behaviors of Ti₃C₂T_x and Ti₂CT_x MXenes against themselves as well as other 2D materials including graphene and MoSe₂. We measured the friction forces using both the bare SiO₂ spherical AFM probes as well as Ti₃C₂T_x and Ti₂CT_x MXenes coated probes on graphene, MoSe₂, Ti₃C₂T_x and Ti₂CT_x MXenes sheets with varying number of monolayers (1-25 monolayers), and normal forces (0-2 mN). We have found that the COFs for all the investigated 2D interfaces are below 0.01 indicating the super lubricant frictional behavior of MXenes on themselves and other 2D materials. A less significant number-of-monolayer effect on the friction was also observed for MXenes. The measured COFs were also found to be independent of the applied normal forces at contacts. These results indicate the great potential of MXenes as excellent lubricating materials for 2D homo-/heterostructured materials, also the mechanical vulnerability of 2D homo-/heterstructured materials containing MXenes under shear forces.

In vertical van der Waals (vdW) homo- or heterobilayers, a finite twist angle between the two layers leads to a moire superlattice that induces periodic modulations of atomic alignments and consequently, a periodic modulation of energy, ground-states, and excited states. While most prior experiments on moire superlattices have been interpreted using a rigid lattice model in which the local atomic stacking is assumed to be determined by rotating pristine two-dimensional lattices, lattice reconstructions can occur in bilayers near a commensurate stacking style, driven by a competition between interlayer coupling and strain.

The lattice reconstructions have been visualized in various scanning probe experiments. Here, we report the signature of lattice reconstruction in phonon spectra in twisted MoS2 bilayers measured by far-field Raman experiments. Over a range of small twist angles (0-6°), the phonon spectra evolve rapidly due to ultra-strong coupling between different phonon modes and atomic reconstructions of the moire pattern. We further develop a new low-energy continuum model for phonons that overcomes the outstanding challenge of calculating properties of large moire supercells and successfully captures essential experimental observations.

The emerging field of twistronics has revolutionized quantum materials research. At the small twist limit, and particularly under strain, as atomic relaxation prevails, the emergent moiré superlattice encodes elusive insights into the local interlayer interaction. In this work we introduce moiré metrology as a combined experiment-theory framework to probe the stacking energy landscape of bilayer structures at the 0.1 meV/atomscale, outperforming the gold-standard of quantum chemistry. Through studying the shapes of moiré domains with numerous nano-imaging techniques, and correlating with multi-scale modelling, we assess and refine first-principle models for the interlayer interaction. We document the prowess of moiré metrology for three representative twisted systems: bilayer graphene, double bilayer graphene and H-stacked MoSe₂/WSe₂. Moiré metrology establishes sought after experimental benchmarks for interlayer interaction. Furthermore, in this talk we extend the previously reported framework of moiré metrology[1] to account for multi-layered structures, and demonstrate a few material systems where such a treatment is essential to describe experimental observations.

[1] Halbertal et al., Nat. Comm. 12, 242 (2021)

Dorri Halbertal¹, Nathan R. Finney², Sai S. Sunku¹, Alexander Kerelsky¹, Carmen Rubio-Verdú¹, Sara Shabani¹, Lede Xian^3,4, Stephen Carr^5,6, Shaowen Chen^1,7, Charles Zhang^1,8, Lei Wang^1,9, Derick Gonzalez-Acevedo^1,7, Alexander S. McLeod¹, Daniel Rhodes^1,10, Kenji Watanabe¹¹, Takashi Taniguchi¹¹, Efthimios Kaxiras¹², Cory R. Dean¹, James C. Hone², Abhay N. Pasupathy¹, Dante M. Kennes^3,13, Angel Rubio^3,14, D. N. Basov<sup>1

1</sup>Department of Physics, Columbia University, New York, NY, USA.
²Department of Mechanical Engineering, Columbia University, New York, NY, USA.
³Max Planck Institute for the Structure and Dynamics of Matter and Center Free-Electron Laser Science, Luruper Chaussee 149, 22761 Hamburg, Germany.
⁴Present address: Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China.
⁵Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.
⁶Present address: Brown University, Providence, RI 02912, USA.
⁷Present address: Department of Physics, Harvard University, Cambridge, MA 02138, USA.
⁸Present address: Department of Physics, University of California at Santa Barbara, Santa Barbara, CA 93106, USA.
⁹Present adress: National Laboratory of Solid-State Microstructures, School of Physics and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, China
¹⁰Present address: Department of Materials Science and Engineering, University of Winsconsin-Madison, WI 53706, USA.
¹¹National Institute for Material Science, Tsukuba, Japan
¹²John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
¹³Institut fur Theorie der Statistischen Physik, RWTH Aachen University, 52056 Aachen, Germany.
¹⁴Center for Computational Quantum Physics, Flatiron Institute, New York, NY 10010 USA.

Plasmons are collective oscillations of electrons. Plasmon resonance occurs when the collective oscillations of electrons are coupled efficiently with the incident photon through light-matter interactions. Graphene is uniquely advantageous for plasmon resonance when compared to conventional plasmonic materials due to extremely high quantum efficiency for light-matter interactions, low loss of the plasmon, and a wide range of tunability. Graphene plasmon resonance has been reported via nano-patterning, selective doping, or defect engineering. Our previous theoretical study has suggested that inducing surface inhomogeneity via wrinkling or crumpling of graphene thin films may offer increased plasmon resonance tunability over conventional electrical doping methods. Despite this significant potential, there has not yet been experimental evidence of topography-induced plasmon resonance on a semi-infinite atomically thin graphene layer. Here, we report experimental studies of plasmonic resonance tunability of a crumpled graphene using photo-induced force microscopy (PiFM). PiFM is an effective technique for probing light-matter interaction at a sub-20 nm spatial resolution, which is not possible with conventional optics-based technique due to the diffraction limit. By applying uniaxial compressive strain, we created crumpled structures of graphene with controlled topography features (e.g., wavelength, aspect ratio) on a polymeric substrate. Using PiFM, we observed a wide tunability of plasmon resonance energy (1260-1732 cm^-1) which we attributed to both the variation in topography as well as efficient coupling with the incident photon. We further investigated the relationship between the plasmon resonance wavelengths and the topography features by spatially mapping the near-field optical responses. Our observed plasmon resonance followed a redshift trend with an increase in wavelength and aspect ratio of the crumples, which agrees well with our previously established theoretical model. Finally, we demonstrated enhanced chemical imaging of a polymethyl methacrylate (PMMA) film aided by the effective plasmon resonance of the underlying crumpled graphene structure. The coupling between PMMA phonon and graphene plasmon showed a two-fold increase in the obtained near-field optical response. Our results not only are the first experimental demonstration of the coupling between topography and plasmon resonance of semi-infinite atomically thin graphene layer but also suggest widely tunable, plasmonically-enhanced biosensing or spectroscopy in the infrared range.

Two distinct stacking orders in ReS₂ are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principle calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm^-1 for AA stacking, and 20 cm^-1 for AB stacking, making a simple tool for determining the stacking orders in ReS₂. Polarized photoluminescence (PL) reveals that AB stacking possesses blue-shifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. Our findings underscore the stacking-order driven optical properties and carrier dynamics of ReS₂, mediate many seemingly contradictory results in literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

E-beams have been suggested to be useful for fabrication on the atomic scale.¹ Here, we leverage techniques that have been developed recently for positioning dopants in single layer graphene^2–5 to pattern arrays of dopants in twisted bilayer graphene (TBG) in a scanning transmission electron microscope. The process is governed by a feedback-controlled algorithm allowing patterning to proceed in an automated fashion. These techniques hold promise for the tailoring of TBG properties by the attachment of foreign atoms to defect sites.⁶
References
(1) Dyck, O.; Ziatdinov, M.; Lingerfelt, D. B.; Unocic, R. R.; Hudak, B. M.; Lupini, A. R.; Jesse, S.; Kalinin, S. V. Atom-by-Atom Fabrication with Electron Beams. Nat. Rev. Mater. 2019, 4 (7), 497–507. https://doi.org/10.1038/s41578-019-0118-z.
(2) Dyck, O.; Zhang, C.; Rack, P. D.; Fowlkes, J. D.; Sumpter, B.; Lupini, A. R.; Kalinin, S. V.; Jesse, S. Electron-Beam Introduction of Heteroatomic Pt–Si Structures in Graphene. Carbon 2020, 161, 750–757. https://doi.org/10.1016/j.carbon.2020.01.042.
(3) Dyck, O.; Zhang, L.; Yoon, M.; Swett, J. L.; Hensley, D.; Zhang, C.; Rack, P. D.; Fowlkes, J. D.; Lupini, A. R.; Jesse, S. Doping Transition-Metal Atoms in Graphene for Atomic-Scale Tailoring of Electronic, Magnetic, and Quantum Topological Properties. Carbon 2021, 173, 205–214. https://doi.org/10.1016/j.carbon.2020.11.015.
(4) Dyck, O.; Kim, S.; Kalinin, S. V.; Jesse, S. Placing Single Atoms in Graphene with a Scanning Transmission Electron Microscope. Appl. Phys. Lett. 2017, 111 (11), 113104. https://doi.org/10.1063/1.4998599.
(5) Dyck, O.; Yoon, M.; Zhang, L.; Lupini, A. R.; Swett, J. L.; Jesse, S. Doping of Cr in Graphene Using Electron Beam Manipulation for Functional Defect Engineering. ACS Appl. Nano Mater. 2020, 3 (11), 10855–10863. https://doi.org/10.1021/acsanm.0c02118.
(6) This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (A.R.L., S.J., O.D.) and by the Center for Nanophase Materials Sciences (CNMS), a U.S. Department of Energy, Office of Science User Facility (D.H.).