Symposium F.MT06—Strain and Defect-Driven Transport Properties in van der Waals Solids

Hexagonal boron nitride (h-BN) has recently emerged as a prominent nanophotonic material due to observation of several unique optoelectronic phenomena such as natural hyperbolicity, strong second order non-linearities and robust deep UV emission. Specifically, its ultra wide bandgap ~ 6eV can host midgap defects that often exhibit intense room temperature quantum emission. Understanding the origin of such defect related emission is crucial to harness their technological potential. Here, we report controlled tailoring of defect states in hBN by focussed helium ions using a helium ion microscope and present a direct correlation of their structural and optical properties using a combination of Raman, photoluminescence (PL), cathodoluminescence (CL) and high resolution electron microscopy. We observe that while end of range defects cause an evolution of sp³ phase in the otherwise sp² hBN leading to bright and previously unreported near infrared emission, near surface defects contribute to strong visible emitters often attributed to single photons. Our results provide a comprehensive overview of defect related colour centres in hBN across a wide spectral range that would serve as a platform to realize next generation quantum optoelectronic devices.

A chemical vapor deposition method is developed for thickness-controlled (one to four layers), uniform, and continuous films of both defective gallium(II) sulfide (GaS): GaS_0.87 and stoichiometric GaS. The unique degradation mechanism of GaS_0.87 with XPS and HAADF-STEM is studied, and it is found that the poor stability and weak optical signal from GaS are strongly related to photo-induced oxidation at defects. Photodetectors based on the two materials are fabricated with short response time (<66 ms), excellent UV photoresponsivity. An enhanced stability of the stoichiometric GaS is demonstrated under laser and strong UV light, and by controlling defects in GaS, the photoresponse range can be changed from vis-to-UV to UV-discriminating.
Besides the role in degradation mechanism, defects in GaS_0.87 also contributed to the formation of a new band sitting within its bandgap, which is confirmed by cathodoluminescence results. As a result, controlling defects in GaS turns the band alignment of the bilayer GaS-WS₂ van der Waals heterostructure from type I to type II. XPS and KPFM characterization demonstrates that the defect band can facilitate the charge transfer from WS₂ to GaS. Devices based on the type-II heterojunction further show that the defect band greatly assisted the carrier transport by lowering the contact resistance, and also demonstrate superior photodetector properties with a large linear dynamic range (>73 dB) at a R of 13 A/W for green light.

[1] Lu, Y., Chen, J., Chen, T., Shu, Y., Chang, R.-J., Sheng, Y., Shautsova, V., Mkhize, N., Holdway, P., Bhaskaran, H., Warner, J. H., Controlling Defects in Continuous 2D GaS Films for High-Performance Wavelength-Tunable UV-Discriminating Photodetectors. Adv. Mater. 2020, 32, 1906958.
[2] Lu, Y., Warner, J. H., et al., Transition of type-I to type-II heterojunction in monolayer GaS-WS₂. In preparation.

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 a path with enormous potential. 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. arxiv:2005.02416 (2020)

Single photon emitters (SPEs), or quantum emitters, are key components in a wide range of nascent quantum-based technologies. A solid state host offers many advantages for realization of a functional system, but creation and placement of SPEs are difficult to control. We describe here a novel paradigm for encoding strain into 2D materials to create and deterministically place SPEs in arbitrary locations with nanometer-scale precision [1]. We demonstrate the direct writing of SPEs in 2D semiconductors based on a materials platform consisting of a WSe₂ monolayer on a deformable substrate using an atomic force microscope nano-indentation process. This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs for facile coupling with photonic waveguides, cavities and plasmonic structures.

The weak interlayer bonding in van der Waals heterostructures (vdWh) enables one to rotate the layers at arbitrary azimuthal angles. For transition metal dichalcogenide vdWh, twist angle has been treated solely through the use of rigid-lattice moiré patterns. No atomic reconstruction has been observed to date, although reconstruction can be expected to have a significant impact on all measured properties, and its existence will fundamentally change our understanding of such systems. Here we demonstrate via conductive AFM and TEM that vdWh of MoSe₂/WSe₂ and MoS₂/WS₂ undergo significant atomic level reconstruction at twist angles ≤ 1° leading to discrete commensurate domains divided by narrow domain walls [2], rather than a smoothly varying rigid-lattice moiré pattern as has been assumed in prior work. We show that this occurs because the energy gained from adopting low energy vertical stacking configurations is larger than the accompanying strain energy [3]. Such reconstruction impacts both the local conductivity and the optical properties.

[1] M.R. Rosenberger et al, ACS Nano 13, 904 (2019). DOI: 10.1021/acsnano.8b08730
[2] M.R. Rosenberger et al, ACS Nano 14, 4550 (2020). https://dx.doi.org/10.1021/acsnano.0c00088
[3] M. Phillips and C.S. Hellberg, ArXiv 190902495 Cond-Mat (2019)

*Work done in collaboration with Matthew R. Rosenberger, Hsun-Jen Chuang, Madeleine Phillips, Kathleen M. McCreary, and C. Stephen Hellberg of the Naval Research Laboratory, Washington DC, and Chandriker Kavir Dass and Joshua R. Hendrickson of the Air Force Research Laboratory, Wright-Patterson AFB, OH.
† This work was supported by AFOSR and core programs at NRL.

The ability to tune the electronic and optical properties of 2D materials by induced defects has wide-ranging applications. For example, patterned graphene surfaces can be used for biosensing, while strained 2D transition metal dichalcogenides (TMDs) could enable the control of excitons for quantum emitters. Here, we first report on electronic band structure calculations of periodically patterned graphene, which can be fabricated using electron-beam chemistry with radiolyzed water (Islam, et al. Carbon 2020, 166, 446-455). Analysis of the effects of alternating strips in the graphene surface, which are pristine or with epoxide defects, illustrate electronic confinement in the graphene ribbon, which leads to one-dimensional quasi-metallic or semiconducting response, dependent on the strip’s width. Electron transport calculations demonstrate electron transmission along the graphene strip direction, minimized in the direction perpendicular to edges terminated by oxygen atoms, while intrinsic current-voltage (I-V) characteristics indicate comparable performance to graphene nanoribbons with perfect edges. The electron transport in a device setting with gold electrodes shows improved conductivity in comparison to a graphene-based device. Next, in an effort to move towards controlled emission properties, such as for single-photon emission applications, we consider strained 2D WSe₂ monolayers with varying wrinkle structures. The tunability we demonstrate in the optical response of the mechanically deformed nanostructures is described in detail. Our results on exciton tunability by wrinkle structure could motivate experimental fabrication.

Stacking two-dimensional layers with slightly different lattice vectors produces a new periodic structure known as moiré lattices. As one of such moiré system, twisted bilayer graphene (tBLG) has been predicted to host a moiré miniband with flat dispersion if the layers are stacked at the ’magic angles’. Recently, magic angle tBLG was reported to exhibit correlated transport phenomena, where the presence of the flat miniband in the system is thought to be essential. Although STM and compressibility measurements in tBLG have found a van Hove singularity consistent with the presence of the flat band, a direct observation of the flat dispersion in the momentum-space of such a moiré miniband in tBLG is still lacking. Here, we report the visualization of this flat moiré miniband by using angle-resolved photoemission spectroscopy with nanoscale resolution (nanoARPES) [1]. Meanwhile, we probed the real-space structure of the moiré lattice with scanning microwave impedance microscopy (sMIM), where the sub-5 nm spatial resolution has also allowed us to image the strain-related inhomogeneity and defect structures in various graphene-based moiré lattices [2]. Our measurements demonstrate the existence of the flat moiré band near the charge neutrality for tBLG close to the magic angle at room temperature.

[1] MIB Utama*, RJ Koch*, K. Lee*, et al. Nature Physics 2020 (in press), ArXiv:1912.00587
[2] K. Lee*, MIB Utama*, S Kahn*, et al, ArXiv:2006.04000 (2020)

Two-dimensional transition metal dichalcogenides (TMDCs) have unique optoelectronic properties in part because of their spin- and valley-selective carrier dynamics at the band edges. However, limited valley coherence times represent a major challenge for application. Here we comprehensively investigate electron-phonon interactions at valleys in both the valence and conduction band edges using a first-principles, spin-resolved approach. We capture the effects of strain by introducing ±2% biaxial strain to each of the four monolayer TMDCs (MoS₂, MoSe₂, WS₂ and WSe₂) studied. We find that the strength of the spin-orbit splitting is strongly sensitive to strain for tungsten-based monolayers relative to molybdenum-based compounds. Additionally, our results show that the direct nature of the bandgap of monolayer TMDCs is sensitive to less than 1% compressive strain. Our results detail the relevant mode- and wavevector-resolved electron-phonon processes responsible for scattering involving valley states, which we find to be highly tunable by strain. Most notably, we find up to an order of magnitude increase in conduction-band lifetimes under tensile strain. Overall we find significant potential in engineering valley dynamics and optoelectronic properties of monolayer TMDCs through the use of applied strain.

Recent measurements of spatially-resolved current flows have revealed that electrons in condensed matter can behave hydrodynamically, exhibiting classical fluid phenomena such as vortices and Poiseuille flow^1,2,3. This occurs when microscopic scattering processes conserve momentum over time- and length-scales significantly exceeding those of the experimental probe⁴. In contrast with graphene, where the hydrodynamic regime persists well into room-temperature, other materials exhibit hydrodynamic flow only within a finite low-temperature window, and as such require temperature-resolved measurements. Here, we report direct observation of temperature-dependent electron hydrodynamic flow in the van der Waals solid WTe2, via measurements of the associated stray magnetic field using nitrogen vacancy echo magnetometry. At low-enough temperatures the onset of ballistic flow competes with the hydrodynamic regime, eventually resulting in a flat current profile.

In this work, we predict temperature-resolved lifetimes for various electron scattering mechanisms in WTe2 from first principles⁵. We use our ab initio microscopic lifetimes as inputs to the electronic Boltzmann transport equation⁶, capturing the non-monotonic behavior in the experimental data. This provides a quantitative estimate for the electron interaction length in WTe2, which seems to be robust against a wide range of impurity concentrations. We find that these results cannot be explained using a direct Coulomb effect alone and that the major contributing momentum-conserving process is a Frohlich-like phonon-mediated electron interaction. We then investigate the effect of strain on the hydrodynamic regime in WTe2, concluding both the temperature-window and the magnitude of the hydrodynamic flow vary.

¹ Ella, L., Rozen, A., Birkbeck, J. et al. Nat. Nanotechnol. 14, 480–487 (2019)
² Sulpizio, J.A., Ella, L., Rozen, A. et al. Nature 576, 75–79 (2019)
³ Ku, M.J.H, Zhou, T.X., Li Q. et al. arXiv:1905.10791 (2019)
⁴ Varnavides G., Jermyn A.S., Anikeeva P. et al. arXiv:2002.08976 (2020)
⁵ Coulter J., Sundaraman R., Narang P., Phys. Rev. B 98, 115130 (2018)
⁶ de Jong M.J.M., Molenkamp L.W. Phys. Rev. B 51, 13389 (1995)

The formation of atomic defects is unavoidable in 2D materials with currently available
growth techniques 1 . Nevertheless, there is a myriad of functionalities in modern optoelectronic
and nanophotonic devices that leverage quantum defects including the recent demonstration of
single photon emitters in 2D materials 2,3 . In parallel, advances in atomic-resolution imaging
techniques provide new opportunities to directly create, manipulate and characterize defects
down to the atomic scale in low-dimensional materials 4 . Therefore, we present theoretical
calculations and analysis of optically active quantum defects in 2D materials. We study the
electron-phonon interactions 5,6 of electronic transitions in defects and quantify their optical
efficiency by calculating the Huang-Rhys factor. This presents a pathway for maximizing the
optical efficiency of designer defect and provides a deterministic choice for defect creation at the
atomic scale using scanning probe techniques 2 .
[1] D. Edelberg, et al, Nano Lett. 19, 7, (2019)
[2] O. Dyke et al, Nat. Mat., 4, 497, (2019)
[3] C. Chakraborty, L. Kinnischtzke, K. Goodfellow, R. Beams, and N. Vamivakas, Nat. Nano.
10, 507 (2015)
[4] G. Grosso, H. Moon, B. Lienhard, S. Ali, D. K. Efetov, M. M. Furchi, P. Jarillo-Herrero, M. J.
Ford, I. Aharonovich, and D. Englund, Nat. Comm. 8, 705, (2017)
[5] C. J. Ciccarino, T. Christensen, R. Sundararaman, and P. Narang, Nano Lett. 18, 5709,
(2018)
[6] C. J. Ciccarino, C. Chakraborty, D. Englund, and P. Narang, Far. Disc. 214, 175, (2018)

Carbon/graphite-based energy storage devices, such as lithium-ion batteries (LIBs), are the most reliable energy storage devices for portable electronics, electric vehicles (EV) and electric grid storage due to their high-power density, stability, low cost, and safety. Despite the wide use of LIBs, the graphitic anode has limited capacity (372 mAh.g^-1) due to the limited rate of lithium diffusion within the commensurately stacked layered structure.
Here we discuss the unique superior capacity of Lithium intercalation of multilayered graphene structures, named as an Incommensurate Multilayer Graphene (IMLG), that consists of closely packed rotated graphene layers. In-plane and out-plane features of graphene are studied by Raman spectroscopy, High-Resolution Transmission Electron Microscopy (HRTEM), and X-ray Diffraction. The commensurate-incommensurate transition of layered graphene is analyzed by Raman spectroscopy and the range of the key parameters is discussed as a significant for efficient Lithium intercalation.
The structural and binding features of graphene anode before and after Lithium insertion is studied, and a new lithium intercalation model is considered. We revealed that weakened interplanar Van der Waals interaction as it occurs within rotated graphene layers enables easy and full penetration of lithium, followed by flexible adjustment of layers for stable and reversible stable cycling without affecting graphene in-plane features. Superior Lithium intercalation provides reversible and stable long-term charge/ discharge of IMLG anodes reaching up over 1500 mAh.g^-1 capacity of LiBs.
Our recent development of IMLG confirms the feasibility of the synthesis technique which allows us the development of high energy density and powerful lightweight secondary batteries in the nearest future.

Monolayer transition metal dichalcogenides (TMDCs), with the general chemical formula MX₂ (M = Mo or W and X = S, Se and Te), are a class of 2D materials that have been predicted to be piezoelectric using Density Functional Theory (DFT) calculations. In addition, some of the TMDCs possess certain advantages in monolayer form – such as a direct bandgap and better Q-factors – over graphene. Here, we present resonance and piezoelectric measurements on monolayer transition metal dichalcogenides (TMDCs) using cavity-interferometry. We also demonstrate that TMDCs are promising candidates for Nanoelectromechanical Systems (NEMS) due to their low mass and intrinsic piezoelectricity.

Cavity-interferometry refers to a displacement transduction technique in nanoelectromechanical systems (NEMS). Most fabrication protocols for NEMS naturally render an optical cavity where the NEMS structure and the underlying substrate constitute the two mirrors. The modulation in the cavity depth due to the motion of the NEMS translates to intensity variations of the reflected light from the optical cavity, which can be measured on an appropriate detector. By optimizing the cavity depth, wavelength of the probe laser and the back-end electronics, exquisite displacement sensitivity (sub pico-meter/Hz^1/2) can easily be achieved [1].

Device fabrication is challenging; hence, we explored various fabrication methods for creation of suspended 2D material devices and established working protocols for both exfoliated and chemical vapor grown TMDCs. We also developed a new transfer method driven by water wettability to facilitate fabrication efforts. To perform device characterization, we developed an integrated in-vacuum Raman/Interferometer setup [1]. By exploiting the cavity formed between suspended 2D TMDC “bridges” and underneath substrates, resonance frequencies of various modes were characterized including exotic line shapes arising due to 1:1 internal resonance coupling in the nonlinear regime. Using a combined experimental-computational approach and the measured shift of fundamental resonance frequencies, induced by applied bias, we extracted piezoelectric properties of monolayer WS₂ and MoS₂. The exfoliated samples display piezoelectric coefficients that are in good agreement with DFT predictions. By contrast, CVD grown monolayers, exhibit a 10-fold reduction in piezoelectricity, likely due to the existence of atomic defects such as vacancies.

Additionally, by driving resonance in the nonlinear regime, we revealed internal resonance pairs in monolayer WS₂. Discovery of Hopf bifurcations linearly varying with driving forces inspired novel sensing modality with ultrahigh force sensitivity (aN) at room temperature [2]. Our efforts in fabricating and experimenting on suspended 2D TMDC devices will support further development in applying 2D materials to sensors and NEMS applications.

[1] S. S. P. Nathamgari, S. Dong, E. Hosseinian, L. J. Lauhon and H. D. Espinosa, "An Experimental Setup for Combined In-Vacuo Raman Spectroscopy and Cavity-Interferometry Measurements on TMDC Nano-resonators" Experimental Mechanics, Vol. 59, p. 349-359, 2019.

[2] S. S. P. Nathamgari, S. Dong, L. Medina, N. Moldovan, D. Rosenmann, R. Divan, D. Lopez, L. J. Lauhon and H. D. Espinosa, "Nonlinear Mode Coupling and One-to-One Internal Resonances in a Monolayer WS2 Nanoresonator" Nano Letters, Vol. 19, No. 6, p. 4052-4059, 2019.

The quantum confinement in atomically-thin transition metal dichalcogenides (TMDCs) has been investigated for single photon emission based on naturally or artificially occurring defects. In particular, the precise control of the position of a single photon emitter by applying mechanical strains to the TMDC is feasible. Strains have been induced in TMDCs using various nanostructures including dielectric pillars, nanobubbles, optical waveguide, metal nanogaps, and metal nanoparticles. However, polarization control and integration with a high-quality (Q) cavity to increase the Purcell factor remain elusive in TMDC-based single photon sources.
In this talk, I will present the deterministic control of both the position and polarization of single photon emitters in atomically thin WSe₂ placed on a nanogap array. Manipulation of the band structure by a local strain gradient generates a potential well at the nanogap site, and position-controlled single photon emission is subsequently achieved. Additionally, directional elongations of the potential well, which are tuned by changing the nanogap size, allow polarization-controlled single photon emission. Moreover, single photons with a g⁽²⁾ of ~0.1 are generated at 4-20 K. To take a full advantage of such deterministic control of the position and polarization of single photon emitters, the emitters are integrated with one-dimensional photonic crystal cavities. Therefore, this approach is a unique way to develop next-generation, deterministic, controllable single photon emitters based on TMDC materials, which outperform the present single photon sources with random occurrence and uncontrolled polarization properties.

Rapid advances in synthesis of graphene and 2D materials, and fabrication methods for functional devices enable sophisticated types of functionality and their application to various emerging electronics, such as flexible, wearable and optoelectronic applications, that cannot be addressed with conventional materials. In this talk, I present that graphene and 2D materials can play critical roles in this context, through demonstrations of complex, mechanically assembled electronic and optoelectronic devices for flexible and wearable applications. Specifically, the mechanics of graphene and MoS₂ can yield various devices in distinct, engineered flexible geometries that cannot be easily reproduced with conventional materials and/or conventional device layouts. Examples of devices include touch, tactile sensors and wearable OLED display.

A pair of electron and hole across the interface of semiconductor heterostructure can form a bound quantum state of the interlayer exciton. In a coupled interface between atomically thin van der Waals layers (vdW), the Coulomb interaction of the interlayer exciton increases further. In this presentation, we will discuss observing interlayer exciton formation in semiconducting transition metal dichalcogenide (TMDC) layers. Unlike conventional semiconductor heterostructures, charge transport in of the devices is found to critically depend on the interlayer charge transport, electron-hole recombination process mediated by tunneling across the interface. We demonstrate the enhanced electronic, optoelectronic performances in the vdW heterostructures, tuned by applying gate voltages, suggesting that these a few atom thick interfaces may provide a fundamental platform to realize novel physical phenomena. Furthermore, complete experimental control of density, displacement and magnetic fields in our graphene double layer system enables us to explore the rich phase diagram of several superfluid exciton phases with the different internal quantum degrees of freedom.

One of the most alluring features of 2D materials is their tunability, as their carrier density, effect mass, band gap, and electronic phases can be tuned in situ by external parameters such as electric field, magnetic field and strain. Here we will present transport studies of high quality 2D materials. For instance, we demonstrate large tunable SOC and zero-field spin-splitting in atomically thin InSe with unprecedented mobility. From beating patterns in quantum oscillations, we establish that the SOC parameter a is thickness-dependent; it can be continuously modulated over a large range by an out-of-plane electric field, achieving zero-field splitting tunable between 0 and 20 meV. Surprisingly, a could be enhanced by an order of magnitude in some devices, suggesting that SOC can be further manipulated by variations in interlayer spacing induced by stacking and/or electrostatic compression. Our work highlights the extraordinary tunability of SOC in 2D materials, which can be harnessed for in operando spintronic and topological devices and applications.

Many-electron interaction effects dominate many spectroscopic properties of reduced-dimensional systems, leading often to manifestation of novel phenomena not seen in the bulk. In this talk, I present some of our recent work on electron interaction effects in atomically thin quasi 2D materials: (1) strongly bounded correlated multi-particle excitations, such as trions and bi-excitons, in quasi 1D and 2D materials; (2) giant excitonic effects in shift currents, a nonlinear optical phenomenon, in non-centrosymmetric quasi 2D semiconductors; and (3) excitons and band renormalization in optical field driven angle-resolved photoemission spectroscopy (ARPES). Studies of these novel phenomena are made possible because of newly developed theoretical methods which incorporate higher-order many-electron correlation effects from first principles, using an interacting Green’s function approach.

In a magnetic topological insulator, nontrivial band topology conspires with magnetic order to produce exotic states of matter that are best exemplified by quantum anomalous Hall (QAH) insulators and axion insulators. Up till now, such magnetic topological insulators are obtained by doping topological insulators with magnetic atoms. The random magnetic dopants, however, inevitably introduce disorders that hinder further exploration of topological quantum effects in the material. We resolve this dilemma by probing quantum transport in MnBi₂Te₄ thin flake—a topological insulator with intrinsic magnetic order. In this layered van der Waals crystal, the ferromagnetic layers couple anti-parallel to each other, so bulk MnBi₂Te₄ is an antiferromagnet. Atomically thin MnBi₂Te₄, however, becomes ferromagnetic when the sample has odd number of septuple layers (a septuple layer represents a single structural unit in the out-of-plane direction). We observe zero-field QAH effect in a five-septuple-layer specimen; an external magnetic field further enhance the QAH quantization by forcing all layers to align ferromagnetically. MnBi₂Te₄ therefore becomes the first intrinsic magnetic topological insulator exhibiting QAH effect.

The availability of intense tabletop terahertz-frequency pulses and simple field-enhancement structures that enable peak electric fields of tens of MV/cm have ked to demonstrations of highly nonlinear THz-induced responses from a wide variety of materials. THz-driven electronic, magnetic, and structural phase transitions and domain switching have illustrated novel possibilities for control over collective properties and dynamics. In experiments on single-layer and few-layer MoTe₂, we have found that irradiation with just one sufficiently strong THz pulse induces an irreversible phase transition [1]. The initial, noncentrosymmetric 2H phase disappears, as shown by the disappearance of characteristic Raman lines and second harmonic generation. After further THz irradiation, the topological insulator phase appears, revealed by its distinct Raman spectrum. Single-shot measurements of the MoTe₂ evolution as a function of time after THz irradiation reveal complex dynamics over many-picosecond time scales. Theoretical calculations show that the transition is induced through THz-induced liberation of carriers which stabilize the new phase and reduce the energy barrier between the two phases. The result illustrates the prospects for THz control over complex multiphase landscapes in quantum materials.

[1] “Terahertz-driven irreversible topological phase transition in two-dimensional MoTe₂,” J. Shi, Y.-Q. Bie, W. Chen, S. Fang, J. Han, Z. Cao, T. Taniguchi, K. Watanabe, V. Bulović, E. Kaxiras, P. Jarillo-Herrero, and K. A. Nelson, arXiv:1901.13609 (2019).

Selective patterning and functionalization of graphene can produce spatially-defined properties. Buckling or wrinkling of graphene on polymeric substrates enables a direct approach to tune the physical properties without lithographic steps, and the resulting curvature of the wrinkles can control local chemical reactivity. However, most buckling methods have been limited by the range of wavelength tunability, only global control of the wrinkled patterns, and delamination and cracks in the graphene. This talk will describe a scalable approach to achieve area-specific reactivity and patterning of conformal graphene wrinkles. We will discuss how a fluoropolymer layer sandwiched between graphene and different polymer substrates can facilitate crack-free and switchable graphene nanostructures. Such patterned areas with different curvatures show different reactivities based on a plasma fluorination reaction. Our approach for large-area, fine control over graphene wrinkle topographies has prospects for other two-dimensional electronic materials and optoelectronics and plasmonics applications.

The two-dimensional transition metal dichalcogenides (2D TMDCs) tend to have basal planes that are chemically relatively inert. Many approaches and reaction schemes have been demonstrated to modify the chemical nature of TMDCs, resulting in new chemical properties and applications. This talk will highlight our recent work on the chemical properties of 2D TMDCs mediated by point defects and edges, which subsequently enable applications in water treatment and antimicrobial treatment. We have shown robust and versatile covalent functionalization of materials across the metal chalcogenides including MoS₂, MoSe₂, WS₂, WSe₂, Bi₂S₃, and Sb₂S₃ using aryl diazonium salts. The reaction mechanism is initiated at point defect sites and edges, but propagates across the chalcogenide surfaces in a chain-like step growth. We have also shown the ability for MoS₂ to remove Pb contamination from water via a reaction with the surface sites to form sulfate precipitates. The reaction mechanism is again initiated at defect sites in the MoS₂. Finally, we have used single stranded DNA to encapsulate MoSe₂ nanosheets and shown their broad-spectrum antimicrobial properties against multidrug resistant bacteria. The choice of ssDNA sequence influences the conformal coating, and the ability for the edges of MoSe₂ to cut through bacterial cell membranes.

Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. We directly evaluate deep levels and their chemical trends in the bandgap of MoS₂, WS₂ and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different host materials, which explains the chemical trend of native electron density in the hosts as well as a persistent photoconductivity up to 400K.

Two-dimensional (2D) materials, such as Graphene, hBN and MoS₂, are promising candidates in a number of advanced functional and structural applications, owing to their exceptional electrical, thermal and mechanical properties. To better meet the requirements of practical applications, property modification and regulation are oftentimes desired. Defect and strain engineering strategies are powerful tools to modulate electronic, optoelectronic and other functional properties of 2D semiconductors.
In this talk, we will report our efforts to employ such strategies to tune functional properties of 2D semiconductors. We first demonstrated how substrate interfacial chemistry can be utilized to tailor the physical properties of single-crystalline molybdenum disulphide (MoS₂) monolayers. We use self-assembled monolayers with a variety of end termination chemistries to functionalize substrates and systematically study their influence on the physical properties of MoS₂. Our data shows that combined interface-related effects of charge transfer, built-in molecular polarities, varied densities of defects, and remote interfacial phonons strongly modify the electrical and optical properties of MoS₂. Tunable friction behavior of the heterostructure of monolayer MoS₂ and different self-assembled organic molecules (SAMs) were also demonstrated, which was attributed to the charge transfer from SAMs to MoS₂ influencing the electron concentration and thus the carrier-mediated friction in MoS₂. In another example, we systematically characterize chemical vapor deposition (CVD)-grown MoS₂ by photoluminescence spectroscopy and mapping and demonstrate non-uniform strain in single-crystalline monolayer MoS₂ and strain-induced bandgap engineering. We further modified our CVD process to create a Janus SMoSe monolayer and demonstrated high basal plane hydrogen evolution reaction (HER) activity in such samples and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

The study of atomic-monolayer materials has been extremely popular within the last decade due to the emergence of a family of two-dimensional (2D) materials with expansive mechanical, electrical and magnetic properties. Subtle changes in these 2D materials at the atomic length scales, ranging from strain fields and dislocations, have large effects for tuning the electronic and mechanical properties, especially when used in real devices. Electron microscopy can provide the insight needed to visualize these effects [1-2]. Recent advancements in detector technology have already pushed the limits of resolution using 2D bilayer molybdenum disulfide using ptychographic reconstruction [3-4]. In my talk, I will discuss ways electron microscopy can provide opportunities for sensitive strain and magnetic measurements of 2D materials, with an emphasis on emerging techniques provided by the electron microscope pixel array detector (EMPAD), a state-of-the-art, high-dynamic range and high-speed electron diffraction detector.

[1] Lee, CH et al, Nanoletters, 20 (5), 3369–3377 (2020).
[2] Han, E et al, Nature Materials, 19 (3), 305-309 (2019).
[3] Tate, MW et al, Microscopy and Microanalysis, 22 (1), 237-249 (2016).
[4] Jiang, Y et al, Nature, 559 (7714), 343-349 (2018).

Tellurium nanostructures are promising candidates for functional electronic and optical nanomaterials. The anisotropic helical crystal structure of trigonal tellurium (t-Te) and the weak van der Waals stacking of its layers; allows the formation of a variety of 1D nanostructures such as nanowires and nanobelts. A kinematically driven growth can promote the formation of 2D tellurium known as tellurene. This recent realization of 2D tellurium has allowed scientists to demonstrate an air-stable monoelemental van der Waals material with remarkable potential for applications in electronics and optoelectronics. The chiral-chain structure of ultrathin Te allows controllable electronic and optical properties due to the non-monotonic carrier mobility and an increase in electronic band-gap as the thickness is reduced. Scalable and accurate production of 2D Te-based functional devices will require detailed knowledge of structural defects and strain field distributions that can be linked to transport properties.
Standard HRTEM images provide localized crystallographic information, which usually leaves out larger-scale structural components. Precise quantification of lattice parameters and additional structural features is often not fully captured in this type of analysis. Modern single electron detectors allow the rapid collection of 2D diffraction patterns of a 2D set of probe positions in the so-called 4D Scanning Transmission Electron Microscopy (4D-STEM). 4D-STEM provides a remarkable tool for structural analysis across many length scales, and enables extracting the crystalline structure, orientation, virtual images, and strain information at the nanometer scale.
In this presentation, we will demonstrate a new microwave-assisted synthesis method for 1D and 2D tellurium nanostructures and the analysis of the micro/nanostructure using conventional XRD and the new 4D-STEM technique. Fast collection of nano beam electron diffraction (NBED) patterns of beam sensitive Te nanowires and flakes was performed using a Gatan K3 direct electron detector, and these data sets were used to perform peak profile analysis over the thousands of NBED patterns to quantify the nanostructure of 1D and 2D Te including chirality and grain boundary analysis.

A moiré lattice is produced by stacking lattice-mismatched 2D materials or by introducing misalignment. The moiré lattices generate the periodic potential that can strongly modify the electronic properties of the constituent 2D layers. In addition, superstructures of moiré lattices can emerge from multiple misaligned lattice vectors or inhomogeneous strain distribution, which offers an extra degree of freedom in engineering electronic band structure. Thus, a high-resolution imaging moiré lattices and superstructures is critical to correlate the moiré structure with the resulting material behavior. Here we report the versatility of ultra-high-resolution implementation of scanning impedance microwave microscopy (sMIM) in imaging the moiré superlattices in various graphitic systems at ambient conditions. We show that an ultra-high spatial resolution better than 5 nm can be achieved although the scanning probe tip has a gross radius of ~100 nm. The high sensitivity of sMIM allows the imaging of the intricate nanostructural details in the superlattice. Furthermore, the imaging capability of sMIM enables the characterization of various composite moiré designs, including the super-moiré pattern and Kagome-like superlattice.

Transport phenomena play an important role in designing and engineering materials with tailored functionalities. Thermal conductivity and spin precession damping, as basic transport properties of materials, can provide a wealth of information on the fundamental scattering processes of phonons and magnons with structural defects and interface imperfection. In this talk, I will share our group’s recent progress on utilizing time-resolved magneto-optical Kerr effect (TR-MOKE) metrology to study the thermal and magnetic properties of functionalized materials spanning a wide range of applications. This will include: (1) revealing the 3D anisotropic thermal transport in black phosphorus, as the next-generation of “wonder materials” for the semiconductor industry; (2) developing low-damping and high-thermal stability materials with perpendicular magnetic anisotropy for spintronic applications; and (3) manipulating spin precession using optically launched acoustic strains via a strong magnon-phone coupling at ultra-high frequencies (~60 GHz). The structure-property relationships of functional materials revealed by the ultrafast pump-probe technique open up opportunities of tailoring material properties by structural engineering at the atomic and molecular levels. Ultimately, such an understanding can be leveraged to guide the design and optimization of materials, as promising building blocks for high-performance electronics, thermal management, hard-disk data storage, and magnetoacoustic devices.

The field of spin transport in two-dimensional has flourished in the past couple of years. Some remarkable features are the reports of large charge-to-spin in graphene-based heterostructures [1], the discovery of the quantum spin Hall effect [2], and ultra-clean spin transport measurements within the ballistic regime [3]. In this talk, we will present predictions on diffusive and ballistic spin transport in different two-dimensional materials. Using complementary large-scale numerical transport methods, we developed a theory showing that contacts have a dramatic effect on the spin transport in systems with a size comparable to the spin relaxation length, leading to underestimating the latter [4]. We then apply this theory and bulk transport methods to unveil the role of broken symmetries in the spin transport of transition metal dichalcogenides in their 1T' and 1Td structural phases, showing they can promote an unconventional spin transport regimen. We believe our predictions will lead to a better understanding of the recent measurements of the quantum spin Hall effect in WTe2 and will help optimize experimental setups for its harvesting.

[1] Nat. Mater. 19, 170–175 (2020).
[2] Science 359,6371, pp. 76-79, (2018).
[3] Phys. Rev. Lett. 124, 177701 (2020).
[4] Phys. Rev. Lett. 124, 196602 (2020).

Tuning physical properties of materials by applying strain opens up wide range of applications. In this presentation, sensitivity of these properties on application of strain will be illustrated for the case of TMDs. Using first principles calculations, the effect of normal compressive (NC), bi-axial compressive (BC), and bi-axial tensile (BT) strain on the electronic properties of few layered MoS₂ has been investigated. Regardless of the type of strain, a reversible semiconductor-to-metal transition is observed MoS₂. The effect of strain as a function of number of layers on the transport properties is investigated. It is found that the 3L and 2L-MoS₂ emerge as the most efficient thermoelectric materials under NC and BT strain, respectively. The study was further extended to semi-metallic TMD TiS₂, which has been in focus due to its application in energy storage and thermoelectric materials. Upon application of uniform biaxial strain, TiS₂ transforms from semi-metal to semiconducting phase. The transport calculations show three-fold enhancement in thermopower for both p- and n-type doped TiS₂. Further, the effect of strain on thermal conductivity of bulk MoS₂ is explored, wherein an increase of cross-plane thermal conductivity from 3.5 to about 25 W/m-K. The giant enhancement arises from the increase in interlayer interaction on application of strain and decrease in phonon lifetimes due to the unbundling effect along the cross-plane direction. The concepts proposed from our studies can be extended to other semiconducting and metallic TMDs.

References:
Semiconductor-metal transition in semiconducting bilayer sheets of transition-metal dichalcogenides, Phys. Rev. B, 86, 075454 (2012)
Pressure-induced semiconducting to metallic transition in multilayered molybdenum disulphide, Nat. Commun., 5, 3731 (2014)
Effect of strain on electronic and thermoelectric properties of few layers to bulk MoS₂, Nanotechnology, 25, 465701 (2014)
Strain-induced electronic phase transition and strong enhancement of thermopower of TiS₂, Phys. Rev. B, 90, 174301 (2014)
Thermal Conductivity Enhancement in MoS₂ under Extreme Strain, Phys. Rev. Lett., 122, 155901 (2019)

The ferromagnetic state in van der Waals two-dimensional (2D) materials has been reported recently in the monolayer limit. Intrinsic CrI₃ and CrGeTe₃ semiconductors reveal ferromagnetism but the Tc is still low below 60K. In contrast, monolayer VSe₂ is ferromagnetic metal with Tc above room temperature but incapable of controlling its carrier density. Moreover, the long-range ferromagnetic order in doped diluted chalcogenide semiconductors has not been demonstrated at room temperature. The key research target is to realize the long-range order ferromagnetism, Tc over room temperature, and semiconductor with gate tunability. Here, Ferromagnetic order is manifested using magnetic force microscopy up to 360K, while retaining high on/off current ratio of ~10⁵ at 0.1% V-doping concentration. The V-substitution to W sites keep a V-V separation distance of 5 nm without V-V aggregation, scrutinized by high-resolution scanning transmission-electron-microscopy. More importantly, the ferromagnetic order is clearly modulated by applying a back gate. We also observe a ferromagnetic hysteresis loop together with oscillatory behavior at room temperature in diluted V-doped WSe₂, while maintaining the semiconducting characteristic of WSe₂ with a high on/off current ratio of five orders of magnitude. Our findings open new opportunities for using two-dimensional transition metal dichalcogenides for future spintronics.

In recent years, transition metal dichalcogenides become emergent thermoelectric materials with potential wide spread in wearable electronics applications. Considering the extensive applications of transition metal dichalcogenides, many research efforts have been paid and good progresses have been made. So far, all of the researches in this field focus on the electrical properties of them. It has not been reported about the transition metal dichalcogenides’ thermal transport properties under mechanical strains. Due to their potential applications, it is of significance to discover the thermal transport properties of transition metal dichalcogenides at large mechanical strains, and thus develop the novel thermoelectric platforms which can sustain the extreme environments in wearable electronics.

In this work, we demonstrate using Raman optothermal technique to measure the thermal conductivities of two dimensional (2D) MoS₂. We first utilize more direct measurements of the optical absorption, and then by comparing the response of the samples using different laser spot sizes, we are able to measure the thermal conductivities of the 2D MoS₂. Then thermal conductivities at large mechanical strains are obtained providing more scientific merits.

The Raman optothermal technique has been the most successful method for measurement of thermal conductivity of 2D materials, and was used to measure the 2D materials at large mechanical strains for the first time in this work. In this technique, a laser is focused at the center of a thin film and used to measure the peak position of a Raman-active mode. As the laser power is increased, the sample is heated which enables red-shift Raman mode due to thermal softening. Another comparison experiment is conducted by placing the samples on a heating platform and monitor the change of Raman-active mode peak position shift. Combining these two sections of experiments provide us the thermal modeling can then be used to extract the thermal conductivity from the measured shift rate. We have used a refined version of the optothermal Raman technique to study thermal conductivity of 2D MoS₂, at large mechanical strains. It is the first thermal measurement on 2D materials at large mechanical strains. This work also addresses several important issues in the measurement of thermal conductivity of 2D materials at large strains using Raman spectroscopy.

In conclusion, we have used Raman optothermal technique to study thermal transport properties of 2D MoS₂ at large mechanical strains. This work addresses several important issues in the measurement of thermal conductivity of the 2D materials MoS₂ using Raman spectroscopy. We derive the thermal conductivity values at large mechanical strains. These results are of significance to discover the thermal transport properties of 2D transition metal dichalcogenides at large mechanical strains, and thus develop the efficient 2D thermoelectric materials which can sustain the large mechanical strains and in turn to use strains to tune their thermal properties.

Understanding the influence of strain on transport properties, such as piezoresistive effects, is important for device applications of 2D materials. Accurate predictions of transport in materials depend on detailed knowledge of electron-phonon interactions, calculations of which have only recently become feasible due to algorithmic improvements. In particular, we can now apply the Electron-Phonon Averaged approximation to reduce the cost of such calculations and automate the prediction of resistivity with respect to variations in strain. Using first-principles electron-phonon calculations, we explore the specific mechanisms underlying piezoresistive effects in systems such as graphene and transition metal dichalcogenides, and provide additional understanding of how to manipulate these effects for practical use.

Active thermal management devices often lack the efficacy and energy efficiency of electronic switches. This is mainly because of the bosonic nature of phonons and the type of interactions they have with external forces. As a result, discovery of materials with unique vibrational properties that provide better control of thermal carriers is necessary. Our research shows that the phonons in group IVB transition metal dichalcogenides such as hafnium disulfide have distinct characteristics and provide remarkable mechanism for control of thermal careers. This is caused by the intermixing of van der Waals forces and covalent bonds in the material structure combined with their highly anisotropic bond properties. In this talk we present our results on vibrational and thermal transport properties of a member of this group, hafnium disulfide (HfS₂). The highly asymmetric bond properties in HfS₂ dominate the structure and vibrational properties of this material. We explore the physics of HfS₂ by developing a comprehensive computational model to study its phonon behavior, anharmonic properties, and phonon transport characteristics. Our findings demonstrate a unique phonon behavior in the material where the absence of a phonon-gap between the acoustic and optical branches grant a more prominent and unique role to the optical modes of HfS₂. As a result HfS₂ has a low thermal conductivity, ranging between 0.3-5W/mK. In additional only one acoustic phonon branch, the ZA branch, contributes up to 80% to the thermal transport of the material in both out-of-plane and in-plane crystal orientations. Our experiments confirm and support the theory and add further insight into an anomalous behavior in the optical modes. Our quasi-harmonic model reveals a structural phase transformation at around room temperature that leads to unique anharmonicity, discernible in the experimental results of the zone center out-of-plane optical mode. This unique phase transformation contributes to both changes in the phonon decay properties and thermal transport behavior. The structural phase transformation combined with the constricted ZA branch phonon carriers and the distinct scattering physics in optical branches of HfS₂ provide unique means for control of thermal transport. We explore a few strategies for control of thermal properties in HfS₂ based on fundamental vibrational properties of the material. We demonstrate that a successful approach for dynamic control of thermal conductivity is the control of van der Waals gap properties. We demonstrate that through intercalation and modification of these gaps a 4-fold control of cross-plane thermal conductivity in hafnium disulfide from 0.35Wm^-1K^-1 to 1.45Wm^-1K^-1 is achievable. We also demonstrate that an out-of-plane strain can be used to make thermal regulators based on HfS₂. This research unveils a unique class of 2D layers with vibrational properties suitable for development of thermal management devices

A pair of electron and hole across the interface of semiconductor heterostructure can form a bound quantum state of the interlayer exciton. In a coupled interface between atomically thin van der Waals layers (vdW), the Coulomb interaction of the interlayer exciton increases further. In this presentation, we will discuss observing interlayer exciton formation in semiconducting transition metal dichalcogenide (TMDC) layers. Unlike conventional semiconductor heterostructures, charge transport in of the devices is found to critically depend on the interlayer charge transport, electron-hole recombination process mediated by tunneling across the interface. We demonstrate the enhanced electronic, optoelectronic performances in the vdW heterostructures, tuned by applying gate voltages, suggesting that these a few atom thick interfaces may provide a fundamental platform to realize novel physical phenomena. Furthermore, complete experimental control of density, displacement and magnetic fields in our graphene double layer system enables us to explore the rich phase diagram of several superfluid exciton phases with the different internal quantum degrees of freedom.

One of the most alluring features of 2D materials is their tunability, as their carrier density, effect mass, band gap, and electronic phases can be tuned in situ by external parameters such as electric field, magnetic field and strain. Here we will present transport studies of high quality 2D materials. For instance, we demonstrate large tunable SOC and zero-field spin-splitting in atomically thin InSe with unprecedented mobility. From beating patterns in quantum oscillations, we establish that the SOC parameter a is thickness-dependent; it can be continuously modulated over a large range by an out-of-plane electric field, achieving zero-field splitting tunable between 0 and 20 meV. Surprisingly, a could be enhanced by an order of magnitude in some devices, suggesting that SOC can be further manipulated by variations in interlayer spacing induced by stacking and/or electrostatic compression. Our work highlights the extraordinary tunability of SOC in 2D materials, which can be harnessed for in operando spintronic and topological devices and applications.

Many-electron interaction effects dominate many spectroscopic properties of reduced-dimensional systems, leading often to manifestation of novel phenomena not seen in the bulk. In this talk, I present some of our recent work on electron interaction effects in atomically thin quasi 2D materials: (1) strongly bounded correlated multi-particle excitations, such as trions and bi-excitons, in quasi 1D and 2D materials; (2) giant excitonic effects in shift currents, a nonlinear optical phenomenon, in non-centrosymmetric quasi 2D semiconductors; and (3) excitons and band renormalization in optical field driven angle-resolved photoemission spectroscopy (ARPES). Studies of these novel phenomena are made possible because of newly developed theoretical methods which incorporate higher-order many-electron correlation effects from first principles, using an interacting Green’s function approach.

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 a path with enormous potential. 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. arxiv:2005.02416 (2020)

Single photon emitters (SPEs), or quantum emitters, are key components in a wide range of nascent quantum-based technologies. A solid state host offers many advantages for realization of a functional system, but creation and placement of SPEs are difficult to control. We describe here a novel paradigm for encoding strain into 2D materials to create and deterministically place SPEs in arbitrary locations with nanometer-scale precision [1]. We demonstrate the direct writing of SPEs in 2D semiconductors based on a materials platform consisting of a WSe₂ monolayer on a deformable substrate using an atomic force microscope nano-indentation process. This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs for facile coupling with photonic waveguides, cavities and plasmonic structures.

The weak interlayer bonding in van der Waals heterostructures (vdWh) enables one to rotate the layers at arbitrary azimuthal angles. For transition metal dichalcogenide vdWh, twist angle has been treated solely through the use of rigid-lattice moiré patterns. No atomic reconstruction has been observed to date, although reconstruction can be expected to have a significant impact on all measured properties, and its existence will fundamentally change our understanding of such systems. Here we demonstrate via conductive AFM and TEM that vdWh of MoSe₂/WSe₂ and MoS₂/WS₂ undergo significant atomic level reconstruction at twist angles ≤ 1° leading to discrete commensurate domains divided by narrow domain walls [2], rather than a smoothly varying rigid-lattice moiré pattern as has been assumed in prior work. We show that this occurs because the energy gained from adopting low energy vertical stacking configurations is larger than the accompanying strain energy [3]. Such reconstruction impacts both the local conductivity and the optical properties.

[1] M.R. Rosenberger et al, ACS Nano 13, 904 (2019). DOI: 10.1021/acsnano.8b08730
[2] M.R. Rosenberger et al, ACS Nano 14, 4550 (2020). https://dx.doi.org/10.1021/acsnano.0c00088
[3] M. Phillips and C.S. Hellberg, ArXiv 190902495 Cond-Mat (2019)

*Work done in collaboration with Matthew R. Rosenberger, Hsun-Jen Chuang, Madeleine Phillips, Kathleen M. McCreary, and C. Stephen Hellberg of the Naval Research Laboratory, Washington DC, and Chandriker Kavir Dass and Joshua R. Hendrickson of the Air Force Research Laboratory, Wright-Patterson AFB, OH.
† This work was supported by AFOSR and core programs at NRL.

The ability to tune the electronic and optical properties of 2D materials by induced defects has wide-ranging applications. For example, patterned graphene surfaces can be used for biosensing, while strained 2D transition metal dichalcogenides (TMDs) could enable the control of excitons for quantum emitters. Here, we first report on electronic band structure calculations of periodically patterned graphene, which can be fabricated using electron-beam chemistry with radiolyzed water (Islam, et al. Carbon 2020, 166, 446-455). Analysis of the effects of alternating strips in the graphene surface, which are pristine or with epoxide defects, illustrate electronic confinement in the graphene ribbon, which leads to one-dimensional quasi-metallic or semiconducting response, dependent on the strip’s width. Electron transport calculations demonstrate electron transmission along the graphene strip direction, minimized in the direction perpendicular to edges terminated by oxygen atoms, while intrinsic current-voltage (I-V) characteristics indicate comparable performance to graphene nanoribbons with perfect edges. The electron transport in a device setting with gold electrodes shows improved conductivity in comparison to a graphene-based device. Next, in an effort to move towards controlled emission properties, such as for single-photon emission applications, we consider strained 2D WSe₂ monolayers with varying wrinkle structures. The tunability we demonstrate in the optical response of the mechanically deformed nanostructures is described in detail. Our results on exciton tunability by wrinkle structure could motivate experimental fabrication.

Carbon/graphite-based energy storage devices, such as lithium-ion batteries (LIBs), are the most reliable energy storage devices for portable electronics, electric vehicles (EV) and electric grid storage due to their high-power density, stability, low cost, and safety. Despite the wide use of LIBs, the graphitic anode has limited capacity (372 mAh.g^-1) due to the limited rate of lithium diffusion within the commensurately stacked layered structure.
Here we discuss the unique superior capacity of Lithium intercalation of multilayered graphene structures, named as an Incommensurate Multilayer Graphene (IMLG), that consists of closely packed rotated graphene layers. In-plane and out-plane features of graphene are studied by Raman spectroscopy, High-Resolution Transmission Electron Microscopy (HRTEM), and X-ray Diffraction. The commensurate-incommensurate transition of layered graphene is analyzed by Raman spectroscopy and the range of the key parameters is discussed as a significant for efficient Lithium intercalation.
The structural and binding features of graphene anode before and after Lithium insertion is studied, and a new lithium intercalation model is considered. We revealed that weakened interplanar Van der Waals interaction as it occurs within rotated graphene layers enables easy and full penetration of lithium, followed by flexible adjustment of layers for stable and reversible stable cycling without affecting graphene in-plane features. Superior Lithium intercalation provides reversible and stable long-term charge/ discharge of IMLG anodes reaching up over 1500 mAh.g^-1 capacity of LiBs.
Our recent development of IMLG confirms the feasibility of the synthesis technique which allows us the development of high energy density and powerful lightweight secondary batteries in the nearest future.

Monolayer transition metal dichalcogenides (TMDCs), with the general chemical formula MX₂ (M = Mo or W and X = S, Se and Te), are a class of 2D materials that have been predicted to be piezoelectric using Density Functional Theory (DFT) calculations. In addition, some of the TMDCs possess certain advantages in monolayer form – such as a direct bandgap and better Q-factors – over graphene. Here, we present resonance and piezoelectric measurements on monolayer transition metal dichalcogenides (TMDCs) using cavity-interferometry. We also demonstrate that TMDCs are promising candidates for Nanoelectromechanical Systems (NEMS) due to their low mass and intrinsic piezoelectricity.

Cavity-interferometry refers to a displacement transduction technique in nanoelectromechanical systems (NEMS). Most fabrication protocols for NEMS naturally render an optical cavity where the NEMS structure and the underlying substrate constitute the two mirrors. The modulation in the cavity depth due to the motion of the NEMS translates to intensity variations of the reflected light from the optical cavity, which can be measured on an appropriate detector. By optimizing the cavity depth, wavelength of the probe laser and the back-end electronics, exquisite displacement sensitivity (sub pico-meter/Hz^1/2) can easily be achieved [1].

Device fabrication is challenging; hence, we explored various fabrication methods for creation of suspended 2D material devices and established working protocols for both exfoliated and chemical vapor grown TMDCs. We also developed a new transfer method driven by water wettability to facilitate fabrication efforts. To perform device characterization, we developed an integrated in-vacuum Raman/Interferometer setup [1]. By exploiting the cavity formed between suspended 2D TMDC “bridges” and underneath substrates, resonance frequencies of various modes were characterized including exotic line shapes arising due to 1:1 internal resonance coupling in the nonlinear regime. Using a combined experimental-computational approach and the measured shift of fundamental resonance frequencies, induced by applied bias, we extracted piezoelectric properties of monolayer WS₂ and MoS₂. The exfoliated samples display piezoelectric coefficients that are in good agreement with DFT predictions. By contrast, CVD grown monolayers, exhibit a 10-fold reduction in piezoelectricity, likely due to the existence of atomic defects such as vacancies.

Additionally, by driving resonance in the nonlinear regime, we revealed internal resonance pairs in monolayer WS₂. Discovery of Hopf bifurcations linearly varying with driving forces inspired novel sensing modality with ultrahigh force sensitivity (aN) at room temperature [2]. Our efforts in fabricating and experimenting on suspended 2D TMDC devices will support further development in applying 2D materials to sensors and NEMS applications.

[1] S. S. P. Nathamgari, S. Dong, E. Hosseinian, L. J. Lauhon and H. D. Espinosa, "An Experimental Setup for Combined In-Vacuo Raman Spectroscopy and Cavity-Interferometry Measurements on TMDC Nano-resonators" Experimental Mechanics, Vol. 59, p. 349-359, 2019.

[2] S. S. P. Nathamgari, S. Dong, L. Medina, N. Moldovan, D. Rosenmann, R. Divan, D. Lopez, L. J. Lauhon and H. D. Espinosa, "Nonlinear Mode Coupling and One-to-One Internal Resonances in a Monolayer WS2 Nanoresonator" Nano Letters, Vol. 19, No. 6, p. 4052-4059, 2019.

The quantum confinement in atomically-thin transition metal dichalcogenides (TMDCs) has been investigated for single photon emission based on naturally or artificially occurring defects. In particular, the precise control of the position of a single photon emitter by applying mechanical strains to the TMDC is feasible. Strains have been induced in TMDCs using various nanostructures including dielectric pillars, nanobubbles, optical waveguide, metal nanogaps, and metal nanoparticles. However, polarization control and integration with a high-quality (Q) cavity to increase the Purcell factor remain elusive in TMDC-based single photon sources.
In this talk, I will present the deterministic control of both the position and polarization of single photon emitters in atomically thin WSe₂ placed on a nanogap array. Manipulation of the band structure by a local strain gradient generates a potential well at the nanogap site, and position-controlled single photon emission is subsequently achieved. Additionally, directional elongations of the potential well, which are tuned by changing the nanogap size, allow polarization-controlled single photon emission. Moreover, single photons with a g⁽²⁾ of ~0.1 are generated at 4-20 K. To take a full advantage of such deterministic control of the position and polarization of single photon emitters, the emitters are integrated with one-dimensional photonic crystal cavities. Therefore, this approach is a unique way to develop next-generation, deterministic, controllable single photon emitters based on TMDC materials, which outperform the present single photon sources with random occurrence and uncontrolled polarization properties.

Rapid advances in synthesis of graphene and 2D materials, and fabrication methods for functional devices enable sophisticated types of functionality and their application to various emerging electronics, such as flexible, wearable and optoelectronic applications, that cannot be addressed with conventional materials. In this talk, I present that graphene and 2D materials can play critical roles in this context, through demonstrations of complex, mechanically assembled electronic and optoelectronic devices for flexible and wearable applications. Specifically, the mechanics of graphene and MoS₂ can yield various devices in distinct, engineered flexible geometries that cannot be easily reproduced with conventional materials and/or conventional device layouts. Examples of devices include touch, tactile sensors and wearable OLED display.

Two-dimensional (2D) materials, such as Graphene, hBN and MoS₂, are promising candidates in a number of advanced functional and structural applications, owing to their exceptional electrical, thermal and mechanical properties. To better meet the requirements of practical applications, property modification and regulation are oftentimes desired. Defect and strain engineering strategies are powerful tools to modulate electronic, optoelectronic and other functional properties of 2D semiconductors.
In this talk, we will report our efforts to employ such strategies to tune functional properties of 2D semiconductors. We first demonstrated how substrate interfacial chemistry can be utilized to tailor the physical properties of single-crystalline molybdenum disulphide (MoS₂) monolayers. We use self-assembled monolayers with a variety of end termination chemistries to functionalize substrates and systematically study their influence on the physical properties of MoS₂. Our data shows that combined interface-related effects of charge transfer, built-in molecular polarities, varied densities of defects, and remote interfacial phonons strongly modify the electrical and optical properties of MoS₂. Tunable friction behavior of the heterostructure of monolayer MoS₂ and different self-assembled organic molecules (SAMs) were also demonstrated, which was attributed to the charge transfer from SAMs to MoS₂ influencing the electron concentration and thus the carrier-mediated friction in MoS₂. In another example, we systematically characterize chemical vapor deposition (CVD)-grown MoS₂ by photoluminescence spectroscopy and mapping and demonstrate non-uniform strain in single-crystalline monolayer MoS₂ and strain-induced bandgap engineering. We further modified our CVD process to create a Janus SMoSe monolayer and demonstrated high basal plane hydrogen evolution reaction (HER) activity in such samples and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Selective patterning and functionalization of graphene can produce spatially-defined properties. Buckling or wrinkling of graphene on polymeric substrates enables a direct approach to tune the physical properties without lithographic steps, and the resulting curvature of the wrinkles can control local chemical reactivity. However, most buckling methods have been limited by the range of wavelength tunability, only global control of the wrinkled patterns, and delamination and cracks in the graphene. This talk will describe a scalable approach to achieve area-specific reactivity and patterning of conformal graphene wrinkles. We will discuss how a fluoropolymer layer sandwiched between graphene and different polymer substrates can facilitate crack-free and switchable graphene nanostructures. Such patterned areas with different curvatures show different reactivities based on a plasma fluorination reaction. Our approach for large-area, fine control over graphene wrinkle topographies has prospects for other two-dimensional electronic materials and optoelectronics and plasmonics applications.

The two-dimensional transition metal dichalcogenides (2D TMDCs) tend to have basal planes that are chemically relatively inert. Many approaches and reaction schemes have been demonstrated to modify the chemical nature of TMDCs, resulting in new chemical properties and applications. This talk will highlight our recent work on the chemical properties of 2D TMDCs mediated by point defects and edges, which subsequently enable applications in water treatment and antimicrobial treatment. We have shown robust and versatile covalent functionalization of materials across the metal chalcogenides including MoS₂, MoSe₂, WS₂, WSe₂, Bi₂S₃, and Sb₂S₃ using aryl diazonium salts. The reaction mechanism is initiated at point defect sites and edges, but propagates across the chalcogenide surfaces in a chain-like step growth. We have also shown the ability for MoS₂ to remove Pb contamination from water via a reaction with the surface sites to form sulfate precipitates. The reaction mechanism is again initiated at defect sites in the MoS₂. Finally, we have used single stranded DNA to encapsulate MoSe₂ nanosheets and shown their broad-spectrum antimicrobial properties against multidrug resistant bacteria. The choice of ssDNA sequence influences the conformal coating, and the ability for the edges of MoSe₂ to cut through bacterial cell membranes.

Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. We directly evaluate deep levels and their chemical trends in the bandgap of MoS₂, WS₂ and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different host materials, which explains the chemical trend of native electron density in the hosts as well as a persistent photoconductivity up to 400K.

The study of atomic-monolayer materials has been extremely popular within the last decade due to the emergence of a family of two-dimensional (2D) materials with expansive mechanical, electrical and magnetic properties. Subtle changes in these 2D materials at the atomic length scales, ranging from strain fields and dislocations, have large effects for tuning the electronic and mechanical properties, especially when used in real devices. Electron microscopy can provide the insight needed to visualize these effects [1-2]. Recent advancements in detector technology have already pushed the limits of resolution using 2D bilayer molybdenum disulfide using ptychographic reconstruction [3-4]. In my talk, I will discuss ways electron microscopy can provide opportunities for sensitive strain and magnetic measurements of 2D materials, with an emphasis on emerging techniques provided by the electron microscope pixel array detector (EMPAD), a state-of-the-art, high-dynamic range and high-speed electron diffraction detector.

[1] Lee, CH et al, Nanoletters, 20 (5), 3369–3377 (2020).
[2] Han, E et al, Nature Materials, 19 (3), 305-309 (2019).
[3] Tate, MW et al, Microscopy and Microanalysis, 22 (1), 237-249 (2016).
[4] Jiang, Y et al, Nature, 559 (7714), 343-349 (2018).

The field of spin transport in two-dimensional has flourished in the past couple of years. Some remarkable features are the reports of large charge-to-spin in graphene-based heterostructures [1], the discovery of the quantum spin Hall effect [2], and ultra-clean spin transport measurements within the ballistic regime [3]. In this talk, we will present predictions on diffusive and ballistic spin transport in different two-dimensional materials. Using complementary large-scale numerical transport methods, we developed a theory showing that contacts have a dramatic effect on the spin transport in systems with a size comparable to the spin relaxation length, leading to underestimating the latter [4]. We then apply this theory and bulk transport methods to unveil the role of broken symmetries in the spin transport of transition metal dichalcogenides in their 1T' and 1Td structural phases, showing they can promote an unconventional spin transport regimen. We believe our predictions will lead to a better understanding of the recent measurements of the quantum spin Hall effect in WTe2 and will help optimize experimental setups for its harvesting.

[1] Nat. Mater. 19, 170–175 (2020).
[2] Science 359,6371, pp. 76-79, (2018).
[3] Phys. Rev. Lett. 124, 177701 (2020).
[4] Phys. Rev. Lett. 124, 196602 (2020).

Transport phenomena play an important role in designing and engineering materials with tailored functionalities. Thermal conductivity and spin precession damping, as basic transport properties of materials, can provide a wealth of information on the fundamental scattering processes of phonons and magnons with structural defects and interface imperfection. In this talk, I will share our group’s recent progress on utilizing time-resolved magneto-optical Kerr effect (TR-MOKE) metrology to study the thermal and magnetic properties of functionalized materials spanning a wide range of applications. This will include: (1) revealing the 3D anisotropic thermal transport in black phosphorus, as the next-generation of “wonder materials” for the semiconductor industry; (2) developing low-damping and high-thermal stability materials with perpendicular magnetic anisotropy for spintronic applications; and (3) manipulating spin precession using optically launched acoustic strains via a strong magnon-phone coupling at ultra-high frequencies (~60 GHz). The structure-property relationships of functional materials revealed by the ultrafast pump-probe technique open up opportunities of tailoring material properties by structural engineering at the atomic and molecular levels. Ultimately, such an understanding can be leveraged to guide the design and optimization of materials, as promising building blocks for high-performance electronics, thermal management, hard-disk data storage, and magnetoacoustic devices.

Tuning physical properties of materials by applying strain opens up wide range of applications. In this presentation, sensitivity of these properties on application of strain will be illustrated for the case of TMDs. Using first principles calculations, the effect of normal compressive (NC), bi-axial compressive (BC), and bi-axial tensile (BT) strain on the electronic properties of few layered MoS₂ has been investigated. Regardless of the type of strain, a reversible semiconductor-to-metal transition is observed MoS₂. The effect of strain as a function of number of layers on the transport properties is investigated. It is found that the 3L and 2L-MoS₂ emerge as the most efficient thermoelectric materials under NC and BT strain, respectively. The study was further extended to semi-metallic TMD TiS₂, which has been in focus due to its application in energy storage and thermoelectric materials. Upon application of uniform biaxial strain, TiS₂ transforms from semi-metal to semiconducting phase. The transport calculations show three-fold enhancement in thermopower for both p- and n-type doped TiS₂. Further, the effect of strain on thermal conductivity of bulk MoS₂ is explored, wherein an increase of cross-plane thermal conductivity from 3.5 to about 25 W/m-K. The giant enhancement arises from the increase in interlayer interaction on application of strain and decrease in phonon lifetimes due to the unbundling effect along the cross-plane direction. The concepts proposed from our studies can be extended to other semiconducting and metallic TMDs.

References:
Semiconductor-metal transition in semiconducting bilayer sheets of transition-metal dichalcogenides, Phys. Rev. B, 86, 075454 (2012)
Pressure-induced semiconducting to metallic transition in multilayered molybdenum disulphide, Nat. Commun., 5, 3731 (2014)
Effect of strain on electronic and thermoelectric properties of few layers to bulk MoS₂, Nanotechnology, 25, 465701 (2014)
Strain-induced electronic phase transition and strong enhancement of thermopower of TiS₂, Phys. Rev. B, 90, 174301 (2014)
Thermal Conductivity Enhancement in MoS₂ under Extreme Strain, Phys. Rev. Lett., 122, 155901 (2019)

The ferromagnetic state in van der Waals two-dimensional (2D) materials has been reported recently in the monolayer limit. Intrinsic CrI₃ and CrGeTe₃ semiconductors reveal ferromagnetism but the Tc is still low below 60K. In contrast, monolayer VSe₂ is ferromagnetic metal with Tc above room temperature but incapable of controlling its carrier density. Moreover, the long-range ferromagnetic order in doped diluted chalcogenide semiconductors has not been demonstrated at room temperature. The key research target is to realize the long-range order ferromagnetism, Tc over room temperature, and semiconductor with gate tunability. Here, Ferromagnetic order is manifested using magnetic force microscopy up to 360K, while retaining high on/off current ratio of ~10⁵ at 0.1% V-doping concentration. The V-substitution to W sites keep a V-V separation distance of 5 nm without V-V aggregation, scrutinized by high-resolution scanning transmission-electron-microscopy. More importantly, the ferromagnetic order is clearly modulated by applying a back gate. We also observe a ferromagnetic hysteresis loop together with oscillatory behavior at room temperature in diluted V-doped WSe₂, while maintaining the semiconducting characteristic of WSe₂ with a high on/off current ratio of five orders of magnitude. Our findings open new opportunities for using two-dimensional transition metal dichalcogenides for future spintronics.

Active thermal management devices often lack the efficacy and energy efficiency of electronic switches. This is mainly because of the bosonic nature of phonons and the type of interactions they have with external forces. As a result, discovery of materials with unique vibrational properties that provide better control of thermal carriers is necessary. Our research shows that the phonons in group IVB transition metal dichalcogenides such as hafnium disulfide have distinct characteristics and provide remarkable mechanism for control of thermal careers. This is caused by the intermixing of van der Waals forces and covalent bonds in the material structure combined with their highly anisotropic bond properties. In this talk we present our results on vibrational and thermal transport properties of a member of this group, hafnium disulfide (HfS₂). The highly asymmetric bond properties in HfS₂ dominate the structure and vibrational properties of this material. We explore the physics of HfS₂ by developing a comprehensive computational model to study its phonon behavior, anharmonic properties, and phonon transport characteristics. Our findings demonstrate a unique phonon behavior in the material where the absence of a phonon-gap between the acoustic and optical branches grant a more prominent and unique role to the optical modes of HfS₂. As a result HfS₂ has a low thermal conductivity, ranging between 0.3-5W/mK. In additional only one acoustic phonon branch, the ZA branch, contributes up to 80% to the thermal transport of the material in both out-of-plane and in-plane crystal orientations. Our experiments confirm and support the theory and add further insight into an anomalous behavior in the optical modes. Our quasi-harmonic model reveals a structural phase transformation at around room temperature that leads to unique anharmonicity, discernible in the experimental results of the zone center out-of-plane optical mode. This unique phase transformation contributes to both changes in the phonon decay properties and thermal transport behavior. The structural phase transformation combined with the constricted ZA branch phonon carriers and the distinct scattering physics in optical branches of HfS₂ provide unique means for control of thermal transport. We explore a few strategies for control of thermal properties in HfS₂ based on fundamental vibrational properties of the material. We demonstrate that a successful approach for dynamic control of thermal conductivity is the control of van der Waals gap properties. We demonstrate that through intercalation and modification of these gaps a 4-fold control of cross-plane thermal conductivity in hafnium disulfide from 0.35Wm^-1K^-1 to 1.45Wm^-1K^-1 is achievable. We also demonstrate that an out-of-plane strain can be used to make thermal regulators based on HfS₂. This research unveils a unique class of 2D layers with vibrational properties suitable for development of thermal management devices

In recent years, transition metal dichalcogenides become emergent thermoelectric materials with potential wide spread in wearable electronics applications. Considering the extensive applications of transition metal dichalcogenides, many research efforts have been paid and good progresses have been made. So far, all of the researches in this field focus on the electrical properties of them. It has not been reported about the transition metal dichalcogenides’ thermal transport properties under mechanical strains. Due to their potential applications, it is of significance to discover the thermal transport properties of transition metal dichalcogenides at large mechanical strains, and thus develop the novel thermoelectric platforms which can sustain the extreme environments in wearable electronics.

In this work, we demonstrate using Raman optothermal technique to measure the thermal conductivities of two dimensional (2D) MoS₂. We first utilize more direct measurements of the optical absorption, and then by comparing the response of the samples using different laser spot sizes, we are able to measure the thermal conductivities of the 2D MoS₂. Then thermal conductivities at large mechanical strains are obtained providing more scientific merits.

The Raman optothermal technique has been the most successful method for measurement of thermal conductivity of 2D materials, and was used to measure the 2D materials at large mechanical strains for the first time in this work. In this technique, a laser is focused at the center of a thin film and used to measure the peak position of a Raman-active mode. As the laser power is increased, the sample is heated which enables red-shift Raman mode due to thermal softening. Another comparison experiment is conducted by placing the samples on a heating platform and monitor the change of Raman-active mode peak position shift. Combining these two sections of experiments provide us the thermal modeling can then be used to extract the thermal conductivity from the measured shift rate. We have used a refined version of the optothermal Raman technique to study thermal conductivity of 2D MoS₂, at large mechanical strains. It is the first thermal measurement on 2D materials at large mechanical strains. This work also addresses several important issues in the measurement of thermal conductivity of 2D materials at large strains using Raman spectroscopy.

In conclusion, we have used Raman optothermal technique to study thermal transport properties of 2D MoS₂ at large mechanical strains. This work addresses several important issues in the measurement of thermal conductivity of the 2D materials MoS₂ using Raman spectroscopy. We derive the thermal conductivity values at large mechanical strains. These results are of significance to discover the thermal transport properties of 2D transition metal dichalcogenides at large mechanical strains, and thus develop the efficient 2D thermoelectric materials which can sustain the large mechanical strains and in turn to use strains to tune their thermal properties.

A chemical vapor deposition method is developed for thickness-controlled (one to four layers), uniform, and continuous films of both defective gallium(II) sulfide (GaS): GaS_0.87 and stoichiometric GaS. The unique degradation mechanism of GaS_0.87 with XPS and HAADF-STEM is studied, and it is found that the poor stability and weak optical signal from GaS are strongly related to photo-induced oxidation at defects. Photodetectors based on the two materials are fabricated with short response time (<66 ms), excellent UV photoresponsivity. An enhanced stability of the stoichiometric GaS is demonstrated under laser and strong UV light, and by controlling defects in GaS, the photoresponse range can be changed from vis-to-UV to UV-discriminating.
Besides the role in degradation mechanism, defects in GaS_0.87 also contributed to the formation of a new band sitting within its bandgap, which is confirmed by cathodoluminescence results. As a result, controlling defects in GaS turns the band alignment of the bilayer GaS-WS₂ van der Waals heterostructure from type I to type II. XPS and KPFM characterization demonstrates that the defect band can facilitate the charge transfer from WS₂ to GaS. Devices based on the type-II heterojunction further show that the defect band greatly assisted the carrier transport by lowering the contact resistance, and also demonstrate superior photodetector properties with a large linear dynamic range (>73 dB) at a R of 13 A/W for green light.

[1] Lu, Y., Chen, J., Chen, T., Shu, Y., Chang, R.-J., Sheng, Y., Shautsova, V., Mkhize, N., Holdway, P., Bhaskaran, H., Warner, J. H., Controlling Defects in Continuous 2D GaS Films for High-Performance Wavelength-Tunable UV-Discriminating Photodetectors. Adv. Mater. 2020, 32, 1906958.
[2] Lu, Y., Warner, J. H., et al., Transition of type-I to type-II heterojunction in monolayer GaS-WS₂. In preparation.