Browsing by Subject "2D materials"

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Ab initio electronic transport : from two-dimensional semiconductors to nanoscale metals
(2024-02-07) Zhang, Chenmu; Liu, Yuanyue; Akinwande, Deji; Wang, Yaguo; Zheng, Yuebing
Over the past few decades, Moore’s Law has been a driving force behind the exponential increase in integrated circuits capabilities, significantly transforming people’s lives. However, the continuation of Moore’s Law is being called into question as the continuous shrinking of electric circuits is approaching physical limitations. Currently, the size of transistors and interconnects, two fundamental components in modern chips, has reached the tens of nanometers. Further downscaling leads to circuit delay, heat dissipation and poor performance of microchips, ultimately prohibiting Moore’s Law. One of the biggest challenges is the materials: conventional semiconductors and metals in transistors and interconnects exhibit undesirable resistivity increases when reduced to the nanoscale. Thus, the need for 2D semiconductors and better metals with fewer dimensional impacts has become increasingly urgent. This underscores the critical importance of understanding electronic transport in small system, which is the focus of this thesis. By utilizing first-principles calculations, the mobility and conductivity of 2D semiconductors and finite-size metals can be predicted, leading to physical insights and high-throughput screening for best candidates for electronic transport. This dissertation is organized as follows: Chapter 2 describes the Boltzmann transport equation (BTE) to study electronic transport in materials. Chapter 3 demonstrates how to obtain the transition rates, necessary for solving the BTE, from first principles. In particular, we developed an efficient approach to interpolate the electron-phonon-coupling matrix in 2D materials, which can also include the effect of free carrier screening. Based on the state-of-the-art calculations, Chapter 4 shows that reduced dimensionality results in a larger “density of scattering” in 2D semiconductors, leading to generally lower carrier mobility. We predict several high-mobility (>1400 cm²V⁻¹s⁻¹) 2D semiconductors with extremely small effective mass and/or weak EPC, thus avoiding the “dimensional curse”. Chapter 5 and 6 demonstrate the computational approaches to high-field transport properties and remote phonon scattering for 2D semiconductors, respectively. Finally, for metal in interconnects, we develop an ab initio approach to incorporating electron-phonon/surface scattering for electronic transport in finite-size metals and demonstrated it in copper films with different surfaces in Chapter 7.
Atomistic simulations of 2D materials and van der Waal’s heterostructures for beyond-Si-CMOS devices
(2017-08) Valsaraj, Amithraj; Register, Leonard F.; Banerjee , Sanjay K.; Tutuc, Emanuel; Yu, Edward T.; MacDonald, Allan H.
The unique electrical and optical properties of two-dimensional (2D) materials has spurred intense research interest towards development of nanoelectronic devices utilizing these novel materials. The atomically thin form of 2D materials translates to excellent electrostatic gate control even at nanoscale channel length dimensions, near-ideal two-dimensional carrier behavior, and perhaps conventional and novel devices applications. Monolayer transition metal dichalcogenides (TMDs) are novel, gapped 2D materials. Toward device applications, I consider MoS₂ layers on dielectrics, in particular in this work, the effect of vacancies on the electronic structure. In density-functional-theory (DFT) simulations, the effects of near-interface oxygen vacancies in the oxide slab, and Mo or S vacancies in the MoS₂ layer are considered. Band structures and atom-projected densities of states for each system and with differing oxide terminations were calculated, as well as those for the defect-free MoS₂-dielectrics system and for isolated dielectric layers for reference. Among the results, I find that with O-vacancies, both the HfO₂-MoS₂ and the Al₂O₃-MoS₂ systems appear metallic due to doping of the oxide slab followed by electron transfer into the MoS₂, in manner analogous to modulation doping. The n-type doping of monolayer MoS₂ by high-k oxides with O-vacancies is confirmed through collaborative experimental work in which back-gated monolayer MoS₂ FETs encapsulated by oxygen deficient high-k oxides have been characterized. Van der Waal’s heterostructures allow for novel devices such as two-dimensional-to-two-dimensional tunnel devices, exemplified by interlayer tunnel FETs. These devices employ channel/tunnel-barrier/channel geometries. However, during layer-by-layer exfoliation of these multi-layer materials, rotational misalignment is the norm and may substantially affect device characteristics. In this work, by using density functional theory methods, I consider a reduction in tunneling due to weakened coupling across the rotationally misaligned interface between the channel layers and the tunnel barrier. As a prototypical system, I simulate the effects of rotational misalignment of the tunnel barrier layer between aligned channel layers in a graphene/hBN/graphene system. Rotational misalignment between the channel layers and the tunnel barrier in this van der Waal’s heterostructure can significantly reduce coupling between the channels by reducing, specifically, coupling across the interface between the channels and the tunnel barrier. This weakened coupling in graphene/hBN/graphene with hBN misalignment may be relevant to all such van der Waal’s heterostructures. TMDs are viable alternatives to graphene and hBN as channel and tunnel barrier layers, respectively, for improved performance in interlayer tunnel FET device structures. In particular, I used DFT simulations to study the bilayer-graphene/WSe₂/bilayer-graphene heterostructure as well as single and multilayer ReS₂-layer systems. Significant roadblocks to the widespread use of TMDs for nanoelectronic devices are the large contact resistance and absence of reliable doping techniques. Hence, I studied substitutional doping of, and evaluated various metal contacts to MoS₂ by computing the density of states for the systems. Metal contacts that pin the Fermi level within the desired band are optimal for device applications. My simulation results suggest that monolayer (ML) MoS₂ can be doped n-type or p-type by substituting for an S atom in the supercell with a group-17 Cl atom or a group-15 P atom, respectively. My simulations also suggest that Sc and Ti would serve as excellent contacts to n-type ML MoS₂ due to the strong bonding and large number of states near the Fermi level. But the theoretical expectations are tempered by the material characteristics, i.e., the extremely reactive nature of Sc and the oxidation prone nature of Ti atoms. I also studied commonly used Ag and Au metal contacts to ML MoS₂, which exhibited medium strength bonding to MoS₂ and an apparent pinning of the Fermi level nearer to the nominal MoS₂ conduction band edge
Building fabrication-structure-application datacubes of 2D heterostructures
(2020-11-30) Wang, Jimi; Akinwande, Deji
Researches on graphene and other two-dimensional (2D) layered materials remain exponential growth, driven by the fundamental interests and potential applications. Isolated atomic layers are intrinsically the building blocks that can be reassembled into vertically stacked heterostructures. Those so-called van der Waals heterostructures exhibit intriguing, unique properties that cannot be found in their single-layer counterparts. Recent deterministic placement methods have further opened up new possibilities to fabricate even more complex heterostructures with high performance. Here, this report provides insights into the recent progress of 2D heterostructures with an emphasis on their fabrication-structure-performance datacubes. First, we introduce a detailed description of state-of-the-art deterministic assembly and fabrication methods. We then compare different approaches, summarize their advantages and limitations, alongside the recommendations on choosing suitable techniques. Next, we will discuss the supreme electrical properties of heterostructures and the electron transfer mechanisms that make them outstanding. Then, we present some typical examples of state-of-the-art high-performance electronic applications. Finally, the perspectives and challenges will be addressed for future developments of 2D heterostructures.
Design, modeling, and control of a roll-to-roll mechanical transfer process for two-dimensional materials and printed electronics
(2022-06-08) Zhao, Qishen; Li, Wei (Of University of Texas at Austin); Chen, Dongmei, Ph. D.; Djurdjanovic, Dragan; Liechti, Kenneth M.
Flexible electronics, as an emerging field, has gained tremendous attention over the past few years with the advancement in areas such as two-dimensional (2D) materials fabrication and transfer printing techniques. Mechanical peeling, where thin film materials or printed patterns are transferred from a donor substrate to a target substrate, is demonstrated to be a promising and key operation to transfer 2D material or fabricating complex flexible electronic devices, and it is shown that in many cases performing the mechanical peeling process at a desired peeling condition is needed for high quality product. However, despite the promise of mechanical peeling techniques, studies on scaling up the process and enabling the process in a high-volume manufacturing setting are lacking. In this study, a R2R mechanical peeling process that enables continuous and high-throughput transfer of 2D materials and printed electronics has been developed. The system allows for speed and tension control and is used for performing 2D materials transfer. A system model of the R2R process is derived by integrating the peeling process with the R2R system dynamics. The proposed model provides fundamental understanding of the physics involved in the process, including the interactions between peeling front and the roller dynamics. The model can be effectively used for control design and simulation. The success of the R2R peeling process is dependent on the peeling front geometry and its stability during the peeling process. A real-time supervisory control strategy is developed to enable the peeling angle control in a R2R mechanical peeling process. The supervisory control strategy utilizes a peeling front model to estimate the peeling speed and the adhesion energy between the donor substrate and the material to be transferred. The information is used to generate reference signals for the web tension controllers at the regulatory level to adjust the web tensions in order to achieve desired peeling angles. The proposed control strategy is demonstrated with both simulation and experimental results. To reject periodic disturbances originating from rotating roller shafts and adhesion energy changes in the laminate, a model-based repetitive controller is developed and demonstrated.
Electron interactions in 2D materials : excitons and quantum hall effect
(2016-08) Wu, Fengcheng; MacDonald, Allan H.; Li, Xiaoqin; Fiete, Gregory A.; Niu, Qian; Tutuc, Emanuel
This dissertation presents studies of the electron interaction effects in two-dimensional materials. In particular, excitonic effect in transition metal dichalcogenides and quantum Hall effect in graphene have been investigated. The common thread that passes through the two topics is the interplay between electron interactions and spin and valley degrees of freedom. Chapter 1 is a brief introduction to the thesis. Chapter 2 addresses the energy and wave function of excitons in monolayer MoS$_2$. It reveals several interesting features, which can be important for exciton dynamics. Chapter 3 describes a theory of spatially indirect exciton condensates in transition metal dichalcogenide heterostructures. A systematic approach is developed to construct an effective exciton model with exciton-exciton interactions. The effective exciton model provides a useful guidance to construct the condensate phase diagram of excitons with multiple flavors. Chapter 4 identifies an SO(5) symmetry in the quantum Hall effect in graphene. The enlarged SO(5) symmetry unifies the spin antiferromagnetic order and valley $XY$ order. The physics of the SO(5) symmetry is explored using exact diagonalization and low-energy effective theory. Chapter 5 speculates about possible SU(3) and SU(4) singlet fractional quantum Hall states at a filling factor $\nu=2/3$ based on finite-size exact diagonalization study. These singlets are surprising because they are not captured by the composite fermion approach. The shift quantum number and the pair correlation function of the new states are presented.
Electron mobility in monolayer WS₂ encapsulated in hexagonal boron-nitride
(2021-05-03) Wang, Yimeng, M.S. in Engineering; Tutuc, Emanuel, 1974-
We report electron transport measurements in dual-gated monolayer WS₂ encapsulated in hexagonal boron-nitride. Using gated Ohmic contacts which operate from room temperature down to 1.5 K, we measure the intrinsic conductivity and carrier density as a function of temperature and gate bias. An intrinsic electron mobility of 100 cm²/(V·s) at room temperature, and 2,000 cm²/(V·s) at 1.5 K are achieved. The mobility shows a strong temperature dependence at high temperatures, consistent with phonon scattering dominated carrier transport. At low temperature, the mobility saturates due to impurity and long-range Coulomb scattering. First principles calculations of phonon scattering in monolayer WS₂ are in good agreement with the experimental results, showing we approach the intrinsic limit of transport in these 2D layers.
Electrostatic effects of inhomogeneous strain in monolayer transition metal dichalcogenides
(2023-08) De Palma, Alex Christopher; Yu, Edward T.; Lu, Nanshu; Shih, Chih-Kang; Akinwande, Deji
Two-dimensional materials are only a few atoms thick, exhibiting novel properties due to their reduced dimensionality. Strain engineering can be used to modify optical and electronic properties, and highly inhomogeneous strain distributions in two-dimensional materials can be easily realized, allowing for their properties to be tuned on the nanoscale. This work is primarily focused on transition metal dichalcogenides, which have received much attention owing to their semiconducting nature, placing them in a key role among two-dimensional materials. Monolayer transition metal dichalcogenides exhibit a significant piezoelectric effect that can couple with spatially inhomogeneous strain distributions to influence electronic and optical behavior. In this work, inhomogeneous strain and piezoelectricity in transition metal dichalcogenides are studied. We first examine the luminescence behavior of monolayer MoS₂ and WSe₂ in the presence of strain and strain gradients generated via nanoindentation. The strain distribution and piezoelectricity resulting from indentation of monolayer MoS₂ and WSe₂ is modeled, and the interaction between the piezoelectric effect and strain distribution is demonstrated to result in charge densities reaching 10¹² e/cm², with electrostatic potential variations on the order of ±0.1V across the suspended monolayer in the modeled geometry. These results have potential implications for luminescence and exciton transport behavior in monolayer transition metal dichalcogenides with spatially varying strain. We then characterize monolayer MoS₂ which is inhomogeneously strained via a nanopatterned substrate. Kelvin probe force microscopy and electrostatic gating are used to measure the spatial distribution of the conduction band edge energy which can be correlated to in-plane hydrostatic strain. This method demonstrates the capability to resolve the strain distribution on length scales less than 100nm. A method for determining the distribution of the full in-plane strain tensor from the in-plane hydrostatic strain distribution is also presented. The combination of these methods is able to successfully calculate the spatial distribution of the electrostatic potential resulting from piezoelectricity that agrees well with experimental results. These methods present a powerful way to characterize inhomogeneous strain distributions and piezoelectricity that can be extended towards characterization of a variety of 2D materials.
Engineering two-dimensional materials : discovery, defects, and environment
(2021-05-21) Holbrook, Madisen A.; Shih, Chih-Kang; Shi, Li; Li, Xiaoqin; Lai, Keji
The discovery of graphene and its unprecedented properties inspired an extraordinary increase in research progress, launching an era of two-dimensional (2D) electronic materials. These stable crystalline atomic layers enable the design of ultrathin 2D devices by combining different 2D materials as the foundational components. In order to control the properties of these devices, materials with a variety of electronic properties must be available. In this dissertation, we explore three distinct paths to achieve this goal: expanding the library of 2D materials, post synthesis defect engineering, and proximity engineering of the electrostatic environment. First, we report the MBE synthesis and STM/S characterization of a new 2D insulator, honeycomb structure BeO. In addition to determining the atomic structure and density of states, we used moiré pattern analysis to demonstrate the high crystallinity of the BeO and determined the work function modulation across the moiré pattern. We illustrate that the scalable growth, weak substrate interactions, and long-range crystallinity make honeycomb BeO an attractive candidate for future technological applications. The next focus of this work was defect engineering of monolayer WS₂ by UHV annealing. A high concentration of S vacancies was generated by UHV annealing of the WS2, leading to S vacancy defect-defect coupling. Using STM/S we determined that the interaction of nearby S vacancies leads to an increase of deep in-gap states for different divacancy geometries. This indicates that vacancy engineering can be a useful tool to controllably manipulate 2D material electronic properties. Finally, we demonstrate the creation of a nanoscale planar p-n junction within a single monolayer of MoSe₂ by modulating the electronic properties of the underlying substrate. By intercalating Se at the interface of the hBN/Ru substrate, the hBN becomes decoupled from the Ru, changing its conductivity and work function. We find that this change in the electronic landscape tunes the band gap of the overlying MoSe₂, by screening and shifting the MoSe₂ work function. Thus, this dissertation shines a light on the vast opportunities 2D materials provide for exploration of novel approaches to materials engineering, and demonstrates a tool set for manipulating the electronic properties of these fascinating materials.
Fabrication and characterization of TMD FET architectures for increased functionality
(2020-12-09) Rodder, Michael Allen; Dodabalapur, Ananth, 1963-; Tutuc, Emanuel; Banerjee, Sanjay K; Li, Xiaoqin; Yu, Edward T
The discovery of ultra-thin, van der Waals bound semi-metal (graphene), transition metal dichalcogenide (TMD) semiconductors, and insulator (h-BN), obtained via mechanical exfoliation and stacked cleanly onto one another via dry-transfers, leads naturally to the study of field effect transistors (FETs) made from these 2D materials. In this dissertation, we fabricate various conventional and newly designed field-effect transistor (FET) architectures comprised of 2D materials (for logic, memory, or synaptic applications) and report on electrical characteristics. The 2D materials used in fabrication of our FET architectures include materials for the channel region (MoS2 or WSe2), gate dielectric (h-BN), or the gate (graphene). We begin studying the FET properties of a simple 2D FET architecture (demonstrated with a MoS2 channel) which could augment Si, namely a 2D FET structure utilizing contact gating to reduce parasitic source-drain series resistance (RSD). We show that if mobility and threshold voltage (VT) are well-extracted, then this contact-gated 2D FET structure can still be easily modeled with basic FET equations, such that e.g. a conventional circuit design algorithm could implement the contact-gated 2D FET as easily as a conventional Si FET. Since contact gating is a basic feature of our 2D FET architectures, we next fabricate and characterize two novel contact-gated 2D FET architectures, with the goal being to reduce RSD, while maintaining as thin-as-possible channel layer for electrostatics and/or small subthreshold slope, for improved overall performance. The first FET architecture (single-gated; demonstrated with a WSe2 channel) improves hole injection into a single layer WSe2 channel by use of multilayer WSe2 (with smaller bandgap compared to single layer WSe2) only underneath the source (hole injecting) metal contact. The smaller bandgap multilayer WSe2 results in a smaller Schottky barrier at the metal-WSe2 interface, reducing contact resistance (RC) and thus reducing overall RSD. The second FET architecture (double-gated; demonstrated with MoS2) reduces RSD by using a MoS2 channel layer both above and below the source/drain metal contacts so that both the top and bottom gate electrostatically dope the contact regions. This reduces RSD by ~factor of 2x, and further improves the gating symmetry in MoS2 DGFETs, thus allowing for circuit design flexibility. We then move on to the fabrication of a significantly different FET architecture (i.e. not focused on a contact-gated FET architecture) that could further augment Si technology. In particular, we fabricate and characterize a device, a double-gate MoS2 field-effect transistor (FET) with hexagonal boron nitride (h-BN) gate dielectrics and a multi-layer graphene floating gate (FG), in multiple operating conditions to demonstrate logic, memory, and synaptic applications, beyond that which could be demonstrated in a single Si-FET architecture. In our work, we noted that some of our fabricated devices exhibited a particular gate-bias-dependent kinking in I-V characteristics, which required explanation. The final chapter of this thesis thus formulates a phenomenological model, accounting for interface states at metal-semiconductor contacts, to explain the I-V kinking. The model highlights that 1) metal-semiconductor interface states need to be accounted for when modeling MoS2 FETs, and 2) the importance of forming metal-semiconductor interfaces with low interface state density to avoid I-V kinks which are detrimental for analog applications.
Flexible high frequency electronics and plasmonics using two dimensional nanomaterials
(2017-12) Nagavalli Yogeesh, Maruthi; Akinwande , Deji; Banerjee , Sanjay; Dodabalapur , Ananth; Zheng , Yuebing; Lai, Keji
In this work, we have demonstrated novel flexible electronics and plasmonic devices using 2-dimensional (2D) nanomaterials (graphene and MoS2). The first part of this work is about design of flexible high frequency electronics using 2D nanomaterials. We report sub-THz graphene transistors with fT ~ 100GHz. We also discuss how to integrate graphene based sub blocks (antenna, mixer and speaker) to fabricate all graphene based wireless receiver. We report for the first time flexible RF transistors with GHz frequency response using CVD grown monolayer MoS2. We also demonstrate flexible low power RF nanosystems (amplifiers, mixers, AM receiver) using CVD MoS2. We have developed MoS2 transistor models for integrated circuit design application. RF MoS2 transistors results are very promising for low power internet of things (IOT) applications. In second part, we have shown design of novel plasmonic devices using 2D nanomaterials. We have demonstrated large area tunable graphene metasurface using moiré nanosphere lithography (MNSL). We have shown novel method to fabricate large area graphene nanoribbons (GNRs) using block copolymer lithography (BCPL) and its potential application towards tunable mid-IR plasmonic sensing. We report for the first time nanopatterning of CVD MoS2 on plasmonic substrate using bubble pen lithography (BPL). We have also shown light enhancement of monolayer CVD MoS2 using plasmonic nanoantenna array (PNA). These results are very useful for design of highly efficient 2D nanomaterial based LEDs, photodetectors, lasers and sensors.
Hybrid systems of plasmonic nanostructures and functional materials for light-matter interactions and active plasmonic devices
(2018-08-15) Wang, Mingsong; Zheng, Yuebing; Ben-Yakar, Adela; Milliron, Delia; Li, Wei
Advances in nanofabrication and characterization of nanomaterials enable the development of plasmonic nanostructures with unique optical properties. Plasmonic nanostructures have been extensively studied for their potential applications in optical sensing, photothermal therapy, photovoltaics, and photocatalysis. In this dissertation, we present studies of light-matter interactions in hybrid systems consisting of plasmonic nanostructures and functional materials. These studies are focused on four major types of light-matter interactions in plasmonic nanostructures: (1) plasmon-induced resonance energy transfer (PIRET); (2) plasmon-enhanced spontaneous emission; (3) Fano interference between plasmonic nanostructures and emitters; and (4) strong plasmon-exciton coupling. We also achieved the tuning of light-matter interactions by modifying the physical properties of functional materials or plasmonic nanostructures. In addition, the active control of light-matter interactions was demonstrated by integrating plasmonic nanostructures with switchable materials, such as photochromic dyes. Specifically, we first demonstrated the blue-shifted PIRET from a single gold nanorod (AuNR) to dye molecules. AuNRs enable the energy transfer from plasmonic donors to dye acceptors with light having a longer wavelength and lower intensity, compared to dye donors. Secondly, we studied the tuning of plasmon-trion and plasmon-exciton resonance energy transfer from a single gold nanotriangle (AuNT) to monolayer MoS₂. We achieved these phenomena by the combination of rationally designed monolayer MoS₂-plasmonic nanoparticle hybrid systems and single-nanoparticle measurements. Thirdly, we realized the large modulation of hybrid plasmonic waveguide mode (HPWM) in single hybrid molecule-plasmon nanostructures through the strong molecule-plasmon coupling. The HPWM features both the capacity of plasmonic nanostructures to manipulate light at the nanoscale and the low loss of dielectric waveguides. Fourthly, we demonstrated the photoswitchable plasmon-induced fluorescence enhancement. This large switchable modulation of fluorescence was derived from the large near-field enhancement at the subnanometer gap between Au nanoparticles and switchable intersystem crossing as a nonradiative decay channel in photochromic dyes. Finally, we achieved tunable Fano resonances and plasmon-exciton coupling in two-dimensional (2D) WS₂-AuNT hybrid structures at room temperature. The tuning of Fano resonances and plasmon-exciton coupling were achieved by the active control of the WS₂ exciton binding energy and dipole-dipole interaction through controlling the dielectric constant of the surrounding medium.
In-situ real-time spectroscopy platform for monitoring gas adsorption and reactions on 2D materials
(2019-12-05) Holt, Milo; Akinwande, Deji; Banerjee, Sanjay; Bank, Seth; Cullinan, Michael; Dodabalapur, Ananth
Thin film gas sensors are at the center of critical areas of research and innovation, with applications in a wide range of important and fast-evolving fields. From environmental monitoring and medical diagnosis to early warning systems and safety, gas sensors are a vital front end technology already ubiquitous in industry. With development of the Internet of Things (IoT), the need for untethered platforms is accelerating the search for gas sensing materials suitable in low-power applications. 2D materials like graphene and 40+ combinations of the transition metal dichalcogenides (TMDs) show particular promise here, owing to a suite of exceptional dimensional, structural and electronic properties. Specifically with respect to gas sensing, 2D thin films provide the ultimate material dimensions and properties for low-power fast-response devices, at the same time providing an excellent interface for analyte-surface observations. This dissertation concerns the sensing properties and stability of the four prototypical semiconducting TMD combinations of molybdenum (Mo), Tungsten (W), Sulfur (S) and Selenium (Se), with a focus on molybdenum disulfide (MoS₂). This work presents a platform solution for the in-situ real-time (dynamic) capture and study of ammonia (NH₃), water vapor (H₂O) and oxygen (O₂) interactions with exfoliated and synthesiszed TMDs in a controlled environment, at both ambient and elevated temperatures
Interlayer excitons in twisted van der Waals heterostructures
(2020-08-07) Choi, Junho; Li, Elaine; Shih, Chih-Kang; Lai, Keji; Roberts, Sean T.
Van der Waals (vdW) heterostructures represent a promising material platform with rich electronic and optical properties highly tunable via a wide selection of layer materials, electric doping, strain, and twist angle. Monolayers of transition metal dichalcogenide (TMD) semiconductors commonly show strong light-matter coupling and direct bandgaps from the infrared to the visible spectral range, making them promising candidates for various optoelectronic applications. Vertically stacking different TMD monolayers allows one to create TMD heterostructures with rich and tunable correlated electronic phases and optical properties. Among different methods to tune the properties of vdW heterostructures, the twist angle is the most unique parameter. In this dissertation, we investigated the twist angle dependent optical properties of interlayer excitons in TMD heterostructures. First, we studied the twist-angle dependent interlayer exciton lifetimes in MoSe₂/WSe₂ heterostructures. We found that the multiple resonances of interlayer excitons subject to strong confinement in the moiré potential. Their properties are consistent with the interpretation that these resonances are ground- and excited state excitons. Our experiments further revealed that the recombination dynamics of interlayer excitons depends strongly on the twist angle. For example, their lifetimes change from ~ 1 ns to hundreds of ns when the twist angle is increased from 1 to 3.5°. In collaboration with a theoretical group, we explored two mechanisms for this drastic dependence. First, a relative rotation between the two layers in real space translates to a rotation in the momentum space. As the twist angle is increased, the interlayer exciton transitions change from direct- to indirect transitions in the momentum spacing, leading to a longer lifetime. Second, the presence of moiré potential also has a significant impact on the lifetime, reducing its angle dependence by relaxing the requirement of momentum conservation. Next, we investigated the influence of moiré potential on interlayer exciton diffusion in MoSe₂/WSe₂ heterostructures. The interlayer exciton diffusion offers a unique channel of energy and information transport in TMD heterostructures. While early studies focused on how mobile excitons are in TMD heterostructures, we find that interlayer exciton diffusion is impeded in the presence of the moiré potential by comparing two types of samples: those prepared by mechanical exfoliation and those grown with chemical vapor deposition. We investigated multiple mechanically stacked samples with accurately controlled twist-angles. We showed that the interlayer exciton diffusion does not depend on the size of the moiré supercell in a simple and monotonic manner. These experiments provide an important and complementary view of the diffusion properties of interlayer excitons from those reported in the literature.
Mechanics of bubbles and tents formed by 2D materials
(2020-06-19) Dai, Zhaohe; Lu, Nanshu; Mangolini, Filippo; Huang, Rui; Ravi-Chandar, Krishnaswamy; Liechti, Kenneth M
Poking and bulging have been standard methods for characterizing the mechanics of thin solids, including biological, metallic, and elastomeric membranes, as well as emergent atomically thin 2D materials. We call the poked and bulged thin solids tents and bubbles, respectively. Besides their broad uses for fundamental mechanics metrologies, 2D material bubbles and tents have seen a surge of interest in the field of condensed matter physics. The interest is triggered by the fact that the out-of-plane deformation associated with bubbles and tents can produce self-sustained, non-uniform in-plane strains, based on which exciting strain-coupled physics (e.g., bandgap engineering and pseudomagnetic fields) and unique quantum applications have been extensively demonstrated. The deterministic control of these experiments brings the necessity to elucidate the kinematics of 2D material bubbles and tents as well as how the geometry is selected by the elasticity and the film-substrate adhesive interaction. However, there are significant gaps between experimental observations and theoretical understanding of these systems, due to the under-appreciation of the atomically smooth/lubricated nature of 2D materials. In particular, 2D material-substrate interfaces are extremely susceptible to shear, while bubbles and tents have been frequently modeled with clamped boundary conditions that prohibit any motions of their edges. In striking contrast, experimental observations have revealed subtle wrinkling instabilities near the edge of 2D material bubbles and tents (even at small deflections before further edge delamination occurs). This gives concrete evidence for the existence of inward slippage/shear of these atomic sheets on their supporting substrates, on which our quantitative understanding has been lacking. As a consequence, many questions arise that are of vital importance to both metrology and functionality applications of 2D material bubbles and tents. How does the shear of 2D material interfaces and the formation of elastic instabilities modify the mechanical responses of the sheet to point and distributed transverse loads on tents or bubbles? How are the strain fields modified? Besides, in many scenarios, 2D material bubbles and tents form spontaneously by trapping liquids and nanoparticles at the 2D material-substrate interfaces, respectively. A natural question is how the elasticity of the 2D material, together with its adhesive and shear interactions with the substrate, select the geometry of those spontaneous bubbles and tents. Notably, the geometry may include not only the global out-of-plane profiles but also local features such as the extent of the elastic instabilities due to edge slippages. This dissertation aims to answer these questions concerning the mechanics of 2D material bubbles and tents. We study a collection of problems in this area with a particular emphasis on the role of the shear deformations associated with the atomic-level interfaces. With the understanding of these questions, we proceed to discuss a number of useful implications for the elasticity and interface metrology of 2D materials as well as the rational design of bubbles and tents for a range of applications with the instabilities being either avoided or exploited.
Memory effect and RF switch applications based on two-dimensional materials
(2020-10-15) Kim, Myungsoo; Akinwande, Deji; Lee, Jack C; Register, Leonard F; Lai, Keji; Chen, Ray T
Since Novoselov and Geim rediscovered the promising characteristics of single-layer graphene at 2004, various two-dimensional (2D) materials such as graphene, transition-metal dichalcogenides (TMDs), hexagonal boron nitride (hBN), Xenes (silicene, phosphorene, germanene) gained a lot of interest due to their unique and fascinating physical properties. Recently, these novel semiconducting 2D materials have led to a variety of promising technologies for nanoelectronics, photonics, sensing, energy storage, and optoelectronics, to name a few. In this dissertation, we report a detailed study of memory effect and RF switch applications based on two-dimensional materials. In chapter 2, we report non-volatile resistive switching (NVRS) in single-layer atomic sheets sandwiched between metal electrodes. We named this device as Atomristor and it shows low switching voltage, forming-free characteristic, high on/off ratio, and record pulse operation. Ab-initio simulations reveal that the NVRS can be attributed to the interactions between metal ions and sulfur vacancies. In chapter 3, RF switches based on the forming-free MoS2 non-volatile memory are discussed to overcome the limitations of conventional emerging RF switch technologies. The MoS2 switches have low insertion loss and high isolation, scalable cutoff frequencies, and displays good linearity. In chapter 4, analog switches made from boron nitride monolayers for application in 5G and terahertz communication systems are examined. The hBN switches achieve a better insertion loss, isolation, and power handling compared to MoS2 RF switches. For data communication systems, eye-diagram and BER measurements were conducted to evaluate the distortion and revealed a good operation at a bit rate of 8.5 Gbit s-1. In chapter 5, single-pole-double-throw (SPDT) RF switches based on 2D memristors are explored for practical circuit applications.
Microwave impedance microscope study of two dimensional materials
(2015-05) Liu, Yingnan, Ph. D.; Lai, Keji, 1978-; Shih, Chih-Kang
In this thesis, I will introduce a unique technique, microwave impedance microscope (MIM), which has shown its potential in characterization of local electrical inhomogeneity of materials. I will also discuss some results about the study of In₂Se₃ and MoS₂ electrical properties with MIM.
Non-volatile resistive switching behavior in two-dimensional monolayer materials
(2021-04-19) Wu, Xiaohan, Ph. D.; Lee, Jack Chung-Yeung; Akinwande, Deji; Dodabalapur, Ananth; Shi, Li; Swartzlander, Earl
Two-dimensional (2D) materials have drawn much attention in the last two decades and have been investigated as an emerging material system in a wide range of applications. Recently, various 2D materials have been reported to exhibit non-volatile resistive switching (NVRS) phenomenon. However, it was not believed to be accessible in monolayer atomic sheets due to excessive leakage current. This dissertation presents the first discovery of resistive switching behavior in 2D monolayer materials in a vertical metal-insulator-metal (MIM) structure with systematic research in electrical characteristics, switching mechanisms, performance improvements, and potential applications. Chapter 2 discusses NVRS in monolayer molybdenum disulfide (MoS₂) and three other transition metal dichalcogenides, featuring low switching voltages, large on/off ratio, fast switching speed, and forming-free characteristics. The electrical performance, including retention, endurance and variability, and applications in flexible non-volatile memory and radio-frequency (RF) switch are investigated. In Chapter 3, the resistive switching phenomenon in the representative 2D insulator, hexagonal boron nitride (h-BN), is studied, which sets the record of the thinnest NVRS material to ~0.3 nm. In Chapter 4, the collection of 2D atomic sheets showing NVRS is further expanded to a dozen materials, indicating its potential universality in ultrathin non-metallic 2D materials. A Dissociation-Diffusion-Adsorption (DDA) model has been proposed to describe the common mechanism behind bipolar NVRS in 2D crystalline monolayers with both experimental support by scanning tunneling microscope (STM) measurements and theoretical support by first-principle calculations. In Chapter 5, a current-sweep method and a constant electric stress method are applied on the 2D-based memory devices to provide more insights into the switching mechanism. Multiple resistance states can be achieved in 2D memory devices, which enables the applications in multi-bit storage and neuromorphic computing. In Chapter 6, defect engineering through electron irradiation treatment has been performed on monolayer MoS₂, showing a significant improvement on yield and endurance of the fabricated devices, which is attributed to the induced sulfur vacancies that promote the formation of “conductive points”.
Optoelectronic, structural, and topological properties of van der Waals layered materials under extreme conditions
(2018-08) Kim, Joonseok; Akinwande, Deji; Lin, Jung-Fu; Banerjee, Sanjay K; Dodabalapur, Ananth; Wang, Yaguo
The concept of Internet of Things (IoT) has been discussed extensively in the recent years, where billions of smart devices and sensors communicate with each other and provide ubiquitous service. Two-dimensional (2D) materials for such application could be exposed to extreme conditions that IoT devices may experience, such as mechanically stressing, chemically reactive, high-temperature, and/or radiative environment. Therefore, it is crucial to understand the materials' properties under extreme conditions, and further engineer the properties from the acquired knowledge. In this dissertation, we focus on the effects of oxygen/moisture condition on air-sensitive 2D materials, and effects of hydrostatic pressure on 2D and other layered materials. In Chapter 2, we report detailed study on air-degradation of few-layer phosphorene films and field effect transistors, as well as an effective encapsulation method that enhances the stability of devices up to several months. In the later parts we explore the effects of hydrostatic pressure on layered materials, where the anisotropic van der Waals structure exhibit remarkably large pressure-modulation of material properties. In Chapter 3, pressure effects on Raman modes in bulk Mo₀.₅W₀.₅S₂ alloy are examined to discover strengthening of inter-layer interactions under pressure. In Chapter 4, pressure-induced structural transition of bulk WTe₂ is discussed, where layer sliding introduces inversion symmetry, similar to the case in monolayer WTe₂. In Chapter 5, evolution of optical band gaps of monolayer WS₂ and Mo₀.₅W₀.₅S₂ are studied, where we show different pressure-behaviors of band edges according to the composition. In Chapter 6, structural, vibrational, and topological electronic properties of Bi₁.₅Sb₀.₅Te₁.₈Se₁.₂ topological insulator alloy is explored, to show that the topological states could be modulated by pressure, without transitions in the crystal structure.
Piezoelectricity and flexoelectricity in 2D transition metal dichalcogenides
(2018-05-01) Brennan, Christopher James; Yu, Edward T.; Lu, Nanshu; Banerjee, Sanjay K; Akinwande, Deji; Huang, Rui
Two-dimensional materials are only on to a few atoms thick, making them the thinnest possible material known to man. Their combination of electrical, optical, and mechanical properties allows for unique electrical devices with a wide range of future applications, from being a post-silicon material option, creating high-speed communication systems, allowing the advancement of flexible electronics, and even creating transparent electronics. Among their amazing characteristics is the coupling of electrical and mechanical properties. Although not unique to 2D materials, electromechanical coupling could be used in 2D materials to create a class of sensors, actuators, and energy harvesters at a scale not previously possible. Specifically, 2D materials could be utilized in flexible, wearable electronics as an energy harvester to convert the motion of the body into electrical energy. In this dissertation, the electromechanical coupling properties known as piezoelectricity and flexoelectricity are studied in 2D materials both to advance the development of 2D materials in general, and to improve the understanding of the relatively novel effect of flexoelectricity. This work focuses on a class of 2D materials known as transition metal dichalcogenides (TMDs), which are semiconducting and intrinsically piezoelectric. To begin, the adhesion between the TMDs and soft substrates is studied. Soft substrates could be used in flexible and wearable electronic systems, so adhesion of TMDs to soft substrates is important. It was found that the adhesions between the TMD molybdenum disulfide and polydimethylsiloxane is roughly 18 mJ m⁻². Next, the out-of-plane electromechanical coupling of molybdenum disulfide and other TMDs was studied. Piezoelectric theory predicts that there should be zero out-of-plane response, but a signal is measured in all TMDs, suggesting the presence of flexoelectricity. The measured effective out-of-plane piezoelectric response is on the order of 1 pm V⁻¹ and the estimated flexoelectric response is on the order of 0.05 nC m⁻¹. Additionally, it was found that the magnitude of the out-of-plane electromechanical response of different TMDs roughly follows a trend predicted by a simple model of flexoelectricity. The work presented in this dissertation provides the first experimental evidence of a flexoelectric effect present in 2D TMDs.
Pressure induced structure-property tuning of two dimensional materials
(2015-05) Nayak Pradeep, Avinash; Akinwande, Deji; Lin, Jung-Fu; Wang, Yaguo; Wang, Zheng; Hall, Neal; Dodabalapur, Ananth
Controlling the band gap by tuning the lattice structure through pressure engineering is a relatively new route for tailoring the optoelectronic properties of two dimensional (2D) materials. Here we investigate the electronic and lattice vibrational dynamics of the distorted monolayer 1T-MoS₂ (1T') and the monolayer 2H-MoS₂ via a diamond anvil cell (DAC) and density functional theory (DFT) calculations. The direct optical band gap of the monolayer 2H-MoS₂ increases by 11.7% from 1.85 eV to 2.08 eV, which is the highest reported for a 2D transition metal dichalcogenide (TMD) material. DFT calculations reveal a subsequent decrease in the band gap with eventual metallization of the monolayer 2H-MoS₂, an overall complex structure-property relation due to the rich band structure of MoS₂. Remarkably, the metastable 1T'-MoS₂ metallic state remains invariant with pressure, with the J₂, A₁[subscript g], and E₂[subscript g] modes becoming dominant at high pressures. This substantial reversible tunability of the electronic and vibrational property of the MoS₂ family can be extended to other 2D TMDs. These results present an important advance toward controlling the band structure and optoelectronic properties of monolayer MoS₂ via pressure, which has vital implications for enhanced device applications.