Symposium NM11 : Deformable Atomically Thin Materials---Mechanics, Materials and Devices

We have assembled a variety of new 3D structures comprised of graphene oxide and/or graphene sheets. I will present their structure and some of their properties, including if our work has progressed that far by the time of the presentation, the electrical, optical, and mechanical properties.

Two thousand years ago, the mass-manufacturing of paper simplified all aspects of information technology: generation, processing, communication, delivery and storage. Similarly powerful changes have been seen in the last century through the development of integrated circuits based on silicon. Monolayers of 2D crystals provide an ideal material platform for realizing these integrated circuits thin and free-standing, which were the key advantages of paper over other medium two thousand years ago. Once realized, these atomically thin circuits will be foldable and actuatable, which will further increase the device density and functionality, allowing them to be used tether-free (or wirelessly) in environments not previously accessible to conventional circuits, such as water, air or in space. In this talk, we will discuss our recent progresses toward building atomically-thin integrated circuits using wafer-scale 2D crystals. In order for this, we developed a series of approaches that are scalable, precise, and modular. We developed wafer-scale synthesis of three atom thick semiconductors, reported a wafer-scale patterning method for one-atom-thick lateral heterojunctions, and showed how atomically thin films and devices can be vertically stacked to form more complicated 3D circuitry. Then we will discuss our most recent efforts to turn these 2D circuits into 3D structures.

The development of new processing schemes that enable 2D materials to be incorporated on soft, organic substrates remains a fundamental challenge for future flexible electronics based on the unique layered structures. Local, transient heating and patterning of amorphous precursors by laser processing may be the key to unlocking the unique attributes of flexible 2D material systems. Initial experiments reveal the successful phase transformation of amorphous transition metal dichalcogenides (TMD) including MoS₂ and WS₂ deposited by physical vapor deposition (PVD) on stretchable polymer substrates to their crystalline van der Waals layered structures. Detailed kinetic studies of crystal formation were accomplished via high throughput in-situ Raman spectroscopy at different surface temperatures and environmental conditions. With this technique, heterostructures were formed incorporating multiple TMD layers that were annealed simultaneously, and insights into the role of surface diffusion and activation energy for crystallization will be discussed. Additionally, large area, wafer-scale crystallization of 2D materials on flexible substrates with the use of a broadband pulsed lamp source demonstrate the potential for optical annealing at multiple length scales for ease of manufacturing directly on soft substrates.

The ultrathin of two-dimensional crystals offers the possibility to use external strain to manipulate, in a controlled manner, their optical and electronic properties. Here, we utilized strain engineering to manipulate the electronic and optical properties of 2D transition metal dichalcogenide materials through controlled synthesis on the patterned substrate. How the curved surface affects the nucleation and growth pathway of 2D crystals will be discussed. The benefits of strain engineering in 2D crystals for applications in nano-electronics and optoelectronics will be discussed.
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division and performed in part as a user project at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Atomically thin graphene exhibits fascinating mechanical
properties, although its hardness and transverse stiffness
are inferior to those of diamond. So far, there has been
no practical demonstration of the transformation of
multilayer graphene into diamond-like ultrahard structures.
Here we show that at room temperature and after nano-indentation,
a two-layer graphene film on SiC(0001) exhibits a transverse
stiffness and hardness comparable to those of diamond,
is resistant to perforation with a diamond indenter and
shows a reversible drop in electrical conductivity upon
indentation [1]. Density functional theory calculations suggest
that, upon compression, the two-layer graphene film transforms
into a diamond-like film, producing both elastic deformations
and sp2 to sp3 chemical changes. Experiments and calculations
show that this reversible phase change is not observed for a
single buffer layer on SiC or graphene films thicker than
three to five layers. Indeed, density functional theory
calculations show that whereas in two-layer graphene
layer-stacking configuration controls the conformation of the
diamond-like film, in a multilayer film it hinders
the phase transformation [1]. Atomistic indentation simulations
show also that a SiC(0001) substrate coated by a stiff diamond-like
film yields a force versus indentation depth curve steeper than
that of the bare SiC substrate, whereas a five-layer graphene
film (which does not undergo any phase change) on SiC leads to
a significant softening of the transverse mechanical response.
Overall, experiments and calculations show that the hardening
effect exhibited by two-layer graphene on SiC(0001) arises
from a pressure-induced phase transformation to a diamond-like
film [1]. Our study opens up new ways to investigate
graphite-to-diamond phase transitions at room temperature in
low-dimensional systems, and it identifies supported two-layer
graphene as an interesting candidate for pressure-activated adaptive
ultrahard and ultrathin coatings and for force-controlled dissipation
switches.

[1] Yang Gao, Tengfei Cao, Filippo Cellini, Claire Berger,
Walter A. de Heer, Erio Tosatti, Elisa Riedo, and Angelo Bongiorno
Nature Nanotechnology (2017) doi:10.1038/s41565-017-0023-9

Two dimensional (2D) layered materials are crystalline materials with layered structures, including Graphene, h-BN, and Transition Metal Di-chalcogenides (TMD’s). Each of their layers is consisting of one or a few atomic layers and they form van der Waals interactions with neighboring layers. Recently, they have been studied intensively due to their extraordinary properties, such as, flexibility and transparency. In addition, they have exceptional electronic, optoelectronic, chemical and mechanical properties. For example, Graphene has high electron mobility, chemical inertness, and thermal conductivity, while TMD has high photo responsivity. Based on their properties, the 2D electron systems have long been building blocks of electronic and photonic devices.
We have been investigated 2D layered materials for Si technology, but not for the active materials. We have focused 2D layered materials as interface materials due to the chemical inertness and their atomically thin nature. Especially, Graphene has been suggested as a promising material for future interconnects because of its unique electrical and chemical properties. For instance, they are good candidates for diffusion barrier.[1] Also, they are good candidates for interface materials between metal and Si to reduce the Schottky barrier heights and contact resistance in source and drain, which is one of the most critical issues for scaling down.[2]
In this talk, we will cover and discuss the possibility of Graphene and other 2D layered materials for interconnects and contact resistance reducer in Si technology. In addition, we will also cover adaptation of these materials into present Si integration process. The direct growth is always the key technology to make all these applications realistic, and a little prospect of wafer scale graphene and 2D material growth will also be presented.

[1] L.Li et al., “Verticle and lateral copper transport through graphene layer”, ACS Nano, 9 (8), pp. 8361-8367 (2015)
[2] K.-E. Byun et al., “Graphene for true ohmic contact at metal-semiconductor junctions”, Nano Letters, 13 (9), pp. 4001-4005 (2013)

Non-Euclidean geometries and metrics have aided in the development of theoretical understandings of nontrivial three-dimensional (3D) structures, which are experimentally driven by mechanical instabilities of soft elastic materials. In recent years, two-dimensional (2D) materials have also been shown to accommodate various imaginable mechanical deformations owing to their atomic thickness. In contrast to most previous examples relying on post-growth deformations, we show non-flat configurations of atomically-thin MX₂ (M = transition metal and X = chalcogen) via a direct and conformal growth onto pre-defined corrugated surfaces with nonzero Gaussian curvature. Lithographically defined corrugations are tunable in size (50 nm - 1 μm) and curvature, and furthermore, are patternable, hence enabling us to construct laterally connected flat and corrugated networks. We also demonstrate that the grown 2D materials retain their original non-Euclidean architecture even after being suspended. We further explore how 2D crystal growth competes with elastic energy on curved templates employing theoretical calculations and structural analysis.

Epitaxial heterostructures and superlattices with coherent heterointerfaces, in which lattices of dissimilar materials are matched without dislocations, enable devices with strain-engineered electronic, optical and thermal properties. Two-dimensional (2D) coherent heterostructures and superlattices can enable the realization of these strain-engineered applications at the atomically-thin limit. These coherent 2D superlattices can further serve as ultrathin building blocks for advanced stacking and hetero-integration with other materials, providing unique opportunities that are not available to their 3D analogs. Monolayer transition metal dichalcogenides (TMDs), many of which share similar crystal structures, provide an ideal material platform with diverse electrical, optical, piezoelectric, and valley properties. However, the synthesis of coherent monolayer TMD superlattices with strain-engineered properties remains an unsolved challenge. Here, we report coherent monolayer TMD superlattices with precisely controlled supercell dimensions and lattice coherence maintained over the entire structure [1]. This strong epitaxial strain is precisely engineered via the nanoscale supercell dimensions, thereby enabling broad tuning of the optical properties and producing photoluminescence peak shifts as large as 250 meV. The epitaxial strain further induces periodic out-of-plane rippling within the monolayer superlattice, a unique deformation of the thin-film material under strain. Despite a large 4% lattice mismatch, strained WS2/WSe2 superlattices show lattice coherence with dislocation-free heterointerfaces, which is directly confirmed on both atomic and micron scale using electron diffraction and newly-developed TEM imaging technique with electron microscope pixel array detector, which provides the atomic scale mapping of lattice constant, magnitude, and direction of strain. We further discuss the 1D coherent epitaxy growth and the formation of out-of-plane ripples in the monolayer TMD superlattices where the monolayer TMD superlattices are vertically confined to the substrate by the strong van der Waals interaction with the growth substrate. This model is further supported by our theoretical calculation that includes multipole van der Waals energy and lattice strain. Our work opens up new possibilities of generating structures with strain-engineered functionalities by design, at the atomically-thin monolayer limit.
[1] S. Xie, L. Tu, Y. Han, L. Huang, K. Kang, K. U. Lao, P. Poddar, D. A. Muller, R. A. DiStasio, J. Park, “Coherent Atomically-Thin Superlattices with Engineered Strain,” arXiv:1708.09539

Ultrathin heterostructures using van der Waals two-dimensional layered materials have recently demonstrated superb electronic and optical properties. Although graphene and other 2D transition metal dichalocogenides (TMdCs) demonstrate electronic properties, their electronic properties are often obscured by gate oxides which involve charge scattering and charge doping and degrades carrier mobility. The use of h-BN in 2D heterostructure devices could resolve such matters, for example, mobility of graphene and TMdCs have been greatly enhanced. In this talk, we would like to introduce the role of h-BN as a gate insulator and passivation layer and further discuss its effect in terms of trap charges, doping, contact resistance, and subthreshold swing, and protection from environment.

Two-dimensional layered semiconductors have attracted considerable research interest, with optical and electronic properties closely related to their layer stackings. Here, we report the detailed investigation on the controllable growth and formation mechanisms of spiral WS₂ nanoplates. Spiral WS₂ nanoplates with controllable number of screw dislocations and defined shapes can be controllably grown by vapor phase deposition under different conditions. These structures were characterized by atomic force microscopy (AFM) and second harmonic generation (SHG) imaging, which reveal the growth mechanisms. “Spiral arm” contrast features were found at the bottom plane of the nanoplates, the number of which correlates with the number of screw dislocations initiated at the bottom plane. Different numbers of screw dislocations and orientation of layers result in the distinct morphologies and different ways of stacking, such as triangular or hexagonal spiral pattern formed on the top of the triangular spiral nanoplates. The supersaturation-dependent growth may generate new screw dislocation from the existing layers or even new layers templated by an existing screw dislocation. The discovery of these spiral WS₂ nanostructures deepen our understanding and control of screw-dislocation-driven growth of two dimensional nanostructures, and offer diverse candidates for probing the physical properties of layered materials and exploring new applications in functional nanoelectronic and optoelectronic devices.

The chemical vapor deposition (CVD) is a powerful technique for synthesizing monolayer materials specifically transition metal dichalcogenides (TMDs). This method has advantages over exfoliation techniques including higher purity and ability to control chemistry of the products. However, controllable and reproducible synthesis of 2D materials using the CVD technique is a challenge, because the complexity of the growth process and its sensitivity to subtle changes in the growth parameters. This will also hinder the extending of the conclusions and growth conditions between different CVD reactors, without exhaustive trial and error experimentations. Here, we developed a generalized multiscale method, where CVD control parameters are linked to morphology, size, and distribution of synthesized 2D materials. The model is further validated experimentally by systematic growth of MoS_2, demonstrating its generality and capabilities. Developing this model, we coupled the reactor-scale governing heat and mass transport equations with the mesoscale phase-field equations of the growth, where edge energies vary as a function of precursor concentration within the chamber. Predicted distribution of 2D materials are also statistically analyzed indicating a perfect match with the density of material growth over the substrate. The simulation results indicate an excellent capability developed model for predicting the morphology and thus characteristics of synthesized 2D materials and can be used for designing new CVD chambers, determining the optimum growth conditions, and control of the morphology and characteristics of synthesized 2D materials.

Zinc oxide has shown great promise due to its favorable properties for piezoelectricity, UV optics, and catalysis as well as its wide array of crystal morphologies and phases. It has been shown [1,2] that below approximately 1 nm in thickness, ZnO will preferentially form a planar honeycomb structure similar to BN or graphene, called graphitic ZnO (gZnO), as a way to counteract the instability of its polar surfaces. These ultrathin sheets of ZnO have been predicted to hold many advantages over conventional atomically thin layers, including a wider bandgap and high temperature stability in ambient conditions. Additionally, its high ionicity relative to most other conventional 2D materials offers intriguing opportunities for study, including stronger electromechanical coupling and enhanced phonon scattering. However, little is known about this phase due to the difficulty of synthesizing large area gZnO for characterization and applications. In this work, we demonstrate solution-based synthesis of polycrystalline ZnO nanoflakes down to monolayer thicknesses and sizes up to 20 µm using a graphite oxide template process. The process can be performed on a variety of non-metal, flat substrates. TEM imaging on suspended structures show polycrystalline samples with grain sizes on the order of 15 nm. X-Ray Absorption Near Edge spectroscopy (XANES) also shows a very distinct change that indicates a large change in the local structure from buckled wurtzite to a graphitic phase. XPS measurements are performed on synthesized gZnO samples, and, in addition to the XANES spectra, show significant changes to the electronic band structure compared to its bulk phase, including an enlarged band gap. The gZnO sheets also exhibit excellent stability at temperatures as high as 800^oC in ambient environment. This new wide band gap, atomically thin material provides us a platform for harsh environment electronic devices and deep ultra-violet optical applications. Also, the growth process is able to easily incorporate dopants and inherently forms a heterostructure of reduced graphene oxide and gZnO, simplifying the fabrication process for heterostructures and offering a unique platform for study.

Acknowledgements:
This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231.
Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
References:
Freeman, C. L., et al. Graphitic nanofilms as precursors to wurtzite films: theory. Physical review letters 96(6), 066102/1-066102/4 (2006).
Ta, H. Q., et al. Graphene-Like ZnO: A Mini Review. Crystals 6(8), 100 (2016).

In this study, graphene is synthesized on Cu foil via CVD and characterized using optical microscope (OM), scanning electron microscope (SEM) and Atomic Force Microscope (AFM). Well-defined bundles of wrinkles are observed on the graphene covered copper after CVD process using atomic force microscopy (AFM). Wrinkle formation is attributed to the non-hydrostatic compression stresses induced on the graphene by the linear thermal expansion coefficient (LTEC) difference between graphene and copper during cooling. X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) studies are also performed in order to understand the effect of Cu surface orientation. (Supported by TUBITAK grant no 216M042.)

Crumpling is a stochastic folding processing that can turn a flat sheet into a paper ball-shaped particle. This unusual morphology is fractal dimensional, and can be strain stiffened under stress, which makes them remarkably aggregation-resistant. It is also a type of hollow structure, of which both the external and internal surface area are accessible. Here I will present a case study of crumpled graphene balls, which are made by capillary compression of graphene-based sheets in evaporating aerosol droplets. The resulting graphene particles exhibits universal solution processability without the need of surface functionalization, and can redisperse in solvent even after being compressed into a pellet. Such properties are advantages for graphene applications in ultracapacitors, batteries, electrocatalysis, solid state extraction and lubrication.

Direct, bottom-up synthesis of graphene with well-defined pores over large areas can transform the fabrication of nanoporous atomically thin membranes (NATMs) and greatly enhance their potential for practical applications. However, scalable bottom-up synthesis of continuous sheets of nanoporous graphene that maintain integrity over large areas has not been demonstrated. Here, we show that a simple change of parameters during chemical vapor deposition (CVD) on Cu induces in-situ formation of nanoscale defects (≤ 2-3 nm) in the graphene lattice, enabling direct and scalable synthesis of nanoporous monolayer graphene. By solution-casting of hierarchically porous polyether sulfone (PES) supports on the as-grown nanoporous CVD graphene, we demonstrate large-area (> 5 cm²) nanoporous atomically thin membranes (NATMs) for dialysis. The synthesized NATMs show size-selective diffusive transport and effective separation of small molecules and salts from a model protein, with ~2-100× increase in permeance along with better/comparable selectivity to state-of-the-art commercially available polymeric dialysis membranes. Our membranes constitute the largest fully functional NATMs reported to date, which can be easily scaled up to large sizes permitted by CVD synthesis. Our approach highlights synergistic benefits in blending traditional membrane casting with bottom-up pore creation during graphene CVD for advancing NATMs towards practical applications.

References
Kidambi et al Advanced Materials 2017
Kidambi et al Nanoscale 2017
Kidambi et al Advanced Materials 2017
Wang et al Nature Nanotechnology 2017

Abstract:
Recent studies have shown that tellurium based 2D crystals undergo dramatic structural, physical, and chemical changes under ambient conditions. This not only adversely impacts their much desired properties, but also is a roadblock for their applications. Here, we introduce diazonium molecule functionalization based surface engineering route that greatly enhances their environmental stability without sacrificing their much desired properties. Spectroscopy and microscopy results show that diazonium groups significantly slow down the surface reactions, and consequently gallium telluride (GaTe), zirconium telluride (ZrTe₃) and molybdenum ditelluride (MoTe₂) gain strong resistance to surface transformation in air or when immersed under water. Density functional theory calculations show functionalizing molecules reduces surface reactivity of Te-containing 2D surfaces by chemical binding followed by electron withdrawal process. While pristine surfaces structurally decompose due to strong reactivity of Te surface atoms, passivated functionalized surfaces retain their structural anisotropy, optical band gap, and emission characteristics as evidenced by our conductive AFM, PL and absorption spectroscopy measurements. Overall, our findings offer an effective method to increase the stability of these environmentally sensitive materials without impacting much of their physical properties.
Keywords: 2D materials, environmental stability, chemical functionalization, spectroscopy

In van der Waals (vdW) materials such as transition metal dichalcogenides, physical properties such as band structures are sensitive to interlayer coupling between neighboring monolayers across the vdW gap. If the interlayer coupling can be artificially enhanced, one can effectively modulate the electronic dimensionality, and study scientific problems of emergent physical behavior of the system that would not arise otherwise. We seek to enable, discover and understand emergent electronic behavior of vdW nanostructures by maximally modulating their interlayer coupling with high pressures utilizing diamond anvil cells. By doing so, we quantify the vdW interlayer pressure, enhance carrier mobility, renormalize vibrational spectra of existing 2D materials, and discover new 2D semiconductors.

Interfacial mechanics between graphene and substrate such as adhesion and friction plays a critical role in the morphology and functionality of graphene-based devices. Here I will present our recent work on adhesion and interfacial shear stress transfer of graphene. In the first part, I will present a new method that can measure adhesion energies between ultraflat graphene and a broad range of materials using atomic force microscopy (AFM) with a microsphere tip. In our experiments, only van der Waals force between the tip and a graphene flake is measured. The Maugis-Dugdale theory is employed to calculate the adhesion energy. The ultraflatness of monolayer graphene on mica eliminates the effect of graphene surface roughness on the adhesion, while roughness of the microsphere tip is addressed by the modified Rumpf model. Adhesion energies of monolayer graphene to SiO2 and Cu are obtained as 0.46 and 0.75 Jm-2, respectively. In the second part, I will present the nonlinear mechanical response of monolayer graphene on polyethylene terephthalate (PET), which is characterised using in-situ Raman spectroscopy and AFM. While interfacial stress transfer leads to tension in graphene as the PET substrate is stretched, retraction of the substrate during unloading imposes compression in the graphene. Two interfacial failure mechanisms, shear sliding under tension and buckling under compression, are identified. Using a nonlinear shear-lag model, the interfacial shear strength is found to range between 0.46 and 0.69 MPa. The critical strain for onset of interfacial sliding is ∼ 0.3%, while the maximum strain that can be transferred to graphene ranges from 1.2% to 1.6% depending on the interfacial shear strength and graphene size. Beyond a critical compressive strain of around −0.7%, buckling ridges are observed after unloading. I will end the presentation with tuning the morphology and multifunctionality of monolayer 2D nanomaterials using mechanical strain.

Selenium and Tellurium are two special elemental materials which are 1D helical atomic structures and formed by van der Waals force between helical atomic chains. In this talk, we will report on the fundamental studies of these two new nano-materials at atomic scale in terms of their electrical, optical, thermal and mechanical properties. The helical atomic structure offers strong anisotropic properties of these 1D van der Waals materials. The band-structures of the materials themselves also offer some excellent materials properties such as highest Seebeck coefficient for selenium and high carrier mobility of 700 cm²/Vs for tellurium with demonstrated field-effect transistor drain current exceeding 1 A/mm. The work is in close collaborations with Prof. Wenzhuo Wu at Purdue University.

Two-dimensional (2D) materials, such as graphene, have mechanical properties that are both new and unique to their class. At the same time, upon exfoliation and/or transfer to a substrate, they are prone to ordinary processes, like folding and wrinkle formation. With the ever-increasing interest on 2D-material based stretchable devices and nano-electromechanical systems, a natural question that arises is how the above mentioned unique properties and ordinary processes couple. In other words, do the exquisite 2D-material properties lend any uniqueness, or universality, to mundane features such as wrinkles? For example, when a wrinkle is compressed vertically, what are the deformation pathways and do they portray any general pattern? Do any unforeseen mechanical properties emerge? The present work brings some answers to these questions. Therefore, we investigate the mechanical deformation of few-layer suspended structures (wrinkles) made of graphene, h-BN and talc. Through their atomic force microscopy-based nanomanipulation, we find that their experimental restoring forces fall into universal functions of their strain. Two distinct, and universal, pathways are revealed: with and without lateral displacement of the wrinkle topmost part. Such universality further enables the investigation of each fold bending stiffness κ as a function of its characteristic height h₀. We observe a more than tenfold increase of κ as h₀ increases in the 10-100 nm range, with power-law behaviors of κ versus h₀ with exponents larger than unity for the three materials. This implies in anomalous scaling of the mechanical responses of nano-objects made of these materials. These reported properties may play a major role in nano(electro)mechanical devices and stretchable electronics.

Emerging atomically thin nanomaterials such as two-dimensional (2-D) graphene have attracted tremendous attention for their many unique properties. However, a single piece of them is too delicate to be useful in most applications, for example, high-performance electrodes in energy storage, filters for waste water/gas treatments in environmental systems, and lightweight structures. Assembling these nanomaterials into three-dimensional (3-D) scaffolds to achieve superior overall performance with multiple functionalities has attracted growing interests, yet this is challenging in manufacturing. In particular, these low-dimensional nanomaterials tend to aggregate/restack due to strong van der Waals attraction between them such as restacking of 2-D flat graphene sheets, which not only results in a tremendous reduction of their accessible surface area and poor mass/ion transport, but also degrades with processing and/or application environments such as mechanical loadings, hence adversely affecting their properties and subsequent applications. A liquid evaporation-assisted manufacturing technique is considered to provide a facile route, where 2-D nanomaterials will experience large deformation and severe instability under evaporation-induced compression to create spacings when assembled, which is highly desirable to minimize restacking and retain the large surface areas of 2-D nanomaterials in the assembled 3-D architectural structures. In this study, we establish a theoretical mechanics framework to quantitatively describe the liquid evaporation-driven deformation and self-assembly of 2-D graphene suspended in a liquid environment. The energy competition among surface energy of liquid, solid-liquid interfacial energy, solid-solid interactive energy, and deformation energy of solids during liquid evaporation is probed and incorporated into the mechanics theory. The critical deformation lengths of graphene sheets, and size and configuration of ultimate stable assembled 3-D particles are predicted and validated with extensive molecular dynamics simulations.

Atomically thin materials such as graphene and other 2D materials are promising for a wide range of applications. Mechanical interactions at the interfaces, including adhesion and friction, are critical for manufacturing (e.g., synthesis and transfer), integration, functional performance and reliability of these atomically thin materials. While van der Waals interactions have been commonly assumed to be the primary mechanism for the 2D materials, recent studies have suggested other mechanisms that may have to be considered, such as the effects of water capillary, reactive defects, and surface roughness. For adhesion and separation, in addition to the adhesion energy, the relation between the normal traction (attraction/repulsion) and the separation has been measured for some interfaces (e.g., graphene/Si), providing further insights into the underlying mechanisms associated with the strength and range of normal interactions. For friction or generally shear interactions, direct measurements are more challenging, but the critical shear strength has been reported for some interfaces (e.g., graphene/PET and graphene/Cu). Both normal and shear interactions are at play and coupled in the mixed-mode fracture experiments. This talk will first summarize the recent experimental efforts to characterize the mechanical interactions at the interfaces of 2D materials (mostly graphene). Theoretical models and MD simulations will be presented to provide qualitative understanding on the effects of surface roughness, water (wet adhesion) and temperature.

The transition metal dichalcogenides exhibit polymorphism; i.e. both 2H and 1T' crystal structures, each with unique electronic properties. These two phases can coexist within the same monolayer microstructure, producing 2H/1T' heterostructure and interfaces/edges. Here we report by applying tensile starin and tailoring edges/interfaces structure, etheir a sufficiently narrow 2H ribbon can be transfered to metastable 1T' ribbon, or a narrow equilibrium 1T' strip can be generated in the edge of large 2H flakes. These effects provide a means to phase engineer transition metal dichalcogenide microstructures.

Two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides, collectively known as MXenes, are a large class of materials that are finding numerous applications ranging from energy storage and electromagnetic interference shielding to water purification and antibacterial coatings. Yet, despite the high technological relevance of MXenes for various applications, their mechanical properties have not been studied yet. In this work, we measured the elastic properties of monolayers and bilayers of Ti₃C₂T_x (T stands for surface termination), which is the most studied MXene material to date. We developed a method for preparing well-strained membranes of Ti₃C₂T_x monolayers or bilayers and performed their nanoindentation with a tip of an atomic force microscope to record the force-displacement curves. We determined the effective Young’s modulus of a single layer of Ti₃C₂T_x to considerably surpass the value previously measured for a monolayer of graphene oxide, a benchmark solution-synthesized 2D material that is often considered for large-scale applications. This study extends the already broad range of technological relevance of MXenes toward composites, protective coatings, nanoresonators and other applications that would utilize exceptional mechanical properties of these materials.

Moiré patterns are widely observed in layered systems of weakly interacting 2D materials such as graphene, h-BN, MoS₂, WSe₂, etc. We explain these patterns using the concept of interlayer or van der Waals (VdW) dislocation arrays. We note that patterns and the defects appearing therein are electronic structure artifacts of the weak interactions between layers, locked into place by the strong in-plane interactions in the constituent layers; they are therefore topological states. We explain several experimentally observed Moiré phenomena, including the distinct Moiré patterns formed by different combinations of 2D materials, such as h-BN and graphene, on the same metal support layer. We also examine point and line defects in Moiré patterns, and explore their connection with defects in the constituent 2D layers. Finally, we examine the link between Moiré patterns in bilayer MoS₂ and out-of-plane deformation. These examples inspire the idea of Moiré engineering, through which it is possible to design topological states for devices, with applications including electron transport, photovoltaics, and charge-based storage.

Single-layer graphene and few-layer graphene are known to exhibit characteristic dynamic ripples as well as static corrugations when suspended. Traditionally, shallow configurations of the ripples and the corrugations have been assumed to be in sinusoidal shape. In contrast, here, we report the discovery of a new, low-energy, subcritical mode shape that produces shallow-kink corrugation in graphene, particularly in multilayer graphene. Our DFT analysis shows that the configuration has a ~2 nm wide boundary layer of highly localized curvature that connects two regions of uniformly but oppositely sheared stacks of flat atomic sheets. The DFT and our flexoelectricity model analyses reveal that periodic kinks, named crinkles, are the lowest energy configuration of multilayer graphene under critical axial pressure. The results show that quantum flexoelectricity leads to emergence of the boundary layer in which curvature is focused primarily within a 0.86 nm fixed band width. Furthermore, the analyses predict high peak-polarization density, e.g. 0.03 -e/nm for 3^o tilt angle. The polarization develops surface electric charge concentration in the fixed band width, along the crinkle ridges and valleys, on the top and bottom free surfaces of multilayer graphene. The surface electric charges are negative on the tops of ridges and positive on the bottoms of valleys. In our experiment, we compressed an assembly of multilayer graphene (~200 layers) attached to a PMMA or a silicon grating of grooves up to ~ 0.1% strain to observe the hinge-mode buckling with an atomic force microscope. The suspended portion of the graphene over the grooves pop out to make crinkle ridges on the PMMA grating, while they sink in to make crinkle valleys on the silicon grating. The ridges generate negative line charges to form N-type crinkles, and the valleys positive line charges to form P-type crinkles. We found that the parity of the crinkles can be controlled by choosing proper elastic moduli mismatch and the strength of adhesion between the graphene and the grating substrate. Our DFT analysis shows that typical adsorption potential bias of a neutral molecule, e.g. O_², on the line charges is ~30 meV, and much larger for charged molecules. Controlling the charge potential depth, the line charge acts as a molecular zipper which attracts and aligns bio-molecules, or nano-particles along the ridges or valleys. The graphene-crinkle molecular zipper is expected to be a powerful tool to study molecular adsorption, and self-organization of molecules and nanoparticles for various applications.

Two-dimensional (2D) materials beyond graphene are attracting much attention due to their unique properties. 2D carbides and nitrides of transition metals (MXenes) have shown very attractive electrical and electrochemical properties, but their mechanical behavior has been poorly characterized. There are no experimental measurements of intrinsic mechanical properties of MXenes reported in the literature and only a handful of theoretical data on strength, fracture modes, Young’s modulus, and bending rigidity for single-layer MXenes.
The mechanical properties of two-dimensional titanium carbides (Ti₂C, Ti₃C₂, and Ti₄C₃) were investigated in this study by performing in silico experiments using large scale classical molecular dynamics. Young’s modulus was calculated from the linear part of strain–stress curves obtained under tensile deformation of the samples. Strain-rate effects were observed for all Ti_n+1C_n samples. Dynamical behavior of the samples under external bending load was simulated via classical molecular dynamics. The central deflection and bending rigidity of the MXene nanoribbons were calculated as functions of applied force. Calculated bending rigidity of the Ti₂C nanoribbon is 5.21 eV at small deflections and nonlinearly increases at larger deflections, reaching the maximum magnitude of 12.79 eV before the onset of disintegration. We discuss these properties in comparison to mechanical properties of other 2D materials and emphasize the importance of the advanced mechanical properties of MXenes in applications.

In experiments, grain boundaries have been shown to change the mechanical and electrical properties of 2D materials by manipulating their structure and periodicity. We investigate the mobility of varying grain boundaries through a multiscale framework on graphene. Atomistic calculations are done in LAMMPS to find the energetics of grain boundary motion through kink formation. A continuum model is used to isolate governing contributions to grain boundary motion by modelling grain boundary kinks as dislocations in the displacement shift complete lattice. Together these show that low sigma boundaries have larger burgers vectors and are more localized in space to account for larger energy barriers to grain boundary motion. These trends are consistent with experimental observations that low sigma boundaries persist in grown graphene.

Metal dichalcogenides have shown interesting physical properties depending on their complex stacking and unique structures. Here, we report a simple chemical method via chemical vapor deposition (CVD) to engineer distinctive microscale kirigami structures of multilayered WSe₂, which have not been previously observed. These intricate three-dimensional architectures of multilayer WSe₂ structures were unraveled by scanning electron microscope and atomic force microscopy images, showing the novel concave edge structures with panhandles. Structure-symmetry relationship of these WSe₂ patterns was elucidated by second-harmonic generation (SHG) imaging and micro-Raman spectroscopies, revealing that the formation process of these intricate WSe₂ kirigami is governed by the magic layer stacking of bottom trilayers. This chemical approach could be applied into other layered metal dichalcogenide materials and opens up new possibility for creating novel and complex nanostructures for studying the rich physical properties of two-dimensional solids.

In flexible electronics, metal electrodes or metal contacts are essential components, but they are most susceptible to external mechanical loading. Metals such as copper or gold intrinsically suffer from poor mechanical flexibility where they can withstand low tensile strains (<2%). Furthermore, metals are often prone to fatigue failure resulting in a significant degradation of device performance during numbers of operation cycles in practical applications. Crucial key aspects of flexible electronic devices, thus, would be strain-tolerance to large deformation and mechanical robustness under the cyclic loading. To achieve advanced strain tolerance and mechanical robustness, many recent works on incorporating the outstanding mechanical properties of atomically-thin materials, especially graphene, as a composite material were reported. However, severe filler agglomeration hinders a realization of comparable electrical conductivity to the conventional metal electrodes. In contrast, several other approaches focused on structuring the conventional metal electrodes. Despite their enhanced flexibility, the resistance of flexible electrodes often sharply increased with even relatively small strains after a few thousands of cycles. In this presentation, we discuss a simple approach of inserting multilayer graphene under the metal electrode, which can be readily applied to versatile flexible electronics. We investigated the underlying mechanism of the mechanical reinforcement via in-situ bending test with environmental scanning electron microscope. The axial bending test results revealed thickness dependency of multilayer graphene on strain tolerance enhancement. Metal electrodes integrated with multilayer graphene sustained larger bending strains (>300%) compared with metal electrodes with single layer graphene. Furthermore, in contrast to a sharp increase measured in bare metal electrode device, the resistance gradually increased in the multilayer graphene integrated electrodes as bending strain increased. This new approach enables flexible electronics to operate without an abrupt performance failure at large strains.

Atomically thin transition metal dichalcogenides (TMDs) are sensitive to mechanical strain, where large changes in the photoluminescence response will occur during an indirect-to-direct band gap transition brought on by elastic tensile strain. Highly localized strain has also been hypothesized to enable single photon emission in tungsten diselenide and related TMDs. Understanding of the mechanisms bringing about strain effects, however, has been limited by difficulty in quantifying atomic-level strain experienced by an atomically thin material. The four-point bending method is widely used for straining these materials, such as graphene, molybdenum disulfide and black phosphorous. Due the air sensitive properties of some layered materials, an encapsulation technique is necessary to prevent material degradation or defect formation during annealing processes or optoelectronic measurements. The selection of a flexible substrate and encapsulation layers is also critical for surface adhesion which governs the strain transferred from the substrate to the atomically thin sample. Furthermore, the encapsulated material experiences an out-of-plane compressive strain due the Poisson effect of encapsulation layers. These factors complicate the strain experienced by the material under investigation which obfuscate quantification of strain-coupled effects and erode confidence in previously reported values of elastic strain. In this study, we report the strain coupled phonon and optoelectronic properties of encapsulated CVD grown few layer graphene and bilayer WSe₂. A large strain value and high strain transfer amount are achieved by selection of appropriate encapsulation materials and annealing treatment. An analytical model based on the three dimensional Poisson effect is also developed for obtaining the Grüneisen parameter of encapsulated atomic-thin materials. By demonstration of vibrational properties of thin graphite in response to four point bending, we present a useful and straightforward approach that will allow verification of the amount of strain transferred to an atomically thin material using the four-point bending method.

Negative Poisson's ratio (NPR) in auxetic materials is of great interest due to the typically enhanced toughness, shear resistance, sound and vibration absorption, which enables plenty of novel applications such as aerospace and defense. Insight into the mechanisms underlying the NPR is significant to the design of auxetic nanomaterials and nanostructures. Currently, the understanding of the NPR phenomena is dominated by the geometry analysis in literature. The auxetic effect is generally thought to be independent of chemical composition and electronic structure, which originates from the special reentrant structures or the rigid building blocks linked by flexible hinges.
In this study, by employing first-principles calculations, we report intrinsic NPR in a class of two-dimensional honeycomb structures (graphene, silicene, h-BN, and h-GaN), which are distinct from all other known auxetic materials. Their honeycomb structures possess no reentrant or hinge-like building block. The electronic effect rather than the mechanical factor is found responsible for the intrinsic NPR. The four 2D materials share the same mechanism for the emerged NPR despite the different components, which lies in the increased bond angle. The increase of bond angle is quite intriguing and anomalous, which cannot be explained in the traditional point of view of the geometry structure and mechanical response, such as in the framework of classical molecular dynamics (MD) simulations based on empirical potential. We attribute the counter-intuitive increase of bond angle and the emerged NPR foundamentally to the strain modulated electronic orbital coupling and hybridization. We further propose that the NPR phenomenon can also emerge in other nanostructures or nanomaterials with similar honeycomb structure. Our study not only make a comprehensive investigation of the intrinsic NPR in the four 2D materials with honeycomb structure, but also reveals the physical origins, which deepens the understanding on the NPR and would shed light on future design of modern nanoscale electromechanical devices with special functions based on auxetic nanomaterials and nanostructures.

Two dimensional (2D) semiconductors such as MoS₂ and WSe₂ have attracted attention for various optoelectronic and electronic applications owing to their good optical, electrical and mechanical properties. However, the lack of efficient methods for their production at levels of quality, uniformity, and reliability needed for practical applications restricts the applicability of 2D semiconductors to optoelectronics and electronics. In this talk, as an alternative 2D semiconductor, I present single-crystal Si nanomembranes (NMs), with precisely defined thicknesses ranging from 1.4 to 10 nm. The Si NMs exhibit high optical transparency, low flexural rigidity and distinctive electrical properties. Deterministic assembly techniques allow integration of this Si NMs into unusual device architectures, including transistors and logic circuits for potential use in transparent and flexible forms of electronics. In addition, I report a new strain engineering approach to induce a tensile strain to the Si NMs, exhibiting a mobility enhancement factor of 1.2−1.4 compared with an unstrained Si TFT without using an additional epitaxial stressor layer.

Two-dimensional (2D) materials exhibit many exciting phenomena that make them promising as materials for future electronic, optoelectronic, and mechanical devices. In addition to the exploration of individual 2D monolayer properties, one emerging area of research is van der Waals heterostructures (vdWh), in which different 2D materials are stacked together to create a hybrid structure with different behavior than the constituent layers. In any device utilizing 2D layers, it is crucial to obtain clean and repeatable interfaces between different 2D layers and between 2D layers and their substrates. The current state-of-the-art is to mechanically transfer 2D layers onto target substrates, which results in trapped contaminants between layers and spatially inconsistent interfaces. These trapped contaminants convolute the intrinsic 2D material behavior with extrinsic factors, such as strain and charge transfer doping, which often leads to ambiguous experimental observations that are difficult to reproduce. Also, trapped contaminants between layers in vdWh prevent strong coupling of the 2D layers, which is essential for achieving the desired vdWh behavior. In this work, we present an atomic force microscope (AFM) based approach for creating clean interfaces between mechanically-transferred 2D layers and their substrates and between 2D layers within vdWh. The operating principle is to use the AFM tip to controllably squeeze contaminants out from between 2D layers and their substrates, similar to a squeegee. We show that there is a critical applied load for successfully squeezing contaminants out from between layers that is typically around 50 nN for a 2D monolayer on hexagonal boron nitride (hBN). We demonstrate that the AFM squeegee technique is superior to annealing because it allows for guided removal of contaminants rather than random redistribution of contaminants as occurs in annealing. Also, the AFM squeegee forces 2D monolayers closer to their substrates than annealing, as observed from AFM height measurements. The reduction of external heterogeneity from our technique leads to a substantial reduction of photoluminescence linewidth. For example, using AFM to remove contaminants from beneath a WS₂ triangle on hBN led to a reduction of room temperature photoluminescence linewidth from 28 meV to 23 meV. We show that the AFM squeegee works even through hBN layers up to 16 nm thick. This allows us to create extremely clean samples encapsulated in hBN on both sides, which is a crucial device structure for realizing optimal 2D material performance in many applications. Finally, we demonstrate that our AFM squeegee forces 2D monolayers into intimate contact with each other. Our evidence for this claim includes AFM height measurements, bilayer-like Raman signature in a MoSe₂/WSe₂ vdWh, and the observance of reproducible, bright, and narrow interlayer exciton features in MoSe₂/WSe₂ vdWh.

Two-dimensional (2D) SnS, one of the group-IV monochalcogenides (MXs, M=Sn/Ge, X=S/Se), has recently attracted interests as a material for piezoelectric nanogenerators because of a remarkable piezoelectric coefficient d~144 pm/V, which is much larger than MoS₂ (d~4 pm/V) and comparable to PZT (d~300 pm/V) [1]. Although the brittle fracture is inevitable for the conventional piezoelectric ceramics (e.g. PZT or BaTiO₃), 2D materials with the structural flexibility enable to realize reliable and flexible IoT devices. For SnS, the piezoelectricity exists only in the odd numbers of layers, since the broken inversion symmetry disappears in the even-number layers. The piezoelectric coefficient increases with decreasing the layers, and is maximized in the monolayer [2]. However, the fabrication of monolayer SnS has not been achieved by mechanical exfoliation. This is probably due to the strong interlayer interaction between SnS layers. The thinnest large-scale SnS crystal has been grown by low temperature physical vapor deposition with the thickness of 5.5 nm (~10 layers), though the crystalline quality should be degraded with low growth temperature [3]. In this work, ultra-thin SnS layers are fabricated by mechanical exfoliation to investigate the crystallinity and chemical stability.
With Au-mediated exfoliation [4], µm-sized ultrathin SnS flakes were obtained with the thickness of ~4 nm, much thinner than tape-exfoliated flakes (several tens of nm). This is probably attributed to the strong semicovalent bond between the Au and S atoms. For the TEM image of the 17.1-nm-thick SnS, an SnO_x amorphous layer with the thickness of ~3 nm was observed at the topmost layer. For the 4.1-nm-thick SnS, the whole region was determined to be SnO_x. These results indicate that SnS is easily oxidized, probably due to the lone pair electrons in the Sn atoms that favor oxygen bonding. However, for the thick SnS layers (>4 nm), the SnS underlayer was intrinsic and the interface between SnS and SnO_x layers was atomically abrupt. Moreover, metal-insulator transition was observed in I_sd-V_bg measurement for the SnS layers with the total thickness of 9.1 nm, suggesting that the crystallinity of SnS underlayer is high and stable because of SnO_x passivation layer. Based on these results, self-passivated SnS monolayer should be realized with controlling the oxidation of the SnS layers.
[1] R. Fei et al., Appl. Phys. Lett. 107, 1 (2015). [2] H. Zhu et al., Nat. Nanotechnol. 10, 151 (2015). [3] J. Xia et al., Nanoscale 8, 2063 (2016). [4] S.B. Desai et al., Adv. Mater. 28, 4053 (2016).

Engineering the structure of materials endows them with novel physical properties across a wide range of length scales. With high in-plane stiffness and strength, but low flexural rigidity, 2-dimensional (2D) materials are excellent building blocks for nanostructure engineering. They can be easily bent and folded to build 3-dimensional (3D) architectures. Taking advantage of the large lattice mismatch between the constituents, we demonstrate a 3D heterogeneous architecture combining a basal Bi₂Se₃ nanoplate and wavelike Bi₂Te₃ edges buckling up and down forming periodic ripples. The balance between bending and in-plane strain energies gives rise to controllable rippling of the material. Our experimental results show clear evidence that the wavelengths and amplitudes of the ripples are dependent on both the widths and thicknesses of the rippled material, matching well with continuum mechanics analysis. The rippled Bi₂Se₃/Bi₂Te₃ heterojunction broadens the horizon for the application of 2D materials heterojunction and the design and fabrication of 3D architectures based on them, which could provide a platform to enable nanoscale structure generation and associated photonic/electronic properties manipulation for optoelectronic and electro-mechanic applications.

The search for new materials that could improve the existent scaling barriers of today’s electronics is in high demand¹. Transition metal dichalcogenides (TMDC) are strong candidates on this matter due to their excellent electronic, mechanical and optical properties². It was shown that mechanical strain can tune the electronic properties of TMDC³. Thus, stretching and bending methods are currently being used to deform these layered materials , utilizing for example atomic force microscopy⁴.
Another strategy to modify the behavior of these structures is by vertically stacking different combinations of TMDC. This is usually accomplished by methods like molecular beam epitaxy, mechanical transfer or chemical vapor deposition⁵. The production of devices made from these structures depends on the understanding of their electrical, optical and mechanical properties.
To shed light on deformation characteristics of vertical heterostructure MoS₂/WS₂, we performed fully atomistic molecular dynamics simulations using LAMMPS⁶. We considered monolayers of WS₂ or MoS₂ stacked on a monolayer MoS₂. Similarities were found regarding the stress x strain in the structures at strains up to 1.5%. As the strain increases, the stress values shown drops at specific points, due to atomic rearrangements. Interestingly, this happens first for MoS₂ bilayer, approximately at 1.6 % of strain. WS₂ showed better resistance to tension and drop in stress was observed at about 2 % of strain. The main cause for this difference is due to interlayer interactions, stronger for WS₂/MoS₂ in comparison to MoS₂/MoS₂.
The sliding between these layered materials were also studied, applying a force on the top layer of the WS₂/MoS₂ layers to make it slide. The systems shown directional dependent plasticity that can be attributed to different levels of roughness at interface between the two layers.
We also analyzed the crack propagation for MoS₂/MoS₂ and MoS₂/WS₂. When the top layer is WS₂, larger strain values are necessary to start altering the atomic arrangements of the structure prior to fracture. When the fracture happens, at first the crack propagates in a straight line and its edges have armchair configuration. This is due to the required force to cut the metal-chalcogenide layer along armchair-like edges, that is lower in comparison to zig-zag ones⁷. However, when the crack reaches the MoS₂/WS₂ interface, a deviation occurs and forms zig-zag edges. This shows that fracture patterns are highly chirality dependent in these heterostructures.

1. Bhimanapati, G. R. et al. ACS Nano 9, 11509 (2015).
2. Fiori, G. et al. 9, 768 (2014).
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7. Manimunda, P. et al. 2D Mater. 4, 45005 (2017).

Recently, graphene grown by chemical vapor deposition(CVD)has been the focus of intense research because of its unique electrical, mechanical, optical and thermal properties[1]. However, the critical step to realize the graphene application is to transfer graphene from the metal substrates to different desired substrates without degrading the quality of graphene, where the most commonly used polymethylmethacrylate(PMMA) mediated technique shows some disadvantages[1]. For examples, the PMMA film is generally removed by organic solvent washing, which inevitably generates residue of polar PMMA on graphene surface due to its large polymer nature and therefore invoke impurity scattering as well as unintentional doping effects; while the high-temperature thermal removal also causes substrate-induced graphene doping because of the interaction between graphene and the substrate with H₂O and O₂ molecules; furthermore, these two ways are not applicable to some flexible substrates that will either dissolve in acetone or degenerate under high temperature, which therefore largely constrains the application of graphene on flexible devices.
To solve these problems, we investigated a self-sacrificing camphor assisted transfer technique which is free from organic solvent washing and high temperature annealing. Due to the sublimation effect, the camphor film is easily removed by either keeping the sample in dry-box or heating under low temperature(~100-200°C) within several hours, and leave no residue on the graphene surface. Our method is cost-effective, reproducible, environmental-friendly and applicable for flexible substrates. The optical microscopy(OM) and scanning electron microscopy(SEM) characterization results show large-area continuous and clean graphene after transfer; Raman spectra shows obvious G and 2D characteristic peaks and no obvious D peak which is related with defects; and the sheet resistance can attain as low as 350Ω/sq, which is much higher than reported result in recent published work(560Ω/sq)[2]. This work is also applicable for other 2D materials grown by CVD, which paves the ways for utilizing 2D materials in electronics and optoelectronics applications.

Reference:
[1] Martins L G P, Song Y, Zeng T, et al. Direct transfer of graphene onto flexible substrates[J]. Proceedings of the National Academy of Sciences, 2013, 110(44): 17762-17767.
[2] Zhang Z, Du J, Zhang D, et al. Rosin-enabled ultraclean and damage-free transfer of graphene for large-area flexible organic light-emitting diodes[J]. Nature Communications, 2017, 8.

Flexible and stretchable photodetector devices based on two dimensional (2D) materials have been demonstrated to exhibit a rare combination of excellent optoelectronic performance with the ability to accommodate large amounts of strain during device operation. This unique coupling is enabled by the broad optical absorption in graphene and other 2D systems, quantum confinement of energy carriers in the 2D plane resulting in ultrafast transport dynamics, the van der Waals bonding between the layers, and the enhanced electromechanical properties that arise due to the extreme thinness of the material. Practical realization of flexible 2D materials in these applications is limited by the lack of large area, transfer-free processing schemes that enable the layered materials to be incorporated on soft, organic substrates and allow for bottom-up device fabrication. Wafer-scale photonic crystallization of amorphous precursors to layered 2D materials directly on soft substrates, as demonstrated herein, can facilitate the future 2D flexible electronic and optoelectronics that are easily processed at low temperatures for devices including ultra-thin photodetectors.
High quality growth of 2D materials typically require high temperature growth in the range of 500~1000°C coupled with an epitaxial template to facilitate the thermodynamic reactions in forming non-defective and stoichiometric materials. To overcome the processing limitations for flexible electronics where the substrate typically cannot accommodate for either of these, the 2D layers must either be grown polycrystalline at a much lower substrate temperature, transferred from a rigid substrate to the flexible substrate of interest, or processed by non-thermal annealing techniques in order to achieve similar device performance. Recently, techniques including additive manufacturing have emerged that allow for the transfer of liquid-based 2D inks onto flexible substrates for device manufacturing. While 3D printing provides for patternability and low temperature solution processing, issues with repeatability and film discontinuity require advances before practical devices can be achieved.
Herein, we present a technique that utilizes large area pulsed light source in order to initiate controllable heating and transformation of amorphous ultrathin MoS₂ on flexible PDMS. The phase transformation in this case occurs through absorption of the incoming light, resulting in local heating just in the absorbate material and allowing for crystallization of materials to occur at temperatures as high as 390°C. Photonic crystallization has been utilized to process materials including graphene inks, metallic nanoparticles, and other nanomaterials, but has yet to be demonstrated in the phase transformation of 2D materials directly on soft substrates. If optimized, the use of large area photonic annealing technique can unlock new device constructs not achievable through conventional deposition processes.

Graphene and semiconductor based Schottky junction solar cells (SJSCs) have recently shown rapid increase of power conversion efficiency with simple structure and easy fabrication process, thereby being considered as a promising alternative to conventional p-n junction solar cells. Such a rapid enhancement of the PEC, yet it is still lower than the best record of the metal/Si Schottky junction solar cells (20 %), illustrates that the junction-potential plays an importance role of the cell performance. Graphene show the electrically tunable Fermi level, optical transparency, and substantial permeability to electric field and it means that remains lot of room for improvement of PCE. In this study, we introduced graphene and graphene mesh sheets as Schottky electrodes on n-Si SJSCs and investigated their power generation characteristics depending on the V_g applied to the cells. Compared with the SJSC with graphene, the cell with graphene mesh exhibited a higher PCE and a more sensitive response to photovoltaic parameters at V_g. The advantages of the graphene mesh can be attributed to the existence of holes that increase the work function and permeability across the electric-field.

Microfluidics has been a versatile platform for sorting and sensing. Most sorting methods are based on optical techniques, such as identifying based on fluorescence. There is a growing field of mechanical-based sorting, through flow pinching, interactions with solid arrays (deterministic lateral displacement) or other methods that exploit density differences. However, these techniques require specialized microfabrication and their throughput will be limited by delicate microstructures. We deployed a novel and highly sensitive sensor based on metallic (Pd) nanoislands grown on graphene, that is imbedded in a flexible material (PDMS). These sensors are low-cost (<$1 per device), inherently low-power but are exquisitely sensitive (0.001%). These sensors measure a range elastohydrodynamic deformation which can result from changes in flow rate, viscosity or presence of particles. We can readily measure flow-rates from a variety of ranges, from 100’s of nL/min to liters/min. We discuss applications of this sensor in particle determination, viscometry and several elastohydrodynamic phenomena.

Chemical vapor deposition (CVD) growth of graphene has intrigued many research efforts as it enables high-quality graphene in very large scale with relatively low cost [1]. However, the transition metal substrate (typically copper) on which the graphene is grown can still degrade the graphene quality in many ways [2, 3]. One imperfection on CVD graphene is the embedded ripple pattern caused by the relaxation of local compressive strain induced by thermal expansion mismatch between graphene and the transition metal [4, 5, 6]. In this work, we show that this ripple pattern remains after wet transfer of graphene onto a SiO₂ substrate and is a major source of disorder. We analyze the non-idealities induced by the ripple pattern using atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. Particularly, Raman mapping of transferred single-layer graphene shows spatially periodic patterns in the 2D and G peak positions. By using the decomposition method described in [7] we show that the ripple pattern creates periodic variations in the (hole) carrier concentration and strain levels and quantify their magnitudes. The potential variation induced by the doping disorder agrees with results from electrical characterization [8]. We also show that thermal annealing of the transferred CVD graphene increases both the strain and the strain-induced disorder, while having little effect on the doping disorder. Finally, the effect of surface functionalization with 1-pyrene-methylamine on the ripple pattern is investigated. Raman mapping shows that the functionalization eliminates the ripple pattern, while also reducing the overall hole carrier concentration. This latter result is consistent with Dirac point shifts obtained from electrical measurements of graphene capacitor structures. These results provide valuable guidance for the optimization of performance and uniformity of a wide range of graphene-based nanodevices. The authors acknowledge funding from Boston Scientific Corporation. Portions of this work were also carried out in the University of Minnesota Characterization Facility, which received capital equipment funding from the University of Minnesota MRSEC under NSF Award DMR-1420013.
[1] X. Li et al., Science, vol. 324, no. 5932, pp. 1312-1314, 2009; [2] A. Reina et al., Nano Lett., vol. 9, no. 1, pp. 30-35, 2009; [3] X. Li et al., Nano Lett., vol. 9, no. 12, pp. 4359-4363, 2009; [4] M. Bronsgeest et al., Nano Lett., vol. 15, no. 8, pp. 5098-5104, 2015; [5] D. Kim et al., J. Mater. Chem. C, vol. 1, no. 47, p. 7819, 2013; [6] G. Troppenz et al., J. Appl. Phys., vol. 114, no. 21, p. 214312, 2013; [7] J. Lee et al., Nat. Commun., vol. 3, p. 1024, 2012; [8] M. Ebrish et al., ACS Appl. Mater. & Interfaces, vol. 6, no. 13, pp. 10296-10303, 2014.

The deterministic confluence of organized 0D/1D plasmonic nanoparticles (NPs) with atomically-thin 2D materials can lead to novel and tunable near-field effects such as plasmonic/exciton coupling in 0D/1D NPs and 2D semiconductors. However, current top-down strategies to fabricate desired nanoscale resonant architectures based on lithography and metal deposition are limited in their spatial resolution, material quality, and flexibility in morphological configurations for both the resonant nanostructures and the substrate. In contrast, colloidal preparations of plasmonic nanoparticles are extremely versatile and highly scalable, which couple directly to the deterministic surface morphologies formed via deformed atomically-thin 2D materials serving as ideal templates for self-assembly of such 0D/1D NPs. Here, we present a generalized method to self-organize a variety of high quality, colloidally-prepared 0D/1D gold NPs of various geometries and surface chemistries onto deterministically deformed graphene and transition metal chalcogenide (TMDC) monolayers. This is achieved via nanoscale convective self-assembly of 0D/1D NPs onto large-scale, heterogeneously strained, deformed 2D materials templates formed via strain induced mechanical surface instabilities (i.e., buckles, wrinkles, creases, etc.). By controlling simple material parameters, we design deformed 2D materials templates with characteristic features spanning a few dozens of nanometers to tens of microns. This allows independent control over the various factors enabling such versatile assembly (morphology, coverage, ordering) including nanoparticle size, concentration, aspect ratio, surface and solvent chemistry, in addition to the conjugate properties in the deformed 2D material template and supporting substrate (isotropy, periodicity, amplitude, surface energy). By optimizing the various material properties, NPs as small as 15nm can be readily self-assembled up to inch scale into well-organized, massively-parallel, sub-100nm single-file arrays onto deterministically deformed monolayer 2D materials. This development represents a first in realizing self-assembly with mixed-dimensionality deformed nanoscale materials and enables not only new tools to study emergent coupled phenomena in mixed low-dimensional heterogeneous systems but also as a general strategy to realize self-assembly based nanomanufacturing of novel low-dimensional material architectures.

This talk will describe a memory-based, sequential wrinkling process that can transform flat thermoplastic sheets into multi-scale, three-dimensional hierarchical textures. Multiple cycles of plasma-mediated polymer skin growth followed by directional strain relief of the substrate can produce hierarchical architectures with independent control over wrinkle wavelength and wrinkle orientation. We will discuss how this bottom-up sequential wrinkling process with soft skin layers can produce conformable deformable two-dimensional electronic materials with new structural properties for diverse applications.

For centuries, practitioners of the paper arts of origami (“ori”=fold) and kirigami (“kiri”=cut) have created beautiful and complex structures from a sheet of paper. Here we show that 2D materials are a perfect starting material for microscale paper arts. We first demonstrate that we can, with the right tools, pick up a single sheet of graphene and manipulate it like a sheet of paper. We then apply ideas from kirigami to pattern the graphene into a variety of shapes and explore their properties. These include stretchable electrodes, springs, and robust hinges. Next, we create nanometer-thick bimorphs by pairing a 2D material with another thin film. These bimorphs are the basis of active hinges that actuate in response to external chemical, electrical, or optical signals. We argue that these materials, when coupled with other electronic and optical devices, can be the basis of a new generation of micron-scale smart machines.

The discovery of atomically thin materials have given rise to intriguing physics in reduced dimensions like graphene as well as outstanding optoelectronic properties like transition metal dichalcogenides. The successful exfoliation of black phosphorus further enriches the two dimensional material family and is seen as a promising candidate for high mobility application due to its ideal band gap (unlike graphene) and smaller effective mass (unlike the transition metal). However, the stability of phosphorene has long remained a critical issue that hinders its further application. To circumvent the problem, we explore the possible growth of 2D metal phosphides that could dramatically enhance the stability. Among various candidates, SiP is chosen for its possible high mobility and compatibility with Silicon substrate. A combined method of high energy milling and chemical vapor deposition is used for the obtaining of SiP nanostructure on Si and SiO₂ substrates. The as-grown high quality SiP, confirmed by Raman spectroscopy, shows an intriguing ripple-like pattern that could be unique to phosphide materials. Efforts have also been made to determine the material's mobility via field effect transistor fabricated on SiP/SiO₂/Si. Our growth on SiP hereby provides knowledge on the exploration of non III-V metal phosphides and its related unique nucleation and growth behavior and shows that metal phosphides could be a promising new member in the two dimensional materials family.

The morphology and properties of 2D materials are strongly tied to their lattice structure. The highly malleable structure and amenable surface chemistry of atomically thin 2D materials open up fertile opportunities to tailor the morphology and properties of 2D materials via simple mechanical deformation, such as stretching, folding and rolling. Ever maturing techniques of 2D material synthesis and functionalization further promise such tailoring of morphology and properties in a programmable manner. In this talk, we demonstrate strong coupling of electro-mechanical properties of graphene and graphene-based hetero-structures by tuning charge carrier dynamics through mechanical strain. The graphene electro-mechanical coupling yields very large pseudomagnetic fields for small strain fields, up to hundreds of Tesla, which offer new scientific opportunities unattainable with ordinary laboratory magnets. Such strain-induced pseudomagnetic fields can enable on-demand and reversible formation of quantum dots in graphene. We further reveal a viable approach to achieving programmable uniform pseudomagnetic fields of extreme intensity by a simple uniaxial stretch.

As more and more 2D atomically thin films are discovered, innovative technologies to assemble them into complex hierarchical structures continue to emerge. Here we report a new approach that using the platform of self-rolled-up membrane (S-RuM) nanotechnology to tune the strain applied to graphene. S-RuM technology relies on the built in strain between interfaces at the atomic level and strain relaxation results in deformation of the membrane. Here we report continuous tuning of the embedded strain in graphene sheet as a function of the pre-stressed S-RuM tube curvature.
To describe the experimental process briefly, monolayer graphene films are grown through the Cu-catalyzed chemical vapor deposition (CVD) technique, followed by direct transfer, via the well-established poly(methyl-methacrylate) (PMMA) transfer process, onto strained bilayer stacks of SiN_x. As the SiN_x bilayer, which is formed by dual-frequency plasma-enhanced CVD, is released from the substrate and rolls-up, the graphene film atop is subjected to uniaxial compression. The degree of strain applied to the SiN_x stack and, therefore, that induced in the graphene sheet, is a function of the tube diameter. The S-RuM curvature is directly tuned according to the relative thickness of the low- and high-frequency deposited SiN_x layers. The graphene Raman signature shows a clear splitting of the G-band in the case of the rolled (compressively strained) membrane stack, whereas no such sub-band signal is detected from the pristine graphene sheet. The separation of G-band allows for quantifiable strain characterization as a function of S-RuM curvature. Across a widely tunable micro-tube diameter range of roughly 1 – 8 µm, over 5.25 % compressive strain has been realized in a monolayer graphene film.
By reducing the microtube diameter further (which can be done using epitaxially strained bilyers), much larger strain values can be achieved. In addition, by changing the rolling direction from rolling up to rolling down, via switching the deposition sequence of the compressive and tensile strained carrier layers (SiN_x or III-V), tensile strain can be realized and tunable in the same manner. This work shows an unprecedented range of strain engineering in graphene and holds promise for similar strain control in other two-dimensional materials, such as MoS2, wherein uniaxial deformation can allow for bandgap energy tunability. 2D sheets rolled-up in tandem with the self-rolled, three-dimension tubular platform highlighted here have potential for applications in ultra-compact inductors, micro-fluidic channel bio-sensors, rolled-up transistors, and micro-cavity optical resonators.

The recent demonstration of piezoelectric phenomena in non-centrosymmetric molybdenum disulfide (MoS₂) provides new opportunities for fast and efficient exfoliation processes [1, 2]. Here we demonstrate the exfoliation of MoS₂ bulk crystals by utilizing the concomitant electric field and mechanical shear force produced in a microcentrifugation surface acoustic wave (SAW) device. We show that excellent exfoliation yields, containing around 60% monolayer MoS₂, can be attained in only 25 minutes of SAW facilitated exfoliation. The short processing time and high monolayer yield are clear advantages of our method. We report that the SAW induced electric field is crucial for enhancing both the exfoliation yield and the fraction of monolayers. Our findings are supported by both experimental data and computational studies. The developed method provides many future opportunities since it is predicted to apply to all layered, non-centrosymmetric transition metal oxides and chalcogenides [3, 4]. Furthermore the method constitutes a direct pathway towards the integration of nanosheet exfoliation processes into complex microfluidic systems.

[1] Wenzhuo Wu, Lei Wang, Yilei Li, Fan Zhang, Long Lin, Simiao Niu, Daniel Chenet, Xian Zhang, Yufeng Hao, Tony F. Heinz, James Hone, and Zhong Lin Wang, Piezoelectricity of single-atomic-layer MoS₂ for energy conversion and piezotronics. Nature, 514, 470-474 (2014).
[2] Hanyu Zhu, Yuan Wang, Jun Xiao, Ming Liu, Shaomin Xiong, Zi Jing Wong, Ziliang Ye, Yu Ye, Xiaobo Yin, and Xiang Zhang, Observation of piezoelectricity in free-standing monolayer MoS₂. Nature Nanotechnology, 10, 151–155 (2015).
[3] Karel-Alexander N. Duerloo, Mitchell T. Ong, and Evan J. Reed, Intrinsic piezoelectricity in two-dimensional materials. Journal of Physical Chemistry Letters, 3, 2871-2876 (2012).
[4] Michael N. Blonsky, Houlong L. Zhuang, Arunima K. Singh, and Richard G. Hennig, Ab initio prediction of piezoelectricity in two-dimensional materials. ACS Nano, 9, 9885-9891 (2015).

Retention of functional properties of a device under high mechanical strain is highly desirable for next generation wearable applications. To date, extensive research has focused on development of thin-film semiconductors coupled with elastomeric substrates for flexible/stretchable electronics. Previous approaches relied on placing semiconductors on neutral axis and/or employing elastic deformation (i.e. crumpling) for enhanced flexibility/stretchability. However, such schemes are still faced with added complexities, such as piezoresistivity and premature failure modes, which lead to strain dependency of functional devices. Here, we present kirigami-inspired flexible graphene devices with enhanced mechanical robustness. Strategic kirigami incisions or notches on two-dimensional (2D) precursor are used to enable strain independent device functions by redistributing the stress concentrations systematically. Under mechanical loading/stretching, 2D kirigami surface architectures could be transformed into morphing three-dimensional (3D) structures in the vicinity of patterned regions owing to mechanical bistability. A chemical vapor deposition (CVD) synthesized graphene is transferred onto ultrathin flexible substrates and patterned into multiple notch/slit kirigami designs via photolithography and reactive ion etching. We observed that designs with packed incisions/notches endure greater stretch distance without compromise of electrical integrity. We further showed control over the deformation of our devices by systematically varying the kirigami unit-cell geometries. Cyclic stretching demonstrated the mechanical robustness of the kirigami structures when subjected to repeated strains. Stress distribution by complementary computational modeling indicated stress being primarily concentrated at the corners of the interior cuts, while remaining relatively insignificant in other regions of the kirigami. The flipping and rotation enable the high ductility of graphene kirigami as the assembly is elongated and twisted without significantly stretching the carbon bonds of graphene, thus sustaining the electrical performance. To highlight strain-independence performance, we demonstrated a flexible/stretchable field effect transistor (FET) based glucose sensor in the kirigami design. Our kirigami graphene FET exhibited similar sensitivity even when highly deformed up to 130% of strain. The enhanced mechanical robustness and potential for continuous real-time monitoring of glucose by kirigami graphene FET holds great promise, especially for future wearable applications.

The conduction of heat in two-dimensional lattices features striking phenomena that have attracted considerable interest from a basic science point of view and for technological applications. The thermal conductance of monolayer materials have been extensively studied with Raman and electrical measurements. However, the thermal transport properties of monolayers remain highly debated. Here, I will discuss a new method to study thermal transport in two-dimensions based on opto-mechanical measurements. These measurements are possible because suspended MoSe₂ monolayers form mechanical resonators that feature high quality factors and can be probed with low laser power [1]. We measure both the thermal conduction and the heat capacity of suspended MoSe₂ monolayers. These measurements reveal ballistic transport of heat when lowering temperature. The new measurement method opens avenues in thermal transport of low-dimensional systems.

[1] Nicolas Morell, Antoine Reserbat-Plantey, Ioannis Tsioutsios, Kevin G. Schadler, Francois Dubin, Frank H. L. Koppens, and Adrian Bachtold, NanoLetters 16, 5102-5108 (2016)

MoS₂ and transition metal dichalcogenides have opened numerous research directions and potential applications for this diverse family of nanomaterials. The bandgap of MoS₂ is highly strain-tunable which results in the modulation of its electrical conductivity and manifests itself as the piezoresistive effect while a piezoelectric effect was also observed in odd-layered MoS₂ with broken inversion symmetry. This coupling between electrical and mechanical properties makes MoS₂ a very promising material for nanoelectromechanical systems (NEMS), with a high piezoresistive gauge factor which is comparable to state-of-the-art silicon strain sensors. I am also going to report on the realization of self-sensing electromechanical resonators based on single-layer MoS₂. The gate voltage tunable resonant frequency allows the extraction of resonator parameters such as mass density and built-in strain. The results open the way to the applications of TMDC-based NEMS as ultra-low power switches, force, mass and strains sensors, resonators for applications in RF-communications, flexible and stretchable electronics and tunable optoelectronics.

In this talk, I will present graphene-based nanolasers and photon-triggered nanowire transistors. First, I will talk about the demonstration of coupled photonic-crystal nanolasers with asymmetric optical gains. We observed the phase transition of lasing modes at exceptional points through tuning of the area of graphene cover on one photonic-crystal cavity and systematic scanning photoluminescence measurements. As the gain contrast between the two identical photonic-crystal cavities exceeds the intercavity coupling, the phase transition occurs from the bonding/anti-bonding lasing modes to the single-amplifying lasing mode, which is confirmed by the experimental measurement of the mode images and the theoretical modeling of coupled cavities with asymmetric gains. In addition, we demonstrated active tuning of exceptional points by controlling the optical loss of graphene through electrical gating. Furthermore, I will present the demonstration of on/off switching of single- and double-cavity photonic crystal lasers by electrical gating of a monolayer graphene sheet on top of photonic crystal cavities. The optical loss of graphene was controlled by varying the gate voltage, with the ion gel atop the graphene sheet.
Second, I will show photon-triggered nanowire transistors, photon-triggered nanowire logic gates and a single nanowire photodetection system. Nanowires are synthesized with long crystalline Si segments connected by short porous Si segments. Exposing the porous Si segment to light triggers a current in the nanowire with a high on/off ratio of >8 x 10⁶. A device that contains two porous Si segments along the nanowire can be triggered using two independent optical input signals. Using localized pump lasers, we demonstrated photon-triggered logic gates including AND, OR and NAND gates. Furthermore, we take advantage of the high photosensitivity and fabricate a submicrometer-resolution photodetection system. We believe that photon-triggered transistors offer a new venue towards multifunctional device applications such as programmable logic elements and ultrasensitive photodetectors.

Semiconductor heterostructures are central for all modern electronic and optoelectronic devices. Traditional semiconductor heterostructures are typically created through a “chemical integration” approach with one-to-one covalent bonds, and generally limited to the materials with highly similar lattice symmetry and lattice constants (thus similar electronic structures) due to lattice/processing matching requirement. Materials with substantially different structure or lattice parameters can hardly be epitaxially grown together without generating too much defects that could seriously alter their electronic properties. In contrast, van der Waals integration, where pre-formed materials are “physically assembled” together through van der Waals interactions, offers an alternative “low-energy” material integration approach (vs. the more aggressive “chemical integration” strategy). The flexible “physical assembly” approach is not limited to materials that have similar lattice structures or require similar synthetic conditions. It can thus open up vast possibilities for damage-free integration of highly distinct materials beyond the traditional limits posed by lattice matching or process compatibility requirements, as exemplified by the recent blossom in van der Waals integration of a broad range of 2D heterostructures. Here I will discuss van der Waals integration as a general material integration approach beyond 2D materials for creating diverse heterostructure (e.g., dielectric/semiconductor and metal/semiconductor) interfaces with minimum integration damage and interface traps, enabling high-performing devices (including high speed transistors, diodes, plasmonic devices) that are difficult to achieve with conventional “chemical integration” approach.

Achieving ultrathin electromechanically active structures for memory and energy applications is severely challenged by size and screening effects that prevent downscaling of classical ferroelectrics. While interfacing ferroelectrics with 2D electronic materials mitigate some of these constraints, fundamental intrinsic and extrinsic polarization compensation mechanisms can still limit performance. Moreover, defects and impurities at interfaces between 2D materials and bulk pseudo-cubic ferroelectrics impose further challenges. Van der Waals ferroelectrics, especially transition metal thiophosphates such as copper indium thiophosphate CuInP₂S₆ (CIPS) might provide a path to overcome these limitations as those layered crystals can be easily exfoliated and exhibit stable surfaces. However, the intrinsic polarization is low (~3.5 µC/cm²), raising questions about realistic prospects in terms of applications. In this work, we use scanning probe microscopy, analytical and first principles modelling to demonstrate giant electrostriction in CIPS. This effect results from the intrinsic mechanical compliance of the van-der-Waals lattice, as well as the low Curie temperature. Giant electrostriction, up to 100-fold larger than those of perovskite ferroelectrics and near the largest value for ferroelectric polymers, facilitates large overall electromechanical response comparable to well established perovskite ferroelectrics. At the same time, the low Curie tempertature provides easy access to the paraelectric state, where we find that the electromechanical response becomes tunable by the applied electric field. The observed dielectric tunability is comparable to widely used BaSrTiO₃ and yields additional functionality in electrically tunable components. Altogether, despite low intrinsic polarization, the van-der-Waals crystals of CIPS exhibit competitive or better electromechanical response when compared to traditional ferroelectric materials. Persistence and even enhancement of susceptibilities above the Curie temperature provides new opportunities for down-scaling of CIPS lattice toward the quasi-2D regime. Finally, new electromechanical devices can be envisioned, utilizing electroactive control of 2D electronic materials via native van-der-Waals interfaces.

This research was conducted the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility (CNMS2017-R49). (PM, MAM, MAS) Research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. The work was supported by Science Foundation Ireland (SFI/14/US/I3113). Manuscript preparation was partially funded by the Air Force Research Laboratory under an Air Force Office of Scientific Research grant (LRIR 14RQ08COR) and the National Research Council. AT acknowledges CICECO (FCT UID/CTM/50011/2013) financed through FCT/MEC and FEDER under the PT2020 Partnership Agreement.

Polycrystalline grain boundaries can have a strong influence on the properties of graphene films, and also allow the preparation of materials with novel properties [1-5]. Herein, a room temperature friction method is developed for the rapid transfer-free production of polycrystalline graphene on flexible substrates. Starting with low-cost commercially available graphite powders, mono- and few-layer graphene were directly fabricated with an average production time of less than one minute (from raw graphite to graphene on substrates). The applications including strain sensing and humidity sensing were investigated.
Firstly, we studied the flexible polycrystalline graphene films on PET substrates for strain sensing. This type of strain sensing is based on percolating networks of graphene flakes, and the sensing mechanism originates from strain-dependent changes to the film morphology, which was exploited to obtain films with a high strain gauge factor. From the results in the curves describing the change in resistance ΔR as a function of mechanical strain, Δε, it demonstrates a strain gauge factor as high as 300 (calculated per the Ref. 6), which reveals the applicability of this material as a transparent strain sensor.
Secondly, we studied the flexible polycrystalline graphene films as transparent humidity sensors. In this section, the curves of resistance, ΔR, changing as a function of relative humidity, were investigaed. The change in resistance rates was approximately 20% for relative humidity values from 30% to 90%, demonstrating the usefulness of this method for transparent flexible humidity sensing. These results indicate that the PET humidity sensor has the advantage of high flexibility during operation.
Thus, considering the highly efficient, scalable and low-cost fabrication process (the total time from raw materials to final products was within minutes, all the procedures were carried out under room temperature, direct fabrication without any transfer step, avoiding chemical wastes, the raw materials were economically low cost.), such friction method and the flexible polycrystalline graphene films is favorable to flexible electronic applications in the near future.

References
1. Coleman, J. N. Acc. Chem. Res. 46, 14–22 (2013).
2. Cummings, A. W., et al. Adv. Mater. 26, 5079–5094 (2014).
3. Batzill, M. Surf. Sci. Rep. 67, 83–115 (2012).
4. Zandiatashbar, A., et al. Nat. Commun. 5, 3186 (2014).
5. Zhang, Z., et al. Adv. Funct. Mater. 25, 367–373 (2015).
6. Hempel, M., et al. Nano Lett. 12, 5714–5718 (2012).

Strain gradient or electric field gradient induced electromechanical response, flexoelectricity, has gained attention owing to its pervasiveness over piezoelectricity. It is present in any dielectric material, whereas piezoelectricity only present in materials with no inversion symmetry. Furthermore, the ease of applying strain gradients at nanoscale without mechanical failure has enabled new opportunities in flexoelectricity based micro or nano electro-mechanical systems. However, inducing flexoelectric effects in two-dimensional materials by inducing strain gradients, especially using substrate wrinkling, has not been investigated. In this presentation, we introduce topography induced tunable flexoelectricity in mono-layer MoS₂. The effective out of plane piezoelectric coefficient (d₃₃) of mono-layer MoS₂ has been investigated on substrates with controlled topographies, flat and wrinkled, using piezoresponse force microscopy. We observed that the deterministically introduced wrinkles exhibit out of plane electromechanical response on mono-layer MoS₂. Since mono-layer MoS₂ does not possess out of plane piezoelectric response, our findings are attributed to the converse flexoelectric effect. The induced wrinkles facilitate electric field gradient, thus exhibit out of plane electromechanical response from MoS₂ mono-layers. We believe our results on characterizing and tuning the out of plane electromechanical response of wrinkled transition metal dichalcogenide structures can lead to advanced micro and nanoscale actuators and sensors based on flexoelectricity.

The internal buckling is ubiquitous in the grown CNT arrays. It is an important factor which makes the measured physical properties of carbon nanotube array further lower than that of Carbon nanotube with the perfect structure. In this work, the evolution and fabrication mechanism of the internal buckling in the vertically grown carbon nanotube arrays have been studied systematically. The formation of internal buckling of CNTs is induced due to the influence of the self-restriction effect, which is formed by the CNT terminals assembled into clusters in the top of the CNT array and the differently increase of the length of CNTs in the arrays. It makes the quasi-straight and bent CNTs coexisted in the array. Considering the interaction between CNTs, we generalized the Euler beam to wave-like beam and the model from quasi-static compression was applied to analyze buckling behavior in carbon nanotube arrays. The calculated self-restriction force of the bent CNT is about 55-60 MPa, while the maximum mean length difference of the CNTs in the arrays reaches approximately 16%. Basing on these, we have prepared the well-organized carbon nanotube array with a 300 μm thickness.

Flexible electronics have recently attracted much attention for use in wearable devices and biomedical applications. For the bendable, stretchable and wearable devices, organic materials have been widely employed as channels due to due to their excellent scalability and flexibility. However, unsatisfactory electrical properties such as low electron mobility, easy degradation and poor integration density have hindered their widespread use. On the other hand, single crystalline inorganic materials have been widely used for high-performance electronic devices. For example, nanowire based field-effect transistor exhibited excellent electronic properties such as large I_max/I_min ratio, high carrier mobility and a small subthreshold swing. Nevertheless, requirements for single crystalline substrates such as SiC or sapphire have hindered to implement them on flexible electronics. To resolve this problem, a hybrid material system composed of one dimensional (1-d) inorganic nanostructures on two dimensional (2-d) nanomaterials such as graphene films have recently been proposed. The well-controlled inorganic nanostructures could serve as efficient channels for electronics with better electron mobility and stability. Great scalability and flexibility can be offered from the graphene substrates. Furthermore, precise position control of each individual nanostructures provides an advantage for realizing the high-density integration of devices, as well as integration with other electronics for flexible and/or wearable device applications. However, to utilize them as electronic devices, three-dimensional architectures are required since channels are grown vertically from the graphene substrates in contrast to conventional devices with lateral channels.
Here, we report the fabrication of vertical field-effect transistor(VFET) arrays using position- and morphology-controlled ZnO nanotube arrays grown on graphene films. For the fabrication of the VFET, single crystalline ZnO nanotubes were heteroepitaxially grown on graphene films with controlled position and dimension. The fabricated devices exhibited good electrical characteristics such as the small subthreshold swing of 110 mV/dec, high I_max/I_min ratio of 10⁶ and a transconductance of 170 nS/um, thanks to its novel surrounding gate structures and single crystalline ZnO nanotube channels. Furthermore, fabricated VFET arrays could be transferred onto flexible substrates via simple mechanical lift-off process. The performance of the devices remained the same on flexible substrates, even at highly bent conditions. This research offers a general route to construct high-performance electronics for flexible and wearable device applications.

Increases in nanoscale device packing density and energy consumption have led to a growing need for improved thermal management technologies. Thermal switches or “thermal transistors” offer a way to dampen sudden thermal transients in order to reduce thermal cycling loads and improve device and system reliability. For example, a 10x thermal switching ratio could reduce the temperature swing of a device by up to 10x and increase its lifetime by 3000x [1].
In this work, we demonstrate the first scalable, CMOS-compatible thermal switches based on suspended graphene. Graphene is selected as the switch membrane due to its high Young’s modulus, and high thermal and electrical conductivity [2-4]. We show reversible switching cycles at low (< 2 V) pull-in voltages. Using voltage pulses, we modulate the electrostatic deflection of suspended graphene to contact it to the underlying substrate. The controlled, partial collapse of graphene micro-ribbons and graphene-supported metal beams directionally channels heat for thermal management applications.
Large area graphene is grown on copper foil via chemical vapor deposition. We fabricate bilayer graphene stacks on 540 nm thick SiO2 on highly doped silicon using sequential PMMA-assisted wet transfers, and pattern devices using optical photolithography. We define graphene ribbon arrays using a copper sacrificial layer and oxygen plasma etching, and deposit 3 nm thick Cr channels on top of the graphene. 5 μm wide Cr/Au or Pt contacts are deposited, clamping the graphene and chrome channels to the oxide, and serving as the etch mask. We use buffered oxide etch to remove approximately 520 nm of underlying oxide, and critical point drying is used to suspend the graphene over oxide pillars.
We electrostatically collapse the graphene by applying a voltage between the metal contact and the highly doped silicon substrate. We observe a graphene pull-in voltage of ~1.8 V, and demonstrate thermal switching for ~10 cycles without showing signs of irreversible collapse. (More cycles could be possible, as devices were not tested to the point of failure.) Using optical pump-probe reflectance techniques., we measure the changes in thermal conductance between the suspended (OFF) and collapsed (ON) state of the device. These represent the first demonstration of nanoelectromechanical thermal switches based on graphene, with potential applications for management of thermal transients and of electronics reliability.

[1] Bayerer et al., Conf. Int. Power Sys. (CIPS), 2008
[2] Lee et al., Science, 2008, 18, 385
[3] Pop et al., MRS Bulletin, 2012, 7, 1273
[4] Milaninia et al., Appl. Phys. Lett., 2009, 95, 183105

Although recent efforts in device designs and fabrication strategies have resulted in meaningful progresses to the goal of soft electronics, significant challenges still remain in fabricating a soft form of the image sensor array. In this presentation, we report our recent achievement in a high-density soft curved image sensor (CurvIS) array by using a heterostructure of inherently soft 2D materials (MoS₂ and graphene), by employing an ultrathin device structure, and by applying strain-isolating/-releasing array designs. This high-density soft CurvIS array with the single-lens optics successfully acquires pixelated images without optical aberration and infrared noises. We also present a human-eye-inspired soft implantable optoelectronic device based on the developed CurvIS array. Theoretical analysis in conjunction with supporting experiments corroborates the validity of the proposed soft materials and device designs.

Two-dimensional materials such as MoS₂ have been shown to exhibit large Seebeck coefficient and are promising candidates for thermoelectric energy conversion [1]. The ab-initio calculations predict a high intrinsic thermoelectric power factor if the substrate effects are excluded [2]. But the overall power factor is typically limited by its low electrical conductivity. This is possibly due to localized states arising from the impurities and the adsorbates introduced from the substrate [2]. In this work, we measure Seebeck coefficient and electrical conductance of CVD-grown monolayer MoS₂ on Si/SiO₂ substrates [3]. We observe hopping transport from the localized states introduced by the substrate at low temperatures and low electrostatic doping. To reduce the observed defect states, we use 2D hexagonal boron nitride (hBN) as a substrate because it provides less charge inhomogeneity and atomically flat surface. The CVD grown MoS₂ monolayer is placed on top of hBN using a dry transfer printing process and a PDMS stamp [4]. We observe an enhanced thermoelectric power factor from MoS₂ on hBN substrate over Si/SiO₂. However, we still find the localized states to be dominating the extended states from Seebeck measurements at low temperatures and doping. We attribute these defect states observed in hBN/MoS₂ to arise from the dry transfer printing process that could introduce strain heterogeneity. We systematically characterize the defect states through electrical measurements on similarly transfer printed monolayer MoS₂ on Si/SiO₂ substrates. This work advances the understanding of substrate effects on transfer printed 2D materials for improved thermoelectric performance.
1. Wu, Jing, et al. Nano letters 14.5 (2014): 2730-2734.
2. Babaei, Hasan, J. M. Khodadadi, and Sanjiv Sinha. Applied Physics Letters 105.19 (2014): 193901.
3. Lee, Gwan-Hyoung, et al. ACS nano 7.9 (2013): 7931-7936.
4. Lee, Chul-Ho, et al. Nature nanotechnology 9.9 (2014): 676-681.

Graphene has drawn great interest as a promising platform for novel electronic systems. For instance, graphene has been intensively studied for use in sensors because its charge carrier transport is sensitively affected by the surrounding chemical and biological environment. However, the sensing characteristics of graphene devices have varied from case to case, and its mechanism has not been satisfactorily determined thus far. Here we describe a silica-assisted chemical vapor deposition (CVD) technique that can directly synthesize graphene meshes. The resulting graphene meshes have a circular hole array and nearly ideal edges, and thus can be exploited as a platform to study the intrinsic response of defects to gaseous and ionic molecules, as well as biological species. Particular interests will be paid on the defect mediated molecular interaction and charge transfer and their effects on the sensitivity and reliability of the graphene-based chemical and biological sensors. Our findings indicated that defects on graphene can be used to chemically active sites and improve charge-transfer across the adsorbed chemical species-graphene interfaces.

Bio-tissues are soft, curvilinear and dynamic whereas wafer-based electronics are hard, planar, and fragile. Such mismatch fundamentally impedes their integration with each other. As an atomically thin, optically transparent, mechanically robust, electrically conductive, and chemically inert material, graphene is an ideal material for skin-mounted soft biometric sensors. We have invented a cost- and time-effective “wet transfer, dry patterning” process for the freeform manufacture of graphene e-tattoos (GETs) ^[1]. Our GET has a total thickness of less than 500 nm, an optical transparency of ~85%, and a stretchability of more than 40%. Tensile fracture of PMMA-supported graphene has been experimentally investigated and compared with PMMA-supported gold thin film ^[2]. GET can be directly laminated on human skin exactly like a temporary transfer tattoo and can fully conform to the microscopic morphology of the skin surface via just van der Waals forces. Analytical models are developed to predict GET-skin conformability even under skin deformation ^[3]. The open mesh structure of GET makes it breathable and its stiffness negligible. As a dry electrode, GET-skin interface impedance is found to be as low as medically used Ag/AgCl gel electrodes. GET has been successfully applied to measure electrocardiogram (ECG), electromyogram (EMG), electroencephalogram (EEG), skin temperature, and skin hydration. When applied around human eyes, imperceptible GET electrooculogram (EOG) sensors can capture eye movement with an angular resolution of 4 degrees, which can be used to wirelessly control a quadcopter in real-time ^[4].

[1] S. K. Ameri, R. Ho, H. W. Jang, L. Tao, Y. H. Wang, L. Wang, D. M. Schnyer, D. Akinwande, N. Lu, Acs Nano 2017, 11, 7634-7641.
[2] H. Jang, Z. Dai, S. K. Ameri, N. Lu, To be submitted 2017.
[3] L. Wang, S. Qiao, S. Kabiri Ameri, H. Jeong, N. Lu, Journal of Applied Mechanics 2017, 84, 111003.
[4] S. K. Ameri, M. Kim, I. A. Kuang, W. K. Perera, M. Alshiekh, H. Jeong, U. Topcu, D. Akinwande, N. Lu, submitted 2017.

Excitation of graphene plasmon in nanostructured graphene with various geometries has been investigated for a broad range of plasmonic applications in near- and mid-infrared frequency regimes. In particular, graphene has drawn considerable attention as a promising platform for guiding infrared light due to its ability to transport graphene plasmons with ultrashort wavelengths and high field confinement. In addition, graphene plasmons can be controlled by changing the carrier density of graphene via electrostatic gating. However, it is challenging to excite the plasmonic waves on a smooth flat graphene by far-field light illumination due to large mismatch between the wavelength of incident light and propagation graphene plasmon. Furthermore, although the plasmonic resonances are gate-tunable, broadband tunability of plasmonic resonance across wavelength ranges from near-infrared to mid-infrared is severely restricted.
Here, we present a novel approach to excitation of graphene plasmon by far-field illumination with broadband tunability based on curved graphene nanostructures with subwavelength periodic modulation. We demonstrate the excitation of strongly confined graphene plasmon in a periodically modulated crumpled graphene nanostructures by far-field illumination and its broadband tunability with varying crumple wavelength and height. The reconfiguration of crumpled graphene structures with varying crumple wavelength and height enables the modulation of plasmonic resonances over a five times broader range of tunable wavelengths than that achievable by conventional electrical tuning of graphene. Furthermore, we show the strong confinement of the excited plasmons in crumpled graphene allows strong near-field enhancements. We further discuss the excitation of graphene plasmon on a conformally graphene coated nanowire through periodic shell modulation of the nanowire. Our curved graphene nanostructures with strong and broadly tunable plasmonic resonances from near-infrared to mid-infrared will find broad applications, including ultrasensitive biological and chemical sensing, photonic and optoelectronic devices.

Graphene has a great potential to replace indium tin oxide (ITO) as transparent conductive films (TCFs) for flexible opto-electronic devices, such as organic photovoltaic (OPV) cells and organic light emitting diodes (OLEDs), due to high electrical conductivity, optical transmittance, mechanical flexibility, chemical stability and the successful synthesis of large-area graphene film. However, the low work function and poor compatibility with hole injection layer (HIL) of graphene make the performance of graphene-based opto-electronic devices to be far from satisfactory and usually worse than those using ITO TCFs,^[1] and large surface roughness causes the active area of devices to be very small. For example, the available lighting area of OLEDs and the active area of OPV cells using graphene TCFs are usually less than 1 and 0.6 cm², respectively.
Here we propose a graphene oxide/graphene (GO/G) vertical heterostructure TCFs, which are fabricated by directly oxidizing the top layer of three-layer graphene films through ozone treatment and show greatly improved optical transmittance, a large work function, and high stability.^{[2, 3]} Moreover, the GO/G heterostructure TCFs have much better compatibility with HIL materials than pristine graphene and ITO, allowing a uniform MoO₃ HIL deposition on its surface with good smoothness. Besides high flexibility, OLEDs with different colors based on the GO/G heterostructure TCFs show much better performance than those based on pristine graphene and ITO TCFs. Furthermore, a natural organic small molecular-assisted wet etching transfer method is developed to fabricate a clean and damage-free graphene TCF with a very low surface roughness with a maximum height of about 15 nm over a large area. Such clean and damage-free graphene greatly improves the current efficiency and power efficiency of OLEDs, with maxima as high as 89.7 cd A^-1 and 102.6 lm W^-1, respectively. More importantly, a 4-inch flexible green OLED with uniform light emitting and high luminance (ca. 10000 cd m^-2 at 16 V) has been successfully fabricated for the first time, showing a strong potential of graphene TCFs for next generation flexible opto-electronics. ^[4]

References

[1] Du J. H. et al., Carbon nanotube- and graphene-based transparent conductive films for optoelectronic devices, Adv. Mater. 2014, 26 (13): 1958.
[2] Jia S and Du J. H. et al., Graphene oxide/graphene vertical heterostructure electrodes for highly efficient and flexible organic light emitting diodes, Nanoscale 2016, 8(10): 10714.
[3] Yuan J. T. and Du J. H. et al., Tuning the electrical and optical properties of graphene by ozone treatment for patterning monolithic transparent electrodes, ACS Nano 2013, 7 (5): 4233.
[4] Zhang Z. K. and Du J. H. et al., Rosin-enabled ultraclean and damage-free transfer of graphene for large-area flexible organic light-emitting diodes, Nat. Commun. 2017, 8:14560.

Atomically thin materials, also known as two dimensional materials including graphene and transition metal dichalcogenide monolayers, have emerged as an attractive material for electronic and optoelectronic devices owing to their exceptional mechanical, optical and electronic properties. Crumple nanostructuring of two dimensional materials allows for the enhancement of such outstanding material properties and enables new functionalities in mechanical, optical, and electrical properties.
Here, we demonstrate that high performance materials and devices can be achieved by the crumple nanostructuring of atomically thin materials including graphene and MoS₂ atomic layers. We demonstrate that engineering crumpling of graphene enhances optical absorption by more than an order of magnitude (≈12.5 times) and therefore allows for enhanced photoresponsivity of a graphene-based photodetector by 370% compared to a flat graphene photodetector. Based on the crumpled graphene nanostructures, we also show highly stretchability for a photodetector with stretching capability to 200% of its original length. Moreover, the photoresponsivity is modulated by about 100% with a 200% applied strain. This advanced photodetector with enhanced, strain-tunable photoresponsivity has potential for application to conformable and flexible optical sensors and dynamic mechanical strain sensors. Most importantly, this crumple nanostructuring approach can be used to enhance photoabsorption and develop a highly responsive photodetector based on other emerging atomically thin materials such as transition metal dichalcogenides. Furthermore, the crumple nanostructuring allows for enhancements of stretchability and flexural strength of atomically thin materials. We believe that the new functionalities in mechanical, optical, electrical properties based on crumple nanostructuring has enormous potential for wearable and flexible electronics and optoelectronics with applications of high performance materials and devices.

Graphene, together with other two-dimensional materials, are promising building blocks for both conventional semiconductor technologies and the nascent flexible nanotechnology. However, due to its intrinsic stiffness and strength, it is challenging to utilize them in stretchable electronics. For example, CVD graphene transferred onto a polydimethylsiloxane (PDMS) elastic substrate can only maintain its conductivity up to 6% strain¹ and transistor can maintain electrical functional at stretching up to 5%.² The above stretchability is far less than the minimum required value (~50%) for wearable health monitoring sensors and electronic skin. To address this challenge, graphene kirigami-approach has recently been explored.³ However, this method requires suspended graphene and is extremely complicated in fabrication and operation.

Here, to achieve highly stretchable large-area graphene devices, we developed an all graphene nanostructure that confines graphene scrolls in between stacked graphene layers, namely multi-layer G/G scrolls (MGG). MGG consists of three-dimensional conductive paths, which bridge the fragmented domains to maintain conductivity of the resulting transparent graphene film. Bi- and tri-layer MGG supported on an elastomer exhibited significantly less reduction in conductivity even at the strain up to 100%. An all-carbon transistor fabricated using MGG as electrodes retained 60% of its original performance at 120% strain. This is the first demonstration of highly stretchable and ultra-transparent graphene-based transistors. The concept reported here should be applicable to other 2D materials and thus opens up a new route toward stretchable 2D electronics.

For more information, please refer to: Science Advances, 2017, 3, 9, e1700159.
Reference:
1. Nature, 2009, 457, 706-710.
2. Nano Letters, 2011, 11, 4642-4646.
3. Nature, 2015, 524, 204-207.

Wearable sensors offer the ability to monitor the rehabilitation of patients outside of the clinic. For patients with cancer of the head and neck who have undergone radiation therapy, diminished swallowing activity and treatment effects can result in disuse atrophy and fibrosis of the swallowing muscles in up to 39% of these patients. This condition causes dysphagia and reduced swallowing functionality. There is a need for earlier detection of radiation-associated dysphagia such that subtle changes in swallowing muscle function can be detected well before irreversible damage has occurred. This presentation describes a highly sensitive, flexible strain sensor comprising palladium nanoislands on single-layer graphene. These sensors were placed on the submental region of the neck in a cohort of 14 cancer-free head and neck cancer patients status post radiation therapy with different levels of swallowing function: from non-dysphagic to severely dysphagic. The patch-like devices successfully detect differences in the consistencies of food boluses when swallowed (i.e. water, yogurt, or cracker), along with differences between dysphagic and non-dysphagic swallows. When electrical activity from surface electromyography (sEMG) is obtained simultaneously with the strain data, it is also possible to differentiate swallowing vs. non-swallowing events (i.e. head turns or coughing). The major features in the plots of resistance (strain sensors) and electrical activity (sEMG) vs. time are correlated to specific events during the course of swallowing a barium paste as recorded by video X-ray fluoroscopy (the current standard of care). Finally, we developed a machine learning algorithm to automate the identification of bolus type being swallowed by a healthy subject (Male, 24 years old) with an accuracy of 86.4%. Moreover, the algorithm was also able to discriminate between swallows of the same bolus from either the healthy subject or a dysphagic patient with an accuracy of 94.7%. Taken together, these results may lead to non-invasive and home-based systems for monitoring of swallowing function and improved quality of life.

Atomically thin semiconducting materials are a promising platform for optoelectronic applications owing to their optical, electronic, and mechanical properties. Monolayer molybdenum disulfide (MoS₂) is one of the most widely investigated materials for photosensors due to its direct bandgap. However, the low optical absorption of monolayer MoS₂, resulting from its single atom thickness, limits higher photosensitivity. We present a mechanically self-assembled, crumpled MoS₂ photodetector which achieves higher photosensitivity through structural deformation. The three-dimensional architecture of the crumpled structure offers several advantages including (i) enhanced sensitivity from material densification, (ii) high mechanical stretchability permitted by relaxation of the crumpled structure, and (iii) strain-induced reduction of the direct bandgap and funneling of photogenerated excitons. We further used graphene as electrical contacts to the crumpled MoS₂ channel as graphene provides mechanical robustness by forming a van der Waals interface with MoS₂. Our crumpled MoS₂ photodetector exhibited an order of magnitude higher photosensitivity compared to a flat MoS₂ photodetector. In addition, the photodetector is stretchable up to 200% without introducing major structural damage, verified by a durability test with 1000 cycles of stretching-releasing. Our approach to mechanical self-assembly of atomically thin semiconductors offers a simple but powerful way to enhance photosensitivity and potentially engineer exciton generation and funneling in two-dimensional materials.

Radiative cooling is an attractive method of passive thermal management that utilizes spectral radiation characteristics in the ambient environment. While the solar spectrum heats up surface areas facing the sun in the wavelength range of 200 nm to 2500 nm, the atmospheric transmission window allows reemission of the heat (i.e., cooling) from the ambient surface to the outer space in the wavelength range of 8 μm to 14 μm. Because of the distinct spectral ranges allowed for heating and cooling phenomena, conventional approaches relying on a simply high or low emissivity material do not offer optimal solutions for temperature control. By selectively controlling the emissivity spectrum, we can govern the thermal energy exchange in the ambient environment and optimally control the surface temperature without running electricity or any active components such as bulky heat exchangers. A novel surface design can lead to breakthroughs in thermal management, energy harvesting, and thermal imaging, particularly for surface systems where radiative heat transfer is critical to performance and reliability.
Here we show reconfigurable modulations of the surface topography of graphene via mechanical straining-induced crumpling and demonstrate how the crumpled graphene offers an innovative approach to control emissivity and temperature. The crumpled graphene maximizes radiative cooling by keeping the emissivity high in the mid-infrared range while keeping the thermal absorption low in the solar spectrum. The crumpled surface design has been inspired by Saharan silver ants, where a dense array of triangular-shaped hairs is found to be highly emissive in the mid-infrared range, which helps them stay cool in the desert. Our emissivity computation based on the rigorous coupled-wave analysis predicts that the crumpled graphene can keep the emissivity below 0.3 in the range of 100 nm and 6000 nm, which minimizes the solar spectrum heating, while keeping the emissivity above 0.7 in the spectral range beyond 8 μm, which maximizes atmospheric cooling. The reconfigurabiliy of the crumpling effect on emissivity has been demonstrated on a stretchable polymer substrate by characterizing the reflectance spectra before and after stretching the substrate by mechanical forces. Our thermal analysis shows that the optimally crumped graphene will achieve a net radiative cooling power of 110 W/m² and a surface temperature reduction 10 K below the ambient air. This work shows the straining-induced crumpling (or uncrumpling) in two-dimensional (2D) materials leads to significant changes in the emissivity and enables a novel temperature control mechanism. These findings advance our knowledge of surface topography-driven radiative properties in 2D materials and enable developments of dynamic and self-regulating temperature control systems.