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Berkeley, CA 94720, University of California, Berkeley


Design and Preparation of Nano-scale Architecture Engineered by Strain

Engineering of materials structures at various scales have been widely shown to give rise to novel material properties and functionalities. 2D materials, due to their high strength and flexibility, can transform into many shapes and 3D structures, therefore bringing new opportunities for nanostructure engineering. Usually, the controllable methods to realize 3D architectures from 2D materials are through the manipulation of substrates, which substantially limits the practicality of such approaches and their compatibility with existing technologies that are mostly based on rigid solid substrates.

We are focusing on a completely new route toward controllable generation of 3D structures based on mismatched 2D material lateral heterostructures. The freestanding 2D lateral Bi2Se3/Bi2Te3 heterojunctions with wavelike structures prepared from a substrate-free solution method. Due to a large lattice mismatch of around 5.9%, Bi2Te3 grows buckling up and down, forming periodic ripples anchored by Bi2Se3 next to it. Unlike the widely used CVD growth where the interaction between the materials and the substrates restrains the out-of-plane deformation, our solution based method set the materials free. The out-of-plane rippling can dramatically reduce the elastic energy from lattice mismatch, and the competition between bending energy and in-plane strain energy results in different rippling wavelengths and amplitudes. Accordingly, the ripples’ parameters are dependent on the width and thickness of the Bi2Te3 region, which have been quantitatively verified in our experimental investigation, agreeing quite well with continuum mechanics analysis. We also observed clear high-resolution electron transmission patterns manipulated by the single-crystalline 3D structure, which demonstrates the potential of such architectures as lithography masks. In addition, bismuth chalcogenides have been well known as topological insulators and excellent thermoelectric materials. The wrinkles heterojunctions may provide a new route to controllably tune both topological surface states and thermoelectric properties across the interface.

As the first experimental demonstration of 3D architecture enabled by strained 2D material heterojunction, our work will not only shed new light on the design of 3D nanostructures in 2D materials, but can also work as a platform to engineer the electronic structure and transport properties.


2D nanosheets-based novel architectures: Synthesis, assembly and applications

Abstract: The discovery of graphene attracted great interest because of the potential prospects of which in both basic and applied research. And other kinds of 2D graphene analogues, including hexagonal BN, carbon nitride, transition metal dichalcogenides, etc. derived from their layered bulk crystals, have also been extensively investigated because of their promising characters and potential applications. It is expected that these 2D nanosheets, in bulk or in composite materials, could maintain their extraordinary properties. However, the irreversible aggregation or accumulating of 2D nanosheets due to the strong van der Waals interactions, extremely decrease their accessible surface area. More recently, with the recognization of the tremendous interest of these 2D structures, scientists noticed that the performance of certain devices could be significantly improved by utilizing 3D architectures and/or aerogels due to the increase of unit activity of the materials. This review summarizes different synthetic process (such as assembly, chemical vapor deposition direct synthesis, in-situ confinement growth and so on) used for preparation of 2D graphene analogues based 3D architectures and/or aerogels containing either any or their composites. And the different fields of application for energy storage (including both supercapacitor and lithium ion battery applications), electrocatalysis, sensing and others are provided a significant enhancement in the efficacy as compared to their 2D analogues or even opened the path to novel application. In addition, some perspectives on the challenges and opportunities in this promising research area are also discussed.

Pub.: 05 Aug '16, Pinned: 29 Jan '18

Graphene kirigami.

Abstract: For centuries, practitioners of origami ('ori', fold; 'kami', paper) and kirigami ('kiru', cut) have fashioned sheets of paper into beautiful and complex three-dimensional structures. Both techniques are scalable, and scientists and engineers are adapting them to different two-dimensional starting materials to create structures from the macro- to the microscale. Here we show that graphene is well suited for kirigami, allowing us to build robust microscale structures with tunable mechanical properties. The material parameter crucial for kirigami is the Föppl-von Kármán number γ: an indication of the ratio between in-plane stiffness and out-of-plane bending stiffness, with high numbers corresponding to membranes that more easily bend and crumple than they stretch and shear. To determine γ, we measure the bending stiffness of graphene monolayers that are 10-100 micrometres in size and obtain a value that is thousands of times higher than the predicted atomic-scale bending stiffness. Interferometric imaging attributes this finding to ripples in the membrane that stiffen the graphene sheets considerably, to the extent that γ is comparable to that of a standard piece of paper. We may therefore apply ideas from kirigami to graphene sheets to build mechanical metamaterials such as stretchable electrodes, springs, and hinges. These results establish graphene kirigami as a simple yet powerful and customizable approach for fashioning one-atom-thick graphene sheets into resilient and movable parts with microscale dimensions.

Pub.: 30 Jul '15, Pinned: 29 Jan '18