Crumpled graphene films are broadly used, for instance in electronics1, energy storage2, 3, composites4, 5, and biomedicine6. Although it is known that the degree of crumpling affects graphene's properties and the performance of graphene-based devices and materials3, 5, 7, the controlled folding and unfolding of crumpled graphene films has not been demonstrated. Here we report an approach to reversibly control the crumpling and unfolding of large-area graphene sheets. We show with experiments, atomistic simulations and theory that, by harnessing the mechanical instabilities of graphene adhered on a biaxially pre-stretched polymer substrate and by controlling the relaxation of the pre-strains in a particular order, graphene films can be crumpled into tailored self-organized hierarchical structures that mimic superhydrophobic leaves. The approach enables us to fabricate large-area conductive coatings and electrodes showing superhydrophobicity, high transparency, and tunable wettability and transmittance. We also demonstrate that crumpled graphene-polymer laminates can be used as artificial-muscle actuators.
A series of two-dimensional (2D) hybrid organic-inorganic perovskite (HOIP) crystals, based on acene alkylamine cations (i.e., phenylmethylammonium (PMA), 2-phenylethylammonium (PEA), 1-(2-naphthyl)methanammonium (NMA), and 2-(2-naphthyl)ethanammonium (NEA)) and lead(II) halide (i.e., PbX, X = Cl, Br, and I) frameworks, and their corresponding thin films were fabricated and examined for structure-property relationship. Several new or redetermined crystal structures are reported, including those for (NEA)PbI, (NEA)PbBr, (NMA)PbBr, (PMA)PbBr, and (PEA)PbI. Non-centrosymmetric structures from among these 2D HOIPs were confirmed by piezoresponse force microscopy-especially noteworthy is the structure of (PMA)PbBr, which was previously reported as centrosymmetric. Examination of the impact of organic cation and inorganic layer choice on the exciton absorption/emission properties, among the set of compounds considered, reveals that perovskite layer distortion (i.e., Pb-I-Pb bond angle between adjacent PbI octahedra) has a more global effect on the exciton properties than octahedral distortion (i.e., variation of I-Pb-I bond angles and discrepancy among Pb-I bond lengths within each PbI octahedron). In addition to the characteristic sharp exciton emission for each perovskite, (PMA)PbCl, (PEA)PbCl, (NMA)PbCl, and (PMA)PbBr exhibit separate, broad "white" emission in the long wavelength range. Piezoelectric compounds identified from these 2D HOIPs may be considered for future piezoresponse-type energy or electronic applications.
The
unique hybrid nature of 2D Ruddlesden–Popper (R–P)
perovskites has bestowed upon them not only tunability of their electronic
properties but also high-performance electronic devices with improved
environmental stability as compared to their 3D analogs. However,
there is limited information about their inherent heat, light, and
air stability and how different parameters such as the inorganic layer
number and length of organic spacer molecule affect stability. To
gain deeper understanding on the matter we have expanded the family
of 2D R–P perovskites, by utilizing pentylamine (PA)2(MA)
n−1Pb
n
I3n+1 (n = 1–5,
PA = CH3(CH2)4NH3
+, C5) and hexylamine (HA)2(MA)
n−1Pb
n
I3n+1 (n = 1–4, HA = CH3(CH2)5NH3
+, C6) as the organic
spacer molecules between the inorganic slabs, creating two new series
of layered materials, for up to n = 5 and 4 layers,
respectively. The resulting compounds were extensively characterized
through a combination of physical and spectroscopic methods, including
single crystal X-ray analysis. High resolution powder X-ray diffraction
studies using synchrotron radiation shed light for the first time
to the phase transitions of the higher layer 2D R–P perovskites.
The increase in the length of the organic spacer molecules did not
affect their optical properties; however, it has a pronounced effect
on the air, heat, and light stability of the fabricated thin films.
An extensive study of heat, light, and air stability with and without
encapsulation revealed that specific compounds can be air stable (relative
humidity (RH) = 20–80% ± 5%) for more than 450 days, while
heat and light stability in air can be exponentially increased by
encapsulating the corresponding films. Evaluation of the out-of-plane
mechanical properties of the corresponding materials showed that their
soft and flexible nature can be compared to current commercially available
polymer substrates (e.g., PMMA), rendering them suitable for fabricating
flexible and wearable electronic devices.
Two-dimensional
(2D) hybrid organic–inorganic perovskites
(HOIPs) are recent members of the 2D materials family with wide tunability,
highly dynamic structural features, and excellent physical properties.
Ultrathin 2D HOIPs and their heterostructures with other 2D materials
have been exploited for study of physical phenomena and device applications.
The in-plane mechanical properties of 2D ultrathin HOIPs are critical
for understanding the coupling between mechanical and other physical
fields and for integrated devices applications. Here we report the
in-plane mechanical properties of ultrathin freestanding 2D lead iodide
perovskite membranes and their dependence on the membrane thickness.
The in-plane Young’s moduli of 2D HOIPs are smaller than that
of conventional covalently bonded 2D materials. As the thickness increases
from monolayer to three-layer, both the Young’s modulus and
breaking strength decrease, while three-layer and four-layer 2D HOIPs
have almost identical in-plane mechanical properties. These thickness-dependent
mechanical properties can be attributed to interlayer slippage during
deformation. Our results show that ultrathin 2D HOIPs exhibit outstanding
breaking strength/Young’s modulus ratio compared to many other
widely used engineering materials and polymeric flexible substrates,
which renders them suitable for application into flexible electronic
devices.
Two-dimensional (2D) layered hybrid organic-inorganic perovskites (HOIPs) have demonstrated improved stability and promising photovoltaic performance. The mechanical properties of such functional materials are both fundamentally and practically important to achieve both high performance and mechanically stable (flexible) devices. Here, we report the mechanical properties of a series of 2D layered lead iodide HOIPs and investigate the role of structural subunits (e.g., variation of the length of the organic spacer molecules, R and the number of inorganic layers, n) in the mechanical properties. Although 2D HOIPs have much lower nominal elastic modulus and hardness than 3D HOIPs, larger n number and shorter R lead to stiffer materials. Density functional theory simulations showed a trend similar to the experimental results. We compared these findings with other 2D layered crystals and shed light on routes to further tune the out-of-plane mechanical properties of 2D layered HOIPs.
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