Hanqing Jiang

Stretchable supercapacitors based on buckled single-walled carbon-nanotube macrofilms.

Adv. Mater. 2009, 21, 4793–4797 2009 WILEY-VCH Verlag G N Stretchable electronic devices, such as p–n diodes, photovoltaic devices, transistors, and functional electronic eyes, have been fabricated using buckled single-crystal (e.g., Si, GaAs) thin films supported by elastomeric substrates. Recently, carbon nanotube (CNT)-based highly conducting elastic composites and stretchable graphene films have been reported, which are suitable as interconnects in stretchable electronic devices. As an indispensable component of stretchable electronics, a stretchable power-source device should be able to accommodate large strains while retaining intact function. Of various power-source devices, supercapacitors have attracted great interest in recent years due to their high power and energy densities compared with lithium-ion batteries and conventional dielectric capacitors, respectively. The most active research in supercapacitors is the development of new electrode materials. Recently, CNTs have been studied as good candidates for electrode materials because of several advantages, including a high surface area, nanoscale dimensions, and excellent electrical conductivity. Here, we report stretchable supercapacitors based on periodically sinusoidal single-walled carbon nanotube (SWNT) macrofilms (a 2D network of randomly oriented SWNTs). The stretchable supercapacitors comprise two sinusoidal SWNT macrofilms as stretchable electrodes, an organic electrolyte, and a polymeric separator. Electrochemical tests were performed and the fabricated stretchable supercapacitors are found to possess energy and power densities comparable with those of supercapacitors using pristine SWNT macrofilms as electrodes. Remarkably, the electrochemical performance of the stretchable supercapacitors remains unchanged even under 30% applied tensile strain. The preparation of the periodically sinusoidal SWNT macrofilms is of primary importance for stretchable supercapacitors. The synthesis of high-quality, purified, and functionalized SWNT macrofilms is, thus, an important preprocess, which has been presented elsewhere. The purified SWNT macrofilm was then shaped to a sinusoidal form by following the steps shown in Figure 1a. The procedure introduced here (step i in Fig. 1a) involves the uniaxial prestretching (epre) of an elastomeric substrate of a poly(dimethylsiloxane) (PDMS) slab (epre1⁄4DL/L for length changed from L to LþDL), followed by a chemical surface treatment to form a hydrophilic surface (see Experimental Section). The exposure of UV light introduces atomic oxygen, an activated species that reacts with PDMS and, thus, changes the

Finite deformation mechanics in buckled thin films on compliant supports

We present detailed experimental and theoretical studies of the mechanics of thin buckled films on compliant substrates. In particular, accurate measurements of the wavelengths and amplitudes in structures that consist of thin, single-crystal ribbons of silicon covalently bonded to elastomeric substrates of poly(dimethylsiloxane) reveal responses that include wavelengths that change in an approximately linear fashion with strain in the substrate, for all values of strain above the critical strain for buckling. Theoretical reexamination of this system yields analytical models that can explain these and other experimental observations at a quantitative level. We show that the resulting mechanics has many features in common with that of a simple accordion bellows. These results have relevance to the many emerging applications of controlled buckling structures in stretchable electronics, microelectromechanical systems, thin-film metrology, optical devices, and others.

Thermal Expansion of Single Wall Carbon Nanotubes

We have developed an analytical method to determine the coefficient of thermal expansion (CTE) for single wall carbon nanotubes (CNTs). We have found that all CTEs are negative at low and room temperature and become positive at high temperature. As the CNT diameter decreases, the range of negative CTE shrinks. The CTE in radial direction of the CNT is less than that in the axial direction for armchair CNTs, but the opposite holds for zigzag CNTs. The radial CTE is independent of the CNT helicity, while the axial CTE shows a strong helicity dependence.

Origami lithium-ion batteries

There are significant challenges in developing deformable devices at the system level that contain integrated, deformable energy storage devices. Here we demonstrate an origami lithium-ion battery that can be deformed at an unprecedented high level, including folding, bending and twisting. Deformability at the system level is enabled using rigid origami, which prescribes a crease pattern such that the materials making the origami pattern do not experience large strain. The origami battery is fabricated through slurry coating of electrodes onto paper current collectors and packaging in standard materials, followed by folding using the Miura pattern. The resulting origami battery achieves significant linear and areal deformability, large twistability and bendability. The strategy described here represents the fusion of the art of origami, materials science and functional energy storage devices, and could provide a paradigm shift for architecture and design of flexible and curvilinear electronics with exceptional mechanical characteristics and functionalities.

Folding Paper-Based Lithium-Ion Batteries for Higher Areal Energy Densities

Paper folding techniques are used in order to compact a Li-ion battery and increase its energy per footprint area. Full cells were prepared using Li4Ti5O12 and LiCoO2 powders deposited onto current collectors consisting of paper coated with carbon nanotubes. Folded cells showed higher areal capacities compared to the planar versions with a 5 × 5 cell folded using the Miura-ori pattern displaying a ~14× increase in areal energy density.

An analytical study of two-dimensional buckling of thin films on compliant substrates

A stiff thin film on a heated compliant substrate may buckle when the system is cooled due to the thermal expansion mismatch between the film and substrate. Highly ordered and disordered herringbone patterns (wavy structures) then emerge as the system continues to cool. We have established an analytic approach to study one-dimensional, checkerboard, and ordered herringbone buckling patterns. The analytical approach gives the buckle wave length and amplitude in terms of the thin film and substrate elastic properties, thin film thickness, and the thermal strain. It is shown that the herringbone mode has the lowest energy, which explains why this mode is frequently observed in experiments. These classes of materials might be interesting as a route to high performance electronics with full, two-dimensional stretchability.

Mechanics of precisely controlled thin film buckling on elastomeric substrate

Stretchable electronics has many important and emerging applications. Sun et al. [Nature Nanotech. 1, 201 (2006)] recently demonstrated stretchable electronics based on precisely controlled buckle geometries in GaAs and Si nanoribbons on elastomeric substrates. A nonlinear buckling model is presented in this letter to study the mechanics of this type of thin film/substrate system. An analytical solution is obtained for the buckling geometry (wavelength and amplitude) and the maximum strain in buckled thin film. This solution agrees very well with the experiments, and shows explicitly how buckling can significantly reduce the thin film strain to achieve the system stretchability.

A finite element method for transient analysis of concurrent large deformation and mass transport in gels

A gel is an aggregate of polymers and solvent molecules. The polymers crosslink into a three-dimensional network by strong chemical bonds and enable the gel to retain its shape after a large deformation. The solvent molecules, however, interact among themselves and with the network by weak physical bonds and enable the gel to be a conduit of mass transport. The time-dependent concurrent process of large deformation and mass transport is studied by developing a finite element method. We combine the kinematics of large deformation, the conservation of the solvent molecules, the conditions of local equilibrium, and the kinetics of migration to evolve simultaneously two fields: the displacement of the network and the chemical potential of the solvent. The finite element method is demonstrated by analyzing several phenomena, such as swelling, draining and buckling. This work builds a platform to study diverse phenomena in gels with spatial and temporal complexity.

Stiffness and thickness of boron-nitride nanotubes

Some of the main experimental observations related to the occurrence of exchange bias in magnetic systems are reviewed, focusing the attention on the peculiar phenomenology associated to nanoparticles with core/shell structure as compared to thin film bilayers. The main open questions posed by the experimental observations are presented and contrasted to existing theories and models for exchange bias formulated up to date. We also present results of simulations based on a simple model of a core/shell nanoparticle in which the values of microscopic parameters such as anisotropy and exchange constants can be tuned in the core, shell and at the interfacial regions, offering new insight on the microscopic origin of the experimental phenomenology. A detailed study of the magnetic order of the interfacial spins shows compelling evidence that most of the experimentally observed effects can be qualitatively accounted within the context of this model and allows also to quantify the magnitude of the loop shifts in striking agreement with the macroscopic observed values.We establish an analytic approach to determine the tensile and bending stiffness of a hexagonal boron-nitride (h-BN) monolayer and single- and multi-wall boron-nitride nanotubes (BNNTs) directly from the interatomic potential. Such an approach enables one to bypass atomistic simulations and to give the tensile and bending stiffness in terms of the parameters in the potential. For single- and multi-wall BNNTs, the stiffness also depends on the (inner most or outer most) wall radius and the number of the walls. The thickness of h-BN monolayer is also discussed.

The influence of mechanical deformation on the electrical properties of single wall carbon nanotubes

Recent experimental studies and atomistic simulations have shown that carbon nanotubes (CNTs) display strong interplay between the mechanical deformation and electrical properties. We have developed a simple and accurate method to determine atom positions in a uniformly deformed CNT via a continuum analysis based on the interatomic potential. A shift vector is introduced to ensure the equilibrium of atoms. Such an approach, involving only three variables for the entire CNT, agrees very well with the molecular mechanics calculations. We then study the effect of mechanical deformation on the band gap change of single wall CNTs under tension, torsion, and combined tension/torsion via the k-space tight-binding method. Prior studies without this shift vector lead to significant overestimation of the band gap change. It is established that the conducting CNTs may easily become semi-conducting ones subject to mechanical deformation, but the semi-conducting CNTs never become conducting ones upon deformation.