Xiao Research Group                

• We develop devices and systems that utilize the advantages of both hard and soft materials, which cannot be achieved through conventional means.
• We develop materials with unique properties that can only be realized through the combination of distinct properties of hard and soft materials.
• We study fundamental mechanics of hybrid hard/soft systems that can advance our understanding of such systems and guide the design and optimization of devices and materials.

Stretchable Electronics: Devices, Materials, and Mechanics

Bio-inspired Bug Eye Cameras (Artificial Compound Eye)

In arthropods, evolution has created a remarkably sophisticated class of imaging systems, with a wide-angle field of view, low aberrations, high acuity to motion and an infinite depth of field. A challenge in building digital cameras with the hemispherical, compound apposition layouts of arthropod eyes is that essential design requirements cannot be met with existing planar sensor technologies or conventional optics. In this study, we combined elastomeric compound optical elements with deformable arrays of thin silicon photodetectors into integrated sheets that can be elastically transformed from the planar geometries in which they are fabricated to hemispherical shapes for integration into apposition cameras. References: Y. M. Song+, Y. Xie+, V. Malyarchuk+, J. Xiao+, I. Jung, K.-J. Choi, Z. Liu, H. Park, C. Lu, R.-H. Kim, R. Li, K. B. Crozier, Y. Huang, and J. A. Rogers, Digital Cameras With Designs Inspired By the Arthropod Eye, Nature 497, 95–99 (2013)  Highlighted on the cover.

Tunable Eyeball Cameras With Zoom

In our previously developed bio-inspired eyeball cameras, photodetectors are distributed on curvilinear surfaces to match the strongly nonplanar image surfaces (i.e., Petzval surfaces) that form with simple lenses. Although these systems provide advantages compared to those with conventional, planar designs, their fixed detector curvature renders them incompatible with changes in the Petzval surface that accompany variable zoom achieved with simple lenses.Here, we put stretchable photodetector arrays on thin elastomeric membranes, capable of reversible deformation into hemispherical shapes with radii of curvature that can be adjusted dynamically, via hydraulics. Combining this type of detector with a similarly tunable, fluidic planoconvex lens yields a hemispherical camera with variable zoom and excellent imaging characteristics.

Flexible, Ultrathin Sensors for Biomedical Applications

In biomedical practices, many electronic devices are used to perform diagnosis, treament and other functions. However, electronics are usually very hard, and cannot comply with the complex geometries and extreme deformabilities of biological tissues. This incompatibility greatly limits the application of electronics in biomedical areas (one extreme example is brain computer interface). We developed electronics that are capable of intimate, non-invasive integration with the soft, curvilinear surfaces of biological tissues, which offer important opportunities for diagnosing and treating disease and for improving interfaces between electronics and biological tissues. As shown on the left are neural sensors on cat's brains (top left and right) and cardiac sensor on pig's heart, for electrophysiology measurement.

Stretchable Inorganic LEDs

Inorganic light-emitting diodes and photodetectors represent important, established technologies for solid-state lighting, digital imaging and many other applications. Eliminating mechanical and geometrical design constraints imposed by the supporting semiconductor wafers can enable alternative uses in areas such as biomedicine and robotics. We developed systems that consist of arrays of interconnected, ultrathin inorganic light-emitting diodes and photodetectors configured in mechanically optimized layouts on unusual substrates. As shown on the left are LEDs poked by a pencil tip (top left), twisted to different angles (top right), LEDs on a thread sutured underneath the skin (bottom left) and LEDs and photodetectors (PDs) integrated on a glove for detecting distance (bottom right). References: R.-H. Kim, D.-H. Kim, J. Xiao, B. H. Kim, S.-I. Park, B. Panilaitis, R. Ghaffari, J. Yao, M. Li, Z. Liu, V. Malyarchuk, D. G. Kim, A.-P. Le, R. G. Nuzzo, D. L. Kaplan, F. G. Omenetto, Y. Huang, Z. Kang, and J. A. Rogers, Waterproof AlInGaP optoelectronics on stretchable substrates with applications in biomedicine and robotics, Nature Materials 9, 929-937 (2010)

Flexible Solar Cells

The high natural abundance of silicon, together with its excellent reliability and good efficiency in solar cells, suggest its continued use in production of solar energy, on massive scales, for the foreseeable future. Although organics, nanocrystals, nanowires and other new materials hold significant promise, many opportunities continue to exist for research into unconventional means of exploiting silicon in advanced photovoltaic systems.Here,we developed modules that use large-scale arrays of silicon solar microcells created from bulk wafers and integrated in diverse spatial layouts on foreign substrates by transfer printing. The resulting devices can offer useful features, including high degrees of mechanical flexibility, user-definable transparency and ultrathin-form-factor microconcentrator designs. References: Baca et al., Compact monocrystalline silicon solar modules with high voltage outputs and mechanically flexible designs, Energy & Environmental Science 3, 208-211 (2010) (cover feature article) Yoon et al., Ultrathin silicon solar microcells for semitransparent, mechanically flexible and microconcentrator  module designs. Nat. Mater. 7, 907-915 (2008). (cover feature article)

Stretchable and Compressible Conductors

We developed stretchable Au thin films on elastomeric substrates of poly dimethylsiloxane that are designed with sinusoidal, “wavy” features of surface relief. This approach eliminates the compressive strains introduced into the thin films through commonly adopted buckling approach, and therefore can provide both high stretchability and compressibility. Such systems can be useful as stretchable conductors in electronic or sensory devices. References: Xiao et al., Stretchable and Compressible Thin Films of Stiff Materials on Compliant Wavy Substrates. Appl. Phys. Lett. 93, 013109 (2008) Xiao et al., Analytical and Experimental Studies of the Mechanics of Deformation in a Solid with a Wavy Surface Profile. Journal of Applied Mechanics-Transactions of the ASME 77, 011003 (2010).

Wrinkling of Thin Films on Soft Substrates

Nonlinear Buckling of Silicon Ribbons on Elastomers

In previous studies of thin film wrinkling on soft substrates, the mechanics models usually assume plane-strain deformation, which was found to disagree with experimental observations for narrow thin films. Systematic experimental and analytical studies are conducted for buckling of finite-width stiff thin films on compliant substrates. Both experiments and analytical solution show that the buckling amplitude and wavelength increase with the film width.The effect of film spacing is also studied via the analytical solutions for two thin films and for periodic thin films. References: H. Jiang, D.-Y. Khang, H. Fei, H. Kim, Y. Huang, J. Xiao, and J. A. Rogers, Finite Width Effect of Thin-Films Buckling on Compliant Substrate: Experimental and Theoretical Studies. J. Mech. Phys. Solids 56, 2585-2598 (2008).

Molecular Scale Buckling Mechanics of Carbon Nanotubes on Elastomers

We have studied for the first time the scaling of controlled nonlinear buckling processes in materials with dimensions in the molecular range (i.e., ~1 nm) through experimental and theoretical studies of buckling in individual single-wall carbon nanotubes on substrates of poly(dimethylsiloxane). The results show not only the ability to create and manipulate patterns of buckling at these molecular scales, but also, that analytical continuum mechanics theory can explain, quantitatively, all measurable aspects of this system. Inverse calculation applied to measurements of diameter dependent buckling wavelengths yields accurate values of the Young’s moduli of individual SWNTs. As an example of the value of this system beyond its use in this type of molecular scale metrology, we implement parallel arrays of buckled SWNTs as a class of mechanically stretchable conductor. References: D.-Y. Khang, J. Xiao, C. Kocabas, S. Maclaren, T. Banks, H. Jiang, Y. Y. Huang, and J. A. Rogers, Molecular Scale Buckling Mechanics in Individual Aligned Single-Wall Carbon Nanotubes on Elastomeric Substrates. Nano Lett. 8, 124-130 (2008). J. Xiao, H. Jiang, D.-Y. Khang, J. Wu, Y. Huang, and J.A. Rogers, Mechanics of buckled carbon nanotubes on elastomeric substrates. J. Appl. Phys. 104, 033543 (2008).

In-plane Buckling of Silicon Nanowires on Elastomers

We combined experimental and theoretical means to study the buckling mechanics in silicon nanowires (SiNWs) on elastomeric substrates. The system involves randomly oriented SiNWs grown using established procedures on silicon wafers, and then transferred and organized into aligned arrays on prestrained slabs of poly(dimethylsiloxane) (PDMS). Releasing the prestrain leads to nonlinear mechanical buckling processes that transform the initially linear SiNWs into sinusoidal (i.e., “wavy”) shapes. We observed for the first time that the displacements associated with these waves lie in the plane of the substrate, unlike previously observed behavior in analogous systems of silicon nanoribbons and carbon nanotubes where motion occurs out-of-plane. Theoretical analysis indicates that the energy associated with this in-plane buckling is slightly lower than the out-of-plane case for the geometries and mechanical properties that characterize the SiNWs. An accurate measurement of the Young’s modulus of individual SiNWs, between ∼170 and ∼110 GPa for the range of  wires examined here. A simple strain gauge built using SiNWs in these wavy geometries demonstrates one area of potential application.

Mechanics of Nanomaterials

Self Folding of Graphene

Graphene, like a sheet of paper, folds under mechanical forces. The folded graphene edges can affect the electrical properties of graphenes.  The stability of folded graphene, however, depends on the folding direction and the resulted graphene stacking. Suspended graphene in liquids folds freely under random ultrasonic stimulations. We determined the structure of ~100 folded graphene edges by electron nanodiffraction. About 1/3 are armchair and 1/3 are zigzag. The results are explained by the energetics of graphene folding and atomic simulation. The zigzag edge has AB stacking, while in the armchair edge, AB stacking is achieved in some areas by a small twist. An analytical model based on finite deformation mechanics was also developed to accurately describe the shapes of folded edges. References: J. Zhang, J. Xiao, X. Meng, C. Monroe, Y. Huang, and J.-M. Zuo, Free Folding of Suspended Graphene Sheets by Random Mechanical Stimulation, Physical Review Letters 104, 166805 (2010) X. Meng, M. Li*, Z. Kang, X. Zhang, J. Xiao*, Mechanics of Self-Folding of Single-layer Graphene, J. Phys. D: Appl. Phys. 46, 055308 (2013)

Mechanics of Graphene Blisters

Graphene membranes are adhered to substrates with patterned microcavities of prescribed volumes. By controlling the gas pressure within the microcavity, the membrane can be made to bulge and delaminate in a stable manner from the substrate. We study the analytical mechanics of this system, which is combined with experimental measurement to determine the elasticity of graphene and the adhesion energy between a substrate and a graphene (or other two-dimensional solid) membrane. A different microcavity configuration with a post in the center was also used, to study the pull-in behavior of graphene, which allows the determination of interfacial forces between two-dimensional nanomaterials and substrates. References: X. Liu, N.G. Boddeti, M.R. Szpunar, L. Wang, M.A. Rodriguez, R. Long, J. Xiao, M.L. Dunn, and J.S. Bunch, Observation of Pull-in Instability in Graphene Membranes under Interfacial Forces, Nano Letters (accepted) N. G. Boddeti, S. P. Koenig, R. Long, J. Xiao, J. S. Bunch, and M. L. Dunn, Mechanics of Pressurized Graphene Blisters, Journal of Applied Mechanics-Transactions of the ASME (accepted)

Alignment Controlled Growth of Carbon Nanotubes on Quartz

Single walled carbon nanotubes (SWNTs) possess extraordinary electrical properties, with many possible applications in electronics.  Dense, horizonally aligned arrays of linearly configured SWNTs represent perhaps the most attractive and scalable way to implement this class of nanomaterial in practical systems.  Recent work shows that templated growth of tubes on certain crystalline substrates yields arrays with the necessary levels of perfection, as demonstrated by the formation of devices and full systems on quartz.  Here, we examine advanced implementations of this process on crystalline quartz substrates with different orientations, to yield strategies for forming diverse, but well-defined horizontal configurations of SWNTs.  Combined experimental and theoretical studies indicate that angle dependent van der Waals interactions can account for nearly all aspects of alignment on quartz with X, Y, Z and ST cuts, as well as quartz with disordered surface layers.  These findings provide important insights into methods for guided growth of SWNTs, and possibly other classes of nanomaterials, for applications in electronics, sensing, photodetection, light emission and other areas. References: J. Xiao, S. Dunham, P. Liu, Y. Zhang, C. Kocabas, L. Moh, Y. Huang, K.-C. Hwang, C. Lu, W. Huang and J. A. Rogers, Alignment controlled growth of single-walled carbon nanotubes on quartz substrates, Nano Letters 9, 4311-4319 (2009).

Collapse and Stability of Carbon Nanotubes

Carbon nanotubes (CNTs) usually possess circular cross sections. As the size increases, van der Waals interaction could cause CNTs to collapse to a dumbbell shape. The collapse and stability of carbon nanotubes (CNTs) have important implications for their synthesis and applications. While nanotube collapse has been observed experimentally, the conditions for the collapse, especially its dependence on tube structures, are not clear. We have studied the energetics of the collapse of single- and multi-wall CNTs via atomistic simulations. The collapse is governed by the number of walls and the radius of the inner-most wall. The collapsed structure is energetically favored about a certain diameter, which is 4.12, 4.96 and 5.76 nm for single-, double- and triple-wall CNTs, respectively. The CNT chirality also has a strong influence on the collapsed structure, leading to flat, warped and twisted CNTs, depending on the chiral angle. References: J. Xiao, B. Liu, Y. Huang, J. Zuo, K.-C. Hwang, and M.-F. Yu, Collapse and Stability of Single- and Multi-wall Carbon Nanotubes. Nanotechnology 18, 395703 (2007).