@article{Jiang2019,

title = {Direct Ink Writing of Poly(tetrafluoroethylene) (PTFE) with Tunable Mechanical Properties},

author = {Zhuoran Jiang and Ozn Erol and Devina Chatterjee and Weinan Xu and Narutoshi Hibino and Lewis H. Romer and Sung Hoon Kang and David H. Gracias},

url = {https://doi.org/10.1021/acsami.9b07279},

doi = {10.1021/acsami.9b07279},

year  = {2019},

date = {2019-07-10},

journal = {ACS Applied Materials and Interfaces},

abstract = {Poly(tetrafluoroethylene) (PTFE) is a unique polymer with highly desirable properties such as resistance to chemical degradation, biocompatibility, hydrophobicity, antistiction, and low friction coefficient. However, due to its high melt viscosity, it is not possible to three-dimensional (3D)-print PTFE structures using nozzle-based extrusion. Here, we report a new and versatile strategy for 3D-printing PTFE structures using direct ink writing (DIW). Our approach is based on a newly formulated PTFE nanoparticle ink and thermal treatment process. The ink was formulated by mixing an aqueous dispersion of surfactant-stabilized PTFE nanoparticles with a binding gum to optimize its shear-thinning properties required for DIW. We developed a multistage thermal treatment to fuse the PTFE nanoparticles, solidify the printed structures, and remove the additives. We have extensively characterized the rheological and mechanical properties and processing parameters of these structures using imaging, mechanical testing, and statistical design of experiments. Importantly, several of the mechanical and structural properties of the final-printed PTFE structures resemble that of compression-molded PTFE, and additionally, the mechanical properties are tunable. We anticipate that this versatile approach facilitates the production of 3D-printed PTFE components using DIW with significant potential applications in engineering and medicine.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{doi:10.1021/acsami.8b17218,

title = {Dual-Gel 4D Printing of Bioinspired Tubes},

author = {Jiayu Liu and Ozan Erol and Aishwarya Pantula and Wangqu Liu and Zhuoran Jiang and Kunihiko Kobayashi and Devina Chatterjee and Narutoshi Hibino and Lewis H. Romer and Sung Hoon Kang and Thao D. Nguyen and David H. Gracias},

url = {https://doi.org/10.1021/acsami.8b17218},

doi = {10.1021/acsami.8b17218},

year  = {2019},

date = {2019-01-29},

journal = {ACS Applied Materials & Interfaces},

abstract = {The distribution of periodic patterns of materials with radial or bilateral symmetry is a universal natural design principle. Among the many biological forms, tubular shapes are a common motif in many organisms, and they are also important for bioimplants and soft robots. However, the simple design principle of strategic placement of 3D printed segments of swelling and nonswelling materials to achieve widely different functionalities is yet to be demonstrated. Here, we report the design, fabrication, and characterization of segmented 3D printed gel tubes composed of an active thermally responsive swelling gel (poly N-isopropylacrylamide) and a passive thermally nonresponsive gel (polyacrylamide). Using finite element simulations and experiments, we report a variety of shape changes including uniaxial elongation, radial expansion, bending, and gripping based on two gels. Actualization and characterization of thermally induced shape changes are of key importance to robotics and biomedical engineering. Our studies present rational approaches to engineer complex parameters with a high level of customization and tunability for additive manufacturing of dynamic gel structures.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Li2019,

title = {Ultrasensitive, flexible, and low-cost nanoporous piezoresistive composite for tactile pressure sensing},

author = {Jing Li and Santiago Orrego and Junjie Pan and Peisheng He and Sung Hoon Kang},

url = {https://pubs.rsc.org/en/content/articlepdf/2014/NR/C8NR09959F?page=search},

doi = {10.1039/C8NR09959F.},

year  = {2019},

date = {2019-01-02},

journal = {Nanoscale},

volume = {11},

pages = {2779-2786},

abstract = {Highly sensitive flexible tactile sensors are of continuing interest for various applications including wearable devices, human- machine interface, and internet of things. Current technologies for high sensitivity piezoresistive sensors rely on costly materials and/or fabrication methods such as graphene-based and micro-structured composites limiting accessibility and scalability. Here, we report a facile sacrificial casting-etching method to synthesize nanoporous carbon nanotube/polymer composites for ultra-sensitive and low-cost piezoresistive pressure sensors. Our synthesis method overcomes the limitations of traditional solution-dip-coating method for adhering nanoscale conductive materials to the nanoscale porous surface. Importantly, we show ultra-high sensitivity with a strain gauge factor over 300, which is ~50 times higher than that of traditional CNT-based piezoresistive sensors and ~10 times higher than most of graphene-based ones. For practical tactile sensing applications, we demonstrate that the sensors can detect both gentle pressures (1 Pa-1 kPa) and low-pressure (1 kPa-25 kPa) with a fraction of cost. Our nanoporous polymer composite could contribute to expanding the scope of using nanocomposites for applications including subtle locomotion sensing, interactive human-machine interface systems, and internet of things from its easy tunability for sensing diverse range tactile signals.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wang2018,

title = {Multifunctional ferrofluid-infused surfaces with reconfigurable multiscale topography},

author = {Wendong Wang and Jaakko V. I. Timonen and Andreas Carlson and Dirk-Michael Drotlef and Cathy T. Zhang and Stefan Kolle and Alison Grinthal and Tak-Sing Wong and Benjamin Hatton and Sung Hoon Kang and Stephen Kennedy and Joshua Chi and Robert Thomas Blough and Metin Sitti and L. Mahadevan and Joanna Aizenberg },

url = {https://www.nature.com/articles/s41586-018-0250-8},

year  = {2018},

date = {2018-06-25},

journal = {Nature},

volume = {559},

pages = {77-82},

abstract = {Developing adaptive materials with geometries that change in response to external stimuli provides fundamental insights into the links between the physical forces involved and the resultant morphologies and creates a foundation for technologically relevant dynamic systems. In particular, reconfigurable surface topography as a means to control interfacial properties has recently been explored using responsive gels, shape-memory polymers, liquid crystals and hybrid composites including magnetically active slippery surfaces. However, these designs exhibit a limited range of topographical changes and thus a restricted scope of function. Here we introduce a hierarchical magneto-responsive composite surface, made by infiltrating a ferrofluid into a microstructured matrix (termed ferrofluid-containing liquid-infused porous surfaces, or FLIPS). We demonstrate various topographical reconfigurations at multiple length scales and a broad range of associated emergent behaviours. An applied magnetic-field gradient induces the movement of magnetic nanoparticles suspended in the ferrofluid, which leads to microscale flow of the ferrofluid first above and then within the microstructured surface. This redistribution changes the initially smooth surface of the ferrofluid (which is immobilized by the porous matrix through capillary forces) into various multiscale hierarchical topographies shaped by the size, arrangement and orientation of the confining microstructures in the magnetic field. We analyse the spatial and temporal dynamics of these reconfigurations theoretically and experimentally as a function of the balance between capillary and magnetic pressures and of the geometric anisotropy of the FLIPS system. Several interesting functions at three different length scales are demonstrated: self-assembly of colloidal particles at the micrometre scale; regulated flow of liquid droplets at the millimetre scale; and switchable adhesion and friction, liquid pumping and removal of biofilms at the centimetre scale. We envision that FLIPS could be used as part of integrated control systems for the manipulation and transport of matter, thermal management, microfluidics and fouling-release materials.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Fang2018,

title = {Piezoelectric polymer thin films with architected cuts},

author = {Lichen Fang and Jing Li and Zeyu Zhu and Santiago Orrego and Sung Hoon Kang},

editor = {Lorenzo Valdevit, Katia Bertoldi, James Guest, Christopher Spadaccini},

url = {https://www.cambridge.org/core/journals/journal-of-materials-research/article/piezoelectric-polymer-thin-films-with-architected-cuts/41109CD493CBADDE85D9446FCE3A95A7},

doi = {10.1557/jmr.2018.6},

year  = {2018},

date = {2018-02-14},

journal = {Journal of Materials Research},

volume = {33},

number = {3},

pages = {330-342},

abstract = {Introducing architected cuts is an attractive and simple approach to tune mechanical behaviors of planar materials like thin films for desirable or enhanced mechanical performance. However, little has been studied on the effects of architected cuts on functional materials like piezoelectric materials. We investigated how architected cut patterns affect mechanical and piezoelectric properties of polyvinylidene fluoride thin films by numerical, experimental, and analytical studies. Our results show that thin films with architected cuts can provide desired mechanical features like enhanced compliance, stretchability, and controllable Poisson’s ratio and resonance frequency, while maintaining piezoelectric performance under static loadings. Moreover, we could observe maximum ∼30% improvement in piezoelectric conversion efficiency under dynamic loadings and harvest energy from low frequency (<100 Hz) mechanical signals or low velocity (<5 m/s) winds, which are commonly existing in ambient environment. Using architected cuts doesn't require changing the material or overall dimensions, making it attractive for applications in self-powered devices with design constraints.},

note = {Invited article on Focus Issue on Architected Materials},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wadhwa16102017,

title = {Motion microscopy for visualizing and quantifying small motions},

author = {Neal Wadhwa and Justin G Chen and Jonathan B Sellon and Donglai Wei and Michael Rubinstein and Roozbeh Ghaffari and Dennis M Freeman and Oral Büyüköztürk and Pai Wang and Sijie Sun and Sung Hoon Kang and Katia Bertoldi and Frédo Durand and William T Freeman},

url = {http://www.pnas.org/content/early/2017/10/11/1703715114.abstract},

doi = {10.1073/pnas.1703715114},

year  = {2017},

date = {2017-10-16},

journal = {Proceedings of the National Academy of Sciences},

volume = {114},

pages = {11639–11644},

abstract = {Although the human visual system is remarkable at perceiving and interpreting motions, it has limited sensitivity, and we cannot see motions that are smaller than some threshold. Although difficult to visualize, tiny motions below this threshold are important and can reveal physical mechanisms, or be precursors to large motions in the case of mechanical failure. Here, we present a “motion microscope,” a computational tool that quantifies tiny motions in videos and then visualizes them by producing a new video in which the motions are made large enough to see. Three scientific visualizations are shown, spanning macroscopic to nanoscopic length scales. They are the resonant vibrations of a bridge demonstrating simultaneous spatial and temporal modal analysis, micrometer vibrations of a metamaterial demonstrating wave propagation through an elastic matrix with embedded resonating units, and nanometer motions of an extracellular tissue found in the inner ear demonstrating a mechanism of frequency separation in hearing. In these instances, the motion microscope uncovers hidden dynamics over a variety of length scales, leading to the discovery of previously unknown phenomena.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{C7NR05163H,

title = {Analytical, numerical, and experimental studies of viscoelastic effects on the performance of soft piezoelectric nanocomposites},

author = {Jing Li and Zhiren Zhu and Lichen Fang and Shu Guo and Ugur Erturun and Zeyu Zhu and James E West and Somnath Ghosh and Sung Hoon Kang},

url = {http://dx.doi.org/10.1039/C7NR05163H},

doi = {10.1039/C7NR05163H},

year  = {2017},

date = {2017-09-15},

journal = {Nanoscale},

volume = {9},

pages = {14215-14228},

publisher = {The Royal Society of Chemistry},

abstract = {Piezoelectric composite (p-NC) made of a polymeric matrix and piezoelectric nanoparticles with conductive additives is an attractive material for many applications. As the matrix of p-NC is made of viscoelastic materials, both elastic and viscous characteristics of the matrix are expected to contribute to the piezoelectric response of p-NC. However, there is limited understanding of how viscoelasticity influences the piezoelectric performance of p-NC. Here we combined analytical and numerical analyses with experimental studies to investigate effects of viscoelasticity on piezoelectric performance of p-NC. The viscoelastic properties of synthesized p-NCs were controlled by changing the ratio between monomer and cross-linker of the polymer matrix. We found good agreement between our analytical models and experimental results for both quasi-static and dynamic loadings. It is found that, under quasi-static loading conditions, the piezoelectric coefficients (d33) of the specimen with the lowest Young's modulus ([similar]0.45 MPa at 5% strain) were [similar]120 pC N-1, while the one with the highest Young's modulus ([similar]1.3 MPa at 5% strain) were [similar]62 pC N-1. The results suggest that softer matrices enhance the energy harvesting performance because they can result in larger deformation for a given load. Moreover, from our theoretical analysis and experiments under dynamic loading conditions, we found the viscous modulus of a matrix is also important for piezoelectric performance. For instance, at 40 Hz and 50 Hz the storage moduli of the softest specimen were [similar]0.625 MPa and [similar]0.485 MPa, while the loss moduli were [similar]0.108 MPa and [similar]0.151 MPa, respectively. As piezocomposites with less viscous loss can transfer mechanical energy to piezoelectric particles more efficiently, the dynamic piezoelectric coefficient (d[prime or minute]33) measured at 40 Hz ([similar]53 pC N-1) was larger than that at 50 Hz ([similar]47 pC N-1) though it has a larger storage modulus. As an application of our findings, we fabricated 3D piezo-shells with different viscoelastic properties and compared the charging time. The results showed a good agreement with the predicted trend that the composition with the smallest elastic and viscous moduli showed the fastest charging rate. Our findings can open new opportunities for optimizing the performance of polymer-based multifunctional materials by harnessing viscoelasticity.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Chen2017,

title = {Simple Triple-State Polymer Actuators with Controllable Folding Characteristics},

author = {Shuyang Chen and Jing Li and Lichen Fang and Zeyu Zhu and Sung Hoon Kang},

url = {http://aip.scitation.org/doi/pdf/10.1063/1.4979560},

doi = {10.1063/1.4979560},

year  = {2017},

date = {2017-03-30},

journal = {Applied Physics Letters},

volume = {110},

pages = {133506},

abstract = {Driven by the interests in self-folding, there have been studies developing artificial self-folding structures at different length scales based on various polymer actuators that can realize dual-state actuation. However, their unidirectional nature limits the applicability of the actuators for a wide range of multi-state self-folding behaviors. In addition, complex fabrication and programming procedures hinder broad applications of existing polymer actuators. Moreover, few of the exiting polymer actuators are able to show the self-folding behaviors with precise control of curvature and force. To address these issues, we report an easy-to-fabricate triple-state actuator with controllable folding behaviors based on bilayer polymer composites with different glass transition temperatures. Initially, the fabricated actuator is in flat state, and it can sequentially self-fold to angled folding states of opposite directions as it is heated up. Based on an analytical model and measured partial recovery behaviors of polymers, we can accurately control the folding characteristics (curvature and force) for rational design. To demonstrate an application of our triple-state actuator, we have developed a self-folding transformer robot which self-folds from a two-dimensional sheet into a three-dimensional boat-like configuration and transforms from the boat shape to a car shape with the increase of the temperature applied to the actuator. Our findings offer a simple approach to generate multiple configurations from a single system by harnessing behaviors of polymers with rational design.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Orrego2017,

title = {Harvesting ambient wind energy with an inverted piezoelectric flag},

author = {Santiago Orrego and Kourosh Shoele and Andre Ruas and Kyle Doran and Brett Caggiano and Rajat Mittal and Sung Hoon Kang},

url = {http://www.sciencedirect.com/science/article/pii/S0306261917302350},

doi = {10.1016/j.apenergy.2017.03.016},

year  = {2017},

date = {2017-03-19},

journal = {Applied Energy},

volume = {194},

pages = {212-222},

abstract = {The paper describes an experimental study of wind energy harvesting by self-sustained oscillations (flutter) of a flexible piezoelectric membrane fixed in a novel orientation called the “inverted flag”. We conducted parametric studies to evaluate the influence of geometrical parameters of the flag on the flapping behavior and the resulting energy output. We have demonstrated the capability for inducing aero-elastic flutter in a desired wind velocity range by simply tuning the geometrical parameters of the flag. A peak electrical power of ∼5.0 mW/cm3 occurred at a wind velocity of 9 m/s. Our devices showed sustained power generation (∼0.4 mW/cm3) even in low-wind speed regimes (∼3.5 m/s) suitable for ambient wind energy harvesting. We also conducted outdoor experiments and harvested ambient wind energy to power a temperature sensor without employing a battery for energy storage. Moreover, a self-aligning mechanism to compensate for changing wind directions was incorporated and resulted in an increase in the temperature sensor data output by more than 20 times. These findings open new opportunities for self-powered devices using ambient wind energy with fluctuating conditions and low speed regimes.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wang2017,

title = {Harnessing Geometric Frustration to Form Band Gaps in Acoustic Channel Lattices},

author = {Pai Wang and Yue Zheng and Matheus C. Fernandes and Yushen Sun and Kai Xu and Sijie Sun and Sung Hoon Kang and Vincent Tournat and Katia Bertoldi},

url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.084302},

doi = {10.1103/PhysRevLett.118.084302},

year  = {2017},

date = {2017-02-21},

journal = {Physical Review Letters},

volume = {118},

pages = {084302},

abstract = {We demonstrate both numerically and experimentally that geometric frustration in two-dimensional periodic acoustic networks consisting of arrays of narrow air channels can be harnessed to form band gaps (ranges of frequency in which the waves cannot propagate in any direction through the system). While resonant standing wave modes and interferences are ubiquitous in all the analyzed network geometries, we show that they give rise to band gaps only in the geometrically frustrated ones (i.e., those comprising of triangles and pentagons). Our results not only reveal a new mechanism based on geometric frustration to suppress the propagation of pressure waves in specific frequency ranges but also open avenues for the design of a new generation of smart systems that control and manipulate sound and vibrations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Zárate2016,

title = {Elastic metamaterials for tuning circular polarization of electromagnetic waves},

author = {Yair Zárate and Sahab Babaee and Sung H. Kang and Dragomir N. Neshev and Ilya V. Shadrivov and Katia Bertoldi and David A. Powell},

url = {http://www.nature.com/articles/srep28273},

doi = {doi:10.1038/srep28273},

year  = {2016},

date = {2016-06-20},

journal = {Scientific Reports},

volume = {6},

pages = {28273},

abstract = {Electromagnetic resonators are integrated with advanced elastic material to develop a new type of tunable metamaterial. An electromagnetic-elastic metamaterial able to switch on and off its electromagnetic chiral response is experimentally demonstrated. Such tunability is attained by harnessing the unique buckling properties of auxetic elastic materials (buckliballs) with embedded electromagnetic resonators. In these structures, simple uniaxial compression results in a complex but controlled pattern of deformation, resulting in a shift of its electromagnetic resonance, and in the structure transforming to a chiral state. The concept can be extended to the tuning of three-dimensional materials constructed from the meta-molecules, since all the components twist and deform into the same chiral configuration when compressed.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Liu2016,

title = {Harnessing Buckling to Design Architected Materials that  Exhibit Effective Negative Swelling},

author = {Jia Liu and Tianyu Gu and Sicong Shan and Sung H. Kang and James C. Weaver and Katia Bertoldi},

url = {http://onlinelibrary.wiley.com/doi/10.1002/adma.201600812/pdf},

doi = {10.1002/adma.201600812},

year  = {2016},

date = {2016-05-17},

journal = {Advanced Materials},

volume = {28},

pages = {6619–6624},

abstract = {Inspired by the need to develop materials capable of targeted and extreme volume changes during operation, numerical simulations and experiments are combined to design a new class of soft architected materials that achieve a reduction of projected surface area coverage during swelling. },

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kang2016,

title = {Patterning via self-organization and self-folding: Beyond conventional lithography },

author = {Sung Hoon Kang and Michael D. Dickey, Guest Editors},

editor = {Sung Hoon Kang and Michael D. Dickey},

url = {http://journals.cambridge.org/download.php?file=%2FMRS%2FMRS41_02%2FS0883769416000038a.pdf&code=ffd6509119daa4a591802b67ab63f032},

doi = {10.1557/mrs.2016.3 },

year  = {2016},

date = {2016-02-01},

journal = {MRS Bulletin},

volume = {41},

number = {2},

pages = {93-96},

abstract = {Conventional photolithography is an effective patterning technique that has enabled modern 

electronics and advanced micro- and nanoscale devices. However, it has limitations, including 

high cost, limited resolution, and poor compatibility with unconventional materials that may be 

soft, nonplanar, or difficult to process. There is active research ongoing to develop unconventional 

patterning methods such as self-organization and self-folding. Self-organization harnesses 

various driving forces to produce patterns without external intervention and includes 

methods such as self-assembly of block copolymers, capillary-driven assembly of micro-/

nanoscale structures, and thin-fi lm instabilities. Self-folding (origami)—and its cousin, 

kirigami—harnesses patterning and materials strategies to convert planar substrates into 

three-dimensional shapes in response to external stimuli. These multidisciplinary approaches 

open many engineering opportunities by providing new and versatile material functionalities. 

This article overviews the field and the topics covered in the articles in this issue of  MRS Bulletin, highlighting recent progress in patterning approaches based on self-organization and self-folding. },

note = {co-Guest Editor of the issue},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{ADMA:ADMA201501708,

title = {Multistable Architected Materials for Trapping Elastic Strain Energy},

author = {Sicong Shan and Sung H. Kang and Jordan R. Raney and Pai Wang and Lichen Fang and Francisco Candido and Jennifer A. Lewis and Katia Bertoldi},

url = {http://dx.doi.org/10.1002/adma.201501708},

issn = {1521-4095},

year  = {2015},

date = {2015-06-18},

journal = {Advanced Materials},

volume = {27},

pages = {4296–4301},

note = {SS, SHK, JRR: equal contribution},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Shan2015,

title = {Design of planar isotropic negative Poisson’s ratio structures},

author = {Sicong Shan and Sung H. Kang and Zhenhao Zhao and Lichen Fang and Katia Bertoldi},

url = {http://www.sciencedirect.com/science/article/pii/S2352431615000759},

doi = {10.1016/j.eml.2015.05.002},

issn = {2352-4316},

year  = {2015},

date = {2015-05-21},

journal = {Extreme Mechanics Letters},

volume = {4},

pages = {96–102},

abstract = {Most of the auxetic materials that have been characterized experimentally or studied analytically are anisotropic and this limits their possible applications, as they need to be carefully oriented during operation. Here, through a combined numerical and experimental approach, we demonstrate that 2D auxetic materials with isotropic response can be easily realized by perforating a sheet with elongated cuts arranged to form a periodic pattern with either six-fold or three-fold symmetry. Moreover, we also show that the auxetic behavior can be easily tuned by varying the length of the cuts and that it is retained even under large levels of applied deformation beyond the limit of small strains. This novel, simple and scalable design can serve as an important guideline for designing and fabricating isotropic auxetic materials that can have a significant impact on a wide range of applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Wang2015,

title = {Locally resonant band gaps in periodic beam lattices by tuning connectivity},

author = {Pai Wang and Filippo Casadei and Sung Hoon Kang and Katia Bertoldi},

url = {http://journals.aps.org/prb/pdf/10.1103/PhysRevB.91.020103},

year  = {2015},

date = {2015-01-26},

journal = {Physical Review B (Rapid Communications)},

volume = {91},

pages = {020103},

abstract = {Lattice structures have long fascinated physicists and engineers not only because of their outstanding functionalities, but also for their ability to control the propagation of elastic waves. While the study of the relation between the connectivity of these systems and their static properties has a long history that goes back to Maxwell, rules that connect the dynamic response to the network topology have not been established. Here, we demonstrate that by tuning the average connectivity of a beam network (z ̄), locally resonant band gaps can be generated in the structures without embedding additional resonating units. In particular, a critical threshold for z ̄ is identified, far from which the band gap size is purely dictated by the global lattice topology. By contrast, near this critical value, the detailed local geometry of the lattice also has strong effects. Moreover, in stark contrast to the static case, we find that the nature of the joints is irrelevant to the dynamic response of the lattices. Our results not only shed new light on the rich dynamic properties of periodic lattices, but also outline a new strategy to manipulate mechanical waves in elastic systems.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Shan2014,

title = {Harnessing Multiple Folding Mechanisms in Soft Periodic and Porous Structures to Design Highly Tunable Phononic Crystals},

author = {Sicong Shan and Sung Hoon Kang and Pai Wang and Cangyu Qu and Samuel Shian and Elizabeth R. Chen and James C. Weaver and Katia Bertoldi},

url = {http://onlinelibrary.wiley.com/doi/10.1002/adfm.201400665/abstract},

year  = {2014},

date = {2014-08-20},

journal = {Advanced Functional Materials},

volume = {24},

pages = {4935–4942},

abstract = {Mechanical instabilities in periodic porous elastic structures may lead to the formation of homogeneous patterns, opening avenues for a wide range of applications that are related to the geometry of the system. This study focuses on an elastomeric porous structure comprising a triangular array of circular holes, and shows that by controlling the loading direction, multiple pattern transformations can be induced by buckling. Interestingly, these different pattern transformations can be exploited to design materials with highly tunable properties. In particular, these results indicate that they can be effectively used to tune the propagation of elastic waves in phononic crystals, enhancing the tunability of the dynamic response of the system. Using a combination of finite element simulations and experiments, a proof-of-concept of the novel material is demonstrated. Since the proposed mechanism is induced by elastic instability, it is reversible, repeatable, and scale-independent, opening avenues for the design of highly tunable materials and devices over a wide range of length scales.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kang2014,

title = {Complex Ordered Patterns in Mechanical Instability Induced Geometrically Frustrated Triangular Cellular Structures},

author = {Sung Hoon Kang and Sicong Shan and Andrej Košmrlj and Wim L. Noorduin and Samuel Shian and James C. Weaver and David R. Clarke and Katia Bertoldi},

url = {http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.098701},

year  = {2014},

date = {2014-03-05},

journal = {Physical Review Letters},

volume = {112},

pages = {098701},

abstract = {Geometrical frustration arises when a local order cannot propagate throughout the space because of geometrical constraints. This phenomenon plays a major role in many systems leading to disordered ground-state configurations. Here, we report a theoretical and experimental study on the behavior of buckling-induced geometrically frustrated triangular cellular structures. To our surprise, we find that buckling induces complex ordered patterns which can be tuned by controlling the porosity of the structures. Our analysis reveals that the connected geometry of the cellular structure plays a crucial role in the generation of ordered states in this frustrated system.},

note = {Selected as Physical Review Letters Editors’ Suggestion and Highlighted in Physics Synopsis.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Shim2013,

title = {Harnessing Instabilities for Design of Soft Reconfigurable Auxetic/Chiral Materials},

author = {Jongmin Shim and Sicong Shan and Andrej Kosmrlj and Sung Hoon Kang and Elizabeth R. Chen and James C. Weaver and Katia Bertoldi},

url = {http://pubs.rsc.org/en/content/articlelanding/2013/sm/c3sm51148k#!divAbstract},

year  = {2013},

date = {2013-05-31},

journal = {Soft Matter},

volume = {9},

pages = {8198-8202},

abstract = {Most materials have a unique form optimized for a specific property and function. However, the ability to reconfigure material structures depending on stimuli opens exciting opportunities. Although mechanical instabilities have been traditionally viewed as a failure mode, here we exploit them to design a class of 2D soft materials whose architecture can be dramatically changed in response to an external stimulus. By considering geometric constraints on the tessellations of the 2D Euclidean plane, we have identified four possible periodic distributions of uniform circular holes where mechanical instability can be exploited to reversibly switch between expanded (i.e. with circular holes) and compact (i.e. with elongated, almost closed elliptical holes) periodic configurations. Interestingly, in all these structures buckling is found to induce large negative values of incremental Poisson's ratio and in two of them also the formation of chiral patterns. Using a combination of finite element simulations and experiments at the centimeter scale we demonstrate a proof-of-concept of the proposed materials. Since the proposed mechanism for reconfigurable materials is induced by elastic instability, it is reversible, repeatable and scale-independent.},

note = {Highlighted on the Soft Matter blog.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lipomi2010b,

title = {Fabrication and Replication of Arrays of Single- or Multi-Component Nanostructures by Replica Molding and Mechanical  Sectioning},

author = {Darren J. Lipomi and Mikhail A. Kats and Philseok Kim and Sung Hoon Kang and Joanna Aizenberg and Federico Capasso and George M. Whitesides},

url = {http://pubs.acs.org/doi/abs/10.1021/nn100993t},

year  = {2010},

date = {2010-06-08},

journal = {ACS Nano},

volume = {4},

pages = {4017–4026},

note = {Featured on the cover and highlighted in the issue.},

keywords = {Fabrication, Multi-Component, Nanoskiving, Nanostructure, Replication},

pubstate = {published},

tppubtype = {article}

}

@article{Pokroy2009,

title = {Self-Organization of a Mesoscale Bristle into Ordered, Hierarchical Helical Assemblies},

author = {Boaz Pokroy and Sung Hoon Kang and L. Mahadevan and Joanna Aizenberg

},

url = {http://www.sciencemag.org/content/323/5911/237.short},

year  = {2009},

date = {2009-01-09},

journal = {Science},

volume = {323},

pages = {237-240},

abstract = {Mesoscale hierarchical helical structures with diverse functions are abundant in nature. Here we show how spontaneous helicity can be induced in a synthetic polymeric nanobristle assembling in an evaporating liquid. We use a simple theoretical model to characterize the geometry, stiffness, and surface properties of the pillars that favor the adhesive self-organization of bundles with pillars wound around each other. The process can be controlled to yield highly ordered helical clusters with a unique structural hierarchy that arises from the sequential assembly of self-similar coiled building blocks over multiple length scales. We demonstrate their function in the context of self-assembly into previously unseen structures with uniform, periodic patterns and controlled handedness and as an efficient particle-trapping and adhesive system. },

note = {Highlighted in the issue, and various media including New York Times, NPR, Discovery, AAAS EurekAlert, C&EN, Technology Review, IEEE Spectrum, Science Daily, and New Scientist. },

keywords = {bio-inspired science and engineering, Helical, Hierarchical, Mesoscale, Nanopillar, Order, Self-Organization},

pubstate = {published},

tppubtype = {article}

}

@article{Mandal2008,

title = {III–VI Chalcogenide Semiconductor Crystals for Broadband Tunable THz Sources and Sensors},

author = {Krishna C. Mandal and Sung Hoon Kang and Michael Choi and Jian Chen and Xi-Cheng Zhang and James M. Schleicher and Charles A. Schmuttenmaer and Nils C. Fernelius},

url = {http://ieeexplore.ieee.org/xpl/articleDetails.jsp?reload=true&arnumber=4481122},

year  = {2008},

date = {2008-04-04},

journal = {IEEE Journal of Selected Topics in Quantum Electronics},

volume = {14},

pages = {284 - 288},

abstract = {The layered chalcogenide semiconductor GaSe has been grown under various crystal growth conditions for optimum performance for tunable terahertz (THz) wave generation and broadband THz detection. Low-temperature photoluminescence (PL), Raman spectroscopy, optical absorption/transmission, electrical charge transport property measurements, and THz time-domain spectroscopy (TDS) have been used to characterize the grown crystals. It is observed that indium doping enhances hardness of the grown GaSe crystals, which is very useful for processing and fabricating large-area devices. GaSe crystals have demonstrated promising characteristics with good optical quality (absorption coefficient les0.1 cm-1 in the spectral range of 0.62-18 mum), high dark resistivity (ges109 Omega cm), wide bandgap (2.01 eV at 300 K), good anisotropic (parand perp) electrical transport properties (mue/h, taue/h, and mutaue/h) and long-term stability. The THz emission measurements have shown that the GaSe crystals are highly efficient for broadband tunable THz sources (up to 40 THz), and sensors (up to 100 THz). Additionally, new THz frequencies (0.1-3 THz) have been observed for the first time from an anisotropic binary and a ternary semiconductor crystal. Details of characterizations as well as optimum crystal growth conditions including simulation and computer modeling are described in this paper.},

keywords = {Broadband, Chalcogenide, Crystal, III-VI, Semiconductor, Sensor, Source, THz, Tunable},

pubstate = {published},

tppubtype = {article}

}

@article{Mandal2007,

title = {Characterization of Low-Defect Cd0.9Zn0.1Te and CdTe Crystals for High-Performance Frisch Collar Detectors},

author = {Krishna C. Mandal and Sung Hoon Kang and Michael Choi and Alireza Kargar and Mark J. Harrison and Douglas S. McGregor and Aleksey E. Bolotnikov and Gabriella A. Carini and Giuseppe C. Camarda, and Ralph B. James},

url = {http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4291754&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D4291754},

year  = {2007},

date = {2007-08-20},

journal = {IEEE Transactions on Nuclear Science},

volume = {54},

pages = {802 - 806 },

abstract = {Low dislocation density, high-purity, and low inclusion concentration Cd0.9Zn0.1Te (CZT) and CdTe crystals were grown by a vertical Bridgman technique using in-house zone refined precursors. The grown crystals were sequentially processed using optimized chemo-mechanical processes to fabricate planar and Frisch collar detectors. Infrared transmission and scanning electron microscopy studies have shown that EIC grown CZT and CdTe crystals have significantly lower Te inclusions and defect densities than commercially available spectrometer grade crystals. The charge transport properties (electron and hole mobility-lifetime products, mutaue & mutauh) of various detectors have been evaluated by Hecht analysis. The detectors have been tested for spectral response using 59.5 and 662 keV gamma-ray sources. The CZT detectors with planar electrodes showed 2.6% FWHM at 662 keV. By adding a Frisch collar, the detectors' spectra improved significantly. The Frisch collar detectors proved to be very promising for assembling large-area arrays with excellent energy resolution at relatively low manufacturing cost.},

keywords = {CdTe, Characterization, Crystal, CZT, Detector},

pubstate = {published},

tppubtype = {article}

}

@article{Chen2007,

title = {High Definition Digital Fabrication of Active Organic Devices by Molecular Jet Printing},

author = { Jianglong Chen and Valerie Leblanc and Sung Hoon Kang and Paul  J. Benning and David Schut and Marc  A. Baldo and Martin  A. Schmidt and Vladimir Bulović},

url = {http://onlinelibrary.wiley.com/doi/10.1002/adfm.200601144/abstract},

year  = {2007},

date = {2007-08-17},

journal = {Advanced Functional Materials},

volume = {17},

pages = {2722–2727},

abstract = {We introduce a high resolution molecular jet (MoJet) printing technique for vacuum deposition of evaporated thin films and apply it to fabrication of 30 μm pixelated (800 ppi) molecular organic light emitting devices (OLEDs) based on aluminum tris(8-hydroxyquinoline) (Alq3) and fabrication of narrow channel (15 μm) organic field effect transistors (OFETs) with pentacene channel and silver contacts. Patterned printing of both organic and metal films is demonstrated, with the operating properties of MoJet-printed OLEDs and OFETs shown to be comparable to the performance of devices fabricated by conventional evaporative deposition through a metal stencil. We show that the MoJet printing technique is reconfigurable for digital fabrication of arbitrary patterns with multiple material sets and high print accuracy (of better than 5 μm), and scalable to fabrication on large area substrates. Analogous to the concept of “drop-on-demand” in Inkjet printing technology, MoJet printing is a “flux-on-demand” process and we show it capable of fabricating multi-layer stacked film structures, as needed for engineered organic devices.},

keywords = {Active, Device, Digital, Fabrication, High Definition, Molecular Jet, Organic, Printing},

pubstate = {published},

tppubtype = {article}

}

@article{Mandal2007b,

title = {Component Overpressure Growth and Characterization of High-Resistivity CdTe Crystals for Radiation Detectors},

author = {Krishna C. Mandal and Sung Hoon Kang and Michael Choi and Jiuan Wei and Lili Zheng and Hui Zhang and Gerald E. Jellison and Michael Groza and Arnold Burger },

url = {http://link.springer.com/article/10.1007%2Fs11664-007-0164-y},

year  = {2007},

date = {2007-07-06},

journal = {Journal of Electronic Materials},

volume = {36},

pages = {1013-1020},

abstract = {Spectrometer-grade CdTe single crystals with resistivities higher than 109 Ω cm have been grown by the modified Bridgman method using zone-refined precursor materials (Cd and Te) under a Cd overpressure. The grown CdTe crystals had good charge-transport properties (μτ e = 2 × 10−3 cm2 V−1, μτ h = 8 × 10−5 cm2 V−1) and significantly reduced Te precipitates compared with crystals grown without Cd overpressure. The crystal growth conditions for the Bridgman system were optimized by computer modeling and simulation, using modified MASTRAPP program, and applied to crystal diameters of 14 mm (0.55′′), 38 mm (1.5′′), and 76 mm (3′′). Details of the CdTe crystal growth operation, structural, electrical, and optical characterization measurements, detector fabrication, and testing using 241Am (60 keV) and 137Cs (662 keV) sources are presented.},

keywords = {CdTe, Characterization, Crystal, Detector, Growth},

pubstate = {published},

tppubtype = {article}

}

@article{Leblanc2007,

title = {Micromachined Printheads for the Evaporative Patterning of Organic Materials and Metals },

author = {Valerie Leblanc and Jianglong Chen and Sung Hoon Kang and Vladimir Bulovic and Martin A. Schmidt},

url = {http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4147590&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F84%2F4147571%2F04147590.pdf%3Farnumber%3D4147590},

year  = {2007},

date = {2007-04-10},

journal = {Journal of Microelectromechanical Systems},

volume = {16},

pages = {394 - 400 },

abstract = {This paper describes the design, fabrication, and testing of electrostatically actuated microshutters used as active shadow masks to pattern evaporated materials. The fabricated microshutters can obstruct a 25-mum-wide aperture at an actuation voltage of 90 V, with a resonant frequency of 4 kHz due to a 400-mum-long actuator. The microshutters integrated with an x-y-z manipulator were used to print patterns of organic material and metal on glass substrates in vacuum with a pixel size of 25 mum. The maximum resolution achievable with this setup is 800 dpi, and we printed active organic light-emitting device arrays of 400 dpi resolution. This printing scheme could enable the patterning of large-area organic optoelectronic devices on diverse substrates.},

keywords = {Evaporation, Fabrication, Metal, Organic, Patterning, Printing},

pubstate = {published},

tppubtype = {article}

}

@article{Mandal2006,

title = {Simulation, modeling, and crystal growth of Cd0.9Zn0.1Te for nuclear spectrometers},

author = {Krishna C. Mandal and Sung Hoon Kang and Michael Choi and Job Bello and Lili Zheng and Hui Zhang and Michael Groza and Utpal N. Roy and Arnold Burger and Gerald E. Jellison and David E. Holcomb and Gomez W. Wright and Joseph A. Williams },

url = {http://link.springer.com/article/10.1007%2Fs11664-006-0250-6},

year  = {2006},

date = {2006-06-01},

journal = {Journal of Electronic Materials},

volume = {35},

pages = {1251-1256},

abstract = {High-quality, large (10 cm long and 2.5 cm diameter), nuclear spectrometer grade Cd0.9Zn0.1Te (CZT) single crystals have been grown by a controlled vertical Bridgman technique using in-house zone refined precursor materials (Cd, Zn, and Te). A state-of-the-art computer model, multizone adaptive scheme for transport and phase-change processes (MASTRAP), is used to model heat and mass transfer in the Bridgman growth system and to predict the stress distribution in the as-grown CZT crystal and optimize the thermal profile. The model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. The grown semi-insulating (SI) CZT crystals have demonstrated promising results for high-resolution room-temperature radiation detectors due to their high dark resistivity (ρ≈2.8 × 1011 Θ cm), good charge-transport properties [electron and hole mobility-life-time product, μτe≈(2–5)×10−3 and μτh≈(3–5)×10−5 respectively, and low cost of production. Spectroscopic ellipsometry and optical transmission measurements were carried out on the grown CZT crystals using two-modulator generalized ellipsometry (2-MGE). The refractive index n and extinction coefficient k were determined by mathematically eliminating the ∼3-nm surface roughness layer. Nuclear detection measurements on the single-element CZT detectors with 241Am and 137Cs clearly detected 59.6 and 662 keV energies with energy resolution (FWHM) of 2.4 keV (4.0%) and 9.2 keV (1.4%), respectively.},

keywords = {Crystal, CZT, Growth, Modeling, Simulation, spectrometer},

pubstate = {published},

tppubtype = {article}

}

@article{Kang2004,

title = {Memory Effect from Charge Trapping in Layered Organic Structures},

author = {Sung Hoon Kang and Todd Crisp and Ioannis Kymissis and Vladimir Bulović},

url = {http://scitation.aip.org/content/aip/journal/apl/85/20/10.1063/1.1819991},

year  = {2004},

date = {2004-11-15},

journal = {Applied Physics Letters},

volume = {85},

pages = {4666-4668},

abstract = {We demonstrate organic light emitting devices(OLEDs) with a charge trap layer that show memory behavior. These OLEDs demonstrate that organic heterojunction structures can controllably trap and release electronic charges. The trap layer is either 5-nm-thick clustered silver islands, or a 10-nm-thick organic laser dye DCM2 ([2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[i,j]quinolizin-9-yl)-ethenyl]-4H-pyran-4-ylidene] propane-dinitrile) doped into TPD (N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine). Predictions of the energy band structure indicate that both DCM2 sites and the metal islands can trap charge, consistent with the measured current–voltage characteristics. Trap sites are charged by applying reverse bias over the OLEDs. For devices with DCM2 traps we observe quenching of DCM2 photoluminescence upon charging, which allows us to quantify the charged trap density as approximately 10% of the trap sites or 10^18cm^−3. From time resolved measurements we observe that the charge retention time exceeds 2h.},

keywords = {Charge Trap, Device, Layer, Memory, Organic},

pubstate = {published},

tppubtype = {article}

}