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Transition metal perovskite chalcogenides (TMPCs) are explored as stable, environmentally friendly semiconductors for solar energy conversion. They can be viewed as the inorganic alternatives to hybrid halide perovskites, and chalcogenide counterparts of perovskite oxides with desirable optoelectronic properties in the visible and infrared part of the electromagnetic spectrum. Past theoretical studies have predicted large absorption coefficient, desirable defect characteristics, and bulk photovoltaic effect in TMPCs. Despite recent progresses in polycrystalline synthesis and measurements of their optical properties, it is necessary to grow these materials in high crystalline quality to develop a fundamental understanding of their optical properties and evaluate their suitability for photovoltaic application. Here, we report the growth of single crystals of a two-dimensional (2D) perovskite chalcogenide, Ba3Zr2S7, with a natural superlattice-like structure of alternating double-layer perovskite blocks and single-layer rock salt structure. The material demonstrated a bright photoluminescence peak at 1.28 eV with a large external luminescence efficiency of up to 0.15%. We performed time-resolved photoluminescence spectroscopy on these crystals and obtained an effective recombination time of ~65 ns. These results clearly show that 2D Ruddlesden-Popper phases of perovskite chalcogenides are promising materials to achieve single-junction solar cells., 4 Figures

Films of the ferromagnetic alloy Co-Fe-B in general perform very nicely at room temperature and have been widely used, but not in magnetoelectric devices or microwave applications, because of unfavorable magnetostriction, Gilbert damping, and thermal tolerance. Here the authors instead use carbon-doped Co${}_{0.5}$Fe${}_{0.5}$ to create a Co-Fe-C film with high magnetostriction, low Gilbert damping, and high thermal stability. This work provides insight into the basic aspects of Co-Fe-C as a promising candidate for use in voltage-tunable magnetoelectric devices and microwave devices.

Parametric resonance is a complex phenomenon that touches many aspects of scientific and technical society, but is still not well understood because of the intensive calculations required to describe the behavior. Thus the importance of developing simple mathematical approaches to describe parametric resonance cannot be overstated. Here a consistent theory of the parametric resonance of a harmonic oscillator under any periodic frequency modulation is constructed. Using a Hamiltonian approach and resonance approximation, simple equations were derived and critical amplitudes for all parametric resonance orders were obtained for any periodic modulation function. The theory agrees with the well-known result for the main resonance and gives correct power dependence on damping. In addition, the theory qualitatively predicts behavior at large modulations. This simplified approach revealed unique features-"safety windows" at large modulation amplitudes where parametric resonance does not occur which were then qualitatively confirmed with numerical simulations. The Hamiltonian approach should serve as a framework that a greater understanding of parametric resonance at large amplitudes and higher orders can be built upon.

The interactions between electric and magnetic subsystems in a ferroelectric-ferromagnetic composite occur through mechanical forces. Here we discuss results of a systematic investigation on the strength of the magnetic response of the composite to an applied electric field, known as the converse magnetoelectric (CME) effect, and its dependence on the ferroic order parameters and volume fraction for the two phases. Studies were carried out on composites of lead zirconate titanate and 2-30-mu m-thick nickel zinc ferrite (NZFO) films grown by liquid phase epitaxy on lattice matched (100) and (111) MgO substrates. Ferromagnetic resonance was utilized to determine the strength of CME from data on electric field E induced shift in the resonance frequency and its dependence on ferrite film orientation and thickness as well as MgO substrate thickness. The CME coupling coefficient A was found to be a factor of 2 to 4 higher in samples with NZFO films with (100) orientation than for (111) films. A decrease in A was measured with increasing ferrite film thickness and a very significant enhancement in the strength of CME was measured for decreasing MgO thickness. A model for CME that takes into consideration the influence of nonferroic MgO substrate was developed, and estimated A values are in very good agreement with the data. The results presented here are also of importance for a new class of electric field tunable ferrite microwave devices. Public domain – authored by a U.S. government employee

Holographic reflection gratings formed by photopolymerization of low concentrations of liquid crystal monomers in nematic liquids demonstrate greater than 150nm of red tuning when a direct current (DC) field is applied across standard liquid crystal cell. The band position is restored when the field is removed. Diffraction efficiencies are both polarization and temperature polarization dependent. Samples are prepared using various LC mixtures, formulated from 2wt% photoinitiator, 5~20 wt% liquid crystal monomers and positive dielectric anisotropy liquid crystals. The basic tuning mechanism will be discussed, along with electric field, temperature effects on the optical performance.

Recently, we have prepared holographic reflection gratings in polymer stabilized cholesteric liquid crystals (H-PSCLCs) prepared by a 365 nm UV laser using a single prism. The H-PSCLC samples are prepared using various CLC mixtures, formulated from 2wt% photoinitiator, 5~20 wt% liquid crystal monomers, chiral dopants, and positive or negative dielectric anisotropy liquid crystals. Higher order reflection peaks, such as the second and third order reflections, are observed from the H-PSCLC samples without application of the electric field. Spectral position and bandwidth of the higher order diffraction peaks in the H-PSCLCs are tuned in blue, green, and red colors by the application of the direct current (DC) field. Tunablity of the higher order reflection band in H-PSCLCs will be discussed.

Perovskite chalcogenides are gaining substantial interest as an emerging class of semiconductors for optoelectronic applications. High-quality samples are of vital importance to examine their inherent physical properties. We report the successful crystal growth of the model system, BaZrS3 and its Ruddlesden–Popper phase Ba3Zr2S7 by a flux method. X-ray diffraction analyses showed the space group of Pnma with lattice constants of a = 7.056(3) Å, b = 9.962(4) Å, and c = 6.996(3) Å for BaZrS3 and P42/mnm with a = 7.071(2) Å, b = 7.071(2) Å, and c = 25.418(5) Å for Ba3Zr2S7. Rocking curves with full width at half maximum of 0.011° for BaZrS3 and 0.027° for Ba3Zr2S7 were observed. Pole figure analysis, scanning transmission electron microscopy images, and electron diffraction patterns also establish the high quality of the grown crystals. The octahedral tilting in the corner-sharing octahedral network is analyzed by extracting the torsion angles., Air Force Office of Scientific Research (Award FA9550-16-1-0335), Army Research Office (Award W911NF-19-1- 0137)

Diffractive waveplates (DWs) are highly efficient optical components realized by means of a polarization holography setup that makes use of UV/blue laser sources. It is more convenient to perform the holographic recording process with green lasers (e.g., continuous wave operating at 532 nm) because they offer compactness, efficiency, and high power. Unfortunately, the photo-alignment materials used for DW fabrication exhibit limited sensitivity at 532 nm. Herein, we report the realization of cycloidal diffractive waveplates (CDWs) by employing a polarization holographic setup using a high-power, diode-pumped solid-state laser operating at 532 nm. The enabling factor is the use of a photo-alignment material with a broad absorption band extending into the visible part of the spectrum. Samples are characterized in terms of morphological and optical properties. They exhibit a well-defined morphology along with very high diffraction efficiency (≈97%). The possibility to realize CDWs with larger apertures is underway, enabling the realization of a new generation of optical components for integration in modern applications.

Next-generation nanoelectronics based on 2D materials ideally will require reliable, flexible, transparent, and versatile dielectrics for transistor gate barriers, environmental passivation layers, capacitor spacers, and other device elements. Ultrathin amorphous boron nitride of thicknesses from 2 to 17 nm, described in this work, may offer these attributes, as the material is demonstrated to be universal in structure and stoichiometric chemistry on numerous substrates including flexible polydimethylsiloxane, amorphous silicon dioxide, crystalline Al2O3, other 2D materials including graphene, 2D MoS2, and conducting metals and metal foils. The versatile, large area pulsed laser deposition growth technique is performed at temperatures less than 200 °C and without modifying processing conditions, allowing for seamless integration into 2D device architectures. A device-scale dielectric constant of 5.9 ± 0.65 at 1 kHz, breakdown voltage of 9.8 ± 1.0 MV cm−1, and bandgap of 4.5 eV were measured for various thicknesses of the ultrathin a-BN material, representing values higher than previously reported chemical vapor deposited h-BN and nearing single crystal h-BN.

The essential step for epitaxial growth of tetragonal Mn3Ge films with high perpendicular magnetic anisotropy (PMA) is to choose suitable substrates with small lattice misfit. The exploration process has involved large efforts on depositing films on single crystalline substrates using buffer layers preferably formed from Cr or Pt, but they lacked a systematically comparative investigation for practical applications. This study investigates the structural, surface and magnetic properties of ultrathin Fe (2 nm) seed layer to induce homoepitaxial Mn3Ge films on MgO (0 0 1) substrates compared with that of the heteroepitaxial Mn3Ge films on three typical buffer layers, such as Cr (40 nm), Cr (20 nm)/Pt (10 nm), Fe (2 nm)/Pt (20 nm). Furthermore, a correlation between film strain and film quality has been established, which is critical for spintronics applications. More importantly, we attribute the homoepitaxial growth of Mn3Ge films on the ultrathin Fe seed layers to the Fe diffusion and formation of Fe-Mn-Ge alloy at the interface, and confirm this supposition with HAADF-STEM characterizations. The Fe-doped Mn3Ge interlayer can act as the gradual buffer layer, and lead to a high-quality crystal structure and extremely high magnetic squareness ratio of Mn3Ge films in a large range of thickness (100 ~ 400 nm). This result offers a new concept of high-quality growth of D022-Mn3Ge films, which may enhance the prospect for tetragonal Mn3Ge thin films in superior spintronics applications.

We develop a theory describing two-beam energy exchange in a cholesteric liquid crystal (CLC) stabilized in the planar state by a helicoidal polymer network. The CLC layer is placed between photore...

Diffractive waveplates (DWs) generally diffract light in different directions for the two circular polarizations of light, and the diffraction efficiency of such devices generally falls when the angle through which light is diffracted becomes large. It would be desirable to be able to make fast (low f-number), high-efficiency lenses with DW type structures. We study the dependence of the efficiency of DWs and polarization volume gratings (PVGs) on the polarization and angle of incidence of light, and identify structures with polarization independent function — the diffraction efficiency and angle of diffraction. The results are obtained for DWs and PVGs periodic in one linear coordinate, however, they are applicable to other optical devices, including lenses based on these structures.

Acoustic wavelengths are about 5 orders of magnitude shorter than EM waves of the same frequency. This can be a critical advantage in the quest to further miniaturize devices that have dimensions linked to their characteristic resonance frequencies. Therefore, many novel multiferroic (MF) devices rely on the transduction of signals from EM radiation to surface and bulk acoustic waves, and yet acoustic wave interactions in complex MF heterostructures are poorly understood. Here, an all-optical interferometric imaging system is developed for two-dimensional scanning imaging of acoustic wave amplitudes with sub-angstrom surface displacement sensitivity and submicron spatial resolution. Measurement capability has been verified with a 50-MHz piezoelectric SAW bandpass filter, with the frequency spectrum showing similar behavior to the S21 transmission measured with a network analyzer, and with a linear dependence on the input power. Spatial dependence of the acoustic waves also behaves as expected, with the largest amplitude near the input electrode. Next, the system has been demonstrated to detect acoustic waves in MF heterostructure-based MEMS devices with frequencies up to 3 GHz. These detections pave the way for fast and reliable troubleshooting of new device designs involving acoustic waves, in which it may not be immediately clear to where or by what mechanism power is being dissipated.

Devices based on diffractive waveplates exhibit diffraction characteristics that are dependent on the polarization and wavelength of light. In applications, switchability, polarization independence, and wavelength independence are often desirable. Here we report on recent developments of diffractive optical elements that have various combinations of switchability, polarization independence, and broad wavelength coverage.

Diffractive waveplates enable stackable ultrathin light-weight large area optics fast-switchable with low-voltage/low power fields. State-of-the-art systems and low-cost manufacturing opportunities will be presented aimed at next generation augmented, virtual reality, flexible, polarizer-free displays, LiDARs, etc.

We experimentally investigate spin-orbit torque and spin pumping in Y$_3$Fe$_5$O$_{12}$(YIG)/Pt bilayers with ultrathin insertion layers at the interface. An insertion layer of Cu suppresses both spin-orbit torque and spin pumping, whereas an insertion layer of Ni$_{80}$Fe$_{20}$ (permalloy, Py) enhances them, in a quantitatively consistent manner with the reciprocity of the two spin transmission processes. However, we observe a large enhancement of Gilbert damping with the insertion of Py that cannot be accounted for solely by spin pumping, suggesting significant spin-memory loss due to the interfacial magnetic layer. Our findings indicate that the magnetization at the YIG-metal interface strongly influences the transmission and depolarization of pure spin current.

Low-loss magnetization dynamics and strong magnetoelastic coupling are generally mutually exclusive properties due to opposing dependencies on spin-orbit interactions. So far, the lack of low-damping, magnetostrictive ferrite films has hindered the development of power-efficient magnetoelectric and acoustic spintronic devices. Here, magnetically soft epitaxial spinel NiZnAl-ferrite thin films with an unusually low Gilbert damping parameter (

We report on the preparation of color-tunable mirrors based on electrically regulated bandwidth broadening of the circularly polarized reflection of polymer-stabilized cholesteric liquid crystals (PSCLCs). A number of improvements relating to the practical implementation of these materials are detailed including color and bandwidth stability, baseline optical properties, and response times. Experimentation reported herein focuses on the contribution of structural chirality, viscoelastic properties of the polymer network architecture, and electro-optic drive schemes. Through the examination of samples prepared in different conditions and compositions, we further elucidate the dominant role of structural chirality as well as the impact of cross-linking of the polymer stabilizing network on the threshold voltage and relative change in bandwidth per voltage (Δ(Δλ)/V). Furthermore, the appearance of nonideal optical properties (scatter and haze) in some samples is shown to be correlated with the polymer/LC com...

Stimuli-induced control of the reflection notch of cholesteric liquid crystals (CLCs) is potentially useful in optics, photonics, and displays. This report briefly reviews and describes a series of related results in which we have observed symmetric bandwidth broadening and reflection notch tuning in polymer stabilized CLCs (PSCLCs) upon application of a DC field. The electro-optic responses are typically observed in the presence of polymer stabilization, in formulations based on negative dielectric liquid crystals hosts, and when subjected to DC field.

Tunable reflective material systems are potentially useful in a range of applications including displays, optical filters, photonics, and sensing. Presented here is a systematic study of thermally induced reflection wavelength tuning in structurally chiral nematic gels. The tuning is enabled by a swelling/de-swelling/re-swelling transition driven by order/disorder transitions in the liquid crystal and the liquid crystalline polymer network. By preparing liquid crystal polymer networks from single component liquid crystal monomers we isolate the contribution of chemical chirality and the crosslink density of the liquid crystal polymer network to thermally-induced reflection wavelength tuning. Further elucidating the fundamental relationships between the composition of the polymer network and the resulting optical properties is a key step towards enabling advancement and optimization necessary to realize the utility of this mechanism in devices such as mirrorless lasers.

Acoustically-driven ferromagnetic resonance (ADFMR) has recently emerged as a powerful scientific test-bed toward understanding complex interactions between phonons and magnons. In this technique, a traditional surface acoustic wave (SAW) delay-line filter interfaces with a ferromagnetic thin-film which can be driven into precession at the ferromagnetic resonance (FMR) frequency by the SAWs. SAW filters are used extensively in industry, but in the context of ADFMR, their design considerations are largely absent from the literature. We produced a variety of ADFMR devices by systematically changing parameters including the material and the number of pairs of interdigital transducers, the ferromagnetic thin-film growth technique, and the presence or the absence of a capping layer on the ferromagnetic thin-film. We quantitatively compare results by adapting traditional ferromagnetic resonance techniques. This work describes the parameters relevant to the development and optimization of SAW-driven FMR.

Light-induced phenomena occurring in nature and in synthetic materials are fascinating and have been exploited for technological applications. Here visible-light-induced formation of a helical superstructure is reported, i.e., a cholesteric liquid crystal phase, in orientationally ordered fluids, i.e., nematic liquid crystals, enabled by a visible-light-driven chiral molecular switch. The cyclic-azobenzene-based chiral molecular switch exhibits reversible photoisomerization in response to visible light of different wavelengths due to the band separation of n-π* transitions of its trans- and cis-isomers. Green light (530 nm) drives the trans-to-cis photoisomerization whereas the cis-to-trans isomerization process of the chiral molecular switch can be caused by blue light (440 nm). It is observed that the helical twisting power of this chiral molecular switch increases upon irradiation with green light, which enables reversible induction of helical superstructure in nematic liquid crystals containing a very small quantity of the molecular switch. The occurrence of the light-induced helical superstructure enables the formation of diffraction gratings in cholesteric films.

We report on the temperature (T), magnetic field (μ0H), and angle (Θ, Φ) dependent resonant absorption of X-band microwaves in spinel ferrite epitaxial films subjected to two distinct states of growth strain. The polar angle (Θ) dependence of the resonance field (Hres) in films with ∼0.5% ab-plane expanded unit cell establishes a distinct perpendicular magnetic anisotropy (PMA). The anisotropy field (Han┴) for Θ = 0° increases monotonically on lowering the temperature from 300 K to 90 K following the behavior of the saturation magnetization (Ms) keeping Han┴/Ms ≈ 1. The narrow resonance linewidth (μ0ΔHres┴ = 3.7 mT at 300 K) and its negligible (±0.3 mT) variation with temperature establish the magnetic softness of these PMA films. The dependence of Hres on Θ, Φ, and T in films subjected to compressive stress shows in-plane cubic anisotropy whose strength is nonmonotonic in temperature. The ∼2.0% compression of the unit cell basal plane also appears to accentuate noncollinearity of sublattice magnetization of such films as indicated by the T-dependence of ΔHres. The thicker films with PMA display spin wave resonances whose position allows determination of the spin wave stiffness constant together with independent determination of Han┴. The resonance characteristics of the PMA films qualify them as potential candidates for frequency agile microwave devices and magnonic circuit elements.

Layered magnetic/piezoelectric heterostructures have drawn a great amount of interest for their potential use in ultra-sensitive magnetoelectric (ME) sensors, ME antennas, voltage tunable inductors, magnetic tunable resonators, etc. It is critically important to characterize the saturation magnetostriction, piezomagnetic coefficient, ΔE effect, and magnetomechanical coupling factor of magnetic thin films, which determine the performance of these ME devices. In this work, a sensitive system has been developed to measure these magnetomechanical properties, on which several different magnetostrictive thin films on the silicon substrate cantilever were characterized. A 0.015 ppm limit of detection of the magnetostriction tester and a frequency resolution of 0.01 Hz of the ΔE tester have been achieved. After magnetic anneal treatment, a record high piezomagnetic coefficient of 12 ppm/Oe, a giant magnetic field induced Young's modulus change of 153 GPa, and a high effective magnetomechanical coupling factor of 0.84 have been measured in FeGaB thin films.

Making small actuators more effective Liquid-crystal molecules orient locally in response to external fields. When long-chain liquid-crystalline molecules are crosslinked together, changes in local orientation can lead to significant volume changes. Ware et al. made efficient microactuators that can change their shape from flat to three-dimensional structures (see the Perspective by Verduzco). By patterning volume elements so that each has a different preferred alignment for the liquid-crystalline molecules, they could fine-tune the volume changes. Science , this issue p. 982 ; see also p. 949

Films subsumed with topological defects are transformed into complex, topographical surface features with light irradiation of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs). Using a specially designed optical setup and photoalignment materials, azo-LCN films containing either singular or multiple defects with strengths ranging from |½| to as much as |10| are examined. The local order of an azo-LCN material for a given defect strength dictates a complex, mechanical response observed as topographical surface features.

State-of-the-art compact antennas rely on electromagnetic wave resonance, which leads to antenna sizes that are comparable to the electromagnetic wavelength. As a result, antennas typically have a size greater than one-tenth of the wavelength, and further miniaturization of antennas has been an open challenge for decades. Here we report on acoustically actuated nanomechanical magnetoelectric (ME) antennas with a suspended ferromagnetic/piezoelectric thin-film heterostructure. These ME antennas receive and transmit electromagnetic waves through the ME effect at their acoustic resonance frequencies. The bulk acoustic waves in ME antennas stimulate magnetization oscillations of the ferromagnetic thin film, which results in the radiation of electromagnetic waves. Vice versa, these antennas sense the magnetic fields of electromagnetic waves, giving a piezoelectric voltage output. The ME antennas (with sizes as small as one-thousandth of a wavelength) demonstrates 1–2 orders of magnitude miniaturization over state-of-the-art compact antennas without performance degradation. These ME antennas have potential implications for portable wireless communication systems., The miniaturization of antennas beyond a wavelength is limited by designs which rely on electromagnetic resonances. Here, Nan et al. have developed acoustically actuated antennas that couple the acoustic resonance of the antenna with the electromagnetic wave, reducing the antenna footprint by up to 100.

We report quantitative measurements of optically detected ferromagnetic resonance (ODFMR) of ferromagnetic thin films that use nitrogen-vacancy (NV) centers in diamonds to transduce FMR into a fluorescence intensity variation. To uncover the mechanism responsible for these signals, we study ODFMR as we 1) vary the separation of the NV centers from the ferromagnet (FM), 2) record the NV center longitudinal relaxation time $T_1$ during FMR, and 3) vary the material properties of the FM. Based on the results, we propose the following mechanism for ODFMR. Decay and scattering of the driven, uniform FMR mode results in spinwaves that produce fluctuating dipolar fields in a spectrum of frequencies. When the spinwave spectrum overlaps the NV center ground-state spin resonance frequencies, the dipolar fields from these resonant spinwaves relax the NV center spins, resulting in an ODFMR signal. These results lay the foundation for an approach to NV center spin relaxometry to study FM dynamics without the constraint of directly matching the NV center spin-transition frequency to the magnetic system of interest, thus enabling an alternate modality for scanned-probe magnetic microscopy that can sense ferromagnetic resonance with nanoscale resolution.

The plasma polymerization of NIPAAM and titanium isopropoxide monomers into responsive ultrathin films with responsive optical properties using plasma enhanced chemical vapor deposition is reported. The composite ultrathin films possess a large window for potential changes in their refractive index from 1.60 to 1.95. We demonstrated that these polymer films exhibit fast (transition time below 2 s), large, reversible, and repeatable changes to their thickness and refractive index as a function of periodic environmental humidity changes.

The magnitude of the color changes induced by heating chiral templated polymer structures formed in spatially heterogeneous cells is examined. The large blue and subsequent red tuning of the selective reflection wavelength, caused by deswelling/reswelling of the network at the transition temperatures, is affected by the network crosslink density, heating/cooling rates, and architecture of the cell. The system is stable to multiple heat/cool cycles and shows large scale coloration changes at heating rates up to 10°C/min. At higher heating rates, a distortion of the selective reflection notch shape occurs during tuning. The magnitude of the color change is not affected by rate. At both high and low crosslink density, no tuning is observed. A window of network elasticity and responsiveness is needed to enable the anisotropic contraction/expansion processes to be observed macroscopically. Finally, the magnitude of tuning relative to the cell thickness and spatial heterogeneity across the thickness direction i...

We report on high reflectivity CLC structures (R > 50%) whose wavelength can be thermally tuned reversibly by a de-swelling/re-swelling transition unique to ordered solvent-gel systems. The system contains no chiral dopant and the coloration is completely induced by a responsive chiral structured gel. The de-swelling transition, leading to blue tuning, occurs at the nematic-isotropic transition of the liquid crystal, which is a result of a mismatch in the orientational energy of the isotropic liquid crystal and the anisotropic gel. The re-swelling transition subsequently occurs at the nematic-isotropic transition of gel, due to the miscibility of the isotropic liquid crystal and the isotropic gel, which induces a red-shift in the coloration. Examination of varying clearing point liquid crystal solvents, contact angle measurements, and white light optical profilometry localized thickness measurements shed light on this de-swelling/re-swelling transition. A dynamic, high reflectivity cell was demonstrated by combining both a left-handed chiral gel with a right handed chiral gel, both of whose initial periodicities were equal. Heating of this so-called hyper-reflective cell drove reversible and large scale wavelength changes (100's of nm) while maintaining large reflectivity (R ∼ 90%).

The development of atomic force microscopy has evolved into a wide variety of microscopy and characterization techniques well beyond conventional imaging. The focus of this review is on characterization methods based on the scanning probe and their application in characterizing physical properties of soft materials. This consideration is broken into three major categories focusing on mechanical, thermal, and electrical/magnetic properties in addition to a brief review of high-resolution imaging. Surface spectroscopy is discussed to great extent and consideration includes procedural information, common pitfalls, capabilities, and their practical application in characterizing soft matter. Key examples of the method are presented to communicate the capabilities and impact that probe-based characterization techniques have had on the mechanical, thermal, and electrical characterization of soft materials.

Mechanical instabilities such as buckling, wrinkling, creasing, and folding are commonplace in both natural and synthetic systems over a wide range of length scales. In this study, we focus on the spontaneous folding behavior of the highly swellable confined nanoscale (thickness below 100 nm) gel films resulting in the formation of a network of regularly folded structures spontaneously emerging in the course of their swelling and drying. We suggest that regular self-folding is originated from periodic instabilities (wrinkles) caused by swelling-initiated stresses under confined conditions. Furthermore, folded gel structures can be organized into regular serpentine-like manner by imposing various boundary conditions on microimprinted surfaces. We suggest that this demonstration of uniform gel to mechanically mediate morphogenesis has far-reaching implications in the creation of complex, large-area, 3D gel nanostructures.

Bioinspired design is an engineering approach that involves working to understand the design principles and strategies employed by biology in order to benefit the development of engineered systems. From a materials perspective, biology offers an almost limitless source of novel approaches capable of arousing innovation in every aspect of materials, including fabrication, design, and functionality. Here, recent and ongoing work on the study of bioinspired materials for sensing applications is presented. Work presented includes the study of fish flow receptor structures and the subsequent development of similar structures to improve flow sensor performance. The study of spider air-flow receptors and the development of a spider-inspired flexible hair is also discussed. Lastly, the development of flexible membrane based infrared sensors, highly influenced by the fire beetle, is presented, where a pneumatic mechanism and a thermal-expansion stress-mediated buckling-based mechanism are investigated. Other areas that are discussed include novel biological signal filtering mechanisms and reciprocal benefits offered through applying the biology lessons to engineered systems.

A novel method for the facile fabrication of conformal, ultrathin, and uniform synthetic amino acid coatings on a variety of practical surfaces by plasma-enhanced chemical vapor deposition is introduced. Tyrosine, which is utilized as an agent to reduce gold nanoparticles from solution, is sublimed into the plasma field and directly deposited on a variety of substrates to form a homogeneous, conformal, and robust polyamino acid coating in a one-step, solvent-free process. This approach is applicable to many practical surfaces and allows surface-induced biometallization while avoiding multiple wet-chemistry treatments that can damage many soft materials. Moreover, by placing a mask over the substrate during deposition, the tyrosine coating can be micropatterned. Upon its exposure to a solution of gold chloride, a network of gold nanoparticles forms on the surface, replicating the initial micropattern. This method of templated biometallization is adaptable to a variety of practical inorganic and organic substrates, such as silicon, glass, nitrocellulose, polystyrene, polydimethylsiloxane, polytetrafluoroethylene, polyethylene, and woven silk fibers. No special pretreatment is necessary, and the technique results in a rapid, conformal amino acid coating that can be utilized for further biometallization.

Using a precision drop-casting method, a bioinspired hydrogel-capped hair sensory system was created, which enhanced the performance of flow detection by about two orders of magnitude and endowed the sensors with threshold sensitivities that rival those of fish.

In order to improve the sensitivity of hair cell sensors for fluid flow detection, poly-ethylene oxide acrylic macromonomer is used as a crosslinkable photo-patterned material capable of being swollen into a hydrogel of different shapes and sizes. We demonstrated that simple arrays of various hydrogel structures can be synthesized by photopatterning with photomasks. The mechanical properties of the hydrogel materials were measured to be in the range of 5–100 Pa under varying crosslinking conditions. Additional support for these high-aspect ratio hydrogel structures was provided with electrospun polycaprolactone microfibers that were deposited onto the microfabricated hairs. These fibers served as scaffolding to support the swollen hydrogel. This approach looks to integrate several key design components in order to create a highly sensitive flow sensor.

Although there are several aspects that contribute to an efficient chemical sensor system, the choice of responsive materials can help to optimize several key attributes critical for their ultimate performance, specifically high sensitivity, selectivity, fast response time, and wide dynamic range. Some common issues reported to date for sensors are limited detection range, slow response time, long recovery period, and fast saturation (limited dynamic range). Most of these issues are directly related to the large volume of bulk porous material that is needed for measurable signal output, which leads to inherently slow sorption, diffusion, and desorption processes. One type of gas sensor that receives only limited attention despite being among the most frequently used sensors for measuring important environmental quantities is the humidity sensor.[2] Important applications of this type of sensor include for respiratory equipment, incubators, chemical gas purification, and surgical operations. The main requirements for humidity sensors are good sensitivity over a wide humidity range, low hysteresis, good reproducibility, and longevity. Often sensors are required to be very small and suitable for integration into arrayed systems, such as odor-sensing arrays.

In contrast, active sonar systemsare widely used for a variety of civil and military applicationsin underwater environment. However, passive sensor systemssimilar to fish provide a huge advantage in “silent” observa-tion of the environment which does not reveal the source anddoes not interfere with moving vehicles and various lifeforms.

Dynamic control of shape can bring multifunctionality to devices. Soft materials capable of programmable shape change require localized control of the magnitude and directionality of a mechanical response. We report the preparation of soft, ordered materials referred to as liquid crystal elastomers. The direction of molecular order, known as the director, is written within local volume elements (voxels) as small as 0.0005 cubic millimeters. Locally, the director controls the inherent mechanical response (55% strain) within the material. In monoliths with spatially patterned director, thermal or chemical stimuli transform flat sheets into three-dimensional objects through controlled bending and stretching. The programmable mechanical response of these materials could yield monolithic multifunctional devices or serve as reconfigurable substrates for flexible devices in aerospace, medicine, or consumer goods.

Polymeric nanolayers are introduced here as active, thermal-stress mediating structures facilitating extremely sensitive thermal detection based upon the thermomechanical response of a bimaterial polymer-silicon microcantilever. To maximize the bimaterial bending effect, the microcantilever bimorph is composed of stiff polysilicon, with a strongly adhered polymer deposited via plasma-enhanced chemical vapor deposition. The polymer layers with thickness ranging from 20 to 200 nm possess a rapid and pronounced response to temperature fluctuations due to intrinsic sensitive thermal behavior. We show that by taking advantage of the thermal stresses generated by the huge mismatch of material properties in the polymer-silicon bimorph, unprecedented thermal sensitivities can be achieved. In fact, the temperature resolution of our bimaterial microcantilevers approaches 0.2 mK with thermal sensitivity reaching 2 nm/mK; both parameters are more than an order of magnitude better than the current metal-ceramic design. This new hybrid platform overcomes the inherently limited sensitivity of current sensor designs and provides the basis to develop the ultimate uncooled IR microsensor with unsurpassable sensitivity.

A method for the creation of a type IV collagen (CNIV) scaffold on polytetrafluoroethylene (PTFE) for endothelial cell attachment is described. This mimic for the basal lamina can be used in the seeding and retention of endothelial cells for blood contacting devices. The CNIV-PTFE production technique can be defined as three processes: (i) creation of a reactive superacidic/ionic PTFE surface with retained hydrophobic characteristics; (ii) activation of this surface via covalent attachment of N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC); and (iii) conjugation of the EDC with human CNIV resulting in the covalent binding of protein to the PTFE surface. This article demonstrates exciting new results showing that a reaction of CNIV to this particular surface results in a unique matrix assembly of CNIV scaffolds similar to those found in the basal lamina. This assembly is concentration-dependant, occurring in a narrow window around 0.435 μM. Following the fabrication of the CNIV matrix assembly, porcine aortic endothelial cells (PAEC) were seeded onto this material. Results described in this article demonstrate that the PAEC subsequently aligned with the direction of shear, filled voids created by dead or detached cells, and divided during the 6 h of experimentation. Under static conditions, cells remained viable for 1 week of testing. This was not observed with PAEC attached to glass with adsorbed Vitrogen. In summary, this article describes a novel biotechnological breakthrough that enables the creation of stable endothelial cell monolayers useful for fabricating blood contacting devices. © 2006 Wiley Periodicals, Inc. J Biomed Mater Res, 2006