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Piezoelectric unimorphs are bilayer structures where a blanket piezoelectric film (with top and bottom electrodes) is uniformly laminated on an inactive but flexible substrate. Because of their simple construction and flexibility, unimorphs are widely used as a key element in flexible sensors and actuators. The response of a unimorph is governed by the material properties of the film and the substrate as well as their geometric parameters. For low frequency biological energy harvesting, structural optimization is critical due to the dimensional confinement imposed by curvilinear and deformable bio-tissues. Here we report a comprehensive theoretical framework to investigate the effects of the film-to-substrate thickness ratio on voltage, charge, and energy outputs when the unimorph is subjected to eight different boundary/loading conditions. A broad class of power generators can be designed using such a framework under the assumption that the unimorph length is very large compared to its thickness, where the only dimensionless variable is the film-to-substrate thickness ratio. We show that the analytical and finite element modeling results are in excellent agreement. For not so thin unimorphs, there is non-zero normal stress in the thickness direction (σ 3) and d 33 can play a significant role in this case. Non-monotonic dependence of voltage and energy generation on thickness ratio has been found in some cases and optimum thickness ratio for unimorph generator can be predicted. When the unimorph is actuated by voltage applied across the piezo-film thickness, non-monotonic maximum deflection versus thickness ratio is also found. This work provides new physical insights on unimorphs and analytical solutions that can be used for the structural design and optimization of unimorphs under different boundary/loading conditions.

We describe the use of semiconductor nanomaterials, advanced fabrication methods and unusual device designs for a class of electronics capable of integration onto the inner and outer surfaces of thin, elastomeric sheets in closed-tube geometries, specially formed for mounting on the fingertips. Multifunctional systems of this type allow electrotactile stimulation with electrode arrays multiplexed using silicon nanomembrane (Si NM) diodes, high-sensitivity strain monitoring with Si NM gauges, and tactile sensing with elastomeric capacitors. Analytical calculations and finite element modeling of the mechanics quantitatively capture the key behaviors during fabrication/assembly, mounting and use. The results provide design guidelines that highlight the importance of the NM geometry in achieving the required mechanical properties. This type of technology could be used in applications ranging from human‐machine interfaces to ‘instrumented’ surgical gloves and many others.

Epidermal electronic system (EES) is a class of integrated electronic systems that are ultrathin, soft, and lightweight, such that it could be mounted to the epidermis based on van der Waals interactions alone, yet provides robust, intimate contact to the skin. Recent advances on this technology will enable many medical applications such as to monitor brain or heart activities, to monitor premature babies, to enhance the control of prosthetics, or to realize human-machine interface. In particular, the contact between EES and the skin is key to high-performance functioning of the above applications and is studied in this paper. The mechanics concepts that lead to successful designs of EES are also discussed. The results, validated by finite element analysis and experimental observations, provide simple, analytical guidelines for design and optimization of EES with various possible functionalities.

The line shape of electromodulation in uniform electric field of δ‐doped GaAs has been calculated and compared with the experiments. The calculations show that the Franz–Keldysh oscillations (FKO) beats observed in experiments are contributed form the interference of heavy‐hole and light‐hole transitions. The calculations also show that the linear equation of extrema is a good approximation to analyze the electric field from the FKO line shape, although the equation only considers the heavy‐hole transition.

The signals from buried layer in Si‐δ‐doped GaAs of different undoped cap thickness have been studied by differential photoreflectance. The first‐derivative‐like line shape of differential photoreflectance is attributed to the energy transitions related to the modulations of two‐dimensional electron gas density. We have observed a change of line shape at low temperatures. This change of line shape is probably due to the change of potential distribution in the conduction band, which is caused by the surface Fermi‐level pinning and surface photovoltaic effect.

We report classes of electronic systems that achieve thicknesses, effective elastic moduli, bending stiffnesses, and areal mass densities matched to the epidermis. Unlike traditional wafer-based technologies, laminating such devices onto the skin leads to conformal contact and adequate adhesion based on van der Waals interactions alone, in a manner that is mechanically invisible to the user. We describe systems incorporating electrophysiological, temperature, and strain sensors, as well as transistors, light-emitting diodes, photodetectors, radio frequency inductors, capacitors, oscillators, and rectifying diodes. Solar cells and wireless coils provide options for power supply. We used this type of technology to measure electrical activity produced by the heart, brain, and skeletal muscles and show that the resulting data contain sufficient information for an unusual type of computer game controller.

Development of advanced surgical tools for minimally invasive procedures represents an activity of central importance to improvements in human health. A key materials challenge is in the realization of bio-compatible interfaces between the classes of semiconductor and sensor technologies that might be most useful in this context and the soft, curvilinear surfaces of the body. This paper describes a solution based on biocompatible materials and devices that integrate directly with the thin elastic membranes of otherwise conventional balloon catheters, to provide multimodal functionality suitable for clinical use. We present sensors for measuring temperature, flow, tactile, optical and electrophysiological data, together with radio frequency (RF) electrodes for controlled, local ablation of tissue. These components connect together in arrayed layouts designed to decouple their operation from large strain deformations associated with deployment and repeated inflation/deflation. Use of such ‘instrumented’ balloon catheter devices in live animal models and in vitro tests illustrates their operation in cardiac ablation therapy. These concepts have the potential for application in surgical systems of the future, not only those based on catheters but also on other platforms, such as surgical gloves.

2D systems have great promise as next generation electronic materials but require intimate knowledge of their interactions with their neighbors for device fabrication and mechanical manipulation. Although adhesion between 2D materials and stiff substrates such as silicon and copper has been measured, adhesion between 2D materials and soft polymer substrates remains difficult to characterize due to the large deformability of the polymer substrates. In this work, a buckling-based metrology for measuring the adhesion energy between few layer molybdenum disulfide (MoS2) and soft elastomeric substrates is proposed and demonstrated. Due to large elastic mismatch, few layer MoS2 flakes can form spontaneous wrinkles and buckle-delaminations on elastomer substrates during exfoliation. MoS2-elastomer interface toughness can therefore be calculated from the buckle delamination profile measured by atomic force microscopy. The thickness of the MoS2 flake is obtained by analyzing coexisting wrinkles on the same flake. Using this approach, adhesion of few layer MoS2 to 10:1 Sylgard 184 polydimethylsiloxane is measured to be 18 ± 2 mJ m−2, which is about an order of magnitude below graphene-to-stiff-substrate adhesion. Finally, this simple methodology can be generalized to obtain adhesion energies between various combinations of 2D materials and deformable substrates.

Germanium p-MOSFETs with a thin high-k dielectric (EOT/spl sim/1.6 nm) were fabricated on bulk Ge and epitaxial germanium-on-silicon substrates. These devices exhibited sub-90 mV/decade subthreshold swing and low gate leakage. The IV and CV characteristics achieved in this work allow for accurate extraction of important device parameters such as transconductance (Gm) and low-field mobility. Results from n-MOSFETs fabricated on bulk Ge substrates are also presented.

In this paper, for the first time, we report high quality CVD hafnium oxynitride (HfOxNy) MOSFETs with conventional self-aligned poly-Si gate. These CVD HfOxNy films deposited using TDEAH (Tetrakisdiethylamino hafnium, C/sub 16/H/sub 40/N/sub 4/Hf) and NH/sub 3/ remain amorphous after 900/spl sim/950/spl deg/C annealing. Compared to HfO/sub 2/, HfOxNy exhibits reduced leakage current by 2/spl sim/3 orders of magnitude, excellent boron penetration immunity, superior thermal stability of both EOT and leakage current after high temperature annealing, and excellent reliability.

We present a technique for measuring the interfacial fracture energy, Ci, between a hard thin film and a soft substrate. A periodic array of hard thin islands is fabricated on a soft substrate, which is then subjected to uniaxial tension under an optical microscope. When the applied strain reaches a critical value, delamination between the islands and the substrate starts from the edge of the islands. As the strain is increased, the interfacial cracks grow in a stable fashion. At a given applied strain, the width of the delaminated region is a unique function of the interfacial fracture energy. We have calculated the energy release rate driving the delamination as a function of delamination width, island size, island thickness, and applied strain. For a given materials system, this relationship allows determination of the interfacial fracture energy from a measurement of the delamination width. The technique is demonstrated by measuring the interfacial fracture energy of plasma-enhanced chemical vapor deposition SiNx islands on a polyimide substrate. We anticipate that this technique will find application in the flexible electronics industry where hard islands on soft substrates are a common architecture to protect active devices from fracture. V C 2013 AIP Publishing LLC .[ http://dx.doi.org/10.1063/1.4810763]

Semiconductors are materials — traditionally inorganic ones — that can act as either conductors or insulators under different conditions. They form the foundation of electronics and play a crucial role in many devices. John Rogers and co-workers discuss recent advances that have made possible the preparation of inorganic semiconductors as nanowires or nanomembranes, their assembly together with organic components into hybrid materials, and their integration into devices. These one- and two-dimensional structures allow for the construction of electronic or opto-electronic devices that are flexible and stretchable — properties that are particularly, albeit not exclusively, promising for biological applications. Systems capable of mapping the electrical activity of the heart and brain have been built, and assemblies whose characteristics mimic those of human skin have led to the construction of epidermal electronic devices. Such devices hold promise for monitoring functional activity and for therapeutic interventions.

The above titled paper (ibid., vol. 58, no. 11, pp. 4074-4078), contains two errors, both relating to nomenclature associated with the Wheatstone bridge layout of the sensors. The corrections are presented here.

We perform in-situ uniaxial tensile tests on polyimide substrates with patterned ceramic islands. The islands fail by either channel cracking or debonding from the substrate, depending on island size and thickness. To understand why different failure modes occur, we have analyzed the fracture and debonding of stiff islands on deformable substrates. Using finite element simulations, we find that the maximum tensile strain in the islands increases with island size, but decreases with island thickness. The maximum energy release rate for island/substrate debonding, in contrast, increases with both island size and thickness. Assuming that the islands do not fracture if the maximum tensile strain in the islands is lower than a critical value and that no debonding occurs if the maximum energy release rate is smaller than the interfacial toughness, the critical substrate strains for island fracture and debonding can be calculated. If the islands are thick and small, the critical debonding strain is small, and the islands fail by debonding. If the islands are large and thin, the critical fracture strain is small, and the islands fail by channel cracking. When the two critical strains are similar, debonding and cracking are expected to co-exist. Experimental observations confirm these findings.

When a freestanding plastically deformable metal film is stretched, it ruptures by strain localization, and the elongation is less than a few percent. When the film is deposited on a polymer substrate, however, strain localization may be retarded by the substrate. This paper reports Cu films deposited on Kapton substrates and stretched up to the rupture of the substrates (at an elongation between 50% and 60%). When Cr adhesion layers are introduced between Cu and Kapton, few microcracks in Cu may be found, and the measured electrical resistance agrees with a theoretical prediction. Micrographs show that the strain localization and debonding coevolve.

A flexible electronic system may consist of a polymeric substrate and an array of stiff islands, on which devices are fabricated. When the substrate is stretched, the devices on the islands experience small strains but the islands may debond if they exceed a critical size. The authors show that a thin layer of polymer coating, covering the islands and the substrate, can markedly increase the critical island size.

We report a study on Ge diffusion and its impact on the electrical properties of TaN∕HfO2∕Ge metal-oxide-semiconductor (MOS) device. It is found that Ge diffusion depends on the amount of GeO2 formed at the HfO2∕Ge interface and can be retarded by surface nitridation. It is speculated that Ge diffusion is in the form of GeO or Ge-riched HfGeO. Effective suppression of Ge diffusion by NH3 nitridation has resulted in improved electrical properties of TaN∕HfO2∕Ge MOS device, including equivalent oxide thickness (EOT), leakage current, hysteresis, and interface state density. The degradation of leakage current after high temperature post metallization anneal (PMA) is found to be due to Ge diffusion.