Radar-absorbing materials are used in stealth technologies for concealment of an object from radar detection. Resistive and/or magnetic composite materials are used to reduce the backscattered microwave signals. Inability to control electrical properties of these materials, however, hinders the realization of active camouflage systems. Here, using large-area graphene electrodes, we demonstrate active surfaces that enable electrical control of reflection, transmission and absorption of microwaves. Instead of tuning bulk material property, our strategy relies on electrostatic tuning of the charge density on an atomically thin electrode, which operates as a tunable metal in microwave frequencies. Notably, we report large-area adaptive radar-absorbing surfaces with tunable reflection suppression ratio up to 50 dB with operation voltages o5 V. Using the developed surfaces, we demonstrate various device architectures including pixelated and curved surfaces. Our results provide a significant step in realization of active camouflage systems in microwave frequencies.

Wearable health and wellness trackers based on optical detection are promising candidates for public health uses due to their noninvasive tracking of vital health signs. However, so far, the use of rigid technologies hindered the ultimate performance and form factor of the wearable. Here, we demonstrate a new class of flexible and transparent wearables based on graphene sensitized with semiconducting quantum dots (GQD). We show several prototype wearable devices that are able to monitor vital health signs noninvasively, including heart rate, arterial oxygen saturation (SpO2), and respiratory rate. Operation with ambient light is demonstrated, offering low-power consumption. Moreover, using heterogeneous integration of a flexible ultraviolet (UV)–sensitive photodetector with a near-field communication circuit board allows wireless communication and power transfer between the photodetectors and a smartphone, offering battery-free operation. This technology paves the way toward seamlessly integrated wearables, and empowers the user through wireless probing of the UV index.

such as diabetes. To satisfy the requirements of such a system, active materials with intrinsic properties, including good mechanical, electrical, optical, and structural properties, are in high demand. [1] The development of suitable flexible pressure sensors for e-skin applications has been a challenge, due to inadequate flexibility, conductivity, large-area manufacturability, and reliable and repeatable performance of the structure, to be applicable in practical robots. [7] In this regard, only very few approaches have been successfully employed in actual robots. [7,8] Further, making the e-skin transparent adds an extra dimension in the functional design space of e-skin, as it enables incorporating photovoltaic (PV)-energy harvesting, electro/thermochromicity, chameleon effect, etc. Along with a new generation of flexible and stretchable solar cells, [9] this will allow fabrication of energy-autonomous, stretchable e-skins. Accordingly, a novel approach is explored in this work, of a vertical-layered-stack structure consisting of a photovoltaic cell attached to the back plane of a transparent tactile skin; where skin transparency is a crucial feature that allows light to pass through, making the building block unique and opening a new, promising line of energy-autonomous devices for flexible electronics. In this regard, graphene is a promising material as it offers key parameters to develop nonplanar, transparent electronic or tactile skin. It has been shown that graphene has a good combination of stiffness (≈1000 GPa) and tensile strength (≈100 GPa). [10] Together with its sunlight blindness [11] and good electrical conductivity, [12] graphene has also emerged as a viable candidate for various flexible, transparent electronic and optoelectronic devices. [13][14][15][16] Moreover, in our recent work, we demonstrated that high-quality graphene can be synthesized and transferred on large area, flexible substrates (400 cm 2 ) with a very low-cost and easy fabrication process. [15] Owing to the intrinsic properties and advances in the synthesis and fabrication of devices, graphene is also a promising candidate for the development of high-performance e-skin, requiring large area device fabrication on nonplanar surfaces.A few flexible pressure sensors reported in literature, based on capacitive, [17][18][19][20] piezoelectric, [21] and piezoresistive sensing mechanisms, [2,[22][23][24][25][26][27][28] use graphene as an active material. Piezoresistive sensors transduce the pressure imposed on the sensor's active area in terms of resistance change, and offer an attractive solution for pressure sensing due to advantages such as low cost and easy signal collection. Graphene-based piezoresistive pressure sensors have been reported in various configurations. For example, Yao et al. demonstrated the fabrication of flexible pressure sensors based on a graphene nanosheet on Energy-Autonomous, Flexible, and Transparent Tactile SkinCarlos García Núñez, William Taube Navaraj, Emre O. Polat, and Ravinder Dahiya* Tactile or el...

Optical modulators are commonly used in communication and information technology to control intensity, phase, or polarization of light. Electro-optic, electroabsorption, and acousto-optic modulators based on semiconductors and compound semiconductors have been used to control the intensity of light. Because of gate tunable optical properties, graphene introduces new potentials for optical modulators. The operation wavelength of graphene-based modulators, however, is limited to infrared wavelengths due to inefficient gating schemes. Here, we report a broadband optical modulator based on graphene supercapacitors formed by graphene electrodes and electrolyte medium. The transparent supercapacitor structure allows us to modulate optical transmission over a broad range of wavelengths from 450 nm to 2 μm under ambient conditions. We also provide various device geometries including multilayer graphene electrodes and reflection type device geometries that provide modulation of 35%. The graphene supercapacitor structure together with the high-modulation efficiency can enable various active devices ranging from plasmonics to optoelectronics.

Flexible electronics has huge potential to bring revolution in robotics and prosthetics as well as to bring about the next big evolution in electronics industry. In robotics and related applications, it is expected to revolutionise the way with which machines interact with humans, real-world objects and the environment. For example, the conformable electronic or tactile skin on robot's body, enabled by advances in flexible electronics, will allow safe robotic interaction during physical contact of robot with various objects. Developing a conformable, bendable and stretchable electronic system requires distributing electronics over large non-planar surfaces and movable components. The current research focus in this direction is marked by the use of novel materials or by the smart engineering of the traditional materials to develop new sensors, electronics on substrates that can be wrapped around curved surfaces. Attempts are being made to achieve flexibility/stretchability in e-skin while retaining a reliable operation. This review provides insight into various materials that have been used in the development of flexible electronics primarily for e-skin applications.

Here we report chemical vapor deposition of graphene on gold surface at ambient pressure. We studied effects of the growth temperature, pressure, and cooling process on the grown graphene layers. The Raman spectroscopy of the samples reveals the essential properties of the graphene grown on gold surface. In order to characterize the electrical properties of the grown graphene layers, we have transferred them on insulating substrates and fabricated field effect transistors. Owing to distinctive properties of gold, the ability to grow graphene layers on gold surface could open new applications of graphene in electrochemistry and spectroscopy. © 2011 American Institute of Physics

We investigate metadevices working in microwave frequencies by integrating passive metamaterials with active graphene devices.

This work demonstrates an attractive low-cost route to obtain large area and high-quality graphene films by using the ultra-smooth copper foils which are typically used as the negative electrodes in lithium-ion batteries. We first compared the electronic transport properties of our new graphene film with the one synthesized by using commonly used standard copper foils in chemical vapor deposition (CVD). We observed a stark improvement in the electrical performance of the transistors realized on our graphene films. To study the optical properties on large area, we transferred CVD based graphene to transparent flexible substrates using hot lamination method and performed large area optical scanning. We demonstrate the promise of our high quality graphene films for large areas with ~400 cm2 flexible optical modulators. We obtained a profound light modulation over a broad spectrum by using the fabricated large area transparent graphene supercapacitors and we compared the performance of our devices with the one based on graphene from standard copper. We propose that the copper foils used in the lithium-ion batteries could be used to obtain high-quality graphene at much lower-cost, with the improved performance of electrical transport and optical properties in the devices made from them.