Ruozhou Li

Direct writing on paper of foldable capacitive touch pads with silver nanowire inks.

Paper-based capacitive touch pads can be fabricated utilizing high-concentration silver nanowire inks needle-printed directly onto paper substrates through a 2D programmable platform. Post deposition, silver nanowire tracks can be photonically sintered using a camera flash to reduce sheet resistance similar to thermal sintering approaches. Touch pad sensors on a variety of paper substrates can be achieved with optimized silver nanowire tracks. Rolling and folding trials, which yielded only modest changes in capacitance and no loss of function, coupled with touch pad functionality on curved surfaces, suggest sufficient flexibility and durability for paper substrate touch pads to be used in diverse applications. A simplified model to predict touch pad capacitance variation ranges with differing touch conditions was developed, with good agreement against experimental results. Such paper-based touch pads have the advantage of simple structure, easy fabrication, and fast sintering, which holds promise for numerous commercial applications including low-cost portable devices where ultrathin and lightweight features, coupled with reliable bending stability are desirable.

High-rate in-plane micro-supercapacitors scribed onto photo paper using in situ femtolaser-reduced graphene oxide/Au nanoparticle microelectrodes

Direct laser-reduction of graphene oxide (GO), as a lithography-free approach, has been proven effective in manufacturing in-plane micro-supercapacitors (MSCs) with fast ion diffusion. However, the power density and the charge/discharge rate are still limited by the relatively low conductivity of electrodes. Here, we report a facile approach by exploiting femtolaser in situ reduction of the hydrated GO and chloroauric acid (HAuCl4) nanocomposite simultaneously, which incorporates both the patterning of rGO electrodes and the fabrication of Au current collectors in a single step. These flexible MSCs boast achievements of one-hundred fold increase in electrode conductivities of up to 1.1 × 106 S m−1, which provide superior rate capability (50% for the charging rate increase from 0.1 V s−1 to 100 V s−1), sufficiently high frequency responses (362 Hz, 2.76 ms time constant), and large specific capacitances of 0.77 mF cm−2 (17.2 F cm−3 for volumetric capacitance) at 1 V s−1, and 0.46 mF cm−2 (10.2 F cm−3) at 100 V s−1. The use of photo paper substrates enables the flexibility of this fabrication protocol. Moreover, proof-of-concept 3D MSCs are demonstrated with enhanced areal capacitance (up to 3.84 mF cm−2 at 1 V s−1) while keeping high rate capabilities. This prototype of all solid-state MSCs demonstrates the broad range of potentials of thin-film based energy storage device applications for flexible, portable, and wearable electronic devices that require a fast charge/discharge rate and high power density.

Femtosecond Laser Fabrication of Cavity Microball Lens (CMBL) inside a PMMA Substrate for Super-Wide Angle Imaging.

Since microlenses have to date been fabricated primarily by surface manufacturing, they are highly susceptible to surface damage, and their microscale size makes it cumbersome to handle. Thus, cavity lenses are preferred, as they alleviate these difficulties associated with the surface-manufactured microlenses. Here, it is shown that a high repetition femtosecond laser can effectively fabricate cavity microball lenses (CMBLs) inside a polymethyl methacrylate slice. Optimal CMBL fabrication conditions are determined by examining the pertinent parameters, including the laser processing time, the average irradiation power, and the pulse repetition rates. In addition, a heat diffusion modeling is developed to better understand the formation of the spherical cavity and the slightly compressed affected zone surrounding the cavity. A micro-telescope consisting of a microscope objective and a CMBL demonstrates a super-wide field-of-view imaging capability. Finally, detailed optical characterizations of CMBLs are elaborated to account for the refractive index variations of the affected zone. The results presented in the current study demonstrate that a femtosecond laser-fabricated CMBL can be used for robust and super-wide viewing micro imaging applications.

Bisphenol A Sensors on Polyimide Fabricated by Laser Direct Writing for Onsite River Water Monitoring at Attomolar Concentration

This work presents an aptamer-based, highly sensitive and specific sensor for atto- to femtomolar level detection of bisphenol A (BPA). Because of its widespread use in numerous products, BPA enters surface water from effluent discharges during its manufacture, use, and from waste landfill sites throughout the world. On-site measurement of BPA concentrations in water is important for evaluating compliance with water quality standards or environmental risk levels of the harmful compound in the environment. The sensor in this work is porous, conducting, interdigitated electrodes that are formed by laser-induced carbonization of flexible polyimide sheets. BPA-specific aptamer is immobilized on the electrodes as the probe, and its binding with BPA at the electrode surface is detected by capacitive sensing. The binding process is aided by ac electroosmotic effect that accelerates the transport of BPA molecules to the nanoporous graphene-like structured electrodes. The sensor achieved a limit of detection of 58.28 aM with a response time of 20 s. The sensor is further applied for recovery analysis of BPA spiked in surface water. This work provides an affordable platform for highly sensitive, real time, and field-deployable BPA surveillance critical to the evaluation of the ecological impact of BPA exposure.

Fabrication of embedded microball lens in PMMA with high repetition rate femtosecond fiber laser

Embedded microball lenses with superior optical properties function as convex microball lens (VMBL) and concave microball lens (CMBL) were fabricated inside a PMMA substrate with a high repetition rate femtosecond fiber laser. The VMBL was created by femtosecond laser-induced refractive index change, while the CMBL was fabricated due to the heat accumulation effect of the successive laser pulses irradiation at a high repetition rate. The processing window for both types of the lenses was studied and optimized, and the optical properties were also tested by imaging a remote object with an inverted microscope. In order to obtain the microball lenses with adjustable focal lengths and suppressed optical aberration, a shape control method was thus proposed and examined with experiments and ZEMAX® simulations. Applying the optimized fabrication conditions, two types of the embedded microball lenses arrays were fabricated and then tested with imaging experiments. This technology allows the direct fabrication of microlens inside transparent bulk polymer material which has great application potential in multi-function integrated microfluidic devices.

Photonic nanomanufacturing of high performance energy devices on flexible substrates

The authors have investigated various photonic processing for various energy devices on flexible substrates with nanoinks. For printable electronics, different conducting nanoinks are developed, including silver nanowires, silver nanoplates, Cu-Ag core-shell nanoparticles, graphene oxide, and graphene. The authors showed that these inks are enabling for direct writing of antenna on paper for radio frequency (RF) energy harvesting, potentially for wireless charging application. For curing printed nanoinks and nanopastes, the authors compared four kinds of methods: chemical activated self-sintering, thermal sintering, photonic sintering with flash light, and athermal sintering with ultrafast fiber laser irradiation. The authors also developed an innovative and facile approach to fabricate supercapacitors on flexible substrates with femtosecond laser writing and photonic reduction. Au-reduced graphene oxide nanocomposite is used for electrical electrodes and collectors. Unlike previous studies, collectors ar...

Photonic nanomanufacturing of high performance energy device on flexible substrate

With the developing of wearable electronics and information society, integrated energy storage devices are urgently demanded to be integrated on flexible substrates. We successfully demonstrated using direct laser-reduction of the hydrated GO and chloroauric acid (HAuCl4) nanocomposite to fabricate in-plane micro-supercapacitors (MSCs) with fast ion diffusion on paper. The electrode conductivity of these flexible nanocomposites reaches up to 1.1 x 106 S m-1, which enhances superior rate capability of micro-supercapacitors, and large specific capacitances of 0.77 mF cm-2 (17.2 F cm-3 for volumetric capacitance) at 1 V s-1, and 0.46 mF cm-2 (10.2 F cm-3) at 100 V s-1. We also have demonstrated that pulsed laser irradiation rapidly converts the polyimide (PI) sheets into an electrically conductive porous carbon structure in ambient conditions. The specific capacitance of single layer surface supercapacitors can reach 20.4 mF/cm2 at 0.1 mA/cm2 discharge current density. Furthermore, we successfully fabricate the multi-layer supercapacitor with the PI substrate using 3D femtosecond laser direct writing, and the specific capacitances of three layers supercapacitors is 37.5 mF/cm2.