Light-material interaction based self-organized nanopatterning and flexible artery pulse sensor

Abstract

In the modern society, flexible electronic materials and devices have great deal of attention for the next-generation information technology (IT) such as internet of things (IoTs), wearable electronics, and bio-medical devices, which can offer convenient lifestyle unprecedented to the previous eras. Although various researches including organic and carbon materials have been conducted for flexible electronics, they have ex-hibited insufficient electrical properties and poor scalability.

For inorganic or nano materials, high temperature thermal conditions including material synthesis, an-nealing and semiconductor process have to be employed for high-performance flexible electronics. However, plastic substrates have an inherent thermal limitation such as low melting point, and heat shrinkage/expansion, requiring new solutions to directly implement high-performance flexible materials and devices on plastics. In this regard, light-material interaction has provided powerful solutions for realizing future flexible materials and devices (e.g., solar cells, smart sensors on plastics, and transparent conductors), via the exceptional capability to stimulate physical/chemical reactions. Here, we introduce ultrafast 2-dimensional material synthesis, sub-10 nm patterning, and inorganic based laser lift off technique via light-material interactions for next-generation technology.

In chapter 2, we report novel synthesis method of multilayer graphene (MLG) using xenon flash lamp, which has never been reported. The custom-designed xenon flash system could quantitatively control the ratio of gas molecules (acetylene/argon/hydrogen), and pulse width/intensity of light to optimize the flash-induced graphene synthesis. The thickness of Ni film was precisely determined to minimize the lattice mismatch between graphene and Ni for high-throughput and structurally homogeneous graphene growth. High-intensity flash light could rapidly raise the temperature of Ni thin film to induce chemical bond breaking of hydrocarbon precursor, enabling synthesis of MLG by time controlled thermal cooling mechanism. The synthesized MLG was successfully transferred onto flexible polyethylene terephthalate film, which shows high quality, and uniform MLG. Finally, the principles of MLG synthesis was theoretically investigated by finite element method, proving that photo-thermal interactions between flash and Ni could sufficiently provide the thermal energy for ultrafast and large-area MLG growth. These results may open up a new feasibility of the xenon flash lamp system for mass production of graphene associating with highly productive roll-to-roll process.

In chapter 3, we introduce wafer-scale reliable, fab-compatible and ultrafast directed self-assembly (DSA) of high-χ BCPs using Xenon flash lamp annealing. Millisecond scale instantaneous heating/quenching process over extremely high temperature (over 600 °C) enabled by flash light irradiation successfully achieved ultrafast large grain growth of sub-10 nm scale self-assembled nanopatterns with minimal defect formation. The underlying mechanism for the rapid high order self-assembly is analyzed based on the kinetics and thermodynamics of BCPs self-assembly. Furthermore, this novel self-assembly process was applied to graphoepitaxial assembly to demonstrate the feasibility of DSA nanolithography with sub-10 nm patterns over a large area.

In chapter 4, we report self-powered real-time arterial pulse monitoring system utilizing epidermal piezoelectric sensor. An ultrathin sensor enables conformal attachment to the rugged skin and response to the tiny human pulse signals. The self-powered piezoelectric sensor showed characteristics with a sensitivity of 0.018 kPa^{-1} and response time of 60 ms, and good mechanical stability. Ultrathin pulse sensor on human wrist and neck detected radial/carotid arterial pulse, respiration rates, and trachea movements for medical health monitoring. Our piezoelectric sensor exhibited good biocompatibility from human and mouse cell cytotoxicity test. Furthermore, we demonstrated a complementary signal processing circuit which amplifies and filters the tiny pulse voltage for real-time health monitoring operation. Finally, radial artery pulse detected by piezoelectric pulse sensor was wirelessly transmitted to smart phone.

In chapter 5, we report band-shaped prototype wearable sphygmomanometer by implementing a piezoelectric based pressure sensor on a flexible printed circuit board (FPCB). With the principle of piezoelectric effect, we examined a correlation between output voltage of the sensor obtained by radial artery pulse and actual systolic/diastolic blood pressure (SBP/DBP) values. The FPCB was designed with various integrated circuits (ICs) and chips that process the pulse signals detected by the sensor including RC-based band pass filtering, charge IC, voltage booster/converter, and operational amplifier and so on. The analogy results showed a linear relationship between output voltage and blood pressure. Furthermore, we presented an algorithm to predict SBP/DBP value for continuous cardiovascular monitoring. Finally, with experimentally acquired correlation and algorithm, we estimated the SBP/DBP from detected radial artery pulse signal, confirming that the SBP/DBP values had an error range of ~2.4% and 9.7 %, respectively. These results will open up a new approach to self-powered wearable active sensor system for continuous health monitoring devices.

In chapter 6, we report flexible lead-free piezoelectric based active sensor via screen-printed LNKN thin film for biocompatible epidermal sensor. The LNKN thin film with tens of microns thickness was obtained by only one squeeze motion, which methods is extremely easy, simple and cost-effective compared to vacuum based deposition methods. The flexible LNKN sensor responded to various mechanical modes including lateral strain and normal pressure, showing characteristics with a sensitivity and response time of 1.015 V$ε^{-1}$ and 70 ms, and those of 0.0062 V$kPa^{-1}$ and 90 ms, respectively. Our active sensor also exhibited a good mechanical durability over a long period of 10 days. Furthermore, flexible LNKN sensor on human neck detected carotid artery pulse and deep respiration actions, which signal was wirelessly delivered to a smart phone after amplification and filtering for reliable real-time medical signal monitoring.