Der-Hsien Lien

Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis

Wearable sensor technologies are essential to the realization of personalized medicine through continuously monitoring an individual’s state of health. Sampling human sweat, which is rich in physiological information, could enable non-invasive monitoring. Previously reported sweat-based and other non-invasive biosensors either can only monitor a single analyte at a time or lack on-site signal processing circuitry and sensor calibration mechanisms for accurate analysis of the physiological state. Given the complexity of sweat secretion, simultaneous and multiplexed screening of target biomarkers is critical and requires full system integration to ensure the accuracy of measurements. Here we present a mechanically flexible and fully integrated (that is, no external analysis is needed) sensor array for multiplexed in situ perspiration analysis, which simultaneously and selectively measures sweat metabolites (such as glucose and lactate) and electrolytes (such as sodium and potassium ions), as well as the skin temperature (to calibrate the response of the sensors). Our work bridges the technological gap between signal transduction, conditioning (amplification and filtering), processing and wireless transmission in wearable biosensors by merging plastic-based sensors that interface with the skin with silicon integrated circuits consolidated on a flexible circuit board for complex signal processing. This application could not have been realized using either of these technologies alone owing to their respective inherent limitations. The wearable system is used to measure the detailed sweat profile of human subjects engaged in prolonged indoor and outdoor physical activities, and to make a real-time assessment of the physiological state of the subjects. This platform enables a wide range of personalized diagnostic and physiological monitoring applications.

Few-Layer MoS2 with High Broadband Photogain and Fast Optical Switching for Use in Harsh Environments

Few-layered MoS2 as Schottky metal-semiconductor-metal photodetectors (MSM PDs) for use in harsh environments makes its debut as two-dimensional (2D) optoelectronics with high broadband gain (up to 13.3), high detectivity (up to ~10(10) cm Hz(1/2)/W), fast photoresponse (rise time of ~70 μs and fall time of ~110 μs), and high thermal stability (at a working temperature of up to 200 °C). Ultrahigh responsivity (0.57 A/W) of few-layer MoS2 at 532 nm is due to the high optical absorption (~10% despite being less than 2 nm in thickness) and a high photogain, which sets up a new record that was not achievable in 2D nanomaterials previously. This study opens avenues to develop 2D nanomaterial-based optoelectronics for harsh environments in imaging techniques and light-wave communications as well as in future memory storage and optoelectronic circuits.

Dual-Gated MoS2/WSe2 van der Waals Tunnel Diodes and Transistors

Two-dimensional layered semiconductors present a promising material platform for band-to-band-tunneling devices given their homogeneous band edge steepness due to their atomically flat thickness. Here, we experimentally demonstrate interlayer band-to-band tunneling in vertical MoS2/WSe2 van der Waals (vdW) heterostructures using a dual-gate device architecture. The electric potential and carrier concentration of MoS2 and WSe2 layers are independently controlled by the two symmetric gates. The same device can be gate modulated to behave as either an Esaki diode with negative differential resistance, a backward diode with large reverse bias tunneling current, or a forward rectifying diode with low reverse bias current. Notably, a high gate coupling efficiency of ∼80% is obtained for tuning the interlayer band alignments, arising from weak electrostatic screening by the atomically thin layers. This work presents an advance in the fundamental understanding of the interlayer coupling and electron tunneling in semiconductor vdW heterostructures with important implications toward the design of atomically thin tunnel transistors.

Probing surface band bending of surface-engineered metal oxide nanowires.

We in situ probed the surface band bending (SBB) by ultraviolet photoelectron spectroscopy (UPS) in conjunction with field-effect transistor measurements on the incompletely depleted ZnO nanowires (NWs). The diameter range of the NWs is ca. 150-350 nm. Several surface treatments (i.e., heat treatments and Au nanoparticle (NP) decoration) were conducted to assess the impact of the oxygen adsorbates on the SBB. A 100 °C heat treatment leads to the decrease of the SBB to 0.74 ± 0.15 eV with 29.9 ± 3.0 nm width, which is attributed to the removal of most adsorbed oxygen molecules from the ZnO NW surfaces. The SBB of the oxygen-adsorbed ZnO NWs is measured to be 1.53 ± 0.15 eV with 43.2 ± 2.0 nm width. The attachment of Au NPs to the NW surface causes unusually high SBB (2.34 ± 0.15 eV with the wide width of 53.3 ± 1.6 nm) by creating open-circuit nano-Schottky junctions and catalytically enhancing the formation of the charge O(2) adsorbates. These surface-related phenomena should be generic to all metal oxide nanostructures. Our study is greatly beneficial for the NW-based device design of sensor and optoelectronic applications via surface engineering.

Photoconductive enhancement of single ZnO nanowire through localized Schottky effects

We demonstrated the Au nanoparticle (NP) decoration as an effective way to enhance both photocurrent and photoconductive gain of single ZnO nanowire (NW) photodetectors (PDs) through localized Schottky effects. The enhancement is caused by the enhanced space charge effect due to the existence of the localized Schottky junctions under open-circuit conditions at the NW surfaces, leading to a more pronounced electron-hole separation effect. Since the band-bending under illumination varies relatively small for an Au NP-decorated ZnO NW, the decay of gain is less prominent with increased excitation power, demonstrating the feasibility for a PD to maintain a high gain under high-power illumination.

ALL-PRINTED PAPER MEMORY

We report the memory device on paper by means of an all-printing approach. Using a sequence of inkjet and screen-printing techniques, a simple metal–insulator–metal device structure is fabricated on paper as a resistive random access memory with a potential to reach gigabyte capacities on an A4 paper. The printed-paper-based memory devices (PPMDs) exhibit reproducible switching endurance, reliable retention, tunable memory window, and the capability to operate under extreme bending conditions. In addition, the PBMD can be labeled on electronics or living objects for multifunctional, wearable, on-skin, and biocompatible applications. The disposability and the high-security data storage of the paper-based memory are also demonstrated to show the ease of data handling, which are not achievable for regular silicon-based electronic devices. We envision that the PPMDs manufactured by this cost-effective and time-efficient all-printing approach would be a key electronic component to fully activate a paper-based circuit and can be directly implemented in medical biosensors, multifunctional devices, and self-powered systems.

Supersensitive, Ultrafast, and Broad-Band Light-Harvesting Scheme Employing Carbon Nanotube/TiO2 Core–Shell Nanowire Geometry

We demonstrate a novel, feasible strategy for practical application of one-dimensional photodetectors by integrating a carbon nanotube and TiO(2) in a core-shell fashion for breaking the compromise between the photogain and the response/recovery speed. Radial Schottky barriers between carbon nanotube cores and TiO(2) shells and surface states at TiO(2) shell surface regulate electron transport and also facilitate the separation of photogenerated electrons and holes, leading to ultrahigh photogain (G = 1.4 × 10(4)) and the ultrashort response/recovery times (4.3/10.2 ms). Additionally, radial Schottky junction and defect band absorption broaden the detection range (UV-visible). The concept using metallic core oxide-shell geometry with radial Schottky barriers holds potential to pave a new way to realize nanostructured photodetectors for practical use.

Engineering Light Outcoupling in 2D Materials

When light is incident on 2D transition metal dichalcogenides (TMDCs), it engages in multiple reflections within underlying substrates, producing interferences that lead to enhancement or attenuation of the incoming and outgoing strength of light. Here, we report a simple method to engineer the light outcoupling in semiconducting TMDCs by modulating their dielectric surroundings. We show that by modulating the thicknesses of underlying substrates and capping layers, the interference caused by substrate can significantly enhance the light absorption and emission of WSe2, resulting in a ∼11 times increase in Raman signal and a ∼30 times increase in the photoluminescence (PL) intensity of WSe2. On the basis of the interference model, we also propose a strategy to control the photonic and optoelectronic properties of thin-layer WSe2. This work demonstrates the utilization of outcoupling engineering in 2D materials and offers a new route toward the realization of novel optoelectronic devices, such as 2D LEDs and solar cells.

Solar-Blind Photodetectors for Harsh Electronics

We demonstrate solar-blind photodetectors (PDs) by employing AlN thin films on Si(100) substrates with excellent temperature tolerance and radiation hardness. Even at a bias higher than 200 V the AlN PDs on Si show a dark current as low as ~ 1 nA. The working temperature is up to 300°C and the radiation tolerance is up to 1013 cm−2 of 2-MeV proton fluences for AlN metal-semiconductor-metal (MSM) PDs. Moreover, the AlN PDs show a photoresponse time as fast as ~ 110 ms (the rise time) and ~ 80 ms (the fall time) at 5 V bias. The results demonstrate that AlN MSM PDs hold high potential in next-generation deep ultraviolet PDs for use in harsh environments.

High Luminescence Efficiency in MoS2 Grown by Chemical Vapor Deposition

One of the major challenges facing the rapidly growing field of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is the development of growth techniques to enable large-area synthesis of high-quality materials. Chemical vapor deposition (CVD) is one of the leading techniques for the synthesis of TMDCs; however, the quality of the material produced is limited by defects formed during the growth process. A very useful nondestructive technique that can be utilized to probe defects in semiconductors is the room-temperature photoluminescence (PL) quantum yield (QY). It was recently demonstrated that a PL QY near 100% can be obtained in MoS2 and WS2 monolayers prepared by micromechanical exfoliation by treating samples with an organic superacid: bis(trifluoromethane)sulfonimide (TFSI). Here we have performed a thorough exploration of this chemical treatment on CVD-grown MoS2 samples. We find that the as-grown monolayers must be transferred to a secondary substrate, which releases strain, to obtain high QY by TFSI treatment. Furthermore, we find that the sulfur precursor temperature during synthesis of the MoS2 plays a critical role in the effectiveness of the treatment. By satisfying the aforementioned conditions we show that the PL QY of CVD-grown monolayers can be improved from ∼0.1% in the as-grown case to ∼30% after treatment, with enhancement factors ranging from 100 to 1500× depending on the initial monolayer quality. We also found that after TFSI treatment the PL emission from MoS2 films was visible by eye despite the low absorption (5-10%). The discovery of an effective passivation strategy will speed the development of scalable high-performance optoelectronic and electronic devices based on MoS2.