Dry Lithography Patterning of Monolayer Flexible Field Effect Transistors by 2D Mica Stamping.

Organic field-effect transistors (OFETs) based on 2D monolayer organic semiconductors (OSC) have demonstrated promising potentials for various applications, such as light emitting diode (LED) display drivers, logic circuits, and wearable electrocardiography (ECG) sensors. To date, the fabrications of this class of highly crystallized 2D organic semiconductors (OSC) are dominated by solution shearing. As these organic active layers are only a few molecular layers thick, their compatibilities with conventional thermal evaporated top electrodes or sophisticated photolithography patterning are very limited, which also restricts their device density. Here, an electrode transfer stamp and a semiconductor patterning stamp are developed to fabricate OFETs with channel lengths down to 3 µm over a large area without using any chemicals or causing any damage to the active layer. 2D 2,9-didecyldinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (C10 -DNTT) monolayer OFETs developed by this new approach shows decent performance properties with a low threshold voltage (VTH ) less than 0.5 V, intrinsic mobility higher than 10 cm2 V-1 s-1 and a subthreshold swing (SS) less than 100 mV dec-1 . The proposed patterning approach is completely comparable with ultraflexible parylene substrate less than 2 µm thick. By further reducing the channel length down to 2 µm and using the monolayer OFET in an AC/DC rectifying circuit, the measured cutoff frequency is up to 17.3 MHz with an input voltage of 4 V. The newly proposed electrode transfer and patterning stamps have addressed the long-lasting compatibility problem of depositing electrodes onto 2D organic monolayer and the semiconductor patterning. It opens a new path to reduce the fabrication cost and simplify the manufacturing process of high-density OFETs for more advanced electronic or biomedical applications.

Effectiveness of pelvic floor muscle training alone or combined with either a novel biofeedback device or conventional biofeedback for improving stress urinary incontinence: A randomized controlled pilot trial.

PURPOSE: To (i) compare the acceptance of a newly developed, novel biofeedback device (PelviSense) with that of conventional biofeedback (CB) using an intravaginal probe for the treatment of stress urinary incontinence (SUI) in women, (ii) examine the feasibility and safety of using the PelviSense device as a pelvic floor muscle (PFM) training (PFMT) adjunct, and (iii) compare the PFMT adherence and effectiveness of CB, the PelviSense device, with PFMT alone for women with SUI. METHODS: An assessor-blinded, three-arm, randomized controlled pilot trial was conducted among 51 women with SUI. Women were randomly allocated to one of three study groups (PelviSense-assisted PFMT, CB-assisted PFMT, or PFMT alone [control]). Outcome measures included the International Consultation on Incontinence Questionnaire-Short Form, the 1-h pad test, and the Modified Oxford Scale. RESULTS: Participants in the PelviSense-assisted PFMT group expressed good device acceptance. PFMT adherence was greater in the PelviSense-assisted PFMT group than in the unassisted or CB-assisted PFMT groups. Between-groups analysis revealed significant effects on improved SUI symptoms, urine loss severity, and PFM strength for the PelviSense-assisted PFMT group compared with the CB-assisted and PFMT alone groups. CONCLUSIONS: The pilot trial results demonstrated moderate to high PFMT adherence in the PelviSense-assisted PFMT group and supported the safety of using the PelviSense device. The preliminary results of the pilot trial showed that PelviSense-assisted PFMT was more effective for reducing SUI symptoms among women than unassisted or CB-assisted PFMT. TRIAL REGISTRATION: This trial was registered in http://ClinicalTrials.gov (reference number: NCT04638348) before the recruitment of the first participant.

Intrinsically Stretchable Organic Electrochemical Transistors with Rigid-Device-Benchmarkable Performance.

Intrinsically stretchable organic electrochemical transistors (OECTs) are being pursued as the next-generation tissue-like bioelectronic technologies to improve the interfacing with the soft human body. However, the performance of current intrinsically stretchable OECTs is far inferior to their rigid counterparts. In this work, for the first time, the authors report intrinsically stretchable OECTs with overall performance benchmarkable to conventional rigid devices. In particular, oxygen level in the stretchable substrate is revealed to have a significant impact on the on/off ratio. By employing stretchable substrates with low oxygen permeabilities, the on/off ratio is elevated from ≈10 to a record-high value of ≈104 , which is on par with a rigid OECT. The device remained functional after cyclic stretching tests. This work demonstrates that intrinsically stretchable OECTs have the potential to serve as a new building block for emerging soft bioelectronic applications such as electronic skin, soft implantables, and soft neuromorphic computing.

Pushing OECTs toward Wearable: Development of a Miniaturized Analytical Control Unit for Wireless Device Characterization.

Organic electrochemical transistors (OECTs) have emerged as a next-generation biosensing technology because of their water-stability, cost-effectiveness, and ability to obtain high sensitivity at low operation voltage (mV). However, a miniaturized readout unit that can wirelessly characterize the overall performance of an OECT is still missing, which hinders the assembling of truly wearable OECT systems for continuous health-monitoring applications. In this work, we present a coin-sized analytical unit for remote and wireless OECT characterization, namely, a personalized electronic reader for electrochemical transistors (PERfECT). It has been verified that PERfECT can measure the transfer, output, hysteresis, and transient behavior of OECTs with resolution and sampling rate on par with the bulky equipment used in laboratories. PERfECT is also capable of characterizing other low-voltage transistors. An integrated board for multiplexed OECT characterizations (32 channels) has also been demonstrated. This work provides a missing building block for developing next-generation OECT-based bioelectronics for digital wearable applications.

Organic Field-Effect Transistor Fabricated on Internal Shrinking Substrate.

In the development of flexible organic field-effect transistors (OFET), downsizing and reduction of the operating voltage are essential for achieving a high current density with a low operating power. Although the bias voltage of the OFETs can be reduced by a high-k dielectric, achieving a threshold voltage close to zero remains a challenge. Moreover, the scaling down of OFETs demands the use of photolithography, and may lead to compatibility issues in organic semiconductors. Herein, a new strategy based on the ductile properties of organic semiconductors is developed to control the threshold voltage at close to zero while concurrently downsizing the OFETs. The OFETs are fabricated on prestressed polystyrene shrink film substrates at room temperature, then thermal energy (160 °C) is used to release the strain. The OFETs conformally attached to the wrinkled structure are shown to locally amplify the electric field. After shrinking, the horizontal device area is reduced by 75%, and the threshold voltage is decreased from -1.44 to -0.18 V, with a subthreshold swing of 74 mV dec-1 and intrinsic gain of 4.151 × 104 . These results reveal that the shrink film can be generally used as a substrate for downsizing OFETs and improving their performance.

Mimicking associative learning using an ion-trapping non-volatile synaptic organic electrochemical transistor.

Associative learning, a critical learning principle to improve an individual's adaptability, has been emulated by few organic electrochemical devices. However, complicated bias schemes, high write voltages, as well as process irreversibility hinder the further development of associative learning circuits. Here, by adopting a poly(3,4-ethylenedioxythiophene):tosylate/Polytetrahydrofuran composite as the active channel, we present a non-volatile organic electrochemical transistor that shows a write bias less than 0.8 V and retention time longer than 200 min without decoupling the write and read operations. By incorporating a pressure sensor and a photoresistor, a neuromorphic circuit is demonstrated with the ability to associate two physical inputs (light and pressure) instead of normally demonstrated electrical inputs in other associative learning circuits. To unravel the non-volatility of this material, ultraviolet-visible-near-infrared spectroscopy, X-ray photoelectron spectroscopy and grazing-incidence wide-angle X-ray scattering are used to characterize the oxidation level variation, compositional change, and the structural modulation of the poly(3,4-ethylenedioxythiophene):tosylate/Polytetrahydrofuran films in various conductance states. The implementation of the associative learning circuit as well as the understanding of the non-volatile material represent critical advances for organic electrochemical devices in neuromorphic applications.

Sub-thermionic, ultra-high-gain organic transistors and circuits.

The development of organic thin-film transistors (OTFTs) with low power consumption and high gain will advance many flexible electronics. Here, by combining solution-processed monolayer organic crystal, ferroelectric HfZrOx gating and van der Waals fabrication, we realize flexible OTFTs that simultaneously deliver high transconductance and sub-60 mV/dec switching, under one-volt operating voltage. The overall optimization of transconductance, subthreshold swing and output resistance leads to transistor intrinsic gain and amplifier voltage gain over 5.3 × 104 and 1.1 × 104, respectively, which outperform existing technologies using organics, oxides and low-dimensional nanomaterials. We further demonstrate battery-powered, integrated wearable electrocardiogram (ECG) and pulse sensors that can amplify human physiological signal by 900 times with high fidelity. The sensors are capable of detecting weak ECG waves (undetectable even by clinical equipment) and diagnosing arrhythmia and atrial fibrillation. Our sub-thermionic OTFT is promising for battery/wireless powered yet performance demanding applications such as electronic skins and radio-frequency identification tags, among many others.

Crystallized Monolayer Semiconductor for Ohmic Contact Resistance, High Intrinsic Gain, and High Current Density.

The contact resistance limits the downscaling and operating range of organic field-effect transistors (OFETs). Access resistance through multilayers of molecules and the nonideal metal/semiconductor interface are two major bottlenecks preventing the lowering of the contact resistance. In this work, monolayer (1L) organic crystals and nondestructive electrodes are utilized to overcome the abovementioned challenges. High intrinsic mobility of 12.5 cm2 V-1 s-1 and Ohmic contact resistance of 40 Ω cm are achieved. Unlike the thermionic emission in common Schottky contacts, the carriers are predominantly injected by field emission. The 1L-OFETs can operate linearly from VDS  = -1 V to VDS as small as -0.1 mV. Thanks to the good pinch-off behavior brought by the monolayer semiconductor, the 1L-OFETs show high intrinsic gain at the saturation regime. At a high bias load, a maximum current density of 4.2 µA µm-1 is achieved by the only molecular layer as the active channel, with a current saturation effect being observed. In addition to the low contact resistance and high-resolution lithography, it is suggested that the thermal management of high-mobility OFETs will be the next major challenge in achieving high-speed densely integrated flexible electronics.

Achieving Ultralow Turn-On Voltages in Organic Thin-Film Transistors: Investigating Fluoroalkylphosphonic Acid Self-Assembled Monolayer Hybrid Dielectrics.

The properties of organic thin-film transistors (TFTs) and thus their ability to address specific circuit design requirements depend greatly on the choice of the materials, particularly the organic semiconductor and the gate dielectric. For a particular organic semiconductor, the TFT performance must be reviewed for different combinations of substrates, fabrication conditions, and the choice of the gate dielectric in order to achieve the optimum TFT and circuit characteristics. We have fabricated and characterized organic TFTs based on the small-molecule organic semiconductor 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene in combination with an ultrathin hybrid gate dielectric consisting of aluminum oxide and a self-assembled monolayer. Fluoroalkylphosphonic acids with chain lengths ranging from 6 to 14 carbon atoms have been used to form the self-assembled monolayer in the gate dielectric, and their influence on the TFT characteristics has been studied. By optimizing the fabrication conditions, a turn-on voltage of 0 V with an on/off current ratio above 106 has been achieved, in combination with charge-carrier mobilities up to 0.4 cm2/V s on flexible plastic substrates and 1 cm2/V s on silicon substrates.

Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors.

The contact resistance in organic thin-film transistors (TFTs) is the limiting factor in the development of high-frequency organic TFTs. In devices fabricated in the inverted (bottom-gate) device architecture, staggered (top-contact) organic TFTs have usually shown or are predicted to show lower contact resistance than coplanar (bottom-contact) organic TFTs. However, through comparison of organic TFTs with different gate-dielectric thicknesses based on the small-molecule organic semiconductor 2,9-diphenyl-dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene, we show the potential for bottom-contact TFTs to have lower contact resistance than top-contact TFTs, provided the gate dielectric is sufficiently thin and an interface layer such as pentafluorobenzenethiol is used to treat the surface of the source and drain contacts. We demonstrate bottom-contact TFTs fabricated on flexible plastic substrates with record-low contact resistance (29 Ωcm), record subthreshold swing (62 mV/decade), and signal-propagation delays in 11-stage unipolar ring oscillators as short as 138 ns per stage, all at operating voltages of about 3 V.

Smart Surgical Catheter for C-Reactive Protein Sensing Based on an Imperceptible Organic Transistor.

Organic field-effect transistors (OFETs)-based sensors have a great potential to be integrated with the next generation smart surgical tools for monitoring different real-time signals during surgery. However, allowing ultraflexible OFETs to have compatibility with standard medical sterilization procedures remains challenging. A novel capsule-like OFET structure is demonstrated by utilizing the fluoropolymer CYTOP to serve both encapsulation and peeling-off enhancement purposes. By adapting a thermally stable organic semiconductor, 2,10-diphenylbis[1]benzothieno[2,3-d;2',3'-d']naphtho[2,3-b;6,7-b']dithiophene (DPh-BBTNDT), these devices show excellent stability in their electrical performance after sterilizing under boiling water and 100 °C-saturated steam for 30 min. The ultrathin thickness (630 nm) enables the device to have superb mechanical flexibility with smallest bending radius down to 1.5 µm, which is essential for application on the highly tortuous medical catheter inside the human body. By immobilizing anti-human C-reactive protein (CRP) (an inflammation biomarker) monoclonal antibody on an extended gate of the OFET, a sensitivity for detecting CRP antigen down to 1 µg mL-1 can be achieved. An ecofriendly water floatation method realized by employing the wettability difference between CYTOP and polyacrylonitrile (PAN) can be used to transfer the device on a ventricular catheter, which successfully distinguishes an inflammatory patient from a healthy one.

Dual-Gated Transistor Platform for On-Site Detection of Lead Ions at Trace Levels.

On-site monitoring of heavy metals in drinking water has become crucial because of several high profile instances of contamination. Presently, reliable techniques for trace level heavy metal detection are mostly laboratory based, while the detection limits of contemporary field-based methods are barely meeting the exposure limits set by regulatory bodies such as the World Health Organization (WHO). Here, we show an on-site deployable, Pb2+ sensor on a dual-gated transistor platform whose lower detection limit is 2 orders of magnitude better than the traditional sensor and 1 order of magnitude lower than the exposure limit set by WHO. The enhanced sensitivity of our design is verified by numerically solving PNP (Planck-Nernst-Poisson) model. We demonstrate that the enhanced sensitivity is due to the suppression of ionic flux. The simplicity and the robustness of the design make it applicable for on-site screening, thereby facilitating rapid response to contamination events.

Investigation of interfacial thermal transport across graphene and an organic semiconductor using molecular dynamics simulations.

The interfacial thermal transport across graphene and an organic semiconductor, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), is investigated using molecular dynamics simulations. The average thermal boundary resistance (TBR) of graphene and DNTT is 4.88 ± 0.12 × 10-8 m2 K W-1 at 300 K. We find that TBR of a graphene-DNTT heterostructure possesses as high as 83.4% reduction after the hydrogenation of graphene. Moreover, as the graphene vacancy increases from 0% to 6%, the TBR drops up to 39.6%. The reduction of TBR is mainly attributed to the coupling enhancement of graphene and DNTT phonons as evaluated from the phonon density of states. On the other hand, TBR keeps a constant value while the vacancy in the DNTT layer increases. The TBR would decrease when the temperature and coupling strength increase. These findings provide a useful guideline for the thermal management of the graphene-based organic electronic devices, especially the large area transistor arrays or sensors.

Phonon thermal transport in silicene-germanene superlattice: a molecular dynamics study.

Two-dimensional (2D) hybrid materials have drawn enormous attention in thermoelectric applications. In this work, we apply a molecular dynamics (MD) simulation to investigate the phonon thermal transport in silicene-germanene superlattice. A non-monotonic thermal conductivity of silicene-germanene superlattice with period length is revealed, which is due to the coherent-incoherent phonon conversion and phonon confinement mechanisms. We also calculate the thermal conductivity of a Si-Ge random mixing monolayer, showing a U-shaped trend. Because of the phonon mode localizations at Ge concentration of <20% and >80%, thermal conductivity varies dramatically at low doping regions. By changing the total length (L total), the infinite-length thermal conductivities of pure silicene, pure germanene, silicene-germanene superlattice, and Si-Ge random mixing monolayer are extracted as 16.08, 15.95, 5.60 and 4.47 W/m-K, respectively. The thermal boundary conductance (TBC) of the silicene-germanene is also evaluated, showing a small thermal rectification. At L total = 274.7 nm, the TBC of silicene to germanene is 620.49 MW/m2-K, while that of germanene to silicene is 528.76 MW/m2-K.

Highly Sensitive Glucose Sensor Based on Organic Electrochemical Transistor with Modified Gate Electrode.

An organic electrochemical transistor (OECT) with a glucose oxidase (GOx) and poly(n-vinyl-2-pyrrolidone)-capped platinum nanoparticles (Pt NPs) gate electrode was successfully integrated with a microfluidic channel to act as a highly sensitive chip-based glucose sensor. The sensing mechanism relies on the enzymatic reaction between glucose and GOx followed by electrochemical oxidation of hydrogen peroxide (H2O2) produced in the enzymatic reaction. This process largely increases the electrolyte potential that applies on PEDOT:PSS channel and causes more cations penetrate into PEDOT:PSS film to reduce it to semi-conducting state resulting in lower electric current between the source and the drain. The extremely high sensitivity and low detection limit (0.1 µM) of the sensor was achievable due to highly efficient Pt NPs catalysis in oxidation of H2O2. Pt NPs were deposited by a bias-free two-step dip coating method followed by a UV-Ozone post-treatment to enhance catalytic ability. A polydimethylsiloxane (PDMS) microfluidic channel was directly attached to the OECT active layer, providing a short detection time (~1 min) and extremely low analyte consumption (30 µL). Our sensor has great potential for real-time, noninvasive, and portable glucose sensing applications due to its compact size and high sensitivity.

Molecular dynamics study of thermal transport in a dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) organic semiconductor.

The thermal transport in a high-mobility and air-stable small molecule organic semiconductor, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), is simulated by using non-equilibrium molecular dynamics. We find that the thermal conductivity of DNTT has a strong dependence on crystal size and orientation directions (a*, b* and c*). The bulk thermal conductivities of DNTT along the a*, b* and c* directions are 0.73, 0.33 and 0.95 W m-1 K-1, respectively. The polycrystalline nature of the DNTT thin film in the experiment means that it is essential to consider the effects of thermal boundary resistance (TBR) and vacancy on the thermal conductivity. The TBRs across different interfaces are calculated as 7.00 ± 0.26, 6.15 ± 0.13 and 3.20 ± 0.09 × 10-9 m2 K W-1 for the a*-b*, a*-c* and b*-c* interfaces, respectively. On the other hand, the thermal conductivities of DNTT with a vacancy concentration of 6% can be reduced by 44%, 33% and 35% in the a*, b* and c* directions. Our findings indicate that the boundary and defect scattering of phonons has significant effects on the thermal conductivity of organic semiconductors. This work contributes fundamental knowledge to control the thermal properties of organic semiconductors in organic electronic devices.

High Sensitivity, Wearable, Piezoresistive Pressure Sensors Based on Irregular Microhump Structures and Its Applications in Body Motion Sensing.

A pressure sensor based on irregular microhump patterns has been proposed and developed. The devices show high sensitivity and broad operating pressure regime while comparing with regular micropattern devices. Finite element analysis (FEA) is utilized to confirm the sensing mechanism and predict the performance of the pressure sensor based on the microhump structures. Silicon carbide sandpaper is employed as the mold to develop polydimethylsiloxane (PDMS) microhump patterns with various sizes. The active layer of the piezoresistive pressure sensor is developed by spin coating PEDOT: PSS on top of the patterned PDMS. The devices show an averaged sensitivity as high as 851 kPa(-1) , broad operating pressure range (20 kPa), low operating power (100 nW), and fast response speed (6.7 kHz). Owing to their flexible properties, the devices are applied to human body motion sensing and radial artery pulse. These flexible high sensitivity devices show great potential in the next generation of smart sensors for robotics, real-time health monitoring, and biomedical applications.

A Low-Operating-Power and Flexible Active-Matrix Organic-Transistor Temperature-Sensor Array.

An organic flexible temperature-sensor array exhibits great potential in health monitoring and other biomedical applications. The actively addressed 16 × 16 temperature sensor array reaches 100% yield rate and provides 2D temperature information of the objects placed in contact, even if the object has an irregular shape. The current device allows defect predictions of electronic devices, remote sensing of harsh environments, and e-skin applications.

Modifying the thermal conductivity of small molecule organic semiconductor thin films with metal nanoparticles.

Thermal properties of organic semiconductors play a significant role in the performance and lifetime of organic electronic devices, especially for scaled-up large area applications. Here we employ silver nanoparticles (Ag NPs) to modify the thermal conductivity of the small molecule organic semiconductor, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT). The differential 3-ω method was used to measure the thermal conductivity of Ag-DNTT hybrid thin films. We find that the thermal conductivity of pure DNTT thin films do not vary with the deposition temperature over a range spanning 24 °C to 80 °C. The thermal conductivity of the Ag-DNTT hybrid thin film initially decreases and then increases when the Ag volume fraction increases from 0% to 32%. By applying the effective medium approximation to fit the experimental results of thermal conductivity, the extracted thermal boundary resistance of the Ag-DNTT interface is 1.14 ± 0.98 × 10(-7) m(2)-K/W. Finite element simulations of thermal conductivity for realistic film morphologies show good agreement with experimental results and effective medium approximations.

High performance organic transistor active-matrix driver developed on paper substrate.

The fabrication of electronic circuits on unconventional substrates largely broadens their application areas. For example, green electronics achieved through utilization of biodegradable or recyclable substrates, can mitigate the solid waste problems that arise at the end of their lifespan. Here, we combine screen-printing, high precision laser drilling and thermal evaporation, to fabricate organic field effect transistor (OFET) active-matrix (AM) arrays onto standard printer paper. The devices show a mobility and on/off ratio as high as 0.56 cm(2)V(-1)s(-1) and 10(9) respectively. Small electrode overlap gives rise to a cut-off frequency of 39 kHz, which supports that our AM array is suitable for novel practical applications. We demonstrate an 8 × 8 AM light emitting diode (LED) driver with programmable scanning and information display functions. The AM array structure has excellent potential for scaling up.