Flexible Field-effect Transistors Research Articles

High mobility graphene field effect transistors on flexible EVA/PET foils

Monolayer graphene is a promising material for a wide range of applications, including sensors, optoelectronics, antennas, EMR shielding, flexible electronics, and conducting electrodes. Chemical vapor deposition (CVD) of carbon atoms on a metal catalyst is the most scalable and cost-efficient method for synthesizing high-quality, large-area monolayer graphene. The usual method of transferring the CVD graphene from the catalyst to a target substrate involves a polymer carrier which is dissolved after the transfer process is completed. Due to often unavoidable damage to graphene, as well as contamination and residues, carrier mobilities are typically 1000–3000 cm2(Vs)−1 , unless complex and elaborate measures are taken. Here, we report on a simple scalable fabrication method for flexible graphene field-effect transistors that eliminates the polymer interim carrier, by laminating the graphene directly onto office lamination foils, removing the catalyst, and depositing Parylene N as a gate dielectric and encapsulation layer. The fabricated transistors show field- and Hall-effect mobilities of 7000–10 000 cm2(Vs)−1 with a residual charge-carrier density of 2×1011 1 cm−2 at room temperature. We further validate the material quality by terahertz time-domain spectroscopy and observation of the quantum Hall effect at low temperatures in a moderate magnetic field of ∼5 T. The Parylene encapsulation provides long-term stability and protection against additional lithography steps, enabling vertical device integration in multilayer electronics on a flexible platform.

Non-volatile memory characteristics of flexible ferroelectric field-effect transistors consisting of Sm-doped BiFeO3 thin films

This study investigates the impact of Sm doping on the structural, electrical, and functional properties of BiFeO3 (BFO) thin films. The aim is to improve their non-volatile memory characteristics for use in flexible ferroelectric field-effect transistors (FeFETs). We fabricated Sm-doped BFO (SBFO) thin films on flexible mica substrates with Sm concentrations ranging from 0 to 15 %. The FeFET device structures, which include MoS2 monolayer channels and SBFO thin films with Pt bottom electrodes, were analyzed. The SBFO thin films displayed a (111) preferential orientation due to the influence of underlying Pt electrodes. Notably, the SBFO thin film doped with a 10 % Sm concentration showed optimal enhancements, including a reduction in leakage current density and an increase in remanent polarization to 55.8 μC/cm2, compared to undoped thin film (35.5 μC/cm2). Furthermore, the thin film exhibited superior fatigue resistance for up to 1012 switching cycles and maintained stable values under significant bending stress for ferroelectric polarizations. On the other hand, SBFO thin film with 15 % Sm doping displayed relaxor antiferroelectric behavior, highlighting the importance of doping concentration. Therefore, for non-volatile memory application reliability, the FeFET device featuring a 10 % Sm-doped SBFO thin film not only enhanced the memory window but also showed superior fatigue characteristics over 1012 switching cycles, compared to its undoped counterpart. These findings demonstrate the effect of Sm doping in customizing the properties of BFO thin films for advanced, flexible non-volatile memory devices, offering a novel insight to optimize ferroelectric field-effect transistor performance.

High-performance ferroelectric field-effect transistors with ultra-thin indium tin oxide channels for flexible and transparent electronics

With the development of wearable devices and hafnium-based ferroelectrics (FE), there is an increasing demand for high-performance flexible ferroelectric memories. However, developing ferroelectric memories that simultaneously exhibit good flexibility and significant performance has proven challenging. Here, we developed a high-performance flexible field-effect transistor (FeFET) device with a thermal budget of less than 400 °C by integrating Zr-doped HfO2 (HZO) and ultra-thin indium tin oxide (ITO). The proposed FeFET has a large memory window (MW) of 2.78 V, a high current on/off ratio (ION/IOFF) of over 108, and high endurance up to 2×107 cycles. In addition, the FeFETs under different bending conditions exhibit excellent neuromorphic properties. The device exhibits excellent bending reliability over 5×105 pulse cycles at a bending radius of 5 mm. The efficient integration of hafnium-based ferroelectric materials with promising ultrathin channel materials (ITO) offers unique opportunities to enable high-performance back-end-of-line (BEOL) compatible wearable FeFETs for edge intelligence applications.

High-Performance Ni3(HHTP)2 Film-Based Flexible Field-Effect Transistor Gas Sensors.

Conductive metal-organic frameworks (MOFs) have received increasing attention in recent years and present high application potential as sensing elements in electronic sensors. In this study, flexible field-effect transistor (FET) sensors based on conductive MOF, i.e., Ni3(HHTP)2, have been constructed. This Ni3(HHTP)2 sensor has high sensitivity (detection limit of 56 ppb) as well as superior selectivity for NO2 detection at room temperature, which is demonstrated by accurate gas detection in a mixed gas atmosphere. Moreover, by employing six flexible substrates, i.e., polyimide (PI), tape (PET), facemask, paper cup, tablecloth, and take-out bag (textile), we successfully demonstrate the universality of the flexible sensor construction with conductive MOF as sensing film on various substrates. This study of conductive MOF-based flexible electronic sensors offers a new opportunity for a wide range of sensing applications with wearable and portable electronic devices.

Flexible field-effect transistors with high-quality and uniform single-layer graphene for high mobility

In this work, a fully flexible graphene field-effect transistor with high carrier mobility is reported. Patterned high-quality and uniform single-layer graphene films are successfully realized by combining the selective growth on a patterned copper foil and the direct transfer method to minimize degradation factors. The selectively grown single-layer graphene is directly transferred to the target substrate through the deposition of poly-para-xylylene (Parylene) C. The quality of the graphene films is confirmed by Raman spectroscopy. The analysis reveals that the use of Parylene C as the substrate, gate dielectric, and encapsulation layer has the advantage of reducing the scattering by the optical phonons and charge puddles. The estimated residual carrier density is 1.72 × 1011 cm−2, and the intrinsic hole and electron carrier mobilities are found to be as high as 10 260 and 10 010 cm2 V−1 s−1, respectively. This study can pave the way for the development and mass production of high-performance and fully flexible graphene electronics.

Ultrathin Flexible Encapsulation Materials Based on Al2O3/Alucone Nanolaminates for Improved Electrical Stability of Silicon Nanomembrane-Based MOS Capacitors.

Ultrathin flexible encapsulation (UFE) using multilayered films has prospects for practical applications, such as implantable and wearable electronics. However, existing investigations of the effect of mechanical bending strains on electrical properties after the encapsulation procedure provide insufficient information for improving the electrical stability of ultrathin silicon nanomembrane (Si NM)-based metal oxide semiconductor capacitors (MOSCAPs). Here, we used atomic layer deposition and molecular layer deposition to generate 3.5 dyads of alternating 11 nm Al2O3 and 3.5 nm aluminum alkoxide (alucone) nanolaminates on flexible Si NM-based MOSCAPs. Moreover, we bent the MOSCAPs inwardly to radii of 85 and 110.5 mm and outwardly to radii of 77.5 and 38.5 mm. Subsequently, we tested the unbent and bent MOSCAPs to determine the effect of strain on various electrical parameters, namely the maximum capacitance, minimum capacitance, gate leakage current density, hysteresis voltage, effective oxide charge, oxide trapped charge, interface trap density, and frequency dispersion. The comparison of encapsulated and unencapsulated MOSCAPs on these critical parameters at bending strains indicated that Al2O3/alucone nanolaminates stabilized the electrical and interfacial characteristics of the Si NM-based MOSCAPs. These results highlight that ultrathin Al2O3/alucone nanolaminates are promising encapsulation materials for prolonging the operational lifetimes of flexible Si NM-based metal oxide semiconductor field-effect transistors.

Flexible Bio-Electronic Hybrid Metal-Oxide Channel FET as a Glucose Sensor.

A high-sensitivity flexible field-effect transistor (FET) based glucose sensor is fabricated that can surpass the conventional electrochemical glucometers in terms of sensitivity, limit of detection, and other performance parameters. The proposed biosensor is based on the FET operation with the advantage of amplification which provides a high sensitivity and a very low limit of detection. Hybrid metal oxide (ZnO and CuO) nanostructures have been synthesized in the form of hollow spheres (ZnO/CuO-NHS). The FET was fabricated by depositing ZnO/CuO-NHS on the interdigitated electrodes. Glucose oxidase (GOx) was immobilized successfully on the ZnO/CuO-NHS. Three different outputs of the sensor are examined, the FET current, the relative current change, and the drain voltage. The sensitivity of the sensor for each output type has been calculated. The readout circuit can convert the current change to the voltage change that has been used for wireless transmission. The sensor has a very low limit of detection of 30 nM with satisfactory reproducibility, good stability, and high selectivity. The electrical response of the FET biosensor towards the real human blood serum samples demonstrated that it can be offered as a potential device for glucose detection in any medical application.

High‐Performance Flexible MoS2 Transistors Using Au/Cr/Al/Au as Source/Drain Electrodes

Abstract2D transition metal dichalcogenides such as molybdenum disulfide (MoS2) are promising candidates for flexible electronics because of their bandgap tunability, high carrier mobility, and mechanical flexibility. However, flexible MoS2 field effect transistors (FETs) are typically faced with high contact resistance which is identified as a critical limiting factor to their potential applications. The present work successfully addresses this challenge by using Al contacts without any annealing process. It is found that the contact resistance is strongly coupled to the Al thickness, increasing the Al thickness is beneficial to reduce the contact resistance. The observed variation in the device electrical characteristics can be associated with the formation of the natural aluminum oxide (AlxOy) film at the interface. An ultrathin Au insert layer can further advance the device performance. An optimal contact resistance of 2.03 kΩ and an on/off current ratio of 1.57 × 109 can be achieved in the flexible MoS2 FETs using Au/Cr/Al/Au as source/drain electrodes. Furthermore, no apparent degradation in the device properties is observed under systematic cyclic bending testing with bending radii of 18 and 15 mm. The successful integration of Au/Cr/Al/Au source/drain electrodes into MoS2 FETs promises its potential application in flexible electronic devices.

Scaling carbon nanotube field effect transistors to 30 nm channel length on pretreated PET

With the extensive application of flexible circuits to artificial intelligence (AI) and the internet of things (IoT), high requirements have been promoted for the integration and miniaturization of flexible circuits. However, the vast majority of transistors on flexible substrates reported so far have micrometer-level channel length (Lch), seriously affecting the integration abilities of flexible circuits. In this work, we explore the scaling behaviors of flexible carbon nanotube field effect transistors (CNT FETs) by fabricating p-type flexible CNT FETs with Lch varying from 2 μm to 70 nm on polyethylene terephthalate (PET) substrates. The improved width-normalized on-state current (Ion/Wch), transconductance (gm) and near-constant gate efficiency indicate that CNTs have great potential in constructing ultra-small flexible FETs. Additionally, we fabricate a CNT FET with a 30-nm channel length on a cleaned PET substrate using a self-aligned top-gate structure; this CNT FET can achieve a high on-state performance of 103 μA/μm when Vds = −0.8 V. The pitch size of the device is 230 nm, which is the smallest size of flexible transistors so far. Even with deep submicron and nanometer channel lengths, the yields of flexible CNT FETs can reach 60% (33/56); which is benefiting from the pretreated substrate, demonstrating the potential for large-scale production.

Strain-enabled defect migration and defect activation in monolayer MoS2

Two-dimensional (2D) materials possess remarkable strain tolerance and exhibit strain-tunable properties, making them highly promising for flexible device applications. Defects within these materials significantly impact their optoelectronic response to strain. In this study, we investigate the influence of strain on the electrical properties of monolayer MoS2, emphasizing the pivotal role played by intrinsic defects in shaping the material’s electrical and optoelectronic response under strain. We observed an enhancement in photocurrent and persistent photoconductivity at specific strains, indicating the activation of defects at these strain values, thus enhancing the photoresponse. Moreover, our device exhibits diodic behavior at specific strain values after prolonged measurements under a static field, suggesting a reduction in the migration energy of defects caused by the applied strain. This finding holds significant implications for memory, logic, and flexible devices. Additionally, we observe an increase in electron mobility under tensile strain, with our flexible field-effect transistor exhibiting higher mobility (∼38 cm2 (V·s)−1) at 0.4% strain. Our study provides insight into the role of strain in the activation and migration of defects in monolayer MoS2 and opens up new avenues for the development of multifunctional ultra-thin flexible devices and memory applications.

Low-temperature growth of MoS2 on polymer and thin glass substrates for flexible electronics.

Recent advances in two-dimensional semiconductors, particularly molybdenum disulfide (MoS2), have enabled the fabrication of flexible electronic devices with outstanding mechanical flexibility. Previous approaches typically involved the synthesis of MoS2 on a rigid substrate at a high temperature followed by the transfer to a flexible substrate onto which the device is fabricated. A recurring drawback with this methodology is the fact that flexible substrates have a lower melting temperature than the MoS2 growth process, and that the transfer process degrades the electronic properties of MoS2. Here we report a strategy for directly synthesizing high-quality and high-crystallinity MoS2 monolayers on polymers and ultrathin glass substrates (thickness ~30 µm) at ~150 °C using metal-organic chemical vapour deposition. By avoiding the transfer process, the MoS2 quality is preserved. On flexible field-effect transistors, we achieve a mobility of 9.1 cm2 V-1 s-1 and a positive threshold voltage of +5 V, which is essential for reducing device power consumption. Moreover, under bending conditions, our logic circuits exhibit stable operation while phototransistors can detect light over a wide range of wavelengths from 405 nm to 904 nm.

Advances in flexible graphene field-effect transistors for biomolecule sensing.

With the increasing demand for biomarker detection in wearable electronic devices, flexible biosensors have garnered significant attention. Additionally, graphene field-effect transistors (GFETs) have emerged as key components for constructing biosensors, owing to their high sensitivity, multifunctionality, rapid response, and low cost. Leveraging the advantages of flexible substrates, such as biocompatibility, adaptability to complex environments, and fabrication flexibility, flexible GFET sensors exhibit promising prospects in detecting various biomarkers. This review provides a concise summary of design strategies for flexible GFET biosensors, including non-encapsulated gate without dielectric layer coverage and external gate designs. Furthermore, notable advancements in sensing applications of biomolecules, such as proteins, glucose, and ions, are highlighted. Finally, we discuss the future challenges and prospects in this field, aiming to inspire researchers to address these issues in their further investigations.

Bending Stability of Ferroelectric Gated Graphene Field Effect Transistor for Flexible Electronics.

In this work, we explored the potential of the ferroelectric gate of (Pb0.92La0.08)(Zr0.30Ti0.70)O3 (PLZT(8/30/70)) for flexible graphene field effect transistor (GFET) devices. Based on the deep understanding of the VDirac of PLZT(8/30/70) gate GFET, which determines the application of the flexible GFET devices, the polarization mechanisms of PLZT(8/30/70) under bending deformation were analyzed. It was found that both flexoelectric polarization and piezoelectric polarization exist under bending deformation, and their polarization direction is opposite under the same bending deformation. Thus, a relatively stable of VDirac is obtained due to the combination of these two effects. In contrast to the relatively good linear movement of VDirac under bending deformation of relaxor ferroelectric (Pb0.92La0.08)(Zr0.52Ti0.48)O3 (PLZT(8/52/48)) gated GFET, these stable properties of the PLZT(8/30/70) gate GFETs make them have great potential for applications in flexible devices.

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.

High-mobility electrolyte-gated perovskite transistors on flexible plastic substrate via interface and composition engineering

Perovskite has emerged as a promising semiconductor for flexible electronics. However, perovskite-based flexible field-effect transistors (FETs) have typically exhibited a low performance owing to their use of conventional polymer dielectrics. To address this, interfacial and compositional engineering has been employed in emerging perovskite transistors to boost their charge-carrier transport. Here, we introduce the interfacial engineering of a perovskite surface using solution-processed poly(3-hexylthiophene) (P3HT) to enable the use of an electrolyte dielectric. Among the fabricated lead iodide-based perovskite devices (methylammonium (MA) lead triiodide (MAPbI3), formamidinium (FA) lead triiodide (FAPbI3), and mixed A-cation lead triiodide (FA0.2MA0.8PbI3)), the P3HT-capped FAPbI3 FETs exhibited the best hole mobility of 24.55 cm2 V−1 s−1 (average ≈ 16.83 ± 4.86 cm2 V−1 s−1) on a plastic substrate at sub-2 V. This notable performance was attributed to an increase in the charge carrier density in the perovskite-P3HT hybrid channel owing to the high capacitance of the electrolyte dielectric and better injection properties of the FAPbI3 perovskite. These findings demonstrate the potential of the proposed approach for achieving high mobility and low-voltage operated flexible perovskite-based transistor devices.

Electrolyte-Gated Graphene Field Effect Transistor-Based Ca2+ Detection Aided by Machine Learning.

Flexible electrolyte-gated graphene field effect transistors (Eg-GFETs) are widely developed as sensors because of fast response, versatility and low-cost. However, their sensitivities and responding ranges are often altered by different gate voltages. These bias-voltage-induced uncertainties are an obstacle in the development of Eg-GFETs. To shield from this risk, a machine-learning-algorithm-based LgGFETs' data analyzing method is studied in this work by using Ca2+ detection as a proof-of-concept. For the as-prepared Eg-GFET-Ca2+ sensors, their transfer and output features are first measured. Then, eight regression models are trained with the use of different machine learning algorithms, including linear regression, support vector machine, decision tree and random forest, etc. Then, the optimized model is obtained with the random-forest-method-treated transfer curves. Finally, the proposed method is applied to determine Ca2+ concentration in a calibration-free way, and it is found that the relation between the estimated and real Ca2+ concentrations is close-to y = x. Accordingly, we think the proposed method may not only provide an accurate result but also simplify the traditional calibration step in using Eg-GFET sensors.

Direct self-assembled monolayers treatment on the polar polymer dielectric towards ultra-flexible organic field-effect transistors

• A direct SAM treatment on the polar polymer gate dielectric without any preprocess is introduced. • A hysteresis-free OFET with a high mobility of 1.51 cm 2 V −1 s −1 was achieved. • An ultra-flexible OFET was fabricated and presented a stable electrical performance under deformations. Surface treatment for the polymer gate dielectric plays a significant role in achieving the high-performance flexible field-effect transistors. In this work, the direct SAM treatment on the polar polymer gate dielectric is introduced and the effects on the electrical performance of OFETs are investigated. After an appropriate SAM treatment for the polar polymer dielectric, the mobility can be improved and the hysteresis and interfacial trap states are decreased obviously. Moreover, the SAM treated polymer dielectric is used to achieve the ultra-flexible OFET successfully, with a stable electrical performance under various deformations. These results demonstrate that the direct SAM treatment on polar polymer dielectric is a promising strategy for the high-performance flexible organic transistors, presenting the huge potential for the next-generation flexible electronics.

Low temperature CVD growth of WSe2 enabled by moisture-assisted defects in the precursor powder

Two-dimensional transition metal dichalcogenides (TMDs) have been proposed for a wide variety of applications, such as neuromorphic computing, flexible field effect transistors, photonics, and solar cells, among others. However, for most of these applications to be feasible, it is necessary to integrate these materials with the current existing silicon technology. Although chemical vapor deposition is a promising method for the growth of high-quality and large-area TMD crystals, the high temperatures necessary for the growth make this technique incompatible with the processes used in the semiconductor industry. Herein, we demonstrate the possibility of low-temperature growth of TMDs, using tungsten selenide (WSe2) as a model, by simply using moisture-assisted defective tungsten oxide (WO3) precursor powders during the growth of these materials. Density functional theory calculations reveal the mechanism by which moisture promotes the defect formation on the precursor crystal structure and how it dictates the reduction of the temperature of the growth. The results were compared with the standard growth at high temperatures and with a precursor mixture with alkali salts to show the high quality of the WSe2 grown at temperatures as low as 550 °C. To conclude, the work improves the understanding of nucleation and growth mechanisms of WSe2 at low temperatures and provides a useful strategy for the growth of TMDs at temperatures required for the back-end-of-line compatibility with current silicon technology.

Electrotuneable Radical Polymers for Thin-Film Electronic Device Applications

Polyradicals – organic radical polymers in which each repeating unit contains a singly occupied molecular orbital (i.e. an unpaired electron spin) – are unique alternatives to their π-conjugated and semiconducting counterparts for several applications.[1] Unique of polyradicals are tunable charge states at their repeating units (see figure) which enable multi-stable charge transport regimes at the nanoscale. In this talk, we will present the use of thin films of polyradicals in transparent and flexible thin-film nanoelectronics. Although field-effect thin-film transistors (FETs) and flash memory devices ("memristors") based on radical polymers have been often proposed, memristor stability was frequently limited to a few writing cycles, in spite of the excellent quality of the active layer, and no FETs have been demonstrated, even though evidence of polyradical doping has been offered.[2]Here, the design criteria for flash memory devices are reviewed.It will be shown, using a combination of Kelvin-probe force microscopy (KPFM), electrical transport and optical measurements, that single-layer flash memory devices can be demonstrated from 6-oxoverdazyls, a class of radical polymers from which ultra-thin and ultra-smooth organic thin flims are advantageously processable.[3] As a case study, ultrathin devices in which the active layer is formed by a 15-nm homogeneous film of a poly-norbornene-6-oxoverdazyl (PN-6OV) polyradical synthesized by a dry vacuum polymer deposition technique are presented and compared with the corresponding devices of poly-6-oxoverdazyls (P6OV) synthesized by wet chemistry. [4] We will show that high performance is associated to the presence three tunable charge states in each monomer: positive, neutral, and negative, and also depends on the length of the pendant groups to which the radical repeating units are attached. We will demonstrate that careful engineering of the anode and cathode work functions, specifically aligning them with the negative and positive energy levels of the polyradical, is vital to maximize the on/off current ratio and ensure flash operation. The possibility to achieve electro-tunable poly-6-oxoverdazyl radical polymers by different techniques offer uniques opportunities for their use in a variety of different contexts, for example in transparent and/or flexible electronics, and where compatibility with different substrates is required.In the last part of our talk, we will present how a vertical device architecture, with drain-source contacts sandwiching the active layer of a strongly correlated 6-oxoverdazyl polyradical, leads to on/off ratios >103 in p-type PR-FETs. [4] Hole injection thus occurs by contact doping via tunable charge states at the polyradical-electrode interface. PRFETs are superior to existing organic FETs as they combine memristor and transistor functions in one mem-transistor device, offering unique potential for synaptic and spintronic applications.[1] Joo, Y.; Agarkar, V.; Sung, S. H.; Savoie, B. M.; Boudouris, B. W. A Nonconjugated Radical Polymer Glass with High Electrical Conductivity. Science 2018, 359, 1391-1395[2] Nguyen, T. P.; Easley, A. D.; Kang, N.; Khan, S.; Lim,S-M.; Rezenom, Y. H.; Wang, S.; Tran, D. K.; Fan, J.; Letteri, R. A.; He, X.; Su, L.; Yu, C-H.; Lutkenhaus, J. L.; Wooley K, L. Polypeptide organic radical batteries. Nature, 2021, 593, 61[3] Ezugwu, S.; Paquette, J. A.; Yadav, V.; Gilroy, J. B.; Fanchini, G. Design Criteria for Ultrathin Single-Layer Flash Memristors from an Organic Polyradical. Adv. Electron. Mater. 2016, 2, 1600253[4] Singh, D.; Magnan, F.; Gilroy, J. B.; Fanchini, G. Transparent and flexible field-effect transistors and mem-transistors with electroactive layers of solution-processed organic polyradicals, https://arxiv.org/abs/1910.10212 Figure 1

Tailored Uniaxial Alignment of Nanowires Based on Off-Center Spin-Coating for Flexible and Transparent Field-Effect Transistors.

The alignment of nanowires (NWs) has been actively pursued for the production of electrical devices with high-operating performances. Among the generally available alignment processes, spin-coating is the simplest and fastest method for uniformly patterning the NWs. During spinning, the morphology of the aligned NWs is sensitively influenced by the resultant external drag and inertial forces. Herein, the assembly of highly and uniaxially aligned silicon nanowires (Si NWs) is achieved by introducing an off-center spin-coating method in which the applied external forces are modulated by positioning the target substrate away from the center of rotation. In addition, various influencing factors, such as the type of solvent, the spin acceleration time, the distance between the substrate and the center of rotation, and the surface energy of the substrate, are adjusted in order to optimize the alignment of the NWs. Next, a field-effect transistor (FET) incorporating the highly aligned Si NWs exhibits a high effective mobility of up to 85.7 cm2 V−1 s−1, and an on-current of 0.58 µA. Finally, the single device is enlarged and developed in order to obtain an ultrathin and flexible Si NW FET array. The resulting device has the potential to be widely expanded into applications such as wearable electronics and robotic systems.