High-Performance Field-Effect Transistor-Type Sensors Based on Nanoscopically Engineered Organic Semiconductors

Department of Energy EngineeringSensors based on organic field-effect transistor (OFET) platforms show great promise for use in chemical and biological sensors due to their prominent advantages, including high sensitivity, light-weight, low-cost, simple platforms, and flexible applications. Functional properties of active organic semiconductor layers can be tailored by material design or/and surface functionalization to enhance selectivity. To date, a large number of sensors for chemical and biogenic substances have used high-cost immobilization methods and high-end technologies. OFET-based sensors are particularly attractive for applications in simple, cost-effective, high-performance electronics. Furthermore, the sensitivity, selectivity, response time, stability, reproducibility, and limit of detection of sensors can be optimized by choosing or engineering more suitable fabrication techniques and materials for the active layers. Such on-demand, structure-engineered, and surface-engineered organic semiconducting layers are highly desirable for the practical uses of OFETs. In my thesis, commendable molecular engineering, process engineering and interface engineering are highlighted to demonstrate the feasibility of high-performance nanoscopically engineered organic-transistor-based sensors. Here, I begin with an introduction to OFET and organic sensors, with an emphasis on the organic semiconductor engineering strategies in chapter 1. In detail, in chapter 1, typical properties of organic semiconductors, a discussion of OFET operation, and a working principles of this OFET-type sensors are introduced. Chapter 2 presents molecular engineering strategies to enable the fabrication of n-channel-dominant ambipolar OFETs. The electrical charge transport through fluorine-substituted semiconducting materials is investigated. These investigations are easily applied to demonstrate complementary inverters with a reasonable performance. In chapter 3, I focus on the device design and fabrication of high mobility OFETs made by using organic???organic heterointerface. Pentacene is used as an active layer above, and m-bis(triphenylsilyl)benzene is used as the bottom layer. Sequential evaporation process without breaking vacuum of these materials results in high-quality organic semiconductor thin films with far fewer grain boundaries. In addition, the pentacene film exhibits myriad nanometre-sized pores in the organic layers. This surprising structure, the pore-rich structure improves the sensitivity of organic-transistor-based chemical sensors. This approach demonstrates a conceptually novel methodology for the fabrication of ???structurally engineered??? organic semiconducting thin films and our work has a significant impact in the fields of materials science as well as organic electronics. Furthermore, organic semiconductor engineering strategies to improve sensitivity and selectivity for biogenic substances by direct semiconductor surface functionalization and to enhance sensitivity and selectivity towards psychostimulants by modification with specific selective sensing layer are given in chapter 4 and 5, respectively. In chapter 4, highly sensitive organic-transistor-based sensors that can selectively detect a neurotransmitter acetylcholine without enzyme immobilization are fabricated using organic thin films functionalized with a synthetic receptor, a cucurbit[6]uril (CB[6]) derivative. The liquid-phase sensing experiments are successfully performed by using organic semiconductor layer with high operational stability in water. The findings provide a low-cost, simple, and feasible method for the fabrication of high-performance water-stable sensors for biogenic substances. In addition, the results obtained herein describe the first demonstration of acetylcholine sensing without any enzymatic reactions using the synthetic receptor-functionalized OFET-platform. In chapter 5, the direct detection of amphetamine-type-stimulants (ATS) is suggested for the illicit and designer drugs sensing OFET platforms. Their novel sensing system and sensing mechanism are studied using other CB homologues, a cucurbit[7]uril (CB[7]) derivative decorated OFET-based sensors. By synergistic combination of a highly selective synthetic host molecule and a highly sensitive OFET device, the first ATS sensors with specific synthetic receptor-engineered OFET-platform are demonstrated flexible polymer substrates. These sensors in physiological buffer system and even in urine samples show highly sensitive sensing behaviors.ope

Enhancing 2D Growth of Organic Semiconductor Thin Films with Macroporous Structures via a Small-Molecule Heterointerface

The physical structure of an organic solid is strongly affected by the surface of the underlying substrate. Controlling this interface is an important issue to improve device performance in the organic electronics community. Here we report an approach that utilizes an organic heterointerface to improve the crystallinity and control the morphology of an organic thin film. Pentacene is used as an active layer above, and m-bis(triphenylsilyl) benzene is used as the bottom layer. Sequential evaporations of these materials result in extraordinary morphology with far fewer grain boundaries and myriad nanometre-sized pores. These peculiar structures are formed by difference in molecular interactions between the organic layers and the substrate surface. The pentacene film exhibits high mobility up to 6.3 cm(2)V(-1)s(-1), and the pore-rich structure improves the sensitivity of organic-transistor-based chemical sensors. Our approach opens a new way for the fabrication of nanostructured semiconducting layers towards high-performance organic electronics.X116049Nsciescopu

Ultra-narrow-bandgap thienoisoindigo polymers: structure-property correlations in field-effect transistors

From a structural point of view, the newly conceived thienoisoindigo (TIIG) moiety can serve as an ideal building block for the synthesis of high-performance polymers. To expand the range of available TIIG-based conjugated polymers, herein we report the synthesis and characterization of two new TIIG-based donor-acceptor polymers (PTIIG-TT and PTIIG-TVT), containing either the thieno[3,2-b] thiophene (TT) or the (E)-2-(2-(thiophen-2-yl) vinyl) thiophene (TVT) moiety. In addition, we conducted a systematic investigation on the relationship between the microstructure of the polymer film and charge transport in organic field-effect transistors (OFETs) fabricated using these polymers. It was observed that the incorporation of a TVT moiety into the TIIG backbone imparts higher crystallinity and increases the molecular packing density, leading to an increased hole mobility (similar to 0.45 cm(2) V-1 s(-1)) in PTIIG-TVT, compared with PTIIG-TT. When an Al electrode is used instead of a Au electrode in the OFET devices, both polymers exhibit outstanding ambipolar characteristics. This study advances the understanding of the structural features of TIIG-based polymers, which will potentially accelerate the improvement in the mobility of TIIG-based polymers.clos

Point-of-Use Detection of Amphetamine-Type Stimulants with Host-Molecule-Functionalized Organic Transistors

In recent years, there has been a rapid increase in the abuse of amphetaminetype stimulants (ATS). One way to tackle this problem is to develop an easy, sensitive, rapid, and cheap ATS detection platform. Here, a strategy that synergistically combines the selectivity of supramolecular chemistry and the sensitivity of organic field-effect transistors is used as the basis of an ATS sensor. Cucurbit[7]uril derivatives that can selectively detect ATS have been synthesized and used as a functional material. The fabricated amperometric sensors exhibited unprecedented sensitivity toward ATS, with a detection limit of nanomolar concentrations in urine and picomolar concentrations in water or a physiologic buffer. The feasibility of this strategy was further demonstrated through the preparation of flexible and wearable devices with a wireless sensing platform. This sensing system offers rapid and sensitive detection of trace amounts of ATS in urine and other samples at the point of use.2017 (c) 2017 Elsevier Inc.11Nsciescopu

Point-of-Use Detection of Amphetamine-Type Stimulants with Host Molecule Functionalized Organic Transistors

In recent years, there has been a rapid increase in the abuse of amphetamine-type stimulants (ATS). One way to tackle this problem is to develop an easy, sensitive, rapid, and cheap ATS detection platform. Here, a strategy that synergistically combines the selectivity of supramolecular chemistry and the sensitivity of organic field-effect transistors is used as the basis of an ATS sensor. Cucurbit[7]uril derivatives that can selectively detect ATS have been synthesized and used as a functional material. The fabricated amperometric sensors exhibited unprecedented sensitivity toward ATS, with a detection limit of nanomolar concentrations in urine and picomolar concentrations in water or a physiologic buffer. The feasibility of this strategy was further demonstrated through the preparation of flexible and wearable devices with a wireless sensing platform. This sensing system offers rapid and sensitive detection of trace amounts of ATS in urine and other samples at the point of use.1110sciescopu

Fluorinated Benzothiadiazole (BT) Groups as a Powerful Unit for High-Performance Electron-Transporting Polymers

Over the past few years, one of the most remarkable advances in the field of polymer solar cells (PSCs) has been the development of fluorinated 2,1,3-benzothiadiazole (BT)-based polymers that lack the solid working principles of previous designs, but boost the power conversion efficiency. To assess a rich data set for the influence of the fluorinated BT units on the charge-transport characteristics in organic field-effect transistors (OFETs), we synthesized two new polymers (<b>PDPP-FBT</b> and <b>PDPP-2FBT</b>) incorporating diketopyrrolopyrrole (DPP) and either single- or double-fluorinated BT and thoroughly investigated them via a range of techniques. Unlike the small differences in the absorption properties of <b>PDPP-FBT</b> and its nonfluorinated analogue (<b>PDPP-BT</b>), the introduction of doubly fluorinated BT into the polymer backbone induces a noticeable change in its optical profiles and energy levels, which results in a slightly wider bandgap and deeper HOMO for <b>PDPP-2FBT</b>, relative to the others. Grazing incidence X-ray diffraction (GIXD) analysis reveals that both fluorinated polymer films have long-range orders along the out-of-plane direction, and π–π stacking in the in-plane direction, implying semicrystalline lamellar structures with edge-on orientations in the solid state. Thanks to the strong intermolecular interactions and highly electron-deficient π-systems driven by the inclusion of F atoms, the polymers exhibit electron mobilities of up to 0.42 and 0.30 cm² V^–1 s^–1 for <b>PDPP-FBT</b> and <b>PDPP-2FBT</b>, respectively, while maintaining hole mobilities higher than 0.1 cm² V^–1 s^–1. Our results highlight that the use of fluorinated BT blocks in the polymers is a promising molecular design strategy for improving electron transporting performance without sacrificing their original hole mobility values

Wafer‐Scale Synthesis of Mixed‐Dimensional Heterostructures via Manipulating Platinization Conditions

Abstract 2D van der Waals (vdW) hetero integration, which features exotic interplanar interactions derived from mixed‐dimensional heterostructures, is an emergent platform for implementing high‐performance electronics and broadband/wavelength‐tunable photodetectors. However, the production of large‐area 2D spatially homogeneous transition‐metal dichalcogenides (TMDs) and elucidation of the electrostatic dynamics governing the interlayer interactions are two paramount prerequisites for realizing practical 2D‐TMD‐heterostructure‐based photodetectors. Here, a wafer‐scale synthesis of mixed‐dimensional Pt–MoS2‐based vdW heterostructures is unprecedentedly demonstrated by manipulating the platinization conditions. The rationally designed platinization yields dimensionality‐tailored Pt, including Pt nanofilm, Pt nanoparticles, and Pt atoms, with MoS2 as host platform. From density functional theory calculations, this study insights that Mo vacancy sites on the MoS2 surface are thermo‐dynamically favorable sites for Pt with an adsorption energy of −2.25 eV, then Pt clusters are sequentially formed neighboring the specific Pt‐substituted position with a formation energy of 1.30 eV. Intensive microscopic and spectroscopic analyses reveal the structural, chemical, and electrical features, validating the proposed dynamics‐related mechanism. The dimensionality‐tailored vdW heterostructures exhibit outstanding optoelectrical properties with excellent photoresponsivity (2.04 mA W−1) and highly sensitive detectivity (9.82 × 106 cm Hz1/2 W−1)

PVDF-stimulated surface engineering in ZnO for highly sensitive and water-stable hydrazine sensors

Sensors based on multifunctional n-type metal oxide semiconductors are attracting significant interest in environmental monitoring owing to their distinct characteristics including low production cost, high detection response to different noxious analytes, nontoxic nature, and acceptable biocompatibility. Herein, we present an innovative approach that utilizes surface functionalization on ZnO thin-film transistor (TFT)-type sensors with a fluompolymer, poly (vinylidene fluoride-co-hexafluoropmpylene) (PVDF-HFP) to realize highly sensitive and water-stable liquid-phase sensors. ZnO sensors laminated with PVDF-HFP thin films demonstrate exceptional repeatable stability to DI water and liquid-phase hydrazine, indicating excellent sensitivity in addition to low hydrazine-detection limits approaching 0.01 nM (sub-ppt level) under ambient conditions. This detection limit is five orders of magnitude less than that of the legal limit for an 8 h exposure time-weighted average for hydrazine. Moreover, relatively acceptable repeatability and reproducibility of the sensors were guaranteed over 96% of their initial base current with hydrazine for a month. This outstanding sensing performance is attributed to the enhanced surface interaction between PVDF-HFP with a strong dipole moment and hydrazine, which is completely discriminated from the universal detection mechanism associated with oxygen ion species in ZnO

Rational surface modification of ZnO with siloxane polymers for room-temperature-operated thin-film transistor-based gas sensors

High demands for and rapid development of technologies related to the Internet of Things (IoT) call for a pertinent technological breakthrough in sensing devices to effectively detect various external stimuli or target analytes. Advanced sensing platforms utilizing thin-film transistors (TFTs) are essential for realizing cost-effective and high-performance chemical sensors. Here, it is reported that the utilization of a gas-selective layer based on polymeric chromatographic stationary phases is an unprecedented and facile method to establish simultaneously the desired gas selectivity and responsivity of ZnO thin films at room temperature. With the aid of computational studies, in-depth analysis and comparison of gas-sensing and the charge transfer mechanism between the gas and the resulting sensor devices are performed. ZnO with cyanopropylmethyl-phenylmethyl polysiloxane films provide excellent selective sensing with gas mixtures, and the achieved response to vaporized ethanol is nearly three times higher than the response of pristine ZnO at ~22 ??C and atmospheric pressure. This effective enhancement of sensing performance under ambient conditions is attained through the transition from chemisorption to physisorption based on intermolecular interactions between gas molecules and gas-selective polymers. This work demonstrates a potent yet cost-effective method to fabricate low power consumption gas sensor systems based on metal oxide TFT