Liyong Niu

Photosensitive Graphene Transistors

High performance photodetectors play important roles in the development of innovative technologies in many fields, including medicine, display and imaging, military, optical communication, environment monitoring, security check, scientific research and industrial processing control. Graphene, the most fascinating two-dimensional material, has demonstrated promising applications in various types of photodetectors from terahertz to ultraviolet, due to its ultrahigh carrier mobility and light absorption in broad wavelength range. Graphene field effect transistors are recognized as a type of excellent transducers for photodetection thanks to the inherent amplification function of the transistors, the feasibility of miniaturization and the unique properties of graphene. In this review, we will introduce the applications of graphene transistors as photodetectors in different wavelength ranges including terahertz, infrared, visible, and ultraviolet, focusing on the device design, physics and photosensitive performance. Since the device properties are closely related to the quality of graphene, the devices based on graphene prepared with different methods will be addressed separately with a view to demonstrating more clearly their advantages and shortcomings in practical applications. It is expected that highly sensitive photodetectors based on graphene transistors will find important applications in many emerging areas especially flexible, wearable, printable or transparent electronics and high frequency communications.

Production of Two-Dimensional Nanomaterials via Liquid-Based Direct Exfoliation.

Tremendous efforts have been devoted to the synthesis and application of two-dimensional (2D) nanomaterials due to their extraordinary and unique properties in electronics, photonics, catalysis, etc., upon exfoliation from their bulk counterparts. One of the greatest challenges that scientists are confronted with is how to produce large quantities of 2D nanomaterials of high quality in a commercially viable way. This review summarizes the state-of-the-art of the production of 2D nanomaterials using liquid-based direct exfoliation (LBE), a very promising and highly scalable wet approach for synthesizing high quality 2D nanomaterials in mild conditions. LBE is a collection of methods that directly exfoliates bulk layered materials into thin flakes of 2D nanomaterials in liquid media without any, or with a minimum degree of, chemical reactions, so as to maintain the high crystallinity of 2D nanomaterials. Different synthetic methods are categorized in the following, in which material characteristics including dispersion concentration, flake thickness, flake size and some applications are discussed in detail. At the end, we provide an overview of the advantages and disadvantages of such synthetic methods of LBE and propose future perspectives.

Waterproof, Ultrahigh Areal-Capacitance, Wearable Supercapacitor Fabrics

High-performance supercapacitors (SCs) are promising energy storage devices to meet the pressing demand for future wearable applications. Because the surface area of a human body is limited to 2 m2 , the key challenge in this field is how to realize a high areal capacitance for SCs, while achieving rapid charging, good capacitive retention, flexibility, and waterproofing. To address this challenge, low-cost materials are used including multiwall carbon nanotube (MWCNT), reduced graphene oxide (RGO), and metallic textiles to fabricate composite fabric electrodes, in which MWCNT and RGO are alternatively vacuum-filtrated directly onto Ni-coated cotton fabrics. The composite fabric electrodes display typical electrical double layer capacitor behavior, and reach an ultrahigh areal capacitance up to 6.2 F cm-2 at a high areal current density of 20 mA cm-2 . All-solid-state fabric-type SC devices made with the composite fabric electrodes and water-repellent treatment can reach record-breaking performance of 2.7 F cm-2 at 20 mA cm-2 at the first charge-discharge cycle, 3.2 F cm-2 after 10 000 charge-discharge cycles, zero capacitive decay after 10 000 bending tests, and 10 h continuous underwater operation. The SC devices are easy to assemble into tandem structures and integrate into garments by simple sewing.

Salt-assisted high-throughput synthesis of single- and few-layer transition metal dichalcogenides and their application in organic solar cells

Transition metal dichalcogenides (TMDs) such as MoS 2 , MoSe 2 , WS 2 and WSe 2 have the common chemical formula of MX 2 , where the transition metal M is sandwiched between two layers of chalcogen X. Although atoms in-plane are strongly linked by covalent bonds, the adjacent layers outof-plane are weakly held together by van der Waals force, which allows the exfoliation of bulk TMDs into atomically thin, singleand few-layer two dimensional (2D) materials. Compared with zero-bandgap graphene, these TMD 2D materials possess obvious semiconductor bandgaps, [ 1 ] which show remarkable advantages for a wide range of applications including fi eld effect transistors, [ 2–4 ] sensors, [ 2 ] energy storage devices, [ 5–8 ] and optoelectronics. [ 9–11 ]

Salt-assisted direct exfoliation of graphite into high-quality, large-size, few-layer graphene sheets

We report a facile and low-cost method to directly exfoliate graphite powders into large-size, high-quality, and solution-dispersible few-layer graphene sheets. In this method, aqueous mixtures of graphite and inorganic salts such as NaCl and CuCl2 are stirred, and subsequently dried by evaporation. Finally, the mixture powders are dispersed into an orthogonal organic solvent solution of the salt by low-power and short-time ultrasonication, which exfoliates graphite into few-layer graphene sheets. We find that the as-made graphene sheets contain little oxygen, and 86% of them are 1-5 layers with lateral sizes as large as 210 μm(2). Importantly, the as-made graphene can be readily dispersed into aqueous solution in the presence of surfactant and thus is compatible with various solution-processing techniques towards graphene-based thin film devices.

Organic electrochemical transistors with graphene-modified gate electrodes for highly sensitive and selective dopamine sensors

Organic electrochemical transistors (OECTs) are successfully used as highly sensitive and selective dopamine sensors. The selectivity of the OECT-based dopamine sensors is significantly improved by coating biocompatible polymer Nafion or chitosan on the surface of the gate electrodes. The interference induced by uric acid and ascorbic acid is effectively eliminated especially after the modification of Nafion. The sensitivity of the devices is improved by graphene flakes co-modified on the gate electrodes. The detection limit of the devices to dopamine is down to 5 nM, which is much lower than that of conventional electrochemical approaches. Because the OECT-based dopamine sensors are solution processable, they are suitable for low-cost and disposable sensing application.

Highly selective and sensitive glucose sensors based on organic electrochemical transistors with graphene-modified gate electrodes

The sensitivity of glucose sensors based on organic electrochemical transistors (OECT) is increased by co-modifying graphene or reduced graphene oxide (rGO) and enzyme (glucose oxidase) on the gate electrodes for the first time. The optimized device shows linear responses to glucose in a broad concentration region from 10 nM to 1 μM and with a detection limit down to 10 nM, which is two orders of magnitude better than that for the device without the graphene modification. The selectivity of the device is systematically studied for the first time. The device selectivity is dramatically improved when the gate electrode is modified with biocompatible polymers (chitosan or Nafion). The interfering effect caused by uric acid and l-ascorbic acid is almost negligible for practical applications. Therefore, highly sensitive and selective OECT-based glucose sensors can be realized by functionalizing the gate electrodes. In addition, the devices are solution processable and low-cost, and are thus suitable for disposable sensing applications.

Full‐Solution Processed Flexible Organic Solar Cells Using Low‐Cost Printable Copper Electrodes

Full-solution-processed flexible organic solar cells (OSCs) are fabricated using low-cost and high-quality printable Cu electrodes, which achieve a power conversion efficiency as high as 2.77% and show remarkable stability upon 1000 bending cycles. This device performance is thought to be the best among all full-solution-processed OSCs reported in the literature using the same active materials. This printed Cu electrode is promising for application in roll-to-roll fabrication of flexible OSCs.

Printed light-trapping nanorelief Cu electrodes for full-solution-processed flexible organic solar cells

Light-trapping nanorelief metal electrodes have been proven to be an effective approach to improve the absorption performance of flexible organic solar cells (FOSCs). These nanorelief electrodes have been made by conventional vacuum deposition techniques, which are difficult to integrate with roll-to-roll fabrication processes. To address this challenge, this paper reports, for the first time, the fabrication of highly conductive nanorelief Cu electrodes on the flexible substrates through solution printing and polymer-assisted metal deposition at room temperature in the air. FOSCs made with these printed nanorelief Cu electrodes possess not only much improved power conversion efficiency, by 13.5%, but also significant enhancement in flexibility when compared with those made with flat Cu electrodes. Because of the low material and fabrication cost, these printed nanorelief Cu electrodes show great promise in roll-to-roll fabrication of FOSCs in the future.

Interfacial engineering of printable bottom back metal electrodes for full-solution processed flexible organic solar cells

Printing of metal bottom back electrodes of flexible organic solar cells (FOSCs) at low temperature is of great significance to realize the full-solution fabrication technology. However, this has been difficult to achieve because often the interfacial properties of those printed electrodes, including conductivity, roughness, work function, optical and mechanical flexibility, cannot meet the device requirement at the same time. In this work, we fabricate printed Ag and Cu bottom back cathodes by a low-temperature solution technique named polymer-assisted metal deposition (PAMD) on flexible PET substrates. Branched polyethylenimine (PEI) and ZnO thin films are used as the interface modification layers (IMLs) of these cathodes. Detailed experimental studies on the electrical, mechanical, and morphological properties, and simulation study on the optical properties of these IMLs are carried out to understand and optimize the interface of printed cathodes. We demonstrate that the highest power conversion efficiency over 3.0% can be achieved from a full-solution processed OFSC with the device structure being PAMD-Ag/PEI/P3HT:PC61BM/PH1000. This device also acquires remarkable stability upon repeating bending tests.