Rongjin Li

Electrochemically Exfoliated Graphene as Solution-Processable, Highly Conductive Electrodes for Organic Electronics

Solution-processable thin layer graphene is an intriguing nanomaterial with tremendous potential for electronic applications. In this work, we demonstrate that electrochemical exfoliation of graphite furnishes graphene sheets of high quality. The electrochemically exfoliated graphene (EG) contains a high yield (>80%) of one- to three-layer graphene flakes with high C/O ratio of 12.3 and low sheet resistance (4.8 kΩ/□ for a single EG sheet). Due to the solution processability of EG, a vacuum filtration method in association with dry transfer is introduced to produce large-area and highly conductive graphene films on various substrates. Moreover, we demonstrate that the patterned EG can serve as high-performance source/drain electrodes for organic field-effect transistors.

Graphene nanoribbon heterojunctions

Despite graphenes remarkable electronic properties, the lack of an electronic bandgap severely limits its potential for applications in digital electronics. In contrast to extended films, narrow strips of graphene (called graphene nanoribbons) are semiconductors through quantum confinement, with a bandgap that can be tuned as a function of the nanoribbon width and edge structure. Atomically precise graphene nanoribbons can be obtained via a bottom-up approach based on the surface-assisted assembly of molecular precursors. Here we report the fabrication of graphene nanoribbon heterojunctions and heterostructures by combining pristine hydrocarbon precursors with their nitrogen-substituted equivalents. Using scanning probe methods, we show that the resulting heterostructures consist of seamlessly assembled segments of pristine (undoped) graphene nanoribbons (p-GNRs) and deterministically nitrogen-doped graphene nanoribbons (N-GNRs), and behave similarly to traditional p-n junctions. With a band shift of 0.5 eV and an electric field of 2 × 10(8) V m(-1) at the heterojunction, these materials bear a high potential for applications in photovoltaics and electronics.

Transparent Conductive Electrodes from Graphene/PEDOT:PSS Hybrid Inks for Ultrathin Organic Photodetectors

A novel solution fabrication of large-area, highly conductive graphene films by spray-coating of a hybrid ink of exfoliated graphene (EG)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (PH1000) is demonstrated. The fabricated graphene films exhibit excellent mechanical properties, thus enabling their application as bottom electrodes in ultrathin organic photodetector devices with performance comparable to that of the state-of-the-art Si-based inorganic photodetectors.

Bioinspired Wafer-Scale Production of Highly Stretchable Carbon Films for Transparent Conductive Electrodes†

Flexible transparent and conductive films (TCFs) are essential elements of the next-generation flexible devices, including touch screens and displays, organic light-emitting diodes (OLEDs), solar cells (SCs), organic field-effect transistors (OFETs), and sensors. Indium tin oxide (ITO) has been the most widely used TCF in optoelectronic devices for almost four decades, owing to its low sheet resistance (Rs 10 W/&) coupled with high transmittance (T 80%). However, ITO suffers from inherent brittleness, which makes it unsuitable for flexible devices. In the race to replace ITO, networks of metal nanowires and carbon nanotubes are leading the pack; however, their high costs of synthesis and high surface roughness hamper their commercial applications. 4] Graphene and graphene-based thin films have been recognized as attractive alternatives because of their outstanding electronic, optoelectronic, and mechanical properties. Although sheet resistance as low as 30 W/& (at T= 90 %) has been obtained for graphene grown on metallic substrates, the multitransfer process associated with the additional chemical doping increases the costs dramatically. The establishment of facile, yet controllable methods for the large-area production of flexible TCFs at low costs remains a challenging task. The molecular precursor approach, namely the production of carbon-based TCFs using aromatic-rich precursors as the carbon source seems to be much less explored. Through the thermal treatment of properly selected organic precursors, this synthetic method offers an intriguing means to lowcosts processing and structural control of highly conductive carbon/graphene films, which are difficult to achieve using methods such as chemical vapor deposition (CVD), epitaxial growth, liquid exfoliation, and reduction of graphene oxide (GO) film. Nevertheless, the bottleneck associated with this method lies in the relatively low electrical conductivity of the produced TCFs (e.g., 206 Scm 1 for a pyrolyzed film of a giant polycyclic aromatic hydrocarbon). Recently, inspired by the adhesive proteins secreted by marine mussels, polydopamine (PDA) has proven to be a material for facile and universal surface coating. By selfpolymerization of dopamine, PDA that sticks to the surface of virtually all types of solid materials regardless of their chemical nature, can be produced. 14] The self-polymerization reaction is so mild that simple immersion of substrates in an aqueous solution of dopamine results in the spontaneous deposition of PDA film. 14] The film is layer-structured and the thickness can be tailored at the nanometer scale. PDA is mainly composed of cross-linked indolequinone units (Figure 1, the definitive atomic scale structure of

Organic single crystal field-effect transistors based on 6H-pyrrolo[3,2-b:4,5-b ]bis[1,4]benzothiazine and its derivatives.

[*] Z. Wei, H. Geng, C. Wang, Dr. Y. Liu, Dr. R. Li, Dr. W. Xu, Prof. Z. Shuai, Prof. W. Hu, Prof. D. Zhu Beijing National Laboratory for Molecular Sciences Key Laboratory of Organic Solids, Institute of Chemistry Chinese Academy of Sciences Beijing 100190 (P. R. China) E-mail: wxu@iccas.ac.cn; zgshuai@iccas.ac.cn; huwp@iccas.ac.cn Z. Wei, C. Wang Graduate University of the Chinese Academy of Sciences Beijing 100039 (P. R. China)

Dibenzothiophene derivatives as new prototype semiconductors for organic field-effect transistors

New prototype semiconductor materials based on dibenzothiophene (DBT) derivatives were successfully synthesized by a convergent approach using palladium catalyzed Stille coupling reactions. Thermogravimetric analysis, UV-vis spectra and electrochemistry results indicated these materials had good thermal and photooxidation stability. X-Ray diffraction measurements of the vacuum-evaporated films showed enhanced crystalline order with increasing substrate deposition temperature. The ordered vacuum-evaporated films with charge carrier mobility as high as 7.7 × 10−2 cm2 V−1 s−1 and an on/off ratio of nearly 1 × 107 had been achieved with 3,7-bis(5′-hexyl-thiophen-2′-yl)-dibenzothiophene (3,7-DHTDBTT). These results suggest that the 3,7-substituted DBT system is a good prototype for new type organic semiconductors and will play a more important role in organic semiconductors.

Electrochemical Functionalization of Graphene at the Nanoscale with Self-Assembling Diazonium Salts

We describe a fast and versatile method to functionalize high-quality graphene with organic molecules by exploiting the synergistic effect of supramolecular and covalent chemistry. With this goal, we designed and synthesized molecules comprising a long aliphatic chain and an aryl diazonium salt. Thanks to the long chain, these molecules physisorb from solution onto CVD graphene or bulk graphite, self-assembling in an ordered monolayer. The sample is successively transferred into an aqueous electrolyte, to block any reorganization or desorption of the monolayer. An electrochemical impulse is used to transform the diazonium group into a radical capable of grafting covalently to the substrate and transforming the physisorption into a covalent chemisorption. During covalent grafting in water, the molecules retain the ordered packing formed upon self-assembly. Our two-step approach is characterized by the independent control over the processes of immobilization of molecules on the substrate and their covalent tethering, enabling fast (t < 10 s) covalent functionalization of graphene. This strategy is highly versatile and works with many carbon-based materials including graphene deposited on silicon, plastic, and quartz as well as highly oriented pyrolytic graphite.

New type of organic semiconductors for field-effect transistors with carbon-carbon triple bonds

A series of thiophene-phenylene semiconductors containing –CC– bonds were designed and synthesized. Their thermal, optical, electrochemical and FET properties were fully characterized. The crystal structure of 5,5′-bis(phenylethynyl)-2,2′-bithiophene (1c) revealed that the introduction of carbon-carbon triple bonds efficiently eliminated the steric repulsions between adjacent aromatic rings. TGA, UV-vis spectra and electrochemical measurements showed that all compounds had good thermal and environmental stability. FET devices based on these materials exhibited high performance and stability. All the results suggested the introduction of carbon-carbon triple bonds provided an efficient route to high performance organic semiconductors.

Single crystal ribbons and transistors of a solution processed sickle-like fused-ring thienoacene

A fused-ring thienoacene with sickle-like molecular shape was synthesized and examined in self-assembly and in transistors for the first time. Different from the linear fused-ring thienoacenes with herringbone packing motif, the sickle-like thienoacene exhibits a π–π molecular packing motif and could self-assemble into one-dimensional single crystalline ribbons efficiently. Moreover, the compound demonstrates not only excellent solubility in common organic solvents, but also high mobility and stability in transistors, indicating the great application prospect of the compound in organic electronics.

2D Organic Materials for Optoelectronic Applications

The remarkable merits of 2D materials with atomically thin structures and optoelectronic attributes have inspired great interest in integrating 2D materials into electronics and optoelectronics. Moreover, as an emerging field in the 2D-materials family, assembly of organic nanostructures into 2D forms offers the advantages of molecular diversity, intrinsic flexibility, ease of processing, light weight, and so on, providing an exciting prospect for optoelectronic applications. Herein, the applications of organic 2D materials for optoelectronic devices are a main focus. Material examples include 2D, organic, crystalline, small molecules, polymers, self-assembly monolayers, and covalent organic frameworks. The protocols for 2D-organic-crystal-fabrication and -patterning techniques are briefly discussed, then applications in optoelectronic devices are introduced in detail. Overall, an introduction to what is known and suggestions for the potential of many exciting developments are presented.