Zhongming Wei

Nanowire crystals of a rigid rod conjugated polymer.

In this paper, we show that well-defined, highly crystalline nanowires of a rigid rod conjugated polymer, a poly(para-phenylene ethynylene)s derivative with thioacetate end groups (TA-PPE), can be obtained by self-assembling from a dilute solution. Structural analyses demonstrate the nanowires with an orthorhombic crystal unit cell wherein the lattice parameters are a approximately = 13.63 A, b approximately = 7.62 A, and c approximately = 5.12 A; in the nanowires the backbones of TA-PPE chains are parallel to the nanowire long axis with their side chains standing on the substrate. The transport properties of the nanowires examined by organic field-effect transistors (OFETs) suggest the highest charge carrier mobility approaches 0.1 cm(2)/(V s) with an average value at approximately 10(-2) cm(2)/(V s), which is 3-4 orders higher than that of thin film transistors made by the same polymer, indicating the high performance of the one-dimensional polymer nanowire crystals. These results are particular intriguing and valuable for both examining the intrinsic properties of PPEs polymer semiconductors and advancing their potential applications in electronic devices.

Fluorination-enabled optimal morphology leads to over 11% efficiency for inverted small-molecule organic solar cells.

Solution-processable small molecules for organic solar cells have attracted intense attention for their advantages of definite molecular structures compared with their polymer counterparts. However, the device efficiencies based on small molecules are still lower than those of polymers, especially for inverted devices, the highest efficiency of which is <9%. Here we report three novel solution-processable small molecules, which contain π-bridges with gradient-decreased electron density and end acceptors substituted with various fluorine atoms (0F, 1F and 2F, respectively). Fluorination leads to an optimal active layer morphology, including an enhanced domain purity, the formation of hierarchical domain size and a directional vertical phase gradation. The optimal morphology balances charge separation and transfer, and facilitates charge collection. As a consequence, fluorinated molecules exhibit excellent inverted device performance, and an average power conversion efficiency of 11.08% is achieved for a two-fluorine atom substituted molecule.

Photoresponsive and Gas Sensing Field-Effect Transistors based on Multilayer WS2 Nanoflakes

The photoelectrical properties of multilayer WS2 nanoflakes including field-effect, photosensitive and gas sensing are comprehensively and systematically studied. The transistors perform an n-type behavior with electron mobility of 12 cm2/Vs and exhibit high photosensitive characteristics with response time (τ) of <20 ms, photo-responsivity (Rλ) of 5.7 A/W and external quantum efficiency (EQE) of 1118%. In addition, charge transfer can appear between the multilayer WS2 nanoflakes and the physical-adsorbed gas molecules, greatly influencing the photoelectrical properties of our devices. The ethanol and NH3 molecules can serve as electron donors to enhance the Rλ and EQE significantly. Under the NH3 atmosphere, the maximum Rλ and EQE can even reach 884 A/W and 1.7 × 105%, respectively. This work demonstrates that multilayer WS2 nanoflakes possess important potential for applications in field-effect transistors, highly sensitive photodetectors, and gas sensors, and it will open new way to develop two-dimensional (2D) WS2-based optoelectronics.

Millimeter-sized molecular monolayer two-dimensional crystals.

The discovery of graphene has shocked the world because it proves the existence of 2D crystals, which theoretically predicted could not exist. The peculiar electronic structure of graphene suggests unique properties of 2D crystals which may be promising for applications in many fi elds such as nanoelectronics and sensors. [ 1–3 ] The bottleneck locates at the growth of 2D crystals, except graphene, and the preparation of 2D crystals (including graphene) on arbitrary substrates, in large size and high quality. [ 3 ]

Solution‐Processed Ultrathin Chemically Derived Graphene Films as Soft Top Contacts for Solid‐State Molecular Electronic Junctions

A novel method using solution-processed ultrathin chemically derived graphene films as soft top contacts for the non-destructive fabrication of molecular junctions is demonstrated. We believe this protocol will greatly enrich the solid-state test beds for molecular electronics due to its low-cost, easy-processing and flexible nature.

Electric-Field Tunable Band Offsets in Black Phosphorus and MoS2 van der Waals p-n Heterostructure

The structural and electronic properties of black phosphorus/MoS2 (BP/MoS2) van der Waals (vdW) heterostructure are investigated by first-principles calculations. It is demonstrated that the BP/MoS2 bilayer is a type-II p-n vdW heterostructure, and thus the lowest energy electron-hole pairs are spatially separated. The band gap of BP/MoS2 can be significantly modulated by external electric field, and a transition from semiconductor to metal is observed. It gets further support from the band edges of BP and MoS2 in BP/MoS2 bilayer, which show linear variations with E⊥. BP/MoS2 bilayer also exhibits modulation of its band offsets and band alignment by E⊥, resulting in different spatial distribution of the lowest energy electron-hole pairs. Our theoretical results may inspire much interest in experimental research of BP/MoS2 vdW heterostructures and would open a new avenue for application of the heterostructures in future nano- and optoelectronics.

High‐Performance Langmuir–Blodgett Monolayer Transistors with High Responsivity

Shrinking the dimensions of organic field-effect transistors (OFETs) down to the nanometer scale offers new, defect-free charge transport regimes. This may lead to the improvement of the device performance, such as larger carrier mobilities, increased device speed, lower power dissipation, and enhanced on/off ratios. Therefore, much effort has recently been made to scale down both the thickness of the OFET devices (the semiconductor and/or insulator layers) and their lateral dimension (source–drain distance). With size reduction, however, the device performance is mainly hampered by the parasitic contact resistances with a high injection barriers and the poor long-range order of organic semiconductors. Most commonly, gold source–drain (S/D) electrodes are used as the charge-injecting metal in organic electronic devices. Gold is used because of its chemical stability and its work function that matches the energy level of organic semiconductors in most cases, thus lowering the Schottky barriers. To reduce the contact resistance, several alternative materials, including carbon nanotubes (CNTs), carbon nanotube/polymer nanocomposites, graphene multilayers, and conductive polymers, have been utilized as potential substitutes for the expensive gold S/D electrodes. On the other hand, molecular organization can be improved by forming dense and well-ordered self-assembled monolayers through bottom-up approaches, as illustrated by the work of Smits et al. To optimize the performance of OFETs, device fabrication should be considered as a holistic process. The electrode materials, the contact surface, and device fabrication are so closely interrelated that they cannot be optimized independently. To date, only few examples of OFETs have been demonstrated to achieve high-performance by holistic consideration of all these parameters. With this in mind, herein we present a new class of highperformance photoresponsive molecular field-effect transistors formed from Langmuir–Blodgett (LB) monolayers of copper phthalocyanine (CuPc), using two-dimensional (2D) ballistically-conductive single-layer graphene as planar contacts. The unique feature detailed herein is the integration of LB techniques with the fabrication of nanogap electrodes to build functional molecular electronic devices. LB techniques offer a promising and reliable method to prepare large-area ordered ultrathin films with well-defined architectures. In previous work, we and others have demonstrated the successful applications of the LB technique to CuPc and conjugated polymers in producing ultrathin film OFETs. However, the charge carrier mobilities m in these devices were low, namely about 10 –10 3 cmV 1 s . This might be mainly ascribed to the high contact resistance when gold was used for S/D electrodes and to the defects in the micrometer-long channels. For this study, we employed single-layer graphene as S/D nanoelectrodes to overcome these difficulties. This choice is because graphene, a new class of 2D carbon nanostructure, holds a set of remarkable electronic and physical properties, such as ballistic transport with low resistivity, high chemical stability, and high mechanical strength. Recently, we developed a reliable method of oxidatively cutting single-layer graphene as ideal 2D contacts for producing nanoscale organic transistors. Due to the reduced hole-injection barrier and the large contact area, these devices showed bulk-like FET properties (m 10 3 cmV 1 s ). In the current work, we present a combined method based on the holistic consideration mentioned above. The significant improvement is that this method gives almost 100 percent yields of working monolayer transistors with higher carrier mobility, higher on/off ratio, and reliable reproducibility. We also present the details of their very sensitive photoresponsive behavior, which has never been reported before in monolayer transistors. The structure of CuPc monolayer transistors is illustrated in Figure 1. The fabrication of cut 2D graphene nanogap electrodes was detailed in our previous work. In brief, three-terminal graphene-based transistors were first made by the combination of a peeling-off technique and electron beam [*] Y. Cao, S. Liu, L. Gan, Prof. X. Guo, Prof. Z. Liu Beijing National Laboratory for Molecular Sciences State Key Laboratory for Structural Chemistry of Unstable and Stable Species College of Chemistry and Molecular Engineering, Peking University Beijing 100871 (P. R. China) Fax: (+86)10-6275-7789 E-mail: guoxf@pku.edu.cn zfliu@pku.edu.cn

Enhanced rectification, transport property and photocurrent generation of multilayer ReSe2/MoS2 p–n heterojunctions

Van der Waals (vdW) heterojunctions are equipped to avert dangling bonds due to weak, inter-layer vdW force, and ensure strong in-plane covalent bonding for two-dimensional layered structures. We fabricated four heterojunctions devices of different layers based on p-type distorted 1T-MX2 ReSe2 and n-type hexagonal MoS2 nanoflakes, and measured their electronic and optoelectronic properties. The device showed a high rectification coefficient of 500 for the diode, a high ON/OFF ratio and higher electron mobility for the field-effect transistor (FET) compared with the individual components, and a high current responsivity (Rλ) and external quantum efficiency (EQE) of 6.75 A/W and 1,266%, respectively, for the photodetector.

Ultrathin Reduced Graphene Oxide Films as Transparent Top‐Contacts for Light Switchable Solid‐State Molecular Junctions

A new type of solid-state molecular junction is introduced, which employs reduced graphene oxide as a transparent top contact that permits a self-assembled molecular monolayer to be photoswitched in situ, while simultaneously enabling charge-transport measurements across the molecules. The electrical switching behavior of a less-studied molecular switch, dihydroazulene/vinylheptafulvene, is described, which is used as a test case.

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)