Jiahui Tan

Low Temperature Growth of Highly Nitrogen-Doped Single Crystal Graphene Arrays by Chemical Vapor Deposition

The ability to dope graphene is highly important for modulating electrical properties of graphene. However, the current route for the synthesis of N-doped graphene by chemical vapor deposition (CVD) method mainly involves high growth temperature using ammonia gas or solid reagent melamine as nitrogen sources, leading to graphene with low doping level, polycrystalline nature, high defect density and low carrier mobility. Here, we demonstrate a self-assembly approach that allows the synthesis of single-layer, single crystal and highly nitrogen-doped graphene domain arrays by self-organization of pyridine molecules on Cu surface at temperature as low as 300 °C. These N-doped graphene domains have a dominated geometric structure of tetragonal-shape, reflecting the single crystal nature confirmed by electron-diffraction measurements. The electrical measurements of these graphene domains showed their high carrier mobility, high doping level, and reliable N-doped behavior in both air and vacuum.

Influence of D/A Ratio on Photovoltaic Performance of a Highly Efficient Polymer Solar Cell System

A new copolymer PIDTDTQx based on indacenodithiophene and quinoxaline is synthesized and characterized. The correlation between the D/A ratio, mobility, and photovoltaic properties, as well as morphology of the D/A blend based on a PIDTDTQx:PC(70) BM system is investigated. The power conversion efficiency of the polymer solar cells based on PIDTDTQx/PC(70) BM (1:4, w/w) reaches 7.51%.

Sulfur-bridged annulene-TCNQ co-crystal: a self-assembled ''molecular level heterojunction'' with air stable ambipolar charge transport behavior.

and two-component transistors with the combination of p -type and n -type semiconductors. [ 4–6 ] Ambipolar transport materials are particularly desirable concerning the construction of complementary-like circuits, as the fabrication processes could be signifi cantly simplifi ed when two unipolar materials were replaced with an ambipolar material. Despite the huge progresses in organic semiconductors, the ambipolar transport materials are still rare with respect to unipolar materials, and it is still a great challenge in design and synthesis of π -conjugated systems that could fulfi ll the requirement for achieving both stable p -type and n -type transport in ambient condition. As an alternative choice, construction of heterostructures with p -type and n -type semiconductors constitutes an effi cient way to realize ambipolar operation. For example, bilayer heterojunction, [ 4 ] bulk heterojunction [ 5 ] and lateral heterostructure [ 6 ] have been successfully demonstrated to fabricate

Fullerene/Sulfur-Bridged Annulene Cocrystals: Two-Dimensional Segregated Heterojunctions with Ambipolar Transport Properties and Photoresponsivity

Fullerene/sulfur-bridged annulene cocrystals with a two-dimensional segregated alternating layer structure were prepared by a simple solution process. Single-crystal analysis revealed the existence of continuing π-π interactions in both the donor and acceptor layers, which serve as transport paths for holes and electrons separately. The ambipolar transport behaviors were demonstrated with single-crystal field-effect transistors and rationalized by quantum calculations. Meanwhile, preliminary photoresponsivity was observed with the transistor configuration.

Anisotropic Photoresponse Properties of Single Micrometer-Sized GeSe Nanosheet

Micrometer-sized single-crystal GeSe nanosheets have been synthesized by a solution method. The single GeSe nanosheet exhibits novel anisotropic photoresponse properties in two photodetectors based on individual nanosheet. The on/off switching ratio of the photodetector perpendicular to the nanosheet is 3.5 times higher than that parallel to the nanosheet.

Substitution effect on molecular packing and transistor performance of indolo[3,2-b]carbazole derivatives

In this manuscript, two chloro-substituted derivatives of indolo[3,2-b]carbazole (ICZ), 2,8-dichloro-indolo[3,2-b]carbazole (CICZ) and 2,8-dichloro-5,11-dihexyl-indolo[3,2-b]carbazole (CHICZ) were designed and synthesized. The only difference between CICZ and CHICZ is at the N-5 and N-11 positions with or without long alkyl side chains. Interestingly, CICZ and CHICZ exhibited similar thermal, optical, and electrochemical properties, while their molecular packing motifs in solid state and corresponding charge transport properties were significantly different. The alkyl chains at N-5 and N-11 positions were proved not only enhancing the solubility and self-organization of the compounds but also shifting the molecular packing from herringbone (CICZ) to one-dimensional π–π stacking (CHICZ). Moreover, nearly no field-effect performance was observed for CICZ, while the mobility of CHICZ was as high as 0.14 cm2 V−1 s−1 for its thin films and 0.5 cm2 V−1 s−1 for its single crystals. These results confirmed that the chemical substitutions are a powerful molecular design tool to tune the molecular packing motifs of organic semiconductors and their corresponding electronic properties.

Copolymers of benzo[1,2-b:4,5-b′]dithiophene and bithiazole for high-performance thin film phototransistors

A series of conjugated polymers based on benzo[1,2-b:4,5-b′]dithiophene and bithiazole were systematically investigated for applications of polymer field-effect transistors and thin film phototransistors. Grazing incidence X-ray diffraction and atomic force microscopy were employed to investigate the solid state molecular organization and film morphology. The solution processability, solid state molecular organization, film morphology, charge transport and phototransistor performance can be tuned by elaborately optimizing the polymer backbone and side chains synchronously. For the optimized polymer (P5), a charge-carrier mobility of 0.194 cm2 V−1 s−1, a threshold voltage of −6 V and an on/off current ratio of 106 were achieved. Phototransistors based on this polymer showed a highest photoresponsivity of 132 A W−1 and a photocurrent/dark current ratio of 2 × 105.

Molecular Crystal Lithography: A Facile and Low‐Cost Approach to Fabricate Nanogap Electrodes

A novel cost-efficient and facile technique, molecular crystal lithography, to fabricate nanogap electrodes efficiently is reported. The gap width of the electrodes can be tuned from ∼9 nm to several micrometers. Organic field-effect transistors based on the nanogap electrodes all exhibit a high performance, indicating the effectiveness and practicability of molecular crystal lithography for mass production of nanogap electrodes.

Solution-processed high-performance flexible 9, 10-bis(phenylethynyl)anthracene organic single-crystal transistor and ring oscillator

Organic semiconductor of 9, 10-bis(phenylethynyl)anthracene (BPEA) single crystal ribbon with ultra-long length has been prepared by solution drop casting method, where the growth direction was controlled with the seed crystal. The BPEA single crystal ribbon based field-effect transistors show high hole mobility up to 3.2 cm2/V·s, and the inverters exhibited the highest gain of 92. The complex device such as 5-stage ring oscillator consisting of 10 transistors was also constructed on a single crystal ribbon. This straightforward methodology was applied to fabricate plastic transistors on the flexible substrate, showing high performance even after repeatedly bending of 300 times.

Tuning charge transport from unipolar (n-type) to ambipolar in bis(naphthalene diimide) derivatives by introducing π-conjugated heterocyclic bridging moieties

A series of acceptor–donor–acceptor (A–D–A) conjugated molecules based on naphthalene diimide dimers bridged with different π-conjugated heterocyclic units (NDI–π–NDI) have been designed and synthesized. By an ingenious design strategy, the LUMO (the lowest unoccupied molecular orbital) of the NDI-based small molecules is well controlled to a relatively constant value of −3.8 to −3.9 eV, whereas their HOMO (the highest occupied molecular orbital) could be tuned over a wide range, from −6.5 eV (compound 1) to −5.5 eV (compound 5), leading to varied band gaps from 2.6 eV to 1.5 eV. Organic field-effect transistor (OFET) characterization of these NDI–π–NDI molecules shows that compounds 1, 2, and 3 have good n-type semiconducting properties in a N2 atmosphere with the maximum electron mobilities up to 0.15 cm2 V−1 s−1, 0.46 cm2 V−1 s−1 and 0.57 cm2 V−1 s−1, respectively. Compounds 4 and 5, due to the high-lying HOMO levels and reduced energy band gaps, have ambipolar semiconducting properties and OFETs based on 5 show the highest electron and hole mobilities up to 1.23 cm2 V−1 s−1 and 0.0074 cm2 V−1 s−1, respectively. Moreover, the performances are enhanced under thermal treatment because of the increased crystallinity as evidenced by X-ray diffraction (XRD) and atomic force microscopy (AFM). The easily tunable electronic energy levels make the NDI-based semiconductors promising n-channel and ambipolar components in organic devices.