Blue phosphorus, a previously unknown phase of phosphorus, has been recently predicted by theoretical calculations and shares its layered structure and high stability with black phosphorus, a rapidly rising two-dimensional material. Here, we report a molecular beam epitaxial growth of single layer blue phosphorus on Au(111) by using black phosphorus as precursor, through the combination of in situ low temperature scanning tunneling microscopy and density functional theory calculation. The structure of the as-grown single layer blue phosphorus on Au(111) is explained with a (4 × 4) blue phosphorus unit cell coinciding with a (5 × 5) Au(111) unit cell, and this is verified by the theoretical calculations. The electronic bandgap of single layer blue phosphorus on Au(111) is determined to be 1.10 eV by scanning tunneling spectroscopy measurement. The realization of epitaxial growth of large-scale and high quality atomic-layered blue phosphorus can enable the rapid development of novel electronic and optoelectronic devices based on this emerging two-dimensional material.
Black phosphorus, a fast emerging two-dimensional material, has been configured as field effect transistors, showing a hole-transport-dominated ambipolar characteristic. Here we report an effective modulation on ambipolar characteristics of few-layer black phosphorus transistors through in situ surface functionalization with caesium carbonate (Cs 2 CO 3 ) and molybdenum trioxide (MoO 3 ), respectively. Cs 2 CO 3 is found to strongly electron dope black phosphorus. The electron mobility of black phosphorus is significantly enhanced to B27 cm 2 V À 1 s À 1 after 10 nm Cs 2 CO 3 modification, indicating a greatly improved electrontransport behaviour. In contrast, MoO 3 decoration demonstrates a giant hole-doping effect. In situ photoelectron spectroscopy characterization reveals significant surface charge transfer occurring at the dopants/black phosphorus interfaces. Moreover, the surface-doped black phosphorus devices exhibit a largely enhanced photodetection behaviour. Our findings coupled with the tunable nature of the surface transfer doping scheme ensure black phosphorus as a promising candidate for further complementary logic electronics.
In situ bottom-gated molybdenum disulfide (MoS2) field effect transistors (FETs) device characterization and in situ ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy measurements were combined to investigate the effect of surface modification layers of C60 and molybdenum trioxide (MoO3) on the electronic properties of single layer MoS2. It is found that C60 decoration keeps MoS2 FET performance intact due to the very weak interfacial interactions, making C60 as an ideal capping layer for MoS2 devices. In contrast, decorating MoO3 on MoS2 induces significant charge transfer at the MoS2/MoO3 interface and largely depletes the electron charge carriers in MoS2 FET devices.
In this work, efficient bulk heterojunction (BHJ) organic solar cells (OSC) in inverted configuration have been demonstrated. Power conversion efficiency (PCE) of 3.7% is reported for OSC employing silver top electrodes, molybdenum trioxide (MoO3) as the hole-transport interlayer (HTL), active layer comprising of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as well as a nanocrystalline solution-synthesized zinc oxide (ZnO) nanoparticle (NP) film as the electron-transport layer (ETL). By using solution-processable ZnO crystalline NPs as ETL, we can eliminate the typical high temperature processing/annealing step, which is widely adopted in the conventional ZnO ETL fabrication process via the sol-gel method. Such highly crystalline ZnO NP films can enhance charge collection at the electrodes. It is also found that inverted OSCs exhibit greater air stability and lifetime performance compared to the OSC employing the normal structure.
The interface properties of organic-organic heterojunctions (OOHs), such as interface energy level alignment (ELA), interfacial charge transfer, interface nanostructuring, molecular orientation and so on, play an essential role in determining the device performance for some technologically important organic electronic devices, encompassing organic solar cells, bipolar organic field-effect-transistors, and organic light-emitting-diodes. The aim of this article is to provide a balanced assessment on the understanding of the ELA at the small-molecule based OOH interfaces with well-defined molecular orientation, with particular emphasis on the role of gap states in organic thin films. A generalized picture of gap states determined ELA at the OOH interfaces is provided and their implications in relevant organic electronic devices have been discussed.
We demonstrate the use of chemical-vapor-deposited (CVD) graphene as an effective indium-tin-oxide (ITO) electrode surface modifier to engineer the organic donor-acceptor heterojunction interface properties in an inverted organic solar cell device configuration. As revealed by in situ near-edge X-ray adsorption fine structure measurement, the organic donor-acceptor heterojunction, comprising copper-hexadecafluoro-phthalocyanine (F16CuPc) and copper phthalocyanine (CuPc), undergoes an obvious orientation transition from a standing configuration (molecular π-plane nearly perpendicular to the substrate surface) on the bare ITO electrode to a less standing configuration with the molecular π-plane stacking adopting a large projection along the direction perpendicular to the electrode surface on the CVD graphene-modified ITO electrode. Such templated less-standing configuration of the organic heterojunction could significantly enhance the efficiency of charge transport along the direction perpendicular to the electrode surface in the planar heterojunction-based devices. Compared with the typical standing organic-organic heterojunction on the bare ITO electrode, our in situ ultraviolet photoelectron spectroscopy experiments reveal that the heterojunction on the CVD graphene modified ITO electrode possesses better aligned energy levels with respective electrodes, hence facilitating effective charge collection.
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