Zhongming Wei

IT University of Copenhagen, København, Capital Region, Denmark

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Publications (55)328.94 Total impact

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    ABSTRACT: Electrical transport properties of molecular junctions are fundamentally affected by the energy alignment between molecular frontier orbitals (highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO)) and Fermi level (or work function) of electrode metals. Dithiafulvene (DTF) is used as substituent group to the oligo(phenylene ethynylene) (OPE) molecular wires and different molecular structures based on OPE3 backbone (with linear to cruciform framework) are achieved, with viable molecular orbitals and HOMO–LUMO energy gaps. OPE3, OPE3–DTF, and OPE3–tetrathiafulvalene (TTF) can form good self-assembled monolayers (SAMs) on Au substrates. Molecular heterojunctions based on these SAMs are investigated using conducting probe–atomic force microscopy with different tips (Ag, Au, and Pt) and Fermi levels. The calibrated conductance values follow the sequence OPE3–TTF > OPE3–DTF > OPE3 irrespective of the tip metal. Rectification properties (or diode behavior) are observed in case of the Ag tip for which the work function is furthest from the HOMO levels of the OPE3s. Quantum chemical calculations of the transmission qualitatively agree with the experimental data and reproduce the substituent effect of DTF. Zero-bias conductance, and symmetric or asymmetric couplings to the electrodes are investigated. The results indicate that improved fidelity of molecular transport measurements may be achieved by systematic studies of homologues series of molecular wires applying several different metal electrodes.
    Advanced Functional Materials 02/2015; 25(11). DOI:10.1002/adfm.201404388 · 10.44 Impact Factor
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    ABSTRACT: Synthesis of large-scale highly crystalline two-dimensional alloys is significant for revealing properties. Here, we have investigated the vapor growth process of high-quality bilayer CoxMo1-xS2 (x = 0.16) hexagonal nanosheets systematically. As the initial loading of the sulfur increases, the morphology of the CoxMo1-xS2 (0 < x ≤ 1) nanosheets becomes hexagons from David stars step by step at 680 °C. We find that Co atoms mainly distribute at the edge of nanosheets. When the temperature increases from 680 to 750 °C, high-quality cubic pyrite-type crystal structure CoS2 grows on the surface of CoxMo1-xS2 nanosheet gradually and forms hexagonal film induced by the nanosheet. Electrical transport measurements reveal that the CoxMo1-xS2 nanosheets and CoS2 films exhibit n-type semiconducting transport behavior and half-metallic behavior, respectively. Theoretical calculations of their band structures agree well with the experimental results.
    ACS Nano 01/2015; DOI:10.1021/nn505048y · 12.03 Impact Factor
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    ABSTRACT: High-quality Bi2S3 nanowires are synthesized by chemical vapor deposition and their intrinsic photoresponsive and field-effect characteristics are explored in detail. Among the studied Au–Au, Ag–Ag, and Au–Ag electrode pairs, the device with stepwise band alignment of asymmetric Au–Ag electrodes has the highest mobility. Furthermore, it is shown that light can cause a sevenfold decrease of the on/off ratio. This can be explained by the photoexcited charge carriers that are more beneficial to the increase of Ioff than Ion. The photoresponsive properties of the asymmetric Au–Ag electrode devices were also explored, and the results show a photoconductive gain of seven with a rise time of 2.9 s and a decay time of 1.6 s.
    ChemPhysChem 01/2015; 16(1). DOI:10.1002/cphc.201402594 · 3.36 Impact Factor
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    ABSTRACT: SnS nanoparticles were synthesized with a facile hydrothermal method and characterized by X-ray diffraction (XRD), Raman, transmission electron microscope (TEM) and scanning electron microscope (SEM) in details. The red light photoresponse of the SnS based devices in different gas environments were also systematically investigated, which revealed that the adsorbed gas molecules play important roles in photosensitive properties. Compared with that in vacuum, the photosensitivity were enhanced in O2 (or air) and reduced in NH3. The dynamic response time was much longer in gas environment. These influences were ascribed to the charge transfer between the adsorbed gas molecules and SnS.
    12/2014; DOI:10.1039/C4TC02574A
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    ABSTRACT: ZnSb2O4 nanoparticles with an average size of about 53 nm are synthesized by a facile hydrothermal method. The humidity sensing characteristics of the devices based on our ZnSb2O4 nanoparticles are investigated systematically. Such humidity sensors show excellent performance with ultra-high sensitivity, fast response/recovery speed, a wide range of relative humidity (RH) response, and excellent stability and reversibility. The responsive mechanisms in the low and high humidity ranges are also analyzed and simulated.
    RSC Advances 12/2014; 5(4). DOI:10.1039/C4RA13398F · 3.71 Impact Factor
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    ABSTRACT: The triazatriangulene (TATA) ring system was investigated as binding group for tunnel junctions of molecular wires on gold surfaces. Self-assembled monolayers (SAMs) of TATA platforms with three different lengths of phenylene wires were fabricated and their electrical conductance recorded by both conducting probe-atomic force microscopy (CP-AFM) and scanning tunneling microscopy (STM). Similar measurements were performed for phenylene SAMs with thiol anchoring groups as references. It was found that despite the presence of a sp3 hybridized carbon atom in the conduction path, the TATA platform, displays a contact resistance only slightly larger than the thiols. This surprising finding has not been reported before and was analyzed by theoretical computations of the transmission functions of the TATA anchored molecular wires. The relatively low contact resistance of the TATA platform along with its high stability and directionality makes this binding group very attractive for molecular electronic measurements and devices.
    Langmuir 11/2014; DOI:10.1021/la504056v · 4.38 Impact Factor
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    ABSTRACT: Cruciform-like molecules with two orthogonally placed π-conjugated systems have in recent years attracted significant interest for their potential use as molecular wires in Molecular Electronics. Here we present synthetic protocols for a large selection of cruciform molecules based on oligo(phenyleneethynylene) (OPE) and tetrathiafulvalene (TTF) scaffolds, end-capped with acetyl-protected thiolates as electrode anchoring groups. The molecules were subjected to a comprehensive study of their conducting properties as well as their photophysical and electrochemical properties in solution. The complex nature of the molecules and their possible binding in different configurations in junctions called for different techniques of conductance measurements, 1) conducting-probe atomic force microscopy (CP-AFM) measurements on self-assembled monolayers (SAMs), 2) mechanically controlled break-junction (MCBJ) measurements, and 3) scanning tunneling microscopy break-junction (STM-BJ) measurements. The CP-AFM measurements showed structure-property relationships: from SAMs of series of OPE3 and OPE5 cruciform molecules, the conductance of the SAM increased with the number of dithiafulvene (DTF) units (0, 1, 2) along the wire, and it increased when substituting two arylethynyl end-groups of the OPE3 backbone with two DTF units. The MCBJ and STM-BJ stud-ies on single molecules both showed that DTFs decreased the junction formation probability, but, in contrast, no significant influ-ence on the single-molecule conductance was observed. We suggest that the origins of the difference between SAM and single-molecule measurements lie in the nature of the molecule|electrode interface as well as in effects arising from molecular packing in the SAMs. This comprehensive study shows that for complex molecules care should be taken when directly comparing single-molecule measurements and measurements of SAMs and solid-state devices thereof.
    Journal of the American Chemical Society 11/2014; 136(47). DOI:10.1021/ja509937k · 11.44 Impact Factor
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    ABSTRACT: Recently, molybdenum disulfide (MoS2) has become a popular material due to its unique electrical and chemical properties, and its use as a potential substitute for graphene. Herein, we report a new two-step method by utilizing thermal evaporation-sulfurization to synthesize MoS2 which possesses an innovative micro-ring structure. The average statistical values of the height, width and external diameter were 69 nm, 0.3 μm and 5.0 μm, respectively. Then the mechanism for the growth of such MoS2 micro-rings was proposed. A device based on the MoS2 micro-ring was prepared by electron beam lithography, and its electrical transport properties were determined at different temperatures.
    Nanoscale 11/2014; 6(24). DOI:10.1039/c4nr05111d · 6.74 Impact Factor
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    ABSTRACT: Van der Waals heterostructures designed by assembling isolated two-dimensional (2D) crystals have emerged as a new class of artificial materials with interesting and unusual physical properties. Here, the multilayer MoS2–WS2 heterostructures with different configurations are reported and their optoelectronic properties are studied. It is shown that the new heterostructured material possesses new functionalities and superior electrical and optoelectronic properties that far exceed the one for their constituents, MoS2 or WS2. The vertical transistor exhibits a novel rectifying and bipolar behavior, and can also act as photovoltaic cell and self-driven photodetector with photo-switching ratio exceeding 103. The planar device also exhibits high field-effect ON/OFF ratio (>105), high electron mobility of 65 cm2/Vs, and high photo­responsivity of 1.42 A/W compared to that in isolated multilayer MoS2 or WS2 nanoflake transistors. The results suggest that formation of MoS2–WS2 heterostructures could significantly enhance the performance of optoelectronic devices, thus open up possibilities for future nanoelectronic, photovoltaic, and optoelectronic applications.
    Advanced Functional Materials 11/2014; 24(44). DOI:10.1002/adfm.201401504 · 10.44 Impact Factor
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    ABSTRACT: MoO3 nanoflakes/graphene heterocomposite was synthesized by a water bath method and showed abnormal photoelectric property with the resistance increasing under irradiation of visible light in atmosphere. The origin of such property is the physical contact between the two moieties lead to hole doping in graphene. Thus, the increase of electrons in the MoO3 nanoflakes induced by oxygen molecules desorption from its surface under visible light was considered as the dominant reason of resistance change. This view was confirmed by resistance of the heterocomposite in vacuum was 102 times larger than that in air and the photoelectric measurement took place in Ar. The MoO3 nanoflakes/graphene heterocomposite also exhibited great application potential in oxygen gas sensing.
    RSC Advances 09/2014; 4(91). DOI:10.1039/C4RA08557D · 3.71 Impact Factor
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    ABSTRACT: Bi2S3 single-crystalline nanowires are synthesized through a hydrothermal method and then fabricated into single nanowire photodetectors. Due to the different contact barrier between the gold electrode and Bi2S3 nanowires, two kinds of devices with different electrical contacts are obtained and their photoresponsive properties are investigated. The non-ohmic contact devices show larger photocurrent gains and shorter response times than those of ohmic contact devices. Furthermore, the influence of a focused laser on the barrier height between gold and Bi2S3 is explored in both kinds of devices and shows that laser illumination on the AuBi2S3 interface can greatly affect the barrier height in non-ohmic contact devices, while keeping it intact in ohmic contact devices. A model based on the surface photovoltage effect is used to explain this phenomenon.
    ChemPhysChem 08/2014; 15(12). DOI:10.1002/cphc.201402201 · 3.36 Impact Factor
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    ABSTRACT: Flowerlike MoS2 microspheres were synthesized through a hydrothermal method. 2H-MoS2 nanoparticles, MoS2/MoO3 heterojunctions, and α-MoO3 nanoplates were prepared by annealing the MoS2 microspheres under different reaction conditions. The formation and growth mechanism of the samples from flowerlike MoS2 microspheres to α-MoO3 nanoplates is explained in detail. The photocatalytic properties of the four samples for the degradation of rhodamine B (RhB) under visible-light irradiation were studied. The results showed that the flowerlike MoS2 microspheres, MoS2/α-MoO3 heterojunctions, and α-MoO3 nanoplates all have excellent photocatalytic activities. In particular, the flowerlike MoS2 microspheres exhibit the highest photocatalytic activity for the degradation of RhB.
    European Journal of Inorganic Chemistry 07/2014; 2014(20):3245-3251. DOI:10.1002/ejic.201402079 · 2.97 Impact Factor
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    ABSTRACT: 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 cm(2)/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 × 10(5)%, 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.
    Scientific Reports 06/2014; 4:5209. DOI:10.1038/srep05209 · 5.08 Impact Factor
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    ABSTRACT: MoSe2 was fabricated by a facile hydrothermal method, and a simple device based on it was prepared to investigate the low temperature electrical and photo-responsive (PR) properties. PR current of MoSe2 under 650 nm red illumination is 2.55 × 10−5 A and remains approximately at low temperatures, which demonstrates its fine PR property. As the temperature became lower, electrical conductivity of MoSe2 first decreased from 300 to 43 K and then increased at temperatures from 43 to 13 K. Mechanisms of such electrical and PR phenomenon were proposed. Our findings revealed potential method to adjust band gap of transition metal dichalcogenides and demonstrated their potential applications under special environment.
    Applied Physics Letters 05/2014; 104(20):202105-202105-5. DOI:10.1063/1.4878837 · 3.52 Impact Factor
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    ABSTRACT: α-MoO3 nanosheets were synthesized by a water bath method using ammonium heptamolybdenum tetrahydrate and concentrated nitric acid as precursors. Hydrogen was doped by a chemical reduction in aqueous acidic media, with hydrazine hydrate used as the reducing agent. Temperature dependent resistance showed that the low temperature Peierls transition of H-doped MoO3 nanosheets breaks below 50 K, and its resistance is satisfied at temperatures lower than 37 K (37–10 K). This phenomenon was induced by thermal disturbance and the dominance of defects in low temperature transport, which was confirmed by photoresponse measurements taken before and after the break of the new phase.
    01/2014; 2(6). DOI:10.1039/C3TC32142H
  • Chao Fan, Zhongming Wei, Shengxue Yang, Jingbo Li
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    ABSTRACT: Novel MoSe2 flower-like nanostructures were produced by a facile hydrothermal method. Devices containing such nanostructures were also fabricated and reveal obvious photo-responsive characteristics. SEM and HRTEM images show that the as-prepared products have a flower-like structure, and the diameter of a single particle is about 500 nm. The average atomic ratio between Se and Mo is 2.68 according to the results of the EDS measurements. The photo-responsive characteristics responding to red illumination of a device with MoSe2 flower-like nanostructures were investigated for the first time and a mechanism for the photoresponse was proposed. Our findings reveal that such nanostructures have excellent electric conductivity and new photo-responsive properties.
    RSC Advances 01/2014; 4(2):775. DOI:10.1039/c3ra42564a · 3.71 Impact Factor
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    ABSTRACT: Bi2Se3–ZnS nanocomposites were synthesized with different morphologies and their photoluminescence were investigated. The compounds formed hexagonal rods as the thickness of Bi2Se3 sheets stayed about a few nanometers; while the Bi2Se3 sheets’ thickness increased to tens of nanometers, the compounds formed novel morphologies of Bi2Se3–ZnS nanocomposites with small ZnS nanoparticles randomly decorated onto Bi2Se3 sheets. The formation mechanism was proposed based on the different thickness of Bi2Se3 sheets used in experimental processes. In addition, the significant fluorescence quench and obvious improvement in photoresponsive characteristic were shown after the integration of ZnS with Bi2Se3 sheets, which showed potential application in optoelectronic devices.
    Journal of Materials Science Materials in Electronics 11/2013; 24(11). DOI:10.1007/s10854-013-1383-z · 1.97 Impact Factor
  • Renxiong Li, Qu Yue, Zhongming Wei
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    ABSTRACT: High-quality Bi2S3 nanowires are synthesized and their photoresponses are investigated in detail. Our results show that the photoresponsive curves have distinctly different characteristics at low-temperature (50 K) compared to those at room temperature (290 K). The transferred-electron effect is believed to cause this difference. A first principle calculation shows that Bi2S3 has many energy valleys, which agree with our experimental analysis. At low temperature, due to the lack of sufficient phonon energy, the photoexcited electrons in Bi2S3 mainly aggregate at the bottom of the conduction band. When this electron concentration increased to a high enough level after illumination, an electron transfer between the energy valleys happened and the photocurrent began to decrease slowly after the rapid increase in the first stage. After the transfer process reaches equilibrium, the photocurrent reaches a minimum, thus the trap states play a dominant role and the photocurrent rises slowly again. Furthermore, photocurrent curves at different temperatures were recorded to estimate the phonon energy value needed to assist the electron transitions. The required phonon energy is calculated to be about 16.3 meV (corresponding to 190 K), which fits well with previous results.
    08/2013; 1(37). DOI:10.1039/C3TC30943F

Publication Stats

429 Citations
328.94 Total Impact Points


  • 2014
    • IT University of Copenhagen
      København, Capital Region, Denmark
  • 2008–2011
    • Chinese Academy of Sciences
      • • Key Laboratory of Organic Solids
      • • Institute of Chemistry
      Peping, Beijing, China
  • 2009
    • Fudan University
      • Department of Chemistry
      Shanghai, Shanghai Shi, China
    • Technical Institute of Physics and Chemistry
      Peping, Beijing, China