研究目的
To investigate the transport properties, including conductivity, mobility, and carrier density, of spray-coated composite networks of graphene and WS2 nanosheets as a function of graphene content, and to assess the viability of adding graphene to improve transistor performance.
研究成果
The addition of graphene to WS2 nanosheet networks increases conductivity and carrier density in accordance with percolation theory, but only modestly improves mobility due to similar junction resistances. The on-off ratio decreases significantly at the percolation threshold, making this approach not viable for enhancing transistor performance. Carrier density percolation is more fundamental, reflecting properties of the conductive nanoparticles.
研究不足
The study assumes network porosity is constant (~0.5) and independent of graphene content, which may not hold precisely. Mobility values for graphene networks are lower than reported in other studies, possibly due to morphological differences from spray coating and lack of annealing. Hysteresis effects in electrolytic gating may affect accuracy of threshold voltage and bandgap measurements. The homogeneity of graphene dispersion is not perfect, with some clumping observed.
1：Experimental Design and Method Selection:
The study uses electrolytic gating to characterize transport properties. Composite networks are formed by spray-coating mixtures of liquid-exfoliated WS2 and graphene nanosheets. Percolation theory is applied to analyze conductivity, mobility, and carrier density dependencies on graphene volume fraction.
2：Sample Selection and Data Sources:
WS2 and graphene powders are produced via liquid phase exfoliation in N-Methyl-2-pyrrolidone (NMP), followed by centrifugation to optimize nanosheet size distributions. Dispersions are transferred to isopropanol (IPA) for deposition. Composite inks are created by mixing WS2 and graphene inks in various ratios.
3：List of Experimental Equipment and Materials:
Equipment includes a horn tip sonicator (Sonics Vibra-cell VCX-750), centrifuge (Hettich Mikro 220R), spray coater, metal evaporation system (Temescal FC2000), probe station, spectrometer (Perkin Elmer Lambda 1050), Raman spectrometer (Horiba Jobin Yvon LabRAM HR800), TEM (JOEL 2100), and electrical measurement instruments (Keithley 2612A). Materials include WS2 powder (Sigma Aldrich, CAS: 12138-09-9), graphene powder (Asbury, grade 3763), NMP, IPA, ionic liquid (EMIM-TFSI from Sigma Aldrich), and substrates (Al2O3-coated PET from Mitsubishi Paper Mills).
4：Experimental Procedures and Operational Workflow:
Inks are prepared by sonication and centrifugation. Films are spray-coated onto substrates, followed by deposition of gold electrodes. Ionic liquid is added for gating experiments. Electrical measurements (conductivity, output and transfer curves) are performed under vacuum. Optical spectroscopy, Raman spectroscopy, and TEM are used for characterization.
5：Data Analysis Methods:
Data is analyzed using percolation theory equations. Mobility is derived from transfer curves, carrier density from conductivity and mobility. Fitting is done to percolation models, and statistical analysis is performed on multiple devices.
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Keithley 2612A
2612A
Keithley
Used for electrical measurements, including output and transfer curves.
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Perkin Elmer Lambda 1050 spectrometer
Lambda 1050
Perkin Elmer
Used for measuring extinction spectra of the dispersions.
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JOEL 2100 TEM
2100
JOEL
Used for transmission electron microscopy to analyze nanosheet dimensions.
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Sonics Vibra-cell VCX-750 ultrasonic processor
VCX-750
Sonics
Used for sonication during liquid phase exfoliation of nanomaterials.
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Hettich Mikro 220R centrifuge
Mikro 220R
Hettich
Used for centrifugation to separate and size-select nanosheets.
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Temescal FC2000 metal evaporation system
FC2000
Temescal
Used for depositing gold electrodes on the sprayed networks.
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Horiba Jobin Yvon LabRAM HR800 Raman spectrometer
LabRAM HR800
Horiba Jobin Yvon
Used for Raman spectroscopy to characterize the composite networks.
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N-Methyl-2-pyrrolidone
HPLC grade solvent
Solvent used for liquid phase exfoliation of WS2 and graphene powders.
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Isopropanol
HPLC grade solvent
Solvent used for transferring nanosheets before deposition.
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EMIM-TFSI ionic liquid
Sigma Aldrich
Used as the ionic liquid for electrolytic gating experiments.
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Al2O3-coated PET substrate
Mitsubishi Paper Mills
Substrate for spray-coating the nanosheet networks.
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WS2 powder
CAS: 12138-09-9 Ref:243639
Sigma Aldrich
Source material for producing WS2 nanosheets via exfoliation.
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Graphene powder
grade 3763
Asbury
Source material for producing graphene nanosheets via exfoliation.
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