1.3 Nanophotonics: light-,atter interaction in TMDCs materials
2.1.5 Work function analysis on tBLG
the other half of the BLG has = 30, as confirmed by point Raman spectrum. In the case of 14- tBLG Raman mapping, the R-band indicates inter-valley scattering, which only appears in the BLG region.
we do not find any discernible surface potential differences for tBLG with different interlayer twist angles.
We have also performed UPS measurements on large MLG and BLG films, and found that their work functions are 4.65 eV and 4.72 eV, respectively, as shown in Figure 2.7 (d). The work function measurements for MLG and BLG large films are performed via normal emission UPS by using monochromatic He I radiation with 21.2 eV photon energy as the UV source. The UPS characterization has been carried out in ultra-high vacuum (UHV), and the work function difference between MLG and BLG is found to be ~ 80 meV, which is on the same order of magnitude as the results obtained from KPFM. The slight difference between the results from KPFM and UPS may be attributed to environmental issues, such as atmospheric humidity for KPFM studies in ambient environment.
Figure 2.7. (a) Mapping of the surface potential of a large graphene film by KPFM. The inset shows the corresponding Raman map of the G-band intensity of the same sample. (b) Raman point spectra taken on an MLG area (green cross) and a nearly AB-stacking BLG area (blue solid-circle). (c) Spatial evolution of the work function measured along the red line depicted in (a), showing an apparent increase of the work function in the BLG area. (d) UPS measurements of the work functions of both MLG and BLG films, yielding values of 4.65 eV and 4.72 eV, respectively.
The graphene films were grown on 25μm thick poly crystalline Cu foils in a similar process80 described earlier in Section 2.1. In a standard polymer-based graphene transfer method, such as using PMMA as the temporary supporting layers via spin-coating, a rigid support is needed for transferring graphene from Cu substrates to another substrate in order to prevent destroying the atomically thin graphene. Another interesting approach for graphene transfer involves using an amorphous carbon (a-C) transmission electron microscopy (TEM) grid as supporting layers81. To bond the graphene and the a-C TEM grid, a drop of isopropanol (IPA) is placed on top of the grid to wet both the a-C TEM grids and the underlying graphene film. As the IPA evaporates, surface tension draws the graphene and a-C film together into close contact. Therefore, in our new transfer method, surface tension is an important factor. The polymer-free transfer method developed in this study is shown in Figure 2.8. To control the surface tension of our etching solution, we mixed an IPA solution to reduce the surface tension as low as possible. For the same purpose of reducing surface tension around the graphene sample, we also designed a graphite holder to reduce the external force from ambient or solution that would apply on the graphene and to prevent it from degrading (folding or tearing) during the transfer process. All comparisons discussed below are made between the layers from the same CVD-grown graphene sample, but transferred with different methods.
Figure 2.8 illustrates the experimental set-up for the preparation of large-area monolayer graphene which can be applied on any substrates. A clean Petri dish was filled with 1: 10 mixed etchant, which is made by 1 part isopropyl alcohol (IPA) and 9 parts 0.1M ammonium persulphate solution ((NH4)2S2O8). A thin graphite holder with a diameter of 1 cm was then carefully placed at the etchant-air boundary, serving as a confinement area for the monolayer graphene and preventing it from attaching to the edge of the holder. The copper was etched with mixed etchant, and the monolayer graphene film floated on the surface of the solution. Two syringes, one empty and the other containing a mixture of DI water and isopropyl alcohol solution, were loaded into the syringe pump before the pumps were turned on. To control the surface tension for graphene in the solution, the etchant was pumped through the influent transfer lines at a rate of 0.3 ml/min, and the mixed
water solution was simultaneously injected at the same rate. After the etchant became an overall mixture of water and isopropyl alcohol, the substrate was soaked in the solution. The solution was then pulled out with the syringe to lower the graphene onto the substrate, and the sample was heated at 60 C in air for over 10 minutes to enable graphene flattening.
Figure 2.8. Schematic illustration of the polymer-free transfer process.
Since graphene can be used as a transparent conductive electrode, high conductivity and optical transmittance should not be compromised after transfer. The residues left from the conventional polymer-assisted transfer method may degrade the conductivity and transmittance of the graphene sheet. Accordingly, Figure 2.9 shows the electrical and optical properties of graphene after transfer to glass and SiO2 (300nm)/Si substrates. The transmittance of the graphene transferred by this polymer-free method was measured on glass by layers, from a single layer to multiple stacked layers.
Figure 2.9 (a) shows the photos of stacked graphene layers on glasses sequentially created using the polymer-free transfer process. The quality of the graphene transferred on glass and SiO2 is checked with Raman spectroscopy, as shown in Figure 2.9 (b). The sharp 2D peaks at around 2700 cm-1 indicate that the single-layer graphene sheets are still intact with the polymer-free transfer method.
As the layer number increases, the intensity of the 2D peak decreases as expected. The transmittance of the stacked graphene layers was measured using a spectrophotometer (Jasco, V-670) with bare glass as a reference. The transmittance data, as a function of wavelength for the stacked graphene layers, is shown in Figure 2.9 (c). In addition, the inset shows the transmittance at λ= 550 nm as a function of the number of stacked graphene layers in each stacked layer. By fitting the data to Beer’s law, we find that the attenuation coefficient α is 2.65% per layer, which is near the theoretical value of 2.3%82,83. The slightly higher attenuation for our stacked graphene layers may result from the wrinkles induced during the transfer process. Figure 2.9 (d) shows a comparison of sheet resistance between the stacked graphene layers transferred by the conventional PMMA method and the polymer-free transfer method on SiO2 substrates. The sheet resistance of monolayer graphene and four-layer graphene transferred by the PMMA method on the SiO2 substrate are 2.2 kΩ/ and 450/ , respectively. On the other hand, the sheet resistance of monolayer graphene is 810/ , and that of four stacked graphene layers is 230/ on SiO2 substrates. Although the sheet resistance will also be affected by the quality of the original graphene layers grown on Cu, we demonstrated that, from the same sample of CVD-grown graphene on Cu, the resistance of graphene sheets produced from the polymer-free transfer method is always lower than that from the PMMA transfer method.
The difference in the sheet resistance of graphene transferred with these two methods may be attributed to PMMA residues on the graphene and the better surface quality of graphene transferred with the polymer-free method. To investigate these points, the chemical composition and surface quality of the graphene are studied with scanning tunneling microscopy (STM), alongside X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS).
Figure 2.9. Optical and electrical properties of graphene sheets. (a) Photographs of 1.21.0 cm2 films with 1 to 4 layers of stacked graphene film on glass. (b) Raman spectra of graphene with 1, 2, 3, and 4 layer(s). (c) Transmittance of n-layer graphene films shown in (a). The inset is the transmittance, T(%), at λ = 550 nm as a function of the number of stacked graphene layers, n. (d) The sheet resistance of graphene with different numbers of layers on SiO2.