High throughput secondary electron imaging of organic residues on a graphene surface

Abstract

Surface organic residues inhibit the extraordinary electronic properties of graphene, hindering the development of graphene electronics. However, fundamental understanding of the residue morphology is still absent due to a lack of high-throughput and high-resolution surface characterization methods. Here, we demonstrate that secondary electron (SE) imaging in the scanning electron microscope (SEM) and helium ion microscope (HIM) can provide sub-nanometer information of a graphene surface and reveal the morphology of surface contaminants. Nanoscale polymethyl methacrylate (PMMA) residues are visible in the SE imaging, but their contrast, i.e. the apparent lateral dimension, varies with the imaging conditions. We have demonstrated a quantitative approach to readily obtain the physical size of the surface features regardless of the contrast variation. The fidelity of SE imaging is ultimately determined by the probe size of the primary beam. HIM is thus evaluated to be a superior SE imaging technique in terms of surface sensitivity and image fidelity. A highly efficient method to reveal the residues on a graphene surface has therefore been established.

Figure 1. Low magnification images of the measured sample.

(a) An optical image of the graphene sample. Only the 3 × 3 hole arrays are shown on the Si substrate and no graphene contrast can be observed. (b) A 5 keV SEM image of the graphene sample. The existence of graphene (marked as ‘G' in the image) can be identified from the substrate (‘S') and the bare holes (‘H'). (c) A 35 keV HIM image of the graphene sample. The image also shows the existence of graphene, substrate and bare holes. The graphene has a better contrast in HIM in comparison with SEM. (d) An AFM phase image of the graphene sample. Holes and freestanding graphene can be identified. The covering of graphene on the substrate is difficult to identify in the image at the selected magnification. All the images have the same field of view (2 μm scale bar shown in Figure 1a).

Figure 2. High magnification SE images of freestanding graphene.

(a) A 35 keV HIM image of the freestanding graphene. The featured structures such as bright spots, clusters and dark dots are marked by the arrows to show their existence. The dark dots can be observed more clearly from a selected and magnified area as shown in the white square frame. (b) A 0.5 keV SEM image of the freestanding graphene. (c) A 10 keV SEM image of the freestanding graphene. (d) A 20 keV SEM image of the freestanding graphene. The featured structures can also be observed in all these SEM images. All the SE images have the same FOV of 400 nm. (e) The intensity distributions for 35 keV HIM, 0.5 keV SEM and 20 keV SEM images. The 35 keV HIM and 20 keV SEM images exhibit similar and symmetric peaks. The peak of the 0.5 keV SEM image is asymmetric and has an additional peak at the high intensity value, as marked by the grey arrow. (f) The size distribution of the dark dots observed in the different imaging conditions. The dots observed in a finely focused 35 keV HIM image (Figure 2a) have small size values. The dots observed in a 0.5 keV SEM image (Figure 2b) and an over-focused 35 keV HIM image (Figure 3d) have similar size values due to their close probe size values.

Figure 3. Discussion of the similarities and differences between SEM and HIM images.

(a) The relationship between the beam probe size and the dark dot size. A linear relationship (black dashed line) can be fitted for both the conditions of e-beam (red squares) and He⁺ beam (blue circles). (b) A finely focused 35 keV HIM image of the freestanding graphene. The image shows a small dot size similar to Figure 2a. (c) A de-focused HIM image of Figure 3b. The observed dot size becomes larger than that of Figure 3b. (d) A finely focused 0.5 keV SEM image. The dot size is quite close to that of Figure 3c because of the close beam probe sizes for these two images.

Figure 4. The PMMA nanoparticle stacking model for the formation of surface roughness and dark dots.

(a) The three-dimensional AFM height image of the substrate-supported graphene with a FOV of 700 nm. A white dashed line is drawn across two peaks (marked as A and B by the arrows) to show its height variation. Inset: A demonstration of the stacking model for the PMMA nanoparticles. PMMA nanoparticles can be stacked as monolayer or multilayers (clusters and dots) on the graphene surface. (b) The height profile of the dashed line in Figure 4a. Spots A and B are shown as two peaks that have large height intensities. (c) The simulated interaction volume of the electrons in the top three layers of PMMA on a graphene surface. The interaction volume of the 0.5 keV electrons in the region exhibits a pear-like shape, while the interaction volume of the 20 keV electrons is a narrow conical shape (see the dashed shapes in the figure). The volume ratio of the top PMMA layer to the total PMMA layers is smaller for the interaction with the 0.5 keV electrons than the 20 keV electrons.

Figure 5. Comparison of other graphene surface features.

(a) The observation of the graphene folds in different microscopes. Graphene folds show different contrast in the 35 keV HIM, 0.5 keV SEM and 20 keV SEM images. (b) The observation of the graphene ridges in different microscopes. (c) The observation of graphene corrugations in different microscopes.