HD-KFM Mode

HS-KFM: the most advanced single-pass KFM mode

Figure 1: schematics of Bimodal HD-KFM setup
On the contrary to standard single-pass setups, Bimodal HD-KFM is characterised because the electric feedback is tuned to the second eigenmode frequency of the cantilever. Figure 1 shows the concept on which HD-KFM is based: the first flexural eigenmode of the cantilever is excited mechanically and the second flexural mode is excited electrically. In the past decade, multi-frequency approach has become one of the front-end topics amongst the AFM community as it has shown many potential applications.

High sensitivity and robustness

Figure 3 : Topgography (3a - left image), surface potential (3b - middle image) and phase (3c - right image) channels of a graphene aggregate on SiO2 substrate showing flakes of 1, 2 and 3 layers
Graphene has emerged as a nanomaterial for the future due to its bidimensional structure and electronic properties. However, synthesis and transfer processes are not easy to implement especially on large areas where a single layer of graphene may have impurities or flakes of multiple layers of graphene. Although Raman spectroscopy is most commonly used to characterise monolayers of graphene, HD-KFM measurements have also proved to be useful to distinguish between single monolayer and multiple layers of graphene. Figure 3 shows both topography (Fig. 3a) and HD-KFM surface potential images (Fig 3b) of a graphene on Si sample. An ANSCM-PT probe (AppNano, USA) with an spring constant of 3 nN/nm was used. The imaging parameters were A01 = 18.5 nm, ASP1 = 18 nm, A02 = 1.9 nm, ASP1 = 1.6 nm. In the topographic image, it can be seen an aggregate of several flakes which form nearly a continuous film. Some areas show higher height values due to multiple layer stack or self bending of the layers (white stripes in the image).

Simultaneously, the surface potential image (Fig 3b.) provides information of the layers with higher contrast. Silicon oxides areas are depicted as red areas (with the colouredscale chosen). The graphene monolayers correspond to the blue areas, double layer to green areas and triple layers of graphene appear as orange areas. In addition, it can also seen that some single monolayer areas in the bottom corner of the structure have slightly higher potential values (notice the dark blue colour) which can also indicate some surface charging of the SiO2 substrate below the graphene monolayer. Another channel typically used in resonant mode is the phase signal, which is related to energy dissipation mechanisms between tip and sample.

Image (Fig 5c.) shows the corresponding phase signal channel obtained simultaneously to the surface potential, where no contrast among different layers is shown, only between substrate and graphene aggregate.

Figure 5 : Topography (top), surface potential (middle) and cross-section (bottom) of molibdene disulfide on gold substrate. High sensitivity of bimodal HD-KFM reveals a transition region between silicon and aluminium. Values are in good agreement with the bulk values of work functions
Figure 5 shows both topography and SP images and a cross-section. It is important to notice that the SP cross-section (white line) shows no bending due to capacitive influences of tip and cantilever or averaging with the surrounding areas (Au and Al regions are 100 nm high). Even more, HD-KFM shows its high sensitivity as an intermediate region between silicon and Aluminium can be resolved. This region might be an effect of the lithography process or the fabrication of the sample as the topography reveals that the Aluminium area is not as flat as the gold area. The differences in the SP values between the Au-Si-Al areas are consistent with standard values of the work function for each material.