MICROSCOPY & IMAGING
between the cantilever/tip-system and the sample at varying sample biases with the aid of a current-voltage preamplifier. Since C-AFM is a contact mode AFM method, the use of soft cantilevers with low spring constants is advantageous to minimise tip wear or sample damage. Tis article compares C-AFM
measurements on the TMD-material molybdenum disulfide (MoS2
) in air
and in a high vacuum environment of less than 1 · 10-5 Torr achieved by the Park NX-Hivac AFM (Fig. 1). Generally, a thin layer of water forms on surfaces in ambient conditions, which can reduce the quality and sensitivity of the conductivity measurement in cAFM. Additionally, water induces a p-doping in MoS2
, therefore limiting the electrical conductivity of the material.
ON MOS2 Fig. 2 summarises topography measurements on three MoS2
ATOMIC FORCE MICROSCOPY samples
with different layer thicknesses: one sample with 1-2 MoS2
sample with 3-4 MoS2 layers (b), layers (a), one
and another multi-layer sample with pyramidal surface structures (c). Te 1-2-layer sample consists of one closed MoS2
Fig. 3. Direct comparison of cAFM measurement in air and in high vacuum on a 3-4-layer sample. a) and b) topography, c) and d) current signal. Scale bars are 500nm
monolayer with additional single- additional single-layer islands. Figure 2d
layered islands, featuring a step height of 0.6 nm – the layer thickness of a single monolayer. Te islands represent the onset of the growth of the second layer and can be recognised in Fig. 2 as bright regions in the topography. Similarly, the 3-4-layer sample comprises a closed three-layered MoS2
film with
schematically shows the basic structure of the 3-4-layer sample as well as the C-AFM experimental setup for the following measurements. Here, each green layer represents one MoS2
monolayer, while
the grey arrangement on the left side is the electrical contact required to apply of the sample bias during the C-AFM
measurement. Te multi-layered MoS2 sample differs strongly from the former in its surface structure. Tis sample exhibits three-dimensional (3D) pyramid-shaped
features on top of a closed 3-layer MoS2 film. Te formation of the pyramid-
AFM imaged steps in the topography of the 1-2-layer and 3-4-layer samples that run diagonally across both images in Fig 2a and b. Tese lines originate from the terrace structure of the underlying sapphire substrate, which the AFM detected through the MoS2
films.
However, the height of the steps allows a clear distinction between sapphire terraces and MoS2
steps: While the
characteristic step height on the c-plane of the sapphire substrate is 0.2nm, a monolayer MoS2
has a thickness of
0.6nm. Te height profile in Fig. 2e visualised the different step heights of the substrate and the MoS2
sample.
Fig. 4. cAFM measurement in high vacuum on 1-2-layer sample. a) sample topography and b) current signal. Scale bars are 500nm
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CONDUCTIVITY IN AIR AND HIGH VACUUM To demonstrate the advantageous effects of high vacuum on the quality of C-AFM, comparative measurements in air and in vacuum <1·10-5
Torr were carried out on the same sample with the same
shaped features originates from a change of the growth mechanism from layer- by-layer to a 3D growth at thicknesses beyond 4 layers. In addition to the MoS2
islands, the
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