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Thinning out MoS2 crystals to atomically thin layers results in the transition from an indirect to a direct bandgap material. This makes single layer MoS2 an exciting new material for electronic devices. In MoS2 devices it has been observed that the choice of materials, in particular for contact and gate, is crucial for their performance. This makes it very important to study the interaction between ultrathin MoS2 layers and materials employed in electronic devices in order to optimize their performance. In this work we used NC-AFM in combination with quantitative KPFM to study the influence of the substrate material and the processing on single layer MoS2 during device fabrication. We find a strong influence of contaminations caused by the processing on the surface potential of MoS2. It is shown that the charge transfer from the substrate is able to change the work function of MoS2 by about 40 meV. Our findings suggest two things. First, the necessity to properly clean devices after processing as contaminations have a great impact on the surface potential. Second, that by choosing appropriate materials the work function can be modified to reduce contact resistance.