We study single wall carbon nanotubes (SWNTs) deposited on quartz. Their Raman spectrum depends on the tube−substrate morphology, and in some cases, it shows that the same SWNT-on-quartz system exhibits a mixture of semiconductor and metal behavior, depending on the orientation between the tube and the substrate. We also address the problem using electric force microscopy and ab initio calculations, both showing that the electronic properties along a single SWNT are being modulated via tube−substrate interaction.