Interplay of Substrate Adhesion, Axial Strain and Doping in the Raman Response of Metallic Carbon Nanotubes Grown on Quartz Monocrystal

Single-walled carbon nanotubes (SWCNTs) grown on single-crystal quartz exhibit complex Raman features that reflect interactions between the nanotube, substrate, and environment. Here, we combine Raman spectroscopy and DFT calculations to study individual metallic SWCNTs on quartz. As often observed but previously unexplained, the G-band displays four components rather than one or two, with both its shape and position varying along the nanotube length. Our analysis shows that these variations arise from a combination of chemical doping and axial compression, which are strongly correlated. The gradual upshift of the G- and D-bands from the nanotube ends toward the center is primarily due to axial compression (up to ∼1%) that develops upon cooling and cannot relax because of strong substrate adhesion. This adhesion gradually weakens under ambient conditions, allowing partial relaxation of axial stress from the ends, or can be partially suppressed by a strain-releasing treatment. DFT calculations reveal that the G-band splitting originates from strong SWCNT-quartz interactions that induce radial deformation and activate localized, low-symmetry C-C stretching modes: two predominantly circumferential and two predominantly axial. The evolution of the G-band and RBM in metallic SWCNTs reveals a substrate-induced increase in p-type doping originating from a reduction in the SWCNT work function due to quartz interaction, further enhanced by axial strain. Together, these findings elucidate the intertwined effects of adhesion, strain, and doping in determining the Raman response of quartz-grown SWCNTs, highlighting the importance of substrate effects when interpreting Raman data or integrating horizontally aligned SWCNTs into electronic devices.