One-dimensional heterostructures, based on functionalities of dissimilar materials within a monolithic structure, are promising building blocks for different applications. Herein, utilizing surface decoration of multiwalled carbon nanotubes (MWCNTs) with hydrogenated graphitic carbon layers (HGCLs), the realization of a vertically aligned MWCNT/amorphous-silicon (a-Si) core/shell heterostructure is reported. The proposed method enables the formation of conformal, continuous, and all-around silicon deposition on the carbon nanotube and eliminates any signature of line-of-sight deposition problem, even for thicknesses as low as a few nanometers. Precise elaboration using comparative Raman analysis reveals that the HGCLs play a major role in the construction of such structures. Evidence of direct binding between Si and C, a missing remarkable feature in previous reports, has been observed in high-resolution transmission electron microscope images and X-ray photoelectron spectroscopy. Monitoring time evolution during the formation of the silicon shell declares a diffusive mechanism for the deposition of Si on the surface of MWCNTs. Furthermore, the electro-optical proficiency of the MWCNT/a-Si heterostructure was studied by fabrication of a photodiode. Unlike previous attempts, a naturally formed Schottky junction at the high-quality a-Si/nanotube interface is exploited for charge separation in this photodiode, which provides a sensitivity of >10⁷% in the reverse saturation current for a wavelength of λ = 405 nm.