Dense nanowire films can be considered as quasi-porous structures with a high surface-to-volume ratio. From this perspective, structural, optical and electrical properties of axial Si-Ge nanowire heterojunctions produced by the vapor-liquid-solid growth method using Au nanoclusters as catalysts are analyzed. The lattice mismatch induced strain is partially relieved due to spontaneous SiGe intermixing at the heterointerface and lateral expansion of the Ge segment of the nanowire. The mismatch in Ge and Si coefficients of thermal expansion and low thermal conductivity of Si-Ge nanowire heterojunctions are found to be responsible for the thermally induced stress detected under intense laser radiation in photoluminescence and Raman scattering measurements. In electrical measurements, the observed non-linear current-voltage characteristics, strong flicker noise, and damped current oscillations with frequencies of 20-30 MHz are explained using a proposed SiGe nanowire heterojunction energy band diagram that includes energy states associated with structural imperfections at the nanowire surface.