In the present report, the structures, energetics and electronic properties of neutral and cationic Nb-doped Ge-n (n=7-18) clusters are systematically investigated under the first-principles density functional theory approach. The isomers in which the Nb atom is encapsulated inside a germanium cage are relatively stable compared to the exohedral surface doping. The thermodynamic stability and chemical activity of the ground-state isomers are analyzed through various energetic parameters. The results highlight the enhanced stability of the neutral NbGe12 hexagonal prism-like structure with D-6h symmetry and cationic NbGe16 fullerene isomers. The negative nucleus-independent chemical shift can explain the enhanced stability of neutral NbGe12. However, the enhanced stability of cationic NbGe16 is explained by shell closing model associated with the quasi-spherical geometry with a sequence 1S(2)1P(6)1D(10)1F(6)1G(12)2S(2)2P(6)IF(8)IG(6)2D(10) following Hund's rule. To understand the effect of hybridization on stability, we have calculated density of states (DOS) and projected DOS (PDOS). From PDOS, it is clear that Nb-p and Ge-s and p orbitals are mainly take part in hybridization; however, near below Fermi level, the dominating contribution comes from Nb-d orbitals. In addition, IR and Raman spectra of clusters are also calculated to explain their vibrational properties of the isomers. Specifically, IR spectrum of the clusters in the range of 12-16 shows the possible application of these clusters in the IR sensing device.