The structural analysis and electronic properties of unpassivated and hydrogen (H)-passivated germanium (Ge) nanowires (NWs) oriented along , , , and  directions using a sp3 tight binding model have been investigated in this paper. It has been observed that the unit cell and complete structures of a GeNW is different for different growth directions, therefore, the total number of Ge atoms and H atoms within each unit cell and complete structure, length of unit cell, and diameter of a GeNW are changed with the change in their growth direction. Further, the electronic properties of both the unpassivated and H-passivated GeNWs have been investigated with the help of their band structures obtained for different growth directions. The band structures show that the unpassivated  -oriented and  -oriented GeNWs are indirect band gap semiconductors with band gap magnitude of 0.40 eV and 1.49 eV while the unpassivated  -oriented GeNW is metallic because of zero band gap and  -oriented GeNW is a direct band gap semiconductor with a very small band gap magnitude of 0.22 eV. Further, it has been observed that the nature of a GeNW is changed from indirect to direct when their entire surface dangling bonds are passivated with H atoms, and it has been investigated that the H-passivated GeNWs oriented along , , ,  directions are direct band gap semiconductors with band gap magnitude of 3.66 eV, 2.73 eV, 2.96 eV, 2.51 eV which are larger than the band gaps of an unpassivated GeNWs. Finally, it has been observed from these results that the H-passivated GeNWs are useful for designing novel nanoelectronic and optoelectronic devices.

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