  By using density functional theory, we have studied vibrational features,
Raman activities, and electronic properties of ultrathin hydrogen-passivated
diamond nanowires (H-DNWs). Confinement imposes the softening of transverse
acoustic modes and the hardening of the low-optical modes. Higher frequencies
optical modes are more influenced by the surface termination effect. Surface
effect splits transverse (TO) and longitudinal (LO) optical modes with a
redshift from the bulk diamond. Electronic inspection shows that the surface
terminated hydrogen atoms have the main contribution to the conduction bands of
H-DNWs. This causes, contrary to our expectations from the quantum confinement
effect, the electronic bandgap of H-DNWs becomes smaller than bulk diamond.
