  We performed first principles calculations on work functions of single wall
carbon nanotubes (SWNTs), which can be divided into two classes according to
tube diameter (D). For class I tubes (D > 1 nm), work functions lie within a
narrow distribution (~0.1 eV) and show no significant chirality or diameter
dependence. For class II tubes (D < 1 nm), work functions show substantial
changes, with armchair tubes decrease monotonously with diameter, while zigzag
tubes show opposite trend. Surface dipoles and hybridization effects are shown
to be responsible for the observed work function change.
