  For semimetal nanowires with diameters smaller than a few tens of nanometers,
a semimetal-to-semiconductor transition is observed as the emergence of an
energy band gap resulting from quantum confinement. Quantum confinement in a
semimetal results in either lifting of the degeneracy of the conduction and
valence bands in a zero gap semimetal, or shifting of bands with a negative
energy overlap to form conduction and valence bands. For semimetal nanowires
with diameters below 10 nanometer, the magnitude of the band gap can become
significantly larger than the thermal energy at room temperature resulting in a
new class of semiconductors relevant for nanoelectronics with critical
dimensions on the order of a few atomic lengths. The smaller a nanowire's
diameter, the larger its surface-to-volume ratio thus leading to an increasing
impact of surface chemistry on its electronic structure. Energy level shifts to
states in the vicinity of the Fermi level due to the electronegativity of
surface terminating species are shown to be comparable in magnitude to quantum
confinement effects at nanowire diameters of a few nanometer; these two effects
can be used to counteract one another leading to semimetallic behavior for
nanowire cross sections at which the quantum confinement effect would otherwise
dominate. Abruptly changing the surface terminating species along the length of
a nanowire leads to an abrupt change in the surface electronegativity. This can
result in the formation of a semimetal-semiconductor junction within a
monomaterial nanowire, without the need for impurity doping nor requiring the
formation of a heterojunction.
