  We report the composition and gate voltage induced tuning of transport
properties in chemically synthesized Bi2(Te1-xSex)3 nanoribbons. It is found
that increasing Se concentration effectively suppresses the bulk carrier
transport and induces semiconducting behavior in the temperature dependent
resistance of Bi2(Te1-xSex)3 nanoribbons when x is greater than ~10%. In
Bi2(Te1-xSex)3 nanoribbons with x ~20%, gate voltage enables ambipolar
modulation of resistance (or conductance) in samples with thickness around or
larger than 100nm, indicating significantly enhanced contribution in transport
from the gapless surface states.
