  We present a photoluminescence study of freestanding and Si/SiO2 supported
single- and few-layer MoS2. The single-layer exciton peak (A) is only observed
in freestanding MoS2. The photoluminescence of supported single-layer MoS2 is
instead originating from the A- (trion) peak as the MoS2 is n-type doped from
the substrate. In bilayer MoS2, the van der Waals interaction with the
substrate is decreasing the indirect band gap energy by up to ~ 80 meV.
Furthermore, the photoluminescence spectra of suspended MoS2 can be influenced
by interference effects.
