  The band structure of MoS$_2$ strongly depends on the number of layers, and a
transition from indirect to direct-gap semiconductor has been observed recently
for a single layer of MoS$_2$. Single-layer MoS$_2$ therefore becomes an
efficient emitter of photoluminescence even at room temperature. Here, we
report on scanning Raman and on temperature-dependent, as well as time-resolved
photoluminescence measurements on single-layer MoS$_2$ flakes prepared by
exfoliation. We observe the emergence of two distinct photoluminescence peaks
at low temperatures. The photocarrier recombination at low temperatures occurs
on the few-picosecond timescale, but with increasing temperatures, a
biexponential photoluminescence decay with a longer-lived component is
observed.
