Atomically thin MoS2 has recently emerged as a very attractive material for
nanoscale optoelectronic devices. While n-type transport in MoS2 devices has
been demonstrated, hole conduction has been more challenging. Here we show
work-function engineering to be an effective approach for controlling the
polarity of MoS2 devices. Gated multi-layer MoS2 transistors with Au
source/drain contacts exhibit n-type operation, while those with Pd contacts
are shown to have p-type behavior. Devices with one Au and one Pd contact
exhibit asymmetric ambipolar behavior and diode characteristics over a wide
range of gate voltage, as well as a sizable photovoltaic effect. We argue that
the photovoltaic effect arises from the built-in potential of the space charge
accumulated at the source and drain contacts.Comment: 11 pages, 5 figure
