  Thermoelectric devices that utilize the Seebeck effect convert heat flow into
electrical energy and are highly desirable for the development of portable,
solid state, passively-powered electronic systems. The conversion efficiencies
of such devices are quantified by the dimensionless thermoelectric figure of
merit (ZT), which is proportional to the ratio of a device's electrical
conductance to its thermal conductance. High ZT (>2) has been achieved in
materials via all-scale hierarchical architecturing. This efficiency holds at
high temperatures (700K~900K) but quickly diminishes at lower temperatures. In
this paper, a recently-fabricated two-dimensional (2D) semiconductor called
phosphorene (monolayer black phosphorus) is assessed for its thermoelectric
capabilities. First-principles and model calculations reveal that phosphorene
possesses spatially-anisotropic electrical and thermal conductances. The
prominent electrical and thermal conducting directions are orthogonal to one
another, enhancing the ratio of these conductances. As a result, ZT can reach
2.5 (the criterion for commercial deployment) along the armchair direction of
phosphorene at T=500K and is greater than 1 even at room temperature given
moderate doping (~2 x 10^16 m-2). Ultimately, phosphorene stands out as an
environmentally sound thermoelectric material with unprecedented qualities:
intrinsically, it is a mechanically flexible material that converts heat energy
with high efficiency at low temperatures (~ 300K) - one whose performance does
not require any sophisticated engineering techniques.
