  Heat engines (HEs) made of low dimensional structures offer promising
applications in energy harvesting due to their reduced phonon thermal
conductance. Many efforts have been devoted to the design of HEs made of
quantum-dot (QD) superlattice nanowire (SLNW), but only SLNWs with uniform
energy levels in QDs were considered. Here we propose a HE made of SLNW with
staircase-like QD energy levels. It is demonstrated that the nonlinear Seebeck
effect can lead to significant electron transports for such a nanowire with
staircase-like energy levels. The asymmetrical alignment of energy levels of
quantum dots embedded in nanowires can be controlled to allow resonant electron
transport under forward temperature bias, while they are in off-resonant regime
under backward bias. Under such a mechanism,the power output and efficiency of
such a SLNW are better than SLNWs with uniform QD energy levels. The SLNW HE
has direction-dependent power output and heat current. In addition, the HE has
the functionality of a heat diode with impressive negative differential thermal
conductance under open circuit condition.
