  In this work we present a theoretical study of transport properties of a
double crossbar junction composed by segments of graphene ribbons with
different widths forming a graphene quantum dot structure. The systems are
described by a single-band tight binding Hamiltonian and the Green's function
formalism using real space renormalization techniques. We show calculations of
the local density of states, linear conductance and I-V characteristics. Our
results depict a resonant behavior of the conductance in the quantum dot
structures which can be controlled by changing geometrical parameters such as
the nanoribbon segments widths and relative distance between them. By applying
a gate voltage on determined regions of the structure, it is possible to
modulate the transport response of the systems. We show that negative
differential resistance can be obtained for low values of gate and bias
voltages applied.
