  The generation of highly-confined plasmons through far-field optical
illumination appears to be impractical for technological applications due to
their large energy-momentum mismatch with external light. Electrical generation
of plasmons offers a possible solution to this problem, although its
performance depends on a careful choice of material and geometrical parameters.
Here we theoretically investigate graphene-based structures and show in
particular the very different performance between (i) two layers of graphene
separated by a dielectric and (ii) metal$|$insulator$|$graphene sandwiches as
generators of propagating plasmons assisted by inelastic electron tunneling.
For double-layer graphene, we study the dependence on the relative tilt angle
between the two sheets and show that the plasmon generation efficiency for
$4^\circ$ twist angle drops to $\sim20$\% of its maximum for perfect stacking.
For metal$|$insulator$|$graphene sandwiches, the inelastic tunneling efficiency
drops by several orders of magnitude relative to double-layer graphene,
regardless of doping level, metal$|$graphene separation, choice of metal, and
direction of tunneling (metal to or from graphene), a result that we attribute
to the small fraction of the surface-projected metal Brillouin zone covered by
the graphene Dirac cone. Our results reveal a reasonable tolerance to graphene
lattice misalignment and a poor performance of structures involving metals,
thus supporting the use of double-layer graphene as an optimum choice for
electrical plasmon generation in tunneling devices.
