  Addressing the optical properties of a single nanoparticle in the infrared is
particularly challenging, thus alternative methods for characterizing the
conductance spectrum of nanoparticles in this spectral range need to be
developed. Here we describe an efficient method of fabricating single
nanoparticle tunnel junctions on a chip circuit. We apply this method to narrow
band gap nanoparticles of HgSe, which band structure combine the inverted
character of the bulk semimetal with quantum confinement and self-doping. Upon
tuning the gate bias, measurement reveals the presence of two energy gaps in
the spectrum. The wider gap results from the interband gap, while the narrower
gap results from intraband transitions. The observation of the latter near zero
gate voltage confirms the doped character of the nanoparticle at the single
particle level, which is in full agreement with the ensemble optical and
transport measurements. Finally we probe the phototransport within a single
quantum dot and demonstrate a large photogain mechanism resulting from
photogating.
