Single-particle structure of silicon isotopes approaching $^{42}$Si

$^{42}$Si would seem to be a candidate for a magic nucleus with a filled N=28 neutron shell and a Z=14 proton subshell closure, however, experimental evidence has shown it to be collective [1]. It has been proposed that this breakdown is due to the monopole effect of the tensor force at large isospin, and that both proton excitations across Z=14 and neutron excitations across N=28 contribute to the increased collectivity of $^{42}$Si [1,2]. To experimentally investigate the relative contributions of protons and neutrons to the collectivity, we have performed one-proton and one-neutron knockout reactions on $^{36,38,40}$Si using the GRETINA array and S800 spectrometer at the NSCL. We will present preliminary results in comparison to large-scale shell model calculations.\\[4pt] [1] B. Bastin et al, Phys. Rev. Lett. 99, 022503 (2007)\\[0pt] [2] T. Otsuka et al, Phys. Rev. Lett. 95, 232502 (2005)