  Si nanoparticles (NPs) have been actively developed as a hyperpolarized
magnetic resonance imaging (MRI) agent with an imaging window close to one
hour. However, the progress in the development of NPs has been hampered by the
incomplete understanding of their structural properties that correspond to
efficient hyperpolarization build up and long polarization decays. In this work
we study dynamic nuclear polarization (DNP) of single crystal porous Si (PSi)
NPs with defined doping densities ranging from nominally undoped to highly
doped with boron or phosphorus. To develop such PSi NPs we perform low-load
metal-assisted catalytic etching for electronic grade Si powder followed by
thermal oxidation to form the dangling bonds in the Si/SiO$_2$ interface, the
$P_b$ centers. $P_b$ centers are the endogenous source of the unpaired electron
spins necessary for DNP. The controlled fabrication and oxidation procedures
allow us to thoroughly investigate the impact of the magnetic field,
temperature and doping on the DNP process. We argue that the buildup and decay
rate constants are independent of particle size. Instead, the rates are limited
by the polarization transfer across the nuclear spin diffusion barrier
determined by the large hyperfine shift of the central $^{29}$Si nuclei of the
$P_b$ centers. The size-independent rates then lead to a weak dependence on
doping degree for low and moderately doped Si although slight doping is
required to achieve the highest polarization. Thus, we find the room
temperature relaxation of low boron doped PSi NPs reaching $75 \pm 3$ minutes
and nuclear polarization levels exceeding $\sim 6$ % when polarized at 6.7 T
and 1.4 K. Our study thus establishes solid grounds for further development of
Si NPs.
