  The nitrogen-vacancy (NV) centre in diamond has emerged as a candidate to
non-invasively hyperpolarise nuclear spins in molecular systems to improve the
sensitivity of nuclear magnetic resonance (NMR) experiments. Several promising
proof of principle experiments have demonstrated small-scale polarisation
transfer from single NVs to hydrogen spins outside the diamond. However, the
scaling up of these results to the use of a dense NV ensemble, which is a
necessary prerequisite for achieving realistic NMR sensitivity enhancement, has
not yet been demonstrated. In this work, we present evidence for a polarising
interaction between a shallow NV ensemble and external nuclear targets over a
micrometre scale, and characterise the challenges in achieving useful
polarisation enhancement. In the most favourable example of the interaction
with hydrogen in a solid state target, a maximum polarisation transfer rate of
$\approx 7500$ spins per second per NV is measured, averaged over an area
containing order $10^6$ NVs. Reduced levels of polarisation efficiency are
found for liquid state targets, where molecular diffusion limits the transfer.
Through analysis via a theoretical model, we find that our results suggest
implementation of this technique for NMR sensitivity enhancement is feasible
following realistic diamond material improvements.
