Abstract Nanodiamonds containing negatively charged nitrogen vacancy (NV − ) centres are highly promising biolabels due to NV − photostability and spectral range. For effective cell tracking we require NV − to be stable in this charge state, but it is known that nanodiamond surfaces may alter the NV charge state; intermittent fluorescing and conversion to NV 0 are frequently observed. Different models have been proposed linking surface termination type to the resultant NV charge state, but a full understanding has not yet been reached. For the work presented in this paper, we use density functional theory to examine how the NV electronic structure in nanodiamond clusters changes with different surfaces. We move beyond examining fully terminated surfaces and focus on the role of surface radicals, based on recent research showing pH also affects NV charge. Our work shows that surface radicals can explain the intermittent fluorescence observed for hydrogenated surfaces, and that different absorbates on the surface influence the resultant NV charge for a specified termination type. We have found that both the termination type, and the absorbates on the surface, play important roles in determining NV charge and should be considered together when predicting surface coverage that will stably produce NV − .

中文翻译：

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纳米金刚石表面自由基和氮空位电荷波动的计算研究

摘要 由于 NV - 光稳定性和光谱范围，含有带负电荷的氮空位 (NV - ) 中心的纳米金刚石是非常有前途的生物标记物。对于有效的电池跟踪，我们要求 NV - 在该电荷状态下保持稳定，但众所周知，纳米金刚石表面可能会改变 NV 电荷状态；经常观察到间歇性荧光和转换为 NV 0。已经提出了将表面终止类型与所得 NV 电荷状态联系起来的不同模型，但尚未完全理解。对于本文中提出的工作，我们使用密度泛函理论来研究纳米金刚石簇中的 NV 电子结构如何随不同表面变化。我们超越检查完全终止的表面，并专注于表面自由基的作用，基于最近的研究表明 pH 值也会影响 NV 电荷。我们的工作表明，表面自由基可以解释在氢化表面上观察到的间歇性荧光，并且表面上的不同吸收会影响特定终止类型的所得 NV 电荷。我们发现终止类型和表面上的吸收物在确定 NV 电荷方面起着重要作用，并且在预测将稳定产生 NV - 的表面覆盖率时应该一起考虑。