Nano-diamond heat capacities, C_V(T), plotted as a function of temperature in the dimensionless units C_V(T)∕(3 N_A k) where N_A is the Avogadro number and k is the Botlzmann constant. The dashed black line shows the C_V (T) data for diamond assuming a standard Debye temperature, T_D of 2250 K. The 50 − 300 K data points (left hand zoom plot) delineate the laboratory-measured (nano-)diamond heat capacities, as shown by Vasiliev et al. (2010a,b, 2011), for detonation nano-diamonds (red diamonds) and synthetic diamond (black diamonds); the latter fitted with T_D = 1875 K (black line). The fit to the detonation nano-diamond data (red line) is described in the text. The other C_V (T) data shown are for: fully surface-hydrogenated nano-diamonds with radii of 0.54, 1.20, 2.66, 5.88, and 13.01 nm (from top to bottom, solid purple lines). The green lines show the size-dependent contribution of the surface H atoms.