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In the present study, we have investigated the phonon and thermodynamic properties of bulk zinc-blende CdS by first-principle calculations within the density functional theory (DFT) and the density functional perturbation theory (DFPT) method using the quasi harmonic approximation (QHA). We calculated the phonon dispersion at several high symmetry directions, density of phonon state, temperature dependence feature of Helmholtz free energy (F), internal energy, bulk modulus, constant-volume specific heat, entropy, coefficient of the volume thermal expansion and Grüneisen parameter estimated with the local density approximation (LDA) and generalized gradient approximation (GGA) for the exchange-correlation potential and compared them with each other. For internal energy, Helmholtz free energy, constant volume heat capacity and phonon entropy the LDA and GGA results are very similar. But, the LDA gives lattice constants that are smaller than GGA while phonon frequencies, bulk modulus and cohesive energies are larger. On the other hand, the results obtained through the GGA approximation for the coefficient of the volume thermal expansion and Grüneisen parameter are larger than those obtained from LDA.

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Density functional theory, as implemented in SIESTA code, was utilized in this study to investigate the structural, electronic and stability properties of bare and hydrogenated small nanoclusters of diamond. The results obtained by studying different nanoparticles of diamond composed of 19, 50, and 104 carbon atoms, revealed that while the gap energy of hydrogenated nanodiamonds reduced from 8.2 to 6.5 eV by increasing the size of nanoparticles (number of carbon atoms), the bare nanodiamonds showed almost no gap energy except C19 sample which has the highest occupied molecular orbital-lowest unoccupied molecular orbital of about 0.33 eV. Electron affinity of hydrogenated samples was calculated and it was found that hydrogenated nanodiamond exhibits negative electron affinity. The quantum confinement effect found to be still significant for the sample larger than 1 nm i.e. the largest hydrogenated sample of C104H90 with a diameter of about 1.2 nm which showed greater gap energy in comparison to the bulk diamond. This achievement was explained considering electron affinity and partial density of states analysis. The calculated formation energy of the nanoparticles confirmed that the small hydrogenated nanoclusters of diamond have more stability compared to the bare nanodiamonds.

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