Although it is recognized that dynamic behavior of adsorbing molecules strongly affects the entropic contribution to adsorption free energy, detailed studies of the adsorption entropy of large organic molecules at insulating surfaces are still rare. We compared adsorption of two different functionalized organic molecules, 1,3,5-tri-(4-cyano-4,4 biphenyl)-benzene (TCB) and 1,4-bis(cyanophenyl)-2,5-bis(decyloxy)benzene (CDB), on the KCl (001) surface using density functional theory (DFT) and molecular dynamics (MD) simulations. The accuracy of the van der Waals corrected DFT-D3 was benchmarked using Møller-Plesset perturbation theory calculations. Classical force fields were then parameterized for both the TCB and CDB molecules on the KCl (001) surface. These force fields were used to perform potential of mean force (PMF) calculations of adsorption of individual molecules and extract information on the entropic contributions to adsorption energy. The results demonstrate that entropy losses upon adsorption are significant for flexible molecules, such as considered here, and even at relatively low temperatures (e.g. 400 K) can match the enthalpy contribution to adsorption energy.

The electronic and geometric structures of a range of intrinsic and extrinsic defects in black phosphorus (BP) are calculated using Density Functional Theory (DFT) and a hybrid density functional.

International audienceWe studied the effect of molecular flexibility on the morphology and growth mechanisms of self-assembled films on an insulating substrate using a combination of experimental and theoretical methods. 1,3,5-Tri-(4-cyano-4,4 biphenyl)-benzene (TCB) and 1,4-bis(cyanophenyl)-2,5-bis-(decyloxy) benzene (CDB) molecules were deposited on a KCl (001) surface and imaged using noncontact atomic force microscopy (NC-AFM). Both molecules were designed to contain the same anchoring groups and benzene ring structures, yet CDB self-assembled structures were observed to grow from step edges, while TCB self-assembled structures grew as islands on the clean terrace and from step edges. Density functional theory (DFT) and atomistic molecular dynamics simulations were performed to understand this qualitative difference in growth modes. Calculations of free energies of dimer formation and step adhesion provide an insight into the role played by entropy loss in the morphology and growth modes of self-assembled films

Nominally pure black phosphorus (BP) is commonly found to be a p-type semiconductor, suggesting the ubiquitious presence of impurity species or intrinsic, charged defects. Moreover, scanning tunneling microscopy (STM) images of black phosphorus reveal the presence of long-range doublelobed defect features superimposed onto the surface atomic lattice. We show that both the p-type doping of BP and the defect features observed in STM images can be attributed to substitutional tin impurities. We show that black phosphorus samples produced through two common synthesis pathways contain tin impurities, and we demonstrate that the ground state of substitutional tin impurities is negatively charged for a wide range of Fermi level positions within the BP band gap. The localized negative charge of the tin impurities induces hydrogenic states in the band gap, and it is the 2p level that sits at the valence band edge that gives rise to the double-lobed features observed in STM images.

Density functional theory simulations are used to investigate the formation and mobility of Ti interstitial ions, Tii, at the (110) surface of rutile TiO2.