Direct observation of narrow electronic energy band formation in 2D molecular self-assembly

Surface-supported molecular overlayers have demonstrated versatility as platforms for fundamental research and a broad range of applications, from atomic-scale quantum phenomena to potential for electronic, optoelectronic and catalytic technologies. Here, we report a structural and electronic characterisation of self-assembled magnesium phthalocyanine (MgPc) mono and bilayers on the Ag(100) surface, via low-temperature scanning tunneling microscopy and spectroscopy, angle-resolved photoelectron spectroscopy (ARPES), density functional theory (DFT) and tight-binding (TB) modeling. These crystalline close-packed molecular overlayers consist of a square lattice with a basis composed of a single, flat-adsorbed MgPc molecule. Remarkably, ARPES measurements at room temperature on the monolayer reveal a momentum-resolved, two-dimensional (2D) electronic energy band, 1.27 eV below the Fermi level, with a width of ∼20 meV. This 2D band results from in-plane hybridization of highest occupied molecular orbitals of adjacent, weakly interacting MgPc's, consistent with our TB model and with DFT-derived nearest-neighbor hopping energies. This work opens the door to quantitative characterisation – as well as control and harnessing – of subtle electronic interactions between molecules in functional organic nanofilms

Van der Waals tiheys-funktionaalin implementaation FHI-aims ohjelmassa

Van der Waals (vdW) interactions play an important role in the bonding of sparse biological matter. However, standard Density Functional Theory (DFT), which is commonly used to simulate solid-state systems, fails to properly describe vdW interactions. This is because the common generalized-gradient or local density approximations of electron exchange and correlation fail to account for nonlocal electron correlations which are the cause of vdW forces. Schemes for restoring van der Waals interactions to the framework of DFT are currently the subject of much attention. This work describes the implementation of the Langreth-Lundqvist Van der Waals Density Functional (vdW-DF) into the electronic structure code FHI-aims. The vdW-DF has been implemented in numerous other codes successfully and has been shown to greatly increase the accuracy of calculations with systems in which vdW interactions are prominent. Implementation of vdW-DF into FHI-aims, however, will require special considerations due to the use of all-electron density rather than pseudopotentials. Among these, a method is developed to allow accurate interpolation of all-electron density from an adaptive octree grid. The implementation is tested in calculations of the interaction energies of noble gas dimers and the S22 set of biological complexes. The accuracy achieved is comparable to that of other implementations and is a significant improvement over standard DFT. This allows for the prospect of simulations of important biological systems with vdW-DF in FHI-aims. The computational performance of the algorithm is assessed and options for continuing development are outlined

Hydration Structure of Brookite TiO₂ (210)

The interface of TiO₂ and water has been heavily researched due to the photocatalytical capabilities of this system. Whereas the majority of existing work has targeted the rutile and anatase phases of TiO₂, much less is known about the brookite phase. In this work, we use first-principles molecular dynamics simulations to find the hydration structure of the brookite (210) surface. We find both pure water and an aqueous solution of KCl to order laterally at the sites of surface Ti cations due to electrostatic and chemical considerations, qualitatively in agreement with experimental high-resolution atomic force microscopy measurements. A significant fraction of surface oxygens is hydroxylated for all cases, with up to 40% realized for the aqueous solution at bulk coverage, a result originating in orientational constraints placed on water near the solvated K and Cl ions. Proton transfer is nearly equally frequent between the surface and liquid regions and within the liquid region, but the presence of K and Cl ions makes proton transfer less efficient