Showing papers by "Fernando Flores published in 2011"

C6H6/Au(111): Interface dipoles, band alignment, charging energy, and van der Waals interaction

Abstract: We analyze the benzene/Au(111) interface taking into account charging energy effects to properly describe the electronic structure of the interface and van der Waals interactions to obtain the adsorption energy and geometry. We also analyze the interface dipoles and discuss the barrier formation as a function of the metal work-function. We interpret our DFT calculations within the induced density of interface states (IDIS) model. Our results compare well with experimental and other theoretical results, showing that the dipole formation of these interfaces is due to the charge transfer between the metal and benzene, as described in the IDIS model.

Simulating the organic‐molecule/metal interface TCNQ/Au(111)

Abstract: A density functional calculation is performed to analyze the tetracyanoquinodimethane (TCNQ)/Au(111) interface, including the effect of the molecular charging energy on the transport gap. We find that the adsorbed TCNQ molecules are bent so that the edge N atoms are closer to the Au(111) surface. Theoretical scanning tunneling microscopy (STM) imaging is carried out and compared with the experimental STM for the TCNQ/Au(111) self-assembled structure, finding good agreement, and validating the interface geometry obtained in our calculations. We show that the alignment between the metal and the organic levels is controlled by the charge transfer between the two materials and the dipole created in the molecule because of its deformation when adsorbed in Au(111). The calculated transport gap is 3.1 eV.

Charging energy and barrier height of pentacene on Au(111): a local-orbital hybrid-functional density functional theory approach.

Abstract: We analyze the pentacene/Au(111) interface by means of density functional theory (DFT) calculations using a new hybrid functional; in our approach we introduce, in a local-orbital formulation of DFT, a hybrid exchange potential, and combine it with a calculation of the molecule charging energy to properly describe the transport energy gap of pentacene on Au(111). Van der Waals forces are taken into account to obtain the adsorption geometry. Interface dipole potentials are also calculated; it is shown that the metal/pentacene energy level alignment is determined by the potential induced by the charge transfer between the metal surface and the organic material, as described by the induced density of interface states model. Our results compare well with the experimental data.

Barrier height formation for the PTCDA/Au(1 1 1) interface

Abstract: A Density Functional calculation is performed to analyze the PTCDA/Au(1 1 1) interface, including the effect of the molecular charging energy on the transport gap. The calculated transport gap is 2.9 eV, and the lowest unoccupied molecular orbital, or LUMO, level is found at 4.25 eV below the vacuum level. We show that the alignment between the metal and the organic levels is controlled by the charge transfer between the two materials, as determined by the difference between the molecule Charge Neutrality Level, or CNL, (located 0.3 eV below the LUMO level) and the metal work function. We compare with independent General Gradient Approximation–Density Functional calculations, DFT–GGA, yielding an energy gap of 1.4 eV; we find, however, good agreement between both approaches for the CNL and the interface potential step. Good agreement is also found between our results and the experimental evidence.

Giant alkali-metal-induced lattice relaxation as the driving force of the insulating phase of alkali-metal/Si(111):B

Abstract: Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2sqrt[3]×2sqrt[3]R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effects.

Barrier height formation for the PTCDA/Au(111) interface

Abstract: A Density Functional calculation is performed to analyze the PTCDA/Au(1 1 1) interface, including the effect of the molecular charging energy on the transport gap. The calculated transport gap is 2.9 eV, and the lowest unoccupied molecular orbital, or LUMO, level is found at 4.25 eV below the vacuum level. We show that the alignment between the metal and the organic levels is controlled by the charge transfer between the two materials, as determined by the difference between the molecule Charge Neutrality Level, or CNL, (located 0.3 eV below the LUMO level) and the metal work function. We compare with independent General Gradient Approximation–Density Functional calculations, DFT–GGA, yielding an energy gap of 1.4 eV; we find, however, good agreement between both approaches for the CNL and the interface potential step. Good agreement is also found between our results and the experimental evidence.