Finite-temperature phase diagram and critical point of the Aubry pinned-sliding transition in a 2D monolayer

The Aubry unpinned--pinned transition in the sliding of two incommensurate lattices occurs for increasing mutual interaction strength in one dimension ($1D$) and is of second order at $T=0$, turning into a crossover at nonzero temperatures. Yet, real incommensurate lattices come into contact in two dimensions ($2D$), at finite temperature, generally developing a mutual Novaco-McTague misalignment, conditions in which the existence of a sharp transition is not clear. Using a model inspired by colloid monolayers in an optical lattice as a test $2D$ case, simulations show a sharp Aubry transition between an unpinned and a pinned phase as a function of corrugation. Unlike $1D$, the $2D$ transition is now of first order, and, importantly, remains well defined at $T>0$. It is heavily structural, with a local rotation of moir\'e pattern domains from the nonzero initial Novaco-McTague equilibrium angle to nearly zero. In the temperature ($T$) -- corrugation strength ($W_0$) plane, the thermodynamical coexistence line between the unpinned and the pinned phases is strongly oblique, showing that the former has the largest entropy. This first-order Aubry line terminates with a novel critical point $T=T_c$, marked by a susceptibility peak. The expected static sliding friction upswing between the unpinned and the pinned phase decreases and disappears upon heating from $T=0$ to $T=T_c$. The experimental pursuit of this novel scenario is proposed.Comment: 9 pages, 9 figure

Friction Boosted by Equilibrium Misalignment of Incommensurate Two-Dimensional Colloid Monolayers

Colloidal 2D monolayers sliding in an optical lattice are of recent importance as a frictional system. In the general case when the monolayer and optical lattices are incommensurate, we predict two important novelties, one in the static equilibrium structure, the other in the frictional behavior under sliding. Structurally, realistic simulations show that the colloid layer should possess in full equilibrium a small misalignment rotation angle relative to the optical lattice, an effect so far unnoticed but visible in some published experimental moir\'e patterns. Under sliding, this misalignment has the effect of boosting the colloid monolayer friction by a considerable factor over the hypothetical aligned case discussed so far. A frictional increase of similar origin must generally affect other incommensurate adsorbed monolayers and contacts, to be sought out case by case.Comment: 9 pages, 11 figures (including Supplemental Material

Ferrocene Molecular Architectures Grafted on Si(111): A Theoretical Calculation of the Standard Oxidation Potentials and Electron Transfer Rate Constant

The standard oxidation potential and the electron transfer (ET) rate constants of two silicon-based hybrid interfaces, Si(111)/organic-spacer/Ferrocene, are theoretically calculated and assessed. The dynamics of the electrochemical driven ET process is modeled in terms of the classical donor/acceptor scheme within the framework of Marcus theory. The ET rate constants, k(ET), are determined following calculation of the electron transfer matrix element, V-RP, together with the knowledge of the energy of the neutral and charge separated systems. The recently introduced Constrained Density Functional Theory (CDFT) method is exploited to optimize the structure and determine the energy of the charge separated species. Calculated ET rate constants are k(ET) = 77.8s(-1) and k(ET) = 1.3 x 10(-9) s(-1), in the case of the short and long organic-spacer, respectively

An Integrated Experimental/Theoretical Study of Structurally Related Poly-Thiophenes Used in Photovoltaic Systems

In this work, a series of eight thiophene-based polymers (exploited as “donors” in bulk heterojunction photovoltaics cells), whose structures were designed to be suitably tuned with the electronic characteristics of the [6,6]-Phenyl C61 butyric acid methyl ester (PCBM), is considered,. The electronic properties of the mono-, di-, trimeric oligomers are reckoned (at the Hartree-Fock and DFT level of the theory) and compared to experimental spectroscopic and electrochemical results. Indeed, electrochemical and spectroscopic results show a systematic difference whose physical nature is assessed and related to the exciton (electron-hole) binding energy (Je,h). The critical comparison of the experimental and theoretical band gaps, i.e., the HOMO-LUMO energy difference, suggests that electrochemical and DFT values are the most suited to being used in the design of a polythiophene-based p-n junction for photovoltaics

Growth Dynamics of Ultrathin Films of Benzo[1,2-b:4,5-b']dithiophene Derivatives on Au(111): A Photoelectron Spectroscopy Investigation

: Ultrathin films of a stereoisomeric mixture of benzo[1,2-b:4,5-b']dithiophene derivatives were grown by thermal evaporation in vacuum on Au(111), and they were studied in situ by photoelectron spectroscopy. X-ray photons from a non-monochromatic Mg Kα conventional X-ray source and UV photons from a He I discharge lamp equipped with a linear polarizer were used. He I photoemission results were compared with density functional theory (DFT) calculations: density of states (DOS) and 3D molecular orbital density distribution. Au 4f, C 1s, O 1s, and S 2p core-level components suggest a surface rearrangement as a function of film nominal thickness, with the variation of the molecular orientation, from flat-laying at the initial deposition to tilted toward the surface normal at coverages exceeding 2 nm. Eventually, the DFT results were exploited in assigning of the valence band experimental structures. Moreover, polarization-dependent photoemission confirmed the tilted arrangement of the molecules, starting at 2 nm. A variation of the work function of 1.4 eV with respect to the clean substrate was measured, together with a valence band offset of 1.3 eV between the organic layer and gold

Sondeos arqueológicos Cueva Pintada Corte 6 cierre sur [Material gráfico]

Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte. Subdirección General de Coordinación Bibliotecaria, 201

An Integrated Theoretical/Experimental Study of Quinolinic-Isoquinolinic Derivatives Acting as Reversible Electrochromes

A series of compounds, featuring an ethenylic bridge and quinoline and isoquinoline end capping units possessing systematically varied substitution patterns, were prepared as molecular materials for electrochromic applications. The different structures were optimized in order to maximize the electrochromic contrast in the visible region, mostly by achieving a completely UV-absorbing oxidized state. Density functional theory (DFT) calculations are exploited in order to rationalize the correlation between the molecular structure, the functional groups' electronic properties, and the electrochemical behavior. It is shown that the molecular planarity (i.e. ring/ring pi conjugation) plays a major role in defining the mechanism of the electrochemical charge transfer reaction, while the substituent's nature has an influence on the LUMO energy. Among the compounds here studied, the (E)-10-methyl-9-(2-(2-methylisoquinolinium1- yl)-vinyl)-1,2,3,4-tetrahydroacri-dinium trifluoromethanesulfonate derivative shows the most interesting properties as an electrochromophore

Structure, stoichiometry, and charge transfer in cocrystals of perylene with TCNQ-F_x

Semiconductor charge transfer (CT) cocrystals are an emerging class of molecular materials which combines the characteristics of the constituent molecules in order to tune physical properties. Cocrystals can exhibit polymorphism, but different stoichiometries of the donor-acceptor (DA) pair can also give different structures. In addition, the structures of the donor and acceptor as pristine compounds can influence the resulting cocrystal forms. We report a structural study on several CT cocrystals obtained by combining the polyaromatic hydrocarbon perylene with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its fluorinated derivatives having increasing electronegativity. This is achieved by varying the amount of fluorine substitution on the aromatic ring, with TCNQ-F2 and TCNQ-F4. We find structures with different stoichiometries. Namely, the system perylene:TCNQ-F0 is found with ratios 1:1 and 3:1, while the systems perylene:TCNQ-Fx (x = 2, 4) are found with ratios 1:1 and 3:2. We discuss the structures on the basis of the polymorphism of perylene as pure compound, and show that by a judicious choice of growth temperature the crystal structure can be in principle designed a priori. We also analyze the structural motifs taking into account the degree of charge transfer between the perylene donor and the TCNQ-Fx acceptors and the optical gap determined from infrared (IR) spectroscopy. This family of materials exhibits tunable optical gaps in the near-IR (NIR), promising applications in organic optoelectronics