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

Probing the thermal resistance of solid-liquid interfaces in nanofluids with molecular dynamics

The significance of interfacial thermal resistance in the thermal conductivity of nanofluids is not well understood, in part because of the absence of measurements of this quantity. Here, we study the interfacial thermal resistance for metal-oil nanofluids of interest as heat transfer fluids for concentrating solar power, using density functional theory and molecular dynamics simulations. Insights on the role of chemical interactions in determining the interfacial thermal resistance are revealed. The results presented here showcase a general picture in which the stronger the chemical interactions between species at the interface, the lower the associated interfacial thermal resistance. The implications toward nanofluid design are discussed. We show that, for this important family of metal-oil nanofluids, the interfacial thermal resistance values are low enough so that it is possible to afford a reduction in particle size, minimizing stability and rheological issues while still offering enhancement in the effective thermal conductivity with respect to the base fluid.

Substituent effects on the regium-π stacking interactions between Au6 cluster and substituted benzene

The regium-π stacking interactions in the Au₆···PhX (X = H, CH₃, OH, OCH₃, NH₂, F, Cl, Br, CN, NO₂) complexes are studied using quantum chemical methods. The present study focuses on the different effects of electron-donating and electron-withdrawing substituent. The structure and binding strength of the complexes are examined. The interactions between Au₆ cluster and various substituted benzene become strengthened relative to the Au₆···benzene complex. The interaction region indicator analysis was performed, and the interaction region and interaction between the substituent and Au₆ cluster are discussed. It is found that the substituent effects on the regium-π stacking interactions between Au₆ cluster and substituted benzene are different from π···π interactions of benzene dimer. Energy decomposition analysis was carried out to study the nature of regium-π stacking interactions, and the substituent effects are mainly reflected on the electrostatic interaction and dispersion.

Unravelling the Open-Shell Character of Peripentacene on Au(111)

Polycyclic aromatic hydrocarbons (PAHs) are a family of organic compounds comprising two or more fused aromatic rings which feature manifold applications in modern technology. Among these species, those presenting an open-shell magnetic ground state are of particular interest for organic electronic, spintronic, and non-linear optics and energy storage devices. Within PAHs, special attention has been devoted in recent years to the synthesis and study of the acene and fused acene (periacene) families, steered by their decreasing HOMO-LUMO gap with length and predicted open-shell character above some size. However, an experimental fingerprint of such magnetic ground state has remained elusive. Here, we report on the in-depth electronic characterization of isolated peripentacene molecules on a Au(111) surface. Scanning tunnelling spectroscopy, complemented by computational investigations, reveals an antiferromagnetic singlet ground state, characterized by singlet-triplet inelastic excitations with an experimental effective exchange coupling (J_eff) of 40.5 meV. Our results deepen the fundamental understanding of organic compounds with magnetic ground states, featuring perspectives in carbon-based spintronic devices.

Dipole controlled Schottky barrier in the blue-phosphorene-phase of GeSe based van der Waals heterostructures

Controlling the interface structure is of utmost importance to regulating the nanoscale Schottky barrier height (SBH). Herein, by using first-principles calculations, the electronic properties of the graphene (G) based blue-phosphorene-phase of GeSe van der Waals (vdW) heterostructures, including M/G and X/G interfaces (M = Ge; X = Se), are systematically investigated. When the layer spacing exceeds the vdW gap, n-type Schottky contacts are formed for both MX/G and XM/G heterojunctions. With the layer spacing decreasing to equilibrium distances, due to different charge transfer across the interface, MX/G and XM/G heterojunctions display n- and p-type Schottky contacts, respectively. Further decreasing the layer distance makes both heterojunctions transit into p-type ones. The layer-spacing-dependent SBHs can be rationalized by the increased charge transfer across the interface and the resulting interfacial dipole enhancement. Enlightened by the finding of dipole-controlled SBHs, using MX as building blocks, two different stacking patterns, i.e., nMX-MX-G and nXM-XM-G (n = 1 and 2), are designed to further modulate the SBH. Interestingly, due to the presence of the intrinsic dipole of MX, it is found that the magnitude and orientation of the interfacial dipole can be artificially engineered. With n increasing from 0 to 2, nMX-MX-G with an X/G interface changes from the n-type Schottky contact to Ohmic contact. The Fermi level meets the conduction band and G shows a p-type doping feature finally. Likewise, transition from p-type Schottky contact to Ohmic contact is observed for the nXM-XM-G with M/G interface, accompanied by the Fermi level touching the valence band and the feature of n-type doping for G. The role of nMX stacking seems like the role of applying an external electric field (E-field): applying positive E-field is equivalent to the increase of dipole moment while negative E-field corresponds to the offset of dipole moment. In brief, the SBHs of GeSe/G contact are found to be tunable which originates from the intrinsic dipole of MX. The predictable SBHs for these kinds of charming built-in dipole systems are expected to be highly desirable in electronic devices.

2D vertical field-effect transistor

Within the framework of 2D materials, we present four theoretical models of a vertical field-effect transistor (FET) composed of simple alternate graphene and MoS₂ layers. The electronic transport properties at a specific graphene/MoS₂ interface in each configuration are investigated by focusing in particular on the current as a function of the gate voltage. The gate voltage, simulated with a shift of the bands of a specific layer, allows us to tune the current at the interface and the charge transfer between the planes. This analysis of the charge transfer as a function of the gate voltage reveals a strong connection with the transport characteristics as the slope of the current curve. The analysis of physical phenomena at the graphene/MoS₂ interface can further improve the 2D vertical FET performance and contribute to the development of new 2D nanotechnology.

First-principles insights on the electronic and optical properties of ZnO@CNT core@shell nanostructure

In recent years, various kinds of ZnO-based core@shell nanomaterials have been paid much attention due to their widespread applications in the fields of physics, chemistry and energy conversion. In this work, the electronic and optical properties of a new type of ZnO-based one-dimensional core@shell nanostructure, which is composed of inner ZnO nanowire and outer carbon nanotube (CNT), is calculated based on the first-principles density functional theory (DFT). Calculation results suggest that the ZnO nanowire encapsulated in (9, 9)-CNT is the most stable structure from the view of formation energy. The interaction between the inner ZnO nanowire and the outer (9, 9) CNT belongs to a weak van der Waals type. The complex structure is found to possess metallicity for the outer (9, 9) CNT and maintain the wide band gap nature for the inner ZnO nanowire. Under the different external strains, the charge redistribution between inner ZnO nanowire and outer CNT caused by electron tunneling leads to the shift of Dirac point and the band narrowing of inner ZnO nanowire. The inner ZnO nanowire only has light absorption in the UV region, which is consistent with its optical property originating from its wide bandgap nature.

Theoretical insights into the effect of a conjugated core on the hole transport properties of hole-transporting materials for perovskite solar cells

An improved hole transport ability is obtained by introducing a conjugated core into the structure of HTMs.

Unveiling universal trends for the energy level alignment in organic/oxide interfaces

Organic/oxide interfaces exhibit an energy-level-alignment universal behaviour when a bias is applied. Coulomb-blockade regime is ruled by the organic electronegativity.

Densely-packed ZnTPPs Monolayer on the Rutile TiO2(110)-(1×1) Surface: Adsorption Behavior and Energy Level Alignment

The adsorption of a densely packed Zinc(II) tetraphenylporphyrin monolayer on a rutile TiO₂(110)-(1×1) surface has been studied using a combination of experimental and theoretical methods, aimed at analyzing the relation between adsorption behavior and barrier height formation. The adsorption configuration of ZnTPP was determined from scanning tunnel microscopy (STM) imaging, density functional theory (DFT) calculations and STM image simulation. The corresponding energy alignment was experimentally determined from X-ray and UV-photoemission spectroscopies and inverse photoemission spectroscopy. These results were found in good agreement with an appropriately corrected DFT model, pointing to the importance of local bonding and intermolecular interactions in the establishment of barrier heights.

Role of the Pinning Points in epitaxial Graphene Moiré Superstructures on the Pt(111) Surface

The intrinsic atomic mechanisms responsible for electronic doping of epitaxial graphene Moirés on transition metal surfaces is still an open issue. To better understand this process we have carried out a first-principles full characterization of the most representative Moiré superstructures observed on the Gr/Pt(111) system and confronted the results with atomically resolved scanning tunneling microscopy experiments. We find that for all reported Moirés the system relaxes inducing a non-negligible atomic corrugation both, at the graphene and at the outermost platinum layer. Interestingly, a mirror “anti-Moiré” reconstruction appears at the substrate, giving rise to the appearance of pinning-points. We show that these points are responsible for the development of the superstructure, while charge from the Pt substrate is injected into the graphene, inducing a local n-doping, mostly localized at these specific pinning-point positions.

Organics on oxidic metal surfaces: a first-principles DFT study of PMDA and ODA fragments on the pristine and mildly oxidized surfaces of Cu(111)

Using van der Waals corrected density-functional theory calculations, we study the fundamental physico-chemical properties of the molecular fragments of pyromellitic dianhydride oxydianiline (PMDA–ODA) on pristine and oxidized Cu(111) to investigate the effect of mild oxidation of the metal substrate on PMDA–ODA adsorption.

Chemical Interaction, Space-charge Layer and Molecule Charging Energy for a TiO2/TCNQ Interface

Three driving forces control the energy level alignment between transition-metal oxides and organic materials: the chemical interaction between the two materials, the organic electronegativity and the possible space charge layer formed in the oxide. This is illustrated in this study by analyzing experimentally and theoretically a paradigmatic case, the TiO₂(110) / TCNQ interface: due to the chemical interaction between the two materials, the organic electron affinity level is located below the Fermi energy of the n-doped TiO₂. Then, one electron is transferred from the oxide to this level and a space charge layer is developed in the oxide inducing an important increase in the interface dipole and in the oxide work-function.

Solvent Control of Surface Plasmon-Mediated Chemical Deposition of Au Nanoparticles from Alkylgold Phosphine Complexes

Bottom-up approaches to nanofabrication are of great interest because they can enable structural control while minimizing material waste and fabrication time. One new bottom-up nanofabrication method involves excitation of the surface plasmon resonance (SPR) of a Ag surface to drive deposition of sub-15 nm Au nanoparticles from MeAuPPh3. In this work we used density functional theory to investigate the role of the PPh3 ligands of the Au precursor and the effect of adsorbed solvent on the deposition process, and to elucidate the mechanism of Au nanoparticle deposition. In the absence of solvent, the calculated barrier to MeAuPPh3 dissociation on the bare surface is <20 kcal/mol, making it facile at room temperature. Once adsorbed on the surface, neighboring MeAu fragments undergo ethane elimination to produce Au adatoms that cluster into Au nanoparticles. However, if the sample is immersed in benzene, we predict that the monolayer of adsorbed solvent blocks the adsorption of MeAuPPh3 onto the Ag surface because the PPh3 ligand is large compared to the size of the exposed surface between adsorbed benzenes. Instead, the Au-P bond of MeAuPPh3 dissociates in solution (Ea = 38.5 kcal/mol) in the plasmon heated near-surface region followed by the adsorption of the MeAu fragment on Ag in the interstitial space of the benzene monolayer. The adsorbed benzene forces the Au precursor to react through the higher energy path of dissociation in solution rather than dissociatively adsorbing onto the bare surface. This requires a higher temperature if the reaction is to proceed at a reasonable rate and enables the control of deposition by the light induced SPR heating of the surface and nearby solution.

Modeling adsorption and reactions of organic molecules at metal surfaces

CONSPECTUS: The understanding of adsorption and reactions of (large) organic molecules at metal surfaces plays an increasingly important role in modern surface science and technology. Such hybrid inorganic/organic systems (HIOS) are relevant for many applications in catalysis, light-emitting diodes, single-molecule junctions, molecular sensors and switches, and photovoltaics. Obviously, the predictive modeling and understanding of the structure and stability of such hybrid systems is an essential prerequisite for tuning their electronic properties and functions. At present, density-functional theory (DFT) is the most promising approach to study the structure, stability, and electronic properties of complex systems, because it can be applied to both molecules and solids comprising thousands of atoms. However, state-of-the-art approximations to DFT do not provide a consistent and reliable description for HIOS, which is largely due to two issues: (i) the self-interaction of the electrons with themselves arising from the Hartree term of the total energy that is not fully compensated in approximate exchange-correlation functionals, and (ii) the lack of long-range part of the ubiquitous van der Waals (vdW) interactions. The self-interaction errors sometimes lead to incorrect description of charge transfer and electronic level alignment in HIOS, although for molecules adsorbed on metals these effects will often cancel out in total energy differences. Regarding vdW interactions, several promising vdW-inclusive DFT-based methods have been recently demonstrated to yield remarkable accuracy for intermolecular interactions in the gas phase. However, the majority of these approaches neglect the nonlocal collective electron response in the vdW energy tail, an effect that is particularly strong in condensed phases and at interfaces between different materials. Here we show that the recently developed DFT+vdW(surf) method that accurately accounts for the collective electronic response effects enables reliable modeling of structure and stability for a broad class of organic molecules adsorbed on metal surfaces. This method was demonstrated to achieve quantitative accuracy for aromatic hydrocarbons (benzene, naphthalene, anthracene, and diindenoperylene), C60, and sulfur/oxygen-containing molecules (thiophene, NTCDA, and PTCDA) on close-packed and stepped metal surfaces, leading to an overall accuracy of 0.1 Å in adsorption heights and 0.1 eV in binding energies with respect to state-of-the-art experiments. An unexpected finding is that vdW interactions contribute more to the binding of strongly bound molecules on transition-metal surfaces than for molecules physisorbed on coinage metals. The accurate inclusion of vdW interactions also significantly improves tilting angles and adsorption heights for all the studied molecules, and can qualitatively change the potential-energy surface for adsorbed molecules with flexible functional groups. Activation barriers for molecular switches and reaction precursors are modified as well.

The role of charge transfer in the energy level alignment at the pentacene/C60 interface

Understanding the mechanism of energy level alignment at organic-organic interfaces is a crucial line of research to optimize applications in organic electronics. We address this problem for the C60-pentacene interface by performing local-orbital Density Functional Theory (DFT) calculations, including the effect of the charging energies on the energy gap of both organic materials. The results are analyzed within the induced density of interface states (IDIS) model. We find that the induced interface potential is in the range of 0.06-0.10 eV, in good agreement with the experimental evidence, and that such potential is mainly induced by the small, but non-negligible, charge transfer between the two compounds and the multipolar contribution associated with pentacene. We also suggest that an appropriate external intercompound potential could create an insulator-metal transition at the interface.

Signatures in vibrational and UV-visible absorption spectra for identifying cyclic hydrocarbons by graphene fragments

To promote possible applications of graphene in molecular identification based on stacking effects, in particular in recognizing aromatic amino acids and even sequencing nucleobases in life sciences, we comprehensively study the interaction between graphene segments and different cyclic organic hydrocarbons including benzene (C6H6), cyclohexane (C6H12), benzyne (C6H4), cyclohexene (C6H10), 1,3-cyclohexadiene (C6H8(1)) and 1,4-cyclohexadiene (C6H8(2)), using the density-functional tight-binding (DFTB) method. Interestingly, we find obviously different characteristics in Raman vibrational and ultraviolet visible absorption spectra of the small molecules adsorbed on the graphene sheet. Specifically, we find that both spectra involve clearly different characteristic peaks, belonging to the different small molecules upon adsorption, with the ones of ionized molecules being more substantial. Further analysis shows that the adsorptions are almost all due to the presence of dispersion energy in neutral cases and involve charge transfer from the graphene to the small molecules. In contrast, the main binding force in the ionic adsorption systems is the electronic interaction. The results present clear signatures that can be used to recognize different kinds of aromatic hydrocarbon rings on graphene sheets. We expect that our findings will be helpful for designing molecular recognition devices using graphene.

Probing the electronic properties of trimesic acid nanoporous networks on Au(111)

Nowadays molecular nanoporous networks have numerous uses in surface nanopatterning applications and in studies of host-guest interactions. Trimesic acid (TMA), a benzene derivative with three carboxylic groups, is a marvelous building block for forming 2D H-bonded porous networks. Here, we report a low-temperature study of the nanoporous "chicken-wire" superstructure formed by TMA molecules adsorbed on a Au(111) surface. Distinct preferential orientations of the porous networks on Au(111) lead to the formation of peculiar TMA polymorphs that are stabilized only at the boundary between rotational molecular domains. Scanning tunneling microscopy (STM) and spectroscopy are used to investigate the electronic properties of both the molecular building blocks and the pores. Sub-molecular-resolution imaging and spatially resolved electronic spectroscopy reveal a remarkable change in the appearance of the molecules in the STM images at energies in the range of the lowest unoccupied molecular orbital, accompanied by highly extended molecular wave functions into the pores. The electronic structure of the pores reflects a weak confinement of surface electrons by the TMA network. Our experimental observations are corroborated by density-functional-theory-based calculations of the nanoporous structure adsorbed on Au(111).

Tailored formation of N-doped nanoarchitectures by diffusion-controlled on-surface (cyclo)dehydrogenation of heteroaromatics

Surface-assisted cyclodehydrogenation and dehydrogenative polymerization of polycyclic (hetero)aromatic hydrocarbons (PAH) are among the most important strategies for bottom-up assembly of new nanostructures from their molecular building blocks. Although diverse compounds have been formed in recent years using this methodology, a limited knowledge on the molecular machinery operating at the nanoscale has prevented a rational control of the reaction outcome. We show that the strength of the PAH-substrate interaction rules the competitive reaction pathways (cyclodehydrogenation versus dehydrogenative polymerization). By controlling the diffusion of N-heteroaromatic precursors, the on-surface dehydrogenation can lead to monomolecular triazafullerenes and diazahexabenzocoronenes (N-doped nanographene), to N-doped oligomeric or polymeric networks, or to carbonaceous monolayers. Governing the on-surface dehydrogenation process is a step forward toward the tailored fabrication of molecular 2D nanoarchitectures distinct from graphene and exhibiting new properties of fundamental and technological interest.

On the organic energy gap problem

In conjugated organic molecules, the difference between the HOMO and LUMO Kohn-Sham eigenvalues is significantly smaller than the transport gap measured experimentally. We discuss here, within a local-orbital formulation of DFT, how this problem can be corrected using appropriate hybrid potentials, that add a fraction of Hartree-Fock exchange interaction in the DFT calculation. We illustrate this approach presenting calculations for two simple systems: H(2) and C(6)H(6); then, we discuss how to implement this hybrid approach in a general local-orbital calculation, adjusting the hybrid contribution to yield the correct experimental HOMO/LUMO energy gap for the molecule. We also consider the case of an organic molecule on a metal and analyze the effect of the molecule-metal interaction on the organic energy gap. In particular, we discuss how to introduce in this hybrid-potential scheme the effect of the image potential, and present results for the organic molecules PTCDA, TTF, benzene and pentacene on the metal surfaces Au(111), Ag(111) and Cu(111).

Dynamical screening of the van der Waals interaction between graphene layers

The interaction between graphene layers is analyzed combining local orbital DFT and second order perturbation theory. For this purpose we use the linear combination of atomic orbitals-orbital occupancy (LCAO-OO) formalism, that allows us to separate the interaction energy as the sum of a weak chemical interaction between graphene layers plus the van der Waals interaction (Dappe et al 2006 Phys. Rev. B 74 205434). In this work, the weak chemical interaction is calculated by means of corrected-LDA calculations using an atomic-like sp(3)d(5) basis set. The van der Waals interaction is calculated by means of second order perturbation theory using an atom-atom interaction approximation and the atomic-like-orbital occupancies. We also analyze the effect of dynamical screening in the van der Waals interaction using a simple model. We find that this dynamical screening reduces by 40% the van der Waals interaction. Taking this effect into account, we obtain a graphene-graphene interaction energy of 70 ± 5 meV/atom in reasonable agreement with the experimental evidence.