Surface Stabilization to Enhance Single Molecule Toroidal Behavior in {Dy3} Molecules: the Impact of Au(111), MgO, and Graphene Surfaces

Single-molecule toroids (SMTs), with vortex-like magnetic anisotropy axes, hold promise for quantum technologies, but controlling their toroidal states on the surface remains challenging. To address this, the SMT behavior of [Dy3(µ3-OH)2L3Cl(H2O)5]Cl3 (where L = ortho-vanillin) grafted onto Au(111), MgO has been studied, and graphene surfaces in pristine form (1) and with pyrene (2) and (CH2)8S (3) linkers, using periodic density functional theory and ab initio CASSCF/RASSI-SO methods. Both pristine and chemically functionalized molecules are stable on Au(111) and graphene surfaces; however, functionalization provides higher binding energies and, in some cases, enhances the SMT properties. The MgO surface, however, is found to be unsuitable as it abstracts an H atom from the molecule, leading to the loss of its SMT characteristics. The energy gap (ΔE) between the toroidal (nonmagnetic) and spin-flip (magnetic) states in complex 1 on Au(111) and graphene surfaces are 6.9 and 6.6 cm-1, respectively. Complexes 2 on Au(111) and 3 on graphene exhibit ΔE and toroidal blocking fields of 9.8 cm-1/1.2 T and 6.8 cm-1/0.83 T, respectively, representing the highest recorded values for this class of SMTs. These findings demonstrate the potential of surface stabilization to improve the functionality and applicability of SMTs in advanced quantum technologies.

On-Demand Catalysed n-Doping of Organic Semiconductors

A new approach to control the n-doping reaction of organic semiconductors is reported using surface-functionalized gold nanoparticles (f-AuNPs) with alkylthiols acting as the catalyst only upon mild thermal activation. To demonstrate the versatility of this methodology, the reaction of the n-type dopant precursor N-DMBI-H with several molecular and polymeric semiconductors at different temperatures with/without f-AuNPs, vis-à-vis the unfunctionalized catalyst AuNPs, was investigated by spectroscopic, morphological, charge transport, and kinetic measurements as well as, computationally, the thermodynamic of catalyst activation. The combined experimental and theoretical data demonstrate that while f-AuNPs is inactive at room temperature both in solution and in the solid state, catalyst activation occurs rapidly at mild temperatures (~70 °C) and the doping reaction completes in few seconds affording large electrical conductivities (~10-140 S cm-1). The implementation of this methodology enables the use of semiconductor+dopant+catalyst solutions and will broaden the use of the corresponding n-doped films in opto-electronic devices such as thin-film transistors, electrochemical transistors, solar cells, and thermoelectrics well as guide the design of new catalysts.

Orchestration of ferro- and anti-ferromagnetic ordering in gold nanoclusters

The unpaired electron in the gold clusters (Aun, n = no. of Au atoms) with an odd number of total electrons is solely responsible for the magnetic properties in the small-sized Au nano-clusters. However, no such unpaired electron is available due to pairing in the even number of atom gold clusters and behaving as a diamagnetic entity similar to bulk gold. In this work, we unveiled the spin-density distribution of odd Aun clusters with n = 1 to 19 that reveals that a single unpaired electron gets distributed non-uniformly among all Au-atoms depending on the cluster size and morphology. The delocalization of the unpaired electron leads to the spin dilution approaching a value of ∼1/n spin moments on each atom for the higher clusters. Interestingly, small odd-numbered gold clusters possess spin-magnetic moments similar to the delocalized spin moments as of organic radicals. Can cooperative magnetic properties be obtained by coupling these individual magnetic gold nanoparticles? In this work, by applying state-of-the-art computational methodologies, we have demonstrated ferromagnetic or anti-ferromagnetic couplings between such magnetic nanoclusters upon designing suitable organic spacers. These findings will open up a new avenue of nanoscale magnetic materials combining organic spacers and odd-electron nano-clusters.

The thermodynamics of self-assembled monolayer formation: a computational and experimental study of thiols on a flat gold surface

A methodology based on molecular dynamics simulations is presented to determine the chemical potential of thiol self-assembled monolayers on a gold surface. The thiol de-solvation and then the monolayer formation are described by thermodynamic integration with a gradual decoupling of one molecule from the environment, with the necessary corrections to account for standard state changes. The procedure is applied both to physisorbed undissociated thiol molecules and to chemisorbed dissociated thiyl radicals, considering in the latter case the possible chemical potential of the produced hydrogen. We considered monolayers formed by either 7-mercapto-4-methylcoumarin (MMC) or 3-mercapto-propanoic acid (MPA) on a flat gold surface: the free energy profiles with respect to the monolayer density are consistent with a transition from a very stable lying-down phase at low densities to a standing-up phase at higher densities, as expected. The maximum densities of thermodynamically stable monolayers are compared to experimental measures performed with reference-free grazing-incidence X-ray fluorescence (RF-GIXRF) on the same systems, finding a better agreement in the case of chemisorbed thiyl radicals.

Effects of chain-chain interaction on the configuration of short-chain alkanethiol self-assembled monolayers on a metal surface

Surface-specific sum frequency generation vibrational spectroscopy is applied to study the molecular configuration of short-chain n-alkanethiol self-assembled monolayers (SAMs with n = 2-6) on the Au surface. For monolayers with n≥ 3, the alkanethiols are upright-oriented, with the CH3 tilt angle varying between ∼33° and ∼46° in clear even-odd dependency. The ethanethiol monolayer (n = 2) is, however, found to exhibit a distinct lying-down configuration with a larger methyl tilt angle (67°-79°) and a smaller CH2 tilt angle (56°-68°). Such a unique configurational transition from n = 2 to n≥ 3 discloses the steric effect owing to chain-chain interaction among neighboring molecules. Through density functional theory calculations, the transition is further confirmed to be energetically favorable for thiols on a defective reconstructed Au(111) surface but not on the pristine one. Our study highlights the roles of the chain-chain interaction and the substrate surface atomic structure when organizing SAMs, offering a strategic pathway for exploiting their applications.

Chiral-Induced Spin Selectivity and Non-equilibrium Spin Accumulation in Molecules and Interfaces: A First-Principles Study

Electrons moving through chiral molecules are selected according to their spin orientation and the helicity of the molecule, an effect known as chiral-induced spin selectivity (CISS). The underlying physical mechanism is not yet completely understood. To help elucidate this mechanism, a non-equilibrium Green's function method, combined with a Landauer approach and density functional theory, is applied to carbon helices contacted by gold electrodes, resulting in spin polarization of transmitted electrons. Spin polarization is also observed in the non-equilibrium electronic structure of the junctions. While this spin polarization is small, its sign changes with the direction of the current and with the handedness of the molecule. While these calculations were performed with a pure exchange-correlation functional, previous studies suggest that computationally more expensive hybrid functionals may lead to considerably larger spin polarization in the electronic structure. Thus, non-equilibrium spin polarization could be a key component in understanding the CISS mechanism.

pH-Induced Changes in the SERS Spectrum of Thiophenol at Gold Electrodes during Cyclic Voltammetry

Thiophenol is a model compound used in the study of self-assembly of arylthiols on gold surfaces. In particular, changes in the surface-enhanced Raman scattering (SERS) spectra of these self-assembled monolayers (SAMs) with a change of conditions have been ascribed to, for example, differences in orientation with respect to the surface, protonation state, and electrode potential. Here, we show that potential-induced changes in the SERS spectra of SAMs of thiophenol on electrochemically roughened gold surfaces can be due to local pH changes at the electrode. The changes observed during the potential step and cyclic voltammetry experiments are identical to those induced by acid-base switching experiments in a protic solvent. The data indicate that the potential-dependent spectral changes, assigned earlier to changes in molecular orientation with respect to the surface, can be ascribed to changes in the pH locally at the electrode. The pH at the electrode can change as much as several pH units during electrochemical measurements that reach positive potentials where oxidation of adventitious water can occur. Furthermore, once perturbed by applying positive potentials, the pH at the electrode takes considerable time to recover to that of the bulk solution. It is noted that the changes in pH even during cyclic voltammetry in organic solvents can be equivalent to the addition of strong acids, such as CF3SO3H, and such effects should be considered in the study of the redox chemistry of pH-sensitive redox systems and potential-dependent SERS in particular.

Chemisorption of nitronyl-nitroxide radicals on gold surface: an assessment of morphology, exchange interaction and decoherence time

A combined Tof-SIMS, XPS and STM characterization has been performed to study the deposition of a sulphur-functionalized nitronyl nitroxide radical on Au(111) clearly demonstrating the chemisorption of intact molecules. Continuous -wave EPR characterization showed that the radical molecules maintain their paramagnetic character. Pulsed EPR measurements allowed to determine the decoherence time of the nanostructure at 80 K, which turned out to be comparable to the one measured in frozen solution and longer than previously reported for many radicals and other paramagnetic molecules at much lower temperatures. Furthermore, we conducted a state-of-the-art ab initio molecular dynamics study, suggesting different possible scenarios for chemisorption geometries and predicting the energetically favoured geometry. Calculation of the magnetic properties indicates a partial non-innocent role of the gold surface in determining the magnetic interactions between radicals in packed structures. This suggests that the observed EPR spectrum is to be attributed to low-density domains of disordered radicals interacting via dipolar interactions.

Quantum dynamics of a single molecule magnet on superconducting Pb(111)

Magnetic materials interfaced with superconductors may reveal new physical phenomena with potential for quantum technologies. The use of molecules as magnetic components has already shown great promise, but the diversity of properties offered by the molecular realm remains largely unexplored. Here we investigate a submonolayer of tetrairon(III) propeller-shaped single molecule magnets deposited on a superconducting lead surface. This material combination reveals a strong influence of the superconductor on the spin dynamics of the single molecule magnet. It is shown that the superconducting transition to the condensate state switches the single molecule magnet from a blocked magnetization state to a resonant quantum tunnelling regime. Our results open perspectives to control single molecule magnetism via superconductors and to use single molecule magnets as local probes of the superconducting state.

Deciphering the origin of variation in the spin ground state and oxidation state of a {Mn19} cluster on a Au(111) surface: is the Au(111) surface innocent?

Periodic DFT calculations on a {Mn19} cluster possessing S = 83/2 ground state and its reduced variant on a Au(111) surface unravel the importance of structural distortions that triggered drastic variations in the J values leading to a large reduction in the spin ground state. Reduction of MnIII ions leads to antiferromagnetic Js with a very small spin ground state manifesting the non-innocent behavior of the Au(111) surface.

Combined DFT and XPS Investigation of Cysteine Adsorption on the Pyrite (1 0 0) Surface

The adsorption of cysteine on the pyrite (1 0 0) surface was evaluated by using first-principles-based density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS) measurements. The frontier orbitals analyses indicate that the interaction of cysteine and pyrite mainly occurs between HOMO of cysteine and LUMO of pyrite. The adsorption energy calculation shows that the configuration of the -OH of -COOH adsorbed on the Fe site is the thermodynamically preferred adsorption configuration, and it is the strongest ionic bond according to the Mulliken bond populations. As for Fe site mode, the electrons are found transferred from cysteine to Fe of pyrite (1 0 0) surface, while there is little or no electron transfer for S site mode. Projected density of states (PDOS) is analyzed further in order to clarify the interaction mechanism between cysteine and the pyrite (1 0 0) surface. After that, the presence of cysteine adsorption on the pyrite (1 0 0) surface is indicated by the qualitative results of the XPS spectra. This study provides an alternative way to enhance the knowledge of microbe–mineral interactions and find a route to improve the rate of bioleaching.

Fullerene faraday cage keeps magnetic properties of inner cluster pristine

Any single molecular magnets (SMMs) perspective for application is as good as its magnetization stability in ambient conditions. Endohedral metallofullerenes (EMFs) provide a solid basis for promising SMMs. In this study, we investigated the behavior of functionalized EMFs on a gold surface (EMF-L-Au). Having followed the systems molecular dynamics paths, we observed that the chemically locked inner cluster inside fullerene cage will remain locked even at room temperature due to the ligand-effect. We have located multiple possible minima with different charge arrangements between EMF-L-Au fragments. Remarkably, the charge state of the EMF inner cluster remained virtually constant and so magnetic properties are expected to be untouched. © 2018 Wiley Periodicals, Inc.

SCC-DFTB parameters for simulating hybrid gold-thiolates compounds

We present a parametrization of a self-consistent charge density functional-based tight-binding scheme (SCC-DFTB) to describe gold-organic hybrid systems by adding new Au-X (X = Au, H, C, S, N, O) parameters to a previous set designed for organic molecules. With the aim of describing gold-thiolates systems within the DFTB framework, the resulting parameters are successively compared with density functional theory (DFT) data for the description of Au bulk, Aun gold clusters (n = 2, 4, 8, 20), and Aun SCH3 (n = 3 and 25) molecular-sized models. The geometrical, energetic, and electronic parameters obtained at the SCC-DFTB level for the small Au3 SCH3 gold-thiolate compound compare very well with DFT results, and prove that the different binding situations of the sulfur atom on gold are correctly described with the current parameters. For a larger gold-thiolate model, Au25 SCH3 , the electronic density of states and the potential energy surfaces resulting from the chemisorption of the molecule on the gold aggregate obtained with the new SCC-DFTB parameters are also in good agreement with DFT results.

Carbon dioxide interaction with isolated imidazole or attached on gold clusters and surface: competition between σ H-bond and π stacking interaction

Using first principle methodologies, we investigate the subtle competition between σ H-bond and π stacking interaction between CO2 and imidazole either isolated, adsorbed on a gold cluster or adsorbed on a gold surface. These computations are performed using MP2 as well as dispersion corrected density functional theory (DFT) techniques. Our results show that the CO2 interaction goes from π-type stacking into σ-type when CO2 interacts with isolated imidazole and Au clusters or surface. The balance between both types of interactions is found when an imidazole is attached to a Au20 gold cluster. Thus, the present study has great significance in understanding and controlling the structures of weakly-bound molecular systems and materials, where hydrogen bonding and van der Waals interactions are competing. The applications are in the fields of the control of CO2 capture and scattering, catalysis and bio- and nanotechnologies.

Self-assembled monolayers of CH3S from the adsorption of CH3SSCH3 on Au(111)

By performing density functional theory calculations, we have examined the adsorption of dimethyl disulfide (CH3SSCH3) on Au(111). In addition, we have also charted the reaction paths leading to dissociation of the S-S bond and the formation of thiolate-adatom species. Further, we have simulated the scanning tunneling microscopic (STM) images of the key adsorption intermediates and products and compared them with the experimental data available in the literature. In summary, our computations show that CH3SSCH3 adsorbs on Au(111) non-dissociatively with a moderate adsorption energy 0.42 eV for the trans-conformation which is 0.24 eV more stable than its cis-isomer. When the adsorbed trans-CH3SSCH3 dissociates at low temperature, the two CH3S fragments are retained by the closest neighboring sites and the trans-conformation of them prior to S-S dissociation can be preserved. The activation barrier to rotating the CH3S fragments on Au(111) is 0.17 eV so a moderate temperature rise can facilitate the transformation of the trans-conformation and generate a mixture of the trans- and cis-conformation. Our calculations also show that intrinsic defects such as gold adatoms (Auad) are active in CH3SSCH3 dissociation in two mechanisms: a direct mechanism in which CH3SSCH3 dissociation drives Auad formation and CH3S-Auad formation simultaneously, with leftover vacancies as by-products; a sequential mechanism in which Auad is present in advance and subsequently interacts with a nearby CH3S fragment to form CH3S-Auad. The respective activation barriers are 0.15 and 0.32 eV for the trans- and cis-CH3S-Auad-SCH3 complex along the direct mechanism, and 0.21 and 0.18 eV along the sequential mechanism.

Effects of intrinsic defects on methanthiol monolayers on Cu(111): a density functional theory study

Density functional theory calculations were used to examine the effects of intrinsic surface defects of Cu(111) on the adsorption of methylthiol (CH3SH). The examination covers both the initial non-dissociative adsorption and the subsequent dissociation reaction pathways to form intermediate and final reaction products. By comparing the most probable adsorption structures likely formed after the adsorption of CH3SH on Cu(111) with and without the presence of adatoms (Cu(ad)) and vacancies, this computational work offers new insights about the geometry and thermodynamic stability of these structures. Particularly, it reveals a new type of surface complexes having two CH3S bonding to one Cu(ad) (referred therein as CH3S-Cu(ad)-CH3S). In addition, this work also yields new reaction dynamics results on transition states and activation barriers. The results reveal that the presence of Cu(ad) indeed significantly changes the kinetics of adsorption and dissociation of CH3SH on Cu(111). The most kinetically favorable reaction pathway turns out to be that involving the formation of a special surface complex formed by one Cu(ad) plus two CH3S fragments from the dissociation of CH3SH, with the two S atoms located at the bridge sites of Cu(111). Finally, this work also gives simulated scanning tunneling microscopic images for the most important adsorption species in the course of the transition from CH3SH∕Cu(111) to CH3S∕Cu(111), which may stimulate future experimental studies of self-assembled monolayers on practical metal substrates such as thiols on copper.

Effect of coverage and defects on the adsorption of propanethiol on Au(111) surface: a theoretical study

Periodic density functional calculations have been carried out to investigate both the thiol adsorption on Au(111) surface and the reaction mechanism for the formation of the self-assembled monolayers, taking propanethiol as a representative example. The effect of coverage and surface defects (adatoms and vacancies) has been analyzed. It is found that the most stable physisorption (undissociated) site is an adatom site, whereas the chemisorption site for the thiol is a vacancy site or protrusion consisting of a pair of adatoms, followed by one adatom site. The results point out that the thiolate self-assembled monolayer adsorption process occurs preferentially on step edges.