Monolayer Islands Research Articles

Initial gas exposure effects on monolayer pentacene field-effect transistor studied using four gallium indium probes

Abstract In this study, we report that the electrical transport property in monolayer pentacene is highly sensitive to a small amount of exposure to gaseous molecules of oxygen, nitrogen, and argon, by using independently-driven four gallium indium (GaIn) probes. Liquid metal GaIn probes have been used as non-destructive conductivity electrodes for monolayer OFET films in a vacuum chamber. We carried out the fabrication of monolayer pentacene and the four-probe measurement without exposure to atmospheric air. Four GaIn probes were used in situ to measure conductivity in the channel separately from the overall characteristics including contact resistance at the electrode probes. The results demonstrate that oxygen exposure of 1 L ( 10 − 6 Torr s) reduces mobility to 8% of the original value in the monolayer film, and the reduction is irreversible, i.e., mobility does not recover its original value after the evacuation of gaseous molecules. Physisorption of gaseous molecules is involved in the gas exposure effects, because chemically inert gases of nitrogen and argon also reduce the mobility. Taking these findings together with the results of photoelectron spectroscopy and atomic force microscopy measurements, we assume that the reduction is attributed to the physisorption at grain boundaries formed by the coalescence of monolayer islands.

Ni Alloy Monolayer Electrodeposition

The morphology and the microstructure of alloys grown by co-electrodeposition generally result from kinetics and depart from equilibrium. On the atomic scale key parameters include binding energies (e.g. adatom – surface, adatom – adatom etc.) and also on adatom mobility. In this presentation we will examine the impact of above parameters on Ni alloy monolayers electrodeposition. An unreconstructed AuPd(111) substrate is used to co-deposit Ni and Ag, Au [1] and Pd [2]. These elements were chosen because they present different bulk mixing enthalpies with Ni, from phase separation (Ag) to perfect mixing (Pd). These three elements present also quite different surface mobility. Complementary experiments in which the two metals are sequentially deposited [3] [4] as well as dealloying experiments (selective dissolution of Ni) [5] will be presented in support of discussion. References : [1] F. Lecadre et al. AuNi alloy monolayer films electrodeposited on Au(111): An in situ STM study, Surf. Sci. 607, 25 (2013). [2] A. Damian et al. Electrodeposition of NiPd monolayer on Au(111): An in situ scanning tunneling microscopy study, Electrochim. Acta 112, 824 (2013). [3] A. Damian et al. Selective Growth and Dissolution of Ni on a PdAu Bimetallic Surface by In Situ STM: Determining the Relative Adsorbate-Substrate Interaction Energy, Phys. Rev. Lett. 102, 196101 (2009). [4] F. Lecadre et al. Electrodeposition of Ag, Pd and Au on Ni monolayer islands on (1 × 1)-Au(111) by in-situ scanning tunneling microscopy, Electrochim. Acta 197, 241 (2016). [5] A. Damian et al. Electrochemical de-alloying in two dimensions: role of the local atomic environment, Nanoscale 8, 13985 (2016).

Polarization-dependent Total Reflection Fluorescence X-ray Absorption Fine Structure (PTRF-XAFS) Studies on the Structure of a Pt Monolayer on Au(111) Prepared by the Surface-limited Redox Replacement Reaction

We studied the initial stage of a Pt monolayer produced by surface-limited redox replacement (SLRR) using polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). Different from the widely accepted understanding that a metallic monolayer island is formed, our XAFS showed that the Pt monolayer, initially present on the Au(111) substrate, was mainly in the form of a planar [PtCl4]2- complex with its molecular plane parallel to the Au(111). This result provides a new insight into the mechanism of SLRR.

Stranski–Krastanov mechanism of growth and the effect of misfit sign on quantum dots nucleation

The thermodynamics of the Stranski–Krastanov mode of epitaxial growth and the effect of the sign of the lattice misfit are discussed. The Stranski–Krastanov mode of growth represents a sequence of layer-by-layer or Frank-van der Merwe growth followed by the formation of three-dimensional (3D) islands or Volmer–Weber growth. The occurrence of both growth modes mentioned above is in compliance with the wettability criterion of Bauer. The positive wetting function required for the occurrence of the Volmer–Weber growth is originated by the vertical displacements of the atoms close to the edges of the two-dimensional (2D) islands as a result of the relaxation of the lattice misfit. The monolayer high islands become unstable against bilayer islands, bilayer islands in turn become unstable against trilayer islands, etc. beyond some critical islands sizes. Monolayer islands appear as necessary precursors of three-dimensional (3D) islands. The critical island size for mono-bilayer transformation increases steeply with decreasing lattice misfit and diverges at a critical value of the misfit. This value divides the regions of Frank-van der Merwe and Stranski–Krastanov modes in a phase diagram of coordinates wetting-misfit. The transformation of monolayer to multilayer islands takes place either by consecutive nucleation and growth of 2D islands (layer-by-layer transformation), or by nucleation and lateral (2D) growth of multilayer islands (multilayer 2D transformation). The former occurs in the case of “stiff” overlayer materials and mostly in compressed overlayers. The latter takes place in the case of “soft” materials like Pb and In, mostly in tensile overlayers. Tensile films show non-nucleation transformation compared with the nucleation-like behavior of compressed films.

Adsorption characteristics of Er3N@C80on W(110) and Au(111) studied via scanning tunneling microscopy and spectroscopy.

We performed a study on the fundamental adsorption characteristics of Er3N@C80 deposited on W(110) and Au(111) via room temperature scanning tunneling microscopy and spectroscopy. Adsorbed on W(110), a comparatively strong bond to the endohedral fullerenes inhibited the formation of ordered monolayer islands. In contrast, the Au(111)-surface provides a sufficiently high mobility for the molecules to arrange in monolayer islands after annealing. Interestingly, the fullerenes modify the herringbone reconstruction indicating that the molecule–substrate interaction is of considerable extent. Investigations concerning the electronic structure of Er3N@C80/Au(111) reveals spatial variations dependent on the termination of the Au(111) at the interface.

Influence of Step and Island Edges on Local Adsorption Properties: Hydrogen Adsorption on Pt Monolayer Island Modified Ru(0001) Electrodes

The influence of steps and island edges on the local electronic structure of a (bi-)metallic single crystalline electrode surface and on the local, site-specific adsorption energy of adsorbed species, the so-called structural effects, was studied by periodic density functional theory based calculations, focusing on longer-range effects. Using hydrogen adsorption energies as a local probe, calculations were performed both for partly Pt monolayer covered planar Ru(0001) surfaces and for a stepped Ru( $10\bar {19}$ ) surface decorated with a row of Pt atoms. The calculations demonstrate that the steps/island edges affect not only the nearest neighbor adsorption sites but also more distant ones with the extent depending on the particular structure. This longer-range effect is in excellent agreement with recent temperature-programmed desorption and spectroscopy experiments (Hartmann et al. Phys. Chem. Chem. Phys. 14, 10919, 2012). For the interaction of water molecules with partly Pt monolayer covered Ru(0001), similar trends as in the hydrogen adsorption have been found. In addition, hydrogen adsorption energies as a function of coverage have been used to derive the hydrogen coverage as a function of the electrode potential, exhibiting a broad range of stable hydrogen adsorption structures.

A combined UHV-STM-flow cell set-up for electrochemical/electrocatalytic studies of structurally well-defined UHV prepared model electrodes.

We describe the construction and discuss the performance of a novel combined ultrahigh vacuum (UHV)-electrochemistry set-up, allowing the controlled preparation and structural characterization of complex nanostructured electrode surfaces by high resolution scanning tunnelling microscopy (STM) under UHV conditions on the one hand and, after electrode transfer under clean conditions, electrochemical measurements under continuous, controlled electrolyte mass transport conditions on the other. Electrochemical measurements can be coupled with online product detection, either using an additional collector electrode or by differential electrochemical mass spectrometry (DEMS). The potential of the set-up will be illustrated in two electrocatalytic reactions on complex, but structurally well-defined bimetallic electrode surfaces, O2 reduction on PtxAg1-x/Pt(111) monolayer surface alloys and bulk CO oxidation on Pt monolayer island modified Ru(0001) electrodes. We will particularly demonstrate the importance of structural characterization after the electrochemical measurements for identifying structural modifications induced by the electrochemical environment and thus avoiding misleading conclusions about the structure-activity relationships.

Heteroepitaxial growth of cadmium carbonate at dolomite and calcite surfaces: Mechanisms and rates

The systematic variation of rates and the mechanism of cadmium uptake on the (104) surface of dolomite (CaMg(CO3)2) were investigated using in situ and ex situ atomic force microscopy (AFM), ex situ specular X-ray reflectivity (XR), and ex situ X-ray fluorescence (XRF). Selected experiments were performed on the calcite (CaCO3) (104) surface for comparison. Aqueous solutions of CdCl2, CaCl2, and NaHCO3, undersaturated with respect to calcite and supersaturated with respect to otavite (CdCO3) and the (CdxCa1−x)CO3 solid solution, were reacted with dolomite surfaces for minutes to days. Calcite substrates were reacted with solutions containing 1–50μM CdCl2, and with no added Ca or CO3. Thin carbonate films following the Stranski–Krastanov growth mode were observed on both substrates. Specular XR and XRF revealed the formation of nm-thick Cd-rich carbonate films that were structurally ordered with respect to the dolomite (104) plane. Epitaxial films adopted the calcite crystal structure with a d104-spacing (3.00Å) larger than those of pure dolomite (2.88Å) and otavite (2.95Å) indicating either a solid solution with x≈0.5, or a strained Cd-rich carbonate with a composition near that of otavite. The growth rate r of this phase increases with the initial supersaturation of the solution with respect to the solid solution, βmax, and follows the empirical relationship, as determined from XRF measurements, given by: r=10-4.88±0.42(βmax2.29±0.24-1),(in units of atoms of Cd/Å2/h).The morphology of the overgrowth also varied with βmax, as exemplified by AFM observations. Growth at step edges occurred over the entire βmax range considered, and additional growth features including 3Å high monolayer islands and ∼25Å high tall islands were observed when logβmax>1. On calcite, in situ XR indicated that this phase is similar to the Cd-rich overgrowth formed on dolomite and images obtained from X-ray reflection interface microscopy (XRIM) reveal the existence of laterally variable Cd-rich domains.

Water-Activated Lattice Oxygen in FeO(111) Islands for Low-Temperature Oxidation of CO at Pt–FeO Interface

The role of water in low-temperature oxidation of CO catalyzed at metal–oxide interfaces is of great interest. In this paper, we prepared Fe16O(111)/Pt(111) and Fe18O(111)/on Pt(111) inverse model catalysts with the same coverage of FeO(111) monolayer islands and comparatively studied C16O oxidation with 18O2 or D218O on Fe16O(111)/Pt(111) inverse model catalyst and C16O oxidation with 16O2 or D216O on Fe18O(111)/Pt(111) inverse model catalyst. Lattice oxygen in FeO does not participate into CO oxidation with O2 but participates into CO oxidation with water via the mechanism of CO oxidation with hydroxyl groups formed by water dissociation at the FeO(111)–Pt interface. These results provide unambiguous experimental evidence for the unique role of water in activating surface lattice oxygen of oxides to oxidize CO at low temperatures, advancing the fundamental understandings of low-temperature CO oxidation involving hydroxyl groups catalyzed by metal–oxide interfaces.

A Combined Monte Carlo and Hückel Theory Simulation of Orientational Ordering in C60 Assemblies

Orientational ordering of C60 molecules within monolayer and multilayer islands is a regularly observed phenomenon in scanning tunneling microscopy (STM) studies. Here we simulate the orientational ordering seen in STM images via a novel combination of Monte Carlo and Huckel theory methods and compare to experimental data. A measure of the repulsive interaction energy between two adjacent C60 molecules is precalculated by estimating and processing the electron density distribution between them. Many combinations of molecular orientations are considered to encompass all the details of the molecular orbitals. Precalculated intermolecular interaction energies are inputted into a simulated C60 island. Here, the center position of each molecule is fixed, but the molecules are allowed to rotate freely around their centers. A minimum in the total island free energy is sought by sequentially picking molecules at random and rotating them according to their neighbors. Results show significant correlation with exper...

Electrodeposition of Ag, Pd and Au on Ni monolayer islands on (1 × 1)-Au(111) by in-situ scanning tunneling microscopy

We study the initial stage of the electrodeposition of Ag, Pd and Au on Ni submonolayers deposited on unreconstructed Au(111) by in–situ scanning tunneling microscopy. At submonolayer coverage all the deposited Ag forms islands on the Au substrate. Ag partially wets the Ni island edges but does not nucleate on top of the Ni islands. On the other hand, we found a completely different behavior for Pd and Au which nucleate on top of the Ni islands, undergo place exchange with the Ni atoms and also form rims wetting the entire Ni island edges. By selectively dissolving the Ni, we evidence an unexpected behavior: the initial stage of Pd place exchange occurs at the minima of the Ni moiré whereas it occurs at the maxima in the case of Au. The origin of the difference between Pd and Au preferential place exchange site is attributed to the presence of a hydrogen overlayer.

Nanoscale plasmonic phenomena in CVD-grown MoS(2) monolayer revealed by ultra-broadband synchrotron radiation based nano-FTIR spectroscopy and near-field microscopy.

Nanoscale plasmonic phenomena observed in single and bi-layers of molybdenum disulfide (MoS(2)) on silicon dioxide (SiO(2)) are reported. A scattering type scanning near-field optical microscope (s-SNOM) with a broadband synchrotron radiation (SR) infrared source was used. We also present complementary optical mapping using tunable CO(2)-laser radiation. Specifically, there is a correlation of the topography of well-defined MoS(2) islands grown by chemical vapor deposition, as determined by atomic force microscopy, with the infrared (IR) signature of MoS(2). The influence of MoS(2) islands on the SiO(2) phonon resonance is discussed. The results reveal the plasmonic character of the MoS(2) structures and their interaction with the SiO(2) phonons leading to an enhancement of the hybridized surface plasmon-phonon mode. A theoretical analysis shows that, in the case of monolayer islands, the coupling of the MoS(2) optical plasmon mode to the SiO(2) surface phonons does not affect the infrared spectrum significantly. For two-layer MoS(2), the coupling of the extra inter-plane acoustic plasmon mode with the SiO(2) surface transverse phonon leads to a remarkable increase of the surface phonon peak at 794 cm(-1). This is in agreement with the experimental data. These results show the capability of the s-SNOM technique to study local multiple excitations in complex non-homogeneous structures.

Deposition and Self-Assembly of Large Magnetic Molecules

We have used two different unconventional preparation techniques, namely, pulse injection and rapid heating, to deposit large trinuclear transition-metal complexes onto a Au(111) surface under ultrahigh-vacuum conditions. Both techniques turn out to provide a clean preparation and leave the molecules intact upon deposition. While pulse injection leads to isolated molecules on the surface, rapid heating yields close-packed ordered monolayer islands. From a comparison of topographies and electronic structures we conclude that both molecule–substrate and intermolecular interactions are relatively weak. We discuss the reasons for the appearance of the different structures and give a detailed comparison of both deposition techniques which allows us to evaluate their specific advantages and drawbacks. Our results show that these relatively simple and cost-efficient preparation techniques are suitable for nanoscale studies of large molecules such as single-molecule magnets.

Antiferromagnetic Spin Coupling between Rare Earth Adatoms and Iron Islands Probed by Spin-Polarized Tunneling.

High-density magnetic storage or quantum computing could be achieved using small magnets with large magnetic anisotropy, a requirement that rare-earth iron alloys fulfill in bulk. This compelling property demands a thorough investigation of the magnetism in low dimensional rare-earth iron structures. Here, we report on the magnetic coupling between 4f single atoms and a 3d magnetic nanoisland. Thulium and lutetium adatoms deposited on iron monolayer islands pseudomorphically grown on W(110) have been investigated at low temperature with scanning tunneling microscopy and spectroscopy. The spin-polarized current indicates that both kind of adatoms have in-plane magnetic moments, which couple antiferromagnetically with their underlying iron islands. Our first-principles calculations explain the observed behavior, predicting an antiparallel coupling of the induced 5d electrons magnetic moment of the lanthanides with the 3d magnetic moment of iron, as well as their in-plane orientation, and pointing to a non-contribution of 4f electrons to the spin-polarized tunneling processes in rare earths.

Changes of the molecular structure in organic thin film transistors during operation

Thin films of organic semiconductors have been widely studied at different length scales for improving the electrical response of devices based on them. Hitherto, a lot of knowledge has been gained about how molecular packing, morphology, grain boundaries, and defects affect the charge transport in organic thin film transistors. However, little is known about the impact of an electric field on the organic semiconductor microstructure and the consequent effect on the device performances. To fill this gap, we investigated the evolution of the structure of pentacene thin film transistors during device operation by in situ real time X-ray diffraction measurements and theoretical calculations. We observed for the first time the occurrence of a reversible structural strain taking place during the bias application mainly due to reorientation at the terrace edges of monolayer islands under the effect of electrical field. Strain exhibits the same trend of the threshold voltage hinting to the existence of a direct ...

Ag on Pt(111): Changes in Electronic and CO Adsorption Properties upon PtAg/Pt(111) Monolayer Surface Alloy Formation.

The electronic and chemical (adsorption) properties of bimetallic Ag/Pt(111) surfaces and their modification upon surface alloy formation, that is, during intermixing of Ag and Pt atoms in the top atomic layer upon annealing, were studied by X-ray photoelectron spectroscopy (XPS) and, using CO as probe molecule, by temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), respectively. The surface alloys are prepared by deposition of sub-monolayer Ag amounts on a Pt(111) surface at room temperature, leading to extended Ag monolayer islands on the substrate, and subsequent annealing of these surfaces. Surface alloy formation starts at ≈600-650 K, which is evidenced by core-level shifts (CLSs) of the Ag(3d5/2 ) signal. A distinct change of the CO adsorption properties is observed when going to the intermixed PtAg surface alloys. Most prominently, we find the growth of a new desorption feature at higher temperature (≈550 K) in the TPD spectra upon surface alloy formation. This goes along with a shift of the COad -related IR bands to lower wave number. Surface alloy formation is almost completed after heating to 700 K.

Changes of the Molecular Structure in Organic Thin Film Transistors during Operation

Thin films of organic semiconductors have been widely studied at different length scales for improving the electrical response of devices based on them. Hitherto, a lot of knowledge has been gained about how molecular packing, morphology, grain boundaries, and defects affect the charge transport in organic thin film transistors. However, little is known about the impact of an electric field on the organic semiconductor microstructure and the consequent effect on the device performances. To fill this gap, we investigated the evolution of the structure of pentacene thin film transistors during device operation by in situ real time X-ray diffraction measurements and theoretical calculations. We observed for the first time the occurrence of a reversible structural strain taking place during the bias application mainly due to reorientation at the terrace edges of monolayer islands under the effect of electrical field. Strain exhibits the same trend of the threshold voltage hinting to the existence of a direct correlation between the phenomenon of bias stress and the structural modification.

Dynamics driven by lipophilic force in Langmuir monolayers: In-plane and out-of-plane growth.

While monolayer area fraction versus time (A(n)-t) curves obtained from surface pressure-area (π-A) isotherms for desorption-dominated (DD) processes in Langmuir monolayers of fatty acids represent continuous loss, those from Brewster angle microscopy (BAM) also show a two-dimensional (2D) coalescence. For nucleation-dominated (ND) processes both techniques suggest competing processes, with BAM showing 2D coalescence alongside multilayer formation. π enhances both DD and ND processes with a lower cutoff for ND processes, while temperature has a lower cutoff for DD but negligible effect on ND processes. Hydrocarbon chain length has the strongest effect, causing a crossover from DD to ND dynamics. Imaging ellipsometry of horizontally transferred films onto Si(100) shows Stranski-Krastanov-like growth for ND process in an arachidic acid monolayer resulting in successive stages of monolayer, trilayer, and multilayer islands, ridges from lateral island coalescence, and shallow wavelike structures from ridge coalescence on the film surface. These studies show that lipophilic attraction between hydrocarbon chains is the driving force at all stages of long-term monolayer dynamics.

Activating Nonreducible Oxides via Doping.

Nonreducible oxides are characterized by large band gaps and are therefore unable to exchange electrons or to form bonds with surface species, explaining their chemical inertness. The insertion of aliovalent dopants alters this situation, as new electronic states become available in the gap that may be involved in charge-transfer processes. Consequently, the adsorption and reactivity pattern of doped oxides changes with respect to their nondoped counterparts. This Account describes scanning tunneling microscopy (STM) and photoelectron spectroscopy (XPS) experiments that demonstrate the impact of dopants on the physical and chemical properties of well-defined crystalline oxide films. For this purpose, MgO and CaO as archetypical rocksalt oxides have been loaded either with high-valence (Mo, Cr) or low-valence dopants (Li). While the former generate filled states in the oxide band gap and serve as electron donors, the latter produce valence-band holes and give rise to an acceptor response. The dopant-related electronic states and their polarization effect on the surrounding host material are explored with XPS and STM spectroscopy on nonlocal and local scales. Moreover, charge-compensating defects were found to develop in the oxide lattice, such as Ca and O vacancies in Mo- and Li-doped CaO films, respectively. These native defects are able to trap the excess charges of the impurities and therefore diminish the desired doping effect. If noncompensated dopants reside in the host lattice, electron exchange with surface species is observed. Mo ions in CaO, for example, were found to donate electrons to surface Au atoms. The anionic Au strongly binds to the CaO surface and nucleates in the form of monolayer islands, in contrast to the 3D growth prevailing on pristine oxides. Charge transfer is also revealed for surface O2 that traps one Mo electron by forming a superoxo-species. The activated oxygen is characterized by a reinforced binding to the surface, an elongated O-O bond length, and a reduced barrier for dissociation, and represents an important intermediate for oxidation reactions. The charge-transfer processes described here are quenched if Li is inserted into the oxide lattice, neutralizing the effect of the extra electrons. The specific behavior of doped oxides has been explored on a mechanistic level, i.e. on thin-film model systems at ultrahigh vacuum and low temperature. We believe, however, that our results are transferrable to realistic conditions and doping might thus develop into a powerful method to improve the performance of nonreducible oxides in surface-catalyzed reactions.

Synergetic effect and oscillatory behavior of CO oxidation over a bimetallic composite catalyst

This article describes the results of simulations of CO oxidation under low pressure conditions over a bimetallic Pt/Rh composite catalyst. The model is represented by a system of four differential–algebraic equations (DAEs) for the coverages of COads and Oads, on the two parts of the composite surface. Surface diffusion of both adsorbates is assumed to be fast. The experimentally observed synergetic effect concerning CO oxidation on a Rh surface partly covered by Pt monolayer islands is reproduced in the simulations. It is shown that this synergetic effect is due to the difference in the CO sticking coefficients on Pt and Rh surfaces. In addition, a new type of kinetic oscillations, namely “synergetic kinetic oscillations” are predicted by the simulations. These oscillations exist under conditions at which no oscillations can exist over Pt and Rh monometallic catalysts. The driving force of this new type of oscillations is connected to the large difference in binding energy of adsorbed oxygen atoms on Rh and Pt.