Organic Overlayers Research Articles

Surface and interface states of gallium-polar versus nitrogen-polar GaN: Impact of thin organic semiconductor overlayers

Using ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy, we have investigated the electronic properties of interfaces between copper phthalocyanine (CuPc) films and gallium-polar (Ga-polar) and nitrogen-polar (N-polar) GaN surfaces, respectively. Prior to the deposition of CuPc films, the clean Ga-polar and N-polar surfaces exhibited about 0.6 and 0.13 eV upward band bendings, respectively, showing the influence of electronic states on the surface termination and growth direction. With the deposition of ultrathin layers of CuPc, no additional band bending or charge displacement was observed for the Ga-polar heterointerface. In contrast, the N-polar interface exhibited an additional 0.54 eV band bending upon deposition of only a CuPc monolayer, attributed to a partial electron displacement from GaN to CuPc. The difference between the two cases is interpreted in terms of the difference between the electron affinity of the N-polar and Ga-polar GaN surfaces. With potential device use in mind, GaN/CuPc hybrid photodetector devices were fabricated and their photocurrent responses were investigated for possible applications as photosensors.

A simple erosion dynamics model of molecular sputter depth profiling

AbstractA simple model which describes the essential features commonly observed in a molecular sputter depth profile is presented. General predictions of the dependence of measured molecular ion signals on the primary ion fluence are derived for the specific case where a mass spectrometric technique such as SIMS or secondary neutral mass spectrometry (SNMS) is used to analyze the momentary surface. The results are compared with recent experimental data on molecular depth profiles obtained by cluster‐ion‐initiated SIMS of organic overlayers. Copyright © 2008 John Wiley & Sons, Ltd.

Molecular Depth Profiling using a C(60) Cluster Beam: the Role of Impact Energy.

Molecular depth profiling of organic overlayers was performed using a mass selected C(60) ion beam in conjunction with time-of-flight (TOF-SIMS) mass spectrometry. The characteristics of sputter depth profiles acquired for a 300-nm Trehalose film on silicon were studied as a function of the kinetic impact energy of the projectile ions. The results are interpreted in terms of a simple model describing the balance between sputter erosion and ion induced chemical damage. It is shown that the efficiency of the projectile to clean up the fragmentation debris produced by its own impact represents a key parameter governing the success of molecular depth profile analysis.

Molecular depth profiling of trehalose using a C 60 cluster ion beam

Molecular depth profiling of organic overlayers was performed using a mass selected fullerene ion beam in conjunction with time-of-flight (TOF-SIMS) mass spectrometry. The characteristics of depth profiles acquired on a 300-nm trehalose film on Si were studied as a function of the impact kinetic energy and charge state of the C 60 projectile ions. We find that the achieved depth resolution depends only weakly upon energy.

Molecular dynamics simulations of sputtering of organic overlayers by slow, large clusters

The ion-stimulated desorption of organic molecules by impact of large and slow clusters is examined using molecular dynamics (MDs) computer simulations. The investigated system, represented by a monolayer of benzene deposited on Ag{1 1 1}, is irradiated with projectiles composed of thousands of noble gas atoms having a kinetic energy of 0.1–20 eV/atom. The sputtering yield of molecular species and the kinetic energy distributions are analyzed and compared to the results obtain for PS4 overlayer. The simulations demonstrate quite clearly that the physics of ejection by large and slow clusters is distinct from the ejection events stimulated by the popular SIMS clusters, like C 60, Au 3 and SF 5 at tens of keV energies.

Desorption of organic overlayers by Ga and C 60 bombardment

This paper reviews our recent work on computer simulations of Ga and C 60 bombardment of thin organic overlayers deposited on metal substrate. A multilayer of benzene, a monolayer of PS4 on Ag{1 1 1} and a self-assembled monolayer of octanethiol molecules on Au{1 1 1} were irradiated with 15 keV monoatomic (Ga) and polyatomic (C 60) projectiles that are recognized as valuable sources for desorption of high mass particles in secondary ion and neutral mass spectrometry (SIMS/SNMS) experiments. The results indicate that the sputtering yield decreases with the increase of the binding energy and the average kinetic energy of parent molecules is shifted toward higher kinetic energy. Although the total sputtering yield of organic material is larger for 15 keV C 60, the impact of this projectile leads to a significant fragmentation of ejected species. As a result, the yield of the intact molecules is comparable for C 60 and Ga projectiles. Our results indicate that the chemical analysis of thin organic films performed by detection of sputtered neutrals will not benefit from the use of C 60 projectiles.

Sputtering of thin benzene and polystyrene overlayers by keV Ga and C 60 bombardment

The mechanisms of ion-stimulated desorption of thin organic overlayers deposited on metal substrates by mono- and polyatomic projectiles are examined using molecular dynamics (MD) computer simulations. A monolayer of polystyrene tetramers (PS4) physisorbed on Ag{1 1 1} is irradiated by 15 keV Ga and C 60 projectiles at normal incidence. The results are compared with the data obtained for a benzene overlayer to investigate the differences in sputtering mechanisms of weakly and strongly bound organic molecules. The results indicate that the sputtering yield decreases with the increase of the binding energy and the average kinetic energy of parent molecules is shifted toward higher kinetic energy. Although the total sputtering yield of organic material is larger for 15 keV C 60, the impact of this projectile leads to a significant fragmentation of ejected species. As a result, the yield of the intact molecules is comparable for C 60 and Ga projectiles. Our data indicate that chemical analysis of the very thin organic films performed by detection of sputtered neutrals will not benefit from the use of C 60 projectiles.

ADVANCED MODELLING OF EPITAXIAL ORDERING OF ORGANIC LAYERS ON CRYSTALLINE SURFACES

A fine balance between weak molecule-molecule interactions and usually only weakly laterally varying molecule-substrate interactions governs the physical structure of organic-inorganic and organic-organic heteroepitaxial thin films. Therefore, it is important to investigate the energetics of realistically large overlayer domains. So far, Potential calculations for large domains of organic overlayers on crystalline substrates were computationally demanding due to the vast number of atoms to be considered. Here, we present a technique which for the first time enables the calculation of both the molecule-substrate interaction potential and the molecule-molecule interaction potential for large molecular domains (up to several thousands of rather large molecules) by utilizing potential energy grid files. This technique does not only allow the investigation of the substrate influence but also full Monte-Carlo based structural optimizations, if applied to 2D crystalline overlayers. Furthermore, it provides an estimate for the energetic gain combined with the differently aligned overlayer domains. In several examples we will discuss the usefulness of this method. As a general result, we will provide evidence that energetically favorable lattice structures in OMBE systems are not restricted to commensurate unit cells or coincident super cells.

Mechanisms of secondary ion emission from self‐assembled monolayers and multilayers

AbstractAlkanethiol self‐assembled monolayers/multilayers (SAMs) have been applied as model organic systems with which to investigate secondary ion formation and emission processes during kiloelectronvolt ion bombardment. Self‐assembled monolayer and multilayer films of 11‐mercaptoundecanoic acid capped with 1‐dodecanethiol were prepared on gold‐coated substrates. Samples with varying number of thiolate layers were studied using static secondary ion mass spectrometry to investigate the origin of molecular secondary ions and the influence of surface chemistry and structure. The nature of the thiolate bonding affects the type and abundance of the observed ions. The intensity of atomic and cluster ions derived from the substrate decreases exponentially with increasing number of thiolate overlayers because of losses in transmission through the organic overlayers. Intact molecular and cluster ions can escape from >100 Å below the surface of these structures. The variation of molecular‐ion yields with multilayer thickness suggests that a significant proportion of molecular ions originate from subsurface thiolate layers. The detection of ions comprised species from the substrate or bottom of the multilayer associated with species from the top layer supports the view that chemical association at or near the surface is a viable mechanism of formation for molecular secondary ions. Copyright © 2005 John Wiley & Sons, Ltd.

Modeling keV particle interactions with molecular and polymeric samples

Organic surfaces are locally submitted to extreme, out of equilibrium conditions when they are bombarded by kiloelectronvolt particles (atoms, ions, clusters). The time scale of the energy transfer is from tens of femtoseconds to several picoseconds depending on the material and the average energy per atom in the energized volume is of the order of a few eV, i.e. sufficient to break bonds in the solid. As a result, atoms, molecules and their fragments are released in the gas phase, which makes sputtering/desorption methods useful for surface treatment (ion beam patterning) and analysis (mass spectrometry). The radicals created in the sample also induce branching and cross-linking reactions that can be useful for surface modification purposes. Molecular dynamics simulations have provided an invaluable help for the elucidation of keV particle-induced processes in organic overlayers and, most recently, bulk materials. In this review, I illustrate the various mechanisms at play using case studies taken from our recent investigations and from the literature. They include the Ar-induced sputtering of a large polymeric molecule on a metal substrate and a molecular sample made of polystyrene oligomers. The emphasis is placed on the understanding of the energy transfer processes in the disturbed surface region and the mechanisms of molecule desorption, fragmentation and recombination, crucial for ion beam-based analytical methods.

Organic surfaces excited by low-energy ions: atomic collisions, molecular desorption and buckminsterfullerenes

This article reviews the recent progress in the understanding of kiloelectronvolt particle interactions with organic solids, including atomic displacements in a light organic medium, vibrational excitation and desorption of fragments and entire molecules. This new insight is the result of a combination of theoretical and experimental approaches, essentially molecular dynamics (MD) simulations and secondary ion mass spectrometry (SIMS). Classical MD simulations provide us with a detailed microscopic view of the processes occurring in the bombarded target, from the collision cascade specifics to the scenarios of molecular emission. Time-of-flight SIMS measures the mass and energy distributions of sputtered ionized fragments and molecular species, a precious source of information concerning their formation, desorption, ionization and delayed unimolecular dissociation in the gas phase. The mechanisms of energy transfer and sputtering are compared for bulk molecular solids, organic overlayers on metal and large molecules embedded in a low-molecular weight matrix. These comparisons help understand some of the beneficial effects of metal substrates and matrices for the analysis of molecules by SIMS. In parallel, I briefly describe the distinct ionization channels of molecules sputtered from organic solids and overlayers. The specific processes induced by polyatomic projectile bombardment, especially fullerenes, are discussed on the basis of new measurements and calculations. Finally, the perspective addresses the state-of-the-art and potential developments in the fields of surface modification and analysis of organic materials by kiloelectronvolt ion beams.

Self‐assembly of stearic acid on aluminium: the importance of oxide surface chemistry

AbstractThe influence of ambient atmospheric exposure on the chemistry of the magnetron‐sputtered aluminium surface has been characterized as hydroxyl incorporation to the oxide surface and concurrent adsorption of airborne carbonaceous contamination. Here, the consequence of these changes in oxide surface chemistry upon the adsorption of stearic acid from solution is investigated. Water contact angle and polarization modulation infrared refection–absorption spectroscopy (PM‐IRRAS) reveal a strong dependence of stearic acid monolayer order upon ambient exposure time prior to assembly. It is proposed that hydroxyl formation in the ambient atmosphere increases the stearic acid adsorption density and thus the self‐assembled monolayer (SAM) order, whereas airborne carbonaceous material blocks these adsorption sites and introduces disorder in the monolayers through a decrease in adsorption density.Monolayer adsorption has been correlated with the aluminium oxide surface chemistry. It is proposed that this phenomenon represents an explanation for the irreproducible results often reported for assembly on metal oxide substrates. Furthermore, it indicates that in this broad class of material surfaces assembly can be used as a means of estimating the reactivity of surfaces with respect to organic overlayers such as paints and adhesives. Removal of adsorbed carbonaceous material from the aluminium surface using an oxygen plasma resulted in a significantly increased order of the stearic acid monolayer, as assessed by water contact angle. This observation is rationalized as the removal of carbonaceous material blocking surface adsorption sites by the plasma, but retention of the underlying hydroxyl functionality. This is predicted to have important implications in the preparation of aluminium for painting and adhesive bonding. Copyright © 2004 John Wiley & Sons, Ltd.

Sputtering simulations of organic overlayers on metal substrates by monoatomic and clusters projectiles

This paper reviews our recent work on computer simulations of monoatomic and cluster bombardment of metal and organic surfaces. The investigated surfaces are irradiated with keV monoatomic (C, Ar, Ga) and polyatomic (C 60) projectiles that are recognized as valuable sources for desorption of high mass particles in secondary ion and neutral mass spectrometry (SIMS/SNMS) experiments. The analysis of the results reveals that the C 60 ion beam enables to perform chemical imaging with higher sensitivity, better depth resolution, and lower contamination than monoatomic projectiles with similar kinetic energy. For monoatomic projectiles, the development of a linear collision cascade is the predominant mechanism responsible for the ejection of particles. In contrast, strongly nonlinear, collective processes occur during C 60 bombardment. These nonlinear processes lead to the enhanced removal of material and the formation of a macroscopic crater. This paper presents theoretical insight into possible mechanisms responsible for the observed behavior with the emphasis on the phenomena important to the SIMS/SNMS community.

Enhancement of sputtering yields due to C60 versus Ga bombardment of Ag[111] as explored by molecular dynamics simulations.

The mechanism of enhanced desorption initiated by 15-keV C60 cluster ion bombardment of a Ag single crystal surface is examined using molecular dynamics computer simulations. The size of the model microcrystallite of 165,000 atoms and the sophistication of the interaction potential function yields data that should be directly comparable with experiment. The C60 model was chosen since this source is now being used in secondary ion mass spectrometry experiments in many laboratories. The results show that a crater is formed on the Ag surface that is approximately 10 nm in diameter, a result very similar to that found for Au3 bombardment of Au. The yield of Ag atoms is approximately 16 times larger than for corresponding atomic bombardment with 15-keV Ga atoms, and the yield of Ag3 is enhanced by a factor of 35. The essential mechanistic reasons for these differences is that the C60 kinetic energy is deposited closer to the surface, with the deeply penetrating energy propagation occurring via a nondestructive pressure wave. The numbers predicted by the model are testable by experiment, and the approach is extendable to include the study of organic overlayers on metals, a situation of growing importance to the SIMS community.

Molecular dynamic simulations of the sputtering of multilayer organic systems

Sputtering of organic overlayers has been modeled using molecular dynamics computer simulations. The investigated systems are composed of benzene molecules condensed into one, two and three layers on an Ag{1 1 1} surface. The formed organic overlayers were bombarded with 4 keV Ar projectiles at normal incidence. The development of the collision cascade in the organic overlayer was investigated. The sputtering yield, mass, internal and kinetic energy distributions of ejected particles have been analyzed as a function of the thickness of the organic layer. The results show that all emission characteristics are sensitive to the variation of layer thickness. Although most of the ejected intact benzene molecules originate from the topmost layer, the emission of particles located initially in second and third layers is significant. The analysis indicates that the metallic substrate plays a dominant role in the ejection of intact organic molecules.

Mechanistic changes in cut edge corrosion induced by variation of inhibitor pigmentation in organically coated galvanised steel

AbstractCorrosion at the exposed cut edges of organically coated galvanised steel (OCS) used for construction is one of the primary failure mechanisms. The scanning vibrating electrode technique (SVET) has shown that corrosion is driven by differential aeration when materials are coated with organic over-layers of widely differing thickness and primer layers are pigmented with strontium chromate (SrCrO4 ). In such materials anodic activity is localised on the zinc layer proximal to the thicker organic coating with cathodic activity focused on the steel and zinc proximal to the thinner organic coating. To overcome the localisation of anodic and cathodic activity we have prepared novel OCS panels in which cathodic inhibitor primer systems based on rare earth metal cation exchanged pigments have been employed beneath the thinner organic coating and conventional strontium chromate anodic inhibitor systems beneath the thicker organic coating. The differential inhibitor loading overrides the effects of differen...

A system based on metal alkyl species that forms chemically bound organic overlayers on hydroxylated planar surfaces

We report the growth of chemically-bound organic overlayers on planar hydroxylated oxide surfaces, oxidized silicon and glass, using dimethyl-zinc plus alkane-thiols. Infrared spectroscopy of the layers indicates that they are similar in density and ordering to self-assembled octadecyl-trichlorosilane layers. Similar reactions can also be used to grow multilayer structures. When multilayers are grown, significant smoothing of the surface is noted. The strategies reported here lend themselves to the production of nanocomposite materials consisting of alternating layers of organic and inorganic material.

Surface-Enhanced Raman Scattering from Substrates with Conducting or Insulator Overlayers: Electromagnetic Model Predictions and Comparisons with Experiment

Model electromagnetic (EM) calculations are presented of the surface-enhanced Raman scattering (SERS) intensities expected for gold and silver particles coated with conducting as well as insulating dielectric films, with the objective of assessing their thickness-dependent properties and hence the anticipated scope of such “overlayer SERS” tactics to chemically diverse interfacial materials. Most calculations refer to Raman enhancements at the film outer edge, relevant to species adsorbed on the overlayer. Spheroidal and spherical metal particles are treated by using the electrostatic approximation, with a first-order electrodynamic correction for “radiation damping”, in vacuum and aqueous environments. The presence of simple insulating dielectric (such as organic) overlayers yields progressive decays in the calculated Raman enhancement factor, G, at the film edge with increasing thickness, d, throughout the visible optical region of experimental significance, even though the largest G values are necessarily obtained close to the plasmon resonance energy. These G–d dependencies are markedly milder than predicted for bare metal particles in water or vacuum, suggesting the potential broad-based analytical utility of nanoscale (∼ 1–10 nm thick) molecular dielectric overlayers in SERS. Furthermore, Raman enhancements increasing with film thicknesses are typically obtained at locations close to the inner film edge, relevant to moieties imbedded in dielectric overlayers. Sharper G–d decays are predicted at the film edge of transition-metal overlayers, especially near the plasmon resonance; unlike dielectric insulators, the metal overlayers progressively “quench” the optical frequency-dependent enhancement at the film edge arising from substrate plasmon resonance. Reasonable agreement is obtained in comparison with the Raman intensity-thickness dependence measured for chemisorbed carbon monoxide on rhodium films electrodeposited onto gold. The model calculations can also account qualitatively for the nonmonotonic Raman intensity-thickness dependencies observed for phonon bands from semiconducting (cadmium chalcogenide) overlayers on gold, attributed to film-induced redshifts in the substrate plasmon resonance. More general implications for the analytical utility of “overlayer SERS” tactics are also pointed out.

New method to correct for the influence of organic contamination on intensity ratios in quantitative XPS

A new method is proposed to correct for attenuation effects by adventitious organic contamination in quantitative XPS, The corrected intensity ratio I-X(o)/I-Y(o) involving two different substrate peaks is determined as the slope of the plot of I-X/I-C1s as a function of I-Y/I-C1s for samples covered with organic overlayers of varying thickness. The thickness of deposited organic overlayers does not need to be perfectly uniform provided that it does not vary by more than +/-40%. The sample surface can either be flat or present a random roughness (e.g. powders), Shadowing effects accompanying overlayer non-uniformity are taken into account by a simple model involving effective thickness and fractional coverage, which is shown to enhance greatly the accuracy of computed intensity ratios. Constraints of the method and possible implementations were examined with an NaCl powder covered with adventitious contamination, polystyrene (PS) and polydimethylsiloxane (PDMS). The most straightforward implementation of the method would be to record repeatedly the peaks of interest while decreasing the adventitious contamination overlayer thickness by thermal desorption, The error on I-Na2s(o)/I-Na1s(o) is +10%, with the apparent overlayer thicknesses varying from 1.1 to 0.3 nm, The method is applied to check the reliability of the spectrometer transmission functions reported previously by Weng et al, Copyright (C) 1999 John Wiley & Sons, Ltd.

Kelvin probe liquid-surface potential sensor

The design of a surface potential sensor is described in detail. It uses the Kelvin vibrating-probe principle and is specifically designed to characterize organic overlayers on poorly conducting liquid substrates. A novel manual compensation control allows the systematic measurement offsets originating in the electronics to be nulled. The head is based on a commercially available piezo-driven tuning-fork assembly with a 6 mm diameter gold-plated electrode, and includes a high-gain low-noise preamplifier to maximize sensitivity and minimize parasitic coupling. The input stage of the preamplifier uses balanced junction field-effect transistors mounted in a transimpedance configuration, giving low noise and minimizing phase shifts of the Maxwell-offset signal related to low substrate conductivity. The reproducibility of the measured surface potential is ±1 mV.