Thickness Of Self-assembled Monolayers Research Articles

Disordered Phase-Assisted Growth of Organic Semiconductor Crystals on Self-Assembled Monolayer Templates.

Achieving high mobility and bias stability is a challenging obstacle in the advancement of organic thin-film transistors (OTFTs). To this end, the fabrication of high-quality organic semiconductor (OSC) thin films is critical for OTFTs. Self-assembled monolayers (SAMs) have been used as growth templates for high-crystalline OSC thin films. Despite significant research progress in the growth of OSC on SAMs, a detailed understanding of the growth mechanism of the OSC thin films on a SAM template is lacking, which has limited its use. In this study, the effects of the structure (thickness and molecular packing) of SAM on the nucleation and growth behavior of the OSC thin films were investigated. We found that disordered SAM molecules assisted in the surface diffusion of the OSC molecules and resulted in a small nucleation density and large grain size of the OSC thin films. Moreover, a thick SAM with disordered SAM molecules on the top was found to be beneficial for the high mobility and bias stability of the OTFTs.

Electrochemical Stability of Thiolate Self-Assembled Monolayers on Au, Pt, and Cu.

Self-assembled monolayers (SAMs) of thiolates have increasingly been used for modification of metal surfaces in electrochemical applications including selective catalysis (e.g., CO2 reduction, nitrogen reduction) and chemical sensing. Here, the stable electrochemical potential window of thiolate SAMs on Au, Pt, and Cu electrodes is systematically studied for a variety of thiols in aqueous electrolyte systems. For fixed tail-group functionality, the reductive stability of thiolate SAMs is found to follow the trend Au < Pt < Cu; this can be understood by considering the combined influences of the binding strength of sulfur and competitive adsorption of hydrogen. The oxidative stability of thiolate SAMs is found to follow the order: Cu < Pt < Au, consistent with each surface's propensity toward surface oxide formation. The stable reductive and oxidative potential limits are both found to vary linearly with pH, except for reduction above pH ∼10, which is independent of pH for most thiol compositions. The electrochemical stability across different functionalized thiols is then revealed to depend on many different factors including SAM defects (accessible surface metal atom sites decrease stability), intermolecular interactions (hydrophilic groups reduce the stability), and SAM thickness (stability increases with alkanethiol carbon chain length) as well as factors such as SAM-induced surface reconstruction and the ability to directly oxidize or reduce the non-sulfur part of the SAM molecule.

Boosting the Brightness of Thiolated Surface-Enhanced Raman Scattering Nanoprobes by Maximal Utilization of the Three-Dimensional Volume of Electromagnetic Fields.

Self-assembled monolayers (SAMs) of thiols on plasmonic nanoparticles constitute one of the most common methods for fabricating surface-enhanced Raman scattering (SERS) nanoprobes with wide applications. However, this method greatly limits the sufficient utilization of electromagnetic fields derived from plasmon excitation of the nanoparticles, because the thickness of SAMs (<1 nm) is usually much smaller than the attenuation length (>10 nm) of the fields. To overcome this, we propose a three-dimensional (3D) volume-active SERS (VASERS) technique to break the SAM limit, which integrates large amounts of thiol reporters into polydopamine shells on silver nanoparticles via Michael addition and allows sufficient utilization of 3D electromagnetic fields, leading to a dramatic increase in the intensity of the signal of the nanoprobes by about one order of magnitude. We demonstrate the universality of this strategy on various thiol reporters and plasmonic substrates. We also show that orthogonal VASERS nanoprobes with alkyne readout allow for high-precision in vivo tumor targeting and margin delineation.

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

Determination of pseudo-refractive index in self-assembled ligand layers from spectral shift of surface plasmon resonances in colloidal silver nanoplates

Abstract We examined systematically how self-assembled monolayers (SAMs) of different mercaptoacids affect the spectral shift of the localized surface plasmon resonance in silver nanoplates and nanospheres. We observed a clear trend in the magnitude of a redshift with a molecular length or the SAM thickness within a homologous series of aliphatic mercaptoacids: the thicker shell the stronger the red shift. Using classic Mie theory for plasmonic core-dielectric shell spheres and oblate spheroids we developed the method for determination of a pseudo-refractive index in SAM of different molecules and obtained a good correlation with the reference refractive indices for bulk long-chain aliphatic acids, but only in case of silver nanoplates. Calculations for silver core–shell nanospheres gave overestimated values of refractive index perhaps due to restrictions of Mie theory on the minimum particle size.

Embedded Dipole Self‐Assembled Monolayers for Contact Resistance Tuning in p‐Type and n‐Type Organic Thin Film Transistors and Flexible Electronic Circuits

AbstractBased on the powerful concept of embedded dipole self‐assembled monolayers (SAMs), highly conductive interfacial layers are designed, which allow tuning the contact resistance of organic thin‐film transistors over three orders of magnitude with minimum values well below 1 kΩ cm. This not only permits the realization of highly competitive p‐type (pentacene‐based) devices on rigid as well as flexible substrates, but also enables the realization of n‐type (C60‐based) transistors with comparable characteristics utilizing the same electrode material (Au). As prototypical examples for the high potential of the presented SAMs in more complex device structures, flexible organic inverters with static gains of 220 V/V and a 5‐stage ring‐oscillator operated below 4 V with a stage frequency in the range of the theoretically achievable maximum are fabricated. Employing a variety of complementary experimental and modeling techniques, it is shown that contact resistances are reduced by i) eliminating the injection barrier through a suitable dipole orientation, and by ii) boosting the transmission of charge carriers through a deliberate reduction of the SAM thickness. Notably, the embedding of the dipolar group into the backbones of the SAM‐forming molecules allows exploiting their beneficial effects without modifying the growth of the active layer.

Ionic Strength, Surface Charge, and Packing Density Effects on the Properties of Peptide Self-Assembled Monolayers.

The various environmental parameters of packing density, ionic strength, and solution charge were examined for their effects on the properties of the immobilized peptide mimotope CH19 (CGSGSGSQLGPYELWELSH) that binds with the therapeutic antibody Trastuzumab (Herceptin) on a gold substrate. The immobilization of CH19 onto gold was examined with a quartz crystal microbalance (QCM). The QCM data showed the presence of intermolecular interactions resulting in the increase of viscoelastic properties of the peptide self-assembled monolayer (SAM). The CH19 SAM was diluted with CS7 (CGSGSGS) to decrease the packing density as CH19/CS7. The packing density and ionic strength parameters were evaluated by atomic force microscopy (AFM), ellipsometry, and QCM. AFM and ellipsometry showed a distinct conformational difference between CH19 and CH19/CS7, indicating a relationship between packing density and conformational state of the immobilized peptide. The CH19 SAM thickness was 40 Å with a rough topology, while the CH19/CS7 SAM thickness was 20 Å with a smooth topology. The affinity studies showed that the affinity of CH19 and CH19/CS7 to Trastuzumab were both on the order of 107 M-1 in undiluted PBS buffer, while the dilution of the buffer by 1000× increased both SAMs affinities to Trastuzumab to the order of 1015 M-2 and changed the binding behavior from noncooperative to cooperative binding. This indicated that ionic strength had a more pronounced effect on binding properties of the CH19 SAM than packing density. Electrochemical impedance spectroscopy (EIS) was conducted on the CH19/CS7 SAM, which showed an increase in impedance after each EIS measurement cycle. Cyclic voltammetry on the CH19/CS7 SAM decreased impedance to near initial values. The impact of the packing density, buffer ionic strength, and local charge perturbation of the peptide SAM properties was interpreted based on the titratable sites in CH19 that could participate in the proton transfer and water equilibrium.

Striped gold nanoparticles: New insights from molecular dynamics simulations.

Recent simulations have improved our knowledge of the molecular-level structure and hydration properties of mixed self-assembled monolayers (SAMs) with equal and unequal alkyl thiols at three different arrangements, namely, random, patchy, and Janus. In our previous work [V. Vasumathi et al., J. Phys. Chem. C 119, 3199-3209 (2015)], we showed that the bending of longer thiols over shorter ones clearly depends on the thiols' arrangements and chemical nature of their terminal groups. In addition, such a thiol bending revealed to have a strong impact on the structural and hydration properties of SAMs coated on gold nanoparticles (AuNPs). In this paper, we extend our previous atomistic simulation study to investigate the bending of longer thiols by increasing the stripe thickness of mixed SAMs of equal and unequal lengths coated on AuNPs. We study also the effect of stripe thickness on the structural morphology and hydration of the coated SAMs. Our results show that the structural and hydration properties of SAMs are affected by the stripe thickness for mixtures of alkyl thiols with unequal chain length but not for equal length. Hence, the stability of the stripe configuration depends on the alkyl's chain length, the length difference between the thiol mixtures, and solvent properties.

(Invited) Effects of Sensitization Conditions on Photovoltaic and Photophysical Properties of Porphyrin-Sensitized Solar Cells

Elucidating the structural details of molecular assemblies formed on surfaces is essential to understand a variety of fundamental surface and interface phenomena as well as develop new types of functional surfaces and interfaces. Examples include molecular electronic devices and adsorption of biological molecules and materials. In the past the vast majority of such studies have been performed for thiol-based self-assembled monolayers (SAMs) which can be formed on gold surfaces by immersion. In contrast, the use of thiol-based SAMs on other metal and semiconducting surfaces has been limited technologically. Meanwhile, carboxylic and phosphonic acids have been frequently employed as an anchoring group for the immobilization of functional molecules on semiconducting surfaces. However, their studies have focused mainly on function rather than structure. As such, adsorption/desorption and exchange behaviour of functional molecules on semiconducting surfaces and their structure-function relationship have not been fully addressed. As a promising alternative to silicon-based solar cells, considerable efforts have been devoted to the development of dye-sensitized solar cells (DSSCs) where a SAM of dye is formed on a highly porous semiconducting electrode. In particular, porphyrin-sensitized solar cells (PSSCs) have drawn much attention in DSSCs because of potential advantage of porphyrins over ruthenium dyes in terms of cost, molecular design, and light-harvesting properties. On the basis of push-pull concept as well as asymmetrical pi-elongation, cell performances of PSSCs have been improved rapidly. In particular, push-pull porphyrins have been developed intensively in recent years with the aim of better matching between the absorption and solar spectra, reaching a power conversion efficiency more than 10%. To achieve further improvement of the cell performances, it is also important to elucidate close relationship between the dye structure, film structure, photophysics and photovoltaic properties, providing a guideline for the rational molecular design of high-performance dyes. During the course of our studies, we accidently encountered “memory effects” of the photovoltaic properties in the porphyrin SAMs on the TiO2 that could be regulated by choice of the sensitization solvents (MeOH vs. t-BuOH/MeCN) with and without the porphyrins. Although there are some examples of memory effects of SAMs induced by external stimuli, “memory effects” of photovoltaic properties by SAMs have not been reported in organic photovoltaics devices. Herein we report the first example of the memory effects in porphyrin SAMs on TiO2 electrodes that can be controlled by sensitization solvents, which have been evaluated by the photovoltaic properties of PSSCs, complemented by measurements of electron transfer kinetics and SAM composition and thickness. CN-labeled ZnP carboxylic acid (CNMP) and the reference porphyrin (MP) were used in this study.

Structural Investigations of Self-Assembled Monolayers for Organic Electronics: Results from X-ray Reflectivity

Self-assembled monolayers (SAMs) have been established as crucial interlayers and electronically active layers in organic electronic devices, such as organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), organic thin film transistors (OTFTs), and nonvolatile memories (NVMs). The use of self-assembling functionalized organic molecules is beneficial due to mainly three advantages compared with common thin film deposition approaches. (1) Molecular self-assembly occurs with surface selectivity, determined by the interaction between the functional anchor group of the organic molecules and the target surface. (2) The film thickness of the resulting layers is perfectly controllable on the angstrom scale, due to the self-terminating film formation to only a single molecular layer. And finally, (3) the wide variability in the chemical structure of such molecules enables different SAM functionalities for devices, ranging from electrical insulation to charge storage to charge transport. The SAM approach can be further expanded by employing several functionalized molecules to create mixed SAMs with consequently mixed properties. The function of SAMs in devices depends not only on the chemical structure of the molecules but also on their final arrangement and orientation on the surface. A reliable and nondestructive in-depth characterization of SAMs on nonconductive oxide surfaces is still challenging because of the very small thickness and the impracticality of methods such as scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In this Account, we illustrate how X-ray reflectivity (XRR) provides analytical access to major questions of SAM composition, morphology, and even formation by means of investigations of pure and mixed SAMs based on phosphonic acids (PAs) of various chain structures on flat alumina (AlOx) surfaces. XRR is an analytical method that provides access to spatially averaged structural depth profiles over a relatively large area of several square micrometers. The key outcome of XRR, the surface-normal electron density profile of the SAMs, leads to precise information on the SAM thickness with subangstrom resolution and allows for the determination of molecular tilt angles and packing densities. We have systematically increased the chemical complexity of PA molecules and the resulting SAMs, utilizing XRR to provide insight into the SAM structures. In SAMs composed of functionalized molecules or complex chain structures, the distribution of electron rich and electron poor signatures is detected and thus the molecular order within the SAM is determined. In mixed SAMs of two different molecular species, electron density profiles reveal the morphology and how the surface-normal structure changes if one component of the mixed SAM is altered. Furthermore, XRR was applied to investigate in situ the self-assembly of functionalized PA molecules from solution by tracking the monolayer growth over time. Even though the results provided by XRR on in-plane molecular arrangement are limited, it presents excellent information on the molecular scale along the surface normal and in addition allows for drawing conclusions on the intermolecular interactions within the SAM.

Characterization of alkylsilane self-assembled monolayers by molecular simulation.

Self-assembled monolayers (SAM) of dodecyltrichlorosilane (DTS) and octadecyltrichlorosilane (OTS) on silica are studied by molecular dynamics simulations at 298 K and 1 bar. The coverage (number of alkylsilane molecules per surface area) is systematically varied. The results yield insight into the properties of the alkylsilane SAMs, which complement experimental studies from the literature. Relationships are reported between thickness, tilt angle, and coverage of alkylsilane SAMs, which also hold for alkylsilanes other than DTS and OTS. They are interpreted based on the information on molecular ordering in the SAMs taken form the simulation data. System size and simulation time are much larger than in most former simulation works on the topic. This reduces the influence of the initial configuration as well as the periodic boundary conditions and hence minimizes the risk of artificial ordering. At the same time, more reliable statistics for the calculated properties can be provided. The evaluation of experimental data in the field is often based on strongly simplified models. The present simulation results suggest that some of these lead to errors, concerning the interpretation of experimental results, which could be avoided by introducing more realistic models.

Low energy ion scattering: Determining overlayer thickness for functionalized gold nanoparticles.

With the widespread use of engineered nanoparticles for biomedical applications, detailed surface characterization is essential for ensuring reproducibility and the quality/suitability of the surface chemistry to the task at hand. One important surface property to be quantified is the overlayer thickness of self-assembled monolayer (SAM) functionalized nanoparticles, as this information provides insight into SAM ordering and assembly. We demonstrate the application of high sensitivity low energy ion scattering (HS-LEIS) as a new analytical method for the fast thickness characterization of SAM functionalized gold nanoparticles (AuNPs). HS-LEIS demonstrates that a complete SAM is formed on 16-mercaptohexadecanoic acid (C16COOH) functionalized 14 nm AuNPs. HS-LEIS also experimentally provides SAM thickness values that are in good agreement with previously reported results from simulated electron spectra for surface analysis (SESSA) analysis of X-ray photoelectron spectroscopy (XPS) data. These results indicate HS-LEIS is a valuable surface analytical method for the characterization of SAM functionalized nanomaterials.

Self-Assembled Monolayers of Vinyltriethoxysilane and Vinyltrichlorosilane Deposited on Silicon Dioxide Surfaces

This study was aimed at deposition of self-assembled monolayers (SAMs) using vinyltriethoxysilane (VTES) and vinyltrichlorosilane (VTCS) molecules chemisorbed on silicon dioxide surfaces. The kinetics of SAM formation on planar glass substrates and silicon wafers was characterized by contact angle measurements. The surface free energy and its dispersion and polar components enabled to estimate the time of immersion required to deposit compact SAMs. Adsorption of organosilane molecules as a function of immersion time was characterized by X-ray photoelectron spectroscopy. The SAM thickness was evaluated by spectroscopic ellipsometry. Surface topography of deposited layers was investigated by atomic force microscopy (AFM). The VTCS/glass combination exhibited the fastest kinetics but the deposit was not uniform and included local agglomerates. The hydrophobic vinyl groups at deposit surface resulted in a surface free energy of 32 mJ/m2.

Controlled Thicknesses of Vaporized Self-Assembled Multilayers on Copper Nanopowders Under Ultra-High Vacuum (UHV)

Copper nanoparticles were coated with 1-octanethiol self-assembled monolayers (SAMs) using the dry-coating method for oxidation prevention. In this study, thicknesses of 1-octanethiol SAMs were successfully controlled, and the stability of SAMs as a passivation layer on copper nanoparticles was examined. Thicknesses of 1-octanethiol SAMs varied with vacuum levels and coating cycles. Under low-vacuum conditions, the thickness was 10 nm, regardless of the coating conditions. In contrast, various thicknesses resulted under ultra-high vacuum (UHV) and ranged from 4 nm to 10 nm. SAMs that were nearly a monolayer thick (4 nm) resulted from two coating cycles of 1.5 min, and the oxidation inhibition period was 15 days. Thus, the dry-coating method successfully controlled the thicknesses of SAMs with satisfactory oxidation inhibition properties under ultra-high vacuum.

Plasmon-Enabled Study of Self-Assembled Alkanethiol Ordering on Roughened Ag Substrates

Self-assembled monolayers (SAMs) have been employed for years on plasmonic nanostructured substrates to facilitate surface-enhanced Raman detection of nontraditional molecules. To date, no experiments have been conducted to study the monolayer formation or crystallinity on traditional roughened silver film over nanosphere (AgFON) substrates or to clarify what fundamental properties make a partition layer effective. The work presented herein utilizes surface-enhanced Raman spectroscopy (SERS) band analysis and localized surface plasmon resonance (LSPR) monitoring to examine formation of alkanethiol self-assembled monolayers on AgFONs, with SERS band analysis yielding structural information and LSPR spectral shifts yielding insight about the SAM formation and thickness. Herein, two short chain monolayers and two long chain monolayers were employed to look for SAM thickness and ordering effects over a 72 h time period and under potential control. Overall, the results demonstrate that both the distance a mole...

Electric-field effects on the interfacial electron transfer and protein dynamics of cytochrome c

Time-resolved surface enhanced resonance Raman and surface enhanced infrared absorption spectroscopy have been employed to study the interfacial redox process of cytochrome c (Cyt-c) immobilised on various metal electrodes coated with self-assembled monolayers (SAMs) of carboxyl-terminated mercaptanes. The experiments, carried out with Ag, Au and layered Au–SAM–Ag electrodes, afford apparent heterogeneous electron transfer constants (krelax) that reflect the interplay between electron tunnelling, redox-linked protein structural changes, protein re-orientation, and hydrogen bond re-arrangements in the protein and in the protein/SAM interface. It is shown that the individual processes are affected by the interfacial electric field strength that increases with decreasing thickness of the SAM and increasing difference between the actual potential and the potential of zero-charge. At thick SAMs of mercaptanes including 15 methylene groups, electron tunnelling (kET) is the rate-limiting step. Pronounced differences for kET and its overpotential-dependence are observed for the three metal electrodes and can be attributed to the different electric-field effects on the free-energy term controlling the tunnelling rate. With decreasing SAM thickness, electron tunnelling increases whereas protein dynamics is slowed down such that for SAMs including less than 10 methylene groups, protein re-orientation becomes rate-limiting, as reflected by the viscosity dependence of krelax. Upon decreasing the SAM thickness from 5 to 1 methylene group, an additional H/D kinetic isotope effect is detected indicating that at very high electric fields re-arrangements of the interfacial or intra-protein hydrogen bond networks limit the rate of the overall redox process.

Phonon interference at self-assembled monolayer interfaces: Molecular dynamics simulations

Using molecular dynamics simulations, we expose phonon interference effects in thermal transports across a self-assembled monolayer (SAM) of alkanethiol molecules covalently bonded to (111) gold substrate and physically bonded to silicon. In particular, we show that the thermal conductance of SAM-Au interface depends on the bonding strength at the SAM-Si interface and that the phonon transmission coefficients show strong and oscillatory dependence on frequency, with oscillatory features diminishing with increasing SAM thickness. To explore the generality of this behavior we analyze a simple model of point junction on a one-dimensional chain using the scattering boundary method.

The morphology of integrated self-assembled monolayers and their impact on devices – A computational and experimental approach

We report on a combined experimental and computational study on self-assembled monolayers (SAMs). The dielectric properties of SAMs based on n-alkyl phosphonic acids in large area thin film devices (organic field-effect transistors with good hole mobilities up to 0.3 cm 2 V −1 s −1 at −1 V and capacitors) depend on their chain length, but not consistently to theoretical pictures of tunneling through saturated n-alkanes. An unexpected saturation of current density with increasing chain length was obtained in devices, what impact on the leakage current in transistors and current density in capacitors. This is attributed to different self-assembled monolayer morphology ranging from an amorphous state for short alkyl chains to a quasi-crystalline state for longer alkyl chains. Molecular dynamics (MD) simulations provide a deeper insight into the nature of the three-dimensional intermolecular interactions and support the proposed SAM morphology. The change in morphology leads to a reduced effective SAM thickness in devices, which could be described by a Simmons approach quantitatively. The morphological aspect of self-assembled molecules is of enormous importance beyond the applications of SAMs in low-voltage, high mobility organic transistors because of it’s relevance for the hole field of molecular scale electronics. It demonstrates that even small changes in the molecular design can change the molecular interactions and monolayer assembly.

Molecular Anchors for Self-Assembled Monolayers on ZnO: A Direct Comparison of the Thiol and Phosphonic Acid Moieties

Two of the most promising schemes for attaching organic molecules to metal oxides are based on the chemistry of the thiol and phosphonic acid moieties. We have made a direct comparison of the efficacy of these two molecular anchors on zinc oxide by comparing the chemical and physical properties of n-hexane derivatives of both. The surface properties of polycrystalline ZnO thin films and ZnO(0001̅)-O crystals modified with 1-hexanethiol and 1-hexanephosphonic acid were examined with a novel quartz crystal microbalance (QCM)-based flow cell reactor, angle-resolved and temperature-dependent photoelectron spectroscopy, and contact angle measurements. A means of using ammonium chloride as a probe of molecule-ZnO interactions is introduced and used to ascertain the relative quality of self-assembled monolayers (SAMs) based on thiols and phosphonic acids. QCM data shows that a phosphonic acid-anchored alkyl chain only six carbons long can provide significant corrosion protection for ZnO against Brønsted acids, reducing the etch rate relative to the bare ZnO surface by a factor of more than nine. In contrast, we find that monolayers from the analogous molecule hexanethiol are more defective as revealed by their higher ionic permeability and lower hydrophobicity. Substrate attenuation X-ray photoelectron spectroscopy (XPS) experiments were used to determine the thickness of SAMs formed by the two hexane derivatives and it was found that SAMs from phosphonic acids were approximately twice as thick as those formed by hexanethiol. The thermal stability of the two linking groups was also explored and we find that previous claims of highly stable alkanethiolate monolayers on ZnO are suspect. Taken as a whole, our results indicate that the phosphonic acid moiety is preferred over thiols for the attachment of short alkyl groups to ZnO.

The impact of self-assembled monolayer thickness in hybrid gate dielectrics for organic thin-film transistors

We have investigated the electrical characteristics of hybrid dielectrics with a thickness of 6 nm or less that are composed of a plasma-grown aluminum oxide (AlO x ) layer and a self-assembled monolayer (SAM) of an aliphatic phosphonic acid. The impact of the quality of the AlO x layer on the insulating properties of the double-layer dielectrics was assessed by comparing two different oxidation procedures, and the influence of the thickness of the organic SAM was evaluated by employing molecules with five different chain lengths. In order to decouple the relative contributions of the oxide and the SAM to the performance of the double-layer dielectrics we have also performed cyclic voltammetry measurements on indium tin oxide (ITO)/SAM devices without AlO x layer. Finally, we have evaluated how the quality of the AlO x layer and the thickness of the SAM affect the performance of low-voltage organic thin-film transistors (TFTs) that employ the thin AlO x /SAM dielectrics as the gate dielectric. The results confirm the important role of the SAM in determining the breakdown voltage, in limiting the current density, and in compensating the somewhat lower quality of AlO x layers produced under mild plasma conditions.