Electron transport through rectifying self-assembled monolayer diodes on silicon: Fermi-level pinning at the molecule-metal interface.

We report the synthesis and characterization of molecular rectifying diodes on silicon using sequential grafting of self-assembled monolayers of alkyl chains bearing a pi group at their outer end (Si/sigma-pi/metal junctions). We investigate the structure-performance relationships of these molecular devices, and we examine the extent to which the nature of the pi end group (change in the energy position of their molecular orbitals) drives the properties of these molecular diodes. Self-assembled monolayers of alkyl chains (different chain lengths from 6 to 15 methylene groups) functionalized by phenyl, anthracene, pyrene, ethylene dioxythiophene, ethylene dioxyphenyl, thiophene, terthiophene, and quaterthiophene were synthesized and characterized by contact angle measurements, ellipsometry, Fourier transform infrared spectroscopy, and atomic force microscopy. We demonstrate that reasonably well-packed monolayers are obtained in all cases. Their electrical properties were assessed by dc current-voltage characteristics and high-frequency (1-MHz) capacitance measurements. For all of the pi groups investigated here, we observed rectification behavior. These results extend our preliminary work using phenyl and thiophene groups (Lenfant et al., Nano Lett. 2003, 3, 741). The experimental current-voltage curves were analyzed with a simple analytical model, from which we extracted the energy position of the molecular orbital of the pi group in resonance with the Fermi energy of the electrodes. We report experimental studies of the band lineup in these silicon/alkyl pi-conjugated molecule/metal junctions. We conclude that Fermi-level pinning at the pi group/metal interface is mainly responsible for the observed absence of a dependence of the rectification effect on the nature of the pi groups, even though the groups examined were selected to have significant variations in their electronic molecular orbitals.

Evidence of a charge-density threshold for optimum efficiency of biocidal cationic surfaces.

The deposition of organic monolayers containing quaternary ammonium groups has been shown by many authors to confer biocidal properties on a large variety of solid surfaces. In a search for the controlling factors, the authors have grafted quaternized poly(vinylpyridine) chains on glass surfaces by two different methods and varied the charge density within the organic layer between 10(12) and 10(16) positive charges per cm2. The measurements show that this parameter has a large influence on the killing efficiency. Bacterial death occurs in less than 10 min in the quiescent state above a threshold value. The value is smaller for bacteria in the growth state. It also depends on the bacterial type. An electrostatic mechanism based on the exchange of counterions between the functionalized cationic surface and the bacterial membrane is proposed and appears consistent with the results.

Plasma surface modification of organic materials: comparison between polyethylene films and octadecyltrichlorosilane self-assembled monolayers.

Low-pressure low-frequency NH3 plasmas have been used for the surface modification of bulk polyethylene films and of octadecyltrichlorosilane (OTS) self-assembled monolayers deposited on oxidized silicon wafers. The incorporation of nitrogen-containing groups by the plasma treatment has been followed by contact angle measurements and by X-ray photoelectron spectroscopy. The surface degradation of the OTS monolayers due to plasma etching has been measured separately by optical ellipsometry with subnanometric accuracy. Our data show clear evidence for the existence of an optimum treatment time, yielding a high density of NH2 functional groups without significant variation of the structural features of the organic material. Self-assembled monolayers appear as excellent model systems to characterize the effects of plasma discharges on polyolefins. In particular, they allow testing the influence of molecular orientation, packing density, and crystallinity on the final results.

Influence of multivalent counterions adsorption on Langmuir films.

It has been shown recently by neutron and X-ray reflectivity that nanometer-sized multivalent counterions (Keggin salts) can assemble as a dense monomolecular sublayer beneath a charged Langmuir monolayer of opposite sign. We have conducted experiments that examine the surface pressure isotherms of docosamine surfactant monolayers under such conditions and have shown that they undergo dramatic modifications when the Keggin salts are added. We model these experimental results by a close-packed sublayer of counterions on which charged surfactants can organize and form complexes. We then provide a thermodynamic description of the surface/sublayer system by giving an expression for surface free energy and surface pressure. We compare the results of this discrete model to traditional mean-field descriptions where the counterions form a diffuse continuous layer. The new features are: i) modifications in the shape of the surface pressure isotherm; ii) appearance of a phase separation in the surfactant layer. Finally, we show that the model is in satisfactory agreement with the experimental isotherms and a best fit yields numerical estimates of the different interaction parameters.

Vesicles and particles of sodium bis(2-ethylhexyl) phosphate in binary and ternary systems.

Vesicles were identified in aqueous solution of pure sodium bis(2-ethylhexyl) phosphate, a short branched chain surfactant. Superficial tension measurements show that the vesicles appear above a molality of 0.02 (0.69 %w). These aggregates are equilibrium structures. The "packing parameter' theory established by Israelachvili et al. allows the prediction of the occurrence of such vesicles. If an organic solvent, such as xylene or ethylhexanoate, is added to the binary system, a different type of aggregate appears, the size of which is determined by several methods including electron microscopy and light scattering. Interfacial tension measurements show that these aggregates would be expected to form above a molality of 0.02. According to our experimental results, the microstructure of these aggregates can be described as micelles and/or vesicles, swollen or not.

A NMR study of the ionization of fatty acids, fatty amines and N-acylamino acids incorporated in phosphatidylcholine vesicles.

The ionization of fatty acids, fatty amines and N-acylamino acids incorporated in phosphatidylcholine single-walled vesicles has been measured. The guest molecules have been specifically enriched with 13C and titrated by using NMR spectroscopy. The apparent pKa of fatty acids in phosphatidylcholine bilayers if 7.2-7.4 and those of fatty amines are approx. 9.5. These pKa values depend on many different parameters related to the structure of the lipid/solution interface, to the composition of the aqueous medium and to the localization of the ionizable groups. A special sensitivity to the ionic strength and to the surface charge has been found. A positive surface charge decreases the pKa value whereas a negative one increases it, the total range of variation being 2.5-3 units. In a qualitative macroscopic interpretation, it is proposed that pKa is essentially determined by the low polarity of the lipidic matrix.