Self assembled monolayers on silicon for molecular electronics

We present an overview of various aspects of the self-assembly of organic monolayers on silicon substrates for molecular electronics applications. Different chemical strategies employed for grafting the self-assembled monolayers (SAMs) of alkanes having different chain lengths on native oxide of Si or on bare Si have been reviewed. The utility of different characterization techniques in determination of the thickness, molecular ordering and orientation, surface coverage, growth kinetics and chemical composition of the SAMs has been discussed by choosing appropriate examples. The metal counterelectrodes are an integral part of SAMs for measuring their electrical properties as well as using them for molecular electronic devices. A brief discussion on the variety of options available for the deposition of metal counterelectrodes, that is, soft metal contacts, vapor deposition and soft lithography, has been presented. Various theoretical models, namely, tunneling (direct and Fowler-Nordheim), thermionic emission, Poole-Frenkel emission and hopping conduction, used for explaining the electronic transport in dielectric SAMs have been outlined and, some experimental data on alkane SAMs have been analyzed using these models. It has been found that short alkyl chains show excellent agreement with tunneling models; while more experimental data on long alkyl chains are required to understand their transport mechanism(s). Finally, the concepts and realization of various molecular electronic components, that is, diodes, resonant tunnel diodes, memories and transistors, based on appropriate architecture of SAMs comprising of alkyl chains (sigma- molecule) and conjugated molecules (pi-molecule) have been presented.     

Transmission infrared spectroscopy of methyl- and ethyl-terminated silicon(111) surfaces

Transmission infrared spectroscopy (TIRS) has been used to investigate the surface-bound species formed in the two-step chlorination/alkylation reaction of crystalline (111)-oriented Si surfaces. Spectra were obtained after hydrogen termination, chlorine termination, and reaction of the Cl-Si(111) surface with CH(3)MgX or C(2)H(5)MgX (X = Cl, Br) to form methyl (CH(3))- or ethyl (C(2)H(5))-terminated Si(111) surfaces, respectively. Freshly etched H-terminated Si(111) surfaces that were subsequently chlorinated by immersion in a saturated solution of PCl(5) in chlorobenzene were characterized by complete loss of the Si-H stretching and bending modes at 2083 and 627 cm(-1)(,) respectively, and the appearance of Si-Cl modes at 583 and 528 cm(-1). TIRS of the CH(3)-terminated Si(111) surface exhibited a peak at 1257 cm(-1) polarized perpendicular to the surface assigned to the C-H symmetrical bending, or "umbrella" motion, of the methyl group. A peak observed at 757 cm(-1) polarized parallel to the surface was assigned to the C-H rocking motion. Alkyl C-H stretch modes on both the CH(3)- and C(2)H(5)-terminated surfaces were observed near 2900 cm(-1). The C(2)H(5)-terminated Si(111) surface additionally exhibited broad bands at 2068 and 2080 cm(-1), respectively, polarized perpendicular to the surface, as well as peaks at 620 and 627 cm(-1), respectively, polarized parallel to the surface. These modes were assigned to the Si-H stretching and bending motions, respectively, resulting from H-termination of surface atoms that did not form Si-C bonds during the ethylation reaction.     

Nanometric protein arrays on protein-resistant monolayers on silicon surfaces

We present a novel approach for preparation of nanometric protein arrays, based on binding of avidin molecules to nanotemplates generated by conductive AFM lithography on robust oligo(ethylene glycol)-terminated monolayers on silicon (111) surfaces that are protein-resistant. We showed that only biotinated-BSA but not the native BSA bind to the avidin arrays and that the resulting arrays of biotinated BSA could bind avidin to form protein dots with a feature size of approximately 30 nm. This result demonstrates that the avidin array may serve as templates for preparation of nanoarrays of a wide variety of biotin-tagged proteins for studying their interactions with other protein molecules at nanoscale.     

Engineering silicon oxide surfaces using self-assembled monolayers

Although a molecular monolayer is only a few nanometers thick it can completely change the properties of a surface. Molecular monolayers can be readily prepared using the Langmuir-Blodgett methodology or by chemisorption on metal and oxide surfaces. This Review focuses on the use of chemisorbed self-assembled monolayers (SAMs) as a platform for the functionalization of silicon oxide surfaces. The controlled organization of molecules and molecular assemblies on silicon oxide will have a prominent place in "bottom-up" nanofabrication, which could revolutionize fields such as nanoelectronics and biotechnology in the near future. In recent years, self-assembled monolayers on silicon oxide have reached a high level of sophistication and have been combined with various lithographic patterning methods to develop new nanofabrication protocols and biological arrays. Nanoscale control over surface properties is of paramount importance to advance from 2D patterning to 3D fabrication.     

Molecular printboards: monolayers of beta-cyclodextrins on silicon oxide surfaces

Monolayers of beta-cyclodextrin host molecules have been prepared on SiO2 surfaces. An ordered and stable cyano-terminated monolayer was modified in three consecutive surface reactions. First, the cyanide groups were reduced to their corresponding free amines using Red Al as a reducing agent. Second, 1,4-phenylene diisothiocyanate was used to react with the amine monolayer where it acts as a linking molecule, exposing isothiocyanates that can be derivatized further. Finally, per-6-amino beta-cyclodextrin was reacted with these isothiocyanate functions to yield a monolayer exposing beta-cyclodextrin. All monolayers were characterized by contact angle measurements, ellipsometric thickness measurements, Brewster angle Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry, which indicate the formation of a densely packed cyclodextrin surface. It was demonstrated that the beta-cyclodextrin monolayer could bind suitable guest molecules in a reversible manner. A fluorescent molecule (1), equipped with two adamantyl groups for complexation, was adsorbed onto the host monolayer from solution to form a monolayer of guest molecules. Subsequently, the guest molecules were desorbed from the surface by competition with increasing beta-cyclodextrin concentration in solution. The data were fitted using a model. An intrinsic binding constant of 3.3 +/- 1 x 10(5) M(-1) was obtained, which corresponds well to previously obtained results with a divalent guest molecule on beta-cyclodextrin monolayers on gold. In addition, the number of guest molecules bound to the host surface was determined, and a surface coverage of ca. 30% was found.     

Functionalization of silicon surfaces with Si-C linked beta-cyclodextrin monolayers

Vinyl-terminated heptapodyl beta-cyclodextrins react with hydrogenated silicon surfaces to generate covalently-bound molecular recognition devices.     

Covalent attachment of organic monolayers to silicon carbide surfaces

This work presents the first alkyl monolayers covalently bound on HF-treated silicon carbide surfaces (SiC) through thermal reaction with 1-alkenes. Treatment of SiC with diluted aqueous HF solutions removes the native oxide layer (SiO2) and provides a reactive hydroxyl-covered surface. Very hydrophobic methyl-terminated surfaces (water contact angle theta = 107 degrees ) are obtained on flat SiC, whereas attachment of omega-functionalized 1-alkenes also yields well-defined functionalized surfaces. Infrared reflection absorption spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy measurements are used to characterize the monolayers and show their covalent attachment. The resulting surfaces are shown to be extremely stable under harsh acidic conditions (e.g., no change in theta after 4 h in 2 M HCl at 90 degrees C), while their stability in alkaline conditions (pH = 11, 60 degrees C) also supersedes that of analogous monolayers such as those on Au, Si, and SiO2. These results are very promising for applications involving functionalized silicon carbide.     

Self assembled molecular monolayers on oxidized inhomogeneous aluminum surfaces

The adsorption and self organization of phosphonic acids on oxidized aluminum surfaces has been investigated by surface sensible techniques: ARXPS, AES and FTIR. The analysis has been performed on Al 99.999 samples and on surfaces of a technical alloy whose surfaces contain iron inclusions. The results obtained lead to the conclusion that self organization is not only possible on perfect defect free surfaces but also on inhomogeneous ones. Received: 24 June 1996 / Accepted: 21 January 1997     

Grazing angle mirror-backed reflection (GMBR) for infrared analysis of monolayers on silicon

An easy handling infrared measurement, grazing angle mirror-backed reflection (GMBR), has been established to analyze the silicon-based organic monolayer. Theoretical prediction gave the optimal configuration with p-polarized irradiation near a grazing angle 78.1 degrees of incidence. Experimental measurement of hydrogen-terminated, undecylenic acid (UA) and N-hydroxysuccinimide (NHS) functionalized silicon (111) surfaces showed good signal peaks and reproducibility.     

pH sensitivity of Si--C linked organic monolayers on crystalline silicon surfaces.

The electrochemical behavior of Si--C linked organic monolayers is studied in electrolyte-insulator-Si devices, under conditions normally encountered in potentiometric biosensors, to gain fundamental knowledge on the behavior of such Si electrodes under practical conditions. This is done via titration experiments, Mott-Schottky data analysis, and data fitting using a site-binding model. The results are compared with those of native SiO(2) layers and native SiO(2) layers modified with hexamethyldisilazane. All samples display pH sensitivity. The number of Si--OH groups on the alkylated samples is calculated to be less than 0.7 % of that of a pure SiO(2) insulator, which still causes a pH sensitivity of approximately 25 mV per pH unit in the pH range: 4-7. The alkylated samples hardly suffer from response changes during up- and down-going titrations, which indicates that very little oxide is additionally formed during the measurements. The pK(a) values of all samples with monolayers (4.0-4.4) are lower than that of native SiO(2) (6.0). The long-term drift (of approximately 1 mV h(-1)) is moderate. The results indicate that biosensors composed of alkylated Si substrates are feasible if a cross-sensitivity towards pH in the sensor signal is taken into account.     

Reversible covalent patterning of self-assembled monolayers on gold and silicon oxide surfaces

This paper describes the generation of reversible patterns of self-assembled monolayers (SAMs) on gold and silicon oxide surfaces via the formation of reversible covalent bonds. The reactions of (patterned) SAMs of 11-amino-1-undecanethiol (11-AUT) with propanal, pentanal, decanal, or terephthaldialdehyde result in dense imine monolayers. The regeneration of these imine monolayers to the 11-AUT monolayer is obtained by hydrolysis at pH 3. The (patterned) monolayers were characterized by Fourier transform infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, contact angle and electrochemical measurements, and atomic force microscopy. Imines can also be formed by microcontact printing of amines on terephthaldialdehyde-terminated substrates. Lucifer Yellow ethylenediamine was employed as a fluorescent amine-containing marker to visualize the reversible covalent patterning on a terephthaldialdehyde-terminated glass surface by confocal microscopy. These experiments demonstrate that with reversible covalent chemistry it is possible to print and erase chemical patterns on surfaces repeatedly.     

Screening and surface states in molecular monolayers adsorbed on silicon

We performed density functional theory calculations of the atomic and electronic structure of a dense monolayer of phenyl-terminated alkyl chains chemisorbed onto the (100) Si surface. Different adsorption sites were characterized for both the pristine and (2 x 1) reconstructed surface. A strong effect on the ordering and alignment of the molecular energy levels with respect to the Fermi level of silicon is observed, consequent to intermolecular screening in the monolayer and of the appearance of surface localized states, as a function of the different bonding arrangements. Some possible consequences of these findings are discussed in the framework of the experimental synthesis of such monolayers as molecular current rectifiers in silicon-integrated nanoscale electronics.     

Electrical properties of diamond surfaces functionalized with molecular monolayers

Recent studies have shown that semiconductor surfaces such as silicon and diamond can be functionalized with organic monolayers, and that these monolayer films can be used to tether biomolecules such as DNA to the surfaces. Electrical measurements of these interfaces show a change in response to DNA hybridization and other biological binding processes, but the fundamental nature of the electrical signal transduction has remained unclear. We have explored the electrical impedance of polycrystalline and single-crystal diamond surfaces modified with an organic monolayer produced by photochemical reaction of diamond with 1-dodecene. Our results show that, by measuring the impedance as a function of frequency and potential, it is possible to dissect the complex interfacial structure into frequency ranges where the total impedance is controlled by the molecular monolayer, by the diamond space-charge region, and by the electrolyte. The results have implications for understanding the ability to use molecularly modified semiconductor surfaces for applications such as chemical and biological sensing.     

The titration of carboxyl-terminated monolayers revisited: in situ calibrated fourier transform infrared study of well-defined monolayers on silicon

The acid-base equilibrium at the surface of well-defined mixed carboxyl-terminated/methyl-terminated monolayers grafted on silicon (111) has been investigated using in situ calibrated infrared spectroscopy (attenuated total reflectance (ATR)) in the range of 900-4000 cm (-1). Spectra of surfaces in contact with electrolytes of various pH provide a direct observation of the COOH <--> COO (-) conversion process. Quantitative analysis of the spectra shows that ionization of the carboxyl groups starts around pH 6 and extends over more than 6 pH units: approximately 85% ionization is measured at pH 11 (at higher pH, the layers become damaged). Observations are consistently accounted for by a single acid-base equilibrium and discussed in terms of change in ion solvation at the surface and electrostatic interactions between surface charges. The latter effect, which appears to be the main limitation, is qualitatively accounted for by a simple model taking into account the change in the Helmholtz potential associated with the surface charge. Furthermore, comparison of calculated curves with experimental titration curves of mixed monolayers suggests that acid and alkyl chains are segregated in the monolayer.     

Controlled modulation of conductance in silicon devices by molecular monolayers

We have controllably modulated the drain current (I(D)) and threshold voltage (V(T)) in pseudo metal-oxide-semiconductor field-effect transistors (MOSFETs) by grafting a monolayer of molecules atop oxide-free H-passivated silicon surfaces. An electronically controlled series of molecules, from strong pi-electron donors to strong pi-electron acceptors, was covalently attached onto the channel region of the transistors. The device conductance was thus systematically tuned in accordance with the electron-donating ability of the grafted molecules, which is attributed to the charge transfer between the device channel and the molecules. This surface grafting protocol might serve as a useful method for controlling electronic characteristics in small silicon devices at future technology nodes.     

Si-C linked organic monolayers on crystalline silicon surfaces as alternative gate insulators.

Herein, the influence of silicon surface modification via Si-C(n)H(2n+1) (n=10,12,16,22) monolayer-based devices on p-type 100 and n-type 100 silicon is studied by forming MIS (metal-insulator-semiconductor) diodes using a mercury probe. From current density-voltage (J-V) and capacitance-voltage (C-V) measurements, the relevant parameters describing the electrical behavior of these diodes are derived, such as the diode ideality factor, the effective barrier height, the flatband voltage, the barrier height, the monolayer dielectric constant, the tunneling attenuation factor, and the fixed charge density (Nf). It is shown that the J-V behavior of our MIS structures could be precisely tuned via the monolayer thickness. The use of n-type silicon resulted in lower diode ideality factors as compared to p-type silicon. A similar flatband voltage, independent of monolayer thickness, was found, indicating similar properties for all silicon-monolayer interfaces. An exception was the C10-based monolayer device on p-type silicon. Furthermore, low values of N(f) were found for monolayers on p-type silicon (approximately 6 x 10(11) cm(-2)). These results suggest that Si--C linked monolayers on flat silicon may be a viable material for future electronic devices.     

Allergen microarrays on high-sensitivity silicon slides

We have recently introduced a silicon substrate for high-sensitivity microarrays, coated with a functional polymer named copoly(DMA-NAS-MAPS). The silicon dioxide thickness has been optimized to produce a fluorescence intensification due to the optical constructive interference between the incident and reflected lights of the fluorescent radiation. The polymeric coating efficiently suppresses aspecific interaction, making the low background a distinctive feature of these slides. Here, we used the new silicon microarray substrate for allergy diagnosis, in the detection of specific IgE in serum samples of subjects with sensitizations to inhalant allergens. We compared the performance of silicon versus glass substrates. Reproducibility data were measured. Moreover, receiver-operating characteristic (ROC) curves were plotted to discriminate between the allergy and no allergy status in 30 well-characterized serum samples. We found that reproducibility of the microarray on glass supports was not different from available data on allergen arrays, whereas the reproducibility on the silicon substrate was consistently better than on glass. Moreover, silicon significantly enhanced the performance of the allergen microarray as compared to glass in accurately identifying allergic patients spanning a wide range of specific IgE titers to the considered allergens.     

One-step photochemical attachment of NHS-terminated monolayers onto silicon surfaces and subsequent functionalization

N-Hydroxysuccinimide (NHS)-ester-terminated monolayers were covalently attached in one step onto silicon using visible light. This mild photochemical attachment, starting from omega-NHS-functionalized 1-alkenes, yields a clean and flat monolayer-modified silicon surface and allows a mild and rapid functionalization of the surface by substitution of the NHS-ester moieties with amines at room temperature. Using a combination of analytical techniques (infrared reflection absorption spectroscopy (IRRAS), extensive X-ray photoelectron spectroscopy (XPS) in combination with density functional theory calculations of the XPS chemical shifts of the carbon atoms, atomic force microscopy (AFM), and static contact angle measurements), it was shown that the NHS-ester groups were attached fully intact onto the surface. The surface reactivity of the NHS-ester moieties toward amines was qualitatively and quantitatively evaluated via the reaction with para-trifluoromethyl benzylamine and biotin hydrazide.     

Dynamic chiral flipping within strongly chemisorbed molecular monolayers at surfaces

Unexpected local chiral switching events are shown to occur within strongly chemisorbed homochiral domains, in which a pair of surface-bonded molecules dynamically switches their chiral configuration for a short period of time.