Measuring the Diameter of Single-Wall Carbon Nanotubes Using AFM.

In this work, we identify two issues that can significantly affect the accuracy of AFM measurements of the diameter of single-wall carbon nanotubes (SWCNTs) and propose a protocol that reduces errors associated with these issues. Measurements of the nanotube height under different applied forces demonstrate that even moderate forces significantly compress several different types of SWCNTs, leading to errors in measured diameters that must be minimized and/or corrected. Substrate and nanotube roughness also make major contributions to the uncertainty associated with the extraction of diameters from measured images. An analysis method has been developed that reduces the uncertainties associated with this extraction to <0.1 nm. This method is then applied to measure the diameter distribution of individual highly semiconducting enriched nanotubes in networks prepared from polyfluorene/SWCNT dispersions. Good agreement is obtained between diameter distributions for the same sample measured with two different commercial AFM instruments, indicating the reproducibility of the method. The reduced uncertainty in diameter measurements based on this method facilitates: (1) determination of the thickness of the polymer layer wrapping the nanotubes and (2) measurement of nanotube compression at tube-tube junctions within the network.

Surface chemistry of metal oxide nanoparticles: NMR and TGA quantification.

Surface functionalization is widely used to control the behavior of nanomaterials for a range of applications. However, methods to accurately quantify surface functional groups and coatings are not yet routinely applied to nanomaterial characterization. We have employed a combination of quantitative NMR (qNMR) and thermogravimetric analysis (TGA) to address this problem for commercial cerium, nickel, and iron oxide nanoparticles (NPs) that have been modified to add functional coatings with (3-aminopropyl)triethoxysilane (APTES), stearic acid, and polyvinylpyrrolidone (PVP). The qNMR method involves quantification of material that is released from the NPs and quantified in the supernatant after removal of NPs. Removal of aminopropylsilanes was accomplished by basic hydrolysis whereas PVP and stearic acid were removed by ligand exchange using sodium hexametaphosphate and pentadecafluorooctanoic acid, respectively. The method accuracy was confirmed by analysis of NPs with a known content of surface groups. Complementary TGA studies were carried out in both air and argon atmosphere with FT-IR of evolved gases in argon to confirm the identity of the functional groups. TGA measurements for some unfunctionalized samples show mass loss due to unidentified components which makes quantification of functional groups in surface-modified samples less reliable. XPS provides information on the presence of surface contaminants and the level of surface hydroxylation for selected samples. Despite the issues associated with accurate quantification using TGA, the TGA estimates agree reasonably well with the qNMR data for samples with high surface loading. This study highlights the issues in analysis of commercial nanomaterials and is an advance towards the development of generally applicable methods for quantifying surface functional groups.

A Multi-Method Approach for Quantification of Surface Coatings on Commercial Zinc Oxide Nanomaterials.

Surface functionalization is a key factor for determining the performance of nanomaterials in a range of applications and their fate when released to the environment. Nevertheless, it is still relatively rare that surface groups or coatings are quantified using methods that have been carefully optimized and validated with a multi-method approach. We have quantified the surface groups on a set of commercial ZnO nanoparticles modified with three different reagents ((3-aminopropyl)-triethoxysilane, caprylsilane and stearic acid). This study used thermogravimetric analysis (TGA) with Fourier transform infrared spectroscopy (FT-IR) of evolved gases and quantitative solution 1H nuclear magnetic resonance (NMR) for quantification purposes with 13C-solid state NMR and X-ray photoelectron spectroscopy to confirm assignments. Unmodified materials from the same suppliers were examined to assess possible impurities and corrections. The results demonstrate that there are significant mass losses from the unmodified samples which are attributed to surface carbonates or residual materials from the synthetic procedure used. The surface modified materials show a characteristic loss of functional group between 300-600 °C as confirmed by analysis of FT-IR spectra and comparison to NMR data obtained after quantitative release/extraction of the functional group from the surface. The agreement between NMR and TGA estimates for surface loading is reasonably good for cases where the functional group accounts for a relatively large fraction of the sample mass (e.g., large groups or high loading). In other cases TGA does not have sufficient sensitivity for quantitative analysis, particularly when contaminants contribute to the TGA mass loss. X-ray photoelectron spectroscopy and solid state NMR for selected samples provide support for the assignment of both the functional groups and some impurities. The level of surface group loading varies significantly with supplier and even for different batches or sizes of nanoparticles from the same supplier. These results highlight the importance of developing reliable methods to detect and quantify surface functional groups and the importance of a multi-method approach.

Formulation of Screen-Printable Cu Molecular Ink for Conductive/Flexible/Solderable Cu Traces.

Screen printing is the most common method used for the production of printed electronics. Formulating copper (Cu) inks that yield conductive fine features with oxidation and mechanical robustness on low-temperature substrates will open up opportunities to fabricate cost-effective devices. We have formulated a screen-printable Cu metal-organic decomposition (MOD) ink comprising Cu formate coordinated to 3-(diethylamino)-1,2-propanediol, a fractional amount of Cu nanoparticles (CuNPs), and a binder. This simple formulation enables ∼70-550 µm trace widths with excellent electrical [∼8-15 mΩ/□/mil or 20-38 µΩ·cm] and mechanical properties with submicron-thick traces obtained by intense pulse light (IPL) sintering on Kapton and poly(ethylene terephthalate) (PET) substrates. These traces are mechanically robust to flexing and creasing where less than 10% change in resistance is observed on Kapton and ∼20% change is observed on PET. Solderable Cu traces were obtained only with the combination of the Cu MOD precursor, CuNP, and polymer binder. Both thermally and IPL sintered traces showed shelf stability (<10% change in resistance) of over a month in ambient conditions and 10-70% relative humidity, suitable for day-to-day fabrication. To demonstrate utility, light-emitting diodes (LEDs) were directly soldered to IPL sintered Cu traces in a reflow oven without the need for a precious metal interlayer. The LEDs were functional not only during bending and creasing of the Cu traces but even after 180 min at 140 °C in ambient air without losing illumination intensity. High definition television antennas printed on Kapton and PET were found to perform well in the ultrahigh frequency region. Lastly, single-walled carbon nanotube-based thin-film transistors on a silicon wafer were fabricated with a screen-printed Cu source and drain electrodes, which performed comparably to silver electrodes with mobility values of 12-15 cm2 V-1 s-1 and current on/off ratios of ∼105 and as effective ammonia sensors providing parts per billion-level detection.

The impact of processing on the cytotoxicity of graphene oxide.

In-house prepared graphene oxide (GO) was processed via base washing, sonication, cleaning and combinations of these processing techniques to evaluate the impact on the flake morphology, composition and cytotoxicity of the material. The flakes of unprocessed GO were relatively planar, but upon base washing, the flakes became textured exhibiting many folds and creases observed by AFM. In addition to the pronounced effect on the topography, base washing increased the C/O ratio and increased the cytotoxicity of GO on all four cell lines studied determined via the WST-8 assay. Sonicating the unprocessed and base washed samples resulted in smaller flakes with a similar topography; the base washed flakes lost the texture previously observed upon sonication. The sonicated samples were more toxic than the unprocessed sample, attributed to the smaller flake size, but were interestingly less toxic than the base washed, unsonicated sample despite the base washed unsonicated sample having a larger flake size. This unexpected finding was confirmed by a second analyst using the same, and a different source of GO and resulted in the conclusion that the morphology of GO greatly impacts the cytotoxicity. Cleaning the GO reduced the amount of nitrogen and sulfur impurities in the sample but had no significant impact on the cytotoxicity of the material. It was observed that nutrient depletion via nanomaterial adsorption was not the route of cytotoxicity for the GO samples studied.

Quantification of amine functional groups on silica nanoparticles: a multi-method approach.

Surface chemistry is an important factor for quality control during production of nanomaterials and for controlling their behavior in applications and when released into the environment. Here we report a comparison of four methods for quantifying amine functional groups on silica nanoparticles (NPs). Two colorimetric assays are examined, ninhydrin and 4-nitrobenzaldehyde, which are convenient for routine analysis and report on reagent accessible amines. Results from the study of a range of commercial NPs with different sizes and surface loadings show that the assays account for 50-100% of the total amine content, as determined by dissolution of NPs under basic conditions and quantification by solution-state 1H NMR. To validate the surface quantification by the colorimetric assays, the NPs are modified with a trifluoromethylated benzaldehyde probe to enhance sensitivity for quantitative 19F solid state NMR and X-ray photoelectron spectroscopy (XPS). Good agreement between the assays and the determination from solid-state NMR is reinforced by elemental ratios from XPS, which indicate that in most cases the difference between total and accessible amine content reflects amines that are outside the depth probed by XPS. Overall the combined results serve to validate the relatively simple colorimetric assays and indicate that the reactions are efficient at quantifying surface amines, by contrast to some other covalent modifications that have been employed for functional group quantification.

High-Purity Semiconducting Single-Walled Carbon Nanotubes: A Key Enabling Material in Emerging Electronics.

Semiconducting single-walled carbon nanotubes (sc-SWCNTs) are emerging as a promising material for high-performance, high-density devices as well as low-cost, large-area macroelectronics produced via additive manufacturing methods such as roll-to-roll printing. Proof-of-concept demonstrations have indicated the potential of sc-SWCNTs for digital electronics, radiofrequency circuits, radiation hard memory, improved sensors, and flexible, stretchable, conformable electronics. Advances toward commercial applications bring numerous opportunities in SWCNT materials development and characterization as well as fabrication processes and printing technologies. Commercialization in electronics will require large quantities of sc-SWCNTs, and the challenge for materials science is the development of scalable synthesis, purification, and enrichment methods. While a few synthesis routes have shown promising results in making near-monochiral SWCNTs, gram quantities are available only for small-diameter sc-SWCNTs, which underperform in transistors. Most synthesis routes yield mixtures of SWCNTs, typically 30% metallic and 70% semiconducting, necessitating the extraction of sc-SWCNTs from their metallic counterparts in high purity using scalable postsynthetic methods. Numerous routes to obtain high-purity sc-SWCNTs from raw soot have been developed, including density-gradient ultracentrifugation, chromatography, aqueous two-phase extraction, and selective DNA or polymer wrapping. By these methods (termed sorting or enrichment), >99% sc-SWCNT content can be achieved. Currently, all of these approaches have drawbacks and limitations with respect to electronics applications, such as excessive dilution, expensive consumables, and high ionic impurity content. Excess amount of dispersant is a common challenge that hinders direct inclusion of sc-SWCNTs into electronic devices. At present, conjugated polymer extraction may represent the most practical route to sc-SWCNTs. By the use of polymers with a π-conjugated backbone, sc-SWCNTs with >99.9% purity can be dispersed in organic solvents via a simple sonication and centrifugation process. With 1000 times less excipient and the flexibility to accommodate a broad range of solvents via diverse polymer constructs, inks are readily deployable in solution-based fabrication processes such as aerosol spray, inkjet, and gravure. Further gains in sc-SWCNT purity, among other attributes, are possible with a better understanding of the structure-property relationships that govern conjugated polymer extraction. This Account covers three interlinked topics in SWCNT electronics: metrology, enrichment, and SWCNT transistors fabricated via solution processes. First, we describe how spectroscopic techniques such as optical absorption, fluorescence, and Raman spectroscopy are applied for sc-SWCNT purity assessment. Stringent requirements for sc-SWCNTs in electronics are pushing the techniques to new levels while serving as an important driver toward the development of quantitative metrology. Next, we highlight recent progress in understanding the sc-SWCNT enrichment process using conjugated polymers, with special consideration given to the effect of doping on the mechanism. Finally, developments in sc-SWCNT-based electronics are described, with emphasis on the performance of transistors utilizing random networks of sc-SWCNTs as the semiconducting channel material. Challenges and advances associated with using polymer-based dielectrics in the unique context of sc-SWCNT transistors are presented. Such transistor packages have enabled the realization of fully printed transistors as well as transparent and even stretchable transistors as a result of the unique and excellent electrical and mechanical properties of sc-SWCNTs.

Versatile Molecular Silver Ink Platform for Printed Flexible Electronics.

A silver molecular ink platform formulated for screen, inkjet, and aerosol jet printing is presented. A simple formulation comprising silver neodecanoate, ethyl cellulose, and solvent provides improved performance versus that of established inks, yet with improved economics. Thin, screen-printed traces with exceptional electrical (<10 mΩ/□/mil or 12 µΩ·cm) and mechanical properties are achieved following thermal or photonic sintering, the latter having never been demonstrated for silver-salt-based inks. Low surface roughness, submicron thicknesses, and line widths as narrow as 41 µm outperform commercial ink benchmarks based on flakes or nanoparticles. These traces are mechanically robust to flexing and creasing (less than 10% change in resistance) and bind strongly to epoxy-based adhesives. Thin traces are remarkably conformal, enabling fully printed metal-insulator-metal band-pass filters. The versatility of the molecular ink platform enables an aerosol jet-compatible ink that yields conductive features on glass with 2× bulk resistivity and strong adhesion to various plastic substrates. An inkjet formulation is also used to print top source/drain contacts and demonstrate printed high-mobility thin film transistors (TFTs) based on semiconducting single-walled carbon nanotubes. TFTs with mobility values of ∼25 cm2 V-1 s-1 and current on/off ratios >104 were obtained, performance similar to that of evaporated metal contacts in analogous devices.

Fully Printed and Encapsulated SWCNT-Based Thin Film Transistors via a Combination of R2R Gravure and Inkjet Printing.

Fully printed thin film transistors (TFT) based on poly(9,9-di-n-dodecylfluorene) (PFDD) wrapped semiconducting single walled carbon nanotube (SWCNT) channels are fabricated by a practical route that combines roll-to-roll (R2R) gravure and ink jet printing. SWCNT network density is easily controlled via ink formulation (concentration and polymer:CNT ratio) and jetting conditions (droplet size, drop spacing, and number of printed layers). Optimum inkjet printing conditions are established on Si/SiO2 in which an ink consisting of 6:1 PFDD:SWCNT ratio with 50 mg L-1 SWCNT concentration printed at a drop spacing of 20 µm results in TFTs with mobilities of ∼25 cm2 V-1 s-1 and on-/off-current ratios > 105. These conditions yield excellent network uniformity and are used in a fully additive process to fabricate fully printed TFTs on PET substrates with mobility values > 5 cm2 V-1 s-1 (R2R printed gate electrode and dielectric; inkjet printed channel and source/drain electrodes). An inkjet printed encapsulation layer completes the TFT process (fabricated in bottom gate, top contact TFT configuration) and provides mobilities > 1 cm2 V-1 s-1 with good operational stability, based on the performance of an inverter circuit. An array of 20 TFTs shows that most have less than 10% variability in terms of threshold voltage, transconductance, on-current, and subthreshold swing.

Analysis Method for Quantifying the Morphology of Nanotube Networks.

While atomic force microscopy (AFM) is a powerful technique for imaging assemblies and networks of nanoscale materials, approaches for quantitative assessment of the morphology of these materials are lacking. Here we present a volume-based approach for analyzing AFM images of assemblies of nano-objects that enables the extraction of relevant parameters describing their morphology. Random networks of single-walled carbon nanotubes (SWCNTs) deposited via solution-phase processing are used as an example to develop the method and demonstrate its utility. AFM imaging shows that the morphology of these networks depends on details of processing and is influenced by choice of substrate, substrate cleaning method, and postdeposition rinsing protocols. A method is outlined to analyze these images and extract relevant parameters describing the network morphology such as the density of SWCNTs and the degree to which tubes are bundled. Because this volume-based approach depends on accurate measurements of the height of individual tubes and their networks, a procedure for obtaining reliable height measurements is also discussed. Obtaining quantitative parameters that describe the network morphology allows going beyond qualitative descriptions of images and will facilitate optimizing network preparation methods based on measurable criteria and correlating performance with morphology.

Enrichment of large-diameter semiconducting SWCNTs by polyfluorene extraction for high network density thin film transistors.

A systematic study on the use of 9,9-dialkylfluorene homopolymers (PFs) for large-diameter semiconducting (sc-) single-walled carbon nanotube (SWCNT) enrichment is the focus of this report. The enrichment is based on a simple three-step extraction process: (1) dispersion of as-produced SWCNTs in a PF solution; (2) centrifugation at a low speed to separate the enriched sc-tubes; (3) filtration to collect the enriched sc-SWCNTs and remove excess polymer. The effect of the extraction conditions on the purity and yield including molecular weight and alkyl side-chain length of the polymers, SWCNT concentration, and polymer/SWCNT ratio have been examined. It was observed that PFs with alkyl chain lengths of C10, C12, C14, and C18, all have an excellent capability to enrich laser-ablation sc-SWCNTs when their molecular weight is larger than ∼10 000 Da. More detailed studies were therefore carried out with the C12 polymer, poly(9,9-di-n-dodecylfluorene), PFDD. It was found that a high polymer/SWCNT ratio leads to an enhanced yield but a reduced sc-purity. A ratio of 0.5-1.0 gives an excellent sc-purity and a yield of 5-10% in a single extraction as assessed by UV-vis-NIR absorption spectra. The yield can also be promoted by multiple extractions while maintaining high sc-purity. Mechanistic experiments involving time-lapse dispersion studies reveal that m-SWCNTs have a lower propensity to be dispersed, yielding a sc-SWCNT enriched material in the supernatant. Dispersion stability studies with partially enriched sc-SWCNT material further reveal that m-SWCNTs : PFDD complexes will re-aggregate faster than sc-SWCNTs : PFDD complexes, providing further sc-SWCNT enrichment. This result confirms that the enrichment was due to the much tighter bundles in raw materials and the more rapid bundling in dispersion of the m-SWCNTs. The sc-purity is also confirmed by Raman spectroscopy and photoluminescence excitation (PLE) mapping. The latter shows that the enriched sc-SWCNT sample has a narrow chirality and diameter distribution dominated by the (10,9) species with d = 1.29 nm. The enriched sc-SWCNTs allow a simple drop-casting method to form a dense nanotube network on SiO2/Si substrates, leading to thin film transistors (TFTs) with an average mobility of 27 cm(2) V(-1) s(-1) and an average on/off current ratio of 1.8 × 10(6) when considering all 25 devices having 25 µm channel length prepared on a single chip. The results presented herein demonstrate how an easily scalable technique provides large-diameter sc-SWCNTs with high purity, further enabling the best TFT performance reported to date for conjugated polymer enriched sc-SWCNTs.

Highly sensitive electrical detection of TCNE on chemically passivated silicon-on-insulator.

Adsorption of tetracyanoethylene (TCNE) onto hydrogen terminated, n-type silicon-on-insulator is shown to cause significant depletion of majority carriers. Employing an ambient pseudo-MOSFET, ppm levels of TCNE vapour rapidly decrease the n-channel saturation current by at least two orders of magnitude. Covalent passivation with a decyl monolayer improves the reversibility of the response while only slightly decreasing the sensitivity.

Electro-optic investigation of the surface trapping efficiency in n-alkanethiol SAM passivated GaAs(001).

The electro-optic characteristics of the semi-insulating and n(+)-type GaAs(001) surfaces passivated with n-alkanethiol self-assembled monolayers were investigated using Kelvin probe surface photovoltage (SPV) and photoluminescence (PL) techniques. Referencing the equilibrium surface barrier height established in an earlier report, SPV measurements demonstrated a significant (>100 mV) increase in the non-equilibrium band-bending potential observed under low-level photo-injection. Modeling of the SPV accounts for these observations in terms of a large (>10(4)) decrease in the hole/electron ratio of surface carrier capture cross-sections, which is suggested to result from the electrostatic potential of the interfacial dipole layer formed upon thiol chemisorption. The cross-section effects are verified in the high-injection regime based on carrier transport modeling of the PL enhancement manifested as a reduction of the surface recombination velocity.

Diazonium-derived aryl films on gold nanoparticles: evidence for a carbon-gold covalent bond.

Tailoring the surface chemistry of metallic nanoparticles is generally a key step for their use in a wide range of applications. There are few examples of organic films covalently bound to metal nanoparticles. We demonstrate here that aryl films are formed on gold nanoparticles from the spontaneous reduction of diazonium salts. The structure and the bonding of the film is probed with surface-enhanced Raman scattering (SERS). Extinction spectroscopy and SERS show that a nitrobenzene film forms on gold nanoparticles from the corresponding diazonium salt. Comparison of the SERS spectrum with spectra computed from density functional theory models reveals a band characteristic of a Au-C stretch. The observation of this stretch is direct evidence of a covalent bond. A similar band is observed in high-resolution electron energy loss spectra of nitrobenzene layers on planar gold. The bonding of these types of films through a covalent interaction on gold is consistent with their enhanced stability observed in other studies. These findings provide motivation for the use of diazonium-derived films on gold and other metals in applications where high stability and/or strong adsorbate-substrate coupling are required.

Molecular modulation of conductivity on H-terminated silicon-on-insulator substrates.

The adsorption of a range of molecular species (water, pyridine, and ammonia) is found to reversibly modulate the conductivity of hydrogen-terminated silicon-on-insulator (H-SOI) substrates. Simultaneous sheet-resistance and Hall-effect measurements on moderately doped (10(15) cm(-3)) n- and p-type H-SOI samples mounted in a vacuum system are used to monitor the effect of gas exposure in the Torr range on the electrical-transport properties of these substrates. Reversible physisorption of "hole-trapping" species, such as pyridine (C(5)H(5)N) and ammonia (NH(3)) produces highly conductive minority-carrier channels (inversion) on p-type substrates, mimicking the action of a metallic gate in a field-effect transistor. The adsorption of these same molecules on n-type SOI induces strong electron-accumulation layers. Minority/majority channels are also formed upon controlled exposure to water vapor. These observations can be explained by a classical band-bending model, which considers the adsorbates as the source of a uniform surface charge ranging from +10(11) to +10(12)q cm(-2). These results demonstrate the utility of DC transport measurements of SOI platforms for studies of molecular adsorption and charge-transfer effects at semiconductor surfaces.

Direct stabilization of a phospholipid monolayer on H-terminated silicon.

This Article describes a strategy to stabilize a phospholipid monolayer directly on the surface of a H-terminated silicon substrate in order to provide a useful platform for silicon based biosensors. The stabilization of an acrylated phospholipid monolayer is obtained by two-dimensional chain polymerization. As the formation of the lipid monolayer in aqueous solution competes with the oxidation of the silicon surface, several cycles of oxide removal and lipid exposure are necessary to densify the lipid layer. Lipid monolayer formation is followed by Fourier transform infrared spectroscopy. The resulting monolayer is denser than corresponding alkyl monolayers formed on H-terminated silicon via photochemical or thermally initiated reactions.

Surface dipole layer potential induced ir absorption enhancement in n-alkanethiol SAMs on GaAs(001).

The work function of n-alkanethiol self-assembled monolayers (SAMs) prepared on the GaAs(001) surface was measured using the Kelvin probe technique yielding the SAM 2D dipole layer potential (DLP). Direct n-dependent proportionality between the DLP values and the C-H stretching mode infrared (IR) absorption intensities was observed, which supports a correspondence of reported IR enhancements with the electrostatic properties of the interface. X-ray photoelectron spectroscopy is also used to verify the work function measurements. In addition, the principal components of the refractive index tensor are shown to be n-invariant in the ordered SAM phase. Our results suggest that a local field correction to the transition dipole moment accounts for the observed increase in IR activity through an increase to the electronic polarizability.

Evidence for initiation of thermal reactions of alkenes with hydrogen-terminated silicon by surface-catalyzed thermal decomposition of the reactant.

New insights into the mechanism of thermal reactions of alkenes with hydrogen terminated silicon are presented. Scanning tunneling microscopy (STM) imaging at the early stages of the reaction of 1-decene with H/Si(111) at 150 degrees C confirm this reaction occurs via a propagating radical chain mechanism. In addition, evidence is presented for an initiation mechanism involving degradation of hydrocarbon molecules catalyzed by the silanol surface of Schlenk tubes commonly used in carrying out these reactions. Hydrogen-terminated silicon surfaces are found to be unstable in the "inert" solvent dodecane when heated at 150 degrees C in a Pyrex Schlenk tube. By contrast, the surfaces were significantly more stable at the same temperature when reactions were carried out in Teflon (polytetrafluoroethylene or PTFE). The thermal reaction of decene with H/Si(111) was found to proceed more rapidly in Pyrex than in PTFE, consistent with an impurity-based initiation mechanism.

Sequential reactions with amine-terminated monolayers and isolated molecules on H/Si(111).

A simple method for preparing monolayers with terminal amine functionality is demonstrated. A gas-phase photochemical reaction of 1,3-diaminopropane with a H-terminated Si(111) surface results in the molecules covalently attaching to the surface, primarily through the formation of a Si--N bond. These monolayers are characterized by scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS). The reactivity of the terminal amine is confirmed by exposing the monolayer to benzaldehyde, resulting in the formation of an imine link and the grafting of phenyl rings onto the surface. For short irradiation times, this reaction leads to the formation of isolated amine groups on an otherwise pristine H-terminated surface. STM and HREELS studies of the benzaldehyde reaction on these low-coverage surfaces (less than 0.005 monolayers) indicate that the reaction is restricted to the reactive amine groups, leaving the remainder of the surface unaffected. This simple approach for a sequential coupling reaction is expected to facilitate attachment of more complex molecules (molecular switches, biomolecules) for single-molecule STM studies.

Reaction of alkenes with hydrogen-terminated and photooxidized silicon surfaces. A comparison of thermal and photochemical processes.

Reagentless micropatterning of hydrogen-terminated Si(111) via UV irradiation through a photomask has proven to be a convenient strategy for the preparation of ordered bicomponent monolayers. The success of this technique relies upon the differential rate of reaction of an alkene with the hydrogen-terminated and photooxidized regions of the surface. Monolayer formation can be accomplished under either thermal or photochemical conditions. It was observed that, after 3 h, reaction in neat alkene solution irradiation (Rayonet, 300 nm) afforded the expected patterned surface, while thermal conditions (150 degrees C) resulted in a partial loss of pattern fidelity. Monolayer properties and formation were studied on oxidized and hydrogen-terminated silicon under thermal and photochemical initiation, by contact angle, ellipsometry, Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy. Results show that alkenes add to silanol groups on the silica surface in a manner consistent with acid catalysis: once attached to the surface, the silica oxidized the hydrocarbon.