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A major challenge in upscaling and commercializing dye-sensitized solar cells (DSSC) is to simultaneously rely on a robust photoconversion efficiency (PCE) along with long-term device stability. Two-dimensional (2D) nanocarbon materials are known to improve the PCE and stability of devices made of hybrid nanocomposite photoanodes. However, there is inadequate knowledge on how the morphology of 2D nanocarbon materials plays an important role in improving photovoltaic (PV) performance and stability. Here we report the comparative effect on the PV performance and the long-term stability of DSSCs made of optimal loadings of few-layer graphene (FLG) flakes and graphene nanoribbons (GNR) into the mesoporous TiO₂ active layer. DSSCs were fabricated by using standard nanocrystalline TiO₂, GNR-TiO₂ and FLG-TiO₂ hybrid mesoporous films as anodes and subjected to continuous visible light irradiation to monitor their long-term stability. Results show that the optimized incorporation of GNR (0.005 wt%) and FLG (0.010 wt%) in the mesoporous TiO₂ active layer enhances the PCE by 34% and 21% compared to pristine TiO₂, respectively. Moreover, the operational stability of hybrid devices shows a 51.6% (for GNR-TiO₂) and a 10% (for FLG-TiO₂) sustained higher PCE than the device based on bare TiO₂, after 273 h of continuous one sunlight soaking. The electrochemical impedance spectroscopy (EIS) and transient photovoltage decay were studied to investigate how GNR strikingly reduces degradation of device performance as compared to its counterpart FLG-TiO₂ and TiO₂ based cells. This significant enhancement can be attributed to the structural modifications induced by GNR within the TiO₂ photoanode, which improves the electron lifetime and reduces non-radiative carrier recombination.

A series of well-defined polymerized ionic liquid (PIL) statistical and block copolymers consisting of ionic liquid monomer, 1-(4-vinylbenzyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide, ...

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 (

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.

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.

Colloidal semiconductor quantum dots (QDs) are considered as promising building blocks to fabricate efficient and stable optoelectronic devices, thanks to their size/shape/composition-dependent electronic and optical properties based on the quantum confinement effect. However, inadequate light absorption and undesirable charge recombination events still limit the efficiency and long-term stability of solar energy to fuel conversion in QD-based photoelectrochemical (PEC) cells. In this work, we engineer the optoelectronic properties and band alignment in colloidal heterostructured CdS/CdSe core/shell QDs by tuning the shell thickness. Starting with a 3.0 nm CdS QD core (in diameter), we investigate the changes in structural and optical properties as a function of CdSe shell thickness (0.6–1.9 nm). We show that the optimization of the shell thickness can significantly broaden the light absorption range towards longer wavelengths and enhance the rate of photoelectron separation and transport in photoanodes made of QDs sensitized TiO2 mesoporous film. Complemented by theoretical modeling, we find that the band alignment in this heterostructured system can be engineered from quasi type-II (electrons are localized over the entire QDs but holes are mostly delocalized in shell) to inverted type-I (both electrons and holes are largely delocalized in shell) by changing the shell thickness. As a proof-of-concept, employing QDs with optimal shell thickness of 1.6 nm together with carbon nanotube doped TiO2 as photoanode in PEC cells, we obtained a very high photocurrent density of ~ 16.0 mA/cm2 (at 0.9 V vs. the reversible hydrogen electrode, RHE), which is the highest value ever reported for PEC cells based on CdS/CdSe QDs. We also observed an excellent long-term stability (maintaining 83% of its initial value after four hours) under one sun illumination (AM 1.5G, 100 mW/cm2). These results indicate that optimizing the design and band engineering of heterostructured core/shell QDs is a facile and efficient approach to enhance the performance of QDs-based PEC cells and other optoelectronic devices.

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&ndash, 600 &deg, 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.

Atomic scale defects in semiconductors enable their technological applications and realization of novel quantum states. Using scanning tunneling microscopy and spectroscopy complemented by ab-initio calculations we determine the nature of defects in the anisotropic van der Waals layered semiconductor ReS$_2$. We demonstrate the in-plane anisotropy of the lattice by directly visualizing chains of rhenium atoms forming diamond-shaped clusters. Using scanning tunneling spectroscopy we measure the semiconducting gap in the density of states. We reveal the presence of lattice defects and by comparison of their topographic and spectroscopic signatures with ab initio calculations we determine their origin as oxygen atoms absorbed at lattice point defect sites. These results provide an atomic-scale view into the semiconducting transition metal dichalcogenides, paving the way toward understanding and engineering their properties., 9 pages, 4 figures; Supp 5 pages, 4 figures

Improving the conversion efficiency of dye-sensitized solar cells (DSSCs) requires enhancing the photogeneration of charge carriers as well as facilitating their transport to electrodes before charge recombination or quenching can occur. Here we describe a simple, fast and large-area scalable procedure for the preparation of a nanocomposite made of functional gold nanoparticles (AuNPs) and multiwall carbon nanotubes (MWCNTs) to improve the performance of DSSCs. We fabricated AuNP/MWCNT inlaid mesoporous TiO2 films as photoanodes in DSSCs, to improve crucial factors including light absorption, charge-carrier generation, collection and transport. By using a AuNP/MWCNT nanocomposite directly inlaid in TiO2 as the working electrode, a power conversion efficiency (PCE) of 6.61% and short-circuit photocurrent density (Jsc) of 12.26 mA cm−2 were obtained, representing an enhancement of ~31% in PCE and ~19% in Jsc compared to a control cell based on TiO2 alone. In addition, DSSCs based on the TiO2/AuNP/MWCNT photoanode remained remarkably stable compared with the control device, retaining 92% of the initial PCE value after ten days of continuous illumination.

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 with99.9% 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.

Cyanoethylated pullulan (CEP), a high-k solution processable polymer gate dielectric, is used to fabricate bottom gated single wall carbon nanotube (SWCNT) network thin film transistors (TFTs). Both aqueous and organic dispersions of highly semiconducting enriched SWCNTs are used as the channel material. Use of CEP as the dielectric enables fabrication of devices operating at low voltage (1 Hz) is found to significantly decrease the transconductance. This is shown to be related to a significant frequency dependence of the capacitance associated with a slow polarization response of the dielectric. Despite this limitation, CEP could be a useful dielectric in SWCNT TFTs for applications such as sensors and low frequency amplifiers.

Al-doped ZnO (AZO) is a promising earth-abundant alternative to Sn-doped In2O3 (ITO) as an n-type transparent conductor for electronic and photovoltaic devices. We have deposited AZO films with resistivities as low as 1.1 × 10−3 Ω·cm by atomic layer deposition (ALD) using trimethylaluminum (TMA), diethylzinc (DEZ), and water at 200 °C. The work functions of the films were measured using a scanning Kelvin probe (sKP) to investigate the role of aluminum concentration. The work function of AZO films prepared by two different ALD recipes were compared: a “Al-terminated” recipe and a “ZnO-terminated” recipe. As aluminum doping increases, the Al-terminated recipe produces films with a consistently higher work function than the ZnO-terminated recipe. The resistivity of the Al-terminated recipe films shows a minimum at a 1:16 Al:Zn atomic ratio and using a ZnO-terminated recipe, minimum resistivity was seen at 1:19. The film thicknesses were characterized by ellipsometry, chemical composition by EDX, and resistivity by a four-point probe.

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 (104 were obtained, performance similar to that of evaporated metal contacts in analogous devices.

Al-doped ZnO (AZO) is a promising earth-abundant alternative to Sn-doped In2O3 (ITO) as an n-type transparent conductor for electronic and photovoltaic devices; AZO is also more straightforward to deposit by atomic layer deposition (ALD). The workfunction of this material is particularly important for the design of optoelectronic devices. We have deposited AZO films with resistivities as low as 1.1 x 10-3 Ωcm by ALD using the industry-standard precursors trimethylaluminum (TMA), diethylzinc (DEZ), and water at 200◦C. These films were transparent and their elemental compositions showed reasonable agreement with the pulse program ratios. The workfunction of these films was measured using a scanning Kelvin Probe (sKP) to investigate the role of aluminum concentration. In addition, the workfunction of AZO films prepared by two different ALD recipes were compared: a “surface” recipe wherein the TMA was pulsed at the top of each repeating AZO stack, and a interlamellar recipe where the TMA pulse was introduced halfway through the stack. As aluminum doping increases, the surface recipe produces films with a consistently higher workfunction as compared to the interlamellar recipe. The resistivity of the surface recipe films show a minimum at a 1:16 Al:Zn atomic ratio and using an interlamellar recipe, minimum resistivity was seen at 1:19. The film thicknesses were characterized by ellipsometry, chemical composition by EDX, and resistivity by four-point probe.

We report on a fast and simple protocol for the electrodeposition of Fe-MnOx films used as catalysts for the water oxidation (WO) reaction at neutral pH, and showing the beneficial effect of iron in terms of both activity and stability of the catalyst. While most electrodeposited MnOx WO catalysts are obtained starting from Mn(II) precursors, the proposed protocol consists in the galvanostatic cathodic deposition of Fe-MnOx onto conductive FTO glasses, using KMnO4 and Fe(NO3)3 as Mn and Fe precursors, respectively. In the absence of Fe, the Tafel slope drastically increases from 103 to 270 mV dec−1 when passing from low to high overpotentials. The slope change, instead, is progressively reduced when the Fe precursor is added to the deposition solution and a constant slope of 105 mV dec−1 is obtained in the whole overpotential range with an optimal Fe concentration of 1.00 mM, Accordingly, Electrochemical Impedance Spectroscopy (EIS) shows that Fe addition improves both charge transfer and transport properties of the electrodeposited films. In particular, a five-fold decrease in charge transfer resistance at the catalyst/electrolyte interface was observed, suggesting a more facile oxygen evolving kinetics for Fe containing samples. Furthermore, the lower the iron content, the lower the film stability, as pointed out by chronopotentiometric measurements, and confimed by FESEM analysis and EIS as measured both before and after water electrolysis experiments. To establish structure activity relationships, an extended characterization of the electrodeposited films was carried out by means of Raman Microscopy, Transmission Electron Microscopy, UV–vis and X-ray Photoelectron Spectroscopies. The ensemble of the characterization results suggests that Fe3+ ions are actually incorporated within the electrodeposited film, with limited effects on the final Fe-MnOx structure, consisting in a defective MnO2 birnessite-type structure with significant fraction of surface Mn3+ species.

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.

For the first time, a CMOS imager has been used to detect the presence of radio-labeled target biomolecules captured on a functionalized surface. A CMOS image sensor from an SLR camera was used. Using aptamer functionalization the system successfully detected $^{32}{\rm P}$ labeled adenosine triphosphate (ATP) at concentrations well below those typically associated with fluorescence-based sensor architectures. The system has also demonstrated its amenability to multiplexed/spotted biomolecule detection. GEANT4, a Monte Carlo toolkit for simulating the passage of radiation through matter, was used to analyze the detector. Radiolabel detection offers the potential for direct quantitative measurement of bound biomolecules which is advantageous compared to the use of fluorescent tags.

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.

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. © 2011 American Chemical Society.

Field-effect sensors with hafnium silicate gate insulators were fabricated following standard CMOS processes and compatible post-processes. The floating gates were functionalized with glycidoxypropyltrimethoxy silane and amine-terminated oligonucleotide probes. The pH sensitivity of the silicate surface decreased from 31.4 mV/pH to 24.6 mV/pH after functionalization due to passivation of a significant fraction of the amphoteric proton binding sites. A threshold voltage shift of 33 mV was observed as the buffer solution concentration changed from 0.015 M to 0.3 M after DNA probe molecule attachment. The hybridization caused a net increase in this shift of 3mV. With the help of a model which includes the solution of the Poisson-Boltzmann and proton site-binding at the gate, methods of achieving higher sensitivity are discussed., 214th ECS Meeting, October 12-17, 2008, Honolulu, HI

A simple chemical strategy is described to produce branched alkyl chains on Si(111) from the reaction of an ester-terminated silicon surface. The stability of the silicon surfaces with linear and branched monolayers is characterized by electrochemical impedance, Kelvin probe, and high-resolution electron energy loss spectroscopy (HREELS). The direct observation of surface states in capacitance-voltage plots can be used to monitor the growth of electrically active defects associated with oxidation of the silicon substrate. We find that the total surface state density of the freshly made surfaces increases in the following order: Si-B < Si-UDE < Si-C10 (where B is the branched structure, UDE is ethyl undecanoate, and C10 is decyl) in aqueous and organic solvent/electrolyte systems. After 24 h in the electrolyte solution, the surface state densities increase but the order remains the same. The branched structure is significantly more resistant to oxidation. These observations are consistent with the results of other characterization techniques including HREELS and surface photovoltage and indicate that the branched alkyl chain-modified surfaces are considerably more stable, especially in aqueous electrolytes, making them suitable for future use in biological sensor applications.

A new method for the formation of molecular monolayers on silicon surfaces utilizing gas-phase photochemical reactions is reported. Hydrogen-terminated Si(111) surfaces were exposed to various gas-phase molecules (hexene, benzaldehyde, and allylamine) and irradiated with ultraviolet light from a mercury lamp. The surfaces were studied with in situ Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and scanning tunneling microscopy. The generation of gas-phase radicals was found to be the initiator for organic monolayer formation via the abstraction of hydrogen from the H/Si(111) surface. Monolayer growth can occur through either a radical chain reaction mechanism or through direct radical attachment to the silicon dangling bonds.

The modification of Si(111) surfaces with Si-C linked alkyl monolayers terminated by electrochemically polymerizable di(2-thienyl)carbinol moieties has been investigated by different spectroscopic techniques including IR spectroscopy, X-ray photoelectron spectroscopy (XPS), and high-resolution electron energy loss spectroscopy (HREELS). These surfaces were prepared from the photochemical reaction of ethyl undecylenate with hydrogen-terminated Si(111), followed by the conversion of the produced terminal ester groups into the tertiary alcohol using 2-thienyllithium. Despite the steric constraints induced by the aromatic rings in the monolayer, the absolute surface coverage of the organic chains was found to be high, ca. 40%. Differential capacitance measurements revealed that the derivatization procedure introduced surface states probably due to some oxidation of the Si(111) surface occurring during the photochemical step. The photopotentiodynamical oxidation of the modified Si(111) surfaces in the presence of thiophene yielded strongly adherent and smooth conducting polythiophene films. These photoelectrogenerated hybrid polythiophene-silicon junctions were then characterized by ex situ electrical measurements (dc transport, ac impedance, and surface potential measurements). The expected diode behavior due to the formation of a Schottky barrier at the alkyl/Si(111) interface was clearly observed although the rectification was rather weak and the characteristics highly nonideal. The surface states at the silicon interface were found to play a major role in determining the electrical properties of such junctions.

Covalent chemical modification of Si(111) surfaces by attachment of organic monolayers with terminal tert-butylhydrazide groups is described. Second harmonic generation studies of the hydrazide-modified surfaces in air show the presence of an electric field induced second harmonic (EFISH) response, indicative of charge trapping at the surface. Charge trapping is associated with the hydrazide group and is not observed for other organic monolayers or oxidized silicon surfaces. Charging is reversible and can be modulated by variation of the gaseous ambient of the sample, including particularly oxygen and water vapor. An interpretation is given in terms of ionosorption of O 2 as an electron trap, with strong stabilization of the electron affinity level of O 2 in the monolayer by polar hydrazide groups and adsorbed water molecules. There is also an indication that the hydrazide group may become ionized, as an electron donor. From the dependence of EFISH on laser irradiance and the observation of a transient EFISH response, it is suggested that ionosorption of O 2 may occur spontaneously in the dark and, in addition, is transiently enhanced by laser photoexcitation of silicon. By comparison with EFISH and high-resolution electron energy loss measurements on oxidized silicon surfaces, the density of charge trapped at hydrazide-modified surfaces has been estimated.

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.

We compare scanning tunneling microscope (STM) measurements on the adsorption of benzene on Si(1 0 0) with first principles density functional methods. Adsorption energies and preferred adsorption sites are determined by DFT calculations. The tunnel current is calculated within the transfer-Hamiltonian picture. We show that adsorption energies are reproduced remarkably well by simulations. STM images and line scans are reproduced qualitatively. We discuss the chemical nature of the tip, and we show that the tip used in experiments is consistent with a tungsten terminated tip. (C) 2001 Elsevier Science B.V. All rights reserved.

The current carrying capacity of individual organic molecules covalently bound to silicon has been studied. Adsorbates consisting exclusively of saturated CC bonds were found to be entirely stable whereas adsorbates containing π bonds could be controllably dislodged under modest conditions. The π bonds act as a chromophore, taking energy from a scattered electron, energy that can be selectively channeled into SiC bond breaking. The class of adsorbates that are dislodged is closely related to unsaturated molecules widely investigated for enhanced molecular wire character. It is predicted that molecular devices containing such molecules will fail when operated. Measures to avoid current-induced bond breaking are described. New processes based upon the controlled bond breaking phenomenon are suggested.

Organic molecular adsorption on Si(100) is modelled using a combination of semi-empirical, Hartree–Fock and density functional theory molecular orbital methods together with a cluster model of a surface. Various adsorption configurations for benzene on Si(100) are considered. For each, fully optimized structures and absolute adsorption energies are calculated. Valence region occupied state current density iso-surfaces are calculated to simulate constant current STM images. Detailed comparison with experimental energetics and STM images allows unambiguous identification of three room temperature stable structures.

Electron energy loss spectroscopy (EELS) has been used to observe size dependent changes in the electronic states of isolated, nanocrystalline carbon particles supported on SiO{sub 2} substrates. The structure and size distribution of the particles have been characterized by Raman scattering, Auger electron spectroscopy, and electron microscopy. EELS observations indicate that a semimetal to semiconductor transition occurs for particles smaller than 1nm. In addition, hydrogen adsorption is found to significantly alter the electronic states, demonstrating that both finite size and dangling bond effects modify the properties of small carbon nanocrystallites. {copyright} {ital 1998} {ital The American Physical Society}

Scanning tunneling microscopy (STM), Fourier-transform infrared spectroscopy and semiempirical quantum cluster calculations have been used to investigate the chemisorption of benzene on the Si(100)(2×1) surface. Room temperature adsorption results in the occupation of multiple bonding configurations including on-top of a single Si dimer and two different bridging geometries involving interaction with two Si dimers. While the single dimer state is populated preferentially upon adsorption, it is observed to be metastable with respect to the bridging configuration. The single dimer to bridge conversion is activated, with a barrier of 0.94 eV. The single dimer geometry can be repopulated via a process assisted by the STM tip.

Scanning tunneling microscopy (STM) has been used to investigate the adsorption of benzene on the Si(100)(2×1) surface. At room temperature two types of bonding configurations are observed; on-top of a single Si dimer and bridging two dimers. Upon dosing the single dimer state is populated preferentially but is seen to be metastable with respect to the bridging configuration, in agreement with semi-empirical quantum cluster calculations. The single dimer to bridge conversion is activated, with a barrier of 0.95 eV. The single dimer state can be re-populated via a process assisted by the STM tip.

The gas phase anhydrous reaction of glycidoxypropyldimethylethoxysilane (GPDMES) with a model hydroxylated surface has been investigated using high-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM). Water dissociation on the clean reconstructed (2 × 1)-Si(100) surface was used to create an atomically flat surface with ~ 0.5 ML of hydroxyl groups. Exposure of this surface to GPDMES at room temperature under vacuum was found to lead to formation of covalent Si–O–Si bonds although high exposures (6 × 10 8 L) were required for saturation. STM images at the early stages of reaction indicate that the reaction occurs randomly on the surface with no apparent clustering. The STM images together with semi-empirical (AM1) calculations provide evidence for hydrogen bonding interactions between the oxygen atoms in the molecule and surface hydroxyl groups at low coverage.

We report a silicon-on-insulator ring resonator biosensor array with one output port, using wavelength division multiplexing as the addressing scheme. With the use of on-chip referencing for environmental drift cancellation, simultaneous monitoring of multiplexed molecular bindings is demonstrated, with a resolution of 0.3 pg/mm(2) (40 ag of total mass) for protein concentrations over 4 orders of magnitude down to 20 pM. Reactions are measured over time periods as long as 3 h with high stability.

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.

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. © 2011 The Royal Society of Chemistry.

Accumulation mode pseudo-MOSFETs formed on hydrogen terminated silicon-on-insulator (SOI-H) were used to probe molecular adsorption and reaction events. Current-voltage characteristics of such n-channel devices are found to be sensitive to the environment, with the accumulation threshold voltage, or flat-band voltage, exhibiting large reversible changes upon cycling between ambient atmosphere, high vacuum (≤10-5 Torr), and exposure to water and pyridine vapor at pressures in the Torr range. The field-effect mobility is found to be comparatively less affected through these transitions. Oxidation of the H-terminated surface in ambient conditions leads to irreversible shifts in both the flat-band voltage and the field-effect mobility. A photochemical gas phase reaction with decene is used to form a decyl monolayer on the SOI(100)-H surface. Formation of this monolayer is found to result in a relatively small shift of the threshold voltage and only a slight degradation of the field effect mobility, suggesting that alkyl monolayer dielectrics formed in this way could function as good passivating dielectrics in field effect sensing applications. © 2011 American Institute of Physics.

We report the surface chain reaction of styrene with hydrogen-terminated Si(111), H-Si(111) initiated at isolated surface dangling bonds. Electrons from a scanning tunneling microscope in ultrahigh vacuum are used to create surface isolated dangling bonds on H-Si(111) prepared by aqueous ammonium fluoride etch. Exposure of dangling bonds on an otherwise hydrogen terminated surface to as little as 5 langmuirs of styrene leads to the formation of compact islands containing multiple styrene adsorbates bonded to the surface through individual C-Si bonds. From this observation we conclude that these adsorbate islands are the result of a surface chain reaction of styrene with H-Si(111). This result suggests that the key radical-chain propagation step of hydrogen atom abstraction from the silicon surface for the proposed mechanism of reaction of liquid phase terminally unsaturated organic molecules with hydrogen-terminated silicon under free-radical conditions readily occurs.

We discuss several applications of both resonant and non-resonant subwavelength gratings (SWGs) for silicon photonics. We present results of evanescent field molecular sensing using the transverse magnetic mode of a 0.22 mm thick silicon slab waveguide with a resonant SWG, which couples a free space laser beam to the silicon waveguide mode. The optical readout of this configuration is almost identical to the established surface plasmon resonance sensing technology. Using calibrated sucrose solutions, we demonstrate a bulk refractive index sensitivity of 111 nm/RIU in good agreement with rigorous coupled wave analysis calculations. The binding of a monolayer of streptavidin protein on the waveguide surface is monitored in real time with a signal-to-noise ratio of ~500. In another application, non-resonant SWGs are used to create effective dielectric media with a refractive index that can be tuned between the values of silicon (3.48) and SU-8 polymer used for the cladding (1.58). For example, we present SWG waveguides with an effective core index of approximately 2.65, which exhibit lower propagation loss than photonic wire waveguides of similar dimensions. We use these SWG waveguides to demonstrate highly efficient fiber-chip couplers., Silicon Photonics V, January 24-27, 2010, San Francisco, California, USA, Series: SPIE Proceedings

We present an SOI biosensor microarray chip that allows multiple molecular binding reactions to be simultaneously monitored. The individual biosensors are formed using 0.26 × 0.45 μm 2 silicon photonic wire waveguides, which are arranged in compact Mach-Zehnder interferometer geometries with near temperature independent response. The sharp bend radius permitted by the photonic wires is exploited to form dense spiral waveguide structures that provide several millimeters of path length in a compact 130 μm diameter circular area. This design provides the high sensitivity of a long waveguide, while maintaining compatibility with commercial microarray spotting tools. For low volume analyte delivery the sensor array chip contains a monolithically integrated microfluidic channel formed in an SU-8 overlayer. Multiple antibody-antigen reactions are observed in real-time by using an infrared camera to monitor the optical powers emerging from the sensor array output waveguides.

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.

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.

We present experimental and theoretical results of label-free molecular sensing with the TM mode of a 0.22 µm thick silicon slab waveguide used in a guided mode resonance configuration. Due to the strong overlap of the evanescent field of the waveguide mode with a molecular layer attached to the surface, these sensors exhibit high sensitivity, while their fabrication and packaging requirements are minimal. Experimentally, we demonstrate a resonance wavelength shift of ~1 nm when a monolayer of streptavidin is attached to the surface, in good agreement with calculations based on rigorous coupled wave analysis.

We demonstrate Mach-Zehnder interferometer and ring resonator evanescent field sensors fabricated on the silicon-on-insulator material platform. These devices exploit the strong evanescent field of the transverse magnetic mode of a high index contrast, submicron dimension waveguide to obtain strong interaction of the guided mode with biomolecules adsorbed to the waveguide surface. We utilize the extremely small bend radius achievable with silicon photonic wire waveguides to fabricate spiral and folded waveguide sensors that maintain the high sensitivity of a long waveguide, while providing compact footprint. These devices offer a suitable geometry for the development of sensor arrays and provide compatibility with commercial spotting functionalization systems. We demonstrate sensors containing waveguides of up to 5.5 mm length that are contained within a 150 μm × 150 μm area.

We demonstrate a new silicon photonic wire waveguide evanescent field (PWEF) sensor that exploits the strong evanescent field of the transverse magnetic mode of this high-index-contrast, submicrometer-dimension waveguide. High sensitivity is achieved by using a 2 mm long double-spiral waveguide structure that fits within a compact circular area of 150 microm diameter, facilitating compatibility with commercial spotting apparatus and the fabrication of densely spaced sensor arrays. By incorporating the PWEF sensor element into a balanced waveguide Mach-Zehnder interferometer circuit, a minimum detectable mass of approximately 10 fg of streptavidin protein is demonstrated with near temperature-independent response.

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.

The sheet resistance of hydrogen terminated silicon-on-insulator substrates increases significantly with time in air due to depletion of free carriers, attributed to the growth of electrically active defects as the surface oxidizes. Surprisingly, physisorbed water (via adsorption from ambient or controlled exposure in vacuum) causes an increase in the conductivity. This effect is largely reversible when the water layer is displaced by inert gas purging, heating, or pumping. The observed conductivity changes are correlated with Hall voltage changes, indicating that the adsorbed water layer induces accumulation of majority carriers on n-doped substrates.

The adsorption of organic molecules on solid substrates is a fundamental step in many important heterogeneous catalytic processes, and is also becoming an increasingly significant aspect of surface modification in microelectronics and sensing technology. The conformation of adsorbed molecules not only influences the outcome of surface-catalysed reactions but also becomes important for recognition processes involved in chemical sensors. The scanning tunnelling microscope (STM) is uniquely able to monitor surface structures at the individual-molecule level1, and has been shown previously to be capable of distinguishing between different molecular conformations on a surface2. Here we show that the geometric configuration (cis or trans) of several simple alkenes chemisorbed on the silicon (100) surface can be determined using the STM, through its ability to identify individual methyl groups. Because both the position and the orientation of these groups can be seen, we can determine the absolute configuration (R or S) for each of the chiral centres formed on chemisorption. Thus the STM can probe enantioselective processes at surfaces at the single-molecule level, and may assist in the development of structured chiral surfaces capable of complex recognition tasks.