Growth Of Self-assembled Monolayers Research Articles

Grating-Coupled Surface-Plasmon-Resonance Biosensor Discs with a C-Type Fluidic Channel for Monitoring Growth of Self-Assembled Monolayer

In this paper, we present a grating-coupled surface plasmon resonance (SPR) biosensor with a c-type fluidic channel on a grating disc. For obtaining the kinetic information of molecular interaction, we combined a c-type fluidic channel on disc to drive testing solution forward by gravity via turning the grating disc. The disc biosensor can react with probe molecules on the different sensing surfaces in the following time. The repeatability testing shows the resolution of angle of the measuring system was enhanced by threefold comparing to that without the automation improvement. In order to identify the accuracy of angle changes of SPR due to the chemical reaction on surface, experiment for monitoring a growth self-assembled monolayer (SAM) immobilized on gold surface was performed. The measured response curves show that the saturation time of the SAM formation, which thickness is smaller than 2 nm, is roughly 5 hours when 1 mM MHDA growing on gold at room temperature. The demonstration reveals that the disc biosensor with c-type fluidic channels can be a promising tool for a kinetic analysis of bimolecular interaction without any external fluid pumping systems.

Grating-Coupled Surface-Plasmon-Resonance Disc Biosensor for Monitoring Growth of Self-Assembled Monolayer

Grating-Coupled Surface-Plasmon-Resonance Disc Biosensor for Monitoring Growth of Self-Assembled Monolayer

In-Situ Probe of Gate Dielectric-Semiconductor Interfacial Order in Organic Transistors: Origin and Control of Large Performance Sensitivities

Organic thin film transistor (OTFT) performance is highly materials interface-dependent, and dramatic performance enhancements can be achieved by properly modifying the semiconductor/gate dielectric interface. However, the origin of these effects is not well understood, as this is a classic "buried interface" problem that has traditionally been difficult to address. Here we address the question of how n-octadecylsilane (OTS)-derived self-assembled monolayers (SAMs) on Si/SiO(2) gate dielectrics affect the OTFT performance of the archetypical small-molecule p-type semiconductors P-BTDT (phenylbenzo[d,d]thieno[3,2-b;4,5-b]dithiophene) and pentacene using combined in situ sum frequency generation spectroscopy, atomic force microscopy, and grazing incidence and reflectance X-ray scattering. The molecular order and orientation of the OTFT components at the dielectric/semiconductor interface is probed as a function of SAM growth mode in order to understand how this impacts the overlying semiconductor growth mode, packing, crystallinity, and carrier mobility, and hence, transistor performance. This understanding, using a new, humidity-specific growth procedure, leads to a reproducible, scalable process for highly ordered OTS SAMs, which in turn nucleates highly ordered p-type semiconductor film growth, and optimizes OTFT performance. Surprisingly, the combined data reveal that while SAM molecular order dramatically impacts semiconductor crystalline domain size and carrier mobility, it does not significantly influence the local orientation of the overlying organic semiconductor molecules.

MOKE study of hybrid magnetic thin films: Permalloy on molecular self-assembled monolayer

This paper reports the magneto optical Kerr effect (MOKE) study of magnetic thin film (of Permalloy-Py) deposited on molecular self-assembled monolayer (SAM) of different organic functional groups (OFGs), polar (–COOH/NH2) and non-polar (–CH3) on Au and Si/SiO2 substrates. MOKE study of these hybrid films demonstrated the change in magnetic behavior of Py due to its initial uniform growth on polar SAM and cluster like growth on non-polar SAM. Clear difference in MOKE response in terms of coercivity, shape of hysteresis loops, and even paramagnetic response was observed for ferromagnetic thin film (∼5nm) of Py deposited on polar and non-polar SAM. For thick (∼10nm) film Py deposited on SAM MOKE hysteresis loops were almost square on all SAM (polar and non-polar) with some difference in coercivity due to observed contrast in initial morphology arising from the difference in reactive nature of the OFG of SAM. These results suggest an alternative way for tuning the magnetic properties of the ferromagnetic thin film by OFG of SAM. Furthermore, with patterning flexibility of SAM at micron/nano scale, these finding can be used to create template of magnetic thin film.

Self-Assembled Monolayers (SAMs): Which Perspectives in Implant Dentistry?

Self-assembled monolayers (SAMs) are ordered organic films formed by adsorption of an active organic coating on a solid surface. Their formation provides an alternative, highly innovative, to current traditional chemical treatments of the titanium surfaces. For this reason the structural phases, the formation and the growth of SAMs is described from a surface science point of view. Particulars are given to SAMs on titanium concerning surface morphology, chemical composition and affinity of specific head group for Ti surfaces (silanes, siloxane, phosphonates and phosphates). Preparation, coating methodologies, limitations and techniques used for the characterization of SAMs are reported. For their physicochemical characteristics and micro-nano scale features some perspectives of using SAMs in biomedical application are outlined.

Microphase-separated block copolymer film anchored on ITO substrate with newly designed self-assembled monolayer

ABSTRACTThe surface of Indium-tin-oxide (ITO) substrate was modified with a newly designed silane coupling molecule bearing azobenzene moiety. The silane coupling molecules formed self-assembled monolayer (SAM) on pretreated ITO surface. The SAM growth and coverage were quantified by contact angle measurement and X-ray photoelectron spectroscopy (XPS). The silane coupling molecules improved the adhesion between the ITO surface and an amphiphilic block copolymer (BC) thin film, which consists of poly(ethylene oxide) (PEO) and poly(methacrylate) (PMA) with azobenzene mesogens, because the azobenzene moieties of the SAM anchor the liquid crystalline PMA azobenzene domains of BC.

Surface Potential Mapping of SAM‐Functionalized Organic Semiconductors by Kelvin Probe Force Microscopy

Previously, it has been shown that formation of self assembled monolayers (SAMs) of alkyl-silanes on the surfaces of π -conjugated organic molecular crystals results in an orders-ofmagnitude increase in the surface conductivity of these semiconducting materials. [ 1 ] The magnitude of the effect depends on the chemical structure of the alkyl tail of the SAM: a fl uorinated SAM (tridecafl uoro-1,1,2,2-tetrahydrooctyl)trichlorosilane, or FTS) results in a 10 4 -fold increase in surface conductivity of rubrene crystals, for example, while a non-fl uorinated SAM (n-octyltrichlorosilane, or OTS) raises the conductivity only by a factor of 10. These numbers are quoted relative to the surface conductivity of the built-in conduction channel for pristine rubrene crystals, σ 0 ∼ 10 − 9 –10 − 8 S per square, with carrier densities n 0 ∼ (0.6–6) × 10 9 cm − 2 . [ 2 , 3 ] Generally alkyl-silanes functionalized with stronger electron withdrawing groups induce a greater surface conductivity change in p-type organic semiconductors. This effect is a result of interfacial charge-transfer doping where the silane derivative withdraws electrons from the organic semiconductor surface, thereby inducing mobile charge carriers (holes in the case of rubrene) for transport. The interfacial charge-transfer doping that occurs when silanes bind to rubrene will result in surface dipoles and work function shifts for the rubrene surface. Indeed, it is well known that SAMs modify the work functions of both metals and semiconductors by surface dipoles. [ 4 , 5 ] Measuring the resulting work function shifts provides a way to quantify the effects of silane treatment in terms of the band edge offsets from the Fermi level, which can then be related to doping levels in the organic semiconductor. Accordingly, in this work we have quantifi ed the degree of charge-transfer doping and the magnitude of the rubrene band energy modulation induced by OTS and FTS SAMs by performing surface potential measurements with Kelvin probe force microscopy (KFM). [ 6–10 ] In addition we have utilized the sub-50 nm spatial resolution of KFM to visualize important aspects of SAM nucleation and growth on rubrene crystals. Our results are the fi rst quantitative assessment of charge-transfer doping of organic semiconductors by silanes.

Surface‐enhanced resonance Raman spectroscopic characterization of cytochrome c immobilized on 2‐mercaptoethanesulfonate monolayers on silver

AbstractSingle‐component self‐assembled monolayers (SAMs) of mercaptoethanesulfonate (MES) on Ag surfaces were studied with surface‐enhanced resonance Raman scattering (SERRS) spectroscopy with a view to their application to immobilize (ferro)cytochrome c (cyt c). SERS studies revealed that MES molecules adopt primarily trans (T) conformation even at early stages of the SAM growth and over wide range of pH values. High accessibility of the negatively charged groups for (bio)molecules in solution makes single‐component MES SAMs suitable linkage monolayers for electrostatic attachment of cyt c, which was verified with SERRS. Cyt c was successfully anchored to MES‐coated Ag at natural (∼5), neutral, and isoelectric point (10.6) pH. At pH = 7.0 and 10.6, SERRS bands characteristic of native six‐coordinated low‐spin (6cLS) heme iron configuration were detected. Lack of buffering resulted in additional appearance of five‐coordinated high‐spin (5cHS) SERRS markers and the presence of bands indicating ferric ion. An electrostatic attraction between protein and SAM was confirmed to exclude the hydrophobic interactions involved in cyt c binding. Cyt c attached to MES SAM on Ag was found to be electroactive at neutral pH, and protein oxidation was assisted with reversible conversion of 6cLS to the non‐native 5cHS state. Alteration of heme orientation deduced from SERRS spectra upon change of redox state allowed us to propose the protein dynamics as the electron transfer rate‐limiting step. Copyright © 2010 John Wiley & Sons, Ltd.

Ion-scattering analysis of self-assembled monolayers of silanes on organic semiconductors

We describe new Rutherford Backscattering Spectroscopy (RBS) measurements to explore the surface chemistry associated with the growth of self-assembled monolayer (SAM) molecules on conducting organic films. The report includes a description of the optimization of both substrates and RBS scattering parameters to appropriately analyze these novel and damage susceptible structures. Our RBS measurements reveal that the final surface stoichiometry is consistent with a specific model of hydrolyzed and crosslinked trichlorosilanes that form a dense two-dimensional network (a monolayer) at the surface of small-molecule organic semiconductors and a bulk SAM network in the case of conjugated polymer films. Organic semiconductors used in this study are thin films of rubrene (a small molecule semiconductor (C42H28)) and poly(3-hexyl)thiophene (P3HT) (a conjugated polymer (C10H18S)n). As a substrate we used a thick (1 μm) film of parylene (a non-conjugated polymer (C8H8)n) deposited on Si (1 0 0) wafers. The SAM molecules used to functionalize the organic semiconductor films are fluoroalkyl trichlorosilane (FTS) (C8(H4F13)SiCl3) and octyltrichlorosilane (OTS) (C8H17SiCl3). Quantitative detection of medium and small-mass elements, such as O, F, Si, S and residual Cl is demonstrated and used to elucidate the surface chemistry in these novel organic systems.

Surface engineering for high performance organic electronic devices: the chemical approach

Self-assembled monolayers (SAMs) are rapidly becoming an essential part of organic electronics such as light emitting diodes (LEDs), organic field-effect transistors (OFETs), and complementary circuits, where they are employed to control the morphology and energetics of the interfaces. This review will focus on interface engineering and its influence on such devices. We will describe the growth of SAMs of organic molecules on various surfaces of interest for this purpose (mainly metals and oxides). For the growth of SAMs on metal electrodes, the most common approach makes use of thiols to modify the metal surface, but the preparation of SAMs on dielectrics requires other reactive groups such as carboxylic or phosphonic acids. We review also the control of the interfacial properties by appropriate molecular design of the SAMs and their effect on device performance. SAMs can modify the morphology of the organic semiconductor, the dielectric properties of the insulator, the electronic states at the dielectric interface and the level alignment at the electrode interfaces. All these factors can influence current transport mainly by modifying the charge mobility, the contact resistance and the trap density at the interfaces.

Digital biomagnetism: Electrodeposited multilayer magnetic barcodes

A novel magnetic encoding technique for performing high-throughput biological assays is presented. Electrodeposited Ni/Cu and Co/Cu multilayer pillar structures with a diameter of 15 μm and a thickness up to 10 μm are presented as “magnetic barcodes”, where the number of unique codes possible increases exponentially with a linear increase in length. A gold cap facilitates the growth of self-assembled monolayers (SAMs), while microdrop printing allows efficient generation of large libraries of tagged probes. Coercivity-tuning techniques are used to exploit a non-proximity encoding methodology compatible with microfluidic flow.

Controlled growth and formation of SAMs investigated by atomic force microscopy

Recently we reported a simple method for obtaining both monolayer thickness and surface patterning using self-assembled monolayers (SAMs). Here we presented a straightforward method for controlling the formation of SAMs over surfaces useful for both chemical and biological applications. Atomic force microscopy (AFM) has been used to investigate the growth mechanism and formation of octadecylsiloxane (ODS) films obtained using a less-reactive silane; octadecyltrimethoxysilane (OTMS). SAMs formation from both OTMS and octadecyltrichlorosilane (ODTS) differ in the hydrolysis step where ODTS results in hydrochloric acid formation, which may affect the delicate features on surfaces. On the other hand, OTMS does not show this behavior. In contrast to monolayer formation from chlorosilane precursors, methoxysilane SAMs have been studied less extensively. Our observations highlight the importance of controlling water content during the formation of ODS monolayers in order to get well-ordered SAMs. We have also seen that, like ODTS, OTMS exhibits monolayer growth through an island expansion process but with a comparatively slow growth rate and different island morphology. The average height of islands, surface coverage, contact angle and root-mean-square (RMS) roughness increase with OTMS adsorption time in a consecutive manner.

Self-Assembled Monolayers Directly Attached to Silicon Substrates Formed from 1-Hexadecene by Thermal, Ultraviolet, and Visible Light Activation Methods

Using 1-hexadecene as a precursor, self-assembled monolayers (SAMs) were fabricated on hydrogen-terminated silicon (111) [H-Si(111)] surfaces without forming an interfacial oxide layer on the basis of thermal (180 °C, 2 h), ultraviolet (UV; 500 mW cm-2, 10 h), and visible-light activation (330 mW cm-2, 16 h) processes. As characterized by water contact angle measurements, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and ellipsometry, the hexadecyl SAM fabricated by the visible-light process had a highly-ordered molecular arrangement and a closely-packed methyl-terminated surface similar to the SAMs prepared by the thermal and UV activation processes. The photo-irradiation wavelength dependence of the visible-light activation process was further studied at irradiation wavelengths of 400, 550, and 700 nm. The SAM formation reaction was certainly promoted at all the wavelengths, even at 700 nm. However, oxidation of the Si surface became apparent due to the slow rate in SAM growth, and thus the monolayer coverage of SAM at 700 nm became smaller. The reaction rate became faster with the decreasing wavelength for activation, probably due to the increase in the light adsorption coefficient of Si. The excitation of Si, namely, the generation of hole/electron pairs at the Si substrate, is assumed to be the rate-controlling step of the visible-light activation process.

Fast displacement and structural transition of cyclohexanethiol self-assembled monolayers by octanethiols on Au (1 1 1)

Displacement of cyclohexanethiols (CHTs) self-assembled monolayers (SAMs) by octanethiols on Au (1 1 1) (OTs) was examined by scanning tunneling microscopy (STM) and contact angle measurements. We revealed that the fast displacement of CHT by OT takes place within a few minutes, and then displacement proceeds slowly to form the closely packed OT SAMs. The main driving force for the unusual fast displacement is due to the large increase of chemical interactions between the sulfur and gold atoms, and the van der Waals interactions between alkyl chains after displacement of CHT by OT. STM imaging clearly demonstrated the structural transitions from the (5×2√10) R48° structure of CHT SAMs to the (√3×√3) R30° or c(4×2) structures of OT SAMs via an intermediate phase that were often observed during alkanethiol SAM growth on gold. In this study, we found that CHT SAMs can be used as a new transient layer for the fabrication of nanostructures on a surface.

Organosilane Self-Assembled Monolayer Growth from Supercritical Carbon Dioxide in Microstructured Optical Fiber Capillary Arrays

Microstructured optical fibers form a new class of extreme aspect ratio templates that are well-suited for precise, designed spatial organization of materials and molecules at dimensions down to the nanoscale. The extreme aspect ratios of the nanoscale to microscale pores in the templates necessitates new approaches to fabrication of nanowires, nanotubes, and self-assembled monolayers within them. High-pressure fluids, which have lower viscosities than liquids and no surface tension, are well-suited for penetrating such extreme aspect ratio capillaries. Here we report an approach to fabricating self-assembled monolayers within microstructured optical fibers using near supercritical or supercritical carbon dioxide. An AFM-based "shaving" technique has been developed to characterize the monolayers formed in capillaries that are too small to allow for characterization by conventional approaches.

Growth of self-assembled monolayer on tetrahedral amorphous carbon film coated magnetic head

The magnetic heads overcoated by tetrahedral amorphous carbon (ta-C) films have been immersed in the 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) solvent at different predetermined dipping time. The growth of FDTS self-assembled monolayer (SAM) on the ta-C surface has been confirmed by time-of-flight secondary ion mass spectrometry. Four distinct stages were observed in the growing process: (1) low coverage (nucleation), (2) growing up (3) coalescence and (4) complete monolayer, similar to the theoretical models of epitaxial film growth at low temperatures. Contact angle measurement using de-ionized water is carried out to study the time dependence of contact angle during the growth process of FDTS monolayer. The surface morphology, profiles and roughness (rms) of FDTS monolayer were determined by atomic force microscopy (AFM). Furthermore, the variation of monolayer thickness and distribution was revealed by transmission electron microscopy, which is in agreement with the AFM and contact angle results.

Vapor-Phase Self-Assembled Monolayers for Anti-Stiction Applications in MEMS

We have investigated the anti-stiction performance of self-assembled monolayers (SAMs) that were grown in vapor phase from six different organosilane precursors: CF3(CF2)5(CH2)2SiCl3 (FOTS), CF3(CF2)5(CH2)2Si(OC2H5)3 (FOTES), CF3(CF2)5(CH2)2Si(CH3)Cl2 (FOMDS), CF3(CF2)5(CH2)2Si(CH3)2Cl (FOMMS), CF3(CF2)7(CH2)2SiCl3 (FDTS), and CH3(CH2)17(CH2)2SiCl3 (OTS). The SAM coatings that were grown on silicon substrates were characterized with respect to static contact angle, surface energy, roughness, nanoscale adhesive force, nanoscale friction force, and thermal stability. The best overall anti-stiction performance was achieved using FDTS as precursor for the SAM growth, but all coatings show good potential for solving in-use stiction problems in microelectromechanical systems devices.

Electronic functionalization of the surface of organic semiconductors with self-assembled monolayers

Self-assembled monolayers (SAMs) are widely used in a variety of emerging applications for surface modification of metals and oxides. Here, we demonstrate a new type of molecular self-assembly: the growth of organosilane SAMs at the surface of organic semiconductors. Remarkably, SAM growth results in a pronounced increase of the surface conductivity of organic materials, which can be very large for SAMs with a strong electron-withdrawing ability. For example, the conductivity induced by perfluorinated alkyl silanes in organic molecular crystals approaches 10(-5) S per square, two orders of magnitude greater than the maximum conductivity typically achieved in organic field-effect transistors. The observed large electronic effect opens new opportunities for nanoscale surface functionalization of organic semiconductors with molecular self-assembly. In particular, SAM-induced conductivity shows sensitivity to different molecular species present in the environment, which makes this system very attractive for chemical sensing applications.

Orientation-controlled molecule-by-molecule polymer wire growth by the carrier-gas-type organic chemical vapor deposition and the molecular layer deposition

The carrier-gas-type organic chemical vapor deposition (CG-OCVD) and the molecular layer deposition were developed for orientation-controlled molecule-by-molecule growth of conjugated polymer wires on a surface with self-assembled monolayer (SAM), which provides a feasible way to construct self-organized three-dimensional polymer wire networks. It was also found that, in CG-OCVD, growth saturation occurs for a substrate without SAM while no growth saturation for a substrate with SAM. The analysis of results in terms of reaction site densities on surfaces suggests vertical wire growth on a substrate with SAM and horizontal wire growth on a substrate without SAM.

Monitoring the real time growth of a self-assembled monolayer on a polymer electrolyte surface

The growth of a self-assembled monolayer (SAM) at the surface of a polymer electrolyte has been shown to inhibit the formation of the passivating layer that forms when the polymer is in contact with lithium metal. In this work, ac impedance spectroscopy was used to monitor the formation of SAM layers on polyethylene oxide (PEO) polymer electrolyte thin films as a function of time. To monitor SAM growth, thin PEO films were cast onto interdigitated electrodes. The electrodes were subsequently immersed in a saturated SAM solution and the film impedance was measured. SAM molecules with the general formula: H (CH 2) 32 (CH 2CH 2O) y H ( y = 2, 10, 40) were used. Growth occurred due to interactions with the ethylene oxide portion of the SAM molecules with the PEO surface. To visualize SAM growth impedance data at a single frequency sensitive to changes at the solution interface was plotted verses time. At the point of immersion, a sharp increase in impedance was observed. With time, the rate at which the impedance increased slowed and ultimately leveled off presumably indicating the point at which a nearly complete monolayer had formed. SAM growth was verified using attenuated total reflectance infrared spectroscopy (ATR-IR).