Electrochemical Self-Assembly of Alkanethiolate Molecules on Ni(111) and Polycrystalline Ni Surfaces

Fabrication of ultra-smooth and oxide-free molecule-ferromagnetic metal interfaces for applications in molecular electronics under ordinary laboratory conditions

Direct self-assembly of n-alkanethiolate SAMs on ferromagnetic metal surface was fabricated. The stability and tunnelling characteristics of SAMs were investigated.

Self-assembled monolayers based spintronics: from ferromagnetic surface functionalization to spin-dependent transport

Chemically functionalized surfaces are studied for a wide range of applications going from medicine to electronics. Whereas non-magnetic surfaces have been widely studied, functionalization of magnetic surfaces is much less common and has almost never been used for spintronics applications. In this article we present the functionalization of La2/3Sr1/3MnO3, a ferromagnetic oxide, with self-assembled monolayers for spintronics. La2/3Sr1/3MnO3 is the prototypical half-metallic manganite used in spintronics studies. First, we show that La2/3Sr1/3MnO3 can be functionalized by alkylphosphonic acid molecules. We then emphasize the use of these functionalized surfaces in spintronics devices such as magnetic tunnel junctions fabricated using a nano-indentation based lithography technique. The observed exponential increase of tunnel resistance as a function of alkyl chain length is a direct proof of the successful connection of molecules to ferromagnetic electrodes. For all alkyl chains studied we obtain stable and robust tunnel magnetoresistance, with effects ranging from a few tens to 10 000%. These results show that functionalized electrodes can be integrated in spintronics devices and open the door to a molecular engineering of spintronics.

Case studies on the formation of chalcogenide self-assembled monolayers on surfaces and dissociative processes

This report examines the assembly of chalcogenide organic molecules on various surfaces, focusing on cases when chemisorption is accompanied by carbon-chalcogen atom-bond scission. In the case of alkane and benzyl chalcogenides, this induces formation of a chalcogenized interface layer. This process can occur during the initial stages of adsorption and then, after passivation of the surface, molecular adsorption can proceed. The characteristics of the chalcogenized interface layer can be significantly different from the metal layer and can affect various properties such as electron conduction. For chalcogenophenes, the carbon-chalcogen atom-bond breaking can lead to opening of the ring and adsorption of an alkene chalcogenide. Such a disruption of the π-electron system affects charge transport along the chains. Awareness about these effects is of importance from the point of view of molecular electronics. We discuss some recent studies based on X-ray photoelectron spectroscopy that shed light on these aspects for a series of such organic molecules.

Self-assembled monolayers on a ferromagnetic permalloy surface

Self-assembled monolayers (SAMs) are nowadays broadly used as surface protectors or modifiers and play a key role in many technological applications. This has motivated the study of their formation in all kind of materials; however, and despite the current interest in molecular spintronics, the study of SAMs on ferromagnetic surfaces remains almost unexplored. In this paper, we report for the first time a methodology for the formation of SAMs of n-alkylphosphonic acids on permalloy in ambient conditions. The formed monolayers have been fully characterized by means of contact angle measurements, atomic force microscopy, X-ray photoelectron spectroscopy, matrix assisted laser desorption ionization time-of-flight mass spectrometry, infrared reflection absorption spectroscopy, and X-ray reflectometry. Additionally, the magnetic stability of the modified permalloy after the solution process required for the SAM formation has been confirmed by magneto-optical Kerr effect magnetometry. Moreover, by means of microcontact printing lithography, very accurate SAM patterns have been transferred onto permalloy surfaces and used as resist mask in a chemical etching process giving rise to submicrometric permalloy surface patterns with potential interest in nanomagnetism, spintronics, and storage technologies.

New one-step thiol functionalization procedure for Ni by self-assembled monolayers

This article reports on a facile and fast strategy for the self-assembled monolayer (SAM) functionalization of nickel surfaces, employing cyclic voltammetry (CV) cycling of a suitable tailored solution containing the species to be adsorbed. Results are presented for ultrathin films formed on Ni by 1-hexadecanethiol (C16), L-cysteine (L-cys), and the poly{methyl (2R)-3-(2,2'-bithiophen-4-ylsulfanyl)-2-[(tert-butoxycarbonyl)amino]propanoate} (PCT-L) thiophene-based chiral polymer. The effective formation of high-quality ultrathin organic films on the nickel was verified both electrochemically and by exploiting typical surface characterization techniques such as contact angle, ellipsometry, atomic force microscopy (AFM), polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS).

Ionic liquid based approach for single-molecule electronics with cobalt contacts

An electrochemical method is presented for fabricating cobalt thin films for single-molecule electrical transport measurements. These films are electroplated in an aqueous electrolyte, but the crucial stages of electrochemical reduction to remove surface oxide and adsorption of alkane(di)thiol target molecules under electrochemical control to form self-assembled monolayers which protect the oxide-free cobalt surface are carried out in an ionic liquid. This approach yields monolayers on Co that are of comparable quality to those formed on Au by standard self-assembly protocols, as assessed by electrochemical methods and surface infrared spectroscopy. Using an adapted scanning tunneling microscopy (STM) method, we have determined the single-molecule conductance of cobalt/1,8-octanedithiol/cobalt junctions by employing a monolayer on cobalt and a cobalt STM tip in an ionic liquid environment and have compared the results with those of experiments using gold electrodes as a control. These cobalt substrates could therefore have future application in organic spintronic devices such as magnetic tunnel junctions.

Preparation and characterization of alkylphosphonic acid self-assembled monolayers on titanium alloy by chemisorption and electrochemical deposition

Ti-6Al-4V alloy is the most commonly used alloy for dental and orthopedic implants. In order to improve osseointegration, different surface modification methods are usually employed, including self-assembled monolayers (SAMs). This study presents an investigation of both active (electroassisted) and passive (adsorption) approaches for the modification of Ti-6Al-4V using alkylphosphonic acid. The monolayers were characterized by cyclic voltammetry, double-layer capacitance, contact angle measurements, X-ray photoelectron spectroscopy, polarization modulation infrared reflection adsorption spectroscopy, electrochemical impedance spectroscopy, and corrosion potentiodynamic polarization measurements. It is shown that the electrochemically assisted monolayers, which are assembled faster, exhibit better control over surface properties, a superior degree of order, and a somewhat higher packing density. The electrosorbed SAMs also exhibit better blockage of electron transfer across the interface and thus have better corrosion resistance.

Self-assembled monolayers of thiolates on metals: a review article on sulfur-metal chemistry and surface structures

A review article on fundamental aspects of thiolate self-assembled monolayers (SAMs) on the (111) and (100) surfaces of the Cu and Ni groups is presented.

New findings for the composition and structure of Ni nanoparticles protected with organomercaptan molecules

Here we explore the synthesis of alkanethiol-coated Ni NPs following the one-phase reaction method by Brust et al. The reduction of NiCl2 with NaBH4 in the presence of dodecanethiol (C12SH) yields a complex product that is difficult to identify as illustrated in the figure of merit. We synthesized Ni(II) dodecanethiolate (C12S) (without the addition of NaBH4) for comparison and performed an exhaustive characterization with TEM, HR-TEM, AFM, MFM, XPS, XRD, UV-vis, magnetism, and FT-IR. It is found that the organic coating is not quite a well-organized self-assembled monolayer (SAM) surrounding the Ni cluster as previously reported. XPS and XRD data show slight differences between both syntheses; however, Ni(II) thiolate appears to be more stable than reduced Ni when exposed to ambient air, indicating the propensity of metallic Ni to oxidize. It has been shown that irradiating with TEM electrons over various metal thiolates leads to nanoparticle formation. We irradiated over Ni(II) thiolate and observed no evidence of NP formation whereas irradiating a reduced Ni sample exhibited an ~3.0 nm nanoparticle diameter. Magnetism studies showed a difference between both samples, indicating ferromagnetic character for the reduced Ni sample. According to our results, the product of the synthesis is comprised of ultrasmall metallic clusters embedded in some form of Ni(II) C12S. In this work, we open a discussion of the chemical nature of the core and the shell in the synthesis of Ni NPs protected with organomercaptan molecules.

From the bottom up: dimensional control and characterization in molecular monolayers

Self-assembled monolayers are a unique class of nanostructured materials, with properties determined by their molecular lattice structures, as well as the interfaces with their substrates and environments. As with other nanostructured materials, defects and dimensionality play important roles in the physical, chemical, and biological properties of the monolayers. In this review, we discuss monolayer structures ranging from surfaces (two-dimensional) down to single molecules (zero-dimensional), with a focus on applications of each type of structure, and on techniques that enable characterization of monolayer physical properties down to the single-molecule scale.

Electrochemistry of redox-active self-assembled monolayers

Redox-active self-assembled monolayers (SAMs) provide an excellent platform for investigating electron transfer kinetics. Using a well-defined bridge, a redox center can be positioned at a fixed distance from the electrode and electron transfer kinetics probed using a variety of electrochemical techniques. Cyclic voltammetry, AC voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are most commonly used to determine the rate of electron transfer of redox-activated SAMs. A variety of redox species have been attached to SAMs, and include transition metal complexes (e.g., ferrocene, ruthenium pentaammine, osmium bisbipyridine, metal clusters) and organic molecules (e.g., galvinol, C(60)). SAMs offer an ideal environment to study the outer-sphere interactions of redox species. The composition and integrity of the monolayer and the electrode material influence the electron transfer kinetics and can be investigated using electrochemical methods. Theoretical models have been developed for investigating SAM structure. This review discusses methods and monolayer compositions for electrochemical measurements of redox-active SAMs.

Atom probe microscopy of self-assembled monolayers: preliminary results

We have achieved three-dimensional imaging of decanethiol self-assembled monolayers (SAMs) on metal surfaces by atom probe tomography (APT). The present Letter provides preliminary results on Ni [001] and Au [111], shows the analytical potential of APT analysis of SAMs, and details developments in specimen preparation and in data-treatment methodologies. Importantly, the investigation of the mass spectra from analysis of the SAMs revealed no combination of sulfur and hydrogen at the interface between the metal substrates and the organic materials, potentially providing insight about the bonding of the thiols on the substrate.

Self-assembled monolayers of thiols and dithiols on gold: new challenges for a well-known system

Self-assembled monolayers (SAMs) of alkanethiols and dialkanethiols on gold are key elements for building many systems and devices with applications in the wide field of nanotechnology. Despite the progress made in the knowledge of these fascinating two-dimensional molecular systems, there are still several "hot topics" that deserve special attention in order to understand and to control their physical and chemistry properties at the molecular level. This critical review focuses on some of these topics, including the nature of the molecule-gold interface, whose chemistry and structure remain elusive, the self-assembly process on planar and irregular surfaces, and on nanometre-sized objects, and the chemical reactivity and thermal stability of these systems in ambient and aqueous solutions, an issue which seriously limits their technological applications (375 references).

Deprotecting thioacetyl-terminated terphenyldithiol for assembly on gallium arsenide

We characterize the assembly of terphenyldithiol (TPDT) on gallium arsenide (GaAs) from ethanol (EtOH) and tetrahydrofuran (THF) as a function of ammonium hydroxide (NH4OH) concentration. NH4OH facilitates the conversion of thioacetyl end groups of the TPDT precursor to thiolates in the assembly solution. The final structure of TPDT assembled on GaAs is sensitive not only to the assembly solvent but also to NH4OH concentration. In the presence of low concentrations of NH4OH (1 mM), TPDT assemblies from EtOH are oriented upright. The same assemblies are less upright when adsorption is carried out at higher NH4OH concentrations. In THF, TPDT does not adsorb significantly on GaAs at low NH4OH concentrations. The surface coverage and structural organization of these assemblies improve with increasing NH4OH concentrations, although these assemblies are never as organized as those from EtOH. The difference in the final structure of TPDT assemblies is attributed to differences in the thiolate fraction in the assembly solution at the point of substrate immersion.