Self-assembled Molecular Monolayers Research Articles

Donor/Acceptor Mixed Self-Assembled Monolayers for Realising a Multi-Redox-State Surface.

Mixed molecular self-assembled monolayers (SAMs) on gold, based on two types of electroactive molecules, that is, electron-donor (ferrocene) and electron-acceptor (anthraquinone) molecules, are prepared as an approach to realise surfaces exhibiting multiple accessible redox states. The SAMs are investigated in different electrolyte media. The nature of these media has a strong impact on the types of redox processes that take place and on the redox potentials. Under optimised conditions, surfaces with three redox states are achieved. Such states are accessible in a relatively narrow potential window in which the SAMs on gold are stable. This communication elucidates the key challenges in fabricating bicomponent SAMs as electrochemical switches.

(Invited) Engineering Interfaces for Added Functionality of Nanoelectronics

Future electronic components will be vastly different in response to two critical technical drivers; the increasing complexity to make smaller, faster high performance components and the “electronics everywhere” phenomena requiring portable, low power, niche roles of electronic components. New materials, devices, and architectures are actively being explored to meet these demands. A common theme among these approaches is the need for fundamental innovations and understanding of electronic processes at interfaces. I will describe advances made to manipulate and measure organic and molecular interfaces at the nanometer scale and the impact on the resulting electronic performance. In particular, I will focus on the measurement advances made in molecular electronics, highlighting our understanding of the science and characterization of the critical buried interfaces. This talk will contain two illustrative case studies demonstrating the impact of interface engineering on charge trapping memory devices and spin-based paradigms.Organic molecules are advantageous for trapped charge memory devices due to their small size, discrete nature, and tunability. Moreover, because each molecule can trap one (and sometimes more) charges, they enable very large trapped charge densities. I will describe a series of advances in creating and characterizing molecular memory devices incorporating a novel ruthenium based redox-active molecule as the charge storage layer.(1) Spin-based paradigms are increasingly important for future electronics. Of special interest is the manipulation of spin in organic materials. I will describe a series of advances that examine the impact of organic monolayers on the ferromagnetic cobalt and cobalt oxide interfaces and the role interface engineering can play in organic spintronics.(2,3) 1. “Non-volatile memory devices with redox-active diruthenium molecular compound” S. Pookpanratana, H. Zhu, E. G. Bittle, S. Natoli, T. Ren, D. J. Gundlach, C. A. Richter, Q. Li, and C. A. Hacker J. Phys. Chem.: Condens. Matter, a special issue on “Molecular functionalization of surfaces for device applications” (invited) Oct 2015 accepted 2. “Modifying spin injection characteristics of Co/Alq3 system by using a molecular self-assembled monolayer”, Hyuk-Jae Jang, J.-S Lee, S. J. Pookpanratana, I. C. Tran, C. A. Hacker, and C. A. Richter, J. Phys. Chem. C 119, 12949 (2015) 3. “Self-Assembled Monolayers Impact Cobalt Interfacial Structure in NanoElectronic Junctions” S. Pookpanratana, L. K. Lydecker, C. A. Richter, and C. A. Hacker, J. Phys. Chem. C, 119(12), 6687-6695 (Mar 2105)

On-chip molecular electronic plasmon sources based on self-assembled monolayer tunnel junctions

Molecular electronic control over plasmons offers a promising route for on-chip integrated molecular plasmonic devices for information processing and computing. To move beyond the currently available technologies and to miniaturize plasmonic devices, molecular electronic plasmon sources are required. Here, we report on-chip molecular electronic plasmon sources consisting of tunnel junctions based on self-assembled monolayers sandwiched between two metallic electrodes that excite localized plasmons, and surface plasmon polaritons, with tunnelling electrons. The plasmons originate from single, diffraction-limited spots within the junctions, follow power-law distributed photon statistics, and have well-defined polarization orientations. The structure of the self-assembled monolayer and the applied bias influence the observed polarization. We also show molecular electronic control of the plasmon intensity by changing the chemical structure of the molecules and by bias-selective excitation of plasmons using molecular diodes. Self-assembled molecular monolayers between metal electrodes are demonstrated as an electronic plasmon source.

Inhibition of aluminum alloy corrosion in electrolytes by self‐assembled fluorophosphonic acid molecular layer

This paper presents the use of self‐assembled molecular (SAM) layers of fluorophosphonic acids developed on aluminum surface. The SAM monolayers were adsorbed from ethanol solution, characterized by surface morphology visualized by atomic force microscopy, and by the wettability by contact angle measurements. The anti‐corrosion efficiency of the nanolayers was determined by ex situ AFM measurements and by electrochemical techniques, which indicated the importance of the fluorophosphonic acid concentration and of the film formation time in the molecular deposition process. The morphological change with or without nanolayers caused by the corrosive environment was in accordance with the electrochemical results.

Simulations of molecular self-assembled monolayers on surfaces: packing structures, formation processes and functions tuned by intermolecular and interfacial interactions.

Surfaces modified with a functional molecular monolayer are essential for the fabrication of nano-scale electronics or machines with novel physical, chemical, and/or biological properties. Theoretical simulation based on advanced quantum chemical and classical models is at present a necessary tool in the development, design, and understanding of the interfacial nanostructure. The nanoscale surface morphology, growth processes, and functions are controlled by not only the electronic structures (molecular energy levels, dipole moments, polarizabilities, and optical properties) of building units but also the subtle balance between intermolecular and interfacial interactions. The switchable surfaces are also constructed by introducing stimuli-responsive units like azobenzene derivatives. To bridge the gap between experiments and theoretical models, opportunities and challenges for future development of modelling of ferroelectricity, entropy, and chemical reactions of surface-supported monolayers are also addressed. Theoretical simulations will allow us to obtain important and detailed information about the structure and dynamics of monolayer modified interfaces, which will guide the rational design and optimization of dynamic interfaces to meet challenges of controlling optical, electrical, and biological functions.

Probing molecular interactions with methylene blue derivatized self-assembled monolayers

The emergence of stratified and personalised medicine and the associated need for highly multiplexed detection strategies are driving the development of innovative sensor technology. Electronic immunosensor arrays capable of label-free and highly parallel monitoring of ligand binding have emerged as a particularly promising technology capable of meeting these new diagnostic challenges. In this study, we present an approach for interrogating molecular interactions electronically using redox active molecular monolayers. Specifically, we have synthesised self-assembled molecular monolayers assembled from long-chain alkanethiols (LCAT) incorporating oligoethyleneglycol (OEG) linkers that can be derivatized with a range of functional groups, including the redox active molecule methylene blue. Critically, we show that the electron transport properties of this redox-active monolayer are highly sensitive to the electrochemical environment, including the local concentration of protons and the electrostatic potential at the plane of electron transfer. Using a combination of cyclic voltammetry and QCM-D to study in detail the behaviour of the monolayer during functionalisation and analyte binding, we demonstrate that these redox properties can be exploited for the electrochemical sensing of molecular interactions (biotin–avidin in our case) on SAMs. Given the versatility of LCAT-OEG monolayers, in terms of linker lengths, choice of functional group, and ability to create mixed component layers and the straight-forward assembly of mixed SAMs of high quality, our electrochemical sensing approach forms an excellent and generic label-free platform for probing a wide range of molecular interactions.

Metamaterial Absorbers for Infrared Detection of Molecular Self-Assembled Monolayers.

The emerging field of plasmonic metamaterials has introduced new degree of freedom to manipulate optical field from nano to macroscopic scale, offering an attractive platform for sensing applications. So far, metamaterial sensor concepts, however, have focused on hot-spot engineering to improve the near-field enhancement, rather than fully exploiting tailored material properties. Here, we present a novel spectroscopic technique based on the metamaterial infrared (IR) absorber allowing for a low-background detection scheme as well as significant plasmonic enhancement. Specifically, we experimentally demonstrate the resonant coupling of plasmonic modes of a metamaterial absorber and IR vibrational modes of a molecular self-assembled monolayer. The metamaterial consisting of an array of Au/MgF2/Au structures exhibits an anomalous absorption at ~3000 cm−1, which spectrally overlaps with C-H stretching vibrational modes. Symmetric/asymmetric C-H stretching modes of a 16-Mercaptohexadecanoic acid monolayer are clearly observed as Fano-like anti-resonance peaks within a broad plasmonic absorption of the metamaterial. Spectral analysis using Fano line-shape fitting reveals the underlying resonant interference in plasmon-molecular coupled systems. Our metamaterial approach achieves the attomole sensitivity with a large signal-to-noise ratio in the far-field measurement, thus may open up new avenues for realizing ultrasensitive IR inspection technologies.

English

Bacteriophages (or phages) are parasites that infect specific bacteria. This host-selectivity can be useful to identify bacterial contaminants in food, water, environment etc. In the present study, phage was isolated from stagnant water and cultivated by overlay method against host bacteria that is E. coli. Phage titer was calculated to be 107 pfu/ml using 10-fold dilutions. Plaque reaction activity was observed within 4 to 6 h against host bacteria by spot test. Morphological identification of phage by transmission electron microscopy (TEM) using uranyl acetate staining revealed about 78 nanometers (nm) in wide phage capsid and tail length (527 nm). The isolated phage was classified into order Caudovirales since it possessed a long non-contractile tail and icosahedral capsid head, thus is a member of the family Siphoviridae. Also, the phage identified followed a lytic life cycle since plaque reaction activity was observed within 4-6 h against host bacteria. Gold immuno-functionalization using self-assembled monolayers (SAM) has been widely used for the detection of small targets, but there are limited reports available describing the detection systems for bacteria by using phages. Thus, in order to develop a suitable detection system for identification of specific bacteria, it is suitable to exploit the close association and selectivity between bacteria and bacteriophage. Bacteriophages were immobilized onto gold surface by SAM using a stable acyl amino ester intermediate generated by 1-ethyl-3(3-dimethylaminoproply) carbodiimide (EDC) and N-hydrosuccinimide (NHS) to condense the bacteriophage. Fourier transformation infrared (FTIR) microscopy presence of different functional groups present in each layer formation. Bacteriophage immobilization over the gold surface was verified through by scanning electron microscope (SEM). Electrochemical analysis was performed for a rapid and specific detection of E. coli cell. The present bio-sensing system comprises of quick and specific detection of host bacteria and possesses a very low detection limit (104 cfu/ml). We propose phages utilization as a bio-component in biosensor development for bacteria capture. Key words: Bacteriophage, biosensors, electrochemical, microscopy, immune-functionalisation and Self-assembled molecular monolayer.

Electrochemical impedance immunosensor for rapid detection of stressed pathogenic Staphylococcus aureus bacteria.

In this work, we report the adaptation of bacteria to stress conditions that induce instability of their cultural, morphological, and enzymatic characters, on which the identification of pathogenic bacteria is based. These can raise serious issues during the characterization of bacteria. The timely detection of pathogens is also a subject of great importance. For this reason, our objective is oriented towards developing an immunosensing system for rapid detection and quantification of Staphylococcus aureus. Polyclonal anti-S. aureus are immobilized onto modified gold electrode by self-assembled molecular monolayer (SAM) method. The electrochemical performances of the developed immunosensor were evaluated by impedance spectroscopy through the monitoring of the charge transfer resistance at the modified solid/liquid interface using ferri-/ferrocyanide as redox probe. The developed immunosensor was applied to detect stressed and resuscitate bacteria. As a result, a stable and reproducible immunosensor with sensitivity of 15 kΩ/decade and a detection limit of 10 CFU/mL was obtained for the S. aureus concentrations ranging from 10(1) to 10(7) CFU/mL. A low deviation in the immunosensor response (±10 %) was signed when it is exposed to stressed and not stressed bacteria.

Modifying Spin Injection Characteristics in the Co/Alq3 System by Using a Molecular Self-Assembled Monolayer

We present the results of experiments that explore the influence of molecular self-assembled monolayers (SAMs) on characteristics of spin injection into an organic semiconductor, Alq3 [tris(8-hydroxyquinoline)aluminum], from a ferromagnetic metal, Co. Two different SAMs, MHA (16-mercaptohexadeconic acid, HS(CH2)15CO2H) and ODT (1-octadecanethiol, CH3(CH2)17SH), are inserted between Alq3 and Co layers, and their effects on electronic structure hybridization and related changes in energy levels and spin-dependent properties at the interface are investigated. X-ray photoelectron measurements of the surface structure for the MHA, ODT, and Alq3 organic layers provide bonding and chemical conformational information. Ultraviolet photoelectron spectroscopy (UPS) measurements reveal that both MHA and ODT treatments lower the work function of Co. X-ray magnetic circular dichroism (XMCD) spectra imply that SAMs reduce the hybridization between Co and Alq3, and furthermore, they enhance the spin magnetic moment of Co.

(Invited) Importance of Interface Engineering in Organic Spintronics

Organic semiconductors provide a route to revolutionary changes in many areas of device technology with their unique electronic properties and functionalities, which can be easily tailored by chemical synthesis and molecular engineering.[1] In particular, an emerging research area referred to as organic spintronics has initiated a range of device concepts by exploiting spin-related properties in organic semiconductors.[2] Currently, there are two major interests in organic spintronics technology.[3] One of them is to realize a system enabling the injection/transport/detection of spin polarized charge carriers in organic semiconductors.[3] The other one is to understand and explore the magnetic field effects (MFEs) of organic semiconductors for the development and improvement of organic-based devices such as organic light emitting diodes and magnetic sensors.[3] It has been known that in organic-based devices, the interfaces between active materials play a critical role in their performance and functionality because the interfaces greatly affect the injection/transport/extraction of charge carriers in organic semiconductors.[4] Recent findings suggest that the interface engineering is also very crucial in organic spintronics technology such as enabling efficient spin-polarized charge carrier injection and tuning of organic MFEs.[5-7] In this presentation, we will discuss our efforts towards the realization of efficient spin injection and transport in organic semiconductors as well as the modification of organic MFEs by interface engineering. We experimentally show the importance of band alignment between a metal electrode and an organic semiconductor at their interface for efficient spin injection/detection.[6] We will also show how one can utilize a molecular self-assembled monolayer by inserting it between a metal electrode and an organic semiconductor in order to change the characteristics of spin injection into an organic semiconductor from a ferromagnetic metal as well as in order to tune the organic MFEs.[7] 1. Editorial, Nat. Mater. 12, 591 (2013). 2. C. Boehme and J. M. Lupton, Nat. Nano. 8, 612 (2013). 3. W. Wagemans, et al., SPIN, 1, 93 (2011). 4. H. Ma, et al., Adv. Funct. Mater. 20, 1371 (2010). 5. S. Sanvito, Nat. Phys. 6, 562 (2010). 6. H.-J. Jang, et al., Appl. Phys. Lett. 101, 102412 (2012). 7. H.-J. Jang, et al., ACS Nano 8, 7192 (2014).

Uses of Alkanethiol Effects on Surface Functionalization of Gold Membranes for Biosensors

The surface properties and performance of a biosensor's operative element plays a decisive role in medical applications by offering a universal platform for biological and chemical sensing. It is extremely important for the surface stress biosensors, which operate via free energy change, an underlying principle in all bonding reactions. A biosensors' surface specificity to a chosen analyte is determined by the chemical functionalization. Ultraspecialized surface functionalizations can be achieved using molecular self-assembled monolayers (SAMs). We applied five different alkanethiols to functionalize the gold/polydimethylsiloxane (Au/PDMS) membrane of capacitive surface stress biosensors to research the surface properties and performance. Glucose was used as the adsorbed target analyte of the SAM/Au/PDMS membrane. The functionalized effects were characterized by scanning electron microscopy and Raman spectroscopy, then confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. Results indicate that test membranes were functionalized successfully by the alkanethiols. Moreover, our findings reveal that extended alkyl chain length not only increases the distance of electron transfer pathway between the electron donor and acceptor, but also alters the surface properties of the SAMs. The functional end group of SAMs critically affects the interfacial action of adsorbed glucose molecules as well as electron transfer from glucose to electrode. This comprehensive study may provide a good understanding to functionalize biosensor surfaces and improve surface performance, especially in the case of surface stress biosensors.

Nanooptics of molecular-shunted plasmonic nanojunctions.

Gold nanoparticles are separated above a planar gold film by 1.1 nm thick self-assembled molecular monolayers of different conductivities. Incremental replacement of the nonconductive molecules with a chemically equivalent conductive version differing by only one atom produces a strong 50 nm blue-shift of the coupled plasmon. With modeling this gives a conductance of 0.17G0 per biphenyl-4,4′-dithiol molecule and a total conductance across the plasmonic junction of 30G0. Our approach provides a reliable tool quantifying the number of molecules in each plasmonic hotspot, here <200.

Characterization of new molecular self-assembled monolayers on gold electrode by QCM, EIS, SEM and CV techniques: application for electrocatalytic determination of dopamine in the presence of acetaminophen

In this work, molecular self-assembled monolayers (SAM) of 2-(2,3-dihydroxy phenyl) benzothiazole (HPB) on gold electrode are prepared for the first time and described by cyclic voltammetry (CV), electrochemical impedance spectroscopy, scanning electron microscopy (SEM) and quartz crystal microbalance techniques. The redox properties of the modified electrode were investigated by CV at various scan rates, and the apparent charge transfer rate constant (k s) and transfer coefficient (α) were calculated for HPB-SAM/Au. Then, electro oxidation of dopamine (DA) on a HPB-SAM-modified gold electrode has been studied by cyclic voltammetry, chronoamperometry and differential pulse voltammetry (DPV). The mediated oxidation of DA at the modified electrode in CV occurs at a potential of about 180 mV less positive than that of the unmodified gold electrode. The values of electron transfer coefficients (α), catalytic rate constant (k) and diffusion coefficient (D) were calculated for DA, using electrochemical methods. DPV exhibited a linear dynamic range over the concentration range of 0.1–500.0 µM and a detection limit (3 s) of 0.03 µM for DA in pH 7.0.

Stereoselective photopolymerization of tetraphenylporphyrin derivatives on Ag(110) at the sub-monolayer level.

We explore a photochemical approach to achieve an ordered polymeric structure at the sub-monolayer level on a metal substrate. In particular, a tetraphenylporphyrin derivative carrying para-amino-phenyl functional groups is used to obtain extended and highly ordered molecular wires on Ag(110). Scanning tunneling microscopy and density functional theory calculations reveal that porphyrin building blocks are joined through azo bridges, mainly as cis isomers. The observed highly stereoselective growth is the result of adsorbate/surface interactions, as indicated by X-ray photoelectron spectroscopy. At variance with previous studies, we tailor the formation of long-range ordered structures by the separate control of the surface molecular diffusion through sample heating, and of the reaction initiation through light absorption. This previously unreported approach shows that the photo-induced covalent stabilization of self-assembled molecular monolayers to obtain highly ordered surface covalent organic frameworks is viable by a careful choice of the precursors and reaction conditions.

Organic Electronics: A New Route to Low Resistance Contacts for Performance‐Enhanced Organic Electronic Devices (Adv. Mater. Interfaces 5/2014)

In the macroscopic world smart mechanical engineering is utilized to overcome potential steps. The Falkirk wheel lock system represents one of these ingenious solutions to connect waterways at different altitudes. In article 1300130, P. Schulz and co-workers employ molecular self-assembled monolayers to mitigate the energy barrier for electrons between high work function metals and n-type organic semiconductors on the nanoscale. Tailored dithiocarbamates serve as molecular gearing to align the respective energy levels at the organic/metal interface. The cover design is by Taek-Bong Kim.

Interface engineering to control magnetic field effects of organic-based devices by using a molecular self-assembled monolayer.

Organic semiconductors hold immense promise for the development of a wide range of innovative devices with their excellent electronic and manufacturing characteristics. Of particular interest are nonmagnetic organic semiconductors that show unusual magnetic field effects (MFEs) at small subtesla field strength that can result in substantial changes in their optoelectronic and electronic properties. These unique phenomena provide a tremendous opportunity to significantly impact the functionality of organic-based devices and may enable disruptive electronic and spintronic technologies. Here, we present an approach to vary the MFEs on the electrical resistance of organic-based systems in a simple yet reliable fashion. We experimentally modify the interfacial characteristics by adding a self-assembled monolayer between the metal electrode and the organic semiconductor, thus enabling the tuning of competing MFE mechanisms coexisting in organic semiconductors. This approach offers a robust method for tuning the magnitude and sign of magnetoresistance in organic semiconductors without compromising the ease of processing.

Atomic Scale Modeling of Two-Dimensional Molecular Self-Assembly on a Passivated Si Surface

The self-assembly of two-dimensional (2D) molecular structures on a solid surface relies on the subtle balance between noncovalent intermolecular and molecule–surface forces. The energetics of 2D molecular lattices forming different patterns on a passivated semiconductor surface are here investigated by a combination of atomistic simulation methods. Density-functional theory provides structure and charges of the molecules, while metadynamics with empirical forces provides a best guess for the lowest-energy adsorption sites of single molecules and dimers. Subsequently, molecular dynamics simulations of extended molecular assemblies with empirical forces yield the most favorable lattice structures at finite temperature and pressure. The theoretical results are in good agreement with scanning tunneling microscopy observations of self-assembled molecular monolayers on a B-doped Si(111) surface, thus allowing to rationalize the competition of long-range dispersion forces between the molecules and the surface. Such a result demonstrates the interest of this predictive approach for further progress in supramolecular chemistry on semiconductor surfaces.

Topological defects in cholesteric liquid crystals induced by monolayer domains with orientational chirality.

Unless stabilized by colloids or confinement with well-defined boundary conditions, defects in liquid crystals remain elusive short-lived objects that tend to disappear with time to minimize the medium's free energy. In this work we use multimodal three-dimensional imaging to visualize cholesteric director structures to show that self-assembled chiral molecular monolayer domains can stabilize topologically constrained defect configurations when in contact with a cholesteric liquid crystal. The cholesteric liquid crystal, having features of both coarse-grained lamellar and nematic liquid crystal with chiral symmetry breaking, allows us to explore the interplay of chirality and implications of layering on the formed defects and director configurations.

Quantum Plasmon Resonances Controlled by Molecular Tunnel Junctions

Quantum tunneling between two plasmonic resonators links nonlinear quantum optics with terahertz nanoelectronics. We describe the direct observation of and control over quantum plasmon resonances at length scales in the range 0.4 to 1.3 nanometers across molecular tunnel junctions made of two plasmonic resonators bridged by self-assembled monolayers (SAMs). The tunnel barrier width and height are controlled by the properties of the molecules. Using electron energy-loss spectroscopy, we directly observe a plasmon mode, the tunneling charge transfer plasmon, whose frequency (ranging from 140 to 245 terahertz) is dependent on the molecules bridging the gaps.