Molecular self-assembly at bare semiconductor surfaces: cooperative substrate-molecule effects in octadecanethiolate monolayer assemblies on GaAs(111), (110), and (100)

Chiral-Molecule-Based Spintronic Devices

Spintronics and molecular chemistry have achieved remarkable achievements separately. Their combination can apply the superiority of molecular diversity to intervene or manipulate the spin-related properties. It inevitably brings in a new type of functional devices with a molecular interface, which has become an emerging field in information storage and processing. Normally, spin polarization has to be realized by magnetic materials as manipulated by magnetic fields. Recently, chiral-induced spin selectivity (CISS) was discovered surprisingly that non-magnetic chiral molecules can generate spin polarization through their structural chirality. Here, the recent progress of integrating the strengths of molecular chemistry and spintronics is reviewed by introducing the experimental results, theoretical models, and device performances of the CISS effect. Compared to normal ferromagnetic metals, CISS originating from a chiral structure has great advantages of high spin polarization, excellent interface, simple preparation process, and low cost. It has the potential to obtain high efficiency of spin injection into metals and semiconductors, getting rid of magnetic fields and ferromagnetic electrodes. The physical mechanisms, unique advantages, and device performances of CISS are sequentially clarified, revealing important issues to current scientific research and industrial applications. This mini-review points out a key technology of information storage for future spintronic devices without magnetic components.

Structural characterization of α,ω-DH6T monolayer films grown at the liquid-liquid interface

The molecular self-organization of α,ω-dihexylsexithiophene (α,ω-DH6T) monolayers prepared at the solvent-water interface is investigated by complementary microscopy techniques. Our study focuses on the influence of solvents and initial droplet volume on the resulting film morphology. Long-range extended domains in the monolayer regime are detected by visible light microscopy only for toluene. Small-area electron diffraction (SAED) proves the formation of single-crystalline monolayers with structural parameters identical to the organic bulk crystals. In comparison with conventional vacuum sublimated thin films a deviant molecular orientation, derived from near-edge-X-ray absorption fine structure (NEXAFS) in combination with a lower step height measured by atomic-force-microscopy (AFM), indicates a different behaviour of the flexible terminal hexyl chains during growth in a liquid surrounding. Furthermore, a structural degradation over time is observed which is caused by residual solvent molecules that are incorporated during the transfer procedure.

Porous Honeycomb Self-Assembled Monolayers: Tripodal Adsorption and Hidden Chirality of Carboxylate Anchored Triptycenes on Ag

Molecules with tripodal anchoring to substrates represent a versatile platform for the fabrication of robust self-assembled monolayers (SAMs), complementing the conventional monopodal approach. In this context, we studied the adsorption of 1,8,13-tricarboxytriptycene (Trip-CA) on Ag(111), mimicked by a bilayer of silver atoms underpotentially deposited on Au. While tripodal SAMs frequently suffer from poor structural quality and inhomogeneous bonding configurations, the triptycene scaffold featuring three carboxylic acid anchoring groups yields highly crystalline SAM structures. A pronounced polymorphism is observed, with the formation of distinctly different structures depending on preparation conditions. Besides hexagonal molecular arrangements, the occurrence of a honeycomb structure is particularly intriguing as such an open structure is unusual for SAMs consisting of upright-standing molecules. Advanced spectroscopic tools reveal an equivalent bonding of all carboxylic acid anchoring groups. Notably, density functional theory calculations predict a chiral arrangement of the molecules in the honeycomb network, which, surprisingly, is not apparent in experimental scanning tunneling microscopy (STM) images. This seeming discrepancy between theory and experiment can be resolved by considering the details of the actual electronic structure of the adsorbate layer. The presented results represent an exemplary showcase for the intricacy of interpreting STM images of complex molecular films. They are also further evidence for the potential of triptycenes as basic building blocks for generating well-defined layers with unusual structural motifs.

Polymer Brush-GaAs Interface and Its Use as an Antibody-Compatible Platform for Biosensing

Despite evidence showing that polymer brushes (PBs) are a powerful tool used in biosensing for minimizing nonspecific interactions, allowing for optimization of biosensing performance, and the fact that GaAs semiconductors have proven to have a remarkable potential for sensitive biomolecule detection, the combination of these two robust components has never been considered nor evaluated as a platform for biosensing applications. This work reports different methodologies to prepare and tune PBs on the GaAs interface (PB-GaAs) and their potential as useful platforms for antibody grafting, with the ultimate goal of demonstrating the innovative and attractive character of the PB-GaAs interfaces in the enhanced capture of antibodies and control of nonspecific interactions. Three different functionalization approaches were explored, one "grafting-to" and two "grafting-from," in which atom transfer radical polymerization (ATRP) was performed, followed by their corresponding characterizations. Demonstration of the compatibility of Escherichia coli (E. coli) and Legionella pneumophila (Lp) antibodies with the PB-GaAs platform compared to the results obtained with conventional biosensing architectures developed for GaAs indicates the attractive potential for operation of a sensitive biosensor. Furthermore, these results showed that by carefully choosing the nature and preparation methodology of a PB-GaAs interface, it is possible to effectively tune the affinity of PB-GaAs-based sensors toward E. coli and Lp antibodies ultimately demonstrating the superior specificity of the developed biosensing platform.

Monitoring damage of self-assembled monolayers using metastable excited helium atoms

The breaking of molecular bonds during exposure to ionizing radiation and electron beams creates irreversible damage in the molecular structure. In some cases, such as lithography, controlled damage of a molecular resist is a desirable process and is the basis for the entire semiconductor industry. In other cases, such as environmental exposure or probing of the molecular structure, the induced damage is a major problem that has limited advances in science and technology. We report here the use of an in situ probe that is minimally invasive to detect real-time damage induced in organic materials. Specifically, we use metastable excited helium atoms in the ³S₁ state to characterize the damage caused by a low-energy electron beam ∼30 eV on an organic self-assembled monolayer of 11-bromo-1-undecanethiol on a gold substrate. We were able to monitor the damage caused by the electron beam without introducing any additional observed damage by the probing metastable atoms.

Exploration of the Interaction Strength at the Interface of Anionic Chalcogen Anchors and Gold (111)-Based Nanomaterials

Nowadays, the use of sulfur-based ligands to modify gold-based materials has become a common trend. Here, we present a theoretical exploration of the modulation of the chalcogenides-gold interaction strength, using sulfur, selenium, and tellurium as anchor atoms. To characterize the chalcogenide-gold interaction, we designed a nanocluster of 42 gold atoms (Au₄₂) to model a gold surface (111) and a series of 60 functionalized phenyl-chalcogenolate ligands to determine the ability of electron-donor and -withdrawing groups to modulate the interaction. The analysis of the interaction was performed by using energy decomposition analysis (EDA), non-covalent interactions index (NCI), and natural population analysis (NPA) to describe the charge transfer processes and to determine data correlation analyses. The results revealed that the magnitudes of the interaction energies increase following the order S < Se < Te, where this interaction strength can be augmented by electron-donor groups, under the donor-acceptor character the chalcogen-gold interaction. We also found that the functionalization in meta position leads to better control of the interaction strength than the ortho substitution due to the steric and inductive effects involved when functionalized in this position.

Modified Organosilane Monolayers With Enhanced Radiation Stability

Organosilane monolayers containing long carbon chains are susceptible to damage when exposed to X-rays and other radiation during characterization or processing. The origin of the damage has been attributed to both energetic photons and photoelectrons. These particles can break bonds in the molecules comprising the layer altering its composition and structure and can create silanol groups at the interface between the monolayer and the silicon dioxide surface. A common practice to minimize damage is to keep the X-ray beam time as short as possible. In this study, we report a method to deposit an organosilane monolayer on silicon dioxide that withstands X-ray beam damage to the interface. Adding the small monofunctional trimethylchlorosilane to a layer composed of the long-chain trifunctional octadecyltrichlorosilane prevented X-ray beam damage for at least 4 h. Interfacial damage was monitored by titrating silanol groups with TiCl₄ and water atomic layer deposition and measuring the TiO₂ deposited by X-ray photoelectron spectroscopy (XPS). The small monofunctional silane reacted with residual silanol groups capping the siloxane (-Si-O-Si-O-) chains at the interface. The low silanol group concentration minimized X-ray beam damage and prevented TiO₂ from depositing to the detection limit of XPS. Using a small molecule to terminate reactive groups could improve the stability of monolayers not only during characterization and processing, but also when the monolayer is a component in an electrical device by reducing sources of interfacial charge traps.

n-Alkanethiols Directly Grown on a Bare Si(111) Surface: From Disordered to Ordered Transition

We observed the growth phase transition of n-alkanethiols (AT), CH₃(CH₂)_n-1SH, n = 4-16, directly implanted on a bare Si(111) surface, forming an AT monolayer. These monolayers were characterized with static water-contact angle, high-resolution X-ray photoelectron spectroscopy, near-edge X-ray fine-structure spectroscopy, and grazing-angle reflection absorption Fourier-transform infrared spectroscopy. The integrated spectral results indicated that the implanted n-AT molecules formed a self-oriented and densely packed monolayer through formation of an S-Si bond. With the number of carbons in the alkyl chain at six or more, namely beginning at hexanethiol, the molecular monolayer began to develop an orientation-ordered structure, which is clearly shorter than that for AT monolayers on Au and Ag. This result implies that, with a stronger molecule-substrate interaction, an ordered molecular monolayer can form with a short chain.

Molecular Patterning and Directed Self-Assembly of Gold Nanoparticles on GaAs

The ability to create micro-/nanopatterns of organic self-assembled monolayers (SAMs) on semiconductor surfaces is crucial for fundamental studies and applications in a number of emerging fields in nanoscience. Here, we demonstrate the direct patterning of thiolate SAMs on oxide-free GaAs surface by dip-pen nanolithography (DPN) and microcontact printing (μCP), facilitated by a process of surface etching and passivation of the GaAs. A quantitative analysis on the molecular diffusion on GaAs was conducted by examining the writing of nanoscale dot and line patterns by DPN, which agrees well with surface diffusion models. The functionality of the patterned thiol molecules was demonstrated by directed self-assembly of gold nanoparticles (Au NPs) onto a template of 4-aminothiophenol (ATP) SAM on GaAs. The highly selective assembly of the Au NPs was made evident with atomic force microscopy (AFM) and scanning electron microscopy (SEM). The ability to precisely control the assembly of Au NPs on oxide-free semiconductor surfaces using molecular templates may lead to an efficient bottom-up method for the fabrication of nanoplasmonic structures.

The correlations of the electronic structure and film growth of 2,7-diocty[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) on SiO2

Combining ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), atomic force microscopy (AFM) and small angle X-ray diffraction (SAXD) measurements, we perform a systematic investigation on the correlations of the electronic structure, film growth and molecular orientation of 2,7-diocty[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) on silicon oxide (SiO₂). AFM analysis reveals a phase transition of disorderedly oriented molecules in clusters in thinner films to highly ordered standing-up molecules in islands in thicker films. SAXD peaks consistently support the standing-up configuration in islands. The increasing ordering of the molecular orientation with film thickness contributes to the changing of the shape and lowering of the leading edge of the highest occupied molecular orbital (HOMO). The end methyl of the highly ordered standing molecules forms an outward pointing dipole layer which makes the work function (WF) decrease with increasing thickness. The downward shift of the HOMO and a decrease of WF result in unconventional downward band bending and decreased ionization potential (IP). The correlations of the orientation ordering of molecules, film growth and interface electronic structures provide a useful design strategy to improve the performance of C8-BTBT thin film based field effect transistors.

Observation of Shape, Configuration, and Density of Au Nanoparticles on Various GaAs Surfaces via Deposition Amount, Annealing Temperature, and Dwelling Time

Metallic nanoparticles have been widely witnessed in many applications: serving as the catalysts for various nanowire systems, as the active mediums of various device applications, and also for the nanoscale templates for hybrid quantum structures. In the performance of devices and configurations of the resulting nanostructures, the size and density of nanoparticles play critical roles. In this paper, the control of self-assembled Au droplets on GaAs (100), (110), and (111) is systematically investigated through the variation of deposition amount (DA), annealing temperature (AT), and dwelling time (DT). Based on the Volmer-Weber growth model, the formation of Au droplets and dramatic evolution of Au nanostructures on various GaAs surfaces is observed from the Au clusters to the round-dome shapes with the AT variation between 250 and 550 °C. With the systematic DA control, a radical size and density evolution of Au droplets shows the size expansion of over 400 % in average height and 800 % in average lateral diameter, while the density shows over two orders of decrease. With the DT variation, the self-assembled Au droplets tend to grow larger due to the Ostwald ripening while a clear distinction among the surface indexes is observed.

Effects of Immersion Solvent on Photovoltaic and Photophysical Properties of Porphyrin-Sensitized Solar Cells

Memory effects in self-assembled monolayers (SAMs) of zinc porphyrin carboxylic acid on TiO2 electrodes have been demonstrated for the first time by evaluating the photovoltaic and electron transfer properties of porphyrin-sensitized solar cells prepared by using different immersion solvents sequentially. The structure of the SAM of the porphyrin on the TiO2 was maintained even after treating the porphyrin monolayer with different neat immersion solvents (memory effect), whereas it was altered by treatment with solutions containing different porphyrins (inverse memory effect). Infrared spectroscopy shows that the porphyrins in the SAM on the TiO2 could be exchanged with the same or analogous porphyrin, leading to a change in the structure of the porphyrin SAM. The memory and inverse memory effects are well correlated with a change in porphyrin geometry, mainly the tilt angle of the porphyrin along the long molecular axis from the surface normal on the TiO2, as well as with kinetics of electron transfer between the porphyrin and TiO2. Such a new structure-function relationship for DSSCs will be very useful for the rational design and optimization of photoelectrochemical and photovoltaic properties of molecular assemblies on semiconductor surfaces.

Thiolate versus Selenolate: Structure, Stability, and Charge Transfer Properties

Selenolate is considered as an alternative to thiolate to serve as a headgroup mediating the formation of self-assembled monolayers (SAMs) on coinage metal substrates. There are, however, ongoing vivid discussions regarding the advantages and disadvantages of these anchor groups, regarding, in particular, the energetics of the headgroup-substrate interface and their efficiency in terms of charge transport/transfer. Here we introduce a well-defined model system of 6-cyanonaphthalene-2-thiolate and -selenolate SAMs on Au(111) to resolve these controversies. The exact structural arrangements in both types of SAMs are somewhat different, suggesting a better SAM-building ability in the case of selenolates. At the same time, both types of SAMs have similar packing densities and molecular orientations. This permitted reliable competitive exchange and ion-beam-induced desorption experiments which provided unequivocal evidence for a stronger bonding of selenolates to the substrate as compared to the thiolates. Regardless of this difference, the dynamic charge transfer properties of the thiolate- and selenolate-based adsorbates were found to be nearly identical, as determined by the core-hole-clock approach, which is explained by a redistribution of electron density along the molecular framework, compensating the difference in the substrate-headgroup bond strength.

Jones-Ray effect on the organization of lysozyme in the presence of NaNO3 at an air/water interface: is it a cause or consequence?

Interfacial rheology confirms the Jones-Ray effect resulting from a synergy between lysozyme and NaNO₃ at an air/fluid interface.

Conformational transitions of cytochrome c in sub-micron-sized capsules at air/buffer interface

This work presents the design of sub-micron-sized capsules of Cytochrome c (cyt c) in the range 300-350 nm and the conformational transitions of the protein that occur when the films of these capsules spread at the air/buffer interface are subjected to repeated compression-expansion cycles. Steady state fluorescence, time-resolved fluorescence, and circular dichroic (CD) spectra have been used to study the highly compact native conformation (70% helicity) of the protein in the capsules and its stability has been analyzed using cyclic voltammetry. The capsules have been characterized using zeta sizer and high resolution transmission electron microscopy (HRTEM). Surface concentration-surface pressure (Γ-π) isotherms of the films of the capsules spread at air/buffer interface following compression-expansion show destabilizing effect on cyt c. FTIR and CD spectra of these films skimmed from the surface show that the protein transitions gradually from its native helical to an anomalous beta sheet aggregated state. This results from a competition between stabilizing hydrated polar segments of the protein in the capsule and destabilizing nonspecific hydrophobic interactions arising at the air/buffer interface. This 2D model could further our understanding of the spatial and temporal roles of proteins in confined spaces and also in the design of new drug delivery vehicles using proteins.

Influence of a Thiolate Chemical Layer on GaAs (100) Biofunctionalization: An Original Approach Coupling Atomic Force Microscopy and Mass Spectrometry Methods

Widely used in microelectronics and optoelectronics; Gallium Arsenide (GaAs) is a III-V crystal with several interesting properties for microsystem and biosensor applications. Among these; its piezoelectric properties and the ability to directly biofunctionalize the bare surface, offer an opportunity to combine a highly sensitive transducer with a specific bio-interface; which are the two essential parts of a biosensor. To optimize the biorecognition part; it is necessary to control protein coverage and the binding affinity of the protein layer on the GaAs surface. In this paper; we investigate the potential of a specific chemical interface composed of thiolate molecules with different chain lengths; possessing hydroxyl (MUDO; for 11-mercapto-1-undecanol (HS(CH₂)₁₁OH)) or carboxyl (MHDA; for mercaptohexadecanoic acid (HS(CH₂)₁₅CO₂H)) end groups; to reconstitute a dense and homogeneous albumin (Rat Serum Albumin; RSA) protein layer on the GaAs (100) surface. The protein monolayer formation and the covalent binding existing between RSA proteins and carboxyl end groups were characterized by atomic force microscopy (AFM) analysis. Characterization in terms of topography; protein layer thickness and stability lead us to propose the 10% MHDA/MUDO interface as the optimal chemical layer to efficiently graft proteins. This analysis was coupled with insitu MALDI-TOF mass spectrometry measurements; which proved the presence of a dense and uniform grafted protein layer on the 10% MHDA/MUDO interface. We show in this study that a critical number of carboxylic docking sites (10%) is required to obtain homogeneous and dense protein coverage on GaAs. Such a protein bio-interface is of fundamental importance to ensure a highly specific and sensitive biosensor.

Adsorption site determination of a molecular monolayer via inelastic tunneling

We have combined scanning tunneling microscopy with inelastic electron tunneling spectroscopy (IETS) and density functional theory (DFT) to study a tetracyanoethylene monolayer on Ag(100). Images show that the molecules arrange in locally ordered patterns with three nonequivalent, but undeterminable, adsorption sites. While scanning tunneling spectroscopy only shows subtle variations of the local electronic structure at the three different positions, we find that vibrational modes are very sensitive to the local atomic environment. IETS detects sizable mode frequency shifts of the molecules located at the three topographically detected sites, which permits us to determine the molecular adsorption sites through identification with DFT calculations.

Solid-State Densification of Spun-Cast Self-Assembled Monolayers for Use in Ultra-Thin Hybrid Dielectrics

Ultra-thin self-assembled monolayer (SAM)-oxide hybrid dielectrics have gained significant interest for their application in low-voltage organic thin film transistors (OTFTs). A [8-(11-phenoxy-undecyloxy)-octyl]phosphonic acid (PhO-19-PA) SAM on ultrathin AlOx (2.5 nm) has been developed to significantly enhance the dielectric performance of inorganic oxides through reduction of leakage current while maintaining similar capacitance to the underlying oxide structure. Rapid processing of this SAM in ambient conditions is achieved by spin coating, however, as-cast monolayer density is not sufficient for dielectric applications. Thermal annealing of a bulk spun-cast PhO-19-PA molecular film is explored as a mechanism for SAM densification. SAM density, or surface coverage, and order are examined as a function of annealing temperature. These SAM characteristics are probed through atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure spectroscopy (NEXAFS). It is found that at temperatures sufficient to melt the as-cast bulk molecular film, SAM densification is achieved; leading to a rapid processing technique for high performance SAM-oxide hybrid dielectric systems utilizing a single wet processing step. To demonstrate low-voltage devices based on this hybrid dielectric (with leakage current density of 7.7×10-8 A cm-2 and capacitance density of 0.62 µF cm-2 at 3 V), pentacene thin-film transistors (OTFTs) are fabricated and yield sub 2 V operation and charge carrier mobilites of up to 1.1 cm2 V-1 s-1.

Origin of the contact angle hysteresis of water on chemisorbed and physisorbed self-assembled monolayers

Self-assembled monolayers (SAMs) are known to form on a variety of substrates either via chemisorption (i.e., through chemical interactions such as a covalent bond) or physisorption (i.e., through physical interactions such as van der Waals forces or "ionic" bonds). We have studied the behavior and effects of water on the structures and surface energies of both chemisorbed octadecanethiol and physisorbed octadecylamine SAMs on GaAs using a number of complementary techniques including "dynamic" contact angle measurements (with important time and rate-dependent effects), AFM, and electron microscopy. We conclude that both molecular overturning and submolecular structural changes occur over different time scales when such SAMs are exposed to water. These results provide new insights into the time-dependent interactions between surfaces and colloids functionalized with SAMs when synthesized in or exposed to high humidity or bulk water or wetted by water. The study has implications for a wide array of phenomena and applications such as adhesion, friction/lubrication and wear (tribology), surfactant-solid surface interactions, the organization of surfactant-coated nanoparticles, etc.

RECONSTITUTION OF A PROTEIN MONOLAYER ON THIOLATES FUNCTIONALIZED GaAs SURFACE

In the aim to realize an efficient resonant biosensor, gallium arsenide (GaAs) presents many advantages. In addition to its properties of transduction, GaAs is a crystal for which microfabrication processes were developed, conferring the possibility to miniaturize the device and integrate electronic circuit. Moreover, the biofunctionalization could be realized on the crystalline surface without layer deposition, constituting a real advantage to perform reusable sensor. The functionalization of GaAs surface was engaged in order to immobilize a protein monolayer on this substrate. Functionalization was done using a mixed self assembled monolayer of thiolate molecules. Characterizations at micro and nanoscale were performed to control the surface state, the establishment of thiolates self-assembled monolayer, the surface atomic composition and the topography of the GaAs substrate at the different steps of the process. Protein immobilization on thiolates modified GaAs was revealed through a detailed AFM study and in situ MALDI-TOF MS and MS/MS modified surface interrogations.

Directing substrate morphology via self-assembly: ligand-mediated scission of gallium-indium microspheres to the nanoscale

We have developed a facile method for the construction of liquid-phase eutectic gallium-indium (EGaIn) alloy nanoparticles. Particle formation is directed by molecular self-assembly and assisted by sonication. As the bulk liquid alloy is ultrasonically dispersed, fast thiolate self-assembly at the EGaIn interface protects the material against oxidation. The choice of self-assembled monolayer ligand directs the ultimate size reduction in the material; strongly interacting molecules induce surface strain and assist particle cleavage to the nanoscale. Transmission electron microscopy images and diffraction analyses reveal that the nanoscale particles are in an amorphous or liquid phase, with no observed faceting. The particles exhibit strong absorption in the ultraviolet (∼200 nm), consistent with the gallium surface plasmon resonance, but dependent on the nature of the particle ligand shell.

Direct assembly of hydrophobic nanoparticles to multifunctional structures

We present a general process that allows convenient production of multifunctional composite particles by direct self-assembly of hydrophobic nanoparticles on host nanostructures containing high-density surface thiol groups. Hydrophobic nanoparticles of various compositions and combinations can be directly assembled onto the host surface through the strong coordination interactions between metal cations and thiol groups. The resulting structures can be further conveniently overcoated with a layer of normal silica to stabilize the assemblies and render them highly dispersible in water for biomedical applications. As the entire fabrication process does not involve complicated surface modification procedures, the hydrophobic ligands on the nanoparticles are not disturbed significantly so that they retain their original properties such as highly efficient luminescence. Many complex composite nanostructures with tailored functions can be efficiently produced by using this versatile approach. For example, multifunctional nonspherical nanostructures can be efficiently produced by using mercapto-silica coated nano-objects of arbitrary shapes as hosts for immobilizing functional nanoparticles. Multilayer structures can also be achieved by repeating the mercapto-silica coating and nanoparticle immobilization processes. Such assembly approach will provide the research community a highly versatile, configurable, scalable, and reproducible process for the preparation of various multifunctional structures.