AVS2015 Session TF+AS+SS-MoM: Self-Assembled Monolayers, Layer-by-Layer, etc.

In recent years, observations of the presence of a spontaneous and powerful static electric field within thin films of molecular solids have been reported by Field and co-workers [1]. These electric fields, which can approach 10⁸ V m^-1 or more, are believed to arise from alignment of the molecular dipoles in the thin films. Seeking to provide an independent means of observing this phenomenon of the "spontelectric phase", the first new electrically-unique, structural phase to have emerged in decades, we have used reflection-absorption infrared spectroscopy (RAIRS) to investigate thin films of nitrous oxide (N₂O). The presence of a static electric field within the thin film, the defining characteristic of spontelectrics, is demonstrated through the observed temperature dependence of longitudinal-transverse optical (LO-TO) splitting in RAIR spectra, using an analysis based on the vibrational Stark effect [2]. Tentative evidence for the surface-templating of the growth of the spontelectric phase will be presented from RAIRS studies of solid carbon monoxide (CO) on a range of water substrates (porous amorphous solid water, compact amorphous solid water and crystalline water) [3].

[1] Spontaneous electric fields in solid films: spontelectrics. D. Field, O. Plekan, A. Cassidy, R. Balog, N.C. Jones and J. Dunger, Int. Rev. Phys. Chem., 2013, 32, 345-392.

[2] Spontaneously electrical solids in a new light. J. Lasne, A. Rosu-Finsen, A. Cassidy, M. R. S. McCoustra and D. Field, Phys. Rev. Lett., submitted.

[3] Templating dipole alignment in solid carbon monoxide on water ice surfaces. A. Rosu-Finsen, J. Lasne, A. Cassidy, D. Field and M. R. S. McCoustra, Phys. Rev. Lett., in preparation.

Layer-by-Layer (LbL) deposition, combined with metal-ligand coordination, has served as a powerful tool for generating functional architectures.^[2] Such systems might find interesting applications in molecular electronics, sensor, solar cells and data storage. More significantly, owing to their interesting electrochromic (EC) behavior, they are promising candidates for use in smart windows and display devices. In this study we used a dip-coating process to generate molecular assemblies (MA) from metal polypyridyl complexes cross-linked with PdCl₂. These polypyridyl complexes are considered ideal chromophores for fabricating electrochromic materials, due to their excellent stability and light absorption that greatly depends on their oxidation state.^[3],[4] The number of pyridine moieties of the chromophores is varied to control (i) the materials’ stability, (ii) color, (iii) redox-chemistry, and (iv) the film growth (i.e., linear vs. exponential). We also observed that minor structural differences (i.e., the pyridine-bipyridine bond order) at the molecular level become apparent when the stability and electrochromic properties are examined (Figure 1). The MAs exhibit high coloration efficiencies and are extremely stable: they are thermally robust and have exceptionally high (spectro)electrochemical activity. Furthermore, we demonstrated the formation of a first-generation solid-state set-up.^[1],[5]

The use of N-heterocyclic carbenes (NHCs) to modify homogeneous metal catalysts is widespread, however despite the versatility of these complexes, the high metal–NHC bond strength and oxidative stability of NHC–ligated metals, and the ease of synthesis of NHCs, there have been only a handful of reports of mostly ill-defined surfaces funtionalized by NHCs.

We will describe the the use of NHCs to form self-assembled monolayers on gold surfaces. In particular, films prepared by the deposition of 1,3-dihydro- 1,3-bisisopropylbenzimidazol-2-ylidene, show molecular ordering on the surface and remarkable stability. They show no decomposition upon heating for 24 hrs in THF, in boiling in water for 24hrs or upon treatment with acid (pH 2) or base (pH 12). Incredibly, they even survive largely after 24 hr exposure to hydrogen peroxide. This remarkable increase in stability relative to thiol-based SAMs will greatly increase the number of reagents and conditions to which the SAMs can be exposed.

The use of these films in SPR-based biosensing will be described, as will novel methods for the preparation of such films that can be carried out in air on the bench top.

Due to excellent physical properties, Ag thin films have been used as low-e coating, optical mirror, and so on. It is necessary to prevent degradation of the Ag films in air or humid atmosphere. For the purposes, it has been reported that several metal oxide nanolayers and organic monolayers are effective to make Ag thin films stable. Previously, we reported that modification of Ag films with 3-mercaptopropyltrimethoxysilane (MPTMS) monolayer can improve durability of the Ag films after environmental tests because strong bonds were formed between thiol moiety and Ag films, and also between silanol moiety and glass substrate. In the present study, we attempted to use other type of organic molecules, namely straight chain alkylthiols, such as 1-octadecanthiol (1-ODT), 1-dodecanthiol (1-DT) for protection of Ag film surface and compared with MPTMS.

Ag thin films (10nm) were deposited on clean glass substrates by vacuum evaporation. Then monolayer of 1-ODT and 1-DT were formed over the Ag thin films by solution method. The samples were kept in a constant temperature and humidity chamber (40 degree Celsius and 90 RH%) for a week. The electrical resistance, surface morphology, optical transmittance were measured before and after the test. Ag film without the monolayer, and that with MPTMS were also examined for comparison.

The surface roughness of the Ag film without the monolayer drastically increased from 2.7 nm to 27 nm after the test. However, the increase was within 1nm on Ag films with 1-DT and 1-ODT surface layers. These changes were smaller than that on Ag film with MPTMS surface layer. By the measurement of electrical resistivity, it was found that increase in the resistivity after the test was very much suppressed in the Ag film with these monolayers. In addition, optical transmittance measurement showed that transmittance spectrum of Ag films with these monolayers did not change after the test. These results accord with the Ag film morphology change. Consequently, excellent passivation effect of 1-DT and I-ODT surface monolayers on Ag films was confirmed.

We assume the biaxial strain parallel to the GaSb (111) plane. For the bulk with a direct band gap at ambient pressure, GaSb becomes an indirect band gap material under the compressive biaxial strain. The biaxial strain makes the twofold-degenerate heavy-hole and light-hole bands split into two bands at the Gamma point of the valence band. Interestingly, under the biaxial tensile strain, the effective mass of holes becomes anisotropic. We will also report on changes in electronic properties of the GaSb (111) ultrathin films under the biaxial strain.

[1] J. A. del Alamo, Nature 479, 317 (2011)

[2] A. Ohtake, T. Mano, N. Miyata, T. Mori, and T. Yasuda, Appl. Phys. Lett. 104, 032101 (2014)

[3] J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys. 118, 8207 (2003); ibid 124, 219906€ (2006)

Organic monolayers or polymer brushes, often in combination with surface structuring, are widely used to prevent nonspecific adsorption of polymeric or biological material on sensor and microfluidic surfaces. Here we show for the first time how robust, covalently attached alkyne– derived monolayers or ATRP-produced polymer brushes, with a varying numbers of fluorine atoms, on atomically flat Si(111), effectively repel a wide range of apolar polymers without the need for micro– or nanostructuring of the surface. We have studied the antifouling property of fluoro-hydro monolayers and of fluorine-containing polymer brushes towards a range of commonly used polymers/plastics with comparable molecular weight in non– aqueous solvent, and have investigated the effect of polymer molecular weight on the fouling behavior. These studies relied on a range of characterization methods: wettability studies, ellipsometry, X– ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). We developed a novel surface morphology survey by AFM characterization that can accurately quantify the degree of fouling.

These studies consistently displayed that especially the mono–fluorinated (F1) monolayer shows excellent anti– fouling behavior, even more so than e.g. corresponding monolayers with perfluorinated alkyl tails. In this presentation the causes of this unprecedented and surprising finding are discussed. Second, we will focus on polymer brush properties that further reduce the adsorption of polymers. These findings and analysis offer significant potential for antifouling applications of ultrathin and covalently bound fluorine– containing coatings for a range of micro– and nanotechnological applications.

A SAM ideal for this purpose should exhibit a high vertical conductivity. In view of this, we investigated several aromatic and araliphatic SAMs, e.g. terphenyl-terminated monolayers on gold [1]. In a more recent ansatz, we have examined SAMs bearing annulated moieties such as anthracene, which in contrast to oligophenyls are entirely planar [2]. However, as a consequence of the asymmetric substitution of anthracene to the thiol anchor group it is hardly possible to alter the tilt angle of these thiolate molecules within the SAMs [2].

To overcome this drawback, we extended the annulated system of the SAM-forming thiols with an oxazole unit, which allows for a quasi-symmetric attachment to the ancor group. This restores the possibility to influence the tilt angle of the aromatic units using the so-called odd-even effect in SAMs.

Here, we like to present preliminary results on preparation and structural properties of SAMs made from aromatic oxazole thiols on gold surfaces.

REFERENCES

[1] Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Grunze, M.; Zharnikov, M./J. Phys. Chem. B/108, 14462-14469 (2004).

[2] Dauselt, J.; Zhao, J.; Kind, M.; Binder, R.; Bashir, A.; Terfort, A.; Zharnikov, M./ J. Phys. Chem. C/115, 2841-2854 (2011).