Construction of Binding Group Terminated Self-Assembled Monolayers for Multilayer Formation

Introduction.

       The self-assembly technique is one of the best methods to arrange the molecule in order onto the solid substrate surface and the alkylthiol self-assembled monolayers (SAMs) on gold and alkyl SAMs on silicon have been widely studied because of their high stability, high packing density, and high orientation [1,2]. Thus, they are expected to apply to the modern nanotechnology such as sensors, wetting control, corrosion inhibition, and biomolecular and molecular electric devices [1,2]. Moreover, using the binding group terminated SAM as a substrate, multilayer with various functionalities can be constructed. In order to control the multilayer thickness and functionality, we have to construct highly oriented and well-packed SAMs with binding group as a terminal group [3].

       In this study, we constructed four kinds of binding group terminated SAMs such as 4-mercaptobenzoic acid (4-MBA) and 3-mercaptopropionic acid (3-MPA) SAMs on Au(111) and 2-methoxy ethyl (2-ME) and 11-ethoxy undecyl (11-EU) SAMs on Si(111) by various preparation conditions and their characteristics were estimated by electrochemical measurements for the reductive desorption, angle-resolved X-ray photoelectron spectroscopy (ARXPS), attenuated total reflection - Fourier transformed infrared (ATR-FTIR) spectroscopy, and contact angle measurements.

Experimentals.

       After Au(111) single crystal disks were annealed by a gas burner and quenched with ultrapure water, they were immersed in ethanol solution containing 1 mM 4-MBA or 1 mM 3-MPA for certain periods at room temperature. From the results of the electrochemical measurements for the reductive desorption and of ARXP specta, density, stability, and orientation of the constructed 4-MBA and 3-MPA SAMs were investigated in order to find the optimum preparation conditions of the densely-packed and well-oriented SAMs on Au(111).

       After etching of n-typed Si(111) prisms, 2-ME and 11-EU SAMs were prepared on Si(111) by dipping Si prism into liquid state of 2-ME or 11-EU under UV light irradiation and nitrogen atmosphere for several hours. From the results of the ATR-FTIR spectra and contact angle measurement, density, stability, and orientation of the constructed 2-ME and 11-EU SAMs were investigated in order to find the optimum preparation conditions of the densely-packed and well-oriented SAMs on Si(111).

Results and Discussion.

       Linear sweep voltammograms (LSVs) measured in a deaerated 0.1 M KOH at Au(111) electrodes modified with the 4-MBA SAMs, which were prepared for several dipping periods, showed cathodic peaks due to the reductive desorption of 4-MBA, around -0.60 V. With increasing of the dipping period, the peak potential shifted to negative, namely stability increased, and the surface coverage of 4-MBA, namely density, increased. However, full width of half maximum (fwhm) of the cathodic peak, which shows homogeneity of SAM’s orientation, decreased until the dipping period of 1 h and then increased with increasing of the dipping period. This result indicated that repulsion interaction between carboxylate groups should increase with increasing of the dipping period, namely density. For 5 h dipping, the surface coverage was saturated to 0.52 nmol cm^-2. From the XP spectrum of the 4-MBA SAM, we confirmed that the 4-MBA SAM prepared by the dipping period of 5 h is highly stable. As compared with 4-MBA SAM on Au(111) prepared for the same dipping period, 3-MPA SAM on Au(111) was less stable, less densely-packed, and less highly oriented, because van der Waals interaction between 4-MBA molecules, namely hydrophobic interaction between phenyl groups, should be stronger than that between 3-MPA, namely hydrophobic interaction between propyl chains. Modification of metal complex molecules such as metallo-porphyrin and metallo-phthalocyanine on the 4-MBA SAM modified Au(111) is now under investigation.

       On the other hand, 2-ME and 11-EU SAMs on Si(111) was relatively stable, because C-Si bonding energy is higher than S-Au bonding energy. As compared with 2-ME SAM on Si(111) for the same preparation period, 11-EU SAM was more stable, more densely-packed, and more highly oriented on Ai(111), because van der Waals interaction between 11-EU, namely hydrophobic interaction between longer chains, should be higher than those between shorter chains. Now we are trying to find preparation condition to construct more stable, densely-packed, and highly oriented 2-ME SAM on Si(111) because shorter chain SAM is easy to use as a substrate for multilayer formation.

References.

 [1] A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly, Academic Press, New York (1990).

[2]   T. Kondo, R. Yamada, and K. Uosaki, in Organized Organic Ultrathin Films – Fundamentals and Applications, Wiley-VCH, Chap 2 (2012).

[3]   T. Arisaka, B. Zhang, N. Aoki, and T. Kondo, 224th ECS Meeting, San Francisco, #44 (2013).