Self-Assembled Monolayers on Aluminum and Copper Oxide Surfaces: Surface and Interface Characteristics, Nanotribological Properties, and Chemical Stability

Abstract

The formation of self-assembled monolayer (SAM) films onto aluminum and copper oxide surfaces by reaction with 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octadecylphosphonic acid (ODP), decylphosphonic acid (DP), octylphosphonic acid (OP), and 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS) is discussed in this chapter. The properties and chemical stability of the films have been investigated using complementary surface analysis techniques: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, contact anglemeasurements, and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS). XPS data confirm the presence of alkylphosphonate (perfluorinated and nonperfluorinated) and perfluorosiloxy molecules in the PFDP/Al, ODP/Al, DP/Al, OP/Al, and PFMS/Al SAMs formed on aluminum oxide surfaces. The sessile drop static contact angles of deionized water on PFDP/Al and PFMS/Al are typically more than 130◦ and on ODP/Al, DP/Al, and OP/Al typically more than 125◦, indicating that Al surfaces reacted with alkylphosphonic acids and alkylsilanes are very hydrophobic. The surface roughness for PFDP/Al, ODP/Al, DP/Al, OP/Al, PFMS/Al, and unmodified Al is approximately 35 nm, as determined by AFM. The critical surface tension for PFDP/Al has been determined to be approximately 11 mJm−2 (mNm−1) by the Zisman plot method compared with 16, 20, 21, and 25 mJm−2 for PFMS/Al, ODP/Al, DP/Al, and OP/Al, respectively. PFDP/Al gives the lowest adhesion and friction force, while unmodified Al gives the highest. The adhesion and friction forces for ODP/Al and DP/Al SAMs are in-between those of PFDP/Al and Al. The influence of relative humidity, temperature, and sliding velocity on the friction and adhesion behavior has also been studied. Failure mechanisms of SAMs have been investigated by wear tests. The chemical stability of ODP/Al, PFDP/Al, DP/Al, OP/Al, and PFMS/Al SAMs has been tested by exposure to warm nitric acid (pH 1.8, 30 min, 60– 95 ◦C). The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid or silane molecule and the oxidized Al surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60–95 ◦C) show ODP/Al SAMs to be most stable, followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al. Hydrophobic, low adhesion, and robust Al surfaces have useful applications for microelectromechanical/nanoelectromechanical systems (MEMS/NEMS), such as the digital micromirror device. These studies are expected to aid in the design and selection of proper lubricants and antistiction coatings forMEMS/NEMS. The PFMS SAM on Cu is found to be extremely hydrophobic, typically having sessile drop static contact angles of more than 130◦ for deionized water and a critical surface tension of 14 mJm−2. FFM shows a significant reduction in the adhesive force and friction coefficient of PFMS-modified Cu (PFMS/Cu) compared with unmodified Cu. Treatment by exposure to harsh conditions shows that a PFMS/CuSAMcan withstand boiling nitric acid (pH 1.8), boiling water, and warm sodium hydroxide (pH 12, 60 ◦C) solutions for at least 30 min. Furthermore, no SAM degradation is observed when PFMS/Cu is exposed to warm nitric acid solution for up to 70 min at 60 ◦C or 50 min at 80 ◦C. XPS and FT-IRRAS data reveal a coordination of the PFMS Si atom with a cuprate (CuO) molecule present on the oxidized Cu substrate. The data give good evidence that the stability of the SAM film on the PFMS-modified oxidized Cu surface is largely due to 236 E. Hoque et al. the formation of a siloxy–copper (–Si–O-Cu–) bond via a condensation reaction between silanol (–Si–OH) and copper hydroxide (CuOH). Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs films could be useful for surface passivation, corrosion inhibition and/or as antiwetting/low-adhesion promoters in microelectronical/nanoelectromechanical devices or on heat-exchange surfaces (dropwise condensation).