Applied Surface Science 288 (2014) 98–108 Contents lists available at ScienceDirect Applied Surface Science jou rn al h omepa g e: www.elsevier.com/locate/apsusc Investigating phosphonate monolayer stability on ALD oxide surfaces Brittany Branch a , Manish Dubey b , Aaron S. Anderson c , Kateryna Artyushkova a , J. Kevin Baldwin d , Dimiter Petsev a , Andrew M. Dattelbaum d,∗ a Nanoscience & Microsystems Engineering and Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131, USA b Lujan Center, Los Alamos National Laboratory, Los Alamos, NM 87545, USA c Physical Chemistry and Applied Spectroscopy, Los Alamos National Laboratory, Los Alamos, NM 87545, USA d Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM 87545, USA a r t i c l e i n f o Article history: Received 20 May 2013 Received in revised form 20 September 2013 Accepted 21 September 2013 Available online 29 September 2013 Keywords: Self-assembled monolayer SAMs Phosphonate stability Phosphonate SAMs Atomic layer deposition Hafnium oxide ALD Alumina ALD a b s t r a c t We report a series of studies aimed at investigating the stability of phosphonate self-assembled mono- layers (SAMs) made from octadecylphosphonic acid (ODPA) or a perfluorinated phosphonic acid (PFPA) on hafnium and aluminum oxide surfaces deposited by atomic layer deposition (ALD). The monolayers were deposited by a series of techniques including self-assembly from solution, tethering by aggregation and growth, and the Langmuir–Blodgett (LB) method. SAMs prepared by LB method were primarily used in our stability investigations because they were found to be the most uniform and reproducible. All films deposited on ALD oxide-coated substrates were characterized by means of water contact angle measure- ments, spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). XPS data conclusively showed covalent phosphonate formation on both substrates. SAMs formed on both Al 2 O 3 and HfO 2 were stable upon exposure to water. PFPA SAMs on HfO 2 were found to be the most stable SAMs studied here in either water or phosphate buffer (PBS) at room temperature. We also show that similar silane-based SAMs made from octadecyltrichlorosilane (OTS) on silicon oxide (SiO 2 ) are less stable in PBS than phosphonate SAMs on atomic layer deposited HfO 2 substrates. These data suggest that phosphonate SAMs should be considered for use in (bio)molecular sensing and actuator devices that utilize ALD and require longer-term stability under aqueous conditions. © 2013 Elsevier B.V. All rights reserved. 1. Introduction In the progress of microelectromechanical (MEMS) fabrication techniques the use of ultrathin film, high-dielectric constant oxides, such as Al 2 O 3 or HfO 2 has become essential [1]. Among many depo- sition methods for ultrathin oxide films, atomic layer deposition technique has attracted the most attention due to its excellent thickness and conformity control [2]. Typically ALD processes are based on binary reaction cycles where two surface reactions occur independently and deposit a film containing two distinct elements. This process differs in thermal oxide growth where the oxide is grown at high temperatures through a bottom up process, where oxygen atoms penetrate into the surface and combine to form an oxide layer. Although thermal oxidation can produce high-quality oxide films, ALD can take place at much lower temperatures and is less sensitive to the underlying substrate to form uniform films with the desired electrical properties. The functionalization of oxide lay- ers that are stable under aqueous conditions is important for a ∗ Corresponding author. Tel.: +1 505 665 0142. E-mail address: amdattel@lanl.gov (A.M. Dattelbaum). number of applications including microelectronics, (bio)sensors, lab-on-a-chip devices and micro-total analytical systems [3–8]. Self-assembled monolayers (SAMs) have been extensively stud- ied as coatings in the above mentioned fields due to their ability to tailor surface properties and relative ease of film formation [9,10]. Molecules capable of forming monolayer-type thin films usually consist of a head group, which binds to the solid surface, a functional tail group that allows the tailoring of the chemical and physical properties of the interface, and a chain group, usually an alkyl chain, which connects the two. SAMs are ordered assemblies of such molecules that are formed spontaneously by the specific adsorp- tion of the head group onto a solid surface while organizing the molecules into dense layers, which is usually due to weaker inter- actions (hydrophobic attraction, Van der Waals forces) between adjacent chains [11,12]. Thiols deposited on gold surfaces are a model SAM system extensively used due to their ease of preparation and well-defined packing order dictated by the metal lattice [13]. They are used for a wide range of applications, but lack long-term stability due to oxidation under ambient conditions or removal from the surface in aqueous environments over time [14,15], and are limited to cer- tain metal surfaces that react with the thiol head group [16]. For some applications, SAM formation on oxide surfaces is preferred 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.09.128