## 1 Introduction  
The self-assembled monolayers (SAMs) of organic molecules on a substrate usually refer to a single layer of the molecules, in which their headgroups are anchored to the substrate and the molecular chains are aligned in an orderly manner [1; 2; 3; 4]. Derivatizing a substrate with SAMs of organic molecules with tailored end groups is an effective approach to controlling the surface properties of a substrate because it only requires minimal amounts of chemicals to totally alter the surface properties of the substrate. SAMs have aroused enormous interest in interdisciplinary research areas as diversified as the molecular engineering of surfaces [5; 6], materials science [7; 8] and organic electronics [9; 10; 11; 12; 13]. SAM formation normally requires that the molecules have strong interactions with the substrate on which the monolayer is supported so that they can be anchored to the substrate covalently and eventually form close-packed monolayers via van der Waals forces between the molecular chains. For example, one of the most studied SAM systems is alkanethoils on coinage metals with an S-metal bond formation [2]. Another type of molecule used for SAM formation on oxide surface is organosilanes [3; 14; 15; 16; 17; 3], among which octadecyltrimethoxysilane (OTMS) is a popular one. The methoxy groups in OTMS headgroups, denoted as -Si(OCH\({}_{3}\))\({}_{3}\), are hydrolyzed and the silanols (-Si(OH)\({}_{3}\)) are (1) anchored to the oxide surface via the condensation reaction and (2) polymerized between silanols themselves via homocondensation to form a -Si-O-Si- network, which is responsible for SAM formation [18; 19]. More recently, the SAMs of organophosphombic acids, especially octadecylphosphombic acid (OPA or ODPA), formed on oxides have also been explored [2; 20; 21; 22]. In this system, the phosphonic headgroups are anchored to an oxide substrate via the condensation reaction. However, when solvents having a dielectric constant of \(\sim\)4 are used to deliver OPA molecules onto a substrate, H-bonding OPA SAMs can be formed, for example, on a Si wafer [23]. The most frequently used technique to characterize the SAMs of molecules terminated by an alkyl group are contact angles (CAs) and Fourier transform infrared (FTIR) spectroscopy [1]. These SAMs with complete coverage tend to have fixed water CAs, which is due to the surface energy largely determined by the terminating methyl groups. Water CAs are simple in concept, yet extremely useful in assessing the surface chemistry of methyl-terminated SAMs. For example, for SAMs of OTMS, static water CAs are 105\({}^{\circ}\)-113\({}^{\circ}\)[14; 15; 16], which can be used as a reference to gauge the quality of one's own SAMs. FTIR provides crystallinity information on the methylene chains via IR absorption peaks for the methylene stretching modes [1; 13; 24]. It also provides information on headgroup-substrate interactions [19; 25]. Another technique used for SAMs is atomic force microscopy (AFM) for evaluating their coverage of the monolayer and measuring their thickness when they are not made complete coverage [13; 17; 23]. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) [26] is especially powerful in revealing the chemical information of a surface [27; 28; 29]. In this technique, a pulsed (primary) ion beam (e.g., Bi\({}_{3}\)\({}^{\circ}\)) is used to bombard the surface of a sample to generate (secondary) ions from the topmost monolayer (1-3 nm). The secondary ions are extracted, mass separated and detected in parallel, providing a powerful approach to understanding surface chemistry. The probing depth makes ToF-SIMS extremely surface sensitive and useful for exploring the interface chemistry of SAMs because their thickness is commonly around 2 nm [1; 2; 3; 4]. The generation of ions in ToF-SIMS may include molecular information, which depends on how the molecules are attached to a substrate. The superior chemicals selectivity of ToF-SIMS provides a unique opportunity to explore the interface chemistry of SAMs.