INTRODUCTIONSelf-assembled monolayers (SAMs) offer well-controlled and structurally ordered surfaces that can be used in a variety of studies and applications, including adhesion, wetting, corrosion inhibition, and lubrication. 1 SAMs have also been widely investigated and used in biological systems, optical systems, nanoelectronics, information storage, and the fabrication of biosensors. 1-3 To determine the chemical and physical properties of SAMs (e.g., thickness, roughness, molecular structure, and chemical composition), the monolayers have been characterized by a variety of techniques, such as ellipsometry, contact angle goniometry, spectroscopy (electronic and vibrational), and imaging. [4][5][6][7] One of the more revealing techniques used to characterize SAMs is sum frequency generation imaging microscopy (SFGIM). 8-10 This technique can be used to provide the chemical identification and distribution of the adsorbed molecules on the substrates. This paper examines the chemical properties of multidentate alkanethiolate SAMs in terms of packing density, distribution, and conformational order using mapping analysis with SFGIM.Three types of alkanethiolate SAMs with different alkyl-tosulfur atomic ratios ( Figure 1) were analyzed to explore the influence of the degree of chelation on monolayer structure.Self-assembled monolayers (SAMs) consist of ordered molecular assemblies by chemisorption of molecule on a substrate. 11 To vary the properties of SAMs, the substrate can be changed and/or the adsorbate molecules can be chemically modified for a desired application or study. For the latter, one can vary the nature of the headgroup and/or tailgroup of the molecule before deposition. Moreover, one can also study the reaction or response of the monolayers by exposing the chemisorbed monolayers to selected reagents or reaction conditions. 12 The preparation conditions can also be modified by changing the temperature, solvent, and duration of deposition.Recently, nonlinear optical imaging has become a useful technique in the analysis of SAMs. In particular, second-harmonic generation (SHG) has been effectively used in studies of biological molecules. 13-19 SHG microscopy is a far field imaging method and a second-order nonlinear spectroscopy, which makes it a surface-sensitive technique; however, it provides electronic structure information on the interface, which is not always easy to interpret without specific labeling or any initially surface-active system. Coherent anti-Stokes Raman spectroscopy (CARS) 20-24 is also useful in providing spatially resolved vibrational spectra of the system, but being a third-order nonlinear process, CARS is not an interface-specific technique. More recently, sum frequency generation imaging microscopy (SFGIM) has been developed and used for imaging solutiondeposited and patterned SAMs. This technique is advantageous because it is a vibrational spectroscopy, where external labeling is not required to achieve sensitivity, and it is inherently surface specific. [8][9][10][25][26][...