Chemical Imaging and Distribution Analysis of Mono-, Bi-, and Tridentate Alkanethiol Self-Assembled Monolayers on Gold by Sum Frequency Generation Imaging Microscopy

Abstract

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][...

Surface assembly and nanofabrication of 1,1,1-tris(mercaptomethyl)heptadecane on Au(111) studied with time-lapse atomic force microscopy

“…The interplay of a wider intermolecular spacing between adjacent backbones and the larger geometry of the tridentate “foot” provide the rationale for a less dense arrangement of TMMH films. The tridentate adsorbates formed a monolayer in which the alkyl chains are highly disordered on the surface as compared to SAMs derived from monodentate n -alkanethiols reported from studies with sum frequency generation imaging microscopy [ 38 ]. The packing density followed the trend monodentate > bidentate > tridentate.…”

Quantitative Orientation Analysis by Sum Frequency Generation in the Presence of Near-Resonant Background Signal: Acetonitrile on Rutile TiO2 (110)

“…Controversially, the uncertainty of tilt angles (less than ±1°) deduced from the distinct polarization and polarization mapping method are too small to consider them as an orientation distribution of acetonitrile, since even well-ordered monolayers such as selfassembled monolayers exhibit about 10° to 20° of tilt angle distribution. 64 In fact, the small uncertainty in the tilt angle obtained from distinct polarization and polarization mapping method is because the geometry of the SFG configuration leads to the loss of tilt angle sensitivity for the molecules perpendicular to the surface as shown in Figure 3 (a).…”

Advanced Electrochemical Analysis for Energy Storage Interfaces

“…It also currently suffers from issues with local heating and insensitivity to the SEI thickness. As with the other vibrational spectroscopies, SFG mapping is highly desirable but has yet to be applied for surface species in energy storage interfaces. , …”