Sum Frequency Generation Imaging Microscopy Research Articles

Direct imaging of electric field behavior in 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene organic field-effect transistors by sum-frequency generation imaging microscopy.

Sum-frequency generation imaging microscopy combined with compressive-sensing (CS-SFG) is a powerful micro-spectroscopic technique for probing interfaces and surfaces with a spatial resolution where contrast is based on the chemical functional groups. We reported the use of the CS-SFG technique to probe the electric field due to charge accumulation and the internal electric field in operating organic field-effect transistors (OFETs) with the aluminum oxide and octadecylphosphonic acid (ODPA) self-assembled monolayer as the gate dielectric layer and 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT) as the semiconductor layer. In addition, the electric field behavior was discussed by a difference in the electric field induced SFG intensity between the open-circuit and the voltage application conditions. The SFG peak of CH stretching mode derived from methyl groups of ODPA and phenyl groups of DPh-BTBT could be observed at each interface of ODPA/DPh-BTBT or DPh-BTBT/Au, respectively. Moreover, the electric field induced SFG coming from ODPA/DPh-BTBT shows the presence of intense electric field due to charge injection and accumulation near the drain and source electrode edges under the operation of OFETs. Our studies show that the electric field-induced SFG imaging technique is useful for probing the local electric field distribution or charge accumulation behavior in OFETs under operating conditions.

Multicolor Chemical Imaging by Sum Frequency Generation Imaging Microscopy of Monolayers on Metal Surfaces

Sum frequency generation imaging microscopy (SFG-IM) is a unique surface-specific technique that can detect the spatial distributions of differing monolayer species based on chemical contrast. Here...

Sum Frequency Generation Imaging Microscopy of Self-Assembled Monolayers on Metal Surfaces: Factor Analysis of Mixed Monolayers

Sum frequency generation (SFG) images of microcontact patterned self-assembled alkanethiol monolayers on metal surfaces were analyzed by factor analysis (FA) to determine the spatial distribution of the patterned monolayers over the images. Additionally, each significant abstract factor produced by FA was assessed to determine the information contained within it. These results indicate that FA of the SFG spectra is a promising method to determine the composition and identities of mixed alkanethiol systems that show different vibrational spectra and image contrast. Factor analysis has successfully been applied to SFG images obtained with low signals, which reduces the time required for full spectral SFG imaging.

Surface-Induced Heterogeneity Analysis of an Alkanethiol Monolayer on Microcrystalline Copper Surface Using Sum Frequency Generation Imaging Microscopy

An octadecanethiol (ODT) self-assembled monolayer on microcrystalline copper is investigated by sum frequency generation (SFG) imaging microscopy and electron backscattering diffraction (EBSD). The strong SFG signal contrast across the domain boundary indicates the existence of grain structures on copper surface, which is further verified by EBSD measurements. The nonresonant contribution of SFG response shows anisotropy with respect to the in-plane rotation of the sample relative to the surface normal and varies in each crystal domain area. The resonant contribution of the monolayer, such as the amplitude ratio of CH3-sym/CH3-asym, is azimuthally nearly isotropic. Since the zzz tensor component of nonlinear susceptibility dominates, the resonant part of SFG spectra on the metal surface does not show any anisotropy. Further, a strong correlation between the local metal structures with the top monolayer packing behaviors is identified based on the statistical distribution analysis. Using the methyl group a...

Roles of oxygen for methanol adsorption on polycrystalline copper surface revealed by sum frequency generation imaging microscopy

The adsorption of atmospheric pressure methanol on the polycrystalline copper surface has been studied by a combination of sum frequency generation imaging microscopy (SFGIM) and temperature programmed desorption (TPD). Methoxy species can be generated by exposing the polycrystalline copper surface to methanol vapor at room temperature. SFGIM results demonstrate that oxygen promotes the surface adsorption of methanol and the increase in the amount of methoxy produced on copper surface. SFGIM orientation analysis suggests the methoxy monolayer is oriented closer to the surface normal with introduction of oxygen. Employing the image statistical analysis approach, the heterogeneities and conformation distribution of methoxy monolayers on copper surface with and without oxygen adsorption are compared. These results illustrate SFGIM indeed could provide more insight for understanding the heterogeneous metal/metal oxide surface in the molecular level.

Monitoring Localized Initial Atmospheric Corrosion of Alkanethiol-Covered Copper Using Sum Frequency Generation Imaging Microscopy: Relation between Monolayer Properties and Cu2O Formation

In order to evaluate the interfacial monolayer order on copper surfaces in two-dimensions, the tilt angle distribution and orientation-conformation correlation maps were investigated using sum frequency generation imaging microscopy (SFGIM). In this paper, these methods of analyses were further utilized in evaluating the effect of the formation of a thin oxide film on the monolayer order of octadecanethiol (ODT) deposited on Cu. By chemically stripping the native oxide of the copper surface using amidosulfonic acid (ASA), the spontaneous atmospheric oxidation of Cu from an initial reduced state was observed as a change in the nonresonant phase response of the sum frequency generation (SFG) spectral line shape obtained from the SFGIM setup configured with a 1064 nm probe beam. Statistical calculations and contour maps for the CH-stretch modes of the terminal methyl and methylene group showed the overall decrease in the mean tilt angle and increase in the amount of alkyl gauche defects as a result of the ox...

Image Contrast in Sum Frequency Generation Microscopy Based on Monolayer Order and Coverage

Sum frequency generation imaging microscopy (SFGIM) was used to study a binary system of self-assembled monolayers (SAMs) on evaporated gold. A two-step process that consists of microcontact printing (μCP) and the solution backfilling method is used to generate a sample surface with two distinct SAMs domains. The high contrast chemical identification and the homogeneity map of the SAMs for two different head groups are revealed by processing the three-dimensional images (xy-surface and infrared wavenumbers) obtained from SFGIM. For conformation order analysis, the amplitude ratios of the methylene and methyl symmetric resonance obtained by a nonlinear curve fit of the spatially and spectrally resolved SFG images are used to reconstruct spatial image maps. The conformation order map shows the distribution of gauche defects in SAMs with improved contrast across monodentate and bidentate alkanethiol monolayer regions. The results show that variation of the head group attached on the surface dominantly influences the conformation disorder and surface coverage of the molecules, while the terminal methyl group in both monodentate and bidentate SAMs shows a similar orientation angle.

Scale Dependence of the Orientation and Conformation Distribution Analysis of a Molecular Monolayer Using Sum Frequency Generation Imaging Microscopy

A systematic approach of experimentally measuring the average orientation and distribution of octadecanethiol (ODT) monolayer on gold was performed by using sum frequency generation imaging microscopy (SFGIM). By constructing maps from region of interests (ROIs) of different length scales, measuring from 5 to 1000 μm, two-dimensional domains could be distinguished to contain less-oriented monolayers. By generating the tilt angle distributions of the scaled ROIs, SFGIM determines the experimental orientation, θ, and distribution width, σ0, as dictated by the SFGIM resolution limit. Using the same statistical methods, the heterogeneities of ODT monolayers on Au and Cu were compared. The quantitative analysis of these orientation parameters is essential for future work on characterizing monolayer integrity on more complex surfaces such as reactive metals and alloys. Composite maps that merge orientation and conformational information were demonstrated as potential tools for spectromicroscopic measurement of the reactivity of metal substrates to surface processes such as corrosion and wettability.

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

Sum frequency generation (SFG) microscopy was used to investigate the structure and orientation of multidentate alkanethiolate monolayers on gold surfaces. Adsorbate molecules in which one, two, or three sulfur atoms coordinated to the surface of gold afforded self-assembled monolayers (SAMs) with controlled spacing between the alkyl chains. SFG imaging was used to determine the orientation of the molecules on the surface and to visualize spatial variations across the surface. Using SFG imaging, the average orientation of the terminal methyl group is measured as well as the standard deviation. Further, the presence of film defects is measured by the CH2/CH3 intensity ratio and the distribution quantified through the standard deviation. The results demonstrate that the tridentate adsorbate forms a monolayer in which the alkyl chains are highly disordered on the surface when compared to SAMs derived from normal alkanethiols (i.e., monodentate adsorbates). The results are consistent with a model in which the...

Chemical Microscopy of Surfaces by Sum Frequency Generation Imaging

Sum frequency generation imaging microscopy has been developed and implemented in different systems from self-assembled monolayer on metal surfaces to reaction of a metal surface upon exposure to a...

Sum Frequency Generation Imaging Microscopy of Patterned Self-Assembled Monolayers with Terminal −CH3, −OCH3, −CF2CF3, −C═C, −Phenyl, and −Cyclopropyl Groups

Vibrational spectroscopic imaging is demonstrated for a variety of organic monolayer-functionalized surfaces patterned using microcontact printing. The images from sum frequency generation imaging microscopy (SFGIM) are analyzed using different contrast mechanisms in the interpretation of the transition from stamped to backfilled regions of interest. For this experiment, microcontact printing is used to spatially control the surface monolayers by using a patterned stamp and by varying the terminal functional group of the backfilling solutions. Analysis by the three different methods suggests that significant mixing occurs between the stamped and backfilled regions, which influence the contrast in the images at the resonant peaks. In addition, the interference between the resonant peaks and nonresonant background also has an effect on the appearance of the image.

Chemical Imaging of Corrosion: Sum Frequency Generation Imaging Microscopy of Cyanide on Gold at the Solid−Liquid Interface

Sum frequency generation (SFG) imaging is used to monitor, in situ, the reaction of cyanide ions with gold surface. Spatial and chemical variations across the surface are observed as a function of time. The initial period resulted in the formation of linearly bound cyanide to gold, and continuous exposure of gold film to cyanide solution led to the presence of higher-coordinated gold-cyanide complexes. These species were identified by their specific position in the SFG vibrational spectrum (2105, 2140, 2170, and 2225 cm(-1)). The relative amounts of these gold-cyanide species varied across the surface as resolved by SFG microscopy.

Adsorption and Structure of Octadecanethiol on Zinc Surfaces As Probed by Sum Frequency Generation Spectroscopy, Imaging, and Electrochemical Techniques

Octadecanethiol (ODT) adsorbed onto zinc has been studied with sum frequency generation (SFG), sum frequency generation imaging microscopy (SFG-IM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and electrical impedance spectroscopy (EIS) in order to investigate its corrosion protective ability and conformational ordering. SFG shows that ODT forms an ordered adsorbate on both reduced and oxidized zinc within short times after immersion in 1 mM ODT/ethanol solution. The corrosion protection, deduced by EIS, is also improved after immersion in the ODT solution. After longer immersion times, the corrosion protection decreases as well as the conformational order of the adsorbed ODT. Increasing the ODT concentration avoids this degradation with prolonged immersion time. The ODT is seen in the XPS spectra to adsorb to the reduced as well as the oxidized zinc by forming a Zn−S bond for both short and long immersion times. The SFG-IM completes the picture, showing a heterogeneous surface with a...

Spatially Resolved Surface Analysis of an Octadecanethiol Self-Assembled Monolayer on Mild Steel Using Sum Frequency Generation Imaging Microscopy

Sum frequency generation imaging microscopy has been used to investigate a self-assembled monolayer of an alkanethiol (octadecanethiol, ODT) on a mild steel surface. The images are used to analyze the orientation of the alkanethiol monolayer and the distribution of orientational angles as well as defects in the film. The results show that, on average, ODT forms an ordered monolayer on mild steel when compared to the same monolayer on gold. However, the image analysis suggests that the distribution of tilt angles and conformational defects is greater for ODT on a mild steel surface compared to ODT/Au.

Sum Frequency Generation Imaging Microscopy of CO on Platinum

Sum frequency vibrational spectroscopy is utilized as an imaging technique to distinguish and compare the local response of carbon monoxide (CO) covered platinum (Pt) polycrystalline surface versus the average response of the investigated area. The Pt electrode was prepared using the standard method and was exposed to approximately 1 atm of CO(g). SFG images and vibrational spectra were obtained where the contrast is based on the intrinsic nature of each peak in the CO vibrational spectrum. The illustration of the images and the chemical maps of CO on the platinum surface showed the distribution of the CO across the observed area. The results obtained by comparing the local and the average response confirmed the spatial distributions of the CO on the platinum sample which are due to several reasons such as dipole-dipole coupling and surface coverage. This finding has a significant contribution toward recognizing that surfaces usually considered homogeneous may in fact be quite heterogeneous.

Sum Frequency Generation Microscopy of Microcontact-Printed Mixed Self-Assembled Monolayers

Sum frequency generation imaging microscopy (SFGIM) is used to image the chemically distinct regions of a microcontact-printed monolayer surface. The contrast in the images is based on the vibrational spectrum of each component in the monolayer. Mixtures of C16 thiols on gold with CH3 and phenyl termination are imaged with a resolution of approximately 10 microm. Microcontact printing produces films that are different compared to the immersion procedure of forming self-assembled monolayers. The SFGIM technique is able to obtain a vibrational spectrum at each point on the surface and demonstrate that the stamped area has significant mixing with the molecules deposited from the backfilling solution.