Adsorption of Alkanethiols on Gold Surfaces: PM-IRRAS Study of the Influence of Terminal Functionality on Alkyl Chain Orientation

Phosphonic acid (PA) self-assembled monolayers (SAMs) are utilized at critical interfaces between transparent conductive oxides (TCO) and organic active layers in organic photovoltaic devices (OPVs). The effects of PA deposition method and time on the formation of close-packed, high-quality monolayers is investigated here for SAMs fabricated by solution deposition, micro-contact printing, and spray coating. The solution deposition isotherm for pentafluorinated benzylphosphonic acid (F5BnPA) on indium-doped zinc oxide (IZO) is studied using polarization modulation-infrared reflection-absorption spectroscopy (PM-IRRAS) at room temperature as a model PA/IZO system. Fast surface adsorption occurs within the first min; however, well-oriented high-quality SAMs are reached only after ~48 h, presumably through a continual process of molecular adsorption/desorption and monolayer filling accompanied by molecular reorientation. Two other rapid, soak-free deposition techniques, micro-contact printing and spray coating, are also explored. SAM quality is compared for deposition of phenyl phosphonic acid (PPA), F13-octylphosphonic acid (F13OPA), and pentafluorinated benzyl phosphonic acid (F5BnPA) by solution deposition, micro-contact printing and spray coating using PM-IRRAS. In contrast to micro-contact printing and spray coating techniques, 48-168 h solution deposition at both room temperature and 70 °C result in contamination-and surface etch-free close-packed monolayers with good reproducibility. SAMs fabricated by microcontact printing and spray coating are much less well ordered.

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“…PM-IRRAS is the technique of choice for this work because of its high surface sensitivity and molecular specificity. [36][37][38][39]…”

Section: Introductionmentioning

confidence: 99%

Effect of time and deposition method on quality of phosphonic acid modifier self-assembled monolayers on indium zinc oxide

Sang

Knesting

Bulusu

et al. 2016

Applied Surface Science

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“…It should be noted that peak positions of methylene stretching bands shift to lower wavenumbers for well-packed monolayers. ,, The vibrational Stark effect critically depends on dipole moment orientation; therefore, the appearance of significant wavenumber shift indicates that methylene stretching vibration motion is oriented somewhat along the surface normal. It should be noted that the transition dipole moment direction for ν s (CH 2 ) mode lies in the CCC plane and perpendicular to the hydrocarbon axis . Thus, we speculate that a majority of WC14 molecules adopt horizontal orientation with respect to the surface.…”

Section: Resultsmentioning

confidence: 73%

“…It should be noted that the transition dipole moment direction for ν s (CH 2 ) mode lies in the CCC plane and perpendicular to the hydrocarbon axis. 69 Thus, we speculate that a majority of WC14 molecules adopt horizontal orientation with respect to the surface. In fact, the previously discussed predominant gauche orientation of headgroup (S−C−C) complements such a picture.…”

Section: ■ Results and Discussionmentioning

confidence: 82%

Potential-Induced Structural Alterations in the Tethered Bilayer Lipid Membrane-Anchoring Monolayers Revealed by Electrochemical Surface-Enhanced Raman Spectroscopy

2020

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Mixed self-assembled monolayers (SAMs) are widely used to anchor tethered lipid bilayer membranes (tBLMs) that mimic biological membranes and are useful platforms for fundamental studies and construction of biosensors. Electrode potential is an important variable in the construction of biosensors based on tBLMs and fundamental studies. In this work, potential-induced structural changes in mixed SAMs composed of lipid-like long-chain WC14 molecules and short surface backfiller 2-mercaptoethanol (ME-D4) was probed in situ by isotope-edited electrochemical surface-enhanced Raman spectroscopy (EC-SERS) coupled with first-principles calculations, reductive desorption voltammetry, and generalized two-dimensional correlation spectroscopy (2DCOS). We show that negative electrode polarization leads to the lengthening of Au–S and Au–O bonds and reorganization of molecules in the headgroup region. Calculations predicted the preferential formation of Au–O bonds of ME-D4 at more positive electrode potentials. Alkane chains of WC14 form potential-induced clusters at more negative electrode polarization in mixed monolayers, a phenomenon that is witnessed from all-trans vibrations of the C–C group at 1129 cm–1. The evidence of the formation of tBLM on mixed WC14/ME-D4 SAMs is documented by SERS. Specifically, the peak position of olefin group stretching vibration suggests a fluid-like structure of tBLM. This work provides vibrational spectroscopy evidence for the potential-induced formation of phase-segregation clusters of the hydrophobic thiols in mixed monolayers, a phenomenon that should be considered in preparation of tBLMs, construction of membrane-based biosensors, bioengineering developments, and studies of biomolecular processes using model membranes.

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“…PM-IRRAS can detect differences in monolayer ordering that impact alignment of alkyl C−H stretch dipoles. 22 Figure 6 illustrates the relationship between molecular ordering and PM-IRRAS signal strength in the C−H stretching region. For previous studies of 2D and 3D crystals of long-chain alkanes, the CH 2 asymmetric stretch (Figure 6a right inset, ν(CH 2a ) ∼ 2925 cm −1 ) and the orthogonal CH 2 symmetric stretch (ν(CH 2s ) ∼ 2850 cm −1 ) have been used to assess alkyl chain orientation and ordering.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Spectroscopic Metrics for Alkyl Chain Ordering in Lying-Down Noncovalent Monolayers of Diynoic Acids on Graphene

et al. 2018

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Noncovalent monolayer chemistries are widely used to control physical properties of 2D materials. For many applications (e.g., energy conversion, sensing), molecular ordering across a range of length scales is important in determining the physical properties of the interface. Scanning probe microscopy can resolve details of molecular packing and orientation over nanoscopic areas of graphene, graphite, and other 2D materials; however, evaluating molecular ordering over larger scales is also key. Such ordering is especially challenging to characterize at large scales for lying-down phases (thickness <0.5 nm) on topographically rougher materials such as graphene (vs flatter graphite). Here, we combine scanning electron microscopy and polarization-modulated IR reflection absorption spectroscopy to evaluate alkyl chain ordering in lying-down monolayers of diynoic acids on few-layer graphene and graphite substrates with areas ∼1 cm2. The ability to assess ordering in this widely used class of molecules reinforces the potential utility of spectroscopic metrics for evaluating structure in noncovalently functionalized 2D materials at micro- and macroscopic scales.

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Adsorption of Alkanethiols on Gold Surfaces: PM-IRRAS Study of the Influence of Terminal Functionality on Alkyl Chain Orientation

Cited by 13 publications

References 52 publications

Effect of time and deposition method on quality of phosphonic acid modifier self-assembled monolayers on indium zinc oxide

Effect of time and deposition method on quality of phosphonic acid modifier self-assembled monolayers on indium zinc oxide

Potential-Induced Structural Alterations in the Tethered Bilayer Lipid Membrane-Anchoring Monolayers Revealed by Electrochemical Surface-Enhanced Raman Spectroscopy

Spectroscopic Metrics for Alkyl Chain Ordering in Lying-Down Noncovalent Monolayers of Diynoic Acids on Graphene

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