Modification of Alkanethiolate Self-Assembled Monolayers by Ultraviolet Light: The Effect of Wavelength

Abstract

The effect of the wavelength upon the modification of the archetypical nonsubstituted alkanethiolate (AT) selfassembled monolayers (SAMs) on Au(111) by ultraviolet (UV) light was investigated. As a test system, the dodecanethiolate monolayer was selected, while the wavelength of the UV light was varied in two steps from 254 to 375 nm, which is a relevant range for a variety of inexpensive UV sources as well as custom-made and commercial lithography setups. The absolute cross sections of the most prominent and application-relevant UV-induced processes, above all photooxidation of the thiolate anchoring groups to sulfonates, were determined. They were found to decrease exponentially with increasing wavelength, with an effective "wavelength attenuation parameter" of ∼24 nm. This behavior was rationalized within a model assuming the primary role of UV-induced hot electrons, originating from the substrate and triggering an excitation and subsequent oxidation of the sulfur atoms by their reaction with oxygen molecules, penetrating to the SAM−substrate interface across the SAM matrix. The observed behavior and the absolute values of the photooxidation cross sections represent a useful "tool" for the estimation of a proper dose or dose range in the applications involving modification of AT SAMs and thiolate SAMs in general by UV light.

The Effect of Ultraviolet Light on Biorepulsive Hydrogel Poly(ethylene glycol) Films

“…The films were exposed to UV light with a wavelength of 254 nm provided by a short-wave (UV-C) Hg vapor lamp (Benda Konrad Laborgeräte). UV light with this wavelength is known to be very efficient for modification of ultrathin organic films, including the OEG-substituted SAMs in particular. ,,, The UV treatment was performed at ambient conditions and room temperature, with a temperature increase by just few degrees during irradiation. The distance between the UV source and samples was ∼2.5 cm.…”

Amide-Assisted Polymerization of 1,3-Butadiyne Containing Thiolate Ligands on Small Gold Nanoparticles

“…UV irradiation of self-assembled monolayers of alkyl thiolates on 2D gold surfaces and AuNPs is known to cause cleavage of the Au–S bond through photooxidation of the thiolate and other mechanisms. − Degradation mechanisms and rates strongly depend on conditions, such as wavelength and intensity of the light source, type of solvent, structure of the alkylthiolate ligand, and presence of oxygen. , A systematic study on the photodegradation of small AuNPs in chloroform solutions during irradiation with a nanopulsed laser and in a Luzchem photoreactor at different wavelengths (UV-A and -C) was conducted by Pocoví-Martínez et al UV-C irradiation led to aggregation and fusion of the nanoparticle cores within minutes, whereas UV-A irradiation led to aggregation and precipitation when a laser was used and no change when conducted in the photoreactor. It is also noted that Workentin and co-workers successfully used small alkylthiolate-protected AuNPs as substrates for photochemical reactions at an excitation wavelength of 350 nm. − The high stability of the gold cores in these cases may be explained by the filtering effect of the used photoreactive groups.…”

Location-Selective Work Function Engineering by Self-Assembled Monolayers

“…30−32 For the exchange reaction, the samples were immersed in the solutions of the secondary SAM molecules in ethanol (EtOH) or tetrahydrofuran (THF) for 2 h, which is considered as an optimal time for this purpose. 25,27 The monitoring of the WF of the SAMs and the outcome of the exchange reaction was performed by Kelvin probe measurements (a UHV Kelvin Probe 2001 system; KP technology Ltd.) under UHV conditions. As reference, we used a hexadecanethiolate SAM on Au(111) with a WF value of 4.3 eV according to the literature.…”

“…We have used a quite long wavelength (365 nm) of UV light, which simplifies the adaptation of the approach in both research and industrial laboratories. The use of shorter wavelengths, which is the current trend in the industry, should lead to better spatial resolution and allow a significant reduction of the necessary irradiation dose, because the cross section of the respective process increases exponentially with wavelength (e.g., by a factor of ∼150 upon going from 375 to 254 nm) . We hope that this approach will be adapted soon to device studies, providing advantages for model OTFTs and electronic circuits.…”

“…The suggested approach is illustrated in Figure and is primarily based on promoting the exchange reaction between a SAM covering the substrate and a potential molecular substituent by ultraviolet irradiation, a strategy that has been used so far in the context of mixed SAMs, chemical lithography, and biointerfaces. − As the first step, all electrodes at the given substrate are covered with a SAM of specific polarity. Second, the electrodes requiring the opposite polarity are irradiated with ultraviolet light, which results in the oxidation of the anchoring thiolate groups, , diminishing their coupling to the substrate and promoting, thus, the exchange reaction. − Third, the entire substrate is immersed in a solution of molecules exhibiting the opposite polarity. Fourth, the promoted exchange reaction within the areas exposed previously to ultraviolet (UV) light takes place, resulting in the nearly complete “extinction” of the primary SAM and the assembly of the SAM with the opposite polarity within these areas.…”