This article reviews recent progress in the characterization of self-assembled monolayers (SAMs) with a chalcogen headgroup by synchrotron-based high-resolution X-ray photoelectron spectroscopy ...(HRXPS). We present reference data for archetypical, most frequently used SAM systems and discuss specific effects and SAM properties which can only be observed at high energy resolution. We show that not only the emissions related to a SAM but also those related to the substrate can provide important information on the system under study. We demonstrate that the standard chemical shift framework is not always sufficient to explain photoemission from SAMs, but, in some selected cases, electrostatic effects should be taken into account as well. General aspects of XPS and HRXPS experiments on SAMs are discussed, including X-ray induced damage and proper calibration procedures.
This article reviews recent progress in the application of core hole clock approach in the framework of resonant Auger electron spectroscopy to the monomolecular assembles of alkyl, oligophenyl, and ...oligo(phenylene–ethynylene) based molecules on Au(111) substrates, referring mostly to the work by the author et al. The major goal was to study electron transfer (ET) dynamics in these systems serving as prototypes of molecular electronics (ME) devices. The ET pathway to the conductive substrate was unambiguously defined by resonant excitation of the nitrile tailgroup attached to the molecular backbone. Characteristic ET times within the femtosecond domain were determined, along with the attenuation factors for the ET dynamics, analogous to the case of the static transport. The above parameters were found to exhibit strong dependence on the character of the molecular orbital which mediates the ET process. In addition, certain spectral features, which can be associated with an inverse ET from the molecular backbone to the excitation site, were observed upon exchange of the nitrile group by strongly electronegative nitro moiety. The reported results represent a valuable input for theory and a certain potential for applications such as ME devices where optimization of ET can have significant technological impact.
Self‐assembled monolayers (SAMs) can serve as versatile resist/template materials for surface engineering and electron beam lithography (EBL), making possible a new type of lateral patterning: ...chemical lithography (CL). Whereas CL has been well established for aromatic SAMs, it is hardly possible for aliphatic monolayers, because of extensive irradiation‐induced damage excluding selective modification of specific chemical groups. Turning this drawback into an advantage, the irradiation‐promoted exchange reaction approach is developed, which is described in detail in the present review. The key idea of the approach is tuning the extent of the exchange reaction between a primary aliphatic SAM covering the substrate and a potential molecular substituent, which is capable of building a SAM on the same support, by electron irradiation. The major advantages of the approach are low irradiation doses (≤1 mC cm−2) and flexible choice of SAM‐forming molecules, with a broad pool available commercially. Consequently, a large variety of binary SAMs with controlled compositions can be prepared and, in combination with EBL, complex chemical patterns can be fabricated, serving in particular as templates for subsequent area‐selective chemical reactions, surface‐initiated polymerization, attachment of nanoparticles, non‐specific and specific proteins adsorption, and growth of 3D DNA nanostructures.
The electron‐irradiation promoted exchange reaction approach, enabling flexible preparation of binary self‐assembled monolayers (SAMs) and chemical litho‐graphy with aliphatic SAMs, is presented. Complex chemical patterns can be fabricated, serving, for example, as templates for subsequent area‐selective chemical reactions, surface‐initiated polymerization, attachment of nanoparticles, non‐specific and specific protein adsorption, and growth of 3D DNA nanostructures.
The focus of the present article is on understanding the insight that X-ray photoelectron spectroscopy (XPS) measurements can provide when studying self-assembled monolayers. Comparing density ...functional theory calculations to experimental data on deliberately chosen model systems, we show that both the chemical environment and electrostatic effects arising from a superposition of molecular dipoles influence the measured core-level binding energies to a significant degree. The crucial role of the often overlooked electrostatic effects in polar self-assembled monolayers (SAMs) is unambiguously demonstrated by changing the dipole density through varying the SAM coverage. As a consequence of this effect, care has to be taken when extracting chemical information from the XP spectra of ordered organic adsorbate layers. Our results, furthermore, imply that XPS is a powerful tool for probing local variations in the electrostatic energy in nanoscopic systems, especially in SAMs.
Free-standing poly(ethylene glycol) (PEG) membranes were prepared from amine- and epoxy-terminated four-arm STAR-PEG precursors in a thickness range of 40-320 nm. The membranes feature high stability ...and an extreme elasticity, as emphasized by the very low values of Young's modulus, varying from 2.08 MPa to 2.6 MPa over the studied thickness range. The extreme elasticity of the membranes stems from the elastomer-like character of the PEG network, consisting of the STAR-PEG cores interconnected by crosslinked PEG chains. This elasticity is only slightly affected by a moderate reduction in the interconnections at a deviation from the standard 1:1 composition of the precursors. However, both the elasticity and stability of the membranes are strongly deteriorated by a strong distortion of the network imposed by electron irradiation of the membranes. In contrast, exposure of the membranes to ultraviolet (UV) light (254 nm) does not affect their elastic properties, supporting the assumption that the only effect of such treatment is the decomposition of the PEG material with subsequent desorption of the released fragments. An analysis of the data allowed for the exclusion of so called "hot electrons" as a possible mechanism behind the modification of the PEG membranes by UV light.
The effective detection of hydrogen peroxide (H2O2) in different environments and, above all, in biological media, is an important practical issue. To this end, we designed a novel electrochemical ...sensor for H2O2 detection by introducing gold nanoparticles (AuNPs) into the porous poly(ethylene glycol) (PEG) matrix formed by the thermally activated crosslinking of amino- and epoxy-decorated STAR-PEG precursors. The respective composite PEG-AuNP films could be readily prepared on oxidized Si substrates, separated from them as free-standing nanosheets, and transferred as H2O2 sensing elements onto the working electrode of the electrochemical cell, with the performance of the sensing element relied on the established catalytic activity of AuNPs with respect to H2O2 decomposition. The sensitivity, detection limit, and the operation range of the composite PEG-AuNP sensors were estimated at ~3.4 × 102 μA mM−1 cm−2, 0.17 μM of H2O2, and 20 μM–3.5 mM of H2O2, respectively, which are well comparable with the best values for other types of H2O2 sensors reported recently in literature. The particular advantages of the composite PEG-AuNP sensors are commercial source materials, a simple fabrication procedure, the bioinert character of the PEG matrix, the 3D character of the AuNP assembly, and the possibility of transferring the nanosheet sensing element to any secondary substrate, including the glassy carbon electrode of the electrochemical cell. In particular, the bioinert character of the PEG matrix can be of importance for potential biological and biomedical applications of the designed sensing platform.
Abstract
The controlled functionalization of single-walled carbon nanotubes with luminescent
sp
3
-defects has created the potential to employ them as quantum-light sources in the near-infrared. For ...that, it is crucial to control their spectral diversity. The emission wavelength is determined by the binding configuration of the defects rather than the molecular structure of the attached groups. However, current functionalization methods produce a variety of binding configurations and thus emission wavelengths. We introduce a simple reaction protocol for the creation of only one type of luminescent defect in polymer-sorted (6,5) nanotubes, which is more red-shifted and exhibits longer photoluminescence lifetimes than the commonly obtained binding configurations. We demonstrate single-photon emission at room temperature and expand this functionalization to other polymer-wrapped nanotubes with emission further in the near-infrared. As the selectivity of the reaction with various aniline derivatives depends on the presence of an organic base we propose nucleophilic addition as the reaction mechanism.
Tunnelling currents through tunnelling junctions comprising molecules with cross-conjugation are markedly lower than for their linearly conjugated analogues. This effect has been shown experimentally ...and theoretically to arise from destructive quantum interference, which is understood to be an intrinsic, electronic property of molecules. Here we show experimental evidence of conformation-driven interference effects by examining through-space conjugation in which π-conjugated fragments are arranged face-on or edge-on in sufficiently close proximity to interact through space. Observing these effects in the latter requires trapping molecules in a non-equilibrium conformation closely resembling the X-ray crystal structure, which we accomplish using self-assembled monolayers to construct bottom-up, large-area tunnelling junctions. In contrast, interference effects are completely absent in zero-bias simulations on the equilibrium, gas-phase conformation, establishing through-space conjugation as both of fundamental interest and as a potential tool for tuning tunnelling charge-transport in large-area, solid-state molecular-electronic devices.
Based on the powerful concept of embedded dipole self‐assembled monolayers (SAMs), highly conductive interfacial layers are designed, which allow tuning the contact resistance of organic thin‐film ...transistors over three orders of magnitude with minimum values well below 1 kΩ cm. This not only permits the realization of highly competitive p‐type (pentacene‐based) devices on rigid as well as flexible substrates, but also enables the realization of n‐type (C60‐based) transistors with comparable characteristics utilizing the same electrode material (Au). As prototypical examples for the high potential of the presented SAMs in more complex device structures, flexible organic inverters with static gains of 220 V/V and a 5‐stage ring‐oscillator operated below 4 V with a stage frequency in the range of the theoretically achievable maximum are fabricated. Employing a variety of complementary experimental and modeling techniques, it is shown that contact resistances are reduced by i) eliminating the injection barrier through a suitable dipole orientation, and by ii) boosting the transmission of charge carriers through a deliberate reduction of the SAM thickness. Notably, the embedding of the dipolar group into the backbones of the SAM‐forming molecules allows exploiting their beneficial effects without modifying the growth of the active layer.
Based on the powerful concept of embedded dipole self‐assembled monolayers, highly conductive interfacial layers are designed. They allow tuning the contact resistance of bottom contact organic thin‐film transistors over three orders of magnitude, enable the realization of p‐type and n‐type transistors using Au electrodes, and facilitate the fabrication of highly efficient flexible inverter and ring‐oscillator structures.
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