Articles you may be interested inElectron-beam-induced alteration of the dielectric properties of sandwiched self-assembled organic monolayers Modification of thiol-derived self-assembling monolayers by electron and x-ray irradiation: Scientific and lithographic aspects

No abstract

Chemically defined surface patterning has been achieved via irradiation with low‐energy electrons. Irradiation of nitro‐arene‐thiol monolayers leads to the reduction of the nitro groups to amino groups, which can then be further chemically modified. The Figure shows such a monolayer that was irradiated through a copper grid followed by acylation with trifluoroacetic acid anhydride.

We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a ‘hands-on’ approach, providing practical details and procedures as derived from literature as well as from the authors’ experience, in order to enable the reader to reproduce the results. Section is devoted to ‘bottom up’ approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section covers ‘top down’ techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers’ and modified Hummers’ methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by ...

No abstract

Free-standing nanomembranes with molecular or atomic thickness are currently explored for separation technologies, electronics, and sensing. Their engineering with well-defined structural and functional properties is a challenge for materials research. Here we present a broadly applicable scheme to create mechanically stable carbon nanomembranes (CNMs) with a thickness of ~0.5 to ~3 nm. Monolayers of polyaromatic molecules (oligophenyls, hexaphenylbenzene, and polycyclic aromatic hydrocarbons) were assembled and exposed to electrons that cross-link them into CNMs; subsequent pyrolysis converts the CNMs into graphene sheets. In this transformation the thickness, porosity, and surface functionality of the nanomembranes are determined by the monolayers, and structural and functional features are passed on from the molecules through their monolayers to the CNMs and finally on to the graphene. Our procedure is scalable to large areas and allows the engineering of ultrathin nanomembranes by controlling the composition and structure of precursor molecules and their monolayers.

When aromatic self-assembled monolayers (SAMs) are electron-irradiated, intermolecular cross-links are formed and the SAMs transform into carbon nanosheets with molecular thickness. These nanosheets have a very high mechanical stability and can withstand temperatures above 1000 K. In this report, we investigate the electron induced cross-linking of 1,1'-biphenyl-4-thiol (BPT) SAMs on gold by combining X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (NEXAFS), thermal desorption spectroscopy (TDS), and UV photoelectron spectroscopy (UPS). The experimental data were acquired as a function of electron dose and temperature and compared with quantum chemical calculations. Details of the intermolecular cross-linking, the microstructure of cross-linked films, and their structural transformations upon heating were obtained to derive a view of the mechanisms involved. Our analysis shows that room-temperature electron irradiation causes a lateral cross-linking via the formation of C-C linked phenyl species as well as a new sulfur species. The thermal stability of the BPT films increases with the electron dose and saturates at approximately 50 mC/cm2. Nevertheless, nonlinked fragments in the thermal desorption spectra indicate an incomplete cross-linking even at high doses, which can be attributed to steric reasons and quenching due to the reduced band gap of partially linked molecules. At temperatures above 800 K, all sulfur species are thermally desorbed, while the remaining film reveals an onset of carbonization.

Self-assembly of amphiphilic molecules, alkanethiols CH3(CH2) n SH (n = 15, 17), carboxylic acids CH3(CH2) n COOH (n = 16, 18) and bifunctional SH(CH2)15COOH on indium tin oxide (ITO) surfaces was investigated by X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies, contact angle measurements, and atomic force microscopy. These molecules were found to form homogeneous, well-oriented monolayers on ITO. XPS indicates that at low coverages, thiol molecules adsorb as thiolates, whereas at saturation coverage ∼30% of the molecules adsorb as unbound thiols. From NEXAFS, we determined average alkyl chain tilts of 33° for thiol from the neat liquid, 44° for thiols prepared via CVD, 38° for carboxylic acids from hexadecane, and 43° for COOH(CH2)15SH from ethanol. Adsorption from a mixed hexadecane solution results in a preferred adsorption of carboxylic acids over thiol. Thus, COOH(CH2)15SH is found to bind to ITO via the carboxylic acids, forming a thiol-terminated SAM.