Monolayer surface analysis techniques

An alternate route to formation of alkyl monolayers is via Lewis acid catalyzed reactions of alkenes with the hydrogen terminated surface. In this approach, a catalyst such as ethyl aluminum dichloride is used to mediate the hydrosilylation reaction of an alkene (or alkyne), resulting in the same type of product as in the case of the photochemical or thermal reactions. This type of reaction is well known based on molecular organosilane chemistry and has also been used successfully to alkylate porous silicon [31]. Although this route has been shown to work on H/Si(lll), the resulting monolayers are found to have lower coverages than those achieved using the photochemical or thermal approach [29], Another concern with this approach is the possibility of trace metal residues from the catalyst that could adversely affect the electronic properties of these surfaces (even when present at levels below the detection limit of most common surface analysis techniques). 

Only exposed atoms on the surface, that is, the top monolayer of atoms, contributes to the signal in ISS. It is therefore the most surface sensitive of the surface analysis techniques. ISS is one technique that provides isotopic information on surface atoms. ISS can be used to determine all elements with an atomic number greater than that of the bombarding ion. The elemental and isotopic compositions of the surface can be determined both qualitatively and quantitatively. Sensitivity is about 1% of a monolayer for most elements. 

Surface analysis techniques specifically probe the surface of a material and are capable of detecting sub-monolayer surface coverage. However, charging in polymeric systems and the chemical similarity between the graft polymer and the substrate complicate the analysis in entirely polymeric systems. 

Auger spectroscopy Auger electron spectroscopy is a powerful surface analysis technique, as the analytical signals come from the top few monolayer of the specimen. In this technique, a primary beam of electrons is used to produce inner shell vacancies in atoms of the specimen. This method has been used for the determination of sulfur, e.g., on GaAs surfaces and to study sulfur surface segregation in Ni-Al solid solutions and the adsorption on a GaAs (001) surface by hydrogen sulfide exposure and heat treatment. 

A single monolayer of contamination can obscure the results of surface analysis techniques. 

Principles and Characteristics There are many types of samples for which the transmission approach is not optimum, desirable or even practicable, e.g. urethane foams, polymer laminates, and surface coatings. To obtain spectra from these types of sample it is more usual to employ a reflection technique. Reflection measurements are often also needed when materials are to be measured in their original form, except for thin films. This essentially turns IR spectroscopy into a surface analysis technique, but of low sensitivity compared to high vacuum spectroscopic techniques such as XPS, LEFT), EELS and SIMS. Since the advent of FTIR spectrometers, infrared sensitivity has so much improved that nowadays a measurable spectrum can be produced from even a single monolayer on a flat surface interfaces are also commonly examined. 

Batts and Paul [101b] used time-of-flight secondary-ion mass spectrometry (ToF-SIMS) to investigate the competitive adsorption of a cationic fluorinated surfactant (FC-134) at the gelatin-air interface. ToF-SIMS is a very sensitive surface analysis technique. In the static mode, the sampling depth of ToF-SIMS is only one to two monolayers. However, the ToF-SIMS data are difficult to interpret in quantitative terms and experimental conditions must be carefully controlled. Batts and Paul used positive secondary-ion spectra only, although negative-ion spectra may have been used as well. 

X-ray photoelectron spectroscopy (XPS), which is synonymous with ESCA (Electron Spectroscopy for Chemical Analysis), is one of the most powerful surface science techniques as it allows not only for qualitative and quantitative analysis of surfaces (more precisely of the top 3-5 monolayers at a surface) but also provides additional information on the chemical environment of species via the observed core level electron shifts. The basic principle is shown schematically in Fig. 5.34. 

We have recently modified U7) one of the several radiochemical methods (U5) which have been used for surface electrochemistry investigations in order to characterize adsorption on well-defined, single crystal electrodes. Below, we will describe the technique and identify some challenging issues which we will be able to address. The proposed method is sensitive to a few percent of a monolayer at smooth surfaces, is nondestructive and simple to use. The radiochemical measurements can be made with all compounds which can be labelled with reasonably long-lived, preferably g- emitting radioisotopes. We believe this technique will fulfill the quantitative function in in situ surface analysis as Auger spectroscopy currently does in vacuum, ex situ characterization of electrodes. 

The higher order alkylchlorosilanes (C8 and C18) have historically been treated in the same way as organosilanes. The reaction inevitably occurs in the liquid phase and is usually followed by a curing step. The extremely low surface of the silicon wafers and the deposited Si02 layers, used for self-assembled-monolayers does not allow a spectroscopic quantification of the surface species. A completely different type of analysis techniques is used here mainly to determine the quality (roughness and uniformity), the adherence (parallel or at random) and the hydrofobicity of the coated layer. 

Determination of lateral periodicities in the self-assembled layer is an important goal in surface analysis. 2D surface crystal structures are best studied with low energy electrons, since their escape depth, contrary to X-rays, is basically limited to the top-most atomic layers. Consequently, LEED has become the most important method in surface monolayer crystallography. However, single-crystalline substrates are required. Via this technique, 2D supramolecular chiral lattice structures on single crystal surfaces had already been observed in 1978 [19]. 

It will be going beyond the scope of the present review to give a detailed description of the techniques available, or to provide a comprehensive list of all methods used in surface analysis. Such information could be found in a number of monographs and review papers414,15. Analytical Chemistry publishes reviews every two years on the latest developments in surface characterization methods16. We will attempt to discuss briefly the most powerful and most readily available methods applicable to routine gold-thiol monolayer analysis with particular emphasis on the kind of information which can be obtained from these systems. 

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