SAMs heterogeneous

Modern spectrometers only require electron beam currents in the range 0.1 lOnA and hence probe sizes of 20-200 nm may be readily achieved with thermionic sources and 5-15 nm with a FEG. Spatially resolved compositional information on heterogeneous samples may be obtained by means of the Scanning Auger Microprobe (SAM), which provides compositional maps of a surface by forming an image from the Auger electrons emitted by a particular element. 

In SAM the electron beam can be focussed to provide a spatial resolution of < 12 nm, and areas as small as a few micrometers square can be scanned, providing compositional information on heterogeneous samples. For example, the energy resolution is sufficient to distinguish the spectrum of elemental silicon from that of silicon in the form of its oxide, so that a contaminated area on a semiconductor device could be identified by overlaying the Auger maps of the two forms of silicon obtained from such a specimen. 

Metal UPD at the SAM/substrate interface is of interest for several reasons. Firstly, from an application point of view as the intercalation of another metal alters the thiol-substrate bond and, thus, the stability of a SAM that can be exploited to generate heterogeneous and patterned SAMs, a point we will return to later. Secondly, the intercalation and alteration of the thiol-substrate bond changes the morphology of a... 

In many supported catalytic systems, it is nearly impossible to determine either the specific species, responsible for the observed catalytic activity, or the mechanistic pathway of the reaction. Using a defined SAM system in which careful molecular design is followed by controlled deposition into a solid-supported catalyst of known morphology, surface coverage, mode of binding and molecular orientation, allows direct correlation of an observed catalytic activity with the structure on the molecular scale. SAM and LB-systems allow detailed and meaningful studies of established surface bound catalysts to understand their behavior in heterogeneous... 

Fig. 9.6 Tailored SAMs for surface engineering provides the control of the surface physical properties, chemical reactivity and heterogeneity on the molecular level, a) Self-assembly of one kind of surface active compound results in homogeneous monolayers, b) Adsorption of two components give ride to mixed SAMs, combining the physical and...

Fig. 9.10 Comparison of the formation and wetting behavior of the aliphatic HUT/DDT (a,c,e) and the aromatic HMB/MMB (b,d,f) mixed monolayer system on Au(lll). (a,b) Composition of the solution and surface composition of the resulting SAM. (c,d) Plot of the cos 6>= (7sv-7si)/7iv) of the advancing (and additionally in d) receding) water contact angle as a function of the surface OH concentration. The straight line represents the Cassie equation [104], in c) the grey line is calculated after the equation from Israelachvili [105] describing the contact angle on heterogeneous surfaces. (e,f)...

The formation of mixed SAMs composed of two components provides unique possibilities in the control of the physical and chemical surface properties. Besides homogeneously mixed SAMs, a directed deposition of the components results in surfaces of controlled heterogeneity. One example reported by Liedberg et ah, forming SAM gradients (Fig. 9.6d) by controlled diffusion has already been mentioned [73]. 

In the second part of this Chapter the thickness of the organic layer under discussion is slightly increased and a closer look at recent developments of more complex surface-bonded systems involving polymers is outlined. Despite the introduction of flexible polymer chains, the surface coating should still be defined and uncontrolled heterogeneities minimized. Here, especially, polymer brush-type layers where self-assembled monolayers (SAMs) are used as two-dimensional template systems for the preparation of well-defined surface coatings will be subject of a more detailed discussion. 

Besides homogeneous and uniform SAMs or polymer brushes, systems of tailored heterogeneity such as mixed monolayers of two or more compounds, gradients, block copolymer brushes etc. are now under investigation. Especially, the development of patterned surfaces offers the exciting possibility to perform multiple parallel experiments on a single substrate or cascade reactions. 

Finally, self-assembled monolayers (SAMs) on gold electrodes constitute electrochemical interfaces of supramolecular structures that efficiently connect catalytic reactions, substrate and product diffusion and heterogeneous electron transfer step when enzymes are immobilised on them. Resulting enzyme-SAM electrodes have demonstrated to exhibit good performance and long-term enzyme stability. 

Luminescence has proven a useful probe of structure and dynamics in a broad range of heterogeneous media, from zeolites to micelles to biomaterials. The sensitivity of this process to its environment, and its ease of detection, make it a highly versatile analytical tool. To date, luminescence as a probe of solid-liquid interfacial processes in SAMs is still relatively limited. However, with the development of fluorescence-based analytical methods of increasing spatial and temporal resolution, it is likely to be used increasingly in answering fundamental questions regarding monolayer behavior. 

Electrochemical studies on SAMs have proven invaluable in elucidating the impact of various molecular parameters such as bridge structure, molecular orientation or the distance between the electroactive species and electrode surface. As described above in Section 5.2.1, the kinetics of heterogeneous electron transfer have been studied as a function of bond length for many systems. Similarly, the impact of bridge structure and inter-site distances have been studied for various supramolecu-lar donor-acceptor systems undergoing photoinduced electron transfer in solution. In both types of study, electron transfer is observed to increase as the distance between the donor and acceptor decreases. As discussed earlier in Chapter 2, the functional relationship between the donor-acceptor distance and the electron transfer rate depends on the mechanism of electron transfer, which in turn depends on the electronic nature of the bridge. 

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