Monolayer deposition mode

Films may be formed only in downstroke (X-type, Fig. 4b). The deposition speed may affect the deposition mode (16,17). If deposition occurs only when films are formed in upstroke Z-type films result (Fig. 4c). These are cases where the head group is not as hydrophilic, eg, COOCH3 (18), or where the alkyl chain is terminated by a weak polar group, eg, N02 (19). In both cases the interactions between adjacent monolayers are... 

Figure 15. (a) A schematic representation of organic amphiphilic molecules at air-water interface, (b) Schematic representation of the various deposition modes for monolayer films and the resulting L-B assemblies. (Reproduced with permission from H. Kuhn, D. Mobius, and H. Bucher, Physical Methods of Chemistry, Vol. I, Part IIIB, A. Weissberger and B. W. Rossiter, Eds., Wiley, New York, 1972, p. 577.)... 

Although this is the most frequently encountered situation, it is also possible for monolayer deposition to occur only when the substrate enters the subphase, depending on the nature of the monolayer, substrate, subphase, and the surface pressure. These deposition modes are called X-type (monolayer transfer on the down-stroke only) and Z-type (transfer on the upstroke only). The ideal structures resulted from the deposition modes are illustrated in Fig. 9. 

To be deposited successfully on a solid substrate in an LB film, a monolayer should be in the condensed phase. The most common deposition mode is termed Y-type. Here the monolayer is picked up on a hydrophilic substrate (often glass) as it is pulled from the monolayer. The hydrophilic head groups attach on the substrate, leaving a surface that is now covered by hydrophobic chains. The substrate plus monolayer is then dipped back into the water, picking up the monolayer in the reversed orientation. The resulting LB film is therefore a stack of bilayers. 

Fig. 8. Surface phonon dispersion along [ 110 ] for an incommensurate Xe monolayer deposited on Cu(llO) measured with HATOF. The modes Hj and H2 are generated by hybridisation of the vertical vibration of the adsorbate with the RW. The mode L corresponds to the longitudinal motion of the Xe adatoms. The upper dispersionless feature ( 2) corresponds to the double excitation of the vertical vibration. The short dashed lines are the calculated dispersion curves for vertical (1) and longitudinal (L) motion assuming a perfectly hexagonal Xe lattice. The solid lines are the dispersion curves with the actual lattice distortion taken into account. The long dashed line indicates the substrate RW[94Zep].

Monolayers can be transferred onto many different substrates. Most LB depositions have been perfonned onto hydrophilic substrates, where monolayers are transferred when pulling tire substrate out from tire subphase. Transparent hydrophilic substrates such as glass [18,19] or quartz [20] allow spectra to be recorded in transmission mode. Examples of otlier hydrophilic substrates are aluminium [21, 22, 23 and 24], cliromium [9, 25] or tin [26], all in their oxidized state. The substrate most often used today is silicon wafer. Gold does not establish an oxide layer and is tlierefore used chiefly for reflection studies. Also used are silver [27], gallium arsenide [27, 28] or cadmium telluride wafer [28] following special treatment. 

Vandamme N, Snauwaert J, Janssens E, Vandeweert E, Lievens P, Van Haesendonck C (2004) Visualization of gold clusters deposited on a dithiol self-assembled monolayer by tapping mode atomic force microscopy. Surf Sci 558 57-64... 

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