Page 68 - Nanotechnology an introduction

Figure 6.8 Scanning force micrograph (contact mode) of a mixed behenic acid/pentadecanoic acid film transferred horizontally onto muscovite mica at a surface pressure of 15 mN/m. Image size: 20 × 20 μm (reproduced with permission
from [3]).
Although there has been recurrent industrial interest in Langmuir–Blodgett (LB) films for numerous applications in microelectronics and integrated
optics, the cumbersome nature of the fabrication methodology, well suited to laboratory research but difficult to automate, and the difficulty of
creating defect-free films, have militated against its adoption. The problem of defects (e.g., pinholes in the insulating layer of a field-effect transistor
(FET), Figure 7.6) can, however, be overcome by reducing the component area (Section 10.4), suggesting that nanoscale electronics might see a
revival of interest. One attractive feature is the versatile chemistry allowing environmentally responsive films to be synthesized; bio-inspired films in
a chemFET configuration can be responsive to both total captured particle number and their state of aggregation (see, e.g., [138]). Bio-inspired LB
films are typically in the liquid crystalline phase, in which defects are self-annealed. Moreover, for electronics applications only very thin films are
needed, whereas an optical waveguide requires ~ 100 monolayers, the defect-free assembly of which is cumbersome.
Post-processing LB films expands the fabrication possibilities. Blodgett herself explored “skeleton” films prepared by selectively dissolving away
one component of a mixed film. Any multivalent metal or metal compound nanoplate can be prepared from a metal–fatty acid LB film (e.g.,
dysprosium behenate) which is then pyrolysed to remove the organic moiety.
Under certain conditions, regular striated patterns can be formed during LB deposition [156]. The cause is dynamic instability of substrate wetting.

6.3.3. Self-Assembled Monolayers (SAMs)

If particles randomly and sequentially added to the solid/liquid interface are asymmetrical, i.e. elongated, and having affinity for the solid substratum
at only one end (but the ability to move laterally at the interface), and with the “tail” (the rest) poorly solvated by the liquid, they will tend to adsorb in a
compact fashion, by strong lateral interaction between the tails (Figure 6.9). This is a practical procedure of some importance for modifying the
surfaces of objects fabricated by other means. The precursors are molecules of general formula XR, where X is (typically) an apolar chain (e.g.
alkyl), and R is a ligand capable of binding to a substratum. Addition of XR to the metal surface results in a closely packed array of XR. The film is
stabilized by hydrogen or chemical bonds to the substrate, and lateral LW forces between the X.

Figure 6.9 A (fragment of a) self-assembled monolayer. The component molecules have the general formula LXR, where X is an apolar chain (e.g., alkyl), and R is a reactive group capable of binding to the substratum S. X can be
functionalized at the end opposite from R with a group L to form molecules L–XR; the nature of L can profoundly change the wetting properties of the SAM.
Self-assembled monolayers were discovered by Bigelow et al. [18]. Currently the two main types of R are −SH (thiol or mercaptan), which binds
strongly to Au, Ag, Pt, Cu, Hg, etc.), and organosilanes, which bind strongly (bond covalently) to silica. These chemical requirements are the main
constraints limiting the versatility of the technology. The original example was eicosyl alcohol (C H OH) dissolved in hexadecane (C H )
20 41
16 34
adsorbing on silica glass. Later, molecules with R = −SH and the organosilanes were investigated. If the tail moiety X is poorly solvated by the
liquid, its flexibility may enable it to be compactified while the molecule is in the bulk solution, tending to prevent self-aggregation, and only unfurling
itself after R is attached to the solid surface—a rudimentary form of programming (cf. programmable self-assembly, Section 8.2.8). SAMs provide
a very convenient way to change the wettability of a solid surface. Bigelow et al.'s monolayers were both hydrophobic and oleophobic. An
octadecanethiol (R = −SH) film adsorbed on gold would be both oil and water repellent; if L = −OH it will be hydrophilic (see Section 3.2.1).
X can be functionalized at the end opposite from R with reactive groups to form molecules RXL. These can profoundly change the wetting
properties of the assembled monolayer. For example, whereas octadecanethiol (L = −H) films are both oil and water repellent, if L = −OH then oil