Page 67 - Nanotechnology an introduction

Figure 6.6 Langmuir–Blodgett films. (a) A monolayer; (b) a bilayer; (c) a Y-type multilayer. The circles represent the polar heads and the squares the apolar tails of the amphiphilic molecule. (d) A micelle, which can form spontaneously
upon dispersal in water if the amphiphilic molecules have a smaller tail than the head (see Section 8.2.9).

Figure 6.7 The Langmuir–Schaefer (LS) technique. (a) The polar substrate is slowly drawn upwards through the Langmuir film to deposit a monolayer. (b) The coated substrate, now apolar, is pushed horizontally through the Langmuir film to
deposit a second monolayer (and is caught by the receptacle R). This operation is equivalent to a very rapid vertical descent. The rapidity is needed to ensure that the mensicus curves downwards during deposition.
According to the nanocriterion of “absence of bulk” (Section 2.2), a few layers (Figure 6.6(b)) would be nanoscale. But even though the multilayer
(Figure 6.6(c)) could in principle be of microscopic or even macroscopic thickness, it has no bulk equivalent. Simply throwing amphiphilic
molecules into a large vessel and stirring would lead to a highly disordered material and it would take an inordinately long time to thermally anneal it
into any kind of ordered structure. Consider a Langmuir–Blodgett film made with two types of fluorescent polar head, let us call them A and B
(suppose that B is represented by the black circles in Figure 6.6), with differing optical absorption and emission spectra, such that A's emission
can be absorbed by B (the process is called Förster energy transfer), which in turn would emit light at wavelength . Then, exciting A (with light of
wavelength λ ) would result in emission of wavelength . On the other hand, if A and B were simply mixed at random, irradiating the mixture with
A
λ would mostly result in emission of wavelength , since A and B have to be very close to each other in order for Förster energy transfer to occur:
A
nanoscale structure results in qualitatively different behavior. Amphiphilic molecules can also form a bicontinuous cubic phase, which is pervasively
nanostructured, although it can be of indefinite (i.e., macroscopic) extent.
If two different immiscible amphiphiles are mixed on the Langmuir trough, the resulting variegated structures can be imaged using scanning probe
microscopy after transferring the Langmuir film to a solid support using the Langmuir–Schaefer technique (Figure 6.8). These experiments have
shown (a) how difficult it is to find convenient mixtures, and (b) how difficult it is to predict theoretically what patterns result. There are moreover
experimental difficulties in imaging texture (see Chapter 5) at scales below a few tens of nanometers. The lack of metrology techniques suited for
this purpose is in fact one of the main current hindrances to progress in the field.