Assembly of charged gold nanocrystals at electrodes and liquid/liquid interfaces

Abstract

It has long been a wish in nanoscience to manipulate matter at the molecular level. However, current lithography cannot easily produce ordered structures smaller than 100 nm. The gap between the length scale accessible by lithography and the molecular scale might be bridged by self-assembly techniques. Interactions at the molecular level determine the assembly of nanometre-scaled building blocks (e.g. molecules, colloidal particles or nanocrystals) into well-defined, periodic structures of varying length scale (10-1000 nm). This thesis reports on the self-assembly of charge-stabilised gold nanocrystals (diameter 4-40 nm) on chemically modified gold substrates and at the aqueous/heptane interface.

In chapter 2 the structure and electrochemical properties of gold (substrate)/oligo(cyclohexylide-ne) SAM/gold nanocrystal junctions were studied. Such a junction was built up layer by layer. Application of a monosulfide or disulfide cyclohexylidene results in the formation of a robust SAM on the Au(111) surface. Strikingly, the oxime-sulfide cyclohexylidene did not produce a well-ordered SAM. The nature of the oxime functionality seems to prevent the formation of the well-ordered SAM, showing that slight changes in the chemical nature of the molecules can have a strong effect on molecular self-assembly. On the gold (111)/disulfide cyclohexylidene SAM system individual particles attached to the surface were observed. Aggregates did not occur. The maximum coverage of the layer was close to 10 %. It was shown that the gold nanocrystals can be addressed electrically through the oligo(cyclohexylidene) layer, which hence acts as a molecular electronic bridge between the gold substrate and the nanocrystal.

In chapter 3 we report on the self-assembly of colloidal gold nanocrystals at the water/heptane interface. When the aqueous colloidal gold solution (16 nm diameter particles) is brought into contact with heptane, no significant changes are observed. However, after addition of several millilitres of ethanol a blue interfacial layer is formed at the water/heptane interface. When the water/heptane interface is full, the layer can extend up the glass for several centimetres without breaking. The interfacial gold nanocrystal layer was studied with both in-situ and ex-situ methods. TEM images show a dense interfacial layer which consists of unordered, individual nanocrystals separated by large voids. It was shown that the particle surface charge density was reduced after addition of ethanol. Also lowering of the pH of the colloidal gold solution, addition of salt, and addition of thiols or sulfide produced dense interfacial layers. With a decrease in the width of the double-layer, three-dimensional aggregation occurs simultaneously with the formation of an interfacial layer, leading to a ‘clumpy layer’.

In chapter 4 a model is described aimed at explaining the observations made in chapter 3. In this model the chemical potential of colloidal particles in the bulk (aqueous) phase µbulk and at the interface µsurf are calculated. Using the equilibrium condition, µbulk = µsurf an adsorption isotherm was derived. It was found that the reduction of surface charge is the driving force for surface attachment. Large particles require a lower surface charge to reach the same surface coverage at the water/heptane interface than small particles. Despite somewhat crude approximations, the model was able to explain the results in a semi-quantitative way.