Self-assembly and increased salt content assist preparation of thin films of nanoparticles
Glass and semiconductors coated with uniform thin film of nanoparticles find more and more applications. Researchers at the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw have developed two methods for preparing gold monolayers – coatings as thin as a single nanoparticle. One of them makes use of surprising properties of highly concentrated salt solutions, while the other is based on self-assembly. Both methods allow preparing for the first time uniform, positively charged monolayers.
In contact with water, glass and silicon surfaces may spontaneously generate a negative electric charge. Deposition of thin and uniform layers of positively charged gold nanoparticles from solution on such surfaces seemed impossible so far. The nanoparticles aggregated and instead of developing a uniform layer, they formed numerous clusters. To decrease the effective range of electrostatic interactions between the nanoparticles, chemists have usually added salt to the working solution. The salt anions shielded in part the nanoparticles, which then could deposit more densely.
“The solution is far from ideal, as the more salt we add the entire system is getting more destabilized. As a result, at certain level of salt concentration the nanoparticles start to precipitate from solution in a form of aggregates”, notices Katarzyna Winkler, a PhD student at the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw. It was commonly believed that the phenomenon was an insurmountable obstacle. “We decided to check how the nanoparticles would respond if we increase the salt concentration in spite of all. And here we faced a surprise”, says Winkler.
The researchers from the IPC PAS have noticed that in certain range of high salt concentrations the working solution regains stability and the nanoparticles in the solution stop to aggregate while still being able to interact with the substrate. “Most likely we are dealing with an electrostatic effect. The discovery, however, is so fresh that the exact nature of the phenomenon has not been recognised by us”, says Dr Marcin Fiałkowski from the IPC PAS.
In the second method for preparing uniform nanoparticle coatings on substrates the key role is played by self-assembly. Gold nanoparticles were first covered with thiols – RSH type molecules containing an R substituent and a group composed of one sulphur S and one hydrogen H atom. In experiments, electrically charged hydrophilic thiols and uncharged hydrophobic ones were used. Nanoparticles surrounded by the thiols were then placed on a water surface. “If we properly adjust the proportions between charged and uncharged thiols, the nanoparticles present on the water surface start to behave like floats. They don’t sink, but form spontaneously a monolayer”, describes Dr Volodymyr Sashuk from the IPC PAS.
A layer of nanoparticles formed on water surface due to self-assembly can be compressed mechanically with an apparatus called Langmuir balance. As a result of compression, the nanoparticles, even when covered by thiols with the same charge, approach each other to a much shorter distance. “Now, to transfer such a monolayer on a glass substrate it’s enough to make use of the Langmuir–Blodgett method. To do so, a glass plate is placed in a computer-controlled dipping arm and slowly immersed in water or raised above the surface. Then, the nanoparticles from the surface layer just attach to the plate”, says Dr Sashuk.
As the direction of the plate displacement through the monolayer decides which side is the one the nanoparticles use to attach to the glass, the method allows preparing hydrophobic or hydrophilic surfaces. Moreover, it is possible to cover glass surfaces with monolayers of either negatively or positively charged nanoparticles – just depending on what is needed.
The first of the above methods is very simple, and also very cheap. It can be used when the charge and order of nanoparticles do not matter. The recipe allows to form layers not only on glass but also on semiconductor substrates. With the method one can fabricate, for instance, silicon surfaces coated with gold nanoparticles that in further processing would initiate growth of nanowires. Self-assembly based production of nanoparticle layers is more complicated. The layers obtained are, however, highly ordered, and their electric charge can be precisely controlled.
Polish Academy of Sciences