Functionalising AFM tips for pick and place manipulation of micro and nanoparticles
Keywords: Kelvin probe force microscopy
Nanoparticles
Nanofabrication
Nanomanipulation
Atomic force microscopy

Nanofabrication techniques have always been at the heart of realising novel functional nanodevices. Modern nanomanufacturing relies heavily on lithography, which is the patterning of two-dimensional structures on radiation sensitive polymers in multiple steps to create nanodevices. Although suitable for mass manufacturing, lithography lacks the capability to precisely manipulate a single to few particles. The main goal of my research is to develop new technologies that are capable of manipulating single to few particles ranging from nanometer to micrometer scale. With the development of these new particle manipulation technologies, we demonstrate new applications that would have been difficult to realise with current existing nanomanufacturing methods. Here, we will look at how the atomic force microscope (AFM) can be used as a particle manipulator. The cantilever of the AFM acts as a robotic arm with nanometer precision in all three spatial dimensions. We show that by appropriately functionalising or modifying the cantilever, we can manipulate different types of particles (from nano- to micrometer sized) for various applications.
We demonstrate that gold nanoparticles can be picked up by an AFM tip modified with a suitably attractive monolayer. However, picking up a single nanoparticle with this method proved challenging due to monolayers diffusing through each other over time. Hence, the picking up could not be localised, which led to the use of dielectrophoresis. We show that by coating the tip with specific coatings (i.e. GST, carbon) that are capable of forming conductive filaments, the electric field can be further localised resulting in the picking up of single 20 nm gold particles — a significant improvement compared to tips without the coating. We show that these filamentary probes also have applications in high-resolution Kelvin probe force microscopy. We achieved a spatial resolution improvement over commercial platinum probes by ∼48%.
Besides suitable coatings, microchannels can be fabricated within the cantilever. Such cantilevers are commercially available and are often used to deliver small amounts of fluids with a pressure controller. We use this technology to aspirate and dispense 1 µm particles that are larger than the aperture size in a liquid medium. Moreover, we can also print nanoparticle inks with the hollow cantilever in air. We developed methods to monitor the deposition process in-situ as well as image the substrate before and during deposition. This enables us to align the deposition of nanoparticles into sub-500 nm gaps. This have direct applications in fabrication of microLEDs and hybrid photonic nanostructures.
Furthermore, we demonstrate that microparticles of liquid metal can be picked up by coating the cantilever with gold, which is more adhesive to the liquid metal compared to the polystyrene substrate. We are able to pick up and place liquid metal particles at will. Moreover, the contact area can be controlled by adjusting the applied force. We demonstrate an application of such a liquid metal droplet probe system by making small area (~4 – 12 µm2) molecular junctions in the weak-coupling regime.
There are various ways an AFM cantilever can be modified to facilitate the manipulation of nano- and microparticles. These are general techniques and the concepts presented here can be extended and applied to many different systems. With this understanding, experimental parameters such as coating material and voltages can be modified accordingly depending on the particle type and application. The ability to manipulate single to few particles will enable interdisciplinary researchers to study novel interactions between functional particles and nanostructures.