Charge manipulation of single nanoparticles on insulating surfaces – CHAMAN

Electron or hole transfer processes in semiconductor or insulating interfaces, which are sandwiched between metallic electrodes, are important elementary processes in any kind of micro- or nano-electronic component. In particular, they can be responsible for the limitation of device performance due to leakage currents that may appear along defects in the thin insulating layer - a major issue in the device industry of today. Until now the study of charge transfer (CT) processes has mainly been done macroscopically by electrical measurements of the entire device or by atomic force microscopy related techniques in air such as scanning capacitance microscopy or electrostatic force microscopy/spectroscopy (EFM, EFS). However, these latter techniques cannot be used to study CT processes at the single charge level. To reach this level of information it is necessary to work (1) under ultra-high vacuum (UHV) where absolute cleanness can be guaranteed and where the highest control can be exercised on a sample leading to an utmost resolution in charge, (2) on model systems composed of metallic 3D nanoparticles or 2D islands (indistinctly labelled NP in the following) which are supported on an insulating layer, and (3) with non-contact AFM (nc-AFM) and related techniques like EFM and Kelvin probe force microscopy (KPFM) which allow explicitly charging metal NPs and quantifying their charge via measuring the electrostatic interaction between the tip and the surface.
The objective of CHAMAN is to study CT processes (1) between a metallic NP and a conducting support through an insulating thin film of thickness t (1 nm< t <100 nm) and (2) between two NPs under the influence of the insulating film. Such CT processes will bestudied in dependence on the film thickness and quality (monocrystalline, polycrystalline, amorphous), and on the NP morphology (e.g., size). In an ideal, defect-free film, CT mechanisms involve tunnelling or internal field emission. They depend on the electronic structure of the insulating film but also on the metal-insulator interfaces where, even in the absence of extrinsic defects, specific states (e. g., metal induced gap states (MIGS) or insulator surface states) can play a role. But thin insulating films generally include defects of different types (point defects, dislocations, grain boundaries, etc.) which in most cases determine their electrical properties. EFM/EFS and KPFM will be used to control and characterize the CT of single NPs at the single electron level. The influence of defects in the insulating film will be investigated by choosing the metal growth parameters to vary the number of defects covered by a single island. A third type of experiments that will be performed in CHAMAN consists in transport measurements, where an electrical current instead of a CT will be monitored. This will be done either by STM measurements for thin enough films or by using a four STM/nc-AFM probe low temperature microscope.
Modelisation and theoretical calculations will be used to assist experiments and to describe and predict charge/discharge phenomena of NP. In particular the calculations will consider two systems: 2D Au islands on AlN(0001) and Pd NPs on MgO(001) for which preliminary experiments suggest that charge can be injected from the AFM tip in a controlled way. The understanding gained in the study of these systems will be used to interpret similar experiments for metals deposited on amorphous hafnia (HfO2) films.
The three types of systems that will be investigated in CHAMAN have been chosen because (1) their preparation is at least partially mastered in the consortium, (2) they span of large range of interesting properties and (3) they are relevant for important technological issues.