An EPSRC Funded Research Project

KPFM on single crystal facets and polymer surfaces under both dry and humid conditions

Charge characterisation on crystal facets of model crystals and surfaces of PP and PE, with and without charge modifiers, and the quantification of the effect of humidity will be carried out to validate WP1, using Kelvin Probe Force Microscopy (KPFM) at the School of Physics and Astronomy of UoL.  The Figure here shows examples of KPFM data acquired in the Leeds AFM Facility. Each KPFM image on the bottom row corresponds to the AFM topographic image above. Samples (a) and (b) show circles of methyl terminated self-assembled monolayer micro-contact printed (uCP) onto template stripped gold (TSG), surrounded by a backfilled F-terminated thiol.  The electronegativity of the terminating group alters the molecular dipole and hence surface potential.  Sample (c) is a methyl terminated silane polymerised on silicon, revealing the variation in density.  Absolute values of work function Φ are found using calibration sample (d), consisting of interdigitated stripes of gold and aluminium on silicon, providing three independent work functions.  Frequency-Modulation (FM)-KPFM, a recently available mode will be used, sensitive to force gradient thus minimising capacitive coupling from the probe base and cone, and hence more accurate ΔVCPD values. Contact electrification is minimised/eliminated by operating in the attractive tapping regime, at small amplitude and monitoring of phase shift. Pt/Ir coated SCM-PIT-V2 (Bruker Probes) and ARROW-NCPt (NanoAndMore) conductive probes will be used. Humidity can lead to surface water.  This together with adsorbates form surface dipoles alter surface potential.  Differences in published values are partly attributed to this environmental gating.  Regular calibration of the work function of the probe during experiments is therefore essential for accuracy, and we will use Au/Al electrodes deposited on Si (see figure), with additional biasing, giving 3 standard materials in one rapid scan. The work function of Au decreases by only 3% from 0-40% RH, and humidity can be kept < 40%, or controlled to a desired humidity with saturated salt solutions, in a sealed environmental chamber.  The external collaborator, Prof. Burgo, will provide complementary support for the study of the effect of humidity and will visit over the first year for two months for data exchange and corroboration of results.  New surfaces of fractured crystals will also be assessed for charging and compared with the combined impact charge transfer and fracture to elucidate the troublesome issue of charge accumulation during milling.  Large model crystals will be obtained from JH’s laboratory, ICL, and PP and PE beads from the industrial partner LyondellBasell.


Effective work function data for the validation of predictions from WP1, and single facet data for use in WP3.




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