Electrohydrodynamic jet printing (EHD) has emerged as a competitive technique in the field of additive nanomanufacturing 1. With the wide range of materials it can accommodate, it has been shown to produce printed features of up to 50 nm in resolution 2. Whilst this is remarkable, the printing speed used to achieve this is not. One way to improve this important parameter (for upscaling purposes) is to better control the wetting properties of the substrate being printed on. Their modification is a key step in the quest for high resolution printing. A reliable technique to achieve this is by using self-assembled monolayers (SAMs) to modify the surface energy.
It has been shown that the presence of a SAM can increase the resolution obtained during EHD printing 3. However, there has been insufficient analysis into whether this monolayer survives the printing process. With electric fields of the order of MV/m passing through several nanometres of soft matter, does the dielectric SAM experience breakdown? Previous studies suggest that the monolayer does suffer breakdown 4, but this has not been demonstrated with a scanning probe microscopy technique.
In this study, we present our findings to these questions. We use conductive atomic force microscopy (C-AFM) to probe the electrical characteristics of four different SAM layers grown on an indium tin oxide (ITO) surface to determine properties such as conductivity and breakdown threshold. We do this by studying I-V plots over a certain potential window and assessing any deviations from normal Ohmic behaviour of the ITO. We relate the observed results to the physical properties of the SAMs such as chain length and terminal groups. We also use Kelvin Probe Force Microscopy (KPFM) to study the surface potential of SAM coated substrates which have been exposed to high electric fields using EHD. We find that there is a difference in contrast between E-field exposed regions and those not exposed, suggesting that field effects do influence the monolayer. Finally, we suggest ways to preserve the monolayer integrity during the printing process.
1. Porter, B. F., Mkhize, N. & Bhaskaran, H. Nanoparticle assembly enabled by EHD-printed monolayers. Microsystems Nanoeng. 3, 17054 (2017).
2. Galliker, P. et al. Direct printing of nanostructures by electrostatic autofocussing of ink nanodroplets. Nat. Commun. 3, 890 (2012).
3. Jeong, Y. J. et al. Directly drawn poly(3-hexylthiophene) field-effect transistors by electrohydrodynamic jet printing: Improving performance with surface modification. ACS Appl. Mater. Interfaces 6, 10736–10743 (2014).
4. Haag, R., Rampi, M. A., Holmlin, R. E. & Whitesides, G. M. Electrical Breakdown of Aliphatic and Aromatic Self-Assembled Monolayers Used as Nanometer-Thick Organic Dielectrics Electrical Breakdown of Aliphatic and Aromatic Self-Assembled Monolayers Used as Nanometer-Thick Organic Dielectrics. 7895–7906 (1999).