SPM characterisation of nanomechanical proprieties of C60 monolayer formed by LB

Lamantia, Angelo and Robson, Alexander James and Pinter, Gergo and Underwood, Kaycee L. and Roberts, Jonathan and Cao, Yameng and Young, Robert James and Robinson, Benjamin James (2017) SPM characterisation of nanomechanical proprieties of C60 monolayer formed by LB. In: MMC2017, 2017-07-03 - 2017-07-06, Manchester, UK.

Abstract

C60 is a fascinating material due to its unusual and sometimes spectacular mechanical and optoelectronic properties, allowing a series of interesting applications in the fields of nanoscience, material science, optics and electrics [1,2,3,4]. However, since its discovery a large number of experiments have been carried out to study the intriguing proprieties of this exciting material but methods for preparing macroscopic quantities of monolayer C60 [5] in a facile and scalable way has proved challenging [6]. Here we report a potential method for achieving monolayer (ML) production of C60 by Langmuir-Blodgett(LB) technique. We have characterised large area films produced by assembly at the air-water interface and transferred by a modified Langmuir-Blodgett (LB) method onto cleaned SiO2 substrates using a variety of scanning probe methods. We have mapped the mechnaical, thermal and electrical properties with nanoscale resolution. We investigated optimisation of the deposition through monitoring of the mean molecular area (Π - MMA) as a function of surface pressure (isotherms), Brewster angle microscopy, Raman spectroscopy and optical microscopy. Results were obtained using deionised water on a commercial KVS-NIMA trough; optimal solvent for monolayer formation was found to be toluene and methanol in 5:1 ratio by volume. C60 solution was spread on the water surface using a custom built electrospray system, enabling the formation of stable C60 thin films with high stability. The transferred samples, on SiO2 substrates, were allowed to dry in controlled atmosphere and the amount of C60 on the substrate was monitored using a quartz crystal microbalance (QCM). Optical microscopy images of transferred samples showed large area coverage of the C60, additionally Raman spectroscopy confirmed the presence of C60 on the sample surface. The height of the obtained monolayer and its mechanical and thermal proprieties were measured by ultrasound force microscopy (UFM), quantitative nano-mechanics atomic force microscopy (AFM-QNM) and scanning thermal microscopy (SThM). Low noise measurements were made by TappingMode AFM in the new state-of-the-art ISOLAB facilities at Lancaster University. Investigations revealed a step height of about 0.76 nm ± 0.06 nm, which is in agreement with the expected molecular dimension of a single C60 layer. Furthermore, measurements conducted at the ISOLAB have shown a uniform and homogeneous C60 ML, suggesting the validity of this technique as a viable method for the deposition of large area ML of C60 and other fullerene moieties on a hydrophilic/hydrophobic substrate. QNM-AFM and UFM were used to study the mechanical proprieties of C60 ML, showing an high degree of layer stability under repeated scanning, resistance to mechanical deformation and a stiffness lower than that of the SiO2 substrate. Young's modulus of 7 GPa for the C60 ML were obtained through QNM-AFM, in a good agreement with other similar studies. Preliminary high resolution AFM measurements made in the ISOLAB have allowed us to observe the close packed molecular structure of the C60 ML, and confirm that this methodology is ideally suited to the deposition of such films. In conclusion, we present a straight forward, rapid and scalable way to produce large area ML of C60 using the Langmuir-Blodgett technique as an alternative to other methods such as evaporation or drop cast film. Analyzing the samples with a range of scanning probe microscopy techniques have afforded a wealth of vital information about the condition, topography and properties of C60 monolayers.