• Young, Robert
• Lamantia, Angelo
• Cao, Yameng
• Roberts, Jonathan
• Robson, Alexander James
• Underwood, Kaycee L.
• Robinson, Bj
• Pinter, Gergo

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].<br/>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.<br/>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).<br/>Optical microscopy images of transferred samples showed large area coverage of the C₆₀, additionally Raman spectroscopy confirmed the presence of<br/>C₆₀ on the sample surface.<br/>The height of the obtained monolayer and its mechanical and thermal proprieties were measured by ultrasound force microscopy (UFM), quantitative<br/>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.<br/>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<br/>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.<br/>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.<br/>

Topics

• Deposition
• impedance spectroscopy
• surface
• experiment
• thin film
• atomic force microscopy
• optical microscopy
• Raman spectroscopy
• evaporation
• molecular structure
• monolayer formation