What are Nanodroplets?
When a drop of any type of liquid lands on the surface of a material, various different physical effects that range from sticking to rebounding can occur. In fact, the exact physical mechanisms that follow the impact of small droplets, as well as their sources, have been well studied in an effort to understand better the transition that takes place between both macroscopic and nanoscopic domains of soft matter science. For example, the two properties of vapor pressure and surface tension both are essential features that play a role in the formation of nanodroplets, as well as their ability to be used in materials engineering, catalysis and both environmental and atmospheric chemistry. While significant, this area of research still requires a great deal of study to fully understand the physical properties of nanodroplets to derive equations for future applications.
To this end, a recent study investigating the role of vapor pressure on nanodroplets in the range of 9 to 960 molecules. In their study, the researchers found that the Kelvin equation, as shown in Figure 1, is an applicable mathematical model to determine the average density of water droplets of a homogeneous profile. By determining this relationship within that of nanodroplets, the researchers found that both time and thermodynamic approaches are valid for investigating this property of nanodroplets1.
Figure 1: The Kelvin equation provides the vapor pressure (Pv) of a droplet as a function of the radius of curvature of the interface.
Herein, the following symbols are utilized and explained for the Kelvin equation:
- P0 represents the vapor pressure of the bulk substance
- σ represents the surface tension
- ρ represents the density of the condense phase
- R represents the gas constant
- T represents temperature
What are Nano-Ribbons?
Nanoribbons, particularly those composed of graphene (GNRs) initially emerged in 2012 when Genorio and colleagues successfully split carbon nanotubes to isolate this novel material. GNRs, which are the most widely studied nanoribbons to date, exhibit a quasi-one-dimensional structure which in turn exhibits a greater aspect ratio as compared to their bulk material, as well as an ability to lower the percolation threshold when present in both conductive films and polymer composites. Current nanoribbon synthesis methods include lithography, bottom-up fabrication, and a longitudinal opening, or unzipping, of multi-walled carbon nanotubes (MWCNTs)2.
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The Emerging Relationship between Nanodroplets and Nano-Ribbons
Researchers are continually looking towards different methods of developing rapid ways in which they can produce semiconductor materials. Scientists from the National University of Singapore have developed a novel technique of growing atomically thin semiconductor ribbons, otherwise known as nanoribbons, that can be potentially applied for high-performance nanoelectronic devices.
In their design, the researchers initially reacted sulfur vapor with a mixture of molybdenum trioxide and sodium chloride salt at approximately 700 C. As the salt reacted with the molybdenum trioxide, a tertiary compound that contained sodium, molybdenum, and oxygen formed small droplets. These microscopic droplets then reacted with the sulfur components of the mixture to form ultrathin ribbon-shaped molybdenum disulfide through a phenomenon known as vapor-liquid-solid (VLS) growth3. This mechanism shows that vapor phase precursors of a material can condense into a liquid intermediate immediately before forming a solid product.
While the nanoribbon structure was an unexpected final product of this type of reaction, the researchers found that the nanodroplets demonstrated a method that can be compared to painting with an ink droplet. As the mixture nanodroplet moved across the substrate surface, it left behind a track of ultrathin crystals that were able to successfully form nanoribbons of high crystalline quality. The researchers of this study are hopeful that this method of nanoribbon synthesis presents a new avenue to investigate the way in which the interface growth of nanomaterials occurs.
1. Factorovich, M. H., Molinero, V., & Scherlis, D. A. (2014). Vapor pressure of water nanodroplets. Journal of The American Chemical Socie 136, 4508-4514. DOI: 10.1021/ja405408n.
2. “Graphene Nanoribbons: Production and Applications” – Sigma Aldrich
3. Shisheng Li et al. Vapour–liquid–solid growth of monolayer MoS2 nanoribbons, Nature Materials (2018). DOI: 10.1038/s41563-018-0055-z