Research thrust I: Intrinsic properties of graphitic carbon materials

Graphite and other forms of graphitic carbon materials have found widespread applications in areas such as electrochemical energy storage, water purification, and catalyst support. These applications involve a carbon surface in contact with either air or water, making the interfacial properties of graphitic materials an important topic of research. Beyond the applications, understanding the interfacial properties of carbon has far-reaching implications for fundamental research. For example, the wettability data of graphite has been used to calibrate the carbon-water force field for molecular dynamic simulations; graphite is also one of the most widely used models in tribology and surface science studies, such as adsorption, adhesion, and friction.

Our work reveals a strong environmental influence on the surface properties of carbon materials. Unintentional surface contamination by the environment masks the intrinsic properties of carbon materials but had not been recognized by the community prior to our work. As such, our work has overturned a number of long-held views about graphitic materials.

Since the 1940s conventional wisdom held that graphite is hydrophobic (i.e., it does not ‘like’ water). However, we discovered that clean graphite and graphene surfaces are in fact mildly hydrophilic (Nature Mater. 2013). The previously observed hydrophobicity was entirely caused by unintentional contamination from airborne hydrocarbons; this contamination occurs within several minutes of exposing a graphitic surface to air and had been overlooked for the previous 70 years! More importantly, this initial discovery suggests that many other surface properties of carbon materials (e.g., surface energy, double layer capacitance, and heterogeneous electron transfer rate) are likely impacted as well and that their literature values are likely not intrinsic. Indeed, we showed that the intrinsic surface energies of graphite and graphene are ca. 30% higher than those previously documented in the literature (Langmuir 2014).

Since 2016, our group has expanded our carbon research to electrochemistry and electrochemical energy storage. We showed that the interfacial contamination could significantly reduce the double layer capacitance of carbon electrodes and the performance of carbon-based supercapacitor (Carbon 2018, 2019). In collaboration with Prof. Amemiya, we also showed that the basal plane of graphite, although long deemed to have very low electrochemical activity, becomes highly active if kept free of surface contamination. We showed that a clean graphite electrode has a heterogeneous electron transfer rate constant that is ca. 3 orders of magnitude higher than that of the ‘dirty’ counterparts, achieving performance similar to that of Pt electrode (Anal. Chem. 2016). Related to these discoveries, we have developed a method to reduce the rate of surface contamination by using a coating of water layer (ACS Nano 2016). This approach was especially useful in preserving the electrochemical activity of carbon electrodes.

These results highlight the potential to improve the performance of carbon materials, in some cases beyond literature records, by simply keeping them ‘clean’. To realize this goal, the research community needs to be better informed of the surface contamination issue and change the way carbon materials are handled in the lab. To this end, we contributed a protocol paper to Chemistry of Materials (Chem. Mater. 2019), recommending guidelines for handling carbon materials in research labs. 

 
Research thrust II: DNA-based nanofabrication.

Self-assembled DNA nanostructures are an attractive template for ultra-high resolution (< 10 nm) and low-cost (< $10s/m2) nanofabrication. DNA nanostructures can be made into both 2D and 3D shapes with a resolution down to ca. 5 nm and sizes up to micrometer range. Although DNA materials are often perceived to be expensive, surface pattering only requires a monolayer amount of template and hence the cost of DNA tempalte can be as low as <$1/m2. We believe that DNA-based fabrication combines the best of both top-down and bottom-up fabrication approaches: scalable, low cost, compatible with non-flat substrates, and capable of producing designer patterns.

  Our research has focused on the development of new pattern transfer methods for DNA-based nanofabrication. Because of the low chemical and mechanical stability of DNA templates, they are not compatible with most of the pattern transfer methods used in traditional lithography. Our early work discovered that DNA templates could change the amount of molecular catalyst or precursor that can be adsorbed by the substrate, which in turn changes the rate of etching or deposition reactions on the surface. This mechanism of pattern transfer is conceptually very different from that of the traditional lithography, which is based on physical masking. This fundamental research has resulted in two novel pattern transfer methods (DNA-mediated HF vapor etching of SiO2 and chemical vapor deposition) that can produce sub-20 nm resolution patterns using unmodified DNA templates (J. Am. Chem. Soc. 2011, 2013; Chem. Mater. 2015). Both pattern transfer methods are compatible with Si and thus have significant implications to semiconductor industry.

Since 2016, we have further expanded the depth and scope of our work. We have developed DNA-based patterning of self-assembled monolayers (Chem. Comm. 2016) and polymers (ACS Nano 2017). These substrates are highly relevant to industrial coatings (e.g., paint) and our long-term goal is to use DNA-based fabrication for large scale surface engineering applications, e.g., patterning the hull of a ship to reduce biofouling and hydrodynamic drag. As an example, we recently demonstrated that the DNA-patterned SiO2 surface significantly reduces bacterial adhesion and biofilm growth (Langmuir 2019). At the same time, we have been continuously improving our fundamental understanding of the DNA-mediated surface reactions. We found that the contrast of pattern transfer in the HF-etching reaction depends on the local chemical composition of DNA template. As a result, we can control the contrast of the pattern transfer by using chemically modified DNA templates. This approach allows the fabrication of 3D nanostructures using a 2D DNA template in one step. More recently work demonstrated doping of Si using the phosphorous atoms in DNA (Adv. Func. Mater. 2020).