The objective of this EArly-Concept Grant for Exploratory Research (EAGER) project is to improve physical understanding of Graphene - the thinnest possible sheet of just atomic thickness. In addition to its remarkable mechanical strength, Graphene has tantalizing electronic, magnetic, and optical properties which could lead to a host of applications ranging from nanoelectronics, highly sensitive sensors, strong materials and energy storage devices among others. One property missing from Graphene is the so-called "piezoelectricity," which allows a voltage to be developed when a material is mechanically deformed and vice-versa. Thus the specific research objective of this investigation is to elucidate "if" and "how" Graphene can be endowed with piezoelectricity (even though it may not be expected to exhibit such behavior ...). Advanced quantum mechanical calculations and state-of-the-art fabrication and testing techniques will be employed to realize such goals.
This project puts forth a high-risk high-payoff concept and, if successful, will establish a new paradigm in creating multifunctional materials and the use of Graphene as the thinnest material capable of a variety of functions - sensing and actuation, among others. Piezoelectricity is important in a host of applications where both sensing and actuation are needed - piezoelectric Graphene, for example, could be used to create artificial muscles. Other potential applications range from biomedical to space. Graduate and undergraduate students working on the project will develop a strong foundation in the highly multidisciplinary areas of nanotechnology, computational materials science and mechanics. Plans are in place to reach out to school-children through the concept of "Science behind Harry Potter."
The goal of the proposed experimental and computational research was to modify graphene to act as a piezoelectric material.Graphene, although has several fascinating properties, is non-piezoelectric due to the zero band gap and the centrosymmetry of the crystal structure. The EAGER project aimed to explore possible ways of inducing piezo-electricity in graphene layers. Following are the experimental efforts that brought modification to the graphene structure via chemical approaches, both by creating fluorinated graphene and carbon nitride layers. The synthesis of the carbon-nitride structure is particularly relevant since theoretical prediction shows that this structure could be piezoelectric unlike graphene. Experimental Outcomes a. Fabrication of graphitic carbon nitride (g-C3N4) sheets/films: Graphitic carbon nitride (g-C3N4) is graphite-like layered material, where there are tri-s-triazine units connected with planar amino groups in each layer and weak van der Waals force between its layers. Recent theoretical investigations reveal that g-C3N4 sheets exhibit unique piezoelectric properties. Moreover, as a fascinating semiconductor material, g-C3N4 powder can be synthesized by polycondensation of various organic precursors containing both carbon and nitrogen and hence, producing g-C3N4 sheets is a potential route to synthesize a 2D piezoelectric material. In this project, two approaches are developed for the synthesis of free-standing g-C3N4 sheets as described below. Liquid phase exfoliation method to produce g-C3N4 sheets: As illustrated in Figure 2, bulk g-C3N4 was exfoliated in various solvents such as IPA, N-methyl-pyrrolidone (NMP), water, ethanol and acetone to create g-C3N4 sheets. As shown in the figure, numerous sheets with laminar morphology can be observed after exfoliation in IPA. Their typical AFM image and thickness analyses reveal the morphology, SEM and TEM displayed, showing uniform thickness of ?2 nm. The composition of g-C3N4 sheets (C/N=1.33) is confirmed by XPS and elemental mapping analysis. Our preliminary results reveal that the resulting g-C3N4 sheets have a very good piezo-response when an AC bias applied. Detailed study to understand the piezoelectric properties of this material is in progress. CVD method to produce g-C3N4 films: To synthesize thin and continuous g-C3N4 film, cyanamide was chosen as precursor. As shown in Figure 3, the optimized CVD process resulted in thin and continuous g-C3N4 films; AFM, SEM and TEM measurements and thickness analyses revealed the similar morphology to the exfoliated samples and have uniform thickness of 2-3 nm. The further characterization of the CVD grown layers including XPS, Raman and piezoelectric properties is ongoing. b. Synthesis of Fluorinated Graphene Oxide and its Applications: Realizing a chemical approach to fluorinate graphene has several advantages. Since fluorine is the most electronegative element the high polarity of the C-F bond makes it an attractive alternative to the C-O bond, especially in light of previous reports that exploit the electronegativity of oxygen. The low surface energy of the C-F bond also enables one to tailor the surface characteristics and charge, in this case, by chemically altering the C/O and C/F ratio on the graphene basal plane. We developed a one-pot methodology to synthesize bulk quantities of 2-dimensional (2D) nanoflakes of fluorinated graphene oxides (FGOs) with different C/O and C/F ratios. Fluorine is covalently bonded to the graphitic domain in the form of aliphatic C-F bonds, in addition to epoxy, hydroxyl and carbonyl moieties that typically exist on the surface of GO. Ease of the solution processing leads to fabrication of FGO-based ‘inks’ in volatile, organic solvents, that have been sprayed on a range of porous and smooth surfaces to create thin films (1 – 100 µm). In addition to the simple spray-painting, thin films of superhydrophobic FGO were also obtained via drop-casting and spin coating on various substrates. We characterized the wetting behavior of the modified surfaces, and we are in the process of studying the electrical properties of these new material including piezoelectric behavior. Broader Impacts: During this short Eager project we created two graphene-based, non-centrosymmetric materials that have the possibilities to become piezoelectric and characterized these materials revealing their exact composition and crystal structure. The work on elucidating their piezo-electric properties, is under way. The understanding of new properties like piezoelectricity in graphene will lead to new frontiers in materials science and nanotechnology. The graduate student involved in the project received interdisciplinary training on various aspects of materials synthesis, chemistry and atomic scale characterization. Figure 1: The non-symmetric void gives rise to high polarization of C3N4 under tensile stress. The model used above was developed by the PI (Pradeep Sharma) who has done theoretical modeling of the material. Figure 2: Fabrication of g-C3N4 nanosheets by a liquid exfoliation method from bulk g-C3N4 powders. (a) Typical FE-SEM and (b) TEM images reveal the flexible g-C3N4 nanosheets with size of 500 nm to several micrometers. Figure 3: Fabrication of g-C3N4 film by CVD method: a representative SEM image showing a thin and continuous g-C3N4 film.
