Publications
Single-Crystalline Graphene by Low-Pressure CVD Method: Nucleation Limited Growth Transfer and Characterization
Withanage Sajith Madushan Wijayarathna
Georgia State University
Abstract
Graphene has attracted enormous attention due to its unique characteristics. However, the LPCVD graphene grown on copper turns out to be polycrystalline because of the high nucleation density (ND) on the copper foil surface. In order to realize better quality LPCVD graphene, this ND needs to be significantly reduced. Based on the observations from our initial graphene growths on as-received copper, we figured that the uneven Cu surfaces with defects produce large NDs. At a large ND, the graphene flakes nucleated at different sites coalesced to produce polycrystalline graphene. Due to such issues, we have implemented an electropolishing technique to smoothen the native surface of the copper foil. We will discuss the successful implementation of the surface smoothening process to reduce nucleation site formation while limiting the surface defects (which leads to wrinkle formation). The annealing process was also helpful to flatten the surface during the growth process further. We have also observed that graphene grows across Cu grain boundaries and, in the process, produces an additional surface area for graphene growth. That later causes to form wrinkles, which affect graphene properties negatively.
In the next project, the effect of multi-step copper surface oxidization, base pressure vacuum in the middle of the process, and integration of Cu enclosures on suppressing the ND will be discussed. The technique is based on the self-cleaning characteristics of copper oxides and the metal evaporation in a high vacuum at high temperatures. The ND has reduced to ~5 nucleations/𝑐𝑚2 on average (an improvement compared to the previously reported minimum value, ten nucleations/𝑐𝑚2 which was obtained using copper enclosures), and the graphene/copper surface has become smoother. The self-aligned graphene island geometry and shape of the flakes have reflected the symmetry and the single crystallinity of graphene.
The final project will discuss the growth of cm-scale graphene flakes on Cu and 3Dmultilayered graphene on 3D-Ni foams and used Ni’s gettering carbon diffusion effect to make the Cu foil carbon-free. The Ni-foam/Cu enclosure was oxidized in situ to assist with the self-cleaning process of metal oxides. The ND has been reduced to ~0.57 nucleation/𝑐𝑚2 and obtained cm-scale graphene flakes.
Official Download: https://scholarworks.gsu.edu/bitstreams/2b7f539d-faad-4e7b-aafe-e44c27ba7ba5/download
DOI: https://doi.org/10.57709/23981486
Citation: Withanage, Sajith Madushan Wijayarathna. “Single-Crystalline Graphene by Low-Pressure CVD Method: Nucleation Limited Growth, Transfer, and Characterization.” 2021. Dissertation, Georgia State University. https://doi.org/10.57709/23981486
The role of surface morphology on nucleation density limitation during the CVD growth of graphene and the factors influencing graphene wrinkle formation
Sajith Withanage, Tharanga Nanayakkara, U. Kushan Wijewardena, Annika Kriisa, and R. G. Mani
Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, U.S.A.
Abstract
CVD graphene growth typically uses commercially available cold-rolled copper foils, which includes a rich topography with scratches, dents, pits, and peaks. The graphene grown on this topography, even after annealing the foil, tends to include and reflect these topographic features. Further, the transfer of such CVD graphene to a flat substrate using a polymer transfer method also introduces wrinkles. Here, we examine an electropolishing technique for reducing native foil defects, characterize the resulting foil surface, grow single-crystal graphene on the polished foil, and examine the quality of the graphene for such defects.
Official Download: https://par.nsf.gov/servlets/purl/10177827
DOI: https://doi.org/10.1557/adv.2020.73
Citation: Withanage, S., Nanayakkara, T., Wijewardena, U. K., Kriisa, A., & Mani, R. G. (2019). The role of surface morphology on nucleation density limitation during the CVD growth of graphene and the factors influencing graphene wrinkle formation. MRS Advances, 4(61-62), 3337-3345.
Strain relaxation in different shapes of single crystal graphene grown by chemical vapor deposition on copper
Tharanga R. Nanayakkara, U. Kushan Wijewardena, Sajith M. Withanage, Annika Kriisa, Rasanga L. Samaraweera, Ramesh G. Mani
Department of Physics and Astronomy, Georgia State University, Atlanta, Georgia 30303, USA
Abstract
The chemical vapor deposition (CVD) growth of single-crystal graphene on polycrystalline copper foils is a complex process affected by thermodynamics, kinetics, and growth conditions. These factors lead to the diversity of island shapes of single crystal graphene. Here, we present an experimental atomic force microscopy (AFM) study of the different shapes of single-crystal graphene grown on the inner surface of copper enclosures using the low pressure CVD technique. Most remarkably, this study indicates that graphene single crystal appears to form below the adjacent copper foil surface. This feature is revealed in cross sectional AFM scans of the height, which indicate that the graphene surface lies below the neighboring foil surface by ~15- 30 nm. Our results also show that an impurity assisted growth mechanism governs the growth of single crystal graphene via isotropic diffusion, producing two-fold, four-fold, and six-fold symmetries in the resulting flakes. In addition, single crystal graphene produced via anisotropic diffusion is also present here, but they do not exhibit signs of an impurity assisted growth mechanism. Finally, we find that strain relaxation in two-fold and four-fold symmetric graphene structures via isotropic diffusion are more complicated than the six-fold structures via isotropic diffusion, which results in multiple steps orientations in low symmetry structures.
Official Download: https://www.sciencedirect.com/science/article/am/pii/S0008622320306862
DOI: https://doi.org/10.1016/j.carbon.2020.07.025
Citation: Nanayakkara, T. R., Wijewardena, U. K., Withanage, S. M., Kriisa, A., Samaraweera, R. L., & Mani, R. G. (2020). Strain relaxation in different shapes of single crystal graphene grown by chemical vapor deposition on copper. Carbon, 168, 684-690.
Effects of Long-Time Current Annealing to the Hysteresis in CVD Graphene on SiO2
U. Kushan Wijewardena, Tharanga Nanayakkara, Rasanga Samaraweera, Sajith Withanage, Annika Kriisa, Ramesh G. Mani
Department of Physics & Astronomy, Georgia State University, Atlanta, GA 30303, USA
Abstract
Graphene specimens produced by chemical vapor deposition usually show p-type characteristics and significant hysteresis in ambient conditions. Among many methods, current annealing appears to be a better way of cleaning the sample due to the possibility of in-situ annealing in the measurement setup. However, long-time current annealing could increase defects in the underlying substrate. Studying the hysteresis with different anneal currents in a graphene device is, therefore, a topic of interest. In this experimental work, we investigate electron/hole transport in a graphene sample in the form of a Hall bar device with a back gate, where the graphene was prepared using chemical vapor deposition on copper foils. We study the hysteresis before and after current annealing the sample by cooling down to a temperature of 35 K from room temperature with a back-gate bias in a closed cycle refrigerator.
Official Download: https://par.nsf.gov/servlets/purl/10177829
DOI: https://doi.org/10.1557/adv.2019.366
Citation: Wijewardena, U. K., Nanayakkara, T., Samaraweera, R., Withanage, S., Kriisa, A., & Mani, R. G. (2019). Effects of long-time current annealing to the hysteresis in CVD graphene on SiO2. MRS Advances, 4(61-62), 3319-3326.