KAJIAN STRUKTUR GEOMETRI DAN SIFAT ELEKTRONIK BILAYER GRAPHENE DENGAN DOPING KALSIUM: KOMPUTASI DENSITY FUNCTIONAL THEORY (DFT)

SEFTY YUNITASARI, Sholihun, S.Si., M.Sc., Ph.D.Sc.; Drs. Pekik Nurwanto, M.S., Ph.D.

2019 | Skripsi | S1 FISIKA

Telah dilakukan penelitian mengenai pengaruh penambahan atom kasium (interstitial doping) pada pristine bilayer graphene menjadi calcium-doped bilayer graphene baik konfigurasi AA-stacking dan AB-stacking beserta supersel 2x2 menggunakan pendekatan komputasi density functional theory (DFT). Dalam penelitian ini juga ditinjau stabilitas seperti energi formasi dan frekuensi phonon. Hasil analisa geometri menunjukkan konfigurasi AA-stacking memiliki point group D2h dan AB-stacking memiliki point group CS. Sistem AA-stacking memiliki energi formasi Ef = -0.11 eV. Sedangkan sistem AB-stacking memiliki energi formasi Ef = -0.12 eV. Terjadi penurunan energi Fermi pada penambahan doping kalsium pada kedua konfigurasi yang berpengaruh pada kecepatan Fermi. Pada tinjauan struktur elektronik, teramati celah tenaga akibat penambahan atom kalsium sehingga semilogam graphene berubah menjadi semikonduktor. Sedangkan pada supersel 2x2 hanya mengalami penurunan pita tenaga saja tanpa ada celah, sehingga semilogam graphene berubah menjadi logam. Terakhir penambahan atom kalsium juga berpengaruh pada penurunan rapat keadaan phonon yang mengindikasikan adanya perubahan fase pada kristal.

Geometries and electronic properties for bilayer graphene interstitially doped by calcium atom and its 2x2 super-cell have been studied by first-principles calculation density functional theory. This study focus on energetic stabilities and dynamic stabilities such as formation energy and phonon frequency. The results show that the point group symmetry of calcium-doped bilayer graphene is D2h for AA-stacking and CS for AB-stacking configuration. The formation energies of AA and AB-stacking configuration are Ef = -0.11 eV and -0.12 eV, respectively. The Fermi energy of calcium-doped bilayer graphene both AA- and AB-stacking configuration have decreased, consequently will affect the Fermi velocities. There is small energy gap in the band structure of AA and AB-stacking makes the semi-metal graphene becomes a semiconductor. While in the band structure of super-cell, there is no gap energy and it becomes metal. At phonon frequency calculation, it's obtained that calcium-doped bilayer graphene has a lower phonon density of states than pristine bilayer graphene. This phenomenon called phonon softening which indicates phase transition in the crystals.

Kata Kunci : DFT, struktur geometri, sifat elektronik, calcium-intercalated bilayer graphene, semikonduktor