Edge Saturation effects on the magnetism and band gaps in multilayer graphene ribbons and flakes

Using a density functional theory based electronic structure method and semi-local density approximation, we study the interplay of geometric confinement, magnetism and external electric fields on the electronic structure and the resulting band gaps of multilayer graphene ribbons whose edges are saturated with molecular hydrogen (H$_2$) or hydroxyl (OH) groups. We discuss the similarities and differences of computed features in comparison with the atomic hydrogen (or H-) saturated ribbons and flakes. For H$_2$ edge-saturation, we find \emph{shifted} labeling of three armchair ribbon classes and magnetic to non-magnetic transition in narrow zigzag ribbons whose critical width changes with the number of layers. Other computed characteristics, such as the existence of a critical gap and external electric field behavior, layer dependent electronic structure, stacking-dependent band gap induction and the length confinement effects remain qualitatively same with those of H-saturated ribbons.Comment: 9 pages, 10 figures, submitte

Effects of edge magnetism and external electric field on energy gaps in multilayer graphene nanoribbons

Using first-principles density-functional theory, we study the electronic structure of multilayer graphene nanoribbons as a function of the ribbon width and the external electric field, applied perpendicular to the ribbon layers. We consider two types of edges (armchair and zigzag), each with two edge alignments (referred to as alpha- and beta-alignments). We show that, as in monolayer and bilayer armchair nanoribbons, multilayer armchair nanoribbons exhibit three classes of energy gaps which decrease with increasing width. Nonmagnetic multilayer zigzag nanoribbons have band structures that are sensitive to the edge alignments and the number of layers, indicating different magnetic properties and resulting energy gaps. We find that energy gaps can be induced in ABC-stacked ribbons with a perpendicular external electric field while in other stacking sequences, the gaps decrease or remain closed as the external electric field increases.Comment: 7 pages, 9 figures, text revised, last version before publicatio

Ab Initio Theory of Gate Induced Gaps in Graphene Bilayers

We study the gate voltage induced gap that occurs in graphene bilayers using \textit{ab initio} density functional theory. Our calculations confirm the qualitative picture suggested by phenomenological tight-binding and continuum models. We discuss enhanced screening of the external interlayer potential at small gate voltages, which is more pronounced in the \textit{ab initio} calculations, and quantify the role of crystalline inhomogeneity using a tight-binding model self-consistent Hartree calculation.Comment: 7 pages, 7 figures; the effect of r3 coupling included; typo correcte

Density Functional Study of Ternary Topological Insulator Thin Films

Using an ab-initio density functional theory based electronic structure method with a semi-local density approximation, we study thin-film electronic properties of two topological insulators based on ternary compounds of Tl (Thallium) and Bi (Bismuth). We consider TlBiX$_2$ (X=Se, Te) and Bi$_2$$X$_2$Y (X,Y= Se,Te) compounds which provide better Dirac cones, compared to the model binary compounds Bi$_2$X$_3$ (X=Se, Te). With this property in combination with a structurally perfect bulk crystal, the latter ternary compound has been found to have improved surface electronic transport in recent experiments. In this article, we discuss the nature of surface states, their locations in the Brillouin zone and their interactions within the bulk region. Our calculations suggest a critical thin film thickness to maintain the Dirac cone which is significantly smaller than that in binary Bi-based compounds. Atomic relaxations or rearrangements are found to affect the Dirac cone in some of these compounds. And with the help of layer-projected surface charge densities, we discuss the penetration depth of the surface states into the bulk region. The electronic spectrum of these ternary compounds agrees very well with the available experimental results.Comment: 9 pages, 11 figures, 1 table, Accepted for publication in Physical Review

Uji Aktivitas Antibakteri Tumbuhan Caesalpinia sappan L Berdasarkan Studi Etnobotani Di Hutan Lereng Gunung Wilis Pada Bakteri Shigella dysenteriae

Masyarakat di Indonesia telah memanfaatkan teknik pengobatan tradisional sejak dahulu. Hasil dari studi etnobotani salah satu tanaman yang digunakan sebagai obat tradisional adalah tanaman kayu Secang (Caesalpinia sappan L). Tanaman kayu Secang (Caesalpinia sappan L) merupakan salah satu obat tradisional yang tidak hanya di manfaatkan sebagai seduhan minuman melainkan mengobati berbagai penyakit karena tanaman kayu secang memiliki kandungan kimia flavonoid, brazilin, alkaloid, saponin,tanin, fenil propane, dan terpenoid. Tujuan dari penelitian ini adalah untuk menguji aktivitas antibakteri kayu secang (Caesalpinia sappan L) dan sebagai langkah awal penemuan obat baru. Penelitian ini adalah penelitian kuantitatif dengan metode eksperimental laboratorium yang menggunakan rancangan True Experimental post test control design dengam pengujian antibakteri dengan menggunakan metode difusi sumuran. Hasil penelitian ini menunjukan bahwa semua konsentrasi 2,5 mg/ml, 5 mg/ml, 10 mg/ml, 20 mg/ml, 40 mg/ml ekstrak etanol kayu secang Caesalpinia sappan L memiliki aktivitas antibakteri pada bakteri Shigella dysenteriae. pada konsnetrasi 40 mg/ml memiliki nilai rata-rata tertinggi yaitu 14,36 mm. Berdasarkan uji one way anova, menunjukan adanya pengaruh aktivitas antibakteri pada bakteri Shigella dysenteriae dengan nilai sig 0,00 (a <0,05). Hal ini menunjukan adanya perbedaan aktivitas antibakteri tumbuhan kayu secang (Caesalpinia sappan L) pada berbagai konsentrasi

Ethnoscience Phenomenon: The Chemical Element Glucose as a Symbol of Human Enjoyment of Life in The Nyadran Celebration in Magelang

The purpose of this study is to determine the symbolic meaning represented by the food that must be present in every nyadran performance in Magelang City. This study is a case study with ethnographic approach. With the guidance of symbolic interactionism theory, the meaning of the taste of the chemical elements of the food will be analyzed so that there is a scientific explanation present. The informants in this study are artists, art observers and nutritionists. The results of the study, ethnoscience representations appear when the previous artists even though they cannot parse the chemical elements of coffee that create bitterness. Likewise, in the chemical elements of bananas that cause sweetness. Bitter is a flavor that in chemistry is caused by the compound trigonelline (niacin), which is found in coffee. Sweetness is a flavor that, according to chemistry, is caused by glucose in bananas. According to the informants, the sweet and bitter flavors in the food that must be present at the event represent how human life is always filled with two dominant feelings, namely bitter which is a representative of sadness, difficulty, suffering and sweetness which represents pleasure, happiness