بررسي خواص مغناطيسي نانو ساختارهاي گرافيني

چكيده انگليسي :

Study of the Magnetic Properties of Graphene Nano Structures Behnaz Bagheri Varnosfaderani b bagherivarnosfaderani@ph iut ac ir 27 07 2011 Department of Physics Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language PersianSupervisor Dr Farhad Fazileh fazileh@cc iut ac irAbstractCarbon forms many allotropic forms with different dimensionality three dimensional diamond quasi two dimensional graphite one dimensional nanotubes and zero dimensional fullerenes These mate rials are well known to exhibit remarkable electronic properties depending on their geometry Thereexists another form of carbon material graphene which is a monolayer graphite Graphene can the oretically be considered the basic material for the other forms of carbon that can be obtained fromit either by stacking graphite wrapping nanotubes or creation of topological defects fullerenes Graphite is made out of stacks of graphene layers that are weakly coupled by van der Waals forces A pencil is a typical graphite When we use it we are actually producing graphene stacks on a sheetof paper which may well contain individual graphene layers Nevertheless it was only in 2004 whengraphene was experimentally isolated The discovery of graphene and its remarkable electronic andmagnetic properties initiated great research interest in this material They produced high quality large up to 100 m in size graphene crystallites which immediately triggered enormous theoretical andexperimental studies Moreover the quality of the samples produced are so good that ballistic trans port and quantum Hall effects QHE have been observed Among graphene derivatives graphenenanoribbons constitute a fascinating object due to a rich variety of band gaps from metals to wide gap semiconductors In particular the half filled zero energy states emerge in all zigzag nanoribbonsand hence they are metallic Another basic element of graphene derivatives is a graphene nanodisk It is a nanometer scale disk like material which has a closed edge It is also referred to as nanoisland nanoflake nanofragment or graphene quantum dot Nanoribbons and nanodisks correspond to quan tum wires and quantum dots respectively They are candidates of future carbon based nanoelectron ics and spintronics alternative to silicon devices Theoretical studies have shown that magnetism canarise in various situations such as point defects disorder and reduced dimensionality Using a meanfield Hubbard model we studied the appearance of magnetic textures in zero dimensional graphenenanofragments one dimensional graphene nanoribbons and bulk graphene Among nanofragments triangular shape bowtie and coronene are studied We discussed how the shape of these materials the imbalance in the number of atoms belonging to the graphene sublattices the existence of zero energy states and the total and local magnetic moments are related A vacancy induces around itselfa quasilocalized impurity state at the fermi level and leads to formation of local magnetic moments The emerge of quasilocalized impurity state at the fermi level and local magnetic moments due topoint defects and vacancies in armchair nanoribbons and bulk graphene are also studied A simple setof rules based on Lieb theorem which determines the total spin of a bipartite system described by theHubbard model applied on these systems and compared with the results of numerical calculations In graphene nanoribbons with zigzag edges the spin polarized state and charge polarized state dueto on site U and nearest neighbor V Coulomb repulsions within the extended Hubbard model underthe maen field approximation were studed At the end we focused on the effects of a model disor der potential Anderson type and illustrated how density of states of zigzag nanoribbons have beenaffected Keywords Graphene nanofragment graphene nanoribbon bulk graphene spin polarized state charge polarized state spintronic

باقري ورنوسفادراني، بهناز

بررسي خواص مغناطيسي نانو ساختارهاي گرافيني

ريز ساختارهاي نانو متري گرافيتي , نانو روبان گرافيتي , حالت هاي اسپين قطبيده و بار قطبيده , اسپين الكترونيك , بي نظمي اندرسون