| Since its advent,graphene has taken the nanotechnology platform by storm,and its applications are growing day by day.The unique and outstanding properties of graphene make it a “magic bullet” for the composite world.Even being considered as a wonder material of the 21 st century,it contains few drawbacks such as zero band gap,nonmagnetic nature and absorbs 2% of light in the visible region of spectrum.The electronic,magnetic and optical properties of monolayer graphene can be manipulated by chemically doping foreign atoms and/or clusters in its two-dimensional（2D）lattice structure.In this study,we try to introduce individual foreign atoms（i.e.Boron（B）,Nitrogen（N）,Alkaline earth metal（AEM）and 5d transition metal（TM）elements）and clusters（i.e.3d Transition metal trioxide and tetraoxide（TMO3（4）））in graphene lattice,in order to overcome the mentioned drawbacks prevailing in pure graphene and introduce new trends in its properties to make it functional for real engineering applications.The structural,electronic,magnetic and optical properties of pure graphene and different foreign atom and/or cluster doped graphene complex structures were investigated using first-principles calculations based on density functional theory（DFT）method.The optical properties for pure and foreign atom doped graphene complexes have been calculated within the random phase approximation（RPA）approach.All the calculations were performed using default temperature T = 0 K parameter.In case of BN co-doping,it was observed that,BN rings doped graphene structure shows a direct band gap opening which increases by increase in the number of BN rings present in the graphene sheet,also a significant red shift in absorption towards visible region is found to occur and the absorption peak at 14 e V reduces by increasing number of BN rings in graphene sheet.In case of AEM atoms（i.e.Be,Mg,Ca,Sr and Ba）substitution in graphene,it is found that,Be,Mg and Ca atom substituted graphene structures exhibit half metallic behavior with 0.00 μB,1.86 μB,and 4.00 μB magnetic moment,respectively.While,Sr and Ba atom-doped graphene structures display indirect band gap semiconductor behavior with 3.16 μB and 0.46 μB magnetic moment,respectively.Through density of states（DOS）plots we found that,the sp orbitals of impurity atoms give rise to magnetic moments in graphene complexes.It is also found that,AEM atom substitution into graphene produces an increase in absorption spectrum in the energy range of 0 to 3 e V and reduction in absorption peak at 14 eV.Moreover,a third minimum absorption peak appears in the energy interval of 7 to 11 e V,which is not present in the absorption spectrum of pure graphene.A significant red shift in absorption towards visible range of radiation is also obtained.An increase in reflectivity peak in the low energy region is observed after AEM atom substitution into graphene.Similarly,in case of 5d series TM atom（i.e.Hf,Ta,W,Os,Re,Ir and Pt）substitution in divacancy（DV）graphene,it is found that,Hf,Ta and W substituted graphene structures show band gap at high symmetric K-point during their spin up and spin down channels with 0.783 μB,1.65 μB,and 2.00 μB,magnetic moment,respectively.Ir and Pt substituted structures show indirect band gap semiconductor behavior.Interestingly,Os substituted graphene shows direct band gap semiconductor behavior during their spin up channel with magnetic moment of 1.5 μB.Through DOS plots,we can predict that d orbitals of 5d TM atoms can be responsible for introducing ferromagnetism in doped graphene layer.All the impurity atoms（i.e.AEM and 5d TM elements）are tightly bonded with graphene,having significant formation energy and direction of charge transfer is from impurity atoms to the graphene.Later,different 3d TMO3（TM=Ti,V,Cr,Fe,Co,Mn and Ni）clusters were embedded in graphene using two different approaches.Firstly,TMO3 cluster was embedded in monolayer graphene substituting four carbon（C）atoms.In second approach,three C atoms were substituted by three oxygen（O）atoms in one graphene ring and TM atom was adsorbed at the hollow site of O atoms substituted graphene ring.In case of Cr,Fe,Co and Ni atoms substitution in between the O atoms,leads to Fermi level（EF）shifting to conduction band thereby causing the Dirac cone to move into valence band,and a band gap appears at high symmetric K-point.In case of Ti O3 and VO3 substitution in graphene,systems exhibit semiconductor properties.Interestingly,Ti O3-substituted system shows dilute magnetic semiconductor（DMS）behavior with 2.00 μB magnetic moment.On the other hand,the substitution of Co O3,Cr O3,Fe O3 and Mn O3 induced 1.015 μB,2.347 μB,2.084 μB and 3.584 μB magnetic moment,respectively.In second case of O3 atom doping and TM atom adsorption at the hollow site of graphene,leads the Fermi level（EF）to shift into conduction band and Dirac cone moves to valence band for all the given structures.In case of Cr and Ni adsorption,system displays indirect band gap semiconductor properties with 0.0 μB magnetic moment.Co adsorption exhibits DMS behavior producing 0.916 μB magnetic moment.Fe,Mn,Ti and V adsorption introduces band gap at high symmetric K-point also inducing 1.54 μB,0.9909 μB,1.912 μB,and 0.98 μB magnetic moments,respectively.Lastly,we substituted 3d TMO4 clusters in SV and DV graphene and their effects were analyzed.It is found that,Cr O4 and Mn O4 substituted SV graphene structures exhibit DMS behavior in their spin down channel with 2.15 μB and 3.51 μB magnetic moment,respectively.However,Co O4,Fe O4,Ti O4 and Ni O4 substitution into SV graphene,leads to EF level shifting to conduction band,thereby causing the Dirac cone to move into valence band and a band gap appears at high symmetric K-point.Interestingly,Co O4,CrO4,Fe O4 and Mn O4 substituted DV graphene structures exhibit DMS behavior in their spin up channel with 1.74 μB,3.27 μB,3.09 μB and 1.99 μB magnetic moment,respectively.We found that,for all cases of TMO3（4）cluster substitution in graphene,all the impurity atoms are tightly bonded with graphene,having significant formation energy and large charge transfer occurs from graphene to TMO3（4）clusters.Detailed analysis of DOS plots for both TMO3（4）cluster-doped graphene structures indicate that d orbitals of 3d TM atoms should be responsible for inducing magnetic moments in graphene.The fundamental understanding and the obtained results from this dissertation will boost the progress for designing and optimization of graphene based devices for applications of nanoelectronics,spintronic,energy storage and optoelectronic device systems.Future research possibilities in this area have also been suggested in this thesis work.We also recommend to carry out some experimental studies on the graphene structures provided in this dissertation and compare the measured results with our predicted outcomes. |
