Polymerization Of The Hetero Fullerene-like Clusters And Functionalization Of The Product Polymer

In recent years,nanomaterials,as new type material structures,have attracted considerable attention because of their unique properties and potential applications. The fullerenes,since the discovery of C₆₀in 1985 and successful synthesis of it in 1990,have attracted a great deal of interest in the community of physics,chemistry and material engineering due to their remarkable structural,electronic,optical and magnetic properties.One of the most important attempts is to change their electronic and mechanical properties by doping and synthesizing novel fullerene-based materials with special physical and chemical properties.The research on the structures and properties of the doped fullerenes and fullerene-based nanomaterials is essential for better understanding of forming mechanism of the materials and exploring their potential applications.Computational simulation,which is the third extremely powerful tool to study physical world following experiments and theories,plays a very important role in science and technology.It provides a bridge between theory and experiment and helps us not only to understand and interpret the experiments at the microscopic level,but also to study regions which are not accessible experimentally,or too expensive to approach experimentally,e.g.extremely high pressure and high temperature conditions to be required.There are two kinds of computational methods to study nanoscale materials.One is empirical or semi-empirical method;the other is ab initio or first-principle calculations,such as density functional theory(DFT)calculation. Empirical method can predict some properties of materials through analytic potential functions,while ab initio calculation starts with first-principle quantum mechanics and solves Schrodinger or Kohn-Sham equations self-consistently.In the present work,by using DFT calculations and molecular dynamics(MD) simulations,we have studied the doping of fullerene C₆₀with Si atoms,the polymerization of the Si-C hetero fullerene to form nanotubes,the functionalization of polymerized Si-C hetero fullerene-based nanotubes(Si-CHFBNT),and the polymerization of the noncarbon fullerene-like cage clusters.This dissertation includes two parts.The first part introduces the theoretical fundamentals that we used in our research work.The second part introduces the main work done during my Ph.D. degree studies.The following gives a brief outline of the main contents of this dissertation.â… .The Theoretical Fundamentals Used in Our Research WorkChapterâ…¡gives a brief introduction of fundamental aspects for the ab initio calculations and MD simulations used in this work.In this chapter,the theoretical fundamentals of the first principle calculations,the molecular dynamics(MD)simulations and ab initio MD simulations are described briefly.At present,DFT calculation is one of the most important investigation methods that can describe the structural,mechanical,electronic,magnetic,and optic properties of small systems,without need of any empirical parameters.However,due to its large computational demands,DFT calculation can only deal with the small systems which contain no more than several hundreds of atoms.Classical MD simulation is an effective tool for material design and the studies of the material properties.But only the structures,energies of the materials and some related properties,such as mechanical properties,can be obtained.We have to use the first principle calculations to study the electronic properties and optoelectronic properties of the materials.So in the practical calculations,we are trying to combine the two methods together to obtain as much information as possible concerning the system under study.â…¡.The Main Work Done during My Ph.D.Degree StudiesChapterâ…¢Substitutionally doping C₆₀with Si atoms and polymerization of the Si-C hetero fullerene derivativesIn recent years,doped fullerenes with various elements have attracted considerable research efforts due to the interest in synthesizing fullerene-based nanomaterials with special physical and chemical properties.In addition to endohedrally and exohedrally adsorption of foreign species on the wall of fullerenes, the substitutionally doped fullerenes,which have the C atoms substituted by the foreign atoms,are also intriguing for their novel geometric and electronic structures. Silicon should be an ideal candidate to substitute the C atoms on the fullerene cages due to the similarity of their valent electron configurations.In this chapter,we use DFT calculations to investigate the energetics,structural and electronic properties of the Si-C hetero fullerene-based materials obtained by doping C₆₀with different numbers of Si atoms.The results indicate that,among the different Si-C hetero fullerene isomers obtained,the one with the C atoms and the Si atoms located in separated region,i.e.,with a phase-separated structure is more stable.The energy gaps of these Si-C hetero fullerene-based materials demonstrate that the HOMO-LUMO gaps are greatly modified and show a decreasing trend with increasing the size of the clusters.The structures of the fully optimized Si-C hetero fullerene-based nanotubes (SiCFBNT)are especially regular and exhibits interesting dumbbell-shaped chain structures.The Si-C hetero fullerene-based nanotubes have narrow and direct energy band gaps,implying that it is a narrow gap semiconductor and may be a promising candidate for optoelectronic devices.Chapterâ…£Functionalization of the polymerized one-dimensional Si-C hetero fullerene-based nanotube with H adsorption on the sidewallNanotubes are always functionalized via different approaches to tune their electronic structures and thus the relevant electronic and optoelectronic properties by using the methods,such as adsorption of foreign atoms or molecules on the interior or exterior-wall of the nanotube.It provides a great potential of tailoring the properties of one-dimensional nanomaterials to meet the needs of application in nanotechnology. In view of the very narrow band gap of the one-dimensional Si-C hetero fullerene-based nanotube(Si-CHFBNT)predicted in the previous chapter,in this chapter,we investigate hydrogen atom being adsorbed on the Si-C hetero fullerene-based one-dimensional nanotube using density functional theory(DFT).The results indicate that,the electronic structures of the Si-CHFBNT can be drastically changed by the H atom adsorbed on the exterior-wall.The functionalized Si-CHFBNT generated conducting properties at room temperature independent of the adsorption sites of H atom on the Si-CHFBNT.This result may be utilized for band structure engineering.Chapterâ…¤Polymerization of the fullerene-like cage(SiC)₁₂to novel silicon carbide nanowiresIn recent years,the exploration of possible fullerene-like structures or nanotubes composed of noncarbon elements has attracted much attention.Very recently, theoretical investigations addressing the stability of SiC nanostructures based on ab initio calculations using density functional theory(DFT)were reported.The structures and stability of fullerene-like cages(SiC)_n(n=6-36)were studied and it was suggested that the fullerene-like cage(SiC)₁₂was energetically the most stable cluster among those cage structures and would be possibly synthesized under certain condition.In the previous theoretical studies on the fullerene-like cages(BN)_n and (AlN)_n,the fullerene-like cages(BN)₁₂and(AlN)₁₂were also predicted to be the most stable ones.Therefore,the fullerene-like cage structure(XY)_n may be a magic cluster when n is equal to 12.Considering the promising applications of the silicon carbide nanomaterials in optoelectronic devices,in this chapter,we performed ab initio studies on the stabilities and structural,as well as,electronic properties of the fullerene-like cage(SiC)₁₂,(SiC)₁₂-(SiC)₁₂dimers and(SiC)₁₂-based SiC nanowires obtained from the(SiC)₁₂clusters.The results indicate that the stable fullerene-like cage(SiC)₁₂can form a new family of SiC nanomaterials,including(SiC)₁₂-based dimers and nanowires,which are energetically more stable than the(SiC)₁₂cluster. The fully optimized structures of the two(SiC)₁₂-based nanowires are regular and exhibit interesting dumbbell-shaped chain structures.The calculated HOMO-LUMO gaps of all these configurations of(SiC)₁₂-based materials are not recognized to have a remarkable difference from that of the(SiC)₁₂cluster.They have the similar semiconducting electronic properties as SiC nanotubes.We hope this finding could motivate further studies on the(SiC)₁₂-based materials,for instance,on the synthesis methods,the applications and functionalizations of this novel material family.Chapterâ…¥Theoretical prediction of the(AlN)₁₂fullerene-like cage based nanomaterialsGenerally,â…¢-â…¤compounds,especially the groupâ…¢nitrides,are found to be important source of nanoscale materials for their direct band gaps affording optical and electrooptical properties that are of considerable importance to technology applications.Among these,aluminum nitride semiconducting nanostructures are especially promising materials and have attracted considerable attention due to their large band gap,low electron affinity,and excellent physical and chemical properties, e.g.,high thermal-conductivity,low thermal-expansion coefficient,and chemical inertness.Recently,the theoretical investigations addressing the stability of aluminum nitride nanostructures based on ab initio calculations were reported and it was suggested that the fullerene-like cage(AlN)₁₂is energetically the most stable cluster in the family of(AlN)_n(n=2-41).Based on the study in the former chapter,in this chapter,we performed ab initio calculations on the stability and structural and electronic properties of the fullerene-like cage(AlN)₁₂and the polymerized dimers and nanowires obtained from it.The results indicat that the(AlN)₁₂-(AlN)₁₂dimers and aluminum nitride(AlN)₁₂-based nanowires polymerized from the(AlN)₁₂cage are more stable than the(AlN)₁₂.The optimized configurations of the nanowires are especially regular and exhibit an interesting dumbbell-shaped chain structure.We also calculated the electronic structures of all the constructed nanostructures.The two novel(AlN)₁₂-based aluminum nitride nanowires have band gaps of 2.844eV and 3.085eV,respectively,implying that they are both wide-gap semiconductors and may be promising candidates for nanotechnology as novel AlN nanomaterials.