The First-principles Study Of Structural Stabilities And Electronic Properties Of Carbon Group Nanocrystals

Group IV nanocrystals include carbon,silicon,germanium nanocrystals,carbon silicon,silicon germanium nanocrystals and so on.Because of the biocompatibility and ultra-high hardness,carbon nanocrystals can be used in the biology,biomedicine industry,such as fluorescent labeling,drug delivery and so on.Owing to high luminous efficiency,silicon nanocrystals show potential applications in optoelectronic devices,including field effect transistor,light emitting diodes,molecular electronics,nano-sensors and photoelectronic chemical.Compared with silicon nanocrystals,germanium nanocrystals have narrow energy gap and higher luminous efficiency,so it can be used in infrared optics and optoelectronics.New functional devices which are made of SiGe/Si heterojunction will cover the shortage of original silicon optoelectronic devices.In this work,we mainly investigate the structural stabilities and electronic properties of carbon,silicon,germanium nanocrystals and silicon germanium alloy nanocrystals.In chapter 1,we briefly introduce the sizes,shapes,characteristic,and applications of clusters.Then we mainly introduce structural features of the C,Si,Ge and SiGe alloy nanocrystals.Next,the application and research status of C,Si,Ge and Si Ge alloy nanocrystals were analyzed.In the end of the chapter,we elaborate the main contents and methods in our work.In chapter 2,we introduce the first-principles method based on Density Functional Theory?DFT?and the software used in this work.In addition,we introduce the way to calculate phonon spectrum and Wang-Landau algorithm which is used to search candidates.In chapter 3,we importantly investigate the three isomers of X₂₂H₂₈?X=C,Si,Ge?.Based on ab initio molecular dynamics simulations,we have theoretically investigated the structural stabilities and electronic properties of X₂₂H₂₈ nanocrystals,as a function of temperature with consideration of vibrational entropy effects.To compare the relative stabilities of X₂₂H₂₈ isomers,the vibration free energies are obtained according to the calculated phonon spectrum,where the typical modes are shown to be dominant to the structural stabilities.In addition,there is a significant gap reduction as the temperature increases from 0K to 300 K,where the decrements are 0.2/0.5/0.6eV for C/Si/Ge nanocrystals,respectively.The dependence of energy gap on the varience of bond length is also analyzed according to the corresponding atomic attributions to the HOMO and LUMO levels.In chapter 4,we mainly investigates the structural stabilities and electronics properties of SiGe alloy clusters,including Si_xGe_10-xH₁₆,Si_xGe_14-xH₂₀,Si_xGe_18-xH₂₄,Si_xGe_22-xH₂₈.Firstly,we search all the possible configurations based on the condition of fixed total atom numbers of Si and Ge using different bond number combination.Next,using bond energy model and first-principles calculations,we find the stable structures.On these system mentioned above,GenHy?n=10,14,18,22?,SinHy,and the structure characterized that Si atoms connected with two H atoms and Ge atoms don't connected with two H atoms are stable.In addition to Si_xGe_10-xH₁₆,other systems have a common structure that some Si atoms connected with two hydrogen atoms,and other Si atoms connected with one H atom and three Si atoms.There are similar structure in Si_xGe_18-xH₂₄ and Si_xGe_22-xH₂₈ system,where all the Ge atoms don't connect with H atom,while all the Si atoms connect with one or two H atoms.Lastly,we have calculated the energy gap of all possible structures that have different bond number combinations.