عنوان البحث(Papers / Research Title)
Ab initio restricted Hartree-Fock
الناشر \ المحرر \ الكاتب (Author / Editor / Publisher)
نور هادي عيسى عباس الشمري
Citation Information
نور,هادي,عيسى,عباس,الشمري ,Ab initio restricted Hartree-Fock , Time 6/15/2011 10:07:45 AM : كلية الصيدلة
وصف الابستركت (Abstract)
Ab initio restricted Hartree-Fock (RHF)
الوصف الكامل (Full Abstract)
Ab initio restricted Hartree-Fock (RHF)
Abstract
Ab initio restricted Hartree-Fock (RHF) method coupled with the large unit cell method (LUC) is used to simulate five sizes of Ge nanocrystal (8, 16, 54, 64, and 128) atoms and determine the electronic structure and physical properties of these nanocrystals with core and surface parts such as energy gap, cohesive energy, lattice constant, and valence bandwidth for two geometrical shapes: cubic and parallelogram.
Cohesive energy increased with the core size reaching the value ( 10.79 eV) which represent approximately twice the experimental value, whereas energy gap decreased as the core size becomes near from the experimental value when we add surface effect so we obtain the value (0.29 eV) for 64 atoms which equals half the experimental value and we expect that we have an accurate value when we study high number of surface atoms.
We take into account relativistic and zero-point corrections. The introduce of oxygenated (001)-(1×1) surface is effected and according to quantum confinement reduced the energy gap and enlarged valence bandwidth of (20.29 eV) greater than that of bulk and in agreement with most theoretical researches.Valence bands are wider on the surface part due to the splitting of energy levels and oxygen atoms.
lattice constant with 0.54 nm is in good agreement with the experimental value. These properties seem to be larger than that obtained from semiempirical methods using the complete neglect of differential overlap (CNDO) cause it computes all integrals.
The results show that the surface states are found mostly few-degenerate because of the effect of surface discontinuity and oxygen atoms. Nanocrystalline materials, can be used in a wide variety of new, unique and existing applications.
It is also very evident that nanomaterials outperform their conventional counterparts because of their superior chemical, physical, and mechanical properties and of their exceptional formability.
Semiconductor nano crystals have been intensively investigated in the last decade for numerous applications. They are crystals with dimensions on the order of nanometers. Once crystal size is decreased to the nano scale, the continuous bands of the bulk semiconductor turn into discrete atomic-like energy levels due to quantum confinement [1].
This property makes it possible to tune nanocrystals to desired absorption or emission wavelengths. This tunability and many other optical and electronic properties favor quantum dots for applications in light emitting diodes LEDs, lasers, electronic devices, and photovoltaic.
In particular their narrow emission line-width and stability against bleaching have made them popular as near-infrared fluorescent tags for biological studies, where they can be used for single-molecule experiments.
In recent years, Ge has become material of choice for different applications in the microelectronics industry. The widespread applications of Ge nanocrystals (NCs) are imminent and expected in the near future.
Because of its low field mobility, Ge is used for high speed devices, and by using (SiGe) crystals, the electrical properties of Si substrates can be tuned to obtain very efficient device performance. As a result of the growing applications of Ge nanocrystals in device production, there is an emerging need to study the electronic structure of these crystals [2].
The large unit cell (LUC) [3], was suggested and first applied for the investigation of electronic band structure of bulk materials first, then in particularly elemental semiconductors in the 70s of the last century. After its success in describing the electronic structure of bulk semiconductors, the method was also applied to a variety of systems.
The method was usually coupled with semiempirical methods to overcome ab initio computational difficulties of large scale and deformed systems[4]. In the present work we introduce the ab initio version of large unit cell method and apply the method to germanium nanocrystals.
We shall discuss the various benefits of this method that can be gained in this application such as reaching higher number of germanium atoms, reducing computational efforts, investigating nanocrystals physical properties that were not investigated earlier.
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