Time filter

Source Type

Abdulsattar M.A.,Ministry of Science and Technology
Silicon | Year: 2013

Infrared vibrational modes were calculated using ab initio density functional theory and coupled perturbed Hartree-Fock equations for Si nanocrystals of approximately 1.57 nm length. Frequencies of modes were analyzed against intensities, reduced masses and vibrational force constants. The present calculations were able to reproduce the red shift associated with size reduction from the original 521 cm-1 of the bulk optical Si-Si vibrations to the present nanocrystal 493 cm-1 high intensity mode. In order to investigate size dependence of infrared spectra a comparison is given with bulk silicon crystals and sila adamantane. A comparison revealed several changes as the nanocrystal size goes from bulk to the molecular limit. This includes a blue shift of radial breathing mode which is an acoustical mode and red shift of the optical modes. Different surface SiH2 and SiH3 vibrational modes were matched with experimental symmetric, asymmetric, wagging, scissor, rocking and twisting modes. © 2013 Springer Science+Business Media Dordrecht. Source

Abdulsattar M.A.,Ministry of Science and Technology
Superlattices and Microstructures | Year: 2011

Ab initio density functional theory is used to simulate electronic structure of hydrogenated SiGe nanocrystal superlattice pure and doped with substitutional P single atom. The results of electronic structure calculations are compared to the same size silicon and germanium nanocrystals. The comparison reveals that the energy gap of the three kinds of nanocrystals is nearly the same in non-relativistic and relativistic limits. Because of large width of gap in the present small nanocrystals the relativistic corrections are not as much important as in the case of bulk crystals. The doping of SiGe nanocrystal with P single atom introduced an impurity level at 4 eV below original conduction band edge. This result is much larger than comparable silicon bulk and nanocrystal doping with P atoms. Results also show that the deep internal angles and bonds in SiGe nanocrystals reach approximately the angles and structure of bulk crystals after nearly three surface layers. A double positively charged layer is located at the Ge terminated surface of SiGe nanocrystal. This layer is enhanced and is accompanied with a large increase of the dipole moment of the nanocrystal in the case of P doped nanocrystal. Due to oscillatory lattice potential in SiGe superlattice, density of states show that bands are broken up to sub-bands in comparison with silicon nanocrystal density of states especially at the conduction band. © 2011 Elsevier Ltd. All rights reserved. Source

Abdulsattar M.A.,Ministry of Science and Technology
Beilstein Journal of Nanotechnology | Year: 2013

Infrared spectra of hydrogenated diamond nanocrystals of one nanometer length are calculated by ab initio methods. Positions of atoms are optimized via density functional theory at the level of the generalized gradient approximation of Perdew, Burke and Ernzerhof (PBE) using 3-21G basis states. The frequencies in the vibrational spectrum are analyzed against reduced masses, force constants and intensities of vibration. The spectrum can be divided into two regions depending on the properties of the vibrations or the gap separating them. In the first region, results show good matching to several experimentally obtained lines. The 500 cm-1 broad-peak acoustical branch region is characterized by pure C-C vibrations. The optical branch is centered at 1185 cm-1. Calculations show that several C-C vibrations are mixed with some C-H vibrations in the first region. In the second region the matching also extends to C-H vibration frequencies that include different modes such as symmetric, asymmetric, wagging, scissor, rocking and twisting modes. In order to complete the picture of the size dependence of the vibrational spectra, we analyzed the spectra of ethane and adamantane. The present analysis shows that acoustical and optical branches in diamond nanocrystals approach each other and collapse at 963 cm-1 in ethane. Variation of the highest reduced-mass-mode C-C vibrations from 1332 cm-1 of bulk diamond to 963 cm-1 for ethane (red shift) is shown. The analysis also shows the variation of the radial breathing mode from 0 cm-1 of bulk diamond to 963 cm-1 for ethane (blue shift). These variations compare well with experiment. Experimentally, the above-mentioned modes appear shifted from their exact positions due to overlap with neighboring modes. © 2013 Abdulsattar; licensee Beilstein-Institut. Source

Germanium silicide diamondoids are used to determine electronic, structural, and vibrational properties of GeSi superlattice nanocrystals and bulk as their building block limit. Density functional theory at the generalized gradient approximation level of Perdew, Burke, and Ernzerhof (PBE) with 6-31G(d) basis including polarization functions is used to investigate the electronic structure of these diamondoids. The investigated molecules and diamondoids range from GeSiH6 to Ge63Si63H92. The variation of the energy gap is shown from nearly 7 eV toward bulk value which is slightly higher than the average of Si and Ge energy gaps. Variations of bond lengths, tetrahedral, and dihedral angles as the number of atoms increases are shown taking into account the effect of shape fluctuations. Localized and delocalized electronic charge distribution and bonds for these molecules are discussed. Vibrational radial breathing mode (RBM) converges from its initial molecular value at 332 cm-1 to its bulk limit at 0 cm-1 (blue shift). Longitudinal optical-highest reduced mass mode (HRMM) converges from its initial molecular value 332 cm-1 to experimental bulk limit at 420.7 cm-1 (red shift). Hydrogen vibrational modes are nearly constant in their frequencies as the size of diamondoids increases in contrast with lower frequency Ge-Si vibrational modes. GeSi diamondoids can be identified from surface hydrogen vibrational modes fingerprint, while the size of these diamondoids can be identified from Ge-Si vibrational modes. © 2014 Springer Science+Business Media New York. Source

Abdulsattar M.A.,Ministry of Science and Technology
Solid State Sciences | Year: 2011

In order to reduce computational efforts, and separate surface and core properties, diamond nanocrystals in the present model is represented by a heterojunction between the surface and the core in which the surface represents the outer most four layers and the core by the rest of the internal region of nanocrystal. Ab initio restricted Hartree-Fock (RHF) method coupled with the large unit cell method (LUC) is used to determine the electronic structure and physical properties of diamond nanocrystals core part with different sizes. The use of STO-3G basis choice is made to be able to compare to semiempirical methods using the complete neglect of differential overlap (CNDO) that uses Slater type orbitals (STO). The oxygenated (001)-(1 × 1) facet that expands with larger sizes of nanocrystals is also investigated to determine the rule of the surface in nanocrystals electronic structure. The results show that the present method agrees with semiempirical method contraction of lattice constant with increasing nanocrystal size but disagrees with energy gap variation with nanocrystal size in some regions. After nearly 1.4 nm the energy gap which is controlled by surface states begins to rise. The lowest unoccupied molecular orbital (LUMO) is attributed to surface states that largely reduce the value of energy gap. The sources of disagreement between semiempirical and ab initio results are discussed. The present method shows a maximum increment of the lattice constant by 3.3% over the calculated bulk for the smallest diamond nanocrystals. The surface states are found mostly non-degenerated because of the effect of surface discontinuity and oxygen atoms. Valance and conduction bands are wider on the surface due to splitting and oxygen atoms. The method also shows fluctuations in the converged energy gap, valence band width and cohesive energy of the core part of nanocrystal. These fluctuations might partially explain the controversial experimental results for diamond nanocrystals greater than 1.4 nm in size. The method of the present model has threefold results; it can be used to obtain the electronic structure of bulk, surface, and nanocrystals. © 2011 Elsevier Masson SAS. All rights reserved. Source

Discover hidden collaborations