China
Africa
Explore chapters and articles related to this topic
HEMT Material Technology and Epitaxial Deposition Techniques
Published in D. Nirmal, J. Ajayan, Handbook for III-V High Electron Mobility Transistor Technologies, 2019
The electron affinity of an atom is the amount of energy released when an electron is added to a neutral atom to form a negative ion. Generally, nonmetals have more positive electron affinity values than metals. Electron affinity generally increases across a period the and decreases while going down groups.
Molecular engineering of the efficiency of new thieno[3,2-b]thiophene-based metal-free dyes owning different donor and π-linkers groups for use in the dye-sensitised solar cells: a quantum chemical study
Published in Molecular Physics, 2021
Hossein Roohi, Nafiseh Motamedifar
To further elucidation of the charge transfer property of the dyes, ionisation potential (IP) and electron affinity (EA) are computed for the injection of holes and electrons mechanisms [70]. IP and EA are calculated in two ways. (1) Via difference between total energies of the neutral (Eneutral) and ionic systems (Ecation, Eanion) obtained from optimised molecular structures [71] and (2) based on Koopman’s theorem that IP and EA are approximated as the negative value of the energy for the lowest unoccupied molecular orbital (HOMO) and the negative value of the highest occupied molecular orbital, respectively [72]. The calculated results at DFT/B3LYP/6–31++G(d,p) level of theory are listed in Table 2. Ionisation potential is referred to the energy required for the removal of an electron from the neutral molecule, i.e. the lower the IP, the easier is to release an electron and thus to create a hole [73]. The electron affinity is defined as the difference between the energies of the neutral molecule and the anionic form of the molecule in their lowest states, i.e. it denotes the binding energy of an electron to the molecule [74]. The EA of the dyes allows the recombination process between the injected electron and the oxidised dye species, where an electron in the CB conductor can be captured by the adsorbed sensitiser entity [75]. Therefore, the small EA and IP values for dyes are favourable for the migration of electron efficiently to the conduction band.
Oxidation behavior with quantum dots formation from amorphous GaAs thin films
Published in Philosophical Magazine, 2018
Srikanta Palei, Bhaskar Parida, Keunjoo Kim
The amorphous GaAs thin film with nanodots was also studied for unoccupied states of the conduction band by inverse photoemission spectroscopy (IPS) to understand the band structure and electron affinity. Figure 8(b) shows the results. IPS measurement in isochromat mode was carried out by a low energy electron gun with a BaO cathode and a photon detector of the bandpass filter (9.5 eV). The electron affinity is the amount of energy gained by dropping an electron from vacuum level to the lowest unoccupied state or conduction band minimum (CBM). The photon energy calibration was obtained experimentally from the Fermi level of the evaporated Au and Al films. The CBM was located above the Fermi level with the energy difference of 1.15 eV. Therefore, the electron affinity was 2.88 eV, and the energy band gap was 2.73 eV. This band structure provided the slightly n-type semiconductor character on the amorphous GaAs thin film with nanodots. To understand the wide energy band structure, we further performed photoluminescence measurement.
Accurate density functional prediction of molecular electron affinity with the scaling corrected Kohn–Sham frontier orbital energies
Published in Molecular Physics, 2018
DaDi Zhang, Xiaolong Yang, Xiao Zheng, Weitao Yang
Electron affinity (EA) is a fundamental property of a chemical species, which is the energy released when an electron is added to the neutral system to form an anion. To evaluate the EA A and the ionisation potential I in density functional theory (DFT) [1], the directly way is where Ev(N) is the ground-state total electronic energy of an N-electron system in an external field v(r). Here, the values of A and I are ‘vertical’, which means the geometry of the system remains unchanged upon electron addition or depletion. Equations (1) and (2) are referred to the ΔSCF method, because A and I are obtained by performing self-consistent-field (SCF) calculations on N-electron and (N ± 1)-electron systems. However, it is rather challenging to obtain the accurate Ev(N + 1) in DFT, if the (N + 1)-electron system is an anionic species. For a molecular anion which has a higher total energy than the neutral specie, its EA is negative. This means the anion is unstable, which typically has a lifetime about 10−15 to 10−12 s [2]. In the gas phase, the states of anions can be experimentally characterised by the electron transmission spectroscopy, which is also used to measure the EA [3–5].