Electron Affinity Values Research Articles

Experimental studies of electron affinity and work function from titanium on oxidised diamond (100) surfaces

Sub-monolayers of titanium were deposited onto oxidised (100) single-crystal diamond surfaces and annealed in vacuo at temperatures up to 1000 °C to find a temperature-stable termination procedure that produces a surface with Negative Electron Affinity (NEA). The samples were analysed by X-ray Photoelectron Spectroscopy, Ultraviolet Photoelectron Spectroscopy and Energy-Filtered Photoemission Electron Microscopy to determine their electron affinity and work function values. NEA values were observed on samples following annealing above 400 °C, with the largest NEA value being –0.9 eV for a sample coated with a half-monolayer of Ti annealed at 400 °C. Work function values were ∼4.5 eV for all samples annealed at temperatures between 400 and 600 °C, then rose at higher temperatures due to the loss of substantial amounts of O from the surface. Work-function maps indicated that the surface was uniform over areas 5700 μm2, suggesting that the deposition and annealing steps used are reliable methods to produce films with homogeneous surface properties.

Passivation of Mid-Gap Electronic States at Calcium Aluminosilicate Glass Surfaces upon Water Exposure: An Ab Initio Study.

When a clean glass surface is exposed to humid air, a thin water layer forms on the hydrophilic surface. Using ab initio molecular dynamics, we simulate the changes in the electronic structure of a CaO-Al2O3-SiO2 glass model upon vacuum fracture and subsequent exposure to H2O. When the glass is fractured, dangling bonds form, which lower the band gap of the surface by ∼1.8 eV compared to the bulk value due to mid-gap surface states. When H2O adsorbs onto the vacuum-fractured surface, the band gap increases to a value closer to that of the bulk band gap. Using two different hydroxylation methods, we find that the calculated band gap of the glass surface depends on the hydroxylation state. Surfaces with ∼4.5 OH/nm2 have smaller band gaps due to unfilled surface states, and surfaces with ∼2.5 OH/nm2 have larger band gaps with no apparent unfilled surface states. The resulting changes in the electronic structure, quantified by electron affinity and work function values, are hypothesized to play an important role in the electrostatic charge transfer based on the principles of surface state theory, which posit that the density of electronic surface states determines the amount of electronic charge transfer to or from material surfaces.

Designing limiting-efficiency BaSi[formula omitted] solar cells by device simulation and computational material screening

A double heterojunction solar cell consisting of a solar absorber and electron/hole transport layers (ETL/HTL) is ideally efficient and can contribute to the exploitation of solar energy. BaSi2 is an attractive solar absorber material possessing a band gap of 1.3 eV and high absorption coefficients. However, finding proper ETL/HTL materials remains challenging. Here, limiting efficiency calculation, device simulation, and computational material screening are combined to design BaSi2 solar cells of near-limiting efficiency (> 28%). The limiting efficiency is calculated considering radiative and trap-assisted Auger recombinations, which shows that the Lambertian limit efficiency (32%) exceeds that of Si cells. Device simulations of double heterojunction BaSi2 cells were performed with different values of band gap, ionization potential, and electron affinity, which yields criteria for band-edge energy levels of the ETL and HTL. Candidate materials for the ETL and HTL are identified from the Materials Project database by three-step screening. The first step screens the materials for band gap, interface reactivity, lattice matching, etc. The second step is a literature survey to exclude air-sensitive or unstable materials under ambient conditions. In the third step, band-edge levels are calculated using density functional theory, and eight ETL and seven HTL candidate materials are identified. This device design method is applicable to any solar absorber material and promotes accelerated solar cell development.

Negative Ion Photoelectron Spectra of Deprotonated Benzonitrile Isomers via Computation of Franck-Condon Factors.

Negative ion photoelectron spectra of ortho (o-), meta (m-), and para (p-) deprotonated benzonitrile (o-, m-, p-C6H4(CN)-) isomers as well as the associated thermochemical values corresponding to deprotonation at o-, m-, and p-positions in C6H5(CN) are presented. Quantum mechanical results based on the density functional theory (DFT) utilizing the aug-cc-pVQZ basis set indicate that the o-, m-, p-C6H4(CN)● radicals have electron affinity values (EAs) of 1.901, 1.778, and 1.789 eV, respectively. The computed Franck-Condon (FC) factors give rise to o-, m-, and p-C6H4(CN)- negative ion spectra with FC active ring distortion vibrational modes with harmonic vibrational frequencies of ∼450, 760, and 1000 cm-1 as the dominant vibrational progressions. Deprotonation at the o-, m-, and p-positions in C6H5(CN) results in calculated gas-phase acidity values (ΔacidH298Ko) of 383.9, 385.7, and 385.3 kcal mol-1, respectively. The calculated ΔacidH298Ko is in close agreement with the previously reported high-pressure mass spectrometry experimental value of 383.4.0 ± 4.4 kcal mol-1. The computed ΔacidH298Ko and EAs are utilized to estimate the bond dissociation energy (DH298(H-C6H4CN)) associated with the formation o-, m-, and p-C6H4(CN)● using the negative ion thermochemical cycle: DH298(C6H5CN) = ΔacidH298Ko (H-C6H4(CN) + EA (C6H5CN)● - IP(H). The respective values of DH298(H-C6H4CN) corresponding to the formation of ortho, meta, and para C6H4(CN) radicals are 114.15, 113.11, and 113.51 kcal mol-1.

Stability, Energetic, and Reactivity Properties of NiPd Alloy Clusters Deposited on Graphene with Defects: A Density Functional Theory Study.

Graphene with defects is a vital support material since it improves the catalytic activity and stability of nanoparticles. Here, a density functional theory study was conducted to investigate the stability, energy, and reactivity properties of NinPdn (n = 1–3) clusters supported on graphene with different defects (i.e., graphene with monovacancy and pyridinic N-doped graphene with one, two, and three N atoms). On the interaction between the clusters and graphene with defects, the charge was transferred from the clusters to the modified graphene, and it was observed that the binding energy between them was substantially higher than that previously reported for Pd-based clusters supported on pristine graphene. The vertical ionization potential calculated for the clusters supported on modified graphene decreased compared with that calculated for free clusters. In contrast, vertical electron affinity values for the clusters supported on graphene with defects increased compared with those calculated for free clusters. In addition, the chemical hardness calculated for the clusters supported on modified graphene was decreased compared with free clusters, suggesting that the former may exhibit higher reactivity than the latter. Therefore, it could be inferred that graphene with defects is a good support material because it enhances the stability and reactivity of the Pd-based alloy clusters supported on PNG.

A systematic computational study of acridine derivatives through conceptual density functional theory.

A detailed computational analysis of acridine derivatives viz. acridone, 9-amino acridine hydrochloride hydrate, proflavin, acridine orange and acridine yellow is done in terms of conceptual density functional theory (CDFT). CDFT-based global descriptors-ionization potential, electron affinity, HOMO-LUMO gap, hardness, softness, electronegativity and electrophilicity index of acridine derivatives for ground state as well as excited state are estimated with the help of different hybrid functionals B3LYP/6-31G (d, p), B3LYP/6-311G (d, p), B3LYP/DGDZVP and B3LYP/LANL2DZ. Acridine derivatives show higher values of ionization potential and electron affinity in excited state as compared to ground state, indicating that these compounds are willing to accept electrons in excited state rather than donating electron. Acridone shows the maximum HOMO-LUMO energy gap in ground and excited state which implies that one-way electron transfer is most feasible with this compound. Our computed results emphasize the pronounced electron acceptor behaviour of the acridine derivatives in the excited state which has already been experimentally verified. It is observed that the trend in the computed values of the descriptors is not much improved on refinement of the basis set.

Photoelectric, optical and microstructural characterization of thin palladium silicide (Pd2Si) layers fabricated by magnetron sputtering from a stoichiometric target

The properties of thin Pd2Si layers fabricated by means of magnetron sputtering deposition from a stoichiometric target was examined. Optical parameters were determined using spectroscopic ellipsometry. The refraction indexes (n) and extinction coefficients (k) of a Pd2Si layer as a function of light wavelength were obtained. The microstructural properties of the Pd2Si layer were investigated using Transmission Electron Microscopy (TEM), Energy-Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), X-ray reflectivity (XRR) and Secondary Ion Mass Spectrometry (SIMS). The work function of the silicide layer was measured on the fabricated metal(Pd2Si)-insulator-semiconductor (MIS) structures using Internal Photoemission Spectroscopy (IPS). The IPS technique was used to determine the barrier energy heights at the gate/dielectric (EBG) and dielectric/substrate (EBS) interfaces of the investigated Pd2Si/SiO2/Si system. The appropriate model allowing calculations of fractions of light absorbed by the gate and transmitted to the substrate was developed. The barrier energy from the Fermi level of the silicide gate and from the substrate valence band to the dielectric conduction band are estimated to be EBG = 4.11 ± 0.05 eV and EBS = 4.34 ± 0.05 eV, respectively. Taking the measured barrier height values and reported values of Si electron affinity (χSi = 4.05 eV) and Si band gap (EG = 1.12 eV), the electron affinity of the thermally grown oxide (SiO2) was determined to be χSiO2 = 0.83 ± 0.05 eV and the Pd2Si work function equal to ϕM = 4.94 ± 0.1 eV. It is shown that the application of a single, stoichiometric sputtering target allows to obtain thin Pd2Si layers with work function value resembling that for the bulk material.

A First-Principles Study on the Electronic, Thermodynamic and Dielectric Properties of Monolayer Ca(OH)2 and Mg(OH)2.

We perform first-principles calculations to explore the electronic, thermodynamic and dielectric properties of two-dimensional (2D) layered, alkaline-earth hydroxides Ca(OH) and Mg(OH). We calculate the lattice parameters, exfoliation energies and phonon spectra of monolayers and also investigate the thermal properties of these monolayers, such as the Helmholtz free energy, heat capacity at constant volume and entropy as a function of temperature. We employ Density Functional Perturbation Theory (DFPT) to calculate the in-plane and out-of-plane static dielectric constant of the bulk and monolayer samples. We compute the bandgap and electron affinity values using the HSE06 functional and estimate the leakage current density of transistors with monolayer Ca(OH) and Mg(OH) as dielectrics when combined with HfS and WS, respectively. Our results show that bilayer Mg(OH) (EOT∼0.60 nm) with a lower solubility in water offers higher out-of-plane dielectric constants and lower leakage currents than does bilayer Ca(OH) (EOT∼0.56 nm). Additionally, the out-of-plane dielectric constant, leakage current and EOT of Mg(OH) outperform bilayer h-BN. We verify the applicability of Anderson’s rule and conclude that bilayers of Ca(OH) and Mg(OH), respectively, paired with lattice-matched monolayer HfS and WS, are effective structural combinations that could lead to the development of innovative multi-functional Field Effect Transistors (FETs).

The Evolution of Geometric Structures, Electronic Properties, and Chemical Bonding of Small Phosphorus-Boron Clusters

We report a comprehensive theoretical investigation on phosphorus–boron mixed neutral, anionic, and cationic clusters P2Bn/P2Bn−/P2Bn+ (n = 3–7) with two phosphorus atoms and three to seven boron atoms. We reveal the common character of all the structures (i.e., the phosphorus atoms choose to occupy the peripheral position), whereas the boron atoms tend to be in the central and inside position of the ground state phosphorus—boron mixed clusters at each stoichiometry. Any three atoms preferentially form a stable triangle and grow with zigzag shape in a planar network. Interestingly, a series of planar motifs (including tetra-, penta-, and hexa-coordination) have been discovered in the phosphorus–boron clusters. The large binding energies (3.6 to 4.6 eV/atom) and quite large HOMO–LUMO gaps (5 to 10 eV) indicate the high stability of the clusters. The energy differences Δ1E, Δ2E, and energy gaps display oscillating behavior with increasing numbers of boron atoms. The electron affinity (EA) and ionization potential (IP) generally have small variations, with the EA values ranging from 2 to 3 eV, and the IP values ranging from 7 to 9 eV. Chemical bond analysis shows that the existence of multi-center delocalized bonds stabilize the clusters.

H2 storage capacity of Li-doped five member aromatic heterocyclic superalkali complexes; an in silico study

Hydrogen adsorption characteristics of different Li dopped aromatic five-membered heterocyclic (Ar) systems; have been studied [Ar = Imidazole, Pyrazole, Oxazole, Isoxazole, Thiazole, and Isothiazole]. The Lithiated heterocyclic systems are stabilized as cation (Ar-Li)+ and behave as superalkali (the vertical electron affinity (VEA) value ranges from 4 − 1.99 eV). The bonding nature between Ar and Li as well as that between Li and adsorbed H2 molecule has been analyzed through the electron localization function (ELF) study. The first principle calculation reveals that (Ar-Li)+ complexes can adsorb up to five molecular hydrogens in a quasi-sorption way with the binding energy of ∼ −2.17 eV. The spontaneity of the hydrogen adsorption in (Ar-Li)+ complexes is studied to find in which temperature Gibbs free energy change (ΔG) for all H2 adsorbed systems will be negative. At 80 K temperature, it will be satisfied.

Superior p‐Type Surface Doping of Cubic Boron Nitride via MoO3 Adsorption

AbstractCubic boron nitride (c‐BN) is a potential candidate material for electronic and optoelectronic devices under extreme conditions, while the difficulty in conventional doping severely hinders its applications. Herein, by first‐principles calculation, an efficient p‐type doping on the c‐BN surface (areal hole density of 8.82 × 1013 cm−2 and hole mobility of 826 cm2 V−1 s−1 at room temperature) is realized by the surface charge transfer mechanism with the MoO3 molecule having a high electron affinity as the dopant. The MoO3 molecule draws electrons from the c‐BN, making the hole accumulation on the c‐BN surface. The areal hole density of c‐BN enhances with the increase of MoO3 molecule density and reaches the maximum (1.24 × 1014 cm−2) with MoO3 monolayer adsorption. The optical absorption coefficient significantly increases in the infrared and visible regions. The superior p‐type doping for c‐BN is also achieved by adsorbing transistor metal oxides with the high electron affinity values (such as ReO3, CrO3, WO3, and V2O5). This study opens a novel avenue for promoting c‐BN applied in optoelectronic devices.

ThAu2 -, ThAu2O-, and ThAuOH- anions: Photoelectron spectroscopic and theoretical characterization.

The thorium-gold negative ions ThAu2 -, ThAu2O-, and ThAuOH- have been observed and experimentally characterized by anion photoelectron spectroscopy. These experiments are accompanied by extensive ab initio electronic structure calculations using a relativistic composite methodology based primarily on coupled cluster singles and doubles with perturbative triples calculations. The theoretical electron affinities (EAs) at 0K agree with the experimental adiabatic EAs to within 0.02eV for all species. Two separate isomers were located in the calculations for ThAuOH-, and detachment from both of these appears to be present in the photoelectron spectrum. Excited electronic states of the neutral molecules are reported at the equation of motion-coupled cluster singles and doubles level of theory. Atomization energies and heats of formation are also calculated for each neutral species and have expected uncertainties of 3 and 4 kcal/mol, respectively. The σ bonds between Th and Au are determined by natural bond orbital analysis to consist of predominately sd hybrids on Th bonding with the Au 6s orbital. In order to investigate the correspondence between the bonding in Th-Au and Th-F molecules, a limited number of calculations were also carried out on most of the F-analogs of this study. These results demonstrate that Au does behave like F in these cases, although the Th-F σ bonds are much more ionic compared to Th-Au. This results in an EA for ThF2 that is 10 kcal/mol smaller than that of ThAu2. The EA values for the Th(IV) species, i.e., ThX2O and ThXOH, only differed, however, by 3-4 kcal/mol.

Effect of FA: X+3 (X = B, Al, and Ga) Color Centers on the Electronic and Optical Properties of LiF (001) Surface

The density functional theory and configuration interaction single excitations methods were used to study the effect of FA color centers on the electronic and optical properties of the LiF (001) surface. Three types of FA color centers were used, FA: B+3, FA: Al+3, and FA: Ga+3. The evaluated values of the ionization potential, chemical hardness, and softness indicate the stability of the investigated color centers. FA: B+3 is the most stable one. While the electron affinity values reveal that the FA: Al+3 center is the highest reactive one. The Ultraviolet-Visible spectra for FA: B+3, FA: Al+3, and FA: Ga+3 centers showed that the scrutinized centers have two absorption peaks. The highest absorption peak recorded for FA: B+3 center is located at 285 nm, while the highest absorption peaks recorded for FA: Al+3, and FA: Ga+3 centers are located at 173 and 165 nm, respectively. According to the light-harvesting efficiency values, FA: Al+3, and FA: Ga+3 centers have the highest efficiency to gather the energy during the pumping process than FA: B+3 centers. Stokes-shift values for FA: B+3, FA: Al+3, and FA: Ga+3 centers were 0.16, 0.37, and 0.33 eV, respectively, consequently, FA: Al+3 and FA: Ga+3 centers, are more suitable for laser production than FA: B+3 center.

Powerful Organic Molecular Oxidants and Reductants Enable Ambipolar Injection in a Large-Gap Organic Homojunction Diode.

Doping has proven to be a critical tool for enhancing the performance of organic semiconductors in devices like organic light-emitting diodes. However, the challenge in working with high-ionization-energy (IE) organic semiconductors is to find p-dopants with correspondingly high electron affinity (EA) that will improve the conductivity and charge carrier transport in a film. Here, we use an oxidant that has been recently recognized to be a very strong p-type dopant, hexacyano-1,2,3-trimethylene-cyclopropane (CN6-CP). The EA of CN6-CP has been previously estimated via cyclic voltammetry to be 5.87 eV, almost 300 meV higher than other known high-EA organic molecular oxidants. We measure the frontier orbitals of CN6-CP using ultraviolet and inverse photoemission spectroscopy techniques and confirm a high EA value of 5.88 eV in the condensed phase. The introduction of CN6-CP in a film of large-band-gap, large-IE phenyldi(pyren-1-yl)phosphine oxide (POPy2) leads to a significant shift of the Fermi level toward the highest occupied molecular orbital and a 2 orders of magnitude increase in conductivity. Using CN6-CP and n-dopant (pentamethylcyclopentadienyl)(1,3,5-trimethylbenzene)ruthenium (RuCp*Mes)2, we fabricate a POPy2-based rectifying p-i-n homojunction diode with a 2.9 V built-in potential. Blue light emission is achieved under forward bias. This effect demonstrates the dopant-enabled hole injection from the CN6-CP-doped layer and electron injection from the (RuCp*Mes)2-doped layer in the diode.

Structural and electronic properties of pristine and hydrogen-terminated c-BN(100) surfaces.

Semiconductor surfaces are crucially important for electronics, but it is difficult to directly image their surface structures. In this work, the surface structures and electronic properties of pristine and H-terminated c-BN(100) surfaces are predicted by first principles calculation. It is found that the surfaces with reconstructed dimers and staggered dimers are thermally and dynamically stable. When the surface N and B atoms are saturated with the virtual H of 0.5 e and 1.5 e, the surface states near Fermi level are nearly removed, and have the wide bandgaps. Meanwhile, after surface hydrogenation, the electron affinity value changes from positive to negative. Our findings could provide a theoretical guidance for designing c-BN-based electronic devices.

Benzo[de]isoquinoline-1,3-dione condensed asymmetric azaacenes as strong acceptors.

We have designed and synthesized three novel benzo[de]isoquinoline-1,3-dione (BQD) condensed asymmetric azaacenes, BQD-TZ, BQD-AP and BQD-PA, with different end groups of 1,2,5-thiadiazole, acenaphthylene and phenanthrene. The triisopropylsilylethynyl groups were grafted to increase the solubility and crystallinity of the three molecules. These molecules have high electron affinity values of 3.87, 3.69 and 3.74 eV for BQD-TZ, BQD-AP and BQD-PA, respectively as confirmed by cyclic voltammetry measurements. Single-crystal X-ray diffraction revealed that these molecules have strong π–π stacking with distances of 3.31–3.41 Å and different stacking arrangements.

Theoretical study of new promising conjugated psoralens in psoralen ultraviolet A therapy

AbstractIn our previous report (J. Phys. Org. Chem., 2019), we discussed novel silicon photosensitizers (PSs) in psoralen ultraviolet A (PUVA) therapy. Continuing this quest, we have scrutinized a new brand of PSs, via density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) methods. These PSs show intense narrow absorption peaks in the therapeutic window with very high molar absorption coefficients ( ). Calculated vertical ionization potential (VIP) and vertical electron affinity (VEA) values indicate that our PSs can produce reactive oxygen species (ROS) through type I mechanism. Also, energies of the first triplet excited states (T1) and spin–orbit coupling (SOC) constants verify the possibility and efficiency of type II mechanism for the generation of the singlet excited state of molecular oxygen (1O2). The results introduce new potential PSs in PDT and PUVA therapy.

Rational Design and Characterization of Symmetry-Breaking Organic Semiconductors in Polymer Solar Cells: A Theory Insight of the Asymmetric Advantage.

Asymmetric molecule strategy is considered an effective method to achieve high power conversion efficiency (PCE) of polymer solar cells (PSCs). In this paper, nine oligomers are designed by combining three new electron-deficient units (unitA)—n1, n2, and n3—and three electron-donating units (unitD)—D, E, and F—with their π-conjugation area extended. The relationships between symmetric/asymmetric molecule structure and the performance of the oligomers are investigated using the density functional theory (DFT) and time-dependent density functional theory (TD–DFT) calculations. The results indicate that asymmetry molecule PEn2 has the minimum dihedral angle in the angle between two planes of unitD and unitA among all the molecules, which exhibited the advantages of asymmetric structures in molecular stacking. The relationship of the values of ionization potentials (IP) and electron affinities (EA) along with the unitD/unitA π-extend are revealed. The calculated reorganization energy results also demonstrate that the asymmetric molecules PDn2 and PEn2 could better charge the extraction of the PSCs than other molecules for their lower reorganization energy of 0.180 eV and 0.181 eV, respectively.

BAl4Mg−/0/+: Global Minima with a Planar Tetracoordinate or Hypercoordinate Boron Atom

We have explored the chemical space of BAl4Mg−/0/+ for the first time and theoretically characterized several isomers with interesting bonding patterns. We have used chemical intuition and a cluster building method based on the tabu-search algorithm implemented in the Python program for aggregation and reaction (PyAR) to obtain the maximum number of possible stationary points. The global minimum geometries for the anion (1a) and cation (1c) contain a planar tetracoordinate boron (ptB) atom, whereas the global minimum geometry for the neutral (1n) exhibits a planar pentacoordinate boron (ppB) atom. The low-lying isomers of the anion (2a) and cation (3c) also contain a ppB atom. The low-lying isomer of the neutral (2n) exhibits a ptB atom. Ab initio molecular dynamics simulations carried out at 298 K for 2000 fs suggest that all isomers are kinetically stable, except the cation 3c. Simulations carried out at low temperatures (100 and 200 K) for 2000 fs predict that even 3c is kinetically stable, which contains a ppB atom. Various bonding analyses (NBO, AdNDP, AIM, etc.) are carried out for these six different geometries of BAl4Mg−/0/+ to understand the bonding patterns. Based on these results, we conclude that ptB/ppB scenarios are prevalent in these systems. Compared to the carbon counter-part, CAl4Mg−, here the anion (BAl4Mg−) obeys the 18 valence electron rule, as B has one electron fewer than C. However, the neutral and cation species break the rule with 17 and 16 valence electrons, respectively. The electron affinity (EA) of BAl4Mg is slightly higher (2.15 eV) than the electron affinity of CAl4Mg (2.05 eV). Based on the EA value, it is believed that these molecules can be identified in the gas phase. All the ptB/ppB isomers exhibit π/σ double aromaticity. Energy decomposition analysis predicts that the interaction between BAl4−/0/+ and Mg is ionic in all these six systems.

Metal-Metal Bonding in Actinide Dimers: U2 and U2.

Understanding direct metal-metal bonding between actinide atoms has been an elusive goal in chemistry for years. We report for the first time the anion photoelectron spectrum of U2-. The threshold of the lowest electron binding energy (EBE) spectral band occurs at 1.0 eV, which corresponds to the electron affinity (EA) of U2, whereas the vertical detachment energy of U2- is found at EBE ∼ 1.2 eV. Electronic structure calculations on U2 and U2- were carried out with state-of-the-art theoretical methods. The computed values of EA(U2) and EA(U) and the difference between the computed dissociation energies of U2 and U2- are found to be internally consistent and consistent with experiment. Analysis of the bonds in U2 and U2- shows that while U2 has a formal quintuple bond, U2- has a quadruple bond, even if the effective bond orders differ only by 0.5 unit instead of one unit. The resulting experimental-computational synergy elucidates the nature of metal-metal bonding in U2 and U2-.