Positive Electron Affinity Research Articles

Field emission mechanism of oxidized highly phosphorus-doped homoepitaxial diamond (111)

Spatially resolved electron field emission experiments on oxidized highly phosphorus-doped homoepitaxial diamond (111) were applied at room temperature. The diamond layer shows hopping conductivity. Field emission properties have three distinct regions. We attribute the variation in emission currents to: (a) Electron emission from conduction-band minimum (Region I), (b) Depletion of conduction-band electrons at the surface (Region II), and (c) emission from the phosphorus level (Region III). From these data, we calculate an effective positive electron affinity for the oxidized surface of 1–1.5eV.

Ab initio study of structures and energies of Al2H4 and Al2H 4 −

The low-lying isomers of Al2H4 and their anions are investigated with the hybrid density functional B3LYP, the coupled-cluster CCSD and CCSD(T) methods, and the electron propagator theory. The positive adiabatic electron affinities 5,798 and 10,112 cm−1 are predicted for the neutral C2v and D2d symmetric isomers, respectively. The D2h symmetric anion is more stable by 852 cm−1 than the C2v symmetric anion. The photodetachment spectra for Al2H4− anions at the C2v and D2h symmetries are simulated on the basis of the Franck–Condon factor calculations, indicating a reasonable way to study the transition state of the intramolecular torsion process

Photochemical Functionalization of Hydrogen-Terminated Diamond Surfaces: A Structural and Mechanistic Study

Hydrogen-terminated diamond surfaces can be covalently modified with molecules bearing a terminal vinyl (C=C) group via a photochemical process using sub-band-gap light at 254 nm. We have investigated the photochemical modification of hydrogen-terminated surfaces of nanocrystalline and single-crystal diamond (111) to help understand the structure of the films and the underlying mechanism of photochemical functionalization. A comparison of the rates of photochemical modification of single-crystal diamond and nanocrystalline diamond films shows no significant difference in reactivity, demonstrating that the modification process is not controlled by grain boundaries or other structures unique to polycrystalline films. We find that both single-crystal and polycrystalline hydrogen-terminated diamond samples exhibit negative electron affinity and are functionalized at comparable rates, while oxidized surfaces with positive electron affinity undergo no detectable reaction. Gas chromatography-mass spectrometry (GC-MS) analysis shows the formation of new chemical products in the liquid phase that are formed only when the alkenes are illuminated in direct contact with H-terminated diamond, while control experiments with other surfaces and in the dark show no reaction. Our results show that the functionalization is a surface-mediated photochemical reaction and suggest that modification is initiated by the photoejection of electrons from the diamond surfaces into the liquid phase.

Hydrogenation and oxygenation of the (100) diamond surface and the consequences for transfer doping

AbstractAb initio density‐functional‐theory calculations have been performed to determine the structural and electronic properties of the (100) diamond surface with various hydrogen and oxygen terminations. Total energies indicate that an –OH‐terminated surface is favoured over an oxygenated surface plus gas‐phase hydrogen. Ionisation potentials and electron affinities (EAs) are reported for the different systems, and the distinction is made between bulk‐ and surface‐related properties. A first‐order correction is used to offer estimated surface EAs. A negative bulk EA is found for surfaces terminated with –H and –OH groups, although many surfaces have deep traps which can act as positive EA in the absence of band bending. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and Energetic Characterization of a DNA Nucleoside Pair and Its Anion: Deoxyriboadenosine (dA) − Deoxyribothymidine (dT)

The geometries of the DNA nucleoside pairs between 2'-deoxyriboadenosine (dA) and 2'-deoxyribothymidine (dT) and its anion (dAdT-) were fully optimized using carefully calibrated density functional methods. The addition of an electron to dAdT results in remarkable changes to the two hydrogen bonding distances, the H...O distance decreasing by 0.303 angstroms and the N...H distance increasing by 0.229 angstroms. The electron affinity of the dAdT pair was studied to reveal the correct trends of adiabatic electron affinity (EA(ad)) under the influence of the additional components to the individual bases. The consequence of negative charge in terms of structural variations, energetic changes, and charge distribution were explored. The EA(ad) of dAdT is predicted to be positive (0.60 eV), and it exhibits a substantial increase compared with those of the corresponding bases A and T and the nucleic acid base pair AT. The effects of pairing and the addition of the sugar moiety on the EA(ad) are well described as the summation of the individual influences. The influence of the pairing on the EA is comparable to that of the addition of 2-deoxyribose. The excess charge is mainly located on the thyminyl moiety in the anionic dAdT pair. The positive vertical electron affinity (VEA = 0.20 eV) for dAdT suggests that it is able to form a stable anion through electron attachment. A large vertical detachment energy (VDE = 1.14 eV) has been determined for the anionic dAdT nucleoside pair. Therefore, one may expect that the stable anionic dAdT nucleoside pair should be able to undergo the subsequent glycosidic bond cleavage process.

Electron affinity and field emission characteristics of boron carbon nitride film

Boron nitride (BN) and boron carbon nitride (BCN) films with various carbon composition ratios are synthesized by plasma-assisted chemical vapor deposition (PACVD). The electron affinity is estimated from the difference between the threshold energy of the photoelectron yield and the bandgap energy. The negative electron affinity is observed for the BN film. The electron affinity varies from the negative value of the hexagonal boron nitride (h-BN) to 1.2 eV of the BCN film with increasing carbon (C) composition ratio of 29%. The positive electron affinity is estimated to be as low as 0.7 eV for the BCN film with C composition ratio of 18%. Field emission characteristics of BN and BCN nanofilms are investigated. No degradation of the field emission characteristics occurs for the BCN nanofilms with C composition ratio lower than 18%. On the other hand, the turn-on electric field increases rapidly for the BCN nanofilms with C composition ratio higher than 20%. An increase of turn-on electric field is possibly due to an enhancement of the surface potential barrier height.

Gas-Phase Ion/Molecule Reactions in C5F8

Gas-phase ion-molecule reactions in octafluorocyclopentene (C5F8) were studied with a pulsed electron beam mass spectrometer. When a few Torr of major gas, CH4, Ar, or N2, containing approximately 10 mTorr C5F8 was ionized by 2 keV electrons, C5F8+, C5F7+, C4F6+, C4F5+, and C3F3+ were formed as major fragment ions. The interaction between those ions and C5F8 is found to be a weak electrostatic interaction. The cation...C5F8 bonding energies are around 10 kcal/mol, which were reproduced well by (U)B3LYP/6-311+G(d) calculations. The proton affinity of C5F8 (=148.6 kcal/mol by B3LYP/6-311+G(d)) was found to be smaller than that of C2H4 (=162.8 kcal/mol). In the negative mode of operation, the intense signal of C5F8- was observed during the electron pulse. This indicates that C5F8 has a positive electron affinity (1.27 eV by (U)B3LYP/6-311+G(d)). The C5F8- ion was quickly converted to a complex C10F16-. This complex did not react further with C5F8 down to 170 K. The theoretical calculation revealed that a C5F7-F-...C5F8 interaction mode in (C5F8)2- was converted to a C5F7*...C5F9- one via fluoride-ion transfer. The F- ion was found to form a strong covalent bond with C5F8, but the interaction in F-(C5F8)- - -C5F8 is a weak electrostatic interaction due to the charge dispersal in F-(C5F8). The halide ions except F- interact with C5F8 only weakly. Thermochemical stabilities for the cluster ions I-(CH3I)n (n = 1, 2) were also determined.

Origin of the magic numbers of water clusters with an excess electron

Electron-bound water clusters [e(-)(H(2)O)(n)] show very strong peaks in mass spectra for n=2, 6, 7, and (11), which are called magic numbers. The origin of the magic numbers has been an enigma for the last two decades. Although the magic numbers have often been conjectured to arise from the intrinsic properties of electron-bound water clusters, we attributed them not to their intrinsic properties but to the particularly weak stability of the corresponding neutral water clusters (H(2)O)(n=2,6,7, and (11)). As the cluster size increases; this nonsmooth characteristic feature in stability of neutral water clusters is contrasted to the smooth increase in stability of e(-)-water clusters. As the magic number clusters have significant positive adiabatic electron affinities, their abundant distributions in atmosphere could play a significant role in atmospheric thermodynamics.

The negative ion states of c-C 4F 8

Multiple negative ion states account for three different positive electron affinities, E a of c-C 4F 8 reported in the literature. From published data and a kinetic model the following are obtained: anion neutral partition function ratios; Log [ Q an ( 0 ) ] = − 3.2 ( 3 ) , Log [ Q an ( 1 ) ] = 0.0 ( 3 ) and Log [ Q an ( 2 ) ] = 0.0 ( 3 ) ; electron affinities E a(0)=1.05(8), E a(1)=0.66(3) and E a(2)=0.50(5) eV; activation energies for electron attachment E 1(0)=0.25(2), E 1(1)=0.02(2) and E 1(2)=0.00 eV. The electron affinities correspond to the lowest unoccupied molecular orbitals A 2u=−0.75 eV and two degenerate E u −0.42 eV. Semi-empirical multiconfiguration configuration interaction calculations using the CURES-EC procedure support the E a. Bond dissociation energies, frequencies and internuclear distances are calculated for the neutral and the anions. Herschbach ionic Morse person empirical curves are calculated with these values and electron impact data.

Effect of charge on the stability of single-walled carbon nanotubes

By using density-functional-theory based DMol3 code, the structure optimizations are performed on a short charged single-walled carbon nanotube. Results show that the total energy of the nanotube exhibits a parabolic variation with respect to the amount of extra charge, and one negatively charged nanotube has the lowest total energy; thus the carbon nanotube has a positive electron affinity. When the charge is small, the variation of the atomic structure of the nanotube is also small, and neglecting the atomic structure variation leads to the qualitatively correct properties of the total energy and the energy of the highest occupied molecular orbital. When the extra charge is large, the end structure of the nanotube will be first affected and form into a trumpet shape. With the increasing of the extra charge, the nanotube end gradually becomes unstable, and this may lead to the ultimate destruction of the nanotube.

Theoretical study on isomeric stabilities of C2H2Si and its ionization potentials and electron affinities.

The geometric structures and isomeric stabilities of various stationary points in C(2)H(2)Si neutral and its cation and anion are investigated at the coupled-cluster singles, doubles (triples) [CCSD(T)] level of theory. For the geometrical survey, the basis sets used are of the Dunning's correlation consistent basis sets of triple-zeta quality (cc-pVTZ) for the neutral and cation. For the anions, the cc-pVTZ basis sets with diffuse functions (aug-cc-pVTZ) are used. The final energies are calculated by the use of the CCSD(T) level of theory with the aug-cc-pVTZ basis set at their optimized geometries. To lower lying neutrals and cations, the Dunning's correlation consistent basis sets of quadruple-zeta quality (cc-pVQZ) are also applied. Both the global minima of the C(2)H(2)Si neutral and cation, N-1 (C(2v):(1)A(1)) and C-1 (C(2v):(2)B(2)), respectively, are silacyclopropenylidene conformers, having a CCSi ring with a C[Double Bond]C double bond. No competitive stable isomers exist in the present C(2)H(2)Si neutral. In the cation, however, the second lowest lying isomer C-2 lies 10.8 kJ/mol above the most stable C-1. The vertical and adiabatic ionization potentials from the lowest lying neutral N-1 are 9.83 and 8.97 eV, respectively, at the CCSD(T)/cc-pVQZ level of theory. The electron addition to the N-1 does not result in the anion with positive (real) electron affinities. On the other hand, the electron addition to the N-2 isomer produces the global minimum anion A-1 (C(2v):(2)B(1)) with the positive electron affinities of 1.13 eV. The second lowest lying anion isomer A-2 with silylenylacetylene conformer, produced from an electron addition to the N-3 neutral, very well competes with the A-1 after the zero-point vibrational energy corrections. The energy difference between the two lowest lying isomers of the neutral and its anion, N-1 and A-1, is only 0.39 eV.

Annealing of CVD un‐doped diamond: a photoemission investigation

AbstractThe temperature evolution of the surface region of an undoped chemical vapour deposited (CVD) diamond film is characterized via UV and x‐ray induced photoemission. Methods are proposed to diagnose and to overcome possible electrostatic charging problems connected with the poor conductivity of the film. The position of the relevant energy levels is measured and the transition from a negative to a positive electron affinity surface is observed. For the bare diamond surface, an electron affinity χ = 0.4 eV is derived while the film conductivity is shown to decrease on going from the surface towards the bulk. Copyright © 2004 John Wiley & Sons, Ltd.

An ab initio MO study on structures and energetics of CH 2Si 2, CH 2Si 2+, and CH 2Si 2−

The geometric structures and isomeric stabilities of various stationary points in CH 2Si 2 neutral, cation and anion are investigated at the coupled-cluster singles, doubles (triples) (CCSD(T)) level of theory. For the geometrical survey, the basis sets used are of the cc-pVTZ for the neutral and cation. The final energies are calculated by the use of the CCSD(T) level of theory with the aug-cc-pVTZ basis set at their optimized geometries. To the competitive two-anion isomers, the aug-cc-pVTZ basis sets are applied. The global minimum ( N-1) of the CH 2Si 2 neutral has a quite different framework from those of the C 3H 2 (cyclopropenylidene) and Si 3H 2 (trisilacyclopropenylidene) neutrals. No competitive low-lying isomers are found in the CH 2Si 2 neutral. The attractive conformer ( C-1) is predicted for the most stable cation, where its framework is quite different from that of the neutral N-1. Both H atoms are connected to the same C atom, but each C–H bond length is different from each other. Two competitive anion isomers with positive (real) electron affinities are predicted. The framework of the most stable anion A-1 is quite similar to that of the cation C-1, whereas both H atoms are equally connected to the same C atom. The framework of the anion isomer A-2 is the same as that in the neutral N-1. The vertical and adiabatic ionization potentials from the most stable neutral N-1 are 9.02 and 8.71 eV, respectively. The adiabatic electron affinity of the lowest lying isomer N-1 is only 0.43 eV and the vertical electron detachment energy form the global minimum anion ( A-1) is 2.02 eV. The multi-centered Si–H–Si bonds are found in the neutral, cation, and anion.

Positive electron affinity of fullerenes: Its effect and origin.

The universal variation pattern of the total energy of various fullerenes including single-walled carbon nanotubes with respect to their extra charge is revealed by the density-functional-theory calculations. The parabolic energy-charge curve with its lowest energy value corresponding to a negatively charged fullerene indicates that these carbon materials have positive electron affinity and are not in the most stable state. The positive electron affinity seems to originate from the pi-electrons and is found to be related to the aggregation property of fullerenes.

1,3,2‐Dioxaborines as potential components in advanced materials—a theoretical study on electron affinity

Abstract1,3,2‐Dioxaborines with fluorine and other substituents at boron were calculated by first‐principles methods based on density functional theory (DFT: B3‐LYP) or many‐body perturbation theory at the second order [MBPT(2): MP2]. 1,3,2‐Dioxaborines are derivatives of 1,3‐dicarbonyl compounds also known as 1,3‐diketoborates. According to quantum chemical calculations, 2,2‐difluoro‐1,3,2‐dioxaborines are of puckered structure with a low barrier to inversion. The calculated charge distribution does not reflect well the traditional formula description. The boron atom carries a positive rather than a negative charge. Some compounds have a pronounced zwitterionic character accompanied by relatively high dipole moments. The calculated positive electron affinities (EAs) classify 1,3,2‐dioxaborines as organic electron‐acceptor compounds. Depending on the type of substitution, the EAs vary between about 0.5 and 3.5 eV (DFT calculations). The EAs of various substituted 1,3,2‐dioxaborines are of the same order of magnitude as those of quinones and close to electron affinities of strong organic acceptor compounds such as tetracyanoethylene (TCNE). Numerical predictions are verified by comparison with EAs of a series of well‐known medium‐sized organic compounds studied experimentally and theoretically at the same level of theory. In good agreement with results reported for other series of compounds, the average absolute error between theoretical and experimental EAs was 0.19 eV. Because of the ability of 3,5‐diaryl‐2,2‐difluoro‐1,3,2‐dioxaboranes to fluoresce efficiently and to accept electrons easily, these compounds are potential candidates for future applications. Copyright © 2004 John Wiley & Sons, Ltd.

Specific features of the resonant electron attachment to the chlorodibenzo-p-dioxin molecules

The processes of resonant dissociative electron attachment to the molecules of dibenzo-p-dioxin and its chlorinated derivatives containing one to four chlorine atoms (totally eight compounds) were investigated. It was established that 2,3,7-trichlorodibenzo-p-dioxin; 1,2,3,4-tetrachlorodibenzo-p-dioxin; 1,3,7,8-tetrachlorodibenzo-p-dioxin, and 2,3,7,8-tetrachlorodibenzo-p-dioxin molecules are chatacterized by positive electron affinities. At electron energies below 2 eV, the electron attachment is caused by the shape resonances. Based on the energy correlation between the negative ion resonance peaks at 3—4 eV and the UV band maxima, it was suggested that electron attachment in this energy region occurs by the mechanism of inter-shell resonance with the molecular singlet-excited states as parents. The possibility for the rearrangement processes resulting in oxy-anionic structures to occur is substantiated.

The electronic quenching rates of NO(A, v′=0–2)

The electronic quenching rates of NO(A 2 Σ + , v ′ =0–2) are measured for the gases He, Ar, Xe, N 2 , O 2 , CO 2 , N 2 O, and SF 6 . The variations of the fluorescence intensity were measured for the (0,0), (1,0), and (2,0) bands of the γ band system when the quencher gases were added. The quenching rates were determined by using the Stern–Volmer plots with the known radiative lifetimes of the excited states. The electronic quenching rate constants are fast for the group of gases of O 2 , CO 2 , N 2 O, and SF 6 , whose quenching rate constants are in the order of 10 −10 cm 3 /s. The quenching rate constants are slow for the group of gases including He, Ar, Xe, and N 2 whose rate constants are in the order of 10 −14 cm 3 /s. For the slow group, the quenching rate constants increase rapidly for v ′ =2 compared with those of v ′ =0 and 1. The charge transfer model and collision complex model are used to understand the quenching mechanism. For the fast group which mainly consists of gases with positive electron affinities, the charge transfer model adequately describes the mechanism. For the slow quenching group, a theoretical background is provided by consider the coupling of initial and final states in the complex potential surfaces.

Effect of charge on the stability of single-walled carbon nanotubes

By using density-functional-theory based DMol3 code, the structure optimizations are performed on a short charged single-walled carbon nanotube. Results show that the total energy of the nanotube exhibits a parabolic variation with respect to the amount of extra charge, and one negatively charged nanotube has the lowest total energy; thus the carbon nanotube has a positive electron affinity. When the charge is small, the variation of the atomic structure of the nanotube is also small, and neglecting the atomic structure variation leads to the qualitatively correct properties of the total energy and the energy of the highest occupied molecular orbital. When the extra charge is large, the end structure of the nanotube will be first affected and form into a trumpet shape. With the increasing of the extra charge, the nanotube end gradually becomes unstable, and this may lead to the ultimate destruction of the nanotube.

Negative ion-atmospheric pressure photoionization-mass spectrometry

The ionization mechanism in the novel atmospheric pressure photoionization mass spectrometry (APPI-MS) in negative ion mode was studied thoroughly by the analysis of seven compounds in 17 solvent systems. The compounds possessed either gas-phase acidity or positive electron affinity, whereas the solvent systems had different polarities and gas-phase acidities and some of them positive electron affinities. The analytes that possessed gas-phase acidity formed deprotonated ions in proton transfer; in addition, fragments and solvent adducts were observed. The compounds of positive electron affinity formed negative molecular ions by electron capture or charge exchange and substitution products of form [M − X + O] − by substitution reactions. The efficiency of deprotonation was decreased if the solvent used possessed higher gas-phase acidity than the analyte. Solvents of positive electron affinity captured thermal electrons and deteriorated the ionization of all the analytes. Also, the proportion of substitution products was affected by the solvent. Finally, the performances of negative ion APPI and negative ion APCI were compared. The sensitivity for the studied compounds was better in APPI, but the formation of substitution products was lower in APCI.

Low energy electron interaction with free and bound SF5CF3: Negative ion formation from single molecules, clusters and nanofilms

The interaction of free electrons with the potent greenhouse molecule SF5CF3 is studied under different degrees of aggregation: single molecules at collision free conditions, clusters within a supersonic molecular beam and condensed molecules. Electron collisions with single molecules are dominated by SF5− formation produced via dissociative electron attachment (DEA) within a resonance located below 2 eV. In clusters, undissociated parent anions SF5CF3− (and larger complexes containing undissociated anions) are observed in addition to the fragment ions. This indicates that (i) SF5CF3 possesses a positive adiabatic electron affinity and (b) low energy attachment is partly channeled into nondissociative processes when the molecule is coupled to an environment. Electron impact to condensed phase SF5CF3 exhibits a remarkably strong F− desorption signal appearing from a pronounced resonance located at 11 eV while in the gas phase at 11 eV only a weak DEA signal is observed. Electron induced desorption from sub-monolayers of SF5CF3 on an amorphous H2O ice surface is found to be more efficient compared to desorption of SF5CF3 from a Xe surface. The implications of these results for the heterogeneous photochemistry of SF5CF3 adsorbed on ice or dust particles in the Earth’s atmosphere are discussed.