Large Electron Affinity Research Articles

Theoretical studies of structural, electronic, and magnetic properties for small V2Fn0,− (2 ≤ n ≤ 7) clusters using first-principles calculations

The structural, electronic, and magnetic properties of V2Fn0,− (2 ≤ n ≤ 7) clusters are investigated by using density functional theory (DFT) calculations. The spin multiplicities of the lowest isomers of the optimized V2Fn0,− (2 ≤ n ≤ 7) clusters are generally high, and V2F6 exhibits the highest stability. The largest electron affinity (≈ 4.976 eV) of V2F7 among the systems may reflect superhalogen nature of this cluster. The polarizability analysis reveals that V2F2− or V2F5− cluster has the largest or smallest polarizability anisotropy invariant, corresponding to the strongest or weakest anisotropic response to the external field, respectively. The total magnetic moments of mixed V2Fn0,− clusters are in the range of 3–7 μB, mainly arising from the local magnetic moments of V atoms with unfilled 3d shell. The atoms at the symmetrical positions of the clusters have the same magnetic moments. As for the calculated IR and Raman spectra, the larger number n of F atoms of the clusters is associated with the more vibration peaks, with the highest peaks largely attributable to the stretching vibrations of F atoms.

Soft X-ray absorption near-edge structures of B/C and B/C/N materials and the analysis of their electronic state using the first-principle calculations

Graphite-like layer materials composed of boron/carbon (B/C) and boron/carbon/nitrogen (B/C/N) have been prepared by chemical vapor deposition (CVD). In order to evaluate the electronic states of these materials, the X-ray absorption near-edge structures (XANES) of the materials have been measured and analyzed by first-principle calculation using the “discrete variational (DV)-Xα molecular orbital method”. Absorption intensities of the CK-edge XANES spectra for B/C and B/C/N materials started to increase at energies 1.1 eV lower than that of graphite, which suggests that the energy levels of the conduction bands of B/C and B/C/N materials were lower than that of graphite by 1.1 eV. DV-Xα calculation suggests that covalent bonds between boron and carbon atoms lower the bottoms of the conduction bands of these materials, compared with that of graphite. Since electron affinity is defined to be the difference between the vacuum level and the bottom of the conduction band, these results indicate that B/C and B/C/N materials have larger electron affinities than that of graphite. Consequently, the present study supports the experimental results which have been reported for the donor-type intercalation of Na, Mg and Ca into B/C and B/C/N materials.

The stability and electronic properties of Si-doped ZnO nanosheet: a DFT study

This work deals with stability, structural and electronic properties of perfect ZnO nanosheet and substiutionally doped ZnO nanosheet with Si are simulated and optimized successfully using density functional theory (DFT) with the help of SIESTA program in the generalized gradient approximation (GGA). The substitution atoms have been replaced on the oxygen site in line and zigzag doping. The stability of perfect ZnO nanosheet and ground state structures of Sin-ZnO (n = 1–6) are studied in terms of binding energy, show that a maximum stabilized of one Si in line doping and two Si in zigzag doping due to the dopant located in the center of nanosheet is a more stable. The electronic properties of ZnO nanosheet and Si-doped are discussed using ionization potential, electron affinity, HOMO–LUMO gap, electronegativity, and hardness. The results showed the presence of silicon atoms substitution expands the bond length with respect to perfect ZnO nanosheets. The obtained values of HOMO and LUMO are slightly different and this suggests that different of position dopant play significant roles on electronic properties and large electron affinity at four silicon atoms doped ZnO nanosheet in two cases that it improved the electron more accepting ability. The study of HOMO-LUMO gap reveals that the gap decreases with the increase in number of Si dopant atoms in ZnO nanosheet. These results global gave molecular electronics important electronic applications and help us to replace some oxygen atoms instead of silicon atoms in ZnO nanosheet.

Germanium Fluoride Nanocages as Optically Transparent n-Type Materials and Their Endohedral Metallofullerene Derivatives.

Carbon- and silicon-based n-type materials tend to suffer from instability of the corresponding radical anions. With DFT calculations, we explore a promising route to overcome such challenges with molecular nanocages which utilize the heavier element Ge. The addition of fluorine substituents creates large electron affinities in the range 2.5-5.5 eV and HOMO-LUMO gaps between 1.6 and 3.2 eV. The LUMOs envelop the surfaces of these structures, suggesting extensive delocalization of injected electrons, analogous to fullerene acceptors. Moreover, these Ge nF n inorganic cages are found to be transparent in the UV-visible region as probed with their excited states. Their capacitance, linear polarizabilities, and dielectric constants are computed and found to be on the same order of magnitude as saturated oligomers and some extended π-organics (azobenzenes). Furthermore, we explore fullerene-type endohedral isomers, i.e., cages with internal substituents or guest atoms, and find them to be more stable than the parent exohedral isomers by up to -206.45 kcal mol-1. We also consider the addition of Li, He, Cs, and Bi, to probe the utility of the exo/ endo cages as host-guest systems. The endohedral He/Li@F8@Ge60F52 cages are significantly more stable than their parent exohedral isomers He/Li@Ge60F52 by -182.46 and -49.22 kcal mol-1, respectively. The energy of formation of endohedral He@F8@Ge60F52 is exothermic by -10.4 kcal mol-1, while Cs and Bi guests are too large to be accommodated but are stable in the exohedral parent cages. Conceivable applications of these materials include n-type semiconductors and transparent electrodes, with potential for novel energy storage modalities.

Tetracyanocorannulene – an Easily Accessible and Strongly Electron‐Deficient Compound

Herein we report the synthesis of tetracyanocorannulene by a simple and clean reaction that in principle can be performed on a multigram scale. The tetranitrile can be transformed into corannulene‐fused imides with fluorinated substituents. The cyano compound as well as the imides were investigated structurally by X‐ray diffraction and electrochemically by cyclic voltammetry. Tetracyanocorannulene as well as the derived bis[(pentafluorophenyl)imide] proved to be corannulene compounds with a large electron affinity.

Electron Affinity Control of Amorphous Oxide Semiconductors and Its Applicability to Organic Electronics

AbstractEnergy level matching is a key factor for realizing efficient optoelectronics, such as organic light‐emitting diodes (OLEDs). Transition metal oxides (TMOs) are widely used as hole‐injection layers (HILs) because of their large electron affinities, corresponding to conduction band minimum levels (ECBM). However, the poor electrical properties and chemical instabilities of TMOs restrict device performance and fabrication processes. Conversely, amorphous oxide semiconductors (AOSs) exhibit superior electrical properties and chemical stability but cannot be applied to HILs because of their small electron affinities. In this study, a novel method is proposed to tune the electron affinity of conventional AOSs and an example material design of AOSs for HILs. The electron affinity deepening phenomenon that is attributable to the higher oxidation states of transition metal cations achieves a very deep ECBM of 5.7 eV for the proposed amorphous In–Mo–O (a‐IMO). Furthermore, an a‐IMO exhibits a high mobility of ≈1 cm2 V−1 s−1 and high chemical stability against various solvents. As a result, it is demonstrated that a very thick a‐IMO layer is applicable to OLEDs as an HIL with no operating‐voltage increase. This study provides a new approach for efficient organic optoelectronics by tuning the electron affinity of AOSs.

Chemical Bonding of Crystalline LnB6 (Ln = La-Lu) and Its Relationship with Ln2B8 Gas-Phase Complexes.

Solid-state lanthanide (Ln) borides of the simple LnB6 composition not only exhibit exciting physical behavior, in particular magnetic properties, but their electronic structure and chemical bonding are particularly intriguing as well. To shed more light on the latter, we have performed quantum-chemical (DFT+ U) electronic-structure calculations and bonding analyses of the entire LnB6 series with Ln from La to Lu. Trivially, the boron framework is held together by the B 2sp orbitals, and this framework bonds to the Ln atoms via covalent-ionic interactions. The Ln 4f electrons, however, are decisive for the magnetic properties. In more detail, the effective charges of the Ln atoms as calculated by (Mulliken or Löwdin) occupation numbers of the 6s/5d/4f orbitals are compatible with experimentally assigned oxidation numbers. The shorter inter-octahedral B-B bonds, dominated by 2s-2p interactions, turn out to be stronger than the intra-octahedral B-B bonding with a more 2p-2p-like character. Interestingly, there are strong structural similarities between the LnB6 motif studied here and gas-phase Ln2B8 species showing inverse sandwich structures, and these similarities are also reflected in the electronic structure. In particular, Ln2B8 is predicted to have a large electron affinity. Hence, this work aims at providing an intrinsic link between gas-phase complexes and solid-state crystal structures in order to better understand the former species.

Strategic fluorination of polymers and fullerenes improves photostability of organic photovoltaic blends

The photobleaching dynamics of a series of three organic photovoltaic (OPV) donor polymer blends with five different fullerenes are presented. The fullerenes studied include PC60BM and four perfluoroalkylfullerenes with relatively large electron affinities, namely C-60(CF3)(2), C-60(i-C3F7)(2), C-60(CF3)4(,) and C-60(CF3)(8). The donor polymers were all based on cyclopentadithiophene (CPDT) and thienopyrrolodione (TPD), but the TPD side chains were designed to include alkyl, partially fluorinated alkyl, and fluorinated phenyl groups to improve miscibility of the active layer components. Exciton harvesting was probed with photoluminescence quenching measurements. Accelerated photodegradation studies of polymer:fullerene blends were then carried out under white light illumination at similar to 1.2 suns in air. A strong correlation was observed between the polymer donor photobleaching rate and the electron affinity of the fullerene. The most dramatic effect was observed for a blend of C-60(CF3)(8) with the donor containing fluorinated phenyl groups: the blend required 150 times the dosing of photons to bleach to 80% of its initial optical density than an analagous blend of PC60BM and non-fluorinated donor polymer. These results ultimately suggest that appropriate fluorination strategies applied to both the donor and acceptor can be a viable route toward a new paradigm of intrinsically photo- and phase-stable OPV active layers.

Design of a Novel Series of Donor-Acceptor Frameworks via Superalkali-Superhalogen Assemblage to Improve the Nonlinear Optical Responses.

Presently, many researches are directed toward the design of novel superatoms with high nonlinear optical responses. Inspired by a fascinating finding of superatoms which were designed by bonding superhalogen (Al13 nanocluster) with superalkalis (M3O, M = Na and K), we suggest an effective strategy to form a series of typical donor-acceptor frameworks with high nonlinear optical responses via bonding the superalkalis M3O (Li3O, Na3O, K3O, Li2NaO, Li2KO, Na2LiO, Na2KO, K2LiO, K2NaO, and LiNaKO) with low ionization potential to the superhalogen Al13 with large electron affinity. The ionization potential, electronic spatial extent, electric field gradient tensors of 17O nuclei, and natural bond orbital charge values of the superalkalis M3O were also calculated. We found that the M ligands have the remarkable effect on the ionization potential as well as 17O nuclear quadrupole resonance parameters of the superalkalis M3O. Our results also represented that the bonding superalkalis can efficiently narrow wide HOMO-LUMO gap and considerably enhance first hyperpolarizability of the pristine Al13, due to electron transfer in this type of superatom. Also, the effect of oriented external electric fields on the nonlinear optical responses of the superatoms M3O-Al13 has been systematically explored. We found that the first hyperpolarizability of the superatom compounds can be gradually increased by increasing the imposed oriented external electric field from zero to the critical external electric field along the charge transfer direction (M3O → Al13). In this respect, this work reveals an effective approach to gradually enhance the nonlinear optical responses of the superatoms through applying oriented external electric fields.

Exploring Low Internal Reorganization Energies for Silicene Nanoclusters

High-performance materials rely on small reorganization energies to facilitate both charge separation and charge transport. Here, we performed DFT calculations to predict small reorganization energies of rectangular silicene nanoclusters with hydrogen-passivated edges denoted by H-SiNC. We observe that across all geometries, H-SiNCs feature large electron affinities and highly stabilized anionic states, indicating their potential as n-type materials. Our findings suggest that fine-tuning the size of H-SiNCs along the zigzag and armchair directions may permit the design of novel n-type electronic materials and spinctronics devices that incorporate both high electron affinities and very low internal reorganization energies.

Be- and Mg-Based Electron and Anion Sponges.

By using G4(MP2) high-level ab initio methods, we show that Be and Mg derivatives of cyclopropane exhibit very large electron and anion affinities, reflecting the electron-deficient nature of the -BeX and -MgX (X=CH3 , F, Cl, CN) substituents. In particular, these compounds present electron affinities among the largest reported for neutral closed-shell systems. Their anion affinities are also among the largest reported for single neutral molecules, indeed higher than the 1,8-diBeX-naphthalene (X=F, Cl, CN) derivatives, recently shown to behave as anion sponges. Quite unexpectedly, our results indicate that the intrinsic anion affinity of the Mg-containing compounds is higher than that of the Be-containing analogs.

New insights in low-energy electron-fullerene interactions

The robust Regge-pole methodology has been used to probe for long-lived metastable anionic formation in Cn (n = 20, 24, 26, 28, 44, 70, 92 and 112) through the calculated electron elastic scattering total cross sections (TCSs). All the TCSs are found to be characterized by Ramsauer-Townsend minima, shape resonances and dramatically sharp resonances manifesting metastable anionic formation during the collisions. The energy positions of the anionic ground states resonances are found to match the measured electron affinities (EAs). We also investigated the size-effect through the correlation and polarization induced metastable resonances as the fullerene size varied from C20 through C112. The C20 TCSs exhibit atomic behavior while the C112 TCSs demonstrate strong departure from atomic behavior attributed to the size effect. Surprisingly C24 is found to have the largest EA among the investigated fullerenes making it suitable for use in organic solar cells and nanocatalysis.

Band offset and electron affinity of MBE-grown SnSe2

SnSe2 is currently considered a potential two-dimensional material that can form a near-broken gap heterojunction in a tunnel field-effect transistor due to its large electron affinity which is experimentally confirmed in this letter. With the results from internal photoemission and angle-resolved photoemission spectroscopy performed on Al/Al2O3/SnSe2/GaAs and SnSe2/GaAs test structures where SnSe2 is grown on GaAs by molecular beam epitaxy, we ascertain a (5.2 ± 0.1) eV electron affinity of SnSe2. The band offset from the SnSe2 Fermi level to the Al2O3 conduction band minimum is found to be (3.3 ± 0.05) eV and SnSe2 is seen to have a high level of intrinsic electron (n-type) doping with the Fermi level positioned at about 0.2 eV above its conduction band minimum. It is concluded that the electron affinity of SnSe2 is larger than that of most semiconductors and can be combined with other appropriate semiconductors to form near broken-gap heterojunctions for the tunnel field-effect transistor that can potentially achieve high on-currents.

Spin Control Induced by Molecular Charging in a Transport Junction.

The ability of molecules to maintain magnetic multistability in nanoscale-junctions will determine their role in downsizing spintronic devices. While spin-injection from ferromagnetic leads gives rise to magnetoresistance in metallic nanocontacts, nonmagnetic leads probing the magnetic states of the junction itself have been considered as an alternative. Extending this experimental approach to molecular junctions, which are sensitive to chemical parameters, we demonstrate that the electron affinity of a molecule decisively influences its spin transport. We use a scanning tunneling microscope to trap a meso-substituted iron porphyrin, putting the iron center in an environment that provides control of its charge and spin states. A large electron affinity of peripheral ligands is shown to enable switching of the molecular S = 1 ground state found at low electron density to S = 1/2 at high density, while lower affinity keeps the molecule inactive to spin-state transition. These results pave the way for spin control using chemical design and electrical means.

Mechanism Underlying the Nucleobase-Distinguishing Ability of Benzopyridopyrimidine (BPP).

Benzopyridopyrimidine (BPP) is a fluorescent nucleobase analogue capable of forming base pairs with adenine (A) and guanine (G) at different sites. When incorporated into oligodeoxynucleotides, it is capable of differentiating between the two purine nucleobases by virtue of the fact that its fluorescence is largely quenched when it is base-paired to guanine, whereas base-pairing to adenine causes only a slight reduction of the fluorescence quantum yield. In the present article, the photophysics of BPP is investigated through computer simulations. BPP is found to be a good charge acceptor, as demonstrated by its positive and appreciably large electron affinity. The selective quenching process is attributed to charge transfer (CT) from the purine nucleobase, which is predicted to be efficient in the BPP-G base pair, but essentially inoperative in the BPP-A base pair. The CT process owes its high selectivity to a combination of two factors: the ionization potential of guanine is lower than that of adenine, and less obviously, the site occupied by guanine enables a greater stabilization of the CT state through electrostatic interactions than the one occupied by adenine. The case of BPP illustrates that molecular recognition via hydrogen bonding can enhance the selectivity of photoinduced CT processes.

Bonding the superalkali M3O (M = Li and K): An effective strategy to improve the electronic and nonlinear optical properties of the inorganic B40 nanocage

Inspired by the fascinating finding of all-boron fullerene B40 (Nat Chem, 2014, 6, 727), we propose a new and effective strategy to construct a series of typical Donor-Acceptor (D-A) frameworks via linking the superalkali M3O (M = Li and K) unit with the low ionization potential to the B40 nanocage with large electron affinity. By means of the density functional theory computations, we have systematically investigated the structures, electronic properties, the first and second hyperpolarizabilities of these modified B40 nanocage systems. Owing to the formation of a B–O chemical bond, these composite systems (M3O)n-B40 (M = Li and K, n = 1 and 2) can possess the considerably large binding energy ranging from 57.0 to 99.8kcal/mol, indicating their high structure stabilities. Compared with the pristine B40 nanocage, linking the superalkali M3O can effectively narrow the wide energy gap from the original 2.86eV to 0.61–1.11eV, and significantly increase the first and second hyperpolarizabilities to as large as 5.00 × 104–2.46 × 105 au and 1.48 × 107–4.85 × 108 au, respectively, owing to the occurrence of evident electron transfer process in this kind of typical D-A framework. These fascinating findings will be advantageous for promoting the potential applications of the inorganic boron-based nanosystems in the new type of electronic nanodevices and high-performance nonlinear optical materials.

Charge transfer from and to manganese phthalocyanine: bulk materials and interfaces.

Manganese phthalocyanine (MnPc) is a member of the family of transition-metal phthalocyanines, which combines interesting electronic behavior in the fields of organic and molecular electronics with local magnetic moments. MnPc is characterized by hybrid states between the Mn 3d orbitals and the π orbitals of the ligand very close to the Fermi level. This causes particular physical properties, different from those of the other phthalocyanines, such as a rather small ionization potential, a small band gap and a large electron affinity. These can be exploited to prepare particular compounds and interfaces with appropriate partners, which are characterized by a charge transfer from or to MnPc. We summarize recent spectroscopic and theoretical results that have been achieved in this regard.

Nitrogen substitution and vacancy mediated scandium metal adsorption on carbon nanotubes

First-principle calculation reveals that N containing carbon nanotubes (CNTs) can support the functionalization of transition metals such as Sc on the CNT surface. For N-substituted CNTs without a vacancy, the enhanced adsorption results from large electron affinity difference of the N adjacent to C atom. In this case, the N atom activates nearby C atom and enhances its interaction with the Sc metal on the CNT surface. Meanwhile, the formation of a vacancy in CNTs causes local reconstruction of the surface near the vacancy site. Simulation and analysis show that vacancy mediated N substitution is a more favored scheme for Sc functionalization on the surface of CNTs that suppresses the clustering of Sc. The enhanced Sc adsorption in N-doped CNTs with mono- and di-vacancy defects was attributed to strong hybridization between the Scandium d orbital and nitrogen p orbital. The results explain theoretically the mechanism of enhanced functionalization of metals on N doped CNTs and suggests that Sc functionalized nitrogen doped CNTs with vacancies is an excellent candidate for the adsorption of small molecules.

Mechanistic elucidation of the origins of the hydrogen-abstraction reactivity of hydroxyimide organocatalysts and its application in catalyst design

The mechanistic origin of the hydrogen-abstraction reactivity of hydroxyimide organocatalysts in three types has been theoretically explored by intrinsic reaction coordinate and distortion/interaction model. All these H-abstraction processes can be divided into two parts: CH bond approaching to catalyst and subsequent breaking via a cross-point structure. Generally, the H-abstraction reactivity increase when a catalyst has small LUMOcatalyst-HOMOsubstrate gap, CH bond approaching energy and distortion energy required to achieve its transition state (TS) geometry, and large electron affinity. The TS structures for 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and N-methyl benzohydroxamic acid (NMBHA) are inclined to be product-like, but much reactant-like for N-hydroxyphthalimide (NHPI)-type catalysts.

Singlet open-shell diradical nature and redox properties of conjugated carbonyls: a quantum chemical study

In the present work, a set of 90 conjugated carbonyls based on cyclic hydrocarbons with different ring sizes are studied by means of theoretical methods of quantum chemistry in order to analyze their singlet open-shell diradical character and its influence in different molecular properties. In particular, we have investigated the molecular structures, singlet–triplet gaps, ionization energies (IE), electron affinities (EA), first (E 1) and second (E 2) redox potentials and coordination of the reduced species to lithium cation. The diradical character is estimated using the diradical index from the spin-projected unrestricted Hartree–Fock theory (y PUHF). Non-negligible diradical character is accounted for most of the studied molecules, and trends may be devised in relation to their molecular properties. Thus, in those groups where several or all molecules show certain diradical character, this feature accomplishes larger EAs, E 1 and E 2. In this study, 35 carbonyls display E 1 values greater than 3 V, and the largest calculated value is 4.22 V. Regarding the IEs, no clear relationship may be established, although in some cases with large values of y PUHF small IEs are recorded. Finally, the binding energy to Li+ mainly depends on the relative position of the carbonyls, and the lithiation process is favored when the carbonyl moieties are adjacent or close in the molecule. These results highlight the importance of the diradical character on conjugated carbonyls and its effect in molecular properties such as redox potentials, a characteristic that may be exploited both theoretically and experimentally to provide with a deeper insight on the application of these diradicals as organic cathode materials.