Large Electron Affinity Research Articles

Metallo-boranes: a class of unconventional superhalogens defying electron counting rules.

Superhalogens are a class of highly electronegative atomic clusters whose electron affinities exceed those of halogens. Due to their potential for promoting unusual reactions and role as weakly coordinating anions as well as building blocks of bulk materials, there has been considerable interest in their design and synthesis. Conventional superhalogens are composed of a metal atom surrounded by halogen atoms. Their large electron affinities are due to the fact that the added electron is distributed over all the halogen atoms, reducing electron-electron repulsion. Here, using density functional theory with a hybrid exchange-correlation functional, we show that a new class of superhalogens can be developed by doping closo-boranes (e.g., B12H12) with selected metal atoms such as Zn and Al as well as by replacing a B atom with Be or C. Strikingly, these clusters defy electron counting rules. For example, according to the Wade-Mingos rule, Zn(B12H12) and Al(BeB11H12) are closed-shell systems that should be chemically inert and, hence, should have very small electron affinities. Similarly, Zn(B12H11), Al(B12H12), and Zn(CB11H12), with one electron more than needed for electronic shell closure, should behave like superalkalis. Yet, all these clusters are superhalogens. This unexpected behavior originates from an entirely different mechanism where the added electron resides on the doped metal atom that is positively charged due to electron transfer.

A Combined Theoretical and Photoelectron Spectroscopy Study of Al3Hn− (n=1–9) clusters

AbstractCombined photoelectron spectroscopic experiments and computational studies have been performed on Al3Hn− (n=1–9) clusters. Three modes of hydrogen bonding to the Al3 moiety have been observed: terminal, bridging, and capping. Among various hydrides, Al3H5− and Al3H8− clusters have highest HOMO‐LUMO gap and largest electron affinity, respectively. Our studies indicate that as the number of hydrogen atoms increase the presence of AlH2 groups, representing the tetrahedral coordination of the Al atom, which in turn led to the stoichiometric ring structure.

Band‐tailored van der Waals heterostructure for multilevel memory and artificial synapse

AbstractTwo‐dimensional (2D) van der Waals heterostructure (vdWH)‐based floating gate devices show great potential for next‐generation nonvolatile and multilevel data storage memory. However, high program voltage induced substantial energy consumption, which is one of the primary concerns, hinders their applications in low‐energy‐consumption artificial synapses for neuromorphic computing. In this study, we demonstrate a three‐terminal floating gate device based on the vdWH of tin disulfide (SnS2), hexagonal boron nitride (h‐BN), and few‐layer graphene. The large electron affinity of SnS2 facilitates a significant reduction in the program voltage of the device by lowering the hole‐injection barrier across h‐BN. Our floating gate device, as a nonvolatile multilevel electronic memory, exhibits large on/off current ratio (~105), good retention (over 104 s), and robust endurance (over 1000 cycles). Moreover, it can function as an artificial synapse to emulate basic synaptic functions. Further, low energy consumption down to ~7 picojoule (pJ) can be achieved owing to the small program voltage. High linearity (<1) and conductance ratio (~80) in long‐term potentiation and depression (LTP/LTD) further contribute to the high pattern recognition accuracy (~90%) in artificial neural network simulation. The proposed device with attentive band engineering can promote the future development of energy‐efficient memory and neuromorphic devices.image

Electronic structure and reactivity of tirapazamine as a radiosensitizer

Tirapazamine (TP) has been shown to enhance the cytotoxic effects of ionizing radiation in hypoxic cells, thus making it a candidate for a radiosensitizer. This selective behavior is often directly linked to the abundance of O2. In this paper, we study the electronic properties of TP in vacuum, micro-hydrated from one up to three molecules of water and embedded in a continuum of water. We discuss electron affinities, charge distribution, and bond dissociation energies of TP, and find that these properties do not change significantly upon hydration. In agreement with its large electron affinity, and bond breaking triggered by electron attachment requires energies higher than 2.5 eV, ruling out the direct formation of bioactive TP radicals. Our results suggest, therefore, that the selective behavior of TP cannot be explained by a one-electron reduction from a neighboring O2 molecule. Alternatively, we propose that TP’s hypoxic selectivity could be a consequence of O2 scavenging hydrogen radicals.

Indium tin oxid/germanium Schottky photodetectors modulated by ultra-thin dielectric intercalation

Germanium (Ge) photodetectorhas been considered as one of the promising optoelectronic devices for optoelectronic integration. So far, most of reported Ge photodetectors with bulk Ge show high dark currents and low responsivities. In this paper, ultra-thin dielectric interlayer-modulated indium tin oxid (ITO)/Ge Schottky photodetectors with high responsivities and low dark currents are investigated, in which the ultra-thin dielectric interlayers are deposited through atomic layer deposition. The characteristics of ITO/Al<sub>2</sub>O<sub>3</sub> (or MoO<sub>3</sub>)/Ge Schottky photodiodes fabricated on bulk Ge wafers with various doping concentrations and Ge epilayer on silicon substrates are comparatively studied. It is found that the 2-nm-thick Al<sub>2</sub>O<sub>3</sub> intercalation between ITO transparent electrode and Ge can effectively enhance the Schottky barrier heights of the photodetectors and trap holes at interface states, rendering their dark currents low and responsivities high. The effective Schottky barrier heights increase from 0.34 eV (ITO/i-Ge) to 0.55 eV (ITO/Al<sub>2</sub>O<sub>3</sub>/i-Ge), and from 0.24 eV (ITO/n-Ge) to 0.56 eV (ITO/Al<sub>2</sub>O<sub>3</sub>/n-Ge). While MoO<sub>3</sub> intercalation between ITO and Ge has no significant effect on the characteristics of all of the photodetectors due to its large electron affinity. The best performance is realized on the ITO/Al<sub>2</sub>O<sub>3</sub>/i-Ge photodetector with a low dark current of 5.91 mA/cm<sup>–2</sup> at –4 V, sharply dropping by two orders of magnitude, compared with that of the ITO/i-Ge photodetector without the Al<sub>2</sub>O<sub>3</sub> interlayer, and the responsivity is significantly improved to 4.11 A/W at 1310 nm. The ITO/Al<sub>2</sub>O<sub>3</sub>/epi-Ge photodetector fabricated on 500 nm Ge epilayer on a silicon substrate also shows the improved performance with a dark current density of 226.70 mA/cm<sup>2</sup> at –3 V and a responsivity of 0.38 A/W at 1310 nm, compared with ITO/epi-Ge photodetector. Finally, experiment studies of single-point infrared images at 1310 nm and 1550 nm are carried out with the ITO/Al<sub>2</sub>O<sub>3</sub>/i-Ge photodetector on a two-dimensional <i>XY</i> displacement platform, which contains 25 pixels and a total detection size of 1750 μm × 1750 μm. The clear and distinguishable images of the infrared spot position are obtained. Consequently, these results suggest that the dielectric interlayer- modulated Schottky photodetectors are competitive in low power consumption and high responsivity, and have great potential applications in the civil field of short wave infrared imaging.

Th2O-, Th2Au-, and Th2AuO1,2- Anions: Photoelectron Spectroscopic and Computational Characterization of Energetics and Bonding.

The observation and characterization of the anions: Th2O-, Th2Au-, and Th2AuO1,2- is reported. These species were studied through a synergetic combination of anion photoelectron spectroscopy and ab initio correlated molecular orbital theory calculations at the CCSD(T) level with large correlation-consistent basis sets. To better understand the energetics and bonding in these anions and their corresponding neutrals, a range of smaller diatomic to tetratomic species were studied computationally. Correlated molecular orbital theory calculations at the CCSD(T) level showed that in most of these cases, there are close-lying anions and neutral clusters with different geometries and spin states and are consistent with the experimentally observed spectra. Thus, comparison of experimentally determined and computationally predicted vertical detachment energies and electron affinities for different optimized geometries and spin states shows excellent agreement to within 0.1 eV. The structures for both the neutrals and anions have a significant ionic component to the bonding because of the large electron affinity of the Au atom and modest ionization potentials for Th2, Th2O, and Th2O2. The analysis of the bonding for the Th-Th bonds from the molecular orbitals is consistent with this ionic model. The results show that there is a wide variation in the bond distance from 2.7 to 3.5 Å for the Th-Th bonds all of which are less than twice the atomic radius of Th of 3.6 Å. The bond distances encompass bond orders from 4 to 0. There can be different bond orders for the same bond distance depending on the nature of the ionic bonding suggesting that one may not be able to correlate the bond order with the bond distance in these types of clusters. In addition, the presence of an Au atom may provide a unique probe of the bonding in such clusters because of its ability to accept an electron from clusters with modest ionization potentials.

Influence of Fluorination on Energetic Parameters of Silole, Phosphole, Thiophene, Oligomers of Silole and Related Acenes

The effects of fluorination on the energetic and thermochemical properties of a series of five-membered rings such as silole, phosphole and thiophene, and oligomers of silole and acenes were evaluated using density functional theory computations. Computed results indicate that substitution by fluorine atoms induces an increase of ionization energy and electron affinity and a decrease of proton affinity and HOMO-LUMO gap. For five-membered rings, fluorination effects are more significant on phosphole and silole as compared to those on thiophene. Changes in energetic properties of the oligomers of silole and acenes follow non-linear curves with increasing monomeric ring numbers. Effects of fluorination on energetic properties of the five-membered rings and oligomers are decreasing in the sequence of P-H > Si-H > C-H bonds. Replacement of H by F atom affects more strongly at Si-H bond than at C-H bond of siloles. Perfluorination of silole oligomers leads to derivatives with large electron affinity (> 4 eV). The anisotropy of induced current density (ACID) plots point out that fluorine replacement largely affects the electron delocalization at the C-C peripheral bonds in oligomers of silole and acenes. The silole unit emerges as a promising five-membered heterocycle to be used to tune up properties of oligomers.

Unusual and Conventional Dative Bond Formation by s2 Lone Pair Donation from Alkaline Earth Metal Atoms to BH3, AlH3, and GaH3.

Using ab initio electronic structure methods with flexible atomic orbital basis sets, we examined the nature of the bonding arising from donation of an ns2 electron pair on an alkaline earth atom (Mg or Ca) into a vacant n'p orbital on the group 13 atom of BH3, AlH3, or GaH3. We also examined what happens when an excess electron is attached to form corresponding molecular anions. Although the geometries of MgBH3, MgAlH3, MgGaH3, and CaBH3 are found to be much as one would expect for datively bound molecules, CaAlH3 and CaGaH3 were found to have very unusual geometries in that their Al-H or Ga-H bonds are directed toward the Ca atom rather than away, as in the other compounds. Internal electrostatic Coulomb attractions between the partially positively charged Ca center and the partially negatively charged H centers were suggested as a source of these unusual geometries. The other novel finding is that the electron affinities (EAs) of all six M'-MH3 species lie in the 0.7-1.0 eV range, which is suggestive of ionic electronic structures for the neutrals even though the partial charges on the alkaline earth centers are as low as 0.3 atomic units. Partial positive charge on the alkaline earth atoms combined with substantial electron affinities of the BH3, AlH3, and GaH3 groups, but only when distorted from planar geometries, were suggested to be the primary contributors to the large EAs.

Geometric and electronic structures of AlnCum (n = 5–9, m = 1–3) clusters: genetic algorithm combined with ab initio models

The geometries, stabilities and electronic structures of Al n Cu m (n = 5–9, m = 1–3) clusters were explored by using the genetic algorithm combined with ab initio methods. The geometric structures are almost spherical when the valence electrons are around the magic number 20, otherwise the structures are oblate or prolate. The stabilities of the clusters are related to both the Cu/Al ratios and the electronic configurations. The clusters with lower Cu/Al ratios have high stabilities. The molecular orbitals are in accord with the shell structures predicted by the jellium model. The 3d orbitals of the Cu atoms are localised, although their orbital energies are between the 1P and 1D jellium orbitals. The Al6Cu2 with 20 valence electrons forms closed 1S21P61D102S2 shells, and shows large binding energy and removal energy, large ionisation potential and small electron affinity. For the no-magic clusters, the structure deformation leads to crystal-field-like splitting of the degenerate shells and stabilises the clusters.

Are Anions of Cyclobutane Beryllium Derivatives Stabilized through Four-Center One-Electron Bonds?

High-level G4 ab initio calculations allowed us to show that C4H4(BeX)4 (X = H, Cl) derivatives behave as rather efficient electron capturers due to their ability to trap the extra electron through the formation of a four-membered beryllium ring. This finding is in agreement with previous work showing the ability of highly electron-deficient atoms, such as beryllium, to lead to multicenter one-electron bonds. In our particular case, the formation of the four-center bond is characterized, in very good harmony, by different topological methods such as quantum theory of atoms in molecules (QTAIM), the electron localization function (ELF), and the noncovalent interactions (NCI) approach and is accompanied by large electron affinity values, around 300 kJ·mol-1, in the gas phase. Preliminary results may anticipate that the ability of groups of beryllium atoms to trap electrons decays on going to bigger systems.

Structural properties of possible interstellar valence anions of the series HCnN- (n = 3, 5, 7, 9).

In this contribution we investigate the structural properties of stable anions of small carbon clusters, with one nitrogen and one hydrogen atoms attached to the C-cluster, to surmise their possible existence in the Interstellar Medium (ISM). Many possible configurational (geometrical) isomers with positive vertical electron detachment values are presented, and arranged according to their relative energy. Specific attention is paid to the structures of the lowest-energy valence isomers, the chain-structures of HC7N- and HC9N- anions with quasilinear and linear geometry, respectively. They exhibit relatively large permanent dipole moments (2.697 and 5.034 Debye) and adiabatic electron affinities (AEAs) of 1.13 and 1.35 eV. Isomers of the HNCn- family and many branched structures are possible stable species viable for detection in the ISM.

Si surface passivation by using triode-type plasma-enhanced chemical vapor deposition with thermally energized film-precursors

We fabricated hydrogenated amorphous Si (a-Si:H) passivation layers on the surfaces of Si wafers by using triode-type plasma-enhanced chemical vapor deposition with gas-heating, and discussed high-quality surface passivation for Si heterojunction solar cells. The sample with the a-Si:H layers corresponding to the highest proportion of SiHx(x=2,3) content in SiHx(x=1–3) content exhibited the minimum surface recombination velocity (S) after annealing. This suggests that using SiHx(x=2,3)-rich a-Si:H grown at low-temperature as a passivation layer is advantageous to inhibit an epitaxial growth at the a-Si:H/crystalline Si interface, and that a structural relaxation of the a-Si:H takes place during post-deposition annealing, drastically improving passivation quality. Also, the importance to use a low Tsub and to optimize gas-heating and the triode technique, for obtaining simultaneously higher film quality and abrupt interface, is suggested. Low S obtained for our unoptimized samples implies the potency of this deposition technique. Nevertheless, further studies are needed to elucidate the impact of gas-heating and the triode technique on Si surface passivation. Temperature-dependent effective carrier lifetime for our samples might suggest relatively large electron affinity for an a-Si:H, which might be one possible reason for high-quality surface passivation.

An asymmetric hot carrier tunneling van der Waals heterostructure for multibit optoelectronic memory

Novel optoelectronic memory is fabricated using a van der Waals heterostructure of PtS2/h-BN/graphene with asymmetric hot carrier tunneling barriers.

Nitrogen-doped graphene oxide as a catalyst for the oxidation of Rhodamine B by hydrogen peroxide: application to a sensitive fluorometric assay for hydrogen peroxide.

The authors report that nitrogen-doped graphene oxide (NGO) catalyzes the oxidative decomposition of the fluorophore Rhodamine B (RhB) by hydrogen peroxide. The catalytic decomposition of hydrogen peroxide yields free hydroxyl radicals that destroy RhB so that the intensity of the yellow fluorescence is reduced. Nitrogen doping enhances the electronic and optical properties and surface chemical reactivities of GO such as widening of bandgap, increase in conductivity, enhanced quenching and adsorbing capabilities etc. The catalytic properties of NGO are attributed to its large specific surface and high electron affinity of nitrogen atoms. The chemical and structural properties of GO and NGO were characterized by XRD, FTIR, SEM, UV-visible and Raman spectroscopies. The method was optimized by varying the concentration of RhB, nitrogen dopant and hydrogen peroxide. The fluorescent probe, best operated at excitation/emission wavelengths of 554/577nm, allows hydrogen peroxide to be determined in concentrations as low as 94 pM with a linear range spanning from 1nM to 1μM. Graphical abstract Schematic illustration of a fluorescence quenching method for the determination of H2O2. Upon addition of H2O2, nitrogen-doped graphene oxide (NGO) catalyzes the oxidation of Rhodamine B dye due to hydroxyl radical generation, which leads to a sensitive quenchometric methd for H2 O2.

Endohedral Metallofullerene M2@C80: A New Class of Magnetic Superhalogen

Superhalogen is a special class of atomic clusters whose electron affinity is larger than that of chlorine atom, which has the largest electron affinity in the periodic table of elements. In this p...

Electron and radical cation of sulfur-substituted thymine derivatives produced near photoionization threshold can alter and distort double-helix DNA structure

Results from DFT calculations show that the aqueous adiabatic ionization energies for thymine, 2-thiothymine, 2,4-dithiothymine, and 4-thiothymine are 4.95, 4.99 eV, 4.94 eV and 4.81 eV, respectively. The calculated absorption spectra for the radical cations of thymine and 4-thiothymine exhibit significant red-shift and show characteristics of noncanonical structures. The calculated large aqueous adiabatic electron affinity for 2,4-dithiothymine (AEA 2.67 eV) suggests that it can be a potent radio chemotherapeutic agent in cancer treatment.

Greatly Enhanced Electron Affinities of Au2n Cl Clusters (n = 1-4): Effects of Chlorine Doping.

Au2nCl– (n = 1–4) clusters are investigated by both laser ablation mass spectrometry and theoretical calculations. It is interesting to find that the electron affinities of neutral Au2nCl (n = 1–4) clusters are much larger than those of corresponding pure Au2n clusters. Among them, the electron affinity of Au2Cl is 4.02 eV, which can be defined as a very unique superhalogen that is quite different from classical ones of MnXm (M = metal, X = halogen, and n < m). Natural bond orbital and highest occupied molecular orbital analyses indicate that the extra electron is predominantly delocalized over the positively charged metal moiety in these anionic Au2nCl– clusters, which is the main reason for the large electron affinities of the corresponding neutral species.

Rational molecular design of polymeric materials toward efficient triboelectric energy harvesting

In the periodic table, halogenic elements have a larger electron affinity (EA = −270~−349 kJ/mol) than any other group, allowing them to accommodate extra electrons for triboelectric energy harvesting. However, because halogenic elements are not homopolymerizable, carbon (EA = −122 kJ/mol)-based polymers (i.e. PTFE, PVDF) are often employed for structural use and mechanical integrity at the expense of reduced electron affinity, intrinsically sacrificing triboelectric energy harvesting. Herein, we report the first example of triboelectric energy harvesting with sulfur backbone-based inorganic polymers synthesized via inverse-vulcanization process of elemental sulfur, a by-product of petroleum refining process. Fluorinated polymeric sulfur demonstrated 6-fold and 3-fold increase in triboelectric energy outputs in voltage and currents respectively in comparison with commercial PTFE film. Such high energy harvesting was achieved by high electron affinity of sulfur (−200 kJ/mol) and by its hypervalency via expanded-octet which provided coordination of two additional bonds with halogens compared to carbon. The triboelectric open-circuit voltage output reached 1366 V and demonstrated direct powering of 630 LEDs under the minimal force of ~30 N. This yellow chemistry-based molecular engineering paves a way for a new class of triboelectric materials toward low-cost, eco-friendly, and scalable triboelectric energy harvesting applications.

Modeling of silicon- and aluminum-doped phosphorene nanoflakes.

The electronic structure of phosphorene nanoflakes (PNFs) doped with Al and Si has been explored using hybrid functional BHandHlyp/def2-SVP and complete active space (CASSCF) methods. Doping increases the bond length alternation and changes the overall PNF shape. Doping also decreases singlet-triplet splitting in the PNFs. This effect is most notable for Si doping where singlet and triplet states become virtually degenerated. Doping also reduces band gaps and changes the nature of the ground states for Si-doped systems. The ground state of Si-doped PNFs becomes polyradicalic. In general, dopants with even number of valence electrons promote polyradicalic ground state. Doped systems show increased electron affinities (EAs), while the ionization potentials are much less affected. Larger EAs are related with the delocalization of an extra electron over the empty or partially empty 3p orbitals of the dopants. Doping increases the reorganization energies in all cases. Al-doped PNFs are the hole transport materials while Si-doped nanoflakes tend to be electron transport systems. Graphical abstract.

Magnesium-Based Clusters as Building Blocks of Electrolytes in Lithium-Ion Batteries.

Superhalogens, owing to their large electron affinity (EA, exceeding those of any halogen atom), play an essential role in physical chemistry as well as new material design. They have applications in hydrogen storage and lithium-ion batteries. Owing to the unique geometries and electronic features of magnesium-based clusters, their potential to form a new class of lithium salts has been investigated here theoretically. The idea is assessed by conducting ab initio computations on Li+ /Mgn F2n+1-2m Om- compounds (n=2, 3; m=0-3) and analyzing their performance as potential Li-ion battery electrolytes. The Mg3 F7- cluster, with large electron binding energy (EA of 7.93 eV), has been proven to serve as a building block for lithium salts. It is shown that, apart from high electronic stability, the new superhalogen-based electrolytes exhibit a set of desirable properties, including a large band gap, high electrolyte stability window, easy mobility of the Li+ , and favorable insensitivity to water.