Playing Catch with Electrons: A Dynamic Metaphor for Chemical Bonding with an Extension of the Tunneling Model to Gas-Phase Molecules.

Quantum mechanical electron tunneling is explored as the mediator of chemical bonding. Three topics are addressed. First, a baseball game-of-catch metaphor is employed to elucidate the physics of chemical bonding. According to this metaphor, a baseball player, a baseball, and the act of throwing a baseball are analogous to an atom, an electron, and the act of electron tunneling. Using this framework, the tunneling physics of covalent, polar covalent, ionic, and hydrogen bonding is clarified. Second, gas-state energies associated with frontier orbital positioning are established and are used in the chemical bond tunneling assessment of four homopolar bonds, H-H, N≡N, O=O, and F-F, and four heteropolar bonds, H-F, H-Cl, H-Br, and H-I. Third, a new procedure for estimating solid-state tunneling parameters is introduced, involving quantum confinement of gas-state energy frontier orbitals.

Chemical Bonding with or without Tunneling.

A solid-state tunneling analysis is performed in order to assess whether a given chemical bond type is mediated by quantum mechanical electron tunneling. Four bond types are found to involve tunneling-covalent, ionic, polar covalent, and transition metal bonding. Two bond types do not rely on tunneling-free electron metal and van der Waals bonding. Cohesive energy is large for the four bonds involving tunneling due to tunneling-induced Coulombic energy storage, while it is small for the two bonds that do not involve tunneling. Coulombic energy storage is dynamic for covalent and strong polar covalent bonding, static for ionic bonding, and quasi-static for weak polar covalent bonding, where quasi-static pertains to tunneling times longer than ∼160 fs, the room-temperature vibrational attempt time. The cohesive energy of tungsten (W) is anomalously large, suggesting that chemical bonding in W is mediated by a two-electron d-d tunneling process in which charge polarity flips between W+W- and W-W+ with every two-electron tunneling event. All six bonds just listed are directly connected bonds, in contradistinction to a hydrogen bond, which is a bridge bond linking two adjacent atoms. A hydrogen bond is mediated by quantum mechanical electron tunneling. However, its cohesive energy is variable and can be either relatively large or very small depending on interatomic spacing.

New Perspective on Hydrogen Bonding.

A new model is proposed for hydrogen bonding in which an intermediate hydrogen atom acts as a bridge bond connecting two adjacent atoms, X and A, via quantum mechanical tunneling of the hydrogen electron. A strong hydrogen bond (X-H-A) is formed when the X-H and H-A interatomic distances are short and symmetric, thereby facilitating intense electron tunneling to and from both adjacent atoms. The hydrogen bond weakens (X-H···A) as the H···A interatomic distance lengthens compared to that of X-H since the H···A tunneling intensity degrades exponentially with increasing distance. Two modes of electron tunneling are distinguished. When an electron tunnels from H to either X or A (forward tunneling), the X-H···A bond is initially charge neutral but after tunneling is charged as either X--H+···A or X-H+···A-. In contrast, electron tunneling from either X- or A- back to H+ (reverse tunneling) discharges the X-H···A bond, resetting it back into its neutral charge state. Reverse tunneling is central to understanding the nature of a hydrogen bond. When the H···A interatomic distance is sufficiently short, reverse tunneling occurs through a triangular energy barrier (Fowler-Nordheim tunneling) such that the reverse tunneling probability is almost 100%. Increasing the H···A interatomic distance leads to a decreasing H···A reverse tunneling probability, as tunneling occurs through an asymmetric trapezoidal energy barrier (direct tunneling) until finally the H···A interatomic distance is so large that the bond persists indefinitely in the X-H+···A- charge state such that it is incapable of acting as a bridge bond linking X and A.

Bonding and Tunneling.

Quantum mechanical electron tunneling is proposed as the mediator of chemical bonding. Covalent, ionic, and polar covalent bonds all rely on quantum mechanical tunneling, but the nature of tunneling differs for each bond type. Covalent bonding involves bidirectional tunneling across a symmetric energy barrier. Ionic bonding occurs by unidirectional tunneling from the cation to the anion across an asymmetric energy barrier. Polar covalent bonding is a more complicated type of bidirectional tunneling, consisting of both cation-to-anion and anion-to-cation tunneling across asymmetric energy barriers. Tunneling considerations suggest the possibility of another type of bond-denoted polar ionic-in which tunneling involves two electrons across asymmetric barriers.

Demonstration of Fowler-Nordheim Tunneling in Simple Solution-Processed Thin Films.

The production of high-quality thin-film insulators is essential to develop advanced technologies based on electron tunneling. Current insulator deposition methods, however, suffer from a variety of limitations, including constrained substrate sizes, limited materials options, and complexity of patterning. Here, we report the deposition of large-area Al2O3 films by a solution process and its integration in metal-insulator-metal devices that exhibit I- V signatures of Fowler-Nordheim electron tunneling. A unique, high-purity precursor based on an aqueous solution of the nanocluster flat-Al13 transforms to thin Al2O3 insulators free of the electron traps and emission states that commonly inhibit tunneling in other films. Tunneling is further confirmed by the temperature independence of device current.

Structural convergence properties of amorphous InGaZnO4 from simulated liquid-quench methods.

The study of structural properties of amorphous structures is complicated by the lack of long-range order and necessitates the use of both cutting-edge computer modeling and experimental techniques. With regards to the computer modeling, many questions on convergence arise when trying to assess the accuracy of a simulated system. What cell size maximizes the accuracy while remaining computationally efficient? More importantly, does averaging multiple smaller cells adequately describe features found in bulk amorphous materials? How small is too small? The aims of this work are: (1) to report a newly developed set of pair potentials for InGaZnO4 and (2) to explore the effects of structural parameters such as simulation cell size and numbers on the structural convergence of amorphous InGaZnO4. The total number of formula units considered over all runs is found to be the critical factor in convergence as long as the cell considered contains a minimum of circa fifteen formula units. There is qualitative agreement between these simulations and X-ray total scattering data - peak trends and locations are consistently reproduced while intensities are weaker. These new IGZO pair potentials are a valuable starting point for future structural refinement efforts.

Lanthanum aluminum oxide thin-film dielectrics from aqueous solution.

Amorphous LaAlO3 dielectric thin films were fabricated via solution processing from inorganic nitrate precursors. Precursor solutions contained soluble oligomeric metal-hydroxyl and/or -oxo species as evidenced by dynamic light scattering (DLS) and Raman spectroscopy. Thin-film formation was characterized as a function of annealing temperature using Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray reflectivity (XRR), scanning electron microscopy (SEM), and an array of electrical measurements. Annealing temperatures ≥500 °C result in thin films with low leakage-current densities (∼1 × 10(-8) A·cm(-2)) and dielectric constants ranging from 11.0 to 11.5. When incorporated as the gate dielectric layer in a-IGZO thin-film transistors (TFTs), LaAlO3 thin films annealed at 600 °C in air yielded TFTs with relatively low average mobilities (∼4.5 cm(2)·V(-1)·s(-1)) and high turn-on voltages (∼26 V). Interestingly, reannealing the LaAlO3 in 5%H2/95%N2 at 300 °C before deposition of a-IGZO channel layers resulted in TFTs with increased average mobilities (11.1 cm(2)·V(-1)·s(-1)) and lower turn-on voltages (∼6 V).

Atomic solid state energy scale.

A plot of electron affinity (EA) and ionization potential (IP) versus energy band gap (E(G)) for 69 binary closed-shell inorganic semiconductors and insulators reveals that E(G) is centered about the hydrogen donor/acceptor ionization energy ε(+/-). Thus, ε(+/-), or equivalently the standard hydrogen electrode (SHE) energy, functions as an absolute energy reference, determining the tendency of an atom to be either a cation or anion in a compound. This empirical trend establishes the basis for defining a new solid state energy (SSE) scale. This SSE scale makes possible simple approaches for quantitatively assessing electronegativity, chemical hardness, and ionicity, while also providing new insight into the periodic trends of solids.

Aqueous inorganic inks for low-temperature fabrication of ZnO TFTs.

A simple, low-cost, and nontoxic aqueous ink chemistry is described for digital printing of ZnO films. Selective design through controlled precipitation, purification, and dissolution affords an aqueous Zn(OH)(x)(NH(3))(y)((2-x)+) solution that is stable in storage, yet promptly decomposes at temperatures below 150 degrees C to form wurtzite ZnO. Dense, high-quality, polycrystalline ZnO films are deposited by ink-jet printing and spin-coating, and film structure is elucidated via X-ray diffraction and electron microscopy. Semiconductor film functionality and quality are examined through integration in bottom-gate thin-film transistors. Enhancement-mode TFTs with ink-jet printed ZnO channels annealed at 300 degrees C are found to exhibit strong field effect and excellent current saturation in tandem with incremental mobilities from 4-6 cm(2) V(-1) s(-1). Spin-coated ZnO semiconductors processed at 150 degrees C are integrated with solution-deposited aluminum oxide phosphate dielectrics in functional transistors, demonstrating both high performance, i.e., mobilities up to 1.8 cm(2) V(-1) s(-1), and the potential for low-temperature solution processing of all-oxide electronics.