Ab-initio calculations for a realistic sensor: A study of CO sensors based on nitrogen-rich carbon nanotubes

The use of nanoscale low-dimensional systems could boost the sensitivity of gas sensors. In this work we simulate a nanoscopic sensor based on carbon nanotubes with a large number of binding sites using ab initio density functional electronic structure calculations coupled to the Non-Equilibrium Green's Function formalism. We present a recipe where the adsorption process is studied followed by conductance calculations of a single defect system and of more realistic disordered system considering different coverages of molecules as one would expect experimentally. We found that the sensitivity of the disordered system is enhanced by a factor of 5 when compared to the single defect one. Finally, our results from the atomistic electronic transport are used as input to a simple model that connects them to experimental parameters such as temperature and partial gas pressure, providing a procedure for simulating a realistic nanoscopic gas sensor. Using this methodology we show that nitrogen-rich carbon nanotubes could work at room temperature with extremely high sensitivity. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4739280]

Klassische und ab initio Molekulardynamik-Untersuchungen zu Germaniumdioxidschmelzen

Germaniumdioxid (GeO2) ist ein Glasbildner, der wie das homologe SiO2 ein ungeordnetes tetraedrisches Netzwerk ausbildet. In dieser Arbeit werden mit Hilfe von Molekulardynamik-Computersimulationen die Struktur und Dynamik von GeO2 in Abhängigkeit von der Temperatur untersucht. Dazu werden sowohl Simulationen mit einem klassischen Paarpotentialmodell von Oeffner und Elliott als auch ab initio-Simulationen gemäß der Car-Parrinello-Molekulardynamik (CPMD), bei der elektronische Freiheitsgrade mittels Dichtefunktionaltheorie beschrieben werden, durchgeführt. In der klassischen Simulation werden dazu ein Temperaturen zwischen 6100 K und 2530 K betrachtet. Darüberhinaus ermöglichen Abkühlläufe auf T=300 K das Studium der Struktur des Glases. Zum Vergleich werden CPMD-Simulationen für kleinere Systeme mit 60 bzw. 120 Teilchen bei den Temperaturen 3760 K und 3000 K durchgeführt. In den klassischen Simulationen kann die im Experiment bis 1700 K nachgewiesene, im Vergleich zu SiO2 starke, Temperaturabhängigkeit der Dichte auch bei höheren Temperaturen beobachtet werden. Gute Übereinstimmungen der Simulationen mit experimentellen Daten zeigen sich bei der Untersuchung verschiedener struktureller Größen, wie z.B. Paarkorrelationsfunktionen, Winkelverteilungen, Koordinationszahlen und Strukturfaktoren. Es können leichte strukturelle Abweichungen der CPMD-Simulationen von den klassischen Simulationen aufgezeigt werden: 1. Die Paarabstände in CPMD sind durchweg etwas kleiner. 2. Es zeigt sich, daß die Bindungen in den ab initio-Simulationen weicher sind, was sich auch in einer etwas stärkeren Temperaturabhängigkeit der strukturellen Größen im Vergleich zu den klassischen Simulationen niederschlägt. 3. Für CPMD kann ein vermehrtes Auftreten von Dreierringstrukturen gezeigt werden. 4. In der CPMD werden temperaturabhängige Defektstrukturen in Form von Sauerstoffpaaren beobachtet, die vor allem bei 3760 K, kaum jedoch bei 3000 K auftreten. Alle strukturellen Unterschiede zwischen klassischer und CPMD-Simulation sind eindeutig nicht auf Finite-Size-Effekte aufgrund der kleinen Systemgrößen in den CPMD-Simulationen zurückzuführen, d.h. sie sind tatsächlich methodisch bedingt. Bei der Dynamik von GeO2 wird in den klassischen Simulationen ebenfalls eine gute Übereinstimmung mit experimentellen Daten beobachtet, was ein Vergleich der Diffusionskonstanten mit Viskositätsmessungen bei hohen Temperaturen belegt. Die Diffusionskonstanten zeigen teilweise ein verschiedenes Verhalten zum homologen SiO2. Sie folgen in GeO2 bei Temperaturen unter 3000 K einem Arrheniusgesetz mit einer deutlich niedrigeren Aktivierungsenergie. Darüberhinaus werden die Möglichkeiten der Parametrisierung eines neuen klassischen Paarpotentials mittels der Kräfte entlang der CPMD-Trajektorien untersucht. Es zeigt sich, daß derartige Parametrisierungen sehr stark von den gewählten Startparametern abhängen. Ferner führen sämtliche an die Schmelze parametrisierten Potentiale zu zu hohen Dichten im Vergleich zum Experiment. Zum einen liegt dies sehr wahrscheinlich daran,daß für das System GeO2 Kraftdaten allein nicht ausreichen, um grundlegende strukturelle Größen, wie z.B. Paarkorrelationen und Winkelverteilungen, der CPMD-Simulationen gut reproduzieren zu können. Zum anderen ist wohl die Beschreibung mittels Paarpotentialen nicht ausreichend und es ist erforderlich, Merkörperwechselwirkungen in Betracht zu ziehen.

Electronic structure and physical properties of Heusler compounds for thermoelectric and spintronic applications

This thesis focuses on synthesis as well as investigations of the electronic structure and properties of Heusler compounds for spintronic and thermoelectric applications.rnThe first part reports on the electronic and crystal structure as well as the mechanical, magnetic, and transport properties of the polycrystalline Heusler compound Co2MnGe. The crystalline structure was examined in detail by extended X-ray absorption fine structure spectroscopy and anomalous X-ray diffraction. The low-temperature magnetic moment agrees well with the Slater-Pauling rule and indicates a half-metallic ferromagnetic state of the compound, as is predicted by ab-initio calculations. Transport measurements and hard X-ray photoelectron spectroscopy (HAXPES) were performed to explain the electronic structure of the compound.rnA major part of the thesis deals with a systematical investigation of Heusler compounds for thermoelectric applications. Few studies have been reported on thermoelectric properties of p-type Heusler compounds. Therefore, this thesis focuses on the search for new p-type Heusler compounds with high thermoelectric efficiency. The substitutional series NiTi1−xMxSn and CoTi1−xMxSb (where M = Sc, V and 0 ≤ x ≤ 0.2) were synthesized and investigated theoretically and experimentally with respect to electronic structure and transport properties. The results show the possibility to create n-type and p-type thermoelectrics within one Heusler compound. The pure compounds showed n-type behavior, while under Sc substitution the system switched to p-type behavior. A maximum Seebeck coefficient of +230 μV/K (at 350 K) was obtained for NiTi0.26Sc0.04Zr0.35Hf0.35Sn, which is one of the highest values for p-type thermoelectric compounds based on Heusler alloys up to now. HAXPES valence band measurement show massive in gap states for the parent compounds NiTiSn, CoTiSb and NiTi0.3Zr0.35Hf0.35Sn. This proves that the electronic states close to the Fermi energy play a key role for the behavior of the transport properties. Furthermore, the electronic structure of the gapless Heusler compounds PtYSb, PtLaBi and PtLuSb were investigated by bulk sensitive HAXPES. The linear behavior of the spectra close to εF proves the bulk origin of Dirac-cone type density of states. Furthermore, a systematic study on the optical and transport properties of PtYSb is presented. The compound exhibits promising thermoelectric properties with a high figure of merit (ZT = 0.2) and a Hall mobility μh of 300 cm2/Vs at 350 K.rnThe last part of this thesis describes the linear dichroism in angular-resolved photoemission from the valence band of NiTi0.9Sc0.1Sn and NiMnSb. High resolution photoelectron spectroscopy was performed with an excitation energy of hν = 7.938 keV. The linear polarization of the photons was changed using an in-vacuum diamond phase retarder. Noticeable linear dichroism is found in the valence bands and this allows for a symmetry analysis of the contributing states. The differences in the spectra are found to be caused by symmetry dependent angular asymmetry parameters, and these occur even in polycrystalline samples without preferential crystallographic orientation.rnIn summary, Heusler compounds with 1:1:1 and 2:1:1 stoichiometry were synthesized and examined by chemical and physical methods. Overall, this thesis shows that the combination of first-principle calculations, transport measurements and high resolution high energy photoelectron spectroscopy analysis is a very powerful tool for the design and development of new materials for a wide range of applications from spintronic applications to thermoelectric applications.rn

A New Crystalline LiPON Electrolyte: Synthesis, Properties, and Electronic Structure

The new crystalline compound, Li2PO2N, was synthesized using high temperature solid state methods starting with a stoichiometric mixture of Li2O, P2O5, and P3N5. Its crystal structure was determined ab initio from powder X-ray diffraction. The compound crystallizes in the orthorhombic space group Cmc2(1) (# 36) with lattice constants a = 9.0692(4) angstrom, b = 53999(2) angstrom, and c = 4.6856(2) angstrom. The crystal structure of SD-Li2PO2N consists of parallel arrangements of anionic chains formed of corner sharing (PO2N2) tetrahedra. The chains are held together by Li+ cations. The structure of the synthesized material is similar to that predicted by Du and Holzwarth on the basis of first principles calculations (Phys. Rev. B 81,184106 (2010)). The compound is chemically and structurally stable in air up to 600 degrees C and in vacuum up to 1050 degrees C. The Arrhenius activation energy of SD-Li2PO2N in pressed pellet form was determined from electrochemical impedance spectroscopy measurements to be 0.6 eV, comparable to that of the glassy electrolyte LiPON developed at Oak Ridge National Laboratory. The minimum activation energies for Li ion vacancy and interstitial migrations are computed to be 0.4 eV and 0.8 eV, respectively. First principles calculations estimate the band gap of SD-Li2PO2N to be larger than 6 eV. (C) 2013 Elsevier B.V. All rights reserved.

Quantum molecular dynamics and ab initio studies of the crystal structure of hydrogen and deuterium

Hydrogen isotopes play a critical role both in inertial and magnetic confinemen Nuclear Fusion. Since the preferent fuel needed for this technology is a mixture of deuterium and tritium. The study of these isotopes particularly at very low temperatures carries a technological interest in other applications. The present line promotes a deep study on the structural configuration that hydrogen and deuterium adopt at cryogenic temperatures and at high pressures. Typical conditions occurring in present Inertial Fusion target designs. Our approach is aims to determine the crystal structure characteristics, phase transitions and other parameters strongly correlated to variations of temperature and pressure.

Electronic structure and transport properties of atomic NiO spinvalves

Ab initio quantum transport calculations show that short NiO chains suspended in Ni nanocontacts present a very strong spin-polarization of the conductance.The generalized gradient approximation we use here predicts a similar polarization of the conductance as the one previously computed with non-local exchange, confirming the robustness of the result. Their use as nanoscopic spinvalves is proposed.

Mécanismes de transfert de proton d’une réaction acido-basique en phase aqueuse : une étude ab-initio

Les réactions de transfert de proton se retrouvent abondamment dans la nature et sont des processus cruciaux dans plusieurs réactions chimiques et biologiques, qui se produisent souvent en milieu aqueux. Les mécanismes régissant ces échanges de protons sont complexes et encore mal compris, suscitant un intérêt des chercheurs en vue d’une meilleure compréhension fondamentale du processus de transfert. Le présent manuscrit présente une étude mécanistique portant sur une réaction de transfert de proton entre un acide (phénol fonctionnalisé) et une base (ion carboxylate) en phase aqueuse. Les résultats obtenus sont basés sur un grand nombre de simulations de dynamique moléculaire ab-initio réalisées pour des systèmes de type « donneur-pont-accepteur », où le pont se trouve à être une unique molécule d’eau, permettant ainsi l’élaboration d’un modèle cinétique détaillé pour le système étudié. La voie de transfert principalement observée est un processus ultra-rapide (moins d’une picoseconde) passant par la formation d’une structure de type « Eigen » (H9O4+) pour la molécule d’eau pontante, menant directement à la formation des produits. Une seconde structure de la molécule d’eau pontante est également observée, soit une configuration de type « Zündel » (H5O2+) impliquant l’accepteur de proton (l’ion carboxylate) qui semble agir comme un cul-de-sac pour la réaction de transfert de proton.

Mécanismes de transfert de proton d’une réaction acido-basique en phase aqueuse : une étude ab-initio

Les réactions de transfert de proton se retrouvent abondamment dans la nature et sont des processus cruciaux dans plusieurs réactions chimiques et biologiques, qui se produisent souvent en milieu aqueux. Les mécanismes régissant ces échanges de protons sont complexes et encore mal compris, suscitant un intérêt des chercheurs en vue d’une meilleure compréhension fondamentale du processus de transfert. Le présent manuscrit présente une étude mécanistique portant sur une réaction de transfert de proton entre un acide (phénol fonctionnalisé) et une base (ion carboxylate) en phase aqueuse. Les résultats obtenus sont basés sur un grand nombre de simulations de dynamique moléculaire ab-initio réalisées pour des systèmes de type « donneur-pont-accepteur », où le pont se trouve à être une unique molécule d’eau, permettant ainsi l’élaboration d’un modèle cinétique détaillé pour le système étudié. La voie de transfert principalement observée est un processus ultra-rapide (moins d’une picoseconde) passant par la formation d’une structure de type « Eigen » (H9O4+) pour la molécule d’eau pontante, menant directement à la formation des produits. Une seconde structure de la molécule d’eau pontante est également observée, soit une configuration de type « Zündel » (H5O2+) impliquant l’accepteur de proton (l’ion carboxylate) qui semble agir comme un cul-de-sac pour la réaction de transfert de proton.

Electronic structure methods applied to gas-carbon reactions

A review is given on the fundamental studies of gas-carbon reactions using electronic structure methods in the last several decades. The three types of electronic structure methods including semi-empirical, ab initio and density functional theory, methods are briefly introduced first, followed by the studies on carbon reactions with hydrogen and oxygen-containing gases (non-catalysed and catalysed). The problems yet to solve and possible promising directions are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.

The location of Ti atom in sodium alanate : an ab initio spin-polarised study

In this work, ab initio spin-polarised Density Functional Theory (DFT) calculations are performed to study the interaction of a Ti atom with a NaAlH4(001) surface. We confirm that an interstitially located Ti atom in the NaAlH4 subsurface is the most energetically favoured configuration as recently reported (Chem. Comm. (17) 2006, 1822). On the NaAlH4(001) surface, the Ti atom is most stable when adsorbed between two sodium atoms with an AlH4 unit beneath. A Ti atom on top of an Al atom is also found to be an important structure at low temperatures. The diffusion of Ti from the Al-top site to the Na-bridging site has a low activation barrier of 0.20 eV and may be activated at the experimental temperatures (∼323 K). The diffusion of a Ti atom into the energetically favoured subsurface interstitial site occurs via the Na-bridging surface site and is essentially barrierless.

Electron capture of tetracyanoethylene oxide in the gas phase. Rearrangement of the parent radical anion to form [(NC)(3)C](-). A joint experimental and ab initio study

Capture of an electron by tetracyanoethylene oxide can initiate a number of decomposition pathways. One of these decompositions yields [(NC)3C]− as the ionic product. Ab initio calculations (at the B3LYP/6-31+G∗ level of theory) indicate that the formation of [(NC)3C]− is initiated by capture of an electron into the LUMO of tetracyanoethylene oxide to yield the anion radical [(NC)2C–O–C(CN)2]−· that undergoes internal nucleophilic substitution to form intermediate [(NC)3C–OCCN]−·. This intermediate dissociates to form [(NC)3C]− (m/z 90) as the ionic product. The radical (NC)3C· has an electron affinity of 4.0 eV (385 kJ mol−1). Ab initio calculations show that [(NC)3C]− is trigonal planar with the negative charge mainly on the nitrogens. A pictorial representation of this structure is the resonance structure formed from three degenerate contributing structures (NC)2–CCN−. The other product of the reaction is nominally (NCCO)·, but there is no definitive experimental evidence to indicate whether this radical survives intact, or decomposes to NC· and CO. The overall process [(NC)2C–O–C(CN)2]−· → [(NC)3C]− + (NCCO)· is calculated to be endothermic by 21 kJ mol−1 with an overall barrier of 268 kJ mol−1.