Control of Electronic and Optical Properties of Two-Dimensional Materials by Functionalization

Since the exfoliation of graphene in 2005, two-dimensional (2D) materials have become the subject of exploiting interest. In particular, graphene is regarded as a serious alternative to many conventional materials in various applications. The rapid and prosperous development of graphene stimulates numerous research interests on other 2D materials, such as transition metal dichalcogenides (TMDCs). Although the 2D materials have been continuously refreshing and enriching their family, a pure material may not meet the demands for versatile applications. Therefore, this thesis focuses on the modulation of the properties of 2D materials by means of different methods. In this thesis, three approaches are presented to control the properties of 2D systems involving stacking, covalent functionalization and defect engineering in combination with molecule adsorption. To fully understand the effect of these methods on the 2D materials, we explore the structural, electronic and optical properties of the monolayered graphene and MoS2 (a traditional representative of TMDCs) by state-of-the-art ab-initio computational methods. Specifically, we study (a) the intercalation of surface and subsurface Co-Ir alloy between graphene/Ir(111); (b) the covalent functionalization of 2H-MoS2, 1T'-MoS2 and graphene by various chemical groups such as -F, -CH3, -C6H5 and so on; (c) the physisorption and chemisorption of small molecules on the pristine and defective 2H-MoS2 monolayer. Our results reveal that stacking is an effective method to tune the moire superstructure and electronic properties of graphene and the interaction strength between graphene and substrate is strongly influenced by the composition and nature of an alloy; covalent functionalization results in dramatic changes to the electronic and optical properties of MoS2 and graphene, achieving semiconductor-to-metal or metal-to-semiconductor transition; the band gap and optical absorption display a strong dependence on the covalent functionalization coverage, suggesting that the ability to accurately select the coverage of groups attached to the monolayer surfaces, may be an effective way to engineer the optoelectronic properties of graphene and MoS2 for selected device applications; defects in the MoS2 are active centers for the molecule adsorption and chemical functionalization; the chemisorption and dissociation of O2 on the defective surface tend to passivate S defect states, while the physisorption of O2 and NO molecules on the defective and pristine MoS2 could enhance the optical absorption peak and the excitonic binding energy. Our work confirms the tunability of properties of the considered systems and further indicates the possibility of artificially controlling the properties of 2D materials. The deep insights into the functionalized graphene and MoS2 from this thesis are expected to provide a useful guide for the design of 2D-based devices.Seit der Exfoliation von Graphen im Jahr 2005 sind zweidimensionale (2D) Materialien Gegenstand von wissenschaftlichem Interesse geworden. Insbesondere Graphen wird in verschiedenen Anwendungen als ernsthafte Alternative zu vielen herkömmlichen Materialien diskutiert. Die schnelle und florierende Entwicklung von Graphen stimuliert das groß Forschungsinteresse an anderen 2D-Materialien, wie z.B. den Übergangsmetalldichalkogeniden (TMDCs). Obwohl die Familie der 2D-Materialien kontinuierlich wächst, erfüllt ein pure Material möglicherweise nicht die Anforderungen für vielseitige Anwendungen. Daher konzentriert sich diese Arbeit auf die Modulation der Eigenschaften von 2D-Materialien mit Hilfe verschiedener Methoden. In dieser Dissertation werden drei Ansätze vorgestellt um die Eigenschaften von 2D-Systemen zu steuern: Stapelung, kovalente Funktionalisierung und Defekt-Engineering in Kombination mit Moleküladsorption. Um die Auswirkungen dieser Methoden auf die 2D-Materialien vollständig zu verstehen, untersuchen wir die strukturellen, elektronischen und optischen Eigenschaften des einschichtigen Graphens und MoS2 (eines traditionellen Vertreters von TMDCs) mit modernsten ab-initio Berechnungsmethoden. Insbesondere untersuchen wir (a) die Interkalation von Oberflächen- und Suboberflächen von Co-Ir-Legierungen zwischen Graphen/Ir(111); (b) die kovalente Funktionalisierung von 2H-MoS2, 1T'-MoS2 und Graphen durch verschiedene chemische Gruppen wie -F, -CH3, -C6H5 und ähnliche; (c) die Physisorption und Chemisorption kleiner Moleküle auf der idealen und defekten 2H-MoS2-Monoschicht. Unsere Ergebnisse zeigen, dass das Stapeln eine effektive Methode ist, um die Moiré-Überstruktur und die elektronischen Eigenschaften von Graphen abzustimmen, und dass die Wechselwirkungsstärke zwischen Graphen und Substrat stark von der Zusammensetzung und Art einer Legierung beeinflusst wird; kovalente Funktionalisierung führt zu dramatischen Änderungen der elektronischen und optischen Eigenschaften von MoS2 und Graphen, wodurch ein Halbleiter-zu-Metall- oder Metall-zu-Halbleiter-Übergang erreicht wird; die Bandlücke und die optische Absorption zeigen eine starke Abhängigkeit von dem ausmaß an kovalenter Funktionalisierung; dies deutet darauf hin, dass die Fähigkeit die Konzentration der Funktionalisierung auf den Monoschichtoberflächen genau zu bestimmen ein effektiver Weg sein könnte, um die optoelektronischen Eigenschaften von Graphen und MoS2 für gezielt Anwendungen zu optimieren; Defekte in MoS2 sind aktive Zentren für Moleküladsorption und chemische Funktionalisierung; die Chemisorption und Dissoziation von O2 auf der defekten Oberfläche tendiert dazu, S-Defektzustände zu passivieren, während die Physisorption von O2 und NO-Molekülen auf dem defekten und idealen MoS2 den optischen Absorptionspeak und die exzitonische Bindung verstärken können. Unsere Arbeit bestätigt die Einstellbarkeit der Eigenschaften der betrachteten Systeme und weist ferner auf Möglichkeiten hin die Eigenschaften von 2D-Materialien künstlich zu steuern. Die tiefen Einblicke in funktionalisiertes Graphen und MoS2 aus dieser Dissertation sollen einen nützlichen Leitfaden für das Design von 2D-Material-basierten Bauelementen darstellen

Cluster Formation Effect of Water on Pristine and Defective MoS2 Monolayers

The structure and electronic properties of the molybdenum disulfide (MoS2) monolayer upon water cluster adsorption are studied using density functional theory and the optical properties are further analyzed with the Bethe–Salpeter equation (BSE). Our results reveal that the water clusters are electron acceptors, and the acceptor tendency tends to increase with the size of the water cluster. The electronic band gap of both pristine and defective MoS2 is rather insensitive to water cluster adsorbates, as all the clusters are weakly bound to the MoS2 surface. However, our calculations on the BSE level show that the adsorption of the water cluster can dramatically redshift the optical absorption for both pristine and defective MoS2 monolayers. The binding energy of the excitons of MoS2 is greatly enhanced with the increasing size of the water cluster and finally converges to a value of approximately 1.16 eV and 1.09 eV for the pristine and defective MoS2 monolayers, respectively. This illustrates that the presence of the water cluster could localize the excitons of MoS2, thereby greatly enhance the excitonic binding energy

Modulating electronic and optical properties of monolayered MoS2 by covalent mono- and bisfunctionalization

By employing first-principles simulations, we present theoretical predictions regarding the modification of structural, electronic and optical properties of 2H- and 1T′-MoS2 monolayers by covalent mono- and bisfunctionalization. Specifically, non-aromatic groups (–F, –NH2, –CH3, –CH2CH2 CN and –CH2CH2 OH) and aromatic (–Ph, –PhNO2 and PhOH) groups are utilized for monofunctionalization, and –F/–NH2, –NH2/–CH3 and –CH3/–Ph for bisfunctionalization. The stability of functionalized 2H- and 1T′-MoS2 monolayers mainly depends on the bonded groups and their surface coverage. In particular, the mixed bisfunctionalization with –F/–CH3 and –NH2/–CH3 groups enhances the stability of 2H-MoS2 through the formation of intermolecular hydrogen bonds. Both 2H- and 1T′-MoS2 can serve not only as electron donors, but also as electron acceptors, subject to the charge transfer behavior of the attached groups. Furthermore, mono- and bisfunctionalization are predicted to be efficient approaches to control the electronic band gaps in 2H- and 1T′-MoS2, where the corresponding values can be tuned by varying the coverage of the absorbed groups. At the same time, the choice of the chemical groups and their coverage also effectively determines the optical adsorption range and intensity. Therefore, our work shows that chemical functionalization of 2D materials with varying coverage can be an important approach to extend the scope of 2D materials in specific electronic and optoelectronic applications

Probing Active Sites on Pristine and Defective MnPX3 (X: S and Se) Monolayers for Electrocatalytic Water Splitting

The state-of-the-art density functional theory approach was used to study the structural and electronic properties of pristine and defective MnPX3 monolayers as well as their activity toward water and hydrogen evolution reaction (HER) catalytic performance. The adsorption behavior of H2O on a pristine MnPX3 structure is of physisorption nature, whereas the adsorption energy is significantly increased for the defective structures. At the same time, the water dissociation process is more energetically favorable, and the reactivity of MnPX3 is determined by the vacancy configuration. Following Nørskov’s approach, the HER catalytic performance is evaluated by calculating the hydrogen adsorption free energy on the respective MnPX3 surface. Our calculation results demonstrate that defective 2D MnPX3 with low coordinated P shows significantly higher HER performance compared to the pristine counterpart

Mixed-potential-function-based large-signal stability analysis of DC microgrid with constant power loads

With the development of power grid technology, a large number of constant power loads (CPL) are introduced into the power grid system. The negative damping characteristic of CPL poses a great challenge to the stability of power grid under the condition of large signal disturbance. This paper focuses on the large signal stability and dynamic response characteristics of DC microgrid with CPL. The mixed potential function (MPF) method is adopted to model the large signal characterstic of load converter controlled by the traditional PI control strategy and the new virtual DC motor (VDM) control strategy. The large signal stability criteria and the asymptotic stability region of the DC microgrid system under the two control strategies are derived. The dynamic response characteristics of the DC microgrid are compared under three large signal operation conditions of the load step, load linear change and the load fault. The results show that VDM control strategy can effectively improve dynamic response characteristics of the DC microgrid system compared with those of PI control, and enhance the stability and anti-interference ability of the DC microgrid system

Sex-related difference in food-anticipatory activity of mice

The expression of food-anticipatory activity (FAA) is induced by restricted feeding (RF), and its entrainment requires food-entrainable oscillators, the neuroanatomical basis of which is currently unclear. Although RF impacts various hormones, sex-related differences in FAA are unclear. 'Here, we report significantly more food-anticipatory wheel-running activity in male than in female mice during RF. In parallel with the sex-related difference in FAA, male and female mice display different food intake and body weight in response to RF. Since gonadal hormones could be involved in the sex-specific difference in FAA, we compared sham and gonadectomized male and female wild-type mice. In gonadectomized mice, the sex difference in FAA was abolished, indicating a role for gonadal hormones in FAA. Further, plasma concentrations of the hormone ghrelin were higher in female than in male mice during ad libitum (AL) feeding, and RF induced a temporal advance in its peak in both sexes. RF also shifted the expression peak of the circadian gene mPer1 in the hippocampus and liver, although no sex difference was found in either the level or the cyclic phase of its expression. Per1(Brdm1) mutant mice were still sexually dimorphic for FAA, but diminished FAA was noted in both male and female Per2(Brdm1) mutant mice. In summary, our results imply that gonadal hormones contribute to the sex difference in FAA, possibly through modulating ghrelin activity. (C) 2015 Published by Elsevier Inc

Influence of Si addition on the corrosion behaviour of 9 wt% Cr ferritic/martensitic steels exposed to oxygen-controlled molten Pb-Bi eutectic at 550 and 600 °C

Three 9 wt% Cr ferrite/martensite steels (two alloyed with Si) have been exposed to oxygen-controlled LBE at 550 and 600 °C, respectively. The passivating oxide scale consists of a spinel layer plus internal oxidation zone (IOZ). By adding Si, the thickness of spinel layer is decreased while the IOZ is enhanced. Moreover, a Si-rich oxide layer is observed underneath the spinel layer on Si-containing samples after 2000 h exposure at 600 °C. Besides, the less visible cracks/exfoliations on Si-containing samples indicate the positive role of Si addition on scale adherence