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

The impact of experience on satisfaction and revisit intention in theme parks: An application of the experience economy

In recent years, theme park is becoming a more and more popular destination for tourists around the world. Moreover, theme parks provide visitors with various experiences. It is crucial to understand visitors’ experiences in the theme park since experience is an important factor that is related to tourist behavior. The experience economy is widely used in the tourism setting to measure experience from four dimensions: education, entertainment, escapism and esthetic (4Es). However, limited research has examined visitors’ experiences in the theme park setting. Therefore, the purpose of this study is to measure visitors’ experience in the theme park applying 4Es and investigate the relationship between experience, and satisfaction and revisit intention. Furthermore, the differences on the means of these variables and differences on the impact of experience on satisfaction and revisit intention were also tested. In this study, 486 visitors who have been to Disney parks in the U.S. within the past twelve months took the online survey. Independent sample t-tests and multiple linear regression analysis were adopted. The results showed that experience has a significant impact on satisfaction and revisit intention in the theme park and differences existed between the groups. Overall, findings of this study enhanced the concept of experience in the theme park literature and offered practical implications to the theme park operators

The loss of taste genes in cetaceans

Background: Five basic taste modalities, sour, sweet, bitter, salt and umami, can be distinguished by humans and are fundamental for physical and ecological adaptations in mammals. Molecular genetic studies of the receptor genes for these tastes have been conducted in terrestrial mammals; however, little is known about the evolution and adaptation of these genes in marine mammals. Results: Here, all five basic taste modalities, sour, sweet, bitter, salt and umami, were investigated in cetaceans. The sequence characteristics and evolutionary analyses of taste receptor genes suggested that nearly all cetaceans may have lost all taste modalities except for that of salt. Conclusions: This is the first study to comprehensively examine the five basic taste modalities in cetaceans with extensive taxa sampling. Our results suggest that cetaceans have lost four of the basic taste modalities including sour, sweet, umami, and most of the ability to sense bitter tastes. The integrity of the candidate salt taste receptor genes in all the cetaceans examined may be because of their function in Na+ reabsorption, which is key to osmoregulation and aquatic adaptation. Electronic supplementary material The online version of this article (doi:10.1186/s12862-014-0218-8) contains supplementary material, which is available to authorized users

Impact of Osmotic Stressors on the Metabolic Activity of Methylocystis sp. Strain SC2

Proteobacterial methane-oxidizing bacteria, or methanotrophs, have the unique ability to grow on methane as their sole source of carbon and energy. Among these, Methylocystis spp. belong to the family Methylocystaceae within the Alphaproteobacteria. Their key enzyme is the particulate methane monooxygenase (pMMO), which oxidizes methane to methanol. Methylocystis spp. are among the ecologically most relevant methanotroph populations in terrestrial environments and are widely distributed in diverse habitats. In consequence, Methylocystis spp. require a range of physiological capabilities that allow them to respond and acclimatize to fluctuations in abiotic and biotic factors in their native environment. However, to date there still exist major gaps in our knowledge of their metabolic potential, in particular with regard to their ability to acclimatize to environmental change and to cope with abiotic stress. In my first project, we used a recently developed proteome workflow to elucidate the cellular mechanisms underlying the acclimatization of Methylocystis sp. strain SC2 to high NH4+ load (added as NH4Cl). Relative to 1 mM NH4+, high (50 mM and 75 mM) NH4+ load under CH4-replete conditions significantly increased the lag phase duration required for proteome adjustment, while the addition of 100 mM NH4+ completely inhibited growth of strain SC2. The number of differentially regulated proteins was highly significantly correlated to the increase in NH4+ load. The cellular responses involved the significant upregulation of stress-responsive proteins, the K+ “salt-in” strategy, the synthesis of compatible solutes (glutamate and proline), and the glutathione metabolism. The apparent Km value for CH4 oxidation significantly increased with the NH4+ load. This observation was indicative of an increased pMMO-based oxidation of NH3 to toxic hydroxylamine. In consequence, the detoxifying activity of hydroxlyamine oxidoreductase (HAO) increased with the NH4+ concentration and led to a significant accumulation of NO2− and, with delay, N2O. Significant production of N2O occurred only after the oxygen concentration had dropped to low or unmeasurable levels. Thus, high NH4+ load had a dual effect on the activity of strain SC2, with one being general phenomenon of ionic-osmotic stress and the other being the competitive inhibition effect of NH3 on pMMO-based methane oxidation. Although strain SC2 precisely rebalanced enzymes and osmolyte composition in response to the increase in NH4+ load, the need to simultaneously combat both ionic-osmotic stress and the toxic effects of hydroxylamine may be the reason why its acclimatization capacity is limited to 75 mM NH4+. Starting point of my second project was the knowledge that the growth of strain SC2 is completely inhibited at medium concentrations  1.5% NaCl. Sodium chloride is an important ionic-osmotic stressor in bulk and rhizosphere soils. We therefore tested various amino acids and other osmolytes for their potential to act as a compatible solute or osmoprotectant under otherwise inhibitory NaCl conditions. The addition of 10 mM asparagine to the growth medium had the greatest stress relief effect under severe salinity (1.5% NaCl), leading to a partial growth recovery of strain SC2. The analysis of the exo-metabolome revealed that asparagine was taken up quantitatively by strain SC2. This resulted in an intracellular concentration of 264 ± 57 mM asparagine. Under severe salinity (1.5% NaCl), the uptake of asparagine induced major proteome rearrangements related to the KEGG level 2 categories energy metabolism, amino acid metabolism, and cell growth and death. In particular, various proteins involved in cell division and peptidoglycan synthesis showed a positive expression response. The incorporation of asparagine-derived 13C-carbon into nearly all amino acids indicated that asparagine acted as a source for cell biomass under severe salinity (1.5% NaCl), with glutamate being a major hub between central carbon and amino acid pathways

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

Modeling of counter-current spontaneous imbibition in independent capillaries with unequal diameters

Spontaneous imbibition is a crucial process for oil recovery from fractured and unconventional reservoirs. Herein, with the assumption of capillaries being independent, a new mathematical model for spontaneous imbibition is proposed and solved using a numerical method. The simulated results show that the wetting phase preferentially enters smaller capillaries where the advancement velocity is higher than that in larger ones, while the non-wetting phase can be displaced out in the larger capillaries. In addition, the effect of fluid viscosity ratio on counter-current imbibition is analyzed. The results show that imbibition velocity becomes higher with the increase in the viscosity ratio. When the viscosity of the non-wetting phase is larger than that of the wetting phase, the end pressure gradually increases as the imbibition front advances. In contrast, when the viscosity of the non-wetting phase is less than that of the wetting phase, the end pressure decreases with the infiltration. With a higher viscosity ratio of non-wetting and wetting phase, the heterogeneity of the interface advancement among different capillaries increases.Cited as: Chen, K., Xu, H., Zhang, Z., Meng, Q., Zhang, T. Modeling of counter-current spontaneous imbibition in independent capillaries with unequal diameters. Capillarity, 2022, 5(6): 115-122. https://doi.org/10.46690/capi.2022.06.0

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