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Two dimensional (2D) materials, including graphene, hexagonal boron nitride and layered transition metal dichalcogenides (TMDs), have been a revolution in condensed matter physics and they are at the forefront of recent scientific research. They are being explored for their unusual electronic, optical and magnetic properties with special interest in their potential uses for sensing, information processing and memory. Molybdenum disulfide (MoS2) has been the flagship semiconducting TMD over the past ten years due to its unique electronic, optical and mechanical properties. In this thesis, we grow mono- to few layer MoS2 films using ambient pressure chemical vapor depositions (AP-CVD) to obtain high quality samples. We employ low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) to study the effect of layer number on the electronic density of states (DOS) of MoS¬2. We find a reduction of the magnitude of the quasiparticle band gap from one to two monolayers (MLs) thick. This reduction is found to be due mainly to a shift of the valence band maxima (VBM) where the conduction band minimum (CBM) does not change dramatically. Density functional theory (DFT) modeling of this system shows that the overlap of the interfacial S-pz orbitals is responsible for shifting the valence band edge at the Γ-point toward the Fermi level (EF), reducing the magnitude of the band gap. Additionally, we show that the crystallographic orientation of monolayer MoS2 with respect to the HOPG substrate can also affect the electronic DOS. This is demonstrated with five different monolayer regions having each with a unique relative crystallographic orientation to the underlying substrate. We find that the quasiparticle band gap is closely related to the moiré pattern periodicity, specifically the larger the moiré periodicity the larger the band gap. Using DFT, we find that artificially increasing the interaction between the film and the substrate means that the magnitude of the band gap reduces. This indicates that the moiré pattern period acts like a barometer for interlayer coupling. We investigate the effect of defects, both point and extended defects, on the electronic properties of mono- to few layer MoS¬2 films. Atomic point defects such including Mo interstitials, S vacancies and O substitutions are identified by STM topography. Two adjacent defects were investigated spectroscopically and found to greatly reduce the quasiparticle band gap and arguments were made to suggest that they are Mo-Sx complex vacancies. Similarly, grain boundaries were found to reduce the band gap to approximately ¼ of the gap found on the pristine film. We use Kelvin probe force microscopy (KPFM) to investigate the affect of annealing the films in UHV. The work function measurements show metastable states are created after the annealing that relax over time to equilibrium values of the work function. Scanning transmission electron microscopy (STEM) is used to show that S vacancies can recombine over time offering a feasible mechanism for the work function changes observed in KPFM. Lastly, we report how strain affects the quasiparticle band gap of monolayer MoS2 by bending the substrate using a custom built STM sample holder. We find that the local, atomic-scale strain can be determined by a careful calibration procedure and a modified, real-space Lawler Fujita algorithm. We find that the band gap of MoS2 reduces with strain at a rate of approximately 400 meV/% up to a maximum strain of 3.1%, after which the film can slip with respect to the substrate. We find evidence of this slipping as nanoscale ripples and wrinkling whose local strain fields alter the local electronic DOS. / Physics

Abstract Density functional theory (DFT) is a well-established tool for calculating the properties of materials. The volume of DFT-related publications doubles every 5–6 years, which has resulted in the appearance of continuously growing open material databases, containing information on millions of compounds. Furthermore, the results of DFT computations are frequently coupled with experimental ones to strengthen the computational findings. In this thesis, several applications of DFT related to physics and chemistry are discussed. The conductivity between MoS₂ and transition metal nanoparticles is evaluated by calculating the electronic structure of two different models for the nanoparticles. Chemical bonding of Ni to the MoS₂ host is proven by the system’s band alignment. To meet the demand for cleaner fuel, the applicability of the (103) edge surface of molybdenum disulfide in relation to the early stages of the hydrodesulfurization (HDS) reaction is considered. The occurrence of the (103) edge surface of molybdenum disulfide in the XRD patterns is explained. A method for calculating oxidation states based on partial charges using open materials databases is suggested. We estimate the applicability of the method in the case of mixed valence compounds and surfaces, showing that DFT calculations can be used for the estimation of oxidation states. / Tiivistelmä Tiheysfunktionaaliteoria (density functional theory, DFT) on yleisesti käytetty työkalu laskennallisessa materiaalitutkimuksessa. DFT:llä tuotettujen julkaisujen määrä kaksinkertaistuu 5–6 vuoden välein, minkä johdosta käytettävissä on jatkuvasti kasvava määrä avoimia materiaalitietokantoja, joihin on talletettu miljoonien yhdisteiden ominaisuuksia. DFT-laskujen tuloksia täydennetään myös usein kokeellisilla tuloksilla. Tässä työssä tarkastellaan tiheysfunktionaaliteorian sovelluksia fysiikassa ja kemiassa. MoS₂:n ja metallisten nanopartikkelien välistä johtavuutta on tutkittu mallintamalla erilaisia nanopartikkeleita. Nikkelin ja MoS₂:n välinen kemiallinen sidos selittyy systeemin energiavöiden kohdistumisella. MoS₂:n (103)-pinnan soveltuvuutta rikinpoistoreaktion varhaisissa vaiheissa on tutkittu tarkoituksena löytää uusia menetelmiä puhtaan polttoaineen tuottamiseksi. Myös (103)-pinnan esiintyminen röntgendiffraktiokuvissa selitetään. Työssä on myös esitetty menetelmä hapetustilojen laskemiseksi tietokannoista löytyvien laskettujen varausjakaumien avulla. Menetelmän soveltuvuutta on tarkasteltu erilaisille yhdisteille ja pinnoille. Tämä tarkastelu osoittaa, että DFT-tuloksia voidaan käyttää hapetustilojen laskemiseen.

ALENCAR, Rafael Silva. Estudo de fônons em nanotubos de carbono e dissulfeto de molibdênio: efeito do acoplamento entre camadas. 2016. 116 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2016. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2016-07-04T18:55:57Z No. of bitstreams: 1 2016_tese_rsalencar.pdf: 15208196 bytes, checksum: 0879a7d3b08f7d02d899e3d78888e828 (MD5) / Approved for entry into archive by Edvander Pires (edvanderpires@gmail.com) on 2016-07-04T18:56:56Z (GMT) No. of bitstreams: 1 2016_tese_rsalencar.pdf: 15208196 bytes, checksum: 0879a7d3b08f7d02d899e3d78888e828 (MD5) / Made available in DSpace on 2016-07-04T18:56:56Z (GMT). No. of bitstreams: 1 2016_tese_rsalencar.pdf: 15208196 bytes, checksum: 0879a7d3b08f7d02d899e3d78888e828 (MD5) Previous issue date: 2016 / In this work we present the studies on Double (DWCNTs) and Triple Wall Carbon Nanotubes, on molybdenum disulfide in the bulk form and on few layer of MoS_2 under hydrostatic high pressure conditions. Theoretical calculations were performed in collaboration to support the experimental results. For the DWCNTs samples, changes in the G-band frequency vs. pressure plot and the disappearance of the radial breathing modes (RBM) between 2 GPa and 5 GPa indicate the beginning and ending of the radial collapse of the nanotubes. Theoretical calculations based on Density-Functional Tight-Binding (DFTB) shown that the collapse pressure (P_c) for DWCNTs follows a d^{-3}_{in} law, in excellent agreement with the experimental results. The P_c dependence on number of tube-walls and on the inter-wall distance is also investigated. For the TWCNTs samples, pressure screening effects are observed for the innermost tubes of TWCNTs similar to what has been already found for DWCNTs. However, using the RBM pressure coefficients in conjunction with the histogram of the diameter distribution, we were able to separate the RBM Raman contribution related to the intermediate tubes of TWCNTs from that related to the inner tubes of DWCNTs. By combining Raman spectroscopy and high pressure measurements, it was possible to identify these two categories of inner tubes even if the two tubes exhibit the same diameters, since their pressure response is different. Furthermore, it was possible to observe similar RBM profiles of the innermost tubes of TWCNTs using different resonance laser energies but also under different pressure conditions. This is attributed to changes in the electronic transition energies caused by small pressure-induced deformations. Theoretical calculations based on ab initio were performed for support the experimental results. By using Raman spectroscopy, it was possible to estimate the displacement of the optical energy levels with pressure. For the exfoliated MoS_2 samples, we studied the effect of the stacking on the E^1_{2g} and A_{1g} vibrational modes at high pressures. New components for both modes were observed with increasing pressure. It was also observed that the pressure coefficient of the E^1_{2g} mode decreases exponentially with MoS_2 thickness is increased, differently of the A_{1g} mode and the new components, which do not present a significant dependence on the variation of the number of layers. These results were attributed to deformations in the MoS_2 structure induced by a biaxial strain (dependent on the number of layers), resulting from the deformation of the SiO_2 substrate. Such adhesion decreases with the increasing of the MoS_2 thickness due to the increasing on the unbinding regions between MoS_2 and SiO_2. As result, a higher pressure is needed to improve the adhesion and consequently, a higher pressure is required to achieve the biaxial strain. For the MoS_2 microcrystalline powder, except for the B_{1u}, E^2_{2g}, E_{1g}, E^1_{2g} and A_{1g} modes, the behavior of all other modes was studied for the first time in high pressure conditions. For all modes, a linear variation of the Raman frequency and positive pressure coefficient was observed. Moreover, the differences in the behavior of the intensity profiles of the A_{1g}, 2LA(M) and A_{2u} modes in resonance and off-resonance were attributed to variations in the energy of direct optical transitions induced by pressure. / Nesta Tese apresentamos os estudos de espectroscopia Raman em condições extremas de pressão hidrostática realizados em nanotubos de carbono de parede dupla (DWCNTs) e tripla (TWCNTs), em dissulfeto de molibdênio na forma bulk e em poucas camadas. Cálculos teóricos foram usados para dar suporte aos resultados experimentais. Para as amostras de DWCNTs, as mudanças no coeficiente de pressão da banda G e o desaparecimento dos modos de respiração radial (RBMs) entre 2 GPa e 5 GPa foram interpretados como um indicativo do início e fim do colapso radial dos nanotubos de carbono (CNTs). Os cálculos teóricos usando Tight-Binding baseado no Funcional da Densidade (DFTB) mostraram que a pressão de colapso (P_c) para os DWCNTs segue uma lei de potência do tipo d^{-3}_{in}, em excelente acordo com os resultados experimentais. A dependência de P_c em relação ao número de paredes do tubo, como também a distância inter-paredes também foram investigadas. Para a amostra contendo TWCNTs, através da análise dos coeficientes de pressão dos modos RBMs em conjunto com o histograma da distribuição de diâmetros da amostra, foi possível separarmos as contribuições dos RBMs nos espectros Raman relacionados aos tubos internos dos TWCNTs e DWCNTs, embora possuam a mesma distribuição de diâmetro, a resposta das propriedades vibracionais à pressão são diferentes. Adicionalmente, foi possível observar perfis de intensidades semelhantes para os modos RBMs dos tubos mais internos dos TWCNTs usando diferentes energias de LASER, mas sob diferentes condições de pressão. Atribuímos este resultado à mudanças nas energias de transições eletrônicas causadas por pequenas deformações estruturais nos nanotubos induzidas pela pressão. Cálculos teóricos baseados em ab initio foram realizados para dar suporte às interpretações dos resultados experimentais. Para as amostras de MoS_2 esfoliadas, estudamos o efeito do empilhamento nos modos vibracionais E^1_{2g} e A_{1g} em altas pressões. Novas componentes para esses modos foram observadas com o aumento da pressão. Foi também observado que o coeficiente de pressão do modo E^1_{2g} diminui exponencialmente com o aumento do número de camadas, diferentemente do modo A_{1g} e das novas componentes, que não apresentam uma dependência significativa com a variação da espessura do MoS_2. Atribuímos estes resultados às deformações estruturais do MoS_2 induzidas por uma tensão biaxial (dependente da adesão entre SiO_2 e MoS_2) resultante da deformação do substrato de SiO_2. O aumento do número de camadas diminui a adesão entre o MoS_2 e o SiO_2 devido ao aumento da porcentagem de regiões em não-contato com o substrato, e como consequência, uma pressão mais elevada é necessária para aumentar a adesão, resultando no aumento da pressão para deformar a estrutura do MoS_2. Para o pó microcristalino de MoS2, com exceção dos modos B_{1u}, E^2_{2g}, E1g, E^1_{2g} e A_{1g}, o comportamento de todos os outros modos foi também estudado em condições de altas pressões hidrostáticas. Todos os modos apresentaram uma variação linear de suas frequências Raman com a pressão e coeficientes de pressão positivos. Além disso, as diferenças no comportamento dos perfis de intensidade dos modos A_{1g}, 2LA(M) e A_{2u} em ressonância e fora de ressonância foram interpretados como sendo devido às variações nas energias das transições ópticas direta induzidas pela pressão.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / Nesta Tese apresentamos os estudos de espectroscopia Raman em condiÃÃes extremas de pressÃo hidrostÃtica realizados em nanotubos de carbono de parede dupla (DWCNTs) e tripla (TWCNTs), em dissulfeto de molibdÃnio na forma bulk e em poucas camadas. CÃlculos teÃricos foram usados para dar suporte aos resultados experimentais. Para as amostras de DWCNTs, as mudanÃas no coeficiente de pressÃo da banda G e o desaparecimento dos modos de respiraÃÃo radial (RBMs) entre 2 GPa e 5 GPa foram interpretados como um indicativo do inÃcio e fim do colapso radial dos nanotubos de carbono (CNTs). Os cÃlculos teÃricos usando Tight-Binding baseado no Funcional da Densidade (DFTB) mostraram que a pressÃo de colapso (P_c) para os DWCNTs segue uma lei de potÃncia do tipo d^{-3}_{in}, em excelente acordo com os resultados experimentais. A dependÃncia de P_c em relaÃÃo ao nÃmero de paredes do tubo, como tambÃm a distÃncia inter-paredes tambÃm foram investigadas. Para a amostra contendo TWCNTs, atravÃs da anÃlise dos coeficientes de pressÃo dos modos RBMs em conjunto com o histograma da distribuiÃÃo de diÃmetros da amostra, foi possÃvel separarmos as contribuiÃÃes dos RBMs nos espectros Raman relacionados aos tubos internos dos TWCNTs e DWCNTs, embora possuam a mesma distribuiÃÃo de diÃmetro, a resposta das propriedades vibracionais à pressÃo sÃo diferentes. Adicionalmente, foi possÃvel observar perfis de intensidades semelhantes para os modos RBMs dos tubos mais internos dos TWCNTs usando diferentes energias de LASER, mas sob diferentes condiÃÃes de pressÃo. AtribuÃmos este resultado à mudanÃas nas energias de transiÃÃes eletrÃnicas causadas por pequenas deformaÃÃes estruturais nos nanotubos induzidas pela pressÃo. CÃlculos teÃricos baseados em ab initio foram realizados para dar suporte Ãs interpretaÃÃes dos resultados experimentais. Para as amostras de MoS_2 esfoliadas, estudamos o efeito do empilhamento nos modos vibracionais E^1_{2g} e A_{1g} em altas pressÃes. Novas componentes para esses modos foram observadas com o aumento da pressÃo. Foi tambÃm observado que o coeficiente de pressÃo do modo E^1_{2g} diminui exponencialmente com o aumento do nÃmero de camadas, diferentemente do modo A_{1g} e das novas componentes, que nÃo apresentam uma dependÃncia significativa com a variaÃÃo da espessura do MoS_2. AtribuÃmos estes resultados Ãs deformaÃÃes estruturais do MoS_2 induzidas por uma tensÃo biaxial (dependente da adesÃo entre SiO_2 e MoS_2) resultante da deformaÃÃo do substrato de SiO_2. O aumento do nÃmero de camadas diminui a adesÃo entre o MoS_2 e o SiO_2 devido ao aumento da porcentagem de regiÃes em nÃo-contato com o substrato, e como consequÃncia, uma pressÃo mais elevada à necessÃria para aumentar a adesÃo, resultando no aumento da pressÃo para deformar a estrutura do MoS_2. Para o pà microcristalino de MoS2, com exceÃÃo dos modos B_{1u}, E^2_{2g}, E1g, E^1_{2g} e A_{1g}, o comportamento de todos os outros modos foi tambÃm estudado em condiÃÃes de altas pressÃes hidrostÃticas. Todos os modos apresentaram uma variaÃÃo linear de suas frequÃncias Raman com a pressÃo e coeficientes de pressÃo positivos. AlÃm disso, as diferenÃas no comportamento dos perfis de intensidade dos modos A_{1g}, 2LA(M) e A_{2u} em ressonÃncia e fora de ressonÃncia foram interpretados como sendo devido Ãs variaÃÃes nas energias das transiÃÃes Ãpticas direta induzidas pela pressÃo. / In this work we present the studies on Double (DWCNTs) and Triple Wall Carbon Nanotubes, on molybdenum disulfide in the bulk form and on few layer of MoS_2 under hydrostatic high pressure conditions. Theoretical calculations were performed in collaboration to support the experimental results. For the DWCNTs samples, changes in the G-band frequency vs. pressure plot and the disappearance of the radial breathing modes (RBM) between 2 GPa and 5 GPa indicate the beginning and ending of the radial collapse of the nanotubes. Theoretical calculations based on Density-Functional Tight-Binding (DFTB) shown that the collapse pressure (P_c) for DWCNTs follows a d^{-3}_{in} law, in excellent agreement with the experimental results. The P_c dependence on number of tube-walls and on the inter-wall distance is also investigated. For the TWCNTs samples, pressure screening effects are observed for the innermost tubes of TWCNTs similar to what has been already found for DWCNTs. However, using the RBM pressure coefficients in conjunction with the histogram of the diameter distribution, we were able to separate the RBM Raman contribution related to the intermediate tubes of TWCNTs from that related to the inner tubes of DWCNTs. By combining Raman spectroscopy and high pressure measurements, it was possible to identify these two categories of inner tubes even if the two tubes exhibit the same diameters, since their pressure response is different. Furthermore, it was possible to observe similar RBM profiles of the innermost tubes of TWCNTs using different resonance laser energies but also under different pressure conditions. This is attributed to changes in the electronic transition energies caused by small pressure-induced deformations. Theoretical calculations based on ab initio were performed for support the experimental results. By using Raman spectroscopy, it was possible to estimate the displacement of the optical energy levels with pressure. For the exfoliated MoS_2 samples, we studied the effect of the stacking on the E^1_{2g} and A_{1g} vibrational modes at high pressures. New components for both modes were observed with increasing pressure. It was also observed that the pressure coefficient of the E^1_{2g} mode decreases exponentially with MoS_2 thickness is increased, differently of the A_{1g} mode and the new components, which do not present a significant dependence on the variation of the number of layers. These results were attributed to deformations in the MoS_2 structure induced by a biaxial strain (dependent on the number of layers), resulting from the deformation of the SiO_2 substrate. Such adhesion decreases with the increasing of the MoS_2 thickness due to the increasing on the unbinding regions between MoS_2 and SiO_2. As result, a higher pressure is needed to improve the adhesion and consequently, a higher pressure is required to achieve the biaxial strain. For the MoS_2 microcrystalline powder, except for the B_{1u}, E^2_{2g}, E_{1g}, E^1_{2g} and A_{1g} modes, the behavior of all other modes was studied for the first time in high pressure conditions. For all modes, a linear variation of the Raman frequency and positive pressure coefficient was observed. Moreover, the differences in the behavior of the intensity profiles of the A_{1g}, 2LA(M) and A_{2u} modes in resonance and off-resonance were attributed to variations in the energy of direct optical transitions induced by pressure.

To achieve sustainable production of H2 fuel through water splitting, low-cost electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are required to replace Pt and IrO2 catalysts. Here, for the first time, we present the interface engineering of novel MoS2/Ni3S2 heterostructures, in which abundant interfaces are formed. For OER, such MoS2/Ni3S2 heterostructures show an extremely low overpotential of ~218 mV at 10 mA cm-2, which is superior to that of the state-of-the-art OER electrocatalysts. Using MoS2/Ni3S2 heterostructures as bifunctional electrocatalysts, an alkali electrolyser delivers a current density of 10 mA cm-2 at a very low cell voltage of ~1.56 V. In combination with density function theory (DFT) calculations, this study demonstrates that the constructed interfaces synergistically favor the chemisorption of hydrogen and oxygencontaining intermediates, thus accelerating the overall electrochemical water splitting.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) show great potential for the future electronics, optoelectronics and energy applications. But, the studies unveiling their interactions with the host substrates are sparse and limits their practical use for real device applications. We report the facile nano-scratch method to determine the adhesion energy of the wafer scale MoS2 atomic layers attached to the SiO2/Si and sapphire substrates. The practical adhesion energy of monolayer MoS2 on the SiO2/Si substrate is 7.78 J/m2. The practical adhesion energy was found to be an increasing function of the MoS2 thickness. Unlike SiO2/Si substrates, MoS2 films grown on the sapphire possess higher bonding energy, which is attributed to the defect-free growth and less number of grain boundaries, as well as less stress and strain stored at the interface owing to the similarity of Thermal Expansion Coefficient (TEC) between MoS2 films and sapphire substrate. Furthermore, we calculated the surface free energy of 2D MoS2 by the facile contact angle measurements and Neumann model fitting. A surface free energy ~85.3 mJ/m2 in few layers thick MoS2 manifests the hydrophilic nature of 2D MoS2. The high surface energy of MoS2 helps explain the good bonding strength at MoS2/substrate interface. This simple adhesion energy and surface energy measurement methodology could further apply to other TMDs for their widespread use.