The isolation of single layer graphene in 2004 by Geim and Novoselov introduced a method that researchers could extend to other van der Waals materials. Interesting and new properties arise when we reduce a crystal to two dimensions where they are often different from their bulk counterpart. Due to the van der Waals bonding between layers, these single sheets of crystal can be combined and stacked with diferent sheets to create novel materials. With the goal to study the interesting physics associated to these stacks, the focus of this work is on the fabrication and characterization of van der Waals heterostructures. In this work, we first present a brief history of 2D materials, the fabrication of heterostructures, and the various tools used to characterize these materials. We then give a description of the custom-built instrument that was used to assemble various 2D heterostructures followed by the findings associated with the optimization of the cleanliness of the stack's interface and surface. Finally, we discuss the results related to the twisting of adjacent layers of stacked MoS2 and its relation to the interlayer coupling between said layers.

ESPINOZA HERRERA S.J. - DOCTORAL THESIS - FUNCTION OF MAGNESIUM IONS IN THE FORMATION OF BIOLOGICALLY-ACTIVE NUCLEIC ACIDS STRUCTURES ____________________________________________________________________ ABSTRACT Interaction of magnesium ions, the divalent ions with the highest concentration inside living cells, with the key biomolecules of nucleic acids was studied with aim to find out structural details of the interaction mechanism and effect of the magnesium ions presence on the nucleic-acid structure and structural transitions. Raman spectroscopy was chosen as the main experimental method, some complementary measurements of UV absorption spectra were also performed. Basic analyses of the spectral series obtained as a function of temperature and/or for various concentrations of Mg2+ ions were done by means of factor analysis. Where possible the factor analysis outputs were used for fits of appropriate thermodynamic equations. For the studies, various nucleic-acid molecular models were employed, in particular complementary RNA homopolynucleotides polyA and polyU, DNA homopolynucleotides polydA and polydT and RNA 14-mer oligonucleotide representing the apical hairpin of the TAR segment of HIV-1 genomic RNA. We have confirmed via Raman spectroscopy that magnesium ions added to a solution of polyA-polyU...

Guanine quadruplexes are a class of unusual nucleic acids conformations based on stacked planar guanine tetrads stabilized via Hoogsteen pairing and cation coordination. They are implicated in numerous cellular processes including replication, recombination or transcription. Guanine quadruplexes are widespread within the human genome but their occurrence is highest in the single stranded guanine-rich regions at telomeres. Telomeric guanine quadruplexes are gaining growing interest due to their ability to inhibit the activity of the telomerase enzyme, which is responsible for the proliferation of tumor cells. Specifically, we investigated the conformational polymorphism of the human core telomeric sequence G3(TTAG3)3 conditioned by the concentration of DNA, metal cations (K+ , Na+ ) and/or annealing. Raman spectroscopy was employed as the primary method for this study because, unlike common spectroscopic methods, it allowed us to monitor the quadruplex structure at very high DNA concentrations mimicking molecular crowding conditions in the cell. We demonstrate that the G3(TTAG3)3 quadruplex switch between the antiparallel and parallel strand alignment as a function of nucleoside and potassium concentration. In addition, we demonstrate that cationic porphyrins can be used as sensitive probes of the quadruplex...

In this thesis the properties of graphene and its few-layers are engineered to make them highly conductive. Two different approaches were implemented to achieve this goal. One approach was to increase the concentration of charge carriers by intercalation of acceptor FeCl3 molecules between graphene planes. This resulted in a highly conductive yet transparent material which can be useful for applications. Another approach was to increase the mobility of carriers by means of removing surface contamination in the current annealing process. Optimal annealing parameters were found and a reproducible cleaning method was suggested.

For the past three decades, newly discovered carbon nanostructures such as fullerenes, graphene and carbon nanotubes (CNTs) have revolutionized the field of nanoscience, introducing many practical and potential applications pertaining to their exceptional structural, mechanical, thermal, and optoelectronic properties. Raman spectroscopy has been an instrumental technique for characterizing these materials due to its non-destructive nature and high sensitivity to the material responses. While Raman spectroscopy is broadly used for identifying specific material types and quality, it has also been increasingly useful as a tool for probing the electronic and excitonic properties, as well as their interplay with the vibrational properties in the aforementioned carbon nanomaterials. In this dissertation, we present our Raman-related research on carbon nanotubes and a new member of the nano-carbon family - carbon nanohoops (cycloparaphenylenes, or CPPs). We discuss our new findings on the resonance Raman spectroscopy (RRS) of various semiconducting CNTs, with the focus on the Raman excitation profiles (REPs) for the G-band. The asymmetric lineshapes observed in the G-band REPs for the second excitonic (E22) transition of these CNTs contradict a long-held approximation, the Franck-Condon principle, for the vibronic properties of the carbon nanotubes. In addition, the G-band REPs from the closely spaced E33 and E44 transitions are investigated, and we demonstrate that these excitonic levels exhibit significant quantum interference effects between each other. We also present the first comprehensive study of Raman spectroscopy of CPPs. Analogously to CNTs, we show that Raman spectroscopy can be used to identify CPPs of different sizes. A plethora of Raman modes are observed in these spectra, including modes that are comparable to those of CNTs, such as the G-band, as well as Raman peaks that are unique for CPPs. Calculated Raman spectra using density functional theory (DFT) are compared with the experimental results for the assignment of different modes. Furthermore, we refine our knowledge of the CPP Raman modes by concentrating on the even-numbered CPPs. By taking advantage of the symmetry arguments in the even [n]CPPs, we are able to utilize group theory and accurately identify the size dependences of different Raman-active modes.

Surface-Enhanced Raman Scattering (SERS) has the potential to be a rapid disease diagnostic platform. SERS is a well-known ultrasensitive, label-free method for the detection and identification of molecules at low concentrations. The Raman cross-sections are primarily enhanced by plasmonic effects for molecules close to (< 5 nm) the surface of nanostructured metal substrates. Due to the unique Raman vibration features that provide molecular signatures, we have shown that SERS can provide a rapid (< one hour), label-free, sensitive and specific diagnosis for a number of diseases. This work demonstrates the capability of SERS to be an effective optical diagnostic approach, in particular, for bacterial infectious diseases such as urinary tract infections (UTI) and sexually transmitted diseases (STD), and cancer cell identification. More specifically, this work demonstrates the ability of SERS to distinguish different vegetative bacterial cells with species and strain specificity based on their intrinsic SERS molecular signatures. With the exception of C. trachomatis - the causative agent of chlamydia - whose SERS molecular signatures are found to be aggregated proteins on the cell membrane, all bacterial SERS molecular signatures are due to purine molecules resulting from nucleic acid metabolism as part of the rapid onset of the starvation response of these pathogens. The differences in relative contribution of different purine metabolites for each bacterium gives rise to the SERS strain and species specificity. The ability of SERS to distinguish cancer and normal cells grown in vitro based on changes of SERS spectral feature as a function of time after sample processing is also demonstrated. Furthermore, the difference of spectral features on the gold and silver SERS substrate of the same bacteria can be used as additional attribute for identification. This work demonstrate the potential of SERS platform to provide antibiotic-specific diagnostics in clinical settings within one hour when combined with a portable Raman microscopy instrument, an effective enrichment procedure, multivariate data analysis and an expendable SERS reference library with drug-susceptibility profile for each bacterial strain determined a priori, as well as the ability of SERS platform as a powerful bioanalytical probe for learning about near cell membrane biochemical processes.

>Magister Scientiae - MSc / The efficiency of organic photovoltaics is influenced by the structure of the polymer, the morphology of the film, the interfaces between the layers, the choice of electron acceptor material and the ratio between the electron acceptor material and the polymer. In this project, we have used regioregular poly (3-hexylthiophene) (rr-P3HT) as the electron donor material and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), a derivative of the C60 fullerene, as the acceptor material. Different weight-ratios of rr-P3HT to PCBM were prepared by stirring for several hours in a chloroform solution and subsequently spin coated onto crystalline silicon and transparent conductive oxide/glass substrates. The effect of the PCBM concentration on the photo-induced optical emission and absorption properties of rr-P3HT was investigated by photoluminescence and ultraviolet-visible spectroscopy, respectively. Changes in the structural properties, as a function of the weight-ratio, were probed by high-resolution transmission electron microscopy, x-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. Results show that the structural integrity and crystallinity of rr-P3HT is compromised with the addition of excessive amounts of PCBM, which has a negative impact on the optical absorption of rr-P3HT and the photo-induced charge transfer mechanism between the rr-P3HT and PCBM. This work illustrates that blending rr-P3HT with an equal weight of PCBM results in an optimum configuration for improved performance in organic photovoltaic devices.

Submitted by Suelen Reis (suziy.ellen@gmail.com) on 2013-04-23T17:39:54Z No. of bitstreams: 1 Tese - Gilberth Acioli.pdf: 3536665 bytes, checksum: a37b496b111b3eb77680d5e81b8fac30 (MD5) / Approved for entry into archive by Rodrigo Meirelles(rodrigomei@ufba.br) on 2013-05-08T12:01:41Z (GMT) No. of bitstreams: 1 Tese - Gilberth Acioli.pdf: 3536665 bytes, checksum: a37b496b111b3eb77680d5e81b8fac30 (MD5) / Made available in DSpace on 2013-05-08T12:01:42Z (GMT). No. of bitstreams: 1 Tese - Gilberth Acioli.pdf: 3536665 bytes, checksum: a37b496b111b3eb77680d5e81b8fac30 (MD5) Previous issue date: 2010 / O objetivo deste estudo foi avaliar o efeito da fotobiomodulação laser (780nm, 50mW, 4x4J/cm2 = 16J/cm2, ϕ 0,5cm2, CW) associada ou não a implante de Enxerto Ósseo Cerâmico Bifásico e Reparação Óssea Guiada através da técnica de histologia, histomorfometria, espectroscopia Raman e Fluorescência laser no reparo de fraturas cirúrgicas fixadas com o sistema de fixação rígida (miniplacas) ou semi-rígida (fio de aço) em tíbias de coelhos. Foram utilizados 27 coelhos Oryctolagus que foram divididos em nove grupos e mantidos em gaiolas individuais em temperatura média de 22°C, ambientação dia/noite, alimentação sólida e água ad libidum. As fraturas foram produzidas sob anestesia geral (Ketamina 0,4ml/Kg IP e Xilazina 0,2ml/Kg IP). No período pós operatório os mesmos receberam em dose única, como terapia antimicrobiana (Pentabiótico 0,2ml/Kg IM) e como terapia antiinflamatória e analgésica (Banamine 0,1ml/Kg IM). Nos grupos II, III, IV e V foram realizadas as fraturas e os cotos ósseos fixados com sistema rígido (FIR). Nos grupos VI, VII, VIII e IX a fratura foi realizada e logo depois fixada com sistema semi-rígido (FISR). Em seguida, foi feita a colocação do enxerto e da membrana nos grupos III, V, VII e IX. Os animais dos grupos IV, V, VIII e IX foram irradiados durante 14 (catorze) dias, a cada 48 horas com uma dose de 16J/cm2, de forma pontual em 4 (quatro) regiões adjacentes a área da fratura óssea (4 x 4J/cm2). Os animais foram sacrificados no 30° dia pós-operatório através de overdose de anestesia geral (Ketamina e Xilazina IP) e administração de Cloreto de Potássio (5ml/Kg, IV). Em seguida os espécimes foram removidos, sendo metade encaminhado para análises histológica e histomorfométrica e a outra metade para análise por espectroscopia Raman. Antes da cirurgia e da morte animal a fluorescência laser foi medida. Histologicamente, observou-se um preenchimento das fraturas por um trabeculado ósseo maduro nos grupos onde houve o uso da associação laser, HATCP e ROG nos grupos tratados com FIR e FISR. Histomorfometricamente verificou-se maior neoformação óssea e maior deposição de colágeno, menor quantidade reabsorção óssea e de infiltrado inflamatório nos grupos nos quais o laser foi associado a HATCP. As análises por fluorescência laser (DIAGNOdent®) e por espectroscopia Raman, observaram-se diferenças significantes entre os grupos (p<0.001) entre os grupos tratados com FIR e FISR. A correlação de Pearson evidenciou uma correlação negativa entre as medidas de fluorescência e deslocamento Raman. Concluiu-se que a fotobiomodulação Laser infravermelho acelerou o reparo de fraturas ósseas e que quando o laser foi associado a HATCP e ROG esta causou aumento da deposição da HAC. Adicionalmente, o uso do DIAGNOdent® como instrumento de biópsia óptica pode ser útil. / Salvador

PEREIRA, José Enedilton Medeiros. Espectroscopia vibracional em cristais do ácido trans-cinâmico complexado com európio sob condições extremas. 2014. 90 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-09-12T19:42:18Z No. of bitstreams: 1 2014_dis_jempereira.pdf: 4086204 bytes, checksum: f746c208810c91b5b87b4ef1b2f4de4a (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-09-12T19:50:42Z (GMT) No. of bitstreams: 1 2014_dis_jempereira.pdf: 4086204 bytes, checksum: f746c208810c91b5b87b4ef1b2f4de4a (MD5) / Made available in DSpace on 2014-09-12T19:50:42Z (GMT). No. of bitstreams: 1 2014_dis_jempereira.pdf: 4086204 bytes, checksum: f746c208810c91b5b87b4ef1b2f4de4a (MD5) Previous issue date: 2014 / This work is focused on characterization of a polycrystalline sample of the coordination compound with ions Eu3+ using as a binder the trans-cinnamic acid , [Eu(cin)3]. The interest in coordination with lanthanide compounds is due to their large technological applications. The process of luminescence of coordination compounds containing lanthanide, also called antenna effect, has a main objective the use of new ligands with various applications, both in increasing the efficiency of energy receptors in the ultraviolet region and the development of biomarkers. In this work, to characterize the polycrystalline material, Raman spectroscopy was used to determine the vibrational behavior of the material at ambient temperature and pressure. It was also shown the behavior of the compound at low temperatures, in which variations in various vibrational modes were observed, indicating thus the occurrence of phase transition. Using a diamond anvil cell the material was studied by high hydrostatic pressure Raman spectroscopy. The purpose of this methodology was to map possible phase transitions and discover the occurrence of possible polymorphism. However, until the pressure reached in the experiments - about 5.2 GPa - it was found that the original phase remained stable. A discussion of the luminescence of the sample is also provided in this work. / O presente trabalho está focado na caracterização de uma amostra policristalina do composto de coordenação com íons Eu3+ utilizando como ligante o ácido trans-cinâmico, [Eu(cin)3]. O interesse em compostos de coordenação com lantanídeos deve-se às suas amplas aplicações tecnológicas. O processo de luminescência dos compostos de coordena- ção contendo lantanídeo, também chamado de efeito antena, tem como principal objetivo o uso de novos ligantes com aplicações diversas, tanto no aumento da eficiência de re- ceptores de energia na região do ultravioleta quanto no desenvolvimento de marcadores biológicos. Neste trabalho, para realizar a caracterização do material policristalino, foi utilizada a espectroscopia Raman, a fim de determinar o comportamento vibracional do material à temperatura e pressão ambiente. Também foi verificado o comportamento do composto sob baixas temperaturas, no qual foram observadas variações em vários modos vibracionais, indicando, por conseguinte, a ocorrência de transição de fase. Utilizando uma célula de pressão a extremos de diamante o material foi estudado a altas pressões hidrostáticas através de espectroscopia Raman. O intuito desta metodologia era mapear possíveis transições de fase e descobrir a ocorrência de eventual polimorfismo. Entretanto, até a pressão atingida nos experimentos – cerca de 5,2 GPa – verificou-se que a fase ori- ginal permaneceu estável. Uma discussão sobre a luminescência da amostra também é fornecida neste trabalho.

MAIA, José Robson. Estudo da cafeína anidra sob condições extremas através de Espectroscopia Raman. 2010. 82 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2010. / Submitted by francisco lima (admir@ufc.br) on 2014-06-30T17:14:06Z No. of bitstreams: 1 2010_dis_jrmaia.pdf: 2070388 bytes, checksum: 91f8d25cd08ec0e46e3c280ebd10e9d0 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-08-06T17:23:39Z (GMT) No. of bitstreams: 1 2010_dis_jrmaia.pdf: 2070388 bytes, checksum: 91f8d25cd08ec0e46e3c280ebd10e9d0 (MD5) / Made available in DSpace on 2014-08-06T17:23:39Z (GMT). No. of bitstreams: 1 2010_dis_jrmaia.pdf: 2070388 bytes, checksum: 91f8d25cd08ec0e46e3c280ebd10e9d0 (MD5) Previous issue date: 2010 / In this work anidrous cafeine (C8H10N4O2) crystals were investigated through the Raman spectroscopy technique, with change of temperature and pressure, in the spectral region between 30 and 3200 cm-1 In the temperature experiments the thermodynamic parameter changed from 23 and 170 0C. From them it was possible to realize that anidrous cafeine undergoes a structural phase transition at about 140 0C, as pointed by the disappearance of three bands associated to external modes. After sample goes back to room temperature, it is observed that the original Raman spectrum was not recovered, indicating that the transition is not reversible. Thermal analysis experiment confirms the structural phase transition undergone by cafeine. In order to perform high pressure experiments we have used a diamond anvil cell, being possible to arrive to pressures up to 10.6 GPa. In the high pressure Raman experiments it was possible to observe a phase transition at about 1 GPa, as indicated by the appearance of a band at 37 cm-1 as well as a second phase transition between 6.2 and 7.5 GPa. This last phase transition was characterized by the discontinuity in the frequency band of two low energy. In the pressure range 7.3-10.6 GPa, the Raman scattering experiments performed on anidrous cafeine showed indicious of lost of long range order, which was interpreted as a possible amorphization of the sample. / No presente trabalho policristais de cafeína anidra (C8H10N4O2), foram investigados por meio de experimentos de espectroscopia Raman, com variação dos parâmetros termodinâmicos temperatura e pressão, em uma região espectral entre 30 e 3200 cm-1. Nos experimentos de espectroscopia Raman realizados variando a temperatura entre 23 e 170 °C, verificou-se que a cafeína anidra sofreu uma transição de fase estrutural em torno de 140 °C, acompanhada pelo desaparecimento de três dos modos externos. A análise dos espectros Raman obtidos após o retorno a temperatura ambiente revelaram que a cafeína não recupera a fase beta. O experimento de análise térmica confirma a transição de fase estrutural sugerida pelos experimentos de espectroscopia Raman a altas temperaturas. Nos experimentos de altas pressões utilizando-se uma célula de pressão a extremos de diamantes os cristais foram submetidos a pressões de até 10,6 GPa. Em tais experimentos pode-se observar uma transição de fase, em torno de 1 GPa devido ao surgimento de uma banda Raman em aproximadamente 37 cm-1 e uma outra transição em um intervalo de pressão compreendido entre 6,2 e 7,5 GPa, relacionado a uma descontinuidade na freqüência de duas bandas de baixa energia. Para o intervalo de pressão entre 7,3 e 10,6 GPa os experimentos de espectroscopia Raman realizadas sobre policristais de cafeína anidra mostraram fortes indícios de perda da ordem de longo alcance o que foi interpretado como uma possível amorfização da amostra.