The goal of this thesis work is to investigate the possibility to grow graphene by Chemical Vapor Deposition (CVD) on copper foil with acetylene as a precursor and varigon (5\% H$_2$ in Ar) as a carrier gas. The possibility of nitrogen doping by ammonia treatment during the growth process is also investigated. The possibility of graphene transfer, with the use of Poly(Methyl Metacrylate) (PMMA), from the copper onto another target substrate, Flourine doped Tin Oxide (FTO), is also explored. The main technique of characterization of the grown and transfered graphene is Raman spectroscopy, a great tool for investigating the number of graphene layers and amount of defects. Other characterization methods used are Scanning Electron Microscopy (SEM) X-ray Photoelectron Spectroscopy (XPS) to investigate morphology and elemental composition, respectively. The result of this thesis study is that graphene growth is entirely possible with acetylene as a precursor, as shown by the Raman spectroscopy, XPS and SEM. The grown graphene has a high quality with few layers and a low number of defects. The ammonia treatment, however, doesn't seem to have an immediate effect on the graphene growth. The XPS data indicates that there are no nitrogen doping in the graphene, though there might be a correlation between the ammonia and the number of layers, but further investigations has to be made. Transfer is also proven possible with the method developed. However, improvements to the transfer method can be done since there are both larger tares, caused by the transfer onto the FTO, as well as microscopic tares, possibly caused by thermal expansion of the PMMA.

Graphene base transistors (GBTs) have been, recently, proposed to overcome the intrinsic limitations of the graphene field effect transistors (GFETs) and exploit the graphene unique properties in high frequency (HF) applications. These devices utilize single layer graphene as the base material in the vertical hot-electron transistors. In an optimized GBT, the ultimate thinness of the graphene-base and its high conductivity, potentially, enable HF performance up to the THz region.  This thesis presents an experimental investigation on the GBTs as well as integration process developments for the fabrication of graphene-based devices. In this work, a full device fabrication and graphene integration process were designed with high CMOS compatibility considerations. To this aim, basic process modules, such as graphene transfer, deposition of materials on graphene, and formation of tunnel barriers, were developed and optimized. A PDMS-supporting graphene transfer process were introduced to facilitate the wet/dry wafer-scale transfer from metal substrate onto an arbitrarily substrate. In addition, dielectric deposition on graphene using atomic layer deposition (ALD) was investigated. These dielectric layers, mainly, served as the base-collector insulators in the fabricated GBTs. Moreover, the integration of silicon (Si) on the graphene surface was studied. Using the developed fabrication process, the first proof of concept devices were demonstrated. These devices utilized 5 nm-thick silicon oxide (SiO2) and about 20 nm-thick aluminum oxide (Al2O3) as the emitter-base insulator (EBI) and base-collector insulator (BCI). The direct current (DC) functionality of these devices exhibited &gt;104 on/off current ratios and a current transfer ratio of about 6%. The performance of these devices was limited by the non-optimized barrier parameters and device manufacturing technology. The possibility to improve and optimize the GBT performance was demonstrated by applying different barrier optimization approaches. Comparing to the proof of concept devices, several orders of magnitude higher injection current density was achieved using a bilayer dielectric tunnel barrier. Utilizing the novel TmSiO/TiO2 (1 nm/6 nm) dielectric stack, this tunnel barrier prevents defect mediated tunneling and, simultaneously, promotes the Fowler-Nordheim tunneling (FNT) and step tunneling (ST). Furthermore, it was shown that Si/graphene Schottky junction can significantly improve the current gain by reducing the electron backscattering at the base-collector barrier. In this thesis, a maximum current transfer ratio of about 35% has been achieved. / <p>QC 20160503</p>

[pt] Nesta tese de doutoramento apresentamos a fabricação e a caracterização de dispositivos fotovoltaicos orgânicos (OPVs) fabricados a partir de eletrodos de grafeno e de substratos híbridos flexíveis à base de polímeros recobertos com um filme fino condutor. Para isso, inicialmente sintetizamos filmes de grafeno através da técnica de deposição química em fase de vapor (CVD), seguido de modificações no processo de transferência do grafeno para o substrato desejado. Nesta etapa, desenvolvemos uma nova metodologia utilizando uma blenda condutora de EPDM-PAni que simplifica o processo de transferência e melhora as propriedades elétricas do grafeno. Em outro momento, otimizamos diferentes substratos híbridos à base de polímeros de PVC, PVA e celulose bacteriana (BC) recobertos com um filme fino condutor de ITO. Tanto os substratos híbridos flexíveis, quanto os filmes de grafeno, foram investigados por transmitância ótica e resistência de folha a fim de avaliar os seus potenciais uso para as aplicações em OPVs. Por fim, fabricamos diversas estruturas de OPVs, tanto com o grafeno como eletrodo condutor, quanto usando os substratos híbridos flexíveis. Estes dispositivos foram caracterizados principalmente através das suas curvas características JxV, no escuro e sob iluminação. Além disso, realizamos ciclos de flexão/extensão de alguns dispositivos a fim de avaliar seu comportamento frente aos esforços mecânicos a estes submetidos. Os resultados obtidos mostraram que os filmes de grafeno fabricados são promissores para a aplicação como eletrodo condutor transparente em OPVs e que os substratos híbridos investigados podem ser utilizados em dispositivos flexíveis, visto que apresentaram comportamento semelhante aos substratos inorgânicos comumente utilizados. / [en] In this doctoral thesis we present the fabrication and characterization of organic photovoltaic devices (OPVs) assembled onto graphene electrodes and flexible hybrid polymers-based substrates coated with a conductive thin film. For this, initially the graphene films were synthesized by chemical vapor deposition (CVD) technique, followed by modifications in the transfer process of the graphene to the desired substrate. In this step, we developed a new methodology using an EPDM-PAni conductive blend that simplifies the transfer process and improves the electric properties of graphene. We also used another approach which consists in optimizing different hybrid substrates based on PVC, PVA and bacterial cellulose (BC) polymers coated with an ITO conductive thin film. The flexible hybrid substrates as well as the graphene films were investigated by optical transmittance and sheet resistance in order to evaluate their potential use for OPVs applications. Finally, we fabricate various structures of OPVs, using graphene as a conducting electrode, well as using flexible hybrid substrates. Such devices were characterized mainly through their dark and light J×V characteristic curves. In addition, we performed flexion/extension cycles in some devices in order to evaluate their behavior against the mechanical stresses submitted to them. The results showed that the graphene films are a promising material for the application as a transparent conductive electrode in OPVs and the hybrid substrates investigated can be used in flexible devices, since they presented similar behavior to the commonly used inorganic substrates.

CASTRO, Manuela Oliveira de. Síntese de grafeno pelo método CVD. 2011. 84 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2011. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-11-13T20:03:00Z No. of bitstreams: 1 2011_dis_mocastro.pdf: 3411512 bytes, checksum: 64f6579c926c263cd9391dfb058954bb (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-11-13T20:35:29Z (GMT) No. of bitstreams: 1 2011_dis_mocastro.pdf: 3411512 bytes, checksum: 64f6579c926c263cd9391dfb058954bb (MD5) / Made available in DSpace on 2014-11-13T20:35:29Z (GMT). No. of bitstreams: 1 2011_dis_mocastro.pdf: 3411512 bytes, checksum: 64f6579c926c263cd9391dfb058954bb (MD5) Previous issue date: 2011 / The advancement and improvement of synthesis techniques and handling of materials are fundamental to understand their properties and possible forms of production and use. However, in the case of nanomaterials, problems such as structural defects, high cost and difficulty of achieving production on a large scale have yet to be solved. Inserted in this panorama is graphene, a two-dimensional nanomaterial whose morphology, consisting of carbon atoms arranged in hexagonal form, is responsible for unprecedented properties that have revolutionary relevance for both basic and applied research. There are different methods of synthesis of graphene. The method of Chemical Vapor Deposition (CVD) is among the most advantageous ones. This method consists in breaking the bonds of the molecules of a gas subjected to high temperatures so that the atoms from the gas are deposited on a given substrate. In this work, we used the CVD method for the synthesis of graphene on oxidized silicon substrates (Si/SiO2) coated with a 500 nm thick film of nickel (Ni), which served as the catalyst. Methane gas (CH4) was used as the source of the carbon atoms and the synthesis was carried out using different sets of parameters. Experiments were performed, firstly, using parameters es-tablished in the literature and the results were compared with those obtained by other authors. The influence of the synthesis parameters and the characteristics of the films of Ni catalysts on the properties of the graphene films was studied. The samples were characterized using Scanning Electron Microscopy, Confocal Raman and Optical Microscopy, and Atomic Force Microscopy. In agreement with results from the literature, it could be observed that thin films are synthesized and they are composed of graphitic flakes with a non-uniform thickness, which is strongly dependent of the morphology of catalyst film. Larger regions with characteristic Raman spectra of monolayer and few layer graphene could be obtained by combining thermal treatment of Ni film during the sputtering process with low gas flow and time of exposure to CH4 in the CVD experiment. Variations in the Raman spectra of the flakes could be observed, including the emergence of the D-band and the displacement of the peaks. These variations, which reveal the influence of substrates on the synthesized films, were more intense the smaller the number of graphene layers. Next, we combined methods reported in the literature for estimating the number of layers on the basis of the characteristics of the Raman spectra with AFM analysis to obtain the thickness of the graphene layer. The results obtained from our analysis show that monolayer graphene could be successfully synthesized in the experiments. / O avanço e o aperfeiçoamento das técnicas de síntese e manipulação de materiais são fundamentais para o entendimento de suas propriedades e das possíveis formas de produção e utilização. Porém, no caso dos nanomateriais, principalmente, cujas extraordinárias capacidades são bastante celebradas, problemas como defeitos estruturais, alto custo de obtenção e dificuldade de produção em larga escala ainda necessitam ser solucionados. Inserido neste panorama está o grafeno, um nanomaterial cuja morfologia bidimensional, constituída por átomos de carbono dispostos de forma hexagonal, é responsável por propriedades sem precedentes que apresentam revolucionária relevância, tanto para a pesquisa básica quanto para a pesquisa aplicada. Neste sentido, existem diferentes métodos de síntese de grafeno, estando entre os mais vantajosos o método de deposição química em fase de vapor (Chemical Vapor Deposition - CVD). Este método consiste na quebra das ligações das moléculas de um gás submetido a altas temperaturas de modo que os átomos provenientes do gás sejam depositados sobre um determinado substrato. Neste trabalho, utilizou-se o método CVD para a síntese de grafeno sobre substratos de silício oxidado (Si/SiO2) recobertos por filmes de níquel (Ni) com, aproximadamente, 500nm de espessura, os quais funcionaram como catalisadores. O gás metano (CH4) foi utilizado como a fonte dos átomos de carbono depositados e os processos de síntese tiveram diferentes conjuntos de parâmetros executados. A síntese de grafeno pelo método CVD teve como objetivo geral verificar os resultados divulgados na literatura e aperfeiçoá-los, relacionando os parâmetros utilizados nas sínteses e as características dos filmes de Ni catalisadores com aquelas apresentadas pelos filmes de grafeno obtidos nos experimentos. As amostras foram caracterizadas por meio de Microscopia Eletrônica de Varredura, Microscopia Óptica e Raman Confocal e Microscopia de Força Atômica. Em consistência com os resultados publicados na literatura, observou-se que são sintetizados filmes finos compostos por flakes de material grafítico com espessura não uniforme, e que a obtenção de filmes mais uniformes é fortemente dependente da morfologia do filme catalisador. Regiões apresentando espectro Raman característico de monocamadas de grafeno e de grafeno de poucas camadas foram maiores quando combinados o tratamento térmico do filme de Ni com o baixo fluxo e menor tempo de exposição ao CH4. Verificaram-se, ainda, variações nos espectros Raman dos flakes. Estas variações apresentaram-se mais intensas, quanto mais reduzido é o número de camadas de grafeno e incluem o aparecimento da banda D, além do deslocamento dos picos, revelando a influência dos substratos sobre os filmes sintetizados. Esta pesquisa considerou métodos de estimativa do número de camadas por características do espectro Raman, divulgados na literatura, aliados à análise da espessura por AFM que mostraram ser possível a síntese de monocamadas de grafeno.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / O avanÃo e o aperfeiÃoamento das tÃcnicas de sÃntese e manipulaÃÃo de materiais sÃo fundamentais para o entendimento de suas propriedades e das possÃveis formas de produÃÃo e utilizaÃÃo. PorÃm, no caso dos nanomateriais, principalmente, cujas extraordinÃrias capacidades sÃo bastante celebradas, problemas como defeitos estruturais, alto custo de obtenÃÃo e dificuldade de produÃÃo em larga escala ainda necessitam ser solucionados. Inserido neste panorama està o grafeno, um nanomaterial cuja morfologia bidimensional, constituÃda por Ãtomos de carbono dispostos de forma hexagonal, à responsÃvel por propriedades sem precedentes que apresentam revolucionÃria relevÃncia, tanto para a pesquisa bÃsica quanto para a pesquisa aplicada. Neste sentido, existem diferentes mÃtodos de sÃntese de grafeno, estando entre os mais vantajosos o mÃtodo de deposiÃÃo quÃmica em fase de vapor (Chemical Vapor Deposition - CVD). Este mÃtodo consiste na quebra das ligaÃÃes das molÃculas de um gÃs submetido a altas temperaturas de modo que os Ãtomos provenientes do gÃs sejam depositados sobre um determinado substrato. Neste trabalho, utilizou-se o mÃtodo CVD para a sÃntese de grafeno sobre substratos de silÃcio oxidado (Si/SiO2) recobertos por filmes de nÃquel (Ni) com, aproximadamente, 500nm de espessura, os quais funcionaram como catalisadores. O gÃs metano (CH4) foi utilizado como a fonte dos Ãtomos de carbono depositados e os processos de sÃntese tiveram diferentes conjuntos de parÃmetros executados. A sÃntese de grafeno pelo mÃtodo CVD teve como objetivo geral verificar os resultados divulgados na literatura e aperfeiÃoÃ-los, relacionando os parÃmetros utilizados nas sÃnteses e as caracterÃsticas dos filmes de Ni catalisadores com aquelas apresentadas pelos filmes de grafeno obtidos nos experimentos. As amostras foram caracterizadas por meio de Microscopia EletrÃnica de Varredura, Microscopia Ãptica e Raman Confocal e Microscopia de ForÃa AtÃmica. Em consistÃncia com os resultados publicados na literatura, observou-se que sÃo sintetizados filmes finos compostos por flakes de material grafÃtico com espessura nÃo uniforme, e que a obtenÃÃo de filmes mais uniformes à fortemente dependente da morfologia do filme catalisador. RegiÃes apresentando espectro Raman caracterÃstico de monocamadas de grafeno e de grafeno de poucas camadas foram maiores quando combinados o tratamento tÃrmico do filme de Ni com o baixo fluxo e menor tempo de exposiÃÃo ao CH4. Verificaram-se, ainda, variaÃÃes nos espectros Raman dos flakes. Estas variaÃÃes apresentaram-se mais intensas, quanto mais reduzido à o nÃmero de camadas de grafeno e incluem o aparecimento da banda D, alÃm do deslocamento dos picos, revelando a influÃncia dos substratos sobre os filmes sintetizados. Esta pesquisa considerou mÃtodos de estimativa do nÃmero de camadas por caracterÃsticas do espectro Raman, divulgados na literatura, aliados à anÃlise da espessura por AFM que mostraram ser possÃvel a sÃntese de monocamadas de grafeno. / The advancement and improvement of synthesis techniques and handling of materials are fundamental to understand their properties and possible forms of production and use. However, in the case of nanomaterials, problems such as structural defects, high cost and difficulty of achieving production on a large scale have yet to be solved. Inserted in this panorama is graphene, a two-dimensional nanomaterial whose morphology, consisting of carbon atoms arranged in hexagonal form, is responsible for unprecedented properties that have revolutionary relevance for both basic and applied research. There are different methods of synthesis of graphene. The method of Chemical Vapor Deposition (CVD) is among the most advantageous ones. This method consists in breaking the bonds of the molecules of a gas subjected to high temperatures so that the atoms from the gas are deposited on a given substrate. In this work, we used the CVD method for the synthesis of graphene on oxidized silicon substrates (Si/SiO2) coated with a 500 nm thick film of nickel (Ni), which served as the catalyst. Methane gas (CH4) was used as the source of the carbon atoms and the synthesis was carried out using different sets of parameters. Experiments were performed, firstly, using parameters es-tablished in the literature and the results were compared with those obtained by other authors. The influence of the synthesis parameters and the characteristics of the films of Ni catalysts on the properties of the graphene films was studied. The samples were characterized using Scanning Electron Microscopy, Confocal Raman and Optical Microscopy, and Atomic Force Microscopy. In agreement with results from the literature, it could be observed that thin films are synthesized and they are composed of graphitic flakes with a non-uniform thickness, which is strongly dependent of the morphology of catalyst film. Larger regions with characteristic Raman spectra of monolayer and few layer graphene could be obtained by combining thermal treatment of Ni film during the sputtering process with low gas flow and time of exposure to CH4 in the CVD experiment. Variations in the Raman spectra of the flakes could be observed, including the emergence of the D-band and the displacement of the peaks. These variations, which reveal the influence of substrates on the synthesized films, were more intense the smaller the number of graphene layers. Next, we combined methods reported in the literature for estimating the number of layers on the basis of the characteristics of the Raman spectra with AFM analysis to obtain the thickness of the graphene layer. The results obtained from our analysis show that monolayer graphene could be successfully synthesized in the experiments.

Made available in DSpace on 2016-03-15T19:38:52Z (GMT). No. of bitstreams: 1 Henrique Guimaraes Rosa.pdf: 6486656 bytes, checksum: 6fc9fe7875ed2cceb89c9f5179b9f028 (MD5) Previous issue date: 2015-02-25 / Fundo Mackenzie de Pesquisa / In this thesis, we present results on the fabrication, transfer and characterization of chemical vapor deposition (CVD) graphene and exfoliated graphene over glass and optical fiber substrates, to study optical properties of graphene and its application as a saturable absorber for Erbium-doped fiber laser (EDFL). Monolayer CVD graphene and stacked CVD graphene samples were fabricated and characterized, transferred to the transverse face of optical fibers, and a study on the relation between the optical properties of graphene samples and the properties of ultrashort laser pulses generated in (EDFL) was performed. Furthermore, we have developed a technique for transferring exfoliated nanomaterials which allowed us to transfer exfoliated graphene onto optical fiber s faces and align the graphene flake to the fiber core. With this transfer technique it is possible to fabricate samples with controlled number of graphene layers onto optical fiber faces. As application, we demonstrate ultrashort pulse generation in Erbium-doped fiber laser with exfoliated monolayer graphene samples as saturable absorber. This is the first time that ultrashort laser pulses are generated with a single exfoliated monolayer graphene sample. / Nesta tese, apresentamos resultados sobre a fabricação, transferência e caracterização de grafeno CVD (grafeno fabricado por deposição química de vapor chemical vapour deposition) e de grafeno esfoliado em substratos de vidro e em fibras ópticas, para o estudo das propriedades ópticas do grafeno e sua aplicação como absorvedor saturável em laser à fibra dopada com Érbio (EDFL). Foram fabricadas e caracterizadas amostras de grafeno CVD monocamada e de grafeno CVD empilhado, transferidas para a face transversal de fibras ópticas, e com estas amostras foram feitos estudos sobre a relação entre as propriedades ópticas do grafeno e as propriedades de pulsos ultracurtos gerados em EDFL. Além disto, desenvolvemos uma técnica para a transferência de nanomateriais esfoliados que permitiu a transferência de grafeno esfoliado para fibras ópticas e seu alinhamento com o núcleo da fibra. Com esta técnica de transferência é possível fabricar amostras com controlado número de camadas de grafeno em fibra óptica. Como aplicação, demonstramos a geração de pulsos em EDFL com uma amostra de grafeno esfoliado monocamada como absorvedor saturável. Esta é a primeira vez que pulsos ultracurtos são gerados em lasers à fibra com amostra de grafeno esfoliado de uma única camada sobre a face transversal da fibra óptica.