Scalable Graphene Coatings for Enhanced Condensation Heat Transfer

Water vapor condensation is commonly observed in nature and routinely used as an effective means of transferring heat with dropwise condensation on nonwetting surfaces exhibiting heat transfer improvement compared to filmwise condensation on wetting surfaces. However, state-of-the-art techniques to promote dropwise condensation rely on functional hydrophobic coatings that either have challenges with chemical stability or are so thick that any potential heat transfer improvement is negated due to the added thermal resistance of the coating. In this work, we show the effectiveness of ultrathin scalable chemical vapor deposited (CVD) graphene coatings to promote dropwise condensation while offering robust chemical stability and maintaining low thermal resistance. Heat transfer enhancements of 4× were demonstrated compared to filmwise condensation, and the robustness of these CVD coatings was superior to typical hydrophobic monolayer coatings. Our results indicate that graphene is a promising surface coating to promote dropwise condensation of water in industrial conditions with the potential for scalable application via CVD.United States. Office of Naval ResearchNational Science Foundation (U.S.) (Major Research Instrumentation Grant for Rapid Response Research (MRI-RAPID))National Science Foundation (U.S.). Graduate Research Fellowship Program (Grant 1122374

Unraveling the interlayer-related phonon self-energy renormalization in bilayer graphene

In this letter, we present a step towards understanding the bilayer graphene (2LG) interlayer (IL)-related phonon combination modes and overtones as well as their phonon self-energy renormalizations by using both gate-modulated and laser-energy dependent inelastic scattering spectroscopy. We show that although the IL interactions are weak, their respective phonon renormalization response is significant. Particularly special, the IL interactions are mediated by Van der Waals forces and are fundamental for understanding low-energy phenomena such as transport and infrared optics. Our approach opens up a new route to understanding fundamental properties of IL interactions which can be extended to any graphene-like material, such as MoS2, WSe2, oxides and hydroxides. Furthermore, we report a previously elusive crossing between IL-related phonon combination modes in 2LG, which might have important technological applications.United States. Office of Naval Research (ONR-MURI-N00014-09-1-1063)Conselho Nacional de Pesquisas (Brazil)Japan. Ministry of Education, Culture, Sports, Science and Technology (grant No.20241023)Japan. Ministry of Education, Culture, Sports, Science and Technology (grant No.23710118

Mass-related inversion symmetry breaking and phonon self-energy renormalization in isotopically labeled AB-stacked bilayer graphene

A mass-related symmetry breaking in isotopically labeled bilayer graphene (2LG) was investigated during in-situ electrochemical charging of AB stacked (AB-2LG) and turbostratic (t-2LG) layers. The overlap of the two approaches, isotopic labeling and electronic doping, is powerful tool and allows to tailor, independently and distinctly, the thermal-related and transport-related phenomena in materials, since one can impose different symmetries for electrons and phonons in these systems. Variations in the system's phonon self-energy renormalizations due to the charge distribution and doping changes could be analyzed separately for each individual layer. Symmetry arguments together with first-order Raman spectra show that the single layer graphene (1LG), which is directly contacted to the electrode, has a higher concentration of charge carriers than the second graphene layer, which is not contacted by the electrode. These different charge distributions are reflected and demonstrated by different phonon self-energy renormalizations of the G modes for AB-2LG and for t-2LG.Czech Republic. Ministry of Education, Youth, and Sports (LH-13022)Czech Science Foundation (P208-12-1062)Conselho Nacional de Pesquisas (Brazil)National Science Foundation (U.S.) (NFS-DMR 10-04147

Using gate-modulated Raman scattering and electron-phonon interactions to probe single-layer graphene: A different approach to assign phonon combination modes

Gate-modulated and laser-dependent Raman spectroscopy have been widely used to study q = 0 zone center phonon modes, their self-energy, and their coupling to electrons in graphene systems. In this work we use gate-modulated Raman of q ≠ 0 phonons as a technique to understand the nature of five second-order Raman combination modes observed in the frequency range of 1700–2300 cm[superscript −1] of single-layer graphene (SLG). Anomalous phonon self-energy renormalization phenomena are observed in all five combination modes within this intermediate frequency region, which can clearly be distinguished from one another. By combining the anomalous phonon renormalization effect with the double resonance Raman theory, which includes both phonon dispersion relations and angular dependence of the electron-phonon scattering matrix elements, and by comparing it to the experimentally obtained phonon dispersion, measured by using different laser excitation energies, we can assign each Raman peak to the proper phonon combination mode. This approach should also shed light on the understanding of more complex structures such as few-layer graphene (FLG) and its stacking orders as well as other two-dimensional (2D)-like materials.National Science Foundation (U.S.) (DMR 10-04147)Conselho Nacional de Pesquisas (Brazil) (SM Projeto LTDA)Conselho Nacional de Pesquisas (Brazil)United States. Office of Naval Research. Multidisciplinary University Research Initiative (N00014-09-1-1063

Ambient-pressure CVD of graphene on low-index Ni surfaces using methane: A combined experimental and first-principles study

The growth of large area single-layer graphene (1-LG) is studied using ambient pressure chemical vapor deposition on single-crystal Ni(111), Ni(110), and Ni(100). By varying both the furnace temperature in the range of 800–1100 °C and the gas flow through the growth chamber, uniform, high-quality 1-LG is obtained for Ni(111) and Ni(110) single crystals and for Ni(100) thin films. Surprisingly, only multilayer graphene growth could be obtained for single-crystal Ni(100). The experimental results are analyzed to determine the optimum combination of temperature and gas flow. Characterization with optical microscopy, Raman spectroscopy, and optical transmission support our findings. Density-functional theory calculations are performed to determine the energy barriers for diffusion, segregation, and adsorption, and model the kinetic pathways for formation of different carbon structures on the low-index surfaces of Ni.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0001088

Rational Use of Medication in Pregnant Women

Introduction: the use of medication in pregnancy is a worldwide health challenge, since it can cause complications for both the pregnant woman and the fetus, and this risk is potentially increased in the first gestational trimester. Objectives: The objective of this article is to conduct a literature review on the rational use of medication in pregnant women. Materials and methods: The articles published from 2013 to 2019 in Portuguese - Brazil were used for the development of this literature review through an electronic search in the platforms: SCIELO - Scientific Electronic Library Online and Google Academic. Results and Discussion: 14 articles were analyzed, which showed a reflection on the effects of the use of drugs during pregnancy, where the perceptions of the authors were divergent, between the certainty that everything is harmful or everything is beneficial. Conclusion: Great variations were observed regarding the use of medication, in the same way, that the need for intervention promoting an increase in the rational use of medication is evident

Using inelastic scattering of light to understand the nature of electron-phonon interactions and phonon self-energy renormalizations in graphene materials

Exportado OPUSMade available in DSpace on 2019-08-14T02:55:49Z (GMT). No. of bitstreams: 1 tese_daniela_lopes_mafra.pdf: 23983901 bytes, checksum: 6ebd5c0e45403780200a53b58aa5a38f (MD5) Previous issue date: 13Na última década, muitos avanços teóricos e experimentais foram alcançados na física do grafeno. Em particular, a Espectroscopia Raman tem sido muito importante para elucidar propriedades físicas e químicas em sistemas de grafeno. Nessa tese nós usamos a Espectroscopia Raman para estudar alguns dos efeitos do acoplamento elétron-fônon no grafeno de camada única e de dupla camada e para obter informações sobre a estrutura eletrônica e vibracional do grafeno de camada dupla. As renormalizações das energias dos fônons tem sido estudadas basicamente para fônons com vetor de onda nulo (q=0). Aqui, nós combinamos a Espectroscopia Raman com aplicação de tensão de porta, para estudar, em grafeno de camada única, as bandas originadas do processo Raman com dupla ressonância (DDR) com etores de onda q0. Nós observamos os efeitos de decaimento dos fônons com o aumento da tensão de porta e esse efeito é o oposto do que é observado para os fônons com q=0. Nós mostramos que esse tipo de renormalização é uma assinatura dos fônons com vetor de onda q2K que vem de um processo de camada única, os modos de fônons que contribuem para a banda Raman G*, em ~2450cm-1 e para outros cinco picos provenientes de combinação de modos na região de frequência 1700-2300cm-1. Combinando a teoria do processo DRR com o efeito de renormalização de fônons, nós mostramos uma nova técnica para usar a Espectroscopia Raman para identificar cada modo Raman apropriadamente. Nó também estudamos o comportamento dos modos ópticos do grafeno de camada dupla combinando o espalhamento Raman e a aplicação de tensão de porta em dispositivos desse material. Nós observamos que a banda G se divide em duas quando o nível de Fermi da amostra é mudado e isso é explicado em termos da mistura dos modos de fônon Raman (Eg) e infravermelho (Eu) devido a diferença de concentração de carga nas duas camadas. Nós mostramos que a comparação entre os dados experimentais e o modelo teórico não dá apenas informação sobre a concentração de carga total no dispositivo de grafeno de camada dupla, mas também nos permite quantificar separadamente a quantidade de cargas não intencionais provenientes da camada de cima e de baixo do sistema e, portanto caracterizar a interação do grafeno de camada dupla com o ambiente a sua volta. Na segunda parte dessa tese, a dispersão de elétrons e fônons perto do ponto K do grafeno de camada dupla é investigada atravé do estudo da banda G' usando várias energias de excitação de laser na região do infravermelho e do visível. A estrutura eletrônica foi analisada dentro da aproximação de ligações-forte e os parâmetros Slonczewski-Weiss-McClure (SWM) foram obtidos através do comportamento dispersivo da banda G' considerando-se tanto o processo DRR interno, quanto o externo. Nós mostramos que os parâmetros SWM obtidos considerando-se que o processo DRR interno está em melhor acordo com os valores obtidos por outras técnicas experimentais, sugerindo fortemente que o processo interno é o principal responsável pela banda G' no grafeno. Além disso, a dependência da intensidade dos quatro picos que compõe a banda G' do grafeno de camada dupla com a energia de excitação de laser e com a potência do laser é explorada e explicada em termos do acoplamento elétron-fônon e do relaxamento dos elétrons foto-excitados. Nós mostramos que o relaxamento dos elétrons ocorre predominantemente pela emissão de fônons acústicos de baixa energia e as diferentes combinações dos processos de relaxamento determinam as intensidades relativas dos quatro picos que dão origem à banda G'. Esse efeito nos fornece informações importantes sobre a dinâmica dos elétrons e fônons e precisa ser levado em conta para aplicações do grafeno de camada dupla do campo nanotecnológico.In the last decade, many theoretical and experimental achievements have been made in the physics of graphene. In particular, Raman spectroscopy has been playing an important role in unraveling the properties of graphene systems. In this thesis we use the Raman spectroscopy to study some effects of the electron-phonon coupling in monolayer and bilayer graphene and to probe the electronic and vibrational structure of bilayer graphene. Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center (q = 0) wavevectors. Here, we combine Raman spectroscopy and gate voltage to study phonons of monolayer graphene for the features originated from a double-resonant Raman (DRR) process with q .= 0 wavevectors. We observe phonon renormalization effects in which there is a softening of the frequency and a broadening of the decay width with increasing the gate voltage, that is opposite from what is observed for the zone-center q = 0 case. We show that this renormalization is a signature for the phonons with q . 2k wavevector that come from both intravalley and intervalley DRR processes. Within this framework, we resolve the identification of the phonon modes contributing to the G. Raman feature, at ¡­ 2450 cm.1, and also forfive second order Raman combination modes in the frequency range of 1700 . 2300 cm.1 of monolayer graphene. By combining the DRR theory with the anomalous phonon renormalization effect, we show a new technique for using Raman spectroscopy to identify the proper phonon mode assignment for each combination mode. We also study the behavior of the optical phonon modes in bilayer graphene devicesby applying top gate voltage, using Raman scattering. We observe the splitting of the Raman G band as we tune the Fermi level of the sample, which is explained in terms of mixing of the Raman (Eg) and infrared (Eu) phonon modes, due to different doping in the two layers. We show that the comparison between the experiment and theoretical model not only gives information about the total charge concentration in the bilayer graphene device, but also allows to separately quantify the amount of unintentional charge coming from the top and the bottom of the system, and therefore to characterize the intrinsic charges of bilayer graphene with its surrounding environment. In the second part of this thesis, the dispersion of electrons and phonons near the K point of bilayer graphene was investigated in a resonant Raman study of the G¡Ç band using different laser excitation energies in the near-infrared and visible range.The electronic structure was analyzed within the tight-binding approximation, and the Slonczewski-Weiss-McClure (SWM) parameters were obtained from the analysis of the dispersive behavior of the G¡Ç band considering both the inner and the outer DRR processes. We show that the SWM parameters obtained considering the inner process are in better agreement with those obtained from other experimental techniques, strongly suggesting that the inner process is the main responsible for the G¡Ç feature in graphene. Additionally, the dependence of the intensity of the four peaks that compose the G¡Ç band of bilayer graphene with laser excitation energy and laser power is explored and explained in terms of the electron-phonon coupling and the relaxation of the photon-excited electron. We show that the carrier relaxation occurs predominantly by emitting a lowenergy acoustic phonon and the different combinations of relaxation processes determine the relative intensities of the four peaks that give rise to the G¡Ç band. Some peaks show an increase of their intensity at the expense of others, thereby making the intensity of the peaks both different from each other and dependent on laser excitation energy and on power level. This effect gives important information about the electron and phonon dynamics and needs to be taken into account for certain applications of bilayer graphene in the field of nanotechnology

Intra- and Interlayer Electron-Phonon Interactions in [superscript 12/12]C and [superscript 12/13]C BiLayer Graphene

This review focuses on intra- and interlayer (IL) electron-phonon interactions and phonon self-energy renormalizations in twisted and AB-stacked bilayer graphene (2LG) composed either only of 12C or a mixing of 12C and 13C isotopes. A simple way to imagine a 2LG is by placing one monolayer graphene (1LG) on top of another 1LG. The orientation of one of the layers with relation to the other may originate a twisted 2LG system (known as turbostratic) as well as a AB-stacked system, also known as Bernal stacking. By rotating the layers of a 2LG one can departure from a fully misoriented system to achieve the AB-stacked configuration and their IL interactions can be dramatically different being close to zero in a fully misoriented system and maximum in an AB-stacked system. Interlayer interactions are expected to slightly perturb the intralayer phonons and they also govern the low-energy electronic and vibrational properties, which are of primary importance to phenomena such as transport, infrared (IR) optics and telecommunication bands in the IR range. Therefore, a comprehensive discussion combining intra- and interlayer phenomena is necessary and addressed throughout the text. Keywords: bilayer graphene; interlayer interaction; electron-phonon couplingConselho Nacional de Pesquisas (Brazil