Design and implementation of a device based on an off-axis parabolic mirror to perform luminescence experiments in a scanning tunneling microscope

We present the design, implementation, and illustrative results of a light collection/injection strategy based on an off-axis parabolic mirror collector for a low-temperature Scanning Tunneling Microscope (STM). This device allows us to perform STM induced Light Emission (STM-LE) and Cathodoluminescence (STM-CL) experiments and in situ Photoluminescence (PL) and Raman spectroscopy as complementary techniques. Considering the \'Etendue conservation and using an off-axis parabolic mirror, it is possible to design a light collection and injection system that displays 72% of collection efficiency (considering the hemisphere above the sample surface) while maintaining high spectral resolution and minimizing signal loss. The performance of the STM is tested by atomically resolved images and scanning tunneling spectroscopy results on standard sample surfaces. The capabilities of our system are demonstrated by performing STM-LE on metallic surfaces and two-dimensional semiconducting samples, observing both plasmonic and excitonic emissions. In addition, we carried out in situ PL measurements on semiconducting monolayers and quantum dots and in situ Raman on graphite and hexagonal boron nitride (h-BN) samples. Additionally, STM-CL and PL were obtained on monolayer h-BN gathering luminescence spectra that are typically associated with intragap states related to carbon defects. The results show that the flexible and efficient light injection and collection device based on an off-axis parabolic mirror is a powerful tool to study several types of nanostructures with multiple spectroscopic techniques in correlation with their morphology at the atomic scale and electronic structure.Comment: 19 pages, 14 Figure

Estudo de propriedades estruturais e óticas de heteroestruturas formadas com materiais bidimensionais

Exportado OPUSMade available in DSpace on 2019-08-09T17:11:06Z (GMT). No. of bitstreams: 1 ingrid_barcelos___tese.pdf: 9954985 bytes, checksum: 076497eab605c0262fa3508c704c8059 (MD5) Previous issue date: 27Nesta tese apresentamos a fabricação de heteroestruturas formadas a partir de materiais bidimensionais com a finalidade de estudar suas propriedades estruturais e ópticas. Os resultados mostram que é possível integrar materiais bidimensionais em heteroestruturas com interfaces bem definidas. Duas metodologias independentes foram utilizadas. A primeira metodologia consiste no enrolamento de filmes finos para fabricar micro- e nano-tubos de InGaAs/Cr/grafeno. Neste sistema estudamos as interações e as propriedades estruturais de mono- e multicamadas de grafeno inseridas em heteroestruturas com simetria cilíndrica. Fomos capazes de recuperar os valores para as constantes elásticas do grafeno por meio de um método que impõeuma curvatura homogênea e não-local e mostramos que os efeitos apenas de curvatura não modificam a assinatura Raman do grafeno até um limite inferior de raio da ordem de 600 nm. A segunda metodologia utiliza um sistema de transferência discreta de camadas atômicas de materiais bidimensionais esfoliados micromecanicamente, formando heteroestruturas de Van der Waals (VdW) de grafeno/nitreto de borohexagonal (hBN). Apresentamos um estudo das propriedades ópticas destas estruturas com ênfase nos efeitos devido à polaritons utilizando a técnica de espalhamento de infravermelho por microscopia de campo próximo (IR s-SNOM). Ao menos duas classes diferentes de propagação de polaritons são conhecidas em sistemas grafeno/hBN: plasmons-polaritons de superfície (SPP ou SP2) no grafeno, efônons-polaritons hiperbólicos (HPP ou HP2) no hBN. Aqui relatamos a observação de uma hibridização destes modos, resultando em modos próprios da heteroestrutura grafeno/hBN. Estes modos serão designados por plasmon-fônons polaritons de superfície (SPPP) e são produzidos por oscilações coerentes da densidade de elétrons no grafeno e vibrações da rede do hBN.In this thesis we describe the fabrication of two-dimensional materialheterostructures for the study of structural and optical properties. Our results show that it is possible to build heterostructures with well-defined interfaces in which twodimensional materials are integrated. Two invependent methods were used. The first method consists in rolling thin films to produce InGaAs/Cr/Graphene micro- and nanotubes. In such system the interactions and structural properties of single and multilayer graphene inserted into cylindrical heterostructures are investigated. We were able to retrieve elastic constant values for graphene through a method that imposes homogeneous and non-local curvature. We show that curvature effects solely do not modify the Raman signature of graphene down to a lower limit of 600 nm radius. The second method uses a discrete atomic layer transfer system that is used to build Van der Waals heterostructures of exfoliated two-dimensional materials such asgraphene on hexagonal boron nitride (hBN). We present scattering-scanning near-field optical microscopy (s-SNOM) results in the infrared region that allow the study of optical properties of these structures with emphasis on the effects generated by polaritons. Two distinct polariton propagation phenomena are well known in thegraphene/hBN system: (i) surface plasmon-polaritons (SPP or SP2) on graphene and hyperbolic phonon-polaritons (HPP or HP2) on hBN. We describe the observation of mode hibridization, resulting on specific graphene/hBN heterostructure modes. These modes are named surface plasmon-phonon polaritons (SPPP) and are induced by electron density oscillations on graphene and lattice vibrations on hBN

Observation of ferromagnetism in PdCo alloy nanoparticles encapsulated in carbon nanotubes

INCT - INSTITUTO NACIONAL DE CIÊNCIA E TECNOLOGIA METROLOGIA DE RADIAÇÃO EM MEDICINACNPQ – CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP – FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISLNLS - LABORATÓRIO NACIONAL DE LUZ SÍNCROTRONCarbon nanotubes terminated by PdCo catalyst nanoparticles were grown by plasma-enhanced chemical vapor deposition. Transmission electron microscopy reveals that these nanoparticles have a droplike shape and are completely encapsulated inside multiwalled nanotubes. Magnetization measurements showed the existence of a permanent magnetization with a small shape anisotropy effect. The magnetization of both cobalt and palladium was confirmed by x-ray magnetic circular dichroism. These results show that nanotubes ended by magnetic PdCo nanoparticles can be grown and could be used in magnetic storage media and electrical spin injection.9625INCT - INSTITUTO NACIONAL DE CIÊNCIA E TECNOLOGIA METROLOGIA DE RADIAÇÃO EM MEDICINACNPQ – CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP – FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISLNLS - LABORATÓRIO NACIONAL DE LUZ SÍNCROTRONINCT - INSTITUTO NACIONAL DE CIÊNCIA E TECNOLOGIA METROLOGIA DE RADIAÇÃO EM MEDICINACNPQ – CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP – FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOFAPEMIG - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE MINAS GERAISLNLS - LABORATÓRIO NACIONAL DE LUZ SÍNCROTRO

Exfoliation and characterization of a two-dimensional serpentine-based material.

We report on an experimental investigation of serpentine, an abundant phyllosilicate, as an alternative source of two-dimensional (2D) nanomaterials. We show, through scanning probe microscopy (SPM) measurements, that natural serpentine mineral can be mechanically exfoliated down to few-layer flakes, where monolayers can be easily resolved. The parent serpentine bulk material was initially characterized via conventional techniques like XRD, XPS, FTIR and Raman spectroscopies and the results show that it is predominantly constituted by the antigorite mineral. From ab initio calculations using density functional theory, we also determine the geometry and electronic structure of antigorite, the observed structural form of serpentine. Additionally, we further characterized electrical and mechanical properties of the obtained 2D material flakes using SPM and broadband synchrotron infrared nanospectroscopy. Wavelength tuning of the serpentine vibrational resonances, assigned to in- and out-of-plane molecular vibrations, are observed and compared with the FTIR characterization of the parent bulk material. They show that there is no degradation of serpentine's structural properties during its mechanical exfoliation down to nanometer-thin sheets. Therefore, our results introduce the serpentine mineral as an attractive low-cost candidate in 2D materials applications

Brazilian Flora 2020: Leveraging the power of a collaborative scientific network

International audienceThe shortage of reliable primary taxonomic data limits the description of biological taxa and the understanding of biodiversity patterns and processes, complicating biogeographical, ecological, and evolutionary studies. This deficit creates a significant taxonomic impediment to biodiversity research and conservation planning. The taxonomic impediment and the biodiversity crisis are widely recognized, highlighting the urgent need for reliable taxonomic data. Over the past decade, numerous countries worldwide have devoted considerable effort to Target 1 of the Global Strategy for Plant Conservation (GSPC), which called for the preparation of a working list of all known plant species by 2010 and an online world Flora by 2020. Brazil is a megadiverse country, home to more of the world's known plant species than any other country. Despite that, Flora Brasiliensis, concluded in 1906, was the last comprehensive treatment of the Brazilian flora. The lack of accurate estimates of the number of species of algae, fungi, and plants occurring in Brazil contributes to the prevailing taxonomic impediment and delays progress towards the GSPC targets. Over the past 12 years, a legion of taxonomists motivated to meet Target 1 of the GSPC, worked together to gather and integrate knowledge on the algal, plant, and fungal diversity of Brazil. Overall, a team of about 980 taxonomists joined efforts in a highly collaborative project that used cybertaxonomy to prepare an updated Flora of Brazil, showing the power of scientific collaboration to reach ambitious goals. This paper presents an overview of the Brazilian Flora 2020 and provides taxonomic and spatial updates on the algae, fungi, and plants found in one of the world's most biodiverse countries. We further identify collection gaps and summarize future goals that extend beyond 2020. Our results show that Brazil is home to 46,975 native species of algae, fungi, and plants, of which 19,669 are endemic to the country. The data compiled to date suggests that the Atlantic Rainforest might be the most diverse Brazilian domain for all plant groups except gymnosperms, which are most diverse in the Amazon. However, scientific knowledge of Brazilian diversity is still unequally distributed, with the Atlantic Rainforest and the Cerrado being the most intensively sampled and studied biomes in the country. In times of “scientific reductionism”, with botanical and mycological sciences suffering pervasive depreciation in recent decades, the first online Flora of Brazil 2020 significantly enhanced the quality and quantity of taxonomic data available for algae, fungi, and plants from Brazil. This project also made all the information freely available online, providing a firm foundation for future research and for the management, conservation, and sustainable use of the Brazilian funga and flora