Photovoltaic effect in an electrically tunable van der Waals heterojunction

Semiconductor heterostructures form the cornerstone of many electronic and optoelectronic devices and are traditionally fabricated using epitaxial growth techniques. More recently, heterostructures have also been obtained by vertical stacking of two-dimensional crystals, such as graphene and related two- dimensional materials. These layered designer materials are held together by van der Waals forces and contain atomically sharp interfaces. Here, we report on a type- II van der Waals heterojunction made of molybdenum disulfide and tungsten diselenide monolayers. The junction is electrically tunable and under appropriate gate bias, an atomically thin diode is realized. Upon optical illumination, charge transfer occurs across the planar interface and the device exhibits a photovoltaic effect. Advances in large-scale production of two-dimensional crystals could thus lead to a new photovoltaic solar technology.Comment: 26 pages, 14 figures, Nano Letters 201

Optical imaging of strain in two-dimensional crystals

Strain engineering is widely used in material science to tune the (opto-)electronic properties of materials and enhance the performance of devices. Two-dimensional atomic crystals are a versatile playground to study the influence of strain, as they can sustain very large deformations without breaking. Various optical techniques have been employed to probe strain in two-dimensional materials, including micro-Raman and photoluminescence spectroscopy. Here we demonstrate that optical second harmonic generation constitutes an even more powerful technique, as it allows to extract the full strain tensor with a spatial resolution below the optical diffraction limit. Our method is based on the strain-induced modification of the nonlinear susceptibility tensor due to a photoelastic effect. Using a two-point bending technique, we determine the photoelastic tensor elements of molybdenum disulfide. Once identified, these parameters allow us to spatially image the two-dimensional strain field in an inhomogeneously strained sample.Comment: 13 pages, 4 figure

Microcavity-integrated graphene photodetector

The monolithic integration of novel nanomaterials with mature and established technologies has considerably widened the scope and potential of nanophotonics. For example, the integration of single semiconductor quantum dots into photonic crystals has enabled highly efficient single-photon sources. Recently, there has also been an increasing interest in using graphene - a single atomic layer of carbon - for optoelectronic devices. However, being an inherently weak optical absorber (only 2.3 % absorption), graphene has to be incorporated into a high-performance optical resonator or waveguide to increase the absorption and take full advantage of its unique optical properties. Here, we demonstrate that by monolithically integrating graphene with a Fabry-Perot microcavity, the optical absorption is 26-fold enhanced, reaching values >60 %. We present a graphene-based microcavity photodetector with record responsivity of 21 mA/W. Our approach can be applied to a variety of other graphene devices, such as electro-absorption modulators, variable optical attenuators, or light emitters, and provides a new route to graphene photonics with the potential for applications in communications, security, sensing and spectroscopy.Comment: 19 pages, 4 figure

Mechanisms of Photoconductivity in Atomically Thin MoS₂

Atomically thin transition metal dichalcogenides have emerged as promising candidates for sensitive photodetection. Here, we report a photoconductivity study of biased mono- and bilayer molybdenum disulfide field-effect transistors. We identify photovoltaic and photoconductive effects, which both show strong photogain. The photovoltaic effect is described as a shift in transistor threshold voltage due to charge transfer from the channel to nearby molecules, including SiO₂ surface-bound water. The photoconductive effect is attributed to the trapping of carriers in band tail states in the molybdenum disulfide itself. A simple model is presented that reproduces our experimental observations, such as the dependence on incident optical power and gate voltage. Our findings offer design and engineering strategies for atomically thin molybdenum disulfide photodetectors, and we anticipate that the results are generalizable to other transition metal dichalcogenides as well

É possível prever o número de nebulizações e o uso de corticosteróide intravenoso em crianças com crise asmática na unidade de emergência? Is it possible to predict the number of nebulizations and the use of intravenous corticosteroid in children with asthmatic attack at the emergency room?

OBJETIVO: identificar o número de nebulizações necessárias e a demanda de corticosteróide intravenoso em crianças em crise asmática, a partir de características clínicas e funcionais observadas no momento da admissão na unidade de emergência. MÉTODOS: Foram avaliadas prospectivamente 130 crianças em crise asmática, na faixa etária de 1 a 13 anos. No momento da admissão, as crianças foram avaliadas por meio de escore clínico e medidas de saturação arterial de oxigênio (por oximetria de pulso) e de pico de fluxo expiratório. Em seguida, receberam um tratamento padronizado e foram acompanhadas em relação à necessidade de nebulizações consecutivas com b2 agonista e costicosteróide intravenoso. Através de análise de regressão buscou-se uma correlação dos parâmetros avaliados com o número de nebulizações realizadas e o uso de corticosteróide. RESULTADOS: Oitenta e oito crianças (67,7%) receberam de uma até três nebulizações, e 42 crianças (32,3%) receberam seis nebulizações. Sessenta e oito crianças (52,3%) receberam corticosteróide. Os valores iniciais de escore clínico, saturação arterial de oxigênio e pico de fluxo expiratório mostraram uma correlação significativa com o número de nebulizações realizadas e com a necessidade do uso de corticosteróide. CONCLUSÃO: Com base em nossos resultados, é possível predizer e antecipar, no momento da admissão das crianças em crise asmática na Unidade de Emergência, a necessidade do uso de corticosteróide e de um maior número de nebulizações, o que pode alterar o prognóstico e o tempo de evolução da crise.<br>OBJECTIVE: to identify the number of nebulizations needed and the demand for intravenous corticosteroids in children with asthmatic attack, considering clinical and functional characteristics presented at the moment children were admitted to the emergency room. METHODS: we prospectively evaluated 130 children with asthmatic attack and from 1 to 13 years of age. At the moment children were admitted, they were evaluated according to clinical score, arterial oxygen saturation (pulse oximetry), and peak expiratory flow. Next, children received a standard treatment and were observed for the number of consecutive nebulizations required and the use of corticosteroid. Using regression analysis, we assessed the parameters evaluated for a correlation with the number of nebulizations and the use of intravenous corticosteroid. RESULTS: Eighty-eight children (67.7%) received one to three nebulizations, and 42 children (32.3%) received six nebulizations. Sixty-eight children (52.3%) received corticosteroids. The initial values for the clinical score, arterial oxygen saturation, and peak expiratory flow showed a significant correlation with the number of nebulizations required and the need for corticosteroids. CONCLUSIONS: According to our results, it is possible to predict and anticipate the need for corticosteroids and more nebulizations on admission of asthmatic children to the emergency room, allowing changes both in prognosis and in the time of evolution of the asthma attack