1,720,989 research outputs found
Switchable, Tunable, and Directable Exciton Funneling in Periodically Wrinkled WS2
No full textThe extreme elastic strain of monolayer transition metal dichalcogenides provides an ideal platform to achieve efficient exciton funneling via local strain modulation; however, studies conducted thus far have focused on the use of substrates with fixed strain profiles. We prepared 1L-WS2 on a flexible substrate such that the formation of topographic wrinkles could be switched on or off, and the depth or the direction of the wrinkle could be modified by external strain, thereby providing full control of the periodic undulation of the band gap profile of 1L-WS2 in the range 0-57 meV. Nanoscale photoluminescence (PL) imaging unambiguously evinced that the photoexcited excitons of 1L-WS2 were accumulated at the top regions of the wrinkles with less band gap than the valley region. Our results of broad tunability of the two-dimensional (2D) exciton funneling suggest a promising route to control exciton drift for enhanced optoelectronic performances and future 2D exciton devices11Nsciescopu
Identifying Defect-Induced Trion in Monolayer WS2 via Carrier Screening Engineering
No full textUnusually high exciton binding energies (BEs), as much as similar to 1 eV in monolayer transition-metal dichalcogenides, provide opportunities for exploring exotic and stable excitonic many-body effects. These include many-body neutral excitons, trions, biexcitons, and defect-induced excitons at room temperature, rarely realized in bulk materials. Nevertheless, the defect-induced trions correlated with charge screening have never been observed, and the corresponding BEs remain unknown. Here we report defect-induced A-trions and B-trions in monolayer tungsten disulfide (WS2) via carrier screening engineering with photogenerated carrier modulation, external doping, and substrate scattering. Defect-induced trions strongly couple with inherent SiO2 hole traps under high photocarrier densities and become more prominent in rhenium-doped WS2. The absence of defect-induced trion peaks was confirmed using a trap-free hexagonal boron nitride substrate, regardless of power density. Moreover, many-body excitonic charge states and their BEs were compared via carrier screening engineering at room temperature. The highest BE was observed in the defect-induced A-trion state (similar to 214 meV), comparably higher than the trion (209 meV) and neutral exciton (174 meV), and further tuned by external photoinduced carrier density control. This investigation allows us to demonstrate defect-induced trion BE localization via spatial BE mapping in the monolayer WS2 midflake regions distinctive from the flake edges.11Nsciescopu
Gate-Tunable Spin Exchange Interactions and Inversion of Magnetoresistance in Single Ferromagnetic ZnO Nanowires
No full textElectrical control of ferromagnetism in semiconductor nanostructures offers the promise of nonvolatile functionality in future semiconductor spintronics. Here, we demonstrate a dramatic gate-induced change of ferromagnetism in ZnO nanowire (NW) field-effect transistors (FETs). Ferromagnetism in our ZnO NWs arose from oxygen vacancies, which constitute deep levels hosting unpaired electron spins. The magnetic transition temperature of the studied ZnO NWs was estimated to be well above room temperature. The in situ UV confocal photoluminescence (PL) study confirmed oxygen vacancy mediated ferromagnetism in the studied ZnO NW FET devices. Both the estimated carrier concentration and temperature-dependent conductivity reveal the studied ZnO NWs are at the crossover of the metal-insulator transition. In particular, gate-induced modulation of the carrier concentration in the ZnO NW FET significantly alters carrier-mediated exchange interactions, which causes even inversion of magnetoresistance (MR) from negative to positive values. Upon sweeping the gate bias from 40 to +50 V, the MRs estimated at 2 K and 2 T were changed from 11.3% to +4.1%. Detailed analysis on the gate-dependent MR behavior clearly showed enhanced spin splitting energy with increasing carrier concentration. Gate-voltage-dependent PL spectra of an individual NW device confirmed the localization of oxygen vacancy-induced spins, indicating that gate-tunable indirect exchange coupling between localized magnetic moments played an important role in the remarkable change of the MR.clos
Plasmon-enhanced phosphorescence of hybrid thin films of metal-free purely organic phosphor and silver nanoparticles
No full textWe present phosphorescence enhancement of 2,5-dihexyloxy-4-bromobenzaldehyde (Br6A), a metal-free organic phosphor, by means of Ag nanoparticles (NPs) through surface plasmon excitation. The emission enhancement and lifetime reduction was observed in the fluorescence and phosphorescence suggesting that the phosphorescence enhancement can be achieved in the same manner as in fluorescence, through the increase of photoabsorption and the enhanced emission rate by the field enhancement around the metal nanostructures. Our results help to improve the understanding of the phosphorescence enhancement mechanism of a new class of purely organic phosphors. © 2017 Elsevier B.V1111sciescopu
Biexciton Emission from Edges and Grain Boundaries of Triangular WS2 Monolayers
No full textMonolayer tungsten disulfides (WS2) constitute a high quantum yield two-dimensional (2D) system, and can be synthesized on a large area using chemical vapor deposition (CVD), suggesting promising nanophotonics applications. However, spatially nonuniform photoluminescence (PL) intensities and peak wavelengths observed in single WS2 grains have puzzled researchers, with the origins of variation in relative contributions of excitons, trions, and biexcitons to the PL emission not well understood. Here, we present nanoscale PL and Raman spectroscopy images of triangular CVD-grown WS2 monolayers of different sizes, with these images obtained under different temperatures and values of excitation power. Intense PL emissions were observed around the edges of individual WS2 grains and the grain boundaries between partly merged WS2 grains. The predominant origin of the main PL emission from these regions changed from neutral excitons to trions and biexcitons with increasing laser excitation power, with biexcitons completely dominating the PL emission for the high-power condition. The intense PL emission and the preferential formation of biexcitons in the edges and grain boundaries of monolayer WS2 were attributed to larger population of charge carriers caused by the excessive incorporation of growth promoters during the CVD, suggesting positive roles of excessive carriers in the PL efficiency of TMD monolayers. Our comprehensive nanoscale spectroscopic investigation sheds light on the dynamic competition between exciton complexes occurring in monolayer WS,, suggesting a rich variety of ways to engineer new nanophotonic functions using 2D transition metal dichalcogenide monolayers. © 2016 American Chemical Society169751sciescopu
Simultaneous Hosting of Positive and Negative Trions and the Enhanced Direct Band Emission in MoSe2/MoS2 Heterostacked Multilayers
No full textHeterostacking of layered transition-metal dichalcogenide (LTMD) monolayers (1Ls) offers a convenient way of designing two-dimensional exciton systems. Here we demonstrate the simultaneous hosting of positive trions and negative trions in heterobilayers made by vertically stacking 1L MoSe2 and 1L MoS2. The charge transfer occurring between the 1Ls of MoSe2 and MoS2 converted the polarity of trions in 1L MoSe2 from negative to positive, resulting in the presence of positive trions in the 1L MoSe2 and negative trions in the 1L MoS2 of the same heterostacked bilayer. Significantly enhanced MoSe2 photoluminescence (PL) in the heterostacked bilayers compared to the PL of 1L MoSe2 alone suggests that, unlike other previously reported heterostacked bilayers, direct band transition of 1L MoSe2 in heterobilayer was enhanced after the vertical heterostacking. Moreover, by inserting hexagonal BN monolayers between 1L MoSe2 and 1L MoS2, we were able to adjust the charge transfer to maximize the MoSe2 PL of the heteromultilayers and have achieved a 9-fold increase of the PL emission. The enhanced optical properties of our heterostacked LTMDs suggest the exciting possibility of designing LTMD structures that exploit the superior optical properties of 1L LTMDs. © 2016 American Chemical Society202211sciescopu
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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