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《Identification of exfoliated monolayer hexagonal boron nitride films with a digital color camera under white light illumination》

  • 来源专题：现代化工
  • 编译者： 武春亮
  • 发布时间：2024-06-19
  • Optical microscopy with white light illumination has been employed when obtaining exfoliated monolayer hexagonal boron nitride (1L hBN) films from a large number of randomly placed films on a substrate. However, real-time observation of 1L hBN using a color camera under white light illumination remains challenging since hBN is transparent in the visible wavelength range. The poor optical constant of 1L hBN films in microphotographs is significantly improved using a Si substrate coated with a SiNx thin-film (SiNx/Si). When observing hBN thin films on SiNx/Si using a color digital camera in an optical microscope under white light illumination, the clarity of the captured images depends on the thickness of the SiNx film (d). For direct real-time observation, the d was optimized based on quantitative chromatic studies tailored to Bayer filters of a color image sensor. Through image simulation, it was determined that the color difference between 1L hBN and the bare substrate is maximized at d = 59 or 70 nm, which was experimentally verified. The SiNx/Si with optimized d values visualized 1L hBN films without requiring significant contrast enhancement via image processing under white light illumination in real-time. Furthermore, the captured color photographs facilitate the reliable determination of the number of layers in few-layer hBN films using the contrast of the green channel of the images.
  • 原文来源：http://iopscience.iop.org/article/10.1088/1361-6528/ad58e7
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  • 《High-quality boron nitride grown at atmospheric pressure》
    • 来源专题：后摩尔
    • 编译者：shenxiang
    • 发布时间：2020-05-12
    • Graphene Flagship researchers at RWTH Aachen University, Germany and ONERA-CNRS, France, in collaboration with researchers at the Peter Grunberg Institute, Germany, the University of Versailles, France, and Kansas State University, US, have reported a significant step forward in growing monoisotopic hexagonal boron nitride at atmospheric pressure for the production of large and very high-quality crystals. Hexagonal boron nitride (hBN) is the unsung hero of graphene-based devices. Much progress over the last decade was enabled by the realisation that 'sandwiching' graphene between two hBN crystals can significantly improve the quality and performance of the resulting devices. This finding paved the way to a series of exciting developments, including the discoveries of exotic effects such as magic-angle superconductivity and proof-of-concept demonstrations of sensors with unrivalled sensitivity. Until now, the most widely used hBN crystals came from the National Institute of Material Science in Tsukuba, Japan. These crystals are grown using a process at high temperatures (over 1500°C) and extremely high pressures (over 40,000 times atmospheric pressure). "The pioneering contribution by the Japanese researchers Taniguchi and Watanabe to graphene research is invaluable," begins Christoph Stampfer from Graphene Flagship Partner RWTH Aachen University, Germany. "They provide hundreds of labs around the world with ultra-pure hBN at no charge. Without their contribution, a lot of what we are doing today would not be possible." However, this hBN growth method comes with some limitations. Among them is the small crystal size, which is limited to a few hundred μm, and the complexity of the growth process. This is suitable for fundamental research, but beyond this, a method with better scalability is needed. Now Graphene Flagship researchers tested hBN crystals grown with a new methodology that works at atmospheric pressure, developed by a team of researchers led by James Edgar at Kansas State University, US. This new approach shows great promise for more demanding research and production. "I was very excited when Edgar proposed that we test the quality of his hBN," says Stampfer. "His growth method could be suitable for large-scale production." The method for growing hBN at atmospheric pressure is indeed much simpler and cheaper than previous alternatives and allows for the isotopic concentration to be controlled. "The hBN crystals we received were the largest I have ever seen, and they were all based either on isotopically pure boron-10 or boron-11" says Jens Sonntag, a graduate student at Graphene Flagship Partner RWTH Aachen University. Sonntag tested the quality of the flakes first using confocal Raman spectroscopy. In addition, Graphene Flagship partners in ONERA-CNRS, France, led by Annick Loiseau, carried out advanced luminescence measurements. Both measurements indicated high isotope purity and high crystal quality. However, the strongest evidence for the high hBN quality came from transport measurements performed on devices containing graphene sandwiched between monoisotopic hBN. They showed equivalent performance to a state-of-the-art device based on hBN from Japan, with better performance in some areas. "This is a clear indication of the extremely high quality of these hBN crystals," says Stampfer. "This is great news for the whole graphene community, because it shows that it is, in principle, possible to produce high quality hBN on a large scale, bringing us one step closer to real applications based on high-performance graphene electronics and optoelectronics. Furthermore, the possibility of controlling the isotopic concentration of the crystals opens the door to experiments that were not possible before." Mar García-Hernández, Work Package Leader for Enabling Materials, adds: "Free-standing graphene, being the thinnest material known, exhibits a large surface area and, therefore, is extremely sensitive to its surrounding environment, which, in turn, results in substantial degradation of its exceptional properties. However, there is a clear strategy to avoid these deleterious effects: encapsulating graphene between two protective layers." García-Hernández continues: "When graphene is encapsulated by hBN, it reveals its intrinsic properties. This makes hBN an essential material to integrate graphene into current technologies and demonstrates the importance of devising new scalable synthetic routes for large-scale hBN production. This work not only provides a new and simpler path to produce high-quality hBN crystals on a large scale, but it also enables the production of monoisotopic material, which further reduces the degradation of graphene when encapsulated by two layers." Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, adds: "This is a nice example of collaboration between the EU and the US, which we fostered via numerous bilateral workshops. Devising alternative approaches to produce high-quality hBN crystals is crucial to enable us to exploit the ultimate properties of graphene in opto-electronics applications. Furthermore, this work will lead to significant progress in fundamental research." Explore further Graphene, perovskites, and silicon—an ideal tandem for efficient solar cells More information: Jens Sonntag et al, Excellent electronic transport in heterostructures of graphene and monoisotopic boron-nitride grown at atmospheric pressure, 2D Materials (2020). DOI: 10.1088/2053-1583/ab89e5
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  • 《White nanolight source for optical nanoimaging》
    • 来源专题：后摩尔
    • 编译者：shenxiang
    • 发布时间：2020-06-24
    • Nanolight sources based on resonant excitons of plasmons near a sharp metallic nanostructure have attracted great interest in optical nanoimaging. However, the resonant phenomenon only works for one type of wavelength that resonates with plasmons. Compared to plasmonic resonance, the alternative plasmon nanofocusing method can generate a source of nanolight by propagating and compressing plasmons on a tapered metallic nanostructure, independent of wavelength, due to its reliance on propagation. In a new report on Science Advances, Takayuki Umakoshi and a research team in applied physics and chemistry in Japan generated a white nanolight source spanning across the entire visible light range through plasmon nanofocusing. Using the process, they demonstrated spectral bandgap nanoimaging of carbon nanotubes (CNTs). The experimental demonstration of the source of white nanolight will enable diverse research fields to progress toward next-generation, nanophotonic technologies. The co-existence of multiple wavelengths of light in a confined nanometric volume can constitute an interesting optical effect. The unique nanolight is therefore a promising platform for diverse research fields by providing opportunities to probe a sample across a range of wavelengths, or induce light-light interactions between different wavelengths at the nanoscale. Optical antennas have played an important role in recent decades to confine light at the nanoscale through localized plasmon resonances in metallic nanostructures, leading to unprecedented research in nanolight, including light-field enhancement. Since plasmon resonance is a resonant phenomenon, it cannot facilitate broadband nanolight generation, therefore, as a result, plasmon nanofocusing has gained wider attention as an alternative to generate sources of nanolight. During the process, a nanoscale light source can be engineered by propagating and superfocusing surface plasmon polaritons (SPPs) at the apex of a metallic, tapered superstructure. The work led to enormous enhancement of the light field at the nanoscale, at the apex and resulted in background-free illumination. Scientists have explored the resulting broadband property for four-wave mixing with a high nonlinear conversion efficiency. The plasmon-nanofocused broadband light source is a powerful tool across diverse research fields. Broadband property of plasmon nanofocusing evaluated by FDTD simulations. (A) Electric field distribution maps in the vicinity of the apex of the tapered silver structure produced by FDTD simulations. Scale bars, 100 nm. The plasmon coupler slit, where white light was illuminated, is not shown, as it is out of the frame. (B) Simulated near-field spectrum detected 6 nm below the apex. Credit: Science Advances, doi: 10.1126/sciadv.aba4179 In this work, Umakoshi et al. introduced a white nanolight source spanning across the entire visible wavelength range—generated via plasmon nanofocusing. They showed broadband energy bandgap optical imaging of carbon nanotubes using the white nanolight source. Although plasmon nanofocusing can be excited in a broad wavelength range, researchers have only used it in the near-infrared range due to limitations of materials constituting the tapering structure. They had used gold as a material to form conical tapered structures and lower ohmic losses, but such experiments remained in the near-infrared range and not in the visible or ultraviolet range. Umakoshi et al. had also recently developed an efficient fabrication method to form tapered metallic structures based on thermal evaporation, where the construct included a commercially available silicon cantilever with a pyramidal tip. Using a surface of the pyramid as a base, they obtained a two-dimensional metallic taper and created an extremely smooth metallic coating applicable to a range of metal types, including silver. Using the silver taper, the team obtained highly efficient plasmon nanofocusing with 100 percent reproducibility at 642 nm and conducted white plasmon nanofocusing across a broad range of visible wavelengths. Fabrication of a tapered silver structure on a cantilever tip. (A) Schematic of fabrication process of the tapered silver structure on a cantilever tip. (B) Scanning electron microscopy image of the fabricated tapered silver structure on the cantilever tip. The inset shows a side view of the silver layer. Scale bars, 2 μm (inset, 200 nm). Credit: Science Advances, doi: 10.1126/sciadv.aba4179 Umakoshi et al. developed a tapered metallic structure to maintain a broadband white nanolight source on an oxidized silicon pyramidal tip with a thin silver layer coated on a surface of the pyramid. Using a single slit of 200 nanometer (nm) in silver they coupled light in the visible range, and calculated the electric field distributions in the vicinity of the apex at multiple excitation wavelengths using the finite-difference-time domain (FDTD) method. The team observed strong electric fields confined at the apex tip at excitation wavelengths ranging from 460 nm to 1200 nm. The work showed how a 200-nm-wide slit generated a broadband nanolight source spanning across the entire visible region to even reach the near-infrared region. During the fabrication process, the scientists used a commercially available silicon cantilever tip with a pyramidal shape. They oxidized the silicon cantilever and developed a smooth silver coating of 1 nm surface roughness to reduce energy loss during SPP (surface plasmon polariton) propagation. Optical observation of a white nanolight source generated through plasmon nanofocusing. (A) Optical image of a tapered silver structure under illumination by supercontinuum laser at its slit. The locations of the boundaries of the tip as well as the slit are indicated by dashed lines. The inset shows a zoomed image of the apex. Incident polarization was normal to the slit as indicated by the arrow. (B and C) Optical images of the same tapered silver structure with supercontinuum laser illumination at different incident polarizations, as indicated by the arrows. (D) Polar graph of the light spot intensity at the apex with respect to the incident polarization; 0° and 90° correspond to parallel and perpendicular polarizations, respectively. (E) Optical images of the tapered silver structure illuminated with a supercontinuum laser, observed through a series of band-pass filters indicated by their central wavelengths. (F) Scattering spectrum of the optical spot at the apex of the tapered silver structure. a.u., arbitrary units. (G) Simulated near-field spectrum calculated at the tip apex. Scale bars, 2 μm (A and E). Credit: Science Advances, doi: 10.1126/sciadv.aba4179 Generating a white light source via plasmon nanofocusing and conducting spectral bandgap imaging To understand the process of confined white light production through the tapered structure based on plasmon nanofocusing, the team illuminated the slit structure with a coherent supercontinuum laser that spanned across a wide range of wavelengths. When the incident polarization was perpendicular to the slit, they noted the best coupling in the setup in agreement with simulations. As the wavelength shortened, the scattering efficiency increased. Therefore, the team experimentally observed a higher intensity in the shorter wavelength range. They used the plasmon-nanofocused white light source to perform spectral nanoanalysis of CNTs (carbon nanotubes). The white nanolight source localized at the tip of the apex interacted with CNT bundles containing multiple bandgaps during the experiment. The scattering signal increased during the experiment to indicate photons with the same energy that corresponded to the bandgaps of the CNTs. Umakoshi et al. then combined the approach with Raman spectroscopy to examine chirality of the CNT sample. Optical nanoimaging of CNTs using the white nanolight source. (A) An AFM image of CNT bundles. The structures observed on the left and the right parts of the image are the metallic (m-CNTs) and semiconducting (s-CNTs) CNTs, respectively, as identified during the sample preparation process. Scale bar, 100 nm. (B) Near-field spectra of s-CNTs and m-CNTs, obtained from the locations indicated by the blue and red crosses, respectively, in (A). (C) Near-field spectra obtained pixel by pixel along the dotted line in (A). (D to F) Bandgap images constructed at 620, 680, and 730 nm, respectively. Scale bars, 100 nm. Credit: Science Advances, doi: 10.1126/sciadv.aba4179 The plasmon-focussed white light source in this work is a fundamental and effective state of light for bandgap nanoimaging. This work will pave the way for a variety of possible applications, including probing biomolecules to understand their absorption properties at nanoscale spatial resolution. A mid-infrared broadband nanolight source will also be productive across materials science and molecular biology. This technique can also boost the analytical capability of surface-enhanced Raman spectroscopy to investigate molecular vibrations. In this way, Takayuki Umakoshi and colleagues generated a white nanolight source at the apex of a tapered silver structure using plasmon nanofocusing to perform nanoanalysis of carbon nanotubes. The team designed and engineered a tapered structure that induced plasmon nanofocusing across a broad wavelength range. The spectral bandgap technique will have wide-ranging applications at the nanoscale across materials science and biological research. The demonstrated work is only a single example, with diverse applications possible based on a powerful and fundamental nanoscale optical tool with excellent wavelength flexibility.
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