Hydrogen is an important building block in global strategies towards a future green energy system. To make this transition possible, intense scientific efforts are needed, also in the field of ...materials science. Two-dimensional crystals, such as hexagonal boron nitride (hBN), are very promising in this regard, as it was demonstrated that micrometer-sized exfoliated flakes are excellent barriers to molecular hydrogen. However, it remains an open question whether large-area layers fabricated by industrially relevant methods preserve such promising properties. In this work we show that electron beam-induced splitting of water creates hBN bubbles that effectively store molecular hydrogen for weeks and under extreme mechanical deformation. We demonstrate that epitaxial hBN allows direct visualization and monitoring of the process of hydrogen generation by radiolysis of interfacial water. Our findings show that hBN is not only a potential candidate for hydrogen storage, but also holds promise for the development of unconventional hydrogen production schemes.
The recent progress in the growth of large-area boron nitride epilayers opens up new possibilities for future applications. However, it remains largely unclear how weakly attached two-dimensional BN ...layers interact with their substrate and how their properties are influenced by defects. In this work, we investigate hBN layers grown by Metal Organic Vapor Phase Epitaxy (MOVPE) using Fourier-transform Infrared (FTIR) spectroscopy in the temperature range of 160-540 K. Our measurements reveal strong differences in the character of layer-substrate interaction for as-grown and delaminated epitaxial layers. A much weaker interaction of as-grown layers is explained by wrinkles formation that reduces strain at the layer-substrate interface, which for layers transferred to other substrates occurs only in a limited temperature range. The most striking result is the observation of a giant increase in the \(E_{1u}\) phonon energy of up to \(\sim6\) cm\(^{-1}\) in a narrow temperature range. We show that the amplitude and temperature range of the anomaly is strongly modified by UV light illumination. The observed giant effect is explained in terms of strain generation resulting from charge redistribution between shallow traps and different defects, which can be interpreted as a result of strong electron-phonon coupling in hBN. The observed narrow temperature range of the anomaly indicates that the effect may be further enhanced for example by electrostrictive effects, expected for sp\(^2\) boron nitride.
Transition metal dichalcogenides (TMDs) are materials that can exhibit intriguing optical properties like a change of the bandgap from indirect to direct when being thinned down to a monolayer. ...Well-resolved narrow excitonic resonances can be observed for such monolayers, however only for materials of sufficient crystalline quality, so far mostly available in the form of micrometer-sized flakes. A further significant improvement of optical and electrical properties can be achieved by transferring the TMD on hexagonal boron nitride (hBN). To exploit the full potential of TMDs in future applications, epitaxial techniques have to be developed that not only allow to growlarge-scale, high-quality TMD monolayers, but allow to perform the growth directly on large-scale epitaxial hBN. In this work we address this problem and demonstrate that MoSe2 of high optical quality can be directly grown on epitaxial hBN on an entire two-inch wafer. We developed a combined growth theme for which hBN is first synthesized at high temperature by Metal Organic Vapor Phase Epitaxy (MOVPE) and as a second step MoSe2 is deposited on top by Molecular Beam Epitaxy (MBE) at much lower temperatures. We show that this structure exhibits excellent optical properties, manifested by narrow excitonic lines in the photoluminescence spectra. Moreover, the material is homogeneous on the area of the whole two-inch wafer, with only +/-0.14 meV deviation of excitonic energy. Our mixed growth technique may guide the way for future large-scale production of high quality TMD/hBN heterostructures.
Hexagonal boron nitride is a promising material for many applications ranging from deep UV emission to an ideal substrate for other two dimensional crystals. Although efforts towards the growth of ...wafer-scale, high quality material strongly increased in recent years, the understanding of the actual growth mechanism still remains fragmentary and premature. Here, we unveil fundamental growth mechanisms by investigating the growth of hBN by metalorganic vapor phase epitaxy (MOVPE) in a wide range of growth conditions. The obtained results contradict the widespread opinion about the importance of parasitic gas-phase reactions decreasing the growth efficiency. Two different growth mechanisms that depend on ammonia flow and reactor pressure can be distinguished. Both mechanisms are effective in the case of polycrystalline growth, but the growth of highly ordered, flat layers, is strongly hindered. The problem is caused by a low efficiency of boron chemisorption on N-terminated edges of sp2-BN sheets forming the atomic steps on the surface of the layer. Two-dimensional growth can be activated and sustained by the flow modulation epitaxy (FME) method, an alternate switching of ammonia and TEB flows. The success of the FME method is explained in terms of periodic changes between N- and B-terminated reconstructions at the edges of sp2- BN sheets, which restore boron chemisorption. The presented results identify the fundamental growth mechanisms, which is the prerequisite for any further deterministic development of efficient, high-quality, large-scale hBN growth with MOVPE.
Provider: - Institution: - Data provided by Europeana Collections- artykuł w: Annales Universitatis Mariae Curie-Skłodowska, Sectio B, Geographia, Geologia, Mineralogia et Petrographia Vol. 69 ...(2014), 1, s. 49-60- streszcz. pol.- tyt. równol. w jęz. pol.: Monitoring transportu fluwialnego w małych zlewniach wyżynnych - metody i wstępne wyniki- All metadata published by Europeana are available free of restriction under the Creative Commons CC0 1.0 Universal Public Domain Dedication. However, Europeana requests that you actively acknowledge and give attribution to all metadata sources including Europeana
