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Abstract: Tin oxide (SnOx) by atomic-layer deposition (ALD), in combination with fullerene, is widely employed as an electron transport layer in p–i–n perovskite solar cells. This study investigates the direct deposition of ALD SnOx on top of formamidinium (FA)-based perovskites, as a step toward the elimination of the fullerene interlayer and its poor effect on solar cell’s long-term stability. The interfacial chemistry between FA-based perovskites (FAPbI3 and FAPbBr3) and ALD SnOx was studied using soft and hard X-ray photoelectron spectroscopy (SOXPES and HAXPES) with a focus on investigating the separate roles FA and different halides play during interface formation. FAPbI3 and FAPbBr3 solar cell structures solely containing ALD SnOx resulted in s-shaped current–voltage characteristics, indicating the formation of a transport barrier at the interface. Both SOXPES and HAXPES measurements revealed the emergence of additional nitrogen states at the interface during the ALD SnOx deposition on FAPbI3 and FAPbBr3, where these states are linked to the decomposition of FA+. The FAPbI3/ALD SnOx interface also showed the presence of lead iodide (PbI2) through additional lead states other than that from FAPbI3 by using SOXPES measurements. Concerning the FAPbBr3/ALD SnOx interface, no additional lead states were observed; however, measurements instead revealed the formation of Sn–Br bonds at the interface along with the migration of bromine ions into the bulk of the ALD SnOx. Thus, FAPbI3 and FAPbBr3 undergo distinct reaction pathways upon direct deposition of ALD SnOx on top of them. We reason that the decomposition of FA+ in both perovskites and the formation of PbI2 at the FAPbI3/ALD SnOx interface and the incorporation of Br in SnOx at the FAPbBr3/ALD SnOx interface prove detrimental toward device performance. Therefore, careful interfacial engineering that can mitigate the formation of these products should be utilized to enhance the performance of perovskite solar cells.

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Abstract: Li-rich manganese-based oxides (LRMO) are promising cathode materials for next-generation lithium-ion batteries due to their high-capacity and low-cost merits. However, the low initial coulombic efficiency (ICE) and irreversible oxygen release of LRMO severely hinder their commercialization processes. Here, we employ glyoxal treatment to modulate the hybridization between transition metal (TM) 3d and oxygen (O) 2p orbitals in LRMO. This approach is found to reduce the Co/Mn t2g-O 2p hybridization in LRMO while simultaneously activating the Co2+/Co3+ redox below the Fermi level. Our findings demonstrate that tuning TM 3d-O 2p orbital hybridization can be a viable approach to improve the ICE of LMRO. Specifically, the ICE of LRMO can be elevated from 85.3 % to 102.5 %, and a high specific capacity of 291.2 mAh g−1 can be achieved at 0.1 C. Moreover, the treated LRMO cathodes exhibit significantly enhanced capacity retention.

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Abstract: Chromium ditelluride, CrTe2, is an attractive candidate van der Waals material for hosting 2D magnetism. However, how the room-temperature ferromagnetism of the bulk evolves as the sample is thinned to the single-layer limit has proved controversial. This, in part, reflects its metastable nature, vs. a series of more stable self-intercalation compounds with higher relative Cr:Te stoichiometry. Here, exploiting a recently developed method for enhancing nucleation in molecular-beam epitaxy growth of transition-metal chalcogenides, we demonstrate the selective stabilisation of high-coverage CrTe2 and Cr2+εTe3 epitaxial monolayers. Combining X-ray magnetic circular dichroism, scanning tunnelling microscopy, and temperature-dependent angle-resolved photoemission, we demonstrate that both compounds order magnetically with a similar TC. We find, however, that monolayer CrTe2 forms as an antiferromagnetic metal, while monolayer Cr2+εTe3 hosts an intrinsic ferromagnetic semiconducting state. This work thus demonstrates that control over the self-intercalation of metastable Cr-based chalcogenides provides a powerful route for tuning both their metallicity and magnetic structure, establishing the CrxTey system as a flexible materials class for future 2D spintronics.

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Abstract: Epitaxial heterostructures integrating thin Fe3O4 films hold great potential for spintronics, magnetoionics, and multifunctional device development. In this work, the morpho-structural and magnetic properties of all-spinel Fe3O4/MgCr2O4/Fe3O4 trilayers grown on a MgCr2O4 buffer-layer, exhibiting very close lattice matching, were investigated by using both surface and bulk sensitive techniques. The close lattice match between Fe3O4 and MgCr2O4 enables the growth of epitaxial heterostructures with magnetically decoupled Fe3O4 layers for spacer thicknesses ≥ 1.6 nm, while reducing the formation of antiphase boundaries. Despite localized interphase diffusion, which leads to the formation of a mixed Cr/Fe spinel oxide with magnetically polarized Cr ions at the Fe3O4/MgCr2O4 interfaces, the overall magnetic properties remain largely consistent with those of the individual Fe3O4 layers. This study sheds light on the magnetic interactions within Fe3O4 layers mediated by a MgCr2O4 spacer, and demonstrates the feasibility of the approach in preserving the properties of thin Fe3O4 films, in complex heterostructures, thus offering a promising pathway for designing advanced all-spinel oxide devices.

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Abstract: Single-crystalline LiNiO2 (SC-LNO), a high-energy-density Li-ion cathode material, suffers from poor long-term electrochemical performance when cycled above 4.2 V (vs Li+/Li). In this study, this degradation is evaluated using SC-LNO–graphite pouch cells electrochemically aged within a stressful voltage window (2.5–4.4 V) using a constant-current constant-voltage (CC-CV) protocol. Notable capacity fade is observed after one hundred cycles at C/3 rate, in addition to an increase in the overall electrochemical cell impedance. Operando X-ray diffraction data reveal that, despite no significant long-range bulk structural changes, (de-)lithiation of the aged SC-LNO becomes kinetically hindered after 100 cycles. Aging-induced changes in the short-range structure and charge compensation are evaluated through a multi-model quantitative analysis of the operando X-ray absorption spectroscopy data. While the electrochemical aging does not result in particle cracking, soft X-ray absorption spectroscopy data revealed the reconstruction of the cathode surface to a dense rock salt-like layer after long-term cycling, which acts as a kinetic trap for Li+ diffusion. Therefore, even under stressful conditions, it is the surface reconstruction that dominates the overall cathode degradation by reducing the Li+ mobility and leading to the capacity fade. Cathode surface engineering will therefore be key to improving the long-term electrochemical performance of SC-LNO cathodes.