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1. Stabilizing Ti 3 C 2 T x MXene flakes in air by removing confined water

   https://doi.org/10.1073/pnas.2400084121  

   Fang, Hui ; Thakur, Anupma ; Zahmatkeshsaredorahi, Amirhossein ; Fang, Zhenyao ; Rad, Vahid ; Shamsabadi, Ahmad A ; Pereyra, Claudia ; Soroush, Masoud ; Rappe, Andrew M ; Xu, Xiaoji G ; et al ( July 2024 , Proceedings of the National Academy of Sciences)

   MXenes have demonstrated potential for various applications owing to their tunable surface chemistry and metallic conductivity. However, high temperatures can accelerate MXene film oxidation in air. Understanding the mechanisms of MXene oxidation at elevated temperatures, which is still limited, is critical in improving their thermal stability for high-temperature applications. Here, we demonstrate that Ti${}_3$C${}_2$T${}_x$MXene monoflakes have exceptional thermal stability at temperatures up to 600${}^{°}$C in air, while multiflakes readily oxidize in air at 300${}^{°}$C. Density functional theory calculations indicate that confined water between Ti${}_3$C${}_2$T${}_x$flakes has higher removal energy than surface water and can thus persist to higher temperatures, leading to oxidation. We demonstrate that the amount of confined water correlates with the degree of oxidation in stacked flakes. Confined water can be fully removed by vacuum annealing Ti${}_3$C${}_2$T${}_x$films at 600${}^{°}$C, resulting in substantial stability improvement in multiflake films (can withstand 600${}^{°}$C in air). These findings provide fundamental insights into the kinetics of confined water and its role in Ti${}_3$C${}_2$T${}_x$oxidation. This work enables the use of stable monoflake MXenes in high-temperature applications and provides guidelines for proper vacuum annealing of multiflake films to enhance their stability.

    

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2. Structural phase purification of bulk HfO 2 :Y through pressure cycling

   https://doi.org/10.1073/pnas.2312571121  

   Musfeldt, J. L. ; Singh, Sobhit ; Fan, Shiyu ; Gu, Yanhong ; Xu, Xianghan ; Cheong, S.-W. ; Liu, Z. ; Vanderbilt, David ; Rabe, Karin M. ( January 2024 , Proceedings of the National Academy of Sciences)

   We combine synchrotron-based infrared absorption and Raman scattering spectroscopies with diamond anvil cell techniques and first-principles calculations to explore the properties of hafnia under compression. We find that pressure drives HfO${}_2$:7%Y from the mixed monoclinic ($P{2}_1/c$)$+$antipolar orthorhombic ($\mathit{Pbca}$) phase to pure antipolar orthorhombic ($\mathit{Pbca}$) phase at approximately 6.3 GPa. This transformation is irreversible, meaning that upon release, the material is kinetically trapped in the$\mathit{Pbca}$metastable state at 300 K. Compression also drives polar orthorhombic ($Pca{2}_1$) hafnia into the tetragonal ($P{4}_2/nmc$) phase, although the latter is not metastable upon release. These results are unified by an analysis of the energy landscape. The fact that pressure allows us to stabilize targeted metastable structures with less Y stabilizer is important to preserving the flat phonon band physics of pure HfO${}_2$.

    

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3. Development of broad modulus profile upon polymer–polymer interface formation between immiscible glassy–rubbery domains

   https://doi.org/10.1073/pnas.2312533120  

   Gagnon, Yannic J ; Burton, Justin C ; Roth, Connie B ( January 2024 , Proceedings of the National Academy of Sciences)

   Interfaces of glassy materials such as thin films, blends, and composites create strong unidirectional gradients to the local heterogeneous dynamics that can be used to elucidate the length scales and mechanisms associated with the dynamic heterogeneity of glasses. We focus on bilayer films of two different polymers with very different glass transition temperatures ($T_{\text{g}}$) where previous work has demonstrated a long-range (∼200 nm) profile in local$T_{\text{g}}(z)$is established between immiscible glassy and rubbery polymer domains when the polymer–polymer interface is formed to equilibrium. Here, we demonstrate that an equally long-ranged gradient in local modulus$\tilde{G}(z)$is established when the polymer–polymer interface ($\approx$5 nm) is formed between domains of glassy polystyrene (PS) and rubbery poly(butadiene) (PB), consistent with previous reports of a broad$T_{\text{g}}(z)$profile in this system. A continuum physics model for the shear wave propagation caused by a quartz crystal microbalance across a PB/PS bilayer film is used to measure the viscoelastic properties of the bilayer during the evolution of the PB/PS interface showing the development of a broad gradient in local modulus$\tilde{G}(z)$spanning$\approx$180 nm between the glassy and rubbery domains of PS and PB. We suggest these broad profiles in$T_{\text{g}}(z)$and$\tilde{G}(z)$arise from a coupling of the spectrum of vibrational modes across the polymer–polymer interface as a result of acoustic impedance matching of sound waves with$\lambda \sim 5$nm during interface broadening that can then trigger density fluctuations in the neighboring domain.

    

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4. Tuning metal/superconductor to insulator/superconductor coupling via control of proximity enhancement between NbSe 2 monolayers

   https://doi.org/10.1088/1361-648X/acbf92  

   Chiatti, Olivio ; Mihov, Klara ; Griffin, Theodor U. ; Grosse, Corinna ; Alemayehu, Matti B. ; Hite, Kyle ; Hamann, Danielle ; Mogilatenko, Anna ; Johnson, David C. ; Fischer, Saskia F. ( March 2023 , Journal of Physics: Condensed Matter)

   The interplay between charge transfer and electronic disorder in transition-metal dichalcogenide multilayers gives rise to superconductive coupling driven by proximity enhancement, tunneling and superconducting fluctuations, of a yet unwieldy variety. Artificial spacer layers introduced with atomic precision change the density of states by charge transfer. Here, we tune the superconductive coupling between$\text{NbS}{\text{e}}_{\text{2}}$monolayers from proximity-enhanced to tunneling-dominated. We correlate normal and superconducting properties in${\left[{\left(\text{SnSe}\right)}_{1+\delta }\right]}_m{\left[\text{NbS}{\text{e}}_{\text{2}}\right]}_1$tailored multilayers with varying SnSe layer thickness ($m=1-15$). From high-field magnetotransport the critical fields yield Ginzburg–Landau coherence lengths with an increase of$140\mathrm{\%}$cross-plane ($m=1-9$), trending towards two-dimensional superconductivity for$m>9$. We show cross-overs between three regimes: metallic with proximity-enhanced coupling ($m=1-4$), disordered-metallic with intermediate coupling ($m=5-9$) and insulating with Josephson tunneling ($m>9$). Our results demonstrate that stacking metal mono- and dichalcogenides allows to convert a metal/superconductor into an insulator/superconductor system, prospecting the control of two-dimensional superconductivity in embedded layers.

    

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5. Nano-acoustic resonator with ultralong phonon lifetime

   https://doi.org/10.1126/science.abc7312  

   MacCabe, Gregory S. ; Ren, Hengjiang ; Luo, Jie ; Cohen, Justin D. ; Zhou, Hengyun ; Sipahigil, Alp ; Mirhosseini, Mohammad ; Painter, Oskar ( November 2020 , Science)

   The energy damping time in a mechanical resonator is critical to many precision metrology applications, such as timekeeping and force measurements. We present measurements of the phonon lifetime of a microwave-frequency, nanoscale silicon acoustic cavity incorporating a phononic bandgap acoustic shield. Using pulsed laser light to excite a colocalized optical mode of the cavity, we measured the internal acoustic modes with single-phonon sensitivity down to millikelvin temperatures, yielding a phonon lifetime of up to${\tau }_{\mathrm{ph},0}\approx 1.5$seconds (quality factor$Q=5\times {10}^{10}$) and a coherence time of${\tau }_{\mathrm{coh},0}\approx 130$microseconds for bandgap-shielded cavities. These acoustically engineered nanoscale structures provide a window into the material origins of quantum noise and have potential applications ranging from tests of various collapse models of quantum mechanics to miniature quantum memory elements in hybrid superconducting quantum circuits.

    

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