Effect of Element Substitution on Structural Transformation and Optical Performances in I2BaMIVQ4 ( I = Li, Na, Cu, and Ag; MIV = Si, Ge, and Sn; Q = S and Se).

In the exploration of new infrared nonlinear optical (IR NLO) materials, element substitution has been developed as an effective way to adjust the structural features and material performances. A series of new IR NLO materials have been discovered in the I-Ba-MIV-Q system ( I = Li, Na, Cu, and Ag; MIV = Si, Ge, and Sn; Q = S and Se), and they undergo interesting structural transformation with different element substitution except Li analogues. Herein, we have successfully synthesized three selenides with different space groups (Ag2BaSiSe4: I4̅2 m; Ag2BaGeSe4 and Ag2BaSnSe4: I222) in the above system and studied their properties through experimental and theoretical methods. Remarkably, the detailed analysis on the structural changes and properties comparison was also systematically investigated in the I-Ba-MIV-Q system and the results indicate that the distortion degrees of different IQ4 tetrahedra play the critical role to cause the structural transformation with the M or Q elements substitution. More importantly, we have also found that the structural changes have the close relationship with the distance d( I- I) between adjacent I cations in the I2BaSnSe4 system, which makes the four-membered rings formed by edge-sharing BaSe8 units change from the square to rhombus with the increase of d( I- I). The properties comparisons (band gap and NLO effect) in this system have been also systematically studied.

Superconductivity in Freestanding Infinite-Layer Nickelate Membranes.

The observation of superconductivity in infinite-layer nickelates has attracted significant attention due to its potential as a new platform for exploring high-Tc superconductivity. However, thus far, superconductivity has only been observed in epitaxial thin films, which limits the manipulation capabilities and modulation methods compared to two-dimensional exfoliated materials. Given the exceptionally giant strain tunability and stacking capability of freestanding membranes, separating superconducting nickelates from the as-grown substrate is a novel way to engineer the superconductivity and uncover the underlying physics. Herein, this work reports the synthesis of the superconducting freestanding La0.8Sr0.2NiO2 membranes ( T c zero = 10.6 K ${T}_{\mathrm{c}}^{\mathrm{zero}}\ =\ 10.6\ \mathrm{K}$ ), emphasizing the crucial roles of the interface engineering in the precursor phase film growth and the quick transfer process in achieving superconductivity. This work offers a new versatile platform for investigating superconductivity in nickelates, such as the pairing symmetry via constructing Josephson tunneling junctions and higher Tc values via high-pressure experiments.

Sr4Al2O7: A New Sacrificial Layer with High Water Dissolution Rate for the Synthesis of Freestanding Oxide Membranes.

Freestanding perovskite oxide membranes have drawn great attention recently since they offer exceptional structural tunability and stacking ability, providing new opportunities in fundamental research and potential device applications in silicon-based semiconductor technology. Among different types of sacrificial layers, the (Ca, Sr, Ba)3Al2O6 compounds are most widely used since they can be dissolved in water and prepare high-quality perovskite oxide membranes with clean and sharp surfaces and interfaces; However, the typical transfer process takes a long time (up to hours) in obtaining millimeter-size freestanding membranes, let alone realize wafer-scale samples with high yield. Here, a new member of the SrO-Al2O3 family, Sr4Al2O7 is introduced, and its high dissolution rate, ≈10 times higher than that of Sr3Al2O6 is demonstrated. The high-dissolution-rate of Sr4Al2O7 is most likely related to the more discrete Al-O networks and higher concentration of water-soluble Sr-O species in this compound. This work significantly facilitates the preparation of freestanding membranes and sheds light on the integration of multifunctional perovskite oxides in practical electronic devices.

Synthesis of Oxide Interface-Based Two-Dimensional Electron Gas on Si.

Two-dimensional electron gas (2DEG) at the interface of amorphous Al2O3/SrTiO3 (aAO/STO) heterostructures has received considerable attention owing to its convenience of fabrication and relatively high mobility. The integration of these 2DEG heterostructures on a silicon wafer is highly desired for electronic applications but remains challanging up to date. Here, conductive aAO/STO heterostructures have been synthesized on a silicon wafer via a growth-and-transfer method. A scanning transmission electron microscopy image shows flat and close contact between STO membranes and a Si wafer. Electron energy loss spectroscopic measurements reveal the interfacial Ti valence state evolution, which identifies the formation of 2D charge carriers confined at the interface of aAO/STO. This work provides a feasible strategy for the integration of 2DEG on a silicon wafer and other desired substrates for potential functional and flexible electronic devices.