The isolation of 2D materials from mother bulks into nanoscale is of vital importance for fulfilling their applications in many technological fields. Among known methods, mechanical exfoliation is one of the most widely utilized ways due to its simplicity. For a given 2D material, both its interlayer and its intralayer bonding strengths need to be taken into account for understanding the exfoliation process as the former dominates the ease level for cleaving adjacent molecular layers while the latter regulates the ability to resist cracking. In this regard, strong intralayer but weak interlayer bonding interactions respectively lead to large and thin nanosheets and, hence, facile exfoliation (and vice versa). As the bonding forces can be directly reflected through elastic properties of materials, here we propose to use the ratio between the in-plane and out-of-plane elastic modulus (E) as a universal index, A In/Out (= E In-plane /E Out-of-plane ), to quantify the ease level of a 2D material's mechanical exfoliation. Such ratios, which can be facilely obtained from routine computational and mechanical experiments, could provide useful information for estimating suitable exfoliation methods of 2D materials.
Here we study the Jahn-Teller (JT) effect on framework flexibility of two analogous hybrid organic-inorganic perovskites, [C(NH)][Zn(HCOO)] (1-Zn) and [C(NH)][Cu(HCOO)] (2-Cu). Single-crystal nanoindentation measurements show that the elastic moduli and hardnesses of 1-Zn are up to ∼52.0% and ∼25.0% greater than those of the JT active 2-Cu. Temperature-dependent X-ray diffraction measurements indicate that the thermal expansion along the b-axis is switched from negative to positive by replacing Zn with Cu on the B-site. These stark distinctions in framework flexibility are primarily attributed to the ∼10.0% elongation of Cu-O bonds induced by the JT effect and associated alterations in octahedral tilting and hydrogen-bonding. Our results demonstrate the prominence of the JT effect in the emerging hybrid perovskites and highlight the possibilities of tuning materials' properties using orbital order.
A Cd(II)-based metal-organic framework, [Cd2(DPDC)2(BTB)]∞ (Cd-MOF, DPDC = 2,2'-diphenyldicarboxylate and BTB = 1,4-bis(1,2,4-triazol-1-yl)butane) was successfully constructed via a hydrothermal reaction. Structural analysis shows that the synthesized Cd-MOF is a three-dimensional (3D) architecture crystallized in the hexagonal system with a chiral space group P61. Powder X-ray diffraction experiments and thermogravimetric analysis reveal that the constructed Cd-MOF has a high chemical and thermal stability. A study of additional mechanical properties indicates that it exhibits a moderate stiffness with the average values of Young's modulus (E) and H as 11.3(2) and 0.9(7) GPa, respectively. The luminescence properties of the Cd-MOF were further studied. The result shows that it could be an effective sensor to the organic nitrobenzene molecule via a strong quenching effect, and also to the inorganic Tb(III) ion by a strong green emission effect. Moreover, when loading bimetal ions (Eu(III) and Tb(III) into the Cd-MOF/methanol suspension, tunable visible luminescence can also be achieved by carefully adjusting the excitation wavelengths.
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