Achieving Superior Tensile Performance in Individual Metal-Organic Framework Crystals.

Rapid advances in the engineering application prospects of metal-organic framework (MOF) materials necessitate an urgent in-depth understanding of their mechanical properties. This work demonstrates unprecedented recoverable elastic deformation of Ni-tetraphenylporphyrins (Ni-TCPP) MOF nanobelts with a tensile strain as high as 14%, and a projected yield strength-to-Young's modulus ratio exceeding the theoretical limit (≈10%) for crystalline materials. Based on first-principles simulations, the observed behavior of MOF crystal can be attributed to the mechanical deformation induced conformation transition and the formation of helical configuration of dislocations under high stresses, arising from their organic ligand building blocks in the crystal structures. The investigations of the mechanical properties along with electromechanical properties demonstrate that MOF materials have exciting application potential for biomechanics integrated systems, flexible electronics, and nanoelectromechanical devices.

The Limits of Electromechanical Coupling in Highly-Tensile Strained Germanium.

Speculations regarding electronic and photonic properties of strained germanium (Ge) have perpetually put it into contention for next-generation devices since the start of the information age. Here, the electromechanical coupling of <111> Ge nanowires (NWs) is reported from unstrained conditions to the ultimate tensile strength. Under tensile strain, the conductivity of the NW is enhanced exponentially, reaching an enhancement factor of ∼130 at ∼3.5% of strain. Under strains larger than ∼2.5%, the electrical properties of Ge also exhibit a dependence on the electric field. The conductivity can be further enhanced by ∼2.2× with a high bias condition at ∼3.5% of strain. Cyclic loading tests confirm that the observed electromechanical responses are repeatable, reversible, and related to the changing electronic band structure. These tests reveal the excellent prospects for utilizing strained Ge NWs in photodetector or piezoelectronic transistor applications, but significant challenges remain to realize strict direct band gap devices.

Performance Recovery in Degraded Carbon-Based Electrodes for Capacitive Deionization.

Limitations of capacitive deionization (CDI) and future commercialization efforts are intrinsically bound to electrode stability. In this work, thermal treatments are explored to understand their ability to regenerate aged CDI electrodes. We demonstrate that a relatively low thermal treatment temperature of ∼500 °C can sufficiently recover the lost salt adsorption capacity of degraded electrodes. Furthermore, a systematic study of electrode replacement clarifies that the desalination ability loss and regeneration for a CDI cell are isolated to the aged anode, as expected. Characterizations of surface functionalities support that the acidic oxygen-containing functional groups formed in situ during cycling undergo thermal decomposition during treatment. The modified Donnan model quantitatively confirms that the surface charges originate from the formation/decomposition of functional groups. Accordingly, the lost pore volume and the increased resistance are recovered during thermal treatments, while the surface morphologies and pore structure of the electrodes are well-preserved. Therefore, thermal treatment can be applied practically to extend the lifetime of aged electrodes. This study also offers insights into strategies for minimizing electrode degradation or in situ regeneration such that the technology gains momentum for future commercialization.

Leveraging Titanium to Enable Silicon Anodes in Lithium-Ion Batteries.

Silicon is a promising anode material for lithium-ion batteries because of its high gravimetric/volumetric capacities and low lithiation/delithiation voltages. However, it suffers from poor cycling stability due to drastic volume expansion (>300%) when it alloys with lithium, leading to structural disintegration upon lithium removal. Here, it is demonstrated that titanium atoms inside the silicon matrix can act as an atomic binding agent to hold the silicon atoms together during lithiation and mend the structure after delithiation. Direct evidence from in situ dilatometry of cosputtered silicon-titanium thin films reveals significantly smaller electrode thickness change during lithiation, compared to a pure silicon thin film. In addition, the thickness change is fully reversible with lithium extraction, and ex situ post-mortem microscopy shows that film cracking is suppressed. Furthermore, Raman spectroscopy measurements indicate that the Si-Ti interaction remains intact after cycling. Optimized Si-Ti thin films can deliver a stable capacity of 1000 mAh g-1 at a current of 2000 mA g-1 for more than 300 cycles, demonstrating the effectiveness of titanium in stabilizing the material structure. A full cell with a Si-Ti anode and LiFePO4 cathode is demonstrated, which further validates the readiness of the technology.