We report high optical gain from freestanding, optically stable, and mechanically robust films that are loaded with cross-linked CdSe/Cd1–x Zn x Se1–y S y core/alloyed shell quantum dots (QD). These solid films display very high net optical gain as high as 650 cm–1 combined with a low pump excitation gain threshold of 44 μJ/cm2. The functionalization of the QDs using short-chain bifunctional cross-linkers not only significantly improves the net optical gain by allowing for a nearly 2-fold increase in QD loading but also provides stable passivation of the QDs which imparts excellent thermal stability, mechanical robustness, and stability under harsh chemical environments. The gain achieved here is up to 3-fold higher than that typically reported for traditional drop-cast QD films. Moreover, stable photoluminescence over long shelf storage time is a distinguished characteristic of the films. The QD films fabricated here span large areas (several cm2), can be readily micropatterned and sustain multiple harsh chemical treatment. Furthermore, they can be readily transferred onto different substrates without compromising their structural integrity and without diminishing optical activity that opens the paths to design complex and robust gain–loss optical structures.

Twisted bilayer graphene exhibits many intriguing behaviors ranging from superconductivity to the anomalous Hall effect to ferromagnetism at a magic angle of ∼1°. Here, using a first-principles approach, we reveal ferromagnetism in a twisted bilayer graphene nanoflex. Our results demonstrate that when the energy gap of a twisted nanoflex approaches zero, electronic instability occurs and a ferromagnetic gap state emerges spontaneously to lower the energy. Unlike the observed ferromagnetism at a magic angle in the graphene bilayer, we notice the ferromagnetic phase appearing aperiodically between 0 and 30° in the twisted nanoflex. The origin of electronic instability at various twist angles is ascribed to the several higher-symmetry phases that are broken to lower the energy resulting from an aperiodic modulation of the interlayer interaction in the nanoflex. Besides unraveling a spin-pairing mechanism for the reappearance of the nonmagnetic phase, we have found orbitals at the boundary of nanoflex contributing to ferromagnetism.

An antiferromagnet offers many important functionalities such as opportunities for electrical control of magnetic domains, immunity from magnetic perturbations, and fast spin dynamics. Introducing some of these intriguing features of an antiferromagnet into a low dimensional semiconductor core−shell nanowire offers an exciting pathway for its usage in antiferromagnetic semiconductor spintronics. Here, using a quantum mechanical approach, we predict that the Cr-doped Ge-core/Sishell nanowire behaves as an antiferromagnetic semiconductor. The origin of antiferromagnetic spin alignments between Cr is attributed to the superexchange interaction mediated by the p z orbitals of the Ge atoms that are bonded to Cr. A weak spin−orbit interaction was found in this material, suggesting a longer spin coherence length. The spin-dependent quantum transport calculations in the Cr-doped nanowire junction reveals a switching feature with a high ON/OFF current ratio (∼41 times higher for the ON state at a relatively small bias of 0.83 V).

Incorporating spin functionality into a semiconductor core–shell nanowire that offers immunity from the substrate effect is a highly desirable step for its application in next generation spintronics.

Quantum dot (QD) assemblies are nanostructured networks made from aggregates of QDs and feature improved charge and energy transfer efficiencies compared to discrete QDs. Using first-principles many-body perturbation theory, we systematically compare the electronic and optical properties of two types of CdS QD assemblies that have been experimentally investigated: (i) QD gels, where individual QDs are covalently connected via di- or polysulfide bonds, and (ii) QD nanocrystals, where individual QDs are bound via van der Waals interactions. Our work illustrates how the electronic and optical properties evolve when discrete QDs are assembled into 1D, 2D, and 3D gels and nanocrystals, as well as how the one-body and many-body interactions in these systems impact the trends as the dimensionality of the assembly increases. Furthermore, our work reveals the crucial role of the di- or polysulfide covalent bonds in the localization of the excitons, which highlights the difference between QD gels and QD nanocrystals.