Size control is critical in the synthesis of quantumconfined semiconductor nanocrystals, otherwise known as quantum dots. The achievement of size-uniformity and narrow spectral line-width in quantum dots conventionally relies on a very precise kinetic control of the reactions, where reaction time plays a significant role in defining the final crystal sizes and distribution. Here, we show that synthesis of quantum-confined perovskite nanostrips could be achieved through a thermodynamically controlled reaction, using a low-temperature and ligand-rich approach. The nanostrip growth proceeds through an initial onedimensional (1D) nanorod stage, followed by the lateral widening of the rod to form a two-dimensional (2D) nanostrip. The spectral characteristics of the final product remain unchanged after prolonged reaction, indicating no signs of crystal ripening and confirming the thermodynamic nature of this reaction. The CsPbBr 3 perovskite nanostrips were highly uniform and emit at a deep-blue wavelength of 462 nm with a remarkably narrow line-width of 13 nm. This corresponds to color coordinates of (0.136, 0.049) on the CIE 1931 color space, which fulfils the stringent Rec. 2020 standard for next-generation color displays. The well-passivated nanostrips also possess negligible defects and provide a near-unity photoluminescence quantum yield at 94%. Crucially, the achievement of blue emission through a pure-halide perovskite circumvents the problems of spectral instability that are frequently experienced in mixed-halide perovskite systems. The convenience and scalability of our thermodynamic approach, coupled with the excellent optical attributes, would likely enable these quantum-confined perovskite systems to be the preferred method toward color control in trichromatic display applications.

The ability for a magnetic field to penetrate biological tissues without attenuation has led to significant interest in the use of magnetic nanoparticles for biomedical applications. In particular, active research is ongoing in the areas of magnetically guided drug delivery and magnetic hyperthermia treatment. However, the difficulties in tracing these optically nonactive magnetic nanoparticles hinder their usage in medical research or treatment. Here, a new perovskite‐based magneto‐fluorescent nanocomposite that allows the in situ, real‐time optical visualization of magnetically induced cellular movements is reported. A swelling–deswelling technique is employed to capture a cesium lead halide perovskite and magnetite nanoparticles within a biocompatible polyvinylpyrrolidone matrix, to produce a water‐dispersible composite that possesses a combination of strong magnetic response and intense fluorescence. The wavelength‐tunability of perovskite nanocrystals is taken advantage of to demonstrate simultaneous multicolor fluorescent tagging of cancer stem cells. The magneto‐directed motion of the cancer stem cells through a microfluidic channel is also imaged as a proof‐of‐concept toward an optically traceable magnetic manipulation of biological systems. These dual‐functional nanocomposites could find promising applications in advanced biotechnologies, such as in optogenetics, cellular separation, and drug delivery studies.

Near‐infrared (NIR) lighting plays an increasingly important role in new facial recognition technologies and eye‐tracking devices, where covert and nonvisible illumination is needed. In particular, mobile or wearable gadgets that employ these technologies require electronic lighting components with ultrathin and flexible form factors that are currently unfulfilled by conventional GaAs‐based diodes. Colloidal quantum dots (QDs) and emerging perovskite light‐emitting diodes (LEDs) may fill this gap, but generally employ restricted heavy metals such as cadmium or lead. Here, a new NIR‐emitting diode based on heavy‐metal‐free In(Zn)As–In(Zn)P–GaP–ZnS quantum dots is reported. The quantum dots are prepared with a giant shell structure, enabled by a continuous injection synthesis approach, and display intense photoluminescence at 850 nm with a high quantum efficiency of 75%. A postsynthetic ligand exchange to a shorter‐chain 1‐mercapto‐6‐hexanol (MCH) affords the QDs with processability in polar solvents as well as an enhanced charge‐transport performance in electronic devices. Using solution‐processing methods, an ITO/ZnO/PEIE/QD/Poly‐TPD/MoO3/Al electroluminescent device is fabricated and a high external quantum efficiency of 4.6% and a maximum radiance of 8.2 W sr−1 m−2 are achieved. This represents a significant leap in performance for NIR devices employing a colloidal III–V semiconductor QD system, and may find significant applications in emerging consumer electronic products.

Across the different classes of sodium‐ion battery cathodes, Prussian Blue holds the greatest promise because of its high working potentials, abundance, low‐toxicity and ease of synthesis. However, its performance as a sodium‐ion battery cathode has generally been limited to less than 120 mAh g−1, which is inferior compared to commercial lithium‐based counterparts. Here, sodium–Prussian Blue rechargeable batteries with a remarkably high discharge specific capacity of 153 ± 6 mAh g−1 at 1C is reported. This is achieved through the employment of excess ascorbic acid during the colloidal preparation of Prussian Blue crystals, followed by a 200 °C heat‐vacuum drying process. The optical, structural and thermogravimetric investigations show that the chelation of ascorbic acid to the iron ions disrupts the growth of Prussian Blue, and lead to the formation of a useful nano‐porous crystal structure. This allows for deeper percolation of sodium ions into the Prussian Blue crystals, and successfully unlocked useful inner volumes that are otherwise unreachable, thereby leading to an outstanding 47% elevation in specific capacity. This development brings sodium‐based battery technology significantly closer to the incumbent lithium‐ion batteries, and marks an important early step towards its practical application in commercial devices.

Mobile and wearable devices are increasingly reliant on near-infrared (NIR) covert illumination for facial recognition, eye-tracking or motion and depth sensing functions. However, these small devices offer limited spatial real estate that is typically already occupied by their full-area electronic color displays. Here, we report a transparent perovskite light-emitting diode (LED) that could be overlaid across a color display to provide an efficient and high-intensity NIR illumination. Our transparent devices are constructed with an ITO/AZO/PEIE/FAPbI 3 /poly-TPD/MoO 3 /Al/ITO/Ag/ITO architecture, and offer a high average transmittance of more than 55% across the visible spectral region. In particular, our Al/ITO/Ag/ITO top transparent electrode was designed to offer a combination low sheet resistance and low plasma damage upon electrode deposition. The devices emit at 799 nm with a high total external quantum efficiency of 5.7% at a current density of 5.3 mA cm −2 and a high radiance of 1.5 W sr −1 m −2 , and possess a large functional device area of 120 mm 2 . The efficient performance is ideal for battery-powered wearable devices, and could enable advanced security and sensing features on future smart-watches, phones, gaming consoles and augmented or virtual reality headsets.