Seven Bit Nonvolatile Electrically Programmable Photonics Based on Phase-Change Materials for Image Recognition

With the rapid development of the Internet of Things, how to efficiently store, transmit, and process massive amounts of data has become a major challenge now. Optical neural networks based on nonvolatile phase change materials (PCMs) have become a breakthrough point due to their zero static power consumption, low thermal crosstalk, large-scale, and high efficiency. However, current photonic devices cannot meet the multilevel requirements in neuromorphic computing due to their limited storage capacity. Here, a new way for increasing storage capacity is paved from the perspective of modulation of the crystallization kinetics of PCMs. A more progressive transition from the amorphous to the crystalline states is achieved through the grain-refinement phenomenon induced by nitrogen (N) element doping in Ge2Sb2Te5 (GST), giving precise access to more multibit states. By integrating N-doped Ge2Sb2Te5 (N-GST) with a waveguide, a high-capacity nonvolatile photonic device enabling over 7 bits (∼222 levels) storage is achieved for the first time. The storage capacity is increased nearly by 7 times compared to the state-of-the-art device (∼32 levels). An optical convolutional neural network is successfully established for the MINIST handwritten digit recognition task by mapping synapse weight to the multiple optical levels, and a recognition accuracy of up to 96.5% is achieved. Our work provides a new strategy for the development of integrated photonic devices with multilevel and demonstrates enormous application potential in the field of large-scale photonic neural networks

Variations of Local Motifs around Ge Atoms in Amorphous GeTe Ultrathin Films

Phase-change materials, the highly promising candidate for nonvolatile data recording, present a different phase-change property when film thickness shrinks to very deep submicron scale. The local structure of amorphous GeTe ultrathin films, which contributes to the characteristics of phase change, is examined using X-ray absorption measurements. Ge atoms are found to be low-coordinated when the film thickness decreases. Ge atoms are linked to neighbor atoms by covalent bond, and the weaker Ge–Te bonds are more easily broken, which suggests that Ge atoms are located in the defective Ge₂Te₃ local arrangement. The mixture of sp³/sp² hybridization and 3-coordinated Ge found in ab initio molecular dynamics simulations also supports this local motif. The exponential rise of crystallization temperatures of ultrathin films with decreasing film thickness, which is a vital parameter for phase change process, can be well explained by the proposed defective GeTe local arrangement

Combination of Cation Exchange and Quantized Ostwald Ripening for Controlling Size Distribution of Lead Chalcogenide Quantum Dots

A new strategy for narrowing the size distribution of colloidal quantum dots (QDs) was developed by combining cation exchange and quantized Ostwald ripening. Medium-sized reactant CdS­(e) QDs were subjected to cation exchange to form the target PbS­(e) QDs, and then small reactant CdS­(e) QDs were added which were converted to small PbS­(e) dots via cation exchange. The small-sized ensemble of PbS­(e) QDs dissolved completely rapidly and released a large amount of monomers, promoting the growth and size-focusing of the medium-sized ensemble of PbS­(e) QDs. The addition of small reactant QDs can be repeated to continuously reduce the size distribution. The new method was applied to synthesize PbSe and PbS QDs with extremely narrow size distributions and as a bonus they have hybrid surface passivation. The size distribution of prepared PbSe and PbS QDs are as low as 3.6% and 4.3%, respectively, leading to hexagonal close packing in monolayer and highly ordered three-dimensional superlattice

Impact of Pressure on the Resonant Bonding in Chalcogenides

Resonant bonding has been appreciated as an important feature in some chalcogenides. The establishment of resonant bonding can significantly delocalize the electrons and shrink the band gap, leading to low electrical resistivity and soft optical phonons. Many materials that exhibit this bonding mechanism have applications in phase-change memory and thermoelectric devices. Resonant bonding can be tuned by various means, including thermal excitations and changes in composition. In this work, we manipulate it by applying large hydrostatic-like pressure. Synchrotron X-ray diffraction and density functional theory reveal that the orthorhombic lattice of GeSe appears to become more symmetric and the Born effective charge has significantly increased at high pressure, indicating that resonant bonding has been established in this material. In contrast, the resonant bonding is partially weakened in PbSe at high pressure due to the discontinuity of chemical bonds along a certain lattice direction. By controlling resonant bonding in chalcogenides, we are able to modify the material properties and tailor them for various applications in extreme conditions

Electrostatic Gating of Spin Dynamics of a Quasi-2D Kagome Magnet

Electrostatic gating has emerged as a powerful technique for tailoring the magnetic properties of two-dimensional (2D) magnets, offering exciting prospects including enhancement of magnetic anisotropy, boosting Curie temperature, and strengthening exchange coupling effects. Here, we focus on electrical control of the ferromagnetic resonance of the quasi-2D Kagome magnet Cu(1,3-bdc). By harnessing an electrostatic field through ionic liquid gating, significant shifts are observed in the ferromagnetic resonance field in both out-of-plane and in-plane measurements. Moreover, the effective magnetization and gyromagnetic ratios display voltage-dependent variations. A closer examination reveals that the voltage-induced changes can modulate magnetocrystalline anisotropy by several hundred gauss, while the impact on orbital magnetization remains relatively subtle. Density functional theory (DFT) calculations reveal varying d-orbital hybridizations at different voltages. This research unveils intricate physics within the Kagome lattice magnet and further underscores the potential of electrostatic manipulation in steering magnetism with promising implications for the development of spintronic devices

Manipulating Exchange Bias in 2D Magnetic Heterojunction

The exchange bias effect has been instrumental in the development of a variety of spintronic devices. Here, we use pressure to tune the exchange bias effect in all van der Waals heterostructures composed of FePSe3/Fe3GeTe2.   </p