Traditional von Neumann computing architecture with separated computation and storage units has already impeded the data processing performance and energy efficiency, calling for emerging neuromorphic electronic and optical devices and systems which can mimic the human brain to shift this paradigm. Material-level innovation has become the key component to this revolution of information technology. Chalcogenide phase-change material (PCM) as a well-acknowledged data-storage medium is a promising candidate to tackle this challenge. In this review, the use of PCMs to implement artificial neurons and synapses from both the electronic and optical respects is discussed, and in particular, the structure-property physics and transition dynamics that enable such brain-inspired and in-memory computing applications are emphasized. Recent advances on the atomic-level amorphous and crystalline structures, transition mechanisms, materials optimization and design, neural and synaptic devices, brain-inspired chips, and computing systems, as well as the future opportunities of PCMs, are summarized and discussed.

Using external stimulants to tune the structural and physical properties of chalcogenides has become a hot research topic due to the potential applications in high-temperature superconductors and geological physics. The typical 2D-like trichalcogenide CrSiTe3 has special atomic, electronic, and magnetic features, expected to exhibit interesting structure and property transformations under pressure. Using the synchrotron X-ray diffraction and first-principles calculations, we discover an interesting pressure-induced 2D-3D polymorphic transformation in CrSiTe3 at 15 GPa followed by an amorphization process, which may originate from the disordering of the chemical bonding rearrangement in “Si2Te6” clusters. The pressure-induced metallization and spin-reorientation transition of CrSiTe3 in the low-pressure range are predicted through simulations. Our results shed light on the complicated bonding mechanism that leads to special structural transformations in CrSiTe3 and would encourage further investigations on trichalcogenides as well as other 2D-like materials under extreme conditions.

technologies encodes digital information via fast and reversible transition between amorphous and crystalline phases of chalcogenide phase-change materials. [1][2][3][4][5][6][7] The phase transitions are precisely controlled by electrical pulses to realize the SET (crystallization) and the RESET (melt-quenched amorphization) operations. PCM has the characteristics of fast switching speed (<1 ns), [8] long data retention (>10 years), [9,10] and good scalability, [11,12] bridging to some extent the performance gap between the slow solid-state drive (SSD) and the fast dynamic random access memory (DRAM).The large resistance contrast (over 2-3 orders of magnitude) between amorphous and crystalline phases allows intermediate resistance states to be populated in, and thus multiple bits of data can be stored in a single memory cell, also known as multilevel cell (MLC) technology. The MLC increases the storage capacity of PCMs exponentially without changing the physical volume, and thus the cost per bit of data is remarkably reduced. Over the past years, many methods have been developed to enable MLC PCMs, including: 1) stacking multiple layers: multiple resistance levels are obtained owing to the designed heat profile inducing phase transition of chalcogenide layers sequentially; [13] 2) write programming strategy: modulating the programming pulse protocols to systematically control the volume ratio of amorphous to crystalline phases; [14,15] and 3) materials engineering: to design novel materials with multiple crystallization stages to achieve MLC. [16,17] Prototypical phase change material Ge 2 Sb 2 Te 5 (GST) has been explored for multi-bit date storage. However, the relatively low thermal stability (T c = 150 °C) and narrow voltage programming margin (i.e., the resistance is very sensitive to the change of voltage) are key issues that limit the adoption of GST in MLC applications. [18] It has been recently reported that phase change material based on pseudobinary Ga-Sb-Ge system shows fast switching speed (80 ns), good endurance (10 9 cycles), and high reliability (10 years at 220 °C). [10] Moreover, Ge-Sb, Ga-Sb, and Ge-Ga-Sb alloys were found to have a certain degree of phase separation at different temperatures, [19][20][21][22][23] which may result in variations of resistance. Based *on the above features of Phase-change memory (PCM), using the fast and reversible transition between crystal and glass to store binary data, is a promising candidate for next-generation information storage and computing technologies. Recording more than one bit of information on each memory cell, known as multilevel cell (MLC) technology, can greatly increase the data density of PCMs. In this paper, the MLC capability in a phase change material Ge-Ga-Sb (GGS) is explored. Using the "SET" operation with increasing voltage amplitudes on this PCM cell in a 250 nm pillar structure device, three resistance levels are achieved and can be stabilized within large operating voltage windows, allowing large tolerance of SET voltage variation ...