Nanoscale phase change memory materials

Chemical and Physical Properties of Photonic Noble-Metal Nanomaterials

Colloidal noble metal nanoparticles (NPs) are composed of metal cores and organic or inorganic ligand shells. These NPs support size- and shape-dependent plasmonic resonances. They can be assembled from dispersions into artificial metamolecules which have collective plasmonic resonances originating from coupled bright and dark optical electric and magnetic modes that form depending on the size and shape of the constituent NPs and their number, arrangement, and interparticle distance. NPs can also be assembled into extended 2D and 3D metamaterials that are glassy thin films or ordered thin films or crystals, also known as superlattices and supercrystals. The metamaterials have tunable optical properties that depend on the size, shape, and composition of the NPs, and on the number of NP layers and their interparticle distance. Interestingly, strong light-matter interactions in superlattices form plasmon polaritons. Tunable interparticle distances allow designer materials with dielectric functions tailorable from that characteristic of an insulator to that of a metal, and serve as strong optical absorbers or scatterers, respectively. In combination with lithography techniques, these extended assemblies can be patterned to create subwavelength NP superstructures and form large-area 2D and 3D metamaterials that manipulate the amplitude, phase, and polarization of transmitted or reflected light.

BODIPY-Based Nanomaterials—Sensing and Biomedical Applications

Cancerous diseases are rightfully considered among the most lethal, which have a consistently negative effect when considering official statistics in regular health reports around the globe. Nowadays, metallic nanoparticles can be potentially applied in medicine as active pharmaceuticals, adjustable carriers, or distinctive enhancers of physicochemical properties if combined with other drugs. Boron dipyrromethene (BODIPY) molecules have been considered for future applications in theranostics in the oncology field, thus expanding the potential of conceivable applicability. Hence, taking into account positive practical features of both metal-based nanostructures and BODIPY derivatives, the present study aims to gather recent results connected to BODIPY-conjugated metallic nanoparticles. This is with respect to their expediency in the diagnosis and treatment of tumor ailments as well as in sensing of heavy metals. To fulfill the designated objectives, multiple research documents were analyzed concerning the latest discoveries within the scope of BODIPY-based nanomaterials with particular emphasis on their utilization for diagnostical sensing as well as cancer diagnostics and therapy. In addition, collected examples of mentioned conjugates were presented in order to draw the attention of the scientific community to their practical applications, elucidate the topic in a consistent manner, and inspire fellow researchers for new findings.

Reconfigurable label-free shape-sieving of submicron particles in paired chalcogenide waveguides

Up-to-date particle sieving schemes face formidable challenges for sieving label-free submicron molecules with similar sizes and dielectric constants but diverse shapes. Herein, optical sorting of polystyrene particles with various shapes is illustrated in optofluidic nanophotonic paired waveguide (ONPW) composed of chalcogenide semiconductor Sb₂Se₃. The Sb₂Se₃-ONPW creates the coupling length (C_L) between the neighboring hot spots that can be actively modulated via the transition of Sb₂Se₃ between amorphous (AM) and crystalline (CR) phases. Submicron particles interfere with the coupled hotspots, which can exert various optical torques on the particles according to their profiles. In the model system, spherical (diameter of 0.5 μm) and rod-shaped (diameter of 0.5 μm, length of 1.5 μm) polystyrene particles were employed to mimic two types of bacteria, namely, Staphylococcus aureus and rod-shaped Escherichia coli, respectively. For the AM state, the C_L value is ∼7.0 μm, enabling the structure to trap the sphere stably in the hot spots. For the CR state, the C_L value becomes ∼25 μm, leading to stable trapping of the rod-shaped particle. In this work, the working wavelength was fixed at 1.55 μm at which both AM- and CR-Sb₂Se₃ are transparent. Our scheme may offer a paradigm shift in shape-selective sieving of biomolecules and fulfill the requirements of the new-generation lab-on-chip techniques, where the integrated manipulation system must be much more multifunctional and flexible.

Phase-Change-Memory Process at the Limit: A Proposal for Utilizing Monolayer Sb2Te3

One central task of developing nonvolatile phase change memory (PCM) is to improve its scalability for high-density data integration. In this work, by first-principles molecular dynamics, to date the thinnest PCM material possible (0.8 nm), namely, a monolayer Sb2Te3, is proposed. Importantly, its SET (crystallization) process is a fast one-step transition from amorphous to hexagonal phase without the usual intermediate cubic phase. An increased spatial localization of electrons due to geometrical confinement is found to be beneficial for keeping the data nonvolatile in the amorphous phase at the 2D limit. The substrate and superstrate can be utilized to control the phase change behavior: e.g., with passivated SiO2 (001) surfaces or hexagonal Boron Nitride, the monolayer Sb2Te3 can reach SET recrystallization in 0.54 ns or even as fast as 0.12 ns, but with unpassivated SiO2 (001), this would not be possible. Besides, working with small volume PCM materials is also a natural way to lower power consumption. Therefore, the proposed PCM working process at the 2D limit will be an important potential strategy of scaling the current PCM materials for ultrahigh-density data storage.

Microwave AC Resonance Induced Phase Change in Sb2Te3 Nanowires

Scaling information bits to ever smaller dimensions is a dominant drive for information technology (IT). Nanostructured phase change material emerges as a key player in the current green-IT endeavor with low power consumption, functional modularity, and promising scalability. In this work, we present the demonstration of microwave AC voltage induced phase change phenomenon at ∼3 GHz in single Sb₂Te₃ nanowires. The resistance change by a total of 6-7 orders of magnitude is evidenced by a transition from the crystalline metallic to the amorphous semiconducting phase, which is cross-examined by temperature dependent transport measurement and high-resolution electron microscopy analysis. This discovery could potentially tailor multistate information bit encoding and electrical addressability along a single nanowire, rendering technology advancement for neuro-inspired computing devices.

The optimization effect of titanium on the phase change properties of SnSb4 thin films for phase change memory applications

Titanium-doped SnSb₄ phase-change thin film has been experimentally investigated for phase-change random access memory (PCRAM) use.

Design and modeling of a transmission and reflection switchable micro-focusing Fresnel device based on phase-change materials

In this paper, a switchable micro-focusing Fresnel device based on phase-change materials (PCMs) is proposed, which can selectively display the functions of transmission and reflection without the use of mechanical adjustment on micro scale. The switchable function is realized by combining Fresnel structure with PCM. A four-level switchable Fresnel device consisting of a typical PCM Ge₃Sb₂Te₆ (GST-326) is designed to focus light into a focal length of 30 µm at wavelength of 3.1 µm. The optical performance of the switchable device has been analyzed by using finite-difference time-domain (FDTD) method, showing bright convergence point near pre-designed focal length with focusing efficiencies larger than 18%, depth of focus (DOF) less than 4.65 µm and the full width at half-maximum (FWHM) not larger than 1.30 µm. Furthermore, by precisely manipulating the variation of PCM thickness, we also obtain a device that possesses the characteristics of a transmission-reflection focusing beam splitter. The devices show good potential for the combination of traditional binary optical devices and PCM to produce new functions, and provides a promising innovative approach for miniature focal length switching device.

Phase change thin films for non-volatile memory applications

The rapid development of Internet of Things devices requires real time processing of a huge amount of digital data, creating a new demand for computing technology. Phase change memory technology based on chalcogenide phase change materials meets many requirements of the emerging memory applications since it is fast, scalable and non-volatile. In addition, phase change memory offers multilevel data storage and can be applied both in neuro-inspired and all-photonic in-memory computing. Furthermore, phase change alloys represent an outstanding class of functional materials having a tremendous variety of industrially relevant characteristics and exceptional material properties. Many efforts have been devoted to understanding these properties with the particular aim to design universal memory. This paper reviews materials science aspects of chalcogenide-based phase change thin films relevant for non-volatile memory applications. Particular emphasis is put on local structure, control of disorder and its impact on material properties, order-disorder transitions and interfacial transformations.

Density dependent structural phase transition for confined copper: origin of the layering

Confinement presents the opportunity for novel structural transition scenarios not observed in three-dimensional systems. Here, we report a comprehensive molecular dynamic (MD) study of the structural phase transition induced by density for an ordinary metal copper (Cu) confined between two parallel panel walls. At 4.19 g cm-3 < ρ < 4.66 g cm-3, a notable structural phase transition occurs between the triangle unit cell structure and quasi-square unit cell structure upon densification. Both the bond order parameter (BOP) and angular distribution function (ADF) can provide evidence for the transition. We highlight the fact that when the sole decrease of the atom distance cannot adapt to the further densification, the system starts to adjust the neighboring bond angle and promote the layering transition, thus inducing the structural phase transition. At the metastable coexistence zone, the viscosity exhibits a remarkable drop and the diffusion coefficient shows a notable increase, both facilitating the accomplishment of the structural transition. Our results will trigger more interest on the phase transition under confinement in a metallic system.

Metasurfaces Based on Phase-Change Material as a Reconfigurable Platform for Multifunctional Devices

Integration of phase-change materials (PCMs) into electrical/optical circuits has initiated extensive innovation for applications of metamaterials (MMs) including rewritable optical data storage, metasurfaces, and optoelectronic devices. PCMs have been studied deeply due to their reversible phase transition, high endurance, switching speed, and data retention. Germanium-antimony-tellurium (GST) is a PCM that has amorphous and crystalline phases with distinct properties, is bistable and nonvolatile, and undergoes a reliable and reproducible phase transition in response to an optical or electrical stimulus; GST may therefore have applications in tunable photonic devices and optoelectronic circuits. In this progress article, we outline recent studies of GST and discuss its advantages and possible applications in reconfigurable metadevices. We also discuss outlooks for integration of GST in active nanophotonic metadevices.

Size-dependent and tunable crystallization of GeSbTe phase-change nanoparticles

Chalcogenide-based nanostructured phase-change materials (PCMs) are considered promising building blocks for non-volatile memory due to their high write and read speeds, high data-storage density, and low power consumption. Top-down fabrication of PCM nanoparticles (NPs), however, often results in damage and deterioration of their useful properties. Gas-phase condensation based on magnetron sputtering offers an attractive and straightforward solution to continuously down-scale the PCMs into sub-lithographic sizes. Here we unprecedentedly present the size dependence of crystallization for Ge₂Sb₂Te₅ (GST) NPs, whose production is currently highly challenging for chemical synthesis or top-down fabrication. Both amorphous and crystalline NPs have been produced with excellent size and composition control with average diameters varying between 8 and 17 nm. The size-dependent crystallization of these NPs was carefully analyzed through in-situ heating in a transmission electron microscope, where the crystallization temperatures (T_c) decrease when the NPs become smaller. Moreover, methane incorporation has been observed as an effective method to enhance the amorphous phase stability of the NPs. This work therefore elucidates that GST NPs synthesized by gas-phase condensation with tailored properties are promising alternatives in designing phase-change memories constrained by optical lithography limitations.

Revisiting the Local Structure in Ge-Sb-Te based Chalcogenide Superlattices

The technological success of phase-change materials in the field of data storage and functional systems stems from their distinctive electronic and structural peculiarities on the nanoscale. Recently, superlattice structures have been demonstrated to dramatically improve the optical and electrical performances of these chalcogenide based phase-change materials. In this perspective, unravelling the atomistic structure that originates the improvements in switching time and switching energy is paramount in order to design nanoscale structures with even enhanced functional properties. This study reveals a high- resolution atomistic insight of the [GeTe/Sb₂Te₃] interfacial structure by means of Extended X-Ray Absorption Fine Structure spectroscopy and Transmission Electron Microscopy. Based on our results we propose a consistent novel structure for this kind of chalcogenide superlattices.

Heat collection and supply of interconnected netlike graphene/polyethyleneglycol composites for thermoelectric devices

The key challenges in thermoelectric power conversion are creating a significant temperature difference and obtaining more heat energy through a thermoelectric device. Herein, graphene/polyethyleneglycol composites (G-PEGs) were proposed as a heat supply for thermoelectric devices. The G-PEGs not only afford a lot of conductive pathways for heat transfer but also act as highly thermally conductive reservoirs to hold phase-change materials for thermal energy collection, storage and release. The concept described in this study holds great promise in designing multifunctional composites for heat collection, transport, and supply in thermoelectric power conversion.

Phase-change-induced martensitic deformation and slip system in GeSbTe

Phase-change stress induced martensitic deformation on GeSbTe.

An optoelectronic framework enabled by low-dimensional phase-change films

The development of materials whose refractive index can be optically transformed as desired, such as chalcogenide-based phase-change materials, has revolutionized the media and data storage industries by providing inexpensive, high-speed, portable and reliable platforms able to store vast quantities of data. Phase-change materials switch between two solid states--amorphous and crystalline--in response to a stimulus, such as heat, with an associated change in the physical properties of the material, including optical absorption, electrical conductance and Young's modulus. The initial applications of these materials (particularly the germanium antimony tellurium alloy Ge2Sb2Te5) exploited the reversible change in their optical properties in rewritable optical data storage technologies. More recently, the change in their electrical conductivity has also been extensively studied in the development of non-volatile phase-change memories. Here we show that by combining the optical and electronic property modulation of such materials, display and data visualization applications that go beyond data storage can be created. Using extremely thin phase-change materials and transparent conductors, we demonstrate electrically induced stable colour changes in both reflective and semi-transparent modes. Further, we show how a pixelated approach can be used in displays on both rigid and flexible films. This optoelectronic framework using low-dimensional phase-change materials has many likely applications, such as ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent 'smart' glasses, 'smart' contact lenses and artificial retina devices.

Graphene based non-volatile memory devices

With the continuous advance of modern electronics, the demand for non-volatile memory cells rapidly grows. As a promising material for post-silicon electronic applications, graphene non-volatile memory cells have received renewed interest due to its outstanding electronic and other physical properties. This research news briefly summarizes the recent progress in this area. Appealing research activities and technical trends are highlighted. Afterwards, requirements and aims of future graphene non-volatile memory cells that may possibly influence their commercialization are also discussed.

Structure and assembly of filamentous bacteriophages

Filamentous bacteriophages are interesting paradigms in structural molecular biology, in part because of the unusual mechanism of filamentous phage assembly. During assembly, several thousand copies of an intracellular DNA-binding protein bind to each copy of the replicating phage DNA, and are then displaced by membrane-spanning phage coat proteins as the nascent phage is extruded through the bacterial plasma membrane. This complicated process takes place without killing the host bacterium. The bacteriophage is a semi-flexible worm-like nucleoprotein filament. The virion comprises a tube of several thousand identical major coat protein subunits around a core of single-stranded circular DNA. Each protein subunit is a polymer of about 50 amino-acid residues, largely arranged in an α-helix. The subunits assemble into a helical sheath, with each subunit oriented at a small angle to the virion axis and interdigitated with neighbouring subunits. A few copies of "minor" phage proteins necessary for infection and/or extrusion of the virion are located at each end of the completed virion. Here we review both the structure of the virion and aspects of its function, such as the way the virion enters the host, multiplies, and exits to prey on further hosts. In particular we focus on our understanding of the way the components of the virion come together during assembly at the membrane. We try to follow a basic rule of empirical science, that one should chose the simplest theoretical explanation for experiments, but be prepared to modify or even abandon this explanation as new experiments add more detail.

Redox-gated three-terminal organic memory devices: effect of composition and environment on performance

The performance of redox-gated organic nonvolatile memory (NVM) based on conducting polymers was investigated by altering the polymer structure, composition, and local environment of three-terminal devices with a field-effect transistor (FET) geometry. The memory function was dependent on the presence of a redox active polymer with high conducting and low conducting states, the presence of a redox counter-reaction, and the ability to transport ions between the polymer and electrolyte phases. Simultaneous monitoring of both the "write" current and "readout" current revealed the switching dynamics of the devices and their dependence on the local atmosphere. Much faster and more durable response was observed in acetonitrile vapor than in a vacuum, indicating the importance of polar molecules for both ion motion and promotion of electrochemical reactions. The major factor determining "write" and "erase" speeds of redox-gated polymer memory devices was determined to be the rate of ion transport through the electrolyte layer to provide charge compensation for the conducting polarons in the active polymer layer. The results both confirm the mechanism of redox-gated memory action and identify the requirements of the conducting polymer, redox counter reaction, and electrolyte for practical applications as alternative solid-state nonvolatile memory devices.

In silico optimization of phase-change materials for digital memories: a survey of first-row transition-metal dopants for Ge₂Sb₂Te₅

Phase-change materials are the alloys at the heart of an emerging class of next-generation, non-volatile digital memory technologies. However, the widely studied Ge-Sb-Te system possesses several undesirable properties, and enhancing its properties, e.g. by doping, is an area of active research. Various first-row transition-metal dopants have been shown to impart useful property enhancements, but a systematic study of the entire period has yet to be undertaken, and little has been done to investigate their interaction with the host material at the atomic level. We have carried out first-principles computer simulations of the complete phase-change cycle in Ge2Sb2Te5 doped with each of the ten first-row transition metals. In this article, we present a comprehensive survey of the electronic, magnetic and optical properties of these doped materials. We discuss in detail their atomic-level structure, and relate the microscopic behaviours of the dopant atoms to their influence on the Ge2Sb2Te5 host. By considering an entire family of similar materials, we identify trends and patterns which might be used to predict suitable dopants for optimizing materials for specific phase-change applications. The computational method employed here is general, and this materials-discovery approach could be applied in the future to study other families of potential dopants for such materials.

Size-dependent chemical transformation, structural phase-change, and optical properties of nanowires

Nanowires offer a unique approach for the bottom up assembly of electronic and photonic devices with the potential of integrating photonics with existing technologies. The anisotropic geometry and mesoscopic length scales of nanowires also make them very interesting systems to study a variety of size-dependent phenomenon where finite size effects become important. We will discuss the intriguing size-dependent properties of nanowire systems with diameters in the 5 - 300 nm range, where finite size and interfacial phenomena become more important than quantum mechanical effects. The ability to synthesize and manipulate nanostructures by chemical methods allows tremendous versatility in creating new systems with well controlled geometries, dimensions and functionality, which can then be used for understanding novel processes in finite-sized systems and devices.