Hasan Hayat

Design of practicable phase-change metadevices for near-infrared absorber and modulator applications

Phase-change chalcogenide alloys, such as Ge2Sb2Te5 (GST), have very different optical properties in their amorphous and crystalline phases. The fact that such alloys can be switched, optically or electrically, between such phases rapidly and repeatedly means that they have much potential for applications as tunable photonic devices. Here we incorporate chalcogenide phase-change films into a metal-dielectric-metal metamaterial electromagnetic absorber structure and design absorbers and modulators for operation at technologically important near-infrared wavelengths, specifically 1550 nm. Our design not only exhibits excellent performance (e.g. a modulation depth of ~77% and an extinction ratio of ~20 dB) but also includes a suitable means for protecting the GST layer from environmental oxidation and is well-suited, as confirmed by electro-thermal and phase-transformation simulations, to in situ electrical switching. We also present a systematic study of design optimization, including the effects of expected manufacturing tolerances on device performance and, by means of a sensitivity analysis, identify the most critical design parameters.

In vitro toxic effects of reduced graphene oxide nanosheets on lung cancer cells

The intriguing properties of reduced graphene oxide (rGO) have paved the way for a number of potential biomedical applications such as drug delivery, tissue engineering, gene delivery and bio-sensing. Over the last decade, there have been escalating concerns regarding the possible toxic effects, behaviour and fate of rGO in living systems and environments. This paper reports on integrative chemical-biological interactions of rGO with lung cancer cells, i.e. A549 and SKMES-1, to determine its potential toxicological impacts on them, as a function of its concentration. Cell viability, early and late apoptosis and necrosis were measured to determine oxidative stress potential, and induction of apoptosis for the first time by comparing two lung cancer cells. We also showed the general trend between cell death rates and concentrations for different cell types using a Gaussian process regression model. At low concentrations, rGO was shown to significantly produce late apoptosis and necrosis rather than early apoptotic events, suggesting that it was able to disintegrate the cellular membranes in a dose dependent manner. For the toxicity exposures undertaken, late apoptosis and necrosis occurred, which was most likely resultant from limited bioavailability of unmodified rGO in lung cancer cells.

Simulation of ultrahigh storage densities in nanoscale patterned probe phase change memories

Phase Change Memory (PCM), based on the reversible transitions of chalcogenides such as Ge 2 Sb 2 Te 5 (GST) between a high resistance amorphous state (binary ‘0’) and low resistance crystalline state (binary ‘1’), is one of the leading contenders to complement or even replace existing technologies such as Flash, DRAM and HDD [1]. This is attributed to its size scalability (sub-10nm [2-4]), ultrahigh storage densities (Tb/in2 [5-6]), fast programming speeds (picoseconds [7]), and potentially low programming currents (∼μA [3]).

The Scaling of Phase-Change Memory Materials and Devices

In this chapter we address the basic question of the extent to which phase-change memories can be scaled down in size. Fundamental physical limits, along with material properties and device design considerations, all affect the smallest phase-change devices that might be achieved; we explore each of these issues in turn. We also examine the effects of scaling on key performance parameters, in particular on device switching currents and energies and device switching speeds. Finally, we summarize device performance attributes for production-oriented and research-oriented cell designs and provide some perspectives for possible future developments.

A Self-Resetting Spiking Phase-Change Neuron

Neuromorphic, or brain-inspired, computing applications of phase-change devices have to date concentrated primarily on the implementation of phase-change synapses. However, the so-called accumulation mode of operation inherent in phase-change materials and devices can also be used to mimic the integrative properties of a biological neuron. Here we demonstrate, using physical modelling of nanoscale devices and SPICE modelling of associated circuits, that a single phase-change memory cell integrated into a comparator type circuit can deliver a basic hardware mimic of an integrate-and-fire spiking neuron with self-resetting capabilities. Such phase-change neurons, in combination with phase-change synapses, can potentially open a new route for the realisation of all-phase-change neuromorphic computing.

Emerging Nanoscale Phase-Change Memories: A Summary of Device Scaling Studies

Phase-change materials have generated very significant interest in recent years due to their potential for data storage and memory applications. For the exploration of phase-change memory many different types of device structures and materials have been presented and investigated by numerous researchers, academic and industrial, using a variety of experimental and theoretical approaches. Among all the properties, size-scaling, reductions in power consumption and switching speeds, improvements in endurance and possibilities for multilevel storage have been the most prominent topics of research. Key approaches to achieving such properties, with a particular focus on size-scaling, ie, how small phase-change devices can be shrunk while still working effectively, are here presented in detail.