Hussein Nili

Elemental Analogues of Graphene: Silicene, Germanene, Stanene, and Phosphorene

The fascinating electronic and optoelectronic properties of free-standing graphene has led to the exploration of alternative two-dimensional materials that can be easily integrated with current generation of electronic technologies. In contrast to 2D oxide and dichalcogenides, elemental 2D analogues of graphene, which include monolayer silicon (silicene), are fast emerging as promising alternatives, with predictions of high degree of integration with existing technologies. This article reviews this emerging class of 2D elemental materials - silicene, germanene, stanene, and phosphorene--with emphasis on fundamental properties and synthesis techniques. The need for further investigations to establish controlled synthesis techniques and the viability of such elemental 2D materials is highlighted. Future prospects harnessing the ability to manipulate the electronic structure of these materials for nano- and opto-electronic applications are identified.

Flexible metasurfaces and metamaterials: A review of materials and fabrication processes at micro- and nano-scales

The ability to bend, stretch, and roll metamaterial devices on flexible substrates adds a new dimension to aspects of manipulating electromagnetic waves and promises a new wave of device designs and functionalities. This work reviews terahertz and optical metamaterials realized on flexible and elastomeric substrates, along with techniques and approaches to lend tunability to the devices. Substrate electromagnetic and mechanical characteristics suitable for flexible metamaterials are summarized for readers, followed by fabrication and processing techniques, and finally novel approaches used to-date to attain tunability. Future directions and emerging areas of interests are identified with these promising to transform metamaterial design and translate metamaterials into practical devices.

Reduced impurity-driven defect states in anodized nanoporous Nb2O5: the possibility of improving performance of photoanodes.

Anodized nanoporous Nb2O5 films are synthesized using two different types of electrolyte compositions onto transparent conductive glasses and their impurities induced during the anodization process are assessed. These films are incorporated as photoanodes in dye sensitized solar cells (DSSCs). The one with no traces of impurity-driven defects exhibits higher current density and longer electron lifetimes, and consequently, an improvement in photoconversion efficiencies compared to the one that contains impurities.

Microstructure and dynamics of vacancy-induced nanofilamentary switching network in donor doped SrTiO3-x memristors

Donor doping of perovskite oxides has emerged as an attractive technique to create high performance and low energy non-volatile analog memories. Here, we examine the origins of improved switching performance and stable multi-state resistive switching in Nb-doped oxygen-deficient amorphous SrTiO3 (Nb:a-STO x ) metal-insulator-metal (MIM) devices. We probe the impact of substitutional dopants (i.e., Nb) in modulating the electronic structure and subsequent switching performance. Temperature stability and bias/time dependence of the switching behavior are used to ascertain the role of substitutional dopants and highlight their utility to modulate volatile and non-volatile behavior in a-STO x devices for adaptive and neuromorphic applications. We utilized a combination of transmission electron microscopy, photoluminescence emission properties, interfacial compositional evaluation, and activation energy measurements to investigate the microstructure of the nanofilamentary network responsible for switching. These results provide important insights into understanding mechanisms that govern the performance of donor-doped perovskite oxide-based memristive devices.

Nanoscale electro-mechanical dynamics of nano-crystalline platinum thin films: An in situ electrical nanoindentation study

Here, we present a detailed methodology for the study of nano-electromechanical properties of thin films through in situ electrical nanoindentation. The nanomechanical properties of nano-crystalline platinum thin films have been accurately evaluated via nullifying multiple phenomena and artefacts that can introduce errors in interpreting nanoindentation experimental data. To gain quantified insights from in situ electrical measurements, an empirical equation is introduced to model the resistance imposed by the conductive probe at the nanoscale contact as a function indentation depth and load. Using the empirical model, nanoscale electrical properties of nano-crystalline platinum films are quantitatively evaluated. It is observed that the resistivity of the platinum increases subject to high contact pressure, which is also associated with substantial structural deformations around the nano-contact area.

A Physical Unclonable Function With Redox-Based Nanoionic Resistive Memory

Emerging non-volatile reduction-oxidation (redox)-based resistive switching memories (ReRAMs) exhibit a unique set of characteristics that make them promising candidates for the next generation of low-cost, low-power, tiny, and secure physical unclonable functions (PUFs). Their underlying stochastic ionic conduction behavior, intrinsic nonlinear current-voltage characteristics, and their well-known nano-fabrication process variability might normally be considered disadvantageous ReRAM features. However, using a combination of a novel architecture and special peripheral circuitry, this paper exploits these non-idealities in a physical one-way function, nonlinear resistive PUF, potentially applicable to a variety of cyber-physical security applications. We experimentally verify the performance of valency change mechanism (VCM)-based ReRAM in nano-fabricated crossbar arrays across multiple dies and runs. In addition to supporting a massive pool of challenge-response pairs (CRPs), using a combination of experiment and simulation our proposed PUF exhibits a reliability of 98.67%, a uniqueness of 49.85%, a diffuseness of 49.86%, a uniformity of 47.28%, and a bit-aliasing of 47.48%.

Alkali ratio control for lead-free piezoelectric thin films utilizing elemental diffusivities in RF plasma

High performance piezoelectric thin films are generally lead-based, and find applications in sensing, actuation and transduction in the realms of biology, nanometrology, acoustics and energy harvesting. Potassium sodium niobate (KNN) is considered to be the most promising lead-free alternative, but it is hindered by the inability to control and attain perfect stoichiometry materials in the thin film form while using practical large area deposition techniques. In this work, we identify the contribution of the elemental diffusivities in the radio frequency (RF) plasma in determining the alkali loss in the KNN thin films. We have also examined the effect of the substrate temperature during the RF magnetron sputtering deposition on the crystal structure of the substrate and KNN thin films, as well as the effect of the post-annealing treatments. These results indicate the need for well-designed source materials and the potential to use the deposition partial pressure to alter the dopant concentrations.