Maguy Abi Jaoude

Capillary monolithic titania column for miniaturized liquid chromatography and extraction of organo-phosphorous compounds

A new sol–gel protocol was designed and optimized to produce titanium-dioxide-based columns within confined geometries such as monolithic capillary columns and porous-layer open-tubular columns. A surface pre-treatment of the capillary enabled an efficient anchorage of the monolith to the silica capillary wall during the synthesis. The monolith was further synthesized from a solution containing titanium n-propoxide, hydrochloric acid, N-methylformamide, water, and poly(ethylene oxide) as pore template. The chromatographic application of capillary titania-based columns was demonstrated with the separation of a set of phosphorylated nucleotides as probe molecules using aqueous normal-phase liquid chromatography conditions. Capillary titania monoliths offered a compromise between the high permeability and the important loading capacity needed to potentially achieve miniaturized sample preparations. The specificity of the miniaturized titania monolithic support is illustrated with the specific enrichment of 5′-adenosine mono-phosphate. The monolithic column offered a ten times higher loading capacity of 5′-adenosine mono-phosphate compared with that of the capillary titania porous-layer open-tubular geometry.

Chromatographic behavior of xanthines in aqueous normal phase chromatography using titania stationary phase

The chromatographic behavior of native titania was investigated in aqueous normal phase chromatography using a set of N-methylated xanthines as polar test solutes. In agreement with a hydrophilic interaction on a polar bed, the retention of xanthine models increased mainly along their molecular polarity. Adsorption of these molecules on the hydrated surface of titania prevailed as a retention mechanism for low water contents in the mobile phase. Several N-methylated xanthines could be easily discriminated along the number and position of their methyl groups while the nitrogen atom at position 3 was found deeply involved in the retention on titania. To get further insights on the interactions occurring on the surface of titania, the retention of xanthine derivatives based on ligand-exchange was evaluated as a function of the buffer concentration and type. The separation efficiency of native titania for the set of N-methylated xanthines was comparable to that observed on zirconia but lower than that obtained on native silica due to mixed-mode interactions. However, titania exhibited a superior ability to recognize several isomeric positions of xanthine derivatives in comparison to zirconia and silica.

State of the art of metal oxide memristor devices

Abstract Memristors are one of the emerging technologies that can potentially replace state-of-the-art integrated electronic devices for advanced computing and digital and analog circuit applications including neuromorphic networks. Over the past few years, research and development mostly focused on revolutionizing the metal oxide materials, which are used as core components of the popular metal-insulator-metal memristors owing to their highly recognized resistive switching behavior. This paper outlines the recent advancements and characteristics of such memristive devices, with a special focus on (i) their established resistive switching mechanisms and (ii) the key challenges associated with their fabrication processes including the impeding criteria of material adaptation for the electrode, capping, and insulator component layers. Potential applications and an outlook into future development of metal oxide memristive devices are also outlined.

Retention of β blockers on native titania stationary phase

In recent years, metal oxides such as titania have been commercially available as chromatographic beds that can potentially be used to achieve novel separations of polar compounds. For example β blockers, which are more often encountered in environmental sciences, have a wide range of polarity, and their basic character leads to difficult sample treatment and separation on conventional silica-based sorbents. The contribution of titania to the selective analysis of nine β blockers was evaluated in terms of retention mechanisms observed in hydrophilic interaction LC using acetonitrile/water mobile phases with various additives. The mobile phase additives enabled to control the β blocker charge as well as the titania surface charge. Depending on their respective ionic state, various retention mechanisms were identified at low water contents (<40%), including mainly adsorption mixed with hydrophilic interaction LC partition, ion exchange and ion exclusion. An unexpected retention was also observed for high water content and high pH, changing the selectivity of the support.

Novel secret key generation techniques using memristor devices

This paper proposes novel secret key generation techniques using memristor devices. The approach depends on using the initial profile of a memristor as a master key. In addition, session keys are generated using the master key and other specified parameters. In contrast to existing memristor-based security approaches, the proposed development is cost effective and power efficient since the operation can be achieved with a single device rather than a crossbar structure. An algorithm is suggested and demonstrated using physics based Matlab model. It is shown that the generated keys can have dynamic size which provides perfect security. Moreover, the proposed encryption and decryption technique using the memristor based generated keys outperforms Triple Data Encryption Standard (3DES) and Advanced Encryption Standard (AES) in terms of processing time. This paper is enriched by providing characterization results of a fabricated microscale Al/TiO2/Al memristor prototype in order to prove the concept of the proposed approach and study the impacts of process variations. The work proposed in this paper is a milestone towards System On Chip (SOC) memristor based security.

Resistive switching in sol-gel derived microscale memristors

This paper describes the synthesis of micro-thick hafnium-oxide (∼ 60 μm) memristors using a low-cost sol-gel drop-coating technique, and emphasizes on key parameters to be considered in the development of the microscale technology. The impact of electrode material on the device I-V electrical behavior is screened with the use of different metals, with different work-functions and oxygen affinities such as aluminum (Al), copper (Cu), titanium (Ti) and palladium (Pd) under a symmetric metal-insulator-metal arrangement. The type of metal contact was found to play a vital role in the switching behavior of the devices, highlighting a potential chemical interplay between the drop-coating solution and the electrode surface during the fabrication process. The effect of the sol-gel drying temperature on the I-V curve characteristics is also investigated with similar devices made at 60 °C and 80 °C. For example, the Roff/Ron ratio of Al/Al-type memristors was found to increase by approximately nine folds when the drying temperature was increased from 60 °C to 80 °C. As depicted from the changes observed in the maximum flowing current within an I-V curve, the drying temperature was generally found to mainly affect the internal resistance of the devices, allowing external control over the Roff/Ron ratio.

Novel microscale memristor with uniqueness property for securing communications

A rewritable micro-thick low power memristor device is presented. The memristor is fabricated based on a novel structure that is composed of Al/TiO2/Cu. The device switches at 3X lower voltage compared to the existing bipolar memristor state of the art at the microscale. The fabricated device exhibits unique I-V characteristic among similar devices when operated without prior electroforming. Thus, a new secure communication technique is proposed based on the existence of third trust party (TTP). As each host has unique memristor device, messages can be encrypted and decrypted securely using memristor based keys. In addition, Scyther analysis are provided to prove the security of the proposed technique. This work takes advantage of the inherited variation of memristor device electric characteristics to strengthen security algorithm in cryptographic application.

Physics model of memristor devices with varying active materials

This paper presents a physics-based model for memristors with different active layer materials. The model predicts the effect of changing the active material on the electrical characteristics of the devices. It captures the essential characteristics of the memristor such as coupling between ion mobility and electron current in addition to the nonlinear effects of electric fields. The parameters in the model depend on material (metal-oxide) properties that have impact on the device behavior. These properties are activation energy, escape attempt frequency, hopping parameter and relative permittivity. In this work, the effect of each parameter is highlighted and explained. In addition, the physics-based Matlab model is used to analyze the electrical characteristics of simulated memristor device using the following oxide materials; ZnO, TiO2 and Ta2O5. The simulation results of the model are validated with experimental data reported in the literature. The value of this contribution is to enable the selection of suitable oxide materials for the target memristor using correlated mathematical models.

Novel hafnium oxide memristor device: Switching behaviour and size effect

Unipolar RRAM devices are of high interest due to their high resistance ratio and simple selector circuit. In this paper, we report on a measurements from nano-thick memristor featuring a novel Pd/Hf/HfO2/Pd stack. The fabricated device exhibits a unipolar switching behavior, due to the asymmetric device structure and the existence of the Pd metal as a bottom electrode. The forming voltage of the proposed memristive stack (Hf-10nm/HfO2-10nm) is found to be size dependent at the microscale and its average forming voltage decreases by 20% when the active area increases from 30×30 to 1000×1000 μm2. The findings presented in this work highlight the impact of device geometry on its electrical performance and power, which provide guidance to the design tradeoffs (size, power, resistance ratio) and fabrication process of memristor devices.

Separation of xanthines in hydro-organic and polar-organic elution modes on a titania stationary phase.

Hydrophilic interaction LC was investigated in hydro-organic and nonaqueous elution modes on a titania column by using a set of N-methyl xanthines as neutral polar probes. To get information regarding the mechanisms that are behind the discrimination of these analytes in hydrophilic interaction, we focused our study on the type and amount of organic modifier as a critical yet rarely explored mobile phase parameter. Several alcohols such as methanol, ethanol, and isopropanol were studied as substitutes to acetonitrile in hydro-organic elution mode. Compared to silica, the investigation of the eluotropic series of these alcohols on titania highlighted a different implication in the retention mechanism of the xanthine derivatives. At low amounts of protic solvents, the adsorption mainly characterized the retention of analytes on bare silica; whereas mixed interactions including adsorption and ligand exchange were identified on native titania. To investigate the peculiar behavior of alcohols on the metal oxide, methanol, ethanol, and ethylene glycol were tested in replacement of water in polar-organic elution mode. Distinctive effects on the chromatographic retention and selectivity of xanthines were noticed for the dihydric alcohol, which was found to be a stronger eluting component than water on titania.