Gabriela P. Kissling

Tuning Electrochemical Rectification via Quantum Dot Assemblies

A novel approach to tuning electrochemical rectification using 2D assemblies of quantum dots (QDs) is presented. Asymmetric enhancement of the oxidation and reduction currents in the presence of the Fe(CN)(6)(3-/4-) redox couple is observed upon adsorption of QDs at thiol-modified Au electrodes. The extent of the electrochemical rectification is dependent on the average QD size. A molecular blocking layer is generated by self-assembling 11-mercaptoundecanoic acid (MUA) and an ultrathin film of poly(diallyldimethylammonium chloride) (PDADMAC) on the electrode. The polycationic film allows the electrostatic adsorption of 3-mercaptopropionic acid (MPA)-stabilized CdTe QDs, generating 2D assemblies with approximately 0.4% coverage. The QD adsorption activates a fast charge transfer across the blocking layer in which the reduction process is more strongly enhanced than the oxidation reaction. The partial electrochemical rectification is rationalized in terms of the relative position of the valence (VB) and conduction band (CB) edges with respect to the redox Fermi energy (ε(redox)). Quantitative analysis of the exchange current density obtained from electrochemical impedance spectroscopy demonstrates that the enhancement of charge transport across the molecular barrier is strongly dependent on the position of the QD valence band edge relative to ε(redox). The average electron tunneling rate constant through the QD assemblies is estimated on the basis of the Gerischer model for electron transfer.

Non-aqueous electrodeposition of functional semiconducting metal chalcogenides: Ge2Sb2Te5 phase change memory

We report a new method for electrodeposition of device-quality metal chalcogenide semiconductor thin films and nanostructures from a single, highly tuneable, non-aqueous electrolyte. This method opens up the prospect of electrochemical preparation of a wide range of functional semiconducting metal chalcogenide alloys that have applications in various nano-technology areas, ranging from the electronics industry to thermoelectric devices and photovoltaic materials. The functional operation of the new method is demonstrated by means of its application to deposit the technologically important ternary Ge/Sb/Te alloy, GST-225, for fabrication of nanostructured phase change memory (PCM) devices and the quality of the material is confirmed by phase cycling via electrical pulsed switching of both the nano-cells and thin films.

Non-aqueous electrodeposition of p-block metals and metalloids from halometallate salts

A versatile electrochemical system for the non-aqueous electrodeposition of crystalline, oxide free p-block metals and metalloids is described, and it is demonstrated that by combining mixtures of these reagents, this system is suitable for electrodeposition of binary semiconductor alloys. The tetrabutylammonium halometallates, [NnBu4][InCl4], [NnBu4][SbCl4], [NnBu4][BiCl4], [NnBu4]2[SeCl6] and [NnBu4]2[TeCl6], are readily dissolved in CH2Cl2 and form reproducible electrochemical systems with good stability in the presence of a [NnBu4]Cl supporting electrolyte. The prepared electrolytes show a wide potential window and the electrodeposition of indium, antimony, bismuth, tellurium and selenium on glassy carbon and titanium nitride electrodes has been demonstrated. The deposited elements were characterised by scanning electron microscopy, energy dispersive X-ray analysis and powder X-ray diffraction. The compatibility of the reagents permits the preparation of a single electrolyte containing several halometallate species which allows the electrodeposition of binary materials, as is demonstrated for InSb. This room temperature, ‘bottom-up’ electrochemical approach should thus be suitable for the one-pot deposition of a wide range of compound semiconductor materials.

Conductive-bridge memory cells based on a nano-porous electrodeposited GeSbTe alloy

We report on the fabrication of memory devices based on a nanoporous GeSbTe layer electrodeposited inbetween TiN and Ag electrodes. It is shown that devices can operate along two distinct electrical modes consisting of a volatile or a non-volatile resistance switching mode upon appropriate preconditioning procedures. Based on electrical measurements conducted in both switching modes and physical analysis performed on a device after electrical stress, resistance switching is attributed to the formation/dissolution of a conductive filament from the Ag electrode into the GST layer whereas the volatile/non-volatile resistance switching is attributed to the presence of an interface layer between the GST and the Ag top electrode. Due to their simple, low-cost and low-temperature fabrication procedure, these devices could be advantageously exploited in flexible electronic applications or embedded into the back-end of line CMOS technology.