Anomalous insulator metal transition in boron nitride-graphene hybrid atomic layers

The study of two-dimensional (2D) electronic systems is of great fundamental significance in physics. Atomic layers containing hybridized domains of graphene and hexagonal boron nitride (h-BNC) constitute a new kind of disordered 2D electronic system. Magneto-electric transport measurements performed at low temperature in vapor phase synthesized h-BNC atomic layers show a clear and anomalous transition from an insulating to a metallic behavior upon cooling. The observed insulator to metal transition can be modulated by electron and hole doping and by the application of an external magnetic field. These results supported by ab-initio calculations suggest that this transition in h-BNC has distinctly different characteristics when compared to other 2D electron systems and is the result of the coexistence between two distinct mechanisms, namely, percolation through metallic graphene networks and hopping conduction between edge states on randomly distributed insulating h-BN domains.Comment: 9 pages, 15 figure

MIS capacitor studies on silicon carbide single crystals

Cubic SIC metal-insulator-semiconductor (MIS) capacitors with thermally grown or chemical-vapor-deposited (CVD) insulators were characterized by capacitance-voltage (C-V), conductance-voltage (G-V), and current-voltage (I-V) measurements. The purpose of these measurements was to determine the four charge densities commonly present in an MIS capacitor (oxide fixed charge, N(f); interface trap level density, D(it); oxide trapped charge, N(ot); and mobile ionic charge, N(m)) and to determine the stability of the device properties with electric-field stress and temperature. The section headings in the report include the following: Capacitance-voltage and conductance-voltage measurements; Current-voltage measurements; Deep-level transient spectroscopy; and Conclusions (Electrical characteristics of SiC MIS capacitors)

Bistability and Electrical Characterisation of Two Terminal Non-Volatile Polymer Memory Devices.

Polymer blended with nanoparticle and ferroelectric materials in two terminal memory devices has potential for electronic memory devices that may offer increased storage capacity and performance. Towards understanding the memory performance of a combination of an organic polymer with a ferroelectric or unpolarised material, this research is concerned with testing the memory programming and capacitance of these materials using two-terminal memory device structures. This research contributes to previous investigation into the internal working mechanisms of polymer memory devices and increases understanding and verifies the principles of these mechanisms through testing previously untested materials in different material compositions. This study makes a novel contribution by testing the electrical bistability of new materials; specifically, nickel oxide, barium titanate and methylammonium lead bromide and considers their properties which include nanoparticles, ferroelectric, perovskite structures and organic-inorganic composition. Due to their material properties which have different implications for internal switching and memory storage. Nanoparticles have a greater band gap between the valence band and conduction band compare to bulk material which is exploited for memory storage and ferroelectric properties and perovskite materials have non-volatile properties suitable for switching mechanisms. Specific attributes of memory function which include charging mechanism, device programming, capacitance and charge retention were tested for different material compositions which included, blend and layered with a PVAc polymer, and as a bulk material with a single crystal structure using MIM memory devices and MIS device structures. The results showed that nickel oxide was the most effective material as a blend with the polymer for memory performance, this was followed by barium titanate, however, methylammonium lead bromide performed poorly with polymer but showed promise as a single crystal structure. The results also showed that an increase in concentration of the tested material in a blend composition resulted in a corresponding increase in memory function, and that blend compositions were much more effective than layered compositions

Nano-engineering and Simulating Electrostatic Capacitors for Electrical Energy Storage

Electrical energy storage solutions with significantly higher gravimetric and volumetric energy densities and rapid response rates are needed to balance the highly dynamic, time-variant supply and demand for power. Nanoengineering can provide useful structures for electrical energy storage because it offers the potential to increase efficiency, reduce size/weight, and improve performance. While several nanostructured devices have shown improvements in energy and/or power densities, this dissertation focuses on the nanoengineering of electrostatic capacitors (ESC) and application of these high-power electrostatic capacitors in electrical energy storage systems. A porous nano-template with significant area enhancement per planar unit area coated with ultra-thin metal-insulator-metal (MIM) layers has shown significant improvements in areal capacitance. However, sharp asperities inherent to the initial nano-template localized electric fields and caused premature (low field) breakdown, limiting the possible energy density (E = ½ CV2/m). A nanoengineering strategy was identified for rounding the template asperities, and this showed a significant increase in the electrical breakdown strength of the device, providing rapid charging and discharging and an energy density of 1.5 W-h/kg - which compares favorably with the best state-of-the-art devices that provide 0.7 W-h/kg. The combination of the high-power ESC with a complementary high-energy-density electrochemical capacitor (ECC) was modeled to evaluate methods resulting in the combined power-energy storage capabilities. While significant improvements in the ESC's energy density were reported, the nanodevices display nonlinear leakage resistance, which directly relates to charge retention. The ECC has distinctly different nonlinearities, but can retain a greater density of charge for significantly longer, albeit with slower inherent charging and discharging rates than the ESC. The experimentally derived dynamic model simulating the nonlinear performance of the ESC and ECC devices indicated this hybrid-circuit reduces the time required to charge the ECC to near-maximum capacity by a factor of up to ~ 12