Christopher J. Hawley

Magnetostrictive stress reconfigurable thin film resonators for near direct current magnetoelectric sensors

The magnetic response of microdevices is significantly enhanced at structural resonance allowing for improved sensitivity and signal-to-noise ratio. Here, free-standing thin film CoFe bridge resonators have been fabricated and investigated. It is shown that the strong magnetic field dependence of the fundamental resonance frequency is a function of magnetic field orientation due to stress-induced anisotropy. These devices may offer a new approach for developing fully integrated resonant magnetic field sensing technology.

Surface chemically-switchable ultraviolet luminescence from interfacial two-dimensional electron gas

We report intense, narrow line-width, surface chemisorption-activated and reversible ultraviolet (UV) photoluminescence from radiative recombination of the two-dimensional electron gas (2DEG) with photoexcited holes at LaAlO3/SrTiO3. The switchable luminescence arises from an electron transfer-driven modification of the electronic structure via H-chemisorption onto the AlO2-terminated surface of LaAlO3, at least 2 nm away from the interface. The control of the onset of emission and its intensity are functionalities that go beyond the luminescence of compound semiconductor quantum wells. Connections between reversible chemisorption, fast electron transfer, and quantum-well luminescence suggest a new model for surface chemically reconfigurable solid-state UV optoelectronics and molecular sensing.

Subsurface imaging of coupled carrier transport in GaAs/AlGaAs core-shell nanowires.

We demonstrate spatial probing of carrier transport within GaAs/AlGaAs core-shell nanowires with nanometer lateral resolution and subsurface sensitivity by energy-variable electron beam induced current imaging. Carrier drift that evolves with applied electric field is distinguished from a coupled drift-diffusion length. Along with simulation of injected electron trajectories, combining beam energy tuning with precise positioning for selective probing of core and shell reveals axial position- and bias-dependent differences in carrier type and transport along parallel conduction channels. These results indicate how analysis of transport within heterostructured nanomaterials is no longer limited to nonlocal or surface measurements.

Resonant domain-wall-enhanced tunable microwave ferroelectrics

Ordering of ferroelectric polarization1 and its trajectory in response to an electric field2 are essential for the operation of non-volatile memories3, transducers4 and electro-optic devices5. However, for voltage control of capacitance and frequency agility in telecommunication devices, domain walls have long been thought to be a hindrance because they lead to high dielectric loss and hysteresis in the device response to an applied electric field6. To avoid these effects, tunable dielectrics are often operated under piezoelectric resonance conditions, relying on operation well above the ferroelectric Curie temperature7, where tunability is compromised. Therefore, there is an unavoidable trade-off between the requirements of high tunability and low loss in tunable dielectric devices, which leads to severe limitations on their figure of merit. Here we show that domain structure can in fact be exploited to obtain ultralow loss and exceptional frequency selectivity without piezoelectric resonance. We use intrinsically tunable materials with properties that are defined not only by their chemical composition, but also by the proximity and accessibility of thermodynamically predicted strain-induced, ferroelectric domain-wall variants8. The resulting gigahertz microwave tunability and dielectric loss are better than those of the best film devices by one to two orders of magnitude and comparable to those of bulk single crystals. The measured quality factors exceed the theoretically predicted zero-field intrinsic limit owing to domain-wall fluctuations, rather than field-induced piezoelectric oscillations, which are usually associated with resonance. Resonant frequency tuning across the entire L, S and C microwave bands (1–8 gigahertz) is achieved in an individual device—a range about 100 times larger than that of the best intrinsically tunable material. These results point to a rich phase space of possible nanometre-scale domain structures that can be used to surmount current limitations, and demonstrate a promising strategy for obtaining ultrahigh frequency agility and low-loss microwave devices.The domain-wall structure and dynamics are found to enhance, rather than inhibit, the high-frequency performance of an intrinsically tunable material, obtaining ultralow loss and exceptional frequency selectivity.

High capacity lithium ion batteries composed of cobalt oxide nanoparticle anodes and Raman spectroscopic analysis of nanoparticle strain dynamics in batteries

Cobalt nanoparticle thin films were electrophoretically deposited on copper current collectors and were annealed into thin films of hollow Co3O4 nanoparticles. These thin films were directly used as the anodes of lithium ion batteries (LIBs) without the addition of conducting carbons and bonding agents. LIBs thus fabricated show high gravimetric capacities and long cycle lives. For ≈1.0 μm thick Co3O4 nanoparticle films the gravimetric capacities of the batteries were more than 800 mAh g-1 at a current rate of C/15, which is about 90% of the theoretical maximum. Additionally, the batteries were able to undergo 200 charge/discharge cycles at a relatively fast rate of C/5 and maintain 50% of the initial capacity. In order to understand the electrochemistry of lithiation in the context of nanoparticles, Raman spectra were collected at different stages of the electrode cycles to determine the chemical and structural changes in the nanomaterials. Our results indicate that initially the electrode nanoparticles were under significant strain and as the battery underwent many cycles of charging/discharging the nanoparticles experienced progressive strain relaxation.