Steve Cronin

Anomalous Kink Behavior in the Current-Voltage Characteristics of Suspended Carbon Nanotubes

AbstractElectrically-heated suspended, nearly defect-free, carbon nanotubes (CNTs) exhibiting negative differential conductance in the high bias regime experience a sudden drop in current (or “kink”). The bias voltage at the kink (Vkink) is found to depend strongly on gate voltage, substrate temperature, and gas environment. After subtracting the voltage drop across the contacts, however, the kink bias voltages converge around 0.2 V, independent of gate voltage and gas environment. This bias voltage of 0.2 V corresponds to the threshold energy of optical phonon emission. This phenomenon is corroborated by simultaneously monitoring the Raman spectra of these nanotubes as a function of bias voltage. At the kink bias voltage, the G band Raman modes experience a sudden downshift, further indicating threshold optical phonon emission. A Landauer model is used to fit these kinks in various gas environments where the kink is modeled as a change in the optical phonon lifetime, which corresponds to a change in the non-equilibrium factor that describes the existence of hot phonons in the system.

Atomically Thin Femtojoule Memristive Device

The morphology and dimension of the conductive filament formed in a memristive device are strongly influenced by the thickness of its switching medium layer. Aggressive scaling of this active layer thickness is critical toward reducing the operating current, voltage, and energy consumption in filamentary-type memristors. Previously, the thickness of this filament layer has been limited to above a few nanometers due to processing constraints, making it challenging to further suppress the on-state current and the switching voltage. Here, the formation of conductive filaments in a material medium with sub-nanometer thickness formed through the oxidation of atomically thin two-dimensional boron nitride is studied. The resulting memristive device exhibits sub-nanometer filamentary switching with sub-pA operation current and femtojoule per bit energy consumption. Furthermore, by confining the filament to the atomic scale, current switching characteristics are observed that are distinct from that in thicker medium due to the profoundly different atomic kinetics. The filament morphology in such an aggressively scaled memristive device is also theoretically explored. These ultralow energy devices are promising for realizing femtojoule and sub-femtojoule electronic computation, which can be attractive for applications in a wide range of electronics systems that desire ultralow power operation.

Electron transport of suspended quasi-metallic carbon nanotube transistors in split gate configuration

We investigate the electron transport of quasi-metallic carbon nanotube field-effect-transistors using split gate structures. The transistor exhibit metallic behavior at room temperature. At cryogenic temperatures, the device starts to exhibit semiconducting behavior with band-to-band tunneling. We characterize the device performance and show that the schottky contacts of quasi-metallic nanotube devices cannot be ignored. Our measurements give insights into the magnitude of the band gap of these quasi-metallic carbon nanotubes which can have potential narrow band gap device applications in the future.

Improved Temperature Determination from Plasmon Energy Shifts in Aluminum

1. Center for Electron Microscopy and Microanalysis, University of Southern California, Los Angeles, California 90089 USA. 2. Molecular Foundry, Lawrence Berkley National Laboratory, Berkeley, California 94720 USA. 3. Department of Physics and Astronmony and California NanoSystems Institute, University of California Los Angeles, Los Angeles 90095 USA. 4. Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089 USA

Unique One‐ and Two‐Dimensional Phenomena Observed in Carbon Nanotubes and Graphene

Our ability to fabricate nearly defect-free, suspended carbon nanotubes (CNTs) has enabled us to observe several phenomena never seen before in CNTs, including breakdown of the Born-Oppenheimer approximation[1], mode selective electron-phonon coupling[2], leading to negative differential resistance (NDR) and non-equilibrium phonon populations, and a Mott insulator transition[3]. In this work, Raman spectroscopy is used to measure individual, suspended CNTs under applied gate and bias potentials. Raman spectroscopy of periodic ripple formation in suspended graphene will also be reported. As will be shown, preparing clean, defect-free devices is an essential prerequisite for studying the rich low-dimensional physics of CNTs and graphene.