Tristan DeBorde

Identifying Individual Single-Walled and Double-Walled Carbon Nanotubes by Atomic Force Microscopy

We show that the number of concentric graphene cylinders forming a carbon nanotube can be found by squeezing the tube between an atomic force microscope tip and a silicon substrate. The compressed height of a single-walled nanotube (double-walled nanotube) is approximately two (four) times the interlayer spacing of graphite. Measured compression forces are consistent with the predicted bending modulus of graphene and provide a mechanical signature for identifying individual single-walled and double-walled nanotubes.

Photothermoelectric Effect in Suspended Semiconducting Carbon Nanotubes

We have performed scanning photocurrent microscopy measurements of field-effect transistors (FETs) made from individual ultraclean suspended carbon nanotubes (CNTs). We investigate the spatial-dependence, polarization-dependence, and gate-dependence of photocurrent and photovoltage in this system. While previous studies of surface-bound CNT FET devices have identified the photovoltaic effect as the primary mechanism of photocurrent generation, our measurements show that photothermoelectric phenomena play a critical role in the optoelectronic properties of suspended CNT FETs. We have quantified the photothermoelectric mechanisms and identified regimes where they overwhelm the photovoltaic mechanism.

Origins of Charge Noise in Carbon Nanotube Field-Effect Transistor Biosensors

Determining the major noise sources in nanoscale field-effect transistor (nanoFET) biosensors is critical for improving bioelectronic interfaces. We use the carbon nanotube (CNT) FET biosensor platform to examine the noise generated by substrate interactions and surface adsorbates, both of which are present in current nanoFET biosensors. The charge noise model is used as a quantitative framework to show that insulating substrates and surface adsorbates are both significant contributors to the noise floor of CNT FET biosensors. Removing substrate interactions and surface adsorbates reduces the power spectral density of background voltage fluctuations by 19-fold.

Terahertz imaging of inhomogeneous electrodynamics in single-layer graphene embedded in dielectrics

We investigate electron transport properties in large-area, single-layer graphene embedded in dielectric media, using free-space terahertz (THz) imaging and time-domain spectroscopy. Sandwiched between a thin polymethyl methacrylate (PMMA) layer and a Si substrate, graphene layers of different growth recipes exhibit distinctive spatial inhomogeneity of sheet conductivity. The non-contacting, non-destructive THz probe reveals that the PMMA layer induces a small, yet noticeable reduction in conductivity.

Controlling the function of carbon nanotube devices with re-writable charge patterns

We use atomic force microscopy lithography to write charge patterns in close proximity to carbon nanotube field-effect transistor devices. The silicon dioxide substrate retains the charge for days, allowing various charge configurations to be tested. We show that the written charge can move the Fermi level in the nanotube by 1 eV and we use this charge lithography to reconfigure a field-effect transistor into a pn junction. The substrate charge can be erased and rewritten, offering a new tool for prototyping nanodevices and optimizing electrostatic doping profiles.

A spectrally-tunable photocurrent microscope for characterizing nanoelectronic devices

Scanning photocurrent microscopy is a unique tool that facilitates both device characterization and the study of fundamental properties of optoelectronic nanomaterials. We have built a scanning photocurrent microscope that incorporates a super continuum laser as the light source. The microscope illuminates nanoelectronic devices with a micron-scale light spot and a photon energy that is tunable from 0.67 eV to 2.7 eV. We describe the design of our microscope and present measurements of carbon nanotube transistor devices. These measurements highlight the features of our microscope, particularly the advantages of combining spatial and spectral resolution when characterizing nanoelectronic devices.

Terahertz Imaging and Spectroscopy of Large-Area Single-Layer Graphene

THz imaging and spectroscopy using broadband THz pulses map out the THz carrier dynamics of large-area single-layer graphene. Non-contacting, non-destructive THz probing reveals the local sheet conductivity of the graphene samples.