Tal Sharf

Scalable graphene field-effect sensors for specific protein detection

We demonstrate that micron-scale graphene field-effect transistor biosensors can be fabricated in a scalable fashion from large-area chemical vapor deposition derived graphene. We electrically detect the real-time binding and unbinding of a protein biomarker, thrombin, to and from aptamer-coated graphene surfaces. Our sensors have low background noise and high transconductance, comparable to exfoliated graphene devices. The devices are reusable and have a shelf-life greater than one week.

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.

Single Electron Charge Sensitivity of Liquid-Gated Carbon Nanotube Transistors

Random telegraph signals corresponding to activated charge traps were observed with liquid-gated CNT FETs. The high signal-to-noise ratio that we observe demonstrates that single electron charge sensing is possible with CNT FETs in liquids at room temperature. We have characterized the gate-voltage dependence of the random telegraph signals and compared to theoretical predictions. The gate-voltage dependence clearly identifies the sign of the activated trapped charge.

Terahertz transmission ellipsometry of vertically aligned multi-walled carbon nanotubes

We demonstrate time-resolved terahertz transmission ellipsometry of vertically aligned multi-walled carbon nanotubes. The angle-resolved transmission measurements reveal anisotropic characteristics of the terahertz electrodynamics in multi-walled carbon nanotubes. The anisotropy is, however, unexpectedly weak: the ratio of the tube-axis conductivity to the transverse conductivity, σz/σxy≅2.3, is nearly constant over the broad spectral range of 0.4–1.6 THz. The relatively weak anisotropy and the strong transverse electrical conduction indicate that THz fields readily induce electron transport between adjacent shells within multi-walled carbon nanotubes.

Fabrication of low-noise carbon nanotube field-effect transistor biosensors

We report the fabrication of suspended carbon nanotube field effect transistors (CNT FETs) designed for biological sensing applications. We compare the electrical properties of our suspended CNT FET design to standard surface based CNT FETs. In particular, we show a reduction in environmental noise, suggesting that the new sensor design is a promising candidate for low-noise sensing applications.

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 ellipsometry of vertically grown carbon nanotubes

THz ellipsometry with broadband THz pulses reveals anisotropic THz responses from closely packed, vertically grown CNTs. Non-negligible conductivity in a direction normal to the CNT axis indicates carrier transport between adjacent CNTs.

Terahertz Ellipsometry of Vertically Grown Carbon Nanotubes

THz ellipsometry with broadband THz pulses reveals anisotropic THz responses from closely packed, vertically grown CNTs. Non-negligible conductivity in a direction normal to the CNT axis indicates carrier transport between adjacent CNTs.