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

Electrical Monitoring of sp³ Defect Formation in Individual Carbon Nanotubes

Many carbon nanotube (CNT) applications require precisely controlled chemical functionalization that is minimally disruptive to electrical performance. A promising approach is the generation of sp³ hybridized carbon atoms in the sp²-bonded lattice. We have investigated the possibility of using a carboxylic acid-functionalized diazonium reagent to introduce a defined number of sp³ defects into electrically contacted CNTs. Having performed real-time measurements on individually contacted CNTs, we show that the formation of an individual defect is accompanied by an upward jump in resistance of approximately 6 kΩ. Additionally, we observe downward jumps in resistance of the same size, indicating that some defects are unstable. Our results are explained by a two-step reaction mechanism. Isolated aryl groups, formed in the first step, are unstable and dissociate on the minute time scale. Stable defect generation requires a second step: the coupling of a second aryl group adjacent to the first. Additional mechanistic understanding is provided by a systematic investigation of the gate voltage dependence of the reaction, showing that defect formation can be turned on and off. In summary, we demonstrate an unprecedented level of control over sp³ defect formation in electrically contacted CNTs, and prove that sp³ defects are minimally disruptive to the electrical performance of CNTs