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We report the capacitive spring softening effect observed in single-walled carbon nanotube (SWNT) nanoelectromechanical (NEM) resonators. The nanotube resonators adopt a dual-gate configuration with both bottom-gate and end-gate capable of tuning the resonance frequency through capacitive coupling. Interestingly, downward resonance frequency shifting is observed with increasing end-gate voltage, which can be attributed to the capacitive softening of the spring constant. Furthermore, in-plane vibrational modes exhibit a much stronger spring softening effect than out-of-plan modes. Our dual-gate design should enable the differentiation between these two types of vibrational modes and open up the new possibility for nonlinear operation of nanotube resonators. [ABSTRACT FROM AUTHOR]

Conspectus: Nanoelectronic devices based on nanomaterials such as nanowires, carbon nanotubes, graphene, and other 2D nanomaterials offer extremely large surface-to-volume ratios, high carrier mobility, low power consumption, and high compatibility for integration with modern electronic technologies. These distinct advantages promise great potential for nanoelectronic devices as next generation chemical and biological sensors. Currently, majority of existing nanoelectronic sensors are direct current (DC) sensors, which rely ubiquitously on detection of conductance change associated with molecular adsorption. However, despite the simplicity of the conventional DC sensing technology, it also has severe limitations such as the Debye screening effect in ionic solutions, and the speed-sensitivity trade-off for the detection of charge-neutral molecules. Hence, the development of nanoelectronic sensors calls for new sensing platform technologies that can truly showcase the advantages of electronic sensors. In this Account, we will summarize recent efforts from our group on the development of a new electronic sensing paradigm, the nanoelectronic heterodyne sensors. Unlike conventional charge-detection based sensors, the heterodyne sensor explores the frequency mixing response between molecular dipoles and a nanoscale transistor. As an example, we first discuss the capability of heterodyne sensing in gas sensing applications by using graphene devices. Rapid (down to 0.1 s) and sensitive (down to 1 ppb) detection of a wide range of vapor analytes is achieved, representing orders of magnitude improvement over state-of-the-art nanoelectronic sensors. Furthermore, the heterodyne sensing technique enables electrical probing and tuning of the noncovalent physisorption of polar molecules on graphene surface for the first time. These results provide insight into small molecule-nanomaterial interaction dynamics and signify the ability to electrically tailor interactions, which can lead to rational designs of complex chemical processes for catalysis and drug discovery. Finally, we discuss the application of heterodyne sensing in solution for chemical and biological sensors by using carbon nanotube devices. The fundamental ionic screening effect can be mitigated by operating carbon nanotube field effect transistor as a heterodyne biosensor. Electrical detection of streptavidin binding to biotin in 100 mM buffer solution can be achieved at a frequency beyond 1 MHz. The results should promise a new biosensing platform for point-of-care detection, where biosensors functioning directly in physiologically relevant condition are desired. [ABSTRACT FROM AUTHOR]

We demonstrate a fully tunable diode structure utilizing a fully suspended single-walled carbon nanotube. The diode’s turn-on voltage under forward bias can be continuously tuned up to 4.3 V by controlling gate voltages, which is ∼6 times the nanotube band gap energy. Furthermore, the same device design can be configured into a backward diode by tuning the band-to-band tunneling current with gate voltages. A nanotube backward diode is demonstrated for the first time with nonlinearity exceeding the ideal diode. These results suggest that a tunable nanotube diode can be a unique building block for developing next generation programmable nanoelectronic logic and integrated circuits. [ABSTRACT FROM AUTHOR]

The discovery of electric field induced band gap opening in bilayer graphene opens a new door for making semiconducting graphene without aggressive size scaling or using expensive substrates. However, bilayer graphene samples have been limited to μm2size scale thus far, and synthesis of wafer scale bilayer graphene poses a tremendous challenge. Here we report homogeneous bilayer graphene films over at least a 2 in. × 2 in. area, synthesized by chemical vapor deposition on copper foil and subsequently transferred to arbitrary substrates. The bilayer nature of graphene film is verified by Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. Importantly, spatially resolved Raman spectroscopy confirms a bilayer coverage of over 99%. The homogeneity of the film is further supported by electrical transport measurements on dual-gate bilayer graphene transistors, in which a band gap opening is observed in 98% of the devices. [ABSTRACT FROM AUTHOR]

In a modern wireless communication system, signal amplification is critical for overcoming losses during multiple data transformations/processes and long-distance transmission. Common mode and differential mode are two fundamental amplification mechanisms, and they utilize totally different circuit configurations. In this paper, we report a new type of dual-gate graphene ambipolar device with capability of operating under both common and differential modes to realize signal amplification. The signal goes through two stages of modulation where the phase of signal can be individually modulated to be either in-phase or out-of-phase at two stages by exploiting the ambipolarity of graphene. As a result, both common and differential mode amplifications can be achieved within one single device, which is not possible in the conventional circuit configuration. In addition, a common-mode rejection ratio as high as 80 dB can be achieved, making it possible for low noise circuit application. These results open up new directions of graphene-based ambipolar electronics that greatly simplify the RF circuit complexity and the design of multifunction device operation. [ABSTRACT FROM AUTHOR]

The ability to tune the molecular interaction electronically can have profound impact on wide-ranging scientific frontiers in catalysis, chemical and biological sensor development, and the understanding of key biological processes. Despite that electrochemistry is routinely used to probe redox reactions involving loss or gain of electrons, electrical probing and tuning of the weaker noncovalent interactions, such as molecular physisorption, have been challenging, primarily due to the inability to change the work function of conventional metal electrodes. To this end, we report electrical probing and tuning of the noncovalent physisorption of polar molecules on graphene surface by using graphene nanoelectronic heterodyne sensors. Temperature-dependent molecular desorptions for six different polar molecules were measured in real-time to study the desorption kinetics and extract the binding affinities. More importantly, we demonstrate electrical tuning of molecule-graphene binding kinetics through electrostatic gating of graphene; the molecular desorption can be slowed down nearly three times within a gate voltage range of 15 V. Our results provide insight into small molecule-nanomaterial interaction dynamics and signify the ability to electrically tailor interactions, which can lead to rational designs of complex chemical processes for catalysis and drug discovery. [ABSTRACT FROM AUTHOR]

Thephoto-Dember effect arises from the asymmetric diffusivityof photoexcited electrons and holes, which creates a transient spatialcharge distribution and hence the buildup of a voltage. Conventionally,a strong photo-Dember effect is only observed in semiconductors witha large asymmetry between the electron and hole mobilities, such asin GaAs or InAs, and is considered negligible in graphene due to itselectron–hole symmetry. Here, we report the observation ofa strong lateral photo-Dember effect induced by nonequilibrium hotcarrier dynamics when exciting a graphene–metal interface witha femtosecond laser. Scanning photocurrent measurements reveal theextraction of photoexcited hot carriers is driven by the transientphoto-Dember field, and the polarity of the photocurrent is determinedby the device’s mobility asymmetry. Furthermore, ultrafastpump–probe measurements indicate the magnitude of photocurrentis related to the hot carrier cooling rate. Our simulations also suggestthat the lateral photo-Dember effect originates from graphene’s2D nature combined with its unique electrical and optical properties.Taken together, these results not only reveal a new ultrafast photocurrentgeneration mechanism in graphene but also suggest new types of terahertzsources based on 2D nanomaterials. [ABSTRACT FROM AUTHOR]