Nanosensors based on the unique electronic properties of nanotubes and nanowires offer high sensitivity and have the potential to revolutionize the field of Point-of-Care (POC) medical diagnosis. The direct current (dc) detection of a wide array of organic and inorganic molecules has been demonstrated on these devices. However, sensing mechanism based on measuring changes in dc conductance fails at high background salt concentrations, where the sensitivity of the devices suffers from the ionic screening due to mobile ions present in the solution. Here, we successfully demonstrate that the fundamental ionic screening effect can be mitigated by operating single-walled carbon nanotube field effect transistor as a high-frequency biosensor. The nonlinear mixing between the alternating current excitation field and the molecular dipole field can generate mixing current sensitive to the surface-bound biomolecules. Electrical detection of monolayer streptavidin binding to biotin in 100 mM buffer solution is achieved at a frequency beyond 1 MHz. Theoretical modeling confirms improved sensitivity at high frequency through mitigation of the ionic screening effect. The results should promise a new biosensing platform for POC detection, where biosensors functioning directly in physiologically relevant condition are desired.

Nearly all existing nanoelectronic sensors are based on charge detection, where molecular binding changes the charge density of the sensor and leads to sensing signal. However, intrinsically slow dynamics of interface-trapped charges and defect-mediated charge-transfer processes significantly limit those sensors' response to tens to hundreds of seconds, which has long been known as a bottleneck for studying the dynamics of molecule-nanomaterial interaction and for many applications requiring rapid and sensitive response. Here we report a fundamentally different sensing mechanism based on molecular dipole detection enabled by a pioneering graphene nanoelectronic heterodyne sensor. The dipole detection mechanism is confirmed by a plethora of experiments with vapour molecules of various dipole moments, particularly, with cis-and trans-isomers that have different polarities. Rapid (down to B0.1 s) and sensitive (down to B1 ppb) detection of a wide range of vapour analytes is achieved, representing orders of magnitude improvement over state-of-the-art nanoelectronics sensors.

In modern communication systems, modulation is a key function that embeds the baseband signal (information) into a carrier wave so that it can be successfully broadcasted through a medium such as air or cables. Here we report a flexible all-graphene modulator circuit with the capability of encoding a carrier signal with quaternary digital information. By exploiting the ambipolarity and the nonlinearity in a graphene transistor, we demonstrate two types of quaternary modulation schemes: quaternary amplitude-shift keying and quadrature phase-shift keying. Remarkably, both modulation schemes can be realized with just 1 and 2 all-graphene transistors, respectively, representing a drastic reduction in circuit complexity when compared with conventional modulators. In addition, the circuit is not only flexible but also highly transparent (~95% transmittance) owing to its all-graphene design with every component (channel, interconnects, load resistor and source/drain/gate electrodes) fabricated from graphene films.

Substantial effort has been dedicated to understand the intrinsic electronic properties of molybdenum disulfide (MoS2). However, electron transport study on monolayer MoS2 has been challenging to date, especially at low temperatures due to large metal/semiconductor junction barriers. Herein, we report the fabrication and characterization of the monolayer MoS2 single-electron transistor. High performance devices are obtained through the use of low work function metal (zinc) contact and a rapid thermal annealing step. Coulomb blockade is observed at low temperatures and is attributed to single-electron tunneling via two tunnel junction barriers. The nature of Coulomb blockade is also investigated by temperature-dependent conductance oscillation measurement. Our results hold promise for the study of novel quantum transport phenomena in 2D semiconducting atomic layer crystals.

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.