Sensors based on graphene and metal nanowires

Graphene based chemical sensors:

We have been exploring ultra-sensitive material sensors based on graphene. We have demonstrated that graphene sensors, based on our digital signal processing platform, have extremely high-sensitivity and specificity towards chemicals. Being a pure two-dimensional system, graphene represents the ultimate NEMS system with all its atoms exposed to the surface. This makes the conductance of graphene extremely sensitive to the ambient and the presence of a single adsorbed molecule on its surface can significantly modify its electrical characteristics. Several unique properties of graphene make it exceptionally suitable for making sensors.  Firstly, it is highly conductive even in very low carrier density regimes and hence has extremely low levels of Johnson-Nyquist thermal noise as compared to semiconductor based sensors. It also has fewer kinds of defects and hence has intrinsically low levels of 1/f noise arising out of thermal switching of defects. It is relatively easy to make four terminal measurements on graphene strips making contact resistances much easier to deal with than for example in carbon nanotubes which share almost all other advantages of graphene. All these factors combine to give a very large signal-to-noise ratio in graphene sensors even at room temperatures giving it the ability to detect changes in local charge concentration of less than the charge of a single electron. Properties like atomically thin layers, very high absorption coefficient, high mobility of charge carriers and high mechanical strength make it an ideal candidate for use as radiation sensors. Making an effective sensor requires interface accessibility, good transduction, mechanical/electrical robustness, ease of preparation and integration into existing technologies. Graphene seems to satisfy this entire list of criterion and hence has the potential to emerge as the sensor material of choice in future.

Reduced graphene-oxide based pressure sensors:

Sensing of mechanical stimuli forms an important communication pathway between humans/environment and machines. The progress in such sensing technology has possible impacts on the functioning of automated systems, human machine interfacing, health-care monitoring, prosthetics and safety systems. The challenges in this field range from attaining high sensitivity to extreme robustness. We have demonstrated the sensing of complex mechanical stimuli with a patch of taped crumpled reduced graphene oxide (rGO). The sensor can can typically be assembled under household conditions. The ability of this sensor to detect a wide variety of pressures and strains in conventional day-to-day applications has been demonstrated. An extremely high gauge factor (∼103) at ultra-low strains (∼10⁻⁴) with fast response times (~ 20.4 ms) could be achieved with such sensors. Pressure resulting from a gentle touch to over human body weight could be sensed successfully. The capability of the sensor to respond in a variety of environments could be exploited in the detection of water and air pressures both below and above atmospheric, with a single device.

Ultra-thin nanowires (Au and Te) based chemical sensors:

Te-nanowire: Band structure engineering is a powerful technique both for the design of new semiconductor materials and for imparting new functionalities to existing ones. In this article, we present a novel and versatile technique to achieve this by surface adsorption on low dimensional systems. As a specific example, we demonstrate, through detailed experiments and ab initio simulations, the controlled modification of band structure in ultra-thin Te nanowires due to ₂ adsorption. Measurements of the temperature dependence of resistivity of single ultra-thin Te nanowire field-effect transistor (FET) devices exposed to increasing amounts of NO₂ reveal a gradual transition from a semiconducting to a metallic state. Gradual quenching of vibrational Raman modes of Te with increasing concentration of NO₂ supports the appearance of a metallic state in ₂ adsorbed Te. Ab initio simulations attribute these observations to the appearance of mid-gap states in NO₂ adsorbed Te nanowires. Our results provide fundamental insights into the effects of ambient on the electronic structures of low-dimensional materials and can be exploited for designing novel chemical sensors.

Au-nanowire: We have demonstrated that, contrary to expectations, the adsorption of common chemicals like methanol and acetone has a profound impact on the electrical transport properties of the ultra-thin gold nanowires of diameter ~ 4nm. Our measurements and subsequent calculations establish conclusively that in gold nanowires, semiconductor-like sensitivity to the ambient arises because of changes induced in its local density of states by the surface adsorbed molecules. The extreme sensitivity of the resistance fluctuations of the gold nanowires to ambient suggests their possible use as solid-state sensors.