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Research Alert: July 16, 2014

A nanosensor to identify vapors based on a graphene/silicon heterojunction Schottky diode

Among other carbon-based nanomaterials, graphene represents a great promise for gas sensing applications. In 2009 the detection of individual gas molecules of NO2 adsorbed onto graphene surface was reported for the first time. This initial observation has been successfully explored during the recent years. The Nanobioelectronics & Biosensors Group at Institut Català de Nanociència i Nanotecnologia (ICN2), led by ICREA Research Professor Arben Merkoçi, published in Small a work showing how to use a Graphene/Silicon Heterojunction Schottky Diode as a sensitive, selective and simple tool for vapors sensing. The work was developed in collaboration with researchers from the Amirkabir University of Technology (Tehran, Iran).

The Graphene/Silicon heterojunction Schottky diode is fabricated using a silicon wafer onto which Cr and Au were deposited to form the junction between graphene and silicon (see the attached figure). The adsorbed vapor molecules change the local carrier concentration in graphene, which yields to the changes in impedance response. The vapors of the various chemical compounds studied change the impedance response of Graphene/Silicon heterojunction Schottky diode. The relative impedance change versus frequency dependence shows a selective response in gas sensing which makes this characteristic frequency a distinctive parameter of a given vapor.

The device is well reproducible for different concentrations of phenol vapor using three different devices. This graphene based device and the developed detection methodology could be extended to several other gases and applications with interest for environmental monitoring as well as other industries.

A cool approach to flexible electronics

A nanoparticle ink that can be used for printing electronics without high-temperature annealing presents a possible profitable approach for manufacturing flexible electronics.

Printing semiconductor devices are considered to provide low-cost high performance flexible electronics that outperform amorphous silicon thin film transistors currently limiting developments in display technology. However the nanoparticle inks developed so far have required annealing, which limits them to substrates that can withstand high temperatures, ruling out a lot of the flexible plastics that could otherwise be used. Researchers at the National Institute of Materials Science and Okayama University in Japan have now developed a nanoparticle ink that can be used with room-temperature printing procedures.

Developments in thin film transistors made from amorphous silicon have provided wider, thinner displays with higher resolution and lower energy consumption. However further progress in this field is now limited by the low response to applied electric fields, that is, the low field-effect mobility. Oxide semiconductors such as InGaZnO (IGZO) offer better performance characteristics but require complicated fabrication procedures.

Nanoparticle inks should allow simple low-cost manufacture but the nanoparticles usually used are surrounded in non-conductive ligands – molecules that are introduced during synthesis for stabilizing the particles. These ligands must be removed by annealing to make the ink conducting. Takeo Minari, Masayuki Kanehara and colleagues found a way around this difficulty by developing nanoparticles surrounded by planar aromatic molecules that allow charge transfer.

The gold nanoparticles had a resistivity of around 9 x 10-6 Ω cm – similar to pure gold. The researchers used the nanoparticle ink to print organic thin film transistors on a flexible polymer and a paper substrate at room temperature, producing devices with mobilities of 7.9 and 2.5 cm2 V-1 s-1 for polymer and paper respectively – figures comparable to IGZO devices.

As the researchers conclude in their report of the work, “This room temperature printing process is a promising method as a core technology for future semiconductor devices.”

Graphene grain boundaries reviewed

Graphene has attracted overwhelming attention both for exploring fundamental science and for a wide range of technological applications. Chemical vapor deposition (CVD) is currently the only working approach to grow high-quality graphene at very large scale, which is required for industrial applications such as high-frequency devices, sensors, transparent electrodes, etc. Unfortunately, the produced graphene is typically polycrystalline, consisting of a patchwork of grains with various orientations and sizes, joined by grain boundaries of irregular shapes. The understanding of the relation between polycrystalline morphology and charge transport is crucial for the development of applications, as recently reported by SAMSUNG.

Researchers from the ICN2 Theoretical and Computational Nanoscience Group curated a review article in Advanced Materials to determine whether graphene grain boundaries are a blessing or a curse. ICREA Research Professor Stephan Roche, Group Leader at ICN2, together with Dr Aron CummingsJose Eduardo Barrios Vargas and Van Tuan Dinh, from the same Group, share the authorship of the review with researchers from Sungkyunkwan University. The review article not only provides guidelines for the improvement of graphene devices, but also opens a new research area of engineering graphene grain boundaries for highly sensitive electro-biochemical devices.

The review analyses the challenges and opportunities of charge transport in polycrystalline graphene, which means summarizing the state-of-the-art knowledge about graphene grain boundaries (GGBs). The review is divided in the following sections: Structure and Morphology of GGBs; Methods of Observing GGBs; Measurement of Electrical Transport across GGBs; Manipulation of GGBs with Functional Groups.

The work describes how TEM and STM, combined with theory and simulation, can provide information for the observation and characterization of GGBs at the atomic scale. These boundaries have interesting properties, such as the fact that they can be a good template for the synthesis of 1D materials, might be useful to design sensors for detecting gases and molecules or allow selective diffusion of limited gases and molecules. Controlling the atomic structure of GGBs by CVD is a big challenge from a scientific point of view, but would be a huge step forward in the realization of next-generation technologies based on this material.

The work has been partly funded by SAMSUNG within the Global Innovation program.

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