Graphene-based humidity sensors: the origin of alternating resistance change.

The response of a graphene-based humidity sensor is considered as a function of film structures. Analysis of the resistance changes due to water molecule adsorption on the graphene or multi-layer graphene (MLG) surface is performed for films with different structures and resistivities from hundreds of ohms/sq to hundreds of kilo-ohms/sq. The results revealed possible increase, decrease and non-monotonous behavior of resistance with changes in film structure. Adsorption of water molecules at grain boundary defects is assumed to lead to an increase in film resistivity due to the donor property of water and the p-type conductivity of graphene. Another type of conductive center with a higher capture cross-section is realized in the case of water molecule adsorption at edge defects in MLG films (the formation of conductive chains with ionic conductivity). If these chains form a continuous network the film resistivity decreases. The result of the competition between the opposite effects of the conductivity compensation and formation of the water-based conductive chains depends on the film structure and determines the response of humidity sensors. Sensor sensitivity is found to increase when only one type of defect determines water adsorption (edge defects or grain boundary defects).

Fluorinated graphene suspension for inkjet printed technologies.

The possibility to control the size of the flakes of graphene suspension in the course of their fluorination in an aqueous hydrofluoric acid solution was demonstrated. The effect of the suspension composition, the fluorination time, temperature and thermal stress on the fragmentation process was investigated. The corrugation of suspension flakes, which occurs at fluorination due to a difference in the constants of graphene and fluorographene lattices, leads to the appearance of nonuniform mechanical stresses. The fact that the flake size after fragmentation is determined by the size of corrugation allows the assumption that the driving force of fragmentation is this mechanical stress. This assumption is confirmed by the break of the corrugated layers from flakes under thermal stress. Moreover, fluorination treatment at elevated temperatures (∼70 °C) significantly accelerates the fragmentation process. Suspensions of fluorinated graphene with nanometer size flakes are of interest for the development of 2D ink-jet printing technologies and production of thermally and chemically stable dielectric films for nanoelectronics. The printed fluorinated graphene films on silicon and flexible substrates have been demonstrated and the charges in metal-insulator-semiconductor structures have been estimated as the ultra low values of (0.5-2) × 10(10) cm(-2).

Fluorinated graphene dielectric films obtained from functionalized graphene suspension: preparation and properties.

In the present study, we have examined the interaction between a suspension of graphene in dimethylformamide and an aqueous solution of hydrofluoric acid, which was found to result in partial fluorination of suspension flakes. A considerable decrease in the thickness and lateral size of the graphene flakes (up to 1-5 monolayers in thickness and 100-300 nm in diameter) with increasing duration of fluorination treatment is found to be accompanied by a simultaneous transition of the flakes from the conducting to the insulating state. Smooth and uniform insulating films with a roughness of ∼2 nm and thicknesses down to 20 nm were deposited from the suspension on silicon. The electrical and structural properties of the films suggest their use as insulating elements in thin-film nano- and microelectronic device structures. In particular, it was found that the films prepared from the fluorinated suspension display rather high breakdown voltages (field strength of (1-3) × 10(6) V cm(-1)), ultralow densities of charges in the film and at the interface with the silicon substrate in metal-insulator-semiconductor structures (∼(1-5) × 10(10) cm(-2)). Such excellent characteristics of the dielectric film can be compared only to well-developed SiO2 layers. The films from the fluorinated suspension are cheap, practically feasible and easy to produce.

A New Approach To the Diagnosis of Point Mutations in Native DNA Using Graphene Oxide.

Development of new methods for the diagnosis of point mutations is a pressing issue. We have developed a new approach to the design of graphene oxide-based test systems for the diagnosis of point mutations in native DNA. This new approach is based on the use of graphene oxide for the adsorption and quenching of fluorescently labeled primers in a post-amplification PCR mixture followed by detection of fluorescently labeled PCR products. It is possible to detect fluorescently labelled amplicons in the presence of an excess of primers in a PCR product solution due to the different affinities of single-stranded and double-stranded DNA molecules to graphene oxide, as well as the ability of graphene oxide to act as a quencher of the fluorophores adsorbed on its surface. The new approach was tested by designing a graphene oxide-based test system for the DNA diagnosis of the point mutation associated with the development of the 3M syndrome in Yakuts. The developed approach enables one to design graphene oxide-based test systems suitable for the diagnosis of any point mutations in native DNA.