Thin layers of black phosphorus have recently raised interest owing to their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of phosphorus atoms can be exfoliated down to monolayer 2D-phosphane (also called phosphorene) using procedures similar to those used for graphene. Probing the properties has, however, been challenged by a fast degradation of the thinnest layers on exposure to ambient conditions. Herein, we investigate this chemistry using in situ Raman and transmission electron spectroscopies. The results highlight a thickness-dependent photoassisted oxidation reaction with oxygen dissolved in adsorbed water. The oxidation kinetics is consistent with a phenomenological model involving electron transfer and quantum confinement as key parameters. A procedure carried out in a glove box is used to prepare mono-, bi- and multilayer 2D-phosphane in their pristine states for further studies on the effect of layer thickness on the Raman modes. Controlled experiments in ambient conditions are shown to lower the A(g)(1)/A(g)(2) intensity ratio for ultrathin layers, a signature of oxidation.

Graphene field effect transistors (FETs) are extremely sensitive to gas exposure. Charge transfer doping of graphene FETs by atmospheric gas is ubiquitous but not yet understood. We have used graphene FETs to probe minute changes in electrochemical potential during high-purity gas exposure experiments. Our study shows quantitatively that electrochemistry involving adsorbed water, graphene, and the substrate is responsible for doping. We not only identify the water/oxygen redox couple as the underlying mechanism but also capture the kinetics of this reaction. The graphene FET is highlighted here as an extremely sensitive potentiometer for probing electrochemical reactions at interfaces, arising from the unique density of states of graphene. This work establishes a fundamental basis on which new electrochemical nanoprobes and gas sensors can be developed with graphene.

The back-gated field-effect transistor (FET) configuration has been central to the study of the electronic transport properties of organic and nanoscale materials. [1][2][3] Three-terminal transport measurements using this geometry are facilitated by using a degenerately doped silicon wafer covered with a thin silicon oxide layer as the substrate. It has been widely assumed that the substrate did not influence the measurement of the intrinsic properties of the material under study. However, Chua et al. [4] recently demonstrated that changing the nature of the dielectric led to ambipolar behavior in FETs made from organic semiconductors that were previously thought to be exclusively hole (p-type) conductors. Silanol groups present on the SiO 2 surface were singled out as the culprits for generating electron traps responsible for suppressing electron (n-type) conduction in these devices. Here we show that the substrate-induced suppression of n-type behavior is not unique to organic FETs, but influences the measurements of all devices fabricated on SiO 2 /Si substrates. By using carbon nanotubes as the testbed, we investigated the impact of the chemical nature of the substrate and of ambient adsorbates on the field-effect switching behavior of both nanoscale and thin-film FETs. Our study revealed that the reduction of n-type conduction occurs when an adsorbed water layer containing solvated oxygen is present on the SiO 2 surface. This finding demonstrates that an electrochemical charge transfer reaction between the semiconducting channel and the aqueous oxygen redox couple is the underlying phenomenon governing the suppression of electron conduction in carbon nanotube devices. This effect should be taken into account when interpreting three-terminal measurements conducted on SiO 2 /Si substrates. We anticipate that the design of electronic devices, [5] chemical sensors, [6] and biosensors [7] that are based on the FET configuration will be largely influenced by the charge transfer mechanism that has been brought to light by this study.Individual carbon nanotube field-effect transistors (CFETs) are the most extensively studied molecular-scale FETs to date.

Graphene can be transformed from a semimetal into a semiconductor if it is confined into nanoribbons narrower than 10 nm with controlled crystallographic orientation and well-defined armchair edges. However, the scalable synthesis of nanoribbons with this precision directly on insulating or semiconducting substrates has not been possible. Here we demonstrate the synthesis of graphene nanoribbons on Ge(001) via chemical vapour deposition. The nanoribbons are self-aligning 3° from the Ge〈110〉 directions, are self-defining with predominantly smooth armchair edges, and have tunable width to <10 nm and aspect ratio to >70. In order to realize highly anisotropic ribbons, it is critical to operate in a regime in which the growth rate in the width direction is especially slow, <5 nm h−1. This directional and anisotropic growth enables nanoribbon fabrication directly on conventional semiconductor wafer platforms and, therefore, promises to allow the integration of nanoribbons into future hybrid integrated circuits.