Carbon in the sp¹ hybridization (carbyne) is able to form atomic chains that constitute the elementary one-dimensional phase of carbon [1]. Despite many efforts, the synthesis of carbon chains in appreciable quantities remains difficult and even the existence of bulk carbyne is subject of an ongoing controversy. However, the existence of individual carbon chains is undisputed since they have been observed in in-situ TEM studies. The present work allowed, for the first time, to measure the electrical properties of carbon chains by using a dedicated specimen stage for establishing electrical contacts in the TEM.

Carbon chains are unusual conductors that may occur either as metallic cumulene with double bonds or as semiconducting polyyne with alternating single and triple bonds. Now, as it became possible to characterize them electrically, these two electronic configurations can be distinguished. A piezo-driven tip (Nanofactory), integrated into the specimen holder of a TEM, allowed to establish contacts to graphenic material and, by controlled retraction of the electrodes, to unravel chains of carbon atoms (fig. 1a). At the same time, the electrical properties could be measured [2, 3]. By recording current-voltage curves of individual carbon chains, both cumulene and polyyne were identified (fig. 1b). It was found experimentally and by quantum conductance calculations (fig. 1c) that transport through narrow resonant states makes the conductivity much lower than predicted in previous theoretical work. At high applied bias, however, a sudden rise in current occurs, showing the absence of conduction channels at lower energy and their presence at higher energy.

When the 1D system is under strain, the chains exhibit a semiconducting behavior, corresponding to polyyne. Conversely, when the chain is unstrained, an ohmic behavior, corresponding to cumulene, is observed (fig. 1b) [4]. This confirms a recent theoretical prediction, namely that the Peierls distortion, which would stabilize polyyne, is suppressed by zero-point vibrations in an unstrained chain so that cumulene is the stable configuration. In the presence of strain, however, polyyne is favoured by the Peierls instability. Thus, a metal-insulator transition can be induced by adjusting the strain. Furthermore, it is shown that these atomic chains can act as rectifying diodes when they are in a non-symmetric contact configuration, i.e., between a carbon and a metal contact or between two carbon contacts of different type.

Acknowledgements:

Financial support by the LABEX program “Nanostructures in Interaction with their Environment” (NIE) and different projects of the Agence Nationale de Recherche (ANR, France) is gratefully acknowledged.

 References:

1. F. Banhart, Beilstein J. Nanotech. 6, 559 (2015).

2. O. Cretu, A. R. Botello-Mendez, I. Janowska, C. Pham-Huu, J.-C. Charlier, F. Banhart, Nano Lett. 13, 3487 (2013).

3. A. La Torre, F. Ben Romdhane, W. Baaziz, I. Janowska, C. Pham-Huu, S. Begin-Colin, G. Pourroy, F. Banhart, Carbon 77, 906 (2014).

4. A. La Torre, A. Botello-Mendez, W. Baaziz, J.-C. Charlier, F. Banhart, Nature Comm. 6, 6636 (2015).

Figures:

Figure 1: (a): Principle of the experiment: a carbon chain is unraveled by a STM tip from a graphene layer. The current through the chain is measured at an applied voltage. (b): TEM images and corresponding current-voltage measurements of an unstrained (left) and a strained chain (right). The unstrained (warped) chain shows metallic conductivity (cumulene) whereas the strained (straight) chain shows semiconducting behaviour (polyyne). (c): The model shows the calculated electron distribution (pink) along a chain with sp2-contact at a graphene edge and sp3-contact at a carbon nanotube._x000D_

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