Time-Dependent Fields for a New Breed of Carbon-Based Nanodevices

The 1st of May of 1893 should have been a bitter day for Thomas Edison. That day, the Chicago World’s Fair, one of the biggest international expositions ever, was officially opened to the public. The event featured a large area of electric exhibits powered by alternating currents (ac) provided by Edison’s competitors. Edison’s last minute attempt to prevent the use of light bulbs had failed and the public was able to appreciate the benefits of alternating currents for the first time. It did not take very long until ac finally dominated over the direct current (dc) sources supported by Edison, thereby allowing for longer electrical networks, sparking a revolution that changed our world forever.

Nevertheless, many appliances including the electronic devices we have today use mostly dc power. Something similar occurs with the devices investigated at the nanometer scale: With a few exceptions [1,2], most of the attention has been given to nanodevices under dc conditions. But something started to change in recent years and a wealth of phenomena involving the use of time-dependent fields, such as ac gate voltages or lasers, started to flourish here and there [3], adding a whole new dimension of possibilities. In this brief note I will try to convince you that there are good reasons that make the use of time-dependent fields at the nanoscale attractive and sometimes unique. To such end I will use some examples from my own research, but before that I will introduce a few facts about our favorite nanoscale materials: carbon nanotubes [4] and graphene [5].

The properties of these materials are in many regards very similar, graphene being the two-dimensional younger cousin of carbon nanotubes. They rank among the best conductive materials [4,5,6], conduct heat better than any other material on earth [7] with a thermal conductivity of about 5x103 W/mK for graphene (about ten times that of copper), and they show exceptional mechanical strength [8] (with a breaking strength about 200 times that of steel). Carbon nanotubes for example, show anomalously low sensitivity to disorder-induced backscattering and are very robust to acoustical-phonon-induced backscattering with inelastic mean free paths on the order of the micrometer [9] in a regime which extends up to bias voltages on the order of the optical phonon energies (aprox. 200 meV) when other mechanisms enter into play leading to current saturation [10-12]. Applications for the case of nanotubes range from transistors [13] that take advantage of the high electron mobilities (which would allow even us to triple the frequency of microprocessors) to a promising future as an energy efficient replacement for copper in nanoscale interconnects [14,15].

With these carbon-based materials in mind, our vision is that ac fields could be used for achieving control of the electrical response (current and noise) as well as the heat dissipation, and perform useful functions that could lead to a new breed of carbon-based devices. At the heart of this vision is the fact that owing to their reduced dimensionality and exceptional coherence of the electrons, these materials offer an outstanding arena for tuning the interplay between a unique electronic structure, the driving parameters and inelastic processes induced by the ac fields [16]. In the following, this will be illustrated with a few examples from our own research.

Control of the Conductance and Noise in Carbon-Based Resonators

By applying an ac gate to a carbon nanotube or graphene nanoribbon device, new parameters enter into the game: the driving frequency and amplitude. By tuning these parameters, we have shown that it is possible to achieve control over the current and its fluctuations [17,18]. “Give me the desired response and I will tell you the magnitude of the driving parameters necessary to obtain it.” Adding a static magnetic field leads to even richer features [19].

A Light on the Horizon of Graphene? Tuning Laser-Induced Band Gaps

In spite of the impressive list of promising prospects, graphene has an Achilles heel: it does not have a band-gap, once it is conducting it cannot be switched-off. This hinders applications in active electronic devices where the ability to switch it on and off is crucial. In a recent highlighted article, we reported on the first atomistic simulations of electrical conduction through a micrometer-sized graphene sample illuminated by a laser field [20]. Our simulations show that a laser in the mid-infrared can open an observable band-gap in this material, thereby opening promising prospects for graphene-based optoelectronic devices.

Achieving a dc Current Without Bias Voltage through Time-Dependent Potentials

A direct current (dc) usually requires the application of a bias voltage, with no bias voltage applied between, say, left and a right electrodes, no current flows. However, in systems at the nanoscale a dc current can be generated even at zero bias due to a quantum coherent effect called quantum pumping [21,22]. Originally, it was thought that quantum pumping required at least two time-dependent potentials (such as ac gate voltages applied to the device) but more recent theoretical [23] and experimental [24] studies suggest that it is indeed possible to achieve it with only one ac field as well, thereby avoiding the clutter associated with additional gates. Besides, having no bias voltage applied between the electrodes, a quantum pump could have very low power dissipation.

Up until now, quantum pumps were made mostly of traditional semiconducting materials. Using carbon-based materials would bring many benefits: higher frequency operation and the possibility of having almost perfect contacts, leaving us in a regime far from the one of isolated resonances and poor conductance usually explored. In this field of much current interest [25,26], our contributions try to bring these devices closer to reality, both in the high frequency (non adiabatic) [27] and low frequency (adiabatic) [28] regimes. Besides, its intrinsic interest, these devices could provide a clue on a different kind of active devices with unprecedented low energy dissipation.

References

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