The Project

The purpose of NANOSIM_GRAPHENE is to establish a multi-scale simulation methodology of graphene-based nanomaterials and nanodevices.

Graphene is a two-dimensional monolayer of sp2 bonded carbon atoms in a dense honeycomb crystal structure which behaves electronically as a zero-gap semiconductor. Exceptional electronic properties of this material, resulting for instance in carrier mobilities as large as several thousands of cm2/Vs, make this material a main driver of the beyond-CMOS nanoelectronics research. A key issue lies however in the strategy to design graphene-based materials with enhanced energy (or transport) gap, allowing for field effect efficiency and device capability. In this respect, the chemical complexity of graphene demands in-depth quantum simulation analysis for understanding of the material properties and further optimization of graphene-based device performance.

It is the ambition of NANOSIM_GRAPHENE to provide national cutting-edge expertise and international leadership on this nanomaterial through the development of advanced computational tools enabling both: the material characterization at the nanoscale (STM simulations) as well as the performances of graphene-based nanodevices (such as graphene field effect transistors), with the exploration of its true innovation potential.

The consortium gathers academic partners who are experts in the main simulation techniques: ab initio calculations, non-equilibrium Green’s functions and simulation of quantum nano-devices, analytical approaches and STM simulation. An important aim of NANOSIM_GRAPHENE is to contrast and bridge these different techniques and develop novel multiscale schemes to improve the realism and predictability efficiency of simulation tools. To that end, the results of atomistic (ab initio) simulations will serve as a basis for elaborating effective models used for STM and devices simulation. A more accurate evaluation of the potential of graphene for various applications including logic circuits, as well as spintronics devices and chemical nano-sensors will be undertaken. Calculations will also be compared to experiments performed at the Néel Institute and CEA/LETI laboratories (Grenoble), LPA (ENS-Paris) and LPS-Orsay laboratory, which are in close contact with NANOSIM_GRAPHENE partners. Ultimately, several models will be integrated into commercial software for STM/AFM simulation developed by the NANOTIMES Company. This will allow NANOTIMES to include state-of-the-art developments into user-friendly software interfaces, more easily accessible to a larger community, beyond the specialists of the field.

Background

Since its emergence in 2003-2004 graphene is a rapidly evolving and growing field of research [Fuchs 2008, Daninos 2009, Geim 2008, Neto 2009]. There are many reasons for this enormous interest; for instance the high structural and chemical quality of the samples that preserve quantum coherence properties on large length scales. The electronic structure is also unique, being characterized by a linear dispersion relation, and the so-called chirality of the wavefunction. The 2D character of graphene, and its location at the surface make it easily accessible to modern lithographic techniques which is also of great interest. Notwithstanding, many key issues still need to be addressed in order to fully exploit graphene for technological applications, especially for making efficient graphene-based devices. Graphene can be used either in its two-dimensional (2D) form or as graphene nanoribbons (GNRs) with width in the range 2nm-50nm [Neto 2009].

Due to its zero energy band-gap, 2D graphene exhibits poor field effect efficiency but the design of GNRs using state of the art lithographic techniques or more advanced chemical synthesis strategies allows for bang gap engineering [Kim 2007]. As a consequence, GNRs can provide the natural semiconducting channel of a field effect device, and large ION/IOFF performances of GNR-based field effect transistors (GNR-FETs) have already been reported [Dai 2008]. However, in order to understand the true behavior and limitations of GNR-FETs, one seriously needs to tackle the nature and the effects of the edges, which can display unsaturated bonds and high edge chemical reactivity.

The first simulations of clean GNR-FETs based on diffusion-drift approaches suggest that GNR-FET performance (voltage and subthreshold-slope characteristics) are superior to ultrathin body-SOI MOSFETs due to their extremely thin channel layer [Sano 2009]. The simulations suggest that GNR-FETs could be at least several times faster than SOI MOSFETs, thus appearing very promising for beyond-CMOS technology. However those simulations fail to describe rigorously the quantum transport phenomena, and cannot take into account the high degree of chemical complexity that emerges at the nanoscale in the real materials.

Indeed, the usual approximation of GNR boundary conditions assumes hydrogenation of edges, and a well documented literature already provides effective parameters to deal with the electronic properties of pure GNRs. However, the situation becomes less clear when one tries to account for the complexity of edge chemistry. As shown in recent STM and TEM experiments [Enoki07, Liu 2008], edges are far from being geometrically perfect, and those imperfections are likely to drive enhanced transport gaps, and further degrade charge mobilities and device performances. The real potential of graphene-based materials for FET and related applications thus requires advanced modelling methods, including ab initio techniques.

Another way to tailor the properties of graphene in order to be able to use it in carbon-based nanoelectronics is via intentional and controlled chemical modifications. Stable molecular compounds can be used as dopants and functionalizing agents on graphene. As dopants, these compounds can be used to modify the potential profile in the channel region of graphene devices, reduce parasitic resistances in these devices, and produce large charge mobility gaps. Recently the CEA group has reported a strong electron-hole transport asymmetry induced by chemical doping [Biel 2009-a, Biel 2009-b]. The substitution of carbon atoms either by boron or nitrogen atoms was shown to result in anomalous doping effects, with the occurrence of mobility gaps in the order of 1 eV for graphene ribbons with widths several tens of nanometers large. These theoretical results have opened unprecedented possibility to engineer efficient graphene-field effect transistors, with widths within the current lithographic reach. As functionalizing agents, molecular compounds can also serve as nucleation sites for the uniform growth of thin high-k gate dielectrics, allowing for enhanced capacitive coupling with the graphene channel. The impact of those chemical modifications on graphene need thus to be explored, and related device characteristics and ultimate performances clarified.

In the quest to produce graphene-based devices, for example to understand and test how one can open a gap, it is crucial to study fundamental physical properties of graphene such as the local density of states (LDOS). Given graphene's two-dimensional structure, the LDOS can be obtained via STM experiments. STM experiments that have access to the atomically resolved LDOS in 2D systems have been developed only recently. Measurements of the LDOS were successfully obtained in high-temperature superconductors [McElroy 2003, Pasupathy 2008]. For graphene, the LDOS measurements have at this point been used to extract information about the atomic structure, the number of layers, the linearity of the spectrum, the effect of phonons, and the presence of a gap. Very recent experiments proposed in [Bena 2008] also showed that STM experiments focusing on the LDOS in the presence of impurity scattering gives access to other fundamental properties of graphene such as the chirality of its quasiparticules [Brihuega 2009]. Also in Ref. [Simon 2009] it was shown that STM measurements of the LDOS close to a localized impurity can provide information on the exact location of the impurity, as well as on the validity of the Fermi liquid theory. The field of STM in graphene is thus very new but developing very fast, in particular on the experimental side. Many experimental groups are developing this type of experiment, for example the groups of L. Simon (Mulhouse), E. Andrei (Rutgers), P. Mallet, J. Y. Veuillen (Grenoble), K. Kern (Stuttgart), J. Stroscio (NIST), P. First (Georgia Tech). On the theoretical side, a few studies have investigated the effects of disorder on the LDOS. However, a concerted theoretical effort of analyzing the STM experiments in graphene is to date crucially missing.

Additionally, besides the potential of 2D graphene and GNRs for electronic device applications, transport mechanisms involving the spin of the carriers have very recently received particular attention. First, although spin injection through ferromagnetic metal/semiconductor interfaces remains a great challenge, the spectacular advances made in 2007 converting the spin information into large electrical signals using carbon nanotubes [Hueso 2007] has opened a promising avenue for future carbon-based spintronics applications. The further demonstration of spin injection in graphene [Ohishi 2007] and the observation of long spin relaxation times and length [Tombros 2007] have suggested that graphene could add some novelty to carbon-based spintronics. For instance, taking advantage of the long electronic mean free path and negligible spin-orbit coupling, the concept of a spin field-effect transistor with a 2D graphene channel has been proposed with an expectation of near-ballistic spin transport operation [Semenov 2007]. A gate-tunable spin valve has been has been experimentally demonstrated [Cho 2007]. Finally, a ferromagnetic insulator, such as EuO, may be used to induce ferromagnetic properties in graphene by the proximity effect [Haugen 2008] and also to control the spin polarization of current by a gate voltage [Yokoyama 2008, Do 2008]. This configuration does not make use of any ferromagnetic metallic contact to inject spin-polarized electrons. Thus, it could be a way to circumvent the problem of “conductivity mismatch” [Schmidt 2000, Rashba 2000, Fert 2001] which possibly limits the current spin injection efficiency into a conventional semiconductor from a ferromagnetic metal. These phenomena and the corresponding devices need to be investigated using the appropriate models of relativistic-like electron transport in 2D graphene structures. Additionally, the presence of spin states at the edges of zigzag GNRS has also been demonstrated [Lee 2005, Pisani 2007, Topsakal 2008], and may also be exploited for spintronics. Using first-principles calculations, very large values of magnetoresistance have been predicted in GNR-based spin valves [Kim 2008]. This may help to overcome the performance of conventional materials and of the conventional technology for spin valve devices.

Finally, the management of heat dissipation is a crucial issue for designing modern nanodevices. Extremely high thermal conductivity has been demonstrated for suspended graphene [Ghosh 2008], which makes this material of great interest for controlling the temperature increase in ultra-large scale integrated CMOS circuits. A giant Seebeck coefficient has been predicted for graphene [Dragoman 2007] whose thermoelectric properties are thus utilizable for innovative applications in the field of energy harvesting.

References

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