Ab initio transport fingerprints for resonant scattering in graphene

We have recently shown that by using a scaling approach for randomly distributed topological defects in graphene, reliable estimates for transmission properties of macroscopic samples can be calculated based even on single-defect calculations [A. Uppstu et al., Phys. Rev. B 85, 041401 (2012)]. We now extend this approach of energy-dependent scattering cross sections to the case of adsorbates on graphene by studying hydrogen and carbon adatoms as well as epoxide and hydroxyl groups. We show that a qualitative understanding of resonant scattering can be gained through density functional theory results for a single-defect system, providing a transmission “fingerprint” characterizing each adsorbate type. This information can be used to reliably predict the elastic mean free path for moderate defect densities directly using ab initio methods. We present tight-binding parameters for carbon and epoxide adsorbates, obtained to match the density-functional theory based scattering cross sections.Peer reviewe

Generalized tight-binding transport model for graphene nanoribbon-based systems

An extended tight-binding model that includes up to third-nearest-neighbor hopping and a Hubbard mean-field interaction term is tested against ab initio local spin-density approximation results of band structures for armchair- and zigzag-edged graphene nanoribbons. A single tight-binding parameter set is found to accurately reproduce the ab initio results for both the armchair and zigzag cases. Transport calculations based on the extended tight-binding model faithfully reproduce the results of ab initio transport calculations of graphene nanoribbon-based systems.Peer reviewe

Sensing sulfur-containing gases using titanium and tin decorated zigzag graphene nanoribbons from first-principles

Atom implantation in graphene or graphene nanoribbons offers a rich opportunity to tune the material structure and functional properties. In this study, zigzag graphene nanoribbons with Ti or Sn adatoms stabilised on a double carbon vacancy site are theoretically studied to investigate their sensitivity to sulfur-containing gases (H2S and SO2). Due to the abundance of oxygen in the atmosphere, we also consider the sensitivity of the structures in the presence of oxygen. Density functional theory calculations are performed to determine the adsorption geometry and energetics, and nonequilibrium Green's function method is employed to compute the current–voltage characteristics of the considered systems. Our results demonstrate the sensitivity of both Ti- and Sn-doped systems to H2S, and the mild sensitivity of Ti-doped sensor systems to SO2. The Ti-doped sensor structure exhibits sensitivity to H2S with or without oxidation, while oxidation of the Sn-doped sensor structure reduces its ability to adsorb H2S and SO2 molecules. Interestingly, oxygen dissociates on the Ti-doped sensor structure, but it does not affect the sensor's response to the H2S gas species. Oxidation prevents the dissociation of the H–S bond when H2S adsorbs on the Ti-doped structure, thus enhancing its reusability for this gas species. Our study suggests the potential of Ti- and Sn-doped graphene in selective gas sensing, irrespective of the sensing performance of the bulk oxides

Charge Transport in Polycrystalline Graphene: Challenges and Opportunities

Graphene has attracted significant interest both for exploring fundamental science and for a wide range of technological applications. Chemical vapor deposition (CVD) is currently the only working approach to grow graphene at wafer scale, which is required for industrial applications. Unfortunately, CVD graphene is intrinsically polycrystalline, with pristine graphene grains stitched together by disordered grain boundaries, which can be either a blessing or a curse. On the one hand, grain boundaries are expected to degrade the electrical and mechanical properties of polycrystalline graphene, rendering the material undesirable for many applications. On the other hand, they exhibit an increased chemical reactivity, suggesting their potential application to sensing or as templates for synthesis of one-dimensional materials. Therefore, it is important to gain a deeper understanding of the structure and properties of graphene grain boundaries. Here, we review experimental progress on identification and electrical and chemical characterization of graphene grain boundaries. We use numerical simulations and transport measurements to demonstrate that electrical properties and chemical modification of graphene grain boundaries are strongly correlated. This not only provides guidelines for the improvement of graphene devices, but also opens a new research area of engineering graphene grain boundaries for highly sensitive electrobiochemical devices

Elektronien kuljetusteoria grafeeninanorakenteissa

Since its first synthesis and characterization in 2004 graphene has been the focus of an intense research effort. Charge carriers in graphene are massless Dirac fermions that behave fundamentally differently than electrons in conventional semiconductor heterostructures. For applications the most interesting factor is the very high quality of the graphene lattice leading to ballistic transport over micron length scales. The transport of electrons in graphene has thus been widely studied for applications in electronics. Although bulk graphene is gapless, a gap can be generated by breaking the graphene into finite strips, graphene nanoribbons. Our work concerns the study of electron transport in graphene and graphene nanoribbons using first principles density functional theory (DFT) and semiempirical tight-binding (TB) methods. The TB and DFT approaches are complementary in that DFT makes it possible to study small graphene structures with an accurate accounting for effects such as ionic relaxation, charge transfer and magnetism while TB can be used for fast calculations of large, disordered samples. By combining DFT and TB very accurate TB parameterizations can be generated. The parameterizations can also be generalized for magnetic systems by using the Hubbard model. We have used these methods to study the effect various structural defects such as vacancies, adatoms and disordered edges have on the transmission properties of graphene nanostructures. We have shown that even single defect calculations are enough for estimating the conducting properties of large samples with the aid of a scaling approach to transport. This makes it possible to characterize different defects in graphene based on a scattering cross section that can be calculated directly by DFT for small systems.Grafeenia on tutkittu intensiivisesti sen jälkeen, kun sitä valmistettiin ja karakterisoitiin ensi kertaa 2004. Normaaleista puolijohteista poiketen grafeenin varauksenkuljettajat ovat massattomia Dirac-fermioneja, jotka käyttäytyvät huomattavan eri tavalla normaalien puolijohderakenteiden elektroneihin verrattuna. Grafeeni on myös herättänyt toiveita elektroniikan sovellutuksista, sillä grafeenihilan hyvästä laadusta johtuen elektronit etenevät parhaimmillaan siroamatta mikrometrin matkoja. Vaikka grafeenin vyörakenne on normaalisti aukoton, grafeenin leikkaaminen nanonauhoiksi synnyttää aukon. Tämä väitöskirja käsittelee elektronien kuljetusteoriaa grafeenissa ja grafeeninanonauhoissa tiheysfunktionaaliteoriaan (DFT) ja semiempiiriseen tight-binding-malliin (TB) perustuen. DFT ja TB tukevat toisiaan siinä mielessä, että DFT mahdollistaa pienten rakenteiden tarkat laskut mukaanlukien ionisen relaksaation, varauksensiirron ja magneettiset ilmiöt, ja TB:llä puolestaan voidaan tutkia isoja epäjärjestäytyneitä rakenteita. Yhdistämällä DFT ja TB voidaan TB parametrisoida siten, että saavutetaan hyvin tarkkoja DFT:tä vastaavia tuloksia. TB voidaan myös yleistää magneettisille järjestelmille käyttämällä Hubbard-mallia. Olemme käyttäneet näitä menetelmiä tutkiaksemme erilaisten rakenteellisten defektien kuten vakanssien, adatomien ja reunaepäjärjestyksen vaikutusta elektronien transmissioon. Olemme osoittaneet, että jopa yhdelle defektille tehdyistä laskuista voidaan arvioida suurten systeemien johtavuusominaiuuksia käyttämällä skaalautumiseen perustuvaa lähestymistapaa. Tämä mahdollistaa eri defektien karakterisoinnin niiden sirontavaikutusalojen perusteella. Sirontavaikutusalan puolestaan voi laskea DFT:llä suoraan yhdelle defektille