Gunst, Tue

Grafen, et to-dimensionelt lag af carbon, er grundlaget for en række fremspirende nanomaterialer med lovende elektroniske og varmeledende egenskaber. Denne afhandling omhandler atomistiske simuleringer af opvarmning, termiske og elektroniske egenskaber for nanoskala-komponenter baseret på nanostruktureret grafen. Efterhånden som elektroniske komponenter nedskaleres til nanometer-dimensioner, vil virkemåden afhænge af den detaljerede atomare opbygning.De termiske egenskaber og effekter relateret til vekselvirkningen mellem elektronstrøm og vibrationer er et relativt nyt forskningsområde for nanoskala-komponenter baseret på nanostruktureret grafen. Vi viser, hvordan en systematisk nanogennemhulning af grafen - et grafen antidot gitter (GAL) - kan være med til at gøre grafen til et alsidigt materiale for elektronik og til kontrol af termisk energi. Vi finder, at en opskalering af hullerne kan reducere den termiske ledningsevne relativt mere end den elektroniske ledningsevne, hvilket er et vigtigt for termoelektriske anvendelser af grafen.Vi foreslår også to nye systemer til at opnå en-dimensionel transport i grafen. For det første præsenteres GAL bølgeledere, hvor en region af uberørt grafen indeklemmes imellem GAL regioner. Herved udnyttes det elektroniske båndgab til at opnå lokaliserede bølgeledende tilstande. For det andet kan man anvende den øgede kemiske reaktionsevne af bøjet grafen til elektronisk at isolere pseudo-ribbons vha. hydrogen absorption langs med bukket. Disse systemer kan udgøre en fordelagtig metode til at realisere elektroniske ledninger i integrerede grafen kredsløb.Når en stærk strøm passerer igennem nanostrukturer, vil de opvarmes pga. energitilførsel til lokaliserede vibrationer (fononer). Vi finder, at det elektroniske spektrum for nanostruktureret grafen indeholder elektroniske resonanser med en forbredning i størrelsesordenen af fonon-frekvenserne. Disse resonanser kræver en beskrivelse udover, hvad der er muligt med etablerede simuleringsmetoder. Der er en kraftig tendens til at ramme dette parameterområde i grafen-teknologi pga. de høje fonon-frekvenser, som nærmer sig 0.2eV. Afhandlingen præsenterer atomistiske simuleringsmetoder, som inkorporerer effekten fra den elektroniske energivariation på fonon energiskalaen i modellering af den elektroniske ledningsevne og den lokale opvarmning. Vi finder en stærkt ulineær opvarmning af fononer i en grafen nano-indsnævring (GNC) med spænding pga. negativt dæmpede fononer drevet af strømmen. Denne effekt kan begrænse stabiliteten og kapaciteten for GNC'er til at trække en stærk strøm. Endvidere undersøges strøminduceret termisk udvidelse af en GNC med semiklassisk molekylær dynamik. Carbon nanosystemer som forbinder elektrisk gatede grafen-elektroder kan blive en interessant testbænk for disse effekter.This thesis is concerned with the heating and electronic properties of nanoscale devices based on nanostructured graphene. As electronic devices scale down to nanometer dimensions, the operation depends on the detailed atomic structure. Emerging carbon nano-materials such as graphene, carbon nanotubes and graphene nanoribbons, exhibit promising electronic and heat transport properties. Much research addresses the electron mobility of pristine graphene devices. However, the thermal transport properties, as well as the effects of e-ph interaction, in nanoscale devices, based on nanostructured graphene, have received much less attention. This thesis contributes to the understanding of the thermal properties of nanostructured graphene. The computational analysis is based on DFT/TB-NEGF. We show how a regular nanoperforation of a graphene layer - a graphene antidot lattice (GAL) - may be a solution for making graphene a versatile material for electronics, as well as thermal management. One of the main results is that both electronic and thermal transport properties converge fast with the number of antidot rows between the leads. Increasing the antidot dimensions is found to reduce the thermal conductance relatively more than the electronic conductance. This trend may, together with other thermal scattering mechanisms such as disorder or isotopes, lead to fair thermoelectric performance of graphene with an otherwise high ability to conduct heat. We also propose two novel materials that can lead to pseudo 1D transport in graphene. These systems may be a favorable way of realizing electronic wires in integrated graphene circuits. Firstly, GAL waveguides, where a region of pristine graphene is sandwiched between GAL regions, utilize the electronic band gaps to obtain localized waveguide modes. Secondly, the increased chemical reactivity of graphene with a bend may be used to electronically isolate pseudo-ribbons by absorption of hydrogen along the bends. When a large current is passed through such nanostructures they will heat up due to the excitation of the local vibrations. This thesis further addresses effects of having a large finite bias voltage. In particular, we find that the electronic spectrum of nanostructured graphene features sharp variations in energy. Electronic resonances with a broadening on the scale of the phonon frequencies need a special treatment beyond the common praxis in theoretical modelling. There is a strong tendency to enter this regime in graphene technology due to the large phonon frequencies approaching 0:2 eV. The aim is to develop atomistic simulation methods that incorporate the effect from the electronic energy variation on the phonon energy scale in electronic conductance and local heating modelling. The methods we have developed are applied to graphene devices but are of much more general validity. We find a strongly nonlinear heating of phonons in a graphene nanoconstriction (GNC) with bias when the Fermi level is tuned close to a resonance in the electronic structure. The behavior is traced back to the presence of negatively damped phonons driven by the current. This effect may limit the stability and capacity of GNCs to carry high currents. Carbon nanosystems bridging electrically gated graphene electrodes may offer an interesting test-bed for these effects

Online Research Database In Technology

Transport and dynamics of nanostructured graphene

http://orbit.dtu.dk/ws/files/98206459/TGphdProject.pdf

Transport and dynamics of nanostructured graphene

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