Abstract
Lattice-matched graphene on hexagonal boron nitride is expected to lead to the formation of a band gap but requires the formation of highly strained material and has not hitherto been realized. We demonstrate that aligned, lattice-matched graphene can be grown by molecular beam epitaxy using substrate temperatures in the range 1600-1710 °C and coexists with a topologically modified moiré pattern with regions of strained graphene which have giant moiré periods up to ∼80 nm. Raman spectra reveal narrow red-shifted peaks due to isotropic strain, while the giant moiré patterns result in complex splitting of Raman peaks due to strain variations across the moiré unit cell. The lattice-matched graphene has a lower conductance than both the Frenkel-Kontorova-type domain walls and also the topological defects where they terminate. We relate these results to theoretical models of band gap formation in graphene/boron nitride heterostructures.
Highlights
The symmetry between the A and B graphene sublattices is broken locally and, depending on whether there is a cancellation of this effect across the unit cell, can give rise to an energy gap at the Dirac point.[4,12−18] A more robust route to forming an energy gap has been predicted for G/hexagonal boron nitride1−3 (hBN) heterostructures in which the layers are latticematched;[14] in that case, the A/B sublattice symmetry is broken globally rather than locally
°C, higher than the growth temperatures typically used for epitaxial growth of graphene on hBN,[20−28] we have previously reported moiré periods up to ∼30 nm, corresponding to a strain, ΔaG/aG ∼ 0.9%
Note that here ΔaG is the deviation of the lattice constant averaged over the moiré period; Woods et al.[6] have proposed that a commensurate−incommensurate transition occurs in aligned G/hBN heterostructures leading to a small variation of the graphene lattice constant across the unit cell and this variation has recently been observed directly.[29]
Summary
Letter °C, higher than the growth temperatures typically used for epitaxial growth of graphene on hBN,[20−28] we have previously reported moiré periods up to ∼30 nm, corresponding to a strain, ΔaG/aG ∼ 0.9%. This larger scale morphology is typical of the regions where we find the lattice-matched material, and as we have suggested previously[19] it is possible that these aggregates provide pinning sites which maintain the strain in the epitaxial graphene when cooling down from the high growth temperatures employed in this growth process.
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