We report a novel synthesis to ultra high purity 7,14-bis­((trimethylsilyl)­ethynyl)­dibenzo­[b,def]-chrysene (TMS-DBC) and the use of this material in the growth of single crystals by solution and vapor deposition techniques. We observe that the substrate temperature has a dramatic impact on the crystal growth, producing two distinct polymorphs of TMS-DBC; low temperature (LT) fine red needles and high temperature (HT) large yellow platelets. Single crystal X-ray crystallography confirms packing structures where the LT crystals form a 1D slipped-stack structure, while the HT crystals adopt a 2D brickwork motif. These polymorphs also represent a rare example where both are extremely stable and do not interconvert to the other crystal structure upon solvent or thermal annealing. Single crystal organic field-effect transistors of the LT and HT crystals show that the HT 2D brickwork motif produces hole mobilities as high as 2.1 cm2 V–1 s–1, while the mobility of the 1D structure is significantly lower, at 0.028 cm2 V–1 s–1. Electronic-structure calculations indicate that the superior charge transport in the brickwork polymorph in comparison to the slipped-stack polymorph is due to the presence of an increased dimensionality of the charge migration pathways.

We report measurements of the bias dependence of the Fano factor in ensembles of atomic-scale Au junctions at 77 K. Previous measurements of shot noise at room temperature and low biases have found good agreement of the Fano factor with the expectations of the Landauer-Büttiker formalism, while enhanced Fano factors have been observed at biases of hundreds of mV (Chen et al 2014 Sci. Rep. 4 4221). We find even stronger enhancement of shot noise at 77 K with an 'excess' Fano factor up to ten times the low bias value. We discuss the observed ensemble Fano factor bias dependence in terms of candidate models. The results are most consistent with either a bias-dependent channel mixing picture or a model incorporating noise enhancement due to current-driven, nonequilibrium phonon populations, though a complete theoretical treatment of the latter in the ensemble average limit is needed.

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Two-dimensional topological insulators are of great interest, with predicted topological protection of one-dimensional helical edge states at their boundaries. Shot noise, the fluctuations in driven current due to the discreteness of charge carriers, has been proposed as a way of distinguishing between trivial and nontrivial edge state conduction, as well as a means of assessing back-scattering mechanisms in the latter. Such measurements require an understanding of possible contributions to the noise from contacts and conduction in the 2D bulk. We present noise measurements in Corbino structures based on InAs/Ga(In)Sb quantum well interfaces over a broad temperature and applied current range. As the temperature is lowered and the bulk transport is gapped out, shot noise becomes detectable in these two-terminal devices, in both high- and low-frequency measurement techniques. Quantitative comparison with a noise model shows that the total applied voltage drop is split among the contacts and the bulk and that the devices have some intrinsic asymmetry. Within that model, the magnitude of the shot noise appears to be anomalously large, implying the contacts to the 2D bulk are nontrivial in this system.

We report a novel synthesis to ultra high purity 7,14-bis((trimethylsilyl)ethynyl)dibenzo[b,def]-chrysene (TMS-DBC). Single crystals of this material were grown by solution and vapor deposition techniques and structural determination by single crystal X-ray crystallography was then carried out at the Microfocus Crystallography beamline at the Australian Synchrotron. Two polymorphs of TMS-DBC were observed; low temperature (LT) fine red needles and high temperature (HT) large yellow platelets. These polymorphs represent a rare example where both are extremely stable and do not interconvert to the other crystal structure upon solvent or thermal annealing. Single crystal X-ray crystallographic studies identified two distinct packing structures where the LT crystals form a 1D slipped-stack structure, while the HT crystals adopt a 2D brickwork motif. Single crystal organic field-effect transistors of the LT and HT crystals showed the impact of packing motif on charge transport.[1] Stevens, L. A. et al. (2015). Chem. Mater., 27,[112][113][114][115][116][117][118]