“Transport phenomena in nano systems; transport fluctuations in 2-dimensional systems and high current carrying capacity in metal-carbon nanotube composite fibers”
Dr. DongSu Lee
Applied Quantum Composites Research Center, KIST

Dec. 1 (Fri.), 04:00 PM
E6-2. 1st fl. #1323

In the first part of my talk, I will discuss transport fluctuations observed in 2-dimensional systems [1]. Transport measurements normally provide a macroscopic, averaged view of the sample, so that disorder frequently avoids developing metrologically precise quantum Hall resistance as well as prevents the observation of fragile interaction induced states in the conductance. Producing high quality sample is therefore highly desired, but it is a very tricky task. Here, we note that graphene magnetotransport traces are frequently cluttered with fluctuations and demonstrate that these fluctuations reflect processes that occur very locally. They are a manifestation of charge localization and are more pronounced in the transconductance. A systematic study allows observing higher order fractional quantum Hall states in the Hall or longitudinal resistance traces in graphene and in bilayer graphene [2]. They make phenomena on the nanometer scale visible in macroscopic transport experiment despite significant disorder. The phenomena can be explained by charging effect into the compressible quantum dots which form near incompressible quantum states. The fluctuations appear when conduction channels are influenced by the compressible quantum dots which form at hills or valleys of the disordered potential landscape in graphene.

In the second part of my talk, I will present a systematic transport study to reveal the mechanism of high current carrying capacity in carbon nanotube (CNT)-Cu composite fibers [3]. With the growth of nanoelectronics, the importance of thermal management in device packaging and improvement in high current-carrying interconnects/wires have been emphasized to avoid the premature failure of devices. Upon characterization of a material at the nanometer level, bulk heat and electrical transport properties may not hold because the phenomena at the interfaces and grain boundaries become dominant. The failure mechanism of bulk metal interconnects has been understood in the context of electromigration; however, in nanoscale materials, the effect of heat dissipation that occurs at nanointerfaces may play an important role. In this talk, a high current-carrying capacity of continuous CNT-Cu composite fiber that possesses Cu nanofibrillar structures will be discussed. Various shaped CNT-Cu microfibers with different Cu grain morphologies were produced via Cu electroplating on continuous CNT fibers. The Cu fibril structures were embedded in the void inside the CNT fiber during an earlier stage of electrodeposition. Temperature- and magnetic field-dependent electrical properties and the ampacity were measured for the produced CNT-Cu microfibers. The failure mechanism of the fibers will be discussed. The interconnection of Cu nanograins on the surface of the individual CNTs notably contributed to an enhancement in the fiber charge-carrying ability. The effective ampacity of Cu nanofibrils was estimated to be ~ 1 × 107 A/cm2, which is approximately 50 times larger than the ampacity measured from a bulk Cu microwire.

[1] D. S. Lee et al., “Transconductance fluctuations as a probe for interaction induced quantum Hall states in graphene”, Phys. Rev. Lett. 109, 056602 (2012).
[2] Y. Kim, D. S. Lee et al., “Fractional Quantum Hall States in Bilayer Graphene Probed by Transconductance Fluctuations”, Nano Lett. 15, 7445 (2015).
[3] H. K. Rho et al., “Metal nanofibrils embedded in a long free-standing carbon nanotube fibre with a high critical current density”, submitted.