Sample size effects on the transport characteristics of mesoscopic graphite samples

Abstract

In this work we investigated correlations between the internal microstructure and sample size (lateral as well as thickness) of mesoscopic, tens of nanometer thick graphite (multigraphene) samples and the temperature (T ) and field (B) dependence of their electrical resistivity ρ(T, B). Low energy transmission electron microscopy reveals that the original highly oriented pyrolytic graphite material -from which the multigraphene samples were obtained by exfoliation -is composed of a stack of ∼ 50 nm thick and micrometer long crystalline regions separated by interfaces running parallel to the graphene planes. We found a qualitative and quantitative change in the behavior of ρ(T, B) upon thickness of the multigraphene samples, indicating that their internal microstructure is important.The overall results indicate that the metallic-like behavior of ρ(T ) at zero field measured for bulk graphite samples is not intrinsic of ideal graphite. The results suggest that the interfaces between crystalline regions may be responsible for the superconducting-like properties observed in graphite. Our transport measurements also show that reducing the sample lateral size as well as the length between voltage electrodes decreases the magnetoresistance, in agreement with recently published results. The magnetoresistance of the multigraphene samples shows a scaling of the form ((R(B) − R(0))/R(0))/T α = f (B/T ) with a sample dependent exponent α ∼ 1, which applies in the whole temperature 2 K ≤ T ≤ 270 K and magnetic field range B ≤ 8 T.

Length dependence of the resistance in graphite: Influence of ballistic transport

“…The multigraphene sample was prepared by a rubbing and a ultrasound method described in Ref. 12 and from a HOPG sample with 0.35 • rocking curve width. Micro-Raman measurements indicate that even samples of 10 nm thickness are of good quality without showing any contribution of the defect-related D-peak at 1350 cm −1 [13].…”

The influence of Ga+irradiation on the transport properties of mesoscopic conducting thin films

“…At distances smaller than ∼ 3 nm we do not expect a large increase in the carrier density with further irradiation [11]. [12]. Note that the MR shows a quasi linear field behavior at low temperatures.…”

“…The semiconducting behavior is mainly due to the increase in carrier concentration with temperature because most of the carriers are thermally activated and the Fermi energy increases linearly with temperature [11]. The metallic behavior below 50 K is not intrinsic of graphite but comes from internal interfaces between crystalline regions parallel to the graphene layers but of slightly different orientation [12]. The mentioned interfaces originate during the pyrolysis process [18].…”

Evidence for room temperature superconductivity at graphite interfaces

“…This field driven MIT in graphite was speculated in the past to be related to superconductivity [45,46]. Nowadays we know that the MIT in graphite as well as the metalliclike behavior of its resistance are not intrinsic but are due to the contribution of certain interfaces in parallel to the graphene layers of the graphite structure [18,20,21]. If one measures the resistance with care, see [19] for more details, one can identify an anomaly at T ∼ 350 K for that sample.…”

“…Several electrical resistance measurements of graphite samples of different thickness published in the last years suggest that graphite samples are not homogeneous and that the temperature as well as the absolute value of the electrical resistivity and Hall effect are not unique but thickness- [18,[20][21][22] and sample-length dependent [23,24]. Whereas the last dependence is due to the increasing contribution of ballistic transport to the total resistance the smaller the sample length, the thickness dependence observed in the transport properties is mainly due to the existence of interfaces between the two stacking orders, hexagonal or Bernal (2H) and rhombohedral (3R), see Fig.…”