Session: 03-05-01: Nanomaterials

Paper Number: 108595

108595 - Electrical Conduction Mechanism in Van Der Waals Flake Thin Film 

Graphene flake thin films, as assemblies of nanoscale semicrystalline flakes, feature wide sample by sample variation in electrical property, which is comprehensible considering the variance in distribution of size and disorder in each flake as well as the random nature of the flake assembly.  Over a wide range of temperature from less than 10K up to even room temperature, the electricity in flake thin films conducts dominantly via a variable range hopping (VRH) mechanism. Both Mott VRH and Efros-Shiklovskii (ES) VRH have been reported depending on samples. Secondly, most studies reported a transition in the conduction mechanism from the mentioned VRH to a thermally assisted Arrhenius type mechanism as temperature increases. The transition temperature, although varying widely on samples, lies between 150K and the room temperature.  These two qualitative features are shared with other 2D van der Waals flake thin films such as molybdenum disulfide [1,2] and even with three-dimensional bulk amorphous semiconductors.

Although being phenomenological and macroscopic, those common features mentioned above allow indirect assessments of the microscopic information, especially of the electronic density of states (DOS) of the sample which is hidden under the hierarchical disorder in structure, such as in flake thin films.  Typically, a lightly doped crystalline semiconductor features a transition from ES VRH to Mott VRH around 10K due to Coulomb gap in DOS disappearing with increasing thermal energy. However, such transition between different types of VRH’s has not been reported in graphene flake thin films. Instead, a graphene flake thin film features either ES VRH or Mott VRH throughout the entire VRH regime without the transition between them.  This lack of transition implies that the defect DOS in flake thin films is dominated by the intrinsic defect energy which is temperature independent, not by the Coulomb interaction energy of charge distribution which depends on temperature. We can also deduce that those samples featuring ES VRH is predominantly populated by defect sites whose DOS has a temperature independent intrinsic gap near Fermi level.  Nevertheless, these two VRH models could not explain the trend of the electrical conductivity changing with increasing temperature. Our analysis of multi-channel conduction (VRH added with Thermal Activated (TA) Arrhenius conduction) is however able to predict the trend of the changing conductivity with temperature, as explained below.

The thermally activated (TA) Arrhenius type conduction is attributed to either the nearest neighbor hopping (NNH) in some literatures or the band conduction (BC) in others.  Although both NNH and BC are thermally activated Arrhenius processes, they are fundamentally distinguishable because NNH involves the defect band exclusively, while BC regards inter-band thermal activation of carriers from the defect band to nearby delocalized conduction band.  Incorporation of the multi-channel conduction (VRH+TA) mechanism in the reduced activation energy, W, defined as , the anomalous behavior the reduced activation energy (W) increasing with the temperature ( > 0) around the transition temperature was captured.  It is to note that the VRH models are based on the assumption of W decreasing with temperature ( < 0), thus fails to explain W increase in the transition temperature domain, observed in graphene thin film conductivity measurement. The multi-channel (VRH+TA) model captures the changing conductivity with temperature, whereas the VRH model fails to correlate with the experimental data below room temperature.

References

[1]      Hao Qiu, Tao Xu, Zilu Wang, Wei Ren, Haiyan Nan, Zhenhua Ni, Qian Chen, Shijun Yuan, Feng Miao, Fengqi Song, Gen Long, Yi Shi, Litao Sun, Jinlan Wang, and Xinran Wang, “Hopping transport through defect-induced localized states in molybdenum disulphide,” Nature Communications 4, 2642 (2013).

[2]      [1]      Jianhong Xue, Shaoyun Huang, Ji-Yin Wang, and H. Q. Xu, “Mott variable-range hopping transport in a mos2 nanoflake,” RSC Adv. 9, 17885–17890 (2019).

Presenting Author: Ajit Roy Air Force Research Laboratory

Presenting Author Biography: Dr. Roy's research and leadership experience is in materials innovations and development in structural, thermal, and electronic materials. Extensive expertise in integrating multiscale computational methods to materials processing for accelerated materials development and technology transition. Pioneered micro- and nano-porous carbon materials. Transitioned micro-porous carbon foam materials technology from inception to establishing manufacturing base of national importance in 2x reduced development cycle and inserted technology to space radiators, aircraft heat exchangers, and composite manufacturing tools. Nano-porous carbon invention invigorated research worldwide and attracted venture capital investment in its processing technology scale-up. Integrated multi-scale computational methods in innovative and pragmatic way to develop durable thermal interface materials - resulted in commercial heat spreader products for electronics thermal management within 5 years, a 3x reduction in development cycle. Widely recognized scientific technical expertise with 300 publications, Fellow of AFRL and three professional societies (AIAA, ASME, ASC), journal editorial boards, services in national and international review panels, awards and executive committees in professional societies, advisory committees in service agencies, and mentoring S&Es. Received his PhD from University of Minnesota.

Authors:

Ajit Roy Air Force Research Laboratory
Jonghoon Lee ARCTOS
Dhriti Nepal Air Force Research Laboratory
John Ferguson Air Force Research Laboratory