What makes charge transport in 2D materials special?

Mario Hofmann^1*, Ya-Ping Hsieh², Kevin Chang³, Spencer Chuang¹

¹Department of Physics, National Taiwan University, Taipei, Taiwan
²Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei City, Taiwan
³Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan

* Presenter:Mario Hofmann, email:mario@phys.ntu.edu.tw

Two-dimensional materials have shown the potential to outperform traditional bulk materials in applications such as sensing, computing, and energy storage. However, the two-dimensional nature of materials such as graphene and MoS2 imparts them with unique properties that deeply affect their carrier transport and enable new and promising applications.
Since the charge carriers in 2D materials are confined to the surface, we observe a significant effect of charged adsorbates on their carrier transport. Employing in-situ characterization tools we identify distinct scattering mechanisms at low and high adsorbate coverage regimes. A low adsorbate density was found to result in surprisingly low charge transfer between the dopant and the 2D material, which was found to originate from the limiting geometrical capacitance of small adsorbate clusters. An increase of the cluster size was found to increase the charge transfer efficiency thirtyfold yielding enhanced performance of 2D materials as electrodes. Unfortunately, the achievable enhancement in performance upon increase of adosbate coverage is limited by a competing effect of charge carrier scattering. We observe a capacitive coupling between doped and undoped regions of graphene that suppress ambipolar transport and result in a persistent p-type transfer characteristic. This effect is dominating the carrier scattering and can result in a six-fold decrease in mobility. When dopant clusters reach dimensions where they start merging, charge transport proceeds through percolation in the pristine graphene phase. This transition results in an increase in sheet resistance and a surprising behavior of the Hall effect that had been previously misinterpreted as change in doping type and mobility enhancement.
The efficient charge transfer between 2D materials and adsorbates was demonstrated to enable several novel applications. Electrostatic control over graphene’s carrier concentration allowed change of the band alignment in graphene/insulator/semiconductor heterojunctions. Thus, vertical tunneling-injection light-emitting transistors (VtiLET) were produced where gating allows adjustment of the light emission process. This advance enabled arbitrary color light emission from one single emitter. Moreover, we show that the electrochemical properties of graphene are intimately linked to the position of its Fermi level. Thus, an electrochemical transistor was realized that permits the dynamic modification of the potential and reaction rate of an electrochemical reaction over a wide range.

Keywords: 2D materials, charge transport, doping, graphene, MoS2