Modeling and Engineering the Interfacial Properties of Two-Dimensional Materials
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Author
Date
2019Type
- Doctoral Thesis
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Abstract
Two-dimensional (2D) materials, the crystalline films with one- to few-atom thickness, have attracted the spotlight of material research in recent years. Due to the quantum-confined electronic structures, the 2D materials serve as ideal platforms for studying low-dimensional physics and chemistry. Despite extensive research focusing on the intrinsic characteristics of 2D materials, little has been studied regarding their interfacial properties, which are essential for the integra- tion into modern technology platforms. The aim of this thesis to provide fundamental insights into the atomically thin interfaces by developing theoretical frameworks that bridge multiscale phenomena and providing guidelines for experimental demonstrations. The thesis is organized in four main parts as follows:
The first part gives a brief introduction of the 2D materials and their interfaces. The focus lies on the various types of interactions involved at the interfaces, as well as their connection with the fundamental electronic structures of 2D materials. In particular, several open questions in the context of 2D material interfaces are also reviewed.
The second part focuses on the properties of 2D materials interfaces under static electric field, which is governed by the quantum capacitance (CQ) of the 2D materials. Two studies are presented in this part to demonstrate the practical impact of quantum capacitance: (i) Using a self-consistent Poisson-Boltzmann model, the penetration of field effect through 2D material is studied. The field-effect transparency ηFE of a 2D material is quantified. The non-linear dependency of ηFE on CQ and other parameters are studied in order to guide practical design of 2D material-based vertical field effect devices. (ii) A multilayer quantum capacitor model is proposed to study the penetration of field effect in 2D heterostructures. Using relatively simple parameters including density of states (DOS), band alignment and interlayer dielectric constant, the model captures the experimentally observed asymmetric electrostatic screening with similar accuracy compared with full-scale ab initio simulations.
The third part focuses on another fundamental aspect of 2D materials interfaces: the dielectric properties. Two examples are demonstrated in this part: (i) Using first principles calculations, we show that instead of the macroscopic dielectric constant, the 2D electronic polarizability α2D, is the true descriptor of the dielectric properties for a 2D material. Using high-throughput material database screening, two universal scaling relations for α2D are proposed, linking the polarizability of a 2D material with its electronic and structural information. The idea of α2D is further to be valid for heterostructures and even bulk systems, allowing quantifying the dielectric anisotropy for any dimension. (ii) Using a modified Lifshitz model and knowledge of frequency-dependent dielectric properties, the van der Waals (vdW) interactions at the 2D material interfaces are studied. The dielectric anisotropy of a 2D material selectively screens the vdW interactions at low frequency regime. More interestingly, by proper engineering dielectric properties of 2D and bulk materials, repulsive vdW interactions are predicted by the model, and validated by experimental investigating using molecular epitaxy.
Based on these fundamental understandings, several studies on multiscale phenomena at the 2D material interfaces are shown in the last part. (i) By combining multiscale phenomena including quantum capacitance, interfacial molecular reorientation, electrical double layer (EDL), the wetting phenomena of a 2D material upon doping are studied. The molecular reorientation effect is found to dominate the 2D-liquid interfacial tension. (ii) Using self-consistent transport theory based on Poisson-Nernst-Planck equation and Quantum capacitance, the ionic transport through nanopores in a gated graphene sheet is studied. Gating is found to enable close-to-unity rejection of ionic species, which is in good agreement with experimental observations. (iii) Taking advantage of multiscale phenomena on 2D material interfaces, a novel electronic device named as interfacial field effect transistor (IFET), is proposed and fabricated. The IFET has ultra-sensitive pressure response down below 10 Pa, due to extremely low elastic modulus of liquid metal droplet. Mechanical response is harnessed by deformation on superhydrophobic nanowires assembled on graphene interface.
The studies presented in thesis aim to provide insights into the atomically thin interfaces, as well as providing guidelines and design rules for novel electronic devices and applications. Show more
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https://doi.org/10.3929/ethz-b-000401340Publication status
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ETH ZurichOrganisational unit
02020 - Dep. Chemie und Angewandte Biowiss. / Dep. of Chemistry and Applied Biosc.
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