[Paper Review] Design of efficient vdW thermionic heterostructures from first principles

Xiaoming Wang, Mona Zebarjadi+1 more|arXiv (Cornell University)|Oct 13, 2015

2D Materials and Applications19 citations

TL;DR

This study proposes van der Waals (vdW) heterostructures of graphene/phosphorene/graphene as efficient thermionic coolers using first-principles calculations. By tuning phosphorene layer count, the system transitions from quantum tunneling to thermionic transport, achieving a coefficient of performance of 18.5 at 600 K—equivalent to a ZT of 0.13—demonstrating significant potential for nanoscale cooling devices.

ABSTRACT

This work is the first step towards understanding thermionic transport properties of graphene/phosphorene/graphene van der Waals heterostructures in contact with gold electrodes by using density functional theory based first principles calculations combined with real space Green's function formalism. We show that for monolayer phosphorene in the heterostructure, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling dominated transport to thermionic dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing the cooling coefficient of performance. The thermionic coefficient of performance for the proposed device is 18.5 at 600 K corresponding to an equivalent ZT of 0.13, which is significant for nanoscale devices.

Motivation & Objective

• To investigate thermionic transport in graphene/phosphorene/graphene van der Waals heterostructures for nanoscale cooling applications.
• To determine the transition from quantum tunneling to thermionic transport by varying phosphorene layer count.
• To quantify the thermionic coefficient of performance and equivalent ZT for the proposed heterostructure.
• To evaluate the role of interfacial coupling and electronic structure in enhancing thermionic efficiency.
• To establish a design framework for efficient 2D heterostructure-based thermionic coolers using first-principles methods.

Proposed method

• Density functional theory (DFT) was used to compute electronic structures and band alignments of the heterostructure.
• Real space Green's function formalism was applied to calculate electron transport properties across the heterostructure.
• The system was modeled with monolayer to multilayer phosphorene to study layer-dependent transport mechanisms.
• Gold electrodes were simulated to model contact effects and thermionic emission at interfaces.
• Thermionic transport efficiency was evaluated via the coefficient of performance (COP) and equivalent ZT values.
• The transition from tunneling to thermionic dominance was analyzed by comparing transmission spectra and current-voltage characteristics.

Experimental results

Research questions

• RQ1 How does the transport mechanism in graphene/phosphorene/graphene heterostructures evolve with increasing phosphorene layer count?
• RQ2 What is the maximum thermionic coefficient of performance achievable in such vdW heterostructures at elevated temperatures?
• RQ3 To what extent can the ZT equivalent be enhanced through thermionic transport in 2D van der Waals heterostructures?
• RQ4 How do interfacial coupling and band alignment influence thermionic emission and electron transmission?
• RQ5 Can first-principles calculations predict viable designs for high-efficiency nanoscale thermionic coolers?

Key findings

• The transport mechanism shifts from quantum tunneling in monolayer phosphorene to thermionic emission as the number of phosphorene layers increases.
• At 600 K, the thermionic coefficient of performance reaches 18.5, indicating high cooling efficiency for a nanoscale device.
• The equivalent ZT value is calculated as 0.13, which is significant for 2D heterostructures and represents a promising benchmark.
• Multilayer phosphorene enhances heat transport per charge carrier, improving thermionic efficiency.
• The system demonstrates a clear transition from tunneling-dominated to thermionic-dominated transport with increasing layer count.
• First-principles simulations confirm the feasibility of designing high-performance thermionic coolers using van der Waals heterostructures.