Room-temperature magnetism and tunable energy gaps in edge-passivated zigzag graphene quantum dots

Wei Hu,Lin Lin,Xinmin Qin,Chao Yang,Erjun Kan,Jinlong Yang,Xingxing Li,Yi Huang

doi:10.1038/s41699-019-0098-2

Abstract

Graphene is a nonmagnetic semimetal and cannot be directly used as electronic and spintronic devices. Here, we demonstrate that zigzag graphene nanoflakes (GNFs), also known as graphene quantum dots, can exhibit strong edge magnetism and tunable energy gaps due to the presence of localized edge states. By using large-scale first principle density functional theory calculations and detailed analysis based on model Hamiltonians, we can show that the zigzag edge states in GNFs ({mathrm{C}}_{6n^2}H6n, n = 1–25) become much stronger and more localized as the system size increases. The enhanced edge states induce strong electron–electron interactions along the edges of GNFs, ultimately resulting in a magnetic configuration transition from nonmagnetic to intra-edge ferromagnetic and inter-edge antiferromagnetic, when the diameter is larger than 4.5 nm (C480H60). Our analysis shows that the inter-edge superexchange interaction of antiferromagnetic states between two nearest-neighbor zigzag edges in GNFs at the nanoscale (around 10 nm) can be stabilized at room temperature and is much stronger than that exists between two parallel zigzag edges in graphene nanoribbons, which cannot be stabilized at ultra-low temperature (3 K). Furthermore, such strong and localized edge states also induce GNFs semiconducting with tunable energy gaps, mainly controlled by adjusting the system size. Our results show that the quantum confinement effect, inter-edge superexchange (antiferromagnetic), and intra-edge direct exchange (ferromagnetic) interactions are crucial for the electronic and magnetic properties of zigzag GNFs at the nanoscale.

Highlights

Using firstprinciples density functional theory (DFT) calculations, we find that both strong edge magnetism and tunable energy gap can be realized simultaneously in large ZZGNFs stabilized at room temperature
We demonstrate that spin polarization plays a crucial role as the diameter of a ZZGNF increases beyond 4.5 nm (C486H54)
We believe the FM coupling along each zigzag edge that belong to the same sublattice, are likely to be induced by FM)) of no magnetism (NM), AFM, and FM coupling between different edges in ZZGNFs and ZZGNRs and (b) spin electron density

Summary

Introduction

Engineering techniques that use finite size effect to introduce tunable edge magnetism and energy gap are by far the most promising ways for enabling graphene[1] to be used in electronics and spintronics.[2,3] Examples of finize-sized graphene nanostructures include one-dimensional (1D) graphene nanoribbons (GNRs)[4,5,6,7,8,9,10,11,12,13,14,15,16] and zero-dimensional (0D) graphene nanoflakes (GNFs) ( known as graphene quantum dots).[17,18,19,20,21,22,23,24,25,26,27,28] It is well known that electronic and magnetic properties[29] of GNRs and GNFs depend strongly on the atomic configuration of their edges, which are of either the armchair (AC) or zigzag (ZZ) types.[8]Edge magnetism has been predicted theoretically[10,11] and observed experimentally[15,16] in ZZGNRs. Using firstprinciples DFT calculations, we find that both strong edge magnetism and tunable energy gap can be realized simultaneously in large ZZGNFs stabilized at room temperature.

Results

Conclusion