Fluorination-Enriched Electronic and Magnetic Properties in Graphene Nanoribbons

TL;DR

This study uses first-principles calculations to explore how fluorine doping alters the electronic and magnetic properties of graphene nanoribbons, revealing diverse behaviors including metallic, semiconducting, and magnetic states.

Contribution

It provides a detailed analysis of fluorination effects on graphene nanoribbons' electronic and magnetic properties, highlighting the roles of chemical bonds, quantum confinement, and edge structures.

Findings

Fluorine adatoms induce p-type metallic or semiconducting behavior.

Multiple magnetic states including ferromagnetic and antiferromagnetic are observed.

Electronic properties depend on fluorine concentration and distribution.

Abstract

The feature-rich electronic and magnetic properties of fluorine-doped graphene nanoribbons are investigated by the first-principles calculations. They arise from the cooperative or competitive relations among the significant chemical bonds, finite-size quantum confinement and edge structure. There exist C-C, C-F, and F-F bonds with the multi-orbital hybridizations. Fluorine adatoms can create the p-type metals or the concentration- and distribution-dependent semiconductors, depending on whether the $\pi$ bonding is seriously suppressed by the top-site chemical bonding. Furthermore, five kinds of spin-dependent electronic and magnetic properties cover the non-magnetic and ferromagnetic metals, the non-magnetic semiconductors, and the anti-ferromagnetic semiconductors with/without the spin splitting. The diverse essential properties are clearly revealed in the spatial charge distribution, the spin density, and the orbital-projected density of states.

Tables

Table 1. Table 1: Ground state energy, magnetic moment and magnetism, energy gap, free holes in a unit cell, and geometric parameters for N = A 12 {}_{A}=12 armchair and N = Z 8 {}_{Z}=8 zigzag GNRs under single- and double-side fluorinations. NM, FM and AFM, respectively, correspond to non-magnetism, ferro-magnetism and anti-ferro-magnetism.

GNRs	Adsorption configurations	$E_0$ (eV)	Magnetic moment (${\mu }_B$ )/ magnetism	$E_g^{d\left(i\right)}$ (eV)/ Metal	Num- ber of holes	F-C (Å)	C height (Å)	Nearest C-C (Å)
AGNR N${}_A{}^{}=12$	Pristine	-234.7419	0/NM	$E_g^d=0.6$	0	0	0	1.428
	${\left(13\right)}_s$	-237.0912	0/NM	M	1	1.547	0.053	1.483
	${\left(1\right)}_s$	-237.4201	0.47/FM	M	1	1.471	0.133	1.489
	${(6,21)}_s$	-239.4138	0/NM	M	2	1.545	0.055	1.491
	${(6,21)}_d$	-239.4261	0/NM	M	2	1.546	0.054	1.491
	${(1,23)}_s$	-240.4703	0.76/FM	M	1	1.468	0.132	1.488
	${(1,23)}_d$	-240.4839	0.76/FM	M	1	1.469	0.131	1.488
	${(1,6)}_d$	-241.0514	0/NM	$E_g^i=0.64$	0	1.443	0.161	1.493
	${(6,9,14,17)}_d$	-245.3591	0/NM	M	2	1.503	0.097	1.497
	${(2,9,18,23)}_d$	-245.8006	0.56/FM	M	1	1.454	0.15	1.491
	${(1,2,23,24)}_d$	-250.2258	0/NM	$E_g^i=0.98$	0	1.43	0.17	1.496
	(C:F=24:6)d	-250.4901	0/NM	M	2	1.532	0.068	1.497
	(C:F=24:6)d	-251.897	0.57/FM	M	1	1.443	0.161	1.493
	(C:F=24:6)d	-255.5256	0/NM	$E_g^i=0.85$	0	1.414	0.186	1.506
	(C:F=24:8)d	-258.6336	0/NM	M	2	1.446	0.154	1.503
	(C:F=24:8)d	-257.9218	0.59/FM	M	1	1.473	0.127	1.498
	(C:F=24:8)d	-261.6414	0/NM	$E_g^i=0.12$	0	1.416	0.184	1.506
	(C:F=24:10)d	-268.196	0/NM	$E_g^i=0.56$	0	1.416	0.184	1.507
	(C:F=24:14)d	-283.013	0/NM	$E_g^i=2.25$	0	1.413	0.187	1.53
	(C:F=24:20)d	-303.9926	0/NM	$E_g^i=2.69$	0	1.408	0.192	1.505
	(C:F=24:24)d	-319.2921	0/NM	$E_g^d=3.2$	0	1.395	0.205	1.544
ZGNR N${}_Z{}^{}=8$	Pristine	-308.0119	0/AFM	$E_g^d=0.46$	0	0	0	1.428
	${\left(3\right)}_s$	-311.9155	0.42/FM	M	1	1.462	0.138	1.485
	${\left(13\right)}_s$	-310.4937	0.4/FM	M	1	1.55	0.05	1.481
	${(3,14)}_d$	-314.6181	0.37/FM	M	1	1.45	0.15	1.488
	${(3,6)}_d$	-314.8808	0.37/FM	M	1	1.432	0.168	1.492
	${(11,14)}_d$	-313.5967	0/AFM	$E_g^i=0.2$	0	1.501	0.099	1.494
	${(19,22)}_d$	-313.3396	0/AFM	$E_g^i=0.2$	0	1.504	0.096	1.486
	${(3,30)}_s$	-315.9246	0/NM	$E_g^d=0.46$	0	1.454	0.146	1.487
	${(3,30)}_d$	-315.9294	0/NM	$E_g^d=0.46$	0	1.455	0.145	1.487
	(C:F=32:32)d	-420.1127	0/NM	$E_g^d=2.97$	0	1.381	0.219	1.515

Full text

Keywords: Multi-orbital hybridizations, fluorination, top-site doping, buckled structures, strong electron affinity

National Cheng Kung University] Department of Physics, National Cheng Kung University, 701 Tainan, Taiwan

National Pingtung University] Department of Applied Physics, National Pingtung University, 900 Pingtung, Taiwan

\phone+886-6-275-7575 \fax886-6-274-7995 National Cheng Kung University] Department of Physics, National Cheng Kung University, 701 Tainan, Taiwan

1 Introduction

A new scientific era has been opened since the discovery of graphene 1. This 2D system exhibits a lot of novel and unusual electronic properties 2. However, there exist few obstacles in the potential applications of graphene-based materials 3. To overcome the gapless feature, the direct method is to create one-dimensional (1D) strips of graphene, usually referred to as graphene nanoribbons 4. GNRs are one of the main-stream nanomaterials, mainly owing to the complex relations among honeycomb lattice, one-atom thickness, finite-size quantum confinement and edge structure. Each GNR could be regarded as a finite-width graphene strip or an unzip carbon nanotube 5. Up to now, GNRs have been successfully synthesized by the various experimental methods including both top-down and bottom-up schemes 6. From the geometric point of view, graphene cutting seems to be the simplest and intuitive method, in which the available routes cover lithographic patterning 7 and etching of graphene 8, sonochemical breaking 9, metal-catalyzed cutting of graphene 10, and oxidation cutting of graphene. Another approach is to unzip carbon nanotube using metal nano-clusters as scalpels 11, and a wet chemical method based on acid reactions 12. The chemical vapor deposition is utilized to massively produce GNRs to meet the essential requirement of semiconductor industry 13. GNRs are expected to have high potential applications in nano-electronic 14 and spintronic 15 devices, gas sensor 16, and nanocomposites 17.

Interestingly, electronic properties of GNRs can be easily modulated by chemical doping 18, mechanical strain 19, 20, layer number 21, curved surface 22, edge-passivation 23, 24, stacking configuration 25; electric 26, 27 and magnetic 28, 29 fields. Among these modulations, the chemical modification on ribbon surface is the most effective one in creating the dramatic changes between the semiconducting and metallic behaviors (the non-magnetic and magnetic configurations). The previous theoretical studies clearly show the geometry- and doping-enriched electronic and magnetic properties. Two typical achiral GNRs, armchair and zigzag ones (AGNRs and ZGNRs), present the width-dependent energy gaps 30, 31, and the latter possess the anti-ferromagnetic spin configuration across the ribbon center 32. The chemical dopings of transition metal Co/Ni adatoms will induce the metallic band structures with free conduction electrons 33, in which the spin-split energy bands correspond to the ferromagnetic configuration. However, alkali adatoms can create the non-magnetic metals in AGNRs and the ferromagnetic ones in ZGNRs under specific distributions 34. The ligand-protected aluminum clusters adsorbed AGNRs lead to the semiconducting or metallic band structures, depending on their kinds 35. As for molecule adsorptions, (CO, NO, NO2, O2, N2, CO2) do not change the semiconducting behavior 36, while NH3 presents the n-type doping. On the experimental side, the adsorption of hydrogen molecules on the Pd-functionalized multi-layer GNRs are successfully obtained 37, and tin oxide nanoparticles are also synthesized on GNRs to form a composite material 38. These results indicate that surface chemical adsorption may serve as a tool for controlling the electronic properties of GNRs 39. However, a systematic theoretical study on the halogen-adsorbed GNRs is absent up to now. Fluorine adatoms have very strong electron affinity; they are thus expected to present the significant chemical bondings with carbon atoms and greatly diversify the essential properties.

This work is focused on the essential geometric, electronic and magnetic properties of fluorine-doped GNRs. They are explored in detail by using the density functional theory. The bond lengths, positions of adatoms, ground state energies, energy bands, spatial charge distributions, free carrier densities, spin densities, magnetic moments, and density of states (DOS) are evaluated using the first-principles calculations. The dependence on concentration, distribution of adatoms, and edge structure is fully included in the calculations. By the detailed analyses, the critical orbital hybridizations in chemical bonds are identified from atom-dominated energy bands, the spatial charge distribution, and the orbital-projected DOSs. The current study shows that they are responsible for the diverse electronic and magnetic properties, covering the ferromagnetic and non-magnetic metals, the non-magnetic semiconductors, and the anti-ferromagnetic semiconductors with/without spin splitting. Furthermore, the feature-rich band structures are reflected in a lot of prominent peaks in DOSs. The predicted optimal geometries, energy bands and DOSs could be verified by scanning tunneling microscopy (STM) 40, angle-resolved photoemission spectroscopy (ARPES) 41 and scanning tunneling spectroscopy (STS) 42, respectively.

2 Computational methods

The essential properties of F-doped GNRs are investigated by using the Vienna ab initio simulation package 43 within the spin-polarized density functional theory. The exchange and correlation energies, which come from the many-particle Coulomb interactions, are evaluated from the Perdew-Burke-Ernzerhof functional 44 under the generalized gradient approximation. Furthermore, the projector-augmented wave pseudopotentials can characterize the electron-ion interactions 45. Plane waves, with an maximum energy cutoff of $400$ eV, are utilized to calculate wave function and state energies. The 1D periodic boundary condition is along $\hat{x}$, and the vacuum spacing associated with $\hat{y}$ and $\hat{z}$ is larger than $15$ Å  to avoid the interactions between two neighboring nanoribbons. The Brillouin zone is sampled by $15\times 1\times 1$ and $100\times 1\times 1$ k point meshes within the Monkhorst-Pack scheme for geometric optimizations and further calculations on electronic structures, respectively. The convergence for energy is set to be $10^{-5}$ eV between two simulation steps, and the maximum Hellmann-Feynman force acting on each atom is less than $0.01$ eV/Å  during the ionic relaxations.

3 Results and discussion

The geometric, electronic and magnetic properties of fluorine-adsorbed GNRs are investigated for various distributions and concentrations of adatoms in zigzag and armchair systems. The widths of AGNR and ZGNR, as shown in Figs. 1(a) and 1(b), are characterized by the number of dimers lines and zigzag lines (NA and NZ) along $\hat{y}$, respectively, in which the periodical lengths in a unit cell along $\hat{x}$ are $3b$ and $2\sqrt{3}b$ ($b$ the C-C bond length). In general, the double-side adsorptions have the lower ground state energies $E_{0}$’s , compared to the single-side cases (Table 1). The optimal adatom position is situated at the top site, regardless of any doping cases. Fluorination can induce the buckled GNR structure. Carbon atoms nearest to F deviate from the graphene plane, being sensitive to distributions and concentrations. For single adatom adsorption, the carbon heights are, respectively, $0.053$ Å  and $0.133$ Å  at center and edge of GNR. They obviously grow with F-concentration, e.g., $0.205$ Å  for $100\%$ adsorption. F adatoms are very close to C, in which the shortest and longest F-C bond lengths ($1.395$ Å  and $1.547$ Å ), respectively, correspond to the highest concentration and single adatom near ribbon center. Moreover, the nearest C-C bond lengths are lengthened in the range of $0.06-0.12$ Å , compared with those of pristine GNRs. This indicates the $\sigma$-bonding changes due to the strong fluorination. The critical F-C chemical bondings, being responsible for the featured geometric structures, are expected to dominate the other essential properties.