Cuicui Sun‡
a,
Yingjie Jiang‡*b,
Yanmin Wanga,
Xiao-Cun Liua,
Yanling Wua,
Yongling Dinga and
Guiling Zhangc
aSchool of Civil Engineering, Shandong Jiaotong University, Jinan 250300, China
bState Key Laboratory for Turbulence and Complex System, Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China. E-mail: yingjiejiang@pku.edu.cn
cSchool of Materials Science and Chemical Engineering, Harbin University of Science & Technology, Harbin 150080, China
First published on 4th November 2021
The electronic and transport properties of fluorographane (C2HF) nanoribbons, i.e., bare (B-C2HF) and hydrogen-passivated (H-C2HF) C2HF nanoribbons, are extensively investigated using first-principles calculations. The results indicate that edge states are present in all the B-C2HF nanoribbons, which are not allowed in the H-C2HF nanoribbons regardless of the directions. The spin splitting phenomenon of band structure only appears in the zigzag direction. This behavior mainly originates from the dehydrogenation operation, which leads to sp2 hybridization at the edge. The H-C2HF nanoribbons are semiconductors with wide band gaps. However, the band gap of B-C2HF nanoribbons is significantly reduced. Remarkably, the phase transition can be induced by the changes in the magnetic coupling at the nanoribbon edges. In addition, the B-C2HF nanoribbons along the zigzag direction show optimal conductivity, which is consistent with the band structures. Furthermore, a perfect spin filtering controller can be achieved by changing the magnetization direction of the edge C atoms. These results may serve as a useful reference for the application of C2HF nanoribbons in spintronic devices.
Although several experimental and theoretical studies have focused on the synthesis and properties of fluorographane,21,23,24 the C2HF nanoribbons have been hardly investigated.19 After the formation of one-dimensional (1D) nanoribbons, the electronic and transport properties have been tuned by altering the width, doping hetero atoms, applying external field or strain, edge modification, chemical adsorption, etc.25–27 Among them, edge modification is one of the most popular methods. This is because the basic physical properties of nanoribbons are strongly related with their edge shapes, which may induce the edge states and further result in the spin polarization. Consequently, the effect of various edge passivation states in different nanoribbons has attracted considerable research attention.28,29 For example, the sp3 hybridization on the edge atoms of zigzag silicon carbon nanoribbons after the dual-hydrogenation led to a perfect spin filtering behavior (nearly 100% spin filtering efficiency).30 The edge dehydrogenation caused a transition from antiferromagnetic (AFM) state to ferromagnetic (FM) state and effectively improved the spin filtering efficiency.31 Numerous studies have revealed the influence of edge passivation states on the electronic structure and magnetic properties of nanoribbons. However, the electronic/magnetic properties of the C2HF nanoribbons with different edge passivation states have not been investigated in detail. Therefore, in this study, a series of C2HF nanoribbons with different edge states are investigated to obtain a deeper insight into the quantum behavior of the spin-resolved transport properties. Here, the band structures and transport properties of bare or hydrogen-passivated C2HF nanoribbons with armchair and zigzag edges are presented. Our results show that the spin filtering can be controlled by changing the magnetization direction of edge C atoms. The effectiveness of this method stems from the fact that it provides an easy and precise control of spin filtering properties. Combining with the advantages of 1D nanoribbons, it is potentially useful for the transfer and treatment of information, allowing faster, low-energy operations in very small and complex devices.
After geometrical optimization, the relaxed structure of C2HF is a puckered surface, and the perfectly planar graphene plane is locally buckled, as shown in Fig. 1. Here, C2HF nanoribbons with different edges are considered, including bare and hydrogen-passivated nanoribbons, which are abbreviated as B-C2HF and H-C2HF nanoribbons, respectively. The primitive nanoribbons cells with minimum number of atoms marked in Fig. 1 are used in the electronic structure calculations. The lengths of C–C, C–H, and C–F bonds are 1.53, 1.11, and 1.37 Å, respectively. The two-probe models are constructed for the electronic transport simulation, as depicted in Fig. 1. The graphene was selected as the electrode materials of the device along zigzag direction. Since the pristine armchair graphene nanoribbon is a semiconductor, the N-doped armchair graphene nanoribbons36 are used as electrode materials along the armchair direction. The buffer layers extending from the electrode are used to shield the interaction between the electrodes.
Fig. 4 Bloch states of edge-related bands at Γ for B-C2HF-zigzag-FM, B-C2HF-zigzag-AFM, and B-C2HF-armchair nanoribbons. |
By artificially setting the spin polarizabilities (same or opposite to each other, i.e., FM and AFM state) of the C atoms at the edge of the nanoribbons to simulate an external magnetic field, an interesting phase transition phenomenon is observed in the B-C2HF-zigzag nanoribbons: a spin gapless semiconductor state 1 to a spin gapless semiconductor state 2. Although, they are all spin gapless semiconductors.39–41 But, the B-C2HF-zigzag nanoribbons with the FM state display a spin gapless semiconductor state 1 character: there is a gap between the conduction and valence bands for both the majority and minority electrons, while there is no gap between the majority electrons in the valence band and the minority electrons in the conduction band, as shown in Fig. 2d. However, that with the AFM state behave as a spin gapless semiconductor state 2 character: one spin channel is gapless, while the other spin channel is semiconducting, as shown in Fig. 2e. In other words, the features of the electronic properties can be manipulated by the magnetic field (the variation in the magnetic field indicates the variation in the magnetic coupling mode of the edge C atoms), which means that this transition is experimentally feasible. In addition, another important electronic effect: spin splitting of both AFM and FM states, is shown in Fig. 2. The difference is that both the spin-up and spin-down states show gapless properties in the FM state and the band edge touch at the Fermi level (Ef) at Γ point. However, the charge carriers in the spin-up state are mainly holes, while that in the spin-down state are primarily electrons. Interestingly, the AFM state exhibits different electronic characteristics: the spin-down state presents a gapless character, and it turns into semiconductor with a band gap of 0.72 eV in the spin-up state. The obvious spin splitting near the Ef originates from the change in the electronic configuration of edge C atoms. As mentioned above, the dehydrogenation changes the hybridization state of edge C atoms from sp3 into sp2. The unpaired electrons in the adjacent sp2-hybridized C atoms result in spin splitting. The spin polarization caused by the edge C (C8 and C9, as shown in Fig. 1) can also be seen from the magnetic moment in Table S5.† Clearly, the magnetic moment of B-C2HF-zigzag nanoribbon of both the FM and AFM state mainly arises from the C at the edge. However, a completely different spin splitting is observed in B-C2HF-armchair nanoribbon. Although such kinds of C atoms are also present in B-C2HF-armchair nanoribbon, no spin splitting happens due to the different configuration, as shown in Fig. 3b. To determine the reason for this phenomenon, we calculated the Mulliken population of BC2HF-armchair and B-C2HF-zigzag nanoribbons. Comparing with the results, it is found that the Mulliken population of the edge C atoms in B-C2HF-armchair nanoribbons is similar to that in the central region, which has sp3 hybridization, as seen in Table S3.† In this case, the electrons are paired and no unpaired electron exits. In contrast, the Mulliken population of the edge C atoms in B-C2HF-zigzag nanoribbons is special: there is almost one charge difference between spin-up and spin-down atoms, indicating the presence of unpaired electrons, as shown in Table S4.†
Another interesting issue is the conductive mechanism of the nanoribbons with various edges. Therefore, the transmission functions were calculated to clarify the quantum behavior and electron transmission channels. We can intuitively find that the most obvious distinction between the five transmission functions is the intensity, which can vary by four or five orders of magnitude or more (Fig. 5). From the analysis of the band structures, it is concluded that all the C2HF nanoribbons along the armchair direction have a very large band gap. After these nanoribbons are combined with the electrode material, due to the resonance between the electronic wave functions, some energy levels appear near the Ef (as shown in Fig. 6), but these energy levels are highly localized at the interface and cannot promote electron transmission. Consequently, both H-C2HF-armchair and B-C2HF-armchair nanoribbons show extremely small transmission coefficients. On the other hand, in the zigzag direction, the hydrogenation of the H-C2HF-zigzag nanoribbon causes the edge C atoms to become sp3 hybridized and the electrons are in inert state, causing the energy levels to move away from the Ef. The only few energy levels near Ef in the spin down state of the H-C2HF-zigzag nanoribbon (Fig. 7c) are also localized in the buffer layers. Correspondingly, the bare nanoribbons B-C2HF-zigzag show a large transmission value, and it originates from the active single electron in sp2 hybridization, which introduces more energy levels near Ef, as shown in Fig. 7a and b. Contrary to H-C2HF-zigzag nanoribbon, these energy levels are all completely delocalized through the scattering region. Of course, this is consistent with the band structures, i.e., B-C2HF-zigzag nanoribbon shows spin gapless semiconductor character, while H-C2HF-zigzag nanoribbon shows semiconductor characteristics with a larger band gap. From these results, it can be inferred the conductivity of B-C2HF-zigzag nanoribbon is optimal. More interestingly, the spin polarization of electrons makes the transport phenomenon more complex. Hereafter, we mainly focus on the spin-related transport of B-C2HF-zigzag nanoribbons.
Fig. 5 Transmission spectra of (a) B-C2HF-zigzag-AFM, (b) B-C2HF-zigzag-FM, (c) H-C2HF-armchair, (d) B-C2HF-armchair, and (e) H-C2HF-zigzag nanoribbons. |
Fig. 7 Molecular energy spectra of (a) B-C2HF-zigzag-AFM, (b) B-C2HF-zigzag-FM, and (c) H-C2HF-zigzag nanoribbons. |
Consistent with the band structures, both the AFM and FM states of the B-C2HF-zigzag nanoribbons are calculated. The conductance of both AFM and FM states is highly spin-polarized. The difference is that the frontier molecular orbitals of the AFM state are dominated almost entirely by the spin-down state, while those of the FM state are equally controlled by spin-up and spin-down states, i.e. the highest occupied molecular orbital (HOMO) is provided by the spin-up state, and the lowest unoccupied molecular orbital (LUMO) is dominated by the spin-down state. Specifically, the spin-down electronic state governs the transport in the low voltage range (when the energies of electrons are localized in the range of −0.5 to 0.5 eV) for AFM state, but that is controlled by both spin-up and spin-down for the FM state, which causes the perfect spin filtering, as shown in Fig. 5a and b. The energy level distributions and orbital wave functions in Fig. 7a can provide a deeper perspective to understand this phenomenon: the wave function distributions of frontier orbitals for the two polarization directions are completely different. For the AFM state, these spin-up orbitals are highly localized, but the spin-down orbitals exhibit excellent delocalization. The delocalization may facilitate the construction of effective electron transmission channels. However, for the FM state, taking the Ef as the dividing line, the composition of the electron transmission channels is completely different. The channels with energy above and below the Ef are independently constructed by spin-up and spin-down orbitals, respectively, as shown in Fig. 5b. This is because there are a large number of single electrons with parallel spins in the system. The Hund's rule governs the arrangement of electrons in the orbitals, i.e., each orbital must contain one electron, each spinning in the same direction, before the electrons can be paired in the orbitals. The occupation of single electrons splits the original degenerate spin orbitals, one of which is an occupied spin-up orbital, and the others are empty spin-down orbitals. The partial orbital wave functions intuitively reveal the contribution of related molecular orbitals to the electron transmission. As seen in Fig. 7b, the occupied spin-up orbitals and empty spin-down orbitals display good delocalization characteristics, which can construct effective electron transmission channels.
In addition, the conductance of a device is determined by the spatial distribution of the frontier molecular orbitals.42 For example, one frontier molecular orbital has a high possibility to resonate with the electronic states in the electrodes if the orbitals spread over the entire device, resulting in a more beneficial conductivity. Therefore, we calculated the eigenstates of the transmission peaks (a, b, c, and d) near the Ef. Obviously, the transmission eigenstates of a, b, c and d, which originate from the edge of nanoribbons, are delocalized across the central region and strongly coupled with two electrodes, as shown in Fig. 8, which is an essential prerequisite for the resonant transmission.
Fig. 8 Transmission energy eigenstates corresponding to the transmission peaks near Ef of B-C2HF-zigzag nanoribbons. |
To recapitulate, the edge states play an important role in the transport mechanism. The change in the hybridization of edge C atoms leads to spin splitting. For the FM state, the occupied spin-up orbitals and empty spin-down orbitals can all construct effective electron transmission channels. However, for the AFM state, the spin-down electronic state governs the transport in the low voltage range. Obviously, the spin-up transmission is filtered out during the transition from FM to AFM state. Therefore, we can realize a perfect spin filtering controller through switch the magnetization at any time. This effect can be achieved by changing the magnetization direction of edge C atoms, and the corresponding schematic diagram is shown in Fig. 9. Obviously, the frontier molecular orbitals of the FM state are equally controlled by spin-up and spin-down states that lead to two effective electron transmission channels. While for the AFM state, the frontier molecular orbitals are dominated almost entirely by the spin-down state which result in only one effective electron transmission channels. Compared with graphene nanoribbons, the novel B-C2HF-zigzag nanoribbons exhibit almost perfect spin current switching due to the synergistic effect between the sp2 and sp3 hybridization. In the B-C2HF-zigzag nanoribbons, the sp3 hybridization mode of the C atoms in the central region causes the system to have a large band gap. On the other hand, the sp2 hybridization originating from the dehydrogenation of edge C atoms introduces several active bands, which are gapless near Ef. In general, from the perspective of the band structures, the edge-related bands not only exhibit excellent dispersion characteristics similar to graphene nanoribbons, but also display extremely obvious spin splitting. In terms of transport, the spin transmission channels can be easily tuned from on to off state (or vice versa) by an external magnetic field under low voltage region.
Fig. 9 Schematic diagram of the modulation of system transport properties by external magnetic field. |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra07161k |
‡ Cuicui Sun and Yingjie Jiang contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2021 |