Spin Switch of the Transition-Metal-Doped Boron Nitride Sheet

Apr 12, 2014 - Such spin state switching can open a new route to realize the applications of TM-doped BN for spintronics and quantum information. View...
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Spin Switch of the Transition-Metal-Doped Boron Nitride Sheet through H/F Chemical Decoration Junjie He,† Na Jiao,† Chunxiao Zhang,‡ Huaping Xiao,*,‡ Xiaoshuang Chen,§ and Lizhong Sun*,† †

Key Laboratory of Low-Dimensional Materials and Application Technology and Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan 411105, China ‡ Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan 411105, China § National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China ABSTRACT: The effect of H/F chemical decoration on the spin switch of a single 3d transition-metal (TM = Mn, Fe, Co) doped boron nitride (BN) sheet is systematically studied using density functional theory plus Hubbard U (DFT+U). It is found that the ground spin state of a TM embedded in a BN sheet is sensitive to the value of the on-site Coulomb energy. Interestingly, we find that the spin of the Fe−BN system is switched from “spin ON (S = 5/2)” to “spin OFF (S = 0)” for H decoration and from “spin high (S = 2)” to “spin low (S = 1/2)” for H-decorated Mn−BN and Fdecorated Co−BN systems. Such spin state switching can open a new route to realize the applications of TM-doped BN for spintronics and quantum information.

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between “spin on” and “spin off” through adsorption of NO on Fe/Co-(II) tetraphenylporphyrin, which is of eminent interest in the field of quantum information. Liu et al.13 show that the Kondo resonance of Mn-phthalocyanine (MnPc) can be reversibly switched off and on through absorption and desorption of a single H atom to the TM center of the MnPc molecule. Moreover, the theoretical calculations14 have revealed that the magnetic and electronic properties of TMdoped graphene are sensitive to the adsorption of the O2 molecule. Therefore, it is interesting to study the response of the spin state of the TM-doped BN sheet to chemical decoration by H or F. TM doping on the B vacancy of the BN sheet can be achieved in experiments because the B-vacancy defect can be controllably produced; for example, Jin et al.14 obtained the boron-vacancy doping through boron-deficient growth conditions. In comparison with weak binding between TM and N vacancy, the binding between the TM and B vacancy of the BN sheet demonstrates very high stability, which makes the TMdoped B vacancy easily achieved experimentally without the clustering effect of TMs.4,8 Previous works have revealed that the BN sheet with Mn, Fe, and Co doping on B vacancy shows a low-spin ground state by means of density functional theory (DFT) at the GGA level.4,8 However, for TM-porphyrins,15 TM-absorbed graphene,16−18 and graphyne19,20 systems, the DFT shows a general failure to accurately determine their

INTRODUCTION Two-dimensional (2D) single atomic layer sheets have attracted much interest due to their great potential applications in nanoelectronics and spintronics. Among them, monolayer boron nitride (BN) sheets show tremendous possible applications in nanoscale devices1−3 due to their unique properties, such as wide gap nature, high thermal stability, and chemical inertness. When such a wide band gap semiconductor is doped with transition metal (TM), its electronic and magnetic properties are primarily determined by the distribution of TM d states around the Fermi level because the energies of TM 3d orbitals may exist inside the band gap of the host material behaving as in-gap states.4,5 Therefore, the charge and spin states of TM-doped BN can be possibly controlled by an external electronic field.4 Moreover, some novel electronic structures can be achieved in TM-doped BN systems, such as the half-semiconductor antiferromagnets6 and half-metal5,7,8 characteristics. These findings as well as the controllable spin properties suggest the potential of TM-doped BN systems in spintronics. The electronic and magnetic properties of TM-doped BN sheets will be sensitive to the external chemical environment because the TM doping site is reactive to adsorb atoms or molecules around it. Such an effect on the one hand will influence the spin stability of the system. On the other hand, the reactive TM doping site provides the possibility to control the charge and spin states of the systems at the atom level which is crucial for designing spin-based devices.9,10 Experimentally, Wäckerlin et al.11,12 have recently achieved a reversible spin switch between “spin high” and “spin low” or © 2014 American Chemical Society

Received: October 30, 2013 Revised: April 6, 2014 Published: April 12, 2014 8899

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moment of the three N atoms within the B vacancy indicates 1 μB, 1 μB, and −1 μB, respectively, due to the Jahn−Teller effect. The N−N distance between the two N atoms with opposite spin is 2.71 Å, whereas the distance between the two N atoms with the same spin is 2.78 Å due to Coulomb repulsion. The results are in good agreement with a previous report.4 We take the Fe-doped system (Fe-BN) as an example to explore the dependence of the spin state on the Hubbard U. The total spin magnetic moment of Fe−BN as a function of Ueff is shown in Figure 1(a). The results indicate that there is a critical Ueff value

electronic and magnetic properties. The DFT+U approach has been used to investigate the electronic and magnetic properties of TM−porphyin complexes, TM-adsorbed graphene, and graphyne systems15−19 because it can partially correct the artificially delocalized d state of TM atoms and inherent selfinteraction errors for DFT. Due to the complicated spin configuration of the TM-doped BN sheet, ignorance of the onsite Coulomb energy of the TM d orbit may lead to an incorrect spin state for the TM−BN nanostructure. Therefore, it is necessary to understand the spin state of the TM-doped BN nanostructure based on DFT+U. In the present study, we perform first-principles calculations based on DFT plus the self-consistent Hubbard U approach to explore the influence of the on-site Coulomb correlation effects on the electronic and magnetic properties of TM (where TM = Mn, Fe, and Co) doped on the B vacancy of BN sheets and discuss the possibility of controlling the spin state of the TM dopant by H/F chemical decoration.

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METHOD AND CALCULATION DETAILS A 7 × 7 supercell of a BN sheet with TM doping on B vacancy is adopted as the computational model, where a vacuum space of 15 Å perpendicular to the BN plane is chosen to avoid the interactions between the neighboring images. The firstprinciples calculations are performed using the Vienna ab initio simulation package (VASP)21,22 within spin-polarized DFT. The electronic exchange correlation interaction is described within the generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhof (PBE)23 plus the Hubbard U correction (DFT+U). The interactions between the ion core and valence electrons are modeled with projector-augmented wave (PAW) potentials.24,25 A plane-wave basis set with the kinetic cutoff energy of 500 eV is employed. The Brillouin zone (BZ) is sampled using 5 × 5 × 1 and 9 × 9 × 1 Gammacentered Monkhorst−Pack grids for the calculations of relaxation and electronic structures, respectively. The criteria of energy and atom force convergence are set to 10−5 eV/unit cell and 0.015 eV/Å, respectively. Dipole corrections are considered to deal with the impact of variety of potential distribution introduced by TM doping. All calculation parameters in the present work are systematically optimized. To evaluate the stability of the TM-doped BN sheet, binding energy per TM atom Eb is defined as E b = E BN + E TM − E TM + BN (1)

Figure 1. (a) Calculated spin state and magnetic moment of the Fedoped BN sheet as a function of Ueff. The insets in (a) are the spinpolarized charge density (SCD) of the system under LS (S = 1/2) and HS (S = 5/2) states, respectively. (b), (c), and (d) PDOS of the Fe atom for the Fe-doped BN sheet with U = 0, U = 1.5, and U = 2 eV, respectively.

(around 1.6 eV) that the spin state of the Fe atom transforms from the low spin (LS) state (S = 1/2) to the high spin (HS) state (S = 5/2). As for Mn- and Co-doped systems, they have similar transition from LS to HS state when the Ueff increases over 1.4 and 2.2 eV for Mn−BN and Co−BN, respectively. The results indicate that if we consider the correction of the Hubbard U the TM-doped BN sheet will show HS ground state rather than LS predicted by GGA. Generally, to understand the origin of the spin state, the crystal field theory based on Griffith’s theory of TM ions26 is an effective approach. On the basis of the theory, the occupation of d states of the TM atom is determined by the relative strength between crystal field splitting (Ecf) and exchange splitting (JH). Ecf can be obtained by the splitting between A1 states and E1−E2 states in the same (spin up in present work) channel. JH can be obtained by the splitting of A1 occupied states between spin-up and spindown channels. The details can be found in Figure 1. The correction-induced spin state transition for Fe−BN is due to the Hubbard U which enhances Hund’s exchange energy JH which makes the HS state more favorable. To understand the mechanism of the correlation effects on the spin state of the Fe−BN system, we show the partial density of states (PDOS) with Ueff = 0 eV, Ueff = 1.5 eV, and Ueff = 2.0 eV as shown in Figure 1(b), (c), and (d), respectively. When Ueff = 0 and Ueff = 1.5 eV, the highest occupied molecular orbital (HOMO) mainly comes from the E1 (dyz, dxz) and E2 (dxy, dx2−y2) states, and the lowest unoccupied molecular orbital (LUMO) comes form the A1 (dz2) state. From the insets of Figure 1(a), one can find that the SCD of LS Fe−BN demonstrates the dz2 orbital

where ETM+BN denotes the spin-polarized total energy of the TM-doped BN sheet; EBN is the total energy of the BN sheet with boron vacancy; and ETM is the spin-polarized total energy of the corresponding free TM atom. To have a better understanding of the magnetism of the TM−BN system, we use the map of spin-polarized charge density (SCD) for all systems. The SCDs are defined as ρ(r)⃗ = ρ↑(r)⃗ − ρ↓(r)⃗ , where the terms ρ↑(r)⃗ and ρ↓(r)⃗ are the spinup and the spin-down charge density of the TM−BN system, respectively.

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RESULTS AND DISCUSSIONS Influence of Correlation Effects on the Spin State of TM−BN. Here, we perform GGA as well as GGA+U calculations to investigate the influence of correction of Hubbard U on the spin state of the TM doped on B vacancy of the BN sheet. Our results indicate that a single B vacancy in the BN sheet shows 1 μB magnetic moment. The magnetic 8900

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Table 1. Calculated Height (h) from the TM Atom to the Plane of the BN Sheet, Binding Energy (Eb), Magnetic Moment (M) per Unit Cell, and the Feature of the Electronic Structures (E.S.)a h (Å) Mn(U = 0) Mn(U = 3.71) Fe(U = 0) Fe(U = 3.87) Co(U = 0) Co(U = 4.06)

1.463 1.498 1.481 1.671 1.332 1.369

Eb (eV) 8.269 5.991 8.664 8.415 8.826 7.425

M (μB) 2 4 1 5 0 4

E.S.

Egap (eV)

Ecf (eV)

JH (eV)

EHL

sp-HSC ss-HSC ss-HSC sp-HSC SC sp-HSC

0.94 0.14 0.16 1.63 1.10 0.73

2.13 1.09 1.38 0.51 1.27 1.32

1.20 2.54 0.21 4.37 0 3.09

−0.104 −0.838 −0.116 −0.923 0.625 −0.298

a Egap is defined as the difference between HOMO and LUMO, namely, Egap = EHOMO − ELUMO. The EHL = EHS − ELS defines the energy difference between high-spin and low-spin states.

characteristics. When Ueff = 2 eV, the results in Figure 1(d) show that its majority spin is fully occupied, whereas its minority spin is nearly empty, resulting in HS states. The results also show that along with the increase in Hubbard U the minority A1 states shift to higher energy as shown in Figure 1. Such an energy shift causes the Hunds coupling JH to increase about 0.21, 1.25, and 2.48 eV for U = 0 eV, U = 1.5 eV, and U = 2.0 eV, respectively. The JH will exceed the crystal field splitting Ecf when the Hubbard U correction is adopted as listed in Table 1 and then produce the ground state of the systems changes from the LS state (S = 1/2) to HS state (S = 5/2). Similar results can be found in Co- and Mn-doped systems. Further analysis indicates that the spin state as well as the geometry and electronic structures of the TM-doped BN sheet are sensitive to the value of the on-site Coulomb energy of 3d TM. In general, the local Coulomb interaction U may result in the change of exchange splitting of the TM 3d orbit, which will promote the magnetic state of the TM atom to HS. Meanwhile, the increased localization of d states of the TM atom will reduce the hybridization strength between the TM and BN sheet, resulting in weaker absorption energy. Such results are similar to the reports of TM-adsorbed graphene and graphyne.16,19 The results in previous reports4,8 as well as in our present work reveal that when Mn, Fe, and Co dope on B vacancy the ground state is low spin (LS) at the GGA level. However, when we consider appropriate on-site Hubbard U, the ground state will turn to high spin state (HS). To quantitatively clarify such conclusions, we performed the fixed spin moment (FSM)27 calculations, including full atomic relaxation, to trace the magnetic ground state of the systems. We define the energy difference between HS and LS as EHL = EHS − ELS, and the results based on GGA and GGA+U are listed in Table 1. The results indicate that the LS for GGA is 0.625 eV more stable than that of HS for the Co-doped system. Although the GGA self-consistent calculations of Fe- and Mn-doped systems always give a LS stable ground state, the HS is 0.116 and 0.104 eV more stable than the LS with FSM method. Such a difference may be introduced by the arbitrary fixed spin configurations in the FSM method. Nevertheless, the difference between HS and LS is small, which may not be distinguished by the self-consistent calculations for GGA. When we consider the Hubbard U (the U parameter derived from the linear response approach), the HS is 0.838, 0.923, and 0.298 eV more stable than those of the LS for Mn-, Fe-, and Co-doped systems, respectively. The Hubbard U enlarges the energy difference between HS and LS for Fe and Mn and turns the HS more stable than that of LS for the Co-doped system. On the basis of the FSM results, we can conclude that the Hubbard U approach is essentially important to describe the ground state of the TMdoped BN system.

As mentioned above, to accurately describe the magnetic properties of the TM-doped BN sheet, the use of the DFT+U method and accurately accounting for the electron correlation are crucial. In the present work, the effective on-site Coulomb repulsion parameter Ueff of the TM-doped BN system is calculated by the linear response theory (LRT) introduced by Cococcioni et al.28 The Hubbard U can be calculated directly by response function as follows χIJ =

∂n ∂ 2E = I ∂αI ∂αJ ∂αJ

(2)

In the LRT approach, the χIJ is obtained from the response of d state occupations to a small localized perturbation potential α, and then the parameter Ueff can be obtained from the formula

Ueff = (χ0−1 − χ −1 ) χ0−1

(3)

−1

where and χ represent the Kohn−Sham and the noninteracting inverse density response functions of the system with respect to localized perturbations. We take Fe and Mn as examples, and the calculated results are shown in Figure 2. By

Figure 2. Linear response of d orbital occupations to the change of potential shift α. The lines of blue and red represent the Kohn−Sham and the noninteracting inverse density response functions, respectively.

changing the rigid potential shifts α, we obtain the bare and self-consistent occupation regression response functions. The interacting (χ−1) and the Kohn−Sham (χ−1 0 ) inverse matrix are the slopes of bare and self-consistent regression response functions, respectively, as shown in Figure 2. The parameter U is calculated from the formula 3. The calculated Hubbard U values for Mn-, Fe-, and Codoped BN sheets are listed in Table 1. The results indicate that the calculated Ueff’s are all larger than the critical Ueff value; the ground states of Mn-, Fe-, and Co-doped BN sheets are all HS. Namely, the DFT plus linear response Hubbard U approach favors high spin state for Mn-, Fe-, and Co-doped BN sheets. For comparison purposes, we perform GGA as well as GGA+U (U values are obtain from the LRT approach) calculations of Mn-, Fe-, and Co-doped BN. The results of binding energy and 8901

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our recent report20 of Mn-doped graphyne due to the small radius of the Mn ion. Finally, the Hubbard U correction severely influences the feature of the electronic structures of TM−BN systems, and the comparing results are shown in Figure 3. As for the Mn−BN system, the results at the GGA level as shown in Figure 3(a) indicate that it is a spin-polarized half-semiconductor (sp-HSC) similar to the systems reported by Prigodin et al.29 with full spin-polarization in the opposite spin channel at the valence band maximum (VBM) and the conduction band minimum (CBM). When considering the onsite Coulomb energy, the system turns to be spin-selected halfsemiconductor (ss-HSC), i.e., a fully spin-polarized CBM and VBM with the same spin polarization. We also find that when considered the Hubbard U correction Fe−BN system also shows ss-HSC nature, whereas it is sp-HSC without the correction. The concept of ss-HSC was first proposed by Zhu et al.,30,31 which is considered as a promising candidate for spintronics. In the present work, we find that ss-HSC can be achieved in Mn−BN and Fe−BN systems. As for the Co-doped BN system, it is a nonmagnetic semiconductor and sp-HSC at the DFT and DFT+U level, respectively. H/F Chemical Decoration Induced Spin Switch. On the basis of the DFT+U method, we obtain the HS ground state for Mn-, Fe-, and Co-doped BN systems. We further investigate the modulation effect of single H/F atom chemical decoration on the spin state of the systems when a single H/F atom chemically adsorbs on the TM dopant, and such systems are denoted as H−TM−BN/F−TM−BN for short. According to our calculations, only three systems, H-decorated Fe−BN, Hdecorated Mn−BN, and F-decorated Co−BN, show spin switch. The following study is mainly focused on these three systems. The feature of the electronic structures, magnetism, and structural parameters for the three H−TM−BN/F−TM− BN systems are listed in Table 2. To evaluate the stability of the H/F decorated systems, we calculate the absorption energy of H/F as Ea(H/F) = EH/F + ETM−BN − EH/F−TM−BN, where EH/F and ETM−BN, EH/F−TM−BN represent the energy of isolated H/F atom, TM−BN, and H/F−TM−BN system, respectively. The results are listed in Table 2. All the adsorption energies of the three systems are negative and smaller than −1.0 eV, indicating that the reactions are exothermic and can be realized experimentally. The binding energy for H and F on the pristine BN sheet is −0.46 and −1.89 eV, respectively, which is much smaller than H/F adsorbed on the corresponding TMdoped systems, as listed in Table 2. The results indicate that H/ F is favorable for binding on the TM-doped site resulting in the spin switch. The results as listed in Table 2 indicate that upon the H and F chemical decoration the height of the TM atom is reduced. The reduction of the height of the TM dopant will cause stronger interaction between the TM and the host atoms of the BN sheet, which will change the crystal fields. Thorough investigation indicates that when the H and F atoms adsorb on

TM height and the magnetic properties of TM-doped systems are summarized in Table 1. The total DOS under HS (GGA +U) and LS (GGA) are shown in Figure 3. The results as listed

Figure 3. Total DOS of Mn-, Fe-, and Co-doped BN systems under HS and LS states. The positive (red) and negative (green) region represent the majority and minority spin channels, respectively. The Fermi level is set to zero.

in Table 1 show that the Hubbard U correction produces HS states of Mn-, Fe-, and Co-doped systems. Moreover, the correction also reduces the binding energy of the TM. For example, the binding energy of Mn−BN is reduced from 8.269 to 5.991 eV due to the correction. The results also show that although the Hubbard U correction tends to reduce the binding energy of the systems the binding energies of the three systems are all larger than 5.9 eV, which guarantees the stability of the doping systems. The Hubbard U correction also influences the equilibrium height of the TM atom. For instance, as for the Fe−BN system, with the spin state of Fe increased from S = 1/ 2 to S = 5/2, the height of the Fe atom to the plane of the BN sheet increases from 1.481 to 1.671 Å. The other two systems also show similar results that the height of the TM under HS is higher than that under the LS state. The geometry change will reduce the hybridization strength between the TM and BN sheet and lead to lower binding energy. It is worth noting that the slight change of the height of Mn in Table 1 is similar to

Table 2. Calculated Height (h) from the TM Atom to the BN Sheet, Bond Length between H/F Atoms and TM Atoms (dTM−H/F), Absorption Energy (Ea), Magnetic Moment (M) per Unit Cell, Magnetic Moment of the H/F Atom (MH/F), and the Feature of the Electronic Structure (E.S.)a Mn−H Fe−H Co−F a

h (Å)

dTM−H/F

Ea (eV)

M (μB)

MH/F

E.S.

Egap (eV)

Ecf (eV)

JH (eV)

1.365 1.491 1.251

1.642 1.526 1.765

−2.816 −1.526 −5.185

1 0 1

0.054 0 0.079

sp-HSC SC ss-HSC

1.21 1.94 1.01

2.29 2.57 2.38

1.26 0 0.38

Egap defined the difference between HOMO and LUMO as Egap = EHOMO − ELUMO. 8902

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the TM dopant the original crystal field of the TM atom changes to a triangle pyramidal one. Such chemical environment variation implies that the crystal field splitting and intraatomic Hund’s exchange sequence will be changed as listed in Table 2, and the spin state will be switched upon the H/F atom absorption. Excitingly, as listed in Table 2, we find that H chemical decoration can switch the total spin magnetic moment per unit cell of the Mn−BN system from 4 μB to 1 μB, and F decoration can switch the total spin magnetic moment per unit cell of Co−BN from 4 μB to 1 μB. Most interestingly, the Hdecorated Fe−BN system indicates a spin state transition from “spin ON” to “spin OFF” corresponding to the total magnetic moments per unit cell decrease from 5 μB to 0 μB. The spin is completely quenched, and the DOS is now equally distributed over the two spin channels as shown in Figure 4(b). The above results suggest that the H/F chemical decoration can effectively modulate the spin state of TM−BN sheets. The SCD results of the three systems are shown in Figure 5, from which we can see that the H/F decoration not only influences the feature of the localization of the polarized state of the TM but also the magnetic coupling between TM and its nearest neighbor (NN) N; the details will be discussed below. The SCD of H-decorated Fe−BN systems clearly shows that the decoration will totally quench its magnetic properties. Moreover, the total DOS of H/ F−TM−BN systems as shown in Figure 4 indicate that the H/ F decoration changes the electronic feature of TM−BN systems. The Mn−BN (Fe−BN) system changes from ssHSC (sp-HSC) to SC with H atom decoration. The electronic structures of the Co−BN system transit from sp-HSC to ssHSC upon the F decoration. Huang et al.4 proposed that because of the wide energy gap nature of BN the TM behaves as a TM3+ ion after interacting with the in-gap defect states of the boron vacancy. To understand the spin state switching of TM−BN upon the H and F atom decoration, we calculate the PDOS of d states of the central TM3+ ion, s states of H, and p states of the F atom as shown in Figure 6. For the H-absorbed Mn3+, we can see that the s states of the H atom strongly couple with the d states of the Mn atom within the [−5 eV, 0 eV] energy window. As the H decoration, the Mn dopant transforms from its original Mn3+(d4) valence state to Mn4+(d3) valence state through transferring one electron to a H atom. Such charge transfer can be confirmed by the change of the proportion of occupied s states in the systems. As the H decoration, the occupied majority d states of Mn mainly lie in the [−6 eV, −1 eV] energy window, whereas the occupied minority d state peaks locate in the [−4 eV, −1 eV] energy window and around −0.4 eV, respectively. Such localization of the d states of Mn clearly shows strong crystal field splitting of about 2.29 eV for the majority spin of d states and a small JH of about 1.26 eV, as listed in Table 2. As mentioned above, the H decoration reduces the height between the Mn and BN sheets about 0.133 Å. Such reduction increases the interaction between the TM and host atoms of BN leading to larger crystal field splitting. Therefore, the H−Mn−BN system turns from HS to LS state with total magnetic moments 1 μB. A similar spin switch mechanism can also be observed in the H-decorated Fe−BN system. The Fe atom transfers one electron to the H atom, its valence state, and then changes from Fe3+(d5) to Fe4+(d4) along with the H decoration. The H−Fe−BN system exhibits strong crystal field splitting of about 2.57 eV as shown Figure 6(b). The symmetric PDOS of H−Fe−BN for majority and minority spin states indicates zero JH with vanishing magnetism.

Figure 4. Total DOS of H/F−TM−BN systems. The positive (red) and negative (green) DOS represent the majority and minority spin channels, respectively. The Fermi level is set to zero.

Therefore, the H−Fe−BN turns from HS to LS state with 0 μB magnetic moments. The F atom decorated Co−BN system shows a mechanism similar to that of H-absorbed Fe− and Mn−BN systems. The mechanism of the spin switching of the TM−BN systems with H/F decoration can be briefly summarized as two aspects: (1) H/F draws one electron from the TM3+ dopant and (2) the decoration enhances the crystal field splitting of the TM3+ d states through decreasing the height between the TM and BN sheet. Experimentally, Liu et al.13 have revealed that a single H atom absorbed on the Mn−Pc molecule can lead to the intra-atomic rearrangement of d states with the spin state transition from S = 3/2 to S = 1. The H absorption induces the 8903

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Figure 5. Spin-polarized charge density (SCD) for H/F−TM−BN systems. The light (red) and dark (blue) isosurfaces denote the spinup and spin-down charge density, respectively. The isosurface of SCD is 0.005 e/Å3.

change of the distance between Mn−PC and the substrate. By applying voltage pulses or thermal annealing on H−MnPc molecules, they successfully realize atomic manipulation of adsorption and desorption of the H atom with the reversible spin state transition. Our calculations indicate that it is feasible to reversibly control the spin state by adsorption and desorption of the H atom on the Fe/Mn−BN systems due to the comparable adsorption energy with that reported by Liu et al.13(The binding energy for H-absorbed MnPc is −1.75 eV.). Such a reversibly controllable spin state of TM−BN provides a new route to realize the applications of BN for future spintronic devices or quantum computing building blocks. However, the reversal control may be hardly achieved for the Co−BN system with F. Nevertheless, the spin switch of F-adsorbed Co−BN provides us an effective method to controllably synthesize magnetic materials with the TM-doped BN sheet. The above conclusion is obtained based on free-standing TM -doped BN systems. In view of the application, the influence of external chemical conditions such as substrate is worth discussing. In our present work, we take a Mn-doped BN sheet system as an example to investigate the influence of the substrate on its spin switch. In previous work,32 we found that graphene adsorbed on an ideal clean Si(111) surface tends to induce 2 × 1 Si(111) reconstruction. Considering the analogue characteristics of graphene and the BN sheet, we choose the 2 × 1 reconstructed Si(111) surface as the substrate. The thicknesses of the inner-bulk Si atomic layers are chosen to be 8 Si atom layers with the H-terminated bottom surface for the purpose of retaining their bulk properties. The vacuum layer in our models is set to be larger than 12 Å to avoid the interactions between the adjacent images. The parameters of the optimized 2 × 1 reconstructed Si(111) surface can be found in ref 32. Considering the difference in lattice constants, the 6 × 6 Mn−BN supercell is adopted to match the 4 × 2 supercell of

Figure 6. Partial density of states (PDOS) of H/F−TM−BN systems. PDOS of the d states of TMs, s states of H atom, and p states of F atom are also presented. The positive and negative values denote majority and minority channels, respectively. The Fermi level is set to zero.

the 2 × 1 reconstructed Si(111) surface. The stretch of the 6 × 6 BN supercell lattice is within 0.75%. We place the Mn−BN sheet on the 2 × 1 reconstruction Si(111) substrate, and the adsorption configurations are fully relaxed. The results are shown in Figure 7(a). The absorption energy of the Mn−BN sheet on 2 × 1 reconstructed Si(111) substrates is −0.696 eV. Figure 7(b) indicates the fully relaxed configuration of H− Mn−BN on the Si(111) substrate. The H atom absorption induces about 0.009 Å change of the distance between the BN sheet and substrate. From the above discussion, the magnetic coupling between Mn and the nearest-neighbor N atoms for the free Mn−BN system is hybrid AFM and FM coupling due to the Jahn−Teller effect. However, the Si(111) substrate produces AFM coupling between Mn and the neighbor N atoms for both systems. Moreover, the substrate reduces the distance between the H atom and Mn atom from 1.642 to 1.634 Å, and the magnetic moment of the H atom is 0.054 and 0.0473 μB for free and with substrate systems, respectively. When the Hubbard U correction is adopted, the magnetic moment of the system changes from LS (1.98 μB) to HS (3.99 μB). When the H atom absorbs on Mn, the magnetic moment of the system decrease to 3 μB but not 1 μB for the free-standing 8904

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ACKNOWLEDGMENTS This work is supported by the Program for New Century Excellent Talents in University (Grant No. NCET-10-0169), the National Natural Science Foundation of China (Grant Nos. 11032010, 11304263, and 11304264), Hunan Provincial Innovation Foundation for Postgraduate (Grant No. CX2013B263), and the computational support from Shanghai Supercomputer Center.

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Figure 7. Top view and side view of equilibrium adsorption structure for Mn−BN (a) and H−Mn−BN (b) adsorbed on a reconstructed Si(111) surface with the SCD. The isosurface of SCD is 0.005 e/Å3.

system. The change of the spin switch is derived from the interaction between BN and the substrate. The result suggests that whether a spin transition between HS and LS states can be realized for a given TM-doped BN sheet is dependent on the choice of substrate. Moreover, the result also implies that the spin switch observed in our present work is sensitive to the external chemical environment and perturbation, such as substrate, temperature, external strain, and light irradiation. To realize the spin switch proposed in our present work in experiment will need strictly controlled conditions. Such an issue deserves to be studied further.

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CONCLUSION In the present work, we perform DFT plus scf-consistent Hubbard U calculations to systematically investigate the controllable spin state switching of a single 3d transitionmetal (TM = Mn, Fe, Co) doped boron nitride (BN) sheet through H/F decoration. First, we find that the spin state of TM is sensitive to the value of the on-site Coulomb energy. When considering the Hubbard U correction, the TM−BN systems show high spin ground state, and then, through H/F chemical decoration, we find that the spin state of TM−BN systems can be controllably switched; i.e., the spin state switches from “spin ON (S = 5/2)” to “spin OFF (S = 0)” for the H-decorated Fe−BN system and from “spin high (S = 2)” to “spin low (S = 1/2)” for H-decorated Mn−BN and the F absorbed Co−BN system. Such a novel switching of spin state of TM−BN opens a new route to realize the applications of BN in future spintronics and quantum information.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected]. Notes

The authors declare no competing financial interest. 8905

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