Article pubs.acs.org/JPCC
Transition Metal Doped Phosphorene: First-Principles Study Arqum Hashmi and Jisang Hong* Department of Physics, Pukyong National University, Busan 608-737, Korea ABSTRACT: Recently, a newly fabricated two-dimensional layer structured material so-called phosphorene is receiving great research interests due to its peculiar physical properties. So far, mostly electrical properties are explored because it has a direct band gap and its nonmagnetic behavior in pristine layer. In this report, we propose that the transition metal doped phosphorene layer can have dilute magnetic semiconductor properties. Here, we investigated the structural property, binding energy, and magnetic property by changing the TM impurity dopants in the substitutional site. Our results demonstrate that spin polarized semiconducting state is realized in phosphorene by substitutional doping of Ti, Cr, and Mn, while a half-metallic state is obtained by V and Fe doping. In particular, we suggest that the most promising dopants are Cr and Mn because the spin polarized state is achieved with a band gap larger than 0.5 eV. The magnetic interaction between two same types of TM atoms is also checked. Magnetic ground state was obtained with GGA and GGA+U approaches are quite consistent, except for Mn- and Fe-doped systems. These findings indicate that the TM-doped phosphorene can be used as a potential next-generation spintronics material.
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and defect-assisted doping by electron beam irradiation.20,21 In this perspective, we aim to explore the magnetic properties of transition metal (TM)-doped phosphorene layer system. Our study may provide useful information regarding the potential dilute magnetic semiconductor applications. Indeed, many attempts to use the conventional semiconducting materials for spintronics applications were not successful due to various fundamental issues.22 Since the phosphorene layer is a newly fabricated 2D semiconductor, we will explore whether the phosphorene layer can be utilized as another potential dilute magnetic semiconductor system because of the gap found in pristine phosphorene. Besides, the possibility of half-metallicity induced by TM impurity for spintronics will be studied as well. In this regard, we will consider substitutional doping of a series of 3d TM atoms such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu in the phosphorene monolayer.
INTRODUCTION Two-dimensional (2D) layer-structured materials manifest many interesting physical properties not found in bulk materials because the electronic band structure is substantially modified in low-dimensional geometry. Among various types of 2D materials, extensive studies have been focused on graphene, MoS2, or graphitic carbon nitride1−4 because they display unique physical properties for potential applications in photonics, electronics, or diverse fields of material sciences. Usually, most of the 2D layer-structured materials are nonmagnetic because they consist of nonmagnetic entities. Nonetheless, numerous reports suggest that the magnetic state can be induced by various factors such as vacancy defect, adatom defect, substitutional doping, or edge effect. For instance, one may find several reports for the induced magnetism in graphene or BN layer owing to defects.5−8 Also, the possibility of magnetism in graphene nanoribbon and in carbon nanotube due to the edge effect has also been discussed.9−12 Very recently, a new two-dimensional system, so-called phosphorene, was mechanically exfoliated by scotch tape based microcleavage from layered bulk black phosphorus.13−15 Phosphorene has stacked puckered 2D honeycomb layers and shows a direct band gap feature. The phosphorene-based FET exhibits high mobility and high on/off ratios.14,16 Besides, the electrical transport and optical properties of phosphorene are anisotropic.17−19 However, the magnetism of phosphorene still remains an open question. The most appealing and conventional technique to induce a magnetism in a nonmagnetic material is transition metal (TM) impurity doping or creation of vacancy defects in host material. To date, several methods have proved that it is possible to introduce dopants into a nonmagnetic material such as pulsed laser deposition (PLD), intercalation, chemical modification, low-energy ion implantation, © XXXX American Chemical Society
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NUMERICAL METHOD We have performed all the calculations using the spin-polarized density functional theory (DFT) by Vienna ab initio simulation package (VASP).23−25 Valence electrons are treated explicitly, and their interactions with ionic cores are described by projector augmented wave pseudopotentials.26,27 In our report, the generalized gradient approximation (GGA)28 was used for an exchange functional and the van der Waals (vdW-DF) interaction29 was included for a correlation functional. It has been known that the GGA is very accurate to predict the magnetic state of TM atoms.20,21,30,31 On the other hand, the Received: November 19, 2014 Revised: April 6, 2015
A
DOI: 10.1021/jp511574n J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Schematic illustration of TM-doped phosphorene sheet: single TM-doped phosphorene (a) top view and (b) side view. Two TM-doped phosphorene (c) top view and (d) side view.
Table 1. Optimized Lattice Constants [(a,b) in Å], Bond Lengths (M−P1 = M−P2 and M−P3 = M−P4) in Å), Binding Energy (Eb in eV), Magnetic Moment of the TM-Doped Phosphorene System (in μB) a, b pure phosphorene Sc Ti V Cr Mn Fe Co Ni Cu
13.40, 13.28, 13.30, 13.28, 13.27, 13.29, 13.29, 13.27, 13.30, 13.54,
13.86 13.33 13.22 13.20 13.22 13.10 13.09 13.05 13.17 12.75
M-P1
M-P3
M-P5
P6−P7
Eb
μB (GGA)
μB (GGA+U)
2.234 2.602 2.491 2.416 2.380 2.290 2.208 2.207 2.195 2.201
3.560 2.783 2.602 2.630 2.695 2.530 2.572 2.497 3.001 3.274
2.277 2.498 2.371 2.314 2.332 2.248 2.183 2.188 2.174 2.250
2.277 2.263 2.250 2.249 2.262 2.249 2.257 2.260 2.263 2.272
− −5.15 −5.48 −4.42 −2.97 −2.96 −3.98 −4.64 −4.15 −1.75
− 0.00 1.00 2.00 3.00 2.00 1.00 0.00 0.92 0.00
− 0.00 1.00 2.00 3.00 2.00 1.00 0.00 1.00 0.00
48 atoms exist in the unit cell. With this structure, we doped single TM atom in the substitutional site. This approximately corresponds to 2.08% impurity doping. In order to determine the magnetic interaction between two impurities, we doped two TM atoms in a 4 × 3 supercell by assuming a uniform distribution. Then, the TM impurity concentration was about 4.2%. In addition, ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations were also considered to find the magnetic ground state. To check that the Brillouin-zone integration was performed using a Monkhorst−Pack k-point sampling with a 5 × 5 × 1 k-mesh. The convergence criteria for energy and force were set to 0.1 meV and 0.01 eV Å−1, respectively. We have imposed a vacuum region of 15 Å in the z direction. It is worth noting that the supercell is optimized to obtain the most stable lattice constant for each TM atom. We also performed charge transfer analysis between the TM atom and the phosphorene layer using Bader formalism.37−39
nonempirical vdW-DF method provides accurate binding energy and bond length between TM and 2D sheet.32−34 The electron correlation effect may play a role for magnetic properties of TM elements due to the localized d-orbital. Therefore, we also performed GGA+U calculations to check the magnetic state of TM-doped systems. Here, the Hubbard U term was explicitly added to the DFT Hamiltonian, and we treated it within the mean-field approximation using the formulation of Dudarev et al.35 Previous studies of TM-doped graphene indicated that a reasonable value of U (∼2 eV or larger) was adequate to obtain a magnetic solution for TM atoms.36 So, we considered a moderate value of Ueff = 2.5 eV to investigate the effect of U on TM-doped phosphorene. All results reported have been obtained with high plane-wave energy cutoff 600 eV. We choose a 4 × 3 supercell along the x and y directions of the phosphorene layer. Thus, lattice constants of a = 13.40 Å and b = 13.86 Å are used, and B
DOI: 10.1021/jp511574n J. Phys. Chem. C XXXX, XXX, XXX−XXX
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NUMERICAL RESULTS Structural Properties and Binding Energy. We first investigated the structural properties such as bond length, binding energy, and lattice constant. Figure 1 (panels a and b) and (panels c and d) shows the schematic illustration of top and side views of single and two substitutional TM-doped phosphorene structures, respectively, and the TM dopant is represented by red color. The phosphorus (P) atoms near the impurity site are indicated by numbers to calculate the bond length between metal impurity (M) and neighboring P atoms. Table 1 shows the calculated results of lattice constants, bonding distances, binding energy, and magnetic moment of single TM-doped phosphorene. The upper and lower phosphorus atoms are displaced from their original positions after TM doping. Consequently, a noticeable internal distortion in phosphorene layer is observed, and this resulted in the change of lattice constant. We found inward relaxation (metal atom moved inside from top phosphorene layer to bottom phosphorene layer) from the phosphorene surface, except for Co and Cu. The nearest phosphorus atoms are also displaced from their original positions and showed buckling feature. Thus, a bond length between the metal atom and the nearest neighbor P atom is increased. Due to this downward displacement of the metal atom, the down phosphorus atom denoted by P5 moved downward from the bottom phosphorene layer. In contrast, the late TM atoms from Fe to Cu display different behavior. In this case, the P5 atom does not show downward displacement, but it just stays on the down phosphorene layer. We also calculated the binding energy using the relation
The situation becomes more complicated when the TM atom has more than 5 electrons because some of the nonbonding orbitals are filled. For single TM-doped system, the magnetic state as well as magnetic moments with GGA and GGA+U are quite consistent with each other. To check the magnetic interaction between two same types of TM atoms, we doped two TM atoms in phosphorene. Since both Sc- and Cu-doped systems displayed nonmagnetic state for single impurity doping, we excluded these two systems. We presented the calculated results in Table 2. Both GGA and GGA+U methods produced Table 2. Energy Difference in mev between FM and AFM State (mev) and Magnetic Moment (μB) per Impurity Atom in 2 TM-Doped Phosphorene energy difference (FM − AFM)
magnetic moment per impurity atom
two impurities
GGA
GGA+U
GGA
GGA+U
Ti V Cr Mn Fe Co Ni
NM-only AFM-only +40 +24 −124 NM-only −0.4
NM-only +20 +203 −16 +284 NM-only −0.7
0.00 1.00 2.30 2.80 1.40 0.00 0.4
0.00 1.4 3.00 3.5 2.2 0.00 0.5
the same magnetic ground state, except for Mn- and Fe-doped systems. In these two systems, the opposite behavior was found. This feature should be verified by experimental measurement although there are no available experimental data so far. Charge Analysis and Magnetization Density. To investigate charge transfer between the metal atom and phosphorene sheet, we calculated the difference charge density. Figure 2
E b = E P/M − Ev − EM
where Ev and EM mean the total energy of phosphorene layer with vacancy and isolated transition metal atom, respectively. Generally, the binding energies are in the range of 3−5 eV, except for Cu. We found a maximum binding energy in Tidoped system, while the Cu-doped system has the lowest value. Magnetic Properties. As displayed in Table 1, both Scand Cu-doped systems show no magnetic state. Surprisingly, we found no magnetic moment in the Co-doped system as well. For other impurities, it is clearly shown that the magnetic moment strongly deviates from its bulk value. For instance, the magnetic moment of the Fe-doped system is greatly suppressed, and the similar behavior is observed in the Mn-doped case. These findings indicate that each TM impurity atom has different bonding characteristics. We further performed Bader charge analysis to understand the magnetic properties in a qualitative manner. In most of the systems, the electron charge transfer from the d orbital of TM atom to the p orbital of a phosphorus atom is found, except for Co- and Ni-doped systems. In these Co- and Ni-doped atoms, no meaningful charge transfer is observed. Of course, this was dependent on the specific impurity atom. For instance, in the Sc- and Ti-doped systems, the electron transfer was 1.27 and 1.1, respectively. From V to Mn, it was in the range of 0.76−0.88, while we found charge transfer of 0.3 electron in Fe- and Cu-doped systems. Due to this charge transfer, the Sc, Ti, and V have 0, 1, and 2 unpaired electrons, respectively, and this charge transfer gives rise to a magnetic moment of 0, 1, and 2 μB. In Cr- and Mn-doped layers, both charge transfer and intra-atomic charge redistribution within Cr and Mn atoms resulted in magnetic moments of 3 and 2 μB, respectively.
Figure 2. Difference charge density plots induced by substitutionaldoped TM atom. The gold color (i.e., 0.004 e Å−3) in the plot indicates an increase in the electron density after bonding, and the cyan color (i.e., 0.004 e Å−3) indicates an electron density loss. C
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Figure 3. Calculated spin charge density using GGA functional with an iso-surface value of 0.02 e/A3 of TM-doped phosphorene (a) Ti-doped system, (b) V-doped system, (c) Cr-doped system, (d) Mn-doped system, (e) Fe-doped system, and (f) Ni-doped system.
Figure 4. Magnetization density for the two TM-doped phosphorene within the GGA+U framework of the (a) V-doped system, (b) Cr-doped system, (c) Mn-doped system, (d) Fe-doped system, (e) NiFM-doped system, (f) NiAFM-doped system. The gold color and cyan color have iso-values of ±0.02 e/A3.
interaction between these metal atoms and nearest phosphorus atoms. In Cr, Co, Ni, and Cu, the interaction between the metal atom and phosphorene seems a mixture of covalent and ionic interaction. Figure 3 (panels a−f) presents the spin density of single TM-doped phosphorene using GGA. Spin density suggests that the magnetism originates entirely from the metal atom and no significant spin density is induced, even in the nearest phosphorus atoms. It is clearly displayed that the spectral shape of spin density depends on atomic type. This result implies
displays the difference in charge density (i.e., the difference between the density of the metal-doped phosphorene system and its separated constituents). The gold color represents charge accumulation while the cyan color represents the charge depletion zone. We have observed that most of the charges are depleted from the metal atoms, although each atom has a different trend. In Sc-, Ti-, V-, Mn-, and Fe-doped systems, the region with a higher bonding charge between metal and phosphorus atoms is found and this may indicate covalent D
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Figure 5. Calculated band structure of single TM-doped phosphorene systems. Arrow indicates the major orbital characteristics for a given band. Fermi energy is indicated by the solid black line.
energy gap, but the gap is extremely small because a 1 meV gap is achieved. As indicated in Table 1, the Co-doped system shows no magnetic state, and here, the direct gap of 0.497 eV is found. In V- and Fe-doped systems, we observe narrow spin majority bands which cross the Fermi level, while the minority spin bands have no states at the Fermi level. Consequently, the half metallic state is found. With regard to the spintronics applications, interestingly, Ti-, Cr-, Mn-, and Ni-doped systems show an interesting feature because we found an energy gap in these structures. This finding is a peculiar characteristic found in the phosphorene layer system because most of the conventional dilute magnetic semiconductors show metallic band structure after TM doping. In the Ni-doped case, we obtained an indirect band gap of 22 meV, but the band gap is rather small for device applications. Thus, the Ni-doped phosphorene may not be a favorable structure for a spin polarized semiconductor. However, except for the Ni doping, we believe that the size of the calculated band gap is meaningful because it is at least larger than 0.5 eV. Overall, our result may imply that the TM-doped phosphorene can be used a potential spin polarized dilute semiconductor. To discover the orbital characteristics, we analyzed the spin polarized bands located near the Fermi level, and they are indicated by arrows. Due to the low TM-doping concentration, the bandwidth of 3d orbitals is rather narrow and the band structure near the Femi level reveals this feature. Also, it is clearly shown that each system has a different origin of spin polarization. In Figure 6, we present the partial density of states (PDOS). As indicated in the band structure, the 3d
that the spin polarization in each impurity atom originates from different d orbitals. Figure 4 (panels a−f) presents the spin density of two TM impurities-doped phosphorene using the GGA+U method. The gold color represents spin up, while the cyan color represents spin down densities. Like in the single impurity-doped system, the magnetic moments purely originated from the TM atoms, while phosphorene atoms showed insignificant spin polarization. Figure 4 showed that the V, Cr, and Fe had an AFM coupling. For the Mn- and Fe-doped system, we already indicated in Table 2 that the GGA and GGA+U methods predicted the opposite magnetic ground state. Here, we presented the spin densities using the GGA+U approach. Thus, we observed AFM interaction for the Fe-doped system, and the FM coupling was found in the Mn-doped system. Nonetheless, the spectral shape of spin density was the same as found in a single impurity-doped structure, and only the relative spin direction was switched. For the Ni-doped case, we displayed for the FM and AFM configuration because they were degenerated. Overall, Figures 3 and 4 showed that the same d orbitals were responsible for the magnetic moment. Magnetic Properties: Band Structures and Density of States. Figure 5 shows the calculated band structure of each system for single TM-doped phosphorene. Blue lines imply the majority spin bands while red lines indicate the minority spin bands. First of all, we find that the band structures substantially depend on the specific impurity atom. In the Sc-doped system, we find no magnetic state, and an indirect band gap of 0.826 eV is obtained. In the Cu-doped case, we still find the indirect E
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Figure 6. Spin-polarized PDOS of TM atom in single TM-doped phosphorene systems. The Fermi energy is shifted to zero energy as indicated by the vertical solid black line.
also shows a similar behavior. However, we find the half-metallic state in the V- and Fe-doped structure. Surprisingly, the energy gap with spin polarization is found in Ti-, Cr-, Mn-, and Ni-doped systems, although the band gap changes as the TM impurity changes. As a result, we have obtained a genuine spin polarized semiconducting feature and this is the most noticeable finding in TM-doped phosphorene. Both GGA and GGA+U have produced the same magnetic ground state, except for Mn- and Fe-doped systems. The half-metallicity or magnetic semiconductor characteristic in the presence of the TM doping may suggest that the phosphorene layer system will be a new dilute magnetic semiconductor material, or we can use this phosphorene for potential spintronics applications. In particular, we propose that the most promising dopants are Ti, Cr, and Mn to make a true magnetic semiconductor because a finite size of the band gap is achieved in these systems.
orbitals have a rather narrow spectral width, and this is due to the localized character of impurity atom. No spin asymmetric PDOS is found in Sc-, Co-, and Cu-doped systems, and this is a straightforward result. In spin polarized systems, we found that the major source of spin polarization of each TM atom originated from different d-orbitals. For instance, the major contribution to the magnetic moment in Ti doping stems from the dx2−y2 orbital. In the Ni-doped system, mostly the dxy orbital is the major source of spin polarization. In Mn doping, three different orbitals such as dyz, dz2−r2, and dzx give rise to the magnetic moment of the Mn atom. These results agree with the calculated band structure and spin densities.
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CONCLUSION In summary, we have explored structural, electronic structure, and magnetic properties of TM impurity-doped phosphorene by using density functional calculations. The impurity doping has induced lattice distortion, and consequently, the interatomic distance has changed. Among the 3d single transition metal series, we found no spin polarized state in the Sc- and Cu-doped systems. Moreover, the Co-doped layer also has no magnetic state. In other systems, we have found the spin polarized state, but the magnitude of the magnetic moment of each TM atom is suppressed from its bulk value. In addition, the band structure is strongly dependent on the TM-impurity type. For instance, the phosphorene layer still has a semiconducting band gap in the Sc- and Cu-doped case. The Co-doped system
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: 82-51-629-5537. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea F
DOI: 10.1021/jp511574n J. Phys. Chem. C XXXX, XXX, XXX−XXX
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(NRF) funded by the Ministry of Education, Science and Technology (Grant 2013R1A1A2006071) and by the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources including technical support (KSC-2014-C3-071).
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