MAGNETIC PROPERTIES OFMOLECULAR NITROGEN ADSORBED
PHOSPHORENE
Govinda Kharal
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 1 / 22
Introduction
Phosphorus and Phosphorene•Phosphorus (P) exists in different allotropic forms:
Red P, Black P, Yellow P, White P, Blue P.•Black phosphorus is a layered crystalline allotrope.•An individual layer of black phosphorus is named phosphorene.
(a) Black Phosphorus (b) Phosphorene
• 2D material first isolated in 2014 by Han Liu and his team.Fig. source: www.plasmamaterials.com
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 2 / 22
Structure of Phosphorene
Fig.(a): Unit cell of Phosphorene ( a = 4.57A , b = 3.31A )Fig.(b): Propagation of unit cell to form a supercell
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 3 / 22
Properties
Superior to graphene due to its semiconducting behavior andvarious anisotropic properties1.
Direct and tunable band gap depending upon number of layers2.
High hole mobility3; µ ∼10,000 cm2/Vs (Si: ∼500 cm2/Vs;MoS2: ∼100 cm2/Vs).
p-type transistor characteristics complementary to n-type MoS2
transistors4.
1L. Kou, T. Frauenhelm, C. Chen, J. Phys. Chem. Lett. 4, 2675 (2014).2J. Qiao, X. Kong, Z. Hu, F Yang, and W. Ji, Nature Communications 5, (2014).3D. Akinwande et al., Nature Communications 5, 5678 (2014).4Liu, Han, et al. arXiv preprint arXiv:1401.4133 (2014).
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 4 / 22
Theoretical Background
Density Functional Theory• Uses electron density [ρ(r)] as basic variable.Kohn-Sham (K-S) Equations∗
1 vxc(r) = δExc [ρ(r)]δρ(r) (Exchange-correlation potential)
2 veff(r) = vext(r) +∫ ρ(r′)|r−r′|dr′ + vxc(r) (Effective potential)
3
(− ~2
2m∇2 + veff(r)
)φi (r) = εi φi (r) (K-S single e− equation)
4 ρ(r) =∑N
j=1 |φj(r)|2 (Electron density)•Equations (1)−(4) are self-consistent K-S equations.
5 Ground State Energy:
E =∑
j εj + Exc [ρ(r)]−∫vxc(r) ρ(r) dr − 1
2
∫ ρ(r) ρ(r′)|r−r′| dr dr′
•Exc [ρ(r)] is unknown and needs to be approximated.
∗W. Kohn and L. J. Sham Phys. Rev. 140 A1133 (1965)
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 5 / 22
Theoretical Background
Flowchart for Self Consistency Procedure
Fig. Source: M. P. Marder,Condensed Matter Physics, John Wiley & Sons (2010).
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Theoretical Background
Approximation for Exc [ρ(r)]
Generalized gradient approximation(GGA)•GGA assumes Exc as
EGGAxc =
∫ρ(r) εxc [ρ(r), |∇ρ(r)|] dr
where, εxc is approximated functional.Magnetic Moment (µ)µ = mµB , m → number of unpaired electronsm =
∫ +∞−∞ (D↑(ε)− D↓(ε))dε
where, D↑= DOS for electrons with up spin & D↓= DOS for electronswith spin down
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 7 / 22
Computational Tool
First-principles calculations based on Density FunctionalTheory (DFT) were carried out using Quantum-ESPRESSO
• QUANTUM ESPRESSO: Computational SoftwareQuantum opEn-Source Package for Research in ElectronicStructure, Simulation, and Optimization
• Based on plane wave basis set and Pseudopotentials.
Fig. Source: www.quantum-espresso.org
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Details of Simulation
Construction of unit cell
Experimental data used as the referencer1(A) r2(A) θ1 θ2 a(A) b(A)
Experimental (bulk)? 2.22 2.28 96.37◦ 101.91 ◦ 4.38 3.31
? A. Brown et al. Acta Cryst. 19, 684 (1965)
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 9 / 22
Details of Simulation
Cutoff kinetic energy (ecut) and k points
Figure: Convergence tests for ecut (left) and k points (right)
• ecut = 42 Ry • k points = 20×20×1
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 10 / 22
Details of Simulation
Optimization of lattice parameters a & b
Figure: Variation of energy with lattice parameters a (left) and b (right)
• a = 4.57 A • b = 3.31 A
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 11 / 22
Details of Simulation
Selection of k path to study band structure
Chosen path : Γ-X-S-Γ-Y-S such that Γ-X represents zigzag direction andΓ-Y represents the armchair direction
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Results and Discussion
Band Structure and Total Density of States(DOS) ofPhosphorene for 3x3 supercell
(a) Band structure (b) Density of States
• Band Gap = 0.87 eV • No magnetizationG. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 13 / 22
Results and Discussion
Combined form of Band Structure and DOS of purePhosphorene for spin-up states
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Results and Discussion
Test of favorable site for adsorption
(a) Top (b) Hollow (c) Bridge
Top Hollow Bridge
Relaxed Energy (Ry) -599.347618 -599.341265 -599.347875
Adsorption Energy (eV) 0.062 -0.024 0.066
Bridge site is more stable than other two sites.
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Results and Discussion
Band Structure and Total DOS of Molecular NitrogenAdsorbed Phosphorene
(a) Band structure(b) Density of States
• Band Gap = 0.85 eV • No magnetization
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Results and Discussion
Combined form of Band Structure and DOS of N2 adsorbedPhosphorene for spin-up states
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 17 / 22
Results and Discussion
Comparison of DOS of molecular nitrogen adsorbedphosphorene with pristine phosphorene
(a) Pristine (b) N2 adsorbed
G. Kharal (USC, Columbia) Molecular Nitrogen Adsorbed Phosphorene November 9, 2018 18 / 22
Results and Discussion
Comparison of DOS of molecular nitrogen adsorbedphosphorene with atomic nitrogen adsorbed phosphorene
∗P. Srivastava, K.P.S.S. Hembram, H. Mizuseki, K. Ryeol, S.S. Han, and S. Kim, J. Phys. Chem. C 119, 6530-6538 (2015).
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Conclusion
Magnetic properties of pure and N2 adsorped phosphorene werestudied systematically.
Adsorption of molecular nitrogen decreases band gap negligiblypreserving the direct nature of band gap.
The system under study (i.e., N2 adsorbed phosphorene) remainednon-magnetic.
N2 molecule adsorption behaves differently than that of N-atomadsorption on phosphorene.
One can extend this work by creating vacancies and
increasing number of N2 molecules. In addition, the
optical and thermal properties can also be studied.
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Acknowledgements:
Prof. Narayan Pd. Adhikari (Advisor)
University Grants Commission, Nepal
Prof. Ralf Gothe
Friends and Family
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THANK YOU
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