Molecular Nitrogen Adsorbed Phosphoreneboson.physics.sc.edu/~gothe/730-F18/talks/Govinda-2.pdf ·...

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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

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

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).

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)

(− ~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

∫ 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)

Flowchart for Self Consistency Procedure

Fig. Source: M. P. Marder,Condensed Matter Physics, John Wiley & Sons (2010).

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

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

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)

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

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

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

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

Combined form of Band Structure and DOS of purePhosphorene for spin-up states

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.

Band Structure and Total DOS of Molecular NitrogenAdsorbed Phosphorene

(a) Band structure(b) Density of States

• Band Gap = 0.85 eV • No magnetization

Combined form of Band Structure and DOS of N2 adsorbedPhosphorene for spin-up states

Comparison of DOS of molecular nitrogen adsorbedphosphorene with pristine phosphorene

(a) Pristine (b) N2 adsorbed

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).

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.

Acknowledgements:

Prof. Narayan Pd. Adhikari (Advisor)

University Grants Commission, Nepal

Prof. Ralf Gothe

Friends and Family

THANK YOU

Molecular Nitrogen Adsorbed Phosphoreneboson.physics.sc.edu/~gothe/730-F18/talks/Govinda-2.pdf ·...

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