Doped strontium vanadate: Computational design of a stable, low

work function material

Ryan Jacobs (rjacobs3@wisc.edu)

John Booske, Dane Morgan

2016 IEEE-IVEC MeetingSession 10: Scandate/Dispenser

cathodes April 20th, 2016

Jacobs, R. M., Booske, J., Morgan, D. “Understanding and controlling the work function

of perovskite oxides using Density Functional Theory”, Advanced Functional Materials (2016)

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Current cathodes suffer from shortcomings• W-BaO dispenser1, mixed-matrix2, top layer3, Scandate2,3,4

• All rely on coated metal or oxide layers and volatile surface species

Sc2O3

Sc2O3

+BaO

• Shortcomings that can be mitigated with materials change: lifetime, off gassing, emission nonuniformity, etc.

• Perovskite oxides: possible intrinsic, low work function without needing to replenish surface dipole emission layer

[1] Vlahos, V., Booske, J.H., Morgan, D., Phys. Rev. B. 81 (2010).[2] H. Yuan, X. Gu, K. Pan, Y. Wang, W. Liu, K. Zhang, J. Wang, M. Zhou, and J. Li, Appl. Surf. Sci. 251, (2005).[3] J.M. Vaughn, C. Wan, K.D. Jamison, and M.E. Kordesch. IBM J. Res. & Dev. 55 (2011)[4] Jacobs, R.M. , Booske, J.H., Morgan, D., J. Phys. Chem. C (2014)

Sc O Ba

[011]

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Perovskites: tunable properties, including Φ

• Composition space: {Sr,La}{Sc,Ti,V,Cr,Mn,Fe,Co,Ni}O3

• Perovskites: wide composition range = tunable properties

• Density Functional Theory (with VASP)1,2: obtain Φ for 20 systems (40 surfaces)

• HSE functionals for accurate electronic structure3,4

• (001) surfaces with BO2- and AO- orientations5,6

[1] Y. Wang and J.P. Perdew, Phys. Rev. B 44 (1991).[2] Kresse, G. and J. Furthmuller, Phys. Rev. B, 54 (1996).[3] Franchini, C., J. Phys.: Condens. Matter 26 (2014).

[4] He, J., Franchini, C., Phys. Rev. B, 86 (2012) [5] Kilner, J., Druce, J., et. al., Energy & Env. Sci (2014).[6] Liu, F., Ding, H., et. al., Phys. Chem. Chem. Phys. (2013).

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Work functions of 20 (001)-oriented perovskites

• SrVO3 and LaMnO3 have low AO Φ’s of 1.86 and 1.76 eV, respectively

[1] Suntivich, J., Hong, W. T., Lee, Y.-L., Rondinelli, J. M., Yang, W., Goodenough, J. B., Dabrowski, B., Freeland, J. W., and Shao-Horn, Y. Journal of Physical Chemistry C 118 (2014).

• Band insulators have nearly the same work functions, and have highest values• BO2 Φ’s increase with increased 3d band filling and increased hybridization of 3d and O 2p bands1

• AO Φ’s nearly insensitive to 3d band filling, instead dominated by positive surface dipoles

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Work functions of 20 (001)-oriented perovskites

• AO Φ’s are dominated by positive surface dipoles• Suggests Φ can be lowered if electropositive dopants constrained to surface layer

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SrVO3: a low Φ, stable, conductive material

• SrVO3 is the most promising new emission material, Φ = 1.86 eV• Experiments on powders3,4 and (001) films1 show high conductivity on order of Pt1

and good stability at T > 1000 °C in reducing H2/Ar atmosphere2,3,4

Ba-doped SrVO3 has ultra-low Φ of 1.07 eV, Ba segregates due to larger size, enhances dipole

[1] Engel-Herbert, R., et. al.. Advanced Materials (2013).[2] Hui, S., Petric, A. Solid State Ionics (2001).

• Dope SrVO3 with alkaline metals to investigate if Φ can be lowered further:

[3] Maekawa, T, et. al., Journal of Alloys and Compounds (2006).[4] Nagasawa, H., at. Al., Solid State Communications (1991).

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Ba in SrVO3 is more stable than W, scandate cathodes

• Compare Ba residence lifetime on representative cathode surfaces (Ba Ebind)• SrVO3 : Ba binds more strongly than W and scandate cathodes.• Possibility for ultra low Φ, long lifetime thermionic emitters with SrVO3.

[1] Vlahos, V., Booske, J.H., Morgan, D., Phys. Rev. B. 81 (2010).[2] Jacobs, R. M. , Booske, J. H., Morgan, D., J. Phys. Chem. C (2014)[3] Jacobs, R. M., Booske, J. H., Morgan, D., Advanced Functional Materials (2016)

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Optimizing SrVO3: can we stabilize further?• Materials analysis with Python modules1,2

• Grand potential phase diagram analysis• Examine stability under operating conditions:

(T, P) = (1073 K, 10-10 Torr)

[1] Ong, S. P., et. al. Computational Materials Science (2013)[2] Jain, A., et. al. Applied Physics Letters: Materials (2013)

• Transition metals Cr, Fe, Mn, Mo, Nb and Ta may increase the stability of SrVO3 under operating conditions

• These elements support high oxidation states (give off e-), stabilize under reducing conditions.

Future outlook: Materials design in silico

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Generate thousands of perovskite

compositions and calculate properties

with Density Functional Theory and high-

throughput methods

Work function less than 2 eVHigh bulk stability in vacuum at

1000°C

Stable low work function

surfaces

Elimination of potential compounds. Converge on most promising for

testing

SrVO3

What other potential

compounds exist?

??

Experimental evaluation

New computationally predicted,

experimentally validated material

1000s

100s

<10

10

Summary

• Ba in SrVO3 surface segregates and binds more strongly than in W and scandate cathodes, opening the possibility for very long cathode lifetimes.

• Φ of 20 perovskite systems (40 surfaces) calculated. SrVO3 has pure (Ba doped) Φ of 1.86 eV (1.07 eV).

• Bulk SrVO3 may be further stabilized with other transition metal dopants, such as Mo, Nb, Ta, and Fe

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Summary

• Ba in SrVO3 surface segregates and binds more strongly than in W and scandate cathodes, opening the possibility for very long cathode lifetimes.

• Φ of 20 perovskite systems (40 surfaces) calculated. SrVO3 has pure (Ba doped) Φ of 1.86 eV (1.07 eV).

• Bulk SrVO3 may be further stabilized with other transition metal dopants, such as Mo, Nb, Ta, and Fe

High chemical stability

High emitted current density

Ultra-long lifetimesLower operating temperature

Reduced operational and replacement costs

Low work function

Materials design of novel perovskite cathodes

AcknowledgementCOMPUTATIONAL MATERIALS GROUP

Faculty* Izabela Szlufarska * Dane Morgan

Postdocs* Georgios Bokas * Guangfu Luo * Henry Wu * Jia-Hong Ke * Mahmood Mamivand * Ryan Jacobs * Shipeng Shu * Wei Xie * Yueh-Lin Lee

Graduate Students

* Amy Kaczmarowski * Ao Li * Austin Way* Benjamin Afflerbach

* Chaiyapat Tangpatjaroen * Cheng Liu* Franklin Hobbs * Hao Jiang * Huibin Ke * Hyunseok Ko * Jie Feng * Lei Zhao* Mehrdad Arjmand * Shenzen Xu * Shuxiang Zhou * Tam Mayeshiba * Xing Wang * Yipeng Cao * Zhewen Song

Visiting and Undergraduate Students

* Aren Lorenson * Benjamin Anderson * Haotian Wu

* Jason Maldonis * Josh Perry* Jui-Shen

Chang * Liam Witteman * Tom Vandenberg* Zachary Jensen

We gratefully acknowledge funding from the US Air Force Office of Scientific Research through Grant #FA9550-11-1-

0299, NSF Software Infrastructure for Sustained

Innovation (SI2) award #1148011, and compute

resources of the UW-Madison Center for High Throughput

Computing (CHTC)