Presentation at the IEEE-IVEC conference in 2016 on computational modeling of perovskite work function physics and discovery of low work function materials

1. Doped strontium vanadate:
Computational design of a stable, low
work function material
Ryan Jacobs
(rjacobs3@wisc.edu)
John Booske, Dane Morgan
2016 IEEE-IVEC Meeting
Session 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)

2. Current cathodes suffer from shortcomings
2
• 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]

3. Perovskites: tunable properties, including Φ
3
• 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).

4. Work functions of 20 (001)-oriented perovskites
• SrVO3 and LaMnO3 have low AO Φ’s of 1.86 and 1.76 eV, respectively
4
[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

5. Work functions of 20 (001)-oriented perovskites
5
• AO Φ’s are dominated by positive surface dipoles
• Suggests Φ can be lowered if electropositive dopants constrained to surface layer

6. 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
6
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).

7. Ba in SrVO3 is more stable than W, scandate cathodes
7
• 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)

8. Optimizing SrVO3: can we stabilize further?
8
• 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.

9. Future outlook: Materials design in silico
9
Generate thousands of
perovskite
compositions and
calculate properties
with Density Functional
Theory and high-
throughput methods
High bulk stability
in vacuum at
1000°C
Elimination of potential compounds.
Converge on most promising for
testing
SrVO3
What other
potential
compounds
exist?
??
Experimental
evaluation
New computationally
predicted,
experimentally
validated material

10. Summary
10
• 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

11. Summary
11
• 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 lifetimes
Lower operating temperature
Reduced operational and replacement costs
Low work function
Materials design of novel perovskite cathodes

12. Acknowledgement
COMPUTATIONAL 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)