Polymer diodes require cathodes that do not corrode the polymer but do have
low work function to minimize the electron injection barrier. First-principles
calculations demonstrate that the work function of the (1000) surface of the
compound Ca2N is half an eV lower than that of the elemental metal Ca (2.35 vs.
2.87 eV). Moreover its reactivity is expected to be smaller. This makes Ca2N an
interesting candidate to replace calcium as cathode material for polymer light
emitting diode devices.Comment: 3 pages, 4 figures, accepted by J. Appl. Phy

G. A. de Wijs

M. A. Uijttewaal

Perdew J. P.

R. A. de Groot

English

arXiv

Uijttewaal, M. A.

de Wijs, G. A.

de Groot, R. A.

University of Groningen Research Database

  
 University of Groningen
Low work function of the (1000) Ca2N surface
Uijttewaal, M. A.; de Wijs, G. A. ; de Groot, R. A.
Published in:
Journal of Applied Physics
DOI:
10.1063/1.1767611
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2004
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Uijttewaal, M. A., de Wijs, G. A., & de Groot, R. A. (2004). Low work function of the (1000) Ca2N surface.
Journal of Applied Physics, 96(3), 1751-1753. https://doi.org/10.1063/1.1767611
Copyright
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Download date: 21-05-2019
Low work function of the 1000 Ca2N surface
M. A. Uijttewaal, G. A. de Wijs, and R. A. de Groot
NSRIM, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands
(Received 17 November 2003; accepted 12 May 2004)
Polymer diodes require cathodes that do not corrode the polymer but do have low work function to
minimize the electron injection barrier. First-principles calculations demonstrate that the work
function of the s1000d surface of the compound Ca2N is half an eV lower than that of the elemental
metal Ca (2.35 vs 2.87 eV). Moreover, its reactivity is expected to be smaller. This makes Ca2N an
interesting candidate to replace calcium as cathode material for polymer light emitting diode
devices. © 2004 American Institute of Physics. [DOI: 10.1063/1.1767611]
One of the great challenges for polymer light emitting
diodes is the electron injection barrier as well as the perfor-
mance degradation caused by chemical reactions of the cath-
ode with the polymer.1 State-of-the-art devices2 use
PPV(poly phenylene vinylene) as electro luminescence ma-
terial. Often the cathode is made out of calcium, because of
its low work function3 s2.87 eVd and the presumed align-
ment of its Fermi level with the lowest unoccupied molecular
orbital of PPV (electron affinity 2.73−2.8 eV).4,5 Biases ac-
tually used are much higher than the optical band gap of PPV
s2.4 eVd (Ref. 6), namely, in the order of 10 V,1 which sug-
gests that there is some barrier formation between the cath-
ode and the polymer. This is further confirmed by the obser-
vation that the decrease of performance is much less for
devices in which the cathode is formed at a small residual
oxygen pressure.7 In such a device some oxidation prevents
reaction of the cathode with the polymer.
It would be a major breakthrough when stable, low work
function metals are found. Subnitrides are fascinating candi-
dates because, as in the case of cesium suboxides,8,9 it is
expected that through quantum confinement their work func-
tions are lower than those of the elemental metals while at
the same time ionic bonding reduces reactivity.
The Ca2N crystal10 and electronic structure (bulk and
single slab) (Ref. 11) are known. It is an ionic compound,
built out of slabs of alternating sCa-N-Cad hexagonal layers.
The interslab distance between Ca layers is 3.81 Å and Ca
layers inside a slab are only 2.45 Å apart. This is much
smaller than the ordinary fcc Ca s111d layer distance
s3.18 Åd and can be attributed to the ionic binding. The band
structure shows fully occupied N 2s and N 2p states and a
quasi two-dimensional (2D) free-electron state in the space
between the slabs which further confirms the idea that this is
an ionic compound.
In this paper the Ca2N work function sFd and surface
energy sESd are determined for both the s0001d and the
s1000d surface. An indication of the materials stability is
obtained from its binding energy and degree of surface re-
laxation.
First-principles calculations were carried out based on
density functional theory in the local density
approximation12,13 with generalized gradient corrections.14
The total energy and molecular dynamics program VASP (Vi-
enna Ab initio Simulation Package)15,16 was used which has
the projector augmented wave method17,18 implemented.
Nonlinear core corrections19 were applied for calcium. The
unit cells contained 18 and 42 atoms for the s0001d and the
s1000d surfaces, respectively. The Kohn-Sham orbitals were
expanded in plane waves with cutoffs of 37 Ry. 12312
31 and 131234 Monkhorst-Pack21 k point grids, respec-
tively, were used to sample the Brillouin zones resulting in
74 and 26 k points, respectively, in their irreducible parts.
F is defined as the minimum amount of energy it costs
to extract an electron from a metal. The fact that it depends
on the type of surface is somewhat counter intuitive, it seems
to contradict the conservation of energy. However, the differ-
ences in work function are compensated by differences in
kinetic energy of the electron at large distances.22
In order to determine F, two numbers are needed: the
maximum energy of an electron inside the material, defined
to be the Fermi level sEFd, and the minimum energy outside
the material, the vacuum potential. Because in a finite slab
calculation EF cannot be accurately determined, this number
is first obtained from a bulk calculation. Then for the finite
slab the average potential of the middle layers (which is an
accurate quantity) must be aligned with that of the bulk.23 In
this way the work function for the s0001d surface is deter-
mined using six slabs of Ca-N-Ca and 19 Å of vacuum.24 It
can be extracted from Fig. 1 and equals 3.43 eV.
Another unit cell was made to find F for the s1000d
surface of Ca2N. It contained 14 layers and 20 Å of vacuum.
At this surface several bonds are severed and it is necessary
to relax the atomic positions. The result is shown in Fig. 2
where the view is along the displacement vector from one
slab to the next. The nitrogen atoms at the surface are dis-
placed a little s0.24 Åd inwards, the nitrogens in the second
layer a little s0.17 Åd outwards, while the surface calciums
tend to move in the direction of the surface nitrogens. This
can be understood from an ionic point of view. Nitrogen
favors an environment of high electron density while cal-
cium shows the opposite trend. Although the structure has
changed very little, the effect of the relaxation on F is sub-
stantial (Fig. 3). Before relaxation we find a work function of
2.56 eV, already lower than that of pure calcium, but after
relaxation it has even decreased to 2.35 eV. Not only is this
comparable to the smallest work function for an element (Cs,
JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 3 1 AUGUST 2004
0021-8979/2004/96(3)/1751/3/$20.00 © 2004 American Institute of Physics1751
Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
2.14 eV),3 but also the difference of 1.08 eV with the s0001d
surface is larger than that between any two surfaces of
tungsten,25 which is known for its large work function aniso-
tropy. Both the drop in F after relaxation and the large
surface-anisotropy can be explained by the Smoluchowski
model26 and not by quantum confinement as this is a bulk
model that does not say anything about anisotropies and/or
relaxation. According to Smoluchowski on the other hand,
the more “open” a surface is the more the charge density can
be smoothed, and this lowers F. However, more-open sur-
faces tend to have higher surface energies27 and this makes
them less stable. For the s0001d and (relaxed) s1000d surface
we find ES is 2.7 and 5.3 eV/nm2, respectively, which con-
firms the trend.
Because in Ref. 10 it is suggested Ca2N can also be
made with a larger c axis (interslab separation), we examined
whether this changes the s1000d work function. After enlarg-
ing c with 2% and subsequent relaxation of the atoms, the
structural changes are small and the trends already indicated
are a little more pronounced. The slab thickness, however,
has decreased to almost s+0.5%d its former value. The aver-
aged charge density and local potential are plotted in Fig. 4.
Before relaxation F is decreased from 2.35 to 2.24 eV, but
relaxation increases it again to 2.35 eV. Again this is in ac-
cordance with the Smoluchowski trend because a thicker
slab makes the surface more open. We conclude that the
work function does not (significantly) change when c is en-
larged.
Finally, the reactivity of Ca2N is briefly discussed. In-
spection of the binding energy shows that it is 5.6 eV per
formula unit (with respect to half a nitrogen molecule and
two calcium atoms) for a single slab and 0.6 eV more for the
bulk. This gives evidence that the reactivity of Ca2N is
smaller than that of calcium, of which the binding energy is
just 1.9 eV per atom. It was shown earlier that relaxation of
the atomic positions after bonds are cut or c is enlarged, does
not have a substantial effect on the structure. These facts
taken together give an indication that the reactivity of this
subnitride is less than that of pure calcium, although it is still
air sensitive.10
In summary, using first-principles calculations, we have
FIG. 2. Atomic positions after relaxation of the s1000d surface as seen from
the s1206d direction. Dark circles are nitrogens and light ones calcium.
FIG. 3. Charge densities averaged over s1000d planes (arbitrary units) and
potentials (relative to EF, eV) averaged over s1000d planes and bulk unit
cells, for both the relaxed (solid lines) and the unrelaxed (dashed lines)
structure as function of position sÅd. The potentials converge to 2.35 eV and
2.56 eV in the vacuum, respectively. The horizontal line is the average bulk
potential.
FIG. 4. Charge densities averaged over s1000d planes (arb. units) and po-
tentials (relative to EF, eV) averaged over s1000d planes and bulk unit cells,
for both the relaxed (solid lines) and the unrelaxed (dashed lines) structure
for a 2% larger c axis as function of position sÅd. The potentials converge to
2.35 eV and 2.24 eV in the vacuum, respectively. The horizontal line is the
average bulk potential and it can be seen that relaxation has no observable
effect on the charge density.
FIG. 1. Charge density (dashed line, arb. units) averaged over s0001d planes
and potentials (relative to EF, eV) averaged over s0001d planes and bulk unit
cells (solid/dotted lines, respectively) as function of position sÅd. The hori-
zontal line at −3.83 eV refers to the average bulk potential.
1752 J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de Groot
Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp
shown that the work function of Ca2N has an anisotropy of
more than 1 eV, with a minimum of 2.35 eV (cf. Ca, F
=2.87 eV) for the s1000d surface. Its surface energy
s5.3eV/nm2d, however, is almost the double of that of the
s0001d surface. The low work function does not (signifi-
cantly) depend on the interslab separation and we argued that
this material is not as reactive as calcium. The lower work
function and reduced reactivity make Ca2N a promising
metal to replace Ca in cathodes of polymer light emitting
diodes.
The authors wish to thank Dr. C. M. Fang (TUE) and
Professor Dr. R. Coehoorn (Philips) for helpful discussions.
This work was part of the research programme of the Stich-
ting voor Fundamenteel Onderzoek der Materie (FOM) with
financial support from the Nederlandse Organisatie voor
Wetenschappelijk Onderzoek (NWO).
1J. R. Sheats and D. B. Roitman, Synth. Met. 95, 79 (1998).
2G. G. Andersson, H. H.P. Gommans, A. W. Denier van der Gon, and H. H.
Brongersma, J. Appl. Phys. 93 6, 3299 (2003).
3CRC Handbook of Chemistry and Physics, 77th edn. (CRC, Boca Raton,
FL, 1997).
4I. D. Parker, J. Appl. Phys. 75, 1656 (1994).
5J. L. Brédas and A. J. Heeger, Chem. Phys. Lett. 217, 507 (1994).
6S. Tokito, T. Tsutsui, R. Tanaka, and S. Saito, Jpn. J. Appl. Phys., Part 2
25, L680 (1986).
7J. Birgerson, M. Fahlman, P. Bröms, and W. R. Salaneck, Synth. Met. 80,
125 (1996).
8M. G. Burt and V. Heine, J. Phys. C 11, 961 (1978).
9G. Ebbingaus and A. Simon, Chem. Phys. 43, 117 (1970).
10C. F. Baker, M. G. Barker, and A. J. Blake, Acta Crystallogr. E57, i6
(2001).
11C. M. Fang, G. A. de Wijs, R. A. de Groot, H. T. Hintzen, and G. de With,
Chem. Mater. 12, 1847 (2000).
12P. Hohenberg and W. Kohn, Phys. Rev. B 136, 864 (1964).
13W. Kohn and L. J. Sham, Phys. Rev. A 140, 1163 (1965).
14J. P. Perdew and Y. Wang, Phys. Rev. A 45, 13244 (1992).
15G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993); 49, 14251 (1994).
16G. Kresse and J. Furthmüller, Comput. Mater. Sci. 6, 15 (1996).
17P. E. Blöchl, Phys. Rev. B 50, 17953 (1994).
18G. Kresse and D. Joubert, Phys. Rev. B 59, 1758 (1999).
19S. G. Louie, S. Froyen, and M. L. Cohen, Phys. Rev. B 26, 1738 (1982).
20G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11169 (1996).
21H. J. Monkhorst and J. D. Pack, Phys. Rev. B 13, 5188 (1976).
22C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. Lett. 88, 156802
(2002).
23C. J. Fall, N. Binggeli, and A. Baldereschi, J. Phys.: Condens. Matter 11,
2689 (1999).
24Periodicity is required in all directions. This has the advantage that there is
only one surface and no possible influences from other ones as is the case
in experiments.
25C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. B 65, 045401
(2001).
26R. Smoluchowski, Phys. Rev. 60, 661 (1941).
27ES is defined as the energy of the slab minus the energy of the equivalent
bulk, divided by two times the surface area because a slab has two sur-
faces.
J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de Groot 1753
Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp


Low work function of the (1000) Ca2N surface

Polymer diodes require cathodes that do not corrode the polymer but do have low work function to minimize the electron injection barrier. First-principles calculations demonstrate that the work function of the (1000) surface of the compound Ca2N is half an eV lower than that of the elemental metal Ca (2.35 vs 2.87 eV). Moreover, its reactivity is expected to be smaller. This makes Ca2N an interesting candidate to replace calcium as cathode material for polymer light emitting diode devices

ARTS repository - University of Groningen

   University of GroningenLow work function of the (1000) Ca2N surfaceUijttewaal, M. A.; de Wijs, G. A. ; de Groot, R. A.Published in:Journal of Applied PhysicsDOI:10.1063/1.1767611IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Uijttewaal, M. A., de Wijs, G. A., & de Groot, R. A. (2004). Low work function of the (1000) Ca2N surface.Journal of Applied Physics, 96(3), 1751-1753. https://doi.org/10.1063/1.1767611CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 20-06-2022Low work function of the „1000… Ca2N surfaceM. A. Uijttewaal, G. A. de Wijs, and R. A. de GrootNSRIM, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands(Received 17 November 2003; accepted 12 May 2004)Polymer diodes require cathodes that do not corrode the polymer but do have low work function tominimize the electron injection barrier. First-principles calculations demonstrate that the workfunction of thes1000d surface of the compound Ca2N is half an eV lower than that of the elementalmetal Ca(2.35 vs 2.87 eV). Moreover, its reactivity is expected to be smaller. This makes Ca2N aninteresting candidate to replace calcium as cathode material for polymer light emitting diodedevices. ©2004 American Institute of Physics. [DOI: 10.1063/1.1767611]One of the great challenges for polymer light emittingdiodes is the electron injection barrier as well as the perfor-mance degradation caused by chemical reactions of the cath-ode with the polymer.1 State-of-the-art devices2 usePPV(poly phenylene vinylene) as electro luminescence ma-terial. Often the cathode is made out of calcium, because ofits low work function3 s2.87 eVd and the presumed align-ment of its Fermi level with the lowest unoccupied molecularorbital of PPV(electron affinity 2.73−2.8 eV).4,5 Biases ac-tually used are much higher than the optical band gap of PPVs2.4 eVd (Ref. 6), namely, in the order of 10 V,1 which sug-gests that there is some barrier formation between the cath-ode and the polymer. This is further confirmed by the obser-vation that the decrease of performance is much less fordevices in which the cathode is formed at a small residualoxygen pressure.7 In such a device some oxidation preventsreaction of the cathode with the polymer.It would be a major breakthrough when stable, low workfunction metals are found. Subnitrides are fascinating candi-dates because, as in the case of cesium suboxides,8,9 it isexpected that through quantum confinement their work func-tions are lower than those of the elemental metals while atthe same time ionic bonding reduces reactivity.The Ca2N crystal10 and electronic structure(bulk andsingle slab) (Ref. 11) are known. It is an ionic compound,built out of slabs of alternatingsCa-N-Cad hexagonal layers.The interslab distance between Ca layers is 3.81Å and Calayers inside a slab are only 2.45Å apart. This is muchsmaller than the ordinary fcc Cas111d layer distances3.18Åd and can be attributed to the ionic binding. The bandstructure shows fully occupied N 2s and N 2p states and aquasi two-dimensional(2D) free-electron state in the spacebetween the slabs which further confirms the idea that this isan ionic compound.In this paper the Ca2N work function sFd and surfaceenergy sESd are determined for both thes0001d and thes1000d surface. An indication of the materials stability isobtained from its binding energy and degree of surface re-laxation.First-principles calculations were carried out based ondensity functional theory in the local densityapproximation12,13 with generalized gradient corrections.14The total energy and molecular dynamics programVASP (Vi-ennaAb initio Simulation Package)15,16 was used which hasthe projector augmented wave method17,18 implemented.Nonlinear core corrections19 were applied for calcium. Theunit cells contained 18 and 42 atoms for thes0001d and thes1000d surfaces, respectively. The Kohn-Sham orbitals wereexpanded in plane waves with cutoffs of 37 Ry. 1231231 and 131234 Monkhorst-Pack21 k point grids, respec-tively, were used to sample the Brillouin zones resulting in74 and 26k points, respectively, in their irreducible parts.F is defined as the minimum amount of energy it coststo extract an electron from a metal. The fact that it dependson the type of surface is somewhat counter intuitive, it seemsto contradict the conservation of energy. However, the differ-ences in work function are compensated by differences inkinetic energy of the electron at large distances.22In order to determineF, two numbers are needed: themaximum energy of an electron inside the material, definedto be the Fermi levelsEFd, and the minimum energy outsidethe material, the vacuum potential. Because in a finite slabcalculationEF cannot be accurately determined, this numberis first obtained from a bulk calculation. Then for the finiteslab the average potential of the middle layers(which is anaccurate quantity) must be aligned with that of the bulk.23 Inthis way the work function for thes0001d surface is deter-mined using six slabs of Ca-N-Ca and 19Å of vacuum.24 Itcan be extracted from Fig. 1 and equals 3.43 eV.Another unit cell was made to findF for the s1000dsurface of Ca2N. It contained 14 layers and 20Å of vacuum.At this surface several bonds are severed and it is necessaryto relax the atomic positions. The result is shown in Fig. 2where the view is along the displacement vector from oneslab to the next. The nitrogen atoms at the surface are dis-placed a littles0.24Åd inwards, the nitrogens in the secondlayer a little s0.17Åd outwards, while the surface calciumstend to move in the direction of the surface nitrogens. Thiscan be understood from an ionic point of view. Nitrogenfavors an environment of high electron density while cal-cium shows the opposite trend. Although the structure haschanged very little, the effect of the relaxation onF is sub-stantial(Fig. 3). Before relaxation we find a work function of2.56 eV, already lower than that of pure calcium, but afterrelaxation it has even decreased to 2.35 eV. Not only is thiscomparable to the smallest work function for an element(Cs,JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 3 1 AUGUST 20040021-8979/2004/96(3)/1751/3/$20.00 © 2004 American Institute of Physics1751Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp2.14 eV),3 but also the difference of 1.08 eV with thes0001dsurface is larger than that between any two surfaces oftungsten,25 which is known for its large work function aniso-tropy. Both the drop inF after relaxation and the largesurface-anisotropy can be explained by the Smoluchowskimodel26 and not by quantum confinement as this is a bulkmodel that does not say anything about anisotropies and/orrelaxation. According to Smoluchowski on the other hand,the more “open” a surface is the more the charge density canbe smoothed, and this lowersF. However, more-open sur-faces tend to have higher surface energies27 and this makesthem less stable. For thes0001d and(relaxed) s1000d surfacewe find ES is 2.7 and 5.3 eV/nm2, respectively, which con-firms the trend.Because in Ref. 10 it is suggested Ca2N can also bemade with a largerc axis(interslab separation), we examinedwhether this changes thes1000d work function. After enlarg-ing c with 2% and subsequent relaxation of the atoms, thestructural changes are small and the trends already indicatedare a little more pronounced. The slab thickness, however,has decreased to almosts+0.5%d its former value. The aver-aged charge density and local potential are plotted in Fig. 4.Before relaxationF is decreased from 2.35 to 2.24 eV, butrelaxation increases it again to 2.35 eV. Again this is in ac-cordance with the Smoluchowski trend because a thickerslab makes the surface more open. We conclude that thework function does not(significantly) change whenc is en-larged.Finally, the reactivity of Ca2N is briefly discussed. In-spection of the binding energy shows that it is 5.6 eV performula unit (with respect to half a nitrogen molecule andtwo calcium atoms) for a single slab and 0.6 eV more for thebulk. This gives evidence that the reactivity of Ca2N issmaller than that of calcium, of which the binding energy isjust 1.9 eV per atom. It was shown earlier that relaxation ofthe atomic positions after bonds are cut orc is enlarged, doesnot have a substantial effect on the structure. These factstaken together give an indication that the reactivity of thissubnitride is less than that of pure calcium, although it is stillair sensitive.10In summary, using first-principles calculations, we haveFIG. 2. Atomic positions after relaxation of thes1000d surface as seen fromthe s1206d direction. Dark circles are nitrogens and light ones calcium.FIG. 3. Charge densities averaged overs1000d planes(arbitrary units) andpotentials(relative toEF, eV) averaged overs1000d planes and bulk unitcells, for both the relaxed(solid lines) and the unrelaxed(dashed lines)structure as function of positionsÅd. The potentials converge to 2.35 eV and2.56 eV in the vacuum, respectively. The horizontal line is the average bulkpotential.FIG. 4. Charge densities averaged overs1000d planes(arb. units) and po-tentials(relative toEF, eV) averaged overs1000d planes and bulk unit cells,for both the relaxed(solid lines) and the unrelaxed(dashed lines) structurefor a 2% largerc axis as function of positionsÅd. The potentials converge to2.35 eV and 2.24 eV in the vacuum, respectively. The horizontal line is theaverage bulk potential and it can be seen that relaxation has no observableeffect on the charge density.FIG. 1. Charge density(dashed line, arb. units) averaged overs0001d planesand potentials(relative toEF, eV) averaged overs0001d planes and bulk unitcells (solid/dotted lines, respectively) as function of positionsÅd. The hori-zontal line at −3.83 eV refers to the average bulk potential.1752 J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de GrootDownloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspshown that the work function of Ca2N has an anisotropy ofmore than 1 eV, with a minimum of 2.35 eV(cf. Ca, F=2.87 eV) for the s1000d surface. Its surface energys5.3eV/nm2d, however, is almost the double of that of thes0001d surface. The low work function does not(signifi-cantly) depend on the interslab separation and we argued thatthis material is not as reactive as calcium. The lower workfunction and reduced reactivity make Ca2N a promisingmetal to replace Ca in cathodes of polymer light emittingdiodes.The authors wish to thank Dr. C. M. Fang(TUE) andProfessor Dr. R. Coehoorn(Philips) for helpful discussions.This work was part of the research programme of the Stich-ting voor Fundamenteel Onderzoek der Materie(FOM) withfinancial support from the Nederlandse Organisatie voorWetenschappelijk Onderzoek(NWO).1J. R. Sheats and D. B. Roitman, Synth. Met.95, 79 (1998).2G. G. Andersson, H. H.P. Gommans, A. W. Denier van der Gon, and H. H.Brongersma, J. Appl. Phys.93 6, 3299(2003).3CRC Handbook of Chemistry and Physics, 77th edn.(CRC, Boca Raton,FL, 1997).4I. D. Parker, J. Appl. Phys.75, 1656(1994).5J. L. Brédas and A. J. Heeger, Chem. Phys. Lett.217, 507 (1994).6S. Tokito, T. Tsutsui, R. Tanaka, and S. Saito, Jpn. J. Appl. Phys., Part 225, L680 (1986).7J. Birgerson, M. Fahlman, P. Bröms, and W. R. Salaneck, Synth. Met.80,125 (1996).8M. G. Burt and V. Heine, J. Phys. C11, 961 (1978).9G. Ebbingaus and A. Simon, Chem. Phys.43, 117 (1970).10C. F. Baker, M. G. Barker, and A. J. Blake, Acta Crystallogr.E57, i6(2001).11C. M. Fang, G. A. de Wijs, R. A. de Groot, H. T. Hintzen, and G. de With,Chem. Mater.12, 1847(2000).12P. Hohenberg and W. Kohn, Phys. Rev. B136, 864 (1964).13W. Kohn and L. J. Sham, Phys. Rev. A140, 1163(1965).14J. P. Perdew and Y. Wang, Phys. Rev. A45, 13244(1992).15G. Kresse and J. Hafner, Phys. Rev. B47, 558(1993); 49, 14251(1994).16G. Kresse and J. Furthmüller, Comput. Mater. Sci.6, 15 (1996).17P. E. Blöchl, Phys. Rev. B50, 17953(1994).18G. Kresse and D. Joubert, Phys. Rev. B59, 1758(1999).19S. G. Louie, S. Froyen, and M. L. Cohen, Phys. Rev. B26, 1738(1982).20G. Kresse and J. Furthmüller, Phys. Rev. B54, 11169(1996).21H. J. Monkhorst and J. D. Pack, Phys. Rev. B13, 5188(1976).22C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. Lett.88, 156802(2002).23C. J. Fall, N. Binggeli, and A. Baldereschi, J. Phys.: Condens. Matter11,2689 (1999).24Periodicity is required in all directions. This has the advantage that there isonly one surface and no possible influences from other ones as is the casein experiments.25C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. B65, 045401(2001).26R. Smoluchowski, Phys. Rev.60, 661 (1941).27ES is defined as the energy of the slab minus the energy of the equivalentbulk, divided by two times the surface area because a slab has two sur-faces.J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de Groot 1753Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

NARCIS 

   University of GroningenLow work function of the (1000) Ca2N surfaceUijttewaal, M. A.; de Wijs, G. A. ; de Groot, R. A.Published in:Journal of Applied PhysicsDOI:10.1063/1.1767611IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Uijttewaal, M. A., de Wijs, G. A., & de Groot, R. A. (2004). Low work function of the (1000) Ca2N surface.Journal of Applied Physics, 96(3), 1751-1753. https://doi.org/10.1063/1.1767611CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 01-11-2023Low work function of the „1000… Ca2N surfaceM. A. Uijttewaal, G. A. de Wijs, and R. A. de GrootNSRIM, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands(Received 17 November 2003; accepted 12 May 2004)Polymer diodes require cathodes that do not corrode the polymer but do have low work function tominimize the electron injection barrier. First-principles calculations demonstrate that the workfunction of thes1000d surface of the compound Ca2N is half an eV lower than that of the elementalmetal Ca(2.35 vs 2.87 eV). Moreover, its reactivity is expected to be smaller. This makes Ca2N aninteresting candidate to replace calcium as cathode material for polymer light emitting diodedevices. ©2004 American Institute of Physics. [DOI: 10.1063/1.1767611]One of the great challenges for polymer light emittingdiodes is the electron injection barrier as well as the perfor-mance degradation caused by chemical reactions of the cath-ode with the polymer.1 State-of-the-art devices2 usePPV(poly phenylene vinylene) as electro luminescence ma-terial. Often the cathode is made out of calcium, because ofits low work function3 s2.87 eVd and the presumed align-ment of its Fermi level with the lowest unoccupied molecularorbital of PPV(electron affinity 2.73−2.8 eV).4,5 Biases ac-tually used are much higher than the optical band gap of PPVs2.4 eVd (Ref. 6), namely, in the order of 10 V,1 which sug-gests that there is some barrier formation between the cath-ode and the polymer. This is further confirmed by the obser-vation that the decrease of performance is much less fordevices in which the cathode is formed at a small residualoxygen pressure.7 In such a device some oxidation preventsreaction of the cathode with the polymer.It would be a major breakthrough when stable, low workfunction metals are found. Subnitrides are fascinating candi-dates because, as in the case of cesium suboxides,8,9 it isexpected that through quantum confinement their work func-tions are lower than those of the elemental metals while atthe same time ionic bonding reduces reactivity.The Ca2N crystal10 and electronic structure(bulk andsingle slab) (Ref. 11) are known. It is an ionic compound,built out of slabs of alternatingsCa-N-Cad hexagonal layers.The interslab distance between Ca layers is 3.81Å and Calayers inside a slab are only 2.45Å apart. This is muchsmaller than the ordinary fcc Cas111d layer distances3.18Åd and can be attributed to the ionic binding. The bandstructure shows fully occupied N 2s and N 2p states and aquasi two-dimensional(2D) free-electron state in the spacebetween the slabs which further confirms the idea that this isan ionic compound.In this paper the Ca2N work function sFd and surfaceenergy sESd are determined for both thes0001d and thes1000d surface. An indication of the materials stability isobtained from its binding energy and degree of surface re-laxation.First-principles calculations were carried out based ondensity functional theory in the local densityapproximation12,13 with generalized gradient corrections.14The total energy and molecular dynamics programVASP (Vi-ennaAb initio Simulation Package)15,16 was used which hasthe projector augmented wave method17,18 implemented.Nonlinear core corrections19 were applied for calcium. Theunit cells contained 18 and 42 atoms for thes0001d and thes1000d surfaces, respectively. The Kohn-Sham orbitals wereexpanded in plane waves with cutoffs of 37 Ry. 1231231 and 131234 Monkhorst-Pack21 k point grids, respec-tively, were used to sample the Brillouin zones resulting in74 and 26k points, respectively, in their irreducible parts.F is defined as the minimum amount of energy it coststo extract an electron from a metal. The fact that it dependson the type of surface is somewhat counter intuitive, it seemsto contradict the conservation of energy. However, the differ-ences in work function are compensated by differences inkinetic energy of the electron at large distances.22In order to determineF, two numbers are needed: themaximum energy of an electron inside the material, definedto be the Fermi levelsEFd, and the minimum energy outsidethe material, the vacuum potential. Because in a finite slabcalculationEF cannot be accurately determined, this numberis first obtained from a bulk calculation. Then for the finiteslab the average potential of the middle layers(which is anaccurate quantity) must be aligned with that of the bulk.23 Inthis way the work function for thes0001d surface is deter-mined using six slabs of Ca-N-Ca and 19Å of vacuum.24 Itcan be extracted from Fig. 1 and equals 3.43 eV.Another unit cell was made to findF for the s1000dsurface of Ca2N. It contained 14 layers and 20Å of vacuum.At this surface several bonds are severed and it is necessaryto relax the atomic positions. The result is shown in Fig. 2where the view is along the displacement vector from oneslab to the next. The nitrogen atoms at the surface are dis-placed a littles0.24Åd inwards, the nitrogens in the secondlayer a little s0.17Åd outwards, while the surface calciumstend to move in the direction of the surface nitrogens. Thiscan be understood from an ionic point of view. Nitrogenfavors an environment of high electron density while cal-cium shows the opposite trend. Although the structure haschanged very little, the effect of the relaxation onF is sub-stantial(Fig. 3). Before relaxation we find a work function of2.56 eV, already lower than that of pure calcium, but afterrelaxation it has even decreased to 2.35 eV. Not only is thiscomparable to the smallest work function for an element(Cs,JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 3 1 AUGUST 20040021-8979/2004/96(3)/1751/3/$20.00 © 2004 American Institute of Physics1751Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp2.14 eV),3 but also the difference of 1.08 eV with thes0001dsurface is larger than that between any two surfaces oftungsten,25 which is known for its large work function aniso-tropy. Both the drop inF after relaxation and the largesurface-anisotropy can be explained by the Smoluchowskimodel26 and not by quantum confinement as this is a bulkmodel that does not say anything about anisotropies and/orrelaxation. According to Smoluchowski on the other hand,the more “open” a surface is the more the charge density canbe smoothed, and this lowersF. However, more-open sur-faces tend to have higher surface energies27 and this makesthem less stable. For thes0001d and(relaxed) s1000d surfacewe find ES is 2.7 and 5.3 eV/nm2, respectively, which con-firms the trend.Because in Ref. 10 it is suggested Ca2N can also bemade with a largerc axis(interslab separation), we examinedwhether this changes thes1000d work function. After enlarg-ing c with 2% and subsequent relaxation of the atoms, thestructural changes are small and the trends already indicatedare a little more pronounced. The slab thickness, however,has decreased to almosts+0.5%d its former value. The aver-aged charge density and local potential are plotted in Fig. 4.Before relaxationF is decreased from 2.35 to 2.24 eV, butrelaxation increases it again to 2.35 eV. Again this is in ac-cordance with the Smoluchowski trend because a thickerslab makes the surface more open. We conclude that thework function does not(significantly) change whenc is en-larged.Finally, the reactivity of Ca2N is briefly discussed. In-spection of the binding energy shows that it is 5.6 eV performula unit (with respect to half a nitrogen molecule andtwo calcium atoms) for a single slab and 0.6 eV more for thebulk. This gives evidence that the reactivity of Ca2N issmaller than that of calcium, of which the binding energy isjust 1.9 eV per atom. It was shown earlier that relaxation ofthe atomic positions after bonds are cut orc is enlarged, doesnot have a substantial effect on the structure. These factstaken together give an indication that the reactivity of thissubnitride is less than that of pure calcium, although it is stillair sensitive.10In summary, using first-principles calculations, we haveFIG. 2. Atomic positions after relaxation of thes1000d surface as seen fromthe s1206d direction. Dark circles are nitrogens and light ones calcium.FIG. 3. Charge densities averaged overs1000d planes(arbitrary units) andpotentials(relative toEF, eV) averaged overs1000d planes and bulk unitcells, for both the relaxed(solid lines) and the unrelaxed(dashed lines)structure as function of positionsÅd. The potentials converge to 2.35 eV and2.56 eV in the vacuum, respectively. The horizontal line is the average bulkpotential.FIG. 4. Charge densities averaged overs1000d planes(arb. units) and po-tentials(relative toEF, eV) averaged overs1000d planes and bulk unit cells,for both the relaxed(solid lines) and the unrelaxed(dashed lines) structurefor a 2% largerc axis as function of positionsÅd. The potentials converge to2.35 eV and 2.24 eV in the vacuum, respectively. The horizontal line is theaverage bulk potential and it can be seen that relaxation has no observableeffect on the charge density.FIG. 1. Charge density(dashed line, arb. units) averaged overs0001d planesand potentials(relative toEF, eV) averaged overs0001d planes and bulk unitcells (solid/dotted lines, respectively) as function of positionsÅd. The hori-zontal line at −3.83 eV refers to the average bulk potential.1752 J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de GrootDownloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspshown that the work function of Ca2N has an anisotropy ofmore than 1 eV, with a minimum of 2.35 eV(cf. Ca, F=2.87 eV) for the s1000d surface. Its surface energys5.3eV/nm2d, however, is almost the double of that of thes0001d surface. The low work function does not(signifi-cantly) depend on the interslab separation and we argued thatthis material is not as reactive as calcium. The lower workfunction and reduced reactivity make Ca2N a promisingmetal to replace Ca in cathodes of polymer light emittingdiodes.The authors wish to thank Dr. C. M. Fang(TUE) andProfessor Dr. R. Coehoorn(Philips) for helpful discussions.This work was part of the research programme of the Stich-ting voor Fundamenteel Onderzoek der Materie(FOM) withfinancial support from the Nederlandse Organisatie voorWetenschappelijk Onderzoek(NWO).1J. R. Sheats and D. B. Roitman, Synth. Met.95, 79 (1998).2G. G. Andersson, H. H.P. Gommans, A. W. Denier van der Gon, and H. H.Brongersma, J. Appl. Phys.93 6, 3299(2003).3CRC Handbook of Chemistry and Physics, 77th edn.(CRC, Boca Raton,FL, 1997).4I. D. Parker, J. Appl. Phys.75, 1656(1994).5J. L. Brédas and A. J. Heeger, Chem. Phys. Lett.217, 507 (1994).6S. Tokito, T. Tsutsui, R. Tanaka, and S. Saito, Jpn. J. Appl. Phys., Part 225, L680 (1986).7J. Birgerson, M. Fahlman, P. Bröms, and W. R. Salaneck, Synth. Met.80,125 (1996).8M. G. Burt and V. Heine, J. Phys. C11, 961 (1978).9G. Ebbingaus and A. Simon, Chem. Phys.43, 117 (1970).10C. F. Baker, M. G. Barker, and A. J. Blake, Acta Crystallogr.E57, i6(2001).11C. M. Fang, G. A. de Wijs, R. A. de Groot, H. T. Hintzen, and G. de With,Chem. Mater.12, 1847(2000).12P. Hohenberg and W. Kohn, Phys. Rev. B136, 864 (1964).13W. Kohn and L. J. Sham, Phys. Rev. A140, 1163(1965).14J. P. Perdew and Y. Wang, Phys. Rev. A45, 13244(1992).15G. Kresse and J. Hafner, Phys. Rev. B47, 558(1993); 49, 14251(1994).16G. Kresse and J. Furthmüller, Comput. Mater. Sci.6, 15 (1996).17P. E. Blöchl, Phys. Rev. B50, 17953(1994).18G. Kresse and D. Joubert, Phys. Rev. B59, 1758(1999).19S. G. Louie, S. Froyen, and M. L. Cohen, Phys. Rev. B26, 1738(1982).20G. Kresse and J. Furthmüller, Phys. Rev. B54, 11169(1996).21H. J. Monkhorst and J. D. Pack, Phys. Rev. B13, 5188(1976).22C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. Lett.88, 156802(2002).23C. J. Fall, N. Binggeli, and A. Baldereschi, J. Phys.: Condens. Matter11,2689 (1999).24Periodicity is required in all directions. This has the advantage that there isonly one surface and no possible influences from other ones as is the casein experiments.25C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. B65, 045401(2001).26R. Smoluchowski, Phys. Rev.60, 661 (1941).27ES is defined as the energy of the slab minus the energy of the equivalentbulk, divided by two times the surface area because a slab has two sur-faces.J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de Groot 1753Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

University of GroningenLow work function of the (1000) Ca2N surfaceUijttewaal, M. A.; de Wijs, G. A. ; de Groot, R. A.Published in:Journal of Applied PhysicsDOI:10.1063/1.1767611IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Uijttewaal, M. A., de Wijs, G. A., & de Groot, R. A. (2004). Low work function of the (1000) Ca2N surface.Journal of Applied Physics, 96(3), 1751-1753. https://doi.org/10.1063/1.1767611CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 23-12-2025Low work function of the „1000… Ca2N surfaceM. A. Uijttewaal, G. A. de Wijs, and R. A. de GrootNSRIM, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands(Received 17 November 2003; accepted 12 May 2004)Polymer diodes require cathodes that do not corrode the polymer but do have low work function tominimize the electron injection barrier. First-principles calculations demonstrate that the workfunction of thes1000d surface of the compound Ca2N is half an eV lower than that of the elementalmetal Ca(2.35 vs 2.87 eV). Moreover, its reactivity is expected to be smaller. This makes Ca2N aninteresting candidate to replace calcium as cathode material for polymer light emitting diodedevices. ©2004 American Institute of Physics. [DOI: 10.1063/1.1767611]One of the great challenges for polymer light emittingdiodes is the electron injection barrier as well as the perfor-mance degradation caused by chemical reactions of the cath-ode with the polymer.1 State-of-the-art devices2 usePPV(poly phenylene vinylene) as electro luminescence ma-terial. Often the cathode is made out of calcium, because ofits low work function3 s2.87 eVd and the presumed align-ment of its Fermi level with the lowest unoccupied molecularorbital of PPV(electron affinity 2.73−2.8 eV).4,5 Biases ac-tually used are much higher than the optical band gap of PPVs2.4 eVd (Ref. 6), namely, in the order of 10 V,1 which sug-gests that there is some barrier formation between the cath-ode and the polymer. This is further confirmed by the obser-vation that the decrease of performance is much less fordevices in which the cathode is formed at a small residualoxygen pressure.7 In such a device some oxidation preventsreaction of the cathode with the polymer.It would be a major breakthrough when stable, low workfunction metals are found. Subnitrides are fascinating candi-dates because, as in the case of cesium suboxides,8,9 it isexpected that through quantum confinement their work func-tions are lower than those of the elemental metals while atthe same time ionic bonding reduces reactivity.The Ca2N crystal10 and electronic structure(bulk andsingle slab) (Ref. 11) are known. It is an ionic compound,built out of slabs of alternatingsCa-N-Cad hexagonal layers.The interslab distance between Ca layers is 3.81Å and Calayers inside a slab are only 2.45Å apart. This is muchsmaller than the ordinary fcc Cas111d layer distances3.18Åd and can be attributed to the ionic binding. The bandstructure shows fully occupied N 2s and N 2p states and aquasi two-dimensional(2D) free-electron state in the spacebetween the slabs which further confirms the idea that this isan ionic compound.In this paper the Ca2N work function sFd and surfaceenergy sESd are determined for both thes0001d and thes1000d surface. An indication of the materials stability isobtained from its binding energy and degree of surface re-laxation.First-principles calculations were carried out based ondensity functional theory in the local densityapproximation12,13 with generalized gradient corrections.14The total energy and molecular dynamics programVASP (Vi-ennaAb initio Simulation Package)15,16 was used which hasthe projector augmented wave method17,18 implemented.Nonlinear core corrections19 were applied for calcium. Theunit cells contained 18 and 42 atoms for thes0001d and thes1000d surfaces, respectively. The Kohn-Sham orbitals wereexpanded in plane waves with cutoffs of 37 Ry. 1231231 and 131234 Monkhorst-Pack21 k point grids, respec-tively, were used to sample the Brillouin zones resulting in74 and 26k points, respectively, in their irreducible parts.F is defined as the minimum amount of energy it coststo extract an electron from a metal. The fact that it dependson the type of surface is somewhat counter intuitive, it seemsto contradict the conservation of energy. However, the differ-ences in work function are compensated by differences inkinetic energy of the electron at large distances.22In order to determineF, two numbers are needed: themaximum energy of an electron inside the material, definedto be the Fermi levelsEFd, and the minimum energy outsidethe material, the vacuum potential. Because in a finite slabcalculationEF cannot be accurately determined, this numberis first obtained from a bulk calculation. Then for the finiteslab the average potential of the middle layers(which is anaccurate quantity) must be aligned with that of the bulk.23 Inthis way the work function for thes0001d surface is deter-mined using six slabs of Ca-N-Ca and 19Å of vacuum.24 Itcan be extracted from Fig. 1 and equals 3.43 eV.Another unit cell was made to findF for the s1000dsurface of Ca2N. It contained 14 layers and 20Å of vacuum.At this surface several bonds are severed and it is necessaryto relax the atomic positions. The result is shown in Fig. 2where the view is along the displacement vector from oneslab to the next. The nitrogen atoms at the surface are dis-placed a littles0.24Åd inwards, the nitrogens in the secondlayer a little s0.17Åd outwards, while the surface calciumstend to move in the direction of the surface nitrogens. Thiscan be understood from an ionic point of view. Nitrogenfavors an environment of high electron density while cal-cium shows the opposite trend. Although the structure haschanged very little, the effect of the relaxation onF is sub-stantial(Fig. 3). Before relaxation we find a work function of2.56 eV, already lower than that of pure calcium, but afterrelaxation it has even decreased to 2.35 eV. Not only is thiscomparable to the smallest work function for an element(Cs,JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 3 1 AUGUST 20040021-8979/2004/96(3)/1751/3/$20.00 © 2004 American Institute of Physics1751Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp2.14 eV),3 but also the difference of 1.08 eV with thes0001dsurface is larger than that between any two surfaces oftungsten,25 which is known for its large work function aniso-tropy. Both the drop inF after relaxation and the largesurface-anisotropy can be explained by the Smoluchowskimodel26 and not by quantum confinement as this is a bulkmodel that does not say anything about anisotropies and/orrelaxation. According to Smoluchowski on the other hand,the more “open” a surface is the more the charge density canbe smoothed, and this lowersF. However, more-open sur-faces tend to have higher surface energies27 and this makesthem less stable. For thes0001d and(relaxed) s1000d surfacewe find ES is 2.7 and 5.3 eV/nm2, respectively, which con-firms the trend.Because in Ref. 10 it is suggested Ca2N can also bemade with a largerc axis(interslab separation), we examinedwhether this changes thes1000d work function. After enlarg-ing c with 2% and subsequent relaxation of the atoms, thestructural changes are small and the trends already indicatedare a little more pronounced. The slab thickness, however,has decreased to almosts+0.5%d its former value. The aver-aged charge density and local potential are plotted in Fig. 4.Before relaxationF is decreased from 2.35 to 2.24 eV, butrelaxation increases it again to 2.35 eV. Again this is in ac-cordance with the Smoluchowski trend because a thickerslab makes the surface more open. We conclude that thework function does not(significantly) change whenc is en-larged.Finally, the reactivity of Ca2N is briefly discussed. In-spection of the binding energy shows that it is 5.6 eV performula unit (with respect to half a nitrogen molecule andtwo calcium atoms) for a single slab and 0.6 eV more for thebulk. This gives evidence that the reactivity of Ca2N issmaller than that of calcium, of which the binding energy isjust 1.9 eV per atom. It was shown earlier that relaxation ofthe atomic positions after bonds are cut orc is enlarged, doesnot have a substantial effect on the structure. These factstaken together give an indication that the reactivity of thissubnitride is less than that of pure calcium, although it is stillair sensitive.10In summary, using first-principles calculations, we haveFIG. 2. Atomic positions after relaxation of thes1000d surface as seen fromthe s1206d direction. Dark circles are nitrogens and light ones calcium.FIG. 3. Charge densities averaged overs1000d planes(arbitrary units) andpotentials(relative toEF, eV) averaged overs1000d planes and bulk unitcells, for both the relaxed(solid lines) and the unrelaxed(dashed lines)structure as function of positionsÅd. The potentials converge to 2.35 eV and2.56 eV in the vacuum, respectively. The horizontal line is the average bulkpotential.FIG. 4. Charge densities averaged overs1000d planes(arb. units) and po-tentials(relative toEF, eV) averaged overs1000d planes and bulk unit cells,for both the relaxed(solid lines) and the unrelaxed(dashed lines) structurefor a 2% largerc axis as function of positionsÅd. The potentials converge to2.35 eV and 2.24 eV in the vacuum, respectively. The horizontal line is theaverage bulk potential and it can be seen that relaxation has no observableeffect on the charge density.FIG. 1. Charge density(dashed line, arb. units) averaged overs0001d planesand potentials(relative toEF, eV) averaged overs0001d planes and bulk unitcells (solid/dotted lines, respectively) as function of positionsÅd. The hori-zontal line at −3.83 eV refers to the average bulk potential.1752 J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de GrootDownloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspshown that the work function of Ca2N has an anisotropy ofmore than 1 eV, with a minimum of 2.35 eV(cf. Ca, F=2.87 eV) for the s1000d surface. Its surface energys5.3eV/nm2d, however, is almost the double of that of thes0001d surface. The low work function does not(signifi-cantly) depend on the interslab separation and we argued thatthis material is not as reactive as calcium. The lower workfunction and reduced reactivity make Ca2N a promisingmetal to replace Ca in cathodes of polymer light emittingdiodes.The authors wish to thank Dr. C. M. Fang(TUE) andProfessor Dr. R. Coehoorn(Philips) for helpful discussions.This work was part of the research programme of the Stich-ting voor Fundamenteel Onderzoek der Materie(FOM) withfinancial support from the Nederlandse Organisatie voorWetenschappelijk Onderzoek(NWO).1J. R. Sheats and D. B. Roitman, Synth. Met.95, 79 (1998).2G. G. Andersson, H. H.P. Gommans, A. W. Denier van der Gon, and H. H.Brongersma, J. Appl. Phys.93 6, 3299(2003).3CRC Handbook of Chemistry and Physics, 77th edn.(CRC, Boca Raton,FL, 1997).4I. D. Parker, J. Appl. Phys.75, 1656(1994).5J. L. Brédas and A. J. Heeger, Chem. Phys. Lett.217, 507 (1994).6S. Tokito, T. Tsutsui, R. Tanaka, and S. Saito, Jpn. J. Appl. Phys., Part 225, L680 (1986).7J. Birgerson, M. Fahlman, P. Bröms, and W. R. Salaneck, Synth. Met.80,125 (1996).8M. G. Burt and V. Heine, J. Phys. C11, 961 (1978).9G. Ebbingaus and A. Simon, Chem. Phys.43, 117 (1970).10C. F. Baker, M. G. Barker, and A. J. Blake, Acta Crystallogr.E57, i6(2001).11C. M. Fang, G. A. de Wijs, R. A. de Groot, H. T. Hintzen, and G. de With,Chem. Mater.12, 1847(2000).12P. Hohenberg and W. Kohn, Phys. Rev. B136, 864 (1964).13W. Kohn and L. J. Sham, Phys. Rev. A140, 1163(1965).14J. P. Perdew and Y. Wang, Phys. Rev. A45, 13244(1992).15G. Kresse and J. Hafner, Phys. Rev. B47, 558(1993); 49, 14251(1994).16G. Kresse and J. Furthmüller, Comput. Mater. Sci.6, 15 (1996).17P. E. Blöchl, Phys. Rev. B50, 17953(1994).18G. Kresse and D. Joubert, Phys. Rev. B59, 1758(1999).19S. G. Louie, S. Froyen, and M. L. Cohen, Phys. Rev. B26, 1738(1982).20G. Kresse and J. Furthmüller, Phys. Rev. B54, 11169(1996).21H. J. Monkhorst and J. D. Pack, Phys. Rev. B13, 5188(1976).22C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. Lett.88, 156802(2002).23C. J. Fall, N. Binggeli, and A. Baldereschi, J. Phys.: Condens. Matter11,2689 (1999).24Periodicity is required in all directions. This has the advantage that there isonly one surface and no possible influences from other ones as is the casein experiments.25C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. B65, 045401(2001).26R. Smoluchowski, Phys. Rev.60, 661 (1941).27ES is defined as the energy of the slab minus the energy of the equivalentbulk, divided by two times the surface area because a slab has two sur-faces.J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de Groot 1753Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

University of Groningen

University of GroningenLow work function of the (1000) Ca2N surfaceUijttewaal, M. A.; de Wijs, G. A. ; de Groot, R. A.Published in:Journal of Applied PhysicsDOI:10.1063/1.1767611IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Uijttewaal, M. A., de Wijs, G. A., & de Groot, R. A. (2004). Low work function of the (1000) Ca2N surface.Journal of Applied Physics, 96(3), 1751-1753. https://doi.org/10.1063/1.1767611CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 30-12-2025Low work function of the „1000… Ca2N surfaceM. A. Uijttewaal, G. A. de Wijs, and R. A. de GrootNSRIM, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands(Received 17 November 2003; accepted 12 May 2004)Polymer diodes require cathodes that do not corrode the polymer but do have low work function tominimize the electron injection barrier. First-principles calculations demonstrate that the workfunction of thes1000d surface of the compound Ca2N is half an eV lower than that of the elementalmetal Ca(2.35 vs 2.87 eV). Moreover, its reactivity is expected to be smaller. This makes Ca2N aninteresting candidate to replace calcium as cathode material for polymer light emitting diodedevices. ©2004 American Institute of Physics. [DOI: 10.1063/1.1767611]One of the great challenges for polymer light emittingdiodes is the electron injection barrier as well as the perfor-mance degradation caused by chemical reactions of the cath-ode with the polymer.1 State-of-the-art devices2 usePPV(poly phenylene vinylene) as electro luminescence ma-terial. Often the cathode is made out of calcium, because ofits low work function3 s2.87 eVd and the presumed align-ment of its Fermi level with the lowest unoccupied molecularorbital of PPV(electron affinity 2.73−2.8 eV).4,5 Biases ac-tually used are much higher than the optical band gap of PPVs2.4 eVd (Ref. 6), namely, in the order of 10 V,1 which sug-gests that there is some barrier formation between the cath-ode and the polymer. This is further confirmed by the obser-vation that the decrease of performance is much less fordevices in which the cathode is formed at a small residualoxygen pressure.7 In such a device some oxidation preventsreaction of the cathode with the polymer.It would be a major breakthrough when stable, low workfunction metals are found. Subnitrides are fascinating candi-dates because, as in the case of cesium suboxides,8,9 it isexpected that through quantum confinement their work func-tions are lower than those of the elemental metals while atthe same time ionic bonding reduces reactivity.The Ca2N crystal10 and electronic structure(bulk andsingle slab) (Ref. 11) are known. It is an ionic compound,built out of slabs of alternatingsCa-N-Cad hexagonal layers.The interslab distance between Ca layers is 3.81Å and Calayers inside a slab are only 2.45Å apart. This is muchsmaller than the ordinary fcc Cas111d layer distances3.18Åd and can be attributed to the ionic binding. The bandstructure shows fully occupied N 2s and N 2p states and aquasi two-dimensional(2D) free-electron state in the spacebetween the slabs which further confirms the idea that this isan ionic compound.In this paper the Ca2N work function sFd and surfaceenergy sESd are determined for both thes0001d and thes1000d surface. An indication of the materials stability isobtained from its binding energy and degree of surface re-laxation.First-principles calculations were carried out based ondensity functional theory in the local densityapproximation12,13 with generalized gradient corrections.14The total energy and molecular dynamics programVASP (Vi-ennaAb initio Simulation Package)15,16 was used which hasthe projector augmented wave method17,18 implemented.Nonlinear core corrections19 were applied for calcium. Theunit cells contained 18 and 42 atoms for thes0001d and thes1000d surfaces, respectively. The Kohn-Sham orbitals wereexpanded in plane waves with cutoffs of 37 Ry. 1231231 and 131234 Monkhorst-Pack21 k point grids, respec-tively, were used to sample the Brillouin zones resulting in74 and 26k points, respectively, in their irreducible parts.F is defined as the minimum amount of energy it coststo extract an electron from a metal. The fact that it dependson the type of surface is somewhat counter intuitive, it seemsto contradict the conservation of energy. However, the differ-ences in work function are compensated by differences inkinetic energy of the electron at large distances.22In order to determineF, two numbers are needed: themaximum energy of an electron inside the material, definedto be the Fermi levelsEFd, and the minimum energy outsidethe material, the vacuum potential. Because in a finite slabcalculationEF cannot be accurately determined, this numberis first obtained from a bulk calculation. Then for the finiteslab the average potential of the middle layers(which is anaccurate quantity) must be aligned with that of the bulk.23 Inthis way the work function for thes0001d surface is deter-mined using six slabs of Ca-N-Ca and 19Å of vacuum.24 Itcan be extracted from Fig. 1 and equals 3.43 eV.Another unit cell was made to findF for the s1000dsurface of Ca2N. It contained 14 layers and 20Å of vacuum.At this surface several bonds are severed and it is necessaryto relax the atomic positions. The result is shown in Fig. 2where the view is along the displacement vector from oneslab to the next. The nitrogen atoms at the surface are dis-placed a littles0.24Åd inwards, the nitrogens in the secondlayer a little s0.17Åd outwards, while the surface calciumstend to move in the direction of the surface nitrogens. Thiscan be understood from an ionic point of view. Nitrogenfavors an environment of high electron density while cal-cium shows the opposite trend. Although the structure haschanged very little, the effect of the relaxation onF is sub-stantial(Fig. 3). Before relaxation we find a work function of2.56 eV, already lower than that of pure calcium, but afterrelaxation it has even decreased to 2.35 eV. Not only is thiscomparable to the smallest work function for an element(Cs,JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 3 1 AUGUST 20040021-8979/2004/96(3)/1751/3/$20.00 © 2004 American Institute of Physics1751Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp2.14 eV),3 but also the difference of 1.08 eV with thes0001dsurface is larger than that between any two surfaces oftungsten,25 which is known for its large work function aniso-tropy. Both the drop inF after relaxation and the largesurface-anisotropy can be explained by the Smoluchowskimodel26 and not by quantum confinement as this is a bulkmodel that does not say anything about anisotropies and/orrelaxation. According to Smoluchowski on the other hand,the more “open” a surface is the more the charge density canbe smoothed, and this lowersF. However, more-open sur-faces tend to have higher surface energies27 and this makesthem less stable. For thes0001d and(relaxed) s1000d surfacewe find ES is 2.7 and 5.3 eV/nm2, respectively, which con-firms the trend.Because in Ref. 10 it is suggested Ca2N can also bemade with a largerc axis(interslab separation), we examinedwhether this changes thes1000d work function. After enlarg-ing c with 2% and subsequent relaxation of the atoms, thestructural changes are small and the trends already indicatedare a little more pronounced. The slab thickness, however,has decreased to almosts+0.5%d its former value. The aver-aged charge density and local potential are plotted in Fig. 4.Before relaxationF is decreased from 2.35 to 2.24 eV, butrelaxation increases it again to 2.35 eV. Again this is in ac-cordance with the Smoluchowski trend because a thickerslab makes the surface more open. We conclude that thework function does not(significantly) change whenc is en-larged.Finally, the reactivity of Ca2N is briefly discussed. In-spection of the binding energy shows that it is 5.6 eV performula unit (with respect to half a nitrogen molecule andtwo calcium atoms) for a single slab and 0.6 eV more for thebulk. This gives evidence that the reactivity of Ca2N issmaller than that of calcium, of which the binding energy isjust 1.9 eV per atom. It was shown earlier that relaxation ofthe atomic positions after bonds are cut orc is enlarged, doesnot have a substantial effect on the structure. These factstaken together give an indication that the reactivity of thissubnitride is less than that of pure calcium, although it is stillair sensitive.10In summary, using first-principles calculations, we haveFIG. 2. Atomic positions after relaxation of thes1000d surface as seen fromthe s1206d direction. Dark circles are nitrogens and light ones calcium.FIG. 3. Charge densities averaged overs1000d planes(arbitrary units) andpotentials(relative toEF, eV) averaged overs1000d planes and bulk unitcells, for both the relaxed(solid lines) and the unrelaxed(dashed lines)structure as function of positionsÅd. The potentials converge to 2.35 eV and2.56 eV in the vacuum, respectively. The horizontal line is the average bulkpotential.FIG. 4. Charge densities averaged overs1000d planes(arb. units) and po-tentials(relative toEF, eV) averaged overs1000d planes and bulk unit cells,for both the relaxed(solid lines) and the unrelaxed(dashed lines) structurefor a 2% largerc axis as function of positionsÅd. The potentials converge to2.35 eV and 2.24 eV in the vacuum, respectively. The horizontal line is theaverage bulk potential and it can be seen that relaxation has no observableeffect on the charge density.FIG. 1. Charge density(dashed line, arb. units) averaged overs0001d planesand potentials(relative toEF, eV) averaged overs0001d planes and bulk unitcells (solid/dotted lines, respectively) as function of positionsÅd. The hori-zontal line at −3.83 eV refers to the average bulk potential.1752 J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de GrootDownloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspshown that the work function of Ca2N has an anisotropy ofmore than 1 eV, with a minimum of 2.35 eV(cf. Ca, F=2.87 eV) for the s1000d surface. Its surface energys5.3eV/nm2d, however, is almost the double of that of thes0001d surface. The low work function does not(signifi-cantly) depend on the interslab separation and we argued thatthis material is not as reactive as calcium. The lower workfunction and reduced reactivity make Ca2N a promisingmetal to replace Ca in cathodes of polymer light emittingdiodes.The authors wish to thank Dr. C. M. Fang(TUE) andProfessor Dr. R. Coehoorn(Philips) for helpful discussions.This work was part of the research programme of the Stich-ting voor Fundamenteel Onderzoek der Materie(FOM) withfinancial support from the Nederlandse Organisatie voorWetenschappelijk Onderzoek(NWO).1J. R. Sheats and D. B. Roitman, Synth. Met.95, 79 (1998).2G. G. Andersson, H. H.P. Gommans, A. W. Denier van der Gon, and H. H.Brongersma, J. Appl. Phys.93 6, 3299(2003).3CRC Handbook of Chemistry and Physics, 77th edn.(CRC, Boca Raton,FL, 1997).4I. D. Parker, J. Appl. Phys.75, 1656(1994).5J. L. Brédas and A. J. Heeger, Chem. Phys. Lett.217, 507 (1994).6S. Tokito, T. Tsutsui, R. Tanaka, and S. Saito, Jpn. J. Appl. Phys., Part 225, L680 (1986).7J. Birgerson, M. Fahlman, P. Bröms, and W. R. Salaneck, Synth. Met.80,125 (1996).8M. G. Burt and V. Heine, J. Phys. C11, 961 (1978).9G. Ebbingaus and A. Simon, Chem. Phys.43, 117 (1970).10C. F. Baker, M. G. Barker, and A. J. Blake, Acta Crystallogr.E57, i6(2001).11C. M. Fang, G. A. de Wijs, R. A. de Groot, H. T. Hintzen, and G. de With,Chem. Mater.12, 1847(2000).12P. Hohenberg and W. Kohn, Phys. Rev. B136, 864 (1964).13W. Kohn and L. J. Sham, Phys. Rev. A140, 1163(1965).14J. P. Perdew and Y. Wang, Phys. Rev. A45, 13244(1992).15G. Kresse and J. Hafner, Phys. Rev. B47, 558(1993); 49, 14251(1994).16G. Kresse and J. Furthmüller, Comput. Mater. Sci.6, 15 (1996).17P. E. Blöchl, Phys. Rev. B50, 17953(1994).18G. Kresse and D. Joubert, Phys. Rev. B59, 1758(1999).19S. G. Louie, S. Froyen, and M. L. Cohen, Phys. Rev. B26, 1738(1982).20G. Kresse and J. Furthmüller, Phys. Rev. B54, 11169(1996).21H. J. Monkhorst and J. D. Pack, Phys. Rev. B13, 5188(1976).22C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. Lett.88, 156802(2002).23C. J. Fall, N. Binggeli, and A. Baldereschi, J. Phys.: Condens. Matter11,2689 (1999).24Periodicity is required in all directions. This has the advantage that there isonly one surface and no possible influences from other ones as is the casein experiments.25C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. B65, 045401(2001).26R. Smoluchowski, Phys. Rev.60, 661 (1941).27ES is defined as the energy of the slab minus the energy of the equivalentbulk, divided by two times the surface area because a slab has two sur-faces.J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de Groot 1753Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

Polymer diodes require cathodes that do not corrode the polymer but do have low work function to minimize the electron injection barrier. First-principles calculations demonstrate that the work function of the (1000) surface of the compound Ca2N is half an eV lower than that of the elemental metal Ca (2.35 vs 2.87 eV). Moreover, its reactivity is expected to be smaller. This makes Ca2N an interesting candidate to replace calcium as cathode material for polymer light emitting diode devices. (C) 2004 American Institute of Physics

Uijttewaal, M.A.

Wijs, G.A. de

Groot, R.A. de

   University of GroningenLow work function of the (1000) Ca2N surfaceUijttewaal, M. A.; de Wijs, G. A. ; de Groot, R. A.Published in:Journal of Applied PhysicsDOI:10.1063/1.1767611IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2004Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Uijttewaal, M. A., de Wijs, G. A., & de Groot, R. A. (2004). Low work function of the (1000) Ca2N surface.Journal of Applied Physics, 96(3), 1751-1753. https://doi.org/10.1063/1.1767611CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 25-04-2023Low work function of the „1000… Ca2N surfaceM. A. Uijttewaal, G. A. de Wijs, and R. A. de GrootNSRIM, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands(Received 17 November 2003; accepted 12 May 2004)Polymer diodes require cathodes that do not corrode the polymer but do have low work function tominimize the electron injection barrier. First-principles calculations demonstrate that the workfunction of thes1000d surface of the compound Ca2N is half an eV lower than that of the elementalmetal Ca(2.35 vs 2.87 eV). Moreover, its reactivity is expected to be smaller. This makes Ca2N aninteresting candidate to replace calcium as cathode material for polymer light emitting diodedevices. ©2004 American Institute of Physics. [DOI: 10.1063/1.1767611]One of the great challenges for polymer light emittingdiodes is the electron injection barrier as well as the perfor-mance degradation caused by chemical reactions of the cath-ode with the polymer.1 State-of-the-art devices2 usePPV(poly phenylene vinylene) as electro luminescence ma-terial. Often the cathode is made out of calcium, because ofits low work function3 s2.87 eVd and the presumed align-ment of its Fermi level with the lowest unoccupied molecularorbital of PPV(electron affinity 2.73−2.8 eV).4,5 Biases ac-tually used are much higher than the optical band gap of PPVs2.4 eVd (Ref. 6), namely, in the order of 10 V,1 which sug-gests that there is some barrier formation between the cath-ode and the polymer. This is further confirmed by the obser-vation that the decrease of performance is much less fordevices in which the cathode is formed at a small residualoxygen pressure.7 In such a device some oxidation preventsreaction of the cathode with the polymer.It would be a major breakthrough when stable, low workfunction metals are found. Subnitrides are fascinating candi-dates because, as in the case of cesium suboxides,8,9 it isexpected that through quantum confinement their work func-tions are lower than those of the elemental metals while atthe same time ionic bonding reduces reactivity.The Ca2N crystal10 and electronic structure(bulk andsingle slab) (Ref. 11) are known. It is an ionic compound,built out of slabs of alternatingsCa-N-Cad hexagonal layers.The interslab distance between Ca layers is 3.81Å and Calayers inside a slab are only 2.45Å apart. This is muchsmaller than the ordinary fcc Cas111d layer distances3.18Åd and can be attributed to the ionic binding. The bandstructure shows fully occupied N 2s and N 2p states and aquasi two-dimensional(2D) free-electron state in the spacebetween the slabs which further confirms the idea that this isan ionic compound.In this paper the Ca2N work function sFd and surfaceenergy sESd are determined for both thes0001d and thes1000d surface. An indication of the materials stability isobtained from its binding energy and degree of surface re-laxation.First-principles calculations were carried out based ondensity functional theory in the local densityapproximation12,13 with generalized gradient corrections.14The total energy and molecular dynamics programVASP (Vi-ennaAb initio Simulation Package)15,16 was used which hasthe projector augmented wave method17,18 implemented.Nonlinear core corrections19 were applied for calcium. Theunit cells contained 18 and 42 atoms for thes0001d and thes1000d surfaces, respectively. The Kohn-Sham orbitals wereexpanded in plane waves with cutoffs of 37 Ry. 1231231 and 131234 Monkhorst-Pack21 k point grids, respec-tively, were used to sample the Brillouin zones resulting in74 and 26k points, respectively, in their irreducible parts.F is defined as the minimum amount of energy it coststo extract an electron from a metal. The fact that it dependson the type of surface is somewhat counter intuitive, it seemsto contradict the conservation of energy. However, the differ-ences in work function are compensated by differences inkinetic energy of the electron at large distances.22In order to determineF, two numbers are needed: themaximum energy of an electron inside the material, definedto be the Fermi levelsEFd, and the minimum energy outsidethe material, the vacuum potential. Because in a finite slabcalculationEF cannot be accurately determined, this numberis first obtained from a bulk calculation. Then for the finiteslab the average potential of the middle layers(which is anaccurate quantity) must be aligned with that of the bulk.23 Inthis way the work function for thes0001d surface is deter-mined using six slabs of Ca-N-Ca and 19Å of vacuum.24 Itcan be extracted from Fig. 1 and equals 3.43 eV.Another unit cell was made to findF for the s1000dsurface of Ca2N. It contained 14 layers and 20Å of vacuum.At this surface several bonds are severed and it is necessaryto relax the atomic positions. The result is shown in Fig. 2where the view is along the displacement vector from oneslab to the next. The nitrogen atoms at the surface are dis-placed a littles0.24Åd inwards, the nitrogens in the secondlayer a little s0.17Åd outwards, while the surface calciumstend to move in the direction of the surface nitrogens. Thiscan be understood from an ionic point of view. Nitrogenfavors an environment of high electron density while cal-cium shows the opposite trend. Although the structure haschanged very little, the effect of the relaxation onF is sub-stantial(Fig. 3). Before relaxation we find a work function of2.56 eV, already lower than that of pure calcium, but afterrelaxation it has even decreased to 2.35 eV. Not only is thiscomparable to the smallest work function for an element(Cs,JOURNAL OF APPLIED PHYSICS VOLUME 96, NUMBER 3 1 AUGUST 20040021-8979/2004/96(3)/1751/3/$20.00 © 2004 American Institute of Physics1751Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp2.14 eV),3 but also the difference of 1.08 eV with thes0001dsurface is larger than that between any two surfaces oftungsten,25 which is known for its large work function aniso-tropy. Both the drop inF after relaxation and the largesurface-anisotropy can be explained by the Smoluchowskimodel26 and not by quantum confinement as this is a bulkmodel that does not say anything about anisotropies and/orrelaxation. According to Smoluchowski on the other hand,the more “open” a surface is the more the charge density canbe smoothed, and this lowersF. However, more-open sur-faces tend to have higher surface energies27 and this makesthem less stable. For thes0001d and(relaxed) s1000d surfacewe find ES is 2.7 and 5.3 eV/nm2, respectively, which con-firms the trend.Because in Ref. 10 it is suggested Ca2N can also bemade with a largerc axis(interslab separation), we examinedwhether this changes thes1000d work function. After enlarg-ing c with 2% and subsequent relaxation of the atoms, thestructural changes are small and the trends already indicatedare a little more pronounced. The slab thickness, however,has decreased to almosts+0.5%d its former value. The aver-aged charge density and local potential are plotted in Fig. 4.Before relaxationF is decreased from 2.35 to 2.24 eV, butrelaxation increases it again to 2.35 eV. Again this is in ac-cordance with the Smoluchowski trend because a thickerslab makes the surface more open. We conclude that thework function does not(significantly) change whenc is en-larged.Finally, the reactivity of Ca2N is briefly discussed. In-spection of the binding energy shows that it is 5.6 eV performula unit (with respect to half a nitrogen molecule andtwo calcium atoms) for a single slab and 0.6 eV more for thebulk. This gives evidence that the reactivity of Ca2N issmaller than that of calcium, of which the binding energy isjust 1.9 eV per atom. It was shown earlier that relaxation ofthe atomic positions after bonds are cut orc is enlarged, doesnot have a substantial effect on the structure. These factstaken together give an indication that the reactivity of thissubnitride is less than that of pure calcium, although it is stillair sensitive.10In summary, using first-principles calculations, we haveFIG. 2. Atomic positions after relaxation of thes1000d surface as seen fromthe s1206d direction. Dark circles are nitrogens and light ones calcium.FIG. 3. Charge densities averaged overs1000d planes(arbitrary units) andpotentials(relative toEF, eV) averaged overs1000d planes and bulk unitcells, for both the relaxed(solid lines) and the unrelaxed(dashed lines)structure as function of positionsÅd. The potentials converge to 2.35 eV and2.56 eV in the vacuum, respectively. The horizontal line is the average bulkpotential.FIG. 4. Charge densities averaged overs1000d planes(arb. units) and po-tentials(relative toEF, eV) averaged overs1000d planes and bulk unit cells,for both the relaxed(solid lines) and the unrelaxed(dashed lines) structurefor a 2% largerc axis as function of positionsÅd. The potentials converge to2.35 eV and 2.24 eV in the vacuum, respectively. The horizontal line is theaverage bulk potential and it can be seen that relaxation has no observableeffect on the charge density.FIG. 1. Charge density(dashed line, arb. units) averaged overs0001d planesand potentials(relative toEF, eV) averaged overs0001d planes and bulk unitcells (solid/dotted lines, respectively) as function of positionsÅd. The hori-zontal line at −3.83 eV refers to the average bulk potential.1752 J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de GrootDownloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspshown that the work function of Ca2N has an anisotropy ofmore than 1 eV, with a minimum of 2.35 eV(cf. Ca, F=2.87 eV) for the s1000d surface. Its surface energys5.3eV/nm2d, however, is almost the double of that of thes0001d surface. The low work function does not(signifi-cantly) depend on the interslab separation and we argued thatthis material is not as reactive as calcium. The lower workfunction and reduced reactivity make Ca2N a promisingmetal to replace Ca in cathodes of polymer light emittingdiodes.The authors wish to thank Dr. C. M. Fang(TUE) andProfessor Dr. R. Coehoorn(Philips) for helpful discussions.This work was part of the research programme of the Stich-ting voor Fundamenteel Onderzoek der Materie(FOM) withfinancial support from the Nederlandse Organisatie voorWetenschappelijk Onderzoek(NWO).1J. R. Sheats and D. B. Roitman, Synth. Met.95, 79 (1998).2G. G. Andersson, H. H.P. 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Furthmüller, Comput. Mater. Sci.6, 15 (1996).17P. E. Blöchl, Phys. Rev. B50, 17953(1994).18G. Kresse and D. Joubert, Phys. Rev. B59, 1758(1999).19S. G. Louie, S. Froyen, and M. L. Cohen, Phys. Rev. B26, 1738(1982).20G. Kresse and J. Furthmüller, Phys. Rev. B54, 11169(1996).21H. J. Monkhorst and J. D. Pack, Phys. Rev. B13, 5188(1976).22C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. Lett.88, 156802(2002).23C. J. Fall, N. Binggeli, and A. Baldereschi, J. Phys.: Condens. Matter11,2689 (1999).24Periodicity is required in all directions. This has the advantage that there isonly one surface and no possible influences from other ones as is the casein experiments.25C. J. Fall, N. Binggeli, and A. Baldereschi, Phys. Rev. B65, 045401(2001).26R. Smoluchowski, Phys. Rev.60, 661 (1941).27ES is defined as the energy of the slab minus the energy of the equivalentbulk, divided by two times the surface area because a slab has two sur-faces.J. Appl. Phys., Vol. 96, No. 3, 1 August 2004 Uijttewaal, de Wijs, and de Groot 1753Downloaded 17 Feb 2006 to 129.125.25.39. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

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                  60523.pdf (publisher's version ) (Open Access)Polymer diodes require cathodes that do not corrode the polymer but do have low work function to minimize the electron injection barrier. First-principles calculations demonstrate that the work function of the (1000) surface of the compound Ca2N is half an eV lower than that of the elemental metal Ca (2.35 vs 2.87 eV). Moreover, its reactivity is expected to be smaller. This makes Ca2N an interesting candidate to replace calcium as cathode material for polymer light emitting diode devices. (C) 2004 American Institute of Physics

Low work function of the (1000) Ca2N surface

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