Segregation and interlayer relaxation at the NiPd(111) surfaceG. N. Derry, R. Wan, F. Strauch, and C. English Citation: Journal of Vacuum Science & Technology A 29, 011015 (2011); doi: 10.1116/1.3520434 View online: http://dx.doi.org/10.1116/1.3520434 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/29/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Surface structures of In-Pd intermetallic compounds. II. A theoretical study J. Chem. Phys. 141, 084703 (2014); 10.1063/1.4892409 Surface structures of In-Pd intermetallic compounds. I. Experimental study of In thin films on Pd(111) and alloyformation J. Chem. Phys. 141, 084702 (2014); 10.1063/1.4892408 Growth shapes of Ag crystallites on the Si(111) surface J. Vac. Sci. Technol. B 20, 2492 (2002); 10.1116/1.1523372 Temperature effect on growth of well-ordered thin Al 2 O 3 film on NiAl(110) J. Vac. Sci. Technol. A 20, 2027 (2002); 10.1116/1.1515907 Manganese surface segregation in NiMnSb Appl. Phys. Lett. 76, 2349 (2000); 10.1063/1.126342

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Segregation and interlayer relaxation at the NiPd„111… surfaceG. N. Derrya�

Department of Physics, Loyola University Maryland, Baltimore, Maryland 21210

R. Wanb�

Department of Physics, University of Maryland-Baltimore County, Baltimore, Maryland 21250

F. Strauchc� and C. Englishd�

Department of Physics, Loyola University Maryland, Baltimore, Maryland 21210

�Received 12 August 2010; revised manuscript received 28 September 2010;accepted 30 October 2010; published 4 January 2011�

The composition and structure of the NiPd�111� alloy surface are measured using low energyelectron diffraction �LEED�. The diffracted intensities are analyzed using tensor-LEED methodswith the average t-matrix approximation. The major composition results of the study are that thesurface is enriched in palladium ��80% Pd� while the second layer is enriched in nickel ��28% Pd�.No significant structural changes were found, with all measured interlayer displacements beingsmaller than the corresponding measurement uncertainties. Comparisons of these results are madewith literature for other NiPd low-index surfaces, for the well-studied NiPt�111� surface, and withthe growing body of theoretical literature for NiPd alloy surfaces. © 2011 American VacuumSociety. �DOI: 10.1116/1.3520434�

I. INTRODUCTION

A great deal of progress has been made in understandingthe structure and composition of alloy surfaces. Simpleideas, based on surface energy and lattice strain, are stillconceptually useful but have now been incorporated intomore sophisticated theoretical treatments.1,2 Accurate experi-mental information, once rare, has become increasinglyavailable for a variety of alloy systems.3,4 One such system,for which good experimental results and theoretical under-standing are beginning to emerge, is the surface of Ni/Pdalloys.5–19 The present article is a contribution to this litera-ture, detailing experimental results for the NiPd�111� surface.

NiPd is a substitutionally disordered alloy, which crystal-lizes in a fcc lattice. The large lattice constant mismatch�about 10%� between the constituent elements presumablygives rise to significant strain in the lattice, while the chemi-cal interaction between the Ni and Pd atoms would be ex-pected to be similar to the Ni–Ni and Pd–Pd interactions,consistent with the lack of ordered bulk phases. Our researchprogram has been to make a systematic study of the surfacebehavior of NiPd low-index planes in order to illuminate therelative roles of strain and chemical interactions in drivingsegregation in alloys and its accompanying structural effects.Our hope was that this might be particularly valuable incomparison with the extensive data on the similar �but notidentical� Ni/Pt system. Low energy electron diffraction�LEED� is an extremely useful tool for this purpose, since it

provides both the composition and the structure of severallayers of atoms near the surface. The present article is thelast in a series of LEED studies8,10 for low-index NiPd sur-faces performed in our laboratory.

II. EXPERIMENT

The primary experimental tool for this work was LEED.Data were collected using a standard four grid LEED opticswith a charge-coupled device video camera and analyzedwith the tensor-LEED method.20,21 The sample was a diskcut from a 50 at. % NiPd single crystal, oriented to the �111�face. The sample was mechanically polished, and thencleaned in ultrahigh vacuum with sputter/anneal cycles untilno contamination was observable using Auger spectroscopy.The annealing temperature was �800 K, but the sample wascooled to room temperature for data acquisition. Sample po-sitioning is accomplished with a manipulator allowing trans-lational motion in three orthogonal axes, and rotations abouttwo orthogonal axes to vary the polar angle � �between thesurface normal and the incident beam� and the azimuthalangle � �for rotations about an axis along the surface nor-mal�. Further details concerning the experimental apparatushave been published elsewhere.8,10,22 Since NiPd is a substi-tutionally disordered alloy, the tensor-LEED analysis wasperformed using the average t-matrix approximation,21 in es-sence implying chemically disordered layers in the analysis.Eight phase shifts and an energy-dependent inner potential,calculated with Rundgren’s algorithms,23 were used in thedata analysis, which was implemented using the TENSERLEED

package.21

The experiment and analysis were performed indepen-dently for three different incidence angles �=0°, �=0°; �=10.4°, �=−45.3°; and �=18.6°, �=−45.3°�in order to con-firm the correctness of the structure/composition model by

a�Electronic mail: gderry@loyola.edub�Present address: Kunming University of Science and Technology, Kun-

ming 650093, China.c�Present address: Department of Physics, Williams College, Williamstown,

MA 01267.d�Present address: Department of Physics, Rensselaer Polytechnic Institute,

Troy, NY 12180.

011015-1 011015-1J. Vac. Sci. Technol. A 29„1…, Jan/Feb 2011 0734-2101/2011/29„1…/011015/4/$30.00 ©2011 American Vacuum Society

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demonstrating that it is reproducible, and also to improve theprecision of the composition and displacement measure-ments by increasing the total amount of LEED data beingfitted to the model. In all, data were acquired and analyzedfor 16 inequivalent beams with a total energy range of 2390eV. Multiple runs were performed at each angle, LEED IVdata were averaged over all identical runs, and symmetryequivalent data were also averaged. Both the Pendry R factorand the R2 R factor were minimized in each analysis as afurther reproducibility check; this strategy also extracts moreinformation from the data since R2 and RPendry are sensitiveto different features of the IV curves.

III. RESULTS

The agreement between the calculated and experimentalIV curves was fairly good in all cases for the best-fit param-eters. Results for the best fits using R2 are shown in Fig. 1 forall three angles to illustrate the quality of the agreement. Theactual R2 values for these fits are given in Table I, along withthe comparable RPendry values and the best-fit parameter val-ues in all cases. Overall, the fitting quality is sufficient to

assure the reliability of the results. Results for the composi-tions of the surface and of the second layer are quite consis-tent for all incidence angles and for both R factors. The sur-face layer is enriched in palladium, with an average value of80%�7% palladium in the surface compared to the 50%bulk value. There is also segregation in the second layer, butin contrast to the surface we find that the second layer isenriched in nickel, with an average value of 28%�9% pal-ladium in this layer. The diffracted intensities were relativelyinsensitive to the composition of the third layer, so this wasfixed at 50% and only two composition parameters were var-ied in the final fits.

The first two interlayer distances were also varied in thesefits, with the results shown in Table I. The values for theserelaxations are less consistent than the composition results.All of the measured relaxations are small, but the signs aredifferent for different runs, depending on the incidence angleand R factor �the sign convention used here is that + meansinward toward the bulk, as is typical in LEED work�. Theuncertainties, as described below, are substantial comparedto these measured values, so a large spread is not unreason-able, and all of the values in Table I are consistent with theaverage values for each layer within these measurement un-certainties. Nevertheless, all of the individual interlayer re-laxations reported in Table I do not agree with each otherwithin their uncertainties, and the reasons for this are notentirely clear. The overall conclusion that can be drawn fromthese results, however, is more apparent: Any interlayer re-laxations that occur for this surface are not large. The aver-age values for the relaxation in both the first and the secondinterlayer distances turn out to be �−0.001�0.003 nm, sothe surface structure is consistent with a bulk termination.

Three methods were employed to estimate the uncertain-ties of the compositions and interlayer displacements. Thestandard method, using the variations of the Pendry R factorwith changes in parameter values,24 was used; a relatedmethod exploiting the relationship between R2 and the �2

statistic, similar to that used by Adams et al.,25–27 was used;and these two methods were checked by comparison to thestandard deviations of the average values based on Table I.An example of the method using R2 is shown in Fig. 2. Allthree estimates were reasonably consistent with each other,and the estimated uncertainties are presented along with thefinal averaged values for each parameter in Table II. Hence,the major qualitative conclusions of this work are that thereis significant Pd segregation to the surface and Ni segrega-tion to the second layer, but that structurally the surface ex-hibits very little interlayer relaxation �conclusions consistentwith an earlier preliminary report22�.

IV. DISCUSSION

Palladium enrichment at the surface of Ni/Pd alloys hasbeen observed many times.5–11 LEED studies on aPd8Ni92�111� �Ref. 6� surface as well as on Pd50Ni50�100��Ref. 10� and Pd50Ni50�110� �Ref. 8� surfaces have all mea-sured Pd enrichment. Auger spectroscopy studies5,7 and ionscattering measurement9 of polycrystalline samples have also

FIG. 1. LEED IV curves for the NiPd�111� surface. The solid lines areexperimental data, whereas the dashed lines are the calculated curves for thebest-fit model.

TABLE I. Best-fit values of the parameters for surface composition, secondlayer composition, first interlayer distance change, and second interlayerdistance change.

Trialc1

�%Pd�c2

�%Pd�d1

�nm�d2

�nm� R factor

0°, R2 75 16 �0.003 �0.003 0.0660°, Rp 78 31 �0.005 �0.004 0.144

10.4°, R2 83 26 0.002 0.002 0.05210.4°, Rp 73 25 0.003 0.003 0.25618.6°, R2 87 29 �0.004 �0.004 0.09118.6°, Rp 86 40 0.003 0.000 0.266

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J. Vac. Sci. Technol. A, Vol. 29, No. 1, Jan/Feb 2011

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shown Pd enrichment. X-ray photoelectron spectroscopy andion scattering measurements on Pd8Ni92�111� andPd8Ni92�110� surfaces11 have likewise found significant Pdenrichment. This Pd enrichment is well understoodtheoretically,13–19 and is consistent with simple argumentsbased on surface energies. Looking in more detail, however,it is interesting to note that the amount of segregation mea-sured in this study is almost the same as the 80% Pd found inthe NiPd�100� surface,10 in disagreement with simple argu-ments based on the number of broken bonds. More sophisti-cated theoretical treatments by Noebe and co-workers,17,18

Poyurovskii et al.,15 and Rousset et al.13 have, however, pre-dicted high Pd concentrations in the NiPd�111� surface, com-parable to that of NiPd�100�, in good agreement with thepresent findings. The Ni enrichment in the second layer ofNiPd�111� observed here, resulting in an oscillatory segrega-tion profile, is also qualitatively similar to the results previ-ously found for NiPd�100�,10 though the amount of Ni is lessin the present case �the second layer of NiPd�100� is 100%nickel�. This result, indicative of chemical ordering, waspuzzling when first observed because the Ni/Pd bulk alloyphase diagram exhibits no ordered phases. Theoretical workdone since then has resolved this puzzle, showing clearlythat Ni and Pd have a weak attractive interaction and that Nienrichment of the second layer lowers the energy of the sys-tem, results that are found consistently using many different

theoretical frameworks13–19 �though one study disagrees28�.The lack of ordered bulk phases was found to result fromkinetic hindering.17 Hence, a consistent picture regarding os-cillatory near-surface segregation in NiPd alloys has emergedin both experiment and theory.

There are important differences in the structural behaviorof NiPd�111� compared to NiPd�100�. In the present study ofNiPd�111�, very small interlayer relaxations were observed,whereas in the NiPd�100� case there are large contractions inboth the first �0.005 nm� and the second �0.007 nm� inter-layer distances.10 Such double contractions are uncommon,so it is quite intriguing that NiPd�111� turns out to have analmost bulklike surface termination, a result that was alsofound for Ni92Pd8�111�.6 The more close-packed arrange-ment of these layers is certainly relevant to any explanationof this difference, and the lower concentration of Ni in thesecond layer may also be germane. To obtain further insightinto this situation, we have performed computations of thealloy’s energy at various values of interlayer distance forboth the NiPd�100� and NiPd�111� surfaces using the methodof Bozzolo, Ferrante, and Smith �BFS�.2,29 The layerwisecomposition profile was fixed at the experimentally mea-sured values in all cases, and either the first interlayer dis-tance only or else both the first and second interlayer dis-tances were varied in different trials. The results of theseBFS computations are presented in Fig. 3. The decrease inenergy as the interlayer distance decreases is greater forNiPd�100� than for NiPd�111�, in agreement with the experi-mental observation of more significant contractions for thatsurface. The amount of contraction for which the energyreaches a minimum is also smaller for NiPd�111� than forNiPd�100�, in qualitative agreement with the above observa-tion, though not in quantitative agreement with the

0

0.05

0.1

0.15

0.2

-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06

R2

interlayer displacement (angstroms)

0

0.05

0.1

0.15

0.2

-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06

FIG. 2. Illustration of the method by which uncertainties are estimated. Rfactor R2 is plotted versus vertical displacement, and the range of displace-ments for a change in R2 corresponding to one standard deviation is indi-cated by the horizontal line.

TABLE II. Overall best final values of the compositions and interlayer dis-tance changes, taking all data into account, along with their uncertainties.

c1

�%Pd�c2

�%Pd�d1

�nm�d2

�nm�

Average value 80 28 �0.001 �0.001Uncertainty 7 9 0.003 0.003

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0 1 2 3 4 5 6 7

one layer contracted, (100) surfacetwo layers contracted, (100) surfacetwo layers contracted, (111) surface

computedBFSenergychange(eV/atom)

interlayer contraction distance (10-3nm)

FIG. 3. BFS computation results comparing the lowering of energy for in-terlayer contractions in NiPd�111� and NiPd�100�. These results are qualita-tively consistent with the observed experimental differences in behavior forthese alloys.

011015-3 Derry et al.: Segregation and interlayer relaxation 011015-3

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NiPd�111� distance measurement. The additional decrease inenergy obtained by letting the second layer contract is con-siderable in the NiPd�100� case, where a large double con-traction is seen, but is negligible in the NiPd�111� case.Looking at the energetics for individual Pd and Ni atoms inthe surface and second layer, we find that the major termdriving these energy changes is the BFS strain decrease in Niin the second layer. This is true for both crystal faces, but theenergy gain per Ni atom is much larger in the �100� face andthis effect is further amplified by the higher Ni concentrationfor this case, consistent with the small interlayer distancechanges in NiPd�111� that we have observed here.

An interesting comparison can be made to the analogousalloy surface of NiPt�111�, which has been well studied.30

The atomic size mismatch in the two alloys is almost thesame, and the electronic structures are also comparable. InNiPt�111�, a similar compositional oscillation is observed,with high Pt enrichment at the surface and equally high Nienrichment in the second layer. The segregation in this caseis even stronger than that of NiPd�111�, with higher concen-trations in the surface and second layer and a further Pt en-richment measured in the third layer. In addition, contrac-tions ��0.004 nm� were observed in both the first and secondinterlayer distances, similar to the NiPd�100� results butlarger than the relaxations seen in the present study ofNiPd�111�. Given the large uncertainties in the measured re-laxations, these results taken collectively suggest that thepossibility of very small contractions in the interlayer dis-tances at the surface of NiPd�111� cannot be ruled out, de-spite the sign of the average value measured here.

V. CONCLUSION

The surface of the NiPd�111� alloy is significantly en-riched in segregated Pd, a phenomenon that has been ob-served for a variety of Ni/Pd alloy surfaces. Quantitatively,the composition of the surface is 80%�7% Pd compared tothe 50% bulk composition. The second layer, however, isenriched in Ni, exhibiting a compositional oscillation thathas also been observed previously in the Ni/Pd system. Thecomposition of the second layer is 28%�9% Pd. Both thePd segregation to the surface and the Ni segregation to thesecond layer are consistent with present theoretical modelsfor this alloy as well as the existing experimental literature.Interlayer relaxations for the first two layers of NiPd�111�were measured to be small, with the deviations from a bulktermination being smaller than the measurement uncertain-ties �−0.001�0.003 nm in both cases�. All of these resultsfor both the composition and the structure near the surface ofNiPd�111� are consistent with the premise that strain relief inthe second layer is an important driving mechanism for theobserved changes, and this idea is also qualitatively consis-tent with BFS energy computations for the system.

ACKNOWLEDGMENTS

The authors wish to thank V. Blum and K. Heinz for pro-viding the TENSERLEED code package, and additionally thankBlum for sharing his expertise in the proper use of the code.They also thank J. Rundgren for sharing his phase shift cal-culation code and advice on its use. Conversations with P.Rous have provided valuable insights in the interpretation ofthe LEED data, and G. Bozzolo provided the authors withcode to perform BFS calculations, as well as discussions oftheir interpretation. Finally, the authors wish to thank theNational Science Foundation for funding this work underGrant Nos. DMR-9903108 and DMR-9021959.

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