During the fracture of nanocontacts gold spontaneously forms freely suspended
chains of atoms, which is not observed for the iso-electronic noble metals Ag
and Cu. Au also differs from Ag and Cu in forming reconstructions at its
low-index surfaces. Using mechanically controllable break junctions we show
that all the 5d metals that show similar reconstructions (Ir, Pt and Au) also
form chains of atoms, while both properties are absent in the 4d neighbor
elements (Rh, Pd, Ag), indicating a common origin for these two phenomena. A
competition between s and d bonding is proposed as an explanation

A. Filippetti

A. I. Yanson

B. Ludoph

C. Untiedt

D. Sánchez-Portal

E. Scheer

E. Tosatti

H. Häkkinen

H. Ohnishi

J. C. Cuevas

J. M. van Ruitenbeek

J. Torres

K. W. Jacobsen

L. De Maria

M. Okamoto

M. P. A. Fisher

M. Sørensen

N. Takeuchi

O. Gülseren

O. Y. Kolesnychenko

P. Pyykkö

R. H. M. Smit

S. Bahn

T. Kizuka

V. Rodrigues

Y. Kondo

English

arXiv

During the fracture of nanocontacts gold spontaneously forms freely suspended chains of atoms, which is not observed for the isoelectronic noble metals Ag and Cu. Au also differs from Ag and Cu in forming reconstructions at its low-index surfaces. Using mechanically controllable break junctions we show that all the 5d metals that show similar reconstructions (Ir, Pt, and Au) also form chains of atoms, while both properties are absent in the 4d neighbor elements (Rh, Pd, and Ag), indicating a common origin for these two phenomena. A competition between s and d bonding is proposed as an explanation.This work is part of the research program of de “Stichting FOM,” which is financially supported by NWO, and has been supported by a European Community Marie Curie Fellowship

Smit, R.H.M.

Untiedt, Carlos

Yanson, A.I.

Ruitenbeek, J.M. van

Repositorio Institucional de la Universidad de Alicante

VOLUME 87, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 24 DECEMBER 200126Common Origin for Surface Reconstruction and the Formation of Chains of Metal AtomsR. H. M. Smit, C. Untiedt, A. I. Yanson,* and J. M. van RuitenbeekKamerlingh Onnes Laboratorium, Leiden University, Postbus 9504, 2300 RA Leiden, The Netherlands(Received 3 July 2001; published 4 December 2001)During the fracture of nanocontacts gold spontaneously forms freely suspended chains of atoms, whichis not observed for the isoelectronic noble metals Ag and Cu. Au also differs from Ag and Cu in formingreconstructions at its low-index surfaces. Using mechanically controllable break junctions we show thatall the 5d metals that show similar reconstructions (Ir, Pt, and Au) also form chains of atoms, whileboth properties are absent in the 4d neighbor elements (Rh, Pd, and Ag), indicating a common originfor these two phenomena. A competition between s and d bonding is proposed as an explanation.DOI: 10.1103/PhysRevLett.87.266102 PACS numbers: 68.35.Bs, 73.22.– f, 73.40.JnIt has recently been discovered that nanowires of goldspontaneously evolve into chains of single atoms [1,2],which are surprisingly stable. They form metallic wireswith a nearly ideal quantum value of the conductanceG  2e2h, and are able to sustain enormous current den-sities. Various numerical calculations on these chains havebeen presented, both in regular and distorted configurations[3], but the question as to why these chains form specifi-cally for Au, and, e.g., not for Cu or Ag, was not addressed[4]. An understanding of the mechanism of the formationof these chains may help to improve our ability to controlthe fabrication process. This may lead to the formationof chains for different materials with interesting properties(magnetism, superconductivity) or of longer chains. Apartfrom a possible technical interest longer chains will also in-spire new fundamental research as such atomic chains arethe closest approximation to ideal one-dimensional metal-lic systems. These 1D systems are expected to undergo aPeierls distortion and ultimately Tomanaga-Luttinger Liq-uid effects [5] could appear.In search for properties distinguishing Au from the othernoble metals, that can be linked to the chain formation,we are particularly interested in surface effects, where thebonding between the atoms is modified by the reduceddimensions. Among the special features of Au that havebeen extensively studied are the reconstructions of the low-index surfaces. The (110) surface shows a missing-rowreconstruction, where every second row of atoms on thesurface is removed; the (001) surface has a quasihexagonalreconstruction, where the top layer of the sample contractsto form a hexagonal layer on top of the square structure ofthe bulk.Since these surface reconstructions distinguish Au fromAg and Cu, it is worth looking at the mechanism thathas been put forward to explain them [6]. In fact, theend-of-series 5d elements Ir, Pt, and Au have similar sur-face reconstructions, which are absent in the related 4delements Rh, Pd, and Ag, suggesting that the explanationfor the reconstructions cannot lie in any particular detail ofd band electronic structure. There appears to be a grow-ing consensus that a stronger bonding of low-coordinationatoms of the 5d metals with respect to the 4d metals is6102-1 0031-90070187(26)266102(4)$15.00a result of sd competition caused by relativistic effects inthe electronic structure [7,8]. From the numerical workthat uses relativistic local-density-functional calculationsto evaluate the various contributions to the atomic bindingenergies qualitatively the following picture emerges. Theeffective Bohr radius a0  4pe0 h¯2Zme2 for 1s elec-trons of the heavier (5th row) elements contracts due to therelativistic mass increase m  m0p1 2 yc2. Highers shells also undergo a contraction, both because they haveto be orthogonal against the lower ones and because theyfeel the same mass-velocity terms directly. As Takeuchiet al. explain [7] the contraction of the s shell reduces itsenergy, increasing the s occupation at the expense of thed electrons. Since at the top of the d band the states withantibonding character are partially depleted, the d bondbecomes stronger. While the d electrons thus tend to com-press the lattice the s electrons exert an opposing Fermipressure. At the surface, the spill-out of the s electroncloud into the vacuum relieves some of the electron pres-sure, and allows a contraction of the interatomic distanceand a strengthening of the bonds at the surfaces, giving riseto the observed reconstructions.It is now natural to formulate the hypothesis that theconsequences of these relativistic modifications in the me-tallic bonding will be even more pronounced for the one-dimensional chain structures. This suggests that we shouldlook for possible chain formation in Pt and Ir, which havea reconstructed surface similar to Au, while we expect thatRh, Pd, and Ag, which do not reconstruct, should not showthis tendency to chain formation.For reliable results on surface effects it is essential toprepare a clean sample surface. This can be achieved verysimply with the use of mechanically controllable breakjunctions (MCBJ) [9]. Starting with a macroscopic wire,of purity better than 99.98%, a notch is formed by inci-sion with a surgical knife. For Rh and Ir due to the ex-treme hardness of the materials spark erosion was used tocut the notch. The samples are mounted inside a vacuumcontainer and pumped overnight to a pressure below 5 31027 mbar. Then we cool to 4.2 K in order to attain a cryo-genic vacuum. Two hours after cooling the cryostat, oncedrifts due to thermal gradients are reduced to a minimum,© 2001 The American Physical Society 266102-1VOLUME 87, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 24 DECEMBER 2001the sample wire is broken at the notch by bending of thesubstrate onto which it has been fixed. The clean, freshlyexposed fracture surfaces are then brought back into con-tact by slightly relaxing the bending. With the use of apiezoelement the bending can then be fine tuned to adjustthe contact to atomic size.A thick copper finger provides thermal contact to thesample inside the container. Helium thermal-exchange gasis avoided since adsorbed helium severely modifies thework function of the material [10] and the latter is usedto calibrate the displacement of the two electrodes withrespect to each other [11]. Surface corrugation and varioussurface orientations slightly modify the effective tunnelbarrier, giving rise to a 10% uncertainty in the calibration.The contact is characterized by the conductance, mea-sured with an accuracy of 1%, at a bias voltage of 10–25 mV. As the contact is slowly broken the conductancedecreases in a stepwise fashion (Fig. 1), where the last stepvalue before the junction goes into tunneling typically cor-responds to a contact of a single atom. We will use thelength of the conductance plateau at the level of the con-ductance of a single atom as a measure for the length ofthe atomic chain being formed.The formation of chains of metal atoms in the junctioncan be analyzed by the technique developed in Ref. [1] forAu, which we briefly summarize. The distribution of thelengths of the last plateaus over a large number of contact-breaking cycles can be represented in a length-histogram(similar to the one for Pt in Fig. 2). As the conductanceof a single atom of Au has been shown to be close to thefundamental unit of conductance 1G0  2e2h [12], andsince the conductance of the last plateaus is found veryclose to 2e2h, the length of the plateau can be associ--6 -5 -4 -3 -2 -1 0 1051015202530[2e2 /h]HgCdZnCuNiCoFeOsRuAuPtIrAgPdRh Conductance Piezo Voltage [V]-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5PtElectrode Displacement [nm]FIG. 1. Typical traces of the conductance as a function of thepiezovoltage for Pt at 4.2 K. The thick curve is recorded whilebreaking the contact, and the thin curve is taken while returninginto contact. The formation of a chain of atoms can be recog-nized in the thick curve from the length of the last conductanceplateau. The inset shows a fragment of the periodic table of theelements highlighting the transition metals used in this study.266102-2ated directly with the length of a chain of atoms. Strongsupport for this interpretation was found in the peaks ofthe length histogram. Their regular positions at multiplesof 0.25 6 0.03 nm [11] indicate that the average lengths atwhich the chain breaks comes in multiples of a length com-parable to the atomic diameter (0.288 nm for bulk Au), andagrees with the numerical calculations for the interatomicspacing in stable atomic chains [3]. Further arguments aregiven in Ref. [1]. Conclusive evidence for the formationof chains was obtained from the images of high-resolutionTEM [13–15], although the interatomic distances foundhere are usually bigger, between 0.3 and 0.4 nm.We now describe the application of the same MCBJtechnique for Pt, followed by a description of the othertransition metals in this study. Figure 1 shows an exampleof a conductance trace for Pt. It shows a gradual decreaseof the conductance, interrupted by sudden jumps when theatomic structure rearranges, as seen for most metals, fol-lowed by a long-stretched plateau around a reproducibleconductance value equal to that expected for a single atom,and then a sudden break of the contact into the tunnelregime. In contrast to Au, the last conductance plateau hasa value well above 1G0, and it is much less smooth andconstant than the ones for Au. This is expected from theelectronic structure for Pt, where the sd orbitals provide up0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40102030 Normalized number of counts [a.u.]Plateau length [nm]0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4024681012PdPt    FIG. 2. Histogram of the distribution of lengths for the lastconductance plateau for platinum contacts (top panel) and forits isoelectronic neighbor palladium (bottom panel). Pd breaksessentially at the level of a single atom, while Pt spontaneouslyforms chains of atoms, which is apparent from the tail of thedistribution towards long plateau lengths, and from the peaks atmultiples of the interatomic distance. The area of the histogramabove 1.5 times the chain length of the first peak is shaded gray,and defines the fraction P1.5 used in Fig. 4.266102-2VOLUME 87, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 24 DECEMBER 2001to 5 channels in the last atom [12,16]. The total con-ductance through these channels is strongly influenced bythe local atomic configuration and the strain on the con-tact [17].In order to determine experimentally the various pos-sible conductance values for the last plateau we recordeda conductance histogram (Fig. 3). The histogram is simi-lar to the one in Ref. [18] and the dominant peak in thishistogram is attributed to the conductance for a contact of asingle atom [19]. Based on this observation we start count-ing the length of a conductance plateau as soon as the con-ductance drops below a value, Gstart, situated at the highend of the conductance peak in Fig. 3, and we stop count-ing when it drops below a value, Gstop, at the low end ofthe peak. The two boundary values are illustrated in Fig. 3.The histogram accumulated in this fashion for the plateaulengths in Pt is shown in Fig. 2 (top). It shows a similarregular peak structure as was found for Au. We emphasizethat the shape of this histogram does not change signifi-cantly for different values of Gstart and Gstop. A simi-lar histogram is found also for Ir [20]. The plateau-lengthhistograms give evidence that Pt and Ir, similar to Au, formchains of atoms at the last stages of contact breaking.When we apply the same method to Pd, however, weobtain a strikingly different histogram, with a much shortertail towards long last plateau lengths than for Pt, Fig. 2(bottom). Here only one peak is clearly observable, at amuch shorter length, and in contrast to Pt there is a highprobability for breaking the contact at very short lengths.Similar results have been obtained for Rh and Ag. Clearly,Rh, Pd, and Ag, which do not show the mentioned surfacereconstructions, also do not favor the formation of atomicchains.In order to obtain an objective quantity that measuresthe tendency towards chain configurations we define P1.50 2 4 6 8 10010002000300040005000[2e 2/h]CountsConductanceFIG. 3. Histogram of the number of times each conductancevalue is obtained for a large number (2000) of individual digi-tized conductance traces similar to the one shown in Fig. 1,recorded while repeatedly breaking the Pt contact. The high-lighted area indicates the range between the Gstart and Gstopvalue used to define the length of the last plateau when record-ing the length-histogram in Fig. 2.266102-3as the fraction of plateaus above 1.5 times the length ofthe first peak in the plateau-length histogram, as in Fig. 2.The values obtained are shown in Fig. 4 for all 4d and5d metals considered in this study. The 5d metals clearlyhave a significantly larger fraction of long last plateaus.The lengths obtained for the 4d metals are dominantlyin the range of lengths of only a single atom. The cleardistinction between 4d and 5d metals in Fig. 4 is not verysensitive to the precise definition for the fraction P1.5.The results demonstrate a one-to-one correspondencebetween the metals showing the (110) missing row andthe (001) quasihexagonal reconstructions and those show-ing the formation of chains of atoms. This correlation be-comes meaningful only when we can identify a commonorigin for the two properties. As for the reconstructions,the bonding in atomic chains will be influenced by a tiltingof the balance between s and d electrons by relativistic ef-fects. This difference in bonding becomes revealed whenthe Fermi pressure of the s electrons can be released byspill-out of the wave functions into the vacuum. As a re-sult, there is a gain in energy from the stronger d bondsand a reduction of the interatomic distance. Clearly, the 1Dchain geometry allows for an even larger s pressure releasethan at the surface. The effect of this can be recognized inthe interatomic distances for the chains, obtained from thepeak distances in the length histograms: 0.22 6 0.02 nmfor Ir, 0.23 6 0.02 nm for Pt, and 0.25 6 0.03 nm forAu are all smaller than the bulk values (0.271, 0.277,and 0.288 nm, respectively). The important role of the d0.00.10.20.30.40.50.60.7Au/AgIr/Rh4d5dPt/Pdfraction of long plateausFIG. 4. The fraction P1.5 of plateaus longer than 1.5 times thelength at the first maximum in the length histogram for thevarious metals, in the order of their position in the periodic table.The solid squares and open triangles indicate the fractions forthe 4d and 5d metals, respectively. The error bars indicated inboth graphs show the standard deviation of the result obtainedby combining all histograms for the same material. These resultsshow that only the 5th row elements have a preference to formlong atomic chains. The number of histograms recorded for eachmaterial, and the values Gstart and Gstop used to determine theplateau lengths were 8, 3.4, and 1.0 for Rh; 6, 2.4, and 0.5 forPd; 14, 1.1, and 0.5 for Ag; 6, 2.0, and 0.5 for Ir; 12, 2.5, and1.0 for Pt; 12, 1.2, and 0.5 for Au.266102-3VOLUME 87, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 24 DECEMBER 2001orbitals in the formations of Au atomic chains has re-cently been demonstrated by Häkkinen et al. using scalar-relativistic local density calculations [3].New structures in Au nanowires for larger diametershave recently been observed in HR-TEM experiments, inthe form of helical multishell nanowires [21]. Numericalsimulations for atomic-size metallic nanowires confirmedthe stability of these structures for Au, but did not findthem in Ag [22]. This supports the idea that propertiesof metal surfaces are strongly related to the behavior ofmetallic nanowires. We propose that Pt and Ir are likelycandidates to show similar weird wire helical structures[23] as found for Au. From surface science we learn thatsurface reconstructions can be induced for Rh, Pd, andAg by deposition of a low density of specific adsorbates,which modifies the balance between the s and d bonds inthe metal. It would therefore be interesting to test whetherthe 4d metals can also be induced to form chains under theinfluence of similar adsorbates. Experiments to verify thisare in progress.This work is part of the research program of de “Sticht-ing FOM,” which is financially supported by NWO, andhas been supported by a European Community Marie CurieFellowship. We thank K. W. Jacobsen, S. Bahn, E. Tosatti,and A. P. Sutton for informative discussions, and L. Y.Chen for assistance in the experiments.*Present address: Department of Physics, 510 Clark Hall,Cornell University, Ithaca, NY 14853.[1] A. I. Yanson et al., Nature (London) 395, 783 (1998).[2] H. Ohnishi, Y. Kondo, and K. Takayanagi, Nature (London)395, 780 (1998).[3] M. Sørensen, M. Brandbyge, and K. W. Jacobsen, Phys.Rev. B 57, 3283 (1998); J. Torres et al., Surf. Sci. Lett.426, L441 (1999); M. Okamoto and K. Takayanagi, Phys.Rev. B 60, 7808 (1999); D. Sánchez-Portal et al., Phys.Rev. Lett. 83, 3884 (1999); H. Häkkinen, R. N. Barnett,and U. Landman, J. Phys. Chem. B 103, 8814 (1999);L. De Maria and M. Springborg, Chem. Phys. Lett. 323,293 (2000); H. Häkkinen, R. N. Barnett, A. G. Scherbakov,and U. Landman, J. Phys. Chem. B 104, 9063 (2000).266102-4[4] This problem was recently addressed by model calculations[S. Bahn and K. W. Jacobsen, preceding Letter, Phys. Rev.Lett. 87, 266101 (2001)].[5] M. P. A. Fisher and L. I. Glazman, in Mesoscopic ElectronTransport, edited by L. L. Sohn et al. (Kluwer, Dordrecht,2001).[6] K. W. Jacobsen and J. K. Nørskov, in The Structure of Sur-faces II, edited by J. F. van der Veen and M. A. van Hove,Springer Series in Surface Sciences Vol. 11 (Springer-Verlag, New York, 1988), p. 118.[7] N. Takeuchi, C. T. Chan, and K. M. Ho, Phys. Rev. Lett.63, 1273 (1989); , Phys. Rev. B 43, 14 363 (1991).[8] P. Pyykkö, Chem. Rev. 88, 563 (1988); A. Filippetti andV. Fiorentini, Surf. Sci. 377– 379, 112 (1997).[9] For a review, see J. M. van Ruitenbeek, in MesoscopicElectron Transport, edited by L. L. Sohn et al. (Kluwer,Dordrecht, 1997).[10] O. Y. Kolesnychenko, O. I. Shklyarevskii, and H. van Kem-pen, Phys. Rev. Lett. 83, 2242 (1999).[11] The calibration in the original paper by Yanson et al. [1]quotes a number for the peak-to-peak distance of 0.36 nmwith an accuracy of only 30%. Working without Heexchange gas has improved the accuracy, and reducedthe value to 0.25 6 0.03 nm. C. Untiedt et al. (to bepublished).[12] E. Scheer et al., Nature (London) 394, 154 (1998).[13] T. Kizuka, S. Umehara, and S. Fujisawa, Jpn. J. Appl. Phys.40, L71 (2001).[14] V. Rodrigues, T. Fuhrer, and D. Ugarte, Phys. Rev. Lett.85, 4124 (2000).[15] V. Rodrigues and D. Ugarte, Phys. Rev. B 63, 073405(2001).[16] J. C. Cuevas, A. Levy Yeyati, and A. Martin-Rodero, Phys.Rev. Lett. 80, 1066 (1998).[17] J. C. Cuevas et al., Phys. Rev. Lett. 81, 2990 (1998).[18] A. I. Yanson, thesis, Universiteit Leiden, The Netherlands,2001.[19] B. Ludoph and J. M. van Ruitenbeek, Phys. Rev. B 61,2273 (2000).[20] Iridium is very brittle and the highly corrugated electrodesthat result after a break make it difficult to find a propercontact. It was necessary to frequently verify the calibra-tion to obtain reliable results.[21] Y. Kondo and K. Takayanagi, Science 289, 606 (2000).[22] E. Tosatti et al., Science 291, 288 (2001).[23] O. Gülseren, F. Ercolessi, and E. Tosatti, Phys. Rev. Lett.80, 3775 (1998).266102-4

Common origin for surface reconstruction and the formation of chains of metal atoms

Smit, R. H. M.

Untiedt, C.

Yanson, A. I.

van Ruitenbeek, J. M.

Eldorado - Ressourcen aus und für Lehre, Studium und Forschung

Common Origin for Surface Reconstruction and the Formation of Chains of Metal Atoms

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Common Origin for Surface Reconstruction and the Formation of Chains of
  Metal Atoms

Common Origin for Surface Reconstruction and the Formation of Chains of Metal Atoms

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Repositorio Institucional de la Universidad de Alicante

Eldorado - Ressourcen aus und für Lehre, Studium und Forschung

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