Stability of Supported Lead Nanoparticles: Five-Fold Twinned

Article pubs.acs.org/JPCC

Stability of Supported Lead Nanoparticles: Five-Fold Twinned Pyramids versus Single Crystals Lise Serrier-Garcia,* François Debontridder, Dominique Demaille, Tristan Cren, and Dimitri Roditchev Institut des Nanosciences de Paris, Université Pierre et Marie Curie (UPMC) and CNRS-UMR 7588, 4 place Jussieu, 75252 Paris, France S Supporting Information *

ABSTRACT: Many metals adopt a face-centered cubic structure in their bulk form, but they often exhibit important structural and morphological deviations when confined to nanoscale or interacting with the environment. In this paper, the growth of lead nanoislands on silicon(111) has been investigated in ultrahigh vacuum for different evaporation conditions: temperature, flux, annealing time, and source or surface condition. Unexpected Pb icosahedral nanoparticles of a very large, up to 100 nm, size have been revealed to grow on Si(111) substrate. The coexistence between these 5-fold twinned Pb pyramids and expected face-centered cubic (fcc) Pb single crystals has been investigated in situ by Scanning Tunneling Microscopy (STM) and ex situ by Scanning Electron Microscopy (SEM). We found that the growth of the Pb icosahedral particles only occurs when very high Pb diffusion conditions are met, with a high quality of the silicon surface and a purified lead source. The icosahedral pyramids have been observed to be more stable upon ripening at room temperature than fcc single crystals.

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INTRODUCTION At nanometer scale, many fundamental properties of materials, such as catalytic activity1 or superconductivity,2,3 among many others, strongly depend on particle size and morphology; they often have no macroscopic analogue.4 Tailoring these emerging properties requires to fully understand and control the nanostructure formation. In this frame, the supported growth of Pb on Si(111) has been extensively studied, motivated by exciting phenomena such as the quantum size effects, that is, stabilization of some particular island thicknesses due to electron confinement5 or the peculiar electronic and superconducting properties of the Pb nanoislands.5,3,6 Pb on Si(111) 7 × 7 growth follows the Stranski−Krastanov mode, consisiting of the completion of a wetting layer of one to a few monolayers (ML),7,8 followed by island nucleation and growth.9 The case of Pb/Si(111) is peculiar since the initial wetting layer is amorphous, while it is epitaxial in the usual Stranski-Krastanov growth.8,10 At the early stage of the growth, small Pb clusters of pyramidal or domelike shapes11 form and then undergo a 3D to 2D transition12 leading to flat top islands. The biggest islands then ripen very rapidly to reach stable heights, and the smallest ones suffer from a “sudden death”.13 The remaining stable Pb islands were reported to be (111)-oriented fcc single crystals. A detailed kinetic phase diagram as a function of temperature and coverage was proposed by Hupalo et al.:14 for a few ML coverages and for a temperature low enough (130−250 K), two-dimensional Pb nanocrystals are stabilized. Their thickness shows a prevalence of “magic heights” 5, 7, and 9 ML5 attributed to quantum size effects. Despite numerous works on Pb/Si(111) growth, little is known about high temperature deposition and/or larger © 2015 American Chemical Society

coverage, deviating from the kinetic phase diagram of Hupalo et al. According to this work, higher temperature deposition produces large islands of hundreds of nm still having steep edges and sharp height distribution. Some other works have shown that the islands may have an incomplete terrace on top,15,16 and further annealing at room temperature is commonly employed to fill the craters and flatten the top terrace of the islands.17−19 At the contrary, Stepanovsky et al. state the narrow temperature range of stability of uniform height flattop Pb islands and report on the formation of multiheight “mounds” upon room temperature annealing.20 Results obtained for deposition at room temperature also show some discrepancy: multiple step heights islands with craters have been observed,21 whereas large meanders formed by coalescence of neighboring islands still exhibit flat tops for up to 210 ML.8 Also, all of the reported Pb nanoislands grown on Si(111) crystallize into a face centered cubic (fcc) structure, as for bulk lead. Even on vicinal Si surfaces, Pb islands tend to grow parallel to the (111) plane and exhibit tilted flat pancakelike islands, assisted by twinning boundaries.22 In addition, the nucleation, growth, and ripening of Pb islands deposited on Si(111) are largely conditioned by the surface mobility of Pb adatoms. Precisely, the wetting layer forming at the early stage of growth appears to play a crucial role in the formation of the islands, due to its unusually high mobility properties.23 It has been stated that only a rapid collective mass transport could explain the fast kinetic growth Received: January 15, 2015 Revised: May 5, 2015 Published: May 7, 2015 12651

DOI: 10.1021/acs.jpcc.5b00441 J. Phys. Chem. C 2015, 119, 12651−12659

Article

The Journal of Physical Chemistry C

Figure 1. Different kinds of Pb islands grown on Si(111). The 2.0 × 2.0 μm2 STM topography in (a) shows (111)-oriented fcc single crystals with irregular edges and some rims all around. The 1.2 × 1.2 μm2 STM image in (b) shows typical (111)-oriented flat and faceted fcc single crystals. The 4.4 × 4.4 μm2 STM image in (c) shows a new kind of pyramidal island with pentagonal base; in inset is a zoomed pyramid (320 × 320 nm2). The 3.9 × 3.9 μm2 SEM image performed ex situ in (d) shows that (111)-oriented flat fcc islands coexist with 3D islands and icosahedral pyramids (surrounded by green circles); in inset is a zoomed pyramid (160 × 160 nm2). The images a−d were acquired at ambient temperature, the sample preparation was the following: (a) 5 ML@3 ML/min@290 K, (b) 3.5 [email protected] ML/min@260 K, (c) 12 [email protected] ML/min@260 K, and (d) 6 ML@ 1.5 ML/min@290 K.

of Pb islands,24 and a collective liquid-like motion of Pb atoms in the dense wetting layer has been proposed to explain this extraordinary behavior.25 Therefore, any mechanism that would impair the Pb diffusion might have a strong effect on the nucleation and growth of Pb nanoislands. Among possible mechanisms, one might think about atomic impurities in the Pb flux, surface defects such as step-edges that limit the diffusion from one terrace to another. Free standing nanoparticles of fcc metals may deviate from the single crystalline form, adopting, for instance, specific twinrelated regular polycrystalline structures. Such nanoparticles are known as Multiply Twinned Particles (MTPs).26−31 The basic structure of MTPs can be described as an assembly of singlecrystal tetrahedral blocks, which are twin-related on their common facets. MTPs with icosahedral or decahedral shapes are quite common for free-standing and surface grown fcc metallic clusters.29,32,33 Already in 1967, Ino et al. reported the

observation of MTPs of gold on NaCl and KCl crystals. Allpress et al. also observed icosahedral MTPs for gold deposited on mica of sizes up to 30 nm.27 More recently, whereas free-standing regular Ag icosahedra of up to 3 μm size can be obtained,34 only small icosahedra of 5−20 nm size have been observed for Ag on SrTiO3, as for Au and Cu.35−37 The difference between the morphology of free-standing and supported nanoparticles may be partially explained by additional energy terms due to the interaction with the substrate such as epitaxial strain and stress.29,38−40 In addition, modified Wulff models taking into account kinetic parameters have been developed to explain a large variety of MTP shapes.41 Concerning Pb, icosahedral particles of up to 1.5 μm have been reported when grown by electrodeposition on HOPG,2 and MTPs of 8 nm with a decahedral structure have been observed by HRTEM.42 To our knowledge, no observation of surface grown Pb icosahedral islands were reported yet. 12652


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faceting, as shown on Figure 1b. Such structures have been reported by several groups and have been used for the study of confinement effects in superconductors.2,3 After several preparation/evaporation cycles, the Pb source purity improves further; the resulting fcc islands grow more and more regular and faceted from elongated (Figure 1a) to hexagonal shapes (Figure 1b). When the Pb source becomes well outgassed the shape of the Pb islands we get almost perfect (111)-orientated fcc single crystal (truncated fcc cube-octahedra), but surprisingly, a new kind of island also starts to grow (Figures 1d and 4). The shape of the islands is very different from the (111)-oriented fcc single crystals: the particles are pyramidal with a pentagonal base and identified to be a truncation of icosahedron (see below). Figure 1c is evidence for a relatively monodisperse distribution of icosahedral pyramidal particles, densely yet homogeneously spread on the surface over microns. A zoomed STM image of one selected particle (inset of Figure 1c) shows a regular 5-fold icosahedral shape representative for almost all particles in Figure 1c. The icosahedral pyramids are stable for hours at room temperature. We systematically observed them within the explored range of growth conditions (see Materials and Methods), while they are not referred to in the canonical phase diagram.14 Most of the observed icosahedral pyramids exhibit a height up to 50 nm, but some of them can grow up to 95 nm (Figure 3). To our knowledge, such a big size for supported icosahedra is unprecedented: the largest supported icosahedral MTPs reported until now were less than 20 nm high.26,27,32,33,35−37,44 We found that the icosahedral pyramids grow either as a single phase (Figure 1c) or along with other phases (Figure 1d), which are common truncated cuboctahedral fcc single crystals and other 5-fold symmetry Pb structures that we analyze later on. The different kind of structure we measured is shown in Figure 2. This figure proposes a classification of the whole variety of structures observed in this study by STM (left column) or by SEM (central column), along with their virtual 5- or 3-fold models truncated by the substrate, presented in gray (right column). A regular icosahedral pyramid is exposed in Figure 2a, while on Figure 2b we show a pyramid with five nonequivalent facets. The particle presented in Figure 2c exhibits two equivalent vertices where five facets meet; the vertices are connected together by a common edge parallel to the substrate. The structures in Figure 2a−c belong to the 5fold symmetry family. The last nanostructure, exposed in Figure 2d, is the expected 3-fold symmetry fcc single crystal of epitaxially grown Pb/Si islands. By extracting the angles between different facets of observed icosahedral pyramids from the STM data and properly taking into account the STM tip spatial convolution effect (see Figure 3a−c), we successfully identified each particle structure. They correspond to truncations of the icosahedron by the substrate plane at different angles: pyramids in Figure 2a (left and central panels) correspond to the truncation of regular icosahedrons (right panel), and the tilted particles in Figure 2b correspond to the truncation of icosahedrons. The particles in Figure 2c are regular icosahedrons truncated by the substrate plane parallel to one of its edges. Thus, all particles in Figure 2a−c are MTPs with a regular icosahedral structure. In order to understand how the icosahedral geometry is associated with the current supported 5-fold nanostructures, let

Here, we report a combined in situ Scanning Tunneling Microscopy (STM) and ex situ Scanning Electron Microscopy (SEM) investigation of Pb nanocrystal growth on a (7 × 7) reconstructed Si(111) substrate. Starting from the usual fcc single crystal growth conditions, we demonstrate that when the deposition involves a very pure Pb source and a well reconstructed 7 × 7 Si-substrate, unexpectedly, icosahedral particles nucleate together with the usual fcc single crystals. The icosahedra may grow surprisingly large, up to 100 nm. The factors governing the different growth modes are addressed.

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MATERIALS AND METHODS In situ sample preparation and STM experiments were carried out in two interconnected ultrahigh vacuum chambers of base pressure P = 2.10−11 mbar. Si(111) surfaces (phosphorusdoped Si, 0.001−0.005 W·cm) were prepared by outgassing at 600 °C for 12 h, rapid heating by direct current up to 1200 °C for a few seconds, and followed by a controlled cool down to room temperature (RT). Clean 7 × 7 reconstruction was systematically verified by STM prior lead depositions (see S1). Pb (99.99+% pure lead, Goodfellow) was deposited using an ebeam evaporator with flux monitoring (Focus EFM3T). A molybdenum crucible was used for the Pb evaporation that was refilled with freshly cut Pb wire several times. The flux monitor was calibrated in situ using a quartz crystal microbalance, prior to each deposition. During depositions, the substrate was kept at a controlled temperature ranging from 220 to 300 K. The deposition rate was varied in the range 0.1−10 ML/min, the amount of deposited lead, 4.5−14 ML. We found that the Pb particles are very stable in time; they survive even when exposed to air, thus, authorizing, in addition with in situ STM, for a complementary ex situ SEM analysis. STM images were acquired at RT; electrochemically etched Tungsten tips were used. The SEM experiments were performed ex situ, the samples being exposed to air for less than 15−20 min prior to their installation to the SEM chamber.

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RESULTS AND DISCUSSION

Several types of Pb islands on Si(111) with very different structures can be obtained depending on the growth conditions. A common structure reported in the literature5,43 is represented on Figure 1a: just after the growth, the islands exhibit irregular lateral shapes and various sizes. After several hours of ripening at RT, the islands edges develop facets. In this growth mode, the islands tops are generally nonuniform, a flat central part of a “magic” thickness (5−7−9 ML) is frequently surrounded by a thicker rim. By analyzing the statistics of several depositions, we noticed that irregular islands, as shown in Figure 1a, grow when the deposition is made onto a slightly carbon-contaminated 7 × 7 Si(111) surface or onto a yet lower quality surface obtained from chemically hydrogenated Si(111) in situ flashed at low temperature (850 °C). Furthermore, we found that the shape of the islands strongly depends on the filling level of the Pb crucible. Indeed, the same irregular island morphology of Figure 1a is obtained when a new Pb source or an almost empty crucible is used, even if 7 × 7 Si(111) surface has no defects. We anticipate that a poor substrate quality or an impure Pb source alter the Pb adatom diffusion and lead to such irregular growth. When the growth conditions are improved, by using a clean 7 × 7 Si(111) substrate and a well outgased new Pb source, the resulting fcc islands are flat-top and start demonstrating lateral 12653


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us first focus on the observed pyramid shape, Figure 2a. In all supposed cases of a regular decahedron, Marks decahedron or icosahedron, the ratio between the height (h) and the base width (w) does not depend on the pyramid size (w is the distance between one edge and the opposite facet). Equations give the formula for pentagonal pyramids obtained from a regular decahedron (eq 1.1), icosahedron (eq 1.2), and MTP Marks decahedron (eq 1.3). Here γ is the angle formed by the truncation plane corresponding to the substrate and the facets (Figure 3c). h = w h w h w

4 (5 − 5 (3 +

5) |sin(γ )| 5)

(1.1)

≅ 0.3416, with |sin(γ )| = ico

= deca

3(5 − 5 ) 40

1 ≅ 0.6325, with |sin(γ )| = 5

2 3

(1.2)

(1.3)

Thus, the structure of observed MTPs, as in Figure 2a, for instance, may be determined by calculating the h/w ratio. However, it is not a straightforward operation: While in STM experiments the height of pyramids and angles γ are measured with a relatively high precision (which depends mainly on the STM scanner calibration), there is a systematic overestimate of the particle widths (Figure 3) due to the convolution of the island and the STM tip shapes.18 This phenomenon produces an apparent increase of the particle widths by lateral translation of the facets in STM images. For the same reason, the sharp top vertices of MTPs all appear as the same rounded shape, which simply represents the morphology of the STM tip apex. If the island is anticipated to be an ideal 5-fold pyramid, taking into

Figure 2. From left to right: STM topography, SEM image, and model of truncated icosahedral Pb MTPs in the vertex orientation (a), tilted vertex orientation (b), edge orientation (c), and cuboctaedral Pb monocrystal (d; 210 × 210 nm2).

Figure 3. Identification of the icosahedral pyramids of Figure 2a as truncations of a regular icosahedron. (a) Scheme of the tip convolution effect. (b) The real shape is separated to the tip convolution effect in the STM image. (c) The height h and the real base size wreal are given by the profile in red line. The yellow line corresponds to the measured profile. (d) Ratio between the height and base size in function of the height (h/ wreal = f(h)) of MTPs for the different preparation conditions of this study: the geometry of the observed pyramids corresponds to the regularly truncated icosahedron. Icosahedron (eq 1.2) and Mark decahedron (eq 1.3) values are represented by a gray line as a guide for eyes. Gray line. The experiments a−f were acquired at ambient temperature, the sample preparation was the following: (a) 7 [email protected] ML/min@220 K, (b) 15 [email protected] ML/min@255 K, (c) 12 [email protected] ML/min@260 K, (d) 7 [email protected] ML/min@260 K, (e) 6 [email protected] ML/min@270 K, (f) 7 [email protected] ML/min@ 300 K. 12654


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Figure 4. STM images (1.6 × 1.6 μm2) of fcc single crystals islands (in black), icosahedral pyramids (in white), and intergrowth Pb islands (in gray) just after deposition (a) and after 20 h of ripening at RT (b), showing the death (islands marked by blue circles) of most of the fcc single crystals and the higher stability of the icosahedral pyramids. SEM images (d) and (e) show regular icosahedral pyramids (green circles), fcc single crystals and large clusters (d, SEM image 3.1 × 4.5 μm2; e, SEM image 3.2 × 4.2 μm2). The disappeared islands leave a fingerprint in the wetting layer like as a dark trace in the STM images and a bright trace in the SEM images. (c) STM cross section of Pb islands (blue curve) disappearing due to ripening and leaving imprints in the Pb wetting layer (red curve × 10). The images a−e were acquired at ambient temperature; the sample preparation was the following: (a, b) 15 [email protected] ML/min@255 K, (d, e) 6 [email protected] ML/min@270 K.

⎛ 5⎞ θico = π arctan⎜ − ⎟rad ≅ 138.19° ⎝ 3 ⎠

account the tip convolution effect is rather easy. The plane equations are determined for each individual facet of a selected pyramid, and its real shape is reconstructed knowing the measured particle height (see Figure 3c). In this way, the real width of each pyramid becomes accessible. For instance, the real width of the pyramid shown in Figure 3a,b is wreal = 95 nm, instead of the apparent one, wmeasured = 160 nm. In Figure 3c we report the measured h/wreal|exp ratio of pyramids like Figure 2a observed in our set of experiments. Despite a significant dispersion of the results provoked by the limited precision of width correction (about 5%), the h/w ratio is very close to h/w|ico for all experiments: h/w = 1.03 (±0.09) h/w|ico. The method described above is quite indirect; the best way to identify regular icosahedra consists in directly measuring the dihedral angles formed by two adjacent facets of the pyramid. This could be done with only a few pyramids exhibiting five well-defined facets. The dihedral angles formed by two adjacent facets of the pyramid were found, θexp ≅ 138° (±4°),45 quite identical for all five edges. This value is very close to θico, as expected for icosahedrons:

(2.1)

This result strongly suggests that the observed 5-fold pyramids are truncated regular icosahedra, as presented in the right column of Figure 2a. Let us now examine the shape of tilted icosahedral particles presented in Figure 2b. The inclination angle of the symmetry axis is inferred from the angle γ between the largest unit front and the substrate. We typically measure two different angles whatever the height is, γ1 = 21.8° (±2.6°) and γ2 = 9.8° (±2.1°), which should correspond to inclination 15.6° (±2.6°) and 27.6° (±2.6°), respectively. In all experiments, the 5-fold symmetry axis belongs to a symmetry plane perpendicular to the substrate and crossing through an edge and the middle of the opposite face. A cut through this symmetry plane defines a vertex angle α (Figure 3c). Equation 3 gives this vertex angle α defined for a regular icosahedron:

(

α|th = arcos

(

+ arcos 12655

4(5 − (5 −

5 )/30

5 )/10

)

) ≅ 111°

(3)


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Figure 5. Scanning electron microscopy images, such as on image g (1.7 × 1.2 μm2), permit to perform a statistical analysis of the orientation of islands relative to the substrate. The three symmetry axes of the Si(111) substrate are revealed by the lateral 3-fold fcc single crystal-edge orientations (histogram in (a)). A simulated histogram in (c) is done assuming an uncertainty on angle measurement of 2.1° (panel c). The histogram of angles for the icosahedral pyramids is shown in (b). Simulations of such histogram supposing one edge of the pyramids oriented along one axes of the substrate are shown in (f−h) for two different angle dispersion. In (f) a standard deviation of 4.5° on the angle is assumed and no preferential orientation appears due to this high angle dispersion (shown in panel d). The image g was acquired at ambient temperature; the sample preparation was the following: 6 [email protected] ML/min@270 K.

On the same islands on which we measured γ1 and γ2, we found, respectively, α1 = 1.10 (±0.05)α|th and α2 = 1.39 (±0.05)α|th. It appears that α1 corresponds to the value expected for a regular icosahedron up to 10% while α2 departs quite a lot from this value. The first class fits thus in the class of tilted regular icosahedra, while α2 indicates that the second kind of island belongs to a class of irregular truncated icosahedra. Thus, the combined analysis of γ and α angles indicated that the slightly tilted pyramids are truncated regular icosahedra,

while the strongly inclined pyramids correspond to truncated irregular icosahedra. We now focus on the evolution of the islands in time. Figure 4 shows two STM images acquired just after lead deposition (a) and 20 hours later (b), evidencing the ripening process: almost all fcc single crystals (colored in black) disappear, while it affects only about half of icosahedral pyramids (colored in white). This reflects the higher stability of icosahedral particles in a complex dynamical process46 instead of the usual sizedependence of thermodynamically stabilized particles. Remark12656


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icosahedral particles leads to a very different picture. As shown on Figure 5b, no peak dominates in the histogram. It would be tentative to suggest that 5-fold island edges do not reflect the 3fold symmetry of the substrate. Indeed, with the angle uncertainty found previously, σ = 2.1°, we would expect a histogram similar to the simulation shown on Figure 5h if one of the edges was aligned with the substrate axes. However, the standard deviation of the five angles determined from the pentagonal base presents a large uncertainty due to small irregularity of the MTPs. Thus, we expect a higher standard deviation on the measured angle, one part being due to the measurement uncertainty σ = 2.1° and another part due to the real irregularity of the icosahedra. The simulated histogram of Figure 5f uses a standard deviation σ = 4.5° (see Figure 5d for the angular dispersion); with such angular precision, the predicted histogram does not show any peak. Thus, this statistical analysis does not allow us to conclude if one of the five unit crystals of MTPs is orientated along one 3-fold direction of the substrate. However, if one of the units is aligned, the others have to be rearranged in order to minimize the twin boundaries and, thus, lose the exact 3-fold memory.44 Let us recall now the observed evolution of the island morphology. The use of a newly installed Pb source results in irregular fcc crystals, as in Figure 1a, even if the deposition is done onto a perfectly reconstructed 7 × 7 Si(111). Increasing the number of depositions with the same source while keeping a high substrate quality results in more and more regular single crystals, which are flat top and laterally faceted, as in Figure 1b. Increasing the number of preparation cycles, the fcc phase declines; one gets predominantly icosahedral particles. At the end of life of a given Pb source, the situation reverses: the fcc single crystal phase reappears and becomes dominant; the islands grow more and more irregular and rimmed, like those of Figure 1a. A tentative explanation is that the concentration of contaminants in the almost empty source increases, thus, leading to a loss of the surface mobility of deposited Pb atoms. Our guess is consistent with the fact that a high density of steps at the Si(111) surface also alters the Pb diffusion from one terrace to another and favors the fcc single crystal phase (Figure 3d). Finally, while we identified the discovered 5-fold nanoparticles as multiply twin particles with a truncated icosahedra structure, their true internal structure remains unknown. Twins, dislocations or liquid interfaces49 can stabilize 5-fold symmetry, but straightforward experiments or precise simulations, including a huge number of atoms involved in the observed particles, are lacking and require further investigations. Transmission electron microscopy (TEM) on cross sections could help elucidate the internal atomic arrangement.

ably, disappeared particles (marked by blue circles) leave an imprint on the wetting layer: the shape of the dips is triangular or irregular pentagonal, reflecting the shape and orientation of disappeared particles. In STM images, the imprint is characterized by an atomically irregular structure with different roughness as compared to the naked amorphous wetting layer, and a tiny average depletion of 0.15 Å. The two line profiles of Figure 4c show that three of five dips observed after 20 h of ripening correspond to the imprints of three islands. This indicates that the wetting layer structure is modified by the presence of an island above it and that it does not recover fully the disordered structure after the disappearance of the island. Feng et al. have shown that the structure of the wetting layer under the islands is a well-ordered fcc Pb.10 This ordered structure is thus definitely lost upon the island death. Many imprints in the wetting layer can already be observed in Figure 4a. Most of them are of triangular shape, thus, corresponding to well-known fcc single crystals. It is deduced that the icosahedral particles (Figure 2a−c) are more stable than the fcc single crystals 3-fold islands, and the latter rapidly disappear. In other experiments, only icosahedral particles (as in Figure 2a or b) are directly observed without further ripening. In all experiments, however, the wetting layer keeps traces of the initial growth of mainly fcc single crystals. This leads one to conclude that during the early stage growth, islands are created by kinetic growth, but then the ripening process rapidly decreases the number of unstable islands to the benefit of icosahedral particles pyramidal islands or massive crystals (Figure 4d,e). As the mobility of Pb atoms in the wetting layer is very high at temperatures above 190 K, there is a very fast atom exchange between growing islands and wetting layer. Depending on temperature and density of islands, the energetically less stable islands consequently disappear very rapidly. The major role played by the Pb diffusion on the stabilization of different types of islands is illustrated on Figure 4e. It shows a SEM image acquired close to the macroscopic edge of a Si(111) sample, where the Si step heights and density significantly increase. One can see that far from the sample edge, where the substrate exhibit very large atomic terraces, only massive crystals and pyramids grow, while near the edge the very high step density favors small fcc single crystals. We attribute this phenomenon to the fact that the step edges strongly limit the surface diffusion of deposited Pb atoms from one terrace to another, thus, confining the Pb diffusion within strips. That, in turn, modifies the nucleation, growth, and ripening processes. The growth of Pb/Si(111)-(7 × 7) is well-known to be governed by hcp or fcc stacking.47 The 3-fold symmetry revealed by the islands edge-orientation follows thus the symmetry of the substrate. As the nucleation of 3- and 5-fold symmetry fcc islands begins from the formation of a common primitive undifferentiated aggregate,48 there is a good reason to think that MTPs orientation conserves preferential directions relative to the substrate. A statistical study of the edge orientation for triangular and icosahedral islands was carried out with our SEM measurements, as reported in Figure 5. The histogram of Figure 5a reproduces exactly the three expected directions for the fcc single crystals (blue bars in Figure 5e). The precision on the angle measurement is estimated by simulating a similar histogram (Figure 5e) based on a standard deviation on the measured angle of σ = 2.1° (see Figure 5c for the simulated error). Doing the same angle histogram analysis for the

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CONCLUSION AND OUTLOOK We revealed the existence of Pb multiply twinned particles of icosahedral structure on Si(111) that may grow up to 100 nm large. Depending on the substrate and the Pb-source cleanness, icosahedral particles grow in competition with the usual (111)oriented fcc single crystals. We found that a higher lead mobility induced by a purified source and a low density of defects on the silicon surface leads to the prevalence of icosahedral particles. Different orientations of icosahedra have been observed, with tilted vertices or edge orientation. Pb icosahedra are surprisingly stable, and a monodisperse homogeneous distribution can even be obtained. The Pb icosahedra could potentially present interesting physical and 12657


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(14) Hupalo, M.; Kremmer, S.; Yeh, V.; Berbil-Bautista, L.; Abram, E.; Tringides, M. C. Uniform Island Height Selection in the Low Temperature Growth of PbSi(111) 7 × 7. Surf. Sci. 2001, 493, 526− 538. (15) Li, M.; Wang, C. Z.; Evans, J. W.; Hupalo, M.; Tringides, M. C.; Ho, K. M. Competition Between Area and Height Evolution of Pb Islands on a Si(111) Surface. Phys. Rev. B 2009, 79, 113404. (16) Zhang, X.; Liu, J.; Li, B.; Wanga, K.; Ming, F.; Wang, J.; Xiao, X. Effect of Substrate Doping Concentration on Quantum Well States of Pb Island Grown on Si(111). Surf. Sci. 2010, 604, 175−180. (17) Nishio, T.; Ono, M.; Eguchi, T.; Sakatai, H.; Hasegawa, Y. SizeDependent Superconductivity of Pb Islands under Magnetic Fields Studied by Low-Temperature Scanning Tunneling Microscopy/ Spectroscopy. Jpn. J. Appl. Phys. 2007, 46, L880−L882. (18) Nishio, T.; An, T.; Nomura, A.; Miyachi, K.; Eguchi, T.; Sakata, H.; Lin, S.; Hayashi, N.; Nakai, N.; Machida, M.; Hasegawa, Y. Superconducting Pb Island Nanostructures Studied by Scanning Tunneling Microscopy and Spectroscopy. Phys. Rev. Lett. 2008, 101, 167001. (19) Liu, J.; Wu, X.; Ming, F.; Zhang, X.; Wang, K.; Wang, B.; Xiao, X. Size-Dependent Superconducting State of Individual Nanosized Pb Islands Grown on Si(111) by Tunneling Spectroscopy. J. Phys. Condens. Mater. 2011, 23, 265007. (20) Stepanovskyy, S.; Yeh, V.; Hupalo, M.; Tringides, M. C. Stabilization of the Pb Si(1 1 1) (7 × 7) Uniform Height Islands to Higher Temperatures with Oxygen Adsorption. Surf. Sci. 2002, 515, 187−198. (21) Nishio, T.; Ono, M.; Eguchi, T. Superconductivity of Nanometer-Size Pb Islands Studied by Low-Temperature Scanning Tunneling Microscopy. Appl. Phys. Lett. 2006, 88, 113115. (22) Fokin, D. A.; S. I. Bozhko, S. I.; Dubost, V.; Debontridder, F.; Ionov, A. M.; Cren, T.; Roditchev, D. Electronic Growth of Pb on the Vicinal Si Surface. Phys. Status Solidi C 2010, 7, 165−168. (23) Gramlich, M. W. R.; Hayden, S. T.; Chen, Y.; Kim, C.; Conrad, E. H.; Tringides, M. C.; Miceli, P. F. Nonequilibrium Behavior of the Pb Wetting Layer on Si(111)7 × 7. Phys. Rev. B 2011, 84, 075433. (24) Li, M.; Evans, J. W.; Wang, C. Z.; Hupalo, M.; Tringides, M. C.; Chan, T.-L.; Ho, K. H. Strongly-Driven Coarsening of Height-Selected Pb Islands on Si(111). Surf. Sci. 2007, 601, L140−L144. (25) Huang, Li; Wang, C. Z.; Li, M. Z.; Ho, K. M. CoverageDependent Collective Diffusion of a Dense Pb Wetting Layer on Si(111). Phys. Rev. Lett. 2012, 108, 026101. (26) Ino, S. Epitaxial Growth of Metals on Rocksalt Faces Cleaved in Vaccum. II. Orientation and Structure of Gold Particles Formed in Ultrahigh Vacuum. J. Phys. Soc. Jpn. 1966, 21, 346−362. (27) Allpress, J. G.; Sanders, J. V. The Structures and Orientation of Crystals in Deposits of Metals on Mica. Surf. Sci. 1967, 7, 1−25. (28) Ino, S.; Ogawa, S. Multiply Twinned Particles at Earlier Stages of Gold Film Formation on Alkalihade Crystals. J. Phys. Soc. Jpn. 1967, 22, 1365−1374. (29) Gillet, M. Structure of Small Metallic Particles. Surf. Sci. 1977, 67, 139−157. (30) Marks, L. D. Surface Structure and Energetics of Multiply Twinned Particles. Philos. Mag. A 1984, 49, 81−93. (31) Marks, L. D. Experimental Studies of Small Particle Structures. Rep. Prog. Phys. 1994, 57, 603−649. (32) Ogawa, S.; Ino, S. Formation of Multiply-Twinned Particles in the Nucleation Stage of Film Growth. J. Vac. Sci. Technol. 1969, 6, 527−534. (33) Hayashi, T.; Ohno, T.; Yatsuya, S.; Uyeda, R. Formation of Ultrafine Metal Particles by Gas-Evaporation Technique. IV. Crystal Habits of Iron and Fcc Metals, Al, Co, Ni, Cu, Pd, Ag, In, Au, and Pb. J. Appl. Phys. Jpn. 1977, 16, 705−717. (34) Li, C. R.; Lu, N. P.; Xu, Q.; Mei, J.; Dong, W. J.; Fu, J. L.; Cao, Z. X. Decahedral and Icosahedral Twin Crystal of Silver: Formation and Morphology Evolution. J. Cryst. Growth 2011, 319, 88−95. (35) Silly, F.; Castell, M. R. Growth of Ag Icosahedral Nanocrystals on a SrTiO3(001) Support. Appl. Phys. Lett. 2005, 87, 213107.

catalytic properties. For instance, the superconducting properties of these large multiply twinned particles are expected to be peculiar;2 indeed, the superconducting vortices that usually adopt a hexagonal arrangement will be frustrated by the 5-fold symmetry.

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ASSOCIATED CONTENT

S Supporting Information *

Additional supporting figure. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b00441.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from French ANR Project Electrovortex and University Pierre and Marie Curie Grant Emergence.

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REFERENCES

(1) Leary, R.; Saghi, Z.; Armbrüster, M.; Wowsnick, G.; Schlögl, R.; Thomas, J. M.; Midgley, P. A. Quantitative High-Angle Annular DarkField Scanning Transmission Electron Microscope (HAADF-STEM) Tomography and High-Resolution Electron Microscopy of Unsupported Intermetallic GaPd2 Catalysts. J. Phys. Chem. C 2012, 116, 13343−13352. (2) Engbarth, M.; Milošević, M. V.; S. J. Bending, S. J.; Nasirpouri, F. Geometry-Guided Flux Behaviour in Superconducting Pb Microcrystals. J. Phys. Conf. Ser. 2009, 150, 052048. (3) Cren, T.; Serrier-Garcia, L.; Debontridder, F.; Roditchev, D. Vortex Fusion and Giant Vortex States in Confined Superconducting Condensates. Phys. Rev. Lett. 2011, 107, 097202. (4) Cavo, F. Nanoalloys, From Fundamentals to Emergent Applications; Elsevier: New York, 2013; ISBN 9780123946164. (5) Michailov, M. In Nanophenomena at Surfaces, Fundamentals of Exotic Condensed Matter Properties; Ertl, G., Lüth, H., Mills, D. L., Eds.; Springer: New York, 2011. (6) Liu, J.; Wu, X.; Ming, F.; Wang, K.; Xiao, X. Superconductivity of Individual Pb Islands on Si(111): Pseudogap, Critical Region, Density of States and Island Size. Supercond. Sci. Technol. 2013, 26, 085009. (7) Ganz, E.; Hwang, I.-S.; Xiong, F.; Theiss, S. K.; Golovchenko, J. Growth and Morphology of Pb on Si(111). Surf. Sci. 1991, 257, 259− 273. (8) Weitering, H. H.; Heslinga, D. R.; Hibma, T. Structure and Growth of Epitaxial Pb on Si(111). Phys. Rev. B 1992, 45, 5991−6002. (9) Tringides, M. C.; Hupalo, H.; Yakes, M. Novel Diffusion Pathways in Low Temperature Self-Organization of Nanostructures. Phys. A 2005, 357, 216−249. (10) Feng, R.; Conrad, E. H.; Tringides, M. C.; Kim, C.; Micelli, P. F. Wetting-Layer Transformation for Pb Nanocrystals Grown on Si(111). Appl. Phys. Lett. 2004, 85, 3866. (11) Prieto, J. E.; Markov, I. Forbidden Island Heights in StressDriven Coherent Stranski-Krastanov Growth. Phys. Rev. Lett. 2007, 98, 176101. (12) Su, W. B.; Lin, H. Y.; Chiu, Y. P.; Shih, H. T.; Fu, T. Y.; Chen, Y. W.; Chang, C. S.; Tsong, T. T. Correlation Between Morphological Transition and Preferred Thickness of Pb and Ag Islands on Si(111)7 × 7. Phys. Rev. B 2005, 71, 073304. (13) Zhang, G. P.; Hupalo, M.; Li, M.; Wang, C. Z.; Evans, J. W.; Tringides, M. C.; Ho, K. M. Stochastic Coarsening Model for Pb Islands on a Si(111) Surface. Phys. Rev. B 2010, 82, 165414. 12658


Article

The Journal of Physical Chemistry C (36) Silly, F.; Castell, M. R. Bimodal Growth of Au on SrTiO3(001). Phys. Rev. Lett. 2006, 96, 086104. (37) Silly, F.; Castell, M. R. Temperature-Dependent Stability of Supported Five-Fold Twinned Copper Nanocrystals. ACS Nano 2009, 3, 901−906. (38) Howie, A.; Marks, L. D. Elastic Strains and the Energy Balance for Multiply Twinned Particles. Philos. Mag. A 1984, 49, 95−109. (39) Müller, P.; Kern, R. Equilibrium Nano-Shape Changes Induced by Epitaxial Stress (Generalised Wulf-Kaishew Theorem). Surf. Sci. 2000, 457, 229−253. (40) Henry, C. R. Morphology of Supported Nanoparticles. Surf. Sci. 2005, 80, 92−116. (41) Ringe, E.; Van Duyne, R. P.; Marks, L. D. Kinetic and Thermodynamic Modified Wulff Constructions for Twinned Nanoparticles. J. Phys. Chem. C 2013, 117, 15859−15870. (42) Kofman, R.; Cheyssac, P.; Aouaj, A.; Lereah, Y.; Deutscher, G.; Ben-David, T.; Penisson, J. M.; Bourret, A. Surface Melting Enhanced by Curvature Effects. Surf. Sci. 1994, 303, 231−246. (43) Yakes, M. K. Self Assembled Nano-Structures of Pb on Si(111) Studied by SPA-LEED: Quantum Size Effect Driven Pb Islands and the ″Devil’s Staircase″, Retrospective Theses and Dissertations. 2006, Paper 1983. (44) Wu, Y.; Chen, Q.; Takeguchi, M.; Furuya, K. High-Resolution Transmission Electron Microscopy Study on the Anomalous Structure of Lead Nanoparticles with UHV-MEB-TEM System. Surf. Sci. 2000, 462, 203−210. (45) Note that the value in brackets indicates the standard deviation of statistical measurements. (46) Silly, F.; Castell, M. R. Selecting the Shape of Supported Metal Nanocrystals: Pd Huts, Haxagons, or Pyramids on SrTiO3(001). Phys. Rev. Lett. 2005, 94, 046103. (47) Binz, S. M.; Hupalo, H.; Tringides, M. C. Height-Dependent Nucleation and Ideal Layer by Layer Growth in Pb/Pb(111)/Si(111). Phys. Rev. B 2008, 78, 193407. (48) Gillet, E.; Gillet, M. Formation et Croissance des Édifices Atomiques Présentant des Arrangements Pentagonaux. Thin Solid Film 1973, 15, 249−257. (49) Henry, S. C.; Hall, B. D. Molecular-Dynamics Simulations of Lead Clusters. Phys. Rev. B 2001, 64, 085425.

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