YbO Nanoparticles Created by Oxidation

Article pubs.acs.org/JPCC

Radial Structure of Free Yb/YbO Nanoparticles Created by Oxidation Before or After Aggregation with Divalent Instead of Trivalent Oxide Chaofan Zhang,† Tomas Andersson,† Olle Björneholm,† Xiaojun Xu,‡ Maxim Tchaplyguine,*,§ and Zejin Liu*,‡ †

Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden College of Optoelectronic Science and Engineering, National University of Defense Technology, 410073 Changsha, China § MAX-lab, Lund University, Box 118, 22100 Lund, Sweden ‡

ABSTRACT: Nanoparticles consisting of Yb and its divalent oxide YbO have been created by two different oxidation approaches based on the gas-aggregation method with magnetron sputtering. In one type of nanoparticles, the Yb oxide molecules, created by reactive sputtering before the aggregation, agglomerate predominantly in the interior of mixed-composition nanoparticles. In the other type, the oxide is formed by exposing the preformed metallic Yb nanoparticles to oxygen, which at certain conditions is believed to oxidize primarily the surface of such nanoparticles. Such segregated stoichiometry has been disclosed using Yb 4f core-level photoelectron spectroscopy at a series of oxidation conditions for each type of production. In contrast to a typical macroscopic case where Yb is trivalent, in both production cases, Yb is divalent in the oxide. By using the production methods suggested, it becomes possible to tailor the electronic and thus the physical and chemical properties of such nanoparticles, which are discussed in the literature as building blocks for photonic, electronic, and magnetic nanoscale devices.

Y

devices.14 The problem with silicon oxide is that, at nanoscale, the leakage current in it (due to a low dielectric constant of 3.9) increases to a degree that it practically cannot be used as a gate in transistors. In another field (in nanophotonics), Yb-doped garnet nanoparticles open perspectives for lasing at higher photon energies.15,16 In magnetism, Yb-based compounds are among those in which heavy-fermion and Kondo-effect related phenomena take place.7,17−20 The combination of technological perspectives and fundamental peculiarities gives the question of ytterbium valency in such compounds as oxides and alloys, at nanoscale, an increased relevance. The present study deals with ytterbium oxide nanoparticles, and a report on Yb−Al and Yb−Al-oxide nanoparticles will follow. The method used in the present work to produce such nanoparicles opens one more degree of freedom for tailoring their properties, namely, their stoichiometry and radial element distribution. The method based on vaporization by sputtering and subsequent vapor aggregation allows one to create elemental distribution in which the oxide either dominates at the surface of the, to a large extent, metallic interior, or vice versa; is the main substance in the inner part of a nanoparticle, the later structure being similar to that realized in our recent study on lead/lead-oxide nanoparticles.21 In the oxidized

tterbium is one of those peculiar rare-earth elements that can have mixed valency in a material.1−4 Free ytterbium atoms have two outermost electrons in the 6s shell, which become itinerant in metallic Yb, making it divalent with the 4f146s2 ground-state electronic configuration. At the same time, the fully filled 4f core−shell is so close in energy to the valence region that in some compounds, such as Yb2O3 or some Yb alloys, one of the 4f electrons is promoted to the 5d shell and another electronic configuration, 4f135d16s2, is realized, and one 5d and two 6s electron can take part in the chemical bonding with the valence electrons of the other constituent element of a compound,2,3 making Yb trivalent. The latter situation is the norm for the bulk oxide Yb2O3 and for the bulk Yb intermetallic compounds like, for example, novel Yb−Al alloys.5−10 Opening of the Yb 4f shell in these compounds leads to the emergence of very specific and sophisticated magnetic properties.11,12 However, there have been reports that in the surface monolayer of such compounds, where the dimensionality is reduced, or even in few outermost monolayers, no transition of one of the 4f electrons to the 5d shell takes place, and ytterbium valency remains equal to “two”, like in Yb metal.13 Nowadays, ytterbium oxide and ytterbium alloys enter at a high speed the group of advanced materials used for modern electronic, optical, and magnetic devices, all approaching nanoscale dimensions. For example, due to its very high dielectric constant (≈15), Yb oxide is discussed as a replacement for SiO2 in complementary metal oxide semiconductor (CMOS) © 2013 American Chemical Society

Received: April 16, 2013 Revised: June 5, 2013 Published: June 7, 2013 14390

dx.doi.org/10.1021/jp4037556 | J. Phys. Chem. C 2013, 117, 14390−14397

The Journal of Physical Chemistry C

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ytterbium nanoparticles, we find no significant sign of trivalent oxide, which should have implications for various properties of these nanoparticles. In the present work we have first created and characterized pure metallic Yb nanoparticles formed using the gas-aggregation method, which implements magnetron sputtering for creating primary atomic vapor.21,22 This vapor is produced and then condenses inside a liquid-nitrogen-cooled cryostat (T ≈100 K) in an atmosphere of argon and helium mixture.21,22 An in-house built vapor-aggregation nanoparticle source has been used in the experiments described below. The setup has been designed following the principles outlined in early works23,24 and followed also by several other research groups, for example.25,26 Magnetron sputtering of a solid material as a primary process was what distinguished these works from the classical cluster gas-aggregation approaches developed, for example, in the groups of Recknagel27 and Martin,28 where an oven was used for solid vaporization. The use of liquid nitrogen as coolant allows one to obtain a dense beam of nanoparticles with about 10 nm diameter. As will be discussed below in more detail, the high density of such beam is a vital property for the probing method used in the present work. It has been previously demonstrated27 that the cluster production intensified when a high mass-flow through the aggregation volume was provided. Such a flow can be naturally achieved by increasing the buffer/ carrier gas pressure. The gas is usually a mixture of helium and argon,28 and, as mentioned above, this has also been the case in our experiments. In the source with magnetron sputtering, argon carries one more function: being the main component of the discharge plasma. In our setup, argon is injected via a special homemade arrangement just in front of the so-called target: a 50 mm diameter metal disc out of the solid material to be vaporized. Helium is injected separately, through an input in the rear wall of the cryostat. For both gases, the input pressures are a few millibars. We establish empirically the values for both gases for each material being sputtered. In the case of Yb, the argon input pressure of ∼7 mbar and He of ∼11 mbar have been used. Such values provided the gas flow through the cryostat of ∼10 sccm. Apart from the input pressure values, the flow is defined by the cryostat exit nozzle made out of copper and having a 20 mm long 2 mm diameter channel. The condensation lengththe distance between the target and the exit of the nozzlehas been ≈20 cm. The magnetron head itself is a commercial 2″ device in our case. In the experiments with ytterbium, the DC magnetron power was set to 170 W. Thus formed free metallic Yb nanoparticles, the mean diameter of which has been estimated to be somewhat below 10 nm, have been probed in a beam using synchrotron-based photoelectron spectroscopy. The photoelectron spectra recorded for metallic Yb nanoparticles have been found to resemble to a great extent the response of a deposited Yb polycrystalline film at the substrate temperature of ∼100 K.29 Two different oxidation approaches have been implemented in the present work: (1) doping of the preformed metallic nanoparticles by letting them through a volume with oxygen after the cryostat (Figure 1, upper panel); and (2) reactive sputtering,21 using a mixture of argon and oxygen as sputter-gas inside the cryostat (Figure 1, lower panel). As briefly mentioned above, the latter method has recently been shown to result in metal vapor oxidation at an early stage, predominantly at the single-atom level, prior to the agglomeration into nanoparticles.21

Figure 1. Schematics of the sputtering-based gas-aggregation methods: (a) Yb-core/YbO-shell nanoparticles created by doping of metallic particles outside the cryostat at its exit; (b) YbO-core/Yb-shell particles’ production using reactive sputtering and oxide molecules’ aggregation yet in the liquid-nitrogen cooled cryostat.

Photoelectron spectroscopy (PES) is a unique element- and site-sensitive probing tool capable of distinguishing different geometric sites and chemical states in nonsupported nanoscale particles.21,30−32 In the PES signal of metallic Yb nanoparticles with fully filled 4f-level (i.e., divalent), the 4f bulk and surface responses, appearing as doublets, are well resolved (Figure 2),

Figure 2. Yb 4f photoelectron spectrum of metallic Yb nanoparticles with ≈7 nm diameter. The spectrum has been fitted assuming three different sites in the nanoparticles: bulk, high-coordination surface atoms (HCS), and low-coordination surface (LCS) atoms.

making the situation ideal to investigate the site-specific chemical composition with submonolayer (as shown below) resolution for particles of a few nanometers in dimensions. For that range of sizes, such a probing capability can be compared only with the most advanced electron microscopy. There is, however, an important difference in our PES approach relative to microscopy: we create and study nanoparticles in a beam propagating in vacuum, thus minimizing the influence of any environment first on the self-assembling process and then at the instant of “probing”. The wide X-ray radiation range accessible at the I411 beamline of MAX-lab, the Swedish national synchrotron radiation facility, has allowed us to choose the photon energy providing maximal ionization cross-section for the Yb 4f-level, 14391



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≈100 eV. This is also the value where the flux of the beamline is most intense. The Scienta R4000 electron energy analyzer mounted on the permanent end-station of the beamline has been used to record the photoelectron spectra. The spectrometer has been as a rule set to relatively high collection efficiency mode providing the moderate instrumental contribution to the spectra of ≈0.13 eV. The monochromator of the beamline has been tuned to give a matching spectral width of the radiation. A reliable comparison with macroscopic Yb spectra and between the spectra of metallic and oxidized nanoparticles has been possible due to an accurate absolute energy calibration of the spectra relative to the vacuum level. Such calibration has been facilitated by the absence of any support for free nanoparticles and has been made using the response from the uncondensed Ar in the particle beam. The Ar 3p signal appears in the electron binding energy region just above the response from Yb 4f level. In order to compare the binding energy values, which for our nanoparticles are measured relative to the vacuum level and for supported solids, relative to the Fermi edge, one has to add the work-function value to the 4f energies found in literature for macroscopic ytterbium. There is usually some span in the work-functions given by different sources, and this limits the accuracy of the comparison. In our work we have used the value of 2.6 eV for ytterbium presented in ref 33. Comparing the 4f PES responses for Yb polycrystalline film29 and for Yb nanoparticles, one can conclude that the characteristic property for both is the presence of two distinctly different coordinations at the surface. Indeed, while the bulk response of each of two spin−orbit components can be well described by a narrow single spectral peak, the corresponding surface spectral response of both thin films and nanoparticles requires at least two peaks separated by almost 0.5 eV from each other to be well described (Figure 2). The nanoparticle surface peak, which is furthest separated in binding energy E from the bulk (ΔE is 0.8−0.9 eV), corresponds to the lowercoordination surface atoms (LCS).29 The closer-to-bulk surface peak is due to the higher-coordinated surface atoms (HCS). It is worth noting here that the bulk of metallic Yb nanparticles created in the present work should have a well-ordered structure, since its response is a peak practically as narrow as for the thin films.29 The metallic Yb nanoparticle spectrum in Figure 2 has been fitted with the Doniach−Sunjic spectral lineshapes characteristic for metallic Yb2, with the asymmetry parameter fixed at 0.15. The 7/2−5/2 spin−orbit splitting of the Yb 4f level in nanoparticles has been derived to be 1.2 eV (Figure 2). The detailed results of the fitting are shown in Table 1. The difference of ≈0.3 eV for the core-level binding energy between the macroscopic metal and metallic nanoparticles has been used to estimate the nanoparticle diameter to be ≈7 nm implementing the so-called metallic sphere

approximation.34 The number of atoms in metallic Yb nanoparticles of such a diameter is a few thousands. We will now proceed to the oxidized Yb nanoparticles. As briefly mentioned above, we have produced them in two different ways, using variations of the gas-aggregation method: (1) with oxygen doping of the preformed metallic Yb nanoparticle (Figure 1, upper panel), and (2) implementing reactive sputtering of Yb metal with an oxygen/argon mixture (Figure 1, lower panel). In macroscopic ytterbium oxide (Yb2O3), Yb is trivalent. The presence of trivalent ytterbium in this compound can be unambiguously detected as a certain set of relatively narrow spectral features in the photoelectron signal centered at higher binding energy than the divalent Yb 4f response (see Figure 3). As can be seen, the responses of the

Figure 3. Yb 4f photoelectron spectra of Yb/YbO nanoparticles. The upper spectrum shows the response of the nanoparticles produced by doping method in the case of the highest oxygen pressure, and the lower one shows that produced by reactive sputtering, also at the highest possible oxygen fraction. The spectral region where the trivalent Yb signal would appear is also shown.

oxidized Yb nanoparticles are in both cases strongly dominated by divalent Yb, corresponding to an YbO composition for both production methods. Thus the nanoparticles created here have a different composition and Yb valency compared to the macroscopic ytterbium oxide. We will now proceed to discuss the structure of the oxidized nanoparticles in more details, starting with the doping case. In this case, the preformed metallic nanoparticles were exposed to oxygen at their exit from the cryostat (Figure 1, upper panel). Here a ring-formed reservoir with regulated backing pressure could produce a locally high concentration of O2 on the pathway of the nanoparticle beam. Our results demonstrate that in such a way oxygen does bind at least to the surface of Yb nanoparticles, and that such doping of them creates oxidized surfaces with Yb valency in the oxide equal to 2 and thus different from the bulkoxide value of 3. This has been possible to establish by comparing our nanoparticle 4f spectra (Figure 4) with those obtained from macroscopic ytterbium surface exposed to oxygen.35 There the corresponding supported oxidized Yb exhibits two broad spin−orbit split features in the 4f photoelectron spectrum, in practically the same binding energy region as metallic Yb. Figure 4, case c, shows a nanoparticle spectrum obtained in the present work with the oxygen-doping degree so high that no response from metallic bulk Yb is seen. There are only two broad features shifted downward by about

Table 1. The Fitting Results for the Spectra of Metallic Yb Nanoparticlesa BE (eV) ΔBE (eV) width (eV)

bulk

HCS

LCS

4.1

4.6 0.5 0.4

4.9−5.0 0.8−0.9 0.4

0.3

a

The data is for 4f7/2 spin-orbit component: the bulk binding energy and separations from bulk to LCS, and from bulk to HCS, as well as the width for the bulk and two surface peaks. 14392



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two peaks are almost as sharp and narrow as in the metallic bulk case a, and are close to the latter in their binding energy positions. From that (similar shapes and specific positions) we consider that the probability is high that these peaks are indeed due to the response from the bulk, and from metallic bulk. The peaks due to the oxidized bulk would rather appear as in the discussed below oxide created by reactive sputtering on the lower binding energy side from the original metallic bulk. As also observed in ref 13, it is below the metallic bulk responses where the main part of the extra intensity is observed in divalent Yb oxides. In ref 13, the authors give another example of a negative shift in the divalent oxide, for Ba, an element that has the same valence electronic configuration as Yb. As for the abundance of metallic Yb in such nanoparticles, the following observations can be made. The dominating contribution to the total bulk response should be from the outermost bulk layers (the bulk-surface interface), since the intensity of the photoelectron signal exponentially decreases with the probing depth. The metallic-bulk-like intensity is relatively high in case b, so we conclude that there is a substantial amount of metal atoms coordinated mainly to the other metal atoms just under the oxidized surface in case b. Thus there is a high probability for the segregated structure to have been created in the doping case b. Indeed, it is not very likely that under a metallic layer or two there is an oxidized part again: in the doping method the nanoparticles are preformed metallic and come out from the cryostat with the temperature preventing any significant mobility or diffusion. At the same time, for case c, it is not possible to make a judgment (just from our spectroscopic observations) on how deep the oxidation has happened. In Figure 6 we present a schematic illustration for the connection between the different Yb sites in various chemical states and the corresponding 4f electron binding energies. Our other approach to produce nanoparticles containing oxidized Yb involves formation of YbO molecules by reactive sputtering prior to the nanoparticle aggregation. Reactive sputtering is known to be an efficient method of bulk oxide production14,21 due to the presence of dissociated, ionized, and excited oxygen in the vicinity of the target where Yb exists mostly in the form of atomic vapor. In our setup, the O2 gas, premixed with argon, was let into the sputtering area close to the magnetron target. The initial input Ar pressure has been kept at about 7 mbar, as in the metallic nanoparticle production described above. The amount of O2 in the O2/Ar mixture in present experiments has been stepwise increased to around 5% with each situation characterized by photoelectron spectroscopy. The spectra recorded at two different sets of conditions with the highest and “intermediate” oxygen fractions are shown in Figure 5. At higher than 5% oxygen fractions, the signal intensity decreased significantly due to the introduction of nonactive gas (oxygen) into the discharge volume and due to the target surface partial oxidation and consequent decrease of DC-sputtering efficiency. The magnetron plasma is strongly localized to the target, and the probability is high that YbO molecules are predominantly formed here. Further away from the target, along the 20-cm condensation path in the cryostat, the concentration of the charged, excited, and/or dissociated oxygen decreases drastically (Figure 1). At the same time, free metal atoms are distributed much more uniformly being carried along the cryostat by the flow of the neutral gas toward the exit. It should be mostly the relatively inert ground-state oxygen molecules which are present, together with argon and agglomerating nanoparticles, outside the localized plasma

Figure 4. The Yb 4f photoelectron spectra of Yb/YbO nanoparticles created by O2 doping of the preformed metallic Yb nanoparticles. From the bottom to the top, the amount of O2 in the doping process increases. Spectrum a was recorded when no oxygen was admixed in the sputtering gas. In spectrum c, no metallic-bulk response is seen anymore.

0.5 eV relative to the metallic surface responses if one compares the center of gravity positions for the better resolved surface 4f5/2 features in case a and case c. The width of the surface response in case c (again judging from 4f5/2) is about 2 times larger than that of metallic one. Such spectral changes, together with the above-mentioned results on polycrystalline Yb film oxidation,35 bear a strong witness that at least several outer monolayer of nanoparticles are oxidized. Even in this case of the strongest exposure to oxygen, no apparent intensity has been detected in the binding energy region where the trivalent Yb oxide would manifest itself (see Figure 3, upper spectrum, recorded in an extended binding energy range). In Figure 4 we show the spectra in a shorter range to visually emphasize the changes (in the divalent region) taking place due to the doping. A combination of the fine doping-pressure control and 4fresponse “on-the-fly” monitoring has allowed us to create and characterize nanoparticles with another, intermediate oxidation degree using the metallic-bulk-like response in the spectra as a measure of oxidation. Figure 4, case b, presents a spectrum recorded at an oxygen doping pressure about 2 times lower than that in case c. In this intermediate case, the metallic bulklike response is again observed but is shifted by ≈0.2 eV to higher binding energy relative to case a (no oxygen). The surface peaks in spectrum b are similar to case c: they are broader and have maxima at lower binding energies than the metallic surface ones (case a). We can assume, from the spectroscopic arguments, the presence of oxidized surface here, in case b, too. And, indeed, we can expect the oxidation by doping in case b, since we know for sure from case c that this method works. Thus, while there is little doubt whether the surface is oxidized, the question of what happens to the bulk of nanoparticles is more uncertain. An important consideration to bear in mind here is that our nanoparticles come out from the cryostat cryogenically cold, and it has been established13 that oxygen diffusion under the Yb surface is practically absent at such temperatures. Let us now have a closer look at the spectrally well-defined bulk-like doublet in spectrum b. These 14393



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been disclosed. However, while there can be probably little doubts that the core of Yb nanoparticles created with reactive sputtering consists of, to a considerable extent, the oxide, the question of how the surface of such particles looks like is more uncertain. To repeat our preliminary considerations here, the surface is finalized far away from the intense-plasma region at the later stage of aggregation. Would it be possible that the particles have the segregated distribution of components, like in the case the lead/lead-oxide,21 the one inverse to the doping case? One can argue that Yb atoms agglomerating at the later stage can be efficiently oxidized even by the ground-state oxygen molecules, as we see in the doping case. Is there an answer to the question about the particle outermost layer dominant composition, which we could extract from the spectra? In both reactive-sputtering cases b and c, the surface 4f5/2 response (the better resolved one) has its center of gravity at the very same 6 eV as for the metallic case, while for the surface oxide it is at ≈0.5 eV lower binding energy and is visually broader. As for the bulk response, first of all, we see again in both cases b and c some signals at the very same energy positions as metallic bulk (Figures 5, 6). The most natural assumption is then that there is at least a monolayer of metallic ytterbium under the surface monolayer, the latter also metallic. Indeed, as discussed above, the bulk response should be mostly due to its outer layers. The most notable change in the nanoparticle response is seen in spectrum c, in the region to the lower binding energy side from the 4f5/2 bulk-peak position of the pure-metal nanoparticle − between ≈5.3 and ≈5.0 eV. There is a “flat” part in the spectrum, which we explain by the appearance of a new narrow peak related to the 4f5/2 level. If so, there should be a corresponding second spin−orbit component to the lower energy side of 4f7/2 bulk peak. And, indeed, this bulk peak, centered at around 4 eV, has a shoulder on the lower binding energy side. This shoulder, as well as the increase of intensity below the 4f5/2 bulk peak (the latter centered at 5.3 eV) is also present in the intermediate case b. Such a doublet to the lower binding energy side of Yb metallic bulk peaks has been interpreted as being due to the divalent monoxide YbO in the high-resolution studies of a macroscopic Yb film exposed to

Figure 5. Spectra of YbO/Yb nanoparticles produced by reactive sputtering with two different oxygen fractions in argon ∼3% (spectrum b) and ∼5% (spectrum c). Metallic Yb spectrum a is shown at the bottom. The thick vertical bars denote the positions of the 4f5/2 peaks due to the assigned to the bulk YbO peaks rising with the O2 fraction increase. The argon input pressure, relative to which the oxygen fraction is given, is ≈7 mbar.

volume. We consider it rather probable that further away from the target the aggregation proceeds via the condensation of nonoxidized metal atoms on the partly already agglomerated oxide “seeds”. It is also energetically favorable for such nanoparticles to place the oxide in the interior and the metal on the surface, since an oxide is usually a stronger bound substance.36 In our previous experiments on multicomponent vapor condensation in the same source,37 there has been always enough mobility in the nanoparticles during their stay inside the cryostat, enough to reach the energetically favorable distribution of components. For example, in our recent study in which reactive sputtering was used to create nanoparticles containing lead-oxide, the energetically favorable distribution21 with the oxidized interior and metal-dominated surface has

Figure 6. The assignment of Yb 4f5/2 energies for various sites in the mixed composition nanoparticles. The spectrum from Figure 4, case b (doping), and the spectrum from Figure 5, case c (reactive sputtering), are placed vertically. The solid black horizontal bars represent the binding energy of different sites. The lengths of the bars provide some information on the site propensity. 14394



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oxygen.13 However, while in ref 13 this extra doublet has been explained by the surface oxide formation, we assign it to the “bulk” oxide. This assignment is based on two observations: (1) the reactive-sputtering favors oxidation at an early stage of nanoparticle formation;21 and (2) in the doping experiments Yb surface monoxide has been shown to be at a higher binding energy. Since the extra doublet in spectrum c can be only explained by rather narrow peaks, it can hardly be due to the surface atoms, the latter always showing up in Yb as broad peaks. Another general consideration is that it would be rather unlikely if we did not see any bulk-oxide response with reactive sputtering, and the only signal that can be due to the bulk oxide is then these peaks below the binding energy position of the metallic outermost bulk layer. Since we do not see very deep into the nanoparticles, the extra peaks (the oxide signal) should be from the next pair of layers under the metallic subsurface. One can add here that the oxide must be buried relatively deep in this case so the interface monolayer might not be well seen, in contrast to the doping case. What structure arises from such observations? Could it be that there is a pair of metallic layers grown upon the oxide core and then again oxide at the surface? In principle, yes, partially oxidized surface cannot be excluded, as mentioned above. However, to repeat it once more, the surface response seems to be mainly metallic from its spectral position and the shape. The reasons for that could be, for example, that it has not been enough molecular oxygen in the final phase of aggregation. Indeed, we have been able to admix only up to 5% of oxygen in argon in Yb reactive sputtering case before the discharge could not be sustained anymore, while it has been possible to reach 15% with lead21 and 20% with silver.38 The above analysis allows us to conclude, with certain probability, that we can produce two different types of nanoparticles via the two different procedures. In both cases, we can obtain, at specific oxidation conditions, the segregated distribution of components, but of the opposite radial orders. In this sense, Yb happens to be a case that allows creating both types of ordering using different modifications of the gas aggregation method based on sputtering. While the situation with metal covered by its oxide (although here by an unusual oxide) is common, the nanoparticle structure with the oxide dominating the inner part and with metallic surface is rather peculiar. Such a geometry can become possible due to a combination of thermodynamic and kinetic conditions realized in the gas-aggregation method with reactive sputtering, as discussed in ref 21. As mentioned above, the formation of oxide molecules at an early stage of the nanoparticle aggregation, in the vicinity of the target, is favored by the presence of charged, excited, and dissociated oxygen in the dense magnetron plasma here. The atomic oxygen should facilitate the formation of monoxide molecules. Yb and O atoms strongly bound at the initial stage of aggregation have no energy reasons to rearrange themselves significantly at a later stage into Yb2O3 at the conditions realized in the method. Further away from the target, along the long pathway in the cryostat, the concentration of reactive forms of oxygen decreases, so, as mentioned above, the probability that the nonoxidized metal atoms condense on the “seeds” of the oxide nanoparticles increases. At the oxidation conditions studied in the present work, the interaction of Yb with molecular oxygen in the process of aggregation seems to have no impact on the surface composition of the nanoparticles, which responds as a metallic one. Apart from the low oxygen concentration, it can

also be explained by the presence of enough mobility in the system striving for the most energy-favorable distribution of components. There is little knowledge about Yb monoxide, since it is not the type of oxide met in nature. However, the crystal structure of Yb compounds with other group-VI elements, for example selenium, is known to be of NaCl-like (rocksalt) ionic type.39 In such a structure, the metal atoms are stripped off of their valence electrons, just like in metallic samples, so the initial states of the core-level ionization should be similar for both cases, metallic and ionic. In the final states of ionization, the multiple electrons of the negative nodes of rocksalt ionic crystals screen the positive metal ions practically as strong as in metallic samples.40,41 Thus for metallic Yb and Yb monoxide, the 4f electron binding energies, being the difference between the final and the initial state energies, can indeed be close. As also briefly mentioned above, we interpret the metallicbulk-like narrow peaks in the spectra recorded at reactivesputtering conditions (Figures 5 and 6), as due to the metallic interface between the monoxide-dominated interior and metallic surface. Their binding energies, which are practically the same as those of bulk metal, as well as the shape and maximum positions of the surface peaks, bear witness to the fact that there is more than one monolayer of metallic material around the inner part of the particles. As mentioned above, in view of the closeness of the surface-oxide and metallic-surface responses and their large spectra widths, one cannot, in principle, exclude the presence of some oxide molecules/islands in the outermost layer of the nanoparticles created by reactive sputtering. However, their fraction should not be large, since the radial sandwich-like distribution of the type “oxide-core/ metal/oxidized surface”, would be the case then, and we consider it too complex to be the explanation for the observations. In summary, we report on a novel flexible method allowing us to create and “on-the-fly” monitor the production of Yb/ YbO nanoparticles with divalent oxide. At certain oxidation conditions, the spectra responses are consistent with two opposite radial orders in the distribution of constituent substances: either with metal inside and oxide covering the surface, or vice versa, with oxide inside and a metal-dominated surface. Neither on the surface nor in the bulk is the oxide of the typical chemical composition known for macroscopic ytterbium oxide, Yb2O3. It is the PES observations that in both cases lead to such a conclusion. The specific chemical composition should have implications on the practical applications of Yb-oxide at the nanoscale level, e.g. in electronics, photonics, and magnetism. A different electronic structure realized at nanoscale, which is also connected with a different crystalline structure, would make the properties of this novel material far from expectations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.T.); zejinliu@ vip.sina.com (Z.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the China Scholarship Council (CSC) and National University of Defense Technol14395



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(18) Cho, E.-J.; Oh, S.-J.; Olson, C. G.; Kang, J.-S.; Anderson, R. O.; Liu, L. Z.; Park, J. H.; Allen, J. W. High Resolution Photoemission Study of YbAl3 at Low Temperature. Phys. B 1993, 186−188, 70−73. (19) Kumar, R.; Svane, A.; Vaitheeswaran, G.; Kanchana, V.; Bauer, E.; Hu, M.; Nicol, M.; Cornelius, A. Pressure-Induced Valence Change in YbAl3: A Combined High-Pressure Inelastic X-ray Scattering and Theoretical Investigation. Phys. Rev. B 2008, 78, 1−7. (20) Vyalikh, D.; Danzenbächer, S.; Kucherenko, Y.; Krellner, C.; Geibel, C.; Laubschat, C.; Shi, M.; Patthey, L.; Follath, R.; Molodtsov, S. Tuning the Hybridization at the Surface of a Heavy-Fermion System. Phys. Rev. Lett. 2009, 103, 137601. (21) Zhang, C.; Andersson, T.; Svensson, S.; Björneholm, O.; Tchaplyguine, M. Core-Shell Structure in Self-Assembled Lead/LeadOxide Nanoclusters Revealed by Photoelectron Spectroscopy. Phys. Rev. B 2013, 87, 035402. (22) Andersson, T.; Zhang, C.; Tchaplyguine, M.; Svensson, S.; Mårtensson, N.; Björneholm, O. The Electronic Structure of Free Aluminum Clusters: Metallicity and Plasmons. J. Chem. Phys. 2012, 136, 204504. (23) Haberland, H. Filling of Micron-Sized Contact Holes with Copper by Energetic Cluster Impact. J. Vac. Sci. Technol., A 1994, 12, 2925. (24) Hoffmann, M.; Wrigge, G.; Issendorff, B. Photoelectron Spectroscopy of Al−32000: Observation of a “Coulomb Staircase” in a Free Cluster. Phys. Rev. B 2002, 66, 041404. (25) Pratontep, S.; Carroll, S. J.; Xirouchaki, C.; Streun, M.; Palmer, R. E. Size-Selected Cluster Beam Source Based on Radio Frequency Magnetron Plasma Sputtering and Gas Condensation. Rev. Sci. Instrum. 2005, 76, 045103. (26) Peters, S.; Peredkov, S.; Balkaya, B.; Ferretti, N.; Savci, A.; Vollmer, A.; Neeb, M.; Eberhardt, W. Inner-Shell Photoionization Spectroscopy on Deposited Metal Clusters Using Soft X-ray Synchrotron Radiation: An Experimental Setup. Rev. Sci. Instrum. 2009, 80, 125106. (27) Sattler, K.; Mühlbach, J.; Recknagel, E. Generation of Metal Clusters Containing from 2 to 500 Atoms. Phys. Rev. Lett. 1980, 45, 821−824. (28) Zimmermann, U.; Malinowski, N.; Näher, U.; Frank, S.; Martin, T. P. Producing and Detecting Very Large Clusters. Z. Phys. D: At., Mol. Clusters 1994, 31, 85−93. (29) Schneider, W.-D.; Laubschat, C.; Reihl, B. TemperatureDependent Microstructure of Evaporated Yb Surfaces. Phys. Rev. B 1983, 27, 6538−6541. (30) Tchaplyguine, M.; Legendre, S.; Rosso, A.; Bradeanu, I.; Ö hrwall, G.; Canton, S.; Andersson, T.; Mårtensson, N.; Svensson, S.; Björneholm, O. Single-Component Surface in Binary Self-Assembled NaK Nanoalloy Clusters. Phys. Rev. B 2009, 80, 9−12. (31) Lindblad, A.; Bergersen, H.; Rander, T.; Lundwall, M.; Ohrwall, G.; Tchaplyguine, M.; Svensson, S.; Björneholm, O. The Far from Equilibrium Structure of Argon Clusters Doped with Krypton or Xenon. Phys. Chem. Chem. Phys. 2006, 8, 1899−905. (32) Lindblad, A.; Rander, T.; Bradeanu, I.; Ö hrwall, G.; Björneholm, O.; Mucke, M.; Ulrich, V.; Lischke, T.; Hergenhahn, U. Chemical Shifts of Small Heterogeneous Ar/Xe Clusters. Phys. Rev. B 2011, 83, 1−7. (33) Nikolić, M. V.; Radić, S. M.; Minić, V.; Ristić, M. M. The Dependence of the Work Function of Rare Earth Metals on Their Electron Structure. Microelectron. J. 1996, 27, 93−96. (34) Peredkov, S.; Sorensen, S.; Rosso, A.; Ö hrwall, G.; Lundwall, M.; Rander, T.; Lindblad, A.; Bergersen, H.; Pokapanich, W.; Svensson, S.; et al. Size Determination of Free Metal Clusters by Core-Level Photoemission from Different Initial Charge States. Phys. Rev. B 2007, 76, 1−4. (35) Takakuwa, Y.; Takahashi, S.; Suzuki, S.; Kono, S.; Yokotsuka, T.; Takahashi, T.; Sagawa, T. Photoemission Study on the Surface Components of the 4f Levels in Yb Metal. J. Phys. Soc. Jpn. 1982, 51, 2045−2046.

ogy (NUDT) for the graduate fellowship of Ch. Zhang. The work presented here has been supported by the Swedish Research Council (VR), the Göran Gustafsson Foundation, the Knut and Alice Wallenberg Foundation, the Crafoord Foundation, Nordforsk, and the Swedish Foundation for Strategic Research. We would also like to thank the MAX-lab staff for their assistance during the experiments.

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REFERENCES

(1) Nilsson, A.; Eriksson, B.; Mårtensson, N.; Andersen, J.; Onsgaard, J. Core-Level Binding-Energy Shifts During Metal Adsorption and Compound Formation: Yb/Ni(100). Phys. Rev. B 1988, 38, 10357− 10370. (2) Nyholm, R.; Chorkendorff, I.; Schmidtmay, J. Surface Segregation and Mixed Valency in Dilute Yb−Al Interdiffusion Compounds. Surf. Sci. 1984, 143, 177−187. (3) Domke, M.; Laubschat, C.; Sampathkumaran, E.; Prietsch, M.; Mandel, T.; Kaindl, G.; Middelmann, H. Bulk and Surface Valence in YbPdx Compounds. Phys. Rev. B 1985, 32, 8002−8006. (4) Schnelle, W.; Leithe-Jasper, A.; Schmidt, M.; Rosner, H.; Borrmann, H.; Burkhardt, U.; Mydosh, J.; Grin, Y. Itinerant Iron Magnetism in Filled Skutterudites CaFe4Sb12 and YbFe4Sb12: Stable Divalent State of Ytterbium. Phys. Rev. B 2005, 72, 020402. (5) Lee, S.; Hong, S.; Fisher, I.; Canfield, P.; Harmon, B.; Lynch, D. Optical Properties and Electronic Structure of Single Crystals of LuAl2 and YbAl2. Phys. Rev. B 2000, 61, 10076−10083. (6) Onsgaard, J.; Chorkendorff, I.; Ellegaard, O.; Sørensen, O. The Yb/Al(110) Interface Studied by Electron Spectroscopy. Surf. Sci. 1984, 138, 148−158. (7) Oh, S.-J.; Suga, S.; Kakizaki, A.; Taniguchi, M.; Ishii, T.; Kang, J.S.; Allen, J.; Gunnarsson, O.; Christensen, N.; Fujimori, A.; et al. Observation of Kondo Resonance in YbAl3. Phys. Rev. B 1988, 37, 2861−2866. (8) Rowe, D. M.; Kuznetsov, V. L.; Kuznetsova, L. A.; Min, G. Electrical and Thermal Transport Properties of Intermediate-Valence YbAl3. J. Phys. D: Appl. Phys. 2002, 35, 2183−2186. (9) Kaindl, G.; Reihl, B.; Eastman, D. E.; Pollak, R. A.; Mårtensson, N.; Barbara, B.; Penney, T.; Plaskett, T. S. Surface Core-Level Shifts and Surface Valence Change in Mixed-Valent YbAl2. Solid State Commun. 1982, 41, 157−160. (10) Oh, S.-J. Electronic Structures of Yb Intermetallic Compounds Studied by Photoemission Spectroscopy. Phys. B 1993, 186−188, 26− 30. (11) Hiess, A.; Boucherle, J. X.; Givord, F.; Schweizer, J.; LelièvreBerna, E.; Tasset, F.; Gillon, B.; Canfield, P. C. Magnetization Density in the Intermediate Valence Compound YbAl3. Phys. B 1997, 234− 236, 886−887. (12) Oh, S.-J.; Allen, J. W.; Torikachvili, M. S.; Maple, M. B. Temperature-Induced Valence Change of YbAl2 Studied by XPS and BIS. J. Magn. Magn. Mater. 1985, 52, 183−186. (13) Meier, R.; Weschke, E.; Bievetski, A.; Schüßler-Langeheine, C.; Hu, Z.; Kaindl, G. On the Existence of Monoxides on Close-Packed Surfaces of Lanthanide Metals. Chem. Phys. Lett. 1998, 292, 507−514. (14) Pan, T.-M.; Huang, W.-S. Physical and Electrical Characteristics of a High-k Yb2O3 Gate Dielectric. Appl. Surf. Sci. 2009, 255, 4979− 4982. (15) Yoo, S.; Kalita, M. P.; Boyland, A. J.; Webb, A. S.; Standish, R. J.; Sahu, J. K.; Paul, M. C.; Das, S.; Bhadra, S. K.; Pal, M. YtterbiumDoped Y2O3 Nanoparticle Silica Optical Fibers for High Power Fiber Lasers with Suppressed Photodarkening. Opt. Commun. 2010, 283, 3423−3427. (16) Zhu, J.; Wang, X.; Ma, Y.; Du, W.; Dong, X.; Zhou, P.; Xu, X. Experimental Study on the Polarization Extinction Ratio Degradation in High Power Hybrid Fiber Amplifier Chains Employing PM/nonPM Yb-Doped Fibers. Opt. Laser Technol. 2012, 44, 35−38. (17) Walter, U.; Holland-Moritz, E.; Fisk, Z. Kondo Resonance in the Neutron Spectra of Intermediate-Valent YbAl3. Phys. Rev. B 1991, 43, 320−325. 14396



Article

(36) Ferrando, R.; Jellinek, J.; Johnston, R. L. Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles. Chem. Rev. 2008, 108, 845−910. (37) Tchaplyguine, M.; Andersson, T.; Zhang, C.; Björneholm, O. Core-Shell Structure Disclosed in Self-Assembled Cu−Ag Nanoalloy Particles. J. Chem. Phys. 2013, 138, 104303. (38) Tchaplyguine, M. et al. to be submitted for publication. (39) Assoud, A.; Kleinke, H. Ytterbium Sesquiselenide Yb2Se3. Acta Crystallogr., Sect. E: Struct. Rep. Online 2003, 59, i103−i104. (40) Zhang, C.; Andersson, T.; Svensson, S.; Björneholm, O.; Huttula, M.; Mikkelä, M.-H.; Tchaplyguine, M.; Ö hrwall, G. Ionic Bonding in Free Nanoscale NaCl Clusters as Seen by Photoelectron Spectroscopy. J. Chem. Phys. 2011, 134, 124507. (41) Zhang, C.; Andersson, T.; Svensson, S.; Bjorneholm, O.; Huttula, M.; Mikkelä, M.-H.; Anin, D.; Tchaplyguine, M.; Ohrwall, G. Holding on to Electrons in Alkali-Halide Clusters: Decreasing Polarizability with Increasing Coordination. J. Phys. Chem. A 2012, 116, 12104−11.

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YbO Nanoparticles Created by Oxidation

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