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Inducing Ferromagnetic Behaviour in Cu Nanoparticles and Thin Films via Non-Stoichiometric Oxidation Armando J. Marenco, David Brondum Pedersen, and Simon Trudel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00699 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 27, 2016

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The Journal of Physical Chemistry

Inducing Ferromagnetic Behavior in Cu Nanoparticles and Thin Films via Non-stoichiometric Oxidation Armando J. Marenco,† David B. Pedersen,∗,‡ and Simon Trudel∗,† †Department of Chemistry and Institute for Quantum Science and Technology, University of Calgary, 2500 University Dr NW, Calgary, AB, Canada, T2N 1N4 ‡Defence R&D Canada - Centre for Security Science, 222 Nepean St, 11 floor, Ottawa, ON, Canada K1A 0K2 E-mail: [email protected]; [email protected]

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Abstract The unconventional magnetism of Cu nanomaterials has been studied in highly-pure and capping-ligand-free nanoparticles and thin films. Gas-phase synthesis of these materials allows to study the size and surface effects independently and their capacity for inducing magnetism in Cu. Superconducting quantum interference device (SQUID) room-temperature (300K) measurements displayed no size correlation to the ferromagnetic behavior observed in Cu nanoparticles ranging from 4.5 ± 1.0 nm to 9.0 ± 1.8 nm in diameter. Moreover, magnetic quartz crystal microbalance in situ tests of 4.5 ± 1.0 nm nanoparticles under vacuum conditions showed magnetic behavior only after the onset of oxidation. SQUID analysis conducted on Cu thin films exposed to several heat treatments demonstrated minor oxidation inducing higher ferromagnetic responses compared to extended oxidation. Further analysis of nanomaterial samples exhibiting the highest magnetic responses indicated an atomic ratio of ∼3-5:1 Cu:O suggesting non-stoichiometric oxidation as the source of the ferromagnetic signature.

Introduction Recent years have seen an increasing number of examples of ferromagnetic behavior being induced in diamagnetic noble metals, 1–3 along with an increasing amount of debate over the chemistry and physics responsible for the observed magnetism. The coinage metals (i.e. Cu, Ag and Au) have a d10 s1 electronic configuration and are well known to be diamagnetic due to delocalization of the free electron density in the s band. 4,5 Contrary to expectation, however, hysteretic magnetization loops have been reported for nanoparticles (NPs) of Au, 6,7 Ag 7 and Cu 7 and for nm-thin films of Au 8 whose surfaces were functionalized with self-assembled monolayers (SAMs) of sulfur-containing ligands. 6–8 Crespo et al. 6 described how 1.4 nm Au NPs behave ferromagnetically at room temperature and discussed the importance of capping with strongly binding dodecanethiol to induce magnetism. Similar results were observed by Suda et al. 9 who found that Au thin films modified with a SAM of 2 ACS Paragon Plus Environment

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4-[6(hexyldithio)hexyloxy]azobenzene also exhibit ferromagnetic-like behavior. This latter study purposely demonstrated the importance that the SAM – more specifically its molecular dipole moment – has on the observed surface magnetization, by monitoring a reversible modulation of the magnetism and work function while inducing a cis-trans photoisomerization of the azo moiety. More recently, organic molecule-metal interfaces have gathered a growing interest due to the potential in developing novel organic-based spintronic devices. 10 As a result of combining nm-thick layers of Cu with those of buckminsterfullerene C60 molecules in a multilayer fashion, researches have been able to measure room-temperature ferromagnetic behavior due to substantial electron transfer from the Cu to the C60 layers. 11 Although the origin of the observed magnetism in nanodimensioned noble metals is not well understood and still being debated, 1,2 many models invoke the presence of surface-bound molecules as conditional to the occurrence of magnetism. 12–15 However, the requirement of capping molecules has been called into question by a set of recent experiments wherein ferromagnetism was observed in uncapped nanocrystalline Au films prepared by cluster deposition. 16 A second set of results that questions capping molecules as the source of magnetism in nanodimensioned materials is the study of oxide-metal interfacial systems. For instance, Gamelin and co-workers have reported room-temperature ferromagnetism in several diluted magnetic semiconductors. 17–19 Using trioctylphoshine oxide-capped 0.48% Ni2+ -doped SnO2 , which is a paramagnet as freestanding nanocrystals, they were able to induce ferromagnetic behavior by gently annealing under a N2 atmosphere. 20 Ferromagnetic deactivation could also be achieved if the annealing was performed under aerobic conditions. 20 Activation of ferromagnetism was attributed to an increase in formation of grain-boundary defects upon nanocrystal-nanocrystal interaction, while deactivation occurred due to the passivation of oxygen vacancies when air-annealing is performed. 20 Closer to the system of interest here, Gao et al. 21 has shown that solution-synthesized air-annealed pure ∼26 nm CuO NPs show room-temperature ferromagnetism, which was attributed to the presence of oxygen vacancies,



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specifically at the NP’s surface. Further studies by the same team demonstrated ferromagnetism of bulk CuO/Cu2 O composites 22 and of CuO/Cu2 O microspheres. 23 In both latter cases, the ferromagnetic behavior is attributed to the interface formed between the two oxides and is explained in terms of the indirect double-exchange model. 22,23 The notion of atypical electronic structures at interfaces leading to magnetism is supported by theoretical studies that predict magnetic behavior in small unmodified Ag clusters. The most magnetic of these in the range of 2-22 atoms is predicted to be the icosahedralshaped Ag13 cluster. 24 In this cluster, the central atom’s 4d orbital overlaps with those of its 12 outer-shell neighboring atoms resulting in a charge transfer favoring the outer-shell atoms. 24 Thus, the resulting Ag13 cluster contains an inner atom with a magnetic moment of 0.35 µB while the outer-shell atoms exhibit 0.39 µB /atom. Since 12 of the constituent Ag atoms are surface atoms, they are in effect interfacial. Consistent with theory, experiments on Pt13 clusters supported in zeolite were shown to display magnetic behavior. 25 Notice that these results speak to magnetism in non-functionalized noble metal NPs and underscore the relatively immature nature of the current state of understanding. While it seems clear that interfaces are important, the role of functionalization versus the role of size, noting that most magnetic structures observed have been nanodimensioned, is not yet well understood. Ideally, a study to distinguish between size effects and interface effects on magnetism in coinage metals would control those two parameters independently. To this end, in the present work, Cu NPs and thin films were fabricated in the gas phase under vacuum conditions (i.e., free of capping ligands). With the ability to control the average size of sub-12 nm particles, the size dependence of magnetic properties in pristine Cu NPs was examined. Through oxidation of both nanomaterials, the introduction of oxide-metal interfaces in partially oxidized surfaces was observed to induce ferromagnetism, and the induction of ferromagnetism in nanomaterials of pure metallic Cu could be correlated with non-stoichiometric oxidation at the surface. The systems are complex, however, with further oxidation resulting in a decrease in their ferromagnetic signal.


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Experimental Section In order to distinguish between the independent effects of size-dependence and surface oxidation on magnetism in Cu nanostructures, the gas-phase synthesis approach has been adopted. Using this approach, pristine Cu NPs of high purity were fabricated with control over NP diameter in the 4.5 ± 1.0 nm to 9.0 ± 1.8 nm range. All samples were prepared under vacuum using a Mantis NanoGen sputtering system as described elsewhere. 26 The metal target was cooled with a closed 10◦ C water loop system at all times. During standard use, as seen in Figure 1, NPs generated in the aggregation chamber are swept out to adjacent chambers due to interchamber pressure differentials. Adjacent to the aggregation chamber is a quadrupole mass spectrometer, followed by a collection chamber where NPs are deposited onto substrates. As the produced metal NPs in the aggregation chamber are negatively charged, 27 mass spectrometric methods were used to analyze the NPs produced. Thus, through mass analysis preparative conditions yielding specific size distributions of NPs were determined. Note that during NP collection (site a in Figure 1), the mass spectrometer chamber was removed and the aggregation chamber connected directly to the collection chamber to maximize NP yield. It is important to specify that bypassing the mass spectrometer chamber does not affect the resultant NP size, as the gas-phase nucleation and growth of NPs is terminated by the time the NPs exit the aggregation chamber. 28 In addition to collecting NP samples in the deposition chamber, thin films of Cu were also collected by placing a substrate directly onto the metal target away from the plasma etching location (site b in Figure 1). The process of redeposition is experienced by some of the sputtered material when multiple collisions with, for example, gas atoms a short distance from the target result in a loss of energy and their inability to travel further leading to backscattering. 29,30 Both NPs and thin films were collected on either Si(100) wafers (∼5 × 5 mm in size) or NaCl crystals (∼10 × 7 mm in size). Following preparation, samples were stored under Ar or O2 until further analysis. Deposition times were done in 5 min intervals (i.e. 5 min deposition followed by 5 min rest) to avoid overheating of the metal target. 5 ACS Paragon Plus Environment


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Figure 1: (color online) Schematic of deposition system showing only its major components. In the aggregation chamber, the red holder represents the dc magnetron, on which the Cu metal target sits. Site a (b) refers to the NP (thin film) sampling location. See text for full description. Magnetic properties were measured using superconducting quantum interference device (SQUID) magnetometry with a Quantum Design MPMS XL-7S system. Samples were mechanically exfoliated from their substrates (either Si or NaCl) with Kapton tape which was subsequently inserted in a clear diamagnetic plastic straw for measurement. Isothermal magnetization M as a function of field strength µ0 H measurements were carried out at 300 K and 5 K (latter data not shown) by cycling the applied magnetic field strength between 1 and -1 T. Although SQUID is recognized as definitive in determining magnetic properties, one of the experimental challenges with creating samples under vacuum inside our apparatus is to retrieve them for further analysis and, while in transit, still be considered pristine. Accordingly, although NP samples and thin films were prepared under vacuum conditions and kept under either Ar or O2 gas, the reactive nature of NPs combined with the unavoidable exposure to ambient air during sample transfer (i.e. during sample retrieval after initial synthesis or during SQUID sample preparation and loading) could lead to partial oxidation of NPs, even for those stored under Ar gas. To study the effect of inevitable surface oxidation, the magnetic properties of pristine NPs were also studied in situ under vacuum conditions using the magnetic quartz crystal microbalance (MQCM) technique described by Janata and co-workers. 31,32 MQCM was im6 ACS Paragon Plus Environment

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plemented by initially depositing NPs onto a fresh QCM crystal and monitoring the change in frequency due to mass change. For MQCM experiments, QCM crystals with a frequency of 5.000 MHz (Lap-Tech Inc.) were loaded onto a Quartz Crystal Microbalance Analog Controller (Stanford Research System, QCM 100 model) and measurements were recorded with a Universal Counter (Agilent, 53131A model). Magnetic fields were measured using a Hall Probe (LakeShore, HMNT-4E04-VR) connected to a LakeShore 475 DSP gaussmeter. The resulting NPs and crystal were immediately exposed to a nonhomogeneous magnetic field strength of 0.1 T via a bar magnet. Thus, the change in frequency (∆f MQCM in a MQCM experiment) is defined as the difference between the frequencies in the presence and in the absence of a magnetic field (ZF), i.e. ∆f MQCM = f0.1T - fZF . This experiment provided information about the magnetic properties of the pristine Cu material. Then, the vacuum in the NanoGen instrument was broken when ambient air was slowly leaked into the system and, once again MQCM measurements performed. Further oxidation of these NPs was achieved by mild heat treatments ex situ followed by MQCM. The MQCM technique has the added advantage of allowing for precise monitoring of the mass of oxygen uptake by the Cu NPs as a result of exposure to air. To further explore the effect, more extensive oxidation was driven by incubating thin films in air at mild temperatures (≤100 ◦ C), and then analyzing them with SQUID magnetometry. All MQCM measurements were performed at room temperature. Oxidation states and sample purity were tested via X-ray photoelectron spectroscopy (XPS). A Physical Electronics PHI VersaProbe 5000-XPS instrument was used to record the XPS spectra. The spectra were collected using a monochromatic Al Kα source (1486.6 eV) with a beam diameter of 200.0 µm. Some samples were further analyzed in situ after Ar sputtering at a rate of 10 nm/min for 5 min to reveal a fresh surface. All spectra analyses were performed with the CasaXPS software. 33 Spectra were corrected by calibrating all peaks to adventitious C 1s signal at 284.8 eV. A Shirley-type background was used, and curve fitting was performed using a combination of Gaussian and Lorentzian functions (GL(85)) profiles.



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The analysis of the O 1s region followed a methodology presented elsewhere. 34 Atomic force microscopy (AFM) topography images were collected using a Veeco Innova AFM mounted on an acoustic-vibration isolation system. Images were collected at ambient conditions in tapping mode using a silicon cantilever with a spring constant of ∼50 N/m.

Results Cu NPs The size distribution of NPs produced in the gas phase were determined using mass spectrometry and are illustrated in Figure 2a. Source conditions were adjusted in order to obtain NP size distributions with mean diameters of 4.5 ± 1.0 nm, 6.5 ± 1.0 nm, and 9.0 ± 1.8 nm. Efforts to obtain smaller nanoparticles to increase the size range studied were unsuccessful. These three source conditions were used to generate and deposit all of the NPs studied in this work. For each of these source conditions NPs were deposited for 30 min onto NaCl substrates and stored in sealed containers under Ar until ready for SQUID analysis. SQUID magnetometry data of these NPs are presented in Figure 2b. As seen, the M vs µ0 H loops show a clear magnetic response. Note that a linear diamagnetic response has been subtracted from all hysteresis loops (Figure S1). Closer inspection of the loop near the origin (see Figure 2b inset) shows a weak coercivity and remnant magnetization (e.g., µ0 Hc = 25.7 mT and Mr /M1T = 9.20 % respectively for 4.5 ± 1.0 nm particles), which is a clear indication that the Cu NPs behave as a soft ferromagnet at room temperature. There is no significant size effect observed over the investigated size range, however, and all three sizes of NPs show a similar hysteresis loop (i.e., shape) and saturation magnetization (i.e., intensity). To better understand the impact of oxidation on magnetic properties, a number of NP samples (mean diameters of 4.5 ± 1.0 nm) were deposited onto NaCl for a total of 30 min and stored under either Ar or O2 atmospheres for 6 days. The data in Figure 3 are SQUID data collected for the two types of samples. As seen, exposure to oxygen caused a significant 8 ACS Paragon Plus Environment

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decrease in the saturation magnetization. The lower inset shows a close up of the origin; both coercivity and reduced remanence are virtually unchanged between the samples stored in Ar or O2 gases indicating a loss of magnetic centers, not a change in type of magnetic centers. The upper inset shows an AFM image of Cu NPs illustrating the monodispersed topography of the sample. Interestingly, the NP size based on AFM is ∼10 nm. This could result from the well-known lateral resolution limitations of AFM due to the cantilever tip size and sample roughness when trying to distinguish between two distinct NPs 35,36 (in our case sub-10 nm particles). This is compounded by the high NP density of our samples which could lead to spontaneous restructuring, for example due to capillary condensation between particles, 36 in an attempt to reduce surface energy among particles. 37 15

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Figure 3: (color online) Room temperature SQUID magnetometry measurements of Cu NPs stored in either Ar (•) or O2 (N) atmospheres. Bottom inset shows a close up of the origin. Lines are a guide to the eye. Top inset shows an AFM image with a 50 nm bar.

Cu films At first glance, the results obtained suggest that pristine NPs are ferromagnetic, that there is no size dependence to the magnetism, and that prolonged exposure to oxygen attenuates the 10 ACS Paragon Plus Environment

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ferromagnetic behavior. To explore further if the ferromagnetic properties were broadly sizespecific to sub-12 nm particles, the magnetic properties of Cu films prepared under identical source conditions, but collected onto Si substrates at the target (location a in Figure 1) for a total of 10 min, were also examined. SQUID magnetometry and AFM data are shown in Figure 4. From AFM, it is seen that the Cu films have a very coarse topography and consist of relatively large faceted crystals with dimensions exceeding 100 nm. From the SQUID data it is seen that Cu films display the same ferromagnetic behavior as the Cu NPs. Accordingly, we observe Cu films behaving ferromagnetically, which is puzzling since bulk Cu is diamagnetic. Also, we observe no evidence of size-dependent ferromagnetism since both NPs and thin films behave similarly. As was observed for the Cu NPs (Figure 3), exposure to O2 diminishes the saturation magnetization of the samples. To further explore the effect of oxidation, several Cu films deposited onto Si were heattreated in air (i.e., 50 ◦ C for 30 min, 70 ◦ C for 90 min and 100 ◦ C for 11 hr). The temperatures and times were selected such that the magnetization is roughly halved between each subsequent treatment. The SQUID traces shown in Figure 5 illustrate how the saturation magnetization steadily decreased with increasing temperature. The results are analogous to those seen in Figure 3 and Figure 4 for Cu NPs and films, respectively, which collectively indicate that oxidation leads to a decrease in ferromagnetism. Accordingly, this trend would suggest that the pure Cu NPs, as first produced in the vacuum apparatus and prior to any oxygen exposure, would have an even higher saturation magnetization.

Cu NPs - in situ MQCM To measure the magnetic properties of pristine NPs under vacuum, Cu NPs were deposited onto a QCM and their qualitative magnetic behavior (i.e., magnetic vs non-magnetic) evaluated using the MQCM technique. The general MQCM trends for different magnetic materials can found in Figure S2. The notable trend in Figure S2 is that of ferromagnetic materials 11 ACS Paragon Plus Environment


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Figure 4: (color online) SQUID magnetometry measurements of Cu films stored in either Ar (•) or O2 (N) atmospheres. Bottom inset shows a close up of the origin. Lines are a guide to the eye. Top inset shows an AFM image with a 500 nm bar. which results in a frequency increase during MQCM experiments (i.e. positive ∆f MQCM ). MQCM measurements are inherently convoluted because current flow through the QCM crystal itself is sensitive to the presence of magnetic fields and to proximity effects. 32,38 Accordingly, when the bar magnet was brought into close proximity of the the blank QCM to exert a field, a decrease in QCM oscillation frequency was observed. Janata and co-workers discuss the origins of the sign of the frequency elsewhere. 31 During in situ MQCM of nascent Cu NPs under vacuum conditions, negative frequency changes were observed when the sample was exposed to a magnetic field (Figure 6a). Furthermore, the frequency was comparable to that observed for the blank QCM prior to deposition of Cu NPs on the QCM face (Figure S3). The lack of effect in the presence of Cu NPs is indicative of the pristine NPs being a diamagnetic material as may be expected for bulk Cu. Note that this data also suggests parasitic magnetic impurities are not responsible for the observed magnetic response, as a magnetic response would have been observed here. As seen in Figure 6, however, with the introduction of air into the vacuum system there is a


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Figure 5: (color online) SQUID magnetometry measurements of heat-treated Cu films. Films were heat-treated in air at 50◦ C (“), 70◦ C (•) and 100◦ C (H). An as-prepared sample stored at room temperature under O2 is also shown (N). positive response to the magnetic field that manifests itself as a decrease in the magnitude of the frequency change – albeit still negative. Oxidation was further induced with several ex situ heat-treatments while the magnitude of the negative frequency change is steadily reduced suggesting that uptake of oxygen by the NPs induces magnetism. In contrast to the data above, the MQCM results demonstrate an increase in magnetic behavior as a result of oxidation of the Cu NPs. An advantage of MQCM is that the QCM crystal is a microbalance that permits for the simultaneous measure of the magnetic properties and the quantification of the mass of oxygen uptake by the Cu NPs. In Figure 7a, a plot of the frequency-change response of the Cu NP-coated QCM against the mass of oxygen (as determined from the absolute QCM frequency) uptake by the NPs is shown. Initial exposure to oxygen causes a positive change in the oscillation frequency (i.e. ∆f MQCM becomes less negative). The trend completely reverses, however, near 1 µg of oxygen uptaken for 62 µg of Cu. After this point, further absorption of oxygen manifests itself as a steady decrease (increasingly negative ∆f MQCM ) 13 ACS Paragon Plus Environment


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Figure 7: (color online) a. MQCM experiment of nascent Cu NPs (—) with subsequent oxidation with ambient air (›) and heat-treatment (•). All measurements performed at room temperature. Line is a guide to the eye. b. Saturation magnetization from SQUID measurements of heat-treated Cu films. Line is a guide to the eye. Inset shows SQUID measurements from 0 to 1 T of the Cu films. in the QCM oscillation frequency response to the presence of a magnetic field. That is, with oxygen uptake the sample initially becomes magnetic, but as oxygen pick up continues past a certain threshold the Cu NPs become less and less magnetic. To illustrate the trend more clearly, the SQUID saturation magnetization values of the heat-treated Cu films shown in Figure 5 have been presented beside the MQCM data (Figure 7b). In this case, as the films are exposed to higher temperatures, additional oxidation takes place leading to a further reduced saturation magnetization. To test for potential impurities and to analyze the oxidation state of the Cu films, XPS analysis was performed on several samples. The Cu films kept in Ar and O2 , as well as


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the heat-treated samples shown in Figure 4 and Figure 5, were all analyzed by XPS. As an example, Figure 8a shows the XPS spectrum for the Cu film kept in O2 . The sample is of highly pure Cu with no measurable amounts of ferromagnetic elements (i.e., Fe, Co or Ni). Other elements such as C, O and S are also present. Interestingly, S is absent in all heat-treated samples and sputtered surfaces suggesting to be only physisorbed on the O2 and Ar-kept samples (Figure S4). Accordingly, there is no evidence of an impurity effect and the ferromagnetic signature observed at room temperature can be attributed to the Cu films, specifically. Furthermore, films kept in Ar and O2 as well as the 50 ◦ C-treated sample were sputtered in situ inside the XPS instrument for analysis (sample surveys are shown in Figure S5). Fresh surface was exposed after removing 50 nm of material. To study the oxidation state of Cu, the Cu 2p 3/2 peak was monitored (See Figure S6a and Figure S7a). However, the binding energies of the Cu 2p 3/2 peak for both Cu(I) and Cu(0) overlap within standard deviation of each other. 39 Thus, the modified Auger parameter was adopted for characterization. 39,40 An Auger electron is created when a core-level hole created after the photo-ejection of a core electron (as in the case of XPS analysis) is filled with an outer-shell electron while simultaneously ejecting a third Auger electron. The modified Auger parameter (α’) is defined as the addition of the kinetic energy of the Auger electron (EK ) and the binding energy of the photoelectron (EB ), α’ = EK + EB . 40–42 It is of great aid in determining the chemical state of materials; 40 accordingly, (the modified Auger parameter was utilized to identify the present Cu species more reliably) our results and literature comparison are shown in Table 1. Based on literature average peak positions, 39 all non-sputtered films are comprised of surface Cu oxides with metallic Cu underneath after sputtering, with the exception of the 50 ◦ C heat-treated sample. This latter result is not surprising as the oxidation layer is expected to thicken (due to oxygen diffusion) with temperature. 43 In order to quantify the different Cu species present in our films, the method described by Biesinger et.al. was utilized. 39,44 This method does not make a distinction quantifying Cu(0) and Cu(I) as separate species; thus, their aggregated percentage is presented as a ratio



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in Figure 8b. In the same figure, the atomic percentage of Cu(II), quantified with the help of the shake up satellite peaks in the Cu 2p region (Figure S7a), and of O are also shown. The O 1s region curve-fitting for all samples is shown in Figures S6b and S7b. Table 1: Literature comparison to our results of the Cu 2p 3/2 peak positioning and the modified Auger parameter. Sample Cu 2p 3/2 Modified Auger parameter (nm) (eV) (eV) Cu(0) literature avg. 932.61 ± 0.21 1851.23 ± 0.16 Cu(I) literature avg. 932.43 ± 0.24 1849.19 ± 0.32 Cu(0) experimental 932.63 ± 0.025 1851.24 ± 0.025 Cu(I) experimental 932.18 ± 0.12 1849.17 ± 0.03 Ar-kept 932.74 1849.20 Ar-kept + sputtering 932.70 1851.10 O2 -kept 932.58 1849.57 O2 -kept + sputtering 932.76 1851.10 50 ◦ C treatment 932.76 1849.42 ◦ 50 C treatment + sputtering 932.69 1850.52 70 ◦ C treatment 932.47 1849.28 ◦ 100 C treatment 932.51 1849.28

Reference 39 39 39 39 this work this work this work this work this work this work this work this work

Discussion Size dependence Size-dependent properties are a well-known characteristic of NPs and are highly debated as contributors to the unconventional magnetism that has been observed in nano-dimensioned noble metal systems. The sizes of NPs studied in this work were well-defined by mass spectrometry, as seen in Figure 2a, and ranged from as little as 3 nm to as large as 12 nm with mean diameters of 4.5 ± 1.0 nm, 6.5 ± 1.0 nm, and 9.0 ± 1.8 nm. For all of these size ranges, SQUID magnetometry data (Figure 2b) are very similar for all samples displaying coercivity and remanence which are characteristic of soft ferromagnetic materials. Thus, there is no evidence of size correlating to observed magnetic properties over the investigated


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Cu 3d

Cu 3s Cu 3p

S 2p

C 1s

5

S 2s

O 1s

Cu LMM

10

Cu LMM

Cu LMM

Cu 2s

15

C KLL

4

CPS (x10 )

20

Cu LMM

O KLL

25

Cu 2p3/2

Cu 2p1/2

30

a.

0 1200

1000

800

600

400

200

0

binding energy / eV

b.

100

sputtered

heat-treated

80

atomic percent / %

60

40

20

s

p u tt

e re d

o

C

C

o

7 0

1 0 0

o

C

5 0

p u tt s

C

o

5 0

2

O

-

A r

e re d

e re d

2

s

p u tt

O

0

A r

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8: (color online) a. XPS of Cu films kept under O2 atmosphere. Spectrum shows Cu as the major component with common atmospheric elements at trace levels such as O, C and S. b. Atomic percentages of Cu and O via XPS in several Cu films. Cu(0) and Cu(I) species (“), Cu(II) (N), and O (•). Dark shaded areas represent sputtered samples while light shaded areas are heat-treated samples.



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range. We note, however, that thiol-capped Cu NPs with a 2.3 nm diameter show a significantly stronger magnetization. 7 It is not clear whether this discrepancy arises from the presence of capping molecules, or the further reduced dimensionality. Similarly, a comparison of the SQUID data in Figure 3 and Figure 4, for NPs and Cu films respectively, reveals very similar saturation magnetization, coercivity and remanence indicating that ferromagnetism can be observed in nanostructures with sizes as large as ∼500 nm (see AFM inset in Figure 4). MQCM experiments where changes in magnetism were monitored in situ as source conditions were varied, revealed no indication that certain diameters of NPs were more “magnetic” than others (data not shown). In fact, as noted above, pristine NPs generated under vacuum conditions displayed no magnetism at all, regardless of size (Figure 7a). As discussed below, the ferromagnetic properties observed cannot be specifically associated with nanodimensionality of the Cu metal component of these systems.

Oxidation MQCM data of pristine NPs generated in vacuum and then exposed to air, shown in Figure 7a, demonstrate a rapid increase in magnetic response during initial oxidation of the NPs but then a decrease with further oxidation by heat-treatment in air. The trend follows through in the SQUID data (Figure 7b) with a decline in saturation magnetization with heat-treatment in air. By contrast, XPS analysis of the oxidation states of several Cu films shown in Figure 8b reveal a decline in low oxidation states of Cu (i.e., Cu(0) and Cu(I)), and a steady increase in oxygen content with increased heat-treatment temperatures. Also, Cu(II) is a minor contributor to the overall make up of heat-treated films. This oxidation trend is consistent with previous studies for the formation of Cu@CuOx core-shell NPs. 43 The immediate onset of magnetism upon exposure of pristine Cu NPs to air, as seen in Figure 7a, is most consistent with a surface oxidation process being responsible for generating magnetic centers. The fact that sputtering of Cu films effectively removes all oxide and generates ∼90% pure Cu surfaces, as seen in the XPS data shown in Figure 8b, is completely 18 ACS Paragon Plus Environment

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consistent with surface oxidation being the dominant process while interstitial oxidation within the interiors of nanomaterials being negligible.

Our magnetic system vs other systems In general, according to our data (Figure 8b), there is no obvious evolution of a specific Cu oxidation state consistent with the rise-fall nature of ferromagnetism displayed in Figure 7a. However, others have found certain ratios of specific oxides in CuO/Cu2 O microspheres showing a maximum saturation magnetization when the ratio reaches 73% CuO and 27% Cu2 O. 23 In contrast, our maximum saturation magnetization is found in Cu NPs and films (Figure 3 and Figure 4 respectively) kept under Ar or O2 with undetectable CuO presence according to our XPS analysis (Figure 8b). The absence of Cu(II) in our non-heated samples is not surprising as it has been found that surface formation of a layer of native Cu2 O protects the rest of Cu material against further oxidation. 45 Moreover, conversion of Cu to Cu2 O is more facile than the conversion to CuO. 46 This is due to O diffusion into tetrahedral sites in fcc Cu leading to lattice expansion in the case of cubic Cu2 O, as opposed to atom rearrangement during the formation of monoclinic CuO. 46 Another example in which oxidation plays a role in magnetization consists of nanocrystalline powders of 10-100 nm in size of Cu2 O1+x where x >0 has been observed to show hysteresis connected to the appearance of Cu+2 having a 3d9 electronic configuration; 47 however, the authors state that the reason of the ferromagnetism is unclear. Interestingly, computational studies have proposed to explain ferromagnetism in Cu oxides with specific atomic ratios. 48,49 In our case, Cu:O atomic ratios from experimental MQCM and XPS data were calculated. Utilizing the MQCM data, assuming a spherical NP and using the experimental NP mean diameter of 4.5 nm together with the total mass of Cu NPs deposited on the QCM during the MQCM experiment (before air exposure), the overall number of surface Cu atoms was determined. Also, the point of highest magnetism for pristine 4.5 ± 1.0 nm Cu NPs exposed to air occurs at ∼1 µg of oxygen uptake, as seen in 19 ACS Paragon Plus Environment


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Figure 7, which allows for the number of oxygen atoms to be calculated. The resulting Cu:O atomic ratio was determined as 5:1. Similarly from XPS analysis (Figure 8a), we estimated a Cu:O ratio of approximately 3:1 (i.e., 3.2:1 and 2.9:1 for the Ar- and O2 -kept Cu films respectively). Comparing our values with theoretical studies, ferromagnetism is believed to be induced in a 48-atom Cu2 O supercell model by interstitial oxygen defects at both octahedral (i.e., an interstitial O atom surrounded by nearest six Cu atoms or a 6:1 Cu:O ratio) and tetrahedral (i.e., a 4:1 Cu:O ratio) sites. 48 In contrast, theoretical studies with planar Cu:O clusters found that 4:1 and 4:2 behave diamagnetically while a 4:5 cluster (together with non-planar 16:15 and 28:27 clusters) are ferromagnetic. 49 Although both theoretical studies cite spin density localization and polarization as the source of ferromagnetism, 48,49 and while the compositions are not in accordance with each other, they suggest that specific non-stoichiometry plays a role in the magnetic behavior. Accordingly, the overall description of our ferromagnetic Cu system does not show size dependency which is evident by the mass spectrometric sizing of NPs and their corresponding SQUID measurements. Furthermore, the pure Cu NPs (under vacuum) display no magnetism according to our in situ MQCM experiment. The ferromagnetic behavior in NPs only appears after exposure to oxygen. Cu thin films prepared in a similar fashion corroborate the lack of size dependency and the ferromagnetic behavior after oxygen exposure with further oxidation decreasing the saturation magnetization in both nanomaterials. Based on MQCM calculations with Cu NPs, the highest magnetic signal is obtained when a 5:1 Cu:O atomic ratio is achieved. Similarly, XPS analysis of Cu thin films show higher magnetization when the ratio is ∼3:1 Cu:O. XPS also provides evidence of surface oxidation while maintaining a metallic Cu interior. All these evidence reveal a clear trend in magnetism for Cu nanomaterials as a function of oxidation in which partial oxidation is key to maximizing the ferromagnetic signal. Based on our MQCM and XPS evidence, we propose an intermediate step during the oxidation reaction of fcc metallic Cu to cubic stoichiometric Cu2 O (both of which are non-magnetic 50 ) in which intercalating oxygen atoms form a non-stoichiometric


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oxide resulting in the observed magnetic response (Figure 9). Although previous studies on the early stages of Cu oxidation involve nucleation and growth of metal oxide nanoislands 51–53 (as opposed to a uniform oxide distribution), these studies were performed at 350 ◦ C and higher temperatures. On the contrary, our samples showing the highest magnetization were prepared at room temperature. It is known that oxidation temperatures will effect O diffusion, interfacial strain, surface and interface energies all of which play a role in the nucleation and development of oxide morphologies. 51 Thus at low oxidation temperatures, diffusion and strain for example, should be at a minimum resulting in surface oxidation and a cubic lattice (albeit Cu bonds are expected to elongate when O is incorporated into the unit cell).

Cu (fcc) non-magnetic

non-stoichiometric CumOn magnetic

Cu2O (cubic) non-magnetic

Figure 9: (color online) Proposed oxidation stages of Cu from in its native fcc cell to cubic Cu2 O. Both metallic Cu and Cu2 O are non-magnetic. We also propose a mechanism during the early stages of oxidation of Cu nanomaterials in which magnetic centers are formed during partial oxidation of the material with decreasing magnetic response upon further oxidation (Figure 10). Since the in-vacuum Cu NPs behaved non-magnetically during the in situ MQCM experiment, it is also expected that the thin films will behave identically. This is collaborated by the fact that both nanomaterials show the highest magnetization with partial oxidation and declining magnetic signal upon further oxidation.



Cu Cu2O nanoparticles

magnetic center

in vacuum

partial oxidation after air bled in chamber OR in air OR heat treatment

further oxidation

thin films

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non-magnetic

magnetic

decreased magnetic response

Figure 10: (color online) Proposed growth mechanism for the early stages of Cu oxide on nanoparticles and thin films, and their observed magnetic behavior.

Conclusion In conclusion, we have shown the fabrication of highly pure Cu NPs and thin films which behave ferromagnetically at room temperature. Our findings add valuable information to the current debate on the requirements to induce magnetic behavior into (diamagnetic) coinage metals. The gas-phase procedure for the preparation of our nanomaterials enables the separation of size and surface chemistry effects on the possible impact in inducing magnetic behavior. We have shown that there is no size effect on the observed magnetism; however, our nanodimensioned systems are dominated by surface chemistry and surface effects. Moreover, it is clear that magnetism in our systems comes from non-stoichiometric oxidation. We propose this process as a fundamental source of magnetism in all Cu nanodimensioned systems and potentially all noble metal systems.


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Associated Content Supporting Information. The Supporting Information contains a graphical representation of diamagnetic correction; the general behavior of materials and raw frequency data during MQCM experiments; the XPS spectra of the sulfur region, the complete survey, and the Cu 2p and O 1s regions for several Cu film samples; and sample calculations for the Cu:O ratio calculated via MQCM. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement The authors thank the Natural Sciences and Engineering Research Council of Canada (Discovery Grant), Defence R&D Canada, and the University of Calgary for funding. This research uses infrastructure funded by the Canadian Foundation for Innovation.

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TOC Figure M / Ms

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1.0

0.5

Cu Cu2O magnetic centre

0.0

non-stoichiometric oxidation of Cu NPs & thin films leads to room- temperature ferromagnetism

-1

0 -0.5

-1.0


1

µ0H / T

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