Cu/Cu2O core-shell nanoparticles with diameters around 8–9 nm have been fabricated by magnetron sputtering pure Cu targets with subsequent annealing in oxygen. Room-temperature ferromagnetism (FM) was observed in the samples annealed at 150 °C for 10–120 min. The maximum of saturated magnetization is as high as 19.8 emu/cc. The photoluminescence spectra show solid evidence that the FM originates from Cu vacancies in the Cu2O shell of the Cu/Cu2O core-shell nanoparticles. Furthermore, the FM can be modulated by the amount of Cu vacancies through the Cu/Cu2O core-shell interface engineering. Fundamentally, the FM can be understood by the charge-transfer ferromagnetism model based on Stoner theory.

Over the last decades, oxides diluted magnetic semiconductors (ODMS) have attracted great attention due to their prospective applications in spintronic devices by utilizing both charge and spin.1,2 Doped with transition metal elements in wide bandgap oxides, the room-temperature ferromagnetism (FM) has been observed in various oxides, such as Fe:ZnO, Cr:TiO2.3,4 More interestingly, the unusual FM also exhibits in some oxide semiconductors without magnetic transition metal dopants such as HfO2 thin film5 and other nano-structures of TiO2, In2O3, and ZnO.6–8 Additionally, the abundant and low-cost cuprous oxide (Cu2O) (bandgap 2.17 eV) has been extensively explored because of the potential applications in ODMS and other fields.9 

Cu2O systems doped with and without magnetic transition metal elements have been investigated both experimentally10–12 and theoretically.13 The FM has been discovered in undoped Cu2O fine powder14 and nanowires.15 However, the origin of room-temperature FM still remains controversial. Both Cu vacancies and O defects have been proposed to be the potential source to generate the FM.14,16 So far, there is no solid evidence distinguishing which one essentially associates with the FM mechanism. Without fully understanding the origin of the FM, it is inaccessible to achieve an even higher magnetization. In order to clarify the FM mechanism and systematically study the contribution of defects in the undoped Cu2O, Cu/Cu2O core-shell nanoparticles with diameters of around 8–9 nm are synthesized by magnetron sputtering. The volume ratio of core to shell could be modulated by controlling the annealing time in oxygen. At the same time, the total amount of the Cu vacancy is accordingly changing. Based on the experimental observations, we find that the variation of Cu vacancies has a strong correlation with the magnetization of Cu/Cu2O core-shell nanoparticles. It is thus believed that the room-temperature FM originates from Cu vacancies. Moreover, the FM could be explained by the charge-transfer model based on Stoner theory.

The Cu nanoparticles were fabricated by sputtering pure Cu targets (at 63 W) in Ar (1.0 Pa) at room temperature and the sputtering time was controlled in 20 s. The base pressure was lower than 8 × 10−5 Pa before deposition. After sputtering of Cu, pure oxygen gas (0.2 Pa) was continuously flushed into the chamber (flow rate at 10 sccm) in order to create Cu2O shell at 150 ° C. To prevent the samples from being over-oxidized, the chamber's vacuum was kept below 8 × 10−5 Pa during the cooling stage. Kapton and carbon coated copper grid were attached on the substrate for measurement of magnetic and structural properties, respectively. According to Honjo's verification, under the present annealing temperature and oxygen pressure in our experiment, only Cu and Cu2O phases were expected to exist.17 The structure was characterized by TECNAI F30 field emission transmission electron microscopy (TEM). The magnetic properties were measured by Quantum Design superconducting quantum interference device (SQUID). The optical properties measurements were performed with spectrofluorometer (FL3-2-IHR221-NIR-TCSPC, HORIBA Jobin Yvon Inc.) for photoluminescence (PL) spectra and spectrophotometer (UV-Visible/NIR, U-4100, Hitachi Inc.) for absorption spectra.

The TEM bright field images are shown in Fig. 1, in which the samples are annealed for different durations. Figs. 1(a), 1(c), and 1(e) show samples under annealing time of 10, 30, and 120 min, respectively, while (b), (d), and (f) are their corresponding high resolution TEM (HRTEM) images. In Fig. 1(a), the electron diffraction pattern indicates that the content of Cu2O is too small to identify its presence due to the short annealing time. Thus, no complete and obvious oxidized shell can be observed in Fig. 1(b). It should be noted that the existence of a small fraction of Cu2O is proved by PL spectra later. When the annealing time is increased to 30 min, the typical diffraction pattern of Cu2O appears in Fig. 1(c) and an evident Cu2O shell formation surrounds the Cu core. The two d-spacing values of the core are 0.208 nm and 0.181 nm, which accord to Cu (

$\bar{1}11$
1¯11⁠) and (020) faces as shown in Fig. 1(d). The outer shell Cu2O has different d-spacing values of 0.246 nm ((
$\bar{1}11$
1¯11
) plane) and 0.213 nm ((020) plane) with thickness of about 1.544 nm. When the annealing time is prolonged even up to 120 min, the thickness of Cu2O shell still remains roughly 1.596 nm. As an estimate, the volume ratio of core and shell reduces from 60% for 20 min annealed nanoparticles, to around 30% for 30 min annealed and 120 min annealed nanoparticles, respectively.

FIG. 1.

The figures (a), (c), and (e) show the TEM images of the samples annealed for 10, 30, and 120 min and (b), (d), (f) are their HRTEM images, respectively.

FIG. 1.

The figures (a), (c), and (e) show the TEM images of the samples annealed for 10, 30, and 120 min and (b), (d), (f) are their HRTEM images, respectively.

Close modal

To further obtain the size distribution of the samples, the particle size histograms for 10 min, 30 min, and 120 min annealed samples are shown in Fig. 2. Over 1200 nanoparticles are counted to obtain a more accurate size distribution. The mean diameters of nanoparticles can be calculated by Gaussian fitting,

\begin{equation}f(d_i) = \frac{A}{\sigma \sqrt{2\pi }} \mathrm{exp} [-\frac{{(d_i-d)^2}}{2\sigma ^2}] ,\end{equation}
f(di)=Aσ2π exp [(did)22σ2],
(1)

where A, σ, d, and di represent the area, the standard deviation, the mean value, and the specific particle size of the fitting curve, respectively. Assuming the particles are of spherical shape, the average size of the particles d is calculated from the fitting curve, and the d values of 10 min, 30 min, and 120 min annealed samples are 7.92 nm, 8.99 nm, and 8.93 nm, respectively. The size of the particles increases slightly during the first 30 min annealing process. As discussed above, the little content and the incomplete shell of the Cu2O in 10 min annealed sample make the nanoparticle size smaller. When the annealing time is prolonged to 30 or more than 30 min, the size of nanoparticles increases to around 9 nm. However, the magnetic properties of samples vary greatly although there is no significant increase in both shell thickness and particle size.

FIG. 2.

The size distribution of samples annealed for 10 min, 30 min, and 120 min and their Gaussian fit curves are shown in (a), (b), and (c). The average sizes are 7.92 nm, 8.99 nm, and 8.93 nm, respectively.

FIG. 2.

The size distribution of samples annealed for 10 min, 30 min, and 120 min and their Gaussian fit curves are shown in (a), (b), and (c). The average sizes are 7.92 nm, 8.99 nm, and 8.93 nm, respectively.

Close modal

To characterize the magnetization, the nominal thickness of the nanoparticle film is calibrated with thicker films sputtered for 5 min and 10 min. Fig. 3(a) shows the room-temperature magnetic moment of the samples with different annealing time, and the diamagnetic background of the substrate is preserved. In this figure, the pure Cu sample is completely diamagnetic, which indicates that the samples are uncontaminated by any magnetic impurities. All annealed samples possess ferromagnetism with different saturated magnetization. The magnetization curves after subtracting background are plotted in Fig. 3(b). Different from the nanoparticle size evolution, the saturated magnetization initially increases and then decreases after 30 min, as shown in Fig. 3(c). More importantly, the largest saturated magnetization of 19.8 emu/cc (2.97 emu/g) is achieved in the sample annealed for 30 min, larger than 0.19 emu/g of pure Cu2O powder,14 0.5 emu/g in Mn-doped,10 0.58 emu/g in Ni-doped, and 0.33 emu/g in Co-doped Cu2O nanorods.11 The zero-field-cooling (ZFC) and field-cooling (FC) curves of the 30 min annealed sample are shown in Fig. 3(d). The substrate signal of Kapton was not subtracted from the ZFC and FC curves. It can be seen that a divergence occurs between the ZFC and FC curves at 350 K, implying that the Curie temperature of the annealed Cu/Cu2O nanoparticles is above 350 K.

FIG. 3.

Room-temperature magnetization curves of samples under different annealing durations. (a) shows the raw data. (b) exhibits data without diamagnetic background. A saturated magnetization versus annealing time curve is plotted in (c). (d) is the ZFC/FC curve of the 30 min annealed sample.

FIG. 3.

Room-temperature magnetization curves of samples under different annealing durations. (a) shows the raw data. (b) exhibits data without diamagnetic background. A saturated magnetization versus annealing time curve is plotted in (c). (d) is the ZFC/FC curve of the 30 min annealed sample.

Close modal

It is known that defects can result in magnetism in undoped ODMS.7,16,18 In order to explore the species and energy levels of the defects, optical absorption spectra, and PL spectra are utilized to examine the present samples at room temperature. To obtain the optical absorption edges of Cu/Cu2O core-shell, a (αhν)1/n versus hν plot is shown in Fig. 4, where α is the absorption coefficient and hν is the energy of the incident photon. For the direct bandgap semiconductor Cu2O, n = 1/2 is adopted. The Eg calculated from Fig. 4 is relevant to the dipole-allowed transition between the top of the valence band (VB) and the second-lower level in conduction band (CB), which corresponds to the blue emission series in Cu2O.19 The dipole-allowed bandgaps of all samples are about 2.9 ± 0.1 eV, which are enlarged due to the quantum confinement effects. The blueshift of the bandgap agrees with references, such as 2.6–3.8 eV in Cu2O thin films.20 

FIG. 4.

Tauc plot of different samples. The optical bandgap can be estimated by the tangent along the linear part of the curve.

FIG. 4.

Tauc plot of different samples. The optical bandgap can be estimated by the tangent along the linear part of the curve.

Close modal

To analyze the location of the impurity level, PL spectra for the samples annealed 10, 30, 40, 120 min are presented in Fig. 5. The laser wavelength of excitation source in PL measurement is 350 nm. The main peak around 2.9 eV with blue solid line is a result of the near band-edge transition. It is known that an acceptor level introduced by Cu vacancies is about 0.4 eV above the VB.21,22 The green peak marked by a dashed-dotted line around 2.4 eV may be produced by the transition from CB to VCu level. The orange peak over 3.0 eV with a dotted line and the red peak over 3.2 eV with a dashed line should be associated with the indigo exciton series in Cu2O,19 where the second-lowest state in CB is dominated by Cu 4p orbitals.9,23 On one hand, it is worthy to note that the VCu contributes a relative large part in the 10 min annealed sample (26.8%), as shown in Fig. 5(a). However, its intensity is much weaker than that of the other samples. Such PL spectra prove the existence of a small fraction of Cu2O. Thus, the FM property in 10 min annealed sample could be introduced by Cu2O as well. On the other hand, the ratio of Cu and Cu2O can also be detected by the x-ray photoelectron spectroscopy (XPS) measurement. Our results indicate that the ratio of Cu1 +/Cu0 rapidly increases at the first stage of below 30 min annealing and then the growth rate of Cu2O is greatly depressed, which is in accordance with the TEM measurement.

FIG. 5.

Experimental data and corresponding Gaussian fit curves of room-temperature PL spectra of samples annealed for (a) 10 min, (b) 30 min, (c) 40 min, (d) 120 min. The color lines represent the Gaussian fitting of sub-peaks.

FIG. 5.

Experimental data and corresponding Gaussian fit curves of room-temperature PL spectra of samples annealed for (a) 10 min, (b) 30 min, (c) 40 min, (d) 120 min. The color lines represent the Gaussian fitting of sub-peaks.

Close modal

In Fig. 6, curves of relative proportion of VCu peak, Cu 4p peaks over 3.0 eV and core-shell volume ratio versus annealing time are plotted. The relative content is the area of certain peak divided by the total area of the corresponding PL curve. Combined with Fig. 6, the outer Cu2O shell keeps growing at the initial stage (<30 min), leading to an increase of VCu until 30 min due to its native p-type defect formation.22 Accordingly, Cu–Cu interaction in Cu2O (Cu 4p orbital related) increases as well.24 When the annealing time is prolonged to more than 30 min, the outer shell thickness remains unchanged. Due to the nanoscale core-shell structure, core Cu atoms may diffuse into outer Cu2O shell and compensate the VCu through the Cu/Cu2O interface. Therefore, VCu intensity begins to decline. Conversely, the diffused Cu further increases the Cu–Cu interaction (Cu 4p) and leads to a continuous increase of the Cu 4p peak during the whole annealing process. Moreover, we find that VCu concentration has a direct relationship with the saturation magnetization (see Figs. 3(c) and 6). Thus, Cu vacancy could be the origin of the FM property in the undoped core-shell Cu/Cu2O nanoparticles. In other words, Cu/Cu2O interface plays an important role to tune the VCu and eventually control the magnetization of the nanoparticle.

FIG. 6.

Annealing time dependent relative proportion of VCu peak, Cu 4p peaks, and core-shell volume ratio.

FIG. 6.

Annealing time dependent relative proportion of VCu peak, Cu 4p peaks, and core-shell volume ratio.

Close modal

According to the charge-transfer ferromagnetism model proposed by Coey et al., when impurities introduce the defect states into ODMS and the Fermi level is pinned at these specific gap states, the Stoner criterion for ferromagnetism is satisfied.25–27 The Cu/Cu2O contact in core-shell particles provides the possible conditions for the generation of charge-transfer ferromagnetism. Even in 10 min annealed sample, such contact also exists due to the incomplete shell proved by PL spectra. Fig. 7 is the schematic band diagram to illustrate the charge transfer at the Cu/Cu2O interface. Electrons transfer from Cu to Cu2O, which is driven by the 0.4 eV higher Fermi level of Cu than that of Cu2O.9,28–30 The equivalent EF in Cu2O meets a narrow peak of density of states introduced by VCu, which is labeled as EA in Fig. 7. According to the Stoner criterion, D(EF)J > 1, where D(EF) is the density of states at Fermi level and J is the exchange interaction strength, ferromagnetism can be observed when this specific criterion is satisfied. Hence, the FM is established in the Cu/Cu2O core-shell structure and it can be modulated by the concentration of VCu in the Cu2O side through the Cu/Cu2O interface engineering.

FIG. 7.

Schematic diagrams of band alignment around Cu/Cu2O interface.

FIG. 7.

Schematic diagrams of band alignment around Cu/Cu2O interface.

Close modal

In summary, the room-temperature FM is observed in Cu/Cu2O core-shell structures. The maximum saturated magnetization at room temperature approaches 19.8 emu/cc, which is much greater than other undoped Cu2O nano-structures.14,15 During the annealing process, the diameters of core-shell particles initially increase from around 8 nm to 9 nm and then keep about 9 nm size unchanged. We find that Cu/Cu2O interface plays an important role in controlling Cu vacancies and essentially tuning the FM. This finding provides an insight to understand the origin of FM in the Cu/Cu2O system. Furthermore, this method also offers a practical way to achieve a large FM in Cu2O related ODMS.

The authors thank Mr. C. Gong at UT Dallas for the valuable discussions. This work was supported by the National Natural Science Foundation of China (Grant Nos. 11104148, 51101088, 51171082, and 11204161), Tianjin Key Technology R&D Program (Grant No. 11ZCKFGX01300), the Specialized Research Fund for the Doctoral Program of Higher Education (Grant Nos. 20100031120035 and 20110031110034), and Fundamental Research Funds for the Central Universities.

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