Abstract
We report an application of lowtemperature Raman scattering on inplane CuO nanowires, in which an overview of the characteristic parameter of spinphonon coefficient, the interaction of incident light with the spin degrees of freedom, and size effects will be given. The appearance of spinphonon coefficient decrease reflects the existence of finite size effect.
Background
Lowdimensional nanosized effects in CuO systems, especially their different physical properties such as spinspin [1,2], electron–phonon [3], spinphonon interactions [4], and giant negative thermal expansion have recently received a lot of attention [5]. The spinspin superexchange interaction occurs via the oxygen orbital [4,6]. The magnetic interactions and Néel transition temperature (T_{N}) of the CuO system are strongly dependent on the exchange interaction and the number of neighboring atoms. A transition from a firstorder transition to a commensurate antiferromagnetic state near T_{N} ~ 213 K reported for bulk CuO from neutron scattering experiments [7,8] is well understood. Controlling the size of CuO nanocrystals resulted in shortrange correlation and commensurate antiferromagnetic (AFM) ordering, where the T_{N} decreased from the bulk value of 213 K [911], with decreasing particle size, down to 40 K for 6.6nm nanoparticles [1,2] and 13 K for 2 to 3nm nanorods [12]. It is known that spinphonon coupling is usually weak and undetectable because symmetric vibrations of relevant atoms will cancel the contributions from negative and positive displacements. The main feature of cupric oxide is the lowsymmetry monoclinic lattice, which differs from the other transition metal monoxides, e.g., MnO, FeO, CoO, and NiO with rock salt structure [13]. The low symmetry of the CuO lattice and the anisotropic dispersion curves indicated lattice vibration which caused a modulation of the spinphonon interaction. This originated from slight changes in the interionic distances and bond angles, leading to spinphonon coupling that can be detected in the Raman spectrum, to produce a weak feature at about 230 cm^{−1} below T_{N}[14,15]. The discovery of spinphonon coupling in CuO nanocrystals has led to renewed interest in this phenomenon. Up to now, there have been few experimental alternatives for the determination of the size effect of spinphonon coupling of CuO nanowires. In this study, lowtemperature Raman spectroscopy is employed to investigate the size effects of spinphonon coupling in inplane CuO nanowires. Lowtemperature Raman spectroscopy has the high spatial resolution and sensitivity necessary for probing the local atomic vibrations of nanowires. Our results reveal that below Néel temperature there is a ready shift of the spinphonon coefficient λ_{sp} decreases as the mean diameter of inplane CuO nanowire decreases, exhibiting a long to shortrange spinphonon coupling that can be nicely described with the expected theoretical order parameter as due to antiferromagnetic ordering in inplane CuO nanowires.
Methods
A series of inplane CuO nanowires with various diameters were fabricated. The samples were prepared by a process where a pure copper grid was placed in a ceramic boat inside a quartz tube, which was then evacuated to about 10^{−3} Torr using a mechanical pump. They were then heated in a tube furnace at about 200°C for 2 h for degassing, after which the samples were heated to various temperatures ranging from 300°C to 600°C for 2 h under mixed argon (100 sccm) and oxygen (10 sccm) gas. Details of specimen preparation and characterization have been described in a previous paper [16]. Transmission electron microscopy (TEM) and highresolution transmission microscopy (HRTEM) images from a JEM3010 transmission electron microscope (JEOL Ltd., Tokyo, Japan) were obtained to study the crystalline structure. The results of an early study show that the prepared nanowires are crystalline [16], revealing a monoclinic unique Y structure with lattice parameters of a = 4.63 Å, b = 3.55 Å, c = 5.16 Å, and β = 99°52′. The morphology of the prepared nanowires was characterized using fieldemission scanning electron microscopy (FESEM; JEOL JSM6500 F). The SEM images in Figure 1a,b,c,d show the morphology of the CuO nanowires with various diameters which were synthesized at T = 600°C, 500°C, 400°C, and 300°C, respectively. It can be seen that the inplane CuO grew homogeneously on the copper grid substrate to form straight nanowires. Observation of uniform nanowires (with lateral dimensions in the nanoscale order of tens to hundreds nanometers) shows that they grew up to a few microns in length. Figure 1e shows that the distribution of the nanowires was quite asymmetric. The solid lines represent the fitting curves assuming the lognormal function^{a}. The mean diameters obtained from the fits of lognormal distribution are <d> = 210 ± 15 nm, 120 ± 8 nm, 52 ± 3 nm, and 15 ± 1 nm, respectively. The value obtained for the standard deviation of the distribution profile σ reveals that the increase with broadening was presumably due to the crystalline effects.
Figure 1. Morphology of the inplane CuO nanowires. SEM images of the inplane CuO nanowires synthesized at various temperatures (a, b, c, d). The distributions of the mean diameter of the nanowires obtained from a portion of the SEM image (e). The solid lines represent the fitting curves assuming the lognormal function, where <d> is the mean diameter of the nanowires.
Results and discussion
All lowtemperature Raman spectra were measured using a Jobin Yvon 64000 Raman microscope (HORIBA, Minamiku, Kyoto, Japan) equipped with a Linkam optical DSC system (THMS600; Linkam Scientific Instruments, Surrey, UK). The results were utilized to investigate the spectroscopic properties of CuO nanowire at various temperatures. The specimens were mounted on a nonbackground sample holder fixed to a cold head in a highvacuum (<10^{−3} Torr), lowtemperature (approximately 80 K) chamber. The CuO nanowire was excited by focusing a 514.5nm Ar ion laser (Coherent Inc., Santa Clara, CA, USA) with a 5mW laser power on the sample to form a spot size of approximately 1 μm in diameter, giving a power density of 10^{2} W/cm^{2}. From the factor group analysis of the zone center modes for the monoclinic structure, given by Rousseau et al. [17], there are three Raman active modes (A_{g}, B_{g}^{1}, and B_{g}^{2}) predicted in the spectra of CuO nanowires. Figure 2 shows an example of a series of Raman spectra taken at various temperatures, covering the antiferromagnetic transition temperature, with a mean diameter of 120 ± 8 nm. There are two phonon modes revealed in the Raman spectra taken of the CuO nanowires at T = 193 K at 300.2 and 348.8 cm^{−1}[18], which are related to A_{g} and B_{g}^{1} symmetries [19,20]. The peak position is lower than the value of the bulk CuO (A_{g} = 301 cm^{−1} and B_{g}^{1} = 348 cm^{−1}) [21], reflecting the size effect which acts to confine the lattice vibration in the radial directions resulting in a shift in the A_{g} and B_{g}^{1} symmetries. As the temperature decreases to 83 K, it can be clearly seen that the peak positions of the A_{g} and B_{g}^{1} modes around 301.8 and 350.9 cm^{−1}, shown at the top of Figure 2, shifted toward higher Raman frequencies. While the temperature increased from 83 to 193 K, the peak position of the A_{g} mode softened by 0.7%. Since the frequency of the phonon mode is related to CuO stretching, it is expected that the frequency will downshift with increasing temperature, primarily due to the softening of the force constants that originate from the thermal expansion of the CuO bonds, resulting from the change in vibrational amplitude [22,23]. In the study, the high resolution of our spectrometer allowed detection of relative change as small as 0.5 cm^{−1}, and the vibrational frequency of a phonon mode can be used to determine the spinphonon interaction. A phononphonon effect originates from the dynamical motion of lattice displacements, which are strongly coupled to the spin degrees of freedom dynamically below the magnetic ordering temperature. This coupling between the lattice and the spin degrees of freedom is named as spinphonon. As shown in Figure 2, with decreasing temperature, a welldefined peak developed at 231 cm^{−1} signifying the spinphonon coupling [8,19] which shows that a noticeable shift to lower frequency is sensitive to the temperature variation.
Figure 2. Series of Raman spectra taken at various temperatures of CuO nanowires with a mean average diameter <d> = 120 ± 8 nm. Two main phonon modes corresponding to the A_{g} and B_{g}^{1} symmetries, respectively, are revealed. As the temperature was reduced to143 K, a welldefined peak at 238 cm^{−1} developed, signifying the spinphonon coupling.
Figure 3 shows the temperature dependence of the spinphonon mode for inplane CuO nanowires of various diameters. Typical examples for bulk CuO are shown in Figure 3, indicated by open and solid squares [8]. It has been suggested in previous reports that the temperature dependence of the spinphonon mode (the origin of the peak at 228 cm^{−1}) might be associated with magnetic ordering, the frequency shift corresponding to the spincorrelation function times a spinphonon coupling coefficient λ_{sp}. The temperature dependence of the spinphonon peak can be represented as , where is the Raman shift in the absence of spinphonon coupling at T_{N} and ϕ(T) is the order parameter estimated from the mean field theory [24]. The order parameter can be described as ϕ(T) = 1 − (T/T_{N})^{γ}, where the order parameter γ varied from 3.4 ± 0.2 to 20 ± 5. The solid curves indicate the theoretical fitting, and the corresponding parameters are presented in Table 1. The size effect acts to confine the spinphonon coupling by increasing the T_{N} from 210 to 88 K, as shown in Figure 4a, when the size is reduced from bulk to 15 ± 1 nm (see for comparison T_{N} = 213 K for CuO single crystal and powder [8,16]). The obtained spinphonon coupling coefficient λ_{sp} also tends to decrease with decreased phonon amplitudes as the diameter decreased, as shown in Figure 4b, revealing the existence of shortrange coupling. This result is consistent with past reports which state that the magnetic transition temperature of Cr_{2}O_{3}[25,26] and CuO nanoparticles (open square) is reduced [12], which can be attributed to the fact that the ground state fails to develop longrange antiferromagnetic ordering. This occurs because of quantum lattice fluctuations and being energetically favorable to some kinds of shortrange order state, resulting in a lower spinphonon coefficient with reduced size [27,28]. The magnitudes of these obtained λ_{sp} values are intermediate compared to approximately 1 cm^{−1} for FeF_{2} and MnF_{2}[24], and approximately 50 cm^{−1} for bulk CuO [8], indicating that the size effects could result in a tendency to weaken the strong spinphonon coupling. A minimum spinphonon coefficient of λ_{sp} = 10 cm^{−1} was obtained in <d> = 15 ± 1 nm inplane CuO nanowires, which was found to be weaker by a factor of 0.018 than the nearest neighbor spinspin coupling strength of J = 552 cm^{−1} for onedimensional antiferromagnetic Heisenberg chain [29]. In general, the spinorbit interaction will induce a small orbital moment, which couples the magnetic moment to crystalline axes of the phonon vibration. Anharmonic effects are expected and caused the phonon and spin contribution to mix because the λ_{sp} decreases as the diameter of the CuO nanowires decreases.
Figure 3. Temperature variations of the spinphonon modes of CuO nanowires with various mean diameters. The solid line represents the fit by the ordering parameter.
Figure 4. Size effects of Néel temperature and spinphonon coupling coefficients. The obtained Néel temperature (a) and spinphonon coupling coefficients (b) as a function of mean diameter, which showed a tendency to decrease with reduction in diameter.
Table 1. Summary of the fitting results of the inplane CuO nanowires
Conclusions
In conclusion, we investigate the size dependence of CuO nanowires and the nanosized spinphonon effects. Raising the temperature and decreasing the diameter of CuO nanowires result in the weakening of spinphonon coupling. The temperature variations of the spinphonon mode at various diameters are in good agreement with the theoretical results. We found that the spinphonon mode varies with the size of the CuO nanowires and in corroboration with the strength of spinphonon coupling. Our result reveals that lowtemperature Raman scattering techniques are a useful tool to probe the shortrange spinphonon coupling and exchange energy between antiferromagnetic nextnearest neighboring magnons in nanocrystals below the Néel temperature. The application of lowtemperature Raman spectroscopy on magnetic nanostructures represents an extremely active and exciting field for the benefit of science and technology at the nanoscale. The rising new phenomena and technical possibilities open new avenues in the characterization of shortrange spinphonon interactions but also for the understanding of the fundamental process of magnetic correlation growth in nanomaterials.
Endnote
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
SYW wrote, conceived of, and designed the experiments. PHS grew the samples and analyzed the data. CLC contributed the Raman experimental facility and valuable discussions. All authors discussed the results, contributed to the manuscript text, commented on the manuscript, and approved its final version.
Acknowledgements
This research was supported by a grant from the National Science Council of Taiwan, the Republic of China, under grant number NSC1002112M259003MY3.
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