Chem. Mater. 2005, 17, 3615-3619
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Structural and Magnetic Characterization of Ge0.99Mn0.01 Nanowire Arrays Jaideep S. Kulkarni,† Olga Kazakova,‡ Donats Erts,§ Michael A. Morris,† Matthew T. Shaw,| and Justin D. Holmes*,† Materials Section and Supercritical Fluid Centre, Department of Chemistry, UniVersity College Cork, Cork, Ireland, National Physical Laboratory, Teddington, UK, Institute of Chemical Physics, UniVersity of LatVia, LV-1586, Riga LatVia, and Intel Ireland, Co. Kildare, Ireland ReceiVed February 16, 2005. ReVised Manuscript ReceiVed May 10, 2005
A supercritical fluid inclusion-phase technique has been developed to embed Ge0.99Mn0.01 nanowires, with size-monodisperse diameters, within the pores of anodized aluminum oxide hosts. This article describes the synthesis and structural and magnetic characterization of these nanocomposite materials. The experimental results indicate the presence of both para- and ferromagnetic phases in Ge0.99Mn0.01 nanowires. Paramagnetism could be related to individual Mn atoms in the nanowire core. The ferromagnetic phase yields a high Curie temperature, Tc > 300 K, which is surprisingly high for this class of materials. Extended X-ray atomic fine structure (EXAFS) analysis confirmed that the ferromagnetism observed originates from the manganese atoms located at the nanowire-membrane interface.
Introduction Dilute magnetic semiconductors (DMSs) are materials which demonstrate semiconducting properties and long-range ferromagnetic ordering.1-3 Such materials are expected to play a vital role in the integration of spintronic devices with traditional semiconductor technology.4-6 Significantly, roomtemperature ferromagnetism was recently observed in Mndoped ZnO7 films and the epitaxial growth of Mn-doped Ge (Ge1-xMnx) films, in which the Curie temperature (Tc) was found to increase linearly with increasing Mn concentration from 25 to 116 K, has been reported.8 Bulk Ge1-xMnx single crystals with a Tc of 285 K9 and other group IV semiconductors such as Mn-doped Si10 and Cr-doped Ge11 have also been synthesized. However, for the integration of DMSs with * To whom correspondence should be addressed. Tel: +353 (0)21 4903608. Fax: +353 (0)21 4274097. E-mail:
[email protected]. † University College Cork. ‡ National Physical Laboratory. § University of Latvia. | Intel Ireland.
(1) Ohno, H.; Munekata, H.; Penny, T.; Von Molnar, S.; Chang, T. T. Phys. ReV. Lett. 1992, 68, 2664. (2) Shioida, R.; Ando, K.; Hayashi, T.; Tanaka, M. Phys. ReV. B 1998, 58, 1100. (3) Dietl, T. Nat. Mater. 2003, 2, 646. (4) Fertig, H. A. Science 2003, 301, 1335. (5) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (6) Prinz, G. A. Science 1998, 282, 1660. (7) Sharma, P.; Gupta, A.; Rao, K. V.; Owens, F. J.; Sharma, R.; Ahuja, R.; Osorio Gullen, J. M.; Johansson, B.; Gehring, G. A. Nat. Mater. 2003, 2, 673. (8) Park, Y. D.; Hanbicki, A. T.; Erwin, S. C.; Hellberg, C. S.; Sullivan, J. M.; Mattson, J. E.; Ambrose, T.; Wilson, A.; Spanose, G.; Jonker, B. T. Science 2002, 295, 651. (9) Cho, S.; Choi, S.; Hong, S. C.; Kim, Y.; Ketterson, J. B.; Kim, B. J.; Kim, Y. C.; Jung, J. H. Phys. ReV. B 2002, 66, 033303. (10) Kim, H. M.; Kim, N. M.; Park, C. S.; Yuldashev, S. U.; Kang, T. W.; Chung, K. S. Chem. Mater. 2003, 15, 3964. (11) Choi, S.; Hong, S. C.; Cho, S.; Kim, Y.; Ketterson, J. B.; Jung, J. H.; Rhie, K.; Kim, B. J.; Kim, Y. C. Appl. Phys. Lett. 2002, 81, 3606.
current CMOS technology to be practically viable, group IV materials which display ferromagnetism at room temperature are required. The synthesis of DMS materials as nanostructures within ordered architectures would also be particularly beneficial for the construction of devices, as strong confinement can often cause advantageous changes in their general properties, for example, leading to the enhancement of magnetic moments. Mesoporous12-15 solids and anodized aluminum oxide (AAO)16,17 membranes containing uni-directional arrays of pores running throughout the material have been used as templates for synthesizing nanomaterials by chemical vapor deposition (CVD),18-20 electrodeposition,21,22 and incipient wetness techniques.23-26 These techniques have met with some success, but often these methods have been unable to achieve complete filling of the pores with nanowires or nanoparticles as a result of pore plugging. In our laboratories, (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)
Schuth, F. Chem. Mater. 2001, 13, 3184. Ciesla, U.; Schuth, F. Microporous Mesoporous Mater. 1999, 27, 131. Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature 1995, 378, 366. Zhao, D. Y.; Sun, J. Y.; Li, Q. Z.; Stucky, G. D. Chem. Mater. 2000, 12, 275. Li, Y.; Xu, D.; Zhang, Q.; Chen, D.; Huang, F.; Xu, Y.; Guo, G.; Gu, Z. Chem. Mater. 1999, 11, 3433. Miao, Z.; Xu, D. S.; Ouyang, J. H.; Guo, G. L.; Zhao, X. S.; Tang, Y. Q. Nano Lett. 2002, 2, 717. Dag, O.; Ozin, G. A.; Yang, H.; Reber, C.; Bussiere, G. AdV. Mater. 1999, 11, 474. Leon, R.; Margolese, D.; Stucky, G.; Petroff, P. M. Phys. ReV. B 1995, 52, R2285. Okumura, M.; Tsubota, S.; Iwamoto, M.; Haruta, M. Chem. Lett. 1998, 315. Banerjee, S.; Dan, A.; Chakravorty, D. J. Mater. Sci. 2002, 37, 4261. Zhang, Z.; Dai, S.; Blom, D. A.; Shen, J. Chem. Mater. 2002, 14, 965. Han, Y. J.; Kim, J. M.; Stucky, G. D. Chem. Mater. 2000, 12, 2068. Huang, M. H.; Choudrey, A.; Yang, P. D. Chem. Commun. 2000, 1063. Dapurkar, S. E.; Badamali, S. K.; Selvam, P. Catal. Today 2001, 68, 63. Brieler, F. J.; Froba, M.; Chen, L.; Klar, P. J.; Heimbrodt, W.; von Nidda, H. A. K.; Loidl, A. Chem. Eur. J. 2002, 8, 185.
10.1021/cm050352a CCC: $30.25 © 2005 American Chemical Society Published on Web 06/14/2005
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we have developed a supercritical fluid (SCF) solution-phase method for forming metal, metal oxide, and semiconductor nanowires within mesoporous materials27 and AAO28 membranes. The high diffusivity, high precursor solubility, and reduced surface tension of a SCF results in rapid nucleation and growth of nanowires within the pores, reducing the reaction time for pore inclusion by at least an order of magnitude compared to chemical vapor deposition (CVD). Additionally, SCFs cannot be condensed to a liquid phase, thereby reducing the problems of pore plugging and incomplete inclusion seen with other pore-filling techniques. Although Mn-doped CdS nanowires26 have been synthesized within mesoporous silica powders, so far the synthesis of templated group IV DMS nanowires has not been reported. Here we report the synthesis of surface-doped Ge1-xMnx nanowires within anodic aluminum oxide (AAO) membranes using SCF methodologies. Experimental Section Ge0.99Mn0.01 nanowires were synthesized within AAO membranes through the degradation of diphenylgermane and dimanganese dodecacarbonyl in supercritical carbon dioxide (sc-CO2). The porous membranes were placed inside a 25-mL high-pressure reaction cell. Diphenylgermane and dimanganese decacarbonyl, premixed in a 65:1 proportion, were placed in an open-top quartz boat adjacent to the membrane, in an inert atmosphere. To obtain a higher Mn content, the amount of dimanganese decacarbonyl was proportionately increased. The reaction cell was attached via a three-way valve to a stainless steel reservoir (∼48 mL). A high-pressure ISCO pump (Lincoln, NE) was used to pump CO2 through the reservoir into the reaction cell. The reaction cell was placed in a tube furnace and heated to 873 K using a platinum resistance thermometer and temperature controller. The pressure was simultaneously raised to 37.5 MPa, and the reaction proceeded at these conditions for 60 min. The concentration of manganese within the nanowires was determined by X-ray fluorescence (XRF). Transmission electron microscope (TEM) images were obtained on a Philips CM200 Super TWIN FEG TEM operating at 200 kV. X-ray diffraction (XRD) measurements were carried out on a Philips PW3710 MPD diffractometer. EXAFS and XANES measurements were undertaken at the Daresbury Laboratory (Cheshire, UK). XPS data were collected on a Vacuum Generators 310F apparatus using an Al KR anode at 30-W power. Samples were mounted as whole AAO membranes onto conducting double-sided carbon tape after a light surface polish to remove excess material from the preparation. Data were collected at an emission angle of 90° to the surface plane and at pass energy of 50 eV. Binding energy values were referenced to a C1s feature at 1219.8 eV or a Ge2p3/2 feature 1219.8 eV. Magnetization measurements were performed using a commercial SQUID magnetometer (MPMS XL, Quantum Design) at temperatures between 1.8 and 300 K and in fields up to 10 kOe. As the AAO matrix is very brittle, the samples were mounted onto Si substrates (3 × 3 mm2 size) in order to perform SQUID measurements. A magnetic field was applied along the nanowire long axis, that is, perpendicular to the sample surface. Correction for a diamagnetic contribution from the substrate and AAO membrane was done on all of the data. Additionally, the magnetic properties (27) Holmes, J. D.; Lyons, D. M.; Ziegler, K. J. Chem. Eur. J. 2003, 9, 2144. (28) Ziegler, K. J.; Polyakov, B.; Kulkarni, J. S.; Crowley, T. A.; Ryan, K. M.; Morris, M. A.; Erts, D.; Holmes, J. D. J. Mater. Chem. 2004, 14, 585.
Figure 1. (a) TEM image of a Ge0.99Mn0.01 nanowire liberated from an AAO membrane by dissolving in KOH (scale bar ) 75 nm), and (b) magnified TEM image of the above nanowire showing the Ge(111) lattice plane with a d-spacing ) 3.3 Å.
of a reference sample containing undoped Ge nanowires were measured. All magnetization measurements data for the GeMn nanowires were corrected for this input. Moreover, the lowtemperature data were additionally corrected for a paramagnetic contribution which appeared due to the presence of small amounts of paramagnetic impurities in the Si substrate and AAO matrix. Precautions were taken to exclude oxygen in the sample chamber, as its strong paramagnetism may mask the low-temperature properties of the nanowires.
Results and Discussion Figure 1a shows the TEM image of a Ge0.99Mn0.01 nanowire liberated from an AAO matrix after dissolving the AAO with KOH. The wire is approximately 50 nm in diameter, which is the average pore size of the AAO membranes used in these experiments. A magnified image of this nanowire is shown in Figure 1b. The lattice planes observed correspond to the crystalline (111) plane of the germanium lattice. The nanowires appear to be polycrystalline, and the crystallite orientation appears to be random. The XRD pattern of the Ge0.99Mn0.01 nanowires within the AAO membrane is shown in Figure 2. Peaks at 2θ values of 27.45, 45.51, 53.88, 66.28, and 73˚ were observed, which match the (111), (220), (311), (400), and (331) planes for a cubic germanium lattice. The XRD pattern shows no evidence for the presence of secondary phase ferromagnetic alloys. XRD data did not show any evidence of lattice expansion for samples with up to 5% Mn content. To further probe the structural and chemical environment of the nanowires, extended X-ray absorption fine-edge
Ge0.99Mn0.01 Nanowire Arrays
Figure 2. XRD pattern from Ge0.99Mn0.01 nanowires within AAO. The (111), (220), (311), (400), and (331) reflections of the cubic Ge lattice are clearly observed.
Figure 3. (a) Ge K-edge EXAFS of Ge0.99Mn0.01, (b) Mn K-edge EXAFS of Ge0.99Mn0.01. Both plots show good fits of the experimental data with the theoretical models.
structure (EXAFS) and X-ray absorption near-edge structure (XANES) measurements were undertaken on the nanocomposite materials. Figure 3a shows the Ge K-edge EXAFS spectrum of the Ge0.99Mn0.01 nanowires within the AAO membranes. The Mn K-edge EXAFS spectrum for the Ge0.99Mn0.01 nanowires is shown in Figure 3b. The solid lines represent the experimental data. Best-fit results are summarized in Table 1. Excellent agreement exists between the experimental and theoretical curves. Ge is surrounded by a shell of O atoms at a distance of 1.74 Å, suggesting that the Ge atoms are anchored to oxygen atoms on the AAO membrane walls. The next nearest atoms to Ge are other Ge atoms at a bond distance of 2.44 Å, corresponding to the Ge-Ge distance in the Ge nanowires. Mn atoms are surrounded by a shell of O atoms at a distance of 2.11 Å. The next-nearest neighbor for Mn is Ge at a bond distance of 3.03 Å. The third-nearest neighbor for Mn is also Ge at a distance of 4.06 Å. Mn is therefore surrounded by O and Ge atoms rather than other Mn atoms, which implies that Mn atoms are spatially separated from each other. But from the EXAFS data, it is not possible to determine the precise position of Mn in the nanowire-AAO structure. However, the EXAFS data do confirm the absence of secondary phases
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of ferromagnetic alloys such as Ge3Mn5, Ge5Mn3, and Ge8Mn11. Typical XPS spectra collected from a Ge0.99Mn0.01 sample are shown in Figure 4a. The peak shape is indicative of Mn in a single oxidation state (this was confirmed by data collected at 10 eV). The determined binding energy of the 2p3/2 feature was 641.9 eV. This feature can be assigned to the presence of R-Mn2O3, in close agreement with previous work.29 Further confirmation for the oxidation-state assignment can be found by inspection of the XANES region of the Mn K-edge X-ray absorption spectra shown as Figure 4b. The edge position, preedge structure, and postedge structures are very strongly reminiscent of data previously reported for a Mn2O3 standard. Furthermore, edge positions of the same Ge0.99Mn0.01 sample were compared to values measured from a manganese metal foil and MnO/MnO2 powders. Figure 4c shows the plot of oxidation state against the Mn K-edge potential (E) which was extracted from the XANES data. The linear plot was used to obtain the oxidation state of Mn in Ge0.99Mn0.01. The calculation from this method30 gives a mean oxidation state of 2.7 for Mn within the Ge0.99Mn0.01 nanowires, suggesting that Mn is predominantly present in the +3 state. Together the EXAFS, XPS, and XANES data are fully consistent with the majority of manganese species being present in a Mn2O3-like state. It is worth noting that since the surface-sensitive XPS and bulksensitive EXAFS experiments are in agreement, the data suggest that there is no difference in the oxidation state of Mn in the bulk and surface of the nanowires. Magnetic properties of Ge0.99Mn0.01 nanowires at different temperatures were investigated by means of SQUID magnetometry. The saturation of the magnetic moment was not reached, even at the highest applied field of 10 kOe. The magnetic moment dropped significantly as the temperature increased. Both facts would normally point to a paramagnetic behavior of the material. However, even at room temperature, the sample possessed a coercivity field Hc ) 47 Oe, which is too large to be attributed to paramagnetic properties and usually signifies ferromagnetic behavior of the system (bottom inset in Figure 5). The coercivity of the sample is temperature-independent, being in the range 47-60 Oe, see top inset in Figure 5. The remanence is in the order of 1013% and does not decrease with time, suggesting ferromagnetic ordering rather than a spin glass system. Thus, Ge0.99Mn0.01 nanowires exhibit properties which might be attributed to both ferro- and paramagnetic materials and can be explained by the coexistance of two types of magnetism in the system. Simple analysis of the m vs H curves allows separation of ferro- and paramagnetic contributions. A highfield linear paramagnetic contribution was subtracted from the original data set, and saturated hysteresis loops typical for a ferromagnetic material were obtained. The result of this separation is shown in Figure 5, which demonstrates ferromagnetic components of the total magnetic moment. The saturation field Hs weakly depends on temperature, being HS ) 3 kOe at 300 K and Hs ) 5 kOe at T e 10 K. The (29) Ardizzone, S.; Bianchi, C. L.; Tirelli, D. Colloids Surf. 1998, A134, 305. (30) Arcon, I.; Mirti_, B.; Kodre, A. J. Am. Ceram. Soc. 1998, 81, 222.
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Table 1. Fitted Results for Ge and Mn K-Edge EXAFS Measurements of Ge0.99Mn0.01 Nanowire Arraysa
a
sample
T1
R1 (Å)
T2
R2 (Å)
T3
R3 (Å)
Ge K-edge Ge0.99Mn0.01 Mn K-edge Ge0.99Mn0.01
O O
1.74661 ( 0.00435 2.11738 ( 0.00834
Ge Ge
2.44346 ( 0.00575 3.04261 ( 0.02145
Ge
4.06410 ( 0.11342
T1, T2, and T3 represent the atom that best fits the EXAFS data; R1, R2, and R3 are the corresponding distances from the central atom.
Figure 4. (a) XPS data from the Mn 2p region of the photoelectron spectra, (b) the XANES region of the X-ray absorption spectra, and (c) the measured XANES edge position of the Mn standards (9) and a Ge0.99Mn0.01 sample (b).
Figure 5. The ferromagnetic component m vs H curves for Ge0.99Mn0.01 nanowires at various temperatures. The top inset shows the temperature dependence of coercivity, Hc. The bottom inset demonstrates the m vs H loops in the low field range.
summary of this analysis is shown in Figure 6 as temperature dependences of both components of the magnetic moment. The plot clearly demonstrates that, while the ferromagnetic component is virtually temperature-independent, the paramagnetic response is inversely proportional to the temperature. The later result perfectly satisfies the Curie-Weiss law for paramagnetic materials (eq 1): χ ) C/(T-Θ)
(1)
where χ is the magnetic susceptibility and C and Θ are constants.31 At the same time, a very weak temperature dependence of the magnetic moment is typical for ferromagnetic materials far below their Curie temperature. Thus,
Figure 6. Temperature dependence of both ferro- and paramagnetic components of the magnetic moment for Ge0.99Mn0.01 nanowires. The ferromagnetic component is virtually temperature-independent, whereas the paramagnetic response is inversely proportional to the temperature.
it implies that at least one of the phases in the investigated Ge0.99Mn0.01 nanowires possesses ferromagnetism with a high Curie temperature, Tc > 300 K. To demonstrate that magnetic properties of GeMn nanowires are related to Mn doping, the reference sample containing undoped Ge nanowires was prepared using the technique described above. The reference sample showed a clear diamagnetic behavior with a negative m(H) slope even in low fields and the absence of hysteresis. The magnetic moment was nearly temperature-independent with a slight increase ( 300 K, the possible formation of ferromagnetic MnO nanoclusters cannot be entirely responsible for the ferromagnetism observed in the Ge0.99Mn0.01 nanowires. In general, various manganese oxides, for example, MnO and MnO2, are antiferromagnetic in their bulk state.32 However, the typical antiferromagnetic temperature dependence of magnetization was never observed in our experiments. The strongly diluted states of manganese oxide in a Ge matrix, leading to the unusual magnetic properties, should be considered and require additional studies. Coexistance of ferro- and paramagnetic phases might be explained by a concentration gradient of Mn ion concentration. It is possible that during the growth the Mn ions are gradually migrating toward the nanowire surface, that is, the nanowire center is Mn-depleted. Then in the nanowire core, Mn ions are strongly spatially separated and hence do not possess a long-range ferromagnetic ordering. Thus, the central part of nanowire is assumed to be paramagnetic due to localized magnetic ions. As the concentration of Mn ions increases toward the nanowire surface, the exchange interaction becomes stronger and a ferromagnetic ordering may take place.
(32) Pinto, N.; Morresi, L.; Murri, R.; D’Orazio, F.; Lucari, F.; Passacantando, M.; Picozzi, P. Phys. Status Solidi C 2004, 1, 1748. (33) Park, Y. D.; Wilson, A.; Hanbicki, A. T.; Mattson, J. E.; Ambrose, T.; Spanos, G.; Jonker, B. T. Appl. Phys. Lett. 2001, 78, 2739.
(34) Lee, G. H.; Huh, S. H.; Jeong, J. W.; Choi, B. J.; Kim, S. H.; Ri, H.-C. J. Am. Chem. Soc. 2002, 124, 12094. (35) Jana, N. R.; Chen, Y.; Peng, X. Chem. Mater. 2004, 16, 3931. (36) Nayak, S. K.; Jena, P. J. Am. Chem. Soc. 1999, 121, 644.
Conclusions To summarize, we have fabricated Ge0.99Mn0.01 nanowires with a crystalline germanium lattice structure as confirmed by XRD. The XANES and XPS results together prove that the Mn atoms are predominantly present in the +3 state. The EXAFS results indicate that the Mn ions are spatially separated from each other, thus ruling out the presence of GeMn ferromagnetic alloys. The SQUID results indicate the presence of both para- and ferromagnetic interactions in Ge0.99Mn0.01 nanowires. Paramagnetism could be related to individual Mn atoms in the nanowire core. The ferromagnetic phase yields a high Curie temperature, Tc > 300 K, which is surprisingly high for this class of material. The nature of such ferromagnetic behavior is not clearly understood, though the existence of various GeMn ferromagnetic alloys can be excluded. A more extended experimental work and theoretical calculations are obviously required. The observation of ferromagnetic properties in Ge0.99Mn0.01 nanowires at 300 K is very promising as it opens the way for room-temperature spintronic devices. Acknowledgment. The authors acknowledge financial support for their work from the Higher Education Authority (HEA) in Ireland under PRTLI 3 funding, Intel (Ireland) Ltd, NMS Quantum Metrology Program, Project QMP04.3.4 in UK, the Council of Science of Latvia, and the University of Latvia. CM050352A
