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Irradiation-induced nanostructures in cadmium niobate pyrochlores
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 250 (2006) 188–191 www.elsevier.com/locate/nimb
Irradiation-induced nanostructures in cadmium niobate pyrochlores W. Jiang
a,*
, W.J. Weber a, J.S. Young a, L.A. Boatner b, J. Lian c, L.M. Wang c, R.C. Ewing c a
Pacific Northwest National Laboratory, Richland, WA 99352, USA b Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA c University of Michigan, Ann Arbor, MI 48109, USA Available online 9 June 2006
Abstract This paper reports the formation processes of crystalline Cd nanostructures on ion-cut surfaces of cadmium niobate pyrochlores (Cd2Nb2O7). Irradiation with 3 MeV He+ ions has been performed at low temperatures (6295 K) to induce material decomposition and aggregation of host atoms. The irradiation also leads to surface exfoliation due to rupture of gas (He and O2) filled blisters. Nanoparticles and nanowires are observed on the ion-cut surfaces at low and higher doses, respectively. These structures are examined and characterized using a suite of experimental tools. Both the particles and wires are found to be single crystals that primarily consist of metallic Cd. Published by Elsevier B.V. PACS: 61.82.Ms; 81.07.Ta; 81.07.Vb; 68.37.Hk; 68.37.Lp; 82.80.Yc Keywords: Ion irradiation; Decomposition; Nanostructures; Pyrochlores
1. Introduction Nanostructured materials represent a subject that has attracted worldwide attention in recent years. This high level of interest is primarily attributed to their unique properties, such as the quantum confinement of electrons and exceptionally large surface areas that make the materials excellent candidates for technological applications in a variety of fields, including materials science, chemistry, electronics, photonics and biomedicine. Nanoparticles of elements or compounds have been successfully synthesized in various substrates using high-dose ion implantation, followed by thermal annealing at high temperatures [1]. The formation of the embedded nanoparticles (precipitates) in the near-surface region is due to an Ostwald ripening process of the implanted species in the host materials. Onedimensional nanostructures, such as nanowires, are often *
Corresponding author. Tel.: +1 509 376 5471; fax: +1 509 376 5106. E-mail address: [email protected] (W. Jiang).
0168-583X/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nimb.2006.04.106
fabricated by vapor–liquid–solid processes [2,3]. Template-directed syntheses utilizing lithography [4], pores [5]; dislocations [6] or cracks [7] also have been demonstrated. Recently, we have reported the formation of both nanoparticles and nanowires of host atoms on ion-cut surfaces of cadmium niobate pyrochlores (Cd2Nb2O7) [8,9]. This paper presents additional results and discussion regarding the nanostructure formation. 2. Experimental procedures The single-crystal wafers of Cd2Nb2O7 used in this study were grown, cut and Syton polished on the (1 0 0) plane. During an ion-channeling study of damage recovery in Cd2Nb2O7 irradiated at 150 K with Au2+ ions [10], a virgin spot on the sample was used to monitor damage accumulation caused by the analyzing beam of 3 MeV He+ ions in the near-surface region. Following each annealing cycle over a temperature range from 180 to 295 K for 20 min each, Rutherford backscattering spectrometry (scattering
W. Jiang et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 188–191
Temperature (K)
Ion fluence (He+/nm2)
Duration (s)
140 180 141 210 170 240 174 270 223 295 195 295 295
1560 0 2340 0 1560 0 1560 0 1560 0 2340 0 1560
349 1200 594 1200 297 1200 336 1200 354 1200 546 36,000a 270b
a b
Vacuum storage. Surface exfoliation.
angle = 150) along the h1 0 0i channeling direction (RBS/ C) was performed at temperatures ranging from 140 to 295 K. The experimental conditions for the He+-irradiation and thermal annealing are summarized in Table 1. The sample was then stored in air at room temperature for about one year. Following the storage, secondary electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) methods were employed to study the morphology and composition in the He+ irradiated area. The microscope (LEO 982) used in this study was operated in a voltage range from 2 to 20 kV for imaging. During the EDS experiment, 8 or 10 keV electrons were used for the characterization of nanostructures in order to reduce the excitation volume (300 nm deep). In all cases, the focused electron beam for the EDS experiment had a size on the order of 1 nm. In addition, transmission electron microscopy (TEM) was also used to characterize the atomic-scale structures. 3. Results and discussion The h1 0 0i-aligned spectra for Cd2Nb2O7 sequentially irradiated to various fluences at different temperatures (see Table 1) are shown in Fig. 1, together with a random spectrum. The ion fluence indicated in the figure is estimated based on the total integrated charge for the corresponding spectrum. Initially, the backscattering yields from the surface increase slightly with the increase of ion fluence, as expected; following irradiation to a total ion fluence of 1.1 · 104 He+/nm2 at various temperatures, the spectrum shows an abrupt increase in the scattering yield. This is an indication that the crystal in the irradiated area has become misaligned with the incident beam. A similar result was also observed for H+ ion implanted SiC [11]. During the irradiation of Cd2Nb2O7, the implanted He and some O2 accumulate at the peak in the irradiationinduced vacancy concentration and near the projected range of the implanted He. Prior to surface exfoliation,
+
1000
Random Nb
800 600 400
2
He /nm (TIrrad.) 1560 (140 K) 2340 (141 K) 1560 (170 K) 1560 (174 K) 1560 (223 K) 2340 (195 K) 1560 (295 K) 1560 (295 K)
Cd2Nb2O7
Scattering Yield
Table 1 Sequential irradiation (with 3 MeV He+ ions) and thermal annealing for Cd2Nb2O7
189
Cd
Surface Exfoliation <100>-Aligned
200 0
660
680
700 720 Channel Number
740
Fig. 1. A sequence of 3 MeV He+ RBS/C spectra for a h1 0 0i-oriented Cd2Nb2O7 wafer (beam-spot size: 0.5 · 0.6 mm2). The abrupt increase in backscattering yield is due to blistering that causes misalignment of the crystal relative to the incident beam.
there is a stage of gas accumulation and bubble formation, which is well described by ion-cutting processes [12,13] that form a blister-like structure in which the upper surface layer begins to separate from the underlying substrate and bows outward under the high gas pressure. When the critical gas pressure is reached, the blisters rupture and the surface exfoliates, as shown in Fig. 2(a). The ion-cut surface exhibits a ridged morphology. This structure is due to brittle failure (parallel to the (1 0 0) surface) from the aggregation of high concentrations of He and the possible accumulation of O2 from material decomposition. The simultaneous ion-cutting of the surface at different depths provides a convenient means of examining the radiation effects at different doses, although the dose value cannot be specified without an exact knowledge of the depth (the dose varies with depth along the trajectory of the incident ion [14]). The thickness of an exfoliated layer for the irradiation conditions has been determined to be 6.8 lm, as shown in the insert of Fig. 2(a) [15]. This value is consistent with the depth (6.8 lm) of the maximum vacancy production predicted by SRIM-2003 [16], where nucleation of He and O2 bubbles is promoted. The RBS results for the ion-cut area (indicated in Fig. 2(a)) are shown in Fig. 2(b). Since the beam spot for the RBS analysis had a dimension of 0.2 · 0.2 mm2, the RBS spectrum reflects the average concentration of the elements over the analyzed area. Also included in Fig. 2(b) is a random RBS spectrum for Cd2Nb2O7. The channel number has been converted to depth using the stopping power database from SRIM-2003 [16]. Depth scales for Cd and Nb, which are referenced to the density of the Cd2Nb2O7 single crystal (6.2 g/cm3), are indicated on the bottom and top axes, respectively. The RBS results suggest that the concentration of Cd increases moderately and that of Nb decreases significantly at the ion-cut surface, as com-
190
W. Jiang et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 188–191
Fig. 3. (a) Plan view of nanoparticles on an ion-cut surface of Cd2Nb2O7 single crystal. (b) A high-resolution TEM image of Cd2Nb2O7 that was amorphized with a focused Ga+ ion beam near room temperature. Formation of nanocrystalline Cd particles with random orientations is observed. Fig. 2. (a) SEM image (plan view parallel to the (1 0 0) surface) of an ioncut surface resulted from He+-ion irradiation. Insert to (a) is a 70-tilted view of an exfoliated layer for the thickness measurement. (b) Backscattering yield as a function of Cd and Nb depths for the ion-cut and unirradiated areas.
pared to the bulk Cd2Nb2O7 crystal. However, there are little variations in both the Cd and Nb concentrations over the depth (>300 nm). Evidently, there are efficient decomposition and fast diffusion processes that occur during the irradiation at or below 295 K. It is believed that under these experimental conditions, the material decomposition is primarily caused by the He+ ionization process, while the surface exfoliation is mainly associated with the accumulation of He and O2 gas. In contrast to some other oxides (e.g. SrTiO3 [17]) that only show surface roughness, nanoparticles are observed on the ion-cut surface of the Cd2Nb2O7, as shown in Fig. 3(a). At low doses, the nucleation of nanoparticles occurs and well-separated small particles are dominant [8]. With increasing dose, more and more material is decomposed and the particles grow to larger sizes through an ion-beam-assisted Ostwald ripening process that consists of ion-beam-enhanced diffusion and aggregation.
Small particles can merge into larger ones upon contact. Eventually, the surface becomes completely granulated at higher doses (Fig. 3(a)). In a separate study, small particles are also observed in Cd2Nb2O7 that was amorphized near room temperature with a focused 30 keV Ga+ ion beam (FIB), as shown in Fig. 3(b). The high-resolution TEM image shows clear fringes within the nanoparticles, indicating that the particles are single crystals. The relative orientation of the crystallites is random with respect to each other. Furthermore, the distance between the fringes is 0.28 nm, which is equal to the (0 0 0 2) interplanar spacing of the bulk Cd crystal. Compositional analysis based on the EDS methods also has suggested that the nanoparticles primarily consist of metallic Cd [8]. With increasing dose, nanowires are formed, as illustrated in Fig. 4(a). The wires are randomly distributed on the ion-cut surface (70-tilted view). There are granular knobs at the top of the nanowires. In a higher-dose irradiated region [9], the nanowires exhibit a variety of diameters, lengths and shapes; they can be as long as several tens of microns with diameters ranging from less than 100 nm to nearly 1 lm. The composition of the nanowires
W. Jiang et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 188–191
191
that is pushed off the ion-cut surface. The Cd wires also have been studied using TEM and electron diffraction, and have been found to be mono-crystalline [9]. 4. Summary In summary, nanometer-scale particles and wires of metallic Cd are formed as a result of irradiation-induced decomposition and phase segregation in Cd2Nb2O7 pyrochlores. The particles nucleate and grow with increasing dose. At higher doses, nanowires of soft metallic Cd are extruded/pulled through the pores in the He+-ion irradiated cadmium niobate. The wire consists of a crystalline Cd core with a thin shell of CdO that is formed upon exposure to air. Acknowledgements This work was supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, US Department of Energy under Contract DE-AC0576RL01830. The accelerator and SEM operations within the Environmental Molecular Sciences Laboratory at the Pacific Northwest National Laboratory were supported by the Office of Biological and Environmental Research, US Department of Energy. The FIB and TEM experiments were completed in the Electron Microbeam Analysis Facility at the University of Michigan. References Fig. 4. (a) 70-tilted view of nanowires on a granulated surface. (b) EDS spectra for the tip material and nanowire. The nanowire contains no detectable Nb, and there is a significantly smaller amount of oxygen in the wire than in the tip material.
was analyzed using EDS, and the results are shown in Fig. 4(b). The C peaks in the spectra originated from a few atomic layers of C deposited onto the sample surface prior to the SEM analysis in order to avoid charging effects. Fig. 4(b) suggests that the wire is primarily composed of Cd; the relative oxygen content in the wire is significantly smaller than that in the tip material that has a similar composition as the ion-cut surface [9]. The Nb-concentration in both the nanowire and tip material is below the detection limit. These soft metallic Cd nanowires are believed to be extruded/pulled through pores in the phase-segregated material, and the small amount of oxygen is due to surface oxidation of the wires when exposed to air [9]. The driving force for extruding/pulling the wires stems from the high compressive stress induced by various contributions, including mechanical bending of the top layer during blistering, amorphization induced volume swelling and thermal expansion of the metallic Cd from 140 to 295 K. The granular knobs at the wire tip are a mass of material
[1] A. Meldrum, R.F. Haglung Jr., L.A. Boatner, C.W. White, Adv. Mater. 13 (2001) 1431. [2] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [3] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [4] F. Cerrina, C. Marrian, MRS Bull. 21 (1996) 56. [5] C.R. Martin, Science 226 (1994) 1961. [6] A. Nakamura, K. Matsunaga, J. Tohma, T. Yamamoto, Y. Ikuhara, Nature Mater. 2 (2003) 453. [7] R. Adelung, O.C. Aktas, J. Franc, A. Biswas, R. Kunz, M. Elbahri, J. Kanzow, U. Schu¨rmann, F. Faupel, Nature Mater. 3 (2004) 375. [8] W. Jiang, W.J. Weber, J.S. Young, L.A. Boatner, Appl. Phys. Lett. 80 (2002) 670. [9] W. Jiang, W.J. Weber, C.M. Wang, J.S. Young, L.A. Boatner, J. Lian, L.M. Wang, R.C. Ewing, Adv. Mater. 17 (2005) 1602. [10] W. Jiang, W.J. Weber, L.A. Boatner, Nucl. Instr. and Meth. B 241 (2005) 372. [11] W. Jiang, W.J. Weber, S. Thevuthasan, R. Gro¨tzschel, Nucl. Instr. and Meth. B 166–167 (2000) 374. [12] M. Bruel, Electron. Lett. 31 (1995) 1201. [13] J.K. Lee, M. Nastasi, N.D. Theodore, A. Smalley, T.L. Alford, J.W. Mayer, M. Cai, S.S. Lau, J. Appl. Phys. 96 (2004) 280. [14] W. Jiang, J.W. Weber, Phys. Rev. B 64 (2001) 125206. [15] W. Jiang, W.J. Weber, S. Thevuthasan, L.A. Boatner, Nucl. Instr. and Meth. B 207 (2003) 85. [16] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. Available from:
Nuclear Instruments and Methods in Physics Research B 250 (2006) 188–191 www.elsevier.com/locate/nimb
Irradiation-induced nanostructures in cadmium niobate pyrochlores W. Jiang
a,*
, W.J. Weber a, J.S. Young a, L.A. Boatner b, J. Lian c, L.M. Wang c, R.C. Ewing c a
Pacific Northwest National Laboratory, Richland, WA 99352, USA b Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA c University of Michigan, Ann Arbor, MI 48109, USA Available online 9 June 2006
Abstract This paper reports the formation processes of crystalline Cd nanostructures on ion-cut surfaces of cadmium niobate pyrochlores (Cd2Nb2O7). Irradiation with 3 MeV He+ ions has been performed at low temperatures (6295 K) to induce material decomposition and aggregation of host atoms. The irradiation also leads to surface exfoliation due to rupture of gas (He and O2) filled blisters. Nanoparticles and nanowires are observed on the ion-cut surfaces at low and higher doses, respectively. These structures are examined and characterized using a suite of experimental tools. Both the particles and wires are found to be single crystals that primarily consist of metallic Cd. Published by Elsevier B.V. PACS: 61.82.Ms; 81.07.Ta; 81.07.Vb; 68.37.Hk; 68.37.Lp; 82.80.Yc Keywords: Ion irradiation; Decomposition; Nanostructures; Pyrochlores
1. Introduction Nanostructured materials represent a subject that has attracted worldwide attention in recent years. This high level of interest is primarily attributed to their unique properties, such as the quantum confinement of electrons and exceptionally large surface areas that make the materials excellent candidates for technological applications in a variety of fields, including materials science, chemistry, electronics, photonics and biomedicine. Nanoparticles of elements or compounds have been successfully synthesized in various substrates using high-dose ion implantation, followed by thermal annealing at high temperatures [1]. The formation of the embedded nanoparticles (precipitates) in the near-surface region is due to an Ostwald ripening process of the implanted species in the host materials. Onedimensional nanostructures, such as nanowires, are often *
Corresponding author. Tel.: +1 509 376 5471; fax: +1 509 376 5106. E-mail address: [email protected] (W. Jiang).
0168-583X/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.nimb.2006.04.106
fabricated by vapor–liquid–solid processes [2,3]. Template-directed syntheses utilizing lithography [4], pores [5]; dislocations [6] or cracks [7] also have been demonstrated. Recently, we have reported the formation of both nanoparticles and nanowires of host atoms on ion-cut surfaces of cadmium niobate pyrochlores (Cd2Nb2O7) [8,9]. This paper presents additional results and discussion regarding the nanostructure formation. 2. Experimental procedures The single-crystal wafers of Cd2Nb2O7 used in this study were grown, cut and Syton polished on the (1 0 0) plane. During an ion-channeling study of damage recovery in Cd2Nb2O7 irradiated at 150 K with Au2+ ions [10], a virgin spot on the sample was used to monitor damage accumulation caused by the analyzing beam of 3 MeV He+ ions in the near-surface region. Following each annealing cycle over a temperature range from 180 to 295 K for 20 min each, Rutherford backscattering spectrometry (scattering
W. Jiang et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 188–191
Temperature (K)
Ion fluence (He+/nm2)
Duration (s)
140 180 141 210 170 240 174 270 223 295 195 295 295
1560 0 2340 0 1560 0 1560 0 1560 0 2340 0 1560
349 1200 594 1200 297 1200 336 1200 354 1200 546 36,000a 270b
a b
Vacuum storage. Surface exfoliation.
angle = 150) along the h1 0 0i channeling direction (RBS/ C) was performed at temperatures ranging from 140 to 295 K. The experimental conditions for the He+-irradiation and thermal annealing are summarized in Table 1. The sample was then stored in air at room temperature for about one year. Following the storage, secondary electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) methods were employed to study the morphology and composition in the He+ irradiated area. The microscope (LEO 982) used in this study was operated in a voltage range from 2 to 20 kV for imaging. During the EDS experiment, 8 or 10 keV electrons were used for the characterization of nanostructures in order to reduce the excitation volume (300 nm deep). In all cases, the focused electron beam for the EDS experiment had a size on the order of 1 nm. In addition, transmission electron microscopy (TEM) was also used to characterize the atomic-scale structures. 3. Results and discussion The h1 0 0i-aligned spectra for Cd2Nb2O7 sequentially irradiated to various fluences at different temperatures (see Table 1) are shown in Fig. 1, together with a random spectrum. The ion fluence indicated in the figure is estimated based on the total integrated charge for the corresponding spectrum. Initially, the backscattering yields from the surface increase slightly with the increase of ion fluence, as expected; following irradiation to a total ion fluence of 1.1 · 104 He+/nm2 at various temperatures, the spectrum shows an abrupt increase in the scattering yield. This is an indication that the crystal in the irradiated area has become misaligned with the incident beam. A similar result was also observed for H+ ion implanted SiC [11]. During the irradiation of Cd2Nb2O7, the implanted He and some O2 accumulate at the peak in the irradiationinduced vacancy concentration and near the projected range of the implanted He. Prior to surface exfoliation,
+
1000
Random Nb
800 600 400
2
He /nm (TIrrad.) 1560 (140 K) 2340 (141 K) 1560 (170 K) 1560 (174 K) 1560 (223 K) 2340 (195 K) 1560 (295 K) 1560 (295 K)
Cd2Nb2O7
Scattering Yield
Table 1 Sequential irradiation (with 3 MeV He+ ions) and thermal annealing for Cd2Nb2O7
189
Cd
Surface Exfoliation <100>-Aligned
200 0
660
680
700 720 Channel Number
740
Fig. 1. A sequence of 3 MeV He+ RBS/C spectra for a h1 0 0i-oriented Cd2Nb2O7 wafer (beam-spot size: 0.5 · 0.6 mm2). The abrupt increase in backscattering yield is due to blistering that causes misalignment of the crystal relative to the incident beam.
there is a stage of gas accumulation and bubble formation, which is well described by ion-cutting processes [12,13] that form a blister-like structure in which the upper surface layer begins to separate from the underlying substrate and bows outward under the high gas pressure. When the critical gas pressure is reached, the blisters rupture and the surface exfoliates, as shown in Fig. 2(a). The ion-cut surface exhibits a ridged morphology. This structure is due to brittle failure (parallel to the (1 0 0) surface) from the aggregation of high concentrations of He and the possible accumulation of O2 from material decomposition. The simultaneous ion-cutting of the surface at different depths provides a convenient means of examining the radiation effects at different doses, although the dose value cannot be specified without an exact knowledge of the depth (the dose varies with depth along the trajectory of the incident ion [14]). The thickness of an exfoliated layer for the irradiation conditions has been determined to be 6.8 lm, as shown in the insert of Fig. 2(a) [15]. This value is consistent with the depth (6.8 lm) of the maximum vacancy production predicted by SRIM-2003 [16], where nucleation of He and O2 bubbles is promoted. The RBS results for the ion-cut area (indicated in Fig. 2(a)) are shown in Fig. 2(b). Since the beam spot for the RBS analysis had a dimension of 0.2 · 0.2 mm2, the RBS spectrum reflects the average concentration of the elements over the analyzed area. Also included in Fig. 2(b) is a random RBS spectrum for Cd2Nb2O7. The channel number has been converted to depth using the stopping power database from SRIM-2003 [16]. Depth scales for Cd and Nb, which are referenced to the density of the Cd2Nb2O7 single crystal (6.2 g/cm3), are indicated on the bottom and top axes, respectively. The RBS results suggest that the concentration of Cd increases moderately and that of Nb decreases significantly at the ion-cut surface, as com-
190
W. Jiang et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 188–191
Fig. 3. (a) Plan view of nanoparticles on an ion-cut surface of Cd2Nb2O7 single crystal. (b) A high-resolution TEM image of Cd2Nb2O7 that was amorphized with a focused Ga+ ion beam near room temperature. Formation of nanocrystalline Cd particles with random orientations is observed. Fig. 2. (a) SEM image (plan view parallel to the (1 0 0) surface) of an ioncut surface resulted from He+-ion irradiation. Insert to (a) is a 70-tilted view of an exfoliated layer for the thickness measurement. (b) Backscattering yield as a function of Cd and Nb depths for the ion-cut and unirradiated areas.
pared to the bulk Cd2Nb2O7 crystal. However, there are little variations in both the Cd and Nb concentrations over the depth (>300 nm). Evidently, there are efficient decomposition and fast diffusion processes that occur during the irradiation at or below 295 K. It is believed that under these experimental conditions, the material decomposition is primarily caused by the He+ ionization process, while the surface exfoliation is mainly associated with the accumulation of He and O2 gas. In contrast to some other oxides (e.g. SrTiO3 [17]) that only show surface roughness, nanoparticles are observed on the ion-cut surface of the Cd2Nb2O7, as shown in Fig. 3(a). At low doses, the nucleation of nanoparticles occurs and well-separated small particles are dominant [8]. With increasing dose, more and more material is decomposed and the particles grow to larger sizes through an ion-beam-assisted Ostwald ripening process that consists of ion-beam-enhanced diffusion and aggregation.
Small particles can merge into larger ones upon contact. Eventually, the surface becomes completely granulated at higher doses (Fig. 3(a)). In a separate study, small particles are also observed in Cd2Nb2O7 that was amorphized near room temperature with a focused 30 keV Ga+ ion beam (FIB), as shown in Fig. 3(b). The high-resolution TEM image shows clear fringes within the nanoparticles, indicating that the particles are single crystals. The relative orientation of the crystallites is random with respect to each other. Furthermore, the distance between the fringes is 0.28 nm, which is equal to the (0 0 0 2) interplanar spacing of the bulk Cd crystal. Compositional analysis based on the EDS methods also has suggested that the nanoparticles primarily consist of metallic Cd [8]. With increasing dose, nanowires are formed, as illustrated in Fig. 4(a). The wires are randomly distributed on the ion-cut surface (70-tilted view). There are granular knobs at the top of the nanowires. In a higher-dose irradiated region [9], the nanowires exhibit a variety of diameters, lengths and shapes; they can be as long as several tens of microns with diameters ranging from less than 100 nm to nearly 1 lm. The composition of the nanowires
W. Jiang et al. / Nucl. Instr. and Meth. in Phys. Res. B 250 (2006) 188–191
191
that is pushed off the ion-cut surface. The Cd wires also have been studied using TEM and electron diffraction, and have been found to be mono-crystalline [9]. 4. Summary In summary, nanometer-scale particles and wires of metallic Cd are formed as a result of irradiation-induced decomposition and phase segregation in Cd2Nb2O7 pyrochlores. The particles nucleate and grow with increasing dose. At higher doses, nanowires of soft metallic Cd are extruded/pulled through the pores in the He+-ion irradiated cadmium niobate. The wire consists of a crystalline Cd core with a thin shell of CdO that is formed upon exposure to air. Acknowledgements This work was supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, US Department of Energy under Contract DE-AC0576RL01830. The accelerator and SEM operations within the Environmental Molecular Sciences Laboratory at the Pacific Northwest National Laboratory were supported by the Office of Biological and Environmental Research, US Department of Energy. The FIB and TEM experiments were completed in the Electron Microbeam Analysis Facility at the University of Michigan. References Fig. 4. (a) 70-tilted view of nanowires on a granulated surface. (b) EDS spectra for the tip material and nanowire. The nanowire contains no detectable Nb, and there is a significantly smaller amount of oxygen in the wire than in the tip material.
was analyzed using EDS, and the results are shown in Fig. 4(b). The C peaks in the spectra originated from a few atomic layers of C deposited onto the sample surface prior to the SEM analysis in order to avoid charging effects. Fig. 4(b) suggests that the wire is primarily composed of Cd; the relative oxygen content in the wire is significantly smaller than that in the tip material that has a similar composition as the ion-cut surface [9]. The Nb-concentration in both the nanowire and tip material is below the detection limit. These soft metallic Cd nanowires are believed to be extruded/pulled through pores in the phase-segregated material, and the small amount of oxygen is due to surface oxidation of the wires when exposed to air [9]. The driving force for extruding/pulling the wires stems from the high compressive stress induced by various contributions, including mechanical bending of the top layer during blistering, amorphization induced volume swelling and thermal expansion of the metallic Cd from 140 to 295 K. The granular knobs at the wire tip are a mass of material
[1] A. Meldrum, R.F. Haglung Jr., L.A. Boatner, C.W. White, Adv. Mater. 13 (2001) 1431. [2] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [3] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [4] F. Cerrina, C. Marrian, MRS Bull. 21 (1996) 56. [5] C.R. Martin, Science 226 (1994) 1961. [6] A. Nakamura, K. Matsunaga, J. Tohma, T. Yamamoto, Y. Ikuhara, Nature Mater. 2 (2003) 453. [7] R. Adelung, O.C. Aktas, J. Franc, A. Biswas, R. Kunz, M. Elbahri, J. Kanzow, U. Schu¨rmann, F. Faupel, Nature Mater. 3 (2004) 375. [8] W. Jiang, W.J. Weber, J.S. Young, L.A. Boatner, Appl. Phys. Lett. 80 (2002) 670. [9] W. Jiang, W.J. Weber, C.M. Wang, J.S. Young, L.A. Boatner, J. Lian, L.M. Wang, R.C. Ewing, Adv. Mater. 17 (2005) 1602. [10] W. Jiang, W.J. Weber, L.A. Boatner, Nucl. Instr. and Meth. B 241 (2005) 372. [11] W. Jiang, W.J. Weber, S. Thevuthasan, R. Gro¨tzschel, Nucl. Instr. and Meth. B 166–167 (2000) 374. [12] M. Bruel, Electron. Lett. 31 (1995) 1201. [13] J.K. Lee, M. Nastasi, N.D. Theodore, A. Smalley, T.L. Alford, J.W. Mayer, M. Cai, S.S. Lau, J. Appl. Phys. 96 (2004) 280. [14] W. Jiang, J.W. Weber, Phys. Rev. B 64 (2001) 125206. [15] W. Jiang, W.J. Weber, S. Thevuthasan, L.A. Boatner, Nucl. Instr. and Meth. B 207 (2003) 85. [16] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. Available from: