Letter pubs.acs.org/NanoLett
Ultrahigh-Density sub-10 nm Nanowire Array Formation via SurfaceControlled Phase Separation Yuan Tian,†,‡ Pinaki Mukherjee,‡,§ Tanjore V. Jayaraman,†,‡ Zhanping Xu,†,‡ Yongsheng Yu,‡,§ Li Tan,†,‡ David. J. Sellmyer,‡,§ and Jeffrey E. Shield*,†,‡ †
Department of Mechanical and Materials Engineering, University of Nebraska, Lincoln, Nebraska 68588, United States Nebraska Center for Materials and Nanoscience, Lincoln, Nebraska 68588, United States § Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, United States ‡
S Supporting Information *
ABSTRACT: We present simple, self-assembled, and robust fabrication of ultrahigh density cobalt nanowire arrays. The binary Co−Al and Co−Si systems phase-separate during physical vapor deposition, resulting in Co nanowire arrays with average diameter as small as 4.9 nm and nanowire density on the order of 1016/m2. The nanowire diameters were controlled by moderating the surface diffusivity, which affected the lateral diffusion lengths. High resolution transmission electron microscopy reveals that the Co nanowires formed in the face-centered cubic structure. Elemental mapping showed that in both systems the nanowires consisted of Co with undetectable Al or Si and that the matrix consisted of Al with no distinguishable Co in the Co−Al system and a mixture of Si and Co in the Co−Si system. Magnetic measurements clearly indicate anisotropic behavior consistent with shape anisotropy. The dynamics of nanowire growth, simulated using an Ising model, is consistent with the experimental phase and geometry of the nanowires. KEYWORDS: Nanowires, phase-separation, magnetic anisotropy, Ising model
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arrays with a range of sizes.29,30 There have been numerous reports describing magnetic materials fabricated in nanowire arrays using an AAO template, such as Fe, Co, Ni, FeCo, FeNi, and CoNi.5,31−36 In addition, various multilayered nanowires including Co/Cu, Ni/Cu, NiFe/Cu, CoNi/Cu, and Ag/Co37,38 have been investigated due to their interesting magnetic and transport properties. Heterogeneous ferromagnetic−nonmagnetic alloy nanowire composites such as CoCu39and CoAg40 grown by a variety of methods have also been investigated. Nanowire diameters less than 10 nm are desired for a number of applications.11 One promising method for synthesizing sub-10 nm nanowires/nanowire arrays takes advantage of naturally occurring phase separation.41−43 For example, during eutectic solidification fibrous or lamellar structures result from the lateral diffusion of atoms perpendicular to the growth direction, and the scale of the structure is proportional to the square root of the diffusivity in the liquid in front of the growing solid interface.44 Similarly, phase separation during vapor-to-solid growth, as occurs during physical vapor deposition (PVD), is controlled by the surface diffusion of adatoms during film growth. Atzmon et al.45 predicted the occurrence of fibrous phase morphologies during film growth in systems displaying phase immiscibility. The resulting scale of
ne-dimensional nanostructures such as nanowires, nanowire arrays, nanotubes, and nanorods have attracted a great deal of attention due to their tremendous potential in various fields such as solar cells, field sensors, bioseparation, and medical therapy.1,2 The design and control of size, morphology, and growth of the nanowires are key to enhanced electrical, magnetic, and photonic properties that can be utilized in sensor technology, ultrahigh density magnetic storage, drug delivery vehicles, catalysis/electro-catalysis, selective separation, and thermoelectric devices.3−12 Miniaturization of nanowirebased devices with integrated capabilities13−15 requires a small diameter as well as a high density of nanowires. High-density nanowires/nanowire arrays are primarily synthesized by various template-assisted methods using diblock copolymer templates,7,9 track-etched membranes, or anodized aluminum oxide (AAO) templates.16 Deposition routes include electrodeposition with dc, ac, and pulsed potentials.17−26 Although nontemplate-assisted growth of nanowires in solution of seeded surfaces has been reported,27,28 the template-assisted growth remains a versatile and simple method to fabricate nanowire arrays with wire diameters less than 100 nm and densities greater than 1014 wires/m2. Building such small diameter and high-density nanowire arrays by conventional physical methods such as electron beam and X-ray lithography are generally very expensive and not commercially viable. Among the template-assisted methods, AAO is widely used to fabricate large area chemically and mechanically stable nanowire © XXXX American Chemical Society
Received: March 26, 2014 Revised: May 29, 2014
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estimated from multiple growth rates; a thorough analysis is currently underway using several growth rates for both systems and will be presented elsewhere. A preliminary estimate yields a diffusivity of 10−19 m2/s in the Co−Al system. The smaller diameter nanowires also displayed a narrower size distribution. In addition, we can also control the nanowire size by adjusting the substrate temperature. Compared to 26 °C, a substrate temperature of 500 °C produces a higher surface diffusivity, which increased the nanowires diameters, again described by eq 1 (Supporting Information Figure S1). The packing fraction of Co nanowire arrays in Al matrix is about 62%, which is close to the target composition. For the sample shown in Figure 1a, the nanowires are continuous as revealed by the cross-sectional TEM bright-field micrographs (Figure 2a,b) and high-angle annular dark-field
the structure depends on the diffusion length, which, for processes controlled by surface diffusion at a fixed growth rate, was given by x∼
⎛ Dδ ⎞1/2 ⎜ ⎟ ⎝ u ⎠
(1)
where D is the surface diffusivity, δ is the thickness of the layer in the growth direction, and u is the growth rate. The evolution of laterally segregated structures, and thus the growth of fibers/ nanowires, requires a balance between the growth rate and surface diffusivity; otherwise, vertically segregated (i.e., twophase granular) films are formed. A finer scale phase-separated structure is expected to result from surface diffusion-controlled processing such as PLD and sputtering when compared with solidification processes because of the orders-of-magnitude lower diffusivity on surfaces than in liquids for a given system,46 as well as the ability to control the growth rate in these processes by adjusting the various parameters. Subsequent simulations of the growth of phase-separating systems also suggest the formation of nanowires.47−49 Here, we show the development of Co nanowire arrays do in fact grow during PVD as the result of phase separation of immiscible atomic species, given the correct combination of surface diffusivity and growth rates. This results in a simple, self-assembled, robust fabrication route for ultrahigh density nanowire arrays. The Co−Al and Co−Si systems are ideal systems in which to develop phase-separated nanostructures and ultimately the formation of nanowire arrays using PVD because of the low or nonexistent solubility between the constituent elements.50 The difficulty is in determining the deposition conditions that strike the appropriate balance between lateral diffusion and growth rates. Under appropriate PVDs, we were able to develop nanowire arrays in both systems. For the Co−Al system, the Co nanowires produced were of relatively uniform size, as indicated by the plane-view TEM bright-field micrograph (Figure 1). The average nanowire
Figure 2. Morphology of Co nanowire arrays in Al matrix. (a) Crosssectional view bright-field TEM micrograph. (b) High-resolution cross-sectional view bright-field TEM micrograph. (c,d) Crosssectional view and plane-view of nanowires by HAADF-STEM imaging. The contrast within nanowires show compositional homogeneity.
scanning TEM (HAADF- STEM) (Figure 2c). The nanowires extend the entire thickness of the film (Figure 2a−c), and the thickest films we have grown are 200 nm. The boundaries between the nanowires and the matrix are clearly visible in the high-resolution cross-sectional TEM (Figure 2b). Additionally, HAADF STEM images show uniform contrast (Figure 2c,d), suggesting minimal nanowire-to-nanowire compositional variations. The fast Fourier transform (FFT) of the HRTEM image of a single nanowire reveals that the structure is face-centered cubic (fcc) with a lattice parameter ∼0.35 nm (Figure 3a), within experimental error of the lattice parameter of fcc Co. The selected area electron diffraction (SAED) pattern also reveals an fcc structure with a = 0.346 nm (Figure 3b). No lattice fringes were observed in the Al region during HRTEM, and no additional diffraction maxima were detected in the SAED pattern, suggesting that the internanowire Al regions are amorphous. The HAADF-STEM image reveals distinct atomic number contrast between the nanowires and matrix, with the former appearing brighter indicative of a higher atomic number. The elemental distribution was obtained by X-ray mapping in STEM mode. The false-colored STEM X-ray map shows
Figure 1. Morphology of Co nanowire arrays in Al matrix at different growth rates. Plane-view TEM bright-field micrograph. Inset: size distribution histograms with a Gaussian fit. (a) Low growth rate, average diameter, 9.9 ± 0.16 nm; density, 8.8 × 1015 wires/m2. (b) High growth rate, average diameter, 4.9 ± 0.04 nm; density, 3.2 × 1016 wires/m2. The packing fraction is about 62%.
diameter d with a growth rate of 0.028 nm/s was measured to be 9.9 ± 0.16 nm with a density of 8.8 × 1015 wires/m2. An average diameter of 4.9 ± 0.04 nm and density of 3.2 × 1016 wires/m2 were achieved with a growth rate of 0.033 nm/s. The Co nanowire diameters decreased with higher growth rates, consistent with eq 1. The surface diffusivity, D, can also be B
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Figure 3. Structure and compositional analysis of nanowires. (a) High-resolution TEM image of the cross section of an individual nanowire. Inset: fast Fourier transform of the HRTEM image, which indexes to the [011] zone axis with a lattice parameter about 0.35 nm. (b) The SAED pattern was indexed to an fcc structure with a lattice parameter 0.346 nm, which is consistent with FFT. (c−f) STEM X-ray mapping: (c) STEM HAADF image and (d) STEM X-ray mapping. There is a distinct elemental separation between Co (red) and Al (green) with negligible atomic mixing. (e) Co elemental mapping. (f) Al elemental mapping.
distinct separation of Co in the nanowires (red) and Al in the matrix (green), with negligible atomic mixing (Figure 3c). The X-ray production from the interwire regions was low, resulting in the black regions of Figure 2c, because these regions were preferentially thinned during ion milling (Al atoms are easier to remove than the heavier Co). Individual X-ray spectra showed Al in these regions, as did X-ray maps utilizing longer dwell times. However, the X-ray maps with longer dwell times had slight drift issues that reduced the clarity of the nanowires. Individual X-ray spectra from the two regions also confirmed the negligible elemental mixing between the two phases. The virtually complete segregation of Co and Al is constant with the packing fraction (62%) of Co nanowire arrays mentioned above that mirrors the target composition (Figure 1a). However, the complete segregation is surprising, considering that the equilibrium phase diagram shows a two-phase region between fcc (and hcp) Co and the B2 AlCo phase.50,51 Evidently, the atomic interactions on the surface promote complete segregation that precludes any intermetallic compound or alloy formation whatsoever. It should also be noted that different processing parameters, critically affecting growth rate and surface diffusion-controlled phase separation, resulted in granular films not displaying nanowire arrays. Co nanowire arrays were also successfully produced in the Co−Si system. The plane-view TEM bright-field micrograph showed that the Co nanowires here are also very uniform with diameter d = 5.38 ± 0.04 nm, density of 2.0 × 1016 wires/m2, and packing fraction of ∼44%, which is about 16% lower than the Co atomic percentage in the Co−Si alloy target (Figure 4a). The HAADF STEM image of this system again shows a uniform contrast of the Co nanowires, suggesting compositional homogeneity (Figure 4b). The FFT of the HRTEM image of a single nanowire reveals that the structure is fcc with
Figure 4. Co nanowire arrays in Si-based matrix. (a) Plane-view TEM bright-field micrograph. Inset: size distribution histogram with a Gaussian fit. Average diameter, 5.38 ± 0.04 nm; density, 2.0 × 1016 wires/m2; packing fraction, 44%. (b) Plane-view HAADF STEM image of nanowires. The uniform contrast between nanowires shows compositional homogeneity. (c) High-resolution TEM image of the cross section of an individual nanowire. Inset: fast Fourier transform of the HRTEM image, which indexes to the [011] zone axis with a lattice parameter about 0.35 nm. (d) The SAED pattern was indexed to an fcc structure with a lattice parameter 0.348 nm, which is consistent with FFT.
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Figure 5. Magnetic property of Co nanowire arrays at 10K. The out-of-plane (⊥, blue, the magnetic field is parallel to nanowires axis) and in-plane (//, red, the magnetic field is perpendicular to nanowires axis) hysteresis loops of the Co nanowires, the red and blue arrows (in color version) indicate the magnetic field. The hysteresis loops of (a) Co nanowire arrays in Si-based matrix shown in Figure 4. (b) Co nanowire arrays in Al matrix shown in Figure 1a. (c) Co nanowire arrays in Al matrix shown in Figure 1b.
(Figure 5b,c) is indicative of in-plane easy magnetization, as especially shown by the rapid approach to saturation in the parallel (in plane) orientations. The effective anisotropy is also shown by the difference between the magnetization curves for the in- and out-of-plane measurements. Here, the hysteresis loop for the finer nanowires with a more densely packed array (Figure 5c) shows higher anisotropy than does the hysteresis loop for the larger and less dense array (Figure 5b). The Co−Si nanowires show an even higher anisotropy (Figure 5a), revealing the high shape anisotropy of the nanowires. Previous work has shown that a transition from out-of-plane easy magnetization, governed by the highly anisotropic shape of the nanowires, to in-plane easy magnetization occurs as the density of the nanowires increases.52 The transition is due to the dipolar interactions in closely packed arrays, which effectively counteracts the shape anisotropy that favors magnetization along the nanowire long axis. Physically, the higher the volume fraction of the magnetic phase, the closer the structure resembles that of a continuous thin film with a preferential magnetization direction in plane. This is supported by the different volume fractions of the nanowires, where the Co−Al has a higher packing density (62%) compared to the Co−Si (44%). The coercivity of nanowire arrays has also been shown to be dependent on both size and packing density with a general decrease as the ratio of the nanowire diameter (d) to internanowire spacing (D) increases.53 The observations here for the Co−Al materials is consistent with that work, as the d/D is ∼0.7 for the larger-sized nanowires (Figure 1a) and ∼0.5 for the smaller nanowires (Figure 1b), and respective out-of-plane coercivities of 65 Oe (Figure 5b) and 227 Oe (Figure 5c). The structural, compositional, and magnetic characterization revealed that the controlled phase separation results in uniform, homogeneous, and high-density nanowires. To explain the dynamics of the nanowire growth by phase separation of immiscible elements, an Ising model47 was utilized to simulate the growth process. In this model, no diffusion was allowed in the bulk (here, “bulk” means the material covered by more than one monolayer of additional atoms) and thus phase separation
a lattice parameter about 0.35 nm (Figure 4c), also within experimental error of the lattice parameter of fcc Co. The SAED reveals an fcc structure with lattice parameter of 0.348 nm, which is consistent with FFT above (Figure 4d). No lattice fringes were observed in the matrix region, and no additional diffraction maxima were observed in the SAED pattern, both of which suggest that the internanowire regions are amorphous. STEM X-ray mapping again showed that the nanowires are solely consisting of Co (red) with no Si present (Supporting Information Figure S2). However, in this Co−Si system the matrix consists of both Si (blue) and Co (red), which is in contrast to the Co−Al system described above. Thus, in this case it appears that the growth of the Co nanowires may be controlled more by Si diffusion away from the nanowires than mutual motion of both Co and Si. The presence of both Co and Si in the interwire regions also explains why the packing fraction of Co nanowire arrays does not reflect the composition of the Co−Si alloy target. The magnetic characterization of the nanowires was performed in a superconducting quantum interference device (SQUID) magnetometer at 10K. The hysteresis behavior of the nanowires was studied in two configurations: in-plane (//), with the magnetic field applied parallel to the thin film, and outof-plane (⊥), with the magnetic field applied normal to the thin film but parallel to the length of the nanowires. Measuring in these two orthogonal directions can reveal the presence of magnetic anisotropy. Here, it is expected that the high aspect ratio of the nanowires would lead to a large shape anisotropy so that the easy magnetization direction would be along the length of the wire (i.e., perpendicular to the film plane). The hysteresis loops of the Co−Si material did in fact reveal an out-of-plane easy magnetization direction and an in-plane hard direction (Figure 5a). The out-of-plane measurement showed both a higher remanence and higher coercivity than did the in-plane measurement, suggesting magnetization along the long axis of the wire. However, neither hysteresis loop of the Co−Al samples displayed characteristics of out-of-plane anisotropy. Instead, the magnetic behavior that was observed D
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was restricted to the surface. The “solute” is the nanowireforming phase, while the “solvent” is the matrix phase. The morphology of the phase separation, and the scale of the nanowires, was strongly dependent on growth rates. Wellaligned nanowires were observed at a moderate growth rate (Figure 6a), which is consistent with our experimental results
deposition and compositional analysis of nanowires in Co-Si system, experimental materials and methods, and simulation method. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors appreciate useful discussions with Ralph Skomski concerning the magnetic behavior. This research was supported by the National Science Foundation under Grant DMR0820521. It was performed in part in Central Facilities of the Nebraska Center for Materials and Nanoscience, which is supported by the Nebraska Research Initiative.
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Figure 6. Simulation based on Ising model: (a−c) Perspective views showing morphology of simulated fibers containing 5% vol. Co with deposition rates of (a) 1 × 10−4, (b) 1 × 10−3, and (c) 1 × 10−2 a0/ MCS. Reduced unit is used with a0 as lattice constant and MCS as the time required making exchange attempts for one layer. (d) Top view of the nanowires of (a) showing the diffusion process of the solute particles (cyan, solvent in white). (e) Corresponding 3D view of the nanowires. The solvent phase below top layer is hidden for better illustration.
(Figures 1, 2, and 4a,b). At higher growth rates, the nanowires have smaller diameters and break into long and narrow fibers (Figure 6b), eventually breaking up into small granules (Figure 6c). The simulations also reveal the diffusion process of the solute atoms (Figure 6d). Solute atoms on the surface diffuse to regions of like atoms, which lead to the growth of the nanowires (Figure 6e). The formation of well-assembled Co nanowires is shown to be the result of coupling between the surface diffusion of constituent atoms and growth rate, consistent with both experiment and previous models.45 In summary, Co nanowires with sub-10 nm diameter and high lateral density were formed by phase separation during deposition of two immiscible systems: Co−Al and Co−Si. The ultranarrow nanowires, synthesized by a template-free deposition method, were obtained by controlling the growth in 2D and surface diffusion in each layer of deposition. We also can control the nanowire size by adjusting the processing parameters. The Ising model successfully simulated the dynamics of the phase separation and nanowire growth. The nanowire arrays display excellent shape anisotropy resulting in different in-plane and out-of-plane coercivities, remanences, and approach to saturation. Furthermore, in the Co−Al system, the separated phases are Co with Al with negligible interatomic mixing.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1 and S2 showing morphology of Co nanowire arrays in Al matrix at different substrate temperatures during E
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