Mg Nanostructures Tailored by Glancing Angle Deposition - American

DOI: 10.1021/cg901036a

Mg Nanostructures Tailored by Glancing Angle Deposition Yuping He* and Yiping Zhao

2010, Vol. 10 440–448

Department of Physics and Astronomy, and Nanoscale Science and Engineering Center, University of Georgia, Athens, Georgia 30602 Received August 27, 2009; Revised Manuscript Received September 21, 2009

ABSTRACT: Magnesium nanostructures, with the material characteristic of surface adatom diffusion strongly influencing the nanostructure growth during physical vapor deposition, have been fabricated by a glancing angle deposition (GLAD) technique on flat and patterned substrates at a substrate rotation speed ranging from ω=0 rpm to ω=10 rpm. Depending on the substrate rotation speed, the Mg nanostructures consist of different nanostructure units: a nanoblade at ω = 0 rpm (oblique angle deposition), a bundle of nanoblades at ω=0.1 rpm, an oblate nanorod mixed with a bundle of nanoblades at ω=1 rpm and an oblate nanorod at ω=10 rpm. The formation of different nanostructure units in the samples deposited at different ω values is explained by the competition between the deposition and diffusion processes. In addition, during the oblique angle deposition of Mg, the substrate is rotated 90, and the formed sample is composed of two layers of nanoblade arrays with an included angle γ ≈ 33 between the two layers, resulting from a lattice match at the interface. Macroscopically, the structure parameters, including the number density, width and thickness of the nanostructure units, the height of samples, and the tilting angle of the nanostructures, depend strongly on both the substrate rotation speed and the substrate features.

Introduction Magnesium is very attractive for solid-state hydrogen storage applications due to its nontoxicity, lightweight, low cost, and high reversible hydrogen storage capacity (7.6 wt % in MgH2).1-5 However, its practical application is severely inhibited by its high thermodynamic stability and sluggish hydrogen sorption kinetics.6-12 By tailoring Mg into nanostructures, its hydrogen storage performance could be improved.3,5,10,13 Recently, an oblique angle deposition (OAD) technique has been used to fabricate Mg nanostructures.14,15 OAD is a physical vapor deposition process, where the vapor flux is incident onto a substrate at a large angle R (g70) with respect to the substrate normal. Both Tang et al.14 and He et al.15 have demonstrated that the oblique angle deposition of Mg results in approximately vertical arrays of nanoblades (Figure 1a), and the individual nanoblades show a hexagonal close packed (hcp) single crystal structure by selected area electron diffraction (SAED) with a (1010)[0001] biaxial texture as shown in Figure 1b,14 where (1010) is the top surface of the hexagonal Mg nanoblade and [0001] is the lattice direction of the (0001) crystal plane, which is facing to the incident vapor flux. The thickness of the Mg nanoblade along the incident vapor direction (generally in nanometer scale) is markedly thinner than the blade width perpendicular to the incident vapor direction (generally in micrometer scale). According to the proposed growth model by Tang et al.,14 after Mg atoms land on the (0001) surface, they diffuse isotropically, leading to the fast growth in the width ([0110]) and height ([2130]) directions of the nanoblades. Both the appearance of the surface steps parallel to the [0110] direction and the low surface diffusion on the top surface (1010) of the nanoblades are proposed to cause the thin thickness of the nanoblade along the vapor flux direction. It is well-known that the self-shadowing effect is the dominant growth mechanism for OAD, resulting in columnar *Corresponding author. E-mail: [email protected]. Tel: (706) 5428109. Fax: (706) 542-2492. pubs.acs.org/crystal

Published on Web 10/08/2009

array structures, and these columns will be tilting away from the substrate normal at an angle of β under the condition of limited adatom diffusion.16-20 The column tilting angle β is not the same as the vapor incident angle R, and in general, β < R. The relationship between the two angles obeys two different empirical rules, the tangent rule tan β = 1/2 tan R19 or the cosine rule β=R - arc sin[(1 - cos R)/2],20 or falls in between these two rules. The vertical growth characteristic of the Mg nanoblades violates both the tangent and cosine rules for OAD.14,15 Note that the ratio of the substrate temperature Ts to the melting temperature Tm of Mg is Tr = Ts/Tm ≈ 311 K/924 K ≈ 0.34. According to a three region structure zone model (SZM) for physical vapor deposited films,21 in Zone I (Tr < 0.3), the substrate temperature is too low for any significant diffusion to occur and self-shadowing effect controls the film growth; in Zone II (0.3 < Tr < 0.5), the surface adatom diffusion strongly influences the film growth due to the increased substrate temperature; in Zone III (Tr > 0.5), the thermal energy is high enough for bulk diffusion to occur, leading to recrystallization as the film grows. For Mg, the value of Tr ≈ 0.34 indicates that its growth belongs to Zone II. Thus, during Mg deposition, the surface adatom diffusion and the self-shadowing effect will compete with each other. The shadowing effect leads to the formation of columns inclined toward the vapor source, whereas the surface adatom diffusion could promote a structure whose orientation approaches the substrate normal.22 Therefore, the formation of the approximately vertical Mg nanoblade array structure should be caused by the fast adatom diffusion of light Mg.14,15 In addition, it is noted that under most deposition conditions, the growth of the metals Al (Tm=933 K) and Zn (Tm=692 K) also belongs to Zone II, but our experiments show that only Zn forms a similar nanoblade structure. We believe that this is related to the crystal structure of the metals: both Mg and Zn have a hexagonal close packed (hcp) structure, while Al has a face centered cubic (fcc) crystal structure. Therefore, only those metals in Zone II and with an hcp crystal structure could form an approximately vertical nanoblade structure during OAD. r 2009 American Chemical Society

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Figure 2. The substrate with four typical features (A, B, C, and D) used for the deposition of different Mg nanostructures. The scale bar in the image denotes 50 μm.

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Figure 1. (a) Top view SEM image of a typical OAD Mg nanoblade array deposited at R = 80 and ω = 0 rpm; (b) the texture and orientation of hexagonal Mg nanoblade with respect to the morphology of the nanoblades in (a) and the incident Mg vapor flux F. The decomposition of Mg vapor flux F into a vertical component F^ and a lateral component F is illustrated in the top left corner of (b).

With the understanding of the unusual growth characteristic of those hcp structured metals in Zone II by OAD, it is interesting to explore how their morphologies and structures can be tailored by glancing angle deposition (GLAD). GLAD is an extension of OAD with substrate azimuthal rotation,18,23-33 where the substrate is manipulated by two programmed stepper motors.31,33 One motor controls the incident angle R, and the other motor controls the azimuthal rotation of the substrate with respect to its surface normal. By controlling the azimuthal and/or polar rotation of the substrate, one can sculpture a C-shape, S-shape, L-shape, zigzag, matchstick, vertical or helical nanostructure.18,23-33 The nanostructures not only can be deposited on a flat substrate but also can be deposited on a curved substrate34 or on a patterned substrate by self-alignment.35 Thus, compared to other nanofabrication techniques, OAD/GLAD has the following advantages: (1) it forms an aligned nanostructure array naturally; (2) the size, separation, and density of the nanostructures can be controlled by the vapor incident angle as well as by using templates or patterned substrates to form initial nucleation centers; (3) the shape, alignment, and orientation of the nanostructures can be programmed by substrate rotation; (4) there is virtually no constraint on the materials as long as the materials can be evaporated. Although there are many reports on GLAD-fabricated nanostructures, almost all of them are on the characteristics of film growth in Zone I,21 where the self-shadowing effect is dominant over the limited surface diffusion.18,23-33 However, no report has been found on how to sculpture those materials

with the characteristics of growth in Zone II into nanostructures by the GLAD technique. In this paper, the hcp Mg in Zone II is chosen, and its morphology tailored by GLAD is studied. The results show that, depending on the substrate rotation speed, the GLAD Mg structures consist of different nanostructure units, whose formation is explained by the competition between deposition and diffusion. In addition, during the oblique angle deposition of Mg, the substrate is rotated 90 to form a 90-rotation Mg OAD sample, and the sample is composed of two layers of nanoblade arrays with an included angle γ ≈ 33 instead of 90 between the two layers, which can be microscopically explained from the lattice match principle. Experimental Section A custom designed electron-beam evaporation system (Pascal Technology) was used to tailor Mg nanostructures.33 Before deposition, the chamber was evacuated to a base pressure of 10-7 Torr. The Mg (99.95%, Cerac Inc.) source was evaporated onto the substrate shown in Figure 2 with the features: flat substrate A, linear pattern B, circular pattern C, and dot-matrix pattern D. Typically, the patterns B-D have a pattern width of a ≈ 2 μm and a gap between patterns of b ≈ 5 μm (illustrated by a representative cross-view SEM image in Figure 3e). There are two reasons to use different substrate templates for the study: first, we hope the templates can help reveal the growth behavior of Mg nanostructures under different growth conditions; second, those templates can be used to explore how the substrate features affect the structure and morphology, such as the size, separation, and density, of the formed Mg nanostructures. For the fabrication of different Mg nanostructures, the Mg vapor flux was incident at an angle of R=80 with respect to the substrate normal, the growth rate was fixed at 0.5 nm/s and the overall nominal thickness Ltotal was 5 μm, both monitored by a quartz crystal microbalance (QCM) positioned directly facing the vapor source. Different Mg morphologies were achieved by manipulating the azimuthal rotation of the substrates as follows: OAD Mg Nanoblade Array. The Mg nanoblade array was formed by oblique angle deposition, that is, fixing the vapor incident angle without rotating the substrate azimuthally. 90-Rotation Mg Nanostructure Array. To make this structure, the stepper motor that controls the azimuthal rotation of the substrate was programmed in the following sequence: (1) Grow a layer of Mg nanoblade array until the QCM reads 1/2Ltotal=2.5 μm without rotating the substrate; (2) rotate the substrates 90 clockwise at a fast rotation speed of 5 rpm; (3) Grow the other layer of Mg nanoblade array until the QCM reads 1/2Ltotal = 2.5 μm without rotating the substrate. GLAD Mg Nanostructures. The morphologies of GLAD Mg nanostructures can be tailored by the azimuthal rotation speed of

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Figure 3. The OAD Mg nanoblade array: (a) top view SEM image with one blade ridge marked by an ellipse; (b) the definition of side view and face view of the nanoblades; (c) side view and (d) face view SEM images of the nanoblades grown on the flat substrate A; (e) side view and (f) face view SEM images of the nanoblades grown on Patterns B, C, or D. The Mg vapor flux direction is illustrated with respect to the nanostructure morphology in (a), (c), (e), and the vapor is perpendicular to the paper plane and pointing outward in (d) and (f). The pattern width a, the gap b between the patterns, and the tilting angles β1 and β2 of the nanoblades at the edge of the pattern are illustrated in (e). All the scale bars in the images denote 2 μm. the substrate. During Mg deposition, the substrate rotation speed ω was programmed to be a constant: ω=0.1 rpm, 1 rpm, or 10 rpm, respectively.

Results and Discussion OAD and 90-Rotation Mg Nanostructures. The morphologies of the as-deposited samples were characterized by a field-emission scanning electron microscopy (SEM, FEI Inspect F). First, the OAD and 90-rotation Mg nanostructures are demonstrated. For these two nanostructures, one can regard them as GLAD nanostructures with a substrate rotation speed of ω = 0 rpm for OAD and ω ≈ 0 rpm for 90-rotation case due to the nonrotation substrate during deposition. Figure 3 shows the SEM images of the OAD Mg sample. From the top view in Figure 3a, the oblique angle deposition of Mg results in arrays of well-aligned nanoblades14,15 with a blade thickness t=0.06 ( 0.01 μm (along the incident Mg vapor direction) and blade ridge width w=0.7 ( 0.1 μm (the blade ridge is defined by the illustration in Figure 3a, and its width is perpendicular to the Mg vapor direction). The number density of the blade ridge is estimated to be n ≈ 9.3 108/in2. Because of the anisotropic nature of the nanoblades, there are two different cross-sectional views depending on the way to cut the sample: the side view when the sample is cut parallel to the Mg vapor incident plane, and the face view

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when the sample is cut perpendicular to the Mg vapor incident plane and viewed opposite to the Mg vapor direction, as shown in Figure 3b. Figure 3c,d shows the side view and face view SEM images of the Mg nanoblades grown on the flat substrate A, and Figure 3e,f shows the side view and face view SEM images on the patterns (B, C, or D). From these cross-view SEM images, the height of OAD nanoblade array is h = 13.5 ( 0.2 μm. On the flat substrate, the nanoblades are initially tilting toward the vapor flux direction, and then gradually twist and turn to the direction perpendicular to the substrate surface with a tilting angle of β ≈ 0 (the included angle of the nanostructure and the substrate normal) according to the side view (Figure 3c). On the pattern, the nanoblades have two different growth behaviors: in the center of the pattern, the nanoblades grow in a way similar to the case on the flat substrate; while at the edge of the pattern, the nanoblades are tilted toward the empty space at the two sides at angles of β1 and β2 as illustrated in Figure 3e. It was observed experimentally that the tilting angles at the edge depend on the gap b between the patterns during OAD: the narrower the gap, the smaller the tilting angles. When b=5 μm (Figure 3e), β1 =29 ( 3 and β2=52 ( 4. Here β1 < β2 can be explained as follows: When Mg atoms reach a pattern, they still retain kinetic energy along their incident direction. Under the horizontal component of the kinetic energy, the Mg adatoms will continue to move forward or to the right in Figure 3e until the energy is dissipated. As a result, the formed nanoblades at the left edge have a relatively small tilting angle β1 compared to the β2 for the nanoblades at the right edge. This can also explain why the nanoblades on the flat substrate are initially tilting toward the vapor flux direction (Figure 3c). In addition, from the cross-sectional SEM images of the nanoblades on the patterns (Figure 3e,f), no nanostructures can be found on the substrate in the gap between the patterns, which indicates that the growth of unusual nanoblade structure is still controlled by the shadowing effect besides the fast surface adatom diffusion. Moreover, an obvious nanoblade branching phenomenon can be observed in the face view images (Figure 3d,f). For OAD deposition, it is clear that the nanoblades will grow taller when the deposition is continued. However, during the growth of the OAD Mg nanoblade array, if the deposition process is interrupted, followed by a fast 90 azimuthal rotation of the substrate, and another OAD Mg layer is deposited, does the second Mg layer still form a nanoblade array structure? Are the two Mg layers perpendicular to each other? Figure 4 shows the SEM images of the 90-rotation Mg nanostructure array. The directions of the Mg vapor incidence can be identified by the observed “L”-shape shadow upon a defect site (a particle) on the substrate surface, in the top view SEM image (Figure 4a). One can see that the Mg vapor is first incident in the direction from left to right, as pointed by the arrow “F0”, and then the vapor incidence is changed to the direction “F90”, from bottom to top, after a 90-rotation. Figure 4b shows an enlarged top view SEM image from the framed part in Figure 4a, which reveals two layers of the nanoblade array structure. The nanoblades in the bottom layer, grown directly on a flat substrate, are almost perpendicular to the incident Mg vapor direction “F0”. Those are the nanoblades grown before the 90-rotation. Outside the shadowing region is the top layer nanoblades formed after the substrate was rotated 90. If the

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Figure 5. (a) A hexagonal crystal lattice unit cell corresponding to the bottom nanoblade, and the width orientations [0110] of the formed nanoblades with respect to the Mg vapor directions “F0” and “F90”, respectively; (b) the lattice match diagram of the crystal plane (1010) of the Mg nanoblades in the two layers, where the dark dots denote the lattice of the Mg in the bottom layer, and the gray dots denote the lattice of the Mg in the top layer. The matched lattice forms a super lattice, whose unit cell is indicated by one parallelogram ABCD (AB = 1.0905 nm, BC = 1.9254 nm, — ABC = 73.5).

Figure 4. The 90-rotation Mg nanostructure: (a) top view SEM image shows an “L” shape defect on the sample surface resulting from the shadowing effect. During the deposition, the Mg vapor is first incident onto the substrate in the direction “F0”, then it is rotated 90 and incident in the direction “F90”, as indicated by the arrows; (b) the enlarged top view SEM image from the framed part in (a) reveals that the two-layered nanoblades form an angle γ ≈ 33, shown by the illustration; (c) side view and (d) face view (defined by the Mg structure in the bottom layer) SEM images of the samples grown on the flat substrate A; (e) side view and (f) face view SEM images of the samples grown on patterns B, C, or D. In (c) and (e), the Mg vapor flux F0 is incident from left to right, and the F90 is perpendicular to the paper plane and pointing inward; while in (d) and (f), the F0 is perpendicular to the paper plane and pointing outward, and the F90 is from left to right. The dashed lines in (c-f) indicate the interface of the two Mg layers. The scale bars denote 10 μm in (a) and 2 μm in (b-f).

nanoblades in the top layer were still grown on a flat substrate, their width should spread along a direction perpendicular to that of the first layer, as indicated by the dashed arrow. However, if one takes a close look at Figure 4b, the nanoblades in the top layer actually form an average angle γ ≈ 33 with respect to those in the first layer. The formation of the included angle γ ≈ 33 instead of 90 in the two-layered nanoblades of 90-rotation Mg sample can be explained microscopically by the lattice match principle. By combining Figures 1 and 4a, the width directions of the nanoblades relative to the Mg vapor incident directions observed in Figure 4b are sketched in a hexagonal crystal lattice unit cell as shown in Figure 5a. For simplicity, only one unit cell corresponding to the bottom nanoblade is drawn, and the highlighted crystal plane (1010) in Figure 5a is the top surface of the hexagonal Mg nanoblade. For Mg, the lattice constants along the a, b, and c axes are a= b=0.3209 nm and c=0.5211 nm (c is the long axis of the unit cell), respectively. As viewed from the crystal plane (1010), when Mg vapor is first incident from left to right, the nanoblade width direction [0110] is along the b axis; when Mg vapor incidence is rotated 90, the top nanoblade width

direction [0110] forms an included angle of γ ≈ 33 with respect to the b axis. Figure 5b shows the lattice match diagram of the crystal plane (1010) of the Mg nanoblades in the two layers, where the dark dots denote the lattice of the Mg in the bottom layer, and the gray dots denote the lattice of the Mg in the top layer. If the top layer follows the same direction as that on a flat substrate, that is, the b (short) direction of the gray lattice in Figure 5b is parallel to the c (long) direction of the dark lattice, there is no lattice match. By rotating such a gray lattice clockwise until its b axis is along the direction “AD”, one out of six lattice points of the gray lattice along its b direction is overlapping with one lattice point of the dark lattice, as indicated by the symbol circles. Thus, a super lattice forms at the interface, and its unit cell is a parallelogram, as shown by the parallelogram ABCD, AB=[(2c)2 þ b2]1/2=1.0905 nm in the right triangle AEB, BC=6b=1.9254 nm, — ABC=arc cos(0.5AB/BC) ≈ 73.5 in the isosceles triangle ABC) in Figure 5b. From the super lattice, the included angle of the blade width directions “AC” (the bottom layer) and “AD” (the top layer) can be calculated to be γ ≈ 33 in terms of γ=arc tan(DO/AO)=arc tan(2c/5b) in the right triangle AOD, which agrees with the experimental result (Figure 4b) quite well. Therefore, the oblique but not perpendicular crossing of the two-layered nanoblades of 90-rotation Mg sample is ascribed to their lattice match. In addition, as estimated from the top view SEM image (Figure 4b), the 90-rotation Mg nanostructure array has a blade thickness tB =0.16 ( 0.03 μm, blade ridge width wB = 0.8 ( 0.1 μm, the number density of blade ridge of nB ∼ 9.3 108 /in2 for the bottom layer on the flat substrate A (called substrate AB), and a blade thickness tT = 0.27 ( 0.08 μm, blade ridge width wT =1.2 ( 0.2 μm, the number density of blade ridge of nT ∼ 4.6 108 /in2 for the top layer on the arrays of nanoblades on the flat substrate A (called substrate AT). Thus, the Mg nanoblades in the top layer are thicker and wider than those in the bottom layer, but the number density of blade ridge of the top layer is almost half that of the bottom layer (nT ≈ 1/2nB), which indicates that one top nanoblade approximately crosses two bottom nanoblades. These differences could be caused by their different

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nucleation sites: the bottom layer grows directly on a flat substrate, while the top layer grows on arrays of Mg nanoblades, and the initial nucleation density is halved when grown on the nanoblade layer. Figure 4c,d shows the side view and face view (defined by the Mg structure in the bottom layer) SEM images of the 90rotation Mg sample on the flat substrate A, and Figure 4e,f shows the side view and face view SEM images on the pattern (B, C, or D). From these cross-view images, one can discern the interface of the two layers of nanoblade array structures, as indicated by the dashed lines, and the 90-rotation Mg nanostructure array has a total height of h=9 ( 1 μm high, which is composed of the bottom layer hB=6 ( 1 μm and the top layer hT = 3.0 ( 0.4 μm. Although the nominal thicknesses or QCM readings of both the Mg layers are the same (2.5 μm), the formed top Mg layer is shorter than the bottom Mg layer (hT < hB). Assuming that the nanoblades in both Mg layers are perpendicular to the substrate, one can estimate the volume VT = nTtTwThT ≈ 4.5 108 μm3 for the top Mg and VB = nBtBwBhB ≈ 7.1 108 μm3 for the bottom Mg. Thus, VT < VB. At the same time, considering that the mass of Mg deposited in both layers is the same due to the same nominal thicknesses (2.5 μm), the shorter Mg in the top layer than the bottom layer (hT < hB) implies that the Mg nanoblades in the top layer are tilting away from the substrate normal at a bigger angle β than those in the bottom layer. In addition, it is possible that part of the top layer deposition goes in between the bottom nanoblades. From the side view SEM image in Figure 4c, the 90-rotation Mg nanostructure array on the flat substrate is tilting along the Mg vapor direction “F0” at an angle of β ≈ 25 with respect to the substrate normal, while from the side view SEM image in Figure 4e, on the pattern with a gap of b=0.5 μm, the 90-rotation Mg sample is approximately perpendicular to the substrate, that is, the tilting angle β ≈ 0. From both the side view images, the top Mg structure looks wider than the bottom Mg structure, as viewed from their two-dimensional (2-D) appearance. However, the face view images (Figure 4d,f) show that the bottom Mg layer is approximately perpendicular to the substrate with visible nanoblade branching phenomenon, similar to the case in the OAD Mg sample (Figure 3d,f), and the top Mg layer looks tilting away from the substrate normal. This reveals that either both the Mg layers are obliquely crossing each other or the top Mg layer is not perpendicular to the substrate (note that one cannot obtain much quantitative information on the 3-D structure of the 90-rotation Mg sample from the 2-D SEM images). GLAD Mg Nanostructures. Figure 6 shows the top view SEM images of the GLAD Mg nanostructures deposited on the substrate with the features shown in Figure 2 at different substrate rotation speeds. The images from the top row to the bottom row correspond to the samples on a flat substrate A and patterns B-D, and different columns correspond to the samples deposited at ω=0.1, 1, and 10 rpm, respectively. At first glance, each GLAD Mg nanostructure is composed of many repetitive nanostructure units. These units distribute on the surface of the samples with a random orientation, which can be confirmed by the 2-D power spectra inserted in the top right corner of the SEM images on the flat substrate A for different ω values in Figure 6. Approximately, these power spectra show circular symmetry. From the diameters of the circular patterns, the separation of the nanostructure units on the surface is estimated to be 2.42 ( 0.08 μm for

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ω=0.1 rpm, 1.05 ( 0.09 μm for ω=1 rpm, and 0.80 ( 0.08 μm for ω = 10 rpm. The details and differences of these nanostructure units can be further visualized in the enlarged SEM images shown in Figure 7. The morphology of the nanostructure unit depends strongly on the substrate rotation speed: when ω=0.1 rpm, the unit consists of a bundle of nanoblades; when ω=10 rpm, the unit is an oblate nanorod with many steps on its surface; when ω=1 rpm, the surface has the mixture of above two units, but the oblate nanorod dominates the surface (on average, the number of the oblate nanorods is 20-30 times that of the bundles of nanoblades, and the few bundles of nanoblades units are marked by the arrows in the SEM images in Figure 6). In addition, on the flat substrate A, the nanostructure units almost cover uniformly the whole substrate, while on the patterns, the distributions of the nanostructure units maintain the shapes of the patterns quite well - line distribution on Pattern B, circular or arc distribution on pattern C, and dot-matrix distribution on Pattern D. This feature indicates that there is a shadowing effect during growth. From Figure 6, the number density (n), width (w), and thickness (t) of the nanostructure units of different samples are estimated and summarized in Table 1 and Figure 8a-c. The width w and thickness t are respectively along the long and short directions of the nanostructure units when viewed from their top, as defined in Figure 7a,c. By combining Table 1 with Figure 8, one can see that (1) for all the GLAD samples at different ω values, the density n (Figure 8a) decreases but both the width w (Figure 8b) and thickness t (Figure 8c) increase gradually from Substrate A to D, as indicated by the symbols of hollow circles for A, triangles for B, squares for C, and diamonds for D; (2) for all the GLAD samples on the same kind of substrate, with increasing rotation speed ω, the density n increases (Figure 8a), all the w (Figure 8b) and the t on D (Figure 8c) decreases, but the t on A-C (Figure 8c) decreases first and then increases. Also, when the ω increases from 0.1 to 1 rpm, all the three structure parameters change significantly, while further increasing the ω from 1 to 10 rpm, only slight changes occur to these parameters, indicating a structural similarity. Thus, when rotating the substrate at a lower speed, the structure parameters of the obtained Mg nanostructures are more sensitive to the speed; in other words, there is a qualitative change occurring to the nanostructures from the bundle of nanoblades unit at ω=0.1 rpm to the main oblate nanorod unit at ω=1 rpm. Figure 9 shows the typical cross-section view SEM images of the GLAD Mg nanostructures. When grown on the flat substrate A (Figure 9a for ω=0.1 rpm, Figure 9c for ω=1 rpm, and Figure 9e for ω = 10 rpm), the GLAD Mg nanostructures are approximately perpendicular to the substrates, that is, the tilting angle β ≈ 0 relative to the substrate normal, independent of the substrate rotation speed ω; while grown on the patterns, similar to the OAD nanoblade array, the GLAD Mg nanostructures on the patterned substrates are tilting toward the empty space at the edge sides at angles of β1 and β2 as illustrated in Figure 9d. These tilting angles depend not only on the substrate rotation speed ω but also on the gap b between the patterns. When b =5 μm, the tilting angles of the nanostructures are β1=18 ( 3 and β2=20 ( 3 for ω=0.1 rpm, β1=40 ( 5 and β2=45 ( 8 for ω=1 rpm, and β1 ≈ β2 = 30 ( 1 for ω = 10 rpm according to multiple SEM images obtained in the experiment. Thus, for each GLAD Mg sample, the two tilting angles are very close

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Figure 6. Top view SEM images of GLAD Mg nanostructures at different substrate rotation speeds (left column: ω = 0.1 rpm; middle column: ω = 1 rpm; right column: ω = 10 rpm) on the substrate with four different features shown in Figure 2, top row to bottom row corresponding to the samples on A-D. The insets in the top right corner of the SEM images on flat substrate A are the power spectra with a size of 1.28 μm-1 1.28 μm-1 for ω = 0.1 rpm, 3.2 μm-1 3.2 μm-1 for ω = 1 rpm and ω = 10 rpm.

Figure 7. Enlarged top view SEM images of GLAD Mg nanostructures at different substrate rotation speeds: (a) ω = 0.1 rpm; (b) ω = 1 rpm; (c) ω = 10 rpm. The nanostructure units, a bundle of nanoblades in (a), an oblate nanorod in (c), and both in (b), are marked. The width w and thickness t of the nanostructure units are also marked in (a) and (c). The scale bars in the images denote 1 μm.

to each other, indicating the GLAD Mg nanostructures are almost symmetrically distributed on the patterns, due to the isotropic azimuthal Mg flux during the substrate rotation. This is different from the OAD case, and the two angles in OAD Mg sample are significantly different (β1=29 ( 3 and β2=52 ( 4). In addition, the sample deposited at ω=1 rpm has relatively big tilting angles at the edge of the patterns, followed by that deposited at ω=10 rpm and then at ω=0.1 rpm. The tilting angles decrease with decreasing the gap of the patterns. As shown by an example in Figure 9g, the Mg nanostructures at 10 rpm grown on the pattern with a

narrower gap b=0.5 μm, the nanostructure becomes almost perpendicular to the substrate in the center and at the edge of the pattern, that is, β1 ≈ β2 ≈ 0, as compared to β1 ≈ β2=30 ( 1 of b=5 μm. Moreover, from these cross view images, the height of the GLAD Mg nanostructures is estimated to be 8.45 ( 0.07 μm for ω=0.1 rpm, 5.65 ( 0.09 μm for ω=1 rpm, and 4.75 ( 0.05 μm for ω=10 rpm, which are also listed in Table 1. The substrate rotation speed dependence of the height of all the samples is plotted in Figure 8d, where the symbols correspond to the experimental data and the solid curve is the exponential fitting result. Thus, under the same nominal

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Table 1. The Structure Parameters Obtained from the SEM Images for Various Mg Nanostructuresa nanostructure unit samples

density n (/in2)

width w (μm)

thickness t (μm)

height h (μm)

tilting angle β, β1, β2

0.1 rpm

Substrate A Pattern B Pattern C Pattern D

1.0 10 6.3 107 5.0 107 3.7 107

4(1 4(2 5 (2 6(2

0.5 ( 0.1 0.8 ( 0.2 0.9 ( 0.3 1.2 ( 0.2

8.45 ( 0.07

β ≈ 0 [ β1 = 18 ( 3 β2 = 20 ( 3 (Pattern with a gap of 5 μm)]

1 rpm


6.4 108 2.7 108 1.6 108 1.3 108

0.8 ( 0.2 0.9 ( 0.1 1.0 ( 0.1 1.4 ( 0.5

0.17 ( 0.04 0.39 ( 0.09 0.4 ( 0.2 0.6 ( 0.2

5.65 ( 0.09

β ≈ 0 [β1 = 40 ( 5 β2 = 45 ( 8 (Pattern with a gap of 5 μm)]

10 rpm


7.6 108 3.0 108 2.1 108 1.8 108

0.7 ( 0.1 0.8 ( 0.1 0.9 ( 0.3 1.0 ( 0.3

0.22 ( 0.06 0.5 ( 0.1 0.5 ( 0.2 0.5 ( 0.2

4.75 ( 0.05

β ≈ 0 [β1 = 30 ( 1 β2 = 30 ( 1 (Pattern with a gap of 5 μm)]

8

nanostructure unit (blade ridge)

Substrate A OAD Patterns Substrate AB Substrate AT 90-rotation Patterns

density n (/in2)

width w (μm)

thickness t (μm)

9.3 10

0.6 ( 0.1

0.06 ( 0.01

-

-

-

9.3 108 4.6 108

0.8 ( 0.1 1.2 ( 0.2

0.16 ( 0.03 0.27 ( 0.08

-

-

-

8

13.5 ( 0.2

9(1

β ≈ 0 [ β1 = 29 ( 3 β2 = 52 ( 4 (Pattern with a gap of 5 μm)] β ≈ 25 β ≈ β1 ≈ β2 ≈ 0 (Pattern with a gap of 0.5 μm)

a

Note: Substrate AB and substrate AT are defined by the bottom layer and top layer of the 90-rotation sample on the flat substrate A, respectively. The height of the 90-rotation sample is 9 ( 1 μm, which is composed of the bottom layer of 6 ( 1 μm and the top layer of 3.0 ( 0.4 μm.

Figure 8. The substrate rotation speed ω dependence of the average (a) number density n, (b) width w, and (c) thickness t of the nanostructure units obtained from the top view SEM images, and (d) the average height h of the samples obtained from the cross-sectional view SEM images. Both the OAD and 90-rotation samples are considered to have a substrate rotation speed of 0 rpm.

deposition thickness (5 μm), the sample height decays exponentially with the substrate rotation speed. In addition, the height of the 90-rotation sample (9 ( 1 μm), as indicated by the symbol of hollow square, is located between the OAD sample (13.5 ( 0.2 μm) and the GLAD Mg sample deposited at ω=0.1 rpm (8.45 ( 0.07 μm) but is very close to the latter. It is interesting to explore the possible microscopic formation mechanism for Mg nanostructures formed by GLAD due to different nanostructure units in different GLAD Mg

samples. The nanostructure units, a bundle of nanoblades in Figure 7a, an oblate nanorod in Figure 7c, and a mixture in Figure 7b, have been marked. When the nanostructure units are viewed from different directions, one can obtain different pictures (Figures 7 and 9). For a bundle of nanoblades, both the top and side view (also marked in the cross-sectional view SEM image by a dashed ellipse in Figure 9a) pictures reveal a multilayered stack, whose width and height decrease gradually from the central layer to outer layers, demonstrating a

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vapor flux F to have two components: a vertical component F^, which is normal to the substrate and a lateral component F , which is parallel to the substrate, as shown in the top left corner of Figure 1b.36 The vertical flux is only responsible for the height growth of the nanostructure, while the lateral flux is responsible for the lateral morphology of the structure. Because of the large oblique deposition angle R = 80 with respect to the substrate normal, the vertical component is very small and the main contribution to the growth of nanostructure comes from the lateral component F . Since the GLAD technique imposes a rotation to the substrate, we can define a lateral deposition flux per unit angle as F /ω. For the formation of Mg nanoblade by OAD, previous experiments show that the diffusion is dominated by the Mg adatom diffusion in the (0001) plane.14 The competition between deposition and diffusion in lateral direction can be expressed by a parameter R = (F /ω)/D, where D is the diffusion coefficient of Mg adatoms and is mainly dependent on the substrate temperature T. In our experiment, since F, R, and substrate temperature T are all fixed, R is only determined by ω, R ω-1. Thus, the smaller the ω, the faster the lateral deposition flux F /ω, and the larger the R. Therefore, the deposition may play a more dominant role for GLAD at a slow substrate rotation speed such as ω=0.1 rpm. In this case, initially, many hexagonal crystals will form on the substrate and their (0001) crystal planes are facing toward the incident vapor, as shown in Figure 1. Because of the substrate rotation, these (0001) planes of the initial hexagonal crystals could orient in different directions, resulting in a random orientation distribution of the nanostructures. With further deposition, Mg atoms from all azimuthal directions come to the previously produced crystals and form many new nucleation sites on the (0001), (1010), and (0110) planes. Since the deposition dominates the growth, the relative diffusion length of the Mg adatoms is small. Because of the same incident possibility of Mg atoms to both sides of the (0001) plane of the hexagonal crystal, new layers will gradually form at the two sides. As a result, a multilayer of bundled nanoblades can be formed. In the mean time, the nucleation sites at (1010) and (0110) will act as seeds to form vertical and parallel small blade crystals stacking on the old blade, and fan out along the direction perpendicular to the vapor incident direction. In addition, the geometric shadowing effect37 makes the initially produced central layer(s) accept more Mg atoms than the outer layers, and thus the central layer blade(s) will grow faster than others in both the width and height directions, resulting in a needle shape as viewed from the top and side, as shown in Figures 6, 7, 9 and 10a. With the increase of the substrate rotation speed ω, the lateral deposition flux F /ω slows down, and the Mg adatom diffusion becomes more and more important. When ω=10 rpm, the Mg atoms impinged on the initially produced hexagonal crystal (0001) plane will have enough time to diffuse in the (0001) plane to form a wider plate. However, this diffusion seems anisotropic in the (0001) plane: along the fan-out direction, that is, the direction perpendicular to the Mg vapor direction, the Mg adatom seems to diffuse faster than along the vertical direction, which makes the nanoblade spread along the fan-out direction while forming obvious steps in the vertical direction.38 This is a characteristic of the fan-out effect for OAD. Therefore, only individual blades or nanorods are formed throughout the substrate. The central part of each nanorod grows faster than the outer part in both

Figure 10. The sketches of nanostructure units, (a) a bundle of nanoblades and (b) an oblate nanorod, as viewed from the top, face, and side.

needle shape (Figure 10a). By enlarging the area in the face view of one bundle of nanoblades, as shown in the inset of Figure 9a, one can clearly see that each layer of the multilayered nanostructure unit is composed of many hexagonal crystal plates (Figure 10a). In the GLAD samples deposited at ω=1 rpm (Figure 7b) and ω=10 rpm (Figure 7c), most of the oblate nanorods look like a half convex lens when viewed from their top, and there are many steps formed on their surfaces when viewed from the cross section (face and side view pictures), as sketched in Figure 10b. In addition, from the cross-sectional SEM images in Figure 9c-g, each oblate nanorod has a needle-shaped tip (Figure 10b). For a physical vapor deposition, it is well-known that there are two basic processes, deposition and diffusion, that govern the growth, and the two processes compete with each other. During the GLAD Mg deposition, the Mg vapor is obliquely incident onto the substrate. We can view the Mg

)

)

)

)

)

)

Figure 9. Cross-sectional view SEM images of GLAD Mg nanostructures at different substrate rotation speeds, (a, b) ω = 0.1 rpm, (c, d) ω = 1 rpm, and (e, f, g) ω = 10 rpm, on the substrate with four typical features, (a, c, e) a flat substrate A and (b, d, f) Pattern B, C, or D; (g) shows the Mg nanostructures at 10 rpm grown on the pattern with a narrower separation of b = 0.5 μm. The scale bars in the images denote 2 μm.

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the width and height directions due to shadowing effect, so the formed nanorod is oblate with a sharp tip, as shown in Figures 6, 7, 9 and 10b. While the speed ω=1 rpm is possible in the transition region between the sample composed of bundles of nanoblades deposited at ω = 0.1 rpm and the sample composed of oblate nanorods deposited at ω = 10 rpm, the surface forms the mixture of two nanostructure units, bundle of nanoblades and oblate nanorod, in the sample deposited at ω=1 rpm, as shown in Figures 6, 7 and 9. Clearly, this intriguing phenomenon deserves a more detailed study. Conclusion Different Mg nanostructures have been fabricated on the substrates with four typical features, such as flat substrate, linear pattern, circular pattern, and dot-matrix pattern, by GLAD at the substrate rotation speeds of ω=0.1 rpm, 1 rpm, 10 rpm along with OAD (ω=0 rpm) and 90-rotation OAD (ω ≈ 0 rpm). Magnesium is one of those materials in Zone II of the structure zone model,21 where the fast surface adatom diffusion is competing with the self-shadowing effect. Depending on the substrate rotation speed, the Mg nanostructures are composed of different nanostructure units: a nanoblade (blade ridge) at ω=0 rpm, a bundle of nanoblades at ω = 0.1 rpm, an oblate nanorod mixed with a bundle of nanoblades at ω=1 rpm, and an oblate nanorod at ω=10 rpm. The formation of different nanostructure units in the samples deposited at different ω values can be qualitatively explained by the competition between the lateral deposition rate and diffusion. In addition, the 90-rotation Mg sample is composed of two layers of nanoblade arrays with an included angle γ ≈ 33 between the two layers, which results from super lattice match. Macroscopically, on the flat substrate, the nanostructure units almost cover uniformly the whole substrate; while on the patterns, the distributions of the nanostructure units keep the shapes of the patterns quite well. The structure parameters, including the number density, width, and thickness of the nanostructure units in different Mg samples obtained from the SEM top view images, and the height of samples and the tilting angles of the nanostructures obtained from the SEM cross view images, are estimated. These parameters are found to be strongly related to the substrate rotation speed and substrate feature. While for those materials in Zone I, numerous studies show that the shadowing effect causes the formation of tilted nanorods for OAD, “L” shape nanorods for 90-rotation OAD, helical (slow substrate rotation) and vertical (faster substrate rotation) nanorods for GLAD. Compared to Zone I growth, the phenomenon in Zone II deserves a more detailed study in the future. Acknowledgment. This work was partially supported by the DOE Hydrogen Initiative Award DE-FG02-05ER46251 and NSF CBET-0853130. The authors also would like to thank Prof. Zhengwei Pan for letting us use his SEM equipment and John Gibbs for proofreading the manuscript.

He and Zhao

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Mg Nanostructures Tailored by Glancing Angle Deposition - American

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