Epitaxial Growth and Ordering of GeTe Nanowires on Microcrystals

May 22, 2009 - Chan Su Jung, Han Sung Kim, Hyung Soon Im, Young Seok Seo, Kidong Park, Seung Hyuk Back, Yong Jae Cho, Chang Hyun Kim, Jeunghee ...
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NANO LETTERS

Epitaxial Growth and Ordering of GeTe Nanowires on Microcrystals Determined by Surface Energy Minimization

2009 Vol. 9, No. 6 2395-2401

Hee-Suk Chung,†,‡ Yeonwoong Jung,† Seul Cham Kim,‡ Do Hyun Kim,‡ Kyu Hwan Oh,‡ and Ritesh Agarwal*,† Department of Materials Science and Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, and Department of Materials Science and Engineering, Seoul National UniVersity, Seoul 151-742, Korea Received March 27, 2009; Revised Manuscript Received May 11, 2009

ABSTRACT We report self-assembly of highly aligned GeTe nanowires epitaxially grown on octahedral GeTe microcrystals in two well-defined directions by using one-step vapor transport process. The epitaxial relationship of nanowires with underlying microcrystals along with the growth orientations of nanowires were investigated in detail by electron microscopy combined with atomic unit cell models. We demonstrate that maximizing atomic planar density to minimize energy of the exposed surfaces is the determining factor that governs the unique growth characteristics of micro/nanostructures that evolve from three-dimensional octahedral microcrystals to tetrahedral bases to finally one-dimensional nanowires. The crystallographic understanding of structuring of crystalline nanomaterials obtained from this study will be critical to understand, predict, and control the growth orientation of nanostructures in three-dimensions.

Modern day electronic devices are based on the hierarchical patterning and integration of device elements. The conventional fabrication techniques use multiple levels of lithography and chemical etching processes, which are efficient in precise location and patterning of planar structures on a large scale. However, this process is cumbersome, expensive, inherently two-dimensional, and limited by lithography constraints. In order to overcome these technical limitations, the bottom-up paradigm is being actively pursued, which utilizes self-and directed assembly techniques to fabricate nanoscale devices with sublithographic feature sizes, and often leads to superior and unexpected device properties.1 Nanowires offer a versatile approach for the bottom-up assembly of electronic and photonic devices and have attracted considerable recent interest due to their unique properties and geometry, which allows them to be configured as functional devices and interconnects within the same structure. However, to utilize the full potential of nanowirebased devices, precise control over their composition, structure, morphology, and three-dimensional (3D) alignment must be achieved. New synthetic strategies are being actively pursued to configure nanowires into more ordered forms with control over the diameter,2-4 density,4,5 alignment,6,7 and hierarchical branching of nanowires.8-11 However, it is still * To whom correspondence should be addressed. Tel.: 215-573-3037. Fax: 215-573-2128. E-mail: [email protected]. † University of Pennsylvania. ‡ Seoul National University. 10.1021/nl9009765 CCC: $40.75 Published on Web 05/22/2009

 2009 American Chemical Society

challenging to configure three-dimensional, position-controlled assembly of aligned nanowires. Among a variety of approaches to realize arrays of aligned nanowires,12-17 one of the most prevailing method is an epitaxial growth scheme, which utilizes nanowires grown on structurally compatible substrates.18-21 The epitaxial nanowire growth has been shown to be possible even with structurally mismatched substrates22-24 with the growth direction depending on complex factors such as nanowire diameters and surface energy minimization.25 Therefore, the rational design and control/alignment of nanowire orientation rely on the crystallographic understanding of nanowires and the structural correlation at the interface of nanowires with the substrates. Crystallographic investigation on the nanowire/substrate interface can also elucidate the nucleation/growth mechanism of nanowires as well as guide the selection of appropriate growth substrates. Nevertheless, in many of unconventional semiconducting materials such as chalcogenide (IV-VI and V-VI) systems, epitaxial nanowire growth is difficult to achieve and the factors determining the epitaxial relation have not been investigated. Recently, chalcogenide nanowires such as GeTe and Ge-Sb-Te nanowires have found great potential for novel memory applications due to their nature of reversible phase-change,26-29 which also establishes the demand for the detailed structural understanding of the growth of these systems.

In this letter, we report the epitaxial growth and hierarchical ordering of GeTe nanowires on microcrystals by selfassembly process driven primarily by surface energy minimization via a one-step chemical vapor transport process. Through extensive electron microscopy studies combined with focused ion beam (FIB) lift-out techniques, we report the growth mechanism of aligned nanowires and study their epitaxial relationship with respect to microcrystals. The observed hierarchical ordering of GeTe nanowires on tetrahedral bases that self-assemble on octahedral microcrystals is attributed to surface energy minimization that exposes close-packed planes on the surface, thus providing the driving force for formation of complex three-dimensional structures. This investigation provides insights toward the growth mechanisms of hierarchically ordered nanowires previously reported by several groups.30-33 Epitaxially aligned GeTe nanowire/microcrystals were synthesized via the Au/Pd mediated vapor-liquid-solid (VLS) method in a horizontal vacuum tube furnace. Au/Pd thin film with thickness of ∼100 nm was deposited on a cleaned Si substrate using a DC magnetron sputtering system. The Au/Pd-coated Si substrate was placed at the furnace downstream, and GeTe powder (99.99%, Sigma-Aldrich) was placed at the middle of the quartz tube. Following this step, the quartz tube was sealed and evacuated to a base pressure of 10 mtorr and high-purity Ar gas was fed at 200 SCCM and the final pressure was maintained at 300 Torr. The furnace then was heated up to 690 °C at a rate of 10 °C/min and maintained for 30 min. After the reaction, the furnace was cooled down to room temperature. Figure 1a is a low-magnification scanning electron microscopy (SEM, FEI NOVA) image of GeTe nanowires grown on the growth substrate on a large area (substrate growth temperature ∼370 °C). The diameters of nanowires are in a range of 30-200 nm, and their lengths are typically over 30 µm. Upon closer inspection in SEM images at higher resolution, a few interesting features are revealed (Figures 1b,c). Nanowires grow highly aligned in certain directions only and adjacent nanowires are parallel to each other, suggesting well-defined nanowire growth orientations. In addition, underneath the nanowires 10-30 µm sized microcrystals are almost always found suggesting that these microcrystals serve as growth substrates to guide the nanowires in very specific orientations. In order to investigate the growth characteristics of the hierarchically ordered nanowires with respect to their underlying microcrystals, we first focus our attention on microcrystals with nanowires with low density. Figure 1d clearly shows that each microcrystal possesses eight equivalent facets with triangular shaped surfaces and nanowires grow only in two different directions on each surface of the microcrystals. High-resolution SEM reveals that catalysts are typically present at the ends of the aligned nanowires (Figure 1e), which suggests that the nanowire growth from the microcrystals is due to the VLS mechanism. The structural analysis of the nanowires and the underlying microcrystals was further performed with X-ray diffraction (XRD, Bruker D8 Advance). The XRD diffraction peaks (Figure 1f) can be readily indexed to GeTe rhombo2396

Figure 1. Structural characterization of GeTe nanowires epitaxially grown on GeTe microcrystals. SEM images of (a) GeTe nanowires/ microcrystals on a large area, (b) GeTe nanowires grown in specific directions out of GeTe microcrystals, (c) highly aligned GeTe nanowires, (d) GeTe microcrystals with GeTe nanowires at a low density, and (e) GeTe nanowire with faceted surfaces and catalyst at the end. (f) XRD pattern of as-synthesized GeTe nanowire/ microcrystals.

hedral structure with lattice constants of a ) 8.342 Å and c ) 10.67 Å (JCPDS No. 47-1079), while no other phases are detected. This GeTe structure is also referred as distorted rocksalt structure that is elongated along the 〈111〉 direction with a lattice constant of a ) 5.996 Å and R ) 88.18°.34-36 However, it is more appropriate to depict the structure in the frame of a rhombohedral unit cell, because R * 90° Nano Lett., Vol. 9, No. 6, 2009

Figure 2. (a) SEM image of octahedron-shaped GeTe microcrystals synthesized without Au/Pd catalyst deposition. (b) SEM image of a cross-sectioned GeTe microcrystal prepared by FIB-lift out technique. The arrow denotes the surface of the microcrystal exposed after FIB lift-out. Inset shows low-magnification TEM of the FIB-sliced GeTe thin film with the arrow aligned along the surface normal of the film. (c) Cross-sectional HRTEM image showing rhombohedral [003] growth direction of the GeTe film (as shown in inset of panel b). Inset is FFT corresponding to HRTEM image along [100] zone axis. (d) Illustration of GeTe atomic structure (red, Ge; blue, Te) in a rocksalt unit cell (thin black line depicting the cubic frame) overlapped with a rhombohedral unit cell (gray region outlined by thick black line). The triangle (green line) indicates rocksalt (111) which corresponds to rhombohedral (001).

deviates from the conventional rocksalt structure that is based on the cubic lattice system. To investigate the growth characteristics of GeTe nanowires and their epitaxial relationship with the microcrystals, we further characterized the crystalline structure of GeTe microcrystals that serve as growth substrates. We conducted a control experiment for which we performed the growth of materials under the same synthesis conditions but without depositing Au/Pd film onto Si substrate. The growth products obtained were octahedral-shaped GeTe microcrystals with eight equivalent triangular faceted surfaces but without any nanowires on their surfaces (Figure 2a). The absence of nanowires on microcrystal surfaces also implies an explicit role of Au/Pd catalysts in growing nanowires via the VLS process (detailed mechanism will be suggested later). The octahedral shape of GeTe microcrystals is consistent with recent studies to synthesize GeTe materials via solution-based approach.37 In order to define the crystallographic orientation of the GeTe microcrystal surfaces, we conducted crosssectional high resolution transmission electron microscopy (HRTEM, JEOL-3000F) analysis on a thin slice of GeTe cut from GeTe microcrystals.38 We prepared the GeTe thin film by FIB milling of GeTe microcrystals (Figure 2b), where a thin film (thickness ∼60 nm) was carefully cross-sectioned Nano Lett., Vol. 9, No. 6, 2009

parallel to the surface normal of the GeTe facet (Figure 2b, inset) and transferred to a TEM grid by a lift-out technique. The HRTEM image (Figure 2c) of a FIB-prepared GeTe thin film with its corresponding fast Fourier transformation (FFT) shows lattice fringes parallel to the surface normal of the cross-sectioned GeTe with spacing of ∼0.36 nm which corresponds to (003) rhombohedral crystalline plane of GeTe. The indexed FFT indeed reveals diffraction spots from (003) lattice fringes that are aligned along the GeTe surface normal, indicating that GeTe microcrystals grow by exposing {001} rhombohedral crystalline planes on the surface. Figure 2d illustrates GeTe structure in the framework of a rhombohedral unit cell (gray region outlined by thick black line) overlapped with the unit cell of the distorted rocksalt structure (thin black line). It is noted that {001} crystalline plane in the rhombohedral unit cell corresponds to the {111} plane (green line) of the rocksalt structure, which is a plane of close packing with two-dimensional hexagonal arrangement of atoms. This suggests that the GeTe microcrystals grow in an octahedral shape exposing the surfaces with close packing of atoms, most likely to minimize the surface energy; planes of higher planar atomic density generally possess lower surface energy due to the lower density of broken atomic bonds.39 The next step is to trace the growth evolution of GeTe nanowires from GeTe microcrystals in the presence of Au/ Pd thin film and to identify the growth direction of nanowires. GeTe nanowires at their initial stage of growth (short nanowires) are typically found to grow from tetrahedral structures that form triangular-shaped bases on the surfaces of microcrystals (Figure 3a). Au/Pd catalysts were found at the apex of the tetrahedral bases as characterized with energy dispersive X-ray spectroscopy (EDS) (data not shown), which guides nanowire growth via the VLS reaction through continued supply of vaporized GeTe. Careful SEM inspection (Figure 3b) reveals three triangular facets of the tetrahedral structure with its base lying on {001} planes of microcrystals. These triangular facets correspond to (01j1), (101), and (11j1) planes, all of which are close packing planes in GeTe rhombohedral structure. Figure 3c illustrates a simulated GeTe rhombohedral structure where (01j1), (101), and (11j1) planes (red line) make up tetrahedral structures encompassing (001) base.40 The calculated angles between each (01j1), (101), (11j1) equivalent planes and (001) plane is 36.44°, which is very close to the value obtained from actual measurements based on SEM images of the bottom of GeTe nanowires (Figure 3d,e). Therefore, it is concluded that the highly oriented GeTe nanowires grown on microcrystals are guided by the tetrahedral bases that self-assemble on octahedral microcrystals. We also observe that the nanowires incorporate only two out of the three exposed facets of the tetrahedral bases (Figure 3e-g) while maintaining their tetragonal cross sections. The simulated models provide direct visualization of the atomic structure of the nanowires seen in the Figure 3f,g, illustrating (01j1) and (101) facets of the nanowires. One interesting feature is that the nanowires on each surface of the microcrystal typically grow in two different directions only (Figure 1c,d) despite the 3-fold 2397

Figure 4. (a) Representative SEM image of a GeTe nanowire grown from a tetrahedral base on a GeTe microcrystal. (b) TEM image of the same nanowire in panel a, cross-sectioned and prepared by FIB lift-out technique. (c) SAED pattern corresponding to panel b in [100] zone axis. (d) Simulated model illustrates the atomic structure of the nanowire (yellow line) studied in panels a-c in a rhombohedral unit cell (blue ) Ge, brown ) Te).

Figure 3. (a) SEM image of tetrahedral GeTe structures on the surface of an octahedral GeTe microcrystal. (b) Enlarged SEM image of a tetrahedron GeTe showing Au/Pd catalyst at the apex and three equivalent triangular facets on the surface (c) Simulated structural models to illustrate (01j1), (101), (11j1) planes (red triangles) with (001) base in a rhombohedral GeTe unit cell (blue ) Ge, brown ) Te). (d) The angle between (001) and (101) planes is calculated to be 36.44° (e) SEM side view of a nanowire orientation corresponding to the structural model in (d). (f,g) SEM images of a GeTe nanowire grown from a tetrahedral base exposing two close packing planes. Back (f) and front (g) view of the nanowire and its corresponding simulated models to illustrate the atomic structure. 2398

symmetry that is expected from the tetrahedral base with hexagonal atomic arrangement on {001} plane. This is attributed to the unique atomic arrangement on {001} basal plane in this unique rhombohedral GeTe structure which does not follow ideal hexagonal close packing due to the inherent distortion. The triangular facets of the tetrahedral bases (Figure 3a,b) are not perfect equilateral triangles, but slightly elongated by ∼2% along [111] direction of rocksalt structure as shown in Figure 2d; therefore, the linear atomic density along each base of the triangle will not be equivalent (Supporting Information, Figure S1).34 In order to directly characterize the epitaxial crystallographic relationship of nanowire/microcrystal and to validate the above analysis, we further conducted FIB liftout assisted-TEM analysis on the cross-sectioned GeTe nanowires grown on microcrystals. Figure 4a is a representative SEM image of a GeTe nanowire grown on a facet of an octahedral GeTe microcrystal with a tetrahedral base structure at the bottom. Cross-sectioned TEM sample of the same nanowires was prepared by FIB-lift out technique, and its low magnification TEM image is shown in Figure 4b. Figure 4c is a selective area electron diffraction (SAED) pattern of the same nanowire/microcrystal structure along [100] zone axis. The SAED provides several key features in interpreting the epitaxial relationship of the GeTe nanowire/GeTe microcrystal; the indexed (003) diffraction spot is parallel to the surface normal of GeTe microcrystal, which indeed confirms that octahedral GeTe microcrystals prefer to grow by exposing rhombohedral {001} plane, consistent with the Nano Lett., Vol. 9, No. 6, 2009

Figure 5. Schematic illustrating the evolution from GeTe microcrystals to nanowires under the continued supply of GeTe in vapor phase (orange, Au/Pd; blue, GeTe).

structural analysis discussed earlier (Figure 2a-d). In addition, (02j2) diffraction spots are observed parallel to the normal of a GeTe nanowire facet, indicating that the nanowire is indeed surrounded by (01j1) plane, which is also consistent with the SEM and the analysis of the atom models (Figure 3a-e). The nanowire growth direction is identified to be [024], consistent with other studies of individual GeTe nanowires.41 It is interesting to note that nanowires do not grow vertically on the underlying microcrystals but always grow at 36.44° from the normal of the microcrystal surfaces (Figure 3e). This phenomenon can be explained by considering the atomic planar density of nanowire side surfaces at the initial stage of growth. If the nanowires would grow vertically (along [001] direction) retaining their triangularshaped cross sections, they would expose (100), (01j0), and (1j10) crystalline planes on the side facets of nanowires. However, these planes are of low atomic planar density with noncovalent bonding (Supporting Information, Figure S2) and therefore would inevitably possess higher surface energies in comparison to close packing planes such as (01j1) and (101) clearly observed in our study. This structural analysis strongly suggests that the epitaxial growth of GeTe nanowires on octahedral microcrystals is governed by the atomic planar density (surface energy) of planes exposed through nucleation and growth. Finally, we provide an explanation about the possible growth mechanism of the GeTe nanowires evolving from microcrystals. Thermally evaporated GeTe reacting with Au/ Pd is expected to undergo VLS growth while the supply of excessive amount of GeTe also initiates the vapor-solid (VS)-mediated GeTe crystal growth on a mixture of GeTe/ AuPd. While VS growth of GeTe microcrystals based on the Stranski-Krastanov growth mode becomes dominant, phase separation between Au/Pd and GeTe is also likely to proceed as the outward migration of metals is generally predicted during the solidification of liquid droplets in a eutectic process, such as for Au/Si and Au/Ge.42,43 While Nano Lett., Vol. 9, No. 6, 2009

exposing Au/Pd toward the surface of octahedral GeTe microcrystals, continued reaction of vaporized GeTe with Au/Pd first simultaneously forms tetrahedron base structure in order to reduce the surface energy, and subsequent GeTe nanowire growth results from them. A schematic to explain the growth mechanism of ordered GeTe nanostructures is shown in Figure 5. In order to verify this analysis, we examine the morphology of structures grown on the different regions of a growth substrate where temperature ranges from 610 to 370 °C (Figure 6a-d). At a high-temperature region (∼610 °C, Figure 6a), nucleated Au/Pd/GeTe mixtures are uniformly found over a large area. High-resolution SEM (Figure 6a, inset) shows that in each particle, face-centered cubic faceted (fcc) Au/Pd nanocrystals (100-400 nm) are observed segregated out of surrounding mixture of Ge/Te as confirmed by EDS (Supporting Information, Figure S3) that did not grow into large GeTe microcrystals yet due to a small driving force for nucleation. At a lower temperature region (∼520 °C, Figure 6b), some large GeTe structures are found along with one-dimensional nanowires (upper right), indicative of simultaneous VS and VLS growth, respectively, as suggested above. At a further lowered temperature (∼430 °C, Figure 6c), octahedron GeTe structures start to appear with a mixture of Au/Pd/GeTe clearly seen on their surfaces. Finally, at an optimized temperature (∼370 °C, Figure 6d and Figure 1a), epitaxially grown GeTe nanowire/microcrystal structures dominate over a large area of the substrate. We also observe that these nanowire/microcrystal structures are observed only when the synthesis is performed under relatively high vapor pressures (300 Torr) and high Ar flow rate (200 SCCM or above), both of which are required to introduce fast and excessive supply of precursor materials in vapor phase. Synthesis performed with smaller amount of GeTe powders (0.6-0.8 mg) under low vacuum pressure (below 100 Torr) and Ar flow rate (15 SCCM) realizes preferred conditions for VLS nanowire growth without any emergence of VS2399

the planar atomic density is the critical factor that determines the growth characteristics and complex structuring of crystalline nanomaterials in three-dimensions. This study provides a general framework for understanding the growth mechanism of hierarchically ordered nanostructures using GeTe as a model system and is also useful in designing 3D growth schemes for alignment of nanowires for electronic/photonic devices. Acknowledgment. This work was supported by NSF (DMR-0706381), Penn-MRSEC seed award (DMR0520020), and in part by ONR (Grant N000140910116) and the Korea Science and Engineering Foundation (KOSEF) granted by the Korea government (MOST, NO. R11-2005065). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References Figure 6. Representative SEM images of structures found at different temperature regions on a growth substrate: (a) at 610 °C; nucleated Au/Pd/Ge/Te nanostructures uniformly distributed on the growth substrate. Inset shows faceted fcc Au/Pd nanocrystals segregated out of GeTe (scale bar, 200 nm); (b) at 520 °C; VS grown GeTe microcrystals with some VLS grown-nanowires; (c) at 430 °C; an octahedral GeTe microcrystal with a mixture of Au/ Pd/GeTe on the surface; and (d) at 370 °C; epitaxially grown GeTe nanowire on octahedral microcrystals on the large area of the growth substrate.

grown GeTe microcrystals, even enabling the diametercontrolled synthesis of GeTe nanowires when the temperature of the growth substrate is lowered down to ∼275 °C.44 It is worth mentioning that the dominant growth directions of GeTe nanowires grown via VLS mechanism on growth substrates (Si or SiO2) but not from GeTe microcrystals are [220], [202], or [003],29,36,44 all of which are the surface normals of close-packing planes as studied above (Figure 3 and 4). This observation strongly indicates that growth of GeTe nanowires that are not epitixially guided by microcrystals still prefer to form close-packing surfaces at the nanowire/substrate interface during their very initial nucleation/ growth stage in order to minimize the surface energy of the system. It is also noteworthy that epitaxy-driven dendritic nanowires as well as nanowire/microcrystal structures are often observed in thermal evaporation of chalcogenide powders, particularly Pb-based chalcogenide nanowires,45,46 where the morphology of nanowires varies depending on the gas flow rate, vacuum pressure, and temperature, similarly as discussed above. In conclusion, we demonstrated synthesis of GeTe nanowires epitaxially grown on GeTe microcrystals via one-step chemical vapor transport process. Through extensive TEMFIB analysis and atom models, we identified that GeTe microcrystals grew exposing {001} rhombohedral plane that is a plane of close packing density. Moreover, GeTe nanowires epitaxially grow from the microcrystals also to expose close-packing planes, which is only possible by growing tetrahedral bases again with exposing close-packing planes. This observation clearly indicates that maximizing 2400

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