Epitaxial Growth and Ordering of GeTeNanowires on Microcrystals Determinedby Surface Energy MinimizationHee-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 andEngineering, 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 directionsby using one-step vapor transport process. The epitaxial relationship of nanowires with underlying microcrystals along with the growth orientationsof nanowires were investigated in detail by electron microscopy combined with atomic unit cell models. We demonstrate that maximizingatomic planar density to minimize energy of the exposed surfaces is the determining factor that governs the unique growth characteristics ofmicro/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 hierarchicalpatterning and integration of device elements. The conven-tional fabrication techniques use multiple levels of lithog-raphy and chemical etching processes, which are efficientin precise location and patterning of planar structures on alarge scale. However, this process is cumbersome, expensive,inherently two-dimensional, and limited by lithographyconstraints. In order to overcome these technical limitations,the bottom-up paradigm is being actively pursued, whichutilizes self-and directed assembly techniques to fabricatenanoscale devices with sublithographic feature sizes, andoften leads to superior and unexpected device properties.1

Nanowires offer a versatile approach for the bottom-upassembly of electronic and photonic devices and haveattracted considerable recent interest due to their uniqueproperties and geometry, which allows them to be configuredas functional devices and interconnects within the samestructure. However, to utilize the full potential of nanowire-based devices, precise control over their composition,structure, morphology, and three-dimensional (3D) alignmentmust be achieved. New synthetic strategies are being activelypursued to configure nanowires into more ordered forms withcontrol over the diameter,2-4 density,4,5 alignment,6,7 andhierarchical branching of nanowires.8-11 However, it is still

challenging to configure three-dimensional, position-con-trolled assembly of aligned nanowires. Among a variety ofapproaches to realize arrays of aligned nanowires,12-17 oneof the most prevailing method is an epitaxial growth scheme,which utilizes nanowires grown on structurally compatiblesubstrates.18-21 The epitaxial nanowire growth has beenshown to be possible even with structurally mismatchedsubstrates22-24 with the growth direction depending oncomplex factors such as nanowire diameters and surfaceenergy minimization.25 Therefore, the rational design andcontrol/alignment of nanowire orientation rely on the crystal-lographic understanding of nanowires and the structuralcorrelation at the interface of nanowires with the substrates.Crystallographic investigation on the nanowire/substrateinterface can also elucidate the nucleation/growth mechanismof nanowires as well as guide the selection of appropriategrowth substrates. Nevertheless, in many of unconventionalsemiconducting materials such as chalcogenide (IV-VI andV-VI) systems, epitaxial nanowire growth is difficult toachieve and the factors determining the epitaxial relation havenot been investigated. Recently, chalcogenide nanowires suchas GeTe and Ge-Sb-Te nanowires have found greatpotential for novel memory applications due to their natureof reversible phase-change,26-29 which also establishes thedemand for the detailed structural understanding of thegrowth of these systems.

* To whom correspondence should be addressed. Tel.: 215-573-3037.Fax: 215-573-2128. E-mail: riteshag@seas.upenn.edu.

† University of Pennsylvania.‡ Seoul National University.

NANOLETTERS

2009Vol. 9, No. 62395-2401

10.1021/nl9009765 CCC: $40.75 2009 American Chemical SocietyPublished on Web 05/22/2009

In this letter, we report the epitaxial growth and hierarchi-cal ordering of GeTe nanowires on microcrystals by self-assembly process driven primarily by surface energy mini-mization via a one-step chemical vapor transport process.Through extensive electron microscopy studies combinedwith focused ion beam (FIB) lift-out techniques, we reportthe growth mechanism of aligned nanowires and study theirepitaxial relationship with respect to microcrystals. Theobserved hierarchical ordering of GeTe nanowires ontetrahedral bases that self-assemble on octahedral micro-crystals is attributed to surface energy minimization thatexposes close-packed planes on the surface, thus providingthe driving force for formation of complex three-dimensionalstructures. This investigation provides insights toward thegrowth mechanisms of hierarchically ordered nanowirespreviously reported by several groups.30-33

Epitaxially aligned GeTe nanowire/microcrystals weresynthesized via the Au/Pd mediated vapor-liquid-solid(VLS) method in a horizontal vacuum tube furnace. Au/Pdthin film with thickness of ∼100 nm was deposited on acleaned Si substrate using a DC magnetron sputtering system.The Au/Pd-coated Si substrate was placed at the furnacedownstream, and GeTe powder (99.99%, Sigma-Aldrich) wasplaced at the middle of the quartz tube. Following this step,the quartz tube was sealed and evacuated to a base pressureof 10 mtorr and high-purity Ar gas was fed at 200 SCCMand the final pressure was maintained at 300 Torr. Thefurnace then was heated up to 690 °C at a rate of 10 °C/minand maintained for 30 min. After the reaction, the furnacewas cooled down to room temperature.

Figure 1a is a low-magnification scanning electron mi-croscopy (SEM, FEI NOVA) image of GeTe nanowiresgrown on the growth substrate on a large area (substrategrowth temperature ∼370 °C). The diameters of nanowiresare in a range of 30-200 nm, and their lengths are typicallyover 30 µm. Upon closer inspection in SEM images at higherresolution, a few interesting features are revealed (Figures1b,c). Nanowires grow highly aligned in certain directionsonly and adjacent nanowires are parallel to each other,suggesting well-defined nanowire growth orientations. Inaddition, underneath the nanowires 10-30 µm sized micro-crystals are almost always found suggesting that thesemicrocrystals serve as growth substrates to guide the nanow-ires in very specific orientations. In order to investigate thegrowth characteristics of the hierarchically ordered nanowireswith respect to their underlying microcrystals, we first focusour attention on microcrystals with nanowires with lowdensity. Figure 1d clearly shows that each microcrystalpossesses eight equivalent facets with triangular shapedsurfaces and nanowires grow only in two different directionson each surface of the microcrystals. High-resolution SEMreveals that catalysts are typically present at the ends of thealigned nanowires (Figure 1e), which suggests that thenanowire growth from the microcrystals is due to the VLSmechanism. The structural analysis of the nanowires and theunderlying microcrystals was further performed with X-raydiffraction (XRD, Bruker D8 Advance). The XRD diffractionpeaks (Figure 1f) can be readily indexed to GeTe rhombo-

hedral structure with lattice constants of a ) 8.342 Å and c) 10.67 Å (JCPDS No. 47-1079), while no other phasesare detected. This GeTe structure is also referred as distortedrocksalt structure that is elongated along the ⟨111⟩ directionwith a lattice constant of a ) 5.996 Å and R ) 88.18°.34-36

However, it is more appropriate to depict the structure inthe frame of a rhombohedral unit cell, because R * 90°

Figure 1. Structural characterization of GeTe nanowires epitaxiallygrown on GeTe microcrystals. SEM images of (a) GeTe nanowires/microcrystals on a large area, (b) GeTe nanowires grown in specificdirections out of GeTe microcrystals, (c) highly aligned GeTenanowires, (d) GeTe microcrystals with GeTe nanowires at a lowdensity, and (e) GeTe nanowire with faceted surfaces and catalystat the end. (f) XRD pattern of as-synthesized GeTe nanowire/microcrystals.

2396 Nano Lett., Vol. 9, No. 6, 2009

deviates from the conventional rocksalt structure that is basedon the cubic lattice system.

To investigate the growth characteristics of GeTe nanow-ires and their epitaxial relationship with the microcrystals,we further characterized the crystalline structure of GeTemicrocrystals that serve as growth substrates. We conducteda control experiment for which we performed the growth ofmaterials under the same synthesis conditions but withoutdepositing Au/Pd film onto Si substrate. The growth productsobtained were octahedral-shaped GeTe microcrystals witheight equivalent triangular faceted surfaces but without anynanowires on their surfaces (Figure 2a). The absence ofnanowires on microcrystal surfaces also implies an explicitrole of Au/Pd catalysts in growing nanowires via the VLSprocess (detailed mechanism will be suggested later). Theoctahedral shape of GeTe microcrystals is consistent withrecent studies to synthesize GeTe materials via solution-basedapproach.37 In order to define the crystallographic orientationof the GeTe microcrystal surfaces, we conducted cross-sectional high resolution transmission electron microscopy(HRTEM, JEOL-3000F) analysis on a thin slice of GeTecut from GeTe microcrystals.38 We prepared the GeTe thinfilm by FIB milling of GeTe microcrystals (Figure 2b), wherea thin film (thickness ∼60 nm) was carefully cross-sectioned

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 thinfilm with its corresponding fast Fourier transformation (FFT)shows lattice fringes parallel to the surface normal of thecross-sectioned GeTe with spacing of ∼0.36 nm whichcorresponds 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 2dillustrates GeTe structure in the framework of a rhombohe-dral 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 inthe rhombohedral unit cell corresponds to the {111} plane(green line) of the rocksalt structure, which is a plane ofclose packing with two-dimensional hexagonal arrangementof atoms. This suggests that the GeTe microcrystals growin an octahedral shape exposing the surfaces with closepacking of atoms, most likely to minimize the surface energy;planes of higher planar atomic density generally possesslower surface energy due to the lower density of brokenatomic bonds.39

The next step is to trace the growth evolution of GeTenanowires 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 (shortnanowires) are typically found to grow from tetrahedralstructures that form triangular-shaped bases on the surfacesof microcrystals (Figure 3a). Au/Pd catalysts were found atthe apex of the tetrahedral bases as characterized with energydispersive X-ray spectroscopy (EDS) (data not shown), whichguides nanowire growth via the VLS reaction throughcontinued supply of vaporized GeTe. Careful SEM inspection(Figure 3b) reveals three triangular facets of the tetrahedralstructure 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 GeTerhombohedral structure. Figure 3c illustrates a simulatedGeTe 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 actualmeasurements based on SEM images of the bottom of GeTenanowires (Figure 3d,e). Therefore, it is concluded that thehighly oriented GeTe nanowires grown on microcrystals areguided by the tetrahedral bases that self-assemble onoctahedral microcrystals. We also observe that the nanowiresincorporate only two out of the three exposed facets of thetetrahedral bases (Figure 3e-g) while maintaining theirtetragonal cross sections. The simulated models providedirect visualization of the atomic structure of the nanowiresseen in the Figure 3f,g, illustrating (01j1) and (101) facets ofthe nanowires. One interesting feature is that the nanowireson each surface of the microcrystal typically grow in twodifferent directions only (Figure 1c,d) despite the 3-fold

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

Nano Lett., Vol. 9, No. 6, 2009 2397

symmetry that is expected from the tetrahedral base withhexagonal atomic arrangement on {001} plane. This isattributed to the unique atomic arrangement on {001} basalplane in this unique rhombohedral GeTe structure which doesnot follow ideal hexagonal close packing due to the inherentdistortion. The triangular facets of the tetrahedral bases(Figure 3a,b) are not perfect equilateral triangles, but slightlyelongated by ∼2% along [111] direction of rocksalt structureas shown in Figure 2d; therefore, the linear atomic densityalong each base of the triangle will not be equivalent(Supporting Information, Figure S1).34

In order to directly characterize the epitaxial crystal-lographic relationship of nanowire/microcrystal and tovalidate the above analysis, we further conducted FIB lift-out assisted-TEM analysis on the cross-sectioned GeTenanowires grown on microcrystals. Figure 4a is a representa-tive SEM image of a GeTe nanowire grown on a facet of anoctahedral GeTe microcrystal with a tetrahedral base structureat the bottom. Cross-sectioned TEM sample of the samenanowires was prepared by FIB-lift out technique, and itslow magnification TEM image is shown in Figure 4b. Figure4c is a selective area electron diffraction (SAED) pattern ofthe same nanowire/microcrystal structure along [100] zoneaxis. The SAED provides several key features in interpretingthe epitaxial relationship of the GeTe nanowire/GeTe mi-crocrystal; the indexed (003) diffraction spot is parallel tothe surface normal of GeTe microcrystal, which indeedconfirms that octahedral GeTe microcrystals prefer to growby exposing rhombohedral {001} plane, consistent with the

Figure 3. (a) SEM image of tetrahedral GeTe structures on thesurface of an octahedral GeTe microcrystal. (b) Enlarged SEMimage of a tetrahedron GeTe showing Au/Pd catalyst at the apexand three equivalent triangular facets on the surface (c) Simulatedstructural models to illustrate (01j1), (101), (11j1) planes (redtriangles) with (001) base in a rhombohedral GeTe unit cell (blue) Ge, brown ) Te). (d) The angle between (001) and (101) planesis calculated to be 36.44° (e) SEM side view of a nanowireorientation corresponding to the structural model in (d). (f,g) SEMimages of a GeTe nanowire grown from a tetrahedral base exposingtwo close packing planes. Back (f) and front (g) view of thenanowire and its corresponding simulated models to illustrate theatomic structure.

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

2398 Nano Lett., Vol. 9, No. 6, 2009

structural analysis discussed earlier (Figure 2a-d). In addi-tion, (02j2) diffraction spots are observed parallel to thenormal of a GeTe nanowire facet, indicating that thenanowire is indeed surrounded by (01j1) plane, which is alsoconsistent with the SEM and the analysis of the atom models(Figure 3a-e). The nanowire growth direction is identifiedto be [024], consistent with other studies of individual GeTenanowires.41 It is interesting to note that nanowires do notgrow vertically on the underlying microcrystals but alwaysgrow at 36.44° from the normal of the microcrystal surfaces(Figure 3e). This phenomenon can be explained by consider-ing the atomic planar density of nanowire side surfaces atthe initial stage of growth. If the nanowires would growvertically (along [001] direction) retaining their triangular-shaped 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 withnoncovalent bonding (Supporting Information, Figure S2)and therefore would inevitably possess higher surface ener-gies in comparison to close packing planes such as (01j1)and (101) clearly observed in our study. This structuralanalysis strongly suggests that the epitaxial growth of GeTenanowires on octahedral microcrystals is governed by theatomic planar density (surface energy) of planes exposedthrough nucleation and growth.

Finally, we provide an explanation about the possiblegrowth mechanism of the GeTe nanowires evolving frommicrocrystals. Thermally evaporated GeTe reacting with Au/Pd is expected to undergo VLS growth while the supply ofexcessive 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 onthe Stranski-Krastanov growth mode becomes dominant,phase separation between Au/Pd and GeTe is also likely toproceed as the outward migration of metals is generallypredicted during the solidification of liquid droplets in aeutectic process, such as for Au/Si and Au/Ge.42,43 While

exposing Au/Pd toward the surface of octahedral GeTemicrocrystals, continued reaction of vaporized GeTe withAu/Pd first simultaneously forms tetrahedron base structurein order to reduce the surface energy, and subsequent GeTenanowire growth results from them. A schematic to explainthe growth mechanism of ordered GeTe nanostructures isshown in Figure 5.

In order to verify this analysis, we examine the morphol-ogy of structures grown on the different regions of a growthsubstrate 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 uniformlyfound 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 observedsegregated out of surrounding mixture of Ge/Te as confirmedby EDS (Supporting Information, Figure S3) that did notgrow into large GeTe microcrystals yet due to a small drivingforce for nucleation. At a lower temperature region (∼520°C, Figure 6b), some large GeTe structures are found alongwith one-dimensional nanowires (upper right), indicative ofsimultaneous VS and VLS growth, respectively, as suggestedabove. At a further lowered temperature (∼430 °C, Figure6c), octahedron GeTe structures start to appear with a mixtureof Au/Pd/GeTe clearly seen on their surfaces. Finally, at anoptimized temperature (∼370 °C, Figure 6d and Figure 1a),epitaxially grown GeTe nanowire/microcrystal structuresdominate over a large area of the substrate. We also observethat these nanowire/microcrystal structures are observed onlywhen the synthesis is performed under relatively high vaporpressures (300 Torr) and high Ar flow rate (200 SCCM orabove), both of which are required to introduce fast andexcessive 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 conditionsfor VLS nanowire growth without any emergence of VS-

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).

Nano Lett., Vol. 9, No. 6, 2009 2399

grown GeTe microcrystals, even enabling the diameter-controlled synthesis of GeTe nanowires when the temperatureof the growth substrate is lowered down to ∼275 °C.44 It isworth mentioning that the dominant growth directions ofGeTe nanowires grown via VLS mechanism on growthsubstrates (Si or SiO2) but not from GeTe microcrystals are[220], [202], or [003],29,36,44 all of which are the surfacenormals of close-packing planes as studied above (Figure 3and 4). This observation strongly indicates that growth ofGeTe nanowires that are not epitixially guided by microc-rystals still prefer to form close-packing surfaces at thenanowire/substrate interface during their very initial nucleation/growth stage in order to minimize the surface energy of thesystem. It is also noteworthy that epitaxy-driven dendriticnanowires as well as nanowire/microcrystal structures areoften observed in thermal evaporation of chalcogenidepowders, particularly Pb-based chalcogenide nanowires,45,46

where the morphology of nanowires varies depending on thegas flow rate, vacuum pressure, and temperature, similarlyas discussed above.

In conclusion, we demonstrated synthesis of GeTe nanow-ires epitaxially grown on GeTe microcrystals via one-stepchemical vapor transport process. Through extensive TEM-FIB analysis and atom models, we identified that GeTemicrocrystals grew exposing {001} rhombohedral plane thatis a plane of close packing density. Moreover, GeTenanowires epitaxially grow from the microcrystals also toexpose close-packing planes, which is only possible bygrowing tetrahedral bases again with exposing close-packingplanes. This observation clearly indicates that maximizing

the planar atomic density is the critical factor that determinesthe growth characteristics and complex structuring of crystal-line nanomaterials in three-dimensions. This study providesa general framework for understanding the growth mecha-nism of hierarchically ordered nanostructures using GeTeas a model system and is also useful in designing 3D growthschemes for alignment of nanowires for electronic/photonicdevices.

Acknowledgment. This work was supported by NSF(DMR-0706381), Penn-MRSEC seed award (DMR05-20020), and in part by ONR (Grant N000140910116) andthe Korea Science and Engineering Foundation (KOSEF)granted by the Korea government (MOST, NO. R11-2005-065).

Supporting Information Available: This material isavailable free of charge via the Internet at http://pubs.acs.org.

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Figure 6. Representative SEM images of structures found atdifferent temperature regions on a growth substrate: (a) at 610 °C;nucleated Au/Pd/Ge/Te nanostructures uniformly distributed on thegrowth substrate. Inset shows faceted fcc Au/Pd nanocrystalssegregated out of GeTe (scale bar, 200 nm); (b) at 520 °C; VSgrown 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 GeTenanowire on octahedral microcrystals on the large area of the growthsubstrate.

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