A Novel Sb2Te3 Polymorph Stable at the Nanoscale - ACS Publications

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A Novel Sb2Te3 Polymorph Stable at the Nanoscale Enzo Rotunno,*,† Massimo Longo,‡ Claudia Wiemer,‡ Roberto Fallica,‡ Davide Campi,§ Marco Bernasconi,§ Andrew R. Lupini,∥ Stephen J. Pennycook,⊥ and Laura Lazzarini† †

IMEM-CNR, Parco Area delle Scienze 37/A, 43124 Parma, Italy Laboratorio MDM, IMM-CNR, Unità di Agrate Brianza, Via C. Olivetti 2, 20864 Agrate Brianza, (MB), Italy § Dipartimento di Scienza dei Materiali, University of Milano-Bicocca, Via R. Cozzi, 55, 20125 Milano, Italy ∥ Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ⊥ Department of Materials Science and Engineering, The University of Tennessee, 328 Ferris Hall Knoxville, Tennessee 37996-2200, United States ‡

S Supporting Information *

ABSTRACT: We report on the MOCVD synthesis of Sb2Te3 nanowires that self-assemble in a novel metastable polymorph. The nanowires crystallize in a primitive trigonal lattice (P3̅m1 SG #164) with lattice parameters a = b = 0.422 nm, and c = 1.06 nm. The stability of the polymorph has been studied by first principle calculations: it has been demonstrated that the stabilization is due to the particular side-wall faceting, finding excellent agreement with the experimental observations.

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INTRODUCTION Sb2Te3 is a small band gap (0.28 eV) semiconductor1 of interest for several technological applications. It is a traditional thermoelectric (TE) material that has attracted attention in the past due to its superior TE performance2,3 and its potential for thermal4 and humidity5 sensors. Recently, interest in Sb2Te3 has further increased due to its promising technological applications in spintronics and topological quantum computation. These applications are enabled by its topological insulating properties,6 a new state of quantum matter characterized by an insulating gap in the bulk state and a robust metallic surface or edge state protected by time-reversal symmetry. Ge-doped SbTe has also been reported7 as an interesting alloy for applications in optical media storage (DVDs and Blu Ray disks) and Phase Change Memories (PCM).8 The latter are data storage devices based on the reversible phase switch induced in the active material by nanosecond current pulses.9,10 In PCMs, information is stored by relating binary codes to the significantly different resistivity values of the material in crystalline and amorphous states.11 In general, chalcogenide nanostructures allow a defect-free scaling down in the fabrication of high performance devices, often beyond the range of top-down processes. Moreover, scaling down the cell size positively influences many physical parameters, such as crystallization and melting temperature, crystallization speed, and thermal and electrical resistivity improving the PCM device performances and their endurance.12 A number of pioneering works have already shown nanosecond-level phase switching, suggesting that NWs hold promise for future electrical data storage devices.13−16 © 2015 American Chemical Society

However, there remain considerable difficulties in synthesizing Sb2Te3 NWs with a controlled composition and crystalline phase. Furthermore, to fully understand the basis of NW device operation, it is essential to inspect the crystal structures, which may be different from those of the bulk or thin films counterparts, as already suggested by other works on similar compounds.17,18 In this work, we report on the synthesis and characterization of a novel polytype of Sb2Te3. The ternary Ge−Sb−Te and related compounds have two crystalline polymorphs: the metastable phase crystallizes in a cubic rocksalt-type structure, in which the Te atoms occupy the anion sites, whereas the Ge, Sb, and intrinsic vacancies randomly occupy the cation sites.19−21 The stable phases instead have a layered tetradymite-like structure22−24 in which the c axis length can be directly related to the Ge/Sb composition.25,26 The Sb2Te3 compound crystallizes in the rhombohedral layered structure only. It has space group R3̅m (SG #166) and lattice parameters a = 0.421 nm and c = 3.045 nm.27 The unit cell is composed of three building blocks separated by a van der Waals gap. Each block is rotated by 120° with respect to the previous one and consists of five atomic planes stacked along the [0001] direction of the conventional hexagonal lattice according to the sequence −Te−Sb−Te−Sb−Te−, as reported in Figure 1a). The intrinsic vacancies self-organize and form the Received: March 16, 2015 Revised: May 26, 2015 Published: May 27, 2015 4368

DOI: 10.1021/acs.chemmater.5b00982 Chem. Mater. 2015, 27, 4368−4373

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Chemistry of Materials

between the partial pressures, should not be surprising, given the observed high Sb incorporation efficiency in our Ge−Sb−Te synthesis process. Transmission Electron Microscopy Analyses. In order to perform structural analysis on single nanostructures, the NWs were removed from the as-deposited samples and then dispersed on holey carbon grids for the scanning and conventional transmission electron microscopy (S)TEM observations by means of a high-resolution, (0.18 nm) field emission JEOL 2200FS microscope, equipped with an incolumn Ω energy filter, 2 high-angle annular dark-field (HAADF) detectors, and X-ray microanalysis (EDS). In order to refine the crystal structure of the new polymorph, we performed STEM HAADF experiments in an aberration corrected Nion UltraSTEM operating at 200 keV. Ab-initio Calculations. We performed calculations within the framework of the density functional theory as implemented in the Quantum-Espresso code,36 using norm conserving pseudopotentials and the Perdew−Burke−Ernzerhof (PBE) approximation for the exchange-correlation functional37 with the inclusion of a semiempirical van der Waals correction according to Grimme.38 The Kohn−Sham states were expanded on a plane wave basis up to a 35 Ry cutoff. In bulk calculations, the Brillouin zone was sampled with a uniform Monkhorst−Pack mesh39 of 12 × 12 × 6 k-points for the hexagonal cell of the SG #164 phase and of 12 × 12 × 12 k-points for the elemental rhombohedral cell of the SG #166 phase. The surfaces were modeled by slabs about 3 nm thick with a vacuum 1.5 nm wide separating the periodic replica. The surface Brillouin Zone was integrated with up to 6 × 6 k-point meshes.

gaps, minimizing the system energy. Any other different stacking has been calculated to have higher energy.28

Figure 1. Atomic model of (a) the bulk Sb2Te3 phase and (b) the novel phase refined in this work.

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RESULTS AND DISCUSSION The self-assembled NW growth was performed exploiting the VLS mechanism assisted by Au metal-catalyst colloidal nanoparticles. Although the Au particle size was only 10 nm, the self-assembly of Sb2Te3 NWs with a diameter below 50 nm turned out to be very difficult. In order to obtain thinner wires, a small amount of Germanium was introduced to control the NW growth rate, by altering the rate of atom incorporation into the NW crystal lattice.40 This procedure allowed the formation of NWs with diameter below 40 nm. The chemical analysis performed by EDX both on large area and single NWs confirms Ge incorporation, always lower than 3 atom %, as reported in the Supporting Information, Figure S1. The SEM micrographs showed that our nanowires have a typical zigzag shape due to periodic oscillation of the sidewall facet orientation, as reported in Figure 2a and b. From the SEM investigations (see Supporting Information Figure S2), we recognize that the NWs are composed of many segments having truncated octahedral shape, following the three-

However, the crystal structure we observe in our NWs, reported in Figure 1b, possesses a different symmetry: the fivelayers block is simply repeated identically along the c-axis giving rise to a primitive trigonal lattice with space group P3̅m1 (SG #164). The resulting unit cell is therefore much shorter than the stable bulk phase with c = 1.06 nm. We demonstrate that the SG #164 Sb2Te3 phase appears only in the nanowires as its surfaces have a lower energy than those of the bulk stable SG #166 phase with the same indexes. The minimization of the surface energy results in a complex nanofaceted morphology associated with the formation of an ordered array of twin defects orthogonal to the growth direction, commonly reported as twin super-lattices. This phenomenon was so far observed in zinc-blend compounds only,29−35 mainly III−V semiconductors but, to the best of our knowledge, has never been observed for Sb2Te3. We highlight here that the repetition of the twin defects should not be mistaken with the crystal structure of the new polymorph: the distance between two consecutive twins is 20− 40 nm, one order of magnitude higher than the new phase lattice parameters.

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METHODS

Synthesis of the Nanowires. The self-assembled NW growth was performed in a thermal MOCVD AIX 200/4 reactor, exploiting the vapor-liquid-solid (VLS) mechanism assisted by 10 nm Au metalcatalyst colloidal nanoparticles, inducing the self-assembly of NWs with diameter ranging between 10 and 40 nm. The substrates are 4 in. (100) Silicon wafers on which a 50 nm thick thermal SiO2 layer is present. Electronic grade tetrakisdimethylaminogermanium ([N(CH 3 ) 2 ] 4 Ge, TDMAGe), trisdimethylaminoantimony ([N(CH3)2]3Sb, TDMASb), and diisopropyltelluride ((C3H7)3Te, DiPTe) were used as Ge, Sb, and Te precursors, respectively, due to their comparable vapor pressures. Reactant partial pressure in the vapor phase was 1.5 × 10−2 mbar for TDMAGe, 4 × 10−3 mbar for TDMASb, and 6.5 × 10−2 mbar for DiPTe; total gas flow was 4.2 L/min and deposition time = 20 min. The low Ge content in the NWs, notwithstanding the nominal ratio TDMAGe/TDMASb = 3.8

Figure 2. (a) SEM image of the Sb−Te NWs. (b) High magnification SEM image of a single NW showing the peculiar morphology. (c) Three-dimensional model of the nanowires. The model in c has been shaded taking into account the beam illumination geometry of the picture in b. 4369


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Chemistry of Materials dimensional model proposed by Johansson et al.41 reported in Figure 2c. A 30 nm NW grown along the [0001] direction, as it always happens, is shown in a series of TEM images at increasing magnification (Figure 3a−c). As in Figure 3a, the twinning

The bright and dark contrast in Figure 2b−c is surface morphology related, indicating illuminated or nonilluminated facets. On the contrary, the bright and dark contrast of the segments forming the NW in Figure 3a and b is diffractionrelated, due to a slight misorientation of the nanowire relative to the beam and disappears once the nanowire is perfectly oriented to achieve the best high resolution imaging conditions, as in Figure 3c. Here, the mirror symmetry relationship between two consecutive segments is highlighted by the red dashed lines running parallel to the atomic planes on the two sides of the twin defect (yellow dashed line), which lies in the (0001) plane. The 1.06 nm periodicity along the [0001] growth direction corresponds to the distance across the van der Waals gaps in the SG#164 phase of the Sb2Te3. In Figure 3d the electron diffraction pattern (DP) of the NW in a is shown. In the DP, two sets of orthogonal reflections are present. The shorter reflection, compatible with a lattice periodicity of 1.06 nm, is also consistent with the 0003 reflection of the SG#166 phase of the Sb2Te3 structure. However, the periodicity in the orthogonal direction is 0.36 nm, which can only be attributed to the 1−100 reflection of the SG#164 phase of Sb2Te3. In fact, the 1−100 reflection in the Sb2Te3 bulk phase is forbidden by the SG symmetry. In general, the symmetry observed in the DPs is not consistent with the R3̅m space group of the bulk phase but with a primitive lattice instead. Therefore, the DP in Figure 3d has been indexed as a [1−210] zone axis of the SG #164 phase. A 3D model is reported in Figure 3e. The model is oriented in the same direction as the nanowire but slightly tilted for better rendering. The comparison between the model and the diffraction pattern allowed the NW sidewalls to be identified as belonging to the {1−101} family of planes, as indicated by the red and green arrows in Figure 3. It has to be noted that large area XRD analysis (see Supporting Information, Figure S3) found both the SG#164 phase and the SG #166 bulk phase. The SG #166 bulk phase is most likely due to the always present growth byproducts, usually micrometer sized crystals, and it has never been observed in the tens of NWs studied by TEM. The atomic positions inside the unit cell have been assessed by means of high resolution STEM-HAADF experiments performed in different wire orientations. The STEM-HAADF

Figure 3. (a−c) Series of TEM images at increasing magnification. The yellow dashed line indicates the twin defect. The mirror symmetry at the two sides of the twin planes is highlighted by the red dotted lines in c. (d) Electron diffraction pattern of the wire and (e) 3D model of the wire geometry in the same projection. The model is slightly rotated for clarity.

period has been measured to be 18 nm, and it is constant along the whole length of the NWs, confirming the ordering of the twin defects. Furthermore, the twinning period is roughly proportional to the wire diameter (see Supporting Information, Figure S2).

Figure 4. High Resolution STEM-HAADF images of an Sb2Te3 NW oriented along two different zone axes, as labeled in the image. An intensity line profile is reported along the ⟨1−100⟩ oriented image to identify the stacking sequence. The atomic models of the crystal structure and the image simulations are reported in the inset. The images have been Wiener filtered for noise reduction. 4370


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phase and in the Supporting Information as well as in ref 46 for the SG #166 phase. The internal structure of the penta-layer block is essentially the same in the two phases; the largest structural differences occur in the stacking of the blocks, which results in a different length of the weak Te−Te bond connecting the blocks (see Figure 1). In the SG #164 phase this bond is almost 3% longer than that in the SG #166 phase. As expected, in the bulk the SG #164 phase is higher in energy than the SG #166 phase by about Δμ = 5.399 meV/atom. Moreover, experimentally, the 164 phase is seen only when Sb2Te3 is doped with about 3 at. % Ge. Indeed, we have found that the difference in the bulk energy between the SG #164 and SG #166 phase is reduced to Δμ = 4.8 meV/atom when Sb is substituted by Ge for one Ge atom in a 15-atoms cell (6.6 at. %). Clearly, this reduction facilitates the formation of the metastable phase, but alone would not be enough to stabilize the polymorph. Therefore, we can conclude that the main effect of the Ge is to allow for the growth of thinner wires that are not obtained otherwise. In principle, if thinner NWs could be obtained, the metastable SG#164 phase can be synthesized without Ge doping. To calculate the energy of the surfaces that could be exposed by a nanowire growing along the c axis, we built slabs exposing the (11−20), (1−100), and (1−102) surfaces for the SG #166 phase and the (11−20), (1−100), and (1−101) surfaces for the SG #164 phase. The overall less costly surface is actually the (0001) face for both phases, which however can not be exposed by a wire growing along the c axis. The (11−20) and (1−100) surfaces are the lowest index surfaces parallel to the c-axis; the (1−101) face corresponds to the surface observed experimentally in SG #164 nanowires, while the (1−102) is the surface of the SG #166 phase most similar to the (1−101) of the SG #164 phase. We considered different possible reconstructions of the (1−100), (1−102), and (1−101) surfaces needed to maintain stoichiometry and surface neutrality (see Supporting Information for the surface geometries of the SG #166 and SG #164 phases). However, the (11−20) surface is already neutral and does not need any reconstruction. We calculated the surface energy as the difference between the energy of the slabs and the energy of a bulk with an equivalent number of atoms divided by twice the surface area. The results are summarized in Table 2.

imaging technique has been successfully used to determine crystallographic structures due to the fact that, with respect to other TEM methods, it is free of delocalization phenomena and is very sensitive to the chemical composition.42 Both of these features are due to the geometry of the detector that allows an incoherent image formation and a direct relationship between the intensity produced at each atomic column and its mean square atomic number (Z).43 In Figure 4 we report two STEM-HAADF images of the same NW, taken in two different zone axes, namely, ⟨1−100⟩ and ⟨1−210⟩. These represent the two major symmetry poles around the c axis and are sufficient to determine without ambiguity the position of the atoms inside the unit cell. Since the van der Waals gaps are effectively voids in the structure, they appear as equally spaced dark lines (see Figure 4a) with five atomic layers stacked between them. The stacking sequence of the layers along the c axis can be derived from the image contrast. The intensity line profile of the ⟨1−100⟩ oriented image is reported in yellow along the image. The first, the last, and the central plane of each pentalayer are more intense than the others, indicating that they contain heavier atoms. The stacking sequence is therefore −Te−Sb−Te−Sb− Te− as reported in the labels. The atomic positions in the unit cell are measured from the images in the two projections and compared with corresponding image simulations to check the accuracy of the refined structure. The contrast simulations were performed by using the software STEM_CELL,44 in the framework of the linear STEM image approximation; 45 the parameters of the simulations are Cs = 0, beam energy = 200 keV, and beam convergence semiangle = 30 mrad. The results of the simulations are shown in the inset in the experimental images, superimposed on the models of the crystal structure, in Figure 4. The lattice parameters and the atomic positions of the new polymorph are summarized in Table 1. Table 1. Description of the Refined Unit Cella Space group P3m ̅ 1 (SG#164)

a

a = 0.42 nm

b = 0.42 nm

c = 1.06 nm

(0.420 nm)

(0.420 nm)

(1.043 nm)

atom

x

y

z

Te Te

0 1/3

0 2/3

Sb

1/3

2/3

0 0.6385 (0.6457) 0.1965 (0.1896)

Table 2. Comparison between the Surface Energy of the Bulk and Metastable Phase Surface energy of the SG #166 phase

Theoretical data (this work) are reported in parentheses.

Surface Energy meV/Å2

In order to understand why the SG #164 phase appears in the nanowires with a diameter lower than 40 nm but not in the larger NWs or in the bulk under any experimental conditions, we performed ab initio calculations of the formation energy of different surfaces for the two phases. The formation of the SG #164 phase is caused by the lower formation energy of its surfaces with respect to those of the SG #166 phase, which makes the new phase favored in a nanowire with a large surfaceto-volume ratio. We first computed the theoretical equilibrium cell parameters of Sb2Te3 in the bulk of the SG #166 and SG #164 phases. The structural parameters are in good agreement with experimental data as reported in Table 1 for the SG #164

Surface Energy meV/Å2

(11−20) (1−100) 34.4 32.3 Surface energy of the SG #164 phase (11−20) 34.1

(1−100) 31.0

(1−102) 32.9 (1−101) 27.4

The surface energies of the SG #164 phase are lower than those of the SG #166 phase for all the faces. This is due to two competing effects: first the SG #164 phase is more expanded along the c direction which leads to a larger surface area for the same number of broken bonds; second, the Te−Te bonds broken at the surface are stronger (shorter) in the SG #166 phase than in the SG #164 one. Thus, the NW geometry stabilizes the SG #164 phase instead of the SG #166 one. 4371


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We have attempted to estimate the maximum size that a wire can display in the SG#164 phase (see Supporting Information), and we found that the SG#164 phase should be stable for wires with a radius smaller than 6.2 nm. Although the predicted value is smaller than our NW’s size, due to the inaccuracy of the DFT framework in describing the weak Te−Te bonds, the important result is that the small size accounts for the growth of the new phase. The (1−1 0 1) surface, which was experimentally observed to be exposed by our NWs in the TEM measurements, is found to be the most energetically stable face of the SG #164 phase. The system minimizes its energy by exposing the {1−101} family of planes, and for this reason, the SG#164 is stable in the form of NWs. The surface energy minimization is also responsible for the formation of the ordered array of twin defects. In fact, because the preferred {1−101} sidewall facets do not contain the NW growth direction, the cross-sectional shape of the nanowires is forced to change during the growth. The distortion of the catalyst droplet, in response to the evolution of the NWs shape, produces the twin defects. According to the model proposed by Algra et al.,33 the continuous evolution of the NWs crosssection is responsible for the formation of the periodic twinning. The most stable surface of the SG #166 phase is the (1−100) face, for which several possible reconstructions with very similar energies (within 0.8 meV/Å2) are possible (see Supporting Information). This surface contains the c axis, and thus, the SG #166 NWs can in principle grow, in thicker wires where they are energetically favored, without twinning, which would make them easy to be distinguished from the SG #164 NWs.

This work was performed within the SYNAPSE project (“SYnthesis and functionality of chalcogenide NAnostructures for PhaSE change memories”) which has received funding from the European Union Seventh Framework Programme (FP7/ 2007-2013) under grant agreement no. 310339. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Dr. Toni Stoycheva for her contribution to the MOCVD growth of the nanowires. Mr. Franco Corticelli (IMM-CNR) is acknowledged for the SEM morphological study.

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CONCLUSIONS In summary, we have synthesized Sb2Te3 NWs in a polymorph, which was never found before, it being thermodynamically unstable in the bulk form. The atomic structure of this new polymorph has been identified by means of advanced scanning TEM techniques. The NWs crystallize in a primitive trigonal lattice (P3m ̅ 1 SG #164) with lattice parameters a = b = 0.42 nm and c = 1.06 nm. Ab initio calculations suggest that this polymorph appears when nanostructured as its surface energy is lower than the bulk-structure one.

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ASSOCIATED CONTENT

S Supporting Information *

Energy dispersive X-ray analysis, SEM morphological analysis, XRD analysis, structure of the lowest energy surfaces of the SG #166 phase, and the structure of the lowest energy surfaces of the SG # 164 phase. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b00982.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

M.L. prepared the samples, E.R. and A.R.L. carried out the (S)TEM measurements, C.W. performed the XRD measurements, and D.C. and M.B. performed DFT calculations. L.L. and S.J.P. supervised the work. All authors have contributed to the writing of the paper. 4372


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