Imperial College London · Web viewBismuth telluride is a well-known thermoelectric material operating at room temperature and a three-dimensional topological insulator with potential

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a. Department of Materials, Imperial College London, London SW7 2AZ, UK.Electronic Supplementary Information (ESI) available: experimental details; FTIR characterisation of bismuth precursor; XRD patterns showing degradation of nanosheets in air; quantitative results of the elemental analysis; additional TEM images and SAED patterns illustrating surface termination of the branched nanosheets. See DOI: 10.1039/x0xx00000x


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Received 00th January 20xx,Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

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Room-Temperature Growth of Colloidal Bi2Te3 NanosheetsM. S. Sokolikova,a P. C. Sherrella, P. Palczynski,a V. L. Bemmera and C. Mattevia

In this work, we report the colloidal synthesis of Bi2Te3 nanosheets with controlled thickness, morphology and crystallinity at temperatures as low as 20 oC. Grown at room temperature, Bi2Te3

exhibits two-dimensional morphology with thickness of 4 nm and lateral size of 200 nm. Upon increasing the temperature to 170 oC, the nanosheets demonstrate increased thickness of 16 nm and lateral dimensions of 600 nm where polycrystalline nanosheets (20 °C) are replaced by single crystal platelets (170 °C). Rapid synthesis of the material at moderately low temperatures with controllable morphology, crystallinity and consequently electrical and thermal properties can pave the way toward their large-scale production for practical applications.

Bismuth telluride is a well-known thermoelectric material operating at room temperature and a three-dimensional topological insulator with potential applications in spintronics and quantum computing.1 Nanostructuring can substantially improve thermoelectric performance of the material due to the combined effect of reduced thermal conductivity arising from more efficient phonon scattering on the grain boundaries and of increased density of states in the confined structures providing higher electrical conductivity.2 Moreover, electrical conductivity can be finely tuned by a controllable defect activation.3 Additionally, since the properties of topologically protected states vary considerably depending on the bounding surface, the control over the surface morphology is demanded for fabrication of the material with designed electronic properties.4

Bi2Te3 has anisotropic layered structure with covalently bonded Te-Bi-Te-Bi-Te quintuple layers held together by weak van der Waals forces and therefore can be mechanically cleaved producing single layer Bi2Te3 sheets.5,6 Exfoliation in a liquid phase has been successfully exploited for a large-scale production of dispersions of two-dimensional sheets generally heterogeneous in size and thickness.7 Chemical vapour deposition leads to a wafer-scale production of micron-sized

Bi2Te3 triangles and hexagons, although precise control over thickness remains elusive.8 Moreover, it has been recently predicted that binding to a substrate can dramatically affect the geometry of the growing material thus altering its electronic properties.9 On the other hand, colloidal synthesis, which allows for precise control over the size and shape distribution, has been successfully employed for obtaining two-dimensional nanostructures of layered10,11 and non-layered12,13 materials, but has been seldom reported for bismuth telluride.

Recent examples of colloidal synthesis of bismuth telluride in organic medium include a direct reaction between molecular precursors and a two-step reaction involving initial reduction of bismuth ions to metallic bismuth in presence of a capping ligand followed by treatment with tellurium precursor. The latter approach led to the formation of either spherical Bi2Te3

nanoparticles14 with mean diameter of 7-50 nm or 1 nm thick Bi2Te3 nanodisks15 with lateral size of 10-30 nm if the reduction and tellurisation reactions occurred simultaneously.

A one-step nucleation-growth process without the intermediate reduction of bismuth precursor is advantageous to achieve precise shape control, but remains exceptionally challenging since the direct reaction between bismuth and chalcogen ions is known to be fast. Recently, Chen et al observed the evolution of spherical Bi2Te3 nanoparticles grown at 35 oC into two-dimensional branched nanostructures when the growth temperature was raised higher than 75 oC.16 Similar transformation has been reported by Stavila et al, although they specifically indicated that no reaction was observed at room temperature even after several days.17 In the same synthesis scheme, high quality hexagonal Bi2Te3 platelets were obtained via prolonged growth at 150 oC by Lu et al.18

In this communication, we demonstrate a solution-based method allowing for growth of two-dimensional Bi2Te3

nanosheets with thickness around 4 nm even at room temperature. Nanosheets were transformed into two-dimensional hexagonal platelets by increasing the reaction temperature to 170 oC and decreasing the reaction time to a few seconds, which is significantly shorter than the timescale of any previously reported work.18 Notably, the yield of the high temperature (170 oC) rapid reaction reaches 70%

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potentially allowing for upscaling into a high-throughput technique.

Bi2Te3 nanosheets were synthesized upon a reaction between in situ generated bismuth oleate and trioctylphosphine telluride (TOP:Te) complex at temperatures as low as 20 oC in a non-coordinating organic solvent as described in the Supporting Information. Oleic acid was chosen as capping ligand since it readily reacts with bismuth salts producing bismuth oleate (Supporting Information, Figure S1) and does not reduce bismuth ions to elemental bismuth in contrast to widely used oleylamine. The growth of Bi2Te3 nanosheets was conducted at temperatures ranged from 20 oC to 170 oC. Upon the injection of TOP:Te, a pale-yellow solution of bismuth oleate slowly turned bright-yellow and then black indicating formation of Bi2Te3 nanoparticles, this colour change was almost instantaneous at higher temperatures. The final product was separated from unreacted precursors by centrifugation and, after repetitive washing with ethanol, was redispersed in hexane and stored under vacuum in a drying desiccator to prevent degradation (Supporting Information, Figure S2).

Crucially, in contrast to other approaches we used stoichiometric bismuth oleate as a bismuth source instead of its solution in oleic acid. Due to the dangling bonds present in the lateral planes of growing Bi2Te3 nanostructures, the significant excess of strongly binding capping ligand (free oleic acid) in the reaction medium reduces the growth rate of the lateral planes leading to the formation of smaller nanoparticles as a consequence. This change alone led to formation of Bi2Te3

nanosheets in the whole temperature range from 20 oC to 170 oC differently from Chen et al and Stavila et al.16,17

Low-resolution TEM images shown in Figure 1.a-c are representative images of Bi2Te3 nanosheets grown at 20 oC, 70 oC and 170 oC respectively. Distinct two-dimensional nanosheets emerging from a central core with an approximate lateral size of 200 nm are formed at room temperature. Upon increasing the growth temperature, the material acquires a more regular hexagonal shape, increasing in lateral size to over 600 nm. The thickness of the nanosheets, analysed by TEM, was found to consistently increase with the growth temperature. Specifically, the average thickness, measured across ~150 nanosheets and obtained via the Gaussian fit of the histograms shown in Figure 1.a-c, is of 4 (±1) nm, 7 (±2) nm and 16 (±4) nm at growth temperature of 20 oC, 70 oC and 170 oC respectively. The observed thickness distribution is broadened due to the variation in the inclination angle that is more significant for the rigid hexagonal platelets oriented not ideally perpendicularly to the TEM grid than for the flexible thin nanosheets. Atomic-force microscopy (AFM) analysis of hexagonal Bi2Te3 platelets grown at 170 oC has confirmed the thickness measured by TEM analysis around 14 nm (Figure 1.d). The stoichiometry of 2:3 between Bi and Te has been confirmed by energy dispersive X-ray spectroscopy characterization (Supporting Information, Figure S3).

X-ray diffraction (XRD) data shown in Figure 1.e reveal rhombohedral structure of Bi2Te3 nanosheets (space groupR3m, Supporting Information, Figure S4). The XRD pattern of Bi2Te3 nanosheets grown at room temperature exhibits two broad peaks assigned to (015) and (110) reflections of bulk

bismuth telluride. Significant broadening of (015) and (110) peaks that correspond to the lateral surface might suggest the polycrystalline nature of Bi2Te3 nanosheets. XRD patterns of nanosheets grown at higher temperatures show numerous diffraction peaks with higher intensity denoting improved crystallinity, while their narrow widths confirm the increased thickness and larger crystallite size. In the case of Bi2Te3 grown at 170 oC, anisotropic peak broadening is clearly observed. Considering that the peak width is determined by the crystal dimension in the corresponding direction, we can assume that the material grown at high temperature has two-dimensional morphology with {0001} basal planes and lateral planes being predominantly {1120 }. It should be noted that in all cases the peaks are shifted towards higher diffraction angles relative to the standard reference positions implying the lattice contraction arising presumably due to the wrapping of two-dimensional nanosheets. This shift is less prominent for the rigid platelets grown at high temperature in contrast to the flexible nanosheets. Thus, the (110) and (015) interplanar distances were found to be 2.18 Å and 3.2 Å, 2.17 Å and 3.19 Å, and 2.15 Å and 3.17 Å for the samples grown at 170 oC, 70 oC, and 20 oC respectively that is 0.5-2% smaller than the corresponding bulk value.

Raman spectra of Bi2Te3 nanosheets demonstrate the thickness dependence of the in-plane Eg and out-of-plane A1g

characteristic bands (Figure 1.f). For Bi2Te3 platelets peaks are found at 61.4 cm-1, 102.1 cm-1, and 134.1 cm-1 that is close to the frequencies of the A1g

1 , Eg2 , and A1g

2 vibration modes of bulk bismuth telluride reported by Zhang et al.19 As the thickness of Bi2Te3 nanosheets decreases, peak broadening, with a blue-shift of the A1g

1 and Eg2 peaks and red-shift of the

A1g2 also observed, The peak shift is more pronounced for the

A1g1 and A1g

2 modes arising from the out-of-plane vibrations of the crystal lattice and therefore more sensitive to the number of quintuple layers. For the Bi2Te3 nanosheets grown at room temperature, the A1g

1 , Eg2 , and A1g

2 modes are centred around 56.3 cm-1, 99.7 cm-1, and 139.3 cm-1.

High resolution TEM (HRTEM) and selected area electron diffraction (SAED) analysis were employed to study the surface morphology of the synthesised nanosheets. The polycrystalline nature of Bi2Te3 nanosheets grown at room temperature (as observed via XRD) was confirmed by numerous orientations forming the most prominent (015) and (110) rings present in the SAED pattern acquired from the area shown in the purple circle (Figure 2.a, d). The HRTEM image of a single petal partially suspended on a hole in the carbon film (marked with the green square in Figure 2.a) reveals that Bi2Te3 nanosheets consist of single crystalline domains of 5 nm in lateral size and the corresponding fast Fourier transform (FFT) can be indexed to the [551] crystallographic direction (Figure 2.g and inset). Lattice fringes are (015) planes with the interplanar spacing of 3.16 Å that is smaller than that of bulk bismuth telluride due to the bending of thin flexible nanosheets, this value is in a good agreement with the XRD results described above.

Hexagonal Bi2Te3 platelets grown at 170 oC exhibit excellent crystalline quality. The SAED pattern (Figure 2.e) shows hexagonal symmetry with higher order peaks present in the

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diffraction pattern that are indicative of a long-range translational symmetry. Platelets are oriented in the [001] zone axis and their edges are terminated with {1120 } planes of rhombohedral Bi2Te3. Faint dots observed in the SAED pattern can be attributed to the (100) reflections, which are kinematically forbidden in the bulk crystal, but can arise in the nanostructured material due to antisite defects or incomplete layers.20 The corresponding HRTEM (Figure 2.h) image reveals lattice fringes with interplanar distance of 2.18 Å consistent with the lattice spacing of (110) planes.

Interestingly, flat regions of Bi2Te3 platelets with defined branching points in the structure (Figure 2.c and inset), which were a minor fraction in the high temperature growth, reveal the same {0115 } lateral termination as was found for the nanosheets grown at 20-70 oC (Supporting Information, Figure S5). The SAED pattern (Figure 2.f) taken along the [151] direction demonstrate that single crystalline areas of intersecting plates are enclosed by the {0115 } facets. Accordingly, HRTEM of the area in the vicinity of the edge of a single plate (Figure 2.i) displays lattice fringes with the uniform spacing of 3.2 Å matching well with the distances between (015) planes.

Nanocrystal shape and faceting of its lateral surfaces are defined by the driving force of the growth process. At constant temperature, the driving force is the supersaturation.21 Under low supersaturation, the growth process is controlled by the precursor diffusion to the growing facet, which is slower than the monomer adsorption and migration rates, and eventually nanocrystals with equilibrium shapes are produced. On contrary, under high supersaturation, the growth process is governed by the difference in the facet growth rates resulting in highly-anisotropic metastable particles.22,23 We attribute the difference in the morphology of the nanosheets grown at low (20 oC) and at high (170 oC) temperatures to the fact that at low temperature (high supersaturation) the reaction occurs in the kinetically-controlled mode that explains the observed branching, while at high temperature the branching growth is significantly limited due to the lower supersaturation and thus growing nanocrystals acquire regular equilibrium shapes.

Similar reasoning can explain the difference in the surface morphology. First-principles DFT calculations showed that the excessive surface energies followed the order γ(001) < γ(015) < γ(110) << γ(014) that could be explained by means of the unsaturated bonds on each of these surfaces.9,24 The excessive energies were calculated for pristine surfaces, although presence of a capping ligand in the system can reduce the energy difference between planes through the partial saturation of dangling bonds, but would not significantly alter their order. Grown at high temperature, hexagonal Bi2Te3

platelets exhibit {0001} basal and {1120 } lateral termination that ensures the lowest surface energy. In the low-temperature kinetically-controlled growth, polar high-index facets, like (014), might grow faster than low-energy (001) and (015) facets leading to the considerable extension of both of them and to the {0115 } lateral termination as a result, although the exact mechanism requires more dedicated study.

In summary, we have demonstrated controllable synthesis of Bi2Te3 nanosheets of 4 nm in thickness and lateral size of

hundreds of nanometres at temperatures as low as 20 oC upon a direct reaction between molecular precursors. Increasing the synthesis temperature, controllable conversion from polycrystalline to single crystal nanosheets has been achieved along with an increase thickness of 16 nm with lateral dimensions reaching 600 nm. These results can lay the foundation of engineering crystallinity and morphology of materials, which directly translate into electrical, thermal and optical properties, meeting materials engineering requirements to use materials grown via colloidal synthesis for practical applications.

M.S.S. would like to acknowledge the President’s PhD Scholarship programme for financial support. C.M. would like to acknowledge the EPSRC awards EP/K033840/1, EP/K01658X/1, EP/K016792/1, EP/M022250/1 and the EPSRC-Royal Society Fellowship Engagement Grant EP/L003481/1. C.M. acknowledges the award of a Royal Society University Research Fellowship by the UK Royal Society. P.C.S. would like to acknowledge the funding and support from the European Commission (H2020 – Marie Sklodowska-Curie European Fellowship - 660721).

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10 B. Mahler, V. Hoepfner, K. Liao and G. A. Ozin, J. Am. Chem. Soc., 2014, 136, 14121–14127.

11 G. Almeida, S. Dogan, G. Bertoni, C. Giannini, R. Gaspari, S. Perissinotto, R. Krahne, S. Ghosh and L. Manna, J. Am. Chem. Soc., 2017, 139, 3005-3011.

12 S. Ithurria and B. Dubertret, J. Am. Chem. Soc., 2008, 130, 16504–16505.

13 C. Schliehe, B. H. Juarez, M. Pelletier, S. Jander, D. Greshnykh, M. Nagel, A. Meyer, S. Foerster, A. Kornowski, C. Klinke and H. Weller, Science (80-. )., 2010, 329, 550–553.

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17 V. Stavila, D. B. Robinson, M. A. Hekmaty, R. Nishimoto, D. L. Medlin, S. Zhu, T. M. Tritt and P. A. Sharma, ACS Appl. Mater. Interfaces, 2013, 5, 6678–6686.

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Figure 1. Characterisation of Bi2Te3 nanosheets. Representative TEM images and histograms showing thickness distribution of Bi2Te3 nanosheets grown at 20 oC (a), 70 oC (b), and 170 oC (c). AFM image of two overlapping Bi2Te3 platelets grown at 170 oC (d) and corresponding SEM image (inset) with height profile demonstrating uniform thickness of 14 nm of a single platelet. X-ray diffraction patterns of Bi2Te3 nanosheets and the standard diffraction peaks of the bulk rhombohedral Bi 2Te3 (ICDD 15-863) are shown in panel (e). Raman spectra of Bi2Te3 nanosheets are reported in panel (f), vertical dashed lines denote vibration modes of bulk Bi2Te3 reported in ref. 19. X-ray diffractograms and Raman spectra are offset for clarity.





Figure 2. Surface morphology of Bi2Te3 nanosheets. TEM images (a-c), SAED patterns (d-f) from the areas marked with purple circles in a-c, and HRTEM images (g-i) of the regions marked with green squares in a-c of the Bi2Te3 nanosheets grown at 20 oC (a, d, g) and 170 oC (b, c; e, f; h, i). Inset in panel c shows individual branching Bi2Te3 platelet composed of four intersecting plates, scale bar is 200 nm. In the SAED pattern in panel e, forbidden (100) reflections observed for the Bi 2Te3 platelets are noted with blue circles. Insets in panels g-i are corresponding Fourier transform patterns of HRTEM images.





Table of Contents graphics. Change of morphology of colloidal Bi2Te3 nanosheets upon increasing the growth temperature from 20 oC to 170 oC.


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Imperial College London · Web viewBismuth telluride is a well-known thermoelectric material operating at room temperature and a three-dimensional topological insulator with potential