Impurity controlled crystal growth in low dimensional bismuth telluride

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Impurity controlled crystal growth in low dimensional bismuth telluride Tiva Sharifi, Sadegh Yazdi, Gelu Costin, Amey Apte, Gabriel Coulter, Chandrashekar Tiwary, and Pulickel M. Ajayan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02548 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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Chemistry of Materials

Impurity controlled crystal growth in low dimensional bismuth telluride Tiva Sharifi1,2†*, Sadegh Yazdi1,3†, Gelu Costin4, Amey Apte1, Gabriel Coulter1,5, Chandrashekar Tiwary1, Pulickel M. Ajayan1* 1

Department of Materials Science and Nanoengineering, Rice University, Houston, 77005, USA

2

Department of Physics, Umeå University, Umeå, 90187, Sweden

3

Renewable and Sustainable Energy Institute, University of Colorado Boulder, Boulder, 80309, USA

4

Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, 77005, USA

5

School of Physics and Chemistry, Trinity College Dublin, Dublin 2, Ireland

†

These authors contributed equally to the paper

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ABSTRACT: Topological insulators, such as layered Bi2Te3, exhibit extraordinary properties, manifesting profoundly only at nanoscale thicknesses. However, it has been challenging to synthesize these structures with controlled thickness. Here, control over the thickness of solvothermally grown Bi2Te3 nanosheets is demonstrated by manipulating the crystal growth through select and controlled impurity atom addition. By a comprehensive analysis of growth mechanism and intentional addition of Fe impurity, it is demonstrated that the nucleation and growth of few-layer nanosheets of Bi2Te3 can be stabilized in solution. By optimizing the Fe concentration, nanosheets thinner than 6 nm, and as thin as 2 nm, can be synthesized. Such thicknesses are smaller than the anticipated critical thickness for the transition of topological insulators to the quantum spin Hall regime.

Introduction Topological insulator (TI) is a new state of quantum matter with ordinary insulating behavior in bulk but extraordinary electrical conductivity at edges and surfaces. The presence of topologically-protected, gapless, metallic states at the edges and surfaces of a TI allows electrons to be mobile, making it an excellent conductor at the edges and surfaces.1 The surface states of TIs show Dirac electronic structure similar to that of graphene, although with an exciting difference: TIs have an odd number of Dirac points whereas graphene has two Dirac points in its electronic structure.2-5 Such a unique property opens routes for creating new materials, other than graphene, for quantum computing applications.2, 6 Also, high optical absorption and a unique selection rule are reported for TIs, which may open up for a wide range of photonic applications from terahertz detection to optical communications. Further development in this field in the near future is expected to turn TIs into the material of choice for several key applications such as low-power electronics (spintronic), thermoelectric devices and superconductor systems.7-11 The topological order was first observed in electrons confined to two dimensions12 and later found also in some three-dimensional materials such as BixSb1-x alloy, and group V-VI compounds such as Bi2Se3 and Bi2Te3.13-14 Some properties of 3D TIs are strongly thickness dependent.15 A thickness-dependent transition between quantum spin Hall (QSH) and ordinary insulator phases is observed for Bi2Se3, as also predicted theoretically.16 It has been shown that a Bi2Se3 film is in the QSH regime when its thickness is between 2-5 quintuple layers (QL).16 Also, the optical absorption of Bi2Se3 film significantly improves when it is thinner than 6 QLs.7 The 3D-to-2D crossover of the surface states in Sb2Te3 occurs at 4 QLs17, and there is an optimal thickness for topological insulators for maximum spin torque, which is around 3–5 nm for Bi2Se3 films.18 It is therefore important, both for a fundamental understanding and from an applications point of view, to be able to control the thickness of TIs. Both physical and chemical thin film deposition techniques have been explored for growing desired thicknesses of TI films on different substrates. Single crystalline Bi2Se3 films, as thin as 1 QL, have been grown epitaxially on Si substrates using the technique of molecular beam epitaxy (MBE).19 Also, vapor-liquid-solid synthesis and chemical vapor deposition have been successful in the synthesis of thin film TIs, although with

less control over the film thickness.20-21 Besides film deposition techniques, solution process methods such as the hydro/solvothermal process have been widely used for the synthesis of group V-VI topological insulator nanostructures, such as Bi2Se3 and Bi2Te3.22-24 However, this method has not been successful in synthesizing thin TI nanosheets, specifically below 6 nm.25-26 To control the material thickness resulting from hydro/solvothermal processes, it is important to identify and control parameters driving the nucleation and 2D growth of TI nanosheets while suppressing the growth in the other dimension. One such parameter is the use of impurities, which can be intentionally added to the reacting solution where the structures nucleate and grow. Interestingly, impurities can generate new chemical interactions during the crystal growth process, influencing the morphology, phase and composition without being present in the final product,27 by creating guest-host dynamics during the nucleation and growth process. Impurities can either act as inhibitors during the nucleation process28 or poison the growth in certain crystal directions by creating a “kinetic dead zone”.29 With the help of selective surface binding ligands, impurity elements are shown to be selectively attracted towards the surface, inhibiting the growth and hence enabling the formation of nanoparticles with certain facets.30-31 It has been theoretically shown that the adsorption on certain surfaces is energetically more favorable for certain impurity elements.32-33 Once the impurity is adsorbed, the growth is interrupted and finally hindered from that surface. With this approach, nanoparticles with remarkable control over morphology and size have been designed and synthesized.34-36 Here, we demonstrate an impurity promoted thickness confinement during the solvothermal nucleation and growth of bismuth telluride (Bi2Te3) nanosheets. Bi2Te3 is one of the most explored TIs with a single Dirac point on its surface. Bi2Te3 is also known as a p-type room temperature thermoelectric material making this material even more interesting. Bi2Te3 has a layered structure with each QL consists of five atoms: Te(1)-Bi-Te(2)-Bi-Te(1). The Te(1)-Te(1) and the bond between the QLs has van der Waals nature.37 We used iron ions as an impurity to interrupt the growth process and to control the thickness of the nanosheets. Prior to introducing iron ions in the system, we thoroughly studied the growth mechanism of Bi2Te3 nanosheets by monitoring the crystal formation at different stages of the crystal formation using advanced


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electron microscopy and spectroscopy techniques. After understanding Bi2Te3 growth mechanism, by adding a controlled amount of iron precursor to the solution we succeeded in reducing the thickness of nanosheets stepwise from an average of 20 nm (20 QLs) to below 6 nm with nanosheets as thin as 2 nm (2 QLs). Experimental Section Synthesis of Bi2Te3 nanosheets: 0.48 g of Polyvinylpyrrolidone (MW:360000 g/mol-from SigmaAldrich) was dispersed in 30 ml ethylene glycol (anhydrous- from Sigma-Aldrich) by vigorous magnetic stirring at 100 ˚C. Then 15 mM sodium hydroxide (NaOH), 1.7 mM tellurium oxide (TeO2-from Sigma-Aldrich) and 0.6 mM bismuth oxide (Bi2O3-from Sigma-Aldrich) was added to the dispersion and stirred till completely dissolved. The dispersion was transferred to 40 ml Teflon container and sealed in a stainless steel autoclave. The autoclave was then maintained at 180 ˚C for different time intervals ranging from 1.5 h to 40 h (1.5 h, 3.5 h, 5h, 10 h, 15 h, 24 h and 40 h). Thereafter, the material was washed with water several times and then freeze-dried. To avoid any surface oxidation, the as-synthesized material was kept in an argon filled glovebox. Synthesis of Fe-Bi2Te3 nanosheets: Similar synthesis procedure as that of Bi2Te3 with an additional iron precursor was followed. Iron chloride (FeCl3-from SigmaAldrich) was added along with other precursors with various concentrations of 0.06 mM, 0.18 mM, 0.36 mM and 0.55 mM. The synthesis time was the same and kept at 15 h for all the syntheses. Material characterization: The synthesized materials were characterized by scanning electron microscopy (JEOL 6500F, acceleration voltage = 20 kV, beam current = 250 pA and Helios acceleration voltage = 2 keV using the through lens detector with the beam current = 25 pA), high-resolution transmission electron microscopy (JEOL 2100F, voltage = 200 keV), Energy Dispersive Spectrometry analysis (JEOL Silicon Drift (SD) X-ray Detector, active area=10mm², resolution=133 eV) and Electron Probe Micro-Analysis (EPMA) data acquisition (field emission Jeol JXA 8530F Hyperprobe, using 5 Wavelength Dispersive Spectrometers (WDS) and Bi and Te metal standards). WDS elemental mapping was carried out using 30 msec dwell time, in beam mode. Atomic resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) measurements were carried out in a double spherical aberration corrected FEI Titan Themis3 operated at 300 kV. The probe semi-angle and the inner collection angle were set at 30 mrad and 65 mrad, respectively. Crosssectional TEM specimens along [11͞20] of individual Bi2Te3 nanosheets were prepared using focused ion milling in an FEI Helios dual beam microscope. The specimens were thinned down below 50 nm using 30 keV Ga ion beam followed by 2 keV Ga ion polishing for minimizing the effects of ion beam damage. AFM images were acquired on a Bruker Multimode 8 microscope in tapping mode. Results and discussion

Bismuth telluride nanosheets were solvothermally grown at 180 ˚C from TeO2 and Bi2O3 precursors with the stoichiometry between the present elements to be Bi2Te3 established from energy-dispersive X-ray spectroscopy (EDS) and electron probe microanalysis (EPMA) data. XRD pattern of the as-synthesized material is shown in Figure S1 confirming the formation of Bi2Te3. Bi2Te3 grows in the form of hexagonal nanosheets (Figure S2a and b) and exhibits a highly crystalline layered structure confirmed by selected area electron diffraction (SAED) and scanning transmission electron microscopy (STEM) (Figure S2c and S3). The nanosheets’ growth process was investigated by monitoring the product at different stages. For this, isothermal and isobaric experimental conditions with various time intervals (from 1.5 h to 40 h) were conducted to elucidate the mechanism of growth. For analyzing the effect of impurity, in a separate batch, iron was introduced to the system in the form of FeCl3 with different concentrations along with the Bi and Te precursors. A constant growth time of 15 h was chosen for this series of experiments and the resulting material was compared with Bi2Te3 with similar growth time; the iron content was the only variable in this case. No significant change in the pH of the solution was measured after adding FeCl3; for the highest concentration of FeCl3, 0.55 mM, the reduction in pH was ~0.4. We observed a relative decrease in nanosheet thickness by increasing iron content while the structure remained undisturbed (explained later). The process of Thickness confinement is schematically elucidated in Figure 1.

Figure 1. Schematic illustration of thickness confinement process. Two growth paths, with Fe and without Fe, are shown for comparison. The nucleation and growth of Bi2Te3 nanosheets start on the body of Te nanorods in a-b direction, shown in the most left structure. The reaction proceeds and nanosheets grow larger. Without Fe, the top path, the nanosheets tend to attach to each other via van der Waals force and form thicker sheets. The inset STEM image from a corner of a nanosheet parallel to the electron beam shows the layered structure of Bi2Te3; its crystal structure is shown schematically with red and blue spheres representing Te and Bi atoms respectively. With Fe, the bottom path, as the growth proceeds, the iron-surfactant complexes approach the surfaces of nanosheets and inhibit joining of thin individual sheets. Therefore, thinner nanosheets are formed



as the final product in the presence of Fe precursor. Fe atoms do not contribute to the final structure and form iron-rich nanoclusters that can be washed away from the product. The scale bar on the STEM image is 5 nm.

Electron energy loss spectroscopy (EELS) data and Xray photoelectron spectroscopy (XPS) did not show any iron in the nanosheets (Figure S4 and S5). Separate ironrich amorphous clusters and nanoparticles were however found, particularly in the sample with the highest concentration of FeCl3 (0.55 mM).

Figure 2. Typical SEM images acquired at different stages of the growth. a) Formation of Te nanorods, b) and c) Formation of Te nanorods-Bi2Te3 nanosheet HNSs and d) Detachment of the formed Bi2Te3 nanosheets from nanorods. Scale bars are 500 nm and SEM images are false-colored.

To unravel the growth mechanism of Bi2Te3, we first compared the synthesis products at different growth stages. The scanning electron microscopy (SEM) images in Figure 2, acquired at different stages of the growth, illustrate the growth steps. After 1.5 h (the earliest monitored stage), no nanosheets were found. However, a significant number of nanorods were observed in SEM analysis in addition to the precursor salts (Figure 2a and Figure S6). At 3.5 h of the growth, heterogeneous nanostructures (HNSs) which are visualized as nanosheets on the body of nanorod skewers started to form (Figure 2b). Nanorod-nanosheet HNSs have been previously observed in the synthesis of Bi/Sb2Te3 and an improvement in the thermoelectric property of the material has been reported at this stage compared with pure nanosheets.38-39 At 5 h of growth, a larger quantity of nanosheets was grown on each nanorod (Figure 2c and Figure S7), motivating the idea of nanorods being the first product in the growth process as has been shown before.40-41 Wavelength Dispersive Spectrometry (WDS) elemental mapping showed (Figure S8) that while nanorods are purely made of tellurium, both bismuth and tellurium are present in nanosheets regardless of being isolated or attached to the nanorods. In later stages of the growth, more isolated nanosheets can be distinguished while there still exist both HNSs and isolated nanorods (Figure 2d). Finally, nanorods can be rarely found in the product after 40 hours of the growth which was the last analyzed sample (compare Figure S9a and S9b). Based on

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detailed characterization carried out at each stage, the following growth scenario is suggested. TeO2 reduces to Te(0) in the presence of ethylene glycol. This reaction between TeO2 and ethylene glycol promotes the generation of Te(0) in the solution and works as a Te(0) buffer, e.g. feeding the solution with a constant amount of Te(0) as TeO2 is constantly consumed in this redox reaction. The immediate result is that the Te(0) concentration in the liquid increases until the tellurium (Te) nanorods start to nucleate homogeneously in the solution due to the high concentration of Te(0).42-43 Tellurium exhibits hexagonal crystal structure and the rhombohedral lattice would develop mostly in the cdirection, as the a-b plane has a higher atomic density and therefore higher adsorption energy and higher free energy. The dominant growth along the c axis is also favored by the existence of a screw axis along c, motivating the formation of acicular nano-crystals, or nanorods, and not bulky crystals.44 The high growth rates in the incipient growing favor the development of screw dislocations which in turn enhances the growth rate along the c direction. The first stage of the high growth rate of Te nanorods is mainly controlled by the suprasaturation of Te(0) in the solution, which promotes the growth along the screw c axis.45 As the suprasaturation of Te in solution decreases due to the crystallization of Te nanorods, the growth rate along the c axis is inhibited. Te(0) goes through another reduction step and forms , due to the presence of in the solution by reaction (1):43 3 6 → 2 3

(1)

Even tellurium is subjected to two successive stages of reduction, bismuth remains as Bi3+ in solution throughout the experiment (forming metal-surfactant complex which will be explained later), as no Bi(0) was detected. However, in the presence of NaOH and by generation of H2O (through reaction 1), dissolution rate of Bi2O3 is promoted and more Bi3+ is available in solution. By increasing concentration of Bi3+ relative to Te2-, the relative activity of Te in solution is constantly decreasing. Therefore, the growth of Te nanorods is interrupted at this stage by the appearance of first Bi2Te3 which happens when the : ratio in the solution reaches the 2 : 3, or the relative activity of Te reaches to 0.60. The Bi2Te3 phase heterogeneously nucleates on the Te nanorods; this nucleation can be seen, for example, in Figure S10 as a brighter contrast in the STEM image of a HNS. At this stage, the crystallographic and energetic properties of Te and Bi2Te3 strictly control both nucleation and growth. Bi2Te3 can heterogeneously nucleate anywhere on the rod, however, as the a-c plane of Bi2Te3 is bonded by the a-c plane of the nanorod, the growth will always develop along the a-b plane perpendicular to the nanorod (see Figure S11). The lattice mismatch between Bi2Te3 and Te in the a-b plane is negligible, only 1.615 % (a = b = 4.4572 Å for Bi2Te3 and 4.3852 Å for Te), therefore the epitaxial growth is favored. The nuclei of Bi2Te3 can form on the


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tips of the nanorods, where the c axis of the rhombohedral Bi2Te3 coincides with the c axis of the nanorod. However, the A3 axis of the rhombohedral lattice of Bi2Te3 is not a screw axis as in Te, and the growth rate along the c axis will be much slower.

Figure 3. a) Low magnification HAADF-STEM image from a cross-sectional TEM specimen prepared using Focused Ion Beam (FIB) from the middle of a Bi2Te3 nanosheet. The nanosheet is split into two nanosheets on the left while maintained its integrity on the other side. The lattice mismatch between the Te core remained from the parent Te nanorod and the surrounding Bi2Te3 can be seen from the contrast difference in higher magnification HAADF-STEM image taken from the center of the nanosheet. The Bi2Te3 QLs on the top surface and in the surrounding area of the core are marked with dotted lines, b) Atomic resolution HAADF-STEM image from the brown box in “a”, showing the gradual transition from the P3121 Te lattice to the lattice of Bi2Te3 and c) Atomic resolution HAADF-STEM image from the blue box in “a” showing the trace of metallic Te layer on the surface of Bi2Te3 nanosheets. Bi atoms are heavier and therefore appear brighter than Te atoms in HAADF-STEM images. Bi and Te atoms are marked with blue and red circles on the images, respectively. The lattice structure of Bi2Te3 and Te are shown schematically for

comparison. Scale bars in ‘a’ and the zoon-in from the center region are 20 nm and 100 nm respectively. Scale bars in images ‘b’ and ‘c’ are 2 nm.

Relative activity of Te in solution vs. concentration of the two formed components of the system is shown in Figure S12. During the growth of Te nanorods, from the initial point A to the final point R the concentration (and suprasaturation) of Te in liquid solution decreases from A to R which is the point where the relative activity of Te to Bi in solution reaches the value of 0.60. At this point, the first Bi2Te3 nanosheets nucleate and start to grow. During the growth of Bi2Te3, the concentration of Bi and Te ions will reduce in the liquid, in their stoichiometric ratio. It means that the concentration of Te which was already reduced in solution during the growth of the nanorods will be even more reduced. Therefore, in order to continue to nucleate and grow, the Bi2Te3 will feed with the Te from the nanorods by diffusion. At this stage the epitaxial growth would change to a topotactic growth which means that the orientations of the Bi2Te3 crystals are determined by the orientation of the initial/support crystal (Te nanorods). As the reaction is now fed by the Te nanorod itself, the crystallographic orientation of Bi2Te3 does not change, but it partially to totally replace the Te, keeping its former lattice orientation. A crosssectional HAADF-STEM image of a typical Bi2Te3 nanosheet is shown in Figure 3a. This nanosheet seems to detach from its Te nanorod parent shortly before we interrupted the synthesis for the characterization purpose, and a core of Te surrounded by Bi2Te3 is visible in high magnification image from the center area. This observation is a clear evidence that nanosheets are grown on Te nanorods and consume the available Te in the nanorods. The gradual changes of P3121 lattice of Te into the 3 lattice of Bi2Te3 as can be seen in Figure 3b (an atomic resolution image from the brown rectangular area in Figure 3a). In this image, whilst moving away from the center of the nanosheet, the Te lattice with 3 Te atoms46 changes into a QL of Bi2Te3 with 2 Bi and 3 Te atoms matching the schematic crystal structure in Figure 3. Bi atoms are heavier than Te, so they are brighter in HAADF images. The lack of an abrupt interface between the Te core and Bi2Te3 shows that Te nanorods are consumed by Bi2Te3 nanosheets. The co-existance of nanorods and isolated nanosheets implies that the consumption of Te nanorods by Bi2Te3 is not the only reason for the detachment of the nanosheets from their parent nanorods. The early nanosheet detachments can be explained by the small crystallographic mismatch between the a-b planes of the two solid phases. The mismatch between the two a-b planes necessarily creates defects in the lattice that tend to heal over time. The “healing” is related to the chemical potential difference between the two phases. Such chemical potential difference causes Te atoms to diffuse out from the nanorods towards and through the Bi2Te3 in order to feed the growth of the nanosheets. As Te atoms diffuse into the Bi2Te3, the Bi atoms from solution also diffuse along the reverse direction in order to equilibrate the two



different chemical potentials and reach equilibrium. Once the defects are healed on the top layer and the first QL of Bi2Te3 is formed all the way through the surface of nanosheet, the surface has a different atom density of Te from that of the nanorod and hence the nanosheet can easily detach where van der Waals bonding is the only effective force. At this stage many HNSs that are broken from the junction are observed using SEM. Even though the SEM imaging is carried out in the dry state and there is a chance, although very small, that HNSs break during the drop casting and drying steps, the fact that the HNSs break from the junctions confirms the brittleness of these junctions (see Figure S13). As can be seen in higher magnification of the center region of Figure 3a, Bi2Te3 QLs are distinguishable on the top surface of the nanosheet (marked with dotted lines). During the healing process, extra Te atoms which have not contributed in the lattice need to find their way to the liquid phase by diffusion. The most energetically favored path is formed by breaking the van der Waals force between two Bi2Te3 QLs in a random position within the nanosheet. The Te ejection from the system results in splitting the nanosheets to thinner nanosheets, while leaving a trace of Te in the form of one metallic layer of Te with similar 3 atom lattice structure (see Figure 3c). This phenomenon explains the appearance of the Te layers only on one surface of the isolated Bi2Te3 nanosheets. The reaction consuming Te continues until the Te nanorods are completely dissipated. At this time, the reaction stops, as it reaches equilibrium between Bi2Te3 nanosheets and the liquid phase and all the nanosheets are detached. The main role of NaOH in this process is to catalyze the reduction of Te(0) with reaction (1).43 However, it is shown by Wang et. al. 47 starting with Te(0) and in the absence of NaOH, Bi2Te3 can form through a direct combination of metals which can only occur if Bi(0) is also available. In our case, in the absence of NaOH, we observed the formation of Te nanorods at 15 h of the growth, indicating that Te nanorods nucleate from Te(0). Besides Te nanorods, bulk Bi2Te3 structures (confirmed by XRD) were also formed in the absence of NaOH as shown in Figure S14. This confirms the previous results showing that alkaline additives are not necessary to form Bi2Te3 in solvothermal process but its presence strictly define the morphology.48 Additionally, in order to investigate the possible effect that NaOH might have on the thickness and morphology of nanosheets, we repeated the synthesis with a significantly lower concentration of NaOH (7.5 mM compared with 15 mM used for all other experiments) and as a result pH reduced by 4. At this condition, considerably larger number of nanorods were observed compare to the sample prepared by 15 mM of NaOH, at the same growth stage at 15 h (see Figure S15). In addition, while the shape and size of nanosheets are comparatively similar, much less number of them was formed at this stage. This test confirms the formation of nanorods from Te(0) as the first product. Due to the lower amount of NaOH, more Te(0) have a chance to form nanorods without being reduced to Te2- and hence nanorods can

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grow longer as can be seen in Figure S16. In addition, we need to emphasize that, in the presence of PVP as an organic structure-directing agent, the single crystalline Bi2Te3 seed with a highly anisotropic structure tends to grow into a 2D sheet.42-43

Figure 4. a) Relationship between the number of Bi2Te3 QLs and the initial concentration of FeCl3 in the solution. Data is extracted by statistical thickness measurements from AFM and cross-sectional STEM images, b) Comparison of thickness distribution between Bi2Te3 and Fe-Bi2Te3 with 0.55 mM FeCl3 in the starting solution and c) Low magnification HAADF-STEM image from a cross-sectional TEM specimen prepared using FIB from a Fe-Bi2Te3 nanosheet with 0.55 mM FeCl3, along with an atomic resolution image from the center showing the thickness of the nanosheet to be only 3 QLs. The lattice structure is fully preserved. Te and Bi atoms are marked by red and blue circles. Scale bars are 100 nm and 2 nm.

We then investigated the effect of iron impurity in the manipulation of the Bi2Te3 crystal growth. We introduced different quantities of Fe precursor in the initial solution and kept the other growth parameters exactly the same as before. After the growth, we could not detect any Fe in the nanosheets and nanorods by EELS and XPS, confirming that if there is any Fe in these structures, its concentration is negligible (see Figure S4 and S5). Although Fe is not present in the final growth products, it has a significant effect on their thickness. Cross-sectional HAADF-STEM images and AFM results show that the final thickness of the nanosheets is strongly affected by the presence of Fe precursor in the growth solution. AFM images of typical Bi2Te3 and Fe-Bi2Te3 nanosheets (with 0.36 mM FeCl3) are shown in Figure S17. Our statistical analysis on AFM images of nanosheets suggests that the final thickness of the nanosheets is directly related to the quantity of the Fe precursor as demonstrated in Figure 4a. The average thickness of nanosheets is reduced by ~73 % by introducing 0.55 mM of FeCl3 in the growth environment. Thickness distribution analysis, shown in Figure 4b, reveals a well-separated thickness distribution


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centering at ~20 nm for Bi2Te3 and at ~5 nm for Fe-Bi2Te3 (0.55 mM FeCl3) from the Gaussian fit. Nanosheets with the thickness as low as 2 QL were grown by using 0.36 mM and more FeCl3, as confirmed by AFM and crosssectional HAADF-STEM imaging shown in Figure 4c and S18. In addition, as can be seen in Figure S19 the thickness distribution was more homogenous for the sample with 0.36 mM FeCl3 (with an average thickness of 5.8 nm from statistical analysis of 46 nanosheets) which suggests that there is a limitation in impurity content for which a higher amount introduces structural damages. To ensure that pH variation does not contribute to the thickness confinement, we measured the pH of each solution before the growth. We observed that the pH reduced by a maximum of 0.4 by addition of 0.55 mM FeCl3 and hence any contribution from pH variation should be negligible.49-50 Surfactants are usually used in the nanocrystal growth to moderate the growth rate by passivating the dangling bonds on the surface. In fact, surfactants and ionic impurities play conceptually similar roles during the crystal growth and neither incorporates in the structure of the final material. Ionic impurities usually incorporate with surfactants to form impurity-surfactant complexes. A similar complex is formed by the interaction of the main element and surfactant only if both impurity and main element have similar chemical valence.27, 51 In our case Bi and Fe are both trivalent and hence surfactant molecules form chemical bonds not only to elements of the growing crystal but also to Fe ions in the solution. Both complexes are attracted to the surface of nucleated crystals. When the impurity-surfactant complex approaches the surface, if the binding energy within the complex is weak, dissociative adsorption of the impurity occurs. A compatible element impurity would influence the composition of the nanosheets, forming doped or alloy structure. However, Fe is not a compatible element for substitutional doping of Bi2Te3 and also diffusion of Fe in Bi2Te3 nanocrystal seems unlikely at the low-temperature synthesis condition.52-53 With this argument, the growth rate along a certain direction is inversely correlated with the ability of impurity to incorporate into the crystal and hence the choice of impurity element is crucial for the manipulation of the growth process. To be adsorbed on the surface, the impurity element needs to overcome a large energy barrier which strongly depends on the surface of the growing unit. This is not energetically favored and in most cases, impurity elements are ejected from the crystal surface.54 The other scenario occurs when the binding within the impurity-surfactant complex is stronger than the adsorption energy and hence the impurity does not have any chance to be adsorbed on the surface of the growing crystal. They stay in the complex form and later either remove during washing or form nanoparticles due to micelle formation.27 In our case, we could not detect iron in the structure of nanosheets based on EELS analysis. If there is any iron in the structure, its concentration is below the detection limit of the EELS technique. Regardless of which scenario occur in our case,

the presence of iron only influences the crystal growth. As can be seen in the HAADF-STEM image of nanosheets at an early stage of the growth (3.5 h) in Figure S20, thin layers of Bi2Te3, if nucleated in the close vicinity of previously formed nanosheets, attach to them due to the Van der Waals attraction force. This phenomenon can be seen in the SEM image of the HNS shown in Figure S13 and many more (see Figure S21 and S22 as some examples). The two nanosheets that are still attached to the parent Te rod, are each comprised of at least two stacked nanosheets. They are attached to each other forming thicker nanosheets (compared to the individual ones). The stacked nanosheets are easily distinguishable as they have slightly different orientation and lateral size. This observation can explain the formation of thinner nanosheets when the iron is present in the system. In the presence of iron, nanosheets are exposed to the iron-rich environment and hence their surface is attacked by ironsurfactant complex (schematically shown in Figure 1). Iron impurities either dissociate on the surface or remain in the complex form, inhibit the joining of the newly formed layers to the previous ones and hence depending on the amount of iron, nanosheets with fewer number of QL are formed. Fe-Bi2Te3 nanosheets have a larger lateral size distribution and appear as elongated hexagons compared to Bi2Te3 (see Figure S23). This change in the shape can be easily explained based on the abovementioned argument. In the case of pristine Bi2Te3, the stacked layers have usually dissimilar lateral size and sometimes the top layers (consist of few QLs) have more elongated hexagonal or trigonal shape as can be seen from the top view STEM image shown in Figure S24a. As iron impurity inhibits the attachment of the layers during the growth, they appear as isolated nanosheets with more random size distribution than Bi2Te3 nanosheets and more nanosheets appear as deformed hexagonal and even trigonal (compare Figure S24a and S24b). Indeed, that has not been contributed into the structure of the nanosheets can be further reduced in the reducing environment and form () nanoparticles in the presence of . It is also plausible that FeCl3 partially react with NaOH forming () from the beginning, especially at higher concentrations of FeCl3. Interestingly, the Te monolayer forms on the surface of Bi2Te3 nanosheets in the presence of iron impurity, confirming the same growth mechanism explained before. Figure S25 shows cross-sectional STEM image of two Fe-Bi2Te3 nanosheets on top of each other synthesized with 0.36 mM of FeCl3 during the growth. A monolayer Te is easily recognizable on the surface of both nanosheets. Similar Te monolayer is also observed in vapor transport epitaxial growth of Te-seeded ultrathin Bi2Te3 nanoplates.55 Dissimilar to many other 2D materials such as graphene and transition metal dichalcogenides, Te cannot be exfoliated from bulk and single layer Te can only be grown on a substrate. The Te layer in our case forms on the surface of Bi2Te3 and orientation of Te atoms implies the formation of 2D -Te.56 -Te is a semiconductor with three to four times smaller number



of charge carriers than molybdenum disulfide and it is suggested as a better candidate for electronic applications.57 The interface between the few layer of Bi2Te3 and 2D Te could have interesting physical properties that can be applied in electronic devices. Conclusion We developed a solution process synthesis pathway to confine the thickness of Bi2Te3 topological insulator in the form of nanosheets during crystal formation. The idea is based on a thorough understanding of the growth mechanism and then manipulating the crystal growth by intentional addition of impurity elements (iron) during the growth. We observed a linear relationship between the thickness of nanosheets and concentration of iron precursor in the solution while iron was not detected in the final product and was fully expelled from the nanosheets. By a stepwise addition of FeCl3, we successfully reduced the thickness of nanosheets from an average of 20 nm (20 QLs) to below 6 nm, with nanosheets as thin as 2 nm (2 QLs), while the crystal structure and morphology was preserved to a high extent. In addition, we observed the formation of a monolayer tellurium on the surface of Bi2Te3 with possible interesting properties at the interface.

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large number of Fe-Bi2Te3 nanosheets, Figure S23: A color and contrast modified top view STEM image of a Bi2Te3 nanosheet compared with SEM image of Fe-Bi2Te3 nanosheets, Figure S24: HAADF-STEM image from a cross sectional TEM specimen prepared using FIB from two FeBi2Te3 nanosheets stacked on top of each other.

AUTHOR INFORMATION Corresponding Authors Prof. Pulickel Ajayan, [email protected] Dr. Tiva Sharifi, [email protected]

Author Contributions T.S. performed the experimental part, part of scanning electron microscopy, made most of the figures and was responsible for writing. S.Y. performed the transmission electron microscopy, part of scanning electron microscopy and contributed in writing. G.Costin performed the EPMA measurements, made the analysis for the growth mechanism and contributed in writing. A.A did the AFM measurements. G.Coulter made the schematics. C.T contributed in discussions and P.A supervised the project and contributed to the most of analysis, discussions and the writing. All authors took an active part in scientific discussions and finalization of the manuscript.

ACKNOWLEDGMENT ASSOCIATED CONTENT Supporting Information. Figure S1: XRD pattern of Bi2Te3 sample, Figure S2: a) SEM, AFM and SAED pattern of a Bi2Te3 nanosheet, Figure S3: STEM image from the corner of two stacked nanosheets parallel to the electron beam demonstrating its layered structure, Figure S4: EELS spectra of Fe-Bi2Te3 sample prepared at the highest concentration of FeCl3, Figure S5: XPS spectra of Bi2Te3 and Fe- Bi2Te3, Figure S6 and S7: SEM images of the early stage of Bi2Te3 nanosheet formation, Figure S7: WDS mapping for Bi and Te in nanosheets and nanorods, Figure S8: SEM images of the synthesis products with similar magnification at 10 h and 15 h of the growth, Figure S9: STEM image of a Bi2Te3 HNS, Figure S10: SEM image of a HNS at its early stages of growth, Figure S11: Relative activity of Te in solution vs. Concentration, Figure S12: False-colored SEM image of a HNS at a middle stage of growth, Figure S13. SEM images of the products in the absence of NaOH in the solution with an inset of the XRD pattern, Figure S14: SEM images of the synthesis products with similar magnification but different concentration of NaOH at 15 h of the growth, Figure S15: SEM images of the synthesis products at 15 h of the growth by using 7.5 mM of NaOH, Figure S16: Typical AFM images of Bi2Te3 and Fe-Bi2Te3 nanosheets, Figure S17: Cross-sectional STEM images of a 2 QL Fe-Bi2Te3 nanosheet formed in the presence of 0.55 mM FeCl3, Figure S18: Thickness distribution of Fe-Bi2Te3 using 0.36 mM FeCl3 in the starting solution, Figure S19: STEM image of a Te nanorod-Bi2Te3 nanosheet HNS, Figure S20: a side view SEM image of a Te nanorod-Bi2Te3 nanosheet HNS, Figure S21: SEM image of a Te nanorod-Bi2Te3 nanosheet HNS, Figure S22: SEM and AFM images of a

T.S. acknowledges Swedish Research Council (Grant No. 2015-06462) for the support. The use of the EPMA facility at the Department of Earth, Environmental and Planetary Sciences, Shared Equipment Authority (SEA) and Electron microscopy Center (EMC) at Rice University, Houston, TX, is kindly acknowledged.

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Impurity controlled crystal growth in low dimensional bismuth telluride

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