Bi2Te3 Plates with Single Nanopore: the Formation of Surface Defects

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Bi2Te3 Plates with Single Nanopore: the Formation of Surface Defects and Self-Repair Growth Chaochao Dun, Corey A. Hewitt, Qike Jiang, Yang Guo, Junwei Xu, Yan Li, Qi Li, Hongzhi Wang, and David L. Carroll Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04985 • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 19, 2018

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Chemistry of Materials

Bi2Te3 Plates with Single Nanopore: the Formation of Surface Defects and Self-Repair Growth Chaochao Dun1, Corey A. Hewitt1, Qike Jiang2, Yang Guo1, 3, Junwei Xu1, Yan Li4, Qi Li5, Hongzhi Wang 3, David L. Carroll1 1

Center for Nanotechnology and Molecular Materials, Department of Physics, Wake Forest University, Winston-

Salem NC 27109, USA 2

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

3

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science

and Engineering, Donghua University, Shanghai 201620, P.R. China. 4

Department of Physics, Wake Forest University, Winston-Salem NC 27109, USA

5

Physical Science Division, IBM Thomas J. Watson Center, Yorktown Heights, NY 10598, USA

*Corresponding author: David L. Carroll *Email: [email protected]

Abstract Self-assembly has proven to be a powerful method of preparing structurally intricate nanostructures. In this work, we design a nano-scale “Chinese Coin” based on Bi2Te3 nanoplates (NPs) by using a simple and scalable solution process, i.e. a single pore is introduced on a hexagonal/round plate similar like a fender washer. The diameter of the nanopores is well controlled within the range of 5-100 nm, and depends strongly on the reaction time and heating temperatures, suggesting a kinetics related mechanism. Moreover, the thermal evolution of stable Bi2Te3 plate-pore structures was systematically explored to elucidate the underlying energetics of the V2-VI3 chalcogenides. We found that the nanopore is initiated near the middle of the plate, followed by the successive removal of Bi2Te3 slices from the high edge-energy pore with increased temperatures (70~150 ◦ C), leading finally to the formation of a stable nanopore. The morphology of the pore as well as the local lattice crystallinity were studied using highresolution transmission electron microscopy and First-principles calculations. Based on these observations, a self-repair mechanism for pores under the stability diameter is proposed from the viewpoint of reaction kinetics.

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Introduction Precisely controlled crystal synthesis with novel nanostructures represents an area of intense research because of the promising electrical/chemical properties promised in the unique morphologies

1–4

. This means understanding the co-related formation mechanisms in the local

environments of strain and density fluctuations. Of course, this is particularly true for twodimensional (2D) based nanostructures 5, which have already found a wide range of applications in thermoelectrics

6,7

, superconductor-topological insulators

8,9

and spintronics.

10,11

Such

applications are being pursued now due to the unusual electrical and thermal transport properties derived from the layered atomic structures. Recently, research in the area of nanopores in 2D nanosheets has gained more notice.

12–15

For

instance, the observation of ionic Coulomb blockade effects resulting from the quantization of charge and occupancy, a well-known electrostatic phenomenon controlling conduction in tunnel junctions, is found to be dominant in water-filled nanopores in MoS2 monolayers.

12

Related

theoretical and practical studies of MoS2 nanopores has demonstrated them to be an efficient osmotic generator to power a transistor resulting from a salt concentration gradient

16

, with the

estimated power density reaching up to 106 W/m2. Ultrathin nanopore membranes have also captivated the field of DNA sequencing. 17,18 For instance, graphene is a gapless semiconductor, which allows for the use of nanopores in graphene to electrically detect and quantify DNA translocation using the ionic current through the nanopore. 15 In field of thermoelectric generator, the appearance of voids or nanopores is believed to be beneficial to suppress the lattice contribution of the thermal conductivity by disrupting the crystal's phonon while maintaining a competitive power factor. 19 Experimentally, the most common techniques to create nanopore on membrane include high energy electron-beam lithography (EBL) 16,18,20,21, or electrochemical reaction (ECR) technology 12,15

. For thermoelectric applications, related studies on nanopore are intrinsic inter-particles

voids between grains that are generated in the post-treatment process like spark-plasma-sintering (SPS) or hot-pressing.

19

Alternatively, direct synthesis of ordered nanopores is somewhat

challenging and largely confined to the use of templates. So far, very few studies exist about reaction-engineered single nanopore on nanoplates (NPs).

22,23

Although these are mainly on

metal, controlled chemical synthesis of single nanopore on complex 2D systems such as the 2 ACS Paragon Plus Environment

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metal-chalcogenides could potentially unlock opportunities in new thermoelectric materials

24–26

and topological insulators for quantum information processing. 27,28 In this report, we have developed a solution-based approach to the creation of well controlled, individual nanopore in thin Bi2Te3 platelets. This was done by taking advantage of the nanoplates’ thermal-stability in ethylene glycol (EG). Depending on the reaction time and heating temperature, nano-engineering the specific diameters, morphologies, and “crystallinities” of the nanopore was achieved. We further demonstrate that the process is completely reversible, which provides another template to generate lateral heterojunctions or modulation doping. The “selfrepair” mechanism of reversibility is based upon reaction kinetics and follows the various stages of morphologies seen in initial pore formation. This solution-processing method may well allow for printable formulations of this functional material,

29

and not only highlights the possibilities

in studying anisotropic growth mechanisms in 2D nanostructures, but also provides a new platform

for

nanopore-based

applications

in

bio-sensing,

thermoelectrics and quantum computing.


diagnostics,

sequencing,


Figure 1. (a) Schematic fabrication process of individual nanopore in Bi2Te3 NPs using a wetchemical approach, with (b)-(e) illustrating the bright-field TEM images of the intermediate products obtained at various heating temperatures. The corresponding element mapping of Bi2Te3: (f)-(g) and (h)-(i) are the distributions of Bi/Te for hexagonal and round nanostructures, respectively. The corresponding HAADF images and EDS analysis were given in SI, revealing a similar Te/Bi ratio regardless of the morphology.


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Figure 1 (a) gives a schematic fabrication process of generating single nanopore in Bi2Te3 NPs. This is given from the viewpoint of the processing temperatures in ethylene glycol (EG) solution. To start, perfect hexagonal Bi2Te3 NPs were fabricated, with basal {0001} planes parallel to the plate face. Characterizations of the intact Bi2Te3 NPs are given in Figure S1-S4 including XRD, Raman, XPS, AFM, TEM, and HRTEM analysis. This reveals a high crystallization quality of the as-fabricated plates for our starting materials. Subsequently, the intact NPs were subjected to post-processing at different heating temperature from 25 to 150 ◦ C, resulting in the re-dissolved Bi/Te atoms in EG and nanopores with varied morphology. The yield of Bi2Te3 NPs with single nanopore is greater than 95%. Details of the fabrication procedure can be found in the experimental section of SI. Figure 1(b)-(e) shows the preliminary TEM images of Bi2Te3 nanostructures synthesized at different heating temperature (25, 70, 120 and 150◦ C) for 2 hours, indicating an obvious transformation process from an intact plate to a hexagonal sheet with an irregular pore in the middle, and finally to a round disc with a hexagonal pore like a nano-scale “Chinese Coin”. Therefore, it is safe to conclude that the morphology of this unique nanostructure can be controlled by the heating temperature. Moreover, with the same reaction time, the pore size can also be adjusted. For instance, the size of the nanopore is increased from 20 to 50 nm when heating temperature is increased from 70 to 120 ◦ C for 2 hours. Additionally, the morphology and size can also be controlled by the reaction time. More low-resolution TEM images reflecting the structural evolution of this system are given in Figure S5. In short, nanopores with different diameters can be controlled in the range of 5-100 nm, with a reaction time from 30 minutes to 7 hours, at heating temperature from 70 to 150 ◦ C. The detailed elemental mapping corresponding to the hexagonal (Figure 1(c)) and round Figure 1(d)) nanostructures were given in Figure 1(f)-(i), which confirm the homogeneous distribution of Te and Bi regardless of the morphology, and visualize the morphology of the nanopores in Bi2Te3 plates. Related high-angle annular dark-field images (HAADF), TEM, HRTEM and selected area electron diffraction (SAED) analysis are also shown in Figure S6. Moreover, the corresponding TEM-based energy dispersive spectroscopy (TEM-EDS) data were given in Figure S7, revealing Bi and Te atom ratios that are 2:2.6 and 2:2.7 for hexagon and round Bi2Te3 NPs with nanopores, respectively. Firstly, all nanostructures were Te-deficient, which is an indication of being an n-type semiconductor. Moreover, the approximately unchanged Bi/Te ratio demonstrated that the composition was preserved during the structural evolution process. 5 ACS Paragon Plus Environment


Therefore, under this temperature range (70-150 ◦ C), it is believed that the dismantling occurs to the entire quintuple layers (QLs, each unit cell of Bi2Te3 contains five atomic layers with a stacking sequence of -Te(1)-Bi-Te(2)-Bi-Te(1)-) rather than being limited to individual atoms like observed under higher temperatures reported previously 5. Compared with the preferential sublimation of Te which occurs between 350 and 450 ◦C, in this case, the primary mechanism of the morphology transformation is the re-arrangement of atoms in the (110) plane accompanied with the break of Van der Waals forces between QLs, instead of the break of chemical bonds between Bi and Te along the (0001) or direction of the crystallographic orientation.

Figure 2. (a) Low-resolution TEM and different parts of high-resolution TEM images of a typical hexagonal plate with an irregular nanopore fabricated at 70 ◦C for 2 hours, including the transition area (b), the inter pore (d)-(e), and the outer edge (f). The corresponding SAED pattern is also given (c). The insert scale bar represents 1 nm. To reveal the detailed growth mechanism of pore on the plate, investigation of the edge structure at an atomic scale is extremely important. Figure 2 describes the TEM/HRTEM characterizations at different parts of a typical hexagon plate with nanopore fabricated at 70 ◦C 6 ACS Paragon Plus Environment

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for 2 hour, including the inter pore (part I), the outer edge (part II) and the transition area (part III). A well-resolved lattice fringe space of 0.219 nm is observed at transition areas (Figure 2(b), corresponding to the (110) plane of hexagonal Bi2Te3 phase, which is further confirmed by the diffraction ring in the SAED pattern (Figure 2(c)). Therefore, no change in the crystal structure occurred in the plate except the edges (both outer and inner edges). As described in Figure 2(d), bending contours are observed around the inter pore, which also clearly shows that there is hollow space within the plate. Besides, both the inner and the outer pore exhibit a higher degree of amorphous phase compared with the middle transition area, marked as the disorder layer in Figure 2(e)-(f). The zoomed images in the insert further verified amorphous states near the edges. The size of the pore will be increased slightly with the prolonged reaction time at 70 ◦C, and its morphology gradually approaches regularity during this process. As can be seen in Figure S8, when the reaction time is prolonged to 4 hours, the size of the pore grows up to 45 nm, with a 5 nm disorder layer around the inter edges. At the same time, the round pore gradually becomes closely hexagonal. Further extension of the reaction time will not affect the pore size, which remains around 45 nm (Figure S9). Nevertheless, the amorphous layer with width around 7-10 nm exists nearby all edges of the Bi2Te3 plates at 70 ◦C, demonstrating the release of stress during this stage. Subsequent experiments suggested that the morphology transformation of the nanopore also has a strong dependence on the reaction temperature. Figure 3 describes the TEM analysis of a typical round disc with a hexagonal pore fabricated at 150 ◦C for 4 hours. A similar discussion on the ones with reaction time around 2 hours is given in Figure S10. When the heating temperature was increased up to 150 ◦C, the size of the pore further increased from 20 to 60 and finally 95 nm. Meanwhile, the as-formed nanopore with bend edges would ultimately grow into a perfect hexagon with higher structural regularity. This is because the bend edges containing more atom steps result in a higher surface energy than the straight ones, causing the bend edges to gradually rearrange to a straight cut at certain crystallographic planes to keep a low energy state. As shown in Figure 3(a) and (b), the explored planes were determined as (1100) , (0110) and (1010) facets, which are equivalent to each other for a rhombohedral crystal structure with a space group of R 3m

(Figure 3(c)). It is also noteworthy that the disorder/amorphous layer shown at inner edges

of the plate under 70 ◦C would disappear at 150 ◦C with enough reaction time. Instead, a highly crystalline phase is observed around the pore, as given in the zoomed part in Figure 3(d) and (e). 7 ACS Paragon Plus Environment


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The intersecting angles between the adjacent edges of the pore are around 120 degree, also implying the high crystalline phase. More low-resolution TEM and optical images to verify the high yield are given in Figure S11.

Table 1. Formation energy of two typical lateral surfaces based on density functional theory (DFT). As a comparison, the pristine {0001} surface was also considered. Surface

{0001}

{0110}

{2110}

Energy (eV/ Å2)

0.022

0.079

0.118

First principle calculations based on density functional theory (DFT) were used to rationalize the structure evolution of the nanopore, with calculation details given in SI. Since the surfactant is believed to preferentially bind to the basal outermost Te, only the surface energy of {0110} and

{2110} facets with exposed Te planes were calculated and summarized in Table 1. It is shown that the surface energy is 0.079 and 0.118 eV/Å2 for {0110} and {2110} facets, respectively, confirming that the desorption rate of {0110} planes is indeed more energetically favorable. Therefore, the nanopore will demonstrate a hexagon shape with enough energy at 150 ◦C, which is in agreement with the observed experimental results. At this stage, the disorder layer still exists nearby the outer edges around 3 nm (Figure 3(f)), while a complete transformation from amorphous to crystallization state is observed at the inner edges, resulting in a regular pore.


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Figure 3. Low-resolution TEM ((a) and (b)) and different parts of high-resolution TEM images (d, e and f) of a typical round disc with a hexagonal pore fabricated at 150 ◦C for 4 hours. The corresponding SAED pattern is also given in (c). These detailed temperature-dependent studies on the structural evolution reveal the morphologycontrolled synthesis of Bi2Te3 plates with nanopores in the absence of sacrificial templates, which not only produces a new nanostructure but also enables a new platform for nanoporebased applications: for example, these exposed interfaces may effectively scatter phonons to reduce the thermal conductivity 19. Related works are still ongoing. Under higher temperatures around 185 ◦ C (the boiling point of EG) and enough reaction time (from 2 hours to 5 hours), self-repair is mediated by the reassembly of Bi2Te3 particles onto the edges (both inner and outer sides) of the main Bi2Te3 plate. The detailed self-repair and closure process of the nanopore is systematically investigated on the basis of the TEM images of the intermediate products at different reaction times. As an example, Figure 4(a) shows a typical TEM image for a mid-product with a nanopore around 75 nm fabricated for 2 hours at 185 ◦ C. During this stage, small bulges are observed on the inner and outer edges of the plates as shown 9 ACS Paragon Plus Environment


in Figure 4(c) and (e), suggesting the attachment of small Bi2Te3 nanoparticles which finally fuse to the plate and results in the closure of the pore and constant repair of the defect. The formation of bulges occurs only at high temperature with enough reaction time, which is in agreement with the fabrication process of obtaining the intact Bi2Te3 plate as in previous reports. HRTEM images elucidate more details of the initial stage of the self-repair growth, or the integration of newly generated nanoparticles and the original NPs. Initially, a miss-match of the lattice fringes is observed, which is only metastable since this state possesses a higher binding energy. The re-arrangement of atoms occurs with the increased reaction time to decrease the angle between the newly generated nanoparticles and the main Bi2Te3 plate until all atoms share the same crystallographic orientation. The related HRTEM is described in Figure S12. Actually, it should be noticed that, under high temperature at 185 ◦ C, both the inner and outer edges show a high degree of crystallization Figure 4(d)-(f), which is obviously different from that of the lower temperature; that is, amorphous layers at both edges at 70◦ C, and amorphous outer edges with crystallized inner edges at 150◦ C. This demonstrates that the inner edges have a higher priority of structural transformation than that of the outer ones during all stages. With the help of surfactant, an intact plate having a highly ordered crystallization state without defects can even be formed, similar to the Bi2Se3 nanodiscs reported previously 30. Finally, to maintain a relatively lower surface energy state, the outer edges of the nano-discs will preferentially transform into hexagonal shape with enough governed by the well-known Oswald-Ripening when the reaction time was prolonged to more than 5 hours.


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Figure 4. Self-repair of the Bi2Te3 NPs at 185 ◦ C for 2 hours. (a) Low-resolution TEM images, showing the self-assembled nanoparticles around the edges. The corresponding SAED pattern is given in (b). The zoomed part around the inner pore (c) and high-resolution TEM images (d). The zoomed part around the outer pore (e) and high-resolution TEM images (f). On the basis of the above analysis, we postulate the overall mechanism behind the growth: as the crystal stress is the highest around the center of the Bi2Te3 plates, a defect is created nearby, resulting in the single nanopore under a mild heating temperature at 70 ◦ C. The morphology evolution of the nanopore is triggered with increased reaction time in which a higher amorphous phase exists around the edges. When the heating temperature is high enough around 150 ◦ C, the as-formed nanopore with less structural regularity would ultimately grow into a regular hexagon with highly crystallization to achieve the lowest surface energy. The dismantling of the prismatic {0110} facets is more energetically favorable. Self-repair of the nanopore occurs at a temperature

around 185◦ C, first with aggregation of small Bi2Te3 nanoparticles and then the reorientation of lattice fringes. Therefore, the size of the pore can be well controlled by the reaction time, with the morphology determined by the heating temperature. During all stages, the inner edges have a higher priority than that of the outer ones when it comes to the structure transformation, resulting 11 ACS Paragon Plus Environment


in unique nanostructures including hexagonal Bi2Te3 NPs with irregular or regular pores, and round Bi2Te3 NPs with or without hexagonal pores. To our best knowledge, this is the first example of V2-VI3 2D nanostructures achieved using a simple solution-phase approach without lithographic methods. In particular, the formation of single a pore is only based on a mild reaction condition, i.e. a relatively low heating temperate without using strong acids or a hard template. Moreover, the nanopore fabricated here can be dispersed in water or ethanol, and then spin/dip coated on any substrates without a complex transferring process, which also eliminates the hydrophobic effect compared to grapheme with mechanically introduced nanopore.

Conclusions Inner and outer edge structures of thermally treated Bi2Te3 NPs have been systematically studied by means of atomically resolved high-resolution TEM. A single nonopore with varied morphologies was generated. Between 70 and 150 ◦ C, both heating temperature and reaction time are critical to the growth, including the shape/size and crystallization status of the nanopore. On the contrary, at high temperatures beyond 185◦ C, the attachment of newly generated particles on the pore results in closure of the pore and finally self-repair of the platelets. Self-attachment of small nanoparticles are validated by numerous bulges, followed by epitaxial recrystallization into single crystals that guarantees the nano-discs and the rebirth of intact Bi2Te3 NPs. The observation of pore growth, morphology control and the reverse process are insightful as they reveal mechanisms related to crystal growth of the 2D nanostructured Bi-, Sb-, Cu-, Sn- based chalcogenides. These results are believed to be of great interest to study the topological difference and the related topological electronic phases of the V2-VI3 group.


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Acknowledgements The authors appreciate great help and discussions from Dr. Natalie. A. W. Holzwarth on first principles calculations from Department of Physics, Wake Forest University. The authors acknowledge the U. S. Air Force Office of Scientific Research Grant FA9550-16-1-0328 and NASA/Streamline 1123-SC-01-R0 NASA #NNX16CJ30P for the financial support and the use of equipment from the Center of Nanotechnology and Molecular Materials at Wake Forest University. This work was also partial supported by the National Natural Science Foundation of China (No. 51701201). The authors declare no competing financial interest.

Supporting Information. The detailed fabrication process, computational methodology, and more characterizations include XPS, EDS, AFM, XRD, TEM, HAADF and HRTEM of the materials are supplied as Supporting Information.

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Bi2Te3 Plates with Single Nanopore: the Formation of Surface Defects

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