Solid State Intercalation, Deintercalation, and Cation Exchange in

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Solid state intercalation, deintercalation and cation exchange in colloidal 2D BiSe and BiTe nanocrystals 2

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Joka Buha, and Liberato Manna Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b05440 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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Chemistry of Materials

Solid state intercalation, deintercalation and cation exchange in colloidal 2D Bi2Se3 and Bi2Te3 nanocrystals Joka Buha, Liberato Manna Department of Nanochemistry, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy

KEYWORDS: intercalation, deintercalation, cation exchange, in situ TEM, colloidal nanocrystals, Bi2Se3, Bi2Te3.

ACS Paragon Plus Environment

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ABSTRACT

Intercalation and deintercalation of Cu into and from Bi2Se3 and Bi2Te3 colloidal nanocrystals is demonstrated in solid state. For this purpose, the nanocrystals are deposited on a solid substrate and subjected to moderate heating in the presence of a Cu source or a Cu scavenger. The results of the solid state experiments are consistent with those produced by solution-based intercalation. At moderate levels of solid state intercalation above about 10 at%, Cu forms superlattice structures in both Bi2Se3 and Bi2Te3, while higher intercalation levels ultimately lead to cation exchange. This is particularly the case in Bi2Se3 which transitions to Cu2-xSe whilst maintaining the two-dimensional morphology of the starting nanocrystals. The solid state intercalation is reversible and Cu can also be deintercalated from the Bi2Se3 and Bi2Te3 nanocrystals upon heating and utilized in other reactions on the solid substrate. The extent of intercalation and deintercalation however depends on any interaction between the intercalant and host nanocrystals in response to heating. The concepts explored here hold promise for future development of novel solid state based energy storage nano-devices, or possibly NC-based devices whose properties and/or function could be reversibly modulated through a combination of solid state CE and intercalation/deintercalation.


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INTRODUCTION Colloidal synthesis is emerging as a facile and cost-effective method for the fabrication of various nanomaterials., allowing tuning of the nanocrystals (NC) size, composition and morphology and thereby providing access to a broader range of material’s properties.1,2 Once synthesized, the properties of colloidal NCs can be further tuned via several solution-based methods that can modify e.g. the functionality of the molecules covering the surface of the NCs, or the composition and even the structure of the NCs themselves.1-4 Ultimately however, a widespread application of colloidal NCs would in most cases require their use in solid state, therefore without the presence of the liquid solution and the possibility for facile chemical reactions that such an environment provides. Methods enabling controlled, localized or even reversible modification of the properties of colloidal NCs in solid state therefore need to be developed. With that in mind, the present study explores some of the pathways for solid state chemical reactions in colloidal Bi2Se3 and Bi2Te3 NCs based on two established types of reactions exploited in solution, namely cation exchange (CE) and intercalation. In the solutionbased CE reactions, the cations in the NC are substituted by cations of a different kind present in the solution, while the NC anion framework serves as a template for the compositional and often also structural transformation involved in the process.3,4 CE reactions have been used to produce a number of different NC compounds of varying morphologies, including those not directly accessible by colloidal synthesis 3,4 Solid state CE between colloidal CdSe and Cu2Se or Cu has also been demonstrated recently.5 Intercalation on the other hand involves insertion of ionic, atomic or molecular species into cavities, pores or channels present in the structure of the host material which remains generally unchanged, at least up to a certain level of intercalation.6 Layered materials exhibiting van der Waals gaps or wide cavities in their structure, e.g. clays,


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graphite, metal oxides, di- and tri-chalcogenides such as Bi2Se3 and Bi2Te3, to name a few, are able to host a variety of guest species through intercalation. The latter is generally thought of as being reversible and this aspect has been utilized in several technological applications, most notably in the energy storage devices. For example, lithium ion batteries work on the principle of intercalation and deintercalation of Li+ ions to/from one or both electrodes which are typically layered materials.7 One of the trends in this field too is to move to a solid state operation by replacing the liquid electrolyte with a solid one.8 Bi2Se3 and Bi2Te3 are among the best established thermoelectric materials,9 recently also discovered to be topological insulators.10 The crystal structure of both materials is rhombohedral consisting of quintuple layers (QLs) formed by alternating planes of Bi and Se(Te) atoms in the following sequence Se(Te)-Bi-Se(Te)-Bi-Se(Te).11,12 The QLs are weakly bound together by the van der Waals forces and the gap between the QLs is considered as a suitable intercalation site. Intercalation into Bi–chalcogenides as a mode of doping and tuning of their electrical and thermoelectric properties has been of interest for decades.13-15 More recently, doping with Cu16 or Sr17 was found to induce superconductivity in Bi2Se3, while in Bi2Te3 the same effect is induced by Pd18 or Tl19 doping. This has led to a prediction of topological superconductivity in some of these materials.20 Intercalation of metals into Bi2Se3 and Bi2Te3 bulk single crystals and thin films has usually been achieved by either electrointercalation in an electrolyte containing the metal ions,14 self-intercalation during solidification of a melt containing the intercalant,16 mechanical alloying by means of ball milling,21 hydrothermal route,22 or by diffusion at different temperatures wherein the metal atoms are supplied either by thermal evaporation,15 or by growth on the thin film surface by molecular beam epitaxy.23 Intercalation and deintercalation of Li+ in Bi2Se324,25 and Cu+ in Bi2Te326,27 single crystals from the respective electrolytes using


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electrochemical cells have also been investigated. In each instance the intercalation was found to be reversible though accompanied by structural changes the host lattice.24,25,27,28 In contrast to intercalation of ionic species, a solution-based method for intercalation of zero-valent atomic species and their combinations into Bi2Se3 and several other two-dimensional (2D) layered nanomaterials has also been developed.29-33 The present study demonstrates intercalation and deintercalation into and from 2D Bi2Se3 and Bi2Te3 NCs, achieved in solid state by moderately heating the NCs deposited on a solid substrate. This is in contrast to driving the intercalant in and out of NCs by an externally controlled potential cycling or sweep, as typically done in an electrochemical cell. The processes were monitored in situ inside of a transmission electron microscope (TEM). The findings were verified by additional ex situ experiments. For these experiments, schematically illustrated in Figure 1, the Bi2Se3 and Bi2Te3 NCs prepared by colloidal synthesis were deposited onto a solid substrate, along with colloidal Cu NC as the source of Cu for the intercalation (a-c). Cu was chosen as an intercalant owing to its versatility and remarkable effect on the electronic, thermoelectric and optical properties of Bi2Se3 and Bi2Te3.13-16,20,31 Solid state intercalation was compared with solution-based intercalation. Finally, the Cu-intercalated Bi2Se3 and Bi2Te3 NCs were deintercalated in solid state in the presence of CdSe, acting as a ‘Cu detector’ (d). As a result of deintercalation of Cu from the Bi2Se3 and Bi2Te3 NCs, CdSe reacted with the deintercalated Cu and underwent a CE to Cu2-xSe, proving the reversibility of the intercalation in solid state.


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Figure 1. A scheme of the solid state Cu intercalation and deinterclation into/from Bi2Se3 and Bi2Te3 NCs. For the solid state intercalation (a), the Bi2Se3 or Bi2Te3 NCs exhibiting the form of hexagonal platelets (b) are deposited onto an amorphous C substrate together with Cu NCs. Heating leads to a release of Cu from the Cu NCs and its intercalation into the Bi2Se3 or Bi2Te3 NCs, in some instances also a CE as described in the text. The resulting structural changes in Bi2Se3 or Bi2Te3 crystal lattice are illustrated in c. For the demonstration of solid state deintercalation (d), the Bi2Se3 or Bi2Te3 NCs intercalated with Cu in colloidal solution are deposited onto a solid substrate and heated, leading to a release of Cu from the intercalated Bi2Se3 or Bi2Te3 NCs. The released Cu is detected by CdSe NRs, deposited on the same substrate, which as a result of their interaction with Cu undergo a CE to Cu2Se. EXPERIMENTAL SECTION


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Chemicals. Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, >99.9%), Sodium selenite (NaSeO3, >99%),

Sodium

tellurite

(NaTeO3,

>99.5%),

Sodium

hydroxide

(NaOH,

>75%),

polyvinylpyrrolidone (PVP, average MW 40,000), Ethylene Glycol (EG, ≥99%), copper (II) nitrate

pentahydrate

(Cu(NO3)3

x

5H2O,

98%),

Cadmium

oxide

(CdO,

99.5%),

tetrakis(acetonitrile) copper(I) hexafluorophosphate ([Cu(CH3CN)4]PF6, 99.99%), Oleylamine (OLAM, 70%) and 1-Octadecene (ODE, 90%)

were purchased from Sigma-Aldrich.

Octadecylphosphonic acid (ODPA, 99%) and Hexylphosphonic acid (HPA, 99%) were purchased

from

Polycarbon

Industries.

Trioctylphosphine

oxide

(TOPO,

99%),

Trioctylphosphine (TOP, 97%) and Selenium (Se, 99.99%) were purchased from Strem Chemicals. Anhydrous toluene, methanol, acetone, ethanol and isopropanol were purchased from Carlo Erba Reagents. All chemicals were used as received. Synthesis of Bi2Se3 and Bi3Te3 NCs. The NCs were synthesized following a procedure reported by Lin et al.34 For the synthesis of Bi2Te3 NCs, 1.84 g of Bi(NO3)3·5H2O, 0.135 g of NaTeO3, 0.325 g of NaOH, 0.446 g of PVP and 20 ml of EG were loaded into a 50 ml three-neck flask and stirred for 10 min at ambient temperature and for 20 min at 45°C, before heating to 190°C and holding at this temperature for four hours. For the synthesis of the Bi2Se3 NCs, 0.101 g of Bi(NO3)3·5H2O, 0.057 g of NaSeO3, 0.1632 g of NaOH, 0.224 g of PVP and 10 ml of EG were loaded into a 25 ml three-neck flask and after stirring at ambient temperature for 10 min, the solution was heated to 190°C and kept for 2.5 hours. The NCs were washed three times in a mixture of acetone and isopropanol and were finally dispersed in acetone. Synthesis of Cu NCs. Cu NCs were synthesized following an adaptation of a published procedure.35 A mixture of Cu(NO3)3 x 5H2O (235 mg), OLAM (6 ml) and ODE (6 ml) was degassed in a 25 ml three-neck flask at 140°C for 3 hours before being heated to 300°C and held


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at this temperature for 1 hour under N2 atmosphere. After cooling down to room temperature, the resulting NCs were washed three times in the mixture of methanol and toluene and finally redispersed in toluene. Synthesis of CdSe NCs. CdSe nano-rods (NRs) were synthesized following a published procedure.36 A mixture of 67 mg of HPA, 333 mg of ODPA, 3g of TOPO and 0.1 g of and CdO was first degassed in a 50 ml three-neck flask at ambient temperature and subsequently at 120°C for 60 min. The solution was then slowly heated under N2 atmosphere until it turned clear. When the temperature reached 350°C, a mixture of Se and TOP (prepared by mixing 36 mg of selenium with 0.5 ml of TOP) was rapidly injected into the vigorously stirred Cd precursor solution and kept for 5 minutes. After rapid cooling down, the NRs were washed three times in the mixture of methanol and toluene and finally redispersed in toluene. Cu intercalation in colloidal solution. Solution-based Cu intercalation was carried out following a procedure reported by Koski et al.,29 including specifically the glassware preparation as reported there. 172.7 mg of tetrakis (acetonitrile) copper (I) hexafluorophosphate (the “Cu+ complex”) was dissolved in 12 ml of acetone at ambient temperature and then mixed together in a 50 ml three-neck flask with the dispersion of Bi2Se3 nanoplatelets in 12 ml of acetone, before being heated to 60°C and held at this temperature under reflux. The intercalated samples were taken out after two and three hours and washed in warm isopropanol and acetone, and finally redispersed in isopropanol. Bi2Te3 nanoplatelets were intercalated following a similar process with the only difference being a lower concentration of the Cu+ complex used (75.4 mg dissolved in 12 ml of acetone). The samples were taken out after two, three and four hours, washed in the mixture of isopropanol and acetone and finally redispersed in isopropanol. The characterization of these samples did not reveal significant differences between them.


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Solid state Cu intercalation. Colloidal solutions of as-synthesized Bi2Se3 and Bi2Te3 NCs were drop cast onto gold transmission electron microscopy (TEM) grids with ultrathin carbon on holey carbon support. A solution of Cu NCs was drop cast on these grids. The grids were loaded into a dedicated TEM heating holder and annealed at temperatures between 250° and 450°C for different amounts of time. The annealing was performed in situ in the column of a TEM under the vacuum of 2x10-5 Pa. Except for the brief observation of the samples at low magnification and reduced electron beam doses, the in situ annealing was carried out without the electron beam exposure in order to minimize electron beam-induced heating and other damage to the sample. Additional control annealing experiments were carried out also ex situ under the vacuum conditions comparable to those in the TEM column. The ex situ annealing experiments were performed both in a vacuum furnace and in the TEM heating holder placed in a vacuum chamber outside of the TEM column. Solid state Cu deintercalation. The dispersions of solution-based Cu-intercalated Bi2Se3 (reacted with the Cu+ complex for 2 hours) and Bi2Te3 (reacted with the Cu+ complex for 4 hours) NCs were drop cast onto gold TEM grids with ultrathin carbon on holey carbon support. A solution of CdSe NCs was drop cast on these grids, which were then loaded into a dedicated TEM heating holder and annealed at the different temperatures. For the Cu-intercalated Bi2Se3 NCs the annealing was conducted at 250°C for 6.5 hour followed by 1 hour at 300°C and 1 hour at 350°C. Annealing at 350°C led to a commencement of sublimation of thinner Bi2Se3 platelets. For the Cu-intercalated Bi2Te3 NCs the annealing was conducted at 300°C for 4 hours. The annealing was performed in situ in the column of a TEM under the vacuum of 2×10-5 Pa with minimal exposure to electron beam during the annealing. Additional annealing experiments were also carried out ex situ. The reason why solution-intercalated NCs were used instead of the solid


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state intercalated ones, was to avoid any effect that Cu NCs, that remained on the grids after the solid state intercalation, could have had on this experiment (i.e. served as the source of Cu). By using only the well-washed NCs intercalated with Cu in solution, it is ensured that the only source of Cu would be the Cu-intercalated Bi2Te3 and Bi2Se3 NCs. The solution based Cuintercalated Bi2Se3 and Bi2Te3 NCs were also annealed without the presence of CdSe NRs in order to assess their thermal stability and any structural changes as a result of annealing. The annealing was carried out at 300°C for 4 hours for both samples. The X-ray diffraction (XRD) Characterization. The crystal structure of the pristine and Cuintercalated (in solution) Bi2Te3 and Bi2Se3 NCs was confirmed by XRD measurements, performed in air using the Rigaku SmartLab X-ray diffractometer with the monochromated Cu Kα source operating at 40 kV and 150 mA. The samples were prepared by drop-casting the colloidal solutions of the NCs onto a Si wafer. The in situ annealing and TEM characterization. The in situ annealing experiments were performed in the column of an aberration corrected JEOL JEM 2200FS microscope operated at 200 kV and under the vacuum of 2×10-5 Pa using a dedicated heating holder that can accommodate standard TEM grids. The same microscope was used to characterize the NCs by means of high resolution TEM (HRTEM), scanning TEM (STEM), energy filtered TEM (EFTEM) and energy dispersive X-ray spectroscopy (EDX). The EDX spectra were collected using the Bruker Quantax 400 XFlash 6T silicon drift detector and quantified using the CliffLorimer method. An FEI Tecnai F20 operated at 200 kV and equipped with Gatan Enfinum SE spectrometer was used for the analysis of Cu oxidation state by means of electron energy loss spectroscopy (EELS). The EELS spectra were collected in the TEM mode with a collection semi-angle of 100 mrad. Most of the images following the experiments were recorded using a


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direct detection camera (Gatan K2 Summit) allowing observations with a reduced electron dose on the sample. RESULTS AND DICUSSION Solid state intercalation of Cu into Bi2Te3 and Bi2Se3 NCs. Platelets of colloidally synthesized Bi2Te3 with their [0001] zone axis perpendicular to the substrate (Figure 1b) were deposited onto an amorphous C substrate along with NCs of Cu (Figure 2a), shown in red in the energy filtered transmission electron microscopy (EFTEM) Cu elemental map in Figure 2b. After annealing at 350°C, the Cu NCs had partially or completely sublimated depending on their size (Figure 2c; Figure S1 in the Supporting Information (SI)), releasing Cu which was then inserted into the Bi2Te3 platelet forming a superlattice, which is an indication of intercalation.37

Superlattice diffraction spots at {0110}Bi2Te3 were clearly visible in the TEM electron diffraction (ED) patterns (Figure 2d) at appreciable levels of intercalated Cu. This confirms that Cu inserted into the platelets via a solid state reaction is indeed intercalated and assumes an ordered substructure within the Bi2Te3 structure. The simplest scenario to consider would be Cu intercalation into van der Waals gaps between Te layers, since Cu is expected to be more stable in this arrangement than when intercalated interstitially within the QLs.23 At higher Cu levels interstitial intercalation may be expected,23,29 as well as the formation of Cu2Te.27 No melting of the Cu NC has been observed under the vacuum in the TEM column, although a considerable fraction of the sublimated Cu must have been still adsorbed to surfaces such as the C support, surfactant shell on the Cu NC or the platelets, which served as substrates for its diffusion. Some contribution from the decomposition of Cu from the gas phase may have been possible as well. Around 18 at% of Cu incorporated into the platelet was relatively uniformly distributed (Figure 2e) without notable displacement of Bi or Te (Figure S2) or change in the Bi2Te3 stoichiometry


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(the composition determined by energy dispersive X-ray spectroscopy (EDX) is given in Figure 2d).

Figure 2. Solid state intercalation of Cu into Bi2Te3 (a to f) and Bi2Se3 (g to l) platelets. a) The starting Bi2Te3 hexagonal platelets surrounded by Cu NCs deposited on an amorphous C film, and b) the corresponding Cu EFTEM map. During annealing at 350°C Cu released from the sublimated Cu NCs (c) was incorporated into the platelet and formed a superlattice giving rise to superlattice diffraction spots (circled) in the electron diffraction (ED) pattern (d). The composition of the platelet in at% determined by EDX is also given in d). The corresponding EFTEM map in e) shows that intercalated Cu was uniformly distributed within the platelet. An electron energy loss spectroscopy (EELS) acquired spectrum (f) from one of the Cu-intercalated platelets compared to reference spectra from the elemental (Cu0)38 and monovalent (Cu+)39 Cu


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indicates that the intercalated Cu was mostly in the form of Cu+. Bi2Se3 platelet intercalated with Cu during annealing at 250°C for 2.5 hrs (g) with the corresponding ED pattern exhibiting superlattice spots (circled) and composition in at% given in h). The corresponding EFTEM map (i) shows that the distribution of the intercalated Cu was uniform. An EELS spectrum from one of the platelets (j) indicates that the intercalated Cu was mostly in the Cu+ state. During Cu intercalation experiments at 350°C for 1 hr CE took place in some Bi2Se3 platelets leading to the formation of separate Cu2Se and Bi-rich domains (TEM image in k) and EDX elemental maps of Cu, Bi and Se combined in l); more detailed characterization is given in Figure S4). EELS spectra (Figure 2f) indicate that a considerable amount of Cu intercalated in Bi2Te3 was in the Cu+ state. No chemical shift38,39 associated with Cu2+ was observed. Generally weak signal obtained from the Cu L2,3 edge makes it difficult to assess whether and to what extent C0 was present as well. The Cu intercalation substructure within Bi2Te3 platelets was found to be stable against annealing conditions leading to sublimation of the platelets themselves40 and the partly sublimated platelets still retained their rhombohedral Bi2Te3 structure along with the Cu superlattice (Figure S3b-h). Intercalation of Cu into Bi2Se3 platelets took place during annealing at 250°C (Figure 2g) with evidence of superattice formation discernable in TEM ED patterns at Cu levels around and above 10 at% (e.g. Figure 2h). The arrangement of Cu in the Bi2Se3 structure was different from that in the Bi2Te3 structure observed here, and different from that reported previously for Bi2Se3 intercalated with zero-valent Cu in solution.29 The superlattice diffraction spots appeared at

{2110}Bi2Se3 (Figure 2h) and based on the EELS analysis much of the intercalated Cu was in the form of Cu+ (Figure 2i). At a higher annealing temperature (350°C), solid state Cu intercalation


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led to instances of Bi2Se3 platelets structurally and compositionally undergoing CE to Cu2-xSe (Figures 2k-l and S4). The CE platelets also either contained a substantial amount of Bi, or Bi formed separate domain/s within the platelet (see examples in Figures 2k-l and S4c-d). This is presumably due to a difficulty of completely removing Bi from the platelets into a gas phase, in contrast to intercalation in solution (presented in SI) where such domain formation was rarely observed. Even when the annealing induced sublimation of the Bi2Se3 platelets, examples of both intercalation and CE were observed. This indicates that CE was not driven by the decomposition of the Bi2Se3 structure induced by sublimation, since under similar conditions (see Figure S3) no CE was observed in the Cu-intercalated Bi2Te3 platelets. The ease of CE in Bi2Se3 may instead be related to a higher affinity between Cu and Se as compared to Bi and Se, brought about by a greater electronegativity difference in the former case.41 This difference is not as pronounced in the Bi2Te3 + Cu system. Higher annealing temperature may provide more Cu for the reaction by inducing a greater sublimation of the Cu NCs and by facilitating the diffusion of different atomic/ionic species. Additionally, since the formation energy of Cu2Se is more negative than that of Cu2Te,42-44 under identical experimental conditions, the formation of Cu2Se is expected to be more likely than the formation of Cu2Te. The intercalation was found to be unaffected by the electron beam exposure and equivalent results were obtained in ex situ experiments (examples are shown in Figures S5 and S6). Platelets subjected to a limited Cu supply as compared to e.g. platelets shown in Figure 2, generally exhibited a less uniform distribution of Cu (Figure S7) aggregated near the edges of the platelet, suggesting this to be the preferred entry point for the Cu insertion. This is consistent with earlier computational studies on metal doping which showed that lateral insertion through the steps on the edges of a crystal is energetically less costly than defects-mediated vertical


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diffusion though the layers.23,45 Overall, the amount of intercalated Cu depended on its availability on the substrate. Similar observations were made also when Cu NCs were used as the Cu source for CE reaction with CdSe NRs.5 Ultimately therefore, the amount of Cu or other intercalant could be controlled so that a specific level of intercalation is achieved in a given NC within a device, or so that a specific NC is cation-exchanged to a different material and in this way given a different functionality, etc. This would be feasible particularly with NCs of large lateral dimensions which would be reasonably well accessible by nanofabrication techniques. Importantly, the selectivity that the solid state intercalation or CE could ultimately allow for, is not something accessible by the solution-based processing of colloidal NCs or in the nanostructured bulk materials. Comparison with intercalation in solution. Colloidally synthesized Bi2Te3 and Bi2Se3 platelets were also intercalated with Cu in colloidal solution using a Cu+ complex (see SI for more details), following a procedure reported for the intercalation of zero-valent Cu into Bi2Se3 nanoribbons.29 The results, summarized in Figures S9-S12, were generally consistent with those achieved by solid state intercalation. X-ray diffraction (XRD) characterization confirmed that the Bi2Te3 structure was preserved during the intercalation of Bi2Te3 NCs (Figure S9). The intercalation experiments with Bi2Se3 produced both intercalated and cation exchanged platelets, the latter being more frequent after longer reaction times. The XRD patterns from the Cu+ reacted Bi2Se3 samples (Figure S10) matched the anti-fluorite Cu1.75Se structure.46 Allowing a greater tolerance in the lattice parameter, the spectra could also be indexed according to structures of compounds such as BiCuSe247 and BiCu3Se3.48 The exact location of the Cu atoms in those structures has not been determined however, which is why in this article comparisons are made to a Cu1.75Se structure.46 The Cu intercalated into Bi2Te3 and Bi2Se3 platelets was


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present in the Cu+ state likely mixed with the elemental Cu0 (Figures S9f and S10f),38,39 as reported also in the earlier study on zero-valent Cu intercalation.29 The smaller size of the colloidal NCs used in present work, longer reaction times and/or possibly a relatively higher concentration of the Cu+ complex may be why the intercalation ultimately led also to CE to Cu2xSe.

Superlattices in the solid state Cu-intercalated Bi2Te3 and Bi2Se3 NCs. The TEM diffraction patterns (e.g. Figure 2d) and Fourier transforms (FT) generated from the high resolution HRTEM images of the Cu-intercalated Bi2Te3 nanoplatelets in the [0001] zone axis (e.g. Figure S3g) exhibited

{0110} superlattice reflections, corresponding to interplanar

spacing of 0.76 nm and an in-plane superlattice of a type (2a x 2a) R0°, where a is the “a” lattice parameter of the host Bi2Te3 lattice. A model of FT in Figure 3a (based on Figure S3g) from the Cu-intercalated Bi2Te3 nanoplatelet displays the relative arrangement of the Bi2Te3 diffraction spots (black dots) and superlattice diffraction spots (red dots) believed to be originating from the ordered Cu superlattice (Figure 3b). By extracting from the FT the contributions of the Bi2Te3 and superlattice reflections to the HRTEM image, two images corresponding to in-plane projections of the Bi2Te3 lattice (background grayscale image) and Cu superlattice (a pattern of red dots) are obtained and shown overlaid as inset in Figure 3b. These represent 2D projections of a 3D structure so Cu atoms may not necessarily all be located on a single plane. A hexagonal arrangement of atomic columns believed to be Cu (red dots) with interplanar spacing of 0.76 nm is indeed visible in the superlattice image in inset, where the Cu locations are not overlapped with those of Bi/Te but rather slightly displaced (at approximately (±1/3, ±2/3) positions). A model of a possible Cu arrangement within Bi2Te3 structure is shown in Figure 3c. The superlattice formed in the Bi2Se3 platelets (Figure 2h, Figure 3 d to f) is different from that


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observed in the zero-valent Cu intercalated Bi2Se3 nanoribbons reported before,29 which may impact differently on the properties of the NCs. The difference may due to a different amount of intercalated Cu and/or different temperature at which the intercalation and superlattice ordering took place in the present work.

Figure 3. Superlattices in the solid state Cu-intercalated Bi2Te3 (a to c) and Bi2Se3 (d to f). A model of ED pattern from the Cu-intercalated Bi2Te3 (a) exhibits the Bi2Te3 reflections in the

[0001] zone axis (black dots), and superlattice reflections at 0110 (red dot; based on Figure S3g). The inset in b) is a superposition of the inverse FT (IFT) images generated from the Bi2Te3 reflections (the grayscale background image) and from the superlattice reflections (the red dots arranged in a hexagonal pattern with a 0.76 nm spacing between the individual planes). A model diffraction pattern from the Cu-intercalated Bi2Se3 (d) exhibits the Bi2Se3 reflections in the

[0001] zone axis (black dots) and superlattice diffraction spots at {2110} (red dot; based on Figure S8). An image in e) is a superposition of the IFT images obtained from the Bi2Se3 reflections (background grayscale image where the bright dots correspond to Bi/Se atomic columns) and from the superlattice reflections (red dots slightly displaced from the locations of the Bi/Se atomic columns and arranged in a hexagonal pattern with 0.62 nm spacing between the


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individual planes, and with the side of a hexagon formed by the Cu superlattice projected along the [0001] zone axis of Bi2Se3 being 0.72 nm, or twice the lattice parameter of pure Cu). Planar projections of possible Cu arrangements in the Cu-intercalated Bi2Te3 and Bi2Se3 are illustrated in c) and f, respectively, where the solid lines indicate Bi2Te3 or Bi2Se3 unit cell, while the dashed lines indicate the intercalant unit cell. Based on the HRTEM image in Figure S8, the model of the corresponding FT (Figure 3d)

exhibits superlattice diffraction spots at {2110}, which translates to a plane spacing of 0.62 nm and a unit cell such as (√3 a x √3 a)R30°. The corresponding in-plane projection of the Bi2Se3 lattice image (grayscale background image) and Cu superlattice image (red dots) is shown in Figure 3e, along with a model of possible Cu arrangement within the Bi2Se3 structure in Figure 3f. Structural variations in the intercalated and cation exchanged Bi2Se3 platelets. In addition to Cu-intercalated platelets, the Bi2Se3 platelets that were cation exchanged to Cu2-xSe structure exhibited at least three orientation relationships with the starting Bi2Se3 structure, while still maintaining their 2D morphology. Examples of all these structural variations are given in Figure 4, including the HRTEM images (left panel in each row), the corresponding FTs (middle panel in each row) and models of these FTs (right panel in each row). The composition of the platelets determined by EDX in at% is indicated on the HRTEM images. The examples shown in images a) to f) are from the solid state intercalation experiments; images g) to l) are from the liquid state intercalation experiments. The Cu-intercalated platelets retained the Bi2Se3 structure with [0001] direction perpendicular to the platelet face (a) and reflections in ED/FT patterns arising from the {0110} and {2110} planes (b, c).


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Figure 4. The structures formed in Bi2Se3 subjected to Cu intercalation, in solid (a-f) and liquid (g-l) state Cu intercalation experiments. The first panel in each row is a HRTEM image, with the corresponding FT given in the second panel and a model of the observed primary and


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superlattice reflections in the third panel. The measured interplanar spacings are indicated in the third panel; the composition of the platelets in at % is given in the first panel of each row. (a) to (c) correspond to a platelet intercalated with Cu which exhibited the Bi2Se3 structure with visible intensities of both {0110} and {2110} reflections and superlattice spots at

{2110} ,

originating from the Cu substructure. (d) to (f) correspond to a cation exchanged platelet with a Cu2-xSe structure viewed in the [111] zone axis. Superattice diffraction spots at

{220} are

present as well (e). (g) to (i) corresponds to a cation exchanged platelet oriented in the [110]

Cu2-xSe zone axis exhibiting superlattice spots at {220} and {113} (h). (j) to (l) correspond to a cation exchanged platelet oriented in the [211] Cu2-xSe zone axis, also exhibiting superlattice

spots at {220} and

{113}.

At higher intercalation levels (above about 10 at %) the formation of the Cu superlattice was

evident from the superlattice diffraction spots at {2110}, as described earlier (see Figure 3d-f). The cation exchanged platelets were oriented in the [111] (d-f), [110] (g-i) and less frequently in the [211] (j-l) zone axis of the Cu2-xSe structure. A greater variety of orientations was observed in the sample treated with the Cu+ complex in colloidal solution and after a longer reaction time (3 hrs). Most of the cation-exchanged platelets in these different orientations also exhibited superlattices within their primary Cu2-xSe structures, giving rise to superlattice diffraction spots.

Depending on the orientation, these commonly included the spots at {220}, corresponding to 0.62 nm as observed also in the intercalated Bi2Se3 platelets. This may be an indication of intercalation as an intermediate step in the CE in these platelets, where the extra atomic planes created in the intercalation stage are carried over to the Cu2-xSe structure. Considering the composition of the cation exchanged platelets, where all but one from the examples shown in


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Figure 4 contained substantial amounts of Bi, it is possible that the initially intercalated Cu maintained its arrangement even after the structural transition to a cubic Cu2-xSe structure, or it was Bi that was forced to form a superlattice of its own. Based on the comparison of the Bi2Se3 structure with the Cu2Se orientations observed experimentally (Figure 5), the [111]Cu2Se orientation would require the least amount of restructuring on the part of Se sublattice. The spacing between Se1 (facing the van der Waals gap) and Se2 (in the middle of a QL) layers of Bi2Se3 of 0.3548 nm, slightly larger than the spacing between Se layers along the [111] direction of the cubic anti-fluorite Cu2Se (0.3314 nm), indicates that the Cu layers could be ‘comfortably’ accommodated in place of Bi when starting from the Bi2Se3 structure. For this type of structural transformation where [0001]Bi2Se3║[111]Cu2, additionally also {2110}Bi2Se3║{220}Cu2-xSe with closely matched interplanar spacings, and

xSe

{0110}

Bi2Se3║{224}Cu2-xSe

with d{0110} matching closely with three times the d{112} (Figure 5,

compare panels a and c, and panels b and d). This means that the edges of the platelets which used to be terminated by {0110} planes, are now terminated by the {112} planes of Cu2-xSe. For the orientation relationship where [0001]

Bi2Se3║[110]Cu2-xSe,

also the {2110}Bi2Se3 planes are

parallel with {220}Cu2-xSe planes with closely matched interplanar spacings. Furthermore, similar interplanar spacing between {220}Cu2-xSe (0.2013 nm) and {2110}Bi2Se3 (0.2072 nm), and between {111}Cu2-xSe (0.33 nm) and {0110}

Bi2Se3

(0.3588 nm), favor the structural transformation while

maintaining the morphology of the platelets (Figure 5, compare panels a) and e) and panels b) and f)). In the case where [0001]Bi2Se3║[211] Cu2-xSe (Figure 3 j-l), also {2110}Bi2Se3║{220}Cu2-xSe and {111}Cu2-xSe ║{0110}Bi2Se3, with a close match between all of the corresponding interplanar spacings, so even this orientation relationship favors the transformation. Considering the small


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thickness and the 2D morphology of the platelets with the prismatic {0110}Bi2Se3 planes perpendicular to the platelet face, the matching of the Cu2-xSe orientation with prismatic ( 0110

and {2110}) planes of Bi2Se3 structure, rather than with the Se1-Se2 distances (basal planes) in Bi2Se3, could be what determines the orientation of the cation exchanged platelet. Finally, the most favorable [111]Cu2-xSe orientation is the only one observed in the solid state exchanged plates, possibly because the contact of the platelets with the substrate limits some of the restructuring required for the other orientations.

Figure 5. The comparison of the starting Bi2Se3 structure with the Cu2-xSe structure produced by CE in the most common orientations observed experimentally. a) A side view of a Bi2Se3


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platelet structure (the [2110] zone axis), b) top view of the platelet structure (the [0001] zone axis). Panels c) and d) show side view and top view, respectively, of the structure of a cation exchanged Cu2-xSe platelet having the orientation relationship with the starting Bi2Se3 such that [0001]║[111] and {2110}║{110}. Panels e) and f) show side view and top view, respectively, of the structure of a cation exchanged platelet with the orientation relationship with the starting Bi2Se3 crystal such that [0001]║[110] and {2110}║{110}. The structure in e) is slightly rotated about the vertical axis and a Cu atom is removed from one of the {220} planes to reveal the presence of Se atoms on the same plane. Solid state deintercalation of Cu from Bi2Se3 and Bi2Te3 NCs. Having the Bi2Se3 and Bi2Te3 platelets intercalated with substantial amounts of Cu, the prospects of Cu deintercalation from these platelets in solid state have then been explored (Figures 6 and S13-S15). For this purpose, the platelets Cu-intercalated in liquid solution were deposited on a substrate and subjected to heating (to avoid the effect of Cu NCs remaining on the substrate after solid state intercalation). In order to detect the out-diffusion of Cu from the platelets, CdSe NR were deposited on the same substrate, since CdSe readily reacts with Cu and undergoes CE to Cu2Se even in solid state (Figure 6a).3,5 The Cu-intercalated platelets were also subjected to annealing without the presence of Cu-scavenger in order to assess the effect of annealing alone. Firstly, the Cuintercalated Bi2Se3 platelets underwent restructuring very shortly upon annealing at 250°C, forming Bi-rich domains separated from the Cu-Se rich matrix (Figure 6 a to c). Secondly, when annealed at 350°C in the presence of CdSe NRs, Cu is diffusing out of the platelets and incorporating itself into the NRs. The platelet shown in Figure 6a exhibited a superlattice structure within the Bi2Se3 structure (the first case/row in Figure 4). Many NRs close


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to the platelet underwent a complete structural and compositional transition to Cu2Se (Figure 6 d to f; Figures S13-S14) where [0001]CdSe║[001]Cu2Se. No clear segregation of Bi was observed as a result of annealing the Cu-intercalated Bi2Te3 platelets at 300°C performed ex situ, however an inhomogeneous distribution of Cu was evident but unrelated to the distribution of Te in most cases (Figure S15 a and b). Nevertheless, some Cu did deintercalate from the platelets during the annealing and engaged in the CE within CdSe NRs on the same substrate (Figure S15 c and d). The Cu content in the NRs was found to gradually decrease with the distance away from the Cu source, Cu-intercalated platelets in this case.

Figure 6. Deintercalation of Cu from Bi2Se3 platelets during annealing at 350°C. a) Cuintercalated Bi2Se3 platelet (with the Bi2Se3 structure) before annealing along with CdSe NRs placed on the same substrate. b) Cu-intercalated Bi2Se3 platelet after annealing which resulted in the separation of the solute into Cu-Se and Bi-rich domains (c). A part of Cu was released


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from the platelets during the annealing and got incorporated into the surrounding CdSe NRs. Some of the NRs outlined by a rectangle in (b) and shown in (d) underwent a complete CE to Cu2Se. Bottom panel in (d) is an EDX elemental map of Cd, Se and Cu which shows several NRs free of Cd. One of the NRs outlined in (d) and shown in (e) (an IFT image), structurally transformed to Cu2Se, viewed here in the [100] Cu2Se zone axis. The corresponding HRTEM image is shown in Figure S13. Another cation exchanged NR shown in (f) was oriented in the [110] Cu2Se zone axis relative to the viewing direction. The CE in the CdSe NRs took place so that [0001]CdSe ║[001]Cu2Se. The CE of CdSe NRs to Cu2Se proves that Cu can be thermally deintercalated in solid state from the Cu-intercalated Bi2Se3 and Bi2Te3 NCs. The interaction of the intercalant with the host material may however limit the extent of intercalation and deintercalation, so while ‘charging’ and ‘discharging’ of the host lattice in solid state is possible, the host material and the intercalant need to be carefully selected and their possible interaction under ‘operational conditions’ considered. On the other hand, that interaction may potentially also be utilized, depending on the function of the NCs in a device. Earlier studies showed that the intercalation of Li and Cu in Bi2Se3 and Bi2Te3 bulk single crystals led to irreversible structural changes, such as the formation of Li3Bi,24,25 Cu2Te,27 or CuBi2Te lamellae.15 Those may render a fraction of the intercalant ions unavailable for deintercalation,25-28 or electrically inactive as dopants.14,15 The nature of doping achieved by the present solid state Cu intercalation would have to be assessed experimentally and may be different for the two host materials and, as expected, dependent on the amount and location of Cu in the host structure. High levels of Cu intercalation (within van der Waals gap or interstitially), such as those achieved here, tend to induce an n-type doping in


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both materials.13-15,29 However, in the case of substitutional alloying (Cu replacing Bi), such as that leading ultimately to a CE in the Cu-intercalated Bi2Se3, a p-type doping may be expected. CONCLUSIONS The colloidally synthesized 2D Bi2Se3 and Bi2Te3 NCs deposited on a solid substrate can be thermally intercalated in solid state with Cu, the most versatile dopant known for these two materials. The intercalation is effected by annealing in the presence of Cu source deposited on the same substrate. The solid state intercalated Cu forms ordered superlattice structures in Bi2Se3 and Bi2Te3 NCs and exhibits mostly the Cu+ oxidation state. At higher Cu levels (above about 35 at%), Bi2Se3 undergoes a solid state CE to Cu2-xSe, which often accommodates also substantial amounts of Bi. The NCs that are cation exchanged to Cu2-xSe structure may exhibit a number of orientation relationships with the starting Bi2Se3 structure, however the most commonly observed is the one requiring the least amount of restructuring during the transition, i.e. [0001]Bi2Se3║[111]Cu2-xSe, and {2110}Bi2Se3║{220}Cu2-xSe. The 2D morphology of the NCs is preserved during both the intercalation and CE. Intercalated Cu can also be thermally deintercalated from the Bi2Se3 and Bi2Te3 NCs in solid state, however the deintercalation efficiency strongly depends on any interaction between intercalant and host lattice during heating. The solid state reactions such as CE and inercalation/deintercalation reported here may potentially be used in nano-devices based on colloidal NCs where the properties and function of individual NCs/device components could be altered or fine-tuned during the device fabrication. Such solid state reactions could also be utilized as a part of a device function. Supporting Information Available: The details of all the experimental methods; additional TEM characterization of the solid state intercalation and deintercalation performed in situ and ex


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situ; the characterization of intercalation in liquid solution. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected]; [email protected]

ACKNOWLEDGMENT The authors would like to thank Francesco De Donato for synthesizing the CdSe NRs. This work was supported by the European Union’s Seventh Framework Program FP7/2007-2013 under the ERC research grant TRANS-NANO (contract number 614897).

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