Organobismuth Molecular Crystals for Organic Topological Insulators

Organobismuth Molecular Crystals for Organic Topological Insulators. Soyoung Kim†‡ , Jinyoung Koo†‡ , and Hee Cheul Choi*†‡. † Center fo...
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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Organobismuth Molecular Crystals for Organic Topological Insulators Soyoung Kim,†,‡ Jinyoung Koo,†,‡ and Hee Cheul Choi*,†,‡ †

Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science, Pohang 37673, Republic of Korea Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

‡

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S Supporting Information *

ABSTRACT: Three different types of large and high-quality organobismuth molecular crystals were grown by a physical vapor transport process. The target organobismuth molecules that have similar molecular structures, except for the type and position of the functional group, were crystallized into colorless and wire-shaped crystals having lengths at the centimeter scale with uniform flat surface. The crystal-packing structures of the organobismuth crystals were determined by single-crystal X-ray diffraction. The results show that the molecular packing can be controlled by a slight change of the functional group due to their different intermolecular interactions. Especially, the Bi−Bi distance was successfully controlled to vary from 5.11(1) to 5.71(3) and 5.18(2) Å for triphenylbismuth (TPB), tri-p-tolylbismuthine (p-TTB), and tri-o-tolylbismuthine (o-TTB), respectively. The different crystal structures and Bi−Bi distances can affect the topological behavior of the materials. Moreover, the electrical and optical properties of the target organobismuth crystals were confirmed through the I−V characteristics, density functional theory calculation, and photoluminescence spectroscopy. These findings potentially offer a new route and strategy for the development of organic topological insulators. KEYWORDS: molecular crystal, organobismuth crystal, organic topological insulator, organometallics

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benefits that come from the intrinsic properties and varieties of organic and organometallic materials. Especially, the structural diversity and various functionalization can significantly affect their topological behavior.7,14 Even in the small defect and deference of the crystal structure, the topological behavior highly affected inorganic systems.15,16 The organic or organometallic compound can have a drastic crystal structure change according to their molecular geometry and functional group, which can be one of the big advantages to realization of the desired topological property. In addition, theoretical calculations of bismuth-containing materials have reported that a new topological state can exist in not only inorganic materials but also heavy-element-containing organometallic materials.11 The material predicted to have a topological state generally has a periodic two-dimensional organometallic lattice composed of bismuth and conjugated organic molecule repeating units. However, from an experimental point of view, the bismuthcontaining two-dimensional lattice is extremely difficult to synthesize homogeneously because of the harsh synthetic conditions that require usage of strong acid, organolithium, and air-sensitive Grignard reagents to form the Bi−C bonds.17−19 Nevertherless, we believe that the synthesis of organobismuth molecular crystals is highly important because

he growing interest in topological materials has recently led to the consideration of heavy-atom-containing molecular systems as potential candidates.1,2 The heavyatom-containing molecules are expected to induce a big spin−orbit coupling that can change their electronic states and generate novel topological phases.3,4 Among them, especially bismuth (the heaviest group V semimetal element)-containing materials, such as bismuth selenide/telluride, bismuth chalcogenide, and bismuth oxide, have been actively studied for their topological insulating and ferroelectric properties.5−7 In general, bismuth-containing inorganic materials are relatively well-known compared to organometallic materials because of their simple atomic lattice, which makes it easy to predict, form, and calculate their optical and electrical properties. While new physical and chemical properties have been continuously discovered in heavy-element-containing organometallic materials,8−10 the incorporation of a heavy metal, especially bismuth, into organic molecular systems to form organometallic topological insulators also has gained great interest because of its diversity in the selection and position of functional groups which can affect the properties of the materials, and plausible Bi−Bi distance control, which could have a significant influence on the topological behavior of the material.11−13 Organic-based bismuth materials (organobismuth) are expected to have not only physical and chemical properties similar to those of inorganic ones but also the structural diversity, flexibility, and various functionalization © XXXX American Chemical Society

Received: September 19, 2018 Accepted: October 5, 2018 Published: October 5, 2018 A

DOI: 10.1021/acsanm.8b01649 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

−CH3 derivatives, which can (1) affect the molecular geometry because of different steric hindrance and (2) induce additional C−H intermolecular interactions that would control the intermolecular distance but still allow molecular interactions similar to those of the TPB molecules. We expected that the increased steric hindrance by the tolyl group would induce a molecular structure change especially in the C−Bi−C bond angle, which may increase the planarity of the molecule with closer molecular packing. Also, another important reason for the selection of the tolyl functional group is the additional C− H intermolecular interactions, which could decrease the intermolecular distance, enabling control of not only the crystal structure but also the Bi−Bi interatomic distance. Especially, the tolyl group attached at the ortho position has an ideal geometry to interact with an adjacent carbon of the benzene ring. Therefore, we expected that o-TTB would have the closest molecular packing structure and shortest Bi−Bi distance. In this study, we first propose a new approach toward topological materials using molecular systems offering possibilities through diverse variation of molecules and their crystal structures. The crystals were obtained by a vapor-phase crystallization method, which is suitable to obtain high-quality crystals. Also, we proved that simple geometry changes of the heavy-element-containing molecule could induce a change of the electronic structure, which will cause different electrical and optical properties. For the PVT experiment of three target molecules, a small amount of precursor powder was located at the center of the heating region of a furnace, marked by red in Figure 1b. Then, a SiO2/Si substrate was placed at the end region of the furnace. It should be noted that the SiO2/Si substrate itself does not participate in the crystallization process but allows an easy collection of the resulting crystals. Even without the SiO2/Si substrate, crystals still can be obtained in bulk scale at the end region of the heating furnace (Figure S1). After the quartz protection tube was flushed with argon gas, the furnace temperature gradually increased up to the target temperature. The sublimation temperatures of three different compounds were determined based on thermogravimetric analysis (TGA) and optimized through several trials (Figure S2). The experiment temperatures were set at 150, 170, and 145 °C for TPB, o-TTB, and p-TTB, respectively. The temperatures of the PVT experiment were set at the point where weight loss started to induce slow evaporation to obtain a large-size crystal. It should be noted that there is a temperature gradient in the furnace. We measured the furnace temperature from the center to the end when the setting temperature was 150 °C (Figure S3). Because of this temperature gradient, the target crystals are obtained mostly at the end region of the furnace. The detailed experimental procedure is described in the Supporting Information. After 1 h of reaction at the target temperature, large, colorless, and wire-shaped crystals were obtained on the SiO2/Si substrate, where vaporized precursors were assembled into a crystal with a thermodynamically most stable structure (Figure 2a,c,e). All resulting crystals show a length of centimeter scale and a width of micrometer scale, which is suitable to manipulate the desired crystals for several applications including device fabrication. To confirm their surface in more detail, scanning electron microscopy (SEM) measurement was performed. As shown in the insets of Figure 2b,d,f, the crystals obtained from the three compounds have uniform, smooth, and defect-free surfaces, which means that the PVT method is suitable for obtaining high-quality heavy-

they would be a great alternative for the in-depth studies about heavy-metal-containing organic topological insulators. The fundamental understanding of organobismuth crystals not only can provide useful information to discover new topological phenomena or synthesize target materials that exhibit expected electrical and optical properties but also can provide a unique chance to clarify the correlation between the crystal structure and topological behavior, to demonstrate the effect of chemical functional groups on their topological properties. With this background, we developed the facile and efficient synthesis of large and highquality organobismuth crystals by the physical vapor transfer (PVT) process. Although a solvent-based crystallization method may grant high-quality crystals, there is the possibility of the incorporation of a solvent or any other chemical species into the resulting crystals, which prohibit accurate characterization and further applications. Thus, the final product crystals have to be composed of only target molecules without any impurities or solvents. Therefore, vapor-phase growth under inert conditions is essential. Triphenylbismuth (TPB), one of our target molecules, has been predicted as a potential heavymetal-containing organic topological insulator monomer,11 and it is the simplest molecule composed of bismuth and conjugated benzene functional groups, which offers a plethora of possibilities of functionalization. Therefore, a series of TPBbased compounds [tri-p-tolylbismuthine (p-TTB) and tri-otolylbismuth (o-TTB)] were chosen as target materials to confirm their crystal structures and to examine whether their electrical/optical properties can be affected by a simple change of the functional groups (Figure 1a). Although the crystal

Figure 1. (a) Molecular structures of target organobismuth molecules. (b) Experimental scheme of the PVT setup for the synthesis of organobismuth crystals.

structures of TPB, p-TTB, and o-TTB grown in the solution phase were reported,18,20,21 their electrical/optical properties and in-depth study about their effect on the functional groups and molecular geometry on their crystal structures are still veiled. Especially, although Rao et al. used various types of TPB moieties, they only focused on the cross-coupling reactions using various types of bismuth-containing molecules under metal catalysts.21 Introducing additional functional groups is expected to induce additional intermolecular interactions. Such changes will eventually affect the structures of the resulting crystals, which can affect their topological behavior. Among various candidates, we chose the simplest B

DOI: 10.1021/acsanm.8b01649 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

showed an opposite trend. The interbismuth distances in the crystal structure of p-TTB and o-TTB are significantly reduced compared to the nonsubstituent-free TPB crystal (Figure 3). Although the obtained crystals uniformly have a trigonalpyramidal coordination environment of the bismuth, they exhibit different packing structures affected by weak intermolecular interactions. The distance for the intermolecular interaction (shortest C···C distance = 3.47(7), 3.61(5), and 3.64(8) Å for TPB, p-TTB, and o-TTB, respectively) implies that the weak intermolecular interactions can affect control of the distance between bismuth atoms in the molecular structure. Furthermore, we confirmed that each unit cell parameter matched very well with that in previous reports18,20,21 (see the Supporting Information). As a result, the interatomic distance of each target organobismuth crystal was measured in the order of TPB [5.71(3) Å] > p-TTB [5.18(2) Å] > o-TTB [5.11(1) Å]. Also, we tried to measure powder XRD (PXRD) to determine the growth direction of each crystal. Figure S5 gives the PXRD results, which show a statistically highly exposed crystal plane of each crystal. The most exposed crystal planes of the TPB, p-TTB, and o-TTB crystals were (20−2), (001), and (010), which means that growth direction of TPB, p-TTB, and o-TTB are [20−2], [001], and [010], respectively. In the case of TPB and p-TTB, the interaction between benzene and hydrogen is mostly affected to the crystal growth. Although p-TTB and o-TTB have the same functional groups, their crystal growth processes were totally different. From single-crystal analysis, we can confirm that a small change of the molecules, such as the functional group and the position of the functional group, can result in totally different crystal structures. This result shows that the optical and electrical properties of target materials can be tuned by changing their molecular structures, while the component species can be designed and synthesized infinitely through established organometallic reactions. To confirm the controllability of the electrical and optical properties of the resulting crystals, we calculated their band structures using the Dmol3 modules in the Accelrys Materials Studio 7.0 software, and also photoluminescence (PL) and UV−vis−IR absorption were measured to verify their optical properties. The TPB crystal has a higher density of states (DOS) in the whole energy region than the p-TTB and o-TTB crystals. This means that there are many states available for occupation (Figure 4a). The calculated band gaps were 3.201, 3.317, 3.266 eV for TPB, pTTB, and o-TTB, respectively. To analyze the accurate orbital contribution in the DOS, we additionally calculated DOS according to the s, p, and d orbitals (Figure S6). The DOSs near the Fermi level of the three crystals are mainly affected by the p orbital, and the TPB crystals have higher DOSs in all orbital cases. The three crystals have similar highest occupied molecular orbital levels at around −0.10 eV, and the TPB crystal has a lower lowest unoccupied molecular orbital energy at around 3.37 eV than the p-TTB and o-TTB crystals, which are located at around 3.7 eV. Also, the high DOS is related to the high dI/dV value of the TPB crystal than the p-TTB and o-TTB crystals, as we can confirm from the I−V characteristics in Figure S7.22 Although we could not observe a drastic difference in their electrical properties, there are many chances to increase or decrease their electrical properties, which are highly affected by intermolecular interactions, which can be changed by the molecular packing structure and types of substituting functional groups.23−25 We also measured the UV-

Figure 2. Photographs and optical microscopy images of the obtained TPB (a and b), o-TTB (c and d), and p-TTB (e and f) crystals by the PVT process. The insets of b, d, and f are SEM images of the crystals showing smooth surfaces.

element-containing organometallic crystals. Also, to clarify the yield and reproducibility information, we performed the PVT experiment 10 times for each compound with 100% success in terms of yielding the corresponding crystals. Also, to compare the crystals grown by vapor-phase crystallization (the PVT process), we additionally attempted the growth of TPB, oTTB, and p-TTB crystals in solution phase using drop-drying (DD) and solvent-vapor-annealing (SVA) methods (see Figure S4a and the experimental details in Supporting Information). Crystallization took 10 min and 12 h, respectively, and acetonitrile was used as the solvent. The resulting crystals were small and nonhomogenous except TPB crystals obtained by the SVA method. To compare the crystal structures, we analyzed the crystals grown by the SVA method. DD crystals were too small to analyze their crystal structures by singlecrystal X-ray diffraction (XRD). Figure S4b shows the unit cell parameters, which show a high similarity with our original crystals grown by the PVT method. This means that solvent molecules are not trapped in the three crystals grown in solution phase. However, there is still a high possibility of solvent being present on the crystal surface, which is unavoidable for the solution-phase reaction but still critical for spectroscopic analysis such as band-structure measurement. Compared with the solvent-based growth method, it is obvious that our PVT method showed advantages in rapid crystal growth (∼1 h) and uniform morphology having a length at the centimeter scale. We used 20 mg of TPB, p-TTB, and o-TTB. On the basis of the amount of crystals collected from the quartz tube after the reaction is completed, the yield of collected crystals was calculated to be 42 ± 8, 34 ± 5, and 49 ± 3% for TPB, p-TTB, and o-TTB crystals, respectively. This high reproducibility proves that the PVT process is suitable for obtaining high-yield and large-scale bismuth-containing organometallic compounds. Single-crystal XRD analysis of the organobismuth crystals revealed the different crystal geometries and interesting variations in the Bi···Bi distances. We expected that the interatomic distance of bismuth would be shortened with an increase of the intermolecular interactions, but the results C

DOI: 10.1021/acsanm.8b01649 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 3. Single-crystal structure of target organobismuth crystals: (a and b) TPB; (c and d) p-TTB; (e and f) o-TTB. Three crystals show different crystal packings and intermolecular interactions. Color code: cyan, Bi; gray, C. Hydrogen atoms are omitted for clarity.

Figure 4. (a) Calculated DOS of organobismuth crystals [TPB (blue), p-TTB (green), and o-TTB (orange)]. (b) PL spectra of TPB (blue), pTTB (green), and o-TTB (orange). The inset of part b is PL images of TPB (left) and p-TTB (right).

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DOI: 10.1021/acsanm.8b01649 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

correlation of the crystals are still under investigation, we suggest that organobismuth molecular crystals would provide an opportunity for systematic studies on the structure− property correlation in topological materials because different C−H interactions and Bi−Bi distances play a critical role in the conductance and PL changes. Although we did not succeed in providing direct evidence that supports the first-time experimental demonstration of an organobismuth topological insulator in this study, we believe that our findings would provide new insight in not only the growth of high-quality heavy-element-containing molecular crystals but also the chance to discover organic topological insulators with the possibility of property modulation.

vis−IR absorption to observe electronic states that are affected by the crystal stacking structure and d−d transition. Molecular crystals having d-orbital-containing elements show d−d transitions that can be changed by intermolecular interactions. All spectra were measured in the diffuse-reflectance absorption mode, and absorption in the Kubelka−Munk units was due to their measured diffuse reflectance using the Kubelka−Munk equation [F(R) = (1 − R)2/2R, where R is the measured diffuse reflectance]. All obtained crystals have a strong absorption peak at around 275 nm, which originated from the π → π* transition. In addition, we observed additional peaks at the near-IR region (Figure S8). Normally, the organometal complex has d−d transition peaks in the IR energy region, which are affected by the metal−metal distance and crystal-packing structure.26 Three bismuth-containing crystals having peaks around 1690 nm show different peak positions and shapes. From these experimental results, we confirmed that the electronic structure could be successfully controlled by simple changes of the functional group and molecular geometry in the molecular crystal system. Meanwhile, the three organobismuth crystals show totally different PL behaviors. Such a different PL is believed to be caused by the different behaviors of the photons due to different intermolecular interactions (Figure 4b). While TPB and pTTB have blue and green PL with peak maxima at 470 and 533 nm, respectively, o-TTB did not show any PL in our excitation condition. The most remarkable difference among the three crystals is no PL observation of the o-TTB crystal. The difference in PL can be attributed to the alignment of organobismuth molecules of the mainly exposed crystal plane, which is highly related to the orientation of the dipole moment. The orientation of the dipole moment relative to the direction of the electric-field component of the incident light is one of the key factors that determine the PL activity, which means that when the electric field of incident light is parallel to the dipole moment, the PL intensity is maximized because of a high absorption cross section, and when the electric field of incident light is closer to perpendicular, the PL intensity is decreased.27,28 In the case of o-TTB, the orientation of the dipole moment is almost perpendicular with the electric-field component, as can be seen in Figure S9, which is the reason why o-TTB has no PL. We presume that this different molecular array caused by different intermolecular interactions highly affects their optical properties. Such different PL properties support that the optical properties can be successfully modulated by simple changes of the functional groups. From these experimental results, we confirmed that the optical properties could be successfully controlled by simple changes of the functional groups and molecular geometry in the molecular crystal system. In conclusion, we successfully synthesized three types of high-quality organobismuth crystals by the PVT process for the first time. Different crystal packings and molecular interactions of the resulting crystals were confirmed by single-crystal XRD measurement, which shows the feasibility to obtain organic-based materials with the capability of interatomic distance control that is important for an topological insulator. Also, different electrical/optical properties were confirmed by DFT calculations and PL measurements, which show the compatibility of heavy-elementcontaining organometallic systems to control not only their crystal structures but also their electrical/optical properties. Although further details regarding the structure−property

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b01649.

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Detailed experimental procedure, structural analysis, PXRD data, TGA, and I−V characteristics of each crystal (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +82 54-279-2130. Fax: +82-54-279-3399. ORCID

Soyoung Kim: 0000-0001-6679-7057 Hee Cheul Choi: 0000-0003-1002-1262 Author Contributions

S.K. and H.C.C. designed the overall experiments and analyzed the data. J.K. analyzed the crystal structures of the obtained crystals. All authors contributed to the discussion of the results. The manuscript was written with contributions by all authors. All authors have given approval to the final version of the manuscript. Funding

The work on the high-quality organobismuth molecular crystal system was supported by Grant IBS-R016-G2. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS XRD study with synchrotron radiation was performed at the Pohang Accelerator Laboratory (Beamline 2D) supported by POSTECH, and surface observation of the obtained crystals by SEM was carried out at the National Institute for Nanomaterials Technology in Pohang, Korea.

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ABBREVIATIONS TPB = triphenylbismuth p-TTB = tri-p-tolylbismuthine o-TTB = tri-o-tolylbismuthine

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DOI: 10.1021/acsanm.8b01649 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX