Emergent Properties of Organic Molecule-Topological Insulator Hybrid

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Emergent Properties of Organic Molecule-Topological Insulator Hybrid Interface: Cu-Phthalocyanine on BiSe 2

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Rejaul Sk, Imrankhan Mulani, and Aparna Deshpande J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06584 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 24, 2018

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The Journal of Physical Chemistry

Emergent Properties of Organic molecule-Topological Insulator Hybrid Interface: Cu-Phthalocyanine on Bi2Se3 Rejaul Sk,† Imrankhan Mulani,† and Aparna Deshpande∗,†,‡ †Scanning Tunnelling Microscopy Laboratory, h cross, Indian Institute of Science Education and Research(IISER), Pune-411008, INDIA, ‡Department of Physics and Center for Energy Science, Indian Institute of Science Education and Research (IISER) Pune 411008, India E-mail: [email protected],Telephone:+91-020-2590-8063 Phone: (+91) 20 25908063

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Abstract We unravel the bidirectional influence of the adsorbate-substrate hybrid interface in the case of CuPc doping of the topological insulator (TI) surface Bi2 Se3 . Using ultra high vacuum (UHV) scanning tunneling microscopy (STM) at low temperature (77 K) we observe that for a dilute concentration single CuPc molecules are dispersed on terraces of Bi2 Se3 as individual entities or as clusters. The site dependent sub-molecular resolution images of CuPc on Bi2 Se3 reveal three different sites for CuPc adsorption. Scanning tunneling spectroscopy (STS) measurements show a rigid shift of the Dirac point toward negative voltage by 336 meV upon CuPc deposition. This is a clear signature that the topological surface state experiences a charge transfer due to CuPc . The HOMO-LUMO energy gap of CuPc on Bi2 Se3 is also measured using STS. It is found to be larger than that on Au(111) substrate due to the reduced screening offered by the TI Bi2 Se3 surface state as compared to the Au noble metal surface state. We also find that a higher concentration of CuPc results in an unconventional standing-up stacking of CuPc molecules at the step edge. This is suggestive of a magnetic coupling between the CuPc layers analogous to the observations in magnetic phthalocyanine thin films. Thus, CuPc-Bi2 Se3 system is a potent combination where CuPc induces a charge transfer, a change in the HOMO-LUMO gap on the TI surface of Bi2 Se3 , and the step edges of Bi2 Se3 act as an anchor to guide the unique vertical alignment of CuPc molecules proposing a new route to harness the TI nature as well as the magnetic behavior of the system.

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Introduction Topological insulator (TI) materials are one of the wonders of the condensed matter world. They are endowed with an insulating bulk and unique metallic surface states. These surface states exhibit a Dirac cone-like dispersion similar to graphene and are topologically protected by time reversal symmetry. 1–4 The strong spin-orbit coupling in these surface states couples the spin degree of freedom with the momentum making the surface states immune to back scattering. 5 Thus the electron transport becomes spin polarized. This makes the TIs promising candidates for the field spintronics and quantum computing. A guided manipulation of such surface states can lead to exploration of new exotic physical properties like unconventional superconductivity, 6 Majorana bound state, 7 spin transport and torque devices, 8–10 fault tolerant quantum computing. 11 Though the non-magnetic metal doping doesn’t affect the surface state, doping with magnetic metal atom 12,13 or metal-organic complex 14 is observed to disrupt the surface state by breaking the time reversal symmetry and opening a gap at the Dirac point. The gap opening at the Dirac point is explained by the observation of band bending and quantization of valence and conduction band for carbon monoxide adsorption over Bi2 Se3 . 15 Other recent experiments with magnetic doping have established the robustness of the surface state and no gap opening was observed. 16–18 This robustness of surface state is the necessary condition for forming hybrid interface between metal atoms/metal-organic molecules and topological insulator surface where the TI surface state will remain intact. The hybrid interface can lead us to design new class of spintronics materials by tailoring the electronic properties. 19–21 Metal Phthalocyanines (MPc) are the most promising and well studied organic molecules which host a transition metal ion with partially filled d orbitals at their center. These molecules are already characterized with their excellent electronic properties, 22 giant magnetoresistance, 23 spin doping 24 and long spin relaxation time. 25 On Bi2 Te3 surface, also known for its TI properties, different Pc molecules form a well ordered self-assembly depending on the interaction strength of the center metal atom and TI surface, thereby enabling the tailoring of the electronic and magnetic proper3



ties of the Pc-TI heterointerface. 26 A strong hybridization between molecular orbitals and substrate is observed in case of MnPc on Bi2 Te3 . 27 An in-plane magnetic anisotropy is also observed for FePc monolayer over Bi2 Te3 . 28 All such studies confirm the unaffected topological surface state after doping with organic molecules. The manipulation of TI surface state mainly due to the charge transfer at the interface is evident for MnPc on Bi2 Te3 28 and CoPc on Bi2 Se3 . 29 A recent theoretical and experimental study shows that the dependence of electronic properties on the hybridization strength for organic -TI interface and Rashba-split interface state can arise for strong hybridization. 30 CuPc, an organic semiconductor, with its ease of availability and amenability to modifications, is a potential choice for organic spintronic devices. Particularly useful is its long spin relaxation time, 25 optoelectronic application 31 and exchange interaction. 32 Combining these attributes of CuPc with the backscattering-immune traits of the topological insulator Bi2 Se3 is a worthwhile strategy to pursue for upcoming quantum technologies. In this letter we report the study of organic-TI interface of CuPc on Bi2 Se3 . Where Bi2 Se3 has a very large band gap of 0.3 eV between bulk conduction and valence band and the Dirac cone-like surface state is located in the gap, 33 we observed that after CuPc deposition the TI surface state remains unaffected, CuPc influences the molecule-Bi2 Se3 interface, shifts the Dirac cone towards negative energy due to the charge transfer, and upon increasing coverage the molecules adsorbed on the second layer experience a stronger intermolecular interaction and form 1D chains with standing-up stacking of CuPc molecules that can host superexchange interaction.

Experimental Methods The experiments were performed using an Omicron ultra-high vacuum (UHV) low temperature (LT) scanning tunneling microscope (STM) at 77 K with a base pressure of 5 × 10−11 mbar. The Bi2 Se3 single crystal was grown using the modified Bridgman technique. First,

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stoichiometric mixtures of high purity elements Bi, 99.999% and Se, 99.999 % were melted in evacuated quartz ampoules at 850◦ C for 16 h. Then the mixture was allowed to cool down to 650◦ C very slowly at the rate of 3◦ C per hrs. The mixture was then kept at 650◦ C for a day and then cooled down to room temperature. The orientation of the Bi2 Se3 single crystal was determined to be Bi2 Se3 (111) using x-ray diffraction (XRD) method. For STM studies the Bi2 Se3 single crystal surface was prepared by cleaving the sample using clean room standard adhesive tape just in the UHV sample preparation chamber. The surface quality was checked with STM at 77 K. The images revealed atomic resolution of hexagonal arrangement of Se atoms, standard triangular defects, and atomic steps of 0.95 nm height that confirmed an atomically clean surface. CuPc molecules were purchased from Sigma Aldrich and used as is for evaporation from a resistively heated quartz crucible attached to the UHV sample preparation chamber at 650 K onto the clean Bi2 Se3 surface held at room temperature. STM and STS measurements were carried out at 77 K. Electrochemically etched tungsten tip was used for imaging. Several trials of imaging and spectroscopy were carried out with different tips. The images were processed using the image analysis software (SPIP 6.0.9, Image Metrology, Denmark). The local density of states (LDOS) was determined by scanning tunneling spectroscopy (STS). Using the standard lock-in technique a modulation voltage of 20 mV at 652 Hz frequency was applied with the SRS lockin amplifier. The tunneling voltage (V) was varied within ±2.5 V and the corresponding variation in the current (I) was obtained as a dI/dV spectrum, thus giving the tunneling conductance for the CuPc/Bi2 Se3 system.

Result and Discussion The adsorption behavior of molecules over solid surfaces strongly depends on the fine tuning between the molecule-substrate interfacial interaction and intermolecular interaction. Most of the 2D planar molecules like phthalocyanine lie flat over the substrate for having the

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maximum overlap between substrate electronic state and molecular orbitals. 34 But they can also show tilted adsorption at mono atomic step edge 35 and upright configuration in the second layer when the molecules in the second layer are decoupled from the substrate owing to the molecule-substrate interaction of the first molecular layer formed by the same molecules 36 or thin insulating layer. 37 Figure 1a shows the STM image of CuPc molecules over Bi2 Se3 at a very dilute concentration where CuPc molecules are evaporated thermally on to the Bi2 Se3 substrate held at room temperature. CuPc molecules are seen to be dispersed over the whole surface. The inset shows the molecular structure of CuPc where Cu atom is located at its center. The 4-lobe configuration is a known characteristic of the planar MPc molecules. Individual CuPc molecules with 4 lobe structure can be seen in the zoomed in STM image in Figure 1b with the inset showing the Se atoms over the surface. The arrows represent three equivalent close-packed directions of the surface. The three equivalent orientations of the 4-fold CuPc are observed with respect to the 6-fold (0001) plane of Bi2 Se3 . In each orientation the two lobes are along one of the close-packed Se direction[¯1¯120] and the other two lobes are oriented along [1¯100] crystallographic direction of the substrate. The STM images of CuPc molecules with atomically resolved Se lattice in background reveal the different adsorption sites of the central Cu atom of CuPc. It sits on the hollow site between 3 Se atoms (Figure 1c), on top of Se atom (Figure 1d), and on the bridge site between 2 Se atoms (Figure 1e). Such images are a direct real space tool for an unambiguous determination of the adsorption site of the CuPc molecule on the Bi2 Se3 surface. Though the molecule sits on a different site the orientational preference of the molecule remains same where one pair of opposite lobes is always directed along the close-packed Se atoms [¯1¯120] direction. This indicates a strong interaction of the ligand with the substrate to pin the orientation of the molecule. In contrast to the earlier study of different MPcs on Bi2 Te3 surface the only adsorption site observed is on top. 26,27 But FePc on Bi2 Te3 is observed to adsorbed on both bridge and top sites. 28 Such an orientational preference provides a symmetry mismatch between 4-fold molecule on a 6-fold substrate and creates a non-identical

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Figure 1: (a) STM image showing CuPc molecules at dilute concentration scattered on Bi2 Se3 surface. (Scale bar = 20 nm). (b) A closer look at the adsorbed CuPc molecules at -1.6 V and 0.2 nA, the inset shows the Se atoms of Bi2 Se3 surface and the crystallographic directions(white arrows)(Scale bar=10 nm). (c)-(e) show the site-specific resolution of a single CuPc molecule, hollow site, top site, bridge site respectively with the registry of Se atoms in the background. -1.4 V and 0.4 nA (Image size 5 nm X 5 nm).

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environment for the isoindole groups of the molecule and can induce a geometrical distortion in the molecule. A similar effect has been seen in MnPc over Bi2 Te3 27 but despite similar orientation of CuPc and CoPc on Bi2 Te3 no geometrical distortion and symmetry reduction like MnPc has been reported. 26

Figure 2: (a) STM image of isolated CuPc molecules on Bi2 Se3 surface at 1.8 V and 0.2 nA (Scale bar=10 nm). (b) A closer look at the symmetry reduced single CuPc molecule, the 8-lobe structure (Scale bar=1 nm) V= 1.8 V , I= 0.2 nA. (c) STS spectra of doped and undoped Bi2 Se3 surface. The Dirac point can be seen at EF in the pristine Bi2 Se3 but upon CuPc doping it is shifted to -336 mV. For the undoped spectra the tip was positioned over molecule free region.

In our investigation of CuPc/Bi2 Se3 hybrid interface the effect of ligand substrate interaction is clearly visible in the STM image recorded at the positive bias. More interestingly a reduction of 4-fold molecular symmetry to 2- fold is observed. The STM image in Figure 2a is recorded at 1.8 V and 0.2 nA and it is evident from the image that two lobes along the ¯ [1¯120] direction appear as bright in contrast to the other two lobes which are perpendicular to the [¯1¯120] direction and appear dark in contrast. This reduces the symmetry of CuPc from C4 to C2 .This symmetry reduction is more evident in the single molecule image as shown Figure 2b. Such symmetry reduction of CuPc has been observed before on metal surfaces Ag(110) 38 and Cu(111). 39,40 The origin of this symmetry reduction is attributed to the non-equivalent underneath atomic environment which induces an asymmetric molecule-

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substrate interaction between the lobes oriented along the close-packed Se direction and in the perpendicular direction. As it is described in case of CoPc on Cu(111) 41 two lobes along [1¯10] direction interact more with the substrate effecting a bending towards the substrate and appear darker in STM image. We observe a similar effect for the CuPc lobes perpendicular to [¯1¯120] direction of Bi2 Se3 . They interact more with the substrate and bend towards the substrate and appear dark while the other two lobes along the close-packed direction appear bright. We believe that the central Cu atom does not play a crucial role as its dz2 orbital is fully occupied. The weak interaction of central Cu atom has been observed on several metal substrates, also on Bi2 Te3 . Despite similar orientational preference of CuPc on Bi2 Te3 at dilute concentration no symmetry reduction is observed. 26 The symmetry reduction of CuPc over Bi2 Se3 strongly depends on the asymmetric hybridization of the ligand with the substrate. To probe CuPc/Bi2 Se3 interface for its local electronic properties we investigated the local density of states(LDOS) by STS measurement. Our STS measurements for Bi2 Se3 surface state before(black curve) and after CuPc doping(red curve) are shown in Figure 2c. A clear downward shift of Dirac point of 336mV to the negative bias is observed after CuPc doping. This shift in Dirac point is attributed to the charge transfer in the interface. The charge transfer from CuPc leaves it positively charged and in turn makes it electrostatically bound to the Bi2 Se3 surface with the CuPc/Bi2 Se3 interface dipole strength. This prohibits any ordered in-plane self-assembly on the surface even for a coverage more than 1ML, unlike the case of CuPc/Cu(111) where the chemisorbed CuPc forms 1D chains or 2D islands on Cu(111) 42 as will be discussed later. Such a shift is previously observed in several moleculeTI interfaces 26,29 and transition metal doping studies over Bi2 Se3 . 18,43 In case of CoPc on Bi2 Se3 this shift in Dirac point is observed to vary with the molecule coverage on the surface. After 0.66 ML the charge transfer decreases and Dirac point shift starts to decrease. Next we compared CuPc/Bi2 Se3 with the CuPc/Au(111). As it is observed from our previous study of CuPc on Au(111) no symmetry reduction is observed in the voltage-

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dependent STM images. But the HOMO-LUMO gap increment is observed due to the pendant group substitution in CuPc. 34 The HOMO-LUMO gap measurement for CuPc using the STS technique for both Au(111) and Bi2 Se3 are shown in Figure 3. The position of HOMO and LUMO for CuPc on Bi2 Se3 was identified at -1.63 V and 2.0 V respectively. In case of CuPc on Au(111) the HOMO and LUMO positions were at -1.56 V and 1.53 V resctively. A clear increment in the gap is observed. For CuPc molecules adsorbed on Bi2 Se3 the HOMO-LUMO gap was found to be 3.67 eV whereas for CuPc on Au(111) this gap was 3.09 eV. The increment is mostly due to the shift of LUMO going away from the Fermi energy whereas the HOMO is slightly shifted towards negative voltage. A molecule upon adsorption on a substrate encounters electronic screening due to the substrate. This influences the energy levels of the molecule-metal system. For PTCDA molecules, the change in the HOMOLUMO gap from the isolated PTCDA value of 5.0 eV, 44 occurs as a function of the substrate - for Au(111) it is seen to be the least at 3.1 eV, for graphite it is 3.49 eV, and when a monolayer of transition metal dichalcogenide WSe2 is deposited on graphite the HOMOLUMO gap changes to 3.73 eV. Thus the effect of screening is pronounced for the metallic Au substrate and it manifests in the reduced HOMO-LUMO gap for PTCDA/Au(111). The HOMO-LUMO gap of PTCDA on WSe2 has been observed to reduce due to the strong charge screening when adsorbed on Au(111). 44 For CuPc/Bi2 Se3 the gap is less on Au(111) due to the metallic nature of the substrate and stronger screening than the bulk insulating Bi2 Se3 . This highlights the fact that the type of substrate can play a major role in the energy level alignment at the molecule-substrate interface. From the STM images of individual CuPc molecules it is clearly observed that the CuPc molecule at dilute concentration binds reasonably well to the substrate with preferred orientation via the ligand-substrate hybridization and evidenced in figure 2a,b. So it is expected for the molecule to not to form a nice and ordered self-assembly via intermolecular interaction. The strong molecule-substrate interaction plays a role here, and due to the charge transfer from CuPc to Bi2 Se3 surface the positively charged CuPc molecules repel each other. Sim-

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Figure 3: (a) HOMO and LUMO levels of CuPc on Bi2 Se3 surface and on Au(111) surface. The set point for CuPc/Au(111) is 1.4 V, 0.2 nA and for CuPc/Bi2 Se3 is 1.8 V, 0.4 nA. The CuPc/Au(111) spectrum is shifted vertically for clarity. ilar substrate mediated repulsive intermolecular interaction that overpowers the attractive interactions has also been reported for SnPc/Ag(111) and CuPc/Ag(111) using SPA-LEED (Spot Profile Analysis-Low Energy Electron Diffraction). 45,46 To probe the structural evolution CuPc over Bi2 Se3 at higher coverage, Bi2 Se3 substrate has exposed for a long time until molecule coverage reached more than 1ML. We don’t see any ordered self assembly even after 1ML coverage. Figure 4 represents the STM image for CuPc coverage more than 1ML. As seen in Figure 4a, over the flat terrace molecules are sitting on the second layer (bright one) over the flat lying molecules in first layer but no ordered self-assembly can be seen. See supporting information for a zoomed in view of the disordered assembly. The step edges nucleate an ordered assembly. Figure 4b represents a zoomed in view over the black square area marked in Figure 4a. It reveals 1D chain formation nucleated from the step edge. In this 1D chain CuPc molecules align in standing-up configuration and stack parallel to each other as shown in the inset of Figure 4b. The bright line corresponds to the Cu atom at the center of the molecule. This standing up configuration with a face-to-face alignment has been observed in case of a coverage dependent study of CuPc on Bi(111). 36 Though there is a strong molecule-substrate interaction for flat-lying CuPc over Bi2 Se3 this transition from 11



Figure 4: (a) STM image at higher coverage more than 1ML of CuPc molecules on Bi2 Se3 . Disordered molecules at the terrace but ordered assembly nucleated from the step edge (Scale bar=10 nm). (b) The black square on the self-assembly in (a) is shown in a magnified scale. V=1.8 V I=0.2 nA. Inset showing the superimposed standing-up molecules in the 1D chain. (c-d) Schematic for side and top view of standing up CuPc 1D chain

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flat-lying to standing-up is due to the lifting of the molecule-substrate interaction for the molecules in the second layer by the first layer. This report for 1.6 ML thin film of CuPc where a tilted adsorption geometry with an upright configuration is seen, is a validation of the intermolecular interaction overpowering the CuPc-Bi(111) interaction. 37 In another study involving SnPc molecules, where a thin NaCl layer was observed to act as a buffer layer to reduce the molecule-substrate interaction, a similar arrangement was seen upon Au(111) substrate modification. SnPc molecules when deposited on Au(111) form a thin film parallel to the surface in the Stranski-Krastanov mode whereas when a partial layer of NaCl is deposited on Au(111) a decoupling occurs between CuPc and Au(111). SnPc molecules adsorb in a tilted configuration on the NaCl layer in the Volmer Weber growth mode forming molecular nanocrystals. 37 For CuPc/Bi2 Se3 the first flat-lying layer acts as a buffer layer to reduce the molecule-substrate interaction and molecules coming next opt for a standing up configuration. But we believe that the step edge is playing a major role to nucleate such 1D chain formation as it is only seen to form at the step edge but not on the terrace. Monoatomic step edge on Au(111) is observed to act as a template to assemble molecules along the step edge in a tilted configuration. 35 The step edge of Bi2 Se3 is completely different from the monoatomic step edge of a conventional metal. It consists of five atomic layers with alternating Se-Bi-Se-Bi-Se stacking and the height is 9.5 ˚ A. 4 The molecule at the step edge experiences a strong support to adsorbed vertically as the Bi and Se atoms at the step edge have fewer co-ordination number. Apart from the fully occupied dz2 orbital of Cu atom there are pyrrole and azamethine nitrogen atoms (see supporting information) which can interact with the layer stacked Bi atom in the step to make the molecule stable in standing up configuration. The inset of Figure 4b shows that the stacking, the line joining the central Cu atoms of each molecule with respect to the molecular plane, is 67◦ . The molecules are stacked in parallel but their center is slightly shifted. Such stacking is similar to the well defined cystalline α -phase of the CuPc molecules. 47 Figure 4b shows a side view of the schematic of 1D chain, and

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Figure 4c shows the top view. The separation between the stacking axis (line joining Cu atoms) i.e the bright lines in STM image in Figure 4b is 15 ˚ A and the separation between two molecules in the 1D chain is 3.6 ˚ A . That the molecules in the chain are very close to each other reflects the strong intermolecular interaction in standing-up configuration. The multilayer α- phase stacking of CoPc with stacking angle 65.8◦ has been observed to have strong antiferromagnetism. 48 The mechanism of superexchange in similar stacking of CoPc on Pb(111) has been studied using spin flip spectroscopy and it is observed that such materials can function as versatile toolkits for composing molecule-based magnetic materials. 49 A 1D chain of CuPc with similar stacking is theoretically proposed to have indirect spin exchange. 32 Such 1D chains can be used to design new class of magnetic materials using phthalocyanines.

Conclusions Our investigation reveals the emergent structural and electronic properties of the hybrid interface formed between CuPc and Bi2 Se3 . Despite a weak interaction of the fully occupied dz2 orbital with the substrate, CuPc is observed to bind stably with the surface via ligandsubstrate interaction and charge transfer in the interface. CuPc orients along one of the close-packed directions of the topmost Se layer. The manipulation of Dirac point from the Fermi energy towards negative energy is observed with CuPc doping but the Dirac surface state remains protected. The STS measurement for HOMO and LUMO orbitals and comparison of HOMO-LUMO gap with CuPc/Au(111) reflects the gap increment mainly due to the shift of LUMO towards the positive voltage. This gap increment is attributed to the reduced screening due to the Bi2 Se3 surface than the metallic Au(111). At higher coverage the CuPc is seen to form a 1D chain with standing-up configuration nucleated from the step edge. Such standing-up stacking opens up channels for indirect spin exchange. Due to the long spin relaxation time of CuPc and the spin dependent transport properties of Bi2 Se3 such a hybrid interface between CuPc and Bi2 Se3 can be harnessed for rich spintronics-related

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phenomenon and for new molecule-based designer magnetic materials. Thus our proposed organic molecule-topological insulator hybrid interface awaits application for advances in spintronic devices.

Supporting Information Available Molecular structure, and STM image at higher coverage.

Acknowledgement A.D., R.S., and I.M thank Dr. Nirmalya Ballav for insightful discussions. We also gratefully acknowledge the facilities in the lab of Prof. Surjeet Singh at the Indian Institute of Science Education and Research (IISER) Pune for growing single crystal Bi2 Se3 . This work was financially supported by the Indian Institute of Science Education and Research (IISER) Pune and by DAE-BRNS (India, Project 2011/20/37C/17/BRNS). R.S. is grateful to IISER Pune for financial support in the form of research fellowship and I. M. is thankful to CSIR, Government of India for JRF.

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Emergent Properties of Organic Molecule-Topological Insulator Hybrid

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