This application note describes the growth of a novel Bi₂Se₃/ C₆₀ heterostructure in the Royce deposition system at the University of Leeds. We also present structural characterisation and transmission electron microscopy data in order to understand nature of the Bi₂Se₃/ C₆₀ interface

Knox, C

Moorsom, T

Rogers, M

White Rose Research Online

This is a repository copy of Van der Waals epitaxy of C  on the topological insulator ₆₀Bi Se₂ ₃.White Rose Research Online URL for this paper:https://eprints.whiterose.ac.uk/181587/Version: Published VersionMonograph:Knox, C orcid.org/0000-0002-4673-5649, Rogers, M orcid.org/0000-0002-4550-2392 and Moorsom, T (2020) Van der Waals epitaxy of C  on the topological insulator Bi Se . ₆₀ ₂ ₃Report. Materials from the Royce Deposition System (2). University of Leeds 10.48785/100/81eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse This article is distributed under the terms of the Creative Commons Attribution (CC BY) licence. This licence allows you to distribute, remix, tweak, and build upon the work, even commercially, as long as you credit the authors for the original work. More information and the full terms of the licence here: https://creativecommons.org/licenses/ Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Application NoteVan der Waals epitaxy of C60 on the topological insulator Bi2Se3C.S.Knox, M.D.Rogers, T.MoorsomThis application note describes the growth of a novel Bi2Se3/ C60 heterostructure in the Royce depositionsystem at the University of Leeds. We also present structural characterisation and transmission electronmicroscopy data in order to understand nature of the Bi2Se3/ C60 interface.1 IntroductionBismuth selenide (Bi2Se3) has attracted much ex-perimental interest as an archetypal 3D topologicalinsulator, where transport is dominated not by the in-sulating bulk band alignment, but by metallic, spin-polarised surface states [1]. As long as time-inversionsymmetry remains unbroken, these surface states areprotected against elastic back-scattering, as such anevent would require a spin-flip [2]. Bi2Se3 grows witha layered crystal structure, where atomic layers in thesequence Se-Bi-Se-Bi-Se (so-called quintuple layers)are separated by a van der Waals (vdW) gap betweenthe selenium atoms [3]. This allows Bi2Se3 to begrown on a wide variety of substrates [1, 4, 5], asthe growth is initiated by a vdW gap between the sub-strate and the Bi2Se3, without the need for a strainedinterfacial layer, in a process known as van der Waalsepitaxy [6]. Additionally, as a Bi2Se3 quintuple layeris terminated by a passivated selenium layer, a vdWgap can form between the Bi2Se3 and a subsequentlydeposited material [7]. It has recently been notedthat buckminsterfullerene (C60) also orders surpris-ingly well on a Bi2Se3 substrate [8].Previously, it has been noted that the electronicstructure of a C60 molecular film is highly substrate de-pendant, due to charge-transfer induced band broad-ening [9–11]. However, recent observations of bulk-like C60 bands on a Bi2Se3 substrate seem to indi-cate that there is very little hybridisation between theBi2Se3 bulk and the C60 [8, 12]. As the molecularorbitals within the C60 can be tuned by an appliedgate bias [13], this presents an interesting opportunityinvestigate the hybridisation between the topologicalsurface states and the bands of C60 [14, 15].Here we report on the novel growth of a bilayerBi2Se3/C60 heterostructure grown in the Royce thinfilm deposition system at the University of Leeds.The bulk crystal structure of the Bi2Se3 and the C60is probed by Raman spectroscopy and X-ray diffrac-tion measurements, which show that both materi-als within this heterostructure are well ordered in astrain-free, bulk-like manner. We also examine theBi2Se3/C60 interface under transmission electron mi-croscopy (TEM) at the LEMAS facility at the Universityof Leeds, and determine that while the C60 is grownby vdW epitaxy on top of completed Bi2Se3 quintuplelayers, the existence of bismuth dangling bonds at theedge of a Bi2Se3 terrace disrupts the vdW epitaxy ofthe C60, leading to the formation of a disordered re-gion at the interface.2 GrowthIn preparation for the growth procedure, 8 × 8 mm[0001] oriented Al2O3 substrates were cleaned underultrasonic agitation in acetone and isopropyl alcohol,in order to remove any possible organic contaminantsfrom the dicing process. The sapphire substrate isthen air-dried before being immediately loaded intothe Royce thin film deposition system. Once loaded,the substrate was transferred to the Royce MBE sys-tem, where it was heated to 500 °C (measured by athermocouple attached to the sample manipulator) inorder to outgas the sample holder and remove any re-maining contaminants from the substrate.The Bi2Se3 layer was grown by evaporating bis-muth and selenium from standard dual-filament Knud-sen cells. The Bi:Se flux ratio, measured by an ion-gauge flux monitor, was maintained at 1:15 through-out the growth, in order to obtain stoichiometricBi2Se3 and to minimise the formation of selenium va-cancies [5]. In order to improve the quality of the as-grown Bi2Se3, a two-step process was adopted [1],whereby a thin nucleation layer of Bi2Se3 was grownat a lower temperature. For this study, an intentionallylow temperature of at 130 °C was chosen, in order tostudy the effects of surface roughness on the order-ing of the C60 molecular film [5]. After this nucleationlayer was grown the rest of the epitaxial Bi2Se3 wasgrown at a temperature of 330 °C.The Bi2Se3 sample was then transferred through✶Application Note(a)(b) (c)Figure 1: Structural characterisation of the Bi2Se3/C60 sample. Panel a) depicts the X-ray diffractionspectrum of the Bi2Se3/ C60 sample, where the po-sitions of the known diffraction peaks are labelled withthe relevant material and lattice planes. Panels b) andc) show the Raman spectra of the Bi2Se3/ C60 sample,where the blue dashed lines represent the expectedpositions of the Raman modes. Panel b) shows thelow frequency range, dominated by the Bi2Se3 modes,as excited by a 633 nm laser. Panel c) shows the highfrequency range, dominated by the C60 modes, as ex-cited by a 473 nm laser.the radial transfer chamber under a pressure of ≈10−11 mbar, to the Royce organics chamber. Fromthere, a thick C60 layer was grown, over the courseof 20 minutes, from a standard Knudsen cell with theBi2Se3 substrate at ambient temperature [16]. Thisstructure was then capped by a 15 nm thick platinumlayer, deposited via electron-beam evaporation. Thethickness and deposition rate of the platinum film wasmonitored by a quartz crystal balance.3 Material PropertiesThe X-ray diffraction (XRD) spectrum for theBi2Se3/C60 heterostructure is shown in Fig. 1a. Thesample shows clear Bi2Se3 peaks ordered in the [001]direction, with a c-axis lattice parameter of 2.861 ±0.007 nm, which is in agreement with the bulk latticeparameter of 2.866 nm [17]. Additionally, we can seethat the inter-molecular spacing of the face centredcubic C60 lattice is 1.415±0.007 nm, which correlatesclosely with the lattice spacing for bulk C60, 1.42 nm[18]. This indicates that not only has the Bi2Se3 grownepitaxially on top of the Al2O3 substrate, but that theC60 has grown in a well ordered and relaxed manneron top of the Bi2Se3.To confirm this observation, the vibrational spec-trum of the Bi2Se3/C60 heterostructure was charac-terised by Raman spectroscopy. Two spectral rangeswere considered, one between 80 and 340 cm−1,which encompasses the principle phonon modes ofBi2Se3 (shown in Fig. 1b)[19], and the other between1200 and 1700 cm−1, detailing the vibrational spec-trum of the C60 [20] (shown in Fig. 1c). Fig. 1bcorroborates the observations from the XRD data, asthe sample shows well defined Bi2Se3 Raman modesat 131 ± 1 cm−1 and 173 ± 1 cm−1, indicating wellrelaxed Bi2Se3[19]. The high frequency regime alsoshows that we have well ordered C60, with C60 Ra-man modes at 1419 ± 1 cm−1, 1460 ± 1 cm−1 and1566±1 cm−1, which correlate well with the expectedpositions of the C60 H7g, A2g and H8g Raman modes,respectively [21].In order to discern why the C60 would order sowell on top of the Bi2Se3 film, the Bi2Se3/C60 inter-face was examined under transmission electron mi-croscopy (TEM) at the LEMAS facility at the Univer-sity of Leeds. A scanning electron microscope im-age of the region examined under TEM is shown inFig. 2a. The clear, terraced, triangular domains of theunderlying Bi2Se3 surface are clearly visible throughthe C60 film. We note that, between these triangu-lar domains, there are ≈ 500 nm wide voids in theBi2Se3 film, formed as a result of the excessivelylow temperature chosen for the nucleation layer ofBi2Se3[5], where the C60 has deposited directly on theAl2O3 substrate. Examining the entire stack structureunder TEM shows that the Bi2Se3 is approximately124 ± 1 nm thick, resulting in an average growth rateof 0.295±0.003 Ås−1 over the course of the 70 minutedeposition procedure. Similarly, the C60 is found to be104 ± 1 nm thick, resulting in an average growth rateof 0.833± 0.008 Ås−1.When the Bi2Se3/C60 interface is examined underTEM (as shown in Fig. 2b), we find that not only arethe lattice planes of the Bi2Se3 and the C60 clearly vis-✷Application Note(a) (b)Figure 2: Electron microscopy images of theBi2Se3/C60 sample. Panel a) shows a scanning elec-tron microscope image of the sample surface, high-lighting the C60 grains, as well as the large triangu-lar terraces, characteristic of Bi2Se3 growth. Panel b)shows a TEM image of the Bi2Se3/C60 interface, high-lighting the ordered nature of both materials, and thesharp interface between them.ible, but the Bi2Se3/C60 interface is sharp, and the C60is well ordered within the first few monolayers, withouta strained and disordered interfacial layer. However, ifwe observe the Bi2Se3/C60 interface at a terrace edgeon top of the triangular Bi2Se3 domains, as shown inFig. 3a, we note that this is no longer the case. Infact, we find that if we take a 2D Fourier transform ofthe C60 where the molecular film is deposited on topof a terrace flat (shown in Fig. 3b), there is a clear0.79± 0.02 nm periodicity, corresponding to the diam-eter of the C60 molecular cage [22]. However, if a sim-ilar Fourier transform is attempted at the step-edge ofa terrace we find that the clear vertical periodicity is re-placed by an amorphous ring with a similar 0.79±0.02nm periodicity, as shown in Fig. 3c.The presence of this disordered layer at theBi2Se3/C60 interface appears to indicate that somestrain is being applied to the C60 by the Bi2Se3. Thiswould point to the existence of dangling bonds at thestep-edge of the Bi2Se3 terrace, which can disruptvdW epitaxy [4, 23]. At the terrace step-edge thereare exposed bismuth atoms, effectively creating a se-lenium vacancy at the step edge. Previously, it hasbeen noted that selenium vacancies act as n-typedonors within Bi2Se3[24, 25]. By freeing additionalelectrons, these selenium vacancies will result in theformation of dangling bismuth bonds [3]. These dan-gling bonds are chemically active, and so will bond tothe C60 molecules, but as the bond angles within thematerial are limited [6], this will apply strain to the C60(a)(b) (c)Figure 3: Electron microscopy images of theBi2Se3/C60 sample at the terrace edge of aBi2Se3 domain. Panel a) shows a TEM image of theBi2Se3/C60 interface around a terrace step. Two re-gions are highlighted, one where the C60 has grownon top of a complete Bi2Se3 layer (blue arrow) andanother where the C60 has grown on top of a terracestep (red arrow). Panels b) and c) show 2D Fouriertransforms of the C60 within the regions highlighted bythe blue and red arrows, respectively.film resulting in the disordered interface layer seen inFig. 3a.We note that our results support those in Refs. [8]and [12]. As C60 grows on top of Bi2Se3 by vdW epi-taxy, only tunnelling events will result in charge trans-fer between the bulk of the topological insulator andthe organic semiconductor [9, 10]. As such, thereshould be very little hybridisation between the C60bands and the Bi2Se3 bulk, as seen for C60 in Ref.[8] and Bi2Se3 in Ref. [12]. We have also highlightedthe need for atomically flat Bi2Se3 substrates in po-tential organic-topological insulator heterostructures,due to the existence of bismuth dangling bonds at theBi2Se3 terrace step-edge.✸Application Note4 Further InformationThe Royce Deposition System is a multichamber,multitechnique thin film deposition tool based at theUniversity of Leeds as part of the Henry Royce Insti-tute. Materials from the Royce Deposition System areavailable as a facility service and for collaborations.References1. A. A. Taskin et al., Physical Review Letters(2012).2. C. X. Liu et al., Phys Rev Lett 100, 236601 (232008).3. B. Yan et al., physica status solidi (RRL) - RapidResearch Letters 7, 148–150, ISSN: 18626254,(❤tt♣✿✴✴❞♦✐✳✇✐❧❡②✳❝♦♠✴✶✵✳✶✵✵✷✴♣ssr✳✷✵✶✷✵✻✹✶✺) (1-2 Feb. 2013).4. C. L. Song et al., Applied Physics Letters 97,143118, ISSN: 00036951, (❤tt♣ ✿ ✴ ✴ ❛✐♣ ✳s❝✐t❛t✐♦♥✳♦r❣✴❞♦✐✴✶✵✳✶✵✻✸✴✶✳✸✹✾✹✺✾✺)(14 Oct. 2010).5. N. Bansal et al., Thin Solid Films 520, 224–229,ISSN: 00406090 (1 Oct. 2011).6. A. Koma, Thin Solid Films 216, 72–76, ISSN:00406090 (1 Aug. 1992).7. R Flammini et al., Nanotechnology, (❤tt♣s✿✴✴❞♦✐ ✳ ♦r❣ ✴ ✶✵ ✳ ✶✵✽✽ ✴ ✶✸✻✶ ✲ ✻✺✷✽ ✴ ❛❛❛✷❝✹)(2018).8. D. W. Latzke et al., Phys. Rev. B (2019).9. S. Braun et al., Advanced Materials 21, 1450–1472, ISSN: 09359648, (❤tt♣✿✴✴❞♦✐✳✇✐❧❡②✳❝♦♠✴✶✵✳✶✵✵✷✴❛❞♠❛✳✷✵✵✽✵✷✽✾✸) (14-15 Apr.2009).10. W. W. Pai et al., Physical Review Letters 104,036103, ISSN: 00319007 (3 Jan. 2010).11. T. Moorsom et al., Science Advances 6,eaax1085, ISSN: 23752548 (12 Mar. 2020).12. S. Jakobs et al., Nano Letters 15, 6022–6029,ISSN: 15306992 (9 Sept. 2015).13. A. Dodabalapur et al., Science 269, 1560–1562,ISSN: 00368075 (5230 Sept. 1995).14. K. S. S. et al., Journal of Physics-CondensedMatter, (❤tt♣s✿✴✴❞♦✐✳♦r❣✴✶✵✳✶✵✽✽✴✶✸✻✶✲✻✹✽❳✴❛❜✻✼✹✶) (2020).15. M. Cinchetti et al., NATURE MATERIALS | 16,25, (✇✇✇ ✳ ♥❛t✉r❡ ✳ ❝♦♠ ✴ ♥❛t✉r❡♠❛t❡r✐❛❧s)(2017).16. A. Manivannan et al., “Van der Waals EpitaxialGrowth of C 60 Film on a Cleaved Face of MoS2”, p. 1892.17. S. Nakajima, Journal of Physics and Chemistryof Solids 24, 479–485, ISSN: 00223697 (3 Mar.1963).18. D. L. Dorset et al., Acta Crystallographica Sec-tion A 50, 344–351, ISSN: 16005724 (3 May1994).19. M. V. W. Barcote et al., Materials Chemistry andPhysics 223, 109–113, ISSN: 02540584 (Feb.2019).20. T. Moorsom et al., Appl. Phys. Lett 105, 22408,(❤tt♣✿✴✴❞①✳❞♦✐✳♦r❣✴✶✵✳✶✵✻✸✴✶✳✹✽✽✺✸✸✻❪)(2014).21. D. Jing et al., European Journal of Mechanics /A Solids 28, 948–954, (❤tt♣✿✴✴♠❡✳ts✐♥❣❤✉❛✳❡❞✉✳❝♥✴③❡♥❣♣✴❞♠❡✸✶✵✴❡♥❣✴❡♥❣❴✐♥❞❡①✳❤t♠)(2009).22. A. Goel et al., (✇✇✇✳❡❧s❡✈✐❡r✳❝♦♠✴❧♦❝❛t❡✴❝❛r❜♦♥) (2004).23. X. H. Zhang et al., Phys Rev Lett 103, 106602(10 2009).24. L. A. Walsh et al., ACS Nano 12, 6310–6318,ISSN: 1936086X (6 June 2018).25. M. Brahlek et al., Applied Physics Letters 99,012109, ISSN: 00036951, (❤tt♣ ✿ ✴ ✴ ❛✐♣ ✳s❝✐t❛t✐♦♥✳♦r❣✴❞♦✐✴✶✵✳✶✵✻✸✴✶✳✸✻✵✼✹✽✹)(1 July 2011).✹

Van der Waals epitaxy of C₆₀ on the topological insulator Bi₂Se₃

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Van der Waals epitaxy of C₆₀ on the topological insulator Bi₂Se₃

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