Temperature Driven Phase Transition at the Antimonene/Bi2se3 Van Der Waals Heterostructure

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Temperature Driven Phase Transition at the Antimonene/Bi2se3 Van Der Waals Heterostructure Temperature driven phase transition at the antimonene/Bi2Se3 van der Waals heterostructure Conor Hogan,y,4 Kris Holtgrewe,z Fabio Ronci,y Stefano Colonna,y Simone Sanna,z Paolo Moras,{ Polina Sheverdyaeva,{ Sanjoy Mahatha,{ Marco Papagno,x Ziya S. Aliev,k Mohammad B. Babanly,? Evgeni V. Chulkov,#,@,r Carlo Carbone,{ and Roberto Flammini∗,y yIstituto di Struttura della Materia-CNR (ISM-CNR), Via del Fosso del Cavaliere, 00133 Roma, Italy zInstitut f¨urTheoretische Physik and Center for Materials Research, Justus-Liebig-Universit¨atGießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany {Istituto di Struttura della Materia-CNR (ISM-CNR), S.S. 14, km 163.5, I-34149 Trieste, Italy xDipartimento di Fisica, Universit`adella Calabria, Via P.Bucci, 87036 Arcavacata di Rende (CS), Italy kAzerbaijan State Oil and Industry University, AZ1010 Baku, Azerbaijan ?Institute of Catalysis and Inorganic Chemistry, Azerbaijan National Academy of Science, AZ1143 Baku, Azerbaijan #Centro de F´ısica de Materiales, CFM-MPC, Centro Mixto CSIC-UPV/EHU, Apdo. 1072, 20080 San Sebasti´an/Donostia,Basque Country, Spain @Saint Petersburg State University, 198504 Saint Petersburg, Russia 4 Dipartimento di Fisica, Universit`adi Roma \Tor Vergata", Via della Ricerca Scientifica 1, 00133 Roma, Italy rDonostia International Physics Center (DIPC), P. de Manuel Lardizabal 4, 20018 San Sebasti´an,Basque Country, Spain E-mail: [email protected] Abstract this van der Waals heterostructure explain the different temperature-dependent stability of the We report the discovery of a temperature- two phases and reveal a minimum energy tran- induced phase transition between the α and β sition path. Although the formation energies of arXiv:1906.01901v1 [cond-mat.mtrl-sci] 5 Jun 2019 structures of antimonene. When antimony is free-standing α- and β-antimonene only slightly deposited at room temperature on bismuth se- differ, the β phase is ultimately favoured in the lenide, it forms domains of α-antimonene hav- annealed heterostructure due to an increased ing different orientations with respect to the interaction with the substrate mediated by the substrate. During a mild annealing, the β phase perfect lattice match. grows and prevails over the α phase, eventually forming a single domain that perfectly matches the surface lattice structure of bismuth selenide. First principles thermodynamics calculations of 1 Introduction (RT) as well as why, and indeed how, the β phase forms upon gentle annealing. Among the known allotropes of two- This study sheds light on the structural sta- dimensional (2D) antimony, β-antimonene bility and structural phase transition undergone (hereafter, β-Sb), defined as the buckled Sb by the class of materials made of group 15 el- analogue of graphene, has drawn the attention ements that possess α and β stable allotropic of the scientific community due to its peculiar phases, i.e. phosphorene, arsenene and bis- electronic band structure which might be ex- muthene, all of which are of great current in- ploited in electronics,1,2 spintronics,3 optoelec- terest.12 Furthermore, from a more applicative tronics4 and even thermal energy conversion.5 point of view, this mild temperature annealing Indeed, its electronic structure is predicted to method used to convert one phase to the other radically change depending on the thickness of is fully compatible with modern electronics and the film.6 As a consequence, the electronic gap may be relevant for the next generation of elec- might be tuned to be negative, zero and positive tronic devices based on three-dimensional van and also show an indirect-to-direct transition der Waals heterostructures.26{28 under biaxial strain.7{9 Beside the potential The manuscript is organized as follows. Us- of β-Sb, a second 2D allotrope (the antimony ing STM and DFT, we identify and character- equivalent of black phosphorus, hereafter α- ize the two antimonene phases and show evi- Sb) is currently attracting much interest. α-Sb dence for the phase transition. Following this, shows a direct instead of an indirect gap as we provide an ab initio thermodynamics anal- found in β-Sb. Furthermore, the estimated ysis of antimonene stability on Bi2Se3 during electronic gap of the α phase is much lower the growth and annealing stages. A transition than that of the β allotrope, although its value path between the two phases is proposed, and is still under debate.10{13 α-Sb also exhibits a the critical role played by lattice matching to high carrier mobility12,14 and is predicted to be the substrate is highlighted. Finally, the signif- an excellent absorber in the visible range.15 icance of this study in the design of other group Such a wealth of properties needs to be pre- 15 vdW heterostructures is discussed. served when dealing with a supporting sub- strate. Recently, van der Waals (vdW) epi- taxy16 has been suggested as a valid solution. Characterization of α and β As a result, the number of reports of inter- antimonene phases faces made from stable α-Sb and β-Sb layers on 17{25 vdW substrates continues to grow . Much When Sb is deposited on bismuth selenide at less has been done, however, to try to control RT, islands of different size and shape form.29,30 and possibly induce a structural transition from In Fig. 1(a) we show a typical island after 4 one allotrope to the other. For instance, driv- minutes of annealing. The surface reveals tiny ing mechanisms such as electrostatic doping, features that we attribute to moir´estructures molecular adsorption and the use of graphene originating from the superposition of the rect- 13 as a supporting substrate were proposed, but angular lattice of the α-Sb phase and the hexag- no experimental counterpart has been reported. onal lattice of the Bi2Se3 sample surface. Apart Here we present a method to induce the α- from the moir´e,it is possible to see darker ar- Sb to β-Sb phase transition on a bismuth se- eas with no stripes. These zones indicate the lenide (Bi2Se3) monocrystal by means of an- presence of β-Sb: indeed, the superposition of nealing. The phase transition at the atomic buckled honeycomb and hexagonal structures scale is observed via scanning tunneling mi- do not generate any moir´epattern because of croscopy (STM), while density functional the- the perfect match between adsorbate and sub- ory (DFT) calculations comprehensively ex- strate lattices. In addition to the α and β struc- plain why the α phase is energetically favoured tures, small protruding areas (white regions in upon growth on Bi2Se3 at room temperature the STM images reported in Fig. 1) are present 2 (a) Free-standing sheets 4.79Å a a2 4.28Å -Sb -Sb β α a1 4.05Å (b) Heterostructures 5.9Å 4.0Å 3 3 Se Se 2 2 -Sb/Bi -Sb/Bi β α Figure 2: (a) Ball and stick models of free- standing α-Sb and β-Sb (top views) Unit cells and theoretical lattice parameters are indi- cated. Dashed lines indicate the four-atom β simulation cell; green shaded areas indicate the hexagonal structure common to both al- lotropes. (b) Adsorbed layers on Bi2Se3 (only first quintiple layer shown). Figure 1: (Color online) Panels (a-d): Sb is- the (α+β) phase as a function of the annealing lands grown at RT on Bi2Se3 and subsequently annealed at 473K for different duration time. time, as shown in Fig. 1(e). The complement to The STM images of 100×100 nm2 size, have the surface area covered by the α phase corre- been taken at +1 V, 2 nA. Bright and dark or- sponds to the β phase. After 34 mins the sur- ange areas correspond to α- and β-Sb, respec- face features the sole β phase (see Fig. 1(d)). tively. The moir´estructures seen in the orange The blue straight line represents the simulated areas are due to the superposition of a rectangu- behaviour of the α to the (α+β) ratio as if the phase transition was perfectly linear as a func- lar lattice (α-Sb) on a hexagonal one (Bi2Se3). (e) Ratio of α to β phase as a function of the tion of the temperature. We qualitatively ob- annealing time. A differentiation filter is ap- served that the higher the annealing tempera- plied to the STM images along the horizontal ture, the faster the phase transition. axes. In order to ascertain the presence of the α-Sb and β-Sb phases, we performed first principles DFT simulations of free-standing antimonene on the islands and are attributed to thicker Sb (Fig. 2(a)) and antimonene/bismuth selenide layers. Note that other 2D Sb allotropes (no- heterostructure (Fig. 2(b)). The β phase has 31 tably γ and δ) are predicted to be unstable a buckled hexagonal form (point group D3d). and have never been reported in experiments. Bulk Sb can be understood as an ABC stacking Moreover, the unbuckled α configuration fea- of β monolayers.10 The α phase has a rectan- tures a higher total energy and has been shown gular buckled washboard structure (point group 10 to be dynamically unstable. C3v). Our computed lattice constants for free- In Fig. 1(b) and (c) we show two islands after standing antimonene (a = 4:05A˚ for β; a1 = 12 and 20 mins of annealing. The moir´epat- 4:79A,˚ a2 = 4:28A˚ for α) are consistent with terns are clearly visible although the β phase previous works9,10,31 that include van der Waals starts to be the prevailing phase. By employ- interactions (the latter are known to be crucial ing a masking procedure provided by Gwyd- for describing geometries of puckered monolay- 32 33 dion, we extracted the surface ratio of the α to ers ).
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