Nanofilm Allotrope and Phase Transformation of Ultrathin Bi Film on Si(111)-7×7

Abstract

Our scanning tunneling microscopy and electron diffraction experiments revealed that a new twodimensional allotrope of Bi forms on the Si111-7 7 surface. This pseudocubic f012g-oriented allotrope is stable up to four atomic layers at room temperature. Above this critical thickness, the entire volume of the film starts to transform into a bulk single-crystal (001) When the size and shape of the materials are downsized to a nanometer scale, they often reveal anomalous atomic structures as well as exotic functional properties that do not exist in bulk. A variety of novel structures discovered in nanoparticles and nanowires have been attracting broad interest [1][2][3][4]. Group V elements are known to show rich allotropic transformation because their semimetallic bonding character can be easily shifted to either the metallic or covalent side, for example, by changing the applied pressure [5][6][7][8]. In this context, a question arises whether a new allotrope can be realized by tuning the dimensionality or size, such as thickness of the film, instead of tuning the pressure or temperature.In this Letter, we report on our finding that ultrathin film of Bi really shows such an allotropic transformation as a function of thickness on the scale of several atomic layers. Our electron diffraction and scanning tunneling microscopy (STM) experiments revealed that, over the wetting layer formed initially on the Si111-7 7 surface, Bi grows with a new f012g-oriented phase whose structure is significantly different from bulk Bi and that it transforms into the bulk-like single-crystal Bi(001) phase above the critical thickness which increases with the substrate temperature. Our ab initio theory revealed that the f012g phase with even-number layers is stabilized by forming a puckered-layer structure. The resulting film is very flat, compared to the growth of any known metal films, reflecting the inherent two-dimensional (2D) structure of this f012g phase. To the best of our knowledge, puckered-layer structure was never observed before except for the case of black phosphorus, which is a famous teratoid phase in group V elements.Bismuth ultrathin films were grown in ultrahigh vacuum (UHV) and characterized in situ using STM and reflection high-energy electron diffraction (RHEED). The spot-profile-analyzing low-energy electron diffraction (SPA-LEED) was performed with momentum reso-

Controlling the growth of Bi(110) and Bi(111) films on an insulating substrate

“…There, the coexistence of both crystallographic orientations was attributed to the minor difference in surface free energy of ultrathin Bi(110) and Bi(111) films and Bi(111) films were observed to be kinetically limited at low temperatures. Here, we observe solely the growth of Bi (110) at low temperatures, transforming to Bi(111) around about 400 K. Surprisingly, the Bi(110) (domains) can be grown up to thicknesses of 14 nm, well beyond the critical thickness reported by both, Nagao [23,26] and Bobaru [38].…”

“…According to Nagao et al [23,26], at low film thickness the (puckered-layer) Bi(110) is more stable as a result of surface effects. As the thickness approaches a critical few layers, the surface effects become less dominant, transforming the film to Bi(111), as it becomes energetically more favorable.…”

“…This choice of substrate is also very beneficial for practical applications, as the interface between film and insulating substrate, expected to reveal topological states, will also be protected from influencing oxidation effects arising from ambient exposure in technological applications [19]. The growth of Bi has been extensively studied on Si(111) [20][21][22][23][24][25][26][27][28][29] and HOPG [13,[30][31][32] as well as other surfaces [3,4,12,[33][34][35][36][37][38], resulting in fabrication of films with a range of different morphologies, orientations, and strain. The fabrication of Bi films has attracted considerable interest in recent years, as their controlled growth, with focus on morphology and crystallographic orientation, on semiconductor and oxide surfaces is not a trivial task.…”

Two- and three-dimensional growth of Bi oni-Al-Pd-Mn studied using medium-energy ion scattering

“…15 However, the fact that only even numbers of layers are preferred may indicate that another relevant factor is the formation of a bilayer type of structure, as reported for Bi growth on Si ͑111͒. 17,18 An investigation of the structure of the Bi islands was therefore carried out.…”

“…16 Although the quantum size effects provide a good explanation of the preference for 4 AL high islands seen for Bi on i-Al 71 Pd 20 Mn 9 and related systems, there is another potential explanation which comes from work carried out on the closely related system of Bi films grown on Si͑111͒. 17,18 In this system, as with growth on quasicrystalline surfaces, a monolayer film of Bi is seen to form, on top of which Bi islands of 4 AL height grow, as demonstrated by STM studies. However, in this case density-functional theory calculations indicated the formation of a puckered "blackphosphorus"-like structure composed of bilayers of Bi atoms.…”