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Nucleation and temperature-driven phase transitions of silicene superstructures on Ag(1 1 1)
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 J. Phys.: Condens. Matter 27 255005 (http://iopscience.iop.org/0953-8984/27/25/255005) View the table of contents for this issue, or go to the journal homepage for more
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Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 27 (2015) 255005 (7pp)
doi:10.1088/0953-8984/27/25/255005
Nucleation and temperature-driven phase transitions of silicene superstructures on Ag(1 1 1) C Grazianetti1 , D Chiappe1 , E Cinquanta1 , M Fanciulli1,2 and A Molle1 1
Laboratorio MDM, IMM-CNR, via C. Olivetti 2, I-20864 Agrate Brianza (MB), Italy Dipartimento di Scienza dei Materiali, Universit`a degli Studi di Milano Bicocca, via R. Cozzi 53, I-20126, Milano (MI), Italy
2
E-mail: [email protected] and [email protected] Received 11 February 2015, revised 1 April 2015 Accepted for publication 15 April 2015 Published 28 May 2015 Abstract
Silicene grown on Ag(1 1 1) is characterized by several critical parameters. Among them, the substrate temperature plays a key role in determining the morphology during growth. However, an unexpected important role is also equally played by the post-deposition annealing temperature which determines the self-organization of silicene domains even in the submonolayer coverage regime and consecutive transitions between silicene with different periodicity. These temperature-driven phase transitions can be exploited to select the desired majority silicene phase, thus allowing for the manipulation of silicene properties. Keywords: Silicene, phase transitions, STM, post-deposition annealing, Ag(1 1 1) (Some figures may appear in colour only in the online journal)
growth proven to occur upon onset values of temperature and coverage [5], but also surface phase diagrams were traced by tuning the growth temperature and the coverage [7, 10, 13]. As a consequence, the capability to select the desired silicene properties by switching among different phases would be extremely attractive either for the fundamental perspective or for the applicative one, because the electronic properties of silicene are expected to be dictated by the structural details of each peculiar phase [1, 14, 15]. Indeed, the electronic properties were recently scrutinized through the realization of a silicene-based transistor showing Dirac-like ambipolar behavior and carrier mobility of ∼100 cm2 V−1 s−1 [16]. A first example of phase transition was reported by Chen et al, who described the dynamic phase √ √ transition experienced at low temperature in the 3 × 3 silicene reconstruction, √ √ namely 4/ 3 × 4/ 3 Ag superstructure, below 40 K due to the symmetry breaking [17]. One may wonder whether also other superstructures might undergo similar phase transition depending on the temperature. Indeed, if so, this would open a novel scenario where the intrinsic properties of silicene phases could be artificially manipulated, thus achieving phase-dependent silicene transistors. Although phase diagrams depending on the growth temperature have
1. Introduction
Recently, the experimental discovery of silicene [1], the graphene counterpart of silicon, has attracted tremendous interest since it paved the way to the realization of synthetized two-dimensional (2D) crystal otherwise not existing in nature. Silicene was predicted in theory as a stable configuration of Si atoms arranged in a honeycomb structure and, in its freestanding configuration, it should share with graphene most of its properties, e.g. Dirac cones and quantum Hall effect, even if a vertical buckling, i.e., the vertical distortion of the silicene lattice, is predicted [1–4]. The first evidence of silicene was obtained by growing a Si monolayer on Ag(1 1 1) substrate, thus forming a surface superstructure with fourfold periodicity [1, 5–8]. However, several others superstructures (denoted as silicene phases in the following) were found thereby demonstrating the intrinsic polymorphism of the 2D Si layer and its flexibility to self-organize conformably with the given thermodynamic conditions [5, 8, 9]. Therefore, the Si on Ag(1 1 1) substrate should be intended as a multiphase system [10]. The buckling acts as an additional degree of freedom that can force the silicon nanosheet to match with the underlying substrate [5, 11, 12]. Not only was silicene 0953-8984/15/255005+07$33.00
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been already reported [7, 10, 13], the thermal dependence of the silicene phases as a function of the post-deposition annealing (PDA) has not been considered yet. These aspects become now important in light of the recent advances obtained on the silicene/Ag(1 1 1) system [16]. For this purpose, a systematic survey of silicene superstructures grown on Ag(1 1 1) by molecular beam epitaxy (MBE) and studied through scanning tunnelling microscopy (STM) by varying the nominal thicknesses and PDA temperatures is here reported.
each other [19]. Figure 1(a) shows the STM topography after 70 min long deposition (0.77 ML) at 523 K, where the presence of dotted spots clearly characterizes the adsorbate morphology [5, 18, 20]. These dots can be recognized as SiN clusters, which are the nuclei of silicene growth, as recently discussed by theoretical works [21, 22]. In these experimental conditions, the growth is dominated by small clusters (figure 1(a)), whose stability is provided by the Si–Ag interaction [21]. These clusters can be regarded as embryos of silicene superstructures. Although it is difficult from STM images to estimate the number of Si atoms composing these clusters, their dimensions (∼1.2 nm in diameter) seem to suggest that N 24. Indeed, this number is consistent with the minimal size of the measured clusters since Gao and Zhao showed that Si24 is composed by seven Si hexagons which form a stable circular structure with diameter of ∼1.2 nm [21]. In this framework, silicene domains are very small and, sometimes, dots are isolated thus confirming the stability of Si24 clusters. As already observed [20], the PDA plays an important role in determining the morphological properties. Indeed, after a PDA at the same growth temperature (523 K) for 30 min, the surface morphology results in the disappearance of SiN clusters and the concomitance emergence √ √ of 4 × 4 and 13 × 13 superstructured islands (figure 1(b)) separated by grain boundaries. In particular, figure 1(b) shows, in blue dashed and dotted contours, two of four possible √ √ differently rotated 13 × 13 phases [23]. The 4 × 4 √ √ and 13 × 13 phases were often observed to coexist, in particular at higher coverage, i.e. 1 ML [6, 10, 15, 20, 24]. This nucleation process can be understood in terms of the low diffusion barrier and high nucleation rate of silicon monomer on Ag(1 1 1) surface even at RT [21, 22], which allows for the aggregation/coalescence of small clusters and small silicene islands into a nearly continuous 2D silicene layer. Therefore, even in the submonolayer coverage regime and in the absence of Si atoms from the source, it is possible to obtain silicene domains by aggregation of small Si clusters, thus demonstrating that temperature is a critical parameter for silicene to grow on Ag. Another critical point is concerned with the presence of step edges in the pristine Ag(1 1 1) surface. Indeed, figure 2 shows a STM image where the role of step edge clearly results in the formation of silicene phases, in particular 4 × 4 and √ √ 13 × 13 phases in the present case, whereas far away from the edge the surface morphology is similar to that of figure 1(a). It is not surprising that, in presence of a low diffusion barrier on flat Ag(1 1 1) terraces, step edges play a key role in determining the surface structure. Shu et al calculated a threshold barrier for the diffusion of Si atoms on the different adsorption sites of Ag(1 1 1) surface amounting to 0.16 eV [22]. Since the diffusion across different terraces is assumed to be unfavourable, this implies that Si atoms are expected to migrate very easily to the terrace edges and then start to nucleate therein increasing the silicene domain size [10, 18, 25]. The reported STM image is representative of several others acquired in similar experimental conditions. Although a systematic survey about the role of edges in the Si growth is beyond the scope of the present work, the
2. Experimental methods
Experiments were carried out in ultra-high vacuum system with base pressure in the 10−10 mbar range. The Ag(1 1 1) crystal was cleaned by several cycles of Ar+ ion sputtering (1 keV—10−6 mbar) and subsequent annealing at 800 K. Si was deposited by MBE from a heated crucible with the substrate held at the constant temperature of 523 K. The deposition rate (1.1 × 10−2 ML min−1 ) was estimated by means of a quartz microbalance and by x-ray photoelectron spectroscopy (XPS). Temperature reading was crosschecked by pyrometer-based calibration of the thermocouple attached under the sample holder. PDA were performed in the temperature range between 523 and 563 K for 30 min. The temperature was constantly monitored during the whole PDA process and no temperature drift was observed. XPS measurements were performed at a take-off angle of 37◦ using a non-monochromatized MgKα source (1253.6 eV). Fit to the XPS lines involved Shirley background removal and decomposition in pseudo Voigt functions (products of Gaussians and Lorentzians). Doublets of Voigt functions were adopted when treating p core levels. STM topographies were obtained at room temperature (RT) using an Omicron STM setup equipped with a chemically etched tungsten tip. Tens of STM images were acquired at every stage. Those reported are the most representative of a thorough STM investigation statistically extended in a wide number of surface regions. 3. Results and discussion
In order to study the evolution from the initial stages and the thermal stability, the silicene phases were studied as a function of the nominal thickness and the PDA temperature. In particular, for the case of silicene on Ag(1 1 1), the growth is characterized by the coexistence of different superstructures which organize in domains with different periodicities and azimuths with respect to the Ag(1 1 1) substrate [5, 8, 10, 18]. Referring to √ √the Ag(1 ◦1 1) substrate, the 4 × 4 [1, 5–8, 10], ( 13 × [5, 6, 8, 10], √ √ √ 13)R13.9 √ ( 7 × 7)R19.1◦ [5], and (2 3 × 2 3)R30◦ [5, 7, 10] superstructures with different translational symmetries have been described by both low energy electron diffraction and STM. Details on the periodicities and angles can be found elsewhere [5, 8, 9, 13]. The coexistence of different phases, with different bond lengths, cell parameters, and buckling values, suggests the important role played by thermodynamics through the adsorbate formation energies of these structures, whose values have been calculated to be very close to 2
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Figure 1. STM topographies of 0.77 ML Si on Ag(1 1 1) 100 × 100 nm2 (a) and after PDA at the same growth temperature (523 K) for 2 30 √ min 70 √ × 70 nm (b). 4 × 4 phase is highlighted in green contour while blue dashed and dotted contours are related to differently rotated 13 × 13 domains, according to [23]. Both images were acquired at Ubias = −1.4 V and I = 0.5 nA.
phenomenological arguments supporting the STM data are the following. Recently, it has been recognized the role of hybridization between the Si and Ag atoms at the interface [14]. This means that the Si stabilization onto Ag(1 1 1) surface is mediated by topmost Ag atoms. In the framework of the surface diffusion theory [26], the nucleation at step edges is driven by the energy gain obtained from the bonding of Si atoms at specific sites on the steps. While quantitative values of a such barrier are well documented in the cases of metal homoepitaxy (for instance, Ag on Ag(1 1 1)) [27], at the moment there are no models for the Si/Ag system. Moreover, the Stranski–Krastanov growth mode proposed for the multilayer silicene [28] is consistent with the reported data, since such a growth regime involves a layer-by-layer growth in the initial stages. Indeed, the formation of smaller 2D Si islands is observed with increasing deposition and finally covering the Ag(1 1 1) terraces completely. It is then interesting to study the effects of PDA on already structured silicene layers. Figure 3(a) shows the STM topography acquired on a sample where the 4 × 4 superstructure (see inset of figure 3(a)) is the most abundant phase. However, 4 × 4 domains, depicted in green contours in figure 3(a), are typically smaller than Ag terraces. This morphology is obtained when the coverage is ∼1 ML (90 min long deposition time) with substrate temperature kept at 523 K. Nonetheless, it must be emphasized that 4 × 4 phase is unavoidably √ √ found to coexist with other phases such as the 13 × 13 one (figure 1(b)) [5, 6, 10, 15, 24]. After PDA at√543 K√for 30 min and quenching at RT, large domains √ √of 2 3 × 2 3 phase can be found (figure 3(b)). The 2 3 × 2 3 phase differs from the more conventional 4 × 4 one in that it exhibits a smaller periodicity (∼1 nm) than 4 × 4 phase (1.14 nm) and different buckling which results in the different STM topographies (see insets and hard-spheres models of figures 3(a) and (b) √ for comparison [5, 15]). In contrast to √ the 4 × 4 case, the 2 3 × 2 3 domains extend for hundreds
Figure 2. STM image 80 × 90 nm2 (Ubias = −1.2 V and I = 0.3 nA) showing the role of step edges in silicene nucleation.
of nm with boundary conditions dictated by the underlying Ag terraces (figure 3(b)). Hence, two different but complementary effects combine. The former is the aggregation of silicene SiN clusters resulting in an increase of phase size since domains √ √ of the 2 3 × 2 3 phase are larger than those of 4 × 4 (figures 3(a) and (b)), as observed in the submonolayer case (figure 1), with boundary conditions dictated by step edges (figure 2). On the other hand, the latter effect is represented by the phase transition, which involves a re-arrangement of the silicene periodicity. While the coalescence effect can be understood in terms of energy minimization, i.e., the increase of the cluster size implies the increase of the number of inner atoms rather than those at the edges [22], the effect of phase transition is not so obvious and must be addressed more in 3
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Figure 3. STM topographies (80 × 80 nm2 , Ubias = −1.4 V and I = 0.5 nA) of 1 ML Si on Ag(1 1 1) (top panel): √ as grown √ (a), after PDA
at 543 K√(b), and after second PDA at 563 K (c). Insets show details of the relative most abundant phases: 4 × 4, 2 3 × 2 3, and √ 13 × 13, respectively. Orange and yellow circles in the structural models of the three superstructures represent top and bottom Si atoms as they are imaged in STM (bottom panel), whereas grey spheres reflect the Ag surface lattice. Green, red, and blue rhombi define the unit cells of the respective silicene superstructures.
The effect of a second PDA at higher temperature (563 K) results in a further surface re-organization characterized by √ √ the presence of large domains of 13 × 13 superstructure (figure 3(c) and inset). In the considered thickness regime, √ √ i.e. 1 ML coverage, the 13 × 13 phase appears again only after the second PDA (figure 3(c)) as a dominant phase whereas it was usually observed to coexist with the 4 × 4 in the asgrown silicene layer. Such coexistence could be ascribed to competing effects [6, 10] that, on the other hand, can be bypassed by the PDA process. In addition, domains obtained in this third process step are comparable in size with those found in figure 3(b), thus indicating that the aggregation effect is saturated. Throughout the two described PDAs there is no chemical modification at the silicene/Ag(1 1 1) interface, as supported by a comparative study of the Si 2p/Ag 4s ratio in the respective XPS lines (data not shown). In summary, at the considered growth temperature and nominal thickness, i.e. 523 K and 1 ML, the starting phase is mainly 4 × 4 √ √ with small other contributions, mostly 13 × 13; then, √ √ after a first PDA at 543 K, it transforms into a 2 3 × 2 3 and lastly, after a second PDA at 563 K, the dominant phase √ √ becomes the 13 × 13. Although the described trend
detail. It is important to emphasize that the PDA process started from RT to a higher temperature (543 K) with respect to the growth one (523 K), thus confirming the (meta)stable character of the new silicene phase obtained after cooling down to RT. Although the presence of other coexisting phases cannot √ √ be ruled out, the observed 2 3 × 2 3 is largely prevailing compared to the as-grown condition in the limit of the STM probed areas, as also confirmed by Raman spectroscopy [15]. √ √ Indeed, the Raman spectrum of the 2 3 × 2 3 obtained after PDA exhibits different features compared to the one registered √ √ on the as-grown sample [15]. Unfortunately, the 2 3 × 2 3 phase, although with larger domain size than the 4 × 4 phase, is also fragmented in non-continuous domains (figure 3(b)). Even though this aspect has not been constrictive for other experimental characterizations, e.g. Raman spectroscopy [15], further improvements are required to synthesize higher quality silicene layers. It is also possible to notice that the number of bright clusters (indicated by yellow arrow in figure 3(b)) increase after PDA. It is likely to argue that a dewetting mechanism can be responsible for the accumulation of small clusters on top of silicene superstructure (see also discussion in the following) [10]. 4
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Figure 4. STM images (10 × 10 nm2 ) obtained after at 563 K when Ubias = +1.2 V (a) and Ubias = −1.2 V (b) with I = 0.5 nA. √ PDA√ (c) structural model of the topmost layer of the 4/ 3 × 4/ 3 phase where the black rhombus indicates the unit cell. (d) line profile along the red line indicated in (a). (e) XPS spectra of Si 2p (red dashed area) and Ag 4s core levels recorded before (top panel) and after (bottom panel) PDA at 563 K.
√ √ √ √ that in the last step ranging from 2 3 × 2 3 to 13 × 13 the biaxial strain changes from compressive to tensile, as well as the Si/Ag atomic ratio changes from high (1.17) to low (1.08) [19]. Furthermore, the observation that the formation energies of the silicene phases are very close to each other [19] supports the evidence of the small range of temperature in which the double phase transition takes place. In the general scheme of the absolute rate theory of the surface kinetic processes [29], consecutive transitions between structurally different phases can be regarded as the temperature-driven minimal path between surface states with very close Gibbs free energy. A still different phase is recovered when the PDA temperature is directly increased up to 563 K without passing through the intermediate stage at 543 K. The newly observed phase is displayed in the STM images of figures and (b) √ 4(a) √ as a superstructure that differs from both the 2 3 × 2 3 and
cannot be easily and linearly related to the main silicene properties, such as buckling, Si/Ag atomic ratios, and growth temperature [15, 18, 19], a qualitative explanation of the observed trend can be drawn on the basis of a recent theoretical work by Pflugradt et al [19]. Indeed, the authors reported the phase diagram of the silicene overlayers depending on the chemical potential, which could be related to excess√and defect √ of Si during growth [19]. It can be argued that 2 3 × 2 3 phase appears after the first PDA because the aggregation effect causes an increase of Si atoms nearby the terrace edge √ √ then inducing Si-rich conditions favourable to 2 3 × 2 3 formation. √ √ Moreover, according to Pflugradt et al [19] the 2 3 × 2 3 silicene is unlikely to grow close to equilibrium conditions rather than the 4 × 4 one. On the other hand, the second PDA could probably induce an additional dewetting, which is related√to Si-poor √ conditions then resulting in the formation of the 13× 13 phase [19]. It is also worth noting 5
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√
temperature. Concomitantly, the PDA is responsible for two additional effects: coalescence of silicene nuclei SiN and phase transition. It is worth noting that all silicene phases showed here are (meta)stable since they have been obtained after PDAs processes and scrutinized by STM at RT after cooling down. On the other hand, in the absence of kinetically limited conditions, dewetting takes place and results in a stable silicene multilayer. These findings point out that a careful tuning of the post-growth thermodynamic condition can be exploited to select majority silicene phase domains with average area larger than that of the as-grown silicene (compare figures 3(a) and (b) and (c)), thus paving the way for an advanced control of extended 2D silicene crystals.
13× 13 phases (see structural models of figures 3 and 4(c)). Indeed, a periodicity of 0.67 nm results from the STM profile in √ figure 4(d), √ which closely resembles that reported for the 4/√ 3 × √ 4/ 3 phase of √ [7,√13, 17, 30]. Similarly √ to domains √ 2 3 × 2 3 and 13 × 13, domains of 4/ 3 × 4/ 3 phase can cover the whole Ag terraces, but their average number is comparatively smaller. By switching the voltage bias from −1.2 (occupied states) to +1.2 V (empty states), to √ it is possible √ atomically resolve the topmost layer of the 4/ 3×4/ 3 phase (figures 4(a) and (b)). In the condition of figure 4(a) (Ubias = +1.2 V), it is possible to atomically resolve the structure of this overlayer by imaging the top Si atoms (depicted as orange √ circles √ in figure 4(c)), thus bringing evidence to the 4/ 3×4/ 3 model proposed in√[7, 17, 28, 30]. To rationalize √ the formation of the 4/ 3 × 4/ 3 phase, the additional role of a partial Si dewetting must be taken into account. To this respect, it can be noticed from figure 4(e) that the intensity ratio between the Si 2p and Ag 4s core level photoemission lines recorded before and after PDA decreases. This can be associated to Si desorption as well as to Si dewetting. While the former case can be ruled out because of the limited temperature √ √range here considered, in the latter case, the 4/ 3 × 4/ 3 phase can plausibly arise from the thermally driven detachment of silicene atoms from the first layer to a topmost layer, which is triggered by the higher thermal budget of the applied PDA process. The reduced Si concentration after PDA can be indirectly confirmed by the experienced ease with STM-probing Ag area with atomic resolution (otherwise hard to sample in the√given conditions). To support this √ picture, recently, the 4/ 3 × 4/ 3 phase was interpreted as a second and multi-layer silicene structure [13, 28, 30–32]. This means that previously mentioned dewetting mechanism leads to the formation of multilayer silicene islands. According to Moras et al [10], kinetically limited conditions lead to formation of Si clusters on top of silicene phases (see yellow arrow in figure 3(b)), otherwise multilayer formation and surface dewetting occur. On the other hand, in the range of temperatures here explored, we never observed complete formation of fully 3D structures as instead observed by Acun et al at higher PDA temperature [33] or by Moras et al at higher growth temperature [10].
Acknowledgments
The present research activity has been carried on within the framework of the EU project 2D-NANOLATTICES. The project 2D-NANOLATTICES acknowledges the financial support of the Future and Emerging Technologies (FET) programme within the Seventh Framework Programme for Research of the European Commission, under FET-Open grant number: 270749. References [1] Vogt P, De Padova P, Quaresima C, Avila J, Frantzeskakis E, Asensio M C, Resta A, Ealet B and Le Lay G 2012 Phys. Rev. Lett. 108 155501 [2] Cahangirov S, Topsakal M, Akturk E, Sahin H and Ciraci S 2009 Phys. Rev. Lett. 102 236804 [3] Guzm´an-Verri G G and Lew Yan Voon L C 2007 Phys. Rev. B 76 075131 [4] Houssa M, Pourtois G, Afanas’ev V V and Stesmans A 2010 Appl. Phys. Lett. 97 112106 [5] Chiappe D, Grazianetti C, Tallarida G, Fanciulli M and Molle A 2012 Adv. Mater. 24 5088 [6] Lin C L, Arafune R, Kawahara K, Tsukahara N, Minamitani E, Kim Y, Takagi N and Kawai M 2012 Appl. Phys. Express 5 045802 [7] Feng B, Ding Z, Meng S, Yao Y, He X, Cheng P, Chen L and Wu K 2012 Nano Lett. 12 3507 [8] Jamgotchian H, Colignon Y, Hamzaoui N, Ealet B, Hoarau Y, Aufray B and Bib´erian J P 2012 J. Phys. Condens. Matter 24 172001 [9] Enriquez H, Vizzini S, Kara A, Lalmi B and Oughaddou H 2012 J. Phys. Condens. Matter 24 314211 [10] Moras P, Mentes T O, Sheverdyaeva P M, Locatelli A and Carbone C 2014 J. Phys. Condens. Matter 26 185001 [11] Fleurence A, Friedlein R, Ozaki T, Kawai H, Wang Y and Yamada-Takamura Y 2012 Phys. Rev. Lett. 108 245501 [12] Meng L, Wang Y, Zhang L, Du S, Wu R, Li L, Zhang Y, Li G, Zhou H, Hofer W A and Gao H J 2013 Nano Lett. 13 685 [13] Arafune R, Lin C L, Kawahara K, Tsukahara N, Minamitani E, Kim Y, Takagi N and Kawai M 2013 Surf. Sci. 608 297 [14] Tsoutsou D, Xenogiannopoulou E, Golias E, Tsipas P and Dimoulas A 2013 Appl. Phys. Lett. 103 231604 [15] Cinquanta E, Scalise E, Chiappe D, Grazianetti C, van den Broek B, Houssa M, Fanciulli M and Molle A 2013 J. Phys. Chem. C 117 16719 [16] Tao L, Cinquanta E, Chiappe D, Grazianetti C, Fanciulli M, Dubey M, Molle A and Akinwande D 2015 Nat. Nanotechnol. 10 227
4. Conclusions
Summarizing, the critical role of PDA temperature in silicene growth was examined. Although the results were obtained on Ag(1 1 1) single crystal substrates, they can be equally extended also to Ag(1 1 1)/mica substrates. These latter substrates enabled the realization of silicene transistor by means of a synthesis-transfer-fabrication process [16]. In the submonolayer coverage regime, PDA induces the selforganization of silicene islands. For monolayer coverage, phase transitions among three main silicene superstructures can be induced by tuning the PDA temperature. This introduces an additional parameter in the silicene synthesis, which can allow to tune the structural properties of silicene phases. Interestingly, the superstructures obtained after PDA show domains larger than those obtained at the growth 6
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[17] Chen L, Li H, Feng B, Ding Z, Qiu J, Cheng P, Wu K and Meng S 2012 Phys. Rev. Lett. 109 056804 [18] Lee G W, Chen H D and Lin D S 2015 Appl. Surf. Sci. (DOI:10.1016/j.apsusc.2015.01.155) [19] Pflugradt P, Matthes L and Bechstedt F 2014 Phys. Rev. B 89 035403 [20] Grazianetti C, Chiappe D, Cinquanta E, Tallarida G, Fanciulli M and Molle A 2014 Appl. Surf. Sci. 291 109 [21] Gao J and Zhao J 2012 Sci. Rep. 2 861 [22] Shu H, Cao D, Liang P, Wang X, Chen X and Lu W 2014 Phys. Chem. Chem. Phys. 16 304 [23] Resta A, Leoni T, Barth C, Ranguis A, Becker, C, Bruhn T, Vogt P and Le Lay G 2013Sci. Rep. 3 2399 [24] Molle A, Grazianetti C, Chiappe D, Cinquanta E, Cianci E, Tallarida G and Fanciulli M 2013 Adv. Func. Mater. 23 4340 [25] Prevot G, Bernard R, Cruguel H and Borensztein Y 2014 Appl. Phys. Lett. 105 213106
[26] Barabasi A L and Stanley H E 1995 Fractal Concept in Surface Growth (Cambridge: Cambridge University Press) [27] Bromann K, Brune H, Roder H and Kern K 1995 Phys. Rev. Lett. 75 677 [28] Vogt P, Capiod P, Berthe M, Resta A, De Padova P, Bruhn T, Le Lay G and Grandidier B 2014 Appl. Phys. Lett. 104 021602 [29] Ibach H 2006 Physics of Surfaces and Interfaces (Berlin: Springer) [30] Salomon E, El Ajjouri R, Le Lay G and Angot T 2014 J. Phys. Condens. Matter 26 185003 [31] De Padova P et al 2013 Appl. Phys. Lett. 102 163106 [32] Shirai T, Shirasawa T, Hirahara T, Fukui N, Takahashi T and Hasegawa S 2014 Phys. Rev. B 89 241403(R) [33] Acun A, Poelsema B, Zandvliet H J W and van Gastel R 2013 Appl. Phys. Lett. 103 263119
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Nucleation and temperature-driven phase transitions of silicene superstructures on Ag(1 1 1)
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 J. Phys.: Condens. Matter 27 255005 (http://iopscience.iop.org/0953-8984/27/25/255005) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 139.184.14.159 This content was downloaded on 01/10/2015 at 04:02
Please note that terms and conditions apply.
Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 27 (2015) 255005 (7pp)
doi:10.1088/0953-8984/27/25/255005
Nucleation and temperature-driven phase transitions of silicene superstructures on Ag(1 1 1) C Grazianetti1 , D Chiappe1 , E Cinquanta1 , M Fanciulli1,2 and A Molle1 1
Laboratorio MDM, IMM-CNR, via C. Olivetti 2, I-20864 Agrate Brianza (MB), Italy Dipartimento di Scienza dei Materiali, Universit`a degli Studi di Milano Bicocca, via R. Cozzi 53, I-20126, Milano (MI), Italy
2
E-mail: [email protected] and [email protected] Received 11 February 2015, revised 1 April 2015 Accepted for publication 15 April 2015 Published 28 May 2015 Abstract
Silicene grown on Ag(1 1 1) is characterized by several critical parameters. Among them, the substrate temperature plays a key role in determining the morphology during growth. However, an unexpected important role is also equally played by the post-deposition annealing temperature which determines the self-organization of silicene domains even in the submonolayer coverage regime and consecutive transitions between silicene with different periodicity. These temperature-driven phase transitions can be exploited to select the desired majority silicene phase, thus allowing for the manipulation of silicene properties. Keywords: Silicene, phase transitions, STM, post-deposition annealing, Ag(1 1 1) (Some figures may appear in colour only in the online journal)
growth proven to occur upon onset values of temperature and coverage [5], but also surface phase diagrams were traced by tuning the growth temperature and the coverage [7, 10, 13]. As a consequence, the capability to select the desired silicene properties by switching among different phases would be extremely attractive either for the fundamental perspective or for the applicative one, because the electronic properties of silicene are expected to be dictated by the structural details of each peculiar phase [1, 14, 15]. Indeed, the electronic properties were recently scrutinized through the realization of a silicene-based transistor showing Dirac-like ambipolar behavior and carrier mobility of ∼100 cm2 V−1 s−1 [16]. A first example of phase transition was reported by Chen et al, who described the dynamic phase √ √ transition experienced at low temperature in the 3 × 3 silicene reconstruction, √ √ namely 4/ 3 × 4/ 3 Ag superstructure, below 40 K due to the symmetry breaking [17]. One may wonder whether also other superstructures might undergo similar phase transition depending on the temperature. Indeed, if so, this would open a novel scenario where the intrinsic properties of silicene phases could be artificially manipulated, thus achieving phase-dependent silicene transistors. Although phase diagrams depending on the growth temperature have
1. Introduction
Recently, the experimental discovery of silicene [1], the graphene counterpart of silicon, has attracted tremendous interest since it paved the way to the realization of synthetized two-dimensional (2D) crystal otherwise not existing in nature. Silicene was predicted in theory as a stable configuration of Si atoms arranged in a honeycomb structure and, in its freestanding configuration, it should share with graphene most of its properties, e.g. Dirac cones and quantum Hall effect, even if a vertical buckling, i.e., the vertical distortion of the silicene lattice, is predicted [1–4]. The first evidence of silicene was obtained by growing a Si monolayer on Ag(1 1 1) substrate, thus forming a surface superstructure with fourfold periodicity [1, 5–8]. However, several others superstructures (denoted as silicene phases in the following) were found thereby demonstrating the intrinsic polymorphism of the 2D Si layer and its flexibility to self-organize conformably with the given thermodynamic conditions [5, 8, 9]. Therefore, the Si on Ag(1 1 1) substrate should be intended as a multiphase system [10]. The buckling acts as an additional degree of freedom that can force the silicon nanosheet to match with the underlying substrate [5, 11, 12]. Not only was silicene 0953-8984/15/255005+07$33.00
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been already reported [7, 10, 13], the thermal dependence of the silicene phases as a function of the post-deposition annealing (PDA) has not been considered yet. These aspects become now important in light of the recent advances obtained on the silicene/Ag(1 1 1) system [16]. For this purpose, a systematic survey of silicene superstructures grown on Ag(1 1 1) by molecular beam epitaxy (MBE) and studied through scanning tunnelling microscopy (STM) by varying the nominal thicknesses and PDA temperatures is here reported.
each other [19]. Figure 1(a) shows the STM topography after 70 min long deposition (0.77 ML) at 523 K, where the presence of dotted spots clearly characterizes the adsorbate morphology [5, 18, 20]. These dots can be recognized as SiN clusters, which are the nuclei of silicene growth, as recently discussed by theoretical works [21, 22]. In these experimental conditions, the growth is dominated by small clusters (figure 1(a)), whose stability is provided by the Si–Ag interaction [21]. These clusters can be regarded as embryos of silicene superstructures. Although it is difficult from STM images to estimate the number of Si atoms composing these clusters, their dimensions (∼1.2 nm in diameter) seem to suggest that N 24. Indeed, this number is consistent with the minimal size of the measured clusters since Gao and Zhao showed that Si24 is composed by seven Si hexagons which form a stable circular structure with diameter of ∼1.2 nm [21]. In this framework, silicene domains are very small and, sometimes, dots are isolated thus confirming the stability of Si24 clusters. As already observed [20], the PDA plays an important role in determining the morphological properties. Indeed, after a PDA at the same growth temperature (523 K) for 30 min, the surface morphology results in the disappearance of SiN clusters and the concomitance emergence √ √ of 4 × 4 and 13 × 13 superstructured islands (figure 1(b)) separated by grain boundaries. In particular, figure 1(b) shows, in blue dashed and dotted contours, two of four possible √ √ differently rotated 13 × 13 phases [23]. The 4 × 4 √ √ and 13 × 13 phases were often observed to coexist, in particular at higher coverage, i.e. 1 ML [6, 10, 15, 20, 24]. This nucleation process can be understood in terms of the low diffusion barrier and high nucleation rate of silicon monomer on Ag(1 1 1) surface even at RT [21, 22], which allows for the aggregation/coalescence of small clusters and small silicene islands into a nearly continuous 2D silicene layer. Therefore, even in the submonolayer coverage regime and in the absence of Si atoms from the source, it is possible to obtain silicene domains by aggregation of small Si clusters, thus demonstrating that temperature is a critical parameter for silicene to grow on Ag. Another critical point is concerned with the presence of step edges in the pristine Ag(1 1 1) surface. Indeed, figure 2 shows a STM image where the role of step edge clearly results in the formation of silicene phases, in particular 4 × 4 and √ √ 13 × 13 phases in the present case, whereas far away from the edge the surface morphology is similar to that of figure 1(a). It is not surprising that, in presence of a low diffusion barrier on flat Ag(1 1 1) terraces, step edges play a key role in determining the surface structure. Shu et al calculated a threshold barrier for the diffusion of Si atoms on the different adsorption sites of Ag(1 1 1) surface amounting to 0.16 eV [22]. Since the diffusion across different terraces is assumed to be unfavourable, this implies that Si atoms are expected to migrate very easily to the terrace edges and then start to nucleate therein increasing the silicene domain size [10, 18, 25]. The reported STM image is representative of several others acquired in similar experimental conditions. Although a systematic survey about the role of edges in the Si growth is beyond the scope of the present work, the
2. Experimental methods
Experiments were carried out in ultra-high vacuum system with base pressure in the 10−10 mbar range. The Ag(1 1 1) crystal was cleaned by several cycles of Ar+ ion sputtering (1 keV—10−6 mbar) and subsequent annealing at 800 K. Si was deposited by MBE from a heated crucible with the substrate held at the constant temperature of 523 K. The deposition rate (1.1 × 10−2 ML min−1 ) was estimated by means of a quartz microbalance and by x-ray photoelectron spectroscopy (XPS). Temperature reading was crosschecked by pyrometer-based calibration of the thermocouple attached under the sample holder. PDA were performed in the temperature range between 523 and 563 K for 30 min. The temperature was constantly monitored during the whole PDA process and no temperature drift was observed. XPS measurements were performed at a take-off angle of 37◦ using a non-monochromatized MgKα source (1253.6 eV). Fit to the XPS lines involved Shirley background removal and decomposition in pseudo Voigt functions (products of Gaussians and Lorentzians). Doublets of Voigt functions were adopted when treating p core levels. STM topographies were obtained at room temperature (RT) using an Omicron STM setup equipped with a chemically etched tungsten tip. Tens of STM images were acquired at every stage. Those reported are the most representative of a thorough STM investigation statistically extended in a wide number of surface regions. 3. Results and discussion
In order to study the evolution from the initial stages and the thermal stability, the silicene phases were studied as a function of the nominal thickness and the PDA temperature. In particular, for the case of silicene on Ag(1 1 1), the growth is characterized by the coexistence of different superstructures which organize in domains with different periodicities and azimuths with respect to the Ag(1 1 1) substrate [5, 8, 10, 18]. Referring to √ √the Ag(1 ◦1 1) substrate, the 4 × 4 [1, 5–8, 10], ( 13 × [5, 6, 8, 10], √ √ √ 13)R13.9 √ ( 7 × 7)R19.1◦ [5], and (2 3 × 2 3)R30◦ [5, 7, 10] superstructures with different translational symmetries have been described by both low energy electron diffraction and STM. Details on the periodicities and angles can be found elsewhere [5, 8, 9, 13]. The coexistence of different phases, with different bond lengths, cell parameters, and buckling values, suggests the important role played by thermodynamics through the adsorbate formation energies of these structures, whose values have been calculated to be very close to 2
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Figure 1. STM topographies of 0.77 ML Si on Ag(1 1 1) 100 × 100 nm2 (a) and after PDA at the same growth temperature (523 K) for 2 30 √ min 70 √ × 70 nm (b). 4 × 4 phase is highlighted in green contour while blue dashed and dotted contours are related to differently rotated 13 × 13 domains, according to [23]. Both images were acquired at Ubias = −1.4 V and I = 0.5 nA.
phenomenological arguments supporting the STM data are the following. Recently, it has been recognized the role of hybridization between the Si and Ag atoms at the interface [14]. This means that the Si stabilization onto Ag(1 1 1) surface is mediated by topmost Ag atoms. In the framework of the surface diffusion theory [26], the nucleation at step edges is driven by the energy gain obtained from the bonding of Si atoms at specific sites on the steps. While quantitative values of a such barrier are well documented in the cases of metal homoepitaxy (for instance, Ag on Ag(1 1 1)) [27], at the moment there are no models for the Si/Ag system. Moreover, the Stranski–Krastanov growth mode proposed for the multilayer silicene [28] is consistent with the reported data, since such a growth regime involves a layer-by-layer growth in the initial stages. Indeed, the formation of smaller 2D Si islands is observed with increasing deposition and finally covering the Ag(1 1 1) terraces completely. It is then interesting to study the effects of PDA on already structured silicene layers. Figure 3(a) shows the STM topography acquired on a sample where the 4 × 4 superstructure (see inset of figure 3(a)) is the most abundant phase. However, 4 × 4 domains, depicted in green contours in figure 3(a), are typically smaller than Ag terraces. This morphology is obtained when the coverage is ∼1 ML (90 min long deposition time) with substrate temperature kept at 523 K. Nonetheless, it must be emphasized that 4 × 4 phase is unavoidably √ √ found to coexist with other phases such as the 13 × 13 one (figure 1(b)) [5, 6, 10, 15, 24]. After PDA at√543 K√for 30 min and quenching at RT, large domains √ √of 2 3 × 2 3 phase can be found (figure 3(b)). The 2 3 × 2 3 phase differs from the more conventional 4 × 4 one in that it exhibits a smaller periodicity (∼1 nm) than 4 × 4 phase (1.14 nm) and different buckling which results in the different STM topographies (see insets and hard-spheres models of figures 3(a) and (b) √ for comparison [5, 15]). In contrast to √ the 4 × 4 case, the 2 3 × 2 3 domains extend for hundreds
Figure 2. STM image 80 × 90 nm2 (Ubias = −1.2 V and I = 0.3 nA) showing the role of step edges in silicene nucleation.
of nm with boundary conditions dictated by the underlying Ag terraces (figure 3(b)). Hence, two different but complementary effects combine. The former is the aggregation of silicene SiN clusters resulting in an increase of phase size since domains √ √ of the 2 3 × 2 3 phase are larger than those of 4 × 4 (figures 3(a) and (b)), as observed in the submonolayer case (figure 1), with boundary conditions dictated by step edges (figure 2). On the other hand, the latter effect is represented by the phase transition, which involves a re-arrangement of the silicene periodicity. While the coalescence effect can be understood in terms of energy minimization, i.e., the increase of the cluster size implies the increase of the number of inner atoms rather than those at the edges [22], the effect of phase transition is not so obvious and must be addressed more in 3
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Figure 3. STM topographies (80 × 80 nm2 , Ubias = −1.4 V and I = 0.5 nA) of 1 ML Si on Ag(1 1 1) (top panel): √ as grown √ (a), after PDA
at 543 K√(b), and after second PDA at 563 K (c). Insets show details of the relative most abundant phases: 4 × 4, 2 3 × 2 3, and √ 13 × 13, respectively. Orange and yellow circles in the structural models of the three superstructures represent top and bottom Si atoms as they are imaged in STM (bottom panel), whereas grey spheres reflect the Ag surface lattice. Green, red, and blue rhombi define the unit cells of the respective silicene superstructures.
The effect of a second PDA at higher temperature (563 K) results in a further surface re-organization characterized by √ √ the presence of large domains of 13 × 13 superstructure (figure 3(c) and inset). In the considered thickness regime, √ √ i.e. 1 ML coverage, the 13 × 13 phase appears again only after the second PDA (figure 3(c)) as a dominant phase whereas it was usually observed to coexist with the 4 × 4 in the asgrown silicene layer. Such coexistence could be ascribed to competing effects [6, 10] that, on the other hand, can be bypassed by the PDA process. In addition, domains obtained in this third process step are comparable in size with those found in figure 3(b), thus indicating that the aggregation effect is saturated. Throughout the two described PDAs there is no chemical modification at the silicene/Ag(1 1 1) interface, as supported by a comparative study of the Si 2p/Ag 4s ratio in the respective XPS lines (data not shown). In summary, at the considered growth temperature and nominal thickness, i.e. 523 K and 1 ML, the starting phase is mainly 4 × 4 √ √ with small other contributions, mostly 13 × 13; then, √ √ after a first PDA at 543 K, it transforms into a 2 3 × 2 3 and lastly, after a second PDA at 563 K, the dominant phase √ √ becomes the 13 × 13. Although the described trend
detail. It is important to emphasize that the PDA process started from RT to a higher temperature (543 K) with respect to the growth one (523 K), thus confirming the (meta)stable character of the new silicene phase obtained after cooling down to RT. Although the presence of other coexisting phases cannot √ √ be ruled out, the observed 2 3 × 2 3 is largely prevailing compared to the as-grown condition in the limit of the STM probed areas, as also confirmed by Raman spectroscopy [15]. √ √ Indeed, the Raman spectrum of the 2 3 × 2 3 obtained after PDA exhibits different features compared to the one registered √ √ on the as-grown sample [15]. Unfortunately, the 2 3 × 2 3 phase, although with larger domain size than the 4 × 4 phase, is also fragmented in non-continuous domains (figure 3(b)). Even though this aspect has not been constrictive for other experimental characterizations, e.g. Raman spectroscopy [15], further improvements are required to synthesize higher quality silicene layers. It is also possible to notice that the number of bright clusters (indicated by yellow arrow in figure 3(b)) increase after PDA. It is likely to argue that a dewetting mechanism can be responsible for the accumulation of small clusters on top of silicene superstructure (see also discussion in the following) [10]. 4
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Figure 4. STM images (10 × 10 nm2 ) obtained after at 563 K when Ubias = +1.2 V (a) and Ubias = −1.2 V (b) with I = 0.5 nA. √ PDA√ (c) structural model of the topmost layer of the 4/ 3 × 4/ 3 phase where the black rhombus indicates the unit cell. (d) line profile along the red line indicated in (a). (e) XPS spectra of Si 2p (red dashed area) and Ag 4s core levels recorded before (top panel) and after (bottom panel) PDA at 563 K.
√ √ √ √ that in the last step ranging from 2 3 × 2 3 to 13 × 13 the biaxial strain changes from compressive to tensile, as well as the Si/Ag atomic ratio changes from high (1.17) to low (1.08) [19]. Furthermore, the observation that the formation energies of the silicene phases are very close to each other [19] supports the evidence of the small range of temperature in which the double phase transition takes place. In the general scheme of the absolute rate theory of the surface kinetic processes [29], consecutive transitions between structurally different phases can be regarded as the temperature-driven minimal path between surface states with very close Gibbs free energy. A still different phase is recovered when the PDA temperature is directly increased up to 563 K without passing through the intermediate stage at 543 K. The newly observed phase is displayed in the STM images of figures and (b) √ 4(a) √ as a superstructure that differs from both the 2 3 × 2 3 and
cannot be easily and linearly related to the main silicene properties, such as buckling, Si/Ag atomic ratios, and growth temperature [15, 18, 19], a qualitative explanation of the observed trend can be drawn on the basis of a recent theoretical work by Pflugradt et al [19]. Indeed, the authors reported the phase diagram of the silicene overlayers depending on the chemical potential, which could be related to excess√and defect √ of Si during growth [19]. It can be argued that 2 3 × 2 3 phase appears after the first PDA because the aggregation effect causes an increase of Si atoms nearby the terrace edge √ √ then inducing Si-rich conditions favourable to 2 3 × 2 3 formation. √ √ Moreover, according to Pflugradt et al [19] the 2 3 × 2 3 silicene is unlikely to grow close to equilibrium conditions rather than the 4 × 4 one. On the other hand, the second PDA could probably induce an additional dewetting, which is related√to Si-poor √ conditions then resulting in the formation of the 13× 13 phase [19]. It is also worth noting 5
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√
temperature. Concomitantly, the PDA is responsible for two additional effects: coalescence of silicene nuclei SiN and phase transition. It is worth noting that all silicene phases showed here are (meta)stable since they have been obtained after PDAs processes and scrutinized by STM at RT after cooling down. On the other hand, in the absence of kinetically limited conditions, dewetting takes place and results in a stable silicene multilayer. These findings point out that a careful tuning of the post-growth thermodynamic condition can be exploited to select majority silicene phase domains with average area larger than that of the as-grown silicene (compare figures 3(a) and (b) and (c)), thus paving the way for an advanced control of extended 2D silicene crystals.
13× 13 phases (see structural models of figures 3 and 4(c)). Indeed, a periodicity of 0.67 nm results from the STM profile in √ figure 4(d), √ which closely resembles that reported for the 4/√ 3 × √ 4/ 3 phase of √ [7,√13, 17, 30]. Similarly √ to domains √ 2 3 × 2 3 and 13 × 13, domains of 4/ 3 × 4/ 3 phase can cover the whole Ag terraces, but their average number is comparatively smaller. By switching the voltage bias from −1.2 (occupied states) to +1.2 V (empty states), to √ it is possible √ atomically resolve the topmost layer of the 4/ 3×4/ 3 phase (figures 4(a) and (b)). In the condition of figure 4(a) (Ubias = +1.2 V), it is possible to atomically resolve the structure of this overlayer by imaging the top Si atoms (depicted as orange √ circles √ in figure 4(c)), thus bringing evidence to the 4/ 3×4/ 3 model proposed in√[7, 17, 28, 30]. To rationalize √ the formation of the 4/ 3 × 4/ 3 phase, the additional role of a partial Si dewetting must be taken into account. To this respect, it can be noticed from figure 4(e) that the intensity ratio between the Si 2p and Ag 4s core level photoemission lines recorded before and after PDA decreases. This can be associated to Si desorption as well as to Si dewetting. While the former case can be ruled out because of the limited temperature √ √range here considered, in the latter case, the 4/ 3 × 4/ 3 phase can plausibly arise from the thermally driven detachment of silicene atoms from the first layer to a topmost layer, which is triggered by the higher thermal budget of the applied PDA process. The reduced Si concentration after PDA can be indirectly confirmed by the experienced ease with STM-probing Ag area with atomic resolution (otherwise hard to sample in the√given conditions). To support this √ picture, recently, the 4/ 3 × 4/ 3 phase was interpreted as a second and multi-layer silicene structure [13, 28, 30–32]. This means that previously mentioned dewetting mechanism leads to the formation of multilayer silicene islands. According to Moras et al [10], kinetically limited conditions lead to formation of Si clusters on top of silicene phases (see yellow arrow in figure 3(b)), otherwise multilayer formation and surface dewetting occur. On the other hand, in the range of temperatures here explored, we never observed complete formation of fully 3D structures as instead observed by Acun et al at higher PDA temperature [33] or by Moras et al at higher growth temperature [10].
Acknowledgments
The present research activity has been carried on within the framework of the EU project 2D-NANOLATTICES. The project 2D-NANOLATTICES acknowledges the financial support of the Future and Emerging Technologies (FET) programme within the Seventh Framework Programme for Research of the European Commission, under FET-Open grant number: 270749. References [1] Vogt P, De Padova P, Quaresima C, Avila J, Frantzeskakis E, Asensio M C, Resta A, Ealet B and Le Lay G 2012 Phys. Rev. Lett. 108 155501 [2] Cahangirov S, Topsakal M, Akturk E, Sahin H and Ciraci S 2009 Phys. Rev. Lett. 102 236804 [3] Guzm´an-Verri G G and Lew Yan Voon L C 2007 Phys. Rev. B 76 075131 [4] Houssa M, Pourtois G, Afanas’ev V V and Stesmans A 2010 Appl. Phys. Lett. 97 112106 [5] Chiappe D, Grazianetti C, Tallarida G, Fanciulli M and Molle A 2012 Adv. Mater. 24 5088 [6] Lin C L, Arafune R, Kawahara K, Tsukahara N, Minamitani E, Kim Y, Takagi N and Kawai M 2012 Appl. Phys. Express 5 045802 [7] Feng B, Ding Z, Meng S, Yao Y, He X, Cheng P, Chen L and Wu K 2012 Nano Lett. 12 3507 [8] Jamgotchian H, Colignon Y, Hamzaoui N, Ealet B, Hoarau Y, Aufray B and Bib´erian J P 2012 J. Phys. Condens. Matter 24 172001 [9] Enriquez H, Vizzini S, Kara A, Lalmi B and Oughaddou H 2012 J. Phys. Condens. Matter 24 314211 [10] Moras P, Mentes T O, Sheverdyaeva P M, Locatelli A and Carbone C 2014 J. Phys. Condens. Matter 26 185001 [11] Fleurence A, Friedlein R, Ozaki T, Kawai H, Wang Y and Yamada-Takamura Y 2012 Phys. Rev. Lett. 108 245501 [12] Meng L, Wang Y, Zhang L, Du S, Wu R, Li L, Zhang Y, Li G, Zhou H, Hofer W A and Gao H J 2013 Nano Lett. 13 685 [13] Arafune R, Lin C L, Kawahara K, Tsukahara N, Minamitani E, Kim Y, Takagi N and Kawai M 2013 Surf. Sci. 608 297 [14] Tsoutsou D, Xenogiannopoulou E, Golias E, Tsipas P and Dimoulas A 2013 Appl. Phys. Lett. 103 231604 [15] Cinquanta E, Scalise E, Chiappe D, Grazianetti C, van den Broek B, Houssa M, Fanciulli M and Molle A 2013 J. Phys. Chem. C 117 16719 [16] Tao L, Cinquanta E, Chiappe D, Grazianetti C, Fanciulli M, Dubey M, Molle A and Akinwande D 2015 Nat. Nanotechnol. 10 227
4. Conclusions
Summarizing, the critical role of PDA temperature in silicene growth was examined. Although the results were obtained on Ag(1 1 1) single crystal substrates, they can be equally extended also to Ag(1 1 1)/mica substrates. These latter substrates enabled the realization of silicene transistor by means of a synthesis-transfer-fabrication process [16]. In the submonolayer coverage regime, PDA induces the selforganization of silicene islands. For monolayer coverage, phase transitions among three main silicene superstructures can be induced by tuning the PDA temperature. This introduces an additional parameter in the silicene synthesis, which can allow to tune the structural properties of silicene phases. Interestingly, the superstructures obtained after PDA show domains larger than those obtained at the growth 6
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[17] Chen L, Li H, Feng B, Ding Z, Qiu J, Cheng P, Wu K and Meng S 2012 Phys. Rev. Lett. 109 056804 [18] Lee G W, Chen H D and Lin D S 2015 Appl. Surf. Sci. (DOI:10.1016/j.apsusc.2015.01.155) [19] Pflugradt P, Matthes L and Bechstedt F 2014 Phys. Rev. B 89 035403 [20] Grazianetti C, Chiappe D, Cinquanta E, Tallarida G, Fanciulli M and Molle A 2014 Appl. Surf. Sci. 291 109 [21] Gao J and Zhao J 2012 Sci. Rep. 2 861 [22] Shu H, Cao D, Liang P, Wang X, Chen X and Lu W 2014 Phys. Chem. Chem. Phys. 16 304 [23] Resta A, Leoni T, Barth C, Ranguis A, Becker, C, Bruhn T, Vogt P and Le Lay G 2013Sci. Rep. 3 2399 [24] Molle A, Grazianetti C, Chiappe D, Cinquanta E, Cianci E, Tallarida G and Fanciulli M 2013 Adv. Func. Mater. 23 4340 [25] Prevot G, Bernard R, Cruguel H and Borensztein Y 2014 Appl. Phys. Lett. 105 213106
[26] Barabasi A L and Stanley H E 1995 Fractal Concept in Surface Growth (Cambridge: Cambridge University Press) [27] Bromann K, Brune H, Roder H and Kern K 1995 Phys. Rev. Lett. 75 677 [28] Vogt P, Capiod P, Berthe M, Resta A, De Padova P, Bruhn T, Le Lay G and Grandidier B 2014 Appl. Phys. Lett. 104 021602 [29] Ibach H 2006 Physics of Surfaces and Interfaces (Berlin: Springer) [30] Salomon E, El Ajjouri R, Le Lay G and Angot T 2014 J. Phys. Condens. Matter 26 185003 [31] De Padova P et al 2013 Appl. Phys. Lett. 102 163106 [32] Shirai T, Shirasawa T, Hirahara T, Fukui N, Takahashi T and Hasegawa S 2014 Phys. Rev. B 89 241403(R) [33] Acun A, Poelsema B, Zandvliet H J W and van Gastel R 2013 Appl. Phys. Lett. 103 263119
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