ZnO Nanostructure Formation on the Mo(001) Surface - The Journal of

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ZnO Nanostructure Formation on the Mo(001) Surface Ilaria Valenti,†,‡ Stefania Benedetti,*,‡ Alessandro di Bona,‡ and Sergio Valeri†,‡ †

Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università di Modena e Reggio Emilia, via Campi 213/a, 41125 Modena, Italy ‡ CNR-Istituto Nanoscienze, S3, via Campi 213/a, 41125 Modena, Italy ABSTRACT: In this paper we have determined the role of oxidizing pressure and annealing temperature in the formation of ZnO nanostructures on the Mo surface. ZnO has been grown on Mo(001) by means of physical vapor deposition of Zn in molecular oxygen pressure. Combining STM, XPS, and XRD we have studied the relation between morphology, composition, and structure. ZnO grows as a film with (101̅1) preferential orientation up to a thickness of 10 Å, catalyzed by the Mo surface that provides oxygen available for oxide formation and limited by the residual polarity of the surface. Increasing the deposited amount, the role of the Mo interface becomes less effective, and part of the Zn remains metallic, forming clusters on the surface that catalyze the further growth of ZnO in the form of polycrystalline nanostructures. Surfactant Zn is removed by annealing, and peculiar oxide nanostructures are left on the surface. Increasing O pressure or reducing Zn flux allows control of nanostructure density and shape, while the continuous film underneath does not modify.

1. INTRODUCTION Zinc oxide has been widely studied for its interesting chemical and optical properties in a range of applications from heterogeneous catalysis to gas sensing, optoelectronics devices, transparent oxide applications for smart windows, and solar cells.1−5 For many applications ZnO can be prepared in a wide variety of nanostructures, grown along the c-axis and preferentially exposing nonpolar surfaces.6 Interests are therefore focused on a deeper knowledge of properties of both polar and nonpolar surfaces, for which model systems are particularly useful. Zinc oxide has been grown as a model system in the form of thin film on a variety of substrates to obtain highly oriented crystals and nanostructures. ZnO is stable in the wurtzite structure, with lattice parameters a = 3.25 Å and c = 5.21 Å. In the bulk ZnO crystal lattice, growth rates V are in the relation V(101̅0) < V(0001) < V(101̅1) < V(112̅0),7 with surface energy E(112̅0) < E(101̅0) < E(101̅1) < E(0001)−Zn,8 indicating as favored the exposure of both polar and nonpolar surfaces, together with oblique surfaces like the semipolar (101̅1), reported in the literature for nanostructure growth.9 Besides the thermodynamic stability in the bulk, other factors strongly influence the surface exposed by films and nanostructures, such as the substrate orientation and its epitaxial relationship with the overlayer,10 the growth conditions that can change the thermodynamic stability of surfaces,11 interfacial interactions that can deeply influence the initial growth mechanisms, and hence composition, chemical properties, and structural evolution of films.12 Studies have mainly focused on ZnO thin films grown along the c-axis on the (111) or (0001) oriented substrates.13−18 Attempts have been made also to obtain a-plane and m-plane ZnO on sapphire19,20 and on (001) cubic supports, like MgO, Mo, Ag, and Rh,21−25 often reporting a coexistence of different orientations, a c-axis growth, or a polycrystalline film.23,24,26 However, in most cases the initial © 2015 American Chemical Society

mechanisms of formation and evolution have not been clarified, reporting in few studies the morphology and rarely the combination with an analysis of composition either with chemical sensitive techniques or diffraction. Previous works have shown that growth conditions (in particular oxidizing agent and pressure) influence the quality of the obtained film. In particular insufficient (or not sufficiently active) oxygen pressure can lead to a noncomplete oxidation of Zn during growth.15 Moreover, it has been shown that reduced oxygen chemical potential determines the formation of nanostructures during the annealing process of a ZnO film, due to nucleation induced by the presence of substrate atoms on the film surface.27 Nevertheless, it has never been clarified if the presence of a metallic component plays a role and can control the surface morphology during growth. In this work ZnO has been grown on Mo(001) by physical vapor deposition of Zn in molecular oxygen atmosphere. Combining STM and XPS we have studied the relation between surface morphology and composition, allowing control of the growth of peculiar ZnO nanostructures. Mo has a bcc structure with a lattice parameter of 3.15 Å and a lattice mismatch of 3.2% with the rectangular ZnO(101̅0) in wurtzite structure, as it occurs for ZnO growth on MgO(001),21 or a ∼5/3 matching with the quasi-square (112̅0). Therefore, the exposure of a nonpolar surface is expected. By means of XRD it has been found that, contrary to what is observed for other cubic supports like MgO and Ag, the film has a preferential (1011̅ ) orientation that results in a small mismatch with the substrate, in spite of its polarity. Received: April 8, 2015 Revised: May 27, 2015 Published: May 27, 2015 13743

DOI: 10.1021/acs.jpcc.5b03391 J. Phys. Chem. C 2015, 119, 13743−13749

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The Journal of Physical Chemistry C

2. EXPERIMENTAL SECTION ZnO has been grown on a Mo(001) single crystal, prepared under UHV conditions by cycles of Ar+ ion bombardment at room temperature (RT), followed by sputtering at 1150 K. The surface is almost free of O and C, and a (1 × 1) LEED pattern is visible. ZnO films have been grown by evaporating Zn with an effusion cell on the clean Mo surface kept at RT in an O2 partial pressure varying from 1 × 10−7 to 5 × 10−6 mbar (base pressure 2 × 10−10 mbar). Gas is introduced by means of a nozzle placed at 1 cm from the sample surface. The Zn atomic flux (RZn) has been measured with a quartz crystal microbalance placed at the sample position and has been varied between 7 × 10−3 and 3 × 10−2 Å/s. XPS and Auger spectra were recorded using a nonmonochromatized Al−Kα X-ray source and hemispherical electrostatic analyzer, with an angular acceptance of ±8° to smear out possible diffraction effects. The overall energy resolution, dominated by the width of the Al-Kα line, is about 1 eV. Normal emission spectra have been recorded at a 0° angle from the normal to the sample surface, while for grazing emission an angle of 65° has been used. XPS intensities have been determined by measuring the areas of the peaks after background subtraction. Standard XPS reference spectra have been measured on a ZnO single crystal cleaned by sputtering and annealing for ZnO reference and on a thick Zn film grown on Mo(001) for Zn reference. The morphology of ZnO films has been investigated in situ by STM (UHV RT Omicron), working in constant current mode, with the W tip kept grounded and a positive bias voltage applied to the sample. STM images have been analyzed by means of the WSxM program.28 X-ray diffraction (XRD) plots have been measured by means of a Cu-Kα X-ray source and a diffractometer (Panalytical X’Pert PRO) with θ−2θ geometry, flat monochromator, and parallel plate.

Figure 1. STM images (100 × 100 nm2) of (a) 7 Å (U = 1 V), (b) 10 Å (U = 1.5 V), and (c) 20 Å ZnO deposited at RT on Mo(001) at P(O2) = 1 × 10−6 mbar (U = 2 V). (d) Same sample as (c) after annealing at 500 K in O2. (e) Nanostructures density versus total thickness (dashed line indicates the transition in the growth regime) and (f) nanostructure height versus diameter distribution for 10 and 20 Å images. Dark line is a linear fit indicating an aspect ratio of 0.5.

3. RESULTS AND DISCUSSION Nanostructure Evolution. Figure 1 shows STM images of the system when an increasing amount of Zn is deposited on the Mo(001) surface at RT in molecular oxygen [P(O2) = 1 × 10−6 mbar, RZn = 3 × 10−2 Å/s]. The most evident feature is a change in growth mechanism at a thickness threshold of 10 Å. Below 10 Å of total deposited material a continuous film entirely covers the metal substrate, with a rms roughness of about 1 Å (Figure 1a). Above this threshold thickness nanostructures appear on top of the continuous film with an apparent average diameter of 3 nm and height of about 1 nm (Figure 1b). For larger amounts an increasing number of larger nanostructures grow on the surface, with a mean diameter of 15 nm and mean height of 7 nm (Figure 1c). A mild annealing at 500 K in O2 (1 × 10−6 mbar) does not modify island shape, dimension, and density (Figure 1d). Nanostructure density increases drastically above the threshold thickness of 10 Å (Figure 1e), reaching the almost complete coverage of the surface above 30 Å. As shown in Figure 1f, both the lateral and vertical size of the nanostructures increase with deposited amount, together with the surface coverage and nanostructure density, while the aspect ratio, defined as the ratio between height and diameter, is constant at a value of about 0.5. This consideration does not take into account convolution with the tip that can increase the apparent diameter. However, these values give an idea of the shape of nanostructures and their abundance on the surface.

To gain insight into these nanostructure formation, Auger spectra of the surface have been measured and reported in Figure 2a. The Zn L3M45M45 Auger line measured with Al Kα photons enables the observation of different Zn oxidation states, due to the marked difference between positions and lineshapes for Zn and ZnO spectral components.29 Up to 10 Å the energy position and line shape of the Auger main peak are very similar to the bulk ZnO reference spectrum, characterized by a main feature at 989 eV kinetic energy and a less pronounced shoulder at about 992 eV, while the Mo 3s peak is visible at 982 eV (Figure 2a). This allows us to assign the continuous film observed in STM images to ZnO (Figure 1 a,b). For thin films a broadening of the Auger peaks partially hides the high kinetic energy shoulder, due to the higher defectivity and the presence of different Zn sites as compared to bulk oxide. Above 10 Å an additional component of increasing intensity is detectable at 992 eV beside the main peak, with a smaller shoulder at 995.5 eV. These features are close to the energy position of the L3M45M45 Auger transition of metallic Zn. This additional component is assigned to a metal Zn phase located on the surface since its intensity increases when spectra are measured in grazing emission configuration. After the 13744


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part of the Zn atoms remains on the surface in a metallic state. This fraction of metal atoms remains as a surfactant on the oxide surface, due to a low surface free energy of 0.94 J/m2 as compared to values between 1 and 2.2 J/m2 for oxide, and acts as a catalyzer, nucleating the further growth of ZnO in the form of nanostructures.8,30 By means of an exponential attenuation model we can get a quantitative estimation of the thickness of the ZnO bottom phase and the Zn top phase in nanostructures, in the assumption of a 10 Å continuous ZnO film underneath. For this purpose the Zn Auger line shape has been fitted as the sum of two contributions, namely, Zn0 and Zn2+. The intensity ratios Zn2+LMM/Mo 3d and Zn0LMM/Zn2+LMM obtained have been used, provided the inelastic mean free paths (IMFPs) of 20 Å for Zn LMM and 24 Å for Mo 3d electrons.31 To take into account growth in the form of nanostructures, the fractional coverage obtained by STM images has been considered in the model. The obtained thicknesses are reported in Figure 2b as a function of total thickness (that is, the total deposited amount). Both phases are linearly increasing with total deposited amount, with a smaller contribution of the metal part and with a total height comparable to that obtained from STM images. We exclude the presence of a mixed Zn−Mo oxide phase at the interface based on the evidence that the XPS Mo 3d line shape suggests a minor Mo surface oxidation and the Mo/Zn intensity ratio shows an angular behavior compatible with an almost sharp ZnO/Mo interface. An interesting point is the effect of annealing at 500 K. The Zn2+LMM/Mo 3d intensity ratio does not change when the metal Zn phase of the nanostructures completely disappears from the spectra. This suggests that Zn is not converted in oxide, as this would lead to an increase of the Zn2+ intensity. Annealing therefore removes the metal capping of the nanostructures, leaving only the oxide grown below the metal phase. Nanostructure Control. To rationalize the mechanism that governs the transition from continuous film to nanostructure formation we consider the parameter N that quantifies the O2to-Zn ratio available on the surface. The number of O2 molecules introduced in the growth system per unit time has been estimated by the product of the O2 partial pressure and the pumping speed of the system (250 L/s). Since a nozzle was used to direct the gas flow to the sample surface, we assume that all the introduced molecules impinge on the surface. For samples in Figure 1 where P(O2) = 1 × 10−6 mbar and RZn = 3 × 10−2 Å/s, nO = 9 × 1015 molecules·cm−2·s−1 for O2 and nZn = 2 × 1013 atoms·cm−2·s−1 for Zn, giving N = nO/nZn = 440. This quantification cannot take into consideration the effective (unknown) sticking coefficient of the two species and the dissociated O2 quantity; however, it can give a clear idea of the trends and help the comprehension of the mechanisms involved in the oxidation of the system. For the surfaces in Figures 1 and 2 N was equal to 440. The influence of N on morphology is reported in Figure 3. Reducing N to 44 [reducing P(O2) to 1 × 10−7 mbar] the density of nanostructures increases, and they cover entirely the ZnO film (Figure 3a). Annealing at 500 K leaves on the surface ZnO nanostructures of peculiar shape, grown below the metal capping and visible after its removal. These peculiar features can be observed also for higher N values (Figure 3c), the quantity depending on the growth conditions. An increase of N from 440 to 1780 (obtained by a reduction of the Zn deposition rate to 7 × 10−3 Å/s) determines a reduction in the density of the nanostructuresreducing the

Figure 2. (a) Zn Auger peak measured for increasing total thickness. Black curves are recorded at normal emission and red dashed curves at grazing emission. Bulk spectra are reported for comparison (dotted line). (b) Zn and ZnO phase thickness in nanostructures estimated through the model in the inset as a function of total thickness.

annealing at 500 K in oxygen [P(O2) = 1 × 10−6 mbar] spectra report a completely oxidized system, although no variation in surface morphology is observed. Combining the surface morphology observed in STM images and the quantitative evaluation by XPS and Auger spectra, we can reasonably identify the model for film growth. We assume that the system grows as shown in the model in Figure 2b. In P(O2) = 1 × 10−6 mbar oxidizing conditions (and RZn = 3 × 10−2 Å/s) a continuous ZnO film of up to 10 Å is formed on the Mo surface. When this threshold thickness is overcome, 13745


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thus completely determined by the interfacial contribution of Mo. To clarify the interfacial mechanisms that induce the formation of ZnO film and the transition to nanostructure growth, the O 1s photoemission peak has been considered (Figure 5a). The Zn2+ component of ZnLMM to O 1s intensity ratio gives an estimation of the overall stoichiometry of the oxide. The system is abundant in O before the transition threshold and approaches 1:1 stoichiometry with increasing thickness (Figure 5b), while often in the literature a ZnO poor in O is reported.23 A closer look to the O 1s peak reveals that beside the main component at 530.2 eV, assigned to O2− in ZnO regular lattice (OZnO) or to O in Mo surface oxidation, an additional feature is present at about 1.5 eV higher binding energy. This component is often assigned to O sites with a lower coordination (OD), indicating the presence of defects in the ZnO lattice, as previously reported in the literature.32 The shape of the O 1s peak changes with increasing thickness, as the shoulder shifts to higher binding energy due to the presence of an increasing third component at a separation of 2.3 eV, assigned to OH− groups on the surface.33 Both the decrease in the OD/OZnO ratio and the increase in the OH−/OZnO ratio occurs in correspondence to the appearance of nanostructures (Figure 5c). Annealing at 500 K does not substantially modify the OD relative area, while it decreases significantly the quantity of OH− groups that desorb from the surface. In the first stages of deposition of Zn on the Mo surface in the presence of oxygen, the ZnO film is formed stimulated by the Mo surface oxidation (as shown by the appearance of a small oxidized Mo component on the Mo 3d peak, not reported here) or by the availability of O dissociated by Mo, therefore favored by interfacial effects that make the ambient rich in reactive oxygen. The film is highly defective due to Zn vacancies and O interstitials typical of ZnO films34,35 or to structural deformations. The quantity of defects does not change with increasing N (arrow in Figure 5c), indicating that they are intrinsic of the formed ZnO and not related to oxidation conditions during growth. Increasing the deposited material and going far from the interface the Mo contribution diminishes, and the film starts to become ZnO with a lower amount of defects and with a stoichiometry that approaches 1:1. The process continues until a threshold thickness is reached. When the interface is too far, the interfacial contributions of Mo disappear, and Zn is not sufficiently oxidized by the molecular oxygen provided by the nozzle. At this point nanostructures start growing, catalyzed by the metallic Zn, with a less defective ZnO phase and a stoichiometry O:Zn that remains substantially constant at 1:1.1 even for a large amount of deposited material. Crystalline Structure. Finally XRD has pointed out the crystalline structure of the ZnO film and nanostructures. While no LEED pattern is visible, due to the small grain size, XRD scans were made on 60 Å of the ZnO. Figure 6a shows the radial θ−2θ scan around the surface normal (with tilt angle ψ = 0). This scan investigates the orientation of film grains along the surface normal. In this case only one peak is clearly visible at 36.71°, corresponding to the position of Bragg diffraction from (101̅1) planes in wurtzite ZnO structure (bulk position at 36.253°, PDF 00-036-1451, ICDD, 2004). No other peaks are detectable at the positions expected for the bulk wurtzite (arrows in Figure 6a). The growth along the normal to the (101̅1) plane can be explained by a small lattice mismatch with the Mo substrate (Figure 6c). The unit cell of this surface is

Figure 3. STM images (100 × 100 nm2) of 30 Å ZnO (a) grown at O2-to-Zn ratio (N) equal to 44 (U = 0.5 V) and (b) after annealing in O2 (U = 1 V), (c) N = 440, and (d) N = 1780 after annealing in O2 (U = 2 V).

covered surface from 90% to 40%and increases the height of the nanostructures from about 5 to 9 nm (Figure 3d). Correspondingly ZnLMM Auger peaks reported in Figure 4 for

Figure 4. Zn Auger peaks measured for increasing O2-to-Zn ratio (N) during deposition. Bulk spectra are reported for comparison (dotted line).

increasing N values show a reduction in the fraction of metal Zn. Increasing N to 8900 [P(O2) = 5 × 10−6 mbar, RZn = 7 × 10−3 Å/s] this component almost disappears. This clearly evidences the progressive oxidation of the system with N. However, this is not obtained through an increase in the thickness of the continuous film and a shift of the transition threshold but in an increase of the oxide phase thickness in the nanostructures. The system therefore evolves toward higher and less dense nanostructures on a 10 Å ZnO film. The threshold for transition from film to nanostructure formation is 13746


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Figure 5. (a) XPS O 1s for ZnO film of increasing thickness, taken at normal emission. Dotted red curve reports spectrum after annealing in O2. Inset shows O 1s Gaussian deconvolution for 7 Å. (b) O content in ZnO film versus total thickness. (c) Relative area of O 1s components for N = 440 (black square: OD/OZnO; orange dot: OH−/OZnO) and N = 1780 (empty markers). Dashed line indicates the transition in the growth regime.

Å (Figure 6b). A small distortion to accommodate strain or an extremely small compression of the O1−O2 or Zn1−Zn2 planes (d1 = 0.4127 Å, d2 = 2.4764 Å, Figure 6c) to make them more planar would lead to a larger reduction in the (1010̅ ) plane distance, compatible with the observed shift. No information is available concerning the in-plane orientation of the grains because of the small signal; however, we can speculate the alignment of the (1011̅ ) unit cell reported in Figure 6c with the Mo[100] directions. In Figure 6b two other peaks are barely detectable in the noise at 34.7° and 36.2°, corresponding to (0002) and (101̅1), almost coincident with bulk wurtzite values. The positions unaffected by strain are the sign of the presence of a polycrystalline phase. We assign the single-crystal (101̅1) phase to the continuous film on Mo, while the oxide nanostructures catalyzed by the metal Zn are polycrystalline. In bulk wurtzite the (101̅1) surface is semipolar. The possible presence of a residual polaritypartially compensated by structural deformationscould be an additional effect that limits the growth of the continuous ZnO film above 10 Å when the interface with Mo is too far. At this point the stabilizing effect of epitaxy is negligible, and the film growth is self-limited. On the contrary the more stable polycrystalline nanostructures can grow without limits guided by the metal residuals.

Figure 6. XRD scans with (a) ψ = 0° and (b) ψ = 28.4° with respect to the surface normal, where ψ is the tilt angle. In bold red peaks of the single crystalline ZnO phase are indicated, while in black peaks are reported from the bulk reference position or polycrystalline phase. (c) ZnO bulk wurtzite crystal structure with evidence of the (101̅1) surface, top and side view of crystal cut along (101̅1).

4. CONCLUSIONS ZnO amounts between 5 and 70 Å have been grown on Mo(001) by physical vapor deposition of Zn in oxygen atmosphere. Combining different techniques we have studied the relation between morphology, composition, and structure, to determine the role of oxidizing pressure and annealing temperature in the system evolution with thickness. Up to a thickness threshold of 10 Å, ZnO grows as a fully oxidized continuous film, with a wurtzite semipolar (1011̅ ) preferential orientation and a small lattice mismatch with Mo(001). The Mo surface provides oxygen available in a reactive form, allowing the growth of a film rich in O with a large amount of defects (Zn vacancies, O interstitials). Increasing the amount of deposited Zn in molecular oxygen pressure and going far from the interface, a fraction of metallic Zn remains on the surface that catalyzes the further growth of ZnO in the form of

rectangular, with a = 3.25 Å and b = 11.84 Å, corresponding to a mismatch with Mo(001) of 3% and 6%, respectively. A surface energy of 1.73 J/m2 has been calculated for (101̅1).8 It is therefore lower than the surface free energy of the most commonly exposed surface (0001)-Zn in thin films, and growth of other surfaces will be especially unfavored by epitaxy. To confirm orientation of the grains, other reflections expected for a wurtzite with this orientation have been checked, in the hypothesis of single crystalline film. Near a tilt angle of 28° (keeping constant the substrate rotation angle), a peak is present at 33.28°, corresponding to the Bragg reflection from (101̅0) of the ZnO film (expected at 31.770° in bulk wurtzite), indicative of a reduction of the distance between planes of 0.12 13747


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(14) Weirum, G.; Barcaro, G.; Fortunelli, A.; Weber, F.; Schennach, R.; Surnev, S.; Netzer, F. P. Growth and surface structure of zinc oxide layers on a Pd(111) surface. J. Phys. Chem. C 2010, 114, 15432− 15439. (15) Xue, M.; Guo, Q.; Wu, K.; Guo, J. Initial oxidation and interfacial diffusion of Zn on faceted MgO(111) films. Langmuir 2008, 24, 8760−8764. (16) Gabasch, H.; Knop-Gericke, A.; Schlögl, R.; Penner, S.; Jenewein, B.; Hayek, K.; Klötzer, B. Zn Adsorption on Pd(111): ZnO and PdZn alloy formation. J. Phys. Chem. B 2006, 110, 11391− 11398. (17) Stavale, F.; Pascua, L.; Nilius, N.; Freund, H.-J. Morphology and luminescence of ZnO films grown on a Au(111) support. J. Phys. Chem. C 2013, 117, 10552−10557. (18) Guo, D.; Xue, M.; Guo, Q.; Wu, K.; Guo, J.; Wang, E. G. Ordered ultra thin ZnO films on metal substrate. Appl. Surf. Sci. 2009, 255, 9015−9019. (19) Vennéguès, P.; Chauveau, J. M.; Korytov, M.; Deparis, C.; Zuniga-Perez, J.; Morhain, C. Interfacial structure and defect analysis of nonpolar ZnO films grown on R-plane sapphire by molecular beam epitaxy. J. Appl. Phys. 2008, 103, 083525. (20) Lin, B. H.; Liu, W.-R.; Lin, C. Y.; Hsu, S. T.; Yang, S.; Kuo, C. C.; Hsu, C.-H.; Hsieh, W. F.; Chien, F. S.-S.; Chang, C. S. Single Domain m-Plane ZnO Grown on m-Plane Sapphire by Radio Frequency Magnetron Sputtering. ACS Appl. Mater. Interfaces 2012, 4, 5333−5337. (21) Cagin, E.; Yang, J.; Wang, W.; Phillips, J. D.; Hong, S. K.; Lee, J. W.; Lee, J. Y. Growth and structural properties of m-plane ZnO on MgO (001) by molecular beam epitaxy. Appl. Phys. Lett. 2008, 92, 233505. (22) Nistor, M.; Mandache, N. B.; Perrière, J.; Hebert, C.; Gherendi, F.; Seiler, W. Growth, structural and electrical properties of polar ZnO thin films on MgO (100) substrates. Thin Solid Films 2011, 519, 3959−3964. (23) Zheng, K.; Guo, Q.; Xue, M.; Guo, D.; Liu, S.; Wang, E. G. Ultra-thin zinc oxide film on Mo(100). Thin Solid Films 2007, 515, 7167−7170. (24) Kourouklis, H. N.; Nix, R. M. The growth and structure of ZnOx overlayers on low index silver surfaces. Surf. Sci. 1994, 318, 104−114. (25) Kato, D.; Matsui, T.; Yuhara, J. Oxidation of ultra-thin zinc films on Rh(100) surface. Surf. Sci. 2010, 604, 1283−1286. (26) Zhou, H.; Wang, H.-Q.; Liao, X.-X.; Zhang, Y.; Zheng, J.-C.; Wang, J.-O.; Muhemmed, E.; Qian, H.-J.; Ibrahim, K.; Chen, X.; et al. Tailoring of polar and nonpolar ZnO planes on MgO (001) substrates through molecular beam epitaxy. Nanoscale Res. Lett. 2012, 7, 184. (27) Pascua, L.; Stavale, F.; Nilius, N.; Freund, H.-J. Autocatalytic growth of ZnO nanorods from flat Au(111)-supported ZnO films. Phys. Chem. Chem. Phys. 2014, 16, 26741−26745. (28) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. WSXM: A software for scanning probe microscopy and a tool for nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (29) Schö n, G. Auger and direct electron spectra in X-ray photoelectron studies of zinc, zinc oxide, gallium and gallium oxide. J. Electr. Spectrosc. Relat. Phenom. 1973, 2, 75−86. (30) Mezey, L. Z.; Giber, J. The surface free energies of solid chemical elements: Calculation from internal free enthalpies of atomization. Jpn. J. Appl. Phys. 1982, 21, 1569−1571. (31) Tanuma, S.; Powell, C. J.; Penn, D. R. Calculations of electron inelastic mean free paths. II. Data for 27 elements over the 50−2000 eV range. Surf. Interface Anal. 1991, 17, 911−926. (32) Dupin, J.-C.; Gonbeau, D.; Vinatier, P.; Levasseur, A. Systematic XPS studies of metal oxides, hydroxides and peroxides. Phys. Chem. Chem. Phys. 2000, 2, 1319−1324. (33) Szörényi, T.; Laude, L. D.; Bertóti, I.; Kántor, Z.; Geretovszky. Zs. Excimer laser processing of indiumtinoxide films: An optical investigation. J. Appl. Phys. 1995, 78, 6211.

polycrystalline nanostructures with a surfactant metal capping. This capping can be removed by subsequent annealing that leaves exposed the oxide peculiar nanostructures. Increasing the oxygen-to-Zn ratio available during growth reduces the density of nanostructures and increases their height. On the contrary, ZnO continuous (101̅1) film is unaffected, probably limited by a residual polarity that makes its growth less favored. We expect that in strong oxidizing conditions the nanostructure growth on the Mo(001) surface is inhibited, while the growth of the continuous film is strongly limited, opening the way to possibly different growth regimes.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +39 059 2055313. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors would like to acknowledge the FIRB project RBAP115AYN “Oxides at the nanoscale: multifunctionality and applications” and the COST ACTION CM1104 “Reducible oxide chemistry, structure and functions” for the financial support. I.V. would like to thank the Global Grant Spinner 2013 funded by Regione Emilia Romagna and European Social Fund for the postgraduate fellowship.

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ZnO Nanostructure Formation on the Mo(001) Surface - The Journal of

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