ZnO Morphology Control by Pulsed Electrodeposition - American

Article pubs.acs.org/JPCC

ZnO Morphology Control by Pulsed Electrodeposition C. V. Manzano,† O. Caballero-Calero,† S. Hormeño,† M. Penedo,† M. Luna,† and M. S. Martín-González†,* †

IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC), Isaac Newton 8, PTM, E-28760 Tres Cantos, Madrid, Spain ABSTRACT: ZnO films have been grown by pulsed electrochemical deposition between a reduction potential and an oxidation potential to improve their quality. Different morphologies like columns, platelets, flowers, and high-quality planar films are obtained changing the reduction potential without further addition of additives. The mechanism behind this control in morphology is ascribed to the OH− concentration at the electrode surface (pH) as a consequence of the reduction potential. The morphology, surface potential, and electrical conduction mechanism of high-quality ZnO films were simultaneously measured by kelvin probe force microscopy (KPFM). As a result of these measurements, one can conclude that the ZnO surface perpendicular to the [0001] direction is positively charged. Also, it becomes evident that the surface potential drop is mainly produced at the grain boundaries for the ZnO films grown by pulsing between −0.5 and 0.9 V, which indicates intergrain contact resistance.

1. INTRODUCTION Zinc oxide (ZnO) is a transparent oxide in the visible range from the II−VI semiconductor family. It is claimed to be one of the most competitive semiconductors for the near future. ZnO is an n-type semiconductor with a large direct band gap of ∼3.37 eV at 300 K and a large exciton binding energy of 60 meV. Different ZnO morphologies can be obtained depending on the growth conditions. ZnO films can exhibit a wide assortment of structures such as nanowires, flowerlike morphologies, nanodots, nanobelts, and so forth. These different shapes1 allow a variety of applications in different fields like optoelectronics, field emission devices, and sensors. For example, ZnO nanorods are interesting due to their lasing properties in the UV.2 ZnO nanowires present high efficiency in dye-sensitized solar cells.3,4 Other different nanostructures such as nanocombs, nanotips, or nanoflowers are used for biosensors.5 High-quality films, when doped, can be used for thermoelectric applications.6 The main growth techniques which allow the possibility to obtain the different ZnO morphologies are the hydrothermal method,7,8 chemical bath deposition,9 and electrochemical deposition.10−13 In particular, for the case of electrodeposition, the most typical morphology obtained for ZnO films are hexagonal single-crystal columns. This morphology is obtained at constant potential in different solutions, such as 5.10−3 M ZnCl2 and 0.1 M KCl,14 0.1 M Zn(NO3)210 and in 0.05 M Zn(NO3)2.15,16 Another typical morphology obtained in ZnO are hexagonal platelets, which were first observed by M. Izaki and T. Omi,10 at constant potential in a 0.1 M Zn(NO3)2 solution, and in a 5.10−4 M ZnCl2 solution with different concentrations of CaCl2 as supporting electrolyte.17 The change in the film morphology, from hexagonal columns to hexagonal platelets, has been observed before in a number of experiments when the © 2012 American Chemical Society

electrochemical bath, usually Zn(NO3)2 aqueous solution, is modified with some additives such as KCl,12,16 NaF and NaCl15 or methanol,11 or when the Zn(NO3)2 is replaced by ZnSO4,18 or when a ZnCl2 solution with hexamethylentetramine (HMT)19 is used instead of Zn(NO3)2. The main explanation for the change in morphology is the adsorption of the additives/counterions to a specific crystallographic direction, which causes an increase in the growth of the other directions. In a more recent study, centered in the optimization of ZnO films grown by electrodeposition, Martin-Gonzalez’s group reported the synthesis of ZnO films at constant potential using two different solutions: nitrate and peroxide.20 Those solutions were used to obtain a comparative study of the influence of different sources of OH− in the characteristics of the films. Photoluminescence studies showed that the ZnO films grown in the nitrate solution presented a higher concentration of OH− than the films grown in the peroxide solution. Moreover, it can be seen that some OH− get trapped in the structure during the deposition. This OH− can be oxidized to O2‑ by annealing the films at 150 °C in air. To further increase the quality of the ZnO films obtained in ref 20, pulsed electrodeposition was performed from a nitrate solution without the presence of additives. The main idea behind was that, by pulsing the potential between a reduction potential and the oxidation potential, the possible OH− trapped in the structure during the reduction time will be oxidized to form ZnO in the oxidation time and therefore the quality of the film will be improved. In this study, no other additives or counterions were intentionally present in the solution to avoid any other external influence. Received: October 30, 2012 Revised: December 21, 2012 Published: December 28, 2012 1502

dx.doi.org/10.1021/jp3107099 | J. Phys. Chem. C 2013, 117, 1502−1508

The Journal of Physical Chemistry C

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2. EXPERIMENTAL SECTION ZnO films were pulsed-electrodeposited in 0.1 M zinc nitrate10 (99.999% from Sigma-Aldrich) aqueous solution using a standard three-electrode cell. No supporting electrolyte was added to the solution. Platinum wire, Ag/AgCl electrode, and Au(111)20 were used as counter electrode, reference electrode, and working electrode, respectively. In the working electrode, 150 nm of Au was deposited by a metal evaporation system (electron beam) on a Si (100) substrate, obtaining a (111) Au layer. The temperature of the solution was maintained at 80 °C thanks to a temperature controlled bath. A potentiostatgalvanostat (Eco Chemie, Model AUT302.0) was used to perform the electrodeposition processes at pulsed potential. The depositions were carried out for 1800 cycles. Each cycle consisted of 2 s at a certain reduction potential versus Ag/AgCl and 1 s at +0.9 V. For this study, the reduction potential was modified from −1.5 to −0.5 V, decreasing 0.2 V from film to film, which makes a total of six different reduction potentials. The structural properties of the grown films were characterized by high-resolution X-ray diffraction (XRD). The measurements were carried out using a Philips X’Pert fourcircle diffractometer system with CuKα radiation. In all cases, the adjustment of the XRD spectra was done taking the Au (111) peak position as reference peak. The surface morphology of the films was studied using a commercial scanning electron microscope (SEM) (Hitachi S-800) with 25 KV accelerating voltage. To further investigate the details of the morphology, atomic force microscopy (AFM) measurements were done (using a Nanotec Electronica S.L. microscope). Kelvin probe force microscopy (KPFM) experiments were performed using a Nanotec Electronica S.L. system21 operated at standard conditions. In these experiments, NT-KP tips from Next-Tip (http://www.next-tip.com) were used. NT-KP tips are silicon-based tips covered by Au nanoparticles that display conductive properties and high spatial resolution of the contact potential difference (CPD) images as well as the topographic ones.22 To obtain these conductive tips, regular silicon tips (Nanosensors, PPP, nominal force constant ∼2.8 N/m and resonance frequency ∼75 kHz) were modified by the deposition of metallic nanoclusters of 2−3 nm diameter under ultrahigh vacuum conditions using an ion cluster source.22 In addition to the improvement in the electrical and topographical resolution, a second advantage of these modified tips relies on the fact that they can be regenerated after a nondesired eventual crash with the surface. This is particularly important for these ZnO films due to the large scanned area and their roughness. Topographic as well as simultaneous electrical contact potential images were acquired while an electric field was applied to the ZnO surface between two patterned gold electrodes. With respect to the electrical excitation, Kelvin probe force microscopy was operated in force gradient mode,23 also known as FM (frequency modulation)-KPFM.24 As far as the mechanical excitation is concerned, noncontact amplitude modulation with a phase-locked loop was implemented in all measurements. Typical parameters for the electrical excitation applied (Vac sin(ωet)) were Vac = ∼750 mV and ωe = ∼7 kHz. The Raman scattering was measured with a high-resolution Raman spectrometer (Horiba Jobin Yvon) with a 532 nm Nd:YAG laser (8.5 mW) in the range between 90 and 600 cm−1 in air at room temperature.

3. RESULTS AND DISCUSSION The electrodeposition process was carried out by pulsing the potential between the oxidation potential +0.9 V for 1 s and different reduction potentials for 2 s. The reduction potentials used in this study were −1.5 V, −1.3 V, −1.1 V, −0.9 V, −0.7 V, and −0.5 V. These conditions were determined from the cyclic voltammogram obtained between −1.6 V and 1.0 V in 0.1 M Zn(NO3)2 aqueous solution. The cyclic voltammogram and the reaction mechanisms that take place at different applied voltages were thoroughly discussed in a previous work.20 Figure 1 shows the X-ray diffractograms of the ZnO films grown in this work. The intensity is shown in logarithmic scale

Figure 1. XRD pattern (in logarithmic scale) vs 2θ of ZnO films pulsed electrodeposited on (111) gold substrates at the same oxidation potential (+0.9 V) and at different reduction potentials ranging from −1.5 V to −0.5 V for 1800 cycles at 80 °C. The intensity is shown in logarithmic scale to enhance the low intensity peaks.

to enhance the lower intensity peaks, so all of the small contributions can be clearly observed. All of the diffraction maxima can be identified as corresponding to ZnO (JCPDS 76−0704) or as peaks associated to the components of the substrate: Au (JCPDS 04−0784) and Si (JCPDS 27−1402). The total intensity of the silicon peak observed in the different diffractograms is related to the thickness and covering of the ZnO layer. From these peaks, the orientations of the substrate’s layers are determined as Au (111) and Si (100). Given that there are not unidentified peaks, the XRD pattern indicates that the films grown are purely ZnO. The intensity of the (002) diffraction peak is observed in all the cases at 2θ = 34.33°, except for the film deposited at −1.3 and 0.9 V, where it is present at 2θ = 34.36°. This shift corresponds to a small cell parameter change, from 5.22 Å to 5.21 Å. The (002) diffraction maximum is in the strongest one indicating that the preferred orientation of all those ZnO films is the [0001] direction, which implies that the c axis grows perpendicular to the substrate’s surface. When the intensity is plotted in logarithmic scale, some weak reflections of (101̅0), (101̅1), (101̅2), (112̅0), (101̅3), and (1122̅ ) are also observed for all films, except for those electrodeposited at −0.5 V reduction potential. That means that the film grown at −0.5 V reduction potential is the most strongly oriented along the [0001] direction. 1503



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Figure 2. SEM images of the ZnO films electrodeposited from a 0.1 M Zn(NO3)2 aqueous solution with same oxidation potential (+0.9 V) and different reduction potentials from −1.5 V to −0.5 V for 1800 cycles at 80 °C. Cross section and top view of films grown at: (a) and (b) −1.5 V, (c) and (d) −1.3 V, (e) and (f) −1.1 V, (g) and (h) −0.9 V, (i) and (j) −0.7 V, (k) and (l) −0.5.

Scanning electron microscopy pictures (Figure 2) show the surface morphology of the ZnO films electrodeposited from 0.1 M zinc nitrate aqueous solution. In the upper part of these images, the cross section of the films can also be seen. From these images, it is obvious that a great change in the morphology can be achieved with the variation of the applied potential. Thus, the tailoring of the structure can be achieved. For the film grown at the most negative potential, −1.5 V (part a of Figure 2), hexagonal columns aligned perpendicularly to the substrate surface are obtained, with a preferential orientation along the [0001] direction, as it was confirmed by X-ray diffraction measurements. The cross-sectional view shows that these hexagonal columns are almost parallel between them and perpendicular to the substrate. The thickness of this film was around 22 μm, and the diameter of the hexagonal columns was around 1.5 μm. The calculated growth rate is 15.7 μm/h. The surface of these single crystalline columns was studied by atomic force microcopy (Figure 3). The top part of the columns (surface perpendicular to the [0001] direction or polar c plane) presents steps of 1.9 nm in height, as it can be seen in parts a and b of Figure 3. Taking into account that the Zn−O bond length is approximately 0.198 nm25 in real space, the value observed will imply that each step consists of ten Zn−O bonds approximately. Regarding the lateral side of the columns, which corresponds to the nonpolar or m-plane, it presents a

roughness of 7 nm in average along the growth direction, as it can be observed in parts d, e, and f of Figure 3. Part b of Figure 2 shows the 2D hexagonal platelets that were obtained in the film electrodeposited at −1.3 V reduction pulse. The diameter of those hexagonal platelets was around 8 μm, which is much bigger than the diameter of the prior hexagonal columns. The films show desert rose type crystallization. This variation of the shape could explain the difference in the cell parameter observed by XRD. According to the cross sectional view, the thickness of the film was around 20 μm, thus the growth rate is 13.3 μm/h. It is worth noting that with a slight change in the reduction potential (only 0.2 V), the hexagonal columns are transformed into hexagonal platelets without further changes in the fabrication method. Moreover, this transformation was obtained without the use of additives in the process. For the film electrodeposited at −1.1 V reduction pulse (part f of Figure 2), a flowerlike morphology was found (formed of needles like the petals in thistle flowers). Each needle had a length of around 20 μm and a width of around 1 μm, which means an aspect ratio of the needles of around 20. The crosssectional view in this figure shows that these needles are not perpendicular to the substrate, but they tend to grow with a certain angle. The thickness of the film was ∼11 μm resulting in a growth rate of 7.3 μm/h. 1504



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Figure 4. Scheme of the Raman active vibrational modes of Wurtzite ZnO structure.

attribution and shifts of the peaks present in the Raman spectra with respect to the theoretical values are described in Table 1.

Figure 3. AFM images of a column of the ZnO film electrodeposited at −1.5 V: (a) polar c plane of the columns where the different growth steps can be observed, (b) profile between different steps along the line marked in (a), (c) simplified scheme of the hexagonal columns, (d) lateral view of the corner of a column (non polar m-plane), (e) lateral view in more detail, and (f) lateral profile.

Part h of Figure 2 displays the morphology of the film electrodeposited at −0.9 V reduction pulse. The cross sectional micrograph of this film shows a denser film with a thickness of ∼6 μm arousing a growth rate of around 4 μm/h. The film grown at −0.7 V reduction pulse (part j of Figure 2) was even denser that the previous one and shows a spaghetti-like morphology. The width of each of those structures was ∼0.20 μm and the thickness of the film was ∼5 μm, as it is shown in the cross sectional view, which gives a growth rate of 3.3 μm/h. The last film was grown at −0.5 V reduction pulse, and it is shown in part l of Figure 2. The surface of the film was the smoothest one obtained in the whole series. The resulting film was extremely compact, with a thickness of ∼2.2 μm, which means the slowest growth rate, of around 1.5 μm/h. For reduction potentials higher than −0.5 V, no deposit was obtained. To study the vibrational properties of the material, Raman spectra have been measured for all the fabricated ZnO films. ZnO has Wurtzite structure and belongs to the space group P63mc, with two formula units per primitive cell. In these high structures, there are six Raman active modes: Elow 2 , E2 , A1(TO), A1(LO), E1(TO), and E1(LO). In Figure 4, a scheme of the Raman active vibration modes of ZnO structure is shown. Their theoretical values are:26 101 cm−1 for Elow 2 , 437 −1 cm−1 for Ehigh for A1(TO), 408 cm−1 for E1(TO), 2 , 380 cm 574 cm−1 for A1(LO), and 584 cm−1 for E1(LO). The measured Raman spectra of ZnO films electrodeposited at different reduction potentials are shown in Figure 5. The

Figure 5. Raman spectra of the ZnO films electrodeposited from a 0.1 M Zn(NO3)2 aqueous solution with same oxidation potential (+0.9 V) and different reduction potentials from −1.5 V to −0.5 V for 1800 cycles at 80 °C.

In all of the ZnO films grown, the most intensive peak appears around 101 cm−1 and corresponds to Elow 2 mode. This peak is attributed to vibrations of the Zn sublattice.27 Moreover, a shift in its value has been related to structural defects in the ZnO films.28 In that sense, the film grown at −0.9 V presents the most shifted Elow 2 peak, and accordingly it should be the film with more structural defects. The second most intense peak in the Raman spectra is the one corresponding to Ehigh 2 , located around 437 cm−1. As it can be seen in Table 1, all ZnO electrodeposited films present a red shift in this peak. This Raman mode is assigned to oxygen vibrations29 in ZnO. These red shifts mean that all of them suffer a certain compressive stress.30 This shift is in the range of what is reported in literature for electrodeposited ZnO films.31 Nevertheless, among the fabricated films, there is one that presents a comparatively low shift: the one fabricated at −0.9 V. This is also the one that presented the highest shift in the Elow 2 mode, 1505



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Table 1. Parameters from the Raman Spectra of the Different ZnO Films Compared with the Theoretical Values; the Positions of the Peaks Are in cm−1 E2low (vibrat. Zn in ZnO) (cm−1) E2high − E2 low (cm−1) E2high (vibrat. O in ZnO)(cm−1) E1(LO)(cm−1)

theoretical values

−0.5 V

−0.7 V

−0.9 V

−1.1 V

−1.3 V

−1.5 V

101 333 437 584

100.1 333.6 438.9 581.6

100.6 335.1 439.5

98.1 333.2 437.4

100.6 331.7 439.5 581.6

100.1 332.7 439.9 581.6

100.6 334.6 440.3 583.0

software,40 we have obtained the equilibrium diagram for our system (Figure 6) at 25 °C.

which was related with a higher number of structural defects. Therefore, this could mean that the increase of structural defects could relax the stress. The last mode observed in our films is the E1(LO), which is present in all films except from those grown at −0.9 V and −0.7 V (Figure 4). The position of this mode is quite near to the A1(LO), being both polar modes which are attributed to the effect of impurities and/or defects. Among them, the E1(LO) mode is more affected by the presence of impurities and/or defects.32 In the films that present it, the peak is found at ∼582 cm−1 meaning that it is red-shifted with respect to the theoretical value of E1(LO). This shift has been reported to be related to an oxygen deficiency in ZnO.33 Moreover, in all the ZnO films fabricated via electrodeposition, one additional Raman mode was found located at 333 cm−1. This peak is due to second-order scattering,26 being 34 and Elow this mode attributed to the difference between Ehigh 2 2 . Therefore, we can conclude that our ZnO films present a Raman spectra coherent with what can be found in the literature for similar structures. Nevertheless, all of them suffer a compressive stress, which has also been reported for ZnO films grown by electrodeposition31 and by sputtering on Silicon substrates.35 This stress is less important for the film grown at −0.9 V, which is also the one that presents more structural defects that might have relaxed the structure. The different morphologies obtained in our films can be explained taking into account the OH− concentration during the growth of ZnO. It is worth mentioning here that, by changing the reduction potentials at the electrode, we are effectively modifying the concentration of OH− at the electrode surface, in other words, the local pH. The more negative the reduction potential is, the more OH− is produced and thus the pH increases. In the case of the ZnO films grown at the most negative potential, that is −1.5 V, a hexagonal columnar growth takes place. Under these voltages, the Faradaic efficiency is quite low. This efficiency is calculated as the ratio between the OH− which reacts with the Zn 2+ divided by the total OH − produced.36 This means that a high amount of OH− ions are produced at the surface of the electrode, and then there is much more OH− than Zn2+ available. As a consequence, the OH−/ Zn2+ ratio is very high (≫1) and all the unused OH− produces an increase of the local pH at the surface of the electrode. In this area, the system is comparable to a heavy alkaline solution.37 In fact, the local pH was measured, whereas the −1.5 V voltage was being applied with a pH-meter located at the surface of the electrode, and the value obtained was around 15.5. For such a high pH, it has been postulated that the basic 38 To growth units become the negatively charged Zn(OH)2− 4 . confirm this assertion, we have studied the equilibrium diagram of the thermodynamically stable Zn species that can be present in our solution under different OH− concentrations. Using the Pourbaix diagram for zinc at 25 °C39 and Hydra and Medusa

Figure 6. Calculation of the logarithm of Zn2+ concentration as a function of the pH at 25 °C and the distribution of zinc hydrolysis species as a function of the pH at 25 °C.

With the distribution of zinc hydrolysis species as a function of the pH at 25 °C, and knowing the local pH to be 15.5 for a reduction potential of −1.5 V, it can be seen that the stable species is Zn(OH)2− 4 . Zn 2 + + 4OH− → Zn(OH)24 −

(1)

It is assumed that the adsorption of negatively charged species in ZnO will be more favorable onto the positively charged surface perpendicular to the ZnO-[0001] direction, also known as the c plane. It is generally assumed that Zn(OH)2− 4 quickly undergoes dehydration by proton transfer, forming Zn2+···O2−···Zn2+ bonds41 according to the reaction: Zn(OH)24 − → ZnO + H 2O + 2OH−

(2)

It is also important to highlight that, at the most negative potential (−1.5 V), the driving forces for the reaction are very strong. It has been shown before that, under excessive driving forces, homogeneous nucleation and growth are enhanced.42 By varying the applied potential toward less negative values, the electrodeposition process becomes more and more efficient. This implies that less OH− ions are produced; in other words, less OH− remains without reacting with the Zn2+ making the local pH less basic. By doing that, the thermodynamically stable Zn2+ species is no longer Zn(OH)2− 4 . Instead, the stable species will change with the decrease of local pH toward Zn(OH)−3 , neutral species like Zn(OH)2, or even toward positively charged species like Zn2+ (Figure 7). For the film grown at −1.3 V, ZnO platelets are observed. These structures are found when the Zn ions segregate 1506



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showed the highest crystallographic quality, as shown in the Xray diffractograms, and also had the smoothest surface. An open question in ZnO is if the electrical conduction is due to the migration of oxygen ions/vacancies, anion migration, or grain boundaries effects. KPFM is a powerful atomic force microscope (AFM)-based technique that allows investigating the electrical properties of materials. KPFM measures the topography and the contact potential difference (CPD) of a substrate and a conducting probe at the same time. In this study, we used KPFM to investigate the CPD of ZnO after connecting an external voltage by means of Au electrodes imprinted on the ZnO surface. KPFM measurements required the fabrication, via electron beam lithography, of Au electrodes deposited on the surface of the film to apply an external voltage. The electrode configuration can be seen in part a of Figure 7, where the red square delimits the approximate area where the topographical and electrical measurements were performed. The distance between electrodes is around 150 μm. Examples of the simultaneously acquired topography and surface potential images of ZnO when the applied voltage between the electrodes was 0 V are shown in parts b and c of Figure 7, respectively. In part b of Figure 7, the topography of the film can be seen. From that image, the root mean square (RMS) roughness can be obtained resulting in 51 nm. This is a very good value for an electrochemically deposited film without any additive. In part c of Figure 7, when no voltage was applied and thus the surface was grounded, a contact potential difference between the gold electrode and the ZnO of ∼0.3 V was found (part d of Figure 7). This potential is positive versus ground, which means that the ZnO surface perpendicular to the [0001] direction is positively charged, as we wanted to verify. Then, different potentials between the two gold electrodes were applied (2.0, 4.0, and ∼7.4 V). For each applied potential, we could observe that, within each grain, the color remains the same indicating that within the grain the surface potential is approximately constant. Also, there is not much change between the gold contact and the first ZnO grain indicating a good electrical contact between both of them. However, the variations in color for different ZnO grains correspond to the different surface potential values between grains. In parts e and f of Figure 7, the data obtained when a difference of potential between the two gold electrodes of ∼7.4 V is applied can be seen (the result for 2.0 and 4.0 V is similar, and it is not shown). A good method to quantify the voltage drop between the two electrodes consists in drawing a VCPD profile line (part f of Figure 7, which represents the drop along the black dotted line showed in part e of Figure 7), from left to right, starting at the Au electrode with a potential value of 7.4 V. It can be observed that the potential does not drop significantly at the Au/ZnO boundary because the ZnO grain that contacts the Au electrode (region I) presents the same surface potential as the Au. For the rest of the ZnO grains, the surface potential drops in plateaus decreasing only significantly (∼0.5 V) between the ZnO grains. A decrease of just ∼0.1 V takes place inside each ZnO grain. Therefore, from the KPFM measurements, it can be concluded that the film grown at pulsed potential of −0.5 V and +0.9 V versus Ag/AgCl presents lateral electronic transport along the surface of polycrystalline ZnO. It is worth noting that the electrical conduction is very different inside each individual grain and at the boundaries between them. These grains seem to have a conduction parallel to the substrate, which is not a

Figure 7. (a) Optical image of the contacted Au electrodes deposited on ZnO, (b) simultaneously taken topography and (c) contact potential difference (CPD) image of the same area when no voltage is applied between the electrodes, (d) line profile of CPD along the dotted line in (c), (e) CPD image of part of the Au electrode connected to the potential source and the adjacent ZnO grains at an applied potential of ∼7.4 V, and (f) line profile of CPD along the Au electrode and ZnO grains (marked as Roman numerals) obtained from the KPFM image in (e).

preferentially over the nonpolar faces or m-faces. This is known to happen in relatively weak basic solutions.38 In this case, the stable species is different, probably Zn(OH)2 which is neutral. In the films grown at even less negative potentials, such as −1.1 V, a flowerlike growth is observed. This type of growth in different directions seems to indicate a change in the nucleation process. In this case, the nucleation changes from homogeneous (one nuclei with one direction) toward heterogeneous (several directions of nuclei). Many researchers have suggested that the quality of the seed layer influences the ZnO alignments and also its density.41 In the literature, this flowerlike growth has been observed when the pH of the solution is kept at 11.43 In our case, this kind of growth may be due to a more inhomogeneous nucleation process. This could be caused by the lower driving force of the system at −1.1 V applied potential compared to the previous potentials. Finally, when the ZnO films are grown at −0.5 V, the efficiency of the electrodeposition process is maximum. In other words, all of the OH− produced at the surface of the electrode is used in the formation ZnO. Then, the pH at the surface of the electrode is close to the pH of the solution (pH 3.7). At this pH, the stable species is Zn2+. In this case, a 2D growth is favored and high crystallographic quality films are obtained (Figure 1) with their surface oriented perpendicular to the [0001] direction, as in the case of the films grown at −1.5 V. To confirm that the surface perpendicular to [0001] direction of the ZnO is positively charged and to study how is the electrical conduction in ZnO films, Kelvin probe force microscopy (KPFM) measurements were performed in the film grown at −0.5 V. Those films were chosen because they 1507



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common configuration for electrodeposited films, although they generally have the c-axis perpendicular to the substrate.44 The surface potential drop is mainly produced at the grain boundaries, what indicates a resistive behavior of this zone.

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4. CONCLUSIONS High-quality ZnO films have been grown by pulsed electrodeposition. Different morphologies have been identified: hexagonal columns, hexagonal platelets, flowers, and 2D growth. These different growth mechanisms are related to the OH−/Zn2+ ratio, the local pH at the electrode surface, and the Faradaic efficiency of the process for each potential. The Raman spectra showed the typical ZnO vibration modes as compared with other ZnO films grown by electrodeposition. It is important to note that the films showed a red shift for the Ehigh mode, which means that the films suffer a compressive 2 stress. The surface perpendicular to [0001] direction of the ZnO is shown to be positively charged by kelvin probe force microscopy (KPFM). Finally, the conduction of the film grown at −0.5 V, which presents high crystalline orientation along the [0001] direction and a smooth surface (51 nm RMS roughness), was studied. It was found that the electrical conduction is very different inside individual grains and at the boundaries between them. These grains seem to be parallel to the substrate which is a less common configuration for electrodeposited films, although they generally have the c axis perpendicular to the substrate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge partial financial support from ERC StG NanoTEC 240497and MICINN Project No. MAT2008-06330. C.V.M. acknowledges a JAE Ph.D. grant and O.C.C. and S.H. acknowledge their JAE postdoctoral positions from CSIC and the European Social Fund.

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REFERENCES

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ZnO Morphology Control by Pulsed Electrodeposition - American

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