Critical Nucleation Effects on the Structural Relationship Between ZnO

Article pubs.acs.org/JPCC

Critical Nucleation Effects on the Structural Relationship Between ZnO Seed Layer and Nanowires Sophie Guillemin,*,†,‡ Vincent Consonni,† Estelle Appert,†,§ Etienne Puyoo,† Laetitia Rapenne,† and Hervé Roussel† †

Laboratoire des Matériaux et du Génie Physique, CNRS − Grenoble INP, 3 parvis Louis Néel 38016 Grenoble, France Institut des Nanotechnologies de Lyon, CNRS - INSA Lyon, 7 avenue Jean Capelle 69621 Villeurbanne, France § Laboratoire de Science et Ingénierie des Matériaux et Procédés, CNRS − Grenoble INP, 1130 rue de la Piscine 38402 Saint-Martin d’Hères, France ‡

ABSTRACT: The effects of the structural morphology of the ZnO thin seed layer composed of nanoparticles grown by dip coating have been investigated on the structural properties of ZnO nanowires grown by chemical bath deposition. It is revealed by scanning electron microscopy that the growth of ZnO nanowires is limited by the mass transport of chemical precursors in solution, leading to the inverse relationship of their average diameter and length with their density. It is shown by transmission electron microscopy and X-ray diffraction measurements that ZnO nanowires epitaxially grow on the seed layer and preferentially nucleate on the free surface of ZnO nanoparticles. The vertical alignment of ZnO nanowires as quantitatively deduced by X-ray pole figures is found to be improved by strengthening the texture of the seed layer along the c axis. Similarly, their density increases, showing that the c polar plane is highly reactive chemically and presents preferential surface nucleation sites. The relationship between the average diameters of ZnO nanoparticles and nanowires is completely driven by the nature of the nucleation site that is strongly dependent upon the growth conditions and upon the structural morphology of the seed layer. The texture, roughness, and porosity of the seed layer are three critical parameters. formation of ZnO NWs prior to the NW growth itself.15−18 The seed layer is usually composed of ZnO nanoparticles (NPs) grown by sol−gel processes like dip coating or by bath deposition.15,17,18 It has been stated that the structural properties of ZnO NWs grown by CBD is strongly related to the structural morphology of the ZnO seed layer.15,17−40 Nevertheless, these morphological and structural relationships are controversial and still not completely understood. The orientation of the seed layer is expected to play a significant role on the vertical alignment of ZnO NWs,20,23,26,27,31,32,36,37,39 but no quantitative analysis has so far been reported. Similarly, the epitaxial relationship between ZnO NPs and NWs has not clearly been shown yet.32,34 Additionally, several correlations in terms of characteristic dimensions have been emphasized: the ZnO NW diameter has been found to increase19,23,27,29,34,37−40 or decrease30 as the ZnO NP size is increased. It has also been suggested that the ZnO NW diameter and length may rather be driven by chemical effects as the solution growth proceeds.23,24,27,30,35,39,41 As a result, the nucleation mechanisms of ZnO NWs on the seed layer are likely to be crucial since they can control their density and position.21,25,30,32,39,40 Both surface32,39,40 and grain boundary21,25,30,39 nucleation sites on the seed layer have been mentioned: it has been proposed that the nucleation regime may depend on the ZnO NP size.39 ZnO

I. INTRODUCTION Semiconductor nanowires (NWs) have emerged over the past years as promising building blocks for a wide variety of optoelectronic devices.1 Although the catalyst-induced approach has been used widely for the growth of silicon, germanium, arsenides, phosphides, and nitrides NWs,2 some compound semiconductors like ZnO also have the ability to grow with the NW morphology according to catalyst-free approaches.3−5 In particular, owing to its large band gap energy of 3.3 eV at room temperature, its high exciton binding energy of 60 meV, and its high mobility, ZnO NWs have received in the past decade increasing interest for the development of lightemitting diodes6 and nanostructured solar cells such as dyesensitized solar cells7,8 or type II heterostructure-based solar cells.9 Additionally, ZnO NWs can easily be grown catalyst-free with a wide number of deposition techniques either in vapor or liquid phases such as metal−organic or standard chemical vapor deposition,4,5 physical vapor deposition,10 thermal evaporation,11 pulsed laser deposition,12 electrodeposition,13 or chemical bath deposition (CBD).14−18 The growth in vapor phase results in the formation of ZnO NWs with a high structural and optical quality but is still quite expensive, not versatile, and operates at high temperature.4−7,10−12 In contrast, the growth in liquid phase by CBD has the advantages of being cost-efficient, surface scalable, and easily implemented through a simple immersion in a chemical precursor solution at moderate temperature.14−18 However the initial presence of a thin seed layer is required in order to induce the nucleation and © 2012 American Chemical Society

Received: August 31, 2012 Revised: October 17, 2012 Published: October 31, 2012 25106

dx.doi.org/10.1021/jp308643w | J. Phys. Chem. C 2012, 116, 25106−25111

The Journal of Physical Chemistry C

Article

Table 1. Structural Properties of the Seed Layers Consisting of ZnO NPs samples S1 S2 S3 S4

NP average diameter (nm) 7.5 17.2 22.4 27.4

± ± ± ±

1.3 2.5 4 5.5

NP average density (nm) 383 176 119 75

± ± ± ±

5 5 5 5

seed layer thickness (nm)

seed layer PMS roughness (nm)

C0002 (for N = 3 peaks)

C0002 (for N = 6 peaks)

± ± ± ±

0.95 1.24 3.98 5.26

46% 72.5% 81%

47.5% 58.7%

7.5 17.2 105 180

1.3 2.5 4 4

coefficients Chkl were then calculated by dividing the absolute texture coefficient by the number of diffraction peaks considered. The 00-036-1451 file of the International Centre for Diffraction Data (ICDD) was used. X-ray pole figures were performed with a Siemens D5000 diffractometer using Cu Kα1 radiation. The apparatus is equipped with a 4 circle goniometer (i.e., for omega, 2theta, phi, and chi), 2.5° Sollers slits, as well as a graphite monochromator and a scintillation detector. The complete X-ray pole figure patterns were initially recorded on the (0002) diffraction peak. Subsequently, the patterns were rebuilt for a chi angle in the range of −90 to +90°. For positive and negative chi angles, the intensity was obtained by integrating the phi corresponding intensity in the range of 0 to 180° and 180 to 360°, respectively. The full-width-at-halfmaximum (fwhm) was determined as a quantitative measurement of the average tilt angle of ZnO NWs.

NPs with a small diameter might give rise to grain boundary nucleation, whereas ZnO NPs with a larger diameter might favor surface nucleation. The seed layer roughness may also have significant effects on the nucleation mechanisms and thereby on the structural properties of ZnO NWs.22,35,39 It is the aim of this paper to accurately determine the relationship between the structural morphology of the seed layer and the structural properties of ZnO NWs such as their orientation, vertical alignment, density, diameter, and length by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) imaging as well as X-ray diffraction (XRD) measurements. In particular, we quantitatively determine the vertical alignment of ZnO NWs by X-ray pole figures.

II. EXPERIMENTAL PROCEDURE The thin seed layers of ZnO NPs were grown on Si(100) substrate by dip coating. The chemical precursor solution was composed of zinc acetate dihydrate (ZnAc2·2H2O) from Merck and monoethanolamine from JT Baker dissolved in absolute ethanol in an equimolar ratio. The experimental growth conditions (mainly the chemical precursor solution concentration, withdrawal speed, and deposit number) were selected in order to deposit four samples S1, S2, S3, and S4 presenting ZnO seed layers with distinct structural properties in terms of thickness, NP size, and density as well as crystallinity (i.e., texture). All of the samples S1, S2, S3, and S4 were initially preheated on a hot plate kept at 300 °C for 10 min and subsequently postheated on another plate at 540 °C for 1 h. ZnO NWs were grown by CBD for 3 h in a chemical precursor solution consisting of zinc nitrate hexahydrate (Zn(NO3)2·6H2O) from Sigma-Aldrich and hexamethylenetetramine (C6H12N4) from Sigma-Aldrich mixed in an equimolar ratio of 0.03 M, dissolved in deionized water and heated at 90 °C. The growth conditions were exactly identical for all four samples S1′, S2′, S3′, and S4′ in order to thoroughly investigate the effects of the structural morphology of the seed layer on the NW growth. Field-emission scanning electron microscopy (FESEM) imaging was recorded with a ZEISS Ultra+ microscope. Highresolution TEM specimens were prepared by mechanical lapping and polishing, followed by argon ion milling according to standard techniques. TEM imaging was achieved with a JEOL-JEM 2010 microscope operating at 200 kV. XRD patterns were collected with a Bruker D8 Advance diffractometer using Cu Kα1 radiation according to the Bragg− Brentano configuration. The θ−2θ XRD measurements were performed between 20° and 70° (in 2θ scale) involving six diffraction peaks: (101̅0), (0002), (101̅1), (101̅2), (112̅0), and (101̅3). The texture analysis was quantitatively carried out from the Kα1 component of each diffraction peak in the framework of the Harris method42,43 by determining the absolute texture coefficients for each crystallographic direction as well as the degree of preferred orientation. The relative texture

III. RESULTS AND DISCUSSION A. Structural Morphology of the ZnO Thin Seed Layer. The structural properties of the ZnO thin seed layers are presented in Table 1 and Figures 1−3. The seed layers are polycrystalline and composed of ZnO NPs with a spherical cap like shape, as shown in Figure 1. The seed layer thickness increases from 7.5 to 180 nm as the cycle numbers in dip coating varies from 1 to 7, as deduced from cross-section view FESEM images. The average diameter of ZnO NPs has been assessed from top-view FESEM image analysis among a population of more than 130 NPs and increases from 7.5 to 27.4 nm as the seed layer thickness increases. The seed layer porosity is low: the empty space region in between ZnO NPs (i.e., intergranular zones) as determined by image analysis lies on average in the range of 5 to 15% of the total surface area. Interestingly, ZnO NPs have a wurtzite structure and are preferentially oriented along the c axis as revealed in Figure 2. The relative texture coefficient C0002 significantly increases with the seed layer thickness until reaching the value of 81 and 58.7%, by taking into account either 3 or 6 (hkl) diffraction peaks in the XRD pattern, respectively. The seed layer texture is thus strengthened as its thickness increases as expected for polycrystalline thin films.44 However, whatever the seed layer thickness, no specific in-plane orientation is expected, as commonly found for polycrystalline thin films.44 Additionally, the seed layer root-mean-square (RMS) roughness, lying in the range of 0.95 to 5.26 nm, increases with its thickness and correlatively with the average diameter of ZnO NPs as shown in Figure 3. The distinct structural properties of the seed layers therefore offer a wide variety of morphology for the subsequent growth of ZnO NWs by CBD under identical conditions. B. Characteristic Dimensions of ZnO NWs and Their Relationship to the Density. The characteristic dimensions of ZnO NWs as well as their structural properties are presented in Table 2 and revealed in Figures 1, 2, and 4. Like ZnO NPs, ZnO NWs present a wurtzite structure and are c-axis oriented, as shown in Figure 2. Similarly to the case of the seed layer, the 25107



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Figure 2. Typical XRD patterns of (a) the seed layer and (b) ZnO NWs.

Figure 1. (a−d) Top-view FESEM images of ZnO NPs composing the seed layer with increasing diameters from 7.5to 27.4 nm (i.e., samples S1 to S4). (e−h) Corresponding top-view FESEM images of ZnO NWs grown on top of them with decreasing diameters from 113 to 54 nm (i.e., sample S1′ to S4′). Figure 3. AFM images of the seed layer with (a) small and (b) large average diameters, respectively. (c) Seed layer RMS roughness as a function of the average diameter of ZnO NPs composing the seed layer.

average diameter, length, and density of ZnO NWs have been determined from cross-section and top-view FESEM image analysis among a population of more than 60 NWs. It is found that the average diameter of ZnO NWs decreases from 113 to 54 nm, whereas their average length correlatively decreases from 1380 to 850 nm. Both decrease in the average diameter and length of ZnO NWs is very specific to the growth by CBD. The average density of ZnO NWs follows the opposite trend, by increasing from 26 to 110 NWs/μm2 as their average diameter and length decrease. Recently, Boercker et al. have pointed out in their theoretical modeling that the ZnO NW growth by CBD should be limited by the mass transport of the chemical precursors in solution.41 In other words, it is expected that the average diameter and length of ZnO NWs are strongly related to their density. It has been emphasized that these characteristic dimensions should be inversely proportional to their density.41 Importantly, our experimental results are in very

Table 2. Structural Properties of ZnO NWs Grown on Top of the Seed Layers Consisting of ZnO NPs samples S1′ S2′ S3′ S4′

NW average diameter (nm) 113 75 56 54

± ± ± ±

37 33 25 20

NW average length (nm) 1380 1054 950 850

± ± ± ±

60 60 60 60

NW average density (μm−2) 26 62 100 110

± ± ± ±

5 5 5 5

NW average tilt angle (°) 30.1 27.5 19.7 19.3

good agreement with the theoretical modeling, as shown in Figure 4. This indicates that the characteristic dimensions of ZnO NWs such as their average diameter and length are 25108



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Figure 4. Evolution of the average length (left axis) and diameter (right axis) of ZnO NWs as a function of their density. The solid lines correspond to the fit to 1/density.

completely driven by their density. Still, it should be noted that the density of ZnO NWs is governed by the nucleation process on the seed layer through the nucleation rate and sites. This reveals that the nucleation process on the seed layer is the key point for controlling the characteristic dimensions of ZnO NWs grown by CBD, since their subsequent growth is likely to retain the initial structural properties of NW nuclei. C. Effects of the Seed Layer Structural Morphology on the Characteristic Dimensions and Vertical Alignment of ZnO NWs. As expected, the structural properties of ZnO NWs drastically depend on the structural morphology of the seed layer. The average diameter of ZnO NWs falls as the average diameter of ZnO NPs increases, as clearly shown in Figures 1 and 5. More precisely, the average diameter of ZnO

Figure 6. (a−d) X-ray pole figures recorded on the (0002) diffraction peak of ZnO NWs grown on top of the seed layers with different structural properties. The insets are the corresponding FESEM images.

decreases from 30.1 to 19.3° as the average diameter of ZnO NPs is increased from 7.5 to 27.4 nm. This indicates that the vertical alignment of ZnO NWs improves when the seed layer thickness is increased. Correlatively, it is remarkable that the tilt angle of ZnO NWs decreases as the relative texture coefficient C0002 of ZnO NPs is increased, as presented in Figure 7. This is strong evidence that the vertical alignment of ZnO NWs enhances when the seed layer texture along the c axis is strengthened. D. Nucleation Process of ZnO NWs on the Thin Seed Layer. In order to account for the influence of the structural morphology of the seed layer, the nucleation mechanisms of ZnO NWs grown by CBD are investigated by TEM imaging. According to the standard nucleation theory, ZnO NWs preferentially nucleate at the seed layer/solution interface. The

Figure 5. Evolution of the average diameter (left axis) and density (right axis) of ZnO NWs with the average diameter of ZnO NPs composing the seed layer.

NWs initially decreases drastically and may subsequently saturate, after approximately reaching the average diameter of ZnO NPs. These experimental results are in agreement with ref 30 and in discrepancy with refs 19, 23, 27, 29, 34, and 37−40. Nevertheless, the discrepancy can be reconciled by considering the specific features of the nucleation process as discussed later. The X-ray pole figures are performed to quantify the tilt angle of ZnO NWs with respect to the normal to the substrate surface, as presented in Figure 6. It is found that the tilt angle of ZnO NWs deduced from the fwhm of X-ray pole figures

Figure 7. Average tilt angle of ZnO NWs deduced from the fwhm of X-ray pole figures as a function of the seed layer relative texture coefficient C0002 (bottom axis) and of the seed layer thickness (top axis). 25109



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seed layer is expected to offer a wide number of nucleation sites. Nevertheless, as pointed out before, the nature of the ZnO NW nucleation sites, mainly the free surface of ZnO NPs or the grain boundary in between ZnO NPs, is still open and may depend on their diameter or orientation.39 TEM images of the ZnO NP/NW interface are presented in Figures 8 and 9 for either large or small diameter of ZnO NPs.

For large diameter of ZnO NPs, it is clearly seen in Figure 8a,b that ZnO NWs nucleate on the free surface of ZnO NPs in our growth conditions. This is in agreement with ref 39, in which the surface nucleation site is prevalent in the range of large diameter. Additionally, the HRTEM image of the ZnO NP/NW interface and its diffraction patterns in Figure 8c,d reveal that ZnO NWs epitaxially grow on the ZnO NPs composing the seed layer. The occurrence of an epitaxial relationship between ZnO NPs and NWs can also be deduced from Figure 7, in which increasing the relative texture coefficient C0002 of the seed layer results in the improvement of the vertical alignment of ZnO NWs. For small diameter of ZnO NPs, the NW nucleation process is expected to follow similar physical mechanisms. In the TEM images of Figure 9a,b, the ZnO NW has likely nucleated epitaxially on the quasicentral ZnO NP. Subsequently, the ZnO NW has grown on top of the other ZnO NPs, keeping its orientation along the c-axis as shown in Figure 9c,d. According to ref 39, this would suggest that the average diameter of 7.5 nm for ZnO NPs is not small enough to change the nature of nucleation sites. It is worth noticing that the surface nucleation site is consistent with the FESEM images in Figure 1, showing that the density of ZnO NWs increases with the average diameter of ZnO NPs. By increasing the ZnO NP size, their free surface area is increased while the density of grain boundary is reduced: if the grain boundary nucleation process had been the ruling mechanism, the average density of ZnO NWs would have decreased accordingly. Furthermore, the strengthening of the seed layer texture along the c-axis as the average diameter of ZnO NPs increases may also account for the higher density of ZnO NWs. The chemical reactivity of the c-polar plane has been found to be favorable for the growth of ZnO NWs in solution and hence these planes may present preferential surface nucleation sites. By comparing Figures 8a and 9a, it turns out that the radial growth of ZnO NWs on the seed layer is also limited by the ZnO NP size (i.e., the seed layer roughness) in the nucleation step in addition to the mass transport of chemical precursors in solution in the growth step. The presence of stacking faults in the ZnO NPs and NWs is also revealed by the TEM images. More importantly, the clear relationship between the nucleation process and the characteristic dimensions of ZnO NPs and NWs could reconcile the discrepancy between refs 30 and 19, 23, 27, 29, 34, and 37−40. It is expected that the structural morphology of the seed layer can result in different nucleation sites, depending on the growth conditions and on the specific features of the empty space region in between ZnO NPs (i.e., intergranular zones). For instance, the seed layer porosity may be critical for the nucleation process, since it leads to the strengthening of the contribution of the grain boundary or intergranular zones. If the porosity is low (respectively high), the surface (respectively grain boundary) nucleation sites may be preferential and depend on the nature of the orientation (i.e., texture) of the ZnO NPs, irrespective of their average diameter. This is clearly seen in the present work for which the low porosity results in the nucleation of ZnO NWs on the free surface of ZnO NPs. Similarly, the seed layer roughness may also be crucial for the nucleation process.

Figure 8. (a−c) Cross-section HRTEM images with different magnifications showing the ZnO NP/NW interface and the surface nucleation site. The rectangular area numbered 1 in panel a refers to panel b, whereas the rectangular area on the image b refers to panel c. (d) SAED pattern collected on the ZnO NP/NW interface along the [010] zone axis, showing the epitaxial relationship.

Figure 9. (a−b) Cross-section HRTEM images with different magnifications showing the ZnO NP/NW interface and the surface nucleation site. (c) Cross-section HRTEM image of the ZnO NW near the interface. The rectangular areas numbered 1 and 2 in panel a refer panels c and b, respectively. (d) SAED pattern recorded on the ZnO NW along the [010] zone axis, showing its orientation along the c axis.

IV. CONCLUSION In conclusion, the growth of ZnO NWs by CBD is limited by the mass transport of chemical precursors in solution, resulting in the inverse relationship between their average diameter and length and their density. Furthermore, the structural morphol25110



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ogy of the seed layer consisting of ZnO NPs plays a significant role on the structural properties of ZnO NWs, mainly on their vertical alignment and density through the nucleation process. It is shown by HRTEM images that ZnO NWs epitaxially grow on top of ZnO NPs and especially nucleate on their free surface: the nature of the nucleation site, either on the free surface or at the grain boundary of ZnO NPs, completely governs the relationship of characteristic dimensions between ZnO NPs and NWs. Therefore, the vertical alignment of ZnO NWs as deduced by X-ray pole figures is improved and their density increases when the seed layer texture is strengthened along the c axis, showing the high chemical reactivity of the c polar plane. The texture, roughness, and porosity of the seed layer are three crucial parameters.

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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

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REFERENCES

The authors would like to warmly thank C. Jimenez and P. Chaudouet for their helpful assistance in growth and AFM experiments as well as D. Bellet for fruitful discussions. This work has been supported by the French Research National Agency (ANR) through the program HABISOL under the project ASYSCOL (Contract No. ANR-08-HABISOL-002). The authors acknowledge funding by the Research Cluster Micro-Nano from the Région Rhône-Alpes.

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Critical Nucleation Effects on the Structural Relationship Between ZnO

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