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Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminium Nitrate Claire Verrier, Estelle Appert, Odette Chaix-Pluchery, Laetitia Rapenne, Quentin Rafhay, Anne Kaminski-Cachopo, and Vincent Consonni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11104 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminium Nitrate

Claire Verrier,a,b Estelle Appert,a Odette Chaix-Pluchery,a Laetitia Rapenne,a Quentin Rafhay,b Anne Kaminski-Cachopo,b and Vincent Consonni.*,a a

b

Université Grenoble Alpes, CNRS, LMGP, F-38000 Grenoble, France

Université Grenoble Alpes, CNRS, IMEP-LAHC, F-38000 Grenoble, France

CORRESPONDING AUTHOR FOOTNOTE: *E-mail: [email protected]

ABSTRACT Mastering the properties of ZnO nanowires grown by the low temperature chemical bath deposition (CBD) is of crucial importance, but is still challenging. We show that the shape, dimensions, and doping of ZnO nanowires can simultaneously be tuned by the addition of aluminium nitrate in the standard chemical system using zinc nitrate, hexamethylenetetramine, and ammonia in aqueous solution. The formation and doping mechanisms of ZnO nanowires are thoroughly investigated by combining chemical, structural, and optical analyses with in situ pH measurements correlated with thermodynamic simulations. We reveal that the electrostatic interactions of Al(OH)4complexes with the positive m-plane sidewalls of ZnO nanowires at a given pH favors

S1 ACS Paragon Plus Environment


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their adsorption as capping agents, reducing the radial growth and promoting the elongation, while favoring the aluminium uniform incorporation. Importantly, the aluminium doping is found to be thermally activated above the low temperature of 200 °C under oxygen atmosphere, as indicated by the occurrence of six related additional modes in the range of 200 to 900 cm-1 in temperature-dependent Raman spectroscopy. These findings show that CBD using aluminium nitrate is of high potential for tuning both the morphology of ZnO nanowires and their physical properties via the aluminium doping, which paves the way for their more efficient use into sensing, electronic, and optoelectronic devices both on flexible and rigid substrates.


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1. INTRODUCTION Controlling the structural and physical properties of ZnO nanowire (NW) arrays is of great interest for their efficient integration into optoelectronic devices,1,2 including light emitting diodes,3 self-powered ultra-violet photodetectors,4,5 dye-sensitized6,7 and extremely thin absorber solar cells.8 Although ZnO NWs can be grown by a large number of physical and chemical vapor deposition techniques,9,10 their formation in aqueous solution using chemical bath deposition (CBD) has widely been developed over the last years, as a low-cost, low-temperature, and easily implemented deposition technique.11-16 In the CBD process, the structural properties of ZnO NWs are strongly dependent upon both the morphology of the polycrystalline ZnO seed layer (SL)17,18 (most commonly deposited by sol-gel process using spin or dip coating) and the growth conditions used in aqueous solution1,2 (i.e., nature and concentration of chemical precursors, pH, temperature, time). As regards the effects of the morphology of the ZnO SLs, ZnO NWs have been shown to homoepitaxially nucleate on c-axis oriented ZnO nanoparticles (NPs),18 their polarity being directly transferred from these NPs.19 A surface nucleation mechanism has most often been reported, especially when the diameter of ZnO NPs exceeds about 10 nm.20 Accordingly, the density of ZnO NWs is closely related to the density of c-axis oriented ZnO NPs.18,21 In the standard growth regime governed by the mass transport of active reactants in aqueous solution,22,23 the diameter and length of ZnO NWs are inversely proportional to their density and can thus be indirectly tuned by the SL morphology (via the density of c-axis oriented ZnO NPs). While the ZnO SLs have been optimized by using high temperature annealing,24 ZnO nucleation surfaces pre-patterned



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by advanced lithography and etching processes have also been used to thoroughly control the structural uniformity and physical properties of ZnO NWs.25,26 Regarding the effects of growth conditions in aqueous solution and especially of the chemical precursors used, hexamethylenetetramine (HMTA) has been found to act as a source of HO- ions and as a pH buffer,27,28 but also to strongly reduce the radial growth of ZnO NWs29-31 and to interact with the ZnO SL, which alters their nucleation process.31 Alternatively, chemical additives such as polyethylene-imine,32 citrate ions,33 and chlorine ions34 have been used to monitor the shape and dimensions of ZnO NWs by acting as a capping agent on their non-polar m-plane sidewalls or polar c-plane top facet, respectively. An even more critical issue regarding the use of ZnO NWs grown by CBD is related to the control of their physical (i.e., optical and electrical) properties, which are related to unintentional and intentional doping. Basically, ZnO is an intrinsically n-type wide band gap semiconductor, for which the p-type doping is still not achieved in a stable and reproducible way.9,35 In non-intentionally doped ZnO thin films and NWs grown by a large number of techniques, a high density of free electrons is typically measured by electrical measurements.36 This is due to the high concentration of intrinsic point defects acting as donors, such as oxygen vacancies and zinc interstitials,37 and very likely of hydrogen interstitials.38,39 In view of the difficulty to control the density of these intrinsic point defects, monitoring extrinsically the doping of ZnO NWs is certainly the most promising approach for improving their optical and electrical properties (i.e., carrier density, carrier mobility, near band edge emission, visible band, …), which is a strong requirement for their efficient integration into nanoscale engineering devices. However, the doping of ZnO NWs by CBD has not been investigated yet in much details, even with


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the very common n-type doping elements such as aluminium, gallium, and indium. As these doping elements have a comparable radius with zinc, they preferentially substitute for zinc sites in the lattice to release an additional free electron conferring the extrinsic ntype doping.36 In the CBD process, the n-type doping with these elements may be carried out by introducing appropriate metallic cation salts, but their reactivity in aqueous solution should carefully be considered. The effects of adding different metallic cation sulfates in aqueous solution to control the morphology of ZnO NWs were thoroughly investigated by Joo et al. in the chemical system using zinc sulfate and ammonium chloride at 50 – 60 °C.40 They found that metallic cation ion complexes with positive or negative charges at a given pH of 11 affect the shape of ZnO nanostructures by inhibiting the development of their facets through electrostatic interactions. The comparison between the n-type doping elements such aluminium, gallium, and indium was further achieved by Kim et al. and aluminium was demonstrated to induce the formation of ZnO NWs with the highest morphological and optical quality.41 The very few attempts of doping ZnO NWs with aluminium have been achieved by adding an aluminium chemical precursor in aqueous solution, Al(NO3)3 being usually reported as the most efficient one to form ZnO NWs.42-45 Nevertheless, the possible incorporation of aluminium into ZnO NWs for the doping has not been studied yet and the effects of Al(NO3)3 on their formation mechanisms using the most standard chemical system involving zinc nitrate Zn(NO3)2, HMTA, and ammonia at 90°C have not been investigated either. In this article, the doping of ZnO NWs by CBD is performed by adding Al(NO3)3 as a chemical precursor in aqueous solution in the typical chemical system using Zn(NO3)2, HMTA, and ammonia. The effects of that addition on the formation mechanisms of ZnO



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NWs are thoroughly investigated by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) imaging, and supported by in situ pH measurements correlated with thermodynamic simulations yielding theoretical solubility plots and speciation diagrams of Zn(II) and Al(III) species. The incorporation of aluminium into ZnO NWs is further characterized by energy-dispersive X-ray spectroscopy (EDS) spectra and mapping using FESEM and scanning TEM (STEM), and by temperature-dependent Raman spectroscopy.

2. EXPERIMENTAL AND THEORETICAL SECTION 2.1. Deposition techniques. Polycrystalline ZnO SLs were grown on silicon substrate by sol-gel process using dip coating. The silicon substrate was initially cleaned with acetone and isopropanol. The solution of chemical precursors for the sol-gel reaction was composed of an equimolar mix of zinc acetate dihydrate [Zn(CH3COO)2.2H2O] (Emsure ACS) and monoethanolamine (MEA, J.T.Baker) with a concentration of 0.375M dissolved in pure ethanol. It was stirred for a couple of hours at 60°C and then at room temperature. After slowly dipping the samples into the solution, they were pulled out at a withdrawal speed of 3.3 mm/s under controlled atmosphere (hygrometry < 15%) to form the xerogel film. The post-deposition heat treatments consisted in successively placing the samples on two hot plates kept at 300 and 500 °C for 10 min and 1h, respectively, for evaporating the solvent and residual organic compounds as well as for crystallizing the ZnO SL. ZnO NWs were formed by CBD by placing for 3h the samples in a sealed beaker containing the aqueous solution of chemical precursors and kept in an oven at 90°C. The aqueous solution of chemical precursors consisted of an equimolar mix of zinc


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nitrate hexahydrate (Zn(NO3)2·6H2O) and HMTA (C6H12N4) with a concentration of 30 mM. For the intentional doping of ZnO NWs, aluminum nitrate nonahydrate (Al(NO3)3·9H2O) was added to the aqueous solution of chemical precursors. The series of 11 samples was grown by varying the concentration ratio [Al(NO3)3]/[Zn(NO3)2)] in the range of 0 to 10% (i.e., 0%, 0.2%, 0.3%, 0.4%, 0.5%, 1%, 3%, 5%, 7%, 9% and 10%). Ammonia (NH3) was systematically used as a chemical additive to set the pH to 10.7 at room temperature prior to the ZnO NW growth and to reduce the homogeneous growth in aqueous solution. Its concentration was set to 500 mM. The effects of the addition of ammonia on the morphology of ZnO NWs were reported in Refs. 46 and 47. 2.2. Thermodynamic computations. Thermodynamic simulations yielding theoretical solubility plots and speciation diagrams of Zn(II) and Al(III) species were achieved by using Visual MINTEQ software. The present simulations were performed at 90°C (i.e., growth temperature) by considering the above single metallic cations interacting with HO- ions and NH3 as ligands to form Zn(II) hydroxide and amine complexes as well as Al(III) hydroxide complexes. The simplified chemical mechanism

is given by: nMx+ + iL ↔ M L with β = [ ] [] , in which L is the ligand (i.e., HO

ions or NH3), M L is the complex considered, i is the coordination number, and β is

the stability constant. The stability constants β were taken at 25°C from NIST and deduced at 90°C by using the Kelley’s equation.47-49 All the equilibrium reactions and related stability constants β at 25°C are summarized in Tables S1 and S2. The following solid compounds were considered for calculating the theoretical solubility plot, as summarized in Table S3: ZnO(s) and the different phases of ZnOH s.



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2.3. Characterization techniques. The morphology and structural properties of ZnO NWs were investigated by top-view and cross-sectional FESEM images using a FEI Quanta 250 FEG-SEM and by XRD measurements with a Bruker D8 Advance diffractometer using CuKα1 radiation according to the Bragg-Brentano configuration. TEM specimens were prepared by scratching the surface of ZnO NWs using a diamond tip and put on a copper grid. TEM images were collected with a JEOL 2010 LaB6 microscope operating at 200 kV with a 0.19 nm point-to-point resolution. The incorporation of aluminium into ZnO NWs was investigated by EDS analyses and temperature-dependent Raman spectroscopy. EDS spectra were recorded on top of ZnO NW arrays with a Bruker detector incorporated in the FEI Quanta 250 FEG-SEM. EDS spectra and mapping were collected on top of single ZnO NWs by STEM using a JEOL 2100F FEG microscope operating at 200 kV with a 0.2 nm resolution in the scanning mode and equipped with the novel JEOL SDD Centurio detector having a large solid angle of up to 0.98 steradians. Temperature-dependent Raman scattering spectra were recorded using a Jobin/Horiba Labram spectrometer equipped with a liquid nitrogen cooled coupled charge device detector. The 488 nm line of an Ar+ laser was used with a power on the sample surface close to 3mW. The laser was focused to a spot size smaller than 1µm2 using 50 times long working distance objective. Spectra were calibrated using a silicon reference sample at room temperature (theoretical position of the Si Raman line = 520.7 cm-1). The annealing from room temperature to 450°C was controlled by using a commercial Linkam heating stage (THMS600) placed under the Raman microscope.

3. RESULTS AND DISCUSSION


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3.1. Effects of Al(NO3)3 on the formation mechanisms of ZnO nanowires. The ZnO NWs were grown by CBD on a compact 40 nm thick ZnO SL consisting of NPs with a diameter of the same order. These ZnO NPs are strongly oriented along the polar c-axis to favor the homoepitaxial nucleation of ZnO NWs on top of their surfaces. For all of the ZnO NW growths, the structural morphology of the ZnO SL was strictly identical. The structural properties of ZnO NWs grown by CBD with the varying [Al(NO3)3]/[Zn(NO3)2)] ratios are presented by top-view and cross-sectional FESEM images in Figure 1. The [Al(NO3)3]/[Zn(NO3)2] ratio was varied in the range of 0 to 10 % and the [Al(NO3)3] + [Zn(NO3)2] sum was kept constant and equal to 30 mM. The evolutions of the length, diameter, and related aspect ratio of ZnO NWs as a function of [Al(NO3)3]/[Zn(NO3)2)] ratios are revealed in Figure 2a-c from cross-sectional FESEM image analysis over a population of more than 200 NWs. In particular, the diameter of ZnO NWs was determined both at their top and at 300 nm below to take into account the possible change of their shape. The deposited volume per ZnO NW as depicted in Figure 2d was deduced from their length and diameter. The in situ pH measurements as well as thermodynamic simulations yielding theoretical solubility plots and speciation diagrams of Zn(II) and Al(III) species are shown in Figures 3 and 4. 3.1.1 Effects of Al(NO3)3 on the length of ZnO nanowires. At 0 %, the ZnO NWs have a typical length of 4 ± 0.4 µm, corresponding to a high axial growth rate of about 22 nm / min, as seen in Figure 2a. The top of the ZnO NWs does not exhibit a flat facet, but instead has a pencil-like shape typical of nanoneedles. By increasing the [Al(NO3)3]/[Zn(NO3)2)] ratio in the range of 0 to 1 %, the length of ZnO NWs is drastically decreased from 4 ± 0.4 µm to 1.5 ± 0.4 µm, corresponding to a much lower



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axial growth rate of about 8 nm / min. The top of the ZnO NWs is also affected and a flat facet is formed and composed of polar c-planes, meaning that the shape of nanoneedles is lost. In addition to that significant change of length, the homogeneous growth in aqueous solution is more and more pronounced. By further increasing the [Al(NO3)3]/[Zn(NO3)2)] ratio from 1 to 10%, the length of ZnO NWs is significantly increased from 1.5 ± 0.4 µm to 4.9 ± 0.4 µm, corresponding to the highest axial growth rate of about 27 nm / min. Similarly, the top of the ZnO NWs is again formed of a pencil-like shape corresponding to nanoneedles from the [Al(NO3)3]/[Zn(NO3)2)] ratio of 5 %. To account for the present variation of length, the pH was measured in situ for the [Al(NO3)3]/[Zn(NO3)2)] ratios of 0, 1, 3, and 7 % and is presented in Figure 3a. While the pH was set to 10.7 prior to the ZnO NW growth, it is found to strongly decrease as the CBD of ZnO NWs proceeds. After 40 min of growth, the aqueous solution approximately reaches 85°C and the corresponding pH is about 9 ± 0.25, depending on the [Al(NO3)3]/[Zn(NO3)2)] ratio. Interestingly, the pH is still decreased during the formation of ZnO NWs down to a decreasing plateau around 8.3 ± 0.1 and the present decrease is significantly dependent upon the [Al(NO3)3]/[Zn(NO3)2)] ratio. The theoretical solubility plot as a function of pH is given in Figure 3b. The supersaturation in aqueous solution is indicated by the added arrows in the inset of the theoretical solubility plot. It is strongly influenced by a small variation of pH, which can account for the variation of the deposited volume and correlatively of their length. In particular, the supersaturation is much

larger

for

the

[Al(NO3)3]/[Zn(NO3)2)]

ratio

of

1

%

than

for

the

[Al(NO3)3]/[Zn(NO3)2)] ratio of 0 %. It is very likely that the metastable limit is accordingly overcome, resulting in a more pronounced homogeneous growth as


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experimentally observed and thus in a decrease in the length of ZnO NWs and in the volume deposited per ZnO NW as shown in Figures 2a and 2d, respectively. In contrast, by further increasing the [Al(NO3)3]/[Zn(NO3)2)] ratio to 7 %, the supersaturation is gradually decreased and hence the heterogeneous growth is progressively favored at the expense of the homogeneous growth, increasing in turn both their length and the deposited volume per ZnO NW. Moreover, it should be noted that the density of ZnO NWs is also strongly increased in the range of 1 to 10 %, getting even higher values at 10% than at 0%, which causes an additional increase in the total deposited volume that is not considered in the underestimated total deposited volume per ZnO NW. Following the solubilization of Zn(NO3)2, Zn2+ ions are formed in aqueous solution: Zn(NO3)2 → Zn2+ + 2 NO3-

(1)

These Zn2+ ions as metallic cations are expected to form Zn(II) hydroxide and amine complexes with the HO- ions and NH3 as ligands, respectively. The speciation diagram of Zn(II) species at 90 °C as a function of pH is presented in Figure 3c. Nine soluble Zn(II) species were considered in aqueous solution: Zn2+ ions, ZnOH, ZnOH aq, ZnOH , ZnOH , ZnNH , ZnNH , ZnNH , and ZnNH . The

"#

simplified chemical mechanism is given by: Zn2+ + iL ↔ ZnL with β = ["# ][] , in

which L is the ligand (i.e., HO- ions or NH3), ZnL is the complex considered, i is the coordination number, and β is the stability constant. To a first approximation, the total concentration of NH3 was taken as 500 mM and the contribution originating from the gradual hydrolysis of HMTA with heat was neglected. In the range of pH considered, Zn(II) species are mostly ZnNH complexes while ZnNH are also formed to a much lesser extent. These Zn(NH3)x2+ complexes solubilize the ZnO and their formation S11 ACS Paragon Plus Environment


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can account for the increase in the solubility in the range of pH considered, as shown in Figure 3b. The solubility increase prevents a too important supersaturation in aqueous solution, which is in favor of the heterogeneous growth of ZnO and at the expense of the homogeneous growth. Interestingly, another aspect related to the axial growth rate is the morphology change of the ZnO NWs: the formation of the pencil-like shape related to nanoneedles is favored when the axial growth rate is higher, which is due to these supersaturation considerations. This particular shape with a diameter decrease along one single ZnO NW is shown on the FESEM and TEM images in Figure S1, in which the stacking of successive terraces with the polar c-planes is revealed. 3.1.2. Effects of Al(NO3)3 on the diameter of ZnO nanowires. At 0%, the diameter of ZnO NWs at their top is about 112 ± 12 nm and is smaller than at 300 nm below, which is related to their pencil-like shape corresponding to nanoneedles, as schematically depicted in Figure 2b. By increasing the [Al(NO3)3]/[Zn(NO3)2)] ratio in the range of 0 to 1%, the diameter of ZnO NWs is strongly reduced down to 59 ± 12 nm, which is mainly due to the drastic decrease in the deposited volume and in their related length originating from a more pronounced homogeneous growth in aqueous solution related to the higher supersaturation

as

previously

discussed.

By

further

increasing

the

[Al(NO3)3]/[Zn(NO3)2)] ratio from 1 to 10 %, the diameter of ZnO NWs remains constant and roughly saturates to 55 ± 12 nm. The difference in the diameters of ZnO NWs at their top with respect to 300 nm below, corresponding to the pencil-like shape related to the formation of nanoneedles, is larger in the range of 7 to 10% than at 0% and even more pronounced at 10 %. The high aspect ratio of more than 90 is thus reached while keeping


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a small diameter of about 50 nm by using Al(NO3)3 with the highest concentration in aqueous solution, as shown in Figure 2c. This significant increase in the aspect ratio of ZnO NWs by increasing the Al(NO3)3 concentration in aqueous solution is schematically explained in Figure 4. Following the solubilization of Al(NO3)3, Al3+ ions are formed in aqueous solution: Al(NO3)3 → Al3+ + 3 NO3-

(2)

These Al3+ ions as metallic cations are expected to form Al(III) hydroxide complexes with the HO- ions as ligands. In contrast to Zn(II) ions, Al (III) ions do not form stable amine complexes with NH3. The speciation diagram of Al(III) species at 90 °C as a function of pH is presented in Figure 4a. Seven soluble Al(III) species were considered in aqueous solution: Al3+ ions, AlOH , AlOH , AlOH , AlOH , Al OH , and

Al OH&. The simplified chemical mechanism is given by: nAl3+ + iL ↔ Al L with

β

=

* +

'()

[()* ] []

,

, in which L is the ligand (i.e., HO- ions), Al L

is the

complex considered, i is the coordination number, and β is the stability constant. Interestingly, in the pH ranging from 8 to 9, the AlOH complexes are very largely predominant in aqueous solution and hence negative Al(III) hydroxide complexes are mostly formed, as shown in Figure 4b. These negative AlOH complexes electrostatically interact with the different surfaces of the ZnO NWs, depending on their electrical charge as defined by the isoelectric point (IEP). Basically, the IEP yields the pH at which the surface of a particle switches from a positive charge (with –OH2+ at the surface) to a negative charge (with -OH- at the surface). It should be noted that the values of the IEP have been reported in a broad range of 8 to 10 in ZnO.50 More importantly, the IEP is dependent upon the surface nature and configuration, as reported for TiO2 and S13 ACS Paragon Plus Environment


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Al2O3.51,52 In the present case, the value of the IEP is also presumably higher for the nonpolar m-planes composing the sidewalls of ZnO NWs than for the polar c-planes forming their top facet, as depicted in Figure 4c. More precisely, at the pH of 8.5, the m-planes are expected to be positively charged while the c-planes are likely to be negatively charged. Accordingly, at this pH, the negative AlOH complexes can be adsorbed on the sidewalls and act as capping agents as depicted in Figure 4c, limiting their development and thus the related radial growth while improving the axial growth of ZnO NWs, as shown in Figures 1 and 2. Different stabilization processes are also expected on the non-polar and polar ZnO planes and thus may play a significant role on the present growth mechanisms.53

3.2 Aluminium incorporation into ZnO nanowires. The possible incorporation of aluminium as a n-type doping elements into the ZnO NWs following the addition of Al(NO3)3 in aqueous solution is investigated by structural, chemical, and optical analyses using EDS in both SEM and STEM, XRD, as well as temperature-dependent Raman spectroscopy. 3.2.1. EDS-SEM, EDS-STEM, and XRD measurements. The EDS-SEM spectra of ZnO NW arrays grown with the [Al(NO3)3]/[Zn(NO3)2] ratios of 0, 1, 5, 7, 9, and 10% are presented in Figure 5. The Kα-peak of aluminium is detected at the energy of 1.486 eV.

Overall,

its

intensity

is

initially

increased

significantly

when

the

[Al(NO3)3]/[Zn(NO3)2] ratio is increased from 0 to 7 %, and may subsequently saturate for the [Al(NO3)3]/[Zn(NO3)2] ratios up to 10%. A quantitative analysis is not achieved here since the deposited volume of ZnO NWs is significantly changed and not


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straightforward to precisely determine, as revealed in Figure 2d with the deposited volume per ZnO NW deduced from their diameter and length. This is typically emphasized by the Kα-peak of silicon at the energy of 1.739 eV, for which the intensity is strongly increased as the deposited volume per ZnO NW is decreased. Accordingly, the EDS-SEM indicates the presence of aluminium, but its location either into the ZnO NW arrays and/or the underneath ZnO SL is not identified. In order to localize the position of aluminium, EDS-STEM measurements on single ZnO NWs grown with the [Al(NO3)3]/[Zn(NO3)2] ratios of 0 and 7 % and dispersed onto a copper grid are shown in Figure 6. The Kα-peak of aluminium is clearly detected in the single ZnO NWs as shown by

EDS-STEM

spectra,

indicating

the

presence

of

aluminium

for

both

[Al(NO3)3]/[Zn(NO3)2] ratios. Interestingly, there is an increasing amount of aluminium in the single ZnO NWs by increasing the [Al(NO3)3]/[Zn(NO3)2] ratio: the atomic ratio of aluminium is typically increased by a mean factor ranging from 2 up to 5 for several analysed single ZnO NWs. The detection of aluminium when no Al(NO3)3 was added in aqueous solution is due to the fact that aluminium as a trace metal (i.e., 0.03 ppm) occurs in most of the chemical precursors used, especially in ammonia. It should be noted here that a large amount of aluminium measured may be adsorbed in the form of AlOH complexes on the non-polar m-planes composing the sidewalls of ZnO NWs as previously discussed; in other words, only a much smaller amount is expected to be incorporated into the bulk of ZnO NWs. Furthermore, the EDS-STEM maps reveal that the signal of aluminium is homogeneous all along the ZnO NWs and thus that aluminium is uniformly distributed. The XRD patterns as reported in Figure S2 indicates that all of the diffraction peaks are attributed to the wurtzite crystalline structure of ZnO coming



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from the SL and NW arrays and hence that no alumina phase is detected, which is favorable for its own incorporation into the bulk of ZnO NWs. 3.2.2. Temperature-dependent Raman spectroscopy. In order to more precisely investigate the incorporation of aluminium into the bulk of ZnO NWs, temperaturedependent Raman scattering spectra of ZnO NWs collected from room temperature to 450 °C under oxygen atmosphere are presented in Figure 7a-d for the [Al(NO3)3]/[Zn(NO3)2] ratios of 0, 0.5, 1, and 7 %, respectively. The ZnO NWs were specifically grown on glass substrate for the Raman spectroscopy investigation instead of silicon substrate to avoid any trouble related to the strong silicon line and thus to the increase in the counting time needed to improve the spectrum quality. As observed in Figure 7, all of the spectra recorded at room temperature contain the ZnO Raman modes typical of the wurtzite crystalline structure and expected for NWs oriented with their c-axis perpendicular to the substrate surface (i.e., parallel to the laser wave propagation direction), meaning that A1(TO), E1(TO), and E1(LO) modes are not allowed.54 The strongest lines are pointed at 99 cm-1 (E2(low)), 438 cm-1 (E2(high)), 581 cm-1 (A1(LO)), 333 cm-1 (E2(high)-E2(low)), and in the range of 1000 to 1200 cm-1 (second-order modes). The frequencies of these ZnO Raman modes are unchanged as the [Al(NO3)3]/[Zn(NO3)2] ratio is increased. Interestingly, the ZnO NWs are free of strain as shown by the position of the strain-sensitive E2(high) mode at 438 cm-1, which is identical to that in ZnO single crystals.54 By raising the temperature, the intensity is continuously increased for all these Raman modes. That increase is even more pronounced for the second-order modes, especially the very weak 2E2(low) mode pointed at about 203 cm-1.55 More importantly, the occurrence of several additional modes (AMs)


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is revealed from 200 °C, their intensities also being increased by raising the temperature and by increasing the [Al(NO3)3]/[Zn(NO3)2] ratio. These Raman modes, labeled as AM(1), AM(2), AM(4), AM(5), and AM(6) in the spectra, are pointed at 276 cm-1, 509 cm-1, 623 cm-1, 643 cm-1, and 856 cm-1, respectively. They are typically associated indistinctly with the incorporation of different doping elements into the ZnO lattice, such as iron, aluminium, gallium, antimony, and nitrogen, as reported in Refs. 56 and 57. Another AM labeled as AM(3) has been reported in the range of 579 to 584 cm-1 when the doping is achieved with these elements.56 The mode pointed at 581 cm-1 in our Raman scattering spectra probably results from the overlapping of the AM(3) and ZnO A1(LO) modes. It is also seen in Figure 7(a) that all of the AMs occur in the Raman scattering spectra for the [Al(NO3)3]/[Zn(NO3)2] ratio of 0 %. This is in agreement with the previous EDS analyses detecting aluminium in the ZnO NWs when no Al(NO3)3 was added in aqueous solution, as a part of trace metals contained in the chemical precursors used. The physical origin and nature of the AMs is still under debate. The AMs are commonly attributed to different doping elements possibly correlated with specific intrinsic host lattice defects, which are either activated as vibrational complexes or related to the increase in the doping element concentration inside the ZnO lattice.56 The electric field induced Raman scattering is also considered due to the high carrier concentration and related disorder provided by the doping element,58 enhancing the Raman active phonons such as the A1(LO) mode and Raman inactive phonons such as the B1 modes and B1-related second-order modes.58,59 The intensity increase of most of the Raman modes with the temperature gives evidence of an improvement of the crystalline quality of the ZnO NWs by raising the



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temperature under oxygen atmosphere. The maximum intensity is typically reached for a temperature above 400°C and that required temperature seems to be raised by increasing the [Al(NO3)3]/[Zn(NO3)2] ratio. This may indicate that more host lattice defects are formed by increasing the [Al(NO3)3]/[Zn(NO3)2] ratio, requiring in turn a higher temperature to be cured. Correlatively, the occurence of the six AMs above 200°C indicates that the doping is thermally activated and thus requires a low threshold temperature. Following the present annealing under oxygen atmosphere up to 450°C, the ZnO NW arrays were cooled down to 27°C and the corresponding Raman scattering spectra

recorded

at

this

temperature

are

shown

in

Figure

8a

for

the

[Al(NO3)3]/[Zn(NO3)2] ratios of 0, 0.5, 1, and 7 %. It should be noted here that all the Raman scattering spectra were normalized in intensity with respect to the E2(high) Raman line to make a direct comparison of the AM intensities. Interestingly, the AMs(1,2,4,5,6) pointing at 276 cm-1, 509 cm-1, 623 cm-1, 643 cm-1, and 856 cm-1, respectively, are very well defined in the Raman scattering spectra recorded after cooling down. The occurrence of the AM(4) at 623 cm-1 is of high interest, as being only assigned to iron, aluminium, and gallium (in contrast to the AMs(1,2,5) that are additionally attributed indistinctly to indium and nitrogen). Still, iron (i.e., 0.1 ppm as a trace metal) is not expected to form negative hydroxide complexes in the range of pH considered and the absence of both AMs at 713 and 720 cm-1 reported in Ref. 56 both exclude clearly its possible incorporation. Similarly, gallium is not either expected to form negative hydroxide complexes in the range of pH considered, also excluding its possible incorporation. The AM(4) together with the AM(1), AM(2), and AM(5) in the spectra are thus attributed to aluminium, given that any of the other doping elements are further in


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much lower concentrations as trace metals. More importantly, it is shown that the AM intensities are systematically increased by increasing the [Al(NO3)3]/[Zn(NO3)2] ratios from 0 to 7 %, which is a direct proof of the increased incorporation of Al into the bulk of ZnO NWs. Such a statement also holds for the AM(6) at 856 cm-1, which has mainly been attributed to the incorporation of nitrogen.60 The use of Al(NO3)3, of Zn(NO3)2, and in particular of a high concentration of ammonia forming Zn(NH3)42+ complexes as the majority Zn(II) species may be favorable for the incorporation of nitrogen in the present ZnO NW arrays. Nevertheless, the concentration of these complexes is not expected to significantly vary with the [Al(NO3)3]/[Zn(NO3)2] ratio, such that the AM(6) may alternatively be assigned here to aluminium, as further discussed in Ref. 56. Moreover, the evolution of the intensity ratios of the A1(LO) over E2(high) modes as a function of temperature is presented in Figure 8b for the [Al(NO3)3]/[Zn(NO3)2] ratios of 0, 0.5, 1, and 7 %. The relative intensity of the A1(LO) mode is typically increased by raising the temperature while decreasing the intensity of the E2(high) mode, as reported in doped ZnO thin films.61 This may further be attributed to the intensity increase in the AM(3) mode. Furthermore, regardless of the [Al(NO3)3]/[Zn(NO3)2] ratio, the intensity ratio of the A1(LO) over E2(high) modes at room temperature is systematically larger after annealing, indicating the occurrence of the AM(3). Also, the intensity ratio of the E2(low) over E2(high) modes is very drastically increased as the temperature is raised. The intensity of the E2(low) mode is even higher than the intensity of the E2(high) mode above 300 °C. In contrast to the A1(LO) mode, the intensity ratio of the E2(low) over E2(high) modes at room temperature is however identical after annealing. This is not the case for the intensity ratios of the E2(high)-E2(low) over E2(high) modes, which are



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systematically larger after annealing, regardless of the [Al(NO3)3]/[Zn(NO3)2] ratio. Interestingly, a Raman line attributed to hydrogen intersitials in bond-center sites acting as donors typically lies around 330 cm-1,39 which may be at the origin of that intensity ratio increase. The growth of ZnO NWs by CBD takes place in aqueous solution where hydroxide complexes are formed and their surfaces are typically saturated by hydrogen bonds, which may be favorable for its direct incorporation into ZnO NWs for the n-type doping. On the basis of these findings and of the formation mechanisms discussed above, the incorporation of aluminium into the bulk of ZnO NWs can be described as follows. As the pH is initially set to 10.7 and keeps a value higher than 8.5 as growth proceeds, negative AlOH complexes are adsorbed on the positive m-plane sidewalls of ZnO NWs and act as a capping agent to inhibit their development. At the same time, a significant amount of aluminium is incorporated into ZnO NWs following the possible dissociation of AlOH complexes during growth and subsequently as the annealing above 200 °C under oxygen atmosphere is achieved. The doping of ZnO NWs with aluminium is thermally activated following such an annealing and aluminium is expected to substitute for zinc sites, leading to the occurrence of six AMs in the range of 200 to 900 cm-1 in the Raman scattering spectra. Nitrogen and hydrogen present in a large amount in aqueous solution may also be incorporated and play a role. The possibility to dope ZnO NWs with aluminium using the low temperature, low cost, and easily implemented CBD technique with the help of Al(NO3)3 is highly promising since their optical and electrical properties including carrier density and mobility for instance should be controlled as much as possible for their efficient integration into nanoscale-


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engineering devices.1,2,9,36 Furthermore, the doping activation is perfomed at the low temperature around 200°C, which is technologically relevant for a wide variety of devices, even on some flexible substrates.

4. CONCLUSIONS The effects of Al(NO3)3 as an additional chemical precursor on the standard CBD of ZnO NWs using Zn(NO3)2, HMTA, and ammonia in aqueous solution kept at 90 °C have thoroughly been investigated by combining thermodynamic simulations with advanced characterization techniques such as in situ pH measurements, FESEM, TEM, XRD, EDSSTEM, and temperature-dependent Raman spectroscopy. It is shown that the shape and dimensions of ZnO nanostructures from nanoneedles to NWs can be monitored by adjusting the [Al(NO3)3]/[Zn(NO3)2] ratio in the typical range of 0 to 10%. The nanoneedles consisting of the stacking of successive terraces with the polar c-planes are systematically formed when the axial growth rate is high and above 9.7 nm / min. The role of Al(NO3)3 in the formation mechanisms of ZnO NWs is discussed and supported by in situ pH measurements correlated with thermodynamic simulations yielding theoretical solubility plots and speciation diagrams of Zn(II) and Al(III) species. The deposited volume per ZnO NW and, to some extent, their length are shown to be directly related to the supersaturation in solution, which depends on the chemical precursor concentrations and pH. More importantly, the diameter of ZnO NWs is drastically reduced as the [Al(NO3)3]/[Zn(NO3)2] ratio is increased from 0 to 10%. ZnO nanostructures with a high aspect ratio of more than 90 while keeping a small diameter of about 50 nm are hence formed for the highest [Al(NO3)3]/[Zn(NO3)2] ratio of 10 %. The



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electrostatic interactions of Al(OH)4- complexes with the positive m-plane sidewalls at the pH of 8.5 favor their adsorption as capping agents. The Al(OH)4- complexes thus inhibit the development of the sidewalls of ZnO NWs and thus the related radial growth while promoting their elongation. More importantly, aluminium is found to be uniformly distributed in the center of ZnO NWs and its content is significantly increased by increasing the [Al(NO3)3]/[Zn(NO3)2] ratio. Its incorporation into ZnO NWs is further shown by temperature-dependent Raman spectroscopy, through the occurrence of six AMs lying at 276, 509, 581, 623, 643, and 856 cm-1 above 200 °C. The intensities of these AMs are continuously increased by increasing the [Al(NO3)3]/[Zn(NO3)2] ratio, indicating that the aluminium incoporation can be tuned to a broad extent. The aluminium incorporation is eventually shown to be thermally activated above the temperature as low as 200°C, which is compatible with many nanoscale engineering devices on rigid and flexible substrates. These findings reveal that both the morphology and doping properties of ZnO NWs can simultaneously be monitored by the low cost and low temperature CBD using Al(NO3)3, which is a very significant step forward for the precise control of their overall physical properties.

Supporting information Possible chemical reactions in aqueous solution and related equilibrium constants K at 25°C, considering the chemical system Zn-OH-NH3. (Table S1). Possible chemical reactions in aqueous solution and related equilibrium constants K at 25°C, considering the chemical system Al-OH. (Table S2). Solubility constants KS of the different Zn(OH)2 solid phases at 25°C. (Table S3). (a) Top-view FESEM image of ZnO nanoneedles for


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the [Al(NO3)3]/[Zn(NO3)2] ratio of 0% after 2h of growth. (b) TEM image of ZnO nanoneedles for the [Al(NO3)3]/[Zn(NO3)2] ratio of 7%. (Figure S1). XRD patterns of ZnO NWs for the [Al(NO3)3]/[Zn(NO3)2] ratios of 0, 1, 5, 7, 9, and 10% (Figure S2).

ACKNOWLEDGMENTS The authors would like to thank Hervé Roussel from LMGP, Grenoble, France, for his assistance in the XRD experiments. This work was partially supported by the LabEx Cemam under the contract ANR-10-LABX-44-01. Funding by the Carnot Institute Energies du Futur through the project CLAPE is also acknowledged.



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(58) Tzolov, M.; Tzenov, N.; Dimova-Malinovska, D.; Kalitzova, M.; Pizzuto, C.; Vitali, G.; Zollo, G.; Ivanov, I. Vibrational Properties and Structure of Undoped and Al-Doped ZnO Films Deposited by RF Magnetron Sputtering. Thin Solid Films 2000, 379, 28-36. (59) Manjón, F. J.; Mari, B.; Serrano, J.; Romero, A. H. Silent Raman Modes in Zinc Oxide and Related Nitrides. J. Appl. Phys. 2005, 97, 053516. (60) Kaschner, A. ; Haboeck, U. ; Strassburg, M. ; Kaczmarczyk, G. ; Hoffmann, A.; Thomsen, C. ; Zeuner, A. ; Alves, H. R. ; Hofmann, D. M. ; Meyer, B. K. NitrogenRelated Local Vibrational Modes in ZnO:N. Appl. Phys. Lett. 2002, 80, 1909-1911. (61) Manouri, A. E.; Manjon, F. J.; Mollar, M.; Mari, B.; Gomez, R.; Lopez, M. C.; Ramos-Barrado, J. R. Effect of Aluminium Doping on Zinc Oxide Thin Films Grown by Spray Pyrolysis. Superlattices Microst. 2006, 39, 185-192.



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FIGURE CAPTIONS

Figure 1 Top-view and cross-sectional FESEM images of ZnO NWs grown by CBD with [Zn(NO3)2] + [Al(NO3)3] = 30 mM, [HMTA] = 30 mM, [NH3] = 500 mM and for the [Al(NO3)3]/[Zn(NO3)2] ratios of 0, 0.2, 0.3, 0.4, 0.5, 1, 5, 7, 9, and 10%. The pH prior to the ZnO NW growth was set to 10.7. The scale bar is 1 µm.

Figure 2 Evolution of the (a) length , (b) diameter at the NW top and at 300 nm from the NW top, and (c) aspect ratio of ZnO NWs as a function of the [Al(NO3)3]/[Zn(NO3)2] ratio. (d) Deposited volume per ZnO NW as inferred from its length and diameter as a function of the [Al(NO3)3]/[Zn(NO3)2] ratio.

Figure 3 (a) Evolution of the pH during the growth as a function of time for the [Al(NO3)3]/[Zn(NO3)2] ratios of 0, 1, 3, and 7%. (b) Theoretical solubility plot at 90°C as a function of pH ranging from 0 to 14 as computed with Visual MINTEQ software. The inset is the theoretical solubility plot over the narrow range of pH from 8.5 to 9. (c) Speciation diagram of Zn(II) species at 90°C as a function of pH ranging from 0 to 14 as computed with Visual MINTEQ software for [Zn2+]i = 30 mM and [NH3]i= 500 mM.

Figure 4 (a) Speciation diagram of Al(III) species at 90°C as a function of pH ranging from 0 to 14 as computed with Visual MINTEQ software for [Al3+]i = 3 mM. (b) Corresponding speciation diagram of negative, neutral, and positive Al(III) species. (c) Schematic diagram showing the effects of Al(NO3)3 on the formation mechanisms of


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ZnO NWs, mainly through the adsorption of AlOH complexes on their vertical sidewalls at the pH of 8.5 (left). Schematic diagram illustrating the expected configuration of differential IEP, such that the polar c-planes and non-polar m-planes are negative and positive, respectively, at the pH of 8.5 (right).

Figure 5 (a) EDS spectra of ZnO NWs collected in a SEM for the [Al(NO3)3]/[Zn(NO3)2] ratios of 0, 1, 5, 7, 9, and 10%.

Figure 6 (a) EDS spectrum of one single ZnO NW collected in a STEM for the [Al(NO3)3]/[Zn(NO3)2] ratio of 0%. (b-e) Corresponding bright-field STEM image and EDS-STEM elemental mapping of the aluminium (orange), oxygen (blue), and zinc (green) elements, respectively. (f) EDS spectrum of one single ZnO NW collected in a STEM for the [Al(NO3)3]/[Zn(NO3)2] ratio of 7%. (g-j) Corresponding bright-field STEM image and EDS-STEM elemental mapping of the aluminium (orange), oxygen (blue), and zinc (green) elements, respectively. The EDS spectra were normalized with respect to the Kα-peak of zinc at the energy of 8.638 keV.

Figure 7 Temperature-dependent Raman scattering spectra of ZnO NWs for the [Al(NO3)3]/[Zn(NO3)2] ratios of (a) 0, (b) 0.5, (c) 1, and (d) 7 % collected from room temperature to 450 °C under oxygen atmosphere (λ = 488 nm).

Figure 8 (a) Raman scattering spectra of ZnO NWs recorded after annealing at 450°C and cooling down to 27°C during the in situ experiments for the [Al(NO3)3]/[Zn(NO3)2]



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ratios of 0, 0.5, 1, and 7 %. (b) Evolution of the A1(LO)_AM(3) / E2(high) intensity ratio of ZnO NWs as deduced from Figure 7 as a function of temperature for the [Al(NO3)3]/[Zn(NO3)2] ratios of 0, 0.5, 1, and 7%.


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 6


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Figure 8


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Tunable Morphology and Doping of ZnO ... - ACS Publications

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