ARTICLE pubs.acs.org/JPCC
Fast Growth of High Work Function and High-Quality ZnO Nanorods from an Aqueous Solution Yang Tang, Jie Chen,* Dieter Greiner, Lorenz Ae, Robert Baier, Jascha Lehmann, Sascha Sadewasser, and Martha Ch. Lux-Steiner Helmholtz-Zentrum Berlin f€ur Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany ABSTRACT: A fast preparation process for high-quality as-grown ZnO nanorods by electrodeposition from an aqueous solution of Zn(NO3)2 and NH4NO3 at 75 °C was established. The vertical growth rate of the ZnO nanorods was enhanced with an increase in the NH4NO3 concentration. The NH3 adsorbed on the ZnO nanorods’ surface results in a higher work function without a significant effect on the nanorods’ optical qualities. The use of NH4NO3 leads to an increase in ZnO nanorods’ mobility while maintaining the carrier density constant. The increase of mobility is ascribed to a decrease of the dislocation and point defects inside grains owing to both fast growth rate and excess OH generation.
’ INTRODUCTION Zinc oxide (ZnO), with a direct wide band gap of Eg ≈ 3.3 eV (at 300 K) and an exciton energy of 60 meV, is cheap, stable, environmentally friendly, and nontoxic. Since ZnO single-crystal nanorods (NRs) or nanowires (NWs) exhibit fewer defects than their thin-film structure, they have captured the attention of many researchers and show good prospects for optoelectronic applications including light-emitting diodes (LEDs)14 and highly structured solar cells.5 ZnO NRs, on the other hand, have a good transparency for the solar spectrum and a subwavelength structure size. Therefore, the ZnO NR arrays are a promising material for both solar cells’ antireflective coating6 owing to their appropriate refractive index of ∼2 and superstrate solar cells. For applications in ZnO-based LEDs, reduction of the barrier due to the work function (Φ) difference between the transparent conductive oxide (TCO) cathode and the channel material of ZnO results in an improvement of the carrier collection and the luminescence onset voltage. As for application in solar cells, an enhancement of carrier collection by lowering the Φ difference between ZnO NRs and the absorber materials is highly demanded. In order to decrease the Φ difference, one approach is to prepare high Φ ZnO without deterioration of its optical qualities and electrical properties. For the purpose of increasing Φ, one can dope ZnO by a small quantity of acceptor dopants, which is expected to slightly lower the Fermi level and increase Φ in ZnO. Apart from doping, the other way of changing Φ is to modify the surface states of ZnO NRs. ZnO NRs have been prepared by various methods,7 but electrochemical and chemical bath depositions are suitable for fabricating devices over large areas due to the use of aqueous solutions in an open atmosphere, and they are compatible with low-temperature substrates owing to a moderate operating temperature. For electrodeposition, it is important to boost the ZnO NRs’ growth rate and keep the ZnO NRs’ average diameter virtually constant without degrading their optical qualities. In this paper, we report the preparation and r 2011 American Chemical Society
characterization of ZnO NRs, electrochemically deposited from an aqueous solution of zinc nitrate (Zn(NO3)2) and ammonium nitrate (NH4NO3) at a temperature down to 75 °C. The vertical growth of the ZnO NRs is enhanced due to an increase of the NO3 ion concentration in the solution. This NH4NO3-assisted electrodeposition approach also leads to an increase in the ZnO NRs’ Φ and mobility.
’ EXPERIMENTAL METHODS In order to investigate the influence of NH4NO3 on the ZnO NRs’ properties and geometry, we prepared five ZnO NRs samples from different solutions, i.e., sample 1a from an aqueous solution (5 mM Zn(NO3)2 3 6H2O) and samples 2a5a from an aqueous solution (5 mM Zn(NO3)2 3 6H2O and NH4NO3 (0.520 mM)). ZnO NRs were grown on fluorine-doped SnO2 substrates covered by a sputtered ∼50 nm undoped ZnO film. All substrates were cut into small 2.5 2 cm2 rectangles and then cleaned in an ultrasonic bath of acetone and ethanol with subsequent rinsing in distilled water. The electrodepositions were operated in a three-electrode electrochemical cell configuration with Pt counter and pseudoreference electrodes. All samples were grown with an optimized potential of 1.34 V. The electrochemical cell was placed in a thermoregulated bath, and the deposition temperature was adjusted to 75 °C. The deposition time for all samples was 3600 s in a potentiostatic mode. The solution was stirred during the preparation. After preparation, the samples were washed with distilled water to remove any residual salt. The morphology was observed by use of a scanning electron microscope (SEM). The optical qualities of excitonic recombination in as-grown ZnO NRs were investigated by photoluminescence (PL) experiments. The PL spectra were Received: September 21, 2010 Revised: January 21, 2011 Published: March 16, 2011 5239
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Table 1. Source Solutions, Average Diameter, Average Length and Work Function of the ZnO Nanorods Samples.
Figure 1. Cross-section SEM micrographs of ZnO nanorods electrodeposited using [NH4NO3] = 0 (a) and 20 mM (b).
measured by a HeCd laser at a wavelength of 325 nm. The lowtemperature PL measurements were carried out in a cryostat cooled by liquid helium. The Φ of as-grown ZnO NRs were determined with Kelvin probe force microscopy (KPFM) using an ultra-high-vacuum atomic force microscope (Omicron). The electrostatic forces are minimized by application of the contact potential difference using the second oscillation mode of the cantilever. A highly oriented pyrolytic graphite sample with a known Φ is used to calibrate the tip and obtain an absolute Φ measurement.8 Frequency-dependent electrochemical impedance spectroscopy (EIS) using the method of Mora-Sero et al.9 was conducted to determine the carrier density in as-grown samples. It was carried out in a three-electrode cell using Pt counter and reference electrodes. The measurement was performed under a 20 mV ac sinusoidal signal with the frequency ranging from 500 kHz to 5 mHz. A 0.1 M LiClO4 electrolyte was used to avoid ZnO decomposition. Transmission (T) and reflection (R) were measured by a Varian Cary 500 UVVIS NIR spectrophotometer with an integrating sphere in the interval of 2502500 nm.
’ RESULTS AND DISCUSSION According to the formations of ZnO by NO3 precursors described in eqs 14, an increase in either an electrode potential or the concentration of Zn(NO3)2 leads not only to a high vertical growth rate but also to a high lateral growth rate which has already been reported by Lee et al.10 Nevertheless, the internal structure of ZnO favors anisotropic growth along the [0001] direction.11 Equations 14 were suggested by Izaki et al.12 ZnðNO3 Þ2 S Zn2þ þ 2NO3
ð1Þ
NO3 þ 2e þ H2 O f 2OH þ NO2
ð2Þ
Zn2þ þ 2OH S ZnðOHÞ2
ð3Þ
ZnðOHÞ2 f ZnO þ H2 O
ð4Þ
Figure 1 shows SEM micrographs of both as-grown ZnO NRs prepared by use of the lowest NH4NO3 concentrations, i.e., 0 mM (Figure 1a), and highest NH4NO3 concentrations, 20 mM (Figure 1b), respectively. The average diameter and average length in five ZnO NRs samples, prepared with different amounts of NH4NO3 (020 mM) in the electrolyte, were estimated from a statistical evaluation of SEM images and are summarized in Table 1. It shows that the ZnO NRs’ length in samples 2a5a is longer than that of sample 1a. Therefore, it is clear that an increase in the concentration of NH4NO3 leads to a considerable increase of the ZnO NRs’ length. The length of ZnO NRs was boosted from ∼400 to ∼800 nm by increasing the concentration
concentration of
diameter
length
work function
sample
NH4NO3(mM)
(nm)
(nm)
(eV)
1a 2a
0 0.5
70 ( 5 70 ( 5
400 ( 20 410 ( 20
4.01 ( 0.05 4.24 ( 0.05
3a
1
80 ( 5
510 ( 20
4.23 ( 0.05
4a
5
80 ( 5
770 ( 20
4.28 ( 0.05
5a
20
80 ( 5
800 ( 20
4.29 ( 0.05
Figure 2. Room-temperature PL spectra of the ZnO nanorod samples.
of NH4NO3 from 0 to 20 mM, which is enlarged by ∼100%. An increase in the NH4NO3 concentration, on the other hand, did not induce a significant variation of the ZnO NRs’ average diameter, and the ZnO NRs keep a virtually constant average diameter between 70 and 80 nm. Since the concentration of NO3 is higher than Zn2þ ions in an aqueous solution with NH4NO3, an increase of the concentration of NO3 raises the yields of OH ions. Hence, the fact that the rate of OH generation is larger than the diffusion rate of Zn2þ ions to the cathode enhances the vertical growth and suppresses the lateral growth as well.13 The density of ZnO NRs for all the samples is ∼(4.2 ( 0.2) 109 cm2, which does not depend on the NH4NO3 concentration. However, ZnO NRs cannot be homogenously prepared on the substrate from electrolytes with NH4NO3 concentration over 20 mM. The reason is that the rate of OH generation from an electrolyte with a too high NH4NO3 concentration is much faster than that of the Zn2þ diffusion which suppresses formation of Zn(OH)2 for growing ZnO. It is noted that the length of NR is mostly dependent on the synthesis method, substrate, and growth atmosphere. For instance, the length of ZnO NR grown on bare FTO substrate was a few micrometers without using NH4NO3, and also, the use of NH4NO3 leads to a higher growth rate. However, the demanded length of ZnO NR depends on real applications, e.g., an optimized length of ∼700800 nm for solar cells’ antireflective coating and ∼700 nm for superstrate thin film solar cells. Figure 2 depicts the room-temperature PL spectra of five samples. A dominant and intense near-band edge (NBE) emission, at ∼379 nm, is observed. The intensity ratio of the NBE emission to the broad defect emission for five as-grown samples is in the range of ∼52 ( 1. Therefore, the use of NH4NO3 did not degrade the optical quality of as-grown ZnO NRs. KPFM measurements of the topography and the Φ for as-grown sample 4a are shown in Figure 3. It demonstrates that Φ on top of the 5240
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Figure 5. Capacitance plot and fitting for sample 4a.
Figure 3. KPFM measurement of sample 4a: (a) topography and (b) simultaneously measured work function. (c) Histogram of the work function distribution from which the values of Φ are extracted.
aqueous solution of Zn(NO3)2 and then immersed into ammonium hydroxide solution was increased. Thus, the change of Φ could be due to adsorption of NH3 on the ZnO surface. Though Φ of the presynthesized sample immersed in ammonium hydroxide solution was increased, during the immersion process the ZnO NRs were etched. However, it has remained a challenge to determine whether adsorption of NH3 on ZnO NRs’ surface is physical or chemical. Hydrolysis of NH4þ in the solution is given in eq 5 NH4 þ þ H2 O S NH3 þ H3 Oþ
ð5Þ
The concentration of NH3 in the solution can be calculated as follows ½NH3 ¼ ð½NH4 þ 3 Ka Þ1=2 þ
Figure 4. Low-temperature (4 K) PL spectra of the ZnO nanorod samples.
ZnO NRs is homogeneous and Φ is (4.28 ( 0.05) eV determined from histograms of Φ measurement (Figure 3c). Φ of asgrown samples is given in Table 1. It reveals that Φ of ZnO NRs prepared using NH4NO3 is higher than that of sample without the use of NH4NO3, i.e., sample 1a. The low-temperature (4 K) PL experiments did not have the emission lines related to either an acceptor exciton (A0X) or a donoracceptor pair (DAP) transition in as-grown samples 2a5a, shown in Figure 4. We also did not observe any signal from other potential dopants such as nitrogen from both experiments of extended X-ray absorption fine structure (EXAFS) and electron paramagnetic resonance (EPR). Thus, the increase of Φ for samples 2a5a is not due to introduction of acceptor centers from doping. However, modification of the surface properties can also lead to an increase in Φ. It was observed that Φ of sample which was prepared from an
ð6Þ
þ
where [NH4 ] is the NH4 concentration in solution and Ka is the acidity constant. The [NH3] is enlarged by 533% with increasing NH4NO3 concentration from 0.5 to 20 mM. A constant Φ in samples 2a5a (given in Table 1) shows that the increase of the ZnO NRs’ Φ by adsorption of NH3 is not proportional to the NH3 concentration, indicating the same effect of NH3 adsorption from electrolytes with different NH4NO3 concentration. The mechanism of NH3 adsorption on ZnO NRs is expected to be determined in future deliberate studies. A MottSchottky (MS) model was developed to determine the carrier density in the ZnO samples. The method is an extension of the MS analysis commonly used to determine both dopant density and flatband potential at flat semiconductor and semiconductor/liquid junctions, which now takes into account the particular geometry of ZnO NRs.9,14 The NR capacitance was obtained from the impedance spectra. The donor density was determined by fitting the capacitance in the range of voltages where it is governed by the size of the depletion region. The potential-dependent capacitance measured by EIS in sample 4a is shown in Figure 5. It exhibits a good agreement between the experimental data and the theoretical model. The EIS experiments reveal that the carrier density in these five as-grown samples has the same order of magnitude, i.e., ∼2 1019 cm3. The RIG-VM program, numerically fitting the T and R spectra of multiple stacked layers,15,16 was used to determine the NRs’ mobility as well as the carrier density obtained from Drude’s model for free charge carrier absorption. In order to use thin-film optics for the optical modeling, ZnO 5241
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’ CONCLUSIONS In summary, we reported a low-temperature electrodeposition process for preparation of high-quality as-grown ZnO NRs from an aqueous solution of Zn(NO3)2 and NH4NO3. The vertical growth rate of ZnO NRs is considerably enhanced using NH4NO3. The NH3 adsorbed on the materials’ surface results in a higher Φ of the ZnO NRs. An increase in the NH4NO3 concentration results in an increase in ZnO NRs’ mobility while keeping a constant carrier density. This is a promising approach for increasing ZnO NRs’ Φ and improving their electrical qualities without a significant effect on their optical qualities which may lead to further improvement of the luminescence onset voltage in LED and the performance of LEDs. It shows also the potential for ZnO NR-based highly structured solar cells. ’ AUTHOR INFORMATION Figure 6. Experimental (Exp.) and Simulated (Sim.) transmission/ reflection spectra for sample 4b. (Inset) SEM micrograph of ZnO nanorods with a high packing density.
Table 2. Source Solutions, Carrier Density and Normalized Mobility of the ZnO Nanorods Samples. The Mobility is Normalized to the Value of Sample 1b. sample
concentration of
carrier density
NH4NO3(mM)
(1020 cm3)
normalized mobility
Corresponding Author
*Phone: þ49 30 806243234. Fax: þ49 30 806243199. E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge Prof. Christian-Herbert Fischer for very valuable discussions, support in EXAFS work by Dr. I. Lauermann, and EPR work by Dr. Y. Ryabchikov and Mr. M. Fehr at the HZB. Part of this work was supported by the German Federal Ministry of Education and Research, funding program Optical Technologies, contract no. 13N9615.
1b
0
1.4
1
2b
0.5
1.1
1.3
3b
1
1.4
1.3
4b
5
1.5
2.6
’ REFERENCES
5b
20
1.3
2.4
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NRs samples 1b5b with a high packing density were prepared and we assumed the NRs to be a compact layer (shown as an inset image in Figure 6). The mobility and carrier density of the ZnO NRs were determined from the plasma frequency and damping frequency of the Drude oscillator. As the calculated Drude mobilty is more affected than the charge carrier concentration (position of the plasma edge) by the validity of using thin-film optics with coplanar layers and by deviations of the measured and calculated spectra (see Figure 6), the mobility is normalized to the value in sample 1b. As shown in Table 2 the carrier density in samples 1b5b has the same order of magnitude, i.e., ∼1 1020 cm3. Both EIS and RIG-VM methods reveal that the ZnO NRs’ carrier density is constant by varying the NH4NO3 concentration. On the other hand, the ZnO NRs’ mobility in samples 2b5b is larger than that in sample 1b, and a considerable increase of the ZnO NRs’ mobility was observed in samples grown from an electrolyte with a concentration of NH4NO3 over 5 mM. The mobility is enlarged by 160% when increasing the NH4NO3 concentration from 0 to 5 mM. The mobility of ZnO NRs prepared from a solution with 20 mM NH4NO3 is not further boosted. Since optical analysis brings out information on the properties within the grains, the increase of the optical mobility is attributed to a lower contribution of electron scattering inside the grains. Both the fast growth rate and excess OH generation lead to a decrease of the dislocation and point defects, which suppresses the contribution of the electron scattering to the optical mobility. Therefore, use of NH4NO3 results in an improvement in ZnO NRs’ electrical properties.
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