- Nano Express
- Open Access
Photoluminescence Study of the Influence of Additive Ammonium Hydroxide in Hydrothermally Grown ZnO Nanowires
© The Author(s). 2018
- Received: 28 June 2018
- Accepted: 9 August 2018
- Published: 22 August 2018
We report the influence of ammonium hydroxide (NH4OH), as growth additive, on zinc oxide nanomaterial through the optical response obtained by photoluminescence (PL). A low-temperature hydrothermal process is employed for the growth of ZnO nanowires (NWs) on seedless Au surface. A more than two order of magnitude change in ZnO NW density is demonstrated via careful addition of NH4OH in the growth solution. Further, we show by systematic experimental study and PL characterization data that the addition of NH4OH can degrade the optical response of ZnO NWs produced. The increase of growth solution basicity with the addition of NH4OH may slowly degrade the optical response of NWs by slowly etching its surfaces, increasing the point defects in ZnO NWs. The present study demonstrates the importance of growth nutrients to obtain quality controlled density tunable ZnO NWs on seedless conducting substrates.
- Zinc oxide
- Ammonium hydroxide
Significant developments in the synthesis of functional nanomaterials via bottom-up approaches are now offering high-quality materials for the development of next-generation efficient electronic devices [1–5]. The ZnO’s field of research has shown resurgence in interest after the successful demonstration of the growth of single-crystalline nanostructures (nanobelt) . Thereafter, the use of high-quality, single-crystalline semiconducting ZnO nanostructures for the assembly of high-performance electronics continues to attract enormous research interest in the field of displays [7, 8], logic circuits [9, 10], sensors [11, 12], and optoelectronics . The renewal of interest in ZnO material has largely been driven by its bio-compatibility, facile nanostructure fabrication, and large family of achievable nanomorphologies [14, 15]. Among the various different ZnO nanoarchitectures, one-dimensional (1D) ZnO nanowires (NWs) and nanorods (NRs) have been investigated comprehensively as an active semiconducting material in nanoscale devices such as field-effect transistors (FETs) , nanogenerators (NGs) , or sensors .
Ideally, a well stoichiometric ZnO is an insulator. However, in its non-stoichiometric form, it can behave as semiconductor or conductor depending on the number of native point defects created and/or by amount of dopant introduced. It has been shown that, in nanostructured ZnO, defects play a central role in defining the electronic device performances, as for sensors  and/or nanogenerators [17, 19], by controlling free charge density, minority carrier life time, and luminescence efficiency. For instance , it has been shown that highly sensitive UV sensors can be obtained by increasing the number of surface defects in ZnO NWs. These surface defects may act as trapping centers for free electrons and results in the formation of surface depletion layer. The greater the depth of depletion region at the NW surface, the higher the UV sensitivity. On the other hand, a too large number of defects have detrimental effects on the NG device performances [17, 19]. Therefore, a perfect control over the quality of ZnO nanomaterial produced is essential to build a high-performance electronic device.
Different bottom-up growth techniques, including flame transport approach [20–23], vapor-liquid-solid (VLS) , electrochemical deposition , and hydrothermal and/or chemical bath deposition [16, 26–29] have been utilized for the synthesis of 1D ZnO NWs. Nevertheless, most of the techniques are limited by their high-temperature process that cannot be scaled up over large device area at very low cost, on plastic substrates for example. The need of a facile, industrially scalable, and substrate-independent synthesis of ZnO NWs has seen significant advancements towards the hydrothermal growth process [16, 17]. Hydrothermal growth (HTG) is a low-temperature process where single-crystalline 1D material can be produced on various substrates, including plastics or even textile fibers . In general, HTG-grown ZnO NWs show intense defect level band peak in photoemission spectra which expands from blue to red color wavelength emission depending on type of defects in the nanomaterial . In the literature, many different point defects such as oxygen and zinc vacancies (VO and VZn) and interstitial (Oi and Zni), antisites (OZn and ZnO), and hydrogen impurities were identified to be the cause of the defect level emission band in photoluminescence (PL) . The visible PL band consists of three Gaussian components at 2.52, 2.23, and 2.03 eV, respectively labeled as blue IB, green IG, and orange IO peak emission . However, even after years of investigations, the origin of these defect states is still a subject of debate. Nevertheless, irrespective of the cause of defects in ZnO, the ratio of the band-to-band transition (UV emission) to the defect-related peak intensity in PL spectrum predicts the optical response of the nanomaterial produced [18, 34].
Growth process with an in situ integration of ZnO NWs over a metal electrode without any ZnO seed layer may improve the charge transport process across the metal-semiconductor (MS) contact interface and, in consequence, may improve device performances . Ammonium hydroxide (NH4OH) has often been employed for the growth of ZnO NWs on Au metal surfaces [35, 36]. For instance, in our previous work, we show that NH4OH can be used for simultaneous tuning of the NW density and electrical properties of the ZnO NWs grown on seedless Au surface . However, report detailing the effect of the addition of NH4OH over the optical response of the produced ZnO nanomaterial on Au surface is rarely found in literature. In the present report, we study the ZnO material optical response by analyzing the defect-related emission and UV emission in PL spectrum of NWs grown in different NH4OH concentrations. Two dominant peaks, noticed in the PL graph, are centered at 3.24 eV (382 nm) and 2.23 eV (556 nm), respectively referred as ultraviolet (UV) emission (IUV) and green defect level emission (IG) peaks. The extracted ratio IUV/IG provides a qualitative index of the radiative defect amount in the produced nanomaterial. The effect of NH4OH is further confirmed by carrying out another series of experiments and PL characterizations. In this second series of experiments, we have grown ZnO NWs without NH4OH and, then, carried out a post-growth treatment of NWs in ammonia solution with different pH. We found out a similar trend of decrease in the ratio IUV/IG for both series of samples, i.e., the ones grown in different NH4OH concentration and the other ones post-growth treated in NH4OH.
The ZnO NWs are grown by hydrothermal growth process on (100) oriented Si wafers. A sample of 2 × 2 cm2 rigid silicon is first cleaned in piranha solution (1:1 H2SO4 and H2O2) for 10 min followed by a 2-min dip in hydrofluoric acid (50%) to remove the thin oxide formed during piranha cleaning and, finally, rinsing in deionized (DI) water. This cleaning step is followed by drying with nitrogen gas, and a final baking step is performed at ~ 200 °C to remove any adsorbed moisture before the metal deposition. A gold layer (~ 200 nm thick) is then deposited by direct current sputtering technique at room temperature. To improve the adhesion between gold and silicon, we deposit a layer of titanium (~ 100 nm) using the same technique. The reactant precursor for HTG consists of 1:1 ratio of zinc nitrate hexahydrate (Zn (NO3)2‚6H2O, 98% Sigma Aldrich) and hexamethylenetetramine (HMTA, Sigma Aldrich). During the growth, the substrates were immersed facing down in a Teflon cup, sealed inside stainless steel autoclave reactor and placed in a preheated convection oven at 85 °C for 15 h. The autoclave is taken out from the oven and cools down naturally. The substrates are then thoroughly rinsed with flowing DI water and dried in N2 gas flow. In the experiments, the concentration of NH4OH is varied from 0 to 50 mM. A Hitachi S-4150 scanning electron microscope (SEM) system is used for the morphological characterization of the ZnO NWs. To follow up the optical response of the obtained NWs with different NH4OH concentrations, photoluminescence (PL) measurements were performed; at room temperature (RT), by pumping at 1.5 mW, the 325 nm line of a He−Cd laser chopped through an acousto-optic modulator at a frequency of 55 Hz. Further experimental details for PL measurements can be found in Ref .
HTG parameters for ZnO nanomaterial produced for each NH4OH concentrations at 85 °C
Zinc nitrate and HMTA concentration (mM)
NH4OH concentration (mM)
Growth time (h)
Photoluminescence study of the influence of NH4OH addition over optical response of ZnO NWs
NH4OH concentration (mM)
Free charge density (/cm3) 
4.3 ± 3.9 × 1016
8.3 ± 4 × 1016
2 ± 1 × 1017
Experimental parameters for the post-growth treatment of NWs in different concentrations of ammonium hydroxide solution and their effect over the optical response of the ZnO nanomaterial as measured by photoluminescence
NH4OH concentration (mM)
The resulting PL data arising from the post-growth treated samples are shown in Fig. 4. Figure 4a shows the PL spectra measured at RT for NWs treated with various ammonium hydroxide concentrations, whereas the extracted IUV/IG plot is shown in Fig. 4b. It is to note that the peak position for both UV and visible emission has not been changed after NH4OH treatment, indicating that no extra point defect with different energy level is formed during the NH4OH treatment. The continuous reduction in the PL intensity of UV emission peak, with the increase of NH4OH concentration, clearly indicates the removal of ZnO nanomaterial due to a slow etching of the NWs in basic medium . Furthermore, it is interesting to notice, from Fig. 4b, a clear and sharp decrease of the IUV/IG ratio, as the NWs are treated in NH4OH solution. It is important to mention here that, for the present study, the experimental conditions such as excitation density, radiation area, initial mass of ZnO nanomaterial, etc. are fixed. Therefore, the observed IUV/IG ratio trend can be entirely related to the effect introduced by the addition of NH4OH and not to changes in experimental conditions . The obtained experimental results clearly support the hypothesis made in the previous section for creation of extra point defects with the addition of NH4OH in the growth solution. We believe that the increase of growth solution basicity with the addition of NH4OH can slowly degrade the optical response of NWs by slowly etching its surfaces, which increases the level of point defects in ZnO NWs.
In summary, we demonstrated a facile, low-cost, and scalable bottom-up process for a seedless growth of ZnO NWs on metallic Au surfaces. With a careful addition of ammonium hydroxide in the growth solution, ZnO NW density can be controlled over two orders of magnitude. Consequences of the addition of NH4OH over the optical response of the obtained NWs were studied using photoluminescence technique. The visible emission spectrum, for each NH4OH concentration, was successfully deconvoluted to the blue, green, and orange defect states. Furthermore, percentage contribution of each defect state was also presented, showing the major contribution of visible emission was from green defect state. Thereby, to follow up the optical response of nanomaterial produced, we compared the intensity ratio of UV emission (IUV) to green defect state (IG). It was observed that the IUV/IG ratio decreases sharply after the addition of 20 mM of NH4OH, hinting the creation of extra point defects with the addition of NH4OH in the growth solution. The experimental results were well supported by the literature data on the increase of free charge density with NH4OH addition. Nevertheless, the proposed hypothesis was further confirmed by performing another series of experiments where the as-grown ZnO NWs, without addition of NH4OH, were treated in solutions with increasing basicity. A clear and sharp decrease of the IUV/IG ratio, as the NWs were treated in NH4OH solution, showed that the increase of growth solution basicity with the addition of NH4OH can slowly degrade the optical response of NWs by etching its surfaces which increases the level of point defects in ZnO NWs. The present study is important to control the optical response of ZnO NWs that can be directly grown on metallic Au electrodes for electronic and optoelectronic applications.
This project has received funding from the ECSEL JU under grant agreement no. 692482. This JU receives support from the European Union’s H2020 research and innovation programme and France, Netherlands, Denmark, Belgium, Germany, Czech Republic, and Spain. The authors are also grateful for the supports from Region Centre (MEPS project 2015–2018) and National Research Agency (ANR-14-CE08-0010-01).
Availability of Data and Materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
ASD and SB designed the experiments. ASD and SB performed the synthesis and structural/morphological analysis of the ZnO NWs. GF and SM performed the photoluminescence characterization. The drafting of the manuscript has been done by ASD. GF, GPV, SM, and DA did critical revisions of the manuscript. All authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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