**Soil Application of Nano Silica on Maize Yield and Its Insecticidal Activity Against Some Stored Insects After the Post-Harvest**

**Mehrez E. El-Naggar 1, \* , Nader R. Abdelsalam 2, \* , Moustafa M.G. Fouda 1, \* , Marwa I. Mackled 3 , Malik A.M. Al-Jaddadi 4 , Hayssam M. Ali 5,6 , Manzer H. Siddiqui <sup>5</sup> and Essam E. Kandil 7**


Received: 9 March 2020; Accepted: 7 April 2020; Published: 12 April 2020

**Abstract:** Maize is considered one of the most imperative cereal crops worldwide. In this work, high throughput silica nanoparticles (SiO2-NPs) were prepared via the sol–gel technique. SiO2-NPs were attained in a powder form followed by full analysis using the advanced tools (UV-vis, HR-TEM, SEM, XRD and zeta potential). To this end, SiO2-NPs were applied as both nanofertilizer and pesticide against four common pests that infect the stored maize and cause severe damage to crops. As for nanofertilizers, the response of maize hybrid to mineral NPK, "Nitrogen (N), Phosphorus (P), and Potassium (K)" (0% = untreated, 50% of recommended dose and 100%), with different combinations of SiO2-NPs; (0, 2.5, 5, 10 g/kg soil) was evaluated. Afterward, post-harvest, grains were stored and fumigated with different concentrations of SiO2-NPs (0.0031, 0.0063. 0.25, 0.5, 1.0, 2.0, 2.5, 5, 10 g/kg) in order to identify LC<sup>50</sup> and mortality % of four common insects, namely *Sitophilus oryzae, Rhizopertha dominica, Tribolium castaneum,* and *Orizaephilus surinamenisis*. The results revealed that, using the recommended dose of 100%, mineral NPK showed the greatest mean values of plant height, chlorophyll content, yield, its components, and protein (%). By feeding the soil with SiO2-NPs up to 10 g/kg, the best growth and yield enhancement of maize crop is noticed. Mineral NPK interacted with SiO2-NPs, whereas the application of mineral NPK at the rate of 50% with 10 g/kg SiO2-NPs, increased the highest mean values of agronomic characters. Therefore, SiO2-NPs can be applied as a growth promoter, and in the meantime, as strong unconventional pesticides for crops during storage, with a very small and safe dose.

**Keywords:** maize; NPK; SiO2-NPs; productivity; fertilizer; mineral; weevils; LC50; toxicity

#### **1. Introduction**

The global population will rise to 9 billion by the year of 2050, and the existing agricultural practices cannot satisfy this growing demand for food without variations in the fertilizer's application. Nanotechnology is currently being applied in abundant fields such as medicine, pharmaceutics, electronics, and agriculture. The size and purity of nanomaterials results significantly in various procedures as well as improvements in the physical and chemical properties of any materials due to their small size which in turn, caused very large surface area [1,2].

Worldwide, *Zea mays* L. is considered as one of the most important cereal crops [3–5]. The area of maize cultivation in Egypt is 1.1 million hectares (average yield about 7.4 t/ha) and in the world 188 million hectares (an average yield about 5.6 t/ha) reported by (FAO, 2007).

Elements such as Nitrogen, Phosphor, and Potassium, abbreviated as NPK, are considered vital macronutrients for meristematic production and several physiological processes in plant [6–15] for instance, shoot, root system, flowers etc., moreover, leading to effective water translocation and nutrition, improve the process of photosynthesis [16]. On the other hand, silicon can be considered as a micro nutrient and it is supportive for plant growth, mainly in dry environments, in order to hold water and bind other nutrients, in addition to increasing the cell strength [17]. Moreover, the utilization of silicon makes the plant shoot system more erect as the effect of a high dose of nitrogen fertilizers, which will improve plant photosynthesis, chlorophyll content, and product quality [18,19] evaluated the effectiveness of nano fertilizers relative to their conventional analogues and the results displayed that nano fertilizers has the largest increase in median efficacy increase (29%). Thus, using SiO2-NPs, as nanofertilizer together with NPK will increase the absorbability of fertilizers by plants and, hence, it will be more effective than conventional chemical fertilizers [20]. Prihastanti et al. [21] noticed that SiO2-NPs are an important nanofertilizer which contains silicon which is essential to the monocotyl plants, such as maize, to increase the growth and productivity as well, rather than, NPK alone, that comprises N, P, and K (macronutrient). The combination between NPK and SiO2-NPs limits the utilization of hazard chemical fertilizers besides its capability to improve maize production [22,23].

There are many ways that extensively used for the production of silica nanoparticles (SiO2-NPs) such as electrochemical, hydrothermal, plasma–metal hydrogen reaction, micro-emulsion, arc discharge, chemical vapor condensation, vapor phase laser pyrolysis, radiation, sonication, laser, biological, and chemical methods [12,24–36]. One of these chemical methods is the sol–gel process which is extensively used in order to produce homogenous silica products in a powder form. The produced silica gels are non-toxic and suitable to be used for several domains particularly, agricultural applications.

As far as post-harvest is concerned, maize grains are considered one of the identical hosts for many stored products insects such as, *Sitophilus* and *Rhizopertha dominica*, *Tribolium castaneum,* and *Orizaephilus surinamenisis*, which resulted in a loss of more than 25%. Fumigants and residual pesticides are widely used to protect the stored grains from infestation by plague [37].

Hereby, this current research work aimed to prepare silica nanoparticles (SiO2-NPs) in high concentration with small size and distribution to be used as an alternative and effective nanomaterial for the protection of stored grains. It is expected that these nanoparticles will reduce the utilization of hazardous chemical pesticides which, in turn, will reduce the health hazard from residual toxicity. Additionally, the prepared SiO2-NPs enhances to solve the insect resistance to the conventional insecticides (phosphine and pyrethroids) too [38–41].

Overall, the main objectives of this current research are dived into three key subjects: a) preparation and characterization of SiO2-NPs using the sol–gel method, b) evaluation of the influence of the combination between SiO2-NPs and mineral NPK, as soil application and their interaction with the plant characteristics of maize, and c) application of SiO2-NPs as an alternative pesticide to combat pests infested maize grains through post-harvest, as well as to resolve insect resistance to the conventional pesticides.

#### **2. Materials and Methods**

#### *2.1. Experiment Place and Design*

The present investigation was carried out at the Experimental Farm, Faculty of Agriculture (Saba Basha), Alexandria University, Alexandria, Egypt and Department of Stored Product Pests, Agriculture Research Center, Sabahia, Alexandria, Egypt, cooperated with Botany and Microbiology Department, College of Science, King Saud University, Riyadh, Saudi Arabia through the two successive summer seasons of 2018 and 2019, in split plot design with 3 replications. The major plot was mineral NPK fertilizers rates ((0% (0:0:0:0), 50% (144:30:30), and 100% (288:60:60)/ha), while sub plots were allocated by silica NPs concentration (0.0, 2.5, 5.0 and 10.0 g/kg autoclaved soil) in both seasons.

#### *2.2. Sol-Gel Synthesis of Silica Nanoparticles (SiO2-NPs)*

For the preparation of SiO2-NPs via sol-gel method, 35 mL of H2O was mixed with 65 mL of absolute alcohol for 5 min under mechanical stirring. After that, 25 mL of tetraethyl orthosilicate (TEOS) was added dropwise to the previous ethanol/water solution and kept under mechanical stirring for 60 min at room temperature. To this end, ammonia solution was added dropwise until the complete formation of gel. Thus, it was noted that the solution was converted to gel (sol-gel process). The formed gel was submitted to ultra-centrifugation for 2 h at 7000 rpm. Finally, the precipitated wet gel was collected and washed three times with distilled water in order to remove the undesired/unreacted compound (TEOS). The wet gel was subjected again to ultracentrifugation. At the end, the obtained gel was left for calcination at 700 ◦C for 5-7 h.

#### *2.3. Physical Characterization of Silica Nanoparticles (SiO2-NPs)*

The sample for transmission electron microscope (TEM) examination was prepared by placing the dispersed SiO2-NPS on a carbon-coated copper grid and left for drying at room temperature before being characterized via TEM instrument (JEOL 200 kV, Tokyo, Japan). The particle size and zeta potential of SiO2-NPs in its colloidal solution and after submission for 15 min of sonication were assessed using particle size analyzer (Nano-Sizer SZ90, Malvern instruments Ltd., Cambridge, UK). The size distribution and zeta potential of the as prepared SiO2-NPs was measured at pH = 7 and 25 ◦C. Scanning Electron Microscopy (SEM; JEOL, JSM-6360LA, Tokyo, Japan) instrument was used to investigate the internal structure and surface morphology of SiO2-NPS. X-ray diffraction (XRD) analysis was performed to examine the crystallinity and the specific peaks for the formed SiO2-NPs using an XRD device (Panalytical Emperian, Istanbul, Turkey) having CuKa radiation and operating with 40 kV and a 2-theta range of 10–80.

#### *2.4. Soil Characterization and Preparing Materials*

A surface sample of soil (0-30 cm) was collected before planting to identify some soil physical and chemical properties, as shown in Table 1. According to Keeney et al. [42], the previous crop was clover (berseem) in both seasons. Nano silica powder were mixed well with autoclaved soil and applied at two times, the first time before the first irrigation (after thinning) and the second time before the second irrigation. The recommended dose of NPK as following; the recommended dose of phosphorus fertilizer was used at rate of 60 kg P2O5/ha (where ha = 0.42 feddan) from calcium super phosphate (12.5% P2O5) and potassium rate of 60 kg K2O/ha from potassium sulphate (48% K2O) with soil preparation. The recommended dose of mineral N at the different rate of 288 kg N/ha was fully given in the form of urea (46.5% N) such as previous adding. Each plot size was 12.60 m<sup>2</sup> , included 6 ridges, each 3 m in length and 0.70 m in width, with the distance between hills of 25 cm. The grains of maize hybrid (TWC 1100) were taken from Maize Research Division, Agriculture Research Center, Ministry of Agriculture, Cairo, Egypt. Theses grains were sown on May 15th and 14th of 2018 and 2019 seasons, respectively.


**Table 1.** Soil physical and chemical properties during both seasons.

#### *2.5. Maize Yield and Yield Compound Characteristics*

The maize yield and yield compound parameters were calculated after harvest and the data were obtained as an average of two ridges from middle of each plot. The protein% was concluded according to the methods of Helrich (1990) by assessing the total nitrogen in the grains and multiplied by 6.25 to obtain the percentage according of grains protein% [43].

#### *2.6. Post-Harvest Experiment*

#### 2.6.1. Insect Culture

Two insects, *Sitophilus oryzae* and *Rhizopertha dominica*, were reared under laboratory conditions (27 ± 1 ◦C and 65 ± 5% R.H.) using autoclaved maize grains which obtained from the first experiment after storage two months in dry conditions, *T. castaneum* and *O. surinamenisis* was reared on maize flour mixed with yeast (10:1, w/w), in 1-L glass jars, which were covered by fine mesh cloth for ventilation as reported by [44]. Adult insects used in toxicity tests were about 1-2 weeks old. All investigational procedures were conducted under the identical conditions as culture.

#### 2.6.2. Contact Film Toxicity Bioassay

The previous SiO2-NPs was evaluated (2.5, 5, and 10 g/kg) and the mortality percentage was 100%, so we decreased the concentrations to the lower doses for obtaining the LC<sup>50</sup> to the four stored insects. Toxicity of the nine evaluated SiO2-NPs (0.0031, 0.0063, 0.25, 0.5, 1.0, 2.0, 2.5, 5, and 10 g/kg) against the weevils of *S. oryzea, R. domonica, T. castanium,* and *O. surinamenisis* (adults) were examined by transferring 20 adults into glass jars (250 mL) containing 100 g of sterilized maize grains and admixed them well with different doses of SiO2-NPs according to the method of Su and Zabik (1972) [45]. Control jars continues maize grains alone. Three replicates were used for all treatment and control. The mortality percentage (M%) were measured according to Finney (1971) after one, two, and three days and LC<sup>50</sup> values were determined according to the method of [46].

#### *2.7. Statistical Analysis*

Obtained data were subjected to the proper system of statistical analysis of variance as defined by [47]. The means were compared using L.S.D. test at 5% probability using a split model as found in CoStat 6.311 program, PMB320, Monterey, CA93940, and USA [48].

#### **3. Results**

insects.

In this current research work, it is aimed to develop a new strategy for the soil applications in term of feeding or fertilizing, and at the same time, as pesticide for combating the different kinds of pests that are found through storing maize grains. Nanotechnology in this research work is implemented through the production of silica nanoparticles, SiO2-NPs, which serve as enhancement agents for the soil application as well as a pesticide agent in post-harvest, for maize grains during storage for long time. As reported previously in the literature; the production of SiO2-NPs is depending on two major chemical steps: the first one is nucleation that occurs by the hydrolysis of tetraethyl orthosilicate to form silanol groups, which is followed by the second step, growth stage, that takes place by the condensation between the silanol groups formed leading to the construction of siloxane bridges (Si–O–Si) that, yield at the end the entire silica nanoparticle formula. The hydrolysis step is carried out in the presence of alkali like ammonia (NH3) that acts as a reaction enhancement for the formation of the end product. Scheme 1, represents the preparation of SiO2-NPs and their application as soil nanofertilizer for maize, as well as an insecticide for the stored maize insects.

**Scheme 1.** steps for the preparation and utilization of SiO<sup>2</sup> -NPs as feeding or fertilizing and in the same time, as pesticide for combating the different kinds of pests that are found thru storing maize grains.

Below is the full analysis for the formed SiO2-NPs by means of TEM, particle size analyzer, zeta potential, SEM, and XRD techniques.

‒ Firstly, TEM was represented for the formed SiO2-NPs in order to clarify the particles shape and their distributions. The TEM images of SiO2-NPs are taken at three different magnifications to clarify the actual shape of the synthesized particles. Figure 1A-C shows that the particles shape is spherical with low disparity which may be attributed to the cluster effect of silica particles. However, these aggregated particles are less than 50 nm.

‒

‒

**Figure 1.** (**A**, **B**, **C**) TEM at low and high magnifications, (**D**) average particle size and (**E**) zeta potential of SiO<sup>2</sup> -NPs.

‒ To confirm the particle size and stability, hydrodynamic average size was examined using dynamic light scattering (DLS) as represented in Figure 1D. It is observed that the average size is around 68 nm. As can be clearly seen, the particle size obtained from DLS is little bit larger than that obtained from TEM figures. This can be claimed in terms of a swelling effect. For the DLS technique, the sample during examination is kept in distilled water for a long time (duration of measurements; 18 run). In this case, the particles are marginally swelled, which, in turn, leads to a slight increase in the size of the examined particles.

In light of stability of surface charge of the produced SiO2-NPs, zeta potential (Figure 1E) was carried out to provide us an information about the particle stability against aggregation. It is well known that value of Zeta potential above +30 mV or -30 mV is considered as good stabilized sample and already protected from further aggregation or agglomeration. Thus, the nominated examination is very important to stand out for the sample stability after its preparation. Therefore, the average zeta potential of SiO2-NPs is evaluated ad plotted in Figure 2B. It is depicted that the zeta potential value of SiO2-NPs is recorded as -40 mV. Such a value means that the particles are kept away from further aggregation, even after a long time.

In order to clarify the morphological surface structure of SiO2-NPs, the sample was scanned at different magnifications using SEM. The scanned SiO2-NPs sample is displayed in Figure 2A,B. As shown in the SEM images, the prepared powder consists of spherical particles with well-defined borders. The calcination process at high temperature (600–700 ◦C) is an important factor for purification and the formation of particles with spherical morphology and regular shape.

In order to outline the crystallinity and purity of the aforementioned powder sample, X-ray diffraction pattern (XRD) was utilized. XRD analysis was carried about between 2 theta degree (10–80). It is disclosed from Figure 3 that SiO2-NPs exhibit three major peaks at 21.88◦ , 38.5◦and 45.9◦ which correspond to (100), (110), and (201) planes. The obtained peak value is in accordance with that of JCPDS Card #850335 for SiO2-NPs. Based on the aforementioned peaks, SiO2-NPs can be prepared successfully using the sol–gel technique.

**Figure 2.** SEM of **SiO2-NPs** at different magnifications (**A**) 13 kx, (**B**) 24 kx.

**Figure 3.** XRD of SiO<sup>2</sup> -NPs.

#### 50 *3.1. Growth and Yield Compounds*

0 10 20 30 40 50 60 70 80 2 T h eta D eg ree The growth and yield characters, such as leaf chlorophyll content, plant height, ear length, grains number/row, grains number/ear, weight of 100 grains, grain yield, straw yield, biological yield, harvest index, and protein content of maize hybrid were significantly affected by a combination of NPK fertilizers and SiO2-NPs concentrations in an average of both 2018 and 2019 seasons as found in Table 2. The results verified that the application of NPK at the recommended dose (RD = 100%) recorded the maximum mean values of leaf chlorophyll content (38.72 SPAD), plant height (195.79 cm), ear length (20.17 cm), grains number/row (41.67 grains/row), grains number /ear (583.33 grains/ear), weight of 100 grains (43.00 g), grains yield (4.79 t/fed), straw yield (6.29 t/fed), biological yield (11.08 t/fed), harvest index (43.23%) and content of protein in grain (10.18%) followed by fertilization by 50% of recommended dose from mineral NPK, while the lowest ones were given by untreated treatment (NPK = 0).

**Table 2.** Plant attributes of maize as affected by mineral NPK fertilizers, Nano silica (SiO2-NPs) concentrations and their interaction in an average of the two seasons 2018 and 2019.


\*: significant difference at LSD at 0.05% level of probability.

Regarding, effect soil application of SiO2-NPs on maize yield and components characters, the results detected that with the increase of SiO2-NPs concentrations from 0 up to 10 g/kg, there is an increase in all the studied characters (Table 2). The highest concentration of SiO2-NPs verified the greatest mean values of leaf chlorophyll content (40.27 SPAD), plant height (201 cm), ear length (20.78 cm), grains number /row (41.77 grains/row), grains number /ear (583.35 grains/ear), weight of 100- grains (44.56 g), grain yield (5.00 t/fed), straw yield (6.24 t/fed), biological yield (11.24 t/fed), harvest index (44.48%) and protein content in grain (10.37%) followed by the concentration 5 g/kg Nano silica as compared with the other concentration.

Combinations between NPK and SiO2-NPs showed a highly significant interaction, as found in Table 2 for all the studied characters. The significant interaction shows that the response of effect of treatments of the first factor dependable on the levels of the other factor.

The results in Table 3 presented the interaction effect between mineral NPK and SiO2-NPs, where the highest mean values of chlorophyll content (45.13 SPAD), plant height (222.33 cm), ear length (22.33 cm), grains number/row (44.00 grains/row), grains number /ear (616.00 grains/ear), weight of 100 grains (47.67 g), grain yield (5.59 t/fed), straw yield (7.09 t/fed), biological yield (12.68 t/fed), and content of protein in grain (12.69%) were attained from fertilizing maize plants by the rate of 50% of recommended dose from mineral NPK and soil application of SiO2-NPs (10 g/kg) except the highest mean of harvest index (46.30%) recorded with 50% of recommended dose from mineral NPK and 5g/kg SiO2-NPs.

In comparison with the other treatments, meanwhile the lowest ones were given with untreated plots (0 NPK + 0 SNPs), that cleared the role of SiO2-NPs in the response of maize crop to NPK. The data found in Table 3 demonstrate the interaction impact of mineral NPK fertilizer and SiO2-NPs application of some maize characters, where the highest values of the studied characters recorded with 50% recommended dose of mineral NPK + 10 g/kg SiO2-NPs.

SiO2-NPs with high surface area produced in the commercial way are employed for the growth and productivity of maize as an unconventional source of fertilizer. Physiological transformations that are due to SiO2-NPs fertilization considerably increase the growth and yield characters in maize plants. 


**Table 3.** The interaction effect between mineral NPK fertilizers and nano silica (SiO2-NPs) concentrations on plant attributes of maize in an average of the two seasons 2018 and 2019.

0% Recommended Dose of NPK=(0:0:0), 50% Recommended Dose of NPK=(60:12.5:12) and 100% Recommended Dose of NPK=(120:25:24).

### *3.2. Toxicity of SiO2-NPs against Stored Products Insects*

Nine concentrations of SiO2-NPs were appraised against four stored products insects *S. oryzae*, *R. dominica*, *T. castaneum* and *O. surinamenisis* (Figure 4), the initial results obtained by the application of SiO2-NPs; 2-10 g/kg displayed 100% mortality %, thus we decreased the concentrations used to get the LC<sup>50</sup> for the SiO2-NPs and three exposer time. ‒ ‒

**Figure 4.** Four stored products used in current study (**1**) *O. surinamenisis*, (**2**) *T. castaneum,* (**3**) *R. dominica,* and (**4**) *S. oryzae.*

Figure 5 shows the difference between healthy and infected maize grains by different stored insects.

**Figure 5.** Difference between healthy and infected maize grains used in the current study.

‒

‒

‒ ‒

‒ ‒

‒

‒

The data in Figures 6–8 show that, after 24 h of the treatment, *R*. *Dominica* became more sensitive to SiO2-NPs followed by *O. surinamenisis;* LC<sup>50</sup> were 0.336 (range, 0.177-0.521) and 0.768 (range, 0.438-1.495) g/kg respectively. While the other LC<sup>50</sup> was 1.240 (range, 0.995-1.662) and 1.450 (range, 0.971-3.290) g/kg registered for *S. Oryzae*, *T. Castañea*. With respect to mortality percentage after 24 h, the lowest concentrations of SiO2-NPs such as 0.0031 and 0.0063 were not successful in the case of four species, while the M% increased with concentration changes. ‒ ‒ ‒ ‒ ‒

‒ ‒ ‒

‒ ‒

‒

‒

**Figure 6.** Morphological effect and mortality of SiO<sup>2</sup> -NPs on the four stored products insects i.e., (**1**) *T. castaneum,* (**2**) *R. dominica*, (**3**) *O. surinamenisis* and (**4**) *S. oryzae* showing that SiO<sup>2</sup> -NPs covering the insects and caused death.

Compared with LC<sup>50</sup> of *R. dominica* and *O. surinamenisis* with M%, findings showed that for both insects, these values ranged from 0.25 to 0.5 g/kg and ranged from 11.66–63.3% and 1036.65% respectively (Figures 6–8). After 24 h, 2 g/kg SiO2- NPs displayed that 100% of mortality for all the species. Meanwhile, 1 g/kg of SiO2- NPs showed 86.6% mortality for *R. dominica* comparing with 5% for *S. oryzae*; 10% for *T. castaneum* and 36.65% for *O. surinamenisis*.

Results for both *R. dominica* and *O. surinamenisis*recorded the lowest LC50; 0.014 (range, 0.005–0.035) g/kg and 0.008 (range, 0.004–0.006) g/kg comparing with the other two insects; 0.270 (range, 0.114–0.676) g/kg for *T. castaneum* and 0.388 (range, 0.158–1.087) g/kg for *S. oryzae* (Figures 6–8). *S. oryzae* showed 30% mortality % at 1.0 g/kg of SiO2-NPs, *R. dominica* (90%); T. castaneum (68.3%) and 100% for *O. surinamenisis*. After 48 h of treatments, the data showed that *O. surinamenisis* was very highly sensitive to the SiO2-NPs compared with other insects.

After 72 h, the effect of SiO2-NPs in Figures 6–8 impact posed that, *R. dominica* and *O. surinamenisis* recorded the lowest LC<sup>50</sup> values were 0.002 (range, 0.0004–0.006) and 0.002 (range, 0.0005–0.007) g/kg, while *T. castaneum* (0.034) (range, 0.015–0.072 g/kg) and *S. oryzae* (0.263) (range, 0.055–0.014). From 0.25 to 2.0 g/kg of SiO2-NPs, the *O. surinamenisis* displayed 100% mortality %, while *S. oryzae* was more

resistance to SiO2-NPs which exhibited 93.3% under 1.0 g/kg (Figure 8) compared to the other insects that disclosed 100%.

**Figure 7.** Insecticidal activity of the Nano silica (SiO<sup>2</sup> -NPs) against *S. oryzae*, *R. dominica*, *T. castaneum* and *O. surinamenisis* as recorded (**a**) after 24 h; (**b**) after 48 h and (**c**) after 72 h.

**Figure 8.** Mortality % of SiO<sup>2</sup> -NPs against *S. oryzae*, *R. dominica*, *T. castaneum* and *O. surinamenisis* (**a**) after 24 h; (**b**) after 48 h and (**c**) after 72 h.

#### **4. Discussion**

The main significance of this current work is to prepare silica nanoparticles in a very high concentration using the sol-gel technology. The prepared silica nanoparticles in their current form are not toxic, since our aim in this current work was to prepare them in a pure form without any impurities. Thus, the calcination process has been carried out to degrade the undesired and unreactive compounds of TEOS or ammonia, ethanol substances. The next step is to use this industrial scale up, of silica nanoparticle in agricultural domain, as both growth promoter for soil and in the meantime, as an alternative nano pesticide, to combat pests infested maize grains thru post-harvest, in order to resolve the insect resistance to the conventional pesticides, which reflect the novelty of this work compared with the traditional relevance, for the pests control. The results concluded and assured that, by feeding the soil with silica nanoparticles up to 10 g/kg, the best growth and yield enhancement of maize crop is noticed. Moreover, the combination between mineral NPK and silica nanoparticles on soil application, had a beneficial effect on photosynthesis, yield enhancement and increased productivity of maize plants too. Also, silica nanoparticles displayed great success in combating the stored maize insects, which reached a 100% mortality rate.

In this current research work, we aimed to develop a new strategy for soil applications in terms of feeding or fertilizing, and at the same time, as a pesticide for combating the different kinds of pests that are found through storing maize grains. These results agree with Kyuma and Suriyaprabha et al. [49,50] and Sommer et al. [17] who showed the role of Si (Silicon) as a micronutrient for helping plants achieve the optimal use of water and other nutrients from soil. Also, was agreeing with [16] who detect the effect of NPK fertilizer on the yield and yield compounds in maize.

The current results observe the same trend as [51], who presented that growth and yield characteristics were much influenced with increasing concentration of SiO2-NPs [51]. They observed that the physiological changes showed that the expression of organic compounds such as proteins, chlorophyll, and phenols, as well as the growth and yield of maize increased by using SiO2-NPs. Also, Farooq and Dietz (2015) showed the role of Silicon as a versatile player in plants [52].

The results are in accordance with Dung et al. (2016) that used SiO2-NPs in different doses and reported that 60 ppm caused an increase in fresh weight, dry weight, and chlorophyll content in chili plants [53]. Another study [54] reported that SiO2-NPs play a great role in the physiological components of maize, thus supporting the use of mineral fertilizers based on the distribution of roots and shoots. In addition, SiO2-NPs are essential in increasing the detailed functional properties of mineral fertilizers [54].

These results in an agreement with [55,56] whose reported that the application of Nano silica (8 g/L) showed significantly increased the growth traits of tomato plants [55,56]. Also, [57–59] presented that SiO2−NPs nutrition decreased the inhibitory outcome of salinity on plant growth by decreasing the Na+ content, nutrient uptake, increasing the cell wall peroxidase activities.

The results showing the efficiency of SiO<sup>2</sup> in maize growth and productivity and these results were the same observation detected for SiO2-NPs that increased plant growth as reported by [55], and plant resistance to hydroponic conditions as reported by [51], as well as increased root length in plants, as stated by [60,61], and induced an improvement in photosynthesis as mentioned by [62].

Our results observed the same trend as other studies which showed the effects of Silica NPs with mineral fertilizers in many crop plants, such as maize as stated by [51,55], common bean as reported by [63], tall wheatgrass as described by [64], tomato as outlined by [65], faba bean as mentioned by [66], wheat as described by [67], rice as disclosed by [68], Glycine max as mentioned by [69], and sweet pepper as displayed by [70]. Also, others showed the effectives role of nanomaterial fertilizers on plant growth and productivity [21]. On the other hand, several research works have been carried out to prove the positive impact of silica nanoparticles to the crops, such as Rastogi et al. (2019) who reported the benefits of SiO2−NPs on physiological features of the plant in which that, they allow them to enter plants and affect its metabolic activities [71]. The same group also claim that the mesoporous nature of

silica nanoparticles can also direct them to be good applicants as nano carriers for several molecules that may support in agriculture [71].

The current results showed that SiO2-NPs disclosed effectives against the stored products insects, which reached to 100% (M%). These data are in agreement with [41], [72,73], and [37] who reported the impact of nanomaterials as an alternative pesticide against stored grains insects. Our results designated that nanoparticles could help to produce new insecticides and this finding agreed with [74] who reported the same fact in addition to yield pesticides and insect repellants. Few researches have been carried out to study the toxicity effect of nanoparticles on insects especially storage insects, [75] stated that nanoparticles loaded with garlic essential oil is useful against *Tribolium castaneum* (Herbst) [75]. So, the use of nanoparticles as unconventional pesticide constitutes a new approach to combat pests which have become resistant to chemical conventional pesticides.

Silicon nanoparticles have enormous application as insecticides on different insects such as aphids" cotton leaf worm, *Sitophilus oryzae* L., *Tribolium castaneum* (Herbst) and *Rhizopertha dominica* F under laboratory conditions [76]. The insect control mechanism is dependent on the structure of cuticular lipids for defending their water barrier, and in that way, prohibit death through dehydration. Meanwhile, silica nanoparticles get absorbed into the cuticular lipids by physio sorption and thus causes insects death. Moreover, Barik et al. [77] also verified the use of SiO2−NPs as a nano-pesticide and clarified the same control mechanism of combating insects

#### **5. Conclusions**

High throughput silica nanoparticles (SiO2-NPs) were synthesized via the sol–gel technique. The SiO2-NPs were obtained in a powder form followed by full characterization using state of the art analysis. TEM displayed that the particle shape was spherical with low disparity due to the cluster effect of silica particles. However, these aggregated particles are less than 50 nm. Dynamic light scattering (DLS) confirmed the particle size and stability, where the average size was around 68 nm. In addition, the zeta potential value of the prepared SiO2-NPs was −40 mV, which affirms the stability of these particles against aggregation, even after long time. Moreover, XRD ascertained that SiO2-NPs were prepared successfully using the sol–gel technique, in pure form and free from any other impurities or unreacted compounds.

To this end, the SiO2-NPs were applied successfully as both nanofertilizer and pesticide against four common pests that infect the stored maize and cause severe damage to them. The results obtained demonstrate that, by feeding the soil with SiO2-NPs up to 10 g/kg, the best growth and yield enhancement of maize crop is noticed. Mineral NPK interacted significantly with SiO2-NPs, whereas the application of mineral NPK at the rate of 50% with 10 g/kg SiO2-NPs, increased the highest mean values of agronomic features. Consequently, it can be concluded that the combination of mineral NPK and SiO2-NPs by soil application, had a beneficial effect on photosynthesis, yield enhancement, and increased the productivity of maize plants. In addition, it improved protein content (%) to 12.59% and chlorophyll content to (45.13 SPAD). This increase emphasizes the metabolic balance between induction of chlorophyll and proteins and cell wall transporters, damping off stress-responsive enzyme activities as a function of SiO2-NPs application in maize plants. Also, SiO2-NPs exhibited effectiveness against the stored products insects, which reached a 100% mortality rate.

Finally, SiO2-NPs can be easily applied as growth promoter and can work as strong unconventional pesticides for crops during storage, with a very small and safe dose in order to combat all kinds of pests harmful to maize during storage.

**Author Contributions:** Data curation, N.R.A.; Formal analysis, M.E.E.-N. and M.M.F.; Funding acquisition, N.R.A.; Investigation, N.R.A. and H.M.A.; Methodology, M.E.E.-N., M.I.M., M.A.A.-J. and E.E.K.; Resources, M.I.M, M.A.A.-J. and E.K.; Software, M.M.F., H.M.A. and M.H.S.; Visualization, E.K.; Writing – original draft, M.E.E.-N. and M.H.S.; Writing—review and editing, N.R.A. and M.M.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by King Saud University, Researchers Supporting Project number (RSP-2019/123), King Saud University, Riyadh, Saudi Arabia.

**Acknowledgments:** The authors gratefully acknowledge the Researchers Supporting Project number (RSP-2019/123) King Saud University, Riyadh, Saudi Arabia.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Optimization of Metal-Assisted Chemical Etching for Deep Silicon Nanostructures**

**Rabia Akan \* and Ulrich Vogt**

KTH Royal Institute of Technology, Department of Applied Physics, Albanova University Center, 106 91 Stockholm, Sweden; ulrich.vogt@biox.kth.se

**\*** Correspondence: rabiaa@kth.se

**Abstract:** High-aspect ratio silicon (Si) nanostructures are important for many applications. Metalassisted chemical etching (MACE) is a wet-chemical method used for the fabrication of nanostructured Si. Two main challenges exist with etching Si structures in the nanometer range with MACE: keeping mechanical stability at high aspect ratios and maintaining a vertical etching profile. In this work, we investigated the etching behavior of two zone plate catalyst designs in a systematic manner at four different MACE conditions as a function of mechanical stability and etching verticality. The zone plate catalyst designs served as models for Si nanostructures over a wide range of feature sizes ranging from 850 nm to 30 nm at 1:1 line-to-space ratio. The first design was a grid-like, interconnected catalyst (brick wall) and the second design was a hybrid catalyst that was partly isolated, partly interconnected (fishbone). Results showed that the brick wall design was mechanically stable up to an aspect ratio of 30:1 with vertical Si structures at most investigated conditions. The fishbone design showed higher mechanical stability thanks to the Si backbone in the design, but on the other hand required careful control of the reaction kinetics for etching verticality. The influence of MACE reaction kinetics was identified by lowering the oxidant concentration, lowering the processing temperature and by isopropanol addition. We report an optimized MACE condition to achieve an aspect ratio of at least 100:1 at room temperature processing by incorporating isopropanol in the etching solution.

**Keywords:** metal-assisted chemical etching; Si nanostructures; high aspect ratio; zone plate

#### **1. Introduction**

Nanostructured Si is the material of choice for a variety of applications such as photonics [1], lithium ion batteries [2], solar cells [3], biosensors [4], microfluidic channels [5] and X-ray optics [6]. Many of these applications require devices with highly vertical and deep Si nanostructures, i.e., high aspect ratios. The smaller the structures are, the more challenging it gets to fabricate high-aspect ratio nanostructures that are mechanically stable.

Both dry and wet etching processes are used for the fabrication of Si devices. Reactive ion etching (RIE) [7] is an example of a commonly used dry etching technique. For Si devices with nanostructures, maintaining a vertical etching profile becomes challenging with RIE and thus limits the achievable aspect-ratios [8]. Therefore, RIE is more suitable for the fabrication of devices with structures in the micrometer range or when extreme aspect ratios are not needed.

A wet etching process that overcomes fabrication challenges is MACE. MACE is an electroless method that is gaining a lot of attention as a pattern transfer technique for the fabrication of deep high-aspect ratio Si nanostructures [9,10]. In MACE, a noble metal catalyst (e.g., gold (Au) that is lithographically defined or in nanoparticle form) is etching its way into a Si substrate in an electrolyte (etching solution) composed of hydrofluoric acid (HF) and a strong oxidizer (e.g., hydrogen peroxide (H2O2)). The noble metal (cathode) catalyzes the reduction of the oxidizer and consequently electrical holes are formed. The holes are injected into the Si substrate (anode) locally on the noble metal site and oxidizes

**Citation:** Akan, R.; Vogt, U. Optimization of Metal-Assisted Chemical Etching for Deep Silicon Nanostructures. *Nanomaterials* **2021**, *11*, 2806. https://doi.org/ 10.3390/nano11112806

Academic Editor: Céline Ternon

Received: 30 September 2021 Accepted: 19 October 2021 Published: 22 October 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the substrate. The HF then dissolves the oxidized Si and the catalyst pattern is transferred into the Si substrate.

Although conceptually simple, MACE can become complicated since many parameters influence the process like etching solution composition [11], etching temperature [12], doping concentration of the Si substrate [13,14], catalyst thickness [15] or catalyst morphology [16,17]. These parameters have to be carefully adjusted to obtain the desired etching performance. In the literature, there are numerous studies investigating many of these parameters and their effects on micro- and nanostructure etching. However, comparing results is often difficult due to the variation in active etching areas and structure sizes. There is a lack of systematic studies that make direct comparison of etching behavior possible.

For etching of catalyst structures in the nanometer range two main challenges exist. Firstly, maintaining a vertical etching direction and secondly, keeping a good mechanical stability for deep etching when reaching extreme aspect ratios. Especially, the catalyst design has a great impact on these two challenges [11]. In this work we choose zone plate structures as model catalyst patterns to find the morphological parameters and etching conditions that are best suited for MACE processing of sub-100 nm high-aspect ratio Si structures.

Zone plates are diffractive imaging and focusing optics commonly used in X-ray microscopes [18]. Their circular grating structures are decreasing in width with the zone plate radii. Two parameters are key for the zone plate performance: imaging resolution and diffraction efficiency. The zone plate resolution is defined by the outermost zone width, whereas the diffraction efficiency is defined by the zone thickness [19]. The X-ray energy and zone plate material will define the required zone thickness. For the use in the hard X-ray regime, thicknesses of several micrometers are often needed. In order to fabricate a high-resolution and high-efficient zone plate, very high aspect ratios are therefore required. Since X-ray zone plates contain structures ranging from micron-sized features in the center to nanometer-sized features in the outer parts, they are ideal model patterns to systematically investigate the MACE process. The obtained results are applicable to other Si devices with nanostructured, lithographically defined catalyst patterns.

We systematically investigate MACE of two catalyst pattern designs at four different MACE conditions in order to find optimum process conditions for obtaining vertical, mechanically stable deep high-aspect ratio Si nanostructures. The first design, called "brick wall" (Figure 1a), is a grid catalyst with all zones interconnected, whereas the second design, called "fishbone" (Figure 1b) is a hybrid catalyst with partly interconnected and partly isolated zones. Both designs have smallest feature sizes of 30 nm (width of the outermost zones), which are as far as we are aware the smallest lithographically defined catalyst structures reported for MACE. To our knowledge, this is the first study that combines the brick wall and fishbone type catalyst designs on the same chip, using the same active catalyst area and exposes them to the same exact reaction conditions, including cold etching and IPA addition. This makes direct comparisons of etching behavior possible.

**Figure 1.** Top-view SEM micrographs of Au zone plate catalyst patterns on Si illustrating (**a**) brick wall (red frame) and (**b**) fishbone designs (green frame). The insets show the outermost parts of the zone plate designs. The zone plates designs have a 150 µm diameter, 1:1 line-to-space ratio and the outermost zone widths are 30 nm. Same scale bars apply to the micrographs.

#### **2. Motivation for the Selection of Catalyst Designs and MACE Conditions**

In the literature, there are several studies reporting the fabrication of Si zone plate nanostructures using MACE [11,20–24]. None of the reported studies contain a detailed motivation for their choice of catalyst design and MACE processing conditions, instead, the MACE pattern transfer has been presented as one in a series of steps for a complete device fabrication. Some of the studies used grid catalyst designs [11,20,24] while others preferred fishbone catalyst designs [21–23,25]. It has to be noted that it is difficult to etch isolated catalyst structures in a controlled way, so some kind of connected catalyst design is necessary in order to achieve vertical etching.

There are benefits and challenges with both grid and fishbone designs. A grid catalyst design has the advantage of maintaining a vertical etching profile owing to the interconnected zones, however, resulting in isolated Si nanostructures. The isolated Si structures will limit the achievable etch depths due to collapse, especially in smallest features [20]. In the fishbone design, the noble metal rings are interrupted forming sections with perpendicular lines crossing each section, resulting in partly interconnected Si after MACE. This interconnection will contribute with mechanical stability, but at the expense of etching verticality. It was recently reported that the fishbone design deviates from its vertical etching path at thicknesses beyond 1 µm due to the free Au ends in the design [23]. It should be noted that the results were obtained for a specific MACE processing condition, and it is therefore not possible to identify if this behavior is due to the catalyst design or the reaction conditions. Here, our aim is to directly compare these two types of catalyst designs by processing them on the same substrate and studying their etching behavior at different MACE conditions.

We base our choices of different MACE conditions on reaction kinetics. We want to control the hole injection rate into the Si, and thus, the overall etching rate by lowering the processing temperature and H2O<sup>2</sup> concentration from our previously optimized MACE condition [11]. In our previous study, a grid catalyst design was used and the MACE condition was optimized based on etch depth and silicon zone roughness. The aim here is to investigate if a slower MACE reaction is beneficial for the etching directionality, as cold etching was used in several other studies without further explanation but showing nice vertical structures [21–23]. We further investigate the effect of isopropanol (IPA) which should also lower the etch rate [26]. Additionally, previous work suggests that the transport of generated gas from the etching location will be improved with the addition of IPA due to lower surface tension [27]. All MACE conditions are studied for different etching times to gain an understanding of to what etch depths the two designs are mechanically stable and if the different reaction conditions affect the mechanical stability of the zones. Our goal is to provide benchmark results for researchers in various fields that want to find optimum MACE processes for Si nanostructure fabrication.

#### **3. Materials and Methods**

Si p-type (100) wafers with resistivity of 1–5 Ω·cm were used for all samples. The zone plate fabrication procedure consisted of a sequence of steps: (1) Ultrasonic cleaning of wafer pieces followed by a oxygen plasma cleaning step, (2) resist spinning and patterning via electron-beam lithography, (3) resist development, (4) short oxygen plasma treatment, (5) electron beam evaporation of catalyst layer, (6) resist lift-off, (7) oxygen plasma cleaning for removal of organics, (8) MACE and (9) critical point drying. An overview of the experimental procedure is presented in Figure 2.

The Si wafers were cut into 1.5 cm × 1.5 cm chips and cleaned ultrasonically in acetone and IPA for removal of Si dust and by an oxygen plasma step (PlasmaLab 80 Plus RIE/ICP system, Oxford Instruments, Abingdon, UK) for removal of organics and further oxidising the Si surface (each cleaning step was typically 5 min). We used 80 nm of the positive resist CSAR 62 (Allresist GmbH, Strausberg, Germany) for the electron beam lithography step (50 kV Voyager EBL system, Raith GmbH, Dortmund, Germany) and the zone plate catalyst designs investigated in this study (brick wall and fishbone) were patterned on each substrate. The zone plate designs had a diameter of 150 µm, 1:1 line-to-space ratio and zone widths ranging from 850 nm (innermost zones) to 30 nm (outermost zones). The interconnects in the brick wall design were 40 nm and brick length of the outermost zones were 460 nm. In the fishbone design, the backbone was 95 nm and free ends were 360 nm of the outermost zones. The resist development was performed in amyl acetate (60 s) followed by rinsing in IPA and n-pentane (10 and 15 s, respectively). The short oxygen plasma treatment step (13 s) after development was to ensure removal of any resist residues in the zone plate patterns. For the catalyst layer, 10 nm Au was electron beam evaporated (in-house Eurovac/Thermionics deposition system) at 1 Å/s on a 2 nm adhesive Ti layer. The resist lift-off was performed in dimethyl succinate under ultrasonication. The final oxygen plasma step (3 min) was necessary to remove any organics on the sample surfaces that might prevent the MACE process.

**Figure 2.** Overview of the experimental procedure. (**a**) Si substrate (grey) preparation by spin-coating 80 nm CSAR 62 resist (pink). (**b**) Zone plate patterning by EBL and development for removal of resist residues. (**c**) Deposition of 2 nm Ti and 10 nm Au (yellow) by evaporation. (**d**) Resist lift-off, leaving the metal zone plate pattern. (**e**) Etching the metal zone plate pattern into the Si via MACE.

The MACE experiments were all performed in a polytetrafluoroethylene (PTFE) container under light protection. Four different MACE conditions were used to investigate the etching behavior of the two designs (Table 1).



To understand the reproducibility of the MACE process, eight substrates with three zone plates per design were processed and each MACE condition was repeated twice. The etching solution compositions were based on previously reported studies [11,27]. The

cold MACE experiments at 8 ◦C (C2) were performed using pre-cooled chemicals in a surrounding cooling bath. Both the patterned Si chips and etching solutions were kept in the cooling bath prior MACE to ensure temperature stabilization. To carefully investigate the etching direction and mechanical stability at different etch depths, the processing time was set to 3, 6, 9 and 12 min for each MACE condition. The samples were transferred to ethanol after MACE processing and further critical point dried (Leica EM CPD300, Leica Microsystems GmbH, Wetzlar, Germany). For etch depth analysis, cross-sections were prepared using FIB milling (Nova 200 NanoLab system, FEI Company, Hillsboro, OR, USA). Additionally, some samples were cleaved manually for a better visualization of cross sections. Unfortunately, this is a complicated procedure and cleaved cross-sections could not be obtained for all etching conditions and designs.

#### **4. Results and Discussion**

We characterize the etching behavior of our catalyst designs based on mechanical stability and etching verticality. The MACE conditions that are favorable for each design in terms of these two characteristics are identified and discussed.

#### *4.1. Mechanical Stability*

As previously reported, a grid-like catalyst design resulted in vertically etched Si structures, almost independently of the used MACE condition [11]. While its strength lay in maintaining etching verticality, the mechanical stability of the remaining Si structures became a limitation at too small structural widths and large etch depths. Zone plates were therefore ideal catalyst structures to identify at what point the mechanical instability started thanks to their broad size range of zones.

Figure 3 shows cross-section micrographs of a ≈ 1.4 µm (average thickness over the zone plate radius) thick brick wall zone plate at three points with different zone widths. The zone width in Figure 3a was 60 nm and no collapse of zones were apparent. At 50 nm zone width, as shown in Figure 3b, some tendency of collapse was starting to show suggesting that a stability limit for the etch depth was reached at this point. At the outermost parts of the zone plate where the zone widths were 30 nm most of the Si nanostructures had collapsed (Figure 3c). It should be noted that the etch depth varied over the zone plate, where the innermost zones were less deep than the outermost parts. The exact depth where the collapse started appearing at 50 nm zone width was 1.5 µm, suggesting that the achievable aspect ratio of a stable brick wall design was 30:1.

The etching behavior of the catalysts at the investigated conditions C1–C4 are summarized in Figure 4, a common etching time point of 6 min was chosen for comparison. The average etch rates for the different conditions are presented in Table 2. Room temperature processing with the higher H2O<sup>2</sup> concentration gave the fastest etch rate (C1) which, based on the well accepted MACE theory [9,28], suggested the highest hole injection rate into the Si. The deepest etching of ≈2.4 µm was obtained at this condition and especially the outermost zones of both the brick wall (Figure 4a) and the fishbone (Figure 4e) design showed deformation.

**Figure 3.** Brick wall catalyst design processed at MACE condition C1. The top micrograph shows where the three micrographs labeled (**a**–**c**) are localized on the zone plate. The cross-sections were prepared by manual cleaving.

**Figure 4.** Brick wall (red frame) and fishbone (green frame) zone plate designs etched for 6 min at MACE conditions (**a**) C1, (**b**) C2, (**c**) C3 and (**d**) C4 and (**e**) C1, (**f**) C2, (**g**) C3 and (**h**) C4, respectively. The two designs shown per MACE condition were processed on the same Si chip. Same scale bars apply to main SEM micrographs and the insets. The insets show the outermost zones.


**Table 2.** Average etch rates for the investigated MACE conditions.

Interestingly, a 10 times lower H2O<sup>2</sup> concentration (C2), and consequently a slower etch rate (Table 2), did not result in a homogeneous and controlled etching. An uneven etching with both catalyst designs was more apparent here than with any other investigated MACE condition (Figure 4b,f). Especially for the fishbone design, local differences in etching within neighboring structures was observed suggesting an uneven H2O<sup>2</sup> distribution over the catalyst area (see Figure 5a). The etch depth was ≈0.5 µm and surprisingly, at this shallow etch depth collapse of the outermost zone plate structures was observed for the brick wall catalyst (Figure 4b). A homogeneous and vertical etching was thus key for mechanical stability of the Si structures. Furthermore, these results showed that verticality was a prerequisite for the stability of the Si structures. It should be noted that we did not implement any stirring of the etching solution during MACE to avoid any disturbance of the process.

Lowering the processing temperature (C3) and adding IPA (C4) to the etching solution resulted as expected in slower etch rates [12,24,29] (Table 2). At etch depths of ≈1.7 µm and ≈1.4 µm with MACE conditions C3 and C4, respectively, a positive impact on the overall etching uniformity of both the brick wall (Figure 4c,d) and the fishbone (Figure 4g,h) catalysts was observed. With the IPA addition, slight broadening of the innermost Si structures could be seen (Figure 4d), but nonetheless, both conditions contributed to a better mechanical stability of the outermost structures.

#### *4.2. Etching Verticality*

Maintaining a vertical etch direction for isolated catalyst structures of nanometer sizes is challenging [30,31]. The fishbone design with partly isolated structures was also found to require more careful control of the MACE chemistry and reaction kinetics to maintain a vertical etching direction throughout the process. At the highest investigated etch rate (C1) deformation of the outermost zones of the fishbone catalyst indicated a deviated etch direction (Figure 4e). This was confirmed from the cross-section micrograph in Figure 6a, where the non-vertical etch profile as well as catalyst deformation is shown (≈2 µm etch depth). The brick wall design processed on the same Si chip maintained a vertical but collapsed etching profile (≈3 µm etch depth, Figure 6b). Similarly to the negative effect on

the Si structure stability, the uneven or limited access of H2O<sup>2</sup> to the catalyst surface also negatively impacted the etching verticality, although etch rates were significantly lowered. This is illustrated in Figure 4f (outermost zones) and Figure 5b.

Compared to conditions C1 and C2, conditions C3 and C4 indicated a more even etching, especially at the outermost parts of the fishbone catalyst design (Figure 4g,h). The impact IPA addition (C4) had on the etching verticality is illustrated in Figure 6c, where the zones are vertical and the etching profile is uniform. As for most other conditions, the brick wall catalyst processed on the same chip also kept a vertical etching profile at C4 (Figure 6d). The etch depth of both zone plate designs in Figure 6c,d was ≈3 µm. This suggests that aspect ratios of at least 100:1 with a maintained vertical etching profile that can be obtained by IPA addition.

**Figure 6.** The effect of IPA on etching verticality. (**a**) fishbone and (**b**) brick wall catalyst designs etched at condition C1 (no IPA, same Si chip), and (**c**) fishbone and (**d**) brick wall catalysts etched at condition C4 (with IPA, same Si chip). The MACE time was 9 min for both samples. The cross-sections were prepared by cleaving.

**Figure 7.** Cross-section SEM micrographs of the fishbone design catalyst processed at MACE condition (**a**) C1 for 9 min, (**b**) C3 for 12 min and (**c**) C4 for 12 min. The cross-sections were made by FIB milling.

Figure 7 shows micrographs of cross-sections prepared by FIB milling comparing the original MACE condition (C1, Figure 7a) with the relatively low processing temperature (C3, Figure 7b) and the addition of IPA (C4, Figure 7c) at average etch depths of ≈4.2 µm, ≈3.2 µm and ≈4 µm, respectively. It should be noted that FIB milling caused deformation of the finer structures, but still could give an indication of the etching directionality and depth. From these micrographs, it is obvious that both C3 and C4 were beneficial with regards to the etching direction. We draw the conclusion that controlling the MACE kinetics by lower processing temperature or by addition of IPA in the etching solution had similar, positive impacts on the etching verticality. However, the effect of IPA addition on the MACE process was not completely clear. If the effect of IPA was limited to reduction of the etch rate or if other mechanisms like easier release of formed gases were important is an open question. From a process handling point of view, we recommend adding IPA to the etching solution instead of temperature regulation of the etching solution. Temperature stabilization using a large volume of etching solution was time consuming and needed pre-cooling of the chemicals. The low heat conductivity of the PTFE container was altering the temperature of the chemicals and limited the number of MACE experiments that could be performed due to long temperature stabilization times in between samples. Therefore, to avoid uneven etch rates due to temperature elevation, room temperature processing with IPA is preferable.

#### *4.3. Process Reproducibility*

Many reports on MACE either use the process as one in a series of steps for microand nanofabrication, or investigate the process performance for a given application. As far as we are aware, there are no reports that discuss the reproducibility of the process. In this study, we explored four different MACE conditions and examined the etching behavior at four time points per condition, which in turn was repeated twice. We believe this large set of experiments gave a good indication of the reproducibility of the MACE process.

Figure 8 shows the average etch depth of one brick wall and one fishbone zone plate catalyst design processed on the same chip as a function of processing time at conditions C1–C4. The error bars represent the standard deviation and indicate the variation in depth between the two zone plates. As previously reported, the decreasing width of the zones over the zone plate radius resulted in relatively deeper etching of the smallest, outermost zones than the largest, innermost zones. This gave a local variation of a few per cent over the same zone plate [11,20]. This variation is not shown here, but was present for all devices. Overall, all conditions gave larger zone thicknesses with longer processing time. However, the etch depth did not increase linearly with time. Even though both zone plates had the same active catalyst area, etch depth variations could be observed for certain time points.

Furthermore, differences in etch depths of zone plates on two different Si chips processed at identical conditions are shown in Figure 9. The error bars represent the standard deviation of the measurements. The variations in etch depth were largest for samples processed at conditions C1 and C2, and smaller for C3 and C4. Even though identical samples with same sizes and same active catalyst areas were processed at identical conditions, etching behavior differed depending on condition.

**Figure 8.** Average etch depths of brick wall and fishbone catalysts processed on the same Si chip as a function of etching time at MACE conditions C1–C4. The error bars show the standard deviation of the measurements.

**Figure 9.** Average etch depths of zone plates on different Si chips processed at conditions C1–C4 for 9 min. The error bars indicate the depth variation between different chips.

With these results, we want to highlight that precise control of the etching process was challenging and that the depth might vary not only between samples from run to run, but between structures on the same Si chip. The outcome of our experiments indicated that conditions C3 and C4 were also preferable for good reproducibility.

#### **5. Conclusions**

We demonstrated the MACE effect of catalyst design and reaction kinetics on fabrication of high-aspect ratio Si structures as a function of mechanical stability and etching verticality. Two zone plate catalyst designs with structural sizes ranging from 850 nm down to 30 nm have been used, brick wall (interconnected) and fishbone (partly connected and partly isolated), and their etching behavior at four different conditions has been systematically studied. Our findings reveal that the Si structures of the brick wall design were mechanically stable up to an aspect ratio of 30:1, while deeper etching resulted in collapse of the outermost zones with the smallest structures. Furthermore, etching verticality could be maintained with the brick wall design, independently of the MACE condition. The fishbone design required more careful control of the reaction kinetics for the catalyst to translate linearly into the Si. Addition of IPA and a lowered processing temperature showed a significant improvement in etching verticality of the fishbone design as opposed to room temperature processing without any additive in the etching solution. The cold MACE required a rather complicated cooling setup, and therefore, we recommend IPA addition to the etching solution for controlled kinetics at room temperature processing. For the future, the influence of other alcohols on the processing of Si nanostructures should be investigated. We also show data indicating the sensitivity of the MACE process. These are the first results showing the reproducibility of the process and more statistics are needed for further characterization.

We think our findings provide relevant information for micro- and nanofabrication of high-aspect ratio Si structures with MACE. The easy processing without any need of complicated experimental setups or tools will be beneficial for applications that require controlled Si fabrication with lithographically defined morphology.

**Author Contributions:** R.A. and U.V. conceived the idea for the study. R.A. performed all lab work. R.A. and U.V. wrote the manuscript together. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Swedish Research Council grant number 2018-04237 and 2019-06104.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We thank Thomas Frisk for his help with designing the zone plates and Hanna Ohlin and Mattias Åstrand for their help with the MACE experiments. Part of this work was performed at the Albanova Nanolab.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


MDPI St. Alban-Anlage 66 4052 Basel Switzerland Tel. +41 61 683 77 34 Fax +41 61 302 89 18 www.mdpi.com

*Nanomaterials* Editorial Office E-mail: nanomaterials@mdpi.com www.mdpi.com/journal/nanomaterials

MDPI St. Alban-Anlage 66 4052 Basel Switzerland

Tel: +41 61 683 77 34

www.mdpi.com

ISBN 978-3-0365-4765-7