Facile deposition of ultrafine silver particles on silicon surface not submerged in precursor solutions for applications in antireflective layer

Using a facile deposition method, the ultrafine silver particles are successfully deposited on the Si surface that is not submerged in precursor solutions.Theultrafine silver particles havemany advantages, such as quasiround shape, uniformity in size,monodisperse distribution, and reduction of agglomeration.The internal physical procedure in the deposition is also investigated.The results show that there are more particles on the rough Si surface due to the wetting effect of solid-liquid interface. The higher concentration of ethanol solvent can induce the increase of quantity and size of particles on Si surface not in solutions.The ultrafine particles can be used to prepare porous Si antireflective layer in solar cell applications.


Introduction
Metallic ultrafine particles (1-100 nm in diameter) have attracted much attention for their anomalous physical properties which are different from the bulk solid phases [1]. The novel properties of ultrafine silver particles, such as surface-enhanced Raman spectroscopy (SERS), surface Plasmon renounce (SPR), and catalyst, have been exclusively studied [2][3][4][5][6]. Recently, many chemical methods have been used to synthesize silver particles [6][7][8][9][10][11][12]. But most of these methods focus on the synthesis of silver nanoparticles (Ag-NPs) instead of the one-step deposition methods on the certain substrate. Meanwhile, in most methods, the substrates need to be dipped into the precursor solutions. In vertical deposition process, the substrates are kept vertical to the surface of the solution, which can be used to prepare colloidal crystal and metal thin film or particles [13][14][15][16][17]. This method can produce the high-quality single crystal with controlled thickness by controlling the balance between ethanol evaporation rate and particle sedimentation [16]. The silver mirror reaction is an "old" chemical route, which can be used to generate reflective mirrors on solid supports. In recent times, studies of nanotechnology have renewed the interest for this reaction. The silver mirror reaction has been used to prepare silver particles and films, core-shell structures for extensive applications [6,12,[18][19][20][21][22]. In this work, ultrafine silver particles are prepared on Si surface out of Tollen's reagent and the internal physical procedure of the deposition is revealed.

Experimental Details
Firstly, AgNO 3 solutions were prepared in the mixed solvent of ethanol and water. The volume percent of ethanol was controlled at 0%, 50%, and 70%. The Tollen's reagent (0.2 M) was obtained by dripping aqueous ammonia continuously into the AgNO 3 solutions while stirring until the mixed solutions became transparent. Then, the rough and polished Si samples of 20 × 10 × 0.5 mm 3 were vertically dipped into the Tollen's reagent, respectively. The depth of immerging was about 2 mm, and the other part of the samples was out of solution. After that, the glucose (0.01 M) was added to Tollen's reagent. In this typical silver mirror reaction, the Ag ions were reduced to Ag particles deposited on the Si 2 Journal of Nanomaterials substrate. The reactions were kept for 2, 5, and 10 minutes at room temperature. The effect of reaction time, concentration of ethanol, and surface state (rough and polished) on Ag particles on the surface in solution and not in solution were investigated by scanning electron microscope (SEM, FEI Quanta 200F). Moreover, the polished Si wafer was put on horizontally above the reaction solutions (including 50% ethanol solvent) away from liquid surface about 1 mm. After that, the Ag nanoparticles on the Si surface above solutions were also investigated. Finally, the Si wafer deposited silver particles were immersed in etching solutions composed of HF, H 2 O 2 , and H 2 O (volume ratio is 1 : 5 : 2) for 5 minutes. Then, nanoporous Si antireflective layers were obtained by etching process. The morphology and antireflection property of porous Si were measured by SEM and ZoLix IPCE/QE system, respectively.

Results and Discussion
Through SEM observation, it is found that the ultrafine silver particles can be deposited on the surface out of solutions. Figure 1 shows the changes of morphology of the silver particles away from the solid-liquid interface after 5 minutes reaction. Compared with that on surface out of solution, there are more silver particles with the larger size on the surface in solution, as shown in Figures 1(a) and 1(b). It can be seen from Figures 1(c)-1(f) that the size and quantity of the ultrafine particles out of solutions display an obvious difference, which degrades with increase of distance away from the solidliquid interface. The size of particles in solution is about 50 nm, whereas that out of solution is ranged from 5 nm to 20 nm. The effective deposition distance out of solutions is about 1 cm. It is considered that the successful deposition of ultrafine silver particles on the surface out of solutions is attributed to the wetting of solid-liquid interface. The wetting effect is related to the surface status and deposition process. The wetting effect is proved by investigating the effect of the roughness on distribution of silver particles. Figure 2 shows the morphology of silver particles deposited on the polished and rough surfaces out of solutions by vertical deposition for 2 min and 5 min. The result shows that, for 5 minutes reaction, the particles in polished surface are very small with size of 5-20 nm, while more particles with the larger size (40-60 nm) are deposited on the rough surfaces out of solutions. As is well known, the wetting of solid-liquid interface is closely correlated with surface roughness. The size and distribution of silver particles can be influenced by wetting force which depends on the surface state [15]. In our experiments, the nanoparticles deposited on rough surface are much more than those on the polished surface, which can be explained by the wetting effect during the vertical deposition. When a part of sample is immersed into the precursor solutions, the wetting effect makes a few precursor solutions being adsorbed slowly to the surface not submerged in solution, resulting in the successful preparation of ultrafine particles. These ultrafine particles may come from the particles of initial reaction in solutions or the subsequent reaction of precursor solutions on the surface out of solution.
In previous studies, the evaporation of ethanol solvent has been usually used to vertically deposit colloidal crystals [14]. In this work, the effect of ethanol solvent on deposition out of solutions is also investigated. Figure 3 shows that the quantity and size of particles on the surface out solution change by increasing the volume ratio of ethanol solvent. It is considered that the ultrafine Ag particles are taken Journal of Nanomaterials out by the evaporation of ethanol solvent during vertical deposition. Moreover, it is found that, in the initial period of reaction, ultrafine Ag particles are more easily taken out of solution by evaporation. To prove the effect of ethanol solvent, we observe the experimental phenomenon when the sample is put horizontally above the solution away from liquid surface about 1 mm. In this condition, a few ultrafine particles are observed on the sample surface not contacted with the solutions, but the distribution of particles is not uniform. The result proves that evaporation of ethanol solvent is another factor which contributed to the deposition of ultrafine particles on the surface out of solution. From the above results, it is found that the ultrafine particles are easily prepared on the surface not in solution and their size and shape are better controlled by the combined interaction of the wetting effect and evaporation of ethanol solvent.
Through investigation of deposition process, we find that the ultrafine silver particles can be obtained in the precursor solution with 50% ethanol solvent (volume ration). In the precursor solution with 70% ethanol solvent, the silver 4 Journal of Nanomaterials In the present work, the ultrafine silver particles are used to prepare porous Si antireflective layer. The result shows that ultrafine porous Si is obtained by catalytic etching ultrafine silver particles, which has smaller and more uniform ultrafine holes. The reflectance of ultrafine porous Si in near infrared waveband range is decreased to below 1.6% by the ultrafine porous Si structure, as shown in Figure 5. The improvement of antireflection is attributed to better light scattering of ultrafine porous Si due to the increase of the  optical absorption path lengths [23,24]. The ultrafine porous Si will be used to antireflective layer in solar cell devices.
Journal of Nanomaterials 5

Conclusion
In this work, we propose a facile vertical deposition, which can be used to prepare ultrafine silver particles on the surfaces that can not be entirely dipped into the precursor solutions.
The experimental results prove that the wetting effect of solid-liquid interface and evaporation of ethanol solvent are the internal physical mechanism in vertical deposition. The wetting effect is more obvious in the rough surface out of solution. Meanwhile, the many larger silver particles are obtained by increasing the concentration of ethanol solvent. Through process optimization, ultrafine silver particles with 10-40 nm in size can be deposited on the surface not in solution. The ultrafine porous Si antireflective layer can be obtained by etching ultrafine silver particles, which has the lower reflectance in a broader spectral range.