Controlled Fabrication Of Silicon Nanocones With The Metal Ag-Assisted Chemical Etching Method

Silicon nanocones(SiNCs)are a kind of important one-dimensional(1D)silicon nanostructures.Compared with other 1D silicon nanostructures such as silicon nanowires(SiNWs),SiNCs have unique merits including strong antireflection properties,easy conformal modification,superhydrophobicity,and so on.Encouraged by the excellent properties,SiNCs have attracted intensive attention and investigations.In reported researches,SiNCs are usually fabricated through dry etching and bottom-up growth.These methods are complexed and high-cost,and noble and toxic gases need to be used,hindering the wide applications of SiNCs.The metal-assisted chemical etching(MACE)method consisting of all-wet processes is a simple,efficient,large-area,and low-cost approach for fabricating 1D silicon nanostructures,which has been applied in solar cell industry.Despite these signs of progress,it remains a challenge to fabricate SiNCs through all-wet processes due to the limited understanding of diameter control in MACE.To solve the above problems,the experimental parameters and reaction mechanism of the all-wet MACE method are deeply explored in this dissertation.A strategy is proposed to realize the radial control of 1D silicon nanostructures by dynamically modulating the sizes of catalyst particles.The main contents are listed below:1.The effect of AgNO3 concentration and reaction time in the seed solution on the size and distribution of Ag particles and the morphology of the 1D silicon nanostructures is studied in the all-wet MACE process.It is found that as the concentration of AgNO3 in the seed solution is increased or the reaction time in the seed solution is prolonged,the size and number of Ag particles on the sample surface will increase.Correspondingly,the obtained samples will change from SiNWs to SiNCs after reacted in the etching solution.Therefore,it is a necessary condition for all-wet MACE fabrication of SiNCs to increase the AgNO3 concentration or prolong the reaction time in the seed solution.2.The effect of the H2O2 concentration in the etching solution on the morphology of the 1D silicon nanostructures is investigated in the all-wet MACE process.When the reaction conditions in seed solution are kept constant,the sample morphology will change from SiNWs to SiNCs with the increase of the H2O2 concentration.Therefore,it is another necessary condition for all-wet MACE fabrication of SiNCs to use an appropriate high H2O2 concentration in etching solution.3.Through experimental characterization by SEM,TEM and so on,and analysis of the basic chemical reaction of Si in AgNO3/HF/H2O2 systems,the forming mechanism of SiNCs can be revealed.When the sizes of catalyst Ag particles are large and the H2O2 concentration in the etching solution is high,the anode reaction of Si oxidation is suppressed,while the cathode reaction of H2O2 reduction on catalyst surface is promoted.An extra anode reaction of Ag oxidation will occur.Therefore,the sizes of Ag particles will decrease with the etching of silicon.Accordingly,the diameters of the obtained Si nanostructures will increase,forming SiNCs.4.Based on the previous experiment and mechanism studies,a two-step etching method is proposed to control the cone angle of SiNCs.SiNCs with different cone angles can be obtained through the following steps.Firstly,a silicon wafer is reacted in the seed solution with optimum conditions.Then the sample is transferred into the original etching solution and reacts for 10 min.Finally,the etching solution is diluted to a certain proportion and the sample reacts in it for 5 min.SiNCs with different cone angles can be got with different dilution ratios.The basic mechanism is to control the relative reactive rates of Ag and Si oxidation through controlling the dilution ratio of the etching solution.The above studies demonstrate the feasibility of radial regulation of 1D silicon nanostructures by modulating the reaction conditions of the all-wet MACE process.This work provides a new avenue to control the morphologies of silicon nanostructures for property investigations and wide applications.