Fabricating and Controlling Silicon Zigzag Nanowires by Diffusion

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Letter pubs.acs.org/NanoLett

Fabricating and Controlling Silicon Zigzag Nanowires by DiffusionControlled Metal-Assisted Chemical Etching Method Yun Chen,†,‡ Cheng Zhang,‡,§ Liyi Li,‡ Chia-Chi Tuan,‡ Fan Wu,‡ Xin Chen,*,† Jian Gao,† Yong Ding,‡ and Ching-Ping Wong*,‡,∥ †

School of Electromechanical Engineering and Key Laboratory of Mechanical Equipment Manufacturing and Control Technology of Ministry of Education, Guangdong University of Technology, Guangzhou, 510006, China ‡ School of Materials Science and Engineering, Georgia Institute of Technology, 711 Ferst Drive, Atlanta, Georgia 30332, United States § School of Materials Science and Engineering, Southeast University, Nanjing, 211189, China ∥ School of Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong S Supporting Information *

ABSTRACT: Silicon (Si) zigzag nanowires (NWs) have a great potential in many applications because of its high surface/volume ratio. However, fabricating Si zigzag NWs has been challenging. In this work, a diffusion-controlled metal-assisted chemical etching method is developed to fabricate Si zigzag NWs. By tailoring the composition of etchant to change its diffusivity, etching direction, and etching time, various zigzag NWs can be easily fabricated. In addition, it is also found that a critical length of NW (>1 μm) is needed to form zigzag nanowires. Also, the amplitude of zigzag increases as the location approaches the center of the substrate and the length of zigzag nanowire increases. It is also demonstrated that such zigzag NWs can help the silicon substrate for selfcleaning and antireflection. This method may provide a feasible and economical way to fabricate zigzag NWs and novel structures for broad applications. KEYWORDS: Silicon zigzag nanowires, metal-assisted chemical etching, diffusion-controlled method

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is the reacting catalyst in the previous VLS process, into the surface of NWs can be finely tuned. Then, the sidewall of NWs can be selectively etched in KOH solution under the protection of gold.17,18 However, the VLS methods require expensive equipment and high temperature.19 Metal-assisted chemical etching (MACE) that is a recently developed fabricating method has the ability to fabricate heterostructures.19−23 As in MACE, etching preferentially maintains the ⟨100⟩-oriented direction on both Si(110) and (111) oriented samples because Si atoms to be removed are the least on (100) plane. Therefore, by changing the amount of carriers (h+) injected and consumed, the etching direction and morphology may be regulated.24,25 By increasing etching temperature, the zigzag nanowires with a constant angle of 90°, 125°, or 150° can be formed on (111)-oriented silicon samples.26 Similarly, by increasing the concentration of oxidant, zigzag nanowires can be fabricated on both Si(111) and Si(110) samples with a constant angle of 115° or 90° between consecutive Si segments.27 However, these methods are limited

ilicon (Si) zigzag nanowires (NWs) that have large surface/ volume ratio can be broadly used in devices including photovoltaic solar cell,1,2 supercapacitor,3 microfluidics, sensors,4 and so forth.5−9 However, controlling the geometry in NWs has always been not easy. Many research efforts have been devoted to tuning the sidewall geometry of NWs, and a few novel methods are developed that mainly consist of vapor−liquid−solid (VLS) method and wet etching method.10 As in the classic VLS process, the nanowire growth has a tight relationship with the droplet, thus, by controlling the droplet geometry, position, or so forth, one can tune the geometry of nanowire.11 Therefore, by iteratively controlling the growth conditions, that is, composition of the reactant, the ratio of growth time to purge duration, and pressure over the nucleation and growth of nanowires, zigzag NWs were fabricated.12,13 In addition, by taking advantage of the Plateau-Rayleigh instability of fluid, periodic shells can be added on the one-dimensional cores.14,15 Similarly, by externally applying an electric field to affect the droplet geometry, position, and the droplet−nanowire contact angle, the diameter and growth direction of the nanowire can be controlled.16 Further, a two-step method consisted of VLS method and postwet etching also has been developed. Through carefully tuning the pressure, the diffusion depth of gold, which © XXXX American Chemical Society

Received: March 29, 2017 Revised: May 24, 2017

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DOI: 10.1021/acs.nanolett.7b01320 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters

theory30 (Figure 1a). The diffusion flux near the edge of the sample was the fastest, combining with the fact that the replenishing rate was larger than the reaction rate, so the ratio of [HF] to [H2O2] was approximately constant. Under this condition, only straight or slightly curved NWs were formed (2.24 μm, standard deviation σ = 0.0967 μm, Figure 1b and Figure S2). The average length was used to represent the length of each segment; details of each measurement are shown in Supporting Information. As the diffusion flux became slower and slower when approaching the middle of the sample, the etchant could not be replenished in time. It was under the dynamic diffusion-reaction process and, thus, zigzag NWs were formed (Figure 1c−e). In addition, as the difficulty of diffusion flux increased along the lateral direction, the period for each diffusion-reaction cycle became longer, thus, the amplitude of zigzag increased as the location approached the center of the sample, that is, screwed NWs (37.8 nm, σ = 0.027 nm, Figure 1c and Figure S3), beadon-string NWs (51.3 nm, σ = 0.037 nm, Figure 1d and Figure S4), sawtooth NWs with long arm length (112.3 nm, σ = 0.0066 nm, Figure 1e and Figure S5). NWs Fabricated in Different Etching Time. From the above results, it can also be noticed that there is always a straight segment at the top of each zigzag NW. This may because of the fast replenishment at the first when the diffusion channel composed by NWs was short. Thus, it can be inferred that there should be a critical diffusion distance to fabricate zigzag NWs by using the proposed diffusion-controlled metalassisted chemical etching method. To produce channels with different length, the etching time was varied as 3, 5, 8, and 10 min. All the etchants were the same, consisting of 10 mL of HF, 2 mL of H2O2, 5 mL of glycerol, and 15 mL of DI water. When the etching time was less than 3 min, only straight or slightly curved NWs of 0.98 μm (σ = 0.037 μm, Figure 2a and Figure S6) were formed. As the etching went on, the length of NWs increased and so was the diffusion channel composed by the NWs. Thus, the diffusion flux gradually decreased and when it finally became smaller than the consuming rate, zigzag NWs began to form. When the etching time increased to 5 min, zigzag segments were formed at the bottom of NWs. The length of the straight segment and

to fabricate zigzag NWs on wafers with orient direction other than (100). Multisteps MACE methods were also developed to fabricate zigzag nanowires with periodical kinks. For example, after forming a thin layer of porous silicon oxide on the surface of straight nanowires by an etchant with a high ratio of [H2O2]/ [HF] ([H2O2] and [HF] denote the concentration of H2O2 and HF, respectively, the same notation used in the following), another etchant with a low ratio of [H2O2]/[HF] was then used to remove this layer of porous silicon oxide, thus, zigzag nanowire can be fabricated.28 In our previous study, kinks NWs can also be fabricated by alternating etching in the etchant with carefully selected composition, and the number of kinks, their locations, and their angles can be precisely controlled.29 However, these methods are technology complicated and time-consuming as etchant changing is needed. In this paper, we present a diffusion-controlled metal-assisted chemical etching method to fabricate zigzag NWs in (100) wafers. This may provide a new choice for various zigzag NWs and structures fabrication in broad applications. Nanowires in the Different Locations of the Sample. The samples (n-type Si, (100)-oriented) were immersed into the etchant that was prepared 3 h earlier (Figure 1a). The

Figure 1. Various NWs fabricated on the sample due to the different diffusion flux in the lateral direction. (a) Various diffusion flux in the sample in the lateral direction. The fastest diffusion flux was near the edge of the sample whereas the slowest one was in the center of the sample. (b) Straight NWs were formed near the edge of the sample. Also, zigzag NWs were formed in intermediate space of the sample, such as (c) screwed NWs, (d) bead-on-string NWs, (e) sawtooth NWs with long arm length. The etching time was 8 min for all the cases.

etchant was composed of 10 mL of HF, 2 mL of H2O2, 5 mL of glycerol, and 15 mL of deionized (DI) water. During the etching process, no stirring was conducted; otherwise, only curved NWs were fabricated (Supporting Information Figure S1). For each silicon sample, the diffusion flux of etchant is different in the lateral direction according to the fluid dynamic

Figure 2. Various NWs formed in the same etchant with different etching time. (a) Curved NWs formed in 3 min. Zigzag NWs form in (b) 5, (c) 8, and (d) 10 min. B

DOI: 10.1021/acs.nanolett.7b01320 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters total NWs were 1.03 μm (σ = 0.025 μm) and 2.33 μm (σ = 0.042 μm), respectively (Figure 2b and Figure S7). Zigzag segments also were formed in the NWs when the etching time continued increased to 8 and 10 min (Figure 2c and Figures S8d and S9). In addition, the length of the straight segment was about 1.03 μm (σ = 0.017 μm) and 1.71 μm (σ = 0.035 μm). It can be concluded that to form zigzag NWs the critical length should be longer than 1 μm. Interestingly, when the length of zigzag segment exceeds 2 μm (2.40 μm, σ = 0.0496 μm in Figure 2c and Figure S8, and 2.49 μm, σ = 0.043 μm in Figure 2d and Figure S9), the diffusion flux seems to be further reduced and the etching time along each direction increased, thus resulting in the larger amplitude of the zigzag. NWs Fabricated with Different Glycerol Concentration. The more glycerol that was added to the etchant, the slower the diffusivity would be. To further study the effects of glycerol concentration on the zigzag NWs fabrication and critical straight segment length, the volume of glycerol was varied. The volume of H2O2 (2 mL) and HF (10 mL) was kept constant. The volume of DI water was correspondingly decreased to keep the total volume of the etchant as 32 mL. When adding only 2.5 mL of glycerol, no zigzag NWs were formed even though the length of the NW was as long as 4.91 μm (σ = 0.025 μm, Figure 3a and Figure S10). However, zigzag

zigzag NWs were formed only when the length of diffusion channel was larger than the critical length (1.60 μm in this case, σ = 0.052 μm). However, the amplitude of the zigzag was reduced rather than increased. It can be inferred that as the glycerol can affect the diffusivity notably, and a narrow window of glycerol volume exists to produce zigzag NWs. Zigzag NWs Forming Mechanism. From the above results, the typical zigzag NWs forming mechanism fabricated by this kind of method can be summarized as follows. The etching process is a multistep reaction. First, holes are formed at the cathode either by the reduction of H2O2 (eq 1.1) or H+ (eq 1.2); then the surface silicon is oxidized by the holes (only silicon tetrafluoride is formed when H2O2 is not sufficient, as shown in eq 1.3; silica is also formed when there is enough H2O2, as shown in eq 1.4); afterward, silicon tetrafluoride and silicon oxidation are rapidly dissolved by HF (as shown in eq 1.5 and 1.6, respectively). The net reaction can be written as eq 1.7 and 1.8.21,22,25,31 Holes formation in cathode H 2O2 + 2H+ → 2H 2O + 2h+

(1.1)

2H+ → H 2 ↑ +2h+

(1.2)

Silicon oxidation formation in anodic Si + 4HF + 4h+ → SiF4 + 4H+

(1.3)

Si + 2H 2O + 4h+ → SiO2 + 4H+

(1.4)

Dissolving silicon oxidation by HF in anodic SiF4 + 2HF → H 2SiF6

(1.5)

SiO2 + 6HF → H 2SiF6 + 2H 2O

(1.6)

Overall reaction Si + H 2O2 + 6HF → H 2SiF6 + 2H 2O + H 2↑

(1.7)

Si + 2H 2O2 + 6HF → H 2SiF6 + 4H 2O

(1.8)

Typically, silicon etching by MACE method in HF−H2O2based mixtures is a mixed diffusion- and kinetic- controlled reaction.32 The diffusion coefficients of both HF and H2O2 in water is about 1.5−1.7 × 10−9 m2 s−1.33,34 However, by adding glycerol in the etchant, the diffusion of H2O2 and HF in the etchant can be distinctly reduced, and the maximum reduction can be larger than 30 000 times.35,36 In addition, there is no stirring during the etching process; therefore, the etchant is replenished by diffusion by advection flux and the reaction shifts from mixed controlled to diffusion controlled.32,37,38 This diffusion controlled etching process can be described by coupled partial differential equations (PDEs) as follows39,40

Figure 3. Various NWs formed in the etchant with different volume of glycerol. (a) Straight NWs formed in the etchant with 2.5 mL of glycerol in 6 min. Zigzag NWs formed in the etchant with (b) 7.5 mL of glycerol in 12 min, (c) 10 mL of glycerol in 12 min, and (d) 20 mL of glycerol in 60 min.

NWs were formed when the volume of glycerol increased to 5 mL (Figure 2c and Figure S5), 7.5 mL (Figure 3b and Figure S11), and 10 mL (Figure 3c and Figure S12). Both the length of the straight segment (1.03 μm (σ = 0.017 μm), 1.52 μm (σ = 0.069 μm), and 1.58 μm (σ = 0.041 μm)) and the amplitude of the zigzag (112.3 nm (σ = 0.0066 nm), 126.9 nm (σ = 0.006 nm) and 201.5 nm (σ = 0.012 nm)) increased as the volume of glycerol increased. It is curious whether the trend is still valid if the volume of glycerol keeps increasing. Therefore, 20 mL of glycerol was purposely added to the etchant and no DI water was used. As the viscosity of etchant was quite large, the diffusivity became extremely slow, thus, the etching rate was significantly reduced. About 60 min was necessary to fabricate NWs of 2.65 μm (σ = 0.030 μm, Figure 3d and Figure S13). As the trend predicted,

∂u1 ∂ 2u = D1 21 + f1 (u1 , u 2) ∂t ∂z

(1.9)

∂u 2 ∂ 2u = D2 22 + f2 (u1 , u 2) ∂t ∂z

(1.10)

where t and z are etching time and diffusion distance, respectively; u1 and u2 are the concentrations of H2O2 and HF, respectively; D1 and D2 are the diffusion coefficients of H2O2 and HF, respectively; and f1(u1,u2) and f 2(u1,u2) are the reaction terms for H2O2 and HF, respectively. The specific forms of the reaction terms are in the first order32,37,38 C

DOI: 10.1021/acs.nanolett.7b01320 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters f1 (u1 , u 2) = −k1u1u 2 − 2k 2u1u 2

(1.11)

f2 (u1 , u 2) = −6k1u1u 2 − 6k 2u1u 2

(1.12)

where k1 and k2 are the reacting coefficients of reaction 1.7 and 1.8, respectively. In particular, when H2O2 is not sufficient (in this work, the concentration ratio of H2O2 to HF is assumed to be less than 0.1 and the absolute concentration H2O2 is larger than 0.1 times of its original concentration), only reaction 1.7 takes places, therefore, k2 equals to zero. The PDEs are numerically solved by using Matlab with the values of each parameter listed in Table 1.32,33,37,38 As the Table 1. Simulation Parameters parameters

value

etching time t initial diffusion length z initial and boundary concentration of H2O2 initial and boundary concentration of HF diffusivity of H2O2 in water D1 diffusivity of HF in water D2 reacting rate of reaction (1.7) k1 reacting rate of reaction (1.8) k2

600 s 1 × 10−3 mm 0.6 mol/mL 9.1 mol/mL 1.7 × 10−9 m2/s 1.68 × 10−9 m2/s 9.35 × 10−4 s−1 46 × 10−4 s−1

etching rate has a direct relationship with the reaction rate of H2O2 or HF, the reaction rate of H2O2 is used to represent the etching rate. It can be seen that when the diffusivity was relative large and the replenishment was very fast, thus, the concentration of H2O2 and HF was almost constant and there was little variation of reaction rate of H2O2 (Figure 4a); however, when the diffusivity was reduced by 100 times, the etchant could not be replenished in time, so there was a slight variation of reaction rate of H2O2 (Figure 4b). As the diffusivity continued to be reduced by 1000 times, there were notable oscillations of reaction rate of H2O2 (Figure 4c), a larger oscillations can also be noticed when the diffusivity was reduced by 10 000 times (Figure 4d). The oscillations between the depletion and diffusion result in oscillating etching rate and directions, and may also oscillations between the two competing reaction pathways (either simultaneous reactions of 1.7 and 1.8 or sole reaction 1.7 depending on the etchant concentration), thus forming zigzag structures.31 It can also be seen that the oscillation period was gradually increased with the time, thus, it may explain why the arm length of zigzag nanowire increased along the wire length. By alternating etching the samples under stirring (60 r/min) and unstirring conditions in etchant with 5 mL of glycerol, nanowires that are alternating between linear (1.365 ± 0.45 μm) and zigzag (0.675 ± 0.065 μm) segments were fabricated (Figure 4e and Figure S14). The corresponding etching time for each stirring and unstirring etching was 2 and 3 min, respectively. Other parameters were not changed. As there is always a small linear segment formed before zigzag segment developing, the length of the first and second segment was larger than that of the third one. All of these is consistent with the simulation results, thus demonstrating the correctness of the proposed mechanism. Therefore, when immersing silicon sample in the etchant at first, as both H2O2 and HF, was sufficient, there was little variation of consuming rate of etchant (demonstrated by the large diffusion case in Figure 4a) and straight segments along [100] were fabricated because the number of Si back-bonds

Figure 4. (a) Simulation results with D1 = 1.7 × 10−9 m2/s and D2 = 1.68 × 10−9 m2/s, the reaction rate of H2O2 is almost constant. (b) Simulation results with D1 = 1.7 × 10−11 m2/s and D2 = 1.68 × 10−11 m2/s, there was a slight variation of reaction rate of H2O2. (c) Simulation results with D1 = 1.7 × 10−12 m2/s and D2 = 1.68 × 10−12 m2/s, there was notable variation of reaction rate of H2O2. (d) Simulation results with D1 = 1.7 × 10−13 m2/s and D2 = 1.68 × 10−13 m2/s; there was large amplitude variation of reaction rate of H2O2. The oscillation period was gradually increased with the time; other parameters were kept constant and listed in Table 1. (e) Nanowires that have alternating linear and zigzag segments fabricated by alternating etching under stirring and unstirring conditions.

that have to be broken is the lowest in this direction (Figure 5a,b). However, after etching for a certain time (typically when the length of NWs > 1 μm), the etchant was consumed more and also slowly was replenished by diffusion at the same time. However, the complement rate was much smaller than the consuming rate, therefore, the etchant concentration at the reacting site changed; also, the reaction was kinetic and diffusion mixed controlled and the etching direction changed from [100] to [120] (Figure 5c). Afterward, when the etchant was further consumed and the diffusion channel also increased, the entrance of diffusion flux became more difficult and the reaction shifted to be diffusion limited. A dynamic oscillation was formed between the diffusion and consumption, so the etching rate was oscillating, and resulted in periodical zigzag NWs (etching direction changes from [110] to [1̅10], Figure 5d,e). As the length of the zigzag NWs increased (typically >2 μm), so did the length of diffusion channels; as the geometric surface of the diffusion channel became complicated, the replenish rate further reduced (demonstrated by the very small diffusion case in Figure 4c or 4d) and thus the replenishment time became D

DOI: 10.1021/acs.nanolett.7b01320 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 5. Crystallographic structure of zigzag silicon NWs after diffusion controlled MACE. (a) Bright-field TEM image of zigzag silicon NWs. There are multiple kinks with different arm length in each NW, labeled as zone I, II, III, and IV. (b) Selected area electron diffraction (SAED) patterns recorded zone I. The SAED patterns were recorded along the [001] zone axis. Lattice-resolved TEM images from (c) Zone I, (d) Zone II, (e) Zone III, and (f) Zone IV. Arrows denote etching directions. Scale bars, 5 nm.

high antireflection, and it can have broad potentials for solar applications.44−46 Conclusion. In summary, a diffusion-controlled metalassisted chemical etching method was developed to fabricate zigzag NWs, and its mechanism was discussed. It is found that such manufacturing process is a typical dynamic diffusionreaction process, and the viscosity of etchant plays a significant role in zigzag NWs fabrication through affecting the diffusion rate further, etching direction, and etching time. In addition, there is a critical length that should be longer than 1 μm to form a zigzag NW. The amplitude of zigzag increases as the location approached the center of the sample or the length of zigzag NW increased. Thus, various zigzag NWs can be fabricated by carefully tailoring the composition of each etchant and etching time. It is also found that such zigzag NWs can turn the silicon substrate into superhydrophobic and high antireflection. This method may provide a feasible and economical way to fabricate zigzag NWs and novel structures for many applications. Methods. Silicon wafers used in the experiment are n-type single-crystalline Si wafers ((100)-oriented, boron doped, resistivity: 1−10 Ω cm) purchased from University Wafer, MA, U.S.A.. The polystyrene (PS) microspheres were purchased from Polysciences Inc. (Warrington, PA). The microspheres (500 nm in diameter) were received in 2.5% (w/ v) aqueous solution. All the other chemicals were purchased from VWR International LLC and used without further processing. Si wafers were first cleaned with a piranha solution (H2SO4 (96 wt %) and H2O2 (30 wt %) with a volumetric ratio of 1:1) at 120 °C for 10 min to remove any oxides. This was followed by rinsing in deionized (DI) water and drying in flowing N2.

longer which means the time etching along one direction became longer. Therefore, zigzag NWs with long arm length were fabricated with etching direction changed (Figure 5a, zone IV, and Figure 5f). Wetting Properties and Reflectivity of Zigzag NWs. The contact angle of the zigzag NWs fabricated by the diffusion-controlled metal-assisted chemical etching method, and the comparisons with those of bare silicon and straight NWs. The contact angle of bare silicon was 54.6°, however, after fabricating NWs on the surface of silicon, the contact angle dramatically increased. As there is a straight segment on the top of each zigzag nanowire, there is little difference between the contact angles of straight NWs and zigzag NWs (158° and 158.3° for straight NWs and zigzag NWs, respectively, Figure S15a). It means the silicon substrate became superhydrophobic after fabricating NWs on it, and such zigzag NWs may help for self-cleaning.41−43 Typically, bare silicon has high reflectivity, especially in the ultraviolet (UV) range. Finite-difference time-domain (FDTD) simulation results find that the zigzag NWs can trap more photon resulting in higher photon absorption that straight NWs in the UV range (Figure S15b). Therefore, the reflectivity of the silicon substrate with or without NWs were experimentally measured and compared (Figure S15c). The reflectivity of the substrate with NWs was significantly reduced. In particular, the reflectivity of the substrate with zigzag NWs (∼6%) was lower than that of substrate with straight NWs (8%) in the UV range (wavelength