Centimeter-Long Single-Crystalline Si Nanowires - Nano Letters (ACS

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Centimeter-Long Single-Crystalline Si Nanowires Bing-Chang Zhang,†,‡ Hui Wang,*,† Le He,‡ Cai-Jun Zheng,†,∥ Jian-Sheng Jie,‡ Yeshayahu Lifshitz,‡,§ Shuit-Tong Lee,*,‡ and Xiao-Hong Zhang*,†,‡ †

Key Laboratory of Photochemical Conversion and Optoelectronic Materials and CityU-CAS Joint Laboratory of Functional Materials and Devices, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China ‡ Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, Jiangsu, People’s Republic of China § Department of Materials Science and Engineering, Technion, Israel Institute of Technology, Haifa 3200003, Israel ∥ School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, People’s Republic of China S Supporting Information *

ABSTRACT: The elongation of free-standing one-dimensional (1D) functional nanostructures into lengths above the millimeter range has brought new practical applications as they combine the remarkable properties of nanostructured materials with macroscopic lengths. However, it remains a big challenge to prepare 1D silicon nanostructures, one of the most important 1D nanostructures, with lengths above the millimeter range. Here we report the unprecedented preparation of ultralong single-crystalline Si nanowires with length up to 2 cm, which can function as the smallest active material to facilitate the miniaturization of macroscopic devices. These ultralong Si nanowires with augmented flexibility are of emerging interest for flexible electronics. We also demonstrate the first single-nanowire-based wearable joint motion sensor with superior performance to reported systems, which just represents one example of novel devices that can be built from these nanowires. The preparation of ultralong Si nanowires will stimulate the fabrication and miniaturization of electric, optical, medical, and mechanical devices to impact the semiconductor industry and our daily life in the near future. KEYWORDS: Ultralong nanostructures, silicon nanowires, VLS, strain sensors, flexible electronics

T

growth is catalyzed by a leading droplet.34−39 In conventional VLS growth, Si nanowires were usually synthesized under wellcontrolled conditions including constant temperatures and pressures to ensure the quasi-static growth with a one-way material flow from vapor to liquid and then solid phases.30 The quasi-static growth conditions are beneficial to controlling the nanowire morphology.40,41 But the growth rate is usually limited (e.g., 1−2 μm/min), which makes it difficult to produce long Si nanowires owing to the difficulty in maintaining constant growth conditions and keeping the catalyst droplets active for a long period of time (e.g., 80 h for growing 1 cm long nanowires).22 By using disilane (Si2H6) as the gas-phase reactant, Lieber et al. achieved an outstanding growth rate of 31 μm/min.22 With such a high growth rate, they were able to obtain millimeters-long Si nanowires with uniform diameters and electronic properties within the growth period of 1 h. The preparation of millimeters-long nanowires opened new opportunities for integrated electronics and these nanowires

he fabrication of free-standing one-dimensional (1D) functional nanostructures with lengths above the millimeter range, as exampled by carbon nanotubes and several semiconductor materials, has brought new practical applications as they combine the remarkable properties of nanostructured materials with macroscopic lengths.1−9 For example, they could be spun into lightweight high-strength fibers for applications in portable solar cells, sensors, and artificial muscles.10−19 The integration of multiple devices onto an individual centimeterlong nanowire offers a convenient route to the fabrication of large-scale ordered arrays of nanoelectronic devices.20−22 Such macroscopic lengths also allow direct visualization, location, and manipulation of individual 1D nanostructures and thereby facilitate device fabrication.23 Moreover, individual 1D ultralong nanostructure could function as the smallest active material to significantly cut down the material cost, reduce the lateral size and improve the performance of diverse macroscopic devices.24 To this end, it is of emerging interest to extend the family of 1D ultralong nanostructures to more functional materials. As one of the most important 1D nanostructures, silicon nanowires have attracted extensive attention.25−33 Vapor− liquid−solid (VLS) growth is one of the most widely applied methods for the preparation of 1D nanostructures, in which the © XXXX American Chemical Society

Received: July 12, 2017 Revised: November 3, 2017 Published: November 29, 2017 A

DOI: 10.1021/acs.nanolett.7b02967 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic illustration of the temperature-gradient-assisted growth of Si nanowires. (b) Schematic illustration of one SCO cycle which includes the following three stages: (1) Evaporation of the leading droplet, which is the slowest step, along with the expansion of the liquid−solid interface; (2) rapid adsorption of the source vapor into the leading droplet along with the increase of the droplet size; and (3) fast precipitation of materials from the droplet to initiate the longitudinal growth.

Figure 2. Synthesis and characterization of ultralong single-crystalline Si nanowires. (a) Schematic of the experimental setup inside the furnace tube. (b) Length statistics of 30 ultralong silicon nanowires. (c) A typical HRTEM image of ultralong Si nanowires after removing the oxide shell. (d) Plot of diameter versus positions along the length of a single HF-treated Si nanowire.

could serve as unique building blocks linking integrated structures from the nanometer through millimeter length scale. Their work represents a milestone in the synthesis of ultralong Si nanowires. Nevertheless, it remains a big challenge to readily prepare silicon nanowires in the centimeter range. Herein we develop a temperature-gradient-assisted nonequilibrium growth route and achieve the unprecedented preparation of single-crystalline Si nanowires with length up to 2 cm by addressing two key issues: “how to increase the growth rate” and “how to keep the growth active as long as possible”. We also demonstrate the use of centimeter-long nanowires for the fabrication of single-nanowire-based wearable joint motion sensors without the need of complicated assembling processes.

The VLS growth of Si nanowires includes three repeating stages according to the surface curvature oscillations (SCO) model in our previous theoretical work (Figure 1 and Discussion S1 in Supporting Information).37 The evaporation of the leading droplet is the slowest stage and limits the overall growth rate owing to the creation of new energetically unfavorable liquid−solid interfaces.37,42 Here we show that the introduction of a suitable temperature gradient could facilitate the evaporation of the leading droplet and, thereby, increase the growth rate of Si nanowires (Movie S1 of the Supporting Information). Meanwhile, the supply of an excess amount of catalysts in the growth area ensures a high catalyst concentration in the vapor phase to reduce the unwanted loss B

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Figure 3. TEM characterization of the steps. TEM images of the steps with sharp diameter changes of a centimeter-long nanowire. Inset in (c) shows the SAED pattern of the step indicated in (c).

Figure 4. Images of single ultralong Si nanowires. (a) Integrated optical microscope images along the length of a single 1.4 cm long Si nanowire. (b) Integrated optical microscope images displaying three patterns fabricated with single ultralong Si nanowires, respectively. (c) The bending radius of the ultralong Si nanowires.

of the catalyst in the droplet and keep the grow active for a long period of time. Experimentally, the centimeter-long Si nanowires were synthesized with SiO powder as the Si source and Sn bar as the catalyst using a tube furnace (detailed setup and temperature distribution are shown in Figure S1−S4 of the Supporting Information). After a growth period of 1 h, about 75 mg of yellowish product was obtained from the two sides of the alumina boat (Figure 2a and Figure S2 of the Supporting Information). The average length of 30 products is 1.45 ± 0.06 cm with the length distribution shown in Figure 2b. The growth rate could reach 330 μm/min with the help of the

temperature gradient (Figure S5 of the Supporting Information), which is 1 order of magnitude higher than the fastest rate previously reported (i.e., 31 μm/min).22 As shown in Figure S2 of the Supporting Information, the growth starting points of the ultralong Si nanowires began to appear at the region with the temperature of 1148 ± 1 °C, while the end points were 2 cm away at the 1211 ± 1 °C region. The temperature gradient was found to be 31.5 °C cm−1 (Figure S3 of the Supporting Information). The weight yield of ultralong Si nanowires is 66% of the products obtained in the temperature region of 1148− 1211 °C and 2.5% of the whole SiO source. C

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Figure 5. Single-nanowire wearable strain sensor for human joint-motion detection. (a) The response of the current to the finger joint motion from a single Si nanowire wearable strain sensor. (b) Optical photograph of the wearable strain sensor. (c) Plot of the relative change in resistance versus strain ε. (d) Response of the single Si nanowire wearable strain sensor under different small tensile strains. Error bars represent standard deviation. (e) Response of the single Si nanowire wearable strain sensor under stretch/release cycles of 0.35% strain.

The X-ray diffraction (XRD) pattern of the as-prepared product (Figure S6 of the Supporting Information) reveals that the sample is dominated by a face-centered cubic Si phase (JCPDS 77-2107). Diffraction peaks corresponding to the body-centered crystalline structure of the Sn catalyst were also observed (JCPDS 02-0709). The morphology and structure of the as-prepared product were further studied with transmission electron microscopy (TEM), which indicates that the surface of the product is covered by a shell of silicon oxides (Figure S7 of the Supporting Information). After removing the silicon oxide shell with HF solution, selected-area electron diffraction (SAED) and high-resolution TEM (HRTEM) investigations show that the cores of the Si nanowires are single crystalline and they grow along the crystal orientation of ⟨110⟩ (Figure 2c and Figure S8 of the Supporting Information). In addition to the above common features of the VLS grown nanowires, the following two specific characteristics were also found: (1) some “steps” with local sharp increase of nanowire diameter could be observed along the growth direction (Figure 3) and (2) the overall diameter of the Si nanowire gradually decreases along the growth direction, leading to a cone shaped morphology. For example, a plot of the diameter at different positions along the length from a single Si nanowire shows a nearly linear diameter change from 900 to 500 nm along its 1.6 cm length (Figure 2d). These characteristics are the consequence of the SCO-regulated growth toward higher temperature (Figure 1). On one hand, when the droplet evaporates (Figure 1, process 1) and the coffee-ring effect

occurs, the longitudinal growth is very slow, while the diameter increases, resulting in the steps.37,42 It is worth noting that the steps exhibit a continuous lattice structure and no apparent plane defects or orientation changes can be observed in the SAED and HRTEM images (Figure 3c and Figure S9 of the Supporting Information). On the other hand, as the nanowire grows toward the high temperature area, slow escape of the catalyst into the gas phase would occur, causing the gradual decrease in the size of the leading droplet at the corresponding stage of every cycle and thereby the diameter of the nanowires. This is evidenced by the differences in size and composition of catalyst droplets depending on the nanowire lengths. As shown in Figure S10 and S11, the catalyst droplet with the diameter of 1.6 μm and 35.55% atomic ratio of Sn was observed in Si nanowires of hundreds of micrometers long, while the catalyst diameter decreased to 200 nm and the atomic concentration of Sn dropped to 1.64% for centimeter-long Si nanowires. The nanowire diameters at the base are similar for Si nanowires with different lengths and the rates of diameter change along the length are consistent without observed radial growth. As shown in Figure S7, a thick SiOx layer was present around the crystalline Si core, which was produced through the disproportionation reaction of SiO. The existence of the SiOx layer might prevent further radial growth. It is worth noting that the diameter variation of these Si nanowires could be noticed only because the product has such an ultralong feature. Indeed, the average rate of change is 1.8 nm in diameter per 100 μm in length, which is so small that might be neglected in D

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The GF values were quite consistent for other single-nanowire sensors (Table S1 of the Supporting Information). Moreover, Figure 5d shows that the present sensor can clearly distinguish different tensile strain under 0.1% strain (0.043%, 0.056%, 0.067%, and 0.080%), indicating that the strain resolution limit of the single Si nanowire strain sensor could be very low (0.01%). It is worth noting that the performance of the single nanowire sensor has negligible dependence on the diameter of Si nanowires because the diameter has ignorable influence on both the strain of nanowires and the piezoresistance effect of single-crystal silicon materials in our device design (Discussion S2 of the Supporting Information). The performance of the wearable finger motion sensor made from a single centimeter-long Si nanowire is compared to those of reported piezoresistance or capacitive devices made from popular materials such as carbon nanotubes, graphene, and Ag nanowires (Table S2 of the Supporting Information). The present sensor based on a single ultralong Si nanowire demonstrates one of the best performances reported with a gauge factor of 52 and a strain resolution of 0.01%, while other reported body motion sensors usually have GF smaller than 30 and a strain resolution worse than 0.1%.44−54 In addition, though some strain sensors have high GF (e.g., graphite-based sensors have the highest GF of 536.6), their GF ranges from 60.7 to 536.6 according to different strains.55 More importantly, the use of a single nanowire as the active material could significantly cut down the material cost, reduce the lateral size, maintain/improve the performance of devices, and ultimately facilitate device miniaturization. Also note that the present single Si nanowire wearable strain sensor shows a similar magnitude of GF to traditional rigid Si wafer-based strain sensors.56,57 Besides the single-nanowire wearable sensor, the electrical property of centimeter-long Si nanowires could also be studied by integrating multiple four-probe devices on a single Si nanowire (Figure S16). It was found that the electrical conductivity is different at different positions along the length of the single Si nanowire. The electrical conductivity approximately follows a linear relationship with the reciprocal of the diameter (1/d), which is caused by the change in the surface-to-volume ratio.58 Compared with bulk silicon materials and previously reported millimeter-long Si nanowires, the as-prepared centimeter-long nanowires possess unique properties for opening up new promising applications in various areas. For example, centimeter-long Si nanowires exhibiting improved flexibility are compatible with flexible electronics and can find applications in flexible batteries, photodetectors, and sensors, whereas the use of conventional bulk silicon materials in these areas is hampered by their rigidity. A single centimeter-long nanowire can replace the Si-based active materials widely used in macroscopic sensing devices where active materials with lengths in centimeter range are usually required (e.g., human joint movement sensors) to significantly cut down the material cost, reduce the lateral size, maintain/improve the performance of devices, and ultimately facilitate device miniaturization, while the use of millimeter-long nanowires in these areas is limited by their length. Moreover, the centimeter-long nanowires with the capability of direct visualization, location, and manipulation could achieve macroscopic devices without the need of complicated assembling process. In conclusion, we report the preparation of centimeter-long Si nanowires through a temperature-gradient-assisted VLS

ordinary nanostructures with the length of hundreds of micrometers. Moreover, when the growing nanostructure reaches the highest temperature area, the evaporation might become so fast that the Si nanowire growth stops and heads of periodic nanostructures might be formed with two characteristic geometrical relationships of the SCO model (Figure S12 of the Supporting Information).37 Figure 4a shows the integrated optical microscope images of a single as-prepared Si nanowire with the length of 1.4 cm. With such a length, a single Si nanowire can be observed by naked eyes under controlled light intensity (Figure S13 of the Supporting Information). Besides, the Si nanowires are so long and flexible that a single Si nanowire can be conveniently drawn out from the bundle of the nanowires and shaped into different forms using tweezers (Figure 4b). The ease of observation and manipulation suggests that single centimeter-long Si nanowire may be conveniently patterned into devices. Moreover, the bending radius of these nanowires could be very small (e.g., ∼30 μm for a nanowire with the diameter of 414 nm, Figure 4c), indicating their suitability for flexible and wearable applications. The products exhibit similar flexibility to reported Si nanowires and their overall diameter are still in nanometer scale so that we call them Si nanowires.32 We now demonstrate the use of centimeter-long Si nanowires as wearable sensors (Figure 5). A single 1.2 cmlong Si nanowire is conveniently fabricated into a wearable finger joint motion sensor by sticking it onto a common laboratory rubber glove with Ag electrodes attaching at each end and leaving a space of 1.07 cm between the two electrodes (Figure 5b and Figure S14 of the Supporting Information). The single nanowire sensor responds to the finger joint motion (Figure 5a and Movie S2 of the Supporting Information) with good flexibility, high sensitivity (Figure 5c and 5d) and excellent reproducibility (Figure 5e). Moreover, the multiplexed detection of strain distribution could be realized by fabricating a multiple-electrode array on a single Si nanowire (Figure S15). The sensitivity of the single Si nanowire sensor was quantitatively characterized with the widely used parameter, the gauge factor (GF),43 which is defined as the ratio of the relative change in electrical resistance (R) to the mechanical strain ε, as shown in eq 1, where I is the current of the strain sensor under a given bias, and I0 is the current at zero strain. The GF of the single Si nanowire sensor was measured using a setup shown in the inset of Figure 5c. The sensor was attached to cylinders with different radii (r) and the strain of the Si nanowire could be estimated by eq 2, in which d is the thickness of the Si nanowire-attached object (Discussion S2 of the Supporting Information). GF =

ε=

ΔR R0

ε

=

d 2r + d

I −I ΔR = 0 R0 I

I0 − I Iε

(1)

(2)

(3)

A plot of the relative change in resistance (eq 3) versus strain (ε) is shown in Figure 5c and the slope corresponds to the GF, which is −52 for compression and −32 for tension (negative GF implies that the resistance decreases under tensile strain). E

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method. The key of our strategy is to increase the growth rate and keep the growth active as long as possible. The growth of Si nanowires is accelerated under obvious nonequilibrium growth conditions by introducing a temperature gradient. The growth is kept active for a long period of time through the supply of an excess amount of catalysts to reduce their unwanted loss in the leading droplet. We also demonstrate the use of centimeterlong nanowires for the fabrication of single-nanowire-based wearable joint motion sensors without the need of complicated assembling processes. These sensors exhibited superior performance to the reported systems, which just represents one example of miniaturized devices with the least amount of active materials building from these nanowires. As a new type of soft Si materials with macroscopic lengths, centimeter-long nanowires are of great interest for the emerging flexible electronics, chemical and biological sensing, micro- and nanoelectromechanics, and artificial intelligence. It is expected that the growth strategy for centimeter-long Si nanowires could be extended to grow other ultralong semiconductor nanowires to stimulate the fabrication and miniaturization of electric, optical, medical, and mechanical devices, which may impact the semiconductor industry and our daily life in the near future.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b02967. Details on the experimental methods, product characterization, device characterization, a comparison against previously reported strain sensors, and additional discussion (PDF) The growth model of silicon nanowires without and with a temperature gradient (ZIP) Joint motion detection with a single Si nanowire wearable sensor (ZIP)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jian-Sheng Jie: 0000-0002-2230-4289 Shuit-Tong Lee: 0000-0003-1238-9802 Xiao-Hong Zhang: 0000-0002-6732-2499 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the Major Research Plan of the National Natural Science Foundation of China (Grant 91333208), the National Basic Research Program of China (973 Program) (Grant 2013CB933500), the National Natural Science Foundation of China (Grant 51373188), and Collaborative Innovation Center of Suzhou Nano Science and Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Project and Qing Lan Project. F

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