Flexible Transparent and Free-Standing Silicon Nanowires Paper

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Flexible Transparent and Free-Standing Silicon Nanowires Paper chunlei pang, hao cui, guowei yang, and Chengxin Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl402234r • Publication Date (Web): 28 Aug 2013 Downloaded from http://pubs.acs.org on August 29, 2013

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Flexible Transparent and Free-Standing Silicon Nanowires Paper Chunlei Pang1†, Hao Cui1,2†, Guowei Yang1 & Chengxin Wang1,2* 1

State key laboratory of optoelectronic materials and technologies, School of Physics Science and Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China 2 The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China Abstract

If the flexible transparent and free-standing paper-like materials that would be expected to meet emerging technological demands, such as components of transparent electrical batteries, flexible solar cells, bendable electronics, paper displays, wearable computers, and so on, could be achieved in silicon, it is no doubt that the traditional semiconductor materials would be rejuvenated. Bulk silicon cannot provide a solution, because it usually exhibits brittleness at below their melting point temperature due to high Peierls stress. Fortunately, when the silicon’s size goes down to nanoscale, it possesses the ultralarge straining ability which results in the possibility to design flexible transparent and self-standing silicon nanowires paper (FTS-SiNWsP). However, realization of the FTS-SiNWsP is still a challenging task due largely to the subtlety in the preparation of a unique interlocking alignment with free-catalyst controllable growth. Herein, we present a simple synthetic strategy by gas flow directed assembly of a unique interlocking alignment of the Si nanowires (SiNWs) to produce, for the first time, the FTS-SiNWsP which is consisted of interconnected SiNWs with the diameter of ~10 nm via simply free-catalyst thermal evaporation in a vertical high-frequency induction furnace. This approach opens up the possibility for creating various flexible transparent functional devices based on the FTS-SiNWsP.

KEYWORDS: Si nanowires, flexible, transparent, free-standing, paper.

†

These authors contributed equally to this work.* Correspondence and requests for materials should be addressed to C. X. Wang. Tel & Fax: +86-20-8411-3901, E-mail: [email protected]

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A material comprising a pliable base that is also transparent and free-standing would open up a world of application opportunities for components of electrical batteries or supercapacitors, electronic or optoelectronic components, and molecular storage. As a cornerstone of modern semiconductor industry, the traditional silicon electronics that are rigid, opaque, and about half a millimeter thick have been difficult to meet the device miniaturization and emerging technological demands. Based on a consideration of device miniaturization, silicon nanowires (SiNWs) have been of increasing interest in past the few years because of their unique physical properties, potential as the building blocks for the next generation electronic devices, as well as the controllable diameter, surface composition, and electronic properties during synthesis1-13. Undoubtedly, if the flexible transparent and free-standing paper-like material that is an integral part of our technological society and is expected to meet emerging technological demands such as components of transparent electrical batteries, roll-up displays, smart electronics, and wearable devices, and so on, could be achieved in Si, it would show more superiority than other inorganic semiconductor materials due to the advantage of being compatible with today’s integrated circuit technology for bulk Si10-13. This makes the preparation of flexible transparent paper-like silicon material becoming increasingly important. However, unlike metal, only when close to its melting temperature, bulk silicon becomes ductile due to the high Peierls stress4, 14, 15. That is to say, room-temperature-plasticity hardly happens for bulk Si. Fortunately, when the silicon’s size goes down to nanoscales, the defect-free structure usually makes the SiNWs in higher fracture stress to survive, and this could ultimately allow they to have the opportunity to overcome the critical resolved shear stresses and nucleate ductile dislocations or make these ductile featured dislocations mobile4, 15-18. 2


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Namely, it is possible to synthesize FTS-SiNWsP due to the ultralarge straining ability of Si on the nanometer scale. Here we report the preparation and characterization of flexible transparent paper, a free-standing Si-based membrane material made by gas flow directed assembly of individual SiNWs. This FTS-SiNWsP outperforms many other paper-like materials in transparency and flexibility. Its combination of macroscopic flexibility and transparency is a result of a unique interlocking alignment of the SiNWs. Furthermore, as an example of active flexible transparent and self-standing applications of FTS-SiNWsP, the FTS-SiNWsP@graphene compounds was prepared by microwave plasma chemical vapor deposition method, it can be assembled into a film which can be as a binder-free and flexible transparent electrode for lithium-ion batteries (LIBs) that does not require any current collectors. The FTS-SiNWsP@graphene electrodes exhibit outstanding lithium storage performance with high-storage capacities and good cycling stability. The synthesis of the FTS-SiNWsP was performed in a smart designed vertical high-frequency induction furnace as seen in Fig.1a which has been described previously19. The furnace can be heated to a high temperature in a very short time. It consists of a fused-quartz tube surrounded by the radio-frequency (RF) coil, an induction heated cylinder made of high-purity graphite, a carbon fiber thermoinsulating layer and a graphite crucible which was mounted inside. The carrier gas is blown into the furnace through the gas path under the graphite crucible and goes through the cavity of the graphite cylinder and then be pumped out through the hole above it. The starting material was SiO powder of 99.99% purity, which was loaded in the graphite tube. Before heating, the furnace was evacuated by a 3


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mechanical pump, and then a constant gas flow of high-purity Ar was introduced, serving as carrier gas at a flow rate of 200 sccm (standard cubic centimeter per minute). The pressure was maintained at 8 KPa throughout the whole experimental process. The graphite crucible was then heated to about 1600℃ and keep for 1 hour. The SiO powder evaporated from the graphite tube when heating begin. SiO is metastable and be decomposed into SiO2 and Si when heated as described in part

of

Fig.1b. Si and SiO2 vapor was transported to the low temperature zone of the graphite cylinder by the carrier gas. Because of the molecular weight of SiO2 is larger than Si, they would stratify under the gravity action when they were transported to the upper part of graphite cylinder as shown in part

of Fig.1b. When SiO2 and Si particles

were transported to the locations with appropriate temperatures, they would nucleation and growth. As shown in part

of Fig.1b, the SiO2 deposited on the upper

inner wall of the graphite cylinder and formed powder sample. The Si vapor can be carried to the orifice of the graphite cylinder where the Si nanowires begin to grow and lead to the formation of FTS-SiNWsP. We propose that the growth mechanism is silicon oxide assisted for there was no detectable metal catalyst or impurity formed on the tips of the nanowire nuclei20-23. The formation of the free-standing flexible Si-based membrane may be related to the flow of the carrier gas. The formation process of FTS-SiNWsP is shown in Fig.1c. First, a few SiO2 vapor reached to the orifice of the graphite cylinder, and deposited on the substrate formed a SiO2 matrix. Si vapor precipitated into the SiO2 matrix. Second, Si nanowire nuclei were formed and the nanowire further growth with continual supply of Si vapor carried by uniform 4


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Ar gas flow. Third, the strength of the flow of the carrier gas will force the nanowires to grow in the direction of the gas flow. While the nanowires growing, they will spontaneously interlock with each other. Finally, a free-standing Si-based membrane material was formed by gas flow directed assembly of individual SiNWs. The inset of Fig. 2a shows the optical image of the formed FTS-SiNWsP. The self-supported cylindrical reticulate structure is about 2 cm high and has a diameter of 2 cm. The self-supporting paper has good transparency and exhibited very good flexibility (see the video S1 in Supporting Information). The low magnification SEM image (Fig. 2a) shows the netted structure of the sample. The image indicates that the product is weaved by nanowires, forming a three-dimensional highly porous structure which can be filled with other functional materials to from novel structures for emerging applications. According to these two images above, it is interesting to notice that the long nanowires are interlaced spontaneously and are aligned in a direction roughly parallel to that of the gas flow. The Si nanowires networks have uniform gaps which lead to the good transparency of the Si nanowires paper. The structure of these nanowires was investigated by TEM. Representative TEM images (Fig. 2b) reveal that the surface of the nanowires is smooth and the nanowires are relatively homogeneous and have no catalysts. The diameter of these silicon nanowires is about 10 nm. High-resolution transmission electron microscopy (HR-TEM) confirms that these are crystal Si nanowires (as showed in the inset of Figure.1b) which consist of a crystalline core surrounded by a thin amorphous sheath. The image shows a group of lattice planes in the crystalline core, which corresponds to {111} with interplanar 5


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spacing of 0.31 nm. The selected-area electron diffraction (SAED) pattern of the nanowires, in which the first-, second-, and third-order rings correspond to {111}, {220}, and {311} lattice plane families of cubic structure, respectively, further confirming the formation of crystalline nanowires. To investigate the transmittance of the FTS-SiNWsP, we fabricated a set of FTS-SiNWsPs with different thickness through changing the experiment conditions. Fig. 3 shows the FTS-SiNWsPs with different thickness and optical transmittance spectra corresponding to them. The FTS-SiNWsPs were attached to thin glasses for transmittance measurements. The inset of Figure 3a shows the photographs of FTS-SiNWsPs with the thickness of 10 µm, 50 µm, 100 µm and 300 µm. The cross section SEM images in Figure 3b shows the thickness of different FTS-SiNWsPs corresponding to the inset of Figure 3a. They exhibit approximately flat optical transmittance spectrum in the visible region despite the change in paper thickness, making them potentially desirable for a variety of optical applications24. Although the shift of transmittance spectra was not completely stepwise, it can be said that the FTS-SiNWsP are controllable with high processability. In addition, as an example of active flexible transparent and self-standing applications of FTS-SiNWsP, the FTS-SiNWsP@graphene compounds were prepared for the application in lithium-ion batteries. As shown in Figure.1d, we preformed graphene growth by microwave plasma chemical vapor deposition and removed the SiO2 layer on the surface of the nanowires in order to enhance the conductivity and achieve the core-shell design of Si nanowires@graphene to improve their 6


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electrochemical performance. Unlike graphite intercalated compounds and exfoliated graphite, the graphene sheets synthesized in MPCVD are metallic impurity free, atomically thin, and does not depend on the substrates25, 26. In our MPECVD system, the H2 and CH4 plasma can extend into the empty spaces of the inner part of the Si nanaowires paper leading to growth of graphene sheets on the surface of the Si nanowires.

Before graphene growth, the Si nanowires paper with the thickness of

about 10 µm was first cut into round pieces with the diameter of 7mm. The FTS-SiNWsP was heated to the desired temperature of 380℃ by the hydrogen plasma. A compressed gas of CH4 balanced in H2 was flowed to achieve graphene growth onto Si nanowires. Flow rates of 30 sccm and 60 sccm were used for the delivery of CH4/H2 gas. The pressure was set at 6.5 torr. The react time was controlled at 30 seconds to ensure that we can get a proper thickness of graphene on the surface of Si nanowires. Then perform graphene growth on the other side of the FTS-SiNWsP with the same method. Next, the FTS-SiNWsP@graphene was immersed in 4 wt% HF aqueous solution for 10 minutes to remove the SiO2 layer on the surface of the Si nanowires, followed by deionized water and ethano washing. Finally the etch paper was transferred to the stainless steel substrate by slowly flowing ethanol while holding it in contact with the substrate and then dried in air. The mass after each step was accurately determined by measuring the mass of the substrate using a microbalance (Mettle Toledo XP2U, 0.1 µg resolution) and the ratio of Si to graphene in the FTS-SiNWsP@graphene compounds electrodes is about 1:1. Figure 4 shows the morphology and structure of the FTS-SiNWsP@graphene compounds electrodes. 7


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As seen from the inset of Figure 4a, the resulting paper-like SiNW@graphene network goes slightly black in color but still has good light transmittance. SEM image (Figure 4a) of side-section of the network reveals that the FTS-SiNWsP is covered by graphene and the paper is flexible. The higher magnification top-surface SEM images (Figure 4b) further showed that the nanowires are surrounded by graphene and the gaps between the interlaced nanowires are smaller because graphene grown on the surface of nanowires and filling into the gaps. The inset of Figure 4b shows the TEM image of a thin Si nanowires network after graphene growth. The coverage is spatially uniform in the entire paper (surface and inner part of the paper). The graphene sheets are large enough and the Si nanowires are completely encased in them. The nanowires with graphene sheath are overlapped and intimately contact with another which assures the conductivity of the material. Figure 4c,d shows TEM images of the fabricated Si nanowires@graphene after etching with HF solution. After removed the SiO2 layers, the Si nanowires cores are enveloped with graphene. It is important to note that there are ample empty space between nanowires and graphene walls as marked by red dashed line in Figure 4d. The empty space surrounding Si nanowires allow the free expansion of Si without mechanical constrain during lithiation and also prevents damage to the graphene layer from Si volume changes. The composition and structure was further confirmed by the Raman spectra of FTS-SiNWsP before and after treatment. As seen in the inset of Fig. 4c, the peak at ~517 cm-1 corresponds to the characteristic band of crystal silicon. For the spectrum of FTS-SiNWsP@graphene, besides the characteristic peak from silicon, the peaks appeared at about 1350, 1586, 8


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2701 cm-1 are assignable as the D, G, and 2D bands of graphene, respectively. The relatively high intensity of D band results from the presence of defects in the graphene grown on the nanowires. In

order

to

investigate

the

electrochemical

performance

of

the

FTS-SiNWsP@graphene anodes, two-electrode coin cells were fabricated with Li metal as the counter electrode, no binder or conducting carbon were used. As no binders or conducting additives which would add extra weight were used, the effective capacity of the compounds electrode was close to its theoretical capacity. The active material utilization achieved 99% within experiment error which is higher than many other Si nanostructures electrodes27-33. The electrochemical properties of the anodes were evaluated using deep galvanostatic charge/discharge cycles from 1 to 0.01 V. All the specific capacity values in this paper are reported using the total mass of the FTS-SiNWsP@graphene electrodes. As shown in Figure 5, the electrodes showed excellent lithium storage performance even if without the addition of electrolyte additives or the control of cutoff voltages. The reversible Li charge and discharge capacity and the Columbic efficiency of the FTS-SiNWsP@graphene electrodes versus cycle number are plotted in Figure 5a, the electrodes still keep a capacity over 1000 mAh/g at a current density of 1A/g after discharging and charging for 100 cycles. This result suggests that the capacity retention of the FTS-SiNWsP@graphene electrodes is greatly improved by the design of the material structure. For the first cycle at a current density of 1A/g, the charge and discharge capacities reach 5336 and 2699 mAh/g, and the Columbic efficiency is over 50.1%. 9


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The first charge capacity is higher than theoretical capacity. This may be ascribing to SEI formation on the additional carbon from the coating34 and the effect of partly exposed stainless steel gasket due to the thin Si nanowires network. In the following cycles at the current density of 1A/g, the Columbic efficiency reaches to above 90%. In addition, the material structure enabled outstanding high power rate capability due to the stable SEI formed outside the graphene sheaths. The electrodes also showed excellent cycle stability under high current density. At high charge/discharge current densities ranging from 0.25 to 3A/g, high and stable capacities in the electrodes were demonstrated (Figure 5b,c). Figure 5b shows the influence of the charge-discharge currents on the capacity retention of the electrodes. With the enhanced constant current, the capacity decreases regularly, but downward trend is gradually slowing. The reversible capacity decreases from 2000 mAh/g at the current density of 250 mA/g to 350 mAh/g at the current density of 3A/g. When the current density returns to 250 mA/g and 1 A/g, the capacity can be largely recovered, indicating that the sample has a good electrochemical reversibility and structural integrity. Figure 5c shows the charge/discharge voltage profile of the FTS-SiNWsP@graphene electrodes under different current densities corresponds to Figure 5b. The plateaus at ~0.2 V during discharge and at 0.45 V during charge are similar to the typical charge/discharge behavior of silicon. The excellent electrochemical performance is believed to be a result of their presently achieved unique configuration. Firstly, the small size of the nanowires is one of the advantages to be used as the electrodes35. The nanowires in the network 10


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have a small diameter of ~ 10 nm. This size of Si showed inconspicuous volume expansion and exhibit high capacity and stable cycling behavior36, 37. Secondly, the surrounded graphene and empty space design effectively prevented the continual formation of solid electrolyte interphase (SEI) on the surface of Si nanowires33, 38, 39. The nanowires were protected by graphene sheaths in case they directly contact with electrolyte. And the empty space between nanowires and graphene sheaths prevented damage to the graphene sheaths from Si volume changes. Thirdly, the nanowires have large graphene sheets and constitute a three-dimensional network with good conductivity. The nanowires are interconnected through surrounding graphene sheets which shorten the electron transport distance from/to silicon. This network structures ensure excellent electrodes conductivity40. Fourthly, thanks to the good conductivity, freestanding of electrodes and its good contact with substrate, the need for binders or conducting additives which may degrade the performance of as-designed anode materials, is eliminated33, 41. In conclusion, a novel kind of flexible transparent and self-standing silicon nanowires paper was synthesized by simply thermal evaporation under the action of the carrier gas flow. It was consisted of interlocked nanowires with the diameter of ~10 nm and has good optical transmittance and be able to bend repeatedly without cracking. So, it has great potential values for flexible transparent electronic and electrochemical applications. On this basis, we successfully fabricated a kind of transparent and self-supporting binder-free silicon membrane anodes via the encapsulation of the silicon nanowires with sufficient graphene. The electrodes 11


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exhibit outstanding lithium storage performance with high-storage capacities and good cycling stability. The proposed approach affords a very facile strategy for the fabrication of next generation flexible transparent functional devices based on the FTS-SiNWsP.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (51125008, 11274392) supported this work.

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Figure Captions

Figure 1. a) Schematic diagrams of the vertical high-frequency induction furnace. The direction of gas flow is marked by the yellow dashed lines. The location where Si nanowires grow is marked by a red circle. b,c) Growth mechanism of Si nanowires and FTS-SiNWsP under the action of carrier gas flow. d) Schematic illustration of the synthesis of FTS-SiNWsP@graphene electrode.

Figure 2. a) SEM image of the flexible transparent and self-standing silicon nanowires paper. The inset shows a Digital image of originally synthesized FTS-SiNWsP with excellent transmission. The cylindrical reticulate structure has a height of about 2 cm and diameter of 2 cm. The cylindrical network was torn from one side and be unfold. b) TEM of the Si nanowires in the network. The inset shows the SAED pattern of a bunch of Si nanowires and HRTEM lattice image of an individual silicon nanowire. The crystalline silicon is surrounded by a thin layer of amorphous silicon oxide.

Figure 3. The FTS-SiNWsPs were attached to thin glasses for transmittance measurements. a) The digital photos of FTS-SiNWsPs with the thickness of 10µm, 50µm, 100µm, 300µm and optical transmittance spectra corresponding to them. The gray spectrum is corresponding to a glass. b) The cross section SEM images of FTS-SiNWsPs with different thickness corresponding to Figure 3a.

Figure 4. a) A side view SEM image of FTS-SiNWsP@graphene compounds. The gap among the nanowires is roughly filled by graphene. The inset picture shows a coin cell-size FTS-SiNWsP@graphene

compounds.

b)

Higher-magnification

SEM

image

of

FTS-SiNWsP@graphene. The inset shows the TEM image of a thin nanowires network. c,d) Low-magnification TEM and HRTEM images of the as-fabricated Si nanowires@graphene paper after etching with HF solution. The inset of c) is the room temperature Raman spectrums of Si nanowires paper before and after graphene coating.

Figure 5. Electrochemical characteristics of the synthesized FTS-SiNWsP@graphene electrodes. a) Capacity and Columbic efficiency of the FTS-SiNWsP@graphene electrodes cycled at the designated rate (1A/g) for 100 cycles. b,c) Capacity retention b) and galvanostatic charge/ discharge profiles c) of the FTS-SiNWsP@graphene electrodes cycled at various current densities ranging from0.25 to 3 A/g.

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Nano Letters

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Figure 1

(a)

(b) quartz tube

carbon fiber thermoinsulating layer

Ⅲ induction-heated cylinder

copper loop

Ⅱ

graphite crucible

Ar gas

Ⅰ

Ar

(c)

cut a piece off the paper

(d) HF etching

graphene growth

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Page 17 of 21

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Nano Letters

Figure 2

(a)

1cm

(b)

0.31nm

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111 220 311

Nano Letters

Figure 3

(a)

Glass

100

Sample 1

Transmittance/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Sample 2

80

Sample 3

60

Sample 4

40 20 0 400

500

(b)

600

700

800

Wavelength/nm Sample2

Sample1

10μm

Sample3

20μm

100μm

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Sample4

300μm

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Figure 4

(a)

(b)

(c)

(d) 0.34nm Si NWs@graphene paper Si NWs paper Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

1350 1586

2701

517

500

1000

1500

2000

2500

Wavenumber(cm-1)

3000

3500

4000

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Figure 5

120

6000

100

Capacity(mAh/g)

(a) Discharge Charge

4000

1A/g

80 60 40

2000

20 0

0 0

20

40

60

80

Cycle number

100

Capacity(mAh/g)

(b) Discharge Charge

4000 3000

0.25A/g 0.25A/g

0.5A/g

2000

1A/g

1A/g

2A/g

1000

3A/g

0 0

10

20

30

Cycles

40

50

60

70

(c) Voltage/V

1.0

3A/g

2A/g

1A/g

0.5A/g

0.25A/g

0.8 0.6 0.4 0.2 0

500

1000

1500

Capacity(mAh/g)

20


2000

2500

Coulombic efficiency(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Nano Letters

Table of Content (TOC)

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Flexible Transparent and Free-Standing Silicon Nanowires Paper

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