Scalable large nanosheets of transition metal disulphides through exfoliation of amine intercalated MS₂ [M = Mo, W] in organic solvents

Abstract

Various alkylamine intercalated MS₂ (M = Mo, W) have been prepared by reacting Li_xMS₂ with aqueous solutions of the amines. The amine intercalated MS₂ exfoliate readily in alcohols. While the shorter amine intercalated MS₂ exfoliate better in lower alcohols the longer amine intercalated MS₂ exfoliate better in higher alcohols. The longer amine intercalated MS₂ exfoliate well even in a nonpolar solvent, toluene. Up to 9 millimoles of MS₂ per liter could be dispersed through exfoliation in organic solvents. The dispersions are quite stable and comprise 2D-nanosheets of MS₂. Hybrids of MoS₂ and WS₂ could be prepared by evaporating the solvent from a mixture of colloidal dispersions of amine intercalated MoS₂ and WS₂. The hybrid exhibits features of a heterostructure in its photoluminescence spectrum.

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Introduction

Two dimensional (2D) materials or nanosheets derived from layered solids are in focus owing to their interesting properties and potential applications in various domains. Subsequent to the discovery of graphene^1,2 and the demonstration of its interesting properties, inorganic nanosheets derived from layered solids such as h-BN,³ transition metal dichalcogenides (TMDCs)^1,3–6 and layered oxides^7,8 have taken prominence. Among these, nanosheets of TMDCs that have strong in-plane bonding and flexibility are of special importance due to their potential applications in electronics, optoelectronics,^4,9 sensing¹⁰ and catalysis.^11–13 While the bulk MS₂ are indirect bandgap semiconductors their 2D analogs have a direct bandgap.^14–16 These nanosheets show excellent photoluminescence,¹⁴ high carrier mobility¹⁷ and high on/off ratios for transistor applications.^18–20 Further, 2D nanosheets are used in the preparation of layered hybrids and heterostructures.²¹ TMDC-graphene hybrids exhibit synergetic effects and are used in photoresponsive devices²² and as battery electrodes.^23,24

While the small scale preparations of TMDC 2D nanosheets are suitable for electronic devices, large scale production of the same is required for applications such as catalysis and electrochemical energy storage and in the fabrication of heterostructures/hybrids.^25–27 Numerous methods have been developed to produce two dimensional TMDCs. These include mechanical exfoliation using scotch tape,¹ chemical vapour deposition,^28,29 direct chemical synthesis,³⁰ electrochemical synthesis,³¹ liquid phase exfoliation^32–35 and exfoliation in solvents through suitable intercalation^36,37 (chemical exfoliation). Among these methods exfoliation of TMDCs through suitable intercalation would be suitable for the large scale synthesis of monolayer to few layer nanosheets. Depending on the solvent type, layered materials can be exfoliated after suitable interlayer modification.^36,38,39 Major advantages of chemical exfoliation are the ease of thin film fabrication, nanodevices fabrication and layered hybrid formation through mere stoichiometric mixing of dispersions containing different 2D compounds.⁵

Lithiated MCh₂ (M = Mo, W, T, Ta) and (Ch = S, Se, Te) exfoliate in water due to the highly exothermic reaction between the intercalated Li atoms and water.^40,41 The colloidal dispersions of the 2D TMDC nanosheets thus obtained are quite stable. The problems with this method are that (i) Li_xMCh₂ are highly unstable and have to be stored under inert atmosphere and (ii) lithium compounds remain as impurities in the hybrids prepared starting from the colloidal dispersion. To overcome these difficulties we recently developed a liquid phase exfoliation method through intercalation of NH₃/NH₄⁺ in transition metal disulphides, MS₂ (M = Mo, W).³⁶ In the synthesis of hybrids, it is important to have the two components dispersed in the same solvent. Thus it is important to develop methods to exfoliate TMDCs in diverse solvents. Keeping this in mind we have prepared organic molecule intercalated TMDCs to help exfoliate them in organic solvents. Amines could be intercalated in TMDCs by the procedure similar to the one employed for ammonia intercalation. In this work, we demonstrate the synthesis of a series of amine intercalated MS₂ (M = Mo, W) and their excellent exfoliation in organic solvents.

Experimental

Synthesis of Li_xMS₂ (M = Mo, W)

Dry molybdenum disulphide (1 g, 6.24 mmol) was stirred with 8 ml of 2.5 M solution of n-butyllithium in hexane under nitrogen atmosphere at room temperature for 48 h to form lithiated MoS₂.⁴¹ After 48 h, the excess n-butyllithium was removed by washing several times with dry n-hexane and the product was preserved in dry n-hexane under inert atmosphere.

Li_xWS₂ was synthesized by a solvothermal method.⁴⁰ 10 ml of 2.5 M n-butyllithium was added to dry tungsten disulphide (1 g, 4.03 mmol) taken in an 80 ml teflon lined autoclave under inert atmosphere and the autoclave was sealed immediately. The autoclave was heated at 90 °C for 24 h in an air oven. After 24 h, the autoclave was opened under nitrogen atmosphere. The lithiated tungsten disulphide was washed several times with dry n-hexane to remove the excess n-butyllithium and the product was preserved in dry n-hexane under inert atmosphere.

Synthesis of amine intercalated MS₂ (M = Mo, W)

Aqueous solutions of a series of small chain to long chain n-alkyl amines like n-butylamine (BA), n-octylamine (OA), n-dodecylamine (DDA), and n-octadecylamine (ODA) were prepared by adjusting the pH of the solutions to 7 by adding necessary amounts of dilute HCl. Freshly prepared amine solution was taken in a 100 ml beaker. Under inert atmosphere, about 0.350 g of Li_xMS₂ (M = Mo, W) in n-hexane was added to the amine solution and the mixture stirred vigorously for 20 min. The black slurry formed was washed using rectified spirit five times and with water–ethanol (50 : 50) mixture twice to remove excess amine, filtered and dried at room temperature. The amine–intercalated MS₂ samples are hereafter referred to as BA–MS₂, OA–MS₂, DDA–MS₂ and ODA–MS₂.

Exfoliation studies

Colloidal dispersion of MS₂ (M = Mo, W) layers in organic solvents were obtained by sonicating 25 mg of amine–MS₂ in 30 ml of the solvent such as ethanol, 1-butanol, 1-hexanol, 1-octanol, and toluene for 2 h. The resultant colloidal dispersion was centrifuged at 1000 rpm for 10 min to remove any undispersed solid. The undispersed solid was washed thrice with acetone, dried in the oven and weighed to constant weight to determine the amount exfoliated in the solvent. The stability of colloidal dispersion was probed by allowing the dispersion to stand undisturbed for several hours to days. At the end of each day, the solid settled was separated from supernatant colloid by centrifugation, washed, dried and weighed. This procedure was repeated until the amount of the settled solid reached greater than or equal to 50% of the weight of the dispersed solid. The time taken for ∼50% of the dispersed solid to settle down is taken as the measure of colloidal stability and is represented as the half-life (t_1/2) of the colloidal dispersion. In some cases, where the stability of the colloidal dispersion was found poor, the settled solids were separated on hourly basis.

Preparation of MoS₂–WS₂ hybrid

The layers from the colloidal dispersions can be recovered by evaporation of the solvent, adding a solvent of different polarity⁴² or by high speed centrifugation.⁴³ We have used the method of evaporation here. About 70 mg each of OA–MoS₂, and OA–WS₂ was dispersed separately in 80 ml of 1-octanol. The resultant colloidal dispersions were mixed and the mixture sonicated for 1 h. The solvent from the mixture was removed using a rotary evaporator and the residue was dried at 65 °C in an air oven.

About 100 mg of the above hybrid was heated at 500 °C for 4 h under inert atmosphere to deaminate it. The product was washed with water, followed by ethanol and dried at 65 °C in an air oven.

Characterization

Samples were analyzed by recording PXRD patterns using PANalytical X'pertpro diffractometer (Cu Kα radiation, secondary graphite monochromator, scanning rate of 1° 2θ min⁻¹). Transmission electron microscopy (TEM) images were recorded using a JEOL F3000 microscope operated at 300 kV. The nanoplatelets of MS₂ obtained through exfoliation were characterized by atomic force microscopy (AFM) using APER A100 atomic force microscope in tapping mode. Sample for AFM analysis was prepared by spin coating an aqueous colloidal dispersion of MS₂ onto a Si substrate. X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ESCALab220i-XL spectrometer using a twin-anode Al Kα (1486.6 eV) X-ray source. All spectra were calibrated to the binding energy of the C1s peak at 284.51 eV. The base pressure was around 3 × 10⁻⁷ Pa. Photoluminescence (PL) spectra were recorded using a LS 55 Fluorescence spectrometer (Perkin Elmer) at excitation wavelength of 350 and 400 nm.

Results and discussion

Alkylamines were intercalated in MS₂ by the process described in Fig. 1. When lithiated MS₂ were reacted with alkylamine solutions a vigorous reaction ensued leading to the formation of amine–intercalated MS₂. The increased basal spacings observed for the amine intercalated MS₂ compared to pristine MS₂ confirm intercalation of amines in the interlayer of MS₂ (Fig. 2). The increase in basal spacing depends on the chain length of the amine and its interlayer orientation. Similar basal spacings are observed for a given amine in both MoS₂ and WS₂. While the basal spacing for the BA–MS₂ is ∼10 Å (an increase of ∼4 Å from the basal spacing of the pristine MS₂), it is ∼33 Å for the ODA–MS₂ (an increase of ∼27 Å from the basal spacing of the pristine MS₂). The non-linear increase in basal spacing with chain length of the amine is due to the fact that the orientations of the intercalated amines in the interlayer are different. While the alkyl chains of long chain amines orient themselves perpendicular to the layers, those of the shorter amines have a tilted orientation. Amine intercalation results in turbostratic disorder as indicated by the broad asymmetric peaks starting at 2θ = 32° and 57° in the PXRD patterns of all the intercalated products.

Fig. 1 Schematic representation of amine intercalation and exfoliation.

Fig. 2 (A) PXRD patterns of the amine intercalated molybdenum disulphide: (a) pristine MoS₂, (b) BA–MoS₂, (c) OA–MoS₂, (d) DDA–MoS₂, (e) ODA–MoS₂; (B) PXRD patterns of the amine intercalated tungsten disulphide: (a) pristine WS₂, (b) BA–WS₂, (c) OA–WS₂, (d) DDA–WS₂, (e) ODA–WS₂.

The results of CHNS analysis of the amine–intercalated MS₂ are shown in Table 1. The approximate chemical formulas were computed by comparing the percentages of carbon and nitrogen with that of sulphur. This approximate calculation suggests that 0.24 to 0.43 moles of the amine get intercalated per mole of MoS₂. The extent of intercalation seems to increase with the increase in the chain length of the amine. The extent of amine intercalation is better in the case of WS₂ as we observe higher number of moles of intercalated amine compared to MoS₂ for any given amine.

Table 1 Compositions of amine intercalated MS₂ (M = Mo, W) derived from CHNS analysis

Sample	Mass percentage			Approximate formula
	Carbon	Nitrogen	Sulphur
BA–MoS2	6.54	1.95	33.17	MoS2·0.27(C4H9N)
OA–MoS2	11.65	1.61	31.40	MoS2·0.24(C8H19N)
ODA–MoS2	34.29	2.03	21.46	MoS2·0.45(C18H39N)
BA–WS2	4.82	1.57	20.39	WS2·0.33(C4H9N)
OA–WS2	8.82	1.36	20.61	WS2·0.29(C8H19N)
ODA–WS2	34.61	2.12	13.69	WS2·0.73(C18H39N)

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In order to understand the state of the intercalated amine – whether it is free amine or protonated – and the extent of defects we carried out XPS analysis of the samples and the results are presented in Fig. 3 and 4. In the case of lower amine intercalated MoS₂ (BA–MoS₂) the extent of defects is quite high. In the Mo core level spectrum (Fig. 3) we observe Mo⁶⁺ related peaks in addition to those due to Mo⁴⁺. In the S core level spectrum, in addition to the peaks due to S²⁻ we observe a peak due to sulphur bonded to oxygen. The intercalated amine exists both as free amine and as alkyl ammonium ion. As we increase the chain length of the amine there are discernible changes in the spectra. The defects decrease with increase in the chain length of the intercalated amine. In the Mo core level spectrum the peaks due Mo⁶⁺ become weaker in the case of OA–MoS₂ and these are totally absent in the case of DDA–MoS₂. Correspondingly in the S core level spectrum the intensity of the peak due to sulphur bonded to oxygen go on decreasing as the chain length is increased and the peak is absent in the case of DDA–MoS₂. However, the nature of the intercalated species is the same in all the cases – the intercalated amine exists both as free amine and alkyl ammonium ion, whatever is the intercalated amine.

Fig. 3 Mo, S and N core level X-ray photoelectrons spectra of (A) BA–MoS₂, (B) OA–MoS₂, (C) DDA–MoS₂.

Fig. 4 W, S and N core level X-ray photoelectrons spectra of (A) BA–WS₂, (B) DDA–WS₂.

As W is more prone to oxidation compared to Mo, we observe more intense peaks due to W⁶⁺ in the W core level spectrum of BA–WS₂ and correspondingly a stronger peak due to sulphur bonded to oxygen in the S core level spectrum (Fig. 4). Though, as in the case of MoS₂, the extent of defects decreases with increase in the chain length of the intercalated amine, the defects do not vanish completely as indicated by weak peaks due to W⁶⁺ in the W core level spectrum of DDA–WS₂.

The control over defects by the chain length of the alkylamine is interesting and it could be used to engineer the extent of defects in amine–MS₂ and hence on the nanosheets derived from them. The decrease in defects with increase in the chain length of amines can be attributed to lower reactivity of longer amines leading to less exothermic conditions.

One of the problems with ammoniated MS₂ is their poor shelf-life.³⁶ These were unstable when stored at room temperature and lost the intercalated ammonia within days. In order to see if instability is an issue for the amine intercalated MS₂ too, we studied the room temperature stability of BA–MS₂ and ODA–MS₂ (M = Mo, W) by allowing the samples to stand several days to months. The PXRD analysis was carried out on daily basis to probe any changes. Fig. 5 shows the PXRD patterns of the BA–MS₂ and ODA–MS₂ (M = Mo, W) systems stored in non-air-tight containers for different durations. From the evolution of the PXRD patterns of BA–MoS₂ with time (Fig. 5A) it is clear that the compound is stable up to 70 days. Beyond 70 days, the PXRD pattern of n-BA/MoS₂ shows a weak peak due to the 002 reflection of pristine MoS₂ (marked in asterisk in Fig. 5A). This stability is far superior to what was observed in the case of ammoniated MoS₂.³⁶ The PXRD pattern of BA–WS₂ remains the same even after 70 days (Fig. 5B) indicating that it is very stable under ambient conditions. Stability increases with the chain length of the intercalated amine. The PXRD patterns of ODA–MS₂ (M = Mo, W) after 120 days is identical to those of the as prepared samples (Fig. 5C and D).

Fig. 5 PXRD patterns of [A] BA–MoS₂, [B] BA–WS₂, [C] ODA–MoS₂ and [D] ODA–WS₂ kept standing for various durations at room temperature.

All the amine intercalated MS₂ exfoliate well in alcohols and the colloidal dispersions of exfoliated layers are quite stable and exhibit Tyndall effect (Fig. 6). We list the number of millimoles of the amine intercalated MS₂ that get dispersed per litre of the solvent and the time taken for 50% of the dispersed material to flocculate (t_1/2) in Table 2. From this data we can arrive at the following conclusions. (1) The shorter amine intercalated MS₂ exfoliate better in lower alcohols and the longer amine intercalated MS₂ exfoliate better in higher alcohols. (2) The stability of the colloidal dispersions of exfoliated WS₂ is higher than that of MoS₂. (3) Longer amine intercalated MS₂ exfoliates in toluene, a non-polar solvent, with the extent of exfoliation and the stability of dispersion increasing with increase in the chain length of the intercalated amine.

Fig. 6 Photographs of the colloidal dispersions of BA–MS₂ (a, b and c) and ODA–MS₂ (d, e and f) in 1-butanol. The dispersions exhibit Tyndall effect (a, b, d and e).

Table 2 Extent of exfoliation and the stability of the colloidal dispersions of amine intercalated MS₂

Amine–MS2	Ethanol		1-Butanol		1-Hexanol		1-Octanol		Toluene
	A^a	T^b	A	T	A	T	A	T	A	T
a A = amount exfoliated (mmol dm^−3).b T = t1/2 (h).
BA–MoS2	3.2	72	2.4	60	2.6	58	2.7	3	—	—
OA–MoS2	2.8	24	3.6	24	3.7	24	5.1	8	1.7	8
DDA–MoS2	2.6	24	3.5	32	5.0	32	8.0	36	2.5	20
ODA–MoS2	2.8	4	4.0	24	7.0	32	9.0	76	3.8	48
BA–WS2	4.1	96	3.5	80	2.5	146	2.9	146	—	—
OA–WS2	2.4	96	2.5	48	2.5	48	3.6	24	0.2	8
DDA–WS2	1.8	48	2.6	48	3.0	288	3.2	408	0.3	8
ODA–WS2	3.0	4	4.0	48	5.0	72	7.0	96	3.2	12

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Exfoliation of amine intercalated MS₂ in alcohols operates through solvation of layers and the alkyl chains of the intercalated amines. The alkyl groups of amine surfactant interact with the nonpolar alkyl chains of the alcohols and the inorganic layer is solvated by the polar hydroxyl groups of alcohol. Increasing the chain length of the intercalated amine increases the interaction with the alkyl chains of the alcohols leading to efficient exfoliation.

The colloidal dispersions of the exfoliated amine intercalated MS₂ comprise monolayers of the layered solid. The TEM images of the sample deposited on the grid from the colloidal dispersions of octylamine intercalated MS₂ (Fig. 6a–d) show thin sheets that are folded around their edges. The high resolution images of these sheets (Fig. 6e and f) reveal the crystalline nature of these sheets. Lattice fringes due to the (100) and (101) lattice planes are observed in the case of octylamine intercalated MoS₂ and WS₂ respectively (Fig. 6e and f). The electron diffraction patterns obtained from these layers (insets in Fig. 7e and f) could be indexed to hexagonal MoS₂ and WS₂ respectively with the observed spots corresponding to the d spacings of 010 and 100 reflections. The AFM images of the samples deposited on a Si substrate from the colloidal dispersions (Fig. 7) further confirm excellent exfoliation. The nanosheets are quite large in the lateral dimensions while the thickness of these sheets is 2–3 nm.

Fig. 7 Bright field TEM images of exfoliated layers of BA–MoS₂ (a and b) and BA–WS₂ (c and d) and the HRTEM images of a layer each of BA–MoS₂ (e) and BA–WS₂ (f).

The nanosheets of MoS₂ and WS₂ could be co-stacked by evaporating the solvent from a mixture of the dispersions of amine intercalated MoS₂ and WS₂ to form MoS₂/WS₂ hybrids. The PXRD pattern of MoS₂/WS₂ hybrid obtained from OA–MS₂ (Fig. 9a) shows a basal spacing of 10.2 Å, which is the same as the basal spacing of OA–MS₂ (Fig. 2A(c) and 2B(c)). When the hybrid was deaminated by heating at 500 °C, the 00[small script l] reflections are absent in the PXRD pattern with only the in-plane hk0 reflections observed (Fig. 8b) indicating an exfoliated state.

Fig. 8 AFM images and thickness profiles of the 2D nanosheets of amine intercalated MS₂ deposited on Si/SiO₂ substrate by evaporating a drop of the colloidal dispersion of the exfoliated layers in 1-butanol. (a) BA–MoS₂, (b) BA–WS₂, (c) OA–MoS₂ (d) OA–WS₂, (e) ODA–MoS₂, and (f) ODA–WS₂.

Fig. 9 PXRD patterns of OA–MoS₂/WS₂ hybrid before (a) and after (b) deamination.

The photoluminescence (PL) spectrum of the deaminated MoS₂/WS₂ hybrid (Fig. 10) shows that the A exitonic emission peak of the hybrid is red-shifted and the emission intensity increased compared to the MoS₂ or WS₂ nanosheets. These features are as expected from heterostructures of MoS₂ and WS₂.^36,44,45

Fig. 10 Photoluminescence spectra of (a) MoS₂ nanosheets (b) WS₂ nanosheets, (c) deaminated MoS₂/WS₂ hybrid.

Conclusions

Amine intercalated MS₂ could be prepared by a procedure adapted from our earlier work on the preparation of ammoniated MS₂.³⁶ The advantages of amine intercalation are three-fold. Firstly it allows exfoliation and nanosheet dispersion in organic solvents. Secondly, the amine–MS₂ have a much longer shelf-life compared to ammoniated MS₂. More importantly, it also allows a control over the extent of chemical defects in the nanosheets. The nanosheets could be assembled to form MoS₂/WS₂ hybrids with heterostructure features.

Acknowledgements

This work was funded by DST, India.

Notes and references

1. K. Novoselov, D. Jiang, F. Schedin, T. Booth, V. Khotkevich, S. Morozov and A. K. Geim, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 10451–10453.
2. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669.
3. M. S. Xu, T. Liang, M. M. Shi and H. Z. Chen, Chem. Rev., 2013, 113, 3766–3798.
4. Q. H. Wang, K. K. Zadeh, A. Kis, J. N. Coleman and M. S. Strano, Nat. Nanotechnol., 2012, 7, 699–712.
5. M. Chhowalla, S. H. Shin, G. Eda, J. L. Li, P. K. Loh and H. Zhang, Nat. Chem., 2013, 5, 263–275.
6. I. Song, C. Park and H. C. Choi, RSC Adv., 2015, 5, 7495–7514.
7. T. Sasaki and M. Watanabe, J. Am. Chem. Soc., 1998, 120, 4682–4689.
8. M. Osada and T. Sasaki, Adv. Mater., 2012, 24, 210–228.
9. W. Liu, X. Yang, Y. Zhang, M. Xu and H. Chen, RSC Adv., 2014, 4, 32744–32748.
10. F. K. Perkins, A. L. Friedman, E. Cobas, P. M. Campbell, G. G. Jernigen and B. T. Jonker, Nano Lett., 2013, 13, 668–673.
11. J. Deng, W. Yuan, P. Ren, Y. Wang, D. Deng, Z. Zhang and X. Bao, RSC Adv., 2014, 4, 34733–34738.
12. X. Sun, J. Dai, Y. Guo, C. Wu, F. Hu, J. Zhao, X. Zeng and Y. Xie, Nanoscale, 2014, 6, 8359–8367.
13. J. Li, X. Liu, L. Pan, W. Qin, T. Chen and Z. Sun, RSC Adv., 2014, 4, 9647–9651.
14. G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen and M. Chhowalla, Nano Lett., 2011, 11, 5111–5116.
15. Q. Ji, Y. Zhang, T. Gao, Y. Zhang, D. Ma, M. Liu, Y. Chen, X. Qiao, P. H. Tan, M. Kan, J. Feng, Q. Sun and Z. Liu, Nano Lett., 2013, 13, 3870–3877.
16. A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli and F. Wang, Nano Lett., 2010, 10, 1271–1275.
17. M. M. Perara, M. W. Lin, H. J. Chuang, B. P. Chamlagain, C. Wang, X. Tan, M. M. C. Cheng, D. Tomanek and Z. Zhou, ACS Nano, 2013, 7, 4449–4458.
18. T. Roy, M. Tosun, J. S. Kang, A. B. Sacid, S. B. Desai, M. Hettick, C. C. Hu and A. Javey, ACS Nano, 2014, 8, 6259–6264.
19. Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen and H. Zhang, ACS Nano, 2012, 6, 74–80.
20. M. Tosun, S. Chuang, H. Fang, A. B. Sacid, M. Hettick, Y. Lin, Y. Zeng and A. Javey, ACS Nano, 2014, 8, 4948–4953.
21. A. K. Geim and I. V. Grigorieva, Nature, 2013, 499, 419–425.
22. L. Yu, Y. H. Lee, X. Ling, E. J. G. Santos, Y. C. Shin, Y. Lin, M. Dubey, E. Kaxiras, J. Kong, H. Wang and T. Palacious, Nano Lett., 2014, 14, 3055–3063.
23. L. Zhang, W. Fan, W. Tjiu and T. Liu, RSC Adv., 2015, 5, 34777–34787.
24. Z. Wang, T. Chen, W. Chen, K. Chang, L. Ma, G. Huang, D. Chen and J. Y. Lee, J. Mater. Chem. A, 2013, 1, 2202–2210.
25. F. Withers, H. Yang, L. Britnell, A. P. Rooney, E. Lewis, A. Felten, C. R. Woods, V. S. Romaguera, T. Georgiou, A. Eckmann, Y. J. Kim, S. G. Yeates, S. J. Haigh, A. K. Geim, K. S. Novoselov and C. Casiraghi, Nano Lett., 2014, 14, 3987–3992.
26. S. Min and G. Lu, J. Phys. Chem. C, 2012, 116, 25415–25424.
27. A. V. Powell, L. Kosidowski and A. Mcdowall, J. Mater. Chem., 2001, 11, 1086–1091.
28. Y. H. Lee, X. Q. Zhang, W. Zhang, M. T. Chang, C. T. Lin, K. D. Chang, Y. C. Yu, J. T. W. Wang, C. S. Chang, L. J. Li and T. W. Lin, Adv. Mater., 2012, 24, 2320–2325.
29. J. K. Huang, J. Pu, C. L. Hsu, M. H. Chiu, Z. Y. Juang, Y. H. Chang, W. H. Chang, Y. Iwasa, T. Takenobu and L. J. Li, ACS Nano, 2014, 8, 923–930.
30. S. Jeong, D. Yoo, J. T. Jang, M. Kim and J. Cheon, J. Am. Chem. Soc., 2012, 134, 18233–18236.
31. Q. Li, J. T. Newberg, J. C. Walter, J. C. Hemminger and R. M. Penner, Nano Lett., 2004, 4, 277–281.
32. J. N. Coleman, M. Lotya, A. O'Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De and R. J. Smith, et al., Science, 2011, 331, 568–571.
33. R. Bari, D. Parviz, F. Khabaz, C. D. Klaassen, S. D. Metzler, M. J. Hansen, R. Khare and M. J. Green, Phys. Chem. Chem. Phys., 2015, 17, 9383–9393.
34. L. Dong, S. Lin, L. Yang, J. Zhang, C. Yang, D. Yang and H. Lu, Chem. Commun., 2014, 50, 15936–15939.
35. D. V. Thanh, C.-C. Pan, C.-W. Chuand and K.-H. Wei, RSC Adv., 2014, 4, 15586–15589.
36. A. A. Jeffery, C. Nethravathi and M. Rajamathi, J. Phys. Chem. C, 2014, 118, 1386–1396.
37. A. U. Liyanage and M. M. Lerner, RSC Adv., 2014, 4, 47121–47128.
38. Z. Tang, Q. Wei and B. A. Guo, Chem. Commun., 2014, 50, 3934–3937.
39. C. Nethravathi, A. A. Jeffery, M. Rajamathi, N. Kawamoto, R. Tenne, D. Golberg and Y. Bando, ACS Nano, 2013, 7, 7311–7317.
40. D. Yang and R. F. Frindt, J. Phys. Chem. Solids, 1996, 57, 1113–1116.
41. M. B. Dines, Mater. Res. Bull., 1975, 10, 287–292.
42. C. Nethravathi, G. Harichandran, C. Shivakumara, N. Ravishankar and M. Rajamathi, J. Colloid Interface Sci., 2005, 288, 629–633.
43. L. Hu, R. Ma, T. C. Ozawa and T. Sasaki, Chem.–Asian J., 2010, 5, 248–251.
44. X. Hong, J. Kim, S. F. Shi, Y. Zhang, C. Jin, Y. Sun, S. Tongay, J. Wu, Y. Zhang and F. Wang, Nat. Nanotechnol., 2014, 9, 682–686.
45. K. Kosḿider and J. F. Rossier, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 075451–075454.

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