Aligned 2D CuSCN nanosheets: a high performance field emitter

Abstract

A low turn-on field of 2.4 V μm⁻¹ is observed for the emission current density of 10 μA cm⁻² from aligned copper thiocyanate (CuSCN) nanosheets. The observed low turn-on field is found to be superior to that of inorganic semiconducting and carbon based nanomaterials reported in the literature. The field emission current stability for the preset value of 1 μA over the period of 3 h is found to be better. Aligned CuSCN nanosheets were grown on fluorine doped tin oxide (FTO) coated glass substrates by a simple, low cost Successive Ion Layer Adsorption and Reaction (SILAR) technique at room temperature. Detail characterization of aligned CuSCN nanosheets has been carried out using Field Emission Scanning Electron Microscopy (FESEM), X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). To the best of our knowledge this is the first report on the field emission studies of aligned CuSCN nanosheets. The simple and inexpensive synthesis route coupled with superior field emission make the present emitter suitable for micro/nano electronic devices.

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1. Introduction

Various properties of nanostructures such as optical, electronic, magnetic etc. make them potential candidates for practical applications.^1–4 In recent years, the scientific community has been intensely focused on two-dimensional (2D) nanostructures due to their special geometry and high aspect ratio. 2D nanostructures have been used for various applications such as energy storage, catalysis, electronic devices, sensing, biomedicine and field emission.^5–8 Because of the continuous and high crystal quality of 2D nanostructures, charge carriers can flow through thousands of interatomic distances without scattering.⁸ This leads to enhanced electrical conductivity and superior field emission properties.⁹

Field emission is purely quantum mechanical tunneling phenomenon where electrons are emitted from the surface of nanomaterial under the action of strong electrostatic field. Since, last few decades, it is observed that various 2D nanomaterials have attracted extensive attention due to their superior field emission behavior.^10–16 Superior field emission behavior in terms of low turn-on field, high emission current density and good emission current stability can be achieved by controlling the morphology or electronic properties of the synthesized materials. Various routes have been explored for the synthesis of 2D nanosheets.^17–20

Copper thiocyanate (CuSCN) is wide band gap p-type semiconductor with direct band gap of 3.9 eV.²¹ Due to its excellent transparency in the visible light spectrum, high hole mobility and relatively good chemical stability it is widely used in perovskite-based p–i–n junction solar cells,²² light emitting diodes,²³ organic photovoltaic cell,²⁴ sensing²⁵ and thin film transistors.²⁶ A vast literature survey including recent reports^27–29 indicates that, field emission studies of CuSCN nanostructures are found to be not reported earlier. Hence, for the scientific and technological advancement it is important to explore the field emission properties of CuSCN nanosheets.

In present paper, we report the synthesis of aligned CuSCN nanosheets by Successive Ion Layer Adsorption and Reaction (SILAR) technique. Superior field emission properties of aligned CuSCN nanosheets have been observed. A good correlation is found between morphology and field emission properties of the aligned CuSCN nanosheets.

2. Experimental details

All the chemicals were purchased from Sigma-Aldrich, India, and were used without further purification.

2.1 Synthesis of aligned CuSCN nanosheets by SILAR technique

The synthesis of aligned CuSCN nanosheets on fluorine doped tin oxide (FTO) coated glass substrate was carried out by SILAR technique at room temperature.³⁰ In detail, the anionic precursor was prepared by adding 100 mM copper sulphate and 100 mM sodium thiosulphate in Doubled Distilled Water (hereafter DDW). Sodium thiosulphate acts as a reducing as well as complexing agent.³¹ Potassium thiocyanate (70 mM) was used as a cationic precursor. The immersion time was 15 s in anionic and cationic precursor. The rinsing time was 10 s in ion exchanged DDW. The immersion cycles were optimized (30 cycles) for uniform coverage of film on the substrate surface. Finally, the deposited specimen was washed with DDW and allowed to dry in air at room temperature.

2.2 Characterization

The surface morphology and phase identification of aligned CuSCN nanosheets were studied by using Field Emission Scanning Electron Microscope (FESEM) (Hitachi S-4800) and X-ray diffraction (XRD) (D8 Advance, Bruker instrument) respectively. The morphology and the crystalline nature of the aligned CuSCN nanosheets were further studied by Transmission Electron Microscope (TEM), High Resolution Transmission Electron Microscope (HRTEM), Selected Area Electron Diffraction (SAED) (Tecnai G² 20 Twin, FEI).

The field emission current density–applied field (J–E) and current–time (I–t) measurements were carried out by field emission microscope in a ‘close proximity’ (planar diode) configuration. The aligned CuSCN nanosheets grown on to FTO coated glass substrate served as cathode and a semitransparent cathodoluminescent phosphor screen as anode. Provision of back contact was done using conducting carbon tape. The separation between anode and cathode was 1 mm. In present study, the current density J is defined as J = I/A, where I is the emission current and A is area of emitter (∼0.25 cm²). The applied field (E) is defined as E = V/d, where V is the applied voltage and d is the separation between anode and cathode. Fig. 1 indicating the schematic of field emission diode assembly. The working chamber was evacuated using an ultrahigh vacuum system comprising of rotary backed turbo molecular pump, sputter ion pump, and titanium sublimation pump. The cathode (aligned CuSCN nanosheets) did not show any appreciable degassing and vacuum was obtained with usual speed. After baking the system at 150 °C for 8 h, a base pressure of ∼1 × 10⁻⁸ mbar was obtained. The J–E measurement was carried out at this base pressure using a Keithley 485 Picoammeter and a Spellman high voltage DC power supply. Special care was taken to avoid any leakage current by using shielded cables with proper grounding.

Fig. 1 Schematic of the field emission diode assembly.

3. Results and discussion

3.1 Structural studies

The XRD pattern of as-synthesized aligned CuSCN nanosheets is shown in Fig. 2. Careful analysis of Fig. 2 indicate the existence of α-phase and β-phase.²⁶ XRD pattern of FTO coated glass substrate is provided as reference and marked as (#).³² The strong diffraction peaks (003), (101), (110) and (119) are indexed to rhombohedral structure of β-CuSCN (JCPDS card no. # 29-0581). In addition, four small peaks such as (120), (121), (042) and (024) were also observed and are indexed to orthorhombic structure of α-CuSCN (JCPDS card no. # 29-0582).

Fig. 2 XRD spectra of CuSCN nanosheets.

3.2 Surface morphology

Nanosheet like morphology of CuSCN has been confirmed from FESEM images. Fig. 3 shows the FESEM images of aligned CuSCN nanosheets. The Fig. 3(a) and (b) depicts large and uniform coverage of well aligned CuSCN nanosheets on entire substrate. Smooth wall of aligned CuSCN nanosheets have been observed from Fig. 3(c). Fig. 3(d) is close up image of aligned CuSCN nanosheets, indicating average thickness of 45 nm and length up to few micrometers.

Fig. 3 (a)–(c) Low magnification, (d) high magnification FESEM images of CuSCN grown on FTO substrate.

3.3 TEM analysis

TEM characterizations were carried out to study the crystallinity of aligned CuSCN nanosheets. Fig. 4(a) and (b) are the bright field images of aligned CuSCN nanosheet. Enlarge view shown in Fig. 4(b) indicate smooth wall of nanosheet. The HRTEM image of CuSCN nanosheet [Fig. 4(c)] reveals distinct lattice fringe spacing, ∼0.29 nm. A detail analysis of Fast Fourier Transformation (FFT) indicate that, the observed lattice fringe spacing, ∼0.29 nm may corresponds to (131) or (112) plane of the α-phase.²⁶ The SAED pattern shown in Fig. 4(d) representing the polycrystalline nature of CuSCN nanosheets.

Fig. 4 (a) and (b) Low magnification bright field images of CuSCN nanosheet, (c) an HRTEM image of individual nanosheet with the FFT pattern as inset and (d) corresponding SAED pattern.

3.4 Growth mechanism

During SILAR deposition, cationic and anionic solutions alternately react via precursor decomposition process on the surface of immersed substrate to yield heterogeneous nucleation and growth of desired film.³³ The reaction mechanism for the growth of aligned CuSCN nanosheets is as follows:

Self decomposition of S₂O₃²⁻ produced SO₃²⁻ ions and S [eqn (1)]. A redox reaction was occurred between Cu²⁺ and SO₃²⁻ ions [eqn (2)]. Thiosulphatocuprate(I) complex [Cu(S₂O₃)]⁻ formed through reaction between Cu⁺ and S₂O₃²⁻ ions [eqn (3)]. The thiosulphatocuprate(I) complex is then react with SCN⁻ ions to form solid CuSCN [eqn (4)]. The powdery material or loosely bounded ions were removed by rinsing the substrate in DDW. Detailed growth mechanism of aligned CuSCN nanosheets is shown schematically in Fig. 5.

S₂O₃²⁻ → 2SO₃²⁻ + S (1)

2Cu²⁺ + 2SO₃²⁻ → 2Cu⁺ + S₂O₆²⁻ (2)

Cu⁺ + S₂O₃²⁻ → [CuS₂O₃]⁻ (3)

[CuS₂O₃]⁻ + SCN⁻ → CuSCN↓ + S₂O₃²⁻ (4)

Fig. 5 Growth mechanism of CuSCN nanosheets.

3.5 Field emission studies

Fig. 6(a) depicts J–E plot of aligned CuSCN nanosheets. The turn-on field, defined as the field required to draw an emission current density of 10 μA cm⁻² is found to be 2.4 V μm⁻¹. Furthermore, high current density of 204 μA cm⁻² has been drawn at an applied field of 4.1 V μm⁻¹. Low turn-on field of aligned CuSCN nanosheets is found to be quite superior than other inorganic semiconducting and carbon based nanomaterials reported in the literature. The comparison of turn-on field with reported literature is summarized in Table 1.^10–16 Since, there is no report on field emission studies of CuSCN nanostructures comparison has been done with various inorganic semiconducting and carbon based nanomaterials which have same morphology. Observation of low turn-on field of CuSCN nanosheets may attributed to nanometric size (average thickness of 45 nm), high density and well aligned nature of CuSCN nanosheets.

Fig. 6 (a) J–E plot of CuSCN nanosheets and (b) corresponding F–N plot.

Table 1 Turn-on field values of the inorganic semiconducting and carbon based nanostructures reported in the literature

Material	Morphology	Turn-on field (V μm^−1) (for J = 10 μA cm^−2)	References
CuSCN	Nanosheets	2.4	Present work
Graphene	Nanosheets	4.6	10
Graphite	Free standing nanosheets	4.7	11
Carbon	Vertically oriented nanosheets	3.0	12
WS2	Nanosheets	5.0	13
MoS2	Nanosheets	10.8 and 11.6 (for J = 1 μA cm^−2)	14
SnS2	Nanosheets	4.6 (for J = 1 μA cm^−2)	15
ZnO	Nanosheets array	2.4 (for J = 0.1 μA cm^−2)	16

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The field emission characteristic is further analyzed by Fowler–Nordheim (F–N) plot. The F–N plot, i.e. ln(J/E²) versus (1/E), derived from the observed J–E characteristic is shown in Fig. 6(b). F–N plot shows overall linear nature. The field enhancement factor (β) has been estimated from the slope of the F–N plot, which is mathematically expressed as,

where, Ø = work function.

By considering the work function of CuSCN as 3.35 eV,³⁴ the field enhancement factor (β) is calculated and is found to be 3067. The observed low turn-on field of CuSCN nanosheets may also attributed to the high value of field enhancement factor (β).

Current stability is one of the decisive parameter in field emission investigation for device application. The I–t plot of aligned CuSCN nanosheets at preset of 1 μA recorded for a period of 3 h is shown in Fig. 7. It is observed that the emission current is almost stable for entire duration of measurement. The appearance of ‘spike’ type fluctuations in the emission current is attributed to various atomic scale process such as adsorption, desorption and diffusion of residual gas species on the emitter surface.³⁵

Fig. 7 I–t plot of CuSCN nanosheets.

4. Conclusions

SILAR technique was employed for the synthesis of aligned CuSCN nanosheets. The nanometric size, high density, single crystalline quality and aligned nature of CuSCN nanosheets leads to superior field emission behavior in terms of low turn-on field 2.4 V μm⁻¹. The observed field emission results are found to be superior than those reported in the literature. A good correlation is found between the morphology and field emission studies of aligned CuSCN nanosheets. Quite inexpensive route of synthesis with excellent field emission results indicate the possible use of aligned CuSCN nanosheets in micro/nano electronic devices.

Acknowledgements

GPP and PGC sincerely thank to DST, SERB, Government of India for financial support through Empowerment and Equity Opportunities for Excellence in Science scheme (Ref. No.: SB/EMEQ-208/2013 dated 23/08/2013). PKB is sincerely thank to DST, SERB, Government of India for financial support through the Fast Track Young Scientist scheme (SR/FTP/PS-063/2012, dated 06/11/2013).

References

1. S. Barth, F. Hernandez-Ramirez, J. D. Holmes and A. Romano-Rodriguez, Prog. Mater. Sci., 2010, 55, 563.
2. A. Datta, D. Mukherjee, M. Hordagoda, S. Witanachchi, P. Mukherjee, R. V. Kashid, M. A. More, D. S. Joag and P. G. Chavan, ACS Appl. Mater. Interfaces, 2013, 5(13), 6261.
3. P. Baviskar, P. Chavan, N. Kalyankar and B. Sankapal, Appl. Surf. Sci., 2012, 258, 7536.
4. M. D. Shinde, P. G. Chavan, G. G. Umarji, S. S. Arbuj, S. B. Rane, M. A. More, D. S. Joag and D. P. Amalnerkar, J. Nanosci. Nanotechnol., 2012, 12, 3788.
5. C. Tan and H. Zhang, Nat. Commun., 2015, 6, 7873.
6. D. Li, M. B. Muller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat. Nanotechnol., 2008, 3, 101.
7. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos and A. A. Firsov, Nature, 2005, 438, 197.
8. V. S. Bagal, G. P. Patil, A. B. Deore, P. K. Baviskar, S. R. Suryawanshi, M. A. More and P. G. Chavan, Chem. Phys. Lett., 2016, 650, 7–10.
9. C. S. Rout, P. D. Joshi, R. V. Kashid, D. S. Joag, M. A. More, A. J. Simbeck, M. Washington, S. K. Nayak and D. J. Late, Sci. Rep., 2013, 3, 3282.
10. C. L. Lian, G. L. Wei, L. Yu, L. Z. Lin, H. Jiao and L. W. Chin, Chin. Phys. B, 2013, 22, 107901.
11. J. J. Wang, M. Y. Zhu, R. A. Outlaw, X. Zhao, D. M. Manos, B. C. Holloway and V. P. Mammana, Appl. Phys. Lett., 2004, 85, 1265.
12. M. Y. Zhu, R. A. Outlaw, M. B. Hansen, H. J. Chen and D. M. Manosa, Carbon, 2011, 49, 2526.
13. S. R. Suryawanshi, P. S. Kolhe, C. S. Rout, D. J. Late and M. A. More, Ultramicroscopy, 2015, 149, 51.
14. R. V. Kashid, D. S. Joag, M. Thripuranthaka, C. S. Rout, D. J. Late and M. A. More, Nanomater. Nanotechnol., 2015, 5:10, 1.
15. S. R. Suryawanshi, S. S. Warule, N. S. Chaudhari, S. B. Ogale and M. A. More, AIP Conf. Proc., 2014, 1591, 342.
16. K. K. Naik, R. Khare, D. Chakravarty, M. A. More, R. Thapa, D. J. Late and C. S. Rout, Appl. Phys. Lett., 2014, 105, 233101.
17. X. Lia and H. Zhua, Journal of Materiomics, 2013, 1, 33.
18. T. Gao and T. Wang, Cryst. Growth Des., 2010, 10, 4995.
19. S. Muralikrishna, K. Manjunath, D. Samrat, V. Reddy, T. Ramakrishnappa and D. H. Nagarajud, RSC Adv., 2015, 5, 89389.
20. H. Li, H. Wu, S. Yuan and H. Qian, Sci. Rep., 2016, 6, 21171.
21. J. E. Jaffe, T. C. Kaspar, T. C. Droubay, T. Varga, M. E. Bowden and G. J. Exarhos, J. Phys. Chem. C, 2010, 114, 9111.
22. K. Zhao, R. Munir, B. Yan, Y. Yang, T. Kim and A. Amassian, J. Mater. Chem. A, 2015, 3, 20554.
23. A. Perumal, H. Faber, N. Yaacobi-Gross, P. Pattanasattayavong, C. Burgess, S. Jha, M. A. McLachlan, P. N. Stavrinou, T. D. Anthopoulos and D. D. C. Bradley, Adv. Mater., 2015, 27(1), 93.
24. N. D. Treat, N. Yaacobi-Gross, H. Faber, A. K. Perumal, D. D. C. Bradley, N. Stingelin and T. D. Anthopoulos, Appl. Phys. Lett., 2015, 107, 013301.
25. R. D. Ladhe, P. K. Baviskar, W. W. Tan, J. B. Zhang, C. D. Lokhande and B. R. Sankapal, J. Phys. D: Appl. Phys., 2010, 43, 245302.
26. P. Pattanasattayavong, G. O. N. Ndjawa, K. Zhao, K. W. Chou, N. Yaacobi-Gross, B. C. O'Regan, A. Amassian and T. D. Anthopoulos, Chem. Commun., 2013, 49, 4154.
27. T. Zhai, L. Li, Y. Ma, M. Liao, X. Wang, X. Fang, J. Yao, Y. Bando and D. Golberg, Chem. Soc. Rev., 2011, 40, 2986.
28. X. Fang, Y. Bando, U. K. Gautam, C. Ye and D. Golberg, J. Mater. Chem., 2008, 18, 509.
29. G. Mittal and I. Lahiri, J. Phys. D: Appl. Phys., 2014, 47, 323001.
30. B. R. Sankapal, E. Goncalves, A. Ennaoui and M. C. Lux-Steiner, Thin Solid Films, 2004, 451, 128.
31. S. L. Dhere, S. S. Latthe, C. Kappenstein, S. K. Mukherjee and A. V. Rao, Appl. Surf. Sci., 2010, 256, 3967.
32. B. Sankapal, A. Tirpude, S. Majumder and P. Baviskar, J. Alloys Compd., 2015, 651, 399.
33. M. A. Becker, J. G. Radich, B. A. Bunker and P. V. Kamat, J. Phys. Chem. Lett., 2014, 5, 1575.
34. N. D. Treat, N. Yaacobi-Gross, H. Faber, A. K. Perumal, D. D. C. Bradley, N. Stingelin and T. D. Anthopoulos, Appl. Phys. Lett., 2015, 107, 013301.
35. G. P. Patil, V. S. Bagal, S. R. Suryawanshi, D. J. Late, M. A. More and P. G. Chavan, Appl. Phys. A, 2016, 122, 560.

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