Facile synthesis of ZnPc nanoflakes for cold cathode emission

Abstract

The challenge of developing two dimensional metal phthalocyanine nanostructures by controlling the reaction protocols is successfully addressed in the present work by synthesizing zinc phthalocyanine (ZnPc) novel nanoflakes using a simple low temperature hydrothermal route. The as synthesized samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ultra-violet visible spectrometer (UV-Vis), X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscope (FESEM). The field emission or cold cathode emission characteristics of these phthalocyanine nanostructures have been reported for the first time here and it is shown that as prepared nanoflakes can act as electron field emitter having a turn-on field 4.7 V μm⁻¹ at a current density of 1 μA cm⁻² for an inter electrode distance of 130 μm. The local electric field distributions around nanoflakes were also further studied theoretically using a finite element method. The obtained results indicate that ZnPc nanoflakes are the potential candidate for electron emission based applications such as vacuum nanoelectronic devices and field emission display devices.

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Introduction

In the last few decades, organic semiconductors have gained significant interest due to its various applications such as non-volatile memory devices, solar cells, light emitting diodes, field effect transistors etc.^1–5 The low temperature deposition technique, mechanically flexible nature and light weight for the processing of the organic nanostructures are advantageous over the inorganic and other counterparts.^6,7 Among the organic semiconductors, metal phthalocyanines (MPcs), consisting of central metallic atom bound to a π conjugated ligand, are of special interest because of their high molecular symmetry, electron delocalization, high thermal, chemical stability and outstanding optical and electronic properties.^8–11 Field induced electron emission properties of these phthalocyanine nanostructures as well as organic nanostructures have garnered an incredible thrust for the development of field emission display (FED) where electrons are emitted from these nanostructures by overcoming surface potential barrier in the presence of an externally applied electric field. Recently we have reported the cold cathode emission property of copper phthalocyanine (CuPc) nanostructures such as CuPc nanowires, nanotubes, nanotips and CuPc nanoparticles functionalized amorphous carbon nanotube.^12–15 In these regards, field emission properties of other kinds of organic nanostructures such as Cu-TCNAQ (tetracyanoanthraquinodimethane) nanowires, CuTCNQ (tetracyanoquinodimethane) nanotubes, CuTCNQ nanorods, aligned PANI nanotubes, Alq₃ {Tris(8-hydroxyquinoline)aluminium} nanowires, FePc, H₂Pc, CuPc nanowires and nanofibers are well documented in literatures.^16–24 To the best of the authors' knowledge, there is no report related to field emission property of any kind of zinc phthalocyanine (ZnPc) nanostructures.

Various methods are employed to the synthesis of ZnPc nanostructures. For example, Tong et al.²⁵ fabricated the ZnPc nanoribbon through organic vapour phase deposition. ZnPc nanoflower with aggregated nanoparticle on gold coated quartz substrates was synthesized by Karan et al.²⁶ through vapour deposition techniques. Chowdhury et al.²⁷ prepared the ZnPc nanorods by thermal evaporation method followed by post annealing technique. All the methods require high temperature and special kind of templates for the development of ZnPc nanostructures. Therefore alternative approaches such as self assembly, hydrothermal and solvothermal are advantageous techniques over the physical deposition routes. Guo et al.²⁸ reported the ZnPc hierarchical nanostructure with hollow interior space by solvothermal route and studied their photocatalytic property. Till now there is no report in the literatures on the synthesis of novel ZnPc nanoflakes through hydrothermal route.

In this work, the authors report the fabrication of ZnPc nanoflakes through hydrothermal route. The chemical structure, phase purity, exact valence state and morphology were analysed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) technique, X-ray photoelectron spectroscopy (XPS) and field emission scanning electron microscopy (FESEM) respectively. Field emission properties of the synthesized nanostructures are reported here. Additionally finite element method is used to justify the experimental results from theoretical point of view.

Experimental

Zinc phthalocyanine (ZnPc) nanoflakes were formed through a simple low temperature hydrothermal route in water at 180 °C. In a typical reaction, zinc powder (0.45 mmol), phthalonitrile (1.8 mmol), sodium dodecyl sulphate (SDS) (20 mg) and ammonium molybdate (10 mg) were dispersed in water (12 ml) and the solution was vigorously stirred for 40 minutes. After that the resulting solution was put in a Teflon lined stainless-steel auto clave (15 ml volume) and kept at 180 °C temperature for 24 h. Finally the blue-purple precipitate was washed with 1 (M) HCl aqueous solution, ethanol and water separately and then dried at 60 °C for over night.

The as-prepared samples were characterized by X-ray diffraction technique (XRD-Rigaku Ultima-III, by Cu K_α radiation and λ = 1.54059 Å). In this measurement, the sample was put on a quartz substrate and 2θ ranging from 5° to 30° with scanning rate 2° min⁻¹. The chemical structure of the synthesized nanoflakes was analysed by the help of Fourier transform infrared (FTIR) spectrometer (Shimadzu IRPrestige-21, spectral resolution 0.5 cm⁻¹). X-ray photoelectron spectroscopy (SPECS, HSA 3500) was used to examine the valence states of the constituent elements in the sample where monochromatic Al K_α with hν = 1486.6 eV was used as X-ray source. Morphological study of the as prepared sample was carried out by field emission scanning electron microscope (FESEM, Hitachi S-4800). Field emission studies were measured in high vacuum field emission set up.

Results and discussion

The crystallinity of the as-prepared ZnPc nanoflakes was analyzed by XRD analysis. The major peaks of ZnPc were obtained at 6.86°, 9.14°, 10.5°, 12.51°, 18.16°, 18.68°, 21.16°, 23.66°, 26.18°, 28.06° values of 2θ, which corresponds to reflection from (1̄01), (101), (002), (200), (3̄01), (202), (1̄12), (211), (212), (311), (3̄13) plane of ZnPc respectively which is shown in the Fig. 1(a). All the peaks are well suited with standard JCPDS data card (number: 39-1882).²⁹ The chemical structure of ZnPc nanoflakes was analysed by Fourier transformed IR spectra shown in the Fig. 1(b). The peaks at 726 cm⁻¹ and 777 cm⁻¹ were assigned to the C–H out of plane deformation and the peaks at 753 cm⁻¹, 1085 cm⁻¹, 1118 cm⁻¹, 1167 cm⁻¹, 1286 cm⁻¹ corresponds to C–H in plane bending. The peaks at 889 cm⁻¹ and 1411 cm⁻¹ are assigned to isoindole stretching. The peaks at 1060 cm⁻¹, 1332 cm⁻¹ and 1486 cm⁻¹ are related to C–H bending, in-plane pyrrole stretching and C=C benzene stretching respectively.^11,30 X-ray spectroscopy technique is used to confirm the exact valence state of the constitute elements present in the ZnPc nanoflakes. The peak located in the Fig. 1(c) shows the core level spectra of Zn 2p and have two spin–orbit split components which are positioned at 1022.5 eV and 1045.6 eV of the electronics states of Zn 2p_3/2 and 2p_1/2 respectively.^9,31 The Fig. 1(d) shows the high resolution spectrum of N 1s at 394.5 to 403.0 eV, consisting of two groups of nitrogen atoms in dissimilar chemical environments in the ZnPc.³¹

Fig. 1 (a) XRD pattern of ZnPc nanoflakes; (b) FTIR spectra of ZnPc nanoflakes; (c) & (d) high resolution spectra of Zn 2p doublet and N 1s respectively.

Fig. 2 shows the UV-Vis absorption spectrum of ZnPc nanoflakes. The absorption band from 300 to 450 nm corresponds to B band arising from the deeper π levels to LUMO transition and the band 550 nm to 750 nm implies the Q-absorption band of ZnPc. In the Q band, the peaks observed at 604 nm and 667 nm are mainly due to the π–π* transition of monomer from the HOMO state to the LUMO state of the Pc²⁻ ring. The peak is found at 637 nm in the Q band which might be responsible for the vibronic band due to the dimers and multimers.²⁸

Fig. 2 UV-Vis spectra of ZnPc nanoflakes.

The FESEM images of the as synthesized ZnPc nanostructures at different magnifications are shown in the Fig. 3. In the Fig. 3(a) and (b) some nanoflakes are interconnected with each other. The diameters of the nanoflakes are around 700 nm and thicknesses of the nanoflakes are 20–40 nm. Fig. 3(d) shows the commercially available potatochips which is similar to our hydrothermally synthesized nanostructures. That's why our synthesized nanoflakes can be called as nanochips. It is clearly shown that the sharp edges are present in the single nanoflake periphery (shown in the Fig. 3(c)) which helps to produce the local field enhancement and field emission. A schematic growth mechanism for nanoflakes formation is shown in the Fig. 4. In the reaction, ammonium molybdate acts as a catalyst to accelerate the hydrothermal reaction and at first small nuclei are formed and after that these small nuclei tend to grow the 2d sheet like morphology in the presence of surfactant at 180 °C.

Fig. 3 (a)–(c) FESEM images of ZnPc nanoflakes at different magnifications; (d) digital image of commercially available chips.

Fig. 4 Schematic formation process of ZnPc nanoflakes.

In the present work, SDS behaves as structure-directing agent and in the solution, SDS decomposes into DS⁻ (C₁₂H₂₅–O–S–O₃⁻) and Na⁺ ion in water medium which pretentious the morphology of the hydrothermally derived nanostructures. At the initial stage of reaction, small ZnPc nuclei are formed where zinc powders acts as the nucleating agent and SDS provides the microenvironment for the synthesis of ZnPc.⁶ When temperature increases, it tends to form one dimensional nanostructure through along c-axis due to its monoclinic unit cell. But DS⁻ ions were anchored with the Zn²⁺ ions of ZnPc structure due to the electrostatic attraction of anions and cations.^32,33 This adsorption of DS⁻ ions blocked the further nucleation and growth of self-assembly process through π–π and van der Waals interaction for the formation of 1D phthalocyanine nanostructure. This soft templating behaviour of SDS in the solution phase profoundly modifies the final 2 dimensional nanoflake like structures³⁴ which are clearly seen in the Fig. 3(c). Another important role of SDS is that SDS acts as an emulsifying agent in the solution phase to provide a hydrophobic environment and hydrolysis of phthalonitrile precursor is prevented by this microenvironment.⁶ When the temperature increases and reaches at 180 °C, surface tension of the system decreases and this reduced surface tension help to lower the aggregation behaviours and form the well resolved nanosheet like morphology. Same kinds of observations are found elsewhere.^35,36

Cold cathode emission characteristics were performed in our home designed high vacuum system at pressure of ∼10⁻⁶ mbar. High vacuum is achieved by using diffusion pump with a liquid nitrogen trap and high vacuum level was measured through a penning gauge. In our experiment, samples glued onto a conducting carbon tape that acted as cathode and a conical shaped stainless steel tip (1.0 mm tip diameter) worked as anode. From macroscopic point of view, it seems conical shaped; however, the bottom of the tip has a flat, smooth circular surface of 1 mm diameter. Thus in nano regime, the anode behaves like a flat plate and a parallel plate configuration is obtained. So, uniform electric field is generated. The electric field was determined from the ratio of applied voltage with the separation distance. The sample size was ∼10 mm × 10 mm and the anode–cathode distance was adjustable to a few hundred micrometers with the help of micrometer screw. The cold cathode emission current voltage characteristics were analysed by the classical Fowler Nordheim (F–N) equation:³⁷

J = (Aβ²E²/φ)exp(−Bφ^3/2/βE) (1)

where A = 1.54 × 10⁻¹⁰ (A V⁻² eV), B = 6.83 × 10⁹ (V m⁻¹ eV^−3/2), J is the current density, E is the applied field, φ is the work function of the emitting materials and β is the enhancement factor relating to the following equation:

β = −Bφ^3/2/S (2)

where S is denoted as the slope of the F–N plot (i.e. a plot of ln(J/E²) versus 1/E) and it is obtained from the F–N eqn (1). The experimental J–E curve and corresponding F–N plot are shown in the Fig. 5(a) and (b). The experimental turn on field of the synthesized ZnPc nanoflakes for 130 μm inter-electrode distance is 4.7 V μm⁻¹ (defined as the field required obtaining current density of 1 μA cm⁻²). The enhancement factor of ZnPc nanoflakes have been calculated by taking the work function 4.7 eV (ref. 38) is 2244 (high field portion of F–N plot). The registered turn-on field and enhancement factor of the present nanostructures are comparable or better than previously reported several organic and inorganic field emitter (shown in the Table 1).^13,39–41

Fig. 5 (a) Current density (J) versus applied field (E) plot of ZnPc nanoflakes at 130 μm inter-electrode distances and inset of this figure indicates the J–E curve in the lower field region; (b) F–N plot of ZnPc nanoflakes; (c) field emission stability test of ZnPc nanoflakes at 130 μm anode–cathode distance; (d) three dimensional perspective of the models of ZnPc nanoflakes along with computed electric field distribution as overlay.

Table 1 Comparison of synthesis route, turn-on field and field enhancement factor of various cold cathode nanostructures

Cold cathode	Synthesis route	Turn-on field (V μm^−1)@1 μA cm^−2	βexperimental	Cathode–anode distance	Ref.
CuPc nanotube	Hydrothermal	3.2	1300	180 μm	13
SiNW–ZnO core–shell arrays	Plasma assisted-atomic layer deposition	7.6	4227	100 μm	39
Carbon nanotube	Plasma enhanced chemical vapour deposition	4.71	460	400 μm	40
ZnO single crystal microtube	Microwave heating	5.6	1548	51 μm	41
ZnPc nanoflakes	Hydrothermal	4.7	2244	130 μm	This work

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Field electron emission behaviour depends on the various parameters such as nanostructure morphology, cathode–anode distance, work function and screening factor etc.^42–47

Sharp geometry of the nanostructure plays the significant role to enhance the cold cathode emission. In our case, nanoflakes with sharp edges develop the local electric field and as a result the electric lines of force easily accumulate at the cathode. Therefore electron emission occurs at lower electric field to cross the reduced surface potential barrier. The potential energy of field emitter can be written as,⁴⁸

U(x) = −e²/4x − βeEx + E_F + ϕ (3)

where e, x, E, E_F and ϕ are the electronic charge, distance from field emitting surface, electric field strength projected on the emitter surface, Fermi energy level of the cold cathode and surface potential barrier of electron respectively. The physical significance of field enhancement factor (β) is nothing but the geometrical effect of nanostructures upon application of electric field. In our case nanoflakes morphology with sharp edges can reduce the potential energy barrier upon application of electric field and enhance the field emission performance. The F–N plot has two distinguished slopes and the non-linear behaviour over the applied field range, arises due to several reasons.^49–53 Due to semiconducting nature of ZnPc, electrons are emitted from the conduction band state at lower electric field and at the higher field strength electrons are tunnelled out from the valance band state. Vacuum space charge is the one of the responsible factor for this kind of non linear behaviour. In the field emission vacuum systems, there may be some residual gases present. Due to electron bombardment, these gases desorbed from the anode and at the same time it desorbed from the cathode due to local heating. These gases are ionized along with cathode emitted electron charge and produce space charge in the inter-electrode gap. Dynamic behaviour of space charge, which is generated in the vacuum chamber may enhance or reduce the local field enhancement factor. In the high field region, more space charges are generated due to ionization of gases near the anode and it is directed toward cathode which increases the local electric field or β value. At low field region, lesser amount of space charge is created in the vacuum gap and β factor is related to nanostructure surface geometry. Therefore our synthesized samples have the higher β value (2244) at high field region in comparison with lower β value (1440) at the lower field region. Field emission stability is one of the major criteria for implementation of field emitting material into the practical applications. Furthermore, we have probed the stability of field emission up to 120 minutes using an applied field of 10.4 V μm⁻¹ that corresponds to current density of 103 μA cm⁻² for an inter electrode distance 130 μm. The stability plot (shown in Fig. 5(c)) display only marginal (less than 7%) degradation/fluctuation of the emission current density as compared to similar organic field emitters.^24,54

We have further computationally investigated the local electric field profile of ZnPc nanoflakes by a finite element method as implemented in ANSYS Maxwell simulation package for theoretical understanding of the cold cathode emission at the level of ZnPc nanoflakes.^55,56 Simulated electric field distributions were carried out for ZnPc nanoflakes as cathode and stainless steel electrode as anode. Simulation parameters were chosen in direct analogy with the experiments performed. The separation between cathode and anode was taken as 130 μm and 2 kV potential was applied between the cathode and anode. The whole system was kept inside a vacuum chamber during all steps of the computation process. A rainbow colour coordinate (red is maximum and blue is minimum) is used to plot the magnitude of the electric field of 2d plane. Fig. 5(d) show the three dimensional perspective models of nanostructures where some distinct nanoflakes are simulated along with computed electric field distribution as overlay. The simulated maximum field strength is found 41.9 V μm⁻¹ and electrostatic field distribution indicates that the sufficient electron emitting sites are present in the nanoflakes like structure which supports the experimentally observed field emission behaviours.

Conclusions

We have reported the field emission characteristics of hydrothermally synthesized ZnPc nanoflakes with the presence of SDS surfactant. The samples were well characterized by XRD, FTIR, XPS and FESEM. The nanoflakes show the 4.7 V μm⁻¹ turn-on field (at a current density of 1 μA cm⁻²) with field enhancement factor 2244. The acquired results are further verified by ANSYS Maxwell simulation analysis. The obtained turn on field and stable cold cathode emission behaviour strongly reflect that the ZnPc nanoflakes can be used as the potential candidate for vacuum nanoelectronics and field emission display devices.

Acknowledgements

The author (MS) wishes to thank Department of Science and Technology for funding during her research work. The authors (UKG, SD) acknowledge the financial support from the Council of Scientific and Industrial Research (CSIR), the Government of India, for awarding a Senior Research Fellowship during the execution of the work. We also wish to thank the University Grants Commission for ‘University with Potential for Excellence scheme’ (UPE-II) the Government of India for financial help.

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Footnote

† These authors contributed equally to this work.

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