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Copyright © 2014 by American Scientific Publishers All rights reserved. Printed in the United States of America

Science of Advanced Materials Vol. 6, pp. 1–6, 2014 (www.aspbs.com/sam)

Investigation of Local Field Enhancement and Hot Electron Injection in Carbon Nano-Tube Doped Phosphor Nano-Composite for Ultra-Bright Electroluminescence Deepika Yadav, Dileep Dwivedi, Savvi Mishra, B. Sivaiah, A. Dhar, Virendra Shanker, and D. Haranath∗ CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110012, India

ABSTRACT

1. INTRODUCTION Electroluminescence (EL) is defined as a phenomenon in which a material produces visible light emission when it is subjected to electric field.1 Alternating current EL takes place under very high electric fields and involves charge carrier injection followed by radiative electron–hole recombination process. An EL device is usually a capacitive type of structure in which a light-emitting phosphor layer is sandwiched between two conductive electrodes. Among various EL device structures available, alternating current driven thick film type of structure gained considerable attention as a source of general illumination and display applications, primarily due to easy fabrication (screen-printing) process for large area, low manufacturing cost and no requirement of vacuum processing etc.2–5 With the advantages like low-power consumption, uniform illumination, wide vision, good contrast and brightness, EL devices are being used in watches, digital assistants, cell phones, exit lighting, signs and many more fields of light ∗

Author to whom correspondence should be addressed. Email: [email protected] Received: xx xxxx xxxx Accepted: xx xxxx xxxx

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engineering.6 7 Currently, some groups8 have reported the use of single walled carbon nanotube (SWCNT) as conducting ‘electrodes’ and others9 used them as ‘extended electrodes’ for alternating current (AC) driven EL devices. Kim et al. have reported an entirely different tandem structure for EL devices by using SWCNTs as external field enhancers before the phosphor layer.10 11 By this way they observed enhanced EL at reduced operating voltages due to the intrinsic field emissive properties of CNTs. Moreover, the unique properties such as low turnon voltage, high current density, high chemical stability and high field enhancement factors12–14 made the CNT based materials even more attractive in electronic applications involving field emission displays,15 backlighting units for liquid crystal displays16 and long-lasting EL devices or lamps.17 However, the only factor inhibiting the popularity of EL devices is the high driving voltage requirement (> 350 V) with relatively low brightness values. This issue could be addressed effectively either by improving the crystallinity of phosphor particles or by reducing the thickness of the emitting layer.18 The threshold voltage requirements of the EL device can also be reduced if some localized conducting paths are introduced inside

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doi:10.1166/sam.2014.1731

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Present work focuses on the effective doping of multi-walled carbon nanotube (CNT) in the ZnS:Cu phosphor nano-composite and thereafter improvement in the optical performance of electroluminescent (EL) device due to increased local field effects. To facilitate doping of CNTs into the phosphor and decrease the operating voltage of the EL device, CNTs were shortened by milling and incorporated effectively using a flux assisted solid-state annealing reaction. Interestingly shorter the length of CNTs used; greater was the local field enhancement, improvement in brightness and efficiencies observed for the EL devices. When the field is applied, adequate charge carriers are tunneled into the ZnS:Cu system through the tips of the CNTs by forming high energy hot spots thus enhancing the local field. The improved device characteristics are due to field enhancement and effective transfer of energy from hot spots to copper activator by impact ionization. The detailed electrical characterization of the novel EL device along with its brightness measurements are presented by considering the hot electron injection model. KEYWORDS:

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Investigation of Local Field Enhancement and Hot Electron Injection in CNT Doped Phosphor Nano-Composite for Ultra-Bright EL

the phosphor grains. Using this concept, we have successfully incorporated multi-walled CNTs into a cathodoluminescent phosphor material and observed smart EL at nominal threshold voltages < 100 V at 1.5 kHz for Orange-red emitting ZnS:Mn phosphor19 and < 50 V at 2.65 kHz for Green emitting ZnS:Cu phosphor.17 But, the CNTs employed in latter case had a very high aspect ratio and the probability of shortening of the EL device also increased due to vertical alignment of CNTs (if any) between the two electrodes while fabricating the device. In the current study, we report a modified methodology by which the CNTs were shortened by milling and doped in comparatively larger amounts; and also able to prevent undesired flow of current by the randomly aligned CNT networks inside the ZnS:Cu phosphor grains. Further, this has improved the stability of the device, luminance and efficiency at reduced driving voltages indicating the importance of local field enhancement in EL devices due to CNTs and proposes a novel device structure to construct highly efficient smart EL devices. Serious efforts have been explored to improve the characteristics of EL devices using nanocomposite based structures such as organic dyephosphor composites,20 multiple-emitting layer structure21 etc., but high brightness with a simple device structure operated at relatively low voltages (< 50 V) and frequency (< 5 kHz) is still considered a grand challenge for EL devices as one of the futuristic flat panel displays.

2. EXPERIMENTAL DETAILS 2.1. Preparation of EL Phosphor The well-known alkaline route22 has been adapted to synthesize green-emitting ZnS:Cu phosphor and was used for making an EL device in the current study. In a typical process, NH4 OH was added to ZnCl2 under constant stirring conditions to form zinc ammonia complex. To this H2 S gas has been bubbled continuously to obtain the required ZnS precipitate. After repeated washing and drying cycles, the aqueous solutions of activators, Cu and Al, have been added as copper acetate and aluminum nitrate. The slurry was then kept in an oven at ∼ 80 C for 5 h in order to completely dry the chemical constituents. All the chemicals used were of analytical grade from Merck and were used without further purification. After thorough milling, the precursor was packed in a quartz boat and fired at 700–1100 C for 1–3 h to get the desired ZnS:Cu phosphor. Interestingly this phosphor did show green photoluminescence under UV (365 nm) irradiations but not electroluminescence upon application of AC voltages up to 500 V. In another step, multi-walled carbon nanotubes (CNTs) grown by chemical vapor deposition technique were procured from M/s JK Impex, India. The crushing of the CNTs was performed using a high energy ball milling machine in a tungsten carbide jar with tungsten carbide balls for about 2 h under argon atmosphere to reduce 2

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their aspect ratio. The ball to CNT ratio and the rotation speed of the rotating jar were kept constant at 5:1 by weight and 75 rpm, respectively. The sizes of the asprepared and ball milled CNTs were estimated using electron microscopy observations (not shown). To remove the undesired presence of amorphous carbon from ball milled CNTs, a facile sonication method suggested by Rinaldi et al. was adopted.23 About 5 mg of crushed CNTs were dispersed in 10 ml of ethyl alcohol via rigorous ultrasonication for 5 h to form stable suspension that has been used for all experiments. The required amount of CNT solution was further added to ZnS:Cu phosphor and allowed to dry at ∼ 50 C. NH4 Br and KBr were added as chemical additives in amounts < 10 wt% for effective and adequate doping of CNTs inside the ZnS:Cu phosphor grains. After complete drying and mixing of the constituents, the mixture was annealed at 450 C under a flowing N2 gas for about an hour. The amount of CNT addition is a critical parameter in all our experiments. However, the advisable quantity to exhibit stable and bright EL was below 0.25 wt% for the ZnS-CNT nanocomposite. 2.2. EL Device Fabrication In general, the layer structure of an EL device is comprised of an glass substrate/reflecting electrode/phosphor/ transparent electrode/glass substrate, and each layer is deposited using wet-chemical, Dr. Blade technique and e-beam evaporation respectively. Figure 1 shows the schematic diagram of the EL device fabricated experimentally using CNT doped ZnS:Cu phosphor nanocomposite and mentioning of an indigenously made driving circuit. Appropriate amount of dielectric (dielectric constant, r ≈ 5) was mixed with the CNT embedded phosphor powder and spread between the two electrodes using Dr. Blade technique. The electrodes used are silver coated glass substrate as back electrode and the other is ITO coated glass substrate to offer transparency and observe light-emission from the EL device under the influence of an applied electric field. The active area of the EL device fabricated was 75 × 25 mm2 . In order to drive the EL

Fig. 1. Schematic diagram of CNT doped ZnS:Cu phosphor EL device structure. The bar indicated in SEM image is of 10 m. Sci. Adv. Mater., 6, 1–6, 2014

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device, an indigenous portable power supply was designed as shown in Figure 2. To reduce the cost and better portability, the circuit was designed in such a way that it could be driven using a conventional 9 V DC battery. As a first step, to convert DC into AC, a 555 timer IC in astable multivibrator mode was used. The resistance and capacitor values are adjusted such that it produces a fixed AC signal of frequency 2.5 kHz. A variable resistor has been included in the circuit to vary the input voltage to the current booster, which further feeds a high voltage amplifier to get the desired operating voltage. At the output side of the circuit, one could get a sinusoidal signal of 2.5 kHz frequency with an option to vary the output voltage from 12–300 VAC with the help of the potentiometer.

3. RESULTS AND DISCUSSION

Fig. 2. Circuit diagram of low-cost portable power supply to drive the EL device.

Fig. 3. Raman spectra of the as-prepared and ball milled CNTs.

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2.3. Characterization The device characteristics were measured by applying a sinusoidal AC voltage of 2.5 kHz under ambient conditions. All the AC measurements reported in the paper refers to the peak-to-peak voltage. AC signal was generated using the circuit described in Figure 2. High voltage was measured using 6.5 digits, Keithley 2100 multi-meter. The brightness of the device was measured using a Luminance Meter (Minolta, LMT 1009). The high energy ball milling of CNTs was performed in a ball milling machine from M/s. Fritsch, Germany (Pulveresette 4). The structure and Raman spectroscopy of as-prepared and ball milled CNTs were studied using Micro-Raman Microscope (Renishaw inVia make) equipped with a He–Ne Laser of wavelength (514 nm). The X-ray diffraction patterns were recorded using (XRD; Rigaku: MiniFlex, Cu K; = 154 Å). The surface morphology and micro-structural characterization was carried out by scanning electron microscopy (SEM, LEO 440). The steady state photoluminescence (PL) excitation and emission spectra were recorded using Luminescence Spectrometer (Edinburgh, F900) at room temperature (∼ 25 C).

As mentioned earlier, the multi-walled CNTs were ball milled in a high energy ball milling machine under argon atmosphere using tungsten carbide jar and tungsten carbide balls at 75 rpm for 2 h. The average lengths of as-prepared and mechanically ball milled CNTs were ∼ 15 m and ∼ 5 m, respectively. The Raman spectral peak at 1325 cm−1 represents the disorder state of carbon atoms in the graphitic structure, known as the D-band. It is clearly seen from the Figure 3 that the D-band intensities of the CNTs increased after ball milling in comparison to the as-prepared CNTs. The higher D-band intensity of the ball milled CNTs is originated from a relatively large amount of amorphous carbon produced through the ball milling process. The ratio of areas under G and D-bands of as-prepared CNTs is approximately 1, where as this ratio for ball milled CNTs is less than one, which indicates that as-prepared CNTs have better graphitic structure as compared to ball milled ones. To remove the undesired presence of amorphous carbon from ball milled CNTs, a facile sonication method was adopted.19 When an EL device is fabricated using as-prepared CNTs with better graphitic structure, there exists a high probability of flow of undesired current in the randomly oriented CNT networks inside the ZnS:Cu grains. The current flowing through the EL device initially heats it up, degrades the phosphor and finally burns out limiting the operating life time of the device. Hence, in the current study deliberately shortened CNTs via ball milling were selected and doped effectively into the ZnS:Cu phosphor grains to overcome the undesired shortening effect of electric field and to get more field enhancement. Usually, an electric field in an AC driven EL device is the key governing factor for the luminance enhancement as the luminescence intensity varies with the strength of the electric field.24 25 Figure 4 shows the current density (J ) versus electric field (E) characteristics of the fabricated EL device operated in the applied field ranging


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Fig. 4. Variation of current density (J ) as a function of the applied field (E) for the AC driven EL device; First inset shows the change in current efficiency of the EL device as a function of applied voltage; second inset shows the photograph of working device at an operating voltage of 100 VAC and 2.5 kHz.

0 to 6 × 106 V/m. The profile clearly indicates that the EL device is Ohmic in nature up to 527 × 106 V/m and beyond this it showed an exponential behavior. This may be due to the fact that the EL device acts as a capacitor up to 527 × 106 V/m and thereafter it starts behaving like a semiconductor due to accumulation of large number of charge carriers in the ZnS-CNT nanocomposite at higher voltages. The reciprocal of the slope of the above curve indicates the resistivity of the EL device, which is ∼ 03779 × 106 -m. The observed behavior of the current density curve with respect to applied field could be explained as follows: For a normal electrical conduction, the conditions are always such that the free charge carriers (electrons) involved have thermal energies that are very much greater than the energy corresponding to the drift velocity under the influence of applied field. This is the primary condition for the validity of Ohm’s law. However, deviations from Ohmic conduction can be expected when the applied fields are very high that in turn results in decrease of effective mobility of the carriers. First inset of Figure 4 shows the variation of current efficiency (CE) of the device as a function of applied voltage. It is usually calculated by taking the ratio of luminance (L) and current density (J ). It has been observed that the CE of the EL device increases gradually with an increase in the applied field. Second inset shows the photograph of fabricated EL device operating at 100 VAC and a frequency of 2.5 kHz. The observed brightness was uniform and color of the EL emission is centered at 515 nm. The underlying mechanism responsible for generating EL from CNT doped ZnS:Cu phosphor can be explained by field enhanced hot electron injection model as shown in Figure 5. The CNT doped ZnS:Cu phosphor grains have ZnS–Cux S hetero-junctions, where ZnS (Cux S) is n-type (p-type) semiconductor, a junction similar to the Schottky.22 When an electric field of 106 –107 V/m 4

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Fig. 5. Appearance of high energy hot-spots in CNT doped EL device due to the tunneling of electrons between the nearest CNTs when electric field (E) greater than the threshold fields (Eth ) are applied across the material. The arrows indicate the direction of flow of electrons.

is applied to the phosphor, high electric field induces tunneling of electrons between the two nearest CNTs which creates high energy hot spots near the tunneling region. Increase in the electric field further energizes the electrons in the hot spots i.e., electrons in the phosphor are accelerated to higher energies. Energy transfer from these ballistic energy electrons to luminescent centers (Cu+ result in promoting the impact excitation of the luminescent centre. Finally, the radiative relaxation of luminescent centers leads to bright green EL emission at 515 nm. Figures 6(a)–(b) show the schematic band model of simultaneous injections of electrons and holes from opposite ends of a Cux S needles into the ZnS system,25–28 without and with the incorporation of CNTs, respectively. When the CNTs were effectively doped inside the ZnS:Cu system, a slight band bending appears across the interface between CNT and ZnS:Cu phosphor (Fig. 6(b)). By considering the Fermi level of CNTs and their band bending, it is evident that the thickness of the tunneling barrier gets reduced to great extent due to which electrons can easily overcome tunneling barrier even at lower bias voltages leading to more charge carrier injection into the ZnS system. As compared to usual EL device, the CNT doped device experiences local field enhancement that causes impact excitation of luminescent centers leading to generation of sufficient number of electrons and holes.25 Holes are being trapped at the Cu recombination centers whereas electrons at the Br donor sites. At the reversal phase of electric field, the more emitted electrons by field enhancement of CNTs can be recombined with the trapped holes to produce efficient EL light emission as shown in Figures 6(a)–(b). The luminance characteristics of the CNT doped EL device were recorded at applied voltages up to 250 VAC and are shown in Figure 7. The power efficiency ( ) has been calculated from the equation = ∗ L/ V ∗ J where L is luminance in cd-m−2 , V is the applied voltage and J is the current density in A-m−2 . The threshold voltage for the EL device to exhibit stable green emission was found to be as low as ∼ 40 VAC . In general, the EL Sci. Adv. Mater., 6, 1–6, 2014

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4. CONCLUSIONS A series of AC driven EL devices have been fabricated using a novel methodology of doping CNTs inside the ZnS:Cu phosphor nanocomposite. The CNTs were crushed to decrease their aspect ratio by high energy ball milling. The EL performance was found to be improved with adequate doping of CNTs, indicating the importance of local field enhancement and prevention of undesired flow of current in the CNT networks inside the ZnS:Cu phosphor nanocomposite. The evaluation of the EL device efficiency highlighted the modified B–V characteristics upon CNT doping. The role of CNT has been understood as a local electric field enhancer and facilitator in the hot carrier injection to produce ultra-bright EL in the nanocomposite.

Fig. 6. (a) Schematic band model of simultaneous injections of electrons and holes from opposite ends of a Cux S needle into the ZnS:Cu surrounding lattice, (b) schematic showing band bending due to incorporation of CNTs.

brightness (B) of the device increases with an increase in the applied voltage (V ) and attains a saturation value. The empirical relation29 that governs the above parameters is given by B = B0 exp −bV −05 where, B0 and b are the constants. The values of these constants depend on the EL material characteristics and the dielectric used. However, the presence of CNTs seems to have modified the B–V relationship, which may be due to efficient intra-CNT impact excitation by hot carriers inside the ZnS grains.30

Fig. 7. Luminance and efficiency of CNT doped EL device as a function of the applied voltages. Sci. Adv. Mater., 6, 1–6, 2014

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Acknowledgments: The authors’ sincerely acknowledge the Department of Science and Technology (DST), Government of India, for the financial support under the scheme SR/FTP/PS-012/2010 and Academy of Scientific and Innovative Research (AcSIR) to carry out the research work. We thank Ms. Shweta Sharma of Carbon section for the micro-Raman measurements.


24. Y. A. Ono, Electroluminescent Displays, World Scientific, River Edge, NJ (1995). 25. A. G. Fischer, J. Electrochem. Soc. 110, 733 (1963). 26. S. Shionoya and W. M. Yen, Phosphor Handbook, I edn., CRC Press, USA (1999). 27. A. G. Fischer, J. Electrochem. Soc. 109, 1043 (1962). 28. A. G. Fischer, Luminescence of Inorganic Solids, edited by P. Goldberg, Academic Press, New York (1966), Chap. 10. 29. P. Zalm, Philips Res. 353, 417 (1956). 30. J. P. Keir and J. F. Wager, Annu. Rev. Mater. Sci. 27, 223 (1997).

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19. D. Yadav, S. Mishra, V. Shanker, and D. Haranath, J. Alloys Compds., In Press, Available online 29 July (2013), DOI: http://dx. doi.org/10.1016/j.jallcom.2013.07.124. 20. T. Satoh, M. Kobayashi, S. Kawamura, and T. Uchida, Electron. Lett. 43, 1048 (2007). 21. J.-Y. Kim, M. J. Bae, S. H. Park, T. Jeong, S. Song, J. Lee, I. Han, J. B. Yoo, D. Jung, and S. Yu, Org. Electron. 12, 529 (2011). 22. H. Chander, V. Shanker, D. Haranath, S. Dudeja, and P. Sharma, Mat. Res. Bull. 38, 279 (2003). 23. A. Rinaldi, B. Frank, D. S. Su, S. B. A. Hamid, and R. Schoogl, Chem. Mater. 23, 926 (2011).

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