Local Field Enhancement-Induced Enriched Cathodoluminescence

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Local Field Enhancement Induced Enriched Cathodoluminescence Behaviour from CuI-RGO Nanophosphor Composite for Field Emission Display Applications Subhajit Saha, Rajarshi Roy, Swati Das, Dipayan Sen, Uttam Kumar Ghorai, Nilesh Mazumder, and Kalyan Kumar Chattopadhyay ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02047 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on May 14, 2016

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ACS Applied Materials & Interfaces

Local Field Enhancement Induced Enriched Cathodoluminescence Behaviour from CuI-RGO Nanophosphor Composite for Field Emission Display Applications Subhajit Saha1, Rajarshi Roy1,2, Swati Das2, Dipayan Sen2, Uttam Kumar Ghorai1, Nilesh Mazumder2 and Kalyan Kumar Chattopadhyay1, 2* 1

School of Materials Science and Nanotechnology, Jadavpur University, Kolkata-700032, India

2

Department of Physics, Jadavpur University, Kolkata-700032, India

ABSTRACT: Field emission displays (FEDs) constitute one of the major foci of the cutting edge materials research owing to the increasingly escalating demand for high resolution display panels. However, poor efficiencies of the concurrent low voltage cathodoluminescence (CL) phosphors have created a serious bottleneck in the commercialization of such devices. Herein we report a novel CuI-RGO composite nanophosphor which exhibits bright red emission under low voltage electron beam excitation. Quantitative assessment of CL spectra reveals that CuI-RGO nanocomposite phosphor leads to the four fold enhancement in the CL intensity as compared to the pristine CuI counterpart. Addition of RGO in the CuI matrix facilitates efficient triggering of luminescence centers that are activated by local electric field enhancement at the CuI-RGO contact points. In addition, conducting RGO also reduces the negative loading problem on the surface of the nanophosphor composite. The concept presented here opens up a novel generic route for enhancing CL intensity of the existing (nano)phosphors as well as validates the bright prospects of the CuI-RGO composite nanophosphor in this rapidly growing field.

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Keywords: CuI-RGO composite, nanophosphor, cathodoluminescence, field emission display, density functional theory INTRODUCTION: Recent advances in high resolution display technology have entailed parallel development in several flat panel display technologies having effective energy saving performances. Among these next generation displays, FEDs define a supreme pinnacle due to their high brightness, low power consumption, large color gamut, wide viewing angle and short response time.1-3 In such devices the accelerated electron beam generated from an array of nanostructured cathode excites a phosphor or group of phosphors to produce the image. Hence, prospective FED phosphors should have some unique properties such as high CL efficiencies at low electron voltages (< 5kV) and good stability at high current density (>100 µA/cm2).4,

5

However, reduced CL

efficiency of the current iteration of phosphors in the low electron voltage regime creates a serious bottleneck in this nascent field. Therefore, to circumvent this problem, development of highly efficient low voltage CL phosphors has recently garnered the researchers’ attention in worldwide scale. Many sulphide based phosphors such as Y2O2S:Eu, Gd2O2S:Tb, ZnS:Ag,Cl, etc. have shown good promise in the low voltage region;6-8 but unfortunately these phosphors often undergo decomposition under electron beam excitation, resulting in evolution of S containing gasses which eventually corrodes the cathode.4 In addition, all red phosphors developed for FED are rare earth element (Eu3+) doped and they have multiple disadvantages. First of all, low abundance of them in earth severely undermines the end product’s economic efficiency. Secondly, most of these phosphors are prepared by high temperature solid state reaction method and even after mechanical milling they yield average grain size of ≥ 2 µm which are not 2 ACS Paragon Plus Environment

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particularly desirable while scaling up the display resolution.9 Furthermore, due to large band gap, these phosphors are mostly insulators and surface accumulation of negative charge significantly reduces their CL efficiencies.1 Consequently, in order to solve the above mentioned problems, it is necessary to develop rare earth free, nanocrystalline semiconducting phosphors. Improvement of conductivity is traditionally one of the most challenging aspect for designing good low voltage CL phosphors. Lately, incorporation of conducting materials on the surface of the phosphor emerged as an effective solution in this front. Few research groups introduced carbon nanotubes (CNTs) on the surface of ZnS:Cu, Al or ZnS:Mn phosphor and successfully achieved enhanced efficiency at low operating voltages.10,

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Additionally, over the past few

years, different alternative routes have emerged as the effective pathways to enhance the FE performance of the concurrent field emitters for display applications. Among them, coating with low work function or low electron affinity material on the surface of field emitter is the most prevailing technique.12-14 In this context, all carbon two dimensional networks such as graphene and derived materials are already well known for their high aspect ratios, novel electrical properties, good catalytic activities and brilliant mechanical properties.15, 16 Intuitively, the sheetlike structure of RGO (Reduced Graphene Oxide) can be expected to provide better wrapping over the nanophosphor particles as compared to CNTs. However, to the best of our knowledge, no prior report in the literature describes an attempt to improve the CL efficiency of any nanophosphor by using RGO adlayer. Here we select CuI as a possible CL nanophosphor, based upon some of its impressive qualities. First of all, CuI is a cost effective solution of existing rareearth activated phosphors. Furthermore, its moderate electrical conductivity17 is very much beneficial for obtaining efficient CL output. Moreover, large band bap of CuI (Eg = 3.2 eV) strongly discards reabsorption problem and thereby minimizes any possibility of emission loss.



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In the current work, we report a comparative, multifaceted investigation on CL performances of CuI and CuI-RGO composite nanophosphors. Experimentally measured field emission characteristics were correlated with finite element method simulations to identify the local field profile and emission sites. Further first-principles investigations were carried out to probe the effective work functions, which, in conjunction with experimentally measured data, were exploited to quantitatively estimate the field enhancement. RESULTS AND DISCUSSION: Structural and morphological analysis: The X-ray diffraction pattern of pristine CuI, RGO and CuI-RGO nanocomposite is shown in the Figure1 (a). Pristine RGO exhibits a broad hump around 2θ=23° whereas CuI-RGO composite reveals similar XRD pattern of pristine CuI. The dominant peaks are indexed to (111), (200), (220), (311) and (331) crystallographic planes of zinc blende CuI which are in good agreement with the JCPDS card no. 06-0246. Due to small amount and low diffraction intensity of RGO in the composite; interestingly, no diffraction peaks of RGO was detected in the CuI-RGO composite. Composite formation has been also verified from the analysis of Raman spectra shown in Figure 1(b). Signature of both CuI and RGO is prominent in the Raman spectra of the CuI-RGO composite. The sharp peak at 124 cm-1 corresponds to the transverse optical (TO) phonon mode of CuI whereas two relatively weak peaks at 1350 cm-1 and 1593 cm-1 designate the D and G band of RGO, respectively.20 The intensity ratio of D and G band, ID/IG, is a direct estimate of local defects and disorders in the sp2 bonded RGO lattice.21 The ratio is found to be 1.08 in the present case which indicates the presence of small amount of defects in the nanocomposite phosphor.


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The morphology of the phosphors before and after composite formation has been analyzed by FESEM images as shown in Figure 2 (a, b). Pristine CuI exhibits regular distribution of aggregated particles with dimension 70-80 nm whereas the composite nanophosphor clearly reveals the existence of sheet-like RGO within these CuI nanoparticles. Moreover, it is interesting to note that even after composite formation sharp thin edges of RGO still exists on the surface of the nanocomposite phosphor. The uniform large area distribution of RGO in the composite is also prominent from the TEM image shown in Figure 2 (c &d). Highly crystalline nature of the nanophosphor is further evidenced by the well resolved lattice fringes with an interplaner spacing of 0.35 nm corresponding to (111) plane of CuI. Spectroscopic analysis and field emission study: In order to check the potential of the phosphors for using in FED, the room temperature cathodoluminescence (CL) spectra of the pristine CuI and CuI-RGO nanocomposite phosphors has been studied which is shown in Figure 3(a). The FESEM image of the micro-region used for acquiring the CL spectra is shown in the Figure S1 of supporting information. Both the samples primarily exhibit two peaks; one weak peak ~ 415 nm and a strong broad peak at 688 nm. The higher and lower energy peak can be directly assigned to near band edge emission (NBE) and defect level emission (DLE), respectively. The basic CL mechanism in the nanophosphor is described in Figure 3(b). Recombination of electrons in the conduction band and holes in the valence band generates the NBE. But, when the electrons from the impurity levels recombine with the holes in the valence band, DLE takes place.18 Furthermore, it is also observed that incorporation of RGO in the CuI matrix leads to a four fold enhancement in the CL intensity as compared to the pristine CuI nanophosphor. The colour co-ordinate of this emitted CL radiation lies in the red region of CIE 1931 chromaticity diagram which is shown in Figure 3(c). 5 ACS Paragon Plus Environment


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Moreover, the stability of CL intensity of the composite nanophosphors was also measured for 1 hour of electron bombardment time. The CL spectra along with their corresponding colour coordinates are shown in Figure S2. Figure S2 clearly reveals that even after such a long time of electron bombardment, the nanophosphor undergoes only ~ 2% degradation of its initial CL intensity and its colour coordinate remains almost unaltered. The obtained colour co-ordinate also lies within the standard HDTV colour triangle which strongly claims the capability of the nanocomposite phosphor for using in high definition FED applications. The observation of improved CL characteristics from the phosphor composite can be explained in terms of the efficient excitation of luminescence centers activated by local electric field enhancement at the phosphor RGO interface. Therefore as a consequence, field emission performance is expected to improve in the composite nanophosphor, which we discuss in the next section. It is believed that the field emission from the RGO sheets is responsible for this enhancement of local electric field inside the nanophosphor composite. Basically, field emission is a unique quantum mechanical effect, in which electrons tunnel through a potential energy under an external electric field. The high electric field sufficiently narrows the surface potential barrier and thereby increases the tunneling probability of the electrons causing sufficient impact excitation of luminescence centers to generate strong CL emission.10 In order to clarify this effect, the field emission characteristics of the pristine CuI and CuI-RGO composite has been explored. The typical current density vs. applied field curve of both the samples is depicted in Figure 3(d). From the Figure it is evident that pristine CuI nanophosphor does not show any field emission phenomenon; but the nanophosphor composite exhibits significant field emission performance. Thin and free RGO edges with different orientation mainly serve as the active emission site in the nanophosphor composite.19, 22, 23


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Generally, the field emission current density (J) is related to the applied electric field (E) by Fowler–Nordheim (F–N) type tunneling formalism24 described asJ = (Aβ2E2/ φ) exp (-B φ 3/2/β E)

(1)

where A and B are constants with values of 1.54 × 10−6 AeVV−2 and 6.83 × 103V µm−1 eV−3/2, respectively, β is the field enhancement factor and φ is the work function of the emitting material. In the F-N relation the field enhancement factor (β) can be estimated from the corresponding F-N plot [ln(J/E2) vs. (1/E)] as-

Bϕ3/ 2d β= slope of F-N plot

(2)

Where, d is the distance between cathode and anode. It is well known that work function is highly dependent on the surface registry of the nanostructured materials. Accordingly, the origin of superior field emission properties from the nanophosphor composite was further probed through calculation of work function with the help of Density Functional Theory (DFT). For this purpose, a 7 × 7 supercell of graphene layer (P3m1, a = b = 17.220 Å) atop a 4 × 4 supercell of CuI (111) surface (P63/mmc, a = b = 17.114 Å) was used to model the CuI-RGO composite phosphor. An initial lattice parameter of a = b = 17.150 Å having minimal lattice mismatch was used to construct the starting model and then full structural optimizations were carried out without imposing any symmetry restriction. The optimized structure was found have slightly higher lattice parameter of 17.223 Å and a modified cm symmetry. After optimization the distance between CuI (111) surface and graphene adlayer was found to be 3.54 Å. Optimized models of the pristine CuI (111) surface and CuI-RGO nanophosphor composite are shown in Figure 4(a) and (b), respectively. The work function (ϕ) 7 ACS Paragon Plus Environment


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of the above structures can be readily calculated as ϕ = Evac-Ef , where Ef is the position of Fermi level and Evac is the position of vacuum level (i.e. the nearly constant value of potential energy in the vacuum region) of each system. The variation of electrostatic potential along c axis for bare CuI (111) surface and CuI-RGO composite with their respective Fermi levels (DFT computed) and vacuum levels (obtained from the plots) are graphically shown in Figure 4(c) and (d). Our calculations yielded ϕ of 4.64 eV for the bare CuI (111) surface and 3.85 eV for the CuI-RGO nanophosphor composite. Therefore, the tunnelling probability is higher in case of CuI-RGO nanophosphor composite and accordingly it can be exploited as a low voltage CL material for FEDs. We use the DFT calculated work function for composite phosphor in the F-N relation shown in the inset of Figure 3(d) and find that in the present case the value of β ~2421 is sufficiently large which confirms the improved field emission and CL performance in the nanophosphor composite. Furthermore, in order to investigate further the origin of exact field emitting sites, the simulated local electric field profiles of both the samples were analyzed. The simulated profiles of electricfield distributions for pristine CuI and CuI-RGO composite are shown in Figure 5. If V is the applied bias voltage then the enhancement of local electric field at the contact point between CuI and RGO is expressed as-

ELocal = β ×

(3)

V d

A rainbow color coordinate is used to map the different electric field intensities. From the figures it is evident that local electric field of the composite nanophosphor is significantly higher as compared to pristine CuI nanophosphor. It is seen that the local electric field at the CuI-RGO


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interface is enhanced significantly which in turn increases the resultant excitation density for the CL emission. Our obtained field enhancement factor is also higher than that of the phosphorCNT composite [10]. The value of maximum local field is found to be ~ 1.72 × 109 and 2.08 × 109 V/m for the phosphor and nanophosphor composite, respectively. From the figure 5 (b) it is also prominent that the strongest electric field exists at the free edges of RGO. Vertical orientation of RGO over the CuI surface is also helpful for this enhanced electric field. As a whole, due to the significant enhancement of local field, the CuI-RGO nanophosphor composite demonstrates high emission current density and field enhancement factor. Additionally, due to efficient wrapping with conducting RGO, effective electron pathway is formed inside the composite nanophosphor. Consequently, less surface charge are accumulated on the surface of the CuI nanophosphor which leads to the reduced repulsion of next incident electron. As a consequence, both the current density and penetration depth of incident electron beam increases significantly. Due to this enhanced penetration depth, volume of excited luminescence center also increases and causes subsequent increase in CL performance. CONCLUSION: In summary, CuI-RGO nanophosphor composite were synthesized by facile hydrothermal technique. The hybrid phosphor retains its nanostructural registry even after composite formation which is extremely beneficial for high resolution display applications. Incorporation of RGO within the CuI matrix resulted in four fold enhancement of the CL intensity and strong red emission under low voltage electron beam excitation (5 kV). This observed CL enrichment is identified to be a consequence of excellent field emission properties of RGO which facilitates local field enhancement at the CuI-RGO interfacial points and thereby activates higher no. of luminescence centers. Moreover, increased electrical conductivity of nanophosphor composite 9 ACS Paragon Plus Environment


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was found to promote electron beam penetration depth which in turn led to higher volume of excited luminescence centers. The obtained high quality CL characteristics highlight the merits of this CuI-RGO nanocomposite phosphor as a low voltage CL material suitable for FEDs. EXPERIMENTAL SECTION: Synthesis of CuI-RGO nanocomposite: CuI nanophosphors were synthesized by using our previous report.18 In brief, the mixture of 0.05 M CuCl2·5H2O, 0.1 M KI and 1.5 mM SDS was stirred in 5mM PEG solution at 40°C till complete dissolution of the reactants. Then the solution was heated at 60°C for 2 hr. till a viscous amaranth solution was obtained. The resultant solution was added dropwise in a 0.05M NaNO3 solution and stirred at 60 °C for 2 hr. which initiates the precipitation of tiny CuI crystals in the solution. Then the solution along with the precipitate was transferred in 100 ml teflon lined stainless steel autoclave and kept at 180 °C for 12 hours. After the reaction, the final product was obtained, washed several times by ethanol and dried in a vacuum oven for overnight. RGO were also synthesized by using one of our previous report described elsewhere.19 Typically, 0.01 gm CuI nanophosphor was mixed with 40 ml of RGO suspension. Then the mixture was ultrasonicated for 30 mins and transferred to a 50 ml teflon lined stainless steel autoclave to react at 180 °C for 12 hours. After that, the product was collected through cetrifugation and washed several times with DI water and ethanol. Finally, the composite the nanophosphor was obtained after vacuum drying the sample at 70°C for overnight.


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Chracterization: At first, the crystallinity and phase purity of the samples were checked by X-ray diffraction (XRD) by using Cu Kα radiation (λ = 1.5406 Å) (XRD, D8 Advanced, Bruker). Different Raman modes of the samples were analyzed by a high resolution Raman spectrometer (WITECH) by using an excitation of 532 nm laser source. The morphologies of the prepared nanostructures were characterized with field emission scanning electron microscope (FESEM, S-4800, Hitachi), and high resolution transmission electron microscope (JEOL, JEM 2100). A Gatan Mono CL3 equipment attached to the FESEM was used to record the room temperature CL spectra using a beam accelerating voltage of 5kV. Field Emission Measurement: The field emission measurement of the samples were carried out

in a high vacuum (2 × 10−6 mbar) chamber, where the phosphors or the phosphor composites served as the cathode and a conical shaped stainless steel electrode served as an anode with a tip diameter (2R) of ∼ 1.5 mm. The separation (d) between the anode and the grounded cathode samples were maintained at 150 µm during the measurement. Electric field simulation details: For different phosphor based cathodes and stainless steel

anodes, calculations of the electric field distribution have been carried out using finite element method as implemented in ANSYS Maxwell simulation package. The separation distance between anode-cathode was maintained at 150 µm in direct analogy to the experiment and the radius of the anode used was 1.5 mm. During the calculation, an equipotential negative voltage of -2 kV was applied to the cathode, while the anode was maintained at a constant potential value of 0 V. During all steps of the computation process, the whole system was kept inside a vacuum chamber.



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DFT calculation details: The spin unrestricted density functional theory based computations

were performed by Vienna ab initio simulation package (VASP) using projector-augmentedwave (PAW) approach to describe the ion cores. Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation (GGA) was used to calculate the exchange and correlation contributions. Plane wave basis up to an energy cut off 400 eV were used in all calculations. Brillouin zone integrations were performed using k-points spacing of ∼0.5/ Å (actual value: 0.423 × 0.423 × 0.251/ Å) centered at the Γ point. All the considered systems were allowed to relax until the forces were converged below 1 × 10−2 eV/ Å and the total energies were converged below 1 × 10−5 eV per atom. Contributions of dispersive forces were taken into account to facilitate improved accuracy by using PBE+D2 forcefield (Grimme’s) method. A vacuum slab having the length of 25 Å was used along the perpendicular direction in all models to ward off the spurious interactions with the structures own periodic images. Supporting Information Available: FESEM image of the micro-region used for obtaining the CL spectra and the stability of CL intensity as well as colour coordinate are demonstrated in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION: Corresponding author: *Email address: [email protected] ACKNOWLEDGEMENT: The authors acknowledge the financial support from the Council of Scientific and Industrial Research (CSIR), the Government of India, for awarding Senior Research Fellowships during the 12 ACS Paragon Plus Environment

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execution of the work. One of us (DS) wishes to thank the West Bengal State Govt. for providing fellowship during the execution of this work. The authors also wish to thank the Department of Science & Technology (DST), the Government of India and the University Grants Commission, for ‘University with Potential for Excellence (UPE-II) scheme. REFERENCES: 1.

Li, G.; Lin, J., Recent Progress in Low-Voltage Cathodoluminescent Materials:

Synthesis, Improvement and Emission Properties. Chem Soc Rev 2014, 43, 7099-7131. 2.

Geng, D.; Li, G.; Shang, M.; Peng, C.; Zhang, Y.; Cheng, Z.; Lin, J., Nanocrystalline

CaYAlO4:Tb3+/Eu3+ as Promising Phosphors for Full-Color Field Emission Displays. Dalton Trans. 2012, 41, 3078-3086.

3.

Shang, M.; Li, G.; Kang, X.; Yang, D.; Geng, D.; Peng, C.; Cheng, Z.; Lian, H.; Lin, J.,

LaOF:Eu3+ Nanocrystals: Hydrothermal Synthesis, White and Color-Tuning Emission Properties. Dalton Trans. 2012, 41, 5571-5580. 4.

Li, G.; Li, C.; Zhang, C.; Cheng, Z.; Quan, Z.; Peng, C.; Lin, J., Tm3+ and/or Dy3+ Doped

Laocl Nanocrystalline Phosphors for Field Emission Displays. J. Mater. Chem. 2009, 19, 89368943. 5.

Li, X.; Zhang, Y.; Geng, D.; Lian, J.; Zhang, G.; Hou, Z.; Lin, J., CaGdAlO4:Tb3+/Eu3+as

Promising Phosphors for Full-Color Field Emission Displays. J. Mater. Chem. C 2014, 2, 99249933.



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

6.

Page 14 of 22

Zhang, F. L.; Yang, S.; Stoffers, C.; Penczek, J.; Yocom, P. N.; Zaremba, D.; Wagner, B.

K.; Summers, C. J., Low Voltage Cathodoluminescence Properties of Blue Emitting SrGa2S4:Ce3+ and Zns:Ag,Cl Phosphors. Appl. Phys. Lett. 1998, 72, 2226-2228. 7.

Yang, S.; Stoffers, C.; Zhang, F.; Jacobsen, S. M.; Wagner, B. K.; Summers, C. J.;

Yocom, N., Green Phosphor for Low-Voltage Cathodoluminescent Applications: SrGa2S4:Eu2+. Appl. Phys. Lett. 1998, 72, 158-160.

8.

Guo, P.; Zhao, F.; Li, G.; Liao, F.; Tian, S.; Jing, X., Novel Phosphors of Eu3+, Tb3+ or

Bi3+ Activated Gd2GeO5. J. Lumin. 2003, 105, 61-67. 9.

Dinsmore, A.; Hsu, D.; Gray, H.; Qadri, S.; Tian, Y.; Ratna, B., Mn-Doped Zns

Nanoparticles as Efficient Low-Voltage Cathodoluminescent Phosphors. Appl. Phys. Lett. 1999, 75, 802-804.

10.

Bae, M. J.; Park, S. H.; Jeong, T. W.; Lee, J. H.; Han, I. T.; Jin, Y. W.; Kim, J. M.; Kim,

J.-Y.; Yoo, J. B.; Yu, S. G., Carbon Nanotube Induced Enhancement of Electroluminescence of Phosphor. Appl. Phys. Lett. 2009, 95, 071901-071903. 11.

Kim, J.-Y.; Park, S. H.; Jeong, T.; Bae, M. J.; Kim, Y. C.; Han, I.; Yu, S., High

Electroluminescence of the Zns:Mn Nanoparticle/Cyanoethyl-Resin Polymer/Single-Walled Carbon Nanotube Composite Using the Tandem Structure. J. Mater. Chem. 2012, 22, 2015820162. 12.

Cui, H.; Gong, L.; Yang, G. Z.; Sun, Y.; Chen, J.; Wang, C. X., Enhanced Field Emission

Property of a Novel Al2O3 Nanoparticle-Decorated Tubular SiC Emitter with Low Turn-on and Threshold Field. Phys. Chem. Chem. Phys. 2011, 13, 985-990.


Page 15 of 22

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


13.

Sun, Y.; Cui, H.; Gong, L.; Chen, J.; She, J.; Ma, Y.; Shen, P.; Wang, C., Carbon-in-

Al4C3 Nanowire Superstructures for Field Emitters. ACS nano 2011, 5, 932-941. 14.

Das, B.; Sarkar, D.; Maity, S.; Chattopadhyay, K. K., Ag Decorated Topological Surface

State Protected Hierarchical Bi2Se3 Nanoflakes for Enhanced Field Emission Properties. J. Mater. Chem. C 2015, 3, 1766-1775.

15.

Wu, Z.-S.; Pei, S.; Ren, W.; Tang, D.; Gao, L.; Liu, B.; Li, F.; Liu, C.; Cheng, H.-M.,

Field Emission of Single-Layer Graphene Films Prepared by Electrophoretic Deposition. Adv. Mater. 2009, 21, 1756-1760.

16.

Sen, D.; Thapa, R.; Chattopadhyay, K. K., Rules of Boron-Nitrogen Doping in Defect

Graphene Sheets: A First-Principles Investigation of Band-Gap Tuning and Oxygen Reduction Reaction Catalysis Capabilities. Chemphyschem 2014, 15, 2542-2549. 17.

Zi, M.; Li, J.; Zhang, Z.; Wang, X.; Han, J.; Yang, X.; Qiu, Z.; Gong, H.; Ji, Z.; Cao, B.,

Effect of Deposition Temperature on Transparent Conductive Properties of γ‐CuI Film Prepared by Vacuum Thermal Evaporation. Phys. Status Solidi A 2015, 212, 1466-1470. 18.

Saha, S.; Das, S.; Sen, D.; Ghorai, U. K.; Mazumder, N.; Gupta, B. K.; Chattopadhyay,

K. K., Bane to Boon: Tailored Defect Induced Bright Red Luminescence from Cuprous Iodide Nanophosphors for on-Demand Rare-Earth-Free Energy-Saving Lighting Applications. J. Mater. Chem. C 2015, 3, 6786-6795.

19.

Roy, R.; Jha, A.; Sen, D.; Banerjee, D.; Chattopadhyay, K. K., Unique Quasi-Vertical

Alignment of RGO Sheets under an Applied Non-Uniform DC Electric Field for Enhanced Field Emission. J. Mater. Chem. C 2014, 2, 7608-7613.



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

20.

Page 16 of 22

Weng, B.; Yang, M.-Q.; Zhang, N.; Xu, Y.-J., Toward the Enhanced Photoactivity and

Photostability of Zno Nanospheres Via Intimate Surface Coating with Reduced Graphene Oxide. J. Mater. Chem. A 2014, 2, 9380-9389.

21.

Tian, G.; Chen, Y.; Zhou, J.; Tian, C.; Li, R.; Wang, C.; Fu, H., In Situ Growth of

Bi2MoO6 on Reduced Graphene Oxide Nanosheets for Improved Visible-Light Photocatalytic Activity. CrystEngComm 2014, 16, 842-849. 22.

Roy, R.; Jha, A.; Banerjee, D.; Sankar Das, N.; Chattopadhyay, K. K., Edge Effect

Enhanced Electron Field Emission in Top Assembled Reduced Graphene Oxide Assisted by Amorphous CNT-Coated Carbon Cloth Substrate. AIP Advances 2013, 3, 012115-012126. 23.

Yamaguchi, H., et al., Field Emission from Atomically Thin Edges of Reduced Graphene

Oxide. ACS Nano 2011, 5, 4945-4952. 24.

Fowler, R. H.; Nordheim, L. In Electron Emission in Intense Electric Fields, Proceedings

of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, The Royal Society: 1928; pp 173-181.


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Figures:

Figure 1. (a) XRD pattern and (b) Raman spectra of CuI, RGO, and CuI-RGO composite nanophosphor



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Figure 2. Typical FESEM image of (a) CuI nanophosphor (b) CuI-RGO composite nanophosphor. Sharp edges of RGO are indicated by the arrows. (c, d) Lower and higher magnification TEM image of CuI-RGO composite nanophosphor. Inset shows typical lattice fringes corresponding to (111) plane of CuI


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Figure 3. (a) CL spectra of the nanophosphors under low voltage electron beam excitation (b) Schematic CL process in the nanophosphors. (c) CIE chromaticity diagram representing the colour coordinate of CuI-RGO composite nanophosphor (d) Field emission current density vs. applied electric field curve for the nanophosphors. Inset shows typical F-N plot of CuI-RGO composite nanophosphor



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Figure 4. (a, b) Optimized supercell structures of the pristine CuI (111) surface and CuI-RGO composite nanophosphor. (c, d) Demonstration of electrostatic potential as a function of fractional coordinates for the respective nanophosphors.


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Figure 5. Calculated electric field distribution profiles for (a) CuI nanophosphor (b) CuI-RGO composite nanophosphor.



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Table of Contents (TOC):

Demonstration of intense cathodoluminescence emission from CuI-RGO nanophosphor composite. Such a phenomenon is accomplished by successful enhancement of local electric field at the CuI-RGO contact points. The obtained results promote this composite nanophosphor as an ultimate choice for designing field emission display devices.


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Local Field Enhancement-Induced Enriched Cathodoluminescence

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