Graphene-Anchored p-Type CuBO2

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Graphene anchored P-type CuBO2 nanocrystals for transparent cold cathode Saswati Santra, Nirmalya Sankar Das, NRIPEN BESRA, Diptonil Banerjee, and Kalyan Kumar Chattopadhyay Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01650 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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Graphene anchored p-type CuBO2nanocrystals for transparent cold cathode S.Santra1†, N. S. Das1#, N. Besra1, D. Banerjee1,$ and K.K. Chattopadhyay1,2* 1

Thin Film &Nanoscience Laboratory, Department of Physics, Jadavpur University, Kolkata 700 032, India

2

School of Materials Science and Nanotechnology, Jadavpur University, Kolkata 700 032, India

Abstract CuBO2nanostructures were synthesized employing low cost hydrothermal technique to combine into CuBO2-RGO nanocomposite for the first time using chemically prepared graphene sheets. The nanohybrid samples were characterized for structural information using X-ray diffraction (XRD) which revealed proper crystalline phase formation of CuBO2 unaltered by composite formation with graphene. Raman spectroscopic studies were employed to confirm the presence of graphene. Morphological study with field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) suggested proper wrapping of RGO sheets over CuBO2nanocubes. Moreover close proximity of lattice planes of CuBO2 and RGO to each other was observed in high resolution TEM studies which were correlated with Raman spectroscopic studies. Finally, the samples were characterized to study the field emission (FE) properties of the same using laboratory made high vacuum field emission set up. Finite element based theoretical simulation studies were carried out to explain and compare the field emission properties with the experimental results. The FE properties of the composite samples were found to be tuned by the nature of wrapping of the RGO sheets over the CuBO2nanocubes which was typically dependent upon the spiky morphology of the nanocubes.

Keywords: CuBO2; RGO; Hydrothermal synthesis; Field emission 1

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----------------------------------------------------------------------------------------------*Corresponding author at: Department of Physics, Jadavpur University, Kolkata-700 032, west Bengal, India.Tel: 91 33 2413 8917; FAX: 91 33 2414 6007; E-mail address: [email protected] (K. K. Chattopadhyay) † Present address: Materials Research Centre, Indian Institute of Science, Bangalore-560012,India # Present Address: Dept. of Physics, Techno India – Batanagar, Maheshtala, Kolkata-700141 $ Present Address: M. N. Dastur School of Materials Science and Nanotechnology, IIEST Shibpur, Howrah-711103 Introduction In the versatile range of different types of advanced materials incorporated in industrial and domestically used devices, TCOs have been claiming the most important contribution since the development of modern scientific research. TCOs are able to offer numerous facilities mainly due to some very important in-built features. For example, appreciable transparency and tunable optical band gap of materials like ZnO, NiO enable them to exhibit exotic optical properties which are important for solar cell technology1 and hence inevitable for future energy solutions. Easily adjustable electrical conductivity within a wide range of ambient temperature facilitates resistive switching,2 electrochemical activity,3 thermoelectric power4 and several other properties necessary for energy related devices. However, applications of those TCOs are not limited only to the traditional devices. With increasing danger of environmental pollution by industrial wastes, disinfection of consumable water using smart technique has become an important issue, especially in the developing countries. TCOs like TiO2, ZnO and CeO2 offer solution also in this field by showing photocatalytic activity under UV/visible light excitation.5-7 However, most of the materials discussed so far are typically in a special group. i.e. most of them exhibit n-type conductivity. Among the elite group of TCOs, there has been a scarcity of novel p-type materials which show identical or comparable optical and electrical properties like their n-type counterparts. Among 2


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the classical p-type TCOs like NiO or CuO, although they are easy to synthesize8-10and show appreciable thermal11-12and structural13-14 stability, they often show inappropriate transmittanc15 or inferior room temperature conductivity15which limits their application in smart devices. It is not at all a new fact that Kawazoe et al.16 reported their interesting idea of chemical modulation of valance band to overcome this barrier. Various p-type copper based delafossites materials were fabricated17-20 using this technology with CuAlO2 being the leader. We have also synthesized CuAlO2 in powder and thin film forms and characterized different optical and electrical properties of the same in our previous works.21-22 However, considerable gap between the expectation and facilities actually achieved from CuAlO2 in view of conductivity and other features lead scientists to make further efforts to understand physical correlation between the properties of such delafossites and the nature of ‘M’ present in general CuMO2 form.23-24 Several metals and non-metals like Ga,25 Fe,26 Cr,27Sc,28 Y,29 B30were used to fabricate such Cu delafossites for further better performance. It is mention-worthy in this regard that theoretical study by Nie et al.31 has indicated that decreasing atomic radius of ‘M’ may result in wider energy gaps of delafossites of CuMO2 structure which was further supported by Snure et al.32 where largest band gap of CuBO2 among such ternary Cu delafossites was established. Since then various attempts were made to study opto-electrical properties of this novel p-type delafossites.30, 33-38 In our earlier works with CuBO2, we have synthesized this novel material in cost effective sol-gel method,39 easy hydrothermal route,40 molten salt technique.41

Those

synthesis methods are well established processes for fabrication of nanoparticles and nanostructure of high aspect ratios of CuBO2 with fine control over dimensions. CuBO2 samples synthesized employing such methods showed marvellous applications in fields of water

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disinfection,30 photoconductivity,34cold electron emission,38thermopower generation38with efficiencies comparable to that of the well-known n-type TCOs. Inspired by such success in obtaining important and efficient application based activities of CuBO2, it has therefore become necessary to further investigate the possibilities of enhancement the optoelectrical properties of CuBO2 by manipulating the same by means of doping or composite formation. Formation of composite has been emerged as a most suitable technique for n-type TCOs in this regard. For example, TiO2nanocomposite has been proven as a better green cleaning agent than pure titania.42-43 Other important n-type TCOs like ZnO, CdO, SiO2 often showed enhanced properties when involved in a composite and become more useful in the fields of device based applications,44-47 display technology,48-50 solar cells51-53 etc. Several reports are there demonstrating better properties of CuAlO2 and some other delafossites achieved by fabrication of composite of the same with other materials.54-55 We fabricated heterojunction of CuBO2nanocubes with ZnO nanostructures resulting in highly efficient diode which indicated easy carrier transfer in between CuBO2 and ZnO.41 However, up to our best knowledge, in spite of several established efficient applications CuBO2, no efforts have been made to investigate the tuning of properties of CuBO2 by formation of composite aiming to even better applications of the same. For example, RGO and graphene has already been known for boosting electrochemical activities,3photocatalytic

performances,42-43photoresponse

capability51

of

many

pristine

semiconductors. So enhancing Cu delafossites’ properties with 2D carbon sheets may lead to better performing cold cathodes, efficient electrochemical cells like NiO based storage devices etc. But studies on RGO-Cu delafossite composite are still rare.The reason behind this may be that, in-situ fabrications of composite of such materials are not easy due to the possibility

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deformation of crystalline phase of the host material by the presence of precursors of the counterpart of the composite in experimental process. The way out of this problem has been suggested in this work, this is also the point which highlights the novelty of this work. To avoid the possible occurrence of impurity phase within the sample, we employed entirely ex-situ process for fabrication of composite of CuBO2and thus the crystallinity and the phase purity of the host delafossite was retained. Another important idea was to choose reduced graphene oxide as the counterpart of composite in place of employing doping by rare earth metal ions or other expensive elements like noble metals keeping in mind the financial aspect of large production. RGO has already been established as a novel carbon material which is easy to synthesise and offer different facilities like high conductivity,56-57 moderate transparency58 etc. and hence has emerged as one of the best candidate for composite formation with advanced TCOs. Exploiting the spiky nature of the surface of the nanocubes of CuBO2 observed in our previous work, we availed the facility of better wrapping of RGO sheets over the CuBO2nanocubes. In addition to the fabrication of CuBO2-RGO composite for the first time, the effect of presence of RGO in field emission properties is investigated in detail. The experimental results were correlated with the results obtained theoretical analysis by ANSYS MAXWELL software. It was observed that the better wrapping of few layers RGO sheets over CuBO2nanocubes result in enhanced FE properties. Experimental Preparation of CuBO2nanocubes:CuBO2nanocubes were synthesized using cost effective easy hydrothermal route. The detail synthesis process is described elsewhere.44 In brief, water solutions of copper acetate (Cu(CH3COO)2.H2O) and borax (Na2B4O7) were used for hydrothermal reaction at 200o C in three different durations of 1h, 2h and 3h. The prepared 5


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samples were preserved separately in a dry atmosphere for further use in composite formation. The details of possible chemical reactions leading to formation of CuBO2 are provided in supporting information. Preparation of RGO sheets: GO sheets were prepared using modified Hummer’s method,59-60 in a brief typical synthesis procedure, commercially available graphite powder, sodium nitrate and potassium permanganate were used as precursor material for synthesis of grapheme oxide employing well known modified Hummer’s method. The synthesized GO sample was further reduced using hydrazine hydrate61 and thus the RGO sample was obtained in result. The prepared RGO was taken in a solution taking DI water as solvent such that 100 ml of such RGO solution contain 1 mg of RGO. The prepared solution was kept undisturbed for 48 h to observe whether any precipitation occurs. No precipitation within RGO solution observed indicating proper dispersion of RGO sheets in DI water. The pH value of the solution was tested using a pH meter which showed pH value of 7.00 which indicated proper neutral nature of the solution. Thereafter the solution having no basic or acidic character was divided into three parts and further used to fabricate nanocomposite with previously synthesized CuBO2 samples. The neutral nature of RGO solution ensured no possible chemical reaction with CuBO2 which restricted the possibility of phase deformation of CuBO2. Preparation of CuBO2-RGOnanocomposite:The CuBO2 and RGO samples obtained in the above mention procedures were further used to fabricate CuBO2-RGO composite by easy ex-city physical stirring method. In atypical synthesis procedure, 0.06 g of each CuBO2 powders (synthesized during 1h, 2h and 3h time) were taken in a solution of 60 ml of RGO solution. They were mixed in separate glass beakers and stirred for 4h in a moderate stirring speed. During 6


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stirring special care was taken so that the temperature never exceeded the room temperature (300k). After stirring, the solutions were allowed to settle down and a black precipitate in each beaker was observed. The precipitates were filtered and the same was washed several times with DI water to avoid any possibility of contamination by any residual hydroxide. The filtered CuBO2-RGO precipitates with CuBO2 powders obtained in 1h, 2h and 3h were collected in a dry container and labelled as sample A, B and C respectively and dried naturally to carry out further characterizations. A brief demonstration of different time intervals of nanocomposite formation can be observed in figureES1. Characterization: Obtained nanocompositesamples A, B and C were firstly studied by X-ray diffractometer (XRD; Bruker, D-8 Advance) with the Cu Kα radiation of wavelength λ = 1.5406 Å to confirm proper phase formation of CuBO2 and additionally to ensure whether formation of nanocomposite caused any phase alteration of the novel delafossite. The morphological features of the samples were characterized by a field emission scanning electron microscope (FESEM, Hitachi, S-4800) to investigate how much wrapping of RGO is covered over different CuBO2 samples. The elemental mapping of the constituent elements was also performed using the EDX attachment of the FESEM equipment. Further, high resolution transmission electron microscopic (JEOL, 200 kV HRTEM) studies were also carried out to reveal whether the RGO sheets wrapped over the nanocubes of CuBO2 loosely or some atomic plane interference occurred in between the two counterparts. Raman spectroscopic (alpha 300, Witec, Germany) studieswere also carried out for each sample to investigate whether the RGO sheets are in proper chemical phase and to find out the possible interaction between the CuBO2nanocubes and RGO sheets. Finally all the samples 7


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were subjected to be characterized by our laboratory made high vacuum field emission set up and cold emission current density and other relevant FE parameters were obtained for all the samples.ANSYS Maxwell software was further used to carry out finite element electrostatic simulations study the effects of wrapping of RGO sheets overCuBO2nanocubes and the outcomes were thoroughly compared with experimental results. Results and discussion Structural analysis: XRD pattern of the pure and composite samples are presented in figure 1. Where we can observe intense diffraction peaks occurring at 2θ angles of 32o, 36o, 38o, 48o, 53o, 58o and 61o were assigned to be arising due to reflection from (006), (100), (012), (106), (107), (018), (0011) planes respectively of CuBO2 lattice [28-1256]. In the pure and composites forms, the peak positions were observed to be rarely altered. The reason may be stated as the effect of RGO sheets to be randomly incorporated into CuBO2nanocubes without any occurrence of selfrestacktion. It is also important to remember that no additional peak corresponding to grapheme/carbon was detected in any of the samples. Song et al., in their similar experiment with ZnO-graphene nanocomposite, indicated that such absence of characteristic peaks of carbon clearly indicates that RGO sheets are completely exfoliated due to loading of the host oxide material.67 Several other reports of RGO-TCO nanocomposite suggest same phenomenon of well exfoliation of RGO sheets into host TCO nanostructure actually cause the disappearance of the characteristic XRD peak corresponding to carbon or RGO. However, it was also inferred from the XRD characterization that in spite of composite formation by CuBO2 with RGO sheets, CuBO2 samples could retain their original crystalline phase. 8


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Morphological analysis by FESEM: High and low resolution FESEM images of the samples are depicted in figure 2.For Sample A and C distorted cube-like structures are found while for sample B the morphology is almost similar to perfect cube.

It can be clearly seen from figure2(a), 2(d) and 2(g) that

theCuBO2nanocubes are not solid in nature; rather they are composed of finer CuBO2 nanostructures of higher aspect ratio. The results are quite expected in regard of our previous work,40 the synthesis process of which is thoroughly followed here. All of the CuBO2nanocubes are made up of smaller CuBO2nanorods. Incase of sample A, the nanorods could not grow sufficiently and they are hardly better than nanoparticles (figure2(a)) in terms of aspect ratio due to shorter growth time.However in sample B, the situation changes drastically. The CuBO2nanocubes here can be observed to be composed of well-defined high aspect ratio CuBO2nanorods (figure2(d)) as they were subjected to optimized growth duration. Again in case of sample C, the constituent CuBO2nanorodsactually collapsed with each other resulting in thicker and deformed nanorods of lower aspect ratio as they were allowed to grow in higher growth time (figure 2(g)). The morphological evidences also varied accordingly in view of wrapping of RGO sheets over the CuBO2nanocube samples. If we keep in mind the process of wrapping of RGO sheets, it is expected that a surface having more spikyfeature i.e. a surface having higher average roughness, should be more able to entrap RGO sheets than surfaces having lesser spiky feature. The surface roughness of the cubes grown in case of pristine samples was qualitatively analyzed using imageJ software figure ES2. It was observed that surface of the nanocubes grown for 1h and 3h durations were smoother than those obtained in 2h synthesis durations figure ES3. The spiky nature of those cubes varied accordingly. Infigures 2(b), 2(e) and 2(h), difference of wrapping of RGO sheet over CuBO2nanocubes are not explicitly visible 9


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but they are clear in figures 2(c), 2(f) and 2(i). We can see that sample A could not attach much area of RGO sheet due to the undergrowth of constituent CuBO2nanorods. Whereas, in case of sample B, the optimized spiky nature of the surface of CuBO2nanocube allowed proper wrapping of RGO sheet over the entire CuBO2 cube as can be observed infigure2(f). Again in figure2(i), we can see half of the CuBO2nanocube surface is wrapped by RGO sheet and the rest is uncovered. This is nothing but the effect of overgrown constituent CuBO2nanorods which gradually caused the inferior spiky nature of host CuBO2nanocube surface. As we postulated in earlier section that the nature of wrapping of RGO sheets over CuBO2nanocubes conclusively determine the FE properties of the samples, the experimental outcome of FE analysis (discussed later) agrees with this morphological study.Based on FESEM results, the sample B, having most favorable morphology for anchoring RGO, was further treated with different loadings of RGO. Thus two more samples having CuBO2: RGO ratio 1:0.25 and 1:0.5 were obtained. EDX analysis was carried out to ensure the proper elemental distribution within the samples. The results are shown in figure 3. The image confirms presence of all the constituent elements in regularly distributed fashion. Additionally, it is well known that carbon is always contributed by chamber atmosphere of EDX and from semi-dried vacuum oil present in the inner walls of the FESEM sample chamber but carbon occurring from RGO showed strong and entirely different sign of its presence. Figure 3(a) shows weak signals of C as overall general distribution in the pure CuBO2 sample but the presence of C can be found in specific shape (same as the RGO) in samples A, B and C. Morphological analysis by HRTEM:

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FESEM analysis clearly cleared the nature of wrapping of CuBO2nanocubes byRGO sheets, but did not reveal any information about crystallographicinteraction between the two counterparts of the nanocomposite. To obtain this information, HRTEM studies were carried out with the sample (sample B) showing most efficient wrapping of RGO sheets over CuBO2nanocubes. General morphological information about the nature of wrapping is in full agreement as observed form FESEM studies and depicted in figure 4(a) and 4 (b). It can be seen that the nanocube is properly wrapped as awhole by RGO sheet. However, nature of wrapping can be clearly seen if we comparefigure4(c) and 4(d). It can be clearly seen that the spiky end of CuBO2nanorod has trapped RGO sheets and is totally wrapped by the same.Figure 4(f) expresses rather more important information regarding the composite formation. We can see that the atomic planes have successfully interfered into the layers of wrapping RGO sheets. This is in fact a notable success of easy ex-situ technique of composite formation. The interfering atomic planes of RGO and CuBO2 clearly indicate easy carrier transfer in between the involved counterparts which in turn expected to result in enhanced FE properties (discussed in FE section). However, the obvious query arise that, inspite of such atomic interaction, why RGO did not affect the crystalline nature of CuBO2 but only contributed to the cold emission property. The probable explanation is that the interference of atomic planes was observed at the pointy end of the constituent CuBO2nanorod only, not on the whole surface of the nanorod. The atomic planes could not overlap other than the pointy end because of close presence of the neighboring nanorod. Again, cold electron emission is mainly contributed by the part of the nanostructure having highest aspect ratio (pointy end in case of our samples). This is why the wrapping of RGO actually did not affect the crystalline nature of host CuBO2 but contributed efficiently to

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the enhancement of FE properties (discussed later).A schematic of the wrapping procedure depending on the spiky nature of the CuBO2nanocube surface has been depicted in figure 5. RAMAN Spectroscopic and transmittance studies: Raman spectroscopy was employed mainly to confirm the compositional purity of the wrapping RGO sheets. The results are presented in figure 6(a). It can be clearly seen that the samples show Raman peak near 300 cm-1. However all the samples A, B and C show characteristic Raman peaks for RGO G and D bands near 1300 and 1700 cm-1. If we carefully look at the Raman spectra, we can observe that all the peaks corresponding to pure CuBO2 samples have been shifted in a higher wave number due to formation of composite. To ensure that this shift is purely occurring due to composite formation, we prepared a sample where RGO was drop casted onto a Si wafer and CuBO2 powders (2h synthesis duration) was also separately placed onto the same Si wafer. The result is depicted in the same figure and Raman peak corresponding to CuBO2 did not suffer any shift to higher wave number. The shifts toward higher wave number were therefore accounted for the composite formation and local lattice interactions in between the counterparts. As a potential transparent cold cathode, all the samples were also tested for their transparency in UV-Vis spectrophotometer. The results are presented in figure 6(b). It can be seen that the samples exhibited ~80% transmittance in the visible region. Field emission studies: Field emission performances of sample A, B and C wereinvestigated using our laboratory-made high vacuum field emission set up and using a diode configuration consisting of a cathode and a stainless steel tip anode of conical shape having a tip diameter of 1mm. The samples worked as 12


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the cathode. The field emissionchamber was evacuated up to ~10-7 mbar base pressure prior to starting measurement. The inter-electrode separation was adjustable to a few hundred micrometers by using a micrometer screw and the measurements were carried out keeping 120 µm effective tip-sample separation. A chamber view port was opened to maintain constant observation on the sample surface to determine any discharge, if occurring at all. No such discharge occurred during the application of high electric field and hence the current observed in these measurements were inferred to be cold field emission of electron from the samples. Theoretically, the emission current I is related to the macroscopic electric field E by62-63 = ( ) exp [− ⁄ ( ) ]

(1)

Where φ and β are the local work function and the field enhancement factor respectively, A the effective emission area, a, b are Fowler-Nordheim constants having values a =1.56 × 10-10 AeVV-2 and b = 6.83 × 103 eV-3/2 V µm-1. The applied macroscopic field E was determined by dividing the external voltage (V) by the actual inter-electrode distance d which was maintained same (120 µm) throughout the experiment. Equation (1) may be rewritten in the following form

)

= (φ

−

#

!" $ %&

(2)

So plot of ln{J/E2} vs. 1/E should be geometrically a straight line. Important information about the field enhancement factor (β) and local work function (φeff) etc. can be derived from its slope (m) and intercept. For this, the following relations were used = −φ/ /( 13


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and φ)** = φ/ /

(3)

Experimentally observed emission current value was further processed to obtain current density and plotted against applied field. The results are presented in figure 7(a-c). We can see that the emission current density is maximum for sample B, which is much higher in value compared to that of sample A and C.Moreover all the samples showed remarkably enhanced emission current density than their pure form. The turn-on field, defined as the external field required to get an emission current density of 25 µA/cm2 was also measured for each samples and are represented in figure 7(d). We can observe that sample B again showed lowest turn on field which is a key factor for any material to be used in emission based devices. However, sample A and C showed much higher turn on field. However, sample C showed comparatively better performance than sample A in this regard. Another important FE parameter is the field enhancement factor. The values of field enhancement factor was calculated for each sample by first drawing the FN curves for the samples which are presented in figure 8(a-c) and values of β were calculated using the slopes of FN plot by using equation (3) and considering the work function of CuBO2 as 4.79eV.38 The comparative results are depicted in figure 8(d). It can be seen that the β values do not show same trend as found in case of emission current density and turn on field. We observe that sample A showed lowest emission current density and sample C shows the highest value of β. This apparent irregularity and all the comparative values of FE parameters can be satisfactorily

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explained by considering the morphology and hence the nature of wrapping of RGO sheets over the CuBO2nanocubes. If we carefully consider figure 2, we may find that the spiky nature of the surfaces of the CuBO2nanocubes are mainly responsible for the success of wrapping of RGO sheets as we discussed earlier in the morphological analysis section. Extending full agreement to this idea, figure 2(b) shows that the RGO sheets are mostly spread over bunches of CuBO2 and rarely covers some or entire part of a single CuBO2. This is quite expected as the nanospikes, the building units of nanocubes are half grown in case of sample A. It is therefore expected that sample A actually fails to entrap any appreciable amount of RGO sheet over its surface. It is to be mentioned that, we suspect this to be the main reason behind failure of pure sample A (before formation of composite with RGO) to show any effective field emission, the fact revealed by the resulting positive slope of FN plot (not shown here) corresponding to the J-E curve obtained for sample A. However, the case is not similar for sample C. Although sample C is morphologically inferior compared to sample B, it however has some spiky nanorods over the nanocube surface. Being overgrown, the constituent nanorods are inferior in view of aspect ratio. But they are still capable of entrapping some RGO sheets over the nanocube surface. The wrapping of RGO sheets over nanocube surface for sample C is not global in nature as it is for sample B but the same fact offer less screening effect compared to sample B. So, according to our best understanding, FE properties of the samples are governed by two factors related to morphology. Firstly, composite formation of CuBO2 with RGO sheets for individual nanocubes and secondly, the screening effect, occurring after wrapping of RGO sheets over the constituent nanorods. In first aspect, sample B is more facilitated having narrower 15


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constituent nanorods and hence result in more number of nanocubes to entrap RGO sheets and in second respect sample C is slightly in a better position to minimize screening effect after wrapping by RGO sheets. The first factor enables sample B to show extraordinarily high emission current density and the second factor enables sample C to show higher field enhancement factor. However, it is also important to explain the novelty of our work in this regard. i.e. what the role RGO is actually playing more effective than the effects of morphology of the pure samples alone. The effect of composite formation can be exclusively understood if we consider the FE properties of samples compared to the pure ones. Figure 7(a-c) clearly demonstrate that the emission current density enhanced as a direct effect of formation of composite of CuBO2 with RGO. To investigate the reason behind such enhancement, we must consider the work functions of the counterparts involved in cold emission process, i.e. CuBO2 and RGO. The work function of RGO is well reported to be considered as 4.53 eV.64 However, being a very new material in this field, work function of CuBO2 is not experimentally established till now. In our earlier studies we theoretically derived the work function of this novel delafossite and it was found to be 3.13eV for (100) plane and 4.79 eVfor (012) plane.38 Now if we look at figure 4(f) we may find that the constituent CuBO2nanorods are mainly composed of (012) plane. Although other crystalline planes may also be present in the samples but it may be stated beyond as doubts that (012) plane of CuBO2 are mainly taking part in cold emission process based on the morphological proof. It is therefore obvious that we should consider the work function as 4.79 eV. Now we may analyze the carrier transfer process across the CuBO2-RGO junction. On application of external electric field, the Fermi level of CuBO2 (CBF) is expected to bend validating carrier from CuBO2 conduction band tunnel through the barrier in junction between 16


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CuBO2 and RGO and enrich RGO sheets with higher carrier density. This process is additionally favored in our sample as we can see clear attachment of CuBO2 atomic planes to the layer RGO wrapping (figure4). Further application of higher electric field causes the carriers tunnel through vacuum barrier and the same are collected by anode resulting in higher current density achieved for composite samples. A schematic of the band interaction process have been represented in figure 9(a). Moreover, as the work functions of CuBO2 and RGO are not too much different we must also consider the morphological changes occurred due to composite formation. The wrinkles present in RGO sheets act as additional emission sites in case of composite samples than the pure ones where nanorods emitters are found to have very smooth surface. Those additional emission sites eventually contribute to much higher emission current density. This possible morphological facility has been depicted schematically in figure 9(b). Like some earlier works65-69 in identical systems, the enhancement of FE properties via morphological (and hence internal) tuning can also be represented via a model. Wrapping of RGO over CuBO2nanorod surface in turn changes the earlier comparatively smooth surface of the rods into rough wrinkled ones. Hence considerable amount of field enhancement and charge accumulation occur there which favors easier electron tunneling. This effect has been demonstrated in form of a model presented in figure 9(c). It was clearly observed from FE studies that RGO has played the most crucial role to enhance cold emission performance of the samples. To support this, two more loadings of RGO were added to sample B to obtain two different ratios (1:0.5 and 1:0.25) of CuBO2 and RGO. It was

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observed, as depicted in fig. ES4, that the emission current densities gradually decreased with decreasing RGO contain. Simulation of FE properties using ANSYS The variation of field emission current density due to contribution of morphology of CuBO2nanocubes (and the constituent nanorods) and the composite formation with RGO were further simulated employing finite element analysis by ANSYS software.62, 70-71We considered 2D model of CuBO2nanocubes and RGO sheets keeping the dimensions as observed in FESEM and TEM micrographs. As per actual basis, the sample to anode distance was kept 120 µm. All the samples A, B and C were modeled in their composite and pure form. The obtained results of simulation proceess are presented in figure 10(a-i). In figure 10(a), single nanocubeemitter representing sample A showed the smallest field density around it which is higher in case of figure 10(g) and much higher in figure 10(d) which represent single nanocube emitters of sample C and sample B respectively. The nanocube emitter models have been shown in higher magnification in figure10(b,e,h) to clearly represent that the spiky natures of the cubes have been taken into account as per micrographic observation. They also show highest emission field density for sample b and lesser for sample C and A. These results are in full agreement with our experimental outcome (figure 7). The simulation also included the possible field density which should occur after wrapping of RGO sheets over the nanocube models. These results are depicted in figure10(c,f,i). If we compare figure10(a)and 10(c) for sample A,we can find that emission field magnitude is much higher covering the vicinity of the sample after wrapping by model RGO sheet. The same is found true in a comparative analysis between figures 10(d) and 10(f), also in figures 10(g) and 10(i). These simulated results are also in full agreement with our 18


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experimental results where we found that all the CuBO2nanocubes showed much enhanced emission current density after they were transformed into composites. A comparison of FE behavior of CuBO2 and other identical copper delafossites systems has been drawn in table ES1 which clearly indicate that our sample in this offers much better performance as cold cathodes compared to previous reports both in view of current density and turn-on field. Conclusion CuBO2 nanocubes of different surface morphology and RGO sheets were synthesized via easy hydrothermal route and modified Hummer’s method respectively. The two samples were used to fabricate CuBO2-RGO nanocomposite by simple stirring method. The XRD studies confirmed proper phase formation and unaltered crystalline structure of CuBO2 even after nanocomposite formation. Morphological analysis were carried out using FESEM and HRTEM facilities where proper attachment of CuBO2 with layers of RGO up to atomic plane level was confirmed, additionally, different degree of wrapping of RGO sheets depending upon the spikyness of CuBO2 surface was also observed. Raman spectroscopic studies confirmed proper composition of the RGO sheets and also indicated local atomic interaction of the same with CuBO2 which was supported by HRTEM studies. Field emission properties of all the samples were studied in detail and the same was theoretically supported by simulation carried out using ANSYS. It was observed that the cold electron emission process of the CuBO2nanocubes was controlled by morphology for the pure samples which enhanced in a large extent after formation of composite with RGO sheets. The composite system involving such ternary delafossites and showing such high current density is therefore an advance addition to the classical emitter systems and surely is a key candidate for futuresmart display fabrication.The same composite system, having the 19


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enhanced properties of both copper delafossites and 2D carbon sheets may also be applicable for future smart electrochemical cells and junction based devices. ACKNOWLEDGMENT One of us (SS) wishes to thank the Dept. of Science and Technology, the Govt. of India for providing her ‘Inspire Fellowship’. The authors also thank the University Grants Commission, the Govt. of India (UGC) for ‘University with potential for excellence (UPE II) scheme. Supporting Information Available Possible chemical reaction involved in formation of CuBO2; Details of roughness analysis of individual pristine nanocubes; 3D images of pristine CuBO2nanocubes simulated using iamgeJ software; variation of simulated roughness of individual pristine CuBO2nanocubes. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Figure captions: Figure 1: XRD pattern for pure and composite samples A, B and C Figure 2:FESEM image of (a) pure sample prepared for 1h, (b) composite sample A and (c) single composite CuBO2 cube for sample A; (d) pure sample prepared for 2h, (e) composite sample B and (f) single composite CuBO2 cube for sample B; (g) pure sample prepared for 3h, (h) composite sample C and (i) single composite CuBO2 cube for sample C; Figure 3:EDX elemental mapping of (a) pure CuBO2 sample and (b) composite sample A, (c) composite sample B and (d) composite sample C Figure 4: TEM image of sample B of (a) single CuBO2nanocube, (b) single CuBO2nanocube after wrapping by RGO, (c) pure constituent CuBO2nanorods (d) magnified image of wrapping of RGO over constituent nanorods; HRTEM image of (e) single CuBO2nanorod, (f) interference atomic planes of CuBO2nanorod with RGO layers Figure 5: Schematic of wrapping of RGO over CuBO2nanocubes for various samples Figure 6: (a)Raman spectra of pure and composite samples; (b) transmittance spectra for pure and composite samples Figure 7: Field emission J-E curves for (a) pure and composite sample A, (b) pure and composite sample B, (c) pure and composite sample C; (d) comparative representation of turn – on fields of sample A, B and C

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Figure 8: Field emission FN curves for (a) composite sample A, (b) pure and composite sample B, (c) pure and composite sample C; (d) comparative representation of field enhancement factors of sample A, B and C Figure 9: (a) Enhancement of field emission properties caused by intraband carrier transport; (b) Enhancement of field emission properties due to morphological modification before and after composite formation; (c) proposed model showing effect of RGO in charge accumulation Figure 10: Simulated emission field distribution for (a) single nanocube of pure form of sample A, (b) magnified image of an edge of pure form of sample A, (c) composite sample A; (d) single nanocube of pure form of sample B, (e) magnified image of an edge of pure form of sample B, (f) composite sample B; (g) single nanocube of pure form of sample C, (h) magnified image of an edge of pure form of sample C, (i) composite sample C

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Figure 1. S. Santra et al.

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Graphical abstract

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