Tailored CsPbX3 Nanorods for Electron-Emission Nanodevices | ACS

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Tailored CsPbX3 Nanorods for Electron-Emission Nanodevices Tufan Paul, Soumen Maiti, Nripen Besra, Biplab Kr Chatterjee, Bikram Kumar Das, Subhasish Thakur, Saikat Sarkar, Nirmalya Sankar Das, and Kalyan Kumar Chattopadhyay ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01379 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 6, 2019

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ACS Applied Nano Materials

Tailored CsPbX3 Nanorods for Electron-Emission Nanodevices Tufan Paul1, Soumen Maiti2, Nripen Besra3, Biplab Kr Chatterjee1, Bikram Kumar Das3, Subhasish Thakur1, Saikat Sarkar3, Nirmalya Sankar Das4 and Kalyan Kumar Chattopadhyay1, 3* 1

School of Materials Science and Nanotechnology, Jadavpur University, Kolkata 700032, India. 2

St. Thomas College of Engineering & Technology Kolkata 700032, India. 3

4

Departments of Physics, Jadavpur University, Kolkata 700032, India.

Department of Basic Science and Humanities, Techno India, Batanagar, Kolkata 700141, India. *Corresponding author Tel.: +91 33 2413 8917. Fax: +91 33 2414 6007. E-mail: [email protected].

Abstract Going beyond much cultivated photovoltaic aspects of all inorganic lead halide perovskite, here we have explored electron field emission of CsPbX3 nanorods where X represents different halides specifically Cl, Br and I. Starting with room temperature preparation of different perovskites nanorods we have examined their electron emission behavior and found CsPbI3 as best electron emitter among the synthesized samples where they exhibited low work function and high aspect ratio driven best emission performance. Aspect ratios of the CsPbI3 nanorods were tailored via alteration of processing temperature. Nanorods prepared at 90°C possessing maximum aspect ratio delivered a current density of 133 µA/cm2 at 8 V/µm applied external field. With an expectation of gaining better emission performance from as synthesized optimized nanorods, reduced graphene oxide (rGO) was further attached to them. Relying on the emission beneficial electronic features of rGO as prepared CsPbI3-rGO composite delivered significantly improved electron emission performance with low turn-on field and high field enhancement value which are much better than the individual structural blocks. Easy electron passage from the 1 ACS Paragon Plus Environment

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composite as a consequence of work function decrement as well as high field amplification at graphene basal plane supported by 1D CsPbI3 nanorod renders improvement in field values. Experimentally observed electron emission result is further verified through electrostatic field distribution calculation using ANSYS software. Such results indicated the potential utility of these kinds of perovskite materials in field electron emission nanodevices.

Keywords: Perovskite, Field emission, DFT, Work function, CsPbI3-rGO composite, ANSYS.

Introduction With the ever-increasing global energy consumption, necessities for renewable energy alternatives and fast development of high-performance energy electronic devices have attracted ever-growing interests among the researchers. [1–3] To date, scientists have dedicated significant efforts to explore new materials [4] and rationally designed coupled structures to improve the efficiencies of such electronic and optoelectronic devices. Recent era has witnessed the development of organic–inorganic metal halide perovskites (OMHP) as building blocks of numerous electronic and optical devices owing to their unique optical and electronic features, , large absorption coefficient, high charge carrier mobility, tunable band-gap and etc. [5-8] OMHP with the general formula of ABX3 comprised of three different species, where A stands for monovalent cation (Cs+, MA+, FA+), B as metal cation (Pb2+; Sn2+; Ge2+), and X denotes halide anion (Cl-; Br-; I-). [9, 10] A notable advancement in the photovoltaic arena has been already observed over the last few years using metal halide perovskite CsPbX3, as the primary semiconductor of interest due to their serval intriguing chemical and physical features [11, 12]. CsPbX3 possess a perovskite structure where Cs+ cations lie within the framework of PbX6- octahedral. In these octahedrons, one Pb2+ 2 ACS Paragon Plus Environment

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cation positions at the center and coordinates with 6 apical I- ions. [13] These semiconductor forms good quality films that yield solar cells performance with an efficiency of 10.77%. [14] More precisely, from CsPbX3 family, its iodine analogue CsPbI3 with narrow band gap showed superior photovoltaics characteristics as it has a wide light absorption window beyond 700 nm. [15] However, most of the potential applications based upon these OMHP have been utilizing their photovoltaic character. Apart from photovoltaics one, have not been investigated properly yet. From our preceding experience, transition metal oxides, both in binary and ternary oxide form often deliver boosted luminescence performance and field emission (FE) features [16] This kind of electron emission has huge potential in field emission displays, infrared imaging devices, dynamic CT scanners etc. Low dimensional semiconductor nanoforms in general have potential candidature as field electron emitters. Some well-known field emitting materials e.g. ZnO, CNT, grapheme, rGO nanocomposites and MoS2 [17-22] are well studied and few has already been commercialized. However, almost each of them has limitations related to synthesis routes, stability, toxicity etc. Recently Besra et al. [23] first reported the field emission property of CH3NH3PbI3 nanorods which delivered considerable emission behavior with a current density reaching 96 A cm-2 and a turn-on field of 4.2 V m-1. Herein, we have reported electron field emission characteristics of the all-inorganic perovskite CsPbX3 nanorod. Initially we have synthesized different kinds of CsPbX3 nanorods and investigated their FE behavior. From the assortment of CsPbX3 nanorod, room temperature synthesized CsPbI3 achieved maximum current density. Expecting improvement in emission behavior, aspect ratio of the CsPbI3 sample was controlled by subtle adjustment over reaction temperature. Not restricting at this point, visualizing further enhancing in emission performance we have attached chemically reduced graphene oxide (rGO) with the optimized CsPbI3 rods. It is 3 ACS Paragon Plus Environment


well documented that rGO, due to its peculiar 2D features, bestows great electrical and thermal conductivity, which helps in electron field emission and heat dissipation. [24, 25] Synergy effect of 1D perovskite and 2D rGO delivered much improved FE activity with low turn-on, high field enhancement value and better stability than the separate components. Additionally, for the identification of the local field profile at the emission sites finite element method was applied and as observed FE results were substantiated. Effective work functions of emitter materials were assessed via first-principles investigations. Effective work functions values were used in conjunction with experimental results to quantitatively estimate overall field enhancement.

Experimental Chemicals Following are the details of the raw materials which are used for the synthesis of the samples: Lead (II) chloride (PbCl2), Lead (II) bromide (PbBr2), Lead (II) iodide (PbI2), cesium chloride (CsCl), cesium bromide (CsBr), cesium iodide (CsI), (Sigma Aldrich) dimethylformamide (DMF), dimethyl sulfoxide (DMSO), hexane, oleic acid (OA, Alfa Aesar), n-octylamine, tertbutanol (Sigma-Aldrich).

Synthesis of CsPbX3 nanorods For the synthesis of CsPbX3 rods we are slightly altered our previous synthesis procedure [26, 27]. In a typical synthesis procedure, two independent solution were prepared by dissolving CsI (1.0 mmol, 0.259 mg) in deionized water (D.I, 0.2 ml) and PbI2 (1.0 mmol, 0.461 mg) in DMF (2 ml) separately. Simultaneously, an oil phase was also prepared by adding hexane (10 ml) with 1ml of oleic acid and 0.5 ml of n-octylamine. Thereafter, the CsI solution was added very slowly into PbI2 solution followed by the final solution was mixed with the oil phase under vigorous stirring for 5 min. Tert-butanol (8 ml) was further added with the aforesaid solution and stirring 4 ACS Paragon Plus Environment

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for 3 minutes. Next, the solution was centrifuged at 10000 rpm for 10 min and the precipitate at the bottom of the tube was collected. During Centrifugation the precipitate was washed with hexane for several times. Finally, it was dried in regular vacuum oven for overnight and collected. An analogous protocol was adopted for the realization of CsPbCl3 and CsPbBr3 except one change for both samples as compared to the above mentioned. For the preparation of CsPbCl3, 2.0 ml of DMSO was used to dissolve PbCl2 whereas for CsPbBr3, 1.0 ml of DMF was used to mix PbBr2. Schematic of the CsPbI3 nanorod synthesis procedure is presented in figure 1.

Figure 1: Schematic representation of synthesis protocol of CsPbI3 rod.

Synthesis of CsPbI3 nanorods with different aspect ratio Initially, PbI2 (1.0 mmol, 0.461 mg) was dissolved in DMF and stirred at different temperature 30°-90°C on a hot plate. Simultaneously, CsI (1.0 mmol, 0.259 mg) was mixed in deionized water (D.I) separately. Then aqueous solution of CsI was added very slowly with PbI2 solution. Further the oil phase added to the solution under vigorous stirring for 5 min. Tert-butanol (8 ml) was added afterwards with the previous solution instantaneously to initialize the demulsification.

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Finally, the sample was collected via centrifugation followed by drying overnight in vacuum oven.

Synthesis of rGO Using modified Hummer’s method graphene oxide were prepared. Details are present in literature [28] For the reduction of GO, 1 mg GO was mixed with 20 ml DI water followed by centrifuged at 15000 rpm. With the supernatant solution 20 ml of DI water, 35 µl ammonia and 7 µl hydrazine hydrate was mixed further and stirred at 90˚C for 1 hour. Finally, the sample was collected and dried in vacuum.

Synthesis of CsPbI3 and rGO composite For the preparation of composite, 1 mg rGO was dissolved in 10 ml DI water via sonication. 10 mg CsPbI3 powder was added with the aforesaid solution and stirred magnetically at moderate speed. After 30 min of stirring the solution was allowed to settle down and a blackish precipitate was observed. The precipitate was filtered and dried at 120 ˚C in vacuum.

Characterization See the supporting information

Results and Discussion Structural and morphological analysis For the confirmation of sample purity and crystal structure, X-ray diffraction (XRD) analysis was performed. XRD profile of the as-synthesized CsPbX3 is presented in Fig. 2a. Sharp peaks at 15.79°, 22.34°, 31.72°, 35.55° and 39.02° are related to (100), (110), (200), (210) and (211) planes of tetragonal CsPbCl3 respectively (JCPDS18-0366) [29]. For CsPbBr3, intense peaks appeared at 2θ =15.45°, 21.84°, 31.01°, 34.64° and 38.05° which can be assigned to (100), (110), (200), (210) and (211) planes respectively of its orthorhombic phase (JCPDF01-072-7929) [30]. 6 ACS Paragon Plus Environment

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Remaining XRD profile shows diffraction peaks related to (100), (110), (200), (210) and (211) planes of orthorhombic CsPbI3 (JCPDF 18-0376) [31]. Such intense peaks in the XRD profiles of CsPbX3 samples not only ensured phase purity but also indicated high crystallinity. No other impurity peaks are detected within the scanning range. Careful observation of XRD profiles further reveal shift in diffraction peak position towards lower 2θ side with gradual alteration of halide from Cl to Br to I. Such shifting in peak position can be ascribed to the lattice contraction resulted from the shrinkage in halide ion radii from Cl˗ to I-. FESEM images of the as-synthesized samples presented in Fig. 2(b-d) depict a typical 1D rod like morphology for all. Very short and blunt edged nanorods of CsPbCl3 with uniform size and thickness is visible from Fig. 2(b). Alteration with other halides (i.e. Br, Fig. 2(c) and I, Fig. 2(d)) did not create much change in their shape; however, their size changed drastically. In both the samples, rods are micrometer long with diameters varying in nano regime. Furthermore, careful comparison of these two images suggests relatively smaller diameter of CsPbI3 than CsPbBr3. Smooth and faceted surface of these nanorods, evident from corresponding FESEM images, indicate high crystallinity of the sample. Despite the randomness in individual length, diameter of rods remained almost fixed for a particular category. Although the length of the rods varies over a range, their diameters are more or less same for a particular sample. Energy dispersive x-ray (EDX) analysis was performed to check the stoichiometry of constituent elements and results are summarized in Table S1 (Supporting information). Fig. 2e-2g reveals the presence of Cs, Pb in all samples along with corresponding halides. Absence of any impurity



Figure 2: (a) XRD patterns of the CsPbX3(X = Cl, Br or I) rods. FESEM images of assynthesized (b) CsPbCl3 (c) CsPbBr3 and (d) CsPbI3 samples. EDX spectra and the elemental distribution of the (e) CsPbCl3 (f) CsPbBr3 and (g) CsPbI3 rods.


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element related peaks in these spectra validate the XRD results of pure phase formation. It is observed that the atomic ratios of constituent elements are in well agreement with their actual stoichiometry. X-ray photoelectron spectra (XPS) analysis was further performed to check the chemical compositions of CsPbX3 nanorod as well as to access the oxidation states of constituent’s elements. XPS spectra of Cs and Pb of CsPbI3 sample are presented in figure 3a and 3b. Cs 3d and Pb 4f both exhibit typical doublet features of Cs and Pb respectively. The binding energy curves of the Cs 3d3/2 and Cs 3d5/2 are observed at ~740.4 eV and ~726.36 eV respectively. Meanwhile the peaks of Pb 4f5/2 and Pb 4f7/2 are located at ~145.46 eV and ~140.57 eV respectively. Doublet peaks of Cl 2P1/2 and Cl

Figure 3: XPS core level spectra with peak fitting for (a) Cs (b) Pb (c) Cl (d) Br (e) I



2P3/2 states are appeared at ~201.21 eV and ~197.80 eV respectively (see figure 3c). Peaks related to Br 3d3/2, Br 3d5/2, I 3d3/2 and I 3d5/2 are positioned at ~72.87, ~69.29, ~ 632.57 and 621.05 eV respectively (see figure 3d and figure 3e). These results suggest that all the constituent elements (Cs, Pb, Cl, Br and I) are in their normal valance i.e. 1+, 2+, 1-, 1- and 1-states respectively which also mimics the literature value [32].

Work function calculation Work function (ϕ), the minimum energy required by the electron to escape from a solid into a point immediate outside of its surface, has a pivotal role to play in electron emission from the sample. It is a surface characteristic of the materials rather than that of the bulk one. In order to estimate the work function of the samples through first principles, computations were performed using Vienna ab-initio simulation package (VASP) [33-36] with projector-augmented-wave (PAW) [37, 38] approach. Driven by the fact of maximum intensity of (110) plane in the XRD profiles we have studied that plane only for work function analysis. Exchange-correlation contributions were taken under consideration via Perdew-Burke-Ernzerhof (PBE) [39] functional within generalized gradient approximation (GGA) [40]. Plane wave basis set up to an energy cut off 400 eV was utilized for entire calculation. For geometrical optimizations, structures were allowed to relax till total energy of this system converged below 10-4 eV/atom. For geometrical optimization of CsPbX3 structure a 2×2×2 k-mesh was used while for work function calculations of (110) surface of the same a k-mesh of 1×5×5 was considered. A vacuum slab of 30 Å was deployed on the CsPbX3 (110) surface to exclude any false interaction. Work function (ϕ) was obtained following formula: Φ=Evac-EF where EF stands for Fermi energy and Evac denotes reference vacuum energy level. EF and Evac are obtained through scf calculation in spin restricted condition. Initially, tetragonal CsPbCl3 unit cell was optimized and assessed lattice parameters 10 ACS Paragon Plus Environment

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are found to be a=8.79 Å and c=12.98 Å. A (2x2) view from z direction of the optimized CsPbCl3 structure and the (110) surface of CsPbCl3 was constructed and relaxed. In our calculation, Evac was assessed from constant value of planar average electrostatic potential at a large distance from the surface and EF was obtained from DFT calculation. EF and Evac for CsPbCl3 (Fig. 4a) are found to be -4.3035 eV and 1.2265 eV respectively, results in a ϕ value of 5.52 eV. ϕ value for orthorhombic CsPbBr3 and CsPbI3 was further calculated using the same procedure. For CsPbBr3, EF and Evac were found to be -4.3035 eV and 1.041 eV (Fig. 4b) respectively which results in a ϕ value of 5.34 eV. Using the same (110) surface, EF and Evac for CsPbI3 (Fig. 4c) were found to be -4.3035 eV and 0.523 eV respectively. Thus, the ϕ value of CsPbI3 is 4.82 eV. Atomic arrangement of CsPbCl3, CsPbBr3 and CsPbI3 in the (110) plane are

Figure 4: Average variation of the electrostatic potential with distance from the surface along (110) plane of (a) CsPbCl3 (b) CsPbBr3 (c) CsPbI3. The (110) optimized surfaces with atomic arrangement of (d) CsPbCl3 (e) CsPbBr3 (f) CsPbI3 perovskite.



depicted in the Fig. 4d-4f respectively. For atomic arrangement of the (100) plane see supporting information in the figure S1.

Field emission study Electric field assisted electron emission property of the as synthesized CsPbX3 nanorods were performed in a homemade field emission setup under a high vacuum with a parallel plate configuration where synthesized samples over conducting carbon tape served as cathode and a conical shaped stainless-steel tip (1.5 mm diameter) as anode. Schematic of the experimental setup is presented in figure S2 (please see supporting information). FE characteristics of the samples were carried out at a fixed inter-electrode distance (200 µm). Electron emission current density (J) vs electric field strength (E) plot in fig. 5a for CsPbX3 nanorods clearly suggests the impact in electron emission behavior due to different halide constitution in the sample. From fig. 5a, maximum current density corresponding to CsPbI3 nanorods is obvious which attained a current density of 90 µA/cm2 at 8 V/µm external fields. At same applied field CsPbCl3 and CsPbBr3 sample attained current density of 66 and 72 µA/cm2 respectively. Careful observation of these curves suggests a gradual reduction in turn-on fields (corresponding to an emission current density of 10 μA/cm2) from CsPbCl3 to CsPbBr3 to CsPbI3. Registered turn on field values for CsPbI3, CsPbBr3 and CsPbCl3 are found to be 4.5, 5.8 and 10.1 V/µm respectively. For quantitative analysis of FE result, Fowler–Nordheim formalism is used. According to this formalism emission current density (J) and applied electric field (E) follows the relation: J  ( A 2 E 2 /  ) exp( B 3 / 2 /  E ) where A and B is constants, ϕ is the work function of the

emitter E=V/d (d is the anode-emitter distance) and β is the geometrical field enhancement factor. F-N plots (ln (J/E2) vs 1/ E) corresponding to all samples presented in figure 5b, shows linearity in high field region which confirms that the emission current arises solely from barrier 12 ACS Paragon Plus Environment

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tunneling of electron. ‘β’ designates amplification ability of the emitter to strengthen the applied electric field at emitting site. [24] Such emission ability of nanostructure depends on several parameters among which nanostructure geometry and work function play most pivotal role. [25] Discernable differences in electron emission behavior registered from the nanorods are accounted for these two factors. FESEM images in figure 2(b-d) discloses minute structural differences among of nanorods. Careful observation of these images suggests maximum aspect ratio for CsPbI3 nanorod which eases the electron emission process and renders high emission performance. Calculated aspect ratios of the pristine samples are found to be ~6, ~11, and ~19 for CsPbCl3, CsPbBr3 and CsPbI3 respectively. Besides aspect ratio, differences in ϕ values also resulted variances in emission behavior. From Fig. 4a-4c it is found that value of ϕ for CsPbCl3 is 5.52 eV whereas the same for CsPbBr3 and CsPbI3 are 5.34 and 4.82 eV respectively. As observed turn on field of CsPbI3 than the other two is be attributed to its relatively lower work function association among all. With low work function, CsPbI3 nanorods emit electrons more easily. Using the computed ϕ values the estimated β values for CsPbCl3, CsPbBr3 and CsPbI3 are found to be 602, 1230, and 1501 respectively.

Figure 5: (a) J–E characteristics of CsPbX3 (b) F-N plots of the as-synthesized CsPbX3 rods. 13 ACS Paragon Plus Environment


Driven by the fact that among all samples under investigations CsPbI3 showed superior emission characteristics, we have tailored CsPbI3 nanorod aspect ratio. More precisely, we have synthesized various types of CsPbI3 nanorods via temperature variation during synthesis. Fig. 6(a-c) shows the FESEM images of CsPbI3 nanorods processed at room temperature, 60°C and 90°C respectively. Henceforth, we will designate the CsPbI3 nanorods prepared at room temperature, 60 and 90°C simply as NR-1, NR-2 and NR-3 respectively. Electron emission performance of these nanorods is presented in figure 6d. Current density curve of these samples as function of electric field strength presented in the aforesaid figure suggests that NR-1 attained current density of 90 µA/cm2 at an external field of 8 V/µm. Nanorod processed at higher temperature showed much higher emission current density at same applied field. NR-2 attained a current density of 116 µA/cm2 whereas NR-3 achieved a current density of 133 µA/cm2. Maximum current density registered from NR-3 sample may accredit to its high aspect ratio. Fig. S3 (see supporting information) displays a comparative plot of aspect ratio of CsPbI3 rods with temperature. Nanostructures with high aspect ratio and curvature point generally inherit a tendency towards the accumulation of electric field to reduce the work function locally and tunnel the electron through the barrier. [42, 43, 17] Such accumulation of field ensures applied electric field enhancement at the emitting site. Thus, the field value at these points are not just E, it enhanced one to βE. FESEM images of these temperature varied nanorods suggest maximum aspect ratio for NR-3 sample. Nanorod with high aspect ratio and faceted surfaces ensures electric lines of force to condensed and pile up near the side edges. [44] These edges acted as efficient emitting site and rendered high emission current. [17] F-N plots (ln (J/E2) vs 1/ E) corresponding to all these samples presented in Fig. 6e, also shows linearity in high field region. Figure 6f presents comparative view of the β values as function of aspect ratio of CsPbI3 rods.


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Figure 6: FESEM images of as-synthesized (a) CsPbI3 @ room temperature (b) CsPbI3 @ 60°C and (c) CsPbI3 @ 90°C. (d) J–E characteristics of CsPbI3 rods at x = 200 m of different temperature. (e) F-N plots of the as-synthesized CsPbI3 rods of different temperature. (f) Aspect ratio vs. field enhancement factor (β) plot. Expecting further improvement in emission characteristics we have attached rGO with the best sample i.e. with NR-3. Owing to its typical electronic features, high aspect ratio and chemical 15 ACS Paragon Plus Environment


stability graphene has already made an impact in FE applications. [25, 41] In graphene-based system, emission takes place from both basal plane and edges. In addition, creation of curvature or corrugations in the basal plane also enhances the local field and thereby helps in electron emission. [24] FESEM image of NR-3 and rGO composite is presented in fig. 7a this suggests the co-existence of sheet like rGO with CsPbI3 nanorods. Crumbling and rippling in basal plane along with protruding edges of rGO are obvious from this image. Basal plane of rGO sheets gains a curvature locally comparable to the diameter of nanorods. [24] Preparation of composite is further verified with the help of Raman spectroscopy. Fig 7b shows Raman spectra of NR-3 and NR-3-rGO composite. For pristine NR-3 sample peaks appearing at ~104, ~178 and ~222 cm-1 agree well with the literature. After the rGO attachment additional peaks appeared at ~1363 and ~1607 cm-1 which can be indexed to D and G band of rGO respectively and may be attributed to the direct charge transfer between NR-3 and rGO. Such attachment not only reduces the contact resistance across the interface but also proliferates the electron transfer mechanism as a whole. In the composite sample, one can expect band bending and alteration in work function values. Work function of the NR-3-rGO composite is evaluated by DFT analysis. For work function analysis of rGO attached CsPbI3, (110) plane of CsPbI3 was considered. To match the symmetry of (110) plane of CsPbI3 a lattice vector of graphene unit cell was rotated along the [210] direction. A (6×7×1) supercell of the rotated graphene unit cell with 160 carbon atoms was considered to minimize the lattice mismatch between CsPbI3 (110) plane and the rotated graphene sheet. The CsPbI3 surface was kept unstrained during the formation of this heterostructure whereas a compressive strain of 0.9% and a tensile strain of 2.4% was generated along the a and b lattice vector of the graphene supercell. After optimization, the distance between CsPbI3 (110) surface and graphene ad-layer is found to be 2.8 Å. Optimized models of pristine CsPbI3 (110) surface 16 ACS Paragon Plus Environment

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and CsPbI3-rGO nanocomposite are shown in Fig. 7c. Calculated work function value of the nanocomposite sample is 4.2 eV, which is lower than the same for CsPbI3. The attachment of CsPbI3-rGO nanocomposite by DFT analysis is shown in Fig. 7d. (for further orientation see supporting information figure S4). FE characteristics of NR-3-rGO composite were further examined and compared with NR-3 sample. Emission comparison of these two samples is displayed in fig.7e. After rGO attachment, not only higher current density was achieved turn on field of the system also downshifted.

Figure 7: (a) FESEM image of CsPbI3_rGO; (b) RAMAN spectra of CsPbI3 and CsPbI3-rGO (c) Average variation of the electrostatic potential with planar distance; (d) Optimized supercell structure of CsPbI3-rGO (e) J–E characteristics of CsPbI3 nanorods and CsPbI3-rGO composite, inset F-N plot of the both sample; (f) Temporal current stability profile for CsPbI3rods and CsPbI3-rGO composite. After the rGO attachment, the composite achieved a current density 900 µA/cm2 at the 8 V/µm. Registered value of turn on field of NR-3 is found to be 5.8 V/μm which downshifted to 4.1 17 ACS Paragon Plus Environment


V/μm after the rGO attachment. β values were also calculated form the slope of the FN plot presented in figure 7e inset. Estimated β value for the composite is found to be 11000 which is almost 3-fold higher than the same of NR-3. A plot of work function vs. β of CsPbI3 and CsPbI3_rGO is shown in fig. S5 (see supporting information). Such high emission behavior registered form composite can be illuminated on the basis of the two factors namely work function and localization of applied field. With the formation of composite, work-function of the overall system decreases drastically than NR-3 sample (evident from DFT analysis). Low work function of the composite system facilitates transfer of electron from the nanostructure to the vacuum. Wrapping of the rGO over the nanorods also ensured easy electron transfer from nanorod to graphene. Furthermore, due to the attachment of rGO with nanorod the basal plane of rGO sheets attains a local curvature comparable to the diameter of the rods. Such high curvature in the basal plane and presence of protruding edges localize and enhance the electric field, thereby easing the electron emission process. [24, 45] To visibly point out the impact of rGO attachment, J vs E characteristics of rGO, CsPbI3 and CsPbI3-rGO composite is presented in same fig. S6 (please see supporting information).This plot suggests very nominal electron emission performance for rGO alone with a turn on field ~8.2 v/μm. Nominal local curvature in the basal plane of rGO sheet in flat configuration, gives rise to such poor field value and very nominal field enhancement. [24] Besides high emission current yield at low applied electric field, emitter should also deliver long term emission stability for their direct usage in advanced electronic applications. Being an all inorganic perovskite material, high vulnerability of the constituent halide under harsh situation might introduce unexpected barriers against their day to day applications. [23] In such scenario stability evaluation of CsPbI3 based emitters is crucial. Temporal variation of the emission current for 2h was observed for NR-3 and composite (fig. 7f) with initial emission 18 ACS Paragon Plus Environment

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current of 135 µA/cm2 corresponding to applied field of 8 V/µm. The as assessed fluctuation in emission current is ~10% for NR-3 whereas it is only ~2% for the composite. Out of numerous critical issues, local resistive heating at the emitting site even to a very small extent may be considered as one prime cause of instability. [24] Long-range ordering in CsPbI3 enables minimal scattering of electrons and thereby less probability of energy dissipation as heat. After rGO attachment, gigantic thermal conductivity of rGO eases the generated heat transmission whereas high surface area of the same helps heat dissipation via surface radiation phenomenon. [25, 40] Thus, the as observed high stability of the composite as compared to NR-3 may be accredited to the high thermal conductivity and surface area of rGO. To judge the figure of merit of the composite, we carefully surveyed the field values of previously reported field emitters of different kinds to date and tabulated in table S2. This table clearly indicates comparable emission performance of the composite than other reported field emitters, particularly highlighting this documentation as the first work on electron emission from all inorganic perovskite CsPbX3. Durability of the emitter, other important parameter, was also tested by reusing the same CsPbI3_rGO sample for four consecutive cycles. Synthesized composite exhibited significant emission stability even after four cycles, as displayed in supporting information (fig. S7). After the fourth operation cycle the emission current decrease to only ~3%.

ANSYS simulation for field emission performance To verify the reliability of experimental observation, theoretical simulation of FE behavior of CsPbI3 and composite system were carried out using ANSYS Maxwell simulation package. As it is very difficult to imitate the exact model of the observed of CsPbI3-rGo sample, we have selected the simulation parameters in such way that it can maintain a direct analogy with the as observed morphology. Dimensions of the nanostructure were considered as per actual values found in FESEM analysis. Inter electrode separations was also maintained in as per experimental 19 ACS Paragon Plus Environment


case. After such modeling, a virtual external macroscopic field of -1.3 kV was applied on the emitters and corresponding output electric field distribution in vacuum column was plotted in appropriate color code. Further, thin sheets of rGO were also modeled as per actual basis and the same was designed to cover the previously modeled nanoform. A virtual external macroscopic field of same magnitude was again applied on the composite system model. The output electric field was again plotted with the same color code. For a comparative view of these two results are presented side by side in fig. 8. It can be clearly seen that output electric field is enhanced after covering CsPbI3 nanostructures with rGO sheet. From these plots, it is obvious that composite system emits higher electric field than NR-3 which corroborates our experimental FE results.

Figure 8: ANSYS simulation of field distributions of (a) CsPbI3 and (b) CsPbI3_rGO composite.

Conclusion


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In summary, we have investigated the field emission features of the all inorganic perovskites CsPbX3 for the first time. Initially different kinds of perovskite nanorods was prepared at room temperature and in-depth characterization like XRD, XPS, FESEM and etc. were performed to establish their successful preparation. As an immediate usage potential beyond photovoltaics, we have examined their field emission performances where CsPbI3 nanorod with high aspect ratio and low work function presented better performance than the others. By controlling the growth temperature, morphology of the CsPbI3 nanorods were further tailored in subtle manner. Optimized nanorod delivered improved emission performance than the room temperature grown sample. Not restricting at that point, expecting further improvement in electron emission, we have prepared CsPbI3_rGO composite. Benefited with unique electronic and thermal features of rGO sheet, the composite delivered much heightened performance which is much better than the same registered from the building blocks. As observed experimental findings on electron emission was further verified with a theoretical analogy by ANSYS simulation. We believe this documentation will open up a new potential aspect of all inorganic perovskite and will motivate the researchers to integrate the functionalities of rGO and other perovskites for practical electronic devices.

Acknowledgement Tufan Paul (IF160045), would like to acknowledge the Department of Science Technology (DST), Govt. of India for the INSPIRE fellowship. The authors also wish to thank the UGC, the Government of India, for the ‘University with potential for excellence (UPEII)’ scheme.

References 21 ACS Paragon Plus Environment


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Tailored CsPbX3 Nanorods for Electron-Emission Nanodevices | ACS

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