In-Situ Growth and Characterization of Indium Tin Oxide Nanocrystal Rods

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Article In-Situ Growth and Characterization of Indium Tin Oxide Nanocrystal Rods

Yan Shen 1,* ID , Youxin Lou 1, Zhihao Wang 1 and Xiangang Xu 1,2,* 1 School of Material Science and Engineering, Qilu University of Technology, Jinan 250353, China; [email protected] (Y.L.); [email protected] (Z.W.) 2 State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China * Correspondence: [email protected] (Y.S.); [email protected] (X.X.); Tel.: +86-(531)-8963-1227 (Y.S.); Fax: +86-(531)-8963-1625 (Y.S.)

Academic Editors: Pietro Mandracci and Paola Rivolo Received: 31 October 2017; Accepted: 22 November 2017; Published: 25 November 2017

Abstract: Indium tin oxide (ITO) nanocrystal rods were synthesized in-situ by a vapor-liquid-solid (VLS) method and electron beam evaporation technique. When the electron-beam gun bombarded indium oxide (In2O3) and tin oxide (SnO2) mixed sources, indium and tin droplets appeared and acted as catalysts. The nanocrystal rods were in-situ grown on the basis of the metal catalyst point. The nanorods have a single crystal structure. Its structure was conﬁrmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The surface morphology was analyzed by scanning electron microscopy (SEM). During the evaporation, a chemical process was happened and an In2O3 and SnO2 solid solution was formed. The percentage of doped tin oxide was calculated by Vegard’s law to be 3.18%, which was in agreement with the mixture ratio of the experimental data. The single crystal rod had good semiconductor switch property and its threshold voltage of single rod was approximately 2.5 V which can be used as a micro switch device. The transmission rate of crystalline nanorods ITO ﬁlm was over 90% in visible band and it was up to 95% in the blue green band as a result of the oxygen vacancy recombination luminescence.

Keywords: indium tin oxide; in-situ growth; Vegard’s law; solid solution

1. Introduction Indium tin oxide (ITO) is a metallic oxide material with good conductivity [1], high transparency [2] and optical nonlinearity [1–3]; As a conductive material, ITO film was widely used in [4] semiconductor devices [5–7], for example liquid crystal displays, solar cells and light emitting diodes [5] etc. Nanostructured ITO [8] has many new applications because of its unique surface [9] and quantum effects [10]. Nanometer-scale structured ITO material has a high specific surface area and can be applied in cells [11] and gas sensors [12,13]; it has also demonstrated gas sensitivity in the field of environmental sensitive exploration [14,15]. Although many other transparent conducting oxide thin-films have been developed to replace ITO film [16,17], ITO films are still the most widely used in commercial applications. As a commercially available film, the ITO research focus is on application fields or its preparation techniques. For example, ITO nanorods have been synthesized by radio frequency magnetron sputtering deposition [18], sol electrophoresis [19] and its characteristic was studied accordingly. Hamid [20] have analyzed the effect of substrate temperature on the electrical conductivity and transparency of ITO films. Canhola [21] studied the role of annealing environment on the performances of large area ITO films. The performance was improved GaSb-based solar cells with ITO nanorod array [22]. The concepts of ITO nanoantenna arrays [23] have been proposed. ITO confronted with the bottleneck in applications of photon conduction and flexible displays [24] for its brittle and polycrystalline natures.

Coatings 2017, 7, 212; doi:10.3390/coatings7120212 www.mdpi.com/journal/coatings Coatings 2017, 7, 212 2 of 8

In this work, ITO single-crystalline nanorods were fabricated at a substrate temperature of 300 ◦C by an electron beam evaporation technique (FU-20TEB-ITO, Fulinten Engineering Co. Ltd., Taiwan). A large area of uniform nanocrystal rod film can be proposed by using electron beam evaporation technique without other special equipment or additional functions. Compared to the existing ITO nanostructures [18,20], the ITO nanorods of we prepared have a single crystal structure and theirs length can be controlled simply by growth time. In-depth discussions and analyses have been carried out on the ITO nanostructured films. The formation mechanism of the single-crystalline nanorods was based on a vapor-liquid-solid (VLS) model. When the electron-beam gun bombarded indium oxide and tin oxide mixed sources, indium and tin droplets appeared and acted as crystal growth auto-catalysts. The nanocrystal rods were in-situ grown from the metal catalyst points. Its crystalline structure was confirmed to a cubic from X-ray diffraction (XRD) and transmission electron microscopy (TEM) results. Indium (In) and tin (Sn) oxide solid solutions have undergone a chemical change and were formed during the VLS process. The percentage of doped tin oxide was calculated by Vegard’s law to be 3.18%, which was responsible for the shift in the XRD peaks and corresponded to the change in lattice parameter. The properties of the crystalline ITO nanorods were measured by conductivity measurement system with SEM image acquisition (ZEISS SUPRA55, Carl Zeiss AG, Oberkochen, Germany) and ultraviolet visible spectrophotometer (EMCLAB EMC-61PC-UV, Duisburg, Germany). The deep mechanism of light transmittance distribution was discussed.

2. Experimental Optical glass substrates were placed in the electron-beam evaporation apparatus after being cleaned. The evaporation source was a homogenously mixed 95% In2O3 and 5% SnO2. A 1 µm thick oxide layer was deposited on the glass substrates after the substrate temperature reached the set point of 300 ◦C in the FU-20TEB-ITO equipment. The Voltage and power were 200 V and 5000 W of the evaporation equipment. Its pressure was set 1 × 10−4 mTorr in the chamber. The distance was 60 cm between the source and sample. The growth rate was kept constant at 1 Å/s. Scanning-electron microscopy (SEM Hitachi S-4800, Hitachi Group, Tokyo, Japan), X-ray diffraction (XRD Bruker D-8 X Advance, Bruker Corporation, Karlsruhe, Germany) and transmission electron microscopy (TEM JEOL JEM-2100F, JEOL, Tokyo, Japan) were employed to characterize the crystallinity and detailed structure of the obtained nano-crystalline ITO. The X-ray source was Cu (Kα = 0.1542 nm). The scanning step width was 0.08◦, and the scan range was from 10◦ to 90◦. The semiconductor switch property was measured by conductivity measurement system with SEM image acquisition (ZEISS SUPRA55). During the current-voltage test, ﬁrst, the single crystalline nanorod was observed by SEM equipment, then the test probe was close to the single crystal rod and gradually applied current-voltage test. The optical transmittance characteristic of crystalline ITO nanorods ﬁlm was analyzed by ultraviolet visible spectrophotometer (EMCLAB EMC-61PC-UV).

3. Results and Analysis Figure1 shows SEM images of the ITO ﬁlms obtained at substrate temperatures of 300 ◦C. Randomly disordered nano-sized ITO rods with good quality single crystals were obtained. The ITO nanometer crystals were formed by the VLS mechanism, yielding single-crystal morphology with a cap. The VLS theory was ﬁrst proposed by Wagner and Ellis in 1964 [25]. In the VLS mechanism, the catalyst forms a liquid alloy at the eutectic temperature and then target material diffuses into the liquid alloy droplet. After reaching the solubility limit at the liquid-solid interface, crystal nucleus precipitates inducing nanocrystal growth. The nucleation with respect to the thermodynamics of adsorption and the kinetics of crystal growth has been given proposed and studied [26,27]. In our experiments, the evaporated ITO source (In2O3 95%-SnO2 5%) decomposed into In and Sn metal vapors under the high voltage electron-beam bombardment. The metal vapor condensed into Coatings 2017, 7, 212

In our experiments, the evaporated ITO source (In2O3 95%-SnO2 5%) decomposed into In and Sn metalCoatings vapors2017 under, 7, 212 the high voltage electron-beam bombardment. The metal vapor condensed3 of 8 into liquidCoatings droplets 2017, 7 ,on 212 the surface of the glass substrate at 300 °C. The liquid metal droplets acted as a metalliquid catalyst droplets and onabsorbed the surface oxide of the vapor, glass substratewhich diffused at 300 ◦C. into The the liquid droplets. metal droplets Nanometer acted assized a metal crystal In our experiments, the evaporated ITO source (In2O3 95%-SnO2 5%) decomposed into In and Sn nucleicatalyst formed and when absorbed the oxide oxide vapor, in the which droplet diffused reached into the saturation. droplets. Nanometer As the liquid sized nuclei crystal constantly nuclei metal vapors under the high voltage electron-beam bombardment. The metal vapor condensed into absorbedformed the when source the oxidevapor in and the droplet reached reached super saturation.-saturation, As theITO liquid crystals nuclei were constantly formed absorbed and began the to liquidsource droplets vapor and on reached the surface super-saturation, of the glass substrat ITO crystalse at 300 were °C. formed The liquid and beganmetal todroplets grow in-situ.acted as a growmetal in-situ. catalyst and absorbed oxide vapor, which diffused into the droplets. Nanometer sized crystal nuclei formed when the oxide in the droplet reached saturation. As the liquid nuclei constantly absorbed the source vapor and reached super-saturation, ITO crystals were formed and began to grow in-situ.

Figure 1. SEM micrographs of ITO films fabricated at a substrate temperature of 300 °C: (a) top view; (b) profile. Figure 1. SEM micrographs of ITO ﬁlms fabricated at a substrate temperature of 300 ◦C: (a) top view; Figure(b 1.) proﬁle.SEM micrographs of ITO films fabricated at a substrate temperature of 300 °C: (a) top view; (b) profile. As shown in Figure 1, the ITO nanocrystal rods exhibited disordered random orientation, and the size ofAsAs the shown droplets in FigureFigure at the1 ,1, the thetips ITO ITO of nanocrystal thenanocrystal dendrites rods rods did exhibited exhibited not exceed disordered disordered 50 nm. random random The orientation, dendrites orientation, andwere and the in the rangethesize of size of40 the– of50 dropletsthenm droplets in diameter. at the at tipsthe oftips the of dendrites the dendrites did not did exceed not exceed 50 nm. 50 The nm. dendrites The dendrites were inwere the in range the rangeofFigure 40–50 of 240–50 nm shows in nm diameter. X in-ray diameter. diffraction patterns of the cubic structure In2O3 and ITO crystalline rods grown atFigure Figuresubstrate 22 showsshows temperatures X-rayX-ray diffractiondiffraction of 300 patterns°patternsC. The ITO ofof thethe nanocrystal cubiccubic structurestructure rods InIn were2O3 and andfound ITO ITO to crystalline have the rodscentered ◦ cubicgrowngrown structure at substrate substrate of In2O temperatures 3. of 300 °C.C. The The ITO ITO nanocrystal nanocrystal rods rods were were found found to to have have the the centered centered cubiccubic structure structure of In 2OO33. .

◦ Figure 2. XRD patterns of In 2OO33 cubiccubic crystal crystal and and ITO ITO single-crystalline single-crystalline rods rods grown grown at at 300 300 °C.C. Figure 2. XRD patterns of In2O3 cubic crystal and ITO single-crystalline rods grown at 300 °C. The diffraction peaks at 2θ = 30.580°, 35.466°, 51.037° and 60.676° were assigned to the (222), The diffraction peaks at 2θ = 30.580◦, 35.466◦, 51.037◦ and 60.676◦ were assigned to the (222), (400),The diffraction (440) and (622) peaks reflections at 2θ = of30.580 cubic° ,In 35.4662O3. Compared°, 51.037° with and that60.676 of the° were In2O assi3 cubicgned crystal, to the the (222), (400), (440) and (622) reﬂections of cubic In2O3. Compared with that of the In2O3 cubic crystal, the (400),XRD (440) spectra and of(622) the reflectionsITO nanocrystal of cubic rods Indisplayed2O3. Compared all the same with peaks that but of shiftedthe In2 Oslightly3 cubic to crystal,higher the XRD2 spectraθ. This peakof the shift ITO can nanocrystal be explained rods by displayed the In2O3 crystalall the changingsame peaks to tin-doped but shifted In2 Oslightly3 during to the higher 2θ. Thisevaporation peak shift process, can although be explained its crystal by thestructure In2O 3remaining crystal changing cubic [28,29]. to tin-doped In2O3 during the evaporation process, although its crystal structure3 remaining cubic [28,29]. 3 Coatings 2017, 7, 212 4 of 8

XRD spectra of the ITO nanocrystal rods displayed all the same peaks but shifted slightly to higher 2θ. This peak shift can be explained by the In2O3 crystal changing to tin-doped In2O3 during the evaporation process, although its crystal structure remaining cubic [28,29]. ◦ CoatingsFrom 2017, the 7, 212 XRD pattern of the ITO nanocrystal rods, the strongest peak was at 2θ = 36.142 , corresponding to the (400) reﬂection. According to the Bragg diffraction Formula (1), From the XRD pattern of the ITO nanocrystal rods, the strongest peak was at 2θ = 36.142°, corresponding to the (400) reflection. According2d sin toθ the= n Brλ agg diffraction Formula (1), (1)

Using the values, θ = 18.071◦ radian, λ =2 1.542푑sinθ Å,= and푛λ n = 4, the parameter d was calculated to(1 be) 9.947Using Å, which the values, is quite θ similar = 18.071 to° theradian lattice, λ = parameter 1.542 Å , and ofITO n = 4 nanocrystal, the parameter rods. d Anwas In calculated2O3 and SnOto be2 substitutional9.947 Å , which solid is quite solution similar was to formed. the lattice The parameter ITO nanocrystal of ITO couldnanocrystal thus be rods. considered An In2O SnO3 and2 doped SnO2 Insubstitutional2O3, as SnO2 solidhas asolution tetragonal was crystal formed. structure. The ITO nanocrystal could thus be considered SnO2 doped In2O3The, as relationshipSnO2 has a tetragonal between mixedcrystal element structure. composition and the lattice constants satisfies Vegard’s law, theThe first relationship approximate between Formula mixed (2), element composition and the lattice constants satisfies Vegard’s law, the first approximate Formula (2), a(ITO) = a(In2O3)·x + a(SnO2)·(1 − x) (2) 푎(ITO) = 푎(In2O3) · 푥 + 푎(SnO2) · (1 − 푥) (2) The lattice parameters of In2O3 and SnO2 respectively were 10.118 Å and 4.738 Å by a look-up PDF The lattice parameters of In2O3 and SnO2 respectively were 10.118 Å and 4.738 Å by a look-up PDF cards (PDF#06-0416 and PDF#41-1445). The parameter for ITO was 9.947 Å in the above calculation. cards (PDF#06-0416 and PDF#41-1445). The parameter for ITO was 9.947 Å in the above calculation. These values were used with Formula (2) to obtain x = 96.82%. The percentage of SnO2 doped in These values were used with Formula (2) to obtain x = 96.82%. The percentage of SnO2 doped in In2O3 was 3.18%, which confirmed the mixture ratio of oxide materials. The recrystallization process In2O3 was 3.18%, which confirmed the mixture ratio of oxide materials. The recrystallization process during the evaporation was a chemical one. In2O3 and SnO2 were broken down by evaporation and during the evaporation was a chemical one. In2O3 and SnO2 were broken down by evaporation and generated an indium tin oxide substitutional solid solution again. Sn atom replaced In atom into the generated an indium tin oxide substitutional solid solution again. Sn atom replaced In atom into the crystal lattice and form alloy. crystal lattice and form alloy. 4+ The ITO also had a cubic crystal structure. Because the ionic radius of Sn 4+ is smaller than the The ITO3+ also had a cubic crystal structure. Because the ionic radius of Sn is smaller than the radius of In in the cubic lattice, the lattice constant of ITO was smaller than that of In2O3, which radius of In3+ in the cubic lattice, the lattice constant of ITO was smaller than that of In2O3, which shifted the diffraction peaks to higher 2θ, as shown in Figure2. shifted the diffraction peaks to higher 2θ, as shown in Figure 2. The strongest peak of the ITO crystal corresponded to the (400) reflection. The other peaks were The strongest peak of the ITO crystal corresponded to the (400) reflection. The other peaks were very weak and nearly disappeared. This indicated that the ITO nanocrystal rods were [001] oriented very weak and nearly disappeared. This indicated that the ITO nanocrystal rods were [001] oriented and that the (100) crystal plane was the major exposed facet. and that the (100) crystal plane was the major exposed facet. Figure3 shows a TEM image of the single nanocrystal ITO, which provides an insight into the Figure 3 shows a TEM image of the single nanocrystal ITO, which provides an insight into the structure of the crystallized ITO. The diameter of the nanocrystal branch was approximately 50 nm. structure of the crystallized ITO. The diameter of the nanocrystal branch was approximately 50 nm. Metal droplets existed on the top of every branch, with diameters of approximately 50 nm. This result Metal droplets existed on the top of every branch, with diameters of approximately 50 nm. This result is in agreement with the crystal growth VLS model. is in agreement with the crystal growth VLS model.

Figure 3. TEM micrograph of a single ITO crystal rod. Figure 3. TEM micrograph of a single ITO crystal rod. The high resolution transmission electron microscopy (HRTEM) image shown in Figure 4a reveals that the ITO nanocrystal rods had a body centered cubic single crystal structure. The crystal lattice was very complete and continuous, and was clearly identifiable as a cubic ITO phase with (001) and (010) inter-planar spacing. This indicated that the growth direction was <100>. The associated selected-area electron diffraction pattern of the imaged ITO shows clear dots corresponding to the (100) planes in Figure 4b, which confirms that the ITO rods were cubic crystals.

4 Coatings 2017, 7, 212 5 of 8

The high resolution transmission electron microscopy (HRTEM) image shown in Figure4a reveals that the ITO nanocrystal rods had a body centered cubic single crystal structure. The crystal lattice was very complete and continuous, and was clearly identiﬁable as a cubic ITO phase with (001) and (010) inter-planar spacing. This indicated that the growth direction was <100>. The associated selected-area electron diffraction pattern of the imaged ITO shows clear dots corresponding to the (100) planes in

FigureCoatings4 2017b, which, 7, 212 conﬁrms that the ITO rods were cubic crystals. Coatings 2017, 7, 212

Figure 4. (a) HRTEM image of a typical single ITO crystal area; (b) an associated selected-area electron Figure 4. (a) HRTEM image of a typical single ITO crystal area; (b) an associated selected-area electron Figurediffraction 4. (a pattern.) HRTEM image of a typical single ITO crystal area; (b) an associated selected-area electron diffraction pattern. diffraction pattern. Figure 5 shows the current-voltage characteristic of the ITO single crystal rod in the voltage rangeFigure of 0–5. 55 0shows showsV. C-1 the theand current-voltagecurrent C-2 are-voltage any two characteristiccharacteristic samples of ITO ofof the thenanorod ITOITO single single in random crystalcrystal selection. rodrod inin thethe Cons voltage voltageistent rangesemiconductor of 0–5.00–5.0 V.pn C-1C junction-1 andand C-2C features-2 areare anyany are twotwo all samplesshownsamples in ofof the ITOITO random nanorodnanorod two inin single randomrandom crystal selection.selection. rods in ConsistentCons Figureistent 5. semiconductorThe ITO nanocrystal pn junction rod exhibited features features are aregood all all semiconductorshown in in the the random random properties two two andsingle single its crystal crystal threshold rods rods voltage in in Figure Figure was 55.. Theapproximately ITO nanocrystal 2.5 V. rod exhibited good semiconductor properties and its threshold voltage was approximately 2.5 V.

Figure 5. Current-voltage curves of a single ITO crystal rod from 0 to 5.0 V. (C- 1 and C-2 are the any two nanorods of ITO). Figure 5. Current-voltageCurrent-voltage curves of a single ITO crystal rod rod from 0 0 to to 5.0 5.0 V. (C-1(C-1 and C-2C-2 are the any two nanorods of ITO).ITO). Figure 6 is the transmission rate curve of the crystalline ITO nanorods film in the wavelength rangeFigure of 300 6– 1000is the nm. transmission From the transmission rate curve of r theate curve,crystalline the transmittance ITO nanorods of film crystalline in the wavelength ITO film is Figure6 is the transmission rate curve of the crystalline ITO nanorods ﬁlm in the wavelength rangeover 90 of% 300 in the–1000 visible, nm. Fromand that the is transmission up to 95% in r theate bluecurve, green the transmittanceband. For many of oxygen crystalline vacancies ITO film exist is range of 300–1000 nm. From the transmission rate curve, the transmittance of crystalline ITO ﬁlm is overin the 90 cubic% in the structure visible, and of ITO that nanocrystals.is up to 95% in The the cavityblue green excited band. by For photons many oxygen and the vacancies electron exist that over 90% in the visible, and that is up to 95% in the blue green band. For many oxygen vacancies inoccupied the cubic the oxygen structure vacancy of ITO recombined nanocrystals. luminescence The cavity to excited enhance by the photons transmission and the rate electron of this band. that oStrongccupied absorption the oxygen occurs vacancy near recombined the wavelength luminescence of 300 nm to because enhance its the high transmission crystallinity rate leads of this to band.weak Strongquantum absorption-confinement occurs-effect near [30]. the wavelength of 300 nm because its high crystallinity leads to weak quantum-confinement-effect [30].

5 5 Coatings 2017, 7, 212 6 of 8 exist in the cubic structure of ITO nanocrystals. The cavity excited by photons and the electron that occupied the oxygen vacancy recombined luminescence to enhance the transmission rate of this band. Strong absorption occurs near the wavelength of 300 nm because its high crystallinity leads to weak quantum-conﬁnement-effectCoatings 2017, 7, 212 [30].

FigureFigure 6. Transmission 6. Transmission rate of rate the ofcrystalline the crystalline ITO nanorods ITO nanorods film in the film wavelength in the wavelength range of 200 range–1000 ofnm. 200–1000 nm. 4. Conclusions 4. ConclusionsIn summary, ITO nanocrystal rods were grown by the VLS growth method during the process of electronIn summary,-beam evaporation. ITO nanocrystal The rodsgrowth were mechanism grown by of the the VLS ITO growth nanocrystal method rods during was theauto process-catalytic of electron-beamin-situ growth evaporation.via the VLS process. The growth Themechanism ITO nanorods of the exhibited ITO nanocrystal a perfect rodscrystalline was auto-catalytic cubic phase in-situstructure. growth XRD viaand theTEM VLS results process. revealed The that ITO the nanorods crystallization exhibited was a [001] perfect oriented crystalline and the cubic presented phase structure.facet was XRDthe (1 and00) TEMcrystal results planes. revealed A binary that (In the2O crystallization3 and SnO2) solid was solution [001] oriented was formed and the during presented the evaporation process. The percentage of doped tin oxide was calculated by the first approximate facet was the (100) crystal planes. A binary (In2O3 and SnO2) solid solution was formed during the evaporationVegard’s law process. to be 3.18%, The which percentage was in of agreement doped tin with oxide the wasmixture calculated ratio of by the the experimental first approximate source. Vegard’sThe ITO law single to be nanocrystal 3.18%, which rod was exhibited in agreement good semiconductor with the mixture switch ratio property of the experimental and its threshold source. Thevoltage ITO was single approximately nanocrystal 2.5 rod V. exhibited The transmittance good semiconductor of ITO nanorods switch film property was over and 90% its thresholdin visible voltagelight band, was which approximately was due to 2.5 the V. oxygen The transmittance vacancy recombination of ITO nanorods luminescence film was in some over 90%specific in visible bands. lightStrong band, absorption which was in the due wavelength to the oxygen of vacancy300 nm recombinationwas from weak luminescence quantum-confinement in some specific-effect bands. of its Stronghigh crystallinity. absorption in the wavelength of 300 nm was from weak quantum-confinement-effect of its high crystallinity. Acknowledgement: This work was supported by Shandong Focus on R&D Projects grant No. 2016GGX102040. Acknowledgments:The author wishes to Thisexpress work the was grateful supported to Shaozhi by Shandong Deng team Focus of Sun on Yat R&D-senProjects University grant for No.providing 2016GGX102040. the IV test. TheAuthor author Contributions: wishes to express The entire the gratefulstudy was to completed Shaozhi Deng by Yan team Shen of Sunwith Yat-senthe assistance University of Youxin for providing Lou, Zhihao the IV test. Wang and Xiangang Xu. The notes as follows: Yan Shen and Xiangang Xu designed and planned the research Authortopic; Yan Contributions: Shen conceivedThe and entire carried study out the was experiments completed, bymeasured Yan Shen and with analyzed the assistance the date, wrote of Youxin the paper; Lou, ZhihaoLou and Wang Wang and performed Xiangang the Xu.experiments. The notes as follows: Yan Shen and Xiangang Xu designed and planned the research topic; Yan Shen conceived and carried out the experiments, measured and analyzed the date, wrote theConflicts paper; of Lou Interest: and Wang The performedauthors declare the experiments. no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References

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