Cost-effective screen printing approach for Ce/Nd-doped ZnAl2O4 films: tuning crystallinity induced by the substrate

This jou nal is © he Owne Socie ies 2023 Phys. Chem. Chem. Phys., 2023, 25, 15829–15838 | 15829 Ci e his: Phys. Chem. Chem. Phys., 2023, 25, 15829 Cos -effec i e sc een p in ing app oach o Ce/Nd-doped ZnAl 2 O 4 ilms: uning c ys allini y induced by he subs a e† Rocio E. Rojas-He nandez, * a Fe nando Rubio-Ma cos, b Jallouli Necib, a Ma i Danilson, c Jose ´F. Fe nandez b and I ina Hussaino a a Nea -in a ed (NIR) emi ing phospho s a e cu en ly ecei ing conside able a en ion owing o hei high demand in a ious applica ions, such as ligh de ec ion and anging (LiDAR), sho - ange communica ions, secu i y, biosensing and nigh ision ligh ing applica ions. The minia u iza ion o pho onic componen s demands he in eg a ion o hin ilms in o exploi able de ices. In his con ex , NIR emi ing ZnAl 2 O 4 :Ce/Nd ilms o hund eds o nanome e hickness a e syn hesized using a scalable and cos -efficien app oach o sc een p in ing. Ce ium co-doping is esponsible o he Nd emission in he NIR h ough ene gy ans e by exci ing he ilms unde UV exci a ion a a ound 360 nm. Th ough he p ope design o ink, dense Nd/Ce doped ZnAl 2 O 4 ce amic ilms we e p oduced using polyc ys alline alumina. The use o polyc ys alline alumina subs a es opens up new oppo uni ies because his ce amic is a cheap and well-known subs a e o op oelec onic packaging. Du ing manu ac u ing, as a di ec effec o p edominan c ys al g ow h o e he polyc ys alline alumina subs a e, an inc ease in emission in ensi y is achie ed. The esul s ob ained by X- ay pho oelec on (XPS) and X- ay abso p ion nea edge spec oscopy (XANES) se e o de e mine he oxida ion s a e o Ce. The indings o his s udy indica e ha a highe concen a ion o Ce 4+ p omo es NIR emission. This s udy may con ibu e o a be e unde s anding o ilm p oduc ion p ocesses o ilms based on he ZnAl 2 O 4 ma ix and guide u u e s udies on ilms o NIR emi e s. In oduc ion Nea in a ed (NIR) luminescen ma e ials ha e eme ged as p omising candida es in many applica ions nowadays owing o hei wide po en ial ange in sola cells, pho onic de ices, ligh sou ces, op o-elec onics and bio-imaging 1–5 To da e, mos NIR luminescen ma e ials ha e been p oduced in he o m o pa icles, anging om mic o o nano sizes. Recen ly, he numbe o esea ch wo ks on nanoscale ma e ials has signi i- can ly inc eased because o hei use as luo escen p obes o biological imaging. Howe e , he e is also a s ong demand o NIR-emi ing ma e ials in he o m o hin ilm. 6 NIR-emi ing pa icles can be embedded in o a ma ix o syn hesized ilms; howe e , his s a egy usually implies a mul i-s ep p ocess and/ o encapsula ion echniques in which he sca e ing phenom- enon and non-homogenei y educe he luminescence esponse. To sol e he issues, magne on spu e ing, pulsed elec on deposi ion, pulsed lase deposi ion and a omic laye deposi ion a e used as he deposi ion echniques o p oducing lumines- cen ilms. 7–9 Howe e , hese s a egies equi e expensi e and complex sys ems. Recen ly, an in ensi e esea ch e o has been di ec ed owa ds he de elopmen o cos -e icien and high-pe o mance p in ed ilms h ough di ec app oaches. Using ae osol–je p in ing and inkje p in ing, ela i ely high accu acy can be achie ed. 10 Howe e , i high scalabili y is he goal, he ilms can be p oduced wi h cos -e ec i e echnology, such as sc een p in ing, 11 which has al eady been employed in a ious ields, such as clo hing, p oduc labels, p in ed elec onics, and sola cells, 12 o p epa e con inuous ilms o pa e ns. 13 Mo eo e , luminescence ilms can be deposi edonbo h igidand lexible subs a es, opening up new applica ion ields. 14,15 Usually, sc een- p in ing is implemen ed o p oduce ilms o mic ome e hick- ness; some s udies ha e demons a ed he possibili y o deposi- ion o submic on ilms by uning he p in ing pas es. 16 Howe e , he s udy o an economical way o ab ica e NIR luminescen ilms by sc een p in ing is a om comple e. a Depa men o Mechanical and Indus ial Enginee ing, Tallinn Uni e si y o Technology, Ehi aja e 5, 19180 Tallinn, Es onia. E-mail: ocio. o[email p o ec ed] b Elec oce amic Depa men , Ins i u o de Ce a ´mica y Vid io, CSIC, Kelsen 5, 28049, Mad id, Spain c Depa men o Ma e ial and En i onmen al Technology, Tallinn Uni e si y o Technology, Ehi aja e 5, 19180 Tallinn, Es onia †Elec onic supplemen a y in o ma ion (ESI) a ailable. See DOI: h ps://doi.o g/ 10.1039/d3cp02005c Recei ed 2nd May 2023, Accep ed 17 h May 2023 DOI: 10.1039/d3cp02005c sc.li/pccp PCCP PAPER Open Access A icle. Published on 18 May 2023. Downloaded on 3/12/2024 8:46:07 AM. This a icle is licensed unde a C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence. View A icle Online View Jou nal | View Issue 15830 | Phys. Chem. Chem. Phys., 2023, 25, 15829–15838 This jou nal is © he Owne Socie ies 2023 F om a undamen al iewpoin , one o he possible effec i e NIR emi e s in he o m o ilms is luminescen ma e ials based on alumina es, in pa icula zinc alumina e ilms doped wi h Nd and Ce. Mo eo e , effo s ha e ecen ly ocused on con olling he composi ion and g ow h o he spinel because o he o ma ion o seconda y phases. Zinc alumina e ilms a e syn hesized using a pho ochemical ou e by ep esen ing ZnAl 2 O 4 as he main phase and ZnO as he seconda y phase a e annealing a 1100 1C. 17 Annealed a 1100 1C, g een- emi ing ZnAl 2 O 4 :Mn 2+ phospho hin ilms a e deposi ed by a sol–gel spin coa ing ou e. The p ocedu e allows ilm hick- ness modula ion om 165 o 850 nm h ough he a ia ion o he numbe o coa ing laye s; he achie ed emission in ensi y has demons a ed a s ong dependency on he annealing empe a u e and ilm hickness. 18 Mo eo e , in e connec ed mic os uc u al ZnAl 2 O 4 ilms a e also ob ained by applying he sol–gel me hod, whe e he opog aphy o ZnAl 2 O 4 ilms can be a ied by adjus ing he hea ing a e. 19 The o ma ion o ZnO/ZnAl 2 O 4 nanocomposi e hin ilms is e alua ed by Ul asonic Sp ay Py olysis on Glass and Si(111) subs a es, demons a ing ha Zn/Al mola a ios affec he inal composi ion h ough he o ma ion o a ious poly- mo phs wi hin he Zn 2 SiO 4 –ZnO–ZnAl 2 O 4 sys em. 20 In addi- ion, DC/RF magne on spu e ing echnique is used o deposi ZnAl 2 O 4 ilms by applying he powe o 80 wa s up o 140 wa s (s ep size 20 wa s) o he aluminum (Al) sou ce and uning he o ma ion o ZnAl 2 O 4 . 21 The ca aly ic ac i i y is e ealed and a ibu ed o he p esence o he spinel ZnAl 2 O 4 phase when he ALD deposi ion o ZnO on g-Al 2 O 3 is add essed. 22 In his s udy, Ce and Nd a e chosen as dopan s o p oduce luminescen ilms based on zinc alumina e because Ce and Nd a e conside ed p omising candida es o p omo e emission by adding ce ium. Speci ically, as a po en ial sensi ize , ce ium is inco po a ed in o diffe en hos s. 23 O he s a egies, such as he in oduc ion o alkali me al ions, can also conside ably p omo e luminescence pe o mance. 24 Ou ecen s udy demons a es ha he ZnAl 2 O 4 is a sui able ma ix o bo h dopan s. 25 Howe e , he hos ZnAl 2 O 4 , which is known as a ca aly ic ma e ial, is anspa en conduc i e oxide and sin e ing addi i e. 26–30 Visible emission is obse ed o he undoped ma e ial; 31–33 ecen ly, deep-ul a iole emission has been epo ed o Ra e-Ea h-F ee ZnAl 2 O 4 nano ibe s, 34 which indica es he po en ial o his hos ma ix. By employing a squeegee, a pas e and a sc een, a ilm is p epa ed on a la subs a e using he sc een p in ing me hod. The o mula ion o he pas e is c ucial, oge he wi h he d ying and annealing p ocesses, o densi y he ilm. The emo al o o ganics du ing he d ying p ocess can esul in a high po osi y o he ilm, which hinde s he necking and g ow h o he g ains. The ink, also called a pas e, mainly comp ises 4 componen s: ce amic loading, sol en , binde and dispe san . The a io be ween componen s affec s he ink heology and, consequen ly, he quali y o he end p oduc . P e iously, we demons a ed he ab ica ion o ilms based on S Al 2 O 4 :Eu 2+ ,Dy 3+ using a sc een p in ing me hod assis ed by a mol en sal lux. 35 In his s udy, he eu ec ic mol en sal mix u e (NaCl–KCl) is employed o de elop ZnAl 2 O 4 sub- mic ome e ilms o e polyc ys alline alumina. The densi ica- ion, homogenei y and phase o ma ion a e enhanced by he mol en sal added, which ac s as a liquid phase boos ing he ea angemen and dissolu ion o ino ganic loading and he solubili y o he subs a e, which is exploi ed as a aw ma e ial o o m he phase based on zinc alumina e. The mol en sal ola ilizes a e he sin e ing p ocess, accomplish- ing he objec i e o inco po a ion. The aim o his s udy is h ee old: (i) o op imize he ink o mula ion in he p esence o absence o mol en sal lux and o es ima e diffe en a ios o Zn in p omo ing he o ma ion o he ZnAl 2 O 4 phase; (ii) o elabo a e a p ocedu e o he deposi- ion o inks on o polyc ys alline alumina-based subs a es, and (iii) o e alua e he effec o dopan s (Ce and Nd) on he luminescence esponse o he ilm. In ou p e ious pape , 36 we demons a ed ha luminescen (hk0)- ex u ed ZnAl 2 O 4 :Nd,Ce sub-mic on ilms o e sapphi e subs a es can be success ully ob ained. To u he exploi he po en ial o he p oposed sc een-p in ing echnique and ob ain be e p ope ies using mo e e sa ile subs a es, polyc ys al- line alumina subs a es a e employed. Conside ing ha he p oposed ou e should be applied o subs a es wi h diffe en eac i i ies, diffe en ink o mula ions we e de ailed, and he effec o he Ce oxida ion s a e on he luminescen esponse o he ilm has been ho oughly s udied. The esul s indica e ha an adequa e composi ional engi- nee ing s a egy plays a undamen al ole in he ab ica ion o homogeneous ZnAl 2 O 4 -based ilms wi h high op ical pe o - mance. P ecise con ol o he phase c ys allini y o he ilms is a manda o y equi emen o ob aining a highly efficien end p oduc . Expe imen al sec ion Film p epa a ion ZnAl 2 O 4 ilms we e p epa ed using a sc een p in ing echnique assis ed by a mol en sal lux. P io o deposi ion, ul asonic cleaning o he subs a e su ace in e hanol (polyc ys alline Al 2 O 3 ,1025 1 mm) and subs a e d ying in a s eam o ai we e ca ied ou . The sc een p in ing ink was p epa ed by mixing 60 w % o he ino ganic ehicle o p ecu so s wi h pa icles om 1 mm o7mm and 40 w % o he o ganic ehicle composed o a- e pineol sol en , 2-(2-bu oxye hoxy) e hyl ace - a e plas icize and DuPon 8250 hinne acco ding o p e- iously epo ed p ocedu e. 35 To homogenize he ink, bo h o ganic and ino ganic compo- nen s we e loaded in o 250 mL zi conia ja s wi h zi conia balls and homogenized by a plane a y ball milling (Model YKM-1). The milling p ocess was pe o med a a o a ion speed o 250 pm o 8 hou s. The chlo ide sal mix u e composed o NaCl and KCl was homogenized be o e, including all he o he ino ganic componen s, such as ZnO (Sym ise GmbH), CeO 2 (E onik Degussa) and Nd 2 O 3 (Me all Ra e Ea h Limi ed, 99.5%). The mol a io o Nd was ixed o x= 0.02, and he Ce Pape PCCP Open Access A icle. Published on 18 May 2023. Downloaded on 3/12/2024 8:46:07 AM. This a icle is licensed unde a C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence. View A icle Online This jou nal is © he Owne Socie ies 2023 Phys. Chem. Chem. Phys., 2023, 25, 15829–15838 | 15831 was a ied be ween 0.02 x 0. 06. Owing o he ini ial high iscosi y o he ink, a sui able p ocessing me hod is equi ed o sufficien ly dispe se he agglome a es and agg ega es in o he p ima y pa icles. The e o e, h ee- oll milling was used o he dispe sion, exploi ing he in e nal shea and impac o ces gene a ed om he mo emen o he h ee olle s (employing a olle ’s gap o 15 mm and a o a ion speed o 20 pm) o b eak down agglome a es and agg ega es. The ea e , he ink was deposi ed by sc een-p in ing (DEK 65 sc een-p in ing machine wi h a polyes e 325 mesh) wi h a snap-off dis ance o 0.5 mm. The deposi ed ink laye was d ied in an o en using a mul i-s ep p ocess a 150, 195, 270, 386 and 425 1C o emo e he o ganic ca ie , main aining a slow hea ing a e o 0.5 1C min 1 . Subsequen ly, he he mal ea - men was conduc ed a 1200 1C in he ai o 2 h. This empe a u e was selec ed o ensu e he comple e emo al o he eu ec ic sal used as lux acco ding o he p e ious p oce- du e de eloped by he au ho s. 37 S uc u al and mic os uc u al cha ac e iza ion X- ay diff ac ion analysis was ca ied ou using a Cu-Ka adia- ion sou ce a 40 kV in (y/2y) B agg–B en ano geome y (Rigaku Sma Lab SE). Scanning elec on mic oscopy (SEM) cha ac e - iza ion was pe o med by employing an FE-SEM Zeiss ULTRA- 55, Ge many. The su aces o he ilms and hei p o iles we e analyzed o de e mine he hickness. A 3D op ical p o ilome e B uke Con ou GT-K0+ was employed o de e mine he ough- ness o he alumina subs a es and ZnAl 2 O 4 ilms. C ys alli e sizes we e calcula ed using he Sche e equa ion as ollows: D¼ Kl Bcos y(1) whe e Dis he c ys alli e size (nm), kis a cons an , in ou case 0.9, lis he wa eleng h o he X- ay adia ion, Bis he ull wid h a hal maximum o he di ac ion peak and yis he B agg’s o di ac ion angle. X- ay abso p ion nea -edge s uc u e spec oscopy (XANES) measu emen s we e pe o med a he ALBA synch o on acili y in he CLAESS beamline. The da a collec ion o he Ce L 3 -edge was done in luo escence mode a oom empe a u e, in which he signal was acqui ed wi h a 6-channel silicon d i de ec o (SDD) Xsp ess3 om a Quan um De ec o . Mul iple spec a we e acqui ed o each ilm, and he a e age o each o hem was ob ained by employing he A hena so wa e. 38 X- ay pho oelec on spec oscopy (XPS) analysis was pe o med using a K a os Analy ical Axis Ul a DLD spec ome e . The de ice has he ollowing monoch oma ic X- ay sou ces: Al Kaand an ach oma ic Mg Ka/Al Kadual anode. The ilms we e placed on a s ainless-s eel sample ba , and he XPS spec a we e acqui ed a a ake-off angle o 901 om he su ace. K a os Vision 2.2.10 so wa e was employed o de e mine he ela i e a omic concen a ions o he elemen s using Shi ley backg ound sub ac ion o calcula e he ela i e a omic concen a ions. Op ical cha ac e iza ion Pho oluminescence measu emen s in he isible ange we e ca ied ou a oom empe a u e using a Fluo olog-3, HORIBA Jobin Y on sys em equipped wi h a xenon lamp, whe e he samples we e exci ed a 359 and 740 nm. The exci a ion spec um was acqui ed by ixing he emission a 1060 nm. Resul s and discussion Effec o zinc oxide con en on ink o mula ion To manu ac u e homogeneous ZnAl 2 O 4 -based ilms, i is neces- sa y o unde s and he eac ion mechanisms o aw ma e ials wi h he subs a e. To achie e his aim, h ee concen a ions o ZnO in he ink we e e alua ed o ensu e an adequa e eac ion o o m he ZnAl 2 O 4 phase. Fig. 1 displays he XRD pa e ns o ilms hea ea ed a 1200 1C, which we e aken om h ee inks con aining diffe en amoun s o zinc oxide, designa ed as 1Zn:1Al, 0.5Zn : 1Al and 0.25Zn:1Al. The PDF s anda d pa - e ns o ZnO, ZnAl 2 O 4 and Al 2 O 3 p o ided by he Join Com- mi ee on Powde Diff ac ion S anda ds (Powde Diff ac ion File (PDF)) co esponding o he ca ds PDF: 04-003-2106, PDF: 00-010-0173 and PDF: 01-074-1138 a e shown in he bo om o Fig. 1. The XRD e lec ions a e ma ked wi h a black ci cle symbol ‘‘J’’, ed squa e symbol ‘‘&’’ and blue diamond symbol ‘‘ } ’’, co esponding o he allowed B agg e lec ions Fig. 1 Effec o zinc oxide con en on he ink o mula ion: XRD o ilms syn hesized a 1200 1C by including h ee con en s o zinc oxide, such as 1Zn : 1Al, 0.5Zn : 1Al and 0.25Zn : 1Al. Red, blue and black symbols indica ed he p esence o diff ac ion peaks o Al 2 O 3 , ZnAl 2 O 4 and ZnO, espec i ely. Each s anda d pa e n o he aw ma e ials, ZnO (PDF: 04-003-2106), he Al 2 O 3 (PDF: 00-010-0173) and he ZnAl 2 O 4 (PDF: 01-074-1138) a e included below he XRD o he ilms. PCCP Pape Open Access A icle. Published on 18 May 2023. Downloaded on 3/12/2024 8:46:07 AM. This a icle is licensed unde a C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence. View A icle Online 15832 | Phys. Chem. Chem. Phys., 2023, 25, 15829–15838 This jou nal is © he Owne Socie ies 2023 o ZnO, and Al 2 O 3 and ZnAl 2 O 4 , espec i ely. I is no ewo hy ha he diff ac ion pa e ns a e ee om he componen s o he mol en sal used, indica ing he effec i e he mal emo al o he lux. A ich o mula ion o zinc oxide (1Zn:1Al) mainly yields he o ma ion o ZnO wi h he insigni ican o ma ion o ZnAl 2 O 4 . ZnO is s ill p esen in he o mula ion wi h a educed con en o zinc oxide (0.5Zn : 1Al). Fu he educ ion o ZnO (0.25Zn:1Al) esul ed in he de elopmen o only ZnAl 2 O 4 as he main phase. Fo all h ee diff ac ion pa e ns p esen ed, he con ibu ion o he subs a e is e ealed. The expe imen al alues o ZnAl 2 O 4 uni -cell pa ame e s o he ilms we e calcula ed o be a= 8.0905 (25) Å and Vol = 528.89 Å 3 ,a= 8.0902 (13) Å and Vol = 529.59 Å 3 and a= 8.0902 (25) Å and Vol = 529.52 Å 3 o he ilms designa ed, such as 0.25Zn : 1Al, 0.5Zn : 1Al and 1Zn : 1Al, espec i ely. The la ice pa ame e aand he olume Vwe e unaffec ed by he inco po a ion o excess ZnO. The e o e, a Zn :Al a io o 0.25Zn : Al in he ink may be conside ed op imal, allowing he p oduc ion o ZnAl 2 O 4 as a single phase wi h cubic symme y. To gain deepe insigh s in o he ex en o spinel in e sion, we conduc ed Rie eld e ine- men , as depic ed in Fig. S1 (ESI†). Th ough me iculous e ine- men o si e occupa ion ac ions, we de e mined a ema kably low spinel in e sion deg ee o 0.082 o he ZnAl 2 O 4 ilm syn hesized wi h a Zn:Al a io o 0.25. This inding a es s o he excep ional c ys alline s uc u e o he ilm, which exhibi s minimal pe u ba ions in he occupa ion o c ys allog aphic si es. The mol en sal sys em p o ides a as p ocess o he dissolu ion–p ecipi a ion o Zn 2+ ca ions. Howe e , he diffu- sion o such ca ions h ough he Al 2 O 3 c ys al la ice is he kine ic limi ing ac o o he o ma ion o he zinc alumina e s uc u e. Thus, un eac ed ZnO emains in ilms wi h a ios o 1Zn:1Al and 0.5Zn:1Al. The ela ionships be ween mic os uc u e and he Zn:Al a io in he ilms we e s udied by FE-SEM, as shown in Fig. 2a–j. Fig. 2a and b p esen s low and high magni ica ion SEM images o he su ace- iew mic os uc u e o he polyc ys alline alu- mina subs a e. The subs a e su ace was polished o mean su ace oughness (R a ) o 0.043 mm. SEM mic og aphs o ilm su aces o di e en ZnO a ios a e shown in Fig. 2c, d and g–i. The op su aces o he ilms wi h 1Zn : 1Al (Fig. 2c) and 0.5Zn : 1Al (Fig. 2d) a ios exhibi an inhomogeneous laye wi h small pa icles loosely a anged on he subs a e. Two a eas a e dis inguished in he op-su ace iew o bo h ilms, and a eas ma ked wi h numbe s 1 and 3 appea o be mo e homogeneous wi hou un eac ed pa icles on he op. Howe e , he a eas ma ked as 2 and 4 on panels c and d depic ed in Fig. 2 mainly comp ise pa icles ha do no eac wi h he subs a e. F om he EDS analysis (Fig. 2e and ) o bo h a eas, i is con i med ha he EDS spec um in he poin o in e es 1 and 3 de ec s Zn, Al and O ela ed o ZnAl 2 O 4 o ma ion. Howe e , he EDS spec a in poin s o in e es 2 and 4 show mainly Zn and O; hese esul s a e co ela ed wi h he XRD, which con i med he o ma ion o ZnO. The op-su ace and c oss-sec ion iew o he ilm wi h he op imized a io o ZnO (0.25Zn:1Al) ha yields he ZnAl 2 O 4 ilm o ma ion is shown in Fig. 2h–i. The highe magni ica ion SEM mic og aph shows a su icien deg ee o ec ys alliza ion as he solubili y limi is su passed. When equilib ium is eached, he ini ial ace ing o he su ace is de ec ed. Consequen ly, ce ain ace ed shapes a e isible on he su ace. The su ace oughness inc eases compa ed wi h he oughness o he alumina subs a e o R a = 0.13 mm. This alue can ep esen a smoo h su ace conside ing ha an unpolished alumina subs a e has a mean oughness o 0.75– 0.9 mm. 39 Su ace ex u ing can likely be achie ed by con olling Fig. 2 Mic os uc u e cha ac e iza ion o he ilms, including h ee con en s o zinc oxide: (a and b) su ace- iew ob ained by FE-SEM o he polyc ys alline Al 2 O 3 subs a e polished. Scale ba s, 10 mmand2mm, espec i ely. (c and d), FE-SEM images o he su ace o he ilms deposi ed wi h a ious concen a ions o ZnO, 1Zn : 1Al (panel c) and 0.5Zn : 1Al (panel d). Scale ba s, 20 mm, espec i ely. (d– ) EDS analysis o he su ace iews in c and e. (g and h), FE-SEM images o he su ace o he ilm deposi ed wi h a concen a ion o ZnO co esponding o 0.25Zn :1Al. Scale ba s, 20 mm and 1mm, espec i ely. (i) C oss-sec ions o he ilm based on zinc alumina e ob ained using he 0.25Zn: 1Al ink o mula ion. Scale ba : 1 mic on. (j) EDS line-scan analysis o he c oss-sec ion in i. Pape PCCP Open Access A icle. Published on 18 May 2023. Downloaded on 3/12/2024 8:46:07 AM. This a icle is licensed unde a C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence. View A icle Online This jou nal is © he Owne Socie ies 2023 Phys. Chem. Chem. Phys., 2023, 25, 15829–15838 | 15833 c ys alliza ion and he di usion–dissolu ion mechanism o ZnAl 2 O 4 o ma ion. The homogeneous and ee o c acks su - ace is a consequence o bo h he op imized sc een p in ing p ocess and he mol en sal -assis ed eac i e sin e ing. Ade- qua e heology o he ink is esponsible o he p oduc ion o he c ack ee g een ilms unde con olled d ying up o 425 1C o he elimina ion o o ganic componen s. The SEM mic o- g aph (Fig. 2i) o he c oss-sec ion depic s a uni o m and dense hin ilm wi h E600 nm in hickness. The EDS line (Fig. 2j) scan p o ile de ec s zinc oxide a dep hs up o 600 nms om he su ace; a a la ge dep h, he EDS de ec o dis inguishes only Al and O ela ed o he Al 2 O 3 subs a e. Thus, o he 0.25Zn : 1Al a io, he p oposed me hodology esul s in homogeneous ilms wi h sui able con ol o e he chemical composi ion, p o iding in o ma ion abou he o ma ion o a ZnAl 2 O 4 phase. The mol en sal con ibu es o he eac i e sin e ing mecha- nism by dissol ing he Zn 2+ ca ions and di using hem in o he alumina subs a e. The su ace ension o he eu ec ic liquid esul s in he de elopmen o a con inuous ilm du ing mel ing ha allows o he spinel eac ion on all subs a e su aces. This p ocess begins when he hea ing empe a u e exceeds he eu ec ic empe a u e o he mol en sal . Wi h an inc ease in he hea ing empe a u e, he componen s o he sal mel e apo a e; a he selec ed empe a u e o 1200 1C, he lux is emo ed, and eac ion sin e ing is comple ed. The p esence o ZnO in ilms wi h highe concen a ions, such as 1Zn : 1Al and 0.5Zn : 1Al, indica es ha he e is a limi ed amoun o Zn 2+ ca ions di using in o he alumina subs a e owing o i s c ys al la ice packaging. I is wo h men ioning he e ha , o he i s ime, dense ZnAl 2 O 4 submic ome ic ilms a e ob ained by applying he cos -e icien sc een-p in ing p ocess p oposed in his s udy. Consequen ly, he op imized a io o Zn : Al employed in he ink is ixed a 0.25Zn : 1Al o he es o his s udy. De elopmen o ZnAl 2 O 4 ilms doped and co-doped wi h Ce and Nd To highligh he e sa ili y o his echnique, ink was added by he a e ea hs, Nd and Ce pai o unde s and he iabili y o hei inclusion in he gahni e hos . By enginee ing doping wi h Ce and Nd, he ZnAl 2 O 4 ma ix was modi ied o acqui e a luminescence unc ional esponse. The phase composi ion and c ys allini y o he ilms doped wi h Ce and Nd and annealed a 1200 1C o 2 hou s in ai we e s udied by XRD, and he diff ac og ams a e shown in Fig. 3a. The o mula ions a e designed by ixing he Nd con en and co-doped wi h diffe en Ce concen a ions, such as ZnAl 2 O 4 :xNd,yCe (x= 0.02, and y= 0.02, 0.04 and 0.06 mol a io). The diff ac ion peaks con i m he Fig. 3 ZnAl 2 O 4 ilms doped and co-doped wi h Ce and Nd: (a) XRD o he ilms syn hesized a 1200 1C by including a ixed con en o Nd and h ee Ce concen a ions: ZnAl 2 O 4 :xNd,yCe (x= 0.02, and y= 0.02, 0.04 and 0.06 mol a io). The XRD o he alumina subs a e ( ed line) is shown a he bo om o igu e a. The blue e ical ma ks a he bo om o he igu e ep esen he e lec ion posi ions o zinc alumina es. (b–d) C oss-sec ional iews o he ilms syn hesized in a. PCCP Pape Open Access A icle. Published on 18 May 2023. Downloaded on 3/12/2024 8:46:07 AM. This a icle is licensed unde a C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence. View A icle Online 15834 | Phys. Chem. Chem. Phys., 2023, 25, 15829–15838 This jou nal is © he Owne Socie ies 2023 polyc ys alline na u e o he ZnAl 2 O 4 ilms. All peaks a e in good ag eemen wi h he diff ac ion s anda d PDF: 01-074-1138. The inse illus a ed in Fig. 3a e eals he wo main diff ac ion e lec ions o he ZnAl 2 O 4 cubic polymo ph loca ed a 31.291 and 36.8612y, co esponding o planes (220) and (311), espec- i ely. The con ibu ion o he alumina subs a e is de ec ed and ma ked wi h he ed squa e symbol ‘‘&’’. The c ys alli e size o he zinc alumina e ilms is 140 7, 138 7and1364nm o he samples co-doped wi h 0.02Nd/0.02Ce, 0.02Nd/0.04Ce and 0.02Nd/0.06Ce, espec i ely. These c ys alli e sizes a e la ge enough o p o ide enhanced luminescence p ope ies because o he low g ain bounda y densi y. 40 The la ice pa ame e (a)and he olumeo heuni cell(Vol)we ecalcula ed o heundoped ilm and he ilms co-doped wi h 0.02Nd/0.02Ce, 0.02Nd/0.04Ce and 0.02Nd/0.06Ce. The alues ob ained a e a= 8.0905 (25) Å and Vol = 528.89 Å 3 ,a= 8.0917 (12) Å and Vol = 529.51 Å 3 , a= 8.0926 (25) Å and Vol = 529.99 Å 3 and a= 8.0928 (17) Å and Vol = 529.86 Å 3 o he ilms undoped and co-doped wi h 0.02Nd/0.02Ce, 0.02Nd/0.04Ce and 0.02Nd/0.06Ce, espec i ely. The la ice pa ame e , a, and he olume o he uni cell, Vol, sligh ly inc ease as a unc ion o Ce concen a ion, which indica es he effec i e doping o he s uc u e. As shown in he c oss-sec ional SEM mic og aphs (Fig. 3b–d), he ilm hickness can be modula ed o c.a. 600 nm o allco-doped o mula ions. The iscosi y (Z) o he inks wi h diffe en concen a ions o dopan s was measu ed in he shea a e ange be ween 1 and 2000 s 1 (ESI†,S2).The lowcu esshow ha allinksexhibi a shea hinning beha io . The deg ee o shea hinning is qui e simila o all inks. The e o e, i is expec ed ha he iscosi y will dec ease o he effec i e ans e o he ink on o he subs a e su ace. Du ing he sc een p in ing p ocess, iscosi y changes as a unc ion o he shea a e and ime. Usually, a shea a e alue a ound 1000 s 1 is adop ed when he ink pene a es he opened sc een mesh. 41 The iscosi y a a low shea a e se es only as a e e ence i he e is any change in he ink p ope ies. Howe e , he measu ed iscosi y as a unc ion o he shea a e is almos simila o all inks p epa ed, demons a ing he ep oducibili y o he pas e o mula ion and ab ica ion p ocess, including homogeni- za ion and dispe sion s eps. In summa y, he absence o seconda y phases in he doped ilms is a ema kable esul ,sugges ing ha heZnAl 2 O 4 ma ix and bo h dopan s o m a sui able and s able solid solu ion. I is impo an o no e ha op imiza ion o he ink o mula ion leads o he o ma ion o ZnAl 2 O 4 wi hou seconda y phases. The c oss- sec ional SEM images show homogeneous and dense ilms wi h- ou c acks, which is a signi ican achie emen because ce amic ma e ials a e usually difficul o sin e in o c ack- ee ilms. Using ou app oach, he misma ch be ween he sin e ing sh inkage o he ilm and he subs a e is minimized. The knowledge acqui ed in his s udy is use ul o designing inks ha can be applied o o he sys ems based on alumina es. Luminescence p ope ies o he ZnAl 2 O 4 ilms co-doped wi h Ce and Nd and hei s uc u al co ela ions The e a e diffe en ways o modula e he emission in a ma e- ial. In his s udy, he enhancemen o he NIR emission o ZnAl 2 O 4 ilms by ene gy ans e om Ce o Nd is conside ed. I is known ha he di ec exci a ion o Nd in he UV is hinde ed owing o he low abso p ion c oss-sec ion in his wa eleng h ange. The e o e, h ee ce ium concen a ions we e s udied; he ilms we e exci ed unde 357 nm exci a ion a RT, as shown in Fig. 4a. The ilms doped only wi h Nd and doped only wi h Ce we e compa ed wi h he co-doped samples. In all samples, h ee emissions a e obse ed and a e cha ac e is ics o Nd 3+ . The emission peaks posi ioned a 850 o 950 nm co espond o he 4 F 3/2 - 4 I 9/2 ansi ion. The main emission loca ed a 1000 o 1175 nm is assigned o he 4 F 3/2 - 4 I 11/2 ansi ion, and o he emissions appea a 1300 o 1400 nm co esponding o he 4 F 3/2 - 4 I 13/2 ansi ion. To add he p oo o ene gy ans e om Ce o Nd, he Nd single-doped ma e ials a e also exci ed by 357nm as a e e ence. I can be obse ed ha he emission o Nd is almos no obse able, as migh be expec ed. The emission spec a unde 740 nm a e p esen ed in Fig. 4b, and he exci a ion spec a b ixing he emission a 1060nm is demons a ed in Fig. 4c. Unde 740 nm exci a ion, he e is he p esence o an emission band loca ed a 1060 nm assigned o he di ec exci a ion o Nd. The emission in ensi y unde 740 nm exci a ion a ies insigni ican ly as a unc ion o he Ce concen a ion. This esul is expec ed because Nd is exci ed di ec ly and no h ough he ce ium co-dopan . Nd 3+ ca ion can also be exci ed by a ious wa eleng hs, as shown in he exci a- ion spec a. In pa icula , he mos in ense band is loca ed a 740 nm in he nea in a ed ange. The in ensi y o he exci a- ion band loca ed a 357 nm sligh ly inc eases wi h Ce concen- a ion om 0.02 o 0.06. Howe e , o he ma e ial only doped wi h Nd, his exci a ion band is no p esen , indica ing ha he exci a ion band loca ed a 357 nm is a ibu ed o Ce. He e, he co-dopan allows o he abso p ion o exci a ion ligh in he UV, enhancing NIR emission. I has al eady been demons a ed ha Ce co-dopan can enhance he NIR emission in he ZnAl 2 O 425 sys em, as well as in o he ma ices, such as y ium aluminium ga ne (YAG), Y 3 Al 5 O 12 doped wi h Nd and Ce, 42,43 K 2 CeO 344 and Li 2 CeO 3 . 45 Thus, we can iden i y ha efficien ene gy ans e om Ce o Nd exis s in his sys em. The schema ic ene gy le els wi h ansi ion, which may be in ol ed in he ene gy ans e p ocess, a e demons a ed in Fig. S3 (see ESI†). Nd 3+ has se e al ene gy le els, and some o hem a e in he UV egion, ha ing good o e lap wi h Ce abso p ion. Thus, he e a e se e al channels o ene gy ans e om Ce o Nd. 2 P j le els o Nd migh be popula ed by ene gy ans e om Ce and Nd can hen elax o a 4 F 3/2 s a e by emi ing phonons in se e al s eps. Consequen ly, he NIR emission is ob ained by ansi- ions om 4 F 3/2 o lowe lying 4 I j le els. Ano he possibili y is ha om 5D s a es, Nd can de-exci e o 4 F 3/2 le el by c oss elaxa ion, aking a nea by Nd ca ion o he exci ed s a e. Howe e , u he analyses a e needed o es ablish he exac mechanism. Howe e , he exci a ion band a 740 nm emains unchanged, as expec ed, because he Nd concen a ion is ixed a 002. Howe e , he e is s ill an open ques ion ela ed o he p esence o Ce 3+ and Ce 4+ ions and hei ole in he enhance- men o NIR emission. Only a ew s udies ha e discussed he Pape PCCP Open Access A icle. Published on 18 May 2023. Downloaded on 3/12/2024 8:46:07 AM. This a icle is licensed unde a C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence. View A icle Online This jou nal is © he Owne Socie ies 2023 Phys. Chem. Chem. Phys., 2023, 25, 15829–15838 | 15835 p esence o i alen and e a alen ce ium in he lumines- cence p ocess o Ce doped scin illa o s. 46 Bea ing in mind assump ions ha Ce 4+ has an abso p ion band wide han Ce 3+ , 47 a highe abso p ion due o he cha ge ans e band o Ce 4+ is expec ed. To es ablish he oxida ion s a e o he ilms, XANES a Ce L 3 -edge (5723 eV) 48 and XPS expe imen s we e ca ied ou . The no malized XANES spec a o he ZnAl 2 O 4 :xNd, yCe (x= 0.02, and y= 0.02, 0.04 and 0.06 mol a io) samples wi h he e e ence spec a o CeO 2 and Ce(NO 3 ) 3 6H 2 O a e shown in Fig. 5a. The CeO 2 and Ce(NO 3 ) 3 6H 2 O s anda ds we e used o pe o m he linea combina ion i ing (LC) o de e - mine he Ce 3+ and Ce 4+ a io. 25 Ce(NO 3 ) 3 6H 2 O e e ence shows a sha p edge ea u e, while CeO 2 and ZnAl 2 O 4 ilms doped wi h diffe en Ce concen a ions display a double . 49 The inco po a- ion o highe concen a ions o Ce up o 0.06 mol a io comes wi h he s abiliza ion o Ce 4+ and u ns in o a highe in ensi y o he emission bands loca ed a 1069 and 1077 nm, as shown in Fig. 5b. To u he de e mine he oxida ion s a e on he su ace, XPS was acqui ed. Bo h i alen Ce 3+ and e a alen Ce 4+ a e de ec ed (Fig. 5c). The XPS spec a o he ilms a e composed o wo mul iples a ibu ed o 3d 3/2 co e holes and 3d 5/2 spin-o bi spli . Six o he peaks co esponding o inal sa es o Ce 4+ and ou o Ce 3+ a e de ec ed. The six peaks loca ed a 915.45, 905.45, 899.9, 897.3, 887, and 881.4 eV belong Fig. 4 Luminescence cha ac e iza ion o he ZnAl 2 O 4 ilms doped and co-doped wi h Ce and Nd: (a) emission spec a unde 357 nm a RT o he ilms syn hesized a 1200 1C by including a ixed con en o Nd and h ee Ce concen a ions, ZnAl 2 O 4 :xNd,yCe (x= 0.02, and y= 0.02, 0.04 and 0.06 mol a io), he ilm only doped wi h Nd and he ilm doped wi h a concen a ion o Ce0.02. (b) Emission spec a unde 740 nm. (c) Exci a ion spec a ixing he emission a 1060 nm. PCCP Pape Open Access A icle. Published on 18 May 2023. Downloaded on 3/12/2024 8:46:07 AM. This a icle is licensed unde a C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence. View A icle Online 15836 | Phys. Chem. Chem. Phys., 2023, 25, 15829–15838 This jou nal is © he Owne Socie ies 2023 o he Ce(IV) s a es; he peak loca ed a 915.45 eV is he inge p in o Ce 4+ . The dis inc line shapes loca ed a 901.6, 898.1, 883.1 and 879.8 eV a e assigned o Ce 3+ s a es. All o he emission peaks a e in good ag eemen wi h p e ious s udies. 50,51 The e o e, doping enginee ing wi h he Ce and Nd o ZnAl 2 O 4 ilms leads o he s abiliza ion o Ce 4+ , p omo ing NIR emission. Mo eo e , inc eased Ce con en enhances NIR emission, leading o a g adual inc ease in he p esence o Ce 4+ , which is associa ed wi h local s uc u al homogenei y. Conclusions To expand he scope o he sc een p in ing echnique applied o he syn hesis o NIR-emi ing submic on ilms, he ein, poly- c ys alline alumina is used o he g ow h o he ZnAl 2 O 4 spinel ma e ial. The ink o mula ion and scalable ab ica ion p oce- du es we e ho oughly e alua ed. We e ealed a mol en sal - d i en g ow h mechanism and designed a g ow h s a egy, hus achie ing c ys alli es wi h an a e age size o 140 nm, much la ge han in p e ious epo s. These c ys alli e sizes a e la ge enough o enhance luminescence p ope ies owing o a low g ain bounda y densi y. The subs a e p ecu so in si u pa ici- pa es in he eac ion; by employing he eu ec ic mol en sal mix u e (NaCl–KCl), which p omo es he su ace eac ion, con- ollable nuclea ion is achie ed. Based on he XANES and XPS esul s, he ce ium in he ilms exhibi s a p edominan e a- alen s a e. Ou p oposed me hod imp o es he emission cen e ed a 1069 and 1077 nm by applying high concen a ions o ce ium and by doping enginee ing wi h Ce and Nd. An adequa e composi ional enginee ing s a egy plays a unda- men al ole in he p oduc ion o homogeneous ilms based on ZnAl 2 O 4 wi h high op ical pe o mance. Ou indings o e no el insigh s in o con ollable syn hesis o hin ilms based on ZnAl 2 O 4 and pa e he way o he de elopmen o e icien NIR emi e s based on alumina e sys ems in nex -gene a ion unc ional de ices. Con lic s o in e es The e a e no con lic s o in e es o decla e. Acknowledgemen s R.E Rojas-He nandez and I. Hussaino a acknowledge he sup- po o he Es onian Resea ch Council (g an s PSG-466, PRG- 643). F. Rubio-Ma cos also acknowledges he MINECO (Spanish Go e nmen ) p ojec PID2020-114192RB-C41 and inancial sup- po om Comunidad de Mad id o he ‘‘Doc o ados Indus- iales’’ p ojec (IND2020/IND-17375), which is co- inanced by he Eu opean Social Fund. The au ho s a e g a e ul o Oli e Ja ¨a ik o allowing us o use he acili ies a he Depa men o Ene gy Technology. We exp ess hanks o he Ma i Kauk- Kuusik g oup o allowing us o use he labo a o y acili ies. M. Danilson acknowledges he Eu opean Union h ough he Eu opean Regional De elopmen Fund, P ojec TK141 and he Es onian Resea ch Council g an (PRG1023). The au ho s ecognized he suppo o ALBA Synch o on o p o ide he beam ime o pe o m he XANES expe imen s included in he esea ch. Re e ences 1 C. T. Jackson, S. Jeong, G. F. Do lhiac and M. P. Land y, Ad ances in enginee ing nea -in a ed luminescen ma e i- als, iScience, 2021, 24, 102156. 2 F. Zhang and B. Z. Tang, Nea -in a ed luminescen p obes o bioimaging and biosensing, Chem. Sci., 2021, 12, 3377–3378. Fig. 5 S uc u al cha ac e iza ion by XANES a Ce L 3 -edge and XPS o he ZnAl 2 O 4 ilms doped and co-doped wi h Ce and Nd: (a) XANES a Ce L 3 -edge o he ilms syn hesized wi h 0.02 Nd mol a io and concen a ion o Ce om 0.02 o 0.06 mol a io. A he bo om, he XANES o wo e e ences o ce ium, ce ium oxide and ce ium ni a e a e p esen ed. The linea combina ion i ing is included in igu e a by adding he do ed ed line. Panel b shows he emission in ensi y o he peaks a 1069 and 1077 nm as a unc ion o he Ce 4+ calcula ed om he XANES a he Ce L 3 -edge esul s. (c). The expe imen al solid line and i ed cu es (do cu es o each peak and solid ed line o he sum o he decon olu ion) o he high- esolu ion XPS spec a o he Ce 3d 3/2 , 5/2 lines. Pape PCCP Open Access A icle. Published on 18 May 2023. Downloaded on 3/12/2024 8:46:07 AM. This a icle is licensed unde a C ea i e Commons A ibu ion-NonComme cial 3.0 Unpo ed Licence. View A icle Online This jou nal is © he Owne Socie ies 2023 Phys. Chem. Chem. 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