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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
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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
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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.
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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.
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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.
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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
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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.
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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
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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.
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