Ci a ion: Misiu e , D.; Kaspa , P.;
Holcman, V. B ie Theo e ical
O e iew o Bi-Fe-O Based Thin
Films. Ma e ials 2022,15, 8719.
h ps://doi.o g/10.3390/
ma15248719
Academic Edi o s: Tamas Va ga and
An onio San aga a
Recei ed: 5 Oc obe 2022
Accep ed: 5 Decembe 2022
Published: 7 Decembe 2022
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Licensee MDPI, Basel, Swi ze land.
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A ibu ion (CC BY) license (h ps://
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ma e ials
Re iew
B ie Theo e ical O e iew o Bi-Fe-O Based Thin Films
Denis Misiu e * , Pa el Kaspa * and Vladimí Holcman
Depa men o Physics, Facul y o Elec ical Enginee ing and Communica ion, B no Uni e si y o Technology,
Technicka 2848/8, 61600 B no, Czech Republic
*Co espondence: [email p o ec ed] (D.M.); [email p o ec ed] (P.K.)
Abs ac :
This pape will p o ide a b ie o e iew o he unique mul i e oic ma e ial Bismu h e i e
(BFO). Conside ing ha Bismu h e i e is a unique ma e ial which possesses bo h e oelec ic and
magne ic p ope ies a oom empe a u e, he uniqueness o Bismu h e i e ma e ial will be discussed.
Fundamen al p ope ies o he ma e ial including elec ical and e omagne ic p ope ies also will be
men ioned in his pape . Elec ical p ope ies include cha ac e iza ion o basic pa ame e s conside ing
he elec ical esis i i y and leakage cu en . Fe omagne ic p ope ies in ol e he desc ip ion o
magne ic hys e esis cha ac e iza ion. Bismu h e i e can be ab ica ed in a di e en o m. The
common o ms will be men ioned and include powde , hin ilms and nanos uc u es. The mos
popula me hod o p oducing hin ilms based on BFO ma e ials will be desc ibed and compa ed.
Finally, he pe spec i es and po en ial applica ions o he ma e ial will be highligh ed.
Keywo ds:
Bismu h e i e; mul i e oic; hin ilm; e oelec ic; magne ic; PDL; ALD; leakage
cu en ; hys e esis
1. In oduc ion
The cu en ends equi e mo e minimalis ic echnologies and highe ene gy e iciency
which leads o an inc easing in e es o de eloping new ypes o ma e ials. Recen ends
o ma e ial science equi e in en ion o new ma e ials which would e ec i ely combine
mul iple p ope ies oge he wi h empe a u e and chemical s abili y. New and popula
ends o designing mode n de ices in ol e con ol o bo h magne ic and elec ical pa s,
hus c ea ing mul i unc ional de ices wi h a combina ion o e oelec ic and e omagne ic
e ec s. This combina ion o e s pe spec i e oppo uni ies o designing new and complex
mic oelec onic sys ems. The ma e ials, which combine bo h e oelec ic and magne ic
p ope ies, a e ex emely a e and i al o sol ing a wide a ie y o p oblems and a e
implemen ed in many applica ions.
Mul i e oic ma e ials s and ou , since hei elec ical and magne ic p ope ies can
be changed by he elec omagne ic ield. Mul i e oic ma e ials aim o sol e he p oblem
ela ed o c ea ing mo e ene gy–e icien mic oelec onics and applica ions wi h as e
speed eac ion, elecommunica ion, e c.
The ac ual de ini ion o he wo d mul i e oic s ands o combina ion o e oelec ic
and e omagne ic e ec o a ma e ial. The cu en de ini ion o mul i e oic also includes
he an i e oelec ic e ec . Nowadays, mul i e oic ma e ials d aw a lo o a en ion as hey
can be implemen ed in a wide a ie y o new de ices and de ec o s. Since mul i e oic
ma e ials exhibi e oelec ic and magne ic p ope ies, hey became an essen ial pa o
many nanos uc u es. These ma e ials also equi e less powe han con en ional sys ems
o ope a e.
A unique ad an age o single phase mul i e oic based hin ilm ma e ials is he
ac ha hey sugges appealing and p ospe ous ways o c ea ing new ma e ials wi h a
unique combina ion o di e en subs a es which emphasize he ad an ages o indi idual
ma e ials. Single phase mul i e oic ma e ials which exhibi s ong magne ic coupling a e
a e. The e o e, bismu h e i e is he only in ensi ely s udied ma e ial.
Ma e ials 2022,15, 8719. h ps://doi.o g/10.3390/ma15248719 h ps://www.mdpi.com/jou nal/ma e ials
Ma e ials 2022,15, 8719 2 o 28
BFO is a mul i e oic ma e ial wi h g ea po en ial. BFO is an ou s anding ma e ial
which possesses e oelec ic and magne ic p ope ies simul aneously. The ma e ial has a
signi ican ad an age in e ms o pho o ol aic de ices due o a na ow bandgap, supp es-
sion o ecombina ion o elec on–hole pai and wide abso p ion ange. Manipula ion o
elec omagne ic coupling by elec ic o magne ic ield makes BFO-based ma e ials po en ial
candida es o spin onic applica ions.
Thin ilms o BFO ma e ial we e and s ill a e e y popula o he de elopmen o
mode n nanoma e ials. Thin ilms allow nume ous modi ica ions o mo phology and hick-
nesses o he inal p oduc . In compa ison o BFO ce amics, hin ilms can be p oduced in a
ela i ely low ange o ope a ional empe a u es, which enhance he o e all pe o mance
o he inal ilm. BFO–based hin ilms we e widely used in e oelec ic memo y s o age
de ices due o high emanen pola iza ion. In addi ion o memo y de ices, hin ilms play
an impo an ole in minia u e an ennas and mic owa e MEMS de ices.
O e he pas decade, BFO has been in ensi ely used in BFO–based ce amics due o i s
high empe a u e esis ance and he coexis ence o pa a– e o elec ic phases. In addi ion
o he mo pho opic phase bounda y, he pe o ski e chemical s uc u e o BFO ma e ial
allows implemen a ion o impu i ies, hus enhancing elec omagne ic coupling. Ano he
conside able ad an age o BFO ma e ial is manipula ion wi h i s e o o oidic sys em by
he elec ic/magne ic ield. These p ope ies combine wi h he high conduc i i y o BFO
and make i a p omising ma e ial o he de elopmen o BFO–based ce amics.
Al hough an ou s anding ma e ial wi h unique p ope ies among o he ma e ials,
he e a e s ill challenges ha need o be add essed in he nea u u e. I is necessa y o ind
po en ial solu ions o dec ease high pa asi ic cu en and inc ease ela i ely low elec o-
magne ic coupling o implemen a ion in o la ge-scale de ices. The physical mechanism o
pho o ol aic esponse is s ill unknown and equi es u he and deepe in es iga ion.
2. Cha ac e iza ion o Bismu h Fe i e Ma e ial
BFO ma e ial is one o a ew mul i e oic ma e ials which shows a simul aneous
e oelec ic and e omagne ic e ec [
1
]. The ma e ial was in ensi ely s udied o e he
pas decade because o BFO e sa ile mul i unc ionali y. I was p o en ha BFO is he
only ma e ial which has he s onges elec omagne ic coupling a oom empe a u e ange
among all po en ial ma e ials making i an excellen candida e [
2
] o e oelec ic senso s.
Conside ing all mul i e oic ma e ials, hin ilms based on BFO ha e been success ully
used o mic oelec onic and op oelec onic de ices due o low band gap. The p incipal
o e oelec ici y a oom empe a u e is based on a lone–pai mechanism, whe e alence
elec ons o Bi
+3
c ea e a localized magne ic dipole and a e in ol ed in SP–hyb idiza ion [
3
].
The local magne ic dipole leads o he c ea ion o spon aneous ( emnan P [
3
]) pola iza ion
a ound 90–100 (
µ
Ccm
−2
) [
4
,
5
] which is a main eason BFO is an excep ional mul i e -
oic ma e ial.
Magne oelec ic ma e ials belong o mul i e oic ma e ials wi h a co ela ion o mag-
ne ic and e oelec ic p ope ies. Magne ic p ope ies (magne iza ion) and elec ic p op-
e ies (pola iza ion) can be con olled by he elec ic and magne ic ield. The ma e ial is
syn hesized in o single and mul i–phase molecules.
BFO is popula no only due o i s he mal s abili y bu also due o i s high ange o
pola iza ion (~90–100 µCcm−2) [4,5].
2.1. S uc u al Cha ac e iza ion o BFO Ma e ial
The mos common c ys alliza ion o m o Bismu h e i e ma e ial is a c ys allog aphic
s uc u e ep esen ed by a symme ic hombohed al (Figu e 1) [
2
,
3
,
6
] cen ally o ien a ed
pe o ski e [
6
] s uc u e whe e pa ame e s a, b a e la ice pa ame e s and c is a hexago-
nal pa ame e . The ypical e omagne ic pe o ski e pa ame e s o BFO ma e ial s and
wi hin
a = b = (5.7–6.7)
Å [
7
,
8
] and hexagonal c = (13–14) Å [
5
,
9
]. The angle
α
is a ound
60 deg ees
[
10
]. The space g oup o ien a ion is R3c. The pa ame e s o he cubic hombohe-
d al s uc u e a e a = b = c ~4 Å and angle αs ays oughly unde 90 deg ees.
Ma e ials 2022,15, 8719 3 o 28
Ma e ials 2022, 14, x FOR PEER REVIEW 3 o 28
2.1. S uc u al Cha ac e iza ion o BFO Ma e ial
The mos common c ys alliza ion o m o Bismu h e i e ma e ial is a c ys allo-
g aphic s uc u e ep esen ed by a symme ic hombohed al (Figu e 1) [2,3,6] cen ally
o ien a ed pe o ski e [6] s uc u e whe e pa ame e s a, b a e la ice pa ame e s and c is a
hexagonal pa ame e . The ypical e omagne ic pe o ski e pa ame e s o BFO ma e ial
s and wi hin a = b = (5.7–6.7) Å [7,8] and hexagonal c = (13–14) Å [5,9]. The angle α is
a ound 60 deg ees [10]. The space g oup o ien a ion is R3c. The pa ame e s o he cubic
hombohed al s uc u e a e a = b = c ~4 Å and angle α s ays oughly unde 90 deg ees.
Figu e 1. Rhombohed al c ys allog aphic s uc u e.
R3c space o ien a ion s ands o hombohed al symme y g oup, whe e he di ec ion
o one symme y is c, and he o he di ec ion o symme y being pe pendicula o c. Rhom-
bohed al o de is ep esen ed by wo symme y di ec ions, which a e c and b. The coo -
dina e sys em o he hombohed al g oup co esponds o [9] wi h la ice pa ame e s a =
b= c and α ≠ 90°.
The ypical Pe o ski e s uc u e (Figu e 2) includes an FeO uni which is inse ed in o
he Bi cubic hombohed al c ys al s uc u e. Oxygen anions and Fe ca ions c ea e oc ahe-
d al o ien a ion closed by Bi ions.
Figu e 2. Bismu h e i e uni in he pe o ski e s uc u e [8].
One o he mos signi ican disad an ages which limi s usabili y o BFO [6] is high
pa asi ic cu en (low elec ical esis ance). High pa asi ic cu en acc ues in he BFO com-
posi e due o de ec s ela ed o he seconda y phase o BFO such as absence o Bi o oxygen
a oms du ing he p epa a ion p ocess. The e a e many easons which cause high pa asi ic
cu en ; howe e , he p ima y cause is a high amoun o impu i ies du ing he deposi ion
p ocess, and he chemis y o he BFO uni . One po en ial de ec is ela ed o he e apo a-
ion o bismu h du ing he p epa a ion p ocess, hus changing he chemical alence o Fe+2
o Fe+3 [11,12] ions due o he e apo a ed Bi+2 [11,12] ca ions and oxygen acancies. Un-
compensa ed acancies lead o a dis ibu ion o spin momen o he en i e cell uni .
The absence o oxygen anions o compensa e he cha ge o lowe chemical alency o
Fe+2 causes a signi ican dec ease o elec ical esis ance, and hus an inc easing leakage
cu en .
The pa asi ic cu en becomes mo e p ominen in mul i–phase composi es (Bi2Fe4O9,
BiFeO3 e c.), hus i is becoming mo e popula o ind me hods o educe pa asi ic cu en .
Figu e 1. Rhombohed al c ys allog aphic s uc u e.
R3c space o ien a ion s ands o hombohed al symme y g oup, whe e he di ec ion
o one symme y is c, and he o he di ec ion o symme y being pe pendicula o c.
Rhombohed al o de is ep esen ed by wo symme y di ec ions, which a e c and b. The
coo dina e sys em o he hombohed al g oup co esponds o [
9
] wi h la ice pa ame e s
a=b=candα6=90◦.
The ypical Pe o ski e s uc u e (Figu e 2) includes an FeO uni which is inse ed
in o he Bi cubic hombohed al c ys al s uc u e. Oxygen anions and Fe ca ions c ea e
oc ahed al o ien a ion closed by Bi ions.
Ma e ials 2022, 14, x FOR PEER REVIEW 3 o 28
2.1. S uc u al Cha ac e iza ion o BFO Ma e ial
The mos common c ys alliza ion o m o Bismu h e i e ma e ial is a c ys allo-
g aphic s uc u e ep esen ed by a symme ic hombohed al (Figu e 1) [2,3,6] cen ally
o ien a ed pe o ski e [6] s uc u e whe e pa ame e s a, b a e la ice pa ame e s and c is a
hexagonal pa ame e . The ypical e omagne ic pe o ski e pa ame e s o BFO ma e ial
s and wi hin a = b = (5.7–6.7) Å [7,8] and hexagonal c = (13–14) Å [5,9]. The angle α is
a ound 60 deg ees [10]. The space g oup o ien a ion is R3c. The pa ame e s o he cubic
hombohed al s uc u e a e a = b = c ~4 Å and angle α s ays oughly unde 90 deg ees.
Figu e 1. Rhombohed al c ys allog aphic s uc u e.
R3c space o ien a ion s ands o hombohed al symme y g oup, whe e he di ec ion
o one symme y is c, and he o he di ec ion o symme y being pe pendicula o c. Rhom-
bohed al o de is ep esen ed by wo symme y di ec ions, which a e c and b. The coo -
dina e sys em o he hombohed al g oup co esponds o [9] wi h la ice pa ame e s a =
b= c and α ≠ 90°.
The ypical Pe o ski e s uc u e (Figu e 2) includes an FeO uni which is inse ed in o
he Bi cubic hombohed al c ys al s uc u e. Oxygen anions and Fe ca ions c ea e oc ahe-
d al o ien a ion closed by Bi ions.
Figu e 2. Bismu h e i e uni in he pe o ski e s uc u e [8].
One o he mos signi ican disad an ages which limi s usabili y o BFO [6] is high
pa asi ic cu en (low elec ical esis ance). High pa asi ic cu en acc ues in he BFO com-
posi e due o de ec s ela ed o he seconda y phase o BFO such as absence o Bi o oxygen
a oms du ing he p epa a ion p ocess. The e a e many easons which cause high pa asi ic
cu en ; howe e , he p ima y cause is a high amoun o impu i ies du ing he deposi ion
p ocess, and he chemis y o he BFO uni . One po en ial de ec is ela ed o he e apo a-
ion o bismu h du ing he p epa a ion p ocess, hus changing he chemical alence o Fe+2
o Fe+3 [11,12] ions due o he e apo a ed Bi+2 [11,12] ca ions and oxygen acancies. Un-
compensa ed acancies lead o a dis ibu ion o spin momen o he en i e cell uni .
The absence o oxygen anions o compensa e he cha ge o lowe chemical alency o
Fe+2 causes a signi ican dec ease o elec ical esis ance, and hus an inc easing leakage
cu en .
The pa asi ic cu en becomes mo e p ominen in mul i–phase composi es (Bi2Fe4O9,
BiFeO3 e c.), hus i is becoming mo e popula o ind me hods o educe pa asi ic cu en .
Figu e 2. Bismu h e i e uni in he pe o ski e s uc u e [8].
One o he mos signi ican disad an ages which limi s usabili y o BFO [
6
] is high
pa asi ic cu en (low elec ical esis ance). High pa asi ic cu en acc ues in he BFO
composi e due o de ec s ela ed o he seconda y phase o BFO such as absence o Bi o
oxygen a oms du ing he p epa a ion p ocess. The e a e many easons which cause high
pa asi ic cu en ; howe e , he p ima y cause is a high amoun o impu i ies du ing he
deposi ion p ocess, and he chemis y o he BFO uni . One po en ial de ec is ela ed o
he e apo a ion o bismu h du ing he p epa a ion p ocess, hus changing he chemical
alence o Fe
+2
o Fe
+3
[
11
,
12
] ions due o he e apo a ed Bi
+2
[
11
,
12
] ca ions and oxygen
acancies. Uncompensa ed acancies lead o a dis ibu ion o spin momen o he en i e
cell uni .
The absence o oxygen anions o compensa e he cha ge o lowe chemical alency
o Fe
+2
causes a signi ican dec ease o elec ical esis ance, and hus an inc easing leak-
age cu en .
The pa asi ic cu en becomes mo e p ominen in mul i–phase composi es (Bi
2
Fe
4
O
9
,
BiFeO
3
e c.), hus i is becoming mo e popula o ind me hods o educe pa asi ic cu en .
To lowe he leakage cu en , he s uc u e o he composi e has o be changed by adding a
small amoun o impu i ies [
13
]. Some pape s p o e a signi ican dec ease in leakage cu en
by implemen ing a e–ea h ma e ials [
6
] such as i anium, ch omium and manganese. The
leakage cu en [
14
,
15
] causes a huge limi a ion o implemen a ion o BFO ma e ial in o
complex senso de ices.
Ma e ials 2022,15, 8719 4 o 28
2.2. Magne ic P ope ies o BFO Ma e ial
The c ys allog aphic s uc u e o BFO [
14
] is ep esen ed by a hombohed al cen ally
o ien a ed pe o ski e s uc u e [
15
]. Oxygen anions c ea e an oc ahed al o ma ion in he
c ys al, hanks o hese bands he sys em showing a nonze o [
8
] e omagne ic esponse.
Fe oelec ic p ope ies o he ma e ial a e highly dependen on hese oxygen bands [
4
,
7
].
Bismu h a oms c ea e a cubic hombohed al s uc u e nex o oc ahed al uni s o FeO,
which is loca ed inside o hombohed al s uc u e. Spin in e ac ion o bismu h a oms and
he esul ed spin momen o he oc ahed al FeO uni leads o he o ma ion o e oelec ic
esponse. Spin momen o he Bi
+3
elec on pai (6s
2
) [
16
] sha es an elec on pai wi h
esidual momen o Fe
+3
caused by a weak magne ic esponse [
16
]—G– ype magne iza ion
o de (Figu e 3).
Ma e ials 2022, 14, x FOR PEER REVIEW 4 o 28
To lowe he leakage cu en , he s uc u e o he composi e has o be changed by adding
a small amoun o impu i ies [13]. Some pape s p o e a signi ican dec ease in leakage
cu en by implemen ing a e–ea h ma e ials [6] such as i anium, ch omium and man-
ganese. The leakage cu en [14,15] causes a huge limi a ion o implemen a ion o BFO
ma e ial in o complex senso de ices.
2.2. Magne ic P ope ies o BFO Ma e ial
The c ys allog aphic s uc u e o BFO [14] is ep esen ed by a hombohed al cen-
ally o ien a ed pe o ski e s uc u e [15]. Oxygen anions c ea e an oc ahed al o ma ion
in he c ys al, hanks o hese bands he sys em showing a nonze o [8] e omagne ic e-
sponse. Fe oelec ic p ope ies o he ma e ial a e highly dependen on hese oxygen
bands [4,7]. Bismu h a oms c ea e a cubic hombohed al s uc u e nex o oc ahed al uni s
o FeO, which is loca ed inside o hombohed al s uc u e. Spin in e ac ion o bismu h
a oms and he esul ed spin momen o he oc ahed al FeO uni leads o he o ma ion o
e oelec ic esponse. Spin momen o he Bi+3 elec on pai (6s2) [16] sha es an elec on
pai wi h esidual momen o Fe+3 caused by a weak magne ic esponse [16]—G– ype mag-
ne iza ion o de (Figu e 3).
Figu e 3. G– ype magne iza ion o de .
Magne iza ion o de cha ac e izes he o ien a ion o magne ic couples and angula
momen o a oms. In G– ype magne iza ion o de , all nea es magne ic dipoles a e o i-
en ed an ipa allelly (Figu e 3), causing an imagne ic dis ibu ion. Recen s udies show
ha magne ic p ope ies o he hombohed al s uc u e o BFO a e size-dependen (Figu e
4) [5,6]. I was p o en ha he e oelec ic could be g ea ly enhanced by changing he size
o BFO [14] pa icles. As BFO c ys als a e smalle , he magne ic esponse is s onge due
o a close and s onge in e ac ion o magne ic dipoles [5]. Fe oelec ic esponse is
s onge when he size o pa icles is smalle due o he lowe in e ac ion o Bi and Fe spin
momen s, esul ing in o e all enhanced e oelec ic esponse. C i ical hickness, in which
he an i e omagne ic esponse domina es o e e oelec ic esponse, s a s a ound 150
nm [17].
Figu e 4. Hys e esis loop o a ious pa icle sizes o BFO ma e ial.
Figu e 3. G– ype magne iza ion o de .
Magne iza ion o de cha ac e izes he o ien a ion o magne ic couples and angula
momen o a oms. In G– ype magne iza ion o de , all nea es magne ic dipoles a e o ien ed
an ipa allelly (Figu e 3), causing an imagne ic dis ibu ion. Recen s udies show ha mag-
ne ic p ope ies o he hombohed al s uc u e o BFO a e size-dependen (
Figu e 4
)
[5,6]
. I
was p o en ha he e oelec ic could be g ea ly enhanced by changing he size o BFO [
14
]
pa icles. As BFO c ys als a e smalle , he magne ic esponse is s onge due o a close and
s onge in e ac ion o magne ic dipoles [
5
]. Fe oelec ic esponse is s onge when he size
o pa icles is smalle due o he lowe in e ac ion o Bi and Fe spin momen s, esul ing in
o e all enhanced e oelec ic esponse. C i ical hickness, in which he an i e omagne ic
esponse domina es o e e oelec ic esponse, s a s a ound 150 nm [17].
Ma e ials 2022, 14, x FOR PEER REVIEW 4 o 28
To lowe he leakage cu en , he s uc u e o he composi e has o be changed by adding
a small amoun o impu i ies [13]. Some pape s p o e a signi ican dec ease in leakage
cu en by implemen ing a e–ea h ma e ials [6] such as i anium, ch omium and man-
ganese. The leakage cu en [14,15] causes a huge limi a ion o implemen a ion o BFO
ma e ial in o complex senso de ices.
2.2. Magne ic P ope ies o BFO Ma e ial
The c ys allog aphic s uc u e o BFO [14] is ep esen ed by a hombohed al cen-
ally o ien a ed pe o ski e s uc u e [15]. Oxygen anions c ea e an oc ahed al o ma ion
in he c ys al, hanks o hese bands he sys em showing a nonze o [8] e omagne ic e-
sponse. Fe oelec ic p ope ies o he ma e ial a e highly dependen on hese oxygen
bands [4,7]. Bismu h a oms c ea e a cubic hombohed al s uc u e nex o oc ahed al uni s
o FeO, which is loca ed inside o hombohed al s uc u e. Spin in e ac ion o bismu h
a oms and he esul ed spin momen o he oc ahed al FeO uni leads o he o ma ion o
e oelec ic esponse. Spin momen o he Bi+3 elec on pai (6s2) [16] sha es an elec on
pai wi h esidual momen o Fe+3 caused by a weak magne ic esponse [16]—G– ype mag-
ne iza ion o de (Figu e 3).
Figu e 3. G– ype magne iza ion o de .
Magne iza ion o de cha ac e izes he o ien a ion o magne ic couples and angula
momen o a oms. In G– ype magne iza ion o de , all nea es magne ic dipoles a e o i-
en ed an ipa allelly (Figu e 3), causing an imagne ic dis ibu ion. Recen s udies show
ha magne ic p ope ies o he hombohed al s uc u e o BFO a e size-dependen (Figu e
4) [5,6]. I was p o en ha he e oelec ic could be g ea ly enhanced by changing he size
o BFO [14] pa icles. As BFO c ys als a e smalle , he magne ic esponse is s onge due
o a close and s onge in e ac ion o magne ic dipoles [5]. Fe oelec ic esponse is
s onge when he size o pa icles is smalle due o he lowe in e ac ion o Bi and Fe spin
momen s, esul ing in o e all enhanced e oelec ic esponse. C i ical hickness, in which
he an i e omagne ic esponse domina es o e e oelec ic esponse, s a s a ound 150
nm [17].
Figu e 4. Hys e esis loop o a ious pa icle sizes o BFO ma e ial.
Figu e 4. Hys e esis loop o a ious pa icle sizes o BFO ma e ial.
The o e all magne ic spin momen o he hombohed al s uc u e is pe pendicu-
la [
7
] o i s cen al axis which leads o he exis ence o small magne iza ion abili y. The
magni ica ion is a ec ed by oxygen bands in he same way as e oelec ic p ope ies.
I is necessa y o men ion ha e omagne ic p ope ies s a o occu by he o e -
all supp ession o cycloid magne ic momen [
18
], o he wise e oelec ic and magne ic
p ope ies a e subdued. The cycloid momen can be b oken unde ce ain condi ions: by
Ma e ials 2022,15, 8719 5 o 28
implemen ing he magne ic ield, magne iza ion, chemical addi i es, and s ain in o hin
ilms o he e os uc u es he cycloid momen is supp essed, allowing e oelec ic and
magne ic p ope ies o occu [18].
The exis ence o cycloid spin momen [
18
] is gi en by lexomagne oelec ic in e ac ion.
Conside ing lexomagne oelec ic [
18
] in e ac ion, he elec ic pola iza ion is o se by
spin modula ion. I was shown ha magne ic p ope ies become negligible i he size o
nanopa icles is g ea e han 65 nanome e s [8].
Ano he signi ican ad an age o BFO is he ac ha BFO has ou s anding empe a u e
s abili y (Figu es 5and 6). The empe a u e, whe e i changes om an i e omagne ic o
pe manen magne ic ma e ial, is also known as Neel empe a u e (Figu e 6). The Neel [
7
]
empe a u e o BFO s ays a ound o 320 o 350
◦
C [
6
,
7
]. The ma e ial is capable o wi hs and-
ing a high ange o empe a u es and e ain i s magne ic p ope ies (Cu ie Tempe a u e is
850
◦
C [
6
,
19
]). Conside ing he ou s anding he mal s abili y o he ma e ial, including high
Cu ie and Neel empe a u es, he ma e ial shows li le change wi h empe a u e di e ences.
Ma e ials 2022, 14, x FOR PEER REVIEW 5 o 28
The o e all magne ic spin momen o he hombohed al s uc u e is pe pendicula
[7] o i s cen al axis which leads o he exis ence o small magne iza ion abili y. The mag-
ni ica ion is a ec ed by oxygen bands in he same way as e oelec ic p ope ies.
I is necessa y o men ion ha e omagne ic p ope ies s a o occu by he o e all
supp ession o cycloid magne ic momen [18], o he wise e oelec ic and magne ic p op-
e ies a e subdued. The cycloid momen can be b oken unde ce ain condi ions: by im-
plemen ing he magne ic ield, magne iza ion, chemical addi i es, and s ain in o hin
ilms o he e os uc u es he cycloid momen is supp essed, allowing e oelec ic and
magne ic p ope ies o occu [18].
The exis ence o cycloid spin momen [18] is gi en by lexomagne oelec ic in e ac-
ion. Conside ing lexomagne oelec ic [18] in e ac ion, he elec ic pola iza ion is o se
by spin modula ion. I was shown ha magne ic p ope ies become negligible i he size
o nanopa icles is g ea e han 65 nanome e s [8].
Ano he signi ican ad an age o BFO is he ac ha BFO has ou s anding empe a-
u e s abili y (Figu es 5 and 6). The empe a u e, whe e i changes om an i e omagne ic
o pe manen magne ic ma e ial, is also known as Neel empe a u e (Figu e 6). The Neel
[7] empe a u e o BFO s ays a ound o 320 o 350 °C [6,7]. The ma e ial is capable o wi h-
s anding a high ange o empe a u es and e ain i s magne ic p ope ies (Cu ie Tempe -
a u e is 850 °C [6,19]). Conside ing he ou s anding he mal s abili y o he ma e ial, in-
cluding high Cu ie and Neel empe a u es, he ma e ial shows li le change wi h empe -
a u e di e ences.
Figu e 5. Tempe a u e-dependen magne ic hys e esis loop o BFO ma e ial.
Figu e 6. Magne ic suscep ibili y s. empe a u e cu e.
In addi ion o e omagne ic p ope ies, BFO also shows massi e spon aneous mag-
ne iza ion [20].
I has always been a end o ind a way o enhance he magne ic and e oelec ic
p ope ies o BFO ma e ial. Since magne ic and e oelec ic p ope ies a e g ea ly a ec ed
by oxygen bands, adding addi ional FeO will g ea ly inc ease he magne ic p ope ies [6].
Figu e 5. Tempe a u e-dependen magne ic hys e esis loop o BFO ma e ial.
Ma e ials 2022, 14, x FOR PEER REVIEW 5 o 28
The o e all magne ic spin momen o he hombohed al s uc u e is pe pendicula
[7] o i s cen al axis which leads o he exis ence o small magne iza ion abili y. The mag-
ni ica ion is a ec ed by oxygen bands in he same way as e oelec ic p ope ies.
I is necessa y o men ion ha e omagne ic p ope ies s a o occu by he o e all
supp ession o cycloid magne ic momen [18], o he wise e oelec ic and magne ic p op-
e ies a e subdued. The cycloid momen can be b oken unde ce ain condi ions: by im-
plemen ing he magne ic ield, magne iza ion, chemical addi i es, and s ain in o hin
ilms o he e os uc u es he cycloid momen is supp essed, allowing e oelec ic and
magne ic p ope ies o occu [18].
The exis ence o cycloid spin momen [18] is gi en by lexomagne oelec ic in e ac-
ion. Conside ing lexomagne oelec ic [18] in e ac ion, he elec ic pola iza ion is o se
by spin modula ion. I was shown ha magne ic p ope ies become negligible i he size
o nanopa icles is g ea e han 65 nanome e s [8].
Ano he signi ican ad an age o BFO is he ac ha BFO has ou s anding empe a-
u e s abili y (Figu es 5 and 6). The empe a u e, whe e i changes om an i e omagne ic
o pe manen magne ic ma e ial, is also known as Neel empe a u e (Figu e 6). The Neel
[7] empe a u e o BFO s ays a ound o 320 o 350 °C [6,7]. The ma e ial is capable o wi h-
s anding a high ange o empe a u es and e ain i s magne ic p ope ies (Cu ie Tempe -
a u e is 850 °C [6,19]). Conside ing he ou s anding he mal s abili y o he ma e ial, in-
cluding high Cu ie and Neel empe a u es, he ma e ial shows li le change wi h empe -
a u e di e ences.
Figu e 5. Tempe a u e-dependen magne ic hys e esis loop o BFO ma e ial.
Figu e 6. Magne ic suscep ibili y s. empe a u e cu e.
In addi ion o e omagne ic p ope ies, BFO also shows massi e spon aneous mag-
ne iza ion [20].
I has always been a end o ind a way o enhance he magne ic and e oelec ic
p ope ies o BFO ma e ial. Since magne ic and e oelec ic p ope ies a e g ea ly a ec ed
by oxygen bands, adding addi ional FeO will g ea ly inc ease he magne ic p ope ies [6].
Figu e 6. Magne ic suscep ibili y s. empe a u e cu e.
In addi ion o e omagne ic p ope ies, BFO also shows massi e spon aneous magne-
iza ion [20].
I has always been a end o ind a way o enhance he magne ic and e oelec ic
p ope ies o BFO ma e ial. Since magne ic and e oelec ic p ope ies a e g ea ly a ec ed
by oxygen bands, adding addi ional FeO will g ea ly inc ease he magne ic p ope ies [6].
E en while possessing he s onges e omagne ic esponse o mul i e oic p ope ies,
he mul i e oic p ope ies o BFO ma e ial emain weak o be implemen ed in o la ge-
scale de ices. In addi ion, molecules o BFO a e exposed o agg essi e oxida ion o he
oxygen a mosphe e. I has been a ho opic o many discussions o igu e ou a po en ial
solu ion o dec ease he pa asi ic cu en and enhance he mul i e oic esponse. The weak
e oelec ic and magne ic esponse (an i e omagne ic esponse) o la ge-scale de ices has
been a signi ican d awback o he ma e ial o wide implemen a ion (Figu e 7). Howe e ,

Ma e ials 2022,15, 8719 6 o 28
in he yea 2003, Ramesh de eloped a i icial he e os uc u es wi h signi ican ly highe
mul i e oic p ope ies [21].
Ma e ials 2022, 14, x FOR PEER REVIEW 6 o 28
E en while possessing he s onges e omagne ic esponse o mul i e oic p ope -
ies, he mul i e oic p ope ies o BFO ma e ial emain weak o be implemen ed in o
la ge-scale de ices. In addi ion, molecules o BFO a e exposed o agg essi e oxida ion o
he oxygen a mosphe e. I has been a ho opic o many discussions o igu e ou a po en-
ial solu ion o dec ease he pa asi ic cu en and enhance he mul i e oic esponse. The
weak e oelec ic and magne ic esponse (an i e omagne ic esponse) o la ge-scale de-
ices has been a signi ican d awback o he ma e ial o wide implemen a ion (Figu e 7).
Howe e , in he yea 2003, Ramesh de eloped a i icial he e os uc u es wi h signi ican ly
highe mul i e oic p ope ies [21].
Figu e 7. An i e omagne ic cu e o BFO.
3. Thin Films o Bismu h Fe i e
Bismu h e i e [14] has been unde in ense s udy because o he s onges and unique
combina ion o magne ic and e oelec ic p ope ies. In addi ion o unique mul i e oic
p ope ies, BFO demons a e a pho oelec ic, weak piezoelec ic esponse and s ong die-
lec ic p ope ies, which could be enhanced by adding addi ional impu i ies o a e–ea h
ma e ials.
3.1. Mo phology and Composi ion o P oduced Thin Film
Thin ilms ha e d awn a lo o a en ion and popula i y o be implemen ed in o many
p ospe ous applica ions and complex mic oelec onic de ices [22]. Thin ilms o BFO a e
ex emely popula nowadays due o he ex eme deg ee o e sa ili y [12,23]. Design o
hin ilms [24] o e s nume ous modi ica ions o mo phology (Figu e 8b), pa icle size
(Figu e 8a) and o e all chemical compound o p oduced ilms. The e sa ili y o hin ilms
is achie ed by a ious deposi ion me hods.
Figu e 8. Mo phology o a BFO hin ilm: (A) 2 µm; (B) 1 µm [25].
Figu e 7. An i e omagne ic cu e o BFO.
3. Thin Films o Bismu h Fe i e
Bismu h e i e [
14
] has been unde in ense s udy because o he s onges and unique
combina ion o magne ic and e oelec ic p ope ies. In addi ion o unique mul i e oic
p ope ies, BFO demons a e a pho oelec ic, weak piezoelec ic esponse and s ong
dielec ic p ope ies, which could be enhanced by adding addi ional impu i ies o a e–
ea h ma e ials.
3.1. Mo phology and Composi ion o P oduced Thin Film
Thin ilms ha e d awn a lo o a en ion and popula i y o be implemen ed in o many
p ospe ous applica ions and complex mic oelec onic de ices [
22
]. Thin ilms o BFO a e
ex emely popula nowadays due o he ex eme deg ee o e sa ili y [
12
,
23
]. Design o
hin ilms [
24
] o e s nume ous modi ica ions o mo phology (Figu e 8B), pa icle size
(Figu e 8A) and o e all chemical compound o p oduced ilms. The e sa ili y o hin ilms
is achie ed by a ious deposi ion me hods.
Ma e ials 2022, 14, x FOR PEER REVIEW 6 o 28
E en while possessing he s onges e omagne ic esponse o mul i e oic p ope -
ies, he mul i e oic p ope ies o BFO ma e ial emain weak o be implemen ed in o
la ge-scale de ices. In addi ion, molecules o BFO a e exposed o agg essi e oxida ion o
he oxygen a mosphe e. I has been a ho opic o many discussions o igu e ou a po en-
ial solu ion o dec ease he pa asi ic cu en and enhance he mul i e oic esponse. The
weak e oelec ic and magne ic esponse (an i e omagne ic esponse) o la ge-scale de-
ices has been a signi ican d awback o he ma e ial o wide implemen a ion (Figu e 7).
Howe e , in he yea 2003, Ramesh de eloped a i icial he e os uc u es wi h signi ican ly
highe mul i e oic p ope ies [21].
Figu e 7. An i e omagne ic cu e o BFO.
3. Thin Films o Bismu h Fe i e
Bismu h e i e [14] has been unde in ense s udy because o he s onges and unique
combina ion o magne ic and e oelec ic p ope ies. In addi ion o unique mul i e oic
p ope ies, BFO demons a e a pho oelec ic, weak piezoelec ic esponse and s ong die-
lec ic p ope ies, which could be enhanced by adding addi ional impu i ies o a e–ea h
ma e ials.
3.1. Mo phology and Composi ion o P oduced Thin Film
Thin ilms ha e d awn a lo o a en ion and popula i y o be implemen ed in o many
p ospe ous applica ions and complex mic oelec onic de ices [22]. Thin ilms o BFO a e
ex emely popula nowadays due o he ex eme deg ee o e sa ili y [12,23]. Design o
hin ilms [24] o e s nume ous modi ica ions o mo phology (Figu e 8b), pa icle size
(Figu e 8a) and o e all chemical compound o p oduced ilms. The e sa ili y o hin ilms
is achie ed by a ious deposi ion me hods.
Figu e 8. Mo phology o a BFO hin ilm: (A) 2 µm; (B) 1 µm [25].
Figu e 8. Mo phology o a BFO hin ilm: (A) 2 µm; (B) 1 µm [25].
BFO hin ilms demons a e di e en c ys allog aphic o ien a ions based on a ion o
la ice pa ame e s wi h a ious o ien a ion, wi h a ion angles being 71
◦
, 109
◦
and 180
◦
,
he ope a ion empe a u e du ing he deposi ion p ocess and subs a e mo phology. Thin
ilm can be syn hesized in o hombohed al, elaxed hombohed al (bulk), e agonal and
s ained e agonal phases. The e is a sligh change in he c ys allog aphic s uc u e o
subs a e s uc u e o bulk phase wi h inc easing ilm hickness caused by s ain elaxa ion.
This change e ec s he symme y o he esul ed BFO ilm [26].
As was men ioned p e iously, o enhance he pe o mance o BFO ma e ial, i is
necessa y o supp ess cycloid momen . Acco ding o undamen al s udies [
27
,
28
] hin ilms
Ma e ials 2022,15, 8719 7 o 28
wi h pa icles smalle han 62 nm ha e a signi ican inc ease in e omagne ic esponse.
Inc eased pe o mance o BFO hin ilms can be desc ibed as ollows: pa icle size s ays
wi hin he ange o 62 nm leads o he modula ion o spi al spin momen s esul ing in
g ea e enhancemen o e omagne ic p ope ies [
27
]. Wi h e en smalle pa icle size, BFO
ma e ial demons a es a negligible e omagne ic esponse due o insu icien compensa ion
o spin momen s. Uncompensa ed momen s esul in a change o bond angle and bond
leng h o e agonal and dec ease o il o oc ahed al FeO uni s. Co esponding o he i s
p incipal calcula ion, he e agonal bond angle is c ucial o he ans o ma ion be ween
e omagne ic and an i e omagne ic phases. The change o bond aFngle sugges s he e is a
hickness dependence o hin ilms simila o bulk.
The easies and comp ehensible way o o se spi al spin momen is by s aining
BFO ma e ial in o hin ilms (Figu e 9). I has been ound, ha unde op imal s ain
coe icien (a ound 4.05%) [
28
] (Figu e 10) p oduced BFO ilms [
28
] unde go symme ical
ans o ma ion which co esponds wi h a change o la ice pa ame e s a io.
Ma e ials 2022, 14, x FOR PEER REVIEW 7 o 28
BFO hin ilms demons a e di e en c ys allog aphic o ien a ions based on a ion o
la ice pa ame e s wi h a ious o ien a ion, wi h a ion angles being 71°, 109° and 180°,
he ope a ion empe a u e du ing he deposi ion p ocess and subs a e mo phology. Thin
ilm can be syn hesized in o hombohed al, elaxed hombohed al (bulk), e agonal and
s ained e agonal phases. The e is a sligh change in he c ys allog aphic s uc u e o
subs a e s uc u e o bulk phase wi h inc easing ilm hickness caused by s ain elaxa-
ion. This change e ec s he symme y o he esul ed BFO ilm [26].
As was men ioned p e iously, o enhance he pe o mance o BFO ma e ial, i is nec-
essa y o supp ess cycloid momen . Acco ding o undamen al s udies [27,28] hin ilms
wi h pa icles smalle han 62 nm ha e a signi ican inc ease in e omagne ic esponse.
Inc eased pe o mance o BFO hin ilms can be desc ibed as ollows: pa icle size s ays
wi hin he ange o 62 nm leads o he modula ion o spi al spin momen s esul ing in
g ea e enhancemen o e omagne ic p ope ies [27]. Wi h e en smalle pa icle size,
BFO ma e ial demons a es a negligible e omagne ic esponse due o insu icien com-
pensa ion o spin momen s. Uncompensa ed momen s esul in a change o bond angle
and bond leng h o e agonal and dec ease o il o oc ahed al FeO uni s. Co esponding
o he i s p incipal calcula ion, he e agonal bond angle is c ucial o he ans o ma ion
be ween e omagne ic and an i e omagne ic phases. The change o bond aFngle sug-
ges s he e is a hickness dependence o hin ilms simila o bulk.
The easies and comp ehensible way o o se spi al spin momen is by s aining BFO
ma e ial in o hin ilms (Figu e 9). I has been ound, ha unde op imal s ain coe icien
(a ound 4.05%) [28] (Figu e 10) p oduced BFO ilms [28] unde go symme ical ans o -
ma ion which co esponds wi h a change o la ice pa ame e s a io.
Figu e 9. BFO sample su ace.
The e is a de ini i e change in loca ion o ions in he hombohed al c ys al s uc u e
(Fe+3 oc ahed al uni s) compa ed o s uc u e wi h Fe+5 ca ions, sugges ing he phase sep-
a a ion in o e agonal and dis o ed phase o he igonal symme y g oup (R3c). Wi h
inc easing s ain, coe icien ilms demons a e me as able R3c–like phase wi h minimal
pe o mance enhancemen and phase sepa a ion [29]. Summa iza ion o hickness de-
pendence o BFO ilms on s ain coe icien is shown in Figu e 10 [28].
Figu e 9. BFO sample su ace.
Ma e ials 2022, 14, x FOR PEER REVIEW 8 o 28
Figu e 10. S ain coe icien dependence on hickness [28].
I was shown ha he emanen pola iza ion o p oduced ilms inc eases wi h e e y
laye o deposi ed BFO ma e ial, c ea ing a complex mul ilaye he e os uc u e (Figu e
11). P e e ed o ien a ion o deposi ed mul ilaye s is 110. Imp o emen o he emanen
pola iza ion alue is desc ibed by he lowe pa asi ic cu en o he mul ilaye sys em,
in e laye s ain and inc eased coupling be ween deposi ed laye s, esul ing in he in-
c ease o dis o ion in he BFO pe o ski e uni .
Figu e 11. Mul i–laye he e os uc u e o BFO BTO ma e ials.
Typical BFO hin ilms (Figu e 12) can be p oduced wi hin a wide ange o hick-
nesses. The mos common and widely sp ead ange o p oduced BFO ilms s ays wi hin
50 o 500 nm [28].
Figu e 12. Pu e BFO ilm su ace mo phology [30].
3.2. Pho oelec ical P ope ies
In addi ion o a unique combina ion o e oelec ic and magne ic p ope ies, i is
necessa y o men ion ha BFO-based hin ilms exhibi s ong pho oelec ic p ope ies.
Conside ing he smalle op ical bandgap (~2.5–2.7 eV) in compa ison o o he e oelec ic
ma e ial (Pb(Z ,Ti)O3, LiNbO3, BaTiO3) and wide ange o isible ligh abso p ion, BFO
Figu e 10. S ain coe icien dependence on hickness [28].
The e is a de ini i e change in loca ion o ions in he hombohed al c ys al s uc u e
(Fe
+3
oc ahed al uni s) compa ed o s uc u e wi h Fe
+5
ca ions, sugges ing he phase
sepa a ion in o e agonal and dis o ed phase o he igonal symme y g oup (R3c). Wi h
inc easing s ain, coe icien ilms demons a e me as able R3c–like phase wi h minimal pe -
o mance enhancemen and phase sepa a ion [
29
]. Summa iza ion o hickness dependence
o BFO ilms on s ain coe icien is shown in Figu e 10 [28].
I was shown ha he emanen pola iza ion o p oduced ilms inc eases wi h e e y
laye o deposi ed BFO ma e ial, c ea ing a complex mul ilaye he e os uc u e (Figu e 11).
P e e ed o ien a ion o deposi ed mul ilaye s is 110. Imp o emen o he emanen pola -
iza ion alue is desc ibed by he lowe pa asi ic cu en o he mul ilaye sys em, in e laye
Ma e ials 2022,15, 8719 8 o 28
s ain and inc eased coupling be ween deposi ed laye s, esul ing in he inc ease o dis o -
ion in he BFO pe o ski e uni .
Ma e ials 2022, 14, x FOR PEER REVIEW 8 o 28
Figu e 10. S ain coe icien dependence on hickness [28].
I was shown ha he emanen pola iza ion o p oduced ilms inc eases wi h e e y
laye o deposi ed BFO ma e ial, c ea ing a complex mul ilaye he e os uc u e (Figu e
11). P e e ed o ien a ion o deposi ed mul ilaye s is 110. Imp o emen o he emanen
pola iza ion alue is desc ibed by he lowe pa asi ic cu en o he mul ilaye sys em,
in e laye s ain and inc eased coupling be ween deposi ed laye s, esul ing in he in-
c ease o dis o ion in he BFO pe o ski e uni .
Figu e 11. Mul i–laye he e os uc u e o BFO BTO ma e ials.
Typical BFO hin ilms (Figu e 12) can be p oduced wi hin a wide ange o hick-
nesses. The mos common and widely sp ead ange o p oduced BFO ilms s ays wi hin
50 o 500 nm [28].
Figu e 12. Pu e BFO ilm su ace mo phology [30].
3.2. Pho oelec ical P ope ies
In addi ion o a unique combina ion o e oelec ic and magne ic p ope ies, i is
necessa y o men ion ha BFO-based hin ilms exhibi s ong pho oelec ic p ope ies.
Conside ing he smalle op ical bandgap (~2.5–2.7 eV) in compa ison o o he e oelec ic
ma e ial (Pb(Z ,Ti)O3, LiNbO3, BaTiO3) and wide ange o isible ligh abso p ion, BFO
Figu e 11. Mul i–laye he e os uc u e o BFO BTO ma e ials.
Typical BFO hin ilms (Figu e 12) can be p oduced wi hin a wide ange o hicknesses.
The mos common and widely sp ead ange o p oduced BFO ilms s ays wi hin 50 o
500 nm [28].
Ma e ials 2022, 14, x FOR PEER REVIEW 8 o 28
Figu e 10. S ain coe icien dependence on hickness [28].
I was shown ha he emanen pola iza ion o p oduced ilms inc eases wi h e e y
laye o deposi ed BFO ma e ial, c ea ing a complex mul ilaye he e os uc u e (Figu e
11). P e e ed o ien a ion o deposi ed mul ilaye s is 110. Imp o emen o he emanen
pola iza ion alue is desc ibed by he lowe pa asi ic cu en o he mul ilaye sys em,
in e laye s ain and inc eased coupling be ween deposi ed laye s, esul ing in he in-
c ease o dis o ion in he BFO pe o ski e uni .
Figu e 11. Mul i–laye he e os uc u e o BFO BTO ma e ials.
Typical BFO hin ilms (Figu e 12) can be p oduced wi hin a wide ange o hick-
nesses. The mos common and widely sp ead ange o p oduced BFO ilms s ays wi hin
50 o 500 nm [28].
Figu e 12. Pu e BFO ilm su ace mo phology [30].
3.2. Pho oelec ical P ope ies
In addi ion o a unique combina ion o e oelec ic and magne ic p ope ies, i is
necessa y o men ion ha BFO-based hin ilms exhibi s ong pho oelec ic p ope ies.
Conside ing he smalle op ical bandgap (~2.5–2.7 eV) in compa ison o o he e oelec ic
ma e ial (Pb(Z ,Ti)O3, LiNbO3, BaTiO3) and wide ange o isible ligh abso p ion, BFO
Figu e 12. Pu e BFO ilm su ace mo phology [30].
3.2. Pho oelec ical P ope ies
In addi ion o a unique combina ion o e oelec ic and magne ic p ope ies, i is
necessa y o men ion ha BFO-based hin ilms exhibi s ong pho oelec ic p ope ies.
Conside ing he smalle op ical bandgap (~2.5–2.7 eV) in compa ison o o he e oelec ic
ma e ial (Pb(Z ,Ti)O
3
, LiNbO
3
, BaTiO
3
) and wide ange o isible ligh abso p ion, BFO
ma e ial a ac s inc eased a en ion o po en ial pho o ol aic and/o pho ogal anic ap-
plica ions [
31
]. The pho o ol aic phenomenon has been encoun e ed in di e en o ms
o BFO ma e ial: c ys al s uc u e, ilms and BFO added ce amics. All o ms abo e o
he ma e ial hin ilms a e ex emely use ul o be implemen ed in o small- and la ge-scale
de ices, since i is uncomplica ed o ob ain signal om BFO ilms. Pho o ol aic p ope ies
o BFO hin ilms we e success ully implemen ed in o a ious de ices, o example, plana
pho ode ec o s [
32
], and complex senso s. Ne e heless, implemen a ion in o la ge-scale
de ices emains limi ed conside ing cons ained pe o mance o he ma e ial. Howe e ,
he pho o ol aic phenomenon is a ec ed by a a ie y o ac o s. A po en ial a ibu e o
pho oelec ic p ope ies is pho ocu en in ensi y, which is ela ed o quali y o p oduced
BFO ilms. Wi h smoo he and mo e homogeneous ilms he e is an inc ease o in e nal e-
combina ion o gene a ed elec on–hole pai s. The inc ease o ecombina ion a e (inc ease
o li e ime) [
33
] is caused by a dec ease o in e molecula (mig a ion) dis ance wi hin he
su ace o p oduced ilm, which, in i s own e ms, leads o a po en ial dec ease o cu en
in ensi y (inc ease o pho oconduc i i y). Some s udies p opose ha he pho o ol aic
esponse is a ec ed by ex e nal elec ic ield (pola iza ion) [
34
]. Di ec ion o pho o ol aic
esponse o p oduced ilms has been a ec ed by applied elec ical ield in di e en di-
Ma e ials 2022,15, 8719 9 o 28
ec ions [
34
,
35
]. As shown by Choi and his eam, pho o ol aic esponse in single c ys al
BFO ilms demons a e nonlinea beha io and he di ec ion o pho o ol aic esponse can
be changed by applied ol age. Fu he mo e, simila obse a ions o he dependance o
pho ocu en and elec ic ield has been pe o med by Yi [
36
]. In addi ion o he obse a ion,
he heo y o he e ec o he domain [
37
] wall angle on pho o ol aic esponse has been
p oposed. Acco ding o he heo y, domain walls, which sepa a e holes and elec ons, a e
es ima ed o be abou 100 nm, which is conside ably smalle in compa ison o con en ional
silicon semiconduc o s [38].
As con i med and obse ed by Choi [
35
] and Yang [
39
] on pu e BFO ma e ial wi h
pe iodical domain walls [
36
], pho o ol age la gely inc eases he bandgap o he BFO
ma e ial [
39
]. Yang has p o en ha he s onges pho o ol aic esponse was ob ained a he
71
◦
domain wall [
34
]. Ji and his eam ha e shown ha he bulk pho o ol aic e ec is i al
o he de e mina ion o pho o ol aic esponse o BFO ilms (Figu e 13) [
40
]. Fu he mo e,
he pho oelec ic esponse is dependen on he s uc u e o p oduced ilm. S ain o BFO
ilm could po en ially p o ide enhanced pho oconduc i i y and con ol o e pho o ol aic
e ec o he inal ilm. Fo example, s ained BFO ma e ial on LAO [
41
] exhibi s enhanced
pho oconduc i i y [42].
Ma e ials 2022, 14, x FOR PEER REVIEW 9 o 28
ma e ial a ac s inc eased a en ion o po en ial pho o ol aic and/o pho ogal anic ap-
plica ions [31]. The pho o ol aic phenomenon has been encoun e ed in di e en o ms o
BFO ma e ial: c ys al s uc u e, ilms and BFO added ce amics. All o ms abo e o he
ma e ial hin ilms a e ex emely use ul o be implemen ed in o small- and la ge-scale
de ices, since i is uncomplica ed o ob ain signal om BFO ilms. Pho o ol aic p ope ies
o BFO hin ilms we e success ully implemen ed in o a ious de ices, o example, plana
pho ode ec o s [32], and complex senso s. Ne e heless, implemen a ion in o la ge-scale
de ices emains limi ed conside ing cons ained pe o mance o he ma e ial. Howe e ,
he pho o ol aic phenomenon is a ec ed by a a ie y o ac o s. A po en ial a ibu e o
pho oelec ic p ope ies is pho ocu en in ensi y, which is ela ed o quali y o p oduced
BFO ilms. Wi h smoo he and mo e homogeneous ilms he e is an inc ease o in e nal
ecombina ion o gene a ed elec on–hole pai s. The inc ease o ecombina ion a e (in-
c ease o li e ime) [33] is caused by a dec ease o in e molecula (mig a ion) dis ance
wi hin he su ace o p oduced ilm, which, in i s own e ms, leads o a po en ial dec ease
o cu en in ensi y (inc ease o pho oconduc i i y). Some s udies p opose ha he pho o-
ol aic esponse is a ec ed by ex e nal elec ic ield (pola iza ion) [34]. Di ec ion o pho-
o ol aic esponse o p oduced ilms has been a ec ed by applied elec ical ield in di e -
en di ec ions [34,35]. As shown by Choi and his eam, pho o ol aic esponse in single
c ys al BFO ilms demons a e nonlinea beha io and he di ec ion o pho o ol aic e-
sponse can be changed by applied ol age. Fu he mo e, simila obse a ions o he de-
pendance o pho ocu en and elec ic ield has been pe o med by Yi [36]. In addi ion o
he obse a ion, he heo y o he e ec o he domain [37] wall angle on pho o ol aic
esponse has been p oposed. Acco ding o he heo y, domain walls, which sepa a e holes
and elec ons, a e es ima ed o be abou 100 nm, which is conside ably smalle in compa -
ison o con en ional silicon semiconduc o s [38].
As con i med and obse ed by Choi [35] and Yang [39] on pu e BFO ma e ial wi h
pe iodical domain walls [36], pho o ol age la gely inc eases he bandgap o he BFO ma-
e ial [39]. Yang has p o en ha he s onges pho o ol aic esponse was ob ained a he
71° domain wall [34]. Ji and his eam ha e shown ha he bulk pho o ol aic e ec is i al
o he de e mina ion o pho o ol aic esponse o BFO ilms (Figu e 13) [40]. Fu he mo e,
he pho oelec ic esponse is dependen on he s uc u e o p oduced ilm. S ain o BFO
ilm could po en ially p o ide enhanced pho oconduc i i y and con ol o e pho o ol aic
e ec o he inal ilm. Fo example, s ained BFO ma e ial on LAO [41] exhibi s enhanced
pho oconduc i i y [42].
Figu e 13. Sample o deposi ed BFO ma e ial.
Thin ilms o BFO ma e ial show an abso p ion a e o he wide isible ligh ange o
350 and 575 nm (Figu e 14). The wide isible ligh spec um means he ma e ial can abso b
huge amoun o ligh ene gy [43].
Figu e 13. Sample o deposi ed BFO ma e ial.
Thin ilms o BFO ma e ial show an abso p ion a e o he wide isible ligh ange o
350 and 575 nm (Figu e 14). The wide isible ligh spec um means he ma e ial can abso b
huge amoun o ligh ene gy [43].
Ma e ials 2022, 14, x FOR PEER REVIEW 10 o 28
Figu e 14. Abso p ion a e o isible ligh wa eleng h o BFO ma e ial.
Thin ilms o BFO ma e ial ha e become mo e popula o po en ial pho o ol aic ap-
plica ions due o he exis ence o anomalously la ge pho o ol age, which o e comes he
low bandgap o BFO ma e ial (~2.7 eV). Wi h he combina ion o pho ogal anic e ec ,
o e all chemical s abili y makes BFO wo hy o implemen a ion in o pho oelec ic and
pho oca aly ic de ices.
4. Deposi ion Me hods o BFO Thin Films
BFO is a unique ma e ial which shows e oelec ic and magne ic p ope ies a same
ime. Howe e , due o he cycloid momen o he cell uni , i is necessa y o o se cycloid
spin momen o ob ain he s onges e oelec ic esponse. One way o o se cycloid mo-
men is s aining ma e ial in o hin [25] ilms. I was a c ucial poin o discussion o ind
he mos cos –e ec i e and sui able me hod o p oducing he e os uc u es based on hin
ilms o BFO ma e ial wi h minimum po en ial de ec s.
The mos cos –e ec i e me hod o enhancing o e all p ope ies o p oduced ilms is
he implemen a ion o impu i ies. I was shown ha a e–ea h ma e ials d ama ically de-
c ease he pa asi ic cu en o BFO ilms because o he compensa ion o e apo a ed Bi
a oms and oxygen acancies due o changed o e all spin momen o a cell uni . E apo a-
ion o Bi a oms and oxygen acancies is an una oidable side e ec o p oduc ion o BFO
ilms. The mos p omising ma e ials o enhance mul i e oic p ope ies and dec ease pa -
asi ic cu en a e Mn [44] and Ni [45] due o simila a omic adius o Fe ca ions and chem-
ical alence s abili y (Table 1).
Table 1. Compa ison able o magne ic pa ame e s o pu e and doped by Mn BFO ma e ial.
Ma e ial
Coe ci i y (Oe)
Magne iza ion (emu)
Re en i i y(emu)
BFO
~70
~0.08
~920µ
BMnFO–0.1
~280
~0.12
~5.5 m
BMnFO–0.2
~130
~0.17
~3 m
These dopan s aim o subs i u e ca ions o Fe+2 o compensa e esidual spin momen ,
hus lowe ing he an i e omagne ic esponse o cell uni . Typical magne ic loop is ep e-
sen ed in Figu e 15. Recen epo s clam he implemen a ion o a e–ea h ma e ials lowe
cell alue, hus c ea ing compac su ace mo phology esul ing in a signi ican dec ease
in cu en densi y.
Figu e 14. Abso p ion a e o isible ligh wa eleng h o BFO ma e ial.
Thin ilms o BFO ma e ial ha e become mo e popula o po en ial pho o ol aic
applica ions due o he exis ence o anomalously la ge pho o ol age, which o e comes
he low bandgap o BFO ma e ial (~2.7 eV). Wi h he combina ion o pho ogal anic e ec ,
Ma e ials 2022,15, 8719 16 o 28
subs a e su ace. The e sa ili y o he ope a ion p ocess allows changing a ge ed ma e ial
sequen ially on a o a able holde which is a key ad an age o he p ocess. Sequen ial
change o a ge ed ma e ial is necessa y o p oduce mul i–laye complex s uc u es and he
de elopmen o new a i icial s uc u es o s able/me as able ma e ials wi hou in lic ing
agg essi e ope a ion condi ions by changing lase pa ame e s. An example o he mul i–
laye s uc u e o BFO ma e ial is ep esen ed in Figu e 22a,b.
Ma e ials 2022, 14, x FOR PEER REVIEW 16 o 28
Scandium Oxide (DyScO3), and S on ium i ana e (S TiO3, STO subs a e). Typical ope -
a ional pa ame e s o he PLD me hod include epe i ion a e, deposi ion ime, subs a e
p ehea empe a u e, lase pulses and main chambe p essu e. The o ien a ion o sub-
s a e will be a decisi e ac o o he esul ed BFO c ys al o ien a ion. Since he subs a e
is a ge ed by a lase , he c ys alliza ion o BFO is s a ed by c ea ing a c ys al ma ix o
Bi2O3 and Fe2O3 ma e ials wi h a di e en o ien a ion. The pulse a e epe i ion s ays
a ound 5–15 Hz [61,62] and may a y based on he ecipe. The wa eleng h o lase adia-
ion usually co esponds o a deep ul a iole colo (200–400 nm), which is he mos used
lase adia ion. To achie e maximum ene gy dis ibu ion on he a ge ed ma e ial he in-
cidence angle should s ay a ound 45° [66]. The en i e p ocess is accompanied by acuum
and/o he p esence o an ine gas o educe po en ial impu i ies. The du a ion o he
deposi ion p ocess is based on he hickness o p oduced ilm and pulses o he lase . The
ypical du a ion o he deposi ion cycle o 100nm ilm wi h 5000 pulses s ays wi hin 20
min. The deposi ion empe a u e o BFO ilms (subs a e) s ays unde 500 °C
Figu e 21. C ys allog aphy o p oduced c ys als based on he o ien a ion o subs a e.
PLD has conside able ad an ages compa ed o he con en ional echniques o he
p epa a ion o hin ilm ma e ials based on BFO ma e ial.
The majo supe io i y o PLD [64] is s oichiome y o p oduced ilms. S oichiome y
is caused by a high condensa ion a e o e apo a ed pa icles o a ge ed ma e ial on he
subs a e su ace. The e sa ili y o he ope a ion p ocess allows changing a ge ed ma e-
ial sequen ially on a o a able holde which is a key ad an age o he p ocess. Sequen ial
change o a ge ed ma e ial is necessa y o p oduce mul i–laye complex s uc u es and
he de elopmen o new a i icial s uc u es o s able/me as able ma e ials wi hou in lic -
ing agg essi e ope a ion condi ions by changing lase pa ame e s. An example o he
mul i–laye s uc u e o BFO ma e ial is ep esen ed in Figu e 22a,b.
Figu e 22. SEM image o a BFO ilm: (a) opog aphy; (b) c oss–sec ion [67].
The ope a ional pa ame e s o PLD, such as lase op ical ene gy [64,67] dis ibu ion,
he dis ance be ween a ge and subs a e, he subs a e empe a u e and g ow a e
g ea ly cons ain he nega i e e ec o mo phology and enhance magne ic and elec ical
p ope ies o he esul ed hin ilm s uc u es.
In compa ison o cos ly, con en ional me hods (selec i e ion implemen a ion) which
ope a e in a highe empe a u e ange, hus p oducing a ious ypes o de ec s and im-
pu i ies o deposi ed composi e, he PLD [64,67] me hod ope a es in a lowe empe a u e
ange.
Figu e 22. SEM image o a BFO ilm: (a) opog aphy; (b) c oss–sec ion [67].
The ope a ional pa ame e s o PLD, such as lase op ical ene gy [
64
,
67
] dis ibu ion,
he dis ance be ween a ge and subs a e, he subs a e empe a u e and g ow a e g ea ly
cons ain he nega i e e ec o mo phology and enhance magne ic and elec ical p ope ies
o he esul ed hin ilm s uc u es.
In compa ison o cos ly, con en ional me hods (selec i e ion implemen a ion) which
ope a e in a highe empe a u e ange, hus p oducing a ious ypes o de ec s and impu i-
ies o deposi ed composi e, he PLD [
64
,
67
] me hod ope a es in a lowe empe a u e ange.
The p ice o used subs a e is ano he disad an age o con en ional me hods. Consid-
e ing ha he PLD can ope a e in a lowe ange o empe a u es, he PLD me hod ope a es
wi h mo e a ailable and cheape polyme base subs a es [67].
I has been epo ed ha ilms p oduced by he PLD me hod show ela i ely low
pa asi ic cu en , conside ing he low amoun o c ys allog aphic de ec s and small pa icles
size [68].
4.4. Compa ison o ALD, PLD and Sol Gel Me hods
By compa ing he mos widesp ead me hods o he syn hesis BFO hin ilms, he sol
gel me hod was p o en o be he mos popula me hod o he p epa a ion o ilms in
huge quan i ies. In compa ison o PLD and ALD, he sol gel me hod (Table 2) is ela i ely
inexpensi e since i does no u ilize a acuum no ine gases du ing he p epa a ion
p ocedu e. The majo downside o he sol gel me hod is a signi ican ly high de ec a e
o he c ys allog aphic and mo phological s uc u e o p oduced ilms. The quali y o
p oduced ilms is based on pu i y o used chemical ing edien s such as wa e –soluble sal s
and chemical sol en s. The p esence o an oxygen a mosphe e has a nega i e e ec on he
p ope ies o BFO ilms due o occu ing oxide side p oduc s. Thin ilms p oduced by he
sol gel me hod exhibi a high pa asi ic cu en , which is gi en by he con amina ed su ace
due o he p esence o oxygen a mosphe e and impu i ies o chemical compounds. In
addi ion, ilms demons a e a negligible e oelec ic esponse gi en by inc eased pa icle
size. The sol gel me hod would be ex emely use ul o he p epa a ion o ilms o
applica ions whe e quali y o mo phology and p ope ies a e ole able such as ange
scale senso de ices. The sol gel me hod does no show e sa ili y o p oducing complex
he e os uc u es o BFO ma e ial, and ins ead allows o he p epa a ion o “sandwich”
s uc u es wi h di e en monolaye s.

Ma e ials 2022,15, 8719 17 o 28
Table 2. Compa ison able o deposi ion me hods.
Syn hesis Me hod Mo phology Thickness Cos Quali y
PLD Thin ilms, mic o/nano c ys als,
he e os uc u es. 1 nm–>1 µm High High
ALD Thin ilms, single laye deposi ion.
mic o/nano c ys als, he e os uc u es.
1 nm–>1 µm High High
Sol gel me hod Thin ilms 100 nm–>1 µm Low–medium Low
The ope a ion p inciples o PLD and ALD me hods sha e many simila i ies. Bo h
me hods use an ex e nal hea sou ce o e apo a e a small amoun o a ge ed ma e ial.
The e apo a ed ma e ial will condensa e on a coole subs a e su ace, c ea ing he de-
manded o ien a ion o hin ilm. Bo h me hods show a high deg ee o quali y o p oduced
ilms by ope a ing wi h he p esence o a acuum and ine gases o u he dec ease he
con amina ion a e and impu i ies o p oduced ilms. The me hods do no in ol e he
ope a ion wi h p esence o agg essi e chemical sol en s, which could ha e a nega i e e ec
on p oduced ilms. One majo ad an age o bo h me hods is hei ope a ion wi hin a low
empe a u e ange (<350–400
◦
C), in compa ison o he sol gel me hod (400–700
◦
C). The
p oduced hin ilms by PLD and ALD me hods show conside ably lowe pa asi ic cu en
and a high deg ee o s oichiome y. The key ad an age o bo h me hods is he e ec i e
deposi ion o complex he e os uc u es wi h di e en c ys allog aphic o ien a ion wi h
di e en ma e ial monolaye s. Bo h me hods o e a wide ange o hicknesses o p oduced
ilms. The hickness o he inal p oduc a ies om se e al angs oms o hund eds o
nanome e s whe e minimal hickness o p oduced ilms by he sol gel me hod is 100 nm
and maximum may inc ease se e al
µ
m. The e sa ili y and high quali y o p oduced ilms
comes wi h inc eased coas s o bo h me hods, which is a signi ican disad an age o hese
me hods. The high cos o bo h me hods is gi en by he u iliza ion o lase echnologies,
acuum and ine gases, expensi e ma e ials, and equipmen . Bo h me hods a e sui able
he p epa a ion o hin ilms in small quan i ies wi h supe io p ope ies and mo phology
o p ecise measu emen de ices o p o o ypes.
5. Side Phases o Bismu h Fe i e
The need o cos –e ec i e ene gy sou ces in ol e seeking new and p ospec i e ma-
e ials and echnologies. Conside ing e e inc easing pollu ion and apid de elopmen
o he manu ac u ing indus y, he use o sola ene gy has become an undi ided pa
o he mode n end o u iliza ion o eco– iendly ene gy sou ces. Nowadays ends o
designing mic oelec onic de ises o e new challenges, in ol ing he design o mic o-
elec onic de ices which would be based on manipula ion by elec ic and magne ic pa s.
These challenges equi e in es iga ion and ecache o new ma e ials. Among all ma e ials,
mul i e oics and especially Bismu h e i e, s ands ou . The BFO ma e ial aims o be a
po en ial solu ion ega ding simul aneous con ol o magne ic and elec ic pa s, since BFO
exhibi s he s onges e oelec ic and magne ic esponse a oom empe a u e.
5.1. Bismu h Mulli e Ma e ial
Du ing in ensi e s udying o Bismu h e i e ma e ial p epa a ion me hods, i was
ound ha du ing he ini ial p epa a ion p ocess o BFO powde s, alongside pu e BFO
ma e ial, o he side–phase ma e ials we e syn hesized. I was shown, due o he agg essi e
chemical na u e [
4
] o bismu h, ha BFO causes byp oduc s. These ma e ials ha e been
ca ego ized as impu i ies o he BFO ma e ial. The wo mos popula byp oduc s o
syn hesis o pu e BFO ma e ial a e bismu h Mulli e and I on Seleni e. Ins ead o c ys alizing
he single–phase o he BFO ma e ial, mul i–phase side p oduc s we e c ys alized [
69
]. The
mechanism o c ys alliza ion side p oduc s is ela ed o he s oichiome ic imbalance [
70
]
o BFO [
71
]. The s oichiome ic imbalance e e s o he asymme ical g ow h o FeO
monome s (some pa icles could e apo a e du ing c ys alliza ion), which a e pa icles
Ma e ials 2022,15, 8719 18 o 28
ha can unde go side eac ions. The chemical equa ion Bi
x
Fe
y
O1.5
x
+ 1.5
y
[
72
] desc ibes
he gene al composi ion o a single–phase BFO [
1
] ma e ial and seconda y phase o new
side p oduc s.
The i s and mos p ominen byp oduc [
73
] o he syn hesis pu e BFO ma e ial
is mulli e e i e Bi
2
Fe
4
O
9
. The ypical cell uni o he mulli e ma e ial is ep esen ed
in Figu e 23. The c ys allog aphic s uc u e o he mulli e is ep esen ed by a cen al
symme ic [
74
], o ho hombic s uc u e [
73
] wi h wo a oms o Bi
+2
, ou a oms o Fe
+2
,
and 9 a oms o oxygen [
75
] wi h di e en alency. The la ice pa ame e s a e a=b=8 Å [
1
],
c=6 Å [
74
] and he cell alue is a ound 400–410 Å [
74
,
76
]. The pa ame e s may di e en ia e
due o s uc u e de ec s du ing he p epa a ion p ocess, and oxygen acancies [
74
]. A
ypical Bi
2
Fe
4
O
9
[
77
] cell uni consis s o FeO
4
e ahed a [
73
,
75
] and FeO
6
oc ahed a [
74
]
o ma ion. The oc ahed al [
78
] bond dis ance is app oxima ely 2 Å, and he e ahed a
dis ance is smalle and s ays wi hin 1.8 Å [74,79] (Figu e 24).
Ma e ials 2022, 14, x FOR PEER REVIEW 18 o 28
disad an age o hese me hods. The high cos o bo h me hods is gi en by he u iliza ion
o lase echnologies, acuum and ine gases, expensi e ma e ials, and equipmen . Bo h
me hods a e sui able he p epa a ion o hin ilms in small quan i ies wi h supe io p op-
e ies and mo phology o p ecise measu emen de ices o p o o ypes.
5. Side Phases o Bismu h Fe i e
The need o cos –e ec i e ene gy sou ces in ol e seeking new and p ospec i e ma-
e ials and echnologies. Conside ing e e inc easing pollu ion and apid de elopmen o
he manu ac u ing indus y, he use o sola ene gy has become an undi ided pa o he
mode n end o u iliza ion o eco– iendly ene gy sou ces. Nowadays ends o designing
mic oelec onic de ises o e new challenges, in ol ing he design o mic oelec onic de-
ices which would be based on manipula ion by elec ic and magne ic pa s. These chal-
lenges equi e in es iga ion and ecache o new ma e ials. Among all ma e ials, mul i e -
oics and especially Bismu h e i e, s ands ou . The BFO ma e ial aims o be a po en ial
solu ion ega ding simul aneous con ol o magne ic and elec ic pa s, since BFO exhibi s
he s onges e oelec ic and magne ic esponse a oom empe a u e.
5.1. Bismu h Mulli e Ma e ial
Du ing in ensi e s udying o Bismu h e i e ma e ial p epa a ion me hods, i was
ound ha du ing he ini ial p epa a ion p ocess o BFO powde s, alongside pu e BFO
ma e ial, o he side–phase ma e ials we e syn hesized. I was shown, due o he agg es-
si e chemical na u e [4] o bismu h, ha BFO causes byp oduc s. These ma e ials ha e
been ca ego ized as impu i ies o he BFO ma e ial. The wo mos popula byp oduc s o
syn hesis o pu e BFO ma e ial a e bismu h Mulli e and I on Seleni e. Ins ead o c ys al-
izing he single–phase o he BFO ma e ial, mul i–phase side p oduc s we e c ys alized
[69]. The mechanism o c ys alliza ion side p oduc s is ela ed o he s oichiome ic imbal-
ance [70] o BFO [71]. The s oichiome ic imbalance e e s o he asymme ical g ow h o
FeO monome s (some pa icles could e apo a e du ing c ys alliza ion), which a e pa i-
cles ha can unde go side eac ions. The chemical equa ion BixFeyO1.5x + 1.5y [72] de-
sc ibes he gene al composi ion o a single–phase BFO [1] ma e ial and seconda y phase
o new side p oduc s.
The i s and mos p ominen byp oduc [73] o he syn hesis pu e BFO ma e ial is
mulli e e i e Bi2Fe4O9. The ypical cell uni o he mulli e ma e ial is ep esen ed in Figu e
23. The c ys allog aphic s uc u e o he mulli e is ep esen ed by a cen al symme ic [74],
o ho hombic s uc u e [73] wi h wo a oms o Bi+2, ou a oms o Fe+2, and 9 a oms o
oxygen [75] wi h di e en alency. The la ice pa ame e s a e a=b=8 Å [1], c=6 Å [74] and
he cell alue is a ound 400–410 Å [74,76]. The pa ame e s may di e en ia e due o s uc-
u e de ec s du ing he p epa a ion p ocess, and oxygen acancies [74]. A ypical Bi2Fe4O9
[77] cell uni consis s o FeO4 e ahed a [73,75] and FeO6 oc ahed a [74] o ma ion. The
oc ahed al [78] bond dis ance is app oxima ely 2 Å , and he e ahed a dis ance is smalle
and s ays wi hin 1.8 Å [74,79] (Figu e 24).
Figu e 23. Cell uni o Bi2Fe4O9 [80].
Figu e 23. Cell uni o Bi2Fe4O9[80].
Ma e ials 2022, 14, x FOR PEER REVIEW 19 o 28
Figu e 24. The cell uni o Bi2Fe4O9 [78].
Fe+3 ca ions a e in bo h e ahed a and oc ahed a o ma ions. FeOn a e dis ibu ed
homogeneously ac oss he uni cell and a e su ounded by Bi ions. Spin momen s o oc-
ahed al [81] FeO6 c ea e couples wi h elec ons o e ahed al he FeO4 spin momen s. The
coupling o FeOn spin momen s lead o a s onge in e ac ion be ween he wo monome s,
hus causing s ong an i– e omagne ic p ope ies [82] o he uni cell (G– ype o de ing).
The an i- e omagne ic esponse is gi en by he in e ac ion o coupled elec on spin mo-
men s o FeO4 and FeO6 monome s and une en spin dis ibu ion, causing o e all geome -
ical dis o ion o spin momen s [82].
I was epo ed ha he mulli e also exhibi s an i– e omagne ic p ope ies close o
he 0 °C [73] empe a u e. The pa amagne ic ansi ion empe a u e o an i e omagne ic
(Neel) o he compound s ays wi hin −23 °C as Cu ie empe a u e was de e mined o be
in he ange o −9 o 0 °C. In addi ion o an i– e omagne ic p ope ies, he mulli e e i e
demons a es e omagne ic o de ing. Fe omagne ic p ope ies a e obse ed by pa allel
o ien a ion o spin momen s ac oss one dimensional axis. Fe oelec ic p ope ies o mul-
li e e i e a e po en ially desc ibed by asymme ical dis ibu ion o hyb idiza ion be-
ween s– and p– o bi als o bismu h ca ions and oxygen acancies.
Howe e , o e all asymme ical us a ion migh be s abilized by he in e ac ion o
SP hyb idiza ion o Bismu h ions and oxygen anions.
Magne ic and e oelec ic p ope ies ( esponse) (Figu e 25) o he mulli e ma e ial
a e size dependen and show simila beha io o pu e BFO ma e ial (Figu e 26). I was
shown ha he smalle pa icles demons a e a s onge e omagne ic esponse, in com-
pa ison o g ea e pa icle size. The inc ease in e omagne ic p ope ies could be de-
sc ibed by a dec eased in e ac ion o e ahed al and oc ahed al spin momen s, changing
he spin layou o a cell uni .
Figu e 25. Hys e esis loop o Mulli e e i e ma e ial a oom empe a u e.
Figu e 24. The cell uni o Bi2Fe4O9[78].
Fe
+3
ca ions a e in bo h e ahed a and oc ahed a o ma ions. FeO
n
a e dis ibu ed
homogeneously ac oss he uni cell and a e su ounded by Bi ions. Spin momen s o oc ahe-
d al [
81
] FeO
6
c ea e couples wi h elec ons o e ahed al he FeO
4
spin momen s. The cou-
pling o FeO
n
spin momen s lead o a s onge in e ac ion be ween he wo monome s, hus
causing s ong an i– e omagne ic p ope ies [
82
] o he uni cell (G– ype o de ing). The
an i- e omagne ic esponse is gi en by he in e ac ion o coupled elec on spin momen s
o FeO
4
and FeO
6
monome s and une en spin dis ibu ion, causing o e all geome ical
dis o ion o spin momen s [82].
I was epo ed ha he mulli e also exhibi s an i– e omagne ic p ope ies close o
he 0
◦
C [
73
] empe a u e. The pa amagne ic ansi ion empe a u e o an i e omagne ic
(Neel) o he compound s ays wi hin
−
23
◦
C as Cu ie empe a u e was de e mined o be
in he ange o
−
9 o 0
◦
C. In addi ion o an i– e omagne ic p ope ies, he mulli e e i e
demons a es e omagne ic o de ing. Fe omagne ic p ope ies a e obse ed by pa allel
o ien a ion o spin momen s ac oss one dimensional axis. Fe oelec ic p ope ies o mulli e
Ma e ials 2022,15, 8719 19 o 28
e i e a e po en ially desc ibed by asymme ical dis ibu ion o hyb idiza ion be ween s–
and p– o bi als o bismu h ca ions and oxygen acancies.
Howe e , o e all asymme ical us a ion migh be s abilized by he in e ac ion o SP
hyb idiza ion o Bismu h ions and oxygen anions.
Magne ic and e oelec ic p ope ies ( esponse) (Figu e 25) o he mulli e ma e ial a e
size dependen and show simila beha io o pu e BFO ma e ial (Figu e 26). I was shown
ha he smalle pa icles demons a e a s onge e omagne ic esponse, in compa ison
o g ea e pa icle size. The inc ease in e omagne ic p ope ies could be desc ibed by
a dec eased in e ac ion o e ahed al and oc ahed al spin momen s, changing he spin
layou o a cell uni .
Ma e ials 2022, 14, x FOR PEER REVIEW 19 o 28
Figu e 24. The cell uni o Bi2Fe4O9 [78].
Fe+3 ca ions a e in bo h e ahed a and oc ahed a o ma ions. FeOn a e dis ibu ed
homogeneously ac oss he uni cell and a e su ounded by Bi ions. Spin momen s o oc-
ahed al [81] FeO6 c ea e couples wi h elec ons o e ahed al he FeO4 spin momen s. The
coupling o FeOn spin momen s lead o a s onge in e ac ion be ween he wo monome s,
hus causing s ong an i– e omagne ic p ope ies [82] o he uni cell (G– ype o de ing).
The an i- e omagne ic esponse is gi en by he in e ac ion o coupled elec on spin mo-
men s o FeO4 and FeO6 monome s and une en spin dis ibu ion, causing o e all geome -
ical dis o ion o spin momen s [82].
I was epo ed ha he mulli e also exhibi s an i– e omagne ic p ope ies close o
he 0 °C [73] empe a u e. The pa amagne ic ansi ion empe a u e o an i e omagne ic
(Neel) o he compound s ays wi hin −23 °C as Cu ie empe a u e was de e mined o be
in he ange o −9 o 0 °C. In addi ion o an i– e omagne ic p ope ies, he mulli e e i e
demons a es e omagne ic o de ing. Fe omagne ic p ope ies a e obse ed by pa allel
o ien a ion o spin momen s ac oss one dimensional axis. Fe oelec ic p ope ies o mul-
li e e i e a e po en ially desc ibed by asymme ical dis ibu ion o hyb idiza ion be-
ween s– and p– o bi als o bismu h ca ions and oxygen acancies.
Howe e , o e all asymme ical us a ion migh be s abilized by he in e ac ion o
SP hyb idiza ion o Bismu h ions and oxygen anions.
Magne ic and e oelec ic p ope ies ( esponse) (Figu e 25) o he mulli e ma e ial
a e size dependen and show simila beha io o pu e BFO ma e ial (Figu e 26). I was
shown ha he smalle pa icles demons a e a s onge e omagne ic esponse, in com-
pa ison o g ea e pa icle size. The inc ease in e omagne ic p ope ies could be de-
sc ibed by a dec eased in e ac ion o e ahed al and oc ahed al spin momen s, changing
he spin layou o a cell uni .
Figu e 25. Hys e esis loop o Mulli e e i e ma e ial a oom empe a u e.
Figu e 25. Hys e esis loop o Mulli e e i e ma e ial a oom empe a u e.
Ma e ials 2022, 14, x FOR PEER REVIEW 20 o 28
Figu e 26. Bi2Fe4O9 ilm: (a) opology; (b) c oss–sec ion [82].
Recen epo s claim he e oelec ic esponse is negligible, conside ing he symme -
ical o ien a ion o uni cell by he cen al axis. The e o e, i is necessa y o ind a sui able
solu ion o enhance he magne ic and e oelec ic p ope ies o he mulli e o u u e im-
plemen a ion in o mul i unc ioning de ices. A po en ial solu ion would be adding impu-
i ies o a e–ea h ma e ial o compensa e oxygen acancies. I was p o en ha a e–ea h
ma e ial, such as lan hanum and magnesium, d ama ically enhance e oelec ic p ope -
ies [73].
Mulli e is widely sp ead [83] in chemis y and senso de ices. The ma e ial is ex-
emely sensi i e on alkali apo s and was success ully used as a gas [84] leakage senso .
The mulli e p o es o be a g ea and cheap ca alyze o chemical eac ions, such as he
decomposi ion o ammonia o ni a e oxide.
Recen esea ch epo s he exis ence o di e en eac ions o isible–ligh abso p ion,
which sugges s he ma e ial possesses pho oca alys [85] and pho oelec ic p ope ies.
Hence, Mulli e can be used o he u iliza ion o sola [86] adia ion. The ma e ial shows
wa eleng hs abso p ion wi hin he ange o 350–700 nm [86] (Figu e 27). The ange o
wa eleng hs co esponds wi h 2–3 eV [79].
Figu e 27. Ligh abso p ion cu e o Bi2Fe4O9.
The pho oca aly ic [85,87] p ope ies p o e ha he ma e ial has a lowe bandgap
po en ial [88], which is a conside able ad an age, and make he ma e ial a ac i e o
ligh de ec ion senso s and pho o ol aic applica ion [88].
Mulli e is a p o en, e y p omising ma e ial o be implemen ed in o new ypes o
senso s. Mulli e is widely used in he a ea o senso s o de ec ing gas leakage due o i s
chemical na u e, and in o ganic chemis y as an enhance o chemical eac ions [1]. The
ull po en ial o he ma e ial is s ill no e ealed; hus, mo e and deepe s udying is e-
qui ed.
5.2. I on Seleni e Ma e ial
Bismu h e i e is ep esen ed in wo di e en s uc u es: pe o ski e cell and Seleni e
ma e ial. I on Seleni e ma e ial (Bi25FeO40) [87,89] has been encoun e ed du ing he syn-
hesis o pu e phase BFO ma e ial. Seleni e d aws a lo o a en ion nowadays due o i s
Figu e 26. Bi2Fe4O9 ilm: (a) opology; (b) c oss–sec ion [82].
Recen epo s claim he e oelec ic esponse is negligible, conside ing he symme i-
cal o ien a ion o uni cell by he cen al axis. The e o e, i is necessa y o ind a sui able
solu ion o enhance he magne ic and e oelec ic p ope ies o he mulli e o u u e
implemen a ion in o mul i unc ioning de ices. A po en ial solu ion would be adding
impu i ies o a e–ea h ma e ial o compensa e oxygen acancies. I was p o en ha
a e–ea h ma e ial, such as lan hanum and magnesium, d ama ically enhance e oelec ic
p ope ies [73].
Mulli e is widely sp ead [
83
] in chemis y and senso de ices. The ma e ial is ex emely
sensi i e on alkali apo s and was success ully used as a gas [
84
] leakage senso . The mulli e
p o es o be a g ea and cheap ca alyze o chemical eac ions, such as he decomposi ion
o ammonia o ni a e oxide.
Recen esea ch epo s he exis ence o di e en eac ions o isible–ligh abso p ion,
which sugges s he ma e ial possesses pho oca alys [
85
] and pho oelec ic p ope ies.
Hence, Mulli e can be used o he u iliza ion o sola [
86
] adia ion. The ma e ial shows
wa eleng hs abso p ion wi hin he ange o 350–700 nm [
86
] (Figu e 27). The ange o
wa eleng hs co esponds wi h 2–3 eV [79].
Ma e ials 2022,15, 8719 20 o 28
Ma e ials 2022, 14, x FOR PEER REVIEW 20 o 28
Figu e 26. Bi2Fe4O9 ilm: (a) opology; (b) c oss–sec ion [82].
Recen epo s claim he e oelec ic esponse is negligible, conside ing he symme -
ical o ien a ion o uni cell by he cen al axis. The e o e, i is necessa y o ind a sui able
solu ion o enhance he magne ic and e oelec ic p ope ies o he mulli e o u u e im-
plemen a ion in o mul i unc ioning de ices. A po en ial solu ion would be adding impu-
i ies o a e–ea h ma e ial o compensa e oxygen acancies. I was p o en ha a e–ea h
ma e ial, such as lan hanum and magnesium, d ama ically enhance e oelec ic p ope -
ies [73].
Mulli e is widely sp ead [83] in chemis y and senso de ices. The ma e ial is ex-
emely sensi i e on alkali apo s and was success ully used as a gas [84] leakage senso .
The mulli e p o es o be a g ea and cheap ca alyze o chemical eac ions, such as he
decomposi ion o ammonia o ni a e oxide.
Recen esea ch epo s he exis ence o di e en eac ions o isible–ligh abso p ion,
which sugges s he ma e ial possesses pho oca alys [85] and pho oelec ic p ope ies.
Hence, Mulli e can be used o he u iliza ion o sola [86] adia ion. The ma e ial shows
wa eleng hs abso p ion wi hin he ange o 350–700 nm [86] (Figu e 27). The ange o
wa eleng hs co esponds wi h 2–3 eV [79].
Figu e 27. Ligh abso p ion cu e o Bi2Fe4O9.
The pho oca aly ic [85,87] p ope ies p o e ha he ma e ial has a lowe bandgap
po en ial [88], which is a conside able ad an age, and make he ma e ial a ac i e o
ligh de ec ion senso s and pho o ol aic applica ion [88].
Mulli e is a p o en, e y p omising ma e ial o be implemen ed in o new ypes o
senso s. Mulli e is widely used in he a ea o senso s o de ec ing gas leakage due o i s
chemical na u e, and in o ganic chemis y as an enhance o chemical eac ions [1]. The
ull po en ial o he ma e ial is s ill no e ealed; hus, mo e and deepe s udying is e-
qui ed.
5.2. I on Seleni e Ma e ial
Bismu h e i e is ep esen ed in wo di e en s uc u es: pe o ski e cell and Seleni e
ma e ial. I on Seleni e ma e ial (Bi25FeO40) [87,89] has been encoun e ed du ing he syn-
hesis o pu e phase BFO ma e ial. Seleni e d aws a lo o a en ion nowadays due o i s
Figu e 27. Ligh abso p ion cu e o Bi2Fe4O9.
The pho oca aly ic [
85
,
87
] p ope ies p o e ha he ma e ial has a lowe bandgap
po en ial [
88
], which is a conside able ad an age, and make he ma e ial a ac i e o ligh
de ec ion senso s and pho o ol aic applica ion [88].
Mulli e is a p o en, e y p omising ma e ial o be implemen ed in o new ypes o
senso s. Mulli e is widely used in he a ea o senso s o de ec ing gas leakage due o i s
chemical na u e, and in o ganic chemis y as an enhance o chemical eac ions [
1
]. The ull
po en ial o he ma e ial is s ill no e ealed; hus, mo e and deepe s udying is equi ed.
5.2. I on Seleni e Ma e ial
Bismu h e i e is ep esen ed in wo di e en s uc u es: pe o ski e cell and Seleni e
ma e ial. I on Seleni e ma e ial (Bi
25
FeO
40
) [
87
,
89
] has been encoun e ed du ing he syn he-
sis o pu e phase BFO ma e ial. Seleni e d aws a lo o a en ion nowadays due o i s unique
and s ong pho oca aly ic [
90
] p ope ies [
91
], which is a popula subjec o esea ch. Selen-
i e belongs o semiconduc o amily and was p o en o be a e y eco– iendly ma e ial o
a wide a ie y o applica ions. In addi ion, Seleni e has become an ou s anding candida e
o sola ligh u iliza ion echnologies due o he slowe a e o sunligh deg ada ion, low
bandgap po en ial [
91
,
92
] and o e all ha sh chemical en i onmen esis ance. A supe io
pa amagne ic esponse is ano he ad an age o he ma e ial, making i easily ob ained and
sepa a ed om pu e BFO ma e ial du ing he syn hesis p ocess. The exis ence o a hys e e-
sis cu e nea oom empe a u e ange sugges s e oelec ic and dielec ic p ope ies o
Seleni e ma e ial.
Seleni e ma e ial (Figu e 28) occu s as ano he side p oduc o he syn hesis o pu e
BFO ma e ial. I was p o en by many expe imen s [
89
,
92
,
93
] ha he mos widesp ead
me hods o ob ain pu e phase Seleni e ma e ial a e he sol gel and hyd o he mal g ow
me hods. Seleni e is usually ob ained om a chemical p ecu so o bismu h ni ide and
i on ni ide wa e –soluble sal s wi h an ope a ional empe a u e a ound 650–750
◦
C [
92
,
94
].
Ma e ials 2022, 14, x FOR PEER REVIEW 21 o 28
unique and s ong pho oca aly ic [90] p ope ies [91], which is a popula subjec o e-
sea ch. Seleni e belongs o semiconduc o amily and was p o en o be a e y eco– iendly
ma e ial o a wide a ie y o applica ions. In addi ion, Seleni e has become an ou s and-
ing candida e o sola ligh u iliza ion echnologies due o he slowe a e o sunligh
deg ada ion, low bandgap po en ial [91,92] and o e all ha sh chemical en i onmen e-
sis ance. A supe io pa amagne ic esponse is ano he ad an age o he ma e ial, making
i easily ob ained and sepa a ed om pu e BFO ma e ial du ing he syn hesis p ocess. The
exis ence o a hys e esis cu e nea oom empe a u e ange sugges s e oelec ic and
dielec ic p ope ies o Seleni e ma e ial.
Seleni e ma e ial (Figu e 28) occu s as ano he side p oduc o he syn hesis o pu e
BFO ma e ial. I was p o en by many expe imen s [89,92,93] ha he mos widesp ead
me hods o ob ain pu e phase Seleni e ma e ial a e he sol gel and hyd o he mal g ow
me hods. Seleni e is usually ob ained om a chemical p ecu so o bismu h ni ide and
i on ni ide wa e –soluble sal s wi h an ope a ional empe a u e a ound 650–750 °C
[92,94].
Figu e 28. I on Seleni e powde c ys als [91].
The c ys allog aphic s uc u e o he Seleni e uni (Figu e 28) is ep esen ed by cubic
asymme ical I23 space o ien a ion wi h la ice pa ame e s a = b = c = 10 Å and he es i-
ma ed c ys al alue is abou 250 Å . In his o ien a ion bismu h ca ions wi h alence Bi+3
[92], oge he wi h su ounding oxygen a oms, o m incomple e BiO5 oc ahed al uni s.
The in e a omic dis ance be ween Bi+3 and sha ed oxygen a oms is a ound 2–2.5 Å . The
oc ahed al uni is comple ed by he ine 6s2 [92] pai . An aniso opic ib a ion is shown
in Bi+3 a oms. Each Fe+3 [92] and Bi+5 [92] ca ion o ms e ahed al [92] uni s. Bi+5 ions cause
dis ibu ion no only o he e ahed al uni , bu also o he en i e chemical s uc u e. Bi+5
[92] ca ions a e p esen in he cell uni which leads o he chemical o mula o
Bi24+3(Bi+5Fe+3)O40 [92] wi h s oichiome ic o ma ion.
Seleni e shows weak e omagne ic ac i i y wi h ansi ional empe a u e, a ound –
5–0 C° [92], and signi ican spon aneous magne iza ion a oom empe a u e.
Seleni e is conside ed a pe ec candida e o he pho o–Fen on [95,96] eac ion en-
hance . The pho o–Fen on [95,96] eac ions s and o ad ance oxida ion whe e o ganic
compounds a e decomposed and disin ec in wa e . Ne e heless, insu icien su ace
ansi ion o Fe+2/Fe+3 [92] limi s he pho o–Fen on [95] (Figu e 29) eac ion enhancing ac-
i i y o he ma e ial. I on Seleni e shows ema kable p ope ies, which a e use ul in pie-
zoelec ic [92] and especially in pho o [97] applica ions.
Figu e 28. I on Seleni e powde c ys als [91].
Ma e ials 2022,15, 8719 21 o 28
The c ys allog aphic s uc u e o he Seleni e uni (Figu e 28) is ep esen ed by cubic
asymme ical I23 space o ien a ion wi h la ice pa ame e s a = b = c = 10 Å and he
es ima ed c ys al alue is abou 250 Å. In his o ien a ion bismu h ca ions wi h alence
Bi
+3
[
92
], oge he wi h su ounding oxygen a oms, o m incomple e BiO
5
oc ahed al
uni s. The in e a omic dis ance be ween Bi
+3
and sha ed oxygen a oms is a ound
2–2.5 Å
.
The oc ahed al uni is comple ed by he ine 6s
2
[
92
] pai . An aniso opic ib a ion is
shown in Bi
+3
a oms. Each Fe
+3
[
92
] and Bi
+5
[
92
] ca ion o ms e ahed al [
92
] uni s. Bi
+5
ions cause dis ibu ion no only o he e ahed al uni , bu also o he en i e chemical
s uc u e. Bi
+5
[
92
] ca ions a e p esen in he cell uni which leads o he chemical o mula
o Bi24+3(Bi+5Fe+3)O40 [92] wi h s oichiome ic o ma ion.
Seleni e shows weak e omagne ic ac i i y wi h ansi ional empe a u e, a ound
–5–0 C◦[92], and signi ican spon aneous magne iza ion a oom empe a u e.
Seleni e is conside ed a pe ec candida e o he pho o–Fen on [
95
,
96
] eac ion en-
hance . The pho o–Fen on [
95
,
96
] eac ions s and o ad ance oxida ion whe e o ganic
compounds a e decomposed and disin ec in wa e . Ne e heless, insu icien su ace
ansi ion o Fe
+2
/Fe
+3
[
92
] limi s he pho o–Fen on [
95
] (Figu e 29) eac ion enhancing
ac i i y o he ma e ial. I on Seleni e shows ema kable p ope ies, which a e use ul in
piezoelec ic [92] and especially in pho o [97] applica ions.
Ma e ials 2022, 14, x FOR PEER REVIEW 22 o 28
Figu e 29. Pho oca aly ic [98] eac ion mechanism o i on Seleni e and Mulli e unde ligh adia-
ion
Conside ing e en he lowe band gap [99] o I on-Seleni e, in compa ison o pu e
Bismu h e i e (2–3 eV [100]), he ma e ial was p o en o be an excep ionally good abso -
ben o ul a iole [101] and isible ligh adia ion a ac ing much a en ion. The band
gap [102] o I on Seleni e s ands unde 2 eV [92], which is much lowe among mos o he
con en ional pho o–ac i e ma e ials. By analyzing he abso p ion [99] ac i i y, i was
ound ha he abso p ion a e o I on Seleni e is e en highe compa ed o Mulli e Fe i e.
The wa eleng h abso p ion [103] ange o Mulli e s ays wi hin 600–850 nm, whe eas o
I on Seleni e i is much wide wi hin 500–900 nm [99,101] (Figu e 30), and wi h a smo he
abso p ion a e a highe wa eleng hs, esul ing in s onge op ical abso p ion capabili-
ies.
Figu e 30. Visible ligh [47] abso p ion spec um [104] o I on Seleni e.
In ecen yea s, I on Seleni e (Figu e 28) was widely used in digi al logic and senso
de ices due o he exis ence o a hys e esis loop. A hys e esis loop sugges s he exis ence
o magne ic and elec ic coupling which could be changed by applying a magne ic and/o
elec ic ield. The pa amagne ic beha io o he ma e ial is ano he aspec which makes
he ma e ial a po en ial candida e o digi al applica ions.
The pe o ski e–like c ys allog aphic o ien a ion o I on Seleni e and chemical s uc-
u e o he ma e ial open new oppo uni ies o he implemen a ion o a wide a ie y o
impu i ies. Impu i ies a e necessa y o compensa e o acancies o e apo a ed oxygen
and/o Bismu h a oms, which is majo issue o con en ional, high– empe a u e deposi ion
me hods. Impu i y changes he o e all spin s uc u e, hus enhancing he unique e o-
magne ic and e oelec ic p ope ies o he compound.
Al hough he Seleni e ma e ial was widely s udied, he ull po en ial o he ma e ial
is no ye e ealed and equi es u he and deepe in es iga ion.
6. Discussion
BFO ma e ial is an ou s anding ma e ial, which combines a a ie y o unique p op-
e ies, including high emnan pola iza ion, na ow bandgap po en ial e oelec ic, mag-
ne ic coupling, e c. In addi ion o unique e omagne ic p ope ies, Bismu h e i e shows
Figu e 29.
Pho oca aly ic [
98
] eac ion mechanism o i on Seleni e and Mulli e unde ligh adia ion.
Conside ing e en he lowe band gap [
99
] o I on-Seleni e, in compa ison o pu e
Bismu h e i e (2–3 eV [
100
]), he ma e ial was p o en o be an excep ionally good ab-
so ben o ul a iole [
101
] and isible ligh adia ion a ac ing much a en ion. The band
gap [
102
] o I on Seleni e s ands unde 2 eV [
92
], which is much lowe among mos o
he con en ional pho o–ac i e ma e ials. By analyzing he abso p ion [
99
] ac i i y, i was
ound ha he abso p ion a e o I on Seleni e is e en highe compa ed o Mulli e Fe i e.
The wa eleng h abso p ion [
103
] ange o Mulli e s ays wi hin 600–850 nm, whe eas o
I on Seleni e i is much wide wi hin 500–900 nm [
99
,
101
] (Figu e 30), and wi h a smo he
abso p ion a e a highe wa eleng hs, esul ing in s onge op ical abso p ion capabili ies.
Ma e ials 2022, 14, x FOR PEER REVIEW 22 o 28
Figu e 29. Pho oca aly ic [98] eac ion mechanism o i on Seleni e and Mulli e unde ligh adia-
ion
Conside ing e en he lowe band gap [99] o I on-Seleni e, in compa ison o pu e
Bismu h e i e (2–3 eV [100]), he ma e ial was p o en o be an excep ionally good abso -
ben o ul a iole [101] and isible ligh adia ion a ac ing much a en ion. The band
gap [102] o I on Seleni e s ands unde 2 eV [92], which is much lowe among mos o he
con en ional pho o–ac i e ma e ials. By analyzing he abso p ion [99] ac i i y, i was
ound ha he abso p ion a e o I on Seleni e is e en highe compa ed o Mulli e Fe i e.
The wa eleng h abso p ion [103] ange o Mulli e s ays wi hin 600–850 nm, whe eas o
I on Seleni e i is much wide wi hin 500–900 nm [99,101] (Figu e 30), and wi h a smo he
abso p ion a e a highe wa eleng hs, esul ing in s onge op ical abso p ion capabili-
ies.
Figu e 30. Visible ligh [47] abso p ion spec um [104] o I on Seleni e.
In ecen yea s, I on Seleni e (Figu e 28) was widely used in digi al logic and senso
de ices due o he exis ence o a hys e esis loop. A hys e esis loop sugges s he exis ence
o magne ic and elec ic coupling which could be changed by applying a magne ic and/o
elec ic ield. The pa amagne ic beha io o he ma e ial is ano he aspec which makes
he ma e ial a po en ial candida e o digi al applica ions.
The pe o ski e–like c ys allog aphic o ien a ion o I on Seleni e and chemical s uc-
u e o he ma e ial open new oppo uni ies o he implemen a ion o a wide a ie y o
impu i ies. Impu i ies a e necessa y o compensa e o acancies o e apo a ed oxygen
and/o Bismu h a oms, which is majo issue o con en ional, high– empe a u e deposi ion
me hods. Impu i y changes he o e all spin s uc u e, hus enhancing he unique e o-
magne ic and e oelec ic p ope ies o he compound.
Al hough he Seleni e ma e ial was widely s udied, he ull po en ial o he ma e ial
is no ye e ealed and equi es u he and deepe in es iga ion.
6. Discussion
BFO ma e ial is an ou s anding ma e ial, which combines a a ie y o unique p op-
e ies, including high emnan pola iza ion, na ow bandgap po en ial e oelec ic, mag-
ne ic coupling, e c. In addi ion o unique e omagne ic p ope ies, Bismu h e i e shows
Figu e 30. Visible ligh [47] abso p ion spec um [104] o I on Seleni e.

Ma e ials 2022,15, 8719 22 o 28
In ecen yea s, I on Seleni e (Figu e 28) was widely used in digi al logic and senso
de ices due o he exis ence o a hys e esis loop. A hys e esis loop sugges s he exis ence o
magne ic and elec ic coupling which could be changed by applying a magne ic and/o
elec ic ield. The pa amagne ic beha io o he ma e ial is ano he aspec which makes he
ma e ial a po en ial candida e o digi al applica ions.
The pe o ski e–like c ys allog aphic o ien a ion o I on Seleni e and chemical s uc-
u e o he ma e ial open new oppo uni ies o he implemen a ion o a wide a ie y o
impu i ies. Impu i ies a e necessa y o compensa e o acancies o e apo a ed oxygen
and/o Bismu h a oms, which is majo issue o con en ional, high– empe a u e deposi-
ion me hods. Impu i y changes he o e all spin s uc u e, hus enhancing he unique
e omagne ic and e oelec ic p ope ies o he compound.
Al hough he Seleni e ma e ial was widely s udied, he ull po en ial o he ma e ial is
no ye e ealed and equi es u he and deepe in es iga ion.
6. Discussion
BFO ma e ial is an ou s anding ma e ial, which combines a a ie y o unique p ope -
ies, including high emnan pola iza ion, na ow bandgap po en ial e oelec ic, magne ic
coupling, e c. In addi ion o unique e omagne ic p ope ies, Bismu h e i e shows his
unique combina ion a oom empe a u e. This ma e ial o e s e sa ili y o explo a ion
o new echnologies wi h complex unc ions. Despi e ha ing a unique combina ion o
e oelec ic and magne ic p ope ies, he high pa asi ic cu en is signi ican downside
o he ma e ial, which limi s i s implemen a ion in o la ge-scale de ices. Many s a egies
we e o e ed o enhance and modi y o e all pe o mance and dec ease pa asi ic cu en .
New low– empe a u e deposi ion me hods (PLD, ALD, e c.), p ope implemen a ion o
a e–ea h ma e ials impu i ies and s aining BFO ma e ial in o hin ilms achie e g ea
esul s o enhancing pe o mance o BFO ma e ial. The implemen a ion o new s a egies
d i es he u iliza ion o BFO ma e ial in di e en ields, such as pho o ol aic, piezoelec ic,
senso ics and memo y de ices. Ne e heless, e en hough g ea esul s ha e been achie ed,
many challenges emain. I is necessa y o u he enhance he magne ic esponse o he
ma e ial be o e inco po a ing i in o la ge-scale de ices. The c ys allog aphic s uc u e o
Bismu h e i e is ep esen ed by a hombohed al pe o ski e s uc u e. In his s uc u e,
he ma e ial exhibi s a e oelec ic esponse due o he esidual magne ic spin o ien a ion
o a single cell uni . The pe o ski e s uc u e o e s oppo uni ies o implemen a ion o
di e en ma e ials o imp o e an o e all spin s uc u e by compensa ion o dec eased
alency o Fe a oms, esul ing in s onge e oelec ic esponse and dec eased pa asi ic
cu en . The pa asi ic cu en is gi en by de ec s o c ys allog aphic s uc u e o Bismu h
e i e du ing he ini ial deposi ion me hod. One o he mos common causes o pa asi ic
cu en is e apo a ion o Bi a oms and a dec ease o alency o Fe a oms. A high ope a ional
empe a u e esul ing in a high e apo a ion a e o chemically ola ile Bismu h, sugges s
ha he modi ica ion (o choice) deposi ion me hod ope a es wi h he lowe empe a u e
ange. The bes sui able deposi ion me hods o p epa a ion o Bismu h e i e ma e ial
a e Pulsed lase Deposi ion (PLD) and A omic Laye Deposi ion (ALD), since hey ope a e
in a ela i ely low empe a u e ange and o e ou s anding con ol o e mo phology
o inal p oduc s. A low ange o ope a ional empe a u es is ex emely impo an o
he e alua ion o e oelec ic p ope ies o he p oduced BFO ma e ial, conside ing he
chemical ola ili y o Bismu h a oms. Conside ing he pe o ski e–like s uc u e, he impu-
i ies o di e en ma e ial aim o enhance elec omagne ic coupling o BFO. Impu i ies o
a e–ea h ma e ials no only s abilize he c ys al s uc u e o BFO bu also d ama ically
inc ease he magne ic esponse due o close in e ac ion o spin momen s o he en i e
cell uni . Implemen a ion o a e–ea h ma e ials conside ably inc ease he ansi ion be-
ween e omagne ic and an i e omagne ic phases due o he elimina ion o he cycloid
spin momen , allowing a s ong magne ic esponse o occu . Mos s udied impu i ies
o a e–ea h ma e ials a e Ti anium, Neodymium, Lan hanum, Sama ium, Eu opium,
P aseodymium; implemen a ion o hese ma e ials causes a signi ican inc ease o elec o-
Ma e ials 2022,15, 8719 23 o 28
magne ic coupling [
105
,
106
]. Ano he po en ial solu ion o enhance e oelec ic esponse
is a dec ease in cell uni size. The dec ease o cell size could be achie ed by s aining he
BFO ma e ial in o hin ilms by applying p essu e, which leads o a ans o ma ion be ween
e omagne ic and an i e omagne ic phases (mo pho opic phase bounda y). Smalle cell
uni s show g ea e e oelec ic esponse due o a s onge in e ac ion o spin momen s
o he single cell uni . The exis ence o he mo pho opic phase ans o ma ion unde
applied mechanical p essu e allows he disco e y o new ways o enhancemen o o e all
e oelec ic p ope ies, which could d ama ically dec ease p oduc ion cos s o new ypes
o e oelec ic ma e ials o po en ial complex de ices. The mos e sa ile o m o BFO
ma e ial, which o e s a wide a ie y o p oduced pa icles and modi ica ions, is hin ilm.
Among o he o ms o BFO ma e ial (powde s, nanopa icles), hin ilms a e excep ionally
e sa ile and can be easily in eg a ed in o la ge-scale de ices. In addi ion o e sa ili y,
di e en ma e ials could be implemen ed in o he hin ilm s uc u e o enhance he unique
p ope ies o BFO ma e ial. Besides hin ilms, BFO ma e ial was ex ensi ely used o he
p epa a ion o BFO-based ce amics due o i s empe a u e s abili y. BFO is a lead– ee,
non– oxic ma e ial wi h he elec ical s uc u e o bismu h molecules being simila o Pb.
The pe o ski e–like s uc u e o BFO allows o he c ea ion o he mo pho opic phase
ans o ma ion enabling he achie emen o maximized piezoelec ic esponse. Ne e he-
less, p oblems wi h he p epa a ion o BFO–based ce amics associa ed wi h high ope a ion
empe a u es and un o una e combina ion o low elec ical esis ance and coe ci e ield
o BFO ma e ial cause di icul ies wi h esea ch o he piezoelec ic esponse and domain
beha io . These easons we e a he beginning o nume ous esea ch a enues o po en ial
modi ica ion o BFO–based ce amics. The mos in e es ing compounds a e BaTiO
3
, Bi–
K–TiO, Bi–Na–Ti–O, and Bi(Zn–Ti)O, since hese compounds enhance Cu ie empe a u e
and piezoelec ic esponse. Un o una ely, he p ope ies o hese compounds ha e no
been sys emized.
Byp oduc s o he BFO ma e ial c ys alliza ion we e disco e ed du ing he ini ial
deposi ion p ocess. Mulli e and I on Seleni e a e wo common byp oduc s, which we e
unde in ense in es iga ion, conside ing he uniqueness o hei p ope ies. The s ong
pho oelec ic esponse and isible ligh abso p ions capabili ies o bo h byp oduc s was
p o en i al o pho o ol aic applica ions and de ices.
7. Conclusions
This pape p o ided a e iew o he BFO ma e ial. The easons o making Bismu h
e i e unique o e o he mul i e oic ma e ial has been desc ibed. The c ys allog aphic
s uc u e o he ma e ial is ep esen ed by a cubic Rhombohed al pe o ski e s uc u e. This
pape desc ibes he c ys allog aphic s uc u e o BFO ma e ial in de ail, including he spin
s uc u e o a single cell uni e and hei e ec on e oelec ic and magne ic p ope ies. The
main disad an age o he ma e ial is he pa asi ic cu en . The eason o he occu ence
o pa asi ic cu en and po en ial solu ions o how o dec ease pa asi ic cu en ha e been
desc ibed. Bismu h e i e is syn hesized in di e en o ms, such as powde s, nanopa icles
and hin ilms. Thin ilms o he BFO ma e ial a e he mos widesp ead and popula o m
o he ma e ial. Mo phology, p ope ies, a ie y o hicknesses and hei e ec on leakage
cu en ha e been desc ibed in de ail.
The deposi ion me hods o hin ilms include: he sol gel me hod, Pulsed Lased
Deposi ion (PLD), and A omic Laye Deposi ion (ALD). Thei ope a ional p inciple, com-
pa ison, ad an ages, and disad an ages o p epa a ion o BFO hin ilms ha e been
men ioned. Among he desc ibed me hods, ALD and PLD a e supe io me hods, allowing
he deposi ion o a single a omic laye s oichiome ic he e os uc u e o hin ilms wi h
minimal con amina ion.
I was disco e ed ha hin ilms include byp oduc s o Bismu h e i e. I on Seleni e
and Bismu h Mulli e a e he ypical byp oduc s o he ini ial syn hesis o BFO ilms. The
chemical and c ys allog aphic s uc u e o bo h byp oduc ma e ials ha e been desc ibed.
Supe io isible ligh abso p ion, and s ong pho oelec ic p ope ies ha e been men ioned.
Ma e ials 2022,15, 8719 24 o 28
Fu u e pe spec i es o he hesis will lie wi h analyzing samples o he BFO ma e ial.
Samples will be p epa ed by he PLD me hod on ce amic subs a es wi h a pla inum bu e
laye . Analyzing me hods include opog aphy analysis by a omic o ce mic oscopy (AFM)
and nea – ield scanning op ical mic oscopy (SNOM), due o hei a ailabili y a he esea ch
acili y and uncomplica ed scanning p ocedu e. In addi ion o AFM and SNOM mic oscopy,
opog aphy o p epa ed samples will by s udied by scanning elec on mic oscopy (SEM)
and ocused ion beam (FIB); bo h me hods will p o ide de ailed in o ma ion o c oss
sec ion and hickness o BFO and pla inum bu e laye o p oduced samples. To de e mine
whe he he e a e any byp oduc s p esen in p oduced samples, Raman spec oscopy will
be ex ensi ely used. In he case o disco e y o po en ial byp oduc s o he BFO ma e ial
he a emp o ex ac ion will be ca ied ou .
Funding:
This esea ch was unded by he Czech ounda ion Agency (GA 19–17457S),
P ojec ID: 29780
.
Ins i u ional Re iew Boa d S a emen : No applicable.
In o med Consen S a emen : No applicable.
Da a A ailabili y S a emen : The s udy did no epo any da a.
Con lic s o In e es : The au ho s decla e no con lic o in e es .
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