Robus Dipola Laye s be ween O ganic Semiconduc o s and Sil e
o Ene gy-Le el Alignmen
TomásK ajnák, Ve onika S a á, Pa el P ocházka, Jakub Plane , TomásSkála, Ma hias Bla nik,
and Jan Cechal*
Ci e This: ACS Appl. Ma e . In e aces 2024, 16, 18099−18111
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ACCESS Me ics & Mo e A icle Recommenda ions *
sı Suppo ing In o ma ion
ABSTRACT: The in e ace be ween a me al elec ode and an
o ganic semiconduc o (OS) laye has a de ining ole in he
p ope ies o he esul ing de ice. To ob ain he desi ed
pe o mance, in e laye s a e in oduced o modi y he adhesion
and g ow h o OS and enhance he e iciency o cha ge anspo
h ough he in e ace. Howe e , he employed in e laye s ace
common challenges, including a lack o elec ic dipoles o une he
mu ual posi ion o ene gy le els, being oo hick o e icien
elec onic anspo , o being p one o in e mixing wi h
subsequen ly deposi ed OS laye s. He e, we show ha monolaye s
o 1,3,5- is(4-ca boxyphenyl)benzene (BTB) wi h ully dep o o-
na ed ca boxyl g oups on sil e subs a es o m a compac laye
esis an o in e mixing while capable o media ing ene gy-le el
alignmen and showing a la ge insensi i i y o subs a e e mina ion. Employing a combina ion o su ace-sensi i e echniques, i.e.,
low-ene gy elec on mic oscopy and di ac ion, X- ay pho oelec on spec oscopy, and scanning unneling mic oscopy, we ha e
comp ehensi ely cha ac e ized he compac laye and p o en i s obus ness agains mixing wi h he subsequen ly deposi ed o ganic
semiconduc o laye . Densi y unc ional heo y calcula ions show ha he obus ness a ises om a s ong in e ac ion o ca boxyla e
g oups wi h he Ag su ace, and hus, he BTB in he i s laye is ene ge ically a o ed. Synch o on adia ion pho oelec on
spec oscopy shows ha his laye displays conside able elec ical dipoles ha can be u ilized o wo k unc ion enginee ing and
elec onic alignmen o molecula on ie o bi als wi h espec o he subs a e Fe mi le el. Ou wo k hus p o ides a widely
applicable molecula in e laye and gene al insigh s necessa y o enginee ing o cha ge injec ion laye s o e icien o ganic
elec onics.
KEYWORDS: cha ge injec ion laye s, sel -assembly, su aces, pho oelec on spec oscopy, ene gy le els, low-ene gy elec on mic oscopy,
scanning unneling mic oscopy
■INTRODUCTION
O ganic elec onics is a signi ican echnology o displays and
illumina ion.
1−3
In o he ields ha u ilize o ganic semi-
conduc o s (OSs), e.g., in o ganic hin- ilm ansis o s
4
and
o ganic pho o ol aics,
5
he la ge-scale indus ial applica ions
a e s ill limi ed. The pe o mance o as -swi ching and high-
powe o ganic elec onic de ices, like OFETs, is o en highly
in luenced by he con ac esis ance
6−9
o igina ing om he
ene gy-le el misalignmen be ween a me al elec ode and an
OS laye .
7,10−12
In oducing o de ed dipola laye s a he me al−OS
in e ace can une he elec ode wo k unc ion (WF) and he
in e acial ene gy-le el alignmen (ELA) wi h he OS on ie
o bi als (highes occupied molecula o bi al (HOMO) o
lowes unoccupied molecula o bi al (LUMO)).
13,14
These so-
called cha ge injec ion laye s (CILs) can hus signi ican ly
educe he con ac esis ance and inc ease he e iciency o he
cha ge-ca ie injec ion in o he OS laye . In his espec ,
molecula laye s exhibi ing elec ic dipoles can ac as CILs
be ween me al elec odes and OS laye s;
15,16
he dipoles can
be ei he in insic o he deposi ed molecules, o med due o
he molecule−subs a e cha ge ans e , o by changing he
molecula con o ma ion (e.g., i s bending).
13
The sel -
assembled monolaye s (SAMs) we e in ensi ely s udied in
his espec .
13,15−18
The in oduc ion o pola segmen s in o
he backbone can p o ide he desi ed elec ic dipoles necessa y
o WF enginee ing,
18
bu he molecula chains also p esen a
decoupling laye ha con ibu es o he con ac esis ance
be ween he me al subs a e and he OS laye deposi ed on he
Recei ed: Decembe 13, 2023
Re ised: Ma ch 13, 2024
Accep ed: Ma ch 13, 2024
Published: Ma ch 29, 2024
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op.
17,19−21
In his espec , he OS monolaye s demons a ed
p omising changes o he WF wi h espec o ELA;
10,13,14,22
howe e , hey a e p one o in e di usion o o ma ion o
mixed phases wi h subsequen ly deposi ed molecula
laye s.
10,14,23−33
A sha p, uni o m, and s able in e ace du ing
he li e ime o he de ice is equi ed o echnological
applica ions o e icien CILs.
Recen ly, we ha e shown ha monolaye s o a oma ic
ca boxylic acids can ac as CILs.
34
In ha sys em, he equi ed
elec ic dipoles a e localized a he me al−o ganic in e ace,
which esul s in emo ing he unneling con ac be ween he
molecula laye and he me al elec ode. Howe e , he
employed molecules sha e he main issue wi h o he molecula
species explo ed o his ole: hey eadily mix wi h he
deposi ed OS o e laye , which would comp omise he
pe o mance o po en ial de ices. He e, we show ha 1,3,5-
is(4-ca boxyphenyl) benzene (BTB, Figu e 1a), an a oma ic
ica boxylic acid, o ms a obus laye ha does no mix wi h
deposi ed OS laye s up o empe a u es a which OSs ei he e-
e apo a e o BTB decompose.
The obus in e ace can be o med by employing molecules
ha s ongly bind o he su ace, like in SAMs. Conce ning Ag
su aces, ca boxyl- e mina ed SAMs
35−40
show highe s uc-
u al o de han adi ionally used hiol-based SAMs.
39,40
He e,
a pa ial cha ge ans e be ween molecule and subs a e
p o ides a physically obus and elec onically s ong
connec ion,
9,12,15,17,41
bu in e mixing wi h deposi ed po phy -
in and ph halocyanine molecules e en below oom empe -
a u e was epo ed.
42
In addi ion, he s ong OS molecule−
me al in e ac ion induces undesi able changes o su ace and
OS ilm mic os uc u e and subs an ial modi ica ion o
in e acial elec onic s uc u e, which can p o oundly impac
con ac and channel esis ance and o e all de ice pe o m-
ance.
7,12,19
Some s ongly in e ac ing small o ganic molecules,
like F4-TCNQ and F6-TCNNQ, may o m an o ganome allic
laye wi h sil e wi h a hickness up o 50 nm, which is s able
wi h espec o subsequen deposi ion o pen acene laye s
43
bu is s ill a om an ideal case.
While p o iding a o able p ope ies wi h espec o ELA,
plana weakly adso bing OS molecules a e mo e p one o
in e mixing wi h subsequen ly deposi ed molecula laye s. One
o he possibili ies is o change molecula unc ional g oups o
hei numbe o s eng hen o ganic−me al in e ac ion and,
hus, he i s laye s abili y. In his espec , changing he
molecula s uc u e o pen acene oxo-de i a i es om 6,13-
pen acenequinone (P2O, ea u ing wo oxygens) and
5,7,12,14-pen acene e one (P4O, 4 oxygens) leads o he
change o adso p ion beha io on Ag(111) om physiso p ion
o P2O o chemiso p ion o P4O.
31
In his case, he P4O laye s
we e esis an o in e mixing wi h subsequen ly deposi ed
coppe ph halocyanine (CuPc). The o he possibili y o ob ain
a semis able bilaye is o use 3,4,9,10-pe ylene- e aca boxylic-
dianhyd ide (PTCDA), which is s able agains he mixing wi h
subsequen ly deposi ed CuPc
44
o in ph halocyanine
(SnPc).
25
In hese cases, a kine ic ba ie exis s ega ding
in e laye exchange in bo h CuPc/PTCDA/Ag and PTCDA/
CuPc/Ag s acking o de s, wi h a p ima y pa ame e go e ning
s abili y a lowe empe a u es being he adso p ion ene gy pe
a ea o he indi idual molecules.
30
Howe e , beyond he onse
o deso p ion, he decisi e pa ame e becomes he adso p ion
ene gy pe molecule, and he p e e ed occupancy o he i s
laye can change.
Ou p e ious s udy in oduced a oma ic ca boxylic acids as
dipola laye s.
34
We ha e shown ha he employed 4,4′-
biphenyl dica boxylic acid (BDA, Figu e 1b) molecule can
g adually dep o ona e in di ec con ac wi h sil e su aces
ei he he mally
45−48
o by low-ene gy elec ons,
49
hus
p o iding a possibili y o inely une he ELA. While
conside able shi s in he WF and ene gy le els o deposi ed
molecules up o 0.8 eV we e induced, ou la e expe imen s
ha e shown ha i is p one o mix wi h pen acene laye s
deposi ed on op. In he p esen pape , we show ha ex ending
he molecule o h ee ca boxylic g oups esul s in a obus
monolaye ha does no mix wi h subsequen ly deposi ed OS
molecules, i.e., pen acene (Figu e 1c), a p o o ypical high
mobili y OS,
50
HM-TP (Figu e 1d), and HAT-CN (Figu e
1e), an elec on dono and accep o , espec i ely. Ou densi y
unc ional heo y (DFT) calcula ions show ha he obus ness
is o a he modynamic o igin: he compac laye p esen s he
lowes ene gy s a e. Thus, he molecula monolaye s o ully
dep o ona ed BTB o m a iable pla o m on he pa h owa d
he ohmic con ac s be ween elec odes on OS laye s.
■RESULTS AND DISCUSSION
We ha e pe o med expe imen s o wo low-ene gy ace s o
he sil e su ace: Ag(111) and Ag(100). As he esul s a e
simila on bo h su aces, we will ocus ou desc ip ion on
Ag(111) and gi e he esul s o he o he ace in he
Suppo ing In o ma ion. In he ollowing, we will i s show
synch o on adia ion pho oelec on spec oscopy esul s o
g adual dep o ona ion o BTB and show ha wi h espec o
WF changes and ELA, he BTB beha es consis en ly wi h ou
ea lie esul s on BDA.
34
Then, we will discuss he ob ained
scanning unneling mic oscopy (STM) and low-ene gy
elec on mic oscopy (LEEM) da a o submonolaye and ull
monolaye co e ages o he ully dep o ona ed molecule
(ma ked as δ-BTB in he ollowing), demons a ing ha ,
con a y o BDA, he compac monolaye o he ully
Figu e 1. Chemical s uc u e o o ganic molecules explo ed in his
wo k. (a) 1,3,5-T is(4-ca boxyphenyl) benzene (BTB); (b) 4,4′-
biphenyl dica boxylic acid (BDA); (c) pen acene; (d) hexame hoxy-
iphenylene (HM-TP); and (e) hexaaza iphenylene-hexaca boni ile
(HAT-CN).
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dep o ona ed BTB molecules co e s he whole subs a e
su ace ( u he e e ed o as compac δ-BTB laye ) and is
easily achie able. The compac δ-BTB laye is s able agains
mixing wi h pen acene, HAT-CN, and HM-TP, ypical
examples o o ganic semiconduc o s: we will show a
he modynamic p e e ence o he o ma ion o pen acene−
BTB mixed phases o submonolaye co e ages and demon-
s a e he obus ness o he compac δ-BTB laye agains
s uc u al and chemical changes. Ou DFT calcula ions e eal
ha he compac δ-BTB laye possesses he lowes ene gy wi h
espec o o he possibili ies, so hey a e obus om he
he modynamic poin o iew unde UHV condi ions.
Pho oelec on Spec oscopy. A oma ic ca boxylic acids
dep o ona e (i.e., lose hyd ogen om ca boxylic−COOH
g oups) upon con ac wi h me al subs a es (excep o gold)
unde UHV condi ions.
51
This chemical eac ion occu s below
oom empe a u e o mos me als, including Cu.
52
The
eac ion is kine ically es ic ed on Ag su aces, and annealing
a ele a ed empe a u es (30−50 °C) is usually equi ed o
ob ain pa ially dep o ona ed molecula phases wi hin
minu es.
48
Howe e , signi ican ly highe empe a u es (∼200
°C) a e necessa y o achie e comple e dep o ona ion because
s able molecula phases hinde he dep o ona ion eac ion.
48
We ha e ollowed he dep o ona ion o BTB on bo h Ag(111)
and Ag(100) subs a es by pho oelec on spec oscopy
employing synch o on adia ion.
The O 1s spec um o 1 monolaye (ML) o as-deposi ed
BTB molecules on Ag(111) shown in Figu e 2a can be i ed
by wo pai s o peaks (ligh blue and blue; ligh g een and
g een). As de ailed in Suppo ing In o ma ion Sec ion 1, we
assign hese peak componen s o ca boxyl g oups in wo
dis inc binding mo i es. The in ensi y a io o hese pai s is
2:1. The highe binding ene gy componen om each pai
(highligh ed by a ligh e colo in Figu e 2a) is associa ed wi h
hyd oxyl oxygen (C−OH) and he da ke one wi h ca bonyl
oxygen (−C�O) o he ca boxyl g oup (−COOH) by
compa ison wi h p e ious wo ks.
45,47,48
Two dis inc pai s o
peaks poin o he exis ence o wo di e en chemical
en i onmen s o he ca boxyl g oups; hese a e p obably
associa ed wi h he ibbon-like s uc u e o he comp essed as-
deposi ed phase (see Figu e S3 in Suppo ing In o ma ion
Sec ion 2).
Du ing he annealing a p og essi ely highe empe a u es, a
new componen associa ed wi h ca boxyla e g oups
45,47,48
appea s in he spec a and g ows in in ensi y ( ed componen
in Figu e 2a). The ela i e in ensi y o his peak is a measu e o
he deg ee o dep o ona ion o ca boxylic g oups (i.e., he
ac ion o dep o ona ed ca boxyl g oups wi h espec o all
ca boxyl g oups) in he BTB laye . Figu e 2b shows he
e olu ion o he deg ee o dep o ona ion wi h annealing
empe a u e o bo h Ag su aces. On bo h su aces, BTB
molecules g adually dep o ona e; on Ag(100), he dep o ona-
Figu e 2. Changes in he elec onic p ope ies o BTB/Ag(111) du ing i s g adual dep o ona ion. (a) Examples o O 1s spec a eco ded on he as-
deposi ed phase a 25 °C, a e annealing a 175 °C, and annealing a 235 °C. (b) Deg ee o dep o ona ion o BTB ca boxylic g oups as a unc ion
o annealing empe a u e o bo h Ag(111) and Ag(100) su aces. The e ical lines ma k he h eshold o deca boxyla ion, beyond which he
dec ease o O 1s peak in ensi y and dec ease in sample WF is obse ed. (c) Sample WF as a unc ion o annealing empe a u e o bo h Ag(111)
and Ag(100) su aces. The e ical lines ma k he deca boxyla ion h eshold; he ho izon al lines ma k he measu ed WF o he ba e subs a e
su ace. (d) Plane-a e aged di e ence in cha ge densi y along he z-di ec ion pe pendicula o he δ-BTB/Ag(111) in e ace. The elaxed s uc u e
and he 3D isosu ace o he cha ge densi y di e ence a e depic ed in he backg ound. Sil e , ca bon, oxygen, and hyd ogen a oms a e in g ay,
b own, ed, and whi e, espec i ely; elec on deple ion is colo ed blue, and accumula ion yellow. (e) Posi ion o he C 1s peak associa ed wi h
phenyl ings wi hin he i s BTB molecula laye plo ed as a unc ion o he sample WF compa ed wi h ea lie esul s o BDA.
34
The line has a
slope o −1, whe eas he i ed expe imen al alues ha e a slope o −1.03 ±0.06.
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ion occu s a lowe empe a u es (consis en ly wi h BDA
48
),
and comple e dep o ona ion is obse ed a 170 °C, whe eas on
Ag(111), i is eached a 240 °C. Fo he Ag(111) subs a e,
his empe a u e is al eady e y close o he h eshold o he
deca boxyla ion o BTB molecules, i.e., a comple e emo al o
ca boxyl g oups ha occu s a ound 250 °C o bo h su aces.
Abo e his h eshold, he X- ay pho oelec on spec oscopy
(XPS) da a show a dec ease o oxygen- ela ed signal, while he
C 1s peak associa ed wi h phenyl ings keeps i s in ensi y and
shi s back o highe binding ene gies, i.e., 284.7 eV a Ag(111)
and 284.9 eV a Ag(100), as he ca boxyla e- ela ed dipoles
cease o exis . Diso de ed polyme -like ne wo ks emain on he
su ace, as obse ed by STM (Figu e S4 in Suppo ing
In o ma ion Sec ion 2). We obse e (Figu e 2b) ha he ully
dep o ona ed δ-BTB phase is s able in a b oad window o
empe a u es o 170−250 °C on Ag(100) bu only in a
ela i ely na ow ange o 235−250 °C on Ag(111).
The WF measu ed a e each annealing is displayed in
Figu e 2c. The WF was de e mined om he posi ion o he
seconda y elec on cu o .
34
Due o he push-back e ec , wi h
inc easing BTB co e age, he WF dec eases below 4.1 eV on
bo h su aces.
14,34
A highe empe a u es, he g adual
dep o ona ion leads o he o ma ion o in e acial dipoles,
and he WF inc eases again,
34
eaching 4.61 eV on Ag(111)
and 4.49 eV on Ag(100). A di e en WF o p is ine su aces
explains his di e ence: he measu ed alues we e 4.38 and
4.48 eV o Ag(100) and Ag(111), espec i ely; hei alues a e
wi hin he unce ain y in e al o epo ed alues, i.e., (4.36 ±
0.05) eV o Ag(100) and (4.53 ±0.05) eV o Ag(111).
53
To gi e a deepe insigh in o he adso p ion-induced WF
change, we cha ac e ized he s uc u al and elec onic
p ope ies o an δ-BTB/Ag(111) in e ace wi h ab ini io
calcula ions ollowing he p ocedu e desc ibed elsewhe e.
34
The change in he WF is a ibu ed o he sum o he su ace
dipoles ac oss he eo ganized Ag subs a e and he δ-BTB
laye and he edis ibu ion o he cha ge densi y a he
in e ace esul ing om molecule−subs a e in e ac ion. The
smalles con ibu ion o −0.06 D pe BTB molecule a ises
om he subs a e ea angemen . As shown in Figu e 2d,
sub le changes in he opmos sil e laye gi e ise o his
con ibu ion. The in amolecula dipole momen caused by a
bending o he molecule and shi o nega i ely cha ged oxygen
a oms owa d he subs a e is calcula ed o be −2.89 D. Finally,
he in e ace dipole momen calcula ed om plane-a e aged
cha ge densi y di e ence con ibu es wi h +3.57 D pe BTB
molecule. This con ibu ion a ises om a cha ge densi y
di e ence plo ed in Figu e 2d, which shows elec on deple ion
om he opmos sil e laye and accumula ion in he oxygen
laye si ua ed 2.2 Å om he subs a e. The o e all su ace
dipole densi y o he δ-BTB laye hus esul s in 0.62 D pe
BTB molecule, causing a 0.14 eV inc ease in WF om 4.49 eV
o he p is ine Ag(111) su ace o 4.63 eV o he δ-BTB laye
o Ag(111) su ace in a pe ec alignmen wi h expe imen al
obse a ions.
In addi ion, we ha e measu ed he shi o ene gy le els o
as-deposi ed (α-BTB) and ully dep o ona ed (δ-BTB) laye s
by analyzing he posi ions o phenyl- ing- ela ed componen s
o he C 1s peak o he i s and second molecula laye s; he
p ocedu e is desc ibed in ou p e ious wo k.
34
In Figu e 2e, we
ha e plo ed he posi ion o C 1s peak wi hin he i s laye o
BTB oge he wi h alues ob ained o se e al BDA molecula
phases ob ained p e iously.
34
The BTB da a i he p e iously
Figu e 3. δ-BTB phase on Ag(111) su ace. (a−d) De ailed STM images o he δ-BTB phase: (a) on a la e ace showing he s uc u e o he
phase wi h he uni cell highligh ed as a black hombus; (b) g ow h o δ-BTB molecules ac oss one s ep edge and an ex ended kink; (c) bounda y
o h ee δ-BTB domains ma ked I, II, and III (I and III a e di e en o a ional domains, whe eas in I and II show a mi o symme y); and (d) he
δ-BTB phase wi h a single-o ien a ion ex ending o e se e al e aces; he inse shows a line scan along he whi e line indica ed. Scanning
pa ame e s o all STM images: 1.4 V, 50 pA. The ull-size images a e gi en in Suppo ing In o ma ion Sec ion 5. (e) DFT-based model o he δ-
BTB phase showing he molecula a angemen on he Ag(111) su ace: C: black, O: ed, H: ligh ed, Ag: g ay. The highligh ed uni cell is
posi ioned in he same way as in (a); i ea u es one molecule pe uni cell and shows he adso p ion posi ions o he h ee e minal ca boxyla e
g oups. Two ca boxyla es a e aligned such ha bo h O a oms adso b in an on- op posi ion. In he hi d ca boxyla e g oup, only one o he O a oms
is in an on- op posi ion, whe eas he second is in a b idge posi ion (highligh ed by a blue a ow).
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es ablished linea end be ween he measu ed WF and co e-
le el posi ions. The posi ion o co e le els expe iences he
same shi as he on ie o bi als in he case o acuum le el
alignmen .
31,54
STM and LEEM In es iga ion o δ-BTB Laye s. STM
and LEEM expe imen s ha e been ca ied ou in ou home
UHV clus e sys em. We ha e explo ed submonolaye and ull
monolaye co e ages o he ully dep o ona ed BTB phase (δ-
BTB) on bo h Ag(111) and Ag(100) su aces. As he esul s
a e e y simila o bo h subs a es, we will p esen only da a
o Ag(111) in he main ex , and he da a o he Ag(100)
su ace a e gi en in Suppo ing In o ma ion Sec ion 3.
To ob ain he compac δ-BTB laye , he as-deposi ed BTB
laye s we e annealed a empe a u es necessa y o he ull
dep o ona ion gi en in he p e ious sec ion; he ull
dep o ona ion was p o en by in si u XPS ia he p esence o
a single O 1s peak componen a 530.5 eV (Figu e S9,
Suppo ing In o ma ion Sec ion 4), which is consis en wi h
he synch o on adia ion da a p esen ed abo e. The s uc u al
e olu ion o molecula phases du ing g adual dep o ona ion
was al eady desc ibed ea lie in an STM wo k by Ruben e al.
55
Ou da a o he as-deposi ed as well as pa ially dep o ona ed
molecules a e gene ally in line wi h hei obse a ions. In
addi ion, we could e eal a high deg ee o complexi y in he
phase ans o ma ions in which he co e age and deposi ion
a e play a signi ican ole. Howe e , a mo e de ailed
desc ip ion o his is beyond he scope o his wo k.
The molecula -scale opog aphy o he δ-BTB phase
ob ained by STM shows he BTB molecules as b igh
p o usions o h ee-poin s a s in a hexagonally close-packed
s uc u e. The ca boxyla e (−COO) g oups si ua ed a he ips
o he s a s he eby poin o he cen e s o neighbo ing
molecules. This is shown in de ail in Figu e 3a, wi h he
supe s uc u e uni cell highligh ed as a black hombus. This
phase was o iginally deno ed as phase III wi h a deg ee o
dep o ona ion o 2/3.
55
Howe e , ou combined STM, XPS,
and LEEM da a clea ly indica e ha his phase is ully
dep o ona ed. Figu e 3b−d shows image de ails o he
molecula s uc u e on s ep edges and domain bounda ies.
Figu e 3b shows he a angemen o he molecules along and
o e a single subs a e s ep edge. All o he molecules a he
uppe side o he s ep edge show he same s uc u e, wi h one
poin o he s a p o usion missing. The a ms o BTB
molecules a e pa ially lexible and hus can bend owa d he
lowe e ace. This beha io is e en mo e p onounced a a kink
si e whe e he BTB seems o ha e los a comple e a m. The
kink also exac ly ollows he BTB shape and hus allows
seamless g ow h o he compac δ-BTB laye o e he s ep
edge. In his way, he single domain ex ends o e se e al
mona omic s eps, as shown in Figu e 3d. This is e iden om a
line scan (see inse o Figu e 3d) along he whi e line ha
shows a s ep heigh o ∼244 pm, which is sligh ly highe bu in
line wi h he s ep heigh o he Ag(111) subs a e (236 pm).
The molecula a angemen a he domain bounda y is shown
in Figu e 3c. In ou STM images, we ha e seen 4 o ien a ions
o molecules. In pa icula , we iden i y wo di e en domain
o ien a ions (see Figu e 3c, egions I and III) and wo
s uc u al domains (I and II) ha sha e he same uni cell bu
consis o molecules wi h o ien a ion mi o ed along he uni
cell’s main diagonal. The calcula ed DFT model shown in
Figu e 3e is ully consis en wi h ou STM da a. I p o ides a
deepe insigh in o he in e ace s uc u e. BTB molecules a e
o a ed by 10.5°wi h espec o he high-symme y di ec ion
o Ag(111) subs a e. The mos common si e o oxygen a oms
Figu e 4. LEEM analysis o he δ-BTB phase on he Ag(111) su ace. (a) La ge-a ea di ac ion pa e n aken a 10 eV p ima y elec on ene gy. (b)
Supe la ice di ac ion model o he δ-BTB laye showing he composi ion om wo single-domain di ac ion pa e ns. (c) B igh - and da k- ield
images aken a he submonolaye BTB co e age showing δ-BTB islands; he g een and ed colo s in he da k- ield image a e associa ed wi h a
pa icula o a ional domain gi en by he mic odi ac ion model in (b). (d) B igh - and da k- ield images o he compac δ-BTB laye ; he colo
coding is he same as in (c).
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o adso b is in he on- op posi ion, while one o he six oxygen
a oms is si ua ed in he b idge posi ion (ma ked wi h a blue
a ow).
The LEEM measu emen s shown in Figu e 4 p o ide eal
and ecip ocal space iews on sample mo phology and
s uc u e a he mesoscale. The la ge-a ea di ac ion pa e n
o he δ-BTB phase is p esen ed in Figu e 4a. The
mic odi ac ion measu emen e eals ha he δ-BTB phase
exis s in wo o a ional domains on he Ag(111) su ace: he
model o he la ge-a ea di ac ion pa e n decomposed in o
wo single-domain di ac ion pa e ns is gi en in Figu e 4b.
The modeling o he δ-BTB di ac ion pa e n p o ides a
( )
1 4
5 1
uni cell (in his wo k, all o he supe s uc u e uni
cells a e gi en in he ma ix no a ion). These wo domain
o ien a ions we e also iden i ied in ou STM images; see
Figu e 3c. In addi ion, each o hese domains has an addi ional
s uc u al domain wi h he same uni cell bu a mi o ed
o ien a ion o molecules wi hin hem (see, e.g., Figu e 3c). In
he mic odi ac ion da a and di ac ion model, hese wo
mi o ed domains a e indis inguishable.
The b igh - ield LEEM image (Figu e 4c) po ays
submonolaye co e age δ-BTB islands as a b igh a ea on he
da k backg ound, which ep esen s he ba e subs a e; he
a e age a ea o he BTB islands is 0.3 ±0.1 μm2. LEEM da k-
ield imaging, in which he image is o med only by elec ons
associa ed wi h a single di ac ion spo di e en om he
(0,0), allows eal-space isualiza ion o he o a ional domains.
Fo submonolaye co e age, indi idual δ-BTB islands g ow in
single-domain o ien a ion. Howe e , i he su ace is
comple ely co e ed (Figu e 4d), we obse e a la ge numbe
o smalle o a ional domains wi hin he δ-BTB laye ; he
uppe bound o he a e age a ea o hese domains is 0.011 ±
0.004 μm2, i.e., much smalle compa ed wi h he island size in
he submonolaye co e age. The smalle domain size is
p obably caused by a limi ed BTB anspo ia su ace
di usion, which is hinde ed in he ull monolaye .
47
S ill, he δ-
BTB su ace shows a supe io long- ange o de wi h a
minimum o de ec s as he wo domains a e well ma ched a
hei bounda y (see Figu e 3c), and single domains ex end
ac oss he s ep edges (see Figu e 3b,d). On he o he subs a e
ace , Ag(100), he s uc u e o he compac δ-BTB laye is
e y simila o Ag(111) p esen ed abo e: he molecula
packing is he same wi h h ee BTB molecules pe uni cell
commensu a e wi h he subs a e and he a ea pe molecule
di e s by 2% (see Suppo ing In o ma ion Sec ion 6 o
de ails).
We ha e es ed he applicabili y o he compac δ-BTB laye
as a CIL o OSs. In he ollowing, we will desc ibe he
expe imen s wi h pen acene; he expe imen s wi h HAT-CN
and HM-TP (Figu e 1c−e) a e gi en in Suppo ing
In o ma ion Sec ion 7.
Fo ma ion o Mixed Pen acene−BTB Phases a
Submonolaye BTB Co e age. A 1 ML co e age, δ-BTB
molecules o m a compac laye , which is s able agains mixing
wi h subsequen ly deposi ed o ganic semiconduc o molecules.
Howe e , his changes in he submonolaye egime, whe e
pen acene o ms mixed phases wi h BTB. Deposi ion o 0.5
ML o pen acene and 0.5 ML BTB molecules on Ag(111)
subs a e and subsequen annealing (170 °C, 30 min) esul s in
he o ma ion o mixed pen acene−BTB phases. Du ing he
annealing, he BTB molecules dep o ona e, and he pen a-
cene−BTB mixed phases appea upon cooling. The b igh - ield
image in Figu e 5a shows molecula islands o he mixed phase
(b igh e a eas) co e ing app oxima ely 1/3 o he subs a e,
Figu e 5. Pen acene−BTB mixed phase on Ag(111). (a) B igh - ield image o he mixed phase o med by deposi ion o 0.5 ML pen acene and 0.5
ML BTB molecules and subsequen annealing a 170 °C. (b) Di ac ion pa e n o igina ing om he mixed phase is shown in (a). (c) Di ac ion
model o he mixed phase shown in (b). (d) STM image o mixed pen acene−BTB phase wi h highligh ed uni cell (solid line) and an appa en
uni cell used o DFT calcula ions (dashed). (e) Schema ics o a angemen o molecules wi hin he uni cell ob ained om STM. ( ) Posi ion o
supe s uc u e uni cell wi h espec o Ag(111) subs a e.
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whose size and shape a e es ic ed by he subs a e s ep edges.
The emaining molecules a e p esen in molecula gas o
diso de ed phases. The di ac ion pa e n (Figu e 5b)
measu ed on hese islands is dis inc om hose obse ed o
pu e BTB phases. Employing P oLEED S udio o model he
di ac ion pa e n (Figu e 5c), we ind he associa ed uni cell
as
( )
3 13
15 8
. The STM analysis e eals ha his phase
comp ises wo close-laying pen acene molecules sandwiched
be ween wo δ-BTB molecules, as isualized in Figu e 5d− ,
gi ing he 1:1 a io o pen acene and BTB. Mo eo e , he pai
o pen acene molecules is il ed a he co ne s o he uni cell
wi h espec o he wo pen acene pai s in he in e io , as
shown in Figu e 5d,e.
We no e ha he esul ing molecula a angemen in mixed
phases can be a ec ed by he ini ial a io o deposi ed
molecules. In ano he expe imen , we deposi ed 0.8 ML o
pen acene BTB and 0.5 ML o BTB molecules and annealed
he sample a 170 °C. A e cooling, a wheel-like mixed phase
wi h a 2:1 a io was o med; see de ails in Suppo ing
In o ma ion Sec ion 8.
Mixed pen acene−BTB phases we e o med in all expe i-
men s wi h a submonolaye co e age o BTB molecules. Mixed
phases can be o med in se e al ways. One way is o deposi
bo h molecules on he su ace and ob ain he mix u e wi h
subsequen annealing. Ano he possibili y is o i s c ea e δ-
BTB, deposi pen acene, and anneal he sys em a e wa d. The
main pa ame e s in luencing he esul ing s uc u e o bo h
p ocedu es a e he concen a ions o bo h ypes o molecules
on he su ace and he annealing empe a u e, which needs o
be high enough o each he ull dep o ona ion o he BTB
molecules o dissol e δ-BTB islands bu s ill below he
deca boxyla ion and deso p ion onse .
These expe imen s wi h submonolaye BTB co e age
indica e a he modynamic p e e ence o o ming mixed
molecula phases om pen acene and BTB o e he sepa a e
pu e molecula phases.
Pen acene Deposi ion on he Compac δ-BTB Laye .
We ha e deposi ed pen acene on a sample co e ed by a
compac δ-BTB laye . A e he pen acene deposi ion, he
LEEM b igh - ield image shows a compac δ-BTB laye
co e ed wi h pen acene islands (Figu e 6a) ha appea as
da ke a eas on a b igh δ-BTB backg ound. A LEEM da k-
ield analysis o δ-BTB spo s gi en in Figu e 6b e eals ha
BTB molecules s ill co e he whole su ace, and he pen acene
o e laye a enua es he δ-BTB signal. Figu e 6c shows a
di ac ion pa e n ha is a supe posi ion o a p onounced
di ac ion pa e n associa ed wi h a c ys alline o e laye , likely
associa ed wi h pen acene, and a ain pa e n associa ed wi h
he δ-BTB laye loca ed below.
Annealing he sample a 100 °C o 15 min induces he
comple e deso p ion o pen acene: he LEEM/low-ene gy
elec on di ac ion (LEED) esul s (Figu e 6d− ) show a
compac δ-BTB laye simila o ha be o e he pen acene
deposi ion. We did no e eal any sign o he o ma ion o
mixed phases comp ising BTB and pen acene. XPS spec a o
C 1s and O 1s aken be o e ( ed) and a e (blue) pen acene
deposi ion and sample annealing (g een) a e gi en in Figu e 7.
A e pen acene deposi ion, we obse e an inc ease in he
in ensi y o he C 1s peak, which dec eases o he o iginal one
a e annealing. The O 1s peak shows only a sligh change bo h
Figu e 6. LEEM analysis o pen acene on compac δ-BTB laye on Ag(111). (a) LEEM b igh - ield image showing δ-BTB domains (b igh e a eas)
pa ially co e ed by pen acene (da ke a eas). (b) Composi ion o da k- ield images measu ed o he wo δ-BTB o ien a ional domains; he
employed di ac ion spo s a e ma ked in (c). Only a eas wi hou o e laye show a conside able in ensi y om he δ-BTB laye spo s. (c)
Di ac ion pa e n measu ed on pen acene deposi ed on he compac δ-BTB laye showing he sum o a ain pa e n associa ed wi h δ-BTB and
he one associa ed wi h he o e laye . (d, e) B igh - and da k- ield images ob ained a e annealing show a compac δ-BTB laye p esen on he
su ace. ( ) Di ac ion pa e n measu ed a e annealing showing a b igh δ-BTB pa e n wi hou any addi ional spo s.
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a e deposi ion and annealing, as pen acene comp ises only
ca bon a oms.
Based on XPS and LEEM obse a ions, we conclude ha he
ull δ-BTB laye is obus agains he mixing wi h pen acene.
This obus ness can be ei he o he modynamic o kine ic
o igin. The ac ha pen acene and BTB o m mixed phases
sugges s ha o ming bonds be ween pen acene and δ-BTB
molecules is a o able, which indica es he kine ic o igin o he
obus ness. Howe e , he DFT analysis gi en below shows he
opposi e, as a co e ages app oaching a ull monolaye , he
adso p ion ene gy pe uni a ea dic a es he he modynamic
s abili y o he compac δ-BTB laye .
DFT Calcula ions: The modynamic S abili y o he
In e mixed Phase and δ-BTB Laye . In he ollowing, we
demons a e he ene ge ic p e e ence o he mixed pen acene−
BTB phase in he submonolaye co e age and he p e e ence
o he δ-BTB phase a ull monolaye co e age. In bo h cases,
he decisi e ac o ha de e mines he s abili y is he
adso p ion ene gy o a molecule pe uni a ea calcula ed as
E E E
S
( )
mol sub mol sub
=
+
+
(1)
whe e Emol+sub is he o al ene gy o a molecula phase on a
subs a e wi h a ea S,Emol deno es gas-phase ene gies o δ-BTB
and pen acene molecules, and Esub is he o al ene gy o a ba e
subs a e (see Suppo ing In o ma ion Sec ion 9 o he esul s
i a p o ona ed BTB in he gas phase is used as an ene gy
e e ence). Monolaye s o pen acene and δ-BTB we e modeled
wi h pe iodic bounda y condi ions using he Ag(111)
supe cells gi en by
( )
6 0
2 3
and
( )
1 4
5 1
, espec i ely. Due
o i s size, he eal supe s uc u e uni cell o he pen acene−
BTB mixed phase
( )
3 13
15 8
is app oxima ed by a smalle ,
appa en uni cell o
( )
6 2
9 10
depic ed in Figu e 5 and in
Figu e S18c. This induces ∼3% s ain in he sho e su ace
ec o and 2.5% angula s ain. Re e ence ene gies o he sil e
subs a e we e calcula ed o each supe cell sepa a ely. The
esul ing s abili ies, i.e., absolu e adso p ion ene gies and
ene gies pe uni a ea, o pen acene, δ-BTB, and he mixed
phase on Ag(111) subs a e a e summa ized in Table 1. We
no e ha , in line wi h expe imen s, ou DFT calcula ions do
no show any su ace eo ganiza ion, which is no a o ed due
o a ela i ely s ong in e molecula in e ac ion, which hinde s
he li ing o Ag a oms ou o he no mal Ag(111) plane. This
conclusion is u he suppo ed by ou benchma k calcula ions
in ol ing ully dep o ona ed imesic acid (TMA), which lacks
a ac i e in e molecula in e ac ions. In he case o TMA,
sil e a oms wi h h ee Ag−O bonds we e li ed up, in line wi h
p e ious wo ks showing sil e clus e s in he molecula laye .
56
Howe e , he di ac ion model o he δ-BTB laye excludes
such scena ios due o s e ic easons: in he case o BTB,
ca boxyl g oups a e oo a away o o m 3- old Ag si es, and
he molecula uni cell is oo small o accommoda e any sil e
ada om/clus e .
Fi s , we will e alua e he p e e ed molecula phase in he
case o he ully co e ed su ace. The e a e wo main
con ibu ions ha dec ease he ee ene gy o he sys em:
molecule−subs a e bonding and in e molecula bonding. The
compu ed ene gies pe uni a ea e eal ha he δ-BTB laye
has by ∼15 meV/Å2lowe ee ene gy pe uni a ea han he
mixed phase, i.e., he δ-BTB laye is mo e s able. This ene gy
p e e ence is elucida ed by ela i ely s ong Ag−O bonds, wi h
a calcula ed binding ene gy o −1.7 eV, and supplemen ed by
Figu e 7. XPS analysis o pen acene on he compac δ-BTB laye on
Ag(111). (a) C 1s and (b) O 1s spec a measu ed on he compac δ-
BTB laye ( ed), a e pen acene deposi ion (blue), and subsequen
sample annealing a 100 °C (g een).
Table 1. Calcula ed Adso p ion Ene gies Pe Molecule
(Eads) and Ene gies Pe Uni A ea (γ) o Pen acene,
Dep o ona ed BTB (δ-BTB), and In e mixed Pen acene−
BTB Laye , Using PBE-D3 and op B86 Func ionals
a
Eads (eV) γ(meV/Å2)
molecula laye PBE-D3 op B86b PBE-D3 op B86b
pen acene −2.60 −2.35 −20.0 −18.0
δ-BTB −9.36 −9.44 −61.7 −62.0
in e mixed ( om exp.
di ac ion)
−12.05
b
−11.79
b
−42.8 −41.7
in e mixed (mos s able) −12.14
b
−11.94
b
−47.3 −46.4
a
Eads o he in e mixed phases is gi en o a pai comp ising one BTB
and one pen acene molecule, gi ing highe s abili y han pu e
molecula coun e pa s, i.e., a sum o he i s wo ows in a column.
b
Pe pen acene−BTB pai .
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he con ibu ion o a ac i e in e molecula in e ac ions ha
s abilize he δ-BTB s uc u e by an addi ional 0.8 eV pe
molecule. The s ong a achmen o he subs a e esul s in he
p e e ence o BTB adso p ion o e he physiso bed pen acene.
Hence, he comple e δ-BTB laye shows a weak he mody-
namic p e e ence o e he mixed phase.
Now, we will add ess he submonolaye co e ages. The
decisi e pa ame e is s ill he su ace ee ene gy pe uni a ea.
Howe e , in his case, he e is a ee subs a e o accommoda e
all o he adso bed molecules i espec i e o hei bonding
s eng h o he subs a e. Since we a e no es ic ed o he
a ailable su ace a ea, he ene gy pe molecule can be used o
assess he p e e ence o o ming ei he pu e o mixed phases.
Ou esul s show ha he o al adso p ion ene gy pe
pen acene−BTB pai is 90 meV (PBE-D3) o 10 meV
(op B86b) lowe o he in e mixed phase compa ed o he
sepa a e phases. Howe e , he calcula ed s abili y is a ec ed by
imposed s ain and he es ic ion o pe iodically epea ing
molecules ha e ain ene ge ically un a o able posi ions. To
assess he alidi y o he esul s o he mixed s uc u e, we
ha e also compu ed i s s abili y using modi ied supe cells o
simila dimensions bu wi h di e en o ien a ions wi h espec
o he subs a e, as shown in Suppo ing In o ma ion Sec ion
10. In his case, he highes s abili y achie ed a o s he mixed
phase by 180 meV (PBE-D3) and 150 meV (op B86b) pe one
pen acene−BTB pai . These alues p esen a lowe limi o
he s abili y o he mixed phase compa ed o he sepa a e
coun e pa s. In summa y, hese esul s poin o he
he modynamic s abili y o he pen acene−BTB mixed phase
o submonolaye co e ages, which is consis en wi h
expe imen al obse a ions.
In he nex s ep, we e alua e he kine ic ba ie o b eaking
he compac δ-BTB laye . Due o he obus Ag−O bonds
linking he BTB molecules o he sil e subs a e, he mos
likely scena io o dis up ing he δ-BTB laye is o ep o ona e
he ca boxyl g oups, hus weakening hei bonds o he su ace,
allowing hei subsequen de achmen om he su ace. The
dep o ona ed s a e is a o ed o a la -laying BTB molecule,
whe eas he p o ona ed ca boxyl g oup is p e e ed o he
BTB molecule de ached om he su ace. In de ail, o a
de ached BTB, he e is a 1.8 eV ee ene gy p e e ence o he
p o ona ed ca boxyl g oup compa ed wi h he dep o ona ed
g oup and 1/2 o H2molecule, aking in o accoun he
chemical po en ial o molecula hyd ogen unde condi ions
ou inely eached du ing ou expe imen s (−1.07 eV a 25 °C,
2×10−10 mba ). On he con a y, o he la -laying BTB
molecule, he o ma ion o he O−H bond om molecula
hyd ogen is no a o ed; he ee ene gy is by 0.2 eV highe
compa ed wi h he molecula hyd ogen unde UHV condi ions
as he p oximi y o he sil e subs a e weakens he O−H
bond. The e o e, he mos p obable way o dis up he δ-BTB
laye in ol es ep o ona ion o one o he ca boxylic g oups
and i s sepa a ion om he su ace, esul ing in a s anding-up
BTB con igu a ion wi h he o he wo ca boxyla e g oups
a ached o he subs a e.
To es ima e he ene gy ba ie o opening he compac δ-
BTB laye , one BTB molecule in he 2 ×2 supe cell was
a anged in he s anding-up con igu a ion, he li ed ca box-
yla e g oup was p o ona ed by addi ional hyd ogen, and he
whole s uc u e was allowed o elax back o he la -lying
con igu a ion. Figu e 8 shows his p ocess as a unc ion o
angle αbe ween he z-axis and a no mal ec o o he plane,
which in e sec s he cen al phenyl ing. The de achmen is
composed o wo modes: Fi s , he nonlinea up o 27°and he
o al ene gy di e ence be ween wo limi ing con igu a ions o
0.87 eV; wi hin his in e al, he a ac i e in e molecula and
molecule−subs a e in e ac ions a e b oken. The second mode
shows a linea end wi h an ene gy s ep o 18 meV pe 1°.
This beha io holds up o 70°, in which he o al ene gy
di e ence is es ima ed o be 1.7 eV. Ini ial and inal s uc u es
a e p o ided in Suppo ing In o ma ion Sec ion 10. On he
Ag(111) su ace, he ac i a ion ene gy o he dissocia ion o
hyd ogen molecules amoun s o 1.3 eV,
57
which is signi ican ly
la ge han he ba ie o 0.87 eV o he laye opening. This
makes he hyd ogen dissocia ion he a e-limi ing s ep and he
δ-BTB laye also kine ically s able a oom empe a u e.
Discussion o he O igin o he Robus ness o he
Compac δ-BTB Laye . Ou expe imen al da a and DFT
calcula ions show he he modynamic p e e ence o he
o ma ion o mixed δ-BTB−pen acene phases. Howe e , a
he ull co e age, he δ-BTB laye becomes p e e ed. This
seemingly con adic o y s a emen comes om he s ong
binding o ca boxyla e g oups o he sil e subs a e, which
de ines he molecula laye s uc u e. Hence, he o he e ec s
can ake place only i all BTB molecules a e bound o he
subs a e. Thus, o submonolaye co e ages, he e is a ee
a ea o sa is y he s abili y condi ion o he o ma ion o he
mixed pen acene−BTB phases, which a e o med in he
p esence o supe c i ical
58
pen acene concen a ion.
The compac δ-BTB laye can be ob ained by deposi ing >1
ML o BTB and subsequen sample annealing a he speci ic
empe a u e. The excessi e BTB deso bs om he su ace,
esul ing in a compac δ-BTB laye wi hou emaining BTB in
he second laye . In con as , ob aining he ull laye o he
BDA molecules (p e ious s udies) was challenging as hey
display signi ican deso p ion om he i s laye a empe -
a u es close o ull dep o ona ion. On Ag(111), he maximum
co e age o he ully dep o ona ed BDA phase was a ound
50%, and on Ag(100), i was be ween 90 and 95%.
In he o ma ion o he compac laye o dep o ona ed
ca boxylic acid molecules, he capabili y o illing he esidual
open si es is essen ial. This can be done by illing he gaps wi h
molecules om he second laye . In he case o BTB, he e a e
Figu e 8. De achmen o one singly p o ona ed BTB molecule om
he δ-BTB laye . Fo cla i y, only he molecule being de ached is
shown. The de achmen p ocess is desc ibed as a unc ion o he angle
αbe ween he z-axis and a no mal ec o o he plane ha in e sec s
he cen al phenyl ing (ma ked as blue in he inse ). This p ocess is
composed o a nonlinea mode up o 27°and 0.87 eV ( ed line).
Abo e 27°, he end is linea up o 70°wi h an ene gy s ep o 18
meV pe 1°(black line).
ACS Applied Ma e ials & In e aces www.acsami.o g Resea ch A icle
h ps://doi.o g/10.1021/acsami.3c18697
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