Robust Dipolar Layers between Organic Semiconductors and Silver for Energy-Level Alignment

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 Read Online 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 Resea ch A iclewww.acsami.o g © 2024 The Au ho s. Published by Ame ican Chemical Socie y 18099 h ps://doi.o g/10.1021/acsami.3c18697 ACS Appl. Ma e . In e aces 2024, 16, 18099−18111 This a icle is licensed unde CC-BY 4.0 Downloaded ia BRNO UNIV OF TECHNOLOGY on May 14, 2024 a 07:42:06 (UTC). See h ps://pubs.acs.o g/sha ingguidelines o op ions on how o legi ima ely sha e published a icles. 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). 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 ACS Appl. Ma e . In e aces 2024, 16, 18099−18111 18100 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. 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 ACS Appl. Ma e . In e aces 2024, 16, 18099−18111 18101 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). 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 ACS Appl. Ma e . In e aces 2024, 16, 18099−18111 18102 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). 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 ACS Appl. Ma e . In e aces 2024, 16, 18099−18111 18103 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. 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 ACS Appl. Ma e . In e aces 2024, 16, 18099−18111 18104 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. 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 ACS Appl. Ma e . In e aces 2024, 16, 18099−18111 18105 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 . 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 ACS Appl. Ma e . In e aces 2024, 16, 18099−18111 18106 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 ACS Appl. Ma e . In e aces 2024, 16, 18099−18111 18107