Nitrogen Nanobubbles in a-SiOxNy Coatings: Evaluation of Its Physical Properties and Chemical Bonding State by Spatially Resolved Electron Energy-Loss Spectroscopy

Depósi o de in es igación de la Uni e sidad de Se illa h ps://idus.us.es/ "This documen is he Accep ed Manusc ip e sion o a Published Wo k ha appea ed in inal o m in ACS CATALYSIS, copy igh ©Ame ican Chemical Socie y a e pee e iew and echnical edi ing by he publishe . To access he inal edi ed and published wo k see h ps://doi.o g/10.1021/acs.jpcc.5b09036” 1 Ni ogen Nanobubbles in a-SiO x N y Coa ings: E alua ion o i s Physical P ope ies and Chemical Bonding S a e by Spa ially Resol ed EELS Be and Lac oix*, Vanda Godinho, and Asunción Fe nández* Ins i u o de Ciencia de Ma e iales de Se illa, CSIC - Uni e sidad de Se illa A . Amé ico Vespucio 49, 41092 Se ille, Spain ABSTRACT Nanopo ous silicon based ma e ials wi h closed po osi y illed wi h he spu e ing gas ha e been ecen ly de eloped by magne on spu e ing. In his wo k he physical p ope ies (densi y and p essu e) o molecula ni ogen inside closed po es in a SiO x N y coa ing a e in es iga ed o he i s ime using spa ially esol ed elec on ene gy-loss spec oscopy (EELS) in a scanning ansmission elec on mic oscope. The pape o e s a de ailed me hodology o eco d and p ocess mul iple EELS spec um images (SIs) acqui ed a di e en ene gy anges and wi h di e en dwell imes. I is demons a ed an adequa e ex ac ion and quan i ica ion o he N-K edge con ibu ion due o he molecula ni ogen inside nanopo es. Co e-loss in ensi y and N chemical bond s a e we e e alua ed o e ie e 2D maps e ealing s able high densi y o molecula ni ogen ( om 40 o 70 a ./nm 3 ) in nanopo es o di e en size (20 o 11 nm). This wo k p o ides new insigh s on he quan i ica ion o molecula N 2 apped in po ous ni ide ma ices ha could be also applied o o he sys ems. I. INTRODUCTION Silicon based ma e ials a e s ill nowadays key o he de elopmen o mode n echnology. Al hough signi ican basic esea ch has been unde aken in he ecen decades, he esea ch in silicon oxyni ide (SiO x N y ) coa ings is s ill a opic o high in e es . 1-4 The abili y o modeling he op ical and elec ical p ope ies by con olling he composi ion makes o SiO x N y a p omising ma e ial o op oelec onic de ices. 5-7 A con inuous change in i s composi ion allows la ge e ac i e index anges om 1.47 o pu e silicon dioxide up o 2.3 o pu e silicon ni ide o e en 4.75 o dense amo phous silicon coa ings 8 which is e y appealing o low and high con as index applica ions. Among o he deposi ion echniques SiO x N y coa ings a e mos commonly g own by plasma enhanced chemical apo deposi ion. Composi ional changes a e achie ed by a ying ni ous oxide (N 2 O) and silane (SiH 4 /N 2 ) p ecu so a ios wi h op ional addi ion o ammonia Page 1 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 (NH 3 ). Howe e , he high hyd ogen con en in hese plasma-enhanced chemical apo deposi ion ilms in he o m o Si-H o N-H bonds causes op ical losses a wa eleng hs a ound 1500nm. 3-4, 9 Addi ional annealing s eps be ween 1000 and 1150°C, o plasma ea men s a 300°C ha e been p oposed o educe he H con en 3-4, 10 howe e limi ing he ype o subs a es o be used. We ha e ecen ly in oduced a new bo om up me hodology o p oduce silicon based po ous coa ings wi h con olled e ac i e index by magne on spu e ing. 8, 11-13 This new app oach gi es he possibili y o ob ain ailo ed mic os uc u es and e ac i e indexes based on he in oduc ion o closed po osi y, illed wi h he spu e ing gas, in coa ings p oduced by magne on spu e ing. 8, 12, 14-15 SiO x N y coa ings wi h simila composi ion and ailo ed e ac i e index 11-12 as well as pu e o doped po ous silicon laye s 8, 16 can be deposi ed wi hou he inco po a ion o H. In addi ion o he ad an ages o easy scale-up o he magne on spu e ing p ocess we ha e demons a ed he easibili y o p oducing silicon pho onic s uc u es by his p ocedu e 16 on a wide a ie y o subs a es om polyme s o glass. Simila closed po es mic os uc u es can be ound in ion-i adia ed o ion-implan ed hin ilms 17-23 which ha e been poin ed ou as ab ica ion ou es o nanos uc u ed ma e ials and su aces. 19-25 Gene ally hese mic os uc u es consis in ew po es si ua ed a a ce ain dis ance om he su ace; while wi h ou me hodology, highly po ous s uc u es wi h uni o m po osi y om he subs a e o he coa ings su ace a e deposi ed, in a wide ange o hicknesses ( om 100 nm o ew mic ons). F om a undamen al poin o iew hese new ma e ials wi h embedded po es, illed wi h he spu e ing gas, ep esen a new class o nanocomposi e ma e ials p esen ing on demand op ical p ope ies in compa ison wi h he ma ix ilm. To ully unde s and and con ol hese p ope ies and also o he s abili y and in eg i y o he coa ings in possible applica ions, he physical pa ame e s (densi y, p essu e) de ining he s a e o he gases inside he po es become an impo an issue. In a ecen wo k we ha e in es iga ed he s a e o Helium gas inside he po es o magne on spu e ed amo phous silicon ilms, by spa ially esol ed elec on ene gy-loss spec oscopy (EELS). 15 Th ough hese 2D spec oscopic maps a he nanoscale, we ha e been able o demons a e ha He is loca ed inside he nanopo es o he coa ings in a condensed s a e, wi h densi ies as high as ens o He a oms/nm 3 and p essu e close o he GPa ange. 15 The way o measu e he He densi y consis ed in ex ac ing he in ensi y o he He-K edge (loca ed a abou 23 eV) and doing he quan i ica ion by conside ing he ze o-loss in ensi y, he inelas ic c oss-sec ion o He and he po e hickness . 15,18 The p essu e was hen e alua ed using a sui able equa ion o s a e. 15 In p inciple, a simila app oach could be used o quan i y o he elemen s apped inside po es, like ni ogen in ou case. Howe e , con a y o he case Page 2 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 o nanobubbles o nanopo es illed wi h helium o which quan i ica ion has al eady been achie ed in a ious s udies, 15, 18, 26-29 only a ew wo ks a emp ed o e alua e he N 2 densi y and p essu e inside closed ca i ies. 30-32 Mos o hem we e done in a semi-quan i a i e way based on he a ia ion o he N-K edge in ensi y o e he ni ide ma ix and o e he po e. 30-31 Indeed, he quan i ica ion o he ni ogen densi y inside small po es is a mo e challenging p oblem han o helium. The main eason is ha he N-K edge posi ion is qui e a om he elas ic peak and due o he limi ed dynamical ange o he CCD de ec o s, he low-loss (LL) and co e-loss (CL) ene gy ange ha e o be eco ded sepa a ely, wi h di e en dwell imes. 33 To ou knowledge, only one wo k o he li e a u e 32 has epo ed he quan i ica ion o ni ogen densi y om EELS based on Re . 18, bu e y ew de ails o he analysis we e gi en (in pa icula he way o eco d and p ocess he EELS da a o ge he ze o-loss and N-K edge in ensi ies). In addi ion, his s udy was pe o med on an ideal case wi h a su ounding ma ix ee o ni ogen. In his con ex , his a icle p esen s o he i s ime he ex ac ion o he N-K signal due o molecula ni ogen and he quan i ica ion o he densi y and p essu e o N 2 condensed inside small po es su ounded by an oxyni ide ma ix. The LL and CL EELS spec um images we e acqui ed consecu i ely and ha e been p ocessed ollowing he ecen wo k by Bobynko e al. in Re . 34. A be e app oach would ha e been o use he so-called DualEELS echnique (no a ailable in ou s udy). Howe e , as an al e na i e o DualEELS, we sugges he e o acqui e he LL and CL SIs sepa a ely and o ealign hem spa ially du ing a pos p ocessing s ep. Ou s udy will ocus on nanopo es illed wi h N 2 and o med in SiO x N y ilms deposi ed by magne on spu e ing acco ding o ou p e ious wo k. 11-12 Howe e , his app oach may also be gene alized o allow o s udy o he sys ems, including ni ide ma e ials, and con aining ca i ies illed wi h molecula ni ogen ei he o med du ing syn hesis 30-31, 35-36 o nanos uc u ing p ocesses like ion i adia ion o implan a ion. 19, 22-23 Finally a compa ison wi h mac oscopic esul s ob ained p e iously om Ru he o d backsca e ing spec ome y (RBS) measu emen s will be p esen ed o con i m ou EELS quan i ica ion a he nanoscale. II. EXPERIMENTAL METHODS In o de o ha e closed po osi y illed wi h N 2 , he SiO x N y coa ing chosen o his s udy was deposi ed on Si (100) subs a e, as desc ibed in Re . 12, by eac i e magne on spu e ing using a mix u e o ni ogen and oxygen as spu e ing gas ( o al p essu e o 1.33 Pa, oxygen pa ial p essu e o 1x10 -4 Pa). A RF powe sou ce a 150 W was used o spu e a pu e Si a ge (Ku J. Leske 99.999%). To ob ain he la ges po es acco ding o ou p e ious wo k, 12 he Page 3 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 sample holde was placed a a dis ance o 10 cm om he magne on head (2” ION’X magne on om Thin Film Consul ing, Ge many). T ansmission elec on mic oscopy (TEM) expe imen s we e ca ied ou a he Ad anced Labo a o y o Nanoscopies and Spec oscopies-LANE a he ICMS (Se ille, Spain) using a FEI Tecnai F30 mic oscope ope a ed a 300 keV. C oss-sec ional TEM specimens we e p epa ed by mechanical polishing and dimple g inding ollowed by A gon ion milling, un il elec on anspa ency was achie ed. Jus be o e inse ion in o he TEM, he specimen inside he holde was cleaned in O 2 /A plasma. The coa ing mo phology was s udied by con en ional b igh ield (BF) and high esolu ion TEM (HRTEM) imaging. To analyze he ma ix and po es composi ion, elec on ene gy-loss spec oscopy spec um images (EELS-SIs) we e eco ded wi h a GATAN GIF Quan um ene gy il e using he scanning TEM high-angle angula da k ield (STEM-HAADF) mode. The EELS spec a we e eco ded o e 2048 channels wi h a dispe sion o 0.25 eV/channel. The con e gence and accep ance semi-angles we e se o 12.4 m ad and 9.6 m ad, espec i ely and he p obe cu en anged be ween 50 and 90 pA. In o de o measu e he densi y o ni ogen apped inside he po es, EELS-SIs we e eco ded consecu i ely in he CL and LL anges, wi hin a ew minu es o in e al. The ene gy anges used o eco d he da a ( om -50 o 462 eV o he LL, and om 210 o 722 eV o he CL) we e se o ge a su icien numbe o channels o o e lapping ene gies o subsequen escaling. Elec on p obe spacing o 1 o 3 nm wi h subpixel scanning (16x16) was used and di e en in eg a ion imes in bo h anges we e se o a oid signal sa u a ion on he CCD ( ypical alues a e 0.5 second/spec a o he CL and 0.02 second/spec a o he LL). The ene gy esolu ion o 1.5 eV was de e mined a he ull wid h a hal maximum o he elas ic peak. Du ing he acquisi ion, spa ial d i co ec ion was applied and he EELS da a we e co ec ed o he da k cu en and he gain a ia ion o he CCD de ec o . Da k cu en co ec ion was imp o ed a e each SI acquisi ion using he unc ion p o ided by he GATAN Mic oscopy Sui e 2.1. The ilm bulk composi ion was de e mined by RBS using he 3 MV andem accele a o o he Na ional Cen e o Accele a o s in Se ille (Spain). The spec a we e measu ed wi h a 2 MeV He + beam and he de ec ion angle was 165°. The expe imen al spec a we e analyzed using he SIMNRA simula ion code. 37 III. RESULTS AND DISCUSSION Mic os uc u e, bonding and composi ional in o ma ion o he coa ing Fo his s udy a pa icula po ous silicon oxyni ide coa ing wi h a bulk composi ion Si:O:N = 35:20:45 a . % as gi en by RBS, p esen ing 26% o po osi y and a e ac i e index o Page 4 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 1.60 was selec ed (mo e de ailed cha ac e iza ion o his coa ing is p esen ed in p e ious wo ks 11-12 ). The BF TEM image in Fig. 1a p esen s an o e iew o he SiO x N y coa ing mic os uc u e whe e he po ous s uc u e can be seen in b igh con as . Po es om abou 3 o 20 nm in size a e obse ed. In he high magni ica ion image shown in Fig. 1b i is possible o obse e in mo e de ail he ound shape o he po es and he absence o c ys al plane ea u es. The Si-L, N-K, and O-K edges p o ide use ul in o ma ion on he bonding s a e o hese elemen s in he SiO x N y ma ix and con i m he p esence o molecula N 2 inside he po es. Figu e 1c shows he ene gy-loss nea -edge s uc u es (ELNES) eco ded ac oss he ma ix and ac oss one po e compa ed o e e ence edges o amo phous SiO 2 , 38 Si 3 N 4 , 39 as well as O 2 40 and N 2 41 . These ELNES we e ex ac ed om he singula sca e ing dis ibu ion by i ing he signal p io o he edge wi h a powe -law unc ion. P io o he edge ex ac ion, he EELS-SIs we e p ocessed ollowing he p ocedu e desc ibed la e in his wo k. As can be obse ed in Fig. 1c.1, he Si-L edges o he ma ix and po e egions exhibi simila shapes, wi h ou main ea u es labelled `a1´, `a2´, `a3´ and `a4´ a abou 105 eV, 113 eV, 127 eV, and 156 eV, espec i ely. Such ea u es a e closed o he Si 3 N 4 e e ence and hus indica e ha Si-N bonds domina e in he p esen SiO x N y coa ing. In SiO 2 , hese ea u es a e sligh ly shi ed owa ds he highe ene gies. The e o e, he la ge peak wid hs obse ed o SiO x N y may also indica e he p esence o Si-O bonds. The O-K ELNES eco ded ac oss he po e and he ma ix does no p esen signi ican di e ences (Fig. 1c.3). The spec a p esen wo main ea u es iden i ied as `c2´ and `c3´ a abou 538 eV and 560 eV and ha e s ong simila i ies wi h he SiO 2 e e ence, which indica es ha oxygen a oms ha e simila local a omic en i onmen han in silica. The shape o N-K edge changes signi ican ly wi h he posi ion o he elec on p obe (Fig. 1c.2). Ac oss he ma ix, he ELNES is e y simila o he inge p in in Si 3 N 4 , wi h wo main b oad ea u es `b2´ and `b3´ a 405 eV and 422 eV. In he po e a ea an addi ional in ense na ow peak `b1´ appea s a he onse a ound 401 eV, co esponding o he signa u e o molecula ni ogen N 2 as obse ed p e iously in Re . 12. These da a con i m ha he po es a e illed wi h molecula ni ogen. The composi ion o he SiO x N y ma ix has been de e mined om hese edges using he Digi al Mic og aph p og am. Acco ding o Re . 42, an in eg a ion window o 25 eV was se o Si-L, while he N-K and O-K edges we e in eg a ed o e 100 eV. An a e age ma ix composi ion Si:O:N 33:25:42 a . % was calcula ed using Ha ee-Sla e c oss-sec ions. I is wo h o no e ha he composi ion ob ained by quan i ica ion o he EELS spec a o he ma ix is in qui e good ag eemen wi h he bulk composi ion o he coa ing measu ed p e iously by Ru he o d Page 5 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 6 backsca e ing spec oscopy. The alue o he ma ix composi ion by EELS quan i ica ion will be used la e o calcula e he inelas ic mean ee pa h o elec ons in he specimen. EELS-SI da a p ocessing o composi ional analysis The nex sub-sec ions desc ibe he s eps necessa y o p ocessing he da a in o de o quan i y he N 2 densi y inside he po es su ounded by he amo phous oxyni ide ma ix. An Ad hoc semi-au oma ed code unning wi h MATLAB has been de eloped o allow he da a analysis o e la ge da ase s. I is a ailable on demand. P io o he analysis, X- ays spikes we e emo ed om he SIs using he ou ine p o ided wi h he GATAN Mic oscopy Sui e 2.1. Spa ial and ene gy alignmen o he SIs and noise educ ion Since he LL and CL EELS-SIs we e eco ded sepa a ely, o ensu e ha each pixel o he wo EELS-SIs co esponds o he same specimen a ea, he wo da ase s we e i s ealigned spa ially. Fo ha pu pose, he STEM-HAADF images eco ded in he low-loss and he co e-loss anges we e used as e e ences and submi ed o c oss-co ela ion. Mo e in o ma ion abou he spa ial alignmen o he LL and CL da ase s can be ound in he Suppo ing In o ma ion. In a second s ep, he da ase s we e ealigned along he ene gy axis. Figu e 2a shows he STEM-HAADF signal o he po ous coa ing acqui ed in pa allel wi h EELS spec um images. On his image, he in ensi y is p opo ional o he a omic numbe and he hickness o he sample, he da k a eas co espond o phases wi h lowe Z, and in his case he po es a e da ke han he su ounding ma ix. Figu es 2b-d p esen wo examples o LL and CL spec a eco ded ac oss he ma ix and ac oss one po e a e he alignmen p ocedu e. In he LL ange, he spec a we e shi ed in o de o ge o each pixel posi ion he ze o-loss peak (ZLP) a he same channel co esponding o ze o ene gy-loss (Fig. 2b). On Fig. 2b, he bulk plasmon can be seen a abou 22.6 eV. An addi ional peak a ound 14 eV, a ibu ed o he ca i y plasmon, 28, 43 is also obse ed when he p obe is loca ed ac oss he po e. An addi ional b oad ea u e can be obse ed be ween 15 o 19 eV, which can be en a i ely a ibu ed o su ace s a es o he SiO x N y ma ix. This ea u e is mo e impo an in he po es egions whe e he ma ix laye is hinne han in he bulk ma ix. The Si-L edge also appea s in he LL ange nea 100 eV, see Fig. 2c. Howe e , he N-K edge a ound 400 eV is no isible on he LL spec a due o he low exposu e ime. In he CL ange, he spec a we e shi ed by aligning he onse o he O-K edge a e e ence ene gy o 532 eV. Figu e 2d clea ly shows he N-K edge, whose signa u e depends on he posi ion o he elec on p obe: ou side he po es, he N-K edge p esen s he wo b oad bands ypical o he SiO x N y ma ix 12 whe eas he signal eco ded ac oss he po e has an addi ional na ow peak a he onse (401 eV) which is a ibu ed o molecula ni ogen N 2 . 12 Page 6 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 The choice o O-K ins ead o N-K as e e ence edge o he alignmen was mo i a ed by he ac ha i s shape emains unchanged wha e e he posi ion o he p obe is. As can be obse ed on he blue cu es o Fig. 2, he EELS spec a con ain a subs an ial noise componen ha can a ec he backg ound sub ac ion o u he quan i ica ion. In o de o educe his con ibu ion, p incipal componen analysis (PCA) 44 was applied on he LL and CL HL da ase s a e he alignmen s ep. The signal was hen econs uc ed wi h a g ea ca e o a oid he loss o ele an spec oscopic in o ma ion by keeping a limi ed (bu su icien ly la ge) numbe o componen s, as shown on he ed plo s on Fig. 2. Mo e de ails on PCA a e gi en in he Suppo ing In o ma ion. Splicing o he LL and CL spec a and mul iple sca e ing decon olu ion As he LL and CL EELS da a we e eco ded wi h di e en exposu e imes, he nex s ep consis s in splicing he spec a oge he pixel-by-pixel. This s ep has been pe o med wi h special ca e because esidual noisy signal in he LL splicing egion (due o low exposu e ime) may in oduce addi ional e o s in he de e mina ion o he scaling ac o . An example o spliced spec um and addi ional de ails ela ed o he splicing p ocedu e a e gi en in he Suppo ing In o ma ion. Elec on mul iple sca e ing can s ongly a ec he shape o he ene gy-loss nea -edge s uc u es (ELNES) especially when he specimen hickness inc eases. 45 In he case o po ous ma e ial wi h po es o se e al nm in size, changes in he specimen hickness c ossed by he elec on beam occu . In o de o compa e he edge shapes and in ensi ies o di e en posi ion o he p obe, i is necessa y o emo e hese plu al sca e ing e ec s. Fo ha pu pose, mul iple sca e ing decon olu ion was applied o he spliced spec a using he Fou ie loga i hmic me hod (see FLOG ou ine in Re . 45). An example o spec um ob ained a e decon olu ion is p o ided in he Suppo ing In o ma ion. Quan i ica ion o molecula ni ogen inside he po es In o de o sepa a e he wo o e lapping signals coming om he ma ix and om po es, he N-K edges we e be o ehand ex ac ed om he decon olu ed spec a by i ing he signal p io o he edge wi h a powe -law unc ion. Figu e 3a illus a es he ex ac ion p ocedu e o he N 2 componen . An a e age con ibu ion o he ma ix was i s de e mined om a se o pixels ou side he po e egions. The ma ix spec um was hen scaled up o ma ch he in ensi y o each ex ac ed N-K edge spec um in he da ase . To ind he adequa e scale ac o , leas squa e i ing was pe o med in wo egions: be o e he edge (390 - 398 eV), and be ween 403.5 and 405.8 eV whe e he con ibu ion o he molecula ni ogen o he N-K Page 7 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 edge spec um is closed o ze o. The scaled SiO x N y ma ix con ibu ion was inally emo ed o ob ain he molecula ni ogen componen . In Fig. 3a, one can no ice ha he shape o he molecula con ibu ion ex ac ed om ou sample is simila o he N 2 e e ence om McLa en e al. 41 Mo eo e , he N 2 con ibu ion is negligible ou side he po e while i is e y s ong ac oss he po e, as expec ed. A con ibu ion o he ma ix is s ill p esen due o he SiO x N y walls su ounding he closed po e. All hese obse a ions allow us o alida e ou ex ac ion p ocedu e. This app oach was pe o med pixel-by-pixel and he in ensi ies we e in eg a ed o e 100 eV o e u n a 2D map ep esen ing he spa ial dis ibu ion o each componen . As can be seen on Fig. 3b, he amoun o molecula ni ogen ac oss he po e a ea is closed o 50% (blue egion) indica ing he p esence o a high N 2 con en . The densi y o molecula ni ogen inside he po es   can now be e alua ed om an app oach simila o ha in oduced in Re . 18 using:   =      ℎ(1) In equa ion (1),   ep esen s he in eg al in ensi y o he molecula con ibu ion on he N-K edge.   is he angle-in eg a ed c oss-sec ion o he N-K edge. I was calcula ed in ou expe imen al condi ions wi h he hyd ogenic model using he SIGMAK3 ou ine. 45 Bo h   and   we e de e mined o e an ene gy ange o 100 eV s a ing a he edge onse . A la ge in eg a ion window was chosen o a e age ou chemical bonding e ec s and maximize he signal, as sugges ed by C ozie and Chenna 46 . The pa ame e ℎ is he local po e (o ca i y) hickness c ossed by he elec on beam, which was de e mined as he complemen o he local hickness measu emen s o he ma ix. 15, 26, 28 Fo ha pu pose, he absolu e specimen hickness  was compu ed by combining he calcula ions o he ela i e specimen hickness / wi h he inelas ic mean ee pa h . / was de e mined by he Log- a io me hod. 45  was ob ained using he pa ame ized app oach desc ibed by Malis e al. 47 aking in o accoun he ma ix composi ion de e mined p e iously. As he highes sou ce o unce ain ies ce ainly comes om he ℎ pa ame e , he po e hickness de e mina ion was c oss-checked wi h he measu emen o he po e size  on he STEM-HAADF images assuming sphe ical po es. The in eg al in ensi y o he elas ic peak   was also de e mined and s o ed o each posi ion du ing he calcula ion o ℎ. Figu e 4 and Table I p esen he esul s ob ained on he la ges po e o he da ase (Po e#1 in he able). The 2D maps in Fig. 4a-d ep esen he no malized HAADF in ensi y, /,   and   alues. The ni ogen densi y map, shown in Fig. 4e, was calcula ed using   =2427 ba n and  =161 nm. An a e age densi y   =45 a oms/nm 3 was de e mined om a se o pixels in he po e cen e , which co esponds o abou 23 N 2 molecules/nm 3 (1.04 g/cm 3 o Page 8 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 FIGURES FIG. 1. (a) BF TEM image o he SiO x N y coa ing showing he nanopo ous s uc u e (in b igh con as ). (b) High magni ica ion image eco ded a he icini y o a ious po es. The image was acqui ed in sligh unde - ocused condi ions in o de o see he con ou o he po es. (c) ELNES eco ded ac oss he ma ix and ac oss he po e o he SiO x N y coa ing compa ed wi h e e ences o Si 3 N 4 and SiO 2 , N 2 and O 2 (c.1) Si-L edge. (c.2) N-K edge. (c.3) O-K edge. Page 15 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 16 FIG. 2. (a) STEM-HAADF image eco ded o e a ious po es (in da k con as ) simul aneously wi h he EELS da a. Labels (1) and (2) indica e he pixels chosen o plo ing he aligned EELS spec a eco ded ac oss one po e and ac oss he ma ix, be o e and a e PCA, in di e en ene gy anges. (b) Low-loss ange wi h he ZLP and plasmons con ibu ions. (c) Middle-end o he low-loss ange wi h he Si-L edge isible a abou 100 eV. (d) High-loss ange wi h he N-K and O-K edges isible a abou 400 and 532 eV. Page 16 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 17 FIG. 3. (a) Ex ac ion o he molecula con ibu ion om he N-K edge (ac oss he po e and ac oss he ma ix). The g een a eas indica es he ene gy anges whe e he ma ix con ibu ion was scaled o he N-K edge. The N 2 e e ence spec um shown o compa ison was ob ained om Re . 41. (b) 2D map showing he a io o in ensi ies (in eg a ed o e 100 eV) o he wo N- K componen s (molecula /ma ix). Page 17 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 18 FIG. 4. 2D quan i ica ion o he molecula ni ogen inside one po e. (a) STEM-HAADF map, (b) ela i e specimen hickness map, (c) ze o-loss in ensi y map, (d) molecula N-K in ensi y map, (e) ni ogen a omic densi y map. Each pixel on he maps co esponds o 2 nm. Page 18 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 19 FIG. 5. E olu ion o he molecula ni ogen densi y as a unc ion o he in e se o he po e adius 1/ (wi h  = ℎ/2) assuming ha he po es a e sphe ical. The ed dashed line ep esen s he linea i o he da a. Page 19 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 20 FIG. 6. (a) E olu ion o he N-K edge eco ded in he cen e o a po e o di e en esidence ime o he elec on p obe. EELS-SIs we e eco ded consecu i ely o e he same po e (abou 25 nm in size) using an in eg a ion ime o 0.5s/pixels. Fo compa ison pu pose, he edges we e aken om he same pixel posi ion ac oss he po e and ex ac ed by powe -law i ing. Excep he use o a dispe sion o 0.1 eV/channel, he o he expe imen al condi ions a e simila o ha p esen ed in Sec. II. The esidence ime o he p obe was de e mined as he numbe o pixels eco ded ac oss he po e mul iplied by he in eg a ion ime pe pixel, as p oposed by Da id e al. (Re . 29). (b) Va ia ion o he in eg al in ensi y o he N-K edge in he [399-403]eV ange. Page 20 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 21 TABLES TABLE I. Ni ogen densi y and p essu e measu ed inside po es o di e en size. The po e size ange  was de e mined om he HAADF images. The mean alues o ℎ,   and   we e es ima ed om STEM-EELS analysis o e a se o pixels aken ac oss he po e cen e . The e o s co espond o he s anda d de ia ion. Po e  (nm)  (nm)   (a ./nm 3 )    (GPa) #1 18 - 20.5 18 ± 2 45 ± 3 0.8 #2 12 - 13.5 11 ± 2 72 ± 5 7.5 #3 13 - 14 14 ± 2 69 ± 6 6.3 #4 14 - 20 20 ± 4 40 ± 12 0.5 #5 11 - 15 16 ± 2 59 ± 10 2.8 Page 21 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 22 Table o Con en s (TOC) Page 22 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 23 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 - 1 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 350 400 450 500 550 600 100 200 300 400 ( 2 ) ( d )( c ) a e a l i g n m e n a e P C A I n e n s i y ( a . u . ) E n e g y l o s s ( e V ) ( a ) ( b ) ( 1 ) p o e ( 1 ) m a i x ( 2 ) I n e n s i y ( a . u . ) E n e g y l o s s ( e V ) a e a l i g n m e n a e P C A p o e ( 1 ) m a i x ( 2 ) a e a l i g n m e n a e P C A I n e n s i y ( a . u . ) E n e g y l o s s ( e V ) p o e ( 1 ) m a i x ( 2 ) Page 24 o 29 ACS Pa agon Plus En i onmen The Jou nal o Physical Chemis y 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60