Effects of Microstructure Formation on the Stability of Vapor-Deposited

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Downloaded by guest on September 27, 2021 www.pnas.org/cgi/doi/10.1073/pnas.1821761116 a Moore R. Alex of stability glasses the vapor-deposited on formation microstructure of Effects tbeglass stable time deposition relevant between move the to on scales. unable domains propose are phase-separated we tails of results, local where these clustering mechanism on to trapping Based surface. due surface. a free the reduced at the markedly molecules to also the tails is fluorocarbon diffusion of the Surface of films due segregation liquid bulk-like, the essentially equilibrium become to times and supercooled relaxation slowed the PVD. are the molecules increased, of during these is surface surface length of the tail free the ability near the as reduced at that, the find equilibrate to We to linked molecules further in is these results films. stability PVD length in reduced This molecules tail The glasses. the molecule of stable stability increasing thermodynamic form decreased that to ability suggests how super- the model the examine influences in to liquid domains microphase-separated PVD cooled the simulated of to presence use tails tendency the We increased fluorocarbon an microstructures. containing coarse-grained to form corresponding study formers length, we glass increasing of work, organic now this of is In molecules models challenge. glass-forming years. emerging of glasses of types an PVD thousands different of properties for with and aged formed structure to the glasses thought over of control properties those Exerting exhibiting inter- to materials, an equivalent are of 2018) be 21, (PVD) class December deposition review new for vapor (received esting physical 2019 15, by February formed approved and Glasses NJ, Princeton, University, Princeton Debenedetti, G. Pablo by Edited 19104 PA Philadelphia, Pennsylvania, nsml oe 2–6 n tmsi tde 2,2)alike. 27) (23, studies atomistic lower- and expected used (24–26) been the model since create simple has protocol to vapor in This able the films. higher-stability is mirror energy, that computationally process to deposition algorithm have (23) Pablo an de and developed Singh detail. study microscopic further in to systems simulations these (MD) dynamics molecular to and turned process glass PVD this of properties of the dynamics 22). on (21, effect surface films large thousands a the have or can to hundreds therefore integral for to are aged (relative rate been equiva- temperature Substrate have be years. to that of thought those films greater to low-energy lent (SME) have energy high-density, equilibration potential may in surface-mediated the results This surface of 20). minima free (19, deeper landscape the at sug- states at been additional to also accessibility molecules sample has being that It of to before (14–18). gested surface able layers positioning subsequent free local are by optimal the constrained molecules find at that and mobility are configurations such enhanced glasses film, the stable the use These to (6–13). thought stability thermodynamic and temperature transition (T deposition temperature the cre- while substrate (PVD), molecules deposition organic vapor of physical using packings Recent ated glass (5). that lithography shown have nano-imprint studies to (4) coatings protective A diffusion surface eateto hmcladBooeua niern,Uiest fPnslai,Piaepi,P 90;and 19104; PA Philadelphia, Pennsylvania, of University Engineering, Biomolecular and Chemical of Department nlgto h icvr fteesal lse,mn have many glasses, stable these of discovery the of light In napiain agn rmognceetois(–)to (1–3) electronics organic from used widely ranging are applications molecules organic in small of films morphous | ufc dynamics surface | a irsrcueformation microstructure eri Huang Georgia , (T dep | g hsclvprdeposition vapor physical eosrt eakbekinetic remarkable demonstrate ), shl eo h aeilsglass material’s the below held is ) b aa Wolf Sarah , T g n deposition and ) b | arc .Walsh J. 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Z.F., new A.R.M., and and G.H., contributed data; A.R.M., analyzed G.H. research; designed research; R.A.R. performed and Z.F., P.J.W., contributions: Author fet fitroeua neatosadtepoest for propensity the and interactions stable intermolecular generate of to effects used be broaden can details and that improve- properties, molecules glasses. substantial mechanistic PVD of film the key classes glass for the PVD-engineered allow elucidate of process, could ment SME study- Further the effects stability. behind losing these microstruc- without these they extent imprinted ing what microstructures be to resulting can even or intermolec- the tures PVD, of and upon effects create general, the can of in nature forces poten- exact ular to glasses, the remains about much stable However, learned mobility. of hydrogen be surface formation limited that to the suggests due tially (35–38), inhibit This, work could stability. recent bonding kinetic other hydro- lowering with increasing who exhibit along with (34), capability derivatives al. bonding triazine et gen Laventure model (30–33). by that films PVD recently showed of demonstrated stability the was on effect This large a have to seem interactions a intermolecular remains particular, molecules In organic challenge. fundamental small of types with different agreement with formed good in mole- were the that of (28) anisotropy (29). surface experiments in the trends near observing cules depo- by the surface during process, molecules liquid the sition equilibrium of orientation molecule’s the demon- a and to properties between simulations link MD the used strate also coworkers and Lyubimov owo orsodnesol eadesd mi:[email protected] Email: addressed. be should correspondence whom To b ufc,rsligi eue iae omto ept tail despite formation and bilayer formation, reduced segregation. free surface SG immediate in for the resulting factor below surface, is only occur SGs the rearrangements form not significant sur- to is seg- that diffusion ability surface demonstrate face the we the Importantly, Combined, reduced. to tails. significantly due the the of slow while regation clustering, dynamics to for- relaxation due microstructure surface slows to microstructure-forming diffusion prone of surface more mation, stability molecules the For explore molecules. the to simulations below used dynamics equilibra- relaxation are molecular surface molecular Coarse-grained in surface. and factors free diffusion two surface of role tion: the deposition. investigate vapor for- physical the We upon in (SGs) critical glasses is stable of equilibration mation surface of rate enhanced An Significance ar Fakhraai Zahra , nti ok eamt ytmtclyeaiethe examine systematically to aim we work, this In glasses PVD of properties the controlling and Predicting PNAS NSlicense.y PNAS a,b | ac 6 2019 26, March n oetA Riggleman A. 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CHEMISTRY a sphere interacting with a Lennard-Jones potential. To represent the different sizes and polarizabilities of the body, b, groups and the tail, t, groups, Lennard-Jones parameters were chosen as σbb = 1.0, bb = 1.0, σtt = 0.6, and tt = 0.05, which are consis- tent with the relative sizes and polarizabilities of the constituents (39). For the cross-interactions between b and t components, standard Lorentz–Berthelot mixing√ rules were applied such that σbt = (σbb + σtt )/2 and bt = bb tt . The substrate was generated by taking a slice of a disordered Lennard-Jones system of density ≈ 1, and substrate interactions were neutral with both b- and t-type particles. For each molecule, Tg was determined by measuring the potential energy during constant pressure cooling ramps of bulk systems of 1,000 molecules in a box with peri- odic boundaries in the x, y, and z directions, and identifying the temperature at which the thermal expansion coefficient changed. For zero-tail and one-tail molecules, Tg = 0.55; for the four-tail molecule, Tg = 0.48; and for the eight-tail molecule, Tg = 0.41. This trend of Tg decreasing with increasing tail length agrees well with experimental differential scanning calorimetry (DSC) results for the one-tail and eight-tail molecules, which show Tg s Fig. 1. Molecular representation of the fluorinated (F) one-tail and eight- of 323 and 298 K, respectively. The DSC curves can be seen in tail organic glass formers (left) and corresponding coarse-grained models SI Appendix, Fig. S1. The PVD process was simulated using a (right). Body Lennard-Jones particles are colored blue, and tail particles are method similar to the one developed by Lyubimov et al. (25). colored gray. The time allowed for a deposited molecule to cool on the surface of the film was varied to achieve the effect of different deposition rates. A “normal” deposition rate allowed 150 τ, and a microstructure formation on the surface mobility and stability “slow” deposition rate allowed 300 τ. To create the equilibrium of simulated PVD glasses. To gain insight on the local struc- liquid, films created via the PVD protocol were heated to well ture and mechanism, we use MD simulations to study empirical above their glass transition temperature, T = 1.5 Tg , allowed to coarse-grained models of organic molecules containing model relax, and subsequently cooled to T = 1.2 Tg . Radial distribution fluorocarbon tails of increasing length: zero, one, four, and eight functions of one-, four-, and eight-tail films in the equilibrium liq- fluorocarbons. The organic glass formers of interest here, shown uid state and the as-deposited state can be found in SI Appendix, in Fig. 1, are made up of two distinct sections: the phenyl “body” Fig. S2. More details of the simulation methods can be found in and the fluorocarbon “tail.” These groups exhibit vastly differ- Materials and Methods. ent dispersion forces, and thus, locally, each component will avoid mixing with each other. Our coarse-grained model represents each benzene and each fluorocarbon as one of two distinct Results Lennard-Jones sites, also shown in Fig. 1. By varying the length of Structure and Stability of PVD Glasses. Several films of the model the tail section, we can tune the degree to which these molecules fluorocarbon tail molecules (zero-, one-, four-, and eight-tail) are able to microseparate, and then examine the impact this has were deposited using the algorithm outlined in Materials and on the resulting PVD film properties. We note that, although the Methods and a Tdep ranging from 0.75 to 0.95 Tg . Fig. 2 demon- energy and length scale of the interactions were chosen based strates the difference in the cluster formation between molecules on the relative polarizabilities and sizes of the phenyl and fluo- of varying tail lengths. Analysis of these microstructures rinated carbon groups, no effort was made to quantitatively map was performed by cooling the as-deposited and transformed between a higher-resolution (e.g., atomistic) model. In Results, we observe a clear trend indicating decreasing PVD film stability and increasing microstructure formation with increasing tail length. We then study the dynamic properties of these model glass formers, namely, the mean squared displacement (MSD) at the free surface and the relaxation time of the equilibrium supercooled liquid as a function of depth from the free surface. These results lead us to propose a trapping mechanism in which the longer-tail molecules are less likely to escape from locally phase-separated domains at the surface, which is supported by the short-time behavior of these molecules just after deposition and observed trends in stability at slower deposition rates. Furthermore, the longer tails tend to segregate to the free surface, thus slowing the dynamics at the layers right below the free surface where the tails are depleted. The tail segregation effect is also observed in the PVD films, but, surprisingly, does not lead to bilayer formation, as molecular orientation and rearrangement farther below the free surface reduces the degree of segregation and prevents the formation of layers. Fig. 2. Visualizations of the tail particles in the aggregate analysis performed on PVD films of one-, four-, and eight-tail systems (left to right). Simulation Details Each of these films was deposited at 0.75 Tg at the typical 150 τ deposition For the coarse-grained models used in this study, each phenyl or rate. Aggregates are defined as tail particles within 1 σbb of each other. CF2/CF3 group in the molecule of interest was represented by Color is generated by a randomly assigned identifier.

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Surface Liquid Supercooled less to lead liquid-like microstructures glasses. resulting PVD more stable the inter- overall the and in potentials differences with the action that films indicate results PVD These structure. form distribution molecules radial tail the in Additionally, shown functions counterparts. tail tails, to and shorter relative glasses temperatures longer poten- onset liquid-quenched their lower with to in demonstrably transformation changes molecules upon film smaller rel- those energy the exhibit of tial clusters, shows, in range larger emerges 3 entire thus the and trend Fig. across clear As that, a find stability. can studied, We as temperatures 3. lengths, Fig. ative subsequent tail in various and seen with heating be glasses upon change as-deposited energy the of potential cooling observe system’s to the simulations in ramping temperature used in found complete be A can S3 Fig. aggregates microstructures. system near-percolating them- of form arrange distribution form to can ability to molecules the larger selves lack while fur- molecules 2 clusters, smaller-tail Fig. widespread glasses. that liquid-quenched shows and ther PVD or between temperatures or deposition different rates, between represented observed was cluster size largest the and 93.4% represented tails, 19.1 cluster contained largest cluster the and average the tails, 13.7% molecules, 3.8 four-tail 0.7% the 1.5 contained just For within was represented cluster system. film the molecules were any in one-tail in tails pair of cluster total the largest films the if observed and all tails, another across as size considered cluster ter was same particle the a 1 that in such be defined to cluster, a 0.75 into to ticle films liquid supercooled of ease for curves energy potential the shift to comparison. added been has constant A V lso ahmlcl yeat type molecule each of films PVD 3. Fig. or tal. et Moore σ oivsiaetesaiiyo h iuae V ls we films, PVD simulated the of stability the investigate To bb fec te.Qatttvl paig h vrg clus- average the speaking, Quantitatively other. each of . eprtr aprsls lte sptnileeg vs. energy potential as plotted results, ramp Temperature ftesse;frteegtti oeue,teaverage the molecules, eight-tail the for system; the of ftesse.N infiatdfeec ntecluster the in difference significant No system. the of i.S2 Fig. Appendix, SI T dep T = g 0.90 n otn ahti par- tail each sorting and , olanmr bu the about more learn To T ugs httelonger- the that suggest g n 150 and T τ 1 = eoiinrate. deposition T IAppendix, SI g .2 hntheir than T T /T g We . g for , of S6 ntemlclrpcigna h nefc.Gvntefavor- particles the body the Given particles, interface. tail the the of near arrangement surface variations packing able the molecular to time the can relaxation we in the 4A, in Fig. four-tail variations with the these comparing for addi- attribute by time In and, relaxation molecules, the surface. eight-tail in the and peaks local from are farther the there in extends tion, decrease that a shows time which in relaxation molecule, layer, zero-tail top the very to the until contrast particular, up in behavior molecule, relaxation eight-tail bulk-like The shows times 4B). increasing relaxation (Fig. of state with liquid plot particles rium the diminishes body in also seen the film as free of the length the into tail at increased depth dynamics increas- with propagation enhanced with of its scale degree and the length surface Thus, in size. and tail deviation fraction ing in in stronger average grows tails system that surface, the zone free from depletion the that tail to seen a migrate one-, to in be tendency the resulting strong can of a show It films films molecules. liquid these eight-tail equilibrium and for 4A layer four-, Fig. each layer. thick- in each of particles in layers sites into body state and equilibrium this 1 in ness film each divided h oa oyrlxto iev.nraie egtfo the from height normalized vs. time (C relaxation and body substrate, local the (B) strate, 4. Fig. i.S5 Fig. liq- equilibrium S4 of Fig. films Appendix, for average, (T system uid representing lines black solid hw alrlxto times. relaxation tail shows = shows oa alpril rcinv.nraie egtfo h sub- the from height normalized vs. fraction particle tail Local (A) σ 1.2 bb T nthe in g B C B A fec oeue ahfimi 50 is film Each molecule. each of ) cldrltv obl eaaintm,and time, relaxation bulk to relative scaled S nthe in MSD ) shows z PNAS ieto n esrdtenme ftail of number the measured and direction A ln ihpril ubrdensity, number particle with along | ac 6 2019 26, March x −y ln ftefe ufc ae,with layer, surface free the of plane z hw h rcino tail of fraction the shows | oiini h equilib- the in position σ o.116 vol. bb o60 to | Fig. Appendix, SI σ bb o 13 no. IAppendix, SI nheight. in | 5939 SI

CHEMISTRY a predominantly surface diffusion-based mechanism for achiev- ing stable PVD glasses. This work demonstrates that relaxation below the free surface plays a role in the ultimate structure of PVD glasses, although it may not necessarily lead to increased stability. While differences between surface relaxation dynamics and surface diffusion in small organic molecules have been highlighted in the past (40), few experiments have highlighted the potential role of surface dynamics other than the surface diffusion in the formation of stable glasses (18). The clear evidence for rearrangement at layers below the free surface in this study suggests that the SME process may involve more than just the Fig. 5. Local tail particle fraction vs. height from the substrate, with solid surface diffusion suggested in the past. Interestingly, this process black lines representing system average, at 20%, 40%, 60%, 80%, and 100% also hinders layering from the free surface, preventing the forma- of one instance of the deposition process for the one-tail and eight-tail tion of bilayers in these systems, despite signiﬁcant clustering of molecules at 0.90 Tg. The color of the line changes from dark to light the the molecules with increased tail length. Thus, we do not observe farther along in the deposition process. any longer-range structures or domains, and, instead, we observe disordered clusters.

show preference to the secondary layer, and this change in pack- The Effect of Cluster Formation on Stability of PVD Glasses. Based ing leads to longer relaxation times in this region. Fig. 4C also on the information gleaned from the equilibrium liquid behav- compares the MSD along the x−y plane for all particles within ior of these molecules, we propose a mechanism for the relative the free surface layer. We can see that the molecules with longer instability of PVD films of molecules with increasing disparity tails exhibit subdiffusive behavior, and the MSD for the eight-tail in the interaction potential. The increasingly bulk-like relax- molecule exhibits a caging plateau with little discernible surface ation time and the subdiffusive surface behavior suggest that, diffusion. as molecules are introduced to the film’s free surface, molecules with longer tails will quickly find a configuration in which the Dynamics of the PVD Glass Surfaces. The tendency for tails to body and tail are matched with like sections, and then become migrate to the surface also impacts the formation of the vapor- trapped, as they find it increasingly difficult to sample addi- deposited glasses, as can be seen in Fig. 5. This figure shows tional configurations. This tendency toward local domain sep- snapshots of the film composition throughout one deposition aration interferes with the enhanced mobility typically present instance at 0.90 Tg . The tail composition is enriched at the sur- at the free surface. Thus, molecules with longer tails will find it face layer, but, as the deposition process continues, this initial more difficult to rearrange into configurations that enable opti- layering is reconfigured. This suggests that the liquid-like mobile mal packing, and the resulting PVD glasses will be generally layer at the film free surface extends beyond the length of these less stable. tails, even for the eight-tail molecules where the free surface To determine whether this phenomenon held on the free sur- dynamics are slowed. SI Appendix, Fig. S7 looks further into the face of a film during the deposition, we monitored the behavior displacement distribution of tails at different locations relative to of the eight-tail molecules in the short time immediately after the free surface. deposition. To get a sense of how the mobility of these molecules The existence of mobility below the free surface in the changes with local configuration, we measured the MSD of the deposited glasses is further demonstrated by the results in Fig. 6, tail particles in the x−y plane as well as a tail coordination num- which show the orientation of the unit vector along the end- ber, Ctail, defined as the number of tail particles within 1.5 σbb to-end axis of the tail of the eight-tail molecule during the of a tail particle, over the 300 τ that the molecules were allowed deposition instance shown in Fig. 5. Here, to characterize this to relax after the initial surface contact. Fig. 7 shows the results orientation, we use the P1 orientational order parameter, calcu- of these measurements for all tail particles, and two subgroups: lated using the dot product of this unit vector, n(z), for a tail a distance z away from the free surface, with the unit normal to the substrate, nz , such that a positive value of P1(z) indicates that AB the tail of the molecule is oriented toward the free surface of the film. Note that the location of the free surface, and thus the ori- gin of this z axis, is evolving throughout the deposition process. Fig. 6A shows that tails in the initial surface layer (red) are the most likely to be oriented toward the free surface during deposition, while those in the secondary layer (blue) are in the process of reorienting to become more isotropic, like those in the bulk of the film (black). Fig. 6B then shows the dynamic behavior of this end-to-end tail vector over the course of a deposition via the P1 bond autocorrelation function, Cb , of tails originating at different distances relative to the free surface. More information on these calculations can be found in SI Appendix. Here, the molecular reorientation beyond the initial surface layer can be directly observed. Tails in the surface layer have greater initial mobility, Fig. 6. (A) P1(z), orientational order parameter, calculated using the dot before plateauing after around 50 molecules as they are forced product of the end-to-end tail vector with the unit normal to the substrate, as a function of the distance of the center of the tail from the free surface of to orient in the positive direction, and then more rapidly reori- the film. A positive value indicates that the tail is oriented toward the free enting as they enter the secondary layer. Those tails beginning in surface of the film, and colors represent the surface, secondary, and bulk

the secondary layer show a clear reorientation from initial posi- regions. (B) P1 bond autocorrelation function (Cb) of this same tail vector tion before then becoming trapped in the bulk-like state. This is plotted vs. number of subsequent molecules deposited for tails originating a remarkable observation given that previous work has suggested in the three distinct regions.

5940 | www.pnas.org/cgi/doi/10.1073/pnas.1821761116 Moore et al. Downloaded by guest on September 27, 2021 Downloaded by guest on September 27, 2021 i.7. Fig. or tal. et Moore of effects the the on separation studying domain to local approach and systematic interactions a intermolecular take we Here proposed Conclusions the using made predictions the results mechanism. with These one- aligned temperatures. the substrate well tested contrast, are each is lowest for In three which improvement the glass. dramatic film of more cooled a exhibit liquid films to molecule a glass tail to unstable an identical the from nearly temperature, only substrate lowest is the improvement for con- and, our intervals, within difference fidence no tested to lowest little the show molecule above temperature films substrate eight-tail all for molecule, eight-tail than sta- the films For film films. molecule PVD one-tail in the impact for significant slower bility more the a by that tem- made see imposed rate deposition We quickly deposition state. higher liquid is supercooled the limit equilibrium thermodynamic to the are a opposed kinetics where as in temperatures, peratures, factor, change these depositions greater limiting at a the the because, exhibit expect energy, to we temperatures potential that substrate Note coarse- lower other 28). well at of (26, depositions compare vapor models molecule simulated one-tail grained in the seen for those in energy with decrease in percent changes the temperatures. deposition of The of spectrum form the the across energy in potential rates deposition be slow vs. would films films. molecule molecule eight-tail one-tail the for the that for than greater gain stability expect would the we while holds, that mechanism rate, exploration trapping the greater slower if allowed Thus, the distance. generally from are molecules mobility one-tail additional the much gain the not 0.75 using temperatures, of molecules 0.95 range eight-tail same to the and across rate one- deposition the slow of films additional a into trapped. already directly therefore trapped deposited are are become and area or and tail-rich region locally mobile tail-rich longer- these a initially that entering either seems upon are it proposed, molecules as deposition tail allowed just the Thus, of end period. the time by plateau to appears MSD smaller the overall an as in well as change mobility the lower from much domain-separated show locally beginning themselves tail-poor find tails. fairly other which than Those the quickly more in a much toward number began evolve coordination and greater which mobility average tails than greater those exhibit regions that (C seen coordinated be highly initially can were which (C tails undercoordinated those initially were which tails those groups. these to which belonging tails for between tails coordination those represents line C blue the particles, initially tail for surface all film represents the with contact 300 initial a the following immediately time, vs. tail AB i.8dmntae h bevdsaiiyted o typical for trends stability observed the demonstrates 8 Fig. generated then we mechanism, proposed our verify further To > 0 (C 60. τ C S nthe in MSD (A) lwdpsto fegtti oeue t0.90 at molecules eight-tail of deposition slow T tail g ceai eosrtn h ifrnei ipaeetand displacement in difference the demonstrating schematic A ) < i.4idctsta h ih-almlclsshould molecules eight-tail the that indicates 4 Fig. . C tail 0 n h e ierpeet hs al o hc initially which for tails those represents line red the and 40, cosalti atce nti ih-alsystem, eight-tail this in particles tail all Across . x −y alcodnto number, coordination tail (B) and plane T g h lc line black The . tail tail < > and 40) .It 60). C tail , ih-almolecules. eight-tail (D) and of ordi- spectrum and the film across PVD glass between nary difference energy potential Relative molecules. vs. energy 8. Fig. as such despite macrostructures such, segregation, As 20). tail (19, into surface falling surface strong from the system the near the ignore from states prevent clusters that results low-energy tail barriers the our energy where interpret large perspective, this could landscape below energy one films an PVD Similarly, the layer. throughout con- surface density a the in tail below resulting average layers reorientation, stant molecular in allow enough to fast PVD surface are free the times molecules, eight-tail relaxation and the surface liquids for the even supercooled remarkably, However, equilibrium in immediately glasses. both resulting relaxation layers surface for down increased, the slow to times is in acts which length segments surface, free tail the body below the of as density surface enhanced free the to from stability of show terms in which time. little rearrangement rates, gain longer deposition molecules depo- eight-tail slower after the at immediately that simulations times and short mobility at sition, eight-tail configuration of mech- local observations This by and position. supported packing further is optimize deposition, anism upon to trapped unable the locally therefore interactions, become and to intermolecular are and in they molecule, disparity likely the more of the tail larger the the longer the thus mea- that Using suggest we surface. films, free the the film in the MSD of on surements molecules the the of in behavior changes and potential glasses by measured liquid-cooled temperature. as onset over glasses, change PVD stable energy corresponds less length generally tail increasing to that separated observe locally we then form length, We to tail clusters. molecules the the increasing for phenyl tendency by the with that, increase immiscible show first are We which groups. length, body increasing coarse-grained fluorocar- of a containing tails using molecules bon done glass-forming organic is of This model glasses. PVD of properties CD A eas bev togtnec ftetist segregate to tails the of tendency strong a observe also We the into insight provides films liquid equilibrium Studying eprtr aprslsat results ramp Temperature T /T g eight-tail (B) and molecules one-tail (A) of film PVD a for , PNAS | x ac 6 2019 26, March −y T dep ln n oa eaaintm in time relaxation local and plane o V lso (C of films PVD for B T dep = 0.85 | o.116 vol. T g lte spotential as plotted , n-almolecules one-tail ) | o 13 no. | 5941

CHEMISTRY bilayers do not form in these systems, and only disordered of three particles, with four possible angles (90◦, 120◦, 180◦, 109.5◦, clusters that can span the system are observed. The enhanced kangle = 500). relaxation below the free surface during PVD suggests that sta- All MD simulations were run using the Large-scale Atomic/Molecular Mas- ble glass formation involves more than just equilibration due to sively Parallel Simulator (LAMMPS) (41) package in the NVT ensemble and time step of 0.002. The simulation box was 15 σ by 15 σ in the − plane the free surface diffusion. bb bb x y and was always allowed at least 10 σbb of vacuum space above the free sur- Understanding the effects of various chemical changes on the face of the ﬁlm. The PVD process was simulated using a method similar to surface mobility and packing efﬁciency of organic glass formers the one developed by Lyubimov et al. (25) using a number of deposition

provides us with valuable insight into the fundamentals of this cycles until the film was grown to ∼50 to 60 σbb. A cycle consists of (i) intro- complex process. The mechanism proposed in this study provides ducing a new, randomly oriented molecule, above the film free surface, (ii) a step toward effectively engineering the structure and stability of linear cooling of the molecule from the high temperature, T = 1.0, to Tdep, PVD glass films. and (iii) minimizing the energy of the system. To create the equilibrium liquid, films created via the PVD protocol were heated to well above their glass transition temperature, = 1.5 , allowed Materials and Methods T Tg to relax, and subsequently cooled to T = 1.2 Tg. Local relaxation time was For the coarse-grained models used in this study, each group in the measured by sorting particles according to their position relative to the z molecule of interest was represented by a sphere interacting with a axis, dividing them into bins of equal size, and then fitting the self part of Lennard-Jones potential truncated and shifted using a linear decay term −1 the intermediate scattering function, Fs(q, t) with q = 7.14, evaluated at e . to ensure the potential and force go continuously to zero at the cutoff. The cutoff distance for the potential is r = 2.5 σ . Substrate par- ACKNOWLEDGMENTS. This work was funded by National Science Founda- c bb tion (NSF) Designing Materials to Revolutionize and Engineer our Future ticles were fixed in their original positions with a harmonic potential (DMREF) Grant DMR-1628407 and partially by Materials Research Science (k = 50). Harmonic bonds were placed between each benzene or fluoro- and Engineering Center (MRSEC) Grant DMR-1720530. The authors grate- carbon pair which are connected in the organic molecule, with lengths fully acknowledge computational resources provided by Extreme Science adjusted accordingly such that l = 1.0, 0.667, or 0.333 and kbond = 50. and Engineering Discovery Environment (XSEDE) facilities through Award Additionally, appropriate harmonic angles were placed between groups TG-DMR150034.

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