Exploiting Physical Vapor Deposition for Morphological Control in Semi-Crystalline Polymer Films

Received: 30 April 2018 Accepted: 12 July 2018 DOI: 10.1002/pcr2.10021

REVIEW

Exploiting physical vapor deposition for morphological control in semi-crystalline polymer films

Yucheng Wang1 | Hyuncheol Jeong1 | Mithun Chowdhury1 | Craig B. Arnold2,3 | Rodney D. Priestley1,3

1Department of Chemical and Biological Engineering, Princeton University, Princeton, Research in semi-crystalline polymer thin films has seen significant growth due to their fascinat- New Jersey ing thermal, mechanical, and electronic properties. In all applications, acquiring precise control 2Department of Mechanical and Aerospace over the film morphology atop various substrates or in the free-standing film geometry is key to Engineering, Princeton University, Princeton, advancing product performance. This article reviews the crystallization of polymer thin films New Jersey processed via physical vapor deposition (PVD). Classical PVD techniques are briefly reviewed, 3Princeton Institute for the Science and Technology of Materials, Princeton University, highlighting their working principles as well as successes and challenges to achieving morpho- Princeton, New Jersey logical control of polymer films. Subsequently, the recent development of a unique PVD tech- Correspondence nique termed Matrix Assisted Pulsed Laser Evaporation (MAPLE) is highlighted. The non- Rodney D. Priestley, Princeton Institute for the destructive technology overcomes the major drawback of polymer degradation by classical Science and Technology of Materials, PVD. Recent advances highlighting how MAPLE can be exploited to control polymer film mor- Princeton University, Princeton, NJ 08544. Email: [email protected] phology in ways not achievable by other methods are presented. Challenges and future scope Funding information of PVD for polymer film deposition concludes the review. Kwanjeong Educational Foundation in South Korea; Division of Materials Research (DMR), KEYWORDS Materials Research Science and Engineering Center, Grant/Award Number: 1420541; confinement, interfacial interactions, matrix assisted pulsed laser evaporation (MAPLE), National Science Foundation (NSF) physical vapor deposition (PVD), polymer crystallization, thin films

1 | INTRODUCTION process of melt-crystallization, the mutually entangled polymer chains are driven by both thermodynamic and kinetic factors into different Polymer crystallization, especially in thin film geometry, is an impor- metastable states comprising both crystalline and amorphous tant topic attracting intensive research efforts to unveil its complex regions.[19] It is strongly influenced by crystallization temperature,[20] nature. The studies are of tremendous importance for functional molar mass,[21] and additives in the bulk composition.[22] In contrast, polymers, which enable a broad range of applications such as in thin films, additional processing parameters such as geometric [1–4] [5–8] [9,10] solar cells, transistors, non-volatile memories, and confinement,[23–25] polymer/substrate interactions,[26–28] and sample sensors.[11–13] Among all the applications, film morphology, arising history of non-equilibrated films[29] can further influence crystalliza- from the complex semi-crystalline nature of polymers, critically deter- tion, thus impacting the resulting film morphology. Furthermore, the mines performance.[8,14–17] For example, it is generally accepted that method in which a thin film is processed, including solution proces- a higher extent of crystallinity in the active layer of semiconducting sing, physical or chemical vapor deposition (CVD), and melt- polymer thin films can enhance device efficiency[8,14,15,18] and a lon- crystallization can profoundly impact the film morphology. In this ger lateral dimension of defect-free crystalline lamellae can reduce gas review article, we highlight recent advances in controlling the film permeability through polymer thin films.[11] Despite its significance, morphology of semi-crystalline polymers produced by physical vapor ways to drive polymer thin film crystallization into a desired morphol- deposition (PVD). We focus on elucidating general processing-struc- ogy has not been fully understood or mastered, especially since the process is known to deviate from bulk crystallization. During the ture-property relationships of films formed by this approach. Com- pared to the more commonly used solution processing techniques, Yucheng Wang and Hyuncheol Jeong contributed equally to this study. that is, spin casting and solution casting, substrate temperature and

Polymer Crystallization. 2018;1:e10021. wileyonlinelibrary.com/journal/pcr2 © 2018 Wiley Periodicals, Inc. 1of13 https://doi.org/10.1002/pcr2.10021 2of13 WANG ET AL. deposition rate can be precisely manipulated during PVD to drive the challenges for each technique in the context of polymer thin film fab- semi-crystalline morphology of thin films.[30–33] It allows for simulta- rication. Next, we summarize previous studies related to vapor depos- neous film growth and crystallization, as recently advocated.[34] Fur- ited semi-crystalline films, highlighting how PVD was applied to thermore, PVD approaches are amenable for co-depositing polymers manipulate film morphology by varying key parameters such as sub- that are insoluble in a common solvent or designing multilayered films strate temperature and substrate type. We then highlight recent [35,36] composing chemically similar or dissimilar polymers. advances in the field by the utilization of MAPLE, with particular The critical roles of geometric confinement, polymer/substrate emphasis on work from our group demonstrating, for instance, the interactions, and crystallization temperature have been reported for ability to control crystal lamellar thickness and extent of crystallinity. [30,37,38] polymer thin film crystallization. To set the context for the Moreover, it has immense potential in the application of polymeric continued need of advancements in top-down PVD, Box 1 provides a semiconducting devices.[48–51] Future opportunities are discussed at brief summary of selected studies exploiting solution-based proces- the end of the review. sing methods to manipulate morphology in semi-crystalline polymer films. Challenges still remain to fully understand and control the crystallization mechanism of polymer thin films. For example, many stud- 2 | PVD TECHNIQUES ies have discovered that the orientation of crystalline lamellae atop attractive substrates would preferentially transition to flat-on as film In this section, we briefly discuss several classical PVD techniques: thickness was decreased.[39–41] Researchers have proposed different sputtering, thermal evaporation, and pulsed laser deposition (PLD). models to explain such experimental observations.[41,42] Nevertheless, We describe the mechanism of film formation and the uniqueness/ understanding how parameters such as polymer/substrate interfacial challenges of each process. Finally, we briefly highlight how each energy affect the crystallization process, for example, crystal orienta- approach has been exploited to deposit semi-crystalline polymer thin tion, still remains a subject of intense debate.[38] Part of the reason films. arise from the difficulty to tune processing parameters in a systematic manner during the fabrication of polymer thin films. In other words, to 2.1 | Sputtering better understand how processing affect morphology, it would be useful to exploit a film fabrication technique that has the ability to A typical polymer sputtering process requires a vacuum chamber [56,57] precisely tune the crystallization conditions at a slow film growth rate. coupled to a radio frequency generator. A target polymer and a For instance, one potential approach is to characterize the nanoscale substrate are placed on the cathode and the anode, respectively. In crystalline morphology of thin films in a layer-by-layer manner, which most cases, deposition is initiated by the creation of energetic parti- would be accessible as a result of top-down film deposition. Such a cles in a chemically inert gas via an ionization process. These particles distinctive approach can be challenging for most processing subsequently bombard the solid target with high kinetic energy, techniques. thereby ejecting molecules from the target which lead to film forma- [57,58] It is, therefore, critical to explore various approaches to proces- tion atop the adjacent substrate. Sputtering is, therefore, a dry sing semi-crystalline polymer thin films. To date, the most fre- process that circumvents the use of solvent. It also supports the fine quently reported film production techniques are solution processes control of deposition rate.[59] Nonetheless, the transfer of kinetic such as drop casting, spin casting,[43] and Langmuir-Blodgett deposi- energy from the particles to the target molecules can result in chemi- tion.[44] In contrast, dry processes such as vapor deposition, includ- cal degradation by random polymer chain cleavage.[60,61] This imposes ing PVD[30] and CVD,[45,46] can provide new perspectives by a big constraint to morphological control and characterization of poly- altering the processing route to which a thin film is achieved. With- mer thin film properties.[62] So far, the employment of sputtering has out incorporating chemical reactions, PVD is a viable way of explor- frequently been used to acquire thin film coatings of fluoropolymers, ing the processing-structure-property relationships of crystalline such as poly(tetrafluoroethylene) (PTFE), for enhancing water-repel- thin films by precisely tuning growth rate, crystallization tempera- lent[63,64] or gas separation properties[65] where polymer degradation ture, and interfacial interactions at distinct stages of film formation. can be tolerated. The major drawback of PVD that limits the ability to undertake systematic studies of polymer thin film has been the prospects of 2.2 | Thermal evaporation polymer degradation during processing due to the high energy input into the target material.[47] Despite this challenge, there are Thermal evaporation deviates from sputtering in that the energy input many examples in which PVD has been successfully applied to con- into the target polymer is in the form of heat. The thermal energy trol the semi-crystalline morphology of polymer films. Furthermore, evaporates the molecules in vacuum. Subsequently, film formation a recently developed PVD technique termed matrix assisted pulsed proceeds by the transfer of evaporated molecules that condense atop laser evaporation (MAPLE) has offered a substantial opportunity to the substrate. In a typical thermal evaporation process, the substrate bring deeper insights into the field by diminishing material temperature is held below the polymer melting temperature to control degradation.[31] the degree of supercooling during deposition. Analogous to sputter- The scope of the present article will be specific to the crystalliza- ing, thermal evaporation is a dry process so that the dissolution of tion of polymer thin films via PVD. We start with a description of clas- polymer is not required. With the control of film thickness spanning sical PVD techniques, including a discussion on the uniqueness and from a monolayer to several microns by varying deposition time, this WANG ET AL. 3of13

[52] BOX 1 crystallization temperature. Similarly, Jeon and co-author observed a morphological transition for low-density polyethyl- CONFINED CYSTALLIZATION OF SOLUTION PROCESSED ene thin films atop silicon wafers when the crystallization tem- POLYMER FILMS [55] perature changed from 90 C to 115 C. As a summary, Polymer thin film crystallization atop substrates can be markedly confinement, polymer/substrate interactions, and crystalliza- different from the bulk process. In thin films, geometric confine- tion temperature are critical factors in determining the final ment and polymer/substrate interactions can play critical roles. film morphology of semi-crystalline polymers. This Box briefly highlights the basic concepts of how confinement/ interfaces can influence polymer thin film crystallization. Confine- ment can be utilized to suppress heterogeneous nucleation of poly- technique has been applied to deposit crystalline polymers such as mer crystals to allow for the attainment of homogeneous polyethylene (PE),[66] poly (vinylidene fluoride) (PVDF),[30] and nucleation. Massa and co-authors created impurity-free π-conjugated polymers.[67–69] The flexibility of substrate type has poly(ethylene oxide) (PEO) droplets atop a thin layer of polystyrene enabled the decoration of thin films atop structured substrates such [23] by dewetting. They successfully achieved and characterized as graphite, polymers, and carbon nanotube (CNT).[70,71] These advan- homogeneous nucleation at large supercooling of micron-sized tages render thermal evaporation as one of the most widely used pro- droplets, as shown in Figure B1A. Homogeneous nucleation was cessing methods for making small molecular weight thin films, observed to occur randomly for equal-sized droplets because the including organic molecules that were found to exhibit exceptional energy barrier to nucleation is only an intrinsic function of PEO properties.[72,73] However, material degradation is commonly reported itself.[23] The nucleation kinetics was determined by the volume of when polymers are used as the target due to the required high energy the droplet, as a faster rate was found for larger droplets.[24] input.[30,66,74,75] The scission of polymer chains may occur by random Interactions at the polymer/substrate interface can influence degradation or a depolymerization mechanism, producing low- the film morphology by changing the crystal orientation.[27] Hu molecular weight fragments that are undesirable for producing a uni- and co-workers applied two substrates, that is, highly oriented form film structure.[47] Despite these challenges, new literature is pyrolytic graphite (HOPG) and amorphous carbon-coated mica, emerging in which in situ polymerization is incorporated into thermal to spin coat poly(di-n-hexylsilane) (PDHS) films.[53,54] Ultrathin evaporation processes to make polymer films[76,77] or the creation of films of PDHS revealed edge-on lamellae atop HOPG as a result [78] of epitaxial crystal growth at the interface of highly ordered sub- low-energy polymer glasses. strates, while flat-on lamellae were found atop substrates coated by amorphous carbon.[27,53] Mareau and Prud'homme also observed flat-on lamellae for ultrathin poly(ε-caprolactone) (PCL) 2.3 | Pulsed laser deposition films atop silicon wafers and carbon-coated substrates, as illus- Pulsed laser deposition is a dry process that utilizes laser pulses at suf- trated in Figure B1B.[39] Additionally, they found a nonlinear ficiently high energy density to ablate a solid polymer target. Upon reduction of the crystal growth rate as film thickness decreased, absorption of photons, macromolecules at the target surface rapidly as the reduction was more pronounced for films thinner than the vaporize, creating a plume composed of polymer that is ejected lamellar thickness. They attributed this suppression of crystalliza- toward the substrate.[79,80] The most frequently used laser sources tion kinetics to a slower diffusion of polymer chains to the growth include ultraviolet (UV),[81] infrared (IR),[82] F ,[83] and CO .[84] By front, imposed by both size confinement and polymer/substrate 2 2 sequentially ablating chemically similar materials, PLD can be readily interactions.[39] Napolitano and Wübbenhorst applied dielectric implemented to design multilayered film structures that are challeng- spectroscopy for spin coated poly(3-hydroxybutyrate) atop alu- ing for solution processing techniques.[35] Unfortunately, material deg- minum coated glass slides.[28] They claimed that the slowing radation by thermal dissociation and photochemical reactions are down of the crystallization kinetics was related to a reduced [47,85] mobility layer, that is, irreversibly adsorbed layer, at the interface commonly observed with direct laser ablation, making PLD between polymer and substrate.[28] unsuitable for applications that require preservation of bulk chemis- [86] Besides the effect of geometric confinement and polymer/ try. Consequently, polymers should be kept at low molecular [31,87] substrate interactions, crystallization temperature also influ- weight for PLD. In order to minimize the destruction of poly- ences the crystallization processes and morphology as it deter- mers, careful selection of the laser source and the corresponding mines chain dynamics. Grozev et al. examined the wavelength should also be considered. For instance, when an ArF crystallization of mono-lamellar PEO crystals in spin coated laser (λ = 193 nm) was used, the deposition of low-molecular weight

poly-2-vinylpyridine-poly(ethylene oxide) films atop silicon (Mw = 1400) polyethylene glycol (PEG) revealed structure modifica- wafers. After performing melt-crystallization for the films in a tion, as identified by Fourier transform infrared spectroscopy.[86] temperature range between 45 C and 57 C, they identified However, no clear observation of chemical modification was found that the crystal structure of PEO changed with temperature, as when the authors switched to a free-electron laser (FEL) with λ = 2.9 can be compared in Figure B1C. Both lamellar thickness and or 3.4 μm, wavelengths that are in resonance with specific bond vibra- width of the side branches increased with increasing tions of PEG.[88,89] 4of13 WANG ET AL.

FIGURE B1 A, Optical microscopy image of a small section of the sample (1000 μm wide) obtained at a crystallization temperature of -2.6C. Amorphous droplets appear dark and semi-crystalline droplets appear white under nearly crossed polarizers. The plot shows the fraction of crystallized droplets as a function of temperature upon cooling (0.4C/min) for homogeneous nucleation. Adapted with permission.[24] Copyright 2004, the American Physical Society. B, Tapping mode atomic force microscopy (AFM) height images of PCL films crystallized at room temperature on carbon-coated substrates at film thicknesses of (1) 15 and (2) 6 nm. Adapted with permission.[39] Copyright 2005, American Chemical Society. C, Typical optical micrographs of crystals in a 40 nm thick film grown at (1) 48C and (2) 52C. The average growth rate of the diagonals from center to tip is about 2.9 μm/min for (1) and 0.9 μm/min for (2), respectively. The size of the images is 65 by 65 μm2. Adapted with permission.[54] Copyright 2008, Springer Nature

3 | CRYSTALLIZATION OF POLYMER THIN achievement of strong piezoelectricity in a single step that would FILMS BY PVD commonly require a multi-step process by other means. Substrate temperature is also known to strongly influence the Here, we discuss priori work that has employed PVD approaches to extent of crystallinity of vapor deposited polymer films. Norton probe or control the crystallization of polymer films deposited atop et al. investigated PTFE films atop glass substrates deposited at different various substrates. By controlling processing parameters such as sub- temperatures by PLD with a KrF excimer laser. By studying the x-ray dif- strate temperature and substrate type, the change in crystal form, fraction (XRD) pattern, they reported the crystalline fraction scaled with extent of crystallinity, or chain orientation has been achieved for substrate temperature from 200Cto350C, while the film remained model polymers such as PTFE, PVDF, and PE. amorphous below 200C.[99,100] Jeong and co-authors also discovered Owing to the slow film growth rate of PVD processes, the crystal- increasing crystallinity of vapor deposited PE crystals with increasing lization of molecules take place at the substrate.[30,90] Therefore, one temperatures in the range from −2Cto100C.[34] Such trend has been major advantage of PVD is that it can effectively tune the degree of related to higher chain mobility enabled at elevated substrate tempera- supercooling by simply altering the substrate temperature during film tures for chain disentanglement and reorganization.[34,100] growth. This key parameter enables tunability toward desired poly- morphism and crystallinity. For example, the ferroelectric polymer, PVDF is known to adopt distinct crystalline forms based on the method of processing.[91,92] PVDF crystalline forms are not perma- nent, as they can be interconverted by applying external heating, electric field, or pressure.[93] Takeno and co-workers produced PVDF films exhibiting α and β forms atop silicon wafers by thermal evaporation at 25C and −190C, respectively.[94] Without the need of extra treatment, the β form showed piezoelectricity because of the in situ supercooled crystallization condition provided.[95] The γ form of PVDF was also made using a substrate temperature of 125C and a much slower deposition rate compared to the previous conditions. The authors further proposed a phase diagram where they claimed the crystal form to be a function of deposition rate and temperature, as illustrated in Figure 1.[96] To enhance the control of dipole orientation FIGURE 1 Phase diagram and molecular orientation in poly for exhibiting piezoelectricity, thermal evaporation can also be (vinylidene fluoride) (PVDF) thin films as a function of the deposition rate and the substrate temperature. α, β, and γ denote the crystal equipped with an electric field or an ionization process.[97,98] The pro- forms in PVDF. The symbols ? and k refer to the molecular longed time scale of film growth by PVD facilitates the control of orientation perpendicular and parallel to the substrate, respectively. chain reorientation under certain conditions, thus allowing for the Adapted with permission from Ref.[30]. Copyright 1994, Elsevier WANG ET AL. 5of13

FIGURE 2 A, Decorated single crystal of a polyethylene fraction. Note that sectorization in four quadrants revealed by the orientation of the decorating rods. Scale bar: 1 μm. Adapted with permission from Ref. [104]. Copyright 1985, John Wiley and Sons. B, Illustration of epitaxial crystal growth of polyethylene on a polyethylene single crystal. Adapted with permission from Ref. [30]. Copyright 1994, Elsevier

Another key parameter that can tune the crystalline film morphol- polymers was also applied to reveal the surface topography of the ogy is the interaction between polymer and substrate. The ease of crystal underlayers, and further be exploited as a means to tune sur- depositing polymers by PVD atop various substrates has been face functionality.[70,102,103] Wittmann and Lotz deposited a decorat- exploited by many researchers to manipulate the molecular orienta- ing layer of PE crystallized atop a solution processed PE single crystal; tion by epitaxial crystallization. The concept of epitaxial growth of a the resulting film structure is shown in Figure 2A.[104] The vapor polymer crystal is often considered as the matching of crystallographic deposited PE aligned as edge-on lamellae atop the crystal substrate, orientation between the guest polymer and the crystalline sub- with chains parallel to the direction of the growth face, as illustrated in [30,104] strate.[26,101] Typical substrates used for this purpose are alkali halide Figure 2B. The crystallization features of the top layer revealed or crystalline polymers.[30,70,74] In 1975, Hattori and co-workers sectorization of the underlying solution processed PE during crystalli- reported the epitaxial growth of low-molecular weight PE by perform- zation. It is worth noting that this type of multiple layered film com- ing thermal evaporation atop KCl substrates.[74] At room temperature, prising the same polymeric material can only be achieved by dry rod-like crystals of PE were formed on the cleaved face of a KCl single processes such as vapor deposition. Applying the same concept but crystal, with the chains preferentially aligned parallel to the substrate. using CNTs as the substrate, Li and co-authors created unique two- dimensional (2D) patterns of PE crystals termed nanohybrid shish The crystal structure developed as ellipse-like and disk-like patterns at kebab (NHSK). Figure 3A is a representative transmission electron substrates temperature of 100C and 150C, respectively. At approxi- microscopy (TEM) micrograph of a PE-coated single-walled carbon mately the same time, the epitaxial growth of thermally evaporated nanotube (SWNT).[71] With a schematic illustration in Figure 3B, the formation of the 2D structure, where the rod-like PE crystals stack with their long axes normal to the nanotube, was attributed to a soft epitaxial mechanism due to the extremely small diameter of the nano- tubes limiting the strict lattice matching of PE crystal chains.[71,105] Film morphology after vapor deposition can also be affected by other parameters including target shape and laser source in the case of PLD. For instance, Li et al. deposited PTFE films atop silicon wafers by PLD from two different targets: polished pellets and pressed powder. After ablation by a KrF (λ = 248 nm) excimer laser at a substrate temperature of 300C, the film made from the pressed powder was

FIGURE 3 A, TEM image of PE-decorated SWNTs. Arrows indicate “nanocentipede-like” 2D nanostructures, which are named 2D nanohybrid shish kebabs. SWNTs form the central shish and PE rods form the kebabs. The inset shows a “nanonecklace” formed by PE decorating on a SWNT loop. Note that PE rods are locally perpendicular to the SWNT axis. (b) Schematic representation of the 2D NHSK. PE chain axes are perpendicular to the PE rod crystals, while they are also parallel to the SWNT axis. 2, 3, and 4 show the PE chain orientation on CNTs with different chirality. No matter whether the CNT possesses an armchair (2), a zigzag (3), or a chiral configuration (4), PE chains are always parallel to the CNT axis, suggesting a soft epitaxy growth mechanism. Both images adapted with permission from Ref. FIGURE 4 Schematic of matrix assisted pulsed laser evaporation [71]. Copyright 2006, American Chemical Society (MAPLE) 6of13 WANG ET AL. smoother than the one from polished pellets due to rapid depolymeri- polymer thin film crystallization in greater details. In the next section, zation of the polished pellets.[106] Jiang and co-authors utilized XRD we discuss the unique film formation mechanism of MAPLE, followed and TEM to characterize the film morphology of PTFE atop glass by highlighting our work in advancing the field by employing MAPLE slides or carbon-coated copper grids by PLD using a KrF (λ = 248 nm) to enhance the understanding of structure-property relationships of laser. At a substrate temperature between 200C and 450C, the films polymer films formed by top-down deposition. comprised both amorphous and crystalline regions. The crystalline structure was highly ordered with the long helical molecular chains oriented parallel to the interface plane with the substrate.[100] A simi- 4 | MATRIX ASSISTED PULSED LASER lar film structure for PTFE atop silicon wafers at a substrate tempera- EVAPORATION ture of 300C was also observed using a femtosecond pulsed Ti: Sapphire (λ = 800 nm) laser.[107] Nonetheless, when PTFE films atop Developed nearly two decades ago, MAPLE is a PVD technique that [111] silicon wafers were made by a synchrotron beam (critical wavelength can effectively minimize polymer damage during deposition. While of 1.5 nm), the chain orientation became perpendicular to the sub- it utilizes a laser sources similar to PLD, such as UV excimer [86,112] [48,113,114] [115] strate surface at room temperature deposition, compared to parallel lasers, IR lasers, and FEL lasers, MAPLE uniquely chain orientation of the same films made using a Yttrium Aluminium adapts a sacrificial solvent in the target composition to absorb the Garnet (YAG) laser (λ = 266 nm).[108] Zhang et al. reasoned the differ- majority of the laser energy. Such design preserves the target polymers ence to be the result of a different deposition mechanism. Employing and produces films with identical chemical structure and molecular [86,116] a quadrupole mass spectroscopy, they found saturated fluorocarbons weight to the starting material. A schematic of MAPLE is illus- were ejected from the target by a photochemical reaction mechanism trated in Figure 4. Deposition is initiated by laser ablation on a cryogen- when a synchrotron beam was used, as opposed to a normal laser ically frozen target comprising a dilute polymer solution. Subsequently, ablation yielding radicals of monomers by thermal unzipping of poly- phase explosion occurs at the target surface, releasing a vaporized mer chains.[108] The larger oligomers from the synchrotron irradiation plume containing both polymer and solvent into the vacuum. Film for- were more susceptible to form lamellar structures with the molecular mation proceeds by the addition of polymer molecules, as the volatile [31] axes normal to the interface.[108–110] solvent gets pumped away. The polymer-friendly technique has The current section has discussed the ability of PVD to control been applied to crystalline polymers such as PEO and PE for systematic [34,112,117] the crystal form, extent of crystallinity, and crystal chain orientation crystallization studies. Furthermore, it allows for fabricating – by exploiting the effects of substrate temperature, substrate type and electronic and photonic devices with polymers.[48 50] The size of these other parameters such as target shape and laser source on film mor- polymers range from 3 to 70 kDa. For example, Ge et al. recently phology. However, many of these approaches are limited toward fully reported a solar-cell performance of over 3% by MAPLE-deposited understanding the processing-structure-property relationships due to poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester using polymer degradation. One way of overcoming such challenge is to an Er:YAG laser (λ =2.94μm).[51] Yet application wise, the major limita- employ newly developed techniques. Recently, a PVD technique tion of MAPLE is the unfavorable surface feature. In many cases, termed MAPLE has been proven to minimize chemical modification of MAPLE-deposited films have notable surface roughness with a large polymers. The gentler deposition approach sets the stage for studying size distribution of polymer globules.[114] Molecular dynamics

FIGURE 5 A, Schematic showing the transport of polymer-matrix clusters during MAPLE process. Film fabrication is achieved by the deposition of concentrated polymer droplets. Adapted with permission from Ref. [34]. Copyright 2018, American Chemical Society. B, AFM height image of MAPLE-PEO film formed on Si substrate with Tdep =25C, featuring the growth of 2-dimensional mono-lamellar crystals (MLC) from nucleating droplets marked with N1-N3. The inset schematically shows the structure of PEO crystalline islands. Adapted with permission from Ref. [112]. Copyright 2016, American Chemical Society. C, AFM height image of MAPLE-PE formed on Si with Tdep =75C, showing the morphology of PE nanocrystals consisting of a single lamella or a few stacked lamellae. The inset schematic shows the structure of the PE nanocrystals. Adapted with permission from Ref. [34]. Copyright 2018, American Chemical Society WANG ET AL. 7of13

FIGURE 6 A, AFM height image magnifying the PE nanocrystals consisting of a single lamella formed by MAPLE deposition at Tdep =75C. Adapted with permission from Ref. [34]. Copyright 2018, American Chemical Society. B, Representative height profiles of PE MLCs formed at various Tdep. The MLC formed at Tdep = 116 C had a uniform thickness of ~27 nm, which corresponds to the fully extended chain length of the [34] target PE (Mn = 3000). Adapted with permission from Ref. . Copyright 2018, American Chemical Society. C, AFM height image showing the

PEO film morphology formed on Si with two sequential MAPLE depositions (1st and 2nd MAPLE) at different Tdep; the substrate temperature was maintained at 25C during 1st MAPLE and maintained at 35C during the following 2nd MAPLE, as schematically described in the inset. D, AFM height profiles of the four PEO MLCs marked with P1-P4 in panel (c). The dashed circle highlights ~3 nm step change in the thickness of the four MLCs, which was caused by the sudden change in Tdep from 25 to 35 C during deposition simulation by Leveugle et al. explained the surface roughness to be a the polymers deposit as confined droplets atop a substrate, as schemati- result of the deposition mechanism itself, which involved cluster forma- callyshowninFigure5A.[112,124,125] The diameter of the droplets typically tion consisting of polymer chains.[118] In order to improve the film qual- ranges from tens of nanometers to a few micrometers. Depending on the ity, researchers have managed different ways such as creating substrate temperature and droplet size, the droplets either self-nucleate, emulsions within the target composition[119] and selecting solvents crystallize from a crystal growth front, or remain in the liquid phase. which have high absorption of the laser source.[120,121] The distribution of size in deposited polymer droplets plays a critical role in crystal nucleation due to finite size effects on the rate of nucleation.[24] The nucleation probability is greater in larger liquid droplets.[24] 5 | CRYSTALLIZATION OF POLYMER THIN When polymer droplets are larger than a critical size, they self-nucleate FILMS BY MAPLE during deposition, while smaller droplets remain in the liquid state.[19,126] By short time MAPLE deposition with slow film growth rates, we could Our investigation on crystallization of polymers thin films by MAPLE has examine the early stage film morphology composed of confined polymer been focused on the governing rules of crystallization kinetics occurring droplets. Figure 5B shows an atomic force microscopy (AFM) height during extremely slow film growth.[34,112,117,122] While MAPLE achieves image of the morphology of a PEO (Mn =4600g/mol,Mw/Mn =1.1) concurrent film growth and crystallization similar to other PVD technolo- film deposited onto a silicon (Si) substrate by MAPLE (MAPLE-PEO).[112] gies, it is distinguished in that (1) the polymer is ejected from the target The substrate temperature during deposition (T ) was 25C, which is in the form of a polymer-matrix clusters rather than a gas phase, and dep much lower than the melting temperature of the target PEO (T ~60C). (2) the chemical nature and molecular weight of the target polymer can m The film comprised three distinct structures: nucleating droplets, 2D be preserved. This results in some unique features in crystal nucleation- dendritic patterns consisting of mono-lamellar crystals, and the sur- growth mechanism atop the substrate, and more importantly, allows the rounding amorphous phase. Figure 5B illustrates three large droplets manifestation of long-chain effects of the molecules in crystallization, denoted N1, N2, and N3, which are nucleating droplets formed by self- that is, chain-folding and adsorption to substrate surface. In the following nucleation. Once nucleating droplets form, they render the crystallization section we will address our recent work in advancing the understanding of small droplets into 2D patterns on the surrounding. The resulting of polymer crystallization in thin film geometry by MAPLE. structure is schematically described in the inset of Figure 5B. The liquid layer is formed by small droplets isolated from any crystalline islands and 5.1 | Crystal nucleation and growth mechanism in is incapable of self-nucleation and epitaxial nucleation from growth MAPLE fronts of the crystalline islands. In MAPLE processing, film fabrication is achieved by the assembly of For strong crystal formers, however, such size-effect on nucleation nano-sized to micron-sized polymer droplets rather than individual mole- may not be evident. Figure 5C shows an AFM height image of nanoscale cules. During laser ablation of a target matrix, polymer chains trapped in polymer crystals formed by the deposition of low-molecular weight PE [123] the matrix are entrained together in the ablation plume. Subsequently, (Mn = 3000, Mw/Mn = 1.10) atop Si at Tdep =75C (MAPLE-PE), which 8of13 WANG ET AL.

FIGURE 7 A, Optical image that shows the sample stage of a flash-DSC chip after MAPLE PE deposition. The white dashed line marks the region of the deposited film. B, Plots comparing normalized flash-DSC first heating thermograms of MAPLE-PE formed with various Tdep, ranging from −2 to 116 C. The dotted curve and dashed curve are the normalized heating thermograms of LN2–quenched bulk PE (5.1 mg) and slowly cooled bulk PE (12.7 mg) with a cooling rate of 0.5C/min, respectively, which were obtained using a conventional DSC (bulk-DSC). C, Plots showing the

Tm-Tdep relationship of MAPLE-PE (MAPLE, green circles) and the Tm-Tc relationship of melt-crystallized PE. For melt-crystallization, two methods were used: Isothermal crystallization using flash-DSC (ISO, blue triangles) and non-isothermal crystallization by cooling with various rates using bulk-DSC (NIC, red squares). The target crystallization temperature in ISO and the peak temperature of the exotherm in NIC were defined as Tc. line 1 was obtained from all MAPLE, NIC, and ISO data ranging from Tdep (or Tc) = 75 to 116 C, while line 2 was obtained from MAPLE-PE data [34] ranging from Tdep = −2to75 C. All images are adapted with permission from Ref. . Copyright 2018, American Chemical Society

is more than 50 C below the typical melting temperature of the target In addition, by manipulating Tdep during MAPLE, crystals with dif- [34] PE (Tm ~127C). Most of the isolated PE nuclei were comprised of ferent lamellar thickness can be formed in one film. Figure 6C shows ~10-nm-thick PE mono-lamellae, as schematically shown in the inset of an AFM height image of PEO lamellar crystals, grown using the tem- Figure 5C, which implied that the nucleation capability of PE was suffi- perature protocol described in the inset of Figure 6C. The deposition cient to form single-lamellar nuclei. As a result, the structural formation started with Tdep =25C (MAPLE 1st), temporarily ceased to raise the of PE films formed by MAPLE was highly driven by nucleation of individ- Tdep to 35 C and continued again with Tdep =35C (MAPLE 2nd). The ual droplets, which was in stark contrast to the case of PEO whose self- rise of Tdep from 25 Cto35 C led to a ~3 nm step increase in lamellar nucleation event was only limited to micron-sized droplets. thickness of the growing PEO crystals, as shown in Figure 6D that measures the height profiles along P1-P4 in Figure 6C. Such a sudden

5.2 | Control of lamellar thickness and melting change of Tc while growing different layers of crystals by top-down temperature approach would not be possible in the case of solution processing and melt-crystallization, where crystals rapidly grow from a consolidated The folded-chain structure of polymer crystals has a critical influence on liquid phase. By MAPLE, a delicate control of polymer crystallization the metastability as well as the mechanical,[127–130] and electrical prop- could be realizable by extremely slow film growth combined with the erties of semi-crystalline polymers.[131–134] Therefore, controlling the capability of tuning T by T . lamellar thickness is of significant interest in polymer processing. Since c dep The control over the film morphology with T during MAPLE chain-folding is strongly governed by crystallization temperature,[135,136] dep leads to different thermal properties of the deposited polymer films. precise tuning of the crystallization temperature (Tc) over a wide range Due to the metastability of lamellar crystallites bounded by amor- is desirable to engineer the lamellar thickness in semi-crystalline poly- phous surfaces, their T strongly varies with lamellar thickness, fol- mer films. PVD has a significant advantage in that it enables a facile con- m lowing the Gibbs-Thomson relationship.[41] The strong correlation trol of the substrate temperature at which the target material condensates; this prevents the occurrence of crystallization at untar- between Tdep and Tm in MAPLE-deposited PE films was successfully [34] geted temperature on slow cooling from a liquid state. Conventional verified using flash-calorimetry (flash-DSC) technology. Flash-DSC PVD techniques, however, were not suitable for systematically studying can measure heat flow from an infinitesimal amount of a sample μ the chain-folding behavior due to the fracture of macromolecules during (<1 g) placed on a flash-DSC chip with ultra-high rates of cooling and [137–139] vaporization. In this sense, the use of MAPLE processing was necessary. heating. By MAPLE, PE was directly deposited onto a flash- Via MAPLE, a strong correlation between lamellar thickness of DSC chip as shown in Figure 7A. The temperature of the chip during deposition (=T ) was controlled by a MAPLE substrate heater. the deposited polymers and Tdep has been identified with two model dep Figure 7B plots the flash-DSC first heating thermograms of polymers, low-molecular weight PE (Mn = 3000, Mw/Mn = 1.10) and MAPLE-PE samples obtained with various T , where melting peaks PEO (Mn = 4600 g/mol, Mw/Mn = 1.1). Figure 6A exhibits the AFM dep morphology of PE single-lamellar crystals atop silicon wafers, whose were clearly captured. The manipulation of Tdep over 118 C(−2-116 C) height corresponds to the lamellar thickness. The height profiles of PE resulted in a dramatic change in Tm over 20.6 C(105.7-126.3C). For single crystals formed at four different Tdep, that is, 75, 80, 100, and reference, Figure 7B also illustrates the melting peaks of the same PE 116C are compared in Figure 6B. While the crystals formed with obtained by melt-crystallization in bulk (~mg) with two cooling rates; Tdep = 116 C exhibited a fully extended chain length of ~27 nm, the one by 0.5 C/min (dashed line in Figure 7B) and the other by quench- [34] lamellar thickness clearly decreased as Tdep was lowered. ing into liquid nitrogen (LN2, dotted line in Figure 7B). Those two WANG ET AL. 9of13

FIGURE 8 A, Optical microscopy images captured with 405 nm-laser showing the growth of 2D crystals after additive PEO deposition atop Si. The same region of the sample was captured after 1st, 2nd, and 3rd MAPLE deposition, each of which was performed at Tdep =25C for 2 hours. B, AFM height image showing the morphology of MAPLE PEO film formed after 6 hours of deposition (upper panel) and the corresponding AFM height profile (lower panel). Dendritic 2D crystals, a surrounding nanolayer, and an area scraped with a razor blade (Si surface) are depicted. No depletion zone exists between 2D crystal growth front and the contacting nanolayer. Both images are adapted with permission from Ref. [117]. Copyright 2016, American Chemical Society

extreme cooling rates only resulted in 1.8 CdifferenceinTm,thatis, nucleation capability of the deposited polymer droplets is low, that is, Tm =128.2Cwith0.5C/min and Tm =126.4C with LN2 quenching. when the droplet is smaller than the critical size for nucleation or Tdep This is mainly because a typical melt-crystallization process is limited to is higher than the temperature allowed for self-nucleation. While achieve nucleation at a large supercooling. This comparison clearly MAPLE of PE does not show a noticeable evidence of adsorption due highlights the exceptional tunability of Tm by MAPLE. to the strong nature of PE to crystallize, the effect of droplet size and

How Tdep governs the crystallization of PE during MAPLE can be Tdep on adsorption can be clearly observed in MAPLE-deposited PEO. further clarified by comparing the correlation between Tdep and the PEO films grown atop Si by MAPLE exhibit an adsorbed layer formed actual crystallization temperature of PE during film formation. by the deposition of small PEO droplets that failed to nucleate.[112,117]

Figure 7C plots the Tdep-Tm relationship of MAPLE-deposited PE and MAPLE of PEO at a temperature slightly below the bulk melting tem- Tc-Tm relationship of the same PE by melt-crystallization. Traditional perature (~60 C) can form a film that remains in liquid phase across DSC (bulk-DSC) and flash-DSC were used to obtain the melt- the deposited area.[112,117] crystallization data. Above 75 CofTdep or Tc, all three data sets were Adsorption during MAPLE can lead to the loss of the crystallizability reasonably fit to one straight line (line 1 in Figure 7C), indicating that of deposited polymer films. In the case of metals and molecular com- the crystallization of PE in MAPLE occurred at Tdep (Tdep = Tc). Such lin- pounds deposited via PVD, any deposited species would eventually crys- ear relationship between Tc and the corresponding Tm was expected tallize by forming isolated nuclei or incorporating into any nearby growing crystals.[143,144] In MAPLE of PEO, however, adsorbed molecules for polymer crystals formed at a relatively low undercooling (ΔT = Tm - [140–142] can stay adsorbed persistently even upon contact with crystal growth Tc), where Tm is the equilibrium melting temperature. In the fronts.[112,117] Figure 8A presents the evolution of 2D PEO crystals during Tdep range below 75 C, however, the Tdep-Tm data followed a different the total 6 hours of MAPLE at T =25C atop Si. The deposition was line (line 2 in Figure 7C), at which Tm depression rate was about 3.9 dep times smaller than the rate on line 1. In this case the PE prematurely divided into three distinct 2 hours depositions to capture optical images of the same region in the intervals. While the 2D crystals formed in the crystallized prior to reaching Tdep (Tc > Tdep), suggesting that the charac- first MAPLE showed clear evidence of growth during the second MAPLE teristic nucleation induction time in PE droplets (τnucl) is smaller than interval, no detectable growth of the crystals was observed during the the time scale of heat equilibration at the substrate (τheat) in such a low temperature range. Considering the dominant droplet size of PE in third deposition. Figure 8B captures the AFM morphology of a PEO film MAPLE as 1 μm or below in radius, the target PE material is expected formed after 6 hours of deposition and the corresponding height profile. Clearly, the growth front of 2D crystals was surrounded by an adsorbed to nucleate prior to thermal equilibration (τnucl < τheat) at a temperature layer with nanometer thickness (nanolayer) without a depletion zone. This below ~63 C, which reasonably supports the deviation of Tdep-Tm [34] suggests that the amorphous PEO in direct contact with the crystalline below Tdep ~75C. While the crystallization temperature in MAPLE is also eventually bounded by the nucleation of PE, the non-zero slope face is kinetically stable against crystallization. of line 2 suggests that the depositing PE chains are still effectively The kinetic stability of the adsorbed PEO nanolayer against crystallization can depend highly on deposition temperature. In the above cooled prior to nucleation at temperatures where τnucl < τheat. study, while adsorbed PEO formed at Tdep =25C was stable against crystallization, MAPLE at T =50C rendered strong crystal growth 5.3 | Adsorption of polymers during MAPLE dep from the nanolayer when the substrate temperature was cooled to [117] At the moment of deposition atop a substrate, polymer droplets may 25 C after the deposition. The effect of Tdep on the stability of physically adsorb to the surface, thereby impeding crystallization. As nanolayer was investigated by post-deposition aging of the two hinted previously, such adsorption would be evident when the MAPLE-PEO samples (Tdep = 25, 50 C) at the same aging temperature 10 of 13 WANG ET AL.

FIGURE 9 A, AFM height images comparing the film morphology of MAPLE-PEO film made with Tdep =25C right after deposition (left panel) and after 27 days of post-deposition aging at 25 C under N2 environment (right panel). The yellow dashed lines indicate the position of crystal growth fronts in as-deposited sample, highlighting that there was no growth of the 2D crystals into the nanolayer region during the 27 days of aging. B, Optical microscopy images captured with 405 nm-laser showing 2D crystal growth during post-deposition aging at 25C in a MAPLE- PEO film made at Tdep =50C. The film was quickly transferred onto a 25 C temperature stage after MAPLE for aging and optical microscopy observation. The upper panel was captured right after crystallization in droplet N1 and the lower panel was captured after 1 hour of aging. C, AFM height image (upper panel) showing 2D crystals grown from a nanolayer during post-deposition aging. Left side of the film was scraped with a razor blade. The measured height of the 2D crystals was ~10 nm as depicted in the lower panel. D, AFM height profile of a residual nanolayer after 20 minutes of toluene washing in MAPLE-PEO film made at Tdep =25C. E, Graph comparing nanolayer thickness in MAPLE-PEO film made at Tdep =25C before and after toluene washing. F, AFM height profile of a residual nanolayer in MAPLE-PEO film after 20 min of toluene washing. The original film was made at Tdep =50C. G, Graph comparing nanolayer thickness in MAPLE-PEO film made at Tdep =50C before and after toluene washing. All images are adapted with permission from Ref. [117]. Copyright 2016, American Chemical Society

(Tag =25C) under N2 environment. In both samples the morphology comparison, the AFM profile of a residual nanolayer deposited at Tdep = of a ~5 nm thick nanolayer, formed by 3 hours of MAPLE deposition, 50C shown in Figure 9F clearly exhibited a noticeable loss of PEO after was monitored during aging. Figure 9A compares the AFM images washing. Figure 9G compares the average nanolayer thickness before and when Tdep =25C captured before (left panel) and after 27 days of after solvent washing, which showed a noticeable decrease from ~5 to aging (right panel), which showed no evidence of the growth of 2D ~1 nm. While the origin of how deposition temperature affects the crystals into the nanolayer region. In contrast, when Tdep =50C crys- degree of adsorption is still unclear, the results here imply that the deposi- tallization in the nanolayer during aging at 25 C was observed; see tion temperature in PVD can critically affect the conformation (or energy Figure 9B. Figure 9C exhibits the AFM image of film when Tdep =50 state) of the deposited polymer chains. Another intriguing fact from the C and after crystallization during aging. The ~10 nm height of the 2D above results was that the crystallization of long-chain molecules in PVD crystals (see lower panel of Figure 9C) provides evidence that the can exhibit anomalies compared to other crystalline solids due to their crystallization occurred during aging at 25C. ability to strongly interact with underlying substrates. The stability of adsorbed PEO against crystallization was correlated In this focused section, we have discussed two model polymers to the degree of attractions between the PEO and the underlying sub- deposited by MAPLE and their differences in film morphology and the strate. The degree of adsorption was conjectured by measuring the resid- resulting thermal properties. We first showed that substrate tempera- ual nanolayer thickness of MAPLE-deposited PEO samples after washing ture during MAPLE has a critical impact on crystal lamellar thickness in toluene. Washing the polymer filminasolventcanuncoverloosely and melting temperature. Deposition temperature-lamellar thickness- adsorbed chains, therefore a positive correlation is expected between the – melting temperature relationships observed in MAPLE of PE agree degree of adsorption and residual film thickness.[145 147] Before solvent well with our conventional understanding of polymer crystallization washing, both Tdep =25Cand50 C samples had an ~5-nm thick uniform nanolayer as a result of MAPLE deposition for 3 hours. Figure 9D shows from solution or liquid phase. Due to the rapid cooling effect during the deposition of confined polymer droplets, tuning of lamellar thick- a representative AFM profile of a residual nanolayer deposited at Tdep = 25C after solvent washing. As compared in Figure 9E, there was no ness and melting temperature could be achieved over a wide tempera- meaningful decrease in the thickness of the nanolayer after washing, ture range. In addition, the extremely slow deposition rate of which proved the strong adsorption of PEO to the substrate. In polymers allows for the manipulation of substrate temperature during WANG ET AL. 11 of 13 deposition, resulting in a non-uniform lamellar thickness of crystals in temperature, or effectively the crystallization temperature, we the same film. Next, we demonstrated that the deposition tempera- showed the tunability of melting temperature by 20.6C for MAPLE- ture could affect the stability of the adsorbed polymers formed at the deposited PE. Additionally, we proposed different kinetic stability of substrate surface. In MAPLE deposition of PEO, the adsorption MAPLE-deposited PEO films at various deposition temperatures due strongly affects the nucleation of deposited PEO droplets as well as to the development of irreversibly adsorbed layers during the deposi- the crystal growth kinetics. Interestingly, the stability of adsorbed tion. Despite the distinct film formation mechanism of each PVD tech- layers against crystallization showed a dependence on deposition nique described above, we believe ways to control film morphology temperature. We anticipate structure-property relationships observed and thermal properties by tunable parameters such as substrate tem- for these two model polymers can be generally applied to other semi- perature and substrate type are generally applicable to other top- crystalline polymer systems processed via PVD. down vapor deposition techniques when polymer degradation is prevented.

6 | FUTURE WORK ACKNOWLEDGMENTS

Future directions of using PVD to study polymer crystallization in thin H.J. acknowledges support from Kwanjeong Educational Foundation film geometry are vast. The development of the technique itself in South Korea. C.B.A. and R.D.P. acknowledge the support of the should focus on addressing the limitation of material degradation, National Science Foundation (NSF) Materials Research Science and unfavorable surface features, and technical concerns in commercial Engineering Center Program through the Princeton Center for Com- [148,149] scale-up. Overcoming these challenges will attract more plex Materials (DMR-1420541). R.D.P. acknowledges the support of research efforts aiming to understand the crystallization of polymer the AFOSR through a PECASE Award (FA9550-15-1-0017). thin films in greater details, and to expand the use of PVD application wise. Furthermore, the fundamental studies of polymer film crystalli- ORCID zation should exploit the film formation mechanism of PVD to design Rodney D. Priestley https://orcid.org/0000-0001-6765-2933 film structures such as multilayered films comprising chemically similar or dissimilar polymers, films made by co-polymers or polymer blends, and polymers atop various substrates. By creating different hard or REFERENCES soft interfaces, the effect of heterointerfaces or confinement on the [1] Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, H. Yan, Nat. Commun. 2014, 5, 5293. crystallization kinetics, and the role of irreversible adsorption on the [2] L. Lu, T. Zheng, Q. Wu, A. M. Schneider, D. Zhao, L. Yu, Chem. Rev. film properties can be further explored. This will inspire ways of engi- 2015, 115, 12666. neering film morphologies and material properties to meet various [3] J. R. Tumbleston, B. A. Collins, L. Yang, A. C. Stuart, E. Gann, W. Ma, application needs. 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YUCHENG WANG received his B.S. in Chemical Engineering from Purdue University in 2015. He is currently a Ph.D. candidate in the How to cite this article: Wang Y, Jeong H, Chowdhury M, Department of Chemical and Biological Engi- Arnold CB, Priestley RD. Exploiting physical vapor deposition neering at Princeton University. His research for morphological control in semi-crystalline polymer films. interests include MAPLE deposition of poly- Polymer Crystallization. 2018;1:e10021. https://doi.org/10. mer thin films to achieve control over film 1002/pcr2.10021 morphology and properties.