Self-Assembly of Organic Ferroelectrics by Evaporative Dewetting: A

May 23, 2017 - (30, 31) In our case, the entire grain was switched without forming sophisticated domain structures. The advantage of using in-plane po...
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Self-Assembly of Organic Ferroelectrics by Evaporative Dewetting: A Case of β‑Glycine Ensieh Seyedhosseini,† Konstantin Romanyuk,†,‡ Daria Vasileva,‡ Semen Vasilev,‡ Alla Nuraeva,‡ Pavel Zelenovskiy,‡ Maxim Ivanov,† Anna N. Morozovska,§ Vladimir Ya. Shur,‡ Haidong Lu,∥ Alexei Gruverman,∥ and Andrei L. Kholkin*,†,‡ †

Department of Physics & CICECO − Materials Institute of Aveiro, University of Aveiro, 3810-193 Aveiro, Portugal School of Natural Sciences and Mathematics, Ural Federal University, 620000 Ekaterinburg, Russia § Institute of Physics, National Academy of Science of Ukraine, 03028 Kyiv, Ukraine ∥ Department of Physics and Astronomy, University of Nebraska, Lincoln, Nebraska 68588, United States ‡

S Supporting Information *

ABSTRACT: Self-assembly of ferroelectric materials attracts significant interest because it offers a promising fabrication route to novel structures useful for microelectronic devices such as nonvolatile memories, integrated sensors/actuators, or energy harvesters. In this work, we demonstrate a novel approach for self-assembly of organic ferroelectrics (as exemplified by ferroelectric β-glycine) using evaporative dewetting, which allows forming quasi-regular arrays of nano- and microislands with preferred orientation of polarization axes. Surprisingly, self-assembled islands are crystallographically oriented in a radial direction from the center of organic “grains” formed during dewetting process. The kinetics of dewetting process follows the t−1/2 law, which is responsible for the observed polygon shape of the grain boundaries and island coverage as a function of radial position. The polarization in ferroelectric islands of β-glycine is parallel to the substrate and switchable under a relatively small dc voltage applied by the conducting tip of piezoresponse force microscope. Significant size effect on polarization is observed and explained within the Landau−Ginzburg−Devonshire phenomenological formalism. KEYWORDS: self-assembly, organic ferroelectrics, glycine, dewetting, size effect

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applications.4 A typical example is a copolymer poly(vinylidene fluoride−trifluoroethylene) [P(VDF-TrFE)] that is especially interesting for memory applications because it has excellent solution processability, low deposition temperature, and outstanding ferroelectric and electromechanical properties.5 It has been already used for a variety of piezoelectric and pyroelectric devices including thin-film field-effect transistors (see, e.g., ref 6). With the continuing demand for miniaturization of ferroelectrics (e.g., for ultrahigh density ferroelectric data storage applications), it is becoming extremely important to scale down their dimensions and to form arrays of self-assembled microand nanoislands in which the bits of information could be individually switched without any cross-coupling effects. In inorganic ferroelectrics this has been done using two different approaches.7 In the top-bottom approach, the excessive material has been removed by using either ion-beam etching or e-beam writing techniques. Ferroelectric polarization could

INTRODUCTION Ferroelectrics are multifunctional materials, in which electric polarization can be switched by a suitable electric field or mechanical force. They found numerous applications because of their tunable properties, high electromechanical coupling, and an ability to store information as polarization bits. Most of practically used ferroelectrics are inorganic oxides of heavy metals as exemplified by barium titanate (BaTiO3) and lead zirconate titanate [Pb(Zr,Ti)O3 (PZT)] ceramics. They have been used as a major working horse for these applications over the last 50 years.1,2 However, their high processing temperature, apparent brittleness, and inability to provide conformal coating limit application of ceramic ferroelectrics in many areas. One of the disadvantages is inevitable cross-contamination with heavy metals during standard CMOS microfabrication procedure. This is a major obstacle for the fabrication of nonvolatile ferroelectric memories based on inorganic oxides.3 Special protective barriers have to be designed that inevitably impede the use of ferroelectrics in modern microelectronics.3 An alternative to inorganic ferroelectrics are organic/polymer ferroelectric materials which provide both flexible and low temperature processable ferroelectric components for many © 2017 American Chemical Society

Received: March 1, 2017 Accepted: May 23, 2017 Published: May 23, 2017 20029

DOI: 10.1021/acsami.7b02952 ACS Appl. Mater. Interfaces 2017, 9, 20029−20037

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Figure 1. (a−c) Optical images of β-glycine films at different magnifications, (d) Schematic of the film structure. Distribution of the effective heights (e) and diameters (f) of β-glycine micro- and nanoislands.

not solve the problem because very disordered structures are formed. One of the possible approaches is concerned with evaporative dewetting that has been recently demonstrated for patterning of peptide nanotubes in which direction could be controlled either by Raleigh/Marangoni convection or by viscous fingering.17 In this work, we propose a simple yet effective approach, in which a spin-coating of a drop of an organic solution is used to create nanoarrays of uniformly distributed and highly oriented islands in which the polarization can be individually switched by the application of an electric field to the tip of piezoresponse force microscopy (PFM). As an object of our studies, we chose the simplest amino acid, glycine, which has been recently shown to be a ferroelectric with many potential applications in medicine and biology.18 The easy absorption of amino acids on metal and oxide surfaces has made them attractive for synthesis of functional nanostructures. For example, glycine was found to absorb in the zwitterionic form on Pt(111)19 and Au20 substrates and in the ionic form on Cu(110).21 In this work, we use the ability of binding the glycine molecules onto Pt(111) substrates to form well oriented ferroelectric nano- and microislands of β-glycine which are stabilized by the contact with Pt.22

be individually switched by applying a voltage to the conductive tip of the atomic force microscope (AFM).8 However, with additional damage due to bombardment by heavy ions or due to etching procedure results in a number of defects, some of them could not always be recovered by the subsequent annealing. In the bottom-up approach, much less damage occurs at the expense of the regularity of the structures. Thus, the bottom-up methods based on self-assembly have the key advantage that they can be easily used for fabrication of electronic devices. However, this method is limited in the degree of uniformity and size control. A typical example is the case of self-assembled PZT nanoislands obtained from diluted sol−gel solution.9 A broad distribution of the sizes has been found which allowed study of the size effect, a principal limitation of using ferroelectrics in nanoelectronics.10 The degree of uniformity and size control are important for synthesis of micro- and nanostructures with predictable characteristics. Therefore, the growth regimes providing the spatial uniformity of the island coverage and island density are desirable. Regarding the organic/polymer ferroelectrics, much less studies have been reported due to the lack of suitable techniques for patterning of soft materials. Though selective etching of a material using a patterned mask is common in complementary metal oxide semiconductor (CMOS)-based memory fabrication, it is not suitable for patterning functional polymer ferroelectrics such as P(VDF-TrFE). To avoid significant damage of the surface, as well as detrimental changes of ferroelectric properties under the harsh patterning conditions, P(VDF-TrFE) is usually patterned with soft lithography such as nanoimprinting. The studies performed by Kang et al.,11 Hu et al.,12 Harnagea et al.,13 and recently by Song et al.14 all target the formation of isolated ferroelectric 3D structures, which can be embedded in the crossbar arrays. Another way to produce the arrays of nanoislands of P(VDFTrFE) is their high temperature annealing which helped to create well crystallized nanomesas.15,16 Obviously, there is a lack of self-assembly techniques that can be applied to organic ferroelectrics using a bottom-up approach. Just using diluted organic solutions of these does

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RESULTS AND DISCUSSION Nanostructured films of glycine were prepared by the spincoating of glycine solution which was dropped in the center of the spinning substrate.23 The process involves the centrifugal force that pushed the solution to flow outward from the center of rotation. In this process, the thickness of the solution layer decreased with time and drying/crystallization occurred simultaneously with dewetting as will be explained in detail below. Figure 1(a−c) shows representative morphology of the films obtained by optical microscopy. The film morphology is dominated by the spherulite-like structures (“grains”) with well distinguishable boundaries [Figure 1(a)] and some amount of open voids that appear randomly over the film due to a dewetting phenomenon.24 Dewetting normally arises from fluctuations in the film thickness and causes the film to rupture 20030

DOI: 10.1021/acsami.7b02952 ACS Appl. Mater. Interfaces 2017, 9, 20029−20037

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ACS Applied Materials & Interfaces at random positions.25 It is seen that each “grain” consists of isolated islands organized in the form of radial rays emanating from the “grain” center [Figures 1(c) and 2(b)]. This center

direction with an average aspect ratio of about 2. The distribution of the effective heights and diameters of selforganized micro- and nanoislands is shown in Figure 1(e,f). The mean effective diameter of the self-assembled structure was indeed in the submicrometer range (deff = 560 ± 10 nm), but the distribution was sufficiently broad, thus allowing study of the dependence of spontaneous polarization on the diameter and height of the microislands (ferroelectric size effect). The height distribution shown in Figure 1(e) was even broader, suggesting the existence of islands with heights down to a few tens of nanometers. Raman spectroscopy was used to identify the structure of individual islands and direction of the crystallographic axes relative to the center of each grain. For the determination of the polymorphic phase we brought our attention to the position of νs(CH2) spectral line, which strongly depends on the glycine phase. This line is localized at 2953 cm−1 in β-phase, and at 2963 and 2972 cm−1 in γ- and α-phases, correspondingly (Figure 2).22,26 It was confirmed that all islands belong to a less stable form of polymorphic glycine, the β phase. Polarized micro-Raman measurements were used for determination of the orientation of the crystallographic axes. The spectra with parallel directions of laser polarization and analyzer along and across the longest axis of the island [Figure 3(c)] were measured. We found that the spectral line ν(C−C) at about 894 cm−1 is observed in along geometry only [Figure 3(d)]. In a crystal of the β-phase this bond is oriented along the c-axis [Figure 3(b)]. The line at 1045 cm−1 corresponds to ν(C−N) vibrations (oriented along the a-axis [Figure 3(b)]) and is absent in both spectra, thus demonstrating that the a-axis is normal to the substrate. This can be confirmed also by the comparison of intensities of symmetrical vibrations of the CO2 group. These vibrations occur in β-glycine mainly in the ac plane [Figure 3(b)], and the line at 1414 cm−1 is stronger in the case of polarization oriented along the c-axis [Figure 3(d)]. A similar conclusion can be made when considering the line at

Figure 2. (a) Raman spectra of different glycine polymorphs; spectral line ν(CH2) for determination of the polymorphic phase is indicated. (b) Position of ν(CH2) in different glycine polymorphs.

does not coincide with the geometrical center of the grain. The grains are separated by distinct grain boundaries that form double skirting layers separated by a gap. This behavior obviously rules out centrifugal force as a driving source for the formation of the rays. The schematic of the entire structure is shown in Figure 1(d). The islands are elongated in the radial

Figure 3. (a) Raman spectra of different glycine polymorphs; spectral region for determination of the orientation of crystallographic axes is indicated. (b) Orientation of glycine molecules in crystal of β-phase. (c) Optical image of ordered islands with marked crystallographic directions of β glycine. (d) Comparison of Raman spectra with the direction of polarization of incoming light along and across the longest axis of an island. 20031

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Figure 4. (a) Optical image of glycine island film in polarized light. (b) AFM topography, (c) LPFM, and (d) VPFM images.

Figure 5. (a) AFM topography. LPFM (b) before and (c) after switching with overlaid PFM phase. (d) Dependence of the average lateral piezoresponse (proportional to polarization) on the thickness of the ferroelectric islands.

1310 cm−1 corresponding to the superposition of wagging and torsional vibrations of the CH2 group oriented in the bc plane [Figure 3(b)]. Therefore, the b-axis (polar) lies preferably in the short (circumferential) direction of each individual island. The schematic of the crystallographic direction in the individual grain is shown in Figure 3(c). It was found that the crystallographic directions are oriented toward the grain centers and thus are not related to Pt-substrate (that does not have any in-plane texture). Moreover, the optical contrast in the grains observed with linearly polarized light [Figure 4(a)] is consistent with the circumferential direction of polarization within the island found by micro-Raman measurements. To provide further insight into the polarization direction in self-assembled glycine structures, we used PFM in both vertical and lateral regimes.27 The topography image [Figure 4(b)] displays three neighboring grains with different polarization directions inside rays. Accordingly, the PFM (mixed signal) lateral image [Figure 4(c)] shows a contrast only for a grain where the polarization direction is perpendicular to the longest axis of the cantilever. In two other grains the contrast is weak because their polarization direction is parallel to the cantilever

axis, and the contrast may arise only through a buckling effect.28 The vertical PFM image [Figure 4(d)] shows negligible contrast consistent with Raman measurements (ferroelectric axis b lies parallel to the substrate). If we inspect PFM images at higher magnification [Figure 5(a−c)], we notice that the same sign of PFM contrast is maintained on almost all islands. This means that the polarization direction is perpendicular to the radial direction. The change of the sign of PFM contrast seen on the right slope of the majority of islands is due to the artifact caused by nonsymmetric distribution of electric field and influence of adsorbates.29 This contrast change was thus ignored in further consideration. The polarization could be switched by the application of a relatively weak bias (about 20 V) to individual islands by the PFM tip [Figure 5(c)]. The change of the contrast from bright to dark means the change of the polarization direction to the opposite one. It has been recently shown that the in-plane polarization direction can be switched by the electrically biased PFM tip due to the lateral component of the electric field distribution.30,31 In our case, the entire grain was switched without forming sophisticated domain structures. 20032

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contribution, not related to polarization (background permittivity),33 electron density is n, ionized donor concentration is N+d , e = 1.6 × 10−19 C is the electron charge, and Zd is the donor charge. Donor concentration is N+d = N0d(1 − f(Ed + Wdijσij − eZdφ + EF)), where the Fermi−Dirac distribution function is introduced as f(x) = (1 + exp(x/kBT))−1 and EF is the Fermi energy level in equilibrium. Electron density is n = NCF1/2((eφ + EF − EC)/kBT), EC is the bottom of conductive band, F−1 1/2 is the function inverse to the Fermi integral, and NC is the effective density of states in the conduction band.34 Below we consider that the frequencies ω of applied voltage are much lower than the optical frequency and dc voltages are much higher than ac ones, |Vdc| ≫ |Vac|. Under these assumptions, the analytical estimate of the effective PFM response is possible in 1D approximation, defined as the derivative of the surface displacement at the applied voltage V, ∂U PR ieff = ∂Vi , is possible in 1D approximation as described in Supporting Information. There we also estimate piezoelectric, flexoelectric, and flexochemical relative contributions to the response using methodology described in refs35 and 36. Figure 6 illustrates the thickness effect of the relative piezoelectric and flexochemical contributions of the local PFM

The advantage of using in-plane polarization bits for memory applications is that there is no influence of electrostatic effects and possible read errors. We note that the deposited islands were stable after storage in a desiccator for about 1 year or more. When exposed to normal lab conditions, some islands were converted to a mixture of β- and γ-glycine26 (Figure S1 of Supporting Information). The islands were then hardly switchable. Finally, we could study the size effect, i.e., the dependence of the average lateral piezoresponse (proportional to in-plane polarization) on the height of ferroelectric islands. The lateral dimensions of the islands were too big to expect any size dependence. This dependence is shown in Figure 5(d) with a clear trend of decreasing piezoresponse for heights below 100− 120 nm. The theoretical prediction of such behavior was made using a phenomenological Landau−Ginzburg−Devonshire (LGD) approximation. The main contributions to the vectorial PFM response of ferroelectrics are piezoelectric (including linearized electrostriction), flexoelectric, and Vegard contributions. The corresponding local strain is ⎛ ∂P uij = ⎜sijklσkl + eijkPk + Q ijklPkPl + Fijkl k ∂xl ⎝ ⎞ + + W ijd(Nd+ − Nd0 )⎟ ⎠

(1)

where sijkl, σkl, Fijkl, eijk, Qijkl, and Pk(r) are elastic compliances tensor, elastic stress, “true” piezoelectric strain tensor, flexoelectric stress tensor, electrostriction stress tensor, and polarization component, respectively. Wdij is the Vegard tensor for mobile donor defects (ions, vacancies, protons, etc.), N+d (r) + is their inhomogeneous concentration, and Nd0 is their equilibrium concentration. Mobile acceptors can be considered in a similar way. Generalized Hook’s relation defined by eq 1 should be supplemented by the equilibrium conditions of bulk and surface forces, namely ∂σij/∂xj = 0 in the bulk and σijnj|S = 0 at the free surface of the system.32 Inhomogeneous spatial distribution of the ferroelectric polarization components Pi(z) can be determined from the LGD-type Euler−Lagrange equations aik Pk + aijk PP j kPl + + aijkmnPP j kPP l mPn − gikmn

Figure 6. Dependence of PFM response (solid curve), normalized spontaneous polarization (dashed curve), and dielectric permittivity (dotted curve) on the nanoparticle height. Symbols are experimental data. Fitting parameters are listed in Supporting Information.

∂ 2Pk ∂xm∂xn

response for molecular ferroelectric glycine. Critical thickness for the existence of ferroelectricity in glycine is about 115 nm, and it is very close to the experimental data obtained by PFM. The general mechanism of the formation of self-assembled glycine islands is thought to be as follows. During the spincoating, the drop of the solution is in the form of a flat liquid layer spreading on the substrate. It starts drying and becomes increasingly thin until a critical thickness is reached at which random fluctuations create break points [centers of the grains shown in Figure 1(c)]. Then the drying front (retracting edge) moves from this point in radial directions in the form of concentric rims. Assuming a homogeneous thickness of the liquid layer, we suggest that these break points, serving as nucleation centers, appear randomly with the same probability over the entire surface. When drying fronts (retracting edge) moving from the break points meet, they form grain boundaries which separate individual grains. As already noted, centrifugal force does not play any role and the formation of grains is governed by the capillary force, dynamics of dewetting process

∂σ + Fk lim kl − 2Q klinσklPn ∂xm = Ei

where aij and aijk are the coefficients of LGD potential expansion on the polarization powers (also called as linear and nonlinear dielectric stiffness coefficients). Quasi-static electric field Ei is defined via electric potential as Ei = −∂φ/∂xi. For a ferroelectric-semiconductor with mixed ionic-electronic conductivity, the electric potential φ can be found self-consistently from the Poisson equation, ε0εijb

∂P ∂ 2ϕ = k − ρ(ϕ) ∂xi ∂xj ∂xk

with corresponding electric boundary conditions. The charge density is ρ(φ) = e(ZdN+d (φ) − n(φ)), ε0 = 8.85 × 10−12 F/m is the dielectric permittivity of vacuum, εbij is the permittivity 20033

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dependence was found. This relationship is important for the further analysis of island condensation. As already mentioned, the specificity of the formation of glycine islands is that they are elongated along the radial direction with average size (size distribution) and coverage θ virtually independent of the distance from the center. The coverage θ(r) as a function of radial position can be found using the above eq 2 and dewetting theory described in ref 38. ∂r The dependence ∂t ∝ t −1/2 (ref 38) was obtained under the

(drying front propagation), and supersaturation near the liquid−substrate interface. The dynamics of the dewetting process could be extracted from the geometry of the grain arrangement and the distribution of distance difference between neighboring centers [Figure 7(a,b)]. Geometry of the grain

assumption that the entire volume moved by the retracting edge is contained in the semicircular thickened edge with the

Figure 8. Schematic presentation of the drying area of radius r with the semicircular rim of radius ρ formed in the film of thickness h (according to ref 38 and according to the analytical model for dewetting void growth via edge thickening and retraction initially proposed by Brandon and Bradshaw41).

radius ρ (Figure 8). That is, we have the following relation between r and ρ:38,41 ρ=

hr π

(3)

Due to evaporation, the derived eq 3 should be modified by the proportionality coefficient (1 − α):

Figure 7. (a) Nearest neighbor separation distribution for nucleation centers. (b) Distribution of the distance difference between two neighboring centers measured from the separating boundary.

πρ2 πr = (1 − α)hπr 2 → ρ =

(1 − α)

hr π

(4)

where α is the fraction of the volume lost due to evaporation. In this case, the calculations made in ref 38 will give the same ∂r result: ∂t ∝ t −1/2 . From eq 4, we can easily estimate the coverage θ as follows. If the total volume loss is αhπr2, then the ∂r rate of the volume loss is 2αhπr ∂t . And consequently the drying

arrangement after the dewetting process can be represented as connected polygons (see geometrical construction in Figure S2 of Supporting Information). The legs of the polygons are straight, and these are formed by two parallel lines of the dense island chains separated by a denuded streak. The observed polygon shape of the grain boundaries with their nucleation centers is seemingly represented by the Voronoi diagram,37 but it is not the case because the nucleation centers of the neighboring grains are not equally distanced from the separating grain boundaries. The measured distance difference is continuously distributed in a wide range with average value of about 70 μm [Figure 7(b)]. The observed distribution means that the drying centers are nucleating continuously during dewetting process. It can be shown (see Supporting Information) that the analytical solution describing the motion of the drying front can be written as

∂r

area increase per unit time is2πr ∂t . Thus, the relation of the volume loss per unit time to the drying area increase per unit time multiplied by glycine concentration in the solvent, n, gives the island coverage θ(r) (in g/cm2 units): θ(r ) = αnh

(5)

From the relation for θ(r) (eq 5) and from the model of ref 38, it follows that the island coverage θ(r) does not depend on the radial position if evaporation in the system is equal to a fraction α, lost due to evaporation (eq 4) of the drying volume (hπr2). Therefore, eq 4 results in the drying front propagation as ∂r(t ) ∝ t −1/2 . Since the island concentration on the surface, C, ∂t and island size (mass), m, are directly related to the coverage, θ(r) = mC, the island coverage dependence θ(r) is an important parameter defining uniformity of the obtained film. Thus, the observed polygon geometry of grain boundaries can serve as an attribute indicating that the experimental conditions during the dewetting process (temperature, vapor pressure, solution concentration) are appropriate for the formation of a uniform ferroelectric island film as in the case of β-glycine.

∂r(t ) ∝ t −1/2 (2) ∂t where r is the radial coordinate of the front relative to the center of the “grain” and t is the time. The dynamics of the dewetting process (eq 2) was obtained from the final geometry and distribution [Figure 7(b)], describing statistics of the distance difference between separating boundary and neighboring grain centers. The derived expression (eq 2) for drying front propagation is in agreement with earlier results38−40 in which the same time 20034

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Figure 9. Schematic representation of the drying front geometry and glycine island growth. (a) Top view of the isolated drying area in the liquid layer. (b) Stages 1−4: the sequence of stages of glycine island growth on the micrometer scale. Due to pinning on glycine islands, the drying front exhibits a wavy contour. Drying front propagation is shown by blue arrows. One: nucleating centers of glycine islands, 2: glycine island growth in the direction (red arrows) of the drying front propagation, 3, 4: isolation of glycine islands. After isolation of the islands, the drying front inherits the previous wavy shape. The points with maximal local evaporation flux are preferable for the nucleation of glycine islands [shown as orange crosses in stage 4 in part b].

curvature, i.e., saddle points in the middle between glycine islands. The points with maximal local evaporation flux are preferable places for the nucleation of glycine islands. They are located on the same radial line with previously formed neighboring islands. Thus, the pinning effect induces coherent nucleation of glycine islands arranged in the form of radial chains.

The observed radial orientation of ferroelectric islands indicates that the nucleation of the ferroelectric islands happens at the contact line. Then the island growth succeeds via drying front propagation. Thus, the observed oriented island growth is provided by both the concentric drying front propagation and by the highest supersaturation and evaporative flux at the contact line. The highest evaporative flux at the contact line, as found out in different evaporative crystallization systems,42−44 is a result of the solution of diffusion problem for the vapor concentration near the solvent surface. Nucleation and growth of the islands can be considered in view of common crystal growth theory.45−47 Nucleation is initiated at the glycine saturation (near the contact line) higher than some critical value, then the glycine concentration in solution is locally decreased and a denuded zone is formed near growing islands so that it suppresses nucleation of new islands. The observed chains of islands indicates that the speed of the drying front propagation should be higher than the speed of growth of the islands in the radial direction. In other words, when the drying front outstrips the islands, they become isolated from the glycine solution and stop growing, at the same time the conditions at the contact line becoming favorable for the following nucleation process [Figure 9(b)]. Due to pinning effect to glycine islands, the drying front is not ideally circular [Figure 9(b)]. On the micrometer scale, it should have a wavy contour with maximal mean curvature of a surface near glycine islands and with minimal curvature in saddle points in the middle between glycine islands [Figure 9(b)]. When the drying front retracts from glycine islands [Figure 9(b), stage 4)], it inherits the previous wavy shape. The places with maximal evaporation flux [shown by orange crosses in Figure 9(b), stage 4] are located at the points with the maximal positive curvature near glycine islands, and places with minimal evaporation flux are located at the points with minimal

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CONCLUSIONS To summarize, we have developed a novel method of selfassembly of ferroelectric crystals of β-glycine on platinized substrates which allows the formation of quasi-regular arrays of nano- and microislands with the preferred orientation of polarization axis. The kinetics of the dewetting process is found to be responsible for the observed polygon shape of the grain boundaries and island coverage as a function of radial position. We suggest that the observation of polygon geometry of grain boundaries can serve as a signature, indicating that the experimental conditions (temperature, vapor pressure, solution concentration) are appropriate for the formation of uniform ferroelectric island films with constant substrate coverage. We infer that the pinning effect is responsible for coherent growth of glycine islands arranged in the form of radial chains. The polarization ferroelectric islands are switchable under the relatively small dc voltage. Organic crystals such as β-glycine offer an advantage of facile fabrication and design variability and thus have potential as a media for high density memories and other applications.

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METHODS

Nanostructured films of glycine were prepared by the spin-coating of glycine solutions on a commercial Pt-coated Ti/SiO2/Si substrate (Inostek, S. Korea). A 0.13 M solution of glycine was prepared by dissolving glycine powder (Sigma-Aldrich, 99%) in deionized water. A 20035

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ACS Applied Materials & Interfaces drop of 50 μL was placed on the substrate by a micropipette, and then the substrate was rotated at a speed of 2000 rpm for 3 min to spread out and to dry the solution. The process involves the centrifugal force that pushes the solution to flow outward from the center of rotation. In this process, the thickness of the solution layer decreases and drying/crystallization occurs simultaneously in different places, resulting in almost regular arrays of ferroelectric micro- and nanoislands. The polymorphic state of the glycine crystals and orientation of crystallographic axes were determined by a confocal Raman microscope Alpha 300AR (WiTec, GmbH, Germany) equipped with a solidstate laser of wavelength 488 nm and maximum power 27 mW. Laser beam was focused at the sample surface by the objectives with 100× magnification and NA = 0.75. A diffraction grating with 600 grids per millimeter was used for the decomposition of the scattered light. Spectral resolution of the grating is 3.19 cm−1 for the laser wavelength used. The spectrum was detected by CCD camera with 1600 × 200 pixels cooled to −60 °C. Multimode optical fiber with diameter 50 μm is used as a confocal pinhole. Polarizer and analyzer built into the optical scheme of the microscope allowed rotate polarization of the incident and scattered light. Micro-Raman spectroscopy was used to identify the structure of individual islands and direction of the crystallographic axes relative to the center of each grain. Analysis of the position of specific peaks in low- and high-frequency ranges confirmed that all islands belong to a less stable polymorphic form of glycine, the β phase. By using Raman spectra measured with a parallel polarizer and analyzer in mutually perpendicular directions, we could prove that b- (polar) and c-axes lie in plane of the substrate, and the c-axis of β-glycine lies preferably along the longest axis of the island (i.e., in the radial direction), while the b-axis is perpendicular to it.

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Partnership. P.Z. is grateful for support from the Ministry of Education and Science of the Russian Federation through the RF President grant for young scientists MK-6554.2015.2. M.I. is grateful to the RFBR through research project no. 16-32-60188 mol_a_dk. The authors are grateful to Ohheum Bak for the help with experimental work.

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(1) Uchino, K. Ferroelectric Devices; Marcel Dekker, Inc.: New York, 2000; p 308. (2) Scott, J. F. Applications of Modern Ferroelectrics. Science 2007, 315, 954−959. (3) Cross, J. S.; Coutsaroff, I. Review on Ferroelectric Thin Film Devices: Fundamental Aspects and Integration Challenges. J. Technol. Assoc. Refract., Jpn. 2010, 62, 162−174. (4) Horiuchi, S.; Tokura, Y. Organic Ferroelectrics. Nat. Mater. 2008, 7, 357−366. (5) Naber, R. C. G.; Asadi, K.; Blom, P. W. M.; de Leeuw, D. M.; de Boer, B. Organic Nonvolatile Memory Devices Based on Ferroelectricity. Adv. Mater. 2010, 22, 933−945. (6) Khan, M. A.; Bhansali, U. S.; Alshareef, H. N. High-Performance Non-Volatile Organic Ferroelectric Memory on Banknotes. Adv. Mater. 2012, 24, 2165−2170. (7) Han, H.; Kim, Y.; Alexe, M.; Hesse, D.; Lee, W. Nanostructured Ferroelectrics: Fabrication and Structure-property Relations. Adv. Mater. 2011, 23, 4599−4613. (8) Alexe, M.; Harnagea, C.; Hesse, D.; Goesele, U. Patterning and Switching of Nanosize Ferroelectric Memory Cells. Appl. Phys. Lett. 1999, 75, 1793−1795. (9) Roelofs, A.; Schneller, T.; Szot, K.; Waser, R. Towards the Limit of Ferroelectric Nanosized Grains. Nanotechnology 2003, 14, 250−253. (10) Gruverman, A.; Kholkin, A. Nanoscale Ferroelectrics: Processing, Characterization and Future Trends. Rep. Prog. Phys. 2006, 69, 2443−2474. (11) Hu, Z.; Baralia, G.; Bayot, V.; Gohy, J. F.; Jonas, A. M. Nanoscale Control of Polymer Crystallization by Nanoimprint Lithography. Nano Lett. 2005, 5, 1738−1743. (12) Hu, Z.; Tian, M.; Nysten, B.; Jonas, A. M. Regular Arrays of Highly Ordered Ferroelectric Polymer Nanostructures for Nonvolatile Low-voltage Memories. Nat. Mater. 2009, 8, 62−67. (13) Harnagea, C.; Alexe, M.; Schilling, J.; Choi, J.; Wehrspohn, R. B.; Hesse, D.; Goesele, U. Mesoscopic Ferroelectric Cell Arrays Prepared by Imprint Lithography. Appl. Phys. Lett. 2003, 83, 1827− 1829. (14) Song, H.; Lu, S.; Li, S.; Tan, L.; Gruverman, A.; Ducharme, S. Fabrication of Ferroelectric polymer Nanostructures on Flexible Substrates by Soft-Mold Reverse Nanoimprint Lithography. Nanotechnology 2016, 27, 015302. (15) Bai, M.; Ducharme, S. Ferroelectric Nanomesa Formation from Polymer Langmuir−Blodgett films. Appl. Phys. Lett. 2004, 85, 3528− 3530. (16) Sharma, P.; Reece, T. J.; Ducharme, S.; Gruverman, A. Highresolution Studies of Domain Switching Behavior in Nanostructured Ferroelectric Polymers. Nano Lett. 2011, 11, 1970−1975. (17) Chen, J.; Qin, S.; Wu, X.; Chu, P. K. Morphology and Pattern Control of Diphenylalanine Self-Assembly via Evaporative Dewetting. ACS Nano 2016, 10, 832−838. (18) Heredia, A.; Meunier, V.; Bdikin, I. K.; Gracio, J.; Balke, N.; Jesse, S.; Tselev, A.; Agarwal, P.; Sumpter, B. G.; Kalinin, S. V.; Kholkin, A. L. Nanoscale Ferroelectricity in Crystalline γ-Glycine. Adv. Funct. Mater. 2012, 22, 2996−3003. (19) Lofgren, P.; Krozer, A.; Lausmaa, J.; Kasemo, B. Glycine on Pt(111): a TDS and XPS study. Surf. Sci. 1997, 370, 277−292. (20) Zhao, X. Y.; Rodriguez, J. Photoemission Study of Glycine Adsorption on Cu/Au(111) Interfaces. Surf. Sci. 2006, 600, 2113− 2121. (21) Nyberg, M.; Hasselstrom, J.; Karis, O.; Wassdahl, N.; Weinelt, M.; Nilsson, A.; Pettersson, L. G. M. The Electronic Structure and

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02952. Theory of the dewetting process and formation of microislands (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +351 234 247025. Fax: +351 234 401 470. E-mail: [email protected]. ORCID

Pavel Zelenovskiy: 0000-0003-3895-4785 Maxim Ivanov: 0000-0001-9155-9288 Andrei L. Kholkin: 0000-0003-3432-7610 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The equipment of the Ural Center for Shared Use “Modern nanotechnology” UrFU was used. This work was funded by the Luso-American Foundation (FLAD) (grant no. 299/2015). K.R. is grateful to the financial support of FCT via his postdoctoral grant SFRH/BPD/88362/2012. Part of this work was developed in the scope of project CICECO-Aveiro Institute of Materials (ref FCT UID/CTM/50011/2013), financed by national funds through the FCT/MEC and, when applicable, cofinanced by FEDER under the PT2020 20036

DOI: 10.1021/acsami.7b02952 ACS Appl. Mater. Interfaces 2017, 9, 20029−20037

Research Article

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