Nanostructure- and Orientation-Controlled Digital Memory Behaviors

Jun 23, 2014 - Nanoscale PCzMA cylinders oriented between the top and bottom electrodes ... First, carbazole moieties can form local hopping paths wit...
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Nanostructure- and Orientation-Controlled Digital Memory Behaviors of Linear-Brush Diblock Copolymers in Nanoscale Thin Films Kyungtae Kim,† Young Yong Kim,† Samdae Park,† Yong-Gi Ko,† Yecheol Rho,† Wonsang Kwon,† Tae Joo Shin,‡ Jehan Kim,‡ and Moonhor Ree*,†,‡ †

Department of Chemistry, Division of Advanced Materials Science, Center for Electro-Photo Behaviors in Advanced Molecular Systems, Polymer Research Institute, and BK School of Molecular Science, Pohang University of Science & Technology, Pohang 790-784, Republic of Korea ‡ Pohang Accelerator Laboratory, Pohang University of Science & Technology, Pohang 790-784, Republic of Korea S Supporting Information *

ABSTRACT: Linear-brush diblock copolymers bearing carbazole moieties in the brush block were synthesized. Various phase-separated nanostructures were found to develop in nanoscale thin films of the copolymers, depending on the fabrication conditions including selective solvent-annealing. This variety of morphologies and orientations means that these block copolymers exhibit digital memory versatility in their devices. Overall, the relationship between the morphology and digital memory performance of these copolymers has several important features. In particular, the carbazole moieties in the vertical cylinder phase with a radius of 8 nm or less can trap charges and also form local hopping paths for charge transport, which opens the mass production of advanced digital memory devices with ultrahigh memory density. Charges can be transported through the layer when the dielectric linear block phase has a thickness of 10.6 nm; however, charge transport is not possible for a dielectric phase with a thickness of 15.9 nm. All the observed memory behaviors are governed by the trap-limited space-charge-limited conduction mechanism and local hopping path (i.e., filament) formation.

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INTRODUCTION Recently, the world market for memory devices has rapidly grown with increases in the demand for the storage of multimedia data.1 Polymer materials exhibit easy processability, flexibility, high mechanical strength, and good scalability, and their properties can be tailored easily through chemical synthesis. Thus, much effort has focused on the development of advanced polymer materials that can deliver the properties and processability required for high performance electrical memory devices.2−8 As a result, some polymer memory systems have been reported that exhibit volatile or nonvolatile digital memories, or both, and several switching mechanisms have been proposed.3−8 However, the switching mechanisms of polymer devices are intrinsically very complicated because of the complexity of their molecular and morphological structures as well as of their electrochemical properties. To facilitate the development of high performance memory polymers, it is therefore necessary to understand the relationships between their electrical memory behavior, chemical structure, and morphological structure. In particular, the relationship of memory behavior and morphological structure has rarely been investigated. At present, the relationships are not well understood. © 2014 American Chemical Society

In this study, we prepared two linear-brush diblock copolymers bearing carbazole moieties in the brush block, poly(styrene-b-9-carbazolylethylenyloxycarbonylethylenylcarbonyloxyethylenyl methacrylate) with different block volume ratios (PS245-b-PCzMA34 (67/33, v/v) and PS1269-b-PCzMA46 (89/11, v/v)) (Figure 1A), and investigated their thin film morphologies, orientations, and electrical memory characteristics in detail. The diblock copolymers in thin films were found to undergo phase separation, forming various nanostructures (random-shaped islands, vertically oriented cylinders, and horizontally oriented lamellae) depending on the fabrication conditions including selective solvent-annealing and substrate surface modification. The variations in the nanostructure and orientation were directly related to the electrical digital memory performances of the block copolymer devices. In particular, a memory mode of the block copolymer devices could be converted to the other memory modes by changing the morphology and orientation. Moreover, this study demonstrated that the carbazole moieties in the vertical cylinder phase Received: April 29, 2014 Revised: June 10, 2014 Published: June 23, 2014 4397

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Figure 1. (A) Chemical structure of PSm-b-PCzMAn polymer in two different compositions. (B) Geometry of GIXS: αi is the incident angle at which the X-ray beam impinges on the film surface; αf and 2θf are the exit angles of the X-ray beam with respect to the film surface and to the plane of incidence, respectively; and qx, qy, and qz are the components of the scattering vector q. (C) 2D GIWAXS pattern, which was measured with αi = 0.146° at room temperature for a CS2-annealed PS245-b-PCzMA34 film deposited on an OTS-Si substrate. 2D GISAXS patterns measured with αi = 0.147° at room temperature: (D) as-cast PS245-b-PCzMA34 film deposited on a Si substrate; (E) as-cast PS245-b-PCzMA34 film deposited on an OTSSi substrate; (F) as-cast PS1269-b-PCzMA46 film deposited on a Si substrate. (G) In-plane scattering profiles extracted along the 2θf direction at a chosen αf from the 2D GISAXS patterns in parts D−F: a, extracted at αf = 0.177° from the pattern in part D; b, extracted at αf = 0.150° from the pattern in part E; c, extracted at αf = 0.168° from the pattern in part F. The black symbols are the measured data, and the color solid lines were obtained by fitting the data using the GIXS formulas. The polymer films had a thickness of 44.8−46.8 nm. The synchrotron X-ray source with a wavelength λ of 0.1380 nm was used. PS245-b-PCzMA34 (67/33, v/v) and PS1269-b-PCzMA46 (89/11, v/v) were prepared from the reactions of 9H-carbazole-9-ethylsuccinic acid (SUC-CBZ) with the PS-b-PHEMA polymers, as shown in Scheme S1 (Supporting Information). SUC-CBZ was synthesized as follows. 9H-Carbazole-9-ethanol (4.220 g, 20.0 mmol) and succinic anhydride (3.000 g, 60.0 mmol) were dissolved in dried dimethylformamide (DMF, 50 mL). The reaction mixture was stirred at room temperature for 12 h. Then, the reaction solution was poured into methylene chloride (MC, 100 mL). The solution was washed several times with deionized, distilled water to remove DMF and the reactants. Thereafter, the reaction product was precipitated slowly from the hexane solution and followed by drying, giving the target product SUC-CBZ. Yield: 93%. 1H NMR (δ (ppm), 300 MHz, deuterated dimethyl sulfoxide (DMSO-d6)): 12.18 (s, 1H, COOH), 8.21−7.09 (m, 8H, Ar−H), 4.56−4.23 (2t, 4H, NCH2CH2), 2.25 (s, 4H, COCH2CH2). PS245-b-PCzMA34 was prepared from SUC-CBZ and PS-PHEMA ( M n ratio of 25 500:4400) as follows. A mixture of SUC-CBZ (0.311 g, 1.0 mmol), PS-b-PHEMA ( M n ratio of 25 500:4400, 0.200 g, 0.36 OH mmol), 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC, 0.191 g, 1.00 mmol), and 4-(dimethylamino)pyridine (DMAP, 0.122 g, 1.0 mmol) in 20 mL of MC was stirred at room temperature. After

with a nanoscale cross section can trap charges and also form local hopping paths for charge transport, which opens up the possibility of mass production of advanced digital memory devices with ultrahigh memory density.

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EXPERIMENTAL SECTION

Materials and Block Copolymer Synthesis. Poly(styrene-b-(2hydroxyethyl methacrylate)) (PS-b-PHEMA) with two different number-average molecular weight ( M n ) ratios, 25 500:4400 (a polydispersity index (PDI) of 1.09) and 132 000:6000 (PDI = 1.15), and poly(2-hydroxyethyl methacrylate) (PHEMA: M n = 6500 and PDI = 1.40) were purchased from the Polymer Source Inc. (Quebec, Canada) and used as received. In addition, two polystyrene samples ( M n = 25 000 and PDI = 1.01; M n = 120 000 and PDI = 1.02) were obtained from Prof. T. Chang at POSTECH. All other reagents and chemicals were purchased from the Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) and were used as received. Proton and carbon nuclear magnetic resonance (1H and 13C NMR) spectra were measured at room temperature using a 300 MHz Bruker Fourier Transform spectrometer (model DPX 300, Rheinstetten, Germany). 4398

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24 h, the reaction solution was concentrated under reduced pressure. The reaction mixture was then poured into methanol to precipitate the reaction product. The collected precipitate was washed with methanol several times. After removal of the solvent by filtration, the remaining white precipitate was dried in a vacuum oven, producing the target polymer product PS245-b-PCzMA34. Yield: 98%. 1H NMR (δ (ppm), 300 MHz, DMSO-d6): 7.90−6.12 (m, Ar−H), 4.56−4.23 (2t, 4H, NCH2CH2) 3.95−3.89 (m, OCH2CH2O), 2.25 (s, COCH2CH2), 1.92−0.91 (m, aliphatic-H). In the same manner, PS1269-b-PCzMA46 was prepared by the reaction of PS-b-PHEMA ( M n ratio of 132 000:6000) with SUC-CBZ. In addition, a PCzMA homopolymer was prepared from the reaction of PHEMA ( M n = 6500 and PDI = 1.40) with SUC-CBZ. Thermal Analysis. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were carried out under a nitrogen atmosphere using a thermogravimeter (model TG/DTA 6200, Seiko Instruments, Japan) and a calorimeter (model DSC 6200, Seiko Instruments, Tokyo, Japan). A rate of 10.0 °C/min was employed for heating and cooling runs. Ultraviolet−Visible Spectroscopy and Cyclic Voltammetry. Ultraviolet−visible (UV−vis) spectroscopy measurements were conducted using a Scinco spectrometer (model S-3100, Sinco, Seoul, Korea). Cyclic voltammetry (CV) measurements were carried out in a 0.1 M solution of tetrabutylammonium tetrafluoroborate in acetronitrile, using an electrochemical workstation (IM6ex impedance analyzer, Zahner-Elektric GmBH & Co., Kronach, Germany) with a platinum gauze counter electrode and a Ag/AgCl (saturated KCl) reference electrode. For the measurement, each polymer was spincoated on a gold (Au) working electrode predeposited on silicon (Si) wafer. The voltage scan rate was 100 mV/s. Nanoscale Thin Film Preparation. Nanoscale thin films of the polymers were prepared as follows. A 0.5 wt % solution of each polymer in chloroform was filtered through polytetrafluoroethylene membrane microfilters with a pore size of 0.45 μm. Precleaned silicon (Si) wafers with a 300 nm thick oxide layer were used as substrates. Some of the Si substrates were further deposited with aluminum (Al) or octyltrichlorosilane (OTS). Here, Al was deposited with a thickness of ca. 30 nm onto the Si substrates through or without using a shadow mask containing open strips (whose widths were 0.1, 0.2, and 0.3 mm) by electron beam sputtering. The OTS was deposited with a thickness of 1.1 nm onto the Si substrates as follows. The Si substrates were dipped into an OTS solution in toluene (1 mL of OTS and 70 mL of toluene) for 1 h and taken out and then followed by drying with nitrogen gas. Here, the thickness of the obtained OTS layer was confirmed by synchrotron X-ray reflectivity analysis (3D beamline9,10 of the Pohang Accelerator Laboratory (PAL) at POSTECH) as well as by spectroscopic ellipsometry (model M2000, Woollam, Lincoln, NE, USA). Onto the prepared substrates, each polymer solution was spincoated at 2000 rpm for 40 s followed by drying in a vacuum at 40 °C for 24 h. Some of the as-cast polymer films were further solventannealed at room temperature under toluene or carbon disulfide (CS2) vapor for 1−24 h. The thicknesses of the obtained polymer films were determined to be 44.8−46.8 nm by spectroscopic ellipsometry. The prepared polymers were used for synchrotron X-ray scattering analysis or contact angle measurements or device fabrications. Contact Angle Measurements. Liquid contact angle measurements were conducted for Si substrates with and without an OTS layer as well as PS and PCzMA polymer films by the sessile drop technique using a contact angle meter (KSV Instruments, Tokyo, Japan). The measurements were carried out at 25 °C using toluene, carbon disulifide, chloroform, water, ethylene glycol, and diiodomethane. In addition, the measurements were performed at 25 °C using PS and PCzMA solutions (0.5 wt %) in chloroform. The contact angle was measured five times per experiment, and the resulting contact angles were averaged out. The measured contact angles with Si substrate with a native oxide layer, OTS-Si substrate, and the homopolymer thin films were listed in Table S1 (Supporting Information). Surface energies of the Si substrates with and without an OTS layer and polymer films were estimated from the contact angles measured with three reference

liquids (water, ethylene glycol, and diiodomethane) in accordance to the method described in the literature.11−14 The determined surface energy values were summarized in Table 1. In addition, the contact angles measured with the PS and PCzMA homopolymer solutions are listed in Table 2.

Table 1. Surface Energies of the Substrates, Polymers, and Solvents Used in This Study

a b

substance

surface energy (mN/m)

Si substrate with an oxide layer OTS-Si substrate PS film PCzMA film chloroform toluene carbon disulfide

46.9 (2.3)a 23.8 (0.9) 43.7 (1.5) 48.9 (1.6) 26.7b 27.9b 31.6b

The numbers in parentheses represent standard deviations. Reference 23.

Table 2. Contact Angles of the PS and PCzMA Homopolymer Solutions (0.5 wt %) in Chloroform onto Si and OTS-Si Substrate Surfaces contact angle (deg) homopolymer solution (0.5 wt %)

a

Si substrate

OTS-Si substrate

PS ( M n = 25 000)

11.90 (3.32)a

17.87 (1.15)

PS ( M n = 120 000)

15.23 (1.47)

20.04 (0.58)

PCzMA ( M n = 21 150)

12.33 (1.84)

34.80 (0.97)

The numbers in parentheses represent standard deviations.

Device Fabrication and Test. Some of the polymer films deposited on the Al-deposited substrates with and without an OTS layer (which were prepared above) were used for the fabrication of memory devices. The Al top electrodes with a thickness of 45 nm were deposited onto the polymer film layers through a shadow mask containing open strips (whose widths were 0.1, 0.2, and 0.3 mm) by means of thermal evaporation in a vacuum. The electrode contact area was 0.1 × 0.1, 0.2 × 0.2, and 0.3 × 0.3 mm2. Current−voltage (I−V) measurements were carried out using a Keithley 4200 semiconductor analyzer (Keithley Instruments, Inc. Cleveland, OH, USA) with a maximum current compliance of 0.105 A. All the experiments were performed at room temperature under air ambient conditions. Synchrotron Grazing Incidence X-ray Scattering. Grazingincidence X-ray scattering (GIXS) measurements (Figure 1B) were performed at the 3C and 9A beamlines15−22 of the PAL, POSTECH. In the measurements at the 3C beamline, the sample-to-detector distance (SDD) was 125 mm for grazing incidence wide-angle X-ray scattering (GIWAXS) and 2165 mm for grazing incidence small-angle X-ray scattering (GISAXS). For the measurements at the 9A beamline, the SDD was 2717 mm for GISAXS. Scattering data were typically collected for 30−60 s using a synchrotron X-ray radiation source with a wavelength λ of 0.1380 nm (3C beamline) or 0.0995 nm (9A beamline) and a two-dimensional (2D) charge-coupled detector (CCD, Mar USA, Inc.). At the 3C beamline, the incidence angle αi of the X-ray beam was set in the range 0.140−0.160°, which is between the critical angles of the polymer film and the silicon substrate (αc,f and αc,s). At the 9A beamline, αi was set in the range 0.100−0.120°. Scattering angles were corrected according to the positions of the Xray beams reflected from the silicon substrate with respect to precalibrated polystyrene-b-polyethylene-b-polybutadiene-b-polystyrene block copolymer or silver behenate powder (TCI, Tokyo, Japan). Aluminum foil pieces were applied as a semitransparent beam stop, because the intensity of the specular reflection from the substrate is much stronger than the intensity of GIXS near the critical angle. 4399

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Table 3. Structural Parameters of As-Cast Nanoscale Thin Films of the Diblock Copolymers random two-phase structure as-cast film/substrate PS245-b-PCzMA34/Si PS245-b-PCzMA34/OTS-Si PS1269-b-PCzMA46/Si

Da (nm)

σDb (nm)

9.5 11.0

3.5 4.2

cylinder structure dc (nm)

Rd (nm)

σRe (nm)

Hf (nm)

σHg (nm)

th (nm)

gi

φ̅ j (deg)

σφk (deg)

Os l

29.5

6.8

1.8

34.9

5.8

45.5

0.210

0

18.5

0.859

a

Domain size of the PCzMA block phase. bStandard deviation of the domain size of the PCzMA block phase. cCenter-to-center distance of the PCzMA cylinders (i.e, d-spacing of the hexagon). dAverage radius of the PCzMA cylinders. eStandard deviation of the cylinder radius. fAverage height of the PCzMA cylinders. gStandard deviation of the cylinder height. hThickness of the polymer thin film. iParacrystal distortion factor of the cylinder structure. jMean polar angle between the n vector of the cylinder structure and the out-of-plane direction of the film; for example, φ̅ is zero when the n vector in the film is oriented normal to the film plane. kStandard deviation of the polar angle φ. lSecond-order orientation factor of the cylinder structure.

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PCzMA34, and −5.350 eV for PS1269-b-PCzMA46. From the band gap and HOMO data, the LUMO level was estimated to be −1.424 eV for PCzMA, −1.714 eV for PS245-b-PCzMA34, and −1.840 eV for PS1269-b-PCzMA46. Both the as-cast and solvent-annealed films were first examined by synchrotron GIWAXS. This analysis found that all of the films produce featureless GIWAXS patterns. Representative 2D GIWAXS scattering patterns are given in Figures 1C and S4 (Supporting Information). The scattering pattern contains two ring scatterings, one at 6.5° (0.97 nm dspacing) and another at 12.7° (0.50 nm d-spacing), that are typical amorphous halo rings. Considering the chemical structures of the polymers, the first scattering ring originates from the mean interdistance of the PCzMA block chains and the second ring is due to the mean interdistance of the PS block chains as well as the mean interdistance of the bristles in the PCzMA block chains. These scattering results confirm that the block copolymer films are amorphous. The polymer films were further examined with GISAXS to investigate the nanostructures formed via separation of the block components during the film formation or post-solventannealing processes. The GISAXS patterns of the as-cast films are almost featureless with only a faint scattering signal around the reflected X-ray beam position, as shown in Figure 1D−F. The in-plane scattering profiles were extracted along the 2θf direction at αf = 0.177, 0.150, or 0.168° and are shown in Figure 1G. The in-plane scattering profiles of the PS245-bPCzMA34 films could be satisfactorily fitted with the GIXS formula derived for a random two-phase domain structural model (Figure 1G-a,b; the derivation of the scattering formula is in the Supporting Information). The in-plane scattering profile of the PS1269-b-PCzMA46 film could be satisfactorily fitted with the GIXS formula derived for a hexagonal cylinder structure model (Figure 1G-c; the derivation of the scattering formula is in the Supporting Information). The scattering data and analysis results indicate that for both of the copolymers the block components undergo very limited phase separation during the film formation process. For both of the copolymers, the PCzMA blocks are the minor component in the volume fraction. The phase-separated domains consist of PCzMA block chains. The obtained structural parameters are given in Table 3. The random-shaped PCzMA domains of the PS245-b-PCzMA34 films are present in very small populations and were estimated to have average sizes of 9.5 nm (=D) on a bare Si substrate and 11.0 nm (=D) on an OTS-Si substrate. Vertically oriented PCzMA cylinder domains are present in a small population in the PS1269-b-PCzMA46 film deposited on a bare Si substrate. The cylindrical PCzMA domains have a height H of 34.9 nm, a

RESULTS AND DISCUSSION PS245-b-PCzMA34 (67/33, v/v) was prepared by the esterification reaction of its mother polymer, PS-b-PHEMA (25 500:4400 and PDI = 1.09), with SUC-CBZ. In the same manner, PS1269-b-PCzMA46 (89/11, v/v) was prepared from the mother polymer, PS-b-PHEMA (132 000:6000; PDI = 1.15). In addition, PCzMA homopolymer was prepared from a PHEMA (6500 and PDI = 1.40). The NMR analysis confirmed that, for these polymers, the incorporations of carbazole moieties to the HEMA blocks were successfully conducted by the esterification reaction of a hydroxyl side group per repeat unit with SUC-CBZ. Thermal properties of the obtained polymers were examined by TGA and DSC analysis. Representative TGA and DSC thermograms were given in Figure S1 (Supporting Information), which were measured from the PS245-b-PCzMA34 product. The diblock copolymer revealed a degradation temperature Td of 258 °C. However, the thermal degradation was found to take place in a two-step manner. The weight loss in the first step (258−428 °C) was found to be equivalent to the weight fraction of the polymer backbone and the linker parts of the side groups, whereas that in the second step (428− 525 °C) corresponded to the weight fractions of the aromatic parts of the side groups. This copolymer further exhibited two glass transitions at 51 and 96 °C. The glass transition temperature Tg of 96 °C is identical to that of polystyrene homopolymer. Thus, the phase transition at 96 °C could be assigned to the glass transition of the PS block, while that at 51 °C could be assigned to the glass transition of the PCzMA block. In particular, the observation of the two glass transitions informed that the PS and PCzMA blocks are immiscible and have undergone phase separation. The diblock copolymers were further characterized by UV− vis spectroscopy and CV analysis. The results were given in Figures S2 and S3 (Supporting Information). From the measured UV−vis spectra, the band gap (i.e., the difference between the highest occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level) was estimated to be 3.510 eV for PCzMA, 3.520 eV for PS245-bPCzMA34, and 3.510 eV for PS1269-b-PCzMA46. The oxidation halfwave potential EOx vs Ag/Ag+ was determined to be 0.664 V for PCzMA, 0.964 V for PS245-b-PCzMA34, and 1.080 V for PS1269-b-PCzMA46. The external ferrocene/ferrocenium (Fc/ Fc+) redox standard potential (E1/2) was measured to be 0.530 V vs Ag/Ag+ in acetonitrile. Assuming that the HOMO level of the Fc/Fc+ standard is −4.800 eV with respect to the zero vacuum level, from the CV data, the HOMO level was estimated to be −4.934 eV for PCzMA, −5.234 eV for PS245-b4400

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Figure 2. Representative GISAXS patterns of solvent-annealed copolymer films (44.8−46.8 nm) measured with αi = 0.117° at room temperature using a synchrotron X-ray of λ = 0.0995 nm: (a, d, g, j) toluene-annealed PS245-b-PCzMA34 film deposited on a Si substrate; (b, e, h, k) CS2-annealed PS245-b-PCzMA34 film deposited on an OTS-Si substrate; (c, f, i, l) toluene-annealed PS1269-b-PCzMA46 film deposited on a Si substrate. Here, the scattering patterns in parts a−c are the experimental data and those in parts d−f are the scattering images reconstructed from the structural parameters in Table 4 using the GIXS formulas. 1D scattering profiles in parts g−l were extracted from the 2D GISAXS patterns in parts a−c: (g, j) extracted along the 2θf direction at αf = 0.121° and along the αf direction at 2θf = 0.220° from the pattern in part a; (h, k) extracted along the 2θf direction at αf = 0.109° and along the αf direction at 2θf = 0.084° from the pattern in part b; (i, l) extracted along the 2θf direction at αf = 0.116° and along the αf direction at 2θf = 0.156° from the pattern in part c. The black symbols are the measured data, and the colored solid lines were obtained by fitting the data using the GIXS formulas.

Table 4. Structural Parameters of Solvent-Annealed Nanoscale Thin Films of the Diblock Copolymers cylinder structure toluene-annealed film/substrate

da (nm)

Rb (nm)

σRc (nm)

PS245-b-PCzMA34/Si PS1269-b-PCzMA46/Si

26.8 38.0

8.0 8.3

1.0 0.8

CS2-annealed film/Substrate

Li (nm)

hPSj (nm)

PS245-b-PCzMA34/OTS-Si

23.4

15.9

a

td (nm)

φ̅ f (deg) 0 0

ge

45.5 0.121 44.8 0.152 lamellar structure

hPCzMAk (nm)

σLl (nm)

tm (nm)

g33n

gr3n

grrn

g3n

7.5

0.2

46.8

0.061

0.014

0.011

0.032

b

c

σφg (deg)

Os h

6.8 4.5

0.979 0.986

φ̅ o (deg) 0

σφp (deg)

Os q

6.7

0.980 d

Center-to-center distance of the PCzMA cylinders. Average radius of the PCzMA cylinders. Standard deviation of the cylinder radius. Thickness of the polymer thin film. eParacrystal distortion factor of the cylinder structure. fMean polar angle between the n vector of the cylinder structure and the out-of-plane direction of the film; for example, φ̅ is zero when the n vector in the film is oriented normal to the film plane. gStandard deviation of the polar angle φ. hSecond-order orientation factor of the cylinder structure. iLong period of the horizontally oriented lamella stacks. jAverage thickness of the PS insulator layer. kAverage thickness of the PCzMA layer. lStandard deviation of the thickness deviation of the PS and PCzMA layers. mThickness of the polymer thin film. nParacrystal distortion factors, defined as gij = Δaij/aj, where aj is the jth component of the fundamental lamellar lattice vector a and Δaij is the displacement of the vector aj in the direction of vector ai. oMean polar angle between the n vector of the lamellar structure and the out-of-plane direction of the film. pStandard deviation of the polar angle φ. qSecond-order orientation factor of the lamellar structure.

structure) and the out-of-plane direction of the film is 0°, but the standard deviation σφ is 18.5°. The positional distortion factor g is relatively large (0.210).

radius R of 6.8 nm, and an interdistance d of 29.5 nm. The second-order orientation factor Os is 0.859; the mean polar angle φ̅ between the orientation vector n (for the hexagonal 4401

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and 15.9 nm thick PS layers (=hPS) arranged in alternating stacks. The second-order orientation factor is Os = 0.980 (φ̅ = 0° and σφ = 6.7°) for the lamellar structure with respect to the out-of-plane direction of the film. The positional distortion factor g is very low (0.011−0.061, depending on the direction). Overall, a well-ordered, horizontally oriented lamellar structure is present in the PS245-b-PCzMA34 film deposited on the OTSSi substrate as a result of phase separation during post-CS2annealing. By comparison, a horizontal lamellar structure was found to form poorly in the toluene-annealed films on OTS-Si substrates (data not shown). Figure 2c shows a representative 2D GISAXS pattern of the toluene-annealed PS1269-b-PCzMA46 films on bare Si substrates. In-plane and out-of-plane scattering profiles were extracted along the 2θf direction at αf = 0.116° and along the αf direction at 2θf = 0.156°, respectively, and are displayed in Figure 2i,l. Overall, the scattering pattern resembles that of the tolueneannealed PS245-b-PCzMA34 film. This result suggests that a vertically oriented hexagonal structure is present in the film. The in-plane scattering profile could be satisfactorily fitted with the GIXS formula derived for a hexagonal cylinder structure. The obtained structural parameters are listed in Table 4. The cylinders have the following dimensions: R = 8.3 nm and d = 38.0 nm; it was confirmed that their height is the same as the film thickness. The second-order orientation factor is Os = 0.986 (φ̅ = 0° and σφ = 4.5°). The positional distortion factor g is low (0.152). The GIXS analysis results show that a wellordered, vertically oriented hexagonal PCzMA cylinder structure is present in the PS1269-b-PCzMA46 film deposited on the bare Si substrate as a result of phase separation during post-toluene-annealing. Such a vertical cylinder structure was also observed to form in the CS2-annealed films on bare Si substrates but was poorly developed (data not shown). Moreover, this block copolymer was found to form a random-shaped island morphology in the toluene- and CS2annealed films on OTS-Si substrates (data not shown). The above results give rise to the following conclusions. First, the block components are intrinsically immiscible with each other. Second, the block components undergo limited phase separation during the conventional solution spin-coating and subsequent drying processes, which produces poorly developed structures. Finally, post-solvent-annealing processes promote phase separation in the block components, which results in well-ordered, preferentially oriented nanostructures (vertical hexagonal or horizontal lamellar structures). Note that the morphologies with preferential orientations of the solvent-annealed films are very interesting. These morphologies and orientations result from the selective and specific interactions of the immiscible block components with the substrate surface in the film casting process as well as with solvent molecules during post-solvent-annealing. Our surface energy analysis found that the surface energy (46.9 mN/m = Es,Si) of the Si substrate is closer to that (48.9 mN/m = Es,PCzMA) of the PCzMA block component than that (43.7 mN/ m = Es,PS) of the PS block component (Table 1). In addition, a 0.5 wt % solution in chloroform of the PCzMA homopolymer was measured to have a contact angle of 12.3° on the surface of the Si substrate, which is comparable to that (11.9°) of a PS homopolymer solution in chloroform (0.5 wt %) (Table 2). Chloroform was employed as a mutual solvent in the preparations of the block copolymer and homopolymer solutions; it has a surface energy of 26.5 mN/m (ref 23) and is a good solvent for both of the block components.

Considering the above results, we attempted to anneal some of the as-cast films at room temperature under the vapors of various solvents in order to develop well-ordered nanostructures via phase separation of the block components. It was found that solvent-annealing with toluene for 18 h produced nanostructures in the films deposited on bare Si substrates and that solvent-annealing with CS2 for 18 h produced nanostructures in the films deposited on OTS-treated Si (OTS-Si) substrates. Figure 2a shows a representative 2D scattering pattern of the toluene-annealed PS245-b-PCzMA34 films on bare Si substrates. From the 2D scattering pattern, in-plane and outof-plane scattering profiles were extracted along the 2θf direction at αf = 0.121° and along the αf direction at 2θf = 0.220°, respectively, and are displayed in Figure 2g,j. This pattern in Figure 2g contains scattering peaks along the 2θf direction with relative scattering angles from the specular reflection position of 1 and 2; there are broad oscillation-like features along the αf direction in the individual peaks, in particular in the first peak. These scattering peaks are characteristic of a vertically oriented hexagonal cylinder structure. The in-plane scattering profile as well as the out-ofplane scattering profile could be satisfactorily fitted with the GIXS formula derived for a hexagonal cylinder structure. The obtained structural parameters are listed in Table 4. The cylinders have the following dimensions: R = 8.0 nm and d = 26.8 nm; it was confirmed that their height is the same as the film thickness. The second-order orientation factor Os is 0.979 (φ̅ = 0° and σφ = 6.8°). The positional distortion factor g is low (0.121). Overall, our detailed GIXS analysis confirmed that a well-ordered, vertically oriented hexagonal PCzMA cylinder structure is present in the PS245-b-PCzMA34 film deposited on a bare Si substrate as a result of phase separation during posttoluene-annealing. In addition, the formation of a vertical cylinder structure was observed in the CS2-annealed films on bare Si substrates; however, such structure was poorly developed (data not shown). Surprisingly, however, the CS2-annealed PS245-b-PCzMA34 film on an OTS-Si substrate produces a significantly different 2D GISAXS pattern, as shown in Figure 2b. In-plane and outof-plane scattering profiles were extracted along the 2θf direction at αf = 0.109° and along the αf direction at 2θf = 0.084°, respectively, and are displayed in Figure 2h,k. The outof-plane scattering pattern in Figure 2k contains scattering spots along the αf direction with relative scattering angles from the specular reflection position of 1, 2, 4, and 5. In contrast, there are no scattering spots along the 2θf direction. Thus, a lamellar structure is present in the film and its lamellae are stacked along a direction normal to the film plane. The lamellar structure was estimated to have a long period L of 23.4 nm from the first-order scattering spot; note that this value is comparable to the interdistance d (26.8 nm) of the PCzMA cylinders formed in the toluene-annealed film. In addition, the third-order scattering spot of the horizontal lamellar structure is missing, which indicates that in the lamellar structure the ratio of the thicknesses of the PS and PCzMA layers is near 2:1. This thickness ratio is in good agreement with the volume ratio of the two block components in the copolymer. The in-plane and out-of-plane scattering profiles could be satisfactorily fitted with the GIXS formula derived for a lamellar structural model (the derivation of the scattering formula is in the Supporting Information). The obtained structural parameters are summarized in Table 4. The lamellar structure has a long period L of 23.4 nm and consists of 7.5 nm thick PCzMA layers (=hPCzMA) 4402

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Figure 3. (a) An optical image and schematic diagram of memory devices fabricated with PS245-b-PCzMA34 and PS1269-b-PCzMA46 films. Schematic representations of the phase-separated morphologies in the as-cast copolymer films: (b) island domain structure poorly developed in the PS245-bPCzMA34 film deposited on Si substrate; (c) island domain structure poorly developed in the PS245-b-PCzMA34 film deposited on OTS-Si substrate; (d) vertical hexagonal cylinder structure poorly developed in the PS1269-b-PCzMA46 film deposited on Si substrate. Typical I−V curves of the devices fabricated with the as-cast copolymer films (45 nm thick), which were measured with a compliance current set of 0.01 A: (e) PS245-b-PCzMA34 film without an OTS layer; (f) PS245-b-PCzMA34 film with an OTS layer (1.1 nm thick); (g) PS1269-b-PCzMA46 film without an OTS layer. Al top and bottom electrodes were used. The electrode contact area was 0.2 × 0.2 mm2.

minor component are promoted to grow up as cylinder domains during the subsequent toluene-annealing via phase separation. Toluene has a surface energy of 28.5 mN/m (ref 23) and was found to be another good solvent for the PS block component, but it is a poor solvent for the PCzMA block component. Thus, during toluene annealing, the toluene vapor selectively mobilizes the PS block chains, which then further

Considering these surface energies and contact angles as well as the volume fractions of the block components, both of the block components cover the silicon substrate surface in the film casting process; in particular, the surface coverage of the substrate due to the PCzMA block component is likely to be proportional to its volume fraction in the copolymer. The PCzMA blocks anchored onto the substrate surface as the 4403

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Figure 4. Schematic representations of the phase-separated morphologies in the solvent-annealed copolymer films: (a) vertical hexagonal cylinder structure well developed in the toluene-annealed PS245-b-PCzMA34 film deposited on Si substrate; (b) horizontal lamellar structure well developed in the CS2-annealed PS245-b-PCzMA34 film deposited on OTS-Si substrate; (c) vertical hexagonal cylinder structure well developed in the tolueneannealed PS1269-b-PCzMA46 film deposited on Si substrate. Typical I−V curves of the devices fabricated with the solvent-annealed copolymer films (45 nm thick), which were measured with a compliance current set of 0.01 A: (d) toluene-annealed PS245-b-PCzMA34 film without an OTS layer; (e) CS2-annealed PS245-b-PCzMA34 film with an OTS layer (1.1 nm thick); (f) toluene-annealed PS1269-b-PCzMA46 film without an OTS layer. Al top and bottom electrodes were used. The electrode contact area was 0.2 × 0.2 mm2.

and the phase separation of the block components is promoted. As a result, a horizontally oriented lamellar structure is formed in the block copolymer film. Taking into account the above considerations, we attempted to reconstruct the 2D GISAXS images by using the structural parameters of the solvent-annealed films in Table 4. The reconstructed scattering images are displayed in Figure 2d−f and are in good agreement with the experimental data. The phase-separated structural models shown in Figures 3b−d and 4a−c were developed for the as-cast and solvent-annealed films on the basis of the analysis results shown in Figures 1 and 2. Memory devices were fabricated with the diblock copolymer films; aluminum (Al) top and bottom electrodes were used in these devices (Figure 3a). Figure 3e−g shows representative current−voltage (I−V) data for the devices fabricated with the as-cast films (45 nm thick). As shown in Figure 3e, the as-cast PS245-b-PCzMA34 film without an OTS layer exhibits a high resistance state (i.e., the OFF-state) and its current level gradually increases with increasing voltage during the initial positive voltage sweep. The film then undergoes an electrical

induces the mobilization of the PCzMA block chains, and the phase separation of the block components is promoted. As a result, a vertically oriented hexagonal PCzMA cylinder structure develops in the block copolymer film. The OTS-Si substrate was found to have a much lower surface energy (23.8 mN/m = Es,OTS‑Si) than the untreated Si substrate (Table 1). Furthermore, a 0.5 wt % solution in chloroform of the PCzMA homopolymer was measured to have a contact angle of 34.8° on the surface of the OTS-Si substrate, which is much higher than that (17.9°) of a PS homopolymer solution in chloroform (0.5 wt %) (Table 2). CS2 has a surface energy of 32.0 mN/m (ref 23) and was found to be a good solvent for the PS block component, but it is a poor solvent for the PCzMA block component. Considering these surface energies and contact angles as well as the volume fractions of the block components, the PS block component is likely to anchor to the OTS-Si substrate surface predominantly in the film casting process. During the subsequent CS2-annealing, the CS2 vapor selectively mobilizes the PS block chains, which then further induces the mobilization of the PCzMA block chains, 4404

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for the ON-states of the as-cast PS245-b-PCzMA34 films with and without an OTS layer (which exhibit poor DRAM behaviors) could also be fitted with SCLC models rather than with typical ohmic conduction models (Figure S6d,e, Supporting Information). The charge-trapping sites in the block copolymer devices are likely to originate in the chemical components of the diblock copolymer. PS homopolymer films with a thickness of 20 nm were found to exhibit dielectric-like behavior rather than electrical switching behavior (Figure S8, Supporting Information). In fact, the PS homopolymer is well-known as a dielectric polymer. Thus, the PS block and its phase could act as a barrier to charge transportation through the block copolymer film layer. Taking these results into account, we conclude that all the observed electrical memory behaviors originate in the PCzMA block component. The carbazole moieties, methylvinyl backbone, and methylenyl units are electron donors that can act as nucleophilic sites. When an electric field is applied to the polymer layer in the device, the carbazole moieties and other nucleophilic units are enriched with holes, becoming holetrapping sites. At the same time, the electrophilic carbonyl ester linker units in the polymer films are enriched with electrons, acting as electron-trapping sites. Considering its induction and resonance effects (which contribute to charge trapping and stabilization), the carbazole moiety is likely to have much higher charge-trapping and stabilization abilities than any of the other groups in the PCzMA block component. Thus, the carbazole moieties are likely to act mainly as charge-trapping sites, whereas the other nucleophilic and electrophilic groups are less likely to contribute significantly to charge trapping. The carbazole moieties are also likely to serve as stepping stones that enable the flow of charge carriers. When the applied bias reaches Vc,ON or higher, the flow of charge carriers (i.e., current) occurs through hopping processes (i.e., filament formation) between the carbazole sites in the PCzMA block phases. Considering the molecular orbitals of the polymers and the work function (4.28 eV) of the Al electrode, the energy barriers to hole injection from the electrodes to the HOMO of the polymers are much lower than the barriers to electron injection from the electrodes to the lowest LUMO of the polymers. Thus, the conduction processes in the polymer devices are likely to be dominated by hole injection. Thus, the memory behaviors of the polymer films can be directly correlated with their nanostructures. As shown in Figure 4, the WORM memory behaviors of the tolueneannealed polymer films are attributed to the vertical PCzMA cylinder domains formed in the dielectric PS matrix, which serve as active physical routes for charge trapping and transport. The PS245-b-PCzMA34 film has more PCzMA cylinders per unit volume than the PS1269-b-PCzMA46 film. Moreover, the total volume of PCzMA cylinders in the PS245-b-PCzMA34 film is much lower than that of the PCzMA homopolymer film, which exhibits similar WORM memory behavior. However, the current levels of the polymer films in the ON-state were found to be independent of the number (or total volume) of PCzMA cylinders in the device cell (Figures 4d,f and S5, Supporting Information). These results give rise to the following conclusions. First, in the diblock copolymer, the PCzMA block component is responsible for the WORM memory behavior. Second, the active physical routes (i.e., hopping paths or filaments) for charge transport consist of the carbazole moieties, which act as stepping stones and chargetrapping sites. Third, the local hopping paths (i.e., filaments)

transition from the OFF-state to a high conductivity state (the ON-state) at +8.7 V (which corresponds to Vc,ON, the critical voltage to switch the device on). The ON-state is retained when the electrical power is turned on. However, the film is returned to the OFF-state when a reverse voltage sweep is applied. Moreover, the ON-state is returned to the OFF-state immediately when the electrical power is turned off. This switching behavior is also evident in subsequent voltage sweep procedures. However, the Vc,ON value varies somewhat with the voltage sweeping procedure. The ON/OFF current ratio is relatively low (approximately 102). The film exhibits similar switching behavior during negative voltage sweeps (data not shown). Overall, the as-cast film exhibits unipolar dynamic random access memory (DRAM) behavior. Furthermore, the as-cast PS245-b-PCzMA34 film with an OTS layer (1.1 nm thick) exhibits similar DRAM behavior; however, its Vc,ON value is slightly high (+8.2 to +10.2 V) (Figure 3f). The as-cast PS1269b-PCzMA46 film without an OTS layer exhibits DRAM behavior, as shown in Figure 3g. However, its memory performance is much better than those of the as-cast PS245-bPCzMA34 films with and without an OTS layer, its Vc,ON value is much lower (+2.4 to +4.1 V), and its ON/OFF current ratio is much higher (approximately 105). On the other hand, the toluene-annealed PS245-b-PCzMA34 film (45.5 nm thick) without an OTS layer exhibits memory characteristics that are significantly different from those of the corresponding as-cast film. As shown in Figure 4d, the polymer film is initially in the OFF-state. However, when a positive voltage is applied, the current level in the film increases abruptly at +6.6 V (=Vc,ON), which indicates that the film undergoes a sharp electrical transition from the OFF-state to the ON-state. Once the device has reached its ON-state, it remains there, even after the power is turned off or during sequential reverse and forward voltage sweeping. The ON/OFF current ratio is in the range 105−107, depending on the reading voltage. The ON current level and the ON/OFF ratio are significantly higher than those of the as-cast film. Similar switching behavior was observed during negative voltage sweeps (data not shown). Overall, the toluene-annealed polymer film exhibits unipolar write-once-read-many (WORM) type memory characteristics. The PS1269 -bPCzMA46 film without an OTS layer exhibits similar WORM memory behavior (Figure 4f). Vc,ON is +4.7 V and the ON/ OFF current ratio is in the range 102−105, depending on the reading voltage. Moreover, similar WORM memory behavior was observed for the PCzMA homopolymer film device (Figure S5, Supporting Information). Vc,ON is +4.0 V and the ON/OFF current ratio is in the range 103−105, depending on the reading voltage. Surprisingly, however, the CS2-annealed PS245-bPCzMA34 film with an OTS layer (1.1 nm thick) exhibits no switching behaviors; instead, the film exhibits only dielectriclike I−V behavior, as shown in Figure 4e. The I−V data for the as-cast and solvent-annealed films in the OFF-states were found to be well fitted with the traplimited space-charge-limited conduction (SCLC) model,24 as shown in Figures S6a−c and S7a−c (Supporting Information). These results indicate that in their OFF-states all the film devices are governed by trap-limited SCLC mechanisms. On the other hand, the I−V data for the ON-states of the polymer film devices with high performance DRAM and WORM memory behaviors are satisfactorily fitted with the ohmic conduction model,25 as shown in Figures S6f and S7d,e (Supporting Information). Surprisingly, however, the I−V data 4405

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bottom electrodes exhibit volatile memory behaviors (i.e., poor or excellent DRAM) and dielectric characteristics, depending on the mean path length of the PS dielectric phase. In electric fields up to 10 V, charges can be transported through a 10.6 nm thick PS dielectric phase but are not transported through a 15.9 nm thick PS phase. All memory behaviors were found to be governed by the trap-limited SCLC mechanism and local hopping path (i.e., filament) formation (that owns ohmic conduction mechanism for high performance WORM memory and DRAM behaviors and trap-limited SCLC mechanism for poor performance DRAM behavior).

have a cross section with a radius less than 8.0 nm. Finally, memory cells with a size (i.e., radius) less than 8.0 nm can be fabricated, and therefore, there is potential for the fabrication of advanced memory devices with extremely high data storage densities. In contrast, the CS2-annealed film exhibits no switching behavior (Figure 4e). The film is composed of dielectric PS lamellae that are alternately stacked with PCzMA lamellae along the out-of-plane directon of the film (Figure 4b). In particular, the horizontally oriented PS lamellae have a thickness of 15.9 nm. Thus, these results show that charges cannot be transported across a 15.9 nm thick PS lamellar layer under an electric field up to ±10 V, which is why there is no switching behavior. On the other hand, the as-cast PS245-b-PCzMA34 films (which consist of random-shaped PCzMA islands in a PS matrix) exhibit poor DRAM characteristics (Figure 3b,c,e,f). The PCzMA islands have an average height of 9.5−11.0 nm, which is much less than the film layer thickness (45 nm). Thus, under an electric field, charges can be transported from one electrode in the memory cell through the PCzMA islands and additionally across a certain length scale of the PS phase to reach the other electrode. For these films, the mean path length of charges in the PS phase is not easily estimated. In such a morphology, charge trapping and stabilization could occur in the PCzMA island domains but is likely to be ineffectual in the dielectric PS phase. In particular, it is likely that the PS phase destabilizes the overall charge trapping and transport across the diblock copolymer film layer between the two electrodes, which leads to volatile DRAM behavior. A better DRAM performance was exhibited by the as-cast PS1269-b-PCzMA46 film (45.5 nm thick), which is composed of vertical PCzMA cylinders in the PS matrix (Figure 3d,g). The vertical PCzMA cylinders have an average height of 34.9 nm, which is less than the film layer thickness. From the film thickness and the height of the vertical PCzMA cylinders, the PS phase between the vertical PCzMA cylinder and the top or bottom electrode is estimated to have an average thickness of 10.6 nm. Taking into account the morphological characteristics, these memory results confirm that charges can be transported through a 10.6 nm thick PS dielectric phase under a relatively low electric field, 2.4−4.1 V.

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ASSOCIATED CONTENT

S Supporting Information *

GIXS data analysis, contact angle data, synthetic scheme, TGA and DSC data, UV−vis spectroscopy data, CV data, GIWAXS data, and I−V data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-54-279-2120. Fax: +82-54-279-3399. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Research Foundation (NRF) of Korea (Doyak Program 2011-0028678 and Center for Electro-Photo Behaviors in Advanced Molecular Systems (2010-0001784)) and the Ministry of Science, ICT & Future Planning (MSIP) and the Ministry of Education (BK21 Plus Program and Global Excel Program). The synchrotron X-ray scattering measurements at the Pohang Accelerator Laboratory were supported by MSIP, POSTECH Foundation, and POSCO Company.

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REFERENCES

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CONCLUSIONS Variations can be obtained in the nanostructures and orientations of the nanoscale thin films of PS245-b-PCzMA34 and PS1269-b-PCzMA46 by altering the film fabrication conditions, particularly by performing selective solventannealing, which promotes the phase separation of the block components. These nanostructures and orientations are directly related to the electrical memory performances of the block copolymer films. Nanoscale PCzMA cylinders oriented between the top and bottom electrodes exhibit high performance WORM memory behavior, as observed for the PCzMA homopolymer film. These results have two important consequences for advanced memory devices. First, carbazole moieties can form local hopping paths with a nanoscale cross section for charge trapping and transport; second, advanced memory devices with extremely high data storage densities can be fabricated with orientation-controlled PCzMA cylinder structures on the nanoscale. Moreover, the WORM memory mode of these devices can be converted to other memory modes by changing the morphology and orientation. Mixed arrangements of PCzMA and PS phases between the top and 4406

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