Switching of Resistive Memory Behavior from Binary to Ternary Logic

Article pubs.acs.org/JACS

Switching of Resistive Memory Behavior from Binary to Ternary Logic via Alteration of Substituent Positioning on the Subphthalocyanine Core Hing Chan, Hok-Lai Wong, Maggie Ng, Chun-Ting Poon, and Vivian Wing-Wah Yam* Institute of Molecular Functional Materials [Areas of Excellence Scheme, University Grant Committee (Hong Kong)] and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China S Supporting Information *

ABSTRACT: Two new axially or peripherally functionalized subphthalocyanines with the decoration of donor−acceptor substituents have been successfully synthesized, characterized and employed in the application of resistive memory device via solution-processable technique. Axially substituted subphthalocyanine shows ternary resistive memory behavior with wellseparated current ratios of 1:106:108 between “OFF”, “ON1” and “ON2” states, while only binary logic is observed for peripherally substituted subphthalocyanine. Computational studies show the presence of two well-separated charge transfer states in the axially substituted subphthalocyanine, while the charge transfer processes between the peripheral substituents and the subphthalocyanine core are found to be very close in energy. This work has demonstrated the impact of the substituent positioning on the subphthalocyanine-based memory device performance, providing a new research dimension for the future design and development of multistate organic resistive memory.

■

INTRODUCTION The foreseeable predicament of conventional electronic memories is certainly motivating the development of new types of electronic memory devices for the upcoming big data needs.1 Rather than dwindling the size of electronic components, scientists have started to design electronic memory devices based on molecular properties.2 Among various types of memory devices, organic-based resistive memory devices have attracted enormous attention due to their tunability for achieving memory devices with low power consumption, low cost, high flexibility and high storage density through judicious molecular designs.3 Resistive memory devices store information by electrically stable low and high conductivity (“OFF” and “ON”) states to realize the “0” and “1” coding system. To date, most of the resistive memory devices exhibit binary logic with storage density of 2n,4 while devices with storage capacity higher than 2n are required to cope with the future needs and challenges.4 Recently, ternary resistive memory effect has been realized by diverse organic small molecules and polymers via different design strategies.5 However, a deeper understanding of the resistive switching processes is needed in order to pull the guiding puzzles together for designing multilevel resistive memory devices with high performances. In order to explore factors that are crucial to the resistive memory behaviors, some efforts have been made to tune the memory effect from binary to ternary via different molecular designs on various structurally related compounds.5e,f,6,7 For example, Lu and co-workers have demonstrated that © 2017 American Chemical Society

the multistate memory behaviors could be regulated through the incorporation of two different electron-accepting moieties6a or strong electron donors onto a molecule.6b These works have demonstrated that small structural alterations could lead to drastic differences in resistive memory performance. However, there has been no report concerning the impact brought about by the modulation of substituent positions toward significant changes in the resistive memory properties. Although various small molecules have been explored for their potential in the field of resistive memory,5−7 it is quite astounding to find that investigations on the utilization of the unique electronic nature of macrocyclic compounds as the active materials are rather limited.5f,8 As one of the contracted porphyrinoid families, subphthalocyanines (SubPcs) possess exclusive photophysical and electrochemical features.9 Due to their versatile modifications at the axial and peripheral positions, SubPcs and their derivatives have been shown to play important roles in various optoelectronics including OPVs,10 OLEDs11 and optical recording applications.12 Nonetheless, to the best of our knowledge, there is no report of any electrical memory devices based on SubPcs or their derivatives. In fact, the positions of the substituents at either axial or peripheral disposition could lead to a drastic difference in the electronic interaction at the SubPc moiety. The axially substituted SubPc generally tends to preserve the independent Received: January 26, 2017 Published: May 16, 2017 7256

DOI: 10.1021/jacs.7b00895 J. Am. Chem. Soc. 2017, 139, 7256−7263

Article

Journal of the American Chemical Society

F) and 5-(7-(5-ethynyl-4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)-3-hexyl-N,N-diphenyl-thiophen-2-amine (Ph2N−hexTh−BTD−hexThCCH), compounds 1 and 2 can be obtained, respectively. Compounds 1 and 2 have been fully characterized by 1H, 11B{1H} and 19F{1H} NMR spectroscopy, FAB mass spectrometry and elemental analysis. Photophysical Studies. Both solution and thin-film state electronic absorption studies have been carried out to investigate the optical properties of compounds 1 and 2 as shown in Figures 1a and b. Compounds 1 and 2 possess

electronic features of both the SubPc core and the substituent while the peripherally modified SubPc would show the properties of an extended electronic conjugation contributed by the two moieties.13 In addition to our recent interest in designing various materials for resistive memory devices,5d−f,7 we believe that, with rationally designed substituents, a simple alteration of the substituent position on the SubPc core may already be sufficient to cause a significant change in the memory behaviors. In the light of this scenario, a new class of axially and peripherally substituted SubPcs, 1 and 2, has been designed, synthesized and characterized to test the hypothesis (Scheme 1). Interestingly, the axially substituted SubPc 1 has Scheme 1. Synthetic Pathways of Compounds 1 and 2

Figure 1. Electronic absorption spectra of 1 and 2 (a) in benzene solution and (b) in neat film. Steady state emission spectra of (c) 1 and 2, and (d) 1 and Ph2N−hexTh−BTD−hexTh−C6H4CC TIPS−p in benzene solution.

donor−acceptor type of substituents with similar electronic nature; however, upon varying the position of the substituent from axial to peripheral, the wavelength of the lowest-lying absorption band has been red-shifted from 569 to 608 nm (ca. 1158 cm−1) from 1 to 2. This observation is mainly contributed by the better electronic communication between the peripheral substituent and the SubPc core as well as the increase in the number of peripheral substituents, which is in accordance with the reported literature.13a,d−f In benzene solution, compounds 1 and 2 show drastic differences in their ground state electronic transitions as shown in the UV−vis absorption spectra (Figure 1a). In the thin-film state, there are observable red shifts of the lowest-energy absorption bands for both compounds (Figure 1b), which could be suggestive of the presence of intermolecular π−π interactions. Other than electronic absorption studies, the emissive properties of compounds 1 and 2 have also been studied (Figure 1c). Compound 1 exhibits a structureless emission band centered at ca. 675 nm with an emission shoulder at ca. 583 nm while compound 2 displays a vibronic-structured emission band at ca. 625 nm. The emissive origin of compounds 1 and 2 are found to be different. The emission of compound 1 at ca. 675 nm is assigned as originated from the emission of the axial substituent (Figure 1d) while that of compound 2 is believed to have arisen from the πextended SubPc core. The emission shoulder at ca. 583 nm in compound 1 is believed to be the residual emission from the SubPc core, which is commonly reported by other axially substituted SubPcs with efficient energy transfer processes.14 It is likely that efficient energy transfer from the SubPc core to the axial substituent occurs in compound 1. This has been

been demonstrated to exhibit promising ternary resistive memory effect while the peripherally substituted SubPc 2 only shows binary behavior. This study not only demonstrates the potential of subphthalocyanine-based materials as promising candidates for resistive memory applications, but also, more importantly, affords new approaches and insights for the design of high-performance multistate memory devices in the future.

■

RESULTS AND DISCUSSION Synthesis and Characterizations. The synthetic pathways of 1 and 2 are summarized in Scheme 1. Through typical Suzuki−Miyaura coupling reaction between 4′(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenylethyynylsubphthalocyaninatoboron(III) (SubPcCCC6H4B(pin)−p) and 3-hexyl-5-(7-(4-hexyl-5-iodo-thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)-N,N-diphen-ylthiophen-2amine (Ph2N−hexTh−BTD−hexTh−I) and Sonogashira coupling reaction between 4′-fluoro(2,9,16-triiodosubphthalocyaninao)boron(III) (C3−(2,9,16−(I)3)−SubPc− 7257

DOI: 10.1021/jacs.7b00895 J. Am. Chem. Soc. 2017, 139, 7256−7263

Article

Journal of the American Chemical Society supported by time-correlated single photon counting (TCSPC) experiments, which show that the residual emission of 1 at ca. 583 nm shows an emission decay lifetime of ca. 0.5 ns, shorter than that commonly observed in SubPcs (∼2−5 ns)9b and that in SubPcCCC6H5, the reference compound with the  CCC6H5 axial substituent (∼2 ns). Similar emission lifetimes of SubPc core are observed in systems showing energy transfer processes.14a Combining the observations of the partial spectral overlap between the absorption profile of the reference axial substituent (Ph2N−hexTh−BTD−hexTh−C6H4C CTIPS−p) and the emission profile of the reference SubPc core (SubPcCCC6H5) (Chart 1 and Figure S1), and the Chart 1. Structures of Reference Compounds SubPcC CC6H5 and Ph2N−hexTh−BTD−hexTh−C6H5CC TIPS−p

Figure 3. Differential pulse voltammograms showing the (a) oxidative scan and (b) reductive scan of compound 1 and (c) oxidative scan and (d) reductive scan of compound 2 in dichloromethane (0.1 M n Bu4NPF6). Ferrocenium/ferrocene couple (FeCp2+/0) was used as internal reference. Scan rate: 40 mV s−1.

compounds are found to be irreversible waves while the second reductions are quasi-reversible couples as shown in Figures 2b and d. On the basis of DPV data of the two target compounds (Figures 3a and c), there is only a 0.03 V difference between the potentials for the first oxidation of 1 and 2, which could be due to the predominant nature of the strong electron-donating diphenylamine moiety.13a,15 However, a 0.22 V difference between the first reduction potentials of 1 and 2 is found (Figures 3b and d). The large deviation of the first reduction potential of 2 relative to 1 could be the result of the peripheral substituent effect which is in agreement with the reported studies.13a,b,d,f With results from DPV measurements, the LUMO energy levels of compound 1 and 2 are calculated to be −3.33 and −3.55 eV, respectively, while the HOMO energy levels are found to be −5.08 and −5.11 eV, respectively. Thermal Stability. The thermal stability of compounds 1 and 2 have been tested by thermal gravimetric analysis (TGA). Both compounds show high thermal decomposition temperatures of over 350 °C as shown in Figure 4. Characterization of Resistive Memory Devices. The resistive memory performance of 1 and 2 have been studied by sandwiching their thin films between ITO and aluminum electrodes as illustrated in Figure 5a. By scanning electron microscopy (SEM) and the corresponding cross-sectional images (Figures 5b and c), the thickness of the thermally deposited aluminum and that of the active layer of the device prepared from 1 are found to be 93 and 77 nm, respectively, while the thicknesses of those prepared from 2 are 103 and 106 nm, respectively. Also, thin films of both compounds have been studied by tapping mode atomic force microscopy (AFM) as shown in Figure 6. The AFM topographic images and the corresponding cross-sectional height profile show that thin film of compound 1 possesses a smooth surface with root-meansquare (RMS) roughness of ca. ± 0.75 nm (Figure 6a), while compound 2 has a slightly less smooth surface with ca. ± 2.00 nm RMS roughness (Figure 6b). The current−voltage (I−V) characteristics of 1 is shown in Figure 7a. When the voltage is swept from 0 to 2 V, a sudden increase in current is observed at a switching threshold voltage of ca. 1.9 V (VTh1), suggesting a transition from a low-conductivity “OFF” state to an

significantly shortened emission lifetime of the SubPc part for compound 1, it is believed that there is an energy transfer process from the SubPc core to the axial substituent. The results of emission studies also unravel significant differences between the excited states of compounds 1 and 2. Electrochemical Studies. In order to provide more insight into the energy levels of compounds 1 and 2, cyclic voltammetry (CV) and differential pulse voltammetry (DPV) have been performed in dichloromethane solution (Figures 2 and 3, respectively). Results from both voltammetric measurements are comparable and have been summarized in Table 1. From CV data, both compounds show at least two quasireversible oxidation couples in the range of +0.74 to +1.03 V as depicted in Figures 2a and c. The first reductions of both

Figure 2. Cyclic voltammograms showing the (a) oxidative scan and (b) reductive scan of 1, and (c) oxidative scan and (d) reductive scan of 2 in dichloromethane (0.1 M nBu4NPF6). Ferrocenium/ferrocene couple (FeCp2+/0) was used as internal reference. Scan rate: 100 mV s−1. 7258

DOI: 10.1021/jacs.7b00895 J. Am. Chem. Soc. 2017, 139, 7256−7263

Article

Journal of the American Chemical Society Table 1. Electrochemical Data for Compound 1 and 2 at 298 Ka medium

oxidation E1/2/Vb vs SCE (ΔEp/mV)

1

CH2Cl2

+0.74(62), +1.00 (76) {+0.70, +0.96}

d

2

CH2Cl2

+0.77 (61), +1.03 (68) {+0.73, +1.01}d

compound

a

n

reduction E1/2/Vb vs SCE (ΔEp/mV) [Epc/Vb vs SCE] [−1.07]c, −1.22 (86) {−1.01, −1.19}d [−0.88]c, −1.16 (80) {−0.79, −1.12}d

b

0.1 M Bu4NPF6 as supporting electrolyte at room temperature. E1/2 = (Epa + Epc)/2; Epa and Epc are anodic peak and cathodic peak potentials from cyclic voltammetry, respectively. Scan rate: 100 mV s−1. cIrreversible reduction waves. dValues in curly parentheses are determined by differential pulse voltammetry. Scan rate: 40 mV s−1.

Figure 4. TGA thermograms of (a) 1 and (b) 2. Heating rate: 20 °C min−1 under a nitrogen atmosphere.

Figure 7. (a) Current−voltage characteristics of an ITO/1/Al device. (b) Retention time of the ITO/1/Al device in “OFF”, “ON1” and “ON2” states under a constant stress (1 V). (c) Current−voltage characteristics of an ITO/2/Al device. (d) Retention time of the ITO/ 2/Al device in “OFF” and “ON” states under a constant stress (1 V).

conductivity “ON2” state. The two abrupt transitions can be regarded as two “writing” processes. Sweep 3 clearly indicates that the “ON2” state can also be retained and cannot be turned to “ON1” or “OFF” states by applying a reverse bias (sweep 4). However, the “ON2” state can be relaxed back to the “OFF” state after switching the power off for several hours (Figure S2), typical of static random access memory (SRAM) devices. The resistive memory device can become operational again by applying the next cycle of voltage sweeps (Sweep 5−8, Figure S2), indicating that the memory performance is not originated from the reductive or oxidative damage of the films. The three distinct conductive states show a current ratios of 1:106:108 (“OFF”, “ON1” and “ON2”). All these I−V behaviors are characteristic performance of a ternary data-storage device Furthermore, constant voltage test at 1 V has been carried out to demonstrate the endurance of the three distinct electrically stable states (Figure 7b). There is no significant current degradation of the three conductive states over 104 seconds. Narrow threshold voltage distributions (VTh1 and VTh2) and clear separation between two threshold voltages, as depicted in Figure S3, indicate the reproducibility of the ternary memory performance of compound 1. The same set of I−V tests has also been applied to the devices prepared from compound 2 (Figures 7c and d). Even though more donor−acceptor substituents are attached to compound 2, only two electrically stable states can be observed in the same range of voltage bias. The switching threshold voltage (VTh) is found to be 2.0 V with “OFF” and “ON” current ratio of 1:106. The two distinct conductive states are also found to be stable under the constant voltage test with no significant drop of current. Such a drastic difference between the memory performance of compounds 1

Figure 5. (a) Schematic illustration of memory device structure. SEM images of the cross sections of the devices prepared from (b) 1 and (c) 2.

Figure 6. Tapping-mode (15 μm × 15 μm) AFM images and the corresponding cross-section profiles of the AFM topographic images of the thin film of (a) 1 and (b) 2.

intermediate-conductivity “ON1” state. Subsequent sweep from 0 to 5 V (sweep 2) shows that the intermediate-conductivity “ON1” state could be retained and displays a second switching threshold voltage at ca. 2.7 V (VTh2), indicating a transition from the intermediate-conductivity “ON1” state to a high7259

DOI: 10.1021/jacs.7b00895 J. Am. Chem. Soc. 2017, 139, 7256−7263

Article

Journal of the American Chemical Society

believed to be responsible for the resistive switching properties.5−7 For 2′, there are, in fact, more than two possible charge transfer processes revealed by the calculations (Table S1). However, the vertical excitation energies of these transitions are very close in energy, leading to the observation of only one high-conductivity state. Therefore, the multistate memory performance of compound 1 is possibly due to the weak electronic interaction between the axial substituent and the SubPc core which leads to the preservation of individual electronic properties of both moieties. The “ON1” and “ON2” states are attributed to the two charge transfer processes between the axial substituent and the SubPc core which possess different voltage barriers to impede the conductivity of the charge carriers. On the contrary, the binary logic behavior of compound 2 is due to the greater electronic coupling between the peripheral substituents and the SubPc core, resulting in electrically indistinguishable charge transfer processes between the peripheral substituents and π-extended SubPc core. To further verify the resistive switching mechanism of 1, a thin layer (ca. 1 nm) of molybdenum trioxide (MoO3) has been deposited on top of the ITO electrode to serve as a surface modifier. Treatment by UV-ozone was then carried out before the film formation of 1 via spin-coating method. With thermally deposited aluminum electrode, similar I−V sweeps are performed to the devices. Multilevel resistive memory behavior with threshold voltages at ca. 1.5 and 3.5 V has been observed (Figure S5). The results of these devices indicate that the switching behavior of 1 is not likely due to the carrier trapping sites caused by the structural defects at the ITO/1 interface.

and 2 has demonstrated the critical influence of electronic conjugation between the central SubPc unit and the substituents at different positions toward the resistive memory behavior. Judging from the energy level distributions of compounds 1 and 2, the hole injections from the ITO electrode to the active layers are possibly the dominant pathway because the energy barriers between the work function of ITO electrode (−4.8 eV) and the HOMO levels of the target compounds are smaller than that of the work function of aluminum electrode (−4.3 eV) and the LUMO energy levels of the target compounds as depicted in Figure 8.

Figure 8. Schematic diagram of the charge injection process in compound 1- or 2-based resistive memory devices.

Computational Studies. In order to explain the distinct difference in the switching behaviors of compounds 1 and 2, theoretical calculations have been carried out for model compounds 1′ and 2′, in which the n-hexyl groups are replaced with methyl groups, with optimized ground-state geometries. The vertical excitations of 1′ and 2′ are in good agreement with the experimental absorption spectra of 1 and 2, respectively (Figure S4 and Table S1). The HOMOs and LUMOs of 1′ are localized on either the axial substituent or the SubPc core (Figure 9) while the HOMOs and LUMOs of 2′ show contributions from both the substituent and the SubPc core (Figure 10). TD-DFT calculations show that 1′ can exhibit predominantly two charge transfer transitions with different excitation energies from HOMO → LUMO (2.08 eV) and HOMO−2 → LUMO+2 (2.92 eV) corresponding to the S0 → S1 and S0 → S9 excitations (Table S1), which could be attributed to the formation of the “ON1” and “ON2” conductivity states as charge transfer processes are in general

■

CONCLUSION The first report of subphthalocyanine-based binary and ternary resistive memory devices has been successfully demonstrated by a new class of thermally stable peripherally and axially substituted subphthalocyanines. In comparison to the binary logic presented by the peripherally substituted subphthalocyanine, the axially substituted subphthalocyanine has been shown to exhibit promising ternary memory effect. It is probably due to the rather weak electronic interaction between the axial substituent and the subphthalocyanine core, which conserves the electronic nature of the axial substituent and the subphthalocyanine core. This present work has provided a

Figure 9. Spatial plots (isovalue = 0.03) of the selected molecular orbitals of 1′. 7260

DOI: 10.1021/jacs.7b00895 J. Am. Chem. Soc. 2017, 139, 7256−7263

Article

Journal of the American Chemical Society

Figure 10. Spatial plots (isovalue = 0.03) of the selected molecular orbitals of 2′. Instruments, Inc. model CHI 620A electrochemical analyzer. Electrochemical measurements were performed in dichloromethane solutions with 0.1 mol dm−3 nBu4NPF6 as supporting electrolyte at room temperature. The reference electrode was a Ag/AgNO3 (0.1 mol dm−3 in acetonitrile) electrode and the working electrode was a glassy carbon electrode (CH Instruments, Inc.) with a platinum wire as the counter electrode (CH Instruments, Inc.). The working electrode surface was first polished with 1 μm alumina slurry (Linde) on a microcloth (Buehler Co.) and then with 0.3 μm alumina slurry. It was then rinsed with ultrapure deionized water and sonicated in a beaker containing ultrapure water for 5 min. The polishing and sonicating steps were repeated twice and then the working electrode was finally rinsed under a stream of ultrapure deionized water. The ferrocenium/ ferrocene couple (Fc+/Fc) was used as the internal reference. All solutions for electrochemical studies were deaerated with prepurified argon gas prior to measurements. The AFM images were obtained using a Digital Instruments Nanoscope III AFM with a scan rate of 1.0 m μs−1. The samples were prepared by spin-coating the sample solution onto a quartz plate. SEM experiments were performed on a Hitachi S-4800 FEG Scanning Electron Microscope at the Electron Microscope Unit of The University of Hong Kong. Fabrication and Measurements of the Memory Devices. The transparent anode indium−tin-oxide (ITO)-coated borosilicate glass substrate (2 cm × 2 cm) was ultrasonicated successively with deionized water, acetone, isopropanol and absolute ethanol for 15 min each and then dried in an oven at 120 °C for an hour. Chloroform solutions of compounds 1 and 2 (ca. 10 mg mL−1) were spin-coated onto the ITO substrate followed by thermal annealing at 75 °C for 15 min. Each organic surfaces were deposited with an aluminum top electrode by thermal evaporation through a shadow mask under a pressure of around 5 × 10−6 mbar. 400 devices were fabricated on each ITO glass platform and the active area of each cell was about 0.25 mm2. The deposition of a thin-layer (ca. 1 nm) of surface modifier, molybdenum trioxide (MoO3), was achieved by thermal evaporation under a pressure of around 5 × 10−6 mbar. Current−voltage (I−V) characteristics of the memory devices were measured with a programmable Keithley model 4200SCS power source in a probe station. All electrical measurements of the device were taken under ambient conditions. Computational Details. All calculations were performed by employing the Gaussian 09 suite of programs.16 Density functional theory (DFT) at the PBE0 level17 was employed to optimize the ground-state geometries of the model compounds of 1 and 2, in which all the hexyl groups were replaced by methyl groups (compounds 1′

new insight on the realization of the multistate memory feature of the subphthalocyanine derivatives by modulating the position of the rationally designed substituents, in which the difference in the degree of electronic interactions between different moieties brought about by the positioning could exert a strong influence on the charge transfer processes. This provides a new research dimension for the future design and development of multilevel organic resistive memory.

■

EXPERIMENTAL SECTION

Physical Measurements and Instrumentation. 1H NMR spectra were recorded on a Bruker DPX-300 or a Bruker AV 400 Fourier transform NMR spectrometer with chemical shifts recorded relative to tetramethylsilane (Me4Si). 19F{1H} NMR spectra were recorded on a Bruker AV 400 NMR spectrometer with chemical shifts reported relative to CFCl3. 11B{1H} NMR spectra were recorded on a Bruker DPX-500 with chemical shifts reported relative to BF3·OEt2. Electron impact (EI) and fast atom bombardment (FAB) mass spectra were recorded using a Thermo Scientific DFS High Resolution Magnetic Sector Spectrometer while electrospray ionization (ESI) mass spectra were recorded on a Finnigan LCQ Spectrometer. Elemental analyses of the newly synthesized compounds were performed on a Flash EA 1112 elemental analyzer at the Institute of Chemistry, Chinese Academy of Sciences, Beijing. The electronic absorption spectra were measured on a Varian Cary 50 spectrophotometer. All measurements were carried out at room temperature unless specified otherwise. Steady state emission spectra at room temperature was recorded using a Spex Fluorolog-3 Model FL3-211 spectrofluorometer equipped with a R2658P PMT detector with or without corning filters. For solution emission measurements, a degassing cell with a 10 cm−3 Pyrex round-bottomed flask connected by a side arm to a 1 cm quartz fluorescence cuvette that was sealed by a Rotaflo HP6/6 quick-release Teflon stopper from the atmosphere was used. The samples were degassed on a high-vacuum line with no fewer than four freeze−pump−thaw cycles before emission meaurement. Time-resolved emission studies were performed by using a Horiba Jobin Yvon FluoroCube based on a time-correlated single photon counting method using a nano-LED with peak wavelength and pulse duration equal to 371 nm with