ZnS Core-Multishell

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Functional Inorganic Materials and Devices

Flexible Memristive Devices Based on InP/ZnSe/ ZnS Core-Multishell Quantum Dot Nanocomposites Do Hyeong Kim, Chaoxing Wu, Dong Hyun Park, Woo Kyum Kim, Hae Woon Seo, Sang-Wook Kim, and Tae Whan Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18817 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Materials & Interfaces

Flexible Memristive Devices Based on InP/ZnSe/ZnS Core-Multishell Quantum Dot Nanocomposites Do Hyeong Kim1, Chaoxing Wu1, Dong Hyun Park1, Woo Kyum Kim1, Hae Woon Seo2, Sang Wook Kim2 and Tae Whan Kim1,* 1

Department of Electronics and Computer Engineering, Hanyang University, Seoul 04763,

Republic of Korea 2

Department of Molecular Science & Technology, Ajou University, Suwon 443-749, Republic

of Korea ABSTRACT The effects of the ZnS shell layer on the memory performances of flexible memristive devices based on quantum dots (QDs) with an InP/ZnSe/ZnS core-multishell structure embedded in a poly(methylmethacrylate) (PMMA) layer were investigated. The ON/OFF ratios of the devices based on QDs with an InP/ZnSe, core-shell, structure and with an InP/ZnSe/ZnS, core-multishell, structure were approximately 4.2 × 102 and 8.5×103, respectively, indicative of an enhanced charge-storage capability in the latter. After bending, the memory characteristics of the memristive devices based on QDs with the InP/ZnSe/ZnS structure were similar to those before bending. In addition, those devices maintained the same ON/OFF ratios for retention times of 1 × 104 s, and the number of endurance cycles was above 1 × 102. The reset voltages ranged from -2.3 to -3.1 V, and the set voltages ranged from 1.3 to 2.1 V, indicative of reliable electrical characteristics. Furthermore, the possible operating mechanisms of the devices are presented on the basis of the electron trapping-andrelease mode.

KEYWORDS: memristive device, memory device, flexible devices, InP/ZnSe/ZnS coremultishell QDs, nanocomposites

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1. INTRODUCTION Memristive devices based on inorganic/organic nanocomposites have attracted great interest because of their advantages of having tailored electric properties, simple fabrication, high flexibility, and low cost in comparison with flash-type organic and ferroelectric memories.1-7 Quantum dots (QDs) or nanoparticles have been widely utilized for electronic devices because their unique electrical and optical properties can be precisely controlled by adjusting their size and thickness.8-12 Especially, nanocomposite multilayer films based on layer-by-layer assembled Fe3O4 QDs can be employed for the fabrication of nonvolatile memories with high memory perforances.11 Furthermore, various kinds of semiconductor QDs containing Cd and Pb atoms have also been employed in highly stable memristive devices.13-16 For example, the memristive device utilizing CdSe/InP core-shell nanoparticles embedded in polystyrene exhibits a current hysteresis behavior with an ON/OFF ratio of 1×107 and an endurance number of 1×105 cycles.10 The device fabricated based on CdS nanocrystals shows bipolar resistance switch with an ON/OFF ratio of around 50 and an endurance number of over 1000 cycles.11 Worth noting is that for potential commercialization of memory devices based on QDs, an ON/OFF ratio larger than 102 and an endurance number of at least 106 might be necessary. In addition to the target performances of the memory devices, the active materials should be taken into consideration when designing nextgeneration memory devices due to the requirements for environment-friendly fabrication and green electronics technology. Because the Cd and the Pb atoms in core-shell QDs are toxic, the use of devices using compound semiconductors containing Cd and Pb is prohibited by the Restriction of Hazardous Substances Directive (RoHS) in Europe.15-17 The InP/ZnSe/ZnS multi-shell QDs used in this work are made of environment-friendly materials. Thus, their use in next-generation memristive devices should be environmentally acceptable. In spite of 2


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this, very few investigations of memristive devices based on non-toxic QDs have been conducted.17,18 Even though some works on the optical properties of InP/ZnSe/ZnS coremultishell QDs have been carried out,19,20 studies of the effects of the ZnS shell on the memory performances of flexible memristive devices have not yet been performed. In this paper, the effects of a ZnS shell on the memory performances of flexible memristive

devices

based

on

InP/ZnSe/ZnS,

core-shell-shell

(core-multishell),

nanocomposites were investigated. Because the ZnSe/ZnS (multishell) heterostructure with a similar lattice constant provides a high confinement effect in comparison with the ZnSe (mono-shell) structure, highly-stable memristive devices based on the InP/ZnSe/ZnS coremultishell QDs may be possible.21 For that reason, ultraviolet photoelectron spectroscopy (UPS) spectra were obtained to determine the energy levels of the highest occupied molecular orbitals (HOMOs) of the InP/ZnSe and the InP/ZnSe/ZnS QDs, and their band-gap energies were determined from the UV-vis and the photoluminescence (PL) spectra. In addition, current-voltage (I-V) measurements were performed to investigate the electrical bistability properties of the memristive devices. The retention and the endurance properties of the memristive devices were also measured to investigate the stabilities and the durabilities of the devices before and after bending. Based on the I-V fitting results, we described the possible operating mechanism with the aid of the energy-band diagram.

2. EXPERIMENTAL In the preparation of the InP/ZnSe/ZnS QDs, the InP core were synthesized first. Firstly, zinc acetate (0.183 mg, 1 mmol), indium acetate (0.070 g, 0.24 mmol), and myristic acid (0.496 g, 2.24 mmol) were dissolved in octadecene (ODE, 6 ml) at room temperature. The resulting solution was then degassed for 2 h at 110ºC, after which it was 3



cooled to room temperature. Subsequently, 0.048 mg of a tris(trimethylsilyl)phosphine (0.19 mmol) solution in 1 ml of ODE and 0.5 ml of trioctylphosphine were injected into the degassed solution at 200ºC, and the solution was kept at 300ºC for 1 min. Thus, the InP colloidal solution was successfully prepared. The ZnSe shell was formed by using a 0.25-M zinc oleate solution (4 ml, 1.0 mmol) in ODE, which was added to the InP solution at 200ºC. After the reaction had been allowed to take place for 10 min, trioctylphosphine selenide (1 ml, 1 mmol) was injected into the solution. The solution was heated at 300ºC for 30 min and then cooled to 200ºC. For the formation of the ZnS shell on InP/ZnSe, a zinc oleate solution (4 ml, 1 mmol) was added to the solution at 200ºC. After 10 min, 1-Dodecanthiol, as a sulfur precursor (0.25 ml, 1 mmol), was injected at 300ºC. The solution was maintained at 300ºC for 20 min to allow the reaction to occur, after which it was cooled to room temperature. The final solution with homogeneous QDs was obtained by utilizing the centrifuge method. For the fabrication of the device, poly(methylmethacrylate) (PMMA) (average Mw ~996000) purchased from Sigma-Aldrich was dissolved in toluene at a concentration of 4 wt %. The synthesized InP/ZnSe and InP/ZnSe/ZnS QDs were added to the PMMA solution at QD concentrations of 0.5, 1, 2, and 3 wt%. The PMMA:InP QDs nanocomposite with an InP QD concentration of 4 wt% was used as the active layer to investigate the effect of multishell (ZnSe/ZnS) QDs on the electrical properties of the memristive devices and to find an optimized QD concentration for the memristive device. The mixed PMMA:QD solutions were sonicated for 30 min. The ITO-coated polyethylene glycol naphthalate (PEN) substrates were ultra-sonicated by using methanol and deionized water for 20 min each. Then, the ITO-coated PEN substrates were dried with N2 gas and treated with an ultraviolet ozone cleaner for 20 min. The mixed solution of PMMA:QDs was spin-coated onto cleaned ITO-coated PEN substrates at 300 rpm for 10 s 4


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and 3000 rpm for 30 s at 300 K. Then, the active layers of PMMA:QDs were baked at 111ºC for 20 min to remove the residual solvent. Finally, the top Al electrodes with diameters of 1 mm and thicknesses of 180 nm were deposited on the PMMA:QDs by using the thermal evaporation method at a chamber pressure of 1 × 10-6 Torr. Note that in order to investigate the effect of the ZnSe/ZnS multishell, we fabricated PMMA:(InP/ZnSe) QD devices by using the same process as that used to fabricate the PMMA:(InP/ZnSe/ZnS) QD devices.

3. RESULTS AND DISCUSSION Figure 1(a) shows a schematic of the Al/PMMA:(InP/ZnSe/ZnS) QDs)/ITO/PEN device, and Figure 1(b) shows the schematic structures of an InP/ZnSe core-shell QD and an InP/ZnSe/ZnS core-multishell QD. Figure 1(c) shows photographs of the InP/ZnSe and the InP/ZnSe/ZnS QD solutions with a QD concentration of 2 wt%. A bright green emission was observed. Figure 1(d) presents a cross-sectional scanning electron microscopy (SEM) image of the memristive device. The thickness of the (PMMA:(InP/ZnSe/ZnS) QDs) layer was about 300 nm. High-resolution transmission electron microscope (TEM) images of the InP/ZnSe QDs with a mono-shell structure and of the InP/ZnSe/ZnS QDs with a multishell structure are shown in Figs. 1(e) and (f), respectively.

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Figure 1. (a) Schematic of the fabricated devices with the structure of Al/PMMA:(InP/ZnSe/ZnS QDs)/ITO/PEN. (b) Schematic structures of QDs with core-shell (mono-shell) and core-shell-shell (multishell) structures. (c) Photographs of the InP/ZnSe and the InP/ZnSe/ZnS solutions. (d) Cross-sectional SEM image of the PMMA:InP/ZnSe/ZnS on an ITO glass. (e, f) TEM image of the InP/ZnSe and the InP/ZnSe/ZnS QDs. Figures 2(a) and (b) illustrate the PL and the UV-vis spectra of the InP/ZnSe and the InP/ZnSe/ZnS QDs, respectively. The energy band gaps of the InP-based QDs obtained by using the UV-vis and the PL spectra were found to be 2.26 eV for the InP/ZnSe QDs and 2.39 eV for the InP/ZnSe/ZnS QDs. The highest occupied molecular orbital (HOMO) levels of the QDs were determined from the UPS spectra.22 The HOMO levels of the InP/ZnSe and the InP/ZnSe/ZnS QDs were found to be 5.6 eV and 6.4 eV, respectively, as shown in Figs. 2(c) and 2(d). On the other hand, the lowest unoccupied molecular orbital (LUMO) levels for the InP/ZnSe and the InP/ZnSe/ZnS QDs were determined by using the experimental results of the HOMO energy level and the energy band gap.23 The energy level of the LUMO for PMMA was provided by Sigma-Aldrich.16 6


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Figure 2. (a) and (b) UV-vis spectra (left) and photoluminescence spectra (right) from the InP/ZnSe QDs and the InP/ZnSe/ZnS QDs. Ultraviolet photoelectron spectroscopy spectra from (c) InP/ZnSe QDs and (d) InP/ZnSe/ZnS QDs. The I-V curves of the devices with various InP/ZnSe/ZnS QD concentrations are shown in Fig. 3(a). In the measurements of the resistive characteristics, the bottom ITO layer electrode was grounded, and the bias voltage was applied to the top Al electrode. When the applied voltage was swept from 0 to +3.5 V, the device was in the high-resistance state (HRS, OFF) initially and switched to low-resistance state (LRS, ON) under the setting voltage. When the applied voltage was swept from +3.5 to -3.5 V, reset switching occurred under negative bias, which showed that the memristive device was rewritable.17 For the insulating PMMA mixtures, the amount of PMMA should be minimal to make the films conductive. Thus, the ON/OFF current margin increases with increasing QD concentration due to the increasing filling fraction of the QDs in the active layer. Note that the active layers of the memristive devices are prepared by directly spin-coating the solution 7



containing hydrophobic ligand-stabilized QDs and PMMA. Due to the unfavorable interfacial energy between PMMA and the QDs, segregation or aggregation of the QDs may occur in the active layer.10 However, by increasing the filling fraction of the QDs in the active layer, a more uniform dispersion of the QD aggregations in the active layer may be possible. As a result, the ON/OFF current margin increases in proportion to the QD concentration. The device with a QD concentration of 2 wt% shows the largest memory margin, as shown in Fig. 3(a). However, when the QD concentration is increased further, the ON/OFF current margin decreases. A possible reason is that the QD aggregations tends to connect with one another with a further increase in the QD concentration, which can result in an effective charge transport between neighboring QDs.24 Figure 3(b) shows the I-V curves for the devices with PMMA:(InP/ZnSe/ZnS) QDs and PMMA:(InP/ZnS) QDs. The ON/OFF current margin of the device using the ZnSe/ZnS multishell is seen to be larger than that of the device using the ZnSe mono-shell, which may have been due to the increased energy level of the LUMO for the multishell QDs. The bending stability of the Al/PMMA:(InP/ZnSe/ZnS) QDs/ITO/PEN device was studied. The LRS current in the device bent with a radius of curvature of 20 mm was larger than that of the device bent with a radius of curvature of 10 mm. The LRS current in the flat state was higher than that in the bent state, which is possibly due to a decrease of the resistance of the bottom ITO electrode. The inset of Fig. 3(c) shows the I-V characteristics of the device with (InP/ZnSe/ZnS) QDs at a QD concentration of 2 wt%. For a 10-mm radius of curvature (Fig. 3(d)), the ON/OFF ratio is slightly decreased. However, the ON/OFF ratio is sufficiently high for memory application.25-27

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Figure 3. (a) I-V curves for the devices with InP/ZnSe/ZnS QDs at QD concentrations of 0.5, 1, 2, and 3 wt%. (b) I-V curves for the devices containing InP/ZnSe QDs and InP/ZnSe/ZnS QDs at a QD concentration of 2 wt% in the 10 mm. (c) ON-state currents of the device with InP/ZnSe/ZnS QDs at a QD concentration of 2 wt% in the flat state and in the bent state with a radius of 20 and 10 mm. The inset shows the I-V curves. (d) Photographs the devices in the bent state with a radius of 20 and 10 mm. Figure 4(a) shows a schematic diagram of a bent flexible device, where R is the bending radius (radius of curvature), θ is the central angle, and D is the shortest distance between the edges of the bent device. Figure 4(b) shows photographs of the bent flexible device for various central angles. The I-V characteristics in Fig. 4(c) confirmed distinct differences between the ON-state and the OFF-state after repeated bending up to 30 cycles for various values of the central angles. In addition, the ON state currents in 10 devices were measured for each central angle. The ON-state currents at the read voltage (-1 V) remained between 1.9 × 10-3 and 4.8 × 10-4, as shown in Fig. 4(d).

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Figure 4. (a) Schematic diagram of a bent flexible device. (b) Photographs of, (c) currentvoltage curves for, and (d) distribution of the ON-state currents after repeated bending cycles up to 30 cycles for the flexible device bent with various central angles θ. The endurance properties of the memristive devices after 100 bending cycles are shown in Fig. 5(a). The retention data were obtained at room temperature and a reading voltage of -1 V for the devices containing both (InP/ZnSe) and (InP/ZnSe/ZnS) QDs. The two types of devices showed a distinct difference in the ON/OFF ratios without significant degradation. The ON-state and the OFF-state of the (InP/ZnSe/ZnS) QD devices maintained currents of 1.4 × 10-3 and 1.7 × 10-7 A , respectively, for up to 104 s without any significant change. The OFF-state and the ON-state of the (InP/ZnSe) QD devices showed high stability similar to that of the (InP/ZnSe/ZnS) QDs device, as shown in Fig. 5(b). Figure 5(c) shows the probability distributions of the reset and the set voltages for the Al/PMMA:(InP/ZnSe/ZnS) QDs/ITO/PEN devices. The values of the reset voltages were distributed between -2.3 and 3.1 V, and the values of the set voltages were distributed between 1.3 and 2 V.

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Figure 5. (a) The ON-state and the OFF-state currents at -1 V as functions of the number of bending cycles up to 100 cycles for the devices with InP/ZnSe and InP/ZnSe/ZnS QDs. (b) Retention characteristics of the devices based on InP/ZnSe and InP/ZnSe/ZnS QDs after 100 bending cycles. (c) Set voltage and reset voltage probability distributions for the setting and the resetting processes. In order to describe the carrier transport mechanism for the memristive devices based on (InP/ZnSe/ZnS) QDs, we fitted the current density-voltage curve, as shown in Figs. 6(a) and 6(b). Ohmic conduction was observed in the LRS on the basis of the slopes of the fitted curves, which suggested well-formed conducting filament due to electron trapping in the LRS.28 The current density-voltage curves in the HRS at low applied bias (0 to -1 V, or 0 to 1 V) were also dominated by Ohmic conduction based on the slopes of the fitted I-V curves. However, because of an increase in the electric field (-1 to -1.8 V or 1 to 1.8 V) in the HRS, all of the traps in the multishell (InP/ZnSe/ZnS) QDs were completely filled.29,30 Therefore, 11



the current for the HRS increases due to space-charge-limited Conduction (SCLC) based on slopes of 2.5 and 3.1, respectively, for the fitted curves.28-30 Thus, the carrier transport mechanisms of the memristive devices based on a PMMA:(InP/ZnSe/ZnS) QD active layer can be described on the basis of the above models.

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Figure 6. Current density-voltage fitting curves for the ON and the OFF states in (a) the negative and (b) the positive regions. Figure 7 illustrates the carrier transport of the memristive device. The LUMO energy levels for the (InP/ZnSe) and the (InP/ZnSe/ZnS) QDs were found to be -3.3 and -4.0 eV, respectively, by using the experimental results shown in Fig. 7(a). Electron, rather than hole, injection is more favorable from the Al electrodes because the energy barrier between the conduction band of the (InP/ZnSe/ZnS) QDs and the Al (1.0 eV) is smaller than the valence band of the (InP/ZnSe/ZnS) QDs and the Al electrodes (2.1 eV).17,30,31 The energy barrier (1.5 eV) between the conduction band of the (InP/ZnSe/ZnS) QDs and the ITO is relatively larger than the energy barrier (1.0 eV) between the Al and the conduction band of the (InP/ZnSe/ZnS) QDs. Thus, the efficiency of electron injection from the Al electrodes is large compared to that from the ITO electrode.30,31 Therefore, differences in the trapping and the release processes were observed in the forward and the reverse voltage sweeps.17,32,33 On the basis of the above discussion, the set/reset operation of the memristive device is 12


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shown in Figs. 7(b)-(d). When a low bias voltage (0 to 1 V) is applied to a memristive device that stays in the HRS, the OFF-state current gradually increases linearly with increasing voltage. This is due to the existence of a small number of carriers in the active layer, as schematically shown in Fig. 7(b). When a higher bias voltage (1 to 1.8 V) is applied, the electrons injected from the Al electrode are transported in the active layer along the direction of the applied electric field, and the electric field between adjacent (InP/ZnSe/ZnS) QDs increases with increasing applied voltage. Due to the low energy level of the InP QDs between PMMA layers, the (InP/ZnSe/ZnS) QDs act as electron trapping sites. Thus, the formation of the space-charge electric field dominates the conduction process.29-33 Consequently, the electronic occupation probability of the QDs increases, and the Fermi level of the QDs moves to the LUMO level of the PMMA; thus, the current can flow easily through the active layer due to the conductive filaments that are formed in the PMMA:QD layer, as schematically shown in Fig. 7(c).17,35 After the transition from the HRS to the LRS, the current in the memristive device remains constant. When a negative voltage higher than reset voltage is applied, the electrons trapped in the InP-based QDs are de-trapped, which is known as an erasing process, as shown in Fig. 7(d). As a result, this causes an increase of the resistance, and the device is switched to the HRS.17,30,33

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Figure 7. (a) Schematic of the energy-level bands for the Al/PMMA:(InP/ZnSe/ZnS QDs)/ITO/PEN memristive device with no bias. (b),(c) Schematic of the carrier transport mechanisms in the set process and (d) in the reset process. 4. CONCLUSION Flexible memristive devices were fabricated utilizing PMMA:InP/ZnSe/ZnS (coremultiple shell) QD nanocomposites. The memory margin of the devices fabricated utilizing the InP/ZnSe/ZnS QDs was larger than that of the devices based on the InP/ZnSe QDs, and the maximum window margins of those devices were approximately 8.5×103 and 4.2 × 102, respectively, indicative of enhanced charge-storage capability for the device based on 14


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multishell QDs. The memory characteristics of the memristive device based on InP/ZnSe/ZnS (core-shell-shell) QDs after bending were similar to those before bending. The Al/PMMA:(InP/ZnSe/ZnS) QDs/ITO/PEN devices showed stable and reproducible operation.

AUTHOR INFORMATION *

E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2016R1A2A1A05005502).

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Graphical abstract

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ZnS Core-Multishell

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