Tunable Nonvolatile Memory Behaviors of PCBM ... - ACS Publications

Jan 29, 2018 - Linlin Jia, Xingxing Tang, Cheng Lin, Lihui Yuwen, Lianhui Wang,. Wei Huang ..... where A*, k, T, ϕ0, q, and ε are the Richardson con...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 6552−6559

Tunable Nonvolatile Memory Behaviors of PCBM−MoS2 2D Nanocomposites through Surface Deposition Ratio Control Wenzhen Lv,† Honglei Wang,† Linlin Jia, Xingxing Tang, Cheng Lin, Lihui Yuwen, Lianhui Wang, Wei Huang,* and Runfeng Chen* Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts and Telecommunications, Wenyuan Road, Nanjing 210023, P. R. China S Supporting Information *

ABSTRACT: Efficient preparation of single-layer two-dimensional (2D) transition metal dichalcogenides, especially molybdenum disulfide (MoS2), offers readily available 2D surface in nanoscale to template various materials to form nanocomposites with van der Waals heterostructures (vdWHs), opening up a new dimension for the design of functional electronic and optoelectronic materials and devices. Here, we report the tunable memory properties of the facilely prepared [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)−MoS2 nanocomposites in a conventional diode device structure, where the vdWHs dominate the electric characteristics of the devices for various memory behaviors depending on different surface deposition ratios of PCBM on MoS2 nanosheets. Both nonvolatile WORM and flash memory devices have been realized using the new developed PCBM−MoS2 2D composites. Specially, the flash characteristic devices show rewritable resistive switching with low switching voltages (∼2 V), high current on/off ratios (∼3 × 102), and superior electrical bistability (>104 s). This research, through successfully allocating massive vdWHs on the MoS2 surface for organic/inorganic 2D nanocomposites, illustrates the great potential of 2D vdWHs in rectifying the electronic properties for high-performance memory devices and paves a way for the design of promising 2D nanocomposites with electronically active vdWHs for advanced device applications. KEYWORDS: two-dimensional (2D) transition metal dichalcogenides, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), nanocomposites, memory, heterojunction improved electronic,20 photonic, and catalytic properties of these integrated MoS 2 nanocomposites were generally observed.21 Nonvolatile memory technologies that can be dated back to 1990s in Si-based electronics have been widely used in current electronic products. However, because of the approaching of the integration limit of the traditional memory devices, new materials for advanced memory technologies are highly desired to break the physical scaling limitations of data storage.22 The 2D-templated vdWHs on an atomically thin MoS2 monolayer and heterojunctions in nanoscale could be a promising electronic memory material, when the heterojunctions were established by properly selecting organic aromatics and binding them tightly on the surface of the MoS2 nanosheets. However, memory application studies of the MoS2-based organic/ inorganic vdWHs are quite rare, not only because the unique characteristics of ultrathin vdWHs are insufficiently explored to

1. INTRODUCTION Two-dimensional (2D) layered materials, including graphenes and semiconducting transition metal dichalcogenides (TMDs), have attracted considerable interest for their unique properties depending significantly on the composition, thickness, and geometry.1−4 Especially, their large and flat surface, after being exfoliated to monolayers, can act as an excellent template for the construction of 2D-templated nanocomposites combining different materials in van der Waals heterostructures (vdWHs); the integration of various materials in 2D vdWHs exhibits unprecedented flexibility for various functions, as exemplified by the recent demonstrations of field-effect transistors,5,6 optical lenses and gratings,7 chemo-/electrocatalysts,8,9 memory devices,10,11 and photovoltaics.12 As a typical TMD material with excellent electrical and optical properties in direct band gap feature, molybdenum disulfide (MoS2) nanosheets have served as effective templates for 2D nanocomposites.13,14 A wide range of materials, such as noble metals,15 inorganic semiconductors,16,17 other 2D nanosheets,18 and polymers,19 have been introduced onto the surface of MoS2 nanosheets to prepare the 2D hybrid functional materials in vdWHs; © 2018 American Chemical Society

Received: November 7, 2017 Accepted: January 29, 2018 Published: January 29, 2018 6552

DOI: 10.1021/acsami.7b16878 ACS Appl. Mater. Interfaces 2018, 10, 6552−6559

Research Article

ACS Applied Materials & Interfaces date, but also due to the fact that nanoheterojunctions of vdWHs on MoS2 nanosheets are very difficult to be prepared efficiently.23 Here, with the aid of our previously proposed solvent transfer and surface deposition (STSD) method,24 we successfully constructed a novel kind of 2D vdWHs with massive and stable nanoheterojunctions by accommodating [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanoaggregates on the surface of MoS2 nanosheets, where PCBM is a famous and widely used electron acceptor especially in organic photovoltaics.25 The memory properties of the obtained PCBM−MoS2 nanocomposites were then investigated in a conventional direct charge transport diode device structure. Various nonvolatile memory behaviors of the devices were observed when the charge transport was controlled to pass through the nanoheterojunctions by adjusting the surface deposition ratio of PCBM on MoS2 nanosheets. WORM and flash-type memory devices based on these composites with varied PCBM deposition ratios exhibit both low turn-on voltage (3.5 V for WORM and 2.0 V for flash) and high on/off resistance ratio (1 × 102 for WORM and 3 × 102 for flash). These particular memory properties of the newly prepared PCBM−MoS2 nanocomposites, not possessed by PCBM and the MoS2 nanosheet alone, are found to be attributed to the efficiently established PCBM−MoS2 nanoheterojunctions, which effectively rectify the electric current passing across the nanojunction with a built-in potential (junction barrier) that can be modulated by electric-field-induced polarization in the ultrathin 2D vdWH.

Figure 1. TEM images of MoS2 single-layer nanosheets in different solvents: (a) water, (b) ethanol, (c) NMP, and (d) IPA.

identical to that found in water-dispersed MoS2 nanosheets.26 The single-layer characteristics of the MoS2 nanosheets dispersed in IPA were further verified by the atomic force microscopy (AFM) images (Figure S3), showing a typical thickness of chemically exfoliated single-layer MoS2 around 1.3 nm.28 2.2. Preparation and Characterization of PCBM−MoS2 Composites. During the surface modification process of MoS2 nanosheets using PCBM via surface deposition in carefully adjusted organic solvent environment by solvent transfer, various amounts of PCBM toluene solution (2.0 mg/mL) were injected into the IPA solution of MoS2 nanosheets (1.0 mg/ mL) to decorate the surface of single-layer MoS2 nanosheets to afford different PCBM−MoS2 nanocomposites with varied PCBM weight deposition ratios from 0, 0.2, 2, 5 to 8%. Figure 2a illustrates that PCBM can form small aggregates with a uniform diameter of about 15 nm when participating from toluene solution; similar-sized PCBM aggregates were also observed on the surface of MoS2 nanosheets and over 80% MoS2 nanosheets were randomly anchored by the PCBM nanoaggregates (Figure 2b), indicating the successful formation of the PCBM−MoS2 hybrid composites in nanoscale. Free PCBM aggregates cannot be observed in the composites because the partially hydrophobic feature of the MoS2 surface is favorable for PCBM deposition. In contrast, a small amount of MoS2 free of PCBM deposition (1.4 eV) to support efficient hole transport.35 These understandings are in line with the above VASP calculations that electron transfer occurs from PCBM to MoS2 (MoS2 has lower LUMO and HOMO) in the junction and the literature reports that MoS2 nanosheets are n-type direct band gap semiconductors.36,37 It should be also noted that the PCBM deposition ratio has limited influence on the film morphology and structures, as revealed from their almost identical SEM images to that of the pure MoS2 nanosheet film (Figure S5). Interestingly, the current−voltage (I−V) characteristics of the devices measured at room temperature and under ambient atmosphere showed varied memory behaviors depending on different deposition ratios of PCBM onto the MoS2 nanosheet surface with high reproducibility (Figure 6). Without PCBM deposition (Figure 6a), the I−V curves cannot show any electrical bistable phenomenon. Even when the PCBM content reaches 0.2% (Figure 6b), no switching phenomenon is observed in the diode devices. However, at a PCBM deposition ratio of 2 wt %, a nonvolatile WORM memory effect appears (Figure 6c), showing characteristic I−V curves with a sharp electrical transition from the initial low conductivity (off state) to a high conductivity (on state) at positive bias larger than 3.5 V; this off-to-on transition can function in a memory device as a writing process, and once the device has reached its on state, it remains there after either positive or negative sweeps and even when the power is turned off, suggesting a nonvolatile nature of WORM memory effect.38 When the deposition ratio of PCBM

Figure 6. I−V characteristics of the ITO/PCBM (x wt %)−MoS2 (50 nm)/Al diode memory devices (x = 0, 0.2, 2, 5, and 8 for panels of (a), (b), (c), (d), and (f), respectively) and the retention time of on and off states of the PCBM (5 wt %)−MoS2-based device at a probe voltage of 1.0 V (e). Note that the first (1), second (2), and third (3) voltage sweeps are from 0 to +6 V, +6 to −6 V, and −6 to 0 V, respectively.

was increased to 5 wt %, the device exhibits a repeatable write− read−erase−read−rewrite cycle for an extraordinary rewritable nonvolatile memory flash characteristic (Figure 6d). For the operation of the flash device swept positively from 0 to 6 V (first sweep), the current density is initially low (off state) but increases progressively with the increasing voltage and an abrupt current jump occurs from ∼10−4 to ∼10−2 A at the switching threshold voltage of 2.0 V to generate on state, representing a “writing” process of the memory device. The high conductivity could be read in the subsequent positive sweep and reverse sweep (second sweep). However, once the negative sweep voltage reaches −4.3 V, the high conductivity on state is switched off to the original low conductivity off state, exhibiting the “erasing” process of memory devices. The off state is read (third sweep) and can be recovered to the on state in the next sweep, showing a rewritable feature in “write−read− erase−read−rewrite” cycles for a rewritable nonvolatile flash memory behavior and high resistive switching endurance with on/off current ratio over 3 × 102 and the persistent time as long as 104 s at the voltage of 1.0 V (Figure 6e).39 The nonvolatile on and off states are very stable, even at elevated temperatures up to 100 °C (Figure S6). These performances are comparable to the best results of MoS2- and PCBM-based diode memory devices (Table S1). When the ratio of PCBM in the composite is increased to 8 wt % (Figure 6f), however, the switching phenomenon disappears and the device is always on with high 6555

DOI: 10.1021/acsami.7b16878 ACS Appl. Mater. Interfaces 2018, 10, 6552−6559

Research Article

ACS Applied Materials & Interfaces conductivity, which is identical to that of the pure PCBM-based devices (Figure S7). The I−V characteristic differences of these devices with varied deposition ratios of PCBM indicate that the electrically bistable behavior of the devices is closely related to the relative content of MoS2 and PCBM or the population of PCBM− MoS2 heterostructures. At the composite film thickness of 50 nm in the diode device, the MoS2 nanosheets should be stacked for more than 10 layers according to the thicknesses of PCBM−MoS2 nanocomposites (∼5 nm) and nondeposited MoS2 nanosheets (1.4 nm). Low PCBM content cannot break the self-stacking of the single-layer MoS2 nanosheets, resulting in facile conductive channels among stacked MoS2 nanosheets for high current densities because of the high conductivity of MoS2. When an appropriate number of PCBM nanoaggregates are deposited on the surface of MoS2 nanosheets to form the nanocomposites, the charge current has to pass through the PCBM/MoS2 nanojunction for several times during the operation of the device, resulting in the special WORM and rewritable nonvolatile memory characteristics with electrical properties of the diode device. The further increase of the PCBM ratio will lead to heavily PCBM-loaded MoS 2 nanosheets with small distance between the deposited PCBM nanoaggregates, which may support the PCBM−PCBM electron transport to bypass the PCBM/MoS2 nanojunction for high charge current, because of the high conductivity of both PCBM and MoS2 nanosheets. Also, the heavy PCBM loading may reduce the junction barriers, leading to low ability in rectifying the I−V curves. Apparently, it is of crucial importance in properly adjusting the ratio of surface-deposited PCBM nanoaggregates to realize the memory performance of the 2D nanocomposites aroused by the existence of a large amount of nanoheterojunctions that the charge current must pass through. To further explore the bistable flash memory behavior of the PCBM (5 wt %)−MoS2 nanocomposites, the influence of the film thickness was also studied (Figure S8). When the thickness of the composite film drops to 36 nm, the bipolar resistance switching behavior can be still observed but with lower switchon voltage (1.5 V) and larger turn-off bias (−6.0 V). In addition, when the film thickness reaches 15 nm, the memory performance disappears, showing only the on state with high current density. Such a thickness-dependent flash memory property confirms that the PCBM/MoS2 nanoheterojunction is the intrinsic unit in rectifying the charge current for memory behaviors. Thinner film results in fewer times of composite nanosheet stacking and charge current passing through the junction during the diode device operation, leading to reduced flash memory performance at 36 nm and disappearance of the memory property at 15 nm when the nanojunction can be bypassed. 2.5. Operation Mechanism of PCBM/MoS2 Nanoheterojunctions. To reveal the extraordinary conductance switching mechanism of the flash memory, the charge transport behavior of the 2D nanocomposite under electronic fields was investigated by model fitting of the I−V data (Figure 7a).40 The off state contains two regions A and B. At region A, the I−V curves can be well-fitted by thermionic emission-limited conduction (TELC) model41,42 in terms of ⎛ qϕ qV ⎞ I ∝ A*T 2 exp⎜ − 0 + q2 ⎟ 4πε ⎠ ⎝ kT

Figure 7. Experimental I−V characteristics of the flash memory device at the positive voltage sweep with fitted lines of I−V curves in regions A (b), B (c), C (c), and D (d). Input (e) and output (f) of the write− read−erase−read (WRER) cycles of the memory device for flash storage applications. Voltages for WRER cycles are +5, 1, −5, and 1 V, respectively.

where A*, k, T, ϕ0, q, and ε are the Richardson constant, Boltzmann constant, absolute temperature, Schottky energy barrier, electronic charge, and dielectric permittivity, respectively. The plot of ln(I) versus V1/2 from 0 to 0.7 V (Figure 7b) is in a straight line, suggesting that the carrier transport mechanism in this region of the off state is the thermionic emission and the charge injection dominates the conduction mechanism. For region B, it is qualitatively consistent with the shallow trap-associated space-charge-limited current (SCLC) model I=

9εμV 2 8d3

(2)

where μ is the charge carrier mobility and d is the film layer thickness. A linear relation was observed in the plot of ln(I) versus ln(V) with a slope of 2.0 when the voltage sweeps from 0.5 to 2.0 V (Figure 7c), indicating a typical SCLC mechanism. As the voltage exceeds 2.0 V, the current increases exponentially with I ≈ Vm (m = 27.5) in the switching region (region C) (Figure 7c).43,44 When the sweep voltage exceeds the switching threshold to reach the on state, the I−V data can be fitted with the following Ohmic conductive equation of ⎛ −ΔEae ⎞ ⎟ I ∝ V exp⎜ ⎝ kT ⎠

(3)

where ΔEae is the activation energy of electron. A linear fitting with a slope of 1.0 can be found in whole region D (Figure 7d), indicating that the carrier transport process at the whole on state is in Ohmic conduction model. Flash-type memory 45

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At high temperatures (>100 °C), the on state starts to become unstable with gradually decreased device current, possibly because of the gradual relaxation of the accumulated electrons in the PCBM/MoS2 nanoheterojunctions (Figure S6). When a thinner composite film is adopted in the device, stronger electric field will be resulted at the same sweep voltage with strengthened electron tunneling, which subsequently results in decreased turn-on voltage; the polarized domain rupture also needs a higher sweep voltage to overcome the facilitated electron tunneling for charge transport (Figure S8). At a lower PCBM deposition ratio, the PCBM/MoS 2 nanojunction is more stable and the relaxation of the accumulated electrons in the MoS2 side is more difficult with deeper junction barrier; thus, no turn-off switching before −6.0 V can be observed (Figure 8a). On the other hand, when the PCBM deposition ratio is too high (>8 wt %), the nanojunction barrier of the heavily PCBM-loaded MoS2 nanosheets may be decreased and become too low to invoke the rectification effects of the vdWHs; therefore, memory behavior cannot be observed at either high PCBM deposition ratios (Figure 8c).

writing−reading−erasing cycles were further performed on continuous voltage scan from 5 V (write), 1 V (read), −5 V (erase), to 1 V (read), respectively (Figure 7e). Stable and fast on/off switching of the diode devices was observed, indicating the reprogrammable transport states of the PCBM−MoS2 nanocomposites (Figure 7f). On the basis of the above observations and previous understandings on memory,46 we can propose here a possible mechanism for the highly tunable memory behavior of these novel nanocomposite materials containing nanoheterojunctions formed by PCBM aggregates and MoS2 nanosheets in a 2D architecture (Figure 8). The PCBM nanoaggregates deposited

3. CONCLUSIONS In conclusion, on the basis of the STSD method to prepare PCBM/MoS2 nanocomposites, we have efficiently constructed stable and massive vdWHs in 2D nanocomposites and the memory behavior of the composites rectified by the organic/ inorganic vdWHs was observed. Impressively, the electronic characteristics of the diode devices were tunable by changing the surface deposition ratio of PCBM nanoaggregates on MoS2 nanosheets, leading to distinctive electrically bistable electrical switching and nonvolatile rewritable memory effects for both WORM and flash memory devices with low turn-on voltage (3.5 V for WORM and 2.0 V for flash) and high on/off resistance ratio (1 × 102 for WORM and 3 × 102 for flash). The strategies implemented here, including the STSD method in 2D composite preparation and surface deposition ratio control, can principally be extended to other systems of 2D nanocomposites and offer unique solutions for adjusting the electronic property of the hybrid nanomaterials. This work would stimulate significantly the investigations of electron rectification effects of organic/inorganic nanojunctions in vdWHs templated on the surface of 2D semiconductors.

Figure 8. Proposed rectification mechanism of the PCBM/MoS2 heterojunction for WORM (a) and flash (b) memory behaviors as well as conductor (c) without memory effects.

on the surface of MoS2 nanosheets can form discontinuous PCBM-rich phase in the nanocomposites; the formed PCBM domains in the MoS2 matrix could be polarized under the applied electrical field, resulting in the build-in localized internal electrical fields;47 the polarized states and the build-in localized internal electrical fields can be maintained, even when the voltage is turned off, leading to the nonvolatile properties of the device.48 Differently polarized states upon different PCBM contents in the composite induce the different memory behaviors. The fast on/off switching rate suggests the quick interchange feature of the different transport states of the composites. Therefore, it was suspected that the tightly deposited PCBM nanoaggregates on MoS2 nanosheets result in massive vdWHs with stable interface contacts and high junction barriers, which can act as the charge traps and dominate the electron transport and make the diode initially at the off state.49,50 With the increase of the applied electric field, the injected electrons in the TELC model will gradually accumulate in the MoS2 side of the heterojunction following the SCLC model, resulting in the formation of the polarized aggregate domains and reduced junction barrier at increased electron density in MoS2. In addition, when the junction barrier is low enough for electron tunneling to support facile electron transportation along the PCBM/MoS2 nanojunction, the diode will be set to the on state. The conductive pathway is stable to remain the on state until the reversely applied electric field is high enough to relax the accumulated electrons and rupture the polarized domains. Then, the junction barriers of PCBM/MoS2 heterojunctions resume the initial height, which destroy the conductive pathway to reset the diode device to the off state (Figure 8b).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16878. Experimental part and characterization of PCBM, MoS2, and their composites; fabrication and measurement of devices; HRTEM and SAED for the monolayer MoS2 nanosheets; effective area of the diode memory devices; SEM images of spin-coated MoS2 nanosheet films and PCBM−MoS2 nanocomposite films; retention characteristics of the on state of the flash memory device at different temperatures; and I−V curves of the devices based on pure PCBM and ITO/PCBM (5 wt %)−MoS2/ Al devices with different layer thicknesses (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.H.). *E-mail: [email protected] (R.C.). 6557

DOI: 10.1021/acsami.7b16878 ACS Appl. Mater. Interfaces 2018, 10, 6552−6559

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ACS Applied Materials & Interfaces ORCID

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Lianhui Wang: 0000-0001-9030-9172 Runfeng Chen: 0000-0003-0222-0296 Author Contributions †

W.L. and H.W. equally contributed as first authors.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported in part by the National Natural Science Foundation of China (21304049, 21674049, 21001065, 21601091, and 61136003), Qing Lan Project of Jiangsu Province, Science Fund for Distinguished Young Scholars of Jiangsu Province of China (BK20150041), Natural Science Foundation of Jiangsu Province of China (BK20160891), 1311 Talents Program of Nanjing University of Posts and Telecommunications (Ding shan), and Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX17_0244).

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DOI: 10.1021/acsami.7b16878 ACS Appl. Mater. Interfaces 2018, 10, 6552−6559

Research Article

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DOI: 10.1021/acsami.7b16878 ACS Appl. Mater. Interfaces 2018, 10, 6552−6559