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Tunable Nonvolatile Memory Behavior 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16878 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Applied Materials & Interfaces

Tunable Nonvolatile Memory Behavior of PCBMMoS2

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. KEYWORDS Two-dimensional (2D) transition-metal dichalcogenides, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), Nanocomposites, Memory, Heterojunction

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

ACS Paragon Plus Environment

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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 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.

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-/electro-catalysts,8,9 memory devices,10,11 and photovoltaics.12 As a typical TMD material with excellent electrical and optical properties in direct bandgap feature, molybdenum disulfide (MoS2) nanosheets have served as effective templates for 2D nanocomposites.13,14 A wide range of materials, such as noble


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metals,15 inorganic semiconductors,16,17 other 2D nanosheets,18 and polymers,19 have been introduced onto the surface of MoS2 nanosheet to prepare the 2D hybrid functional materials in vdWHs; improved electronic,20 photonic, and catalytic properties of these integrated MoS2 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. But, due to 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 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


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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). This particular memory properties of the newly prepared PCBMMoS2 nanocomposites, not possessed by PCBM and 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. 2. RESULT AND DISCUSSION Solvent transfer of MoS2 aqueous solution. With the ultrasonication enhanced lithium intercalation method, bulk MoS2 were facilely exfoliated into single-layer nanosheets for a stable dispersion in water.26 To prepare PCBM-MoS2 nanocomposites by mixing PCBM and MoS2 solutions together, the solvents used to dissolve or disperse PCBM and MoS2 should be miscible. However, PCBM is generally dissolvable in non-polar aromatic organic solvents such as toluene, while MoS2 nanosheets are usually dispersed in water, which is highly polar. To bridge this gap, the MoS2 aqueous solution was transferred to organic solvents that is miscible with toluene. Three possible solvents of ethanol, N-methyl-2-pyrrolidone (NMP), and isopropyl alcohol (IPA) were tested. The transfer is processed by centrifuging the aqueous MoS2 solution (2.0 mg/mL, 3 mL) to remove the water, followed by dispersing the collected MoS2 solids into in various organic solvents through ultrasonication. From the transmission electron microscopy (TEM) images, the well-dispersed single-layer MoS2 nanosheets with uniform morphology and size ranging from 200 to 500 nm in water


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(Figure 1a) become heavily stacked and inhomogeneous in ethanol (Figure 1b), while in NMP (Figure 1c) and IPA (Figure 1d), the dispersion is much better. Impressively, it is almost identical to that in water when transferred to IPA, suggesting that IPA is a good solvent transfer medium for single-layer MoS2 nanosheets.27 Moreover, good long-term colloidal stability of MoS2 nanosheets in IPA was also observed (Figure S1). The high resolution TEM (HRTEM) confirms the hexagonal single crystal structure of MoS2 dispersed in IPA, showing a distance of 0.273 nm for the (100) Mo atom plane and the six-fold selected area electron diffraction (SAED) pattern for both (100) and (110) planes (Figure S2), which are identical to that found in waterdispersed MoS2 nanosheets.26 The single-layer characteristics of the MoS2 nanosheets dispersed in IPA was further verified by the atom force microscopy (AFM) images (Figure S3), showing a typical thickness of chemically exfoliated single-layer MoS2 around 1.3 nm. 28 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 amount 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 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 MoS2 surface is favorable for PCBM deposition. In


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contrast, small amount of MoS2 free of PCBM deposition (1.4 eV) to support efficient hole transport.

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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 bandgap 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 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 behaviours 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


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bistable phenomenon. When the PCBM content arriving 0.2% (Figure 6b), still no switching phenomenon is observed in the diode devices. But 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.

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When the deposition ratio of PCBM was

increased to 5 wt%, the device exhibits a repeatable write-read-erase-read-rewrite cycle for an extraordinary rewritable non-volatile memory flash characteristics (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). But, 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 non-volatile ON and OFF states are very stable, even at elevated temperatures up to 100℃ (Figure S6). These performances are comparable to the best results of MoS2- and PCBM-based diode memory


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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 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 non-deposited 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 due to the high conductivity of MoS2. When 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 non-volatile memory characteristics electrical properties of the diode device. The further increasing of PCBM ratio will lead to heavily PCBM-loaded MoS2 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.


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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 composite film drops to 36 nm, the bipolar resistance switching behavior can be still observed but with lower switch-on voltage (1.5 V) and larger turn-off bias (-6.0 V). And, 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 disappeared memory property at 15 nm when the nanojunction can be bypassed. 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 current-voltage (I-V) data (Figure 7a).40 The OFF state contains two regions of A and B. At region A, the I-V curves can be well-fitted by thermionic emission limited conduction model (TELC)41,42 in terms of  qφ qV  I ∝ A * T 2 exp  − 0 + q 2  4πε   kT

(1)

where A*, k, T, Ф0, 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) vs.V1/2 from 0 to 0.7 V (Figure 7b) is in a straight line, suggesting that the carrier


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transport mechanism in this region of 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-chargelimited current (SCLC) model:

9εµV 2 I= 8d 3

(2)

where µ is the charge carrier mobility and d is the film layer thickness. A linear relation was observed in the plot of In(I) vs In(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 exceeding 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  −∆Eae  I ∝ V exp    kT 

(3)

where ∆Eae is the activation energy of electron. 45 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 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).


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On the basis of the above observations and previous understandings on memory46, we can propose here a possible mechanism for the highly tuneable memory behavior of this novel nanocomposite materials containing nanoheterojunctions formed by PCBM aggregates and MoS2 nanosheets in a 2D architecture (Figure 8). The PCBM nanoaggregates deposited 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 behaviours. 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 results 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 OFF state.49,50 With the increase of the applied electric field, the injected electrons in thermionic emission limited conduction model will gradually accumulate in the MoS2 side of the heterojunction following the SCLC model, resulting in the formations of the polarized aggregates domains and reduced junction-barrier at increased electron density in MoS2. And, when the junction-barrier is low enough for electron tunnelling 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


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junction barriers of PCBM/MoS2 heterojunctions resume the initial height, which destroy the conductive pathway to reset the diode device to OFF state (Figure 8b). At high temperatures (>100℃), the ON state starts to become unstable with gradually decreased device current, possibly due to 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/MoS2 nanojunction is more stable and the relaxation of the accumulated electrons in 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%), 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 behaviour cannot be observed either at high PCBM deposition ratios (Figure 8c).

3. CONCLUSIONS In conclusion, on the basis of 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


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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 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.


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

Figure 2. TEM images of PCBM aggregates (a) and TEM (b) and AFM (c) images of PCBM (5 wt%)-MoS2 nanocomposites with corresponding AFM height profile (d)


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Figure 3. (a) Raman spectra of PCBM (purple), MoS2 nanosheet (black) and PCBM (5 wt%)MoS2 nanocomposite (blue). (b) High-resolution XPS spectra of the PCBM-MoS2 nanocomposites

Figure 4. VASP calculated charge density changes during the PCBM/MoS2 junction formation. The yellow and cyan regions represent electron accumulation and depletion, respectively.


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Figure 5. Diode memory device configuration (a) and energy level diagram (b, in eV). Inset, the material structures of the PCBM-MoS2 2D nanocomposite.


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Figure 6. Current-voltage 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 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.


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Figure 7. Experimental I-V characteristics of the flash memory device at the positive voltage sweep with fitted lines of I-V curves in region 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.


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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.


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ASSOCIATED CONTENT Supporting Information. The

Supporting

Information

is

available

free

of

charge

on

the

ACS Publications website at DOI:xxxxx/x0xx00000x The 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 film and PCBMMoS2 nanocomposite films; Retention characteristics of the ON state of the flash memory device at different temperatures; I−V curves of the devices based on pure PCBM and ITO/PCBM (5 wt%)-MoS2/Al devices with different layer thickness.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], *E-mail: [email protected]

Author Contributions ∆

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

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS


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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).

REFERENCES (1) Zhang, X.; Lai, Z.; Tan, C.; Zhang, H. Solution-Processed Two-Dimensional MoS2 Nanosheets: Preparation, Hybridization, and Applications. Angew. Chem. Int. Edit. 2016, 55, 8816-8838. (2) Cui, X.; Lee, G.-H.; Kim, Y. D.; Arefe, G.; Huang, P. Y.; Lee, C.-H.; Chenet, D. A.; Zhang, X.; Wang, L.; Ye, F.; Pizzocchero, F.; Jessen, B. S.; Watanabe, K.; Taniguchi, T.; Muller, D. A.; Low, T.; Kim, P. ; Hone, J. Multi-terminal Transport Measurements of MoS2 Using a van der Waals Heterostructure Device Platform. Nature Nanotech. 2015, 10, 534-540. (3) Li, H.; Wu, J.; Yin, Z.; Zhang, H. Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS2 and WSe2 Nanosheets. Acc. Chem. Res. 2014, 47, 1067−1075 (4) Zhang, H. Ultrathin Two-Dimensional Nanomaterials. ACS Nano 2015, 9, 9451–9469. (5) He, Q.; Zeng, Z.; Yin, Z.; Li, H.; Wu, S.; Huang, X.; and Zhang, H. Fabrication of Flexible MoS2 Thin-Film Transistor Arrays for Practical Gas-Sensing Applications. Small 2012, 8, 2994-2999.


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

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