Facile Synthesis of Co9Se8 Quantum Dots as Charge Traps for

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Facile Synthesis of CoSe Quantum Dots as Charge Traps for Flexible Organic Resistive Switching Memory Device Peng Zhang, Benhua Xu, Cunxu Gao, Guilin Chen, and Meizhen Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09616 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 20, 2016

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ACS Applied Materials & Interfaces

Facile Synthesis of Co9Se8 Quantum Dots as Charge Traps for Flexible Organic Resistive Switching Memory Device Peng Zhang,a,b, § Benhua Xu,c,§ Cunxu Gao,a,b,* Guilin Chen, a,b and Meizhen Gao a,b, *

a Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University, Lanzhou 730000, P. R. China. b Key Laboratory of Special Function Materials and Structure Design of MOE, Lanzhou University, Lanzhou 730000, P. R. China.

c Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China.

ACS Paragon Plus Environment


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ABSTRACT: Uniform Co9Se8 quantum dots (CSQDs) were successfully synthesized

through a facile solvothermal method. The obtained CSQDs with average size of 3.2 ± 0.1 nm and thickness of 1.8 ± 0.2 nm were demonstrated good stability and strong fluorescence under UV light after being easily dispersed in both of N, N-dimethylformamide (DMF) and deionized water. We demonstrated the flexible resistive switching memory device based on the hybridization of CSQDs and polyvinylpyrrolidone

(PVP)

(CSQDs-PVP).

The

device

with

the

Al/CSQDs-PVP/Pt/poly(ethylene terephthalate) (PET) structure represented excellent switching parameters such as high ON/OFF current ratio, low operating voltages, good stability and flexibility. The flexible resistive switching memory device based on hybridization of CSQDs and PVP has a great potential to be used in flexible and high performance memory applications.

KEYWORDS: resistive switching, flexible organic memory, Co9Se8, quantum dots, hybrid nanomaterials


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INTRODUCTION Resistive random access memory (ReRAM) devices with the configuration of electrode/active layer/electrode capacitor-like structure, have been identified as a promising candidate for future data storage applications because of their fascinating virtues such as ease of production, high stacking density, nonvolatility and low power consumption.1-4 Up to now, a variety of materials exhibit resistive switching behaviors, including oxides,5,6 organic molecules,7,8 polymers,9,10 graphene oxide11,12 and nanocomposites.13 Among the various active materials, nanocomposites comprising polymer/organic-inorganic hybrid nanomaterials with advantageous electrical, optical or mechanical properties14-18 have been used widely to fabricating organic nonvolatile resistive memory devices due to their flexibility and transparency.19-22 In the nanocomposites, inorganic materials with robust electrical properties, such as metal nanoparticles,23-26

metal

oxide

nanoparticles,27,28

two-dimensional

(2D)

nanomaterials29,30 and carbon-based materials31,32 are charge trapping conductive materials in the organic resistive memory device and these materials determine the resistive switches of the memory devices. Thus, to explore materials with interesting properties is significant for fabricating memory devices with excellent switching parameters. In principle, developing new materials with excellent performance depends on their not only constituents but also micro/nanostructures. For example, graphene is unpowered for the electronics and optics applications seemingly due to its zero bandgap energy. Nonetheless, the bandgap of graphene is tunable by chopping their sizes.33 As a consequence, graphene quantum dots, typically containing a few thousand atoms, possess plentiful chemical/physical properties34,35 because of the quantum confinement and edge effects, which make them have extensive use in nanoelectronic,

chemosensors,

bio-imaging.36-39

Recently,

transition

metal

dichalcogenides quantum dots40,41 and black phosphorus quantum dots42 have been successfully synthesized by facile solution based method and uniformly dispersed in polymer matrix to form the quantum dots/polymer hybrid nanomaterials. Resistive switching behaviors have been investigated in the flexible memristor based on the hybrid nanomaterials.



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To date, flexible electronics have attracted much attention due to the widely applications involving wearable systems, biointegrated medical devices, and electronic cameras.43-45 Flexible resistive memory device is one of the important example of a flexible circuit component. Up to now, organic polymers are used to fabricate the flexible memory devices due to their advantages of stretchability, bendability, and twistability. Nevertheless, these devices are faced with several crucial problems, such as complicated switching mechanism, relatively poor stability and reliability in comparison with inorganic memory device.46 On the other hand, inorganic materials are unsuitable for fabricating the flexible memory device because of their poor mechanical flexibility. By this token, considerable effort should be devoted to research on active materials that combine insulating polymers with quantum dots for flexible organic resistive memory devices, possessing both high intrinsic performance and additional flexibility virtues, because hybrid nanomaterials could overcome above contradiction between flexible film-forming ability and high resistive switching performance. Very recently, cobalt selenides have been widely investigated because of their excellent chemical stability and catalytic performance. Cobalt selenides nanosheets have been synthesized47 and utilized for catalysts48 and biomedical applications.49 The ultrathin Co9Se8 nanosheets have atomic thickness of 0.5 nm, which is the thickness of half cubic Co9Se8 structure cell.47 Inspired by that graphene, graphene oxide or MoS2/WS2 nanosheets can be destroyed into quantum dots by a simple hydrothermal process,50-54 it is possible to synthesize Co9Se8 quantum dots (CSQDs) by cutting ultrathin Co9Se8 nanosheets and CSQDs could act as charge trapping materials in polymer matrix for the fabrication flexible organic memory device. Herein, we have successfully synthesized the CSQDs by cutting ultrathin Co9Se8 nanosheets through solvothermal method. The obtained CSQDs have good stability in N, N-dimethylformamide (DMF) and could be redispersed in deionized water. Moreover, write-once read-many (WORM) times memory effect was investigated in the flexible memory device fabricated by hybridization of CSQDs and polyvinylpyrrolidone (PVP). The device with the Al/CSQDs-PVP/Pt/poly(ethylene


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terephthalate) (PET) structure represented excellent switching parameters such as high ON/OFF current ratio, low operating voltages, good stability and flexibility. The flexible resistive switching memory device based on hybridization of CSQDs and PVP has a great potential to be used in flexible memory applications.

Figure 1. Schematic representation of the synthesis procedure to synthesis CSQDs. (a) SEM image of bulk Co9Se8 powder. TEM images of (b) exfoliated Co9Se8 nanosheets, (c) CSQDs interspersed in the exfoliated Co9Se8 nanosheets and (d) CSQDs.

RESULTS AND DISCUSSION Figure 1 schematically describes the synthesis procedure to prepare the CSQDs. In detail, bulk Co9Se8 crystals with lamellar stacking structure were synthesized based on Xie’s method.47 Figure S1 (supporting information (SI)) depicts characterizations of bulk Co9Se8. The experimental section detailedly describes the synthesis procedure. Breaking Co9Se8 powder under bath sonication in DMF for 4 h produces a highly dispersed black suspension. Then the disperse solution of CSQDs interspersed in few-layered Co9Se8 nanosheets was obtained by a solvothermal process, and a subsequent several-minute centrifugation to obtain the transparent light yellow solution consist of suspending CSQDs. The morphology of the CSQDs has been characterized by TEM and AFM. The TEM image clearly shows the CSQDs’ size distribution with an average value of 3.2 ± 0.1 nm (Figure 2a, b and e).



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High-resolution TEM images (Figure 2c and d) of CSQDs exhibit a d-spacing of 0.26 and 0.22 nm corresponding to (400) and (420) of crystalline Co9Se8. Furthermore, AFM image (Figure 2f) confirms the morphology and thickness of the CSQDs. The AFM height profiles (Figure 2g) show the heights of CSQDs are 1.5 and 2.5 nm. Figure 2h shows the thickness distribution of the CSQDs and the average thickness of CSQDs of 1.8 ± 0.2 nm, which is very close to thicknesses of Co9Se8 nanosheets with 3-4 monolayers.

Figure 2. (a, b) TEM, (c, d) High-resolution TEM images of CSQDs. (e) Size distribution of the CSQDs in (a). (f) AFM image and the corresponding height profile of CSQDs. (g) Height profiles along the white lines in (f). (h) Morphologies distributions CSQDs. Scale bar = 2 nm.

The XRD pattern of Co9Se8 bulk (Figure 3a) clearly shows the cubic Co9Se8 structure (JPCDS Card No. 65-3116, a=10.43 Å). For the CSQDs, five peaks can be detected and the intensity of these peaks obviously decreases. A portion of peaks with lower intensity are disappearance, demonstrating the formation of mono-or few-layered CSQDs. The Raman shifts of Co9Se8 bulk samples shown in Figure 3b confirm the synthetic nanosheets with Co9Se8 crystal structure, which are consistent with the reported results.55 The Raman spectrum of the CSQDs reveals weaker


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intensity compared that with Co9Se8 bulk. EDX spectrum (Figure S2. IS) reveals that the CSQDs consists of Co and Se elements. The full-range XPS spectrum (Figure 3c) shows that CSQDs are composed of Co and Se. The EDX spectrum and XPS spectra reveal that CSQDs are purity and the Co/Se stoichiometric ratio is about 1.08, which is close to 1.12 of perfect Co9Se8. The deconvolution of the Co 2p profile (Figure 3d) is achieved with the assumption of three species. The two peaks at 785.6 eV and 803.7 eV are Co2+ shakeup satellite peaks.56,57 The Co 2p3/2 spectrum consists of peaks at 778.4 eV and 781.3 eV, attributed to Co-C, Co-Se.58 As shown in Figure 3e, high-resolution Se 3d spectrum comprises three peaks at 54.3, 58.7, and 59.9 eV. The peak at 54.3 eV is attributed to Co-Se and that at 59.9 eV is assigned to Se 3d3/2.59 It is noteworthy that the peak at 58.7 eV corresponds to Se-O.56,60

Figure 3. (a) XRD spectra of CSQDs and bulk crystalline Co9Se8. (b) Raman spectra of CSQDs



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and bulk crystalline Co9Se8. (c) XPS spectrum of the as-synthesized CSQDs. (d) Deconvoluted Co2p spectrum. (e) Deconvoluted Se 3d spectrum.

Figure 4a shows the UV-vis spectra of CSQDs dispersed in DMF and deionized water with a shoulder peak at about 305 nm. UV-vis diffuse reflectance spectra (Figure S3a, SI) shows that bulk Co9Se8 exhibits a broad and continuous photoabsorption range from 400 to 800 nm, which matches the calculated energy bandgap of bulk Co9Se8.47 For CSQDs, the measured band gap is 4.37 eV, much larger than that of bulk Co9Se8 (1.43 eV) (Figure S3b and c, SI). As shown in Figure 4b, the emission peak is centered at 400 nm when the CSQDs are excited at the absorption feature. The Stokes shift is about 100 nm. Figure 4c shows the red-shift of photoluminescence emission as the excitation wavelength ranges from 270 to 400 nm. The CSQDs could be dispersed in deionized water and exhibit the excitation dependent photoluminescence emission behavior (Figure 4d). Moreover, CSQDs also could

be

dispersed

in

alcohol

and

shows

the

wavelength-dependent

photoluminescence behavior (Figure S3d, SI). Due to the dried CSQDs being easily redispersed in water and exhibiting strong fluorescence, it is believed that the new water-soluble CSQDs can be explored as cell-imaging agents.


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Figure 4. (a) Absorption spectra of CSQDs prepared in DMF and redispersed in deionized water. Inset: photographs of the CSQDs taken under visible light and 365 nm UV radiation. (b) Excitation and emission PL spectra of CSQDs. Emission PL spectra of CSQDs (c) prepared in DMF and (d) redispersed in deionized water.

Based on good stability and dispersibility in various solvent, the CSQDs have the potential for further film preparation through various facile solution based method. As shown in Figure 5a, the CSQDs films were fabricated on the flexible Pt/PET substrates by spin-coating method. The films exhibit highly uniformity and compactness. However, pure CSQDs films on the flexible substrate would be prone to cracks (Figure 5b) after being physically fixed from flat to bent conditions for 50 times at curvature radii of 7 mm. These cracks would lead to large leakage current and break the devices down. To overcome this disadvantage, CSQDs were blended into PVP to form organic/inorganic hybrid nanomaterials which also exhibited high dispersity and stability. Then the hybrid nanomaterials, referred to as CSQD-PVP, were used to deposited uniform film by spin coating. The survey spectra (Figure S4, SI) reveal that the active layer contains Co and Se. The hybrid films are still highly compact and uniform after bending 50 times confirmed by the morphology of the



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films (Figure 5c). The morphology of the hybrid film measured using AFM and the surface roughness of the hybrid films is as small as ~ 2.01 nm. The thickness of the active layer was carried out by cross-sectional SEM measurement. The one-layer CSQDs-PVP film has a thickness of 180 nm (Figure 5d). The thickness of the CSQDs-PVP film was determined through the number of times of spin-coating. When the spin-coating was repeated five times, the thickness of five-layer CSQDs-PVP film was about 1 μm (Figure S5, SI). Thus, the CSQDs-PVP films have potential to be utilized to fabricate the flexible memory device.

Figure 5. SEM image of (a) CSQDs films, (b) CSQDs films after bending operation and (c) CSQD-PVP films (Inset: AFM images of CSQD-PVP films). (d) SEM cross-section image of one layer CSQDs-PVP films.


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As shown in Figure 6a and b, a flexible resistive switching memory device with the Al/CSQDs-PVP/Pt/PET structure is demonstrated. In the device, CSQDs-PVP mixture is sandwiched between Al and Pt electrodes. Figure 6c shows the resistive switching characteristics of the device illustrated by the current-voltage (I-V) curve at room temperature. The applied voltage sweeps following the sequence of arrows. Initially, the flexible device is in low current conductive state (OFF state) during the voltage sweep from zero to operating voltage (1.6 V). After the applied voltage exceeds the operating voltage, the device turns to high current conductive state (ON state). During the subsequent positive and negative sweeps, the ON state does not transfer to OFF state. The I-V characteristics indicate that the flexible Al/CSQDs-PVP/Pt/PET device exhibits a write-once-read-many-times (WORM) type memory effect. Figure 6d shows that the device possesses high ON/OFF ratio which promises a low misreading rate. A high ON/OFF ratio of ∼105 is achieved at a read voltage of 0.4 V. The retention characteristics of the ON and OFF states are displayed in Figure 6e. It is obvious that the ON and OFF states can be kept for more than 3200 s and no obvious degradation can be observed. Figure 6f depicts the endurance cycle test of the device. The device shows good endurance properties up to 200 cycles without significant variation in ON states.



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Figure 6. (a) Photograph of the flexible Al/CSQDs-PVP/Pt/PET resistive switching memory device. (b) Schematic conductive diagrams of experiments. Cathodes were grounded for the tests. (c) The current-voltage curve of the Al/CSQD-PVP/Pt/PET device. (d) The ON/OFF ratio of the device. (e) The retention characteristic of ON and OFF states for the device. (f) Switching cycles for the device. The current values are measured at a read voltage of 0.4 V.

In the Al/CSQDs-PVP/Pt/PET resistive switching memory device, CSQDs-PVP is

the

switching

materials.

Therefore,

the

I-V

characteristics

of

the

Al/CSQDs-PVP/Pt/PET device should be primarily investigated for interpret the resistive switching mechanisms of the Al/CSQDs-PVP/Pt/PET device. Resistive switching effects have been investigated in the Ag/PVP/Indium tin oxide/PET


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device.61 In this device, conductive paths is generated in the active layer under the voltage applied to the Ag electrode. Thus, the formation of metal filaments dominates the resistive switching behavior. Figure 7a shows that no resistive switching effect is observed in the Al/PVP/Pt/PET device which is consistent with the reported results62,63and

WORM

times

memory

effect

is

observed

in

the

Al/CSQDs-PVP/Pt/PET device under a reverse voltage sweep of 0 → -2 V → 0 → 2 V→ 0. This indicates the resistive switching effect in the Al/CSQDs-PVP/Pt/PET device could be put down to the existence of the CSQDs. In addition, the results exclude possibility that the resistance switching is derived from formation of metal filaments which results from the migration of electrodes through the PVP films. More negative voltages are applied to the Al/CSQDs-PVP/Pt/PET devices, the ON state is still retained (Figure 7b). Furthermore, the resistance of the ON state decreases when the measurement temperature rise (Figure S6, SI). This also excludes the possibility that the resistance switching is derived from the conductive metal filament formed in the active layer because the resistance increases with temperature increasing for metallic filaments. As we known, resistive switching effect could be realized in the polymer in which organic/inorganic materials are embedded. The reason is that the organic/inorganic materials in polymer can trap charge or release the trapped charge. Thus, it is obvious that CSQDs distributed in the PVP could serve as trapping centers and be responsible for the resistive switching behavior of the Al/CSQD-PVP/Pt/PET device. To comprehend the conduction mechanisms in ON and OFF state of the Al/CSQD-PVP/Pt/PET device, the I-V curves were replotted in double-Ln scale (Figure 7c and d). Initially, the I-V characteristic in OFF state is linear with a slope of 0.93 indicating the Ohmic conduction (Figure 7c). With the applied voltage increase, the slope of the fitted I-V plot varies from 0.93 to 1.99. This means that conduction mechanism is dominated by typical trap-controlled space charge limited current (SCLC) conduction.64,65 After the applied voltage exceed the operating voltage (1.6 V), the device converts OFF state into ON state. In the ON state (Figure 7d), the current conduction is Ohmic (slope ~ 1.09).



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Figure 7. (a) The I-V curves of the Al/CSQD-PVP/Pt/PET and Al/PVP/Pt/PET devices. (b) The I-V curves of the Al/CSQD-PVP/Pt/PET device when the different negative voltage applied. Log-Log I-V characteristics of the in the device in (c) OFF state and (d) the ON state.

On the basis of the above experimental results, a schematic illustration corresponding to SCLC mechanism is represented to explain the conduction mechanism (Figure 8). CSQDs distributed in the PVP could serve as trapping centers. In low voltage regime, the traps are occupied (Figure 8a). In this regime, the current conduction is Ohmic. Then the traps begin to be filled by charges transferring from PVP to CSQDs accompanying the applied voltage increase (Figure 8b). There are two reasons for producing the charge trapped process. On the one hand, CSQDs have lower energy level than PVP;42,66 On the other hand, the trap density is higher than free carrier. The charge trapped process results in the SCLC model.22,30 Then traps are completely filled with the applied voltage increase (Figure 8c). This process results higher electron concentration in the active layers of the CSQD-PVP based devices. When the applied voltage approaches to the operating voltage, injected carriers in the active layers increase exponentially and conductive paths are formed in the active layer due to the higher electron concentration (Figure 8d). The formation of


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conductive paths switches the device from OFF state to ON state. Because amount of electrons accumulate in the active layer, an internal electrical field is formed. The field is applied in the Al/CSQD-PVP/Pt/PET device.30 When turn off the power, the trapped charge are not discharged because the PVP around the traps is insulator. When a reverse bias is applied to device, a mass of trapped charges are still reserved in the traps. So the high electron concentration retains in the active layer. This process prevents the conductive path rupture (Figure 8d). This is because that the internal electrical field associated with the space charge layer in PVP is high enough to oppose the applied electric field. Thus, charges in the traps are difficult to be neutralized or detrapped due to the protection of the internal electric field. Consequently, the device exhibits WORM memory effect.

Figure 8. Schematic drawings of switching processes for the resistive switching memory device. (a) The traps in unoccupied state. (b) The injected carriers are trapped by CSQDs. (c) Traps are gradually filled and completely filled. (d) The formation of conductive path.



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We then test the flexibility of the device. The ON/OFF ratio retains almost its initial value when the PET substrate (30 × 30 mm2) holding the resistive switching memory device is bent from flat to bending radius of 7.1 mm (Figure 9a). When the bending radius is reduced from 7.1 to 5.8 mm, the device is broken. Figure 9b presents the thickness-dependent changes in the operating voltage and ON/OFF current ratio of the device. When the number of the active layers increase from 1 to 5 layers, the ON/OFF ratio decrease and the operating voltage increase form 1.6 V to 2.75 V. Moreover, Figure 9c shows that the operating voltage and ON/OFF ratio decreases when the measurement temperature increase from 25 to 120 °C. As shown in Table 1, resistive memory devices based on CSQD-PVP nanocomposites have intermediate operating voltages and relatively high ON/OFF current ratio compared with other nanocomposite resistive memory devices that comprise charge trapping conductive materials, such as black phosphorus QDs, MoSe2 QDs, WS2 QDs, NbSe2 QDs and carbon-based materials. The high ON/OFF current ratio, low operating voltages, good repeatability and flexibility of the device based on CSQD-PVP nanocomposites give the advantages of perfect performance control for practical applications.

Table 1 Comparison of performances of resistive switching memory devices fabricated with various charge trapping materials in PVP matrix. Active layer materials

Memory effect

Black Phosphorus QDs

Rewritable

Switching

ON/OFF

voltage

current ratio

1.2 V

6.0×104 5

Ref. 42

MoSe2 QDs

WORM

3.8 V

4.0 × 10

41

WS2 QDs

WORM

1.5 V

1.7×103

41

1.3 V

3

41

NbSe2 QDs

WORM

MoS2

Rewritable

3.5

1.3×10 ＞10

2

30 6

Carbon nanotubes

WORM

＞3 V

2.1 × 10

67

Functionalized carbon nanoshells

WORM

3.9V

4.6x106

68

5

Carbon spheres

WORM

4V

10 −10

Graphene QDs

Rewritable

1.5-1.6 V

14

CSQDs

WORM

1.6 V


10

5

6

68 69 This work

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Figure 9. (a) ON/OFF current ratio distribution of the flexible Al/CSQDs-PVP/Pt/PET devices at different bending radii (inset: photograph of the flexible device at different bending radii). ON/OFF current ratio and operating voltage corresponding to (b) different thicknesses and (c) different temperatures.

CONCLUSION In summary, we have successfully synthesized CSQDs through a facile solvothermal method. The obtained CSQDs are demonstrated good stability and strong fluorescence under UV light after being dispersed in both of DMF and deionized water. WORM type memory is demonstrated in the flexible resistive switching memory device with the Al/CSQDs-PVP/Pt/PET structure. The device represents excellent switching parameters such as high ON/OFF current ratio (105), low operating voltages (1.6 V), good stability and flexibility. The present flexible resistive switching device based on hybridization of CSQDs and PVP has a great potential to be used in flexible and high performance memory applications.



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Experimental Section Materials: Benzyl alcohol, N, N-dimethylformamide (DMF), ethanol, Co(Ac)2·4H2O, SeO2 and polyvinylpyrrolidone (PVP, MW= 400000) were supplied from Sinopharm Chemical Reagent Co. Ltd. Polyethylene terephthalate (PET) substrates were purchase from Xiang Science & Technology. Preparation of Co9Se8 Quantum Dots: Bulk Co9Se8 was synthesized by a solvothermal method. 2.4 mmol Co(Ac)2·4H2O and 2.0 mmol SeO2 were dissolved in 70 mL solution of benzyl alcohol. The purple homogeneous solution was stirred for 1 h. The solution was then poured into a 100 ml Teflon-lined stainless autoclave. The autoclave was placed in a heated oven at 180 °C for 16 h. The obtained black precipitate were centrifuged and washed twice with ethanol. In order to obtain the Co9Se8 nanosheets, 0.5 g of Co9Se8 powder was added in 50 mL of DMF to obtain the mixture. Then the mixture was added in 100 mL beaker and sonicated for 4 h. The power of the ultrasonic cleaner is 180 W. Then the mixture was transfer into flask and heated by oil bath at 140 °C with stirring for 6 h. Then, the suspension was sonicated for 5 h. The temperature in the ultrasonic cleaner was kept at 0 °C. The as-prepared dispersion was centrifuged at 5500 rpm for 20 min. The supernatant containing Co9Se8 quantum dots (CSQDs) was decanted gently. The yield of Co9Se8 quantum dots from nanosheets suspension is about 9.8 wt%. Fabrication of resistive memory devices. 2 mL of CSQDs suspension in ethanol was redispersed in 2 ml ethanol containing 40 mg PVP to obtain hybrid nanomaterials of CSQDs and PVP (CSQDs-PVP). The CSQDs-PVP films were deposited by spin-coating the CSQD-PVP suspension onto the electrodes at 3000 rpm for 30 seconds in atmospheric environment. The Pt line electrodes (width =1 mm, length = 20 mm) were sputtered through a shadow mask on the PET substrate. The obtained film was annealed in heated oven at 80 °C for 1 h. Then Al line electrodes, perpendicular to the Pt bottom line electrodes, were deposited on the CSQD-PVP films by evaporating deposition under a vacuum of ∼10-6 mbar. Pure CSQDs and PVP films were fabricated by the same routines of the preparation of CSQD-PVP films. Characterization. The morphology and structure analyses were evaluated with TEM


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(FEI TecnaiTM F30). The SEM image was taken using scanning electron microscopy (SEM, Hitachi S-4800). The AFM images were scanned in tapping mode by atomic force microscope (AFM, MFP-3D, Asylum Research). X-Ray diffraction (XRD, Phillips X’pert Pro) was used to identify the phase structures of samples. Raman shift was measured with a micro-Raman spectro-scope (JY-HR800). Component of the sample was investigated by X-ray photoelectron spectroscopy (XPS, PHI-5702). The luminescence spectra were investigated by a Hitachi F-7000 spectrophotometer. The UV-vis spectra of the CSQDs and Co9Se8 bulk were collected on an Agilent Cary 5000

spectrophotometer

and

PerkinElmer

Lambda

950

spectrophotometer,

respectively. Current-voltage characteristics were investigated by a Keithley 2400 source measurement unit.



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ASSOCIATED CONTENT

Supporting Information.

Figures S1-S5, which represents characterizations of synthetic bulk Co9Se8 and Co9Se8 nanosheets, EDX spectra of Co9Se8 ultrathin nanosheets, UV-Vis absorption spectra of bulk Co9Se8, emission PL spectra of CSQDs, the band-gap energy of bulk Co9Se8 and CSQDs, SEM cross-section image of five layer CSQDs-PVP films, temperature dependent resistance of ON state. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

[email protected] and [email protected]

Author Contributions §

These authors contributed equally.

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We thank for the National Basic Research Program of China (Grant No. 2012CB933101), National Natural Science Foundation of China (Grant Nos. 11274147, 11674141 and 51371093).


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Figure 1. Schematic representation of the synthesis procedure to synthesis CSQDs. (a) SEM image of bulk Co9Se8 powder. TEM images of (b) exfoliated Co9Se8 nanosheets, (c) CSQDs interspersed in the exfoliated Co9Se8 nanosheets and (d) CSQDs.


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Figure 2. (a, b) TEM, (c, d) High-resolution TEM images of CSQDs. (e) Size distribution of the CSQDs in (a). (f) AFM image and the corresponding height profile of CSQDs. (g) Height profiles along the white lines in (f). (h) Morphologies distributions CSQDs. Scale bar = 2 nm.



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Figure 3. (a) XRD spectra of CSQDs and bulk crystalline Co9Se8. (b) Raman spectra of CSQDs and bulk crystalline Co9Se8. (c) XPS spectrum of the as-synthesized CSQDs. (d) Deconvoluted Co2p spectrum. (e) Deconvoluted Se 3d spectrum.


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Figure 4. (a) Absorption spectra of CSQDs prepared in DMF and redispersed in deionized water. Inset: photographs of the CSQDs taken under visible light and 365 nm UV radiation. (b) Excitation and emission PL spectra of CSQDs. Emission PL spectra of CSQDs (c) prepared in DMF and (d) redispersed in deionized water.



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Figure 5. SEM image of (a) CSQDs films, (b) CSQDs films after bending operation and (c) CSQD-PVP films (Inset: AFM images of CSQD-PVP films). (d) SEM cross-section image of one layer CSQDs-PVP films.


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Figure 6. (a) Photograph of the flexible Al/CSQDs-PVP/Pt/PET resistive switching memory device. (b) Schematic conductive diagrams of experiments. Cathodes were grounded for the tests. (c) The current-voltage curve of the Al/CSQD-PVP/Pt/PET device. (d) The ON/OFF ratio of the device. (e) The retention characteristic of ON and OFF states for the device. (f) Switching cycles for the device. The current values are measured at a read voltage of 0.4 V.



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Figure 7. (a) The I-V curves of the Al/CSQD-PVP/Pt/PET and Al/PVP/Pt/PET devices. (b) The I-V curves of the Al/CSQD-PVP/Pt/PET device when the different negative voltage applied. Log-Log I-V characteristics of the in the device in (c) OFF state and (d) the ON state.


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Figure 8. Schematic drawings of switching processes for the resistive switching memory device. (a) The traps in unoccupied state. (b) The injected carriers are trapped by CSQDs. (c) Traps are gradually filled and completely filled. (d) The formation of conductive path.



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Figure 9. (a) ON/OFF current ratio distribution of the flexible Al/CSQDs-PVP/Pt/PET devices at different bending radii (inset: photograph of the flexible device at different bending radii). ON/OFF current ratio and operating voltage corresponding to (b) different thicknesses and (c) different temperatures.


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Table 1 Comparison of performances of resistive switching memory devices fabricated with various charge trapping materials in PVP matrix.

Active layer materials

Memory effect

Black Phosphorus QDs

Rewritable

MoSe2 QDs

WORM

WS2 QDs

WORM

Switching

ON/OFF

voltage

current ratio

1.2 V

6.0×104 5

4.0 × 10

48

1.5 V

1.7×10

3

48

3

48

WORM

1.3 V

1.3×10

MoS2

Rewritable

3.5

＞102

Carbon nanotubes

WORM

＞3 V

2.1 × 106

WORM

49

3.8 V

NbSe2 QDs

Functionalized carbon nanoshells

Ref.

3.9V

4.6x10 5

36 76

6

77

6

77

Carbon spheres

WORM

4V

10 −10

Graphene QDs

Rewritable

1.5-1.6 V

14

78

Reduced graphene oxide

WORM

＞4 V

105

79

1.6 V

5

CSQDs

WORM


10

This work


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TABLE OF CONTENT FIGURE.


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Facile Synthesis of Co9Se8 Quantum Dots as Charge Traps for

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