Localized Surface Plasmon Resonance-Mediated Charge Trapping

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Localized surface plasmon resonance-mediated charge trapping/ detrapping for core-shell nanorod-based optical memory cells Li Zhou, Su-Ting Han, Shiwei Shu, Jiaqing Zhuang, Yan Yan, Qi-Jun Sun, Ye Zhou, and Vellaisamy A. L. Roy ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07486 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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ACS Applied Materials & Interfaces

Localized surface plasmon resonance-mediated charge trapping/detrapping for core-shell nanorodbased optical memory cells Li Zhou,

†, ‡

Su-Ting Han,*, † Shiwei Shu,

†, ‡

Jiaqing Zhuang,

§

Yan Yan, † Qi-Jun Sun, § Ye

Zhou,*, † and V. A. L. Roy*, §

†

College of Electronics Science & Technology and Institute for Advanced Study, Shenzhen

University, Shenzhen, China.

‡

Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and

Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, China.

§

Department of Physics and Materials Science, City University of Hong Kong, Hong Kong,

China.

KEYWORDS: organic memory, photo-tunable, Au@Ag nanorods, LSPR, multilevel data storage

ABSTRACT: For following the trend of miniaturization as per Moore’s law, increasing efforts have been made to develop single devices with versatile functionalities for Internet of Things 1 ACS Paragon Plus Environment


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(IoT). In this work, organic optical memory devices with excellent dual optoelectronic functionality including light-sensing and data-storage have been proposed. The Au@Ag coreshell nanorods (NRs) based memory device exhibits large memory window up to 19.7V due to the well-controlled morphology of Au@Ag NRs with optimum size and concentration. Furthermore, since the extinction intensity of Au@Ag NRs gradually enhance with the increase in Ag shell thickness, the photo-tunable behaviors of memory device were systematically studied by varying the thickness of Ag shell. Multilevel data storage can be achieved with the light assistant. Finally, the simulation results demonstrate that the photo-tunable memory property is originated from the multimode localized surface plasmon resonance (LSPR) of Au@Ag NRs which is in consistent with the experimental results. The Au@Ag core-shell NRs-based memories may open up a new strategy toward developing high-performance optoelectronic devices.

INTRODUCTION Development of low-cost and high-performance organic memory devices is critical for the next-generation of flexible electronics.1-7 Several types of organic memories including resistorbased, capacitor-based, and transistor-based types have been reported.8-14 Among them, the functional organic thin film transistors (OTFTs)-based architecture, embedded with separated metal nanoparticles (NPs) as floating gate, is commonly used for non-volatile memory device due to its single transistor realization, non-destructive readout property, and ease of integration 2 ACS Paragon Plus Environment

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with complementary metal-oxide-semiconductor (CMOS).14-18 The adjustable threshold voltage (Vth) shift is the main operation mechanism of OTFT-based memory device.19 In another aspect, the sensing of light enables health monitoring, object inspection, and optical communications.2022

Tremendous efforts have been taken to exploit light detection devices with excellent

performance including fast response, high sensitivity and tunable absorption spectrum. For following the trend of miniaturization as per Moore’s law, increasing efforts have been made to develop single devices with versatile functionalities for Internet of Things (IoT). One of major direction is proposed for electronics to construct a kind of system for light-sensing and datastorage. Organic optical memory devices with dual optoelectronic functionality have been proposed to address this issue.23-25 The channel conductance of these devices can be adjusted by both external bias and light, therefore the multilevel data storage in single cell can be easily realized.26,

27

In addition, the functional OTFT devices with dual optoelectronic properties

enabled a straightforward path to future unconventional electronics, such as photo signal memory, photodetector and photosensor.28-31

To date, various possibilities have been employed to fabricate the OFET based optical memory devices: i) polymeric-gate electret layers,32-34 ii) up-conversion nanoparticles35 and quantum dots,36 iii) organic materials including small molecules,37, 38 organometallic complex,39 carbon-based materials,40 iv) photochromic materials,41-43 v) two dimensional materials, such as MoS2. 44-46 However, these materials suffer from limitations in narrow light response range and 3 ACS Paragon Plus Environment


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the synthetic ways of these materials are relatively complicated. It is highly desirable to utilize broad spectrum-responsive materials to construct an optical memory device through a facile fabrication method. Because of the localized surface plasmon resonance (LSPR) property, gold (Au) and silver (Ag) NPs are widely employed in photonic device studies.47-52 The plasmon resonance bands of Au@Ag core-shell nanorods (NRs) can be manipulated by varying either size of Au-core or thickness of Ag-shell to achieve a wide spectrum response range from visible to near-infrared.53 In addition, comparable intensities between the longitudinal and transverse plasmon resonance band can be obtained in Au@Ag NRs with suitable thickness of Ag coating since the refractive index sensitivities of Ag is higher than that of Au which promise highly photon response efficiency.54-56 In another aspect, spherical Au NPs were usually utilized as floating gate in traditional floating gate memory devices. Thus, future endeavors focus on exploring new types of Au@Ag NRs as charge trapping materials is highly valuable.

In this article, we report a novel kind of organic optical memory device by incorporating Au@Ag core-shell NRs into a hydrophilic insulating polymer-polyvinylpyrrolidone (PVPy) as discrete trapping sites. The Au@Ag NRs with well-controlled morphology can be uniformly distributed in PVPy matrix. Thus the relatively smooth interface can be formed between charge trapping layer and semiconductor to ensure large memory window. Another advantage of our memory device is that the tunable multimode LSPR of Au@Ag NRs is well matching with light absorption spectrum of organic semiconductor. As result, the near field enhancement induced by 4 ACS Paragon Plus Environment

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the core-shell NRs modulates the light absorption and exciton generation of semiconductor to achieve multilevel data storage. Finite-difference time-domain (FDTD) simulation was conducted to further study the LSPR effect on memory performance. The devices exhibit good reliability with data retention time longer than 104 s. The electrical performances can be well maintained for at least 102 programming/erasing cycles.

EXPERIMENTAL SECTION Materials. Gold(III) chloride trihydrate (HAuCl4•3H2O), bromide(CTAB),

Sodium

Hexadecyltrimethylammonium

Cetyltrimethylammonium

borohydride(NaBH4),

L-(+)-ascorbic

chloride

Silver

(CTAC),

acid

nitrate(AgNO3)

(AA), and

polyvinylpyrrolidone (average Mw ~1,300,000, PVPy) are purchased from Sigma–Aldrich and used without further purification. We use a Milli-Q purification system to obtain pure water (18.2 MΩ cm-1).

Synthesis of Au@Ag core-shell NRs. Au@Ag core-shell NRs with varying thickness of Ag shell were prepared through a two-step seeded growth method.57-59 The Au core were firstly synthesized by the following procedure: 10.0 mL of HAuCl4 (0.50 mM) was mixed with 10 mL of CTAB (0.20M) under continuous stirring, then 1.2 mL of ice-cold fresh NaBH4 (0.010 M) was quickly added into the mixed solution, the solution turned to brownish yellow immediately and was kept for more than 2 h to be used as seed solution. Subsequently, 25 µL of seed solution was added into a mixture solution which consist of 19.0 mL of CTAB (0.1 M), 1.0 mL of 5 ACS Paragon Plus Environment


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HAuCl4 (0.01 M), 160 µL of AgNO3 (0.01 M) and 110 mL of AA (0.10 M) with continuous stirring. The solution was aged for more than 12 h to ensure the completed growth of Au NRs.

The formation of Ag shell on anisotropic Au NRs was obtained according to the following procedure: First, CTAB capped Au NRs were replaced with CTAC by centrifuging. The washing procedure was repeated for three times to ensure the fully replacement. Then, 10 mL of the obtained CTAC capped Au NRs solution was diluted to 50 mL with 80mM CTAC solution. 2.5 mL of 0.10M AA solution and various amount of AgNO3 solution were added. The Ag shell thickness was verified by changing the molar ratios of Ag/Au. The resultant mixture solution was aged at 60 ºC for 5 h to ensure the complete formation of Au@Ag NRs. Finally, the obtained Au@Ag core-shell NRs were concentrated and re-dispersed in DI water.

Device fabrication. The PVPy-Au@Ag nanocomposite solution was first prepared by mixing 5 mg mL-1 of PVPy aqueous solution with various volume ratio of 6.1x10-9 M Au@Ag NRs solution. Ultrasonication was carried out to obtain a homogeneous dispersion. Bottom-gate topcontact geometry was used for all the devices. Heavily doped silicon wafers with 100 nm thermally grown silicon dioxide were used as substrate. The nanocomposite films were coated on the SiO2 surface by spin coating at 2000 rpm for 40 s. The films were then thermal-annealed at 120 ºC for 30 min to eliminate the solvent. A 50-nm thick pentacene was deposited through thermal evaporation at a rate of 0.2 Å/s. The Au source and drain electrodes were finally deposited on the top of pentacene through a shadow mask of defined channel length (L) and 6 ACS Paragon Plus Environment

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width (W) as 50 and 1000 µm, respectively. All of the devices fabrication procedures and electrical properties measurements were carried out in nitrogen atmosphere. Light illumination was carried out by using a light-emitting diode lamp with emission wavelength peak at 525 nm, 660nm and 739nm on the top of the memory devices based on Au@Ag NRs (A) , Au@Ag NRs(B) and Au NRs, respectively.

Characterization. The size distribution of Au NRs and Au@Ag NRs was verified by transmission electron microscope (TEM, Philips Tecnai 12 BioTWIN). The thickness of the PVPy films was measured through an ellipsometer. The morphology of the PVPy-Au@Ag nanocomposite films was characterized using an atomic force microscope (AFM) (VEECO Multimode V, tapping mode). The UV-Visible spectra were obtained with a PerkinElmer Lambda 750 UV-Visible near infrared spectrophotometer. The electrical characteristics of the devices were measured by Keithley 2612 source meter and Agilent 4155C semiconductor parameter analyzer at room temperature.

Simulation. The extinction spectrum and near-field distribution of the isolated Au@Ag NRs was simulated through the FDTD method, which based on a commercial software package (Lumerical FDTD Solutions 7.5). The wavelength of the light is ranged from 300 to 1000 nm. The mesh size of 0.5 nm was employed. The dielectric function of gold and silver were described by Lorentz–Drude (LD) model.60, 61 Refractive index (n) of PVPy medium was set as 1.53.

RESULTS AND DISCUSSION 7 ACS Paragon Plus Environment


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Figure1a shows a schematic illustration of bottom-gate top-contact organic transistor-based nonvolatile memory devices with PVPy-Au@Ag NRs composites as charge trapping layer. Ptype pentacene was utilized as semiconductor layer. Our design principle for Au@Ag NRs-based organic optical memory device is shown in Figure 1b. With light illumination, the photo-excited holes generated in the semiconductor with assistance of Au@Ag NRs can be efficiently trapped by Au@Ag NRs, resulting in the significant Vth shift.

Figure1. (a) Schematic illustration of organic optical memory device. (b) The mechanism of photo-excited carrier generation, transport and trap in the memory device. TEM image of (c) Au NRs, (d) Au@Ag NRs (B), and (e) Au@Ag NRs (A). (f) UV-vis spectrum of Au NRs and Au@Ag NRs in aqueous solution. 8 ACS Paragon Plus Environment

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The transmittance electron microscopy (TEM) image of pristine Au NRs is shown in Figure 1c. The pristine Au NRs exhibits uniform size distribution with length of about 56 nm, diameter of about 15 nm, and aspect ratio of about 3.7. Two sizes of Au@Ag NRs with different thickness of Ag shell were synthesized based on this prinstine Au NRs, as shown in Figure 1d and 1e. The brightness contrast demonstrates distinguishable boundary between Au core and Ag shell. The average length and diameter of Au@Ag NRs (A) are around 66 and 35 nm with 12 and 5 nm Ag coating in longitudinal and transverse directions. The length and diameter of Au@Ag NRs (B) are approximately 60 and 22nm with 3 and 2 nm Ag shells thickness in longitudinal and transverse directions, respectively.

The extinction spectra of Au NRs, Au@Ag NRs (A) and Au@Ag NRs (B) in the aqueous solution are depicted in Figure 1f. The optical absorption of pure Au NRs was also characterized as reference which displays two surface plasmon resonance (SPR) bands of longitudinal and transverse at 800 nm and 500 nm. When the Ag shell thickness reaches a certain value, the two octupolar plasmon resonance bands appear in the UV region. The longitudinal plasmon resonance band of Au@Ag NRs (A) and Au@Ag NRs (B) diplays an obvious blue shift after coating with Ag shell which is arising from both the reduction in the aspect ratio of the whole nanorods and the increasing effect of the optical properties of silver over those of gold. The light absorption of Au@Ag NRs (A) and Au@Ag NRs (B) are substantially improved in the shortwavelength region compared with reference pristine Au NRs. Increasing thickness of Ag shell 9 ACS Paragon Plus Environment


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further enhance the intensity of the SPR band close to the UV region. The enhancement of shortwavelength absorption in Au@Ag NRs is beneficial for improving the detection and sensitivity of Au@Ag NRs-based optical memory devices.

The surface morphologies of the Au@Ag NRs (A)-PVPy nanocomposites were characterized by atomic force microscope (AFM). The AFM images of the Au@Ag NRs (A)-PVPy nanocomposite films with various doping ratio of Au@Ag NRs (A) from 0 to 20 vol. % are denoted as PVPy, NRs-2, NRs-5, NRs-10, and NRs-20 (shown in Figure2a to 2e), respectively. The density of the NRs on the surface as well as the roughness of the nanocomposite films increase with increase in the Au@Ag NRs (A) doping rate. Large aggregation can be observed in NRs-20 nanocomposite film (Figure 2e). The small-scale AFM image of the single nanorod and aggregated nanorods are shown in Figure S1 and clearly demonstrated the aggregation of Au@Ag NRs for high concentration nanocomposite films. The root mean square roughness of the nanocomposite films with respect to the Au@Ag NRs doping concentration are summarized in Figure 2f. The optical image of the nanocomposite solutions is presented in the inset image. Excellent dispersion of Au@Ag NRs in polymer matrix is of significant importance for device fabrication. The high-quality dispersion of Au@Ag NRs in aqueous is attributed to the stability of CTAC surfactant and great water-solubility of PVPy.

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Figure 2. AFM image of (a) PVPy, (b) NRs-2, (c) NRs-5, (d) NRs-10, (e) NRs-20 films. (f) Roughness of the nanocomposite films with respect to the concentration of Au@Ag NRs (inset: optical image of the nanocomposite solution)

The electrical properties of Au@Ag NRs-PVPy nanocomposites-based optical memory devices and operating mechanism were further studied. The typical IV curves of memory devices at initial state, programmed state and erased state are shown in Figure S2 and Figure 3, the corresponding output curves of the initial states are shown in Figure S3. The electrical characteristics including mobility, on/off ratio, initial Vth, programmed Vth, erased Vth and memory windows are summarized in Table 1. And the device statics are shown in Figure S4.



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Field-effect mobility (µ) was calculated from the saturation region of the OTFT devices through the following equation:

‫ܫ‬஽ௌ =

ܹ ߤ‫ ܥ‬ሺܸ − ܸ௧௛ ሻଶ 2‫ ܮ‬௜ ீௌ

Where IDS is the source-drain current and VGS is the source-gate voltage, W and L are the channel width and length, respectively, Ci is the measured capacitance per unit area of the SiO2/Au@Ag-PVPy nanocomposite dielectric. The frequency-capacitance of the dielectric with different Au@Ag NRs concentrations is shown in Figure S5. The capacitance gradually increases with the increase in Au@Ag NRs concentration, the measured µ of PVPy, NRs-2, NRs-5, NRs-10, and NRs-20 devices are 0.16, 5.1×10-2, 2.2×10-2, 8.2×10-3, and 6.3×10-4 cm2 V-1 s-1, respectively. The mobility of the Au@Ag NRs-PVPy devices evidently decreases with the increased concentration of Au@Ag NRs.

The AFM images of pentacene deposited on various nanocomposite films are shown in Figure S6. Pentacene film on the pristine PVPy layer exhibits typical terraced grains with average grain size of approximately 1 µm. The increased concentration of Au@Ag NRs induces the decrease in the grain size of pentacene while the increase in the grain boundaries of the films. These observations indicate that the inferior film quality of the Au@Ag NRs-PVPy composites hamper the growth of pentacene crystal nucleus. The X-ray diffraction pattern of the pentacene films on Au@Ag NRs-PVPy composites films with various doping rates of Au@Ag NRs demonstrates


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that the crystallinities of pentacene degrade with the increase in NRs concentration(as shown in Figure S7).

Figure 3. Memory characteristics of (a) PVPy, (b) NRs-2, (c) NRs-5, (d) NRs-10, (e) NRs-20 memory devices at programmed state and erased state. (f) Memory windows varied with the concentration of Au@Ag NRs.

Following application of external negative gate-source pulse of −50 V with 1 s duration, which defined as programming process, the entire transfer curves are shifted to the negative direction with a series of programmed Vth. On the contrary, under positive external bias of +50 V for 1 s, the curves are shifted back to initial states, usually defined as erasing operation. All scans 13 ACS Paragon Plus Environment


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of drain current (IDS) versus gate voltage (VGS) were at a fixed drain voltage (VDS) of −30 V. It is worth noting that the Vth of a memory device based on pristine PVPy film exhibits a relative small shift after the programming process. The inferior charge trapping ability of the bulk PVPy and the interface between the SiO2 and PVPy dielectric layers are both revealed (as shown in Figure 3a).

Table 1. Summarization of measured fundamental characteristics of various memory devices Memory

µ

Ion / Ioff

devices (cm2 V-1 s-1)

Vth

Vth

Vth

Memory

Ci

(initial)

(programmed)

(erased)

windows

(V)

(V)

(V)

(V)

(nF cm-2)

PVPy

1.6 × 10-1

4.9×104

-6.6

-10

-4.7

5.3

29.5

NRs-2

5.1× 10-2

3.8×104

-6.9

-17.6

-7.6

10.0

30.2

NRs-5

2.2× 10-2

7.8×104

-7.5

-23.8

-8.2

15.6

30.3

NRs-10

8.2× 10-3

3.1×103

-7.0

-27.8

-8.1

19.7

31

NRs-20

6.3× 10-4

3.4×102

-9.3

-22.5

-7.1

15.4

32.1

In the Au@Ag NRs-PVPy-based devices, the significant shifts of Vth are obtained after programming operation. These shifts are directed toward the negative voltage compared with the initial states, indicating that the Au@Ag NRs act as hole trapping elements. By contrary, Vth are shifted to the identical position of initial state after the application of reverse biases. IDS 14 ACS Paragon Plus Environment

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dramatically increase to high-conductance states, suggesting that the holes can be easily trapped or detrapped in the Au@Ag NRs. The memory window (∆Vth), which refers to the variation of Vth, progressively improves from 10 V to 19.7 V with the increase in Au@Ag NRs doping concentration from 2 vol. % to 10 vol. %. However, further increase in Au@Ag NRs doping concentration lead to the shrink of the memory windows which is probably because of enhanced hole conduction among adjacent NRs and reduced charge trapping efficiency originated from the severe aggregation of NRs at high concentration.

The NRs-10 device exhibits the broadest memory window of 19.7V, while the memory window of device based on Au NRs with the same concentration is 13V which is shown in Figure S8. The stored charge (∆n) of NRs-10 device was estimated to 3.73×1012 cm-2 from the following equation: ∆n=∆Vth×Ceff/e, where Ceff is the effective capacitance of SiO2/Au@Ag NRs-PVPy composite dielectric, e is the elementary charge. The charge trapping density is significantly higher than the Au NRs device and our previously reported memory devices based on small Au NPs floating gate since core-shell structured Au@Ag NRs with relatively large sizes has higher capability of charge maintenances.62-64

To explore the effects of light sensing abilities of optical memory device, prinstine PVPy, Au NRs and Au@Ag NRs with various thicknesses of Ag coating were employed as charge trapping elements. The optimum Au NRs and Au@Ag NRs doping concentration of 10% was selected to ensure better device performances. Light was illuminated on the top of the devices by using a 15 ACS Paragon Plus Environment


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light-emitting diode lamp with relevant emission wavelength, and the photo-excitation intensity is 10 mW cm-2. The emission spectra of various light sources are shown in Figure S9 and S10. Figure 4a and 4c shows the transfer curves and threshold voltage shifts of the memory devices fabricated on Au@Ag NRs (A). The Vth value of programmed state and erased state are −27.8 V and −8.1 V, respectively after applying the external bias pulse of −50 V/+50 V for 1 s in dark condition. During programming and erasing process with light illumination, Vth of programmed state and erased state are shifted to −21.1 V and −4.5 V, respectively. Compared with the programming process in the dark condition, the positive backward shift of Vth indicates inferior hole trapping ability of the device under the light illumination which may be caused by localized surface plasmon resonance (LSPR) effect of Au@Ag NRs.


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Figure 4. Transfer curves of the memory devices with 10% Vol ratio of (a) Au@Ag NRs (A), (b) Au@Ag NRs (B) and threshold voltage shifts diagram of (c) Au@Ag NRs (A), (d) Au@Ag NRs (B) at programmed state and erased states with and without light illumination.

From the AFM images as shown in Figure 2, the Au@Ag NRs with 60 nm length are uniformly dispersed on the surface of the PVPy layer, facilitating the contact with the pentacene layer. The enhanced electric field around the Au@Ag NRs efficiently improves light absorbance of semiconductor layer (Figure S11). Therefore more excitons in the pentacene layer are dramatically generated under light irradiation. The electrons and holes are separated by the



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application of external gate bias. The working principles of optical memory devices are shown in Figure 5. Numerous exctions are generated in the pentacene layer during the light-assisted programming process. Even though the part of photon-generated holes can be trapped by the Au@Ag NRs, the rest amount of holes in the semiconductor channel is still more than that in the device without light illumination. Therefore, the increase in the effective charge carriers in the pentacene channel induce much positive shift of Vth under illumination compared with the Vth shift of memory device under dark programming process. During the light-assisted erasing process, both photo-excited holes and detrapping holes are transferred from Au@Ag NRs back to pentacene layer, which also induce much positive shift of Vth compared with Vth shift of memory device under dark erasing process. The similar photo-tunable memory effect can also be observed in memory device based on Au@Ag NRs (B) as shown in Figure 4b and 4d. The memory window of Au@Ag NRs (B) device is comparable to Au@Ag NRs (A) device of 16.8V. The Vth value of programmed state and erased state are −25.7 V and −8.9 V, respectively after applying the external bias pulse of −50 V/+50 V for for 1 s in dark condition. During programming and erasing process with light illumination, Vth of programmed state and erased state are shifted to −23 V and −5.8 V, respectively. However, the Vth of memory device based on Au NRs shows negligible change after the device is programmed or erased with light illumination or not (Figure S8). Compare to the Au@Ag NRs (A) device, lower sensitivity of light are observed in the Au@Ag NRs (B) and Au NRs device because the extinction intensity of Ag@Au NRs gradually decrease with the decrease in the thickness of Ag shell. These finding are 18 ACS Paragon Plus Environment

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in consistent with previous study.65 Moreover, the light sensing abilities of memory device with pristine PVPy layer is shown in Figure S12. During programming and erasing process with light illumination, Vth of the programmed state and the erased state showed a small shift. Different from the nanocomposite samples, the Vth shift towards the more negative position. The chargestorage ability of the PVPy increased a little after being programmed with light illumination.



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Figure 5. Schematic energy band diagram of memory operated under dark (a) programming; (b) erasing and under light assistant (c) programming; (d) erasing.

To further study the LSPR effect of Au@Ag NRs on multifunctional memory devices, the near-field distributions of Au@Ag NRs (A) in the PVPy matrix were simulated. The simulated extinction spectrum of Au@Ag NRs in PVPy matrix is shown in Figure 6a. The resonance positions for the longitudinal and transverse bands are located at 660 nm and 500 nm, respectively. Figure 6b and 6c show that the electric field distributions around the Au@Ag NRs are significantly improved up to 20 nm for both the longitudinal and transverse modes. Since the Au@Ag NRs are closely attached to the pentacene layer, the strong near-field originated from LSPR distribution can be vertically extended into the pentacene layer which dramatically improve the light absorption ability of the semiconductor layer. The electric field distribution of the longitudinal mode is higher than that of the transverse mode. Hence, light absorption is enhanced mainly originated from the longitudinal mode. Therefore, we can conclude that the integration of Au@Ag NRs into the PVPy layer can noticeably boost the light absorption of the pentacene layer through strong near-field and light scattering.


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Figure 6. (a) Simulated extinction spectrum of isolated Au@Ag NRs (A) in PVPy layer. Simulated electric field intensity distribution of Au@Ag NRs (A) in PVPy layer with the transverse (b) and longitudinal (c) dipole mode at 660 nm and 500 nm, respectively.

Reliable properties, including data retention ability and endurance, are extremely important parameters of memory devices. Figure 7a shows the retention capability with respect to the eclipsed time. The programmed and erased states were obtained after the application of −50 V and +50 V gate pulses for 1 s with or without light assistant. The Vth of the device can be maintained appropriately for at least 104 s. Since the coverage of Au@Ag NRs by the PVPy layer is incomplete, the memory window slightly degraded by the leakage of trapped holes from Au@Ag NRs back to the pentacene layer. Vth as a function of repetitive programming and erasing cycles are shown in Figure 7b. The four states of the device can be maintained accurately after at least 100 cycles with slight fluctuations. The four stable Vth levels promise two-bit storage in one single memory cell through simple light-assisted processes.



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Figure 7. (a) Data retention capability of photonic memories. (b) Endurance property of multilevel photonic memories.

CONCLUSIONS In summary, we have demonstrated a novel type of organic optical memory device by utilizing Au@Ag NRs-PVPy nanocomposite as charge trapping material. The Au@Ag NRs with wellcontrolled morphology and optimum size can be uniformly distributed in PVPy matrix to ensure high quality composite films. The device exhibits a large memory window up to 19.7 V due to the relatively high charge trapping density of Au@Ag NRs. More notably, the memory device is found to possess tunable multilevel data storage with the light assistant. The simulation results demonstrated that the photo-tunable memory behaviour is originated from the multimode LSPR of Au@Ag NRs which is in consistent with the experimental results. The broadband and tunable plasmonic enhancement of Au@Ag NRs may pave the way for their successful integration into other functional material systems. This strategy may help advance other high-performance optoelectronic devices.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:



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Small-scale AFM image of nanocomposite films. Transfer and output curves of memory devices with various Au@Ag NRs concentrations at initial state. The device statistics of mobility and memory window with various nanorods concentration. Capacitance per unit area of nanocomposite dielectrics with respect to frequency. AFM images of pentacene films deposited on (a) PVPy, (b) NRs-2, (c) NRs-5, (d) NRs-10, (e) NRs-20. XRD patterns of pentacene films deposited on various nanocomposite films. Transfer curves of memory device based on Au NRs and pristine PVPy film with/without light illumination. Absorption spectrum of pentacene. Emission spectra of various light sources.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 86-755-26534624

*E-mail: [email protected]. Fax: 86-755-26001352

*E-mail: [email protected]. Fax: 852-34420538

Notes

The authors declare no competing financial interest.


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ACKNOWLEDGMENTS We acknowledge the funding for this research from the National Science Foundation of China under Grant Nos. 61604097 and 61601305, the Shenzhen Science and Technology Projects under

Grant

Nos.

JCYJ20150625102943103,

JCYJ20170302145229928

and

JCYJ20170302151653768, the Department of Education of Guangdong Province (No. 2015KQNCX141 and 2016KTSCX120) and the Natural Science Foundation of SZU.

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