Superionic Modulation of Polymethylmethacrylate-Assisted

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Surfaces, Interfaces, and Applications

Superionic Modulation of PMMA-Assisted Suspended Few-Layer Graphene Nanocomposite for High-Performance Photodetectors Jun Yin, Lin Cong, Yu Liu, Pengfei Wang, Wanyun Ma, Jia-Lin Zhu, Kaili Jiang, Wei Zhang, and Jialin Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21055 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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ACS Applied Materials & Interfaces

Superionic Modulation of PMMA-Assisted Suspended Few-Layer Graphene Nanocomposite for High-Performance Photodetectors Jun Yin1,2, Lin Cong3, Yu Liu1,2, Pengfei Wang1,2, Wanyun Ma1,2, Jia-Lin Zhu1, Kaili Jiang,*,1,2,3 Wei Zhang,*,4 and Jia-Lin Sun*,1,2 1State

Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, P. R. China 2Collaborative

3Department

Innovation Center of Quantum Matter, Beijing, P. R. China

of Physics & Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, P. R. China

4Institute

of Applied Physics and Computational Mathematics, P. O. Box 8009 (28), Beijing 100088, China

KEYWORDS: superionic modulation, RbAg4I5, PMMA-assisted suspended graphene, hybrid photodetector, ion-electron bound states

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT. Graphene is receiving significant attention for use in optoelectronic devices because it exhibits a wide range of desirable electrical properties. Although modified graphene that is fabricated on quantum dots (or similar integration strategies) has shown promise, it has not overcome the requirement for high-speed applications and highly sensitive detection. Herein, we report ion-modulated graphene composite nanostructures that were incorporated into photodetectors. We focus on the dynamical properties of the novel photodetector, and they exhibit extraordinary photoelectric performances (photoresponsivity ~1 A/W, response time ~100 μs) over a broad range of wavelengths from 405 nm to 1064 nm (the maximum external quantum efficiency is greater than 300% at 635 nm with a 10 kHz chopping frequency). A theoretical model was proposed in this paper, and it is in a good agreement with our experimental results. The dynamic analyses further confirmed the dissociation and recombination of ion-electron bound states to be responsible for the fast and sensitive photoresponse

from

the

composite

samples.

Although

ion-modulated

optoelectronic

nanomaterials are rarely studied, they require further exploration as they offer new insights and alternatives in nanomaterial research.

INTRODUCTION Graphene and related materials are intriguing building blocks for use in optoelectronic applications.1 The pristine graphene has attractive optical and electronic properties, in terms of its high mobility, zero bandgap, and the linear dispersion of the Dirac excitions;2 but there is still some undesirable properties such as the limited optical absorption and short photocarrier lifetimes, for ultrafast and ultrasensitive detection over a board range of wavelengths.3-5 While a


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variety of devices have been studied, a group that has recently received significant attention incorporates graphene that has been sensitized with colloidal quantum dots (e.g. SnO2, PbS, etc.).6-7 As a result, very high photoresponses have been achieved. However, a number of challenges remain, including improving the speed, spectral range and large-scale production.3, 8 Alternative strategies to enhance the absorption of incident light involve integrating the graphene into an optical microcavity, in conjunction with an optical waveguide, or exploiting field enhancement by plasmons.9-11 In the field of high-speed applications, the achieved responsivity is on the scale of 0.1 A/W, but at the expense of broad wavelength detection.3 To date, simultaneously achieving high speed, high sensitivity, and broad wavelength detection remains a great challenge for graphene-based photodetectors. Herein, we report a new strategy based on ion modulation to improve the performance of photodetectors, using a combination of graphene and the superionic conductor RbAg4I5. These solid-state ion conductors consist of mobile species (typically Ag+ in RbAg4I5) and their ligands, which form a polyhedral crystal structure.12 The conduction mechanism includes the migration and diffusion of ions passing through crystallographic sites along the local minimum energy pathway in stoichiometric ion conductors.13-14 Ion conductors possess high chemical capacitance and ionic transport,15-16 which have aroused intense interest in their potential applications since the 1960s, including energy conversion and storage systems (e.g. supercapacitors), high-energy batteries, and electrochromic devices (e.g. smart window).17-20 Specifically for RbAg4I5, the silver ions exhibit a room temperature conductivity (0.21 S·cm−1 at 20 °C) that is eight orders of magnitude larger than its electronic conductivity (2.5 × 10−9 S·cm−1).15 And the RbAg4I5 thin films are prepared by vacuum evaporation of a powder containing specific proportions of RbI and AgI.21


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In this work, we have focused on the modulation of electron transport by the ions in the ionic conductor other than the intrinsic ionic conductivity. An ion-electron composite nanostructure that consisted of a number of suspended graphene layers and an RbAg4I5 film was fabricated and incorporated into a photodetector. Unlike the microscopic mechanisms of previous approach,6, 2223

Ag+ ion-modulation proved to be an effective way for which RbAg4I5 serves as a modulating

gate on the transport of charge carriers. The response behaviors of devices, in terms of enhanced responsivity, fast response speed, and so on were attributed to the photoelectric effects of ionelectron bound states (IEBSs) that formed at the interfaces of graphene and RbAg4I5. RESULTS AND DISCUSSION Polymethylmethacrylate (PMMA) assisted few-layer graphene was transferred onto a silicon nitride (Si/Si3N4) substrate that had a 50-μm-wide rectangular trench, which resulted in a section of suspended PMMA-assisted graphene.24 This is shown in Figure 1a. The dark area is the suspended section, while the colored area is the section in which the graphene is directly on the Si/Si3N4 substrate. The squares drawn on the different areas (15 μm × 15 μm) contained 100 pixels, which were measured and compared. The red trace in Figure 1b shows the total average Raman spectrum from the pixels in the red square (Figure 1a), while the blue trace shows the corresponding average Raman spectrum from the blue square. Two peaks that were characteristic of graphene (G peak (~1590 cm−1) and 2D peak (~2685 cm−1)) were clearly visible.25 Also in Figure 1b, the additional peak at 520 cm−1 was the characteristic peak of crystalline silicon, and the peaks at 306 cm−1 and that of 950–985 cm−1 were due to the isolating layers α-Si3N4 or βSi3N4 that formed on a crystalline silicon at different annealing temperatures.26-27 Further analysis showed that the properties of the suspended graphene were different than those of graphene on the substrate. The correlated positions of the G and 2D peaks are given by a


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distribution map, shown in Figure 1c. The average G and 2D peaks in the spectrum from the suspend graphene were 1 and 1.1 cm−1 downshifted when compared with those from the graphene on the substrate, respectively. PMMA-assisted suspended graphene that is moderately strained has been suggested to have a weakened doping.28-29 The sum intensity ratios (I2D/IG) shown in Figure 1d were consistent with this. This occurred because graphene preferentially interacts with residual PMMA as they have similar Hildebrand solubility parameters when compared with graphene and Si/Si3N4.28 The PMMA-assisted suspend graphene used in our experiments also contained the ionic conductor RbAg4I5 (a completed sample’s photograph is shown in Figure S1 and the corresponding schematic is shown in Figure 1e; for more details please see Experimental Section). The current-voltage (I-V) characteristics of composite samples after RbAg4I5 deposition times of 5 hours (black line) and 40 hours (red line) are shown in Figure 1f. With increasing deposition time, the electrons that were trapped by RbAg4I5 (i.e. the formation of IEBSs) in the composite nanostructures resulted in a small increase in resistance. The I-V characteristics of the samples were slightly different because only a small part of the graphene (the area between trench) contained RbAg4I5, which explains the differences observed in previous reports.30 The IV characteristics were measured in a vacuum chamber, and the devices were subsequently covered with a SiO2 passivation layer before removing for further electrical and optical measurements. The current-time (I-t) characteristics of the few-layer graphene/RbAg4I5 (FLG/RbAg4I5) composite nanostructure and the pristine few-layer graphene material are shown in Figure 2. The photocurrent from the pristine few-layer graphene was negative in some measurements and negligible in others. This may have been caused by adsorbents (localized states of O2 and/or OH


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groups) forming on the graphene surface under the ambient conditions.31 Despite any changes in the conductivity of the pristine graphene, the values for all wavelengths that were measured were approximately 1–2 microamperes or less (the most obvious cases are shown in hollow squares in Figure 2). In contrast, the FLG/RbAg4I5 composite nanostructure exhibited photocurrents of tens to hundreds of microamperes under the same illumination conditions (laser wavelengths ranging from violet light to near-infrared light, and laser powers varying between 101–102 μW). When illuminated for 25 ms with a chopping frequency of 20 Hz, the current increased sharply to its on-state values in 1 or 2 sampling points, as shown in Figure 2. The interval between two sampling points was 1 ms, and so the response time must have been less than that (more accurate values were determined using frequency dependence studies discussed next). The applied voltage was 5 V, and which would be the same for the following experiments. The photocurrent of the FLG/RbAg4I5 composite was positive. We have previously proposed a model to describe the photoelectric transient mechanisms of FLG/RbAg4I5.32 This model describes the electrons in graphene combing with the silver ions in RbAg4I5 during fabrication, which leads to the formation of IEBSs. Using the IEBS model, the photoelectric behavior of FLG/RbAg4I5 was attributed to the dissociation and recombination of IEBSs in the presence and absence of laser illumination. The silver ions can modulate the transport of electrons in graphene channels with very fast modulation speeds (ms or even μs timescales). FLG/RbAg4I5 composite nanostructures are expected to perform well at higher chopping frequencies. I-t curves from a composite sample that was illuminated with a range of wavelengths/powers, and was modulated by an increasing chopping frequency (200 Hz, 1 kHz (or 2 kHz), 5 kHz, and 10 kHz) are shown in Figure S2 (Supporting Information). A maximum chopping frequency of 10 kHz was the limit of the apparatus, which highlights the fast response


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speed of the superionic modulation in these FLG/RbAg4I5 samples. The photocurrent as a function of chopping frequency (average of 3–5 different samples, the samples were prepared and measured as equally as possible) are shown in Figure 3. The magnitude of the photocurrent differed from one sample to another. Thus, a statistical description was required. THEORETICAL BASIS Frequency-dependent measurements yielded the response times of the FLG/RbAg4I5 composite nanostructures. The dynamics are quantitatively described below based on the IEBS model. Let x be the fraction of the released electrons that result from the dissociation of IEBSs. The variation of x with time t (dx/dt) is determined by the competition between the dissociation of the IEBSs and the combination of the electrons and Ag ions, which can be described by Equation 1: dx    x    1  x  . dt

(1)

Here, γ is the Ag ion-electron combination rate and β is the IEBSs-dissociation rate. Physically, β is proportional to P × (hν – U0), where U0 is the ion-electron binding energy, hν is the photon energy of the laser, and P is the laser power. The time-dependent solution to Equation 1 is given by Equation 2:

x

1    Ce    t  ,    

(2)

where C is a constant to be determined according to the constraint condition at time t = 0. Furthermore, the current increment with and without illumination should be directly proportion to the density of Ag ions (NAg) in the FLG/RbAg4I5 samples (i.e. ΔI ∝ NAg·x). Considering the condition ΔI |t = 0 = 0, the final expression for the photocurrent is given by Equation 3: I  A 1  e  t /  .

(3)


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A and τ are fitting parameters that follow the relationships A ∝ NAg·β/(β+γ) and τ−1 = β+γ, respectively. Equation 3 has a similar exponential form to that of the Kohlrausch function, which is used to describe the transient behavior in a disordered system in which multiple energy transfers occur.33 The fitting parameters relate to the saturation photocurrent (A) and time constant (τ). To fit the frequency dependence of the photocurrent, shown in Figure 3, the independent time variable (t) in Equation 3 was taken as 0.5f-1 (the reciprocal of double the chopping frequency (f)). The fitting parameters were calculated and are summarized in Table S1 (Supporting Information). The saturation photocurrent A describes how the photocurrent behaves at infinite time when illuminated by the laser at different wavelengths (i.e. the steady solution of Equation 1), while τ characterizes the response time of the FLG/RbAg4I5 samples by presenting the time elapsed until the photocurrent reaches the value that A/e off the saturation current. We observed an agreement between the function that was derived using the IEBS model (hollow squares in black in Figure 3) and the experimental results (solid squares in different colors in Figure 3) when the sample was illuminated with the laser at different wavelengths. The results are shown in Figure 3 (further issues regarding curve fittings are discussed in Supporting Information), while there is some fluctuation in the data due to the defects of samples produced in the transfer process. It can be seen from Figure 3, with the decrease of chopping frequency, the photon current increases and finally saturates (corresponding to the IEBS steady state in the long time limit),30 and the critical point is around 1 kHz. Moreover, it is important to note that the response time was often only slightly above 100 μs (Table S1, Supporting Information), which confirmed the role of the superionic conductor for the fast photoelectric modulation within the FLG/RbAg4I5 composite nanostructures.


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Photocurrent-power (ΔI-P) curves were studied to determine the responsivity of the FLG/RbAg4I5 composite nanostructure, as shown in Figure 4. The plots were created using average values (with standard errors) from a series of repeat measurements. The photocurrent increased with the incident power in an approximately linear fashion within the uncertainty of our measurements. The spectral responsivity (Rλ) is the magnitude of the photocurrent induced by incident light power (Pin) at a certain wavelength (λ), which is given by Equation 4:

R 

I ph  I d Pin

,

(4)

where Iph is the current under the illumination, and Id is the dark current. The difference between the two is the photocurrent, written as ΔI = Iph – Id. The linear fits are plotted as red dashed lines in Figure 4, and their slopes give the responsivities when illuminated at different wavelengths (see Table S2, Supporting Information). The responsivity for visible light (532 nm, 635 nm, 808 nm) was greater than 1 A/W (R532 = 1.04 A/W, R635 = 1.61 A/W, R808 = 1.67 A/W), while the responsivity values at 405 nm and 1064 nm were 0.6 A/W and 0.27 A/W, respectively. Interestingly, these results are related to the optical absorption spectrum of RbAg4I5.34 This can be explained by the fact that the light has to pass through the RbAg4I5 before it interacts with IEBSs at the interface of RbAg4I5 and graphene. As a result, the larger responsivity occurred in the regions where the absorption of RbAg4I5 was less intense. A related performance parameter is the external quantum efficiency (EQE). This is defined as the number of photoelectric carriers induced by each photon that can be calculated using responsivity R, as shown in Equation 5:

ex 

I e

h P ,  R h e

(5)

in which hν denotes the photon energy and e denotes the electron charge. The calculated results (Table S2, Supporting Information) show impressive EQE values that exceeded 100% at


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wavelengths of 405 nm (185%), 532 nm (242%), 635 nm (314%), and 808 nm (256%). These results show another significant advantage of superionic modulation. An analysis of the mechanisms of action indicate that the dissociation of IEBSs under illumination are able to generate more than one carrier per photon, which is attributed to the trapped electrons in the graphene channel being modulated by Ag+ ions in the RbAg4I5 ion conductor. When the wavelength of illumination was extended to the infrared region (1064 nm), the photons were lower in energy, which caused the EQE to drop below 100% (EQE = 32% for 1064 nm). To summarize, Table S2 lists some important parameters of the FLG/RbAg4I5 photodetector, such as responsivity Rλ, EQE ηex, and the detectivity D* (please see the Supporting Information). CONCLUSIONS In this paper, we developed an RbAg4I5-integrated graphene photodetector and gave a comprehensive investigation of it. The fabrication process is relatively facile and should be available for large-production. The experiments for the time-dependence, power-dependence, and the chopping frequency-dependence of the photodetector helped us further understand the mechanisms that involved in FLG/RbAg4I5. It exhibited high-performance photoelectric characteristics (i.e. responsivity, response time, and EQE) over a broad spectral range from violet to near-infrared light (405 nm, 532 nm, 635 nm, 808 nm, and 1064 nm). Two strategies proved crucial: (1) the use of PMMA-assisted suspended graphene as a high-speed channel for electron transportation. Suspending the graphene over a trench reduced undesirable substrate effects (primarily doping levels) when compared with the graphene that was directly on the Si/SiO2 or quartz substrates. (2) The superionic modulation that was caused by RbAg4I5 was a significant advantage for photodetection. The experiments and the theoretical model with focus on the dynamical (time dependent) behavior of IEBSs reveal that the multi-carrier generation process


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occurred in IEBS results in EQE values exceeding 100%, as well as high responsivity and fast response speeds. Importantly, IEBSs have a variety of energy band structures that are determined by the silver-ionic energy and defect formation energy within the RbAg4I5 ion conductor, which extends its photoresponse wavelength range and makes broadband photodetection possible. All these photoresponse performances are superior or on a par with the graphene-based photodetectors that were reported in the top international journals recently (please see Table S3, Supporting Information). In summary, we have developed a graphene-based, ion-modulated photodetector that exhibited excellent photoelectric performance, which has great promise for further improvements and will contribute to the nano-photonics and nano-optoelectronics fields. EXPERIMENTAL SECTION Device Fabrication: The few-layer graphene (a size of ~ 5 cm × 10 cm) on copper foil was purchased from XF NANO-Advanced Materials Supplier. To enable the transfer of the graphene onto an isolating substrate, PMMA was spin-cast onto the graphene-covered copper foil at a speed of 2000 rev/min for 1 minute, and followed by heating at 120 °C for 2 minutes. The Raman signals in the range of 2900~3000 cm-1 (as seen in Figure 1b) were attributed to PMMA, which was always appeared in a majority of organic polymers, because of the origination of stretching vibrations from methyl (-CH3) and methylene (-CH2) in the structure.35-36 The graphene was then cut into small strips of 1 cm to 1.5 cm long by 1.5 mm wide. These strips were then treated with FeCl3 solution (1.28 M, in which conc. HCl was added to avoid the solution hydrolyzing, with a final pH of ~0.9) to remove the copper substrate. The resulting PMMA-assisted graphene was transferred onto a prepared Si/Si3N4 substrate. The substrates were processed by depositing a 100-nm nitride as isolating layer on the top of the silicon wafer, which was typically prepared for etching purpose. The narrow trenches in substrate were etched


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using lithography techniques and shaped into a 1.5-mm-long, 50-μm-wide rectangle. With the help of PMMA, the graphene was transferred onto the etched Si/Si3N4 substrate and suspended across the trench. Before transferring the PMMA-assisted graphene, a pair of electrodes was evaporated on each side of the trench. To enhance attachment to the nitride substrate, a 5-nmthick Ti layer was pre-coated on the substrate, followed by a 50-nm-thick layer of gold. The fabrication of the composite samples involved evaporating a RbAg4I5 film on the PMMAassisted graphene over the etched trench at a vacuum of ~2.0 × 10−3 Pa. The RbAg4I5 powder that was used for evaporation contained a mixture of RbI and AgI, which required grinding for several hours. The thermal evaporation process included two evaporation boats (both containing 150 mg RbAg4I5) that were evaporated successively. The sample plate was mounted at a distance of 30 cm directly above the evaporation boats while keeping their relative distance unchanged. Following the evaporation process, the samples were left in the vacuum chamber for approximately one day to ensure that they reached a stable state. (The crystal quality of RbAg4I5 was dependent on the evaporation and stabilization processes, which affected the response performances of the FLG/RbAg4I5 significantly. Characterization of pure RbAg4I5 is given in the Supporting Information, Figure S3.) The sample needed to be coated with a 20-nm-thick SiO2 passivation layer using an electron beam evaporator to avoid degradation when removed from the chamber. The rate of deposition was maintained in a range of 0.2–0.3 Å/s by controlling the current intensity in the electron gun filament. The deposition process usually took 12–13 minutes. During this process, the sample plate was rotating constantly at 10 rev/minute to ensure uniform coverage. After this, the composite samples could be taken out of the vacuum chamber and measured in an ambient atmosphere. The SiO2-protected samples would typically remain in its best state in the air for several days.


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Characterization and Measurements: The pristine few-layer graphene and the composite materials were characterized using a WITec alpha300R confocal Raman microscopy system. The Raman imaging system was equipped with a 532-nm laser source and a 50X Zeiss lens. The electrical measurements were performed using a Keithley 2400 SourceMeter to measure the I-V characteristics, and an Agilent B2911A to record the photoelectric curves. The fastest speed that the Agilent SourceMeter could operate was 10 μs per point. The optical measurement unit was equipped with five continuous lasers, with monochromatic wavelengths of 1064 nm, 808 nm, 635 nm, 532 nm, and 405 nm, and emitted powers of 500 mW, 100 mW, 200 mW, 300 mW, and 300 mW, respectively. The light power that was received by the samples in the experiments was determined using a light power meter (STARLAB, A: PD300-1W), with a measurement range of 10−9–1 watts and a wavelength range of 400 nm–1100 nm (custom ordered). The light powers given in Figure 2 and Figure S2 (Supporting Information) were measured when the laser light passed directly through an etched trench without any materials on the substrate. In addition, an optical chopper (Thorlabs Inc., Model MC2000B-EC) and its compatible chopper blade (Item# MC1F10HP) were used to modulate the laser beam. Along with the 10/100 blade (Inner/Outer slot numbers), a two-frequency chopping was available: the inner frequency could be continuously set at 20 Hz–1 kHz and the outer blade was running 10 times faster than the inner, i.e. 200 Hz–10 kHz.


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(a)

Si/Si3N4 substrate

Si/Si3N4 substrate

graphene

(b)

600 580 560 540 520 500

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suspended graphene

G-peak

2D-peak

with Si/Si3N4 substrate

4500 4000 600 550 500

0

30 μm

1000

2000

3000

4000

-1

Raman shift (cm )

(c)

( (

(d)

) with sub (ave) ) suspend (ave)

( (

) with sub (ave) ) suspend (ave)

2692

4 2690 2688

3

2686

I2D： IG

2D (cm-1)

2684

2

2682

1

2680 2668 2667

1580

1590

1600

0

G (cm ) -1

(e)

suspend

with sub 5h

(f)

40 h

6

PMMA 4

Graphene Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Intensity (cts)


RbAg4I5

Au/Ti SiO2

Si/Si3N4

2 0 -2 -4 -6 -10

laser beam modulated by a chopper

-5

0

5

10

Voltage (V)

Figure 1. (a) Photomicrograph of the PMMA-assisted graphene covering an etched Si/Si3N4 substrate. The middle area is the suspended graphene (the channel length was ~50 μm) and the


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areas either side are the graphene directly on the substrate. The two squares in the figure are 15 μm × 15 μm and consist of 100 pixels. (b) Total average Raman spectroscopy corresponding to the squares in the same color in Figure 1a. The excitation source (532 nm/2 mW) was focused through a 50 X lens and collected with grating grooves of 600 lines using reflection mode. The integration time for each measurement was 5 s. (c) The correlated plot of peak positions ωG and ω2D for PMMA-assisted suspended graphene (yellow square with an error bar) and the PMMAassisted graphene directly on the substrate (green square with an error bar). The dashed lines indicate strain-induced (red dashed line) and doping-induced (blue dashed line) position changes, along with the position of charge-neutral graphene (hollow diamond symbol). (d) The sum intensity ratio between the 2D peak and the G peak for PMMA-assisted suspended graphene (yellow square with an error bar) and the PMMA-assisted graphene directly on the substrate (green square with an error bar). The intensity was the sum of CCD counts at the center of peaks (1594.5 cm−1 for G and 2685 cm−1 for 2D) with a width of 50 cm−1. The statistics in Figure 1c and 1d were determined using 100 pixels within each square shown in Figure 1a. (e) A schematic diagram of the FLG/RbAg4I5 composite nanostructure. (f) Forward and reverse I-V characteristics of FLG/RbAg4I5 after the deposition of RbAg4I5 for 5 h (black line) and 40 h (red line).


15

1064 nm, ~200 W

Graphene

on off

3000 2900 2800

4002 4000 3998 3996

(b)

3000 2900 2800

0.10

0.15

Time t (s) 4001 4000 3999 3998

0.20

0.25

2950

0.00

0.05

0.10

0.15

Time t (s)

(d)

532 nm, ~20 W

Graphene

on off

2900 2850 2800

Graphene

on off

2950 2900 2850

4001 4000 3999 3998

0.20

FLG&RbAg4I5

0.25

0.05

0.10

0.15

0.20

0.25

Time t (s)

(e)

2950

0.00

405 nm, ~60 W

Graphene

on off

2900 2850 2800

FLG&RbAg4I5

0.00

635 nm, ~8 W

FLG&RbAg4I5

Current I (A)

0.05

4001 4000 3999 3998

2800

FLG&RbAg4I5

0.00

(c)

Graphene

on off

3100

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808 nm, ~50 W

Current I (A)

(a)

Current I (A)

4002 4000 3998 3996

Current I (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.05

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FLG&RbAg4I5

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0.25

Time t (s)

Figure 2. Photoelectric response curves of the FLG/RbAg4I5 composite nanostructure and pristine few-layer graphene when illuminated with different laser wavelengths. (a) 1064 nm, ~200 μW. (b) 808 nm, ~50 μW. (c) 635 nm, ~8 μW. (d) 532 nm, ~20 μW. (e) 405 nm, ~60 μW. The chopping frequency was 20 Hz, the sampling interval was 1 ms, and the applied bias was 5 V. The laser spot diameters are 1.5 mm (1064 nm), 3.5 mm (808 nm), 3.0 mm (635 nm), 1.5 mm (532 nm), and 2.5 mm (405 nm), respectively.


16

0.75

100

0.50

50

0.25

0.01 0.1 1 10 Chopper Frequency f (kHz)

Photocurrent I (A)

(d)

200

4

100

2

0.00

IEBS model fittings experimental values

0

200

532 nm, 20 W 10

150

7.5

100

5.0

50

(e)

2.5 IEBS model fittings experimental values

0

10

40

5 IEBS model fittings experimental values

0


405 nm, 60 W 2.0

90

1.5

60

1.0

30

0.5 IEBS model fittings experimental values

0

15

80

120

0.0


635 nm, 8 W

120

0



IEBS model fittings experimental values

6

Responsivity Rf (AW-1)

0

300

(c)



150

808 nm, 50 W 8

0


1.00


200

400


(b)

1064 nm, 200 W Responsivity Rf (AW-1)


(a)

0.0

Figure 3. Chopping frequency dependence of the photocurrent when illuminated with different laser wavelengths. (a) 1064 nm, ~200 μW. (b) 808 nm, ~50 μW. (c) 635 nm, ~8 μW. (d) 532 nm,

60 40 20

0


0

linear fit 1064 nm 50 100 150 200 Power P (W)

30

120

(b)

90 60 30 0 0

linear fit 808 nm 10 20 30 40 50 60 Power P (W)

(d)

20 10 0 0

linear fit 532 nm 5 10 15 20 25 Power P (W)

80


(a)


80


~20 μW. (e) 405 nm, ~60 μW. The curve fits in each panel were determined using Equation 3.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 17 of 24

16

(c)

12 8 4 0 0

2

4 6 Power P (W)

linear fit 635 nm 8 10

(e)

60 40 20 0 0

linear fit 405 nm 20 40 60 80 Power P (W)


17


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Figure 4. Photocurrent versus light power with linear fits for different wavelengths. (a) 1064 nm. (b) 808 nm. (c) 635 nm. (d) 532 nm. (e) 405 nm. Supporting Information The following files are available free of charge. Photograph of samples (Figure S1), photoelectric response curves (Figure S2), dynamic function fittings (Equation S1), characterization of RbAg4I5 and its composites (Figure S3), fitting parameters based on Equation 3 (Table S1), a summary of the FLG/RbAg4I5’s important parameters Rλ, ηex, and D* (Table S2), comparison of the graphene-based photodetectors (Table S3). Corresponding Author *Email: [email protected], [email protected], [email protected] ACKNOWLEDGMENT This work was partially supported by National Key Research and Development Program of China (Grant No. 2017YFA0303400, 2018YFA0208400), National Science Foundations of China (Nos. 11374172, 11174172, 11774036, 11374039, 51727805), NSFC-RGC (Grant No. 11861161002), and the Research Fund Program of the State Key Laboratory of LowDimensional Quantum Physics (No. ZZ201703). REFERENCES (1)

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Superionic Modulation of Polymethylmethacrylate-Assisted

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