Transient and Flexible Photodetectors - ACS Applied Nano Materials

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Transient and Flexible Photodetectors Shih-Yao Lin, Golam Haider, Yu-Ming Liao, Cheng-Han Chang, Wei-Ju Lin, Chen-You Su, YiRou Liou, Yuan-Fu Huang, Hung-I Lin, Tai-Chun Chung, Tai-Yuan Lin, and Yang-Fang Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01169 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Applied Nano Materials

Transient and Flexible Photodetectors Shih-Yao Lin⊥, †, Golam Haider⊥, †, ¶, Yu-Ming Liao†, Cheng-Han Chang†, Wei-Ju Lin†, ChenYou Su†, Yi-Rou Liou†, Yuan-Fu Huang†, Hung-I Lin†, Tai-Chun Chung‡, Tai-Yuan Lin*,‡, and Yang-Fang Chen*,†,§ †Department of Physics, National Taiwan University, Taipei 10617, Taiwan *E-mail: [email protected] *E-mail: [email protected] KEYWORDS: transient, photodetectors, graphene, chlorophyll, optoelectronics

ABSTRACT: With the rapid development of technology, electronic devices have become omnipresent in our daily life as it brought much convenience in every aspect of human activity. Side-by-side, the electronic waste (e-waste) has become a global environmental burden creating an ever-growing ecological problem. The transient device technology in which the devices can physically disappear completely in different environmental conditions has attracted a widespread attention in recent years owing to its emerging application potential spanning from bio-medical to military use. In this work, we demonstrated the first attempt of a dissolvable eco-friendly flexible photodetector using a hybrid of graphene and chlorophyll on poly(vinyl alcohol) substrate. The whole device can physically disappear in aqueous solutions in a time span of ~30 minutes, while it shows a photoresponsivity of ~ 200 AW-1 under ambient conditions. The high carrier mobility

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of graphene and strong absorption strength of green photon harvesting layer, chlorophyll, result in the photocurrent gain of the device as high as 103 with subsecond response time under the illumination of red light. The newly designed photodetector shown here yields zero-waste with a minimum impact on the environment, which is very useful for the development of the sustainability of our planet.

Introduction With the worldwide thriving demand of new technology and rapid development of electronic gadgets, such as cell phones, computers, cameras, to the solar cells, gas/temperature sensors, and household electronics, etc., e-waste is dramatically increasing.1 This becomes an urgent issue in the 21st century because the waste has an adverse impact on the environment, sanitation, and human health. To overcome this challenge, developing different kinds of devices is necessary. The concept of transience, which is considered as fundamental solutions, has obtained a great attention in recent years.2-9 Unlike traditional devices, transient devices possess the advantage of decomposing in different kinds of external circumstances, for example, different solution, heat, pH, chemicals, etc. In the near future, imaging those defective consumer commodities could disintegrate in flowerpots over time rather than stay in landfills for eons, i.e., the discarded electronic gadgets can become compost rather than dirty trash. Typical envisioned applications span from military uses to biomedical implants.10 For example, in modern warfare, to come without a shadow and leave without a trace, suicidal military hardware (e.g., cameras, sensors, antennas, robots, and other spy gears) can be strategically air-dropped or hiddenly embedded throughout hostile environments for remote monitoring, wireless communication, and


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distributed networks without no one the wiser, which provides sufficient conditions for preemptive tactics and fatal strikes.10 On the other hand, transient features of the devices have an engrossing future in biomedical applications. Some researches have been concentrated along this guideline. For example, sutures originally made materials, used for temporary medical tasks, are supposed to decompose and to be absorbed in the body providing the targeted functions (e.g., targeted drug delivery matrices for a specific therapy, injectable electrophysiological sensors, and consumable digital-imaging devices).11-13 Despite these superior application features, up until now, only a few demonstrations of transient devices have been proposed. Henceforth, a new paradigm of designing transient environmentally friendly devices remains open in order to give a practical means of the transient concept. Photodetectors are greatly amalgamated with omnipresent technology. As is well-known, it is the basic building block of the internet of things (IoT) system, which can be equipped on sensors, actuator networks, smart devices, communication hardware, wireless fidelities, and several biomedical devices. Thus, the demonstration of transient photodetector device could pave a significant step towards the practical implementation of transient technology. Meanwhile, the new trend of future technology impelled opportunities and challenges of designing multi-functional, low-powered, and high-performance devices. Towards the realization of such high-performance devices, recent years, graphene-based optoelectronic devices made an overwhelming progress owing to the several superior advantages of the unique properties of graphene, including thinnest, toughest, and non-toxic nanomaterial with ultra-high charge carrier mobility, superior optical transparency, etc.14-18 These unique properties are intriguing for the development of high-performance low-dimensional optoelectronic devices. Unfortunately, graphene is zero bandgap semimetal and can only absorb 2.3% of incident photons.19 Thanks to


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the low density of states of graphene film around the charge neutrality point that facilitates the Fermi energy with high sensitivity to its external perturbations.20-21 Hence, decorating a thin layer of strongly photon absorbing semiconducting nanomaterials or polymers on top of a single layer graphene, the Fermi energy can be optically tuned. The detection of photon using such graphenesemiconductor nanomaterials composite device possesses an ultra-high performance due to the combined effect of ultra-high mobility of graphene layer and the superior absorption strength of the photon harvesting materials.22 As graphene is inherently flexible, the composite graphene device on soft polymer can endow high flexibility and stretchability. In addition, graphene-based hybrid devices have been studied in diverse applications including gas-sensing, energy harvesting, light emitting, high-speed electronics, etc.22-30 Among these applications, the biomedical application of graphene is one of the latest research fields with tremendous potential, like drug delivery,31 tissue engineering,32 biological imaging,33 biomedical therapy,34 and features biocompatibility. Towards the realization of green technology a suitable composite consisting of graphene and the naturally produced materials, such as graphene quantum dots derived from plants or chlorophyll from plant leaf can serve as excellent alternatives. Especially, chlorophyll is abundant in plants and can be reached easily, and it can absorb sunlight efficiently. Moreover, chlorophyll is a non-toxic and environmentally friendly bio-material, which has been utilized for the development of several kinds of eco-friendly optoelectronic devices, such as solar cells and phototransistors.35 Furthermore, the solution processed chlorophyll layer can be dissolved in water easily, which shows its potential to serve as an active material in transient devices. Recently, a great attention has been given to practical applications of synthetic polymers in different areas of biomedical industry.10,

36-38

Poly(vinyl alcohol) (PVA), which is mainly


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composed of vinyl alcohol groups along with small quantities of vinyl acetate groups. It is a hydrophilic, biodegradable, biocompatible, inexpensive and non-toxic synthetic polymer.39 It is in line with environment and energy issues. Interestingly, PVA can be dissolved by water; it therefore can serve as an excellent substrate for the transient devices.40-41 Finally, designing a dissolvable electrode is a great challenge in transient devices as the conventional electrodes are based on Al, Ag or Au, which are not dissolvable in conventional nontoxic liquids. Fortunately, Mg can serve as a suitable alternative electrode, which is highly dissolvable in water solvent.12 Magnesium is a promising material due to its biocompatibility.42 Replacing the conventional metal with biodegradable metals, like magnesium, tungsten or molybdenum with water-soluble polymers makes the transient electronics possible.43 Taking advantage of these superior qualities, in this article, we have demonstrated a transient photodetector based on graphene, chlorophyll and Mg electrodes on a PVA substrate as shown in Scheme 1. It shows a photocurrent gain as high as ~103, and it can be completely dissolved in water solvent in a time span of ~30 minutes. A device with dissolvable functionality not only can reduce the environmental waste, but it can also induce a great impact on many different areas both in academic and industrial interest. Results and Discussion Figure 1 shows the schematic diagram of the designed transient photodetector consisting of chlorophyll, graphene and Mg electrodes on top of PVA substrate. A detailed synthesis of the constituent materials and device is provided in the Method section. Let us first demonstrate the dissolvability of the device. As both PVA substrate and Mg electrode are highly dissolvable in deionized water, the device starts to decompose as soon as the device is exposed into the water


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droplet. Figure 1a-1c provide a series of images of the dissolvable demonstration. All of the components, including the PVA substrate, chlorophyll, graphene, and magnesium, can be disintegrated and dissolved when the device is immersed in deionized water. It takes 6 minutes for PVA to dissolve in water, whereas, the Mg electrodes take about 30 minutes to dissolve completely. Figure S1 shows a variation of PVA slab’s weight from 0 min to 6 min. Magnesium reacts with water producing magnesium hydroxide and hydrogen gas. Certainly, the graphene layer does not dissolve in water, but being a stable, biocompatible, and atomic layer thin material, graphene does not produce any environmental concern and remain undetected in the decomposed solution. Owing to the robust imbalance of external strain due to the strong reaction of underlying PVA substrate in water, it is expected that the graphene layer is torn into multiple pieces and loose its 2dimensional nature in the end. The optical microscope image of the graphene layer with a flakelike structure on PVA substrate is shown in Figure S2. The 5 nm thick Ti layer under the Mg electrodes also dissolves into the water, because Ti film with only 5 nm thick cannot stay in freestanding form. The UV-Vis absorption and photoluminescence (PL) emission spectra of the light harvesting chlorophyll material are depicted in Figure 2a and b, respectively. Figure 2a shows that there are two strong absorption peaks of chlorophyll in the UV and visible region. The photoluminescence spectrum of chlorophyll in Figure 2b was taken using a 266 nm Nd:YAG pulsed laser as the excitation source. The emission peak is also around 670 nm, which matches well with the absorption spectrum in the visible range as shown in Figure 2a. The quality of the single layered graphene on top of the PVA has been studied by recording the Raman scattering spectrum under the illumination of a 633 nm laser. The appearance of Lorentzian shaped 2D band at 2640 cm-1 and the G band located at 1586 cm-1 confirm the presence of graphene film on PVA.


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The D band of the graphene on PVA around ~1350 cm-1 is insignificant indicating a good quality graphene. The intense peak at 2920 cm-1 in the Raman spectra is due to the PVA substrate. The intensity ratio of G and 2D band of the graphene is found to be 0.71 assuring a single layer graphene.44 A complete description of the synthesis of the single layer graphene has been provided in the Method section. Next, we evaluate the device performance under the illumination of an external 656 nm laser excitation as shown in Figure 3. The photocurrent-voltage (I-V) characteristics and transient photoresponse (I-T) of the device to the sequential irradiation of 656 nm laser with different power are shown in Figure 3a and b, respectively. We restricted the laser spot size on sample to 400000 µm2, in order to excite all the chlorophylls on the active area of the device, which is about 35 µm × 90 µm. We finally calculated the power used by the device by normalizing the incident power with the device active area. The enhancement of current under the excitation of photons is due to the hole doping in the graphene, which can be understood as follows. The CVD grown graphene is p-type by nature. Owing to the favorable band alignment as discussed below, the incoming photogenerated holes from the chlorophyll make the graphene even more p-type, which enhances the current in the graphene channel. The I-T characteristics under the excitation of different power as shown in Figure 3b, depicts a gradual decrease of photocurrent with the reduction of incident power, which is due to the less carrier generation and followed by less p-type doping in the graphene layer. Interestingly, with the reduction of the excitation power, the response time becomes slower. This is because under the reduction of the excitation power, the effect of carrier trapping due to interface defects is more pronounced, which will prolong the charge transfer process. A plot of the variation of the response time of the device with the input power is shown in Figure S3. For the decay curve, it shows two segments, a fast component and a slow component,


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which can be interpreted as follows. After turning off the illumination, the free unbounded charge carriers recombine very fast, whereas the bounded charges takes a longer time for complete recombination making the response time slow. Such behavior of the bound charges primarily depends on the interfacial states, which is a property of the constituents of the device.23 Therefore, for the whole decay curve, it shows two significant segments. Let us now underpin the mechanism of the photodetection of the device. In order to probe the movement of the Fermi energy of the graphene channel under different circumstances, we have designed a phototransistor as shown in Figure S4. Concurrently, we measured the transfer characteristics at the different steps of the device fabrication under dark condition and, under the excitation of the photons on the complete device, as shown in Figure 4a. The transfer characteristics of the device show a dip in the current profile, which is commonly known as the Dirac point. The shift of the Dirac point carries a fingerprint of the Fermi energy change of the graphene layer, which is connected by Equation (1),23 Δ𝐸𝐹 = Sign(Δ𝑉𝐺 )ħ𝑣𝐹 √𝛼𝜋|Δ𝑉𝐺 |

(1)

, where Δ𝐸𝐹 , Δ𝑉𝐺 , ħ, 𝑣𝐹 , and 𝛼 are the change in Fermi energy, change of Dirac point, planks constant, Fermi velocity, and gate capacitance, respectively. The device composed of only graphene channel shows (purple curve) the occurrence of the Dirac point at +19.5 V, which shifts toward left (blue curve) to +6 V after spin coating the chlorophyll. Whereas, it shows a further right shift at +13.25 V under the excitation of 656 nm laser of power 1.7 µW. This is due to the fact that compared to the graphene layer, chlorophyll has a higher Fermi energy. Thus, after coating the chlorophyll, the electron diffusion occurs from chlorophyll to graphene to obtain a thermal equilibrium of the Fermi energy of the composite. As a result, the graphene gets n-type doped and


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the Dirac point shifts toward left. It produces a built-in electric field directed toward graphene that causes the upward band bending of the chlorophyll at the junction. The incidence of photons carrying the energy higher than the bandgap energy produces the electrons at the LUMO, and holes at the HOMO level of the chlorophyll. Owing to the existence of the built-in electric field, the holes get transferred to the underlying graphene channel that makes the graphene more p-type doped further, and it shows the Dirac point right shifted. We have schematically shown these phenomena in Figure 4b, c, and d, respectively, based on the previous reports on the HOMOLUMO levels of the chlorophyll.45 To further evaluate the device performance, we have examined the device responsivity and photocurrent gain. The responsivity of the device can be defined by the current change in graphene per unit power of illumination, as given in the following Equation (2),

𝑅=

∆𝐼(A)

(2)

𝑃(W)

, where 𝑃(𝑊) is the total laser power used by the device, i.e. the total power at the device working area, and ∆I = (IPh - ID) is the photocurrent. IPh and ID are the current under the illumination and the current at the dark condition, respectively. The change of responsivity and photocurrent gain on different illumination power are shown in Figure 5a and b, respectively. A maximum photoresponsivity of ~200 AW-1 at 1 V source to drain voltage is achieved at an illumination power of 50 µW. With the enhancement of the incident power, the photoresponsivity is found to decrease, which is a common phenomenon observed in this kind of devices, and it can be understood as follows. The photogenerated carriers and the transfer of holes to the underlying graphene layer is highly influenced by the existing carrier trapping centers and the surface defects in chlorophyll.46-


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The free carrier density (𝑁) in chlorophyll can be described by the following expression,

Equation (3),

𝑁=

𝜂𝑃 ℎ𝜐

𝑇(𝑃)

(3)

, where 𝜂 is the quantum efficiency for carrier generation per unit photon absorption, 𝑃 is excitation laser power density, ℎ𝜐 is the energy of incident photons, and 𝑇(𝑃) is the carrier lifetime and a function of power, 𝑃. The carrier lifetime 𝑇(𝑃) can be expressed by the following formula, Equation (4) 𝑇(𝑃) = 𝑇 0

1

(4)

1+(𝑃/𝑃0 )𝑛

, where 𝑇 0 is the carrier lifetime at lowest excitation energy. 𝑃0 is the excitation intensity in which all the surface states are completely occupied by the photogenerated charge carriers. The 𝑛 is a phenomenological fitting parameter. At low intensity illumination, the electrons from the induced electron-hole pairs get trapped at the existing defect levels and occupy the surface states of the chlorophylls providing holes enough time for transferring to the underlying graphene channel because of the existing built-in electric field. As the excitation light intensity is increased, more and more electron-hole pairs are generated, that eventually shorten the excited state carrier lifetime because of the saturation of the electrons trapping. This phenomenon causes an immediate recombination of a part of photogenerated excitations. Therefore, lesser fraction of the photogenerated holes transfer to the underlying graphene channel as compared with the fractional carrier transfer under the lower incident power. As a result, we observed the reduction of photoresponsivity with the enhancement of incident power. The photocurrent ΔI can be expressed utilizing Equation (3) and (4), as the following Equation (5) shown below, where 𝑐 is constant, 𝑞


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is the elementary charge, 𝑊 and 𝐿 are the sample width and length, 𝜇 is the carrier mobility, 𝑉 is the working voltage, and 𝑣 = 𝜇𝑉𝐿−1 is the carrier drift velocity in graphene. Similarly, the photoresponsivity can also be expressed by the Equation (6). The experimentally obtained responsivity fits well with Equation (6) (R2=0.9991). The solid curve in Figure 5a is the best fitting with the formula given as Equation (7). ∆𝐼 = 𝑐𝑞𝑣𝑁𝑊 =

𝑅=

𝑐𝑞𝜂𝜇𝑉𝑊 ℎ𝜈𝐿

𝑇0

𝑃 1+(𝑃/𝑃0 )𝑛

∆𝐼(A)

(6)

𝑃(W/cm2 )×𝑊𝐿

𝑅 = 329.93 [

(5)

1 𝑃(W) (1+(0.00004037)0.676 ))

]

(7)

Equation (6) can be utilized to predict the highest responsivity of the device of 330 AW-1, while the excitation power P approaches zero. The obtained photoresponsivity is lower than the previously reported graphene-chlorophyll hybrid devices, which is due to the several factors of our design. For instance, Chen et. al.30 obtained a highest photoresponsivity of ~106. This is firstly because of using mechanically exfoliated high quality graphene. Secondly, the device was designed on a high quality of the rigid Si/SiO2-octadecyltrichlorosilanes (OTS)-functionalized substrates, where the electrical contacts on graphene were made by a resist-free approach to keep the high mobility of the pristine graphene. On the contrary, the graphene we have used is CVD grown, where the device is designed on a dissolvable flexible organic polymer substrate and a dissolvable Mg electrode has been used to achieve our desired functionality, which not only induces defects in the graphene layer but also makes the graphene electrode junction resistive, as the work function of graphene and Mg are found to be 4.7 eV and 3.66 eV, respectively. Moreover, the channel length of our device is ~35 micron that is much larger than the reported device, which


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makes the responsivity further lower. From Equation (7), the carrier trapping saturation power is obtained to be 𝑃0 = 0.00004 W, which is higher than the reported values for the semiconductor nanomaterials.46 It resembles the presence of a large number of trap states at the graphenechlorophyll interface. Furthermore, the value of the phenomenological fitting parameter n = 0.67, also disfavors the detection of lower power excitation.46 Because of the existence of trap states, the device response time is found to be slower, which is consistent with the analysis. The dimensionless photocurrent gain G is defined as the ratio between the number of electrons collected per unit time and the number of absorbed photons per unit time, which can be expressed by the following expression, Equation (8),

G=

|∆𝐼| 𝑃

×

ℎ𝜈

(8)

𝜂𝑞

, where |∆𝐼| is the photocurrent taken in absolute value, q is the electron charge, 𝑃 is the incident laser power. The quantum efficiency is assumed to be 1. The calculated photocurrent gain versus illumination power is shown in Figure 5b. The maximum gain measured is 584 under the power of 50 µW. As the excitation power is decreased, the photocurrent gain also increases. A similar analysis of photoresponsivity is useful to understand this phenomenon. Another description can be given by accounting different electric fields present in the composites. Because of the difference Fermi level of graphene and chlorophyll, there is a built-in electric field in the graphene/chlorophyll interface. After the illumination, electron and hole are generated in chlorophyll. Holes are transferred to the graphene and electrons get settled down in the opposite side on the chlorophyll. This phenomenon causes an additional electric field which is opposite to the built-in electric field. Then the effective built-in electric field can be described as the sum of the existing built-in electric field plus the additional electric field. The opposite sign of


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the additional field screens the existing built-in electric field. As the excitation power goes higher, the screening effect becomes more prominent acting against the built-in field at the interface. Hence, the carrier transfer becomes less efficient. Thus, we observed a gradual reduction of photoresponsivity and photocurrent gain with the enhancement of the excitation power. The flexibility of the device has been examined by recording the device performance under different bending radius as shown in Figure 6 with different curvature radius substrates as depicted in Figure S5. The obtained transient photocurrent change under different bending diameter is provided in Figure 6a. It is found that even the device is bended to the radius of 0.52 cm, the device still yields a significant photoresponse. The slight change of the photocurrent under bending condition is due to the fact that the bending of the device reduces the radiation density of incident photons on the active area for a particular power in compared with its flat counterpart. Figure 6b shows the evolution of photocurrent with the number of bending cycle, keeping the bending radius fixed at 0.52 cm, where a complete bending cycle is defined as bending the device up to a certain radius and returning it back to its original shape. The insignificant change in photoresponse of the device up to 100 times bending resembles the high stability of the device under the bending strain. To determine the stability of the chlorophyll, we cast the chlorophyll solution on a glass substrate and kept it under the continuous illumination of 656 nm laser over 30 minutes at the excitation power density of 2 mWcm-2 under ambient condition. We have recorded PL emission spectra and the UV-Vis absorption spectra before and after the illumination as shown in Figure S6. The consistence of the emission and absorption spectra of chlorophyll resembles the photostability of the material, which makes the device stable under the ambient conditions.


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Conclusion In summary, we have demonstrated an eco-friendly, transient and flexible photodetector consisting of chlorophyll, graphene, Mg electrodes and PVA as the device substrate. The device possesses several unique features including transient, soft, dissolvable, and biodegradable characteristics. This proof-of-concept demonstration of transient photodetector opened up a new window to address the global environmental concerns regarding the e-waste. It is foreseeable that transient devices will play a key role in next generation science and technology in order to establish a sustainable living earth. In view of the importance of the emerging challenge to overcome the massive electronic waste, our first attempt of transient photodetector shown here is very useful and timely. Moreover, plant derived light harvesting layer, chlorophyll used here is yet another example for the realization of green optoelectronic devices.


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Methods PVA Substrate Synthesis: The first step of our device fabrication involves the successful synthesis of smooth PVA substrate. First of all, the solid Poly(vinyl alcohol) grains were dissolved into deionized water with a mass fraction of 4 wt% and stirred for 30 min at room temperature. A silicon/silicon dioxide substrate (1.5 cm × 1.5 cm) with 150 nm silicon dioxide was prepared and cleaned for 10 min in DI water, acetone, and ethanol to remove any contaminant on top of the substrates. Then, we used dropper to cast 1 mL PVA solution on SiO2 substrate to cover it fully. The substrate with PVA solution was kept on a smooth horizontal surface of a hotplate and heated it at 65 °C until the deionized water in PVA solution was evaporated completely. As a result, a smooth substrate of PVA was achieved on SiO2 substrate shown in Figure S7. Another synthetic process has been used for bending experiment, PVA solution was poured into plastic petri dishes (diameter 10 cm) and oven-dried at about 65 °C for 2 hours. After baking, the sample was removed from oven and then cooled to ambient temperature. The dried film shown in Figure S8 can be easily peeled from the petri dish and can be used to proceed for the next step of the device design. As a proof of concept, the device can be placed on any arbitrary substrate, which does not show any significant changes in device performance. Graphene Synthesis: We used standard CVD method to prepare single layer graphene. To achieve better performance of graphene, 99.98% copper foils were used. To avoid graphene getting worse quality, the copper foils were immersed in 85% H3PO4 and polished by an electropolishing method with 1.4 V for 20 min before deposition. The polished copper foil was placed in the CVD furnace and annealed at 1000 °C with 60 sccm H2 flow for 60 min. Then methane was then flown at 3.5 sccm for next 30 min along with 60 sccm of H2 flow at 1000 °C. Afterward, the CVD system was


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cooled to room temperature. This process forms a single layer of graphene on copper foils on the both sides. We used the top side graphene on the copper foil. Electrical Characteristics Measurement: For the photocurrent measurement, 656 nm red light laser is used as the excitation light source. Keithley 236 and 2410 are used to supply DC voltage and record the photocurrent. Device Fabrication: To transfer graphene on PVA substrates, we spin-coated Poly(methyl methacrylate) (PMMA) on the graphene-copper composite. Then, it was floated on 1 M FeCl3 solution to etch the Cu. After etching completely, graphene/PMMA was transferred to DI water to wash out the remaining FeCl3. Because PVA dissolves in water, we have used absolute ethanol to transfer the graphene/PMMA composite on PVA. Then, we washed out PMMA from graphene by keeping the samples in acetone vapor for 20 minutes and immersing it in acetone at 65 °C for 20 minutes. G200 Cu mesh of Electron Microscopy Science was used as a shadow mask to make the electrodes on the graphene/PVA sample. Because of the grid pattern, we can make a 35 µm × 90 µm device with 35 µm channel length. After masking the device, we used thermal evaporator to deposit 5 nm of Ti and 40 nm of Au. In order to demonstrate the dissolvable device, we used 5 nm of Ti and 300 nm of Mg electrodes. The titanium layer used in between magnesium and the PVA substrate improves the strength and the adhesion.12 Finally, the copper mask was taken out and chlorophyll a (extracted from spinach, Sigma–Aldrich Co.) solution (0.02 mg L-1) was spin-coated in the optimized speed of 5000 rpm on the device. The chlorophyll a solution in cuvette has been measured to have similar absorption under continuous excitation with 656 nm laser (3 mW cm−2) for 30 minutes as shown in Figure S6. The variation of the spinning speed is shown in Figure S9. The fabrication process of the device is illustrated in Scheme 1.


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Scheme 1. Schematic diagram of the fabrication process of transient photodetectors.


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Figure 1. (a) Schematic diagram of the transient photodetector. (b) Illustration of dissolving process. (c) Demonstrations for the dissolving process of transient photodetector. Device with no deionized water on it showing the original status of photodetector. After 2 minutes, PVA substrate starts shrinking. After 4 minutes, the edges of the electrodes start shrinking. Then, the electrodes are separated from the PVA substrate and there are a few electrodes dissolved.


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Figure 2. (a) Absorption spectrum of chlorophyll a. (b) Photoluminescence spectrum of chlorophyll a. (c) Raman spectrum of photodetector with and without chlorophyll a.


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Figure 3. (a) Dependence of current versus voltage characteristics of the photodetector on pumping laser power. (b) Temporal photocurrent response of the photodetector under different laser power, where τ is the response time of the device. The temporal response indicates that the higher pumping power gets the shorter response time and the higher photocurrent.


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Figure 4. (a) Dirac point shift under three conditions, including pure graphene, graphene with chlorophyll with and without illumination. (b) Schematic band diagram of graphene. (c) Band diagram after spin-coating chlorophyll. (d) Band diagram with the illumination.


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Figure 5. (a) Photoresponsivity versus different excitation power. The inserted picture is the optical microscopy images of the device. The working area is 35 µm × 90 µm in the middle of two electrodes. (b) Photocurrent gain versus illumination power. The solid curve is the theoretical plot with the best fitting.


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Figure 6. (a) Photoresponse under different bending curvature. (b) Photocurrent change under different bending cycles.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. (PDF) A variation of PVA slab’s weight; The optical image of graphene; The variation of response time of the device with different input power; The pictures of devices and materials; A phototransistor is designed with patterned electrodes; Dependence of photocurrent change on chlorophyll thickness for photodetector device; The pictures of devices and materials. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Present Addresses ¶J Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-18223 Prague 8, Czechia ‡

Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan

§Advanced Research Center for Green Materials Science and Technology, National Taiwan University, Taipei 10617, Taiwan Author Contributions S. Y. L. and G. H. contributed equally to this work. S. Y. L. and G. H. analyzed the data and wrote the manuscript. S. Y. L., C. H. C., Y. R. L., and Y. F. H. developed the CVD-graphene fabrication methods. S. Y. L. and Y. M. L. conceived the idea. W. J. L., C. Y. S. and T. C. C. helped solve the


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technical problem. H. I. L. discussed the results and commented on the manuscript. T. Y. L. and Y. F. C. supervised the project and conceived the study. All authors accepted the final version of the manuscript. ⊥These authors contributed equally. Funding Sources This work was supported by the “Advanced Research Center for Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 107-3017-F-002-001).


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The table of contents Transient devices step into another stage for future science and technology, which play an important role for the development of a sustainable earth and have a wide variety of application, including biomedicine, environmental sensors and military use. In this work, we demonstrate the first dissolvable and flexible hybrid photodetector based on chlorophyll, graphene and poly(vinyl alcohol) with high performance.


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Transient and Flexible Photodetectors - ACS Applied Nano Materials

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