Highly Stretchable and Sensitive Photodetectors Based on Hybrid

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Highly Stretchable and Sensitive Photodetectors Based on Hybrid Graphene and Graphene Quantum Dots Chia-Wei Chiang, Haider Golam, Wei-Chun Tan, Yi-Rou Liou, YingChih Lai, Rini Ravindranath, Huan-Tsung Chang, and Yang-Fang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09373 • Publication Date (Web): 22 Dec 2015 Downloaded from http://pubs.acs.org on December 26, 2015

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ACS Applied Materials & Interfaces

Highly Stretchable and Sensitive Photodetectors Based on Hybrid Graphene and Graphene Quantum Dots

Chia-Wei Chiang†, Haider Golam †, Wei-Chun Tan†, Yi-Rou Liou†, Ying-Chih Lai†, Rini Ravindranath‡, Huan-Tsung Chang‡ and Yang-Fang Chen*†

†

‡

Department of Physics, National Taiwan University, Taipei 106, Taiwan

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan

*Corresponding author: [email protected]

Tel: +886-2-33665125, fax: +886-2-23639984

ABSTRACT: Stretchable devices possess a great potential in a wide range of application, such as bio-medical, wearable gadget, and smart skin, which can be integrated with human body. Due to their excellent flexibility, two-dimensional materials are expected to play an important role for the fabrication of stretchable devices. However, only a limited number of reports have been devoted to investigate stretchable devices based on two-dimensional materials, and the stretchabilities were restricted in a very small strain. Moreover, there is no report related to the stretchable photodetectors derived from two-dimensional materials. Herein, we demonstrate a

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highly stretchable and sensitive photodetector based on hybrid graphene and graphene quantum dots (GQDs). A unique rippled structure of PDMS is used to support the graphene layer, which can be stretched under an external strain far beyond published reports. The ripple of the device can overcome the native stretchability limit of graphene, and enhance the carrier generation in GQDs due to multiple reflections of photons between the ripples. Our strategy presented here can be extended to many other material systems, including other two-dimensional materials. It therefore paves a key step for the development of stretchable electronics and optical devices. Keywords: Graphene; Quantum dots; photodetector; stretchable device; nanocomposites


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Introduction Flexible, stretchable, and deformable electronic devices have a promising potential in bio-medical, portable, and wearable devices, which can be widely applied on human body, biosensors, and textile electronics.1-8

Significant progress works have been reported, such as

stretchable transistors, light emitting diodes, and organic memories.9-16 However, most of the devices were fabricated on nanowires and meandering circuits assembled in elastic substrates.1 Only countable works on stretchable devices using two dimensional (2D) materials have been reported. Until now, there does not exist any work involving stretchable photodetectors based on 2D materials. Even though stretchable graphene transistor deposited on PDMS substrate has been reported by Lee et al.9 However, the device can only be stretched up to 6 % due to the limitation of native stretchability of pristine graphene. To overcome the limitation and protect structural deformation, an alternative design of mechanical engineering is needed while stretching the 2D materials. For example, using the rippled structure of graphene nanoribbon for flexible strain sensors has been demonstrated.17 However, the rippled graphene is partially suspended in air while fully released and is totally flat on PDMS while fully strained. In this way, the graphene Fermi-level is highly influenced by the substrate effect, making the change of the charge carrier conduction in different strain levels.18 Therefore, the stable substrate effect is necessary to build a highly sensitive device because the underlying thin membrane could serve as



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a firm foundation, contributing a mechanical strength to avoid many other possible side effects.

Graphene is a well-established 2D material with ultra-high mobility but poor absorption coefficient.19-20 A composite of graphene with a highly photon absorbing material is an excellent way to design sensitive photodetectors, which has recently been demonstrated on rigid substrates.21 Previously, our group has reported an ultra-high sensitive photodetector using the composite consisting of graphene and graphene quantum dots on a rigid substrate.22 Herein, we demonstrated a stable, stretchable and sensitive hybrid graphene and graphene quantum dots (GQDs) photodetector using PDMS membrane on top of the pre-strain 3M tape as a substrate. After the pre-strain is released, the rippled structure of PDMS can be used to support the graphene layer. Consequently, the fabricated device can be stretched far beyond all the reported values based on 2D materials9, because the stretchability of the device is derived from the rippled graphene which can avoid the limitation of its native stretchability. In addition, the rippled structure can also enhance the carrier generation in GQDs by multiple reflections of photons between the ripples. Therefore, the demonstration of the device on the unique rippled supporting membrane is a major advance in designing stretchable devices. The materials used in our device, including graphene, GQDs and PDMS, are nontoxic and stable, showing the promising potential of our devices in a variety of applications, including biomedical applications. Our approach shown here can be applied to many other substance systems, including 2D materials, and it


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should be able to open up a new avenue for the development of stretchable devices.

Results and discussion The illustration of the fabricating method is shown in Figure 1a. To design the rippled supporting layer, PDMS solution was spin-coated on the sacrificed copper sheet substrate followed by a heat treatment. A bilayer graphene sheet was transferred thereafter on the as-prepared PDMS membrane by the standard graphene transfer method.23 Sacrificed substrate was then etched by FeCl3 solution and PDMS/graphene membrane was transferred to the 25 % pre-strained 3M VHB double side tape as the final substrate. Gallium-Indium-Tin was chosen as the electrode to avoid the stress between electrodes and graphene while applying the external strain to the device. The electrodes were separated by about 200 µm. GQDs were used as the active material for photon harvesting, which were spin-coated on top of graphene. Afterwards, the tape was released, and a regular rippled structure of thin PDMS/graphene/GQD membrane was generated on top of the tape. The optical images of the rippled structure under 0 % strain and 25 % strain are shown in Figures 1b and 1c, respectively. The period of the ripple under 0 % strain is around 30 µm, and under 25 % strain it is around 37.5 µm.



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Figure1. (a) Schematic of fabrication diagram of rippled photodetector. (b) Rippled structure in released state. (c) Rippled structure under 25 % strain.

The transmission electron microscopy (TEM) image in Figure 2a shows that the size distribution of GQDs is between 7 to 10 nm. The absorption spectrum in Figure 2b (blue curve) indicates a strong absorption in UV range starting from 600 nm. The photoluminescence spectrum in Figure 2b (red curve) of GQDs was taken at room temperature using 325 nm HeCd laser as the excitation source, which shows a broad emission centered around 523 nm. The quality and layer number of graphene used were evaluated by analyzing its scattering Raman spectrum, as shown in Figure 2c. The absence of D band around 1,350 cm-1 shows that it is almost defect free. The ratio of G to 2D peak around 0.89 confirms that the graphene is bilayer as reported by Reina et al.24-25 The Raman scattering of bare PDMS confirms that the peaks at 1,270 cm-1 and 1,413 cm-1 are the Raman signals of PDMS.



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Figure2. (a) TEM image of graphene quantum dots (GQDs). (b) Absorption and photoluminescence spectra of GQDs. (c) Micro Raman spectrum of graphene on PDMS and pure PDMS. (d) Schematic diagram of the hybrid photodetector device measurement set up.

The device performance was measured by using 325 nm UV laser as the excitation source, as shown in Figure 2d. The initial laser beam was focused to a spot with radius 100 µm, which was adjusted to fit the channel length while we strained the device. It would become 125 µm


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under 25 % strain. The adjustment of the spot size is used to make sure all the GQDs in the channel

can

absorb

the

incident

laser

illumination.

The

dynamic

photocurrent

(|߂‫ܫ| = |ܫ‬௜௟௟௨௠௜௡௔௧௜௢௡ − ‫ܫ‬ௗ௔௥௞ |) in response to different strains was studied under the illumination of 250 nW laser power as shown in Figure 3a. To verify the stability of the device, we checked the device performance by repeatedly applying strain multiple times as shown in Figure 3b. Here, a complete strain cycle is defined by changing strain from 0 % to 20 %, and then the device is released to 0 % strain. All the measurements were taken under the released state (0 % strain) in the beginning and after different cycles. The device performance remained intact after 30 cycles. The device performance was estimated by studying the |ΔI| versus Vsd measurement under different strains with an illumination power of 50 nW as shown in Figure 3c. The dimensionless photocurrent gain G was calculated using the equation26

‫=ܩ‬

|∆୍| ௤

ௐ

ଵ

÷ ௛ఔ × ொா,

(1)

Here, |∆ࡵ| is the photocurrent taken in absolute value, q is the elementary charge, W is the incident laser power, hν is the energy per photon and QE is the quantum efficiency of the charge carrier generated by per unit photon. To simplify the comparison between strains, we assume QE to be 1. The calculated photocurrent gain was shown in Figure 3d, which clearly indicates that the photocurrent gain decreases with increasing strain. This unique feature can be understood as



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follows. When the height of the ripple decreases with the increase of strain, it will cause less multiple reflections of photons within the ripples, as illustrated in Figure 3e. As a result, GQDs absorbs less photons, which causes a decrease in photocurrent. The most obvious decrease of photocurrent at 25 % strain is due to the crack inside the graphene film as the pre-strain is only up to 25 %. The possible reason behind the stability of the device is the mechanical support of PDMS membrane to the graphene layer, which prevents the graphene from behaving as a free standing graphene with random folds after the strain was released.


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Figure 3. (a) Dynamic photoresponse under different strain. (b) Fatigue test of 1st, 10th and 30th times strain. (c) The photocurrent gain under different strain condition. (d) Dependence of



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photocurrent gain on the external strain. (e) Schematic of multiple reflections of incident light arising from the rippled structure.

The dark conductivity under different strains of the rippled graphene is shown in Figure 4a. The dark conductivity of the graphene increases with increasing strain, showing a maximum conductivity at 25 % strain, which is the pre-strain level. For over than 25 % strain, the graphene layer starts to experience the over pre-strain level, and the conductivity decreases with increasing strain. The increase in the conductivity with decreasing the ripple height under the increase of strain can be explained as scattering effect of charge carriers in the ripple structured graphene film.27 The excess resistivity induced by the rippled structure is related to the ripple height and radius by the equation:

௛

ߜߩ ≈ ସ௘ మ

௭ర ோ మ ௔మ

,

(2)

where, a, z and R are the lattice parameter of graphene, characteristic height and the radius of the ripple, respectively. The radius of the rippled structure increases while the height decreases with applied strain in the device. Hence, the resistance decreases nonlinearly with increasing strain according to the equation. The detailed calculation is shown in the supporting information. Theoretically calculated data is plotted in Figure 4a (blue curve) which is consistent with the experiment results (red curve) up to 25 % strain. After 25 % strain, graphene begins to crack


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which is the main reason responsible for the reduction of the conductivity. This result provides an excellent evidence to support the electron scattering phenomenon in rippled graphene. A similar result has been obtained by Wang et. al.18 Thus, the demonstration of our rippled structure can easily avoid the buckled graphene with cracking and overlaping after the pre-strain is released as shown in previous reports.18 We therefore can infer that the decreased photocurrent gain for the strain below 25 % mainly arises from the fact of less photon absorbance by GQDs, instead of the crack of graphene. The drastic decrease of the conductivity beyond 30 % strain can be attributed to the appearance of micro-crack inside the graphene film.

The photodetector performance has also been examined by the measurement of Isd and Vsd under different illumination powers. The conductance measurements have been carried out under the released state (0 % strain) at different illumination power, as shown in Figure 4b. The calculated photocurrent gain as a function of input power is shown in Figure 4c. A maximum photocurrent gain of 2.8 × 10ଷ can be achieved under 12.5 nW illumination power, and the photocurrent gain decreases with increasing excitation power. The photoresponsivity, Rph which is defined as

ܴ௣௛ =

௱ூ(஺) ௉(ௐ)

.

(3)

The calculated result is shown in Figure 4d, which is in good agreement with the trend shown in Figure 4c and consistent with the result reported by Golam et. al.28 Note that the photocurrent



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gain can be further enhanced by reducing channel length or decreasing laser power until it reaches the saturation region.

Figure 4. (a) The experimental and theoretical results of conductivity ratio versus different strains (b) Photoresponse of graphene / graphene quantum dots (GQDs) under 0 % strain. (c) Log-scale of photocurrent gain versus power. (d) Log-scale of photoresponsivity versus illumination power.


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In order to interpret the observed result, the shift of the Dirac point of the graphene layer under different conditions have been measured as shown in Figure 5a, in which SiO2 / p-type Si was used as the substrate. It is found that after coating the GQDs on graphene, the device shows a higher conductivity than that of pure graphene and the Dirac point shifts toward a larger Vg. UV illumination on graphene/GQDs device results in a decrease of conductivity (red curve), and causes a left shift of the Dirac point compared with that without illumination. According to the result shown in Figure 5a, energy band diagrams of the hybrid photodetector under different conditions are plotted in Figure 5b. CVD grown graphene and GQDs are both p-type in ambient condition.29-30 After coating GQDs on graphene, holes will transfer from GQDs to graphene, which makes the higher conductivity in graphene layer and causes the right shift of the Dirac point. Hence, the built-in electric field generated in the graphene-GQDs interface due to the charge transfer causes a downward band bending of the GQDs. After the illumination of UV light, electron-hole pairs are generated. The photogenerated electrons will transfer to the underlying graphene layer due to the built-in electric field, which will cause the reduction of the number of holes and decrease the conductivity in the graphene layer. The nature of the increased photocurrent gain with decreasing laser power can be understood by the screening of the built-in electric field due to the separation of the photoexcited electrons and holes. Because the built-in electric field decreases with increasing illumination power, the capability to transfer



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photogenerated electrons from GQDs to graphene is reduced, and therefore the photocurrent decreases.

Figure 5. (a) Dirac point shift under different conditions, including pure graphene without graphene quantum dots (GQDs) and without illumination (Graphene/Dark), GQDs deposited on graphene without illumination (Graphene/GQDs/Dark), and GQDs deposited on graphene with illumination (Graphene/GQDs/Light) (b) Schematic band diagram of graphene and GQDs and transport of photogenerated charge carriers.

Conclusion We therefore has successfully demonstrated a highly stretchable and sensitive photodetector by using a ripple-structured hybrid device consisting of graphene and GQDs. The ripple-structured photodetector featured a highly stable conductance characteristics and


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photoresponsivity under different strain conditions. This rippled structure provides an excellent opportunity to achieve flexible materials possessing stretchablility. The stretchability endowing from the rippled structure can not only be used on the composite of graphene and GQDs, but also on many other substance systems, including 2D materials. We believe our new strategy presented here can pave a feasible way to develop different stretchable electronic and optical devices, such as highly stretchable 2D transistors, memories, light emitting diodes, and solar cells. Furthermore, both the materials and the fabrication process introduced here are eco-friendly and suitable for future industrial mass manufacturing.

Methods Graphene quantum dots synthesis According to our previous report published by our team before31, the graphene quantum dots (GQDs) solution was made from Neem leaf extract and ultrapure water. Neem leaf extract was obtained by grinding Neem leaves into powder and heated for 1 hour. And then the solution was put into centrifugation at centrifuge force of 12,000 g for 10 mins to remove big flakes. The collected solution was then filtered by 0.22 µm membrane to remove solid residues. After stirring and sonicating for 30 mins, the solution was baked in an autoclave at 300 0C for 8 hrs. After being cooled down, the sediment of the solution was separated out by centrifugation with the



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force of 25,000 g for 20 mins. The supernatant was collected and washed by ultrapure water twice. The solution was dialyzed with a filter (cutoff 3.5kDa) for 3 hours and dried overnight at 60 0C to get the GQDs solution.

Bilayer graphene synthesis Single layer graphene (SLG) sheet was prepared on copper foil by standard chemical vapor deposition method.32 To obtain good quality single layer graphene, 99.98 % copper foil was used. Polished by electro-polishing method, 85 % H3PO4 with 1.5 V for 20 mins was used to reduce the roughness. The polished copper was put into furnace baked at 1000 0C with 60 sccm hydrogen flow for the first 60 minutes. Then methane was flown at 3.5 sccm for 30 minutes at the same temperature. Keeping the same flow, the furnace was cooled down to room temperature. The illustration of the double layer graphene fabrication process is shown in Figure S1. The bilayer graphene was combined with two CVD graphene with copper as growth substrate. 6 % polymethylmethacrylate (PMMA) in anisole solution was then spin coated on the first graphene-copper sheet as a carrier film. After drying, the sample was put in 1M FeCl3 solution to etch the copper sacrificed layer. Then, the PMMA-graphene film was transferred to the second graphene-copper sheet as a target substrate to obtain bilayer graphene (BLG). In this method, there are no resident PMMA between two graphene layers. Note that the two graphene layer used in our devices is to avoid the Poisson’s effect, which can easily induce the crack in the


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monolayer graphene film.

PDMS membrane synthesis PDMS solution was purchased from Dow Corning sylgard 184, and mixed with cross-linking solution with the ratio of 10. After that, the solution was spin-coated on the sacrificed layer with speed 7000 rpm

Measurement setup All the electrical characterizations of the stretchable photodetectors were measured by using Keithley 236 source measurement unit purchased from Keithley. Keysight 4156c semiconductor parameter was purchased from Keysight, and 325 nm HeCd UV laser was purchased from Kimmon Koha.

Acknowledgements This work was supported by the Ministry of Science and Technology and Ministry of Education of the Republic of China.

Author Contributions Y. F. C. planned the project and supervised the overall project. C.W.C designed the experiments, fabricated the device and performed electrical and optical measurements. C. W. C, G.H and T. W. C grew the graphene. R. R. synthesized the GQD. Y.C.L gave technical and



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conceptual advice. Y. R. L., H.C. and C.W.C. measured the PL emission and absorption. Y. F. C. and C.W.C. analyzed the data and wrote the manuscript. All authors discussed and commented on the manuscript

Supporting Information The detailed bilayer graphene synthesis method and the calculation and approximation of the scattering electron in rippled graphene were supplied as Supporting Information.


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Highly Stretchable and Sensitive Photodetectors Based on Hybrid

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