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Highly Stretchable and Sensitive Photodetectors Based on Hybrid Graphene and Graphene Quantum Dots Chia-Wei Chiang,† Golam Haider,† Wei-Chun Tan,† Yi-Rou Liou,† Ying-Chih Lai,† Rini Ravindranath,‡ Huan-Tsung Chang,‡ and Yang-Fang Chen*,† †
Department of Physics and ‡Department of Chemistry, National Taiwan University, Taipei 106, Taiwan S Supporting Information *
ABSTRACT: Stretchable devices possess great potential in a wide range of applications, such as biomedical and wearable gadgets and smart skin, which can be integrated with the human body. Because of their excellent flexibility, twodimensional (2D) materials are expected to play an important role in the fabrication of stretchable devices. However, only a limited number of reports have been devoted to investigating stretchable devices based on 2D materials, and the stretchabilities were restricted in a very small strain. Moreover, there is no report related to the stretchable photodetectors derived from 2D materials. Herein, we demonstrate a highly stretchable and sensitive photodetector based on hybrid graphene and graphene quantum dots (GQDs). A unique rippled structure of poly(dimethylsiloxane) 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 2D materials. It therefore paves a key step for the development of stretchable electronics and optical devices. KEYWORDS: graphene, graphene quantum dots, photodetector, stretchable device, nanocomposites
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conduction at 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 a firm foundation, contributing mechanical strength to avoid many other possible side effects. Graphene is a well-established 2D material with ultrahigh mobility but a 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 reported an ultrahighly sensitive photodetector using the composite consisting of graphene and graphene quantum dots (GQDs) on a rigid substrate.22 Herein, we demonstrate stable, stretchable, and sensitive hybrid graphene and GQD photodetectors using a PDMS membrane on top of the prestrain 3 M tape as a substrate. After the prestrain 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 of 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
INTRODUCTION Flexible, stretchable, and deformable electronic devices have a promising potential in biomedical, portable, and wearable devices, which can be widely applied on the human body, biosensors, and textile electronics.1−8 Significant progress works have been reported, such as stretchable transistors, lightemitting 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 a stretchable graphene transistor deposited on a poly(dimethylsiloxane) (PDMS) substrate has been reported by Lee et al.,9 the device can only be stretched up to 6% because of the limitation of the native stretchability of pristine graphene. To overcome the limitation and protect the structural deformation, an alternative design of mechanical engineering is needed while the 2D materials are stretched. For example, using the rippled structure of a 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, causing a change of the charge carrier © 2015 American Chemical Society
Received: October 3, 2015 Accepted: December 22, 2015 Published: December 22, 2015 466
DOI: 10.1021/acsami.5b09373 ACS Appl. Mater. Interfaces 2016, 8, 466−471
Research Article
ACS Applied Materials & Interfaces
The period of the ripple under 0% strain is around 30 μm, and under 25% strain, it is around 37.5 μm. The transmission electron microscopy (TEM) image in Figure 2a shows that the size distribution of GQDs is between 7 and 10 nm. The absorption spectrum in Figure 2b (blue curve) indicates a strong absorption in the UV range starting from 600 nm. The photoluminescence spectrum in Figure 2b (red curve) of GQDs was taken at room temperature using a 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 a D band around 1350 cm−1 shows that it is almost defectfree. The ratio of G-to-2D peaks of around 0.89 confirms that the graphene is a bilayer, as reported by Reina et al.24,25 The Raman scattering of bare PDMS confirms that the peaks at 1270 and 1413 cm−1 are the Raman signals of PDMS. The device performance was measured by using a 325 nm UV laser as the excitation source, as shown in Figure 2d. The initial laser beam was focused to a spot with a radius of 100 μm, which was adjusted to fit the channel length while the device was strained. It became 125 μm under 25% strain. The adjustment of the spot size is used to make sure all of the GQDs in the channel can absorb incident laser illumination. The dynamic photocurrent (|ΔI| = |Iillumination − Idark|) in response to different strains was studied under an 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 a change in strain from 0% to 20%, and then the device is released to 0% strain. All of 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
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 the design of 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 should be able to open up a new avenue for the development of stretchable devices.
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RESULTS AND DISCUSSION The illustration of the fabricating method is shown in Figure 1a. To design the rippled supporting layer, a PDMS solution was
G= Figure 1. (a) Schematic of the fabrication diagram of a rippled photodetector. (b) Rippled structure in the released state. (c) Rippled structure under 25% strain.
|ΔI | W 1 ÷ × q hv QE
(1)
Here, |ΔI| 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 per unit photon. To simplify the comparison between strains, we assume QE to be 1. The calculated photocurrent gain is shown in Figure 3d, which clearly indicates that the photocurrent gain decreases with increasing strain. This unique feature can be understood as follows. When the height of the ripple decreases with an increase of strain, it will cause fewer multiple reflections of photons within the ripples, as illustrated in Figure 3e. As a result, GQDs absorb fewer photons, which causes a decrease in the photocurrent. The most obvious decrease of the photocurrent at 25% strain is due to the crack inside the graphene film because the prestrain is only up to 25%. The possible reason behind the stability of the device is the mechanical support of the 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. The dark conductivity under different strains of the rippled graphene is shown in Figure 4a. The dark conductivity of the
spin-coated onto the sacrificed copper sheet substrate, followed by heat treatment. A bilayer graphene (BLG) sheet was transferred thereafter on the as-prepared PDMS membrane by the standard graphene transfer method.23 The sacrificed substrate was then etched by a FeCl3 solution, and the PDMS/graphene membrane was transferred to the 25% prestrained 3 M VHB double-sided tape as the final substrate. Gallium−indium−tin was chosen as the electrode to avoid the stress between electrodes and graphene while the external strain is applied 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. Afterward, the tape was released, and a regular rippled structure of a thin PDMS/graphene/GQD membrane was generated on top of the tape. The optical images of the rippled structure under 0% and 25% strain are shown in parts b and c of Figure 1, respectively. 467
DOI: 10.1021/acsami.5b09373 ACS Appl. Mater. Interfaces 2016, 8, 466−471
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ACS Applied Materials & Interfaces
Figure 2. (a) TEM image of 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 setup.
25% strain, the graphene layer starts to experience the overprestrain level, and the conductivity decreases with increasing strain. The increase in the conductivity with decreasing ripple height under the increase of strain can be explained as the 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 δρ ≈
h z4 4e 2 R2a 2
(2)
where a, z, and R are the lattice parameter of graphene, characteristic height, and radius of the ripple, respectively. The radius of the rippled structure increases while the height decreases with the 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 are plotted in Figure 4a (blue curve), which is consistent with the experimental results (red curve) up to 25% strain. After 25% strain, graphene begins to crack, which is the main reason responsible for the reduction of the conductivity. This result provides 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 overlapping after the prestrain 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 that there is 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 a microcrack inside the graphene film. The photodetector performance was also examined by the measurement of Isd and Vsd under different illumination powers.
Figure 3. (a) Dynamic photoresponse under different strains. (b) Fatigue test of the 1st, 10th, and 30th times strain. (c) Photocurrent gain under different strain conditions. (d) Dependence of the photocurrent gain on the external strain. (e) Schematic of multiple reflections of incident light arising from the rippled structure.
graphene increases with increasing strain, showing a maximum conductivity at 25% strain, which is the prestrain level. Over 468
DOI: 10.1021/acsami.5b09373 ACS Appl. Mater. Interfaces 2016, 8, 466−471
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Experimental and theoretical results of the conductivity ratio versus different strains. (b) Photoresponse of graphene/GQDs under 0% strain. (c) Logarithmic scale of photocurrent gain versus power. (d) Logarithmic scale of photoresponsivity versus illumination power.
Figure 5. (a) Dirac point shift under different conditions, including pure graphene without 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.
In order to interpret the observed result, the shift of the Dirac point of the graphene layer under different conditions was measured, as shown in Figure 5a, in which SiO2/p-type Si was used as the substrate. It was found that, after the GQDs were coated on graphene, the device showed a higher conductivity than that of pure graphene and the Dirac point shifted toward larger Vg. UV illumination on the graphene/ GQDs device resulted in a decrease of the conductivity (red curve) and causeed 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. Chemical vapor deposition (CVD)-grown graphene and GQDs are both of the p type in ambient conditions.29,30 After GQDs were coated on graphene, holes transferred from GQDs to graphene, which made the conductivity in the graphene layer higher and caused a right shift of the Dirac point. Hence, the built-in electric field generated in the graphene−GQDs interface due to
The conductance measurements were 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 the input power is shown in Figure 4c. A maximum photocurrent gain of 2.8 × 103 can be achieved under 12.5 nW illumination power, and the photocurrent gain decreases with increasing excitation power. The photoresponsivity Rph is defined as
R ph =
ΔI(A) P(W )
(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 Haider et al.28 Note that the photocurrent gain can be further enhanced by reducing the channel length or decreasing the laser power until it reaches the saturation region. 469
DOI: 10.1021/acsami.5b09373 ACS Appl. Mater. Interfaces 2016, 8, 466−471
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Note that the two graphene layers used in our devices are to avoid Poisson’s effect, which can easily induce the crack in the SLG film. PDMS Membrane Synthesis. The PDMS solution was purchased from Dow Corning Sylgard 184 and mixed with a cross-linking solution with a ratio of 10. After that, the solution was spin-coated on the sacrificed layer at a speed of 7000 rpm. Measurement Setup. All of the electrical characterizations of the stretchable photodetectors were measured by using a Keithley 236 source measurement unit. A Keysight 4156c semiconductor parameter was purchased, and a 325 nm HeCd UV laser was purchased from Kimmon Koha.
the charge transfer caused a downward band bending of the GQDs. After UV-light illumination, electron−hole pairs were generated. The photogenerated electrons transferred to the underlying graphene layer because of the built-in electric field, which caused a reduction of the number of holes and a decrease of 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 of transferring photogenerated electrons from GQDs to graphene is reduced, and therefore the photocurrent decreases.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09373. Detailed BLG synthesis method and calculation and approximation of the scattering electron in rippled graphene (PDF)
CONCLUSION We therefore have successfully demonstrated a highly stretchable and sensitive photodetector by using a ripplestructured hybrid device consisting of graphene and GQDs. The ripple-structured photodetector featured highly stable conductance characteristics and photoresponsivity under different strain conditions. This rippled structure provides an excellent opportunity to achieve flexible materials possessing stretchablility. The stretchability endowed from the rippled structure can be used not only 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 path to developing different stretchable electronic and optical devices, such as highly stretchable 2D transistors, memories, light-emitting diodes, and solar cells. Furthermore, both the materials and fabrication process introduced here are ecofriendly and suitable for future industrial mass manufacturing.
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ASSOCIATED CONTENT
S Supporting Information *
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +886-2-33665125. Fax: +886-2-23639984. 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, H.G., and W.-C.T. grew the graphene. R.R. synthesized the GQDs. Y.-C.L. gave technical and conceptual advice. Y.-R.L., H.-T.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. Notes
The authors declare no competing financial interest.
METHODS
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GQDs Synthesis. According to our previous report published by our team before,31 the GQDs solution was made from Neem leaf extract and ultrapure water. Neem leaf extract was obtained by grinding Neem leaves into powder and heating for 1 h. Then the solution was centrifuged at a centrifugal force of 12000g for 10 min to remove big flakes. The collected solution was then filtered by a 0.22 μm membrane to remove solid residues. After stirring and sonicating for 30 min, the solution was baked in an autoclave at 300 °C for 8 h. After cooling, the sediment of the solution was separated out by centrifugation with a force of 25000g for 20 min. The supernatant was collected and washed by ultrapure water twice. The solution was dialyzed with a filter (cutoff 3.5 kDa) for 3 h and dried overnight at 60 °C to get the GQDs solution. BLG Synthesis. A single-layer-graphene (SLG) sheet was prepared on copper foil by a standard CVD method.32 To obtain good quality SLG, 99.98% copper foil was used. Polished by an electropolishing method with 1.5 V for 20 min, 85% H3PO4 was used to reduce the roughness. The polished copper was put into a furnace and baked at 1000 °C with 60 sccm hydrogen flow for the first 60 min. Then methane was flowed at 3.5 sccm for 30 min at the same temperature. Keeping the same flow, the furnace was cooled to room temperature. The illustration of the double-layer-graphene fabrication process is shown in Figure S1. The BLG was combined with two CVD-grown graphenes with copper as the growth substrate. A total of 6% poly(methyl methacrylate) (PMMA) in an anisole solution was then spin-coated on the first graphene−copper sheet as a carrier film. After drying, the sample was put in a 1 M 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 the BLG. In this method, there is no resident PMMA between two graphene layers.
ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology and Ministry of Education of the Republic of China.
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