Letter pubs.acs.org/NanoLett
Multifunctional Graphene Optoelectronic Devices Capable of Detecting and Storing Photonic Signals Sukjae Jang,†,∥ Euyheon Hwang,†,‡,∥ Youngbin Lee,† Seungwoo Lee,†,§ and Jeong Ho Cho*,†,§ †
SKKU Advanced Institute of Nanotechnology (SAINT), ‡Department of Physics, and §School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea S Supporting Information *
ABSTRACT: The advantages of graphene photodetectors were utilized to design a new multifunctional graphene optoelectronic device. Organic semiconductors, gold nanoparticles (AuNPs), and graphene were combined to fabricate a photodetecting device with a nonvolatile memory function for storing photonic signals. A pentacene organic semiconductor acted as a light absorption layer in the device and provided a high hole photocurrent to the graphene channel. The AuNPs, positioned between the tunneling and blocking dielectric layers, acted as both a charge trap layer and a plasmonic light scatterer, which enable storing of the information about the incident light. The proposed pentacene-graphene-AuNP hybrid photodetector not only performed well as a photodetector in the visible light range, it also was able to store the photonic signal in the form of persistent current. The good photodetection performance resulted from the plasmonics-enabled enhancement of the optical absorption and from the photogating mechanisms in the pentacene. The device provided a photoresponse that depended on the wavelength of incident light; therefore, the signal information (both the wavelength and intensity) of the incident light was effectively committed to memory. The simple process of applying a negative pulse gate voltage could then erase the programmed information. The proposed photodetector with the capacity to store a photonic signal in memory represents a significant step toward the use of graphene in optoelectronic devices. KEYWORDS: Graphene, nonvolatile memory, photodetector, plasmonic, photogating, flexible optoelectronics
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including high carrier density, high carrier mobility, wavelength-independent light absorption, and broad spectral bandwidth. Even if the weak light absorption cross-section (2.3% saturable absorption), low gain mechanism, and fast recombination rate in graphene have limited the use of pristine graphene in practical optoelectronic applications,20 several efforts have attempted to address these weaknesses, and significant progress has been made. For example, graphene has been coupled to optical structural motifs, such as Fabry− Perot microcavities,21,22 plasmonic gratings,23 and nanoantennas,24−28 to enhance the light−matter interactions and exceed graphene’s saturable absorption profile. Graphene photodetectors based on vertical p−n graphene junctions have also been reported by Marcus et al.29 Meanwhile, graphene may potentially be integrated with other active components of devices, including colloidal quantum dots (QDs),30 organic dyes,31 and perovskites,32 by depositing these components on top of a graphene layer to improve the efficiency of a photodetector’s light absorption and spectral selectivity. To date, nonvolatile memory functionalities fabricated using graphene-based flexible/wearable photodetectors have not yet been successfully demonstrated. Such
hotonic signal storage can potentially enable all-optical logic processing,1−3 ensure the confidential security of 4,5 data, and enable quantum information processing.6−9 The persistent storage of photonic signals, along with photosensitivity across the visible range, could be critical for the development of wearable photosensitive skin with the capacity for measuring accumulated photon absorption.10,11 The conversion of photonic signals into electron−hole pairs and the storage of these electronic systems in semiconductor-based memory cells (collectively referred to as photodetectors or photonic memory) have shown some progress over the past decade;12−15 nevertheless, much of this advancement has been applied toward improving the detection sensitivity using a proof-of-concept device, rather than performing systematic studies of nonvolatile storage. A set of limitations, including the relatively small device area,16,17 narrow spectral range at visible frequencies of interest,18 and the incompatibility of inorganic semiconductor processing methods to flexible/stretchable substrates19 present obstacles to the practical application of photonic memory devices. Rapid growth in the field of graphene-based technology could advance photonic memory devices by exploiting graphene’s exceptional optoelectronic properties and developing new device architectures. Graphene has attracted considerable attention in next-generation high-performance optoelectronic devices due to its exceptional properties, © XXXX American Chemical Society
Received: January 9, 2015 Revised: March 25, 2015
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DOI: 10.1021/acs.nanolett.5b00105 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic diagram of the device structure comprising a graphene photodetector with a pentacene light absorption layer and an AuNP charge trapping layer. In the device, the ITO acts as a gate electrode and Au as source/drain electrode. (b) Photographic image of the graphene photodetector fabricated on a PEN substrate. (c) Vis−NIR absorption spectra of the graphene and pentacene−graphene hybrid films. (d) Optical microscopy image (upper panel) and integrated PL intensity map (lower panel) of the pentacene films deposited onto SiO2 and graphene. (e) Representative PL spectra of the pentacene films deposited onto SiO2 or graphene substrates.
of incident light, the signal information about the incident light (both the wavelength and intensity) was effectively memorized. The simple process of applying a negative pulse gate voltage could erase the programmed information. The proposed photodetector with a photonic signal memory function represents a significant step toward the application of graphene to a variety of optoelectronic devices. Note that in addition to the flexibility and transparency, the usage of 2D materials (e.g., graphene, MoS2, etc.) as an active channel is very important in the device architecture because the charge transferred into graphene from pentacene must be accessible to the charge trapping layer (i.e., AuNPs) via tunneling. Figure 1a shows a schematic diagram of the device structure of a flexible graphene photodetector with a nonvolatile memory functionality. The pentacene layer deposited onto the graphene channel was used as the light absorption layer, whereas the gold nanoparticles (AuNPs) inserted between the blocking and tunneling dielectrics served as a charge trapping layer (Figure S2). An indium tin oxide (ITO)-coated polyethylene naphthalate (PEN) film was used as the flexible substrate, and the ITO acted as a gate electrode. A 30 nm thick Al2O3 blocking dielectric was deposited onto the substrate by atomic layer deposition (ALD). AuNPs with an average diameter of 6.7 nm were then deposited thermally. A 19 nm thick cross-linked poly(4-vinylphenol) (cPVP) tunneling dielectric layer was formed on the AuNP/Al2O3 surface by spin-casting a PVP solution containing 1 wt % PVP (Mw = 20 000 g mol−1) and 0.5 wt % poly(melamine-co-formaldehyde) (PMF, Mw = 511 g mol−1) in propylene glycol monomethyl ether acetate (PGMEA). The film was then thermally annealed at 120 °C for 12 h in a vacuum oven. Separately, single-layer graphene was synthesized using the chemical vapor deposition (CVD) method on a Cu foil and was patterned by photolithography and oxygen plasma etching. A poly(methyl methacrylate) (PMMA) supporting layer was then spin-coated onto the patterned graphene/Cu substrate. The Cu layer was wet-etched using an aqueous ammonium persulfate solution. The patterned graphene was transferred to the
photodetectors generally cannot maintain a high photosensitivity across the visible range, which is of practical importance to high-frequency photonic signaling and the development of wearable photosensitive skin for healthcare. Strategies described previously for preparing high-performance graphene photodetectors have relied heavily on complex manipulation or fabrication of active materials; the quantity of an overlying active component adsorbed onto a graphene layer cannot typically be controlled systematically. The development of versatile but efficient fabrication methods is important for further advances in graphene-based photodetectors with nonvolatile memory functions. This letter describes a new multifunctional graphene optoelectronic device that exhibits excellent photodetection and nonvolatile memory storage of a photonic signal. In the device, a pentacene organic semiconductor (optical band gap of 1.76 eV)33 acts as a light absorption layer, while gold nanoparticles (AuNPs) serve as both a charge trapping layer and a plasmonic light scattering layer to store information about the incident light. The proposed hybrid photodetector displayed a strong photodetection response across the visible range and was capable of storing the photonic signal over long periods of time. Upon illumination with visible light, the pentacene layer absorbed the light and generated electron−hole pairs. The photoinduced hole carriers in the valence band of pentacene were transferred to the graphene layer, and consequently, the effective negative gate voltage is applied to graphene, which induced a hole current in the graphene channel through capacitive coupling (see Figure S1). The resulting pentacene−graphene hybrid photodetector exhibited a photoresponsivity of 700 A/W, a photodetectivity of 1013 Jones, and a broad spectral bandwidth across the UV−visible range. The high performances of our hybrid photodetector were attributed to efficient charge transfer from graphene to the pentacene together with the enhanced light absorption of the pentacene layer, which was boosted by the plasmonic light scattering of the AuNPs. Because the device displayed a characteristic photoresponse that depended on the wavelength B
DOI: 10.1021/acs.nanolett.5b00105 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 2. (a) Hysteresis loop of the transfer characteristics (VD = 0.1 V) of the graphene photodetector with a memory functionality under different illumination wavelengths at a fixed incident illumination power of 1 mW. (b) Photocurrents, at VG = 0 V, of the graphene photodetector as a function of the illumination wavelength. (c) Photoresponsivity (R) and photodetectivity (D*) vs the illumination source wavelength. (d) Numerically calculated electric field intensity (|E2|) within the pentacene layer of the hybrid photodetector prepared with or without AuNPs. (e) Two-dimensional spatial profile of |E2| of the hybrid photodetector with or without AuNPs.
preprepared cPVP layer on the plastic substrate, and the supporting PMMA layer was removed. The p-doped pristine graphene was n-type doped by treating the graphene channel with a poly(ethylene imine) (PEI) solution to shift the Dirac voltage (VDirac) toward negative voltage. Finally, a photoabsorbing pentacene layer with 100 nm thicknesses was thermally deposited onto the graphene channel through a shadow mask by organic molecular beam deposition (OMBD) methods. More details of experimental procedure are described in the Supporting Information. Atomic force microscopy topview images (Figure S3) of the pentacene−graphene hybrid film indicated that the pentacene layer was uniformly deposited onto the graphene layer. Figure 1b shows a photographic image of the graphene photodetector arrays assembled with the pentacene light absorption layer and the AuNP charge trapping layer on a plastic substrate. The device displayed good mechanical flexibility. Graphene displays a nearly uniform light absorption profile across the visible and infrared spectral ranges due to its energy band structure (i.e., saturable absorption). This feature of graphene has limited its applicability to optoelectric devices. Here, we introduced an organic light-absorbing layer with an appropriate band gap to improve the efficiency of visible light absorption and to transfer the generated charges to the graphene. A representative p-type organic semiconductor, pentacene, with an optical band gap of 1.76 eV, was directly deposited onto the graphene channel. Photons with energies that exceed the band gap (wavelength (λ) < 700 nm) can excite electrons from the valence band to the conduction band. The visible (Vis)−near-infrared (NIR) absorption spectra of the pristine graphene and the pentacene−graphene hybrid systems are shown in Figure 1c. The light absorption profile below 700
nm (the pentacene band gap was 1.76 eV) was improved dramatically due to the presence of the deposited pentacene layer. Figure 1d shows optical microscopy images (upper panel) and a local map of the integrated photoluminescence (PL) intensity (lower panel) of the pentacene films prepared on SiO2 or a graphene layer. Each data point in this PL map was collected by integrating the local PL signal over the range 620 to 740 nm. The PL quantum yield of the pentacene film deposited onto the graphene layer (dark color) was much lower than that of the pentacene film deposited onto the SiO2 (bright color). A histogram of the integrated PL intensity map is shown in Figure S4. The average PL spectra obtained across both regions (Figure 1e) exhibited a PL peak at around 682 nm corresponding to the pentacene band gap. The maximal PL intensity of the pentacene−graphene hybrid system was quenched by nearly 54% with respect to the corresponding value of the pentacene layer on SiO2 due to effective charge carrier transfer between the pentacene and graphene layers. The photoinduced electron−hole pairs that formed in the pentacene layer recombined within the lifetime of the photoexcited electrons to increase the PL peak corresponding to the pentacene band gap. By contrast, upon contact with graphene, electrons in graphene were transferred to the proximal pentacene layer to fill the empty states in the pentacene valence band because the chemical potential of graphene was higher than the valence band of the pentacene (Figure S1). The pentacene energy states available to facilitate photoexcited electron−hole pair recombination were limited. As a consequence, the photoexcited electrons in the pentacene remained in the conduction band because there are no available density of states in the valence band for recombination. Photoexcited electron trapping provided the main mechanism C
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Figure 3. (a) Hysteresis loop of the transfer characteristics (VD = 0.1 V) of the graphene photodetector with a memory functionality under different illumination powers at a fixed wavelength of 520 nm. (b) Photocurrents at VG = 0 V of the graphene photodetector as a function of the illumination power. (c) Photoresponsivity (R) and photodetectivity (D*) vs illumination power.
through capacitive coupling. In other words, the light illumination resulted in the p-type doping of the graphene channel. The charge trapping behavior in AuNPs under light illumination is analogous to the charge trapping in the floatinggate transistor memory based on p-doped graphene channel.34 As a result, the higher hole currents (compared with that under dark) were measured during the reverse sweep. The photocurrents Iph (Iillumination − Idark) measured at a zero gate voltage (dotted line in Figure 2a) are shown in Figure 2b. The photocurrent increased dramatically under light illumination at wavelengths shorter than 800 nm. The number of holes transferred from pentacene to graphene is proportional to the absorption of light because the production of electron−hole pairs increases with the light absorption in pentacene. Thus, higher absorption introduced larger effective photogating voltage, and therefore, a larger Dirac point shifted toward positive voltages (i.e., a greater number of hole carriers was generated in the graphene layer). This higher p-doping level of the graphene channel resulted in higher hole currents in the reverse sweep. Figure 2c shows the photoresponsivity, R (black line), of the hybrid photodetector as a function of the wavelength of incident light. The photoresponsivity of a photodetector is defined as Iph/(PWL), where P is the illumination power density of the incident light, and W and L are the channel width and length of the device, respectively. As the wavelength decreased, the photoresponsivity displayed a large step at λ = 650 nm, which was attributed to photon absorption in the pentacene. The position of the photocurrent and photoresponsivity peak at λ = 650 nm corresponded to the absorption peak position of the graphene/pentacene hybrid system (see Figures 1c and S6). Interestingly, the photocurrent and photoresponsivity increased further at λ = 520 nm, in stark contrast with the overall trend in the light absorption spectra measured in the graphene/pentacene hybrid system. We attempted to rationalize this extraordinary behavior by numerical simulating the electromagnetic behavior of the device, including all components (not just the graphene/pentacene hybrid system), as shown in Figure S7 (finite element method, provided by COMSOL Multiphysics). Figure 2d,e presents the enhancement factor and the spatial mapping of light intensity (|E|2) within pentacene layer, respectively. In our numerical simulation of a typical device, AuNPs, thermally deposited, were assumed as a pan-like shape rather than a spherical shape. The radial size and height varied widely (from 15 to 5 nm in diameter; from 1.5 to 5 nm in height). AuNPs with 8 nm in diameter and 5 nm in height were selected as a typical model
for the dramatic PL quenching in the pentacene−graphene hybrid system. The photoresponse characteristics of the pentacene− graphene hybrid photodetector with a nonvolatile memory function were investigated over a range of incident light wavelengths. The black curve shown in Figure 2a indicates the transfer characteristics (drain current (ID) vs gate voltage (VG)) at a fixed drain voltage (VD) of 0.1 V under dark conditions. A large clockwise hysteresis with a double minimum conductance feature was clearly observed. The memory window, which was defined as the difference between the two minimum conductance points (ΔVDirac), was 13.3 V. This current hysteresis behavior originated from the trapping−detrapping of charge carriers in the AuNPs as a result of tunneling through the cPVP layer. The charge trapping in AuNPs was also confirmed by the hysteresis loop of the transfer characteristics of the pentacene memory transistor without graphene layer (Figure S5). These results suggested that the photoinduced charge carriers could be effectively trapped in the AuNPs. Because the additional charge produced by photon absorption could be stored in the AuNPs and the trapped charge could not be released without application of a gate voltage, the light signal could be memorized. Note that this property is totally different from the properties of typical graphene photodetectors that display a prompt current decrease immediately after turning off the incident light. The photoinduced transfer characteristics and memory characteristics were investigated by illuminating the device with light having different wavelengths over the range 400−980 nm. The photocurrent was measured at a fixed incident illumination power of 1 mW. The pentacene−graphene photodetector did not respond significantly to light at wavelengths longer than the optical band gap of the overlying pentacene layer (1.76 eV, λ = 704 nm), but the photocurrent increased significantly at wavelengths shorter than 704 nm. The VDiracs shifted toward positive voltages due to the negative gating effects of the photoexcited electrons in pentacene; however, the change in the Fermi energy of graphene could not significantly impact the light absorption properties of pentacene, as demonstrated in the numerical calculations presented in Figure S6. Electron−hole pairs were generated in the pentacene by photon absorption, and the electrons in graphene were transferred to the valence band of the pentacene (holes in the valence band of the pentacene were transferred to the graphene) as shown in Figure S1. The electrons that remained in the pentacene layer acted as effective negative gate voltages and induced a hole current in the graphene channel D
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Figure 4. (a) Retention time and (b) cycling tests of the graphene photodetector with a memory functionality under various illumination powers. (c) Memory functionality of the graphene photodetector.
gate voltage are summarized in Figure 3b. Figure 3c shows the photoresponsivity (black line) and photodetectivity (blue line) as a function of the illumination power. The values of R and D* exceeded 700 A/W and 1013 Jones at 1 μW, respectively. Finally, we investigated the retention characteristics of the photocurrent (the stored light information), as shown in Figure 4a. Pulsed light with a pulse width of 1 s at different optical powers was directed onto the device channel. The illumination wavelength was fixed at 520 nm. The light information was memorized under an applied gate voltage of +15 V over 1 s during light illumination. A gate voltage of −20 V was applied for 1 s to erase the light information. Under each condition, the ID values were then measured as a function of the retention time. The retention time was determined to exceed 104 s. We performed a cyclic endurance test during the memorization steps under light illumination (VG = +15 V, 1s) and erasing (VG = −20 V, 1s). Figure 4b shows the ID values measured as a function of the number of memorizing/erasing cycles. The ID values remained invariant over the 200 cycles. Also, ID was monitored during pulsed light illumination (λ = 520 nm) at four different optical powers, as shown in Figure 4c. The memorizing and erasing operations were performed under an applied gate voltage pulse of +15 and −20 V, respectively. As the optical power increased, ID was observed to increase stepwise. The preillumination state could be recovered upon application of a gate voltage pulse of −20 V. In conclusion, we developed a new multifunctional graphene optoelectronic device that acted as a graphene photodetector with a nonvolatile memory function. A pentacene organic semiconductor layer with an optical band gap of 1.76 eV (704 nm) was deposited onto a graphene channel and utilized as a light absorption layer. AuNPs were positioned between the tunneling and blocking dielectrics and acted as both a charge trap layer and as a plasmonic light scattering layer. Upon light illumination, photoinduced hole carriers in the valence band of the pentacene layer were transferred to the graphene layer,
structure. The light intensity within pentacene was found to be dramatically enhanced at 500−550 nm by the implementation of the AuNPs, as shown in Figure 2d. Also, the spatial mapping of |E|2 (see Figure 2e) clarifies that this huge enhancement of light absorption originated mainly from plasmonic backward light scattering by the AuNPs. In particular, incoming light with a wavelength of the localized surface plasmon resonance (LSPR) of the AuNPs was scattered in the backward direction owing to the pan-like shape of the AuNPs. This result envisions a distinct but highly exciting prospect for precisely tuning the absorption spectrum of nonvolatile photodetector using plasmonic light scattering. The specific photodetectivity (D*), defined as (AΔf)1/2R/in, where A is the effective area of the detector, Δf is the electrical bandwidth, and in is the noise current, is an important characteristic of a photodetector. The blue curves in Figure 2c show the photodetectivity of the photodetector as a function of the wavelength of incident light. The measured maximum D* was 4.5 × 1010 Jones at 520 nm. The overall photodetectivity trend with respect to the wavelength of incoming light also agreed well with the device absorption spectral profile, as with photocurrent and photoresponsivity (see Figure 2d). The illumination power-dependent transfer characteristics of the pentacene−graphene hybrid photodetector were investigated. Figure 3a shows the transfer characteristics measured under different illumination powers at a fixed VD of 0.1 V and a fixed illumination wavelength of 520 nm. The results obtained at longer wavelengths (e.g., λ = 655 nm) are shown in Figure S8. The drain current increased with the illumination power, which supported the photogating mechanism proposed above. As discussed in Figure 2, the number of holes transferred from pentacene to graphene is proportional to the illumination power because the light absorption in pentacene increases with the illumination power. Thus, the higher p-doping level gave rise to the higher hole current in the reverse sweep. The illumination power-dependent spectral photocurrents at a zero E
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thereby inducing a hole current in the graphene channel. The resulting pentacene−graphene hybrid photodetector exhibited a broad spectral photoresponsivity between 800 and 400 nm due to both the plasmonics-enabled enhancement of the optical absorption and photogating mechanisms. The photoresponsivity and photodetectivity were 700 A/W and 1013 Jones, respectively. Importantly, the embedded AuNPs conveyed both a memory and a plasmonic light scattering functionality to the graphene photodetector. The charge carriers in the graphene layer could be trapped in the AuNPs via tunneling dielectrics under an applied gate voltage pulse. This light signal information was effectively memorized but could be readily erased simply by applying an opposite gate voltage pulse. The device performance strongly suggested that new optoelectronic devices could be developed to detect as well as store information about a photonic signal. The complementary optical properties of the pentacene organic semiconductor layer and the gold nanoparticles enabled the storage of incident light information. The proposed photodetector with the capacity to store a photonic signal represents a significant step forward in the development of optoelectronic devices.
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ASSOCIATED CONTENT
S Supporting Information *
Additional experimental details and figures This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ∥
S.J. and E.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Samsung Research Funding Center of Samsung Electronics under Project Number SRFCMA1402-00.
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REFERENCES
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