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Oct 15, 2015 - Near-Infrared Photodetection via Hot Carrier Injection ... KEYWORDS: photodetector, gold nanorods, graphene, plasmonic, solution-proces...
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Solution-Processed Gold Nanorods Integrated with Graphene for Near-Infrared Photodetection via Hot Carrier Injection Zhouhui Xia, Pengfei Li, Yusheng Wang, Tao Song, Qiao Zhang,* and Baoquan Sun* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, 215123, PR China S Supporting Information *

ABSTRACT: Graphene-based photodetectors have attracted wide interest due to their high-speed, wide-band photodetection and potential as highly energy-efficient integrated devices. However, the inherently low-absorption cross-section and nonselective spectra response hinder its utilization as a high-performance photodetector. Here, we report a solution-processed and high-spectral-selectivity photodetector based on a gold nanorods (Au NRs)−graphene heterojunction with near-infrared (NIR) detection. Au NRs are used as a subwavelength scattering source, and nanoantennas with wide light absorption range from ultraviolet to near-infrared via tuning their geometry. Photons couple into Au NRs, exciting resonant plasmas and generating hot carriers that pump into graphene, resulting in selective NIR photodetection. A flexible NIR photodetector is also demonstrated based on this simple structure. Au NRs can achieve variable resonance frequencies by the design of different aspect ratios as nanoantennae for graphene, which promises the selective amplifying of the photoresponsivity and enables highly specific detection. KEYWORDS: photodetector, gold nanorods, graphene, plasmonic, solution-processed their dimensional structure due to their surface plasma effect.14 the integration of optical metal antenna structure with graphene has demonstrated the efficient conversion of visible and NIR photons into electrons, with photocurrent enhancement of up to eight times.15 The photons coupled into Au NRs generate hot carriers arising from plasmon decaying as well as interband absorption. The hot carriers inject into graphene over a potential barrier and generate a photocurrent, resulting in light detection.16 These devices provide opportunities to enhance absorption cross-section and realize the spectrum-selective response. Here, the optical nanoantennas play a key role on the light properties, which are generally fabricated by the expensive electron-beam lithography (EBL) pattern method. 17 In addition, the complicated optical nanoantennas can only be achieved on the small device area because of the slow process for large-area patterned structure by EBL method. Here, we integrate solution-processed gold nanorods (Au NRs) with graphene as the active optical nanoantenna and realize photodetection with NIR spectral selectivity. Colloidal Au NRs exhibit NIR light harvesting via smart anisotropy dimensional design because electrons move coherently along the short radial and long axial directions. Au NRs have longitudinal plasmon bands that can be tuned from the visible to the NIR region by varying the nanorod aspect ratio. Hence, Au NRs can act as a subwavelength scattering source and nanoantennas to enhance the optical detection and photo-

1. INTRODUCTION Near-infrared (NIR) photodetectors are of significance because they have potential applications in night vision surveillance, remote control, and imaging.1 In the past few years, III−V and II−VI semiconductors such GaAs, InxGa1−xAs, HgS, and PbS have received considerable attention as building blocks for NIR photodetectors.2 However, III−V and II−VI semiconductors are not only high-cost but also environmental unfriendly (including those such as arsenic, mercury, and lead), which limits the widely application.3,4 Meanwhile, graphene is a twodimensional (2D) sheet composed of sp2-bonded single-layer carbon atoms with a honeycomb lattice structure that is emerging as an attractive material candidate for future electronics and optoelectronics because of its advantage of high electron mobility, broadband absorption, atomic layer thickness, and unique mechanical flexibility.5 Multiple graphene layers with the remarkable features of long electron and hole momentum relaxation time have been reported for the application of infrared interband detectors based on p−i−n junctions.6−8 Monolayer graphene with a thickness of only 0.33 nm exhibits remarkable optical absorption of nearly 2.3% over a wide frequency range from ultraviolet to infrared.9,10 However, the total light absorption of a graphene layer is still quite low, which limits the quantum efficiency of graphene-based photodetectors.11 In addition, the graphene-based photodetector is lack of spectral selectivity in intrinsic graphene, which is originated from the wavelength-independent absorption characteristics of graphene.12,13 Optical antennas of metal nanostructure display widespectrum light harvesting from visible light to NIR by tuning © XXXX American Chemical Society

Received: August 7, 2015 Accepted: October 15, 2015

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DOI: 10.1021/acsami.5b07299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of the plasmonic photodetector device with Au NRs integrated with graphene. SEM and TEM images of Au NRs were also inset. SEM samples were prepared by the Au NRs solution dropped onto graphene substrate and dried naturally. (b) UV−vis−NIR absorption spectrum of Au NRs solution, graphene, and graphene−Au NRs on quartz substrates. (c) Raman spectroscopy of graphene and graphene−Au NRs on quartz substrates.

(∼300 nm) substrate. The graphene was prepared by chemical vapor deposition (CVD) method.19 Before the transfer of graphene on copper film, graphene was coated with a thin layer of poly(methyl methacrylate) (PMMA). Graphene was then coated with PMMA and floated onto water by FeCl3 etching. After the transfer of graphene onto the silicon substrate, PMMA was removed by acetone. Photolithography was used to define the Ti−Au electrodes deposited by electron-beam evaporation on graphene. The channel length and width are 12 and 6 μm, respectively. The papered Au NRs solution was dropped onto graphene and dried naturally to form the selfassembly Au NRs pattern. In some devices, dilute 1,3benzenedithiol (BDT) solution in isopropanol was dropped onto Au NRs to replace the long ligand with a short one. The devices were washed by isopropanol an additional time to remove any free ligand on the substrate. 2.3. Characterization. The length and width of Au NRs were characterized by transmission electron microscope (TEM, FEI Tecnai F30). The morphology of the Au NRs on graphene was investigated by scanning electron microscopy (SEM, FEI Quanta 200 FEG). The optical absorption spectra of the films were obtained by a UV−vis−NIR spectrophotometer. Raman spectrum measurements were conducted on a microconfocal Raman system. Electrical characterizations were performed by a probe station (Cascade M150) equipped with a semiconductor property analyzer (Keithley 4200) at room temperature in ambient conditions. Photoelectric measurements at NIR region were measured by an optical microscopy platform (Nea Spec) on which IR light was focused onto a specific location of the sample with a focus-spot size of 5 mm × 5 mm.

response at selected plasmon resonance frequency, therefore enabling graphene photodetectors that respond sensitively to selected light. By using high-aspect-ratio Au NRs, the peak value of the photoresponsivity to NIR light (1310 nm) was 4 × 104 A/W without applying any bias. Light is not only harvested by graphene but also by the plasmonic structure Au NRs, which form an energetic electron−hole pair. This process is an additional contribution to the photocurrent and photoresponsibility as well as the spectral selectivity. The photoresponse results indicate that the integration with Au NRs can greatly enhance the photocurrent by up to 200%, with the external quantum efficiency being much higher than the previously reported pristine graphene-based devices at zero source-drain bias and zero gate voltage.

2. EXPERIMENTAL SECTION 2.1. Au NRs Preparation. The synthesis of high-aspectratio Au NR was based on a seed-growth method according to the previous report.18 High-quality Au NRs were achieved by using the cetyltrimethylammonium bromide (CTAB) and sodium oleate (NaOL) surfactant mixtures. Gold seed solution was prepared by the injecting of 0.6 mL of 0.01 M fresh NaBH4 into 5 mL of 0.2 M CTAB and 5 mL of 0.5 mM HAuCl4 solution with vigorously stirring. Meanwhile, 3.500 g of CTAB and 0.617 g of NaOL were dissolved in 125 mL of H2O, and 12 mL of 4 mM AgNO3 and 125 mL of 1 mM HAuCl4 solution were then added into CTAB−NaOL solution to obtain the growth solution. Hydrochloride acid was added into the growth solution to tune the pH value to ∼0.95. Finally, 0.625 mL of 0.064 M ascorbic acid was mixed and followed by the injecting of 0.4 mL of as-prepared seed solution. The seeds were added into the growth solution to initiate Au NRs growing, and the temperature was kept at 30 °C for overnight. The Au NRs were purified with deionized water by centrifuging three times. Finally, the Au NRs were dispersed with a concentration of 10 mg/mL in deionized water for device fabrication. 2.2. Device Fabrication. The photodetectors were fabricated on highly doped n-type silicon−silicon oxide

3. RESULTS AND DISCUSSION The schematic of the device with an Au NRs array on graphene is shown in Figure 1a. Here, once light is shining upon the device, the generated photocarrier can form a current across the two electrodes via graphene. Between them, Au NRs form a high-ordered self-assembly pattern where all of the rods lie on graphene, as shown in Figure 1a of the SEM image. The lying B

DOI: 10.1021/acsami.5b07299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

performance relative to the antennae-less graphene device. Regarding the as-prepared colloidal Au NRs, long carbon chains of CTAB−NaOL are anchored onto their surface, which hinders the charge-injecting from Au NRs to graphene. After the long ligand of CTAB−NaOL is replaced with the short ligand of BDT, a photocurrent-based graphene device coupled with Au NRs grows twice (Iph = Ilight − Idark) compared with one based on pristine graphene under illumination (1 nW, 1310 nm). The photoresponsivity is as high as 2 × 104 A/W. The enhanced photocurrent under illumination is attributed to the generation of photoexcited charge carriers in graphene as well as to hot carrier injection from Au NRs to graphene. Figure 2b shows the schematic potential profile of heterojunction photodetection. When the light is illuminated onto the device, both graphene and Au NRs can harvest incident light and contribute the photocurrent. The light harvest by Au NRs is via both plasmonic and interband absorption. Once Au NRs and graphene have contact, a built-in electrical field is generated by carrier diffusion due to balancing carrier concentrations at the junction. The resulting effective electric field provides the driving force for the generation of free carriers.23 This field, as well as applied bias, drives the generated carriers from Au NRs to graphene, which enhances photocurrent level. Thus, the graphene−Au NRs photodetector can readily achieve high photoresponsivity without applying a high external bias voltage. The influence of such a carrier-doping effect would provide a high photogain.12 To estimate the photoconductive gain, we first calculate the field-effect carrier mobility of graphene−Au NRs heterostructure from the device transfer curve (Figure 3a). The mobility is estimated by24

Au NRs with ordered pattern are favorable for the current flowing along the horizontal direction between two electrodes. In addition, the contact area between Au NRs and graphene is dramatically enlarged. This pattern is driven by long-chain surfactants of CTAB and NaOL on the surface of Au NRs, which can increase the interaction between adjacent Au NRs. Afterward, the long chain is replaced by short-chain BDT to reduce the physical distance between Au NRs and graphene.20 According to the TEM image of Au NRs in Figure 1a, the diameter and the length of NRs are ∼17 and ∼120 nm, respectively. The aspect ratio for Au NRs is more than 7. The high aspect ratios promise the strong plasmon effect at long wavelength.21 In addition, the nanorods display a uniform size distribution. To characterize absorption and scattering characteristics of the Au NRs−graphene composites, we analyzed the identical structure on quartz substrate with a UV−vis−NIR spectrum. As shown in Figure 1b, as-prepared colloidal Au NRs exhibit two peaks because of the anisotropy in their dimension, which is ascribed to two types of surface plasma band. The short and long wavelength peaks are generated by the coherent motion of conduction-band electrons along the radial and axial directions, respectively. These two peaks are ascribed to the Au NRs width (∼17 nm) for transverse plasmon band and length (∼120 nm) for longitudinal plasmon bands. Meanwhile, the absorption spectrum of the graphene layer shows a decrease from 3.5% to 0.6% with a wavelength increase from 250 to 1500 nm, which accounts for the reflections at the oxide−silicon interface. Once Au NRs are coupled with graphene, which exhibit a strong plasmon-resonance-enhanced absorption and extinction of nearly 30% with pronounced resonance peaks of ∼515 nm and ∼1200 nm. This affords the capability of photodetection under the irradiation of NIR light. Furthermore, the incorporation of Au NRs retains the structure integrity of the graphene, as shown by the well-kept G peak (1590 nm) and 2D peak (2690 nm) in the Raman spectrum (in Figure 1c).22 To investigate the photoresponse characteristics of these devices, we measured the current−voltage (I−V) curves with and without light of the devices based on pristine graphene and graphene−Au NRs before (CTAB, long ligand) and after BDT treatment (short ligand) (Figure 2a). In the presence of Au NRs, the photodetectors exhibit dramatically enhanced

μ=

dID L d 1 × × × dVG W ε VD

where W is the channel width, L is the channel length, d is the thickness of SiO2 (d = 300 nm), and ε is the dielectric constant of SiO2.25 Thus, the carrier (hole) mobility (μ) of 626 (cm2 V−1 s−1) is obtained. The carrier transit time (τtransit) is calculated according to the following equation:26 τtransit =

L L2 = ν μV

where v is carrier velocity, and V is applied bias. Thus, a τtransit value of 4.6 × 10−9 s is obtained. As shown in Figure 3b, the photocurrent decay lifetime measurement is performed to calculate the lifetime of photoexcited carriers. The device dynamic characteristics under illumination could be described by27 I = A e−t/ τlifetime + I0

where I0 is the dark current, and A is a coefficient. τlifetime is considered as the carrier’s lifetime, which is constant. According to curve fitting result, the estimated lifetime of the carriers is ∼8.24 s. According to the following equation:28 τ G = lifetime τtransit

Figure 2. (a) Dependence of photocurrent on source-drain voltage (IDS) with different source-drain voltage (VDS) for the devices with pristine graphene (blue line), with long-chain Au NRs−graphene (black line) and with short-chain Au NRs−graphene (red line) under 1310 nm light illumination at 1 nW. Inset shows a working schematic of the Au NRs−graphene photodetector, where electrons from Au NRs inject into graphene. (b) Cartoon image of charge transmission in the energy-band diagram of graphene under forward bias.

a photogain (G) of 1.8 × 109 is achieved. The ultrahigh gain benefits from graphene’s high mobility and an ultralong lifetime of the light-induced carriers in our device. Alternatively, if we use the transfer curves for the Au NRs−graphene transistor under dark and exposed by 1310 nm light illumination as C

DOI: 10.1021/acsami.5b07299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) Transfer curve (ISD − VGS) at VDS = 50 mV for a back-gated Au NRs−graphene transistor. (b) Time-resolved photocurrent decay for the Au NRs−graphene phototransistor excited by 1310 nm laser at a light power of 0.22 nW. (c) Time-dependent photocurrent excited by pulsed light at 1310 nm with different incident powers. (d) Time-dependent photocurrent excited by pulsed light at 1310 nm. The excitation power is 0.22 nW, and the source-drain bias VDS is 1 V. Inset shows the faster lifetime excited by 2 kHz frequency-pulsed laser.

Figure 4. (a) Photoresponsivity of the Au NRs−graphene device as a function of the photoexcitation wavelength compared with the responding absorption spectrum. (b) FDTD-simulated electric field distribution around Au NRs at 400, 800, and 1200 nm. (c) Dependence of photocurrent and photoresponsivity on incident light power at 1310 nm light. (d) Photocurrent as a function of source-drain bias under illumination with a 1310 nm laser (1 nW) for the flexible device with the same structure. Inset shows photograph of the flexible Au NRs−graphene photodetector on PET substrate.

shown in Figure S1, a photogain of 9.6 × 108 is obtained according to the gate-sweep measurement with and without light, which is comparable with that obtained by the above photocurrent decay method.29,30 Temporal photocurrent response of the photodetector at 1310 nm in different power levels is also investigated, as shown in Figure 3c. The photocurrent can be effectively switched on

and off while the light source is intentionally blinked. Photocurrent can also be modulated by different light-power density, similar to the pristine graphene photodetector measured under visible light.31 In Figure 3d, it exhibits that photocurrent response is dominated by two components under illumination at 0.22 nW with a rise time less than 900 ms and a fall time less than 500 ms (corresponding to 70% decay), which D

DOI: 10.1021/acsami.5b07299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

process. Photocurrent curves dependent on source-drain bias are measured under the illumination of 1310 nm light at 1 nW. It is found that the photocurrent increases while the power of incident light becomes stronger, which indicates that this flexible photodetector operates well.

is associated with charges remaining trapped in the nanorods. A steady photocurrent of about 16 μA can be obtained at the “on” state. We also plot the temporal response of the photodetector excited by a high-frequency (2 kHz) pulsed laser enabling a fast on−off response of the devices, where the measured fast lifetime is 0.1 ms (Figure 3b inset). Therefore, the time response of the photocurrent decay is dominated by two components with faster lifetimes of about 0.1 ms and a slow one of 8.2 s, which then leads to a predicted range of values for the photogain between 104 and 109. To verify that the dominated photoresponsivity originates from the hot carrier generation of Au NRs, we conducted wavelength-dependent photocurrent measurements at zero bias, as shown in Figure 4a. Light is focused on the graphene−Au NRs junction. The spectral-dependent photocurrent shows an obvious peak at ∼1300 nm, which matches the extinction peak of graphene−Au NRs composites well. The device based on pristine graphene displays wide broadband photoresponse spectra without any obvious peak.32,33 This spectral-dependent response at 1300 nm indicates that the enhancement is induced from the plasmon resonance enhanced absorption in graphene devices. This plasmon resonance attributes to both the enhanced near-field oscillation and scattering effect.34 Due to the localized surface plasmon of Au NRs, there is an oscillation of conduction electrons that arises in the metal when it is excited by a specific frequency of electromagnetic radiation.35 With this oscillation, light is trapped around the meal surface, leading to an enhanced local electrical field, as shown in Figure 4b (simulated by finite difference time domain, FDTD). According to the simulation results, calculated intensities of electrical field (|E|2) are 9.5, 13.2, and 19.6 at 400, 800, and 1200 nm, respectively. Au NRs generate strong electric fields near their surface at their respective resonant excitation wavelengths. This field can effectively enhance the absorption of graphene as well as spectral selectivity.36,37 The highly spectrally selective enhancement of the photocurrent in graphene devices can readily allow us to construct multilight-sensitive photodetectors by coupling graphene with Au NRs of designed different plasmon resonance frequency. In addition, the scattering effect can also contribute to the enhanced photocurrent, being similar to the plasmonic enhancement effect. The dependence of photocurrent and photoresponsivity on light power at a zero gate voltage is shown in Figure 4c. The photocurrents grow linearly while promoting the light power at a relatively low power. It is concluded that higher photoresponsivity can be achieved when decreasing the illumination power or increasing the bias voltage. Power-dependence studies show that the photocurrent scales linearly with the excitation power even at zero source-drain bias. The average peak photoresponsivity achieves 104 A/W for the graphene device without Au NRs, which is comparable to previous studies at zero bias.38 Significantly, a larger value up to 4 × 104 A/W is achieved for the device with Au NRs at 1310 nm, with the extraordinary enhancement of the photoresponsivity. Flexible, transparent, highly efficient, and broadbandoperable photodetectors are of wide interest for the development of novel photoelectric devices.39,40 Here, we further investigate the application of Au NRs in flexible photodetector devices by constructing graphene−Au NRs heterojunctions on flexible substrates. As illustrated in Figure 4d, CVD-grown graphene transfers to polyethylene terephthalate (PET) substrate, followed by the Au NR self-assembly deposition

4. CONCLUSION In summary, we have demonstrated colloidal Au NRs integrated with graphene for spectrum-selective photodetectors by hot carrier injection effect. Coupling colloidal Au NRs with the atomically thin graphene creates a dramatically enhanced local optical field near the graphene and results in an overall plasmonic enhancement electrical field. A large gain of amounts from 104 to ∼109 with a photoresponsivity of 4 × 104 A/W at 1310 nm is achieved with this plasmonic enhancement to improve the built-in potential as well as enhance light harvesting with hot carrier injection. Importantly, the responsivity of the present device achieves spectral NIR photoreponse, suggesting the great potential of our device for future NIR photodetector applications. Together with the demonstrated abilities including high responsibility, simple processes, spectral selectivity, and flexibility, the scalable fabrication of plasmon-resonance-enhanced graphene photodetectors can create opportunities for future graphene-based optoelectronics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07299. Details of the calculation of the photogain via carrier concentration; figure showing the transfer curves for the Au NRs−graphene transistor under dark and exposed to 1310 nm light (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; tel: 0086-512-65882641. *E-mail: [email protected]; tel: 0086-512-65880951. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program) (2012CB932402), the National Natural Science Foundation of China (91123005, 61176057, and 61211130358), the Natural Science Foundation of Jiangsu Province of China (BK20130310), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Collaborative Innovation Center of Suzhou Nano Science and Technology.

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REFERENCES

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DOI: 10.1021/acsami.5b07299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.5b07299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX