Research Article www.acsami.org
Printable Nanocomposite FeS2−PbS Nanocrystals/Graphene Heterojunction Photodetectors for Broadband Photodetection Maogang Gong,*,† Qingfeng Liu,*,† Ryan Goul,† Dan Ewing,‡ Matthew Casper,‡ Alex Stramel,‡ Alan Elliot,‡ and Judy Z. Wu*,† †
Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, United States Department of Energy’s National Security Campus, Kansas City, Missouri 64147, United States
‡
S Supporting Information *
ABSTRACT: Colloidal nanocrystals are attractive materials for optoelectronics applications because they offer a compelling combination of low-cost solution processing, printability, and spectral tunability through the quantum dot size effect. Here we explore a novel nanocomposite photosensitizer consisting of colloidal nanocrystals of FeS2 and PbS with complementary optical and microstructural properties for broadband photodetection. Using a newly developed ligand exchange to achieve high-efficiency charge transfer across the nanocomposite FeS2−PbS sensitizer and graphene on the FeS2− PbS/graphene photoconductors, an extraordinary photoresponsivity in exceeding ∼106 A/W was obtained in an ultrabroad spectrum of ultraviolet (UV)-visible-near-infrared (NIR). This is in contrast to the nearly 3 orders of magnitude reduction of the photoresponsivity from ∼106 A/W at UV to 103 A/W at NIR on their counterpart of FeS2/graphene detectors. This illustrates the combined advantages of the nanocomposite sensitizers and the high charge mobility in FeS2−PbS/graphene van der Waals heterostructures for nanohybrid optoelectronics with high performance, low cost, and scalability for commercialization. KEYWORDS: FeS2−PbS, graphene, nanohybrids, van der Waals heterostructures, printable broadband photodetector
1. INTRODUCTION
This has prompted intensive research in the synthesis of pyrite nanocrystals and thin films.3,24−26 In our previous work, we have developed a solution process based on the combination of the Lamer theory and orientation attachment (OA) and obtained pure-phase pyrite nanocrystals (FeS2 NCs) with controlled shape and dimension.5 These FeS2 NCs could act as an excellent photosensitizer when integrated with graphene of high charge mobility 27,28 for nanohybrid heterojunction optoelectronics.29−31 A key design parameter in these nanohybrid devices is a clean interface across the FeS2 NC/graphene heterojunction to facilitate charge transfer driven by the built-in interface electric field. Since the FeS2 NCs obtained from the solution synthesis process typically have an insulating layer consisting of ligands with long carbon chains, charge transfer from the FeS2 NCs to graphene across the FeS2 NC/graphene heterojunction can be blocked by this layer. To resolve this issue, we have developed a ligand exchange process to activate the surface of the FeS2 NCs. As we have revealed in this work, the photoconductive gain can be increased by orders of magnitude as this charge-transfer blocking layer was removed. In addition, a nanocomposite of FeS2 NCs and PbS quantum dots (PbS QDs) was decorated on graphene field transistors (GFETs) as the photosensitizer. The obtained FeS2
Pyrite iron disulfide (FeS2) has attracted much attention recently because of its earth abundancy, eco-friendliness, and band gap in the near-infrared (NIR) spectrum which is suitable for photovoltaics and other optoelectronic applications.1−5 FeS2 has an extraordinary absorption coefficient up to 6 × 105 cm−1,1 which is 2 orders of magnitude higher than that of crystalline silicon. In addition, it has a relatively large minority carrier diffusion length on the order of 100−1000 nm. The theoretical power conversion efficiency of 28% according to the Shockley− Queisser model makes FeS2 a competitive candidate for inexpensive and sustainable photovoltaic, optoelectronic, electrochemical, and other types of devices.4,6−16 Disappointingly, demonstration of high-performance pyrite-based optoelectronic devices in practical applications has been hindered by the difficulties in fabrication of phase-pure pyrites due to the coexistence of impurity phases (such as marcasite FeS2, FeS, and Fe3S4). In addition, the instability of the pyrite in air can cause surface decomposition. Consequently, the pyrite-based solar cells typically have low efficiencies below ∼3%, which is significantly lower than that of solar cells based on other chalcogenides, such as CdS, CdTe, CIGS, and CZTS.17−19 Therefore, a major challenge that must be addressed before the pyrite may be utilized for large-scale applications is the synthesis of pure-phase pyrites with a controlled surface state.8,20−23 © 2017 American Chemical Society
Received: June 8, 2017 Accepted: July 31, 2017 Published: July 31, 2017 27801
DOI: 10.1021/acsami.7b08226 ACS Appl. Mater. Interfaces 2017, 9, 27801−27808
Research Article
ACS Applied Materials & Interfaces
chloroform was used as the ink, which was ultrasound agitated for dispersion immediately before printing. The suspension was printed on the GFET channel using an inkjet microplotter (SonoPlot, Inc.) through sonication of a glass capillary tip. The plotter has a built-in calibration to calibrate and adjust the resonant frequency of glass capillary tips in the range of 200−600 kHz for a given dispensing ink. The tips can also be used to measure the surface inclination and declination for smooth substrates so that smooth automated printing can be done. Ligand Exchange. The ODA and ODE long carbon chain lengths molecularly absorbed in the surface of FeS2 NCs and PbS QDs were replaced by short carbon chain molecules of 3-mercaptopropionic acid (MPA). The MPA was dissolved in methanol with a volume ration of 1:1 and strenuously vibrated for 3 min for intensive mixing. After the FeS2 NCs/PbS QDs were printed on the GFET channel and dried in a glovebox filled with N2, every GFET chip was completely submerged in the MPA solution for 30−90 s at room temperature. The residual MPA was rinsed using methanol, and the device was dried in a glovebox for 10 min at room temperature. Finally, a layer of poly(methyl methacrylate) (PMMA) (dissolved in chlorobenzene with a concentration of 46 mg/mL) was printed on the FeS2−PbS/GFET channel surface to passivate the device. Optical and Optoelectronic Characterization. UV−vis−NIR absorption spectra were measured at room temperature using a UV3600 Shimadzu UV−vis−NIR spectrophotometer. Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were taken on FeS2 NCs, PbS QDs, and their mixture using a field emission FEI Tecnai F20XT with an accelerating voltage of 200 kV. The optical images of the GFET devices were taken on a Nikon Eclipse LV 150 optical microscope with a CCD camera. The source− drain current as a function of back gate voltage (I−VBG) characteristics were found using an Agilent B1505A semiconductor device analyzer in a probe station in the dark and under the illumination of a xenon lamp in the UV−vis spectra and an infrared light source in the near-infrared spectrum. The output optical power density of the light source was calibrated using a commercial Si photodetector.
NCs-PbS QD/GFET photoconductive detectors exhibit ultrahigh photoconductive gain and photoresponse, attributing to the combined advantages of high absorption coefficient of FeS2 NCs, enhanced NIR absorption by PbS QDs, and high carrier mobility of graphene. The FeS2−PbS/GFET photodetectors show a remarkable high photosensitivity in exceeding 1.28 × 106 A/W covering the broadband of the ultraviolet−visiblenear-infrared (UV−vis−NIR) spectral range at a very low bias voltage of 0.1 V. In addition, the photoresponse time under NIR (1100 nm) is about 3.8 s, which is significantly faster than the previously reported value of tens to hundreds of seconds on pyrite optoelectronic devices, illustrating the importance of the high crystallinity of FeS2 NCs and PbS QD sensitizers with activated surfaces to minimize charge trapping.8,32 This result not only sheds light on the mechanism of the optoelectronic process in the heterojunction devices with multicolor nanocomposite sensitizers but also paves the way for practical applications of the pyrites with high performance, low cost, and scalability.
2. EXPERIMENTAL SECTION Fabrication of PbS QDs. Lead(II) acetate trihydrate (760 mg) and 1.4 mL of oleic acid (OA) were mixed together and vacuum degassed/refilled argon three times in a Schlenk line system. After that, 20 mL of 1-octadecene (ODE) was added, and vacuum degas/refill argon was done five times. Then the temperature was raised to 100 °C under argon atmosphere. In another three-neck flask, hexamethyldisilathiane (HMS, 180 mg) was injected into 10 mL of ODE, which was subjected to vacuum degas and argon refill for five times while the temperature was stabilized at 100 °C. When the temperature of the lead acetate trihydrate solution reached 130 °C, the HMS degassed mixture solution was quickly injected, and the reaction took place in the span of 5 min. The reaction was allowed to cool down naturally and slowly (∼1 h). The QDs were precipitated after the addition of acetone and centrifugation. Then the QDs were purified by three successive dispersions in hexane and precipitations with acetone/ ethanol (4:1 volume ratio) and finally dispersed in chloroform. Fabrication of Iron Pyrite FeS2 NCs. FeCl2 (63.4 mg) and octadecylamine (ODA, 12 g) were mixed in a three-neck flask and degassed/refilled with argon five times. The samples were then heated to 120 °C and allowed to decompose for 2 h. In another three-neck flask, 128 mg of sulfur powder was mixed with diphenyl ether (5 mL) and sonicated for 10 min. Afterward this mixture was degassed/refilled with argon five times and heated to 70 °C for 1 h to dissolve. The sulfur-diphenyl ether solution was quickly injected to the Fe-ODA precursor solution at the temperature of 120 °C. After injection, the temperature of the combined solution was raised to 220 °C, and the reaction was allowed to proceed for 90 min. Then the reaction solution was cooled naturally to 100 °C and halted by injection of methanol and precipitated using a centrifuge. The FeS2 nanocrystals were isolated and cleaned up using standard crash out/wash method using ethanol/chloroform (1:3 volume ratio) by the centrifugation in a glovebox filled with N2. After cleaning the nanocrystals were redispersed in chloroform for characterization and storage. FeS2−PbS/GFET Photodetector Fabrication. Monolayer graphene was fabricated via chemical vapor deposition (CVD) and transferred onto heavily doped Si (100) substrates with a 90 nm SiO2 insulting layer. The details of the fabrication can be found in our previous work.33 Annealing in mixed gas of Ar:H2 (500 sccm:500 sccm) at 360 °C for 30 min was employed to remove residues of chemicals and polymers on graphene during the transfer process.34,35 Graphene field effect transistors (GFETs) with channels of 10 μm wide and 5 μm long were patterned using photolithography, and the details of this process have been reported previously.30,36 Patterned Au (80 nm)/Ti (2 nm) source and drain contact electrodes were deposited in an electron-beam evaporator, followed by lift-off. A FeS2 NCs (25 mg/mL) and PbS QDs (25 mg/mL) mixed solution in
3. RESULTS AND DISCUSSION Figure 1(a) shows a schematic image of the FeS2 NCs−PbS QDs/GFET heterojunction photodetectors. The device consists of a GFET made on monolayer graphene on the SiO2 (90 nm, back gate)/Si substrate, which was decorated with the nanocomposite of FeS2 NCs (gray) and PbS QDs (red) (see Experimental Section for the detailed procedure for the GFET fabrication and FeS2 NCs−PbS QDs synthesis and printing on GFET). Figure 1(b) shows the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of the FeS2 NCs, PbS QDs, and their composite. High crystallinity of both FeS2 NCs and PbS QDs was demonstrated in the HRTEM images. The lattice fringe of the FeS2 NC is ∼0.27 nm, matching well with the (200) plane of the pyrite. For the PbS QDs, well-resolved (200) lattice planes with an interplane distance of 0.29 nm can be clearly seen. Based on the TEM data, the average dimension of the FeS2 NCs is about 23.3 nm, which is approximately five times larger than that of the PbS QDs (average size 4.3 nm). Figures 1(c) and (d) depict, respectively, the statistical distribution of FeS2 NC and PbS QD sizes extracted from the TEM analysis. The full-width-at-half-maximum for the former is 10.1 nm, and that for the latter is 1.5 nm. In the nanocomposite of FeS2 NCs and PbS QDs, the larger-size FeS2 NCs are typically surrounded by the smaller-size PbS QDs. When printed to the GFET channel, both FeS2 NCs and PbS QDs may be directly in contact with graphene. Based on the interface band edge alignment shown in Figure 1(e), transfer of photogenerated holes can be facilitated from either FeS2 NCs or PbS QDs to graphene. In addition, a 27802
DOI: 10.1021/acsami.7b08226 ACS Appl. Mater. Interfaces 2017, 9, 27801−27808
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the favorable cascade valence band alignment of the three materials. This nanocomposite sensitizer through mixing of the smaller PbS QDs with larger FeS2 NCs has a unique advantage of enhanced light absorption and interface contact area since the former fills in the physical gaps on the interface between the latter and graphene as depicted schematically in Figure 1(a). The PbS QDs can play an important role in the NIR detection due to its absorption peaks located at ∼940 nm, making the nanocomposite sensitizer unique for high-performance, broadband photodetection in the UV−vis−NIR spectrum. Figure S1 shows the absorption spectra of PbS QDs (red), FeS2 NCs (black), and FeS2−PbS nanocomposite (blue), respectively. The first exciton transition of the PbS QDs is located at λ = 940 nm (∼1.319 eV) in solution, corresponding to a QD size of roughly 4−5 nm, which is consistent with that expected from the average QD size of 4.3 nm (Figure 1(c)). The FeS2 NC absorbance ranges from UV to NIR. It is therefore not surprising that the absorbance of the nanocomposite of FeS2 NCs and PbS QDs is a combination of that of the FeS2 NCs and PbS QDs. In particular, the addition of the PbS QDs is shown to enhance the NIR absorption considerably and will be discussed in detail next. Upon light illumination, excitons (electron−hole pairs) can be generated in both FeS2 NCs and PbS QDs depending on the wavelength of the incident light. Based on the band edge offsets across the heterojunctions illustrated in Figure 1(e), holes will transfer from nanocomposite sensitizer to graphene. This photodoping effect can lead to GFET channel conductance change as a photoconductive response. However, the charge transfer across the heterojunction can be sensitively affected by the interface between FeS2 NC and PbS QD sensitizers and
Figure 1. (a) Schematic image of the FeS2−PbS/GFET nanohybrid photodetectors. The nanocomposite photosensitizer consists of FeS2 NCs (gray) and PbS QDs (red). (b) TEM images of FeS2−PbS nanocomposite (left) and the corresponding HRTEM images of FeS2 NCs (upper right) and PbS QDs (lower right). (c, d) The diameter statistic diagram of FeS2 QDs and PbS nanocrystals. (e) Energy level diagram of the FeS2−PbS/GFET heterojunction and charge transfer process under illumination. Inset (left to right): An image of a GFET chip with 36 GFETs of different graphene channel lengths of 2, 5, 10, and 20 μm as labeled; zoom-in views of a FeS2−PbS/GFET device on the chip before and after the FeS2−PbS was printed on its channel. The scale bar is 5 μm.
cascade hole transfer from FeS2 NCs (−5.6 V), to PbS QDs (−5.0 V), and to graphene (−4.5 V) can also be efficient due to
Figure 2. Schematics of a FeS2−PbS/GFET heterojunction and the corresponding charge transfer process before (a) and after (b) the ligand exchange. Dynamic photoresponse curves of FeS2−PbS/GFET heterojunction photodetectors upon NIR illumination (1100 nm, 12 μW/cm2, VDS = 0.1 V) on/off, before ligand exchange (c) and after (d) the ligand exchange shown in (a) and (b), respectively. The GFET channel length and width is 4.3 μm × 11.4 μm. 27803
DOI: 10.1021/acsami.7b08226 ACS Appl. Mater. Interfaces 2017, 9, 27801−27808
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Figure 3. (a) Photoresponse time and photoresponsivity versus the ligand exchange time. (b) Dynamic photoresponse of FeS2 NC/GFET and FeS2 NC−PbS QD/GFET devices, respectively, at wavelength of 500 nm (12.3 μW/cm2) and 1100 nm (19.6 μW/cm2) under the bias voltage of 0.1 V. The photocurrent of FeS2 NCs/GFET is magnified by 10 times. (c) Spectral photoresponsivity of the FeS2 NC−PbS QD/GFET and FeS2 NC/ GFET photodetectors under VDS = 0.1 V. (d) The ratio of the photoresponsivity of FeS2 NCs−PbS QDs/GFET to that of FeS2 NCs/GFET in (c) at different wavelengths. (e) Dynamic photoresponse Iphoto curves of FeS2−PbS/GFET nanohybrid photodetectors upon NIR (1100 nm, 19.6 μW, VDS = 0.1 V) illumination on/off and wavelength. (b−e) The GFET channel length and width is 1.4 μm × 12.2 μm.
illustrated in the enhanced Iphoto value and shortened response time. Without the ligand exchange, the Iphoto is about 1.21 × 10−6 A, and the response time (10% to 90% of the Iphoto peak amplitude) is ∼452.8 s. This slow response time is comparable to that reported earlier on the FeS2 nanocrystal photodetectors, which is indicative of charge traps on the defects especially those on the nanostructure surface.5 Remarkably, the Iphoto was enhanced by >400% to 6.45 × 10−6 A in the device with the ligand exchange. In addition, the response time of 15 s represents an improvement of more than an order of magnitude, indicating the long-chain organic ligand cap layer indeed serves as a charge-transfer block as well as a charge trap layer that can be effectively removed using ligand exchange. The critical effect of the ligand exchange is further demonstrated through the monotonic decrease of the response time with increasing ligand-exchange time (black) as shown in Figure 3(a). At 90 s ligand exchange in MPA, the response time is shortened to 3.5 s. The photoresponsivity (R) can be calculated using the ratio of Iphoto and the incident absorbed light power by the active area of the device (Pi), R = Iphoto/Pi. A monotonic increase of the R to visible light of 500 nm wavelength with the ligand exchange time can be clearly seen (red) in Figure 3(a). Remarkably, high R up to 1.28 × 106 A/W has been obtained, which is about six times higher than that
graphene. Remarkably, ligand exchange played a critical role in achieving optimal charge transfer and hence photoresponse. Figure 2(a) depicts the as-synthesized FeS2 NCs (gray spheres) and PbS QDs (red spheres) that are capped by organic ligands of long chains of octadecylamine or oleic acid (18 carbons) to ensure their solution processability. These organic ligands form an insulating barrier layer across the heterojunction and militate against an efficient carrier transfer. This issue can be addressed instantly by removing the organic ligands through a robust ligand exchange (Figure 2b, see Experimental Section) using MPA. Figure S1(b) shows the FTIR spectra of the FeS2−PbS nanocomposite without (red) and with the ligand exchange (blue). The former has strong C− H stretching peaks at 2850 and 2921 cm−1 anticipated from the long alkyl chain in the organic ligands.37 In contrast, these signatures were much reduced to a negligible level after the ligand exchange. This ligand exchange generated a profound effect on the photoresponse of FeS2 NCs−PbS QDs/GFET heterojunction photodetectors. The photocurrents (Iphoto = Ilight − Idark) in response to “on” and “off” of a modulated NIR light source (1100 nm, 12 μW/cm2) are compared for the devices without (Figure 2(c)) and with (Figure 2(d)) the ligand exchange in MPA for 60 s. The major effect of the ligand exchange is 27804
DOI: 10.1021/acsami.7b08226 ACS Appl. Mater. Interfaces 2017, 9, 27801−27808
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Figure 4. (a) Comparison of channel conductance of a representative FeS2 NCs-PbS QDs/GFET measured in darkness and under illumination (1100 nm, 3.69 μW/cm2, VDS = 0.1 V). The GFET channel length and width is 4.3 μm × 11.4 μm. (b) Photoresponsivity versus back-gate voltages calculated from (a). (c) Gain versus illuminating power. (d) Detectivity versus bias voltage VDS at wavelengths of 340, 500, and 1100 nm.
shows the ratio of photoresponsivity of FeS2 NCs−PbS QDs/ GFET and FeS2 NCs/GFET. The enhancement of the photoresponsivity increases monotonically with the wavelength from 2.5 times at 600 nm to 263.3 times at 1100 nm. This is remarkable, and the dramatic NIR enhancement can be attributed the high absorption of the PbS QDs in this spectrum. The R values of the nanocomposite FeS2 NC−PbS QD/GFET photodetectors are about 6 orders of magnitude higher than that of the FeS2-based photodetectors (∼3 A/W).32 The broadband photoresponse is supported by the UV−visNIR absorption spectrum (Figure S1). While the crystalline FeS2 NCs are superior as a photosensitizer in the UV−vis−NIR spectrum, the addition of the PbS QDs enhances the responsivity by more than two orders of magnitude in NIR spectrum. Consequently, the nanocomposite FeS2 NC−PbS QD/GFET heterojunction photodetectors exhibit uniform photoresponsivity >106 A/W in the broadband of the UV− vis−NIR spectrum, which is advantageous in broadband photodetection and is difficult to achieve using a single sensitizer.30,38 As shown in Table S1, the response times and responsivity values of the FeS2−PbS/GFET devices are compared with several broadband photodetectors reported previously (Supporting Information). The NIR enhancement is particularly significant since the responsivity typically decreases with increasing wavelength by more than an order of magnitude from visible to NIR using a single photosensitizer. Through the design of nanocomposite sensitizers with complementary electronic structure and microstructure, this result shows this issue could be addressed. Figure 3(e) shows the dynamic Iphoto response of the photodetector while switching the NIR source on and off. A stable Iphoto was maintained after many on/off cycles, demonstrating high photodetection reversibility of the device. The charge transfer across the FeS2 NC−PbS QD/GFET heterojunction can be further tuned via graphene doping using the back gate voltage (VBG). The source−drain current (Id)
without the ligand exchange. The dramatically increased responsivity and decreased response time demonstrate that the long carbon chains surrounding the FeS2 NCs−PbS QDs are detrimental to the critical step of the charge transfer in this optoelectronic procedure in FeS2 NCs−PbS QDs/GFET heterojunction devices. Our result indicates that the MPA ligand exchange developed in this work can provide a viable approach in engineering of the heterojunction to facilitate charge transfer from the nanocomposite FeS2 NC−PbS QD sensitizer to graphene for high-performance optoelectronics. In order to analyze the contribution of the PbS QDs in the nanocomposite FeS2 NC−PbS QD sensitizer, Figure 3(b) compares the dynamic responses of the FeS2 NC/GFET and FeS2 NC−PbS QD/GFET devices with respect to visible (500 nm, 12.3 μW/cm2) and NIR (1100 nm, 19.6 μW/cm2) lights. In the visible case (500 nm), the Iphoto is about 2.96 μA in the FeS2 NC/GFET device, which is enhanced by a factor of 2.3 to ∼6.77 μA with addition of the PbS QDs. This enhancement factor is further increased to >260 in the NIR case. In particular, a high NIR Iphoto of 4.21 μA, comparable to that achieved in visible light was obtained in the FeS2 NC−PbS QDs/GFET devices. This is in contrast to the low Iphoto of 0.016 uA for the FeS2 NCs/GFET devices. This result is important and demonstrates the advantage of the nanocomposite FeS2 NC−PbS QD sensitizer in its ability to achieve a high photoresponse in a broad band of UV−vis−NIR. Figure 3(c) compares the spectral photoresponsivity of the FeS2 NC/GFET and FeS2 NC−PbS QD/GFET nanohybrid photodetectors. In the former, a monotonic decrease of the R values from 1.78 × 106 A/W at 340 nm to 4.87 × 103 A/W at 1100 nm can be clearly seen. A significant enhancement of the overall broadband R values is shown in Figure 3(c) with the FeS2 NC−PbS QD nanocomposite sensitizer. The R values are 4.32 × 106 A/W, 3.27 × 106 A/W, and 1.28 × 106 A/W under 340, 500, and 1100 nm illumination, respectively, at a Vbias = 0.1 V across the source−drain electrodes of the GFET. Figure 3(d) 27805
DOI: 10.1021/acsami.7b08226 ACS Appl. Mater. Interfaces 2017, 9, 27801−27808
Research Article
ACS Applied Materials & Interfaces
In this work, the W = 11.4 μm, L = 4.3 μm, VDS = 0.1 V, and μ ∼ 53.6 cm2 V−1 s−1, and the Tt is estimated to be ∼8.0 ns. It should be noted that the Ttransit is inversely proportional to VDS. Therefore, the gain can be obtained by the following equation:
versus back gate VBG characteristics was tested on representative FeS2 NC−PbS QD/GFET nanohybrid photodetectors under dark and NIR illumination (1100 nm, 3.7 μW/cm2), and the result is shown in Figure 4(a). Under the NIR illumination, the Id−VBG curve shifted to right with a ΔVDirac of 12.0 V, which is anticipated from the photoinducted hole doping shown in Figure 1(e). The change in the minimum GFET conductance is negligible as expected since such hole doping should not introduce an altered charge scattering mechanism to the charge transport in the GFET channel. Figure 4(b) depicts the R versus VBG curve at a wavelength of 1100 nm under an illumination intensity of 3.7 μW/cm2, which exhibits an almost constant R ∼ 1.01 × 106 A/W value in the negative VBG region. In the positive VBG region, the R experienced a small peak of R ∼ 1.13 × 106 A/W at ∼5.5 V, followed with decreasing R with further increasing VBG. While further investigation is necessary to fully understand the effect of the VBG, the observed R−VBG behavior could be attributed to the shift of the Fermi energy of graphene by the applied VBG. As schematically depicted in Figure 1(e), a negative VBG tends to shift the Fermi energy up, which means the valence bands of FeS2 NCs and PbS QDs will remain on the more negative side of the Fermi energy of graphene. This band alignment is the key to high efficiency hole transfer from the sensitizers to the GFET, as shown by the almost constant R values observed at negative VBG. In particular, since the FeS2−PbS/GFET devices were holedoped (black curve in Figure 4(a)), shifting the Fermi energy up by applying a negative VBG is necessary to obtain the required band alignment to enable the hole transfer. In contrast, a positive VBG shifts the graphene Fermi energy downward, and when it is aligned perfectly with the valence band of the PbS QDs at VBG ∼ +5.5 V, a maximum charge transfer occurs and results in a peak of the R−VBG curve. However, at more positive VBG’s, the hole transfer will experience an energy barrier since the Fermi energy of graphene is below the valence band of the PbS, leading to a monotonic decrease of the R value with further increasing VBG beyond its peak at 5.5 V. The photoconductive gain (G) is a critical parameter for evaluating the performance of a photodetector device. In typical nanohybrid photodetectors, the gain is mainly determined from the ratio of carrier lifetime in the nanocrystals (Tlife) to carrier transit time in the graphene channel (Ttransit). The quantum confinement and relaxation effect usually result in a relatively long carrier lifetime, while the transit time of the electrons in the bottom graphene channel is very short due to the high carrier mobility of graphene and the short channel length (4.3 μm of the GFETs used in this study). This time difference allowed the transferred electrons to recirculate between the negatively biased source and the positive drain many times within the long carrier lifetime, resulting in a relativly high gain and responsivity. The photocurrent Iphoto can be expressed as39 Iphoto = αqNVW =
αqηWL ⎛ T0 ⎞ P ⎜ ⎟ hν ⎝ Tt ⎠ 1 + (P /P0)n
G=
⎛T ⎞ 1 = αη⎜ 0 ⎟ qPWL T 1 + ( P + P0)n ⎝ t⎠
Iphotohν
(2)
From eq 2 derived for the gain, we can quantitatively calculate the value of the gain. The lifetime of photogenerated holes can be extracted from an approximation fitted via a biexponential function in Figure 3(b), which yields Tlife ∼ 0.53 s.29,40,41 It is noteworthy that the lifetime of the holes in the nanocrystals (∼0.53 s) is over 107 orders of magnitude greater than the electron transit time in the graphene channel (∼8.0 ns). When VDS increased by 2 orders of magnitude (0.1 to 10 V), the transit time Ttransit was accordingly reduced to 1% of the original time (∼0.08 ns), and this led to a 2 orders of magnitude increase in the gain to ∼109. Based on the experimental measurement values of the response time, Ttransit and Tlife, we certify that the experimental gain is in good agreement with the theoretical value, as shown in Figure 4(c). The FeS2−PbS/GFET device showed a maximum gain of 1.32 × 107 under an illumination power of 2.86 pW and exhibited a monotonic decrease with increasing illuminating power due to the reduced average lifetime of holes and saturation of the surface states. The red curve in Figure 4(c) is the best fit line for the experimentally measured gain with fitting parameters: αη = 0.32, Po = 1.20 pW, and n = 0.90. The theoretical calculation predicts that a maximum gain as high as ∼1.91 × 107 can be reached under a lower excitation light power
