Ambipolar GrapheneâQuantum Dot Hybrid Vertical Photodetector with

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Ambipolar Graphene-Quantum Dot Hybrid Vertical Photodetector with a Graphene Electrode Yongli Che, Yating Zhang, Xiaolong Cao, Haiting Zhang, Xiaoxian Song, Mingxuan Cao, Yu Yu, Haitao Dai, Junbo Yang, Guizhong Zhang, and Jianquan Yao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06629 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 2017

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ACS Applied Materials & Interfaces

Ambipolar Graphene-Quantum Dot Hybrid Vertical Photodetector with a Graphene Electrode Yongli Che,† Yating Zhang,*,† Xiaolong Cao,† §

‡

Haiting Zhang,† Xiaoxian Song,† Mingxuan

∥

Cao,† Yu Yu,† Haitao Dai, Junbo Yang, Guizhong Zhang,† and Jianquan Yao† †

Key Laboratory of Opto-Electronics Information Technology (Tianjin University), Ministry of

Education, School of Precision Instruments and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China;

‡

College of Mechanical and Electronic Engineering, Shandong University of Science and

Technology, Qingdao, 266590, China §

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology,

School of Science, Tianjin University Tianjin, 300072 (China); ∥

Center of Material Science, National University of Defense Technology Changsha 410073,

(China).

KEYWORDS: Vertical photodetector; Graphene electrode; Quantum dot; Graphene-quantum dot hybrid; Schottky barrier; short channel

ACS Paragon Plus Environment

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ABSTRACT: A strategy to fabricate an ambipolar near-infrared vertical photodetector (VPD) by sandwiching a photoactive material as a channel film between bottom graphene and top metal electrodes was developed. The channel length in the vertical architecture was determined by the channel layer thickness, which can provide an ultrashort channel length without the need for a high-precision manufacturing process. The performance of VPDs with two types of semiconductor layers, a graphene–PbS quantum dot hybrid (GQDH) and PbS quantum dots (QDs), was measured. The GQDH VPD showed better photoelectric properties than the QD VPD because of the high mobility of graphene doped in the channel. The GQDH VPD exhibited excellent photoresponse properties with a responsivity of 1.6 × 104 A/W in the p-type regime and a fast response speed with a rise time of 8 ms. The simple manufacture and promising photoresponse of the GQDH VPDs reveal that an easy and effective way to fabricate high-performance ambipolar photodetectors was developed.

1. INTRODUCTION Vertical photodetectors (VPDs) are arousing tremendous interest as promising elements for future photoelectronic devices because of their high driving currents at low operating voltage.1,2 The VPD architecture consists of a drain electrode, photoactive channel layer, and source electrode (SE) vertically stacked with respect to the gate electrode. This structure allows an ultrashort channel length (L), which is simply the thickness of the channel layer. The SE of a VPD should be electrically transparent to direct-current (DC) electric fields so that the gate field is able to penetrate the SE to influence the channel.3 A variety of transparent electrodes, such as


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graphene,4-7 metallic nanowires, metallic grating films, and carbon nanotubes, have been used in vertical field-effect transistors (FETs).8 Graphene has attracted much attention because of its remarkable mechanical strength, transparency, thermal conductivity, and ultrahigh mobility.9 The work function (WF) of graphene can be modulated by the gate voltage (VGS) because of the low state density of graphene. Furthermore, the weak electrostatic screening effect of graphene means that the gate electric field can penetrate graphene to modulate the Schottky barrier (SB) height of the graphene–semiconductor channel and then control the drain current (IDS).6,10 Previous studies of VPDs based on graphene have demonstrated high performance from structures of graphene quantum dots (GQDs) sandwiched between monolayer graphene sheets11 and all-graphene p-n vertical-type tunneling diodes12. These devices achieved a responsivity (R) of 10−1–100 A/W and detectivity (D*) of 1011–1012 Jones. Solution-processed materials have attracted attention for use in optoelectronics because of their low cost and suitability for large-area manufacture processes.8 Photoactive PbS QDs are widely used in photodetection because of their advantages of solution processability, tunable wavelength, high responsivity, and low fabrication cost.13 PbS QDs display an absorption peak in the near-infrared (NIR) region, so they can strongly absorb NIR light and efficiently generate multiple excitons from one incident photon.14 Furthermore, it is notable that graphene has an advantage over PbS QDs in terms of carrier mobility. Therefore, it can be expected that photodetectors based on a graphene–PbS QD hybrid (GQDH) channel will exhibit promising performance. In this study, we describe a facile fabrication process of high-performance ambipolar GQDH


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VPDs and reference QD VPDs with graphene bottom electrodes. The two types of VPDs exhibit ambipolar operation with a current density of 11.7 mA/cm2 for the QD VPD and 15.6 mA/cm2 for the GQDH VPD. The devices display excellent photoresponse properties with R of 9.7 × 103 A/W for the QD VPD and 1.6 × 104 A/W for the GQDH VPD in the p-type regime at a light irradiance of 24 mW/cm2. Furthermore, the VPDs show a reproducible dynamic response and fast response speed with a rise time of 14 ms for the QD VPD and 8 ms for the GQDH VPD. Because of the high mobility of graphene used in the channel, the photoelectric properties of the GQDH VPD are superior to those of the QD VPD. Using GQDH as channel materials, we obtain high-performance ambipolar photodetectors with the advantages of low operating voltage, bipolarity, and excellent photoresponse. Because of their strong light absorption in the NIR region and ambipolar charge transport properties, the VPDs can be integrated into complementary logic circuits, which can be used in imaging, sensing, and optical communications.12 2. RESULTS AND DISCUSSION


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Figure 1. Schematics of the (a) QD VPD and (b) GQDH VPD. Cross-sectional SEM images of the (c) QD VPD and (d) GQDH VPD. (e) Absorbance of the PbS QDs. A TEM image of the PbS QDs is shown in the inset. (f) Raman spectrum of the graphene monolayer grown by CVD.


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The architectures of the QD and GQDH VPDs are schematically illustrated in Figure 1(a) and (b), respectively, which show the organization of the graphene/semiconductor/top electrode vertical stacks. To fabricate the GQDH VPD, a graphene monolayer grown on Cu foil by chemical vapor deposition (CVD) was transferred onto a 300-nm-thick SiO2/Si substrate as the bottom SE. Figure 1(f) shows the Raman spectrum of the graphene monolayer SE, which exhibited two strong peaks at ~1588 cm−1 (G band) and ~2685 cm−1 (2D band) with a small value of their intensity ratio (IG/I2D) of 0.22. Subsequently, ten layers of GQDH were deposited on the graphene monolayer as the channel. The GQDH thickness was determined by L. Finally, the top Au drain electrode was thermally evaporated on GQDH through a shadow mask, which defined the channel width (W) of 2 mm. The dimensions of the channel were defined by the overlapping area between the top drain electrode and bottom graphene SE. For comparison, a QD VPD with a similar structure to that of the GQDH VPD was also fabricated. In the QD VPD, GQDH was replaced by ten layers of PbS QDs. Figure 1(e) displays the absorption spectrum of the PbS QDs as a function of wavelength. The PbS QDs exhibit the first exciton absorption peak at 1310 nm, which gave an optical bandgap of 0.86 eV (Figure S1, Supporting Information). In addition, the PbS QDs absorb over the whole wavelength range from 700 to 1440 nm. A photocurrent can be generated when the incident photon energy is larger than the bandgap of the PbS QDs (0.86 eV). The number of photogenerated excitons will increase when the photon energy of the incident laser exceeds 0.86 eV (shorter wavelength than 1440 nm) and no photocurrent is generated for photon energies below 0.86 eV (longer wavelength than 1440 nm). The average diameter of the QDs determined through transmission electron microscopy (TEM)


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analysis was approximately 5.2 nm (Figure 1(e) inset). GQDH exhibited an absorption peak at 1300 nm that was similar to that of the PbS QDs, demonstrating that the photoinduced carriers were only generated in the PbS QDs (Figure S2, Supporting Information). Figure 1(c) and (d) depict cross-sectional scanning electron microscopy (SEM) images of the QD VPD and GQDH VPD, respectively. These images revealed that the QD layer was 166 nm thick and the GQDH layer was 180 nm thick.


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Figure 2. Output characteristics of the (a) QD VPD and (b) GQDH VPD in the p-type and n-type operating regions. Transfer characteristics of the (c) QD VPD and (d) GQDH VPD in the p-type and n-type regions. (e) On–off current ratio of GQDH VPDs with various channel lengths (thicknesses of


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GQDH). (f) Semilogarithmic transfer characteristics of the GQDH VPD with a channel length of 250 nm in the p-type and n-type operating regions.

The electrical properties of the devices were measured in the dark. Figure 2(a) and (b) show the output characteristics of the QD and GQDH VPDs, respectively, at different gate–source bias (VGS). The output curves exhibit hole accumulation at negative VGS and electron accumulation at positive VGS. The VPDs showed ambipolar behavior with a sharp rise of IDS at low VGS, which was attributed to the SB between the graphene SE and channel layer.15 Because of the linear energy dispersion relation of graphene near the Dirac point, the SB height at the graphene–channel layer interface is more sensitive to VGS than that of the top Au electrode–channel layer interface.16 The SB height of the graphene–channel layer interface can be effectively modulated because of the weak electrostatic screening effect and low state density of graphene. In contrast, the SB height of the top Au electrode–channel layer interface is hardly modulated by VGS because of the strong screening effect of the Au electrode. Therefore, the SB of the graphene–channel layer interface plays a dominant role in IDS modulation, which is similar behavior to that reported previously for vertical FETs.6 The doping type and level of graphene can be adjusted by a back-gate voltage.10 When the negative VGS was increased, the graphene electrode became p-doped and its WF increased. Thus, the SB height between the graphene electrode and p-type channel layer was lowered, leading to a larger negative IDS (left of Figure 2(a) and (b)). Similarly, increasing the positive VGS caused the graphene electrode to become n-doped and its WF decreased. Then, the SB height between the graphene electrode and n-type


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channel layer was lowered, generating a larger positive IDS (right of Figure 2(a) and (b)). The two types of VPDs exhibited U-shaped transfer curves in both the p- and n-type regions (Figure 2(c) and (d)), which confirmed that the ambipolar properties of the VPDs were controlled by VGS. The GQDH VPD exhibited a current density (J) of 15.6 mA/cm2, which exceeded that of the QD VPD (11.7 mA/cm2) at VGS = 1.8 V, VDS (drain–source bias) = 2 V; this was attributed to the high carrier mobility of graphene doped in the channel of the GQDH VPD (Figure S3, Supporting Information). Figure 2(e) shows the room-temperature on–off current ratios of GQDH VPDs with different GQDH thicknesses, which exhibited strong GQDH thickness dependence. The on–off ratios for both holes and electrons increased with GQDH thickness, which can be explained by the short-channel effect. When the GQDH thickness decreases, the potential of the entire channel becomes increasingly dominated by the drain electric field, which will lower the off-state SB height of the graphene–GQDH contact, thereby leading to a smaller on–off current ratio.6 The on–off ratios of the GQDH VPDs reached ~40 and ~20 for holes and electrons, respectively, for a relatively thick GQDH layer of 250 nm. Figure 2(f) illustrates the semilogarithmic transfer characteristics of the GQDH VPD with a channel length of 250 nm in the p- and n-type regions. The asymmetric on–off ratio for holes and electrons can be attributed to the higher SB height for hole injection than that for electron injection in the off state (Figure S4, Supporting Information). The on–off ratio of the GQDH VPD increases as VDS decreases (Figure S5, Supporting Information).


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Figure 3. Output characteristics of the (a) QD VPD and (c) GQDH VPD under different irradiation powers in the p- and n-type regionss. Responsivities of the (b) QD VPD and (d) GQDH VPD in the p- and n-type regions. Band diagrams of the VPDs under illumination at (e) negative and (f) positive VGS.

To investigate the photoresponse properties of the devices, their output characteristics under different irradiances from an 808-nm laser are shown in Figure 3(a) and (c). The two types of


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VPDs exhibited obvious photocurrents in both p-type (left) and n-type (right) regions because of the strong absorption of the PbS QDs at 808 nm. Low photocurrents were obtained at weak irradiance and higher photocurrents were observed as irradiance increased. The photocurrents of the GQDH VPD were higher than those of the QD VPD. This was because of the high charge mobility of graphene used in the channel layer, which resulted in the photoinduced charges flowing in the channel for many cycles until they were neutralized with carriers of opposite polarity. Figure 3(a) and (c) show the asymmetric IDS modulation under negative and positive VDS for the devices. The output characteristics of the vertical devices under different laser irradiance exhibit greater IDS modulation at negative VDS than at positive VDS. Such behavior can be explained by considering the schematic band diagrams illustrated in Figure 3(e) and (f). The difference of the SB heights between the graphene–QD and metal–QD interfaces produced the asymmetry. The negative VGS effectively lowered the SB height at the graphene–QD (p-type) interface, and the barrier height of the QD–metal interface also was low, allowing holes to easily flow from the graphene SE to the drain electrode to enhance IDS. Although a negative gate voltage decreased the SB height at the graphene–QD (n-type) interface, the barrier height at the QD–metal interface was high, making it difficult for electrons to be transported across the vertical stack; therefore, the devices showed a smaller IDS under these conditions. For a photodetector, R is an important figure of merit to evaluate its performance and can be given by the equation:17 =

=

△

,

(1)

where ∆ is the photocurrent, A is the illumination area (L×W), and Ee is the irradiance of


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incident light. Figure 3(b) and (d) plot R as a function of Ee using a double-logarithmic scale. A linear dependence between log(R) and log(Ee) is observed for both kinds of devices. Under a light irradiance of 24 mW/cm2, R was approximately 5.7 × 103 A/W for the QD VPD and 6.3 × 103 A/W for the GQDH VPD in the n-type region, and 9.7 × 103 A/W for the QD VPD and 1.6 × 104 A/W for the GQDH VPD in the p-type region. To further evaluate the photoelectronic properties of the devices, their external quantum efficiency (EQE) was calculated using the equation:18,19

= × × 100%

(2)

where h is Planck’s constant, c is the velocity of light, e denotes the fundamental unit of charge, and λ represents the wavelength of incident light. EQE was 8.8 × 105% for the QD VPD and 1 × 106% for the GQDH VPD in the n-type region, and 1.5 × 106% for the QD VPD and 2.4 × 106% for the GQDH VPD in the p-type region under a light irradiance of 24 mW/cm2. To characterize the device sensitivity, D* values were determined using the following equation:20 ∗

=

"∆#

(3)

$ %

where ∆& is the electrical band width in Hz and A is the effective area of the detector. NEP is a critical parameter for photodetectors, representing the minimum optical input power that a detector can distinguish from the noise. NEP can be expressed as: -

'( =

〈*+ 〉+ .

(4)

where In is the noise current (Figure S6, Supporting Information). NEP reached 1.5 × 10−14 W/Hz1/2 for the QD VPD and 6.3 × 10−15 W/Hz1/2 for the GQDH VPD in the n-type region, and


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8.7 × 10−15 W/Hz1/2 for the QD VPD and 2.2 × 10−15 W/Hz1/2 for the GQDH VPD in the p-type region. Estimated D* values were 1.2 × 1010 Jones for the QD VPD and 3 × 1010 Jones for the GQDH VPD in the n-type region, and 2.1 × 1010 Jones for the QD VPD and 8.6 × 1010 Jones for the GQDH VPD in the p-type region under a light irradiance of 24 mW/cm2.

Figure 4. Photoswitching characteristics of the (a) QD VPD and (c) GQDH VPD. A magnified cycle of the temporal photocurrent response of the (b) QD VPD and (d) GQDH VPD. (e) Energy diagram for the QD VPD at the positive gate in the dark. (f) Energy diagram for the GQDH VPD at


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the positive gate under light illumination.

The response time is another important parameter for phototransistors, which reflects the sensitivity, ensuring the VPDs can follow a switched optical signal. The temporal photoresponses of the QD VPD (VDS = 4 V, VGS = 4 V) and GQDH VPD (VDS = 1 V, VGS = 1.5 V) were measured by cyclical on–off light modulation switching at an irradiance of 335 mW/cm2, as shown in Figure 4(a) and (c). Both devices exhibited good stability and long-time repeatability of their photocurrent increases as the laser was turned on and decreases as the laser was turned off.21 To further investigate the response time of the devices, we magnified one of the response periods in each of Figure 4(a) and (c), which are shown in Figure 4(b) and (d), respectively. The photocurrent of the GQDH VPD increased with illumination time (Figure 4(b)) and could be fitted to the following exponential function: ∆/0 = 1234 + A7 81 − e;/=- > + ?@ 81 − A ;/=+ >,

(5)

where BC is the time constant with the respective amplitude weights ?C . The time constants for the GQDH VPD were B7 = 7.3 ms and B@ = 66 ms. The shot relaxation time B7 is related to the rapid increase of the photocurrent arising from the increased number of photoinduced carriers. The longer response time B@ represents the photogenerated carrier transfer in the channel, which causes the photocurrent to gradually reach saturation. The rise time (the time interval from 0% to 90% of ∆ ) was 8 ms, which is faster than that reported for PbS QD FETs with a conventional planar structure using graphene as the channel layer.13,14 Similarly, the photocurrent decreased when the light was turned off and could be fitted to the exponential function:


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∆/0 = 1234 + AD 8e;/=E > + ?F 8A ;/=G >,

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(6)

giving time constants of BD = 18 ms and BF = 148 ms. The fast decay time BD represents the rapid decrease of the photocurrent originating from the decrease in the number of photogenerated carriers. The longer decay time BF corresponds to the lifetime of the photogenerated carriers before being thoroughly recombined, which reveals that the photocurrent decreases to its initial value. The fall time (the time interval from the peak value to 10% of ∆ ) was 125 ms. The short response time is closely related to the short channel length of the GQDH VPD, which can shorten the transport time of the photogenerated carriers in the channel. Figure 4(d) shows the temporal photocurrent response of the QD VPD, which was fitted to Eq. (5) for the rise process and Eq. (6) for the decay process. For the QD VPD, the estimated rise time was 14 ms and fall time was 148 ms. The response time of the GQDH VPD was shorter than that of the QD VPD because of the high mobility of graphene doped in the channel, which allowed rapid transport of the photogenerated carriers. To further understand the fast photoresponse of the VPDs, we correlated the photoresponse characteristics of the GQDH VPD with its carrier transport properties and energy diagrams based on the above results. Figure 4(e) displays the energy band diagram of the GQDH VPD at positive drain and gate voltages in the dark. Under illumination, a large number of photoinduced excitons were produced in the QDs, divided into free holes and electrons by the positive VDS, and then subsequently transferred to graphene doped in the channel. Because of the high carrier mobility of graphene, photogenerated electrons quickly moved towards the drain electrode, while photogenerated holes rapidly diffused to the interface between the graphene electrode and


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GQDH. Subsequently, the photogenerated holes were injected into the graphene electrode and caused its Fermi level shift downward, resulting in a decrease of the contact resistance to hole injection, and greatly enhanced photocurrent (Figure 4(e)). When laser was switched off, the number of photogenerated holes in the GQDH decreased, causing the Fermi level of the graphene electrode to shift upward, increasing the contact resistance to hole injection, and thus greatly decreasing the photocurrent. The photocurrent decayed to its initial value following the recombination of the photogenerated carriers. 3. CONCLUSIONS We fabricated high-performance VPDs with a GQDH or QD layer and graphene as a transparent electrode. The current was controlled by the SB at the interface of the graphene and channel layers. Both the GQDH and QD VPDs exhibited excellent photoresponses because of the short channel length in the vertical device architecture. Doping graphene in the channel caused the GQDH VPD to exhibit better photoelectric properties (higher R, EQE, and D*) than those of the QD VPD. Additionally, the two types of VPDs exhibited excellent reproducible dynamic responses and short response times with a rise time of 8 ms for the GQDH VPD and 14 ms for the QD VPD. The fabrication of VPDs based on a graphene transparent electrode and light-absorbing GQDH as channel materials provides an approach to produce high-performance ambipolar photoelectric detectors.

4. EXPERIMENTAL Device Fabrication. The PbS QD VPD consisted of the following collinear stack: gate


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electrode, gate dielectric, SE, channel film, and top drain electrode. An n-type silicon (n+ Si) substrate was used as the gate electrode, with 300-nm-thick silicon dioxide (SiO2) acting as the gate dielectric. The n+ Si/SiO2 substrate was ultrasonically cleaned with deionized water, acetone, and ethanol in sequence. The graphene electrode was prepared by CVD and subsequently placed on the n+ Si/SiO2 wafer as the bottom SE, followed with an Au source contact along one side of the graphene layer. Subsequently, ten layers of PbS QDs were deposited on the graphene electrode as a channel film using a layer-by-layer approach.22 Each layer was prepared through the following steps. One drop of PbS QD toluene solution (10 mg/mL) was spin-cast onto the substrate at 2000 rpm for 10 s. Then, three drops of 2% (vol) ethanedithiol (EDT) solution were dripped on the spinning substrate at 2000 rpm to induce ligand exchange, followed by two spin-coating steps with two drops of acetonitrile and two drops of toluene at the same spin speed. Finally, the Au drain electrode was thermally evaporated onto the channel, which defined the channel width of 2 mm. The synthesis process of the PbS QDs is described in the Supporting Information. The GQDH VPD had a similar structure to that of the PbS QD VPD, except that the ten layers of PbS QDs in the QD VPD were replaced by ten layers of a graphene–PbS QD hybrid film in the GQDH VPD. The graphene–QD hybrid solution was prepared by mixing a graphene toluene solution (1 mg/mL) with PbS QD toluene solution (10 mg/mL) with a volume ratio of 1:3. The deposition method of GQDH was similar to that for the PbS QDs. A schematic diagram of the fabrication procedure of the GQDH VPD is shown in Figure S7 of the Supporting Information.


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Measurements. Electrical transport measurements were performed using a source meter (Keithley 2400) in the dark. A VDS was applied between the drain electrodes and bottom graphene SE (grounded), while a VGS was applied between the n+ Si gate and graphene electrodes. IDS flowed between the top drain and bottom graphene electrodes through the semiconductor channel. Photoresponse measurements were performed using lateral photodetection. The wavelength of the laser was 808 nm and its photon energy was 1.5 eV, which is in the absorption spectrum of the PbS QDs. All measurements were conducted at room temperature under ambient conditions. ASSOCIATED CONTENT

Supporting Information Optical bandgap of PbS QDs; absorption spectrum of the graphene/PbS QD hybrid; J−VDS output characteristics of the QD VPD and GQDH VPD; band structure of the GQDH VPD with a channel length of 250 nm; on–off ratio as a function of VDS; noise level of the QD and GQDH VPDs; synthesis of PbS QDs; schematic diagram of the fabrication of the GQDH VPD.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 61675147 and 61605141) and the Foundation of Independent Innovation of Tianjin University (No. 0903065043).

REFERENCES (1) Ben-Sasson, A. J.; Tessler, N. Unraveling the Physics of Vertical Organic Field Effect Transistors through Nanoscale Engineering of a Self-Assembled Transparent Electrode. Nano Lett. 2012, 12, 4729-4733.

(2) Kvitschal, A.; Cruz-Cruz, I.; Hümmelgen, I. A. Copper Phthalocyanine based Vertical Organic Field Effect Transistor with Naturally Patterned Tin Intermediate Grid Electrode. Org. Electron. 2015, 27, 155-159.

(3) Ben-Sasson, A. J.; Tesslera, N. Patterned Electrode Vertical OFET: Analytical Description, Switching Mechanisms and Optimization Rules. SPIE. 2011, 8117, 81170Z.

(4) Yang, H. J.; Heo, J.; Park, S.; Song, H. J.; Seo, D. H.; Byun, K. E.; Kim, P.; Yoo, I.; Chung, H. J.; Kim, K. Graphene Barristor, a Triode Device with a Gate-Controlled Schottky Barrier. Science. 2012, 336, 1140-1143.

(5) Li, N.; Oida, S.; Tulevski, G. S.; Han, S. J.; Hannon, J. B.; Sadana, D. K.; Chen, T. C.


20

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Efficient and Bright Organic Light-Emitting Diodes on Single-Layer Graphene Electrodes. Nat. Commun. 2013, 4, 2294.

(6) Yu, W. J.; Li, Z.; Zhou, H.; Chen, Y.; Wang, Y.; Huang, Y.; Duan, X. Vertically Stacked Multi-Heterostructures of Layered Materials for Logic Transistors and Complementary Inverters. Nat. Mater. 2013, 12, 246-252.

(7) Kang, J.; Jariwala, D.; Ryder, C. R.; Wells, S. A.; Choi, Y.; Hwang, E.; Cho, J. H.; Marks, T. J.; Hersam, M. C. Probing Out-of-Plane Charge Transport in Black Phosphorus with Graphene-Contacted Vertical Field-Effect Transistors. Nano Lett. 2016, 16, 2580-2585.

(8) Ben-Sasson, A. J.; Azulai, D.; Gilon, H.; Facchetti, A.; Markovich, G.; Tessler, N. Self-Assembled Metallic Nanowire-Based Vertical Organic Field-Effect Transistor. ACS Appl. Mater. Interfaces. 2015, 7, 2149-2152.

(9) Tian, H.; Tan, Z.; Wu, C.; Wang, X.; Mohammad, M. A.; Xie, D.; Yang, Y.; Wang, J.; Li, L. J.; Xu, J.; Ren, T. L. Novel Field-Effect Schottky Barrier Transistors Based on Graphene-MoS2 Heterojunctions. Sci. Rep. 2014, 4, 5951.

(10) Liu, Y.; Zhou, H.; Weiss, N. O.; Huang, Y.; Duan, X. F. High-Performance Organic Vertical Thin Film Transistor Using Graphene as a Tunable Contact. ACS Nano. 2015, 9, 11102-11108.

(11) Kim, C. O.; Hwang, S. W.; Kim, S.; Shin, D. H.; Kang, S. S.; Kim, J, M.; Jang, C. W.; Kim, J. H.; Lee, K. W.; Choi, S. H.; Hwang, E. High-performance graphene-quantum-dot


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photodetectors. Sci. Rep. 2014, 4, 5604.

(12) Kim, C. O.; Kim, S.; Shin, D. H.; Kang, S. S.; Kim, J. M.; Jang, C. W.; Joo, S. S.; Lee, J. S.; Kim, J. H.; Choi, S. H.; Hwang, E. High Photoresponsivity in An All-Graphene p-n Vertical Junction Photodetector. Nat. Comm. 2014, 5, 3249.

(13) Sun, Z.; Liu, Z.; Li, J.; Tai, G.; Lau, S. P.; Yan, F. Infrared Photodetectors Based on CVD-Grown Graphene and PbS Quantum Dots with Ultrahigh Responsivity. Adv. Mater. 2012, 24, 5878-5883.

(14) Konstantatos, G.; Badioli, M.; Gaudreau, L.; Osmond, J.; Bernechea, M.; Arquer, F. P. G.; Gatti, F.; Koppens, F. H. L. Hybrid Graphene–Quantum dot Phototransistors with Ultrahigh Gain. Nat. Nanotechnol. 2012,7, 363-368.

(15) Zhou, Y.; Han, S. T.; Sonar, P.; Ma, X.; Chen, J.; Zheng, Z.; Roy, V. A. L. Reversible Conversion of Dominant Polarity in Ambipolar Polymer/Graphene Oxide Hybrids. Sci. Rep. 2015, 5, 9446.

(16) Kim, J. S.; Kim, B. J.; Choi, Y. J.; Lee, M. H.; Kang, M. S.; Cho, J. H. An Organic Vertical Field-Effect Transistor with Underside-Doped Graphene Electrodes. Adv. Mater. 2016, 28, 4803–4810.

(17) Chitara, B.; Panchakarla, L. S.; Krupanidhi, S. B.; Rao, C. N. R. Infrared Photodetectors Based on Reduced Graphene Oxide and Graphene anoribbons. Adv. Mater. 2011, 23, 5419-5424.


22

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18) Yu, H.; Bao, Z.; J. Oh, H. High-Performance Phototransistors Based on Single-Crystalline n-Channel Organic Nanowires and Photogenerated Charge-Carrier Behaviors. Adv. Funct. Mater. 2013, 23, 629-639.

(19) Kramer, I. J.; Sargent, E. H. The Architecture of Colloidal Quantum Dot Solar Cells: Materials to Devices. Chem. Rev. 2014, 114, 863-882.

(20) Wang, H.; Li, Z.; Fu, C.; Yang, D.; Zhang, L.; Yang, S.; Zou, B. Solution-Processed PbSe Colloidal Quantum Dot-Based Near-Infrared Photodetector. IEEE Photonics Technol. Lett. 2015, 27, 612-615.

(21) Li, F.; Ma, C.; Wang, H.; Hu, W.; Yu, W.; Sheikh, A. D.; Wu, T. Ambipolar Solution-Processed hybrid Perovskite Phototransistors. Nat. Commun. 2015, 6, 8238.

(22) Zhang, Y. T.; Cao, M. X.; Song, X. X.; Wang, J. L.; Che, Y. L.; Dai, H. T.; Ding, X.; Zhang, G. Z.; Yao, J. Q. Multiheterojunction Phototransistors Based on Graphene−PbSe Quantum Dot Hybrids. J. Phys. Chem. C. 2015. 19, 21739-21743.


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