High‐Performance UV–Vis–NIR Phototransistors Based on ...ohgroup.snu.ac.kr/data/file/br_21/1914721941_V5RjQc8m_92.pdfNW-based ones and comparable to those of high-performance

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according to its intensity and wave-length.[1] Near-infrared (NIR) phototran-sistors have recently emerged as an important technical platform because of their high potential for a wide range of advanced applications, such as interac-tive communication, remote controlling systems, thermal imaging, night vision surveillance, and healthcare monitoring systems.[2] Organic semiconductors are suitable for use in photoactive layers of flexible photo-sensors because of their mechanical flexibility and tunable opto-electrical properties based on rational molecular design with inexpensive fabri-cation processes. In particular, 1D organic nano/microwires (NWs/MWs) have received a great deal of attention as prom-ising photoactive building blocks for high-performance phototransistors owing to their diverse advantages including intrin-sically high crystallinity, high surface-to-volume ratio, and tunable optoelectrical properties.[3]

Plasmonic nanomaterials have been extensively investigated due to their

unique optical/electrical properties exhibiting light scattering and localized surface plasmon resonance (LSPR), which are induced upon interaction with incident electromagnetic radia-tion (e.g., light at specific wavelengths within the visible and NIR spectral regions).[4] Gold nanocrystals (Au NCs) are a rep-resentative plasmonic nanostructure with unique LSPR prop-erties determined by their shape, which can be used as sub-wavelength scattering elements enhancing the light trapping.[5] Especially, Au nanorods (Au NRs) can exhibit higher near-field enhancement and generate more electron–hole pairs and hot-carriers from plasmon decay under light illumination com-pared with other NCs such as spherical nanoparticles (NPs) and their plasmon wavelength of the longitudinal resonance can readily be controlled by tuning their aspect ratio.[6] There-fore, they have been intensively examined for various applica-tions including plasmon-enhanced spectroscopy,[7] chemical sensors,[8] and optoelectronic devices.[9]

Currently, there is an increasing demand for alterna-tive approaches to enhance the performances of optoelec-tronic devices due to the intrinsic limitations (e.g., low molar

High-Performance UV–Vis–NIR Phototransistors Based on Single-Crystalline Organic Semiconductor–Gold Hybrid Nanomaterials

Ji Hyung Jung, Min Ji Yoon, Ju Won Lim, Yoon Ho Lee, Kang Eun Lee, Dong Ha Kim,* and Joon Hak Oh*

Hybrid materials in optoelectronic devices can generate new functionality or provide synergistic effects that enhance the properties of each component. Here, high-performance phototransistors with broad spectral responsivity in UV–vis–near-infrared (NIR) regions, using gold nanorods (Au NRs)-decorated n-type organic semiconductor and N,N′-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) nanowires (NWs) are reported. By way of the synergistic effect of the excellent photo-conducting characteristics of single-crystalline BPE-PTCDI NW and the light scattering and localized surface plasmon resonances (LSPR) of Au NRs, the hybrid system provides new photo-detectivity in the NIR spectral region. In the UV–vis region, hybrid nanoma-terial-based phototransistors exhibit significantly enhanced photo-responsive properties with a photo-responsivity (R) of 7.70 × 105 A W−1 and external quantum efficiency (EQE) of 1.42 × 108% at the minimum light intensity of 2.5 µW cm−2, which are at least tenfold greater than those of pristine BPE-PTCDI NW-based ones and comparable to those of high-performance inorganic material-based devices. While a pristine BPE-PTCDI NW-based photodetector is insensitive to the NIR spectral region, the hybrid NW-based phototransistor shows an R of 10.7 A W−1 and EQE of 1.35 × 103% under 980 nm wavelength-NIR illumination. This work demonstrates a viable approach to high-performance photo-detecting systems with broad spectral responsivity.

J. H. Jung, Y. H. Lee, Prof. J. H. OhDepartment of Chemical EngineeringPohang University of Science and Technology (POSTECH)Pohang, Gyeongbuk 37673, South KoreaE-mail: [email protected]. J. Yoon, J. W. Lim, Prof. D. H. KimDepartment of Chemistry and Nano ScienceEwha Womans UniversitySeoul 03760, South KoreaE-mail: [email protected]. E. LeeComposites Research DivisionKorea Institute of Materials Science (KIMS)797 Changwondae-ro, Changwon, Gyungnam 51508, South Korea

DOI: 10.1002/adfm.201604528

1. Introduction

Phototransistors modulate charge-carrier density in the photo-active layer by generating photo-current from incoming light

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absorptivity or long response time) of single-component semiconduc-tors as photoactive material.[10] To overcome this limitation, novel hybrid systems have been intro-duced as alternatives through var-ious fabrication methods, including electrospinning,[11] meniscus-guided method,[12] electrostatic assembly,[13] spin-coating,[14] and stirring,[15] because they can sup-plement material weaknesses by combining two components into one system.[16] Furthermore, hybrid platforms can generate new func-tionality or offer synergistic inter-actions by making the best use of the different electrical/optical properties of each component.[17] Therefore, the sophisticated inte-gration of two elements is a viable way for realizing next-generation optoelectronics.

Here, we report high-perfor-mance organic/inorganic hybrid phototransistors that can detect incoming photonic signals in a wide range of spectral regions using Au NRs-decorated organic semiconductor NWs as the photo-active layers. We present a simple fabrication methodology to assemble Au NRs onto the sur-face of n-type organic semiconductor N,N′-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) NWs and investigate photo-generated charge-carrier dynamics in the organic/inorganic phototransistors. Compared with pristine organic NW phototransistors, the organic/inorganic hybrid phototransistors exhibit superior optoelectrical characteristics with a photo-responsivity (R) of 7.70 × 105 A W−1 and external quantum efficiency (EQE) of 1.42 × 108% at the minimum light intensity of 2.5 µW cm−2, which are comparable to those of high-performance inorganic material-based devices. Impor-tantly, the photo-detectivity of organic phototransistor in the NIR spectral region has become feasible using the LSPR and scattering effect of Au NRs. Our in-depth study demonstrates a viable approach for fabricating solution-processable high-performance hybrid phototransistors with broad spectral responsivity.

2. Results and Discussion

2.1. Preparation and Characterization of the BPE-PTCDI NWs/Au NRs Hybrid Nanostructure

Polyethylene glycol (PEG)-capped Au NRs were synthesized using a two-step method comprising a seed formation and a growth step.[18] The typical transmission electron microscopy

(TEM) image of the synthesized PEG-capped Au NRs is shown in Figure 1a. The average length and width estimated from the randomly chosen 34 Au NRs were 92.6 (±5.04) and 19.4 (±1.67) nm, respectively, with an aspect ratio of 4.8 (Figure S1, Supporting Information). They were highly dispersible in var-ious solvents, including acetonitrile, acetone, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), ethanol, and meth-anol.[19] The absorption spectra of the aqueous dispersions of the Au NRs before and after ligand exchange with PEG are shown in Figure S2 (Supporting Information). Each sample had a strong resonance band around 1050 nm corresponding to the longitudinal plasmon oscillation as well as a weak band around 500 nm due to the transverse plasmon oscillation, which confirms the successful synthesis of Au NRs. The longi-tudinal band was slightly red-shifted due to the presence of the PEG ligand.

Owing to the strong cofacial π–π interactions in planar aro-matic molecules, 1D single-crystalline organic NWs can readily be fabricated using BPE-PTCDI (Figure 1b) via a non-solvent nucleation-mediated recrystallization method that utilizes a dramatically increasing solubility of BPE-PTCDI molecules at an elevated solution temperature.[3b,15b,20] The length and width of the NWs can be modulated by controlling the cooling rate of hot homogeneous solution or adjusting the injection amount of non-solvent.[20a] In this report, pristine BPE-PTCDI NWs were fabricated using ethyl alcohol as non-solvent. The length

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a)

c) d)

b)

100 nm

1 µm400 600 800 1000 1200

0.00.10.20.30.40.50.60.7

Absorbance(a.u.)

Wavelength (nm)

Hybrid NWsBPE-PTCDI NWs

Au NRs

Figure 1. a) A TEM image of PEG-capped Au NRs. b) The molecular structure of BPE-PTCDI. c) An HR-TEM image of a hybrid NW. d) UV–vis spectra of pristine BPE-PTCDI NWs, Au NRs, and a hybrid BPE-PTCDI NW/Au NR dispersion in ethanol.

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and width of BPE-PTCDI NWs estimated from the randomly chosen 30 samples were 94.47 (±25.26) and 0.763 (±0.126) µm, respectively. To functionalize BPE-PTCDI NW with Au NRs, PEG-capped Au NR solution was added into BPE-PTCDI NW dispersion four times with an interval of 24 h with the mix-ture being shaken. The details for the fabrication procedures of hybrid nanomaterials are described in the Experimental Section. Figure 1c shows a high-resolution (HR)-TEM image of Au NR-decorated single BPE-PTCDI NW and the density of adsorbed Au NRs on surface of BPE-PTCDI NW was esti-mated to be ≈77 µm−2. Judging from the thermogravimetric analysis (TGA) results, ≈24% of the injected PEG-capped Au NRs was adsorbed on the semiconductor nanowires (Experi-mental Methods and Figure S3, Supporting Information). In order to enhance the dispersibility of ligand-capped Au NRs and achieve better adhesion of Au NRs onto the surface of BPE-PTCDI NW, the ligand was exchanged from a cationic sur-factant cetyltrimethylammonium bromide (CTAB) to a more neutral surfactant PEG before hybridization. Only a few aggre-gated CTAB-capped Au NRs were adsorbed on BPE-PTCDI NW (Figure S4, Supporting Information). In addition, the PEG ligand would be better in terms of device operation since lots of counter anionic species in CTAB may affect as unintentional dopants to the photoactive layer and charge transport would become efficient when the ligand is exchanged from CTAB (≈6 nm thickness) to PEG ligand (≈2.5 nm thickness).[21] UV–vis absorption spectra of pristine BPE-PTCDI NWs, Au NRs, and their hybrid nanomaterials are shown in Figure 1d. Au NRs had a maximum band absorption at 1077 nm corresponding to the longitudinal plasmon oscillation, whereas the pristine BPE-PTCDI NWs were at 469 and 663 nm. Although the overall absorption spectrum of hybrid NWs was similar to that of pris-tine BPE-PTCDI NWs due to the relatively high concentration of BPE-PTCDI NWs, highly enhanced absorption was observed at the NIR region as well as at the UV and visible region due to the LSPR and scattering effect of Au NRs. Pristine BPE-PTCDI NWs and hybrid NW show long tail absorption spectra due to the scattering effect of organic NWs, which is consistent with absorption characteristics of previous reports based on PTCDI derivatives.[6c,22]

2.2. Optoelectrical Characteristics of Hybrid Nanomaterial-Based Phototransistors

To analyze the optoelectrical properties of hybrid BPE-PTCDI NW/Au NRs nanomaterials, their dispersion was spin-coated on n-octadecyltrimethoxysilane (OTS)-modified SiO2/Si wafers with photolithographically patterned source/drain electrodes for the bottom-contact bottom-gate configuration transistors. An intensity- and wavelength-tunable monochromatic light source for the UV (350 nm) and visible region (460 nm [blue], 532 nm [green], 670 nm [red]) and an IR dot beam laser module for NIR light (980 nm) were used as light sources. Typical transfer and output characteristics are shown in Figure 2a,b, respectively. In the transfer curves, the drain current was significantly increased under the light illumination. The photo-current exhibited satu-rated behaviors below a 670 nm wavelength (red light). This phenomenon can be attributed to the lattice thermalization of

excessive photon energy loss higher than 1.60 eV, which is the optical energy bandgap of BPE-PTCDI NWs corresponding to 775 nm wavelength light.[23] A single hybrid NW-based photo-transistor from at least ten devices in dark conditions had a peak electron mobility of 0.324 cm2 V−1 s−1, average mobility of 0.175 (±0.062) cm2 V−1 s−1, and current on/off ratio of 2.13 (±1.62) × 105 with a threshold voltage (VT) of 20.70 (±4.97) V. Pristine BPE-PTCDI NW-based transistors showed an average mobility of 0.257 (±0.103) cm2 V−1 s−1. This result indicates that Au NRs may not give synergy effect on the electrical proper-ties of hybrid system under dark condition. Slightly reduced electrical performance of the hybrid NW-based phototransis-tors under dark condition can be attributed to the lower contact quality of dielectric–semiconductor and electrode–semicon-ductor interfaces in the presence of Au NRs on the surface of BPE-PTCDI NWs.[6c,15a,24]

Representative parameters for the evaluation of photosensi-tivity in optoelectronics, R and photo-current/dark-current ratio (P), can be calculated using the following equations[25]

ph

inc

light dark

inc

RI

P

I I

P= =

−

(1)

light dark

dark

PI I

I=

−

(2)

where Iph is the photo-current, Pinc is the incident illumination power, Ilight the drain current under illumination, and Idark the drain current in the dark condition. The R and P values typi-cally increase with increasing VGS due to the efficient exciton separation under a strong vertical electric field according to Equations (1) and (2).[26] EQE (η) of the phototransistors, the ratio of the number of photo-generated carriers enhancing the drain current to that of the photons irradiated onto the channel area, can be estimated using the following equation[27]

light dark

int peak

I I hc

eP Aη

λ( )=

−

(3)

where h is the Planck constant, c the speed of light, e the fun-damental unit of charge, Pint the incident light intensity (i.e., the incident optical power density), A the area of the transistor channel, and λpeak the peak wavelength of the incident light. The average A of hybrid NWs in phototransistor was 16.10 (±3.41) µm2. In our previous study, the pristine BPE-PTCDI NW-based phototransistor recorded an R of 1.40 × 103 A W−1, P of 4.96 × 103, and EQE of 2.63 × 105%, which is one of the highest performance organic single NW-phototransistors based on perylene diimide (PDI) derivatives.[20a] In this study, hybrid NW-based phototransistors reached a maximum value of R of 3.20 × 104 A W−1, P of 1.87 × 106, and EQE of 8.63 × 106% under blue light (λ = 460 nm) illumination (Figure 2c,d). All para-meters, including R, P, and EQE, were significantly increased in the UV–vis region by at least tenfold after the hybridization of nanomaterials. This result can be attributed to the LSPR and scattering effect of Au NRs injecting hot-carriers to the adja-cent semiconductor, the prolonged exciton diffusion length, and the lowered energetic barrier for charge-carrier transport owing to the intrinsically defect-free single-crystalline nature

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of BPE-PTCDI.[6c,15b,20a] Hot-carriers generated by plasmon resonances in Au NRs with sufficient momentum can be trans-ferred to adjacent semiconducting BPE-PTCDI NWs, leading to enhanced electrical performance of phototransistors under light irradiation.[28] It is also noteworthy that functionalization of Au NRs on the surface of semiconducting materials enabled photo-detection in the NIR region. Owing to the longitudinal oscilla-tion of Au NRs, the hybrid NW-based phototransistor resulted in an R of 10.7 A W−1, P of 9.54 × 104, and EQE of 1.35 × 103% under 980 nm (NIR) wavelength-light illumination. The R value was strongly enhanced as the applied gate–source voltage was increased, which can be explained by an effective charge separation generated by the strong electric field of the gate electrode.[29] The generated hot-electrons arising from plasmon decay and interband absorption in Au NRs can overpass the metal–semiconductor Schottky barrier and be injected into the neighboring lowest unoccupied molecular orbital (LUMO) level of the organic semiconductor, transferring electrons into the active layer.[6d,24,28b] In addition, photo-generated carriers in

the photoactive layer and captured at the semiconductor–metal interface can generate high electric fields, resulting in photo-multiplication (PM) and tunneling electron injection.[30]

As shown in Figure 3a, the real-time photo-responses of hybrid NW-based phototransistors were investigated under pulsed incident light at 30 s intervals in photo-conductor (transistor-off) mode without external gate bias (VGS = 0 V and VDS = 80 V). They exhibited highly sensitive, rapid-switching behaviors under all monochromatic light illumination, including in the NIR region. Compared with the pristine BPE-PTCDI NW-based phototransistors (Figure S5, Supporting Information), the hybrid NW-based ones generated much more photo-current. Furthermore, the novel function of photo-detec-tion in NIR region was generated after the integration of Au NRs due to strong light absorption in the NIR region of Au NRs. The phototransistor based on pristine NWs did not show photo-responses in the NIR region.

To describe the rise/decay photo-responses of the devices, the following equations can be used[31]

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0 20 40 60 800

10

20

30

40

50

60

I D(n

A)

VDS (V)

VGS = -20 to 80 V

-40 -20 0 20 40 60 80100

101

102

103

104

105

106

107

350nm460nm532nm670nm980nm

VGS (V)10-1

100

101

102

103

104

105

R(A

W-1)

P

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10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

350 460 532670 980 Dark

VGS (V)

I D(A

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

(ID ) 1/2(X10

-3A1/2)

c) d)

a) b)

-40 -20 0 20 40 60 80101

102

103

104

105

106

107

EQ

E(%

)

VGS (V)

350 nm460 nm532 nm670 nm980 nm

Figure 2. Optoelectrical characteristics of the hybrid NW-based phototransistors. a) Transfer curves in the dark and under different light irradiations and b) output curves in the dark condition. c) Photo-responsivity (R), photo-current/dark-current ratio (P), and d) external quantum efficiency (EQE).

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light dark/ /r 1 r 2I I Ae Bet t= + +τ τ

(4)

light dark/ /d1 d 2I I Ae Bet t= + +τ τ− −

(5)

where t is the light switching time, A and B are scaling con-stants, and τ1 and τ2 are time constants for fast and slow rising/decaying rate, respectively. The rise time (tr) from Equation (4) is the time required for the current to rise to 90% of the peak value after light illumination, whereas the decay time (td) from Equation (5) is the time required for the current to be decreased to 10% of the maximum value.[32] The tr and td values calculated by bi-exponential fitting, revealed that the tr and td were shorter than 4 s (τr1 = 0.79 ± 0.18 s, τr2 = 1.22 ± 0.98 s) and 7 s (τd1 = 1.59 ± 1.26 s, τd2 = 3.45 ± 3.72 s), respectively, under monochromatic light illumination (Figure 3b–f). The real-time photo-response behaviors of hybrid NW-based phototransistors before nor-malization are shown in Figure S6 (Supporting Information). On the other hand, pristine NW-based phototransistors had tr and td values shorter than 4 s (τr1 = τr2 = 0.97 ± 0.48 s) and

longer than 10 s (τd1 = 2.25 ± 2.06 s, τd2 = 27.96 ± 29.99 s), respectively. Although the measured tr values were similar for pristine and hybrid NW-based phototransistors, hybrid NW-based phototransistors had much higher normalized drain current than that of pristine NW-based phototransis-tors because of the hot-electrons generated on the surface of Au NRs. The noticeably shortened td of hybrid NW-based phototransistors can be explained by the tendency of Au NRs to trap and restore charges toward equilibrium states serving as recombination sites, which results in electron depletion in the channel.[28b,31c,33] Additional charge-trapping sites can be induced at the surface of semiconducting materials, and/or semiconductor–Au NRs, semiconductor–dielectric, and semi-conductor–electrode interfaces.[15b,18] Under the operation condition of hybrid optoelectronic devices, however, continu-ously occurring complex physical effects such as exciton generation, exciton coupling, exciton dissociation, and exciton recombination by the plasmon oscillation of Au nanostruc-tures may cause the large fluctuation of drain current level.[34]

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tr ≤ 3.90 s

td ≤ 1.82 s tr ≤ 1.51 std ≤ 1.84 s

tr ≤ 1.51 s

td ≤ 2.70 s

tr ≤ 1.81 s

td ≤ 6.31 str ≤ 1.92 s

td ≤ 3.41 s

d)

a) b) c)

e) f)

60 80 100 120

1.0

1.5

2.0

2.5

Nor

m. I

D(a

.u.)

Time(s)

Experimental data (980 nm)Fitting line

120 140 160 180

1

2

3

4

5

Nor

m. I

D(a

.u.)

Time(s)


180 200 220 240

1

2

3

4

5

Nor

m.I

D(a

.u.)

Time(s)


240 260 280 300

1

2

3

4

5

Nor

m. I

D(a

.u.)

Time(s)


300 320 340 360

1

2

3

4

5

Nor

m. I

D(a

.u.)

Time(s)


980

670 532 460350 nm

60 120 180 240 300 360

1

2

3

4

5

Nor

m. I

D( a

. u. )

Time(s)

Figure 3. a) The real-time photo-response behaviors of hybrid NW-based phototransistors as a function of time under the illumination of mono-chromatic light with a wavelength of 350 nm (UV), 460 nm (blue), 532 nm (green), 670 nm (red), and 980 nm (NIR). b–f) Bi-exponential fitting results for the photo-response time calculation.

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For a more in-depth study of the optoelectronic characteristics of phototransistors, photo-responses to red incident light (λ = 670 nm) with different light intensities were monitored, as this wavelength corresponds to the maximum light absorption range of hybrid NWs (Figure 1d). The hybrid NW-based photo-transistors responded to the light under an extremely low light intensity of 2.5 µW cm−2, which was the minimum light intensity of the monochromator in the laboratory (Figure 4a,b). The R and EQE values were further increased to an R of 7.70 × 105 A W−1 and EQE of 1.42 × 108%, as the power inten-sity of the monochromic light was decreased to 2.5 µW cm−2. It is noteworthy that these unprecedented R and EQE values of phototransistors based on hybrid nanostructures are com-parable to those of high-performance inorganic material-based phototransistors.[35] The value of P increased following power law relationship of (ID ∼ P0.33) at a given light intensity.

To quantitatively investigate the charge accumulation and release phenomena from the deep traps under light on and off switching, the following equation can be used[20a]

Rated

dtC

d

dt2 d

dtT

sat

1/2D

1/2Q V t LC

W

I t

µ( )( ) ( )

= = = −

(6)

The average accumulation and release rate of charge car-riers were 4.21 × 1013 and 3.66 × 1013 cm2 s−1, respectively (see Figure 4c), which was obtained from the linear fitting of NIR light (980 nm) detection in Figure 3a. The highest accumulation rate was 1.75 × 1014 cm2 s−1 under UV light (350 nm) illumina-tion, possibly because it had the highest photon energy among various irradiated light sources. After the functionalization of BPE-PTCDI NWs with Au NRs, greatly enhanced accumula-tion and release rates were estimated compared with pristine

BPE-PTCDI NW-based phototransistors, which had accumu-lation and release rates of 2.76 × 109 and 1.17 × 109 cm2 s−1, respectively.[20a] Much faster charge accumulation and release occurred after the functionalization of BPE-PTCDI NWs with Au NRs due to the continuous supply of hot-electrons from Au NRs to the adjacent semiconducting material under light illumination and charge trapping by Au NRs in the dark condi-tion, respectively.

2.3. Finite-Difference Time-Domain (FDTD) Simulation

To theoretically investigate the scattering and LSPR effects of the synthesized Au NRs, a FDTD simulation was performed (Figure 4d). The FDTD simulation results indicated that the electric field around the surface of Au NRs was intensified under NIR light (980 nm) irradiation. Correlating with longi-tudinal peaks of Au NRs, the simulated absorption spectrum of the hybrid NWs also demonstrates the enhanced light absorp-tion over the UV–vis and NIR spectral regions (Figure S7, Sup-porting Information), which is consistent with the experimental data shown in Figure 1d.

3. Conclusion

In conclusion, n-channel organic semiconductor NW/Au NR hybrid phototransistors were fabricated and their photo-conducting properties were investigated. Au NRs-decorated BPE-PTCDI NWs were prepared using a simple solution-based approach. Our novel hybrid system allowed the efficient conversion of incoming photons into hot-carriers in a broad

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0 20 40 60 80 1000.0

2.0x104

4.0x104

6.0x104

8.0x104

1.0x105

1.2x105

P R (A W-1) EQE (%)

Light intensity ( W cmµ -2)

104

105

106

107

108

0 5 10 15 20 25 307

8

9

10

11

12

13

14

(ID)1/

201

X(-4A

1/2 )

Time after light on/off (s)

Charge accumulation rate after light on (980 nm) Charge release rate after light off Fitting line

100 µW cm-2

50

10

5

2.5

(c)

(a)

(d)

(b)

60 120 180 240 300 3600.0

1.0x10-8

2.0x10-8

3.0x10-8

4.0x10-8

I ph)

A(

Time (s)

Figure 4. a) Photo-responsivity (R), photo-current/dark-current ratio (P), external quantum efficiency (EQE), and b) photo-switching behavior of hybrid NW-based phototransistor depending on the light intensity of red light (670 nm). c) Charge accumulation/release rate calculation under NIR (980 nm), light switching extracted from Figure 3b before normalization. d) Electric field enhancement under 980 nm light calculated by FDTD simulation.

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spectral range covering the UV–vis and NIR regions. The hybrid nanomaterial-based phototransistors exhibited greatly enhanced photo-responsive properties compared with those of pristine BPE-PTCDI NW-based ones, rivaling those of high-performance inorganic material-based devices. In particular, the detection of NIR region was successfully achieved because of the synergy among the surface plasmon, scattering effect, superb hot-electron-harvesting of Au NRs, and the facile injec-tion of hot-electrons to the neighboring semiconductor. In addition, the functionalization of BPE-PTCDI NWs with Au NRs could shorten the photo-response time and enhance the charge accumulation/release rate. Our approach to fabricate a novel hybrid nanostructure system is expected to provide new possibilities, and represents a viable method to achieve high-performance hybrid optoelectronics.

4. Experimental SectionPreparation of BPE-PTCDI NW/Au NR Hybrid Nanostructures:

Pristine BPE-PTCDI NWs were fabricated by a nonsolvent-mediated recrystallization method as reported previously.[6c] After dissolving 2 mg of BPE-PTCDI in refluxing o-dichlorobenzene (4 mL) at 140 °C, 1 mL of ethyl alcohol was injected into hot solution as a non-solvent. After cooling, recrystallized NWs were then vacuum-filtered with a porous anodized aluminum oxide (AAO) to remove unreacted BPE-PTCDI and re-dispersed in 3 mL of ethanol. For the BPE-PTCDI NW/Au NR hybrid nanostructures, 1 mL of Au NR suspension (4.17 wt%) was added into BPE-PTCDI dispersion four times at intervals of 24 h and kept the mixture shaken and/or stirred. At last, another vacuum filtration was conducted to extract segregated Au NRs.

Fabrication and Characterization of BPE-PTCDI NW/Au NR Phototransistors: Conventional photolithography was adopted to define source and drain gold electrode in bottom-gate bottom-contact device configuration. The hybrid nanomaterials could readily be deposited on OTS treated SiO2/Si wafer by spin-coating technique.[36] A heavily n-doped Si wafer with a thermally grown 300 nm thick SiO2 layer (Ci = 10 nF cm−2) was used as the gate and the dielectric layer, respectively. After spin-coating, the devices were annealed on a hot plate at 60 °C for 3 h in N2-filled glove box to evaporate solvent thoroughly.

Other Materials and Methods: Other materials and methods are available in the Supporting Information.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was supported by the Center for Advanced Soft Electronics under the Global Frontier Research Program (2013M3A6A5073175), the National Research Foundation of Korea (2014R1A2A2A01007467) of the Ministry of Science, ICT & Future Planning, Korea, and the Principal Research Program in the Korea Institute of Materials Science (KIMS). M.J.Y., J.W.L., and D.H.K. acknowledge the financial support by the National Research Foundation of Korea Grant funded by the Korean Government (2014R1A2A1A09005656). J.H.J. acknowledges Cheol Hee Park for his help in photolithographic patterning of source/drain electrodes on SiO2/Si wafers and Moo Yeol Lee for his help in drawing 3D illustrations.

Received: September 1, 2016Revised: November 11, 2016

Published online: December 29, 2016

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