Single Crossed Heterojunction Assembled with Quantum-Dot

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Single Crossed Heterojunction Assembled with Quantum Dot-Embedded Polyaniline Nanowires Xianguang Yang, Dinghua Bao, Yao Zhang, and Baojun Li ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00241 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 26, 2016

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Single Crossed Heterojunction Assembled with Quantum Dot-Embedded Polyaniline Nanowires

Xianguang Yang, Dinghua Bao, Yao Zhang*, and Baojun Li* State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China *E-mail: [email protected]; [email protected]

ABSTRACT: Nanoscale photodetectors are highly attractive for their potential applications in integrated optoelectronic devices. One-dimensional flexible nanowire-based photodetectors are especially important because low-dimensional nanostructures are fascinating platforms for manipulating electrons and photons at the subwavelength scale. Herein, we report an ultravioletvisible photodetector based on a single crossed heterojunction assembled with quantum dotembedded polyaniline nanowires. The quantum dot-embedded polyaniline nanowires are fabricated by a direct drawing method. Based on these inorganic-organic hybrid nanowires, room-temperature, high-performance, high-speed photodetectors are constructed. The fabricated photodetectors show an excellent light response in the wavelength region of 365 to 550 nm, with an external quantum efficiency of 105 %, a responsivity of 103 A/W, an on-off switching ratio of

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10 and a short response/recovery time of 9 ms. These excellent performance characteristics are attributed to the abundant inorganic-organic interfaces and the intermediate radiative reabsorption process. The spectral response range can be readily tuned to a desired waveband by changing the emission wavelength of the quantum dots. This cross heterojunction-based photodetector may have potential applications in integrated photonic and optoelectronic devices.

KEYWORDS:

quantum

dot,

polyaniline

nanowire,

heterojunction,

photodetector,

optoelectronics

Because of their versatile physical characteristics and excellent one-dimensional structural properties, polymer nanowires (NWs) have attracted considerable attention as building blocks for important elements of integrated photonic and electronic devices,1-8 including photodetectors, optical waveguides, nanometer-scale lasers, light-emitting diodes, and field-effect transistors. Among these devices, nanoscale photodetectors that convert optical signals to electrical signals on the nanometer scale have aroused great interest as key functional components for on-chip information

communication

and

processing.

Interestingly,

one-dimensional

organic

nanostructures have been widely used to enhance the external quantum efficiency and to shorten the response time of photodetectors. In particular, some researchers have dedicated their studies to single NW-based photodetectors.9-17 This efficient strategy takes advantage of the large surface-to-volume ratio and low dimensionality of one-dimensional nanostructures. Recently, pn heterojunctions composed of NWs have attracted particular attention because of their excellent photoemission, photoconduction, photovoltaic, and rectification behaviors.18-21 The excellent properties of these NW heterojunctions are attributed to not only the simple superposition of the


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individual contributions from both components but also the unique processes that occur at the interfaces formed between the components. One of these unique processes is intermediate radiative re-absorption. Different low-dimensional nanostructures of inorganic semiconductor materials (e.g., Si, GaN, CdSe, InSe, etc.) and organic semiconducting polymers (e.g., polythiophene, polypyrrole, polyaniline, etc.) have been extensively applied to various photodetectors.22-27 Although inorganic semiconductor material-based photodetectors have been intensely investigated and individually used for three important wavebands, 250−400 nm (ultraviolet), 400−700 nm (visible), and 0.7−1.5 µm (near-infrared), it is quite complicated to span and tune these wavebands. Nevertheless, most of these photodetectors demand a response/recovery time of 0.2−5 s and have an external quantum efficiency of ∼104 %.27 Additionally, inorganic-organic NW heterojunction photodetectors with high external quantum efficiency in both ultraviolet and visible wavebands are highly desirable and challenging to achieve. Therefore, in this work, we demonstrate an ultraviolet-visible photodetector based on a single crossed heterojunction assembled with quantum dot-embedded polyaniline NWs. A relatively short response/recovery time of 9 ms and a high external quantum efficiency of 105 % are obtained, which are due to the abundant inorganic-organic interfaces and the large heterojunction area (∼106 nm2) formed by the crossed NWs, resulting in efficient light absorption. The photodetector has a broad spectral response at 365−550 nm, spanning both ultraviolet and visible wavelength bands. Furthermore, the spectral response range can be readily tuned to a desired waveband by changing the emission wavelength of the quantum dots (QDs). QDs and polyaniline are chosen because semiconductor QDs are an active inorganic material for photodetection devices,28-30 whereas transparent conductive polyaniline is an organic material for optoelectronics.31-34 For example, graphene QD-polyaniline composites are expected to find


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applications in photovoltaic and optoelectronic devices.35-37 Besides the composite-based devices, the NW-based devices are also desirable due to the reduced size of the effectively conducting channel. Different from the template-assisted synthesis of inorganic-organic NWs,38 the QDembedded polyaniline NWs are fabricated by directly drawing the QDs and polyaniline mixture solution. In addition, the rectifying characteristics, photodetection responsivity, switching ratio, and reversible stability of the achieved photodetection device are also evaluated. It is expected that this NW crossed heterojunction photodetector will be a promising component in the construction of integrated optoelectronic devices and circuits.

RESULTS AND DISCUSSION Figure 1a shows a typical field emission scanning electron microscope (SEM, JSM-6330F) image of a straight QD-embedded polyaniline NW with a diameter of 630 nm. A corresponding high-resolution SEM image is shown in the inset. Figure 1b shows the SEM image of a QDembedded NW crossed heterojunction assembled from two 500-nm-diameter NWs. To closely inspect the QD distribution in the polyaniline NW, transmission electron microscopy (TEM, JEM-2010HR), operating at 200 kV, and energy-dispersive X-ray spectroscopy (EDS) were performed, the results of which are shown in Figures 1c and 1d, respectively. Figure 1c shows the TEM image of a 500-nm-diameter polyaniline NW. It can be seen from the TEM image that the CdSe-ZnS core-shell QDs (as indicated by red arrows) were successfully embedded into the polyaniline NW. The measured maximum diameter variation is ∆D ≈ 15 nm over a length L = 1.6 µm. Figure 1d shows the EDS analysis, confirming the presence of S (19.18 wt %), Zn (63.54 wt %), Se (6.07 wt %), and Cd (11.21 wt %) elements.


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Figure 1. SEM, TEM, and EDS characterization of QD-embedded polyaniline NWs. (a) SEM image of a straight QD-embedded NW with a diameter of 630 nm and (inset) a corresponding high-resolution SEM image. (b) SEM image of a QD-embedded NW crossed heterojunction. The diameter of each NW is 500 nm. (c) TEM image of a QD-embedded NW with a diameter of 500 nm. The inset shows a zoom-in image. The red arrows indicate the embedded QDs in the polyaniline NW. (d) EDS spectrum of the QD-embedded NW shown in (c). To construct a crossed heterojunction, CdSe-ZnS core-shell QDs with emissions at 650 and 580 nm were embedded into the polyaniline NWs. Figure 2a shows a TEM image of QDs with emission at 650 nm, where the white circles indicate individual QDs with an average diameter of approximately 6 nm. The absorption and photoluminescence (PL) spectra of the QDs are shown in the inset. For comparison, a TEM image of QDs with emission at 580 nm is shown in Figure 2b. The inset shows the absorption and PL spectra of the QDs. To closely inspect the QD


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diameter, Figure 2c shows a high-resolution TEM image of the QDs shown in (b), and the corresponding zoom-in image is shown in the inset. The lattice fringes can be seen in the white and yellow circles, and the average diameter is approximately 4.5 nm. Figure 2d shows the selected area electron diffraction (SAED) pattern for the QDs shown in (c), and the corresponding Fourier transform pattern is shown in the inset. The above characterization presents the size-dependent optical properties of the CdSe-ZnS core-shell QDs, indicating an opportunity to tune the optical properties by changing the size of the QDs.

Figure 2. TEM and optical characterization of CdSe-ZnS core-shell QDs. TEM images of QDs with emission wavelengths at 650 (a) and 580 (b) nm. The insets of (a) and (b) show the corresponding absorption and PL spectra. (c) High-resolution TEM image of the QDs shown in (b). The white circles in (a) and (c) show individual QDs. The inset shows a zoom-in image. (d)


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Corresponding SAED pattern for the QDs shown in (c). The inset presents the corresponding Fourier transform pattern. Figure 3a shows the bright field optical microscope image of a 630-nm-diameter QDembedded NW with emission at 580 nm. The tip diameter of the fiber taper is approximately 500 nm, and no light was launched. Once 473-nm light launched from the fiber taper into the QDembedded NW, the embedded QDs were excited and emitted 580-nm light. Figure 3b shows a corresponding dark field optical microscope image of excited 580-nm light guided along the NW. For comparison, Figure 3c shows the bright field optical microscope image of a 500-nmdiameter QD-embedded NW with emission at 650 nm, whereas Figure 3d shows the corresponding dark field optical microscope image of excited 650-nm light guided along the NW. To characterize the PL properties of the QD-embedded NWs, Figure 3e shows the PL spectra of the QD-embedded NWs shown in (b) (blue line) and (d) (red line). This result further demonstrates that QDs with emissions at 580 and 650 nm were successfully embedded into polyaniline NWs. It is worth noting that two different QD-embedded NWs were used to construct the crossed heterojunction: one with emission at 580 nm and one with emission at 650 nm. When the crossed heterojunction was irradiated by 473-nm light, two different color PL emissions were observed, as shown in Figure 3f. The 473-nm light was focused onto a spot size of approximately 80 µm through a 60× objective (NA = 0.7). The two PL emissions also confirm the formation of the NW crossed heterojunction. Figure 3g shows the absorption spectra of a QD-embedded NW (blue line) and a pure polyaniline NW (red line). TEM images of the corresponding NWs are shown in the inset. It can be seen from Figure 3g that the absorption of a pure polyaniline NW is less than 10%, whereas the absorption of a QD-embedded NW is come


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from the QDs embedded into polyaniline NW. The result is in agreement with the QD absorption spectra shown in the insets of Figures 2a and 2b.

Figure 3. Optical characterization of QD-embedded polyaniline NWs. Bright field optical microscope images of (a) a 630-nm-diameter QD-embedded NW with emission at 580 nm and (c) a 500-nm-diameter QD-embedded NW with emission at 650 nm. The corresponding dark field optical microscope images of QD-embedded NWs excited by 473-nm blue light launched from a fiber taper are shown in (b) and (d). The excited 580-nm (b) and 650-nm (d) lights were guided along the NW. The scale bar in (a) is also applicable to (b-d). (e) PL spectra of QDembedded NWs with emission at 580 (blue line) and 650 (red line) nm. (f) Dark field optical


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microscope image of a NW crossed heterojunction irradiated by 473-nm blue light with an optical spot size of 80 µm. (g) Absorption spectra of a QD-embedded NW (blue line) and a pure polyaniline NW (red line). The insets present the corresponding TEM images. Figure 4a shows the TEM image of a crossed heterojunction constructed with two 680-nmdiameter QD-embedded polyaniline NWs: one with emission at 580 nm and one with emission at 650 nm. The large diagonal line of the heterojunction is 1.1 µm long (as indicated by the yellow double arrow line), whereas the short diagonal line is 1 µm long (as indicated by the white double arrow line). This crossed heterojunction is promising for high photosensitivity and fast responsivity because of the abundant inorganic-organic interfaces between QDs and polyaniline for significantly enhanced light absorption. The abundant inorganic-organic interfaces avoid the aggregation of QDs and make the QDs fully dispersed in polyaniline, resulting in efficient light absorption. In contrast, the QDs directly deposited on substrate become aggregated very easily so that they are irresponsive to light, which dramatically reduces the light absorption. With a more efficient light absorption by QDs, more electron-hole pairs were excited from the QDs and a higher photocurrent was generated under the electric field. As a result, the performance of the device can be greatly improved in terms of spectral responsivity and external quantum efficiency (see later sections). The inset of Figure 4a shows a dark field optical image taken under irradiation excitation. The photodetector was fabricated using the as-constructed QD-embedded NW crossed heterojunction. A false color SEM image of the fabricated photodetector is shown in the inset of Figure 4b. Figure 4b shows the typical I−V curves of the NW crossed heterojunction photodetector. The inset presents a schematic diagram of the electrical measurements, which were all conducted at room temperature. The obtained I−V results were measured under dark conditions and under the illumination of a beam of monochromatic light (intensity of ∼20


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mW/cm2) at wavelengths of 550, 532, 473, 437, 395 and 365 nm. The I−V curves show that the photodetector has distinct rectification characteristics, which is attributed to the formation of a pn junction within the NW crossed heterojunction. The NW with emission at 580 nm is p-type while the NW with emission at 650 nm is n-type. The rectification behavior comes from the p-n junction, which is located in the transition region near the interface of NWs. Additionally, the results show that the electric conductance of the photodetector under dark conditions (i.e., dark current) is relatively weak, whereas the electric conductance under light illumination is increased for all of the studied wavelengths. The dark current is related to the intrinsic conductivity of polyaniline. To study the wavelength-dependent spectral response, we measured the wavelengthdependent photocurrents for incident light wavelengths ranging from 365 to 550 nm and a light intensity of 20 mW/cm2 under bias voltages of 1.2, 1.8, and 2.4 V. The obtained results are given in Figure 4c, which shows that the photodetector exhibits a monotonic decrease in spectral response, i.e., the photocurrent decreased with increasing wavelength. This phenomenon occurs because the high energy (short wavelength) light can simultaneously excite both types of NWs and can induce more photogenerated carriers than the lower energy (long wavelength) light. These results are also in agreement with the absorption spectrum of a QD-embedded NW, as shown in Figure 3g. The absorption spectrum shows that once the wavelength of incident light exceeds 550 nm, the QDs cannot be effectively excited because of the low efficiency of optical absorption. Thus, the cut-off edge of the spectral response is determined by the optical absorption properties of the QDs embedded into the polyaniline NWs. Thus, the spectral response range can be tuned by changing the emission wavelength of experimentally used QDs. A change in emission wavelength can be readily realized by changing the diameters of the CdSe-


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ZnS core-shell QDs. Furthermore, the sensitivity of the spectral response increases as the bias voltage is increased, as indicated in Figure 4c. To evaluate the conversion capability of optical signals to electrical signals at different wavelengths of light, the external quantum efficiency (EQE) of the photodetector can be calculated by the following equation39-41:

EQE =

I hν × e P

(1)

where I is the photocurrent, e is the elementary charge, hν is the photon energy, and P is the optical power. The measured results for a bias voltage of 2.4 V show that the EQE can reach as high as 1.4, 2.1, 3.4, 4.7, 6.2, and 8.1 × 105 % at wavelengths of 550, 532, 473, 437, 395 and 365 nm, respectively. Figure 4d shows the light intensity-dependent photocurrents at wavelengths of 532, 473, and 365 nm for a working voltage of 1.8 V. The photocurrent and light intensity demonstrate a relatively good linear relationship. The light responsivity, R, at different wavelengths can be calculated according to the following equation42,43:

R=

I EQE × λ (nm) = A/W 1240 P

(2)

where λ is the light wavelength in nm. On the basis of the definition of responsivity (R) and by the use of measured EQE under 2.4 V bias voltage, the calculated R at wavelengths of 532, 473, and 365 nm for this ultraviolet-visible photodetector is 901, 1297, and 2384 A/W, respectively. The responsivity of the NW crossed heterojunction photodetector is higher than that of other nanostructures, such as heterojunction NW,43 graphene,44 and semiconductor alloy nanoribbon.45 For comparison, single cross junction assembled by two NWs with the same emission wavelengths is also studied. The cross junction was illuminated with light of different


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wavelengths (intensity of 20 mW/cm2) or under dark conditions. Figure 4e shows the typical I−V curves of a single cross junction assembled by two QD-embedded NWs with the same emission at 580 nm. Under 365-nm light illumination, the photocurrent is about 340 nA for a bias voltage of 2.4 V and the calculated EQE is of 5.8 × 105 %. Similarly, Figure 4f shows the typical I−V curves of a single cross junction assembled by two QD-embedded NWs with the same emission at 650 nm. Under 365-nm light illumination, the photocurrent is about 424 nA for a bias voltage of 2.4 V and the calculated EQE is of 7.2 × 105 %. Different from the results of the cross heterojunction (Figure 4b), there is no rectification characteristics observed in the cross junction with the same NWs. The same NWs mean the same bandgaps, and cannot form a heterojunction but just a cross junction. Under 365-nm light illumination, the photocurrent of the cross heterojunction (Figure 4b) is about 476 nA for a bias voltage of 2.4 V and the calculated EQE is of 8.1 × 105 %. The photocurrent of 476 nA in the cross heterojunction is larger than the weighted average of cross junctions (382 nA). The difference of 94 nA indicates that the emitted 580-nm light can be re-absorbed and can then induce additional photogenerated carriers. In a traditional single bandgap semiconductor-based photodetector, the conversion efficiency of photon-to-electron is low because the photogenerated carriers can recombine and emit photoluminescence. Thus, some holes at the top of the valence band deplete a few electrons at the bottom of the conductance band. This radiative loss reduces the detection efficiency of the photons. The above problem can be largely improved in heterojunction photodetectors. The radiative energy from a QD-embedded NW with 580-nm emission can be re-absorbed by a QDembedded NW with 650-nm emission because the photon energy of the former is higher than the bandgap of the latter (2.14 eV > 1.91 eV). The radiative energy re-absorption induces additional photogenerated carriers. Therefore, this intermediate radiative re-absorption process results in a


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higher photocurrent and responsivity in heterojunction photodetectors compared to single bandgap photodetectors. Additionally, the EQE is an important factor for a photodetector that indicates the number of electron-hole pairs excited by incident photons. The calculated EQE of 105 % for the cross heterojunction photodetector is one order of magnitude higher than that (104 %) of the single CdS nanoribbon- and single In2Se3 NW-based photodetectors.46,47 This finding further demonstrates that the intermediate radiative re-absorption process in the heterojunction plays an important role in the enhanced performance of NW cross heterojunction photodetectors.


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Figure 4. Electrical characterization of the QD-embedded polyaniline NW crossed heterojunction. (a) TEM image of a NW crossed heterojunction assembled from two 680-nmdiameter QD-embedded NWs. The inset shows a corresponding dark field optical image taken under irradiation excitation. (b) Typical I−V curves of the NW crossed heterojunction photodetector illuminated with light of different wavelengths (intensity of 20 mW/cm2) or under dark conditions. The inset shows a false color SEM image of the NW crossed heterojunction-


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based photodetector. (c) Wavelength-dependent photocurrent response of the photodetector for a light intensity of 20 mW/cm2 at voltages of 1.2, 1.8, and 2.4 V. (d) Light intensity-dependent photocurrent at a bias voltage of 1.8 V for wavelengths of 532, 473, and 365 nm. (e, f) Typical I−V curves of the cross junction assembled by two NWs with the same emission wavelengths of 580 nm (e) and 650 nm (f). Figure 5a shows the time response of the NW crossed heterojunction photodetector, which is measured by periodically turning on and off a 365-nm light at a bias voltage of 1.8 V. The results indicate that the photodetector not only exhibits an Ion/Ioff ratio of 10 but also shows excellent reversible stability of switching performance. In the time domain, the response speed of a photodetector is typically evaluated by the response time, τres, and recovery time, τrec. The response time is defined as the time interval required for the response to rise from 10% to 90% of its maximum photocurrent. Similarly, the recovery time is defined as the time interval required for the response to drop from 90% to 10% of its maximum photocurrent. Figure 5b presents the photocurrent response under periodic on and off illumination of 365-nm light through an optical chopper at a frequency of 25 Hz. It can be seen from Figure 5b that both τres and τrec are 9 ms, corresponding to two orders of magnitude faster than those (280 ms and 550 ms) of the CdS nanoribbon-based photodetectors.45 The response of 9 ms is much faster than that of previously reported photodetectors (200 to 320 ms) based on all-inorganic In2Se3, CdSSe, and CdS NWs fabricated by physical vapor deposition.47-49 Compared with the all-inorganic NWs fabricated by a physical vapor deposition process, the QD-embedded polyaniline NWs are fabricated by a direct drawing from the solution of the QDs and polyaniline mixture. The fabrication is simpler and does not require the control of the temperature, pressure, and gas flow rate. Furthermore, Figure 5c gives the photocurrent response as a function of the frequency of the optical chopper,


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which indicates that the photodetector has a 3-dB response at 90 Hz. The measurement configuration is schematically shown in the left inset. The right inset shows the optical chopper with variable frequency (OCV-6500) used herein.

Figure 5. Time response of the NW crossed heterojunction photodetector. (a) Repetitively switching on and off 365-nm light illumination with an intensity of 20 mW/cm2 at a working voltage of 1.8 V. The time period is set to 20 s, with 10 s on and 10 s off. (b) Photocurrent response under periodic on and off 365-nm light illumination through a mechanical chopper at a frequency of 25 Hz. (c) Photocurrent response versus frequency of the optical chopper,


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indicating a 3-dB response at 90 Hz. The insets present schematic configuration of the measurements (left) and a photo of the experimentally used optical chopper (top right).

CONCLUSIONS In summary, highly responsive, room-temperature, high-performance ultraviolet-visible photodetectors were successfully achieved by a QD-embedded polyaniline NW crossed heterojunction with PL emission wavelengths of 580 and 650 nm. The photodetector exhibits distinct p-n junction-associated rectifying I−V characteristics and a significant spectral response over the wavelength range of 365−550 nm. Furthermore, the spectral response range can be readily tuned to a desired waveband by changing the emission wavelength of the QDs. The responsivity reached 103 A/W, the EQE reached 105 %, the response/recovery time was reduced to 9 ms, and a switching ratio of 10 was achieved with excellent reversible stability. These enhanced performance characteristics can be attributed to the abundant inorganic-organic interfaces and the intermediate radiative re-absorption process. These polyaniline NW crossed heterojunction photodetectors may have potential applications in the construction of integrated optoelectronic devices for next-generation on-chip information communication and processing.

EXPERIMENTAL SECTION Transparent conductive polyaniline and CdSe-ZnS core-shell QDs were purchased from Alfa Aesar and Zkwy Bio-Tech companies, respectively. The emission wavelengths of p- and ntype oil soluble CdSe@ZnS QDs are 580 and 650 nm, respectively. QD-embedded polyaniline NWs were fabricated by a direct drawing method as follows. First, 880 mg of polyaniline was dissolved in 1 mL of dimethylbenzene to form a homogeneous polyaniline dimethylbenzene solution. Second, 450 µL of QD dimethylbenzene solution (concentration 4 µM/L) was diluted


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into the polyaniline dimethylbenzene solution. The mixture was stirred at room temperature for 2 h, followed by 40 min of ultrasonication to form a homogeneous solution with an appropriate viscosity for drawing. Third, the tip of a silica fiber was immersed into the solution for 1−4 s and then removed at a speed of 0.1−3 m/s, leaving a polyaniline wire extending between the solution and the fiber tip upon rapid evaporation of the dimethylbenzene. The diameter of the polyaniline wire varied from 400 to 700 nm. Optical characterization of QD-embedded polyaniline NWs was performed under an optical microscope (HIROX, KH-7700). Blue light at a wavelength of 473 nm was evanescently coupled into the QD-embedded NW using a fiber taper for high-efficiency waveguiding excitation. The PL signals were collected by a 20× objective (numerical aperture, NA = 0.45) and directed through a dichroic filter. The filtered light was split by a beam splitter and directed to a charge coupled device (CCD) camera and a spectrometer for image and spectrum measurement, respectively. A crossed heterojunction composed of two QD-embedded NWs can be assembled by the use of a 50-nm-resolution commercial micromanipulator (Kohzu Precision) equipped with a tungsten probe (300-nm tip diameter) under optical microscopy. After assembly, the crossed heterojunction was transferred to a pre-cleaned SiO2 substrate. Then, the substrate was spincoated with poly(vinyl alcohol) and poly(methyl methacrylate), and electrode patterns were defined by electron beam lithography (JEOL 6510 with NPGS). Cr/Au (15/65 nm) electrodes were prepared by metal evaporation. The electrical characteristics of the NW crossed heterojunction photodetector were measured by the use of a semiconductor characterization system (Keithley 4200).


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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]; [email protected]

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT The authors thank Dr. Chuanxiao Xiao from the National Renewable Energy Laboratory (Golden, CO 80401, USA) for fruitful discussions. This work was supported by the National Natural Science Foundation of China (11274395) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13042).

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Single Crossed Heterojunction Assembled with Quantum-Dot

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