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A Stretchable Nanowire UV–Vis–NIR Photodetector with High Performance Jewon Yoo, Sanghwa Jeong, Sungjee Kim, and Jung Ho Je*

Photodetectors (PDs) with a broad spectral response from the ultraviolet (UV)–visible to the near infrared (NIR) have attracted great attention in a variety of industrial and scientific applications, including image sensing, communication, environmental monitoring and day and nighttime surveillance.[1–4] In particular, nanowire (NW) PDs with UV to NIR response are emerging for potential applications, such as optical communication and interconnects in nanophotonic circuits.[5–7] The performance of NWPDs in the broad spectral range still needs to be significantly enhanced for practical applications.[7] Furthermore, it is still a challenge to achieve stretchable NWPDs with UV to NIR response, which will be in demand for future stretchable/flexible nanophotonic circuits. In this study, we have developed stretchable UV–vis–NIR NWPDs with high performance, which will be an important step for stretchable/flexible nanophotonics in the future. The development of high-performance NWPDs with UV to NIR response is still in its infancy,[5,6] compared to thin film PDs whose performance is comparable to or even better than that of commercial SiC, Si, or InGaAs in the UV, the visible (vis), or the NIR range.[1,2] Recently, stretchable thin film PDs have been developed because of their possible integration in electronic eye cameras[8] and epidermal electronic systems.[9] However, the development of stretchable NWPDs is at a very early stage despite their importance for future stretchable nanophotonics. In fact, reports on stretchable NWPDs with a broad spectral range from UV to NIR have not been published before, except for one NWPD with a narrow spectral range.[10] To realize UV–vis–NIR PDs we have employed a narrow-bandgap material, PbS (ca. 0.4 eV), known to be a NIR sensitizer.[11] Specifically, oleic acid capped PbS quantum dots (QDs) were incorporated in poly(3-hexylthiopehene) (P3HT) to efficiently extract/ transport the photogenerated carriers.[12] Then, by applying a direct writing technology, we achieved single PbS QD-P3HT hybrid NWPDs that showed superior sensitivity and response speed in the UV to NIR range. By employing a stretchable

J. Yoo, Prof. J. H. Je X-ray Imaging Center Department of Materials Science and Engineering Pohang University of Science and Technology Pohang 790–784, South Korea E-mail: [email protected] S. Jeong, Prof. S. Kim Department of Chemistry Pohang University of Science and Technology (POSTECH) San 31, Hyoja-Dong Nam-Gu, Pohang, Gyeong-Buk, 790–784, South Korea

DOI: 10.1002/adma.201404945

Adv. Mater. 2015, DOI: 10.1002/adma.201404945

architecture[13] we also were able to demonstrate an unprecedented stretchable UV–vis–NIR NWPD array without performance degradation under extreme (up to 100%) and repeated (up to 100 cycles) stretching conditions, exhibiting excellent mechanical and photoelectrical stabilities. Our direct-writing approach of inorganic–organic hybrid NWs opens innovative opportunities for the creation of a stretchable PD array with an unusually wide spectral range that can contribute to advanced optical communications and interconnects in stretchable/flexible optoelectronics[14] and photonics.[15] PbS QDs (d ranges from 4.4 to 9.1 nm) were prepared by pyrolysis of organometallic precursors.[16] The size and the rock-salt crystal structure of the as-synthesized PbS QDs were identified by transmission electron microscopy (TEM) and X-ray diffraction (XRD), as shown in Figure S1 and S2 in the Supporting Information, respectively. The blend solution of PbS QDs and P3HT was then prepared in toluene. Using the blend solution, PbS QD-P3HT hybrid NWs were produced by a direct-writing technology, called the meniscus-guiding method.[17] Figure 1a schematically illustrates the direct writing of a hybrid NW arch. When a micropipette filled with the blend solution touches an electrode, a meniscus of the solution is created outside its tip opening. As the micropipette is pulled up in the vertical direction, the meniscus solution is stretched, its cross section decreased down to the nanoscale, and the solvent is rapidly evaporated, yielding a freestanding NW. The micropipette is then moved to another electrode to bond the other end of the NW to it. To grow NW arches we manipulated the micropipette and the substrate individually by using two 3-axis motorized stages (accuracy: 250 nm). The scanning electron microscopy (SEM) image (inset of Figure 1a) shows a successful integration of a single hybrid NW arch on an Au–Al electrode. PbS QDs were uniformly dispersed in single hybrid NWs, as demonstrated by the TEM image of a single NW (Figure 1b). The energy-dispersive X-ray spectroscopy (EDS) image of the NW (inset of Figure 1b) clearly indicates the presence of Pb and S elements originating from the PbS QDs. The Cu peak and most of the C peak are from the copper grid and the carbon mesh, respectively. The photoresponse of our single hybrid NWPDs, produced by directly integrating single 60 wt% PbS QD (4.4 nm)-P3HT QD hybrid NW arches (r ≈ 250 nm and l ≈ 50 µm) on Au–Al electrodes (inset of Figure 2a), was characterized using a broad spectral range from the UV to the NIR, as demonstrated in Figure 2. The photoconducting properties were measured using a twoprobe method for the UV to NIR range (from 365 to 940 nm). All measurements were carried out under ambient conditions. The current–voltage (I–V) curves of the hybrid NWPDs, measured under dark and illumination conditions with

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with UV to NIR response (see Table 1). The slight drop found here in the ON/OFF ratio over time is presumably due to photobleaching.[19] In addition, the hybrid NWPDs showed similar and fast response times (0.11 to 0.58 s) regardless of the illumination wavelength in the UV–vis to the NIR range, which is in sharp contrast to the relatively low response times (>10 s) in the UV range in previous reports (Table 1). The enhanced ON/OFF ratios and response times are first of all related to the photo-induced charge transfer between the PbS QDs and P3HT[12] in the hybrid NWs. The enhancements may also be related to the presence of a Schottky barrier[20] that forms at the P3HT/Al interface, as observed in Figure S3 (Supporting Information). In fact, the hybrid NWPDs exhibited photoresponses spanning the full range from UV–vis to NIR, as confirmed by their photoresponse for nine wavelengths (λ = 365, 400, 455, 530, 625, 680, 740, 850, and 940 nm) (Figure S5a, Supporting Information). We calculated the specific detectivity D* (a figure of merit for comparing different detectors[21] of our hybrid NWPD. If the noise from the dark current is the major contribution, the detectivity can be expressed as:[1] D * = R /(2qJ d )1/2

Figure 1. Direct writing of PbS QD-P3HT hybrid NW arches. a) Scheme of direct writing of a hybrid NW arch by the meniscus-guided approach. Inset: SEM image of a single hybrid NW arch on an Au–Al electrode. b) TEM image of a hybrid NW onto a carbon mesh on a copper grid. Inset: EDS spectrum of the hybrid NW.

light of different wavelengths (365 nm; 0.05 mW cm−2, 625 nm; 0.67 mW cm−2, and 850 nm; 1.70 mW cm−2), showed Schottky diode behavior (Au/P3HT/Al),[18] as demonstrated in Figure S3 in the Supporting Information. The photoresponse of the hybrid NWPDs with the light switched “on” and “off” was measured under the three given wavelengths of 365, 625, and 850 nm, spanning the UV to NIR range, as plotted in Figure 2a–c. The hybrid NWPDs showed a high sensitivity and fast response time in the UV to NIR range. First of all, the hybrid NWPDs showed a broad spectral photoresponse from the UV– vis to NIR (Figure 2a–c) range, which is related to the PbS QDs, as they act as NIR sensitizers.[11] Moreover, the photoresponse of P3HT NWPDs was limited to the UV–vis range (Figure S4, Supporting Information). Remarkably, the ON/OFF ratios were significantly enhanced in the UV–vis range by two orders of magnitude compared to previous results of NWPDs

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(1)

where R is the responsivity (inset of Figure S5a, Supporting Information), q is the absolute value of electron charge (1.6 × 10−19 Coulombs) and Jd is the dark current (Figure S5a). Here R (= Jph/Llight) was calculated from the measured photocurrent (Jph) (Figure S5a) and light intensity (Llight) (Figure S5c). As shown in Figure S4b in the Supporting Information, the detectivities of the hybrid NWPDs are higher in the UV–vis range (up to 2.1 × 1012 Jones in UV), compared to previous results of NWPDs with UV to NIR response (Figure S5b, Supporting Information). The high detectivities can be attributed to the high ON/OFF ratios (Jph/Jd), which is in its turn related to the Schottky barrier[20] between the P3HT and Al electrode (Figure S3a, Supporting Information). We note that the detectivities are higher at low light intensities, regardless of illumination wavelength, as seen in Figure S6 in the Supporting Information. The low detectivity at high intensity can be attributed to a weakened electron–hole pair separation capability by flattening of the band bending at high intensity.[7] The photoresponse (from 365 to 940 nm) of single hybrid NWPDs was significantly affected by the QD concentration, as demonstrated in Figure 3a and Figure S6 in the Supporting Information. The QDs were well dispersed in our single hybrid NWs for all QD concentrations investigated (see TEM images in Figure S8, Supporting Information). The ON/OFF ratios, obtained for our PbS QD (4.4 nm)-P3HT hybrid NWPDs, largely increased with QD concentration for all wavelengths (Figure 3a). This increase can be explained by the progressive enhancement in absorbance with QD concentration, as seen in the UV–vis–NIR absorption spectra for PbS QD (4.4 nm)P3HT hybrid films deposited on quartz substrates (Figure 3c). It should be noted that all of the hybrid films except the P3HT film (short dashed line) show absorption features at a wavelength of about 1150 nm (inset of Figure 3c), which matches well with the absorption peak of the 4.4 nm QDs (Figure S7,

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Adv. Mater. 2015, DOI: 10.1002/adma.201404945

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Stretching = (L − L0 ) / L0 × 100%

Figure 2. Photoresponse of single hybrid NWPDs. The photoresponse of the hybrid NWPDs with the light switched “on” and “off” under the three given wavelengths of a) 365, b) 625, and c) 850 nm. The right panels show the enlarged portion of one response and the reset process.

Supporting Information). This clearly indicates that the photoresponse in the NIR range is due to the QDs. The ON/OFF ratios, obtained for 60 wt% PbS QD-P3HT hybrid NWPDs, increased with decreasing QD size for all wavelengths tested, as shown in Figure 3b. This increase can be explained by photoluminescence (PL) quenching due to photoinduced charge transfer between P3HT and the PbS QDs.[12] We characterized the PL spectra of the blend (solid lines) solutions in the QD emission ranges for various QD sizes (Figure 3d). The PL intensities in the blend solutions were quenched relative to those of the pristine QD solutions (dashed lines in Figure 3d). As the QD size increased the PL quenching systematically decreased, due to the restricted hole transfer from the QDs to P3HT by an unfavorable HOMO P3HT-QD band offset (see the approximate band alignment of the P3HT-QD hybrid material in Figure S10a, Supporting Information). On the other hand, the PL quenching for the blend solutions in the P3HT emission range increased with QD size (Figure S10b), because of an enhanced electron transfer from P3HT to the QDs by a favorable LUMO P3HT-QD band offset (Figure S10a, Supporting Information). It is well known that the fraction of photoexcited excitons that are fluorescent, as probed by PL quenching, is significantly larger in PbS QDs (ca. 20%)[16] than

Adv. Mater. 2015, DOI: 10.1002/adma.201404945

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in P3HT (ca. 1%).[22] This indicates that the hole transfer from the QDs to P3HT contributes dominantly to the photoresponse of the hybrid NWPDs, which explains the increase of the ON/ OFF ratio with decreasing QD size (Figure 3b). We used a meniscus-guiding method to develop a stretchable UV–vis–NIR NWPD array (3 × 3) by individually integrating single 60 wt% PbS QD (4.4 nm)-P3HT hybrid NW arches on Au–Al electrodes embedded in polydimethylsiloxane (PDMS), as schematically illustrated in Figure 4a. The NWPD array was fixed on a home-built stretching stage to enable desired stretching, which is defined as: (2)

where L and L0 are the stretched and unstretched distances between the two feet (see Figure S11a, Supporting Information). The NWPD array showed excellent flexibility and stretchability (up to 100%), as clearly demonstrated by a photograph (Figure 4b) and two series of optical microscopy images (Figure 4b and S11b, Supporting Information). Even under substantial stretching (up to 100%) or repeated stretching (up to 100 cycles), the I–V curves of the NWPDs showed nearly identical behavior (Figure S11c, Supporting Information), demonstrating excellent electrical stability. Strikingly, the photoresponse of the NWPDs was almost unchanged during stretching of up to 100% and even under repeated stretching of up to 100 cycles for the UV–vis to NIR range, as demonstrated in Figure 4c and 4d, respectively. Specifically, the ON/ OFF ratio (Figure 4e) and response (rise/fall) time (Figure 4f) of the NWPD array were almost constant under stretching of up to 100%. These remarkable performances clearly demonstrate the excellent photoelectrical stability under extreme stretching conditions, which is mostly due to the relaxation of the external strain by the stretchable architecture of the NW arches.[10,13] In conclusion, we have developed high-performance, stretchable UV–vis–NIR NWPDs of PbS QD-P3HT hybrid NW arches, using a meniscus-guided, direct-writing technology. Remarkably, the hybrid NWPDs showed very high ON/OFF ratios in the UV-vis range and very fast response times in the UV range. The photoresponse of the hybrid NWPDs could significantly be enhanced by controlling the concentration and size of the QDs. We also demonstrated a novel, stretchable UV–vis–NIR NWPD array created by individually integrating single hybrid NW arches on Au–Al electrodes embedded in PDMS. The NWPDs showed a nearly identical photoresponse under extreme (up to 100%) and repeated stretching (up to 100 cycles), indicating their excellent mechanical and photoelectrical stability. Direct writing of inorganic–organic hybrid NWs opens up possibilities for future stretchable/flexible and high-performance optoelectronic circuitry.

Experimental Section Sample Preparation: Poly(3-hexylthiophene) (P3HT) (regioregularity, 90%), lead(II) acetate trihydrate, octadecene (ODE) (technical grade, 90%), bis(trimethylsilyl)sulfide ((TMS)2S) (synthesis grade), trioctylphosphine (TOP) (technical grade, 90%), and oleic acid (OA) (technical grade, 90%) were purchased from Sigma–Aldrich Korea, and used as received. Oleic acid capped PbS QDs were synthesized according to methods previously reported in the literature.[18] A PbS QD-P3HT blend solution

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www.MaterialsViews.com Table 1. Comparison of the ON/OFF ratios and response (rise/fall) times in the UV–vis to NIR range for single PbS QD-P3HT hybrid NWPD and previously reported NWPDs Photodetector

ON/OFF ratio

Response (rise/fall) time [s]

Reference

UV

Vis

NIR

UV

Vis

NIR

In2Te3 nanowire

350 nm (0.31 mW cm−2) ca. 1.3

550 nm (0.14 mW cm2) ca. 0.8

830 nm (0.22 mW cm−2) ca. 0.8

350 nm (0.31 mW cm−2)

A stretchable nanowire UV-Vis-NIR photodetector with high performance.

A simple direct-writing technique can be used to fabricate a stretchable UV-vis-NIR nanowire photodetector (NWPD) consisting of PbS quantum dot (QD)-p...
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