Research Article www.acsami.org
High-Performance MoS2/CuO Nanosheet-on-One-Dimensional Heterojunction Photodetectors Doo-Seung Um, Youngsu Lee, Seongdong Lim, Seungyoung Park, Hochan Lee, and Hyunhyub Ko* School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea S Supporting Information *
ABSTRACT: van der Waals heterostructures based on stacked two-dimensional (2D) materials provide novel device structures enabling high-performance electronic and optoelectronic devices. While 2D−2D or 2D−bulk heterostructures have been largely explored for fundamental understanding and novel device applications, 2D−one-dimensional (1D) heterostructures have been rarely studied because of the difficulty in achieving high-quality heterojunctions between 2D and 1D structures. In this study, we introduce nanosheet-on-1D van der Waals heterostructure photodetectors based on a wet-transfer printing of a MoS2 nanosheet on top of a CuO nanowire (NW). MoS2/CuO nanosheet-on-1D photodetectors show an excellent photocurrent rectification ratio with an ideality factor of 1.37, which indicates the formation of an atomically sharp interface and a high-quality heterojunction in the MoS2/CuO heterostructure by wet-transfer-enhanced van der Waals bonding. Furthermore, nanosheet-on1D heterojunction photodetectors exhibit excellent photodetection capabilities with an ultrahigh photoresponsivity (∼157.6 A/ W), a high rectification ratio (∼6000 at ±2 V), a low dark current (∼38 fA at −2 V), and a fast photoresponse time (∼34.6 and 51.9 ms of rise and decay time), which cannot be achievable with 1D-on-nanosheet heterojunction photodetectors. The wettransfer printing of nanosheet-on-1D heterostructures introduced in this study provides a robust platform for the fundamental study of various combinations of 2D-on-1D heterostructures and their applications in novel heterojunction devices. KEYWORDS: MoS2, CuO nanowire, nanosheet-on-1D heterojunction, photodetector, wet-transfer printing
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nanosheets have unique functionalities.19,20 For example, heterojunctions between MoS2 and WSe2 layers provide interlayer tunneling behavior and tunable electrical properties depending on the gate voltage.9,13,14 The optical energy gap can be tuned by the thickness modulation of 2D materials, resulting in the transition of the spectral response.21 One-dimensional (1D) nanostructures have also been widely studied in electronics and sensor applications because of their unique 1D structures with quantum-confined carrier transport, tunable optical absorption, and large surface-to-volume ratios.22,23 In addition, significant progress has been made on the large-scale assembly of 1D nanostructures at the predefined regions on desired support substrates,24,25 which ultimately enables the fabrication of high-performance 1D electronic
INTRODUCTION Two-dimensional (2D)-layered materials such as graphene, hexagonal boron nitride, and transition-metal dichalcogenides (TMDCs) have received great interest recently in various research fields because of their unique physical and chemical properties.1−3 In particular, ultrathin TMDC materials with atomically sharp interfaces and tunable band gaps are very attractive for next-generation electronics and optoelectronics such as transistors, memory devices, photodetectors, lightemitting diodes (LEDs), and solar cells.4−11 Moreover, singleor few-layer structures of TMDC nanosheets can be cleaved from bulk TMDC crystals and transferred onto various heterogeneous substrates, which allow the preparation of diverse set of heterostructures such as graphene/InAs nanowire (NW), MoS2/WSe2, or MoS2/Si heterojunctions for novel electronic and optical devices.12−18 Furthermore, upon a comparison with traditional crystalline semiconductors such as silicon or III−V semiconductors, the 2D semiconductor © 2016 American Chemical Society
Received: October 2, 2016 Accepted: November 21, 2016 Published: November 21, 2016 33955
DOI: 10.1021/acsami.6b12574 ACS Appl. Mater. Interfaces 2016, 8, 33955−33962
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic representation of a MoS2/CuO nanosheet-on-1D heterojunction photodiode. (b) AFM and (c) SEM images of a MoS2/ CuO nanosheet-on-1D heterojunction photodiode based on the wet-transfer printing of the MoS2 nanosheet on top of the CuO NW, which forms a p−n heterojunction. (d) Cross-sectional HRTEM image at the interface between the MoS2 nanosheet and the CuO NW.
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devices at low cost. However, in contrast to 2D structures, the formation of heterostructures between 1D nanostructures has been a challenging task because of the lattice mismatch between different crystal structures, which limited the development of novel 1D heterostructure devices.26 Compared to 1D nanostructures, 2D nanosheets can provide more conformable architecture and larger contact area, enabling intimate contact with the heterogeneous materials. Although several interesting properties have been reported with the hybridization of 1D NWs on top of 2D nanostructures,27,28 2D−1D heterostructures have rarely been studied because of the difficulty in achieving an atomically sharp interface and high-quality heterojunction devices. Here, we present a high-performance photodiode based on a MoS2 nanosheet on a CuO NW heterojunction fabricated by using a wet-transfer-printing technique. In this heterojunction photodiode design, the increase of the MoS2 layer thickness can enhance the optical absorption and band gap, resulting in high photoresponsivity and wide spectral responsivity. The CuO NWs enable the formation of a type II heterojunction with MoS2 nanosheets, where the type II band structure effectively separates the electron and hole pairs when the light illuminates the heterojunctions.13,29 In particular, the wet-transfer printing of a MoS2 nanosheet onto the top surface of the CuO NW makes it possible to encapsulate a CuO NW with a MoS2 nanosheet, which enables the formation of an intimate contact and thus the high-quality heterojunction with a sharp interface between MoS2 and CuO heterostructures. The high-quality heterojunction enables high-performance heterojunction photodiodes with an excellent photocurrent rectification ratio (∼6000 at ±2 V) with an ideality factor of 1.37, an ultrahigh photoresponsivity of 157.6 A/W, a low dark current of ∼38 fA at −2 V, and a fast photoresponse time (∼34.6 ms of rise time and ∼51.9 ms of decay time).
EXPERIMENTAL SECTION
Growth of a CuO NW. In order to fabricate the nanosheet-on-1D heterostructure of a MoS2/CuO NW, the CuO NWs were first grown on a Cu foil (Aldrich, 99.99%) by thermal oxidation in a furnace at 500 °C for 10 h (Figure S1).30 In this process, CuO NWs can be easily synthesized by a simple heating method, which has been reported to provide controllable diameters and lengths of CuO NWs.31 The CuO NWs synthesized by thermal oxidation have been known to provide excellent chemical and environmental stabilities, which have advantages in chemical sensor applications.32,33 Fabrication of the Heterojunction Photodiodes. After the grown CuO NWs were harvested in ethanol by a sonication process (bath sonication, 10 s), the CuO NWs were coated on a SiO2 (100 nm)/p+-Si wafer by a spin-coating method. Then, MoS2 nanosheets were mechanically exfoliated from MoS2 bulk crystals (SPI Supplies) using a scotch-tape method. For the efficient exfoliation and transfer of MoS2 nanosheets onto the top of CuO NWs, MoS2 nanosheets on the scotch-tape were first transferred onto the poly(dimethylsiloxane) (PDMS) stamp by a stamping process.34 Then, the MoS2 nanosheets on the PDMS stamp were transferred onto the CuO NWs/Si substrate by a wet-transfer technique (Figure S2). While the dry transfer of exfoliated 2D layers has been successfully used as a quick, efficient, and clean transfer process,35 it is not an effective method when the target substrate is topographically rough such as a nanoparticle- or NWcoated surface because of the low adhesion between the 2D layer and the rough surface.35 However, the wet-transfer process employed in our work enables the tight contact between the MoS2 nanosheet and the CuO NW via the capillary-force-assisted attractive force between the MoS2 nanosheet and the CuO NW, which can ultimately induce encapsulation of the CuO NW with the MoS2 nanosheet (Figure S3).36 In the wet-transfer process, after a small amount of liquid (ethanol, 20 μL) was dropped onto the CuO NWs/Si substrate, the exfoliated MoS2 sheets on the PDMS stamp were covered on top of the CuO NWs/Si substrate (Figure S3a). Here, ethanol was used instead of water in the wet-transfer process to avoid the problem of MoS2 surface degradation. MoS2 has been known to show a poor longterm stability with the degradation problem in air and humidity conditions.37,38 Then, the liquid evaporation can induce the conformal contact between the MoS2 nanosheet and the CuO NW on the Si 33956
DOI: 10.1021/acsami.6b12574 ACS Appl. Mater. Interfaces 2016, 8, 33955−33962
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Figure 2. (a) I−V characteristics of a MoS2/CuO nanosheet-on-1D heterojunction photodiode in a dark state. (b) Photocurrents of MoS2/CuO nanosheet-on-1D heterojunction photodiode curves under light illumination of different wavelengths (560, 600, 700, and 760 nm) at 1 mW power of incident light and dark conditions. (c) Comparison of the I−V characteristics of MoS2/CuO nanosheet-on-1D and CuO/MoS2 1D-on-nanosheet photodiodes in a dark state. (d) Comparison of the photocurrents of MoS2/CuO nanosheet-on-1D and CuO/MoS2 1D-on-nanosheet photodiodes under a light illumination of 600 nm at 1 mW power. substrate (Figure S3b). As the liquid further evaporates, the MoS2 sheets uniformly encapsulate the cylindrical CuO NW on the Si substrate, resulting in a tight bonding interface between MoS2 and the CuO NW (Figure S3c). Here, the diameter of the CuO NW and the thickness of the MoS2 sheet used in this study are ∼150 and ∼50 nm, respectively. Last, Cr (5 nm)/Au (70 nm) metal electrodes were thermally evaporated on the nanosheet-on-1D heterostructure of MoS2/CuO NW.
low dark current below 40 fA, and a low turn-on voltage of ∼0.44 V. These results indicate that the p−n heterojunction diode is effectively formed between the n-type MoS2 nanosheet and the p-type CuO NW after the wet-transfer process. The asymmetric I−V characteristic of rectifying behavior is attributed to the energy band bending in the heterointerface between the MoS2 nanosheet and the CuO NW (Figure S6).17,41−43 In particular, the high rectification ratio indicates the excellent interface quality without impurities or amorphous layers between MoS2 nanosheets and CuO NWs.41 Here, the rectifying I−V curve is expressed as44
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RESULTS AND DISCUSSION Figure 1a shows the schematic representation of a MoS2/CuO nanosheet-on-1D heterojunction photodiode. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) images in parts b and c of Figure 1 show the CuO NW covered with the MoS2 nanosheet after the wet transfer of the MoS2 nanosheet onto the CuO NW. (The magnified AFM and SEM images are shown in Figure S4.) The Raman spectrum in Figure S5 shows two distinctive peaks of MoS2 for in-plane E12g (382.9 cm−1) and out-of-plane A1g (408.4 cm−1) vibration modes. Here, the spacing between in-plane E12g and out-ofplane A1g vibration modes indicates the mono- or multilayer states of the MoS2 sheets. In the case of monolayer MoS2, the distance between E12g and A1g is ∼18.3 cm−1.39 As the number of layers increases, the distance between E12g and A1g increases because of the red shift of E12g and the blue shift of A1g due to the long-range interlayer Coulombic interaction.40 Figure S5 exhibits a spacing of ∼25.5 cm−1 between the E12g and A1g modes, which indicates a multilayer structure of the transferred MoS2 nanosheet.39 Figure 1d shows the cross-sectional highresolution transmission electron microscopy (HRTEM) image of the interface between the multilayer MoS2 nanosheet and cylindrical CuO NW, which indicates a sharp and tight-bonding interface at the MoS2/CuO heterojunction. Figure 2a shows the current−voltage (I−V) curves of a MoS2/CuO nanosheet-on-1D heterojunction photodiode measured at dark conditions. The photodiode exhibits a large rectification ratio of ∼6000 at Vg = 0 V and Vds = ±2 V, a very
⎡ ⎛ qV ⎞ ⎤ ⎟ − 1⎥ I = I0⎢exp⎜ ⎣ ⎝ nkT ⎠ ⎦
n=
q ⎛ ∂V ⎞ ⎜ ⎟ kT ⎝ ∂ ln I ⎠
(1)
(2)
where I0 is the reverse saturation current, k is Boltzmann’s constant, T is the temperature in Kelvin, q is the electron charge, and n is the ideality factor, respectively. Here, the ideality factor is a figure-of-merit of the heterojunction diode, which indicates the quality of the diode compared to an ideal diode.45,46 The ideal diode has an ideality factor of 1, which suggests that the diffusion current is the dominant charge transport without a recombination current. An ideality factor higher than 1 indicates the existence of some defects or impurities at the junction, which induce the generation of the recombination current.47 The ideality factor (n) extracted from the slope of the semilog I−V curve in Figure 2a is 1.37, which indicates that the diffusion current is more dominant than the recombination−generation current, which is attributed to the low defects and impurities in the heterojunction interface.45,48 To shed light on the spectral responsivity of a MoS2/CuO nanosheet-on-1D heterojunction photodiode, the photoresponse properties were investigated under illumination of 33957
DOI: 10.1021/acsami.6b12574 ACS Appl. Mater. Interfaces 2016, 8, 33955−33962
Research Article
ACS Applied Materials & Interfaces different light wavelengths. Figure 2b shows the photocurrent (Ipho) curves at the reverse bias region under the illumination of different wavelengths of light (560, 600, 700, and 760 nm) at 1 mW power (Plight) of the incident light and dark conditions. The photocurrents at the reverse bias region increase with an increase of the reverse bias voltage and clearly depend on the wavelength of light. This photoresponsive characteristic at the reverse bias region can be explained by the energy band diagram of a MoS2/CuO NW heterojunction (Figure S6). When the heterojunction is formed between the n-type MoS2 sheet and the p-type CuO NW (Figure S6a), band bending occurs at the junction region by the band alignment mechanism (Figure S6b), which results in the creation of a built-in potential at the interface. The electron−hole pairs are generated at the junction area under the illumination of light and are swept away by the built-in potential, resulting in the generation of the photocurrent.17 When the reverse bias is applied, the photocurrent is further enhanced by an increase of the built-in potential (Figure S6c). It is worth noting that the wet transfer of MoS2 nanosheets on top of the CuO NWs (nanosheet-on-1D structure) enables a conformal wrapping of the nanosheet around the cylindrical NW and thus an excellent heterojunction interface. In comparison, the transfer of CuO NWs on top the MoS2 nanosheet (1D-on-nanosheet structure) does not provide a good heterojunction interface. In order to compare the device performances depending on the heterojunction structure (nanosheet-on-1D vs 1D-on-nanosheet), we fabricated a photodiode based on the heterojunction of the CuO NW above the MoS2 nanosheet (CuO/MoS2 1D-on-nanosheet heterojunction; Figure S7). As can be seen in Figures 2c,d, although the CuO/MoS2 1D-on-nanosheet heterojunction photodiode also shows rectifying behavior and photosensing capability, it provides a lower rectification ratio, a higher turnon voltage, and a lower photoresponsivity compared to those of the MoS2/CuO nanosheet-on-1D heterojunction photodiode. These inferior photodiode performances of the CuO/MoS2 1D-on-nanosheet device can be attributed to the lower interface quality and smaller contact area between the MoS2 nanosheet and the CuO NW when the cylindrical CuO NW is on top of the MoS2 nanosheet. These comparison results indicate that the wet transfer of nanosheets on the 1D cylindrical NWs is a reliable approach to providing an excellent heterojunction interface between 1D and layered materials and fabricating a high-performance heterojunction device. To investigate the photocurrent generation behavior of our heterojunction photodiode, we performed a photocurrent mapping analysis. In the case of a Schottky junction diode, the photocurrent is generated near the electrode area due to the abrupt potential difference by the energy barrier between the metal and semiconductor.49,50 On the other hand, the photocurrent of the p−n junction diode is generated at the interface region of the p−n junction due to the built-in potential generated by the energy band bending.17 For a better understanding of the photocurrent generation behavior, we performed a photocurrent mapping of MoS2/CuO NW heterojunction photodiodes at a reverse bias voltage of −2 V under a 532 nm wavelength of laser excitation (spot size: ∼2 μm diameter) by using a scanning near-field optical microscopy system and a sourcemeter. Figure 3a shows an optical microscopic image with the positions of the electrodes and heterojunction. The photocurrent mapping of this heterojunction structure is shown in Figure 3b, where the electrodes
Figure 3. (a) Optical microscopy image of a MoS2/CuO nanosheeton-1D heterojunction photodiode showing the positions of the electrodes and heterojunction. (b) Photocurrent mapping of a MoS2/ CuO nanosheet-on-1D heterojunction photodiode under a 532 nm laser showing the photocurrent generation at the heterojunction region of MoS2/CuO NW. The electrodes and heterojunction structure are indicated by the dotted lines.
and heterojunction structure are indicated by the dotted lines. The photocurrent generation was mainly observed at the heterojunction region of MoS2/CuO NW and was rarely observed near the electrode area. This result indicates that the large charge separation occurs at the MoS2/CuO heterojunction area, which is attributed to the built-in potential of the p−n heterojunction.41 The photoresponsivity is one important figure of merit representing the performance of the photodetectors. The photoresponsivity indicates the amount of generated photocurrent in the device in response to the illuminated light. The photoresponsivity can be expressed by the relationship Rλ = Ipho/Plight, where Rλ, Ipho, and Plight are the photoresponsivity, the photocurrent, and the illuminated optical power intensity per unit area, respectively.15,50 Here, the light absorption area was defined as the junction area between the CuO NW and the MoS2 nanosheet (a detailed description is given in Figure S8). The photocurrent is experimentally obtained by subtracting the dark current (Idark) from the total current (Itotal) under the illumination of light (Ipho = Itotal − Idark). Figure 4a shows the photocurrents and photoresponsivity depending on the optical power under the reverse bias (−2 V). The photocurrent increases with an increase of the optical power. The fitting of the photocurrent by the power law relationship (Ipho ∼ Pθ) results in an exponent value (θ) of 0.81. The nonunity exponent value in the power law relationship is related to the complex process of electron−hole generation, recombination, and trapping during the photoresponse process. The photoresponsivity decreases with an increase of the optical power and exhibits a maximum value of 157.6 A/W at an illumination intensity of 55 μW. This photoresponsivity value is significantly (over 20 times) higher than the values obtained for the MoS2/ ReSe2 (2D−2D) heterojunction photodetector (6.75 A/W) and the MoS2/Si (2D−bulk) heterojunction photodetector (7.2 A/W).15,18 The decrease of the photoresponsivity with an increase of the optical power can be attributed to the trap states by the defects or impurities at the heterojunction.44,45 Under the illumination of light, the photogenerated charges can be captured by the trap states, which reduce the recombination rate and thus increase the lifetime of photogenerated charge carriers. When the light power increased, the captured charges are saturated because of the limited trap states, resulting in a decrease of the photoresponsivity. One measure of the sensitivity of a photodetector is the photoresponsive on/off ratio. Figure 4b shows the photo33958
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responsive on/off ratios of metal−semiconductor-metal (MSM)-type photodetectors based on MoS2 nanosheets and CuO NWs, respectively. For the fabrication of MSM-type photodetectors, Cr (5 nm)/Au (70 nm) metals were used as electrodes. The MSM photodetectors exhibited photoresponsive on/off ratios of 3.5 and 48 at a bias voltage of 2 V for MoS2 nanosheets and CuO NWs, respectively. Here, the lower values of the photoresponsive on/off ratio of MSM-type photodetectors compared to the MoS2/CuO heterojunction photodetector can be attributed to the considerably high dark currents, even though the on currents of the MSM photodetectors are higher than that of the heterojunction device (Figure S9). A stable and fast-switching behavior of a photodetector is one of the important parameters for applications in optical communication and sequential imaging system. Figure 5 shows the stability of the photoswitching behavior and the photoresponse speed under the green LED light (Plight = 1.4 mW) and a bias voltage of −2 V for a MoS 2 /CuO NW heterojunction photodiode. The repeated switching behavior was well-retained with a stable and reproducible current (Figure 5a). On the other hand, MSM photodetectors show unstable photoresponsive switching behaviors under repeated light on/ off cycles (Figure S10). The MoS2/CuO NW heterojunction device has a fast photoresponse time (∼34.6 ms of rise time and ∼51.9 ms of decay time), which can be favorably compared to those of previous photodetectors (Table S1). Here, the fast response speed of the MoS2/CuO NW heterojunction photodetector can be attributed to the large built-in potential at the interface region.44 Figure 6 shows the photoresponsivity depending on the applied anode voltage and different wavelengths of light. The photoresponsivity increased under both 600 and 700 nm wavelength laser light with an increase of the anode voltage (Figure 6a). With increasing bias voltage, the photogenerated electron−hole pairs are more quickly separated and move toward the electrode because of the enhanced built-in potential at the interface, resulting in large photocurrents.51 The wavelength-dependent photoresponsivity is shown in Figure 6b. The photoresponsivity of a heterojunction device shows two strong peaks at 600 and 660 nm wavelength. On the other hand, the photoresponsivity becomes very low under light wavelength longer than 700 nm. This behavior can be attributed to the absorption characteristic of a multilayer MoS2 nanosheet, which has strong two absorption peaks at 606 and 670 nm wavelength caused by the spin−orbital splitting of the valence band and a weak absorption tail through the
Figure 4. (a) Photocurrent and photoresponsivity of a MoS2/CuO heterojunction photodiode depending on the optical power under the reverse bias (−2 V). (b) Comparison of the photoresponsive on/off ratios of a MoS2/CuO nanosheet-on-1D heterojunction photodiode, a CuO NW MSM photodetector, and a MoS2 sheet MSM photodetector depending on the applied anode under the green LED light (λ = 570 nm; Plight = 1 mW).
responsive on/off ratios of a heterojunction photodiode under the green LED light (λ = 570 nm; Plight = 1 mW). Here, the photoresponsive on/off ratio is defined as the photocurrent ratio when the light is switched on and off. The photoresponsive on/off ratio of a heterojunction device increased with an increase of the applied bias voltage (−0.5 to −1 V) and then reached the highest value of 3500 at a bias voltage of −1 V. A further increase of the negative bias voltage (−1 to −2 V) resulted in a decrease of the photoresponsive on/off ratio to a value of 1000. This result can be related to the dependence of the dark current and photocurrent on the bias voltage. While the dark current was stable at a low value of 2.8 fA below a bias voltage of −1 V, it began to increase when the bias voltage was increased over −1 V. Because the photocurrent already reached the saturation region around a bias voltage of −1 V, the highest photoresponsive on/off ratio was shown at a bias voltage of −1 V. For a better understanding of the performance of the heterojunction photodetector, we also compared the photo-
Figure 5. (a) Stability of the photoswitching behavior of a MoS2/CuO heterojunction photodiode under repeated on/off light illumination (λ = 570 nm; Plight = 1.4 mW) and a bias voltage of −2 V. (b) Photoresponsive rise and decay times of a MoS2/CuO heterojunction photodiode under light illumination of λ = 570 nm at Plight = 1.4 mW and a bias voltage of −2 V. 33959
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the performances of which can be easily tunable based on various combinations of 2D and 1D heterostructures.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12574. Fabrication methods, additional SEM and AFM images, and additional electrical and optical properties of MoS2/ CuO NW heterojunction devices, a MoS2 sheet, and CuO NW MSM photodetectors (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hyunhyub Ko: 0000-0003-2111-6101 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (Grant 2015R1A2A1A10054152), the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT, and Future Planning as Global Frontier Project (Grant 2015M3A6A5072945), and the Ministry of Trade, Industry and Energy (10064058).
Figure 6. (a) Photoresponsivity of a MoS2/CuO nanosheet-on-1D heterojunction photodiode depending on the applied anode voltages under 600 and 700 nm wavelengths of light. (b) Wavelengthdependent photoresponsivity of a MoS2/CuO nanosheet-on-1D heterojunction photodiode under a bias voltage of −2 V.
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indirect band transition.52 Table S1 summarizes the photodetector performances (rectification ratio, dark current, photoresponsivity, and response speed) of our MoS2/CuO NW heterojunction device in comparison with previous MoS2-based heterojunction devices. There is no previous work that simultaneously outperforms all of the figures of merit of the photodetector. In addition, all of the figures of merit of our photodetector exhibit values comparable to those of the best figures of merit of previous works.
REFERENCES
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CONCLUSIONS In summary, we developed nanosheet-on-1D heterojunction photodetectors with ultrahigh photoresponsivities based on the wet-transfer printing of the MoS2 nanosheet onto the CuO NW. The capillarity-assisted attractive forces during the wettransfer-printing process induce intimate nanosheet-on-1D contacts and thus the formation of a high-quality heterojunction with a sharp interface between the MoS2 nanosheet and CuO NW heterostructures. The photodetectors with a high-quality nanosheet-on-1D heterojunction exhibited an excellent photocurrent rectification ratio (∼6000 at ±2 V) with an ideality factor of 1.37, an ultrahigh photoresponsivity of 157.6 A/W, a low dark current of ∼38 fA at −2 V, and a fast photoresponse time (∼34.6 ms of rise time and ∼51.9 ms of decay time). It is well-known that, as the thickness of MoS2 is increased from monolayer to multilayer, the semiconducting properties change from a direct band gap with 1.9 eV to an indirect band gap with 1.2 eV.53,54 In addition, the carrier mobility and optical absorption properties of semiconducting 1D materials can be adjusted depending on the diameter.55,56 Therefore, further study should be done in the future to understand the effects of the thicknesses and diameters of MoS2 and CuO NW on the device performances. The suggested wettransfer printing of nanosheet/1D heterostructures have a great potential for the development of novel optoelectronic devices, 33960
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Research Article
ACS Applied Materials & Interfaces
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DOI: 10.1021/acsami.6b12574 ACS Appl. Mater. Interfaces 2016, 8, 33955−33962
Research Article
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DOI: 10.1021/acsami.6b12574 ACS Appl. Mater. Interfaces 2016, 8, 33955−33962
