PbS-Decorated WS2 Phototransistors with Fast Response - ACS

Article pubs.acs.org/journal/apchd5

PbS-Decorated WS2 Phototransistors with Fast Response Yu Yu,† Yating Zhang,*,† Xiaoxian Song,† Haiting Zhang,† Mingxuan Cao,† Yongli Che,† Haitao Dai,‡ Junbo Yang,§ Heng Zhang,† and Jianquan Yao† †

Key Laboratory of Optoelectronics Information Technology (Tianjin University), Ministry of Education, School of Precision Instruments and Optoelectronics Engineering, and ‡Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, School of Science, Tianjin University, Tianjin 300072, China § Center of Material Science, National University of Defense Technology, Changsha 410073, China S Supporting Information *

ABSTRACT: Tungsten disulfide (WS2), as a typical metal dichalcogenides (TMDs), has aroused keen research interests in photodetection. Here, field effect phototransistors (FEpTs) based on heterojunction between monolayer WS2 and PbS colloidal quantum dots are demonstrated to show high photoresponsivity (up to ∼14 A/W), wide electric bandwidth (∼396 Hz), and excellent stability. Meanwhile, the devices exhibit fast photoresponse times of ∼153 μs (rise time) and ∼226 μs (fall time) due to the assistance of heterojunction on the transfer of photoexcitons. Therefore, excellent device performances strongly underscore monolayer WS2−PbS quantum dot as a promising material for future photoelectronic applications. KEYWORDS: quantum dots, WS2, phototransistors, fast response

T

However, a common roadback in the employment of WS2 in phototransistors is the limited absorption due to atomically thin profile and limited spectral selectivity determined by its bandgap. To expand the absorption property and the photoresponsivity, efforts should be devoted to design the photodetectors through the combination of WS2 and other materials with strong optical absorption intensity.25 Due to narrow band gap and large Bohr radii which provide a strong quantum confinement effect,24,26,27 lead chalcogenide (PbS, PbSe, and PbTe) colloidal quantum dots (CQDs) give rise to strong optical absorption resonances associated with allowed transitions between discrete electronic states, which 2D WS2 has so far failed to access. Kufer et al.28 reported on a highly sensitive hybrid phototransistor device with very high responsivity of 106 A/W that benefits from the synergism of MoS2 and PbS QDs. Chen et al.29 demonstrated a photoresponsivity of approximately 104 A/W in an n−n type heterostructure photodetector that consisted of a monolayer MoS2 thin film covered with a thin layer of graphene QDs. Song et al.30 demonstrated PbS QDs/2D-WSe2 phototransistors obtained a high responsivity up to 2 × 105 A/W and a response time of ∼7 ms. However, QDs are known to exhibit low mobility and as a result the photoresponse times are limited to the range of 30 ms ∼ 2 s.26−28,31,32 Furthermore, WS2−PbS phototransistors have never been reported as our knowledge.

he ultrafast photodetectors have recently attracted substantial interests in optical communications, environmental monitoring, photometers, day- and night-time surveillance, and biological/chemical sensing.1−4 The basic mechanism is that the active materials can capture optical signals and convert them into electrical signals instantaneously.5 For ultrafast photodetectors, the proper material with the ability of transporting photogenerated carriers and its coupling with other active materials are critical factors.6 Owing to semiconducting properties and relatively high carrier mobilities, tungsten based transition metal dichalcogenides compounds (TMDC) have attracted great interests in photodetectors.7−15 The bulk tungsten disulfide (WS2) is an indirect bandgap (1.4 eV) semiconductor, however, it becomes a direct bandgap (2.1 eV) material after exfoliated into a single layer.16 Since adjacent layers in WS2 crystals are joined together by weak van der Waals forces, a single or few layered WS2 can be fabricated by micromechanical cleavage. Equipped with properties including chemical stability and only having a weak impurity band,17 WS2 provides a unique advantage over other 2D materials.18−21 Especially in field effect transistor (FET), it exhibits high mobility, high thermal stability, no dangling bonds, and electrostatic integrity.4,22 Very recently, Yao et al.23 introduced monolayer WS2 as the active layer of FET phototransistor prepared by pulsed-laser deposition (PLD) method. It demonstrated that the responsivity approaches a value of 0.51 A/W. Chowdhury et al.24 investigated p-WS2 2D/ 3D heterojunction phototransistor with a peak responsivity of 1.11 A/W and a moderate on/off current ratio of ∼103. © 2017 American Chemical Society

Received: December 30, 2016 Published: March 10, 2017 950

DOI: 10.1021/acsphotonics.6b01049 ACS Photonics 2017, 4, 950−956

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Figure 1. (a) Room-temperature Raman spectrum from the monolayer WS2 nanoflakes, using the 532 nm laser. (b) The absorption spectrum of WS2−PbS hybrid, exhibits band absorption peaks at 538, 633, and 1306 nm (inset: the transmission electron microscope (TEM) image of PbS QDs).

Figure 2. Characteristics of the WS2−PbS phototransistor. (a) Output characteristics at different VGS with illumination (solid lines) and in the darkness (dashed lines). (b) Transfer characteristics (IDS vs VGS) at VDS = 0.1 V with illumination (red line) and in the darkness (black line). (c) IDS − VDS output characteristics at different red ecitation ligh intensity (λ = 808 nm) without VGS. (d) The relatinship between of red excitation light intensity at different VDS if VGS is biased at 1 V.

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In this paper, WS2 phototransistors have been fabricated with decoration of PbS QDs. The hybrid WS2−PbS phototransistors

RESULTS AND DISCUSSION

The typical Raman spectrum of the WS2 film shown two phonon modes at Brillouin zone center (E12g(Γ) and A1g(Γ)) and one acoustic mode at the M point (LA(M)).2 Raman spectra of the WS2 film were performed with 532 nm laser excitations shown in Figure 1a. LA(M), E12g(Γ), and A1g(Γ) modes are observed and are located at 175, 356.5, and 420.6 cm−1, respectively. The WS2 film can be presented with a high crystal quality shown from sharp phonon peaks. Other peaks correspond to the second-order Raman modes which are multiphoton combinations of the above three modes. Our

exhibit high responsivity (14 A/W), wide electric bandwidth (∼396 Hz), and excellent stability and reproducibility. Particularly, a fast photoresponse time of less than 226 μs is presented in the device. Therefore, these high performances indicate that the combination of monolayer WS2 with PbS colloidal quantum dots has great potential applications for ultrafast photodetection. 951

DOI: 10.1021/acsphotonics.6b01049 ACS Photonics 2017, 4, 950−956

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Raman results for the monolayer WS2 film are well consistent with the previous reports.2,23,33,34 The absorption of WS2−PbS QDs hybrid is displayed in Figure 1b, and the absorption region ranges from visible to near-infrared wavelength, indicating that this device can be used for broadband spectrum detection. The hybrid shows three absorption peaks, “538 nm”, “633 nm”, and “1306 nm”, between 400 and 1500 nm. The transmission electron microscope (TEM) image of PbS QDs (the detailed manufacture process is shown in SI) is shown in the inset of Figure 1b. The average diameter of PbS QDs is ∼5 nm. The schematic diagram of WS2−PbS hybrid phototransistor is shown in Figure 1c. For the bottom-gate configuration, a highly doped n-type silicon wafer covered with a 300 nm thick SiO2 layer (capacitance Cox of 11.5 nFcm−2) was used as gate electrode. Figure 1d shows the cross-sectional scanning electron microscopy (SEM) of the device with the scale of 300 nm. It depicts that the thickness of SiO2 is 300 nm, then the WS2 film grown on SiO2 has the average thickness of ∼1 nm, and the thickness of PbS QDs film spinning on WS2 film is ∼87.8 nm. Figure 2a describes the I−V characteristics of different gate voltages (0 V, ±0.5 V, ±1 V, ±1.5 V, ±2 V) under 808 nm laser with illumination of 845 mW cm−2 (solid lines) and without irradiation (dashed lines). If both VGS and VDS are negative (in the third quadrant), the device operates in the hole-enhancement mode, while in the first quadrant, the device operates in the electron-enhancement mode. Specially, at low |VGS| (OFFstate), IDS increases rapidly with the increase of VDS, indicating that the Schottky barrier forms in the device. However, at high | VGS| (ON-state), unipolar transport with standard linear-tosaturation I−V transistor characteristics is observed. Comparing with the condition with no irradiation, the values of |IDS| with irradiation become larger. It can be understood that the photoinduced carriers can be generated under the condition of light irradiation. Therefore, the enhancements of IDS are ascribed to the increase of the electron and hole concentrations. Figure 2b shows transfer characteristics of a bottom-gate WS2− PbS QDs phototransistor. The device exhibits typically ambipolar characteristics, and a “V” shape of the transfer curves is particularly presented, manifesting either electrons or holes transport in the n-type or p-type channels of the device. Meanwhile, the field-effect mobility and gate voltage can be extracted with the relationship of

μ=

∂IDS L VDSCoxW ∂VGS

Table 1. Comparison in Field Effect Mobility for Various WS2-Based FEpTs device

materials

charge mobility (cm2 V−1 s−1)

ref 36 ref 37 ref 38 ref 39 ref 14 this work

WS2 WS2 WS2 MoS2/WS2 h-BN/WS2/h-BN WS2/PbS

19 60 0.91 65 214 0.32

increases with the excitation light intensity increasing. It can be concluded that with larger irradiance more photoexcitons can be excited, leading to the decrease of built in potential of the Schottky barrier and have a higher probability to overpass the Schottky barrier, and then flow to the external circuit to generate linearly enhanced photocurrents. However, for higher light irradiances, some photoexcitons remained in the trap states of WS2, thus, cannot be converted to the photocurrent, leading to the saturation of photocurrent. To demonstrate the relationship between responsivity (R) and excitation intensity (Ee) with different drain voltages, the plot of R−Ee is shown in Figure 3a. In the double-logarithmic system, a approximately linear dependence of lg(R) with lg(Ee) is presented. The devices showed the maximum responsivitie is ∼14 A/W under IR illumination (wavelength length: 808 nm). To present photodetector’s performance, some important parameters, like the detectivity (D*; show in the inset of Figure 3a) and the gain (G; described in Figure S5), can be hν given as D* = RA1/2/(2eIDS)1/2 and G = e R , respectively, where R is the responsivity, A is the area of the detector, e is the charge of an electron, and IDS is the dark current. In this work, when maximum photoresponsivity (R) is set to 14 A/W, an impressively high detectivity of 3.9 × 108 Jones has been achieved, indicating that the proposed device is extremely sensitive to incident illumination. Meanwhile, the optical gain of the device. is 21.48. To study the frequency responsivity (Rf) and the working bandwidth, we measured photocurrents via the irradiation of 808 nm light laser using a chopper to control light exposure onto the device (shown in Figure 4a). The illumination time change was determined through the changes of chopper’s frequency. The device exhibits fast response and long-term repeatability in the frequency (f) ranging from 5 to 400 Hz (the detail was shown in SI). Based on the fitting curve, shown in Figure 3b, the frequency responsivity was plotted as a 0.248 function of f as described by R f = 0.65 . The maximum of Rf

(1)

(1 + f )

was 0.228 A/W. Due to the response dependence on frequency, the electric working bandwidth of the photo-

where W, L, and Cox are the channel width, channel length, and the gate capacitance per area, respectively. Therefore, both the electron and hole mobilities can be calculated as 0.68 cm2 V−1 s−1 and 0.32 cm2 V−1 s−1, which are 1 order of magnitude larger than prior studies in pristine PbS QDs phototransistor (0.01 and 0.03 cm2 V−1 s−1),35 respectively. Quantitatively, Table 1 shows the comparison in field effect mobility for various WS2 based phototransistors. Without gate voltage, the typical IDS− VDS output characteristics of the WS2−PbS phototransistor under the dark and different excitation light intensity (λ = 808 nm) are shown in Figure 2c. It is presented that the phototransistor is very sensitive to the weak excitation light intensity and IDS increases with the increasing illumination intensity. Figure 2d describes the relationships between photocurrent (Iph = Ilight − Idark) and the excitation intensity under different drain voltages, showing that the photocurrent

∞

transistor was defined by Δf = ∫ 0

Rf R fmax

2

df .40 The result was

397 Hz. This calculated result is in total agreement with the experimental value. Photoresponse speed determining the capability of a photodetector to follow a fast-switching optical signal is another key factor for photodetector. Particularly, shown in Figure S3, the device exhibits no response for purple and green lights. In order to investigate photoresponse speed of WS2− PbS FEpT, initially we used an excitation wavelength of 808 nm, which is below the absorption edge of WS2−PbS (shown in Figure 1b). Figure 4a shows schematic diagram of experimental setup for studying the detailed photoresponse speed of WS2−PbS FEpTs. A mechanical light chopper was 952

DOI: 10.1021/acsphotonics.6b01049 ACS Photonics 2017, 4, 950−956

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Figure 3. (a) Photoresponsivity R and detectivity D* (the inset) with a relationship of excitation light (Ee) at applied drain bias of 1.5 and 0.5 V and the gate voltage is fixed at 2 V. (b) Frequency photoresponsivity Rf vs frequency f, including reciprocal function fitting line.

Figure 4. (a) Schematic diagram of experimental setup adopted for studying the photoresponse speed of WS2−PbS FEpTs. (b) Temporal photocurrent response of a WS2−PbS FEpT under red light irradiation of 845 mW/cm2. (c) Transient photoresponse characteristics of device at different illumination power density. (d) Photocurrent of a photoswitch during 500 cycles with the relationship of irradiation.

Figure 5. (a) Schematic of the band diagram of WS2−PbS heterostructure, where LUMO and HOMO denote the lowest unoccupied molecular orbital and the highest occupied molecular orbital, respectively. (b) Schematic drawing shows change generation and transfer process under the illumination of 808 nm on the WS2−PbS heterostructure.

used to turn the 808 nm laser (∼845 mW cm−2), and the oscilloscope was used to monitor the time dependence of the photocurrent. The impedance was set at 50 MΩ. Figure 4b displays temporal photocurrent response of a WS2−PbS FEpT under a 400 Hz pulsed illumination if the DC voltage is fixed at 15 V. Moreover, the drain current quickly increases as soon as

the light switch is turned on and then decreases when the light switch is turned off. It indicates that the increased charge intensity will lower the effective barrier height upon illumination, which allows easier charge tunneling and transportation than that of device in darkness. The rise and decay times of the photocurrent are ∼153 and ∼226 μs, respectively, 953

DOI: 10.1021/acsphotonics.6b01049 ACS Photonics 2017, 4, 950−956

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film was transferred to the surface of the Si n+/SiO2 substrate. Source and drain electrodes were thermally evaporated through a sophisticated shadow mask, where the channel length (L) and channel width (W) were defined as 0.1 and 2.5 mm, respectively. Finally, a spin-coated film of PbS QDs was covered on the top. The PbS QDs layer was prepared in the following step: a drop of PbS QDs toluene solution (10 mg/ mL) was first deposited on the spinning substrate at a speed of 2000 rpm/min and left for 15 s to dry. A ligand exchange to much shorter 1,2-ethanedithiol (EDT) has been carried out to improve charge transport and charge transfer from PbS to WS2.32 So, three drops of 2% EDT solution were deposited on the rotating substrate and followed by two drops of acetonitrile and two drops of toluene. Each device was dried for 24 h in vacuum conditions before further measurements. The WS2 film was characterized by DXR Raman Microscope with a laser radiation of 532 nm. The absorption of hybrid was measured using a Zolix Omni-λ300 spectrometer. The TEM images of PbS QDs were measured by JEM-100CXII (JEQI, Japan) operating at 100 kV. The cross-sectional scanning electron morphology images of the device were obtained by SEM (NanoSEM 430). A bias voltage (VDS) was applied between the source (with ground) and drain electrodes by Keithley 2400, and the channel current flowing into the drain electrode (IDS), which was also measured by Keithley 2400. And the gate voltage (VGS) was applied by HP6030A. The photoelectrical measurements were also performed based on this system under illumination of 808 nm laser.

indicating a faster response speed than previous reports.2,23,33 Here, promising device performance was demonstrated by the fast response of WS2−PbS phototransistor to light with a stepwise varying power density of 48.75, 166.2, 492.5, 647.5, and 845 mW cm−2, as shown in Figure 4c. Furthermore, to demonstrate the stability of photoresponse for the device, we measured the photocurrents with the relationship of ON/OFF cycles under different illumination at the chopper frequency of 400 Hz. As shown in Figure 4d, WS2−PbS phototransistors exhibit an increase in photocurrent under illumination of light at a high power density. It shows that the stable photoresponse tuned by different light illuminations is critical for high performance in photoelectronic sensors and rewritable memory devices. The working principle of phototransistor is schematically described in Figure 5. Upon illumination, with a photoenergy larger than the energy gap (Eg), the PbS film absorbs photons to generate a large number of electron−hole pairs. Because the Fermi energy (EF) of PbS is higher than that of WS2 as shown in Figure 5a, resulting in some electrons roll down from lowest unoccupied mortal orbital (LUMO) to that of WS2, and the formation of a built-in electric field in the surface that points toward WS2. Accordingly, holes separated from photoexcitons generated in the heterojunction are transferred from PbS to WS2 due to the existence of the built-in field. Therefore, the shift toward positive VGS (shown in Figure 2b) can be ascribed to the efficient photoexcitons separation and holes transfer at the interface.41 The electrons drift faster in the monolayer WS2 than in the PbS QDs due to the greater electron mobility of the single-layer WS2 than as-deposited PbS and can be quickly collected by the electrodes. In addition, the deposited defective PbS film can act as a recombination center of photogenerated carriers, leading to a short composite lifetime in PbS. Therefore, the photoresponse times are greatly reduced, so as to reach the order of several hundred microseconds. The schematic diagram is shown in Figure 5b, indicating that PbS acts as the layer of photon absorption, while monolayer WS2 functions as charge transport layer. By fully utilizing the key functionality of each individual layer and the assistance of built-in electric field of the heterojunction, the detecting performances are enhanced.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.6b01049. The characterizations and synthesis of PbS QDs, photocurrent response versus frequencies, optical gain via irradiance light intensity, and photocurrent response under different wavelength light (PDF).

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CONCLUSIONS In summary, broadband WS2 phototransistors have been fabricated with decoration of PbS QDs through a low-cost, solution-processed strategy. The phototransistor exhibits bipolar behaviors with wide electric bandwidth (∼396 Hz), high photoresponsivity (∼14 A/W). Particularly, the device presents a rapid response of ∼200 μs with the assistance of the WS2−PbS heterojunction. Therefore, these high performances indicate that the combination of monolayer WS2 with PbS colloidal quantum dots has great potential applications for ultrafast response photodetection, especially in the infrared region.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yu Yu: 0000-0001-6712-6191 Haiting Zhang: 0000-0001-8046-710X Heng Zhang: 0000-0002-5175-7367 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Nos. 61675147 and 61605141).

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METHODS The chemical vapor deposition (CVD) grown monolayer WS2 film on Si n+/SiO2 (300 nm) substrate was purchased from Six Carbon Technology Co., Ltd. PbS quantum dots (QDs; the fabrication progress shown in SI) were synthesized via a wet chemical method.42−44 Devices Fabrication. The details of WS2−PbS hybrid phototransistors were fabricated as follows. Monolayer WS2

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DOI: 10.1021/acsphotonics.6b01049 ACS Photonics 2017, 4, 950−956

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DOI: 10.1021/acsphotonics.6b01049 ACS Photonics 2017, 4, 950−956

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DOI: 10.1021/acsphotonics.6b01049 ACS Photonics 2017, 4, 950−956