High-Temperature Flexible WSe2 Photodetectors with Ultrahigh Photoresponsivity

doi:10.21203/rs.3.rs-1541792/v1

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High-Temperature Flexible WSe2 Photodetectors with Ultrahigh Photoresponsivity

https://doi.org/10.21203/rs.3.rs-1541792/v1

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published 28 Jul, 2022

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Industries such as aerospace or military have promoted the development of high-temperature photodetectors. However, current high-temperature photodetectors exhibit low photoresponsivity (<10 A/W) because heat resistant materials usually have poor optical sensitivity. 2D transition metal dichalcogenides demonstrate outstanding optoelectronic properties, but they cannot survive at elevated temperature. It is challenging to realize photodetector with both high-temperature-resistant capability and high photoresponsivity. Here, we report WSe2 photodetector which can endure record high temperature of 700 ℃ in air (1000 ℃ in vacuum). It is the most heat resistant 2D-material device ever reported. Graphene/h-BN heterostructure with atomically flat interface is the key for high-temperature protection. Unconventional negative photoconductivity (NPC) phenomenon appears at high temperature. Operated in NPC mode, the device exhibits photoresponsivity of 2.2×106 A/W, which is ~105-fold higher than that of state-of-the-art high-temperature photodetectors, and even higher than that of existing WSe2 photodetectors. Furthermore, current high-temperature photodetectors are rigid, which limits their applications. Our device demonstrates excellent flexibility, making it highly adaptive to various shaped surfaces. Our approach can be extended to other 2D materials and may stimulate the research of novel 2D-material devices working in harsh environment.

Industries such as aerospace or military have promoted the development of high-temperature photodetectors. However, current high-temperature photodetectors exhibit low photoresponsivity (<10 A/W) because heat resistant materials usually have poor optical sensitivity. 2D transition metal dichalcogenides demonstrate outstanding optoelectronic properties, but they cannot survive at elevated temperature. It is challenging to realize photodetector with both high-temperature-resistant capability and high photoresponsivity. Here, we report WSe₂ photodetector which can endure record high temperature of 700 ℃ in air (1000 ℃ in vacuum). It is the most heat resistant 2D-material device ever reported. Graphene/h-BN heterostructure with atomically flat interface is the key for high-temperature protection. Unconventional negative photoconductivity (NPC) phenomenon appears at high temperature. Operated in NPC mode, the device exhibits photoresponsivity of 2.2×10⁶ A/W, which is ~10⁵-fold higher than that of state-of-the-art high-temperature photodetectors, and even higher than that of existing WSe₂ photodetectors. Furthermore, current high-temperature photodetectors are rigid, which limits their applications. Our device demonstrates excellent flexibility, making it highly adaptive to various shaped surfaces. Our approach can be extended to other 2D materials and may stimulate the research of novel 2D-material devices working in harsh environment.

Photodetectors are considered to be the core of modern communication components. As the development of aerospace, military, underground exploration, and harsh environment robotics, photodetector capable of operating at harsh environments are highly desired. Current high-temperature photodetectors are mainly based on wide-bandgap materials, such as SiC, GaN, or Ga₂O₃. Their optical sensing capability is barely satisfactory. A typical SiC photodetector can endure 550 ℃ and its photoresponsivity (one of the most essential figures of merit for photodetector) is only 0.54 A/W ¹. GaN photodetector is able to operate in a wide temperature range from -196 ℃ to 527 ℃ ², but it demonstrates very low photoresponsivity of 0.02 A/W. So et al., reported a AlGaN/GaN photodetector with operation temperature of 200 ℃ and photoresponsivity of 5 A/W ³. Zhou et al., developed a Ga₂O₃ photodetector with operation temperature of 200 ℃ and photoresponsivity of 0.1 A/W ⁴. There is a trend that photodetectors which can endure high temperature demonstrate very low photoresponsivity. The photoresponsivity of state-of-the-art high-temperature photodetectors is usually below 10 A/W. As such, weak light detection at high temperature is challenging. Furthermore, flexibility is an important development trend for next generation photodetectors. Flexible high-temperature photodetectors are desired in many applications, such as aeroengine blade with flexible (optical) sensors attached on its curved surface, and fully-adaptive soft robot working in high temperature environment with highly flexible sensors integrated on soft artificial muscle. However, high-temperature photodetectors reported so far are rigid. Limited by the poor thermal stability of traditional flexible materials⁵ (polyethylene terephthalate (PET), polyimide (PI), polydimethylsiloxane (PDMS) and so forth), the temperatures that existing flexible optoelectronic devices can endure are below 300 ℃ ^6,7. Consequently, highly-sensitive high-temperature photodetectors with excellent flexibility are highly desired.

Two-dimensional transition metal dichalcogenides (TMDs) have drawn a great deal of attention in optoelectronics field due to their fascinating physical properties^8,9and facile preparation. They can reach monolayer or few layer by mechanical exfoliation¹⁰, liquid-phase exfoliation¹¹, or chemical vapor deposition method^12,13. Although graphene is the most well-investigated 2D material, TMDs have the advantages of higher sensitivity and lower dark current, and demonstrate remarkable optical sensing capability at room temperature. The photoresponsivity of monolayer MoS₂ photodetector reaches 880 A/W ¹⁴. Xie et al., reported photodetectors based on multilayer WSe₂ flake with photoresponsivity of 1.5×10⁵ A/W¹⁰. Monolayer WSe₂ photodetector demonstrates higher photoresponsivity of 1.8×10⁵ A/W ¹². Additionally, the outstanding mechanical properties (large fracture strain^15,16) make TMDs promising channel materials for flexible photodetector. However, TMD electronic/optoelectronic devices cannot survive at high-temperature. MoS₂ starts to oxidize and degrade at 300 ℃ in air, resulting in many triangular etch pits on its surface^17,18,19. The edge of WSe₂ flake begins to oxidize at 300 ℃ in air²⁰. It hinders the application of TMDs in aerospace and many other harsh environments. Importantly, it hinders the exploration of the fascinating optoelectronic properties of TMDs at ultrahigh temperature.

Here, we developed WSe₂ photodetectors with h-BN (hexagonal boron nitride)/graphene heterostructure protection which can endure 700 ℃ in air and 1000 ℃ in vacuum. Unconventional negative photoconductivity (NPC) phenomenon appeared at high temperature. Operated in NPC mode, the device exhibited photoresponsivity of 2.2×10⁶ A/W, which is ~10⁵-fold higher than that of state-of-the-art high-temperature photodetectors, and even higher than that of existing WSe₂ photodetectors. The photodetector demonstrated excellent flexibility and realized in situ high-temperature optical sensing under bending state for the first time.

WSe₂ photodetector fabrication and characterization. We fabricated WSe₂ field-effect transistor (FET) as photodetector with h-BN encapsulation and graphene source/drain electrodes (Fig. 1a) by mechanically stacking each atomic layer sequentially on freshly cleaved mica surface followed by photolithography, Pt deposition, and lift-off for top gate fabrication. The optical microscope image of a representative WSe₂ device is shown in Fig. 1c (Pt top gate is not shown). Mica substrate with thickness of ~ 100 µm is highly flexible (bending radius < 2 mm, Fig. 1b), high-temperature-resistant, and transparent. Additionally, compared with other flexible substrates, mica provides atomically flat terraces over large areas, so 2D materials can approach the limit of atomic flatness on mica surface and get rid of the microscopic corrugations which result in carrier scattering and degradation of electrical properties²². Multilayer WSe₂ flake was chosen as channel material of high-performance photodetector. Hexagonal boron nitride (h-BN) has been reported in previous work to be excellent oxygen-resistant coating ²³. Two h-BN flakes with thickness of ~ 50 nm were placed on top and bottom of WSe₂ respectively as high-temperature encapsulation, where top h-BN also serves as gate dielectric. The 2D material flakes were all characterized by Raman spectroscopy (Supplementary Fig. S4 and Fig. 2c). Sharp Raman peaks imply that the 2D materials studied in this work have nearly perfect lattice structures. Figure 1d is the high-resolution transmission electron microscopy (HRTEM) cross-sectional image of h-BN/graphene/WSe₂/h-BN van der Waals heterostructures in the WSe₂ FET. The thickness of monolayer WSe₂ (~ 0.7 nm) is consistent with reported values^24,25. It can be seen clearly that the 2D materials are atomically flat and no air gap is observed at interface, indicating excellent encapsulation which is essential for high-temperature protection.

High-temperature-resistant capability. To investigate the high-temperature-resistant capability of our device, the WSe₂ FET was heated at 500 ℃, 600 ℃, and 700 ℃ for 15 min sequentially in an open quartz furnace. The high-temperature experiments in this work were all carried out in air, unless otherwise noted. WSe₂ flake protected by h-BN showed negligible change after heating (Fig. 2b, Pt top gate is not shown). Raman spectra showed E¹_2g peak (~ 249 cm^− 1) and A_1g peak (~ 258 cm^− 1) corresponding to WSe₂, whereas no WO₃ character peaks (~ 700 cm^− 1 or ~ 810 cm^− 1) was observed^26,27,28 (Fig. 2c), implying that WSe₂ was not oxidized. After 700 ℃ heating, flexible mica substrate was in good shape as well. Then the same device was heated at 750 ℃ for 15 min. The WSe₂ channel still showed negligible change (Supplementary Fig. S5), but mica substrate became brittle and less transparent. As control groups, bare WSe₂ and WSe₂ covered with 100 nm Al₂O₃ (a widely used oxidation-resistant coating) by atomic layer deposition were heated at 500 ℃ for 15 min. In both situations, the WSe₂ flakes were strongly oxidized and became almost transparent (Supplementary Fig. S6) inasmuch as WO₃ is transparent under visible light. Raman spectrum shows prominent WO₃ character peaks around 700 cm^− 1 and 810 cm^− 1 (Fig. 2c). Therefore, bare WSe₂ has poor thermal stability which is consistent with previous reports²⁰. The h-BN is significantly superior to Al₂O₃ as high-temperature protection layer.

Next, we investigated the high-temperature-resistant capability in vacuum of our devices. Since mica substrate cannot endure temperature above 750 ℃, we replaced it with Si substrate with 300 nm SiO₂ on top. We heated the WSe₂ FETs at 1000 ℃ for 15 min in vacuum with Argon flow rate of 100 sccm. The WSe₂ FET showed negligible change after heating (Supplementary, Fig. S7). Raman spectrum demonstrates prominent WSe₂ character peaks and no WO₃ character peak, indicating that WSe₂ lattice structure remained intact after heating. As a control group, bare WSe₂ flake on SiO₂/Si substrate completely vanished after 1000 ℃ heating (Supplementary, Fig. S7). Therefore, the h-BN/graphene structure effectively protects WSe₂ at ultrahigh temperature of 1000 ℃ in vacuum. The temperature that our devices can tolerate is much higher than that of current 2D material devices, both in air and vacuum environments (Table 1). Our research greatly expands the working temperature range of 2D materials, allowing the excellent electrical properties of 2D materials to be applied in high temperature environments.

Graphene electrodes are essential to high-temperature protection. We replaced the graphene in WSe₂ FET with conventional (Pt) metal electrodes (Fig. 2d). After heating at 500 ℃ for 10 min, WSe₂ flake within h-BN encapsulation was strongly oxidized (Fig. 2e). This is because Pt film deposited by sputtering has much larger surface roughness than that of graphene. Top h-BN cannot completely conform to the topography of Pt surface (HRTEM cross-sectional image in Fig. 2g), leading to oxygen molecules diffusion into h-BN encapsulation through Pt/h-BN interface. Atomic force microscopy (AFM) characterization demonstrates that the height variation of Pt surface is ~ 6.8 nm (Supplementary Fig. S8), while the height variation of graphene measured (~ 0.6 nm) appears to be limited by instrument noise and is identical to that obtained from the surface of highly oriented pyrolytic graphite (HOPG) which approaches the limit of atomic flatness. As such, good contact is formed between h-BN and graphene (HRTEM cross-sectional image in Fig. 2f) which can effectively prevent oxygen diffusion. To further prove the oxygen diffusion through Pt/h-BN interface, we transferred a WSe₂ flake on Pt surface and covered it with h-BN. After heating at 500 ℃ for 15 min, WSe₂ inside h-BN/Pt encapsulation was strongly oxidized (Supplementary Fig. S9). Graphene/WSe₂/h-BN sandwich structure was prepared and tested under the same experimental condition. The WSe₂ flake inside h-BN/graphene encapsulation was still in good shape after heating. Therefore, graphene electrode prevents oxygen diffusion and plays an important role in high-temperature protection.

High-temperature electrical properties. We systematically investigated the temperature-dependent electrical properties of the WSe₂ FET in dark environment without the influence of photoexcitation. Electrical measurements were carried out below 550 ℃ for safety reason. Figure 3a demonstrates the transfer curves of a representative WSe₂ device with channel width/length of 10 µm/5 µm (source/drain voltage V_ds=100 mV). At room temperature (20 ℃), on/off ratio of 2×10⁶ and subthreshold swing (SS) of 130 mV/dec were obtained (SS = dV_gs/dlgI_ds, where V_gs is gate bias, and I_ds is source/drain current). Carrier mobility derived from p branch reached ~ 35 cm²/V·s. The excellent room-temperature electrical performance is comparable to that of the best WSe₂ field-effect transistors reported^{29, 30}. As temperature increased, larger I_ds for all values of gate voltage from − 3 V to 3 V were observed, and the device demonstrated ambipolar behavior under different temperatures (Fig. 3a). Under 500 ℃, the WSe₂ FET still showed excellent transfer properties. Linear and symmetric I_ds-V_ds curves obtained at 500 ℃ further demonstrated ambipolar behavior and suggested near-ohmic contact between WSe₂ and graphene electrodes (Fig. 3b). Photoluminescence spectra illustrated that the bandgap of WSe₂ decreased from 1.55 eV to 1.40 eV as temperature increased from 150 ℃ to 500 ℃ (Supplementary Fig. S10). Theoretically, smaller bandgap resulted in smaller on/off ratio. As temperature varied from 20 ℃ to 500 ℃, on/off ratio of the WSe₂ FET decreased from 2×10⁶ to 1×10² (Fig. 3c). According to the Eq. 3¹:

$$SS=ln10\bullet \frac{kT}{q}\bullet \frac{{C}_{ox}+{C}_{s}}{{C}_{ox}}$$

where k is Boltzmann constant, T is absolute temperature, q is the charge per carrier, ${C}_{ox}$ and ${C}_{s}$ are dielectric capacity and depletion capacity, respectively, higher temperature leads to larger SS. The SS of our device increased from 130 mV/dec to 600 mV/dec as temperature varied from 20 ℃ to 500 ℃ (Fig. 3c). After 500℃ heating in air, the device was tested at room temperature again. Interestingly, due to the high temperature (500 ℃) annealing which improves WSe₂/graphene contact, the electrical properties of our device did not degrade but slightly improved (SS became smaller, Supplementary Fig. S11).

The “off” state current I_off of WSe₂ FET increased from 10^− 12 A to 10^− 8 A as temperature varied from 20 ℃ to 500 ℃ (Fig. 3d). The h-BN gate dielectric leakage current (I_gs) of the same device measured at the same V_gs and V_ds under the same temperature was 1–2 orders of magnitude smaller than I_off (Fig. 3d), indicating that the “off” state current of WSe₂ FET at high temperature is dominated by the intrinsic turn-off characteristics of WSe₂ instead of h-BN leakage current. Therefore, h-BN is not only a perfect oxygen-resistant coating, but also an excellent high-temperature dielectric layer. The remarkable high-temperature isolation properties of h-BN contribute to the high-performance of our WSe₂ devices.

WSe₂ photodetector with negative photoconductivity. We next explored the photoelectric characteristics of the WSe₂ photodetectors. Under 25 W/m² white light illumination at 20 ℃, the I_ds-V_gs transfer curve moved upward for all values of gate bias from − 3 V to 3 V (Fig. 4b), indicating positive photoconductivity (PPC). Interestingly, under high temperature (400 ℃), white light illumination resulted in a left shift of transfer curve (Fig. 4c). Larger shift was observed as light intensity increased. The N branch of transfer curve mainly demonstrated PPC, while P branch demonstrated negative photoconductivity (NPC). As V_gs was set at a constant value of 3 V, I_ds increased under white light illumination at 400 ℃, and the WSe₂ device act as a PPC photodetector (Fig. 4d). In contrast, as V_gs was set at 0 V, I_ds decreased under illumination at 400 ℃, and the WSe₂ device act as a NPC photodetector (Fig. 4e). Therefore, high-temperature reconfigurable photodetector was realized which can switch between NPC and PPC photodetector under the same temperature by adjusting gate voltage V_gs. NPC and PPC photodetector are building blocks of photoelectric logic gate. Reconfigurability makes it a great advantage for our device to be applied to photoelectric logic gate.

To investigate the origin of the unconventional NPC phenomenon, control groups were prepared and tested under the same experimental condition: 1) WSe₂ with graphene electrodes and mica encapsulation (two mica flakes with thickness of 40–60 nm were placed on top and bottom of WSe₂), 2) Bare WSe₂ with graphene electrodes (without h-BN encapsulation), 3) Bare WSe₂ with Pt electrodes (without h-BN encapsulation), 4) WSe₂ with Pt electrodes and h-BN encapsulation. The high-temperature measurements were done in a short time to minimize the oxidation of WSe₂. V_gs=0 V, or V_gs was not applied. The first 3 types of devices all demonstrated PPC from room temperature to 400 ℃ (Supplementary Fig. S13-15). Only the last type of devices demonstrated NPC at 400 ℃ (Fig. S16), indicating that h-BN is responsible for the NPC phenomenon.

At relatively low temperature, photoexcited electron-hole pairs are restricted in WSe₂ channel due to the excellent insulation of h-BN, and can be extracted by applying V_ds. As such, the current increases after photoexcitation (process 1 in Fig. 4f). The h-BN gate dielectric leakage current increased from 10^− 13 A to 10^− 10 A as temperature varied from 20 ℃ to 500 ℃ (Fig. 3d), implying that the chance for charge carriers to enter h-BN layer significantly increases at elevated temperature. Photogenerated holes trapped by the defect states inside h-BN act as an equivalent positive gate voltage (process 2 in Fig. 4f), leading to a left shift of I_ds-V_gs transfer curve as shown in Fig. 4c.

To figure out the temperature that process 2 starts to dominate and the impact of light wavelength, we utilized 385 nm wavelength purple light (18 W/m²), 440 nm wavelength blue light (480 W/m²), and 532 nm wavelength green light (440 W/m²) to illuminate the WSe₂ photodetector respectively (V_gs=0 V) under different temperatures. The photodetector exhibited PPC at low temperatures (Fig. 5a). For all the three wavelengths, the device switched to NPC as temperature reached ~ 275 ℃, indicating that NPC phenomenon happens in a wide wavelength range. It is worth mentioning that the NPC occurs due to the defect states inside h-BN, which means that different WSe₂ photodetectors may exhibit slightly different PPC/NPC transition temperatures. The absolute value of NPC responsivity was significantly larger than that of PPC at room temperature (under 385 nm illumination, the responsivity at 400 ℃ was ~ 2000-fold higher than that at room temperature), and shorter wavelengths usually resulted in higher responsivity at the same temperature. Our devices exhibited ultrahigh responsivity of 2.2×10⁶ A/W at 400 ℃ under 0.2 W/m² 365nm illumination (Fig. 5b and Fig. S17). At 500 ℃, we also obtained an impressive photoresponsivity of 1.1×10⁶ A/W. The photoresponsivity is not only significantly higher than that of state-of-the-art high-temperature photodetectors, but also higher than that of existing WSe₂ devices (Fig. 5d).

In situ high-temperature optical sensing under bending state. Flexible WSe₂ photodetector with a bending radius of 3 cm was attached on a ceramic heating rod with temperature of 450 ℃. We utilized 18 W/m² 385 nm wavelength light to illuminate the device (V_gs=0 V). Its optical sensing performance is coupled with the impact of strain (~ 0.17%) and high temperature. The dynamic resistance variation is shown in Fig. 5c. The resistance of the device increased rapidly from 1.5 MΩ to 2.5 MΩ within 0.3 s after illumination, indicating NPC phenomenon. As the light was turned off, it took approximately 13 s for the resistance to dropped back to the dark state value. This process was repeated for over 5 times and demonstrated good repeatability. Therefore, our flexible photodetectors can adapt to non-coplanar working conditions with excellent optical sensing performance which cannot be achieved by conventional rigid high-temperature photodetectors.

We developed WSe₂ photodetectors which can endure record high temperature of 700 ℃ in air and 1000 ℃ in vacuum. Our research greatly expands the working temperature range of 2D materials, allowing the excellent properties of 2D materials to be applied in high temperature environments. The device exhibited unconventional NPC phenomenon, and the photoresponsivity in NPC mode (2.2×10⁶ A/W) is ~ 10⁵-fold higher than that of existing high-temperature photodetectors, and also higher than that of existing WSe₂ photodetectors. This work bridges the technology gap between highly-sensitive photodetectors and high-temperature photodetectors. Current high-temperature photodetectors are rigid, which largely limits their applications. Our device is the first photodetector that is both highly flexible and can endure ultrahigh temperature. We realized in situ high-temperature optical sensing under bending state for the first time. Our approach opens up opportunities for novel 2D-material devices working in harsh environment, and may stimulate fundamental research of the fascinating properties and new phenomena of 2D materials at high temperature.

Device fabrication

A freshly cleaved mica substrate was firstly immersed in acetone, alcohol, and deionized water successively for 2 min ultrasonic cleaning in order to remove possible impurities. Then, bottom h-BN (40–60 nm), WSe₂ (10–20 nm), two graphene electrodes and top h-BN (40–60 nm) were mechanically exfoliated using scotch tape and transferred onto mica substrate by PDMS film. The transfer process was carried out using an accurate transfer platform (Metatest, E1-T). Finally, a platinum electrode of 30 nm thick as top gate was fabricated using photolithography, metal deposition, and a lift-off process. As-fabricated devices were annealed in N₂ atmosphere at 250 ℃ for 30 min to remove residuals between the layers.

Device characterizations

The thicknesses of the 2D materials and the surface roughness of graphene and Pt were characterized by AFM (Bruker, Dimension Icon) using ScanAsyst-air mode. The Raman spectra were derived by Raman spectrometer (HORIBA Jobin Yvon, LabRAM HR Evolution) with 532 nm laser (0.325 mW/cm²) and acquisition time of 120s. The cross-sectional images of van der Waals heterostructures and h-BN/Pt interface were characterized by high-resolution transmission electron microscopy (JEOL-2100 TEM) with acceleration voltage of 200 kv. The samples for HRTEM were prepared using Xenon focused ion beam (Helios G4). High-temperature photoluminescence (PL) spectrum was detected on a temperature-control stage (Inspec HCP621G+) by PL spectrometer (Andor KYMERA-328I-B1, grating 300 grooves/ mm), and a 532 nm continuous wave laser (commercial Coherent Genesis MX, intensity = 0.05 mW) was applied.

Device measurements

The transfer characteristics and I-t characteristics of the WSe₂ FETs were measured by semiconductor parameter analyzer (Agilent B1500A). The devices were fixed and tested on a temperature-control stage. For the purpose of investigating electrical characteristics of the devices under strain, we fixed it on the home-made matrix with different bending radius, and then tested it with semiconductor parameter analyzer. The dynamic resistance variation of the photodetectors was measurement by digital multimeter (Agilent 34470A). The devices were fixed and tested on a ceramic heating rod.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51775306) and Beijing Municipal Natural Science Foundation (4192027). The authors thank Prof. Rong Zhao for the discussion.

Author contributions

P. L. conceived the experiments. P. L., Y. Z., and Z. Z. fabricated the devices and performed the electrical measurements. P. L., Y. Z., and J. Y. performed the photoelectrical measurements. L. L. and G. H. performed the PL measurements. P. L. and Y. Z. analysed the results. P. L. and Y. Z. co-wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Peng Li.

Competing interests

The authors declare no competing financial interests.

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Table 1 is available in the Supplementary Files section

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Table1.pdf
Table 1
SupplementaryInformation.docx
Supplementary Information
700degreeCinAirExperiment.mp4
700 degree C in air experiment

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High-Temperature Flexible WSe2 Photodetectors with Ultrahigh Photoresponsivity

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