A Centimeter‐Scale Type‐II Weyl Semimetal for Flexible and Fast Ultra‐Broadband Photodetection from Ultraviolet to Sub‐Millimeter Wave Regime

Abstract Flexible photodetectors with ultra‐broadband sensitivities, fast response, and high responsivity are crucial for wearable applications. Recently, van der Waals (vdW) Weyl semimetals have gained much attention due to their unique electronic band structure, making them an ideal material platform for developing broadband photodetectors from ultraviolet (UV) to the terahertz (THz) regime. However, large‐area synthesis of vdW semimetals on a flexible substrate is still a challenge, limiting their application in flexible devices. In this study, centimeter‐scale type‐II vdW Weyl semimetal, Td‐MoTe2 films, are grown on a flexible mica substrate by molecular beam epitaxy. A self‐powered and flexible photodetector without an antenna demonstrated an outstanding ability to detect electromagnetic radiation from UV to sub‐millimeter (SMM) wave at room temperature, with a fast response time of ≈20 µs, a responsivity of 0.53 mA W−1 (at 2.52 THz), and a noise‐equivalent power (NEP) of 2.65 nW Hz−0.5 (at 2.52 THz). The flexible photodetectors are also used to image shielded items with high resolution at 2.52 THz. These results can pave the way for developing flexible and wearable optoelectronic devices using direct‐grown large‐area vdW semimetals.


Introduction
A flexible photodetector is a crucial component for the development of future optoelectronic devices that form the basis of handheld portable devices, bendable high-speed wireless communications, RF energy harvesting, and wearable electronics. [1,2] However, the challenge lies in finding a material system that enables room-temperature and flexible photodetection with a broadband DOI: 10.1002/advs.202205609 response covering UV to infrared (IR), especially in the THz and SMM regimes (0.3 THz to 10 THz). Such a system can boost the applications of flexible photodetectors in smart wearable sensors, which are at the heart of the Internet of Things. While traditional semiconductors and quantum structures (e.g., quantum wells and quantum dots) have been widely used in photodetection technology, they usually suffer from narrow detection bands limited by their finite bandgaps. [3,4] Additionally, semiconductors (such as HgCdTe and PbSe) used in mid-infrared (MIR) and THz detectors typically require cryogenic temperatures to suppress thermal noises. The THz detectors will be further integrated with antennae to enhance the coupling with THz radiation. This results in devices that have a large footprint and are complex. [5,6] Moreover, conventional semiconductors are typically solid and brittle, which makes them unsuitable for use in mechanically flexible devices. Due to the exotic optical and optoelectronic responses they grant, two-dimensional (2D) vdW crystals have garnered much attention in recent years as a new type of material that might revolutionize photodetection technology. [7][8][9][10][11] Among the various vdW crystals, Dirac/Weyl semimetals exhibiting linear band structures close to the Dirac/Weyl node, such as WTe 2 , [7] PtTe 2 , [8] PdTe 2 , [9] and NiTe 2 , [10] and others, have opened up a new landscape for exploring novel photodetection technologies. First, the gapless band structure can confer the semimetals with a broadband photosensitivity covering the UV to the far-IR spectral regime. Second, the linear band structure can give rise to massless, relativistic charge carriers with ultrahigh mobilities. Therefore, photodetectors with ultrafast responses and broad operation bandwidths can be envisioned. Last but not least, the atomic thickness of the vdW semimetals can enable tunable optical and electronic properties via electrostatic gating and offer excellent mechanical flexibility, which benefits various flexible and tunable photodetection applications. Accordingly, recent years have witnessed a burgeoning interest in exploring photodetectors using various vdW semimetals, which are superior to conventional photodetectors in terms of room-temperature broadband photoresponse from UV to THz regime, [8][9][10] ultrafast response time, [8][9][10][11] as well as polarization-resolved response. [9,10,12,13] The vdW semimetals can generally be categorized into two types, namely type-I and type-II semimetals. Type-II semimetals have strongly tilted Dirac/Weyl cones with open Fermi surfaces, resulting in a large density of states near the Dirac/Weyl node, in contrast to type-I semimetals where the Fermi surfaces are closed. [14,15] This property significantly enhances the electromagnetic absorption and the corresponding response of photodetectors, particularly in the long-wavelength regime. Moreover, the heavily tilted linear electronic dispersion of type-II semimetals allows for the development of ultrafast photodetectors that operate in an unbiased self-powered mode. Despite the development of various broadband photodetectors using type-II vdW semimetals, [7,12,13,16,17] they are mainly reliant on exfoliated vdW flakes with finite lateral size. This limitation severely restricts their application in high-throughput and large-area photodetection, such as the development of flexible and wearable photodetectors. To date, direct synthesis of high-quality and large-area (with a lateral size over a centimeter scale) type-II semimetals remains challenging.
Here, we report on the successful growth of large-scale T d -MoTe 2 vdW thin films, a type-II semimetal, using the molecularbeam epitaxy (MBE) method, with lateral sizes of up to 2.0 × 2.0 cm 2 . Importantly, the films were grown on a flexible mica substrate, enabling the development of a flexible photodetector with ultra-broadband sensitivity ranging from UV to SMM spectral ranges (325 nm to 566.0 μm). Due to the strong coupling of T d -MoTe 2 with long wave irradiation, the photodetector does not require an antenna for detecting THz and SMM waves, thus simplifying the device architecture. With its tilted energy band, the photodetector can operate in a self-driven mode, with a fast response time of ≈20 μs, a responsivity of 0.53 mA W −1 (at 2.52 THz), and a NEP of 2.65 nW Hz −0.5 (at 2.52 THz). Moreover, the flexible photodetector can achieve high-resolution THz-imaging, enabling the detection of shielded items. These results not only reveal the potential application of the semimetal T d -MoTe 2 in future high-performance broadband photodetectors but also pave the way for developing flexible and wearable optoelectronic devices using directly grown large-area vdW crystals.

Results and Discussion
The T d -phase MoTe 2 is a type-II vdW Weyl semimetal with an orthorhombic lattice that breaks inversion symmetry, and it has a tilted three-dimensional Dirac cone band structure (as shown in Figure 1a). [18][19][20][21][22] In our study, we grew epitaxial films of T d -MoTe 2 on high-quality hexagonal mica substrate measuring 2.0 × 2.0 cm 2 using MBE, and the high film uniformity is demonstrated in the optical image presented in Figure 1b. During the growth process, we used reflection high-energy electron diffraction (RHEED) to monitor the film thickness, crystal quality, and lattice relationship between the film and substrate in situ. After 30 min of growth, we obtained a MoTe 2 film with a thickness of ≈4 nm (equivalent to 6 monolayers). Figure 1c illustrates the RHEED patterns of mica along the <110> and <210> azimuths, which confirm the hexagonal symmetry of the mica surface. The RHEED patterns of the MoTe 2 film along <100> and <010> azimuths, shown in Figure 1c, indicate an orthogonal symmetry of the epitaxial film, excluding the formation of the 2H phase. The in-plane lattice constants of MoTe 2 were calculated to be a = 3.73 Å and b = 6.73 Å. Additionally, the HRTEM image and regular spot configuration in fast Fourier transform, as shown in Figure 1d, confirm the orthogonal symmetry of the MoTe 2 surface. To differentiate between the 1T′-phase (monoclinic P21/m) and T d -phase (orthorhombic Pmn21), Raman measurements were conducted at room temperature. The spectra between 150 and 300 cm −1 reveal two dominant modes at 162 cm −1 and 261 cm −1 (blue lines), which are significantly different from those of the 2H phase, indicating the presence of T d phase. [25][26][27][28][29] Moreover, the shear mode at 13 cm −1 and the out-of-plane vibration at 136 cm −1 (red lines) further confirm the T d phase of the epitaxial MoTe 2 on mica. [30][31][32] Raman spectroscopy measurements taken from different locations in the same film confirm that the samples are T d phases of MoTe 2 grown on mica substrates via MBE and illustrate great film uniformity in centimeter scale ( Figure 1e). To further demonstrate the repeatability of the experiment, Raman measurements were carried out for several samples grown in the same way but at different source powers ( Figure S1 in Supporting Information), showing the repeatability of T d -MoTe 2 samples.
The Metal-Semimetal-Metal structure of the T d -MoTe 2 /mica photodetector was fabricated using MBE and masking technology to characterize its spectral response. The schematic and micrographs of the device are shown in Figure 2a,b, respectively, indicating that the device dimensions are 150 μm in length and 35 μm in width. The detector's performance was demonstrated at room temperature using radiation with wavelengths of 325 nm, 532 nm, 633 nm, 785 nm, 1064 nm, 4.6 μm, 8 μm, 9 μm, 10 μm, 4.24 THz (70.8 μm), 3.11 THz (96.5 μm), 2.52 THz (119.0 μm), 1.84 THz (163.0 μm), and 0.53 THz (566.0 μm) and active powers ranging from 0.008 mW to 5.7 mW. The active power was calculated from the photoactive area of the detector, which is shown in Figure 2b and has an area of 150 × 35 μm 2 . Here, the responsivities under irradiation from the UV to THz and SMM regimes are calculated by the formula: R = , where I light is the current under irradiation, I dark is the dark current, and P light is active power of lasers. Theoretically, the photoresponse experiment on T d -MoTe 2 devices should be extended to include wavelengths with lower photon energies, due to the gapless linear dispersion in the type-II vdW Weyl semimetal band structure. [18][19][20]33,34] Therefore, different devices were fabricated at various locations on the same film for terahertz detection, as shown in Figure 2c  To reveal the broadband photoresponse mechanism of T d -MoTe 2 /mica photodetector, the photocurrent distribution in the device area (≈150 × 35 μm 2 ), including the channel and the overlap region of metal-MoTe 2 (Figure 3a), was imaged via scanning photocurrent microscopy at 10.0 μm. The light source used was a laser with a power of 9 mW, focused on a diameter of 21 μm. In situ scanning photocurrent images were captured under zero and 0.1 V bias and are shown in Figure 3b,c, respectively.
Under irradiation without any bias voltage, a positive photoresponse was observed at a distance of 10 μm away from the electrodes, while the opposite photocurrents were generated at the interface of MoTe 2 -metal junctions, as depicted in Figure 3b. In addition, nonzero photocurrents were also observed in the biasdependent photocurrent experiment at 10.0 μm and 2.52 THz (Figure 3d). This phenomenon can be attributed to the photothermoelectric effect (PTE), which is caused by the electron temperature gradient resulting from differential thermoelectric power and optical excitation. [35][36][37] In gapless semimetals, such as T d -MoTe 2 , the Shockley-Ramo mechanism leads to the photocurrent generated by PTE being independent of the proximity of the metal-MoTe 2 junction. [38] Furthermore, in type-II vdW Weyl semimetal T d -MoTe 2 , the strongly tilted Weyl cone induces asymmetric excitation of carriers under irradiation, resulting in the self-powered photocurrent. [39] Therefore, both PTE and the asymmetric excitation of carriers by the tilted Weyl cone consistently generate the photocurrent under 0 V bias.
When subjected to a 0.1 V bias, the photocurrent exhibited linear amplification, as depicted in Figure 3d. Based on the distribution of the positive photocurrent, it was observed that the photocurrent in the center of the channel was significantly higher than in the contact region. The detector's performance was assessed using current noise and noise equivalent power. The total noise current (i N ) was composed of 1/f noise and white noise (i w ), with 1/f noise dominating the noise current at 1 Hz, as shown in   0.1 V bias to demonstrate the speed of photodetection. Figure 4b shows that at a modulation frequency of only 5 Hz, the responsivity was reduced by 34% and 32% at both THz and MIR regimes compared to continuous irradiation (Figure 2e), respectively. In contrast, the remaining responsivity decreased by less than 10% (4.7% at 10.0 μm, 9.2% at 2.52 THz) as the modulation frequency was increased to 3000 Hz. This illustrates that the photocurrent at THz and MIR under 0.1 V bias contains two main mechanisms with different response speeds. Additionally, the accurate response speed of 26.6/20.8 μs (rise/fall time) at 2.52 THz was determined from a single magnified photoresponse curve, which was obtained directly from the high-speed sampling oscilloscope (Figure 4c and Figure S3 in Supporting Information). First, the extremely slow photocurrent response generated by the photobolometric effect (PBE) [35,40,41] is caused by the uniform heating effect resulting from photon absorption in the MIR to THz range, leading to the generation of hot carriers and alteration of MoTe 2 conductivity. This in turn generates photocurrent when a bias voltage is applied. Typically, the slow photocurrent response can be filtered out at high chopping frequencies and does not affect the actual THz imaging speed. On the other hand, the fast pho-toresponse in our detector (faster than 333 μs) is generated by the photoelectric effect of gapless dispersion in three-dimensional momentum space. This effect enables low-energy photons to excite carrier transitions and is critical to the performance of detectors based on other semimetals. [35] Additionally, in the absence of bias, the electron−hole pairs compound rapidly, resulting in increased responsivity by breaking symmetry. Overall, both PBE and photoelectric effect based on type-II Weyl cone consistently generate photocurrent under bias voltage.
To investigate the behavior of the T d -MoTe 2 /mica detector under different laser powers at THz and MIR, we measured the power dependence of the photocurrent under 0.1 V bias, as shown in Figure 5a,b. The photocurrent exhibited linear dependence on the light power in both the MIR and THz bands, which confirms the mechanism as the free electron absorption of the Dirac fermions. [8,42,43] Moreover, the excellent linear property facilitates the fabrication of optical imaging devices. To gain further insight into the performance of T d -MoTe 2 /mica, we measured the polarization-dependent photocurrent response under 0.1 V bias at 10.0 μm and 2.52 THz. The polarization states of the THz and MIR sources were fixed using a linear polarizer, and the   device was rotated with respect to the allowing direction of the polarizer during the measurements. As illustrated in Figure 5c,d, the photodetector exhibited an anisotropic response to polarized irradiation at 10.0 μm, with the maximum photocurrent response along the a-axis of the T d -MoTe 2 crystal. These results are consistent with previous studies on type-II semimetals, [9,10,12,13] showing that the strongly tilted gapless band structure can lead to anisotropy of electromagnetic waves in the type-II vdW Weyl semimetal T d -MoTe 2 , thus causing the anisotropic photocurrent. Moreover, Figure 2d illustrates that the photocurrent is higher in the THz band than in the MIR band. This may be due to the Fermi level approaching the Weyl point (only 5-6 meV), [18,20] indicating that carriers are excited near the Fermi level when the photon frequency is close to ≈3 THz. This increases the contribution of photon absorption to the photocurrent. Additionally, under the same power density of irradiation, the increasing number of low-energy photons leads to an increase in the photoexcited carriers at THz, similar to Weyl semimetal TaAs [12] and Dirac semimetal PtTe 2 [13] at higher frequencies of irradiation. When photon energy is further reduced, the contribution of the photocurrent will transform from the interband transition to the in-traband transition. [44−48] Due to the Pauli blocking, the photocurrent will be suppressed and constrained.
To demonstrate its flexibility, the detector was bent to different bending radii to characterize the photocurrent at 2.52 THz (Figure 6a). As shown in Figure 6b, the T d -MoTe 2 /mica device exhibited stable THz detection performance when bent to a curvature radius of 8.8 mm, indicating that the device is suitable for different parts of the human body's surface structures. The choice of mica and MBE technology consistently realized flexible THz sensors. Next, THz imaging were carried out by raster scanning the detector upon 2.52 THz illumination to verify the practical application of the T d -MoTe 2 /mica detector (Figure 6c). A 2D scanning with a speed of 1 mm s −1 was performed to image shielded items (ferrous letters and a silicon wafer stick with paper) under 0.1 V bias. As shown in Figure 6d (bent situation) and Figure S7, Supporting Information, (flat situation), images with 107 × 107 pixels were obtained, which can identify the shape features of ferrum and silicon, and distinguish ferrous and silicon materials by their color differences. The tape signal can be even identified on the back of silicon. From the comparison shown in Table 1, it can be seen that although our device's various www.advancedsciencenews.com www.advancedscience.com performance indicators are not significantly better than other topological semimetal-based devices, its flexible characteristics and the fact that the material can be prepared in large areas give the device a significant advantage in wearable optoelectronic applications.

Conclusions
In summary, we have developed a MBE-based method for growing centimeter-scale type-II vdW Weyl semimetal T d -MoTe 2 on a flexible mica substrate. On the basis of such large-area semimetal, we developed a flexible photodetector with an ultrabroadband photocurrent response from the UV to SMM regime (325 nm -566.0 μm), either with bias voltage or operated in a selfpowered mode. The photodetection mechanism of the device can be ascribed to a combination of PTE, PBE, and photoelectric effect near the type-II Weyl cone. We further employed such device for THz imaging of shielded items with a high resolution at 2.52 THz. We believe that these results will be important for developing of flexible and wearable optoelectronic devices in the MIR and THz spectral regions.

Experimental Section
Growth Details and Characterization: T d -MoTe 2 films were deposited onto mica substrate using MBE system from OMICRON. The substrate was maintained at a temperature of 255°C during deposition. High purity Te (99.99999%) was evaporated at 330°C from an exuding cell, while Mo rod (99.95%) was evaporated using an e-beam evaporator, resulting in a growth rate of ≈5 min ML −1 (with a thickness of 6 ML for the obtained large-area T d -MoTe 2 film). RHEED was employed to ensure a high surface quality, and further characterization was performed using a highresolution transmission electron microscope (HRTED: Titan G2 60-300). For the HRTEM experiment, samples were grown on a few-layer graphene transferred onto a Mo net prior to film growth. Raman measurements were carried out using a commercial Raman equipment (Renishaw inVia Reflex) with 532 nm (0-100 cm −1 ) and 514.5 nm (100-300 cm −1 ) lasers.
Device Fabrication: To prevent oxidation of the T d -MoTe 2 film, an Al thin film was deposited on top of it and then oxidized in ambient atmosphere to serve as a capping layer. Au metal electrodes were then fabricated using MBE and masking technology to create channels with a length of 25 μm and a width of 35 μm.
Photoresponse Measurements: All measurements were conducted at room temperature. The electrical and photocurrent characteristics were measured using a Keithley 2636 B instrument. The light sources used in the study comprised semiconductor lasers (325 nm, 532 nm, 633 nm, 785 nm, and 1064 nm), quantum cascade lasers (QD4500CM1, Thorlabs: 4.6 μm; MIRCat S/N10016, Daylight: 8.0 to 11.0 μm), and THz lasers (FIRL 100, Edinburgh Instruments: 4.24 THz, 3.11 THz, 2.52 THz, 1.84 THz, and 0.53 THz). To control the chopping frequency in the frequency-dependent photocurrent response measurements, a lock-in amplifier (Stanford SR830) was used. A linear polarizer was employed for fixing the polarization state of the incident electromagnetic waves. Response time of the photodetector was obtained directly from a high-speed sampling oscilloscope (Tektronix DPO 7354C).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.