**5. 2D NTMDs for photodetection**

So far, various NTMDs based photodetectors with diverse constructions and high-performance have been reported [53]. **Table 1** summarizes their characteristic parameters. The strong optical absorption capability and large carrier mobility of NTMDs provide high responsivity (R) and detectivity (D\*) for these photodetectors, while the narrow bandgaps of atomic layered PtS2, PtSe2 and PdSe2 make them inherently suitable for NIR detection. For multi-layer NTMDs (over 5 L for PtSe2), which can be regarded as semimetal materials, they can be combined with other semiconductor materials and construct Schottky heterostructures. By choosing a suitable semiconductor functional layer with a particular bandgap (such as n-Si, III − V, 2D perovskite, 2D MoS2, and so on), the photodetector can work efficiently at a specifical wavelength. In addition, owing to the majority-carrier-dominant current-flow mechanism, photodetectors based on NTMDs heterostructures have advantages in high-speed applications. Combined with other electronic

**137**

**Device structure**

Few-layer PtSe2

Bilayer PtSe2 Few-layer PtS2

Few-layer PdSe2

PtSe2/Si PtSe2/n-Si

PtSe2/Ge PtSe GaAs 2 PtSe2/GaN PtSe2/CdTe PtSe2/FA0.85Cs0.15PbI3

PtS2/PtSe2 PdSe2/n-Si G/PdSe2/Ge PdSe2/FA0.85Cs0.15PbI3

PdSe2/MoS2

**Table 1.**

*Summary of characteristic parameters for NTMDs based photodetectors.*

**Material grown methods**

CVT CVT + ME

CVT

ME TAC TAC CVD CVD CVD CVD CVD CVD CVD CVD CVD CVT

**R (AW−1)**

0.01

4.5 1.56 × 103

708

0.52 0.49

0.602 0.262 0.193 0.51 0.118 0.361

0.3

0.69 0.313 42.1

—

*\*The detectivity D of a photodetector is a figure of merit, defined as the inverse of the noise-equivalent power (NEP). The larger the detectivity of a photodetector, the more it is suitable for detecting week* 

*signals which compete with the detector noise. But the specific detectivity D\* is the detectivity normalized to a unit detector area and detection bandwidth; one can calculate it by multiplying the detectivity* 

*with the square root of the product of detector area (in square centimeters) and the detector bandwidth (in Hz). That term is useful for comparing the performance of different detector technologies.*

3.5/4 μs

∼ 1013 8.21 × 109

6.4/92.5 μs

1.73 × 1013

—

66/75 ms

—

∼ 1013

78/60 ns

2.9 × 1012

8.1/43 μs

4.2 × 1011

45/102 μ

3.8 × 1014

5.5/6.5 μs

∼ 102

7.42/16.71 μs

—

—

6.31 × 1011

55.3/170.5 μs

3.26 × 1013

—

460/460 ms

2.9 × 1011 1.31 × 109

1.1/1.2 ms

—

—

7 × 108

τ**r/**τ**f**

**D\* (Jones)**

**Measurement conditions**

λ = 500 nm, Vg = −80 V

λ = 10 μm, Vb = 0.1 V

λ = 500 nm, Vb = 0.1 V

λ = 1064 nm, Vg = 30 V

λ = 808 nm, Vb = 0 V

λ = 808 nm, Vb = −2 V

λ = 1550 nm, Vb = 0 V

λ = 808 nm, Vb = 0 V

λ = 265 nm, Vb = 0 V

λ = 780 nm, Vb = 0 V

λ = 808 nm, Vb = 0 V

λ = 532 nm, Vb = 0 V

λ = 780 nm, Vb = 0 V

λ = 265 nm, Vb = 0 V

λ = 265 nm, Vb = 0 V

λ = 10.6 μm, Vb = 1 V

**Spectral range**

Visible-NIR Visible-MIR

Visible Visible-MIR Visible-NIR

UV–NIR Visible-NIR

DUV–NIR

DUV DUV–NIR

UV–NIR Visible-NIR

UV–NIR DUV-MIR DUV-NIR Visible-MIR

[65]

[64]

[63]

[62]

[47]

[61]

[60]

[59]

[58]

[56]

[55]

[54]

[39]

[23]

**Ref.**

*Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector*

[57]

*DOI: http://dx.doi.org/10.5772/intechopen.95883*

[48]


**Table 1.**

*Summary of characteristic parameters for NTMDs based photodetectors.*

#### *Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector DOI: http://dx.doi.org/10.5772/intechopen.95883*

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

(> a few cm<sup>2</sup>

side. Then the direct selenization of the Pt film happens under high temperature, low pressure and argon gas protection. In 2015, Wang et al. firstly synthesized monolayer PtSe2 nanosheets [40]. Then Han et al. obtained large area PtSe2 film

of as-grown 2D PtSe2 polycrystal films from 0.75 to 10 nm (corresponding to the layer numbers from 1 L to ~15 L). In 2018, Yuan et al. successfully fabricated PtSe2-PtS2 heterostructure film with wafer-scale and successfully achieved the wafer-scale photodetector application [47]. Besides, CVD method can also synthesize high-quality 2D NTMDs nanocrystals. **Figure 3c** exhibits a schematic of growing 2D nanosheets and through the method, Ma et al. successfully fabricated 2D PtTe2 nanoplates with tunable thickness and a large lateral size up to 80 μm [43]. From **Figure 3f**, the high-angle annular darkfield scanning-TEM (HAADF-STEM) image as well as the EDS mapping analysis shows the well-faceted triangular geometry and the uniformly spatial distribution of Pt and Te elements. The Raman spectrum and High-resolution TEM (HRTEM) furtherly show the high quality of nanosheets and the 6-fold symmetry SEAD pattern shows the hexagonal crystal structure. Type-II Dirac fermions are observed in the highquality nanocrystal platform. Another advantage of the grown method is that 2D materials can be grown on arbitrary substrates, because both the pre-deposition and post-selenization process do not have strict requirements to the substrate. Till now, 2D NTMDs have been fabricated on different substrates including Si, SiO2,

Sapphire, gallium nitride (GaN), fused quartz, and flexible polyimide.

**5. 2D NTMDs for photodetection**

There are some other synthesize ways for atomic TMDs. Mechanical exfoliation (ME) is one of the most extensively adopted approaches for 2D nanoflakes from their bulk counterparts [13]. Therefore, the as-prepared 2D flakes can maintain the intrinsic structure. Nowadays, most of mechanically exfoliated NTMDs thin flakes are from bulk crystals grown by CVT [48] and self-flux method [26, 49]. These typical nanosheets show the extraordinary electronic properties, but their small lateral size and uncontrollability during the fabrication process limit their application potential in practical devices. Molecular beam epitaxy (MBE) has also been applied for 2D NTMDs, including PtSe2 [50], PdTe2 [51] and PdSe2 [52], which shows the merits of large-size monocrystalline, and controllable thickness on different substrates. For example, the high-quality PtSe2 atomic film was epitaxial grown on bi-layer graphene/6H-SiC substrate through MBE method [50]. The as-grown film had controllable thickness from single-layer to over 22 layers, which shows extraordinary thickness-dependent electronic properties and tunable bandgaps.

So far, various NTMDs based photodetectors with diverse constructions and high-performance have been reported [53]. **Table 1** summarizes their characteristic parameters. The strong optical absorption capability and large carrier mobility of NTMDs provide high responsivity (R) and detectivity (D\*) for these photodetectors, while the narrow bandgaps of atomic layered PtS2, PtSe2 and PdSe2 make them inherently suitable for NIR detection. For multi-layer NTMDs (over 5 L for PtSe2), which can be regarded as semimetal materials, they can be combined with other semiconductor materials and construct Schottky heterostructures. By choosing a suitable semiconductor functional layer with a particular bandgap (such as n-Si, III − V, 2D perovskite, 2D MoS2, and so on), the photodetector can work efficiently at a specifical wavelength. In addition, owing to the majority-carrier-dominant current-flow mechanism, photodetectors based on NTMDs heterostructures have advantages in high-speed applications. Combined with other electronic

) with controllable thickness [42]. **Figure 3e** shows the photographs

**136**

characteristics, different photodetectors with wide-spectral, fast-speed, self-powered and anisotropic have been realized. NTMDs based wafer-scalable and flexible photodetectors arrays could be the future development trend. We will comprehensively discuss them in this section.

#### **5.1 2D NTMDs photodetectors**

Due to the great electronic transport and optical properties of NTMDs, various of NTMDs based phototransistors have been studied [23, 39, 48, 54]. Here we illustrate a typical PtS2 phototransistor as an instance [48]. The device schematic is as shown in **Figure 4a**, in which few-layer PtS2 as the channel material on h-BN substrate. The device shows a high field-effect mobility of ~13 cm2 V−1S−1 and the high on/off ratio of 105 . Then the photo-response ability under light illumination at visible wavelength (500 nm) is studied. Both photogenerated conductive and photo-gating effects are observed in the device. **Figure 4b** is the 3D diagram which shows the combined photocurrent with incident light intensity and gate voltage (Vg). By calculation, the detectivity and responsivity are obtained with the function of Vg (**Figure 4c**). when Vg is zero, the responsivity is highly at 1560 AW−1, which shows 106 times higher than that of graphene and 103 times higher than that of BP detectors (~0.5 and 657 mAW−1, respectively). Similarly, the detectivity (D\*), as the inverse of the noise-equivalent power and the key parameter related to the signalto-noise rate of the device, reaches 2.9 × 1011 Jones, which is also higher than that of other 2D-based devices (**Figure 4c**). The photo-gain is about 2 × 106 at 30 V of Vg, which could be the highest gain in 2D-based photodetectors. The few-Layered PtS2 phototransistor shows that NTMDs is outstanding candidate in photodetection area at visible wavelength range.

Mid-IR optoelectronics is fantastic and important because there is an optical transparent window at Mid-IR in the atmosphere. However, in traditional TMDs

#### **Figure 4.**

*2D NTMDs phototransistors. (a)-(c) PtS2 on h-BN for photodetection. (a) Schematic of the device structure. (b) 3D view of photocurrent mapping. (c) the responsivity and detectivity as a function of vg measured at Vds = 0.1 V. (a)-(c) are reproduced with permission [48]. (d)-(f) bilayer PtSe2 for ultrawide spectra photodetection. (d) and (e) time-resolved photo-response curve at the wavelength of 0.63, 1.47 and 10 μm. (d)-(e) are reproduced with permission [39]. (f) Polarized plot diagram which shows the photocurrent of the device as a function of linear polarization rotation. The gate bias is 50 V and the wavelength is 532 nm, reproduced with permission [54].*

**139**

*Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector*

based photodetectors, it is very difficult to realize the effective detection at Mid-IR. NTMDs can overcome the difficulty due to the narrow bandgap. Yu et al. fabricated a bi-layer PtSe2 based phototransistor, which can realize wide-spectral and sensitive detection from 632 nm to 10 μm [39]. As shown in **Figure 4d** and **e**, the time-resolved photo-response results are obtained at 632 nm, 1.47 μm and 10 μm, with photoresponsivity of 6.25 AW−1, 5.5 AW−1 and 4.5 AW−1, respectively. The achieved photocurrent responsivity at 10 μm is 3 orders of magnitude higher than that of graphene and comparable to commercial mid-IR detectors. The rise and fall time are also better than other TMDs based photodetectors owing to the high mobility of PtSe2. Overall, bilayer PtSe2 shows promising potential in mid-IR optoelectronic applications.

For anisotropic detector applications, Liang et al. adopted PdSe2 as the photosensitive material [54]. The photodetector shows effective photo-response covering from 532 nm to 4.05 μm. The responsivity is 708 AW−1, which is five orders larger than graphene and two orders larger than commercial InGaAs near-IR photodetectors. Furthermore, with the unique pentagonal structure of PdSe2, the detector shows anisotropic photo-response for the linear-polarized light with varying polarization angle. In **Figure 4f**, when the increase of rotation angle with the step of 15° from 0° to 360°, the photocurrent clearly shows periodical variation and reaches the maximum value at 120° and 300°, which is coincident with the angle-resolved polarized Raman response results, furtherly showing the lattice effects. The anisotropic detectors as linear dichroism media have potential in optical communication

Overall, with the first realization of PtSe2 photodetectors in 2016 [23], Various of NTMDs and their photodetection abilities are studied, which show great performance. Till now, NTMDs based photodetectors exhibit higher responsivity and photo-gain than that of graphene, conventional TMDs and other 2D photodetectors. The work wavelength has been extended to 10 μm and the anisotropic detection has also been realized. With the development of NTMDs synthesis technique, the optimization of device structure, and the study of NTMDs photo-current mechanism, the narrow bandgap material will be the excellent candidate in the field

NTMDs with widely tunable energy gaps and high carrier mobility have broad prospects in developing high-performance photodetectors. However, the ultrathin thickness nature makes 2D NTMDs relatively low light absorption. Constructing NTMDs based heterostructures can not only enhance the light absorption, but accelerate the separation and transmission of carriers, and invent the high-speed photodetectors. Therefore, different NTMDs heterostructures have been studied for

Due to the atomic thickness, NTMDs is very convenient to from heterostructures with conventional semiconductors such as Si [55], n-Si [56, 62], Ge [57, 63], GaAs [58], GaN [59] and CdTe [60]. Few-layer PtSe2 is semimetal state. By choosing p-doped bulk semiconductors with appropriate work function and bandgaps and contacting them with Few-layer PtSe2 layer, the Schottky junction will be formed. The detection wavelength is determined by the bulk semiconductor. Zeng et al. fabricated the PtSe2-GaAs vertical heterostructure detector [58]. The device schematic and the photocurrent generation mechanism are depicted in **Figure 5a**. Under the light illumination, the electron–hole pairs forms at the interface of the heterojunction, then separates with the function of in-built electric field. The photocurrent generates and gathered by two electrodes. The device shows the broadband work wavelength from200 to 1200 nm and a large photo-response at visible wavelength

fast, broadband, self-powered and polarization-sensitive photodetectors.

*DOI: http://dx.doi.org/10.5772/intechopen.95883*

and structural chemistry analysis.

**5.2 2D NTMDs heterostructures for photodetection**

of photodetection.

#### *Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector DOI: http://dx.doi.org/10.5772/intechopen.95883*

based photodetectors, it is very difficult to realize the effective detection at Mid-IR. NTMDs can overcome the difficulty due to the narrow bandgap. Yu et al. fabricated a bi-layer PtSe2 based phototransistor, which can realize wide-spectral and sensitive detection from 632 nm to 10 μm [39]. As shown in **Figure 4d** and **e**, the time-resolved photo-response results are obtained at 632 nm, 1.47 μm and 10 μm, with photoresponsivity of 6.25 AW−1, 5.5 AW−1 and 4.5 AW−1, respectively. The achieved photocurrent responsivity at 10 μm is 3 orders of magnitude higher than that of graphene and comparable to commercial mid-IR detectors. The rise and fall time are also better than other TMDs based photodetectors owing to the high mobility of PtSe2. Overall, bilayer PtSe2 shows promising potential in mid-IR optoelectronic applications.

For anisotropic detector applications, Liang et al. adopted PdSe2 as the photosensitive material [54]. The photodetector shows effective photo-response covering from 532 nm to 4.05 μm. The responsivity is 708 AW−1, which is five orders larger than graphene and two orders larger than commercial InGaAs near-IR photodetectors. Furthermore, with the unique pentagonal structure of PdSe2, the detector shows anisotropic photo-response for the linear-polarized light with varying polarization angle. In **Figure 4f**, when the increase of rotation angle with the step of 15° from 0° to 360°, the photocurrent clearly shows periodical variation and reaches the maximum value at 120° and 300°, which is coincident with the angle-resolved polarized Raman response results, furtherly showing the lattice effects. The anisotropic detectors as linear dichroism media have potential in optical communication and structural chemistry analysis.

Overall, with the first realization of PtSe2 photodetectors in 2016 [23], Various of NTMDs and their photodetection abilities are studied, which show great performance. Till now, NTMDs based photodetectors exhibit higher responsivity and photo-gain than that of graphene, conventional TMDs and other 2D photodetectors. The work wavelength has been extended to 10 μm and the anisotropic detection has also been realized. With the development of NTMDs synthesis technique, the optimization of device structure, and the study of NTMDs photo-current mechanism, the narrow bandgap material will be the excellent candidate in the field of photodetection.

### **5.2 2D NTMDs heterostructures for photodetection**

NTMDs with widely tunable energy gaps and high carrier mobility have broad prospects in developing high-performance photodetectors. However, the ultrathin thickness nature makes 2D NTMDs relatively low light absorption. Constructing NTMDs based heterostructures can not only enhance the light absorption, but accelerate the separation and transmission of carriers, and invent the high-speed photodetectors. Therefore, different NTMDs heterostructures have been studied for fast, broadband, self-powered and polarization-sensitive photodetectors.

Due to the atomic thickness, NTMDs is very convenient to from heterostructures with conventional semiconductors such as Si [55], n-Si [56, 62], Ge [57, 63], GaAs [58], GaN [59] and CdTe [60]. Few-layer PtSe2 is semimetal state. By choosing p-doped bulk semiconductors with appropriate work function and bandgaps and contacting them with Few-layer PtSe2 layer, the Schottky junction will be formed. The detection wavelength is determined by the bulk semiconductor. Zeng et al. fabricated the PtSe2-GaAs vertical heterostructure detector [58]. The device schematic and the photocurrent generation mechanism are depicted in **Figure 5a**. Under the light illumination, the electron–hole pairs forms at the interface of the heterojunction, then separates with the function of in-built electric field. The photocurrent generates and gathered by two electrodes. The device shows the broadband work wavelength from200 to 1200 nm and a large photo-response at visible wavelength

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

substrate. The device shows a high field-effect mobility of ~13 cm2

times higher than that of graphene and 103

other 2D-based devices (**Figure 4c**). The photo-gain is about 2 × 106

sively discuss them in this section.

**5.1 2D NTMDs photodetectors**

high on/off ratio of 105

at visible wavelength range.

shows 106

characteristics, different photodetectors with wide-spectral, fast-speed, self-powered and anisotropic have been realized. NTMDs based wafer-scalable and flexible photodetectors arrays could be the future development trend. We will comprehen-

Due to the great electronic transport and optical properties of NTMDs, various of NTMDs based phototransistors have been studied [23, 39, 48, 54]. Here we illustrate a typical PtS2 phototransistor as an instance [48]. The device schematic is as shown in **Figure 4a**, in which few-layer PtS2 as the channel material on h-BN

at visible wavelength (500 nm) is studied. Both photogenerated conductive and photo-gating effects are observed in the device. **Figure 4b** is the 3D diagram which shows the combined photocurrent with incident light intensity and gate voltage (Vg). By calculation, the detectivity and responsivity are obtained with the function of Vg (**Figure 4c**). when Vg is zero, the responsivity is highly at 1560 AW−1, which

detectors (~0.5 and 657 mAW−1, respectively). Similarly, the detectivity (D\*), as the inverse of the noise-equivalent power and the key parameter related to the signalto-noise rate of the device, reaches 2.9 × 1011 Jones, which is also higher than that of

which could be the highest gain in 2D-based photodetectors. The few-Layered PtS2 phototransistor shows that NTMDs is outstanding candidate in photodetection area

Mid-IR optoelectronics is fantastic and important because there is an optical transparent window at Mid-IR in the atmosphere. However, in traditional TMDs

*2D NTMDs phototransistors. (a)-(c) PtS2 on h-BN for photodetection. (a) Schematic of the device structure. (b) 3D view of photocurrent mapping. (c) the responsivity and detectivity as a function of vg measured at Vds = 0.1 V. (a)-(c) are reproduced with permission [48]. (d)-(f) bilayer PtSe2 for ultrawide spectra photodetection. (d) and (e) time-resolved photo-response curve at the wavelength of 0.63, 1.47 and 10 μm. (d)-(e) are reproduced with permission [39]. (f) Polarized plot diagram which shows the photocurrent of the device as a function of linear polarization rotation. The gate bias is 50 V and the wavelength is 532 nm,* 

. Then the photo-response ability under light illumination

V−1S−1 and the

at 30 V of Vg,

times higher than that of BP

**138**

**Figure 4.**

*reproduced with permission [54].*

#### **Figure 5.**

*(a) Device schematic and the photocurrent generation mechanism of the PtSe2-GaAs photodetector. (b) Wavelength-dependent specific detectivity and responsivity of PtSe2-GaAs photodetector. (c) Fast photoresponse with the rise/fall time of 5.5/6.5 μs. (a)-(c) are reproduced with permission [58]. (d) Schematic diagram of graphene-PdSe2-Ge based photodetectors. (e) the long-term stability measurement results, where the device still remains stable with continuedly working over 5000 cycles. (f) Normalized photocurrent graphs which obtained by changing illumination polarization angle of linearly polarized light with wavelengths of 365, 650, 980 and 1550 nm. (d)-(f) are reproduced with permission [63].*

(**Figure 5b**). The responsivity and specific detectivity reach to 708 mAW−1 and 2.9 × 1012 Jones at 808 nm, respectively. Moreover, the device achieves the fast response speed, in which the rise and fall time are only 5.5 and 6.5 μs (**Figure 5c**). By choosing the semiconductor layer with relatively large energy gap, *e.g.,* GaN, the deep-UV photodetectors can be realized [59]. The self-powered PtSe2-GaN phototransistor has the responsivity of 193 mAW−1, an ultra-high specific detectivity of 3.8 × 1014 Jones and a fast response time of 45.2/102.3 μs at zero gate voltage. In particular, the calculated linear dynamic range (LDR) exceeds 155 dB, which much higher than all reported 2D based detectors and commercial photodetectors. For infra-wavelength application, Wang et al. designed a near-infrared light photovoltaic detector by constructing few-layer PtSe2-Ge heterostructure [57]. Since the device works at photovoltaic region, the self-start operation can be realized without any external power supply. The device also has high responsivity (602 mAW−1 at 1550 nm, closed to that of commercial device) and long environment stability. Then Wu and the co-workers designed the improved graphene-PdSe2-Ge heterostructure (**Figure 5d**) [63]. With graphene as a transport and protector layer, the device has great stability and can realize the imaging application. In particular, with continuedly working over 5000 cycles, the photo-response still remains stable, showing the practical application potential (**Figure 5e**). Due to the particularity of PdSe2, the device can achieve the dipole anisotropic operation (see **Figure 5f**).

Perovskite is also an emerging material with a large absorption coefficient, long diffusion length and low trapping density, which has aroused extensive research interest in optoelectronics. Zhang et al. reported a new type of detector based on few-layer PtSe2 and FACsPbI3 perovskite heterostructure [61]. The device has broad spectra response from 300 to 1200 nm, with the responsivity of 117.7 mAW−1, high Ilight/Idark ratio of 5.7 × 103 and considerable specific detectivity of 2.6 × 1012 Jones. Especially, due to the extraordinary electronic properties of PtSe2 and the perovskite and the well-designed built-in electric field at Schottky junction interface, the response time is only 78/60 ns, which is one of the fastest reported values

**141**

**Figure 6.**

*reproduced with permission [66].*

*Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector*

in mixed-dimensional 2D-3D van der Waals heterostructures. Zeng and the coworkers chose PtSe2 to construct heterojunction with FA1 − xCsxPbI3 perovskite film, which can realize the self-powered detection operation from 200 to 1550 nm [64]. The device demonstrates high responsivity, large on/off ratio, a good polarization

, and reliable imaging application at 808 nm. The heterostructure between NTMDs with other 2D materials is also fantastic. Here we use PdSe2-MoS2 heterostructure as an example [65]. Both of PdSe2 and MoS2 are multilayer flakes with thickness of ~10 nm. The ultra-thin device can not only a ultrawide spectra working range from 532 nm to 10.6 μm, but contributes an

Due to the industrial demand and the inherent advantages of 2D materials, the development trend of 2D optoelectronics is scalability and flexibility. Yuan et al. has realized the fabrication of wafer-scale PtS2- PtSe2 heterojunctions and devices [47]. They pre-deposited 0.8 nm Pt films as arrays of periodic square, then directly grew PtS2 and PtSe2 2D thin films by CVD method. **Figure 6a** is the photograph and **Figure 6b** shows the schematic illustration of one single device. The photodetector array can work from 405 nm to 2200 nm. The ultrathin device has a large external quantum efficiency (EQE) (1.2% at 1064 nm, 0.2% at 1550 nm, and 0.05% at 2200 nm). The response time is several milliseconds. If the quality of thin film is improved, the response time could be faster. The scalable devices can be adopted for

For the study of NTMDs flexible devices, Su and the co-workers did the pioneer

work [66]. PtSe2 thin films with 2.5 nm thickness (~3 L) on flexible polyimide substrate were directly grown by plasma-assisted selenization process, which show p-doped semiconductor behaviors and the average field effect mobility of

*(a)-(c) Wafer-scale NTMDs photodetection and imaging. (a) Photograph of PtS2- PtSe2 photodetectors array on a SiO2/Si wafer. (b) Schematic illustration of the photodetector device. (a)-(b) are reproduced with permission [47]. (c) high-resolution imaging by NTMDs based photodetectors, reproduced with permission [63]. (d)-(f) Flexible photodetection based on PtSe2. (d) illustration of the PtSe2 thin film based Flexible photodetector. (e)Time-resolved photo-response curve at the wavelength of 408, 515 and 640 nm. (f) Mechanic stability measurement, in which the photocurrent is recorded as a function of bending cycles. (e)-(f) are* 

*DOI: http://dx.doi.org/10.5772/intechopen.95883*

ultrahigh responsivity of 42.1 AW−1 at 10.6 μm.

**5.3 Perspective of 2D NTMDs in photodetectors**

high-resolution imaging, as shown in **Figure 6c**.

sensitivity over 104

*Two-Dimensional Group-10 Noble-Transition-Metal Dichalcogenides Photodetector DOI: http://dx.doi.org/10.5772/intechopen.95883*

in mixed-dimensional 2D-3D van der Waals heterostructures. Zeng and the coworkers chose PtSe2 to construct heterojunction with FA1 − xCsxPbI3 perovskite film, which can realize the self-powered detection operation from 200 to 1550 nm [64]. The device demonstrates high responsivity, large on/off ratio, a good polarization sensitivity over 104 , and reliable imaging application at 808 nm.

The heterostructure between NTMDs with other 2D materials is also fantastic. Here we use PdSe2-MoS2 heterostructure as an example [65]. Both of PdSe2 and MoS2 are multilayer flakes with thickness of ~10 nm. The ultra-thin device can not only a ultrawide spectra working range from 532 nm to 10.6 μm, but contributes an ultrahigh responsivity of 42.1 AW−1 at 10.6 μm.

#### **5.3 Perspective of 2D NTMDs in photodetectors**

Due to the industrial demand and the inherent advantages of 2D materials, the development trend of 2D optoelectronics is scalability and flexibility. Yuan et al. has realized the fabrication of wafer-scale PtS2- PtSe2 heterojunctions and devices [47]. They pre-deposited 0.8 nm Pt films as arrays of periodic square, then directly grew PtS2 and PtSe2 2D thin films by CVD method. **Figure 6a** is the photograph and **Figure 6b** shows the schematic illustration of one single device. The photodetector array can work from 405 nm to 2200 nm. The ultrathin device has a large external quantum efficiency (EQE) (1.2% at 1064 nm, 0.2% at 1550 nm, and 0.05% at 2200 nm). The response time is several milliseconds. If the quality of thin film is improved, the response time could be faster. The scalable devices can be adopted for high-resolution imaging, as shown in **Figure 6c**.

For the study of NTMDs flexible devices, Su and the co-workers did the pioneer work [66]. PtSe2 thin films with 2.5 nm thickness (~3 L) on flexible polyimide substrate were directly grown by plasma-assisted selenization process, which show p-doped semiconductor behaviors and the average field effect mobility of

#### **Figure 6.**

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

(**Figure 5b**). The responsivity and specific detectivity reach to 708 mAW−1 and 2.9 × 1012 Jones at 808 nm, respectively. Moreover, the device achieves the fast response speed, in which the rise and fall time are only 5.5 and 6.5 μs (**Figure 5c**). By choosing the semiconductor layer with relatively large energy gap, *e.g.,* GaN, the deep-UV photodetectors can be realized [59]. The self-powered PtSe2-GaN phototransistor has the responsivity of 193 mAW−1, an ultra-high specific detectivity of 3.8 × 1014 Jones and a fast response time of 45.2/102.3 μs at zero gate voltage. In particular, the calculated linear dynamic range (LDR) exceeds 155 dB, which much higher than all reported 2D based detectors and commercial photodetectors. For infra-wavelength application, Wang et al. designed a near-infrared light photovoltaic detector by constructing few-layer PtSe2-Ge heterostructure [57]. Since the device works at photovoltaic region, the self-start operation can be realized without any external power supply. The device also has high responsivity (602 mAW−1 at 1550 nm, closed to that of commercial device) and long environment stability. Then Wu and the co-workers designed the improved graphene-PdSe2-Ge heterostructure (**Figure 5d**) [63]. With graphene as a transport and protector layer, the device has great stability and can realize the imaging application. In particular, with continuedly working over 5000 cycles, the photo-response still remains stable, showing the practical application potential (**Figure 5e**). Due to the particularity of PdSe2, the

*650, 980 and 1550 nm. (d)-(f) are reproduced with permission [63].*

*(a) Device schematic and the photocurrent generation mechanism of the PtSe2-GaAs photodetector. (b) Wavelength-dependent specific detectivity and responsivity of PtSe2-GaAs photodetector. (c) Fast photoresponse with the rise/fall time of 5.5/6.5 μs. (a)-(c) are reproduced with permission [58]. (d) Schematic diagram of graphene-PdSe2-Ge based photodetectors. (e) the long-term stability measurement results, where the device still remains stable with continuedly working over 5000 cycles. (f) Normalized photocurrent graphs which obtained by changing illumination polarization angle of linearly polarized light with wavelengths of 365,* 

device can achieve the dipole anisotropic operation (see **Figure 5f**).

Perovskite is also an emerging material with a large absorption coefficient, long diffusion length and low trapping density, which has aroused extensive research interest in optoelectronics. Zhang et al. reported a new type of detector based on few-layer PtSe2 and FACsPbI3 perovskite heterostructure [61]. The device has broad spectra response from 300 to 1200 nm, with the responsivity of 117.7 mAW−1,

Jones. Especially, due to the extraordinary electronic properties of PtSe2 and the perovskite and the well-designed built-in electric field at Schottky junction interface, the response time is only 78/60 ns, which is one of the fastest reported values

and considerable specific detectivity of 2.6 × 1012

**140**

**Figure 5.**

high Ilight/Idark ratio of 5.7 × 103

*(a)-(c) Wafer-scale NTMDs photodetection and imaging. (a) Photograph of PtS2- PtSe2 photodetectors array on a SiO2/Si wafer. (b) Schematic illustration of the photodetector device. (a)-(b) are reproduced with permission [47]. (c) high-resolution imaging by NTMDs based photodetectors, reproduced with permission [63]. (d)-(f) Flexible photodetection based on PtSe2. (d) illustration of the PtSe2 thin film based Flexible photodetector. (e)Time-resolved photo-response curve at the wavelength of 408, 515 and 640 nm. (f) Mechanic stability measurement, in which the photocurrent is recorded as a function of bending cycles. (e)-(f) are reproduced with permission [66].*

0.7 cm2 V−1S−1. **Figure 6d** shows the array of devices with the finger-type electrode structure. The flexible photodetector shows good photoresponse with responsivity of 0.4, 0.25 and 0.1AW−1 at 408, 515 and 640 nm, respectively (**Figure 6e**). Moreover, the great mechanic stability is exhibited. Under large bending with different radius over 1000 cycles, the device can still generate stable photocurrent with almost no degradation, which is depicted in **Figure 6f**.
