**3. Photodetectors based on 2D materials**

#### **3.1 Graphene photodetectors**

Graphene is regarded as the original 2D material and has a hexagonal arrangement of atoms. Graphene has a planar geometry contrary to some other 2D materials like Xenes (silicene, germanene stanene etc.). The Xenes, in general, have a buckled geometry wherein the two sub lattices of the hexagonal lattice are slightly displaced with respect to each other. Graphene can absorb light with a wavelength ranging from ultraviolet to mid-infrared [29, 30]. Graphene has small optical absorption due to its atomically thin nature, limiting the photoresponsivity of the photodetectors based on it. A graphene photodetector exhibited a bandwidth of 500 GHz and a photoresponsivity of 0.5 mAW−1 [31]. A metal-graphene-metal (MGM) photodetector having asymmetric electrodes has been investigated for extended operating frequency. This device shows an external photoresponsivity of 6.1 mAW−1.

Some of the essential advantages of graphene photodetectors are high speed, ultra-broadband frequency range, and compatibility to circuits [32]. Compared to conventional semiconductors, graphene photodetectors show low photoresponsivity, which proves to be a significant drawback of such photodetectors. To overcome this and the other drawbacks, some techniques have been proposed to improve graphene photodetectors' optical absorption. For example, the use of nanostructured plasmonics leads to enhanced light concentration in the device via plasmonics resonance [33, 34]. This helps in improving the local electric field [33, 34]. Apart from enhancing the quantum efficiency, the plasmonics can also help in achieving multicolor detection [35]. A graphene photodetector possessing plasmonics nanoantennas sandwiched between two graphene layers shows a quantum efficiency of up to 20%. Though this method may offer quantum efficiency improvements, it reduces the device's operational bandwidth as the nanostructures' resonance determines the working wavelength in these systems.

Another method to improve graphene photodetectors' photoresponsivity is to integrate quantum dots with graphene [36]. The photoresponsivity and photodetection gain of such a device are 107 AW−1 and 108 , respectively. The presence of quantum dots in this device helps the photo-excited carriers (electrons or holes) to reach the graphene sheet while trapping the opposite type of carriers (holes or electrons). This leads to a phenomenon known as field-effect doping. Graphene photodetectors using PbS quantum dots have also been fabricated [37]. The device portrays a photoresponsivity of 107 AW−1. Graphene-quantum dot photodetectors are limited by factors like low operational speed and low operating bandwidth.

Another method to improve the photoresponse in graphene photodetectors is to use micro-cavities [38–42]. Such photodetectors are characterized by high speed, high efficiency, ultra-wide bandwidth and high photoresponsivity. The disadvantage of using micro-cavities is that the device's dimensions are relatively large compared to traditional photodetectors [41].

### **3.2 Molybdenum disulfide (MoS2) photodetectors**

MoS2 in its monolayer form has exciting properties like high carrier mobility 200 cm<sup>2</sup> V−1 s−1 [8, 43, 44], direct bandgap of ≈1.8 eV [43, 45], high On/Off ratio of

**113**

**Figure 2.**

*Model of the exfoliated single layer MoS2 phototransistor [47].*

*Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

value of NEP is associated with a low value of dark current.

current [45], strong light-matter interaction [8, 44], mechanical flexibility, chemical stability and ease of processing etc. Such exciting features of MoS2 in its monolayer and few-layer forms make it the most widely studied 2D semiconductor for optoelectronic applications. A photodetector having a typical field-effect transistor (FET) configuration was first reported by Yin et al. [46]. The device comprises of a mechanically exfoliated monolayer of MoS2 monolayer nanosheet as the effective region. The device shows a unique response with a cut-off wavelength of 670 nm. The cut-off wavelength is consistent with the bandgap of MoS2 in its monolayer form (1.8 eV). The maximum responsivity of this device is 7.5 mAW−1 along with a response speed of 50 ms. A similar photodetector/phototransistor was reported by Lopez-Sanchez et al. [47]. Again, this device is based on an exfoliated MoS2 monolayer but has an improved responsivity of 800 mAW−1 and a cut-off wavelength of 680 nm [47]. The model of the device is shown in **Figure 2**. The improvement in the device performance is attributed to improved mobility of the carriers, quality of the contacts and positioning technique. Apart from improved responsivity, the device portrays a low noise equivalent power (NEP) of 1.5 × 10−15 WHz−1/2. Such a low

Furthermore, the dark current in this device is limited by the bandgap of MoS2, which reduces the role of thermally excited carriers. However, the device is relatively slow in its response time, which is of the order of several seconds. Though the response time can be reduced (to 0.6 s) by using short pulses on the gate terminal to remove trapped charges, the response time is still considerable compared to other devices [46]. The photodetector reported by Lopez-Sanchez et al. shows a sublinear dependence of photocurrent on the intensity of the light. Such behavior and the surrounding dependent response speed of MoS2 indicate that charge trapping in MoS2 and/or at the MoS2-SiO2 interface plays a vital role in the sensing process. Some of the properties and qualities of MoS2 depend on the number of layers; accordingly, the performance of the photodetectors varies with the number of layers of MoS2 [43, 45, 48]. For example, in bulk form, MoS2 is an indirect bandgap semiconductor and is not suitable for optoelectronic applications, whereas, in its monolayer form, it is a direct bandgap semiconductor, making it suitable for optoelectronic applications. The lifetime of the photoexcited carriers is also dependent on the number of layers. Lee et al. have fabricated phototransistors, having single, double and triple layer MoS2 as the effective region. The optical bandgap of monolayer MoS2 is 1.82 eV, whereas, for double and triple layer MoS2, it is 1.65 eV and 1.35 eV, respectively. Based on the observations, it is seen that triple layer MoS2 photodetector shows good detection for the red light, whereas double and monolayer MoS2 photodetectors show good detection for the green light. The layer dependent bandgap in MoS2 allows for its use in wavelength range up to near-infrared (NIR) [49]. Multilayer MoS2

#### *Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

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

based on this effect [27, 28].

**3.1 Graphene photodetectors**

**3. Photodetectors based on 2D materials**

metal layer into consideration. Over the years the original theory of IPE has been refined largely resulting in much better assessment of the performance of the devices

Graphene is regarded as the original 2D material and has a hexagonal arrangement of atoms. Graphene has a planar geometry contrary to some other 2D materials like Xenes (silicene, germanene stanene etc.). The Xenes, in general, have a buckled geometry wherein the two sub lattices of the hexagonal lattice are slightly displaced with respect to each other. Graphene can absorb light with a wavelength ranging from ultraviolet to mid-infrared [29, 30]. Graphene has small optical absorption due to its atomically thin nature, limiting the photoresponsivity of the photodetectors based on it. A graphene photodetector exhibited a bandwidth of 500 GHz and a photoresponsivity of 0.5 mAW−1 [31]. A metal-graphene-metal (MGM) photodetector having asymmetric electrodes has been investigated for extended operating

frequency. This device shows an external photoresponsivity of 6.1 mAW−1.

determines the working wavelength in these systems.

tection gain of such a device are 107

portrays a photoresponsivity of 107

compared to traditional photodetectors [41].

**3.2 Molybdenum disulfide (MoS2) photodetectors**

Some of the essential advantages of graphene photodetectors are high speed, ultra-broadband frequency range, and compatibility to circuits [32]. Compared to conventional semiconductors, graphene photodetectors show low photoresponsivity, which proves to be a significant drawback of such photodetectors. To overcome this and the other drawbacks, some techniques have been proposed to improve graphene photodetectors' optical absorption. For example, the use of nanostructured plasmonics leads to enhanced light concentration in the device via plasmonics resonance [33, 34]. This helps in improving the local electric field [33, 34]. Apart from enhancing the quantum efficiency, the plasmonics can also help in achieving multicolor detection [35]. A graphene photodetector possessing plasmonics nanoantennas sandwiched between two graphene layers shows a quantum efficiency of up to 20%. Though this method may offer quantum efficiency improvements, it reduces the device's operational bandwidth as the nanostructures' resonance

Another method to improve graphene photodetectors' photoresponsivity is to integrate quantum dots with graphene [36]. The photoresponsivity and photode-

AW−1 and 108

quantum dots in this device helps the photo-excited carriers (electrons or holes) to reach the graphene sheet while trapping the opposite type of carriers (holes or electrons). This leads to a phenomenon known as field-effect doping. Graphene photodetectors using PbS quantum dots have also been fabricated [37]. The device

are limited by factors like low operational speed and low operating bandwidth.

Another method to improve the photoresponse in graphene photodetectors is to use micro-cavities [38–42]. Such photodetectors are characterized by high speed, high efficiency, ultra-wide bandwidth and high photoresponsivity. The disadvantage of using micro-cavities is that the device's dimensions are relatively large

MoS2 in its monolayer form has exciting properties like high carrier mobility

V−1 s−1 [8, 43, 44], direct bandgap of ≈1.8 eV [43, 45], high On/Off ratio of

, respectively. The presence of

AW−1. Graphene-quantum dot photodetectors

**112**

200 cm<sup>2</sup>

current [45], strong light-matter interaction [8, 44], mechanical flexibility, chemical stability and ease of processing etc. Such exciting features of MoS2 in its monolayer and few-layer forms make it the most widely studied 2D semiconductor for optoelectronic applications. A photodetector having a typical field-effect transistor (FET) configuration was first reported by Yin et al. [46]. The device comprises of a mechanically exfoliated monolayer of MoS2 monolayer nanosheet as the effective region. The device shows a unique response with a cut-off wavelength of 670 nm. The cut-off wavelength is consistent with the bandgap of MoS2 in its monolayer form (1.8 eV). The maximum responsivity of this device is 7.5 mAW−1 along with a response speed of 50 ms. A similar photodetector/phototransistor was reported by Lopez-Sanchez et al. [47]. Again, this device is based on an exfoliated MoS2 monolayer but has an improved responsivity of 800 mAW−1 and a cut-off wavelength of 680 nm [47]. The model of the device is shown in **Figure 2**. The improvement in the device performance is attributed to improved mobility of the carriers, quality of the contacts and positioning technique. Apart from improved responsivity, the device portrays a low noise equivalent power (NEP) of 1.5 × 10−15 WHz−1/2. Such a low value of NEP is associated with a low value of dark current.

Furthermore, the dark current in this device is limited by the bandgap of MoS2, which reduces the role of thermally excited carriers. However, the device is relatively slow in its response time, which is of the order of several seconds. Though the response time can be reduced (to 0.6 s) by using short pulses on the gate terminal to remove trapped charges, the response time is still considerable compared to other devices [46]. The photodetector reported by Lopez-Sanchez et al. shows a sublinear dependence of photocurrent on the intensity of the light. Such behavior and the surrounding dependent response speed of MoS2 indicate that charge trapping in MoS2 and/or at the MoS2-SiO2 interface plays a vital role in the sensing process.

Some of the properties and qualities of MoS2 depend on the number of layers; accordingly, the performance of the photodetectors varies with the number of layers of MoS2 [43, 45, 48]. For example, in bulk form, MoS2 is an indirect bandgap semiconductor and is not suitable for optoelectronic applications, whereas, in its monolayer form, it is a direct bandgap semiconductor, making it suitable for optoelectronic applications. The lifetime of the photoexcited carriers is also dependent on the number of layers. Lee et al. have fabricated phototransistors, having single, double and triple layer MoS2 as the effective region. The optical bandgap of monolayer MoS2 is 1.82 eV, whereas, for double and triple layer MoS2, it is 1.65 eV and 1.35 eV, respectively. Based on the observations, it is seen that triple layer MoS2 photodetector shows good detection for the red light, whereas double and monolayer MoS2 photodetectors show good detection for the green light. The layer dependent bandgap in MoS2 allows for its use in wavelength range up to near-infrared (NIR) [49]. Multilayer MoS2

**Figure 2.** *Model of the exfoliated single layer MoS2 phototransistor [47].*

phototransistors show a degraded responsivity value of 100 mAW−1. Khan et al. have also demonstrated that parameters like responsivity and response speed show a high dependence on the number of MoS2 layers [50].

The properties of 2D MoS2 are distinctly dependent on the method of preparation. Zheng et al. reported a phototransistor based on chemical vapor deposition (CVD) grown MoS2 [51]. This device has a maximum responsivity of 2200 AW−1 in vacuum operating at a wavelength of 532 nm. The same device shows a responsivity of 780 AW−1 in air. The cause for such a decrease in responsivity is the adsorbates. Due to the large surface-to-volume ratio of MoS2, many adsorbates migrate from ambient air to the surface of MoS2 and the MoS2/substrate interface. These adsorbents act as p-type dopants, leading to carrier scattering and degraded carrier mobility and responsivity in air. The photoresponse could also get affected (decreased) as the adsorbents may act as recombination centers for photoexcited carriers [52].

Perea-Lopez et al. have also fabricated a photodetector based on CVD-grown MoS2 monolayer [53]. The reported device shows a relatively lower responsivity of 1.1 mAW−1 at an illuminating wavelength of 514.5 nm [53]. Such a considerable variation in the two devices' responsivity shows the significant role of contact resistance in these devices. Another study has put CVD-grown few-layer MoS2 to use for a photodetector [54]. The performance of the device has been evaluated under harsh conditions with a wavelength of 532 nm [54]. Even at 200°C, the device portrays a photocurrent to dark current ratio of 10. Photodetectors based on MoS2 employing other methods of synthesis like liquid exfoliation [55], solution synthesis [56] and magnetron sputtering [57] have also been reported. As compared to mechanically exfoliated and CVD grown MoS2 based devices, these devices show degraded values of responsivities.

In photodetectors, based on monolayer and bilayer MoS2, both photoconductive and photogating effects were observed to contribute to the photocurrent [58]. Different response times were observed for the two effects, respectively, making it possible to identify their independent contribution to the photocurrent. The photogating effect shows an obvious dependence on the gate voltage and is a slow process. The slowness of this effect comes from the longer lifetime of the trapped charges at the MoS2-SiO2 interface. In contrast, the photoconductive effect has a negligible dependence on the gate voltage and is a fast process. The fast response of the photoconductive effect arises from the mid-gap states due to structural defects in MoS2. The photoconductive response can be studied independently by varying the illuminating light faster than the photo-gating effect.

In view of the average performance of MoS2 photodetectors, several techniques have been proposed to improve their performance [59–66]. One such technique proposed by Leu et al. involves micro-patterning and localized modification of the MoS2 layer [59]. The device is operated at an illuminating wavelength of 532 nm. The local modification is achieved by surface oxidation and oxygen doping. A photodetector based on such a modified MoS2 layer shows improved photoresponse with a responsivity increase of several folds [59]. Kwon et al. proposed a photodetector based on multilayer MoS2 with a bottom gate configuration [60]. As compared to previously reported global gate counterparts, the device shows much-improved photocurrent [49, 60, 67]. The purpose of a bottom gate in such a device is to impose a large tunnel barrier at ungated channel regions, which helps accumulate holes, thereby reducing the potential barrier for free electrons. Once the potential barrier is reduced, there is an increase in the electron depletion region's thermionic current. Furthermore, photocurrent improvement in the accumulation region arises due to decreased tunnel barrier for photoexcited holes. Also, the dark current is suppressed because of the series resistance from ungated areas.

**115**

*Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

from 10 to 104

AW−1.

**3.3 Other TMDCs photodetectors**

presents photodetectors based on these materials.

responsivity from 18.8 mAW−1 in vacuum to 0.2 μAW−1 in air.

Monolayer and few-layer WSe2 has also been studied for photodetector applications. Zheng et al. have fabricated photodetectors using CVD-grown WSe2

monolayer [84]. The effect of metal contacts having different work functions on the device's photoresponse is studied [84]. The device exhibits the maximum (1.8 × 10 5 AW−1) and minimum responsivity with Pd and Ti contacts at a wavelength of

Consequently, the responsivity shows huge improvements and attains a value of 342.6AW−1. Kufer et al. fabricated a MoS2 photodetector, wherein HfO2 encapsulates the MoS2 layer. Upon encapsulation, it was seen that the electronic and optoelectronic properties of multilayer MoS2 photodetector improved [61]. The encapsulated MoS2, along with negligible hysteresis in the transfer characteristics, showed an enhanced n-type behavior. Encapsulation decreases the number of surface adsorbents, which eventually leads to improved performance. Encapsulation results in an increase in the mobility of carriers and a decrease in the contact resistance. These two effects, in combination, give rise to an increased response speed and responsivity. The device's responsivity can be tuned by the gate voltage and ranges

Apart from MoS2, other TMDCs have been utilized for photodetector applica-

Like MoS2, monolayer MoSe2 has several alluring properties, such as a direct bandgap of 1.5 eV [68], enhanced photoluminescence (PL) [69] and considerable binding energy of excitons [70]. Improvements in the synthesis of MoSe2 via mechanical exfoliation [71, 72] and CVD methods [73–75] have widened their scope of photodetector applications. Chang et al. and Xi et al. have reported monolayer MoSe2 phototransistors [76, 77]. MoSe2 monolayers for the phototransistors were prepared via CVD methods. The responsivities of the phototransistors are of the order of mAW−1, which is lower than the CVD-grown MoS2 monolayer counterparts by a few orders [51]. However, if the density of the charge impurities and defects are reduced, an improved photoresponse of the order of tens of milliseconds is expected. The responsivity of MoSe2 based devices can be improved by using a CVD-grown multilayer MoSe2 [78]. But the improvement comes at the cost of degraded response speed [78]. A phototransistor based on a few-layer MoSe2 has been fabricated by Abderrehmane et al. [72]. MoSe2 layers were obtained by mechanical exfoliation methods [72]. This device has a response time of tens of milliseconds and a responsivity of 97.1 AW−1 operating at a wavelength of 532 nm. Photodetectors based on monolayer and few-layer WS2 obtained via different synthesis methods have been reported [79, 80]. The photoresponse of CVD-grown few-layer WS2 has been studied by Parea-Lopez et al. [81]. The photoresponse reportedly shows a high dependence on photon energy [81]. The responsivity and response speed of the device are reported to be 92 μAW−1 and 5 ms, respectively at a wavelength of 457-647 nm. The dependence of multilayer WS2 devices' responsivity was observed to depend on the surrounding gaseous environment by Huo et al. [82]. The responsivity shows an increase when the environment changes from vacuum (tens of AW−1) to NH3 (884 AW−1) at a wavelength of 633 nm. The increased responsivity is a consequence of the charge transfer between the NH3 gas molecule and WS2. The doping level of WS2 gets modified by the charge transfer, which eventually increases the lifetime of photoexcited carriers and hence the responsivity. Another study conducted by Lan et al. showed a similar surrounding dependent performance of WS2 devices [83]. The device showed a decrease in its

tions. These include MoSe2, WS2, WSe2, MoTe2, ReS2 and ReSe2. This section

*Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

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

dependence on the number of MoS2 layers [50].

carriers [52].

degraded values of responsivities.

the illuminating light faster than the photo-gating effect.

phototransistors show a degraded responsivity value of 100 mAW−1. Khan et al. have also demonstrated that parameters like responsivity and response speed show a high

The properties of 2D MoS2 are distinctly dependent on the method of prepara-

Perea-Lopez et al. have also fabricated a photodetector based on CVD-grown MoS2 monolayer [53]. The reported device shows a relatively lower responsivity of 1.1 mAW−1 at an illuminating wavelength of 514.5 nm [53]. Such a considerable variation in the two devices' responsivity shows the significant role of contact resistance in these devices. Another study has put CVD-grown few-layer MoS2 to use for a photodetector [54]. The performance of the device has been evaluated under harsh conditions with a wavelength of 532 nm [54]. Even at 200°C, the device portrays a photocurrent to dark current ratio of 10. Photodetectors based on MoS2 employing other methods of synthesis like liquid exfoliation [55], solution synthesis [56] and magnetron sputtering [57] have also been reported. As compared to mechanically exfoliated and CVD grown MoS2 based devices, these devices show

In photodetectors, based on monolayer and bilayer MoS2, both photoconductive and photogating effects were observed to contribute to the photocurrent [58]. Different response times were observed for the two effects, respectively, making it possible to identify their independent contribution to the photocurrent. The photogating effect shows an obvious dependence on the gate voltage and is a slow process. The slowness of this effect comes from the longer lifetime of the trapped charges at the MoS2-SiO2 interface. In contrast, the photoconductive effect has a negligible dependence on the gate voltage and is a fast process. The fast response of the photoconductive effect arises from the mid-gap states due to structural defects in MoS2. The photoconductive response can be studied independently by varying

In view of the average performance of MoS2 photodetectors, several techniques have been proposed to improve their performance [59–66]. One such technique proposed by Leu et al. involves micro-patterning and localized modification of the MoS2 layer [59]. The device is operated at an illuminating wavelength of 532 nm. The local modification is achieved by surface oxidation and oxygen doping. A photodetector based on such a modified MoS2 layer shows improved photoresponse with a responsivity increase of several folds [59]. Kwon et al. proposed a photodetector based on multilayer MoS2 with a bottom gate configuration [60]. As compared to previously reported global gate counterparts, the device shows much-improved photocurrent [49, 60, 67]. The purpose of a bottom gate in such a device is to impose a large tunnel barrier at ungated channel regions, which helps accumulate holes, thereby reducing the potential barrier for free electrons. Once the potential barrier is reduced, there is an increase in the electron depletion region's thermionic current. Furthermore, photocurrent improvement in the accumulation region arises due to decreased tunnel barrier for photoexcited holes. Also, the dark current is suppressed because of the series resistance from ungated areas.

tion. Zheng et al. reported a phototransistor based on chemical vapor deposition (CVD) grown MoS2 [51]. This device has a maximum responsivity of 2200 AW−1 in vacuum operating at a wavelength of 532 nm. The same device shows a responsivity of 780 AW−1 in air. The cause for such a decrease in responsivity is the adsorbates. Due to the large surface-to-volume ratio of MoS2, many adsorbates migrate from ambient air to the surface of MoS2 and the MoS2/substrate interface. These adsorbents act as p-type dopants, leading to carrier scattering and degraded carrier mobility and responsivity in air. The photoresponse could also get affected (decreased) as the adsorbents may act as recombination centers for photoexcited

**114**

Consequently, the responsivity shows huge improvements and attains a value of 342.6AW−1. Kufer et al. fabricated a MoS2 photodetector, wherein HfO2 encapsulates the MoS2 layer. Upon encapsulation, it was seen that the electronic and optoelectronic properties of multilayer MoS2 photodetector improved [61]. The encapsulated MoS2, along with negligible hysteresis in the transfer characteristics, showed an enhanced n-type behavior. Encapsulation decreases the number of surface adsorbents, which eventually leads to improved performance. Encapsulation results in an increase in the mobility of carriers and a decrease in the contact resistance. These two effects, in combination, give rise to an increased response speed and responsivity. The device's responsivity can be tuned by the gate voltage and ranges from 10 to 104 AW−1.

#### **3.3 Other TMDCs photodetectors**

Apart from MoS2, other TMDCs have been utilized for photodetector applications. These include MoSe2, WS2, WSe2, MoTe2, ReS2 and ReSe2. This section presents photodetectors based on these materials.

Like MoS2, monolayer MoSe2 has several alluring properties, such as a direct bandgap of 1.5 eV [68], enhanced photoluminescence (PL) [69] and considerable binding energy of excitons [70]. Improvements in the synthesis of MoSe2 via mechanical exfoliation [71, 72] and CVD methods [73–75] have widened their scope of photodetector applications. Chang et al. and Xi et al. have reported monolayer MoSe2 phototransistors [76, 77]. MoSe2 monolayers for the phototransistors were prepared via CVD methods. The responsivities of the phototransistors are of the order of mAW−1, which is lower than the CVD-grown MoS2 monolayer counterparts by a few orders [51]. However, if the density of the charge impurities and defects are reduced, an improved photoresponse of the order of tens of milliseconds is expected. The responsivity of MoSe2 based devices can be improved by using a CVD-grown multilayer MoSe2 [78]. But the improvement comes at the cost of degraded response speed [78]. A phototransistor based on a few-layer MoSe2 has been fabricated by Abderrehmane et al. [72]. MoSe2 layers were obtained by mechanical exfoliation methods [72]. This device has a response time of tens of milliseconds and a responsivity of 97.1 AW−1 operating at a wavelength of 532 nm.

Photodetectors based on monolayer and few-layer WS2 obtained via different synthesis methods have been reported [79, 80]. The photoresponse of CVD-grown few-layer WS2 has been studied by Parea-Lopez et al. [81]. The photoresponse reportedly shows a high dependence on photon energy [81]. The responsivity and response speed of the device are reported to be 92 μAW−1 and 5 ms, respectively at a wavelength of 457-647 nm. The dependence of multilayer WS2 devices' responsivity was observed to depend on the surrounding gaseous environment by Huo et al. [82]. The responsivity shows an increase when the environment changes from vacuum (tens of AW−1) to NH3 (884 AW−1) at a wavelength of 633 nm. The increased responsivity is a consequence of the charge transfer between the NH3 gas molecule and WS2. The doping level of WS2 gets modified by the charge transfer, which eventually increases the lifetime of photoexcited carriers and hence the responsivity. Another study conducted by Lan et al. showed a similar surrounding dependent performance of WS2 devices [83]. The device showed a decrease in its responsivity from 18.8 mAW−1 in vacuum to 0.2 μAW−1 in air.

Monolayer and few-layer WSe2 has also been studied for photodetector applications. Zheng et al. have fabricated photodetectors using CVD-grown WSe2 monolayer [84]. The effect of metal contacts having different work functions on the device's photoresponse is studied [84]. The device exhibits the maximum (1.8 × 10 5 AW−1) and minimum responsivity with Pd and Ti contacts at a wavelength of

650 nm. However, the device with Ti contacts shows a much smaller response time (23 ms) than the device with Pt contacts. The variation in the device's performance results from the considerable difference in Schottky barriers between WSe2 and different materials, highlighting the significant role of metal contacts in these devices. Pradhan et al. have demonstrated a photodetector based on a trilayer WSe2 [85]. The device exhibits the responsivity and response speed of 7AW−1 and 10 μs, respectively at an illuminating wavelength of 532 nm. Other reports involving graphene contacts and doping of a few-layer WSe2 have been observed to improve the performance of WSe2 photodetectors [66, 86, 87].

A newly introduced 2D material, MoTe2 has excellent electronic and optoelectronic properties, due to which it has received a lot of attention recently [88–90]. Yin et al. have reported a phototransistor based on exfoliated few-layer MoTe2 [91]. A study of the effect of different metal contacts on the electrical properties of the MoTe2 phototransistor is presented. The device attains a responsivity of 2.56 × 103 under optimum conditions under an illumination of 473 nm laser.

Re-dichalcogenides are different from the majority of other layered TMDCs due to their high crystal symmetry. ReS2 and ReSe2, in their distorted 1 T in-plane structure are anisotropic semiconductors [92]. The electrical, mechanical, and optical properties of these materials are extremely anisotropic, rendering these materials interesting for optoelectronic and electronic applications. The bandgap and carrier mobility of ReSe2 was found to be dependent on the layer thickness by Yang et al. [93]. This allows modification of the electronic and optoelectronic properties of ReS2 devices. A monolayer ReSe2 phototransistor has an exceptional photoresponse with responsivity and response time of 95 AW−1 and tens of milliseconds, respectively [93]. The operating wavelength for the device is chosen to be 633 nm. Just like MoS2 and WS2 devices, the photoresponse of ReSe2 devices is also found to be dependent on the surroundings [50, 51, 82, 83, 94]. The charge transfer between the surrounding gas and ReSe2 consequently affects the device performance. This charge transfer alters the doping in ReSe2 along with the carrier lifetime [94]. One way to avoid this dependence of performance on surroundings is to use encapsulation or passivation. Though the ReSe2 photodetectors/phototransistors show promising results but an obvious disadvantage of these devices is that the current after removing the illuminating light can not return to dark current levels. This disadvantage is a consequence of the slow recombination rate of the photoexcited carriers. However, this issue may be solved by applying short pulses at the gate terminal to reset the device [36].

Because of the anisotropic crystal structure, ReS2, in particular, can be utilized to detect polarized light [95]. The model of one such photodetector is shown in **Figure 3**. The responsivity of ReS2 photodetectors can be largely improved up to the levels of 3.97 × 103 to 1.18 × 106 by electron doping [96] under illumiation of a 1064 nm laser. Besides improved responsivity, the device portrays a broad range of wavelength detection and fast response speed of the order of tens of milliseconds. Significant enhancement in both the electronic and optoelectronic properties of ReS2 via O2 plasma treatment was observed by Shin et al. [97]. The device exhibits a high responsivity of 2.5 × 107 AW−1 at a laser illumination of 405 nm, which is the highest obtained for a 2D semiconductor based back gated photodetector. Such a high responsivity is a consequence of large thickness (30 nm) and direct bandgap of ReS2 layers. The response time is observed to be inversely proportional to the plasma treatment duration. Prolonged plasma treatment leads to the formation of trap states within the bandgap of ReS2. Such trap states result in enhanced recombination rates of photoexcited carriers, which consequently reduce the response time.

In summary, TMDCs photodetectors/phototransistors show a widely varying performance. Responsivities and the response times range from 10−7 AW−1 to 107

**117**

and 10−5 to 103

*Model of ReS2 photodetector [95].*

**Figure 3.**

, respectively. Generally, trap states affect the performance of TMDCs

photodetectors. An increase in responsivities is observed at the existence of the trap states in TMDCs and/or at TMDC-dielectric interfaces. However, the response speed is found to decrease because of these trap states. Other factors that affect the TMDCs photodetectors/phototransistors are synthesis methods, number of layers,

Phosphorous, in its elemental nature, can exist in many forms. One such form of phosphorous is called black phosphorous (BP). With a formation energy of −395 KJmol−1− black phosphorous is a thermodynamically stable form of phosphorous at room temperature. Black phosphorous is similar to graphite in its appearance, properties and structure. Black phosphorous sheets have a puckered geometry [98]. Black phosphorous was first successfully exfoliated in 2014 and has received a lot of attention since then [99, 100]. In its monolayer form, the phosphorous atoms form covalent bonds with three adjacent atoms, which results in a wrinkled honeycomb structure. The corresponding layers are held together by van der Waals forces [101]. Unlike graphene, black phosphorous is a semiconductor with a direct bandgap. Due to its strong anisotropic interaction with electrons and photons, black phosphorous

The bandgap's direct nature in black phosphorous makes it easy for the carriers to transit to excited states, as there are negligible chances of phonon scattering [102]. The

is a strong candidate for electronic and optoelectronic applications.

contact resistance and surrounding environment.

**3.4 Black phosphorous photodetectors**

*Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

*Photo-Detectors Based on Two Dimensional Materials DOI: http://dx.doi.org/10.5772/intechopen.95559*

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

the performance of WSe2 photodetectors [66, 86, 87].

terminal to reset the device [36].

a high responsivity of 2.5 × 107

to 1.18 × 106

the levels of 3.97 × 103

under optimum conditions under an illumination of 473 nm laser.

650 nm. However, the device with Ti contacts shows a much smaller response time (23 ms) than the device with Pt contacts. The variation in the device's performance results from the considerable difference in Schottky barriers between WSe2 and different materials, highlighting the significant role of metal contacts in these devices. Pradhan et al. have demonstrated a photodetector based on a trilayer WSe2 [85]. The device exhibits the responsivity and response speed of 7AW−1 and 10 μs, respectively at an illuminating wavelength of 532 nm. Other reports involving graphene contacts and doping of a few-layer WSe2 have been observed to improve

A newly introduced 2D material, MoTe2 has excellent electronic and optoelectronic properties, due to which it has received a lot of attention recently [88–90]. Yin et al. have reported a phototransistor based on exfoliated few-layer MoTe2 [91]. A study of the effect of different metal contacts on the electrical properties of the MoTe2 phototransistor is presented. The device attains a responsivity of 2.56 × 103

Re-dichalcogenides are different from the majority of other layered TMDCs due to their high crystal symmetry. ReS2 and ReSe2, in their distorted 1 T in-plane structure are anisotropic semiconductors [92]. The electrical, mechanical, and optical properties of these materials are extremely anisotropic, rendering these materials interesting for optoelectronic and electronic applications. The bandgap and carrier mobility of ReSe2 was found to be dependent on the layer thickness by Yang et al. [93]. This allows modification of the electronic and optoelectronic properties of ReS2 devices. A monolayer ReSe2 phototransistor has an exceptional photoresponse with responsivity and response time of 95 AW−1 and tens of milliseconds, respectively [93]. The operating wavelength for the device is chosen to be 633 nm. Just like MoS2 and WS2 devices, the photoresponse of ReSe2 devices is also found to be dependent on the surroundings [50, 51, 82, 83, 94]. The charge transfer between the surrounding gas and ReSe2 consequently affects the device performance. This charge transfer alters the doping in ReSe2 along with the carrier lifetime [94]. One way to avoid this dependence of performance on surroundings is to use encapsulation or passivation. Though the ReSe2 photodetectors/phototransistors show promising results but an obvious disadvantage of these devices is that the current after removing the illuminating light can not return to dark current levels. This disadvantage is a consequence of the slow recombination rate of the photoexcited carriers. However, this issue may be solved by applying short pulses at the gate

Because of the anisotropic crystal structure, ReS2, in particular, can be utilized to detect polarized light [95]. The model of one such photodetector is shown in **Figure 3**. The responsivity of ReS2 photodetectors can be largely improved up to

1064 nm laser. Besides improved responsivity, the device portrays a broad range of wavelength detection and fast response speed of the order of tens of milliseconds. Significant enhancement in both the electronic and optoelectronic properties of ReS2 via O2 plasma treatment was observed by Shin et al. [97]. The device exhibits

highest obtained for a 2D semiconductor based back gated photodetector. Such a high responsivity is a consequence of large thickness (30 nm) and direct bandgap of ReS2 layers. The response time is observed to be inversely proportional to the plasma treatment duration. Prolonged plasma treatment leads to the formation of trap states within the bandgap of ReS2. Such trap states result in enhanced recombination rates of photoexcited carriers, which consequently reduce the response time. In summary, TMDCs photodetectors/phototransistors show a widely varying performance. Responsivities and the response times range from 10−7 AW−1 to 107

by electron doping [96] under illumiation of a

AW−1 at a laser illumination of 405 nm, which is the

**116**

**Figure 3.** *Model of ReS2 photodetector [95].*

and 10−5 to 103 , respectively. Generally, trap states affect the performance of TMDCs photodetectors. An increase in responsivities is observed at the existence of the trap states in TMDCs and/or at TMDC-dielectric interfaces. However, the response speed is found to decrease because of these trap states. Other factors that affect the TMDCs photodetectors/phototransistors are synthesis methods, number of layers, contact resistance and surrounding environment.
