**4. Heterostructures for photodetection**

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

elastomer stamp (poly dimethyl siloxane, PDMS), as illustrated in **Figure 2b** [30]. The transparent PDMS stamp is then used to handle the exfoliated 2D flakes. Under optical microscope, it is then aligned to a target position, e.g., the position of already transferred 2D layer. The position of the stamp is then fine-tuned in all three dimensions to approach the target substrate, until the full contact. It is then slowly peeled off from substrate leaving 2D material behind. Sometimes slight heating of the substrate is necessary to reduce the viscosity of PDMS stamp and promote the successful transfer of 2D material onto target substrate. Instead of the common used PDMS stamps, thermal release tapes can be also used as the handle [32]. Though no wet-chemical etching processes is adopted in the above procedures, the surface of PDMS is full of silane groups and may contaminate the 2D material during transfer and make the contacting substrate hydrophobic. It may therefore

*Setup and typical dry transfer processes for 2D vdW stacking. (a) Schematic of the experimental setup and (b) the processes employed for the all-dry transfer process, reproduced with permission from Ref. [30], Copyright 2014 IOP publishing Ltd. (c) Schematic of the van der Waals (vdW) technique for polymer-free assembly of layered materials, reproduced with permission from Ref. [33], Copyright 2013 Science.*

An improved polymer-free method was reported by Wang et al., which adopted the clean h-BN as the buffer layer to attach graphene (**Figure 2c**) [33]. This is based on the stronger interaction between graphene and h-BN compared to SiO2, so that elastomer stamps with h-BN layer could pick up graphene layer from substrate. Note that the graphene layer is initially transferred onto SiO2 substrate by tape exfoliation; both the top and bottom surface are free from polymer residuals due to the fresh exfoliation when peeling off the tape. Through this method, the graphene layer during all transfer processes is protected by h-BN and thus could form clean interfaces with both the top and bottom h-BN layers. The as-prepared h-BN-encapsulated graphene

with long ballistic transport distance over 15 μm at 40 K, demonstrating the ultrahigh

Recently, the pick-transfer methods have been also modified to transfer metal electrodes onto 2D materials, avoiding the interdiffusion of elements within the contact interface with 2D materials from traditional physical deposition of metal electrodes (via magnetic sputtering, thermal evaporation, etc.) [34]. Importantly, the formed electrical contact with MoS2 using different metal electrodes displayed

 V<sup>−</sup><sup>1</sup> s<sup>−</sup><sup>1</sup> ,

manifests unprecedented room temperature mobility up to 140,000 cm2

interface quality formed in such polymer-free transfer methods.

deteriorate the material and interface quality in device.

**152**

**Figure 2.**

There are several kinds of photodetectors that convert incident light signal to electrical signals, including detectors that rely on photoelectric effect, pyroelectric effect, and photothermal effects. Among the various detectors, the photoelectric detectors exhibit fast response dynamics based on simple separation of electron– hole pairs and are mostly used in commercial products. The photoelectric detectors can be further categorized into photodiode and phototransistors. In photodiodes, the photogenerated electron–hole pairs are separated by the built-in electric field in space charge region, while in phototransistors, an external electric field is applied to generate photodetection gain >100% for highly sensitive detection.

In 2D heterostructures, both photodiodes and phototransistors can be built up by vdW stacking of different materials. Because of the presence of band offset at the interface, heterostructured junctions tend to enable efficient charge separation compared to homojunctions, which requires deliberate control of their p and n doping states. In this section, we discuss several typical heterostructures in type I–III band alignments and their behavior in photodetection.

## **4.1 Heterostructured diodes**

To fabricate heterostructured diode, one kind of 2D material is exfoliated and transferred onto the other one. For the charge separation in vertical direction, type II band alignment is desired. However, this is not naturally obtained, especially when one adopts a narrow bandgap semiconductor for infrared applications, e.g., BP. However, since the work function of ultrathin 2D materials can be dramatically modulated by electrostatic methods, the behavior of 2D diodes was demonstrated tunable by applying gate voltages [35–37]. As illustrated in **Figure 3a**, BP and WSe2 form type I band alignments [38], with slight conduction band offset (~0.1 eV). When increasing the back-gate voltage from negative to positive, the WSe2 layer is tuned sequentially from p to i and n states by the injection of gate-coupled electrons and then forming, respectively, p–p, pi, and p-n junctions with the p-typed BP. Further increasing gate bias also tunes BP to n type and results in n-n junction. Accordingly, these junctions manifest different rectification ratios under gate bias. **Figure 3b** displays the forward and reverse channel current (at *V*ds = 1 V and −1 V); the different onset threshold gate voltage under forward and reverse bias results in a window of −30 V < Vg < 10 V, in which high rectification ratio is obtained by the formation of p-n junction. The widely tuned doping characteristic of 2D bipolar semiconductor thus renders feasible modification of the diode characteristics in the device via various kinds of field effects, including using ionic liquids and ion gels [39].

By choosing appropriate 2D semiconductors, p-n junctions can be formed directly without gate bias. Wang et al. reported such diode based on p-typed gate and n-type MoS2 [37]. It displays apparent photovoltaic effect under light illumination, as indicated in **Figure 3c**. The extracted ideal factor of the junction is as low as 2 at room temperature, corresponding to Shockley-Read-Hall (SRH) recombination-dominated carrier loss during transport. So far, various kinds of p-n junctions have been made based on such type II band alignments, including BP/MoS2 [38], MoS2/MoTe2 [40], MoS2/WSe2 [41], etc. The open circuit voltage by photovoltaic effect in such type II

#### **Figure 3.**

*2D Heterostructures of different interfacial band alignments and their characteristics. (a) The type I band alignment between BP and WSe2 and (b) the appearance of various junction behaviors (p-p, p-n, n-n) under gate modulation, reproduced with permission from Ref. [38], Copyright 2017 Wiley-VCH. (c) Schematic of the type II heterostructure based on n-type MoS2 and p-type GaTe, and (d) the photovoltaic characteristic under light irradiation, reproduced with permission from Ref. [37], Copyright 2015 American Chemical Society. (e) Schematic diagram of a type III WSe2/SnS2 heterostructure and (f) its* IV *characteristic under dark and light illumination (550 nm), reproduced with permission from Ref. [19], Copyright 2018 Wiley-VCH.*

band heterostructures is limited by the interfacial bandgap determined by the lower conduction and higher valance band. It is therefore usually less than the maximum *V*oc attainable in p-n junction of each component. However, an essential benefit of such heterostructure is the formation of photodiode without deliberate efforts in controlling the p and n-type doping. A strong evidence of the formation of type II band alignment is that the photoluminescence at the junction area is quenched due to the separation of electron–hole pairs at the interface. Another benefit of such type II heterostructure is based on the interlayer transition, which supports sub-bandgap photodetection [42]. For example, MoS2/WS2 heterojunction displays near-infrared response that is beyond both the bandgap limits of MoS2 and WS2 [43].

Tunneling diodes can be formed by heterostructures of type III band alignment [24]. In the case of WSe2/SnS2 heterostructure, due to the high electron affinity of SnS2, type III heterostructure is formed with direct interband transition between valance band of WSe2 and the conduction band of SnS2 [19]. The diode initially displayed high rectification ratio >104 for low dark current under reverse bias, whereas under illumination, the device exhibits dramatically increased light current by direct tunneling, resulting in high responsivity >200 AW<sup>−</sup><sup>1</sup> and excellent detectivity >1013 Jones. Further exploration of the kind of heterostructure using other 2D materials with different bandgap may have the potential to make high-performance tunneling photodiodes for infrared. The heterostructure of narrow bandgap BP and larger bandgap MoS2 has been used to realize multi-value inverters with high gains >150 based on gate-modulated tunneling current [44].

In addition to the two-layer stacking, multilayered heterostructures have been also developed as tunneling diodes. **Figure 4a** illustrates such a heterostructure based on vertically stacked graphene/MoS2/graphene [35]. Because of the work function between top and bottom graphene (due to the unidentical substrate doping effect), the multilayer displayed photovoltaic separation of electron–hole pairs under illumination, reaching a *V*oc~0.3 V under additional gate bias. The device also exhibits wavelength-dependent responsivity that is related to the absorption in

**155**

**Figure 4.**

*Emerging Artificial Two-Dimensional van der Waals Heterostructures for Optoelectronics*

MoS2 (as indicated in **Figure 4b**), demonstrating the working principle of the multilayer junction. By using graphene at both the bottom and top of the junction made by MoS2 and WSe2, Lee et al. demonstrated an efficient p-n junction at the ultimate atomic thin thickness with improved collection of photoexcited carriers [36]. **Figure 4c** and **d** illustrate the structure and *IV* characteristic of the device. Under illumination, tunneling-assisted interlayer recombination of the majority carriers dominates the electronic and optoelectronic behavior of the junction. Alternatively, sandwiching graphene within WSe2 and MoS2 can make a broadband photodetector up to 2 μm based on the absorption of graphene [45]. Such interlayer tunneling can be suppressed by inserting an insulating h-BN layer. Vu et al. fabricated a tunneling heterostructure based on graphene/h-BN/MoS2 in the configuration shown in **Figure 4e** [18]. The dark current in device is greatly suppressed by blocking direct tunneling. However, under illumination, photogenerated carriers may overcome the barrier and contribute significant photocurrent via Fowler-Nordheim (FN) tunneling. Notably, to balance the photodetection performance in terms of the responsivity and detectivity, the thickness of h-BN is optimal ~5–7 nm, as indicated in **Figure 4f**. The thicker the h-BN layer, the lower probability of FN tunneling and thus lesser photocurrent, while the thinner h-BN results in large dark current by

*Various kinds of vertical heterostructures. (a) Schematic of Gr/MoS2/Gr heterojunction, which displays photovoltaic separation of electron–hole pairs and (b) the extracted external quantum efficiency (EQE) under different light power and wavelengths, reproduced with permission from Ref. [35], Copyright 2013 Nature Publishing Group. (c) The schematic diagram of vertical p-n junction made by MoS2/WSe2 sandwiched within two graphene layers and (d) their IV characteristic in dark and illumination, reproduced with permission from Ref. [36], Copyright 2014 Nature Publishing Group. (e) Schematic of a tunneling diode based on graphene/h-BN/MoS3 heterostructure and (f) its photodetection performance when using h-BN with thickness within 1–22 nm, reproduced with permission from Ref. [18], Copyright 2016 American Chemical Society.*

In photodiodes, the photodetection gain is limited due to the maximum attainable quantum efficiency (photon-to-electron conversion efficiency) less than unity [46]. Hence, photodiodes are less sensitive and are usually operated under reverse bias or self-driven mode without external bias. In comparison, when integrating such heterostructure into a photoconductor configuration, phototransistors can be

direct tunneling, therefore less detectivity in photodetection.

**4.2 Heterostructured phototransistors**

*DOI: http://dx.doi.org/10.5772/intechopen.88433*

*Emerging Artificial Two-Dimensional van der Waals Heterostructures for Optoelectronics DOI: http://dx.doi.org/10.5772/intechopen.88433*

#### **Figure 4.**

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

band heterostructures is limited by the interfacial bandgap determined by the lower conduction and higher valance band. It is therefore usually less than the maximum *V*oc attainable in p-n junction of each component. However, an essential benefit of such heterostructure is the formation of photodiode without deliberate efforts in controlling the p and n-type doping. A strong evidence of the formation of type II band alignment is that the photoluminescence at the junction area is quenched due to the separation of electron–hole pairs at the interface. Another benefit of such type II heterostructure is based on the interlayer transition, which supports sub-bandgap photodetection [42]. For example, MoS2/WS2 heterojunction displays near-infrared

*illumination (550 nm), reproduced with permission from Ref. [19], Copyright 2018 Wiley-VCH.*

*2D Heterostructures of different interfacial band alignments and their characteristics. (a) The type I band alignment between BP and WSe2 and (b) the appearance of various junction behaviors (p-p, p-n, n-n) under gate modulation, reproduced with permission from Ref. [38], Copyright 2017 Wiley-VCH. (c) Schematic of the type II heterostructure based on n-type MoS2 and p-type GaTe, and (d) the photovoltaic characteristic under light irradiation, reproduced with permission from Ref. [37], Copyright 2015 American Chemical Society. (e) Schematic diagram of a type III WSe2/SnS2 heterostructure and (f) its* IV *characteristic under dark and light* 

Tunneling diodes can be formed by heterostructures of type III band alignment [24]. In the case of WSe2/SnS2 heterostructure, due to the high electron affinity of SnS2, type III heterostructure is formed with direct interband transition between valance band of WSe2 and the conduction band of SnS2 [19]. The diode initially

whereas under illumination, the device exhibits dramatically increased light current

tivity >1013 Jones. Further exploration of the kind of heterostructure using other 2D materials with different bandgap may have the potential to make high-performance tunneling photodiodes for infrared. The heterostructure of narrow bandgap BP and larger bandgap MoS2 has been used to realize multi-value inverters with high gains

In addition to the two-layer stacking, multilayered heterostructures have been also developed as tunneling diodes. **Figure 4a** illustrates such a heterostructure based on vertically stacked graphene/MoS2/graphene [35]. Because of the work function between top and bottom graphene (due to the unidentical substrate doping effect), the multilayer displayed photovoltaic separation of electron–hole pairs under illumination, reaching a *V*oc~0.3 V under additional gate bias. The device also exhibits wavelength-dependent responsivity that is related to the absorption in

for low dark current under reverse bias,

and excellent detec-

response that is beyond both the bandgap limits of MoS2 and WS2 [43].

by direct tunneling, resulting in high responsivity >200 AW<sup>−</sup><sup>1</sup>

>150 based on gate-modulated tunneling current [44].

displayed high rectification ratio >104

**154**

**Figure 3.**

*Various kinds of vertical heterostructures. (a) Schematic of Gr/MoS2/Gr heterojunction, which displays photovoltaic separation of electron–hole pairs and (b) the extracted external quantum efficiency (EQE) under different light power and wavelengths, reproduced with permission from Ref. [35], Copyright 2013 Nature Publishing Group. (c) The schematic diagram of vertical p-n junction made by MoS2/WSe2 sandwiched within two graphene layers and (d) their IV characteristic in dark and illumination, reproduced with permission from Ref. [36], Copyright 2014 Nature Publishing Group. (e) Schematic of a tunneling diode based on graphene/h-BN/MoS3 heterostructure and (f) its photodetection performance when using h-BN with thickness within 1–22 nm, reproduced with permission from Ref. [18], Copyright 2016 American Chemical Society.*

MoS2 (as indicated in **Figure 4b**), demonstrating the working principle of the multilayer junction. By using graphene at both the bottom and top of the junction made by MoS2 and WSe2, Lee et al. demonstrated an efficient p-n junction at the ultimate atomic thin thickness with improved collection of photoexcited carriers [36]. **Figure 4c** and **d** illustrate the structure and *IV* characteristic of the device. Under illumination, tunneling-assisted interlayer recombination of the majority carriers dominates the electronic and optoelectronic behavior of the junction. Alternatively, sandwiching graphene within WSe2 and MoS2 can make a broadband photodetector up to 2 μm based on the absorption of graphene [45]. Such interlayer tunneling can be suppressed by inserting an insulating h-BN layer. Vu et al. fabricated a tunneling heterostructure based on graphene/h-BN/MoS2 in the configuration shown in **Figure 4e** [18]. The dark current in device is greatly suppressed by blocking direct tunneling. However, under illumination, photogenerated carriers may overcome the barrier and contribute significant photocurrent via Fowler-Nordheim (FN) tunneling. Notably, to balance the photodetection performance in terms of the responsivity and detectivity, the thickness of h-BN is optimal ~5–7 nm, as indicated in **Figure 4f**. The thicker the h-BN layer, the lower probability of FN tunneling and thus lesser photocurrent, while the thinner h-BN results in large dark current by direct tunneling, therefore less detectivity in photodetection.

#### **4.2 Heterostructured phototransistors**

In photodiodes, the photodetection gain is limited due to the maximum attainable quantum efficiency (photon-to-electron conversion efficiency) less than unity [46]. Hence, photodiodes are less sensitive and are usually operated under reverse bias or self-driven mode without external bias. In comparison, when integrating such heterostructure into a photoconductor configuration, phototransistors can be

made with high sensitivity based on the photoconductive gain and vertical photovoltaic effects. The photodetection gain originates from the separation of electron– hole pairs at the heterostructure interface, with one kind of carrier accumulated in the 2D high mobility channel, therefore yielding amplified photoconductive gains by the ratio of injected charges compared to the inherent carrier concentration in 2D channel [47]. A representative example is PbS quantum dot (QD)-sensitized graphene, in which the QDs and 2D surface are coupled by vdW interaction (**Figure 5a**) [48]. Upon illumination, holes are injected into graphene and transport there with dramatically increased mobility compared to QDs that have large amount of grain boundaries and surface states. In this way, ultrahigh responsivity >107 A/W has been demonstrated in such hybrid photodetectors. Notably, based on the gatemodulated Fermi level in graphene, the charge injection from PbS QDs to graphene can be extensively tailored. As indicated in **Figure 5b**, the attained responsivity is sensitive to the applied gate bias; under *V*g = 4 V, the photoresponse gain is tuned even to zero by eliminating the interfacial charge transfer. Such widely tuned gain is potentially useful for intentionally selected sensitivity levels for a detector. However, due to the zero-bandgap nature of graphene, hybrid detectors with graphene as the channel exhibit large dark current and low detectivity. Alternatively, other 2D semiconductors, such as MoS2 and WSe2, have been also explored as the channel, yielding improved on–off ratio in detector [47, 49].

In addition to colloidal quantum dots, 2D heterostructures based on vertically stacked 2D layers can also make up phototransistors. A narrow bandgap semiconductor can be stacked on another 2D material for extended photodetection spectra. As illustrated in **Figure 5c**, BP is stacked on a WSe2 channel [17]. The photoexcited carriers in BP by near-infrared photons are separated by the type II interface, with

#### **Figure 5.**

*Phototransistors based on various heterostructures. (a) The schematic of PbS quantum dots sensitized graphene for infrared photodetection; (b) the back-gate-modulated responsivity of the hybrid photodetector, reproduced with permission from Ref. [48], Copyright 2012 Nature Publishing Group. dependence of the responsivity on the different wavelength. (c) Configuration of a vertically stacked BP/WSe2 heterostructure and (d) its wavelength-dependent gain and detectivity, reproduced with permission from Ref. [17], Copyright 2017 Elsevier Ltd. (d) Detectivity of various photodetector versus wavelength of the incident laser. (e) Illustration of the organic/inorganic vdW heterostructured phototransistor based on ZnPc-decorated MoS2 and (f) its photoresponse behavior, which is greatly improved compared to photoconductors that suffer persistent photoconductance, reproduced with permission from Ref. [16], Copyright 2018 American Chemical Society.*

**157**

*Emerging Artificial Two-Dimensional van der Waals Heterostructures for Optoelectronics*

electrons injected to WSe2. The amount of injected charge is related to the junction capacitance and the photovoltage built across the junction. In **Figure 5d**, the

larger than the photodiodes (<1) by the amplification mechanism in phototransis-

measured wavelength (400–1500 nm) range. Longer wavelength results in low gain and detectivity due to the decrease of light absorption. Instead of BP, a lot of other 2D materials has been also explored to construct such heterostructured phototransistor, in which the photovoltaic separation of photocarriers can be used to gate the

Without complicated stacking processes, 2D vdW heterostructures can be also made by combining organic small molecules with 2D material. As illustrated in **Figure 5e**, Huang et al. recently reported such a vdW phototransistor based on Zinc phthalocyanine (ZnPc, a π-conjugated planar molecule)-decorated monolayer MoS2, which is achieved by simple solution treatment [16]. The formed junction displayed apparent rectification characteristic at the out-plane direction by forming type II band alignment and p-n junction. As a result, the detector displayed remarkably improved response speed and optimal responsivity (**Figure 5f**) with proper Al2O3 passivation. Other molecules, such as pentacene, have been also used to modify the performance of 2D semiconductors (MoS2, ReS2, etc.) in addition to response dynamics but also the response spectra [50, 51]. Considering the huge library of 2D materials and organic molecules, it is believed such hybrid heterostructure holds special promise in achieving scalable high-performance photodetections, in which using existing pick-transfer procedures is apparently challenging.

Optoelectronic memory can transform incident optical signals into stored electric charges [52]. Considering the light program signals can be free from interferences, the optoelectronic memories are particularly attractive for realizing high-throughput data storage, e.g., in parallel computing [53]. A typical optoelectronic memory is consisted of light sensing part and charge storage component, which could be feasibly realized using multilayered 2D stacking. Compared to the conventional 3D counterparts, the 2D devices have the advantages in having high on–off ratio by the ultrathin channel, the conductance of which can be feasibly modulated via slight amount of trapped charges. According to the charge trapping mechanism, in the following we describe two kinds of optoelectronic memories, based on, respectively,

The ultrathin nature makes 2D semiconductors highly suitable as the readout

channel in memory, as their conductance can be modulated greatly by slight charge trapping, including by the inherent trap states in devices. In literatures, the prepared MoS2 often exhibits midgap trap states [54], and the device also suffers from interface defect states, e.g., at the interface with SiO2 [55], which may capture some charges under gate modulation by the shifted Fermi level EF (the trap states below EF are prone to be filled with electrons, while those states above EF tend to be empty). This usually results in large hysteresis in field-effect devices and different conduction states after positive and negative gate stress. However, the limited density of trap states restricted the on–off switch in memory. Lee et al. reported an improved device by introducing localized electronic states in MoS2 using tailored

tor. Therefore, the specific detectivity of the device reaches 1010–1014

semiconductor channel and amplify the photoconductive gain.

the charge trapping in (i) defect energy states or (ii) float gates.

at 1500 nm, which is considerably

Jones at the

*DOI: http://dx.doi.org/10.5772/intechopen.88433*

**5. Optoelectronic memories**

**5.1 Charge trapping in defect levels**

photodetection gain in such device reaches 102

## *Emerging Artificial Two-Dimensional van der Waals Heterostructures for Optoelectronics DOI: http://dx.doi.org/10.5772/intechopen.88433*

electrons injected to WSe2. The amount of injected charge is related to the junction capacitance and the photovoltage built across the junction. In **Figure 5d**, the photodetection gain in such device reaches 102 at 1500 nm, which is considerably larger than the photodiodes (<1) by the amplification mechanism in phototransistor. Therefore, the specific detectivity of the device reaches 1010–1014 Jones at the measured wavelength (400–1500 nm) range. Longer wavelength results in low gain and detectivity due to the decrease of light absorption. Instead of BP, a lot of other 2D materials has been also explored to construct such heterostructured phototransistor, in which the photovoltaic separation of photocarriers can be used to gate the semiconductor channel and amplify the photoconductive gain.

Without complicated stacking processes, 2D vdW heterostructures can be also made by combining organic small molecules with 2D material. As illustrated in **Figure 5e**, Huang et al. recently reported such a vdW phototransistor based on Zinc phthalocyanine (ZnPc, a π-conjugated planar molecule)-decorated monolayer MoS2, which is achieved by simple solution treatment [16]. The formed junction displayed apparent rectification characteristic at the out-plane direction by forming type II band alignment and p-n junction. As a result, the detector displayed remarkably improved response speed and optimal responsivity (**Figure 5f**) with proper Al2O3 passivation. Other molecules, such as pentacene, have been also used to modify the performance of 2D semiconductors (MoS2, ReS2, etc.) in addition to response dynamics but also the response spectra [50, 51]. Considering the huge library of 2D materials and organic molecules, it is believed such hybrid heterostructure holds special promise in achieving scalable high-performance photodetections, in which using existing pick-transfer procedures is apparently challenging.
