Mos2 Based Photodetectors: a Review

Review MoS2 Based Photodetectors: A Review

Alberto Taffelli *, Sandra Dirè , Alberto Quaranta and Lucio Pancheri

Department of Industrial Engineering, University of Trento, Via Sommarive 9, 38123 Trento, Italy; [email protected] (S.D.); [email protected] (A.Q.); [email protected] (L.P.) * Correspondence: [email protected]

Abstract: Photodetectors based on transition metal dichalcogenides (TMDs) have been widely

reported in the literature and molybdenum disulﬁde (MoS2) has been the most extensively explored for photodetection applications. The properties of MoS2, such as direct band gap transition in low dimensional structures, strong light–matter interaction and good carrier mobility, combined with

the possibility of fabricating thin MoS2 ﬁlms, have attracted interest for this material in the ﬁeld of optoelectronics. In this work, MoS2-based photodetectors are reviewed in terms of their main performance metrics, namely responsivity, detectivity, response time and dark current. Although neat

MoS2-based detectors already show remarkable characteristics in the visible spectral range, MoS2 can be advantageously coupled with other materials to further improve the detector performance

Nanoparticles (NPs) and quantum dots (QDs) have been exploited in combination with MoS2 to boost the response of the devices in the near ultraviolet (NUV) and infrared (IR) spectral range. Moreover, heterostructures with different materials (e.g., other TMDs, Graphene) can speed up the response of the photodetectors through the creation of built-in electric ﬁelds and the faster transport of charge carriers. Finally, in order to enhance the stability of the devices, perovskites have been exploited both as passivation layers and as electron reservoirs. Keywords: MoS2; TMD; photodetector; heterostructure; thin ﬁlm Citation: Taffelli, A.; Dirè, S.;

Quaranta, A.; Pancheri, L. MoS2 Based Photodetectors: A Review. Sensors 2021, 21, 2758. https:// 1. Introduction doi.org/10.3390/s21082758 Recent improvements in optoelectronics have been partly focused on the use of two- dimensional materials to produce photodetectors. The possibility of fabricating very thin Academic Editor: optoelectronic devices, having high performance, low production costs and mechanical Guillermo Villanueva flexibility has been emerging in the last decade. Graphene was the first 2D material con- sidered for photodetection applications, thanks to its outstanding electrical properties, in Received: 9 March 2021 particular its impressive planar mobility, reaching 200,000 cm2/(V s), that allows to build Accepted: 8 April 2021 Published: 14 April 2021 photodetectors with bandwidth up to 40 GHz [1,2]. However, one of its mayor limitation for its use as photodetector active layer is the absence of an energy band gap, leading to high

Publisher’s Note: MDPI stays neutral noise contribution to the signal, arising from dark currents. with regard to jurisdictional claims in Therefore, the investigation of 2D materials with finite bandgap has increased in published maps and institutional affil- recent years and transition metal dichalcogenides (TMDs) have aroused more and more iations. interest. Despite the modest mobility reported for these materials, which can reach about 200 cm2/(V s) [3], TMDs possess interesting electro-optical properties. A transition from indirect to direct bandgap has been observed in TMDs by reducing the dimensions from the bulk material to the monolayer limit [4]. Moreover, a strong light–matter interaction is observed for 2D-TMDs, due to the direct band gap and to the strong excitonic nature Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. of their low dimensional structures. For TMDs, absorbance values that are one order of This article is an open access article magnitude higher than Si and GaAs are reported [5], thus providing strong light absorption distributed under the terms and with a very thin layer of the photoactive material. These features, combined with a higher conditions of the Creative Commons mechanical flexibility of 2D-TMDs compared to their bulk structures, allow us to fabricate Attribution (CC BY) license (https:// very thin photodetectors also based on flexible substrates, opening the possibility to realize creativecommons.org/licenses/by/ flexible and wearable devices. Applications for such devices can be related to medicine, 4.0/). biosensing, optical communications and security.

Sensors 2021, 21, 2758. https://doi.org/10.3390/s21082758 https://www.mdpi.com/journal/sensors Sensors 2021, 21, x FOR PEER REVIEW 2 of 24

very thin photodetectors also based on flexible substrates, opening the possibility to realize flexible and wearable devices. Applications for such devices can be related to medi- Sensors 2021, 21, 2758 cine, biosensing, optical communications and security. 2 of 22 Among the TMDs, molybdenum disulfide (MoS2) attracted much interest in the last decade,

also due to the relative abundance of molybdenite in nature. MoS2 belongs to the family of

the groupAmong VI transition the TMDs, metal molybdenum dichalcogenides, disulﬁde where (MoS a 2layer) attracted of transition much interestmetal atoms in the (Mo, last decade, also due to the relative abundance of molybdenite in nature. MoS belongs to the W) is sandwiched between two layers of chalcogen atoms (S, Se, Te) as depicted2 in Figure family of the group VI transition metal dichalcogenides, where a layer of transition metal 1.atoms (Mo, W) is sandwiched between two layers of chalcogen atoms (S, Se, Te) as depicted in Figure1.

Figure 1. Transformation of the hexagonal 2H polymorph of MoS2 into its 1T phase, through electron transfer. Reprinted with permission from ref. [6]. © 2017 AIP Publishing. Figure 1. Transformation of the hexagonal 2H polymorph of MoS2 into its 1T phase, through electron transfer. Reprinted with permission from ref. [6]. © 2017Each AIP MoS Publishing.2 layer is generally stacked onto each other via weak van der Waals forces in an ABA stacking sequence, building a hexagonal structure (2H-TMD), which possesses a semiconductingEach MoS2 layer behaviour. is generally Another stacked metastable onto each phase other of MoS via2 weakis known, van der with Waals a tetragonal forces insymmetry an ABA stacking (1T-MoS sequence,2) and ABC building stacking a hexagonal sequence (Figure structure1). The(2H-TMD), 1T phase which of MoS possesses2 is not astable semiconducting at room temperature behaviour. [7 ],An butother it can metastable be induced phase by several of MoS processes2 is known, such with as a chemical tetrag- onaltreatment symmetry [8], plasmonic (1T-MoS2) hot and electron ABC stacking transfer sequence [9], electron (Figure beam 1). irradiation The 1T phase [6,10 of], throughMoS2 is notcharge stable transfer at room in thetemperature TMD lattice. [7], Abut subsequent it can be induced annealing by processseveral isprocesses then required such as to chemicalrestore the treatment 2H-MoS [8],2 phase plasmonic [6]. hot electron transfer [9], electron beam irradiation [6], [10], throughBulk TMD charge electronic transfer properties in the TMD are dominatedlattice. A subsequent by indirect annealing transition process from the is maxi-then requiredmum of to the restore valence the band, 2H-MoS located2 phase at the [6]Γ. point of the Brillouin zone, and the minimum of theBulk conduction TMD electronic band [ 11properties–13]. For are MoS dominat2, the bulked by electronic indirect structuretransition isfrom characterized the maxi- mumby an of indirect the valence energy band, band located gap of at about the Γ 1.2point eV of [14 the]. AsBrillouin with other zone, group and the VI minimum TMDs, at ofthe the monolayer conduction limit band MoS [11–13].2 modifies For MoS its energy2, the bulk band electronic structure structure towards is a characterized direct electronic by antransition indirect fromenergy the band K and gap K’ pointsof about of 1.2 the eV Brillouin [14]. As zone, with reaching other group an energy VI TMDs, band gapat the of monolayer1.8 eV [14, 15limit]. This MoS behaviour2 modifies its can energy be explained band structure by an increase towards in a thedirect indirect electronic band tran- gap sitiondue to from a considerable the K and K’ quantum points of confinement the Brillouin effect zone, in reaching the out-of-plane an energy direction band gap when of 1.8 the eVdimensions [14,15]. This of thebehaviour material can are be reduced explained to few by an layers. increase On thein the other indirect hand, band the direct gap due transi- to ation considerable remains unaffected, quantum confinement becoming the effect minimum in the energeticout-of-plane band-to-band direction when transition the dimen- [11–13] sions(Figure of2 thea) . material Moreover, are TMDs reduced are reported to few layers. to have On strong the other spin-orbit hand, coupling the direct (SOC), transition associ- remainsated with unaffected, the d-orbitals becoming of transition the minimum metals [6,16 energetic,17]. The band-to-band SOC breaks the transition degeneracy [11–13] in the (Figurevalence 2a). band, Moreover, leading TMDs to two are energetic reported maxima to have located strongat spin-orbit the K and coupling K’ points, (SOC), separated asso- ciatedby an with energy the splittingd-orbitals of of 160 transition meV for metals a monolayer [6,16,17]. MoS The2 SOC[16] breaks (Figure the2b). degeneracy This broken in degeneracy opens for MoS2 the possibility for optoelectronic applications in the field of valleytronics [18,19].

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the valence band, leading to two energetic maxima located at the K and K’ points, sepa- therated valence by an band, energy leading splitting to two of energetic160 meV formaxima a monolayer located MoSat the2 [16]K and (Figure K’ points, 2b). This sepa- broken Sensors 2021, 21, 2758 3 of 22 rateddegeneracy by an energy opens splitting for MoS of 1602 the meV possibility for a monolayer for optoelectronic MoS2 [16] (Figure applications 2b). This in broken the field of degeneracyvalleytronics opens [18,19]. for MoS 2 the possibility for optoelectronic applications in the field of valleytronics [18,19].

(a) (b) (a) (b) FigureFigure 2. Energetic 2. Energetic band band structure structure of of MoS MoS2.2 (.(a)a Transition) Transition from from indirect indirect to direct bandband gapgap moving moving from from the the bulk bulk MoS MoS2 to2 to Figure 2. Energetic band structure of MoS2. (a) Transition from indirect to direct band gap moving from the bulk MoS2 to thethe single single layer layer of ofMoS MoS2. Reprinted2. Reprinted with with permission permission from from ref. ref. [15]. [15]. ©© 2010,2010, American American ChemicalChemical Society.Society. ( b(b)) Degeneracy Degeneracy at at the single layer of MoS2. Reprinted with permission from ref. [15]. © 2010, American Chemical Society. (b) Degeneracy at thethe K point K point in inthe the valence valence band band of of a asingle single MoS MoS2 layer.layer. Reprinted Reprinted with with permission permission fromfrom ref.ref. [ 6[6].]. © © 2017, 2017, AIP AIP Publishing. Publishing. the K point in the valence band of a single MoS2 layer.2 Reprinted with permission from ref. [6]. © 2017, AIP Publishing.

WhenWhen MoS MoS2 is2 isirradiated irradiated with with photon photon energies energies larger larger than than its its bandgap, bandgap, photons photons are When MoS2 is irradiated with photon energies larger than its bandgap, photons are are absorbed and electrons are promoted to the conduction band, leaving holes in the absorbedabsorbed and and electrons electrons are arepromoted promoted to the to cothenduction conduction band, band, leaving leaving holes holes in the in valence the valence valence band. The optical absorption of visible light2 by MoS2 in the monolayer limit is band.band. The The optical optical absorption absorption of visible of visible light light by MoS by MoS2 in the in monolayer the monolayer limit limitis dominated is dominated dominated by the direct transition from the K and K’ points of the valence band. However, byby the the direct direct transition transition from from the theK and K and K’ points K’ points of the of valence the valence band. band. However, However, light ab-light ab- light absorption experiments show peculiar resonant features in 2D structures of MoS2 sorptionsorption experiments experiments show show peculiar peculiar resonant resonant features features in 2D in structures 2D structures of MoS of2 MoS(Figure2 (Figure 3), 3), (Figure3), that can be associated with its strong excitonic nature [ 4,15]. The experimentally thatthat can can be beassociated associated with with its strong its strong excitonic excitonic nature nature [4,15]. [4,15]. The experimentallyThe experimentally observed observed observed absorption peaks at speciﬁc energies (EA = 1.88 eV, EB = 2.03 eV in the monolayer absorptionabsorption peaks peaks at specificat specific energies energies (EA (EA = 1.88 = 1.88eV, EBeV, = EB 2.03 = eV2.03 in eV the in monolayer the monolayer limit limit limit [20]) represent the excitonic energies of MoS2. The relative positions of the A and [20]) represent the excitonic energies of MoS2 2. The relative positions of the A and B peaks [20])B represent peaks are the related excitonic both energies to an increase of MoS . in The the relative SOC and positions to a reduction of the A and in the B peaks bandgap, areare related related both both to anto increasean increase in the in SOCthe SOC and anto da reductionto a reduction in the in bandgap, the bandgap, approaching approaching approaching the bulk structure of MoS2 [20]. thethe bulk bulk structure structure of MoSof MoS2 [20].2 [20].

Figure 3. Absorption spectrum of MoS varying the number of layers from 1 layer (1L) to few layers Figure 3. Absorption spectrum of MoS2 varying2 the number of layers from 1 layer (1L) to few lay- ersFigure (FL),(FL), where where3. Absorption A A and and B B represen spectrum representt the theof excitonicMoS excitonic2 varying peaks peaks the of of MoSnumber MoS22.. Reprinted Reprinted of layers with with from permission permission 1 layer (1L) from from to few ref. [lay-20]. ref.ers [20].2014, (FL), 2014, Creative where Creative A Commons and Commons B represen Attribution Attributiont the 3.0excitonic Unported 3.0 Unported peaks Licence. of Licence. MoS2. Reprinted with permission from ref. [20]. 2014, Creative Commons Attribution 3.0 Unported Licence. For these reasons, in 2D-MoS2, the bandgap measured with optical techniques turns out to be lower than the one measured with electronic techniques [6]. The strong light–matter interaction that characterizes MoS is reflected in a high absorption coefficient that can reach 2 about 106/cm [21], which is at least one order of magnitude higher than standard semicon-

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ductors like Si and GaAs. Moreover, a single MoS2 layer is reported to absorb up to 10% of the sunlight [5]. This superior light absorption makes MoS2 suitable to build photodetectors based on very thin layers of material, still having high light conversion efficiency. Therefore, the production of thin MoS2 films is a central step of the device fabrication and several approaches have been investigated so far. Thanks to the weak van der Waals interaction between MoS2 layers, it is possible to obtain mono- or few-layered structures by a simple mechanical exfoliation of bulk MoS2 [22]. This approach has been extensively used in many studies [23–36] for its intrinsic simplicity, but suffers from some critical issues. In fact, MoS2 mechanical exfoliation generally leads to very small flakes (lateral size < 10 um)[37] and it is a low yield process. Therefore, it is unsuitable for industrially scalable-applications [38]. Other synthesis methods have been investigated to overcome these critical aspects. Chemical vapour deposition (CVD) is a powerful bottom-up approach. This method is the most compatible with the existing semiconductor technology. In CVD, large area films with high uniformity can be grown directly on the substrate, through the chemical reactions involved in the process. Despite the high controllability of the process and the high uniformity that can be achieved, CVD has the limit of being a costly process and requires high temperatures (700–1000 ◦C), making the process less affordable and not suitable for deposition on flexible substrates. Moreover, in most of the CVD processes used to produce MoS2, one of the precursors involved is H2S, which is toxic. As an alternative to standard CVD, plasma-enhanced CVD (PECVD) has also been exploited in order to reduce the temperature needed for the reaction (150–300 ◦C), thus allowing film deposition also on plastic substrates [39]. Among the bottom-up approaches, wet chemical syntheses can also be exploited for the fabrication of large area MoS2 films on different substrates. MoS2 sols can be prepared both at ambient pressure or under hydrothermal [40] or solvothermal [41] synthesis conditions. The sols can be used for coating different substrates by spin-coating or dip- coating. Generally, an annealing process (at 500–800 ◦C) is required in order to improve the crystallinity of the sample and an additional sulphurisation step is needed. Recently, Nardi et al. [42] have obtained MoS2 thin layers on Pt, SiO2 and flexible polyimide substrates by the sol–gel approach, using an aqueous sol prepared at ambient pressure; the coatings were annealed at low temperature (350–400 ◦C) without any additional sulphurisation process. The advantages of the solution methods are represented by the versatility of the deposition technique, the low costs of production and the process scalability. The drawbacks with respect to CVD are instead the lower uniformity of the film and its minimum thickness, which is generally limited to some tens of nanometres. The fabrication methods cited above generally lead to n-type behaviour of MoS2. The n-type character of pristine MoS2 is commonly associated to the electron donor nature of the sulphur atoms [43]. In order to exploit the full potential of MoS2 and to build p–n junctions, doping is required to tune the energetic levels at the interface of MoS2 with other materials. Doping through standard ion implantation is not suitable for 2D materials, and other methods have been investigated in the literature. The most common methods for doping MoS2 rely on substitutional doping and surface doping, beside the electrostatic gating. Substitutional doping consists of the substitution of a sulphur atom with an impurity within the MoS2 lattice. Niobium (Nb) substitutional p-doping has been reported in the literature by both chemical vapour transport (CVT) [43] and CVD [44]. Moreover, laser assisted substitutional phosphorous (P) p-doping was reported in [45] and manganese (Mn) substitutional p-doping has been investigated in [46], via a vapour phase deposition technique. Finally, p-type substitutions with fluorine (F) and oxygen (O) were obtained via plasma assisted doping by [47]. On the other hand, surface doping exploits the difference between the electron surface potential of MoS2 and the redox potential of the chemically adsorbed species. Nicotinamide adenine dinucleotide (NADH) has been reported to cause an n-doping to MoS2, while tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) have been exploited for p-doping [48]. Other Sensors 2021, 21, 2758 5 of 22

molecules exploited as p-dopands for TMDs are O2,H2O and NO2, while potassium (K), benzyl viologen (BV) and bis(triﬂuoromethane) sulfonamide (TFSI) have been used for n-doping [6].

2. MoS2 Photodetectors

The strong light absorption of MoS2, combined with its good mobility and the possibility to fabricate very thin layers, led in the last decade to a great interest in this material for photodetection applications. In all photodetectors based on semiconductors, photons with energy larger than the material bandgap are absorbed and generate electron–hole pairs that can move under the action of an electric field. Devices may rely on different physical mechanisms for what concerns the charge drift and collection, giving rise to different photodetector categories. Most light detectors can be grouped into three classes: photoconductors, phototransistor and photodiodes. This review summarizes the different photodetector structures based on MoS2 presented so far. In photoconductors, the radiation creates electron–hole (e–h) pairs, which are then separated by an external applied bias voltage (Figure4). The charges drift towards the electrodes where they are collected, producing a photocurrent. The mechanism beneath the signal detection is the photoconduction, namely incident photons cause an increase in the charge density and thus in the conductivity of the material. Moreover, a mechanism called photoconductive gain can be exploited in photoconductors to enhance the signal level. The gain is defined as G = τ/t, where τ is the lifetime of one of the charge carriers (e.g., holes) and t is the transit time of the opposite carriers (e.g., electrons). A gain arises when one of the charge carriers recirculate many times before it recombines with his opposite counter- part. Generally, energy states within the bandgap of the semiconductor, often induced by defects, are able to trap one of the two carriers, prolonging their lifetime and leading to multiple recirculation of the opposite carriers. The lifetime of the carriers strongly depends on the presence of trapping center within the material and can vary by several orders of magnitude, from few nanoseconds [23] to milliseconds [35]. In practice, trapping can be achieved by controlling the defects present in the material or by introducing sensitizing centres such as QDs or nanoparticles. Photoconductive gain affects the signal intensity but also its temporal response, which is governed by the carriers’ lifetime. Generally, devices relying on the photoconductive gain reach very high values of responsivity, but present slower response and consequently a lower bandwidth compared to G=1 photoconductors. On the other hand, phototransistors are able to maximize the detector performance by reducing the noise rather than enhancing the signal intensity. In addition to the electrical contacts found in the photoconductors, here called “source” and “drain”, a third terminal (“gate”) electrically isolated from the semiconductor through a thin dielectric layer is present. Gate bias is generally exploited to deplete the semiconductor channel from carriers, in order to suppress dark current signals in the detector and thus maximize its signal-to- noise ratio (SNR). Moreover, the gate also modulates the mobility of the carriers, leading to high ON/OFF ratio values and higher values of the responsivity. Photoconductors Sensors 2021, 21, x FOR PEER REVIEW 6 of 24 and phototransistors unavoidably require an external power supply to sustain a voltage difference between the electrodes, which may become significant in large area detectors.

FigureFigure 4. 4.PhotoconductorPhotoconductor scheme scheme (left (left) )and and phototransistor phototransistor scheme scheme (right).). Adapted with with permission permission from from ref. ref. [49 [49].]. CopyrightCopyright 2020, 2020, Creative Creative Commons Commons A Attributionttribution 4.0 UnportedLicence. Licence.

Photodiodes rely on the photovoltaic (PV) effect to collect charges. A built-in electric field is created at the junction between p- and n-sides of the semiconductor or by a Schottky barrier between a semiconductor and its metal contact. The built-in electric field can reach very high values in proximity of the junction, and thus the photogenerated carriers are driven to opposite contacts through an intrinsic voltage potential rather than an external power supply. Photodiodes can be composed of p–n junctions of the same material (homojunctions), of different materials (heterojunctions) or of metal–semiconductor junctions (Schottky diodes). Moreover, energy band alignment at the heterojunction can be exploited to suppress the drift of charges between the two sides of the junction, thus reducing the dark signals. Photodiodes can be arranged in a horizontal fashion, where two materials are put side by side (in-plane junctions), or vertically stacked, where they are put one on top of each other (out-of-plane junctions) (Figure 5). In the next section we will discuss a selection of photodetectors based on MoS2 reported in the literature, chosen from among the most meaningful ones.

Figure 5. In-plane p–n junction (left) and out-of-plane p–n junction (right) of 2D materials, and their respective electronic band alignment. Reprinted with permission from ref. [6]. © 2017, AIP Publishing.

2.1. Neat MoS2 Photodetectors Neat MoS2-based photoconductors are the simplest photodetectors that will be discussed in this review. They are generally composed of an insulating substrate material (e.g., SiO2, Al2O3, Si3N4) over which MoS2 is deposited. The metal contacts can be created directly on the substrate or can be deposited on the MoS2 surface by physical vapour deposition methods. The metal contacts can be designed properly to maximize the response and the speed of the device, as in the case of interdigitated finger contacts. Gonzalez et al. [27] fabricated a phototransistor, based on a mechanically exfoliated MoS2 monolayer deposited on a Si/SiO2 substrate, which reached a large photoresponsivity (R = 103 A/W) in the visible range. However, such a high response was obtained via a large photogain mechanism, arising from a very long carrier lifetime, which resulted in a response time of 13 s. Their device was also implemented into a nanophotonic circuit to test its applicability in nanophotonics (Figure 6). In fact, a single MoS2 layer was successfully applied over a waveguide to detect the waveguide losses.

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Sensors 2021, 21, 2758 6 of 22 Figure 4. Photoconductor scheme (left) and phototransistor scheme (right). Adapted with permission from ref. [49]. Copyright 2020, Creative Commons Attribution 4.0 Unported Licence.

PhotodiodesPhotodiodes rely rely on on the the photovoltaic photovoltaic (PV) (PV) effect effect to to collect collect charges. charges. A A built-in built-in electric electric fieldﬁeld is is created atat the the junction junction between between p- andp- and n-sides n-sides of the of semiconductor the semiconductor or by a or Schottky by a Schottkybarrier between barrier between a semiconductor a semiconductor and its metal and its contact. metal Thecontact. built-in The electric built-in ﬁeld electric can field reach canvery reach high very values high in values proximity in proximity of the junction, of the junction, and thus and the thus photogenerated the photogenerated carriers car- are riersdriven are todriven opposite to opposite contacts contacts through through an intrinsic an intrinsic voltage voltage potential potential rather than rather an than external an externalpower supply.power supply. Photodiodes Photodiodes can be composedcan be composed of p–n of junctions p–n junctions of the sameof the materialsame mate- (ho- rialmojunctions), (homojunctions), of different of different materials materials (heterojunctions) (heterojunctions) or of metal–semiconductor or of metal–semiconductor junctions junctions(Schottky (Schottky diodes). Moreover,diodes). Moreover, energy band energy alignment band atalignment the heterojunction at the heterojunction can be exploited can beto exploited suppress to the suppress drift of chargesthe drift between of charges the between two sides the of two the junction,sides of the thus junction, reducing thus the reducingdark signals. the dark Photodiodes signals. canPhotodiodes be arranged can in be ahorizontal arranged fashion,in a horizontal where two fashion, materials where are twoput materials side by side are (in-plane put sidejunctions), by side (in-plane or vertically junctions), stacked, or wherevertically they stacked, are put onewhere on topthey of areeach put other one (out-of-planeon top of each junctions) other (out-of-plane (Figure5). junctions) (Figure 5). InIn the the next next section section we we will will discuss discuss a selection a selection of photodetectors of photodetectors based based on MoS on MoS2 re-2 portedreported in the in theliterature, literature, chosen chosen from from among among the the most most meaningful meaningful ones. ones.

Figure 5. In-plane p–n junction (left) and out-of-plane p–n junction (right) of 2D materials, and their respective electronic Figureband alignment.5. In-plane Reprintedp–n junction with (left permission) and out-of-plane from ref. p–n [6]. ©junction 2017, AIP (right Publishing.) of 2D materials, and their respective electronic band alignment. Reprinted with permission from ref. [6]. © 2017, AIP Publishing. 2.1. Neat MoS2 Photodetectors

2.1. NeatNeat MoS MoS2-based2 Photodetectors photoconductors are the simplest photodetectors that will be dis- cussedNeat in MoS2-based this review. photoconductors They are generally are composed the simplest of anphotodetectors insulating substrate that will material be discussed(e.g., SiO in 2this, Al 2review.O3, Si3N They4) over are which generally MoS 2compis deposited.osed of an The insulating metal contacts substrate can bematerial created directly on the substrate or can be deposited on the MoS surface by physical vapour depo- (e.g., SiO2, Al2O3, Si3N4) over which MoS2 is deposited. The2 metal contacts can be created sition methods. The metal contacts can be designed properly to maximize the response directly on the substrate or can be deposited on the MoS2 surface by physical vapour dep- ositionand the methods. speed of The the device,metal contacts as in the can case be of designed interdigitated properly finger to maximize contacts. the response and theGonzalez speed of Marin the device, et al. as [27 in] the fabricated case of interdigitated a phototransistor, finger based contacts. on a mechanically exfoliated MoS monolayer deposited on a Si/SiO substrate, which reached a large pho- Gonzalez et2 al. [27] fabricated a phototransistor,2 based on a mechanically exfoliated toresponsivity (R = 103 A/W) in the visible range. However, such a high response was MoS2 monolayer deposited on a Si/SiO2 substrate, which reached a large photoresponsiv- obtained via a large photogain mechanism, arising from a very long carrier lifetime, which ity (R = 103 A/W) in the visible range. However, such a high response was obtained via a resulted in a response time of 13 s. Their device was also implemented into a nanophotonic large photogain mechanism, arising from a very long carrier lifetime, which resulted in a circuit to test its applicability in nanophotonics (Figure6). In fact, a single MoS layer was response time of 13 s. Their device was also implemented into a nanophotonic2 circuit to successfully applied over a waveguide to detect the waveguide losses. test its applicability in nanophotonics (Figure 6). In fact, a single MoS2 layer was success- A significantly faster device was produced by Tsai et al. [50], based on a few-layered fully applied over a waveguide to detect the waveguide losses. MoS2 structure obtained through a wet-synthesis approach, and deposited on a Si/SiO2 substrate. The Au electrodes were fabricated by photolithography in an interdigitated fashion (Figure7) to produce a metal–semiconductor–metal (MSM) Schottky photodiode. The detector showed very fast response to visible light (t_rise = 70 µs, t_fall = 110 µs) and a responsivity of R = 0.57 A/W. The reason for this performance is the good compromise between modest photogain and smart geometry of the device which speeds up the carrier collection. In fact, Au electrodes produced via photolitograpy in an interdigitated fashion with 8µm finger spacing (Figure7b), play an important role in diminishing the time needed to collect the carriers. Sensors 2021, 21, x FOR PEER REVIEW 7 of 24

SensorsSensors 20212021,, 2121,, 2758x FOR PEER REVIEW 77 of of 2224

A significantly faster device was produced by Tsai et al. [50], based on a few-layered MoS2 structure obtained through a wet-synthesis approach, and deposited on a Si/SiO2 substrate. The Au electrodes were fabricated by photolithography in an interdigitated fashion (Figure 7) to produce a metal–semiconductor–metal (MSM) Schottky photodiode. The detector showed very fast response to visible light (t_rise = 70 μs, t_fall = 110 μs) and a responsivity of R = 0.57 A/W. The reason for this performance is the good compromise between modest photogain and smart geometry of the device which speeds up the carrier collection. In fact, Au electrodes produced via photolitograpy in an interdigitated fashion

with 8μm finger spacing (Figure 7b), play an important role in diminishing the time Figure 6. Scheme of a phototransistor and its implementation in a nanophotonic device. Reprinted with permission from Figure 6. Scheme of a phototransistorneeded to andcollect its implementationthe carriers. in a nanophotonic device. Reprinted with permission from ref. [27]. © 2019, Springer Nature. ref. [27]. © 2019, Springer Nature.

needed to collect the carriers. (a) (b)

(a) (b) (c) (d)

2 FigureFigure 7. ((a) Scheme ofof a a photoconductor photoconductor based based on on MoS MoS2;(b;) ( opticalb) optical microscope microscope top view top view of the of device, the device, where where Au interdigitated Au inter- digitatedcontacts arecontacts clearly are visible; clearly (c) responsivityvisible; (c) responsivity and photogain and for photogain the device for over the the devi NUV-IRce over spectral the NUV-IR range; spectral (d) photoswitching range; (d) time response of the detector. Reprinted with permission from ref. [50]. © 2013, American Chemical Society.

Yore et al. [51] built an array of photodetectors based on MoS2 monolayers through a CVD deposition process, and achieved a response extending towards the ultraviolet

spectral region (λ∼400 nm). The devices exhibited a photoresponsivity of about 1 mA/W, with a fast response, t ∼ 0.5 ms. Moreover, the detectors exhibited an extremely low dark current, Id ≤ 10 fA, attributable both to a bipolar Schottky barrier between the MoS2 and the metal contacts, and to a negligible doping of the sample from charge impuritiesintroduced by the substrate through an efﬁcient deposition process. (c) (d) Lopez-Sanchez et al. [24] developed one of the first phototransistors based on MoS2, Figure 7. (a) Scheme of a photoconductorobtained through based a on mechanically MoS2; (b) optical exfoliated microscope single top layer view deposited of the device, on awhere Si/SiO2 Au inter- substrate. digitated contacts are clearlyThe visible; device (c) showedresponsivity a responsivity and photogain of 880for the A/W devi ince the over visible the NUV-IR range, spectral attributable range; also (d) to a strong photogain mechanism, which also affected the photoswitching time of the detector,

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Yore et al. [51] built an array of photodetectors based on MoS2 monolayers through a CVD deposition process, and achieved a response extending towards the ultraviolet spectral region (λ∼400 nm). The devices exhibited a photoresponsivity of about 1 mA/W, with a fast response, t ∼ 0.5 ms. Moreover, the detectors exhibited an extremely low dark current, Id ≤ 10 fA, attributable both to a bipolar Schottky barrier between the MoS2 and the metal contacts, and to a negligible doping of the sample from charge impurities introduced by the substrate through an efficient deposition process. Lopez-Sanchez et al. [24] developed one of the first phototransistors based on MoS2, obtained through a mechanically exfoliated single layer deposited on a Si/SiO2 substrate.

Sensors 2021, 21, 2758 The device showed a responsivity of 880 A/W in the visible range, attributable also8 ofto 22 a strong photogain mechanism, which also affected the photoswitching time of the detector, which was about 9 s. They reported a very low value of the dark current, Id = 2 pA, although it was achieved with a strong negative bias of the gate, Vg = −70 V. whichA was similar about device 9 s. They was reported fabricated a very by A.R. low valueKlots ofet theal. dark[29] with current, faster Id =response, 2 pA, although in the itorder was of achieved 1 ms. The with light a strong response, negative although bias of it the was gate, lower, Vg =reached−70 V. 50 A/W, due to a pho- togainA contribution similar device of was103 times. fabricated by A.R. Klots et al. [29] with faster response, in the orderAnother of 1 ms. Thelow-noise light response, device is although described it wasin [23], lower, where reached a multilayer 50 A/W, MoS due to2 structure a photogain ob- 3 contributiontained through of 10mechanicaltimes. exfoliation was deposited on Al2O3 (Figure 8). The detector exhibitedAnother a responsivity low-noise deviceof 0.12is A/W described in the invisible [23], region, where awhich multilayer dropped MoS to2 structurevery low obtainedvalues for through near infrared mechanical radiation, exfoliation attributable was deposited to the weak on Al absorption2O3 (Figure in8 ).the The IR detector spectral exhibitedregion. The a detector responsivity responsivity of 0.12 A/W was al inso the affected visible by region, a short whichcarrier droppedlifetime τ to, which very lowwas valuesdetermined for near to be infrared 1.27 ns, radiation, limiting the attributable photogain to mechanism. the weak absorption Finally, the in measured the IR spectral dark region.currents The were detector as low responsivity as 10−11 A. wasThe alsogood affected response, by acombined short carrier with lifetime the veryτ, which low noise was determinedcontribution to of be the 1.27 system, ns, limiting led to a the good photogain detectivity mechanism. of the device Finally, that the reached measured 1011 Jones dark −11 currentsin the visible were region. as low Moreover, as 10 A. low The dark good currents response, were combined achieved with with the a verylow bias low of noise the 11 contributiongate terminal of (Vg the = system, −3 V). led to a good detectivity of the device that reached 10 Jones in the visible region. Moreover, low dark currents were achieved with a low bias of the gate

terminal (Vg = −3 V).

(a) (b)

Figure 8. (a) Scheme of a phototransistor based on MoS2. (b) Responsivity and detectivity of the phototransistor over the Figure 8. (a) Scheme of a phototransistor based on MoS2.(b) Responsivity and detectivity of the phototransistor over the visible-NIRvisible-NIR spectralspectral range.range. ReprintedReprinted withwith permissionpermission fromfrom ref.ref. [[23].23]. ©© 2012,2012, JohnJohn WileyWiley andand Sons.Sons.

Gant et al.al. [[26]26] demonstrateddemonstrated aa strainstrain tunabletunable photodetectorphotodetector basedbased onon aa singlesingle MoSMoS22 layerlayer structure,structure, obtainedobtained byby mechanicalmechanical exfoliation.exfoliation. TheirTheir devicedevice displayeddisplayed twotwo toto threethree orders of magnitude magnitude of of variation variation in in the the responsi responsivityvity when when subjected subjected to a to strain a strain in the in vis- the visibleible range range (Figure (Figure 9a).9 a).The The applied applied strain strain also also caused caused a variation a variation of the of time the time response response from from80 ms 80 to ms1.5 tos. 1.5The s. variation The variation of both of magnitude both magnitude and time and response time response can be can attributed be attributed to the to the creation of trap states in the material during strain. Moreover, the measurements showed a good reproducibility when the device was tested under multiple bending cycles, demonstrating the stability of the material under strain (Figure9b). Neat MoS2 photoconductors can reach large values of responsivity when a sufﬁcient voltage bias is applied, attributable to a large photogain contribution and can achieve relatively fast response times if the detector geometry is designed properly (e.g., with interdigitated electrodes). Most of the neat MoS2 photodetectors are characterized in the visible range (400–700 nm) and only very few devices explore the response to the near ultraviolet (NUV) or near infrared (NIR) spectrum (Figure 10). The IR spectrum generally leads to limited response of the material, since the absorption edge of MoS2 lays in the red (≈660 nm) which act as a cutoff for the longer wavelength radiation. On the contrary, the UV response is expected to increase when decreasing the wavelength, as suggested by absorption experiments. Sensors 2021, 21, x FOR PEER REVIEW 9 of 24

creation of trap states in the material during strain. Moreover, the measurements showed Sensors 2021, 21, 2758 9 of 22 a good reproducibility when the device was tested under multiple bending cycles, demonstrating the stability of the material under strain (Figure 9b).

(a) (b) FigureFigure 9. 9. ((aa)) Responsivity Responsivity values values over over the the spectral spectral range range for for a a ZnS-MoS2-based ZnS-MoS2-based device device when when it it is is subject subject to to various various strains. strains. Sensors 2021, 21, x FOR PEER REVIEW 10 of 24 InIn the the inset inset the the temporal temporal response response for for different strain applications is is shown. ( (bb)) Responsivity Responsivity values values against against the the applied applied strainstrain increasing increasing the the bending bending cycle number. Reprinted with permission from ref. [[26].26]. © 2019, Elsevier.

Neat MoS2 photoconductors can reach large values of responsivity when a sufficient voltage bias is applied, attributable to a large photogain contribution and can achieve relatively fast response times if the detector geometry is designed properly (e.g., with interdigitated electrodes). Most of the neat MoS2 photodetectors are characterized in the visible range (400–700 nm) and only very few devices explore the response to the near ultraviolet (NUV) or near infrared (NIR) spectrum (Figure 10). The IR spectrum generally leads to limited response of the material, since the absorption edge of MoS2 lays in the red (≈ 660 nm) which act as a cutoff for the longer wavelength radiation. On the contrary, the UV response is expected to increase when decreasing the wavelength, as suggested by absorption experiments.

FigureFigure 10. 10.Responsivity Responsivity valuesvalues plottedplotted againstagainst thethe wavelengthwavelength forfor variousvarious MoS2-basedMoS2-based photodetectors.photodetectors. 2.2. MoS + QDs Based Photodetectors 2.2. MoS22 + QDs Based Photodetectors ToTo enhanceenhance thethe photodetectorphotodetector responseresponse bothboth to NUV and near to mid infrared (NIR- (NIR- MIR) light spectrum, the MoS2 layer is combined with sensitized centres, namely quantum MIR) light spectrum, the MoS2 layer is combined with sensitized centres, namely quantum dotsdots (QDs)(QDs) oror nanoparticlesnanoparticles (NPs).(NPs). DevicesDevices belongingbelonging toto this category generally show high responsivity values also in regions outside the visible spectrum (Figure 10), but are also responsivity values also in regions outside the visible spectrum (Figure 10), but are also characterized by slower response, caused by the trap states introduced by the sensitizing characterized by slower response, caused by the trap states introduced by the sensitizing centres, which act as impurities. centres, which act as impurities. A large area carbon QD-MoS2-based photoconductor was reported by [52] following a hydrothermal method to grow MoS2 on a paper substrate. The device is sensitive in the NUV with a response of 8.4 mA/W to a 365 nm light and an external quantum efficiency (EQE) of 3% for the same wavelength. Carbon QDs possess a higher band-gap compared to MoS2, which is suitable for UV detection (Figure 11a). A response time of 0.57 s was reported for this structure, making the device not applicable for high frequency operations. Due to the intrinsic flexibility of the paper substrate, the device was also tested under multiple bending cycles, revealing no appreciable variation of the light response (Fig- ure 11b), thus demonstrating the possibility to build flexible and wearable electronics. As for other photoconductors, the dark current approached high values (Id = 10−6 A), limiting the sensitivity of the device.

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A large area carbon QD-MoS2-based photoconductor was reported by [52] following a hydrothermal method to grow MoS2 on a paper substrate. The device is sensitive in the NUV with a response of 8.4 mA/W to a 365 nm light and an external quantum efficiency (EQE) of 3% for the same wavelength. Carbon QDs possess a higher band-gap compared to MoS2, which is suitable for UV detection (Figure 11a). A response time of 0.57 s was reported for this structure, making the device not applicable for high frequency operations. Due to the intrinsic flexibility of the paper substrate, the device was also tested under multiple bending cycles, revealing no appreciable variation of the light response (Figure 11b), thus demonstrating the possibility to build flexible and wearable electronics. As for other Sensors 2021, 21, x FOR PEER REVIEW −6 11 of 24 photoconductors, the dark current approached high values (Id = 10 A), limiting the sensitivity of the device.

(a) (b) FigureFigure 11. 11.(a ()a Electronic) Electronic band band structure structure alignment alignment for for a photodetector a photodetector based based on MoS2on MoS2 and and CQDs. CQDs. (b) Responsivity(b) Responsivity values values of theof devicethe device over theover bending the bending cycle numbers, cycle numbers, demonstrating demonstratin the stabilityg the ofstability the signal of the over signal multiple over bending multiple cycles. bending Reprinted cycles. withReprinted permission with frompermission ref. [52 ].from © 2018, ref. [52]. Elsevier. © 2018, Elsevier.

Similarly,Similarly, a ZnS-MoSa ZnS-MoS2 structure,2 structure, deposited deposited on paper, on paper, was fabricated was fabricated by [53 ]by and [53] tested and undertested UV, under visible UV, and visible NIR and light NIR (365 light nm, 554(365 nm, nm, 740 554 nm, nm, respectively). 740 nm, respectively). The responsivity The re- ofsponsivity the device of in the that device spectral in that region spectral remained region orders remained of magnitude orders of lower magnitude (10−6–10 lower−5 A/W) (10−6– than10−5 theA/W) previously than the reported previously values, reported with responsevalues, with times response in the order times of in 10 the s. Although order of the10 s. performancesAlthough the were performances low, the multiple were low, bending the multiple test also gavebending a successful test also result gave ina successful this case. resultMuch in this better case. performance has been obtained with phototransistors sensitized with nanoparticles.Much better These performance devices require has been an additional obtained power with phototransistors supply of tens of sensitized volts and arewith basednanoparticles. on toxic compoundsThese devices such require as CdSe an additi ([34]),onal PbS power ([32]) andsupply HgTe of ([tens35]), of but volts the and dark are currentbased canon toxic be reduced compounds by several such ordersas CdSe of ([34]), magnitude PbS ([32]) compared and HgTe to sensitized ([35]), but photocon- the dark ductors.current Noisecan be suppression reduced by makesseveral these orders detectors of magnitude highly compared sensitive, with to sensitized detectivity photocon- values 13 15 reachingductors. 10 Noise–10 suppressionJones. makes these detectors highly sensitive, with detectivity values reachingA CdSe 1013 sensitized–1015 Jones. MoS 2 phototransistor is reported in [34], based on an exfoliated MoS2 Abilayer CdSe oversensitized a Si/SiO MoS2 substrate2 phototransistor (Figure 12is ).reported The device in [34], showed based a responsivityon an exfoliated of × 5 2.5MoS102 bilayerA/W overat 405 a Si/SiO nm of2 incidentsubstrate light, (Figure which 12). isThe the devic higheste showed light response a responsivity of a MoS2- of 2.5 based× 105 photodetectorA/W at 405 nm in of the incident NUV spectral light, which range is to the the highest best of light our knowledge. response ofAlthough a MoS2-based the responsivityphotodetector reached in the very NUV high spectral values, range the timeto th responsee best of remainedour knowledge. as small Although as 60 ms. the The re- reasonssponsivity for suchreached performance very high have values, to bethe attributed time response to the remained n–n heterojunction as small as formed 60 ms. byThe thereasons MoS2 forbilayer such and performance the CdSe nanocrystals have to be attributed as depicted to inthe Figure n–n heterojunction12. In fact, when formed the UV by radiation hits the interface region, the electrons formed in the CdSe nanocrystals are driven the MoS2 bilayer and the CdSe nanocrystals as depicted in Figure 12. In fact, when the UV toradiation the MoS 2hitsside, the while interface the holes region, remained the elec conﬁnedtrons formed in the CdSe in the nanoparticles. CdSe nanocrystals When the are light was switched off, the electrons accumulated in the valley easily recombined and the driven to the MoS2 side, while the holes remained confined in the CdSe nanoparticles. n–nWhen barrier the preventedlight was switched the transfer off, of the carriers electr betweenons accumulated the two regions. in the valley easily recombined and the n–n barrier prevented the transfer of carriers between the two regions.

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Sensors 2021, 21, x FOR PEER REVIEW 12 of 24 Sensors 2021, 21, 2758 11 of 22

(a) (b)

Figure 12. (a) Structure of (aa )phototransistor sensitized with CdSe nanoparticles. (b) Band( balignment) between MoS2 and CdSe nanoparticles and their carrier representation when light in ON or OFF. Reprinted with permission from ref. [34]. © 2009,Figure FigureRoyal 12. 12.Society (a()a )Structure Structure of Chemistry. of of a a phototransistor phototransistor sensitized withwith CdSeCdSe nanoparticles. nanoparticles. ( b()b Band) Band alignment alignment between between MoS MoS2 and2 and CdSeCdSe nanoparticles nanoparticles and and their their carrier carrier repres representationentation when light inin ONON oror OFF.OFF. Reprinted Reprinted with with permission permission from from ref. ref. [34 [34].]. © © 2009, Royal Society of Chemistry. 2009, Royal Society of Chemistry.A phototransistor sensitive to the NIR light was reported in [32], composed by a few- layered MoS2 structure obtained through a micromechanical exfoliation, sensitized with a AA phototransistor phototransistor sensitive sensitive to to the the NIR NIR light light was was reported reported in in[32], [32 composed], composed by by a few- a colloidal solution of PbS QDs (Figure 13a). This system, although its speed is limited to a layeredfew-layered MoS2 MoSstructure2 structure obtained obtained through through a micromechanical a micromechanical exfoliation, exfoliation, sensitized sensitized with a 5 subsecondcolloidalwith a colloidal timesolution response, solution of PbS ofexhibited QDs PbS (Figure QDs a (Figure response 13a). 13 Thisa). of This system,10 A/W system, although to althougha 980 itsnm speed its light, speed isand limited is limiteda detec- to a 14 5 tivitysubsecondto aof subsecond 7 × 10 time Jones. timeresponse, Additionally, response, exhibited exhibited in a this response a case response the of 10junction of5 A/W 10 A/W to(p-PbS a 980 to aand nm 980 n-MoSlight, nm light, and2) plays anda detec- aan importantdetectivity role of in 7 determining× 1014 Jones. the Additionally, response of in the this device case the to junctionthe incident (p-PbS light. and Moreover, n-MoS ) tivity of 7 × 1014 Jones. Additionally, in this case the junction (p-PbS and n-MoS2) plays2 an plays an important role in determining the response of the device to the incident light. theimportant electrons roleare inefficiently determining transferred the response from ofPbS the QDs device to MoSto the2, incidentwhile the light. holes Moreover, remain trappedMoreover, in the the PbS electrons side, leading are efﬁciently to large transferred photogain from and PbS a slow QDs recombination to MoS2, while theof the holes car- the electrons are efficiently transferred from PbS QDs to MoS2, while the holes remain remain trapped in the PbS side, leading to large photogain and a slow recombination of riers.trapped in the PbS side, leading to large photogain and a slow recombination of the car- the carriers. riers.

(a) (b) (a) (b) FigureFigure 13. (a 13.) Representation(a) Representation of a of phototransistor a phototransistor based based on on MoS MoS2 2andand PbS PbS quantum quantum dots; ((bb)) ResponsivityResponsivity of of the the MoS MoS2/Pbs2/Pbs basedFigurebased device 13. device compared(a) Representation compared to the to thevalues of valuesa phototransistor for forthe the neat neat MoS2-based based MoS2-based on MoS device device2 and over overPbS thequantum the visible-NIR visible-NIR dots; ( spectralb) Responsivity range.range. Reprinted Reprinted of the MoS with with2/Pbs permissionbasedpermission device from fromcompared ref. [32]. ref. [ 32© to ].2014, the © 2014, values John John Wiley for Wiley the and neat and Sons. Sons.MoS2-based device over the visible-NIR spectral range. Reprinted with permission from ref. [32]. © 2014, John Wiley and Sons. TheThe main main limitation limitation of ofthese these devices devices lies lies in in the the high high gate biasbias voltagevoltage required required to to achieveachieveThe such suchmain a high alimitation high detectivity detectivity of these(Vg (Vg = −devices80 = V− 80[34], lies V Vg [ 34in], = the Vg−100 high = V− [32]),100 gate V and [bias32]), the voltage and intrinsic the required intrinsic toxicity to of achievecadmiumtoxicity such of and cadmium a highlead. detectivity The and strong lead. (Vg The gate = strong − modulati80 V gate[34], modulation onVg required= −100 V required [32]),can be and attributed can the be intrinsic attributed to the toxicity tofor- the formation of the density of the states within the MoS bandgap. mationof cadmium of the density and lead. of theThe states strong within gate modulatithe MoS2on bandgap. required2 can be attributed to the for- mationSome of efforts the density have beenof the done states to within reduce the the MoS required2 bandgap. gate bias, as in the MoS2 photo- transistorSome sensitized efforts have with been Hg-Te done nanopartic to reduceles the reported required by gate [35]. bias, The asdetector in the MoSshowed2 photo- an extremelytransistor high sensitized responsivity with Hg-Te(105–10 nanopartic6 A/W) overles a reportedvery broad by li [35].ght spectrum,The detector ranging showed from an extremely high responsivity (105–106 A/W) over a very broad light spectrum, ranging from

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Sensors 2021, 21, x FOR PEER REVIEW 13 of 24 Some efforts have been done to reduce the required gate bias, as in the MoS2 phototransistor sensitized with Hg-Te nanoparticles reported by [35]. The detector showed an extremely high responsivity (105–106 A/W) over a very broad light spectrum, ranging from visiblevisible to mid to mid infrared infrared (2000 (2000 nm), nm), with with a fast a fast response response time time in the in theorder order of milliseconds. of milliseconds. SuchSuch high high responsivity responsivity was was achieved achieved throug throughh a high a high photogain photogain mechanism, mechanism, originated originated fromfrom a large a large hole hole lifetime lifetime (τ = ( τ4 =ms) 4 ms) induced induced by the by thepresence presence of QDs, of QDs, which which was was six sixorders orders of magnitudeof magnitude larger larger than than the the transit transit time time of of the the electrons electrons (t (t = = 9 9 ns). ns). The The detectivity detectivity meas- measure- 12– 13 urementsments showedshowed valuesvalues of of about about 10 1012–101013 JonesJones overover thethe same same spectral spectral range range (Figure (Figure 14 14),), also alsodue due to to the the low low measured measured value value of the of darkthe dark current current (≈10 (≈ pA).10 pA). In this In device,this device, a toxic a elementtoxic element(Hg) is(Hg) still is present, still present, but the but detector the detector was operatedwas operated at a gateat a gate bias bias of − of15 − V15 in V depletion in de- pletionregime, regime, which which is much is much lower lower than inthan previous in previous devices. devices. The gain The in gain gate in modulation gate modula- can be attributed to the presence of a TiO buffer layer that reduces the interaction between MoS tion can be attributed to the presence2 of a TiO2 buffer layer that reduces the interaction 2 and HgTe. between MoS2 and HgTe.

(a) (b) (c)

FigureFigure 14. 14.(a) Photodetector(a) Photodetector based based on MoS on MoS2 + HgTe;2 + HgTe; (b,c): (b responsivity,c): responsivity and anddetectivity detectivity values values over over the vis-IR the vis-IR spectral spectral range, range, respectively,respectively, for forthe the same same phototransistor. phototransistor. Reprinted Reprinted with with permission permission from from ref. ref. [35]. [35 ©]. 2017, © 2017, John John Wiley Wiley and and Sons. Sons.

A moreA more recent recent work work [54] [54 has] has explored explored the the deep deep UV UV spectral spectral region region reporting reporting a apho- photo- totransistortransistor based based on on a a thin MoSMoS22 filmfilm depositeddeposited onon a a flexible flexible PET PET substrate, substrate, sensitized sensitized with withZnO ZnO nanoparticles nanoparticles (Figure (Figure 15 a).15a). The The device devic showede showed a remarkable a remarkable responsivity responsivity of 2.7of 2.7 A/W A/Wto to a 254 a 254 nm nm UV UV light light source. source. Moreover, Moreover, the performancethe performance was stablewas stable under under multiple multiple bending bendingcycles. cycles. However, However, the response the response was slow,was slow reaching, reaching tensof tens seconds, of seconds, and the and bias the voltagebias voltagerequired required to obtain to obtain such such a high a high response response was was set to set 40 to V 40 (Figure V (Figure 15b). 15b). These These results results make makeclear clear that that more more efforts efforts have have to to be be undertaken undertaken to to produce produce more more efficient efficient photodetectors,photodetectors,especially especially in in the the deep deep UV UV light light region, region, to broadento broaden the the possible possible applications applications of MoS of MoS2 at low2 at lowphoton photon wavelengths. wavelengths.

2.3. MoS2 + Graphene Based Photodetectors The possibility to fabricate 2D structures of graphene and the astonishing mobility that it can reach (about 200,000 cm2/(V s) [55]) are some of the primary reasons for the interest in this material. However, its semimetallic behaviour, leading to a zero band gap energy, is a limiting factor in optoelectronic applications [56]. In fact, due to the absence of a forbidden band gap, large currents can be injected into the material. This leads to large dark current values, which limit the sensitivity of a photodetector. Thus, for NUV-vis-NIR application, graphene is often used as contact material [6], together with other ﬁnite band gap semiconductors. In this way it is possible to exploit the 2D versatility and high mobility of graphene by combining them with other semiconductor properties.

(a) (b)

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visible to mid infrared (2000 nm), with a fast response time in the order of milliseconds. Such high responsivity was achieved through a high photogain mechanism, originated from a large hole lifetime (τ = 4 ms) induced by the presence of QDs, which was six orders of magnitude larger than the transit time of the electrons (t = 9 ns). The detectivity measurements showed values of about 1012–1013 Jones over the same spectral range (Figure 14), also due to the low measured value of the dark current (≈10 pA). In this device, a toxic element (Hg) is still present, but the detector was operated at a gate bias of −15 V in depletion regime, which is much lower than in previous devices. The gain in gate modulation can be attributed to the presence of a TiO2 buffer layer that reduces the interaction between MoS2 and HgTe.

(a) (b) (c)

Figure 14. (a) Photodetector based on MoS2 + HgTe; (b,c): responsivity and detectivity values over the vis-IR spectral range, respectively, for the same phototransistor. Reprinted with permission from ref. [35]. © 2017, John Wiley and Sons.

A more recent work [54] has explored the deep UV spectral region reporting a phototransistor based on a thin MoS2 film deposited on a flexible PET substrate, sensitized with ZnO nanoparticles (Figure 15a). The device showed a remarkable responsivity of 2.7 A/W to a 254 nm UV light source. Moreover, the performance was stable under multiple bending cycles. However, the response was slow, reaching tens of seconds, and the bias voltage required to obtain such a high response was set to 40 V (Figure 15b). These results make clear that more efforts have to be undertaken to produce more efficient photodetec- Sensors 2021, 21, 2758 tors, especially in the deep UV light region, to broaden the possible applications of13 MoS of 222 at low photon wavelengths.

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Figure 15. (a) Scheme of a phototransistor based on a MoS2-ZnO structure and developed on a flexible PET substrate; (b) response of the phototransistor to an incident UV (254 nm) light over time. Reprinted with permission from ref. [54]. Copyright 2019, Creative Commons Attribution 3.0 Unported Licence.

energy, is a limiting factor in optoelectronic applications [56]. In fact, due to the absence of a( aforbidden) band gap, large currents can be injected(b) into the material. This leads to large dark current values, which limit the sensitivity of a photodetector. Thus, for NUV- Figure 15. (a) Scheme of a phototransistor based on a MoS2-ZnO structure and developed on a ﬂexible PET substrate; (b) response of the phototransistorvis-NIR to application, an incident UV graphene (254 nm) is light often over used time. as Reprinted contact material with permission [6], together from ref. with [54 ].other Copyright 2019, Creative Commonsfinite band Attribution gap semiconductors. 3.0 Unported Licence. In this way it is possible to exploit the 2D versatility and high mobility of graphene by combining them with other semiconductor properties. An exampleexample ofof a a phototransistor phototransistor based based on on MoS MoS2 with2 with graphene graphene is reported is reported by [ 31by]. [31]. The Thedevice device is a lateralis a lateral photodetector, photodetec wheretor, where few layersfew layers of MoS of MoS2 were2 were mechanically mechanically exfoliated exfoli- atedon a sheeton a sheet of graphene. of graphene. Two Two different different metal metal contacts contacts (Pd and(Pd and Ti) were Ti) were used used to collect to collect the thecharges charges (Figure (Figure 16). 16).

(a) (b)

Figure 16.16. ((aa)) SchemeScheme ofof aa phototransistorphototransistor basedbased onon aa graphene-MoSgraphene-MoS22 heterostructure. (b) Time-resolved responseresponse ofof thethe phototransistorphototransistor (Vds(Vds == 0 V). ReprintedReprinted with permission fromfrom ref.ref. [[31].31]. ©© 2015,2015, Elsevier.Elsevier.

The detector exhibited a notablenotable responsivityresponsivity (R(R == 33 A/W)A/W) with a fast response for a lateral phototransistor (t < 0.13 ms). These These results were obtained with zero voltagevoltage biasbias application, due to the asymmetric built-in electric field ﬁeld created at the graphene–electrode interface. In In addition, the the role of the graphene sheet is also to increase the velocityvelocity of thethe carrier transport to the metal contact, due to its very high mobility. Moreover,Moreover, the the ON/OFFON/OFF ratio reached a value of 103,, while while the dark current remained lower than 1 nA.

2.4. MoS + TMDs Based Photodetectors 2.4. MoS2 + TMDs Based Photodetectors Several studiesstudies havehave reportedreported photodetectors photodetectors based based on on heterostructures heterostructures composed composed by bydifferent different TMDs. TMDs. In theseIn these devices, devices, the the different different electronic electronic band band structure structure forms forms a built-ina built- in electric field at the interface between the materials that speeds up the photogenerated carrier separation. Moreover, the spectral range response of these photodetectors is widened since it involves different semiconductor materials. Chen et al. [36] proposed a photodiode based on a vertical van der Waals heterojunction between MoS2 and MoTe2, obtained through mechanical exfoliation (Figure 17a). With respect to MoS2, MoTe2 presents a smaller bandgap, which varies from

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electric ﬁeld at the interface between the materials that speeds up the photogenerated carrier separation. Moreover, the spectral range response of these photodetectors is widened since Sensors 2021, 21, x FOR PEER REVIEWit involves different semiconductor materials. 15 of 24 Chen et al. [36] proposed a photodiode based on a vertical van der Waals heterojunction between MoS2 and MoTe2, obtained through mechanical exfoliation (Figure 17a). With respect to MoS2, MoTe2 presents a smaller bandgap, which varies from 0.9 to 1.1 eV 0.9moving to 1.1 from eV moving bulk structure from bulk to monolayer. structure Theto monolayer. structure exploits The structure a type IIexploits band alignment, a type II band alignment, which favours the injection of electrons into MoS2 and the injection of which favours the injection of electrons into MoS2 and the injection of holes into MoTe2 holes(Figure into 17 b).MoTe2 (Figure 17b).

(a) (b) (c)

Figure 17. ((aa)) Scheme Scheme of of a a photodetector photodetector based based on on an an heterojunction heterojunction between MoS2 andand MoTe MoTe2..( (bb)) Type Type II II band band alignment alignment between MoTe2 and MoS2; (c) Photoswitching behaviour of the photodetector at V = 0 V. Reprinted with permission from between MoTe2 and MoS2;(c) Photoswitching behaviour of the photodetector at V = 0 V. Reprinted with permission from ref. [36]. [36]. © 2018, John Wiley and Sons.

TheThe device device dispalyed dispalyed a responsivity of 0.046 A/W to to a a 637 637 nm nm light, light, with with no voltage biasbias applied. applied. Moreover, Moreover, the the authors authors me measuredasured a a response response time time of of about about 60 60 μµs,s, due to the fastfast separation separation of of carriers carriers that that occurs occurs at at the the junction. junction. In In fact, fact, the the space space charge charge region region near near thethe junctionjunction isis characterized characterized by by a very a very strong strong built-in built-in electric electric field, which field, becomeswhich becomes stronger strongeras the length as the of length the junction of the isjunction scaled is down, scaled as down, it happens as it happens for vertical for herojunctions. vertical herojunc- The tions.strong The interlayer strong built-ininterlayer potential built-in provided potential also provided a barrier also for a thebarrier dark for current, the dark which current, was whichmeasured was tomeasured be about to 3 pA.be about 3 pA. AnotherAnother vertical vertical heterostructure heterostructure was was reported reported by byTan Tan et al. et [57], al. [ 57who], who exploited exploited a type a IItype alignment II alignment between between two monolayers two monolayers of MoS2 of and MoS WS2 2and, both WS obtained2, both obtainedby CVD, with by CVD, two graphenewith two contacts graphene (Figure contacts 18). (FigureThe detector 18). Theexhibited detector remarkable exhibited maximum remarkable photorespon- maximum sivityphotoresponsivity (R = 2340 A/W) (R to = 2340532 nm A/W) light. to In 532 particular, nm light. the In particular,photoresponsivity the photoresponsivity of the heterojunction-basedof the heterojunction-based device is at least device one order is at leastof magnitude one order higher of magnitude than the responsivity higher than ob- the tainedresponsivity with the obtained single TMDs with the(Figure single 18c). TMDs However, (Figure the 18 decayc). However, of the response the decay when of the lightresponse was whenswitched the lightoff lasted wasswitched more than off alasted few seconds. more than This a fewperformance seconds. This is attributable performance to anis attributableefficient charge to antransfer efficient at the charge junction transfer and to at trap the junctionstates for andthe photogenerated to trap states for carri- the 4 ersphotogenerated that lead to a photogain carriers that of leadabout to 3 a× photogain104. In fact, of the about measured 3 × 10 dark. In current fact, the was measured 10−6 A, −6 severaldark current orders was of 10magnitudeA, several larger orders than of the magnitude previously larger reported than thedetector previously [36], due reported to a muchdetector less [36 resistive], due to channel a much in less this resistive case. channel in this case. Long et al. [33] reported on a phototransistor based on stacked MoS2–Graphene–WSe2 layers, obtained by mechanical exfoliation (Figure 19a). This device was tested over a broad light spectrum (400–2400 nm). The detector exhibited a responsivity up to 104 A/W in the visible region, which decayed to 300 mA/W approaching the IR region (940 nm), with no gate bias applied (Figure 19c). The calculated specific detectivity was 1015 Jones in the visible region, while decayed to 1011 Jones in the IR region. Finally, the detector showed a time response to visible light of 54 µs, which indicates a fast response of the system, typical in a detector based on a vertical heterojunction.

(a) (b) (c)

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0.9 to 1.1 eV moving from bulk structure to monolayer. The structure exploits a type II band alignment, which favours the injection of electrons into MoS2 and the injection of holes into MoTe2 (Figure 17b).

(a) (b) (c)

Figure 17. (a) Scheme of a photodetector based on an heterojunction between MoS2 and MoTe2. (b) Type II band alignment between MoTe2 and MoS2; (c) Photoswitching behaviour of the photodetector at V = 0 V. Reprinted with permission from ref. [36]. © 2018, John Wiley and Sons.

The device dispalyed a responsivity of 0.046 A/W to a 637 nm light, with no voltage bias applied. Moreover, the authors measured a response time of about 60 μs, due to the fast separation of carriers that occurs at the junction. In fact, the space charge region near the junction is characterized by a very strong built-in electric field, which becomes stronger as the length of the junction is scaled down, as it happens for vertical herojunctions. The strong interlayer built-in potential provided also a barrier for the dark current, which was measured to be about 3 pA. Another vertical heterostructure was reported by Tan et al. [57], who exploited a type II alignment between two monolayers of MoS2 and WS2, both obtained by CVD, with two graphene contacts (Figure 18). The detector exhibited remarkable maximum photoresponsivity (R = 2340 A/W) to 532 nm light. In particular, the photoresponsivity of the heterojunction-based device is at least one order of magnitude higher than the responsivity obtained with the single TMDs (Figure 18c). However, the decay of the response when the light was switched off lasted more than a few seconds. This performance is attributable to an efficient charge transfer at the junction and to trap states for the photogenerated carriers that lead to a photogain of about 3 × 104. In fact, the measured dark current was 10−6 A, Sensors 2021, 21, 2758 15 of 22 several orders of magnitude larger than the previously reported detector [36], due to a much less resistive channel in this case.

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Figure 18. (a) Representation of a MoS2Graphene–WS2 heterostructure; (b) Band alignment at the heterojunction between MoS2 and WS2; (c) photoresponsivity values for different light power input of the WS2-Gr-MoS2 heterostructure (black dots), compared to the responsivity of the single TMDs. Reprinted with permission from ref. [57]. © 2017, John Wiley and Sons.

with no gate bias applied (Figure 19c). The calculated specific detectivity was 1015 Jones in (a) the visible region, while (decayedb) to 1011 Jones in the IR region.( cFinally,) the detector showed a time response to visible light of 54 μs, which indicates a fast response of the Figure 18. (a) Representation of a MoS Graphene–WS heterostructure; (b) Band alignment at the heterojunction between system, typical2 in a detector2 based on a vertical heterojunction. MoS2 and WS2;(c) photoresponsivity values for different light power input of the WS2-Gr-MoS2 heterostructure (black dots),

(a) (b) (c)

FigureFigure 19.19. ((a)a) SchemeScheme of of a a phototransistor phototransistor based based on on the the vertical vertical heterostructure heterostructure between between MoS MoS2, graphene2, graphene and and MoSe MoSe2. (b2). (Bandb) Band alignmentalignment at the at heterojunction the heterojunction between between TMDs TMDs and graphene; and graphene; (c) responsivity (c) responsivity and detectivity and detectivity values valuesplotted plottedagainst againstthe incident the incident wavelength wavelength for the phototransistor for the phototransistor over the over vis-IR the spectrum. vis-IR spectrum. Reprinted Reprinted with permission with permission from ref. from [33]. ref. © 2016, [33]. American Chemical Society. © 2016, American Chemical Society.

Wi etet al.al. [[30]30] presentedpresented a a vertical vertical photodiode photodiode optimized optimized for for UV UV radiations. radiations. The The structure structure is composed of a homojunction between a n-MoS2 layer and a p-MoS2 layer obtained is composed of a homojunction between a n-MoS2 layer and a p-MoS2 layer obtained sarting sarting from exfoliation (Figure 20). p-doped MoS2 was obtained by CHF3 plasma treat- from exfoliation (Figure 20). p-doped MoS2 was obtained by CHF3 plasma treatment, while 2 n-typement, while MoS2 n-typewas naturally MoS was achieved naturally through achieved the production through the process. production Graphene process. and goldGra- werephene then and used gold aswere contacts. then used The as authors contacts. obtained The authors EQE of obtained 80.7% in EQE the UVof 80.7% range in (300 the nm) UV andrange of (300 51.4 nm) % in and the visibleof 51.4 range% in the (532 visible nm). range (532 nm). Both the responsivity and the response time have been plotted in Figure 21 for several MoS2-based photodetectors, in order to compare their performance. Since, through the photoconductive gain, the responsivity is directly proportional to the lifetime τr [1,58], a reference line representing the points for which R/τr = 1 A/(W us) has also been included in the graph. As can be observed, only three devices are represented by a point above this line.

(a) (b) (c)

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Long et al. [33] reported on a phototransistor based on stacked MoS2–Graphene– WSe2 layers, obtained by mechanical exfoliation (Figure 19a). This device was tested over a broad light spectrum (400–2400 nm). The detector exhibited a responsivity up to 104 A/W in the visible region, which decayed to 300 mA/W approaching the IR region (940 nm), with no gate bias applied (Figure 19c). The calculated specific detectivity was 1015 Jones in the visible region, while decayed to 1011 Jones in the IR region. Finally, the detector showed a time response to visible light of 54 μs, which indicates a fast response of the system, typical in a detector based on a vertical heterojunction.

(a) (b) (c)

Figure 19. (a) Scheme of a phototransistor based on the vertical heterostructure between MoS2, graphene and MoSe2. (b) Band alignment at the heterojunction between TMDs and graphene; (c) responsivity and detectivity values plotted against the incident wavelength for the phototransistor over the vis-IR spectrum. Reprinted with permission from ref. [33]. © 2016, American Chemical Society.

Wi et al. [30] presented a vertical photodiode optimized for UV radiations. The struc- Sensors 2021, 21, x FOR PEER REVIEW 17 of 24 ture is composed of a homojunction between a n-MoS2 layer and a p-MoS2 layer obtained sarting from exfoliation (Figure 20). p-doped MoS2 was obtained by CHF3 plasma treatment, while n-type MoS2 was naturally achieved through the production process. Gra- Figure 20. (a): Scheme of aphene photodiode and gold based were on a then p–n junction used as of contacts. MoS2; (b )The energetic authors band obtained alignment EQE between of 80.7% Graphene/n- in the UV Sensors 2021, 21, 2758 16 of 22 MoS2/p-MoS2/Au composingrange the (300 device; nm) and(c): external of 51.4 quantum% in the visibleefficiency range (EQE) (532 with nm). respect to the incident wavelength for the device depicted on the left (red curve), compared with the EQE obtained with different plasma doping and different combinations of the electrode materials. Reprinted with permission from ref. [30]. © 2014, AIP Publishing.

Both the responsivity and the response time have been plotted in Figure 21 for several MoS2-based photodetectors, in order to compare their performance. Since, through the photoconductive gain, the responsivity is directly proportional to the lifetime τr [1,58], a reference line representing the points for which R/τr = 1 A/(W us) has also been included in the graph. As can be observed, only three devices are represented by a point above this line. In [33] the separation of the carriers is performed by the built in electric field at the vertical junction between the TMDs, while the transport is performed laterally by graphene, resulting in the best combination for fast carrier transport. Otherwise, devices where TMDs are responsible both for the separation of carriers and for the carrier (a) (b) (c) transport may suffer from a slower response. On the other hand, MoS2-based photodetectors sensitized with QDs or NPs described in [34,35], despite their slower response com- Figure 20. (a): Scheme of a photodiode based on a p–n junction of MoS2;(b) energetic band alignment between Graphene/n- MoS2/p-MoS2/Au composingpared the to device; heterojunction (c): external beased quantum structures, efﬁciency (EQE)reached with high respect performance to the incident due wavelength to an efficient for the device depicted on thecarrier left (red transfer curve), and compared a remarkable with the photogain. EQE obtained with different plasma doping and different combinations of the electrode materials. Reprinted with permission from ref. [30]. © 2014, AIP Publishing.

FigureFigure 21. Responsivity values values plotted plotted against against the the response time for various MoS2-based photodetectors. The The solid solid line line r representsrepresents the the points points for for which which R/ R/ττ r= =1 1A/ A/ (W (W μs).µs).

In [33] the separation of the carriers is performed by the built in electric ﬁeld at the vertical junction between the TMDs, while the transport is performed laterally by graphene, resulting in the best combination for fast carrier transport. Otherwise, devices where TMDs are responsible both for the separation of carriers and for the carrier transport may suffer from a slower response. On the other hand, MoS2-based photodetectors sensitized with QDs or NPs described in [34,35], despite their slower response compared to heterojunction Sensors 2021, 21, 2758 17 of 22

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beased structures, reached high performance due to an efﬁcient carrier transfer and a remarkable photogain.

2.5. MoS2 + Perovskites Based Photodetectors 2.5. MoS2 + Perovskites Based Photodetectors MoS2-based photodetectors often suffer from remarkable noise power density (NPD) MoS2-based photodetectors often suffer from remarkable noise power density (NPD) and slow responses due to the defects induced on the surface of the MoS2 during the syn- and slow responses due to the defects induced on the surface of the MoS2 during the thesis or from ambient gas absorption (e.g., H2O, O2) [24,59,60]. Passivation is one of the synthesis or from ambient gas absorption (e.g., H2O, O2)[24,59,60]. Passivation is one of procedures exploited to reduce gas absorption at the MoS2 surface and the introduction the procedures exploited to reduce gas absorption at the MoS2 surface and the introduction ofof halogens halogens have have been been reported reported to to be be effective effective on on metal metal calchogenides calchogenides [61]. [61]. He at al. [25] adopted a layer of methylamine (MA) halide (MA3Bi2Br9) to passivate a He et al. [25] adopted a layer of methylamine (MA) halide (MA3Bi2Br9) to passi- MoS2-based photoconductor. Their detector exhibited a high photoresponsivity (R = 112 vate a MoS2-based photoconductor. Their detector exhibited a high photoresponsivity A/W)(R = 112and A/W) a veryand fast a response very fast (t response = 0.3 ms).(t Moreover, = 0.3 ms). Moreover,the response the of response the device of theachieved device aachieved stability aover stability the time, over compared the time, compared to other passivation to other passivation techniques, techniques, as depicted as depicted in Figure in 22.Figure 22.

(a) (b)

FigureFigure 22. 22. (a(a) )Current Current measurements measurements over over time time for for a photoconductor a photoconductor based based on on MoS MoS2 and2 and passivated passivated with with MA MA3Bi23BrBi9;2 Br(b)9 ; 2 2 3 2 9 Responsivity(b) Responsivity measures measures against against irradiance irradiance for pristine for pristine MoS MoSand 2MoSand passivated MoS2 passivated with MA withBi MABr . 3ReprintedBi2Br9. Reprinted with permis- with sionpermission from ref. from [25]. ref. © 2018, [25]. ©American 2018, American Chemical Chemical Society. Society.

Recently,Recently, WangWang et al.at [28 ]al. have [28] introduced have aintroduced 2D halide perovskite, a 2D (Chalide6H5C2 Hperovskite,4NH3)2PbI4 (C((PEA)6H5C22HPbI4NH4),3) over2PbI4 a ((PEA) multilayer2PbI4), MoS over2 astructure. multilayer They MoS achieved2 structure. a darkThey currentachieved suppres- a dark −11 currentsion of suppression six orders of of magnitude six orders with of magnitude respect to thewith neat respect MoS 2todevice, the neat reaching MoS2 10device,A −11 reaching(Figure 23 10a). TheA (Figure reason 23a). for this The behaviour reason for is this found behaviour in the electron is found diffusion in the electron from MoS diffu-2 to sionthe perovskite:from MoS2 whento the theyperovskite: are put when in contact, they itare decreases put in contact, the charge it decreases carrier density. the charge The carrierdetectivity density. of the The device detectivity was improved of the device as well was and improved estimated as to bewell about and 10estimated13 Jones. More-to be aboutover, 10 the13 perovskiteJones. Moreover, layer actsthe perovskite as a defect layer passivator, acts as whicha defect resulted passivator, in a which faster responseresulted inof athe faster device, response that of decreased the device, by that more decrea thansed 100-fold by more with than respect 100-fold to the with neat respect MoS 2tocase, the neatand MoS showed2 case, a responseand showed speed a response of few milliseconds. speed of few In milliseconds. this work, the In responsivity this work, the of re- the sponsivitydetector was of the calculated detector over was a calculated broad spectral over rangea broad that spectr alsoal included range that the also UV spectrumincluded the(200–1100 UV spectrum nm) and (200–1100 reached nm) maximum and reached values ma inximum the visible values range in the (Figure visible 23 rangeb). (Figure 23b). Furthermore the device was found to operate with a zero applied external bias, due to a built-in electric ﬁeld at the interface between the p-type region of the (PEA)2PbI4 and the n-type region of MoS2, and thus to perform as a photodiode. The response speed measured with zero voltage bias was less than one order of magnitude larger than in the case of the positive bias voltage applied, and reached few tens of milliseconds. Many other MoS2-based devices have been reported in the literature; the performance of a few other devices ([62–75]), together with the photodetectors we have described so far, are reported in the graphs of Figures 10 and 21.

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(a) (b)

FigureFigure 23.23.( (aa)) DarkDark currentcurrent comparison comparison between between pristine pristine MoS MoS2 2and and MoS MoS22passivated passivated with with (PEA) (PEA)2PbI2PbI44;(; (bb)) ResponsivityResponsivity ofof thethe devicedevice with with respect respect to to the the incident incident wavelength wavelength for for a a UV-vis-NIR UV-vis-NIR spectrum. spectrum. Reprinted Reprinted with with permission permission from from ref. ref. [28 [28].]. © 2020,© 2020, John John Wiley Wiley and and Sons. Sons.

3. ConclusionsFurthermore the device was found to operate with a zero applied external bias, due to a built-in electric field at the interface between the p-type region of the (PEA)2PbI4 and Many photodetectors based on MoS2 have been developed in the last decade, and thethe mostn-type representative region of MoS have2, and been thus analyzed to perform in thisas a review.photodiode. The main The response characteristics speed ofmeas- the devicesured with discussed zero voltage so far bias are summarizedwas less than in one Table order1. of magnitude larger than in the case of the positive bias voltage applied, and reached few tens of milliseconds. Table 1. MoS2-based photodetectorMany performance. other FLMoS2-based = few layers, devices ML = multilayer, have been TF = thinreported film, tr =in rise thetime, literature; td = decay the time. performance of a few other devices ([62–75]), together with the photodetectors we have de- Photoactive MoS2 Wavelength Responsivity Response Detectivity Dark Technology Ref. Material scribed soLayers far, are reported(nm) in the graphs(A/W) of FiguresTime (s)10 and 21.(Jones) Current (A)

MoS2 phototransistor 1L 647 1000 13 [27] 3. Conclusions −5 MSM tr 7 × 10 10 MoS2 FL 532 0.57 −5 10 [50] photodiode Many photodetectors based on MoS2 havetd been11 × 10 developed in the last decade, and the MSMmost representative have been analyzed in this review. The main characteristics of the MoS 1L ≈400 10−3 0.5 × 10−3 <10−14 [51] 2 photodiodedevices discussed so far are summarized in Table 1. 2 −12 MoS2 phototransistor 1L 561 880 9 2 × 10 [24]

MoS Table 1. MoS2-based photodetector performance. FL = few layers, ML = multilayer, −TF3 = thin film, tr = rise time, td = decay MoS2 phototransistor 1L 640 50 10 [29] time. 10 11 −11 MoS2 phototransistor ML 633 0.12 10 –10 10 [23] −2 tr 3.7 × 10 Dark MoSPhotoactiv2 photodiodeTechnolog MoS2 FL Wavelengt 365Responsivi 9.3 Response −Detectivity2 [62] td 3.9 × 10 Current Ref e Material y Layers h (nm) ty (A/W) Time (s) (Jones) MSM t 4 × 10−5 MoS 3L 532 1.04 r (A) [68] 2 photodiode × −5 phototransi td 5 10 MoS2 1L 647 1000 −3 13 −6 [27] MoS2 + CQD photoconductorstor FL 365 8.4 × 10 0.57 ≈10 [52] 554 1.79 × 10−5 11 [53] MSM r −5 t 7 × 10 10 MoS2 FL 532 0.57 −6 10 [50] MoS2 + ZnS photoconductor 3L 365 9.50 × 10 22 [53] photodiode td 11 × 10−5 − MSM 780 4.52 × 10 6 31 [53] −3 −3 −14 2 MoS2 1L ≈400 10 5 0.5 × 10 14 <10 [51] MoS2 + CdSe phototransistor 2L 405 2.50 × 10 0.06 1.24 × 10 [34]

+ QDs/NPs photodiode 2 5 × 14 × −7 MoS MoS2 + PbS phototransistorphototransi FL 980 10 0.35 7 10 2.6 10 [32] MoS2 1L 561 880 9 2 × 10−12 [24] MoS stor 635 106 10−11 [35] MoS2 + HgTe phototransistor FL phototransi 2000 105 1012 [35] MoS2 1L 640 50 10−3 [29] MoS2 + ZnO phototransistorstor TF 254 2.7 55 [54] phototransi MoS2 ML 633 0.12 1010 - 1011 10−11 [23] stor

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Table 1. Cont.

Photoactive MoS2 Wavelength Responsivity Response Detectivity Dark Technology Ref. Material Layers (nm) (A/W) Time (s) (Jones) Current (A) −4 −10 MoS2 + Gr phototransistor FL 632.8 3 <1.3 × 10 9 × 10 [31] −2 −5 8 −12 MoS2 + MoTe2 photodiode FL 637 4.60 × 10 6 × 10 1.06 × 10 3 × 10 [36] 11 −6 MoS2 + WS2 phototransistor 1L 532 2340 4.1 × 10 10 [57] −5 15 MoS + Gr + 532 4250 5.4 × 10 10 [33] 2 phototransistor 1L/FL WSe2 940 0.3 1011 [33] Heterostructures

2 −4 12 −9 MoS2 + MA photoconductor 15L 530 112 3 × 10 3.8 × 10 4 × 10 [25] −

MoS 3 MoS2 + tr 6 × 10 13 −11 photoconductor ML 637 1.68 −3 1.06 × 10 10 [28] (PEA)2PbI4 tr 4 × 10

Photoconductors based on MoS2 are the simplest structure of photodetectors and they are able to reach very high responsivity values. On the contrary they generally suffer from a slow response, due to absence of a built-in electric ﬁeld, and from high dark currents that limit the sensitivity of the device. Phototransistors based on MoS2 represent a more mature architecture for detecting light, due to the possibility to suppress dark current signals with an appropriate voltage bias applied at the gate terminal. Although so far very few works report the response of the MoS2 device to the UV and IR spectral regions, they clearly show that there is room for improvement if this material is coupled with other TMDs, graphene or quantum dots. These properties, combined with the thin structures of MoS2 that can be fabricated, allow us to build wide-spectrum bendable detectors. In this direction, devices based on MoS2 sensitized with NPs or QDs are able to boost the response of the detectors both in the NIR-MIR and in the NUV spectrum, and are often characterized by very high responsivity values. However, the defects induced on MoS2 in con- sequence of the introduction of sensitizing centres, cause large dark current and slow response. Heterostructures between MoS2 ond other materials (e.g., TMDs and graphene) are able to extend the response to a wider spectral range as well, and have been demonstrated to be suitable for the fabrication of fast photodiodes, exploiting the built-in electric ﬁeld that forms at the heterointerface. Moreover, these devices are able to work also at zero-bias mode, without requiring an external power supply. The main drawback for photodiodes is instead represented by the lower responsivity that they can reach, due to the absence of the photogain mechanism. In heterostructures based on MoS2, the second material can act as a passivation layer, preventing the adsorption of oxygen and water molecules on the MoS2 surface. In this way halide perovskites have been demonstrated to be a good solution, leading to a stable and faster response of the devices.

Author Contributions: Conceptualization, A.T., S.D. and L.P.; Methodology, A.T. and L.P.; Validation, A.T., S.D., A.Q. and L.P.; Resources, S.D., L.P. and A.Q.; Data Curation, A.T.; Writing—Original Draft Preparation, A.T.; Writing—Review & Editing, A.T., S.D., L.P. and A.Q.; Visualization, A.T.; Supervision, S.D., L.P. and A.Q.; Project Administration, L.P.; Funding Acquisition, S.D., L.P. and A.Q. All authors have read and agreed to the published version of the manuscript. Funding: This work was performed in the frame of the program Departments of Excellence 2018-2022 (DII-UNITN)—Italian Ministry of University and Research (MIUR). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request. Conﬂicts of Interest: The authors declare no conﬂict of interest. Sensors 2021, 21, 2758 20 of 22

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